Recent Advances in Indium-Promoted Organic Reactions

REVIEW 1739

Recent Advances in Indium-Promoted Organic Reactions Indium-PromotedJacques Organic Reactions Augé,* Nadège Lubin-Germain, Jacques Uziel Département de Chimie, UMR CNRS-ESCOM-Université de Cergy-Pontoise, Neuville-sur-Oise, 95031 Cergy-Pontoise, France Fax +33(1)34257071; E-mail: [email protected] Received 28 February 2007; revised 29 March 2007

2 The Oxidation States of Indium Abstract: This review deals with organic reactions which are promoted by indium metal or indium salts, with a focus on recent advances in stoichiometric and catalytic pathways. Applications to What makes indium metal so attractive is its low first ion- transmetalations, cross-coupling reactions and carboindation, in ization potential, along with its inertness towards water which an organoindium species may be postulated, are highlighted, and its lack of toxicity.2 Table 1 gives the ionization po- as well as the reactions in which a radical is the key intermediate. tential values of some common metals.4 It turns out that Special attention is placed on the role of indium metal as a reducer, indium has a first ionization potential almost as low as that and on the Lewis acidity of indium salts in catalytic processes. of the alkali metals, much lower than that of magnesium, 1 Introduction tin, or zinc. Moreover, the advantage of indium, compared 2 The Oxidation States of Indium to aluminium, lies in its low propensity to form oxides in 3 Barbier-Type Additions air. Indium powder, when placed in a Schlenk tube for 4 Carbonyl Alkynylation 5 Carbonyl Additions via Transmetalation half an hour under gentle stirring and vacuum, gives satis- 6 Carboindation of Alkynes factory results in terms of reactivity, so that further activa- 7 Cross-Coupling Reactions tion is often unnecessary. A new protocol that uses 8 Nucleophilic Substitutions granular indium metal as a cheaper form of indium was re- 9 Radical Reactions cently introduced, but mild warming is then required.17 10 Hydroindation 11 Reduction of Functional Groups Table 1 First Ionization Potential of Some Metals 12 Activation of Silicon by Indium(III) Salts 13 Indium(III)-Catalyzed Reactions Metal Potential (eV) 14 Conclusion Lithium 5.39 Key words: indium metal, indium salts, reduction, metalation, Lewis acid Sodium 5.12

Magnesium 7.65 1 Introduction Aluminum 5.98 Indium 5.79 Indium-mediated organometallic reactions have elicited considerable attention since the discovery of the remark- Tin 7.43 1 2 able reactivity of this metal in organic or aqueous media. Zinc 9.39 Several authors have highlighted the special interest in using indium metal in aqueous media as it is not affected by water or by air.3–7 Other aspects of indium(0) or indi- Indium(I) halides are insoluble in common solvents; how- um(III) chemistry were recently reviewed.8–11 Organome- ever, they rapidly disproportionate in the presence of wa- tallic species may be postulated when either indium(0) or ter:18,19 indium(I) is inserted into a carbon–halogen bond,12 but in → 3 InX 2 In + InX3 many cases, indium(0), acting as a reducing agent, may induce a radical intermediate.13 Indium(III) in the pres- The species InX (X = Br, I) and RIn (R = alkyl), original- ence of various hydrides may give rise to a powerful re- ly formed by oxidation of indium in the presence of RX in ducing agent, thereby allowing for new reactivities.14,15 the dark, may again insert into the carbon–halogen bond of RX to give a mixture of organoindium species in which Indium(III) salts are water-tolerant additives and this 20 property has resulted in an increase in the interest of using indium is in the 3+ oxidation state: such Lewis acids in catalytic processes.16 This review fo- RX + 2 In → RIn + InX cuses on the recent advances of indium-promoted organic → RX + RIn R2InX reactions, whatever the oxidation state of indium. → RX + InX RInX2 → By summation, 3 RX + 2 In R2InX + RInX2. SYNTHESIS 2007, No. 12, pp 1739–1764xx.xx.2007 The aggregation of R2InX and RInX2 gives an indium(III) Advanced online publication: 08.06.2007 sesquihalide; such a species was postulated as the reactive DOI: 10.1055/s-2007-983703; Art ID: E17807SS © Georg Thieme Verlag Stuttgart · New York intermediate in the indium(0)-mediated allylation of car- 1740 J. Augé et al. REVIEW bonyl compounds.1 When indium(I) iodide is used as the dimethylformamide, might result from the formation of a promoter in tetrahydrofuran, allylindium(III) diiodide, mixture of allylindium(I) and allylindium(III) dibromide. arising from oxidative addition, is presumably the reactive According to this hypothesis, two successive reactions species, as evidenced by the 1H NMR signal at 2.14 might occur in N,N-dimethylformamide. The fact that ppm.21 Such an oxidative addition of an indium(I) salt in only the first reaction is observed in water is probably a tetrahydrofuran solution was evidenced by a single- consequence of the rapid disproportionation of indium(I) crystal analysis of In(C6F5)Br2·2THF obtained from bro- halide in water. 22 mopentafluorobenzene and indium(I) bromide. RX + 2 In → RIn + InX Chan and Yang have carefully investigated the reaction of RX + InX → RInX indium(0) with allyl bromide.12 The 1H NMR signal at 1.7 2 → ppm observed in water was attributed to an allylindium(I) By summation, 2 RX + 2 In RIn + RInX2 species, since the same signal was observed after trans- Substrates other than alkyl halides may insert indium(I) metalation of diallylmercury with either indium or indi- halides under non-aqueous conditions. Thus, oxidative um(I) iodide. Moreover, the authors suggested that the addition of InX may occur with disulfides or diselenides, two signals observed at 1.7 and 2.15 ppm, when they per- which produce sulfides or selenides as nucleophiles in formed the reaction of allyl bromide with indium in N,N- various reactions.23

Biographical Sketches

Jacques Augé graduated from the post-doctoral work with Professor Lubineau. He was promoted to Pro- Ecole Nationale Supérieure de Chim- George Whitesides at Massachusetts fessor of Organic Chemistry in 1993 ie de Paris, and received his PhD Institute of Technology in Cam- at the Université de Cergy-Pontoise. from the Université de Paris-Sud bridge, Massachusetts, he returned His current interests focus on carbo- (Orsay) in 1978, under the supervi- back to Orsay to develop aqueous or- hydrate chemistry, reaction medium sion of Professor Serge David. After ganic chemistry with Professor André effects and indium chemistry.

Nadège Lubin-Germain studied or- in aqueous media and 2-keto-3- versité de Cergy-Pontoise in 1994 ganic chemistry at the Université de deoxyoctulosonic acid (KDO) syn- and joined the group of Professor J. Paris-Sud (Orsay). In 1992, she ob- thesis. After a postdoctoral fellow- Augé. Her research interests current- tained her PhD under the supervision ship with Professor C. Wandrey and ly deal with organometallic chemis- of Professor André Lubineau where Dr. Claudine Augé at the Forschung- try applied to carbohydrates. she developed projects on chemistry zentrum, Jülich, she moved to Uni-

Jacques Uziel was born in Athens, sité de Cergy-Pontoise, initially in peptide synthesis in the group of Pro- Greece, in 1964. He obtained his PhD the group of Professor Sylvain Jugé, fessor Anna Maria Papini. His cur- in organic chemistry at the Université where he worked on stereogenic rent research activity deals with C- Pierre et Marie Curie (Paris VI) un- phosphorus chemistry and function- glycoside synthesis by organometal- der the supervision of Professor Jean- alized amino acid synthesis. In 2000, lic catalysis. Pierre Genêt. Since 1992, he has been he spent a year at the University of Maître de Conférences at the Univer- Florence working on solid-phase

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York

REVIEW Indium-Promoted Organic Reactions 1741

HF C

2 O Both indium(I) iodide and indium(III) iodide are commer- H

cially available, though in some cases it has proven better 2 HO SiR

O In

InO

Br 2 83–97% to prepare them. To accomplish this, indium(III) iodide, +

Me SiO

HF C SiR

HF C SiR 3 2 3 3 which is very hydroscopic, is prepared as a fine yellow 2

Brook powder, from indium and iodine in xylene at reflux. Indi- 1

HF C

2 um(I) iodide is obtained by addition of indium(III) iodide rearrangement into a suspension of indium powder in xylene; the stable Scheme 2 Indium-mediated allylation of difluoroacetyltrialkyl- complex In(InI4) thus obtained is broken up by addition of silanes diethyl ether to form a mixture of insoluble indium(I) iodide and soluble indium(III) iodide.24 dihydrofurans after their indium(III)-catalyzed hydrolysis into lactols.29 Compared to other metals, indium was particularly efficient in the promotion of the allylation of 3 Barbier-Type Additions fluoral methyl hemiacetal, leading to a-trifluoromethylat- ed homoallylic alcohols in water or N,N-dimethylform- 3.1 Carbonyl Allylation amide.30 The use of indium metal as a reducing agent in Barbier- With a,b-unsaturated carbonyl compounds, the indium- type carbonyl additions was first introduced by Araki and mediated allylation generally leads to the [1,2]-addition Butsugan for the allylation of carbonyl compounds in product, but in some cases the homoallylic indium alkox- 1 N,N-dimethylformamide. Considerable progress was ide intermediates undergo deoxygenative carbon–carbon made by Li and Chan, who reported the first carbonyl al- bond formation to provide cyclopropyl or a,a-diallyl de- lylation mediated by indium in water without any addi- rivatives.31 If an external nucleophile, such as a heterocy- 2 tives or special activation. The aqueous indium-mediated clic enamine, is present, then it is prone to attack the carbonyl allylation is now a powerful tool which is used 25 electrophilic intermediate to bring about the formation of with great efficiency in total synthesis (Scheme 1). a new carbon–carbon bond. Thus, indole-3-carboxalde-

hyde (3) was able to provide symmetrical bisindolyl al-

COCF kane and was also allowed to react with other heterocyclic 3

Ph N

O enamines, such as pyrrole, pyrazole, imidazole and 2-ami-

COCF nouracil. The mechanism of the reaction (Scheme 3) was

MeO C CO Me

In 2 2

CO Me

Ph N

+ O recently investigated, with the results indicating that al- HO

THF–H lylindation of 3 was followed by dehydration and nucleo- 2

MeO C

(1:1) philic addition of the heterocycle, or of another

O 32 a single isomer was obtained

72% allylindium reagent. O

Scheme 1 Key step in the total synthesis of dysiherbaine

CHO OH

In It has been shown that in polar organic solvents, the sto- allyl-Br

ichiometric ratio (indium:allyl bromide:ketone) of the re-

O THF–H N N N

action was 2:3:2.1 In water, the stoichiometric ratio is 2

H H H

3 Het-H 1:1:1, owing to the likely existence of allylindium as a allyl-In transient, but discrete, intermediate.12 However, a parallel process involving the metal surface could not be com- Het

pletely ruled out. Decreases of the amount of metallic in-

dium down to 0.1 and even 0.01 equivalent was made 42%

possible in tetrahydrofuran by the addition of manganese N

H and trimethylsilyl chloride as reducing and oxophile H 26 agents. Albeit providing lower yields, the reaction was Scheme 3 Allylation of indole-3-carboxaldehyde also effective when manganese was replaced by tetrakis(dimethylamino)ethylene, which evidences the regen- 27 The allylation reaction with g-substituted allyl bromides eration of indium(0) by this powerful reducer. is g-selective except when the g-substituent is too bulky. Whereas the allylation of the fluorinated acylsilanes 1 Even with cyclic allylic bromides, such as 4-bromo-2- with Grignard reagents was followed by Brook rearrange- enopyranosides, a total g-regioselectivity was observed in ment and consecutive defluorination, such side-reactions the aqueous indium-promoted allylation of aldehydes were avoided in the aqueous indium-mediated synthesis leading to C-branched sugars and C-disaccharides.33 28 of the fluorinated alcohols 2 (Scheme 2). The reaction generally gave moderate anti diastereoselec- Hemiacetals, as masked aldehydes, have been shown to tivity. High diastereoselectivities were observed, howev- undergo indium-mediated allylation; this was recently ex- er, in the indium-mediated allylation of aldehydes with ploited in the aqueous allylation of unprotected sugars or the chloroallylic sulfones 4 (Scheme 4). The g-substitu-

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York

1742 J. Augé et al. REVIEW ents played a pivotal role in the diastereoselection. With a OH

methyl substituent, the six-membered chair-like transition OH

state led to the homoallylic alcohol anti-5, whereas a phe- R 2

nyl substituent induced a twist-boat transition state and R

-adduct 7 6 γ-adduct α

led to syn-5.34

CHO R

– R CHO

1 1 1

1 R R R

OH OH R

R PhCHO +

– H

PhO S + H

2 OH

O In – H O O O + H O

O 2 Ph + Ph 2

H O–THF

2 R SO2Ph R SO2Ph

1 1 1

R R R R OH

4 yield anti- syn-

Cl 2

2 2 2

R R R = Me98% 13: 1 R R = Ph45% 1: 13 Scheme 5 Rearrangement of g-adducts into a-adducts Scheme 4 Diastereoselective allylation with chloroallylic sulfones diols in good yields through Grignard or Barbier-type pro- Loh et al. have shown that the g-adduct was produced un- tocols; saturated aldehydes afford anti adducts, whereas der kinetic control, whereas the a-adduct could be ob- the conjugated aldehydes preferentially lead to the syn ad- 35 tained after equilibration. They have developed a highly ducts.37 The indium-mediated coupling of 3-bromo-1- a-regioselective preparation of E-configured homoallylic acetoxypropene (8) with aldehydes was advantageously alcohols from g-substituted allylic bromides in the pres- applied in a straightforward stereoselective synthesis of ence of indium in a small amount of water. Table 2 shows 1,4-dideoxy-1,4-L-iminoribitol (Scheme 6).38 With 3-bro- the vital role of the solvent on the regioselectivity of the mopropenyl methylcarbonate, the g-regioselectivity is reaction. Whereas g-regioselectivity was observed in clas- also observed, and the indium alkoxide is prone to cyclize, sical aqueous or non-aqueous conditions, a total a-regio- thereby affording cis and trans 4,5-disubstituted 5-vinyl- selectivity appeared when 6 equivalents of water were 1,3-dioxolan-2-ones in high yields; the cis/trans ratio is used as the solvent; when the reaction was carried out with controlled by the nature of the carbonyl compound.39 12 equivalents of water, no a-adduct was detected, even

after the reaction was stirred for up to 72 hours. To illus-

Boc

trate the importance of the activity of water, Loh and co- N

O Boc O

N H workers proposed that an in situ hemiacetalization be- OH

O Me tween the aldehyde and the g-adduct 6 was immediately Br

CO K

2 3 O

followed by a 2-oxonia[3,3]-sigmatropic rearrangement 8 In, THF MeOH–H O 2

80% with the removal of a molecule of water, which might be OH an indication that the activity of water should not be too Scheme 6 Indium-mediated allylation with 3-bromopropenyl ace- high; in contrast, a molecule of water is required in the tate second stage of the mechanism to bring about the a-ad- 36 duct 7 (Scheme 5). A salient feature in the indium-mediated allylation deals with the facial selectivity towards the aldehyde. The reac- Table 2 Regioselectivity of the Indium-Mediated Allylation of tions with a-hydroxy aldehydes and b-hydroxy aldehydes Cyclohexanecarboxaldehyde with Crotyl Bromide are respectively syn and anti stereoselective as a conse- Solvent Amount Time (h) Yield (%) Ratio g/a quence of chelation control even in water.40 (equiv) Such a chelation was also observed with a-aminoalde- DMF 6 72 65 100:0 hydes, as recently described in the synthesis of castanospermine41 and anti-SARS agents.42 In the latter EtOH67297100:0case, the high diastereoselectivity was interpreted by a second chelation between indium and the nitrogen atom of H2O 127290100:0 the isoxazole group of 9 (Scheme 7).42 H O 6 24 85 1:99 2 Can one exploit the efficiency of indium to coordinate,

DMF–H2O 6:6 72 80 100:0 even in aqueous conditions, oxygen and nitrogen in order to induce enantioselection? Indeed, such an enantioselec- THF–H O 6:6 72 95 100:0 2 tive allylation was achieved in the presence of (–)-cin-

CH2Cl2–H2O 6:6 24 68 1:99 chonidine in the synthesis of a key precursor of antillatoxin.43 More recently, a chiral ligand bearing vicinal hydroxyl and amino groups (10) was found to be effi- The allylation reaction of aldehydes with 3-bromo-1-ace- cient in the indium-mediated allylation of both aromatic toxypropene in the presence of indium in tetrahydrofuran and aliphatic aldehydes (Scheme 8).44 is g-regioselective, affording monoprotected 1-ene-3,4-

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York

REVIEW Indium-Promoted Organic Reactions 1743

O O

O tive species, whereas a mixture of allenylindium(I) and al- N

N N

CO Me 2 lenylindium(III) dibromide (15) was formed in

Br O

O O tetrahydrofuran, with the first species being the more re-

+ active towards an aldehyde. With methyl-substituted pro-

H pargyl bromide, the allenylindium/propargylindium

O EtOH–H 2

4:1 CO Me

OH equilibrium was shifted towards the propargyl species; in O

2 OH CO Me 2

66% tetrahydrofuran, 16 reacted with indium to give propar- syn/anti = 98:2

strongly gylindium(I) (17) and propargylindium(III) dibromide favored

unfavored (18), whereas in aqueous media, propargylindium(I) is the

O first detected species (Scheme 10).

NH N H

O E D O O 2

In In In

+ In

H H O O

E = CO Me

+ 13 THF

Br In In 14 15

Scheme 7 Diastereoselective allylation of a-amino aldehydes Br

D O Me 2

Me + In In

In (1 equiv), pyridine (1 equiv)

16 +

THF

OH –78 °C, 1.5 h, THF–hexane O

In Br

17 Br

+ Br

R H HO NH 2

1 equiv

0.5 equiv Scheme 10 Influence of solvent and methyl substitution on the pro- yield 90–99%

ee up to 93% pargyl–allenylindium system

1 equiv 10 These observations could explain why the homopropar- Scheme 8 Enantioselective allylation of aldehydes gylic alcohols are the major products formed from unsub- stituted propargylic bromide, whereas substituted propargylic bromides preferentially give allenyl alcohols. With the a,b-unsaturated aldehydes 11, indium-mediated In tetrahydrofuran propargylation of aldehydes with un- allylation could be followed by an intramolecular cyclo- substituted propargyl bromide is totally regioselective; propanation; when the reaction was conducted in the pres- enantioselectivities up to 85% were obtained in the presence of (S)-methyl mandelate, enantioenriched products ence of a stoichiometric amount of (–)-cinchonidine.47 were obtained.45 This methodology was applied to the synthesis of enantioenriched 1-styrylnorcarene via com- Whereas the reaction was slightly regioselective in favor pound 12 (Scheme 9). of the allenic compound with trimethylsilyl propargyl

bromide (19) in water, excellent regioselectivities in favor

I of the thermodynamically more stable homopropargylic

H O

In alcohols 20 were obtained when the reactions were carried In, THF

O out at reflux in tetrahydrofuran in the presence of 2 equiv-

Ph R

Ph alents of indium and 0.1 equivalent of indium(III) bro- R = (E)-styryl

R 48

11 mide or indium(III) fluoride. As a hypothesis, a possible CO Me

2 chelation of the silicon with the halide of indium can shift

the reaction towards the formation of the homopropargyl- Ph 1. LiI

O 2. Et ic alcohol 20. Such a chelation was rendered more diffi- 2

O Ph 3. H

metathesis

3 cult with bulky silanes in aqueous media, so that reverse

H regioselectivity in favour of the allenyl alcohol 21 was R 86%

1-styrylnorcarene then obtained with the bulky trialkylsilyl propargylic bro- ee 88%

12 H mide 22 (Scheme 11).

Scheme 9 Enantioselective homoallyl-cyclopropanation of diben-

C H

zylideneacetone 8 SiR Br 3

C H CHO OH 8 17 +

C H

8 17

3.2 Carbonyl Propargylation and Allenylation 3

R Si

19 (R = Me) 20 21

i-Pr) (R = The transient organoindium intermediates formed in the 22

20:21)

reaction of propargyl bromide (13) with indium in water yield (

2 equiv In, 0.1 equiv InBr , THF, reflux

95% (99:1)

1 R = Me 3

and in tetrahydrofuran were investigated by H NMR and 2

, THF, reflux 2 equiv In, 0.1 equiv InF 93% (99:12 R = Me

13C NMR spectroscopy,46 and a strong analogy was found 3 O 1:5, r.t. 2 equiv In, THF–H 52% (5:95) i-Pr R = 2 to the results obtained previously with allyl bromide.12 In 2 water, allenylindium(I) (14) was proposed to be the reac- Scheme 11 Synthesis of homopropargylic and allenic alcohols

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York 1744 J. Augé et al. REVIEW

Indium-mediated propargylation of aldehydes with 1,4- in dry toluene under ultrasonication (Scheme 13).55 It was dibromobut-2-yne (23) in aqueous media gave good suggested from NMR measurements that a low-valent yields of the 1,3-butadien-2-ylmethanols 24 indium species was formed in toluene. (Scheme 12).49 A novel diindium intermediate (25) was

1 O O

evidenced in tetrahydrofuran by H NMR spectroscopy HO

and electrospray ionization mass spectrometry; this re- In powder

EtO OEt

+ R markable, stable compound could be stored in tetrahydro- THF, reflux R

88% :syn = 93:7 furan solution and gave pure anti acetylenic 1,6-diols 26 anti

with various aldehydes in the presence of zinc fluoride; 27 28

O Ph the stereoselection was rationalized by a chelation of the HO

+ EtO InCl

Ph allenylindium with the oxygen of the transient indium al- Ph

50 OEt

Oi-Pr coholate. toluene, ((( O

Oi-Pr

99%

31 syn:anti = 98:2

Br In 2

In, RCHO

O HO Ph

InCl

Ph R

H O + 2

OEt

COOMe Br InO toluene, ((( 23 2

30 COOMe

85%

In, THF

R syn:anti = 98:2

OH Scheme 13 Diastereoselective Reformatsky reactions mediated by

RCHO indium and indium(I)

OInBr

Br In InBr

R 2 2 In anti

assigned by

26 25 3.4 Additions to Carbon–Nitrogen Multiple

NMR and MS Bonds Scheme 12 Propargylation and allenylation with dibromobutyne Owing to the ease of hydrolysis of imines under aqueous conditions, the indium-mediated allylation of imines was In the indium-mediated gem-difluoropropargylation of al- first tested under anhydrous conditions.10 It was found, dehydes, an innovation was recently introduced with the however, that pyridine-2-imines and quinoline-2-imines, combined use of zinc (0.9 equiv) and indium (0.1 equiv) as water-stable activated imines, could undergo indium- 51 in the presence of iodine (0.1 equiv). Although the re- mediated Barbier allylation in aqueous media to provide sulting gem-difluorohomopropargyl alcohols were ob- homoallylic amines in very good yields.56 For hydrolyz- tained in moderate yields, the reaction was highly able imines, it was possible to use anhydrous alcohols as regioselective, as the corresponding fluoroallenyl alco- solvent; thus, chiral aldimines derived from (R)-phenyl- 52 hols were not obtained. glycinol were diastereoselectively allylated in anhydrous methanol.57 With glyceraldimine 32 prepared from the 3.3 Aldol-Type Reactions chiral pool substance (R)-2,3-O-isopropylideneglyceral- dehyde, allylation in N,N-dimethylformamide gave rise to Reformatsky reactions may be considered as aldol-type high diastereoselectivity,58 which was believed to arise reactions in which the metal enolate is formed by the in- through an indium-chelated six-membered transition state sertion of a metal into the carbon–halogen bond of a-ha- (Scheme 14). loesters or a-halonitriles. In the presence of indium metal, The indium-mediated allylation of imines in ionic liquids these derivatives were allowed to react in organic solvents 59 with various carbonyl compounds.10,11 was also investigated. Selective formation of monoally- lated amines was achieved by adjusting the amount of Indium(I) bromide has also been shown to promote a Re- bromide anion in the ionic liquid. The addition of indium formatsky-type reaction; thus a,a-dichloroketones gave to allyl bromide led to allylindium(I) and allylindium(III) the corresponding 2-chloro-3-hydroxypropan-1-one de-

rivatives upon addition to carbonyl compounds.53

Bn Bn

The diastereoselectivity of the indium Reformatsky-type HN

F C Br

reaction of ketones with the a-bromo substituted esters 27 3 H

O O

was carefully investigated (Scheme 13). The reaction led DMF, r.t., 12 h O O CF

to the anti isomers 28 arising from a cyclic chair-like tran- 3

sition state, either with indium metal or indium(I) bro- 61% (>95% de)

mide, the former being advantageous in terms of O

In O

54 O

reactivity. The diastereoselectivity with respect to ke- O N

tones was studied with the a-alkoxy ketone 29 and the b- N

F C H

3 3 keto ester 30, which were exposed to the a-iodo ester 31. H

Excellent syn selectivities arising from a chelated transi- Bn tion state were obtained when indium(I) chloride was used Scheme 14 Diastereoselective allylation of glyceraldimine

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York REVIEW Indium-Promoted Organic Reactions 1745 dibromide, the first species being more reactive than the alkoxide intermediate; the formation of 39 from the aro- second. The dynamic equilibrium between these two spe- matic or non-enolizable aldehydes proceeded smoothly in cies could be shifted by addition of bromide anion; when refluxing dichloroethane (Scheme 16). The propargylic bromide was present in greater than 8.5%, it retarded con- ketones 39 were also obtained by indium-mediated alky- version of allylindium(I) into allylindium(III) dibromide, nylation of acyl chlorides.69 This methodology was ap- which accounts for the formation of bis-allylated amines plied in C-glycoside synthesis.70

as by-products.

OInL

Enol ethers could be used as an aldehyde equivalent for x

RCHO + R' I H O

R the generation of imines in the indium-mediated one-pot 2

2 equiv

CH Cl

2 2

three-component reaction of aromatic amines 33, enol R'

40 °C

38 (up to 99%) ethers 34 and allylic bromides 35, affording N-aryl-substi- 37

60 tuted homoallylamines 36 (Scheme 15). In

O O

RCHO +

R R

H DCE

2 equiv

R R

83 °C

6 R'

R OR R R'

39 (up to 93%)

1 NH

Cl , r.t. In, CH

2 2 R

THF

39 (up to 82%)

RCOCl +

NH 5

2 R

R Br

(up to 94%)

36 Scheme 16 Indium-mediated alkynylation of carbonyl compounds 35

Scheme 15 Indium-mediated three-component reaction 4.2 Indium(III)-Catalyzed Alkynylation Azirines,61 oxime ethers62,63 and hydrazones64 have also The indium(0)-mediated alkynylation requires the prior been allylated under Barbier-type conditions. Oxime formation of alkynyl iodides, but the advantage of this re- ethers derived from 2-pyridinecarboxaldehyde and glyox- action lies in the fact that the alkynylation proceeds in ylic acid gave syn g-allylation products with respect to the neutral conditions. A new angle appeared with the direct chelation by indium.62 With chiral hydrazones derived use of a terminal alkyne which could be metalated by in- from oxazolidinones,64 essentially complete diastereose- dium(III) salts under basic conditions, in order to be added lectivity was obtained as well. Enantioselectivity in the al- to carbonyl compounds. In this strategy, indium acetylide lylation of hydrazones could be realized with the use of a is formed when stoichiometric amounts of indium(III) catalytic amount of BINOL ligands.65 bromide and triethylamine are mixed with an alkyne in di- The sulfinyl group, which can be removed under acidic ethyl ether, thereby giving access to propargylic alcohols from aromatic aldehydes and aliphatic non-enolizable al- conditions, was recently evidenced as a chiral auxiliary in 71 a diastereoselective addition of allyl bromides to N-tert- dehydes. Interestingly, propargylic amines were pre- butylsulfinyl aldimines.66 pared from N,O-acetals under the same conditions. The catalytic version of the indium(III)/tertiary amine re- A salient reaction is the indium(I) iodide promoted 1,2- 72 addition of allyl and benzyl bromides to a,b-unsaturated action of alkynes with aldehydes was recently revisited. nitriles that gives the corresponding conjugated imines; Under the following conditions (10 mol% InBr3, 20% high 1,2-selectivity was observed, and surprisingly only mol% DIPEA, 2 equiv of the terminal alkyne), aromatic 20 mol% of indium(I) iodide was required in the reac- aldehydes were smoothly converted into the correspond- tion.67 ing propargylic alcohols. Except for cyclohexanecarboxaldehyde which reacted with an excellent yield, other aliphatic aldehydes gave unsatisfactory results due to ei- 4 Carbonyl Alkynylation ther their self-condensation (for aldehydes with a primary alkyl group) or their low reactivity (for aldehydes with a 4.1 Indium(0)-Mediated Alkynylation tertiary alkyl group). Optimized catalytic conditions for the alkynylation were determined to be 20 mol% indi- Barbier-type alkynylation – that is, the addition of acety- um(III) triflate and 50 mol% N,N-diisopropylethylamine lenic halide upon carbonyl compounds in the presence of in 1,2-dimethoxyethane. Spectroscopic studies showed a metal – is unusual. The first example of the use of a met- that dual activation of both the terminal alkyne and the al(0) in such a reaction involved indium.68 For the reac- carbonyl group by indium(III) salts occurred tion, alkynyl iodides 37 were first prepared from alkyne (Scheme 17). and iodine in the presence of morpholine. With an excess A slight modification of the catalytic system made the of these reagents and indium metal in refluxing dichlo- alkynylation of aldehydes enantioselective: the addition romethane, aldehydes and ketones afforded the corre- of a chiral ligand resulted in the reaction proceeding via an sponding propargylic alcohols 38. When the aldehyde was indium(III)–BINOL complex. Excellent enantioselectivi- in excess, the propargylic ketones 39 were formed, arising ties for propargylic alcohols were obtained with various from an Oppenauer-type oxidation of the indium(III) aldehydes and alkynes. The reaction was performed in

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York

1746 J. Augé et al. REVIEW

3 81

R tioselectivities were either reduced (BINOL) or en-

In(III) cat. InX In(III)

3 hanced (PYBOX).82,83 O

cat. DIPEA

HO + H R

1 2

neat or DME

R R

1 2

R R

(S)-BINOL, InCl (20 mol%) 40 °C 3

SnBu 3 +

up to 99%

4 A MS, CH Cl R H 2 2

Scheme 17 Dual activation in alkynylation reaction yield 55–76% ee 90–96%

Scheme 19 Enantioselective allylation of aldehydes dichloromethane at 40 °C using dicyclohexylmethyl- amine (50 mol%) as a base, and indium(III) bromide (10 mol%) as a catalyst. For the easily enolizable aldehydes, The indium(III)-catalyzed addition of allylstannanes was slow addition of the aldehyde prevented self-condensa- successfully achieved with imines; with ortho-alkynyl- tion. (R)-BINOL (10 mol%) induced the nucleophilic at- arylaldimines a subsequent activation of the triple bond by tack of the indium acetylide on the Re face of the indium(III) triflate brought about in situ cyclization af- 84 aldehyde; strong positive nonlinear effects were observed, fording 1,2-dihydroisoquinolines. suggesting a bimetallic mechanism in the catalytic cy- A related reaction using allyltrimethylsilane instead of al- cle.73 lyltributylstannane gave b,g-unsaturated ketones from acyl chlorides.85 There was no evidence of a transmetalation step, and the activation of the carbonyl moiety by in- 5 Carbonyl Additions via Transmetalation dium(III) bromide as a Lewis acid was not excluded in that reaction. 5.1 Indium(III) and Indium(I)-Mediated Allyla- The indium(III)-mediated allylation of carbonyl com- tion pounds always requires the preparation of allylic organo- Allyldichloroindium has been prepared from allylstan- metallic species, such as silanes or stannanes, from the nanes and indium(III) chloride in various donor solvents. allylic halides. The Barbier allylation reactions also re- In order to use this reactive species in carbonyl additions, quire the use of halides. A new strategy, the reductive a premixing of the aldehyde and indium(III) chloride be- transmetalation of a p-allylpalladium(II) complex with in- fore the addition of the stannane was found to be crucial. dium(I) salts, has enabled the use of more accessible allyl- With crotyl tri-n-butylstannane, the reaction led to ho- ic compounds, including allylic alcohols and their moallylic alcohols with total g-regioselectivity and excel- derivatives 43 (Scheme 20).86 The reaction is rapid at lent anti stereoselectivity in acetone or acetonitrile, even room temperature for all substrates except allylic alco- at room temperature. With a-alkoxy crotylstannane 40 of hols; the use of nickel(II) acetylacetonate instead of tet- >95% ee, the addition of aldehyde at –78 °C in the pres- rakis(triphenylphosphine)palladium(0) made the reaction ence of a stoichiometric amount of indium(III) chloride easier to perform. In this process, indium(I) iodide was led to the anti homoallylic alcohols 41 with total a-re- used to reduce nickel(II) acetylacetonate into nickel(0) gioselectivity and excellent anti stereoselectivity and which was itself involved in the formation of a p-al- enantioselectivity (Scheme 18).74 lylnickel complex; transmetalation with indium(I) iodide then led to a (Z)-allylindium(III) species and thereby in-

OH duced a syn selectivity after addition of the aldehyde.

InCl OMOM 2

InCl RCHO

3 Noteworthy is the combined use of indium and indi-

OMOM

SnBu 3

up to 90% um(III) instead of indium(I) salt in the palladium-cata-

OMOM

40 41 lyzed allylation in aqueous media of various allylic substrates, including alcohol, chloride, acetate and car- Scheme 18 Synthesis of anti-homoallylic alcohols

bonate.88

InI (1 equiv)

Recently, a catalytic amount (20 mol%) of a chiral com- Ph

PhCHO

75 +

) (5 mol%) plex prepared from (S)-BINOL and indium(III) chloride Pd(PPh

3 4

X = OH, OAc, OPh, OCO Et

76 43 2 [or indium(III) bromide] was used in an efficient enan- up to 100% tioselective allylation of aldehydes with allyltributylstan- nane (42) (Scheme 19). The reaction was first tested under Scheme 20 Transmetalation of p-allylpalladium complexes by indium(I) iodide anhydrous conditions, but it turned out that some moisture could be tolerated.77 With 20 mol% of indium(III) triflate and chiral PYBOX, however, the allylation of aldehydes In another notable reaction, the allylation of glyoxylic and ketones with allyltributyltin required the presence of oxime ether 44 with alcohol derivatives 45 in the presence chlorotrimethylsilane to afford the corresponding homo- of a stoichiometric amount of indium(I) iodide and a cat- allylic alcohols in moderate to high enantioselectivi- alytic amount of palladium(0) complex gave either the ki- ties.78–80 The preceding methodologies were tested in netic g-adducts 46 in aqueous tetrahydrofuran or the ionic liquids; according to the chiral ligand used, the enan- thermodynamic a-adducts 47 in anhydrous tetrahydrofuran (Scheme 21).89

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York REVIEW Indium-Promoted Organic Reactions 1747

5. 3 Indium(III)-Mediated Alkynylation

THF–H O,

O 2 O

20 °C In analogy to the transmetalation of allylstannanes with NOHBn NOBn

N N

3 h indium(III) salts, it is also possible to transmetalate alky-

2 ) Pd(PPh 95% 2 3 4

44 nylstannanes with indium(III) chloride to form alkynyldi- Ph

ee >95%

+ 46 InI chloroindium, which smoothly adds to aldehydes in

acetonitrile. In this reaction, a catalytic amount of indi-

Ph X O

THF, 20 °C um(III) chloride (10 mol%) is sufficient, owing to the

X: OAc, OCO Me NOHBn

20 h

N presence of chlorotrimethylsilane as an oxophile that re-

85% 94 SO

2 generates the indium salt. In the silane reactant series,

Ph 47 ee >95% chlorotrimethylsilane can be omitted, so that the alkynyl- silanes 51 undergo coupling with acid chlorides in the Scheme 21 Allylation of a glyoxylic oxime ether mediated by pal- presence of 5 mol% of indium(III) bromide at 0 °C to af- ladium–indium(I) iodide ford the corresponding a,b-acetylenic ketones 52 (Scheme 23).85

5.2 Indium(III)- and Indium(I)-Mediated

Propargylation O

5 mol% InBr

R SiMe

CH Cl , 0 °C R'

2 2

Cl By SN2¢ displacement of secondary chiral propargylic me- R'

52 51 up to 93% sylates with tributyltin–copper complex, it was possible to R form allenylstannanes which underwent transmetalation Scheme 23 Indium(III)-catalyzed alkynylation of acid chlorides with indium(III) chloride to produce anti homopropargylic alcohols from aldehydes.90 More interesting was the straightforward preparation of such enantiomeric alcohols 6 Carboindation of Alkynes directly from enantiomeric mesylates. A catalytic amount of [1,1¢-bis(diphenylphosphino)ferrocene]dichloropalla- In tetrahydrofuran, even inactivated terminal alkynes un- dium(II) [Pd(dppf)Cl2] and a stoichiometric amount of in- derwent indium-mediated allylation with allyl iodide or dium(I) iodide gave rise to an allenylindium which added bromide to afford the Markovnikov adducts. As expected, to aldehydes with good diastereo- and enantioselectivity. alkynols afforded the anti-Markovnikov adducts. As a Curiously, the reaction worked in the absence of catalyst possible mechanism of the allylindation of terminal but with a lower yield (66%) and no enantioselectivity; 24 alkynes 53, Schmid suggested that an indium acetylide diastereoselectivity was, however, preserved. was first formed, and then the addition of one equivalent Both enantiomers of 4-tri(isopropyl)silylbut-3-yn-2-ol are of allylindium would give rise to a vinylic a,a-bis(indium) easily accessible in >95% enantiomeric purity by reduc- intermediate and subsequently to the branched 1,4-dienes tion of the ynone precursor. After conversion of the mesy- 54; alternatively, the intermediate could be quenched with late derivative 48 into a transient allenylindium reagent in N-bromo(or iodo)succinimide to form the gem-dihalo- the presence of a catalyst derived from palladium(II) ace- 1,4-dienes 55.95 This mechanism could be applied even to tate/triphenylphosphine and indium(I) iodide, the addition alkynols in which the hydroxyl group is remote from the to a,b-unsaturated aldehyde 49 led to the anti homopro- alkyne function. In contrast, propargylic alcohol 56 gave pargylic alcohol 50 (Scheme 22).91 Such an allenylindium the linear 1,4-diene 57, suggesting the involvement of a methodology was used with success in the synthesis of the bicyclic chelation-controlled transition state; quenching polypropionate marine defense substance (+)-membra- of the intermediate with an N-halosuccinimide yielded the none C.92 In a related reaction, the propargylation of gly- monohalogenated dienes 58 (Scheme 24). oxylic oxime ether proceeded under similar conditions, Carbometalation of allenyl alcohols is also possible with but yields were increased with the use of lithium chloride 93 propargyl/allenyl indium compounds under sonication, or lithium bromide as additives. but indium(III) bromide is required as an additive.96 Thus, the reaction between prop-2-ynyl bromide and 1,2-disub-

OMs stituted allenols in the presence of a stoichiometric

H amount of indium, carried out in tetrahydrofuran, gave InI (1.6 equiv) Me

TIPS

Pd(OAc)

–PPh (E)-1,2-disubstituted hepta-2-en-6-yn-1-ol. With but-2- 2 3

Me Me 1.2 equiv

(0.06 equiv)

48 ynyl bromide, the reaction led to (E)-1,2-disubstituted 5- OTBS

3:1 THF–HMPA methylhepta-2,5,6-trien-1-ol. This product could also be TBSO +

TIPS obtained directly from two equivalents of but-2-ynyl bro-

(87%)

50 mide and the corresponding aldehyde.

/syn = 91:9

anti Allylindation was also found to proceed via intramolecu- Me

CHO lar addition: the cyclization of 1-bromo-2,7-enynes and 1- 1 equiv

49 bromo-2,8-enynes mediated by indium in N,N-dimethyl- Scheme 22 Carbonyl addition of a chiral allenylindium reagent formamide at 100 °C produced five- and six-membered cyclic compounds.97 The intramolecular cyclization could

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York 1748 J. Augé et al. REVIEW

um or magnesium reagents with indium(III) chloride, was

first described by Sarandeses and co-workers.101 The reac-

L In

"Allyl-In"

H O In R L

3 x

R 54 tion proceeds with a variety of triorganoindium reagents

In R L

(up to 100%)

x in tetrahydrofuran at reflux temperatures, and in addition

NXS

In, THF, ((( to aryl, alkenyl and alkynyl groups, alkyl groups are trans-

R H ferred without detection of b-elimination products. Indi-

53 R

X um organometallics also react with benzyl bromides or

X = Br, I Br 102

(up to 78%) acyl chlorides. All three of the organic groups attached

OH to the metal are transferred. The reaction was also carried

56 out under a carbon monoxide atmosphere; this led to a car-

In, THF, ((( 103

+ bonylative coupling product. Triorganoindium reagents H O OH 3

(97%)

57 reacted with oligoarenes to afford the multiple cross-cou-

InL

InL x

x pling products in a single operation; it was even possible NXS

X with compound 62 to take advantage of the higher reactiv-

OH H H

OH ity of the aryl–iodine bond over the aryl–bromine bond in (66%)

58 sequential cross-coupling reactions to afford the unsym- Scheme 24 Allylindation of alkynes metrical alkyl-, aryl-, and alkynyl-substituted aromatic compounds 63 in one pot (Scheme 26).104 Sarandeses’ protocol was applied to the synthesis of C-aryl glycals be performed in aqueous tetrahydrofuran or in anhydrous through the formation of dihydropyranylindium re- tetrahydrofuran in the presence of an organic acid; it was agents.105 also mediated by stoichiometric indium(I) or substoichio-

metric indium(III) with zinc as a co-reductant.98

I R

R In (35 mol%) 3

Pd (Ph P)

As an alternative, the Barbier-type pathway was replaced 3

then

Br R'

R' by a transmetalation method. Starting from 8-tributylstan- In (50 mol%)

63 (up to 95%) nyloct-6-en-1-yne 59, the addition of 1 equivalent of indi- 62 um(III) chloride gave an allylindium reagent which Scheme 26 Sequential cross-coupling with indium(III) organo- underwent cyclization to afford a vinylindium species metallics which was prone to hydrolysis into 2-allyl-1-methylene- cyclopentane 60. With only 0.2 equivalent of indium(III) Noteworthy is the beneficial influence of indium(III) chloride, the cyclization led to vinylstannane 61, which chloride as a cocatalyst in the cross-coupling of alkenyl- was isolated and hydrolyzed to give 60 in 87% overall aluminum and alkenylzirconium, prepared by hydrometa- yield (Scheme 25).99 With silanes instead of stannanes, lation of alkynes, with vinyl halides.106 the reaction proceeds via electrophilic activation of the triple bond by indium salt.100 7.2 Coupling of Organoindium Reagents

Prepared from Indium Metal

InCl

InCl 3 E

1 equiv

E As shown in Scheme 24, vinylindium compounds could E

E be prepared from indium metal by allylindation of

H O InCl

3 2 96% alkynes; these species were reported to couple to iodoben-

zene in the presence of palladium(0) catalyst.95 Vinylindi- E: CO Me

SnBu

SnBu +

H um reagents could also be obtained in the allylindation of 3

59 InCl

3 carbon–oxygen and carbon–nitrogen double bonds with E 0.2 equiv E 87%

61 60 3-bromo-1-iodopropene (64) and immediately coupled with aryl, alkenyl or allyl halides in a tandem reaction to Scheme 25 Indium(III)-promoted addition of allylstannanes to al- give 65 or 66 (Scheme 27).107

kynes

R X R R

InL R

RCHO In 2

Pd(0)

+ 65 OInL OH

7 Cross-Coupling Reactions 2

up to 81% I Br

RCH=NR

R 7.1 Coupling of Organoindium Reagents R X R

R InL

Prepared from Indium(III) Chloride 2

Pd(0)

NHR NR L 2

up to 77%

The use of palladium-catalyzed cross-coupling reactions 1 = aryl, alkenyl, allyl of aryl iodides or aryl or vinyl triflates with triorganoindi- R um reagents, prepared by transmetalation of organolithi- Scheme 27 Cascade coupling reactions

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York

REVIEW Indium-Promoted Organic Reactions 1749

7.3 Sonogashira-Type Reaction Ph

H Pd (DPEphos)Cl Me

Ph In +

THF, r.t., 8 h

The indium acetylides formed in situ from terminal OBz H

80%

ee >95% alkynes and a catalytic amount (0.05 equiv) of indium(III) 67 68 bromide in the presence of piperidine were shown to react with aryl iodides in the presence of bis(triphenylphos- Scheme 30 Palladium-catalyzed reactions with propargylic esters phine)palladium(II) dichloride (0.05 equiv) to give the corresponding cross-coupling product (Scheme 28).108 7.5 Other Coupling Reactions

A three-component coupling of triorganoindium reagents (0.05 equiv) 1. InBr

piperidine, r.t., 2.5 h with imines and acid chlorides was recently reported to

R take place under catalysis by copper(I) chloride

2. ArI up to 99% 1.2 equiv

(PPh ) (0.05 equiv)

PdCl (Scheme 31). No coupling reaction was observed when 2 3 2 piperidine, r.t. the triorganoindium reagent was used without any catalyst, nor did a palladium catalyst lead to a reaction. All Scheme 28 Indium(III)-catalyzed Sonogashira-type reaction three organic groups attached to the indium, whether alkyl, alkenyl or aryl, were transferred to the transient N- 7.4 Tsuji–Trost-Type Reaction acyl iminium salt, thereby affording a-substituted amides.113 Triorganoindium and tetraorganoindates with aryl, alke-

nyl and methyl groups reacted with cinnamyl and geranyl O

10 mol% CuCl

N chlorides, bromides and acetates in the presence of 5 mol% 2

+ 1/3 InR 3 +

tris(dibenzylideneacetone)dipalladium(0) [Pd (dba) ] to R

3 2 3 MeCN–THF

R Cl

109 R

up to 98% afford exclusively the SN2 products. For optimal re- 1 R sults, a stoichiometric amount of indium was required. R The reaction proceeded with inversion of configuration, Scheme 31 Three-component coupling of organoindium reagents which is consistent with the mechanistic pathway of oxidative addition, transmetalation and reductive elimination; b-hydride elimination was observed with 8 Nucleophilic Substitutions tributylindium compounds. 8.1 Allylation of Epoxides In an optimized procedure, 0.5 equivalent of triarylindium reagent was used in the nucleophilic substitution of allylic With a Grignard-type protocol, the indium-mediated ally- acetates in carbocyclic derivatives.110 The reaction pro- lation of vinyl epoxide gave bishomoallyl alcohol with to- ceeded with inversion of configuration in the presence of tal regioselectivity.114 This result, which is in 1–3 mol% tris(dibenzylideneacetone)dipalladium(0) and contradiction with the general ring opening of epoxides 4 mol% triphenylphosphine. Cyclization was observed giving rise to 1,2- or 1,4-nucleophilic substitutions, was when allylic acetates were present in the alkyne molecule explained by a transient carbocation which undergoes a (Scheme 29).111 [1,2]-sigmatropic rearrangement. Similar results were observed in the allylation of other epoxides with allylindium

prepared from allyl iodide and indium.115 According to the

AcO authors, the allylindium species had enough Lewis acidity

In Ph 3

5 mol% Pd(dba)

2 to induce the rearrangement of epoxide 69 into aldehyde

E 70 prior to the direct allylation, affording the correspond- DMF, 80 °C, 1 h

E E ing homoallylic alcohols 71 irrespective of the nature of E

S 2 product 4% E = CO Me 90% the epoxide substituent. In contrast, the allylindate species N 2 reacted with epoxides to give the classical ring-opening Scheme 29 Palladium-catalyzed reactions of acetoxyenynes with products 72 or 73, depending on the epoxide substituent

triphenylindium (Scheme 32). In order to prepare allenes, allylic substrates were re- Lewis acid promoted

placed by propargyl starting materials. Thus, stoichiomet- rearrangement of epoxide into

allylation

ric amounts of aryl, alkenyl, alkynyl and methyl H R

CH =CH-CH I R 2 2

triorganoindium reagents reacted with propargylic ace- up to 75% OH

O In

tates, benzoates and carbonates under palladium catalysis 70 71

via an S 2¢ rearrangement to give allenes in good yields R N OH

112 OH

and with high regioselectivity. The reaction of triphe- 69

Li(allyl) Me In

4–n n

nylindium with the enantiopure propargylic esters 67 gave and/or R R up to 86%

2 product S 1 product 68 anti S the enantiopure allenes , consistent with an stereo- 72 73 N selectivity (Scheme 30). N Scheme 32 Allylation of epoxides

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York 1750 J. Augé et al. REVIEW

8.2 Allylation and Propargylation of Sulfonium of C-aryl, C-vinyl and C-alkynyl glycosides from the corresponding triorganoindium reagents obtained through Novel nucleophilic substitutions such as allylation, pro- transmetalation.120 The reactions were performed in dieth- pargylation and allenylation were carried out with 3-(tert- yl ether at room temperature and a-selectivity was pre- butyldimethylsilyloxy)alk-2-enylsulfonium triflate (74). dominantly observed. Under Barbier conditions in These smooth reactions, carried out at –78 °C over 30 refluxing dichloromethane, tri-O-acetyl glucal (77) af- minutes, can be described as dimethylsulfide/indium- forded alkynyl derivatives 78 with a-selectivity promoted conjugate additions (Scheme 33).116

(Scheme 35).121

R Br The same conditions were applied to the direct C-glycosi- OTBS OTBS

O dation of simple acetylated carbohydrates. To avoid the

Me S R

R 2 OTf

1 R

R participation of the O-acetyl protecting group at C-2, only

TBSOTf

Me 2-deoxy sugars were used in the pyranose series. C-Alkyn-

R S –78 °C

2 R

up to 70%

R ylglycosides were obtained in good yields with a-selectiv-

3 4

Me R

R 121

6 ity.

In R OAc OAc

OTBS

O OTBS In

AcO AcO

R I

up to 86% up to 73% AcO

Cl CH R 2 2

reflux

77 78

R = CH OAc, SiMe , (CH ) CN

2 up to 92% 2 3 2 3

R R

R 5

R Scheme 35 Indium-promoted Ferrier rearrangement Scheme 33 Indium-mediated conjugate addition to enones

9 Radical Reactions 8.3 SN2¢ Reactions Trialkylindium and triarylindium reagents are soft nu- Because of the low first ionization potential of indium, in- cleophiles towards cinnamyl and geranyl halides and dium is capable of promoting single-electron processes. phosphates. At –30 °C, in the presence of copper(II) tri- The formation of a reactive species by reduction of indi- flate as a catalyst and triethylphosphite as a copper ligand, um(III) chloride with various hydrides is discussed in the the reaction led to SN2¢ products with good yields and re- following section, hydroindation. gioselectivity.117 Under these conditions, organometallics were able to transfer only one of the groups attached to in- 9.1 Radical Substitutions Initiated by Triethyl- dium. borane A particular case was reported concerning the stoichiometric reaction between allenols 75 and indium(III) ha- The formation of ethyl radical by triethylborane in the 76 presence of dioxygen was exploited in the radical indium- lide, leading to the 2-halo-1,3-dienes ; indium(III) 122 halide coordinates to allenol to form a six-membered tran- mediated substitution of a-halo carbonyl compounds. sition state which delivers the halide anion at the electro- Thus, allylindium reagents prepared by transmetalation of Grignard reagents with indium(III) chloride transferred philic carbon according to an SN2¢ pathway (Scheme 34).118 their allyl moiety to a-iodo (or a-bromo) amides or esters such as 79 in aqueous tetrahydrofuran solution. With al-

kenylindium reagents, the reaction was carried out in di-

X ethyl ether. Starting from an ester such as 80, the

InX (1 equiv)

3 O X stereochemistry of alkenylindiums was essentially re-

R benzene 1

R tained, so that the E/Z configurational ratio of the product

3 °C to r.t.

R 2

R was approximately that of the starting alkenyl bromide. In X = Cl or Br

R (up to 80%) 75 76 the same way, phenylethynylindium dichloride and phe- nylindium dichloride were added to a-iodo esters such as Scheme 34 Indium(III) halide mediated S 2¢ reaction of allenols N 81. In all these reactions, dichloroindium(II) radical was used as an efficient radical mediator (Scheme 36). 8.4 C-Glycosylation Glycal derivatives are readily converted into 2,3-unsatur- 9.2 Radical Cyclizations ated glycosyl compounds with oxygen, carbon, nitrogen or sulfur substituents at the anomeric position.119 This spe- Indium reacts with iodine in aromatic solvents under reflux to produce indium(I), indium(II) and indium(III) cific SN2¢ reaction, named the Ferrier rearrangement, is 24 useful in the preparation of C-glycosides. An indium- salts. As observed recently, low-valent indium species mediated Ferrier rearrangement was recently described to can initiate radical processes. Thus, treatment of io- take place under non-acidic conditions in the preparation doalkynes such as 82 with a catalytic amount of indium

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York

REVIEW Indium-Promoted Organic Reactions 1751

O Intramolecular cyclization of allylindium compounds was O

InCl

I found to be promoted by photolysis; thus allylindium

O Ph O Ph

B, O Et compounds formed from 8-bromo- or 8-iodoocta-1,6- 79

3 2

92%

THF–H O 2 dienes gave the 5-exo-trig products, after photolysis-

126

O induced homolytic cleavage of the carbon–indium.

InCl

O Ph

Et B 3

93% Et O 2

+ R

O H

Ph O

Ph InCl 2

N O

V-70

86 In

Et O

InBr

2 88%

81 up to 80%

Py HBr

⋅ R

N O

DMF

PhInCl

Ph 81

Bu Ar

Et B O

ArI

85 benzene 65%

Pd(acac) 2 O

Scheme 36 Radical substitutions of a-iodo carbonyl compounds N up to 80% 87 Bu

Scheme 38 Reductive radical cyclization (0.1 equiv) and iodine (0.05 equiv) promoted an atom- transfer 5-exo cyclization to give the five-membered alkenyl iodide products such as 83.123 With an excess of in- 9.3 Indium(I) as a Radical Initiator dium, cyclization was followed by reduction of the In the aqueous zinc–copper(I) iodide mediated alkylation alkenyl iodide to give 84 (Scheme 37). When the starting of aromatic aldehydes with unactivated alkyl halides, op- alkyne contained a leaving group at the propargylic posi- 124 timized conditions were obtained in aqueous sodium ox- tion, allenic products were produced selectively. alate with indium(I) chloride as a catalyst (0.1 equiv),

which was presumed to transfer one electron to the carbo-

H 127

H nyl oxygen.

I I

In, I 2

+ In the intramolecular cyclization of d-bromoalkynes 88,

MeOH indium(I) iodide was used in a stoichiometric amount un- O O O O O O

H der sonication, and gave rise to a vinylindium intermedi-

In I (Z/E = 11.5:1) 83 84

2 ate according to a mechanism similar to that observed

0.05 0.1 77%

3% with the stoichiometric indium–iodine or indium–

2 0% 85% 1 bromine protocol. After hydrolysis, the corresponding Scheme 37 Radical cyclization of iodoalkyne promoted by indium substituted 4-methylenetetrahydrofurans 89 were ob- 128

metal and molecular iodine tained in yields ranging from 62% to 80% (Scheme 39).

R O

With iodoalkenes instead of iodoalkynes, the reaction was O InI R

performed with two equivalents of indium and one equiv- Br

MeCN

sonication alent of iodine; 5-exo cyclization also occured in metha- Ar

Ar O O nol, giving the corresponding alkylindium intermediate, up to 80%

which was isolated for analytical purpose.124 This com- 88 89 pound was not further hydrolyzed but gave either the alkyl Scheme 39 Radical cyclization mediate by indium(I) iodide iodide or the alcohol, after the addition of hydrogen per- oxide. With alkenes bearing leaving groups at the allylic 9.4 Indium(0) as a Radical Initiator position, the 5-exo cyclization was accompanied by the elimination of these groups. Indium was used as a radical initiator in aqueous media to In some cases, the indium–molecular bromine couple promote intermolecular alkyl addition to oxime and hy- drazone carbon–nitrogen double bonds and to electron- turned out to be more efficient than the indium–molecular 129 iodine couple for inducing the radical cyclization. Fur- deficient carbon–carbon double bonds. The reaction thermore, it was revealed that pyridinium tribromide gave good yields with five equivalents of secondary alkyl 125 iodides in the presence of seven equivalents of indium in (Py·HBr3) could be used instead of molecular bromine. Such conditions were used in the 5-exo radical cyclization aqueous methanol or dichloromethane–water mixtures. of iodo-ynamide 85 leading to the vinylindium intermedi- Another example of single-electron processes initiated by ate, which could be quenched by a proton to give 86 or indium was the one-carbon ring expansion of the a-io- which could, after addition of aryl iodide, undergo a pal- domethyl cyclic b-keto esters 90 in aqueous tert-amyl al- ladium-catalyzed cross-coupling reaction to afford 87 cohol at reflux; the primary radical formed by reduction of (Scheme 38). the iodide underwent a 3-exo-trig cyclization and a subsequent b-cleavage to afford 91 (Scheme 40).130

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York

1752 J. Augé et al. REVIEW

O O O bond cleavage to afford the corresponding allylic alco-

135

I 3-

exo-trig hols.

In (5.2 equiv)

CO R

CO R CO R

2 2

2 The combination of a catalytic amount of indium(III)

tert-amyl alcohol– n n n

n = 1–6 O (2:1) H 2 chloride and sodium borohydride in acetonitrile was

R = Me, Et

O found to reduce, with transposition, the hydroxyl group of 90 Baylis–Hillman adducts 93 (Scheme 42). This remark-

ably chemoselective reduction which afforded exclusive- up to 74%

CO R

91 2 n ly the E-isomers 94 was applied to the synthesis of two alarm pheromones.136

Scheme 40 Indium-mediated ring expansion

OH O H O

NaBH , InCl

1 3 4 3

1 3 R R

10 Hydroindation R

MeCN

2 93 94 2 R R

Transmetalation between indium(III) chloride and a me- up to 84%

tallic hydride can generate dichloroindium hydride. The 100% ( transient formation of ·InCl2 as a radical intermediate is Scheme 42 Chemoselective reduction of Baylis–Hillman adducts the driving force of the subsequent hydroindation of various functionalities. The same system was used to selectively reduce the carbon–carbon bonds in conjugated alkenes, such as a,a-di- 10.1 Hydroindation from Tributyltin Hydride cyano olefins, a,b-unsaturated nitriles, cyanoesters, and Indium(III) Chloride cyanophosphonate and dicarboxylic esters.137 In activated a,b,g,d-unsaturated alkenes, only the a,b carbon–carbon Dichloroindium hydride was generated for the first time double bond was reduced.138 by transmetalation of indium(III) chloride with tributyltin hydride at –78 °C; the solution was found to be stable in Direct hydroindation of 2-arylalkynylphosphonates with tetrahydrofuran at room temperature and could be used in the sodium borohydride and stoichiometric indium(III) the reduction of carbonyl compounds or alkyl bro- chloride in acetonitrile system gave, after hydrolysis, (E)- 139 mides.131 The hydroindation of 1,3-dienes gave allylic in- 2-arylvinylphosphonates. diums which further reacted with carbonyl or imine Hydroindation of arylalkynylsilanes required tetrahydro- moieties in a one-pot process (Scheme 41).132 furan as solvent to afford the (E)-alkenylsilanes.140 In the

case of the terminal aryl alkynes 95, the transient alkenyl

HInCl

Bu SnH + InCl

2 3 3 indium reagents generated by hydroindation with the sodium borohydride and stoichiometric indium(III) chloride

in acetonitrile system coupled with aryl iodides or bro- InCl Cl In 2

2 mides to give preferentially the (E)-alkenes 96 without the 92 need for a transition metal as catalyst; no coupling product was obtained with aliphatic terminal alkynes

OH 140

HInCl

Cl In

2 RCHO 2 (Scheme 43). In the presence of diaryliodonium salts,

– InCl aryl terminal alkynes underwent cross-coupling via the vi- 2

up to 99% nylindiums generated by hydroindation, but under the Scheme 41 Hydroindation of a 1,3-diene same conditions, the aliphatic terminal alkynes did not react very well.141 It was mentioned, however, that the hy- 10.2 Hydroindation from Sodium Borohydride droindation of various terminal alkynes with this system 142 and Indium(III) Chloride could lead to enynes by dimerization. The carbon–selenium bond in gem-difluorinated orga- The non-toxic couple sodium borohydride/indium(III) noselenium compounds was also reduced by the sodium chloride in acetonitrile turned out to be an interesting al- borohydride and stoichiometric indium(III) chloride in ternative for the preparation of dichloroindium hydride. acetonitrile system. Thus, these compounds could under- Only a catalytic amount of indium(III) chloride was rego reduction or reductive radical cyclization and could be quired in the radical reduction of alkyl halides and in 5- added to styrene.143 exo-trig radical cyclizations.133 With vicinal dibromides,

the reaction gave the (E)-alkenes under mild conditions

InCl , NaBH , MeCN

134 3

–15 °C , 30 min

1 (–10 °C). Ar

Ar H

Ar 2

X, r.t., 15 h

In these reactions, the alkyl halides RX presumably gave Ar (up to 78%) the radical species R, which were prone to evolve. In a 95 96 similar way, the reduction of the bromo moiety of 2,3-ep- Scheme 43 Hydroindation with indium(III) chloride and sodium oxybromides was followed by selective carbon–oxygen borohydride in acetonitrile

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York REVIEW Indium-Promoted Organic Reactions 1753

10.3 Hydroindation from Diisobutylaluminum formation of the dimerization product was avoided.148 The Hydride and Indium(III) Chloride intermediary indium enolates were used for inter- and intramolecular aldol reactions and intramolecular Michael Treatment of indium(III) chloride in tetrahydrofuran with additions. The phenylsilane/indium(III) acetate reduction diisobutylaluminum hydride at 0 °C yields dichloroindi- system was particularly efficient in the reduction of or- um hydride, which can further react with terminal alkynes ganic halides in ethanol containing a small amount of 2,4- 97 in the presence of a catalytic amount of triethylborane lutidine.149 as radical initiator.15 Hydroindation proceeds in an anti fashion, giving (Z)-alkenylindium dichloride 98 and then In addition to dichloroindium hydride, the reaction be- the corresponding Z-product by deuteration, iodolysis or tween silanes and indium(III) chloride afforded chlorosi- palladium-catalyzed cross-coupling reaction with aryl io- lanes, the Lewis acidities of which were enhanced by dide (Scheme 44). (Z)-Alkenylindium dichlorides pre- combination with indium(III) chloride. To overcome this pared in this way also reacted with a-halo esters to give putative problem with acid-sensitive substrates, indi- 144 the corresponding a-alkenyl esters. um(III) chloride was replaced by InCl2OMe in the trans-

metalation with diphenylsilane. In the presence of

I R triethylborane, this system promoted intramolecular radi-

150

DIBAL-H, InCl 2 cal coupling of the enyne 100 to give 101 (Scheme 46). 3

R InCl cat. Et B 2

R H

THF, –78 °C

InCl up to 99% 3

ArI

R Ar Pd(0)

NaOMe

66 °C

InCl 2

InCl OMe

Scheme 44 Hydroindation of terminal alkynes 2

SiH Ph H 2 2

O O

Et B (0.1 equiv) Ph Ph 3

Ph (81%) Dichloroindium hydride prepared from indium(III) chlo- 100 101 ride and diisobutylaluminum hydride in the presence of triethylborane reduced alkyl bromides and induced radi- Scheme 46 Intramolecular radical coupling of enynes cal cyclizations. A catalytic process (20 mol% InCl3) was applied to the 5-exo-trig-cyclization of iodide 99 11 Reduction of Functional Groups (Scheme 45).145

11.1 Reduction of Nitrogen-Containing Groups

DIBAL-H (1.1 equiv) BuO

O C H BuO

5 11

O C H

5 11 (0.2 equiv) InCl

3 The reductive coupling of imines, the reduction of quino-

B (0.1 equiv) Et 3

I lines or other benzo-fused nitrogen heterocycles, the re- 99 THF, r.t., 30 min

72% duction of activated oximes, of nitro compounds, of Scheme 45 Radical cyclization induced by dichloroindium hydride azides into amines, and the deprotection of 4-nitrobenzyl ethers or esters have all been discussed in a review devoted to stoichiometric indium metal as a reducing agent in 10.4 Hydroindation from Silanes and InX3 organic synthesis.151 Dichloroindium hydride has been generated from triethyl- A small amount of indium (25 mol%) in conjunction with silane and indium(III) chloride. In the presence of a cata- zinc as the stoichiometric reducing agent turned out to be lytic amount of triethylborane, this new reagent reduced effective in the reduction of the nitro group in a tetrahy- alkyl or aryl halides, induced 5-exo-trig cyclizations of drofuran and aqueous saturated ammonium chloride solu- aryl iodides and reduced alkynes. With 1,6-enynes, a rad- tion.152 This observation was similar to a result described ical cyclization occurred without the side-reactions, such in the reduction of hydroxylamines into amines which as hydroboration, observed with the sodium borohydride could be effected with a catalytic amount of indium (2–5 146 and indium(III) chloride system. mol%) in conjunction with zinc or aluminum.153 When the Dichloroindium hydride reduced organic azides to the reaction was conducted in tetrahydrofuran in the presence corresponding amines; g-azidonitriles gave rise to five- of acetic anhydride and acetic acid, direct acetylation was membered cyclization processes, thereby affording pyrro- possible. Such a domino reduction–acetylation reaction lidin-2-imines.147 was also described in the preparation of N-aryl acetamides 154 Dibromoindium hydride, generated from triethylsilane from nitroarenes in methanol. and indium(III) bromide, added to enones in a 1,4 manner Other domino reactions have been reported recently. in propionitrile to give the transient indium enolate which Among these was the reduction of nitroarenes into was then quenched with an aldehyde to afford the aldol anilines in the presence of 2,3-dihydrofuran in acidic wa- product with an excellent syn selectivity.14 ter, which was followed by the formation of an imine and The 1,4-reduction of enones with phenylsilane in the pres- a subsequent aza-Diels–Alder reaction to afford new 1,2,3,4-tetrahydroquinoline derivatives in a one-pot reac- ence of a catalytic amount of indium(III) acetate was fa- 155 cilitated by the use of ethanol as solvent, and the tion.

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York 1754 J. Augé et al. REVIEW

For nitroarenes with an appropriate functional group at the additions to styrenes in the presence of a catalytic amount ortho position, reductive heterocyclization gave interest- of zinc chloride (15 mol%).160 The strategy has been used ing results (Scheme 47). The reaction was performed in for the regioselective nucleophilic ring opening of ep- methanol at 50 °C; under these conditions, the combined oxides in the presence of indium(III) chloride, thereby use of indium (3 equiv) and iodine (0.8 equiv) trans- producing the corresponding b-hydroxyphenyl sul- formed 2-nitroacylbenzenes such as 102 into 2,1-benz- fides.161 isoxazoles such as 103, independent of the electronic Arylthiolates generated in this way gave, at room temper- effect of the aromatic substituents. Under similar condi- ature, thioethers and thioesters from alkyl and acyl ha- tions, 2-nitroiminobenzene 104 gave a mixture of 2,1- lides.162 Aromatic vinyl bromides underwent the reaction benzisoxazole 105 and 3-anilino-2-phenyl-2H-indazole 156 when tetrakis(triphenylphosphine) palladium(0) was add- (106). ed as the organometallic catalyst.163 The conversion of (E)-vinyl bromides 108 was remarkably stereoselective in

O giving the (E)-vinyl phenylsulfides 109 (Scheme 49).

H O

InI, PhSSPh

In (3 equiv) + I (0.8 equiv) N

Pd(PPh ) (1.5 mol%)

NO SPh

3 4

2 Br

103 (76%) InI (2 equiv)

102 Ar Ar

+ 9% of NO reduction product THF, r.t., 1.5–4.5 h

2 108 109

70–90%

NHPh Scheme 49 Reduction of disulfides with indium(I) iodide and sub-

In (3 equiv) sequent palladium(0)-catalyzed coupling with vinylic bromides

I (0.8 equiv)

+ O N Ph

N N

NO 2

105 (56%) 106 (27%) 104 11.4 Reduction of Diselenides

Scheme 47 Reductive heterocyclization 11.4.1 Reduction with Indium(I) In the same way as for disulfides, indium(I) iodide reduc- 11.2 Reduction of Oxygen-Containing Groups es diphenyl diselenides to give bis(phenylseleno)iodoindium(III) as a source of PhSe–, which reacts with alkyl and Deoxygenation of epoxides bearing a radical-stabilizing aryl halides to give the parent selenoethers and es- group was performed with indium in ethanol in the pres- ters.162,164 As before, the conversion of (E)-vinyl bromides ence of two equivalents of ammonium chloride. Use of in- produces (E)-vinyl phenylselenides in the presence of a dium(I) chloride (2 equiv) instead of ammonium chloride catalytic amount of palladium(0).163 considerably decreased not only the total time of the reaction, but also the number of equivalents of indium metal In deoxygenated aqueous ethanol, a stoichiometric mix- required (from 7 equiv to 2.5 equiv). The presence of a ture of indium(I) bromide and diphenyl diselenide gave small quantity of water in the solvent was necessary for BrIn(SePh)2, which promoted, alternatively, the Mark- the deoxygenation; for sensitive aryl epoxides that are ovnikov hydroselenation or hydration of terminal alkynes, susceptible to nucleophilic attack by alcohols, aqueous depending on the experimental conditions. Thus, under tert-butyl alcohol is the better solvent system reflux conditions, the terminal aliphatic alkynes 110 gave 165 (Scheme 48).157 the methyl ketones 111 (Scheme 50). Reduction of benzophenones, benzaldehydes and ace- The nucleophilic substitution of glycosyl bromides with tophenones by a stoichiometric indium(III) chloride–alu- indium(III) phenyl and butyl selenolates was found to minum couple in aqueous ethanol at 80 °C gave the constitute a convenient preparation of selenoglyco- 166

corresponding pinacol coupling products.158 sides.

InI, PhSeSePh

O O

R O

EtOH (95%)

SePh

110 In, InCl

O O 25 °C, 3 h

InBr, PhSeSePh

t-BuOH–H O

48 h

EtOH (95%), reflux

107

98%

84 °C, 4 h

24 h

80–90%

InBr

Scheme 48 Indium-promoted deoxygenation of epoxides 3

SePh EtOH (95%) reflux, 24 h

11.3 Reduction of Disulfides 111 Scheme 50 Markovnikov hydration of alkynes mediated by indi- A stoichiometric amount of indium(I) iodide was found to um(I) bromide reduce dialkyl or diaryl disulfides to give bis(thioalkyl/ thioaryl)iodoindium(III) as a source of thiolate anions (2 equiv), which, in refluxing tetrahydrofuran, could under- 11.4.2 Reduction with Indium(0) go Michael additions to a,b-unsaturated ketones, alde- In the indium-promoted synthesis of alkyl phenyl se- hydes, esters and nitriles,159 or anti-Markovnikov lenides from alkyl halides, indium metal is presumed to

REVIEW Indium-Promoted Organic Reactions 1755

InCl

first reduce the alkyl halide. As a matter of fact, alkyl io- 3

dides are more reactive than alkyl bromides, and tertiary InCl

Si Me

Cl HSiMe +

alkyl halides are more reactive than secondary or primary O Me

(Si-O) alkyl halides. The reaction requires only 0.5 equivalent n

each of diphenyl diselenide and indium.167 H

With chlorotrimethylsilane, diphenyl diselenide gave Nu = Ar Friedel–Crafts reaction

Nu = H hydrosilylation PhSeSiMe3, which provided the selenoacetals 112 from H

aliphatic aldehydes (Scheme 51). With aromatic alde- Nu = allyl reductive allylation hydes, the selenoacetals were partially reduced into the Scheme 53 Activation of Lewis acidity of silicium by indium(III) corresponding alkyl phenyl selenides.168 Alternatively, chloride the same intermediate, PhSeSiMe3, underwent Michael addition with a,b-unsaturated carbonyl compounds to by indium(III) chloride was later emphasized in various yield b-phenylselenocarbonyl compounds.169 reactions such as aza-Michael additions of carbamates to

enones,172 Friedel–Crafts alkylation, hydrosilylation and

PhSeSePh reductive allylation (Scheme 53).173 In, TMSCl

RCH(SePh)

up to 80% 2 RCHO MeCN, reflux With indium(III) hydroxide instead of indium(III) chlo-

112 ride, the addition of chlorodimethylsilane to ketones gave Scheme 51 Indium-mediated formation of selenoacetals the corresponding chloroalkanes.174 Chloroalkanes were also prepared from alcohols with stoichiometric chlo- 11.5 Reduction of Carbon–Halogen Bonds rodimethylsilane/stoichiometric benzil/catalytic indium(III) chloride system.175 The driving force in this The reduction of the carbon–halogen bond by indium met- reaction is the fast complexation of benzil, followed by al in organic solvents leads to a mixture of organoindium the release of dihydrogen and the complexation by indi- species.20 Various reactions in which the carbon–halogen um(III) chloride of the chlorosilyl ether that facilitates the bond is reduced by indium metal, including reduction of transfer of chloride and liberation of siloxane vicinal dibromides, aryl-substituted geminal dibromides (Scheme 54). In the presence of indium(III) chloride, and a-halo carbonyl compounds, reductive coupling of there was also release of dihydrogen and formation of si- aryl and alkyl halides, or deprotection of trichloroacetyl- loxane when mixing chlorodimethylsilane with carboxyl- and trichloroethoxycarbonyl-protected alcohols and ic acids. The transient acid chloride could then undergo amines, have been reviewed recently.10 More recently, re- the Friedel–Crafts reaction with aromatic ethers.176

ductive elimination of halohydrins, such as chlorohydrin

or bromohydrin, by indium metal was carefully investi- Ph Ph

Ph Ph

H fast gated (Scheme 52). It was determined that the reaction re- Cl slow

O O

O quired the addition of a catalytic amount of O

Cl HSiMe

HSiMe Cl

2 O O 2

Si Si

tetrakis(triphenylphosphine)palladium(0) (0.02 equiv) Cl

Me Me

and indium(III) chloride (0.5 equiv); the reaction of the 1

R InCl R H 3

acyclic syn- and anti-halohydrins 113 afforded the corre- O

R InCl

+ H

170 InCl 3 3 2 sponding (E)-olefins 114 exclusively. 115

Ph Cl

1 Si

2 R = H, Ac Si

In, InCl , Pd(0)

R R InCl

O O

1 X = Cl, Br

R Me

R R

O (1:1), r.t. THF–H

InCl

2 3

benzil R

O Me up to 98%

2 113 114 R

(Me

SiO) InCl 2 n

Scheme 52 Indium-mediated reductive elimination 3

1 R

(up to 100% from 115) Cl

2 R

12 Activation of Silicon by Indium(III) Salts 116 Scheme 54 Chloration of alcohols catalyzed by indium(III) chlor- The first use of the combination of chlorotrimethylsilane ide and indium(III) chloride as a special catalyst was mentioned in the reaction of O-trimethylsilyl monothioacetals The direct reduction of secondary, benzylic and tertiary with triethylsilane and silylated carbon nucleophiles. It alcohols into the corresponding alkanes was realized with was pointed out that, for these reactions, neither chlorotri- chlorodiphenylsilane and a catalytic amount of indi- methylsilane nor indium(III) chloride was effective alone um(III) chloride. The reaction was carried out in dichlo- and the reactions proceeded only when chlorotrimethylsi- romethane and dichloroethane in order to liberate the 171 lane and indium(III) chloride were combined. Such a insoluble hydrochloric acid formed along with the hydro- remarkable enhancement of Lewis acidity of chlorosilane silyl ether in the first, limiting, step. As before, indi-

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York 1756 J. Augé et al. REVIEW um(III) chloride acted as a Lewis acid to accelerate the thiohydantoins with aromatic aldehydes, giving the 5- deoxygenation of the resulting intermediate that accom- arylidene counterparts.184 panied the hydride transfer and the consecutive formation The indium(III) chloride catalyzed Mukaiyama aldol re- of the siloxane. This system showed high chemoselectiv- 177 action was recently revisited in different solvents. The ity in multifunctional compounds. solvent of choice turned out to be propan-2-ol in water In analogy to this reaction driven by the removal of hydro- (95:5), which gave rise to an excellent syn diastereoselec- chloric acid in chlorinated solvents, allylchlorodimethyl- tivity with (Z)-enolsilanes, but only in moderate yield.185 silane was added to alcohols in the presence of a catalytic The Mukaiyama aldol reaction was found to be more effi- amount of indium(III) chloride. Hydroxyl groups in sec- cient with indium(III) triflate than with indium(III) chlo- ondary and tertiary alcohols were then substituted by allyl ride. Thus the reaction could be carried out at –40 °C in groups.178 The direct substitution of the hydroxy group in dichloromethane in the presence of a chiral PYBOX alcohols could even be performed with non-chlorinated ligand to achieve enantioselectivities up to 92%.186 silanes, such as allyl-, propargyl- and alkynyltrimethylsi- In the indium(III)-catalyzed Mannich-type reaction, the lanes. imine is preformed before the addition of silyl enol ethers. The bromotrimethylsilane–indium(III) chloride couple as High yields and high selectivities were obtained when a strong Lewis acid was able to catalyze, even in hexane, chiral amines were used with indium(III) chloride (20 the nucleophilic substitution of alcohols with silyl nucleo- mol%) in methanol187 or ionic liquids.188 philes such as methallyl-, cinnamyl-, prenyl-, alkynyl-, 179 Another reaction is the domino indium(III)-catalyzed for- propargyl-, allenyl-, and allenylmethylsilanes. The re- mation of enamine followed by its addition to imines.189 action is limited to alcohols that are susceptible to forming An indium(III)-mediated diastereofacial addition to the a stabilized carbocation. Highly chemoselective allyla- carbon–nitrogen double bond occurred in the allylation of tions towards a hydroxyl moiety over both ketone and N 180 chiral -acylhydrazones in the presence of indium(III) tri- acetoxy groups were observed. flate as a chelating agent.190 As a multicomponent reaction, the Biginelli reaction has 13 Indium(III)-Catalyzed Reactions received much attention for its direct preparation of dihydropyrimidinones from 1,3-dicarbonyl compounds, alde- We have seen that indium(III) chloride and indium(III) hydes and urea. A large variety of aldehydes and 1,3- bromide have been added in catalytic amounts to organo- dicarbonyl compounds were mixed with urea or thiourea metallics in order to induce reactive organoindium re- in the presence of indium(III) chloride (10 mol%) in reagents by transmetalation. In addition, a section was fluxing tetrahydrofuran, thereby affording the corre- 191 devoted to the activation of silicon by indium(III) salts to sponding dihydropyrimidinones in good yields. generate new reactivities. This final section deals with all Trichloromethylated tetrahydropyrimidinones 117 were the other indium(III)-catalyzed reactions. Indium(III) recently prepared according to this method, with indi- salts have often been used as water-tolerant Lewis acids, um(III) bromide instead of indium(III) chloride 192 especially in allylations, aldol-type reactions, conjugate (Scheme 55).

additions or cycloadditions.16 Protons could, however, be

R O the active catalysts as evidenced in the indium(III) triflate R

181

InBr (10 mol%)

3 catalyzed aza-Michael addition reactions. O EtO EtO N

NH 2

THF, reflux, 24 h

Indium(III) salts were also effective in ionic liquids, thus Cl

Cl C O Y N

HO H N Y Y = O, S allowing the recycling of a BINOL–In(III) complex in an 2

182

-117 (up to 88%) enantioselective Diels–Alder reaction. rac

In a few cases, indium(III) salts have been used as Bron- Scheme 55 Biginelli reaction catalyzed by indium(III) bromide sted bases; thus, a catalytic amount of indium(III) isopro- poxide was effective as a base in the Mannich-type reaction of N-(2-hydroxyacetyl)pyrrole with o-tosyl- 13.2 a-Amination and a-Alkylation of Carbonyl imines; in the presence of a BINOL ligand, excellent dia- Compounds stereo- and enantioselectivities were obtained.183 By adding silica gel to a solution of indium(III) chloride and then evaporating the solvent, it is possible to obtain a 13.1 Aldol-Type and Related Reactions solid-supported indium catalyst. This catalyst was successfully used in the a-amination of 1,3-dicarbonyl com- In aldol-type and related reactions, indium(III) salts are pounds.193 used as Lewis acids which coordinate the oxygen or the nitrogen atom of the carbonyl or imine compounds. Thus, A fascinating reaction with total atom-economy was re- the use of indium(III) triflate showed significant advan- cently described; it concerned the indium(III)-catalyzed tages in the condensation of 3-substituted 1-methyl-2- addition of active methylene to terminal alkynes (Scheme 56). It is supposed that indium enolates, generated from the b-carbonyl compounds 118 and indium(III)

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York REVIEW Indium-Promoted Organic Reactions 1757 triflate, add across the triple bond of terminal alkynes to 13.3 Prins-Type and Related Reactions give 119. This carboindation process (see section 6) was The discovery of the indium(III)-catalyzed Prins-type re- evidenced by the higher reactivity of electron-rich action arose from the identification of side-products ob- alkynes, suggesting a coordination of the indium atom to served during the investigation of the indium(0)-mediated the alkyne.194 With acetylene gas, which is sensitive to allylation of aldehydes. Indeed, the transient homoallylic acidity, this procedure did not operate, but was successful alcohol reacted with the starting aldehyde to give cis-2,4- after addition of molecular sieves, and thereby allowed for dialkyl-4-halo-tetrahydropyrans.199 Substituted tetrahy- an easy access to a-vinyl keto esters.195 drofurans or thiacyclohexanes were further prepared di-

rectly from homoallylic primary alcohols or thiols with

O O O O

(5 mol%) In(OTf) aldehydes in the presence of a stoichiometric amount of

R OEt R

R OEt

neat indium(III) chloride, according to a mechanism involving

R 2

100–140 °C 200

R an oxonium ion. Excellent diastereoselectivity was ob-

up to 99% 118 119 served with the trans homoallylic alcohols which provid-

ed (up-down-up) 2,3,4-trisubstituted tetrahydropyran

TfO OTf

O O In derivatives.

O O

OEt

1 In contrast with the method of formation of 4-chloro-2,4- R

In(OTf)

2 R OEt

R dialkyltetrahydropyrans that required a stoichiometric

R R amount of indium(III) chloride, the method using allyl- chlorosilane as allylating agent with aldehydes required Scheme 56 Indium(III)-catalyzed a-alkenylation only a catalytic amount of indium(III) triflate.201 Similar- ly, the use of chloro- or bromotrimethylsilane as additive The a-alkylation of b-diketones, b-diesters, and b-ke- was found to be efficient in the synthesis of cross 2,6-di- toesters with simple alcohols was realized thanks to the substituted 4-chloro- or 4-bromotetrahydrofuran products presence of a catalytic amount (5 mol%) of indium(III) from secondary homoallylic alcohols and aldehydes in the chloride. Only water was produced as a side product. The presence of a catalytic amount of indium(III) triflate.202 reaction was performed in toluene at 80 °C for 15 hours Especially noteworthy was the excellent stereoselectivity and was applied to the most reactive alcohols, such as observed where only the all-cis configuration products 196 benzylic or allylic alcohols. As previously suggested, were obtained, except in specific cases.203 In order to ap- an indium enolate might be formed, and this enolate might ply the methodology to an enantioselective synthesis, op- react with the alcohol activated by indium(III) chloride; tically active alcohols were engaged in the process; this direct path competes with a two-step mechanism, indium(III) bromide as a weaker Lewis acid turned out to which goes through the transient formation of an ether be more efficient than indium(III) triflate to reduce the from the alcohol. As a matter of fact, both the formation rate of epimerization of the homoallylic alcohol 120, of an ether from the corresponding alcohol, and the alkyl- which led to an 84% enantiomeric excess of a precursor ation of b-diketones by this ether, were accelerated by the (121) of (–)-centrolobine (Scheme 58). With stoichiomet- presence of indium(III) chloride. ric indium(III) chloride, but in the absence of chlorotri- Another reaction that involves active methylene com- methylsilane, the enantiomeric excess was increased to pounds was discovered recently when ethers were used to- 90%.204

gether with 2,3-dichloro-5,6-dicyanobenzoquinone

Me SiBr

O 3

(DDQ) (Scheme 57). This room-temperature reaction was X

(1.2 equiv)

InBr catalyzed by either indium(III) chloride (10 mol%) or in- 3

dium(III) chloride/copper(II) triflate (5 mol% each); suit- (0.1 equiv)

MeO

or InCl

able solvents were dichloromethane, dichloroethane and 3

nitromethane, and suitable ethers were isochromane de- (1.2 equiv)

121

OBn

197 MeO

rivatives and, to a lesser extent, phthalane. The reac- +

tion, carried out at 100 °C under neat conditions, did not catalytic conditions: X = Br, 83% yield, ee = 84%

stoichiometric conditions: X = Cl, 70% yield, ee = 90% require any metal catalyst and could be applied with sim- OH

ple ketones.198

120

O O O O

OBn

Ar InCl (5 mol%)

Ar 3

MeO OMe

Cu(OTf) (5 mol%) 2

Prins

+ DDQH

O 2

O DDQ O

24 h, r.t.

77% OBn

Scheme 57 Cross-dehydrogenative coupling OBn Scheme 58 Indium(III)-mediated Prins-type reaction

200 205 OH

According to the groups of Li and Loh, the mecha- CHO

In(OTf)

3 +

124 + 125

nism of the indium(III)-mediated Prins-type reaction goes S

CH Cl St 2

through an oxonium intermediate that is formed by attack 2

of the nucleophilic alcohol upon the aldehyde activated by 122 123

indium(III) salts. The subsequent cyclization, which is remote 1,4-stereo-

formerly an intramolecular Prins reaction, is diastereose- communication

H H R

ene

lectively controlled by a chair-like transition state. The si- H

reaction

R-alcohol

R O lyl version of the Prins reaction with 4-methylsilylbut-3- R

O R en-1-ols and aldehydes relies on the stabilization of the S

206 matched pair St

(40–49%) carbocation by the silicon atom at the b-position. The 124

methodology was extended to thiols and to amines in the

OH R H

preparation of unsaturated nitrogen and sulfur heterocy- H H

207 St S-alcohol

H O

S cles. R

125 (22–37%) An intermolecular version of the indium(III) Prins reac- mismatched pair tion was recently carried out in ionic liquids.208 Thus, alkenes underwent condensation with paraformaldehyde in Scheme 59 Chemical kinetic resolution of a bishomoallylic alcohol the presence of 10 mol% indium(III) bromide in 1-butyl- 3-methylimidazolium hexafluorophosphate or tetrafluo- omer that forms a matched pair in the transition state of roborate to afford the corresponding 1,3-dioxane deriva- the ene cyclization is reactive; the other is recovered with tives. an enantioselectivity ranging from 92% to more than When operating with chiral substrates, the main problem 99%.214 that may occur is the epimerization of the homoallylic al- An intermolecular carbonyl-ene reaction can also be cata- cohol under the reaction conditions. Such an epimeriza- lyzed by indium(III)triflate, as recently observed in the re- tion, particularly important with indium(III) triflate,209 action between methylene cyclohexane and aldehydes; comes from a retro-cleavage of the homoallylic alcohol the resulting homoallylic alcohol was not isolated because into an aldehyde, followed by the formation of an oxoni- of the subsequent (2,5) oxonium-ene cyclization, which um intermediate between the homoallylic alcohol and the gives new opportunities to prepare tetrahydropyran liberated aldehyde and then by a 2-oxonia[3,3]-sigmatro- rings.215 pic rearrangement (see Scheme 5). However, since the 2- In contrast with a-prenyl alcohols, the g-prenyl alcohols oxonia[3,3]-sigmatropic rearrangement is mediated by in- 126, when exposed to aldehydes in the presence of a cat- dium(III) triflate, it is possible to transfer an allyl group alytic amount of indium(III) triflate, gave an oxonium in- from a ramified homoallylic alcohol to an aldehyde; at termediate which could not undergo carbonyl-ene low temperature the self-transfer could be avoided in reaction; the unique possibility lay with a 2-oxonia[3,3]- enantioselective a-regioselective allylation of alde- sigmatropic rearrangement, which was then followed by a hydes.210 An enantioselective allyl transfer from a linear facile oxonium-ene cyclization and led to tetrahydrofuran homoallylic alcohol to an aldehyde is even possible via a derivatives.216 In order to suppress this ene reaction, the double 2-oxonia[3,3]-sigmatropic rearrangement, provid- oxonium intermediate was trapped with a hydroxyl group, ed that the substrates are chosen with accord to thermody- installed in a judicious position, to form the cyclic ketal namic considerations.211 127 which, after hydrolysis, gives the desired a-prenyl al- A propargylic transfer, assisted by silicon, was also ob- cohol 128 (Scheme 60). With chiral 1,5-diols, transfer of served when an allenic alcohol was treated with an alde- chirality during the [3,3]-rearrangement was possible, hyde in the presence of 1 mol% of indium(III) triflate; the providing a highly enantioselective prenylation of alde- transfer of chirality was also achieved through a 2-oxo- hydes.

nia[3,3]-sigmatropic rearrangement.212

The reaction of homoallylic alcohols, in which the double O

OH OH

bond is dimethylated at the terminal position (a-prenyl al- RCHO

In(OTf) cohols 122), with aldehydes in the presence of a catalytic 3

127

amount of indium(III) triflate led to an oxonium interme- 126

oxonium

H O diate, which underwent a (3,5) oxonium-ene-type cycliza- 2

tion at 0 °C to afford polysubstituted tetrahydrofurans; trapping

R OH tetrahydropyrans were similarly obtained from the ho- R

213 OH

mologated bishomoallylic alcohols. Based on this O

[3,3] chemical reaction, a kinetic resolution of bishomoallylic OH

alcohols was developed with the aid of the steroidal alde- 128

hyde 123 as the chiral inducer (Scheme 59). Both enantio- 67% mers of the bishomoallylic alcohol can form an oxonium Scheme 60 a-Regioselective prenylation of aldehydes intermediate with the chiral aldehyde, but only the enanti-

13.4 Protection of Carbonyl Compounds, solvent in the indium(III) triflate catalyzed Friedel–Crafts Alcohols and Amines reaction of aniline and anisole derivatives with methyl tri- fluoropyruvate.228 Indium(III) chloride catalyzes the protection of aldehydes and ketones as their corresponding 1,3-dioxolanes or di- In the presence of a catalytic amount of indium(III) salts, alkyl acetals in refluxing cyclohexane. The reverse reac- indoles reacted at the 3-position with a variety of electro- tion, namely the deprotection, is carried out in refluxing philes, such as cyclic allylic acetates,229 acetic anhy- aqueous methanol under catalysis by indium(III) chlo- dride,230 aryl sulfonyl chlorides231 or secondary benzylic ride.217 The acetalization of carbonyl compounds was also alcohols.196 In the latter reaction, water was eliminated; found to be particularly efficient with trialkyl orthofor- such an elimination occurs in the preparation of diaryl- mate and with diols in the presence of indium(III) triflate methanes from trioxane as the electrophile, in the pres- (1 mol%) in dichloromethane.218 ence of indium(III) chloride as the catalyst.232 Thioacetalization of aldehydes and ketones was per- Indium(III) triflate was used to activate the carbon–car- formed under catalysis by indium(III) bromide in dichlo- bon triple bond of a terminal alkyne in the Friedel–Crafts romethane at room temperature; in contrast to the alkenylation of arenes; double addition occurred with het- behavior of diacetals, dithioacetals are stable under aque- erocyclic arenes to afford 2:1 adducts, where two hetero- ous conditions in the presence of indium(III) salts, so that cyclic arenes regioselectively attacked the same alkyne dithioacetalization was even possible in aqueous me- carbon atom.233 With propargyl ethers, 2-arylindoles gave dia!219 Thioacetalization of carbonyl compounds or trans- annulated carbazoles with the use of a Lewis acid stronger thioacetalization of diacetals into dithioacetals were also than indium(III) triflate, such as indium(III) nonaflate performed in the presence of indium(III) triflate.220 (Scheme 62).234

The protection of aldehydes into acylals (geminal diace- IIIIn

tates) was achieved under catalysis by indium(III) bro- OMe mide under solvent-free conditions at room OMe

temperature;221 the deprotection occurred in refluxing wa-

In(ONf)

222 Z

ter with indium(III) bromide as catalyst. Acetylation of (30 mol%)

Z 129

C F ONf = OSO

4 9 alcohols with acetic anhydride was catalyzed by indi- 2

um(III) chloride under microwave conditions,223 or direct- Z = NH, S ly by indium(III) triflate.224 A convenient protection of alcohols as their trimethylsilyl

ethers was performed in dichloromethane with hexameth-

70%

Z yldisilazane in the presence of indium(III) bromide (5 Z

mol%).225 The catalyst formed a complex with hexameth- 130 yldisilazane, making the silylating agent more electro- Scheme 62 Annulation of heterocyclic arenes catalyzed by indi- philic. Both of the trimethylsilyl groups were transferred um(III) nonaflate to the alcohol, thereby liberating a mole of ammonia (Scheme 61). On the other hand, indium(III) bromide and 13.6 Activation of Carbon–Carbon Multiple indium(III) chloride catalyze the N-tert-butyloxycarbony- Bonds lation of amines with Boc anhydride at room temperature under solvent-free conditions.226 Indium(III) chloride and indium(III) triflate were found to

be efficient catalysts for the regiospecific Markovnikov-

Si) NH (Me type addition of thioacetic acid to alkenes. The reactions 3 2

InBr 3 were, for the most part, carried out in refluxing dichloro-

ethane with 5 mol% of catalyst.235 When indium(III) tri- InBr Br

2 flate was used as catalyst in refluxing nitromethane, thiols

H N-InBr

Me Si N H

3 3

3 added to alkenes in a regiospecific manner, by either an

SiMe 236

3 inter- or an intramolecular pathway.

ROH ROSiMe InBr 3 Br

2 In the chemoselective dimerization of vinylarenes at 0 °C,

up to 94%

Me Si N H 3 the first step is the activation of the carbon–carbon double

237 ROH

H bond by indium(III) bromide as catalyst. ROSiMe 3 We have already seen that the triple bond of a terminal Scheme 61 Silylation of alcohols catalyzed by indium(III) bromide alkyne could be activated by indium(III) species in Friedel–Crafts reactions or in the a-alkylation of carbonyl 13.5 Friedel–Crafts Reactions compounds. When propargylamines or alcohols add to ethenetricarboxylates according to a Michael process, the Acylation of activated benzenes was carried out with cat- indium enolate intermediate undergoes a cyclization via alytic amounts of indium in solvent (refluxing dioxane) or the activation of the triple bond by indium(III) bromide, under solvent-free conditions.227 Water was used as the providing methylenepyrrolidine or methylenetetrahydro-

238

furan derivatives. An indium(III) bromide catalyzed X

InCl (20 mol%) R cyclization also occurred with the 2-alkynylaniline 131, 3

239

CH Cl , r.t. 132 E 2 leading to the indole . When the reaction was car- 2

135 ried out using a substrate with a trimethylsilyl group or E

with no substituent group on the terminal carbon, such as Br

136 (79%)

133, dimerization occurred and led to the quinoline deriv- R = Ph; X = Br (97%) R = H; X = Cl ative 134 (Scheme 63).240 137 In the presence of indium(III) chloride as catalyst, 1,6- Scheme 64 Atom-transfer cyclization catalyzed by indium(III) enynes with a terminal triple bond gave 1-vinylcycloal- kenes after activation of the triple bond by indium(III); 13.8 Ferrier Rearrangement and Glycosylation with an alkyl group on the extremity of the triple bond, a skeletal reorganization occurs.241 In the first report of the indium(III) chloride catalyzed

Ferrier rearrangement, it was mentioned that treatment of Ph tri-O-acetyl glucal with various alcohols and phenols at

room temperature in dichloromethane gave the alkyl and InBr (5 mol%)

Ph aryl 2,3-unsaturated glycopyranosides rapidly, with a pre-

toluene, reflux

N 246 1 h

NH 2 ferred a-selectivity (62–92%). The reaction was since

131 132 95% extended to other glycals in acetonitrile under microwave conditions.247 With 2-C-acetoxymethyl glycal derivatives

in the presence of a catalytic amount of indium(III) chlo- InBr (1 equiv) NH 2

3 ride, alcohols gave the corresponding 2-C-methylene gly- MeOH, reflux

N 248

24 h NH

2 cosides with exclusive a-selectivity, except in one case.

89% 134

133 The Ferrier rearrangement applied to silyl nucleophiles, Scheme 63 Cyclization of 2-alkynylanilines promoted by indi- such as allyltrimethylsilane, trimethylsilyl cyanide, tri- 249 250 um(III) bromide methylsilyl azide and alkynyltrimethylsilanes, gave the corresponding 2,3-unsaturated allyl glycosides, glycosyl cyanides, glycosyl azides and alkynyl glycosides with 13.7 Activation of an Oxygen or a Halogen Atom high a-selectivity when the reaction was performed in the in a Single Bond presence of indium(III) bromide (5 mol%) in dichlo- The rearrangement and ring opening of epoxides8 have romethane at room temperature. Under microwave condi- been correlated to the heterophilicity of indium(III) salts, tions, the reaction was achieved in less than one minute.251 allowing, for example, a convenient conversion of ep- Another method for the preparation of a-alkynyl glyco- oxides into thiiranes to take place with thiocyanates.242 sides from alkynyltrimethylsilanes was recently reported; Allylic and benzylic acetates were found to be reactive to- this method used d-hydroxy-a,b-unsaturated aldehydes via indium(III) bromide mediated hemiacetalization fol- wards allyltrimethysilane in the presence of a catalytic 252 amount of indium(III) bromide (5 mol%); this deoxygen- lowed by C-glycosylation. ative allylation was possible even directly from benzylic In the synthesis of C-aryl glycosides with a free amino alcohols,243 owing to the possible activation by silicon functionality, aryl amines were put together with tri-O- (see below). acetyl glucal 77 in the presence of indium(III) bromide In the Beckmann rearrangement, the hydroxyl group of an (10 mol%); the Ferrier rearrangement was followed by an oxime is first activated by an acid; in a recent application intramolecular nucleophilic substitution by the amine function and afforded the benzo-fused heterocycle 138 of this rearrangement catalyzed by indium(III) chloride, 253 oxazoloquinolines were prepared directly from the corre- (Scheme 65).

sponding 3-acyl-4-quinolinone ketoximes.244

OAc OAc

Cyclization between allylic halide and alkyne moieties in O

AcO

InBr 3 O

1,6-enynes 135 occurred with halogen transfer in dichlo- Me (10 mol%)

romethane; indium(III) chloride as catalyst was purported HN

CH Cl H N 2 2

to activate the allylic halogen, whereas the alkyne func- 2

tion acted as the nucleophilic partner.245 Depending on the r.t. (87%) alkyne substitution pattern, a bromine atom was trans- 138 ferred from the substrate, or a chlorine atom was trans- Scheme 65 Cyclization of glycals with aryl amines catalyzed by in- ferred from the solvent, to give either 136 or 137 dium(III) bromide (Scheme 64). In addition to glycals, glycosyl halides can be useful as electrophilic partners for carbon nucleophiles in the synthesis of C-glycosides. Thus, C-glycosyl indoles and pyrroles were prepared within a few minutes through the

Synthesis 2007, No. 12, 1739–1764 © Thieme Stuttgart · New York REVIEW Indium-Promoted Organic Reactions 1761 coupling of acetylated glycosyl bromides with indoles and date, with a low viscosity and a high density, facilitating pyrroles in the presence of indium(III) chloride (10 both the stirring and the product separation process.258,259 mol%).254 Similarly, O-glycosylation was performed through the References coupling of acetylated glycosyl bromides with alcohols in the presence of indium(III) chloride (40 mol%), thereby (1) Araki, S.; Ito, H.; Butsugan, Y. J. Org. Chem. 1988, 53, 1831. providing the corresponding O-glycosides with pro- (2) Li, C.-J.; Chan, T. H. Tetrahedron Lett. 1991, 32, 7017. nounced b-selectivity owing to the presence of a partici- (3) Li, C.-J. Chem. Rev. 1993, 93, 2023. pating group at the C2 position.255 With thiols instead of (4) Li, C.-J. Tetrahedron 1996, 52, 5643. alcohols, peracetylated sugars were reactive enough to af- (5) Lubineau, A.; Augé, J.; Queneau, Y. Synthesis 1994, 741. ford the corresponding b-thioglycosides, but the presence (6) Paquette, L. A. In Green Chemistry: Frontiers in Benign of titanium(IV) chloride (20 mol%) along with indi- Chemical Syntheses and Processes; Anastas, P. T.; um(III) chloride (20 mol%) was required.256 Williamson, T. C., Eds.; Oxford University Press: Oxford, 1998, 250. (7) Lubineau, A.; Augé, J. Top. Curr. Chem. 1999, 206, 1. (8) Ranu, B. C. Eur. J. Org. Chem. 2000, 2347. 14 Conclusion (9) Chauhan, K. K.; Frost, C. G. J. Chem. Soc., Perkin Trans. 1 2000, 3015. This review shows the large diversity of reactions that are (10) Podlech, J.; Maier, T. C. Synthesis 2003, 633. promoted by indium. The low first ionization potential of (11) Nair, V.; Ros, S.; Jayan, C. N.; Pillai, B. S. Tetrahedron 2004, 60, 1959. indium(0) is responsible for its faculty to reduce various (12) Chan, T. H.; Yang, Y. J. Am. Chem. Soc. 1999, 121, 3228. chemical functions even in aqueous media. Thus, indium (13) Miyabe, H.; Naito, T. Org. Biomol. Chem. 2004, 2, 1267. metal is a suitable agent for mediating carbon–carbon (14) Shibata, I.; Kato, H.; Ishida, T.; Yasuda, M.; Baba, A. bond reactions, as it can tolerate oxygen and nitrogen Angew. Chem. Int. Ed. 2004, 43, 711. functionalities. Furthermore, indium does not form the (15) Takami, K.; Mikami, S.; Yorimitsu, H.; Shinokubo, H.; corresponding oxides when exposed to air. Indium is an Oshima, K. J. Org. Chem. 2003, 68, 6627. interesting alternative to toxic tin hydride in radical cy- (16) Loh, T.-P.; Chua, G.-L. Chem. Commun. 2006, 2739. (17) Preite, M. D.; Pérez-Carvajal, A. Synlett 2006, 3337. clizations. The higher chemo-, regio-, stereo- and enantio- (18) Tuck, D. G. Chem. Soc. Rev. 1993, 269. selectivities displayed by allyl-, propargyl-, allenyl- or (19) Pardoe, J. A.; Downs, A. J. Chem. Rev. 2007, 107, 2. alkynylindium reagents, compared to Grignard reagents, (20) Gyname, M. J. S.; Waterworth, L. G.; Worrall, I. J. J. make them useful tools in organic synthesis. For a better Organomet. Chem. 1972, 40, C9. understanding of the reactions, the organoindium species, (21) Araki, S.; Ito, H.; Katsumura, N.; Butsugan, Y. J. which differentiate according to the solvent, were identi- Organomet. Chem. 1989, 369, 291. fied by NMR measurements. (22) Tyrra, W.; Wickleder, M. S. J. Organomet. Chem. 2003, 677, 28. The chemical applications of indium(I) in synthesis is still (23) Peppe, C.; Tuck, D. G.; Victoriano, L. J. Chem. Soc., Dalton in its infancy, probably because of its propensity to dis- Trans. 1982, 2165. proportionate. In the presence of an oxidizing agent, indi- (24) Marshall, J. A.; Grant, C. M. J. Org. Chem. 1999, 64, 8214. um(I) yields the metastable indium(II) species which is (25) Huang, J.-M.; Xu, K.-C.; Loh, T.-P. Synthesis 2003, 755. (26) Augé, J.; Lubin-Germain, N.; Thiaw-Woaye, A. rapidly oxidized to the stable indium(III) species. Tetrahedron Lett. 1999, 40, 9245. In contrast, the reduction of indium(III) chloride with hy- (27) Augé, J.; Lubin-Germain, N.; Marque, S.; Seghrouchni, L. J. drides generates dichloroindium hydride and thereby Organomet. Chem. 2003, 679, 79. paves the way to new reactions, known as hydroindation (28) Chung, W. J.; Higashiya, S.; Oba, Y.; Welch, J. T. Tetrahedron 2003, 59, 10031. reactions. In addition, new reactivities have been demon- (29) Juan, S.; Hua, Z.-H.; Qi, S.; Ji, S.-J.; Loh, T.-P. Synlett 2004, strated since the discovery of a synergistic effect of silici- 829. um and indium atoms, both linked to oxygen or chlorine (30) Harthong, S.; Billard, T.; Langlois, B. Synthesis 2005, 2253. atoms. Lewis acidity is then enforced. Of the indium(III) (31) Kumar, S.; Kumar, V.; Chimni, S. S. Tetrahedron Lett. salts used as Lewis acids, indium(III) bromide is the 2003, 44, 2101. weakest, but the striking difference observed between in- (32) Cravotto, G.; Giovenzana, G. B.; Maspero, A.; Pilati, T.; dium(III) chloride [or indium(III) bromide] and indi- Penoni, A.; Palmisano, G. Tetrahedron Lett. 2006, 47, 6439. (33) Levoirier, E.; Canac, Y.; Norsikian, S.; Lubineau, A. um(III) triflate comes from the lack of nucleophilicity of Carbohydr. Res. 2004, 339, 2737. the triflate anion compared to the halide anion. Although (34) Min, J.-H.; Jung, S.-Y.; Wu, B.; Oh, J. T.; Lah, M. S.; Koo, these salts tolerate water in many reactions, they are more S. Org. Lett. 2006, 8, 1459. easily hydrolyzable than lanthanide salts, as evidenced by (35) Loh, T.-P.; Tan, K.-T.; Hu, Q.-Y. Tetrahedron Lett. 2001, 3+ their pKh (Kh = hydrolysis constant) values (4 for In 42, 8705. compared to 7.7 for Yb3+).257 (36) Tan, K.-T.; Chng, S.-S.; Cheng, H.-S.; Loh, T.-P. J. Am. Chem. Soc. 2003, 125, 2958. A new challenge in indium chemistry might come from (37) Lombardo, M.; Morganti, S.; Trombini, C. J. Org. Chem. the faculty of indium(III) chloride to form binary ionic 2003, 68, 997. liquids, such as butylmethylimidazolium tetrachloroin- (38) Lombardo, M.; Licciulli, S.; Trombini, C. Tetrahedron Lett. 2003, 44, 9147.

(39) Lombardo, M.; Pasi, F.; Trombini, C. Eur. J. Org. Chem. (75) Teo, Y.-C.; Tan, K.-T.; Loh, T.-P. Chem. Commun. 2005, 2006, 3061. 1318. (40) Paquette, L. A. Synthesis 2003, 765. (76) Teo, Y.-C.; Goh, J.-D.; Loh, T.-P. Org. Lett. 2005, 2743. (41) Kim, J. H.; Seo, W. D.; Lee, J. H.; Lee, B. W.; Park, K. H. (77) Teo, Y.-C.; Goh, E.-L.; Loh, T.-P. Tetrahedron Lett. 2005, Synthesis 2003, 2473. 46, 6209. (42) Chng, S.-S.; Hoang, T.-G.; Lee, W.-W. W.; Tham, M.-P.; (78) Lu, J.; Hong, M.-L.; Ji, S.-J.; Loh, T.-P. Chem. Commun. Ling, H. Y.; Loh, T.-P. Tetrahedron Lett. 2004, 45, 9501. 2005, 1010. (43) Loh, T.-P.; Yin, Z.; Song, H.-Y.; Tan, K.-L. Tetrahedron (79) Lu, J.; Hong, M.-L.; Ji, S.-J.; Teo, Y.-C.; Loh, T.-P. Chem. Lett. 2003, 44, 911. Commun. 2005, 4217. (44) Hirayama, L. C.; Gamsey, S.; Knueppel, D.; Steiner, D.; (80) Lu, J.; Ji, S.-J.; Teo, Y.-C.; Loh, T.-P. Org. Lett. 2005, 159. DeLaTorre, K.; Singaram, B. Tetrahedron Lett. 2005, 46, (81) Teo, Y.-C.; Goh, E.-L.; Loh, T.-P. Tetrahedron Lett. 2005, 2315. 46, 4573. (45) Lloyd-Jones, G. C.; Wall, P. D.; Slaughter, J. L.; Parker, A. (82) Lu, J.; Ji, S.-J.; Loh, T.-P. Chem. Commun. 2005, 2345. J.; Laffan, D. P. Tetrahedron 2006, 62, 11402. (83) Lu, J.; Ji, S.-J.; Teo, Y.-C.; Loh, T.-P. Tetrahedron Lett. (46) Miao, W.; Chung, L. W.; Wu, Y.-D.; Chan, T. H. J. Am. 2005, 46, 7435. Chem. Soc. 2004, 126, 13326. (84) Yanada, R.; Obika, S.; Kono, H.; Takemoto, Y. Angew. (47) Loh, T.-P.; Lin, M.-J.; Tan, K.-L. Tetrahedron Lett. 2003, Chem. Int. Ed. 2006, 45, 3822. 44, 507. (85) Yadav, J. S.; Reddy, B. V. S.; Reddy, M. S.; Parimala, G. (48) Lin, M.-J.; Loh, T.-P. J. Am. Chem. Soc. 2003, 125, 13042. Synthesis 2003, 2390. (49) Lu, W. L.; Ma, J.; Yang, Y.; Chan, T. H. Org. Lett. 2000, 2, (86) Araki, S.; Kamei, T.; Hirashita, T.; Yamamura, H.; Kawai, 3469. M. Org. Lett. 2000, 2, 847. (50) Miao, W.; Lu, W.; Chan, T. H. J. Am. Chem. Soc. 2003, 125, (87) Hirashita, T.; Kambe, S.; Tsuji, H.; Omori, H.; Araki, S. J. 2412. Org. Chem. 2004, 69, 5054. (51) Arimitsu, S.; Hammond, G. B. J. Org. Chem. 2006, 71, (88) Jang, T.-S.; Keum, G.; Kang, S. B.; Chung, B. Y.; Kim, Y. 8865. Synthesis 2003, 775. (52) Arimitsu, S.; Jacobsen, J. M.; Hammond, G. B. Tetrahedron (89) Miyabe, H.; Yamaoka, Y.; Naito, T.; Takemoto, Y. J. Org. Lett. 2007, 48, 1625. Chem. 2003, 68, 6745. (53) Peppe, C.; das Chagas, R. P. Synlett 2006, 605. (90) Marshall, J. A.; Palovich, M. R. J. Org. Chem. 1997, 62, (54) Babu, S. A.; Yasuda, M.; Shibata, I.; Baba, A. Org. Lett. 6001. 2004, 4475. (91) Marshall, J. A.; Eidam, P.; Eidam, H. S. J. Org. Chem. 2006, (55) Babu, S. A.; Yasuda, M.; Shibata, I.; Baba, A. J. Org. Chem. 71, 4840. 2005, 70, 10408. (92) Marshall, J. A.; Ellis, K. C. Org. Lett. 2003, 5, 1729. (56) Kumar, S.; Kaur, P. Tetrahedron Lett. 2004, 45, 3413. (93) Miyabe, H.; Yamaoka, Y.; Naito, T.; Takemoto, Y. J. Org. (57) Vilaivan, T.; Winotapan, C.; Banphavichit, V.; Shinada, T.; Chem. 2004, 69, 1415. Ohfune, Y. J. Org. Chem. 2005, 70, 3464. (94) Yasuda, M.; Miyai, T.; Shibata, I.; Baba, A.; Nomura, R.; (58) Chen, Q.; Qiu, X.-L.; Qing, F.-L. J. Org. Chem. 2006, 71, Matsuda, H. Tetrahedron Lett. 1995, 36, 9497. 3762. (95) Klaps, E.; Schmid, W. J. Org. Chem. 1999, 64, 7537. (59) Law, M. C.; Cheung, T. W.; Wong, K.-Y.; Chan, T. H. J. (96) Miao, W.; Chan, T. H. Synthesis 2003, 785. Org. Chem. 2007, 72, 923. (97) Lee, P. H.; Kim, S.; Lee, K.; Seomoon, D.; Kim, H.; Lee, S.; (60) Jang, T.-S.; Ku, W.; Jang, M. S.; Keum, G.; Kang, S. B.; Kim, M.; Han, M.; Noh, K.; Livinghouse, T. Org. Lett. 2004, Chung, B. Y.; Kim, Y. Org. Lett. 2006, 8, 195. 6, 4825. (61) Hirashita, T.; Toumatsu, S.; Imagawa, Y.; Araki, S.; (98) Goeta, A.; Salter, M. M.; Shah, H. Tetrahedron 2006, 62, Setsune, J. Tetrahedron Lett. 2006, 47, 1613. 3582. (62) Bernardi, L.; Cerè, V.; Femoni, C.; Pollicino, S.; Ricci, A. J. (99) Miura, K.; Fujisawa, N.; Hosomi, A. J. Org. Chem. 2004, 69, Org. Chem. 2003, 68, 3348. 2427. (63) Ritson, D. J.; Cox, R. J.; Berge, J. Org. Biomol. Chem. 2004, (100) Miura, K.; Fujisawa, N.; Toyohara, S.; Hosomi, A. Synlett 2, 1921. 2006, 1883. (64) Cook, G. R.; Maity, B. C.; Kargbo, R. Org. Lett. 2004, 6, (101) Pérez, I.; Sestelo, J. P.; Sarandeses, L. A. Org. Lett. 1999, 1, 1741. 1267. (65) Cook, G. R.; Kargbo, R.; Maity, B. Org. Lett. 2005, 7, 2767. (102) Pérez, I.; Sestelo, J. P.; Sarandeses, L. A. J. Am. Chem. Soc. (66) Foubelo, F.; Yus, M. Tetrahedron: Asymmetry 2004, 15, 2001, 123, 4155. 3823. (103) Pena, M. A.; Sestelo, J. P.; Sarandeses, L. A. Synthesis 2003, (67) Ranu, B.; Das, A. Tetrahedron Lett. 2004, 45, 6875. 780. (68) Augé, J.; Lubin-Germain, N.; Seghrouchni, L. Tetrahedron (104) Pena, M. A.; Sestelo, J. P.; Sarandeses, L. A. Synthesis 2005, Lett. 2002, 43, 5255. 485. (69) Augé, J.; Lubin-Germain, N.; Seghrouchni, L. Tetrahedron (105) Lehmann, U.; Awasthi, S.; Minehan, T. Org. Lett. 2003, 5, Lett. 2003, 44, 819. 2703. (70) Picard, J.; Lubin-Germain, N.; Uziel, J.; Augé, J. Synthesis (106) Qian, M.; Huang, Z.; Negishi, E. Org. Lett. 2004, 6, 1531. 2006, 979. (107) Hirashita, T.; Hayashi, Y.; Mitsui, K.; Araki, S. J. Org. (71) Sakai, N.; Hirasawa, M.; Konakahara, T. Tetrahedron Lett. Chem. 2003, 68, 1309. 2003, 44, 4171. (108) Sakai, N.; Annaka, K.; Konakahara, T. Org. Lett. 2004, 6, (72) Takita, R.; Fukuta, Y.; Ohshima, T.; Shibasaki, M. Org. Lett. 1527. 2005, 7, 1363. (109) Rodriguez, D.; Sestelo, J. P.; Sarandeses, L. A. J. Org. (73) Takita, R.; Yakura, K.; Ohshima, T.; Shibasaki, M. J. Am. Chem. 2004, 69, 8136. Chem. Soc. 2005, 127, 13760. (110) Baker, L.; Minehan, T. J. Org. Chem. 2004, 69, 3957. (74) Marshall, J. A.; Hinkle, K. W. J. Org. Chem. 1995, 60, 1920. (111) Metza, J. T.; Terzian, R. A.; Minehan, T. Tetrahedron Lett. 2006, 47, 8905.

(112) Riveiros, R.; Rodriguez, D.; Sestelo, J. P.; Sarandeses, L. A. (148) Miura, K.; Yamada, Y.; Tomita, M.; Hosomi, A. Synlett Org. Lett. 2006, 8, 1403. 2004, 1985. (113) Black, D. A.; Arndtsen, B. A. Org. Lett. 2006, 8, 1991. (149) Miura, K.; Tomita, M.; Yamada, Y.; Hosomi, A. J. Org. (114) Oh, B. K.; Cha, J. H.; Cho, Y. S.; Choi, K. I.; Koh, H. Y.; Chem. 2007, 72, 787. Chang, M. H.; Pae, A. N. Tetrahedron Lett. 2003, 44, 2911. (150) Hayashi, N.; Shibata, I.; Baba, A. Org. Lett. 2005, 7, 3093. (115) Hirashita, T.; Mitsui, K.; Hayashi, Y.; Araki, S. Tetrahedron (151) Pitts, M. R.; Harrison, J. R.; Moody, C. J. J. Chem. Soc., Lett. 2004, 45, 9189. Perkin Trans. 1 2001, 955. (116) Lee, K.; Kim, H.; Miura, T.; Kiyota, K.; Kusama, H.; Kim, (152) Bernardi, L.; Bonini, B. F.; Capito, E.; Dessole, G.; Comes- S.; Iwasawa, N.; Lee, P. H. J. Am. Chem. Soc. 2003, 125, Franchini, M.; Fochi, M.; Ricci, A. J. Org. Chem. 2004, 69, 9682. 8168. (117) Rodriguez, D.; Sestelo, J. P.; Sarandeses, L. A. J. Org. (153) Cicchi, S.; Bonanni, M.; Cardona, F.; Revuelta, J.; Goti, A. Chem. 2003, 68, 2518. Org. Lett. 2003, 5, 1773. (118) Cho, Y. S.; Jun, B. Y.; Pae, A. N.; Cha, J. H.; Koh, H. Y.; (154) Kim, B. H.; Han, R.; Piao, F.; Jun, Y. M.; Baik, W.; Lee, B. Chang, M. H.; Han, S.-Y. Synthesis 2004, 2620. M. Tetrahedron Lett. 2003, 44, 77. (119) Ferrier, R. J.; Zubkov, O. A. Org. React. (N.Y.) 2003, 62, (155) Chen, L.; Li, Z.; Li, C.-H. Synlett 2003, 732. 569. (156) Han, R.; Son, K. I.; Ahn, G. H.; Jun, Y. M.; Lee, B. M.; Park, (120) Price, S.; Edwards, S.; Wu, T.; Minehan, T. Tetrahedron Y.; Kim, B. H. Tetrahedron Lett. 2006, 47, 7295. Lett. 2004, 45, 5197. (157) Mahesh, M.; Murphy, J. A.; Wessel, H. P. J. Org. Chem. (121) Augé, J.; Lubin-Germain, N.; Uziel, J. unpublished results. 2005, 70, 4118. (122) Takami, K.; Usugi, S.; Yorimitsu, H.; Oshima, K. Synthesis (158) Wang, C.; Pan, Y.; Wu, A. Tetrahedron 2007, 63, 429. 2005, 824. (159) Ranu, B. C.; Mandal, T. Synlett 2004, 1239. (123) Yanada, R.; Koh, Y.; Nishimori, N.; Matsumura, A.; Obika, (160) Ranu, B. C.; Mandal, T. Tetrahedron Lett. 2006, 47, 6911. S.; Mitsuya, H.; Fujii, N.; Takemoto, Y. J. Org. Chem. 2004, (161) Ranu, B. C.; Mandal, T. Can. J. Chem. 2006, 84, 762. 69, 2417. (162) Ranu, B. C.; Mandal, T. J. Org. Chem. 2004, 69, 5793. (124) Yanada, R.; Obika, S.; Nishimori, N.; Yamauchi, M.; (163) Ranu, B. C.; Chattopadhyay, K.; Banerjee, S. J. Org. Chem. Takemoto, Y. Tetrahedron Lett. 2004, 45, 2331. 2006, 71, 423. (125) Yanada, R.; Obika, S.; Oyama, M.; Takemoto, Y. Org. Lett. (164) Ranu, B. C.; Mandal, T.; Samanta, S. Org. Lett. 2003, 5, 2004, 6, 2825. 1439. (126) Hirashita, T.; Tanaka, J.; Hayashi, A.; Araki, S. Tetrahedron (165) Peppe, C.; Lang, E. S.; Ledesma, G. N.; de Castro, L. B.; do Lett. 2005, 46, 289. Rego Barros, O. S.; de Azevedo Mello, P. Synlett 2005, (127) Keh, C. C. K.; Wie, C.; Li, C.-J. J. Am. Chem. Soc. 2003, 3091. 125, 4062. (166) Tiwari, P.; Misra, A. K. Tetrahedron Lett. 2006, 47, 2345. (128) Ranu, B.; Mandal, T. Tetrahedron Lett. 2006, 47, 2859. (167) Munbunjong, W.; Lee, E. H.; Chavasiri, W.; Jang, D. O. (129) Miyabe, H.; Ueda, M.; Nishimura, A.; Naito, T. Tetrahedron Tetrahedron Lett. 2005, 46, 8769. 2004, 60, 4227. (168) Ranu, B. C.; Mandal, T. Tetrahedron Lett. 2006, 47, 5677. (130) Sugi, M.; Sakuma, D.; Togo, H. J. Org. Chem. 2003, 68, (169) Ranu, B. C.; Das, A. Adv. Synth. Catal. 2005, 347, 712. 7629. (170) Cho, S.; Kang, S.; Keum, G.; Kang, S. B.; Han, S.-Y.; Kim, (131) Miyai, T.; Inoue, K.; Yasuda, M.; Baba, A. Tetrahedron Y. J. Org. Chem. 2003, 68, 180. Lett. 1998, 39, 1929. (171) Mukaiyama, T.; Ohno, T.; Nishimura, T.; Han, J. S.; (132) Hayashi, N.; Honda, H.; Yasuda, M.; Shibata, I.; Baba, A. Kobayashi, S. Chem. Lett. 1990, 2239. Org. Lett. 2006, 8, 4553. (172) Yang, L.; Xu, L.-W.; Xia, C.-G. Tetrahedron Lett. 2007, 48, (133) Inoue, K.; Sawada, A.; Shibata, I.; Baba, A. J. Am. Chem. 1599. Soc. 2002, 124, 906. (173) Onishi, Y.; Ito, T.; Yasuda, M.; Baba, A. Tetrahedron 2002, (134) Ranu, B. C.; Das, A.; Hajra, A. Synthesis 2003, 1012. 58, 8227. (135) Ranu, B. C.; Banerjee, S.; Das, A. Tetrahedron Lett. 2004, (174) Onishi, Y.; Ogawa, D.; Yasuda, M.; Baba, A. J. Am. Chem. 45, 8579. Soc. 2002, 124, 13691. (136) Das, B.; Banerjee, J.; Chowdhury, N.; Majhi, A.; Holla, H. (175) Yasuda, M.; Yamasaki, S.; Onishi, Y.; Baba, A. J. Am. Synlett 2006, 1879. Chem. Soc. 2004, 126, 7186. (137) Ranu, B. C.; Samanta, S. Tetrahedron 2003, 59, 7901. (176) Babu, S. A.; Yasuda, M.; Baba, A. Org. Lett. 2007, 9, 405. (138) Ranu, B. C.; Samanta, S. J. Org. Chem. 2003, 68, 7130. (177) Yasuda, M.; Onishi, Y.; Ueba, M.; Miyai, T.; Baba, A. J. (139) Wang, C.; Pan, Y.; Yang, D. J. Organomet. Chem. 2005, Org. Chem. 2001, 66, 7741. 690, 1705. (178) Yasuda, M.; Saito, T.; Ueba, M.; Baba, A. Angew. Chem. Int. (140) Wang, C.; Yan, I.; Zheng, Z.; Yang, D.; Pan, Y. Tetrahedron Ed. 2004, 43, 1414. 2006, 62, 7712. (179) Saito, T.; Yasuda, M.; Baba, A. Synlett 2005, 1737. (141) Xue, Z.; Yang, D. Y.; Wang, C. Y. J. Organomet. Chem. (180) Saito, T.; Nishimoto, Y.; Yasuda, M.; Baba, A. J. Org. 2006, 691, 247. Chem. 2006, 71, 8516. (142) Wang, C.-Y.; Su, H.; Yang, D.-Y. Synlett 2004, 561. (181) Wabnitz, T. C.; Yu, J.-Q.; Spencer, J. B. Chem. Eur. J. 2004, (143) Qin, Y.-Y.; Yang, Y.-Y.; Qiu, X.-L.; Qing, F.-L. Synthesis 10, 484. 2006, 1475. (182) Fu, F.; Teo, Y.-C.; Loh, T.-P. Org. Lett. 2006, 8, 5999. (144) Takami, K.; Yorimitsu, H.; Oshima, K. Org. Lett. 2004, 6, (183) Harada, S.; Handa, S.; Matsunaga, S.; Shibasaki, M. Angew. 4555. Chem. Int. Ed. 2005, 44, 4365. (145) Takami, K.; Mikami, S.; Yorimitsu, H.; Shinokubo, H.; (184) Gregg, B. T.; Earley, W. G.; Golden, K. C.; Quinn, J. F.; Oshima, K. Tetrahedron 2003, 59, 6627. Razzano, D. A.; Rennells, W. M. Synthesis 2006, 4200. (146) Hayashi, N.; Shibata, I.; Baba, A. Org. Lett. 2004, 6, 4981. (185) Munoz-Muniz, O.; Quintanar-Audelo, M.; Juaristi, E. J. (147) Benati, L.; Bencivenni, G.; Leardini, R.; Nanni, D.; Minozzi, Org. Chem. 2003, 68, 1622. M.; Spagnolo, P.; Scialpi, R.; Zanardi, G. Org. Lett. 2006, 8, (186) Fu, F.; Teo, Y.-C.; Loh, T.-P. Tetrahedron Lett. 2006, 47, 2499. 4267.

(187) Loh, T.-P.; Chen, S.-L. Org. Lett. 2002, 4, 3647. (225) Yadav, J. S.; Reddy, B. V. S.; Basak, A. K.; Baishya, G.; (188) Chen, S.-L.; Ji, S.-J.; Loh, T.-P. Tetrahedron Lett. 2003, 44, Narsaiah, V. Synthesis 2006, 3831. 2405. (226) Chankeshwara, S. V.; Chakraborti, A. K. Synthesis 2006, (189) Anniyappan, M.; Muralidharan, D.; Perumal, P. T.; Vittal, J. 2784. J. Tetrahedron 2004, 60, 2965. (227) Jang, D. O.; Moon, K. S.; Cho, D. H.; Kim, J.-G. (190) Friestad, G. K.; Korapala, C. S.; Ding, H. J. Org. Chem. Tetrahedron Lett. 2006, 47, 6063. 2006, 71, 281. (228) Ding, R.; Zhang, H. B.; Chen, Y. J.; Liu, L.; Wang, D.; Li, (191) Ranu, B. C.; Hajra, A.; Jana, U. J. Org. Chem. 2000, 65, C. J. Synlett 2004, 555. 6270. (229) Yadav, J. S.; Reddy, B. V. S.; Rao, K. V.; Rao, P. P.; Raj, K. (192) Martins, M. A. P.; Teixeira, M. V. M.; Cunico, W.; Scapin, S.; Prasad, A. R.; Prabhakar, A.; Jagadeesh, B. Synlett 2006, E.; Mayer, R.; Pereira, C. M. P.; Zanatta, N.; Bonacorso, H. 3447. G.; Peppe, C.; Yuan, Y.-F. Tetrahedron Lett. 2004, 45, 8991. (230) Nagarajan, R.; Perumal, P. T. Tetrahedron 2002, 58, 1229. (193) Yadav, J. S.; Reddy, B. V. S.; Venugopal, C.; Padmavani, B. (231) Yadav, J. S.; Reddy, B. V. S.; Krishna, A. D.; Swamy, T. Tetrahedron Lett. 2004, 45, 7507. Tetrahedron Lett. 2003, 44, 6055. (194) Nakamura, M.; Endo, K.; Nakamura, E. J. Am. Chem. Soc. (232) Sun, H.-B.; Hua, R.; Yin, Y. Tetrahedron Lett. 2006, 47, 2003, 125, 13002. 2291. (195) Nakamura, M.; Endo, K.; Nakamura, E. Org. Lett. 2005, 7, (233) Tsuchimoto, T.; Hatanaka, K.; Shirakawa, E.; Kawakami, Y. 3279. Chem. Commun. 2003, 2454. (196) Yasuda, M.; Somyo, T.; Baba, A. Angew. Chem. Int. Ed. (234) Tsuchimoto, T.; Matsubayashi, H.; Kaneko, M.; Shirakawa, 2006, 45, 793. E.; Kawakami, Y. Angew. Chem. Int. Ed. 2005, 44, 1336. (197) Zhang, Y.; Li, C.-J. Angew. Chem. Int. Ed. 2006, 45, 1949. (235) Weïner, M.; Dunach, E. Tetrahedron Lett. 2006, 47, 287. (198) Zhang, Y.; Li, C.-J. J. Am. Chem. Soc. 2006, 128, 4242. (236) Weïner, M.; Coulombel, L.; Dunach, E. Chem. Commun. (199) Yang, J.; Viswanathan, G. S.; Li, C.-J. Tetrahedron Lett. 2006, 332. 1999, 40, 1627. (237) Peppe, C.; Lang, E. S.; Molinos de Andrade, F.; Borges de (200) Yang, X.-F.; Mague, J. T.; Li, C.-J. J. Org. Chem. 2001, 66, Castro, L. Synlett 2004, 1723. 739. (238) Morikawa, S.; Yamazaki, S.; Furusaki, Y.; Amano, N.; (201) Chan, K.-P.; Loh, T.-P. Tetrahedron Lett. 2004, 45, 8387. Zenke, K.; Kakiuchi, K. J. Org. Chem. 2006, 71, 3540. (202) Chan, K.-P.; Loh, T.-P. Org. Lett. 2005, 7, 4491. (239) Sakai, N.; Annaka, K.; Konakahara, T. Tetrahedron Lett. (203) Chan, K.-P.; Seow, A.-H.; Loh, T.-P. Tetrahedron Lett. 2006, 47, 631. 2007, 48, 37. (240) Sakai, N.; Annaka, K.; Konakahara, T. J. Org. Chem. 2006, (204) Lee, C.-H. A.; Loh, T.-P. Tetrahedron Lett. 2006, 47, 1641. 71, 3653. (205) Loh, T.-P.; Tan, K.-T.; Hu, Q.-Y. Angew. Chem. Int. Ed. (241) Miyanohana, Y.; Chatani, N. Org. Lett. 2006, 8, 2155. 2001, 40, 2921. (242) Yadav, J. S.; Reddy, B. V. S.; Baishya, G. Synlett 2003, 396. (206) Dobbs, A. P.; Martinovic, S. Tetrahedron Lett. 2002, 43, (243) Kim, S. H.; Shin, C.; Pae, A. N.; Koh, H. Y.; Chang, M. H.; 7055. Chung, B. Y.; Cho, Y. S. Synthesis 2004, 1581. (207) Dobbs, A. P.; Guesné, S. J. J.; Martinovic, S.; Coles, S. J.; (244) Yoo, K. H.; Choi, E. B.; Lee, H. K.; Yeon, G. H.; Yang, H. Hursthouse, M. B. J. Org. Chem. 2003, 68, 7880. C.; Pak, C. S. Synthesis 2006, 1599. (208) Yadav, J. S.; Reddy, B. V. S.; Bhaishya, G. Green Chem. (245) Cook, G. R.; Hayashi, R. Org. Lett. 2006, 8, 1045. 2003, 5, 264. (246) Babu, B. S.; Balasubramanian, K. K. Tetrahedron Lett. (209) Lee, C.-H. A.; Loh, T.-P. Tetrahedron Lett. 2004, 45, 5819. 2000, 41, 1271. (210) Loh, T.-P.; Hu, Q.-Y.; Chok, Y.-K.; Tan, K.-T. Tetrahedron (247) Das, S. K.; Reddy, K. A.; Roy, J. Synlett 2003, 1607. Lett. 2001, 42, 9277. (248) Ghosh, R.; Chakraborty, A.; Maiti, D. K.; Puranik, V. G. (211) Loh, T.-P.; Lee, C.-L. K.; Tan, K.-T. Org. Lett. 2002, 4, Tetrahedron Lett. 2005, 46, 8047. 2985. (249) Yadav, J. S.; Reddy, B. V. S. Synthesis 2002, 511. (212) Lee, K.-C.; Lin, M.-J.; Loh, T.-P. Chem. Commun. 2004, (250) Yadav, J. S.; Reddy, B. V. S.; Raju, A. K.; Rao, C. V. 2456. Tetrahedron Lett. 2002, 43, 5437. (213) Loh, T.-P.; Hu, Q.-Y.; Tan, K.-T.; Cheng, H.-S. Org. Lett. (251) Das, S. K.; Reddy, K. A.; Abbineni, C.; Roy, J.; Rao, K. V. 2001, 3, 2669. L. N.; Sachwani, R. H.; Iqbal, J. Tetrahedron Lett. 2003, 44, (214) Chen, S.-L.; Hu, Q.-Y.; Loh, T.-P. Org. Lett. 2004, 6, 3365. 4507. (215) Loh, T.-P.; Feng, L.-C.; Yang, J.-Y. Synthesis 2002, 937. (252) Yadav, J. S.; Raju, A. K.; Sunitha, V. Tetrahedron Lett. (216) Cheng, H.-S.; Loh, T.-P. J. Am. Chem. Soc. 2003, 125, 4990. 2006, 47, 5269. (217) Ranu, B. C.; Jana, R.; Samanta, S. Adv. Synth. Catal. 2004, (253) Yadav, J. S.; Reddy, B. V. S.; Rao, K. V.; Raj, K. S.; Prasad, 346, 446. A. R.; Kumar, S. K.; Kunwar, A. C.; Jayaprakash, P.; (218) Smith, B. M.; Graham, A. E. Tetrahedron Lett. 2006, 47, Jagannath, B. Angew. Chem. Int. Ed. 2003, 42, 5198. 9317. (254) Mukherjee, D.; Sarkar, S. K.; Chowdhury, U. S.; Taneja, S. (219) Ceschi, M. A.; de Arunja Felix, L.; Peppe, C. Tetrahedron C. Tetrahedron Lett. 2007, 48, 663. Lett. 2000, 41, 9695. (255) Mukherjee, D.; Ray, P. K.; Chowdhury, U. S. Tetrahedron (220) Muthusamy, S.; Babu, S. A.; Gunanathan, C. Tetrahedron 2001, 57, 7701. 2002, 58, 7897. (256) Das, S. K.; Roy, J.; Reddy, K. A.; Abbineni, C. Carbohydr. (221) Yin, L.; Zhang, Z.-H.; Wang, Y.-M.; Pang, M.-L. Synlett Res. 2003, 338, 2237. 2004, 1727. (257) Kobayashi, S.; Nagaayama, S.; Busujima, T. J. Am. Chem. (222) Zhang, Z.-H.; Yin, L.; Li, Y.; Wang, Y.-M. Tetrahedron Soc. 1998, 120, 8287. Lett. 2005, 46, 889. (258) Yang, J.-Z.; Tian, P.; Xu, W.-G.; Liu, S.-Z. Thermochim. (223) Das, S. K.; Reddy, K. A.; Krovvidi, V. L. N. R.; Mukkanti, Acta 2004, 412, 1. K. Carbohydr. Res. 2005, 340, 1387. (259) Da Silveira Neto, B. A.; Ebeling, G.; Gonçalves, R. S.; (224) Chauban, K. K.; Frost, C. G.; Love, I.; Waite, D. Synlett Gozzo, F. C.; Eberlin, M. N.; Dupont, J. Synthesis 2004, 1999, 1743. 1155.