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Activation of Bonds by Ion in the in the New Millennium: A Review

Edgars Abele

Latvian Institute of Organic Synthesis, 21 Aizkraukles Street, Riga LV-1006, Latvia E-mail: [email protected]

ABSTRACT

Recent advances in the fluoride ion mediated reactions of Si-Η, Si-C, Si-O, Si-N, Si-P bonds containing are described. Application of silicon bonds activation by fluoride ion in the syntheses of different types of organic compounds is discussed. A new mechanism, based on quantum chemical calculations, is presented. The literature data published from January 2001 to December 2004 are included in this review.

CONTENTS Page 1. INTRODUCTION 45 2. HYDROSILANES 46 3. Si-C BOND 49 3.1. Vinyl and Allyl Silanes 49 3.2. Aryl Silanes 52 3.3. Subsituted Alkylsilanes 54 3.4. Fluoroalkyl Silanes 56 3.5. Other Silanes Containing Si-C Bond 58 4. Si-N BOND 58 5. Si-O BOND 60 6. Si-P BOND 66 7. CONCLUSIONS 66 8. REFERENCES 67

1. INTRODUCTION

Reactions of compounds catalyzed by have been under extensive study for more than twenty-five years. In this field two excellent reviews were published 11,21. Recently a monograph dedicated to hypervalent organosilicon compounds was also published /3/. There are also two reviews on

45 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review fluoride mediated reactions of fluorinated silanes /4/. Two recent reviews are dedicated to fluoride ion activation of silicon bonds in the presence of transition metal catalysts 151. Beside this, a review on recent advances in the synthesis and transformation of heterocycles mediated by fluoride ion activated organosilicon compounds was published in 2002 161. Fluoride ion as activator of silicon bonds is widely described in these works. The present work was carried out in continuation of the review published by Corriu et al. in 1993 111 and our review published in 2001 111. Our aim is to describe the modern methodologies in the synthesis of different classes of compounds (for example, , , and ) mediated by silanes activated + by fluoride ion. The influence of different sources of fluoride ion (B114NF, Bu4N [Ph3SnF2]\ KF, CsF, tris(dimethylamino)sulfur ()difluoride (TASF), tetrabutylammonium triphenyldifluorosilicate

(TBAT), diethylaminosulfur trifluoride (DAST), Ph4Bi-F, CdF2, CuF2) on the chemical process will be discussed. A new mechanism of fluoride ion action, based on quantum chemical calculations, is presented. The fluoride ion mediated reactions of organosilicon compounds containing Si —X bonds (X = H, C, N, Ο, P), and their use in organic synthesis published from January 2001 to December 2004, have been reviewed. The advances in the transformations of heterocycles mediated by fluoride ion activated organosilicon compounds are not included in this review and will be published separately.

2. HYDROSILANES

Diastereoselectivity in the reduction of α-substituted 1 to alcohols 2 with polymethylhydrosiloxane in the presence of Bu4NF was investigated /8/. The experiments showed high syn- selectivity for α-alkoxy-, α-acyloxy- and α-dialkylamino-substituted ketones (syn: anti = 73: 23 up to 100:0). Reduction of α-monoalkylamino proceeded in anti-selective manner (Scheme 1).

Me I Ο η / Bu,NF / THF / 0-5 " C OH OH Η R R' R' R'

1 syn-2 anti-2

R = alkyl, aryl; R' = NHR, NR 2, OCOR, OR Scheme 1

Nitroalkenes 3 undergo enantioselective conjugate reduction using poly(methylhydrosiloxane) (PMHS) in the AgF2 (1 mol.%)/ CH3N02 (10 mol.%)/ (tf)-(S>JOSIPHOS (l-[2-(diphenylphosphino)- ferrocenyl]ethyldicyclohexylphosphine) / toluene / H20 system. Nitroalkenes 4 were obtained in 52-88% yields with ee up to 96% (Scheme 2) 191.

46 Edgars Abele Main Group Metal Chemistry

PhSiH, / CUF2 / (/?MS)-JOSIPHOS MeNO, / Η,Ο / PMHS / PhMe / rt NO2 _ ^ γ NO2 R· R'

R = aryl; R' = alkyl Scheme 2

Synthesis of silyl ethers of oximes 6 was carried out using phase transfer catalytic (PTC) system oxime 5/ / CsF / 18-crown-6 / benzene at 50°C (Scheme 3). It has been found that the optimal amount of cesium fluoride is 5 mol% to oxime and hydrosilane. An increase in the amount of fluoride ion diminishes the yield of desired silylated oxime O-ethers. Silylated ketoximes were isolated up to 78 % yields as main products /10/.

NOH , , „„ NOSiR2R3R4 ίΝ^π R2R3R4SiH ! ο,ρ ! !8

5 6

R-R4 = alkyl, aryl Scheme 3

The mechanism of silylation of acetophenone oxime with dimethylphenylsilane was carried out using quantum chemical method AM-1 (Scheme 4). The first reaction step is the interaction of F-ion with the oxime group hydrogen atom. The calculations have shown that the reaction proceeds without any activation barrier and leads to formation of oxime anion and complex HF...Cs+...18-crown-6. The oxime anion and dimethylphenylsilane form complex 7. The positively charged complex [HF...Cs... 18-crown-6]+ serves as a source of proton. The next step of the reaction is interaction of complex 7 with [HF...Cs... 18-crown-6]+. After formation of the hydrogen molecule the approaching of oxime anion and silicon cation takes place. The reaction is completed with formation of the desired product 6.

47 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review

Scheme 4

The catalytic system Me3SiN3 / solid CsF / 18-crown-6, as the most active, was used in the synthesis of unsymmetric 9 from hydrosilanes 8 (molar ratio hydrosilane : Me3SiN3 : CsF : 18-crown-6 = 1:3: 0.1 : 0.1, Scheme 5). The siloxanes 9 were obtained in 45-70% yields. This method was also used in the synthesis of trisiloxane (yield 33%) from diphenylsilane and 6 equivalents of Me3SiN3 /ll/.

DID2D3C- Me SiN / CsF / 18-crown-6 R'R2R3SiOSiMe, R R R SiH 3 3 9 8 H20 /PhMe / 8ffC

R1 - R3 = alkyl, aryl

Scheme 5

Quantum chemical calculations of unsymmetrical siloxanes 9 formation in the PTC system allowed to propose the following reaction mechanism (Scheme 6). The first step is the interaction of complex [HO*— 2(18-crown-6)Cs+-F"—H+] 10 with hydrosilanes 8. The next step is addition of hydroxyl anion to positively charged silicon and formation of silanol molecule. The following interaction between silanol and fluoride ion gives HF molecule and silanol anion 11. The next reaction step is interaction between anion 11 and azidotrimethylsilane, leading formation of the complex of azidotrimethylsilane with silanol anion. This complex reacts with the proton from 2(18-crown-6)Cs+-F"—H+ species. Addition of the proton to the nitrogen bonded to the silicon atom proceeds. The result of this process is the cleavage of Si-N bond, and formation of HN3 and unsymmetrical disiloxane 9.

48 Edgars Abele Main Group Metal Chemistry

3. Si-C BOND

3.1. Vinyl and Allyl Silanes

ß-Silyl alkenals 12 in the presence of Bu4NF afforded 2-(arylmethyl) 13 in 35-71% yields. The mechanism included formation of pentacoordinated silicon intermediate 14, aryl-l,2-anionotropic rearrangement to the adjacent atom giving 15, its possible cyclization and final removal of

49 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review silyl group from 16 in the presence of F" / H2O /12,13/ (Scheme 7).

CHO H BU4NF/THF VK Ar OHC SiMe2Ar Z-12 13

Η F- \r Λ Ar Η Ar Z-12 ".Ph Me-—- OHC F—Si OHC Λ O" p Me Me Me Me 14 15

R

Ar F-/H2O Η A χ 13 Ύ Η Η0-slM e OSiMe2F I Me 16 Scheme 7

Allylsilane 17 in the system B114NF / DMSO / H20 gives a mixture of silanol 18 and 19 observed by GC-MS (Scheme 8) /14/.

Ph Ph^ .OH Ph^ BU4NF/H20 s /Si / i + /^i-o-si-Me Me Me Me Me Me Me Me

17 18 19 Scheme 8

Allylsilanes 20 undergo fluorodesilylation in the presence of Selectfluor™ 21 and afford allylic 22 in yields up to 100% (Scheme 9) /15/. Enantioselective fluorodesilylation of allyl silanes 23 in the presence of Cinchona alkaloid-derived catalyst 24 leads to fluorides 25 with ee up to 96% /16/.

50 Edgars Abele Main Group Metal Chemistry

F + 21 CHjCN

20

MeO SiRRH"

24 21 / MeCN / -20°C

23 25

R-R"' = alkyl, Ph

Scheme 9

Allylation of benzaldehyde was successfully carried out by allyldimethyl(2-pyridyl) 26 in the presence of Bu4NF or AgF (1.2 equivalents) in THF. The product of allylation 27 in the presence of these fluoride ion sources was isolated in 35 or 53% yields, correspondingly (Scheme 10) /17/.

PhCHO/F-

N ;sv Me Me 27 26 Scheme 10

Allyldifluorosilane (£)-28 bearing a 2-(phenylazo)phenyl group with excess of KF / 18-crown-6 in CDC13 at room temperature during 10 minutes, affords tetrafluorosilicate 29 in 82 % yield /18/. However, no changes were observed on treatment of (Z)-28 with this system in the dark. Photoirradiation (λ = 445 nm) of silane (Z)-28 with an excess of KF in CDClj for 30 minutes gave 91 % of the salt 29 (Scheme 11).

51 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review

Ph Ph Ph—N, N=N KF 'N-N- KF / 18-crown-6 Ν 18-crown-6 hv

x ' F CrFis CΓ^F ^ K+, 18-crown-6 (Z)-28 (£)-28 29 KF / 18-crown-6

no reaction

Scheme 11

3.2. Aryl Silanes

Generation of arynes 31 from 2-(trimethylsilyl)phenyl triflates in the presence of fluoride ion was described in some articles. Thus, triflates 30 in the presence of amines and CsF afforded anilines 32 or 33 in 62-99 % yields. Reaction of compounds 30 with sulfonamides led to products 34 or 35 in 80-99% yields (Scheme 12)/19/.

SiMe.

MeCN OTf 30

30

X = Η, OMe; R = alkyl, aryl Scheme 12

52 Edgars Abele Main Group Metal Chemistry

Triflate 30 and phenol or in the presence of CsF afforded ethers 36 or 37, correspondingly (Scheme 13) /20Λ

SiMe, AiCOOH/CsF ArOH /CsF CAr- MeCN OTf 37 30 36 66-98 % 68-98%

Scheme 13

Triflates 37 in the system diethylselenourea / CsF / MeCN gave diseleninides 38 in high yields (Scheme 14) /21/.

R Se R RI I A^ ^SiMe, Λ. R'v •s JΛ r R' R V OTf CsF/ MeCN / rt R"' ^Se-Se R" m V R ? R"· R'" 64-83% 37 38

R-R'" = H, F, OMe Scheme 14

Bistriflate 39 in the presence of ketone 40 and Bu4NF was converted to electroluminescent "twistacene" 41 (Scheme 15) 1221.

^JL ^X. BU4NF / CH2C12

39 Scheme 15

53 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review

Fluoride ion mediated insertion of arenes into S-Sn bound of stannyl sulfides was described in 1231. 2-

(Trimethylsilyl)phenyl triflates 42 in system ArS-SR3'/KF/18-crown-6/THF afford 2-(arylthio)arylstannanes 43 in 39-62% yields (Scheme 16).

.SiMe3 .SnR,' f> ^T ArS-SnR', / KF / 18-crown-6 / THF / 0 ° C IT ^T

OTf v SAr

42 43

Scheme 16

5 5 The indenyl iron compound (Ti -C9H7)Fe(Ti -C5H5) reacts with benzyne, generated from Inflate 42 and CsF, to afford the Diels-Alder adduct l,2-(l,4-dihydro-naphthalen-l,4-diyl)ferrocene /24/.

3.3. Substituted Alkylsilanes

Fluoride mediated ß-elimination of syn- or anti ß-silyl azides 44 led to the corresponding (Z)- or (E)- olefins 45 in yields up to 99 % (Scheme 17) /25, 26/.

N, 44 45

R = alkyl, aryl; R' = Br, C02Et; R" = Me, Ph

Scheme 17

Electronic effects and the stereochemistry in rearrangement-displacement reactions of triaryl(halomethyl)silanes in the presence of fluoride and alkoxide ions were studied in detail 1211. For example, (halomethyl)diphenyl(para-substituted-phenyl)silanes 46 and Bu4NF give fluorodiphenyl(para- substituted-phenylmethyl)silanes 47 and fluorophenyl(phenylmethyl)(para-substituted-phenylmethyl)silanes 48 as the result of the attack on silicon and migrations of the phenyl or para-substituted phenyl groups to C-l with displacement of chloride ion (Scheme 18).

54 Edgars Abele Main Group Metal Chemistry

R

PH JU^ y BU«NF Ph -i ^^ /Su Ph F [ 1 X Ph 46 47 48

X = CI, Br, I; R = H, CI, Me, CF 3, OMe, NMe2 Scheme 18

A highly efficient substitution of the vinyl fluoride of perfluoroketene dithioacetals 49 was achieved using trimethylsilylacetate 50 and Me4NF to give products 51 in 91-94% yields (Scheme 19) /28/.

Rf SEt Me4NF/THF Rf^ ySEt

)>==<^ + Me3Si-"^C02Et F SEt SEt C02Et 49 50 51

R,. = CF3, C2F5 Scheme 19

Reaction of nitrobenzene 52 with ethyl trimethylsilylacetate 50 in the presence of TASF and the subsequent oxidation of intermediate 53 with dimethyldioxirane (DMD) leads to 54 in 62% yield (Scheme 20) /29/.

NO, Me,Si'' ^CO.Et " __ 3 2 OEt 50 DMD

TASF/ -25 °C

CI 52

Selective replacement of silyl group by iodine in the fluorinated substrate 55 was carried out in the system

KF /12 / CH2C12 at room temperature (Scheme 21) /30/.

OBn N-PMP OBn N-PMP I || KF/12 / CHJCIJ/ rt ! j|

Ph^X>SiMe3 Ph^>

55 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review

3.4. Fluoroalkyl Silanes

A review on nucleophilic trifluoromethylation and introduction of fluorinated moieties was published in 2003 /31/. Therefore our review includes data of works published in 2003 and 2004. Some articles are dedicated to addition of (trifluoromethyl)trimethylsilane and other

(perfluoroalkyl)trimethylsilanes to ketones or thioketones. Thus, asymmetric addition of CF3SiMe3 to aldehydes and ketones 57 was efficiently catalyzed by fluoride salts derived from Cinchona alkaloids 58. Reaction products 59 were isolated in high yields with ee up to 92% (Scheme 22) 1321. Asymmmetric catalysis by chiral quaternary ammonium fluorides was recently described in a review /33/.

Ο CFjSiMe·,/ CH2C12 /-50°C ij F,r^C VJO.^yOSiMe, y Λ« " D,/\„ R- "R" R' R" 57 59

Ar = 3,5-(MeO)2-C6H3, 1-naphthyl; R' =alkyl, aryl; R" = H, alkyl

Scheme 22

Reaction of 2-perfluoroalkylchromone 60 with (perfluoroalkyl)trimethylsilanes proceeds as 1,4- nucleophilic perfluoroalkylation and leads to a mixture of silyl ethers 61 and 62 (Scheme 23) /34/.

Rf ~ CF3, C2F5 Scheme 23

Fluoride ion catalyzed reaction of thioketones with (trifluoromethyl)trimethylsilane was described in detail /35/. 2,2,4,4-Tetramethylcyclobutane-l,3-dione (63) in system CF3SiMe3 / Bu4NF / THF afforded cyclobutanone 64 in 74% yield (Scheme 24). In the similar conditions thione 65 afforded thiethane derivative

66 in 70% yield. Dithione 67 in the system CF3SiMe3 / Bu4NF / THF gave dimeric product 68.

56 Edgars Abele Main Group Metal Chemistry

0 CF,SiMe, / Bu 4NF / THF /O

67 68

Scheme 24

Aromatic thioketones 69 and CF3SiMe3 in the presence of dry Bu4NF give the mixture of products 70-73 (Scheme 25).

SCF F Ar CF,SiMe, Arx / 3 At Ar χ. + V Bu NF Ar Ar < Ar Η Ar S Ar Ar

26-30% 5-15% 0-45% 0-3% 69 70 71 72 73

Scheme 25

Trifluoromethylation of aromatic thiones with CF3SiMe3 in the presence of stoichiometric amount of

BU4NF H20 was studied too 1361. Η Nitroarenes 74 in the TASF / CF3SiMe3 / THF / pyridine system at -70°C give σ adducts 75. These adducts were oxidized with DMD to afford substituted trifluoromethyl phenols 76 in overall yields up to 78% (Scheme 26) /37/.

NO, OH DMD

TASF

75 76

X = CN, N02; R = H, 4-C1, 4-Br, 5-N02 Scheme 26

57 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review

3.5. Other Silanes Containing Si-C Bond

In 2004 two articles were published about trimethylsilylcyanation of aldehydes /38/ and ketones /39/ in the system Me3SiCN / CsF (2 or 10 mol.%) / CH3CN at room temperature. The authors wrote in the introduction "we wish to herein report the first example about the silylcyanation of ketones by using a cheap, easy handling and readily available chemical, CsF as catalyst to offer racemic trimethylsilyl ethers in excellent yields in relatively shorter reaction time". However, we published the first example of silylcyanation of ketones catalyzed by CsF in 1999 /40/. In our paper trimethylsilylcyanation of heteroaromatic ketones in the Me3SiCN / CsF (20 mol.%) / 18-crown-6 (10 mol. %) / benzene system was studied (Scheme 27). Products 78 were isolated in 67-95% yields.

Ο Me3SiCN / solid CsF / 18-crown-6 / PhH / 20 ° C / 3-6 h R^.OSiMe3 R'^R R'^TN 77 78

R = alkyl, aryl; R' = Ph, 2-furyl, 2-thienyl, 2-, 3- and 4-pyridyl Scheme 27

Addition of Me3SiCH2CN to benzaldehyde was successfully realized in the presence of KF/ A1203 in THF/41/.

The reaction of chiral sulfinimines 79 with Me3SiCN in the presence of CsF gave α-amino nitriles 80 in 92-99 % yield with ee up to 98% (Scheme 28) I All.

Ο» Ο /S NH Me,SiCN / CsF n-hexane / -50 °C R Λ Η Me R^CN

79 80 R = alkyl, aryl Scheme 28

4. Si-N BOND

Oximes 81 /43/ and carboxylic acids 83 /44/ were easily silylated in the system Me3SiN3 / CsF / 18- crown-6 / benzene or CD2C12 at room temperature. Silylated products 82 and 84 were isolated in yields up to 100% (Scheme 29).

58 Edgars Abele Main Group Metal Chemistry

NOH NOSiMe3

Ar-% Ar^R Me3SiN3 / CsF /18-crown-6 y 82 CD2CI2 or PhH / 20°C A > ^X

R OSiMe3 83 84

Ar = Ph, pyridyl; RSchem = H, NHe 292, alkyl; R' = aryl, hetaryl

Selective removal of one SES (ß-trimethylsilylethanesulfonyl) group of imides 85 using CsF in acetonitrile at 65°C afforded the mono-SES derivatives 86 in 62-91% yields (Scheme 30). Bissulfonimides 85 can be used in efficient one-pot monodeprotection / N- sequence. The treatment of compounds 85 with CsF in the presence of alkylating agent (R"X) leads to N-monoalkylated derivatives 87 in 65-95% yields /45/.

SES- NH NSES2 CsF / MeCN / 65 °C R^R·

85

CsF/R"X/MeCN/65 "C

R, R' = alkyl; R" = alkyl, benzyl, allyl Scheme 30

Coupling reaction of N,N-bis(trimethylsilyl)aniline 88 with nitrosobenzene 89 in the system B114NF on

Si02 (0.05 equiv.) / THF gave a mixture of coupling products 90-92 (Scheme 31) /46/.

/=\ ,SiMe3 Br—(v /)—Ν + Ph-N=0 Si Me, Ο 88 89 90 91 CT I + Ph-N=N-Ph + 92 Scheme 31

59 Vol. 28, No. 2,'2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review

Interaction of trimethylsilyl isothiocyanate (93) with alkyl halides (RBr) in the presence of Bu4NBr in THF leads to alkyl thiocyanates 94 in 77-98% yields (Scheme 32) /47/.

Me / R-Br / BU4NF/THF R Ν Me N=C=S 93 94 R = alkyl Scheme 32

5. Si-OBOND

Selective monodeprotection of silyl groups in the presence of a fluoride ion source was recently described /48/. Therefore deprotection reactions of silyl ethers are not included in this review. Interesting deprotection and elimination of silyl groups in the bis-silyl ethers was described in article /49/.

For example, 95 in the presence of Bu4NF or TASF in DMF afforded 96 (Scheme 33).

OSiBu OSiBu, OH OH Ο OCOPh

Me Me Me Me OMe Me Me Me Me OMe

95 96 Scheme 33

CdF2and AgF can also serve as fluoride ion source in the allylation reactions of aldehydes. /50, 51/. Thus, aldehydes 97 were successfully allylated with allyltrimethoxysilane in the system CdF2 / terpyridine ligand

98 / H20 / THF at 30°C. The catalytic cycle of this reaction included the formation of adduct 101 by reaction of complex 100 with and allyltrimethoxysilane. In the second step, adduct 101 is hydrolyzed to give complex 102 and product 99. Fluorotrimetoxysilane is also hydrolyzed to give product 103 along with generation of MeOH. In the third step, complex 102 reacts with silane 103 to regenerate complex 100 (Scheme 34).

60 Edgars Abele Main Group Metal Chemistry

+ RCHO

102 + R,°v .OR"

F OR'" 103 + MeOH

R'-R"" = H, Si

Scheme 34

Trifluoroacetamides derived from amino alcohols /52/ and trifluoromethanesulfinamide derived from ephedrine /53/ serve as trifluoromethylating agents. Thus, derivatives 104 or 105 and carbonyl compounds in the presence of fluoride ion source afford trifluoromethyl substituted alcohols (Scheme 35).

61 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review

Ph Me „ \ .-- Ο 1) CsF or TBAT (0.1 equiv.) / DME HO CF3

Me3SiO N—Me + 2)Bu4NF(l equiv.) R^R'

Cp 36-93% 104

Ph Me

Ο 1) CsF or TBAT (0.1 equiv.) /DME or THF HO^CFj

Me3SiO o—s /N-Me + 2) Bu4NF (1 equiv.) R R·

N / Cp3 30-90%

105 R, R' = alkyl, aryl, hetaryl Scheme 35

N,N-Bis(trimethylsilyloxy)enamine 106 in the system ether 107 / Bu4NF (2.1 equiv.) / CH2C12 at - 78°C afforded oxime 108 in 81% yield (Scheme 36) /54A

Ο Ο

Ο Si Μ e3 OSiMe3 ir, MeO^ ^OMe ι I J II O D.. MC \AI o N • OMe Bu4NF j

V ^osiM'· + VN-OH Μ f I Me 106 107 108

Scheme 36

Interaction of unsubstituted cyclopropanol silyl ethers 109 with DAST leads to mixture of allylic fluorides 110 and cyclopropyl fluorides 111 (Scheme 37) /55/.

Me,SiO DAST/CH2g2/rt ^ J^ + F^

Κ R R 0-96% 0-63% 109 110 111

R= alkyl, aryl Scheme 37

Michael reactions of trimethylsilyl nitronates 112 with aromatic aldehydes 113 /56/ or α,β-unsaturated aldehydes 114 /57, 58/ in the presence of chiral quaternary ammonium bifluorides 115 afforded addition products 116 or 117, correspondingly, with ee up to 98% (Scheme 38).

62 Edgars Abele Main Group Metal Chemistry

115 NOj υ Ar' R' OH OSSiRi , 116 ο ^ 70-94% anti/syn up to 94:6

112 R" 115/PhMe 2) HCl "rr CHO N02 R" 117 72-99%

CF, Scheme 38 Reactions of silyl enol ethers and ketene silyl acetals in the presence of fluoride ion were widely used in organic synthesis. Thus, 118 in the system CsF / PhNTf / DME at 23°C was transformed to bis-vinyl triflate 119 (Scheme 39) /59/. Me

CsF/PhNTf,/DME

OSiBu,

Ο Me

118 Scheme 39

63 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review

Silyl enol ethers 120 were trifluormethylated by 5-trifluoromethyldibenzothiophenium tetrafluoroborate + (121) in the presence of Bu4N [Ph3SnF2]" or CsF in DMF. α-Trifluoromethyl derivatives of ketones 122 were isolated in yields up to 88% (Scheme 40) /60Λ

Ö OSiMe 3 -R" R" R + R Bu4N [Ph3SnF2]- / DMF R' R' 120 121 R = alkyl, aryl; R', R" = H, alkyl Scheme 40

Fluorotetraphenylbismuth was found to be an efficient reagent for regioselective α-phenylation of carbonyl compounds. Thus, silyl enol ethers 122 and Ph4Bi-F in THF afforded ketones 123 in 83-99% yields (Scheme 41) /61/.

Ph, /Ph ;bi Ph^l Ph /THF /-40 °C to rt Ο OSiMe 3 F ^^ Ph R'

R'

122 123 R = alkyl, alkoxy, aryl; R' = H, alkyl; R,R' = cycloalkyl Scheme 41

Regioselective ring opening of alkylidenecyclopropane silyl acetal 124 in the presence of Bu4NF in CH2CI2 leads to ester 125 as single product in 95% yields (Scheme 42) 1621.

BU NF/CH,CI, 4 OMe

124 Scheme 42 125

N-Benzylcinchonidinium fluoride /63, 64/ was used as catalyst in the of ketene silyl acetal 126 with aldehyde in THF (Scheme 43). The product 127 was isolated in yields up to 70% with ee 30% /64/.

64 Edgars Abele Main Group Metal Chemistry

Me^ -CHO Me' OH Ο Si Me, AZ-benzylcinchonidinium fluoride / THF / r.t. Me OMe Me Me

127 Scheme 43 γ/α 100:0

Chiral quaternary ammonium fluorides 128, prepared in situ from corresponding hydrogensulfates and

KF2.H20, were used as catalysts in Mukaiyama reaction of silyl enol ether 129 with aldehydes (Scheme 44). Reaction products 130 were isolated in 84-90 % yields with ee up to 91% /65/. Anfz-selective asymmetric synthesis of ß-hydroxy-a-amino acid esters by the in situ generated chiral quaternary ammonium fluoride - catalyzed Mukaiyiama-type aldol reaction was described too 1661.

OSiMe

KFHjO 128 128 R'CHO (X = HS04) thf (X = F) -78°C

130

erythro / threo up to 95:5

R = ; R' = Ph, a-naphthyl, 9-phenanthryl

Scheme 44

Nucleophilic substitution of hydrogen in nitroarenes by silyl enol ethers or ketene silyl acetals in the presence of fluoride ion source was described in some articles /29, 67/. Thus, reaction of nitrobenzenes 131 with ketene silyl acetal 132 in the presence of TASF leads to intermediate 133. Interaction of compound 133

with KMn04 or dimethyldioxirane (DMD) leads to esters 134 and 135, correspondingly (Scheme 45) /29/.

65 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review

Me OSiMe NO, NO. Me OMe KMn04 / NH3 132 TASF / -25 °C

131 Me C°2Me 133

Μ = TAS; R = H, 2-C1, 3-C1, 3-CN

COzMe C02Me 64-82% 8-15% Scheme 45 135 134

6. Si-PBOND

Fluoride-mediated phosphination of alkenes 136 by silylphosphine 137 afforded phosphines 138 in 62- 95% yields (Scheme 46) /68/.

BU4NF / DMF / rt .PPh, R""^ + Ph2P-SiMe2Bu-t

136 137 138

R = alkyl, aryl, hetaryl Scheme 46

7. CONCLUSIONS

In our review we have described forty-six different reactions. Unfortunately, only in some cases was the mechanism of the processes studied. Our investigations of the mechanism of different silanes reactions mediated by fluoride ion (AMI method) make it possible to propose that the fluoride ion serves as base in the deprotonation of oxime /10/, silanol /ll/ (Chapters 1) or terminal acetylene /69/ in the first step of the process. The classical formation of pentacoordinated silicon intermediates (R4SiF)" Cs+ leads in these cases to adducts with stable Si-F bond, without the possibility to subsequent transformations.

66 Edgars Abele Main Group Metal Chemistry

However the Tamao oxidation of silanes in the presence of fluoride ion and H202 was investigated using the ab initio method. It has been demonstrated that fluoride ion lowers the' activation barrier of the rearrangement step by shifting the equilibrium to more reactive silylfluorides /70/.

8. REFERENCES

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