Recent Developments in Bisdiborane Chemistry: B—C—B, B—C—C—B, B—C=C—B and B—CC—B Compounds

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View metadata, citation and similar papers at core.ac.uk CORE brought to you by APPLIED ORGANOMETALLIC CHEMISTRY Appl. Organometal. Chem. 2003; 17: 327–345 Main Group Metal Compounds Published online in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/aoc.430

Review Recent developments in bisdiborane chemistry: B–C–B, B–C–C–B, B–C C–B and B–C C–B compounds† Valery M. Dembitsky, Hijazi Abu Ali and Morris Srebnik*

Department of Medicinal Chemistry & Natural Products, School of Pharmacy, PO Box 12065, Hebrew University of Jerusalem, Jerusalem 91120, Israel

Received 18 November 2002; Revised 27 November 2002; Accepted 13 January 2003

An overview of the development of new strategies in organic synthesis with a minimum of chemical steps is becoming increasingly necessary for the efﬁcient assembly of complex molecular structures. Therefore, the combination of multiple reactions in a single operation represents a particularly efﬁcient approach. Among these strategies, the synthesis and reactivity of bisdiboranes has never been reviewed although its popularity for the synthesis of complex architectural molecules has been steadily increasing during the last decade. This review is intended to highlight the use of partly geminated boranes (B–C–B), and also bisdiborane reagents (B–C–C–B, B–C C–B, B–C C–B). Copyright  2003 John Wiley & Sons, Ltd.

KEYWORDS: bisdiborane; geminated boranes; tetraorganodiborane; bis(pinacolato) diborane(4); bis(catecholato) diborane(4); diazoalkanes; insertions; hydroboration; dihydroboration; carbenoids; bis(boryl)alkenes; vinylphosphonates; 1-alkynylboronates; diboration; thermolysis

5,6 INTRODUCTION are present such as amido (NR2)oralkoxy(OR). More recently, as part of the interest in the oxidative addition Bisdiborane derivatives are an important class of compounds chemistry of the B–B bond and metal-catalyzed diborations in boron chemistry. The addition of diboranes (X B–BX )to 2 2 of alkenes7–10 and alkynes,11–16 synthesis of stable, crystalline unsaturated hydrocarbons, first discovered by Schlesinger bis(pinacolato) and bis(catecholato) diborane(4) derivatives and coworkers in 1954,1 is an attractive and straightfor- have been reported.17,18 ward method to introduce two boryl units into organic 2–4 The development of new strategies in organic synthesis molecules. Diborane itself, B2H4,isstableonlywhencom- with a minimum of chemical steps is becoming increasingly plexed by Lewis base ligands such as amines or phosphines. = important for the efficient assembly of complex molecular Although the tetrahalides, B2X4 (X F, Cl, Br, I), have a structures.19,20 The combination of multiple reactions in a reasonably well-established chemistry, they suffer from low single operation represents a particularly efficient approach. thermal stability (with the exception of B2F4) and prepara- Among these strategies,21 geminated organobismetallic tive difficulties. Tetraorganodiborane compounds, B2R4,are stable only when substituted with sterically demanding R derivatives (1,1-bis anions) are becoming increasingly useful.22 During the past decades considerable efforts have groups such as t-Bu, CH2 –t-Bu, and mesityl. The most stable derivatives are those in which good π-donor groups been made to find new routes for the preparation of geminated sp2 organobismetallic derivatives and for their selective reactions with several electrophiles.22,23 *Correspondence to: Morris Srebnik, Department of Medicinal Chemistry & Natural Products, School of Pharmacy, PO Box 12065, This review will concentrate primarily on features of the Hebrew University of Jerusalem, Jerusalem 91120, Israel. chemistry of diborane compounds with particular structures: E-mail: [email protected] B–C–B,B–C–C–B,B–C C–B,andB–C C–B. We will also †Dedicated to Professor Thomas P. Fehlner on the occasion of his 65th birthday, in recognition of his outstanding contributions to survey some of the chemistry of organoboron compounds organometallic and inorganic chemistry. derived from diborane precursors.

OO O O Pt(PPh3)4 B B + CH N B CH B 2 2 ° 2 O O Et2O, 0 C O O 1

Scheme 1.

O R1 O FORMATION OF B–C–B COMPOUNDS OO R1 Pt(PPh3)4 B B + C = N B C B 2 toluene, O O R2 O O Bisdiborane, BCB, compounds are usually prepared 80 °C R2 by double hydroboration of terminal alkynes with 6 dialkylboranes.24–26 They are interesting precursors to gem- bimetallics such as BCLi27–30 and BCMgX.31 Recently, we Me reported that the parent compound (pinacolato)2BCH2B O O O (1) can be prepared in high yield by reaction of B C B O O O O bis(pinacolato)diborane(4) with diazomethane (Scheme 1).32 Me B C B O O This reaction gave good results and had not been described 8a, 76% Me before. 8b, 75% Insertions into (R N) B–B(NR ) ,whereR= Me, Et, n-Pr, O 2 2 2 2 B O O were not successful. Two mechanisms for the insertion are O B C B C possible: (a) addition/1,2-migration to and from boron as O O O in the Hooz reaction33–35 or (b) oxidative addition of B–B B O to a Platinum(0) complex, insertion of CH2 into the Pt-B bond, migration, and reductive elimination.36 Alternatively, 8c, 78% 8d, 75%

formation of 1 from (pin)BCH2I by coupling with metals has been reported.32 Reﬂux of 1 and 3 with HCl gave the stable Scheme 3. boronic acids 4 and 5 respectively (Scheme 2). Novel C1-bridged bisboronate derivatives 8a–d have PPh3 recently been reported by insertion of diazoalkanes into R l Ph PPhPt P bis(pinacolato)diborane(4) by Abu Ali et al.37 (Scheme 3) and R2 3 3 BB O C the proposed catalytic cycle is shown in Fig. 1. A view of 8d B B O PPh3 O is shown in Fig. 2. O PPh Preparation of 1,4-dibora-2,5-cyclohexadienes (10a–c)have 3 8 been demonstrated by Wrackmeyer and Kehr.38 Organobo- BBPt

ration of tetrakis(alkynyl)stannanes leads to the spiro-cyclic PPh3 compound 9 in high yield. The reactions of 9 with MeBBr2, 6 − i-PrBBr2,andPhBCl2 in dichloromethane between 78 and Rl + ◦ 25 CgiveB–C C–B (10a–c) compounds. Interconversion BBCPt Rl N R R2 2 2 7 O I CH2 B Figure 1. The proposed catalytic cycle for formation of O O O B–C–B compounds. B 2 B CH2 M = Na, K, Mg O O 1 + of these compounds gives compounds with B–C–B structures HCl O O reflux 11–13 (Scheme 4). B CH CH B 39 2 2 HO OH B–C–B compounds 15 and 16 could be synthesized O O 40 3 B CH2 B from compound 14,and18 from compound 17 (Scheme 5). HO OH Formation of a 2,5-diborabicyclo[2.1.1]-hexane 20 and its HCl 4 reflux conversion product tetracarbahexaborane (19), which also HO OH have a B–C–B structure, have been reported by Enders et al.40 B CH2 CH2 B Synthesis of carbaboron compounds has also been reviewed OH 41 HO 5 recently. The 1,1-spirobistannoles (22a–d) are an intermediate for Scheme 2. synthesis of borolenes (23); and compounds 24a,b with the

Copyright  2003 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2003; 17: 327–345 Main Group Metal Compounds Recent developments in bisdiborane chemistry 329

= NiPr2 B NiPr2 -2 B + 2 Li BNiPr2 B B

NiPr2 NiPr2 14 15 16

R R B Cl B SiMe2

Me3Si Cl B R B R 17 18 17a. R = tBu 17b. R = 2,4,6-trimethylphenyl

Cl Figure 2. The proposed view of the geminal organoboron B −LiCl B compound 8d, showing the atom-numbering scheme. + 2 Li B B Cl

iPr Me Me iPr

+ Sn( )4 2 B(i-Pr)3 iPr BSnBiPr

iPr iPr Me Me H 9 B Me iPr B B > 20 °C 10a. R = Me B RBX2 10b. R = iPr R B B iPr H H H 10c. R = Ph + Me iPr Me iPr 10 X H Sn B iPr X 19 20

Me iPr Scheme 5. = R R X Cl, Br B B iPr iPr Me B–C–B structure were prepared from the reaction of tetra-1- Me alkynyltin (21) with triethylborane (Scheme 6).42 Another B–C–B compound, 2,5-diborylated 3-borolene Me iPr Me iPr B B (26) could be obtained by reacting 25 and MeBBr2 in a 1 : 2 R R ratio (Scheme 7).42 11 12 The synthesis of heterodiborolanes 27 and 28 withaB–C–B structure has been demonstrated in the papers by Siebert and R R coworkers (Scheme 8).43,44 iPr B iPr B Me

Me iPr Me iPr GEMINAL ORGANOBORON COMPOUNDS B B R R The chemistry of geminated boron compounds has been 13 reviewed recently.22,23 The synthesis of (diethylcyclo- propyl)borane or cyclopropyl-9-BBN was performed by dihy- Scheme 4. droboration of either propargyl chloride or bromide, followed by the addition of methyllithium to the resulting 1,1-diborio compound 29 (Scheme 9).24 Also, the gem-diboron 30 could

Copyright  2003 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2003; 17: 327–345 330 V. M. Dembitsky, H. Abu Ali and M. Srebnik Main Group Metal Compounds

R R B(OMe) B 2 two steps Cl R Sn R Cl B(OMe) B 21 21a. R = Et 2 = R 21b. R n-Pr R 21c. R = i-Pr + Et B 21d. R = Bu 3 − 2ClSiMe3 X(SiMe3)2

Et2B Et R Et Et Et Et Et Br MeBBr RRMeBBr Et Me B B B 2 Sn 2 B B Br Br CD Cl R R CD Cl Br Br B 2 2 2 2 B X Pr Pr −40 °C −40 °C Pr Pr Me Me Et BEt2 B 24a22a-d 24b R + − 4 BX3 SnX4 R = Cl, Dur X = S, NMe Et Et Et B BX X = Cl, Br X 2 B R R X H H 23 B B Scheme 6. B B B R B H H N H S be obtained if tosylate was chosen as the leaving group.11 The Me best synthesis of (E)-alkenyl-9-BBN compounds is an indirect 27 28 route involving oxidative elimination of one boron from the easily formed diboryl intermediate 31.45 Scheme 8. The dihydroboration of 1-hexyne with diborane results in the formation of a highly branched, polymeric product. Since it was anticipated that the reaction product would contain the two boron atoms either on adjacent carbon Br B atoms or on the same carbon, oxidation with alkaline B Br 9BBN hydrogen peroxide should lead either to 1,2-hexanediol or ω- H B hexaldehyde respectively. Unexpectedly, however, oxidation of the dihydroborated 1-hexyne gave, in addition, 1-hexanol 29 as the major product. The formation of 1-hexanol from the dihydroboration intermediate can be rationalized in OTs TsO B terms of a rapid hydrolysis of one boron–carbon bond H 9BBN B B of either the 1,1- (32) or the 1,2-diboron (33)compounds 25 prior to the oxidation step (Scheme 10). The only reasonable 30 explanation for the high yield of primary alcohol is that the hydroboration product, whatever its structure, undergoes 9BBN R a rapid hydrolysis to lose one of the two boron–carbon R B bonds prior to oxidation. Since the alkaline hydrolysis R B involves an attack by a base on the electrophilic boron B ArCHO atom, these substituents, which decrease the acidity of 31

Me Scheme 9. Me Me Me Me Me Sn BMe + MeBBr B B 2 2 −Me SnBr Br Br 2 2 B the boron atom, should greatly reduce the hydrolytic H H Me Me stability of the 1,1-diborio derivative. So, the diborioalkane 25 26 was hydrolyzed ﬁrst and oxidized in a second step with alkaline hydrogen peroxide at pH 8, to give the aldehyde in Scheme 7. 68% yield.26

Copyright  2003 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2003; 17: 327–345 Main Group Metal Compounds Recent developments in bisdiborane chemistry 331

CHO CH2OH

OTs HB B CH3Li NaOH/H2O2 H2O2 O OTs H B 37 B H H

H2O2 32 B H H2O B H [O] 2 4 H H C B OH 9 4 H B(OH) THF 2 H B H B(OH) 38 39 B H 2

33 Scheme 13. H2O H2O2 B(OH)2

CH2OH B(OH)2 that the 1,1-diborane compounds 41 formed by the OH dihydroboration of 1-hexyne, 1-decyne, or phenylacetylene Scheme 10. with tetracyclohexyldiborane undergo transmetalation by the action of two equivalents of butyllithium in heptane at −70 ◦C to form mixed 1-bora-1-lithium compounds. The double hydroboration of 3-hexyne was examined Binder and Koster¨ 52 have found that hydrobora- , ﬁrst with an excess of diborane,46 47 but the oxidation of tion of 3-chloro-1-propenyl-dialkylborane gave gem-1,1- the product from dihydroboration of 3-hexyne gave 1,1- bis(dialkylboryl)-3-chloropropane (42). When 42 was heated diborioalkane (34) as the major product46 (Scheme 11). with sodium tetraethyl borate, it cyclized to cyclopropy- The hydroboration of 1-hexyne produced gem-1,1- ldialkylborane (43; Scheme 15). 43 in the presence of bis(dichloroboryl)hexane (35), which could be converted to the well-known 1,1-bis[2-(1,3,2-dioxaborinyl)]hexane (36) (Scheme 12).48 B(C6H11)2 3-Butyn-1-ol tosylate can be converted by 9-BBN to (C6H11)2BH H9C4 H 9-cyclobutyl-9-bora-bicyclo[3.3.1]nonane (38)viageminal HO C4H9 H9C4 B(C6H11)2 compound 37, which is oxidized to cyclobutanol (39) 40 (Scheme 13).49

Tetracyclohexyldiborane adds to 1-hexenyldicyclohexyl- B(R1)2 COOH + borane mainly with the formation of the geminal 1,1- RH(R1)4B2H4 diborane 40 compound (Scheme 14).50 Cainelli et al.51 found R B(R1)2 R COOH 41

= 41a. R n-C4H9 = H H 41b. R n-C8H17 B BH + BF H 3 3 = B R1 H

34 Scheme 14.

Scheme 11.

R2BRCl 2B Cl R2BH Et3SiH BCl2 H H R2B H BCl3 42

BCl Et SiH O 3 3 HO NaBR3H NaBR4 B O HO BCl2 B OO BCl2 B R2B

36 35 44 43

Scheme 12. Scheme 15.

Copyright  2003 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2003; 17: 327–345 332 V. M. Dembitsky, H. Abu Ali and M. Srebnik Main Group Metal Compounds

Br Li B C H O H C H O H BuLi B-B 2 5 130−170 °C 2 5 B BOCH3 THF/Et2O −110 °C Br − ° Br B H H (2:1, 110 C) 3 2 46 47 45a OO O A B B 47a, Bpin B 93.0% O O O OR OO O H > 99.0% H B B B 47b B O O O = (ROCH CH)2B B(OCH3)2 OO O 15.0% 45 C B B 47c B O O O Scheme 16. OO O D B B 47d B <1.0% O O O compounds with B–H bonds symmetrizes to tricycle- propylborane (44). Later, 44 was formed directly from 42 Scheme 17. 49 by reaction with NaBR3H. Geminated compound 45A could be formed during the

exchange reactions between alkoxyvinylboranes and an Br BuLi or Br Bpin LiTMP A, Scheme 7 alkyl borate, apparently proceeding via the formation of 91% intermediate 45 (Scheme 16).53 Li Bpin 48a 49a 50a A novel and efﬁcient method for gem-dimetalation of Ph Cl Ph Li Ph Bpin 54 carbenoids has been reported by Shimizu and coworkers. 40% Ph Cl Ph Cl Ph Bpin Treatment of alkylidene-type lithium carbenoids with 48b 49b 50b interelement compounds, such as silylborane or diborane MEMO MEMO MEMO to generate the corresponding borate complex, followed Br Li Bpin 65% by warming to room temperature, induced migration Br Br Bpin of the silyl or boryl group from a negatively charged 48c 49c 50c Li Bpin boron atom to the carbenoid carbon to afford 1-boryl- Cl Cl 89% 1-silyl-1-alkenes or 1,1-diboryl-1-alkenes in good yields. Cl Bpin Carbon–carbon-bond forming transformations of the gem- 48d 49d 50d Li Bpin dimetalated compounds mediated by boron or silicon is also 82% Hex Hex Hex described. gem-Diborylation of alkylidene-type carbenoids Cl Cl Bpin 48e 49e 50e with diboranes has been demonstrated. Thus, commercially Hex Hex Li Hex Bpin available diboranes A–D were used in the reactions 48% Cl Cl Bpin (Scheme 17). Bis(pinacolato)diborane (A) and optically active 48f 49f 50f bis[(+)-pinanediolato]-diborane (B) reacted with 46 to give gem-diborylated compounds 47a and 47b in high yields. In Scheme 18. contrast, reaction with bis(neopentanediolato)diborane (C) resulted in low yield of 47c due, probably, to its low solubility under the reaction conditions, whereas no desired diborylated and 49f prepared from conjugated chloroalkenes 48e and 48f compound was obtained when bis(catecholato)diborane (D) proceeded smoothly, producing conjugated compounds 50e was employed. and 50f bearing two boryl groups at the terminal positions.54 By use of A (Scheme 17), various kinds of alkylidene- Some synthetic applications of gem-bismetallic compounds type carbenoid 49a–f were gem-diborylated, as shown shown in Scheme 19. When an equimolar amount of in Scheme 18. Unsubstituted and 2,2-disubstituted car- allylbromidewasusedasthecoupling partner, stepwise benoid 49a gave 1,1-diborylalkene (50a) in high yields. coupling was possible. Thus, when 47a was treated with Dichloroalkene (48b)couldalsobeused,which,afterchlo- an equimolar amount of allyl bromide, the allylated rine–lithium exchange, underwent gem-diborylation, giving alkenylboronate 51 was obtained. Furthermore, the allylation rise to 50b in 40% yield, whereas optically active 1,1- followed by coupling with iodobenzene in one pot gave diborylalkene (50c) was obtained from the corresponding the corresponding bis-coupled product 52 in a good overall dibromide 48c in 65% yield. Double deprotonation of 48d gen- yield. Rhodium-catalyzed Michael-type addition reaction erated 49d, which reacts with A to afford 1,1-diborylbutadiene of 47a to methyl vinyl ketone proceeded smoothly to (50d) in 89% yield. gem-Diborylation of lithium carbenoids 49e give diketone 53 in 74% yield.55 Two C–B bonds in

Copyright  2003 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2003; 17: 327–345 Main Group Metal Compounds Recent developments in bisdiborane chemistry 333

(MeO)2B OCH3 B(OMe)2 + − − 55 [(MeO)2B]3C (MeO)2B B B(OMe)2

Bpin (MeO)2B OCH3 B(OMe)2 52 51 60 Br Br − 1. Pd(PPh3)4 3 Mol % 1. Pd(PPh3)4 3 Mol % −MeO aq. KOH aq. KOH 2. I-Ph, 1 eq 2. dioxane 70 °C, 12 h Bpin 70 °C, 12 h B(OMe)2 (MeO)2B B(OMe)2

71% 83% B (MeO)2B B B(OMe)2 O Bpin 47a B(OMe)2 (MeO)2B OCH3 B(OMe)2 I OMe n 61 1. Ph(acac)(CO2) 1. Pd(PPh3)4 3 Mol % O dppb, 6 Mol % aq. KOH 62 2. MeOH / H2O 2. dioxane 50 °C, 24 h 70 °C, 12 h Scheme 21. 74% 80%

53 54

O (60). The resulting adduct 60 might then lose methoxide, add another carbanion, and continue the process to form polymer Scheme 19. 62 via 61 with a C–B–C–B–C...backbone (Scheme 21). The syntheses of cyclic boronic esters 63 and 64 containing B–C–B bonds (Scheme 22) and tetrametallomethanes con- 47a were simultaneously converted into two C–C bonds, taining Group IV metals have been reported by Matteson and as exempliﬁed by the Palladium-catalyzed cross-coupling Wilcsek.58 reaction, with iodobenzene giving rise to 54. Reaction of 64 with butyllithium followed by Ph3SnCl Castle and Matteson reported the successful preparation of gave good yields of 65, 66 and 67 (Scheme 23).58 Compounds the 1,1,1-triboryl compounds 55 (Scheme 20).56 Octamethyl 69–75 could be prepared via carbanion 68 with two different methanetetraboronate (55) appears stable indeﬁnitely in metal atoms M and M1 if base-catalyzed disproportionation neutral methanol, and with NaOH it degraded to 56,which of 65–67 occurred. The ditin (69), dilead (70), tin–lead (71), under H O conditions transformed to 58 and 59 via 57. 2 2 tin–germanium 72, and lead–germanium (74)compounds According to Castle and Matteson,57 55 reacts with were synthesized in good yields.58,59 carbanion [(MeO) B] C− to form methanetetraboronic ester 2 3 Treatment of tetrakis(dimethoxyboryl)methane (62)with bases such as methyllithium or lithium methoxide leads to the tris(dimethoxyboryl)methide ion 76 (Scheme 24),60 B(OCH )2 8Li 3 which reacted with methyl iodine to give the + (CH O) B B(OCH ) CCl4 4ClB(OCH3)2 3 2 3 2 monomethylation product 77. Loss of a dimethoxyboryl THF B(OCH3)2 55

− (CH O) B (CH3O)2B 3 2 − O O B(OCH ) C B(OCH3)2 3 2 O B O (CH O) B (CH3O)2B 3 2 + 55 4 HOCH2CH2OH B B O B O 56 O C6H5CH2Br O

63 C6H5CH2C[B(OCH3)2]3 57 O O H O 1. RMgBr 2 2 O • O B O 2. C H CH Br BF3 Et2O 6 5 2 + 3. H O 55 4 HOCH2CH2CH2OH B B 2 2 THF COOH O B O O O

58 59 64

Scheme 20. Scheme 22.

Copyright  2003 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2003; 17: 327–345 334 V. M. Dembitsky, H. Abu Ali and M. Srebnik Main Group Metal Compounds

Ph H Ph PhPh Ph Ph O O LiOEt − O M O O M O [(MeO) B] CH [(MeO)2B]2CH BBC BuLi 2 3 O CH O 64 B B B CB- 82 83 2 O B O O O R O O 68 O H O 84. R = CH2C CH BBC 85. R = CH2CH CH2 65. M = Sn O O 86. R = n-C4H9 CH3 66. M = Pb Ph3M1Cl 87. R = CH2C6H5 67. M = Ge 88 H O O O O = = O O BBC 69. M M1 Sn Ph PhPh B B = = O O 70. M M1 Pb M CH = = O O H C CH2C CCH2 C H 2 71. M Sn, M1 Pb = = B CB B B O C 72. M Sn, M1 Ge O O = = O O O O 73. M Ge, M1 Sn M1 OCH3 = = 74. M Pb, M1 Ge PhPh Ph = = 89 90 75. M Ge, M1 Pb Scheme 25. Scheme 23.

to 4,4-bis(1,3,2-dioxaborol-2-yl)-1-butene (85). Other alkyl CH3 − halides reacted with 82 to give the B–C–B products 84, [(MeO) B] C + CH I (MeO) B B(OMe) 2 3 3 2 2 86, 87, and 88–90. 60 B(OMe) 76 2 Also, octamethyl methanetetraboronate (55)andmethyl- 77 lithium in tetrahydrofuran (THF) served as a source of tris(dimethoxyboryl)methide ion 76, which reacts −B(OMe) 2 with dimethoxyboron chloride to give tetramethyl 2- methylpropane-1,1-diboronate (91; Scheme 26);61 reaction of

CH3 CH3 91 with ethylene glycol forms the geminated boron com- CH3I (OMe)2B B(OMe)2 (MeO)2B C− B(OMe)2 pound 92. Reﬂuxing benzophenone with 76 for 6 h followed CH by additional ethylene glycol formed 93 in 48% yield. 3 78 79

O CH3 O + + − BBC 55 MeLi [MeB(OMe)2 [(MeO)2B]3C O O 76 CH3 77 + 79 + HOCH CH OH + 80 2 2 1. CH3COCl 2. ClB(MeO) O CH3 O 2 BBC O O O B H COB (HOCH ) H C B(OMe) O O 3 2 2 3 2 O H C B 81 3 H3C B(OMe)2 O 92 91 Scheme 24.

group from monomethylation product 77 gave 1,1- O O bis(dimethoxyboryl)ethide ion 78, which reacted with ROB B O methyliodide to form 2,2-bis(dimethoxyboryl)propane (79). O O Approximately equimolar mixtures of di- (80) and mono- H B B methylation (81) were obtained in about 50% total yield when O O methyliodide was added to a mixture of 77 and 79 in the presence of ethylene glycol. 93 = Alkylation of tris(dimethoxyboryl)methane 82 by LiOEt 94. R C6H5 = and removal of one dimethoxyboryl cation gave bis(dimetho- 95. R CH3 xyboryl)methide ion 83 (Scheme 25) or its borate ester complex.60 The reaction with allyl bromide lead smoothly Scheme 26.

Copyright  2003 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2003; 17: 327–345 Main Group Metal Compounds Recent developments in bisdiborane chemistry 335

Benzaldehyde led to styrene-β,β-diboronic acid, which reacts Li+ O − O with catechol in benzene to form the catechol esters 94 B CB 61 and 95. O B O Electrophilic displacement of the boronic ester group OO OO O O OO occurred in the reaction of 92 with two equivalents of B B C4H9Li B + B mercuric chloride, and sodium acetate in methanol gave O CCLi O 101 61 − 2-methylpropene-1,1-dimercuric chloride (96; Scheme 27). O B B B O 64 C4H9Li O OOO Deprotonation of bis(trimethylenedioxyboryl)methane C4H9Li (97) with lithium 2,2,6,6-tetra-methylpiperidine yielded the diborylcarbanion 98, which reacts with alkyl halides to give 102 O − O high yields of gem-diboronic esters 99, which also could be Li+ B C4H9 deprotonated and alkylated to give 100 (Scheme 28).62 Car- O O banion 98a reacts with aldehydes and ketones to form enol 103 B CB borate intermediates, which hydrolyzed to ketones. O B O OO Anionic bisdiborane compounds derived from methanetetraboronic (64) and methanetriboronic esters,57–63 gem- diborylalkanes,26,29,64 and deprotonation of trialkyl- Scheme 29. boranes65,66 have been found useful in a variety of syntheses, including condensation with aldehydes and ketones, and alkylation by alkyl halides.67 Matteson et al.68 found that 101 is a very strong base and abstracts protons from dimethyl the lithium salts of some bisdiborane compounds (101–103) sulfoxide, in which it is soluble. could be obtained from 64 with LiBu (Scheme 29). The salt The lithium salt of 98 is a slightly soluble in THF and precipitates, giving an apparent increase of the acidity of 97, which reacts with bases Y− to form borate complexes 105 or O the self-condensation product 104 (Scheme 30).69

H3COB H3C HgCl Hydroboration of 1-hexyne leads to 1,1-bis(dichloroboryl)- + 70 2 HgCl2 hexane (35; Scheme 31) and could be converted to O 69 H3C B H3C HgCl 36. Treatment of 35 with trimethylsilane yielded 1,1-bis- O (diborylhexane)dimer 108. 92 96 The ﬁrst reported synthesis of a bidentate borane (B–C–B)

incorporating C6F5 groups (109) came from the Marks Scheme 27. 71 group in 1994. The borane t-BuCH2CH[B(C6F5)2]2 (109) was prepared by the metathetical route shown in Scheme 32. Also, 110 can be prepared via dihydroboration of terminal O H O O O − 72 B B B CB alkynes using the hydroboration reagent HB(C6F5)2. As an O H O O H O alternative to diborate formation 112a–c, retrohydroboration can be precluded by incorporating unsaturation in the 97 98 backbone using tin–boron (111) exchange/hydroboration as 73 X = Br, I shown in Scheme 32. R - X R = n-Pen

O H O O R O O R O − B CB B CB B CB − Y O H O O O O O R1 H 97 O 100 99 O B − B Y H O O O O C H O BO BO C H − RCO2CH3 3 6 2 2 3 6 B CB 104 = O O R C6H5 R R − R1 = 1 Y R C3H7 = O H O 98a. R1 n-Pen H2O B B O H O R CCH2R1 O 105

Scheme 28. Scheme 30.

Copyright  2003 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2003; 17: 327–345 336 V. M. Dembitsky, H. Abu Ali and M. Srebnik Main Group Metal Compounds

BCl2 B(C F ) HBCl HB(C6F5)2 6 5 2 2 Bu HBCl2 Me Si H Bu BCl 3 2 Bu BCl 2 Me3Si B(C6F5)2 35 113

HO Scheme 33. HO Me3SiH

O O B MeO + 2 HB Bu O H H H O B B BBu OO H 114 (Bcat) H Bu B B H Rh(I) 36 H H 108

MeO Scheme 31. MeO Bcat Bcat 115catB 116 catB Piers and co-workers72,74 found that the 1,1-diboryl alkane

(113) formed upon double hydroboration of Me3SiC CH Bcat (Scheme 33). MeO MeO Geminal organoboron compounds 115–118 can also be Bcat Bcat

prepared using 4-vinyl-anisol (114) and HB-1,2-O2C6H4 with 117catB 118 catB Bcat rhodium(I) as catalysts (Scheme 34).4,75 Reaction of the boriranyllideneboranes 119–121 with Scheme 34. tetrahalodiboranes(4) leads to two types of product: 122, 123 and 124, 125 (Scheme 35).76,77 Compounds 119–121 were The quantity, the solvent and the metal determine the ratio ﬁrst prepared by Berndt more then 20 years ago.78 Treatment of BCCB/BCB, varying from 96 : 4 (with sodium excess of 122 with four equivalents of dimethylaminotrimethylsilane 35%, and yield 94%) to 34 : 66 (sodium excess 15%, yield gave 126a–c. 94%) in benzene.32 Hydroboration of 1,3-butadiene in ether = (diene : BH3 5 : 2) produces a mixture of compounds, where FORMATION AND REACTIVITY OF 128 and 1,2-bis-(1-boronyl)butane (127)werealsofound 80,81 ◦ B–C–C–B, B–C C–B, AND B–C C–B (Scheme 36). When heating 128 at 140–170 Citisomerizes 82 COMPOUNDS to geminal 1,1-bis(1-boronyl)butane (129). B–CC–B compounds 130 and 131 can be obtained Formation of BCCB alkanes 3 and 5 (see Scheme 2) from from the addition of alkenylboronates to diborane 32,63,79 (pin)BCH2I by coupling with metals has been reported. (Scheme 37); hydrolysis of 130 by hydrochloric acid,

F5C6 C6F5 1. 2 HBCl 2 1. KBEt3H B 2. 4 C F Li B(C6F5)2 6 5 2. HNBu3Cl tBu H or tBu B(C6F5)2 tBu B 2 HB(C6F5)2 109 F5C6 C6F5

110 R H B(C F ) Me 1. ClB(C6F5)2 6 5 2 Sn 2. HB(C6F5)2 Me R B(C6F5)2 R 112 111 112a. R = t-Bu = 112b. R C6H5 = 112c. R C6F5

Scheme 32.

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RR O O B H B B B 2 6 O NH B 119. R = CMe NH 3 O B Me Si = O 3 120. R C6Me4H = 130 Me3Si 121. R C6Me3H2 H+ n B2X4 B2Cl4 -C4H9OH

B(OC4H9)2

X R B(OC4H9)2 B B X Cl2B B Cl Me3Si 131 Me3Si B Me Si Me Si 3 BBX 3 B Cl Scheme 37. X R = 122. X = Cl 124. R C6Me4H = = C6H5 B(OC4H9)2 123. X Br 125. R C6Me3H2

Br B(OC4H9)2 Et2NH C6H5 H BBr 133 Me SiNMe 3 3 2 + 3% ο LiN(CH2)4 R H Sn(C H ) 0 C 2 5 3 C H Br B B R 6 5 Me3Si 132

(OC4H9)2B B(OC4H9)2 Me3Si B H = 126a. R NEt2 27% R = 126b. R NMe2 = 126 126c. R N(CH2)4 Scheme 38.

Scheme 35. H C O H C O 3 MeLi 3 B B O O Me3Sn Li + + B2H6 HB BH 134 H3C O B PhCOCH Cl O 3

B2H6 1,3-C4H8 O H C CH B 3 3 H CH3 O + B CH CH CH B + BCH CH B 2 2 2 H3C O H Ph H3C Ph CH3 C2H5 B O 128 39% 6% H3C 135 127 140−170 °C Scheme 39. B

CH CH2 CH2 CH3 B–CC–B compound 135 can also be obtained by B 129 destannylation of trimethyltin (134) according to Scheme 39.85 In 1993, Ishiyama et al.86 reported the synthesis of isomerically pure cis-1,2-bis(boryl)alkenes 136a–140a from Scheme 36. their corresponding alkynes 136–140 (Scheme 40)86,87 via platinum complexes. The solvents did not play an important followed by esteriﬁcation with butanol, gave tetrabutyl 1,2- role in the reaction, but a comparison of the reaction rates at propanediboronate (131).83 50 ◦C revealed that the addition was apparently accelerated ◦ Reaction of 132 with tribromoborane at 0 Cformsamixture in polar solvents: e.g. DMF > CH3CN > THF > toluene. of two products, one of which has B-CC-B structure (133; Synthesis of trans-1-organo-1-alkenyl-boronates was reported Scheme 38).84 when palladium(0) was used as catalyst.88

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O O R B B H13C6 H O O 136 OO R B B O O O O H C H 17 8 OO R R B B R1 R2 137 B B Pt(PPh3)4 = O O O O O O 148. R 4-MeO-C6H4 149. R = SiMe = H7C3 C3H7 B B 3 150. R 4-MeO-C6H4 151. R = SiMe 138 Pt(PPh3)4 O O 3 O O R 136a. 82% B B Ph Ph 137a. 86% O O 138a. 86% R 139 R 139a. 79% R O O 140a. 78% O O B B H B B O O 140 O O 152 = 154. R = 4-MeO-C H . R 4-MeO-C6H4 6 4 153 = 155. R = SiMe . R SiMe3 Scheme 40. 3 O O B B Thesamereactionproduceddifferentalkynes141–146 O O (Scheme 41).89 These reactions were not catalyzed by R R O O rhodium(I) or palladium(0). B B The borates react with chlorodialkylboranes in a steri- O O cally selective manner, forming with good yields cis-1,2- = 156. R 4-MeO-C6H4 = bis(dialkylboryl)alkenes (147), the protonolysis of which with 157. R SiMe3 water or alcohols produces cis-oleﬁns (Scheme 42).90 The formation of B–C C–B compounds 150–153 has Scheme 43. been demonstrated with the use of 1,3-diynes via platinum(II) anions.91 The addition of (pin)B–B(pin) and

(cat)B–B(cat) to 4-MeOC6H4C CC CC6H4-4-OMe and from the [(PPh3)nPt(η-alkyne)] complexes, giving rise to a SiMe3C CC CSiMe3 proceeded smoothly to gave the novel mono(phosphine)Pt intermediate, which serves as the active tetrakis(boronate ester) compounds 154–157 respectively catalyst in these systems. (Scheme 43). An important step in the catalyzed dibora- The diboration reaction of bis(pinacolato)diborane(4) with tion of alkynes appears to be the dissociation of phosphine diethyl 1-hexynyl-phosphonate (158a), diethyl phenylethynyl phosphonate (158b), 1-hexynylpinacolato borane (158c), and phenylethynylpinacolatoborane (158d) in the presence of

CH =CH(CH ) H a catalytic amount of Pt(PPh3)4 (3 mol%) in toluene at 2 2 4 ◦ 141 80 C overnight gave the desired novel cis-1,2-diboronated vinylphosphonate and trisboronated alkene addition prod- Cl(CH2)3 H 142 ucts 159a, 159b, 159c and 159d respectively in high yields 92 O (CH ) H O O R H (Scheme 44). 2 4 B B 1 143 O O O O The structure of 159d was found to be fully isomorphous B B to that of 159c,withtheC6H5 ring located in place of the C4H9 CH3O (CH2)4 H Pt(PPh3)4 O O 144 141a. 85% residue. The reaction was efﬁciently catalyzed by Pt(PPh3)4 O 142a. 83% in toluene at 80 ◦C. The apparent hydroboration followed by N C(CH ) H 143a. 87% 2 3 144a. 89% deboronation when the B–B and platinum compounds are O 145 145a. 79% 146a. 77% not pre-equilibrated may be due to either water or other such protic impurities, since it is known that 1-alkynylboronates (CH2)3 H 146 are very susceptible to cleavage by water. Pre-equilibration

of Pt(PPh3)4 plus excess B–B would lead to (PPh3)2Pt(B)2.

Scheme 41. If excess water were present, (PPh3)Pt(B)2 would react with + H2O to give (PPh3)PtH2 B–O–B. (PPh3)2PtH2 would lose

hydrogen and with excess B–B give more (PPh3)2Pt(B)2, R R + R R −NaCl 1 H 1 which, using PPh and B-B, is in equilibrium with (PPh ) Pt + 3 3 3 Na[R2B R1] ClBR3 + or (PPh3)4Pt B–B, by the way, as well as with the R2B B(R2)2 H H 65−80% (PPh3)Pt(B)2 species. So, in effect, platinum can catalyze the 147 hydrolysis of the B–B bond. The desired products 159a–d were produced in 100% conversion yields and high isolated Scheme 42. yields.

Copyright  2003 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2003; 17: 327–345 Main Group Metal Compounds Recent developments in bisdiborane chemistry 339

Ph P PPh 3 3 overnight R R Pt + 1 2 MeO O O R1 R2 B(OR*) B B 80 °C, toluene O O [Pt(dba) ], 5 mM 2 158 B B + 2 MeO O O * O O 4 °C, toluene, B(OR*) B [(R,R)-OCHPhCHPhO] 2 159 2 2 3 days 168 = = 158a. R1 C4H9, R2 P(O)(OC2H5)2 = = 80%, yield 158b. R1 C6H5, R2 P(O)(OC2H5)2 = = 60%, diastereomeric excess 158c. R1 C4H9, R2 Bpin = = 158d. R C H , R Bpin B(OR)2 1 6 5 2 B2(OR)4 R R * O O [Au(I)] or [Pt(0)] B(OR)2 C4H9 P (OC2H5)2 P (OC2H5)2 169 O O O B B O B B O O B O O O O O B [Rh*] O HBcat * 159a. 82% 159b. 83% BO 170 O O O C H 4 9 B O B O Scheme 46. O B B O O B B O O O O O catalyzed the addition of B2cat2 to terminal alkenes giving 159c. 86% 159d. 87% 1,2-diborane ester 169. Scheme 44. Also, saturated bisdiborane 170 with 72% enantiomeric excess (13% yield) could be obtained at −20 ◦C using a catalyst

system composed of [Rh(NBD2)ClO4/S,S-chiraphos] [NBD: norbornadiene; chiraphos: 2,3-bis(diphenylphosphino)- Unusual diboration of allenes catalyzed by palladium butane].95 93 complexes formed novel B–C–C–B compounds 160–167. A The synthesis of B–C–C–B compounds 173 and 174,and new catalytic pathway other than one involving the oxidative also geminal boron compounds 175–177 using rhodium addition of diborane to palladium(0) species operates in this catalysts, have been reported recently.96 Diboration of the new palladium-catalyzed reaction. The observation that an styrylboronate esters with B2cat2 in the presence of a variety aryl-, alkenyl iodide, or I2 was required to initiate the present of rhodium phosphine catalysts gives predominantly either catalytic reaction is interesting and vital to the understanding p-R-C6H4-CH2C(Bcat)3 (177), which contains three boronate of the catalytic mechanism (Scheme 45). ester groups on one carbon atom, or its isomer p-R- Platinum complexes also catalyzed diboration of terminal C6H4-CH(Bcat)CH(Bcat)2 (174; Scheme 47). The formation 9,75 alkenes with chiral diborane compounds. It has recently of 173 apparently involves regiospeciﬁc insertion of the been shown that addition of certain enantiomerically pure chi- vinylboronates into an Rh–B bond followed by β-hydride ral diborane compounds, such as B2[(R,R)-OCHPhCHPhO]2, elimination, another regiospeciﬁc insertion of the 2,2-vinyl 86 to 4-vinylanisole using the Miyaura [Pt(dba)2] (dba: diben- bis(boronate) into the remaining Rh–B bond followed by zylidene acetone) catalyst system provides chiral 1,2-diborane ester 168 with up to 60% diastereometric excess (Scheme 46). Similarly, and based on earlier stoichiometric data (vide supra) Ar 94 'BH3' promoter Iverson and Smith reported that [Pt(NBE)3]and[Pt(COD)2] Ar H + HBcat ° 22 C Bcat 171. Ar = Ph = 172. Ar 4-MeO-C6H4 O O O O B2cat2 R B B + 1 Pd(dba) R1 B O + O a O 2 I [Rh(PPh3)3Cl] O O ° R2 B B 80 C, toluene R B O O O 2 catB Bcat catB Bcat b O = = 160. R1 R2 Me (E) / (Z) = = Ar Ar Bcat 161. R1 H, R2 n-Butyl ratio = = 173 174 162. R1 H, R2 Ph 5-7% / 93-95% 163. R = H, R = cyclohexyl 1 2 Bcat Bcat = = Bcat 164. R1 H, R2 cyclopentyl = = Bcat 165. R1 H, R2 PhO = = 166. R1 H, R2 o-I-C6H4O Ar Bcat Ar Bcat Ar Bcat = = 167. R1 H, R2 p-I-C6H4O 175 176 177

Scheme 45. Scheme 47.

Copyright  2003 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2003; 17: 327–345 340 V. M. Dembitsky, H. Abu Ali and M. Srebnik Main Group Metal Compounds

C–H reductive elimination leading to 2,2-diboration and a 1,2-hydrogen shift. Wilkinson’s catalyst [Rh(PPh ) Cl] gives 3 3 O the highest yields of 177 with 75% and 71% yields from µ H B 171 and 172 respectively, and [Rh(COE)2( -Cl)]2 with two O equivalents of P(o-Tol)3 gave 174 in best yield with 50% and 49% from 171 and 172 respectively. O Tetra(chloro)diborane reacts with acetylene to yield (Z)-1,2- B H bis(dichloroboryl)ethane (178)97–99 and cyclopentene forms O cis-1,2-bis(dichloroboryl)cyclopentene (179; Scheme 48).100 184 Diphenylacetylene reacts with dibromophenylborane to form cis-β-bromovinylborane, which when reacts with lithium leads to hexaphenyl-1,4-diboracyclohexadiene (180), the deuterolysis of which by deuteroacetic acid leads to heat, 25 °C O 3 days Cl B 101 O cis-dideuterostilbene (Scheme 49). 70% Singleton and Redman102 have found that 1,2-diborylethy- lenes 182, 183 and 184 were readily generated from the

reaction of trans-1,2-bis(tributylstannyl)ethylene (181)with Bu3Sn H haloboranes (Scheme 50).103 Formation of B–C–C–B compounds belonging to the H SnBu bidentate Lewis acid was conducted by Biallas and coworkers 3 more then 30 years ago.104 – 106 The reactions of a bidentate 181

version of the quintessential boron-based Lewis acid (BF3) with simple Lewis bases and anions have been investigated. BrBMe2 Br-BBN CCl4 CH2Cl2 Thus, F2BCH2CH2BF2 (135) was prepared by mixing ethylene − ° − ° 107 78 C 78 C and B2F4 at low temperature (Scheme 51). To obtain diboryl derivative 136 with strongly Lewis acidic boron centers, a weaklybasicreagentssuchasPh3COMe was required. NBB H 108 Me2BH Treatment of the known compound 1,2-(BCl2)2C6H4 with slightly more than two equivalents of Zn(C6F5)2 led to smooth H BBN H BMe2

182 183 Cl2B BCl2 HH+ B Cl 2 4 Scheme 50. H H 178

Cl B 2 F F HH F B BF B + B Cl 2 2 Ph COCH 2 4 + 3 3 + B2F4 OCH3 [Ph3C] Cl2B H H H H B F 179 185 F 186 Scheme 48. Scheme 51.

Ph Ph + Ph Ph PhBBr2 conversion to the desired 1,2-diborane 187 (Scheme 52).109 Br BPhBr Reaction of 187 with potassium salts of OH− and F− in the Ph presence 18-crown-6 produces borate anions 188 and 189 when X is chelated between the two boron centers. Ph B Ph Two different routes, by Piers and coworkers109 and Marks and coworkers110 independently, reported recently, Ph B Ph the formation of compound 190. Marks and coworkers110 Ph treated the organotin precursor 1,2-C6F4(SnMe3)2 with excess 180 BCl3 at high temperatures (Scheme 53). For conversion to 191, 111 organotin C6F5 transfer agent [(C6F5)2SnMe2] or Zn(C6F5)2 Scheme 49. reagents were used.109

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F F C6F5 R2N NR2 C6F5 BB F B(C6F5)2 F B R2N N2R LiCCPh BBr3 KX + BB [(C6F4)Hg]3 X [K(18-C-6)] Cl Cl hexane, LiCl F B(C6F5)2 F B C F F F 6 5 = Ph Ph C6F5 192. R CH3 187 193. R = i-Pr = 188. X = OH 194. R CH3 189. X = F 195. R = i-Pr Cp2Zr THF Scheme 52. Cl3CCCl3 R N NR2 2 B B C6H6

F R2N NR2 Ph Ph Me SiCl B B Zr 3 F SnMe3 4 BCl Cp Cp 3 F Cl F HCl Ph Ph ° F SnMe 180 C 196. R = CH Et O 3 F B F 3 2 SiMe3 SiMe = 3 F 197. R i-Pr F B F xs BCl3 R2N NR2 [(C6F4)Hg]3 F Cl F B B ° = 120 C 198. R CH3 190 199. R = i-Pr Ph Ph F C6F5 F H H F B F Scheme 54. F B F

F C6F5 F derivative 209, and reaction of 208 and mono(1,3,2- 191 benzodioxaborol-2-yl)acetylene gives the tetraborylbenzene derivative 210 (Scheme 56).113 Scheme 53. The catechol-substituted diborane(4) 200 was reacted with

diborylacetylene in the presence of [Pt(PPh3)2(C2H4)] to gave 211 in 71% yield (Scheme 57).114 Catechol-substituted dibo- 22 It is known that zirconocenes mediate cyclization. rylacetylenes 200 or 201 in the presence of [Pt(PPh3)2(C2H4)] 112 Metzler et al. synthesized cis-bis(4,5-phenylmethylidene)- or [Pt(PPh3)4] gave tetra- and the RHF/3–21G-optimized 1,3-bis(dialkylamino)-1,3-boranes 198 and 199 with almost geometries of 212 and 213 and reveal intramolecular stabi- quantitative yield (Scheme 54) via zirconocene species 196 lization of the boron pz orbital. When [Pt(cod)2]isusedas and 197. Bis[(dialkylamino)(phenylethynyl)boryl]methanes catalyst, the tetraborylethene 213 is formed exclusively.114 194 and 195 were obtained in good yields by reacting lithium Derivatives of lithium and tin can be used in phenylacetylide with 192 and 193. the preparation of B–C C–B 215–217, 219, 221, 222 The hexaborylbenzene derivatives 202 and 203 have and B–C C–B 214 and 220 compounds (Scheme 58). { been obtained by transition-metal-catalyzed [CpCo(CO)2], By preparing mono- and bi-dentate organoboron Lewis } [Co2(CO)8], [Ni(cod)2] cyclotrimerization of bis(1,3,2- acids, the scope and limitations of synthesizing the benzodioxaborol-2-yl)acetylene (200) and bis[1,3,2-(2,3- requisite organoboranes by the boron–tin exchange between naphtho)-dioxaborol-2-yl] acetylene (201; Scheme 55).113 a boron halide and the appropriate organostannane 115 AlMe3 and AlEt3 were found to react with 202 and 203 have been observed. Also, organotin derivatives to furnish C6(BMe2)6 (204)andC6(BEt2)6 (205) respectively. can be obtained either from the corresponding

With pyridine, the hexafunctional Lewis acid 204 formed the RMgBr or RLi reagent and MenSnCl4−n or from a · crystalline bis(pyridine) adduct C6(BMe2)6 2(NC5H5)(206). Barbier procedure using the organic halide, Me3SnCl Thermolysis of C6(BEt2)6 (205) resulted in the intramolecular and magnesium metal: 1,2-bis(trimethylstannyl)ethyne, elimination of ethane to give the 1,2,3,4-bis(29,39-dihydro- o-, m-, and p-bis(trimethylstannyl) benzenes, α,o- 19,39-diborole)-5,6-bis(diethylboryl)-benzene compound 207 bis(trimethylstannyl)toluene, α,α-bis-(trimethylstannyl)-o- (Scheme 55). xylene, and 2,2-dimethyl-2-stannaindane.115 Studies of the

Reaction of [Co2(CO)8]with200 yielded the properties of these organoboranes have identiﬁed the bis(1,3,2-dioxaborol-2-yl) dicobaltatetrahedron derivative heightened Lewis acidity of 1,2-bis(diethylboryl)ethyne and

208. Reaction of stoichiometric amounts of 208 and the π-electron delocalization involving the 2pz-boron orbitals diphenylacetylene gives the bis(boryl)tetraphenylbenzene in the 9,10-dihydro- (216) and 9,10-dibora-anthracene (217)

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R O O R B B R O O R ′ 200. R = H BR 2 201. R = C H ′ ′ 4 4 R 2B BR 2

∆T ′ [catalyst] R′ B BR 2 toluene 2 ′ BR 2 180 °C 204. R′ = Me R R ′ = Et ′ 205. R Et AlR 3 B

hexane Et2B B R R pyridine Et Et O O Et BB R B 2 O O R NC H 5 5 B B B Et O O 207 BMe2 O O B B Me2B BMe2 R O B O R O O Me2B BMe2 R R BMe2

R R NC5H5 206 202. R = H = 203. R C4H4

Scheme 55.

O O systems. Finally, an electronic mechanism for the boron–tin B B exchange has been developed to account for the selectivity of O O boron halide attack at unsaturated carbon–tin bonds. 200 The thermolysis of boranes 214 led variously to definite dimers or ill-defined oligomers with B–C C–B structure 219 RT [Co2(CO)8] − CH2Cl2 2 CO (Scheme 59). 1,2-Bis(diethylboryl)ethyne (214) was heated to 120 ◦C and the liquid evolved was permitted to reflux for 2 h. Evaporation of the liquid evolved under reduced pressure O O allowed the condensation of 1.0 equivalent of Et3Banda B O B red–brown viscous oil remained (218). Subsequent heating O ∆T up to 700 ◦C in a thermogravimetric analysis apparatus + 200 204 toluene left a shiny black residue of empirical formula of C3B2H4 115 (CO)3Co Co(CO)3 (219). 208 Recently, Ishiyama and Miyaura116 published an article where they reviewed the metal-catalyzed borylation of catB H Ph Ph alkenes, alkynes, and organic electrophiles with B–B Ph compounds. They also reported that the platinum(0)- catB Ph catB Bcat catalyzed addition of bis(pinacolato)diborane to alkenes and alkynes stereoselectively yielded cis-bis(boryl)alkanes or cis- bis(boryl)alkenes; and the addition of diborane to 1,3-dienes catB Ph catB Bcat with a platinum(0) complex afforded a new access to the cis- Ph 210 1,4-bis(boryl)butene derivatives, which are a versatile reagent 209 for diastereoselective allylboration of carbonyl compounds. Scheme 56. The mechanisms and the synthetic applications of these reactions have also been discussed.

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O O B B O O O O [(Ph3P)2PtC2H4] 200 + Bcat B B toluene, reflux O O 24 h O O B B O O 211

O O R B B R O O [(Ph3P)2PtC2H4] O O 200 + Bcat B B ° = THF, reflux, 55 C O R H 24 h O R = t-Bu 212 R R

O O B B O O [Pt(cod)2] 200 + Bcat ° toluene, 40 C O O 48 h B B O O 213

Scheme 57.

Et2BBBr2 Li Li BB+ BO Br Et B ∆T 700 °C THF BB B − 214 Et2B B 93% 218 214 x B(C2H5)2Br THF 219 Me3Sn SnMe3 − H Lindmar Pd Me3SnBr 2 Et Cl O Et BBEt SnMe3 BCl2 B two steps HH Et O 220 SnMe3 BCl2 B Et2B BEt2 215 Cl 221 H2 R 216 − ∆T B B B CH2 CH2 B − 3 Et2B + = x + R Cl, Br 222 B_ R Scheme 59. 217

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