13 Organoboron Chemistry By K. SMITH and W. E. PAGET Department of Chemistry University College of Swansea Singleton Park Swansea SA2 8PP This Report is concerned with major advances in organoboron chemistry over the past five years. It is perhaps appropriate that in 1979 the importance of the field and the unparallelled contributions of H. C. Brown were recognized by the award of a Nobel Prize. H. C. Brown has himself recently reflected upon forty years of hydride reductions.' There have been many recent reviews covering individual aspects of organoboron chemistry but we cite only one book2" and a review2' because together they contain key references to all areas of organoboron chemistry that are likely to be of interest to organic chemists. 1 Reductions using Organoboron Compounds Alkali-metal trialkylhydridoborates (trialkylborohydrides) are increasingly being recognized as useful reducing agents and detailed studies of their preparations from trialkylboranes and a variety of simple and complex hydrides have a~peared.~ The sterically hindered compounds are particularly useful for stereoselective reduction of carbonyl compounds (e.g.Scheme l).4Some trialkylborohydrides are now com- mercially available. Reagents i Li[Me,CHCH(Me)],BH at -78 "C Scheme 1 cup-Enones which are not encumbered at the p-position generally undergo conjugate reduction to give enolates which can be directly alkylated in situ if required (Scheme 2).' On the other hand branching at the &position generally leads to direct reduction of the carbonyl gro~p.~*~ H.C. Brown and S. Krishnamurthy Tetrahedron 1979,35 567. (a) H. C. Brown 'Organic Synthesis oia Boranes,' Wiley-Interscience New York,1975; (6) A. Pelter and K. Smith in 'Comprehensive Organic Chemistry' ed. D. H. R. Barton and W. D. Ollis Pergamon Oxford 1978 Vol. 3 pp. 689-940. (a) C. A. Brown and S. Krishnamurthy J. Organometaal. Chem. 1978,156,111; (b) C. A. Brown and J. L. Hubbard J. Am. Chem. Soc. 1979 101 3964; (c) H. C. Brown. S. Krishnamurthy and J. L. Hubbard ibid. 1978 100 3343; J. Organometal. Chem. 1979 166 271. S. Krishnamurthy and H. C. Brown J. Am. Chem. SOC.,1976,98,3383. J. M. Fortunato and B. Ganem J. Org. Chem. 1976,41,2194. W. G. Dauben and J. W. Ashmore Tetrahedron Lett.1978,4487. K. Smith and W.E. Paget Reagents i LiBus3BH; ii IACO,Et \ Scheme 2 Trialkylborohydrides are extremely powerful reducing agents and as such are useful for the deoxygenation of alcohols by reduction of their tosylates7 or oxy- phosphonium salts (applied particularly to glycosyl derivatives),8 and for the demethylation of quaternary ammonium salts.’ They also react with elemental sulphur and selenium to give active anhydrous lithium sulphides (Li2S or Li2S2 depending upon the stoicheiometry) and selenides which are useful reagents for introduction of sulphur and selenium into organic molecules. lo Metal carbonyls may be converted into metal formyl complexes,” and dimeric carbonyl compounds cleaved to metal carbonyl anions,” by the action of lithium triethylborohydride.9-Alkyl-9-borabicyclo[3.3. llnonanes (9-alkyl-9-BBN’s) and their 23-alkyl ate- complexes are interesting new types of reducing agents. The boranes (9-alkyl-9- BBN’s) eliminate alkene on treatment with an aldehyde and there is concomitant reduction of the aldehyde. B-Siamyl-9-BBN (siamyl = 1,2-dimethylpropyI) appears to be particularly promising for selective reduction of aldehydes in the presence of ket~nes,’~ whilst optically active B-isopinocampheyl-9-BBN (formed by hydro- boration of a-pinene using 9-BBN) is capable of producing primary alcohols RCHDOH of very high optical purity (>go%) from deuterio-aldehydes RCD0.14 Lithium 9,9-dibutyl-9-boratabicyclo[3.3, llnonane donates a hydride ion from one of the bridging positions and itself undergoes rearrangement (Scheme 3).” However it shows considerable discrimination in the facility with which it gives up its hydride.Thus for example it reacts with 4-methylcyclohexanone (cf.Scheme 1) to give the cis-alcohol (84%) in the presence of methanol or the trans-isomer (90%) in the presence of lithium meth0~ide.l’~ It can also be used for selective Bu Bu Bu Bu \-/ \/ B +EX EH + LiX + Lib4 Scheme 3 7 S. Krishnamurthy and H. C. Brown J. Org. Chem. 1976,41,3064; S. Krishnamurthy J. Organometal. Chem. 1978,156 171. 8 P. Simon J.-C. Ziegler and B. Gross Synthesis 1979 951. 9 M. P. Cooke and R. M. Parlman J. Org. Chem. 1975,40,531. 10 (a) J. A. Gladysz V. K. Wong and B. S. Jick J. Chem.SOC.,Chem. Commun. 1978,838; Tetrahedron 1979,35,2329; (b) J. A. Gladysz J. L. Hornby and J. E. Garbe J. Org. Chem. 1978,43 1204. It J. A. Gladysz and J. C. Selover Tetrahedron Lett. 1978,319; R. L. Pruett R. C. Schoening J. L. Vidal and R. A. Fiato J. Organometal. Chem. 1979,182 C57. 12 J. A. Gladysz G. M. Williams W. Tam D. L. Johnson D. W. Parker and J. C. Selover Inorg. Chem. 1979,18 553. 13 M. M. Midland and A. Tramontano J. Org. Chem. 1978 43 1470. 14 M. M. Midland S. Greer A. Tramontano and S.A. Zderic J. Am. Chem. SOC.,1979,101,2352. 1s (a) G. W. Kramer and H. C. Brown J.Am. Chem. SOC.,1976,98,1964; (b) Y. Yamamoto H. Toi A. Sonoda and S.-I. Murahashi ibid. p. 1965. Organoboron Chemistry 289 dehalogenation of reactive (tertiary alkyl allyl or benzyl) halides,16 or for regio- selective cleavage of epoxides in a manner opposite to that usually enc0~ntered.l~ Acyloxy-borohydrides have also been recommended as novel types of reducing agents," whilst new applications of the established reagents 9-BBN (e.g.reduction of enones to allylic and catecholborane (unsaturated tosylhydrazones to alkenes or allenes)20 have also been reported. 2 Hydroboration Details of the hydroboration of alkenes ard alkynes and of the cyclic hydroboration of dienes using monochloroborane diethyl etherate21 or the more stable and convenient adducts of dimethyl sulphide with monohalogeno-boranes,22 have appeared. The dimethyl sulphide adducts in particular promise to be amongst the more important hydroborating agents in future years.1,3,2-Dithiab0rolan~~ is another new hydroborating agent. In some respects 9-BBN is an unusual hydroborating reagent. For example the rate of its hydroboration of many alkenes is independent of the nature or concentration of the alkene because the rate-limiting step is dissociation of the 9-BBN dimer into monomer.24 Also B-cyclo-octyl-9-BBN derivatives do not undergo the isomerization of other B-cyclo-octyl and 9-BBN unlike other dialkyl-boranes hydroborates most alkenes faster than the corresponding alkynes.26 Details of the general applications of 9-BBN have appea~ed,~' and the preparation of a B-deuterio-derivative has also been reported.28 Much effort has been devoted to the development of a convenient procedure for preparation of monoisopinocampheylborane a relatively unhindered asymmetric hydroborating agent.29 One method involves displacement of a-pinene from di-isopinocampheylborane using tetramethylethylenediamine (TMEDA) and results in a product of very high optical purity much higher than that of the original a-pinene used in the preparation of the di-isopino~ampheylborane.~~~ The TMEDA complex of thexylborane (1,l72-trirnethylpropylborane)reacts with l6 Y.Yamamoto H. Toi S.-I. Murahashi and I. Moritani J. Am. Chem. SOC.,1975 97 2558. 17 Y. Yamamoto H. Toi A. Sonoda and S.-I. Murahashi J. Chem. SOC.,Chem. Commun. 1976 672. " N. Umino T. Iwakuma and N. Itoh Tetrahedron Left. 1976,763,2875; G. W. Gribble W. J. Kelly and S. E. Emery Synthesis 1978 763.l9 S. Krishnamurthy and H. C. Brown J. Org. Chem. 1975,40,1864;H. C. Brown S. Krishnamurthy and N. M. Yoon ibid. 1976 41 1778. 2o G. W. Kabalka D. T. C. Yang and J. D. Baker J. Org. Chem. 1976,41 574; G. W. Kabalka R. J. Newton J. H. Chandler and D. T. C. Yang J. Chem. SOC.,Chem. Commun. 1978,726. H. C. Brown and N. Ravindran J.Am. Chem. SOC.,1976 98 1785 1798 H. C. Brown and M. Zaidlewicz ibid. p. 4917. 22 H. C. Brown N. Ravindran and S. U. Kulkarni J. Org. Chem. 1979,44,2417 H. C. Brown and S. U. Kulkarni ibid. p. 2422. 23 S. Thaisrivongs and J. D. Wuest J. Org. Chem. 1977 42 3243. 24 H. C. Brown C. G. Scouten and K. K. Wang J. Org. Chem. 1979,44,2589. 25 H. Taniguchi L. Brener and H. C. Brown J. Am. Chem. SOC., 1976,98,7107. 26 C.A. Brown and R. A. Coleman J. Org. Chem. 1979,44,2328. 27 H. C.Brown R. Liotta and G. W. Kramer J. Org. Chem. 1978,43,1058;H. C. Brown C. G. Scouten and R. Liotta J. Am. Chem. SOC., 1979,101,96;J. C.-S. Chen J. Organometal. Chem. 1978,156,213. 28 M. M. Midland and S. Greer Synthesis 1978 845. *' (a) H. C. Brown and A. K. Mandal Synthesis 1978 146 (b) H. C. Brown J. R. Schwier and B. Singaram J. Org. Chem. 1978,43,4395;(c) A. Pelter D. J. Ryder J. H. Sheppard C. Subrahmanyam H. C. Brown and A. K. Mandal Tetrahedron Lett. 1979,4777. 290 K. Smith and W. E. Paget alkenes to give complexes of monoalkyl-boranes from which the free borane can easily be liberated.30 Diphenylborane another potentially useful reagent for prep- aration of 'mixed' organoboranes has also been prepared in high yield.31 Hydroboration of allene~~~ is very dependent upon the nature of the hydro- borating agent and reaction with 9-BBN is especially useful for the preparation of allyl-b~ranes.~~"~~ Hydroboration-oxidation of an allene was used as a key step in a total synthesis of isocaryophyllene (Scheme 4).33 Reagents i BH, THF at 160"C for 3 h; ii CrO Scheme 4 Vinylsilanes are hydroborated regioselectively with dialkyl-boranes to give over 95OiCl of the P-dialkylboryl-silane whereas borane-THF favours the formation of the ~u-isomer.~~ Borane converts p-enamino-esters into the corresponding a& unsaturated and vinyl-aziridines into allyl-amine~.~~ Hydroboration of 1-bromo-alkynes with a dialkyl-borane followed by treatment with t-butyl-lithium or with lithium triethylborohydride provides a method of synthesizing cis-alkenyl- boranes i.e.alkenyl-boranes with stereochemistry opposite to that obtained by simple hydroboration of a terminal alk~ne.~' It has finally been proved that hydroboration occurs in a cis manner even in acyclic cases3* 3 Synthetic Applications of Organoboranes Potassium tri-isopropoxyborohydride is a superior reagent for hydride-induced carbonylation of organoboranes a reaction which gives an aldehyde after oxidative work-~p.~~ The reaction occurs only under conditions which allow the existence of a small amount of free trialkylb~rane;~' if acid is added subsequent to the car- bonylation step a second rearrangement occurs giving rise to a secondary alcohol upon work-~p.~~ The carbonylation reaction has been applied in the synthesis of benzaldehyde from triphenylb~rane,~~ 14C-labelled aldehydes ketones and of tertiary and of juvabione and its epimer (Scheme 5).44 30 H.C. Brown J. R. Schwier and B. Singaram J. Org. Chem. 1979,44,465. 31 P. Jacob J. Organometal. Chem. 1978,156 101. 32 (a) L. Chevolot J. Soulit and P. Cadiot Tetrahedron Lett. 1974,3435;(b) H. C. Brown R. Liotta and G. W. Kramer J. Am. Chem. SOC.,1979 101 2966; (c) V. V. Ramana Rao S. K. Agarwal D. Devaprabhakara and S. Chandrasekaran Synth. Commun. 1979,9,437. 33 A. Kumar A. Singh and D. Devaprabhakara Tetrahedron Lett. 1976,2177. 34 J. A. Soderquist and A. Hassner J. Organometal. Chem. 1978 156 C12. 35 J.Froburg G. Magnusson and S. ThorBn Tetrahedron Lett. 1975 1621. 36 R. Chaabouni A. Laurent and B. Marquet Tetrahedron Left. 1976,757. 37 E. Negishi R. M. Williams G. Lew and T. Yoshida J. Organometal. Chem. 1975 92 C4; J. B. Campbell and G. A. Molander ibid. 1978 156 71. 38 G. W. Gabalka R. J. Newton and J. Jacobus J. Org. Chem. 1978,43 1567. 39 H. C. Brown J. L. Hubbard and K. Smith Synthesis 1979,701. 40 H. C. Brown and J. L. Hubbard J. Org. Chem. 1979 44,467. 41 J. L. Hubbard and H. C. Brown Synthesis 1978 676. 42 G. W. Kabalka and J. W. Fernell Synth. Commun. 1979,9,443. 43 G.W. Kabalka E. E. Gooch C. J. Collins and V. F. Raaen J. Chem. SOC.,Chem. Commun. 1979,607. 44 E. Negishi M. Sabanski J.-J. Katz and H. C. Brown Tetrahedron 1976 32 925. Organoboron Chemistry 291 C0,Me 53% overall yield Reagents i (Me,CHCMe,)BHBu’; ii CO 70 atm 50 “C; iii [O] Scheme 5 A number of new reagents for oxidation of organoboranes have been reported of which the most notable are trimethylamine oxide dihydrate4’ and pyridinium chlorochr~mate.~~ The latter oxidizes organoboranes and alkoxyboranes directly to aldehydes and ketones.The reagent PhI(OAc)2 converts alkyl- and vinyl-boron compounds into the corresponding alkyl or vinyl The first quantitative conversion of trialkylboranes into chloro-alkanes utilizes nitrogen trichloride as the reagent.48 Methoxide ion is a superior base to hydroxide ion for the base-induced iodinolysis of organoboranes allowing all three alkyl groups of a primary trialkylborane to be converted into iodo-alkane (two for secondary alkyl groups).49 Base-induced iodinolysis and brominolysis of organoboranes both involve inversion of stereochemistry at the displaced centre.” Iodinolysis of a vinyl-catecholborane has been applied in the synthesis of some substituted prosta- glandins.’ Iron(II1) salts have been used to convert organoboranes into alkyl seleno- cyan ate^'^ and a~ides,’~ whilst Chloramine-T reacts with trialkylboranes to give the corresponding alkyl toluenes~lphonamide.’~ The direct transfer of two alkyl groups from a trialkylborane to a single nitrogen atom (‘nitrogen stitching’) has been accomplished in one case as part of a synthesis of the perhydroazaphenalene ring system which is found in alkaloids of the Coccinellidae (Scheme 6).” Trialkylboranes react with the Grignard reagent derived from 1,5-dibromopen- tane to give a spiroborate complex and the alkyl Grignard ~eagent.’~ Alkenyl groups of alkenyl-boranes may be hydrolysed under the influence of palladium(I1) acetate in THF,57 or coupled to give (E,E)-dienes using a reagent composed of 45 G.W. Kabalka and H. C. Hedgecock J. Org. Chem. 1975.40 1776. 46 V. V. Ramana Rao D. Devaprabhakara and S. Chandrasekaran J. Organometal. Chem. 1978,162 C9; C. Gundu Rao S. U. Kulkami and H. C. Brown ibid. 1979,172,C20; H. C. Brown S. U. Kulkami and C. Gundu Rao Synthesis 1979,702,704. 47 Y. Masuda and A. Arase Bull. Chem. SOC.Jpn. 1978,31,901; Y. Masuda A. Arase and A. Suzuki Chem. Lett. 1978 665. 48 H.C. Brown and N. R. De Lue J. Orgunomeful. Chem. 1977,135 C57. 49 N. R. De Lue and H. C. Brown Synthesis 1976 114. ” H. C. Brown N. R. De Lue G. W. Kabalka and H. C. Hedgecock J. Am. Chem. SOC.,1976,98,1290; D. E. Bergbreiter and D. P. Rainville J. Orgunometul. Chem. 1976,121 19. 51 P. W. Collins E. Z. Dajani M. S. Bruhn C. H.Brown J. R.Palmer and R. Pappo Tetrahedron Lett. 1975,4217. ’* A. Arase and Y. Masuda Chem. Lett. 1976,785. 53 A. Suzuki M. Ishidoya and M. Tabata Synthesis 1976 687. ” V. B. Jigajinni A. Pelter and K. Smith Tetrahedron Lett. 1978 181. 55 R. H. Mueller Tetrahedron Lett. 1976 2925. ” K. Kondo and S.-I. Murahashi Tetrahedron Lett. 1979 1237. 57 H. Yatagai Y.Yamamoto and K. Maruyama J. Chem. SOC.,Chem. Commun. 1978,702. K.Smith and W. E. Paget Reagents i [CINHOC,H,(NO,),]; ii H202, OH-; iii heat Scheme 6 palladium(@ chloride lithium chloride and trieth~lamine.’~ On the other hand they can be coupled with aryl or vinyl halides under the influence of palladium(0) and a base.59 Diboryl-alkanes react with silver nitrate to give substantial amounts of cycloalkanes6’ or (E)-alkenes (from 1,2-dibor~l-alkanes).~~ The previously discovered oxygen-induced conjugate addition of trialkylboranes to enones and similar compounds has been extended to 1-acyl-2-vinyl-cyclo-propanes giving y8-unsaturated ketones (Scheme 7).62 B-Alkenyl-9-BBN deriva- R:B + H,C=CH A COR2 i,ii ,R ‘CH2CH=CHCH,CH2COR2 Reagents i 0,; ii H,O Scheme 7 tives undergo conjugate addition of the alkenyl-boron unit to enones apparently by a non-radical mechanism providing another approach to y8-unsaturated ketones.63 Electrolysis of trialkylboranes in the presence of 2-alkylacrylates is an alternative way of inducing conjugate addition.64 Diethylborylpivalate (or triethylborane in the presence of derivatives of pivalic acid) reacts with hydroxylic compounds relatively readily liberating ethane.Such systems have been extensively utilized for the selective protection of sugars and other polyhydroxy-compounds.65 Inversion occurs at the migration terminus during base-induced rearrangement of a-halogenoalkyl-boranes and during the formation of cyclopropanes from y- chloroalkyl-boranes.66 Kinetic and competition experiments provide evidence for a dehydroboration-rehydroborationprocess in the alkene-alkyl exchange reactions of B-alkyl-9-BBN and by implication also in the isomerization of alkyl-boranes.A potentially useful procedure for selective elimination of the secondary alkyl groups present in a typical tri-n-alkylborane prepared by hydro- boration of a terminal alkene involves heating the borane first with anisole and then with a small amount of DMS0.68 V. V. Ramana Rao C. V. Kumar and D. Devaprabhakara J. Organometal. Chem. 1979,179 C7. s9 N. Miyaura and A. Suzuki J. Chem. Soc. Chem. Commun. 1979,866 N. Miyaura K. Yamada and A. Suzuki Tetrahedron Lett. 1979 3437. 6o R. Murphy and R. H. Prager Tetrahedron Lett. 1976,463. 61 K. Avasthi S. S. Ghosh and D. Devaprabhakara Tetranedron Lett. 1976,4871.62 N. Miyaura M.Itoh N. Sasaki and A. Suzuki Synthesis 1975 317. 63 P. Jacob and H. C. Brown J. Am. Chem. SOC.,1976,98,7832. 64 Y. Takahashi K. Yuasa M. Tokuda M. Itoh and A. Suzuki Bull. Chem. Soc. Jpn. 1978,51,339. 6s W. V. Dahloff and R. Koster J. Org. Chem. 1976,41 2316; and references cited therein. 66 M. M. Midland A. R. Zolopa and R. L. Halterman J. Am. Chem. Soc. 1979,101,248; H. L. Goering and S. L. Trenbeath ibid. 1976 98 5016. 67 M. M. Midland J. E. Petre and S. A. Zderic J. Organometal. Chem. 1979,182 C53. 68 Y. Masuda M. Hoshi and A. Arase Bull Chem. SOC.Jpn. 1979,52,271. Organoboron Chemistry 293 4 Synthetic Applications of Organoborates Probably the most significant development in organoborane chemistry over the past decade has been the realization of the synthetic importance of the reactions of four-co-ordinate borate salts with electrophiles.Full details of the reactions of cyanoborates with electrophiles the first reactions to demonstrate the synthetic importance of organoborates were published at the start of the current review period.69 The most useful processes involve acylation of the cyanoborates with reagents such as trifluoroacetic anhydride leading ultimately to ketones or tertiary alcohols (Scheme 8). The adducts of the cyclic intermediates with water have been characterized by X-ray ~rystallography,~' and the reaction has been applied in the annulation of terpenoid polyenes." R2 R3 'n' .. R'R2R3B -b Na+ [R'R2R3BCN]- 4RIYAL\ N J fCF3 R'R2R3COH & R'R2R3CB < R2COR3 Reagents i NaCN; ii (CF,CO),O; iii HzOz OH-; iv excess (CF,CO),O.Scheme 8 The formation of a tertiary alcohol from cis,cis-perhydro-9b-boraphenaleneis interesting in that it gives the trans,trans,trans-alcohol, in contrast to the cis,cis,cis-alcohol obtained by carbonylation (see Scheme 9).72 Alkynylborates react with acids or alkylating agents to give intermediate vinyl- boranes which may be oxidized to ketones or hydrolysed to alkene~.~~ The reactions with a-nitro-alkenes are Michael-type addition With simple alkylating -HQH iv,v,iii* HQH H Reagents i CO pressure; ii heat; iii HzOZ, OH-; iv KCN; v (CF,CO),O warm Scheme 9 69 A. Pelter K. Smith M. G. Hutchings and K. Rowe J. Chem. SOC.,Perkin Trans.1 1975 129 138; A. Pelter M. G. Hutchings and K. Smith ibid.,p. 142; A. Pelter M. G. Hutchings K. Smith and D. J. Williams ibid.,p. 145. 70 P. R. Mallinson D. N. J. White A. Pelter K. Rowe and K. Smith J. Chem. Res. 1978 (S)234 (M) 3101. 71 R. Murphy and R. H. Prager Aust. J. Chem. 1976,29 617; J. Organometal. Chem. 1978,156 133. " A. Pelter P. J. Maddocks and K. Smith J. Chem. SOC.,Chem. Commun. 1978,805. 73 (a) H. C. Brown A. B. Levy and M. M. Midland J. Am. Chem. SOC.,1975 97 5017; (b) M. M. Midland and H. C. Brown J. Org. Chem. 1975 40 2845; (c) A. Pelter, C. R. Harrison C. Subrahamanyam and D. Kirkpatrick J. Chem. SOC.,Perkin Trans. 1,1976,2435;(d) A. Pelter T. W. Bentley C. R. Harrison C. Subrahmanyam and R. J. Laub ibid. p. 2419; (e) A. Pelter C.Subrah- manyam R. J. Laub K. J. Gould and C. R. Harrison Tetrahedron Lett. 1975,1633; (f A. Pelter and L. Hughes J. Chem. SOC.,Chem. Commun. 1977,913. 294 K. Smith and W.E. Paget agents mixtures of the (E)-and (2)-isomers of the vinylboranes are obtained but with more complex ones (e.g. propargyl halides a-halogenocarbonyl compounds and the like) the reaction is stereospecific giving rise to (2)-alkenyl derivatives on hydrolysis (Scheme The intermediate vinylboranes can also be worked up in other ways; for example by application of the Zweifel alkene synthesis using iodine and base.75 R:B R2 H R2 \/ ii \ / Li' [R:BCrCR2]-c=c _.+ c=c (-LiBr) R' ' R'' 'CH2X 'CH2X Reagents i BrCH,X; ii MeC0,H Scheme 10 The stereoselectivity of production of disubstituted ethenes can be greatly improved by use of Bu;SnCl as the electrophile instead of This gives a stannyl-vinylborane stereospecifically; on subsequent hydrolysis this gives a (2)-alkene.Similar reactions occur on reaction of trialkylboranes directly with 1-trialkylstannyl-alkyne~,'~ whereas the use of dialkynyl-stannanes gives rise to bora- stanna- heterocycle^.^^ Other electrophiles which give rise to useful products include iodine (gives alkyne~)~~ and ethylene oxide (gives alkenylethanol derivatives).80 Substituents in the original alkyne also affect the course of the reaction. Thus (tri-methylsily1)ethynyl-borates give overwhelmingly one geometrical isomer of the vinylborane even on &action with simple alkylating agenk8' The lithiated deriva- tives of lithium ethynyltrialkylborates can be alkylated by alkyl halide and then treated with iodine to give unsymmetrical internal alkynesE2 Lithiated propargyl chlorides and acetates react with trialkylboranes to give allenyl-boranes which equilibrate with the corresponding propargyi-boranes at ambient temperature (Scheme 1l).83The reaction of these species with aldehydes gives homopropargyl and homoallenyl alcohols respectively; these reactions are typical of &-unsaturated organoboranes (see Section 5).R -78°C I 20°C Lif [R3BC=CCH2X]-(-LiX) R2B-C=C=CH2 RzBCHzCrCR ____t Scheme 11 74 A. Pelter K. J. Gould and C. R. Harrison J. Chem. SOC.,Perkin Trans. 1 1976 2428. 75 G. Zweifel and R. P. Fisher Synthesis 1975 376.76 J. Hooz and R.Mortimer Tetrahedron Lett. 1976 805. 77 B. Wrackmeyer and R. Zentgraf J. Chem. SOC.,Chem. Commun. 1978,402. 78 L. Killian and B. Wrackmeyer J. Organometal. Chem. 1978,153 153; and references cited therein. 79 A. Suzuki N. Miyaura S. Abiko M. Itoh H. C. Brown J. A. Sinclair and M. M. Midland J. Am. Chem. Soc. 1973,95 3080. K. Utimoto T. Furubayashi and H. Nozaki Chem. Lett. 1975,397. K. Utimoto M. Kitai M. Naruse and H. Nozaki Tetrahedron Lett. 1975 4233; R.Koster and L. A. Hagelee Synthesis 1976 118. K. Utimoto Y.Yabuki K. Okada and H. Nozaki Tetrahedron Lett. 1976,3969. 83 (a) G. Zweifel S. J. Backlund and T. Leung J.Am. Chem. SOC.,1978,100,5561;(6) M. M. Midland and D. C. McDowell J. Organometal. Chem. 1978,156 C5.Organoboron Chemistry 295 Dialkyl(alkenyl)(alkynyl)borates undergo reactions with electrophiles in the same way as trialkyl(alkynyl)borates except that the products are conjugated dienes rather than mono-enes and so omg4Similarly dialkyl(dialkyny1)boratesreact with iodine to give conjugated di-yne~.~' If the method of preparation of the dialkynyl- borate is chosen so as to allow the synthesis of derivatives with two different alkynyl groups on a single boron atom the conjugated di-ynes are also unsymmetrical thus providing a more convenient alternative to the Cadiot-Chodkiewicz rea~tion.~'~*~ Alkenyl(trialky1)borates undergo the same types of reactions as alkynyl-borates for example with iodine to give alkenes,86 but in general they have been less widely studied.The alkene synthesis works well on alkenyl(alkyl)dimethoxyborates allowing better utilization of alkyl groups and this approach has been applied in the synthesis of prostaglandin model Alkenyl-boranes can be hydrolysed under basic conditions if they are first converted into the corresponding borates by treatment with butyl-lithium.88 Alkenyl-borates are also intermediates in the synthetically useful reactions of trialkylboranes with methoxyvinyl-lithium (Scheme 12).89 iii R2C=CH2 Li+12BiRJ -RCOCH~ Reagents i I, at -80 "C;ii H,O'; iii H20z HO-; iv BF Et20;V 12 HO-Scheme 12 Allyl-borates are formed with considerable regioselectivity and then react very selectively with electrophiles which attack at the position y-to boron.This allows head-to-tail coupling of allylic borates with ally1 halides for e~ample.~' Propargyl-borates are intermediates in the formation of alkynyl- and allenyl-silanes from lithiated 3-phenoxy-1 -trimethylsilylpropyne and trialkylborane~.~' Ate-complexes formed from trialkylboranes and 2-lithiated furan thiophen N-methylpyrrole and N-methylindole undergo reaction with iodine to give the 2-alkylated heterocycle^.^^ Other electrophiles can also be used to induce 84 G. Zweifel and S. J. Backlund J. Organometal. Chem. 1978,156 159. 85 (a) A. Pelter K. Smith and M. Tabata J. Chem. SOC.,Chem. Commun. 1975,857; (b) J. A. Sinclair and H. C. Brown J. Org. Chem. 1976,41,1078; (c) A. Pelter R. J. Hughes K. Smith and M. Tabata Tetrahedron Lett. 1976 4385. 86 N.J. Lalima and A. B. Levy J. Org. Chem. 1978,43,1279. D. A. Evans T. C. Crawford R. C. Thomas and J. A. Walker J. Org. Chem. 1976,41 3947. 88 E. Negishi and K.-W. Chiu J. Org. Chem. 1976 41 3484. 89 A. B. Levy S. J. Schwartz N. Wilson and B. Christie J. Organometal. Chem. 1978 156 123. 90 Y. Yamamoto and K. Maruyama J. Am. Chem. SOC.,1978,100,6282; Y. Yamamoto H. Yatagai and K. Maruyama J. Chem. SOC.,Chem. Commun. 1979,157; Chem. Lett. 1979,385. 91 T. Yogo J. Koshino and A. Suzuki Tetrahedron Lett. 1979 1781. 92 I. Akimoto and A. Suzuki Synthesis 1979,146; T. Sotoyama S. Hara and A. Suzuki Bull. Chem. SOC. Jpn. 1979,52 1865; E. R. Marinelli and A. B. Levy Tetrahedron Lett. 1979,2313; A. B. Levy J. Org. Chem. 1978,43,4684. 296 K.Smith and W.E. Paget rearrangement and in the case of N-methylindole this has been used to prepare 2,3-disubstituted indoles (Scheme 13; EX = e.g. ICH2CN ICH2CONH2 CH2=CHCH2Br et~.).~~ Two heteroaromatic units can be coupled by the reaction of the cyclic ethanolamine-borates of di(heteroary1)borinic acids with bromine or N-bromosuccinimide,94 whilst the presence of the bromine substituent in 6-bromo- 2-lithiopyridine causes rearrangement of its initially formed trialkylborate salts leading ultimately to unsaturated nit rile^.^' Reagents i R,B; ii EX; iii [O] Scheme 13 Addition of CuI to lithium trialkylmethylborates gives uncharacterized species assumed to be Cu'[R,BMe]- which undergo conjugate addition of the alkyl group to a@-unsaturated nitriles esters et~.,'~ and can also be used to alkylate reactive halide^.^' A modification of the latter reaction gives a stereoselective synthesis of ~inylsilanes.~~ Copper(1) species though perhaps not the corresponding borates are also intermediates in the preparation of conjugated (E,E)-dienes from dialkenyl- chloroborane and methyl~opper.~~ Lithium tetra-alkyl- and trialkylaryl-borates transfer an alkyl (or aryl) group to acid chlorides giving ketones as products.100 Thus the trialkylborane acts to moderate the reactivity of the organolithium reagent preventing further alkylation.Reactions of trialkylboranes with 1,l-bis(pheny1thio)alkanes give borates which spontaneously rearrange to give compounds that are oxidizable to ketones. Addi- tion of an electrophile such as HgC12 prior to oxidation results in the formation of tertiary alcohols (Scheme 14)."' If 2-alkylbenzo-1,3-dithioles are used instead of 1,l-bis(phenylthio)alkanes this allows the reactions to be applied to hindered organoboranes.lo2 The reaction of the ate-complex from a trialkylborane and methylthiomethyl-lithium with methyl iodide gives the homologated organoborane R2BCH2R.lo3 93 A. B. Levy Tetrahedron Lett. 1979,4021. 94 G. M. Davies P. S. Davies W. E. Paget and J. M. Wardleworth Tetrahedron Lett 1976 795. 95 K. Utimoto N. Sakai M. Obayashi and H. Nozaki Tetrahedron 1976 32 769. 96 N. Miyaura M. Itoh and A. Suzuki Tetrahedron Lett. 1976,255;K. Yamada N. Miyaura M. Itoh and A. Suzuki Bull. Chem. SOC. Jpn. 1977,50,3431;Y.Yamamoto H. Yatagai and K. Maruyama J. Org. Chem. 1979,44,1744. " K. Yamada T. Yano N. Miyaura and A. Suzuki Bull. Chem. SOC.Jpn. 1979,52,275. 98 K. Uchida K. Utimoto and H. Nozaki J. Org. Chem. 1976 41 2941. 99 Y. Yamamoto H. Yatagai and I. Moritani J. Am. Chem. SOC., 1975 97 5606; Y. Yamamoto H. Yatagai K. Maruyama A. Sonoda and S. Murahashi Bull. Chem. SOC., Jpn. 1977 50 3427. loo E. Negishi K.-W. Chiu and T. Yosida J. Org. Chem. 1975,40,1676; E. Negishi A. Abrarnovitch and R. E. Merrill J. Chem. SOC.,Chem. Commun. 1975 138. lo' R. J. Hughes S. Ncube A. Pelter K. Smith E. Negishi and T. Yoshida J. Chem. SOC., Perkin Trans. 1 1977,1172. lo' S. Ncube A. Pelter and K. Smith Tetrahedron Lett. 1979 1883 1895. lo3 E. Negishi T. Yoshida A. Silveira and B.L. Chiou J. Org. Chem. 1975 40 814. Organ o boron Chemistry HgCIz R:R'COH 1[01 \,B-CR:R2 c--R;BCR'R2SPh X' Scheme 14 R'COR' The reactions of lithium aldimines with dialkyl-chloroboranes followed by treatment with electrophiles offer an alternative method for the synthesis of ketones and tertiary alcohols.104 5 Applications of fly-Unsaturated Organoboron Compounds Allyl-boranes have been extensively studied by Mikhailov's group for many years. In particular their reactions with alkynes allenes and reactive alkenes resulting in addition of the allyl-boron unit (with rearrangement of the ally1 group) across the multiple bond have led to interesting novel boron-containing Reactions of allyl-boron compounds with aldehydes and ketones also involve rearrangement and provide syntheses of homoallylic alcohols,lo6 sometimes with considerable diastereoselectivity .106c*d Propargyl-and allenyl-boron compounds react with carbonyl compounds to give homoallenyl and homopropargyl alcohols respectively.83a*107 Following Mukaiyama's demonstration of crossed aldol reactions between boron enolates (vinyloxy-boranes) and carbonyl compounds,108 much effort has been devoted to studying such reactions.Methods for the synthesis of vinyloxy-boranes include addition of boron compounds to a& unsaturated carbonyl compounds '08*'09 or ketens,*1° reactions of trialkylboranes with a-diazocarbonyl compounds,'" and boron-compound-promoted enolysis of ketones."' By careful choice of method it is often possible to obtain either the (2)-or the (E)-isomer of the vinylborane with considerable ~electivity,"~~"~ and it has been shown that a given geometrical isomer Y.Yamamoto K. Konda and I. Moritani Bull. Chem. SOC.Jpn. 1975,48,3682; J. Org. Chem. 1975 40 3644. B. M. Mikhailov T. K. Kozminskaya and B. I. Bryantsev Zh. Obshch. Khim. 1976 46 87; Yu. N. Bubnov B. A. Kazanskii 0.A. Nesmeyanova T. Yu. Rudashevskaya and B. M. Mikhailov Zzv. Akad. Nauk SSSR,Ser. Khim. 1977,2545. (a) G. W. Krarner and H. C. Brown J. Org. Chem. 1977 42 2292; (b) B. M. Mikhailov Yu. N. Bubnov and A. V. Tsyban J. Organometal. Chem. 1978,154,113; (c)T. Herold and R. W. Hoffmann Angew. Chem. Znt. Ed. Engl. 1978,17 768; (d)R. W. Hoffrnann and H.-J. Zeiss ibid. 1979 18 306. lo' 8.Favre and M.Gaudemar J. Organometal. Chem. 1975,92 17. lo* M. Muraki K. Inomata and T. Mukaiyama Bull. Chem.SOC. Jpn. 1975,48,3200;T. Mukaiyama and T. Inoue Chem. Lett. 1976,559 Io9 W. Fenzl R. Koster and H. J. Zimmermann Justus Liebigs Ann. Chem. 1975 2201; 1976 1116. P. Paetzold and M. Lasch Chem. Ber. 1979 112 663. 'I1 S. Masamune S. Mori D. Van Horn and D. W. Brooks Tetrahedron Lett. 1979 1665. 'I2 W. Fenzl and R. Koster Justus Liebigs Ann. Chem. 1975 1322; W. Fenzl H. Kosfeld and R. Koster ibid. 1976 1370. (a) M. Hirama and S. Masamune Tetrahedron Lett. 1979,2225; (b) D. E. Van Horn and S. Masamune ibid. p. 2229; (c) M. Hirama D. S.Garvey L. D.-L. Lu and S. Masamune ibid. p. 3937. 'I4 D. A. Evans E. Vogel and J. V. Nelson J. Am. Chem. SOC., 1979,101,6120.K. Smith and W.E. Paget reacts in a highly diastereoselective manner with a given carbonyl compound thus allowing the synthesis of diastereoisomers with relative ease (e.g.Scheme 15).113~"4 This approach has allowed a simplified synthesis of the Prelog-Djerassi lactonic acid. '3c (>97%this isomer) (385% this isomer) Reagents i c,BOSO,CF +EtNPr',; ii RCHO; iii BOS02CF + EtNPr' Scheme 15 Potassium trialkyl(vinyloxy)borates formed by addition of potassium enolates to trialkylboranes react more selectively with alkyl halides than do the potassium enolates Vinylamino-boranes also undergo aldol condensation reac- tions,"6 whilst the related reactions of phenylamino-boron compounds allow specific ortha-substitution of aniline derivative~."~ Particularly useful is the formation of ortho-formyl derivatives from the reactions with isocyanides (Scheme 16)."* Related reactions have been used to prepare indoles' l9 and acridines.I2' 6 Novel Types of Organoboron Compound Attempts to synthesize highly hindered tri-t-butylborane have now been success- ful.12' A number of bora-adamantane compounds have been prepared including l-bora-adamantane,122 2-b0ra-adamantane,'~~ the dibora-compound (1),124and the E.Negishi M. J. Idacavage F. DiPasquale and A. Silveira Tetrahedron Lett. 1979 845. T. Sugasawa T. Toyoda and K. Sasakura Synth. Commun. 1979,9,515; T. Sugasawa and T. Toyoda ibid. p. 553 Tetrahedron Lett. 1979 1423. T. Sugasawa T. Toyoda M. Adachi and K. Sasakura J. Am. Chem.SOC.,1978,100,4842. "* T. Sugasawa H. Hamana T. Toyoda and M. Adachi Synthesis 1979 99. T. Sugasawa M.Adachi K. Sasakura and A. Kitagawa J. Org. Chem. 1979,44,578. 120-B. M. Mikhailov V. A. Dorokhov and 0. G. Boldyreva Izu. Akad. Nauk SSSR,Ser. Khim. 1978 2574. H. Noth and T. Taeger J. Organometal. Chem. 1977,142,281. B. M. Mikhailov and T. K. Baryshnikova Dokl. Akad. Nauk SSSR,1978 243 929. 123 B. M. Mikhailov T. A. Shchegoleva E. M. Shashkova and V. G. Kiselev Izu. Akad. Nauk SSSR,Ser. Khim. 1977,894. 124 S. U. Kulkarni and H. C. Brown J. Org. Chem. 1979 44 1747. 12' Organoboron Chemistry R' R' liii R' NHR' X \ Reagents i BCI,; ii RZN=C; iii Et3N; iv aq. AcOH; v aq. HCI NR~ Scheme 16 hexabora-adamantane (2).lZ5Such compounds form extremely strong complexes with donor molecules.122*124 A bora-adamantane with a betaine structure has also been prepared. The reactions of dialkyl-chloroboranes with di-iso-propylcarbamoyl-lithium give rise to other novel types of organoboron betaines with interesting properties. lZ7 For example compound (3) apparently forms stable addition compounds with simple amides. 127a (1) (2) (3) A plethora of transition-metal complexes of boron compounds has been synthesized. Of particular interest are complexes of borabenzene derivatives,"* including a new class of stable twenty-electron sandwich complexes (4),lZ9and multi-decker sandwiches in which cyclic boron compounds such as (5),l3' (6),13'and (7)13' are the ligands. The current multi-decker record holder rivalling the 'Big Mac@' in terms of number of layers is a trinuclear tetra-decker sandwich of (5)with consecutive Co Fe and Co atoms as the n~c1ei.l~~ M.P. Brown A. K. Holliday and G. M. Way J. Chem. Soc. Dalton Trans. 1975 148; I. Rayment and H. M. M. Shearer ibid. 1977 136. W. Kliegel and E. Ahlenstiel Chem. Ber. 1976 109 3547. 127 (a) A. S. Fletcher W. E. Paget K. Smith K. Swaminathan J. H. Beynon R. P. Morgan M. Bozorgzadeh and M. J. Haley J. Chem. SOC..Chem. Commun. 1979 347; (b) A. S. Fletcher W. E. Paget K. Smith K. Swaminathan and M. J. Haley ibid.. p. 573. 12' G. E. Herberich H. J. Becker and G. Engelke J. Organometal. Chem. 1978,153,265; and references cited therein; G. E. Herberich G. Engelke and W. Pahlmann Chem.Ber. 1979,112,607;A. J. Ashe W. Butler and H. F. Sandford J. Amer. Chem. SOC.,1979,101 7066. 129 G. E. Herberich W. Koch and H. Lueken J. Organometal. Chem. 1978,160 17. 130 W. Siebert and W. Rothermel Angew. Chem. Znt. Ed. Engl. 1977 16 333. 131 G. E. Herberich J. Hengesbach U. Kolle G. Huttner and A. Frank Angew. Chem. Int. Ed. Engl. 1976 15,433. 13' W. Siebert and M.Bochmann Angew. Chem. Znt. Ed. Engl. 1977,16,857; W. Siebert J. Edwin and M. Bochmann. ibid.. 1978,17.868. 133 W. Siebert,'W. Rotherme1,C. Bohle C. Kriiger and D. J. Brauer Angew. Chem. Int. Ed. Engl. 1979 18,949. K. Smith and W.E. Paget Et Et Et Et Na+ do MeB\ ,BMe S Ph Cyclopentadien ylboron and 7-borabicyc10[2.2.1]heptadienes'~~ are fluxional molecules.Reactions of polyborylmethyl anions with carbonyl compounds give rise to alkenyl-boron compounds with one less boron unit on the B-attached carbon atom.'36 This type of approach has been applied in the synthesis of homologated aldehyde^,'^^" methyl ketones,'36b borazaroquinolines,'36' and keten dithioacetal~.'~~~ In the last case the reagent is an a-phenylthio-boron compound other examples of which have been used in the synthesis of acetals and mon~thioacetals'~~ and of iodomethaneboronic Several other (a-halo- geno-alky1)boronic acid derivatives have been prepared by various procedures. 139 Amine adducts of cyano- carbamoyl- and carboxy-boranes e.g. (8),are interest- ing as boron-containing analogues of a-amino-acid~.'~~ An isocyanide (9)has also been ~btained.'~' A number of other potentially interesting new compound types have not been so well characterized however.For example the previously claimed Bu2BCOPh has now been shown not to be the product of reaction of 'Bu2BK' with benzoyl chloride and indeed 'Bu2BK' is not even an approximate representation of the species in solution after treatment of Bu2BCl with sodium-potassium alloy.'42 The product is in fact some type of borohydride. However highly interesting cyclic products such as (10)and (1 1) have been claimed without substantial evidence to be produced from the reactions of cyclohexene with the MeBC12-potassium and with the photolysis products of tri-l-naphthylb~rane.'~~ The danger of making assumptions about these latter reactions has been pointed 134 P.Jutzi and A. Seufert J. Organometal. Chem. 1979,169,327 357 373; Chem. Ber. 1979,112,2481; H. D. Johnson T. W. Hartford and C. W. Spangler J. Chem. SOC., Chem. Commun. 1978,242. 135 J. J. Eisch and J. E. Galle J. Organometal. Chem. 1977 127 C9. 136 (a) D. S. Matteson R. J. Moody and P.K. Jesthi J.Am. Chem. SOC., 1975,97,5608;(b) R. J. Moody and D. S. Matteson J. Organometal. Chem. 1978,152,265,(c) D. S. Matteson M. S. Biernbaum R. A. Bechtold J. D. Campbell and R. J. Wilcsek J. Org. Chem. 1978,43,950;(d) A. Mendoza and D. S. Matteson ibid. 1979,44 1352. 137 A. Mendoza and D. S. Matteson J. Chem. SOC., Chem. Commun. 1978,357;J. Organometal. Chem. 1978,156.149. 138 D. S. Matteson and D. Majumdar J. Organometal.Chem. 1979 170 259. 139 M. W. Rathke E. Chao and G.Wu J. Organometal. Chem. 1976,122,145; D. S. Matteson and P. K. Jesthi ibid. 1976,114,l;H. C. Brown N. R. De Lue Y. Yamamoto and K. Maruyama J. Org. Chem. 1977,42,3252. 140 B. F. Spielvogel F. Harchelroad and P. Wisian-Nielson J. Inorg. Nucl. Chem. 1979 41 1223; and references cited therein. 141 J. L. Vidal and G. E. Ryschkewitsch,J. Chem. SOC.,Chem. Commun. 1976,192. 142 K. Smith and K. Swaminathan J. Chem SOC.,Dalton Trans. 1976,2297. 143 S. M. van der Kerk J. Boersma and G. J. M. van der Kerk Tetrahedron Lett. 1976,4765. 144 B. G. Ramsey and D. M. Anjo J. Am. Chem. Soc. 1977,99,3182. 145 J. J. Eisch and H. P. Becker J. Organometal. Chem. 1979,171,141;see also K. Smith Ann.Reports (B) 1976 73 122. Organoboron Chemistry 301 Me $I-BH2C02H Me3$-BH2NC O R a 7 Miscellaneous Organoboron Chemistry Tris(organylthio)boranes,(RS)3B react with carboxylic or ester~'~' to give thioesters and their established application for the synthesis of thioketals has been e~tended.'~~ They are also useful for synthesis of 1,3-bis(organylthio)alkenes from enones,14' and for the formation of 2-hydroxyalkyl sulphides by ring-opening of epoxides.150 The corresponding selenoboron compounds are applicable to selenoketal Dialkyl(methylthio)boranes RiBSMe give dialkyl-bromoboranes on reaction with bromine,1s2 and dialkyl-bromoboranes can be converted into 'mixed' tri- alkylboranes RlR2B by treatment with sodium hydride in the presence of an a1kex1e.l~~9-Bromo-9-BBN is a convenient and selective reagent for cleavage of and catecholborane has been used in the formation of amides and macrocyclic lactones from carboxylic acids.155 146 A. Pelter T. E. Levitt K. Smith and A. Jones J. Chem. SOC.,Perkin Trans. 1 1977 1672. 147 T. Cohen and R. E. Gapinski. Tetrahedron Lett. 1978,4319. 14* D. R.Morton and S.J. Hobbs J. Org. Chem. 1979,44,656; H. A. Klein 2.Naturforsch. Teil B 1979 34,999. 149 T. Cohen D. A. Bennett and A. J. Mura J. Org. Chem. 1976,41 2506. Is' H. A. Klein Chem. Ber. 1979,112 3037. D. L. J. Clive and S. M. Menchen J. Chem. SOC., Chem. Commun. 1978,356; J. Org. Chem. 1979,44 1883. Is' A. Pelter K. Rowe D. N. Sharrocks K. Smith and C. Subrahmanyam J.Chem. SOC.,Dalton Trans. 1976,2087. A. Pelter K. Rowe and K. Smith J. Chem. SOC., Chem. Commun. 1975 532. lS4 M. V. Bhatt J. Organometal. Chem. 1978,156 221. 15' D. B. Collum S.-C. Chen and B. Ganem J. Org. Chem. 1978,43,4393.