Vinylic boranes

The reaction of internal alkynes with diborane provides the trialkenylboranes in moderate yields (Eq. 51). Terminal alkynes yield little or none of the desired alkenyl- 

However, many mono-, dialkylboranes and heterosubstituted boranes (Chart 1) exhibit highly selective behavior in hydroboration of alkynes7). Thus, they appeared attractive for the controlled monohydroboration of alkynes for the synthesis of the desired vinylic boranes from terminal and internal alkynes (Eqs. 53 and 54). 

The steric and reaction temperature effects in the hydroboration of alkynes with representative dialkylboranes in THF are given I05). 

The mildness of these reagents tolerates the presence of various functional groups such as ester, ether, halogen, and nitrile. The stereospecific cis nature of hydroboration gives exclusively the tram alkenylboranes, often also in high regioisomeric purity (Eq. 53). On the other hand, highly pure (Z)-l-alkenyl-dialkylboranes are prepared without any difficulty via the monohydroboration of 1-halo-1-alkynes with disiamyl-borane or dicyclohexylborane, followed by treatment with t-butyllithium (Eq. 55)106). 

Subsequent reactions of the alkenylboranes usually proceed by stereodefined pathway, thus allowing highly stereo- and regiospecific syntheses. 

The monohydroboration of alkynes can also be achieved with BH2CI OEt2 at 0 °C using an internal alkyne or a terminal alkyne (Eqs. 56 and 57, Table 8) while dihydroboration is a significant reaction when THE is used as a solvent. This is due to the reason as the nucleophilic character of THE is higher as compared to diethyl ether. 

Alkyne Amount of alkyne used (mmol) Time h Yield of dialkenyl-chloroborane ( %) 

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Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

Scheme 23. Synthesis of vinyl boranes by hydroboration of alkynes.

A convenient route to both saturated and unsaturated acylsilanes lies in the hydroboration-oxidation of alkynylsilanes (Chapter 7). Recent improvements (6) to this method involve the use of the borane-dimethyl sulphide complex for hydroboration, and of anhydrous trimethylamine A-oxide for the oxidation of the intermediate vinyl boranes. 

Triple bonds can be monohydroborated to give vinylic boranes, which can be reduced with carboxylic acids to cis alkenes or oxidized and hydrolyzed to aldehydes or ketones. Terminal alkynes give aldehydes by this method, in contrast to the mercuric or acid-catalyzed addition of water discussed at 15-4. However, terminal alkynes give vinylic boranes (and hence aldehydes) only when treated with a hindered borane such as 47, 48, or catecholborane (p. 798)," or with BHBr2—SMe2. The reaction between terminal alkynes and BH3 produces 1,1-... 

Vinyl boranes add to conjugated ketones in the presence of a rhodium catalyst (with high asymmetric induction in the presence of BINAP) 7 Alkynyl-boranes also add to conjugated ketones, in the presence of... 

For another exception, involving a vinylic borane, see Satoh, Y Tayano, T. Hara, S. Suzuki, A. Tetrahedron Lett, 1989, 30, 5153. 

In another procedure, the addition of a dialkyIborane to a 1-haloalkyne produces an a-halo vinylic borane (82). Treatment of this with NaOMe gives the rearrangement shown, and protonolysis of the product... 

Treatment of 83 with an electrophile such as methyl sulfate, allyl bromide, or triethyloxonium borofluoride, followed by oxidation of the resulting vinylic borane... 

Reaction between vinylic boranes and iodine or NaOMe... 

If the fl-effect were operative in the ground state of a vinylic borane (31a), the effects would be to enhance the 7r-electron density of the a-carbon (31b) and to reduce C=C bond character (31c). 

The hydroboration of alkynes gives predominantly cis isomer, as in the following example. Diborane adds as BH3 to alkynes in a cis isomer and forms vinyl boranes which react with acetic acid at low temperature to give cis-alkenes. 

Internal vinyl tellnrides, which are not accessible via hydrotellnration of alkynes, have been prepared from alkynes through a vinyl borane ronte. 

An additional protocol involves the reaction of vinyl boranes with diorganyl ditellurides. 

Rh-H / -elimination step in step E gives the vinyl-borane which then dissociates to generate the first intermediate. This cycle explains the production of equimolar quantities of alkane and -vinylborane with unreactive R2BH derived from ephedrines with catecholborane there is an additional shunt pathway — step C in competition with step E leading to the primary regio-isomer of alkylborane. 

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

Albert S.C. Chun of the Hong Kong Polytechnic University reports (J. Org. Chem. 68 1589, 2003) two important transformations. The three-component (Mannich) condensation of 10 with 11 and 12 proceeds with high diastereoselectivity, to give the amino alcohol 13. Hydroboration of the alkyne 14 followed by transmetalation of the intermediate vinyl borane gives a zinc species, which under catalysis by the easily-prepared 3-naphthol 13 adds to aromatic and branched aldehydes with high . The product allylic alcohols are useful intermediates for organic synthesis. 

Rotcrmund Kdstcr Liebigs Ann. Chem. 1965,686,153 Brown Negishi Dickason I. Org.Chem. 1985,50, 520. For a review of hydroboration of triple bonds, see Hudrlik Hudrlik. in Patai, Ref. 70, pt. 1, pp. 203-219. For a review of the preparation and reactions of vinylic boranes, see Brown Campbell Aldrichimica Acta 1981, 14. 1-11. 

Reduction of vinylic boranes hydro-boration of enamines... 

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