Boronate from disubstituted alkenes

The behavior of members of the bicyclo[2.2.1]heptene family is also different from that of other common 1,2-disubstituted alkenes.230 The parent bicy-clo[2.2.1]heptene gives bicyclo[2.2.1]heptane in only 3.5% yield when it is treated with Et3SiH/TFA. The major product is reported to be a 2-bicyclo[2.2.1]heptyl trifluoroacetate of unspecified configuration (Eq. 70).230 The carbocation intermediate is presumably the 2-norbornyl cation. Addition of small amounts of boron trifluoride etherate to the reaction mixture causes the yield of hydrocarbon product to rise to 22% after a reaction time of 24 hours at room temperature. Further... 

In the addition to this 1-alkyne, the boron bonds to the terminal carbon because it is attached to more hydrogens. The resulting vinylborane can be viewed as a disubstituted alkene and is less hindered than the vinylborane produced from an internal alkyne (a trisubstituted alkene). Because the vinylborane is less hindered, it adds a second boron to produce an alkane substituted on the end carbon with two boron groups. 

Ca is always the less sterically hindered carbon. For disubstituted alkenes, Ca is the carbon distal (most distant from) a pedant substituent, a and b refer to the points of attachment of the boron atom. 

An alternative synthesis of (Z)-l-halo-l-alkenes involves hydroboration of 1-halo-l-alkynes, followed by protonolysis (246,247). Disubstituted ( )-and (Z)-a1keny1 bromides can be prepared from ( )- and (Z)-a1keny1 boronic esters, respectively, by treatment with bromine followed by base (248). 

The synthesis of Z-disubstituted double bond was accomplished by Zweifel. The Zweifel s (Z)-alkene synthesis involves the iodination of dialkylvinylboranes in the presence of sodium hydroxide which results in the transfer of an alkyl group from boron and the formation of the (Z)-alkene (Eq. 62) 117118>, although the introduction of alkyl groups is limited. 

Treatment of the intermediate alkenylborane, such as 52 or 53, with iodine in the presence of a base (such as sodium hydroxide or methoxide) forms, stereose-lectively, a Z-1,2-disubstituted or trisubstituted alkene. Transfer of one alkyl group from boron to the adjacent carbon atom occurs stereospecifically, resulting, after anti elimination of boron and iodine, in a new alkene in which the two substituents of the original alkyne become trans to each other (2.53, 2.54). ... 

The order of reactivity of halides and triflates for the cross-coupling reaction of boron reagents is 1 > Br > OTf Cl. Thus, the sequential cross-coupling reactions of 4-bromophenyl triflate with two 9-alkyl-9-BBN derivatives obtained from two different alkenes furnishes the unsymmetrically disubstituted benzene derivative (Scheme 18). ... 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the acetylene. Catechol borane (l,3>2-benzodioxaborole), which is prepared from equimolar amounts of catechol (1,2-dihydroxybenzene) and borane, is a particularly useful reagent for hydroboration of acetylenes.Protonolysis of the adduct with acetic acid results in reduction of the original alkyne to the corresponding c/5-alkene. Oxidative workup with hydrogen peroxide gives ketones via an enol intermediate. Treatment of the vinyl borane with bromine and base leads to the vinyl bromide. The net anh-addition has been rationalized on the basis of anh-addition of bromine followed by a second z/tr/-elimination of bromide and boron but there are exceptions to this generalization. 

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