Hydroboration internal allenes

Internal allenes on hydroboration with 9-BBN again give the allylboranes as the major product, as is seen in the case of 2,3-pentadiene (Eq. 5.39) [7]. Di-cyclohexylborane, on the other hand, gives mixture of both vinyl- and allylboranes. 

In sharp contrast to the behavior of 9-BBN, hydroboration of an internal allene with disiamylborane and dicyclohexylborane reveals preferential attack of the boron at the central carbon of the allenic chain, forming vinylboranes. For example, 2,4-dimethyl-2,3-pentadiene is converted [7] essentially, quantitatively, to the vinylborane with dicyclohexyl- and disiamylboranes, but exclusively to the allylborane with 9-BBN (Chart 5.24). In sharp contrast to acyclic allylborane, these allylborane products of 9-BBN do not react with acetone however, it undergoes usual alkaline hydrogen peroxide oxidation to afford the corresponding allylic alcohol. No protonolysis is observed on oxidation. This anomalous behavior of the allylborane is due to the steric bulk surrounding the boron atom, which prevents the coordination of oxygen of the acetone or water, necessary for allylboration or protonolysis [2, 8]. 

The hydroboration of internal allenes with 9-BBN affords the corresponding allylboranes [21]. These allylboranes when treated with water, leading to the formation of predominantly of (Z)-4-(trimethylsilyl)-2-alkenes (Chart 24.4) [22]. 

The reaction sequence involves (1) the hydroboration of internal allenes 29 and 30 with 9-BBN [17], (2) condensation with aldehyde followed by (3) elimination of hydroxy trimethylsilane by either (a) basic (syn-elimination) or (b) acidic (anh -elimination) workup conditions [14] to afford the Peterson olefina-tion reaction. The reaction sequence is sketched in Scheme 24.9 [30]. 

The hydroboration of heteroallyl derivatives often produces boron-heteroatom rings. These reactions are very closely related to the internal hydroborations discussed in Section 4.17.9.1.2. The hydroboration of 2-vinylpyrrole affords boron-nitrogen heterocycle 189 (Equation 29) <1997JOM181>. Two important hydroborations of allyl methyl sulfide <1971JA2823> and the allene 190 <1989JOC5814> are shown in Equations (30) and (31), respectively. The reduction/ hydroboration of 191 probably occurs via the intermediacy of 192 (Scheme 23) <1992JOC5288>. 

When compared with C2 symmetric ligands, Ci symmetric and bidentate NHCs induced the highest ee values (<99% ee) with unsaturated C-C bonds and [B(pin)]2 in MeOH. For instance, in the p-boration of a- and p-unsatu-rated aldehydes and esters, which are more reactive than other C-C unsaturated substrates, a 213-derived system is more enantioselective than the 214-derived complex (74% ee vs. 59% ee, respectively) (Scheme 13.8). Improved results were obtained by Hoveyda and co-workers, using bidentate 217-220 with acyclic and cyclic internal alkenes, alkynes, 1,6-dienes and various allenes ee s of <98% were achieved (Scheme 13.8). For the regio-and enantioselective hydroboration of a,p-unsaturated carbonyl compounds... 

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