Catalytic Non‐Conventional trans‐Hydroboration: A Theoretical and Experimental Perspective

Abstract We have studied the non‐conventional trans ‐hydroboration reaction of alkynes both experimentally and theoretically. A catalytic system based on the in situ mixture of [{Rh(cod)Cl} 2 ]/PCy 3 (cod=1,5‐cyclooctadiene, Cy=cyclohexyl) has been able to activate pinacolborane and catecholborane and transfer boryl and hydride groups onto the same unhindered carbon atom of the terminal alkynes. The presence of a base (Et 3 N) favored the non‐conventional trans ‐hydroboration over the traditional cis ‐hydroboration. Varying the substrate had a significant influence on the reaction, with up to 99 % conversion and 94 % regioselectivity observed for para ‐methyl‐phenylacetylene. Both DFT and quantum mechanical/molecular mechanical ONIOM calculations were carried out on the [RhCl(PR 3 ) 2 ] system. To explain the selectivity towards the ( Z )‐alkenylboronate we explored several alternative mechanisms to the traditional cis ‐hydroboration, using propyne as a model alkyne. The proposed mechanism can be divided into four stages: 1) isomerization of the alkyne into the vinylidene, 2) oxidative addition of the borane reagent, 3) vinylidene insertion into the RhH bond, and finally 4) reductive elimination of the CB bond to yield the 1‐alkenylboronate. Calculations indicated that the vinylidene insertion is the selectivity‐determining step. This result was consistent with the observed Z selectivity when the sterically demanding phosphine groups, such as PCy 3 and P i Pr 3 , were introduced. Finally, we theoretically analyzed the effect of the substrate on the selectivity; we identified several factors that contribute to the preference for aryl alkynes over aliphatic alkynes for the Z isomer. The intrinsic electronic properties of aryl substituents favored the Z ‐pathway over the E ‐pathway, and the aryl groups containing electron donating substituents favored the occurrence of the vinylidene reaction channel.