β‑Borylalkenyl Z–E Isomerization in Rhodium-Mediated Diboration of Nonfunctionalized Internal Alkynes

The elemental steps for the preparation of (E)-pinBC­(Me)C­(Me)­Bpin (Bpin = pinacolboryl) by means of the anti addition of B2pin2 to 2-butyne, promoted by the boryl complex Rh­(Bpin)­{xant­(PiPr2)2} (1; xant­(PiPr2)2 = 9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene), have been analyzed from a kinetic point of view, and the intermediates of the process, the β-borylakenyl complexes Rh­{(Z)-C­(Me)C­(Me)­Bpin}­{xant­(PiPr2)2} (2) and Rh­{(E)-C­(Me)C­(Me)­Bpin}­{xant­(PiPr2)2} (3), have been isolated and fully characterized. The key step for the formation of the diborylalkene is the transformation of 2 into 3 as a result of the Z–E isomerization of the β-borylalkenyl group, which takes place via metallacyclopropene intermediates. The isomerization is sterically controlled, and the pincer diphosphine adapts its coordination mode to the requirements of the process happening to act as κ2-P,P. The Z–E isomerization of the β-borylalkenyl ligand of 2 is slower than the oxidative addition of the diborane to 1. As a consequence, under catalytic conditions, the formation of the syn-addition product (Z)-pinBC­(Me)C­(Me)­Bpin is favored, although the intermediate Rh­(Bpin)3{xant­(PiPr2)2} is much less stable than 2.