Boronate catalytic diboration

Laitar DS, Tsui EY, Sadighi JP. Catalytic diboration of aldehydes via insertion into the copper-boron hand. J Am Chem Soc. 2006 128 11036-11037. 

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

Miyaura and co-workers reported the platinum-catalyzed diboration of allenes with bis(pinacolato)diboron (Scheme 16.52) [57]. The catalytic cycle involves a sequence of oxidative addition of bis(pinacolato)diboron to Pt(0), insertion of an allene into the B-Pt bond and reductive elimination of an allylic boronate, re-producing the Pt(0) species. (Z)-Allylic boronates are formed stereoselectively in the reaction with monosubstituted allenes, which strongly suggests a pathway via a vinylplatinum species rather than a Jt-allylplatinum species. 

The catalytic asymmetric diboration of allenes provides a-substituted 2-boronyl allylic boronates of type 25 (see Eq. 32). One of them, 91, adds to ben-zaldehyde, albeit with a slight erosion of stereoselectivity (Eq. 65). The major P-hydroxy ketone stereoisomer, isolated after an oxidative work-up, originates from the putative chairlike transition structure 92. 

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