Directed catalytic asymmetric hydroboration (CAHB): An effective method for the enantioselective preparation of chiral organoboronates

The use of chiral organoboronates as synthetic intermediates enroute to other useful functionalites via stereospecific functional group conversion, and as important intermediates in natural product synthesis, is of renewed interest. Recent breakthroughs in the stereospecific conversion of carbon-boron to, most notably, carbon-carbon bonds raise the demand for the enantioselective preparation of these valuable synthetic intermediates. Despite advancements in methods leading to the enantioselective preparation of chiral organoboronates, catalytic asymmetric hydroboration (CAHB) is compromised by a lack of substrate scope. We have found that carbonyl directed enantioselective CAHB of β,γ- and γ,δ-unsaturated amides and esters with simple rhodium catalysts derived from chiral monophosphite and phosphoramidite ligands affords the corresponding β- or γ-dioxaborato products in excellent enantioselectivities (up to 99% ee). The controlled regio- and enantioselectivity is, in part, a consequence of two-point binding from the carbonyl and alkene moieties of the substrate to the rhodium catalyst. The importance of two-point binding is highlighted by the enantioselective CAHB of substitution patterns, such as 1,1-disubstituted and trisubstituted alkenes, that have historically remained unsolved. The enantioselective preparation of both β- and γ-dioxaborato esters has provided entry into useful synthetic intermediates used by others in the preparation of natural products of interest such as lignan β-substituted γ-lactones, (−)-enterolactone and (+)-arctigenin, and the anticonvulsant drug pregabalin. Dramatic changes in regio- and enantioselectivity as well as the stereochemical course of the reaction highlight the interdependency of reagents employed. Theoretical calculations of relevant mechanistic intermediates coupled with experimentation shed light on important mechanistic features of the directed CAHB reaction