Date of this Version
Since the beginning of catalytic asymmetric hydroboration (CAHB) in 1989, many new approaches have been developed. Developing an efficient method of catalytic asymmetric hydroboration to produce useful chiral organoboranes is still a challenge due to limited success with a small range of substrates. Typically, effective CAHB requires the presence of vinylarene moiety or a particular substitution pattern around the alkene. One area of research in the Takacs group has been to expand this substrate scope by incorporating two-point binding to direct the reaction to one regioisomer selectively. CAHB of two-point binding substrates in the presence of simple chiral monophosphite and monophosphoramidite ligands is an attractive approach to overcome this challenge.
One of the long standing challenges is the catalytic asymmetric hydroboration of 1,1- disubstituted alkenes. Although practical and highly enantioselective conjugate addition and hydroboration utilizing stoichiometric amounts of chiral borane of 1,1-disubtituted alkenes by Hoveyda , Mazet, and Soderquist have been demonstrated, CAHB of 1,1 disubstituted alkenes remain a significant challenge. Herein, we report an elegant solution of this problem using two-point binding. For example, this reaction can be carried out by treatment of the β,γ-disubstituted alkene unsaturated amide with Rh(nbd)2BF4 and ligands TADDOL-derived monophosphite or BINOL-derived monophosphoramidite. High catalytic activity (62%), high regioselectivity (> 96%), and enantioselectivities up to 94% were obtained with the β,γ-unsaturated ester framework. The applicability of this method was further highlighted by successfully forming chiral β -substituted butyrolactones, the key precursor for the synthesis of biologically active natural products including lignans. This method also enables for the efficient preparation of trifluoroborate salts to provide chiral reagents for the Suzuki- Miyaura cross coupling reaction.
Adviser: James M. Takacs