Approach to a new model of induction of stereoselectivity in the Nicholas reaction via a chiral 1-alkoxy-propargylium cation

A systematic study on the syn–anti diastereoselectivity of the Nicholas reaction between enantiopure propargyl acetal dicobalt–hexacarbonyl complexes, as precursors of chiral propargyl cobalt-hexacarbonyl cations, and several linear and cyclic silyl enol ethers is presented. A high yield up to 95% and high syn–anti diastereoselectivity (from 85:15 up to >99:1) is observed in the generation of the two new stereocenters. Moderate, but promising, syn(R,R)–syn(S,S), up to 70:30, is also observed in this preliminary work. This syn(R,R)–syn(S,S) diastereoselectivity formally would correspond to the enantioselectivity of the Nicholas reaction once the chiral auxiliary should be removed, in order to be recycled. This is the first approach to the induction of ‘enantioselectivity’ in the Nicholas C–C coupling based on cheap and commercially available enantiopure alcohols as chiral auxiliaries.