Double-Asymmetric Hydrogenation Strategy for the Reduction of 1,1-Diaryl Olefins Applied to an Improved Synthesis of CuIPhEt, a C2-Symmetric N-Heterocyclic Carbenoid

A library of iridium and rhodium phosphine catalysts have been screened for the double-asymmetric hydrogenation of 2,6-di-(1-phenylethenyl)-4-methylaniline to produce the C2-symmetric aniline precursor of the N-heterocyclic carbenoid CuIPhEt. The best catalyst produced the desired enantiomer in 98.6% selectivity. This rare example of a highly selective hydrogenation of a 1,1-diaryl olefin enables a four-step asymmetric synthesis of the C2-symmetric phenylethyl imidazolium ion (IPhEt) from p-toluidine and phenylacetylene and its conversion to the hydrosilylation catalyst CuIPhEt.