Highly Selective and Catalytic Generation of Acyclic Quaternary Carbon Stereocenters via Functionalization of 1,3-Dienes with CO2.

The catalytic asymmetric functionalization of readily available 1,3-dienes is highly important, but current examples are mostly limited to the construction of tertiary chiral centers. The asymmetric generation of acyclic products containing all-carbon quaternary stereocenters from substituted 1,3-dienes represents a more challenging, but highly desirable, synthetic process for which there are very few examples. Herein, we report the highly selective copper-catalyzed generation of chiral all-carbon acyclic quaternary stereocenters via functionalization of 1,3-dienes with CO2. A variety of readily available 1,1-disubstituted 1,3-dienes, as well as a 1,3,5-triene, undergo reductive hydroxymethylation with high chemo-, regio-, E/Z-, and enantioselectivities. The reported method features good functional group tolerance, is readily scaled up to at least 5 mmol of starting diene, and generates chiral products that are useful building blocks for further derivatization. Systemic mechanistic investigations using density functional theory calculations were performed and provided the first theoretical investigation for an asymmetric transformation involving CO2. These computational results indicate that the 1,2-hydrocupration of 1,3-diene proceeds with high π-facial selectivity to generate an (S)-allylcopper intermediate, which further induces the chirality of the quaternary carbon center in the final product. The 1,4-addition of an internal allylcopper complex, which differs from previous reports involving terminal allylmetallic intermediates, to CO2 kinetically determines the E/Z- and regioselectivity. The rapid reduction of a copper carboxylate intermediate to the corresponding silyl-ether in the presence of Me(MeO)2SiH provides the exergonic impetus and leads to chemoselective hydroxymethylation rather than carboxylation. These results provide new insights for guiding further development of asymmetric C-C bond formations with CO2.