Visible Light-Mediated [4+2] Annulation of Silylimines with Olefins to 1-Aminotetralins Enabled by Diradical Hydrogen Atom Transfer of C-H Bonds.

A facile photochemical, one-pot synthesis of highly functionalized 1-aminotetralins derivatives (>70 examples) from readily accessible o-alkyl and o-formyl aryl silylimines with olefins is described. A diradical-mediated hydrogen atom transfer (DHAT) of primary, secondary, and tertiary C(sp)-H bonds of o-alkyl arylsilylimines and C(sp)-H bonds of o-formyl arylsilylimines enabled a [4+2] annulation with olefins in excellent diastereoselectivity. This was accomplished upon irradiation at λ = 420 nm in the presence of thioxanthen-9-one (10 mol %) as the sensitizer via energy transfer. Moreover, sulfur-substituted o-alkyl silylimines can undergo such photochemical process in the absence of an external photosensitizer. This effective protocol is compatible with a variety of functional groups and can be applied to the modification of bioactive molecules. Based on mechanistic evidences and computational studies, it is suggested that the silyl substituent enables an efficient energy transfer leading to the formation of a key C,N-diradical and subsequent [4+2]-cyclization was supported by a better molecular orbital matching between the HSOMO of the 1,4-diradical intermediate and the LUMO of the olefins. Thus, upon irradiation, the excited silylimine unlocks a carbon-to-nitrogen DHAT and subsequent [4+2] cyclization that allows the divergent functionalization of benzylic C(sp)-H bonds and C(sp)-H bonds.