Boron Enabled Directed [2+2]- and Dearomative [4+2]-Cycloadditions Initiated by Energy Transfer.

A strategy for the photosensitized [2+2]-cycloaddition between styrenyl dihaloboranes and unactivated allylamines to access cyclobutylboronates with control of stereochemistry and regiochemistry is presented. The success of the reaction relies on the temporary coordination between in situ generated dihaloboranes and amines under mild reaction conditions. In addition, cyclobutanes with varying substitution patterns have been prepared using N-heterocycles as directing group. Manipulation of the C-B bond allows for the synthesis of a diverse class of cyclobutanes from simple precursors. Moreover, these reactions lead to the synthesis of complex amines and heteroaromatic compounds, which have significant utility in medicinal chemistry. Finally, a dearomative [4+2]-cycloaddition of naphthalenes using a boron-enabled temporary tethering strategy has also been uncovered to synthesize complex 3-dimensional borylated building blocks.