Photochemical Generation of Chiral N,B,X-Heterocycles by Heteroaromatic C-X Bond Scission (X=S, O) and Boron Insertion.

Abstract

Chiral organoboron compounds with a chelate backbone and mesityl/heterocycle substituents (thienyl, furyl, and derivatives thereof) undergo a quantitative phototransformation that yields rare, chiral N,B,X-containing heterocycles, such as base-stabilized 1,2-thiaborinines and 1,2-oxaborinines. Boriranes were observed as intermediates in some of these transformations. The oxaborinines display further reactivity, generating 4a,12b-dihydrobenzo[h][1,2]oxaborinino[4,3-f]quinolines through a sequential conrotatory electrocyclization and a [1,5]-H shift. The N,B,X-containing heterocycles display strong blue-green to orange-red emission in the solid state. Combined DFT//CASP2T calculations suggest that a common biradical intermediate is responsible for the formation of these compounds as well as their interconversion.