Magnesium-Catalyzed Hydroboration of C−C σ-Bonds of Benzylidene Cyclobutanes

Abstract

Selective synthetic routes to prepare boronic esters are highly desirable as they are valuable intermediates in the preparation of pharmaceuticals, agrochemicals, and functional materials. In this paper, we report an approach to generate boronic esters through the ring-opening hydroboration of benzylidene cyclobutanes catalyzed by magnesium(II) complexes. Products are accessed through the formal hydroboration of a single C−C σ-bond of the cyclobutane ring with complete chemoselectivity and high E:Z selectivities (99:1 to 4:1) across a wide range of substrates. Catalysis is proposed to proceed through the hydromagnesiation of the benzylidene cyclobutane, ring-opening by β-alkyl migration, and subsequent Mg-to-B metathesis. Mechanistic experiments, 11B nuclear magnetic resonance spectroscopy, and density functional theory calculations suggest that boronate complexes play a role in catalytic turnover and that ring-closed hydroborated species form as off-cycle intermediates. The catalytic process represents an example in which an inexpensive magnesium catalyst performs in an entirely complementary manner to transition metals.