Magnesium-Catalysed, Hydroboration of C–C sigma-bonds of Benzylidene Cyclobutanes

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 a new approach to generate boronic esters through the ring-opening hydroboration of benzylidene cyclobutanes catalysed by magnesium(II) complexes. For the first time, products were accessed through the selective hydroboration of a single C–C sigma-bond of the cyclobutane ring with complete chemoselectivity and high E:Z selectivities (99:1 to 3:1) across a wide range of substrates. The mechanism is proposed to proceed through the hydromagnesiation of the benzylidene cyclobutane, ring-opening by beta-alkyl migration, and subsequent Mg-to-B metathesis. Probe experiments, 11B NMR spectroscopy, and DFT calculations suggest that boronate complexes play a role in catalytic turnover, and that ring-closed hydroborated species form as off-cycle intermediates. The new process represents an important example in which a sustainable and inexpensive magnesium catalyst performs in an entirely complementary manner to transition metal systems.