Undeniable Confirmation of the syn-Addition Mechanism for Metal-Free Diboration by Using the Crystalline Sponge Method

The stereochemical outcome of the recently developed metal-free 1,2-diboration of aliphatic alkenes has, until now, only been elucidated by indirect means (e.g. derivatization). This is because classical conformational analysis of the resulting 1,2-diboranes is not viable; in the ¹H NMR spectrum the relevant ¹H resonances are broadened by ¹¹B, and the occurrence of the products as oily compounds precludes X-ray crystallographic analysis. Herein, the crystalline sponge method is used to display the crystal structures of the diboronic esters formed from internal E and Z olefins, evidencing the stereospecific syn addition mechanism of the reaction, which is fully consistent with the prediction from DFT calculations. The crystalline sponge method provides conclusive proof of the stereospecific addition in the metal-free diboration of internal aliphatic alkenes (see scheme). The compatibility of common boryl groups with a monoporous ZnI₂ network gives access to the crystal structures of the oily organoborane compounds. The relative stereochemistry of the metal-free diboration products was unequivocally assigned and fully consistent with the DFT calculations.