3-(Dimethylboryl)pyridine: Synthesis, Structure, and Remarkable Steric Effects in Scrambling Reactions

A facile method for the synthesis of 3-(dimethylboryl)pyridine (1a) is described. Compound 1a assembles
into a rigid cyclic tetramer stabilized via intermolecular boron−nitrogen coordination bonds both in the
crystalline state and in solution. The outstanding structural feature of 1a, as compared with previously
reported 3-(diethylboryl)pyridine (2a) (which adopts a cone conformation), is that the tetramer of 1a
adopts a 1,2-alternate conformation. To investigate the effect of substituents at the boron atom on the
stabilities of the oligomers, scrambling experiments of the component molecules using 1, 2, and 3-(di-n-butylboryl)pyridines 3 were carried out. Although heating at 80−90 °C for 20 h was required to attain
the equilibrium of the scrambling reactions when the component molecules of the tetramers were 2 or 3,
the scrambling in 1 proceeded under relatively mild conditions (60 °C, 3 h). This difference in reaction
conditions required for 1, as compared to conditions required for 2 or 3, could not be explained solely
by the stabilities based on bond lengths or THC.1g It appears that whereas only an SN1-type pathway
may be involved in the scrambling of 2 or 3, both SN1- and SN2-type mechanisms operate simultaneously
during scrambling reactions of 1 or an intermediate mechanism between SN1 and SN2 operates, which
was supported by kinetic studies and calculations using model compounds.