A facile method for the synthesis of 3-(dimethylboryl)pyridine (
1a) is described. Compound
1a assemblesinto a rigid cyclic tetramer stabilized via intermolecular boron-nitrogen coordination bonds both in thecrystalline state and in solution. The outstanding structural feature of
1a, as compared with previouslyreported 3-(diethylboryl)pyridine (
2a) (which adopts a cone conformation), is that the tetramer of
1aadopts a 1,2-alternate conformation. To investigate the effect of substituents at the boron atom on thestabilities 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 attainthe 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 reactionconditions required for
1, as compared to conditions required for
2 or
3, could not be explained solelyby the stabilities based on bond lengths or THC.
1g It appears that whereas only an S
N1-type pathwaymay be involved in the scrambling of
2 or
3, both S
N1- and S
N2-type mechanisms operate simultaneouslyduring scrambling reactions of
1 or an intermediate mechanism between S
N1 and S
N2 operates, whichwas supported by kinetic studies and calculations using model compounds.
