Insertion reactions of dicyclohexylcarbodiimide with aminoboranes, -boryls and -borylenes

Insertion reactions of dicyclohexylcarbodiimide with aminoboranes and with aminoboryl and -borylene transition metal complexes have been examined as potential routes to new boron-containing ligand systems. Reactions with systems containing two-coordinate boron centres are found to be significantly more facile than those with three-coordinate substrates. Thus, reaction of (dicyclohexylamino)boron dichloride (1a) with dicyclohexylcarbodiimide over 36 h at 50 °C generates the (structurally authenticated) guanidinate complex Cy₂NC(NCy)₂BCl₂ (2a) via insertion into the BN bond. By contrast, the corresponding reaction with the cationic aminoborylene complex [CpFe(CO)₂(BNCy₂)]⁺[BAr^f₄]⁻ (4a) proceeds rapidly at ca. −30 °C, via initial insertion into the FeB bond to give [CpFe(CO)₂C(NCy)₂BNCy₂]⁺[BAr^f₄]⁻ (5a). Consistent with related studies, a key factor in facilitating such insertion chemistry is thought to be the formation of an initial donor/acceptor complex between the diimide and the group 13 centre. Thus, DFT studies suggest that [CpFe(CO)₂B(NCy₂)(CyNCNCy)]⁺[BAr^f₄]⁻ is a potential intermediate in the reaction of 4a with CyNCNCy, and that further reaction to give the observed product, 5a, is strongly exergic (−183 kJ mol⁻¹). By contrast, DFT calculations for the alternative isomer [CpFe(CO)₂B(CyN)₂CNCy₂]⁺[BAr^f₄]⁻ (5a′), formed by BN insertion, suggest that it is 112 kJ mol⁻¹ less stable than 5a. Such experimental and computational findings imply that under reaction conditions where a suitable isomerisation pathway is available, cationic complexes such as 5a′, which contain a four-membered boron-donor heterocycle are likely to be disfavoured with respect to alternative C-bound isomers.