Dialkylboron guanidinates: syntheses, structures and carbodiimide de-insertion reactions

The synthesis of novel dialkylboron guanidinates is reported: the symmetrical compounds, (Me2N)-C(NR)(2)BR2' [R = Pr-i, R' = Nrb (1); R = Cy, R' = Nrb (2); R = Pr-i, R' = Cy (3); R = R' = Cy (4); R = 2,6-Pr-i(2)-C6H3; R' = Cy (5); Nrb = exo-2-norbornyl] and the asymmetrically coordinated {Pr-i(H)N}C((NPr)-Pr-i)(NAr)BCy2 [Ar = Ph (6), 4-Me-C6H4 (7), 4-Bu-t-C6H4 (8)] were prepared by the salt metathesis method from the appropriate lithium guanidinates and chloroboranes. Moreover, the bis(dicyclohexylboron)guanidinate(-2) {Pr-i(Cy2B)N}C((NPr)-Pr-i){N(4-Bu-t-C6H4)}BCy2 (9) was also prepared from the corresponding dilithium guanidinate Li-2[{N((4)-Bu-t-C6H4)}C((NPr)-Pr-i)(2)] and ClBCy2. The structures of compounds 1, 3, 6 and 9 were confirmed by X-ray diffraction and all displayed a chelate coordination of the guanidinate ligand to the BR2' fragment, the latter displaying an additional BCy2 attached to the exocyclic N atom. Solutions of compounds 1-4 reached an equilibrium with the aminoboranes Me2NBR2' [R' = Nrb (10), Cy (11)] and the corresponding carbodiimides, which was slow at 25 degrees C. The thermodynamic parameters for these equilibria are also reported. The activation parameters for the equilibrium for compound 1 have been calculated after a kinetic study. Compounds 5-8, with one or two N-aryl fragments bound to a B centre, are more robust and need higher temperatures (80 degrees C) and prolonged times to give similar carbodiimide de-insertion reactions.