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

The synthesis of novel dialkylboron guanidinates is reported: the symmetrical compounds, (MeN)C(NR)BR' [R = Pr, R' = Nrb (1); R = Cy, R' = Nrb (2); R = Pr, R' = Cy (3); R = R' = Cy (4); R = 2,6-Pr-CH; R' = Cy (5); Nrb = exo-2-norbornyl] and the asymmetrically coordinated {Pr(H)N}C(NPr)(NAr)BCy [Ar = Ph (6), 4-Me-CH (7), 4-Bu-CH (8)] were prepared by the salt metathesis method from the appropriate lithium guanidinates and chloroboranes. Moreover, the bis(dicyclohexylboron)guanidinate(-2) {Pr(CyB)N}C(NPr){N(4-Bu-CH)}BCy (9) was also prepared from the corresponding dilithium guanidinate Li[{N(4-Bu-CH)}C(NPr)] and ClBCy. 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 BR' fragment, the latter displaying an additional BCy attached to the exocyclic N atom. Solutions of compounds 1-4 reached an equilibrium with the aminoboranes MeNBR' [R' = Nrb (10), Cy (11)] and the corresponding carbodiimides, which was slow at 25 °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 °C) and prolonged times to give similar carbodiimide de-insertion reactions.