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

Abstract

The synthesis of novel dialkylboron guanidinates is reported: the symmetrical compounds, (Me₂N)C(NR)₂BR′₂ [R = ⁱPr, R′ = Nrb (1); R = Cy, R′ = Nrb (2); R = ⁱPr, R′ = Cy (3); R = R′ = Cy (4); R = 2,6-ⁱPr₂-C₆H₃; R′ = Cy (5); Nrb = exo-2-norbornyl] and the asymmetrically coordinated {ⁱPr(H)N}C(NⁱPr)(NAr)BCy₂ [Ar = Ph (6), 4-Me-C₆H₄ (7), 4-^tBu-C₆H₄ (8)] were prepared by the salt metathesis method from the appropriate lithium guanidinates and chloroboranes. Moreover, the bis(dicyclohexylboron)guanidinate(−2) {ⁱPr(Cy₂B)N}C(NⁱPr){N(4-^tBu-C₆H₄)}BCy₂ (9) was also prepared from the corresponding dilithium guanidinate Li₂[{N(4-^tBu-C₆H₄)}C(NⁱPr)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 BR′₂ fragment, the latter displaying an additional BCy₂ attached to the exocyclic N atom. Solutions of compounds 1–4 reached an equilibrium with the aminoboranes Me₂NBR′₂ [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.