Insertion reactions of small unsaturated molecules in the N-B bonds of boron guanidinates.

We report here 1,1- and 1,2-insertion reactions of small unsaturated molecules in the N-B bonds of two boron guanidinates, (Me2 N)C(Ni Pr)2 BCy2 (1) and {i Pr(H)N}C(Ni Pr){N(p-t Bu-C6 H4 )}BCy2 (2), and two bisboron guanidinates(2-), {i Pr(BCy2 )N}C(Ni Pr){N(p-t Bu-C6 H4 )}BCy2 (3) and {i Pr(C8 H14 B)N}C(Ni Pr){N(p-Me-C6 H4 )}BC8 H14 (4), the latter being prepared for the first time by double deprotonation of the corresponding guanidine with the 9-borabicyclo[3.3.1]nonane dimer, (H-BC8 H14 )2 . Compounds 1-4 easily insert aromatic isonitriles, XylNC (Xyl = 2,6-Me2 -C6 H3 ) and (p-MeO-C6 H4 )NC, to give the expected diazaboroles 5-12, some of them being structurally characterised by X-ray diffraction. Interestingly, the BC8 H14 derivatives 11 and 12 are in a fast temperature-dependent equilibrium with the de-insertion products, whose thermodynamic parameters are reported here. A correlation between these equilibria and the puckered heterocyclic structure found in the solid state for 11, and confirmed by DFT calculations, is also established. Reactions of the aforementioned guanidinates with CO are more sluggish or even precluded, and only one product, {i Pr(H)N}C{N(p-t Bu-C6 H4 )}(Ni Pr)(CO)BCy2 (13), could be isolated in moderate yields. The 1,2-insertions of benzaldehyde in compounds 1, 2 and 4 are reversible reactions in all cases, and only one of the insertion products, {i Pr(H)N}C{N(p-t Bu-C6 H4 )}(Ni Pr)(PhHCO)BCy2 (16a), was isolated and diffractrometrically characterised. Likewise, CO2 reversibly inserts into a N-B bond of 2 to give {i Pr(H)N}C{N(p-t Bu-C6 H4 )}(Ni Pr)(CO2 )BCy2 (19) with a conversion of ca. 9%. In all these equilibria, de-insertion is always favoured upon increasing the temperature.