Ambient Temperature Carbene-Mediated Depolymerization: Stoichiometric and Catalytic Reactions of N-Heterocyclic- and Cyclic(Alkyl)Amino Carbenes with Poly(N-Methylaminoborane) [MeNH-BH2]n.

Abstract

The reactions of the N-heterocyclic carbenes (NHCs) IDipp and ItBu and the cyclic(alkyl)amino carbene (CAAC) CAACMe with polyaminoborane [MeNH-BH2]n were investigated. Stoichiometric quantities of each carbene were found to cause rapid and complete depolymerization, with the major B-N-containing product identified as the NHC-aminoborane adduct, IDipp-BH2NMeH (1), cyclic borazane [MeNH-BH2]3, or borazine [MeNBH]3 with IDipp, ItBu, and CAACMe, respectively. With substoichiometric quantities of IDipp and ItBu (down to 10 and 2.5 mol %, respectively), complete loss of high molar mass material was also detected, indicating that the depolymerization is catalytic. The main products of the reaction with substoichiometric IDipp were IDipp-BH2NMeH (1) and [MeNH-BH2]3 and with substoichiometric ItBu, [MeNH-BH2]3, and [MeNBH]3 with product ratios dependent on the quantity of NHC used. Under analogous conditions with CAACMe, high molar mass material persisted alongside the formation of [MeNBH]3. Further reactivity studies with cyclic borazane [MeNH-BH2]3 and MeNH2·BH3 provided insights into depolymerization pathways. IDipp showed no reactivity toward [MeNH-BH2]3, whereas with 3 equiv of ItBu and CAACMe, the dehydrogenation product [MeNBH]3, was formed. With MeNH2·BH3, 2 equiv of carbene were used as the first acts to accept dihydrogen; the major products with IDipp, ItBu, and CAACMe were IDipp-BH2NMeH (1), [MeNBH]3, and (CAACMeH)HB═NMeH (2), respectively. The double E-H (E = B, N) bond activation product (CAACMeH)HB═NMe(HCAACMe) (3) was isolated from the reaction between 3 equiv of CAACMe and MeNH2·BH3. A unified mechanism for donor-mediated depolymerization of [MeHN-BH2]n is proposed.