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

Abstract

The reactions of the <i>N</i>-heterocyclic carbenes (NHCs) IDipp and I<i>t</i>Bu and the cyclic(alkyl)amino carbene (CAAC) CAAC^<i>Me</i> with polyaminoborane [MeNH-BH₂]_<i>n</i> 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 (<b>1</b>), cyclic borazane [MeNH-BH₂]₃, or borazine [MeNBH]₃ with IDipp, I<i>t</i>Bu, and CAAC^<i>Me</i>, respectively. With substoichiometric quantities of IDipp and I<i>t</i>Bu (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 (<b>1</b>) and [MeNH-BH₂]₃ and with substoichiometric I<i>t</i>Bu, [MeNH-BH₂]₃, and [MeNBH]₃ with product ratios dependent on the quantity of NHC used. Under analogous conditions with CAAC^<i>Me</i>, 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 I<i>t</i>Bu and CAAC^<i>Me</i>, 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, I<i>t</i>Bu, and CAAC^<i>Me</i> were IDipp-BH₂NMeH (<b>1</b>), [MeNBH]₃, and (CAAC^<i>Me</i>H)HB═NMeH (<b>2</b>), respectively. The double E-H (E = B, N) bond activation product (CAAC^<i>Me</i>H)HB═NMe(HCAAC^<i>Me</i>) (<b>3</b>) was isolated from the reaction between 3 equiv of CAAC^<i>Me</i> and MeNH₂·BH₃. A unified mechanism for donor-mediated depolymerization of [MeHN-BH₂]_<i>n</i> is proposed.