Thermal and Catalytic Dehydrogenation of the Guanidine-Borane Adducts H3B•hppH (hppH=1,3,4,6,7,8-hexahydro-2H-pyrimidol[1,2-a]pyrimidine) and H3B•N(H)C(NMe2)2: A Combined Experimental and Quantum Chemical Study

Herein thermal and catalytic dehydrogenation of the guanidine-borane adducts H3B center dot hppH (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) and H3B center dot N(H)C(NMe2)(2) are analysed. Thermal decomposition of H3B center dot hppH at 80 degrees C leads to [HB(mu-hpp)](2) and a second boron hydride, which is tentatively identified as [(kappa N-2-hpp)BH2]. Decomposition in boiling toluene (110 degrees C) leads to a mixture of [H2B(mu-hpp)](2) and [HB(mu-hpp)](2), from which [H2B(mu-hpp)](2) can be separated and crystallised. In the presence of a catalyst (with Cp2TiCl2/nBuLi or [Rh(1,5-cod)Cl](2) as precatalysts) dehydrogenation at 80 degrees C leads predominantly to [H2B(mu-hpp)](2). In the case of H3B center dot N(H)C(NMe2)(2) uncatalysed dehydrogenation turns out to be a very slow process even at 110 degrees C. Interestingly, the ultimate product of this process is oligomeric methylimino borane, [HBNMe](n). This pathway can be modelled and understood with the aid of quantum chemical calculations. Faster dehydrogenation can be initiated by addition of a catalyst. Finally, the possible mechanisms for thermal and CP2Ti-catalysed dehydrogenation are analysed for the model compound H3B center dot N(H)C(NH2)(2) by means of quantum chemical (DFT) calculations. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)