The Doubly Base-Stabilized Diborane(4) [HB(μ-hpp)]2 (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate): Synthesis by Catalytic Dehydrogenation and Reactions with S8 and Disulfides

In this work we report on new experiments on the catalytic dehydrogenation of [H2B(mu-hpp)](2) leading to the doubly base-stabilized diborane(4) [HB(mu-hpp)](2) featuring two hpp bridges (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) under mild conditions. Several dehydrogenation (pre)catalysts were tested. The best one turned out to be Ru-3(CO)(12), allowing quantitative dehydrogenation already at 60 degrees C. Subsequently we subjected [HB(mu-hpp)](2) to reactions with S-8 and disulfides (Ph2S2 and Bn2S2, Bn = benzyl). Reaction with S-8 leads to oxidative insertion of one sulfur atom into the B B bond and formation of [HB(mu-hpp)](2)(mu-S). In the case of disulfides, substitution reactions leading to the doubly base-stabilized diborane(4) species [RSB(mu-hpp)](2) and HB(mu-hpp)(2)BSR (R = Ph or Bn) compete with sulfuration again leading to [HB(mu-hpp)](2)(mu-S).