Isolation of an Intermediate in the Catalytic Trimerization of Isocyanates by a Monomeric Calcium Carbene with Chelating Iminophosphorane Substituents

Twofold deprotonation of the bis-iminophosphorano ligand (DIPP−N(Ph2)P)2CH2 (DIPP = 2,6-diisopropylphenyl) by (para-tBu-benzyl)2Ca·(THF)4 gave clean conversion to the monomeric THF adduct (DIPP−N(Ph2)P)2CCa·(THF)2 (2). This monomeric calcium carbene showed the same sluggish reactivity toward unsaturated electrophiles as the dimeric calcium carbene [(Me3Si−N(Ph2)P)CCa]2. No reaction was observed with 1-adamantyl cyanide, and with benzophenone a coordination complex was isolated: (DIPP−N(Ph2)P)2CCa·(THF)·(OCPh2) (4). Reaction with a slight excess of cyclohexyl isocyanate gave the double-insertion product [(DIPP−NPPh2)2C−C(O)−N(Cy)−C(O)−N(Cy)2-][Ca2+· (THF)0.5] (5). The monomeric calcium carbene 2 is a very slow catalyst for the trimerization of cyclohexyl isocyanate but a much faster catalyst for the trimerization of phenyl isocyanate. Crystal structures for 2, 4, and 5 are presented.