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 unsatd. electrophiles as the dimeric calcium carbene [(Me3Si-N:(Ph2)P)CCa]2. No reaction was obsd. 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 Ph isocyanate. Crystal structures for 2, 4, and 5 are presented.
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