Catalytic trimerization, proposed mechanism

Figure 1. Proposed PUSH-PULL mechanism for catalytic trimerization of aromatic nitriles.

The catalytic activity of these novel cationic lanthanide monoporphyrinates has also been studied (Wong et al. 1999). The ytterbiimi complex [Yb(TMPP)(H20)3]Cl, but not the yttrium and erbium analogs, can effectively catalyze the conversion of phenyl isocyanate into its cyclic trimer, l,3,5-triphenyl-Ar-triazine-2,4,6-trione. The complex appears to be a living catalyst as the resulting mixture can be re-used without losing any of its activity in the first three cycles. A plausible mechanism has been proposed as... 

Palladium-catalyzed trimerization of alkynes has been developed, " but simple terminal alkynes undergo dimerization to form enynes. A mechanism for the formation of head-o-tail enynes has been proposed that proceeds through palladium(iv) complexes 202 or 203. Probably, however, the acidic terminal alkyne will cleave the palladium-alkenyl bond to give the enyne product and an alkynylpalladium(ii) species that can enter a new catalytic cycle instead." ... 

In palladium-catalyzed [2 - - 2 - - 2] cycloaddition, arynes can be used as alkyne components. In 1994, Pena, Romero, and co-workers reported palladium-catalyzed homo-[2- -2- -2] cycloaddition of arynes (Scheme 6.9) [12]. The reactions of 2-silylaryl trifluoromethanesulfonates 26 with CsF generated the corresponding arynes 27, which were trimerized in the presence of a catalytic amount of Pd(PPh3)4 or Pd2(dba)3 to afford substituted triphenylenes 28. In the reactions of unsymmetric arynes, moderate to high regioselectivities were observed. A mechanism via pallada-cycle intermediates, generated through the oxidative cyclization of two molecules of arynes, was proposed. 

The Phillips catalyst, based on a Cr/2,5-dimethylpyrrole precursor and TEA as a cocatalyst, is the only commercial catalytic system for ethylene trimerization [164] and has thus been extensively studied [139, 140]. Based on DFT calculations, a redox mechanism involving a Cr(ll)/Cr(lV) couple has been proposed [154, 165]. Also, the pyrrole derivative ligand, able to interact with one or two metal centers through the nitrogen lone pair and/or the aromatic Jt-system, may play a key role for the stabilization of heterodinuclear Cr/Al species formed after activation. 

The ethylene trimerization was calculated on the basis of the metaUacycle mechanism first proposed by Briggs (1989). The catalytic cycle including the formation of 1-butene (7X), 1-hexene (6X), and metaUacycle expansion... 

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