Catalytic trimerization, proposed

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

The generation of methane in the reaction was evidenced by the 111 NMR spectrum of the reaction mixture. It was also shown that the newly obtained complex 29 reacts catalytically with silanol 28 to give the trimer 30 (presumably from trimerization of 31) with the evolution of hydrogen gas. In the presence of MesSiOMe the same reaction resulted in the formation of an insertion product of the intermediate silanone 31 as shown in the lower part of Scheme 12. The proposed catalytic cycle for the dehydrogenation of 28 with 29 is shown in Scheme 13. It should be noted, however, that spectroscopic evidence for the proposed silanones was not presented. 

A cationic nickel-hydride is the proposed catalytically active species. Insertion of norbornene results in the nickel-norbomyl cationic intermediate, where R is H originating from the nickel-hydride. Under the conditions employed, ethylene insertion is competitive with the first norbornene insertion, followed by norbornene insertion to yield the nickel-norbornyl intermediate where R is ethyl. The second norbornene insertion is stereoselective about 96% of the time a meso enchainment results. Insertion of ethylene, followed by -hydrogen ehmination forms dimers 4 and 5 (as well as 4% of the isomeric rac dimers). Insertion of the third norbornene is non-stereoselective or random the trimeric product of m so or rac insertion is isolated after ethylene insertion and /l-hydrogen elimination (trimers 6 and 7, where R=H, along with the trimers where R=ethyl). Extrapolation of the random insertion of norbornene observed in the trimers to the high polymer predicts that the nickel-based poly(norbornene) is best described as atactic which should result in a more soluble polymer than a more regular poly (norbornene). 

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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