Polymerization of isocyanide

In the reaction of Ni(CNBu )4 and methyl iodide oligomerization of the isocyanide was observed the only isolable nickel complex was (I), shown below. This product is believed to arise through sequential insertions of three isocyanides into a nickel-carbon bond. Upon further treatment with additional isocyanide at a temperature greater than 60° C one obtains a polymer (RNC) presumably through multiple isocyanide insertion reactions. The addition of benzoyl chloride to Ni(CNBu )4 gave two isolable compounds Ni(CNBu )3(COPh)Cl (74%) and (II) (8.2%). This latter reaction, and the isolation of (II) in particular, suggests that the proposed mechanism for polymerization of isocyanides is reasonable. 

Suginome, M. and Ito, Y. Transition Metal-Mediated Polymerization of Isocyanides. Vol. 

Polymerization of isocyanides to polyiminomethylenes is best achieved with cationic initiators, although anionic and radical polymerizations also occur [Millich, 1988]. 

SCHEME 72. Asymmetric polymerization of isocyanides. [G. Wulff, Angew. Chem., lnt. Ed. Engl., 28, 21 (1989). Reproduced by permission of Verlag Chemie.]... 

Polymerization of isocyanides is a thermodynamically feasible process, in agreement with the stoichiometric multiple insertion observed in reactions between metal-alkyl complexes and isocyanides. The entropy loss in the case of isocyanides is lower than for insertion of CO. Isocyanide insertions into palladium-alkyl a bonds are faster than those for the platinum(II) analogues. The latter, on the other hand, usually lead to more stable and better defined products. Insertion of isocyanides into platinum-carbon bonds has been studied extensively Reaction (j) is typical the ionic product was strongly suggested by observation that the compounds isolated under mild conditions are 1 1 electrolytes. 

The nickel(II)-catalyzed polymerization of isocyanides proceeds relatively fast, a remarkable observation given the steric crowding that is introduced upon formation of the polymer chain. The driving force for the reaction is the conversion of a formally divalent carbon in the monomer into a tetravalent carbon in the polymer, yielding a heat of polymerization of 81.4 kJ moD1.169 For this polymerization reaction, a merry-go-round mechanism has been proposed. Upon mixing of the isocyanides with the Ni(II) catalyst, a square-planar complex is formed (Scheme 7), which in some occasions can be isolated when bulky isocyanides are used. Subsequent attack by a nucleophile on one of the isocyanide ligands is... 

Several procedures have been established in the past for the helix-sense-selective polymerization of isocyanides.169 For the introduction of a chiral bias, required for the discrimination of the otherwise enantiomeric helical senses, three distinct approaches can be applied. One is the use of a chiral monomer, which makes the left- and right-handed helices diastereotopic, and as a result one of the two will be energetically favorable (Figure 5). A second approach involves the use of a chiral catalyst or a chiral initiator, in which case during the initiation of the polymerization there will be a preference for the formation of one particular helical sense that will be retained upon propagation due to the helix inversion barrier. The former approach has been employed in numerous cases (vide infra) the accounts of the use of optically active Ni catalysts are limited.189-192 The... 

Scheme 8. Polymerization of Isocyanides by a Dinuclear Palladium—Platinum //-Ethynediyl Complex...

Much effort has been expended to realize the efficient polymerization of isocyanides using promoters or catalysts. Such effort can be classified using three criteria those where the polymerization occurs via radical intermediates (Scheme 2), via anionic intermediates (Scheme 3), and via carbocationic intermediates (Scheme 4). Only the third criterion has been fruitful, and has led to the findings of the remarkable catalytic activity of homogenous and... 

IR spectroscopy, elemental analysis, and X-ray diffraction studies. Millich et al. demonstrated that acid-treated ground glass showed remarkable catalytic activity in the polymerization of isocyanides [8, 9]. Their heterogeneous... 

This review focuses on the transition metal mediated polymerization of isocyanides and 1,2-diisocyanobenzenes. Other approaches, including acid-catalyzed polymerization and main-group metal mediated polymerization, are also included, but only if they are closely related to the above topic. 

As has already been described, the nickel-catalyzed-system is currently the most general protocol for the polymerization of isocyanides. An initial report [5] described that Ni(CO)4, Ni(CO)3(PPh3), Cp2Ni, and CpNi(CO)2 show high catalytic activity in the polymerization of cyclohexyl isocyanide in benzene, yielding poly(isocyanide) 2 as a white powder, although the nickel catalysts are a little less active than the corresponding cobalt catalysts (Scheme 7). In a typical experiment, the polymerization of cyclohexyl isocy-... 

Deming and Novak then began to develop a modified nickel-initiated system for a highly efficient polymerization of isocyanides [16]. They used the 73-allylnickel trifluoroacetate dimer 14, whose allyl group may serve as an initial migrating group, thus facilitating the initiation step (Scheme 17). 

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