Nickel catalysts isocyanides

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-... 

The nickel(II) complex 14 is also known as a catalyst for the polymerization of 1,3-butadiene. The versatility of this nickel catalyst was used in the synthesis of butadiene-isocyanide block copolymers [27]. In the so-called change of mechanism block copolymerization, 1,3-butadiene was initially polymerized with 14, yielding a butadiene polymer bearing living 773-allyl-nickel termini (Scheme 20). Addition of terf-butyl isocyanide to this living... 

In Nolte and Drenth s nickel catalyzed system, the polymerization was believed to be initiated by a nucleophilic attack by the alcohol used as a solvent or the halide on the starting complex on the coordinated isocyanides. Successful asymmetric polymerization was achieved using a dicationic tetrakis(isocyanide)nickel(II) complex 46 with enantiopure primary amines, which served as a chiral nucleophile in the initial step (Scheme 35) [58, 59]. In a typical experiment, a catalyst prepared from (f-BuNC)4Ni(II)(C104)2 (46a) (1 mol%) and an optically active amine (1 mol%), was used for polymerization of isocyanides with, or without a solvent, such as n-hexane, in... 

Isocyanides bearing ammonium side-chains 51 and 52 have been polymerized in the presence of nickel catalysts [72, 73]. The amphiphilic isocyanide 51 forms vesicles on dispersion in water. The isocyano groups located in the vesicle bilayers were polymerized by nickel capronate to form polymerized vesicles. The isocyanide 51 was also used in the preparation of polymerized vesicles containing metalloporphyrin components within the bilayer membrane [74]. The redox behavior of this membrane-bound cytochrome P-450 mimic has been investigated in detail. In addition to those bearing cationic side chains, isocyanides 53 and 54 bearing zwitterionic side chains were successfully used [75]. 

AT-Vinyl-substituted isocyanides 57 have been polymerized in the presence of nickel catalysts, yielding poly(vinyl isocyanide)s [79]. The solubility and stability of these polymers very much depends upon the substituent on the vinyl group. Substituted vinyl isocyanide, (CH3)2C=CHNC, afforded polymers that were soluble in chloroform when freshly prepared, although they became insoluble on standing for several days, even at temperatures of -10 °C. The polymerization of vinyl isocyanide proceeded in hydrocarbon solvents, unlike aryl or alkyl isocyanides, which required an alcoholic solvent for efficient polymerization. 

A more complex reaction is involved in the cooligomerization of acetylenes and tert-butyl isocyanide using nickel acetate as the catalyst (Scheme 20)43 the nature of intermediate complexes leading to the formation of 2-cyano-5-terf-butylaminopyrroles has not been established. Cocyclization of tert-butyl isocyanide with coordinated hexafluoro-2-butyne gives rise to coordinated cyclopentadienone anils for molybdenum systems,44 hence the nature of acetylene substitutents and of the organometallic catalyst play crucial roles in these processes. The pyrrole products from the former reaction can be decomposed by sulfuric acid and the overall sequence provides a simple synthesis of 5-amino-2-cyanopyrroles (Scheme 20). 

Homogeneous catalysts have now been reported for hydrogenation of carbon monoxide, a combustion product of coal (see Section VI,B). More effective catalysts will undoubtedly be discovered in the near future. Polynuclear or, at least, binuclear sites are favored for reduction of the triple bond in carbon monoxide (see Section VI,B), and this together with the popular parallelism to heterogeneous systems, has renewed interest in metal clusters as catalysts (see Section VI). A nickel cluster is the first catalyst reported for mild (and selective) hydrogenation of the triple bond in isocyanide (see Section VI,A). The use of carbon monoxide and water as an alternative hydrogen source is reattracting interest (see Section VI,C). 

Similar behaviour to carbon monoxide is displayed by other heterounsatur-ated monomers of carbene-like structure, isocyanides, which homopolymerise in the presence of nickel-based catalysts, yielding polymers with a carbon-carbon main chain, poly(iminomethylene)s [60],... 

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... 

With the nickel complex in an aprotic solvent, such as THF, cyclohexyl isocyanide was polymerized instantaneously, affording the corresponding polymer in a quantitative manner. Furthermore, the successful application of this new catalyst system was demonstrated by the quantitative polymerization of tert-butyl isocyanide (DP=ca. 27), which is one of the least reactive isocyanides. 

Cyclization of enynes, dienynes, and diynes with aryl isocyanides. This combination of reagents (1 2) generates a Ni(0) catalyst (1) which is easier to handle than the air-sensitive bis(l,5-cyclooctadiene)nickel(0), Ni(COD)2. Reaction of 1,6-cnyncs (2) with an aryl isocyanidc in the presence of this Ni(0) catalyst combined with Bu3P (2 cquiv.) results in bicyclic iminocyclopentenes (3) which can be hydrolyzed to the corresponding ketone (4). The overall reaction is an alternative to the Pauson-Khand reaction. 

In the last few years the design and use of various disilane compounds has gained importance because of the reactivity of the Si-Si bond and the large potential for organic synthesis involved with it. Many publications offer us numerous examples of possible reactions at the silicon-silicon bond such as addition reactions with C-C double bonds or C-C triple bonds [1, 2], addition reactions with C-element multiple bonds (e.g. aldehydes, quinones, isocyanides) [3-5] or metathesis [6, 7] and cross-metathesis [8]. In the most cases the existence of a catalyst (palladium, platinum or nickel complexes) for activation of the silicon-silicon a bond is indispensable for a successful transformation [9-11]. 

The oxidation of alkylisocyanides to alkylisocyanates is catalyzed by a variety of metal complexes [82,118,147]. Included among the complexes which have been reported to catalyze this reaction are [Ni(r-BuNC)4], [Ni(r-BuNC)2(02)], and [RhCl(Ph3P)3j. When ether solutions of [Ni(r-BuNC)4] were contacted with oxygen at — 20°C the dioxygen complex (Ni(r-BuNC)2(02)] crystallized from solution. Other catalysts for isocyanide oxidation are Ws(cycloocta-l, 5-diene) nickel [Ni(l, 5-C8Hi2)2], and Ws(cycloocta-l,5-diene)cobalt. 

Special Ligands. As discussed in Sect 2.5.3, the substitution of carbonyl ligands in the cluster often enhances the liability of the clusters. This property has also been used in catalytic processes. Thus for instance, the nickel cluster with isocyanide ligands Ni4(CNR)7 is an efficient catalyst precursor in the hydrogenation of acetylenes. Actually, Ni4(CNR)7 reacts with acetylenes yielding the adduct Ni4(CNR)4 (acetylene)3 which in turn is able to catalyze, under rather mild conditions, the hydrogenation of diaryl and dialkylacetylenes at rates of ca. one turnover per minute. Mononuclear complexes do not catalyze this reaction under the same conditions. 

Drenth and coworkers (20) have proposed a directive mechanism for helix formation specific to the polymerization of isocyanide monomers by their nickel (II) coordination catalysts. 

They further specify rules governing a catalyst-induced stereoregulation for the incorporation of chiral isocyanide monomers into the polymers during the chain-propagation process (21). The controlling features are a consequence of substituent relative size, and, where it occurs, substituent coordination to nickel (II). This catalyst-induced stereoregulation probably does not apply to the sulfuric acid polymerization of isocyanides. 

Kros et al. reported a polymer-peptide conjugate prepared via nickel-mediated NCA polymerization and a subsequent polymerization of an isocyanide, again using the nickel complex as initiator [111]. The active catalyst is attacked by the more electrophilic isocyanide and the coordinated amine reacts with the isocyanide to yield a carbene-like initiator for the isocyanide polymerization (see Fig. 14). The product can be purified from free residual homopolymers by selective solvent... 

The first attempts to synthesis asymmetric polyisocyanides are recent ones [30]. Asymmetric polymerization of racemic isocyanide (Xllla) with nickel chloride or nickel acetylacetonate catalyst was attempted in the presence of chiral solvents, (-)-borneol or (+)-sec-butyl-alcohol, or in the presence of (+)-nickel alaninate with the racemic a-phenyl-ethylisocyanide (Xlllb), but the polymers did not show any activity. 

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