Nickel complexes diazomethane

Diazomethane is also decomposed by N O)40 -43 and Pd(0) complexes43 . Electron-poor alkenes such as methyl acrylate are cyclopropanated efficiently with Ni(0) catalysts, whereas with Pd(0) yields were much lower (Scheme 1)43). Cyclopropanes derived from styrene, cyclohexene or 1-hexene were formed only in trace yields. In the uncatalyzed reaction between diazomethane and methyl acrylate, methyl 2-pyrazoline-3-carboxylate and methyl crotonate are formed competitively, but the yield of the latter can be largely reduced by adding an appropriate amount of catalyst. It has been verified that cyclopropane formation does not result from metal-catalyzed ring contraction of the 2-pyrazoline, Instead, a nickel(0)-carbene complex is assumed to be involved in the direct cyclopropanation of the olefin. The preference of such an intermediate for an electron-poor alkene is in agreement with the view that nickel carbenoids are nucleophilic 44). 

Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophi-licity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO)5W=C(OMe)Ph [509], [Cp(CO)2Fe(THF)][BF4] [510,511], and (CO)5Cr(COD) [52,512]. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes. 

The reactivity of carbonyls of Cr, Mo, and W with alkyl lithium is somewhat lower than that of nickel and iron carbonyls. The most interesting reaction of these metal carbonyls, found and studied extensively by Fischer and his coworkers, is the formation of carbene complexes 98-"). Phenyl or methyl lithium adds readily to W(CO)6 at room temperature to form carbonylate complexes. The tetramethyl ammonium salt of the complex is treated with acid and then with diazomethane to give a neutral complex whose structure was found to be that of a carbene complex. In this way a series of carbene complexes of Cr, Mo, and Mn were synthesized. 

In the presence of a large excess of tetracarbonyl nickel and ethanol diphenyldiazo-methane, diazofiuorene, bis-(4-methoxyphenyl)diazomethane, and ethyl diazoacetate give carbonylation products trapped by ethanol and processed into the corresponding carboxylic acid in 74,38,26, and 8.5% isolated yields, respectively, which presumably arise from a nickel carbene carbonyl intermediate that releases a substituted ketene upon decomposition at 50-66 °C. In the absence of ethanol, by refluxing a solution of 1 mol diphenylketene with 6.4 mol tetracarbonyl nickel in diethylether 35% isolated yield of diphenylketene was observed [86]. The r -(C,C)-ketene complex of nickel (31) was isolated in 17% yield from the reaction of nickelacyclobutane (29) with carbon monoxide (3 bar) at 50 °C (reaction 8.54) [87]. Complex 29 is believed to be in equilibrium with the nickel-carbene-olefrn complex 30 [88]. The nickel-ketene complex 31 was also obtained either by direct reaction of Ni(PPh3)4 with ketene or by carbonylation of the nickel-carbene complexes presumably formed from the reaction of Ni(PPh3)4 and CH2Br2 in the presence of metallic zinc [89]. 

Silyl-substituted diazomethanes are regarded as a silylcarbene precursor or as a diazomethane equivalent, as they coordinate, after loss of nitrogen, to transition metals to generate metal-carbene complexes and participate in a variety of metal-catalyzed reactions and the silyl substituent is readily lost in many cases after the total transformation. For example, nickel-catalyzed cycloaddition reactions of dienynes proceed with incorporation of a silylmethyl group to give seven-membered carbocycles having an allylsilane functionality (Scheme 3-69). ... 

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