Nickel cyclopropanations with

Table 3.2. Cyclopropanation with stoichiometric amounts of cationic iron and nickel carbene complexes.

The nickel(0)-catalyzed codimerizations of methylenecyclopropane (26) or 2,2-dimethylmethylene-cyclopropane with the chiral derivatives of acrylic acid lead to optically active 3-methylenecyclopen-tanecarboxylic esters or amides (39 equation 16) in good yields (Table 3). When (-)-camphorsultam acrylate is used, 3-methylenecyclopentanecarboxylic amides are obtained in up to 98% de. °... 

The reaction of substituted methyienecyciopropanes, such as (1-methylethylidene)- or (diphenylmethylene)cyclopropane, with compound 2 only leads to the formation of [3 -I- 2] cycloadducts. The best combined yield of three isomeric cyclocodimers (93%) is obtained with (1-methylethylidene)cyclopropane in a bis(> -cycloocta-l,5-diene)nickel(0)/tris(2-phenylphenyl) phosphite (1 1) catalyzed reaction at 120°C after 8 hours. ... 

Systems for [2 - - 3] cycloaddition can also be generated from methylene cyclopropanes (Scheme 11.43). Treatment of methylene cyclopropanes with either a nickel or a palladium catalyst in the presence of an alkene trap can lead to useful yields of the five-membered ring cycloaddition product 11.130 through oxidative... 

Even cyclopropanes lacking a methylene or vinyl group can be involved in formal cycloadditions. Cyclopropyl ketones 11.137 undergo cycloaddition to enones under nickel catalysis with an NHC ligand (Scheme 11.47) in the absence of the enone, their dimerization is observed. The reaction is proposed to proceed via a metallacyclic enolate complex 11.140, perhaps after initial oxidative addition to one of the cyclopropyl C-C bonds. 

Similar results are observed in the conjugative addition of CH-acidic methylene compounds with the metal derivatives of 2-nitro-5,10,15.20-tetraphenylporphyrin (6). The nickel porphyrin 6 (M = Ni) yields with an excess of dimethyl malonate the cyclopropane derivative 7 whereas the copper porphyrin 6 (M — Cu) forms with two equivalents of malononitrile the bisadduct 8.111... 

Chan et al. [38] prepared optically active atropoisomeric 2,2 -bipyridine by nickel(0)-catalyzed homo-couphng of 2-bromopyridylphenol derivatives (structure 28 in Scheme 16). Tested in the model test reaction, the copper catalyst led to frans-cyclopropanes as major products with up to 86% ee. 

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). 

Nickel(II) acetylaeetonate has been recommended as a very efficient homogeneous catalyst for intramolecular cyclopropanations for unsaturated diazoketones 171 The yields were better than with activated CuO as catalyst (see Table 10 for examples). The authors of this study seem to combine routinely thermocatalytic with photochemical (tungsten lamp) decomposition of the diazoketones. The benefit of this procedure (higher yields, shorter reaction times) has been communicated in the CuO case, but not for the Ni(acac)2-eatalyzed reaction. 

It is supposed that the nickel enolate intermediate 157 reacts with electrophiles rather than with protons. The successful use of trimethylsilyl-sub-stituted amines (Scheme 57) permits a new carbon-carbon bond to be formed between 157 and electrophiles such as benzaldehyde and ethyl acrylate. The adduct 158 is obtained stereoselectively only by mixing nickel tetracarbonyl, the gem-dibromocyclopropane 150, dimethyl (trimethylsilyl) amine, and an electrophile [82]. gem-Functionalization on a cyclopropane ring carbon atom is attained in this four-component coupling reaction. Phenyl trimethyl silylsulfide serves as an excellent nucleophile to yield the thiol ester, which is in sharp contrast to the formation of a complicated product mixture starting from thiols instead of the silylsulfide [81]. (Scheme 58)... 

Transition metals 172 a-bonded to cyclopropanes, substituted on the a-carbon with a halogen atom, are interesting intermediates for cyclopropylidene complexes 173 or allene ones 174 [88]. The former complexes are also supposed to be precursors of the above-mentioned nickel enolates. (Scheme 65)... 

Sulfonium ylides R2S=CR 2 [672,673] and metallated sulfones [674-676] can cyclopropanate simple alkenes upon catalysis with copper and nickel complexes (Table 3.6). Because of the increased nucleophilicity and basicity of these ylides, compared with diazoalkanes, these reagents are prone to numerous side-reactions,... 

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

Only a limited number of examples are known of applications of thietanes in organic synthesis. Prominent among these examples would be electrophilic ring opening reactions leading to polyfunctional sulfur compounds (33)-(37), utilization of 3-thietanones (55) and metal complexes (87) derived therefrom as oxyallyl zwitterion equivalents in cycloaddition reactions, synthesis of dipeptide (63) with a /3-thiolactone, Raney nickel desulfurization of thietanes (e.g. 120 cf. Table 7) as a route to gem-dimethyl compounds, and desulfurization of thietanes (e.g. 17) in the synthesis of cyclopropanes (also see Table 7). 

The metal-catalysed hydrogenation of cyclopropane has been extensively studied. Although the reaction was first reported in 1907 [242], it was not until some 50 years later that the first kinetic studies were reported by Bond et al. [26,243—245] who used pumice-supported nickel, rhodium, palladium, iridium and platinum, by Hayes and Taylor [246] who used K20-promoted iron catalysts, and by Benson and Kwan [247] who used nickel on silica—alumina. From these studies, it was concluded that the behaviour of cyclopropane was intermediate between that of alkenes and alkanes. With iron and nickel catalysts, the initial rate law is... 

The intermediacy of >/2-cyclopropene complexes of nickel has been proposed in catalyzed 2+1 reactions of free cyclopropene with electron-poor olefins, to give vinylcyclo-propanes. For example, the reaction of fumarate esters with 3,3-disubstituted cyclopropenes in the presence of Ni(COD)2 catalyst gave vinyl-substituted trans-2,3-cyclopropane dicarboxylate esters (equation 235)72 302. However, when maleic esters were used instead, a mixture of both cis and trans vinylcyclopropane diesters is obtained. 

Metal catalysed cyclopropanation using other types of intermediate is also possible. Lithiated tert-butyl alkyl sulphones bring about the cyclopropanation of various nonactivated alkenes under nickel(II) acetylacetonate catalysis (equation 88)131,132. Sulphonium ylides of type 90 react with simple alkenes under copper catalysis to give the corresponding cyclopropane adduct (equation 89)113,134. In this example the ylide (90) is the sulphonium equivalent of ethyl diazoacetate134. 

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