Methylene carbene formation

Structures of type XVIII or XIX as proposed for the diazodicyanomethane complex (Table 12) may also apply to the complexes of diphenyldiazomethane and 9-diazo-fluorene. The formation of the ketenimine complex (45) from the reaction of (CN)2CN2 and Ni(t-BuNC)4 probably occurs via attack of the complexed dicyano-methylene carbene on the isocyanide ligand113 The observation that these complexes112 113 catalyze the formation of ketenimines from isocyanides and diazo compounds, a reaction which does not proceed under same conditions without the transition metal, may be of preparative value 113 ... 

The mechanism of this reaction involves carbene formation by an acetylene-methylene rearrangement. 

Carbonylation of a mixture of ethylene, ethanol, and diphenylacetylene in the presence of rhodium carbonyl gives the butenolide (10). 2 Another formation of a butenolide, that of compound (12), is by flash vacuum pyrolysis of diphenylmethyl propiolate, Ph2CH02C H. It has been suggested that the ester isomerizes to the methylene-carbene (11), which yields the product by intramolecular insertion. Ozonization of tetraphenyl-... 

The isolation of a diamagnetic bridging methylene complex [(OEP-N-yx-CH2) Ru(CH3)](BF4) from decomposition of [(OEP - N - CH3)Ru(CH3)](BE4) was also possible. This complex has been characterized by H NMR and partially by an X-ray structure [145]. Unfortunately, reduction of this complex did not result in formation of an axial methylene carbene complex as was postulated by James and Dolphin [146]. Although M = CH2 species have been prepared [147,148], similar metalloporphyrin complexes are not yet known. Ruthenium carbene complexes which are involved in catalytic reactions will be discussed below. 

Photolysis of the sulphinyl-3H-pyrazole 587 in ether or methylene chloride leads to the formation of a relatively stable carbene 588 that can be identified by physical methods. When the irradiation is performed in ethyl vinyl ether or in furan, the expected cyclopropanes are formed smoothly and stereospecifically683 (equation 374). 

The comparatively small size of the simplest carbene (methylene) ensures that it has a definite mobility in frozen inert matrices, which leads to the formation of dimerization products under these conditions. It became possible only in 1981 to detect in the spectra of the diazomethane photolysis products bands at 1115 cm (Ar matrix) and 1109 cm (Xe matrix) which were attributed to the deformation vibration of methylene in its ground triplet state (Lee and Pimentel, 1981). 

This reaction can proceed by 1,1-proton abstraction to form a carbene radical anion, but can also occur by l,n-abstraction to form the negative ion of a diradical. Thus, reaction of O with methylene chloride results in the formation of CCI2 (Eq. S.Sa), reaction with ethylene gives vinylidene radical anion, H2CC (Eq. 5.8b), and the reaction with acetonitrile gives the radical anion of cyanomethylene, HCCN (Eq. 5.8c) Investigations of these ions have been used to determine the thermochemical properties of dichlorocarbene, CCI2, vinylidene, and cyanomethylene. ... 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

Treatment of Fischer-type carbene complexes with different oxidants can lead to the formation of carbonyl compounds [150,253]. Treatment with sulfur leads to the formation of complexed thiocarbonyl compounds [141]. Conversion of the carbene carbon atom into a methylene or acetal group can be achieved by treatment with reducing agents. Treatment of vinylcarbene complexes with diborane can also lead to demetallation and formation of diols [278]. The conversion of heteroatom-substituted carbene complexes to non-heteroatom-substituted carbene complexes... 

The formation of six-membered or larger rings by intramolecular C-H bond insertion normally requires the attacked position to be especially activated towards electrophilic attack [1157,1158]. Electron-rich arenes or heteroarenes [1159-1162] and donor-substituted methylene groups can react intramolecularly with electrophilic carbene complexes to yield six- or seven-membered rings. Representative examples are given in Table 4.8. 

If chiral catalysts are used to generate the intermediate oxonium ylides, non-racemic C-O bond insertion products can be obtained [1265,1266]. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [1135,1259], an observation consistent with a homolysis-recombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free car-benes [1267,1268] acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [1071,1093]. 

Exclusive formation of furans clearly indicates that the intramolecular 1,5-insertion into the C—H bonds of methylene groups cannot compete with that into OH bonds of enols. However, the stereochemistry of enolized carbene intermediates plays an important role in this reaction as acetylace-tone affords a mixture of furan 37 and cyclopentene 38 in a 64 36 ratio in 61% yield (Scheme 14). 

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