Fragmentation processes, carbenes

Alkyl triflates can also be used as efficient leaving groups for intramolecular cyclization in the process of generating heterocycles, such as thioindenes, cyclic enones, iV-heterocyclic carbene precursors, or tetrahydropyridines via a Grob fragmentation process (93). They can also be reduced to the corresponding alkane in the presence of a copper catalyst. ... 

In another copper-mediated carbene transfer reaction, diazoester 222 has been decomposed in tire presence of bis(triethylsilyl- or -germyl)mercuty (equation 72) it was assumed that the obtained ketenes 223 result from the insertion of ethoxycarbonyl(trimethylsilyl)carbene into a Hg-element bond followed by a cyclic fragmentation process" . 

Fragmentation processes have been reported through the investigation of chloro-and lluoro-cyclopropyl methoxy carbenes. This work is related to the decomposition of both cyclopropyl-based carbenes which has been examined by means of ab initio methods. 

Similar to the intramolecular insertion into an unactivated C—H bond, the intermolecular version of this reaction meets with greatly improved yields when rhodium carbenes are involved. For the insertion of an alkoxycarbonylcarbene fragment into C—H bonds of acyclic alkanes and cycloalkanes, rhodium(II) perfluorocarb-oxylates 286), rhodium(II) pivalate or some other carboxylates 287,288 and rhodium-(III) porphyrins 287 > proved to be well suited (Tables 19 and 20). In the era of copper catalysts, this reaction type ranked as a quite uncommon process 14), mainly because the yields were low, even in the absence of other functional groups in the substrate which would be more susceptible to carbenoid attack. For example, CuS04(CuCl)-catalyzed decomposition of ethyl diazoacetate in a large excess of cyclohexane was reported to give 24% (15%) of C/H insertion, but 40% (61 %) of the two carbene dimers 289). 

The rapid development of the chemistry of transition metal complexes containing terminal carbene (A) or carbyne (B) ligands (7) has been followed more recently by much research centered on bridged methylene compounds (C) (2). The importance of /t-methylidyne complexes, whether in recently established binuclear examples (D), the well-known trinuclear derivatives (E), or the unusual complexes (F), has also become apparent. All are based on one-carbon (C,) fragments, and considerable interest is centered on their possible significance as models for intermediates in surface-catalyzed reactions between carbon monoxide and hydrogen (Fischer -Tropsch reactions) and related processes. These topics have been extensively ... 

A number of papers report investigations of the pyrolytic cleavage of aromatic hydrocarbons. The oxidation and pyrolysis of anisole at 1000 K have revealed first-order decay in oxygen exclusively via homolysis of the O—CH3 bond to afford phenol, cresols, methylcyclopentadiene, and CO as the major products.256 A study of PAH radical anion salts revealed that CH4 and H2 are evolved from carbene formation and anionic polymerization of the radical species, respectively.257 Pyrolysis of allylpropar-gyltosylamine was studied at temperatures of 460-500 °C and pressures of 10-16 Torr. The product mixture was dominated by hydrocarbon fragments but also contained SO2 from a proposed thermolysis of an intermediate aldimine by radical processes.258... 

Thermolysis of 2-acetoxy-2-methoxy-5,5-dimethyl-A3-l,3,4-oxadiazoline affords acetoxy(methoxy) carbene.60 The thermal rearrangement of acetoxy(methoxy) car-bene to methyl pyruvate was studied by DFT at the B3PW91/6-31G(d,p) level. The conformation of the carbene was considered, as were competing fragmentations to radical pairs. The authors concluded that the reaction is a concerted 1,2-migration rather than a fragmentation-recombination process. 

A a and a n C—S bond are broken during the formation of type 2 compounds. The resulting fragments function as two 3er donor ligands. Recently Patin and co-workers found that thermal activation or catalysis by electron transfer can transform a type le compound into a type 2 complex 13). This process involves not only rupture of a C—S bond, but also cleavage of an Fe—C a bond, carbon shift, and formation of a metal-carbene bond with the neighboring iron atom 28). 

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