Carbenes generation carbene reactions

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of a-diazo ketones. When a-diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement.232... 

Interaction of an electrophilic carbene or carbenoid with R—S—R compounds often results in the formation of sulfonium ylides. If the carbene substituents are suited to effectively stabilize a negative charge, these ylides are likely to be isolable otherwiese, their intermediary occurence may become evident from products of further transformation. Ando 152 b) has given an informative review on sulfonium ylide chemistry, including their formation by photochemical or copper-catalyzed decomposition of diazocarbonyl compounds. More recent examples, including the generation and reactions of ylides obtained by metal-catalyzed decomposition of diazo compounds in the presence of thiophenes (Sect. 4.2), allyl sulfides and allyl dithioketals (Sect. 2.3.4) have already been presented. 

Ketocarbenes generated from diazoketones give two main type of reactions. The first one is the conventional carbene reaction, i.e. intramolecular insertion into a C—H or C—C bond, as applied in the synthesis of a... 

In the 1988-1999 period, almost all absolute kinetic studies of carbenic reactions employed LFP with UV detection. Carbenes that contain a UV chromophore (e.g., PhCCl) are easily observed, and their decay kinetics during reaction can be readily followed by LFP.11 However, alkyl, alkylhalo, and alkylacyloxycarbenes are generally transparent in the most useful UV region. To follow their kinetics, Jackson et al. made use of the ylide method, 12 in which the laser-generated carbene (2) is competitively captured by (e.g.) pyridine, forming a chromophoric ylide (3, cf. Scheme 1). The observed pseudo first order rate constants (kobs) for the growth of ylide 3 at various concentrations of pyridine are monitored by UV spectroscopy, and obey Eq. 1. 

Generation and reaction of chloro(phenylthio)carbene and phenylthiocarbene... 

Generation and reaction of chloro(methylthio)carbene Procedure 7.1.15 is used replacing the PhSCHCl2 with MeSCHCl2 (5.24 g, 40 mmol). 

Generation and reaction of dimethylvinylidene carbene and related compounds... 

Flash pyrolysis of l,4-bis(tetrazol-5-yl)benzene at 600°C has been explored as a possible route to the dicarbene, CH-C( H4-CH . The product was phenylethyne which carried a C label from the tetrazole C-5, confirming that a CgH, species had been generated <87JA2534>. Thermal degradation of C-labelled 5-diazotetrazole has provided a source of atomic carbon for carbene reactions... 

Eq. 15). On the other hand, thermolysis of azo compound (45) is known to generate the same radical pair in the singlet state and gives the same products in a different ratio (Eq. 16). The NMR spectrum of the coupling product 31 shows that the sense (AE) of polarization observed for the carbene reaction is opposite to that observed (EA) for the thermolysis of azo compound. This result implies that the coupling product 31 in the photolysis is formed by recombination of benzyl and diphenylmethyl generated from a precursor believed to be triplet DPC. 

While this can lead to interesting new carbene reactions, it may also complicate comparisons with the same carbene generated by conventional methods. 

In contrast to considerations of 50 years ago, today carbene and nitrene chemistries are integral to synthetic design and applications. Always a unique methodology for the synthesis of cyclopropane and cyclopropene compounds, applications of carbene chemistry have been extended with notable success to insertion reactions, aromatic cycloaddition and substitution, and ylide generation and reactions. And metathesis is in the lexicon of everyone planning the synthesis of an organic compound. Intramolecular reactions now extend to ring sizes well beyond 20, and insertion reactions can be effectively and selectively implemented even for intermolecular processes. 

Although not as common as the ylide derived from metal carbenes, the ylide-like species generated from metal nitrene or free nitrene has been attracting increasing attention in recent years. The overall transformation is parallel to that of metal carbene reactions. Progress in this direction is also covered in this chapter. 

The oxygen as heteroatom in ethers or carbonyl compounds is weak to moderate Lewis base. Nevertheless, a highly reactive metal carbene complex can interact with the oxygen to generate oxygen ylide. The interaction between ether and metal carbene functional groups is believed to be rather weak as demonstrated by the facts that other metal carbene reactions, such as G-H insertion and cyclopropanation, can proceed in ethereal solvents." These experiments demonstrate that the formation of the metal ylide is much less favored in the equilibrium shown in Equation (1). ... 

Certain diazo compounds can be photolyzed with UV light to generate highly reactive carbenes (Reaction 48). Similar to nitrenes, carbenes can insert into active C—H or... 

To date, bis(diazomethyl)silanes represent the only precursors to silylene-dicarbenes. It should be noted, however, that the extrusion of N2 from the two diazo functions can occur more or less simultaneously or successively. In the latter case, a diazocarbene is formed in the first place, which is supposed to undergo a fast carbene reaction before the generation of a carbene from the second diazo function occurs. This should be kept in mind for all transformations which are explained mechanistically with the participation of silylene-dicarbenes. There are even examples where only one of the two diazo groups enters carbene chemistry at all (see equations 35 and 36 below). 

Products which can be ascribed to the intermediate formation of radicals have long been observed in carbene reactions. In the gas phase these products could arise by homolytic decomposition of excited primary products before collisional deactivation rather than from radicals generated in the course of insertion. This is not so in solution. It is found that, in the thermal decomposition of diphenyldiazomethane (Bethell et al., 1965) or photolysis of diphenylketene (Nozaki et al., 1966) in toluene solution, the product of insertion of diphenylmethylene into the benzylic carbon-hydrogen bonds, 1,1,2-triphenylethane, is accompanied by substantial amounts of 1,1,2,2-tetraphenylethane and bibenzyl. This is a strong indication that discrete diphenylmethyl and benzyl radicals are formed, and, taken in conjunction with EPR (Section IIB) and other evidence (Etter et al., 1959) that diphenylmethylene is a ground-state triplet, would support the view that equation (20) is an adequate representation of triplet insertion. 

Another reaction that builds a cyclopropane ring onto a carbon-carbon double bond is called the Simmons-Smith reaction. This reaction does not actually involve a car-bene, but rather a carbenoid, an organometallic species that reacts like a carbene. This species is generated by reaction of diiodomethane with zinc metal from a special alloy of zinc and copper ... 

The synthetic applications of acyllithiums, generated by reaction of organolithium compounds with carbon monoxide, by treatment with electrophiles started when Nudelman and coworkers found that phenyllithium reacted with carbon monoxide in the presence of alkyl bromides to yield diphenylalkylcarbinols24,27. a-Hydroxy-a-phenylacetophenone was also obtained resulting from the dimerization of the carbene intermediate of type 3. In the absence of electrophiles a,a-diphenylacetophenone was obtained in 94% yield, attributed to the dimerization of the corresponding aroyl anion radical28. 

Scheme 6 Generation and reactions of imidazoline ylide 30 ( nucleophilic carbene).

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