Carbenoid species

Other potential synthetic routes to these unsaturated aziridine derivatives which involve the addition of nitrenes to allenes <75JOC224), carbenes to imines with subsequent hydrolysis <67JA362), and of carbenoid species to ketenimines <76TL1317,79TL559) have been investigated but are collectively of little or no preparative value. 

Direct metallation of oxirane cycle leading to carbenoid species 98SL337. 

The Clemmensen reduction can be formulated to proceed by a sequence of one-electron and proton transfer reactions. It is a heterogenous reaction, taking place at the zinc surface. Initially an electron is transferred from zinc to the carbonyl group of ketone 1, leading to a radical species 3, which is presumed to react further to a zinc-carbenoid species 4 ... 

It has been widely accepted that the carbene-transfer reaction using a diazo compound and a transition metal complex proceeds via the corresponding metal carbenoid species. Nishiyama et al. characterized spectroscopically the structure of the carbenoid intermediate that underwent the desired cyclopropanation with high enantio- and diastereoselectivity, derived from (91).254,255 They also isolated a stable dicarbonylcarbene complex and demonstrated by X-ray analysis that the carbene moiety of the complex was almost parallel in the Cl—Ru—Cl plane and perpendicular to the pybox plane (vide infra).255 These results suggest that the rate-determining step of metal-catalyzed cyclopropanation is not carbenoid formation, but the carbene-transfer reaction.254... 

The Ru porphyrin complex (8) has also been used as a catalyst for the cyclization of allylic diazoacetates,258 albeit with limited success only the cyclization of F-cinnamyl diazoacetate shows high enantioselectivity (Scheme 81). It is noteworthy that a carbenoid species prepared from allyl o-phenyl-o-diazoacetate and complex (8) has been isolated and subjected to X-ray diffraction analysis, though it does not undergo the desired cyclization. In the structure, the carbene plane lies almost halfway between the two adjacent Ru—N bonds. 

Quite different methods employ carbene intermediates. Spencer and his colleagues generate a carbenoid species from ethyl diazoacetate (CuS04 catalyst) and allow it to attack an enol ether of a 1,3-diketone.102 Scheme... 

It is proposed that the primary step in the formation of olefins involves the formation of a carbenoid species, [ CH2], which then inserts into a C—H bond of either methanol or dimethyl ether ... 

A rationale for the heteroatom effect has recently been provided for the reaction of 1,6-enynes based on detailed mechanistic studies,318 314 especially DFT calculations performed by several groups (Scheme 90) 183>319321 From what has been disclosed so far, an interplay of cationic and carbenoid species is obvious in these processes. Nowadays, there is a good consensus to adopt the involvement of cyclopropyl metallacarbene intermediates. 

An electron-enriched 1,3-diene moiety as in the substrate 381 can act as a nucleophile toward an activated alkyne moiety (Scheme 94). Iwasawa340 has reported an elegant synthesis of a diquinane framework 382, which is catalyzed by various metals and the rhenium(i) complex appears to be the best catalyst among the metal complexes examined. Minor product 384 presumably is formed through an insertion of a carbenoid species into the neighboring activated benzylic C-H bond. The same carbenoid species can undergo a 1,2-H shift to give the major product 383. 

Cyclopropanation reactions can be promoted using copper or rhodium catalysts or indeed systems based on other metals. As early as 1965 Nozaki showed that chiral copper complexes could promote asymmetric addition of a carbenoid species (derived from a diazoester) to an alkene. This pioneering study was embroidered by Aratani and co-workers who showed a highly enantioselective process could be obtained by modifying the chiral copper... 

Intramolecular carbene addition reactions have a special importance in the synthesis of strained ring compounds. Because of the high reactivity of carbene or carbenoid species, the formation of highly strained bonds is possible. The strategy for synthesis is to construct a potential carbene precursor, such as diazo compounds or di- or trihalo compounds, which can undergo intramolecular addition to the desired structure. Section E of Scheme 10.5 gives some representative examples. 

Interesting products have been observed from the reaction of aldehydes and vinyl carbenoid species (Scheme 4.31). 

Doyle et al. (34) were the first group to generate isomiinchnones from diazo imides using Rh(II) catalysis. For example, isomiinchnone 60 was produced from diazo imide 59, but attempts to trap this species with ethyl acrylate were unsuccessful. The only material identified was the isomiinchnone hydrolysis product. This use of Rh(II) to generate a rhodium-carbenoid species from an a-diazo carbonyl compound is reminiscent of the first successful synthesis of... 

In the dimetal species described in Scheme 38, one of the two C—metal bonds can react selectively with a source of Cl+ to give the corresponding sp2-carbenoid species. 

The glyoxime-Co(II)-catalyzed asymmetric cyclopropanation shown in Scheme 94 is noteworthy (226). The results of the detailed kinetic study are consistent with the mechanism of Scheme 92, however, the intermediary Co carbenoid species has substantial radicaloid properties, and only styrene and other conjugated olefins can be used as substrates. Simple alkenes are not cyclopropanated by diazo compounds. The reaction of deuterated styrene proceeds in non-stereospecific manner without retention of geometrical integrity. 

Thermolysis of the ylide (15) in thiophene results in ready rearrangement to dimethyl thiophene-2-malonate (16). The same product is obtained if the thermolysis is carried out in the presence of 2-methylthiophene or cyclohexene, proving that the rearrangement occurs by an intramolecular process (78CC85). However, when 2,5-disubstituted thiophenium ylides are thermolyzed, dissociation to carbenoid species seems to occur. This reaction is further discussed in Section 3.14.2.9. 

The comparison of thiophene with thioethers on the one hand and with enol thioethers on the other, in regard to its behaviour towards conventional electrophiles, has been made in Section 3.02.2.3. Attack on carbon is the predominant mode of reaction (Section 3.14.2.4) reaction at sulfur is relatively rare (Section 3.14.2.5). Carbenes are known to act as electrophiles attack at both carbon and sulfur of thiophene has been reported. The carbene generated from diazomalonic ester by rhodium(II) catalysis attacks the sulfur atom of thiophene, resulting in an ylide. It has also been shown that the carbenoid species derived by thermolysis of such an ylide functions as an electrophile, attacking the a-carbon of a second molecule of thiophene (Section 3.14.2.9). Singlet nitrene is electrophilic. However, in contrast to carbenes, it invariably attacks only the carbon atom (Section 3.14.2.9). 

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