Phenyl carbene, hydrogen

Attempts to detect a thermally populated triplet state ( A ) of 8 by ESR spectroscopy were unsuccessful. This was attributed to the high reactivity of the diradical, which presumably easily abstracts hydrogen atoms in hydrocarbon matrices (to form p-quinodimethane), even at very low temperatures. In this context, the triplet state of /7-phenylenebis(phenylmethylene) has been observed. Apparently, substitution of the carbenic hydrogens of 8 by phenyl groups confers sufficient stability (thermodynamic and perhaps kinetic) to the biradical, which allows its observation. According to variable-temperature ESR spectroscopy, the triplet state of /7-phenylenebis(phenylmethylene) is thermally populated and the singlet state lies 0.5-1 kcal/mol lower in energy [76-79]. 

Since the shift of a hydrogen atom is not possible with the methoxy-(phenyl) carbene ligand, it can only dimerize in reactions with bases and in thermal liberations (75). 

Methyllithium (and likewise BuLi and allyllithium) also adds to the carbene ligand of (CO)5W[C(OMe)Ph]. However, the reaction of the resulting anionic adduct with Si02/pentane at —40°C yields pentacarbonyl(n -olefin)-W complexes, probably via the intermediary formation of the methyl(phenyl)carbene complex and following rearrangement via 1,2-hydrogen shift ... 

This analysis predicts that triplet phenyl nitrene should react much more slowly with hydrogen atom donors than will triplet phenyl carbene. 

As shown in Table 3, triplet lb is computed to be 25-26 kcal/mol lower in enthalpy than triplet lc.77 Table 3 also shows that radicals 8b and 8c, formed by adding a hydrogen atom to lb and lc, respectively, differ in enthalpy by only 1-3 kcal/mol. Therefore, the large enthalpy difference between 3lb and 3lc is not due to a difference between the abilities of the phenyl and pyridyl groups to stabilize an unpaired tt electron. Instead it must reflect an intrinsic enthalpy difference between arylnitrenes and arylcarbenes. Table 3 also shows that aniline (9b) and fl-picoline (9c) are also predicted to have very similar enthalpies, thus providing further evidence that the large enthalpy difference between lb and lc is, indeed, due to the fact that lb is a nitrene, while lc is a carbene. 

If, instead of electrocyclization, electrophilic attack of the closer upper edge of the phenyl group by the carbene carbon atom occurs, a zwitterionic intermediate might result, which upon 1,4-elimination of (COljW would yield a 1-methoxy-1,3-cyclopentadiene. Suprafacial hydrogen migration would finally lead to the formation of the observed diastereomer. 

Stang etal. (94JA93) have developed another alkynyliodonium salt mediated approach for the synthesis of y-lactams including bicyclic systems containing the pyrrole moiety. This method is based on the formation of 2-cyclopentenones 114 via intramolecular 1,5-carbon-hydrogen insertion reactions of [/3-(p-toluenesulfonyl)alkylidene]carbenes 113 derived from Michael addition of sodium p-toluenesulfinate to /3-ketoethynyl(phenyl) iodonium triflates 112 (Scheme 32). Replacing 112 by j8-amidoethynyl (phenyl)iodonium triflates 115-119 provides various y-lactams as outlined in Eqs. (26)-(30). 

On the other hand, kcm> increases only slightly as more chlorine atoms are introduced. This result is somewhat surprising in the light of the marked decrease in the dimerization rate caused by the four meta buttressing chloro substituents. It can be understood to indicate that electrophilicity of the carbene center is increased as more chlorine groups are introduced on the phenyl rings (Table 9.14). Thus, the attack of a bulky substrate (e.g., chlorinated DPC) on the carbenic center must be severely restricted, while the rate of abstraction of a very small hydrogen atom from a very efficient H donor is still controlled by the electrophilicity of the carbenic center. 

Enamine formation occurs by the thermolysis of diazo compounds (Scheme 150)67 109,278 284 288,304 332 453 454 via a carbene-like intermediate.284 332 When R1 = Ph, it enters into competition with hydrogen migration,284,332 and the electrophilic character of the carbene enhances the migration of the dimethylaminophenyl more than the phenyl.332 When triazoline synthesis is carried out at temperatures higher than that at which thermolysis of diazo compounds occurs, enamines are obtained exclusively, as in the addition of phenyl azide to cinnamic nitriles and ketones, with phenyl migration dominating in the nitrile.284 Enamine is also formed quantitatively in the reaction of ethyl diazoacetate with benzylideneaniline at 110°C.455... 

As mentioned above, the electrophilic metal carbene complexes are stabilised by the presence of heteroatoms or phenyl rings at the divalent carbon atom, while hydrogen or alkyl groups stabilise the nucleophilic complexes. Therefore, there is a distinction between carbenoids and alkylidenes when designing carbene ligands corresponding to the former or the latter class. 

Althougir the dehydration mechanism seems rational for the homologation of alcohols having a hydrogen in the position of the hydroxyl function, difficulties arise with alcohols such as methanol and benzylalcohol. Ziesccke proposed carbene or phenyl-carbcnc as intermediates in these cases [56]. No detailed description of the reaction path was given but a mechanism according to Equation (18) is conceivable. 

Klett and Johnson have demonstrated the interrelationship between the photoreactions of allenes (93) and cyclopropenes. Their results indicate that irradiation of (93) leads to the carbenes (94) and (95) such carbenes have been proposed as the key intermediates in the photoreactions of cyclopropenes. Thus the irradiation of (93a) yields the indene (96) as the primary photo-pnoduct via the carbene (94) produced by phenyl migration. With aUene (93b) irradiation affords the carbene (95), by hydrogen migration, which then reacts to form photoproducts (97)—(99). For comparison, the irradiation of cyclo-propene (100) was carried out it yielded the allene (93b) and the two indenes (97) and (99). 

A study of alkyl- or aryl-substituted 1,2-dithiolium salts leads to similar conclusions. One group of these salts undergoes thermolysis to a dithiolyl radical, and in the mass spectrum the parent ion is the dithiolyl ion, which often loses a hydrogen atom. However, sometimes the parent ion is stable enough to be observed in the spectrum. This is the case for 3,5-diphenyl-l,2-dithiolium bromide. Other salts, such as 3- or 4-phenyl-1,2-dithiolium bromide, probably first expel a proton, giving a carbene intermediate. " ... 

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