Pyridylcarbene

Another example of the reversibility of the ring expansion — one of special relevance to this chapter — is found in the interconversion of the isomeric pyridylcarbenes, and their conversion to phenylnitrene (Scheme 4).10,11... 

Wentrup s experimental work on carbene-to-nitrene rearrangements10 suggested that calculations on phenylnitrene (lb) and the isomeric pyridylcarbenes might provide some useful relative energies. Kemnitz et al. carried out calculations on lb and 3-pyridylcarbene (lc).77 The latter molecule was used to provide a link between the energies of not only phenylcarbene (la) and phenylnitrene (lb), but also between CH2 and NH. 

Table 3. Relative Enthalpies (298 K, in kcal/mol) of Triplet Phenylnitrene (31 b), Triplet 3-Pyridylcarbene (3lc), a and Pairs of Molecules Formed from them by Addition of Hydrogen Atoms b...

As illustrated by the methyl derivative, the pyridylcarbene gives high yields of aryl nitrene derived products indicating that phenylnitrene is substantially lower in energy than the isomeric pyridylcarbenes, as first suggested by Wentrup and co-workers. ... 

Indeed, CASPT2/6-31G calculations of Kemnitz et al. find that triplet phenyl nitrene is 26.4 kcal/mol lower in energy than the isomeric triplet m-pyridylcarbene. 

Phenylnitrene is intrinsically more stable than the isomeric pyridylcarbene. Kamey and Borden explained that the reason why the N—H BDEs in RNH radicals are smaller than the C—H BDEs of RCH radicals, that is, rehybridization, is also responsible for the fact that nitrenes are thermodynamically more stable than carbenes.The lone pair of electrons of a nitrene reside in a low-energy sp hybrid orbital. This effect dramatically stabilizes nitrenes relative to carbenes in which the nonbonding electrons reside in either pure p or pseudo sp orbitals. 

However, from elegant labelling studies by Wentrup and coworkers on the gas-phase thermal interconversion of phenylnitrene and 2-pyridylcarbene, it became evident that an alternative and equally viable intermediate in these nitrene ring expansions is the azacycloheptatrienylidene (216) or its tautomer, the cumulated system (217) (81AHC(28)23l). 

Diazomethyl)pyridine (708) which normally exists in the ring-closed form (707) thermolyzes to 2-pyridylcarbene (709) which interconverts in the gas phase with phenylnitrene (711). Photolysis of 2-(diazomethyl)pyridine in an argon matrix allows identification of l-aza-l,2,4,6-cycloheptatetraene (710). 3- and 4-Pyridylcarbenes also interconvert with phenylnitrene (81AHC(28)279). 

A variety of 1-aryl-2-vinylcyclopropanes with a heterocyclic aryl group have been prepared by direct photolysis of the corresponding aryldiazomethanes in the corresponding but-1,3-dienes. All the heteroarylcarbenes employed, viz. pyridylcarbene, furylcarbene and thienylcar-bene, gave low yields (< 15 /o) of cyclopropanes. The main product was the corresponding azine, but minor amounts of stilbene were also obtained. Thus, when 2-furyldiazomethane was irradiated in but-1,3-diene at — 40°C furfuralazine (20c) was isolated in 18% yield, l-(2-furyl)-2-vinylcyclopropane (21c, R = H) was obtained in 8% yield, and 1,2-bis(2-furyl)ethene (22c, R = H) was formed in 0.8 /o yield. ... 

Tetrachloroethene, a much less active carbene trap than other alkenes, forms an adduct with chloro(2-pyridyl)carbene, albeit in very low yield however it does not react with the corresponding 3-pyridylcarbene. T... 

Phenylnitrene and 2-pyridylcarbene interconvert in the gas phase, as evidenced by the formation of identical products from phenyl azides and 2-(diazomethyl)pyridines (Scheme 30).1 99,200 Although the carbene precursor 157 exists as a triazole in the solid state, the diazomethane valence tautomer... 

Crow et a/.201,204 found that also 3- and 4-pyridylcarbenes are converted to phenylnitrene. A 13C-labeling study of 4-pyridylcarbene by Thetaz... 

The proposed intermediate (189) connecting the 2- and 3-pyridylcarbenes was not observed, but in further studies229,230 a novel intermediate absorbing at 1935 cm-1 was produced on long wavelength irradiation of 3-pyridyl-carbene. This intermediate was assigned the zwitterionic structure 190 and was photochemically converted to 187 and then to 183 without further intermediates being detectable by IR spectroscopy. Thus, 190 occupies the... 

It is noteworthy that a return of 2-pyridylcarbene (185) to 3- or 4-pyridyl-carbene has not been observed.228,229 This parallels the observations from the thermal chemistry that 2-pyridylcarbene expands exclusively by insertion into the 2,3-bond, not into the 1,2-bond of pyridine.199,204,232 The nonoccurrence of this latter reaction (185 - 189) is due to the unfavorable nonbonded interaction between the nitrogen lone pair and the filled carbene 7t-orbital during such a rearrangement.202,232 In the extreme case, a bicyclic intermediate in this reaction would be the antiaromatic 1H-azirine 191 (Eq. 46). On the other hand, 3-pyridylcarbene can hardly avoid... 

Flash thermolysis of 3-aryl[l,2,3]triazolo[l,5-a]pyridines (277) under mild conditions (380-500°C, 10-3 torr) affords carbazoles (279, 280) in nearly quantitative yields.232 The triazoles exist as the valence tautomeric diazo compounds 278 in the gas phase.189 The substitution patterns in the products (279 and 280) demonstrate that the reactions take place exclusively by a carbene-nitrene rearrangement in which the pyridylcarbenes insert into the 2,3-bond in pyridine (Scheme 52). 

The assignment of the carrier of the latter spectrum was confirmed by the independent generation of 2-pyridylcarbene from 2-pyridyldiazomethane. 

The observation 49) that 2-pyridylcarbene and phenylnitrene interconvert in the gas-phase via an intermediate which has an arrangement of atoms as in 2-azepi-nylidene [Eq. (1)] led to the prediction that... 

Evidence for two such reaction pathways was obtained by pyrolysis of pyrazolo-[3,4- ]pyiidine 5). Here, the arylcarbene route (b) leads to the pyridylcarbene-... 

The tt-LUMO and p-HOMO interactions are symbolised in 130 and 151. 150 is a nitrene situated in the very electrophilic 4-pyrimidyl position. The 2-p5o azinyl position appears less electron-deficient as judged from the rates of nucleophilic substitution of the corresponding chlorides ii ). Furthermore, nitrene 150 has to undergo electrophilic substitution onto the relatively electron-rich 5-position in pyrimidine. Pyrazinylnitrene 151 has only an electron-poor a-position to attack. On this basis, expansion of 150 should be faster than that of 151. The selectivities observed here are the same as in the pyridylcarbenes (Section III.7). 

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