Nitrene, phenyl

DIFFERENCES BETWEEN PHENYLCARBENE AND PHENYL-NITRENE AND THE RING EXPANSION REACTIONS THEY UNDERGO... 

Mechanism [ill] represents crosslinking due to aziridine ring formation. This mechanism is supported by the decrease of ethylenic double bond of 1,2-polybutadiene and the fact that a large amount of aziridine compound is formed in the reaction of phenyl-nitrene with unsaturated olefine monomers, although the direct observation of it in 1,2-polybutadiene film matrix has not been accomplished in the present study. 

For most aryl azides, the rate constants of singlet nitrene decay and product formation (triplet nitrene and/or ketenimine) are the same. Thus, in all these phenyl-nitrenes cyclization to substituted benzazirines is the rate-limiting step of the process of isomerization to ketenimine, as is the case for the parent phenylnitrene. The only known exception, o-fluorophenylnitrene, will be discussed in the next section. 

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. 

Figs 6 and 7) similar to the trend observed with parent singlet phenyl nitrene (Fig. 3). The temperature-independent rate constant observed at low temperature was associated with kisc- In the case of 4-bromo-, 4-iodophenyl-nitrene (Fig. 6) and 2,2-dimethylphenylnitrene (Fig. 7) the values of koBs re independent of temperature over a very large temperature range ( 120-200 K). This proves that indeed the rate constants of ISC in arylnitrenes are temperature independent in solution over typical temperature ranges. 

Intersystem crossing rate constants of ortho- and meto-substituted singlet phenyl nitrenes are presented in Table 3. Mono- and di-ort/io-fluorine substituents have no influence on ISC rate constants. No effect with meta, metfl-difluoro substitution is observed either. Pentafluoro substitution has no efffect on kisc in pentane although a modest acceleration is observed in the more polar solvent methylene chloride. 

Table 3 Intersystem crossing rate constants of ortho- and meM-substituted phenyl-nitrenes.

However, 56 disappears at 11 K with a half-Ufe of 2.46 h, while the deuterated analog is stable. The WKB-computed half-Ufe (using the path computed at MP2/aug-cc-pVDZ and energies at CCSD(T)/cc-pVTZ) of 56 is 3.3 h and 8700 years for d-56. This fast tunneling rate also acconnts for the lack of rearrangement of 56 into the tropone 58, a reaction analogons to the chemistry of phenyl carbene and phenyl nitrene discussed earlier in this chapter (Section 5.2). 

Morawietz, J. Sander, W. Photochemistry of fluorinated phenyl nitrenes matrix isolation of fluorinated azirines, J. Org. Chem. 1996, 61,4351-4354. 

Gritsan, N. R Likhotvorik, I. Tsao, M.-L. Celebi, N. Platz, M. S. Kamey, W. L. Kannitz, C. R. Borden, W. T. Ring-expansion reaction of cyano-substitnted singlet phenyl nitrenes theoretical predictions and kinetic results from laser flash photolysis and chanical trapping expaiments, J. Am. Chem. Soc. 2001,123, 1425-1433. 

The primary thermochemical and electron impact induced degradation of aryl azides involves loss of N 2 with formation of an aryl nitrene. The nitrene subsequently reacts in the thermolysis system to give a complex mixture of products (48). ) An ion corresponding to phenyl nitrene is also prominent... 

The generation and reactions of arylnitrenes continue to attract attention. Evidence for a photoinitiated autocatalytic chain mechanism in the photodecomposition of phenyl azide has been reported, and the reaction of photochemically generated phenyl nitrene with oxygen has been reexamined. Irradiation of p-azidoaniline in aqueous solution yields triplet p-aminophenyl nitrene, which on reaction with water is converted into the highly reactive p-benzoquinone diimine. Both m- and p-nitrophenyl azides, on photoelimination of nitrogen at 77 K, afford the corresponding nitrenes, whereas o-nitrophenyl azide (93) is converted without the intermediacy of a nitrene into the benzofurazan (94). 4,4 -... 

The generation of two conformationally fixed o-(9-fluorenyl)phenyl nitrenes (95) and (96) has been accomplished by irradiation of the ap- and sp-9-(2-azido-4,6-dimethylphenyl)fluorenes (97) and (98) in methanol-ether at 25 The rates of formation of the azanoracaradiene (99) from ap-nitrene (95) and of the 9-methoxyfluorene (100) from the o-quininoid intermediate (101) have been determined. 

Phenyl isocyanate has been shown to undergo a photoinitiated chain decomposition at 77 K in a 3-methylpentane matrix via fragmentation to phenyl nitrene and carbon monoxide. 

The following isomers are represented 2H, 4H and 3H, 4H. A benzo-1,2-thiazete 447 has been proposed as an intermediate in the reaction of phenyl nitrene (triplet) with the thiazo derivative 446 a sulfurane structure 448 was suggested for one product of the reaction of perfluoropropyliminosulfur difluoride with per-fluoropropene. Stable benzo- and naphthyl-l,2-thiazetyl radicals (e.g., 452) are obtained by thermolysis of 5-amino compounds 449, 450, and 451. Electron-spin resonance studies established the structures. The magnitude of the spin density on nitrogen in derivatives of 452, which carry a substituent para to the nitrogen atom, depends on the nature of the substituent. Loss of sulfur from the naphthyl derivative 453 occurs at temperatures above 150°C. ... 

Aniline cannot participate in the reaction in the initial stages since the rate of nitrogen elimination (in phenyl azide) is the same in nitrobenzene or aniline. It has been suggested that phenyl nitrene is in equilibrium with an ionic form which can lead to azacyclopropene, viz. 

Phenyl azide and di-iron nonacarbonyl react rapidly in benzene at room temperature (as compared with thermolysis of PhNg alone which occurs at temperatures of 140-170° ). The principal product was the orange phenyl nitrene-complex (387) which decomposed spontaneously in solution to give the urea-based complex (388). Also obtained in low yield was the orange complex (389). The yield of azobenzene was reported to be negligible. C)n the other hand, when the decomposition was carried out in benzene under reflux in the presence of Fe3(CO)i2j a significant amount of azobenzene was found . ... 

The second parameter is indicative of the difference in spin density along the x and y axes of the molecule. The low value of E in the alkyl nitrenes is a consequence of the cylindrical symmetry of the / -orbitals about the C—N bond. In phenyl nitrene the / -orbital is locked in the plane of the phenyl ring and the equivalence of the two /(-orbitals is removed. The spin distribution is no longer symmetrical about the z-axis and the value of E is consequently quite high. 

Figure 5. (a) Huckel MO-lcvels of phenyl nitrene (b) Energy levels of... 

Aromatic nitrenes are as selective in their insertion reactions as carbonyl nitrenes. In phenyl nitrene the insertion yields into primary, secondary and tertiary C—H bonds are in the approximate ratios 1 10 100 . Aromatic nitrenes insert into aromatic G—H bonds about as efficiently as into secondary G—H bonds of aliphatic hydrocarbons photolysis and thermolysis of 2-azidobiphenyI produces... 

SJ-l-Phenylethanol, preparation, 12 Phenyl carbene ( CHC6H5), 116, 275 Phenyl nitrene (CeHsN), 118 Phenyl nitrenium ion, 120 [4]Phenylene, 150... 

Thermolysis of phenyl azide leads to loss of nitrogen with formation of phenyl nitrene. This conclusion is well supported by experiment. Walker and Waters [6] studied the pyrolysis of phenyl azide at 130°C and confirmed the observations of Smith and Hall [7] from 2-substituted azido biphenyls that thermolysis of aryl azides, lacking ortho-substituents, in inert solvents leads... 

Clearly, neither singlet nor triplet phenyl nitrene is likely to react directly with amines to form a 3H-azepine. On this basis Huisgen et al. [11] suggested formation of an intermediate benzazirine which, as illustrated in Scheme 2, is,... 

The first triumph of photochemistry occurred in 1962 when Smolinsky et al. [17] reported photolyzing phenyl azide and measuring the EPR spectrum of phenyl nitrene in fluorolube at 77 K. This result established with certainty that the ground state of phenyl nitrene is a triplet and initiated three decades of debate about the relevancy of observations made at low temperature to the chemical reactions that occur at room temperature or above. 

In 1965, Reiser et al. [18] published the first in a series of seminal papers describing the photochemistry and optical spectroscopy of aryl azides in rigid media at low temperature. In particular, irradiation of phenyl azide in a glassy solution at 77 K. resulted in formation of an intermediate with absorption maxima 241, 303, and 368 nm. Thoughtful application of models and control experiments led to tentative assignment of these bands to the triplet state of phenyl nitrene. However, concern about the validity of this assignment is well illustrated by consideration of the chemistry and spectroscopy of 2-azidobiphenyl. 

It is informative to compare methylene and nitrene in terms of simple molecular orbital theory and then to extend this comparison to their aromatic derivatives phenyl carbene and phenyl nitrene. Hoffman et al. [28] have described the molecular orbitals of CH2 in the following way. Methylene has two nonbonding molecular orbitals, one is an in-plane, a type, hybrid orbital, the other is an out-of-plane pure p type n orbital. Singlet methylene has a bond angle of 105° and the in-plane, sigma, orbital is doubly occupied. Triplet methylene has both a singly occupied n and a orbital and a bond angle of 135°. 

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