Nitrenes table

The chemistry of covalent azides is primarily determined by the exceptional weakness of the RN—N2 bond they most readily decompose to yield N2 and RN (nitrenes). Table 7 records the bond... 

The parameter Z) is a measure of the interaction between the two unpaired spins of the triplet. The exceptionally high value of D in the spectra of the nitrenes (Table 4) indicates that the two spins are... 

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. 

In contrast, the much lower enthalpy computed for 3 lb, compared to 3lc, means that the N-H BDE of the anilinyl radical 8b is much lower than the C-H BDE of the 3-pyridylmethyl radical 8c. The results in Table 5 show that this is indeed the case, not only for R=Ph and R =3-pyridyl, but also for R=R =Ph and R=R —H.77 The data in Table 5 indicate that, not just for lb and lc but in general, triplet nitrenes are ca. 20 kcal/mol more thermodynamically stable than comparably substituted triplet carbenes. 

The results in Table 3 show that the explanation of why nitrenes are thermodynamically more stable than carbenes must be the same as the reason why the N-H... 

Our calculations showed that the first step, cyclization of the nitrene to an azabicyclo[4.1.0]heptatriene, is rate-determining. Our calculated barriers for cyclization of four fluorinated derivatives of lb are given in Table 6.87 The CASPT2/cc-pVDZ barrier of 13.4 kcal/mol for cyclization of 2,6-difluoro-phenylnitrene (10b) is 4.1 kcal/mol higher than the barrier computed for lb -+ 2b. In contrast, the calculated barriers to rearrangement of 3,5-difluoro-phenylnitrene (lOe) and 4-fluorophenylnitrene (10c) are very similar to that computed for unsubstituted phenylnitrene (lb). These computational results are consistent with the observed reluctance of pentafluorophenylnitrene (10a) and 2,6-difluorophenylnitrene (10b) to rearrange,48 1 81 83 and with the relative ease... 

For 2-fluorophenylnitrene (lOf), the barrier for cyclization at the fluorinated ortho carbon is computed to be ca. 3 kcal/mol higher than that for cyclization at the unfluorinated ortho carbon (Table 6).87 These results are in agreement with experimental observations that nitrene lOf rearranges rapidly to a ketenimine in solution810 and that 2,4-difluorophenylnitrene (lOd) undergoes ring expansion some 15 times faster than 2,6-difluorophenylnitrene (10b).81 c,d Both lOf and 2,4-difluorophenylnitrene (lOd) can cyclize at an unfluorinated ortho carbon but this is not possible for 2,6-difluorophenylnitrene (10b). 

The following table given carbenes and nitrenes which commonly add to olefines. 

In order to determine if the diminish of ethylenic double bond of the polymer by the attack of nitrene occurs, the ratio of IR absorbances due to v(C=C) of ethylenic double bond(3100cm-1) and due to v(CH) of alkane(2990cm-l) has been determined for the unirradiated film, irradiated film, benzene extract and Soxhlet extract(Table 3). 

Two para-substituents, phenyl and cyano depress and retard the rate of cyclization significantly (Table 11.2)." p-Phenyl and p-cyano are both radical stabilizing substituents. These conjugative substituents reduce the spin density on the carbon ortho to the nitrene nitrogen. The reduced spin density at carbons ortho to the nitrogen lowers the rate at which the 1,3-biradical cychzes. The effect with p-cyano and p-biphenyl singlet phenylnitrene is quite dramatic. The lifetimes of these singlet nitrenes at ambient temperature are 8 and 15 ns, respectively, and the activation barriers to cychzation are 7.2 and 6.8 kcal/mol, respectively. 

Laser flash photolysis of a series of fluorinated aryl azides produces the transient spectra of the corresponding singlet nitrenes. ° With the exception of singlet o-fiuorophenylnitrene (39s), the rate of decay of the singlet nitrene was equal to the rate of formation of the reaction products, for example, didehydroazepines and triplet nitrenes. Values of fejsc and the Arrhenius parameters for azirine formation are summarized in Table 11.5. 

The LFP of p-biphenyl azide produces singlet-p-biphenylnitrene. The phenyl group has little influence on the electronic spectra of either singlet or triplet p-biphenylnitrene (Table 11.1), but it extends the singlet nitrene lifetime from 1 to 17 ns at ambient temperature by diluting the spin density ortho to the nitrene nitrogen. 

The next development in direct detection of nitrenium ions came from McClelland et al. ° who applied the azide method to LFP measurements. This permitted the direct detection of those arylnitrenium ions implicated in carcinogenic DNA damage. McClelland s approach proved to be particularly useful in the study of 4-aryl and 4-alkoxy substituted phenylnitrenium ions. Apparently, the corresponding singlet nitrenes are sufficiently long lived to allow for protonation in aqueous solution. Several arylnitrenium ions studied by this route are described in Table 13.6. 

Analysis of the data of Table 1 verifies the same finding that the shift of the near-UV absorption band of singlet arylnitrenes correlates with the shift of the intense near-UV absorption band of triplet nitrenes. Furthermore, the ort/io-substituents influence the absorption spectra of singlet and triplet phenylnitrenes more significantly than do para-substituents. 

Table 2 Kinetic parameters of para-substituted singlet aryl nitrenes (X— in pentane.

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.

Unlike the case in either 8c or 8f, in the cyclization of meta-cyanophenyl-nitrene (8d) the cyano group resides on a carbon at which the it NBMO in the reactant has a node, and it seems unlikely that radical stabilization will influence whether 8d cyclizes to 9d or lOd and the barrier heights connecting 8d to 9d and lOd are quite comparable (Table 7). 

The work of Leyva and Sagredo demonstrated, in fact, that cyclization of the singlet nitrene 16a proceeds away from the fluorine substituent. The steric argument predicts that a single orf/to-fluorine substituent will have little influence on the rate of conversion of 16a to 17a, since cyclization occurs at the unsubstituted ortho carbon. However, the barrier to this process is larger (outside of experimental error) than that of the parent system (Table 8). In... 

In these routes, the N—C—N moiety of the triazole is first constructed as a substituent in the pyrimidine, and then this is cyclized (Table VIII). The mechanism is described as an electrocyclization of a nitrene formed thermally or by oxidation (90SC2617). 

Solvolysis of MC15 by ammonia was studied as early as 1924, but the products were not conclusively characterized.260,261 Halo monoalkylamides262 and dialkyl amides were prepared. The monoalkylamides undergo facile cr-H abstraction reactions to nitrenes, especially when labile substitutents are present (Section 34.2.3.5) dialkylamides (Table 12) have been extensively studied by Bradley.263-265... 

The first tantalum nitrene was obtained in 1959 by thermolysis of [Ta(NEt2)]5-288 This class of compounds is presently accessible by several routes, including hydrogen abstraction from the mono- or di-alkylamides, reaction of metallacarbenes with organic imines, oxidation of low valent species by organic azides, or reductive coupling of nitriles (Table 13). The tantalum derivatives are usually stabler than those of niobium. 

Table 13 Niobium(V) and Tantalum(V) Nitrene Derivatives (without Cyclopentadienyl Ligands)...

A series of homoleptic Nbrv dialkylamido compounds has been obtained as highly air sensitive liquids by spontaneous reduction of [Nb(NR2)5] (R = Et, Pr", Bu" Table 41). Aminolysis of either Nbv or NbIV dialkylamides provided mixed species.256 The existence of quadrivalent tantalum dialkylamides is still questionable. [Ta(NEt2)4] was reported to have been detected during the thermolysis of [Ta(NEt2)5],263 which led predominantly to the Tav nitrene (2 Section 34.2.3.4.ii), but other workers could not find any evidence for its formation in a similar experiment.282 [Ta(NMeBu)4] has been reported to be formed in the decomposition of the corresponding pentavalent dialkylamide. 

Representative examples of ring syntheses involving carbenoid (Table 6) or nitrenoid (Table 7) intermediates are given. In many cases, the free carbene or nitrene is probably not involved, and the distinction between insertion and addition reactions given in the tables is not always clear cut. Such reactions are particularly useful for the preparation of tricyclic compounds. 

Nitrenes have a short lifetime (only several microseconds)86- 8 and undergo stabilization by the following reactions isomerization to imines, dimerization to azo compounds, hydrogen abstraction followed by ring closure to heterocyclic compounds, bimolecular insertion into C-H bonds to secondary amines, addition to solvent yielding ylids, and addition to unsaturated systems yielding heterocyclic compounds. Table 117-106 includes the reaction products and references for the different classes of nitrenes. 

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