Reactive Intermediate Chemistry

stereospecific singlet intermediate is produced initially, and subsequently relaxes to a less selective, lower energy triplet intermediate.57

The results with carboethoxynitrene are quite different from those obtained with pivaloyl azide 7 and the aroylnitrenes. Furthermore, photolysis of azidoesters 1339 and 1452 in contrast to pivalolyl and aroylazides at cryogenic temperatures produces persistent nitrene EPR signals. Thus, it is clear that the nitrene esters have triplet ground states.

Why does the nitrene ester have a triplet ground state in contrast to the acetyl and benzoyl analogues? This author speculates that the N O bonding interaction in the singlet nitrene ester is weaker than that in the singlet acetylnitrene because such an interaction destroys resonance within the ester group.

Nitrene esters have not yet been detected by matrix IR spectroscopy. IR spectroscopy is less sensitive than EPR spectroscopy and IR observes all the photoproducts, not just the paramagnetic triplet species of interest.

Buron and Platz58 recently studied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1–2 ms. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in arylnitrenes and is comparable to a carbenic process.59 As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19.

Nanosecond time resolved infrared (TRIR) spectroscopy has recently become available to physical organic chemists.60 This spectroscopy is an attractive tool for studying carbonyl nitrenes. Such work is in progress in several laboratories58

and it seems safe to predict that this technique will be greatly contributing to our understanding of carbonylnitrenes in solution in the near future.

3.2. Sulfonylnitrenes

We will follow the lead of Lwowski61 and classify sulfonyland phosphorylnitrenes as a type of carbonylnitrenes.

Pyrolysis or photolysis of methanesulfonyl azide in 2-methyl butane (RH) produces formal C H insertion adducts attributed to reactions of the thermally generated nitrene. The selectivity of this nitrene for 3 /2 /1 C H bonds was 6/2.3/1.0.62

Sulfonylnitrenes, like acylnitrenes, insert into the OH bonds of alcohols.

OMe

O O

However, product studies indicate that the chemistry of sulfonyl azides and nitrenes is complicated by rearrangement to form intermediates, such as 21.61

Fragmentation of arylsulfonyl nitrenes to form an arylnitrene and SO2 is also known.61 Presumably, this process involves a prior rearrangement to a species analogous to 21.

Ph

The dimerization of phenylnitrene to form azobenzene is a characteristic reaction of the triplet state of an arylnitrene. Thus, some intermediate formed along the reaction coordinate must undergo intersystem crossing to the triplet manifold. Photolysis (254 nm) of tosylazide63,64 at 77 K in fluorolube or EPA (2:5:5 ethanol: 2-methylbutane/diethyl ether) glass produces a persistent EPR spectrum assigned to the triplet state of p-toluenesulfonylnitrene jD=hcj ¼ 1:471 cm 1. However, upon extended photolysis of the sulfonylnitrenes, the EPR spectrum of tolylnitrene jD=hcj ¼ 0:9761 cm 1 is detected.

518 NITRENES

Photolysis (254 nm) of tosylazide in EPA produces a species with a sharp absorption maximum at 313 nm that also absorbs broadly between 250 and 600 nm. The carrier of the spectrum was associated with triplet tosylnitrene on the basis of the EPR work.64

Laser flash photolysis (266 nm) of tosylazide in cyclohexane produces a transient absorption (325 nm strong, 450 nm weak) with a lifetime on the order of microseconds.64 The lifetime of the transient is shortened to 328 ns in aerated cyclohexane. The same transient was also observed in ethanol and methanol with lifetimes of 8 and 9 ms, respectively. Oxygen again shortened the transient lifetime. Maloney and co-workers64 attributed the transient signals to tosylnitrene in its triplet ground state.

3.3. Phosphorylnitrenes

Photolysis of azidophosphodiesters 22 releases a nitrene that also inserts efficiently into the CH bonds of alkanes. When R0H ¼ 2;3-dimethylbutane is employed the yield of CH insertion adducts is almost 60%, with the remainder the amide 24.65

When 2,3-dimethylbutane (DMB) is used, both 3 and 1 formal C H insertion reactions are possible. In neat DMB, the 3 /1 ratio obtained with the azidodiethylester precursor 22 is nearly statistical, indicating that a very unselective, highly reactive, and short-lived species has been trapped. The ratio of adducts does not change upon dilution with perfluorohexane, although the yield of adducts drops and there is a small increase in the yield of amide 24. The presence of oxygen has no effect on the yield of CH insertion adducts with DMB, but depresses the yield of amide 24 from 40.1 to 6.3%. Maslak65b attributed the nitrene–alkane insertion adducts to C H insertion reactions of the singlet nitrene 23s, and considered amide 24 to be a diagnostic product derived from triplet nitrene 23t reactions. The data are consistent with initial formation of a short-lived, highly reactive singlet nitrene that inserts rapidly into primary and tertiary C H bonds. Maslak found that a plot of the ratio of singlet-to-triplet nitrene derived products versus the concentration of RH in perfluorohexane was not linear. Increasing the concentration of singlet nitrene trap cannot completely suppress the formation of triplet nitrene

derived products. This observation led Maslak to conclude that there is some intersystem crossing in the singlet excited state of the azide. This viewpoint was strongly supported by the observation that 1–3% isoprene, an excellent quencher of triplet excited states, inhibits the formation of phosphoramide.65

Assuming that singlet nitrene reacts with alkanes at near diffusion controlled rates allowed deduction of a rate constant of singlet-to-triplet nitrene intersystem crossing (ISC) of 2–8 108 s 1. This ISC rate is slower than in carbenes, but significantly faster than with arylnitrenes, which are discussed in a subsequent section.

Maloney and co-workers66 studied diphenoxyphosphorylnitrene by direct observational techniques. Irradiation (75 s, 254 nm) of 25 in EPA glass at 77 K produced persistent triplet EPR signals with zero-field parameters jD=hcj ¼ 1:5148 cm 1 and jE=hcj ¼ 0:00739 cm 1, which are rather similar to those of triplet carboethoxynitrene.39

Photolysis of 25 in ether–isopentane–ethanol (EPA) at 77 K produced a species that absorbs at 336 nm. The carrier of this spectrum is persistent for at least 1 h at 77 K, but disappears upon annealing. Based on the EPR spectrum and the similarity of the UV spectrum to those of triplet alkylnitrenes, the species absorbing at 336 nm was attributed to the triplet phosphorylnitrene.66

520 NITRENES

Laser flash photolysis (266 nm) of 25 in cyclohexane produced a weak transient absorption at 345 nm. The same transient was observed in ethanol, but the signal intensity was stronger. The transient lifetimes in ethanol-d6 were 3.8 and 4.1 ms, respectively.

O

PhO P N

PhO N

C

26

Me

The presence of oxygen had no effect, outside of experimental error, on the transient lifetime in ethanol. On the basis of the solution and glassy matrix work the transient absorbing at 345 nm was assigned to the triplet nitrene. LFP of 25 in acetonitrile produced a longer lived transient (t > 150 ms) also absorbing near 345 nm, which may be the result of formulation of a nitrene–solvent ylide.

4. VINYLNITRENES

A simple picture of the nitrogen-centered orbitals of triplet vinylnitrene is shown below in which pz is used to form the sp hybrid orbital on the nitrene nitrogen, pp conjugates with the p system of the double bond, and ps is orthogonal to that p system. Consequently, the openand closed-shell configurations of singlet vinylnitrene are no longer degenerate because conjugation slightly raises the energy of pp relative to ps.

Parasuk and Cramer’s calculations indicate that triplet vinylnitrene is 15 kcal/mol

more stable than the lowest energy singlet state.67 The lowest energy singlet state is open shell and resembles a 1,3-biradical.67,68

π*

CC

π

sp

N

C C

N

522 NITRENES

Unsurprisingly, singlet vinylnitrene has never been observed by time resolved spectroscopy and triplet vinylnitrene has yet to be observed by either time resolved or matrix spectroscopy.

5. PHENYLNITRENE AND SIMPLE DERIVATIVES

The chemistry of phenylcarbene and related aryland diarylcarbenes has been studied extensively since the 1950s. Analysis of arylcarbene product mixtures revealed the broad mechanistic picture.19,70 The singlet and triplet states of phenylcarbene are in rapid equilibrium. The higher energy singlet carbene reacts rapidly and stereospecifically with alkenes to form cyclopropanes. The triplet ground state reacts less rapidly and without stereospecificity—but it abstracts hydrogen atoms

from hydrocarbons to form radical pairs and reacts very rapidly with oxygen, as do free radicals.19,70

Matrix and time-resolved spectroscopy confirmed the pattern revealed previously by chemical analysis and placed it on a quantitative basis. Singlet and triplet arylcarbenes [X ¼ H, aryl (Ar)] generally have triplet ground states 0–5 kcal/mol below the singlet state.70,71 Intersystem crossing from singlet-to-triplet carbene takes

place in hundreds of picoseconds.59 In phenylcarbene itself, the singlet–triplet splitting is 2–4 kcal/mol in solution70,71 and spin state interconversion in both directions

is faster than bimolecular processes.19,70 Thus, most of the chemistry proceeds through the singlet state while the lower energy, less reactive triplet serves as a reservoir for the more reactive singlet carbene.19,70

Understanding the chemistry of arylnitrenes proceeded more slowly than that of arylcarbenes because product studies were less informative. Unlike carbenes, much

524 NITRENES

Wentrup and co-worker81 studied the gas-phase pyrolysis of various phenylnitrene precursors and discovered several interesting stable products, including cyanocyclopentadiene 31 formed in a ring contraction process.

Reiser et al.82 examined the photolysis of aryl azides in a low-temperature glass. Excitation of the precursor led to the formation of new UV–vis bands that disappeared upon annealing the glass. These transitions were attributed to the ground state of the triplet arylnitrenes. Shortly thereafter, Wasserman39 demonstrated that photolysis of phenyl azide immobilized in an organic glass cooled to 77 K produced the EPR spectrum of triplet phenylnitrene (3PN) in good agreement with the conclusions of the low-temperature UV–vis spectroscopic studies.

However, in 1978, Chapman and LeRoux83 discovered that photolysis of phenyl azide, matrix isolated in argon at 10 K, produces a persistent species with a strong vibrational band at 1880 10 cm 1. The carrier of this species was most reasonably assigned to ketenimine 30 rather than benzazirine 29 or triplet phenylnitrene. This result implies that it is the ketenimine 30 and not benzazirine 29 that is trapped with amines to form the 3H-azepines (27) that had been isolated earlier. It does, however, raise the question as to why two groups observed triplet phenylnitrene by low temperature spectroscopy while a third observed ketenimine 30.

To add to the confusion, various groups reported that gas-phase photolysis of

phenyl azide produced the absorption and emission spectra of triplet phenylnitrene.84,85 These observations were reconciled by the work of Leyva et al.86 who

discovered that the photochemistry of phenyl azide in the presence of diethylamine was very sensitive to temperature. Above 200 K, azepine 30 is formed, but <160 K, azobenzene, the product of triplet nitrene dimerization, is produced. The ketenimine can react with itself or with phenyl azide to produce a polymer,87 which can be converted into an electrically conducting material.75 Gritsan and Pritchina88 pointed out that at high-dilution ketenimine 30 can interconvert with singlet phenylnitrene which eventually relaxes to the lower energy triplet that subsequently dimerizes to form azobenzene.

Levya et al.86 also demonstrated that triplet phenylnitrene is very light sensitive. Thus, triplet phenlynitrene subsequently and inadvertantly absorbs light employed to decompose phenyl azide at low temperature and then the triplet nitrene rearranges to ketenimine 30. This idea explains the formation and detection of ketenimine 30 in argon by Chapman and LeRoux.83 Later Sheridan and Hayes89 found that if phenyl azide is decomposed by 340 instead of 254-nm light, then triplet phenylnitrene is the major persistent primary photoproduct formed in argon

at 10 K, which allowed this group to assign its IR spectrum.

What of the gas-phase results?84,85 Cullin et al. demonstrated that triplet phenylnitrene is not the UV–vis active species detected in the gas phase. That species is actually the cyanocyclopentadienyl radical!90 The UV–photolysis of phenyl azide produces singlet phenylnitrene with excess vibrational energy, which can not be shed by collisions with solvent molecules. Thus, the hot nitrene explores the PhN surface and eventually finds the global minimum, cyanocyclopentadiene, which can shed its excess energy by losing a hydrogen atom to form the cyanocyclopentadienyl radical. These results are in excellent agreement with Wentrup’s

gas-phase pyrolysis studies.81 To date, phenylnitrene has not been detected in the gas phase; neither has phenylcarbene.

Photolysis of phenyl azide (32) produces singlet phenylnitrene (33s), but what happens next depends on temperature and phase. In the gas-phase 33s isomerizes to cyanocyclopentadiene 31, in solution at ambient temperature, it isomerizes to ketenimine 30 and in cryogenic matrices, singlet phenylnitrene isomerizes to triplet phenylnitrene (33t).

5.1. Computational Chemistry

In 1992, the Borden and co-workers91 and Schaefer and co-workers92 predicted that the singlet–triplet splitting of singlet phenylnitrene is 18.5 kcal/mol in the gas phase, a result that was confirmed by the negative ion photodetachment studies of Ellison and co-workers.93 Thus, the phenyl group dramatically lowers the singlet– triplet splitting of the nitrene relative to imidogen ( EST ¼ 36 kcal/mol).11,12 The situation is reminiscent of vinylnitrene, which has a calculated ST splitting of only 15 kcal/mol.67 The lowest energy singlet states of both vinyland phenylnitrene have open-shell biradical-like configurations.

Although EST of phenylnitrene is small in comparison to that of imidogen, it is still very large compared to phenylcarbene.70,71 Thus, while singlet and triplet phen-

ylcarbene interconvert rapidly,19,70 singlet to triplet intersystem crossing of phenylnitrene in solution is irreversible.73,86,88 The differences in EST of singlet and

triplet nitrenes have been discussed above with parent methylene and imidogen, and the same arguments apply to their aryl derivatives.

Singlet phenylcarbene is a closed-shell species, but singlet phenylnitrene has an open-shell electron configuration. This observation explains why singlet