Reactive Intermediate Chemistry

cyclopropene, which subsequently undergoes a cyclohexadiene–hexatriene electrocyclic ring opening to form a symmetric cycloheptatetraene. This species can reform the original cyclopropene or the cyclopropene in which the label has apparently moved into the ring before subsequently forming stable products. The label can eventually migrate into the meta and para positions by additional ‘‘phenylcarbene’’ rearrangements.

Similarly the pyridylcarbenes rearrange to form phenylnitrenes.81

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 coworkers.81

N N

Me

+

CN

Me

Indeed, CASPT2/6-31G* calculations of Kemnitz et al.121 find that triplet phenyl nitrene is 26.4 kcal/mol lower in energy than the isomeric triplet m-pyridylcarbene.

As the singlet–triplet splitting of phenylnitrene is 18 kcal/mol, one can deduce that singlet phenylnitrene is 11.4–13.4 kcal/mol more stable than singlet pyridylcarbene.121 However, the corresponding radicals, anilines, and substituted pyridines

540 NITRENES

have very similar energies by CASPT2/6-31G*. Hence, the large difference in enthalpy between phenylnitrene and pyridylcarbene is not the result of intrinsic differences between benzenes and pyridines and their ability to delocalize an unpaired electron, but must reflect a basic difference in stability between carbenes and nitrenes.121

This finding is further illustrated in the following isodesmic reactions:

Phenylnitrene is intrinsically more stable than the isomeric pyridylcarbene. Karney and Borden explained that the reason why the N H BDEs in RNH radicals are smaller than the C H BDEs of RCH2 radicals, that is, rehybridization, is also responsible for the fact that nitrenes are thermodynamically more stable than carbenes.121 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 sp2 orbitals.

6. POLYCYCLIC ARYLNITRENES

6.1. Naphthylnitrenes

Extrapolation of the chemistry of phenylnitrene leads to qualitatively accurate predictions of the properties of the naphthylnitrenes. The naphthylnitrenes have triplet ground states that have been characterized by low-temperature EPR and UV–vis spectroscopy.39,122 The singlet–triplet splitting has not yet been measured, but Tsao’s CASPT2(12,12) þ ZPE calculations predict energy separations of 13.9 and 16.6 kcal/mol for 1- and 2-naphthylnitrene, respectively.123

One can predict that singlet 1-naphthylnitrene will cyclize more readily than phenylnitrene because the resonance energy per aromatic ring is lower in naphthalene than benzene, but by the same token, 1-naphthylnitrene should cyclize more slowly than vinylnitrene.

In principle, singlet 1-naphthylnitrene can cyclize to two distinct azirines 43 and 44, but 43 is clearly much lower in energy than isomer 44, which lacks aromaticity in either ring. One can therefore predict that 43 will be formed preferentially.

Ring opening of azirine 43, to ketenimine 45 is most likely endothermic, in dramatic contrast to the PhN system, because ring opening of azirine 43 converts a species with one aromatic ring into a ketenimine that has no aromaticity. In the PhN system, a nonaromatic azirine opens to a nonaromatic ketenimine (on the other hand, ring opening of azirine (44) will be exothermic, because aromaticity is restored in one of the rings).

Experimental observations support these views. Photolysis of 1-naphthylazide in the presence of diethylamine and tetramethylethylenediamine (TMEDA) yields azirine, but no ketenimine-derived adducts at ambient temperature.124 In the presence of diethylamine but in the absence of TMEDA, good yields of 1-amino- naphthalene and 1,10-azo-naphthalene, products attributable to the triplet nitrene are observed. Good yields of 46 are also achieved when the photolysis of 1-naphthylazide and diethylamine is performed at 60 C in the absence of TMEDA.125 Presumably, lowering the temperature extends the lifetime of azirine 43 by reducing its rate of reversion to singlet 1-naphthylnitrene more than it retards the rate of its reaction with diethylamine.

Laser flash photolysis of 1-naphthylazide produces a transient with IR bands at 1728 cm 1. The carrier of this signal is attributed to azirine 43.123 The TRIR spectrum of 43 is in excellent agreement with the earlier observation of 43 as a persistent species (1730 cm 1) in argon by Dunkin and Thomson.126 In that study it was not possible to ascertain whether a single azirine or a mixture of azirines was formed. Recently, however, Maltsev et al. have reported a mono complete matrix IR study.127 This group demonstrated that only azirine 43 is formed and its vibrational spectrum is in excellent agreement with the predictions of theory. It is

also clear that photolysis of the matrix-isolated azirine leads to formation of a ketenimine (n ¼ 1910–1930 cm 1).127

542 NITRENES

Tsao has used TRIR spectroscopy to determine that the lifetime of azirine in 43 at ambient temperature is 2.6 ms,123 which is in excellent agreement with the work of Shrock and Schuster,128 who studied the same system years earlier by LFP with UV–vis detection.

No transient absorption >350 nm is detected upon LFP of 1-naphthylazide.128 A band with absorption maxima at 370 nm is formed with a time constant of 2.8 ms after the laser pulse. The carrier of the 370-nm absorption reacts over >100 ms to form azonaphthalene. The carrier of the 370-nm absorption is identified as triplet

1-naphthylnitrene that has previously been characterized as a persistent species at 77 K by UV–vis122 (lmax ¼ 367 nm) and EPR spectroscopy.39 Azirine 43, detected

by TRIR spectroscopy must not absorb significantly >350 nm, a fact that was established later by the matrix isolation studies of Wentrup’s and Bally’s groups.127 Assuming a rapidly equilibrating mixture of azirine and nitrene, and given that

kisc ¼ 1 107 s 1 (determined by Tsao by LFP at 77 K and assumed to have the same value at 298 K),123 then K ¼ ½singlet nitrene&=½azirine 43& ¼ 0:038 at 298 K.

ISC

The 2-naphthylnitrene story is similar. Two azirines (47 and 48) can be formed, but azirine 47 is preferred and this azirine will form ketenimine 48 only reluctantly.

The azirine 47 has been detected as a persistent species by matrix isolation spectroscopy ( ¼ 1708, 1723, 1736 cm 1).126 The same species has also been detected in solution by LFP with IR detection (t ¼ 150 ms).123 Azirine 47 can be intercepted with diethylamine to form 49 in 94% yield when the amine concentration is 1.45 M.128 This yield is a much greater than can be achieved with 1-naphthylazide at ambient temperature. Adduct 49 is not formed upon triplet sensitized photolysis

of 2-naphthylazide and diethylamine, instead azonaphthalene 50 is formed.128 This product is also formed in 45% yield upon direct photolysis in the absence of amine.

In the absence of trapping reagents, azirine 47 reverts to singlet-2-naphthylni- trene, which ultimately relaxes to the triplet. Azirine 47 has a lifetime >150 ms, which indicates that the [nitrene]/[azirine] equilibrium constant is very much smaller in 2-naphthyl than in 1-naphthylnitrene. In fact, upon LFP of 2-naphthyl- azide, no transient signals absorbing >350 nm are observed immediately after the laser pulse. Unlike in the 1-naphthyl case, the transient spectrum of 2-naphthylni- trene does not grow in on the microsecond time scale, but the growth of azo compound is observed on the millisecond time scale.128

Shrock and Schuster also studied the pyrenylnitrenes and obtained results very similar to those described above for the naphthyl systems. Other polycyclic nitrenes can be imagined, but have received far less comprehensive study.128

6.2. Biphenylnitrenes

The LFP of p-biphenyl azide produces singlet-p-biphenylnitrene.112 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.

In 1951, Smith and Brown discovered that the decomposition of o-biphenyl azide produces carbazole in excellent yield.73a,b Swenton et al.129 demonstrated that photochemical formation of carbazole was a singlet nitrene process. In contrast, triplet sensitized photolysis of o-azidobiphenyl produces the corresponding azo compound. Direct photolysis in the presence of diethylamine produces an azepine in addition to carbazole, and the latter product is formed even in neat amine.130

Triplet o-biphenylnitrene has been observed as a persistent species at 77 K by UV– vis131 and EPR spectroscopy.39 Tsao has detected singlet o-biphenylnitrene by LFP at 77 K (lmax ¼ 410 nm, t ¼ 59 ns), which decays cleanly to the triplet nitrene at this temperature.123

The photochemistry of o-azidobiphenyl has been studied by conventional132 and by LFP methods with both UV–vis and IR detection of the intermediates.133

544 NITRENES

Thereby, it was found that the formation of carbazole proceeds on more than a single time scale. The following mechanistic picture accounts for many observations.133

Singlet o-biphenylnitrene 52s cleanly relaxes to the lower energy triplet state of 52t at 77 K, but at ambient temperature it likely partitions to form a mixture of azirine 55 and isocarbazole 53 within a few nanoseconds of a laser pulse. Some azirine 55 expands to ketenimine 56, which can be trapped with diethylamine.

In the absence of amine, the ketenimine–azirine singlet nitrene species can equilibrate and, eventually, the singlet nitrene can cyclize to form carbazole. Berry and co-workers132 independently monitored the growth of carbazole (lmax ¼ 289:4 nm) by this process. In cyclohexane, some carbazole was formed this way with an observed rate constant of 2:2 103 s 1 at 300 K over a barrier of 11.5 kcal/mol. Tsao and co-workers133 recently used TRIR spectroscopy to show that ketenimine decay equals the rate of carbazole formation.

Some of the initially formal singlet nitrene 52s can also cyclize to isocarbazole 53. Isocarbazole 53 absorbs broadly in the visible (lmax 430 nm) and decays to

carbazole by a symmetry allowed 1,5-hydrogen shift with a time constant of 70 ns.133

7. POLYNITRENES

Interest in the preparation of high-spin organic compounds has led to the matrix isolation of polynitrenes, such as 57–61.134 These species have been studied primarily by EPR spectroscopy, but increasing use is being made of matrix IR and UV–vis spectroscopy. Density functional theoretical calculations have been used to assign the vibrational spectra that have been observed. Polynitrenes are under active study by material scientists interested in the development of organic magnets.

Mixed-carbene–nitrene compounds have also been prepared by Tomioka and coworkers (X ¼ H, halogen).135

8. AMINOAND OXONITRENES

In 1984, Sylwester and Dervan found that photolysis of carbamoyl azide 62 in a glass of 2-methyltetrahydrofuran (2MTHF) at 80 K led to the formation of a violet species showing a highly structured absorption band between 500 and 700 nm (vibrational progression of 1300 cm 1). The violet photoproduct is neither trans (lmax ¼ 386 nm) nor cis-diimide (lmax ¼ 260 nm), and was assigned to 1,1-diazene 63.136 Ab initio calculations predict H2NN to have a singlet ground state with a

¼ : ˚

short (r 1 25 A) N N bond, indicating that this species is best regarded as a 1,1-diazene rather than an aminonitrene. The generalized valence bond method indicates that the lowest triplet state is pyramidal and has less N N double-bond

Sylwester and Dervan’s assignment was supported by the observation that the violet photoproduct decomposes to N2 and H2 upon warming the glass or upon further irradiation of the glass. Furthermore, photolysis of 62 in argon at 10 K leads to the disappearance of the IR spectrum of the azide and the appearance of new bands at 2865, 2808, 2141, 1863, 1574, and 1003 cm 1. The 2141-cm 1 band is due to the formation of carbon monoxide. The band at 1574 is assigned to the 14N 14N stretch of 1,1-diazene. Photolysis of 15N labeled 62 led to a new vibration at 1548 cm 1 attributed to the 14N 15N stretch in good agreement with Hooke’s law.

Davis and Goddard137 calculated that the heat of formation (298 K) of singlet H2NN is 90.1 kcal/mol. This value is only 14 kcal/mol below that of molecular nitrogen and two hydrogen atoms, and is 54 kcal/mol above Hf(298 K) for trans-diimide determined experimentally by Willis et al.138 The relative energies of 1,1- and trans-1,2-diazene have been calculated by several methods; these studies have been reviewed by Parsons and Dykstra.139 It was found using the