Reactive Intermediate Chemistry

546 NITRENES

self-consistent electron pairs method with the TZ þ 2p basis set spanning a space up to 176,000 configurations that 1,1-diazene 63 is 24.5 kcal/mol higher in energy than trans-1,2-diazene, but that 63 is protected from decay to trans-diimide by a barrier of 58.1 kcal/mol. Kemper and Buck140 also studied this rearrangement and concluded that bimolecular hydrogen exchange is much easier than a unimolecular 1,2-hydrogen shift. These authors found many similarities between diimide chemistry and formaldehyde photochemistry. The chemistry of aminonitrenes has been reviewed by Lemal141 and by Ioffe and Kuznetsov.142

8.1. Stable Aminonitrenes

The Dervan group discovered that addition of tert-butyl hypochlorite to a solution of 1-amino-2,2,6,6-tetramethylpiperidine and triethylamine in ether at 78 C produces triethylammonium chloride as an insoluble white precipitate and an intense

purple solution. The purple coloration, which is stable for hours at 78 C, fades in minutes at 0 C and was assigned to the 1,1-diazene 64.143,144

Examination of the optical spectrum of the filtered purple solution gave a structured absorption band with maxima at 514 and 543 nm. This position is remarkably close (566 nm) to the n–p* electronic transition predicted by Davis and Goddard137 for the parent system H2N N. As expected for an n–p* transition, the position of the absorption maximum is solvent dependent. In dichloromethane solution, lmax is 541 nm, in 2-propanol it is 526 nm. The blue shift of 15 nm is completely consistent with the n–p* absorptions of isoelectronic carbonyl compounds.

The identity of 64 was established by IR spectroscopy. At 78 C, the IR spectrum has a strong band at 1595 cm 1 that disappears on warming to 25 C. The stretching frequency is similar to that of trans-azo compounds (1576 cm 1), which can, however, only be observed by Raman spectroscopy. Upon 15N labeling of the amino group in the piperidine precursor, the band of the photoproduct shifts to 1569 cm 1, in agreement with a Hooke’s law calculation for 64.

Schultz, Dervan, and co-workers145 subsequently reported the synthesis and characterization of the five-membered ring analogue 65. As before, the oxidation of the appropriate hydrazine at 78 C gave a clear, colored solution of the aminonitrene. This 1,1-diazene absorbs at 497 nm (in CH2Cl2) and 487 nm (in 2-propanol). The IR spectrum shows a strong absorption at 1638 cm 1 that disappears on warming to 25 C. The 15N isotopomer has a vibration at 1612 cm 1, a shift of 26 cm 1.

Both aminonitrenes 64 and 65 were also studied by low-temperature proton nuclear magnetic resonance (1H NMR) spectroscopy.145 The spectrum of the

PHOSPHINIDINES 547

chromatographed six-membered ring diazene 64 at 78 C showed signals due to a tetrazene in addition to absorptions at d 1.15 and 2.15. Warming the samples resulted in the disappearance of the diazene bands with the concomitant increase of the resonances due to the tetrazene. The same experiment with the five-mem- bered ring compound 65 gave resonances at d 1.05 and 2.32 at 78 C. That one can observe an NMR spectrum is convincing evidence that the singlet is the ground state. The NMR of a triplet nitrene most likely can not be observed because of very rapid spin relaxation.

8.2. Oxonitrenes

Oxygen substituted nitrenes have received much less study than aminonitrenes, but there have been some noteworthy recent developments. Toscano and co-workers146 studied the photochemistry of diazenium diolates. The quantum yield of photodegradation was 0.10.

The presence of oxygen significantly altered the distribution of products formed. A major new product was the nitrate ester. Under argon, the yield of benzaldoxime (R ¼ phenyl) was 66%, but in the presence of oxygen, the yield of this product goes to zero. The data indicated the following mechanism involving benzyloxynitrene:

The putative benzyloxynitrene can be intercepted with tetramethylethylene to form the expected aziridine. Time-resolved IR spectroscopy was unable to detect the O-nitrene, but detected the presence of PhCH2N O formed with a time constant of 250 ns after the laser pulse. Thus, the lifetime of benzyloxynitrene is also

250 ns. The TRIR spectroscopic studies indicated that benzyloxynitrene reacts with oxygen a rate constant of 109–10 M 1s 1. This value strongly suggests that

the O-nitrene, in contrast to the N-nitrenes has a triplet ground state.

9. PHOSPHINIDINES

The ground state of PH has triplet multiplicity, as does imidogen, but the singlet–triplet splitting of the phosphorus compound is only 22 kcal/mol,147

548 NITRENES

down 14 kcal/mol from the nitrogen analogue.11,12 The valence electrons of PH reside in 3n levels, which are more diffuse than the 2n level orbitals in NH. Thus, the electron–electron repulsion in singlet–PH is reduced, and this lowers the energy of singlet–PH relative to triplet–PH.

The openand closed-shell configurations of singlet–NH and PH are components of a doubly degenerate state.10 As discussed in Sections 7.4 and 7.5, a vinyl group lowers the symmetry and removes the degeneracy67 and the openand closed-shell singlet states of vinylnitrene lie 15 and 40 kcal/mol, respectively, above the ground triplet state.67 The open-shell singlet resembles a 1,3-biradical. The delocalization of one of the unpaired electrons reduces the electron–electron repulsion and stabilizes this configuration. The closed-shell configuration of singlet vinylnitrene is zwitterionic and does not enjoy decreased electron–electron repulsion.

In vinylphosphinidine, the openand closed-shell singlets lie 17 and 23 kcal/mol above the ground triplet state. The energies of the two singlet states of vinylphosphinidine are comparable because electron–electron repulsion in the 3p orbitals is reduced and because the overlap of 2p and 3p orbitals of carbon and phosphorus is poor and the resulting carbon–phosphorus double bond is relatively weak.67

Mathey148 discovered that flash vacuum pyrolysis of vinylphosphirane 66 at 700 C produced phosphapropyne 68, presumably by rearrangement of vinylphosphinidine 67.

This rearrangement is unusual because hydrogen usually migrates toward electrondeficient centers as calculated for vinylcarbene.

H H H

C H C C C

H C H H

H

This observation led Berger et al.149 to investigate the C2H3P singlet surface by computational methods. Cyclization of vinylphosphinidine to a three-membered ring species and isomerization to a phosphaallene are the kinetic products of rearrangement. The global minimum on the surface is the phosphapropyne. The barrier to phosphapropyne formation is 6.9 kcal/mol greater than the barrier to isomerization of the vinylphosphinidine to the phosphapropyne, which is 20 kcal/mol.

In phenylnitrene, the open-shell singlet lies below the closed-shell singlet, but the opposite trend is observed in singlet phenylphosphinidine. In phenylphosphinidine, the closed-shell singlet configuration has been found by two groups to be 4 kcal/mol lower in energy than the open-shell singlet state.150 It is predicted that the electronic structure of phenylphosphinidine is more reminiscent of singlet phenylcarbene than is singlet phenylnitrene! However, the most recent calculations

PHOSPHINIDINES 549

of Galbraith et al.151 place the open-shell singlet of phenylphosphinidine slightly below the closed-shell singlet.

HP

H3C C P

(0)

The latter group also studied the rearrangements of open-shell singlet phenyland vinylphosphinidine at a common theoretical level – (8/8)CASPT2. The surfaces for X ¼ P and N are quite different. Ring closure of X ¼ P is slightly more endothermic and less rapid than that of phenylnitrene. The major difference is that whereas ring opening of benzazirine to azacycloheptatetraene is both fast and exothermic, the analogous process for X ¼ P is very endothermic and very slow. Theory predicts that singlet phosphinidine may reversibly cyclize to 69. This species should be trappable and detectable, but it will not expand to the seven-membered ring species 70. The difference in these systems can be attributed to the difference in the strengths of the s bonds formed to nitrogen and phosphorus in these reactions.

X = N

X 8.6

X = N

6.8

4.9

X = N 1.6

1A 2- 1

0

X = N

-1.3

550 NITRENES

There is chemical evidence for the formation of phenylphosphinidine in solution.152

Hammond et al.153 reported data that indicate that phenylphosphinidine is relatively long lived and likely has a triplet ground state.

Phospacyclopropanes appear to be excellent precursors but Mathey148 has pointed out that one can write reasonable mechanisms to the formation of these products that do not involve the intermediacy of arylphosphinidines.

Spectacular proof of the existence of arylphosphinidines has been provided by Gaspar and co-workers154 and Weissman et al.155 A frozen solution of 71 in methyl-

cyclohexane glass was exposed to 254-nm radiation. The glass turned yellow upon photolysis. Thawing the glass led to the formation of phosphine 72 in the absence of trap, and to the formation of 73 in the presence of 3-hexyne. When the frozen, previously irradiated sample was placed in an EPR spectrometer a triplet signal at

11,492 G was observed with jD=hcj ¼ 3:521 cm 1. This value is much larger than the jD=hcj value of phenylnitrene of 1 cm 1. This difference is most likely the result of a heavy atom effect of phosphorus, relative to nitrogen, on the secondorder spin–orbit contribution.155

73

Recently, Protasiewicz and co-workers156 reported a new precursor of arylphosphinidines.

Ar = i -Pr

i-Pr

10.CONCLUSION AND OUTLOOK

In recent years, LFP studies of nitrenes by nanosecond time-resolved UV–vis spectroscopy, working closely with theoretical calculations, have provided a comprehensive understanding of the chemistry, kinetics, and spectroscopy of nitrenes. The nuances of arylnitrenes are now very well understood and acylnitrenes are becoming well understood. It can be safely predicted that in the coming years time-resolved IR spectroscopy of acylnitrenes, benzazirines and ketenimines, and matrix spectroscopy of these species will, in concert with theory, provide a deeper appreciation of the properties of these intermediates. There is no doubt that these studies will assist synthetic organic chemists, biochemists performing photoaffinity labeling studies, and materials scientists seeking to functionalize polymer surfaces.

552 NITRENES

Important unresolved questions remain to be answered. For example, are singlet alkyland vinylnitrenes true intermediates with finite lifetimes or do they correspond to nonstationary points on the potential energy surface? Photolysis and pyrolysis of azides occasionally leads to very different distributions of stable products. Is the inability to trap singlet nitrenes upon photolysis of certain alkyl azides (in

contrast to their pyrolysis) due to a conical intersection in the azide excited state that leads directly to product?157,158a Is the failure to trap an acylnitrene upon pyr-

olysis of pivaloyl azide (in contrast to its photolysis) due to a dynamical effect, as discussed in Chapter 21 in this volume?158b Azides absorb strongly in the UV and typically very weakly in the visible region. Exposure of an azide to UV light pro-

vides this precursor with 100 kcal/mol of energy, far more energy than is necessary to break the N N bond of 35 kcal/mol.31,32 This difference leads one to

wonder whether electronically or vibrationally excited nitrenes are produced upon UV photolysis and whether some chemistry long attributed to relaxed singlet nitrene species may actually originate from excited states. Hence, it seems safe to predict that future research will make good use of femtosecond spectroscopy. Thus, more work remains to be done and the next 10 years of research in nitrene chemistry should be just as exciting as the recent past.

ACKNOWLEDGMENTS

The author is pleased to acknowledge the outstanding contributions of graduate and postdoctoral students too numerous to mention and senior collaborators such as Nina Gritsan, Christpher Hadad, Thomas Bally, Jakob Wirz, and Wes Borden. Thanks go to Professor Bally, Professor Borden, Professor Cramer, Professor Smith, and Professor Sundberg, and to Eric Tippmann and Meng-Lin Tsao for a critical reading of the manuscript. The generous support of research in the Platz laboratory by the National Science Foundation, the National Institutes of Health, and the Soros Foundation are gratefully acknowledged.

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N. P. Gritsan and M. S. Platz, Adv. Phys. Org. Chem. 2001, 36, 255.

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