7946
J. Phys. Chem. 1995, 99, 7946-7950
Spectroscopy and Kinetics of Triplet (Diphenoxyphosphory1)nitrene Michael HouserJ Shane KelleyJ Vincent Maloney,* Matthew Marlow2 Kathy Steininger,? and Hao Zhou? Department of Chemistry, Indiana University Purdue University Fort Wayne, Fort Wayne, Indiana 46805-1499 Received: November 15, 1994; In Final Form: January 26, 1995@
Triplet (diphenoxyphosphory1)nitrene (3) was studied by solution phase laser flash photolysis, low-temperature absorption spectroscopy, and low-temperature EPR spectroscopy. In ethanol, an absorption maximum at 345 nm was observed upon laser flash photolysis of diphenyl phosphorazidate (1) and assigned to the triplet nitrene (z = 3.8 f 0.6 ps). After irradiation of 1 in an EPA glass at 77 K, an absorption attributed to the triplet nitrene was observed at 336 nm. The zero field splitting parameters from the EPR spectrum of the nitrene in EPA glass at 77 K were IDlhcl = 1.5408 cm-' and IElhcl = 0.007 39 cm-I. Stronger transient absorptions observed in hydroxylic solvents support the proposal that hydrogen bonding catalyzes intersystem crossing in nitrenes. [(a) Schuster, G. B.; Platz, M. S . In Advances in Photochemistry; Volman, D., Hammond, G., Neckers, D., Eds.; John Wiley & Sons: New York, 1992; Vol. 17. (b) Poe, R.; Grayzar, J.; Young, M. J. T.; Leyva, E.; Schnapp, K. A,; Platz, M. S . J . Am. Chem. SOC.1991, 113, 3209. (c) Schnapp, K. A.; Platz, M. S . Bioconjugate Chem. 1993, 4 , 178.1
Introduction Attempts to observe triplet nitrenes through flash photolytic techniques have proceeded at an increasing pace for over forty years.la-d References 2, 3, and 4 also discuss the laser flash photolysis of nitrenes. Usually such efforts encounter two problems. In most cases, the singlet nitrene reacts so rapidly that intersystem crossing to the triplet becomes a minor pathway yielding insufficient concentrations for detection. When triplet nitrenes have been detected, their reactivity was so sluggish that often absolute rate constants for reactions such as hydrogen abstraction could not be obtained. Transient lifetimes were dominated by triplet nitrene dimerization or dependent on precursor c~ncentration.~.~ A survey of the literature reveals that laser flash photolytic techniques have been applied mostly to phenylnitrene and its derivative^.'"^^^^ In non-nucleophilic solvents, many classes of nitrenes undergo rearrangement of some In the case of arylnitrenes, rearrangement to the corresponding ketenimines occurs, and it is these species that are observed in the transient absorption ~ p e c t r a . ' ~ If . ~ the , ~ chosen solvent is sufficiently nucleophilic, the singlet nitrene is trapped to give an ylide product. Under such conditions, the ylide is observed and not the triplet nitrene in the transient absorption spectra. Hydroxylic solvents constitute an important exception discussed later. Substitution at the ortho positions of arylnitrenes has been shown to inhibit the rearrangement pathway with varying degrees of ~ u c c e s s . ~Nonetheless, ~~,~~ it was this tendency that led to confusion over the assignment of transients observed upon photolytic decomposition of aryl azides.Ia Only recently has the evidence obtained from product studies, low-temperature EPR, IR, absorbance, and luminescence spectra, and laser flash photolysis been reconciled in the case of phenylnitrene. Species previously thought to be arylnitrenes are more likely attributable to ketenimines. Two nitrenes that resist rearrangement that are not ortho substituted are (4-nitrophenyl)nitrene5 and (4-(dimethylamino)phenyl)nitrene.6 As yet, only a handful of triplet arylnitrenes have been observed. Of those reported, only a few had singlet states resistant to rearrangement.
' All coauthors are undergraduate research assistants. @Abstract published in Advance ACS Abstracts, April 15, 1995.
SCHEME 1 0
II
(Ph0)2P-N3
1
-
0
II
hv
(Ph0)2P-N3
-
0
II
(Ph0)2P-N3
*l
*3
I
1
2
3
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k.1 RH
0
0
II
(PhO)zP-NHR
0
0
II
(PhO)PP--'N'H
+
R*
4
I
RH
0
/I
(PhO)ZP-NH*
5
Although such rearrangement reactions are intriguing, it would be desirable to find a nitrene whose singlet state was resistant to this pathway and whose triplet state would be sufficiently reactive for the acquisition of absolute rate constants. This would allow the observation of structure reactivity relationships and the temperature dependence for such triplet nitrene reactions as hydrogen abstraction. Previous product studies by Breslow" and Maslaki2 have shown that the thermal and photochemical decomposition of dialkoxy- and bis(ary1oxy)phosphoryl azides to nitrenes and their subsequent reactions are straightforward. In hydrocarbons, Maslak has shown that photolysis of the diethyl phosphorazidate proceeds along the simple pathways outlined in Scheme 1 for diphenyl phosphorazidate. The azide is cleanly converted to singlet (diethoxyphosphoryl)nitrene, which is extremely reactive. In hydrocarbons, singlet insertion into the C-H bond proceeds at diffusion control. (Diethoxy- and (diphenoxyphosphory1)nitrene are reported to be two of the least selective nitrenes."-I3
0022-365419512099-7946$09.0010 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 20, 1995 7947
Triplet (Diphenoxyphosphory1)nitrene Isolable rearrangement products in hydrocarbons and alcohols were not observed in either case. With (diethoxyphosphory1)nitrene, high yields of nitrene-derived products were observed precluding a rearrangement pathway. Somewhat lower yields of nitrene-derived products were reported for (diphenoxyphosphoryl)nitrene.'Ib The possibility remains that some rearrangement product has formed but had not been isolated. This does not seem likely, since Breslow showed that [bis(o-isopropylphenoxy)phosphoryl]nitrene gave an 82% yield of nitrene-derived products with no evidence for any type of intramolecular reaction product upon photolysis of the corresponding azide in cyclohexane."" In each case, small amounts of diethyl or diphenyl phosphoramidate (5) were obtained. Upon sensitized photolysis of diethyl phosphorazidate, Maslak showed that the phosphoramidate product was solely attributable to the triplet nitrene.'2b Photoreduction by the triplet excited state azide and radical chain processes were ruled out. On the basis of these results, (diphenoxyphosphory1)nitrene fulfilled the first criterion for observation of a triplet nitrene through laser flash photolysis. The resistance to rearrangement and the isolation of triplet-derived products gave sufficient reason to believe the triplet could be observed. Unlike the arylnitrenes observed through flash photolysis, the triplet nitrene would not have the benefit of x stabilization of the nitrenic center by an aromatic ring. On the basis of the aforementioned product studies, the phosphoryl group may not provide much x stabilization. It should exhibit a higher reactivity that might allow the acquisition of absolute rate constants. If this is correct, then the second problem would be surmounted in observing triplet nitrenes through laser flash photolysis. (Diphenoxyphosphory1)nitrene appeared to be a good potential candidate for study.
Experimental Section Materials. Diphenyl phosphorazidate (1) (Aldrich) was either used without further purification or di~ti1led.l~ Ethanol (MCB Reagent) was distilled from Mg under nitrogen. Cyclohexane (Aldrich spectrophotometric grade) was distilled from Na under nitrogen. Tetrahydrofuran (THF) (EM LiChrosolv) and 2-methyltetrahydrofuran(2-MTHF) (Aldrich) were distilled from LAH under nitrogen. Distilled water was passed through a Bamstead NANOpure I1 filter. All other materials were used without further purification. Methanol (Aldrich) and acetonitrile (Burdick and Jackson Laboratories) were spectrograde. IR spectra were obtained with a Perkin Elmer 683 infrared spectrophotometer. 'H NMR were recorded on a Hitachi Perkin Elmer R-24B spectrometer (60 MHz). Absorption spectra were obtained on a Cary 1 spectrophotometer. Melting points were recorded on a Thomas Hoover melting point apparatus. NMR spectra for the methyl carbamate derivative of 7 were recorded on a Bruker AC200 and a Bruker AC250 at The Ohio State University. The mass spectrum was recorded on a VG 70-250s at the same location. Diphenyl Phosphorisocyanate (7). The isocyanate was prepared according to the method described by von GizyckiI5 and isolated by vacuum distillation. Bpo.2 124-126 OC.I6 IH NMR (60 MHz) (acetone-& 6): 7.20 (lOH, broad s). IR (neat, cm-I): 3070 (w), 2285 (vs, N=C=O), 1589 (s), 1302 (s), 1207 (s), 1182 (vs), 1163 (s), 1025 (s), 1010 (s), 965 (vs), 770 (s), 690 (s). Reaction of 7 with methanol gave the corresponding methyl carbamate. Mp 99.5-101.5 "C. IR (KBr pellet, cm-I): 3500 (w), 3115 (s), 2900 (m), 1750 (s), 1595 (m), 1475 (vs), 1433 (s), 1290 (m), 1200 (vs), 965 (s), 895 (m), 780 (s), 765 (s), 695 (m). 'H NMR (200 MHz) (CDCl3, 6): 8.12 (d, J = 13 Hz, 1 H), 7.25 (m, lOH), 3.73 (s, 3H). I3C NMR (200 MHz,
0.025 [
1
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Wavelength (nm) Figure 1. Transient absorption spectrum observed 500-540 ns after 266 nm laser flash photolysis of 1.86 x M diphenyl phosphorazidate (1) in ethanol (deoxygenated) at ambient temperature.
6): 153.20, 153.12, 150.00, 149.89, 129.68, 125.58, 125.55, 120.42, 120.32, 53.33,53.30. 31PNMR (250 MHz, 6): -13.0 (s). MS-EI: precise molecular weight calculated for Cl4Hl4N05P, 307.060 96; found, 307.060 82. Laser Flash Photolysis. Transient absorption spectra and lifetimes were acquired on a laser flash photolysis system consisting of a Continuum Surelite Nd-YAG laser with frequency quadrupling (266 nm, e 3 5 d,5 ns), a Photon Technology monochromator, a pulsed (100 ps) 75 W horizontal xenon lamp, a Hamamatsu R936 photomultiplier, a HewlettPackard 545 10A digitizing oscilloscope, and two Standard Research System DG535 pulse and delay generators for synchronizing the laser, shutters, and data acquisition. The system was interfaced with a Gateway 486 computer for data acquisition and analysis. Samples were irradiated in either suprasil quartz 1 cm path length static cells or flow cells. Solutions were deaerated with a stream of deoxygenated nitrogen passing through the static cells or the reservoir of a flow cell. Low-Temperature Absorption Spectra. Solutions were deaerated by four freeze/pump/thaw cycles in a suprasil quartz 1 cm path length cell. Absorption spectra at 77 K were obtained by placing the cell in a quartz dewar fitted with optical windows filled with liquid nitrogen. Spectra of the sample before and after irradiation (254 nm) in the dewar were then acquired using the Cary 1 spectrophotometer. Low-Temperature EPR Spectra. Solutions (0.1 M) of 1 were deaerated by bubbling nitrogen through them in 4 mm 0.d. suprasil quartz tubes. Samples were placed in a quartz optical dewar filled with liquid nitrogen and irradiated in a Rayonet style reactor. EPR spectra were obtained using either a Varian E-112 X-band spectrophotometeror a Bruker ESP 300 X-band spectrophotometer.
Results and Discussion Laser flash photolysis (LFP) (266 nm) of diphenyl phosphorazidate (1) in cyclohexane produced a transient spectrum with a weak absorption at 345 nm. Platz has suggested that alcohols catalyze intersystem crossing ST in Scheme 1) from singlet to triplet nitrenesla on the basis of enhanced yields of triplet (pentafluorophenyl)nitreneIb and (2,6-difluorophenyl)nitreneIC in methanol. A similar effect is observed upon LFP of 1 in ethanol, where a strong transient spectrum was obtained with maxima at 345 nm (see Figures 1 and 2). The transient signal was much weaker in diethyl ether and THF. In hindsight, a stronger transient signal for the triplet nitrene or a species
Houser et al.
7948 J. Phys. Chem., Vol. 99, No. 20, 1995 0.025 -Q,
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Figure 2. Transient absorption spectrum observed at various time M diphenyl windows after 266 nm laser flash photolysis of 1.86 x phosphorazidate (1)in ethanol (deoxygenated) at ambient temperature. TABLE 1: Lifetimes of the 345 nm Transient in Various Solvents solvent
7Y
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derived from the triplet nitrene in alcohols could have been predicted on the basis of Breslow's product studies.'Ib In cyclohexane, a 12% yield of diethyl phosphoramidate upon photolysis of diethyl phosphorazidate was observed. In tertbutyl alcohol, irradiation of the azide resulted in an increase in the yield of the phosphoramidate to 27%. The transient at 345 nm decays by first-order kinetics (z x 3.8 ps, see Table 1). LFP of 1 in 60% ethanol/40% water produces a similar spectrum. In each case, the transient signal appeared within the rise time of the laser. Irradiation (5 min, 254 nm) of 1 in an EPA ( 2 5 5 diethyl ether, 2-methylbutane, ethanol) glass at 77 K produced the spectrum in Figure 3 with an absorption maximum at 336 nm. The band persisted for 1 h at this temperature but disappeared upon annealing. Prolonged photolysis did not destroy the absorbing species. Similar results were observed in 2-MTHF. In 3-methylpentane, the azide was insoluble at 77 K and the matrix was cloudy. Very poor spectra exhibiting an absorption around 335 nm were obtained. The observed species is shifted to the red 20-45 nm from alkylnitrene~'~ but shifted to the blue relative to arylnitrenes by 30-40 0
It
C
'i:
EtO' 6
Irradiation (75 s, 254 nm) of 1 in an EPA glass at 77 K produced an EPR spectrum with two peaks separated by 222 G. Normally, only one peak is observable for triplet arylnitrenes, which is assigned to the overlap of the X2 and Y2 transitions.I8 If the E zero field splitting (ZFS) parameter is sufficiently large, these transitions will not be equivalent, and
300
400
500
600
700
800
Wavelength (nm) M diphenyl phosphorazidate (1) in EPA at 77 Figure 3. 1.86 x K: (a) before irradiation: (b) after irradiation for 5 min at 254 nm.
two peaks are observed. Carboethoxynitrene (6) displays this behavior, and its ZFS parameters are IDlhcl = 1.603 cm-' and (Elhcl = 0.0215 cm-I.l9 Using this assumption, the ZFS parameters of (diphenoxyphosphory1)nitrene (3) would be ID/ hcl = 1.548 cm-' and IElhcl = 0.007 39 cm-'. In the case of 6 , Wasserman argued that although the D value indicated that the n electron was highly localized, the large E value required some unsymmetrical delocalization onto the carbonyl. l9 The principal magnetic axis would no longer be aligned along the C-N bond, and the p orbitals of the triplet nitrene would no longer be equivalent. In the same vein, the n electron of 3 is delocalized onto the phosphoryl group but to a lesser extent, since the E value is less than half the value observed for 6 . Hyperfine coupling with the I4N nucleus such as that observed for e t h ~ l n i t r e n eand ' ~ ~(trimethylsilyl)nitrene20was not discemable in the 2-MTHF glass. Prolonged irradiation at 254 nm had no effect on the EPR signal intensity. Another species that could be conceivably responsible for the transient absorption spectrum observed in ethanol is the aminyl radical (4). The most convenient method for generating such radicals is to irradiate the corresponding amine in di-tertbutyl peroxide. The tert-butoxy radicals initially formed can abstract hydrogen from the amine to give the aminyl radical, which can be detected in a LFP experiment.*I Unfortunately, as has been noted, strong absorption by the peroxide below 350 nm limits the use of this method. A less satisfactory way to generate the radical is to directly photolyze the amine.22 LFP of 5 in ethanol produced the transient absorption spectrum in Figure 4. The shoulder at ~ 2 8 nm 0 has a second-order decay (2kk = (9.0 f 2.0) x lo6 cm s-l). Laser flash photolysis of a relatively low concentration (0.26 M) of tert-butyl peroxide with 6 x M 5 in tert-butyl alcohol in an attempt to minimize the absorption below 350 nm did not lead to observation of the growth of the aminyl radical (4). Instead, a weak spectrum similar to that observed upon direct LFP of 5 was observed. The species appeared within the rise time of the laser. Despite strong absorption by tert-butyl peroxide at 266 nm, it appears that some light was absorbed by 5, producing the same transient as that obtained in ethanol by direct photolysis. Irradiation of 5 (5 min, 254 nm) in an EPA glass at 77 K resulted in the spectrum shown in Figure 5, in which there is a shoulder at 280 nm on the precursor absorption. If this transient is the aminyl radical, then the 345 nm transient in ethanol may be assignable to the triplet nitrene.
J. Phys. Chem., Vol. 99, No. 20, 1995 7949
Triplet (Diphenoxyphosphory1)nitrene 0.055'
,G0.050.
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Power (mJ) Figure 6. Plot of transient absorption at 340 nm vs laser power (mJ) M diphenyl phosphorazidate (1) upon flash photolysis of 1.86 x in ethanol (deoxygenated) at ambient temperature.
nm (z x 6.8 ps). When 1 was flash photolyzed in acetonitrile, the familiar absorption at 340 nm was observed, but now the transient did not decay appreciably within 50 ps. Two explanations can be advanced. One possibility is that irradiation of 7 does not produce the nitrene and that irradiation of 1 in acetonitrile yields the singlet nitrene which reacts immediately to give a long-lived ylide 8. Support for this possibility comes
ry
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.
Wavelength (nm) diphenyl phosphoramidate (5) in EPA at 77 K: (a) before irradiation; (b) after irradiation for 5 min at 254 nm.
Figure 5. 2.0 x
One issue that remained to be addressed was whether the transient in ethanol was the result of multiphotonic processes. At this point, the experimental evidence could be interpreted as indicating that the 345 nm transient is 3. However, if it is due to a multiphotonic process, then it is unlikely that the transient is the nitrene, and one could postulate that it was even a rearrangement product accessible through multiphoton absorption. A linear relationship between transient signal intensity and the energy of the laser pulse resolves this issue in favor of a monophotonic process. In Figure 6, a plot of transient absorption versus laser energy/pulse for LFP of 1 in ethanol exhibits a linear relationship. The plot of ln(transient absorption) versus ln(energy/pulse) has a slope of 0.66, which approaches the expected value of 1 for a monophotonic process. A preliminary attempt to generate 3 from an alternate precursor led to inconclusive results. Photolysis of isocyanates can cause extrusion of carbon monoxide to give n i t r e n e ~LFP .~~ of diphenyl phosphorisocyanate (7)in acetonitrile produced a 0
II
(PhO)ZP-N=C
=O
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0
-
+
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from the fact that the transient observed in diethyl ether also does not decay within 50 p s . Conceivably, this would be attributable to ylide 9. Ylides have been observed for phenylnitrene and (pentafluoropheny1)nitrene.I" Although ylides with ethers have been reported, none have been observed with acetonitrile. An alternative possibility is that the nitrene is formed upon LFP of 7 and that the long-lived transient is some photoadduct between the excited state azide and acetonitrile. Clearly, product studies of the photolysates of 1 and 7 in acetonitrile are required to resolve this issue. If the 345 nm transient is assigned to the triplet nitrene 3, then the question whether it is sufficiently reactive for the acquisition of bimolecular rate constants can be addressed. Table 1 lists the lifetimes of the 345 nm transient under various conditions. Although the transient decay is first order and unaffected by precursor concentration over the limited range 0.7-7 mM, the lifetimes in ethanol and methanol do not appear to be attributable to simple hydrogen abstraction by the triplet nitrene. The lifetimes of the transient in alcohols and perdeuterated alcohols are essentially equivalent within experimental error. No kinetic isotope effect is observed as might be expected for such a reaction. Schuster found that formation of 4-nitroaniline from (4-nitropheny1)nitreneand Nfl-dimethyl-tert-butylamine had a surprisingly low kinetic isotope effect (KIE).5 It was postulated that a rate-limiting electron transfer from the amine to the highly electron deficient nitrene to produce a radical ion intermediate ultimately led to the aniline product. Small or negligible isotope effects would be expected from such a mechanism. This possibility is currently under investigation. Oxygen has no effect on the lifetimes, as can be seen from the values for air-saturated ethanol and 60% ethanol/40% water. This is in accord with previous observations of triplet nitrenes
Houser et al.
7950 J. Phys. Chem., Vol. 99, No. 20, 1995 with oxygen. Although they do react,24the reactions appear to be surprisingly Liang and Schuster reported that the reaction of triplet (4-nitropheny1)nitrene with 0 2 had a bimolecular rate constant less than 2 x lo5 M-' s-l. If 3 reacts at a comparable rate, then the presence of oxygen would have no effect on the observed lifetimes in Table 1. Transients in cyclohexane were weak, resulting in such poor kinetic data that it was not included in Table 1. However, the lifetime of the 345 nm transient in cyclohexane is roughly 90 ps. In 60% ethanol/40% water, the lifetime is somewhat shorter, indicating that solvent polarity has some effect on the transient lifetimes. The addition of triethylsilane, 1,4-~yclohexadiene,and isoprene did not affect the transient lifetime in ethanol. At high concentrations of these compounds, the transient signal became weaker. Presumably the lower alcohol concentration led to weaker signals for the same reason that weak transients were observed in cyclohexane, diethyl ether, and THF. The 345 nm transient can be tentatively assigned to the triplet nitrene 3. Given that rearrangement products have not been observed for phosphorylnitrenes and that most of the products can be attributed to bimolecular reactions of either state, the possible intermediates responsible for the transient in cyclohexane or ethanol are limited to the triplet nitrene or aminyl radical. This is supported by the linear transient absorption to laser energy/pulse relationship, which indicates that the observed transient is the result of a monophotonic process. Since they were generated under the same conditions, one could assign the triplet nitrene observed by EPR to the 336 nm absorption in the EPA glass. On the basis of the similarities of the absorption spectra, the 345 nm transient observed upon ambient temperature LFP of 1 can also be assigned to 3. The case of phenylnitrene should provide a sufficient warning of the limitations of these two statement^.'^,^ The species observed in the EPR spectrum was a triplet nitrene, but the species observed in the matrix absorption and IR experiments was the ketenimine rearrangement product which was a product of secondary photolysis at low temperature and the singlet rearrangement product at ambient temperature. The contention that rearrangements do not occur for these nitrenes lessens this possibility. Furthermore, the triplet EPR signal was unaffected by prolonged irradiation.
Conclusions The transient absorption produced upon LFP of 1 in ethanol and the species observed in the matrix absorption and EPR spectra are tentatively assigned to the triplet nitrene 3. EPR and absorption spectra suggest that delocalization of the nitrenic electrons is limited and that the character of 3 has more in common with akylnitrenes and acylnitrenes than arylnitrenes. The transient signal enhancement observed upon changing to hydroxylic solvents lends support to the contention that hydrogen bonding catalyzes intersystem crossing in nitrenes although these results do not indicate why this might be true. Acknowledgment. This work was supported by an award from the Research Corporation (C-3258), for which we are grateful. The laser flash photolysis system was funded in part by the National Science Foundation, which we gratefully acknowledge. We also thank Professor Robert Berger of Indiana University Purdue University Ft. Wayne for use of the Cary 1 spectrophotometer, Professor Matthew Platz of The Ohio State University for use of the Varian E l l 2 X band EPR spectrophotometer, and Professor Eric Findsen at the University of Toledo for use of the Bruker ESP 300 EPR spectrophotometer. We also thank Professor Platz for the Nh4R and mass spectra of the carbamate and for useful suggestions.
References and Notes (1) (a) Schuster, G. B.; Platz, M. S. In Advances in Photochemistry; Volman, D., Hammond, G., Neckers, D., Eds.; John Wiley & Sons: New York, 1992; Vol. 17. (b) Poe, R.; Grayzar, J.; Young, M. J. T.; Leyva, E.; Schnapp, K. A,; Platz, M. S. J . Am. Chem. SOC.1991, 113, 3209. (c) Schnapp, K. A,; Platz, M. S. Bioconjugate Chem. 1993, 4, 178. (2) Thrush, B. A. Proc. R . SOC.London Ser. A 1956, 253, 143. (3) Platz, M. S. In Azides and Nitrenes; Scriven, E. F. V., Ed.; Academic Press: Orlando, FL, 1984; Chapter 7 and references therein. (4) Platz, M. S.; Maloney, V. M. In Kinetics and Spectroscopy of Carbenes and Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990; Chapter 8 and references therein. ( 5 ) Schuster, G. B.; Liang, T.-Y. J . Am. Chem. SOC.1987, 109,7803. (6) Kobayashi, T.; Ohtani, H.; Suzuki, K.; Yamaoka, T. J . Phys. Chem. 1985, 89, 776. (7) Scriven, E. F. V., Ed. Azides and Nitrenes; Academic Press: Orlando, FL, 1984. (8) Lwowski, W., Ed. 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