Direct Observation of Singlet Phenylnitrene and Measurement of Its Rate of Rearrangement

Full text of DOI:10.1021/ja963753n

J. Am. Chem. Soc. 1997, 119, 5059-5060
5059
Direct Observation of Singlet Phenylnitrene and
Measurement of Its Rate of Rearrangement
Nina P. Gritsan,*, Tetsuro Yuzawa, and
†,‡
†
Matthew S. Platz*,†
Newman and Wolfrom Laboratory of Chemistry
The Ohio State UniVersity, 100 West 18th AVenue
Columbus, Ohio 43210
Institute of Chemical Kinetics and Combustion
6
30090 NoVosibirsk, Russia
ReceiVed October 28, 1996
1
Aromatic azides are widely used in industry as photoresists
2
and in biochemistry as photoaffinity labeling reagents. Their
photochemistry has been called “wonderfully complex” but has
recently been unraveled.3
Photolysis of phenylazide (1) releases singlet phenylnitrene
(
1
2
2S) which in solution phase (T > 165 K) rapidly rearranges to
,2-azacycloheptatetraene 3.3 At temperatures below ≈165 K,
4
,5
S preferentially relaxes to triplet phenylnitrene 2T instead.
Figure 1. Transient spectrum observed (top) immediately following
the laser pulse and (bottom) 150 ns later. The sample was phenyl azide
1
in pentane at 233 K using 266 nm, 10 mJ, 150 ps/pulse excitation.
4a
In 1984 we assumed that ring expansion of 2S had a normal
pre-exponential factor of 101
2-14
s
-1
and that intersystem
crossing (ISC) of 2S had zero activation energy (Ea) and a rate
similar to that of aryl carbenes (109
-10
s ). On this basis it
-1 8
was possible to deduce that Ea for rearrangement was 2-4 kcal/
4
a
mol and that the lifetime of 2S was 10-100 ps at 298 K. In
subsequent years triplet phenyl nitrene 2T and cyclic ketenimine
Figure 2. The variation in optical density as a function of time, at
different wavelengths, following excitation (266 nm) of phenyl azide
at 225 K.
3
were thoroughly characterized by matrix UV-vis and IR
9
spectroscopy, and 3 was studied in solution by time resolved
UV-vis and IR techniques.10
Herein we are pleased to report the first spectroscopic
1
1
†
‡
detection of singlet phenylnitrene, absolute measurement of
its lifetime in solution, and the Arrhenius parameters describing
The Ohio State University.
Institute of Chemical Kinetics and Combustion.
1) Breslow, D. S. Azides and Nitrenes; Scriven, E. F. V., Ed.; Academic
(
5
its rearrangement. We find that the barrier to rearrangement
Press: Orlando, FL, 1984; p 491.
is larger than previously deduced because ISC is much slower
than originally anticipated.
(2) a) Bayley, H. Photogenerated Reagents in Biochemistry and Molec-
ular Biology; Elsevier Press: New York, New York, 1983. b) Fleming, S.
A. Tetrahedron 1995, 51, 12479.
Phenyl azide 1 was studied by laser flash photolysis (LFP,
Nd-YAG, 150 ps, 266 nm, 10 mJ) in pentane. At room
temperature a transient absorbing at 350 nm is observed. The
formation and decay of this species, at ambient temperature, is
faster than the time resolution of the spectrometer (1 -2 ns).
At lower temperatures (<280 K) the formation of the transient
remains “instantaneous”, but its decay can be resolved. At low
temperatures the decay of the 350 nm absorbing transient is
(
3) (a) Schuster, G. B.; Platz, M. S. AdV. Photochem. 1992, 17, 69 and
references therein. (b) Platz, M. S. Acc. Chem. Res. 1995, 28, 487. (c)
Schrock, A. K.; Schuster, G. B. Am. Chem. Soc. 1984, 106, 5228.
(4) (a) Leyva, E.; Platz, M. S.; Persy, G.; Wirz, J. J. Am. Chem. Soc.
1
986, 108, 3783. (b) Marcinek, A.; Leyva, E.; Whitt, D.; Platz, M. S. J.
Am. Chem. Soc. 1993, 115, 8609.
5) Recent calculations indicate that ring expansion is a two-step
process: Karney, W. L.; Borden, W. T. J. Am. Chem. Soc. 1997, 119, 1378.
(
(
6) (a) Vander Stouw, G. G. Ph. D. Thesis, The Ohio State University,
1
1
964. (b) Joines, R. C.; Turner, A. B.; Jones, W. M. J. Am. Chem. Soc.
969, 91, 7754. (c) Gaspar, P. P.; Hsu, J.-P.; Chari, S.; Jones, M. Jr.
accompanied by the formation of ketenimine 3 which absorbs
Tetrahedron 1985, 41, 1479 and references therein.
9,10
broadly above 300 nm
(Figures 1 and 2).
(
7) (a) Kim, S. J.; Hamilton, T. P.; Schaefer, H. F. III. J. Am. Chem.
The decay of the 350 nm transient was monitored as a
function of temperature. Its decay was well fit to an exponential
function to yield an observed rate constant. An Arrhenius
treatment of these rate constants is given in Figure 3 and yields
Soc. 1992, 114, 8698. (b) Hrovat, D. A.; Waali, E. E.; Borden, W. T. J.
Am. Chem. Soc. 1992, 114, 8699. (c) Travers, M. J.; Cowles, D. C.; Clifford,
E. P.; Ellison, G. B. J. Am. Chem. Soc. 1992, 114, 8699.
(8) (a) Grasse, P. B.; Brauer, B.-E.; Zupancic, J. J.; Kaufmann, K. J.;
Schuster, G. B. J. Am. Chem. Soc. 1983, 105, 6833. (b) Sitzmann, E. V.;
Langan, J.; Eisenthal, K. B.; J. Am. Chem. Soc. 1994, 106, 1868. (c) Platz,
M. S.; Maloney, V. M. Kinetics and Spectroscopy of Carbenes and
Biradicals; Platz, M. S., Ed.; Plenum: New York, New York, 1990; p 239
and references therein.
1
3.6(0.4 -1
Ea ) 6.2 ( 0.4 kcal/mol and A ) 10
s
in pentane.
LFP of phenylisocyanate, in pentane, also produced a rapidly
decaying transient which absorbed at 350 nm. Its decay is
(
9) (a) Reiser, A.; Frazer, V. Nature (London) 1965, 208, 682. (b) Reiser,
A.; Wagner, H. M.; Marley, R.; Bowes, G. Trans. Faraday Soc. 1967, 63,
(11) There are only a few previous reports of the direct detection of
singlet aryl nitrenes. The lifetimes of singlet 4-(dimethyamino)phenylnitrene
has been reported (Kobayashi, T.; Ohtuni, H.; Suzuki, K.; Yamaoka, T. J.
Phys. Chem. 1985, 89, 776) as has that of 1-pyrenylnitrene (Sumitani, M.;
Nagakura, S.; Yoshihara, K. Bull. Chem. Soc. Jpn. 1976, 97, 12674).
2
403. (c) Chapman, O. L.; LeRoux, J.-P. J. Am. Chem. Soc. 1978, 100,
82. (d) Hayes, J. C.; Sheridan, R. S. J. Am. Chem. Soc. 1990, 112, 5879.
2
(
10) Li, Y.-Z.; Kirby, J. P.; George, M. W.; Poliakoff, M.; Schuster, G.
B. J. Am. Chem. Soc. 1988, 110, 8092.
S0002-7863(96)03753-5 CCC: $14.00 © 1997 American Chemical Society

5060 J. Am. Chem. Soc., Vol. 119, No. 21, 1997
Communications to the Editor
Singlet phenylnitrene 2S is known to react with an external
1
4
trap, the proton. Indeed, H2SO4 shortens the lifetime of 2S
in acetonitrile at 243 K. However, we cannot obtain the absolute
rate constant of this process because of the very short lifetime
of 2S in acetonitrile and the overlapping absorption of the
1
4
reaction product, phenylnitrenium ion 6.
The assignment of the 350 nm transient to singlet nitrene 2S
is further supported by the Arrhenius parameters and the position
of the absorption band itself. The Arrhenius parameters
correctly model the known temperature dependence of the
4a
photochemistry of phenylazide. Previous work demonstrated
that the rate of ISC (2S f 2T) becomes equal to that of nitrene
rearrangement at 165 K. The deduced rate of rearrangement,
Figure 3. Temperature dependence of the rate constant of the
rearrangement of singlet phenylnitrene in pentane. Fitting for the data
5
-1
2
.5 × 10 s , at 165 K is comparable to the temperature
1
3.6(0.4
below 0° yields kobs ) 10
exp (-(6200 ( 400)/RT). The decay
6 7
independent rate of ISC determined for 4a,b (2.2 × 10 -10
is measured over a temperature range in which rearrangement is the
only significant decay route of singlet nitrene 2S.
-1 12,15
s ).
Furthermore, the Arrhenius parameters predict that
4
the lifetime of 2S in pentane at 298 K will be 0.1-1.0 ns in
18,19
13.4(1.0
s-1 and Ea
good agreement with the findings of Wirz et al.
theory.
and of
associated with Arrhenius parameters A ) 10
6 ( 1 kcal/mol. The transient signal observed with
5
)
Finally, we note that a λmax of 350 nm of 2S is unsurprising
phenylisocyanate is weaker than that obtained from phenylazide,
hence the experimental error in the Arrhenius parameters is
somewhat larger. There is no doubt, however, that the same
species is formed from both precursors. Thus, the 350 nm
absorbing transient cannot be an excited state of the precursor
but is instead a reactive intermediate which is attributed to
singlet phenylnitrene 2S. Nitrene 2S is expected to be in its
lowest electronic state because it is unlikely that an excited state
of this species will have a lifetime of many ns in solution and
must overcome an enthalpic barrier to relax.
7
for several reasons. Singlet phenylnitrene is predicted to have
an open shell configuration as does the corresponding triplet.
Thus, these two species should have comparable electronic
1
2
spectra. Indeed, this is the case with 4a,b both of which
absorb near 350 nm. The spectrum of triplet phenylnitrene 2T
4
a,20
has been analyzed.
In the UV region between 300 and 400
nm triplet phenyl nitrene has several π, π* transitions related
21
to those of anilino radical 7 but has, in addition, a characteristic
transition on nitrogen in which an electron is promoted from
the nonbonding doubly occupied sp orbital into a singly
occupied p orbital of the nitrene nitrogen. This transition is
This spectroscopic assignment follows closely on our work
with perfluorinated singlet nitrenes 4a,b.12
3
3
-
related to the well-known degenerate A ΠisX Σ transition
3
of NH at 336 nm and the spectra of triplet methylnitrene 8
and triplet 1-norbornylnitrene 9 which have λmax values of 315
2
3,24
and 298 nm, respectively.
Fluorination is known to lengthen the lifetime of singlet aryl
nitrenes which allows their facile capture with a variety of
Additional studies of the rearrangement of various substituted
singlet aryl nitrenes are in progress and will be reported in due
course.
13
reagents. Trapping with pyridine produces isolable ylides 5a,b
which have intense absorptions near 390-400 nm. Thus, in
these cases it is possible to correlate the decay of the singlet
Acknowledgment. Support of this work by the NSF (CHE-
8814950) and the National Research Council (for a travel grant) is
gratefully acknowledged. The authors are indebted to the groups of
Borden and Karney and of Born, Burda, Senn, and Wirz for preprints
of their manuscripts. The authors also wish to thank Drs. McClelland
and Ellison for valuable insights.
nitrene with the formation of an ylide12 or with the formation
of ketenimine.
(12) (a) Gritsan, N; Zhai, H. B.; Yuzawa, T.; Platz, M. S. J. Phys. Chem.
In press. See, also: Marcinek, A.; Platz, M. S.; Chan, S. Y.; Floresca, R.;
Rajagopalan, K.; Golinski, M.; Watt, D. S. J. Phys. Chem. 1994, 98, 412.
and Marcinek, A.; Platz, M. S. J. Phys. Chem. 1993, 97, 12674.
JA963753N
(13) (a) Poe, R.; Schnapp, K.; Young, M. J. T.; Grayzar, J.; Platz, M. S.
J. Am. Chem. Soc. 1992, 114, 5054. (b) Karney, W. L.; Borden, W. T. J.
Am. Chem. Soc. 1997, 119, 3347.
(17) Salem, L.; Rowland, C. Angew Chem. Int. Ed. Engl. 1972, 11, 92.
(18) Born, R.; Burda, C.; Senn, P.; Wirz, T. J. Am. Chem. Soc. 1997,
119, 5061-5062.
(14) McClelland, R. A.; Kahley, M. J.; Davidse, P. A.; Hadzrialic, A. J.
5
Am. Chem. Soc. 1996, 118, 4794.
15) The ISC rates of arylnitrenes (10
magnitude slower than the analogous process in arylcarbenes (10
This is a consequence of the larger singlet-triplet gap of phenylnitrene
(19) The recent calculations of Borden and Karney lead us to believe
5
.4-6.2
s-1) are many orders of
(
that the 6 kcal/mol barrier to disappearance of singlet nitrene 2S we have
determined should be associated with the 6 kcal/mol barrier to cyclization
of the nitrene, predicted by theory.
9
-10 -1
8
7
s
).
1
6
relative to phenylcarbene. It also reflects the greater spin orbit coupling
(20) Berry, R. S. In Nitrenes; Lwowski, W., Ed.; Wiley Interscience:
New York, N.Y., 1970 Chapter 2, p 13.
(21) Fairchild, P. W.; Smith, G. P.; Crosley, D. R.; Jeffries, J. B. Chem.
Phys. Lett. 1984, 107, 181.
(22) Carrick, P. G.; Engelking, P. C. J. Chem. Phys. 1984, 81, 1661.
(23) Radziszewski, J. G.; Downing, J. W.; Wentrup, C.; Kaszynski, P.;
Jawdosiuk, M.; Kovacic, P.; Michl, J. J. Am. Chem. Soc. 1985, 107, 2799.
(24) Leyva, E.; Platz, M. S.; Niu, B. J. Phys. Chem. 1987, 91, 2293.
16
of closed shell singlet states (carbenes) than is present in open shell singlet
7
17
species (phenylnitrene) with the corresponding triplet states.
(
16) (a) Matzinger, S.; Bally, T.; Patterson, E. V.; McMahon, R. J. J.
Am. Chem. Soc. 1996, 118, 1535. (b) Schreiner, P.; Karney, W.; Schleyer,
P. V. R.; Borden, W. T.; Hamilton, T.; Schaefer, H. F. III. J. Org. Chem.
1
996, 61, 7030. (c) Wong, M. W.; Wentrup, C. J. Org. Chem. 1996, 61,
7
022.