1952 J. Am. Chem. Soc., Vol. 123, No. 9, 2001
Gritsan et al.
high dilution.5 In solutions of moderate concentration, keten-
imine 4a reacts with phenyl azide to form a polymeric tar.6
As mentioned previously, singlet phenylnitrene rearranges in
solution at ambient temperature to benzazirine. The rate of this
process decreases as the temperature decreases. At ∼180 K the
rate constant for rearrangement (kR) of singlet phenylnitrene
the carbene.1,15 Although 1PC has a closed-shell electronic
structure,16 calculations17,18 and experiments19 indicate that
singlet phenylnitrene has an open-shell electronic structure and
is 18.5 kcal/mol higher in energy than triplet phenylnitrene. The
diradical 2a cannot smoothly undergo concerted cycloaddition
reactions with alkenes to form azirines.20
equals the rate constant for intersystem crossing (kR ) kISC
,
Scheme 1),7 which is independent of temperature.8 Thus, below
180 K, triplet phenylnitrene (6a) is irreversibly formed in
solution, and it eventually dimerizes to form azobenzene (7a).7,8
In separate experiments the Abramovitch group9 and Banks
and co-workers10 demonstrated that polyfluorinated singlet
nitrenes have rich and useful bimolecular chemistry. The original
observations were extended by the Keana group11 and this
laboratory.12 Polyfluorinated singlet aryl nitrenes insert into the
C-H bonds of hydrocarbons (R-H) but do not react with
methylene chloride or Freon solvents.12 C-H insertion chemistry
is observed when fluorine atom substituents occupy both
positions ortho to the nitrene center, but this reaction has not
been reported for other types of fluorinated aryl nitrenes.2
We have postulated that the fluorine substituents exert an
electronic effect which makes the π2 configuration more
accessible by π-back-bonding.12 Fluorine substituents make the
nitrene look like a carbene from this perspective, which explains
the heightened bimolecular chemistry of polyfluorinated arylni-
trenes.20
The isokinetic temperature, where kR ) kISC, is near 270 K
for singlet perfluorophenylnitrene.12
Although this view has received support from the density
Platz,12 Keana,11 and Bayley13 have predicted that polyfluor-
inated aryl azides might be useful as photoaffinity labeling
reagents, which has prompted several comparative studies of
fluorinated and nonfluorinated aryl nitrenes.14
functional theory (DFT) calculations of Smith and Cramer,21
a
different interpretation has been offered by Karney and Borden
using the CASPT2 method.22 These latter calculations indicate
that ortho-fluorine substituents raise the barrier for cyclization
to the corresponding azirine by 3.5-4.5 kcal/mol, relative to
parent 2a, through a combination of steric and electronic
factors.22
Two groups have recently reported the direct spectroscopic
detection of 2a by laser flash photolysis.23,24 Singlet phenylni-
trene has a sharp absorption band at 350 nm. The lifetime of
1PC reacts rapidly with bonding and nonbonding pairs of
electrons through initial coordination of the empty p orbital of
(3) Carroll, S. E.; Nay, B.; Scriven, E. F. V.; Suschitzky, H.; Thomas,
D. R. Tetrahedron Lett. 1977, 3175.
(4) (a) Doering, W. v. E.; Odum, R. A. Tetrahedron 1966, 22, 81. (b)
DeGraff, B. A.; Gillespie, D. W.; Sundberg, R. J. J. Am. Chem. Soc. 1974,
96, 7491.
(5) (a) Gritsan, N. P.; Pritchina, E. A. J. Inf. Rec. Mater. 1989, 17, 391.
(b) Gritsan, N. P.; Pritchina, E. A. Russ. Chem. ReV. 1992, 61, 500. (c) Li,
Y.-Z.; Kirby, J. P.; George, M. W.; Poliakoff, M.; Schuster, G. B. J. Am.
Chem. Soc. 1988, 110, 8092. (d) Schrock, A. K.; Schuster, G. B. J. Am.
Chem. Soc. 1984, 106, 715. (e) Younger, C. G.; Bell, R. A. J. Chem. Soc.,
Chem. Commun. 1982, 1359.
(6) Meijer, E. W.; Nijhuis, S.; Von Vroonhoven, F. C. B. M. J. Am.
Chem. Soc. 1988, 110, 7209.
(7) Leyva, E.; Platz, M. S.; Persy, G.; Wirz, J. J. Am. Chem. Soc. 1986,
108, 3783.
(8) Gritsan, N. P.; Hadad, C. M.; Zhu, Z.; Platz, M. S. J. Am. Chem.
Soc. 1999, 121, 1202.
(9) (a) Abramovitch, R. A.; Challand, S. R.; Scriven, E. F. V. J. Am.
Chem. Soc. 1972, 94, 1374. (b) Abramovitch, R. A.; Challand, S. R.; Scriven,
E. F. V. J. Org. Chem. 1975, 40, 1541.
(10) (a) Banks, R. E.; Sparkes, G. R. J. Chem. Soc., Perkin Trans. 1
1972, 1964. (b) Banks, R. E.; Venayak, N. D. J. Chem. Soc., Chem.
Commun. 1980, 900. (c) Banks, R. E.; Prakash, A. Tetrahedron Lett. 1973,
99. (d) Banks, R. E.; Prakash, A. J. Chem. Soc., Perkin Trans. 1 1974,
1365. (e) Banks, R. E.; Madany, I. M. J. Fluorine Chem. 1985, 30, 211.
(11) (a) Keana, J. F. W.; Cai, S. X. J. Fluorine Chem. 1989, 43, 151.
(b) Keana, J. F. W.; Cai, S. X. J. Org. Chem. 1990, 55, 2034. (c) Cai, S.
X.; Keana, J. F. W. Bioconjugate Chem. 1991, 2, 28. (d) Cai, S. X.; Glenn,
D. R.; Keana, J. F. W. J. Org. Chem. 1992, 57, 1299.
(12) (a) Poe, R.; Schnapp, K.; Young, M. J. T.; Grayzar, J.; Platz, M. S.
J. Am. Chem. Soc. 1992, 114, 5054. (b) Young, M. J. T.; Platz, M. S. J.
Org. Chem. 1991, 56, 6403. (c) Marcinek, A.; Platz, M. S. J. Phys. Chem.
1993, 97, 12674. (d) Marcinek, A.; Platz, M. S.; Chen, S. Y.; Floresco, R.;
Rajagopalan, K.; Golinsk, M.; Watt, D. J. Phys. Chem. 1994, 98, 412. (e)
Schnapp, K.; Platz, M. S. Bioconjugate Chem. 1993, 4, 178. (f) Schnapp,
K.; Poe, R.; Leyva, E.; Soundararajan, N.; Platz, M. S. Bioconjugate Chem.
1993, 4, 172.
(14) (a) Crocker, P. J.; Imai, N.; Rajagopalan, K.; Bogges, M. A.;
Kwiatkowski, S.; Dwyer, L. D.; Vanaman, T. C.; Watt, D. S. Bioconjugate
Chem. 1990, 1, 419. (b) Drake, R. R.; Slama, J. T.; Wall, K. A.; Abramova,
M.; D’Souza, C.; Elbein, A. D.; Crocker, P. J.; Watt, D. S. Bioconjugate
Chem. 1992, 3, 69. (c) Pinney, K. C.; Katzenellenbogen, J. A. J. Org. Chem.
1991, 56, 3125. (d) Pinney, K. C.; Carlson, K. E.; Katzenellenbogen, S.
B.; Katzenellenbogen, J. A. Biochemistry 1991, 30, 2421. (e) Reed, M.
W.; Fraga, D.; Schwarts, D. E.; Scholler, J.; Hinrichsen, R. D. Bioconjugate
Chem. 1995, 6, 101. (f) Kapfer, I.; Jacques, P.; Toubal, H.; Goeldner, M.
P. Bioconjugate Chem. 1995, 6, 109. (g) Kym, P. R.; Carlson, K. E.;
Katzenellenbogen, J. A. J. Med. Chem. 1993, 36, 1993. (h) Kapfer, I.;
Hawkinson, J. E.; Casida, J. E.; Goeldner, M. P. J. Med. Chem. 1994, 37,
133.
(15) Hoffman, R.; Zeiss, G. D.; Van Dine, G. W. J. Am. Chem. Soc.
1968, 90, 1485.
(16) (a) Matzinger, S.; Bally, T.; Patterson, E. V.; McMahon, R. J. J.
Am. Chem. Soc. 1996, 118, 1535. (b) Wong, M. W.; Wentrup, C. J. Org.
Chem. 1996, 61, 7022. (c) Schreiner, P. R.; Karney, W. L.; Schleyer, P. v.
R.; Borden, W. T.; Hamilton, T. P.; Schaefer, H. F., III. J. Org. Chem.
1996, 61, 7030.
(17) Kim, S.-J. I.; Hamilton, T. P.; Schaefer, H. F., III. J. Am. Chem.
Soc. 1992, 114, 5349.
(18) Hrovat, D. A.; Waali, E. E.; Borden, W. T. J. Am. Chem. Soc. 1992,
114, 8698.
(19) Travers, M. J.; Cowles, D. C.; Clifford, E. P.; Ellison, G. B. J. Am.
Chem. Soc. 1992, 114, 8699.
(20) Platz, M. S. Acc. Chem. Res. 1995, 28, 487.
(21) Smith, B. A.; Cramer, C. J. J. Am. Chem. Soc. 1996, 118, 5490.
(22) Karney, W. L.; Borden, W. T. J. Am. Chem. Soc. 1997, 119, 3347.
(23) Gritsan, N. P.; Yuzawa, T.; Platz, M. S. J. Am. Chem. Soc. 1997,
119, 5059.
(13) Bayley, H. Photogenerated Reagents in Biochemistry and Molecular
Biology; Elsevier: Amsterdam, 1983.
(24) Born, R.; Burda, C.; Senn, P.; Wirz, J. J. Am. Chem. Soc. 1997,
119, 5061.