J. Am. Chem. Soc. 2001, 123, 11329-11330
Reactions of Nitrenium Ions with Arenes: Laser
11329
Flash Photoylsis Detection of a σ-Complex between
N,N-Diphenylnitrenium Ion and Alkoxybenzenes
Sean McIlroy and Daniel E. Falvey*
Department of Chemistry and Biochemistry
UniVersity of Maryland, College Park, Maryland 20742
ReceiVed July 6, 2001
Nitrenium ions are a family of electrophilic reactive intermedi-
ates that are defined by a divalent nitrogen having a formal
+
1-3
positive charge (RR′N ). Typical members of this family are
highly reactive and have solution lifetimes rarely exceeding 1
ms. Interest in the possible roles of nitrenium ions in carcinogenic
4
-6
DNA-damaging reactions,
(
the synthesis of conducting poly-
aniline) derivatives,7,8 and the development of methods for the
9
synthesis of complex molecules have motivated much recent
work. All of these processes are thought to involve the addition
of the nitrenium ion to an aromatic ring. Despite the wide interest,
a general picture of nitrenium ion/arene reactions has yet to
emerge.
Figure 1. Panel A: Transient absorption spectra from laser flash
+
photolysis (390 nm, 8-10 mJ/pulse, 4-6 ns) of Ph
the presence of 24 mM DMB in N -purged CH
2
N
precursor 2 in
One unresolved issue concerns the importance of various
intermediates on the pathway to the products. Most electrophilic
2
3
CN taken 100 ns (circle)
and 1.6 µs (cross) following the laser pulse. Panel B: Kinetic traces taken
at 350 and 440 nm from a LFP experiment (355 nm, 10-12 mJ, 4-6
ns) where 2 was irradiated in the presence of 7.6 mM DMB
substitution reactions are considered to involve π and/or σ
complexes between the electrophile and the arene.1
0,11
The only
systematic examinations of this question relative to the nitrenium
ions are studies by McClelland,12 Novak, and Guengerich on
the addition of arylnitrenium ions to guanosine nucleosides. All
of these researchers agree that some intermediate is formed, but
they differ on its structure and/or the pathway leading to the final
products. One reason for this controversy is that the guanine itself
is a fairly complex nucleophile having at least 6 possible sites
13
14
+
photolysis (LFP) studies which show that Ph
2
N reacts rapidly
with both 1,3,5-trimethoxybenzene (TMB) and 1,3-dimethoxy-
benzene (DMB). An intermediate is detected in these reactions,
which is attributed to the corresponding σ-adducts. A product
+
analysis shows that Ph
2
N is trapped by these arenes to give the
predicted adducts.
for electrophilic attack.15
N,N-Diphenylnitrenium ion (Ph
+
The two arenes employed in this study efficiently trap Ph
Compound 2 was photolyzed in the presence of 87 mM TMB
CH CN solvent N -purged) and the products were isolated by
silica gel chromatography, characterized by the usual spectro-
scopic methods, and quantified by GC analysis. Three isomers
were characterized. The most abundant (44%) was an adduct
2
N .
+
2
N ) is an attractive candidate
for further examination of these reactions as it has been
(
3
2
16
extensively characterized by theoretical calculations, time-
resolved IR,17 and UV-vis spectroscopy.
18,19
Many of its decay
reactions are well-known and it can be generated cleanly in a
variety of solvents from the photolysis of 1-(N,N-diphenylamino)-
+
joining a para carbon on Ph
2
N
to an unsubstituted carbon on
TMB (7). Also formed were adducts joining the nitrenium ion
20
2
,4,6-triphenylpydinium ions (2). Described herein are laser flash
(
1) Abramovitch, R. A.; Jeyaraman, R. In Azides and Nitrenes: ReactiVity
center (11, 30%) and an ortho carbon (9, 24%). DMB also traps
+
and Utility; Scriven, E. F. V., Ed.; Academic: Orlando, FL, 1984; pp 297-
Ph
2
N
to produce adducts. In this case the major products are
3
57.
the ortho adduct (10, 39%), the para adduct (8, 34%), and the
parent amine (Ph NH 36%). A GC/MS analysis also shows trace
(
2) McClelland, R. A. Tetrahedron 1996, 52, 6823-6858.
(
3) Falvey, D. E. In Organic, Physical, and Materials Photochemistry;
2
Ramamurthy, V., Schanze, K., Eds.; Marcel Dekker: New York, 2000; pp
amounts of a third isomer, which we tentatively identify as the
N-adduct (m/z 305).
Both arenes show high rate constants (ktrap) for reaction with
2
49-284.
(
4) Miller, J. A. Cancer Res. 1970, 30, 559-576.
5) Kadlubar, F. F. DNA Adducts Structure and Biological Significance;
(
+
Hemminki, K., Dipple, A., Shuker, D. E. G., Kadlubar, F. F., Segerback, D.,
Eds.; Oxford Press: Oxford, UK, 1994.
Ph
3
N with TMB being more reactive than DMB. LFP (355 or
2
+
90 nm, 8-10 mJ/pulse, 4-6 ns) was used to generate Ph
and its pseudo-first-order decay rate constant kobs was measured
with varying concentrations of either TMB or DMB (N -purged,
CN solutions). Rate constants (ktrap) of 3.1 × 10 (TMB) and
2
N
(
6) Hoffman, G. R.; Fuchs, R. P. P. Chem. Res. Toxicol. 1997, 10, 347-
3
2
1
59.
(
7) Wei, Y.; Tang, X.; Sun, Y. J. Polym. Sci. Part A: Chem. 1989, 27,
2
385-2396.
9
CH
.4 × 10 M
These trapping reactions result in the formation of a longer
3
(
8) Ding, Y.; Padias, A. B.; Hall, H. K., Jr. J. Polym. Sci. Part A: Chem.
8
-1 -1
3
s
(DMB) were determined in this manner.
999, 37, 2569-2579.
9) Abramovitch, R. A.; Ye, X.; Pennington, W. T.; Schimek, G.; Bogdal,
D. J. Org. Chem. 2000, 65, 343-351.
10) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic
(
+
2
lived intermediate following the decay of Ph N . Figure 1A shows
(
the transient absorption spectra that result from pulsed laser
excitation of 2 in the presence of 24 mM DMB. The same
Chemistry; Harper and Row: New York, 1987.
(
11) Hubig, S. M.; Kochi, J. K. J. Org. Chem. 2000, 65, 6807-6818.
(
12) McClelland, R. A.; Ahmad, A.; Dicks, A. P.; Licence, V. J. Am. Chem.
Soc. 1999, 121, 3303-3310.
(17) Srivastava, S.; Toscano, J. P.; Moran, R. J.; Falvey, D. E. J. Am. Chem.
(
13) Kennedy, S. A.; Novak, M.; Kolb, B. A. J. Am. Chem. Soc. 1997,
19, 7654-7664.
14) Humphreys, W. G.; Kadlubar, F. F.; Guengerich, F. P. Proc. Natl.
Acad. Sci. U.S.A. 1992, 89, 8278-8282.
15) Blackburn, G. M.; Gait, M. J. Nucleic Acids in Chemistry and Biology;
Oxford University: Oxford, UK, 1996.
16) Cramer, C. J.; Falvey, D. E. Tetrahedron Lett. 1997, 38, 1515-1518.
Soc. 1997, 119, 11552-11553.
1
(18) Moran, R. J.; Falvey, D. E. J. Am. Chem. Soc. 1996, 118, 8965-
(
8966.
(19) McIlroy, S.; Moran, R. J.; Falvey, D. E. J. Phys. Chem. A 2000, 104,
(
11154-11158.
(20) Moran, R. J.; Cramer, C. J.; Falvey, D. E. J. Org. Chem. 1996, 61,
3195-3199.
(
1
0.1021/ja016557b CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/17/2001