Photogenerated diarylnitrenium ions: Laser flash photolysis and product studies on diphenylnitrenium ion generated from photolysis of 1-(N,N-diphenylamino)pyridinium ions

Full text of DOI:10.1021/ja961086s

J. Am. Chem. Soc. 1996, 118, 8965-8966
8965
Photogenerated Diarylnitrenium Ions: Laser Flash
Photolysis and Product Studies on
Diphenylnitrenium Ion Generated from Photolysis
of 1-(N,N-Diphenylamino)pyridinium Ions
Ricardo J. Moran and Daniel E. Falvey*
Department of Chemistry and Biochemistry
UniVersity of Maryland
College Park, Maryland 20742
ReceiVed April 2, 1996
It has recently become possible to study nitrenium ions
+
1-3
[
R-N-R′]
using laser flash photolysis (LFP) methods.
These experiments have provided valuable new information
about the lifetimes of monoarylnitrenium ions in aqueous
4
media, the roles of the singlet and triplet states in determining
5
their chemical behavior, and the acid-base properties of
primary arylnitrenium ions.6 One objective of our efforts has
,7
+
been to generate and characterize diphenylnitrenium ion (Ph2N ).
This species is of interest for two reasons. First, it is the simplest
example of the diarylnitrenium ions, a structural type that could
Figure 1. Transient absorption spectra obtained from pulsed laser (308
nm, 10 ns, 40-50 mJ/pulse) photolysis of N-(diphenylamino)pyridinium
ion 1 in N -purged CH CN containing 0.012 M CF CO H. Upper right
panel shows the kinetic traces at 425 and 675 nm. Upper left panel
shows the same transient spectra where the sample is 0.10 M in TMDD,
the triplet state quencher. Spectra are taken at 1 µs (open symbols)
and 10 µs (filled symbols) after the laser pulse.
8
not be accessed by previously reported LFP methods. Second,
+
the isoelectronic counterparts of Ph2N , diphenylcarbene
2
3
3
2
9-11
+ 12-14
(Ph2C),
and diphenylcarbenium ion (Ph2CH )
have been
the subject of intensive spectroscopic and kinetic research efforts
+
over the past two decades. Similar information for Ph2N is
desirable as it would permit meaningful comparisons between
nitrenium ions and these other, more familiar, types of inter-
mediates.
+
17
of nitrenium ions as diverse as NH2
ion derived from phthalimide.
and the diacylnitrenium
Reported herein are product
16,18
The currently available methods for LFP studies of nitrenium
ions possess limitations either in the structures of the nitrenium
ions that can be accessed and/or in the solvent media than must
and LFP studies on the photochemistry of 1-(N,N-diphenyl-
amino)-2,4,6-trimethylpyridinium tetrafluoroborate (1).
Photolysis of the pyridinium salt 1 gives stable products that
would be expected from a diarylnitrenium ion intermediate. With
the monoarylnitrenium ions, it had been shown that nucleophiles
4
,6,7,15
16
be employed.
However, recently Abramovitch and
17
Takeuchi reported generation and trapping of various nitrenium
ions through the photolysis of N-aminopyridinium ions (e.g.,
-
1
). This seemed an attractive alternative to the previous methods
such as Cl or CH3OH trap singlet-state nitrenium ions by
1
9-22
because with the pyridinium salts there is no inherent limitation
to the types of nitrenium ions that can be generated. Indeed,
these earlier studies reported generation and chemical trapping
attacking the aromatic ring.
H atom donors such as 1,4-
cyclohexadiene (CHD) scavenge triplet-state arylnitrenium ions
through sequential H atom transfers, ultimately giving the parent
5
,15
-
amine.
Photolysis of CH3CN solutions containing 1, Cl ,
(
1) Abramovitch, R. A.; Jeyaraman, R. In Azides and Nitrenes: ReactiVity
and Utility; Scriven, E. F. V., Ed.; Academic: Orlando, FL, 1984; pp 297-
57.
2) Simonova, T. P.; Nefedov, V. D.; Toropova, M. A.; Kirillov, N. F.
Russ. Chem. ReV 1992, 61, 584-599.
3) Heller, H. E.; Hughes, E. D.; Ingold, C. K. Nature 1951, 168, 909-
10.
4) Davidse, P. A.; Kahley, M. J.; McClelland, R. A.; Novak, M. J. Am.
Chem. Soc. 1994, 116, 4513-4514.
5) Srivastava, S.; Falvey, D. E. J. Am. Chem. Soc. 1995, 117, 10186-
0193.
6) McClelland, R. A.; Kahley, M. J.; Davidse, P. A.; Hadzialic, G. J.
Am. Chem. Soc. 1996, 118, 4794-4803.
7) McClelland, R. A.; Davidse, P. A.; Hadzialic, G. J. Am. Chem. Soc.
995, 117, 4173-4174.
and CHD gives N-phenyl-4-chloroaniline 3a and its 2-chloro
isomer 3b in ca. 75% combined yield along with 11% of
Ph2NH 4. Photolysis of 1 in neat CH3OH also gives the
expected ring adduct, N-phenyl-4-methoxyaniline 5 (58%), along
with the triplet product, Ph2NH (18%).
3
(
(
9
(
Laser flash photolysis (308 nm, 10 ns, 50 mJ) of 1 in CH3-
23
CN containing 0.042 M CF3CO2H gives the transient absorp-
(
tion spectra shown in Figure 1. Following the excitation laser
pulse, transient absorption bands at 425 and 600-700 nm are
observed. The 425 nm signal decays with a first order lifetime
of 1.5 µs in CH3CN. In the high-wavelength region, there is a
broad absorption band that also decays with a 1.5 µs lifetime
1
(
(
1
(
8) Certain highly stabilized diarylnitrenium ions have been detected as
intermediates in the electrochemical oxidation of substituted diarylamine.
For example: Serve, D. J. Am. Chem. Soc. 1975, 97, 432-434.
(18) Abramovitch, R. A.; Beckert, J. M.; Chinnasamy, P.; Xiaohua, H.;
Pennington, W.; Sanjivamurthy, A. R. V. Heterocycles 1989, 28, 623-
628.
(19) Gassman, P. G.; Campbell, G. A. J. Am. Chem. Soc. 1972, 94,
3891-3896.
(20) Novak, M.; Pelecanou, M.; Roy, A. K.; Andronico, A. F.; Plourde,
F.; Olefirowicz, T. M.; Curtin, T. J. J. Am. Chem. Soc. 1984, 106, 5623-
5631.
(
9) Eisenthal, K. B.; Turro, N. J.; Sitzmann, E. V.; Gould, I. R.; Hefferon,
G.; Langan, J.; Cha, Y. Tetrahedron 1985, 41, 1543-1545.
(
(
10) Schuster, G. B. AdV. Phys. Org. Chem. 1986, 22, 311-361.
11) Platz, M. S.; Maloney, V. M. In Kinetics and Spectroscopy of
Carbenes and Biradicals; M. S. Platz, Ed.; Plenum: New York, 1990; pp
2
39-352.
(
12) Dorfman, L.; Sujdak, R. J.; Bockrath, B. Acc. Chem. Res. 1976, 9,
3
52-357.
(21) Gassman, P. G.; Campbell, G. A.; Frederick, R. C. J. Am. Chem.
Soc. 1972, 94, 3884-3891.
(
13) Belt, S. T.; Bohne, C.; Charette, G.; Sugamori, S. E.; Scaiano, J. C.
J. Am. Chem. Soc. 1993, 115, 2200-2205.
(22) Fishbein, J. C.; McClelland, R. A. J. Chem. Soc., Perkin Trans. 2
1995, 663-671.
(
14) Bartl, J.; Steenken, S.; Mayr, H. J. Am. Chem. Soc. 1991, 113, 7710-
7
716.
(23) In the absence of acid, the signals in the 600-700 nm region are
(
15) Anderson, G. B.; Yang, L. L.-N.; Falvey, D. E. J. Am. Chem. Soc.
complicated. It appears that the pyridine leaving group deprotonates Ph2-
•
+
•
1
993, 115, 7254-7262.
NH giving a mixture of the latter and Ph2N , which also has a broad
absorption in this region. The singlet nitrenium ion signal at 425 nm is not
noticeably affected by the acid. A more complete analysis of this issue
will be presented in the full paper.
(
16) Abramovitch, R. A.; Shi, Q. Heterocycles 1994, 37, 1463-1466.
(
17) Takeuchi, H.; Hayakawa, S.; Tanahashi, T.; Kobayashi, A.; Adachi,
T.; Higuchi, D. J. Chem. Soc., Perkin Trans. 2 1991, 847-855.
S0002-7863(96)01086-4 CCC: $12.00 © 1996 American Chemical Society

8966 J. Am. Chem. Soc., Vol. 118, No. 37, 1996
Communications to the Editor
Table 1. Second-Order Rate Constants (knuc) Obtained from Decay
of Transient Absorptions at 425 nm Produced by Laser Flash
Scheme 1
3
Photolysis of 1a in CH CN with Added Nucleophiles
nucleophile
k )
nuc (M-1s-1
Cl-
a
1.0 × 10
5.2 × 10
4.9 × 10
6.1 × 10
10
6
MeOH
EtOH
6
5
2
H O
a
The Cl- source is nBu
4 2
NCl‚H O.
We attribute the appearance of Ph2NH•+ to a parallel triplet
pathway illustrated in Scheme 1. The excited singlet state of
to give longer-lived bands at 680 and 325 nm. The long-lived
peaks are assigned to the diphenylamine cation radical (Ph2-
1
the pyridinium precursor ( 1) partitions between heterolytic bond
•
+
24-26
NH ) by comparison to literature spectra.
Further con-
1
+
scission, yielding (Ph2N ), and intersystem crossing, giving
firmation of this assignment was achieved through independent
3
the excited triplet state of the precursor ( 1). The latter cleaves
to give the triplet state nitrenium ion ( Ph2N ) which in turn
rapidly abstracts a H-atom (presumably from the solvent ) to
give the cation radical Ph2NH . This mechanism is supported
•
+
generation of Ph2NH by photolysis of Ph2NH in the presence
of 1,4-dicyanobenzene, a single electron acceptor (see supporting
information).
3
+
15
•+ 5
The short-lived species that absorbs at 425 and at 660 nm is
by a triplet quenching experiment. Addition of TMDD (0.1
1
+
assigned to the singlet diphenylnitrenium ion, (Ph2N ) on the
basis of the following observations.
35
M), a triplet quencher, to the LFP solutions suppresses the
•+
appearance of the Ph2NH band at 680 nm. This is illustrated
(1) Photolysis of 1 gives stable products that are consistent
in the upper left panel of Figure 1. In contrast, the lifetimes
with formation of a singlet arylnitrenium ion intermediate.
Significantly, high yields of ring adducts (3 and 5) are observed
when the photolysis is done in the presence of nucleophiles.
1
+
and positions of the bands assigned to (Ph2N ) are unaffected
by TMDD.
+
The reactivity of Ph2N toward alcohols is almost three orders
(2) The short-lived transient species is quenched by nucleo-
of magnitude lower than that of the corresponding diarylcar-
philes. The decay rate constants at 425 nm (kobs) were measured
at various nucleophile concentrations. A pseudo-first-order
analysis provides the second-order rate constants (knuc) presented
+ 14
benium ion, Ph2CH . A similar effect has been noted by
4
McClelland and Novak when N-acetyl-2-fluorenylnitrenium ion
is compared to a carbenium ion analog. The lower reactivity
of the nitrenium ions can be attributed to the different sites of
attack. Nucleophiles attack the ring carbons of arylnitrenium
ions, resulting in decreased aromaticity. In contrast, nucleo-
philes attack the exocyclic atom of arylcarbenium ions resulting
in increased aromaticity (because the π-electron localization in
the aromatic ring is increased in the transition state).
5
10
-1 -1
in Table 1. The knuc values of 10 -10
M
s
fall in the
range expected for a nitrenium ion. Moreover, the order of
-
reactivity Cl > MeOH > EtOH > H2O follows that observed
for arylcarbenium ions1
4,27
and monoarylnitrenium ions.
28
(3) Competitive trapping experiments provide relative rate
constants that agree with those from LFP. Pyridinium ion 1
was irradiated in the presence of both MeOH and EtOH. The
+
In summary, Ph2N has been generated through the photolysis
1
ratio of adducts was determined by H NMR. After correcting
of 1-(N,N-diphenylamino)-2,4,6-trimethylpyridium ion 1. Both
its absorption spectrum and its rates of nucleophilic trapping
have been characterized using LFP. In principle any type of
for the concentrations of the traps, a ratio of kMeOH/kEtOH ) 1.26
was determined. This is in reasonable agreement with the LFP
ratio of kMeOH/kEtOH ) 1.05 considering the uncertainties
associated with NMR peak integration.
nitrenium ion should be accessible through N-aminopyridinium
16,17,36,37
ion photolysis.
However, the practical limitations have
(
4) The lifetime of the transient species is unaffected by the
yet to be explored. Future studies will examine the dynamics
2
9
addition of O2, a quencher of excited triplet states.
This
+
of singlet-triplet state interconversion for Ph2N as well as the
excludes the possibility that the transient is due to the excited
triplet state of the pyridinium ion. Also, the transient spectra
are not solvent-specific. Essentially the same spectra are
detected in CH2Cl2 and in CH3CN. This argues against
assignment to a nitrile ylide such as have been observed with
carbenes in CH3CN.30
applications of N-aminopyridinium ion photochemistry to the
study of previously inaccessible classes of nitrenium ions.
Acknowledgment. We thank Profs. Rudolph A. Abramovitch and
Christopher J. Cramer for helpful suggestions. This work was supported
by the National Science Foundation. This paper is dedicated to
Professor Gary B. Schuster on the occasion of his 50th birthday.
(5) Finally, the lifetime of the short-lived species is unaffected
by the addition of an H-atom donor, (Me3Si)3SiH. The triplet
+
5
Supporting Information Available: Table of the product studies
state of Ph2N is ruled out for this reason. It also should be
noted that electronic structure calculations generally predict
under various conditions and transient spectra of independently
•
+
3
1-34
generated Ph
2
NH and from photolysis of 1 under neutral conditions
singlet ground states for simple arylnitrenium ions.
contrast, the diarylcarbenes are typically ground state triplets.
In
(
2 pages). See any current masthead page for ordering and Internet
access instructions.
9
-11
(
24) Weir, D.; Scaiano, J. C.; Schuster, D. I. Can. J. Chem. 1988, 66,
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(
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(
(
1
1
(