Photochemical generation of nitrenium ions from protonated 1,1-diarylhydrazines

Article abstract of DOI:10.1021/ol048250y

(Chemical Equation Presented) Laser flash photolysis experiments, chemical trapping studies, and time-dependent density functional theory calculations demonstrate that photolysis of protonated 1,1-diarylhydrazines generates N,N-diarylnitrenium ions.

Full text of DOI:10.1021/ol048250y

ORGANIC
LETTERS
2004
Vol. 6, No. 25
4671-4674
Photochemical Generation of Nitrenium
Ions from Protonated
1,1-Diarylhydrazines
Arthur H. Winter, Selina I. Thomas, Andrew C. Kung, and Daniel E. Falvey*
Department of Chemistry and Biochemistry, UniVersity of Maryland,
College Park, Maryland 20742
falVey@umd.edu
Received August 31, 2004
ABSTRACT
Laser flash photolysis experiments, chemical trapping studies, and time-dependent density functional theory calculations demonstrate that
photolysis of protonated 1,1-diarylhydrazines generates N,N-diarylnitrenium ions.
The development of photochemical methods for generating
nitrenium ions1,2 led to renewed interest in these short-lived
species. The early 1990s saw the first laser flash photolysis
(LFP) studies applied to these reactive intermediates. The
initial experiments used photoisomerization of anthranilium
ions to generate nitrenium ions (1).3 This route is inherently
limited to producing N-alkyl-N-arylnitrenium ions having a
carbonyl group on the aromatic ring (2). McClelland et al.
developed a more general method. These workers photolyzed
aryl azides (3) in water or other protic media and obtained
a monosubstituted nitrenium ion (i.e., R-N-H+) through
rapid protonation of the singlet arylnitrene4 (4). This route,
though more general, is limited to monosubstituted nitrenium
ions. More recently our group and others have used N-
aminopyridinium ions 6 as photochemical precursors to
nitrenium ions.5,6 In principle, this route allows for the
generation of any nitrenium ion in any solvent system.
Unfortunately, N-aminopyridinium ions are available only
from multistep syntheses.
To expand the available routes to nitrenium ions, we
examined the photochemistry of protonated 1,1-diarylhy-
drazines. Through a combination of LFP experiments,
product analysis, and time-dependent density functional
theory (TD-DFT) calculations, it is demonstrated that pho-
tolysis of protonated 1,1-diarylhydrazines generates the
corresponding nitrenium ions in high yields.
Three 1,1-diarylhydrazine derivatives were studied: 1,1-
diphenylhydrazine 8a, 1,1-di(4-chlorophenyl)hydrazine 8b,
and 1,1-di(4-bromophenyl)hydrazine 8c. Protonation of the
hydrazines using HBF4 to make the hydrazinium ions was
followed by UV spectroscopy. In each case the free bases
have strong absorption bands with maxima in the 295-305
nm region. Upon addition of a stoichiometric amount of
HBF4, this highest wavelength absorption band is replaced
with a long absorption tail that extends from 240 to ca. 300
nm. Consequently, compounds 8a, 8b, and 8c require lower
wavelength light for excitation than do the corresponding
N-aminopyridinium ions 6. The latter can generally be
excited with wavelengths as high as 400 nm. The protonated
hydrazines show only weak fluorescence (Φfl < 0.03) in
CH3CN.
(1) Falvey, D. E. In ReactiVe Intermediate Chemistry; Moss, R. A., Platz,
M. S., Maitland Jones, J., Eds.; Wiley-Interscience: Hoboken, NJ, 2004;
Vol. 1, pp 593-650.
(2) Novak, M.; Rajagopal, S. AdV. Phys. Org. Chem. 2001, 36, 167-
254.
(3) Anderson, G. B.; Falvey, D. E. J. Am. Chem. Soc. 1993, 115, 9870-
9871.
(4) McClelland, R. A.; Kahley, M. J.; Davidse, P. A.; Hadzialic, G. J.
Am. Chem. Soc. 1996, 118, 4794-4803.
(5) Abramovitch, R. A.; Shi, Q. Heterocycles. 1994, 37, 1463-1466.
(6) Srivistava, S.; Toscano, J. P.; Moran, R.; Falvey, D. E. J. Am. Chem.
Soc. 1997, 119, 11552-11553.
-
Photolysis of 8 (BF4 salts) in CH3CN produces stable
products that are characteristic of the nitrenium ions 9.8 The
10.1021/ol048250y CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/10/2004

Scheme 1. Photochemical Generation of Nitrenium Ions
product mixtures were analyzed by GC, and the products
were identified by co-injection with authentic samples.7
Photolysis of diphenylhydrazinium ion (8a) in CH3CN with
n
added Cl- (as Bu4N+Cl-) gives a mixture of 4-chloro-N-
phenylaniline 11 and diphenylamine 12. Likewise, the para-
substituted examples 8b and 8c each give a mixture of
chloride adduct 10 and the reduction product. For the para-
substituted diaryl hydrazinium precursors 8b and 8c, addition
occurs at the unsubstituted ortho-position on the benzene
ring. Laser flash photolysis (266 nm, 4 ns, 10-25 mJ/pulse)
Scheme 2. Photolysis of Hydrazinium Ions
shows that the corresponding nitrenium ions 9 are formed
following photolysis. Figure 1 shows the transient UV-vis
spectra generated from pulsed laser photolysis of protonated
1,1-diphenlyhydrazine 8a in CH3CN. The spectrum im-
mediately following the laser pulse, having bands at 650 and
410 nm, is experimentally indistinguishable from the spec-
trum generated from the corresponding N-aminopyridinium
ion 6a.8-10 In previous work the identity of this intermediate
was established as the diarylnitrenium ion 9a by a variety
of experiments, including trapping studies and time-resolved
infrared6 and resonance Raman spectroscopy.11
Figure 1. Transient UV-vis spectra derived from LFP (266 nm
excitation) of 8a-c in CH3CN solutions. The spectra at early times
(filled circles) are assigned to the corresponding diarylnitrenium
ions 9; those at later times (open circles) are assigned to the
corresponding cation radicals 12+•.
of these initial intermediates decay within 5-7 µs to give a
second, longer-lived intermediate. For reasons discussed
below, this secondary transient is attributed to the radical
cation, deriving from an electron-transfer reaction between
9 and 8.
Two experiments confirm that the absorptions seen im-
mediately after the laser pulse correspond to the nitrenium
ions. First, laser flash photolysis (355 nm, 20-30 mJ, 6 ns)
of the corresponding N-aminopyridinium ions (6b and 6c)
give transient absorption peaks at the same maxima as those
observed for the corresponding hydrazinium ions (Table 1).
With the NAP-generated transients, however, no long-lived
intermediate was detected following the decay of the
arylnitrenium ion. In fact, the lifetimes of the NAP-generated
transients were significantly longer (>100 µs) than the initial
intermediates generated from 8b and 8c.
The other two hydrazinium ions, 8b and 8c, give similar
transient spectra upon laser flash photolysis (Figure 1). Both
(7) The identities of the products determined by the GC studies were
further corroborated by separating the components of the photolysate of
6b (0.0012 M 6b with 1 equiv of HBF4 with 0.015 M Bu4N+Cl-, in CH3-
CN) by flash chromatography and confirming the identity of the chloro
1
adducts 10 and 11 by H NMR.
(8) Moran, R. J.; Falvey, D. E. J. Am. Chem. Soc. 1996, 118, 8965-
8966.
(9) McIlroy, S.; Moran, R. J.; Falvey, D. E. J. Phys. Chem. A 2000,
104, 11154-11158.
(10) McIlroy, S.; Falvey, D. E. J. Am. Chem. Soc. 2001, 123, 11329-
11330.
(11) Zhu, P.; Ong, S. Y.; Chan, P. Y.; Poon, Y. F.; Leung, K. H.; Phillips,
D. L. Chem. Eur. J. 2001, 7, 4928-4936.
4672
Org. Lett., Vol. 6, No. 25, 2004

Table 1. Calculated and Experimental Absorption Maxima
(nm) [Lifetimes (µs)] of 9a
Table 2. Rate Constants for Reaction of 9 with Chloride Ion
precursor
kCl (M-1 s-1
)
9a
9b
9c
6a
8a
6b
8b
6c
8c
1.0 × 1010
5 × 109
8
6
DFT
420, 640 [1.3]
420, 640 [1.2]b
645
440, 680 [6.1]
440, 670 [159]
637
450, 690 [4.5]
440, 690 [125]
647
1.1 × 1010
4.7 × 109
9.8 × 109
5.5 × 109
a Derived from LFP of 6 and 8. b Reference 8.
The assignment of 9 was further verified by kinetic
quenching studies. In each case, addition of Cl- was found
to diminish the lifetime of the initially formed transient
species. A pseudo-first-order analysis of the decay rates with
varying concentrations of the trap provides second-order rate
constants that are near the diffusion limit. In these experi-
ments, we note that the trapping rate constants are slightly
smaller when the nitrenium ions are generated from 8 than
those from the N-aminopyridinium ions 6. This difference
can be traced to the acid (HBF4) that is added to the solutions
of 8 in order to protonate the hydrazine. It is likely that the
acid protonates some of the Cl- ions as well (which are much
more basic in CH3CN than in H2O), reducing their reactivity.
This was supported by repeating the experiment with 8a in
the presence of a 40-fold excess of HBF4, which reduced
the trapping rate constant by a factor of 10.
cal-experimental agreement is excellent for the unsubstituted
system 9a and reasonable for the cases of 9b and 9c, given
the approximations inherent in the formalism. The long-lived
intermediates detected in the LFP experiments on 8b and
8c are assigned to the cation radicals of the corresponding
amines 12. Two experiments confirm this assignment. First,
we generated the same cation radicals by LFP using an
alternative route. Specifically, both 12b and 12c were
photolyzed in the presence of an electron acceptor, 1.4-
dicyanobenzene (DCB).8 It was expected that the excited
singlet states of these amines would transfer an electron to
DCB, following Scheme 3. The resulting radical ion pair
Scheme 3. Photogeneration of Amine Radical Cations
As a final confirmation of the assignments, TD-DFT
calculations were performed to predict the absorption spectra
for nitrenium ions 9a-c.12 TD-DFT calculations13 have been
shown to reasonably predict the UV spectra of organic
compounds14-16 including organic free radicals.17,18 Structures
for the singlet states of each example were optimized at the
RB3LYP/6-31G level. Analytical force field calculations
verified that the optimized structures corresponded to minima
on their respective potential energy surface. In each example,
the TD-DFT calculations predict a strong absorption band
in the 600-700 nm region. As shown in Table 1, theoreti-
can then be detected by LFP. The results from this experi-
ment with 12c are illustrated in Figure 2. The two other
amines, 12a and 12b, both give very similar results. A sharp
absorption band is seen at 720 nm for 12b and at 730 nm
for 12c.
The mechanism by which 12+• is formed is shown in
Scheme 3. It is postulated that nitrenium ion 9 abstracts an
electron from unreacted hydrazinium ion 8. Under the acidic
(12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
revision B.03; Gaussian, Inc.: Pittsburgh, PA, 2003.
(13) Marques, M.; Gross, E. Annu. ReV. Phys. Chem. 2004, 55, 427-
455.
(14) Fabian, J. Theor. Chem. Acc. 2001, 106, 199-217.
(15) Merrer, D.; Ozcetinkaya, S.; Shinnar, A. Tetrahedron Lett. 2004,
45, 4899-4902.
(16) van Gisbergen, S.; Rosa, A.; Ricciardi, G.; Baerends, E. J. Chem.
Phys. 1999, 111 (6), 2499-2506.
(17) Radzisweski, J.; Gil, M.; Gorski, A.; Spanget-Larson, J.; Waluk,
J.; Mroz, B. J. Chem. Phys. 2001, 115 (21), 9733-9738.
(18) Hirata, S.; Head-Gordon, M. Chem. Phys. Lett. 1999, 302, 375-
382.
Figure 2. Transient spectra derived from LFP (355 nm excitation)
of a mixture of 12c and 1,4-dicyanobenzene (50 mM) in CH3CN.
Org. Lett., Vol. 6, No. 25, 2004
4673

a unimolecular cyclization reaction, eventually forming
carbazole.9 This apparently competes with the reaction shown
in Scheme 4.
Scheme 4. Generation of Amine Radical Cation
In conclusion, photolysis of 1,1-diarylhydrazinium ions
generates diarylnitrenium ions via N-N bond heterolysis.19
The chief advantage to this method is these photolytic
precursors are easier to obtain than the NAPs that have been
used previously. On the other hand, these hydrazinium
precursors 8 require the use of lower wavelength UV light
(<300 nm) than NAPs 6. An additional limitation is the rapid
reaction of the photogenerated nitrenium ion intermediate
with unconverted hydrazinium ion.
conditions required to protonate the hydrazine, the resulting
aminyl radical 13 is also protonated to give 12+•. This
mechanism is supported by additional LFP experiments. In
this case the N-aminopyridinium ions 6b and 6c were excited
in the presence of the corresponding hydrazinium ions using
355 nm excitation. At this wavelength, the fomer but not
the latter absorb light. In both cases long-lived absorption
bands at 720 and 730 nm, attributed to 12b and 12c,
respectively, were detected following the laser pulse. This
result confirms that the postulated electron-transfer reaction
(Scheme 4) occurs rapidly. It should be noted that the same
intermediates could also form from a direct H atom abstrac-
tion; further experiments would be necessary to rigorously
distinguish these pathways.
Acknowledgment. We thank the Chemistry Division of
the National Science Foundation for support of this work
through Grant CHE-0203142.
Supporting Information Available: Procedures for
precursor synthesis, product studies, laser flash photolysis,
and computational studies; Cartesian coordinates for DFT-
optimized geometries. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL048250Y
The unsubstituted system, 8a, shows only a very weak
signal for the cation radical 12a following the decay of the
nitrenium ion 9a. This can be traced to the short lifetime of
the latter intermediate in the absence of hydrazine. Previous
studies have shown that this diarylnitrenium ion undergoes
(19) On the other hand, laser flash photolysis of diphenylcarbamoyl
chloride (Ph2NCOCl) at 266 nm in CH3CN showed no transients that
corresponded to the diphenylnitrenium ion. Instead a sharp peak at 680 nm
was observed. This absorption could correspond to a six-electron cyclization
intermediate, which has been observed for diphenylamine in this region of
the spectrum.
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Org. Lett., Vol. 6, No. 25, 2004