Solvent effect on product distribution in photochemical pathways of α C-N versus β C-C cleavage of n,π* triplet-excited azoalkanes

Article abstract of DOI:10.1021/ja963948v

The product distribution in the photolysis of the triplet-excited azoalkanes 1a,b depends markedly on the type of solvent used; in contrast, the azoalkanes 1c,d, which undergo efficient deazatization from the singlet- excited state, display solvent-indepe

Full text of DOI:10.1021/ja963948v

5550
J. Am. Chem. Soc. 1997, 119, 5550-5555
Solvent Effect on Product Distribution in Photochemical
Pathways of R C-N versus â C-C Cleavage of n,π*
Triplet-Excited Azoalkanes
Waldemar Adam,*,† Jarugu N. Moorthy,† Werner M. Nau,‡,§ and J. C. Scaiano‡
Contribution from the Institut fu¨r Organische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany, and the Department of Chemistry, UniVersity of Ottawa,
10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada
ReceiVed NoVember 13, 1996X
Abstract: The product distribution in the photolysis of the triplet-excited azoalkanes 1a,b depends markedly on the
type of solvent used; in contrast, the azoalkanes 1c,d, which undergo efficient deazatization from the singlet-excited
state, display solvent-independent photobehavior. Thus, the aziranes 3 are produced essentially exclusively in polar
protic solvents, while the housanes 2 predominate in nonpolar ones. The excellent correlation (r2 ) 0.963, seven
solvents) of the azirane 3b yield with Gutmann’s AN solvent parameter reveals that a combination of solvent properties
such as the polarity and polarizability of the medium and the hydrogen-bonding ability is decisive for the photoproduct
distribution. That the observed solvent dependence derives from bulk medium effects is borne out by the similar
product distribution for the hydroxy-substituted derivative 1f to that for the azoalkane 1b in benzene, i.e., the
intramolecular hydroxy functionality in the azoalkane 1f is ineffective in influencing the photochemistry of the triplet
azo chromophore. Selective formation of the aziranes 3 from the triplet-excited azoalkanes 1 in the polar protic
solvents is rationalized in terms of solvent stabilization of the more polar transition state for â cleavage (azirane 3)
over that for the R scission (housane 2). In marked contrast to the appreciable deuterium isotope effect for quenching
of the singlet-excited azoalkanes by protic solvents, the lifetimes (laser-flash photolysis) and reactivity (product
analysis) of the triplet azoalkanes are not affected by deuterium substitution.
Introduction
Azoalkanes 1 are unique in that not only do they constitute
the first examples in which the triplet states have been directly
characterized8 but they also lead to the housanes 2 (kC3 N) as
well as the aziranes 3 (k3CC) upon direct photoexcitation;
quenching experiments have established that the latter derive
exclusively from the triplet manifold (Scheme 1).8 Herein, we
report significant solvent effects on the triplet lifetimes and
photochemistry of the azoalkanes 1 (Scheme 1), from which
we conclude that a combination of medium effects (polarity,
polarizability, and hydrogen-bonding ability) may be decisive
in controlling photochemical product formation. Indeed, the
respective photoproducts 2 and 3 may be selectively formed
by appropriate selection of the solvent.
Most striking, an enhancement of azirane 3 formation was
observed in polar protic solvents, which suggests that part of
this photoproduct might be formed through the acid-base
photoreaction (proton abstraction) in Scheme 2. However, the
product distribution of the azoalkane 1f, which contains a
hydroxy functionality capable of interacting intramolecularly
with the n,π*-excited azo chromophore, was virtually the same
as for the derivative 1b without the hydroxy group in nonpolar
solvents. Thus, proton transfer is no common denominator for
the observed solvent effects.
Solvents play an important role in determining the fate of
the electronically excited molecules. In addition to chemical
interactions, the solvent can promote radiationless deactivation
of singlet-excited states,1 it can accelerate intersystem crossing,2
and it can cause the so-called state switching,3 i.e., the reversal
of the n,π* and π,π* excited-state configurations.
The investigation of the photophysical properties of cyclic
azoalkanes has demonstrated that the fluorescence of cyclic
azoalkanes is efficiently quenched by certain solvents like
methanol and chloroform.4,5 Reversible proton transfer4 or,
more recently, coValent hydrogen-bonding interactions5 have
been held responsible for this efficient quenching of singlet-
excited azoalkanes. Despite these detailed investigations of
solvent effects on the fluorescent properties of azoalkanes,
surprisingly little has been reported on the solvent influence of
the photochemical product distribution.6 In contrast, the solvent-
dependent thermal decomposition and isomerization of azoal-
kanes is well documented.7
† Universita¨t Wu¨rzburg.
‡ University of Ottawa.
§ Present Address: Institute of Physical Chemistry, University of Basel,
Klingelbergstrasse 80, CH-4056 Basel, Switzerland.
X Abstract published in AdVance ACS Abstracts, June 1, 1997.
(1) (a) Herbich, J.; Hung, C-Y.; Thummel, R. P.; Waluk, J. J. Am. Chem.
Soc. 1996, 18, 3508. (b) Houten, J. V.; Watts, R. J. J. Am. Chem. Soc.
1975, 97, 3843.
(6) (a) Turro, N. J.; Liu, J.-M.; Martin, H.-D.; Kunze, M. Tetrahdron
Lett. 1980, 21, 1299. (b) Lyons, B. A.; Pfeifer, J.; Carpenter, B. K. J. Am.
Chem. Soc. 1991, 113, 9006. (c) Anderson, M. A.; Grissom, C. B. J. Am.
Chem. Soc. 1995, 117, 5041.
(2) Turro, N. J. Modern Molecular Photochemistry; The Benjamin/
Cummings Publishing Co., Inc.; Menlo Park, CA, 1978; p 125.
(3) Wagner, P. J. Top. Curr. Chem. 1976, 66, 1.
(4) (a) Mirbach, M. J.; Liu, K-C.; Mirbach, M. F.; Cherry, W. R.; Turro,
N. J.; Engel, P. S. J. Am. Chem. Soc. 1978, 100, 5122. (b) Ramamurthy,
V.; Turro, N. J. Ind. J. Chem., Sect. B: Org. Chem. Incl. Med. Chem. 1978,
18, 72.
(7) (a) Schulz, A.; Ru¨chardt, C. Tetrahedron Lett. 1976, 3883. (b)
Neuman, R. C., Jr.; Grow, R. H.; Binegar, G. A.; Gunderson, H. J. J. Org.
Chem. 1990, 55, 2682. (c) Neuman, R. C., Jr.; Gunderson, H. J. J. Org.
Chem. 1992, 57, 1641. (d) Schmittel, M.; Ru¨chardt, C. J. Am. Chem. Soc.
1987, 109, 2750.
(8) (a) Adam, W.; Nau, W. M.; Sendelbach, J.; Wirz, J. J. Am. Chem.
Soc. 1993, 115, 12571. (b) Adam, W.; Nau, W. M.; Sendelbach, J. J. Am.
Chem. Soc. 1994, 116, 7049. (c) Adam, W.; Fragale, G.; Klapstein, D.;
Nau, W. M.; Wirz, J. J. Am. Chem. Soc. 1995, 117, 12578.
(5) Adam, W.; Nau, W. M.; Scaiano, J. C. Chem. Phys. Lett. 1996, 253,
92.
S0002-7863(96)03948-0 CCC: $14.00 © 1997 American Chemical Society

SolVent Control of Product Distribution
J. Am. Chem. Soc., Vol. 119, No. 24, 1997 5551
Scheme 1
Scheme 2
Scheme 3
Results
Synthesis of the Azoalkanes. The azoalkanes 1a-d were
synthesized according to the Hu¨nig route.9 The synthesis of
the azoalkanes 1e,f was accomplished according to the reaction
sequence in Scheme 3. Addition of a molar equivalent of
bromine to the solution of 3,3-dimethylpentane-2,4-dione in
acetic acid furnished a mixture (70:30) of monobromo and
dibromo diketones. Acetoxy substitution of the 1-bromo-3,3-
dimethylpentane-2,4-dione under phase-transfer conditions with
18-crown-6 ether as catalyst afforded 1-acetoxy-3,3-dimethyl-
pentane-2,4-dione in a good yield (84%). Subsequent treatment
of the acetoxy dione with hydrazine hydrate led quantitatively
to the isopyrazole. Acid-catalyzed Diels-Alder cycloaddition
with 2,5-norbornadiene afforded the azoalkane 1e in a poor yield
(16%). Hydrolysis of the azoalkane 1e to 1f was accomplished
under mild conditions with potassium carbonate in methanol.
Photolysis Products of the Azoalkanes. Direct irradiation
of the solutions of azoalkanes 1 (0.1 M) under a nitrogen gas
atmosphere was conducted in a Rayonet photoreactor (λ ) 350
nm). The respective photoproducts 2a,b,d and 3a,b,d of the
azoalkanes 1a,b,d have previously been characterized.8,9 The
photoproducts 2c,e,f and 3e,f from the azoalkanes 1c,e,f were
identified by comparison of the characteristic spectral data. The
analysis of the characteristic and well-resolved methyl and
olefinic signals. GC analysis was also attempted, but the
aziranes 3 decomposed at elevated temperatures. In Table 1
are summarized the results of the product studies for the
photolysis of the azoalkanes 1 in the various solvents.
A quite similar trend was displayed in the product distribution
for the photolysis of the azoalkanes 1a,b in a variety of solvents.
Whereas the irradiation in nonpolar solvents yielded the
housanes 2 (Scheme 1) selectively (Table 1, entries 1, 6, and
7), the aziranes 3 were formed predominantly in polar protic
solvents (entries 3, 10-12, and 16). It is noteworthy that the
photolysis in trifluoroethanol led to the formation of aziranes
3a,b essentially exclusively (entries 5 and 15). Efficient
photoreduction was found to occur in the presence of hydrogen
donors to produce the hydrazines 4a,b in significant yields
(entries 1, 13, and 14). The identity of the hydrazines 4 was
established by comparison of the characteristic 1H and 13C NMR
signals of authentic signals produced upon photolysis of 1a,b
in the presence of the excellent hydrogen donor 1,4-cyclohexa-
1
product distribution was determined in each case by H NMR
(9) (a) Beck, K.; Hu¨nig, S. Chem. Ber. 1987, 120, 477. (b) Adam, W.;
Heidenfelder, T.; Sahin, C. Synthesis 1995, 1163.

5552 J. Am. Chem. Soc., Vol. 119, No. 24, 1997
Adam et al.
Table 1. Product Compositiona and Disappearance Quantum
Yieldsb for the Photolysis of Azoalkanes 1
product
distributionc
azo
solvent
2
3
4
φrd
ANe
1
2
3
4
5
1a n-hexane
75
51
11
50
18
7
benzene-d6
49
89
50
0.59 (0.04)f,g
methanol-d4
acetonitrile-d3
trifluoroethanol
<5 >95
6
7
8
1b n-hexane
88 12
0.67
0.0
8.2
12.5
18.9
benzene-d6
acetone-d6
acetonitrile-d3
methanol
methanol-d4
methanol-d
60 40h
0.55 (0.06)g
0.66
57
51
20
43
49
80
9
0.65
10
11
12
13
14
15
16
17
0.13 (<0.01)g 41.3
19 81h
0.31
0.32
Figure 1. Plot of the yields of azirane 3b from the photolysis of
azoalkane 1b in different solvents versus the “solvent acceptor numbers”
(AN) taken from Table 1. The data points for tert-butyl alcohol and
2-propanol were excluded from the correlation, see the text for details.
24
17
21
76
2-propanol
18 65 0.11
31 48 0.27
2-propanol-d8
trifluoroethanol
tert-butyl alcohol
acetic acid-d4
<5 >95
41 59
12 88i
100
100
95 <5
95 <5
100
0.12
0.26
52.9
52.9
competes (hydrazine 4d) and lowers the azirane yield.10,12b As
the azirane yields are essentially the same in deuterated or
protiated solvents, the correlation is not affected.
18 1c benzene-d6
19
20
21
methanol-d4
trifluoroethanol
acetic acid-d4
The influence of the azo triplet quencher,8a namely piperylene
(1.0 M), on the product distribution was probed for the
photolysis of azoalkane 1b in methanol, trifluoroethanol, and
22 1d benzene-d6
23
24
methanol/benzene (3:1) 100
acetic acid/benzene (3:1) 100
1
25 1e benzene-d6
26 methanol-d4
27 1f benzene-d6
28
29
30
74
28
26
72
acetic acid (data not shown). From H NMR monitoring, the
absence of the azirane 3b was established for the photoreaction
in methanol conducted to >50% conversion of the azoalkane
1b. Unfortunately, the complex nature of the 1H NMR spectra
of the crude photolysates for the azoalkane 1b with piperylene
(1.0 M) in trifluoroethanol and acetic acid made even a reliable
qualitative analysis of the products difficult; nevertheless, the
characteristic 1H NMR resonances of the aziranes 3b were not
observed.
72 28j
52 48j
21 79j
0.60 (0.09)g
acetonitrile-d3
methanol-d4
trifluoroethanol
8
92j
a Photolyses were conducted in a Rayonet photoreactor (350 nm) at
8 °C, except for entries 16 and 24 (ca. 25 °C). b Determined by isolating
the 333 nm line of the argon ion laser; 2,3-diazabicyclo[2.2.1]heptene
(DBH) was employed as actinometer; analysis by GC. c Conversions
and mass balances were >90%; relative yields (normalized to 100%)
determined by 1H NMR analysis of the appropriate methyl and olefinic
signals of the norbornene moiety; error (3%. d Quantum yields for
azoalkane disappearance; error (10%. e Solvent acceptor numbers (AN)
from ref 16 used for the correlation in Figure 1. f In benzene from ref
8c. g Value in parentheses refers to 1.0 M piperylene as additive. h A
similar product distribution was observed for laser photolysis at 333
and 364 nm; note that azoalkanes 1 have their λmax at ca. 360 nm (ref
8). i Represents the sum of the yields of the hydrazones 5b and 6b
(see text). j The yield of the isomeric azirane 3f′ was <6%, except ca.
15% for entry 29.
The dependence of the product distribution on the wavelength
was examined for the azoalkane 1b. Essentially identical
1
product distributions (entries 7 and 11) were revealed by H
NMR spectroscopy for the photolysis of benzene and methanol
solutions of the azoalkane 1b at two distinct wavelengths (333
and 364 nm lines of the argon ion laser). Hence, solvent effects
on the product distribution do not arise from the hypsochromic
shifts of the n,π* absorptions caused by the polar protic solvents.
In contrast to the photoproduct profile displayed by the
azoalkanes 1a,b, the azoalkanes 1c (entries 18-21) and 1d
(entries 22-24) exhibited solvent-independent photobehavior
(Table 1). Even in acetic acid, the photolysis of azoalkane 1c
led essentially exclusively to the housane 2c (entry 21). The
poor solubility of the azoalkane 1d precluded photolysis in neat
methanol and acetic acid, so that benzene was used as a
cosolvent and only the housane 2d was observed in these media
(entries 23 and 24).
The azoalkanes 1e,f exhibited a rather similar photochemistry
to that of the derivatives 1a,b. Thus, the housanes 2e,f were
selectively produced in the nonpolar benzene (entries 25 and
27) and the aziranes 3e,f in polar protic solvents (entries 26,
29, and 30). In acetonitrile, the photolysis of azoalkane 1f (entry
28) yielded a product distribution identical to that observed for
the azoalkane 1b (entry 9).
diene; the hydrazines have been trapped and characterized
spectroscopically.10
The photolysis of azoalkane 1b in acetic acid (entry 17) led
to the hydrazones 5b and 6b (Scheme 4) together with the
housane 2b. The hydrazones 5b and 6b were isolated from a
preparative photolysis and fully characterized. In a control
experiment, the azirane 3b was found to undergo ring-opening
upon treatment with acetic acid to afford quantitatively the
hydrazones 5b and 6b (Scheme 4). Thus, the combined yield
of the hydrazones from the photolysis in acetic acid refers to
the primary photoproduct, namely the azirane 3b (entry 17).
The yield of azirane 3b from the photolysis of azoalkane 1b
in various solvents (Table 1) was found to correlate well (Figure
1) with the Gutmann’s acceptor number (AN) solvent parameters
(r2 ) 0.963).
Disappearance Quantum Yields. The disappearance quan-
tum yields for the azoalkane 1b at 333 nm (argon ion laser)
were determined by employing 2,3-diazabicyclo[2.2.1]hept-2-
ene (DBH) as actinometer, whose decomposition quantum yield
In this correlation, the case of tert-butyl alcohol was excluded
since the photolysis was conducted at a higher temperature (ca.
25 °C), which is known to reduce azirane formation.8b Simi-
larly, the data for 2-propanol were omitted since photoreduction
(12) (a) Engel, P. S.; Kitamura, A.; Keys, D. E. J. Org. Chem. 1987, 52,
5015. (b) Adam, W.; Moorthy, J. N.; Nau, W. M.; Scaiano, J. C. J. Org.
Chem. 1996, 61, 8722.
(10) Adam, W.; Moorthy, J. N.; Nau, W. M. Unpublished results.
(11) Clark, W. D. K.; Steel, C. J. Am. Chem. Soc. 1971, 93, 6347.

SolVent Control of Product Distribution
J. Am. Chem. Soc., Vol. 119, No. 24, 1997 5553
Scheme 4
Table 2. Triplet Lifetimes of Azoalkane 1a-H2 in Various Solvents
solvent
3τ/nsa
solvent
3τ/nsa
485
Nonpolar
Polar Nonprotic
n-pentane
n-hexane
n-heptane
isooctane
cyclohexane
cyclohexane-d12 (>99.5%)
benzene
445
acetonitrile
455
470
545
500
545
600
540
495
acetonitrile-d3 (>99.5%)
methyl tert-butyl ether
tetrahydrofuran
1,4-dioxane
ethyl vinyl ether
acetone
ethyl acetate
N,N-dimethylformamide
dimethyl sulfoxide
500
500
130
400
no signal
500b
515
200
285
benzene-d6 (>99.5%)
toluene
Halogenated
Polar Protic
methylene chloride
methylene chloride-d2 (>99.6%)
chloroform
465
500
505
CH3-OH
CD3-OD (>99.0%)
ethanol
240
220
100
chloroform-d (>99.8%)
carbon tetrachloride
Freon-113
tetrachloroethylene
1,2-dibromoethane
480
630
630
no signal
530
2,2,2-trifluoroethanol
2-propanol
hexafluoro-2-propanol
tert-butyl alcohol
water
no signal
100
no signal
no signal
115c
deuterium oxide (>99.9%)
120c
a Determined by transient absorption spectroscopy on laser excitation (λexc ) 355 nm, ca. 6 ns pulse, λmon ) 310 and 450 nm); error from five
measurments was ca. 30 ns. b Not detected at 310 nm. c Acetonitrile (50%) was added to solubilize.
has been reported to be unity.11 The photolysates were analyzed
by GC with diphenyl ether as internal standard. The instability
of the azirane 3b under GC condition (see above) presented no
complications, since only the disappearance of the azoalkane
1b was monitored. In Table 1 are recorded the disappearance
quantum yields for the azoalkane 1b (entries 6-16) in the
various solvents. The values in parentheses for the entries 7
and 10 refer to the decomposition quantum yields in the presence
of 1.0 M piperylene.
Whereas the quantum yields of azoalkane 1b disappearance
in nonpolar aprotic solvents such as hexane (0.67) and benzene
(0.55) were found to be quite high (entries 6 and 7), a dramatic
reduction was observed for the photolysis in the polar protic
media (entries 10, 13, and 15) methanol (0.13), 2-propanol
(0.11), and trifluoroethanol (0.12). Moreover, the substantial
deuterium isotope effect (φDr /φHr ca. 2.5) is noteworthy for the
photolysis in CD3OD and CH3OD versus CH3OH (entries 10-
12). The quantum yield for disappearance of the azoalkane 1b
in the presence of 1.0 M piperylene in methanol (entry 10) was
found to be essentially negligible (<0.01), and for the azoalkane
1f in benzene (entry 27) with and without added piperylene,
the φr values were found to parallel those observed for the
azoalkane 1b (entry 7).
azoalkane displayed the strongest transient triplet absorption of
all the azoalkanes of this class and was, therefore, preferred for
the laser flash studies. Since its photophysics and photochem-
istry closely resemble those for the azoalkanes 1a,b, the
saturated derivative 1a-H2 serves as a suitable model.
Whereas the triplet lifetime was found to be quite similar in
the various solvents, a significant shortening was observed in
polar protic solvents (Table 2). The absence of a signal in some
solvents is presumably due to efficient quenching of the singlet-
excited states,5 which results in a reduced triplet yield and,
hence, a low signal intensity or even no measurable signal. Note
that the low triplet lifetimes in 2-propanol and tetrahydrofuran
may also be due to chemical reactions with the solvent
(hydrogen abstraction).12
Discussion
The direct photolysis of azoalkanes 1a,b not only affords the
expected housanes 2a,b by the R C-N bond cleavage (k3CN),
but also the aziranes 3a,b from â scission of the C-C bond (
k3CC) (cf. Scheme 1).8 From the triplet quenching experiments
with piperylene, the intersystem-crossing (ISC) quantum yields
for the azoalkanes 1a,b have been estimated8 to be as high as
0.5. Particularly noteworthy is the incipient radical stabiliza-
tion: the better the carbon radical site in the intermediate
diazenyl diradical is stabilized, i.e., Ph (1c,d) . Me (1b) . H
(1a), the more R C-N bond cleavage to the housanes occurs.
Thus, the phenyl substitution in 1c,d is so effective in stabilizing
Triplet Lifetimes of the Azoalkane 1a-H2. The triplet
lifetimes in selected solvents were measured for the azoalkane
1a-H2, a derivative of the azoalkane 1a for which the CdC
double bond was hydrogenated, as a prototypal case. This

5554 J. Am. Chem. Soc., Vol. 119, No. 24, 1997
Adam et al.
the incipient carbon radical site that deazatization from the
singlet-excited state predominates with marginal ISC to the
triplet manifold [φISC (1d) ca. 0.1].8c
formation is promoted (Tables 1 and 2). It is noteworthy in
this context that the triplet lifetimes (Table 2), unlike those of
the singlet lifetimes, are not affected by deuterium substitution
in the solvent, which is analogous to the lack of an isotope effect
on the product distribution (Table 1, entries 10-12). Note also
that the triplet lifetime is not significantly shortened in 1,2-
dibromoethane (Table 2), which provides additional evidence
for the absence of measurable heavy atom effects on the
lifetimes of n,π*-excited azoalkanes.8c
On the basis of the computational results obtained for the
parent 2,3-diazabicyclo[2.2.1]hept-2-ene at the UHF/6-31G* ab
initio level of theory,13 which included a full geometry
optimization at the same level, the transition state for â scission
of the triplet state displays a larger dipole moment (3.45 D)
than that for the R cleavage (3.24 D), while that of the azoalkane
triplet state falls between these values (3.34 D). This calculated
increase or decrease in dipole moments becomes significantly
more pronounced if the primary triplet diradicals derived from
â C-C scission (3.84 D) or R C-N cleavage (2.76 D) are used
as reference (Scheme 1). Hence, â C-C cleavage affords a
diradical with a significantly higher dipole moment, and the
increased polarity is reflected in the transition state, while the
opposite is true for R cleavage. This clear-cut trend in dipole
moments suggests that a polar protic solvent should lower the
energy of the â versus R rupture through better stabilization,
and thus, azirane formation should be promoted, as is experi-
mentally observed. In support of this dipole moment argument,
we cite the solvent-dependent photodimerization of isophorone
(eq 1).14a It was observed that the ratio of the head-to-head
As mentioned at the outset, certain solvents are capable of
promoting efficient radiationless deactivation of singlet-excited
azoalkanes, and the rate constants for this deactivation process
are strongly subject to deuterium isotope substitution of the
solvents.4,5 Since this physical quenching of the singlet-excited
states does not alter the photoproduct distribution, it should
merely show up as a reduction in the quantum yield for
azoalkane decomposition. Indeed, our data (Table 1) exhibit a
dramatic (ca. 5-fold) reduction in the quantum yields for the
disappearance of the azoalkane 1b in the polar methanol (entry
10) versus the nonpolar n-hexane (entry 6). Indicative is the
appreciable isotope effect φDr /φrH (kD/kH) of ca. 2.5 for both
pairs CH3OH versus CD3OD (entries 10 and 11) and CH3OH
versus CH3OD (entries 10 and 12). Clearly, it is the deuterium
substitution of OH rather than CH in methanol which exhibits
a more pronounced effect on the azoalkane photoreactivity.
These results, taken together with the cited literature evidence,4
signify efficient deactivation of the singlet-excited states for the
azoalkane 1b by the hydroxyl group of the alcohol solvent.
Apart from the solvent effects on the disappearance quantum
yields, a perusal of the results in Table 1 shows that pronounced
solvent effects operate on the product profiles for the photolysis
of the azoalkanes 1a (entries 1-5), 1b (entries 6-17), 1e (entries
25 and 26), and 1f (entries 27-30). Let us consider the
photobehavior of the azoalkane 1b as a representative case.
Whereas the azirane 3b is the main product in the polar protic
solvents methanol (entries 10-12), trifluoroethanol (entry 15),
and acetic acid (entry 17), the housane 2b predominates in
n-hexane (entry 6) and benzene (entry 7). Furthermore,
photolysis in benzene as well as methanol with the triplet
quencher piperylene (1.0 M) leads exclusively to the housane
2b. In contrast, the azoalkanes 1c,d, which undergo efficient
deazatization from the singlet-excited state,8 display solvent-
independent photobehavior and produce essentially exclusively
(>95%) the housanes 2c (entries 18-21) and 2d (entries 22-
24). Thus, the pronounced shift of the product distribution for
the azoalkane 1b as a function of solvent (Table 1), i.e., from
housane 2b as main product in the n-hexane and benzene (entries
6 and 7) to mainly azirane 3b in the polar and protic CH3OH,
iPrOH, CF3CH2OH, and CH3COOH (entries 10-17), must be
sought in the triplet-state photoreactivity of the azoalkane 1b.
(1)
Since the photolysis of azoalkane 1b in the presence of the
triplet quencher (1.0 M) leads exclusively to the housanes 2b,
the azirane 3b is unambiguously identified as a characteristic
triplet-state product. Thus, the observed increase or decrease
of azirane in the various solvents is best reconciled as a
manifestation of the solvent effects on the triplet-state reactivity.
The previous studies have established that the triplet-excited
azoalkanes 1a,b produce the respective housanes 2 as well as
the aziranes 3 from R and â cleavages (k3CN and kC3 C, Scheme
1).8 Furthermore, it has been shown that the rate constants for
R and â cleavages are strongly temperature dependent due to
the marked difference in the relative activation energies.8b
Therefore, it is perhaps not surprising that the solvent properties
also have an effect on the relative rates of R and â cleavages.
Since the azirane 3b is favored in polar protic solvents,
presumably such solvents stabilize the transition state for â
cleavage or destabilize the one for R scission. In support of
the enhanced â cleavage in polar protic solvents, we find that
the triplet lifetimes, which reflect the propensity of the triplet-
excited states to undergo chemical transformations, are indeed
significantly shorter in protic solvents for which azirane
and head-to-tail dimers increased from 1:4 to 4:1 when the
solvent was changed from cyclohexane to methanol. As
expected, the higher dipole moment of the head-to-head dimer
is better stabilized in methanol and, hence, preferentially
produced.
(13) Gaussian 94, Revision A.1; Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-
Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski,
J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart,
J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A.; Gaussian Inc.:
Pittsburgh, PA, 1995.
(14) (a) Chapman, O. L.; Nelson, P. J.; King, R. W.; Trecker, D. J.;
Griswold, A. Recl. Chem. Prog. 1967, 28, 167. (b) Hrnjez, B. J.; Mehta,
A. J.; Fox, M. A.; Johnston, K. P. J. Am. Chem. Soc. 1989, 111, 2662. (c)
Challand, B. D.; de Mayo, P. Chem. Commun. 1968, 982. (d) Pappas, S.
P.; Pappas, B. C.; Blackwell, J. E., Jr. J. Org. Chem. 1967, 32, 3066. (e)
Lee, K.-H.; de Mayo, P. J. Chem. Soc., Chem. Commun. 1979, 493. (f)
Wagner, P. J.; Stout, C. A.; Sarless, S.; Hammond, G. S. J. Am. Chem.
Soc. 1966, 88, 1242. (g) Ninomiya, I.; Naito, T. Photochemical Synthesis;
Academic Press: Tokyo, 1989; p 75. (h) Shiloff, J. D.; Hunter, N. R. Can.
J. Chem. 1979, 57, 3301.

SolVent Control of Product Distribution
J. Am. Chem. Soc., Vol. 119, No. 24, 1997 5555
Scattered reports of solvents effects on various photochemical
transformations are documented in the literature.14 The pho-
toproduct control by solvents has in some cases been rationalized
in terms of physical properties of the solvents such as the
polarity14a,b and the dielectric constant.14c,e Interestingly, the
thermal decomposition of azoalkanes has been reported to be
strongly solvent dependent and a remarkable correlation of the
decomposition rates of cis-azoisobutane with the Reichardt’s
ET(30) solvent polarity parameters has been documented.6a
Nevertheless, the presently observed solvent-dependent product
distribution for the photolysis of azoalkane 1b in the solvents
examined does not correlate with solvent parameters such as
viscosity, dielectric constant, R (a measure of hydrogen-bond-
donating ability),15a and π* (solvent polarity and polarizability).15b
However, the azirane 3b yield correlates excellently with
Gutmann’s AN solvent parameter (Figure 1, r2 ) 0.963 for
seven solvents, see also Results),16 while a relatively poor
correlation is found for the ET(30) values17 (r2 ) 0.897). The
AN numbers, derived from P31 NMR chemical shifts produced
through solvent effects on Et3PO, display a linear relationship
with the ET(30) parameters17 and measure the ability of solvents
to interact with electron pairs from suitable electron donors.
The advantage of this solvent parameter is that the donor-
acceptor interactions such as hydrogen bonding, ion-dipole
interactions, and charge transfer are collectively taken into
account. Taft et al.18 have shown that the AN parameter for a
nonpolar solvent is equivalent to π*, while for the polar solvents
AN measures the composite effects of π* and R. Thus, the
dependence of the formation of azirane 3b on the polarity and
polarizability of the medium expressed by π*, in addition to
the hydrogen-bonding ability (R), is evident from the correlation
with the AN numbers.
angle O-H‚‚‚N are 2.62 Å and 127.6°, which fall within the
range observed for significant hydrogen-bonding interaction,
especially if one considers the limiting values for much weaker
and intermolecular C-H‚‚‚O hydrogen bonds.20 Consequently,
analogous to the intermolecular interactions of the azoalkane
1b with the hydroxy groups of protic solvents, the hydroxy-
substituted derivative 1f should promote azirane formation
intramolecularly, if proton abstraction were to play a major role.
However, the azoalkane 1f (entries 27-30) portrays quite similar
product profiles as the methyl-substituted 1b (entries 7, 9, 11,
and 15) and as the acetoxymethyl-substituted derivative 1e
(entries 25 and 26) in a variety of solvents, which range from
the nonpolar benzene to the polar alcohols. Thus, the lack of
a significant influence by the intramolecularly tethered hydroxyl
functionality in the azoalkane 1f compared to 1b in benzene is
convincingly evident both from the product distribution and
disappearance quantum yields with and without the triplet
quencher piperylene (Table 1). Thus, proton abstraction
(Scheme 2) is not responsible for promoting the azirane
formation. In fact, the absence of deuterium isotope effects on
the product distribution (see above) speaks also against such a
proton-mediated process. Clearly, the importance of the proper-
ties intrinsic to the solvent as medium is decisive in affecting
the photobehavior of the azoalkane n,π* triplets.
In conclusion, the present results demonstrate that the triplet
state reactivity of azoalkanes is profoundly influenced by
solvents in that the polar azirane photoproduct is favored by
polar protic solvents. In contrast to the quenching of singlet-
excited azoalkanes, the triplet-state reactivity and the triplet
lifetimes remain unaffected by deuterium substitution of the
solvent. The excellent correlation of the azirane yield with the
AN solvent parameter reveals that a combination of solvent
properties like polarity, polarizability, and hydrogen-bonding
properties govern the product distribution in the azoalkane
photolysis.
That the observed solvent effect on the product distribution
is caused by the “macroscopic” solvent property of the medium
is borne out by the photobehavior of the hydroxymethyl-
substituted azoalkane 1f (Table 1). If the enhancement of
azirane 3 formation in polar protic solvents was due to the acid-
base photoreaction (proton abstraction) in Scheme 2, the product
distribution for the azoalkane 1f should parallel that for the
azoalkane 1b in protic solvents (entries 10-17). In azoalkane
1f, the bridgehead hydroxyl group is sufficiently well disposed
for intramolecular proton abstraction by the azo group as
suggested by the force-field calculations.19 In fact, in the lowest
energy conformation, the O‚‚‚N nonbonding distance and the
Acknowledgment. We thank the Deutsche Forschungsge-
meinschaft, the Volkswagen-Stiftung, and the Natural Sciences
and Engineering Research Council of Canada for financial
support. J.N.M is grateful for a postdoctoral fellowship from
Alexander von Humboldt Foundation and W.M.N for a Canada
International and a NATO postdoctoral fellowship awarded by
NSERC and the DAAD.
Supporting Information Available: Experimental data (5
pages). See any current masthead page for ordering and Internet
access instructions.
(15) (a) Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2887.
(b) Kamlet, M. J.; Abboud, J. L.; Taft, R. W. J. Am. Chem. Soc. 1977, 99,
6027.
(16) (a) Gutmann, V.; Schmid, R. Coord. Chem. ReV. 1974, 12, 263. (b)
Gutmann, V. The Donor Acceptor Approach to Molecular Interactions;
Plenum Press: New York, 1978. (c) Parker, A. J.; Mayer, U.; Schmid, R.;
Gutmann, V. J. Org. Chem. 1978, 43, 1843.
(17) Reichardt, C. Angew. Chem., Int. Ed. Engl. 1979, 18, 98.
(18) Taft, R. W.; Pienta, N. J.; Kamlet, M. J.; Arnett, E. M. J. Org.
Chem. 1981, 46, 661.
JA963948V
(19) Sybyl 6.0, Tripos Associates, 1699 S. Hanley Road, Suite 303, St.
Louis, MO 63144.
(20) (a) Desiraju, G. R. Acc. Chem. Res. 1991, 24, 290. (b) Allen, F. H.;
Goud, S.; Hoy, V. J.; Howard, J. A. K.; Desiraju, G. R. J. Chem. Soc.,
Chem. Commun. 1994, 2729.