Photochemical equilibration/isomerization of p-, m-, and o-methylbenzonitrile

Article abstract of DOI:10.1021/ja9808301

The phototransposition reactions in acetonitrile of p-, m-, and o-methylbenzonitrile have been studied. Any one of the three is converted to the other two by either a 1,2- or 1,3-isomerization in a primary photochemical step. However, the reactivities are quite different with the relative values for para:meta:ortho = 32:4:1. For both the para and meta isomers, extended irradiations approach a calculated steady-state composition of para:meta:ortho = 3:20:77. Quenching of the excited triplet state of the para and meta isomers with 2,4-dimethyl-1,3-butadiene indicates that these reactions are occurring from the excited singlet state. Irradiation of selectively labeled 2,6-dideuterio-4-methylbenzonitrile demonstrates that only the cyano-substituted carbon undergoes migration.

Full text of DOI:10.1021/ja9808301

J. Am. Chem. Soc. 1998, 120, 6443-6450
6443
Photochemical Equilibration/Isomerization of p-, m-, and
o-Methylbenzonitrile
Paula J. MacLeod, Alexandra L. Pincock, James A. Pincock,* and Kim A. Thompson
Contribution from the Department of Chemistry, Dalhousie UniVersity, Halifax,
NoVa Scotia, B3H 4J3 Canada
ReceiVed March 12, 1998
Abstract: The phototransposition reactions in acetonitrile of p-, m-, and o-methylbenzonitrile have been studied.
Any one of the three is converted to the other two by either a 1,2- or 1,3-isomerization in a primary
photochemical step. However, the reactivities are quite different with the relative values for para:meta:ortho
) 32:4:1. For both the para and meta isomers, extended irradiations approach a calculated steady-state
composition of para:meta:ortho ) 3:20:77. Quenching of the excited triplet state of the para and meta isomers
with 2,4-dimethyl-1,3-butadiene indicates that these reactions are occurring from the excited singlet state.
Irradiation of selectively labeled 2,6-dideuterio-4-methylbenzonitrile demonstrates that only the cyano-substituted
carbon undergoes migration.
Introduction
rather the ring carbons themselves are transposed. A plausible
mechanism for the isomerization reaction was then proposed
invoking substituted tricyclo[3.1.0.02,6]hex-3-enes (benzvalenes)
as intermediates.
For benzene itself there is, of course, only one benzvalene
formed by two meta bondings in any set of four adjacent
carbons; CA to CC and CB to CD are shown in eq 2.4 Reversal
The photochemical transposition of substituents on benzene
rings was first observed by Wilzbach and Kaplan1 in 1964 for
the gas-phase photochemical isomerization of o-xylene to
m-xylene and of m-xylene to both o- and p-xylene. A quantum
yield of 0.014 at 250 nm for the ortho to meta isomerization
was reported; for isomerization of the meta isomer the value
was about twice as high. Isomerization of the meta isomer in
the pure liquid and in isohexane solution, with a lower quantum
yield of 0.006, was also observed. This initial publication
reported that photolysis of the ortho isomer in the gas phase
and at very low conversions gave only the meta isomer. In a
later paper,2 the same observation was reported for p-xylene in
the gas phase but in solution the yield of para from ortho was
determined to be 7% that of meta. Therefore, at least in solution,
both 1,2- and 1,3-shifts were occurring as primary photoreac-
tions. Similar isomerizations resulting in photoequilibration of
o-, m-, and p-di-tert-butylbenzenes to a photostationary state
of ortho:meta:para ) 0:1:4 were reported by Burgstahler and
Chen in the same year.3
of this process by breaking CA to CB and CC to CD will
interchange CB and CC. In the absence of substituents, this
interchange is an identity reaction. In the general case, for a
polysubstituted benzene there are six possible benzvalene
isomers. For a 1,2-disubstituted benzene, like o-xylene with
two identical substituents, symmetry reduces the number of
possible benzvalene isomers to four: 1,2-; 2,3-; 3,4-; and 1,6-
dimethyltricyclo[3.1.0.02,6]hex-3-ene, 1-4, Figure 1. The ortho
to meta isomerization will only be observed from one of these
four, the 1,2-dimethyl isomer 1. Invisible carbon transpositions
will occur from the other three. A similar analysis for m-xylene
gives three benzvalene isomers with the 1,4-dimethyl form
resulting in conversion to p-xylene and the 1,5-dimethyl form
to o-xylene. For p-xylene, there are only two benzvalenes and
the 1,3-dimethyl isomer results in para to meta conversions.
Phototranspositions have also been analyzed by a permutation
approach.5 For a six-membered aromatic ring, there are 12
possible permutations of the ring carbons, P1-P12, and nine of
these will convert o-xylene to m-xylene. The benzvalene
mechanism is one of the nine and is classified as P2. In the
absence of labels on the other four carbons, proving that this is
the pathway occurring is not possible. However, the benzvalene
An important clue to the possible mechanism of these
transpositions was provided by the observation that photolysis
of 1,3,5-trimethylbenzene (mesitylene) specifically labeled with
14C at C1, C3, and C5 gave 1,2,4-trimethylbenzene subsequently
labeled at C1, C2, and C4, eq 1.2 Therefore, the alkyl groups
are not detached from the ring during this isomerization, but
* Phone: 902-494-3324. Fax: 902-494-1310. Email: PINCOCK@
CHEM1.CHEM.DAL.CA.
(1) Wilzbach, K. E.; Kaplan, L. J. Am. Chem. Soc. 1964, 86, 2307-
2308.
(2) Kaplan, L.; Wilzbach, K. E.; Brown, W. G.; Yang, S. S. J. Am. Chem.
Soc. 1965, 87, 675-676.
(3) Burgstahler, A. W.; Chien, P.-L.; Abdel-Rahman, M. O. J. Am. Chem.
Soc. 1964, 86, 5281-5290.
(4) The carbons are marked with upper case letters instead of numbers
to avoid confusion with nomenclature numbers. For instance, A, B, C, and
D are C1, C2, C3, and C4 in benzene but C2, C1, C6, and C5, respectively,
in benzvalene.
(5) Barltrop, J. A.; Day, A. C. Chem. Commun. 1975, 177.
S0002-7863(98)00830-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/20/1998

6444 J. Am. Chem. Soc., Vol. 120, No. 26, 1998
MacLeod et al.
One of these was the observation of both 1,3- and 1,2-shifts as
primary photochemical events. Other benzene isomers, Dewar
and Landenburg benzene (prismane),2 were therefore proposed
as possible intermediates to explain the 1,3-shifts. Another
question was how a substituted benzvalene, once formed, was
converted back to a substituted benzene. The process for
thermal return for benzvalene itself, with an activation enthalpy
of 26 kcal/mol and a half-life of 48 h at 30 °C in ether,11 is too
slow. Appropriately substituted benzvalenes would likely have
faster rates of return. A photochemical pathway for return is
also possible although benzvalene absorption6 (ꢀ ) 2500 M-1
cm-1 at 225 nm) is rather weak at 254 nm and should not
compete very effectively with that for benzene itself (ꢀ ) 215
M-1 cm-1 at 255 nm)12 which is at much higher concentration.
This two- or three-photon pathway could also lead to 1,3-
phototranspositions because substituted benzvalenes are known
to photoisomerize. By the permutation analysis,5 these two
successive P2 permutations would be equivalent to a P4
permutation. For instance, benzvalene selectively deuterated
at C1 and C6 isomerizes by a vinylcyclopropane-type pathway
to benzvalene deuterated at C1 and C2, eq 3, upon either direct
Figure 1. Pathways to the four possible benzvalenes, 1-4, formed
from o-xylene and the rearrangement to m-xylene for 1.
Chart 1
irradiation or by sensitization with triplet energy sensitizers of
less than 63 kcal/mol.13 In contrast, higher-energy sensitizers
convert benzvalene back to benzene but in its excited triplet
state. Thus a quantum chain process is initiated. This provides
a mechanism for the return of benzvalene to benzene. Because
benzene has both a reasonably high intersystem crossing
efficiency (ΦISC ) 0.25) and a high triplet energy (ET ) 84
kcal/mol), a photochemical sequence is possible for the trans-
position reaction. Direct irradiation of benzene (or substituted
benzene) creates benzvalene (or substituted benzvalene) from
its excited singlet state. The triplet state of benzene then
sensitizes the isomerization of benzvalene back to benzene.
Again, this is a two-photon process, but because the second
step is a quantum chain process, few secondary photons are
required. Product analysis at very low conversions as a function
of light absorbed could clarify whether the phototransposition
reaction is a primary photochemical event or a two-photon
process.
Another interesting feature of the photochemistry of benzene
is that internal conversion from upper electronic states to S1 is
not rapid enough to preclude distinct photochemistry from S2
or from upper vibrational levels of S1. Thus, the quantum yield,
in the gas phase, for formation of benzvalene from S1 benzene
increases, and the fluorescence intensity decreases as the
wavelength decreases.14 This observation suggests that there
is a barrier to benzvalene formation on the S1 surface. As well,
mechanism had already received considerable support after
benzvalene was demonstrated to be formed (0.01% yield) by
photolysis of liquid benzene at 254 nm.6 The yield could be
increased to 1% by irradiating benzene in dilute solutions of
hydrocarbon solvents. Moreover, photolysis of benzene in
solutions of acidic alcohols (HCl/CH3OH and CF3CH2OH) or
by treatment of benzene after irradiation with HCl/CH3OH
resulted in the formation of the addition products 5 and 6 (Chart
1).7 When, after an independent synthesis was developed and
benzvalene became available in “bounteous quantities”,8 these
and other addition reactions were extensively studied.9-11
Several features of the proposed intermediacy of benzvalene
derivatives in the phototransposition reactions were problematic.
(9) Berson, J. A.; Hasty, N. M., Jr. J. Am. Chem. Soc. 1971, 93, 1549-
1551.
(10) Kaplan, L.; Rausch, D. J.; Wilzbach, K. E. J. Am. Chem. Soc. 1972,
94, 8638-8640.
(11) Christl, M. Angew. Chem., Int. Ed. Engl. 1981, 20, 529-546.
(12) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds; John Wiley & Sons: New York,
1991; p 211.
(6) Wilzbach, K.; Ritscher, J. S.; Kaplan, L. J. Am. Chem. Soc. 1966,
89, 1032.
(7) Kaplan, L.; Ritscher, J. S.; Wilzbach, K. E. J. Am. Chem. Soc. 1966,
(13) Renner, C. A.; Katz, T. J.; Pouliquen, J.; Turro, N. J.; Waddell, W.
H. J. Am. Chem. Soc. 1975, 97, 2568-2570.
(14) Kaplan, L.; Wilzbach, K. E.; J. Am. Chem. Soc. 1968, 90, 3291-
3292.
88, 2881-2882.
(8) Katz, T. J.; Wang, E. J.; Acton, N. J. Am. Chem. Soc. 1971, 93,
3782-3783.

Equilibrium/Isomerization of p-, m-, and o-Methylbenzonitrile
J. Am. Chem. Soc., Vol. 120, No. 26, 1998 6445
Table 1. Photophysical Properties for PMBN, MMBN, and OMBN in Acetonitrile
10-3
ꢀ
λmax
(nm)
10-4
ꢀ
E0,0
(kcal/mol)
109τS
(s)
kQτS
10-10kQ
a
λmax
(nm)
(M-1 cm-1
)
(M-1 cm-1
)
ΦF
(M-1
)
(M-1 s-1
)
PMBN
MMBN
OMBN
268
276
276
0.70
1.91
1.36
232
228
228
2.29
2.00
1.09
0.16
0.22
0.35
101
99
99
9.1
7.0
7.1
214
105
143
2.0
1.5
2.0
a Stern-Volmer slope of fluorescence quenching by 2,3-dimethyl-1,3-butadiene.
excitation of liquid benzene to S2 results in formation of Dewar
benzene along with benzvalene and fulvene.15
Our interest in these isomerization reactions was stimulated
by our observations on the photochemistry of 3-cyanobenzyl
acetate in methanol.26 For this substrate the usual ester
photochemistry involving benzylic carbon-to-oxygen bond
cleavage was very inefficient. However, 4-cyanobenzyl acetate
was an obvious photoproduct. To focus on the transposition
reaction, we chose to examine p-, m-, and o-methylbenzonitrile
(PMBN, MMBN, and OMBN), 8-10, in acetonitrile solvent.
A recent theoretical study16 on benzene at the MC-SCF level
has characterized the minima and transition states for these
conversions on all three of the S0, S1, and S2 surfaces. The
conclusion reached for the formation of benzvalene is that there
is a barrier separating the symmetrical excited-state minimum
on the S1 surface of benzene from a prefulvene biradical, 7,
located on the ground-state surface. A conical intersection
connects these two surfaces. The biradical then partitions
between return to benzene and closure to benzvalene.
Results and Discussion
Photophysical Properties of p-, m-, and o-Methylbenzoni-
trile (8-10). All three isomers have the usual Lb and La bands
in the UV spectrum with the former one being at a longer
wavelength and with a higher molar absorptivity (ꢀ). All three
isomers also have fluorescence spectra that overlap with the
absorbance bands at the expected 0,0 transition. Photophysical
data are given in Table 1. As well, fluorescence quenching
studies were done using 2,3-dimethyl-1,3-butadiene, chosen
because its absorbance in the UV begins at shorter wavelengths
than for most dienes, thereby avoiding any problem of competi-
tive absorbance. Good linear Stern-Volmer plots were obtained
for the methylbenzonitriles and the quenching rate constants
are, within experimental error, at the diffusional limit in
Similar phototranspositions have been observed for both the
monomethyl and the dimethylbiphenyls.17,18 In both cases, the
2(or o)-methyl-substituted compounds isomerized efficiently to
the 3(or m)-methyl isomers. The 3-methyl isomers isomerized
less efficiently to the 4(or para)-methyl isomers, and the
4-methyl isomers were relatively inert. The authors attributed
these observations to 1,2-shifts through benzvalene intermedi-
ates. Two explanations were presented for the higher reactivity
of the ortho isomers. Either the o-methyl groups prevent
rotation of the rings of the biphenyl chromophore to the
unreactive coplanar geometry expected for the excited singlet
state or the steric repulsion of adjacent methyl groups in the
benzvalene intermediate(s) force the isomerizations away from
ortho products. In addition to alkyl-substituted benzenes,
phototranspositions have also been observed for 1-fluoro-2-,
1-fluoro-3-, and 1-fluoro-4-trifluoromethylbenzenes19 and inef-
ficiently for 2-tritioanisole20 (but oddly, not for 3- or 4-tritio-
anisole which, except for the tritium label, are the same
compound). Phototranspositions were not observed for deuterium-
substituted chlorobenzenes.21 Therefore, little is known about
how substituents affect the efficiency and selectivity of these
processes. As stated in a recent review,22 the photochemical
transposition reactions of substituted benzenes attracted con-
siderable interest when first observed in the 1960s and 1970s,
but have received little attention since that time. In contrast,
there have been many studies reporting on photoisomerizations
and phototranspositions in aromatic heterocycles.23-25
acetonitrile (1.8 × 1010 M-1 s-1 27
)
for all three, Table 1.
Attempts were made to detect the triplet states of the three
isomers in acetonitrile by laser flash photolysis (LFP) in order
to make estimates of their lifetimes in solution at room
temperature. A transient absorbance between 350 and 600 nm
previously observed for benzonitrile in EPA glass at 77 K with
a lifetime of 3.5 s has been assigned to the triplet state.28
However, only very weak transient signals were observed at
room temperature for the methylbenzonitriles, and these could
not be assigned to triplet states with any confidence.
Photochemical Interconversions of p-, m- and o-Methyl-
benzonitriles (8-10). Plots for the phototranposition (Vicor-
filtered 450 W medium-pressure Hanovia Hg lamp) of PMBN,
MMBN, and OMBN (∼200 mg) in acetonitrile (280 mL) at 25
°C are shown in Figures 2, 3, and 4, respectively, at both high
and low conversion. The yields are reported in percent
normalized to 100% for the sum of all three. However, the
overall mass balances were excellent, and the sum of the yields
for all three isomers at the end of the photolysis reactions was
still 80% based on the original starting isomer; at 50%
conversion of PMBN and MMBN the total mass balance was
over 90%. Therefore, other processes are inefficient relative
to the phototransposition reactions.
(15) Ward, H. R.; Wishnook, J. S. J. Am. Chem. Soc. 1968, 90, 1085-
1086.
(16) Palmer, I. J.; Ragazos, I. N.; Bernerdi, F.; Olivucci, M.; Robb, M.
A. J. Am. Chem. Soc. 1993, 115, 673-682.
(17) Mende, U.; Laseter, J. L.; Griffin, G. W. Tetrahedron Lett. 1970,
43, 3747-3750.
(18) Abramovitch, R. A.; Takaya, T. J. Chem. Soc., Perkin Trans. 1 1975,
1806-1809.
(19) Al-Ani, K. E. J. Chem. Phys. 1973, 58, 5073-5077.
(20) Lodder, G.; du Me´e, P. E. J.; Havinga, E. Tetrahedron Lett. 1968,
57, 5949-5952.
Three major conclusions are obvious from these plots. First,
the rate of isomerization is clearly isomer-dependent. For
instance, to reach 20% disappearance of the starting material
requires 75 min for the para isomer (PMBN), 330 min for the
meta isomer (MMBN), and >6000 min for the ortho isomer
(OMBN). These relative efficiencies were confirmed more
(21) McDermott, S. D.; Lally, J. M.; Spillane, W. J.; Cronin, D.; Caplan,
P.; Canuto, S. J. Chem. Res. Synop. 1988, 142-143.
(22) Gilbert, A. In CRC Handbook of Organic Photochemistry and
Photobiology; Horspool, W. M., Song, P.-S., Eds.; CRC Press: New York,
1995; 229-236.
(23) Pavlik, J. W. In CRC Handbook of Organic Photochemistry and
Photobiology; Horspool, W. M., Song, P.-S., Eds.; CRC Press: New York,
1995; pp 237-249 and references therein.
(24) Pavlik, J. W.; Kebede, N.; Bird, N. P.; Day, A. C.; Barltrop, J. A.
J. Org. Chem. 1995, 60, 8138-8139 and references therein.
(25) Padwa, A. In Rearrangements in Ground and Excited States; de
Mayo., P., Ed.; Academic Press: New York, 1980; Vol 3, pp 501-547.
(26) Pincock, J. A.; Wedge, P. J. J. Org. Chem. 1994, 59, 5587-5595.
(27) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry, 2nd ed.; Marcel Dekker: New York, 1993; p 207.
(28) Achiba, Y.; Kimura, K. Chem. Phys. Lett. 1977, 48, 107-110.

6446 J. Am. Chem. Soc., Vol. 120, No. 26, 1998
MacLeod et al.
Figure 2. Plots at high and low conversion for the phototransposition
reaction of PMBN (8) in acetonitrile: PMBN (b), MMBN (9), OMBN
(().
Figure 3. Plots at high and low conversion for the phototransposition
reaction of MMBN (9), in acetonitrile: PMBN (b), MMBN (9), and
OMBN (().
quantitatively for the para and meta isomers, giving quantum
yields of disappearance (at less than 3% conversion) of 0.025
( 0.001 and 0.003 ( 0.001, respectively. The ortho isomer
was too unreactive to measure its quantum yield reliably. The
previous studies on alkyl benzenes3 and methylbiphenyls17
suggested that one of the driving forces for phototranspositions
was relief of steric strain in the ortho isomers. For the
methylbenzonitriles studied here, this is not the case because
the ortho isomer is the least reactive and the para the most.
Clearly, electronic effects are the dominant factor.
Second, the low conversion plots for PMBN and OMBN
show that both 1,2- and 1,3-transpositions are occurring as
primary photochemical events. No delay of formation of the
1,3-transposition products is observed as would be expected if
they are formed from the 1,2-transposition product by secondary
photochemistry. At low conversion, for the para isomer, the
limiting ratio of meta:ortho is 4.2:1; for the meta isomer, the
ratio of ortho:para is 1.7:1; and for the ortho isomer, the ratio
of meta:para is 7.1:1.
Third, for the more efficiently reacting para and meta isomers
(Figures 2 and 3), a photostationary state is approached at high
conversions. Because both 1,2- and 1,3-transpositions are
occurring, the isomers are equilibrating through a triangular
pathway as shown in Figure 5. Estimates for the efficiency of
each of these six conversions can be made from the relative
reactivity (at low conversion) of each isomer para:meta:ortho
) 32:4:1; their molar absorptivities (ꢀ) at λmax (Table 1) para:
meta:ortho ) 784:1911:1363; and the relative yield (at low
conversion) of the isomers from each substrate, vide supra.
These relative efficiency values for each conversion are shown
by the reaction arrows in Figure 5, with the least efficient
process (OMBN to PMBN) assigned a value of one. Values
of the isomer composition at complete photoequilibration can
be calculated using the steady-state approximation: PMBN, 3%;
MMBN, 20%; OMBN, 77%. These values are in reasonable
agreement with the product composition plots for the para
(Figure 2) and meta (Figure 3) isomers which gave a PMBN:
MMBN:OMBN ratio of 7:26:67 when the irradiations were
stopped. Longer irradiation times result in yellowing of the
solutions leading to decreased absorbance by the benzonitriles
and therefore lower reactivity as well as decreased mass balance.
Consequently, the steady-state composition was not reached
experimentally.
Reaction Multiplicity. The multiplicity of the excited state
responsible for the phototransposition for both PMBN and
MMBN was determined by doing simultaneous quenched and
unquenched irradiations in a merry-go-round apparatus at 25°
C using a Rayonet reactor and 254 nm lamps. The quencher
used was 2,3-dimethyl-1,3-butadiene. Using the Stern-Volmer
slopes determined from fluorescence quenching, Table 1, the
percentage of the singlet-state excited states quenched can be
calculated, Table 2. Higher quencher concentrations decrease
the efficiency of the phototransposition reaction presumably
because of the known photochemical cycloaddition reactions29
of this diene with the methylbenzonitriles. The mechanism for
(29) Gilbert, A.; Griffiths, O. J. Chem. Soc., Perkin Trans. 1 1993, 1379-
1384.

Equilibrium/Isomerization of p-, m-, and o-Methylbenzonitrile
J. Am. Chem. Soc., Vol. 120, No. 26, 1998 6447
triplet excited states of the methylbenzonitriles could not be
detected by LFP, their lifetimes could not be measured but they
are certainly expected to be longer lived than the singlets.
Assuming triplet lifetimes of 1 µs and diffusional quenching of
the triplet state, the percentage of triplet states quenched can
also be estimated, Table 2. Because the triplet states are likely
longer lived than 1 µs, the actual % T values are likely higher.
As can be seen from the phototransposition results in Table
2, the presence of the quencher decreases the rate of conversion
slightly for PMBN as expected because at a diene concentration
of 10-3 M, 20% of the singlet states are being quenched. For
MMBN at 10-4 M diene, essentially no singlet quenching
results. For both substrates, the ratio of products is independent
of the quencher, the classic “fingerprint” method30 for comparing
direct and quenched or direct and sensitized irradiations.
The conclusion reached from this section is that for the
methylbenzonitriles, as for previously studied substates where
multiplicity was examined, the excited singlet state is the
reactive one in the phototransposition reactions.
Photolysis of 2,6-Dideuterio-4-methylbenzonitrile (11).
This known31 deuterated isomer was synthesized by the pathway
shown in eq 4 with the idea that the deuterium labeling might
provide important information on the mechanism for the
phototranspositions of the methylbenzonitriles. For the para
isomer, there are four possible meta bondings which lead to a
prefulvene biradical and then, possibly, to a benzvalene. For
illustrative purposes, only two of these four possibilities are
shown: in eq 5 (Scheme 1), meta bonding occurs between C2
and C6 and the cyano group is on the single-carbon bridge of
the prefulvene biradical; in eq 6 (Scheme 1), the meta bonding
is between C3 and C5 and the methyl group is situated on the
bridge. The first pathway will then lead to 5,6-dideuterio-3-
methylbenzonitrile (specifically labeled MMBN) and then to
4,5-dideuterio-3-methylbenzonitrile (specifically labeled OMBN),
whereas the second will give the isomeric 2,6-dideuterio-3-
methylbenzonitrile (specifically labeled MMBN) and then 3,6-
dideuterio-2-methylbenzonitrile (specifically labeled OMBN).
Of course, if both these and the other two possible pathways
(not shown) are occurring simultaneously, then a complex
mixture of labeled isomers will be formed.
The position of the deuterium labels in the three isomers was
determined by 1H, 2H, or 13C NMR spectra. All three methods
gave the same result, but because of the increased dispersion,
the 13C NMR spectra provide the clearest proof of the pathway-
(s) occurring. As shown in Figure 6a, a 13C NMR spectrum of
a mixture of the three isomers (41% para, 34% meta, and 25%
ortho) gives 10 signals in the region that corresponds to the
aromatic carbons which are hydrogen substituted; four of these
belong to the ortho isomer, four to the meta and two to the
para. Of the 10, eight can be assigned on the basis of literature
spectra,32 but the choice of assignment of C5 and C6 for the
signals labeled 2 and 3 in Figure 6a at δ 128.9 and 129.2 for
MMBN is not known and proved to be inconsequential. The
Figure 4. Plots at high and low conversion for the phototransposition
reaction of OMBN (10) in acetonitrile: PMBN (b), MMBN (9), and
OMBN (().
Figure 5. Efficiencies for the photoequilibration of PMBM, MMBN,
and OMBN.
Table 2. Photolysis of PMBN and MMPN in the Presence of
2,3-Dimethyl-1,3-butadiene
substrate t (min)a Q (M × 103)b % conv
% S1 % Td
c
meta/ortho
4.25
PMBN
15
15
30
30
0
1
0
1
6.6
4.0
8.2
7.4
0
20
0
0
95
0
4.23
3.93
3.75
20
95
ortho/para
1.74
MMBN
60
60
120
120
0
0.1
0
6.6
6.1
12.1
12.2
0
2
0
2
0
67
0
1.64
1.86
1.88
0.1
67
a Irradiation time. b 2,3-Dimethyl-1,3-butadiene. c Percentage of ex-
cited singlet states quenched. d Percentage of the excited triplet states
quenched assuming a triplet lifetime of 1 µs and a rate constant for
triplet quenching of 2 × 1010 M-1 s-1, the diffusional rate.
(30) Zimmerman, H. E. Angew. Chem., Int. Ed. Engl. 1969, 8, 1-11.
(31) Ripka, W. C.; Applequist, D. E. J. Am. Chem. Soc. 1967, 89, 4035-
this quenching is suggested to be by way of a polarized
intermediate formed between S1 of the arene and S0 of the diene
but not by electron transfer between the addends. Because the
4039.
(32) Haupt, E. T. K.; Leibfritz, D. Spectrochim. Acta 1989, 45A, 119-
121.

6448 J. Am. Chem. Soc., Vol. 120, No. 26, 1998
MacLeod et al.
Scheme 1
spectrum for 11, the selectively dideuterated para isomer, gives
only one strong signal at δ 129.7; the replacement of hydrogen
with deuterium at C2 and C6 greatly reduces the NOE of the
carbon signals so that the one at δ 131.9 is considerably weaker.
The expansion, Figure 6b, shows that the carbon signal can still
be observed as a weak triplet (slightly isotope shifted) because
of coupling to the deuterium; the even weaker signal for the
residual C-H is also apparent. Figure 6c is a 13C NMR
spectrum of a sample of 11 that has been irradiated to a
composition of 36% para, 35% meta, and 29% ortho. Only
five strong signals are present: one from the starting para isomer
and two each from meta and ortho. These signals correspond
to those carbons which are hydrogen-substituted. Only very
weak signals, scarcely above the noise level, can be seen at
other chemical shifts. The transposition is obviously highly
selective and gives a deuterium distribution identical to that
expected from eq 5.
An important observation to make is that by this extent of
irradiation, significant amounts of meta and ortho isomers have
been formed from the starting para compound. Therefore,
photoequilibration has commenced so that some para and ortho
isomers are formed from the meta isomer. Thus, the product
composition also reflects the photochemistry of a selectively
labeled meta isomer. Nevertheless, the para isomer remains
essentially exclusively labeled as it began, that is, 2,6-dideu-
terated. Hence the phototransposition reactions of at least two
of the methylbenzonitrile isomers (para and meta) only involve
migration of the cyano-substituted carbon. Moreover, there is
no evidence that ortho reactivity, which is very low relative to
the other isomers, involves migration of other than the cyano
carbon.
Mechanistic Conclusions. The most probable mechanism
for these phototransposition reactions can be summarized in
Figure 7 and eq 7. Excitation of PMBN in the Lb band gives
the reactive excited state, S1. This is clearly a minimum on
the surface because fluorescence is observed. A barrier
separates this state from the biradical 12, which results from
remarkably selective meta bonding between C2 and C6. On
the basis of the theoretical calculations for benzene,16 this barrier
is a result of a conical intersection and 12 is a ground-state
biradical. For both 1,2- and 1,3-transpositions to be explained
as primary photochemical events, 12 must isomerize by migra-
tion of the cyano-substituted carbon to 13 and 14, which then
collapse back to MMBN and OMBN. In permutation analysis,5
the mechanism proposed is P2 for the 1,2-transposition and P4
for the 1,3-transposition. The energetics of the barriers and
minima on the surface connecting 12, 13, and 14 are not easy
to predict. Using ground-state arguments, 13 should be the most
stable of the three minima and the other two might be of
comparable stability. The structures 12, 13, and 14 are shown
as biradicals, but obviously, as ground-state singlets, they could
have considerable zwitterionic character. In that case, the cyano-
substituted carbon would be negative and the allyl moiety would
be positive.
Similarly, irradiation of MMBN and OMBN produce 13 and
14, respectively, in the primary photochemical event from their
S1 states. Thus, any one of the isomers can produce the other
two. However, the trajectory for arriving at the 12/13/14 surface
may be very dependent on the starting material, and experi-
mentally, the barrier on the excited-state surface is much higher
for OMBN than for PMBN or MMBN. High-level MO
calculations might be used to probe the details of this surface.
Substituted benzvalenes have not been included as intermediates
in these reactions both because of the observation of simulta-
neous single-photon 1,2- and 1,3-transpositions and because
there is no need to include them. Possibly, they are formed
but not detected because of their high reactivity.
These results also have a bearing on the mechanism of the
photochemical cycloaddition of alkenes and dienes to substituted
benzenes. Thus PMBN, MMBN, and OMBN have all been
shown to react with trans-1,2-dichloroethene,33 furan,33 and 2,3-
dimethyl-1,3-butadiene.29 The products for PMBN are shown
(33) Cornelisse, J. Chem. ReV. 1993, 93, 615-669.

Equilibrium/Isomerization of p-, m-, and o-Methylbenzonitrile
J. Am. Chem. Soc., Vol. 120, No. 26, 1998 6449
Figure 7. Energy versus reaction coordinate for the photochemical
equilibration and cycloaddition reactions of PMBN.
tions were made for the adducts from MMBN and OMBN.
Because the phototransposition reactions demonstrate that the
preferred biradical from the excited singlet state of the ben-
zonitriles has the cyano group on the single-carbon bridge, these
results support previous mechanistic conclusions that the
cycloadditions occur by direct reaction of the excited singlet
state of the aromatic with the unsaturated compound and not
from a preformed biradical.29,33
Finally, the effect that other substituents have on the
efficiency and selectivity of these transposition reactions is
obviously of interest. We have confirmed and extended the
previous observations for the xylenes and are currently examin-
ing other cases. At the moment, we have no clear idea as to
why meta bonding in the methylbenzonitriles should be so
selective for the cyano-substituted carbon. Preliminary results
for the irradiation of PMBN in 2,2,2-trifluoroethanol indicate
that a complex set of addition products are formed that are
analogous in structure to 5 and 6. Their structures should give
valuable information on the prefulvene biradicals 12, 13, and
14 and/or the substituted benzvalene intermediates. Results on
these studies will be reported soon.
Figure 6. (a) 13C NMR spectrum of aromatic C-H carbons for a
mixture of PMBN, MMBN, and OMBN. (b) 13C NMR spectrum of
the aromatic C-H/C-D carbons of 2,6-dideuterio-4-methyl benzonitrile
(11). The inset is an expansion of C2tC6. (c) 13C NMR spectrum of
the aromatic C-H/C-D carbons for the mixture of PMBN, MMBN,
and OMBN obtained from photolysis of 11 in acetonitrile.
for each in eqs 8, 9, and 10. For the alkene and furan additions,
meta photocycloadditions are observed but the products are not
those that would result from the prefulvene biradicals 12, 13,
and 14, because the cyano group is not at the single-carbon
bridgehead position. For the diene addition, eq 10, 1,4-adducts
are observed, again not from 12, 13, and 14. Similar observa-

6450 J. Am. Chem. Soc., Vol. 120, No. 26, 1998
MacLeod et al.
cycles. For quantum yield measurements, fluorescence intensities
relative to toluene (ΦF ) 0.13)34 were measured using solutions of
equal absorbance at 254 nm.
Experimental Section
Chemicals. P-, m-, and o-methylbenzonitrile (Aldrich) were bulb-
to-bulb distilled before use. GC analyses were performed on a Perkin-
Elmer Autosystem using a Supelco Alpha-Dex column, 30 m × 0.25
mm, at 185° C using He as gas carrier (split) and FID detection. The
retention time and purity of each isomer was PMBN, 5.04 min (<0.05%
meta, <0.05% ortho); MMBN, 4.93 min (0.48% ortho, 0.80% para);
and OMBN, 4.51 min (1.77% meta, 0.31% para). Naphthalene
(retention time, 5.93 min) was used as an internal standard, and
calibration plots for all three isomers were linear. Acetonitrile
(OMNISolv HPLC, Spectrophotometry and Gas Chromatography grade)
was used without further purification. For quenching studies, 2,3-
dimethyl-1,3-butadiene (Aldrich) was distilled immediately before use.
2,6-Dideuterio-4-methylbenzonitrile was prepared as described previ-
ously31 by exchange of the protons ortho to the ammonium group in
p-toluidine hydrochloride with D2O followed by conversion, after
diazotization, to the nitrile with cuprous cyanide. Mass spectral analysis
Quantum Yield Measurements for PMBN were done using the
Rayonet reactor at 25° C and ferrioxalate (0.06 M) actinometry.35 To
account for the very unequal absorbances of the substrate and the
ferrioxalate actinometer, the previously proposed36 correction method
using three simultaneously irradiated samples in concentric quartz tubes
was used, as follows: Sample 1, outer tube (acetonitrile), inner tube
(PMBN); sample 2, outer tube (acetonitrile), inner tube (ferrioxalate
actinometer); and sample 3, outer tube (PMBN), inner tube (ferrioxalate
actinometer). The dimensions of the tubes were as follows: inner, 4
mm i.d. × 17 cm tall, 2 mL volume; outer, 13 mm i.d. × 17 cm tall,
15 mL volume. The concentration of PMBN was adjusted so that the
absorbance was between 2 and 3. The difference in the ferrioxalate
conversion between sample 2 and sample 3 was used as the light
absorbed by the PMBN in sample 1. The quantum yield for MMBN
was measured relative to that for PMBN.
1
gave the molecular ion for C8H5D2N, and H NMR showed less than
2% protons at C2 and C6.
Acknowledgment. We thank NSERC of Canada for finan-
cial support. P.J.M. thanks Dalhousie University for a graduate
fellowship and NSERC of Canada and the Killam Foundation
for scholarship awards. K.A.T. thanks the Dalhousie University
Department of Chemistry for an undergraduate summer research
scholarship.
Photolyses. Product ratio studies were carried out using either a
Rayonet reactor with 254 nm low-pressure mercury lamps or an
immersion well with a 450 W Hanovia medium-pressure mercury lamp
and a Vycor filter. Samples to be irradiated (∼10-2 M in acetonitrile)
were purged with nitrogen before and during photolysis. For the
Rayonet reactions, the sample was thermostated at 25° C with a
circulating water cooling tube centered in the 100 mL quartz reactor.
For both reactors, the process of photolysis was monitored by GC after
adding the standard naphthalene solutions (10 µL into 1 mL of sample).
Percentage yields were corrected for the small amount of isomeric
impurities in o- and m-methylbenzonitrile.
JA9808301
(34) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry, 2nd ed.; Marcel Dekker: New York, 1993; p 51.
(35) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London 1956, 235A,
518-536.
(36) Bunce, N. J. In Handbook of Organic Photochemistry, Scaiano, T.,
Ed.; CRC Press: Boca Raton, FL, 1989; Vol. I, pp 241-259.
Fluorescence measurements were made with either an Aminco-
Bowman Spectrofluorimeter or a Perkin-Elmer MPF 66 fluorescence
spectometer. All samples were degassed by three freeze-pump-thaw