Conrotatory photochemical ring opening of alkylcyclobutenes in solution. A test of the hot ground-state mechanism

Article abstract of DOI:10.1021/ja003860o

Quantum yields for photochemical ring opening of six alkylcyclobutenes have been measured in hexane solution using 228-nm excitation, which selectively populates the lowest π,R(3s) excited singlet states of these molecules and has been shown previously to lead to ring opening with clean conrotatory stereochemistry. The compounds studied in this work - 1,2-dimethylcyclobutene (1), cis- and trans-1,2,3,4-tetramethylcyclobutene (cis- and trans-5), hexamethylcyclobutene (8), and cis- and trans-tricyclo[6.4.0.0^2,7]dodec-1²-ene (cis- and trans-9) - were selected so as to span a broad range in molecular weight and as broad a range as possible in Arrhenius parameters for thermal (ground-state) ring opening. RRKM calculations have been carried out to provide estimates of the rate constants for ground-state ring opening of each of the compounds over a range of thermal energies from 20 00O to 49 000 cm^-1. These have been used to estimate upper limits for the quantum yields of ring opening via a hot ground-state mechanism, assuming a value of k_deact = 10¹¹ s^-1 for the rate constant for collisional deactivation by the solvent, that internal conversion to the ground state from the lowest Rydberg state occurs with close to unit efficiency, and that ergodic behavior is followed. The calculated quantum yields are significantly lower than the experimental values in all cases but one (1). This suggests that the Rydberg-derived ring opening of alkylcyclobutenes is a true excited-state process and rules out the hot ground-state mechanism for the reaction.

Full text of DOI:10.1021/ja003860o

5188
J. Am. Chem. Soc. 2001, 123, 5188-5193
Conrotatory Photochemical Ring Opening of Alkylcyclobutenes in
Solution. A Test of the Hot Ground-State Mechanism
Bruce H. O. Cook, William J. Leigh,* and Robin Walsh
Contribution from the Departments of Chemistry, McMaster UniVersity, 1280 Main Street West,
Hamilton, Ontario, Canada L8S 4M1, and The UniVersity of Reading, P.O. Box 224,
Whiteknights, Reading RG6 6AD, U.K.
ReceiVed NoVember 2, 2000. ReVised Manuscript ReceiVed March 16, 2001
Abstract: Quantum yields for photochemical ring opening of six alkylcyclobutenes have been measured in
hexane solution using 228-nm excitation, which selectively populates the lowest π,R(3s) excited singlet states
of these molecules and has been shown previously to lead to ring opening with clean conrotatory stereochemistry.
The compounds studied in this works1,2-dimethylcyclobutene (1), cis- and trans-1,2,3,4-tetramethylcyclobutene
(cis- and trans-5), hexamethylcyclobutene (8), and cis- and trans-tricyclo[6.4.0.02,7]dodec-12-ene (cis- and
trans-9)swere selected so as to span a broad range in molecular weight and as broad a range as possible in
Arrhenius parameters for thermal (ground-state) ring opening. RRKM calculations have been carried out to
provide estimates of the rate constants for ground-state ring opening of each of the compounds over a range
of thermal energies from 20 000 to 49 000 cm-1. These have been used to estimate upper limits for the quantum
yields of ring opening via a hot ground-state mechanism, assuming a value of kdeact ) 1011 s-1 for the rate
constant for collisional deactivation by the solvent, that internal conversion to the ground state from the lowest
Rydberg state occurs with close to unit efficiency, and that ergodic behavior is followed. The calculated quantum
yields are significantly lower than the experimental values in all cases but one (1). This suggests that the
Rydberg-derived ring opening of alkylcyclobutenes is a true excited-state process and rules out the hot ground-
state mechanism for the reaction.
Introduction
Other results suggest that the stereospecificity is lost because
the reaction proceeds adiabatically, yielding the disrotatory diene
isomer in an excited state, from which it decays by E,Z-
isomerization.9-11
It has been known for some time that the photochemical
electrocyclic ring opening of alkylcyclobutenes proceeds non-
stereospecifically when these compounds are irradiated in
solution with 185- or 193-nm light.1 In most cases that have
been studied, the formation of the corresponding alkyne and
alkene by (stereospecific) [2 + 2] photocycloreversion competes
with ring opening.2,3 It has been concluded that the two types
of reactions originate from two different excited singlet states:
ring opening from the valence π,π* state and cycloreversion
from the π,R(3s) Rydberg state,4 whose energy is similar to or
lower than that of the π,π* state in alkylcyclobutenes and other
nonconjugated olefins.5
Despite the overall nonstereospecificity of the ring-opening
reaction, time-resolved spectroscopic studies on the parent
molecule6 and steric effects on the quantum yields for 214-nm
ring opening of monocyclic cyclobutenes7 both suggest that the
reaction begins with disrotatory stereochemistry on the π,π*
excited-state surface, as orbital symmetry selection rules predict.8
The precise role of the π,R(3s) Rydberg state in cyclobutene
photochemistry has continued to be of concern, particularly with
respect to the ring-opening reaction. A recent paper described
the results of a more systematic attempt to investigate the
Rydberg-state photochemistry of alkylcyclobutenes, through
study of three derivatives in which the π,R(3s) excited state is
of significantly lower energy than the valence π,π* singlet state
and hence can be populated selectively.12 It was found that
selective excitation at the long-wavelength edge of the Rydberg
absorption band of 1,2-dimethylcyclobutene (1), both in the gas
phase at 1 atm and in hexane solution, results in substantial
yields of both ring-opening and cycloreversion products (eq 1),
indicating that the Rydberg state is, in fact, active toward ring
(1) Clark, K. B.; Leigh, W. J. J. Am. Chem. Soc. 1987, 109, 6086.
(2) Leigh, W. J. Chem. ReV. 1993, 93, 487.
(3) Leigh, W. J. In CRC handbook of organic photochemistry and
photobiology; Horspool, W. G., Song, P.-S., Eds.; CRC Press: Boca Raton,
FL, 1995; pp 123-142.
(4) Leigh, W. J.; Zheng, K.; Nguyen, N.; Werstiuk, N. H.; Ma, J. J. Am.
Chem. Soc. 1991, 113, 4993.
opening. Furthermore, results obtained for cis- and trans-1,2,3,4-
(5) Robin, M. B. Higher excited states of polyatomic molecules, Vol. II;
Academic Press: New York, 1975; pp 1-68.
(9) Leigh, W. J.; Zheng, K. J. Am. Chem. Soc. 1991, 113, 4019.
(10) Leigh, W. J.; Postigo, J. A.; Venneri, P. C. J. Am. Chem. Soc. 1995,
117, 7826.
(11) Leigh, W. J.; Postigo, J. A.; Zheng, K. C. Can. J. Chem. 1996, 74,
951.
(12) Leigh, W. J.; Cook, B. H. O. J. Org. Chem. 1999, 64, 5256.
(13) Kropp, P. J. Org. Photochem. 1979, 4, 1.
(6) Lawless, M. K.; Wickham, S. D.; Mathies, R. A. J. Am. Chem. Soc.
1994, 116, 1593.
(7) Leigh, W. J.; Postigo, J. A. J. Am. Chem. Soc. 1995, 117, 1688.
(8) Woodward, R. B.; Hoffmann, R. The conserVation of orbital
symmetry; Verlag-Chemie: Weinheim, 1970.
10.1021/ja003860o CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/11/2001

Conrotatory Photochemical Ring Opening of Alkylcyclobutenes
J. Am. Chem. Soc., Vol. 123, No. 22, 2001 5189
tetramethylcyclobutene (5; eqs 2 and 3, respectively) indicate
for reaction, at whatever internal thermal energy the molecule
possesses when it arrives in the ground state (and after
intramolecular vibrational energy redistribution, which occurs
on a time scale of 10-11-10-12 s24,25,29,30) must be on the order
of 1010 s-1 or higher. Such processes are especially viable for
small molecules with high excited-state energies, and which are
prone to thermal reaction involving low activation barriers.
Simple alkylcyclobutene derivatives would appear to be par-
ticularly reasonable candidates for this, as they undergo thermal
ring opening with Arrhenius activation energies on the order
of ∼35 kcal/mol (log A ≈ 13.5)31 and absorb below ∼230 nm.
For example, for a molecule such as 1 containing 42 vibrational
degrees of freedom and ES1 ≈ 120 kcal/mol, internal conversion
from S1 produces a ground-state molecule that contains excess
vibrational energy corresponding to an effective temperature
of roughly 2300 K (vide infra); a simple calculation (using this
temperature and the published Arrhenius parameters for thermal
ring opening of the molecule32-34) suggests that it should
isomerize with a rate constant on the order of 2 × 1010 s-1. It
is thus clear that the possibility of a hot ground-state reaction
is a viable one, and so we decided to investigate it in more
detail.
that Rydberg-derived ring opening proceeds with a high degree
of conrotatory stereospecificitysthe stereochemistry associated
with the ground-state process. This led to the suggestion of two
possible mechanisms for the Rydberg-derived process. The first
is as a true excited-state reaction, where the observed conrotatory
stereospecificity might be related to the radical cation character
associated with the olefinic π,R(3s) state;13,14 cyclobutene radical
cations undergo ring opening in the gas phase,15,16 and theoreti-
cal calculations suggest that the reaction should proceed with
preferred conrotatory stereochemistry.17,18 On the other hand,
ring opening of cyclobutene radical cations generally does not
occur in condensed phases19,20 except with photochemical
activation,20-22 but the one known example of solution-phase
ring opening proceeds with conrotatory stereochemistry.23 The
second possibility is that it proceeds via a hot ground-state
mechanism, with the Rydberg excited state merely providing a
conduit for efficient internal conversion to upper vibrational
levels of the ground state, from which reaction ensues in
competition with collisional deactivation of the thermally
activated molecule by the solvent.
The approach we have used involves measurement of
quantum yields for ring opening of a series of 1,2-dialkyl-
cyclobutenes (1, 5, 8, 9) of varying molecular weights and
Arrhenius parameters for thermal ring opening, with the idea
of comparing them to “expected” values, calculated with the
assumption that ring opening proceeds exclusively via the hot
ground-state mechanism. The expected quantum yield for ring
opening by this mechanism is given by eq 4, where kRO is the
rate constant for ground-state ring opening of the molecule at
a total internal energy corresponding to that of a 228-nm photon,
kdeact is the rate constant for its collisional deactivation by the
solvent, and internal conversion is assumed to be the only
pathway for excited-state decay other than cycloreversion. The
latter assumption maximizes the quantum yield for ring opening
that is predicted by this mechanism.
Although photoinduced hot ground-state reactions are com-
mon in the gas phase, they are much less so in solution, where
collisional deactivation of vibrationally excited molecules occurs
on a time scale of approximately 10-11 s.24-28 Thus, for such
processes to be observed in significant yields, the rate constant
Φcalc ) (1 - ΦCR)[kRO/(kRO + kdeact)]
(4)
The extensive time-resolved spectroscopic studies of Troe
and Luther and their co-workers on the collisional deactivation
of vibrationally excited organic molecules, such as cyclohep-
tatriene28 and azulene,35 in hydrocarbon solvents allow an
(14) Kropp, P. J. In CRC handbook of organic photochemistry and
photobiology; Horspool, W. G., Song, P.-S., Eds.; CRC Press: Boca Raton,
FL, 1995; pp 16-28.
(15) Gross, M. L.; Russell, D. H. J. Am. Chem. Soc. 1979, 101, 2082.
(16) Dass, C.; Sack, T. M.; Gross, M. L. J. Am. Chem. Soc. 1984, 106,
5780.
(26) Hippler, H.; Troe, J.; Wendelken, H. J. J. Chem. Phys. 1983, 78,
6709.
(27) Hippler, H.; Troe, J. In Bimolecular Collisions; Ashfold, M. N. R.,
Baggott, J. E., Eds.; Royal Society of Chemistry: London, 1989; pp 209-
262.
(28) Benzler, J.; Linkersdorfer, S.; Luther, K. J. Chem. Phys. 1997, 106,
4992.
(29) Oref, I.; Rabinovitch, B. S. Acc. Chem. Res. 1979, 12, 166.
(30) Diau, E. W. G.; Herek, J. L.; Kim, Z. H.; Zewail, A. H. Science
1998, 279, 847.
(31) Frey, H. M.; Walsh, R. Chem. ReV. 1969, 69, 103.
(32) Frey, H. M. Trans. Faraday Soc. 1963, 59, 1619.
(33) Criegee, R.; Seebach, D.; Winter, R. E.; Borretzen, B.; Brune, H.-
A. Chem. Ber. 1965, 98, 2339.
(34) Frey, H. M.; Montague, D. C.; Stevens, I. D. R. Trans. Faraday
Soc. 1967, 63, 372.
(17) Wiest, O. J. Am. Chem. Soc. 1997, 119, 5713.
(18) Sastry, G. N.; Bally, T.; Hrouda, V.; Carsky, P. J. Am. Chem. Soc.
1998, 120, 9323.
(19) Kawamura, Y.; Thurnauer, M.; Schuster, G. B. Tetrahedron 1986,
42, 6195.
(20) Brauer, B.-E.; Thurnauer, M. Chem. Phys. Lett. 1987, 133, 207.
(21) Gerson, F.; Qin, X.-Z.; Bally, T.; Aebischer, J.-N. HelV. Chim. Acta
1988, 71, 1069.
(22) Aebischer, J. N.; Bally, T.; Roth, K.; Haselbach, E.; Gerson, F.;
Qin, X.-Z. J. Am. Chem. Soc. 1989, 111, 7909.
(23) Miyashi, T.; Wakamatsu, K.; Akiya, T.; Kikuchi, K.; Mukai, T. J.
Am. Chem. Soc. 1987, 109, 5270.
(24) Gottfried, N. H.; Kaiser, W. Chem. Phys. Lett. 1983, 101, 331.
(25) Wild, W.; Seilmeier, A.; Gottfried, N. H.; Kaiser, W. Chem. Phys.
Lett. 1985, 119, 259.

5190 J. Am. Chem. Soc., Vol. 123, No. 22, 2001
Cook et al.
estimate of kdeact ≈ 1011 s-1 for hexane solution, a value which
Table 1. Experimental Quantum Yields for Ring Opening (ΦRO
)
and RO/CR Ratios from 228-nm Photolysis of Alkylcyclobutenes in
Hexane Solution, and Calculated Quantum Yields for Ring Opening
can be assumed to be roughly independent of solute structure
and size.36 The rate constants for thermal ring opening (kRO
)
Assuming the Hot Ground-State Mechanism (ΦCalc
)
have been calculated in one (or both) of two ways. Both
calculations required the prior computation of the complete
vibrational spectrum for each compound, which was ac-
complished by quantum mechanical calculation37 at the DFT/
3-21G* or DFT/6-31G*+d levels of theory. In the first method,
an effective temperature of the molecule (T), energized by the
228-nm photon, was estimated, and the rate constant k(T) was
then calculated from reported31,33,38 Arrhenius data. This was
applied to all six compounds studied. In the second method,
the energy-specific, or microcanonical, rate constant k(E) was
calculated using RRKM theory.39 This was applied to four of
the compounds studied. The results allow a quantitative assess-
ment to be made of the extent to which the hot ground-state
mechanism contributes to the photochemical ring opening of
alkylcyclobutenes under conditions of selective Rydberg-state
excitation.
b
c
compound
λ
ex (nm)
RO/CRa
ΦRO
Φcalc
0.17
0.022
1
228
228
228
0.79 ( 0.07 0.09 ( 0.02
0.69 ( 0.05 0.08 ( 0.01
cis-5
trans-5
0.25 ( 0.02 0.04 ( 0.01 (EE) 0.038 (EE)
0.04 ( 0.01 (ZZ) 0.016 (ZZ)
8
cis-9
228
228
214
228
214
0.70 ( 0.07 0.11 ( 0.02
0.0030
0.0005
2.6 ( 0.3
2.7 ( 0.2
0.13 ( 0.02
0.15 ( 0.02
trans-9
0.39 ( 0.04 0.20 ( 0.02
0.27 ( 0.03 0.06 ( 0.02
0.022
a Ratio of the yield of ring-opening products (all isomers) to
cycloreversion product. b The quantum yield for ring opening of 1 was
measured by uranyl oxalate actinometry and is the average of four
determinations; those for 5, 8, and 9 were measured using 1 as a
secondary actinometer and are the averages of two determinations each.
c Calculated according to eq 4, using RO/CR and ΦRO to estimate ΦCR
,
and k(E) from Table 2 (k(T) for cis- and trans-9), and kdeact ) 1011 s-1
in each case.
Results and Discussion
phy, and the products were identified either by co-injection with
authentic samples that were prepared by known procedures (10,
12) or obtained commercially (11), or after isolation from semi-
preparative-scale irradiations and comparison of spectral proper-
ties to literature data (E- and Z-13).
As we found previously for 1 and 5,12 direct irradiation of
hexamethylcyclobutene (8) and cis- and trans-tricyclo[6.4.0.02,7]-
dodec-12-ene (cis- and trans-9) as 0.05 M solutions in hexane,
with the light from a cadmium resonance lamp (228 nm), results
in the formation of the corresponding ring-opening and cyclo-
reversion products (eqs 5-7). The photolyses were monitored
The quantum yield for ring opening of 1 with 228-nm
excitation (Φexp) was determined by uranyl oxalate actinometry.
Quantum yields for ring opening of the other five compounds
were then determined using 1 as a secondary actinometer, from
the relative slopes of concentration vs time plots for product
formation between 0.5 and 5% conversion (see Supporting
Information). Plots of the relative concentration of ring-opening
(RO) and cycloreversion (CR) products ([10]/[3] for 8 and [12]/
[13] for 9) versus time yielded straight lines with slopes
indistinguishable from zero, showing them to be constant over
the conversion ranges monitored in all cases. Quantum yields
for ring opening of cis- and trans-9 were also determined with
214-nm excitation, again using 1 as a secondary actinometer
(Φ2 ) 0.067). The quantum yield data and RO/CR ratios are
summarized in Table 1. No other products could be detected in
any of the systems studied, within the limits of our analytical
method over the conversion range monitored.
The quantum yields for 228-nm ring opening of the six
cyclobutenes studied in this work vary only slightly throughout
the series, and in most cases are similar to those obtained with
214-nm excitation. The one exception is trans-9, which reacts
3-4 times more efficiently at 228 nm than at 214 nm. Since
12 (as the stable cis,cis-isomer) is the product of conrotatory
ring opening of trans-9, the significantly increased quantum
yield for ring opening of this compound at 228 relative to 214
nm is consistent with the behavior of cis- and trans-5, both of
which undergo clean conrotatory ring opening with 228-nm
excitation.12 Interestingly, the RO/CR ratios for both isomers
of 9 and the quantum yield for ring opening of the cis-isomer
change very little as a function of excitation wavelength.
Vibrational spectra for 1, cis- and trans-5, and 8 were
calculated at the B3LYP/6-31G*+d level of theory, while those
for cis- and trans-9 were calculated at the B3LYP/3-21G* level.
A correction factor of 0.98 was applied to the vibrational
frequencies.37
as a function of irradiation time by capillary gas chromatogra-
(35) Schwarzer, D.; Troe, J.; Votsmeier, M.; Zerezke, M. J. Chem. Phys.
1996, 105, 3121.
(36) Hippler, H.; Troe, J.; Wendelken, H. J. J. Chem. Phys. 1983, 78,
6718.
(37) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
(38) Criegee, R.; Reinhardt, H. G. Chem. Ber. 1968, 101, 102.
(39) Holbrook, K. A.; Pilling, M. J.; Robertson, S. H. Unimolecular
Reactions; John Wiley and Sons: Chichester, 1996.
Estimates of k(T) for the six molecules were calculated from
the published Arrhenius parameters for thermal ring opening,
using “effective temperatures”, Teff, calculated from the statistical
thermodynamical expression for the total internal energy of a

Conrotatory Photochemical Ring Opening of Alkylcyclobutenes
J. Am. Chem. Soc., Vol. 123, No. 22, 2001 5191
Table 2. Thermal Ring Opening of Alkylcyclobutenes: Arrhenius
Parameters,31,33,38 Effective Temperatures, and Calculated Rate
Constants (E ) 43 860 cm-1
)
compound
(3N - 6)
Ea
(kcal/mol) log(A/s-1
Teff/
k(T)/
k(E)/
)
Ka 109 s-1 b 109 s-1 c
1 (42)
cis-5 (60)
trans-5 (60)
36.0
37.7
13.84
14.2
2340 29
1970 10
21
2.2
4.0 (EE)
1.6 (ZZ)
0.30
33.6 (EE) 13.85 (EE) 1815
6.2
3.3
0.8
0.05
2.3
∼38.7 (ZZ) 14.2 (ZZ)
1815
1515
1490
1495
8 (78)
cis-9 (84)
trans-9 (84)
38.7d
44.2
28.9
14.5d
14.2
13.6
a Interpolated from plots of total internal energy ( E ; see eq 8) versus
temperature. b Thermal rate constant from Arrhenius equation at Teff.
c RRKM calculated rate constant. d Changed from the reported values33
of Ea ) 40.5 kcal/mol and log(A/s-1) ) 15.5 (see text).
Figure 1. log(k(E)) (from RRKM theory) vs E for the ring opening
of alkylcyclobutenes 1 (0), cis-5 (]), trans-5 (EE, O; ZZ, b), and 8
(4).
molecule (eq 8), given the computed vibrational spectra and a
3N-6
hν(i)
E )
(8)
∑
exp{hν(i)/kT} - 1
i)1
total internal energy corresponding to that of a 228-nm photon
( E ) 43 860 cm-1). The reported Arrhenius parameters for
thermal ring opening of 8 (Ea ) 40.5 and log(A/s-1) ) 15.5,
based on three data points)33 seemed unrealistically high, so
they were adjusted to values which seemed more reasonable
(Ea ) 38.7 and log(A/s-1) ) 14.5), and which reproduced the
rate constants to within 25% of their reported values over the
180-200 °C temperature range. Arrhenius parameters for ring
opening of trans-5 to ZZ-6 are unknown, and so they were
assumed to be roughly the same as those for ring opening of 8:
Ea )38.7 kcal/mol and log A ) 14.2. Table 2 summarizes the
published Arrhenius parameters for the six compounds, their
effective temperatures, and the values of k(T) estimated by this
procedure.
Figure 2. Comparison of calculated (Φcalc) versus experimental (ΦRO
quantum yields for 228-nm photochemical ring opening of alkyl-
cyclobutenes.
)
Estimates of k(E) were calculated for 1, cis- and trans-5, and
8 using RRKM theory.39 For each compound, the ring C-C
stretching mode at ca. 1150 cm-1 was chosen as the primary
reaction coordinate, and four of the low-frequency modes were
chosen as variables for estimation of the vibrational spectrum
of the activated complex. The remaining portion of the spectrum
was then abbreviated by grouping vibrations of similar frequen-
cies together, to produce spectra containing a maximum of 40
individual frequencies. The spectrum of the activated complex
was assumed to be identical to that of the initial substrate, except
for the primary reaction coordinate and the four modes chosen
as variables; these were adjusted manually so as to reproduce
the entropy of activation corresponding to the reported Arrhenius
A factor for thermal isomerization of the molecule. Values of
k(E) were then calculated for internal energies ranging from
20 000 to 49 000 cm-1. The results of these calculations are
shown graphically in Figure 1; the k(E) values at an internal
energy of 43 860 cm-1 were then obtained by interpolation of
these data and are listed in Table 2. As can be seen from the
table, the RRKM rate constants, k(E), are all quite similar to
the more qualitative thermal rate constants, k(T), obtained using
the Arrhenius equation and effective temperatures calculated
from statistical thermodynamics.
While the experimental quantum yields for ring opening vary
by less than a factor of 5 throughout within the series, the values
calculated from the RRKM data show the expected marked
variation with molecular size. Figure 2 shows a plot of calculated
vs experimental quantum yields for the six compounds studied
in this work, which illustrates the resulting poor fit of the
experimental data to the mechanistic model on which the
calculated values are based. For the larger molecules in the series
(8 and 9), the experimental quantum yields are between 10 and
ca. 300 times higher than the values predicted on the basis of
the hot ground-state model, differences that are well beyond
what might be accommodated by the uncertainties associated
with the various assumptions used in the calculations. Thus,
the results strongly suggest that, in general, Rydberg-derived
ring opening does proceeds not via the hot ground-state
mechanism but as a true excited-state process in which ring
opening is initiated on the π,R(3s) potential energy surface.
Unlike π,π* state ring opening, which orbital symmetry rules
predict to proceed in disrotatory fashion, the π,R(3s) process
appears to proceed with the preferred conrotatory stereochem-
istry that is predicted theoretically for the ring opening of
cyclobutene radical cations.17,18 Simple orbital symmetry con-
siderations lead to the prediction that π,R(3s) excited-state ring
opening should proceed with the same stereochemistry as the
ground-state process, since the highest occupied π-MO of the
diene product is the same for ring opening of the π,R(3s) state
as it is for ring opening in the ground state. The same is true
for ring opening of the radical cation.
Expected values for the quantum yield of ring opening by
the hot ground-state mechanism (Φcalc) were then calculated
according to eq 4, using as estimates for kRO either the k(E)
values from RRKM theory for 1, 5, and 8 or the k(T) values
for 9, together with the value of 1011 s-1 for kdeact. These are
listed in Table 1, along with the experimental quantum yield
data for these compounds.

5192 J. Am. Chem. Soc., Vol. 123, No. 22, 2001
Cook et al.
is generally considerable overlap between the valence and
Rydberg absorptions in the gas- and solution-phase UV absorp-
tion spectra of alkylcyclobutene derivatives, and so it remains
to be determined whether competing reaction from the π,π*
(disrotatory) and π,R(3s) (conrotatory) excited states might
provide a more likely explanation for the overall non-
stereospecificity observed upon shorter wavelength excitation.
This possibility will be addressed in a forthcoming paper.
Experimental Section
The cyclobutenes studied in this work were prepared and purified
as previously reported,7,40 while photoproducts were identified by GC
co-injection with authentic samples or after isolation from semi-
preparative-scale photolyses of the corresponding cyclobutene deriva-
tive. The cyclobutenes were purified to >99% purity by semi-
preparative gas chromatography, using stainless steel columns ([a] 20%
ODPN on 80/100 Chromosorb PNAW, 0.25 in. × 20 ft (compound
1); [b] 3.8% UCW-982 on 80/100 Supelcoport, 0.25 in. × 24 ft (cis-
and trans-5 and 8); [c] 15% Carbowax on 80/100 Chromosorb PNAW,
0.25 in. × 12 ft (cis- and trans-9)). Cyclohexane, hexanes, isooctane,
and decane (BDH Omnisolv) were used as received from the supplier.
UV absorption spectra were recorded using a Cary 50 UV/vis
spectrophotometer in 1-cm Suprasil cells. Analytical gas chromato-
graphic separations were carried out using a Hewlett-Packard 5890 gas
chromatograph equipped with a flame ionization detector and a 0.53-
mm × 30-m DB-1 fused silica column (Chromatographic Specialties,
Inc.) for compounds 1, cis- and trans-5, and 8, or a 0.22-mm × 15-m
DB-17 fused silica column (Chromatographic Specialities, Inc.) for cis-
and trans-9. FID response factors were determined for all compounds
by construction of standard working curves in the usual way.
Samples for photolysis contained the cyclobutene (ca. 0.05 M) and
cyclohexane or decane (0.001 M) in isooctane (1) or hexanes (5, 8, 9)
solution. Aliquots (ca. 0.3 mL) were placed in 5-mm-o.d. Suprasil tubes,
sealed with rubber septa, and deoxygenated with a stream of dry argon.
Photolyses were carried out in a merry-go-round apparatus, using 16-W
Philips 93106E zinc or 16-W Philips 93107E cadmium resonance lamps
for irradiation at 214 or 228 nm, respectively, and irradiating solutions
of 1 and one of the other cyclobutenes simultaneously. Aliquots were
removed at suitable time intervals and analyzed by GC. Product yields
were determined from the slopes of product concentration vs time plots
(see Supporting Information) and compared to the slope of the
analogous plot for the formation of 2 from 1 for the determination of
relative quantum yields.
Figure 3. Sketch of ground, π,π*, and π,R(3s) potential energy
surfaces for con- and disrotatory ring opening in monocyclic cy-
clobutenes (solid lines; e.g., cis-5) and in derivatives such as cis-9
(dashed lines), in which the conrotatory pathway is blocked due to
structural constraints.
It is interesting to note that cis-9 maintains a high degree of
reactivity toward ring opening with 228-nm excitation; one
might have expected otherwise on the basis of the fact that
conrotatory ring opening must produce the highly strained
cis,trans-isomer of the diene 12. This might be explained by a
mechanism in which a pathway for internal conversion to the
π,π* surface is activated by the introduction of a structurally
induced barrier to conrotatory ring opening on the π,R(3s)
surface, analogous to that which exists on the ground-state
surface for ring opening of this molecule.8,38 This is illustrated
in Figure 3. A similar explanation might account for the
significant yields of diene obtained from trans-9 upon shorter
wavelength (π,π*) excitation,40 where structural constraints
toward disrotatory ring opening may activate internal conversion
to the π,R(3s) excited-state surface and ring opening via the
conrotatory pathway. If this explanation is correct, then one
might expect to observe pronounced temperature effects on the
quantum yields for ring opening of structurally constrained
derivatives of this type.
Summary and Conclusions
The quantum yield for ring opening of 1 at 228 nm was determined
in isooctane solution using uranyl oxalate as the actinometer, following
the analytical procedure outlined by Kuhn et al.41 The sample and
actinometer solutions were placed in 4-mL cylindrical Suprasil cells,
deoxygenated with argon, mounted on a merry-go-round apparatus, and
irradiated with the Cd resonance lamp for ca. 30 min. The yield of 2
was measured by GC (as the average of three determinations), while
the conversion of uranyl oxalate was measured by UV spectroscopy.
A sample of uranyl oxalate was also photolyzed in a Pyrex cuvette for
a similar period of time in order to check for the effects of the longer
wavelength Cd emission bands on the photolyses. Conversion of the
actinometer was found to be negligible.
Compound 1 was also used as the actinometer for determination of
the quantum yields for ring opening of cis- and trans-9 with 214-nm
light, using the previously determined value of Φ2 ) 0.06 ( 0.01.7
Theoretical calculations were performed on a Silicon Graphics
Octane computer using Gaussian 94.42 RRKM calculations of the
Selective excitation at the long-wavelength edge of the
π,R(3s) absorption band of a series of 1,2-dialkyl-substituted
cyclobutenes results in competing electrocyclic ring opening
and [2 + 2] cycloreversion. Under these conditions, ring opening
proceeds with clean conrotatory stereochemistry and with an
efficiency that does not vary systematically with either molecular
size or reactivity toward ground-state ring opening. Thus, the
experimental quantum yields do not correlate with those
expected for ring opening via a hot ground-state mechanism.
The latter have been estimated from calculated (RRKM) rate
constants for ground-state ring opening at internal thermal
energies corresponding to the energy of a 228-nm photon. This
lack of a correlation between experimental and calculated
quantum yields suggests that Rydberg-derived ring opening
occurs as a true excited-state process and does not proceed from
upper vibrational levels of the ground state, populated by internal
conversion from the Rydberg state.
(41) Kuhn, H. J.; Braslavsky, S. E.; Schmidt, R. P. Appl. Chem. 1989,
61, 187.
Previous work9-11 suggests that the π,π* ring opening of
alkylcyclobutenes proceeds via an adiabatic mechanism, in
which a single diene isomer is produced in the π,π* excited
state by the disrotatory ring-opening pathway. The overall
nonstereospecificity of the reaction then results from decay of
the excited diene product by E,Z-isomerization. However, there
(42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; 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. Gaussian94; Gaussian, Inc.:
Pittsburgh, PA, 1995.
(40) Leigh, W. J.; Zheng, K. J. Am. Chem. Soc. 1991, 113, 2163.

Conrotatory Photochemical Ring Opening of Alkylcyclobutenes
J. Am. Chem. Soc., Vol. 123, No. 22, 2001 5193
microcanonical rate constants, k(E), were performed on a Sun E250
and Physical Sciences Research Council of Great Britain for
financial support of this work.
workstation using the Marcus expression,39
k(E) ) L+W(Ev+)/N(E*)
Supporting Information Available: Representative con-
centration vs time plots showing the formation of diene products,
from merry-go-round irradiation (228 nm) of alkylcyclobutenes
1, cis- and trans-5, 8, and cis- and trans-9 in deoxygenated
hexane solution at 25 °C (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
where L+ is the path degeneracy, W(Ev+) is the sum of vibrational states
of the complex, and N(E*) is the density of states of the energized
molecule. This assumes that adiabatic rotational effects are unimportant.
Sums and densities of states were calculated by standard procedures
(direct count/Whitten-Rabinovitch approximation).39
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada and the Engineering
JA003860O