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Can. J. Chem. Vol. 81, 2003
bicyclic cyclobutenes cis- and trans-4b and 4c at 228 nm are
only modestly different from those obtained with shorter
wavelength excitation, where in each case there is a very
distinct preference for the formation of the disrotatory diene
isomers. In all cases but cis-4b, there appears to be a consis-
tent wavelength dependence in the product ratios, with the
yield of conrotatory product(s) increasing modestly as the
excitation wavelength increases. As we found previously in
our studies of the photochemistry of 7 at this wavelength
(and as the data of Figs. 2–4 clearly show), secondary photo-
lysis effects are generally very difficult to control, and thus,
the product ratios reported cannot be considered to be quan-
titatively accurate. Nevertheless, the trends are clear; the
corresponding disrotatory diene isomers are the major prod-
ucts of irradiation of both isomers of 4b and 4c at 228 nm,
just as they are at shorter irradiation wavelengths.
Assuming that the Rydberg state is indeed exclusively ex-
cited in these compounds upon absorption of a 228 nm pho-
ton, the fact that similar distributions of diene isomers are
observed for 4b and 4c as with shorter excitation wave-
lengths (where the π,π* state is also directly populated) sug-
gests either that ring opening within the Rydberg state
exhibits similar characteristics to π,π* state ring opening in
these compounds or that (conrotatory) Rydberg state ring
opening is less efficient than in monocyclic derivatives, al-
lowing internal conversion to the π,π* state (at a non-
Franck–Condon geometry), from which ring opening occurs
with predominant disrotatory stereochemistry, to compete
more effectively.
One interpretation of the results for 4b and 4c is that
Rydberg state ring opening, in addition to proceeding with
preferred conrotatory stereochemistry, also involves twisting
about the “central” C=C bond as ring opening proceeds; ac-
cordingly, incorporating the cyclobutenyl C=C bond in a
second ancillary ring would be expected to result in a struc-
turally induced barrier to Rydberg state ring opening.
Scheme 1 shows a hypothetical reaction coordinate diagram
for dis- and conrotatory ring opening on the π,π* and
π,R(3s) excited state surfaces, which incorporates these ideas
and illustrates their possible ramifications on the stereochemi-
cal characteristics of the reaction. Disrotatory (π,π* state)
ring opening is assumed to proceed, at least initially, with
little deviation of the 4-carbon system from planarity, since
short wavelength (≤214 nm) irradiation of 4a–4d proceeds
with high efficiency, with no real variation in quantum yield
throughout the series (22); the π,π* surface falls in energy
along this reaction coordinate, as predicted both by simple
orbital symmetry considerations (8) and recent ab initio cal-
culations (9, 10). If the π,R(3s) surface rises in energy along
the disrotatory reaction coordinate, the result will be a ther-
mally activated crossing of the two surfaces at some inter-
mediate geometry (A). The conrotatory reaction coordinate
is represented as projecting perpendicularly to the disrota-
tory one, as it involves both conrotatory twisting about C1—
C4 and C2—C3 and torsion about the central C1—C2 bond,
leading eventually to s-trans-diene. In monocyclic cyclo-
butenes, the π,R(3s) surface is proposed to fall in energy
along this reaction coordinate, while the π,π* surface in-
creases in energy owing to orbital symmetry effects, thus re-
sulting in preferred conrotatory ring opening upon selective
population of the Rydberg state. In 4b–4d on the other hand,
motions along the conrotatory (+ central bond torsion) reac-
tion coordinate in the π,R(3s) state encounter the structurally
induced barrier, possibly allowing the excited molecule to
traverse the (presumably somewhat smaller) barrier to the π,
π* surface along the disrotatory reaction coordinate, from
which ring opening proceeds with the characteristics of that
excited state. It might further be expected that the structur-
ally induced barrier to Rydberg state conrotatory ring open-
ing should decrease as the size and flexibility of the
ancillary ring increases, and thus, allow reaction via this
pathway to be more competitive with internal conversion to
the π,π* state, where ring opening proceeds with preferred
disrotatory stereochemistry. Unfortunately, the data for cis-
and trans-4d are not sufficiently reliable to address this pos-
sibility in a conclusive way; certainly, there is no evidence
for it in the wavelength dependence associated with the ring
openings of cis- and trans-4b and 4c.
It is relevant to point out the analogy between cyclobutene
Rydberg state ring opening and the ring opening of cyclo-
butene radical cations (CB·+), to the extent that the latter
species constitutes a reasonable model for the “semi-
ionized” olefinic π,R(3s) Rydberg state (14, 15). The reac-
tion has been extensively studied in recent years (33–42),
and is known to proceed with the preferred conrotatory ste-
reochemistry (36) that theory predicts (39, 43–47). Calcula-
tions indicate that the reaction proceeds via two competing
conrotatory pathways: a nonsynchronous concerted pathway
leading to s-cis-1,3-butadiene (s-cis-BD·+) radical cation,
and one involving rotation about the ionized double bond
leading to s-trans-BD·+ via a transition state exhibiting struc-
tural characteristics of the cyclopropylcarbinyl radical cation
(43, 45–47). Interestingly, photolysis of CB·+ in a low-
temperature matrix has been reported to yield s-trans-BD·+
exclusively (39).
This explanation is obviously highly speculative, and
undoubtedly not the only one possible. Nevertheless, it con-
tains a number of features that might be tested experimen-
tally. For example, the idea that ring opening on the π,R(3s)
potential energy surface can be impeded by the introduction
of a structurally induced barrier might be tested through
studies of the temperature dependence of the reaction stereo-
chemistry in these and monocyclic cyclobutenes as a func-
tion of excitation wavelength. Ultrafast time-resolved studies,
along the lines of those recently reported for other aliphatic
alkenes (25, 26), would also offer potentially invaluable in-
formation on the mechanism. Finally, it still remains to be
established to what extent the π,R(3s) state contributes to the
ring opening of alkylcyclobutenes at shorter excitation wave-
lengths (185–214 nm), where the π,π* excited singlet state is
also populated. Further work in this area is clearly necessary.
Summary and conclusions
Alkylcyclobutenes have two low-lying excited singlet states
of similar energy, the π,π* (valence) and π,R(3s) (Rydberg)
state, and both lead to electrocyclic ring opening to the iso-
meric 1,3-dienes. The Rydberg state is markedly lower in
energy than the π,π* state in 1,2-dialkylcyclobutenes, a char-
acteristic that allows the photochemistry associated with this
excited state to be studied under conditions of selective exci-
tation.
© 2003 NRC Canada