Stereospecific (Conrotatory) Photochemical Ring Opening of Alkylcyclobutenes in the Gas Phase and in Solution. Ring Opening from the Rydberg Excited State or by Hot Ground State Reaction?

Article abstract of DOI:10.1021/jo990618v

The photochemistry of 1,2-dimethylcyclobutene and cis- and trans-1,2,3,4-tetramethylcyclobutene has been studied in the gas phase (1 atm; SF₆ buffer) and in hydrocarbon solvents with 193-, 214-, and 228-nm light sources. The major products are the isomeric dienes from electrocyclic ring opening and 2-butyne + alkene (ethylene or E-/Z- 2-butene) due to formal [2+2]-cycloreversion. The total yields of dienes relative to 2-butyne are generally higher in the gas phase than in solution but decrease with increasing excitation wavelength under both sets of conditions. In the case of cis-1,2,3,4-tetramethylcyclobutene, 228-nm photolysis results in the stereospecific formation of E,Z-3,4-dimethyl-2,4-hexadiene - the isomer corresponding to ring opening by the thermally allowed (conrotatory) electrocyclic pathway - in both the gas phase and solution. All three diene isomers are obtained upon 228-nm photolysis of trans-1,2,3,4-tetramethylcyclobutene, but control experiments suggest that the thermally allowed isomers (E,E- and Z,Z-3,4-dimethyl-2,3-hexadiene) are probably the primary products in this case as well. The results are consistent with cycloreversion resulting from excitation of the low-lying π,R(3s) singlet state and with ring opening proceeding by at least two different mechanisms depending on excitation wavelength. The first, which dominates at short wavelengths, is thought to involve direct reaction of the second excited singlet (π,π*) state of the cyclobutene. The second mechanism, which dominates at long wavelengths, is proposed to ensue either directly from the lowest energy (Rydberg) state or from upper vibrational levels of the ground state, populated by internal conversion from this excited state.

Full text of DOI:10.1021/jo990618v

5256
J . Org. Chem. 1999, 64, 5256-5263
Ster eosp ecific (Con r ota tor y) P h otoch em ica l Rin g Op en in g of
Alk ylcyclobu ten es in th e Ga s P h a se a n d in Solu tion . Rin g Op en in g
fr om th e Ryd ber g Excited Sta te or by Hot Gr ou n d Sta te Rea ction ?
William J . Leigh* and Bruce H. O. Cook
Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1
Received April 13, 1999
The photochemistry of 1,2-dimethylcyclobutene and cis- and trans-1,2,3,4-tetramethylcyclobutene
has been studied in the gas phase (1 atm; SF6 buffer) and in hydrocarbon solvents with 193-, 214-,
and 228-nm light sources. The major products are the isomeric dienes from electrocyclic ring opening
and 2-butyne + alkene (ethylene or E-/Z- 2-butene) due to formal [2+2]-cycloreversion. The total
yields of dienes relative to 2-butyne are generally higher in the gas phase than in solution but
decrease with increasing excitation wavelength under both sets of conditions. In the case of cis-
1,2,3,4-tetramethylcyclobutene, 228-nm photolysis results in the stereospecific formation of E,Z-
3,4-dimethyl-2,4-hexadienesthe isomer corresponding to ring opening by the thermally allowed
(conrotatory) electrocyclic pathwaysin both the gas phase and solution. All three diene isomers
are obtained upon 228-nm photolysis of trans-1,2,3,4-tetramethylcyclobutene, but control experi-
ments suggest that the thermally allowed isomers (E,E- and Z,Z-3,4-dimethyl-2,3-hexadiene) are
probably the primary products in this case as well. The results are consistent with cycloreversion
resulting from excitation of the low-lying π,R(3s) singlet state and with ring opening proceeding
by at least two different mechanisms depending on excitation wavelength. The first, which dominates
at short wavelengths, is thought to involve direct reaction of the second excited singlet (π,π*) state
of the cyclobutene. The second mechanism, which dominates at long wavelengths, is proposed to
ensue either directly from the lowest energy (Rydberg) state or from upper vibrational levels of the
ground state, populated by internal conversion from this excited state.
In tr od u ction
230-nm spectral range, and both have been proposed to
contribute directly to the photochemistry.7,9 The UV
spectra of 1,2-disubstituted alkylcyclobutenes are typical
of tetrasubstituted alkenes, showing a prominent (π,π*)
absorption band in the 185-195-nm range and, in the
gas phase, a finely structured band at longer wavelengths
(200-230 nm) due to π,R(3s) absorptions.7,10-12 In con-
densed phases, the position of the Rydberg absorption
band shifts to slightly shorter wavelengths, the fine
structure is absent, and the band is substantially reduced
in intensity. This character, and the fact that its position
correlates with the vertical π-ionization potential of the
alkene, are characteristic of Rydberg absorptions.12
The relative contributions of the π,π* and π,R(3s)
states to the photochemistry of aliphatic alkenes has been
extensively investigated in both the gas and solution
phases.13-16 There appears to be general agreement that
the valence (π,π*) state is responsible for direct cis,trans-
photoisomerization, whereas the Rydberg state is respon-
sible for the formation of skeletal rearrangement prod-
ucts, involving carbene intermediates resulting from
The direct irradiation of alkylcyclobutenes in hydro-
carbon solvents results in competing electrocyclic ring
opening and formal [2+2]-cycloreversion.1 Ring opening
proceeds nonstereospecifically, and it has been shown
that in certain cases, the mixtures of isomeric dienes
produced can be accounted for by a mechanism involving
exclusive adiabatic disrotatory ring opening, with inter-
nal conversion to the ground-state potential energy
surface occurring by cis,trans-isomerization after the
diene is formed.2-4 In contrast, the formal [2+2]-cyclor-
eversion reaction proceeds stereospecifically, yielding (in
addition to the corresponding alkyne) a single alkene
isomer whose stereochemistry corresponds to that at C3-
C4 of the cyclobutene precursor.5-7 Despite the high
degree of stereoselectivity of the process, there is evidence
to suggest that it proceeds nonconcertedly via a cyclo-
propylmethylene intermediate, perhaps in competition
with the pericyclic cycloreversion pathway.8
There are at least two excited singlet states accessible
in alkylcyclobutenes by direct irradiation in the 185-
(9) Leigh, W. J .; Zheng, K.; Nguyen, N.; Werstiuk, N. H.; Ma, J . J .
Am. Chem. Soc. 1991, 113, 4993.
(10) Robin, M. B.; Hart, R. R.; Kuebler, N. A. J . Chem. Phys. 1966,
44, 1803.
(11) Watson, F. H., J r.; McGlynn, S. P. Theor. Chim. Acta 1971, 21,
310.
(12) Robin, M. B. Higher Excited States of Polyatomic Molecules, Vol.
II; Academic Press: New York, 1975; pp 1-68.
(13) Kropp, P. J . Org. Photochem. 1979, 4, 1.
(14) Steinmetz, M. G. Org. Photochem. 1987, 8, 67.
(15) Collin, G. J . Adv. Photochem. 1988, 14, 135.
(16) Kropp, P. J . In CRC Handbook of Organic Photochemistry and
Photobiology; Horspool, W. G., Song, P.-S., Eds.; CRC Press: Boca
Raton, 1995; pp 16-28.
(1) Leigh, W. J . In CRC Handbook of Organic Photochemistry and
Photobiology; Horspool, W. G., Song, P.-S., Eds.; CRC Press: Boca
Raton, 1995; pp 123-142.
(2) Leigh, W. J .; Zheng, K. J . Am. Chem. Soc. 1991, 113, 4019.
(3) Leigh, W. J .; Postigo, J . A.; Venneri, P. C. J . Am. Chem. Soc.
1995, 117, 7826.
(4) Leigh, W. J .; Postigo, J . A.; Zheng, K. C. Can. J . Chem. 1996,
74, 951.
(5) Saltiel, J .; Ng Lim, L. S. J . Am. Chem. Soc. 1969, 91, 5404.
(6) Clark, K. B.; Leigh, W. J . J . Am. Chem. Soc. 1987, 109, 6086.
(7) Leigh, W. J .; Zheng, K.; Clark, K. B. Can. J . Chem. 1990, 68,
1988.
(8) Clark, K. B.; Leigh, W. J . Can. J . Chem. 1988, 66, 1571.
10.1021/jo990618v CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/17/1999

Photochemical Ring Opening of Alkylcyclobutenes
J . Org. Chem., Vol. 64, No. 14, 1999 5257
initial [1,2]-migration of a vinylic substituent.15,16 Long-
wavelength irradiation of tetrasubstituted alkenes in
hydroxylic media leads to the formation of alcohols or
ethers, consistent with nucleophilic trapping of a species
with the radical cation character expected of alkene π,R-
(3s) Rydbergs.17,18
In principle, the Rydberg state could provide an
alternate pathway to ring opening in cyclobutene deriva-
tives and account for the nonstereospecificity of the
reaction if the stereochemistry differed from that of the
π,π* process. Cyclobutene radical cations are known to
undergo thermally activated ring opening,19-22 and there
is one example23 that suggests that the reaction proceeds
with the conrotatory stereochemistry that is predicted
by theory.24-26 In two previous papers, we tentatively
ruled out this possibility on the basis of correlations of
the gas- and solution-phase electronic spectra of two
different series of cyclobutene derivatives with product
distributions obtained from photolyses in solution as a
function of substituent and excitation wavelength.7,9 The
yields of cycloreversion relative to ring-opening products
were found to increase with excitation wavelength be-
tween 185 nm (where π,π* absorption predominates) and
214 nm (where π,R(3s) absorption predominates), with
little variation in isomeric diene distribution. This is
consistent with electrocyclic ring opening arising pre-
dominantly from the π,π* state and [2+2]-cycloreversion
from the Rydberg excited state. Clearly, if these assign-
ments are correct, then it might be expected that selective
excitation of the Rydberg absorption band should lead
only to cycloreversion. Such an experiment should be
possible with 1,2-dialkylcyclobutenes, whose lowest en-
ergy absorption band extends to beyond 230 nm and is
particularly prominent in the gas phase.
F igu r e 1. UV absorption spectra of (a) 1 in pentane solution
(- - -) and in the gas phase (s) at 25 °C and (b) 1 (s), c-2
(- - -), and t-2 (- - -) in the gas phase (SF6 buffer; 1 atm total
pressure).
Perhaps surprisingly, the gas-phase photochemistry of
cyclobutene has not been extensively studied.27,28 Vacuum-
UV (106-147 nm) photolysis of cyclobutene itself at <10
Torr total pressure leads mainly to fragmentation; small
amounts of 1,3-butadiene are also formed but have been
shown to be due to reaction of hydrogen atoms with the
ground-state precursor.28 The only other gas-phase study
reported is from Haller and Srinivasan, who examined
the mercury-sensitized photolysis of the molecule.27 The
photochemistry under these conditions bears little re-
semblance to that obtained in solution.29
length 193, 214, and 228 nm. The compounds chosen for
this investigation, 1,2-dimethyl- (1), cis-1,2,3,4-tetra-
methyl- (cis-2), and trans-1,2,3,4-tetramethylcyclobutene
(trans-2), have all been studied previously in solution
with 193- and 214-nm light sources.7,30 In the gas phase,
In this paper, we report the results of a study of the
photochemistry of three simple monocyclic alkylcy-
clobutene derivatives in the gas phase at atmospheric
pressure, using monochromatic light sources of wave-
all three compounds possess low-lying π,R(3s) states,
which can be selectively populated by direct irradiation
with 228-nm light. Solution-phase experiments with the
three light sources have also been carried out to allow a
full comparison of the wavelength effects on the photo-
chemistry of these compounds in both the gas phase and
solution under standard sets of conditions.
(17) Kropp, P. J .; Reardon, E. J . J .; Gaibel, Z. L. F.; Willard, K. F.;
Hattaway, J . H. J . J . Am. Chem. Soc. 1973, 95, 7058.
(18) Fravel, H. G. J .; Kropp, P. J . J . Org. Chem. 1975, 40, 2434.
(19) Gross, M. L.; Russell, D. H. J . Am. Chem. Soc. 1979, 101, 2082.
(20) Dass, C.; Sack, T. M.; Gross, M. L. J . Am. Chem. Soc. 1984,
106, 5780.
Resu lts
(21) Kawamura, Y.; Thurnauer, M.; Schuster, G. B. Tetrahedron
1986, 42, 6195.
(22) Gerson, F.; Qin, X.-Z.; Bally, T.; Aebischer, J .-N. Helv. Chim.
Acta 1988, 71, 1069.
(23) Miyashi, T.; Wakamatsu, K.; Akiya, T.; Kikuchi, K.; Mukai, T.
J . Am. Chem. Soc. 1987, 109, 5270.
(24) Bellville, D. J .; Chelsky, R.; Bauld, N. L. J . Comput. Chem.
1982, 3, 548.
(25) Wiest, O. J . Am. Chem. Soc. 1997, 119, 5713.
(26) Sastry, G. N.; Bally, T.; Hrouda, V.; Carsky, P. J . Am. Chem.
Soc. 1998, 120, 9323.
(27) Haller, I.; Srinivasan, R. J . Am. Chem. Soc. 1966, 88, 3694.
(28) DeLeon, A.; Doepker, R. D. J . Phys. Chem. 1971, 75, 3656.
(29) Adam, W.; Oppenlander, T.; Zang, G. J . Am. Chem. Soc. 1985,
107, 3921.
The UV absorption spectra of 1 in the gas phase (1 atm
SF6) and in pentane solution are shown in Figure 1a.
Figure 1b shows the gas-phase spectra of all three
cyclobutenes recorded under the same conditions (1 atm;
SF6 buffer). The spectra of cis- and trans-2 are indistin-
guishable from those reported previously at lower total
pressures.7 All three spectra exhibit a weak, structured
band at long wavelengths that is assignable to π,R(3s)
absorptions and a more intense band at shorter wave-
(30) Leigh, W. J .; Postigo, J . A. J . Am. Chem. Soc. 1995, 117, 1688.

5258 J . Org. Chem., Vol. 64, No. 14, 1999
Ta ble 1. P r od u ct Yield s fr om P h otolysis of 1,2-Dim eth ylcyclobu ten e (1) a n d cis- a n d
Leigh and Cook
tr a n s-1,2,3,4-Tetr a m eth ylcyclobu ten e (cis- a n d tr a n s-2) in th e Ga s P h a se (1 a tm SF 6) a n d a s Deoxygen a ted 0.05 M
Solu tion s in Isoocta n e (1) or Hexa n e (2) a t 25 °Ca
gas phase
hydrocarbon solution
compd
λex (nm)
4
3
4
3
1
193
214
228
28 ( 2
31 ( 4
32 ( 1
72 ( 5
64 ( 4
62 ( 3
43 ( 3
50 ( 1
56 ( 4
57 ( 4
45 ( 3
44 ( 4
gas phase
EE-6
hydrocarbon solution
compd
λex (nm)
4
EZ-6
ZZ-6
4
EE-6
EZ-6
ZZ-6
cis-2
193
214
228
39 ( 2
50 ( 1
55 ( 3
20 ( 2
7.0 ( 0.5
<1
41 ( 2
42 ( 1
45 ( 2
<1
<1
<1
34 ( 2
48 ( 2
59 ( 7
40 ( 2
21 ( 2
<1
25 ( 2
31 ( 2
40 ( 2
<1
<1
<1
trans-2
193b
214b
228b
39 ( 1
43 ( 2
57 ( 3
41 ( 2
38 ( 2
22 ( 2
13 ( 2
10 ( 2
12 ( 2
7 ( 1
9 ( 1
8 ( 1
40 ( 1
50 ( 2
76 ( 4c
21 ( 2
14 ( 2
3 ( 1c
34 ( 2
29 ( 2
13 ( 2c
4 ( 1
7.5 ( 0.5
8 ( 2c
a
Based on triplicate determinations of relative product yields at maximum conversion. All solution-phase photolyses employed 0.05 M
solutions and were carried to a maximum conversion of 5-8%, unless otherwise noted. Maximum conversion 0.7-1.5%. c Concentration
) 0.15 M.
b
Ta ble 2. Rin g-Op en in g/Cyclor ever sion Ra tios (3(6)/4) a n d Isom er ic Dien e Distr ibu tion s fr om P h otolysis of 1 a n d 2 w ith
193-228-n m Ligh t in th e Ga s P h a se (1 a tm SF 6) a n d a s Deoxygen a ted 0.05 M Solu tion s in Isoocta n e or Hexa n ea
gas phase
EE:EZ:ZZ
hydrocarbon solution
compd
λex (nm)
3(6)/4
therm/photob
3(6)/4
EE:EZ:ZZ
therm/photob
1
193
214
228
3.0
2.4
2.0
1.3
0.92
0.79
cis-2
193
214
228
1.6
0.90
0.82
1:2:0
2.0
3.3
>20
1.9
1.6:1:0
0.63
1.0
>20
1:3.3:0c
<1:>20:0
1.1c
0.69
1.1:1.0:0c
<1:>20:0
trans-2
pssd
193
214
228
1.6
1.6
1.3
6:2:1
3.5
5.4
3.0
1.5
5.2:8.5:1
2.2:4.2:1
1:2.6:2.3c
0.74
0.76
1.3
4.3:1:1.1
0.80
2.4:1:1c
0.25c
228
1:7:2
1:7.1:4.4
a
b
Photolyses of 1 were carried out in isooctane solution, whereas those of cis- and trans-2 were carried out in hexane solution. Therm/
photo represents the ratio of thermally allowed to photochemically allowed diene isomers, i.e., [EZ-6]/([EE-6] + [ZZ-6]) from photolysis
of cis-2 and the inverse for the trans isomer. c From the intercepts of plots of product ratio ([6]/[4], [EE-6]/[EZ-6] and [ZZ-6]/[EZ-6]) vs
d
conversion, from photolysis of a 0.15 M hexane solution of trans-2. The photostationary state for direct E,Z-photoisomerization, established
starting from pure E,E-6 and pure E,Z-6.
lengths assignable to π,π* absorptions. The long wave-
length bands in the three compounds are reduced in
(1)
intensity, devoid of fine structure, and shifted to shorter
wavelengths in the solution-phase spectra, consistent
with the gas-phase π,R(3s) assignment (see Figure 1a and
ref 7).
(2)
Direct irradiation of samples of 1 and 2 (partial
pressure ≈ 200 Torr) in an SF6 buffer (total pressure )
(3)
760 Torr) was carried out with an (unfocused) ArF
excimer laser (193 nm) and zinc and cadmium resonance
lamps (214 and 228 nm, respectively). The product
distributions did not vary with the laser intensity so long
as it was kept at modest levels, but much higher
Table 1 contains percentage yields of products, deter-
intensities resulted in increased yields of dienes relative
mined by triplicate GC analysis of the product mixtures
to 4.
at maximum conversion, with corrections for differences
The photolyzates were monitored as a function of
photolysis time by capillary gas chromatography (GC),
and products were identified by co-injection of the pho-
tolyzates with authentic samples. Photolysis of the three
compounds gave rise to the products shown in eqs 1-3
at conversions below ∼8%; no others could be detected
in yields greater than ca. 5% under our analytical
conditions. The three compounds were also photolyzed
as deoxygenated 0.05 M solutions in isooctane (1) or
hexane (2) with the three light sources.
in GC response factors. Table 2 contains various product
ratios, calculated from the slopes of concentration vs
conversion plots. These generally showed good linearity
up to conversions of ∼6%. In a few cases (e.g., the 228-
nm photolyses of trans-2), plots of the isomeric diene
yields vs conversion showed mild curvature, suggestive
of secondary photolysis effects. The data were thus also
plotted in the form of product ratios versus conversion
to magnify any systematic variations in product distribu-
tion with photolysis time. Plots of [3]/[4] vs conversion

Photochemical Ring Opening of Alkylcyclobutenes
J . Org. Chem., Vol. 64, No. 14, 1999 5259
F igu r e 2. Plots of product ratios versus time for the 214-nm
photolysis of cis-1,2,3,4-tetramethylcyclobutene (cis-2) in (a)
the gas phase (1 atm SF6) and (b) hexane solution (0.05 M) at
25 °C. The quantity [6]/[4] is the total yield of dienes (all
isomers) relative to that of 2-butyne (4).
F igu r e 4. Plots of isomeric diene ratios versus time for the
228-nm photolysis of trans-1,2,3,4-tetramethylcyclobutene
(trans-2) in (a) the gas phase (1 atm SF6) and (b) hexane
solution (0.05 M) at 25 °C.
phases (Figure 4). In these cases, the primary product
ratios were taken as the intercepts of these plots. The
product distributions obtained from photolysis of 1 and
2 in pentane solution with 193- and 214-nm light (see
Table 1) agree fairly well with those published by us in
earlier studies of these compounds.7,30
For example, Figure 2 shows plots of [6]/[4] and [EZ-
6]/[EE-6] vs conversion for the 214-nm photolyses of cis-2
in the gas and solution phases (ZZ-6 was not formed in
detectable yields at either 214 or 228 nm, at conversions
< 5%). Gas- and solution-phase photolysis of cis-2 at 228
nm led to the formation of only 4 and E,Z-6 at conversions
up to ∼8%. Plots of [EE-6]/[EZ-6] and [ZZ-6]/[EZ-6] vs
conversion from the photolyses of trans-2 in the two
phases at 214 and 228 nm are shown in Figures 3 and 4,
respectively.
The product yields and ratios listed in Tables 1 and 2
for the 228-nm photolysis of trans-2 in hexane were
actually determined using a deoxygenated 0.15 M solu-
tion of the cyclobutene, with monitoring of the photolyz-
ate over the 0.2-0.8% conversion range. Photolysis of this
compound under the same conditions as used for all of
the other experiments summarized in the Tables (i.e.,
0.05 M trans-2), but with monitoring over the 2-7%
conversion range, afforded product ratios that varied
more substantially with conversion; the values extrapo-
lated to 0% conversion are [6]/[4] ) 0.32 and EE:EZ:ZZ-6
) 1:3.2:2.1.
F igu r e 3. Plots of isomeric diene ratios [EE-6]/[EZ-6] (circles)
and [ZZ-6]/[EZ-6] (diamonds) versus % conversion for the 214-
nm photolysis of trans-1,2,3,4-tetramethylcyclobutene (trans-
2) in the gas phase (open symbols) and hexane solution (solid
symbols) at 25 °C.
from the photolysis of 1 were linear, with slopes indis-
tinguishable from zero for all three excitation wave-
lengths in both gas and solution phases. The same was
true of the plots of the total yields of dienes relative to
2-butyne ([6]/[4], where [6] ) [EE-6] + [EZ-6] + [ZZ-6])
vs conversion for photolysis of cis- and trans-2 at the
three excitation wavelengths in the two phases. The zero
slopes of these plots suggest that the total product yields
are unperturbed by secondary photolysis effects in these
cases. However, plots of the individual diene ratios had
nonzero slopes for the photolysis of both cis- and trans-2
in the gas phase at 214 nm (Figures 2 and 3) and for the
228-nm photolyses of trans-2 in both the gas and solution
Table 2 also contains the photostationary state com-
positions for direct irradiation of 6 with 228-nm light,
which were determined under the same conditions as
those employed above for photolysis of the cyclobutenes.
These ratios were determined by exhaustive photolysis

5260 J . Org. Chem., Vol. 64, No. 14, 1999
Leigh and Cook
between them are typical of the UV spectra of tetralkyl-
substituted alkenes.10-12 The gas-phase spectrum shows
the structured long wavelength absorption band, with
vibronic spacings on the order of 1400-1500 cm-1, which
is typical of alkene π,R(3s) absorptions,12 along with a
more intense band below 200 nm which can be assigned
to the valence (π,π*) absorption. The intensity and fine
structure associated with the long wavelength absorption
band is reduced in the solution-phase spectrum, and the
absorption is shifted to shorter wavelengths, consistent
with the Rydberg assignment in the gas-phase spectrum.
The absorption spectra of cis- and trans-2 both show a
set of π,R(3s) absorptions similar to that of 1 but more
pronounced fine structure in (or superimposed on) the
intense π,π* absorption at shorter wavelengths. This
structure manifests itself as three bands at 193, 199, and
203 nm (i.e., vibronic spacings on the order of 1000-1200
cm-1), of which the latter two are considerably more
prominent in the spectrum of the trans isomer than in
that of the cis.7 We previously assigned these to π,R(3p)
absorptions, primarily on the basis of the fact that the
fine structure is washed out in the solution-phase spectra
of the two compounds.
In all three cases, there is a marked trend toward
increased yields of cycloreversion relative to ring-opening
products with increasing excitation wavelength, in both
the gas phase and solution. This is consistent with the
conclusion that cycloreversion arises predominantly from
the lower energy Rydberg state, whereas ring opening
results mainly from the π,π* state.7 However, the obser-
vation of significant yields of dienes in the 228-nm
photolyses, where there is insufficient energy to populate
the π,π* state directly, clearly indicates that there exists
a second mechanism for ring opening that involves the
lower energy excited state. This second mechanism is
generally more important in the gas phase than in
solution, and furthermore, it has significantly different
stereochemical characteristics than that which dominates
at shorter excitation wavelengths. This is most evident
from the results of photolysis of cis-2 with 228-nm light,
where the thermally allowed diene isomer (E,Z-6) is
formed stereospecifically in both the gas phase and
solution. In contrast, photolysis of trans-2 at the same
wavelength produces all three diene isomers, in distribu-
tions which only slightly favor the thermally allowed
diene isomers (EE- and ZZ-6) relative to the other. In
this case, the main effect of increasing excitation wave-
length is a trend toward increased yields of the Z,Z-diene
relative to the E,E-isomer, and it is more pronounced in
solution than in the gas phase.
F igu r e 5. Isomeric diene ratios from the 228-nm photolysis
of t-2 in pentane solution (filled symbols) and from the 228-
nm photolysis of 1 to which a 1:1 solution of E,E- and Z,Z-6 is
slowly added (open symbols).
of samples of E,E- and E,Z-6, which led to the formation
of complex mixtures of cis- and trans-2 and other isomeric
products but common mixtures of E,E-, E,Z-, and Z,Z-6.
A control experiment was performed, in which a
hexane solution of 1 (0.15 M) was photolyzed with the
228-nm light source while a 1:1 mixture of E,E- and Z,Z-6
(total concentration ≈ 0.02 M) was introduced via a
syringe pump, at a rate corresponding roughly to the total
rate of diene formation in the photolysis of trans-2 under
the same conditions. GC analysis of aliquots of the
photolysis mixture as a function of irradiation time
showed E,Z-6 to be formed efficiently under these condi-
tions, even at the shortest irradiation period monitored;
Figure 5 shows plots of E,Z-/E,E- and E,Z-/Z,Z-6 versus
photolysis time for this experiment, overlayed on the
corresponding plots from the photolysis of the 0.15 M
solution of trans-2.
Irradiation of a deoxygenated 0.05 M solution of 1 in
methanol with the 228-nm light source resulted in the
formation of three major products in approximately equal
yields, in addition to small amounts of 3 and 5 (4 coelutes
with the solvent under our GC conditions, so its formation
in this experiment can only be inferred from the fact that
5 is formed). The three major products had similar
retention times under our GC conditions, so that baseline
resolution could not be achieved even by capillary GC.
Consequently, they could only be tentatively identified
1
as 7-9 on the basis of H NMR and mass spectral data
recorded on enriched mixtures (see eq 4). The yields of
Because the dienes absorb considerably more strongly
than the cyclobutenes at 228 nm in solution, the contri-
bution of secondary photolysis effects to the observed
distributions of diene isomers from photolysis of 2
requires careful evaluation. Such effects are clearly
unimportant with cis-2, which yields only a single diene
isomer within the limits of our detection method over the
0.4-7% conversion range. They are a cause of real
concern in the case of trans-2, however, for which the
observed diene distribution changes with conversion and
approaches the photostationary state composition at this
wavelength (EE/EZ/ZZ ) 1:7.1:4.4). If one assumes that
the primary diene distribution from photolysis of trans-2
at 228 nm is analogous to that from cis-2 (i.e., that it
consists only of the thermally allowed (EE- and ZZ-)
diene isomers), then it is clear that greater problems from
(4)
7-9 were reduced to ca. 40% of the total, and those of 3
and 5 increased to 36% and ∼20%, respectively, when a
similar solution was irradiated at 214 nm. Equation 4
shows the results of the 228-nm experiment; product
yields were estimated after ∼20% conversion by GC.
Discu ssion
The gas- and solution-phase UV absorption spectra of
1,2-dimethylcyclobutene (1; Figure 1) and the differences

Photochemical Ring Opening of Alkylcyclobutenes
J . Org. Chem., Vol. 64, No. 14, 1999 5261
secondary photolysis are to be expected for the trans-
isomer. This follows from consideration of the diene 228-
nm pss, which shows that the E,E-diene isomer is the
most sensitive to secondary photolysis at this wavelength,
whereas the E,Z-diene is the least sensitive. It should
be noted that although these effects might skew the diene
isomer distribution away from its true value, they should
have little effect on the cycloreversion/ring-opening ratio
(i.e., [4]/[EE+EZ+ZZ-6]), because the quantum yields for
ring closure of these dienes are considerably lower than
those for cis,trans-photoisomerization. Thus, the 2-bu-
tyne/diene ratios from photolysis of both isomers of 2 can
be safely assumed to be close to their “true” values.
Further evidence for this is obtained from the fact that
there is no detectable formation of trans-2 from cis-2, and
vice versa, in the photolyses at any of the three wave-
lengths.
The syringe pump experiment, in which a 1:1 mixture
of EE- and ZZ-6 was introduced into a continuously
photolyzed solution of 1 (0.15 M) at a rate corresponding
roughly to that of total diene formation in the 228-nm
photolysis of trans-2, was carried out to quantify the
importance of secondary photolysis effects in the pho-
tolysis of this compound. GC analyses of the photolyzate
at various time intervals during photolysis (Figure 5)
show that even after the shortest irradiation period
(corresponding to ca. 0.2% conversion of trans-2 in the
original experiment), the E,Z-diene comprises slightly
more than 50% of the distribution of the isomers of 6.
The relative yield of E,Z-6 continues to increase with
continued photolysis, in parallel fashion to that in the
original experiment. The close correspondence between
the plots of diene-isomer ratio vs time from the original
photolysis of 0.15 M trans-2 and those from the control
experiment strongly suggests that the primary diene
distribution from photolysis of trans-2 in hexane at 228
nm is close to a 1:1 mixture of E,E- and Z,Z-6, with
relatively little of the E,Z-isomer present. We thus
conclude that photolysis of both cis- and trans-2 with the
228-nm light source results in the predominant (or
possibly exclusive) formation of the thermally allowed
diene isomer(s) as the primary products.
The present results indicate that both the π,π* and π,R-
(3s) excited states of alkylcyclobutenes are responsible
for ring opening. Most significantly, it is clear that the
lower energy state reacts via a mechanism that results
in the preferential formation of the thermally allowed
diene isomer(s). The effect is particularly pronounced in
the gas phase, where the relative yields of dienes are
significantly higher than in solution. There are at least
two possible explanations for these results.
The first explanation is that ring opening proceeds
from the Rydberg state as a true excited-state process
but with different stereochemical characteristics than
π,π*-state ring opening. This might be consistent with
the radical cation character of alkene π,R(3s) Rydberg
states13 and the fact that cyclobutene radical cations are
predicted by theory to undergo ring opening with conro-
tatory stereochemistry.24-26 Evidence that the lowest
Rydberg state of 1 possesses the same general electronic
character as those of simpler tetrasubstituted alkenes is
provided by the results of 228-nm photolysis of the
compound in methanol solution, where the major prod-
ucts are the methyl ethers 7-9. Ring-opening and
cycloreversion products (3-5) are formed in only minor
amounts, and the rates of their formation appear to be
substantially lower than in isooctane solution. This is
consistent with a concomitant reduction in the quantum
yields for cycloreversion and ring opening as a result of
competitive quenching of the Rydberg excited state by
the solvent. Compounds 7-9 are the products expected
from disproportionation of the free radical formed by
nucleophilic trapping of the Rydberg state by the solvent
(eq 5). Analogous results have been reported for other
(5)
tetrasubstituted alkenes by Kropp and co-workers.31
Photolysis of 1 in methanol with 214-nm light, where
both the π,π* and π,R(3s) states are populated, results
in substantially lower yields of 7-9 relative to 3. It is
not clear why 7-9 appear to be formed in roughly equal
yields; this mechanism would predict that the yield of 9
should be equal to the sum of those of 7 and 8.
Alkylcyclobutene radical cations have been shown to
undergo fast thermal ring opening when generated in the
gas phase20 but not in solution21,32 or other condensed
phases.22,33 These results are consistent with the theo-
retical indication that the process requires thermal
activation.24 If this requirement holds as well for ring
opening in the π,R(3s) Rydberg state, then this mecha-
nism would not be expected to be a major contributor to
diene formation in either the gas phase or solution under
the conditions of our experiments, assuming that the
intrinsic lifetimes of the π,R(3s) states in 1,2-dialkylcy-
clobutenes are not longer than the value estimated for
the π,R(3s) state of 2,3-dimethyl-2-butene, estimated to
be on the order of ∼15 ps.34,35 It is interesting to note
that a lifetime in this range requires that quenching of
the Rydberg state by methanol proceed at the diffusion-
controlled rate to account for the significant yields of 7-9
formed upon photolysis of 1 in methanol.
The second possibility is that the lower energy excited
state is itself unreactive toward ring opening but con-
tributes to the process by providing a pathway for
internal conversion to upper vibrational levels of the
ground state of the cyclobutene, from which thermal ring
opening ensues. This mechanism is consistent with the
increased total yields of ring opening relative to cyclor-
eversion products in the gas phase compared to those in
solution, where the solvent provides a more efficient
means of vibrational relaxation than 1 atm of SF6. The
buffer gas does appear to quench higher activation energy
hot ground-state processes that would be expected at
much lower pressures, such as C-H and C-C bond
fragmentation.36 Relatively few conclusively established
hot ground-state reactions exist in the solution-phase
photochemistry literature, but it seems reasonable to
expect that such processes might be important for
(31) Fuss, W.; Lochbrunner, S.; Muller, A. M.; Schikarski, T.;
Schmid, W. E.; Trushin, S. A. Chem. Phys. 1998, 232, 161.
(32) Cook, B. H.; Leigh, W. J ., unpublished results.
(33) Aebischer, J . N.; Bally, T.; Roth, K.; Haselbach, E.; Gerson, F.;
Qin, X.-Z. J . Am. Chem. Soc. 1989, 111, 7909.
(34) Hirayama, F.; Lipsky, S. J . Chem. Phys. 1975, 62, 576.
(35) Inoue, Y.; Daino, Y.; Tai, A.; Hakushi, T.; Okada, T. J . Am.
Chem. Soc. 1989, 111, 5584.
(36) Calvert, J . G.; Pitts, J . N., J r. Photochemistry; J ohn Wiley &
Sons: New York, 1966.

5262 J . Org. Chem., Vol. 64, No. 14, 1999
Leigh and Cook
relatively small molecules (for which collisional deactiva-
tion by the solvent is relatively slow37-39), particularly
when they exhibit low excited-state reactivity and possess
particularly low energy pathways for reaction in the
ground state. Monocyclic alkylcyclobutenes, which un-
dergo thermal ring opening with Arrhenius activation
energies in the 32-37 kcal/mol range,40-43 would seem
to be ideal candidates in this regard.
quantum yields (Φ ) 0.4-0.5) that are roughly indepen-
dent of total pressure and buffer gas.50 In solution,
photolysis of 11 results in the formation of nortricyclane
(16), as well as the three products observed in the gas
phase (see eq 7). In contrast to the case of cyclohexene,
(7)
Qualitatively, we can address the viability of this
mechanism by considering cyclobutene in the context of
other cycloalkenes that undergo thermal (pericyclic)
reactions that differ from their excited-state reactions.
Two examples are provided by cyclohexene (10) and
norbornadiene (11), which undergo thermal [4+2]-cyclo-
reversion with Ea ) 65.2 kcal/mol44 and 50.2 kcal/mol,45
respectively. The photochemistry of both of these mol-
ecules has been studied extensively in the gas phase and
the quantum yield for cycloreversion of 11 to cyclopen-
tadiene and acetylene is reduced by a factor of ∼4 in
solution compared to that in the gas phase, but it is not
eliminated. In this case, thermal cycloreversion is con-
siderably more facile (Ea ) 50.2 kcal/mol and log A )
14.6)45 than in cyclohexene. Even though the excess
thermal energy contained in the molecule after internal
conversion is lower (∼110 kcal/mol) in this case than with
cyclohexene, it is evidently sufficient to allow cyclorever-
sion to compete with collisional deactivation by the
solvent.
46-50
in solution.
Direct irradiation of cyclohexene vapor with 185- or
214-nm light affords 1,3-butadiene, ethylene, bicyclo-
[3.1.0]hexane (12), methylenecyclopentane (13), and small
amounts of vinylcyclobutane (14) (see eq 6).46-49 The
The Arrhenius activation parameters for the thermal
ring opening of 1 and 2 are in the range Ea ) 33-37 kcal/
mol (log A ≈ 14),40,41,52 and the lowest excited singlet state
energies (∼125 kcal/mol) are even higher than that of
11. Internal energy redistribution can be expected to be
somewhat more effective in 2 than in 11 because of the
larger number of vibrational degrees of freedom, but as
a result of the higher excited-state energy, one would still
expect the vibrationally excited molecules to be formed
with effective internal temperatures of at least 2000 K.
Clearly, ring opening can be expected to compete quite
significantly with collisional deactivation by the solvent
under these conditions.
Our results suggest that the 228-nm photolysis of
trans-2 leads to the formation of E,E- and Z,Z-6 in nearly
equal amounts, to the exclusion of the thermally forbid-
den E,Z-isomer. Unfortunately, these results afford only
a qualitative estimate of the likely primary diene isomer
distribution, so we are unable to pinpoint the degree of
torquoselectivity in the formation of the two isomers more
precisely. The Z,Z-isomer is not formed in detectable
yields on thermolysis of this compound at 150-200 °C,40,41
consistent with the Arrhenius activation energy for its
formation being on the order of 5-6 kcal/mol higher than
that for formation of the E,E-isomer.53 However, because
of the very high effective temperatures that are presum-
ably involved in photochemically induced hot ground-
state ring opening, one would expect to observe consid-
erably lower torquoselectivity than would be obtained
under conventional thermolytic conditions.
(6)
major products at low pressures are butadiene and
ethylene, which are formed with a quantum yield of Φ ≈
0.85 at 5 Torr.47 The quantum yield for formation of the
latter two products decreases by a factor of ∼2 with
increasing pressure between 5 and 30 Torr.46 Their
formation still persists at pressures as high as 500 Torr48
but does not occur detectably in solution-phase photoly-
ses.46 The process has been ascribed to a hot ground-state
reaction on the basis of its pressure dependence and the
fact that it proceeds with [π4s+π2s]-stereochemistry.51
Immediately after internal conversion from the lowest
excited singlet state, 10 would contain ∼130 kcal/mol of
excess thermal energy (corresponding to an effective
internal temperature in excess of 2000 K39), well above
that corresponding to the activation barrier for ground-
state [4+2]-cycloreversion (Ea ∼ 65 kcal/mol).
The major products of photolysis (254-nm) of norbor-
nadiene (11) in the gas phase are the retro-Diels-Alder
products cyclopentadiene and acetylene, formed with
(37) Elsaesser, T.; Kaiser, W. Annu. Rev. Phys. Chem. 1991, 42, 83.
(38) Owrutsky, J . C.; Raftery, D.; Hochstrasser, R. M. Annu. Rev.
Phys. Chem. 1994, 45, 519.
(39) Benzler, J .; Linkersdorfer, S.; Luther, K. J . Chem. Phys. 1997,
106, 4992.
(40) Criegee, R.; Seebach, D.; Winter, R. E.; Borretzen, B.; Brune,
H.-A. Chem. Ber. 1965, 98, 2337.
(41) Branton, G. R.; Frey, H. M.; Skinner, R. F. Trans. Faraday Soc.
1966, 62, 1546.
In a recent study of the 214-nm photolysis of the
homologous series of methylated cyclobutenes 1, 2, and
17-19 in hydrocarbon solution, we showed that the
(42) Brauman, J . I.; Archie, W. C. J . J . Am. Chem. Soc. 1972, 94,
4262.
(43) Marvell, E. N. In Thermal Electrocyclic Reactions; Academic
Press: New York, 1980; pp 124-213.
(44) Tardy, D. C.; Ireton, R.; Gordon, A. S. J . Am. Chem. Soc. 1979,
101, 1508.
(45) Herndon, W. C.; Lowry, L. L. J . Am. Chem. Soc. 1964, 86, 1922.
(46) Inoue, Y.; Takamuku, S.; Sakurai, H. J . Chem. Soc., Perkin
Trans. 2 1977, 1635.
(47) Collin, G. J .; DesLauriers, H. Can. J . Chem. 1983, 61, 1970.
(48) Collin, G. J .; DesLauriers, H.; Makulski, W. J . Photochem. 1987,
39, 1.
(49) Collin, G. J .; DeMare, G. R. J . Photochem. 1987, 38, 205.
(50) Roquitte, B. C. J . Phys. Chem. 1965, 69, 2475.
(51) Inoue, Y.; Takamuku, S.; Sakurai, H. Bull. Chem. Soc. J pn.
1975, 48, 3101.
quantum yields for ring opening vary systematically with
the degree of methyl substitution at the sp3 carbons of
the cyclobutene ring, in the manner expected if ring
opening proceeds via the adiabatic, orbital symmetry-
(52) Frey, H. M. Trans. Faraday Soc. 1963, 59, 1619.
(53) Rondan, N. G.; Houk, K. N. J . Am. Chem. Soc. 1985, 107, 2099.

Photochemical Ring Opening of Alkylcyclobutenes
J . Org. Chem., Vol. 64, No. 14, 1999 5263
controlled mechanism.30 It is also the trend expected if
ring opening is dominated by the Rydberg-derived hot
ground-state mechanism, because the rate constants for
thermal ring opening of these six compounds vary in
fashion similar to the quantum yields for photochemical
ring opening at this excitation wavelength.
hexanes or isooctane contained the cyclobutene (ca. 0.05 M)
and cyclohexane (0.001 M). Aliquots (ca. 3 mL) were placed in
1-cm Suprasil cuvettes containing a small magnetic stirrer,
sealed with rubber septa, and deoxygenated with a stream of
dry argon. Photolyses were carried out using a Lambda Physik
Compex 120 excimer laser (193 nm; ∼25-ns, ∼30-mJ pulses
delivered with a repetition rate of 0.5-Hz), a 16-W Philips
93106E zinc resonance lamp, or a 16-W Philips 93107E
cadmium resonance lamp. Solutions were stirred vigorously
throughout the photolyses. Aliquots were removed from the
cells by microliter syringe at suitable photolysis intervals and
analyzed by GC. Product formation was monitored as a
function of photolysis time in all cases. The data in Table 1
were determined by triplicate analyses of GC traces at the
maximum conversion investigated, whereas those in Table 2
were determined from the slopes of plots of the product peak
areas relative to that of the internal standard versus %
conversion, after correction of the data with the FID response
factors (determined from working curves). They were also
corrected for secondary diene photoisomerization, as discussed
in the Results section. Conversion scales for these plots were
determined from the GC traces at maximum conversion.
Con clu sion s
The results reported in this paper raise new questions
regarding the origins of the nonstereospecificity of the
photochemical ring opening of cyclobutene in solution.
It remains obvious that there is a much greater selectiv-
ity for formation of the diene isomer(s) corresponding to
orbital symmetry-allowed, disrotatory ring opening with
shorter wavelength excitation, where the π,π* state is
populated. Results reported previously for several bicyclic
cyclobutenes whose isomeric dienes are constrained to
be in the s-cis conformation are quantitatively compatible
with an adiabatic, purely disrotatory ring-opening
mechanism,2-4 but it is now clear that this is not general.
In monocyclic alkylcyclobutenes, the π,R(3s) Rydberg
state also contributes to ring opening, either via direct
reaction with orbital symmetry characteristics identical
to those of the ground-state process or by simply provid-
ing a conduit for thermal ring opening from vibrationally
activated ground-state molecules. To what extent this
second mechanism contributes to ring opening at shorter
wavelengths, and with what degree of generality, re-
mains to be determined.
The photolysis of 1 (0.41 g, 5.0 mmol) in methanol (50 mL)
was carried out with the 228-nm light source in a Suprasil
quartz immersion well apparatus equipped with a magnetic
stirrer, reflux condenser, and nitrogen inlet. Monitoring of the
photolyzate at low conversions by capillary GC revealed the
formation of 3, 5, and three new products, which eluted with
longer retention times and were just well-enough resolved to
determine that they were formed in equal yields. Irradiation
was continued to ∼80% conversion of 1 (14 days), with no
change in the relative yields of the five photoproducts. The
mixture was extracted with pentane (4 × 25 mL), and the
pentane extracts were then concentrated by distillation to a
volume of ca. 1.0 mL. Semipreparative gas chromatography
(UCW-982) allowed fractionation of the three major compo-
nents of the mixture, which each consisted of one major
component (70-80%) contaminated with small amounts of one
or both of the others. The major components of each fraction
were identified as 7-9 (in the order of their elution from the
GC column) on the basis of the following 1H NMR and GC/MS
evidence. 3-Meth oxy-2,3-d im eth ylcyclobu ten e (7): 1H NMR
δ 1.33 (s, 3H), 1.59 (s, 3H), 2.15-2.59 (m, 2H), 3.23 (s, 3H),
5.79 (s, 1H); MS m/e (I) 112 (27), 111 (12), 97 (100), 86 (33), 79
(37), 72 (68), 67 (30), 59 (21), 53 (28), 43 (84), 41 (52), 39 (42),
29 (21). 1-Meth oxy-1-m eth yl-2-m eth ylen ecyclobu ta n e (8).
1H NMR δ 1.35 (s, 3H), 2.39-2.15 (m, 4H), 3.24 (s, 3H), 4.86-
4.94 (dt, 2H); MS m/e (I) (5), 111 (11), 97 (79), 86 (21), 72 (100),
53 (19), 43 (69), 42 (41), 41 (26), 39 (39). 1-Meth oxy-1,2-
Exp er im en ta l Section
The cyclobutenes studied in this work were prepared as
previously reported,3 as were authentic samples of E,E-, E,Z-,
and Z,Z-3,4-dimethyl-2,4-hexadiene.54 The cyclobutenes were
purified to >99% purity by semipreparative gas chromatog-
raphy, using stainless steel columns (20% ODPN on 80/100
Chromosorb PNAW, 0.25′′ × 20′ for 1; 3.8% UCW-982 on 80/
100 Supelcoport, 0.25′′ × 24′ for 2 and 6). Sulfur hexafluoride
(VitalAire, Inc.), methanol (Aldrich, spectrophotometric grade),
cyclohexane, hexanes, and isooctane (all BDH Omnisolv) were
used as received from the suppliers. Gas- and solution-phase
UV absorption spectra were recorded using a Perkin-Elmer
Lambda 9 spectrophotometer in 1 cm Suprasil cells. Analytical
gas chromatographic 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.). FID re-
sponse factors were determined for 3 and the various isomers
of 6 by construction of working curves from solutions of
authentic samples of the dienes.
1
d im eth ylcyclobu ta n e (9): H NMR δ 0.93 (d, 3H), 1.15 (s,
3H), 1.56-2.00 (m, 5H), 3.14 (s, 3H); MS m/e (I) 97 (3), 86 (87),
72 (100), 71 (42), 55 (22), 43 (44), 42 (37), 41 (25), 39 (23).
Photostationary-state compositions for 6 at 228 nm were
determined by photolyzing separate samples of EE- and EZ-6
(7 µL in 1 atm of SF6 for gas-phase experiments or deoxygen-
ated 0.05 M solutions in hexane), with periodic monitoring by
GC, until common diene distributions were obtained.
Gas-phase samples for spectroscopy and photolysis were
prepared by placing a quantity (1 µL for spectra, 8 µL for
photolyses) of the neat cyclobutene derivative and cyclohexane
(0.05 µL) in a 1-cm Suprasil cuvette which had been flushed
for ca. 5-min. with a stream of SF6, and were sealed with
rubber septa. The samples were allowed to vaporize completely
before beginning the experiments. Solution-phase samples in
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council of Canada for its
financial support of this work.
(54) Reeve, W.; Reichel, D. M. J . Org. Chem. 1972, 37, 68.
J O990618V