Photocycloelimination of α,α-dichlorocyclobutanones

Article abstract of DOI:10.1139/v99-110

Direct irradiation of a series of α,α-dichlorocyclobutanones in benzene solutions results in photocycloelimination to give 1,1-dichloroalkenes in yields ranging from 30-65%. The α,α-dichlorocyclobutanones were formed in good yields from the [2+2] cycloaddition of the terminal olefins with dichloroketene. This two-step sequence formally represents a "metathesis" of two olefinic functions and provides an easy access to functionalized 1,1-dichloroalkenes. Irradiation of the dichlorocyclobutanones in the solid state led to poor yields of 1,1-dichloroalkenes and polymeric mixtures, however, photoreactions performed in zeolites gave similar yields as those run in benzene solutions.

Full text of DOI:10.1139/v99-110

1245
Photocycloelimination of ␣,␣-dichlorocyclo-
butanones
Jailall Ramnauth and Edward Lee-Ruff
Abstract: Direct irradiation of a series of α,α-dichlorocyclobutanones in benzene solutions results in
photocycloelimination to give 1,1-dichloroalkenes in yields ranging from 30–65%. The α,α-dichlorocyclobutanones
were formed in good yields from the [2+2] cycloaddition of the terminal olefins with dichloroketene. This two-step
sequence formally represents a “metathesis” of two olefinic functions and provides an easy access to functionalized
1,1-dichloroalkenes. Irradiation of the dichlorocyclobutanones in the solid state led to poor yields of 1,1-
dichloroalkenes and polymeric mixtures, however, photoreactions performed in zeolites gave similar yields as those
run in benzene solutions.
Key words: dichlorocyclobutanones, dichloroalkenes, olefin metathesis, photocycloelimination.
Résumé : L’irradiation directe d’une série de α,α-dichlorocyclobutanones, en solutions benzéniques, provoque une
photocycloélémination qui conduit à la formation de 1,1-dichloroalcènes, avec des rendements allant de 30 à 65%. Les
α,α-dichlorocyclobutanones se formes avec de bons rendements par la cycloaddition [2+2] d’oléfines thermiques avec
le dichlorocétène. Cette séquence en deux étapes correspond à une “métathèse” de deux fonctions oléfiniques et elle
fournit une voie d’accès facile aux 1,1-dichloroalcènes fonctionnalisés. L’irradiation des dichlorocyclobutanones à l’état
solide conduit, avec de mauvais rendements, aux 1,1-dichloroalcènes avec des mélanges de polymères; toutefois, les
photoréactions effectuées dans des zéolites donnent des rendements semblables à ceux obtenus en solution dans le
benzène.
Mots clés : dichlorocyclobutanones, dichloroalcènes, métathèse d’oléfine, photocycloélimination.
[Traduit par la Rédaction] Ramnauth and Lee-Ruff 1248
Introduction
Previously, we reported the photocycloelimination of
bicyclic cyclobutanones in nonpolar aprotic solvents con-
taining a minimal amount of an alcohol to quench the inter-
mediate ketene produced, generating substituted terminally
functionalized carboxylic esters (7). Although the yields are
excellent, the complete elimination of the ring expansion
pathway could not be achieved in some cases. Also, the
[2+2] cycloaddition with ketenes requires activated olefins
such as enol ethers and conjugated dienes.
Herein, we report the direct irradiation of a series of α,α-
dichlorocyclobutanones generated from the [2+2] cycloaddi-
tion of dichloroketene to the corresponding terminal olefins,
giving 1,1-dichloroalkenes. Such a reaction can be formally
represented by Scheme 1 in which terminal chlorination of a
terminal alkene is the net conversion. Such direct chlorina-
tion reactions cannot be effectively performed on terminal
alkenes.
Photocycloelimination is one of three principal reaction
channels from electronically excited cyclobutanones, which
also include decarbonylation and ring expansion (1). If the
photocycloelimination process occurs in an orthogonal fash-
ion with respect to the ketene–olefin cycloaddition to form
cyclobutanones, such a two-step sequence formally repre-
sents a metathesis of two olefinic groups (2). Furthermore,
since a large number of functionalized terminal olefins are
commercially available and dichloroketene adds easily to
terminal olefins (3), such a scheme would constitute a con-
venient route to substituted 1,1-dichloroalkenes.
1,1-Dichloroalkenes are important synthetic intermediates,
especially for the synthesis of arylacetic acids (4) and various
alkynes (5). The commonly used methods for the prepara-
tion of 1,1-dichloroalkenes include the dichloromethylen-
ation of carbon–oxygen double bond of carbonyl compounds
with Wittig-type reagents obtained from organophosphines
and carbon tetrachloride, and Horner–Emmons reagent gen-
erated from cathodic reduction of diethyl trichloromethyl
phosphonate (6).
Scheme 1.
R1
R2
Cl
Cl
R1
H
H
R2
Received June 24, 1998.
J. Ramnauth and E. Lee-Ruff.1 Department of Chemistry,
Results and discussion
York University, Toronto, ON M3J 1P3, Canada.
The dichlorocyclobutanone derivatives 2(a) and 2(b) were
prepared according to literature methods (8, 9) from the
commercially available alkenes 1(a) and (b), respectively. Ir-
radiation of a 6 × 10–3 M solutions of 2(a) and 2(b) in
1Author to whom correspondence may be addressed.
Telephone: (416) 736-5443. Fax: (416) 736-5936.
e-mail: leeruff@yorku.ca
Can. J. Chem. 77: 1245–1248 (1999)
© 1999 NRC Canada

1246
Can. J. Chem. Vol. 77, 1999
Scheme 2.
benzene for 2 h using a medium pressure mercury lamp with
a pyrex filter went to ca. 70% conversion to give the known
compounds 3(a) (10) and 3(b) (11) in 30 and 40% isolated
yields, respectively, with trace amounts (<1%) of starting
olefins 1(a) and 1(b) (see Scheme 2). The structures of
showed 10 signals in accordance with the assigned structure
for 3(f).
In the continuation of our work on the triplet photosensi-
tized reaction of cyclobutanones to form cyclopropanes (13),
we were interested in the possibility of preparing dichloro-
cyclopropanes via the photodecarbonylation of dichlorocy-
clobutanones. Some dichlorocyclopropanes have been
shown to exhibit insecticidal properties (14). Under triplet
sensitized conditions, using acetone as both the sensitizer
and solvent, the dichlorocyclobutanone 2(c) and 2(d) gave
olefins 3(c) and 3(d), respectively, as the major nonpolar
products. These results were somewhat surprising and con-
trary to the behaviour of alkyl substituted cyclobutanones.
To rationalize these results, one must consider the mecha-
nism of cyclobutanone photochemistry. It has been shown
that cycloelimination can occur form a vibrationally excited
ground state of cyclobutanone formed from internal conver-
sion (15). This is based on observations of cycloeliminations
occurring in the thermal reactions of cyclobutanones (16).
The triplet state of cyclobutanone has been implicated in the
decarbonylation process (17). The fact that decarbonylation
does not occur in these α,α-dichlorocyclobutanones can be
explained by a simple but very reasonable model proposed
by Moule (18). The triplet state may be too short-lived due
to efficient intersystem crossing to S° in the presence of a
heavy atom (19). This deactivation process would compete
with decarbonylation and form a hot ground state resulting
in cycloelimination.
Another rationalization for the lack of decarbonylation in
these systems is the increase of the triplet state energy level
brought about by an electron-withdrawing substituent. Ke-
tones possessing an electron-withdrawing α-substituent such
as oxygen and chlorine show a hypsochromic shift of the
n→π* absorption band (20). This electronic effect would
also be expected to increase the triplet energy level, which
may result in inefficient triplet sensitization by acetone
(ET = 78 kcal), resulting in an endothermic process. Thus,
direct excitation of cyclobutanone might be expected to give
only cycloelimination products.
1
3(a) and 3(b) were based on H-NMR spectra and compari-
son with literature data. Compound 3(a) exhibited a single
proton singlet at δ 6.86 ppm assigned to the vinylic proton,
while 3(b) showed only aromatic signals. It was somewhat
surprising to detect the presence of trace amounts of starting
olefins 1(a) and (b), indicating that the photocyclo elimina-
tions also occurred on the less substituted side, however,
only to a minor extent. The low yields of these derivatives
could be attributed to some polymerization of the 1,1-
dichloroalkenes.
The dichloroketene adducts 2(c)–2(f) were prepared from
the cycloaddition of dichloroketene (generated in situ from
trichloroacetyl chloride and activated zinc) to the corre-
sponding olefins 1(c)–1(f) in the presence of 1,2-
dimethoxyethane (DME) (12) as cosolvent in yields ranging
from 74 to 85% after purification. The structures of these
compounds were fully characterized by spectral and analyti-
cal data. The olefins 1(c)–1(f) were prepared from the
benzoylation of the commercially available alcohols in ex-
cellent yields (90–96%). The dichlorocyclobutanones
2(c)–2(f) were irradiated as before (ca. 80% conversions) to
give exclusively the dichloroalkenes 3(c)–3(f) in yields
ranging form 54 to 65% after chromatography. The struc-
tures of these alkenes were confirmed by spectral data. The
1H-NMR data of 3(c) exhibited a one proton triplet (J =
6.7 Hz) at δ 6.21 ppm assigned to the vinylic proton. This
proton appeared at δ 5.9 ppm in the original olefin. The 13C-
NMR spectrum showed the eight unique carbon signals. For
compound 3(d),the 1H-NMR spectrum showed a single
triplet (J = 7.1 Hz) at 6.00 ppm associated with the vinylic
proton, and the 13C-NMR spectrum revealed the nine signals
one would expect for this compound. The 13C-NMR spec-
trum revealed the nine unique carbons of compound 3(e),
1
and the H-NMR spectrum showed sharp singlets at δ
5.01 and δ 2.00 corresponding to the methylene and methyl
protons, respectively. Olefin 3(f) showed two triplets at δ
4.45 (J = 6.6 Hz) and δ 2.77 (J = 6.4 Hz) associated with a
substituted ethyl group and a sharp methyl singlet at δ
2.10 in its 1H-NMR spectrum. The 13C-NMR spectrum
Radicals and diradicals that are produced in solution suf-
fer a different fate from those produced in the solid state. To
explore the possibility that the 1,4-acyl-alkyl diradical might
undergo decarbonylation in the solid state, we explored the
photochemistry of 2 in the crystal lattice.
© 1999 NRC Canada

Ramnauth and Lee-Ruff
1247
Solid state photolysis often differs from solution
photochemistry due to the rigidity associated with the crystal
lattice and has received a great deal of attention recently
(21). The solid state photolyses of 2(a)–2(c) and 2(e) were
done according to a modified literature procedure (22). After
the usual work-up and chromatography, poor yields of the
1,1-dichloroalkenes were obtained but no cyclopropanes
were detected. The photolysate consisted primarily of intrac-
table polymeric materials. This problem did not occur to any
great extent in the solution state photolyses due to the dilute
conditions under which these reactions were conducted.
To minimize the extent of polymerization due to radical
processes, we explored these photochemical reactions in
zeolites (23). For this work, incorporation of the
dichlorocyclobutanones within the supercages was accom-
plished by stirring hexane solutions in the presence of anhy-
drous Na-Y zeolite, followed by thorough washings with
hexane. Analysis of the hexane washings revealed no resid-
ual starting materials, indicating complete incorporation of
the substrate within the zeolite cages. The vacuum-dried
complexes were photolyzed as hexane slurries for 2 h with
stirring. After work-up, which involved extraction with ether
and then chromatography, the yields were about the same as
those in the solution phase. However, the conversions were
on average 10% lower than those in the solution phase.
It has been shown that the photocycloelimination of
α,α-dichlorocyclobutanones can provide an easy route to
1,1-dichloroalkenes. This method formally represents a me-
tathesis of two olefinic groups. With the limited number of
methods available for the synthesis of these derivatives, this
method provides an easy two-step alternative. What is at-
tractive is the generality in terms of the wide range of
functionalized terminal olefins that can add to
dichloroketene.
General photolysis procedures
A solution of dichlorocyclobutanones 2(a)–2(e) (0.4 mmol)
in 70 mL of benzene was degassed with argon for 30 min
and then irradiated for 2 h. The solvent was removed under
reduced pressure, and the residue was chromatographed on
TLC plates (hexane:ethyl acetate, 90:10). The conversions
were ca. 70–80%. The yields reported are the isolated yields
based on starting materials. Solid-phase photolyses were
done by sandwiching the solids between two pyrex micro-
scope slides. The slides were secured with tape and placed
in a quartz tube. After the irradiation, the slides were rinsed
with CH2Cl2, the solvent removed under reduced pressure,
and the residue was chromatographed as before. For the zeo-
lite work, 1 g of Na-Y zeolite was added to the dichloro-
cyclobutanone (0.4 mmol) in 50 mL hexane. This
suspension was stirred overnight at room temperature. After
this time, the suspension was filtered, and the solid washed
with 4 × 100 mL hexane. Evaporation of the hexane
washings revealed no starting materials. The vacuum dried
solids were suspended in 70 mL of hexane and photolyzed
for 2 h with constant stirring. After this time, the solid was
filtered and subjected to a soxhlet extraction overnight. Con-
versions were ca. 60–70%.
General procedure for the preparation of
dichlorocyclobutanones 2(c)–2(e)
A two-neck round-bottom flask, equipped with a magnetic
stirrer, dropping funnel, and argon inlet was charged with
activated zinc (9) (4 equiv.) and the olefin (1 equiv.) in an-
hydrous ether (2 mL/mmol of olefin). To this vigorously
stirred suspension was added dropwise a mixture of tri-
chloroacetyl chloride (2 equiv.), 1,2-dimethoxyethane (DME)
(2 equiv.), and anhydrous ether (1 mL/mmol) over a period
of about 30 min. This mixture was stirred at room tempera-
ture overnight. After this time, hexane was added (-50 mL)
and the suspension stirred for 5 min to precipitate the zinc
salts. The solution was then decanted and treated sequen-
tially with 100 mL H2O, 3 × 100 mL saturated NaHCO3,
100 mL saturated NaCl and then dried over Na2SO4.
Experimental
Melting points (mp) were determined on a Reichert melt-
ing point apparatus and are uncorrected. NMR spectra were
recorded on a Bruker ARX 400 spectrometer in CDCl3 con-
taining 1% TMS or unless otherwise noted. Fourier Trans-
form Infrared (FT-IR) spectra were recorded on a Pye
Unicam SP3-200 spectrometer as thin films or as KBr pel-
lets. Photolyses were carried out using a Hanovia 450W me-
dium-pressure mercury arc lamp in a water-cooled quartz
immersion well. Pyrex tubes containing the samples were
strapped around this well and the assembly immersed in an
ice-water bath. The samples were degassed for 30 min prior
to irradiation. All solvents used in these reactions were dried
and distilled. Analytical thin layer chromatography (TLC)
was done using commercially prepared silica gel 60F 254
plastic sheets (E. Merck & Co.). Preparative TLC was car-
ried out with Aldrich silica gel 60F 254 precoated glass
plates. Elemental analyses were performed by Guelph
Chemical Laboratories Limited. All reactions were per-
formed under inert atmosphere conditions. All precursor
chemicals were purchased from the Aldrich chemical com-
pany. Olefins 1(c)–1(f) were prepared from benzoylation of
the corresponding alcohol using benzoyl chloride in
pyridine. The cyclobutanones 2(a) and 2(b) were prepared
according to literature methods (8, 9).
3-(Benzoyloxy)methyl-2,2-dichlorocyclobutanone (2(c)):
Prepared in 85% yield from olefin 1(c). The crude product
was recrystallized from hexane to give a white solid; mp 71–
1
72°C; H-NMR δ: 8.04–7.26 (m, 5H), 4.71–4.64 (m, 2H),
3.57–3.50 (dd, 1H, J = 9.9, 7.7 Hz), 3.44–3.41 (m, 1H),
3.27–3.21 (dd, 1H, J = 7.7, 9.8 Hz); FT-IR (KBr): 1809,
1713, 1599 cm–1; 13C-NMR δ: 191.1, 166.0, 133.5, 129.7,
129.4, 128.6, 87.4, 63.3, 45.2, 44.3. Anal. calcd. for
C12H10O3Cl2: C 52.77, H 3.69; found: C 52.69, H 3.64.
3-[2-(Benzoyloxy)ethyl]-2,2-dichlorocyclobutanone (2(d)):
Prepared in 80% yield from olefin 1(d). The crude product
was passed through a short plug of silica gel to give a
1
colourless oil. H-NMR δ: 8.06–7.44 (m, 5H), 4.51–4.46
(m, 2H), 3.47–3.39 (m, 1H), 3.17–3.06 (m, 2H), 2.47–2.38
(m, 1H), 2.16–2.04 (m, 1H); FT-IR(neat): 1812, 1718 cm–1;
13C-NMR δ: 192.4, 166.4, 133.2, 129.8, 129.6, 128.5, 88.8,
62.5, 48.0, 43.5, 30.4. Anal. calcd. for C13H12O3Cl2: C 54.38,
H 4.21; found: C 54.33, H 4.30.
© 1999 NRC Canada

1248
Can. J. Chem. Vol. 77, 1999
6.6 Hz), 2.77–2.74 (t, 2H, J = 6.4 Hz), 1.98 (s, 3H);
13C-NMR δ: 166.5, 133.1, 131.4, 130.0, 129.6, 128.4, 116.9,
61.9, 34.7, 20.3.
3-(Benzoyloxy)methyl-2,2-dichloro-3-methylcyclobutanone
(2(e)): Prepared in 84% yield from the olefin 1(e). The
crude product was chromatographed through a short plug of
silica gel to give a viscous oil, which solidified on standing
1
at –10°C overnight; mp 48–49°C. H-NMR δ: 8.02–7.46 (m,
Acknowledgements
5H), 4.59 (s, 2H), 3.34–3.39 (d, 1H, J = 17.0 Hz), 3.16–3.11
(d, 1H, J = 17.6 Hz); FT-IR (KBr): 1811, 1732 cm–1;
13C-NMR δ: 191.4, 165.9, 133.5, 129.7, 129.3, 128.6, 91.0,
68.1, 51.6. 44.9, 20.6. Anal. calcd. for C13H12O3Cl2: C
54.38, H 4.21; found: C 54.37, H 4.07.
We would like to thank the Natural Sciences and Engi-
neering Research Council (NSERC) for financial support.
References
3-[2-(Benzoyloxy)ethyl]-2,2-dichloro-3-methylcyclobutanone
(2(d)): Prepared in 75% yield from olefin 1(f). The crude
product was passed through a short plug of silica gel to give
1. D.R. Morton and N.J. Turro. Adv. Photochem. 9, 197 (1974).
2. K.J. Ivin. In Olefin metathesis. Academic Press, New York.
1983.
3. L. Ghosez, R. Montaigne, A. Roussel, H. Vanlierde, and P.
Mollet. Tetrahedron, 27, 615 (1971).
4. T. Shono. Chem. Abstr. 97, 215 767u (1982).
5. J. Villieras, P. Perriot, and J.F. Normant. Synthesis, 458
(1975).
6. P. Savignac, J. Petrova, M. Dreux, and P. Coutrot. Synthesis,
535 (1975).
1
a colourless oil. H-NMR δ: 8.05–7.44 (m, 5H), 4.55–4.50
(m, 2H), 3.41–3.36 (d, 1H, J = 16.8 Hz), 2.93–2.89 (d, 1H, J
= 15.8 Hz), 2.41–2.30 (m, 2H), 1.48 (s, 3H); FT-IR (KBr):
1811, 1732 cm–1; 13C-NMR δ: 192.5, 166.3, 133.2, 129.9,
129.6, 128.5 92.7, 61.5, 54.7. 43.8, 35.3, 22.7. Anal. calcd.
for C14H14O3Cl 2: C 55.83, H 4.69; found: C 55.48, H 4.41.
1
1,1-Dichloro-2-phenylethene (3(a)) (10): Yield: 30%; H-NMR
7. E. Lee-Ruff, H. Kazarians-Moghaddam, and A.C. Hopkinson.
Tetrahedron Lett. 24, 2067 (1983).
δ: 7.54–7.31 (m, 5H), 6.26 (s, 1H).
8. L.R. Krepski and A. Hassner. J. Org. Chem. 43, 2879 (1978).
9. A. Maercker and W. Berkulin. Chem. Ber. 123, 185 (1990).
10. P. Jubault, C. Feasson, and N. Collignon. Bull. Soc. Chim. Fr.
131, 1001 (1994).
11. S. Braverman and Y. Zafrani. Tetrahedron, 54, 1901 (1998).
12. B.D. Johnston, E. Czyzewska, and A.C. Oehlschlager. J. Org.
Chem. 52, 3693 (1987).
13. J. Ramnauth and E. Lee-Ruff. Can. J. Chem. 75, 518 (1997).
14. M. Elliot and N.F. James. Chem. Soc. Rev. 7, 473 (1978).
15. N.E. Lee and E.K.C. Lee. J. Chem. Phys. 50, 2094 (1969).
16. H.A.J. Carless and E.K.C. Lee. J. Am. Chem. Soc. 92, 4482
(1970).
17. H.O. Denschlag and E.K.C. Lee. J. Am. Chem. Soc. 90, 3628
(1968).
18. D.C. Moule. Can. J. Phys. 47, 1235 (1969).
19. I.V. Khudyakov, Y.A. Serebrennikov, and N.J. Turro. Chem.
Rev. 93, 537 (1993).
1,1-Dichloro-2,2-diphenylethene (3(b)) (11): Yield: 40%;
1H-NMR δ: 7.33–7.29 (m).
3-Benzoyloxy-1,1-dichloro-1-propene (3(c)): Yield: 65%;
1H-NMR δ: 8.06–7.43 (m, 5H), 6.21–6.18 (t, 1H, J = 6.7 Hz),
4.93–4.91 (d, 2H, J = 6.8 Hz); 13C-NMR (acetone-d6) δ:
166.4, 134.2, 130.6, 130.3, 129.5, 126.3, 125.1, 61.9. Anal.
calcd. for C10H8O2Cl 2: C 51.89, H 3.49; found: C 51.12, H
3.53.
4-Benzoyloxy-1,1-dichloro-1-butene (3(d)): Yield: 58%;
1H-NMR δ: 8.05–7.43 (m, 5H), 6.00–5.96 (t, 1H, J =
7.1 Hz), 4.39–4.36 (t, 2H, J = 6.2 Hz), 2.69–2.64 (m, 2H);
13C-NMR (acetone-d6) δ: 166.6, 133.9, 131.1, 130.2, 129.4,
127.9, 121.9, 63.2, 30.2. Anal. calcd. for C11H10O2Cl : C
2
53.90, H 4.11; found: C 53.77, H 3.90.
20. D.L. Pavia, G.M. Lampman, and G.S. Kriz. In Introduction to
spectroscopy. 2nd ed. Saunders College Publishing. 1996.
p. 283.
21. Y. Ito. Synthesis, 1 (1998).
22. A.D. Gudmundsdottir, J.R. Scheffer, and J. Trotter. Tetrahe-
dron Lett. 35, 1397 (1994).
3-Benzoyloxy-1,1-dichloro-2-methyl-1-propene (3(e)): Yield:
1
54%; H-NMR δ: 8.06–7.44 (m, 5H), 5.01 (s, 2H), 2.00 (s,
3H); 13C-NMR (acetone-d6) δ: 166.4, 133.9, 132.3, 130.5,
130.3, 129.5, 119.2, 65.5, 18.1. Anal. calcd. for C11H10O2Cl 2:
C 53.90, H 4.11; found: C 55.14, H 4.60.
4-Benzoyloxy-1,1-dichloro-2-methyl-1-butene (3(e)): Yield:
60%; H-NMR δ: 8.05–7.44 (m, 5H), 4.45–4.42 (t, 2H, J =
23. D.W. Breck. In Zeolite molecular sieves: structure, chemistry,
and use. Wiley, New York. 1974.
1
© 1999 NRC Canada