The photochemistry of trans-1,4,4,4-tetraphenyl-but-2-en-1-one: A highly efficient aryl migration (type B) enone photorearrangement

Article abstract of DOI:10.1139/v03-030

Photolysis of trans-1,4,4,4-tetraphenylbut-2-en-1-one (3) in acetonitrile or benzene leads to trans-cis isomerization (7) along with rearrangement to trans-1-benzoyl-2,2,3-triphenylcyclopropane (8). Formation of the latter product represents a new example

Full text of DOI:10.1139/v03-030

705
The photochemistry of trans-1,4,4,4-tetraphenyl-
but-2-en-1-one: A highly efficient aryl migration
(type B) enone photorearrangement
John R. Scheffer and Kodumuru Vishnumurthy
Abstract: Photolysis of trans-1,4,4,4-tetraphenylbut-2-en-1-one (3) in acetonitrile or benzene leads to trans–cis
isomerization (7) along with rearrangement to trans-1-benzoyl-2,2,3-triphenylcyclopropane (8). Formation of the latter
product represents a new example of the aryl migration (type B) enone photorearrangement reaction first reported by
Zimmerman and co-workers for 4,4-diphenylcyclohex-2-en-1-one (1). The quantum yield in the case of enone 3 (0.4) is
approximately 10 times greater than that for 4,4,-diphenylcyclohex-2-en-1-one, a result that is ascribed to steric acceler-
ation of phenyl migration from the triphenylmethyl group plus greater resonance stabilization of the intermediate biradical.
Key words: photochemistry, mechanism, rearrangement, aryl migration, enone, di-π-methane.
Résumé : La photolyse de la trans-1,4,4,4-tétraphénylbut-2-én-1-one (3) dans l’acétonitrile ou le benzène conduit à une
isomérisation trans–cis (7) ainsi qu’à un réarrangement en trans-1-benzoyl-2,3,3-triphénylcyclopropane (8). La forma-
tion de ce dernier produit représente un nouvel exemple de réaction de photoréarrangement d’énone avec migration
d’aryle (type B) qui avait été observé pour la première fois par Zimmerman et ses collaborateurs pour la 4,4-
diphénylcyclohex-2-én-1-one (1). Le rendement quantique dans le cas de l’énone 3 (0,4) est approximativement dix fois
plus grand que celui observé avec la 4,4-diphénylcyclohex-2-én-1-one; on attribue cette différence à l’accélération sté-
rique associée à la migration du phényle à partir du groupe triphénylméthyle ainsi qu’à la plus grande stabilisation par
résonance du biradical intermédiaire.
Mots clés : photochimie, mécanisme, réarrangement, migration d’un groupe aryle, énone, di-π-méthane.
[Traduit par la Rédaction] Scheffer and Vishnumurthy 708
Introduction
by cyclopropane ring closure. Subsequent studies by the
Zimmerman group used the aryl migration cyclohexenone
photorearrangement reaction to investigate the relative ex-
cited-state migratory aptitudes of aryl groups bearing elec-
tron-donating and electron-withdrawing substituents (4).
Many of the principles of organic photochemistry with
which we are familiar today rest on early studies of the
light-induced behavior of enones and dienones (1). As an ex-
ample, consider the case of 4,4-diphenylcyclohex-2-en-1-one
(1) (eq. [1]), whose photochemistry was first reported by
Zimmerman and Wilson (2) in 1964 and subsequently inves-
tigated in more detail by Zimmerman and Hancock (3) in
1968. In these studies, irradiation of ketone 1 in solution was
shown to afford trans-5,6-diphenylbicyclo[3.1.0]hexan-2-
one (2) in rather low quantum efficiency (Φ = 0.043) but ex-
cellent chemical yield; trace amounts of cis-5,6-diphenyl-
bicyclo[3.1.0]hexan-2-one and 3,4-diphenylcyclohex-2-en-1-
one were also formed. Formally a di-π-methane process in-
volving the enone double bond and a double bond on one of
the phenyl groups, the formation of photoproduct 2 takes
place in the triplet state via a 1,2-phenyl shift accompanied
[1]
Received 10 December 2002. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 18 June 2003.
In seeking variations of the aryl migration enone photo-
rearrangement for the purpose of investigating asymmetric
induction in organic photochemical reactions, we chose
trans-1,4,4,4-tetraphenylbut-2-en-1-one (3, eq. [1]) as
a potential target molecule. This choice is attractive from
several points of view. For one, we anticipated that modifi-
cation of the benzoyl group to accept ionic chiral auxiliaries
(5) would be relatively easy. A second, unrelated motivation
was that preparation of analogs of enone 3 in which the
Dedicated to Professor Donald R. Arnold for his
contributions to Chemistry.
J.R. Scheffer1 and K. Vishnumurthy. Department of
Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, BC V6T 1Z1, Canada.
1Corresponding author (e-mail: scheffer@chem.ubc.ca).
Can. J. Chem. 81: 705–708 (2003)
doi: 10.1139/V03-030
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Can. J. Chem. Vol. 81, 2003
three terminal aryl groups are, for example, phenyl, p-
methoxyphenyl, and p-cyanophenyl, offers the possibility of
investigating the relative excited-state migratory aptitude of
three aryl groups simultaneously, rather than only two at a
time as in the case of cyclic enones. In the present article we
outline the synthesis of the parent compound (3) and de-
scribe its basic photochemical behavior in solution.
same compound prepared via the oxadi-π-methane photo-
rearrangement of β,γ-unsaturated ketone 9 (eq. [3]) (7). The
quantum yield for formation of photoproduct 8 from enone 3
in benzene, extrapolated to 0% conversion, was found to be
0.4 (valerophenone actinometry). Although sensitization–
quenching studies were not carried out, given the high effi-
ciency with which phenyl ketones form triplets it seems vir-
tually certain that the photorearrangement of enone 3 is
triplet-mediated.
Results
The synthesis of enone 3 was accomplished in straightfor-
ward fashion through the known intermediates 5 and 6 (6)
(eq. [2]). The final step, selenium dioxide oxidation of
ketone 6, proceeded in 94% overall yield to afford the target
enone 3 as a colorless crystalline solid, mp 133–136°C. The
assignment of trans stereochemistry to enone 3 followed
from its proton NMR spectrum, which showed the vinyl hy-
drogens as doublets at 8.00 and 6.67 ppm with a 15.6 Hz
coupling constant between them. By way of contrast, the cis
isomer of enone 3 (i.e., compound 7, eq. [3]), which could
be isolated in small amounts when 3 was irradiated for rela-
tively short periods of time (vide infra), exhibited an NMR
spectrum in which the vinyl doublets appeared at 6.79 and
6.32 ppm with a mutual coupling constant of 12 Hz.
[3]
The photochemistry of enone 3 was examined in both ben-
Discussion
[2]
While cyclopropyl ketone 8 has the gross structure ex-
pected of a straightforward 1,2-phenyl migration photo-
rearrangement of enone 3, its stereochemistry differs from
that of the major product 2 formed in the photolysis of 4,4-
diphenylcyclohex-2-en-1-one. In the latter case, the migrat-
ing phenyl group ends up cis to the carbonyl group, whereas
in ketone 8 there is a trans relationship between these two
moieties. Both results are consistent with the concerted
mechanism proposed by Zimmerman and Hancock (3) in
which phenyl migration is synchronous with cyclopropane
ring formation. As depicted in eq. [4], this mechanism pre-
dicts that cis-enones (e.g., 1 and 7) will form products in
which the migrating group (Ar) and the C=O group are cis
to one another (e.g., 2), and that trans-enones (e.g., 3) will
lead to products having a trans-Ar – C=O relationship (e.g.,
8). There is a problem with this picture, however, in that in-
dependent photolysis of cis-enone 7 did not form the ex-
pected cis-Ar – C=O diastereomer; only photoproduct 8 was
observed. This indicates that either the concerted
photorearrangement of cis-enone 7 is slower than its
isomerization to trans-enone 3 (no 3 could be detected, how-
ever), or that both enones rearrange by a non-concerted
mechanism through a common, conformationally equili-
brated intermediate such as 11 (eq. [4]) in which ring clo-
sure with clockwise rotation about the C-2—C-3 bond is
favored, leading to photoproduct 8.2 Zimmerman and Han-
cock (3) also concluded that a non-concerted mechanism is
capable of rationalizing the stereochemical course of the
photorearrangement of cyclohexenone 1. At the moment,
therefore, a definitive choice between the concerted and non-
zene and acetontrile solution and the progress of the reaction
monitored by gas chromatography. This showed the forma-
tion of two photoproducts, subsequently shown to be cis-
enone 7 and trans-1-benzoyl-2,2,3-triphenylcyclopropane (8)
(eq. [3]). Photolysis was stopped when GC analysis indi-
cated the complete disappearance of enone 3, at which point
the 7:8 ratio was approximately 1:3; chromatography of this
reaction mixture allowed small amounts of cis-enone 7 to be
isolated and characterized. Continuing the photolysis until
no further enone 7 could be detected by GC analysis (ca.
2 h) and subsequent silica gel column chromatograpy al-
lowed cyclopropyl ketone 8 to be isolated in 96% yield. The
structure and trans stereochemistry of photoproduct 8 were
assigned on the basis of a comparison of its mp and spectro-
scopic properties with those reported in the literature for the
2 Biradical 11 is expected to be more stable than its conformer 10 in which the benzoyl group interferes with a phenyl group on C-4. Clock-
wise rotation around C-2—C-3 during ring closure of biradical 11 is favored over counterclockwise rotation, since the latter pathway leads
to increased nonbonded interactions between the benzoyl group and the phenyl group on C-3.
© 2003 NRC Canada

Scheffer and Vishnumurthy
707
1
[4]
75 MHz under broadband H decoupling. Low- and high-
resolution mass spectra were obtained from a Kratos MS 50
instrument using electron impact (EI) ionization at 70 eV or
the chemical ionization (CI) method; intensities of the ions
are given in parentheses. Ultraviolet spectra were recorded
on a PerkinElmer Lambda-4B spectrophotometer in the sol-
vent indicated. Gas chromatographic analyses were per-
formed on a Hewlett-Packard HP 5890 instrument.
1,1,1,4-Tetraphenylbutane (5)
The known compound 5 (6) was synthesized by a modi-
fied procedure of Brook and Pierce (6) (8). Mercury (36.5 g,
182 mmol) was placed in a 100 mL two-necked round-
bottomed flask equipped with a magnetic stirrer and a reflux
condenser and the system thoroughly flushed with anhy-
drous nitrogen. Sodium (0.365 g, 15.9 mmol) was carefully
added in several small pieces to the mercury under nitrogen
with cooling by means of a water bath. A solution of 2.0 g
(7.2 mmol) of triphenylmethyl chloride (Aldrich) in 50 mL
of anhydrous ether was then added to the sodium amalgam
under nitrogen with stirring, whereupon a blood red color
developed over a period of approximately 1 h. Stirring was
continued for an additional 2 h after which 1.78 g
(8.9 mmol) of 3-phenyl-1-bromopropane (4) (Aldrich) was
added. The red color disappeared immediately and the re-
sulting solution was stirred at room temperature for 4 h. The
reaction mixture was worked up by hydrolysis with 20 mL
of distilled water followed by extraction with diethyl ether.
The ether layers were combined, dried over anhydrous so-
dium sulphate, and concentrated in vacuo to afford a non-
crystalline residue that was subjected to silica gel column
chromatography using 2% ethyl acetate in petroleum ether
as the eluting solvent. This afforded 1.70 g (53%) of com-
pound 5 as a colorless solid. Recrystallization from ether:pe-
troleum ether (2:1) gave colorless needles, mp 94 to 95°C
(lit. (6) mp 92 to 93.5°C).
concerted pathway cannot be made, although it could be ar-
gued that the triplet nature of the photorearrangement favors
the latter.
A final point of discussion concerns the 10-fold higher quan-
tum yield for the photorearrangement of acyclic enone 3
compared with that of cyclic enone 1. Although quantum
yields are not always a reflection of rates of reaction, it is
tempting to equate the higher quantum yield for the
photorearrangement of enone 3 with relief of steric crowding
in the triphenylmethyl group as well as greater resonance
stabilization of the intermediate biradical 10.
Work is continuing in our laboratory on developing meth-
ods for carrying out the aryl migration enone photo-
rearrangement enantioselectively as well as synthesizing
triarylmethane analogs of enone 3 for use in excited-state
migratory aptitude studies.
1,4,4,4-Tetraphenylbutan-1-one (6)
The known compound 6 (6) was prepared using an oxida-
tion procedure of Muzart (9). To a solution of 0.89 g
(2.5 mmol) of compound 5 in 4.2 mL of dichloromethane
was added 0.074 g (0.74 mmol) of chromium trioxide
(Aldrich) and 6.6 mL of a 70% aqueous solution of tert-
butylperoxide (Aldrich) and the resulting mixture stirred for
36 h at room temperature. The biphasic reaction mixture was
worked up by addition of 8 mL of saturated aqueous sodium
thiosulphate solution until no longer oxidizing to starch-
iodide paper and then extracted with dichloromethane (2 ×
15 mL). The combined organic layers were washed with wa-
ter and brine and then dried over anhydrous magnesium sul-
phate. Removal of the solvent in vacuo followed by radial
chromatography over silica gel (2% ethyl acetate in petro-
leum ether) afforded 0.7 g (75%) of ketone 6 as a colorless
solid. Recrystallization from ethyl acetate gave small prisms,
mp 141–143°C (lit. (6) mp 139 to 140.5°C).
Experimental
Instrumentation and general procedures
Commercial spectral grade solvents were used for photo-
chemical experiments unless otherwise noted. Infrared spec-
tra were recorded on a Perkin-Elmer 1710 Fourier transform
spectrometer. Solid samples were ground in KBr (1–5%) and
liquid samples were run neat as thin films. Melting points
were determined on a Fisher-Johns hot stage apparatus and
are uncorrected. 1H NMR spectra were recorded in deuterated
solvents as noted on a Varian AV-300 (300 MHz) instrument.
13C NMR spectra were run on the same instrument at
trans-1,4,4,4-Tetraphenylbut-2-en-1-one (3)
Following an oxidation procedure of Bernstein and Littell
(10), a solution of 1.0 g (2.7 mmol) of ketone 6 and 1.2 g
(10.8 mmol) of selenium dioxide (Aldrich) in 125 mL of
tert-butanol and 5.6 mL of glacial acetic acid was refluxed
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Can. J. Chem. Vol. 81, 2003
with stirring for 48 h. The reaction mixture was diluted with
25 mL of ethyl acetate, filtered, and the filtrate washed with
water, 10% aqueous sodium hydroxide, water again, brine,
and then dried over anhydrous sodium sulphate. Removal of
the solvent in vacuo followed by GC analysis of the residue
revealed the presence of both ketone 3 and ketone 6 in a 2:1
ratio. This mixture was subjected to a second oxidation iden-
tical to the first but on half the scale until GC analysis indi-
cated the complete disappearance of starting material 6.
Workup as before afforded a crude reaction mixture that was
subjected to radial chromatography over silica gel (5% ethyl
acetate in petroleum ether) to afford 0.94 g (94%) of solid
enone 3. Recrystallization from ethyl acetate:hexane (3:1)
gave colorless needles, mp 133–136°C. IR (KBr) (cm–1):
3054, 1667, 1610, 1490, 1445, 1326, 1304, 1291, 1223,
1179, 1033, 1004, 757, 702, 590. 1H NMR (CDCl3,
300 MHz) δ: 8.0 (d, 1H, J = 15.6 Hz, vinyl), 7.79–7.76 (m,
2H), 7.51–7.46 (m, 1H), 7.4–7.35 (m, 2H), 7.30–7.19 (m,
9H), 7.09–7.06 (m, 6H), 6.67 (d, 1H, J = 15.6 Hz, vinyl).
13C NMR (CDCl3, 75 MHz) δ: 190.7, 154.4, 144.4, 137.9,
132.8, 130.1, 129.1, 128.6, 128.1, 126.9, 126.6, 125.5, 61.3.
LR-MS (EI) m/z: 374 (M+), 269, 191, 165, 105, 91, 77, 51.
HR-MS (EI) m/z calcd. for C28H22O: 374.1671; found:
374.1672. Anal. calcd. for C28H22O: C 89.80, H 5.93; found:
C 89.81, H 5.95.
C28H22O: 374.1671; found: 374.1669. Anal. calcd. for
C28H22O: C 89.80, H 5.93; found: C 89.78, H 5.97.
Quantum yield determination
The quantum yield for formation of cyclopropyl ketone 8
in benzene was determined using valerophenone actinometry
(Φ = 0.33) (12) according to the standard protocol used in
our group (13). n-Tetradecane and n-nonadecane were used
as internal standards for the solutions of actinometer and
ketone 8, respectively. Quantum yields were determined at
varying conversions and plotted against conversion; the re-
ported quantum yield of 0.4 represents the value extrapo-
lated to 0% conversion.
Acknowledgement
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) for financial support.
References
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Photolysis of enone 3
Enone 3 (0.1 g, 0.27 mmol) was dissolved in 30 mL of
acetonitrile and the solution purged with nitrogen for
30 min. The solution was then irradiated for 2 h through Py-
rex using the output of a Hanovia 450 W medium pressure
mercury lamp. GC analysis of an aliquot revealed the com-
plete consumption of starting material and the formation of a
single photoproduct. Removal of the solvent in vacuo fol-
lowed by radial chromatography over silica gel (2% ethyl
acetate in petroleum ether) afforded 0.096 g (96%) of
photoproduct 8 as a colorless solid. Recrystallization from a
mixture of diethyl ether and petroleum ether afforded small
prisms, mp 126–128°C (lit. (7) mp 125–127°C). The
methine hydrogens of compound 8 exhibited a mutual 6 Hz
1
coupling in the H NMR, thus confirming their trans rela-
tionship (11). When 0.02 g of enone 3 in 15 mL of benzene
was irradiated through Pyrex for 1 h, GC analysis indicated
the presence of a mixture of trans-enone 3, its cis isomer 7,
and cyclopropyl ketone 8 in a 3:10:20 ratio. Photolysis was
continued for an additional 15 min until GC analysis showed
the complete consumption of starting material 3. Subsequent
removal of solvent in vacuo followed by radial chromatogra-
phy as before afforded 0.012 g of photoproduct 8 and
0.004 g of cis-enone 7, mp 136–139°C (from ethyl acetate).
IR (KBr): 3058, 2974, 1672, 1597, 1581, 1494, 1447, 1226,
1175, 1036, 991, 901, 842, 738, 702. 1H NMR (C6D6,
300 MHz) δ: 7.59–7.56 (m, 2H), 7.32–7.29 (m, 6H), 7.12–
6.91 (m, 12H), 6.79 (d, 1H, J = 12 Hz, vinyl), 6.32 (d, 1H,
J = 12 Hz, vinyl). 13C NMR (CDCl3, 75 MHz) δ: 192.4,
147.4, 145.5, 136.7, 132.7, 130.4, 129.6, 128.6, 128.4,
128.1, 128.0, 127.7, 126.4, 61.7. LR-MS (EI) m/z: 374 (M+),
269, 191, 165, 105, 91, 77, 51. HR-MS (EI) m/z calcd. for
13. M. Leibovitch, G. Olovsson, J.R. Scheffer, and J. Trotter. J.
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© 2003 NRC Canada