Photoreactions of trans-1-o-Hydroxyphenyl-2-phenylcyclopropane

Article abstract of DOI:10.1021/jo981558g

The photochemistry of trans-1-o-hydroxyphenyl-2-phenylcyclopropane, trans-1, was studied under a variety of experimental conditions. Direct irradiation through quartz in cyclohexane gave rise mainly to ring-expanded products, 2-phenyl-3,4-dihydro-2H-benzopyran, 2, 2-benzyl-2,3-dihydrobenzofuran, 3, and 1-o-hydroxyphenylindan, 4. The major products, 2 and 3, are rationalized by intramolecular proton transfer. However, a significant fraction of 3 is formed via ring-opening to cinnamylphenol, 5. An additional product, o-(α-cyclohexylmethyl)phenol, 7, suggests fragmentation of trans-1 and (formal) insertion of o-hydroxyphenylcarbene into cyclohexane. Direct irradiation in methanol produced methanol adducts 8 and 9 instead of 2, 3,4, or 7. Finally, acetone-sensitized irradiation of trans-1 resulted in geometric isomerization to cis-1; this result can be rationalized via a biradical intermediate.

Full text of DOI:10.1021/jo981558g

J . Org. Chem. 1999, 64, 6541-6546
6541
P h otor ea ction s of tr a n s-1-o-Hyd r oxyp h en yl-2-p h en ylcyclop r op a n e
J ulio Delgado,† Amparo Espino´s,† M. Consuelo J ime´nez,† Miguel A. Miranda,*,†
Heinz D. Roth,*,‡,§ and Rosa Tormos†
Departamento de Quı´mica/ Instituto de Tecnologı´a Quı´mica UPV-CSIC, Universidad Polite´cnica de
Valencia, Camino de Vera s/ n, Apdo 22012, 46071 Valencia, Spain, and Department of Chemistry,
Wright-Rieman Laboratories, Rutgers University, New Brunswick, New J ersey 08854-8087
Received August 3, 1998
The photochemistry of trans-1-o-hydroxyphenyl-2-phenylcyclopropane, trans-1, was studied under
a variety of experimental conditions. Direct irradiation through quartz in cyclohexane gave rise
mainly to ring-expanded products, 2-phenyl-3,4-dihydro-2H-benzopyran, 2, 2-benzyl-2,3-dihy-
drobenzofuran, 3, and 1-o-hydroxyphenylindan, 4. The major products, 2 and 3, are rationalized
by intramolecular proton transfer. However, a significant fraction of 3 is formed via ring-opening
to cinnamylphenol, 5. An additional product, o-(R-cyclohexylmethyl)phenol, 7, suggests fragmenta-
tion of trans-1 and (formal) insertion of o-hydroxyphenylcarbene into cyclohexane. Direct irradiation
in methanol produced methanol adducts 8 and 9 instead of 2, 3, 4, or 7. Finally, acetone-sensitized
irradiation of trans-1 resulted in geometric isomerization to cis-1; this result can be rationalized
via a biradical intermediate.
In tr od u ction
1,2-diarylcyclopropane bearing a hydroxyl function in one
ortho position. This substrate was of interest because its
excited singlet state may react by (excited state) proton
transfer (ESPT)20,21 to the cyclopropane moiety, whereas
its radical cation may react by intramolecular nucleo-
philic capture,22-27 aside from the wealth of other reac-
tions suggested by the analogy to previous studies.
The photochemistry of the diarylcyclopropane, trans-
1, was studied under a variety of experimental conditions,
including (a) direct irradiation in cyclohexane (through
quartz), (b) direct irradiation in methanol, (c) acetone-
sensitized irradiation (through Pyrex), and (d) direct
irradiation in dichloromethane in the presence of oxygen
(Pyrex). Under these conditions, a range of interesting
products were formed. The observed significantly differ-
ent structure types require involvement of several dif-
ferent primary excited states and divergent types of
intermediates.
The photochemistry of cyclopropane and derivatives
has been of interest for many years; numerous deriva-
tives have been studied under a variety of reaction
conditions.1-18 Among the many reaction types estab-
lished are addition/substitution with ring opening,1-3
geometric isomerization,1-3,5-7 ring opening generating
alkenes,1 ring enlargement forming indans,1-3 or frag-
mentation yielding carbenes.1-3 The pathways to the
various products may be formulated via a range of
intermediates, including excited singlet1-3 or triplet
states,6,7,18 radical cations,4-17 triplet biradicals,6,7,18 or
zwitterions.
We have studied the photoreactions of trans-1-o-
hydroxyphenyl-2-phenylcyclopropane,19 trans-1, a trans-
† Universidad Polite´cnica de Valencia.
‡ Rutgers University.
§ Profesor Visitante IBERDROLA de Ciencia y Tecnolog´ıa.
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hexane gave rise to products of several structure types,
including five isomeric products formed with either ring
expansion or ring opening. The ring-expansion products
are formed in the highest yields, viz., 2-phenyl-3,4-
dihydro-2H-benzopyran,28 2 (∼25%), 2-benzyl-2,3-dihy-
drobenzofuran,29 3 (∼35%), and 1-o-hydroxyphenylin-
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10.1021/jo981558g CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/17/1999

6542 J . Org. Chem., Vol. 64, No. 18, 1999
Delgado et al.
Sch em e 1
F igu r e 1. Gas-phase FT-IR spectrum (3800-2800 cm-1) of
trans-1-o-hydroxyphenyl-2-phenylcyclopropane (trans-1).
dan,30 4 (∼8%). Ring-opened products include cis- and
trans-1,3-diarylpropenes, 5 (∼5% each), and a saturated
ring-opened diarylpropane, 6 (∼5%). Finally, the struc-
ture of o-(R-cyclohexylmethyl)phenol31 (∼15%), 7, sug-
gests reaction of a fragment of the starting material with
the solvent (Scheme 1). At 30% conversion, the material
balance was 72%. Direct irradiation of a mixture, consist-
ing of 90% cis-1 and 10% trans-1, gave rise to essentially
the same product distribution. However, the cis isomer
was depleted significantly (∼10 times) faster.
form cyclic ethers. For example, the alkene isomer of
trans-1, 3-(o-hydroxyphenyl)-1-phenylpropene (trans-5),
upon irradiation forms the same two cyclic ethers, 2 and
3, obtained from trans-1; their formation was rationalized
by excitation of two π-complex conformers via two “tight”
zwitterions, 11-zw and 12-zw (Scheme 2).21 Substrates
Sch em e 2
Upon direct irradiation (through quartz) in methanol
as reagent/solvent, the formation of essentially all in-
tramolecular reaction products was suppressed in favor
of two methanol adducts, 8 and 932 (in yields of 55 and
43%, respectively; mass balance 93% at 70% conversion).
Only the saturated ring-opened diarylpropane, 6, was
formed in minor yield (∼2%).
lacking an intramolecular hydroxyl function reacted in
intermolecular fashion; for example, photoexcited phen-
ylcyclopropanes add methanol.2 In light of this and other
precedence, we expected proton transfer from the phe-
nolic hydroxy function to the three-membered ring.
Proton transfer was all the more anticipated, as the FT-
IR spectrum of trans-1 showed a narrow “associated” OH
band (3604 cm-1), indicating a weak intramolecular
complex between the phenolic OH and the cyclopropane
ring (Figure 1). A similar association between the OH
group and an alkene function was observed for trans-
5.21
Acetone-sensitized irradiation of trans-1 (through Pyr-
ex) resulted in geometric isomerization to cis-1, a very
clean reaction with excellent material balance (96% at
40% conversion). Similarly, irradiation of acetone solu-
tions containing trans-1-o-acetyloxyphenyl-2-phenylcy-
clopropane (trans-10) caused geometric isomerization to
cis-10.
Rea ction s of tr a n s-1 In volvin g “Ad d ition ” of th e
OH Gr ou p . The cyclic ethers, 2 and 3, obtained in
cyclohexane and the open-chain ethers, 8 and 9, formed
in methanol formally are in line with the anticipated
reaction. Ethers 8 and 9 are readily explained via
methanol addition across the doubly benzylic bond (vide
infra). For the cyclic ethers 2 and 3, protonation at the
secondary cyclopropane carbon (C3) can be excluded as
it would generate a methyl group, which is clearly not
present. Product 2 is readily explained by protonation
at the tertiary benzylic cyclopropane position next to the
phenol moiety (C1) with opening of the doubly benzylic
bond; cyclization of zwitterion 11-zw accounts for the
formation of product 2 (Scheme 3) in direct analogy to
its formation from trans-5.21 In contrast, the formation
of cyclic ether 3 was unexpected; it poses an interesting
mechanistic problem.
Discu ssion
Direct irradiation of trans-1 generates the excited
singlet state, trans-11*. Phenol excited states are known
to show enhanced acidity;20,21 typically, they react by
(intramolecular) proton transfer, if a suitable proton-
accepting function is present. This type of reaction
generates zwitterionic intermediates that combine to
(29) Gripenberg, J .; Hase, T. Acta Chem. Scand. 1966, 20, 1561-
1570.
(30) Hoelzel, H.; Roehling, T. Z. Chem. 1981, 21, 354.
(31) Miranda, M. A.; Tormos, R. J . Org. Chem. 1993, 58, 3304-3307.
(32) Chiba, P.; Ecker, G.; Schmid, D.; Drach, J .; Tell, B.; Goldenberg,
S.; Gekeler, V. Mol. Pharmacol. 1996, 49, 1122.

Photoreactions of trans-1-o-Hydroxyphenyl-2-phenylcyclopropane
J . Org. Chem., Vol. 64, No. 18, 1999 6543
Sch em e 3
A mechanism involving two different pathways via
protonation at the same carbon (C1) may appear unlikely;
however, two conformers with different orientations of
the phenol moiety may protonate the cyclopropane ring
with different “trajectories”, leading to different inter-
mediates. One conformer may cause cleavage of the C1-
C2 bond, generating the “ordinary” zwitterion, 11-zw.
Another conformer may transfer the proton with cleavage
of the C1-C3 bond; this attack is feasible only if the
phenyl moiety stabilizes the developing primary carboca-
tion, forming the phenonium ion, 17 (Scheme 5).
Sch em e 5
A trivial explanation for its formation involves ring
opening of trans-1 to cis- or trans-5, followed by a
secondary photoreaction of the cinnamylphenols;21 how-
ever, this cannot be the only pathway to 3. Irradiation
of trans-1 produces 3 as the predominant product (35%),
whereas 5 forms larger quantities of 2 (30%) than of 3
(22%).21 Thus, even if the entire yield of 2 (25% from
trans-1) were formed via 5 (which is unlikely), close to
50% of 3 would have to be formed by a separate pathway
not involving 5.
P oten tia l P a th w a ys to 3. Since cyclic ether 3 has two
benzylic methylene groups, its formation must include a
rearrangement, either a hydrogen shift or a reorganiza-
tion of the carbon skeleton. The structure of 3 does not
reveal the position to which the proton is transferred.
Protonation at the benzylic cyclopropane carbon next to
the phenyl ring (C2) would generate a zwitterion with a
benzylic carbocationic site, 13-zw, but this species cannot
explain the formation of 3; cyclization of 13-zw (Scheme
3) would form the highly strained benzooxetene, 14, an
unlikely precursor for 3.
A hydride shift in zwitterion, 13-zw, would yield a
rearranged zwitterion, 15-zw, which is a reasonable
precursor for 3; however, conversion of a benzylic (13-
zw) to a secondary carbocation (15-zw) is hardly favor-
able. A hydride shift in concert with the approach of the
phenoxyl group (transition state 16) lacks precedent.
It is a moot point whether these reactions proceed in
concerted fashion or involve short-lived intermediates.
The key point is the stabilization of a developing primary
carbocation by neighboring group participation of the
phenyl group. This type of stabilization is in the best
tradition of phenonium ion involvement (although these
species do not appear to be in fashion currently).33-35
If the course of the reaction is determined by two
phenol rotamers, the stereochemistry of the phenyl group
may be less important; it appears that a phenonium ion
can be formed by capturing the back lobe of a Walsh
orbital from either face of the ring plane. This would
explain why the photochemistry of the cis isomer gives
rise to the same product distribution, albeit with en-
hanced efficiency.
Thus, 13-zw is an unlikely precursor for 3.
A more appealing alternative involves protonation at
C1 with cleavage of a singly benzylic (C1-C3) bond,
assisted by the phenyl group forming a phenonium ion,
17. Capture of the tertiary carbon of 17 by the phenoxyl
group would give rise to ether 3 (Scheme 4). The key
P a r tition betw een Rin g-Op en in g a n d P h en on iu m
Ion Mech a n ism . The partition between the projected
mechanisms leading to 3, ring-opening to cinnamylphe-
nol, 5, and excited-state proton-transfer involving the
putative phenonium ion, 17, can be evaluated by analyz-
ing the fate of a deuterium label in the cyclic ether. For
this purpose, we synthesized trans-1-o-hydroxyphenyl-
2-phenylcyclopropane-d (trans-1-d) from D-exchanged
1-o-hydroxyacetophenone in a three-step synthesis via
chalcone and pyrazoline. MS analysis indicated a deu-
terium content of ∼50% for trans-1-d. The photoproducts
contained similar levels of D; the molecular ions of the
cyclic ethers, 2 and 3, had ∼55 and ∼45% deuterium,
respectively (Scheme 6).
Sch em e 4
The analysis of the fragment ions derived from 3
indicated the presence of D in both a larger (dihydroben-
zofuranyl) and a smaller fragment ion (benzyl). This
finding supports two different pathways for the formation
of 3, placing D either into the benzyl or the dihydrofuran
moiety. The distribution of the label between the frag-
ments is derived from the intensities of the appropriate
peaks. The analysis of the larger fragment ion(s) (Fl, m/e
) 119+, C8H7O, dihydrobenzofuranyl, - C7H7; Fl - H,
118+, benzofuran+, H transfer to benzyl via McLafferty
rearrangement; - C7H8) is complicated by the coincidence
intermediate postulated has ample precedent; phenonium
ions have been invoked in various solvolysis reactions,
including on primary carbons.33-35
(33) Cram, D. J . J . Am. Chem. Soc. 1949, 71, 3863.
(34) Winstein, S.; Heck, R. J . Am. Chem. Soc. 1956, 78, 4801.
(35) Brookhart, M.; Anet, F. A. L.; Cram, D. J .; Winstein, S. J . Am.
Chem. Soc. 1970, 92, 1402-1403.

6544 J . Org. Chem., Vol. 64, No. 18, 1999
Delgado et al.
Sch em e 6
Sch em e 8
or via a 1,3-bifunctional intermediate of type 18 and
hydrogen migration (Scheme 8). In the case of the ring-
opened intermediate, a hydrogen shift in different con-
formers can explain the formation of the isomeric al-
kenes; concerning the excited state, a [1,3]-sigmatropic
shift can account for the isomeric alkenes, depending on
the identity of the migrating hydrogen, the nature of the
intermediate, and the stereochemistry of the migration.
The ring-opened diarylpropane, 6, requires a reduction
step, in addition to ring opening.
Cyclop r op a n e F r a gm en ta tion . One of the products
formed upon direct irradiation of trans-1, o-(R-cyclohexyl-
methyl)phenol, 7, is significantly different from any other
product or the starting material. It consists of two
components, suggesting that a fragment of trans-1 reacts
with the solvent; product 7 is compatible with fragmen-
tation of an excited state, 1*, generating styrene and
o-hydroxyphenylmethylene, 20:. A biradical of the general
structure type 18 (*,# ) •) is a possible intermediate in
the formation of 20:. The resulting carbene fragment then
reacts with the solvent, cyclohexane, by (formal) C-H
insertion (Scheme 9).
of Fl-13C1 with Fl-d1 peaks and of Fl with (Fl-d1 - H)
peaks. The data suggest an essentially equal distribution
of the label between the fragments, ∼32% in the larger
vs ∼33% in the smaller fragment ion. Although the
numerical data contain uncertainties, they nevertheless
establish unambiguously the existence of two different
pathways leading to 3.
Rea ction s of tr a n s-1 Not In volvin g th e OH Gr ou p .
The third ring expansion product, the indan derivative,
4, as well as the ring-opened alkenes (cis- and trans-5),
and the ring-opened diarylpropane, 6, are generated in
reactions without involvement of the phenolic hydroxyl
group. With the possible exception of 6, products of these
structure types have precedence in photolysis reactions
of cyclopropane systems. Indan derivatives are typical
photolysis products of diarylcyclopropanes;1-3 a reaction
of this type was first observed in the photolysis of the
symmetrical prototype, 1,2-diphenylcyclopropane.1 The
conversion is readily explained via cleavage of the doubly
benzylic cyclopropane bond, yielding a 1,3-bifunctional
species, 18, either a biradical (*, # ) •) or a zwitterion
(* ) +, # ) -). Addition of one benzylic carbon, a free
radical or carbocation, to the second aromatic ring,
followed by a 1,3- (or 1,7-) hydrogen shift in the resulting
exo-methylenecyclohexadiene intermediate, 19, then gives
rise to the observed indan (Scheme 7).
Sch em e 9
Sch em e 7
The generation of phenylmethylene upon photolysis of
diarylcyclopropanes has precedent; irradiation of 1,2-
diphenylcyclopropane in methanol generated benzyl meth-
yl ether, which was rationalized via phenylmethylene.1,3
Similarly, irradiation of 2,3-diphenyloxirane gave rise to
insertion and addition products of phenylmethylene.
When this reaction was carried out at cryogenic temper-
atures, a carbonyl ylide was identified as an intermediate
based on its optical absorption spectra; upon warming
to 140 K, this intermediate underwent thermal fragmen-
tation to phenylmethylene.36
Unsymmetrical substrates, such as trans-1 in our
study, may form two different ring-expansion products
via two different methylenecyclohexadiene species. The
exclusive formation of indan derivative, 4, in the pho-
tolysis of trans-1 documents an interesting regiochemical
preference. It indicates that the o-hydroxybenzyl (cat-
ionic) site is more reactive than the unsubstituted benzyl
function. Conversely, attack on the unsubstituted ben-
zene ring is easier than attack on the phenol ring. This
preference can be rationalized as a result of resonance
electron donation by the o-hydroxy substituent.
Typical arylmethylenes have triplet ground states;34
the corresponding singlet states, when generated in
solution, undergo rapid intersystem crossing. The car-
bene presumably involved in the current system has not
been studied because the acidity of the phenolic OH group
is incompatible with the diazo function in the potential
The formation of the geometric isomers, cis- and
trans-5 and of product 6 also require ring opening. The
formation of alkenes from cyclopropanes may proceed
either in concerted fashion by a [1,3]-sigmatropic shift
(36) Do-Minh, T.; Trozzolo, A. M.; Griffin, G. W. J . Am. Chem. Soc.
1970, 92, 1402-1403.

Photoreactions of trans-1-o-Hydroxyphenyl-2-phenylcyclopropane
J . Org. Chem., Vol. 64, No. 18, 1999 6545
precursor. It is not clear whether the fragmentation
proceeds in one or two steps or whether the singlet state,
120, or the triplet state, 320, of the carbene is the primary
intermediate generated in this process. Interestingly, the
fragmentation generates only one carbene; products
derived from phenylmethylene are not observed. Once
again, as in the selective formation of the indan, 4, the
precursor shows a pronounced regiochemical preference.
In the case of the fragmentation leading to 120, this
preference can be rationalized as a result of resonance
electron donation by the o-hydroxy substituent and,
possibly, by a lone pair contribution.
mol) is sufficiently high to allow energy transfer to
trans-1 (or trans-10). This follows from a comparison with
the triplet energy assigned to 1,2-diphenylcyclopro-
pane,38,39 ET ≈ 53 kcal/mol or to the corresponding
biradical, ET ≈ 29 kcal/mol.18 The resulting triplet state
derived from 1 or its acetate, 10, could be either a
biradical or a phosphorescent triplet-state, viz., 31*
(Scheme 11). In the potential biradical (structure type
Sch em e 11
P h otolysis in Meth a n ol. Upon direct irradiation
through quartz with methanol instead of cyclohexane as
solvent, the intramolecular formation of the cyclic ethers
2 and 3 was suppressed in favor of two methanol adducts,
8 and 9. This observation can be explained by a competi-
tion between intramolecular reactions (proton transfer,
ring-opening, fragmentation) and intermolecular quench-
ing. The alcohol function of the solvent (∼20 M) will fully
solvate the phenol function, replacing the weak intramo-
lecular association between the phenolic O-H function
and the cyclopropane ring. Accordingly, it is not surpris-
ing that the intramolecular proton transfer is efficiently
suppressed. On the other hand, the suppression of ring-
opening and fragmentation by methanol requires an
efficient reaction of the external reagent with the excited
18, *,# ) •), the C1-C2 bond is broken, whereas in the
3
phosphorescent triplet-state, 1*, the C1-C2 bond may
be weakened; in either case, the isomerization would be
facilitated. These results are similar to other cyclopro-
pane compounds undergoing triplet-sensitized2,3 or elec-
tron-transfer-induced6,7,18 geometric isomerization.
The electron-transfer-induced geometric isomerization
of cis- and trans-1,2-diphenylcyclopropane6,7 involves
radical cations and triplet species consecutively. The
radical cations retain a significant degree of bonding in
the doubly benzylic (C1-C2) bonds and fail to undergo
geometric isomerization. The lifetimes of cis and trans
radical cations with respect to isomerization must be
greater than their spin lattice relaxation times (2T1’s
typically fall into the microsecond range).6,7 The failure
of the radical cations to isomerize was recently confirmed
by fast, time-resolved optical spectroscopy.18 On the other
hand, the triplet intermediates contain the substituents
in orthogonal planes; upon deactivation, they may “col-
lapse” to either the cis or trans ground state.6,7
1
singlet state, 1*.
This reaction may proceed either by protonation with
ring opening, generating benzylic carbocations, 21 and
22, or by nucleophilic attack with ring opening, giving
rise to zwitterions, 23 and 24 (Scheme 10). Because
Sch em e 10
Na tu r e of Bifu n ction a l Rea ction In ter m ed ia tes.
1,3-Diarylpropane-1,3-diyl intermediates of the general
structure type 18 have been discussed as potential
intermediates in the formation of four different product
types obtained under different reaction conditions. These
product types include the geometric cyclopropane isomer,
cis-1, the (ring-opened) alkenes, cis- and trans-5, the
(ring-enlarged) indan, 4, and the formal carbene insertion
product, 7. Having delineated the various reaction types,
we can define the nature of one of these intermediates
somewhat more narrowly and define limitations for the
others. Because the (acetone) triplet-sensitized reaction
of trans-1 shows a highly specific reactivity, limited to
the formation of cis-1, we identify the corresponding
intermediate as a triplet biradical, 18••. This assignment
is in keeping with the detailed results of the electron-
alcoholic OH functions are less acidic than phenolic ones,
the intermolecular proton transfer is not expected to be
highly efficient. However, the molar concentration of
methanol is very high.
Aceton e-Sen sitized Ir r a d ia tion . Irradiating solu-
tions of trans-1 in acetone through Pyrex resulted in
geometric (cis,trans) isomerization to cis-1. Similarly,
irradiation of acetone solutions containing trans-1-o-
acetyloxyphenyl-2-phenylcyclopropane (trans-10) caused
clean rearrangement to the cis isomer. Under these
conditions, the direct irradiation of trans-1 or trans-10
(37) Trozzolo, A. M.; Wasserman, E. In Carbenes; Moss, R. A., J ones,
M., J r., Eds.; Wiley-Interscience: New York, 1975; Vol. 2, pp 185-
206.
(38) Becker, R. S.; Edwards, L.; Bost, R.; Elam, M.; Griffin, G. W.
J . Am. Chem. Soc. 1972, 94, 6584-6592.
(39) Ouanne´s, C.; Beugelmans, R.; Roussi, G. J . Am. Chem. Soc.
1973, 95, 8472-6474.
is precluded, and all incident irradiation is absorbed by
acetone. The initially excited singlet state of acetone
undergoes rapid intersystem crossing, populating the
triplet state. The acetone triplet energy (ET ) 78 kcal/

6546 J . Org. Chem., Vol. 64, No. 18, 1999
Delgado et al.
solvent; these were irradiated for 1 h in Pyrex or quartz tubes
surrounding a central quartz cooling jacket with a 125-W
medium-pressure mercury lamp. B. For preparative runs, solu-
tions of 1.00 g of the substrate in 400 mL of freshly distilled
solvent were irradiated at ambient temperature with a 125-W
medium-pressure mercury lamp inside a quartz immersion
well.
Isola tion . Reaction products were isolated and purified by
conventional column chromatography on silica gel Merck 60
(0.063-0.200 mm), by preparative thin layer chromatography
on silica gel Merck 60 PF254, using dichloromethane as eluent,
or by means of isocratic HPLC equipment provided with a
semipreparative Microporasil column, using hexane/ethyl
acetate as eluent.
Id en tifica tion . Products were identified by comparison
with authentic samples by GC retention time and MS and
FTIR spectra. A sample of 1-o-hydroxyphenylindan, 4, was
independently synthesized from indan and phenol according
to a literature procedure.30 IR spectra were recorded on a GC-
FTIR instrument; 1H (13C) NMR spectra (CDCl3) were recorded
at 300 (75) MHz; mass spectra were obtained on a commercial
low-resolution electron impact mass spectrometer; high-resolu-
tion mass spectra were determined at SCSIE with either
electron impact or chemical ionization. Combustion analyses
were performed at the Instituto de Quimica Bio-Orga´nica of
the CSIC in Barcelona.
transfer induced geometric isomerization of 1,2-diphen-
ylcyclopropane.6,7,18
Given the specific reactivity of 18••, the (generic)
bifunctional species considered as intermediates in the
formation of 4, cis- and trans-5, and 7 cannot have biradi-
cal structures. Specifically, the carbene, 20:, is not likely
derived from 18••, because there is no precedent of
carbene formation from 1,2-diphenylcyclopropane triplet
biradical.
Con clu sion
The photochemistry of the trans-diarylcyclopropane,
trans-1, gives rise to an interesting variety of products.
The cyclic ethers, 2 and 3, obtained upon direct irradia-
tion in cyclohexane are rationalized by intramolecular
proton transfer, reflecting the enhanced acidity of phenol
excited states; however, a significant fraction of 3 is
formed via prior ring-opening to cinnamylphenol, 5. The
formation of o-(R-cyclohexylmethyl)phenol, 7, is rational-
ized via fragmentation of trans-1 yielding o-hydroxy-
phenylcarbene and insertion into cyclohexane. Direct
irradiation in methanol produced methanol adducts 8 and
9; all products formed in cyclohexane were suppressed.
Finally, acetone-sensitized irradiation of trans-1 resulted
in geometric isomerization to cis-1; this result is rational-
ized via a biradical intermediate.
Ack n ow led gm en t. M.C.J . thanks the Spanish Min-
istry of Education for a fellowship; M.A.M. thanks the
DGICYT for financial support (Grant No. PB94-0539);
H.D.R. thanks NSF (Grant CHE-9714850) and grate-
fully acknowledges the 1997 IBERDROLA Award. The
authors also express their appreciation to a reviewer
for thoughtful comments.
Exp er im en ta l Section
Ma ter ia ls. Diarylcyclopropane (trans-1) was prepared by
a reaction sequence initiated by Claisen-Schmidt condensa-
tion of o-hydroxyacetophenone with benzaldehyde; condensa-
tion of the resulting chalcone with hydrazine hydrate gener-
ated a pyrazoline,40,41 which was deazetized under basic
conditions.19 A D-labeled sample was prepared from o-hydroxy-
acetophenone-d3 and benzaldehyde (∼50% D incorporation).
Ir r a d ia tion P r oced u r es. A. Exploratory experiments were
carried out with solutions of 0.02 g of the substrate in 7 mL of
1
Su p p or tin g In for m a tion Ava ila ble: IR (νmax, cm-1), H
NMR (300 MHz, CDCl3, δ, ppm downfield of TMS), 13C NMR
(75 MHz, CDCl3, δ, ppm downfield of TMS), and mass spectra
(m/z, relative intensities) for compounds cis-1, 8, and cis- and
trans-10. Synthesis, photoreaction, and mass spectral data of
trans-1. This material is available free of charge via the
Internet at http://pubs.acs.org.
(40) Chattree, N. S.; Sharma, T. C. Asian J . Chem, 1994, 6, 405-
407.
(41) Iksanova, S. V. Geterosikl. Soedin. 1991, 1131-1136; Chem.
Abstr. 1992, 116, 194239s.
J O981558G