Di-π-methane photorearrangement of trans-1,3-diphenylpropene upon excitation to higher singlet states in polar solvents

Article abstract of DOI:10.1039/b006267g

A dramatic enhancement of the di-π-methane rearrangement is observed upon excitation of trans-1,3-diphenylpropene (1) to its higher singlet states in acetonitrile, which leads to trans-1,2-diphenylcyclopropane (3) as a major photoproduct.

Full text of DOI:10.1039/b006267g

Di-p-methane photorearrangement of trans-1,3-diphenylpropene upon
excitation to higher singlet states in polar solvents
M. Consuelo Jiménez, Miguel A. Miranda* and Rosa Tormos
Departamento de Química, Instituto de Tecnología Química UPV-CSIC, Camino de Vera s/n, Apdo. 22012, 46071
Valencia, Spain. E-mail: mmiranda@qim.upv.es
Received (in Liverpool, UK) 27th July 2000, Accepted 16th October 2000
First published as an Advance Article on the web 10th November 2000
A dramatic enhancement of the di-p-methane rearrange-
ment is observed upon excitation of trans-1,3-diphenylpro-
pene (1) to its higher singlet states in acetonitrile, which
leads to trans-1,2-diphenylcyclopropane (3) as a major
photoproduct.
obtained in significant yields (e.g. 30% in THF after 100 min
irradiation at 254 nm).
In order to obtain more reliable quantitative data, the di-p-
methane rearrangement quantum yields were determined using
1
5
the trans–cis isomerization of b-methylstyrene (f = 0.2) as
actinometer. Table 1 shows that, while in a hydrocarbon solvent
2
2
The photochemistry of trans-1,3-diphenylpropene (1) has
attracted considerable attention in the past.1 In addition to the
cis isomer 2, small amounts (ca. 5%) of trans- and cis-
fdi-p was lower than 10 (in agreement with the literature data
–10
23
of 5.10 ), in acetonitrile it was at least one order of magnitude
higher.
1
,2-diphenylcyclopropane (3 and 4) were also detected after
As the di-p-methane rearrangement of 1 and related systems
is thought to occur from excited singlet states, it appeared of
interest to characterise such excited states by carrying out a
steady-state and time-resolved fluorescence study in the two
solvents. The obtained results are shown in Table 1.
The emission spectrum consisted of a band with maximum at
308 nm. It was found to be independent of the excitation
wavelength. The main maximum of the excitation spectra was
located at 254 nm; smaller bands appeared at 285 and 294 nm
(see Fig. 2). Thus, 1 emits from its lowest lying singlet state,
even upon excitation to the higher singlet states.
1
7 h irradiation in benzene or cyclohexane with 254 nm UV-
1
light (Scheme 1). Under these conditions, the quantum yield of
2
3
cyclopropane formation was found to be ca. 5 3 10 , and the
2
5 21 10
reaction rate constant was ca. 8 3 10
s . This cyclization,
1
which in the original paper appeared ‘to be without analogy’,
is considered ‘one of the earliest examples of the di-p-methane
7
rearrangement’, a reaction named according to Zimmerman
and extensively investigated during the last decades.1
In spite of the considerable efforts devoted to the study of
both the synthetic and mechanistic aspects of the di-p-methane
rearrangement, there are two issues that have not been studied in
a sufficiently systematic way and therefore require further
investigation: the wavelength effects and the influence of polar
solvents. We have undertaken a study of these two aspects in the
case of trans-1,3-diphenylpropene (1) and found dramatic
variations of the reaction efficiency. According to the obtained
results, the di-p-methane rearrangement of 1 is markedly
enhanced upon excitation to higher singlet states in polar
solvents. This provides new mechanistic insights into the
reaction and can be used as a tool to increase its preparative
value.
1–13
The fluorescence quantum yields and the singlet lifetimes
were smaller in acetonitrile than in hexane (Table 1). No
significant effect of the excitation wavelength on these
parameters was observed.
Compound 1 was prepared following a known procedure.14
Solutions of 1 (5 mM) in the indicated solvents were irradiated
inside a Luzchem multilamp photoreactor, using the light from
four 8W-lamps with emission maxima at 254 or 300 nm
(
Gaussian distribution), through quartz or Pyrex, respectively.
1
The course of the reaction was followed by GC and H-NMR;
the degrees of conversion, the product distributions and the
mass balances were determined using adequate standards. The
results are shown in Fig. 1.
1
As expected, very minor amounts of trans-1,2-diphenyl-
cyclopropane (3) were formed in cyclohexane as solvent,
whichever the irradiation wavelength employed. Prolonged
irradiation at 254 nm resulted in partial isomerization of 3 to the
cis-cyclopropane 4 (data not shown); in any case the combined
yield of cyclopropanes never exceeded 10%. By contrast, upon
irradiation of 1 in acetonitrile solution at 254 nm the di-p-
methane rearrangement was dramatically enhanced, and 3
became the major product (after 100 min, the chemical yield
was almost 50%). This effect was not observed when irradiation
of 1 was performed at longer wavelengths. The same trend was
found in solvents of intermediate polarity, where 3 was also
Fig. 1 Preparative yields of formation of 2 and 3, upon irradiation of 1 in
acetonitrile and cyclohexane for different times, a) at 254 nm and b) at 300
nm.
Scheme 1
DOI: 10.1039/b006267g
Chem. Commun., 2000, 2341–2342
This journal is © The Royal Society of Chemistry 2000
2341

Table 1 Photophysical and photochemical data of 1 at room temperaturea
Solvent
f
F
1027k
F
/s21
F
t /ns
f
di-p
1026
k
di-p/s21
f
cis
Hexane
Acetonitrile
0.27 (0.36)
0.13 (0.15)
5.5
10
6.1 (6.1)
2.5 (2.6)
< 0.01
0.11 (0.04)
< 1
44
0.12 (0.12)
0.10 (0.11)
a
In general, the data have been obtained upon excitation at 254 nm. The fluorescence quantum yield and lifetimes using 285 nm as the excitation wavelength
are given in brackets.
states of styrene is believed to be S
which possesses ionic character) overlap strongly. A conical
intersection between S /S and S could provide a route to the
di-p-methane product 3. This would explain the need for
excitation at shorter wavelength (to populate S ) and the marked
1 2 3 2 3
< S + S , where S and S
(
3
2
1
3
effect of solvents able to modify the energy surfaces of polar
excited states.
We are indebted to Professor F. D. Lewis for his helpful
comments. Financial support by the DGICYT (PB 97-0339) is
gratefully acknowledged. M. C. J. thanks the European
Commission for a Grant (MCFI-1999-00101).
Notes and references
1
G. W. Griffin, J. Covell, R. C. Petterson, R. M. Dodson and G. Klose,
J. Am. Chem. Soc., 1965, 87, 1410.
2
3
4
5
S. S. Hixson, Tetrahedron Lett., 1971, 4211.
E. W. Valyocsik and P. Sigal, J. Org. Chem., 1971, 36, 66.
E. W. Valyocsik and P. Sigal, J. Phys. Chem., 1971, 75, 2079.
S. S. Hixson, J. Am. Chem. Soc., 1972, 94, 2505.
Fig. 2 Excitation and fluorescence emission spectra of 1 in hexane at room
temperature.
6 S. S. Hixson, J. Am. Chem. Soc., 1972, 94, 2507.
7 S. S. Hixson, Tetrahedron Lett., 1972, 1155.
8
J. M. Figuera and M. T. Serrano, J. Chem. Soc., Faraday Trans. 1, 1976,
2, 1534.
J. M. Figuera and M. T. Serrano, J. Chem. Soc., Faraday Trans. 1, 1976,
2, 2265.
7
All the accumulated experimental data on the photochemistry
of 1 indicate that, although the nature of the emitting singlet
state depends neither on the polarity of the solvent nor on the
excitation wavelength, the di-p-methane rearrangement to 3 is
strongly influenced by these factors. The sharp enhancement of
this reaction upon excitation to higher singlet states in polar
9
7
10 S. S. Hixson, J. Am. Chem. Soc., 1976, 98, 1271.
11 S. S. Hixson, P. S. Mariano and H. E. Zimmerman, Chem. Rev., 1973,
73, 531.
1
2 H. E. Zimmerman and D. Armesto, Chem. Rev., 1996, 96, 3065.
3 H. E. Zimmerman, The Di-p-Methane Rearrangement, in CRC
Handbook of Organic Photochemistry and Photobiology, ed. W. M.
Horspool and P.-S. Song, CRC Press, Boca Raton, 1995, p. 184.
4 E. K. Raunio and W. A. Bonner, J. Org. Chem., 1966, 31, 396.
5 F. D. Lewis, D. M. Bassani, R. A. Caldwell and D. J. Unett, J. Am.
Chem. Soc., 1994, 116, 10 477.
1
solvents is an unexpected behaviour, which is difficult to
explain based on the currently accepted models.1
1–13
In
principle, 1 might photoionize from a higher singlet state in
polar solvents, giving rise to the radical cation; this could be the
actual rearranging species. An alternative possibility would be
to start from the treatment of styrene photoreactivity based on
conical intersections.16 The order of vertically excited singlet
1
1
16 M. J. Bearpark, M. Olivucci, S. Wilsey, F. Bernardi and M. A. Robb,
J. Am. Chem. Soc., 1995, 117, 6944.
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Chem. Commun., 2000, 2341–2342