Photolysis of diarylvinylcyclopropenes for the construction of 1-methylene-8a-aryl-1,8a-dihydroazulene skeletons

Article abstract of DOI:10.1039/b820212e

Photoirradiation of vinylcyclopropenes 1 in a Pyrex tube provides 1-isopropylidene-3,8a-diphenyl-1,8a-dihydroazulene derivatives 2 in moderate to good yields within 4.5-14 h. A plausible mechanism has been discussed for the formation of these interesting

Full text of DOI:10.1039/b820212e

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Photolysis of diarylvinylcyclopropenes for the construction
of 1-methylene-8a-aryl-1,8a-dihydroazulene skeletonsw
Fang-Fang Yu,a Wang-Gui Yanga and Min Shi*ab
Received (in College Park, MD, USA) 12th November 2008, Accepted 14th January 2009
First published as an Advance Article on the web 11th February 2009
DOI: 10.1039/b820212e
Photoirradiation of vinylcyclopropenes 1 in a Pyrex tube
provides 1-isopropylidene-3,8a-diphenyl-1,8a-dihydroazulene
Table 1 Photolysis of vinylcyclopropene 1a in a variety of solvents
and under various conditions
derivatives 2 in moderate to good yields within 4.5–14 h. A
plausible mechanism has been discussed for the formation of
these interesting products.
During the last several decades organic photochemistry has
grown into an important and pervasive branch of chemistry.1
So far, significant progress has been made in the theory of
photochemical reactions including the utilization of photo-
reactions in synthetic sequences and the advancement of
powerful laser techniques to study the mechanism of photo-
reactions.2 Photochemical skeleton rearrangements of highly
strained small rings with multiple bonds and functional groups
have attracted much attention from both synthetic and
mechanistic viewpoints.3 We previously reported the prepara-
tion of a variety of diarylvinylcyclopropenes 1 from simple
isomerization of the corresponding diarylvinylidenecyclo-
propanes under basic conditions.4 These results inspired us
to examine the photochemical behavior of these highly
strained but readily available diarylvinylcyclopropenes. In this
paper, we wish to report an interesting photo-induced skeleton
rearrangement of diarylvinylcyclopropenes 1 for the construc-
tion of the 1-methylene-8a-aryl-1,8a-dihydroazulene motif
under mild conditions.
Entrya
Solvent
Time/h
Yield (%)b
1
2
3
4
5
6
7
8
CH2Cl2
CH3OH
n-Hexane
Benzene
CH3CN
DMF
Toluene
Cyclohexene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
7.0
7.0
7.0
4.5
7.0
14
2.5
7.5
7.5
4.0
6.5
12.0
81
50
51
50
70
52
56
78
70
59
63
68
9c
10d
11e
12f
a
All the reactions were carried out using 1a (0.1 mmol) in the listed
b
solvent (5 mL) at the wavelength (l 4 280 nm). Isolated yields.
Saturated with molecular oxygen. Using MK (10 mol%) as the
c
d
e
f
sensitizer. Using benzophenone (10 mol%) as the sensitizer. In the
presence of styrene.
With the optimized conditions in hand, we next investigated
the substrate scope of this interesting photochemical reaction
of vinylcyclopropenes. The results are summarized in Table 2.
For a variety of symmetrical diarylvinylcyclopropenes 1, the
corresponding 1-methylene-8a-aryl-1,8a-dihydroazulene deri-
vatives 2 were obtained in good yields whether an electron-
donating or electron-withdrawing group was introduced on
the benzene ring (Table 2, entries 1–10). As for unsymmetrical
diarylvinylcyclopropenes 1k and 1l, two regioisomers were
formed with the ratios of 1 : 1.5 (Table 2, entries 11 and 12).
A time-based study was carried out using 1d as substrate by
plotting yield and conversion (%) versus time for irradiation and
the results are shown in Fig. 2. Both the yield of 2d and the
In optimization studies, photolysis of the diarylvinylcyclo-
propene 1a was selected as the model reaction in a Pyrex tube
in a variety of solvents including cyclohexene. We found that
1-isopropylidene-3,8a-diphenyl-1,8a-dihydroazulene 2a was
obtained in moderate to good yields within 4.5–14
h
(Table 1, entries 1–8). The photochemical reaction outcome
was not affected by molecular oxygen, sensitizers such as
Michler’s ketone (MK) (10 mol%) and benzophenone
(10 mol%) as well as triplet quenchers such as styrene
(Table 1, entries 9–12). The structure of 2d was determined
by X-ray diffraction, and its ORTEP drawing is shown in
Fig. 1 (see Table 2, entry 3).5 The best result was obtained in
dichloromethane to give 2a in 81% yield under the standard
conditions (Table 1, entry 1).
a School of Chemistry & Molecular Engineering, Laboratory for
Advanced Materials and Institute of Fine Chemicals, East China
University of Science and Technology, 130 Mei Long Road,
Shanghai, 200237, China
b State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Road, Shanghai, 200032, China
w Electronic supplementary information (ESI) available: Experimental
procedures and spectroscopic data for all compounds. CCDC 615536.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b820212e
Fig. 1 The ORTEP drawing of 2d.
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Table 2 Photolysis of a variety of diarylvinylcyclopropenes in
dichloromethane
Table 2 (continued)
Time/
h
Time/
Entrya 1 (R1/R1/R2/R3/R4) h
Entrya 1 (R1/R1/R2/R3/R4)
Yield (%)b
Yield (%)b
11
1l (C6H5/C6H5/4-
BrC6H4/Me/Me)
(E- and Z-mixture
= 1 : 1)
9.5
1
1b (C6H5/C6H5/
4-BrC6H4/Me/Me)
6.0
2
1c (C6H5/C6H5/
4-MeC6H4/Me/Me)
4.5
a
All the reactions were carried out using 1 (0.1 mmol) in CH2Cl2
b
c
(5.0 mL) at the wavelength l 4 280 nm. Isolated yields. E- and
Z-isomeric mixtures (4
(see ESIw).
d
: 1). Two regioisomers were formed
3
4
1d (C6H5/C6H5/
4-ClC6H4/Me/Me)
7.5
7.5
conversion of 1d increased gradually along with reaction time,
suggesting that this is a clean reaction. The plot provided
quantitative information on the amount of reactant that remains.
The reaction multiplicity of the above cases proved to be
one and it is a singlet excited state with the following control
experiments; (1) a molecular oxygen saturated reaction solu-
tion did not affect the yield of 2 under identical conditions;
(2) similar results were obtained upon direct photo-irradiation
and using triplet sensitizers under identical conditions and
no alternation could be observed when this photochemical
reaction was carried out in olefins.6
1e (4-FC6H4/
4-FC6H4/C6H5/
Me/Me)
5
6
1f (4-ClC6H4/
4-ClC6H4/C6H5/
Me/Me)
9.5
6.5
The formation of 2 can be explained by the photochemical
rearrangement of 2-vinylcyclopropene, which involves a sequence
consisting of ring opening of the excited state to a phenyl carbene
intermediate.3e–g As for 1a, the preferential bond cleavage of
the C–C bond a to give phenyl carbene intermediate A is possible
to produce product 2a via intermediate B7 because phenyl carbene
is more stable than the vinyl carbene intermediate C, which
could also be formed via the cleavage of the C–C bond b under
identical conditions to provide product 3.8a Vinyl carbene itself is
a slightly studied species, which usually undergoes intramolecular
1g (4-MeC6H4/
4-MeC6H4/C6H5/
Me/Me)
7
8
1h (C6H5/C6H5/
C6H5/Me/H)
8.0
6.5
9
5.5
10
1k (4-MeC6H4/C6H5/ 6.0
Me/Me/H)
(E- and Z-mixture
= 1 : 1)
Fig. 2 Yield of 2d and the conversion of 1d as a function of
irradiation time.
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R. H. Hesse, F. Mutterer and M. M. Pechet, Chem. Commun.,
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Scheme
1 A plausible reaction mechanism via vinyl carbene
intermediate.
5 Crystal data for 2d; CCDC number 615536. Empirical formula:
C25H21Cl; formula weight: 356.87; crystal color, habit: colorless,
prismatic; crystal system: monoclinic; lattice type: primitive; lattice
rearrangement to give the cyclopropene derivative rather than
insertion into the aromatic ring.8b–e Moreover, it is well known
that singlet phenyl carbene is far more reactive than triplet phenyl
carbene as well as vinyl carbene and in addition, the singlet and
triplet states of phenyl carbene are very close in energy and are in
rapid thermal equilibrium.9 This is why using MK and benzo-
phenone (10 mol%) as the sensitizers produced the same product
as that upon direct photo-irradiation, and why molecular oxygen
did not quench the reaction process.
parameters:
=
a
12.4925(15) A,
=
11.1389(13) A,
901,
b
=
=
13.3200(16) A,
c
a
=
b
91.325(2)1,
g
=
901,
=
V = 1853.0(4) A3; space group: P2(1)/c; Z = 4; Dcalc
1.279 g cmÀ3; F000 = 752; diffractometer: Rigaku AFC7R;
residuals: R; Rw: 0.0442, 0.0896.
6 The photoreaction outcome is the same as that without addition
of olefin (a triplet quencher) and no additional adduct was
formed, suggesting that the reaction proceeds via a singlet excited
state.
7 (a) W. von E. Doering and L. H. Knox, J. Am. Chem. Soc., 1950,
72, 2305; (b) W. von E. Doering, L. H. Knox and F. Deter, J. Am.
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714 and references cited therein.
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intermediate in organic chemistry, ed. M. Jones, Jr and
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Another explanation to account for the formation of 2 is
outlined in the ESIw on the basis of a biradical species
(Scheme 1),10,11 from which it may be impossible to form 2a.
In conclusion, we have developed an interesting photo-
chemical reaction of diarylvinylcyclopropenes 1 to give
1-isopropylidene-3,8a-diphenyl-1,8a-dihydroazulene deriva-
tives 2 in good yields.12 Efforts to elucidate the mechanistic
details of this photo-induced reaction and to further under-
stand the scope and limitations of photochemical reaction of
vinylcyclopropenes are currently in progress.
We thank the Shanghai Municipal Committee of Science
and Technology (06XD14005, 08dj1400100-2), the National
Basic Research Program of China (973)-2009CB825300, and
the National Natural Science Foundation of China (20872162,
20672127, and 20732008).
10 The formation of E can also be considered through intermediate D
(see ESIw).
11 X. Creary, M. E. Mehrsheikh-Mohammadi and S. McDonald,
J. Org. Chem., 1987, 52, 3254–3263.
Notes and references
1 (a) J. J. Mccullough, Chem. Rev., 1987, 87, 811; (b) G. J. Kavarnos
and N. J. Turro, Chem. Rev., 1986, 86, 401; (c) W. Horspool and
F. Lenci, CRC Handbook of Organic Photochemistry
and Photobiology, CRC, New York, USA, 2nd edn, 2004.
2 (a) R. R. Dasilva, V. G. Toscano and R. G. Weiss, Chem.
Commun., 1973, 567–568; (b) D. H. R. Barton, T. J. Bentley,
12 For the conversion of vinylcarbenes and vinylcarbenoids to hydro-
azulene structures. See (a) M. Kennedy, M. A. McKervey,
A. R. Maguire, S. M. Tuladhar and M. Fiona Twohig, J. Chem.
Soc. Perkin Trans. 1, 1990, 1047–1054; (b) H. Duddeck, J. Chem.
Soc., Perkin Trans. 1, 1990, 1055–1063; (c) P. Panne and J. M. Fox,
J. Am. Chem. Soc., 2007, 129, 22–23.
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This journal is The Royal Society of Chemistry 2009
1394 | Chem. Commun., 2009, 1392–1394