New 6-oxa-2-silabicyclo[2.2.0]hexanes by photochemical conversion of acyl(allyl)(dimethyl)silanes

Article abstract of DOI:10.1039/b812550c

Under UV irradiation in acetonitrile, acyl(allyl)silanes undergo an intramolecular (2+2) cycloaddition (Paterno-Buechi type reaction) to give unprecedented 1-alkyl-6-oxa-2-silabicyclo[2.2.0] hexanes. The Royal Society of Chemistry.

Full text of DOI:10.1039/b812550c

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New 6-oxa-2-silabicyclo[2.2.0]hexanes by photochemical conversion
of acyl(allyl)(dimethyl)silanesw
Catherine Hammaecher and Charles Portella*
Received (in Cambridge, UK) 22nd July 2008, Accepted 26th August 2008
First published as an Advance Article on the web 30th September 2008
DOI: 10.1039/b812550c
Under UV irradiation in acetonitrile, acyl(allyl)silanes undergo an
intramolecular (2+2) cycloaddition (Paterno-Buchi type reaction)
¨
to give unprecedented 1-alkyl-6-oxa-2-silabicyclo[2.2.0] hexanes.
As is the case for their fundamental state chemistry, the
photochemistry of acylsilanes offers, in addition to processes
similar to those of ketones, specific reactions depending on the
substrate structure and reaction conditions. Besides some ex-
amples involving g-hydrogen abstraction (Norrish II)1 the main
chemical processes resulting from acylsilane irradiation are
connected, at least formally, with a CO–Si bond fragmentation,
the reaction products depending strongly on the reaction
medium. In non polar solvents, results are solvent dependant:
acylsilanes react similarly to ketones in tetrachloromethane,2 or
via more complex intermolecular or dismutation routes in
hydrocarbons.3 In protic solvents or in the presence of alde-
hydes, products result from the trapping of a siloxycarbene
formed by 1,2-C to O-silyl migration.4 Trapping by electron
deficient alkenes was disclosed in 1971 by A. G. Brook and
co-workers,5 but it was shown later that the cyclopropane
derivative resulted from a reaction between alkene and the
excited state of the acylsilane.6 Such a carbene may be trapped
by water or alcohol giving an aldehyde or an O-silyl acetal,
respectively. This reaction was exploited to prepare glycoside-
derived O-silylacetals having anti-tumor activity7 and homo-
logated nucleoside derivatives.8
Fig. 1 Acyl(allyl)dimethylsilanes.
modified Katritsky strategy.11 UV absorption characteristics
of the carbonyl chromophore are similar to classical acyl-
silanes, as exhibited by 1a and 1b as representative of the aroyl
and acyl compounds, respectively.12
Irradiation of benzoyl derivative 1a at 420 nm in acetonitrile
induced a decoloration of the starting solution but gave an
intractable reaction mixture. Aliphatic compounds were irra-
diated at 350 nm. The phenylacetylsilane 1b underwent a Norrish
type 1 fragmentation on the acyl side, or a CO–Si fragmentation–
decarbonylation sequence, as the presence of diphenylethane in
the crude product shows (NMR analysis).13 Much more interest-
ingly, irradiation of allyl(dimethyl)undecanoylsilane 1c in the
same conditions induced a rapid, clean and quantitative conver-
sion into
a
new type of heterobicyclo[2.2.0]hexane,
6-oxa-2-silabicyclo[2.2.0]hexane 2c (Scheme 1). Compound 2c is
the result of an intramolecular Paterno-Buchi type cyclo-
¨
addition.14,15 The reaction was extended to the acyl(allyl)silanes
1d and 1e which led to the corresponding bicyclic analogues 2d
and 2e, respectively. In contrast to 2c, 2d and 2e were obtained in
lower yields, the reaction giving an unidentified side-product.
According to NMR spectra, this side-product was the same from
both reactions, and did not contain the alkyl moiety.
To the best of our knowledge, all photochemical investigations
conducted so far have been on simple acylsilane or acylsilanes
functionalised on the acyl moiety. Thus it appeared interesting to
study the photochemical reactivity of acyl(allyl)silanes, which
could be expected to follow several routes. We were particularly
interested in a possible intramolecular trapping of an intermediate
carbene or excited species.
Compounds 2 are colorless liquids. They were isolated and
purified by flash chromatography and their purity was checked
by GLC. Their structure was determined according to NMR
spectra, GCMS under chemical ionisation conditions, and
chemical correlations. The NMR data of compound 2d, the
spectra of which is the best resolved, is depicted in Fig. 2. The
structure of oxetanes 2 were corroborated by chemical
Surprisingly, acyl(allyl)silanes seem to have been reported
only once by Tsai’s group who studied intramolecular radical
addition on the allyl unsaturation.9 We have prepared a series
of acyl(allyl)silanes depicted in Fig. 1. The acyl(allyl)silanes 1a,
1b, 1d and 1e were prepared via a modified Brook–Corey
strategy.10 Undecanoyl(allyl) silane 1c was obtained via a
´
Universite de Reims Champagne-Ardenne, Institut de Chimie
Mole´culaire de Reims, CNRS UMR 6229, UFR Sciences Exactes et
Naturelles, BP 1039, 51687 REIMS Cedex 2, France.
E-mail: charles.portella@univ-reims.fr; Fax: +33 326913234;
Tel: +33 326913166
w Electronic supplementary information (ESI) available: Experimental
part including 1H and 13C NMR spectra of compounds 1a–1e, 2a–2e.
See DOI: 10.1039/b812550c
Scheme 1 Photochemical reaction of acyl(allyl)silanes in acetonitrile.
Chem. Commun., 2008, 5833–5835 | 5833
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Scheme 3 Photochemical reaction of acyl(allyl)silanes in aqueous
acetonitrile.
The photochemical behaviour of acyl(allyl)silanes 1 in aceto-
nitrile resembles more the one of g,d-unsaturated ketones,
which were reported to undergo an intramolecular Paterno-
Fig. 2 1H- and 13C-NMR data (d ppm) for compound 2d.
Buchi type cycloaddition,18 than those of acylsilanes, with a
¨
marked difference as far as regioselectivity is concerned. The
reaction of our acylsilanes proved to be completely regio-
selective toward the oxasilabicyclo[2.2.0]hexane regioisomer,
whereas irradiation of isostructural g,d-unsaturated ketones
also gave the bicyclo[3.1.1]hexane isomer; regioselective
cycloaddition towards either isomer was observed only for
rigid cyclic g,d-unsaturated ketones.19
The solvent proved to have a crucial role in the cyclo-
addition route. Indeed, under irradiation in a 1 : 1 mixture
acetonitrile–water, benzoyl(allyl)silane 1a was quantitatively
converted into benzaldehyde, the expected result of the well
known pathway: CO–Si fragmentation—rearrangement into a
siloxycarbene—trapping of the latter to give the silyl hemi-
acetal. Under the same conditions, the undecanoyl analogue
1c gave a 1 : 1 mixture of the corresponding aldehyde and the
cycloaddition product 2c (Scheme 3). This result is in accor-
dance with the particularly high effectiveness of the intra-
molecular cycloaddition observed for 1c in pure acetonitrile.
This enhanced propensity of 1c to cyclise compared to other
aliphatic analogues remains difficult to explain.
A tentative interpretation of the observed photoreactivity of
acyl(allyl)silanes in acetonitrile is proposed in Scheme 4.
Independently of the primary photoprocesses which have not
been considered here,20 one can assume that longer C–Si bonds
(compared to C–C bond) favour a selective cyclisation between
the oxygen of the excited carbonyl group and the terminal
allylic carbon, leading to the intermediate biradical 6 which
further cyclises into 2.21 Such a process leads to a more stable
secondary radical. The regioisomer radical 7, if its formation
occurred, would undergo b-fragmentation toward the starting
material. The usual CO–Si framentation leading to the radical
pair 8 is not competitive in anhydrous medium, but in the
Scheme 2 Chemical transformations of compound 2c.
transformations involving the cleavage of Si–C bonds. Tamao
type oxidation16 of 2c gave the expected ketodiol 4, via the
intermediate hemiketal 3 derived from Si–C bond oxidative
cleavage (Scheme 2). Fluoride induced protodesilylation of 2c
yielded the expected 2,3-dialkyloxetane 5. A trans configura-
1
tion was ascribed to 5 by comparison of H- and 13C-NMR
spectra with those of very similar compounds reported in the
literature.17
Scheme 4 Proposed mechanism.
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presence of water, the siloxycarbene 9 is trapped to give the
corresponding aldehyde 10. Another remarkable feature of this
reactivity is the competitive cycloaddition versus formation of
siloxycarbene, even in aqueous medium as shown for 1c.
In summary, following the various studies on the photo-
chemistry of acylsilanes reported so far, we report for the first
time the synthesis of acyl(allyl)silanes. The latter undergoes, in
anhydrous acetonitrile, selective intramolecular photocyclo-
addition yielding unprecedented 1-alkyl-6-oxa-2-silabicyclo[2.2.0]-
cyclohexanes.
8 A. Wegert, J.-B. Behr, N. Hoffmann, R. Miethchen, C. Portella
and R. Plantier-Royon, Synthesis, 2006, 2243.
9 K.-H. Tang, F.-Y. Liao and Y.-M. Tsai, Tetrahedron, 2005, 61,
2037.
10 G. Brook, J. M. Duff, P. F. Jones and N. R. Davis, J. Am. Chem.
Soc., 1967, 89, 431; E. J. Corey, D. Seebach and R. Freedman,
J. Am. Chem. Soc., 1967, 89, 434.
11 A. R. Katritzky, H. Lang, Z. Wang and Z. Lie, J. Org. Chem.,
1996, 61, 7551.
12 Absorption spectra were measured in CH3CN [lmax(nm);
e
max(L.molÀ1 cmÀ1]. 1a: 416, 45; 1c: 365, 49.
13 For photolysis of phenylacetyl(triphenyl)silane, see A. G. Brook
et al: ref. 3.
14 E. Paterno and G. Chieffi, Gazz. Chim. Ital., 1909, 39, 341;
We thank the CNRS and the Ministry of Research for
supporting this research, and the Region Champagne-
´
G. Buchi, C. G. Inman and E. S. Lipinsky, J. Am. Chem. Soc.,
¨
1954, 76, 4327.
Ardenne for a PhD fellowship (C.H.). We also express our
thanks to Dr Norbert Hoffmann for his interest in and fruitful
discussions on the research.
15 For a recent review, see: A. G. Griesbeck, in Synthetic Organic
Photochemistry, ed. A. G. Griesbeck and J. Mattay, Marcel
Dekker, New York, 2005, p. 89.
16 K. Tamao, N. Ishida, D. Tanaka and M. Kumada, Organometallics,
1983, 2, 1694.
17 M. Christlieb, J. E. Davies, J. Eames, R. Hooley and S. Warren,
J. Chem. Soc., Perkin Trans. 1, 2001, 2983–2996.
18 N. C. Yang, M. Nussim and D. R. Coulson, Tetrahedron Lett.,
1965, 20, 1525, and references therein.
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2 A. G. Brook, P. J. Dillon and R. Pierce, Can. J. Chem., 1971, 49, 133.
3 A. G. Brook and J. M. Duff, Can. J. Chem., 1973, 51, 352.
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Bourque, P. D. Davis and J. C. Dalton, J. Am. Chem. Soc., 1981,
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5 A. G. Brook, H. W. Kucera and R. Pearce, Can. J. Chem., 1971,
49, 1618.
6 J. C. Dalton and R. A. Bourque, J. Am. Chem. Soc., 1981, 103, 699.
7 S. A. Svarovsky, M. B. Taraban and J. J. Barchi, Jr, Org. Biomol.
Chem., 2004, 2, 3155.
19 V. H. Rawal, A. Fabre
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C. de la Torre, I. Garcıa and M. A. Sierra, J. Org. Chem., 2003, 68,
6611.
´
and S. lwasa, Tetrahedron Lett., 1995, 36,
´
20 For photochemical primary processes of Paterno-Buchi reaction,
¨
see the review mentioned in ref. 15.
21 It has been shown that cyclization of 2-(allyldimethylsilyl)ethyl
radicals proceed exclusively via 6-endo-trig mode: K. Saigo, K.
Tateishi, H. Adachi and Y. Saotome, J. Org. Chem., 1988, 53,
1572.
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