A new synthesis of 1,3-dihydrobenzo[1,2]oxasiloles by a novel
rearrangement of a pentavalent silicon intermediate
George Bashiardes,* Vanessa Chaussebourg, Géraldine Laverdan and Jacques Pornet
Departement de Chimie, Université de Poitiers, SFA-UMR 6514, 40 avenue du Recteur Pineau, 86022
Poitiers cedex, France. E-mail: george.bashiardes@univ-poitiers.fr
Received (in Cambridge, UK) 24th September 2003, Accepted 10th November 2003
First published as an Advance Article on the web 26th November 2003
A new synthesis of benzo[1,2]oxasiloles is described, wherein an
unprecedented intramolecular allylic transposition takes place
probably involving a pentavalent silicon intermediate.
required arylallylsilane 7c (entry 3), spontaneous rearrangement
under the experimental conditions (THF, 20°C), provided 1c
directly (see later). As a test experiment for mechanistic elucida-
tion, we also treated the vinylic analogue 7d in the same manner.
Indeed no reaction was observed and starting material was totally
recovered, which concurs with the proposed mechanism.
Following the thermal reaction, in the cases of 7a and 7b the
required 3-allyl benzo[1,2]oxasiloles 1a (R1 = H; R2 = Me) and
1b (R1 = R2 = Me) were obtained in 96% and 70% yields,
respectively (Table 2). In the latter example, the remainder of the
yield corresponds to recovered starting benzaldehyde 7b. 1c was
obtained in 38% overall yield, directly from benzaldehyde 2,
without isolation of the corresponding ortho-allylsilyl benzalde-
hyde 7c.
Benzoxasiloles are interesting in their structure in that they contain
the chemically labile O–Si and C–Si bonds, allowing for much
derivatizing. Few methods for the synthesis of substituted benzox-
asiloles have been reported, and in each case the preparations
concerned mostly specific examples.1–8
We report herein a new, general synthesis of benzo[1,2]ox-
asiloles 1 (Fig. 1) from easily accessible ortho-silylated benzalde-
hydes 7a–c, for which we also describe new preparations.
The reaction sequence we employed is quite general, allowing
possible substitution on the allylic group and the silicon atom.
Other benzaldehydes could also be employed to prepare new
functionalized analogues. In Scheme 1 is described the preparation
of ortho-allylsilylbenzaldehydes by two different approaches. In
the first case benzaldehyde 2 is silylated with chlorosilanes 3a–c
following orthometallation using N,N,NA-trimethylethylenediamine
and n-butyllithium9 (method A). In the second case, ortho-
bromobenzaldehyde diethylacetal 4 undergoes a halogen–metal
exchange using n-butyllithium, followed by silylation to give
acetals 6. Acid hydrolysis then furnishes the required ortho-
substituted benzaldehydes 7 (method B). In both cases, the overall
yields are good (Table 1), while the latter, two-step approach is
generally higher yielding overall. In addition, a ortho-vinylsilyl
benzaldehyde 7d was also prepared by the above procedures.
Silane derivatives 7a and 7b are then heated in toluene or xylene
to effect the observed rearrangement into benzo[1,2]oxasiloles
(Scheme 2). Interestingly, in one case, during the preparation of
We propose a mechanism (Scheme 3) for the observed
transformation by which an intermediate pentavalent silicon is
formed,1,7 followed by a “nucleophilic” intramolecular allylic
transposition.
From the observed results, it appears that the groups present on
the silicon atom can influence the yield and the ease of reaction. 2b
containing a Z crotyl group gave a single product albeit in lower
yields certainly due to the added steric bulk of the methyl
substituent on the attacking allyl moiety. It is proposed that
pentavalent silicon adopts a trigonal bipyramidal configuration1,7
(Fig. 2) in which the oxygen atom occupies an axial position and
that the allylic group must have an equatorial arrangement in order
to transpose to the cationic benzylic methylene. A pseudo-
equilibrium (rearrangement) would assure that such a relative
Table 1 ortho-Allyl- and vinylsilyl benzaldehydes 7a–d
Yield
Yield
(method A) (method B)
Entry Chlorosilane10,11
Compound
(%)
51
(%)
87
1
Fig. 1 3-Allylbenzo[1,2]oxasiloles.
2
71
n/a
3
4
other
product (1c)
n/a
87
67
Scheme 1 Synthetic scheme for the preparation of 2-silyl benzaldehydes. i)
LiNMeCH2CH2NMe2, THF, 278 °C, ii) nBuLi then iii) chlorosilane 3a–
3c; iv) iPrOH, H2O, THF, TsOH.
Scheme 2
122
C h e m . C o m m u n . , 2 0 0 4 , 1 2 2 – 1 2 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
Bashiardes, George
