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A. Massa et al. / Tetrahedron Letters 44 (2003) 7179–7181
Table 1. Allylation of benzaldehyde (1a) with allyltrichlorosilane (2) using racemic methyl sulfoxides (4)
Entry
4: R2
( )-4 (equiv.)
Temp. (°C)
Time (h)
Yield (%)a
1b
2c,d
3f
4a: p-tolyl
4a: p-tolyl
4a: p-tolyl
4a: p-tolyl
4b: p-MeOC6H4
4c: CH2C6H5
4d: p-NO2C6H4
4a: p-tolyl
4a: p-tolyl
4a: p-tolyl
0.4
0.4
0.4
1.5
1.5
1.5
1.5
1.5
2.0
3.0
−78
−78
−30
−78
−78
−78
−78
−78
−78
−78
7
7
24
7
7
7
7
65
7
33
0e
16
44
44
39
0e
63
50
61
4b
5b
6b
7b
8b
9g
10g
7
a Yields refer to isolated, chromatographically pure compounds, whose structures were confirmed by spectroscopic data (1H and 13C NMR).
b The sulfoxide was recovered in 85% yield.
c The reaction was carried out in the absence of (i-Pr)2NEt.
d The sulfoxide disappeared in the course of the reaction.
e No reaction.
f The sulfoxide was recovered in 22% yield.
g The sulfoxide was recovered in 95% yield.
moderate enantioselectivities were observed with aro-
matic and a,b-unsaturated aldehydes 1 (Table 2).
a rapid decomposition of the sulfoxide (entry 2) and the
consequent inhibition of the allylation reaction.
It is noteworthy that, owing to the mild conditions of
this procedure, the chiral sulfoxide could be recovered
with only a very slightly reduced e.e. (97%) and in
almost quantitative yields (>95%).
As expected, increasing the amount of the sulfoxide
resulted in acceleration of the reaction (entries 4–10). It
is noteworthy that when larger amounts of the additive
were used (entries 9 and 10), it could be recovered from
the reaction mixture almost completely.
In conclusion, racemic and/or enantioenriched homoal-
lylic alcohols can be obtained in satisfactory yields by
allylation of aldehydes with allyltrichlorosilane in the
presence of sulfoxides, as racemic and/or chiral Lewis
bases. The catalysts are easily available both in racemic
and chiral forms and can be almost quantitatively
recovered and recycled. Further studies will be devoted
to the investigation of the influence of different chiral
non racemic sulfoxides on the efficiency and enantiose-
lectivity of the reaction.
Very similar yields were observed by carrying out the
reaction in the presence of methyl p-methoxyphenylsul-
foxide 4b and methyl benzylsulfoxide 4c as Lewis bases
since the corresponding alcohol 3a was obtained in 44
and 39% yield, respectively (entry 5 and 6). On the
other hand, methyl p-nitrophenylsulfoxide 4d proved to
be incompatible with 2 (entry 7); in fact, very fast
formation of a precipitate was observed after the addi-
tion of allyltrichlorosilane to the solution containing
the sulfoxide, and no homoallylic alcohol could be
detected in the reaction mixture after a prolonged reac-
tion time.
Typical experimental procedure: Diisopropylethylamine
(0.44 mL, 2.5 mmol) and allyltrichlorosilane 2 (150 mL,
0.93 mmol) were successively added to a solution of the
sulfoxide 4 (1.50 mmol) in dry CH2Cl2 (4.5 mL) at
−78°C under argon and the mixture was stirred for 5
min. Aldehyde 1 (0.50 mmol) was then added and the
reaction mixture was stirred at −78°C for 7 h. The
mixture was then poured into an ice-cooled mixture of
The enantiopure (R)-methyl-p-tolylsulfoxide 4a was
employed under the optimized reaction conditions
(shown in Table 1, entries 8 or 10) to evaluate the
enantioselectivity of the reaction. Good yields and
Table 2. Allylation of aldehydes 1 using enantiopure (R)-methyl-p-tolylsulfoxide 4a
Entry
1 (R1)
4a (equiv.)
Time (h)
Yield (%)a
% ee of (S)-3b
1c
2e
3e
4e
1a (Ph)
1a (Ph)
1b (2-furyl)
1c (PhCHꢁCH)
1.5
3.0
1.5
1.5
65
7
65
65
70
62
88
79
51d
55d
42f
46d
a See note ‘a’ in Table 1.
b The absolute configuration of 3 was found to be (S) according to the optical rotation reported in the Refs. 5 and 6.
c The sulfoxide was recovered in 85% yield and 85% e.e.
d Determined by HPLC analysis employing a Daicel Chiralcel OB column.
e The sulfoxide was recovered in >95% yield and 97% e.e.
f Determined by HPLC analysis of the corresponding 3,5-dinitrobenzoate (Daicel Chiralcel AD column).