Sulfoxides in the allylation of aldehydes in the presence of silicon tetrachloride and allyltributylstannane

Article abstract of DOI:10.2478/s11532-010-0099-7

SiCl₄ can be conveniently activated by catalytic amounts of dimethyl sulfoxide or other readily-available sulfoxides for the allylation of aromatic, hetero-aromatic and unsaturated aldehydes in the presence of allyltributyl stannane. Chiral ary

Full text of DOI:10.2478/s11532-010-0099-7

Cent. Eur. J. Chem. • 8(6) • 2010 • 1210-1215
DOI: 10.2478/s11532-010-0099-7
Central European Journal of Chemistry
Sulfoxides in the allylation of aldehydes
in the presence of silicon tetrachloride
and allyltributylstannane
Research Article
Antonio Massa*, Laura Capozzolo, Arrigo Scettri
Department of Chemistry, University of Salerno,
84084 Fisciano (Salerno), Italy
Received 04 June 2010; Accepted 20 July 2010
Abstract:
SiCl4 can be conveniently activated by catalytic amounts of dimethyl sulfoxide or other readily-available sulfoxides for the allylation of
aromatic, hetero-aromatic and unsaturated aldehydes in the presence of allyltributyl stannane. Chiral aryl methyl sulfoxides have been
used to develop asymmetric allylation methods, as well as probe the aldehyde substrate scope.
Keywords: Allylation of aldehydes • Sulfoxides • Lewis base • Silicon tetrachloride • Allyltributylstannane
© Versita Sp. z o.o.
enantioselectivities have been achieved by Denmark
1. Introduction
in the presence of the chiral bidentate phosphoramide
[7,9a-c,10,11], while little attention has been paid to the
use of different Lewis bases. Chiral N-oxides [9d,9e] and
phosphine oxides [9f] have been used in asymmetric
ring opening of epoxides with good results. Very
recently chiral phosphine oxides have been used in the
asymmetric phosphonylation of aldehydes with moderate
asymmetric induction [12]. It is important to note that in
the reported SiCl4-mediated transformations, none of the
used Lewis bases are commercially available or easy to
synthesize. This is an important limitation to be overcome
to expand the scope of the system. However, despite
the importance of enantiopure sulfinyl groups in many
asymmetric transformations [13], this chiral controller has
never been exploited as Lewis base in SiCl4 mediated
transformations. This is an important limitation to be
overcome to expand the scope of the system. Moreover,
despite the synthetic usefulness of homoallylic alcohols,
only the chiral bidentate phosphoramide has been used
in the allylation of aldehydes using allyltributylstannane
and SiCl4 [11]. For these reasons, in our continuous
efforts to develop reactions catalyzed by new Lewis bases
[7g,7h,8,14a,14c,14e], we report the first application of
simple and readily-available sulfoxides in allylation of
aldehydes using this system.
The allylation of aldehydes is one of the most important
methods for C-C bond formation. Homoallylic alcohols
are useful building blocks for the synthesis of complex
molecules and natural products [1]. In recent years, a
number of new methods based on allylating reagents
such as allylboranes, allylsilanes and allylstannanes
have been developed [2-4]. Particular attention has
been devoted to the development of enantioselective
versions utilizing chiral allylating reagents, chiral Lewis
acids as catalysts and, more recently, chiral Lewis
base organocatalysts [2-4]. The Lewis base strategy,
pioneered by Kobayashi [5] and Denmark [6], has been
successful in the allylation of aldehydes in the presence
of allyltrichlorosilane. A wide variety of chiral Lewis bases
derived from phosphoramides, N-oxides, sulfoxides,
formamides, phosphine oxides, amines and ureas, has
been demonstrated to be effective in enantioselective
allylation [2a,3,4]. Moreover Lewis base catalytic
activation of weak Lewis acids like SiCl4 has proven to
be a very versatile approach in a great number of aldol-
type reactions with silylenolethers [7,8], the asymmetric
opening of epoxides [9], Passerini reactions [10] and
the allylation of aldehydes using allyltributylstannane
[11]. In SiCl4-mediated reactions, very good yields and
* E-mail: amassa@unisa.it
1210

A. Massa, L. Capozzolo, A. Scettri
anhydrous Na2SO4. After removing the solvent under
reduced pressure, the crude oil was purified by flash
chromatography with a hexane / AcOEt mixture from
9.5/0.5 to 8/2 to afford the pure products 3.
All the compounds 3 are known compounds and
were characterised comparing the spectroscopic data
with those reported in literature [14].
2. Experimental Procedure
All reactions were performed in oven-dried (140°C) or
flame-dried glassware under dry N2. Dichloromethane
(reagent grade) was dried and distilled immediately from
CaH2 before use. Column chromatographic purification
of products was carried out using silica gel 60 (70–230
mesh, Merck). Other reagents (Aldrich and Fluka) were
used without further purification. The NMR spectra were
recorded on Bruker DRX 400, 300, 250 spectrometers
3. Results and Discussion
1
(400 MHz, 300 MHz, 250 MHz, H; 100 MHz, 75 MHz,
62,5 MHz 13C). Spectra were referenced to residual In preliminary experiments, we attempted the allylation
CHCl3 (7.26 ppm, 1H, 77.23 ppm, 13C). Yields are given of benzaldehyde 1a in the presence of dimethyl sulfoxide
for isolated products showing one spot on a TLC plate (DMSO) under the condition described in Scheme 1 and
and no impurities detectable in the NMR spectrum. Table 1. Performing the reaction with varying equivalents
HPLC analyses were performed with Waters Associates of the Lewis base, we observed a higher reactivity when
equipment (Waters 2487 Dual λ absorbance Detector) 0.2 eq. of DMSO were used (Table 1, entries 2, 3),
and using a CHIRALPAK AD, a CHIRALCEL OD column while in the presence of 2 or 3 equivalents, the reaction
with hexane/isopropyl alcohol mixtures and flow rates was slower (Table 1, entries 4, 5). These results are in
as indicated in [14] for chiral homoallylic alcohols 3 and contrast with allylation of aldehydes in the presence of
[15] for chiral sulfoxides 4. The HPLC methods were allyltrichlorosilane, where sulfoxides should be used in
calibrated with the corresponding racemic mixtures. GC excess (≥ 2 eq.) to ensure high reactivity [14].
analysis were performed using an Agilent Technologies
No reaction was observed in the absence of DMSO,
equipment (mod. 6850) with the chiral column Supelco supporting the importance of the Lewis base in the
β-DEX 120. Optical rotations were measured with a activation of the weak Lewis acid SiCl4 [9-11].
JASCO DIP-1000 polarimeter. Mass spectral analyses
Under the optimized reaction conditions (Table 1,
were carried out using an electrospray spectrometer, entry 3), different aldehydes were reacted in order to
Waters 4 micro quadrupole.
probe the scope of the method (Scheme 2, Table 2).
Good to high yields were observed for aromatic, hetero-
aromatic and unsaturated aldehydes in the presence of
both electron-withdrawing and electron-donating groups.
In the presence of a substrate bearing a cyano group in
the ortho- position, the reaction was significantly slower.
3-phenyl propionaldehyde [7] did not react at all.
Next we turned our attention to developing an
asymmetric version of the reaction (Scheme 3,
Tables 3 and 4). Using the commercially-available (R)-
2.1. Typical experimental procedure for
allylation reaction
In a flame-dried, 2-necked, round-bottom flask, allyl
tributyl stannane (1.1 eq., 0.66 mmol) was added to
a solution of sulfoxide, i-Pr2EtN (1.0 eq., 0.60 mmol),
SiCl4 (1.1 eq., 0.66 mL of 1.0 M solution in CH2Cl2) and
aldehyde (0.60 mmol) in dry dichloromethane (2.0 mL)
under nitrogen at -78°C. At the end of the reaction, the
mixture was quenched with saturated aqueous NaHCO3
(5 mL), extracted with 15×3 mL of CH2Cl2 and dried over
O
OH
SiCl4, i-Pr2NEt
Bu3SnCH2CH=CH2
+
R
H
DMSO, CH2Cl2
24 h, -78°C
R
O
OH
1
2
3
SiCl4, i-Pr2NEt
Scheme 2. Allylation of aldehydes in the presence of DMSO
Bu3SnCH2CH=CH2
+
Ph
H
DMSO, CH2Cl2
24 h, -78°C
Ph
Table 2. Allylation of different aldehydes in the presence of 0.2 eq.
1a
2
3a
of DMSO.
Scheme 1. Allylation of benzaldehyde in the presence of DMSO
Entries
Aldehyde 1 (R)
Yield 3 %a
Table 1. Allylation of benzaldehyde 1a under different reaction
1
2
3
4
5
C6H5
87
68
78
82
58
conditions.
p-OMeC6H4
p-NO2C H
p-CNC66H44
o-CNC6H4
PhCH=CH
2-furyl
Entries
Eq. DMSO
Time (h)
Yield 3a (%)a
1
2
3
4
5
6
0.1
0.2
0.2
2
5
5
62
70
6
88
7
70
24
5
87
8
5-nitro-2-furyl
5-nitro-2-thiophenyl
PhCH2CH2
72
60
3
5
57
79
9
--
5
No react.
10
No react.
a Yields refer to chromatographically pure compounds
a Yields refer to chromatographically pure compounds
1211

Sulfoxides in the allylation of aldehydes
in the presence of silicon tetrachloride
and allyltributylstannane
Table 3. Allylation of aldehydes in the presence of several chiral (R)-aryl methyl sulfoxides
Entry
Aldehyde
Sulfoxide (Ar)a,b
Eq.
Time (h)
Yield (%)c
e.e. (%)b
sulfoxide
1
2
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
5-nitro-2-furyl
5-nitro-2-furyl
5-nitro-2-furyl
4a p-tolyl
0.1
0.2
1
5.5
23
5
55
89
7
4a p-tolyld
12
18
34
33
7
3
4a p-tolyl
86
4
4a p-tolyl
2
6
32
5
4a p-tolyle
2
23
23
23
23
23
23
23
70
6f
7
4a p-tolylg
2
49
4a p-tolyl
3
No react.
59
--
8
4b p-MeO-phenyle
4a p-tolyl
2
36
57
60
40
9
2
67
10
11
4b p-MeO-phenyle
4c 2-naphthyle
2
76
2
48
a Sulfoxide 4a is commercially available; 4b and 4c were synthesized according to the procedure described in [15] and were obtained respectively with
97 and 98% e.e.s
b Enantiomeric excesses were determined by chiral HPLC.
c Yields refer to chromatographically pure compounds
d The sulfoxide was recovered in quantitative yield and 50% e.e.
e The sulfoxide was recovered in quantitative yield and 96% e.e.
f Reaction performed without i-Pr2EtN.
g The sulfoxide was recovered in 84% yield and 20% e.e.
O
SiCl4, i-Pr2NEt
OH
methyl p-tolyl sulfoxide 4a, we tried the enantioselective
allylation of benzaldehyde in the presence of different
amount of the Lewis base (Table 3, entries 1-6).
Unfortunately, the enantioselectivity was low even
in the presence of 2 equivalents of the sulfoxide
(Table 3, entries 4 and 5). Sub-stoichiometric amounts of
the Lewis base showed higher reactivity but lower e.e.s
CH2Cl2, -78°C
Bu3SnCH2CH=CH2
+
H
R
R
O
1
2
3
:
Ar
S
H3C
4
Scheme 3. Asymmetric allylation of aldehydes in the presence of
chiral methyl aryl sulfoxides
(Table 3, entries 1 and 2), while the trend was the center with the coordination of three Lewis bases
opposite when 2 equivalents of the chiral sulfoxide were (Table 3, entry 7).
used (Table 3, entries 4 and 5). At the end of the reaction,
We screened other chiral aryl methyl sulfoxides
the chiral sulfoxide was recovered in quantitative yield in order to analyze the effect of the substituent of the
but in varying enantiomeric purities that depended on aromatic ring in the allylation of benzaldehyde and
the number of equivalents of the sulfoxide. When 2 5-nitro-2-furfural. Higherenantioselectivitywasobserved
eq. were used, 4a was recovered in quantitative yield in the presence of 5-nitro-2-furfural with both 4a and
and 97% e.e. (Table 3, entries 4 and 5), while in the 4b (Table 3 entries 9-11), suggesting the possibility of
presence of 0.2 eq., 4a was recovered in quantitative an important electronic effect of the substrate on the
yield but with 50% e.e. (Table 3, entries 1 and 2) [14,16]. enantioselectivity. Among the sulfoxides screened, 4b
When we used fewer equivalents of 4a, this effect was with an electron-donating group on the aromatic ring,
more evident in terms of percentage value on the e.e. showed a slightly higher enantioselectivity (Table 3,
of the recovered sulfoxide. These results can in part entries 8 and 10), while the hindered naphthyl group in
explain the lower enantioselectivity in the presence of 4c gave poorer results (Table 3, entry 11). At the end
catalytic amount of 4a (Table 3, entries 1 and 2).
of the reaction, all the sulfoxides were recovered in
Since the racemization of chiral sulfoxides has been quantitative yields and with e.e.s > 96%.
reported to occur under acidic conditions [17], partial
In the presence of 2 eq. of the commercially-
racemization of 4a can be due HCl, formed by the available (R)-methyl p-tolyl sulfoxide 4a, we examined
hydrolysis of SiCl4 due to trace amounts of water. For the reactivity of a series of aldehydes (Scheme 3,
this reason, it is important to use i-Pr2EtN to neutralize Table 4). As previously observed with 5-nitro-2-furfural,
excess HCl [14,16]. Performing the reaction in the same aldehydes bearing electron-withdrawing groups showed
conditions of entry 5 (Table 3) but without i-Pr2EtN, we a significantly higher enantioselectivities (Table 4,
recovered 4a in 84% yield and 20% e.e., while 3a was entries 5-9), while the other aldehydes gave the final
obtained in significantly lower yield and e.e. (Table 3, products 3 with rather low e.e.s (Table 4, entries 2-4
entry 6).
and 10-12), comparable to benzaldehyde (Table 4,
The observed reactivity reflects what we have entry 1). Moreover, the presence of a group in the
described in the preliminary experiments using different ortho position further decreased the enantioselectivity
amounts of DMSO (Table 1). In the presence of a large (Table 4, entries 10 and 12). According to the measured
excess of 4a we did not observe the formation of the optical rotation, all the homoallylic alcohols were
product presumably due to the saturation of the silicon obtained with an excess of the (R)-enantiomer. This
1212

A. Massa, L. Capozzolo, A. Scettri
is in contrast to the allylation of aldehydes using [14], while SiCl4-mediated transformations proceed via
allyltrichlorosilane, in which (R)-methyl p-tolyl sulfoxide an open transition state [9 -11,18].
4a produced products with opposite configuration [14].
Since the yields vary from moderate to high
This difference could be due to the different pathways independently of the aldehyde, the reactivity
occurring in the two transformations. In the presence cannot be related to the electronic properties of the
of allyltrichlorosilane, the observed stereochemistry is substrates. As observed with DMSO as the Lewis base
well-described by a closed, chair-like transition state (see Table 2, entry 5), the reaction of 2-cyano-
benzaldehyde was even slower, probably due to a
combination of steric and electronic effects (Table 4,
Table 4. Allylation of aldehydes in the presence of (R)-methyl
p-tolyl sulfoxide 4a.
Yield 3 (%)a,b e.e. 3 (%)c
entry 10). Also in the presence of the chiral sulfoxide,
the aliphatic aldehyde did not react (Table 4, entry 13).
As widely accepted in the literature [9 -11,18], the
coordination of the weak Lewis acid SiCl4 by a strong
Lewis base polarizes the silicon-chlorine bonds, leading
to ionization of a chloride and formation of a catalytically
active, five-coordinate trichlorosilyl cation 6 (Scheme 4).
Thegenerationofacationicspeciesresultsinasignificant
increase in the Lewis acidity of the central atom. The
activated Lewis acid allows for the coordination and
activation of the aldehyde in a hypervalent silicon centre
transition state 7. The attack of the nucleophile yields
the homoallylic alcohol 3.
Entry
Aldehyde (R)
1
2
C6H5
p-MeOC6H4
70
52
33
14
3
PhCH=CH
86
29
4
2-furyl
72
22
5
6
7
8
9
5-nitro-2-furyl
5-nitro-2-thiophenyl
p-CF3C6H4
67
79
63
57
54
30
58
57
52
45
45d
50
4
37
4
p-NO2C H
p-CNC 6H 4
10
11
12
13
o-CNC6H4
p-ClC66H44
o-ClC6H4
52
PhCH2CH2
No react.
a Yields refer to chromatographically pure compounds
b Reaction time = 23 h
c Enantiomeric excesses were determined by chiral HPLC.
d E.e. was determined by chiral GC using Supelco β-dex column.
+
Cl-
Cl
Cl
Cl
Cl
LB:
SiCl4
+
LB:
Si Cl
LB:
Si
−δ
+δ
Cl
Cl
6
4
5
O
R
H
+
Cl-
Cl
OH
Cl
LB:
Bu3SnCH2CH=CH2
Si
Cl
*
R
O
H
R
3
7
Scheme 4. Proposed mechanistic pathway
+
Cl-
+
+
Cl-
Cl-
O
Cl
Cl
Cl
Cl
LB:
LB:
LB:
Cl
R
H
Si
Cl
LB:
LB:
LB:
Si
Si Cl
O
Cl
Cl
H
R
6
8
9
Scheme 5. Proposed mechanistic pathway
4
1213

Sulfoxides in the allylation of aldehydes
in the presence of silicon tetrachloride
and allyltributylstannane
could be useful for a better description of the enantio-
determining step of the reaction and allow for a more
rational design of effective sulfoxide-based Lewis
bases. The strong dependence of the observed e.e.s
on the substrate structure should be included in a more
detailed kinetic model.
+
+
Cl-
Cl-
Cl
Cl
LB:
LB:
LB:
LB:
LB:
Cl
Si Cl
Cl
Si
Cl
:LB
10
8
Scheme 6. Proposed mechanistic pathway
The use of two equivalents of the sulfoxide
can lead to a six-coordinated transition state 9
(Scheme 5). This species probably displays lower
reactivity than the five-coordinate transition state 7 due
to increased steric hindrance. However, the coordination
of two chiral Lewis bases leads to a species that exhibits
higher asymmetric induction.
In the presence of three equivalents of methyl p-tolyl
sulfoxide, we did not observe any reaction. The excess
of the sulfoxide could produce a six-coordinate, inactive
complex in which three Lewis bases are bound to the
silicon center (Scheme 6). In this octahedral complex,
there is no possibility for the coordination and activation
of the aldehyde.
4. Conclusions
In conclusion, readily available chiral or achiral
sulfoxides can be conveniently used as Lewis bases for
the activation of SiCl4 in the allylation of aldehydes using
allyltributyl stannane. Good to high yields have been
observed in the presence of aromatic, hetero-aromatic
and unsaturated aldehydes using catalytic amount
of sulfoxides. However in the presence of an excess
of commercially available (R)-methyl p-tolyl sulfoxide
4a, moderate enantioselectivity was observed with
aldehydesbearinganelectron-withdrawinggroups,while
all other aldehydes showed rather low e.e.s. Due to the
novelty of this application, further studies are currently
underway to design more effective chiral sulfoxides and
to develop other sulfoxide-SiCl4 mediated reactions.
Other mechanistic studies are necessary in order to
determine the order of the sulfoxide and describe the
equilibria between the possible silicon complexes. This
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