Chiral sulfoxides as activators of allyl trichlorosilanes in the stereoselective allylation of aldehydes

Article abstract of DOI:10.1039/c002988b

Chiral aryl methyl sulfoxides proved to be efficient activators in the asymmetric allylation of aldehydes with allyl trichlorosilanes. High enantioselectivity was found in the case of electron-poor aldehydes. The high levels of diastereoselectivity and th

Full text of DOI:10.1039/c002988b

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Chiral sulfoxides as activators of allyl trichlorosilanes in the stereoselective
allylation of aldehydes†
Vincenzo De Sio, Antonio Massa* and Arrigo Scettri*
Received 15th February 2010, Accepted 20th April 2010
First published as an Advance Article on the web 17th May 2010
DOI: 10.1039/c002988b
Chiral aryl methyl sulfoxides proved to be efficient activators in the asymmetric allylation of aldehydes
with allyl trichlorosilanes. High enantioselectivity was found in the case of electron-poor aldehydes. The
high levels of diastereoselectivity and the detection of nonlinear effects have allowed the elucidation of
some mechanistic aspects of the reaction.
More conveniently, the easy access to a novel chiral tetradentate
bis-sulfoxide has been exploited for the achievement of a new
Within the last two decades, asymmetric organocatalysis based
procedure characterized by a much lower activator loading
Introduction
(0.2–0.3 eq.) and significantly increased yields and e.e.s.14
Furthermore, taking advantage of our previous report, Liao
et al. examined the catalytic activity of a series of mono- and
bidentate aryl t-butyl sulfoxides;15 it has to be noted that moderate
to high yields and enantioselectivity could again be attained in
the presence of a large amount of Lewis bases (3 eq.) and allyl
trichlorosilane (up to 3 eq.), provided that an a-benzyloxy-phenyl
t-butyl sulfoxide was used as an activator. A mechanistic inves-
tigation indicated that the activation took place by coordination
of only one molecule of aryl t-butyl sulfoxide to the silicon atom,
resulting in the formation of a pentacoordinate Si intermediate in
the stereodetermining step.15
Stimulated by our previous results, the catalytic properties of a
variety of easily available sulfoxides were submitted to an extensive
investigation leading to the achievement of a very satisfactory
procedure for the stereoselective allylation of aldehydes and,
furthermore, to the elucidation of some mechanistic aspects
concerning the effective catalytic species.
on the activation of suitable organosilanes with neutral Lewis
bases has attracted the interest of many research groups.1 One
of the most investigated reactions is represented by the allylation
of carbonyl compounds with allyl trichlorosilane to obtain chiral
homoallylic alcohols.2 In this context, a variety of chiral catalysts,
such as formamides,3 phosphoramides,4 pyridine N-oxides5 and
phosphines oxides,6 have been synthesized allowing the achieve-
ment of efficient and stereoselective procedures.
From a mechanistic point of view, the activation of the allylating
agent by strong Lewis bases was attributed to the formation of
a hypervalent silicon compound capable of reacting with suitable
electrophiles.1,7 Furthermore, the high levels of diastereoselectivity,
often observed with substituted allyl trichlorosilanes (such as
E- and Z-crotyl trichlorosilanes) proved to be in agreement with
a reaction path involving an allylating group transfer through a
cyclic six-membered transition state.8
Rather surprisingly, especially in consideration of their broad
employment in asymmetric synthesis both as chiral ligands and
auxiliaries, poor attention has been paid in the past years to
sulfoxides,9 typical neutral Lewis bases, as possible activators of
allyl trichlorosilanes in allylation reactions.10 In fact, only recently
Massa et al.11 reported an asymmetric procedure, based on the use
of chiral methyl p-tolyl sulfoxide, for the allylation of aldehydes
with allyl trichlorosilane. Nevertheless, a large amount of activator
was required (up to 3 eq.) and moderate yields and enantiomeric
excesses (e.e.s) were observed.
Results and discussion
In the early phase, commercially available racemic methyl p-tolyl
sulfoxide, rac-1, was used as an activator of the allylating agent
8 (Scheme 1) and benzaldehyde was chosen as the representative
substrate.
No significant improvement was obtained by the contemporary
Rowland’s procedure,12 which involved the use of sulfoxides
incorporating oxazoline moieties, since homoallylic alcohols were
isolated in good yields but rather low e.e.s. In the attempt to get a
heterogeneous catalyst, chiral sulfoxides were covalently bonded
to mesoporous silica (SBA-15):13 unfortunately, the resulting solid
catalyst exhibited poor efficiency and enantioselectivity.
Scheme 1
Under the conditions reported in Scheme 1 and Table 1 (entries
1–3) the activator loading proved to be the crucial factor: in fact,
3 eq. of rac-1 were required for the formation of the homoallylic
alcohol 9a with very high efficiency.
The procedure proved to be successful, with aromatic alde-
hydes bearing both electron-donor and electron-withdrawing
substituents (entries 4, 6 and 7), as well as heteroaromatic (entry 5),
a,b-unsaturated (entry 8) and aliphatic aldehydes (entry 9).
Dipartimento di Chimica, Universita` di Salerno, Via Ponte don Melillo,
84084 Fisciano, Salerno, Italy. E-mail: scettri@unisa.it; Fax: (+39)-089-
969603; Tel: (+39)-089-969583
† Electronic supplementary information (ESI) available: General experi-
mental procedures, HPLC, 1H and 13C NMR spectra of new compounds.
See DOI: 10.1039/c002988b
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Table 1 Allylation of RCHO with allyltrichlorosilane catalyzed by
racemic methyl p-tolyl sulfoxide
Table 2 Asymmetric allylation of RCHO with allyltrichlorosilane cat-
alyzed by chiral aryl methyl sulfoxides
Entry R (7)
rac-1(eq.)
t/h
9
Yield (%)a
Activatora
(equiv.)
Entry
R
9
Yield (%)b e.e. (%)c
1
Ph (7a)
1
2
3
3
3
3
3
3
3
3
3
8
8
8
18
4
18
18
18
18
18
28
9a
9a
9a
9b
9c
9d
9e
9f
49
84
99
78
92
64
79
81
80
94
71c
2
Ph (7a)
1
Ph (7a)
R-1 (1)
R-1 (2)
R-1 (3)
R-1 (3)
R-1 (3)
R-2 (3)
R-2 (3)
R-3 (3)
R-3 (3)
R-4 (3)
R-4 (3)
R-6 (3)
9a 59
9a 89
9a 99
9b 78
9c 99
9a 91
9c 97
9a 95
9c 99
9a 91
9c 95
9c 94
56
58
59
53
83
50
85
47
81
50
82
84
3
Ph (7a)
2
Ph (7a)
4
4-MeOC6H4 (7b)
5-NO2-2-Furyl (7c)
4-CNC6H4 (7d)
2-MeOC6H4 (7e)
3
Ph (7a)
5
4
4-MeOC6H4 (7b)
5-NO2-2-Furyl (7c)
Ph (7a)
6
5
7
6
=
8
PhCH CH (7f)
7
5-NO2-2-Furyl (7c)
Ph (7a)
5-NO2-2-Furyl (7c)
Ph (7a)
5-NO2-2-Furyl (7c)
5-NO2-2-Furyl (7c)
9
PhCH2CH2 (7g)
Ph (7a)
9g
9a
9h
8
10b
11
9
Ph(CH3)CHCHO (7h)
10
11
12
a All the yields refer to isolated chromatographically pure compounds
whose structures were confirmed by analytical and spectroscopic data.
b The reaction was carried out by using 3 equiv. of DMSO. c d.r. 65/35,
determined by 1H-NMR and 13C-NMR analysis on the crude reaction
mixture.
◦
a R-5 is not soluble in CH2Cl2 at -78 C. b All the yields refer to isolated
chromatographically pure compounds whose structures were confirmed
by analytical and spectroscopic data. c e.e.s have been determined by chiral
HPLC.
Interestingly, the steric hindrance in proximity of the reaction
site, as in entry 11, did not prevent the formation of the
homoallylic alcohol 9h occurring (71% yield) although a poor
diastereoselectivity was observed (65/35 d.r.). Furthermore, more
prolonged reaction times were required by using dimethyl sulfoxide
as activator (entry 10).
Our efforts were then focused on the achievement of the
asymmetric version of the allylation procedure and, therefore,
the influence exerted by a set of chiral activators was examined
(Scheme 2). It has to be noted that, while methyl p-tolyl sulfoxide
is commercially available in both the enantiomerically pure forms,
highly enantioenriched sulfoxides 2–6 were synthesized by a
catalytic enantioselective oxidation of the corresponding sulfides
performed with a very convenient, recent, modification of the
original Modena’s protocol16 and directly used in the optical purity
indicated in Fig. 1.
efficiency of the process on the amount of activator, while,
conversely, only moderate, comparable e.e.s were usually observed
(entries 1–3, Table 2). More interestingly, the electronic properties
of the used aldehydes played a determining role as regards the
stereochemical aspects. In fact, while the presence of an electron-
donor in the para position of the aromatic ring (entry 4) caused
a slight decrease in the level of enantioselectivity with respect
to entry 3, the reaction on the electron-poor aldehyde 7c (entry
5) took place in a much more satisfactory way, leading to the
homoallylic alcohol 9c in 99% yield and significantly enhanced
e.e. (83% e.e.). The same difference in the stereochemical outcome
could be appreciated by using the more Lewis basic sulfoxides
R-2 (entries 6 and 7) and R-3 (entries 8 and 9) as well as
the more hindered R-4 (entries 10 and 11). Furthermore, the
significantly lower Lewis basicity, deriving from the presence of
a nitro group in the para position, as in R-5, can be considered
responsible of the complete failure of the procedure both on
aldehyde 7a and 7c. Conversely, a milder electron-withdrawing
group, although situated in proximity of the sulfoxide moiety, such
as the carbomethoxy substituent in R-6, did not seem to affect the
preparative and stereochemical outcome (entry 12).
Scheme 2
On the grounds of the results reported in Table 2, most of the
chiral sulfoxides used proved to be employable in the asymmetric
allylation of aldehydes with allyl trichlorosilane, affording the
homoallylic alcohols in not significantly different yields and e.e.s.
Consequently, the scope of the protocol was checked by using
the commercially available and relatively cheap methyl p-tolyl
sulfoxides R-1 and S-1.
Therefore, a variety of aldehydes were submitted to treatment
with 8 in the presence of R-1 under the conditions reported
in Scheme 3 and Table 3. As regards aromatic substrates, very
high yields were usually observed, while electron-poor aldehy-
des afforded the best results from the stereochemical point of
view. Particularly, the presence of a strong electron-withdrawing
substituent on the aromatic (entries 2, 3) and heteroaromatic
ring (entries 6, 7 and 9, 10) resulted in the highest e.e.s (up to
86% e.e.). Interestingly, the steric hindrance caused by the –NO2
group situated in proximity of the reaction site, as in 7j, did not
affect its reactivity, although a slightly lower enantioselectivity
Fig. 1 Structures of aryl methyl sulfoxides.
The experiments performed on benzaldehyde 7a in the presence
of R-1, as Lewis base, confirmed the strict dependence of the
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Table 3 Asymmetric allylation of aldehydes with allyltrichlorosilane
catalyzed by chiral methyl p-tolyl sulfoxide
Table 4 Asymmetric allylation of aldehydes with allyl trichlorosilane
catalyzed by chiral methyl p-tolyl sulfoxide in the presence of (n-Bu)4N+I-
Entry
R
9
Yield (%)a
e.e. (%)b
Entry
R
9
Yield (%)a
e.e. (%)b
1
Ph (7a)
S-9a
S-9i
99
97
97
83
96
99
99
65
80
80
78
72
78
59
82
72
66
62
83
-86
67
82
-84
53
65
72
1
2
3
4
5
6c
Ph (7a)
S-9a
S-9c
S-9d
S-9n
R-9g
S-9g
99
99
76
87
80
82
60
90
72
85
74
-76
2
4-NO2C6H4 (7i)
2-NO2C6H4 (7j)
4-CF3C6H4 (7k)
4-FC6H4 (7l)
5-NO2-2-Furyl (7c)
4-CNC6H4 (7d
5-NO2-2-Thienyl (7m)
PhCH2CH2 (7g)
PhCH2CH2 (7g)
3
S-9j
4
S-9k
S-9l
5
6
5-NO2-2-Furyl (7c)
5-NO2-2-Furyl (7c)
4-CNC6H4 (7d)
5-NO2-2-Thienyl (7m)
5-NO2-2-Thienyl (7m)
4-MeOC6H4(7b)
4-MeSC6H4(7n)
PhCH2CH2 (7g)
S-9c
R-9c
S-9d
S-9m
R-9m
S-9b
S-9n
R-9g
7c
8
a All the yields refer to isolated chromatographically pure compounds
whose structure were confirmed by analytical and spectroscopic data. b e.e.s
have been determined by chiral HPLC. c The reaction was performed by
using S-1.
9
10c
11
12
13
Table 5 Allylation of R-citronellal 10 with allyltrichlorosilane catalyzed
by methyl p-tolyl sulfoxide
a All the yields refer to isolated chromatographically pure compounds
whose structures were confirmed by analytical and spectroscopic data.
b e.e.s have been determined by chiral HPLC. c The reactions were
performed by using S-1
Entry
1
Yield (%)a
(4S,6R)-11/(4R,6R)-12 d.r.b
1
2
3
rac-1
R-1
S-1
57
63
55
58/42
33/67
75/25
a The values reported refer to the overall yield of the diastereoisomeric
mixture, whose efficient separation by chromatography proved to be
unsuccessful in spite of several attempts. b Diastereoisomeric ratios were
determined by1H-NMR and 13C-NMR on the crude mixtures.
Scheme 3
was observed in comparison to the isomeric 4-nitro-derivative 7i
(compare entries 2 and 3).
Indeed, although no modification of the reaction rate was
caused by the additive, the formation of alcohols 9 from aromatic
(entries 1, 3), hetero-aromatic (entries 2, 4) and aliphatic aldehydes
(entries 5, 6) was usually found to occur with comparable yields
and, most of all, slightly enhanced e.e.s (~5%).
Some experiments, performed on the commercially available R-
citronellal 10 and reported in Scheme 5 on Table 5, pointed out
that the stereochemical outcome could be notably affected by the
b-situated stereogenic center.
Furthermore, as regards electron-rich aldehydes 7b and 7n, the
experiments reported in entries 11 and 12, confirmed the notable
influence exerted by the electronic effects of the substituents.
In fact, the attainment of higher e.e.s for substrate 7n can be
reasonably explained with the lower overall electron-donor effect
of the MeS– group with respect to the MeO– group. As is
well known, most of the protocols available for the allylation of
aliphatic aldehydes have often given rather unsatisfactory results
leading to the corresponding homoallylic alcohols in modest
yields and/or e.e.s. To our delight, under the usual conditions,
3-phenyl propionaldehyde 7g was converted into product 9g in
78% yield and 72% e.e., which can be considered among the best
values reported for allylation with allyl trichlorosilane promoted
by mono- and bidentate Lewis bases.
The possibility to recover R-1 in almost quantitative yield and
unchanged optical purity by silica gel column chromatography
was conveniently exploited by using the same sulfoxide batch
more times.
Taking advantage of a recent report pointing out the beneficial
effect exerted by the use of tetrabutylammonium salts on the
rate of allylation of aldehydes promoted by O-donor ligands,17
the improvement of our protocol was attempted by performing
the reaction in the presence of (n-Bu)4N+I- under the conditions
depicted in Scheme 4 and Table 4.
Scheme 5
The usual treatment with allyl trichlorosilane in the presence
of rac-1 furnished the allylation product in overall 57% yield and
poor diastereoisomeric ratio (58/42 dr).
The employment of enantiopure R-1 and S-1 resulted in the
increase of diastereoselectivity and, rather interestingly, reverse
diastereoisomeric ratios in entries 2 and 3. The separation of the
diastereoisomeric mixtures by chromatographic techniques proved
to be very problematic and only in the case of entry 2 a very
laborious silica gel chromatography allowed the isolation of the
less abundant diastereoisomer, as pure compound, in rather low
1
yield (10%). H NMR analysis (400 MHz) of the corresponding
R-(-)- and S-(+)-a-methoxy-(trifluoromethyl)phenylacetic acid
esters allowed the assignment of the 4S, 6R configuration to the
less abundant diastereoisomer 11. The consequent attainment of
the predominant product as the (4R,6R)-12 isomer disclosed the
Scheme 4
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Table 6 Asymmetric allylation of aldehydes with differently substituted
allyltrichlorosilanes catalyzed by chiral methyl p-tolyl sulfoxide in presence
of (n-Bu)4N+I-
deep influence exerted by the stereogenic center, as emphasized by
the reversal of the usual sense of asymmetric induction.
Successively, in order to broaden the scope of the protocol, an
investigation was aimed to evaluate the reactivity of substituted
allyl trichlorosilanes of type 13–15, Scheme 6, easily accessible by
a known procedure4i,5f as predominantly E geometrical isomers.
Preliminarily, the feasibility of the reaction was checked on ben-
zaldehyde 7a by using dimethyl sulfoxide and rac-1 as activators,
and E-crotyl silane, 13, as allylating agent. Rather satisfactorily,
the formation of the corresponding alcohol 16a took place in
rather good yields and very high diastereoselectivity in both cases
(entries 1 and 2, Table 6). The achievement of the asymmetric
version was then attempted under the optimized conditions of
Table 4 and, to our delight, the crotylation occurred with good
to high yields, high diastereoselectivity and moderate to high e.e.s
(up to 87%) with aliphatic aldehydes (entries 12, 13), as well as
hetero-aromatic (entries 4, 5 and 8, 9) and, most importantly, both
electron-rich and electron-poor aldehydes (entries 3, 6, 7, 10, 11). It
has to be noted that the enhanced steric hindrance in proximity of
the reaction site of ethyl- and phenyl-substituted trichlorosilanes
E-14 and E-15 did not affect either the level of diastereoselectivity
or enantioselectivity of the process, although a slight lowering of
efficiency was usually observed (respectively, entries 14–19 and
20–25). From a mechanistic point of view, the results reported in
Table 6 are in complete agreement with the typical reaction path,
already proposed for related Lewis bases, involving closed, chair-
like transition states of type A and B characterized by the presence
of hypervalent penta- or hexacoordinated silicon species (Fig. 2).8
Entry R1
R2 Prod. Yield (%)a
e.e. (%)b d.r.c
1d
Ph
Ph
Ph
Me 16a
Me 16a
Me 16a
Me 16b
Me 16b
Me 16c
Me 16c
Me 16d
Me 16d
Me 16e
Me 16e
Me 16f
Me 16f
Et 17a
Et 17a
Et 17b
Et 17b
Et 17c
Et 17c
Ph 18a
Ph 18a
Ph 18b
Ph 18b
Ph 18c
Ph 18c
81
84
88
70
75
71
79
77
81
67
73
67
69
71
80
78
77
58
58
61
64
68
69
65
67
—
—
68
87
-87
70
-77
82
-84
58
-60
64
-64
72
-72
86
-85
74
-80
68
-70
97
-97
66
-66
3 : 97
3 : 97
3 : 97
3 : 97
3 : 97
3 : 97
3 : 97
3 : 97
3 : 97
3 : 97
3 : 97
3 : 97
3 : 97
5 : 95
5 : 95
5 : 95
5 : 95
5 : 95
5 : 95
<1 : 99
<1 : 99
<1 : 99
<1 : 99
<1 : 99
<1 : 99
2e
3f
4f
5-NO2-2-Furyl
5-NO2-2-Furyl
4-MeOC6H4
4-MeOC6H4
5-NO2-2-Thien.
5-NO2-2-Thien.
2-MeOC6H4
2-MeOC6H4
C6H5CH2CH2
C6H5CH2CH2
Ph
5g
6f
7g
8f
9g
10f
11g
12f
13g
14f
15g
16f
17g
18f
19g
20f
21g
22f
23g
24f
25g
Ph
5-NO2-2-Furyl
5-NO2-2-Furyl
4-MeOC6H4
4-MeOC6H4
Ph
Ph
5-NO2-2-Furyl
5-NO2-2-Furyl
4-MeOC6H4
4-MeOC6H4
a All the yields refer to isolated chromatographically pure compounds
whose structures were confirmed by analytical and spectroscopic data.
b e.e.s have been determined by chiral HPLC and they are referred to
the major diastereoisomer. c Diastereoisomeric ratios are referred to as
syn/anti diastereisomers. d The reaction was carried out using 3 equiv.
of DMSO. e The reaction was performed using rac-1. f The reaction was
carried out using R-1 as activator. g The reaction was carried out by using
S-1 as activator.
the recognition of the effective catalytic species. Consequently,
benzaldehyde was submitted to allylation under the typical
conditions in the presence of variously enantioenriched R-1 and,
differently from Liao’s15 finding, a slightly positive deviation was
observed (Fig. 3).
Scheme 6
Fig. 3
A more convincing result was obtained by reacting the het-
eroaromatic aldehyde 7c with allyltrichlorosilane under the same
conditions as benzaldehyde. In fact, as depicted in Fig. 4, a more
significant positive deviation could be observed.
Therefore, these results can be reasonably considered to be
in agreement with a chair-like transition state involving the
Fig. 2
In the past two decades the detection of non-linear effects
(NLE)18 has proven to be a powerful tool for the elucidation of
mechanistic aspects of an asymmetric process, especially as regards
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Conclusions
An easy and general procedure for the attainment of enan-
tioenriched homoallylic alcohols by the allylation of aldehydes
with allyl trichlorosilanes was based on the use of methyl aryl
sulfoxides as activators. Very high levels of enantioselectivity
could be observed for aromatic and heteroaromatic aldehydes
having a strong electron-withdrawing substituent. The use of
tetrabutylammonium iodide, as additive, resulted in an appreciable
increase of the yields and e.e.s. Furthermore, the protocol was
successfully employed in the case of substituted allyl trichlorosi-
lanes, so that the corresponding alcohols could be isolated in high
diastereoselectivity and moderate to high enantioselectivity (up to
97% e.e.). Finally, as regards the mechanistic aspects, the detection
of (+)-NLE supported the involvement of the coordination of
two sulfoxide molecules to the silicon atom in a close chair-like
transition state.
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financial support.
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