Organocatalytic addition on l,2-bis(sulfone)vinylenes leading to an unprecedented rearrangement

Article abstract of DOI:10.1002/chem.200901535

A study was conducted to demonstrate the use of 1,2-bis(sulfone)vinylenes in enamine catalysis. It was revealed that these sulfones led to a new rearrangement arising from a sulfone shift to the adjacent carbon atom. Investigations also revealed that the use of cheaper (Z)-1,2-bis(sulfone) vinylene lead to full conversion after a short reaction time in dioxane, using H₂O as co-catalyst to accelerate the reaction. A careful NMR investigation indicated that the new product was gem-disulfone, which was supported by authentic sample.

Full text of DOI:10.1002/chem.200901535

COMMUNICATION
DOI: 10.1002/chem.200901535
Organocatalytic Addition on 1,2-Bis
ACHTUNGTRNE(NUNG sulfone)vinylenes Leading to an
Unprecedented Rearrangement
Adrien Quintard and Alexandre Alexakis*[a]
The last decade has seen a rapid and fruitful growth of or-
Herein we report on the use of those 1,2-bis-
ACHTUNGTREN(NUGN sulfone)vinylenes in enamine catalysis. In an unexpected
ganocatalysis. By allowing new modes of molecular activa-
tion, in often mild and non-anhydrous conditions, these or-
ganocatalysed reactions appear as a method of choice for its
future use in industry.[1] Among the numerous catalytic acti-
vation modes, aminocatalysis, one of the most extensively
studied has lead to numerous methodologies notably de-
scribing the asymmetric a-functionalization of carbonyl
compounds. A large part of the research on this subject has
been carried out with the aim to discover new nucleophile/
electrophile combinations notably by conjugated addition
that leads to the discovery of valuable synthons, and in few
steps to natural products precursors.[2] Despite this impres-
sive progress, the enantioselective direct a-alkylation of al-
dehydes and ketones remains a challenging task. Few re-
ports have appeared recently on this subject and they still
remain limited to specific substrates.[3] Another alternative
method toward this a-alkylation has been developed in our
group; this method makes use of the Michael addition on
manner, these sulfones lead to a new rearrangement arising
from a sulfone shift to the adjacent carbon atom. This quite
general and impressively fast rearrangement constitutes an
indirect practical a-alkylation of either aldehydes or
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
compound, a powerful dienophile, did not react with n-va-
leraldehyde in the presence of pyrrolidine to give the ex-
pected Michael adduct 4.[4a,c] However, a close scrutiny of
the reaction mixture showed us that compound 3, resulting
from sulfinic acid elimination was formed (Scheme 1).[6a] In-
terestingly, use of the cheaper (Z)-1,2-bisACTHNUTRGNE(NUG sulfone)vinylene 6
lead to full conversion after a short reaction time in dioxane,
using H2O as co-catalyst to accelerate the reaction.[6b] This
difference between E and Z isomers seemed intriguing,
since Z Michael acceptors are known to isomerize to the
thermodynamically more favoured E isomer in the presence
of amine catalysts.[7] More surprisingly, instead of the ex-
pected Michael adduct 4, a new unexpected product 7 ap-
peared, together with the elimination product 3. A careful
NMR study indicated that this new product was gem-disul-
fone 7, and was corroborated by comparison with an au-
thentic sample[4c] (Scheme 1).
bis-activated substrates, such as 1,1-bisACTHNUTRGNE(NUG sulfone)vinylenes
and 1,1-bis(phosphonate)vinylenes.[4] Indeed, sulfones are
highly valuable functionalities due to the great versatility of
the sulfonyl group, which can undergo numerous transfor-
mations and can easily be cleaved at various synthetic
stages.[5] In view of expanding the range of these highly tun-
able products, we wondered about using more versatile Mi-
chael adducts. Since we had previously shown that a single
sulfone was not sufficient to undergo the enamine attack,
we wondered if a 1,2-bis
ACHTUNGTRENNUNG
{[(E/Z)-2(phenylsulfonyl)ethenyl]sulfonyl}benzene)
accelerate the reaction leading to two different tunable
functionalities.
[a] A. Quintard, Prof. Dr. A. Alexakis
Department of Organic Chemistry, University of Geneva
Quai Ernest Ansermet 30, 1211 Geneva (Switzerland)
Fax : (+41)22-379-32-15
E-mail: alexandre.alexakis@unige.ch
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901535.
Scheme 1. First trials on the Michael addition on 1,2-bis-
AHCTUNGTREG(NNUN sulfone)vinylenes.
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The presence of this product 7 coming from an unknown
rearrangement was unexpected and it intrigued us. Indeed,
to the best of our knowledge, such sulfone rearrangement
had never been observed previously and could lead to inter-
esting gem-disulfone adducts. This lead us to undergo a
complete study on this reaction by starting optimising the
reaction conditions in order to obtain a complete selectivity
in favour of the rearranged product (Table 1).
this catalyst, the cleanest reaction (68% conversion, 40%
NMR spectroscopic yield and 88% ee in 8 h) was in water.
However, this reactivity was limited to this example, since
no reaction at all was observed with isovaleraldehyde as
donor. All these disappointing results lead us to turn our at-
tention to the use of our recently developed aminal–pyrroli-
dine catalysts, which already showed excellent reactivity on
1,1-bisACHTUNGTRNEUNG
(sulfone)vinylenes.[11] To our delight, the three already
published catalysts 9a–9c lead to a cleaner reaction in an
impressive short reaction time (entries 5–7). A short solvent
screening using 10 mol% of the best catalyst 9a and de-
creasing the amount of aldehyde to five equivalents, indicat-
ed that the reaction performed best in toluene or chloro-
form with less then 20% self-aldol product formed (entries 8
and 9). Full conversion was obtained and the elimination
compound 3 was observed in less then 15% using these sol-
vents (entries 8–10).[12] Finally, decreasing the temperature
to À108C considerably slowed down the reaction, while in-
creasing the ee to 81%. To keep an excellent reactivity, a
temperature of 158C was then used in the reaction and
other aldehydes were tested to confirm the generality of this
reaction (Table 2).
Table 1. Catalysts and conditions screening for the addition of aldehyde
to 1,2 bis sulfone 6.[a]
t [h] Conv[b] [%] Yield[c] [%] ee[d] [%]
Interestingly, using catalyst 9a, this rearrangement was
observed with different Michael donors. Indeed, by using
different linear aldehydes, good yields and enantioselectivi-
ties could be obtained in an impressively short time. The
more bulky the aldehyde, the better the yield and enantiose-
lectivity. Indeed, using iPr or tBu as R1 groups, no traces of
the elimination product was observed, while ee up to 87%
could be reached (entries 4 and 5). In contrast, when using
propionaldehyde as Michael donor (R1 =Me), poor conver-
sions were obtained, probably due to the formation of a
large amount of sulfinic acid by the elimination pathway
(result not shown in the table). Changing the sulfone group
to SO2Me instead of SO2Ph also lead to an increased reac-
tivity and excellent yield, while lower ee was observed. This
lower enantioselectivity indicates that the more bulky
SO2Ph must interact strongly with the catalyst in the transi-
tion state, leading to the higher ee. In contrast, the reaction
with the SO2tBu group lead to a slow reaction rate and to a
messy mixture (entry 7). Finally, this process is equally effi-
cient, in terms of yields, when an a-disubstituted aldehyde is
used, while leading to lower enantioselectivity (entry 8). Al-
though low (ee=30%), the ee is still higher then the one al-
Cat Solvent
1
2
3
4
5
6
7
8
9
8a
8b
8c
8d
9a
9b
9c
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
7
24
3
9
3
45
<10
100
25
100
100
100
100
100
100
traces
nd
32
nd
56
26
36
84
83 (71)
77 (49)
92
nd
0
nd
70
43
64
72
70
81
3.5
3
6
4
9a[e] CHCl3
9a[e] toluene
10 9a[e] toluene (À108C) 45
[a] Reactions were performed by using catalyst (20 mol%) and aldehyde
(10 equiv) with bis(sulfone)vinylene (0.1 mmol) in solvent (0.2 mL).
ACHTUNGTRENNUNG
When using 1,4-dioxane as solvent, three equivalents of water as additive
was needed to allow the reaction. [b] Determined by 1H NMR spectros-
1
copy. [c] Determined by H NMR spectroscopy. Isolated yields are shown
in brackets. nd= not determined. [d] Determined by super fluid chroma-
tography. [e] Catalyst (10 mol%) and aldehyde (5 equiv) were used.
The first attempts using commercially available catalysts
were rather disappointing. Indeed, only catalyst 8c lead to
complete conversion, but, unfortunately, gave a racemate
(entry 3). In contrast, diphenylprolinol silyl ether 8a, usually
a powerful catalyst for Michael additions,[8] showed excellent
enantiocontrol (92% ee), but only 45% conversion
(entry 1). Furthermore, with all these catalysts, the reaction
was really messy, leading to many byproducts (such as the
product arising from the sulfinic acid elimination 3). The ab-
solute configuration of the rearranged product was ascribed
by comparison with known compounds arising from the
ready observed in the case of 1,1-bis
Surprisingly, when using the E starting material 2 in DMF
(in order to increase the solubility of bis(sulfone)vinylene),
87% of the Michael adduct 7g could be obtained (entry 9).
The Michael addition of ketones to bis(sulfone)vinylenes
(sulfone)vinylene.[4c]
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
represents an even more challenging task in enamine cataly-
sis. Indeed, using catalyst 9a, the addition of cyclohexanone
A
E
to 1,1-bis
mono- and di-addition product and product arising from the
degradation of the bis(sulfone)vinylene, with poor enantio-
control (Scheme 2). Only recently, Lu et al. described the
addition of six-membered ring ketones to this disulfone,
using a primary amine cinchonidine derivative.[13]
ACHTUNGTREN(NUNG sulfone)vinylene leads to a complex mixture of
in terms of chemical yield failed using catalyst 8a.[9] The use
of chloroform, acidic additives in water (PhCOOH, AcOH),
or 5% ethanol in water as recently used in our group,[10]
lead to no, or dirty, reaction (result not presented). With
ACHTUNGTRENNUNG
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Organocatalysis
COMMUNICATION
Table 2. Addition of aldehydes to 1,2-bis
G
Table 3. Addition of ketones to 1,2-bisACTHNGUTERNNUG
(sulfone)vinylenes 6.[a]
Product
t [h] d.r. (syn/anti)[b] Yield [%][c] ee[d] [%]
R
R1, R2
Product 9a
t
Yield[b] ee[c]
[mol%] [h] [%]
[%]
1
2
9
–
66
69
1[d] Ph nPr, H
7a
7b
10
15
45 49
81
78
2
3
Ph C7H15, H
Ph
3
72
5.5
1.85:1 (1.5:1)[e]
61 (63)[e]
71 (68)[e]
7c
15
3
59
76
4
5
Ph iPr, H
Ph tBu, H
7d
7e
7 f
–
7g
7g
20
20
15
20
20
20
6
9
2
86
89
92
82
87
45
nr
30
28
6[e] Me iPr, H
7[f] tBu nPr, H
8[f] Ph Et, Me
9[f,g] Ph Et, Me
3
9
3.76:1
67
73
24 nr
17 76
14 87
[a] Reactions were performed with bisACTHNUGTRNEUNG(sulfone)vinylene (0.2 mmol) in
[a] Reactions were performed with bisACTHNUGTRNEUNG(sulfone)vinylene (0.2 mmol) in
solvent (0.4 mL). [b] Determined by 1H NMR spectroscopy. [c] Isolated
yields. [d] Determined by super fluid chromatography. [e] Result ob-
tained with catalyst 8c are shown in brackets.
solvent (0.4 mL). [b] Isolated yields; nr=no reaction. [c] Determined by
super fluid chromatography. [d] Reaction performed at À108C in toluene.
[e] Reaction performed with bisACHTUNTRGNEU(GN sulfone)vinylene (0.3 mmol). The enan-
tiomeric excess was determined by derivatisation to the corresponding
imidazolidine (see the Supporting Information). [f] Reaction performed
at room temperature (258C). The results are obtained without reduction
to the alcohol [g] The E starting bisACTHNUTRGNEUGN(sulfone)vinylene 2 was used in DMF
as solvent.
We were therefore interested to know if our methodology
could be a valuable alternative for this addition of ketones
Scheme 3. Observed rearrangement using Bronsted base catalysis
to bis
in Table 3. Gratifyingly, using more polar solvents as DMF,
1,2-bis(sulfone)vinylene 6 did undergo the expected reaction
ACHTUNGTRENNUNG(sulfone)vinylenes. Preliminary results are summarized
ACHTUNGTRENNUNG
In view of the broad generali-
ty of the observed rearrange-
ment, a short mechanistic study
was undergone to better under-
stand what was really going on
in this reaction. Thus, several
observations were made and
are summarized in Scheme 4.
The various results (both in
terms of reactivity and enantio-
Scheme 2. Addition of cyclohexanone to 1,1-bisACHTNUTRGNEUNG(sulfone)vinylene
selectivity)
(sulfone)vinylene and 1,1-bis(sulfone)vinylene, and the ab-
sence of any trace of the 1,1-bis(sulfone)vinylene, while
monitoring the reaction by NMR spectroscopy, let us think
that the rearrangement must occur after the enamine attack.
This was further confirmed by the fact that mixing the cata-
lyst and the 1,2-bisACTHNUTRGNEUNG(sulfone)vinylene without aldehyde leads
to an unidentifiable mixture of products that do not react
with isovaleraldehyde. Furthermore, preformed enamine 15
using
1,2-bis-
in a fast and efficient way. The rearranged product could be
obtained in good yield and moderate enantioselectivity.
Commercially available catalyst 8c also gave similar levels
of yield and ee in this reaction (entry 2).
Finally, in a first test, the reaction of diethyl malonate
with sulfone 6 confirmed that this rearrangement could be
generalized to other nucleophiles (Scheme 3). Indeed, in the
presence of DBU as catalytic base, compound 14 was
formed in 41% yield after 2 h. A control experiment by
mixing a catalytic amount of DBU with the (Z)-sulfone 6
lead to the formation of the E isomer after 24 h, without
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
reacted with the 1,2-bis
ACHTUNGTRENN(UNG sulfone)vinylene, rapidly, affording
a
mixture of compounds 7d and elimination product
(Scheme 4a). An experience of anion trapping in deuterated
water lead to a mixture of the three different deuterated
compounds 16, 17, and 18 (Scheme 4b). We hypothesize
any traces of 1,1-bis
ACHTUNGTRENN(UNG sulfone)vinylene. Furthermore, this E
isomer did not react under these reaction conditions.
Chem. Eur. J. 2009, 15, 11109 – 11113
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11111

A. Alexakis and A. Quintard
by a Michael addition, or by a
[2+2] cycloaddition as already
described on the addition to di-
ethyl fumarate.[14] This [2+2]
cycloaddition is consistent with
the observed high reactivity.
Cyclobutane-opening by water
would then lead to the formal
Michael adducts. Depending on
the substrate, two possibilities
appear with this anion 23 de-
pending on the relative confor-
mation of the sulfone and the
adjacent anion. If the anion is
placed antiperiplanar to the ad-
jacent sulfone, the direct sulfin-
ic acid elimination through
path b leads to the elimination
product 24 and catalyst deacti-
vation. If the anion is placed
syn to the adjacent sulfone, we
postulate that the only possible
reaction is the attack of this
anion on the sulfur atom lead-
ing to the unstable species 26,
which instantaneously rearrang-
es to compound 27. Further fast
reprotonation leads to the final
rearranged compound. This in-
Scheme 4. Mechanistic observations
stantaneous
reprotonation
that these three compounds
may come from the same inter-
mediate anion 19. A control ex-
periment
addition
indicated
the
that
1,1-bis-
to
ACHTUNGTRENNUNG(sulfone)vinylene lead to the
single deuterated compound 18
coming from the anion located
on the carbon bearing the two
sulfonyl groups. The formation
of this anion provides good in-
formation in terms of mecha-
nism, but should not be applied
as a model in terms of stereose-
lection, since lower ee values
are obtained in this solvent.
All these results clearly show
that the 1,2-bisACHTUNGTRENNUNG(sulfone)vinylene
reacts directly with the nucleo-
phile forming an ionic inter-
mediate, which then undergoes
the rearrangement. This lead us
to postulate a plausible mecha-
nism presented in Scheme 5. In
a first part, the enamine reacts
Scheme 5. Possible mechanism and explanation of the observed selectivity between rearranged and elimination
on the 1,2-bis
ACHTUNGTRENNUNG(sulfone)vinylene product.
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Chem. Eur. J. 2009, 15, 11109 – 11113

Organocatalysis
COMMUNICATION
of organocatalysis see: Chem. Rev. 2007, 107, 5413; h) D. W. C. Mac-
Millan, Nature 2008, 455, 304.
should prevent the isomerisation of the product and the
elimination of one of the two sulfone a to the anion.
[2] For selected reviews on aminocatalysis, see: a) B. List, Acc. Chem.
Res. 2004, 37, 548; b) B. List, Chem. Commun. 2006, 819; c) A. Erk-
kilꢁ, I. Majander, P. M. Pihko, Chem. Rev. 2007, 107, 5416; d) S. Mu-
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Chem. 2008, 120, 6232; Angew. Chem. Int. Ed. 2008, 47, 6138; for se-
lected reviews on organocatalysed 1,4-conjugate addition see:
f) S. B. Tsogoeva, Eur. J. Org. Chem. 2007, 1701; g) S. Sulzer-Mossꢂ,
A. Alexakis, Chem. Commun. 2007, 3123; h) D. Almasi, A. Alonso,
C. Najera, Tetrahedron: Asymmetry 2007, 18, 299; i) J. L. Vicario, B.
Dolores, L. Carrillo, Synthesis 2007, 2065.
[3] For selected examples see: a) N. Vignola, B. List, J. Am. Chem. Soc.
2004, 126, 450; b) I. Ibrahem, A. Cordova, Angew. Chem. 2006, 118,
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1339; Angew. Chem. Int. Ed. 2009, 48, 1313.
To explain the fact that the elimination occurs or not de-
pending on the different substrates, we postulate that the
formed anion is pyramidal[15] and relatively long living. This
is consistent with the different results obtained using the
two starting materials E or Z. As depicted in Scheme 4c
and d, by using the same Michael donor in the case of the Z
isomer no elimination is observed, while this elimination
product is formed when using the E isomer. This is consis-
tent with the formation of a chiral anion as shown in
Scheme 5. Indeed, if one of the two isomers is formed
during the reaction, the nucleophile being bulky enough to
À
prevent the rotation around the C C bond, and if the inter-
conversion between the two diastereoisomers is sufficiently
slow, a perfect selectivity in favour of the rearranged prod-
uct is observed. This would indicate that the rearrangement
is faster then the isomerisation of the chiral anion. In the
case of the E isomer a poor diastereocontrol leads to an in-
creased amount of elimination product. In the case of less
hindered nucleophiles (as propionaldehyde), either a lower
[4] a) S. Sulzer-Mossꢂ, A. Alexakis, Org. Lett. 2005, 7, 4361; b) S.
Sulzer-Mossꢂ, M. Tissot, A. Alexakis, Org. Lett. 2007, 9, 3749; c) S.
Sulzer-Mossꢂ, A. Alexakis, J. Mareda, G. Bollot, G. Bernardinelli, Y.
Filinchuk, Chem. Eur. J. 2009, 15, 3204; for more recent examples
see: d) Q. Zhu, Y. Lu, Org. Lett. 2008, 10, 4803; e) A. Landa, M.
Maestro, C. Masdeu, A. Puente, S. Vera, M. Oiarbide, C. Palomo,
Chem. Eur. J. 2009, 15, 1562.
À
diastereoselection or the free rotation in the C C bond is
easier. This leads to a higher amount of the form able to un-
dergo the elimination.
[5] For selected reviews see: N. S. Simpkins, Sulphones in Organic Syn-
thesis, Pergamon, Oxford, 1993; b) C. Najera, M. Yus, Tetrahedron
1999, 55, 10547; c) E. N. Prilezhaeva, Russ. Chem. Rev. 2000, 69, 367.
[6] a) The elimination product was detected by 1H NMR spectroscopy
of the crude mixture by a doublet around d=4 ppm and a triplet
around d=6.5 ppm and also by ESIMS analysis. This type of elimi-
nation product is the commonly observed product of the addition of
À
In conclusion, by using cheap and easily available 1,2-bis-
ACHTUNGTRENNUNG(sulfone)vinylene, an unprecedented rearrangement consist-
ing in a sulfone shift leading to gem-disulfone has been ob-
served. Our newly designed aminal–pyrrolidine catalyst
gave impressively high reactivity in this transformation and
up to 87% ee could be obtained. This unprecedented rear-
rangement is quite general, as long as the nucleophile used
is sufficiently bulky, and as long as a good diastereocontrol
is obtained on the transient anion. It constitutes an interest-
ing alternative to the use of other more expensive bis-
nucleophile (C O nucleophile) to 1,2-bisACHTNUTGRNEUNG(sulfone)vinylene. It was al-
ready observed in the addition of preformed enamine to b-carbonyl-
a,b-unsaturated sulfones : F. Benedetti, S. Fabrissin, R. Fagotto, A.
Risaliti, Gaz. Chim. ital. 1990, 120, 613, and references herein;
b) the use of water is needed to co-catalyse the reaction in dioxane
(see the Supporting Information for details)—for beneficial role of
water see : A. Cꢃrdova, W. Zou, I. Ibrahem, E. Reyes, M. Engqvist,
W-W. Liao, Chem. Commun. 2005, 3586.
ACHTUNGTRENNUNG(sulfone)vinylenes in asymmetric synthesis notably in the
[7] a) O. Andrey, A. Alexakis, A. Tomassini, G. Bernardinelli, Adv.
Synth. Catal. 2004, 346, 1147; b) J. Wang, F. Yu, X. Zhang, D. Ma,
Org. Lett. 2008, 10, 2561.
[8] a) M. Marigo, T. C. Wabnitz, D. Fielenbach, K. A. Jorgensen,
Angew. Chem. 2005, 117, 804; Angew. Chem. Int. Ed. 2005, 44, 794;
b) Y. Hayashi, H. Gotoh, T. Hayashi, M. Shoji, Angew. Chem. 2005,
117, 4284; Angew. Chem. Int. Ed. 2005, 44, 4212; c) A. Mielgo, C.
Palomo, Chem. Asian J. 2008, 3, 922.
case of ketones and constitutes an indirect practical a-alky-
lation of carbonyl compounds. Studies are currently under-
way in our laboratory to expand the scope of such reactions.
Experimental Section
[9] See the Supporting Information for details.
[10] S. Belot, A. Massaro, A. Tenti, A. Mordini, A. Alexakis, Org. Lett.
2008, 10, 4557.
[11] A. Quintard, C. Bournaud, A. Alexakis, Chem. Eur. J. 2008, 14,
7504.
For information of the experimental details please see the Supporting In-
formation.
[12] The difficulty in separating the rearranged product from the elimi-
nation product on small scale explains the observed moderate
yields. A scale up reaction (502 mg of starting sulfone) using isova-
leraldehydes as donor and pyrrolidine as catalyst in toluene lead to
78% yield.
Keywords: asymmetric
organocatalysis · rearrangement · sulfones
synthesis
·
enamines
·
[13] Q. Zhu, L. Cheng, Y. Lu, Chem. Commun. 2008, 6315.
[14] K. C. Brannock, R. D. Burpitt, V. W. Goodlett, J. G. Thweatt, J. Org.
Chem. 1964, 29, 813.
[15] For pionnering finding on chiral stabilised sulfone anion see: E. J.
Corey, E. T. Kaiser, J. Am. Chem. Soc. 1961, 83, 490. For detailed
study see: J. S. Grossert, J. Hoyle, T. S. Cameron, S. P. Roe, B. V.
Vincent, Can. J. Chem. 1987, 65, 1407.
[1] For selected reviews on organocatalysis see : a) P. I. Dalko, L.
Moisan, Angew. Chem. 2004, 116, 5248; Angew. Chem. Int. Ed. 2004,
43, 5138; b) G. Guillena, D. J. Ramon, Tetrahedron: Asymmetry
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selective Organocatalysis, Wiley-VCH, Weinheim, 2007; f) H. Pelliss-
ier, Tetrahedron 2007, 63, 9267; g) special issue on different aspects
Received: June 5, 2009
Published online: September 29, 2009
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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