Novel organic catalysts for the direct enantioselective α-oxidation of carbonyl compounds

Article abstract of DOI:10.1016/j.tetlet.2005.03.085

The proline-derived N-sulfonylcarboxamide-catalyzed direct enantioselective α-oxidation of ketones and aldehydes with nitrosobenzene is presented. The reactions proceed smoothly furnishing the corresponding α-aminoxylated compounds in good yields with up to >99% ee. The proline-derived N-sulfonylcarboxamides were also found to be excellent catalysts for the direct enantioselective nitroso Diels-Alder-type reaction between nitrosobenzene and α,β-unsaturated cyclic ketones yielding the corresponding bicyclic Diels-Alder adduct products with up to >99% ee. The proline-derived N-sulfonylcarboxamides represent a readily available and highly modular novel type of organic catalyst.

Full text of DOI:10.1016/j.tetlet.2005.03.085

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 3385–3389
Novel organic catalysts for the direct enantioselective
a-oxidation of carbonyl compounds
*
Henrik Sunden, Nils Dahlin, Ismail Ibrahem, Hans Adolfsson and Armando Cordova
*
´
´
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
Received 26 December 2004; revised 9 March 2005; accepted 15 March 2005
Available online 2 April 2005
Abstract—The proline-derived N-sulfonylcarboxamide-catalyzed direct enantioselective a-oxidation of ketones and aldehydes with
nitrosobenzene is presented. The reactions proceed smoothly furnishing the corresponding a-aminoxylated compounds in good
yields with up to >99% ee. The proline-derived N-sulfonylcarboxamides were also found to be excellent catalysts for the direct enan-
tioselective nitroso Diels–Alder-type reaction between nitrosobenzene and a,b-unsaturated cyclic ketones yielding the corresponding
bicyclic Diels–Alder adduct products with up to >99% ee. The proline-derived N-sulfonylcarboxamides represent a readily available
and highly modular novel type of organic catalyst.
Ó 2005 Elsevier Ltd. All rights reserved.
Optically active a-hydroxy carbonyl moieties are com-
mon in numerous important natural products.1,2 This
has led to extensive research to find new diastereoselec-
tive and enantioselective routes for their syntheses.3 One
way of preparing these compounds is the asymmetric
a-hydroxylation of enolates employing chiral auxiliaries
or substrates.4 Recently, Momiyama and Yamamoto
reported an efficient catalytic system based on AgX/BI-
NAP-complexes that mediate indirect a-oxidation of
activated tin enolates with nitrosobenzene as the
electrophile.5
During studies on novel organic catalysts, we recently
found that N-sulfonyl-2-aminomethylpyrrolidines were
excellent catalysts for the direct enantioselective a-amina-
tion of aldehydes.11 The organic catalysts furnished
the desired products with good enantioselectivites, how-
ever, their preparation required five synthetic steps. In
contrast, proline-derived N-sulfonylcarboxamides could
be prepared in only two steps from commercially avail-
able N-Cbz protected p-nitrophenol proline esters
(Scheme 1) and potentially could have similar activities
as N-sulfonyl-2-aminomethylpyrrolidines. Based on
these facts and our interest in the development of organo-
catalytic reactions,12 we became interested in whether
proline-derived N-sulfonylcarboxamides would be able
to catalyze the direct, catalytic, enantioselective a-amin-
oxylation reaction. Herein, we disclose that proline-
derived N-sulfonylcarboxamides are excellent catalysts
for direct catalytic asymmetric a-oxidation and hetero
Diels–Alder reactions of carbonyl compounds with nitro-
sobenzene, yielding the corresponding a-aminoxylated
products with up to >99% ee.
Organocatalysis has experienced a renaissance in org-
anic chemistry.6 In this context, Zhong,7c MacMillan and
co-workers,7d Hayashi et al.7e,f,g and ourselves7a,b have
reported that amino acids and their derivatives catalyze
Yamamoto-type a-aminoxylation reactions with excel-
lent stereoselectivities.7 These initial reports were later
followed up by the excellent studies of Yamamoto and
co-workers,8 Blackmond and others.9 Furthermore, we
recently demonstrated that amino acids catalyze the bio-
mimetic, asymmetric, aerobic a-oxidation of aldehydes
and ketones.10
O
O
O
(i), (ii)
N
N
HN S R
O
O
H
Cbz
Keywords: Proline-derived N-sulfonylcarboxamides; Asymmetric cataly-
sis; Nitroso Diels–Alder; Ketones; Aldehydes.
NO2
*
Scheme 1. Two-step synthesis of proline-derived N-sulfonylcarb-
oxamides. Reagents and conditions: (i) 1.5 equiv NaH, 1.3 equiv
R–SO2NH2, DMF, rt; (ii) Pd(C), H2, MeOH.
Corresponding authors. Tel.: +46 8 162479; fax: +46 8 154908; e-mail
addresses: hansa@organ.su.se; acordova@organ.su.se; acordova1@
netscape.net
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.03.085

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H. Sunden et al. / Tetrahedron Letters 46 (2005) 3385–3389
3386
Table 1. Catalyst screen for the direct catalytic asymmetric a-oxidations of 2a with nitrosobenzenea
O
O
O
O
N
Catalyst
ONHPh PhHNO
+
ONHPh
(10-30 mol%)
+
DMSO, rt
1a
2a
3
O
O
O
O
S
O
S
O
S
N
H
N
HN
S
HN
N
N
HN
HN
H
H
H
O
O
O
O
TFA
4
5
6
7
N
HO
N
O
O
O
N
N
N
N
N
H
N
H
OH
H
H
N
OH
H2N
OH
H
TFA
8
9
10
12
11
Entry
Cat.
Time (h)
Prod.
Yield (%)b
ee (%)c
Prod.
Yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
4
2
2
2a
2a
55
52
>99
98
3
27
21
>99
99
5
3
6
16
18
3
2a
2a
Traces
18
70
n.d.
99
3
—
Traces
22
—
7
3
n.d.
>99
n.d.
>99
—
8
2a
2a
>99
>99
>99
n.d.
>99
3
9
16
3
67
66
3
Traces
20
—
10
11
12
2a
2a
3
16
16
Traces
5
3
ent-3
ent-2a
—
—
a To nitrosobenzene (1 mmol) in the presence of catalyst (10–30 mol %) in 2 mL of organic solvent was added ketone 1a (2 mmol). After vigorous
stirring at room temperature the reaction mixture was quenched by addition of brine and extraction with EtOAc. Subsequent purification utilizing
silica-gel chromatography afforded the a-aminoxylated ketones 2a and 3.
b Yield of the isolated pure ketone.
c The ee as determined by chiral-phase HPLC analyses.
In an initial catalyst screen of amino acid derivatives 4–
12, we found that N-methylsulfonylcarboxamide 4 was
an excellent catalyst for the reaction between cyclohexa-
none 1a (3 mmol) and nitrosobenzene (1 mmol) and
gave the corresponding a-aminoxylated ketone 2a and
a,a0-diaminoxylated ketone 3 in 55% and 27% yields
with >99% eeÕs, respectively (Table 1).13
selective catalyst and furnished 2a in 67% yield with
>99% ee. To our surprise organic catalyst 6 provided
only trace amounts of 2a, which was probably due to
the presence of TFA. This was further supported by
the ability of our previously reported catalyst 7, which
did not have a TFA additive, to mediate the a-aminoxyl-
ation reaction to yield 2a with >99% ee. Interestingly,
diamine 12 provided 2a with the opposite stereoselectiv-
ity as compared to all the other proline-derived catalysts.
Encouraged by the discovery that the highly modular
proline-derived N-alkyl- and arylsulfonylcarboxamide
Furthermore, the novel sulfonylcarboxamides 4, 5 and
tetrazole 10 were the most reactive catalysts followed
by proline.14 In addition, hydroxyproline was a highly
Table 2. The direct asymmetric a-oxidations of 1 with sulfonylcarboxamide 4 as the catalysta
O
O
O
O
N
Catalyst, 4
ONHPh PhHNO
+
ONHPh
(10 mol%)
+
DMSO, rt
1a
2a
3
Entry
Time (h)
Prod.
Yield (%)b
ee (%)c
>99
Prod.
Yield (%)b
ee (%)c
1
2
3
2
2
16e
2a
2a
2a
55
3
3
3
27
Traces
Traces
>99
n.d.
n.d.
80d
80d
>99
98
a Nitrosobenzene (1 mmol) in DMSO (1 mL) was slowly added via a syringe-pump to a solution of ketone 1a (2 mmol) in the presence of catalyst 4
(10 mol %) in DMSO (1 mL). After vigorous stirring at room temperature the reaction mixture was quenched by addition of brine and extraction
with EtOAc. Subsequent purification utilizing silica-gel chromatography afforded the a-aminoxylated ketones 2a and 3.
b Yield of the isolated pure ketone.
c The ee as determined by chiral-phase HPLC analyses.
d Slow addition of the nitrosobenzene.
e The reaction was performed in DMF at 4 °C.

´
H. Sunden et al. / Tetrahedron Letters 46 (2005) 3385–3389
3387
were excellent catalysts for the direct catalytic a-amin-
oxylation reaction, we decided to optimize the yield of
2a using 4 as the catalyst (Table 2).
ones could be a-oxidized with nitrosobenzene using or-
ganic catalysts (Table 4).8b,15 The reaction was success-
ful and sulfonylcarboxamides, proline and tetrazole 10
were able to catalyze the reaction between cyclohexe-
none 1e and nitrosobenzene to yield the corresponding
bicyclic product 2e with excellent stereoselectivity.
Methylsulfonylcarboxamide 4-catalyzed the tandem a-
aminoxylation/Michael reaction with the highest stereo-
selectivity and enantiomerically pure 2e was obtained
(entry 1). However, tetrazole catalyst 10 furnished 2e
in a higher yield as compared to catalyst 4 with 99%
ee (entry 3). The reaction was also run on a gram scale
with L-proline as the catalyst without decreasing the
enantioselectivity of the reaction.
We found that the yield of 2a was increased by slow
addition of the electrophile using a syringe pump to
the reaction mixture. For example, the reaction was
completed within 2 h in DMSO and 2a was isolated in
80% yield with >99% ee and only trace amounts of ke-
tone 3. The methylsulfonylcarboxamide 4 also mediated
the a-oxidation reaction with excellent results in DMF.
Next, we investigated the a-aminoxylation reaction with
a variety of ketones and aldehydes (Table 3).
The reactions proceeded smoothly and a-aminoxylated
ketones 2a–c were isolated in good yield with excellent
enantioselectivity (entries 1–3). The methylsulfonylcarb-
oxamide 4 was also able to mediate the direct enantio-
selective a-oxidation of aldehydes. For example, the
sulfonylcarboxamide 4-catalyzed reaction between pro-
pionaldehyde and nitrosobenzene furnished the corre-
sponding a-aminoxylated product 2d (entry 4), which
was reduced in situ with excess NaBH4 to the corre-
sponding alcohol, which was isolated in 66% yield with
>99% ee. We also investigated whether 2-cyclohexen-1-
The stereochemical outcome of the a-oxidations and
tandem a-aminoxylation/Michael reactions of ketones
and aldehydes utilizing sulfonylcarboxamide catalysis
was the same as for L-proline catalysis.7 Thus, organo-
catalysts 4 and 5 catalyzed the formation of (2R)-amin-
oxylated products. We therefore believe that the
nitrosobenzene approaches the si-face of the chiral en-
amine-intermediate via transition state I, which is simi-
lar to our and HoukÕs previous density functional
theory (DFT)-calculations of the transition state II
of the proline-catalyzed a-aminoxylation reaction.7b,9
Table 3. The direct asymmetric a-oxidations of ketones and aldehydes with sulfonylcarboxamide 4 as the catalysta
O
O
O
N
Catalyst 4
ONHPh
(10 mol%)
R
R
+
R1
R1
DMSO, rt
1
2
Entry
1
1
Time (h)
2
Prod.
Yield (%)b
80
ee (%)c
>99
O
O
ONHPh
ONHPh
1a
2a
O
O
2
5
7498
O O
O O
2b
1b
O
O
ONHPh
ONHPh
3
4
3
2
54d
>99
2c
O
1c
O
66e,f
>99e,g
H
H
1d
2d
a Nitrosobenzene (1 mmol) in DMSO (1 mL) was slowly added with syringe-pump to a solution of ketone or aldehyde 1 (2 mmol) in the presence of
catalyst 4 (10 mol %) in DMSO (1 mL). After vigorous stirring at room temperature the reaction mixture was quenched by addition of brine and
extraction with EtOAc. Subsequent purification utilizing silica-gel chromatography afforded the a-aminoxylated product 2.
b Yield of the isolated pure product.
c The ee as determined by chiral-phase HPLC analyses.
d Dr = 1:1 (anti:syn).
e The reaction was performed by simple mix and stirring.
f The yield of the isolated pure alcohol obtained by in situ reduction of 2d with NaBH4 at 0 °C.
g ee determined by chiral HPLC-analyses of the a-aminoxylated alcohol derived from 2d.

´
H. Sunden et al. / Tetrahedron Letters 46 (2005) 3385–3389
3388
Table 4. Amine-catalyzed direct enantioselective reactions with 2-cyclohexen-1-onesa
Catalyst
O
O
N
O
(10 mol%)
O
+
Ph
N
Solvent
Ph
1e
2e
Entry
Catalyst
T (°C)
Solvent
Time (h)
Prod.
Yield (%)b
ee (%)c
1
2
3
4
4
40
Rt
40
40
MeCN
DMSO
MeCN
MeCN
15
12
16
15
2e
2e
2e
2e
22
23
21
65
>99
96
4
L-Proline
10
99
99
a A mixture of nitrosobenzene (1 mmol), ketone 1 (2 mmol) and catalyst (10 mol %) in DMSO (2 mL) was vigorously stirred at room temperature or
40 °C. The reaction mixture was quenched by addition of brine and extraction with EtOAc. Subsequent purification of the crude product by silica-
gel chromatography afforded the Diels–Alder product 2.
b Yield of the isolated pure product.
c The ee as determined by chiral-phase HPLC analyses.
Favourable coloumbic interactions between nitrosobenz-
ene and the pyrrolidine ring of catalyst 12 in transition
state III may explain why ent-2a was obtained.
the chiral enamine. Next, the electrophile approaches
the si-face of the chiral enamine intermediate via transi-
tion state I to furnish the activated iminium salt. The
chiral-activated iminium salts undergoes a subsequent
stereospecific intermolecular Michael-addition, which
results in the bicyclic enamine intermediate. Hydrolysis
of this chiral enamine intermediate provides the desired
bicycle 2e and free organic catalyst.
O
O
Ph
N
Ph
Ph
R2N
N
N
N
O
N
N
O
H
H
N
O
S
R''
R''
O
O
R
O
In summary, we have shown that proline-derived sulfon-
ylcarboxamides are excellent catalysts for the direct
enantioselective a-oxidation of ketones and aldehydes.
The alkyl and arylsulfonylcarboxamides furnished the
corresponding a-aminoxylated products in good yield
with up to >99% ee. The novel organic catalysts are read-
ily prepared in two steps and allows for the generation of
catalyst libraries. Furthermore, proline-derived sulfonyl-
carboxamides are able to catalyze nitroso reactions that
proceed via a tandem a-aminoxylation–Michael reaction
R'
R'
I
II
III
In the case, of the sulfonylcarboxamide-catalyzed nitro-
so reactions of a,b-unsaturated cyclic ketones we believe
that the reaction proceeds via a potential step-wise
mechanism (Scheme 2). Accordingly, the tandem a-
aminoxylation/Michael reaction starts by reaction of
the unsaturated ketone with the organic catalyst to yield
+
X
X
O
N
N
N
O
Ph
Ph
N
-
O
Ph
N
N
O
O
H
N
O
S
R
O
Ph
H2O
I
N
X
O
1e
N
X
N
H
O
O
N
Ph
2e
Scheme 2. Potential catalytic mechanism of the amine-catalyzed tandem a-aminoxylation/Michael reaction and plausible transition state I of the
initial 4-catalyzed a-aminoxylation of ketone 1e.

´
H. Sunden et al. / Tetrahedron Letters 46 (2005) 3385–3389
3389
2004, 43, 1112; (g) Hayashi, Y.; Yamaguchi, J.; Sumiya,
T.; Hibino, K.; Shoji, M. J. Org. Chem. 2004, 69, 5966.
8. (a) Momiyama, N.; Torii, H.; Saito, S.; Yamamoto, H.
Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5374; (b)
Yamamoto, Y.; Momiyama, N.; Yamamoto, H. J. Am.
Chem. Soc. 2004, 126, 5962.
to yield the desired bicyclic adducts with up to >99% ee.
The high modularity in the synthesis of proline-derived
carboxamides makes the likelihood of finding highly
enantioselective reactions mediated by this class of cata-
lysts a real possibility. Efforts in this area are in progress.
9. (a) Mathew, S. P.; Iwamura, H.; Blackmond, D. G.
Angew. Chem., Int. Ed. 2004, 43, 3317; (b) Wang, W.;
Wang, J.; Hao; Li; Liao, L. Tetrahedron Lett. 2004, 45,
7235; (c) Hayashi, Y.; Yamaguchi, J.; Hibino, K.; Sumiya,
T.; Urushima, T.; Shoji, M.; Hashizume, D.; Koshino, H.
Adv. Synth. Catal. 2004, 346, 1435. For density functional
calculations see: Ref. 7b and (d) Cheong, P. H.-Y.; Houk,
K. N. J. Am. Chem. Soc. 2004, 43, 13912.
Acknowledgements
A.C. and H.A. thank the Swedish Research Council and
Wenner-Gren Foundation for financial support.
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13. In a typical experiment, the ketone 1a (2 mmol) was added
to a mixture of nitrosobenzene (1 mmol) in DMSO (4mL)
and organic catalyst (10–30 mol %). After vigorous
stirring at room temperature the reaction mixtures were
quenched by addition of brine followed by extraction with
EtOAc to furnish the corresponding a-aminoxylated
ketone 2a. Pure 2a was isolated by silica-gel column
chromatography (EtOAc/pentane-1:4) and the ee
was determined by chiral-phase HPLC-analysis (see Ref.
7b).
14. For previous use of catalyst 10 see: Ref. 8a,8b,10, and (a)
Cobb, A. J. A.; Shaw, D. M.; Ley, S. V. Synlett 2004, 558;
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43, 1460, and references cited therein.
15. In a typical experiment, the ketone 1e (2 mmol) was added
to nitrosobenzene (1 mmol) and catalyst (10 mol %) in
organic solvent (2 mL) and the reaction mixture was
vigorously stirred. After 14–16 h the reaction mixture was
quenched by addition of brine followed by extraction with
EtOAc to give the bicyclic adduct 2e. Pure 2e was isolated
by silica-gel column chromatography (EtOAc/pentane-
1:10) and the ee was determined by chiral-phase HPLC-
analysis. HPLC (Daicel Chiralpak AD, hexanes/i-
PrOH = 97:3, flow rate 0.5 mL/min, k = 254nm): major
isomer: tR = 25.72 min; minor isomer: tR = 19.81 min;
´
´
7. (a) Bøgevig, A.; Sunden, H.; Cordova, A. Angew. Chem.,
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´
Int. Ed. 2004, 43, 1109; (b) Cordova, A.; Sunden, H.;
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23
½aꢀD ꢁ80.1 (c 0.5, CHCl3).