Catalytic selective oxyamidation of cyclic enamides using nitrenes

Article abstract of DOI:10.1002/chem.201102302

Yes, nitrenes did "N,O"! Intermolecular addition of nitrene to enecarbamates and enesulfonamides gives oxyamidated products in excellent yields of up to 98% and with good levels of stereoselectivity. Complete regioselectivity is also observed, leading to

Full text of DOI:10.1002/chem.201102302

DOI: 10.1002/chem.201102302
Catalytic Selective Oxyamidation of Cyclic Enamides using Nitrenes
Nicolas Gigant,[b] Geoffroy Dequirez,[a] Pascal Retailleau,[a] Isabelle Gillaizeau,*[b] and
Philippe Dauban*[a]
Growing attention has recently been paid to the develop-
ment of catalytic alkene difunctionalization reactions.[1]
These transformations involve the simultaneous installation
of two functional groups onto p bonds and offer the most
straightforward access to various key motifs such as vicinal
diols,[2] amino-alcohols[3] or diamines[4] that are found in a
wide range of natural products, drugs, or chiral auxiliaries.
However, whereas efficient selective protocols are available
for the formation of diols, much work remains to be done in
the field of catalytic aminohydroxylation, which raises the
additional issue of regioselectivity with non-symmetrical ole-
fins. In particular, poor control of the sense of the addition
is observed in many cases thereby limiting its scope.[5]
Recent solutions have nevertheless emerged with the design
of ligands,[6] the development of tethered reactions,[3a,7] or
the discovery of new oxidative conditions.[3a,8]
regiocontrol of the addition could be achieved. Fundamen-
tally, should the latter allow selective introduction of the O
functionality at C2, it will afford the possibility to further
functionalize the enamide via the intermediacy of iminium
ions.[14] These observations thus prompted us to investigate
the oxyamidation of enecarbamates and enesulfonamides
within the framework of a program aimed at generating
small molecule libraries for biological screening. In this
communication, we wish to report the results of our studies
that have led to the development of a process occurring
with high levels of regio- and stereoselectivity and relying
on catalytic nitrene transfers (Scheme 1).
p-Electron-rich olefins such as enamides are useful scaf-
folds for the preparation of complex nitrogenated mole-
cules.[9,10] Compared with enamines, enamides have received
little attention until recently as a consequence of their de-
creased nucleophilicity. Their chemistry, initially centered
around the application of catalytic hydrogenation for the
synthesis of enantiopure amines,[11] has expanded considera-
bly within the last decade following the development of sev-
eral methodologies for their stereoselective synthesis.[12] En-
amides have therefore proved to be efficient nucleophilic
agents as well as versatile substrates in radical or pericyclic
reactions.[9] The use of transition-metal complexes has also
allowed selective transformations such as catalytic cyclopro-
panation or dihydroxylation.[13] In comparison, the develop-
ment of an efficient protocol for aminohydroxylation may
offer additional synthetic opportunities provided that good
Scheme 1. Catalytic oxyamidation.
Most of the metal-catalyzed aminohydroxylations involve
osmium, and to a lesser extent, palladium complexes.[1,3a]
However, the sole examples reported with enamides so far
are based on the use of manganese and osmium, with the
latter surprisingly leading to the formation of diols.[13c] In
this context, the use of dirhodium(II) catalysts provides new
opportunities since it has been demonstrated that they effi-
ciently catalyze the intramolecular oxyamidation of elec-
tron-rich olefins in the presence of iodineACTHNUTRGNEUNG
(III) oxidants.[15]
More recently, one of us has reported that efficient intermo-
lecular oxyamidation of the 2,3-indole p-bond can also be
achieved under these conditions.[16] Strikingly, all of these
transformations rely on the intermediacy of metallanitrenes
[a] G. Dequirez, Dr. P. Retailleau, Dr. P. Dauban
Centre de Recherche de Gif-sur-Yvette
Institut de Chimie des Substances Naturelles
UPR 2301 CNRS, Avenue de la Terrasse
91198 Gif-sur-Yvette (France)
À
better known for giving aziridines from alkenes or C H
aminated products.[17] With the aim of exploring this new
type of reactivity for nitrenes in more detail, we therefore
decided to study their reaction with enamides that are struc-
turally related to indoles.[18]
Fax : (+33)1-6907-7247
E-mail: philippe.dauban@icsn.cnrs-gif.fr
Reactivity of simple cyclic enamides 1[19] under conditions
previously optimized for indoles[16] was first investigated
with either methanol or acetic acid as nucleophiles. Pleas-
[b] N. Gigant, Prof. I. Gillaizeau
Institut de Chimie Organique et Analytiqe
UMR6005 CNRS, Universitꢀ dꢁOrlꢀans
45067 Orlꢀans Cedex 2 (France)
Fax : (+33)2 3841 7281
ingly, the combination of the rhodium(II) catalyst Rh2ACHTUNGTRENNUNG
(esp=a,a,a’,a’-tetramethyl-1,3-benzenedipropionic
(esp)2
acid)
E-mail: isabelle.gillaizeau@univ-orleans.fr
with trichloroethylsulfamate (TcesNH2) (both developed by
Du Bois and co-workers for catalytic alkene aziridination
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102302.
90
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Chem. Eur. J. 2012, 18, 90 – 94

COMMUNICATION
place starting from substrates 3a–d[21] in the pres-
ence of methanol or acetic acid, to give cleanly the
expected oxyaminated products with yields in the
54–98% range and cis/trans ratio of up to 1:9
(Table 2). Complete regioselectivity was observed
as in the previous cases, leading to the formation of
a single regioisomer with two N,O-acetal functional-
ities. Such control in the selectivity was once again
secured by X-ray crystallographic analysis of the
crystalline compound 5b. Moreover, it should be
pointed out that pyridoxazines can be transformed
with good conversions, though the latter are lower
than those found with benzoxazines (Table 2, en-
tries 5 and 6 vs. entries 1 and 2).[22] Worthy of note
is also the reaction successful with a trisubstituted
enamide (Table 2, entry 7), whereas the catalytic
sulfide imination of enamide 3e was not surprising
considering the ability of sulfur compounds to react
with nitrenes (Table 2, entry 8).[23]
Table 1. Oxyamidation of enamides.
Entry
Substrate
R1OH
Product
Yield
[%][a]
cis/trans[b]
1
2
3
4
5
6
1a
1b
1c
1d
1e
1 f
MeOH
MeOH
MeOH
MeOH
MeOH
AcOH
2a
2b
2c
2d
2e
2 f
66
42:58
37:63
30:70
0:100
0:100
0:100
71
94
87
Based on our previous observations with indoles,
88
that is, the replacement of Rh
2ACHTUNGERTN(NUNG esp)2 by Rh2-
AHCTUNGTRENNUNG
mers,[16] we performed the reaction in the presence
of this catalyst to improve the cis/trans ratio, how-
ever with limited success in the case of benzoxa-
zines 3a and 3b. Interestingly, this change, when ap-
plied to benzodioxine 7, proved illuminating be-
cause it gave some clues for the hypothetical mech-
74[c]
[a] Yield of isolated product. [b] Determined by 1H NMR spectroscopy. [c] Product
isolated together with 7% of the 2-OPiv analogue. The same reaction performed with
PhIACHTUNGTRENNUNG(OAc)2 led to the isolation of only 2 f in 84% yield.
[20]
À
and C H amination), in the presence of PhIACTHNURTGNENG(U OCOtBu)2
allowed us to isolate the expected oxyaminated products 2
in very good yields ranging from 66–94% starting from ene-
carbamates or enesulfonamides (Table 1). More importantly,
the reaction proceeds with complete regioselectivity leading
to the clean formation of N,O-acetals with the introduction
of the amino group at the C3 position. In terms of stereose-
lectivity, whereas a roughly 2:1 ratio in favor of the trans
isomer is observed with 6-membered cyclic enamides 1a–c,
this isomer is exclusively obtained in the case of the 7-mem-
bered analogues 1d–e. These conclusions regarding the
1
structures of products 2 were deduced from H NMR spec-
troscopic analysis and then confirmed by X-ray crystallo-
graphic analysis of compound 2 f (Figure 1). The picture
clearly shows the trans arrangement between the OAc
group at C2 and the sulfamate at C3. A stabilizing effect in-
duced by the overlap of the nonbonding lone pair nN with
Figure 1. X-ray structure of trans-isomers of compounds 2 f and 5b.
À
the s* orbital of the exocyclic C O bond can also be ob-
anism of the oxyamidation (Scheme 2). This symmetrical
substrate efficiently reacts with acetic acid and methanol,
under conditions reported in Table 2, to afford, respectively,
the expected products 8a and 8b isolated with cis/trans
served. This can account for the stereoselectivity of the reac-
tion when applied to 7-membered substrates. These excel-
lent results both in terms of efficiency and selectivity then
led us to consider other heterocyclic structures such as a
benzoxazine.
ratios of 13:87 and 26:74. Replacement of Rh
2ACHTUNGERTN(NUNG esp)2 by Rh2-
AHCTUNGTRENNUNG
1
This type of compound, which combines an enamide with
an enol function, raises the issue of regioselectivity since it
had previously been shown that glucals also react under
these conditions to afford C3 amino acetals.[15] We were thus
very pleased to observe that the reaction efficiently takes
slightly improved ratio of 10:90. Importantly, the H NMR
spectrum of the crude mixture reveals the presence of the
sole trans isomer,[24] thereby suggesting that the purification
by chromatography on silica gel induces partial epimeriza-
tion to the cis isomer.
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91

P. Dauban, I. Gillaizeau et al.
Table 2. Oxyamidation of N,X-heterocycles.
A comparable improvement of the stereoselectiv-
ity was seen in the case of methanol since com-
pound 8b was formed with a cis/trans ratio of 15:85.
But unexpectedly, with the aim of growing crystals
suitable for X-ray crystallographic studies, complete
epimerization of the latter occurred after heating in
toluene and the cis isomer was exclusively obtained
as indicated by the X-ray crystal structure
(Scheme 2. This phenomenon could be explained
by the stabilization provided by the combination of
the anomeric effect with the exo-anomeric effect as
clearly indicated in Scheme 2. All these observa-
tions brought us to the hypothesis that catalytic
oxyamidation of enamides might involve an initial
step of aziridination followed by a SN2-type ring-
opening of the aziridine.[25] The resulting 1,2-trans-
difunctionalized product would then partly epimer-
ize under the reaction conditions or during the pu-
rification on silica gel.
With these N,O-acetals in hand, preliminary ex-
periments were performed to confirm their capacity
to act as efficient N-(acyl)iminium species likely to
undergo nucleophilic displacement (Table 3).[26] We
were thus pleased to observe that the trans isomer
of 2c, previously separated from its cis isomer after
careful purification on silica gel, as well as the N-
(Boc)-derivative 2d could react with various nucle-
ophiles in the presence of a Lewis acid, that is,
boron trifluoride etherate or trimethylsilyl triflate,
with yields varying from 58 to 94%. The examples
displayed in Table 3 show that very good diastereo-
selectivities, in favor of the trans isomer, can be ob-
tained as a consequence of steric hindrance provid-
ed by the C3 amino substituent.[27] This transforma-
tion, therefore, gives the opportunity to further
functionalize enamides thereby enhancing the syn-
thetic value of this new catalytic oxyamidation.
Entry
Substrate
R1OH
Product
Yield cis/trans[b]
[%][a]
1
2
3
3a MeOH
3a AcOH
3b MeOH
4a 94[c,d]
34:66
23:77
10:90
5a 82[e,f]
4b 98[c]
4
5
6
7
3b AcOH
3c MeOH
3c AcOH
3d MeOH
5b 83[e,g]
4c 61[c]
5c 54[e]
4d 68[c]
30:70
28:72
12:88
36:64
8
3e MeOH
6
55
–
[a] Yield of isolated product. [b] Determined by 1H NMR spectroscopy. [c] Reaction
performed with PhI(OCOtBu)2. [d] Reaction with Rh2A(NHCOCF3)4 afforded 4a in
81% yield and a cis/trans ratio of 1:4. [e] Reaction performed with PhI(OAc)2. [f] Re-
action with Rh2A(NHCOCF3)4 afforded 5a in 64% yield and a cis/trans ratio of 1:3.
E
CHTUNGTRENNUNG
AHCTUNGTRENNUNG
CHTUNGTRENNUNG
[g] Reaction with Rh
1:2.
2ACHTUNGTRENNUNG(NHCOCF3)4 afforded 5b in 65% yield and a cis/trans ratio of
In conclusion, the application of rhodium-cata-
lyzed nitrene transfers to enamides has allowed us to docu-
ment a new type of reactivity for nitrenes. Intermolecular
addition of nitrene to enecarbamates and enesulfonamides
thus affords oxyamidated products with excellent yields of
up to 98% and, sometimes, high levels of stereoselectivity.
Strikingly, the aminohydroxylation of these non-symmetrical
olefins occurs with complete regioselectivity, leading to the
formation of N,O-acetals, which can be further transformed
following reaction with various C- or N-nucleophiles under
acidic conditions. This oxyamidation confirms the synthetic
potential of enamides as versatile platforms for the genera-
tion of molecular diversity. Work is now in progress to apply
these results to the preparation of a library of small bioac-
tive molecules.
Scheme 2. Oxyamidation of benzodioxine 7 and X-ray structure of cis-8b.
92
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Chem. Eur. J. 2012, 18, 90 – 94

Catalytic Selective Oxyamidation of Cyclic Enamides
COMMUNICATION
Table 3. Reactivity of the N,O-acetals 2c and 2d.
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[%][a]
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Allyl-TMS
BF3·OEt2
À78 to À408C
Allyl-TMS
BF3·OEt2
À788C to À408C
TMSN3
BF3·OEt2
À788C
TMSCN
TMSOTf
À408C
1
2
3
4
5
2c
2d
2d
2d
2d
9
61
58
93
75
94
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10
11
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13
Et3SiH
BF3·OEt2
À788C to À308C
[a] Yield of isolated product. Allyl-TMS=Allyltrimethylsilane.
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Experimental Section
General procedure for the rhodium-catalyzed oxyamidation of enamides:
TcesNH2 (1.5 equiv), Rh2A(esp)2 (0.02 equiv) and the nucleophile
(12 equiv) were successively added to a solution of enamide 1 or 3
(1 equiv) in benzene (c=0.36 molLÀ1). PhI
(OCOR)2 (2 equiv) was added
CTHUNGTRENNUNG
AHCTUNGTRENNUNG
to this mixture and the solution was stirred at room temperature for 2 h.
The reaction was then quenched with saturated aqueous NaHCO3 solu-
tion. The aqueous layer was extracted three times with CH2Cl2 and the
combined organic extracts were washed twice with brine, and then dried
over MgSO4 and concentrated. Products were purified by flash chroma-
tography (silica, petroleum ether/ethyl acetate eluent) to yield the de-
sired products.
Acknowledgements
We wish to thank the Institut de Chimie des Substances Naturelles for fi-
nancial support. MESR (Ministꢃre de lꢁEnseignement Supꢀrieur et de la
Recherche) and ICSN are gratefully acknowledged for doctoral fellow-
ships to N. G. and G. D, respectively.
Keywords: aminohydroxylation
regioselectivity · rhodium
·
enamide
·
nitrene
·
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Thus, whereas the 37:63 cis/trans mixture of 2b reacts with sodium
azide to afford only the trans azido product, the 30:70 cis/trans mix-
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ture of the expected 2-azido-3-amino derivative. A comparable un-
expected observation was also made in the reaction with allyl TMS.
The stereochemical outcome of these nucleophilic additions is cur-
rently under investigation.
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Rh
C
Received: July 26, 2011
mers with a cis/trans ratio of 11:89.
Published online: December 8, 2011
94
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ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 90 – 94