Cerium(IV) ammonium nitrate mediated three-component α-allylation of imine surrogates

Article abstract of DOI:10.1021/ol5004143

A general and practical CAN-mediated oxidative radical α-coupling reaction of various imine surrogates with allylsilanes has been described. This multicomponent process affords β-allylated α-carbamido ethers as stable imine precursors in respectable yields under mild conditions.

Full text of DOI:10.1021/ol5004143

Letter
pubs.acs.org/OrgLett
Cerium(IV) Ammonium Nitrate Mediated Three-Component
α‑Allylation of Imine Surrogates
Mathieu Bekkaye and Geraldine Masson*
́
†Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-Yvette Cedex, France
S
* Supporting Information
ABSTRACT: A general and practical CAN-mediated oxidative
radical α-coupling reaction of various imine surrogates with
allylsilanes has been described. This multicomponent process
affords β-allylated α-carbamido ethers as stable imine precursors
in respectable yields under mild conditions.
N-Carbamoyl α-allylated imine is a privileged chemical scaffold
that is found in nitrogen-containing natural compounds such as
spirolides, pinnatoxins, pteriatoxins, nodimines, and spiropror-
ocentrimines.1 In addition, since N-carbamoylimines can be
employed in various transformations, they are valuable as
versatile synthetic intermediates for the preparation of β-
allylated α-substituted amines.2 However, despite their
importance, few efficient syntheses exist in the literature.
Mainly because of their inherent instability (hydrolysis, imine−
enamine tautomerization), the traditional methods through
condensation of aldehydes and carbamates were not well suited
to their preparation.3 The indirect approach based on the α-
allylation of imines4,5 or β-allylation of enamines6 was not
successful. Therefore, development of a simple and efficient
method for the synthesis of N-carbamoyl α-allylated imine is
highly desirable. In continuation of our interest in the α-
alkylation of imines from stable and valuable enecarbamates,7,8
we envisaged that a β-umpolung addition of enecarbamates to
allylic nucleophiles might provide an effective alternative
approach to access these α-allylimines. Here, we describe a
novel α-allylation of imines which is achieved through cerium
ammonium nitrate (CAN) mediated direct allylation of
enecarbamates involving a radical-cationic domino process.7,9
In the past decades, extensive efforts have been devoted to
the development of efficient oxidative10 α-allylation of 1,3-
dicarbonyls11 and carbonyls.12,13b In this field, MacMillan et
al.13 have developed a direct and enantioselective α-allylation of
carbonyls via singly occupied molecular orbital (SOMO)
catalysis.13,15 This approach involves a one-electron oxidation
of a catalytically generated chiral enamine14 that reacts with an
allylsilane.13 Based on our previous work concerning the FeCl3-
catalyzed enantioselective α-oxyamination of enecarbamates
(Scheme 1, eq 1),7a and inspired by seminal work by
Thomson12 and MacMillan,13,15,16 we hypothesized that
oxidative radical reaction of enecarbamates would enable to
access to a range of α-allylated imines (Scheme 1, eq 2).14,17
Indeed, a well-selected oxidant should be able to initiate the
single-electron transfer (SET) oxidation of enecarbamate 1 to
generate the electron-deficient radical cation 5 (Scheme 1, eq
2).18 Trapping of this intermediate by an appropriate allylsilane
radicophile 2 could give rise to a new radical 6, while a second
Scheme 1. Oxidative Three-Component Reaction of
Enecarbamates6a and Rational Design for α-Allylation of
Carbamoylimines
oxidation and a subsequent elimination of the silyl group could
afford the desired N-carbamoyl α-allylated iminium 7. Then,
the alcohol 3 present in the reaction media would react with 7
to furnish β-allylated α-amido ether 4 as a stable precursor of
7.7,19
To realize a three-component radical/ionic domino proc-
ess,7,9 some intrinsic problems might be encountered. Indeed, a
strong oxidant with high single-electron transfer should be used
to oxidize selectively the enecarbamate 120 without touching
the allylsilane 221 and alcohol 3. Following our previous work
on the FeCl3-catalyzed oxidative process,7a,22 we initially
examined the reaction of (E)-propenylbenzylcarbamate (1a),
allyltrimethylsilane (2a), and ethanol (3a) with FeCl3 as the
oxidant (Table 1). Unfortunately, no reaction took place even
with 2 equiv of FeCl3 8 in the presence of NaNO2 and O2
(entries 1 and 2). Since the oxidation potential of cerium(IV) is
higher, we next examined ceric ammonium nitrate (CAN, 9) as
oxidant in our system.23,24 Detailed mechanistic analysis of
Received: February 9, 2014
Published: February 26, 2014
© 2014 American Chemical Society
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Letter
were obtained in acceptable yields (Scheme 2). Aliphatic
alcohols with different steric properties such as methanol (3b)
Table 1. Survey of Reaction Conditions for the CAN-
Promoted Three-Component Radical/Cationic Reaction
Scheme 2. CAN-Catalyzed α-Allylation of Enamide
Derivatives
a,b
Ox
temp
(°C)
2a
3a
yield
(%)
entry (x equiv)
solvent
(x equiv) (x equiv)
1
8 (0.3)
8 (2)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
EtOH
60
60
3
15
15
15
15
15
15
c
2
3
c
3
9 (3)
20
3
d
e
e
e
4
9 (3)
0
3
63
65
64
c,e
20
54
28
27
<5
<5
,
f
5
9 (2.5)
9 (2.5)
9 (3)
−15
−30
0
3
6
3
7
3
e
e
e
e
8
9 (2.5)
9 (2)
−15
−15
−15
−15
−15
−15
1.5
3
15
15
15
15
15
15
9
,
g
10
11
12
13
9 (2.5)
9 (2.5)
9 (2.5)
9 (2.5)
3
DME
3
e
e
toluene
DMF
3
3
a
Reaction conditions: enecarbamate (1a) (0.1 mmol), 2a, 3, and 8 or
b
c
9, c = 0.1 M. Isolated yield after column chromatography. 1a was
recovered. Benzylcarbamate, polymer, and benzyl(ethoxypropyl)-
carbamate were isolated. A solution of 1a in 1 mL of solvent was
d
e
slowly added over 1 h to a solution of 2a, 3a, and 9 in 1 mL of solvent.
f
g
With 0.1 or 5 mmol of 1a. Benzyl(ethoxypropyl)carbamate was also
isolated.
CAN-promoted enantioselective α-alkylation of aldehydes16
showed that the presence of H2O13,15 was vital to reach
excellent activity in promoting the solubility of CAN. As such,
the amount of alcohol could be beneficial to the oxidative
process envisaged. However, our first attempts were largely
unsuccessful (Table 1, entry 3), giving mainly products derived
from direct degradation, polymerization, and isomerization of
1a. This suggested that the CAN acts more as a Lewis acid
catalyst than an oxidant. To overcome this issue, one must
ensure that the oxidation can occur before the side reactions of
1a. One way to do this is to add slowly the enecarbamate to a
solution containing allylsilane, EtOH, and CAN in CH3CN,
guaranteeing that a small amount of 1 is exposed to oxidant.25
Indeed, this experimental condition afforded the β-allyl α-
carbamoyl ether 4a as a 1:1 mixture of diastereomers in decent
yield (Table 1, entry 4). Although no significant improvement
of yield was observed below 0 °C (Table 1, entries 5 and 6), the
temperature of −15 °C was kept for the rest of our
optimization. Then, a series of individual reactions was carried
out with different amounts of allylsilane 2a, CAN 9, and
ethanol (3a), as well as various solvents (Table 1, entries 8−
13). The best results were obtained using 2.5 equiv of CAN and
3 equiv of 2a in CH3CN (entry 5). It should be pointed out
that no reaction took place in the absence of ethanol, and only
starting materials were recovered (entry 7). This seems to
confirm our assumption that the role of ethanol is not limited
to trap the iminium but also increases the solubility of CAN via
the formation of the ate complex.16 Finally, scale up proved to
be straightforward; product 4a was prepared on a 5 mmol scale
in the same yield as reported with a 0.1 mmol scale (entry 5).
With optimal conditions in hand, we next investigated the
scope of the reaction with representative alcohols and allylic
radicophiles. In all cases, the β-allylated α-amidoethers 4a−f
a
Reaction conditions: enecarbamate (1) (0.2 mmol), allylsilane (2)
(0.6 mmol), CAN (0.50 mmol), 3 (3.0 mmol), c = 0.10 M at −15 °C.
b
d
1
Isolated yield after column chromatography. dr determined by H
e
NMR analysis of the crude mixtures. With allylstannane (2b).
and 2-propanol (3c) participated in the reaction, albeit the
latter one gave a relatively lower yield (Scheme 2, 4b and 4c).
In addition, the benzyl alcohol (3d), which is known to be
sensitive toward oxidation,26 was successfully engaged in this
three-component radical/cationic process. When the allylstan-
nane (2b)27 was used instead of 2a, the desired product 4a was
isolated with a slightly better yield. Allylsilanes with alkyl
substituents at the β-carbon reacted well with 1a to give the
corresponding branched β-allyl α-carbamoyl ethers 4e and 4f in
67 and 61% yield, respectively. The reaction scope was also
examined with respect to the enecarbamates. Various
enecarbamates and enamides bearing linear or branched alkyl
chains were all compatible leading to the respective α-allylated
imine precursors 4g−q in good yields (Scheme 2). Since
hydrogenolysis of N-Cbz-protected aminoethers seems tricky
without touching double bonds, other carbamate protecting
groups were tested in this radical/ionic process. As described in
Scheme 2, ethyl, propargyl, allyl, or 9-fluorenylmethyl
carbamates afforded allylated products 4n−q in respectable
yields, providing an access to orthogonally protected homoallyl
amines.28b Moreover, we were pleased to observe a high degree
of functional group tolerance under these conditions. In fact,
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Letter
various acid- or oxidation-labile groups such as benzyl ether
(4i) and silyl ether (4j) remained intact during the reaction,
further highlighting the mildness of this method.26,29 None-
theless, more labile silane functional groups such as TES-
protected enecarbamate 1m underwent the desired β-allylation
with a concomitant deprotection and cyclization in spite of the
large excess of ethanol used in the process (Scheme 3).30 The
of enecarbamates for the synthesis of natural products is
ongoing, and this work will be disclosed in due course.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures and spectral data for all new
compounds are provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
Scheme 3. Synthesis of β-Allyl α-Aminotetrahydrofurans
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: geraldine.masson@cnrs.fr.
Notes
The authors declare no competing financial interest.
same product was isolated in a better yield when the
unprotected alcohol 1n was employed instead. This provides
a rapid access to β-allyl α-aminotetrahydrofuran 4r, an
important intermediate in the synthesis of furoindolines.31 It
is also noteworthy that the terminal double and triple bonds
were untouched in the CAN-promoted allylation (Scheme 2,
4o and 4q). We found that the reaction of tertiary cyclic or
acyclic enamides afforded products 4m and 4k, respectively, in
slightly better yields. Finally, the β-substituted enantiopur
enecarbamate 4m was successfully engaged in this oxidative
process yielding to a 3:2 mixture of diastereomers in 61% yield.
To demonstrate the synthetic utility of the resulting β-
allylated α-amidoethers, 4a was converted into four common
functional groups, showing the versatility of our imines
precursors (Scheme 4). Reduction using triethylsilane and
ACKNOWLEDGMENTS
We thank ICSN and CNRS for financial support. M.B. thanks
MESR for a doctoral fellowship, University of Paris Sud.
■
REFERENCES
(1) For a relevant overview of these compounds, see: (a) Molgo,
Girard, E.; Benoit, E. In Phycotoxins: Chemistry and Biochemistry;
Botana, L. M., Eds.; Blackwell Publishing: Malden, MA, 2007; p 319.
(b) Kita, M.; Uemura, D. Chem. Lett. 2005, 34, 454. (c) Kuramoto, M.;
Arimoto, H.; Uemura, D. Mar. Drugs 2004, 2, 39. (d) Beaumont, S.;
Ilardi, E. A.; Tappin, N. D. C.; Zakarian, A. Eur. J. Org. Chem. 2010,
■
́
J.;
́
5743. (e) Gueret, S. M.; Brimble, M. A. Nat. Prod. Rep. 2010, 27, 1350.
For the recent discovery of koranimine, an α-branched imine natural
product, see: (f) Evans, B. S.; Ntai, I.; Chen, Y.; Robinson, S. J.;
Kelleher, N. L. J. Am. Chem. Soc. 2011, 133, 7316.
(2) For recent use of α-allylated imines, see: (a) Unsworth, W. P.;
Gallagher, K. A.; Jean, M.; Schmidt, J. P.; Diorazio, L. J.; Taylor, R. J.
K. Org. Lett. 2013, 15, 262. (b) Laws, S. W.; Scheerer, J. R. J. Org.
Chem. 2013, 78, 2422. (c) Unsworth, W. P.; Kitsiou, C.; Taylor, R. J.
K. Org. Lett. 2013, 15, 258. (d) Johannes, J. W.; Wenglowsky, S.; Kishi,
Y. Org. Lett. 2005, 7, 3997.
Scheme 4. Synthetic Transformations of β-Allylated α-
Amido Ethers 4
(3) (a) George, N.; Bekkaye, M.; Masson, G.; Zhu, J. Eur. J. Org.
Chem. 2011, 3695. (b) Kano, T.; Yurino, T.; Asakawa, D.; Maruoka, K.
Angew. Chem., Int. Ed. 2013, 52, 5532.
(4) (a) Aelterman, W.; De Kimpe, N.; Tyvorskii, V.; Kulinkovich, O.
J. Org. Chem. 2001, 66, 53. (b) De Kimpe, N.; Aelterman, W.
Tetrahedron 1996, 52, 12815. (c) De Kimpe, N.; Keppens, M.;
Stevens, C. Tetrahedron Lett. 1993, 34, 4693. (d) De Kimpe, N.;
Sulmon, P.; Moens, L.; Schamp, N.; Declercq, J.-P.; Van Meerssche,
̈
M. J. Org. Chem. 1986, 51, 3839. (e) De Kimpe, N.; Sulmon, P.;
Schamp, N. Angew. Chem., Int. Ed. 1985, 24, 881. (f) Stork, G.; Dowd,
S. R. J. Am. Chem. Soc. 1963, 85, 2178. (g) Stork, G.; Terrell, R.;
Szmuszkovicz, J. J. Am. Chem. Soc. 1954, 76, 2029.
(5) For a recent example of Pd-catalyzed allylic alkylation reaction of
imines, see: Chen, J.-P.; Peng, Q.; Lei, B.-L; Hou, X.-L.; Wu, Y.-D. J.
Am. Chem. Soc. 2011, 133, 14180.
TMSOTf as acid to trigger the formation of N-acyliminium
intermediate resulted in 80% yield of 11. The cyanation and
allylation of 4a could be also carried out using Lewis acid to
afford the β-allylated α-amidonitrile 11 and α−β-allylated 12,
respectively. This terminal olefin is ideally placed for various
metal-catalyzed aminations32 and can also serve as an
anchorage point for selective transformations.28 As illustration
in Scheme 4, a hydroboration/oxidation reaction was
performed, thus offering easy access to valuable 1,5-heptanol-
amine in acceptable yield.33
(6) Examples of alkylation of enamines: (a) Vignola, N.; List, B. J.
̌
Am. Chem. Soc. 2004, 126, 450. (b) List, B.; Coric,
́
I.; Grygorenko, O.
O.; Kaib, P. S. J.; Komarov, I.; Lee, A.; Leutzsch, M.; Pan, S. C.;
Tymtsunik, A. V.; Van Gemmeren, M. Angew. Chem., Int. Ed. 2014, 53,
282.
(7) (a) Drouet, F.; Zhu, J.; Masson, G. Adv. Synth. Catal. 2013, 355,
3563. (b) Courant, T.; Masson, G. Chem.Eur. J. 2012, 18, 423.
(8) (a) Dagousset, G.; Zhu, J.; Masson, G. J. Am. Chem. Soc. 2011,
133, 14804. (b) Alix, A.; Lalli, C.; Retailleau, P.; Masson, G. J. Am.
Chem. Soc. 2012, 134, 10389. (c) He, L.; Laurent, G.; Retailleau, P.;
Folleas, B.; Brayer, J.-L.; Masson, G. Angew. Chem., Int. Ed. 2013, 52,
́
11088. (d) Bekkaye, M.; Su, Y.; Masson, G. Eur. J. Org. Chem. 2013,
3978.
In summary, we have developed an approach toward the
synthesis of highly functionalized β-allyl α-carbamoyl-ether
derivatives via a CAN-mediated intermolecular allylation/
intermolecular hemiaminalization reaction. The scope of this
process shows wide tolerance of functional groups on the
enecarbamates and on the allylsilane derivatives. The ready
accessibility of the N-carbamoyl α-allylated imines makes the
current methodology particularly attractive in organic syn-
thesis.1,2 Further work to use this oxidative radical β-allylation
(9) For representative references on radical and radical-ionic
multicomponent reactions, see: (a) Tojino, M.; Ryu, I.; Sonoda, N.;
1512
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Organic Letters
Letter
Curran, D. P. Chem. Rev. 1996, 96, 177. (b) Godineau, E.; Landais, Y.
Chem.Eur. J. 2009, 15, 3044. (c) Tojino, M.; Ryu, I. In
́
Multicomponent Reactions; Zhu, J., Bienayme, H., Eds.; Wiley-VCH:
(21) The oxidation potential of allylsilane (E1/2 = 1.70−2.82V vs Ag/
Ag+): (a) Ohga, K.; Mariano, P. S. J. Am. Chem. Soc. 1982, 104, 617.
(b) Yoshida, J.-i.; Itoh, M.; Isoe, S. J. Chem. Soc., Chem. Commun. 1993,
547.
Weinheim, 2005; p 169.
(22) (a) Sibi, M. P.; Hasegawa, M. J. Am. Chem. Soc. 2007, 129, 4124.
(b) Van Humbeck, J. F.; Simonovich, S. P.; Knowles, R. R.;
MacMillan, D. W. C. J. Am. Chem. Soc. 2010, 132, 10012. (c) Akagawa,
K.; Fujiwara, T.; Sakamoto, S.; Kudo, K. Org. Lett. 2010, 12, 1804.
(d) Simonovich, S. P.; Van Humbeck, J. F.; MacMillan, D. W. C.
Chem. Sci. 2012, 3, 58.
(23) The reduction potential of Ce(IV)/Ce(III) (E° = +1.72 V):
(a) Shabangi, M.; Flowers, R. A., II. Tetrahedron Lett. 1997, 38, 1137.
(b) Shabangi, M.; Sealy, J. M.; Fuchs, J. R.; Flowers, R. A., II.
Tetrahedron Lett. 1998, 39, 4429. (c) Kuhlman, M. L.; Flowers, R. A.,
II. Tetrahedron Lett. 2000, 41, 8049.
(24) Selected reviews using CAN as a single-electron oxidant:
(a) Nair, V.; Deepthi, A. Chem. Rev. 2007, 107, 1862. (b) Nair, V.;
Balagopal, L.; Rajan, R.; Mathew, J. Acc. Chem. Res. 2004, 37, 21.
(25) Selected examples of radical processes where slow addition is
required: (a) Rendler, S.; MacMillan, D. W. C. J. Am. Chem. Soc. 2010,
132, 5027. (b) Urabe, D.; Yamaguchi, H.; Inoue, M. Org. Lett. 2011,
13, 4778.
(26) (a) Kim, S. S.; Jung, H. C. Synthesis 2003, 2135. (b) Rodkin, M.
A.; Beletskaya, I. P.; Makhn’kov, D. I. Bull. Acad. Sci. USSR, Div. Chem.
Sci. 1985, 34, 206.
(10) Recent review on oxidative coupling of enolates, enol silanes,
and enamines, see: Guo, F.; Clift, M. D.; Thomson, R. J. Eur. J. Org.
Chem. 2012, 26, 4881.
(11) Examples of oxidative allylation of 1,3-dicarbonyls with CAN as
the oxidant, see: (a) Hwu, J. R.; Chen, C. N.; Shiao, S.-S. J. Org. Chem.
2002, 60, 856. (b) Zhang, Y.; Raines, A. J.; Flowers, R. A. Org. Lett.
2003, 5, 2363. (c) Devery, J. J., III; Mohanta, P. K.; Casey, B. M.;
Flowers, R. A., II. Synlett 2009, 1490. Oxidative allylation of 1,3-
dicarbonyls with VO(OEt)Cl2 as the oxidant: (d) Hirao, T.;
Sakaguchi, M.; Ishikawa, T.; Ikeda, I. Synth. Commun. 1995, 25,
2579. Oxidative allylation of β-carbonyl imines with CAN as the
oxidant: (e) Zhang, Y.; Raines, A. J.; Flowers, R. A., II. J. Org. Chem.
2004, 69, 6267.
(12) Example of oxidative allylation of enol ethers, see: (a) Clift, M.
D.; Taylor, C. N.; Thomson, R. J. Org. Lett. 2007, 9, 4667. (b) Avetta,
C. T.; Konkol, L. C.; Taylor, C. N.; Dugan, K. C.; Stern, C. L.;
Thomson, R. J. Org. Lett. 2008, 10, 5621. (c) Konkol, L. C; Jones, B.
T.; Thomson, R. J. Org. Lett. 2009, 11, 5550.
(13) (a) Beeson, T. D.; Mastracchio, A.; Hong, J.-B.; Ashton, K.;
MacMillan, D. W. C. Science 2007, 316, 582. (b) Mastracchio, A.;
Warkentin, A. A.; Walji, A. M.; MacMillan, D. W. C. Proc. Nat. Acad.
Sci. U.S.A. 2010, 107, 20648 and references cited therein. For
highlights, see: (c) Wessig, P. Angew. Chem., Int. Ed. 2006, 45, 2168.
(d) Mukherjee, S.; List, B. Nature 2007, 447, 152. (e) Bertelsen, S.;
Nielsen, M.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2007, 46, 7356.
(14) The first example of electron-transfer oxidation of enamine
mediated by CAN: (a) Narasaka, K.; Okauchi, T.; Tanaka, K.;
Murakami, M. Chem. Lett. 1992, 2099.
(15) Intramolecular α-alkylation of aldehydes: (a) Comito, R. J.;
Finelli, F. G.; MacMillan, D. W. C. J. Am. Chem. Soc. 2013, 135, 9358.
Intramolecular α-allylation of aldehydes: (b) Pham, P. V.; Ashton, K.;
MacMillan, D. W. C. Chem. Sci. 2011, 2, 1470. α-Enolation of
aldehydes: (c) Jang, H.-Y.; Hong, J.-B.; MacMillan, D. W. C. J. Am.
Chem. Soc. 2007, 129, 7004. α-Vinylation of aldehydes: (d) Kim, H.;
MacMillan, D. W. C. J. Am. Chem. Soc. 2008, 130, 398. α-Chlorination
of aldehydes: (e) Amatore, M.; Beeson, T. D.; Brown, S. P.;
MacMillan, D. W. C. Angew. Chem., Int. Ed. 2009, 48, 5121. α-
Arylation of aldehydes: (f) Nicolaou, K. C.; Reingruber, R.; Sarlah, D.;
(27) For a review about allylic stannanes, see: Marshall, J. A. Chem.
Rev. 1996, 96, 31.
(28) Selective cross-metathesis of terminal alkene: (a) Crowe, W. E.;
Zhang, Z. J. J. Am. Chem. Soc. 1993, 115, 10998. Selective
hydrothiolation: (b) Floyd, N.; Vijayakrishnan, B.; Koeppe, J. R.;
Davies, B. G. Angew. Chem., Int. Ed. 2009, 48, 7798.
(29) Use of CAN for the deprotection of silyl ethers: (a) Gupta, A.
D.; Singh, R.; Singh, V. K. Synlett 1996, 69. (b) Hwu, J. R.; Jain, M. L.;
Tsai, F.-Y.; Tsay, S. C.; Balakumar, A.; Hakimelahi, G. H. J. Org. Chem.
2000, 65, 5077.
(30) Aubele, D. L.; Floreancig, P. E. Org. Lett. 2002, 4, 3443.
(31) Sunazuka, T.; Yoshida, K.; Kojima, N.; Shirahata, T.; Hirose, T.;
Handa, M.; Yamamoto, D.; Harigaya, Y.; Kuwajima, I.; Omura, S.
̅
Tetrahedron Lett. 2005, 46, 1459.
(32) Examples of hydroamination of carbamates: (a) Michael, F. E.;
Cochran, B. M. J. Am. Chem. Soc. 2006, 128, 4246. (b) Bender, C. F.;
Widenhoefer, R. A. Org. Lett. 2006, 8, 5303. (c) Zhang, Z.; Bender, C.
F.; Widenhoefer, R. A. Org. Lett. 2007, 9, 2887. (d) Kinder, R. E.;
Zhang, Z.; Widenhoefer, R. A. Org. Lett. 2008, 10, 3157. For examples
of carboamination, see: (e) Mai, D. N.; Wolfe, J. P. J. Am. Chem. Soc.
2010, 132, 12157. (f) Ward, A. F.; Wolfe, J. P. Org. Lett. 2011, 13,
4728. For examples of diamination, see: (g) Sibbald, P. A.; Michael, F.
E. Org. Lett. 2009, 11, 1147. (h) Ingalls, E. L.; Sibbald, P. A.;
Kaminsky, W.; Michael, F. E. J. Am. Chem. Soc. 2013, 135, 8854.
(33) Garrett, J.; Osswald, W.; Moreira, M. G. Br. J. Pharmacol. 1962,
18, 49.
Brase, S. J. Am. Chem. Soc. 2009, 131, 2086. (g) Conrad, J. C.; Kong, J.;
̈
Laforteza, B. N.; MacMillan, D. W. C. J. Am. Chem. Soc. 2009, 131,
11640.
(16) (a) Devery, J. J., III; Conrad, J. C.; MacMillan, D. W. C.;
Flowers, R. A., II. Angew. Chem., Int. Ed. 2010, 49, 6106 and references
cited therein. (b) See ref 27 in ref 9a.
(17) For an elegant example of an oxidative allylation of an enamine
in the synthesis of an alkaloid natural product, see: Chandra, A.; Pigza,
J. A.; Han, J.-S.; Mutnick, D.; Johnston, J. N. J. Am. Chem. Soc. 2009,
131, 3470.
(18) Chanon, M.; Rajzmann, M.; Chanon, F. Tetrahedron 1990, 46,
6193.
(19) For reviews, see: (a) Speckamp, W. N.; Moolenaar, M. J.
Tetrahedron 2000, 56, 3817. (b) Hiemstra, H.; Speckamp, W. N. In
Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon: Oxford, 1991; Vol. 2, p 1047. For selected examples of α-
amido ether as N-acylimine or N-acyliminium precursors, see:
(c) Huang, Y. Y.; Chakrabarti, A.; Morita, N.; Schneider, U.;
Kobayashi, S. Angew. Chem., Int. Ed. 2011, 50, 11121. (d) Terada,
M.; Machioka, K.; Sorimachi, K. Angew. Chem., Int. Ed. 2009, 14, 2553.
(20) Oxidation potential data for secondary enecarbamates or
secondary enamides were not available, only the potential of tertiary
enamide (E1/2 = 1.12−1.55 V vs SCE), see: (a) Han, B.; Jia, X.-D.; Jin,
X.-L.; Zhou, Y.-L.; Yang, L.; Liu, Z.-L.; Yu, W. Tetrahedron Lett. 2006,
47, 3545. (b) Bauld, N. L.; Harirchian, B.; Reynolds, D. W.; White, J.
C. J. Am. Chem. Soc. 1988, 110, 8111.
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