An intermolecular hydroamination of allenamides with arylamines catalyzed by cationic Au(I) salts

Article abstract of DOI:10.1021/jo101035n

(Figure presented) An intermolecular hydroamination of allenamides with arylamines has been achieved under mild Au(I) catalysis conditions delivering allylamino E-enamides stereoselectively and in high yield. The reaction is made possible via a convenient method for conjugated N-acyliminium formation.

Full text of DOI:10.1021/jo101035n

pubs.acs.org/joc
SCHEME 1. The Hydroamination of Allenes
An Intermolecular Hydroamination of Allenamides
with Arylamines Catalyzed by Cationic Au(I) Salts
Anthony W. Hill, Mark R. J. Elsegood,
and Marc C. Kimber*
Department of Chemistry, Loughborough University,
Leicestershire, LE11 3TU, United Kingdom
M.C.Kimber@lboro.ac.uk
Received June 1, 2010
(anti-Markovikov), Hg (Markovnikov), Pt (Markovnikov), and
Pd (Markovnikov) salts.3 Additional to these transition metals,
Au salts have proved to be particularly attractive in hydroami-
nation reactions due to their low toxicity and increased stability
to moisture and air.4 Consequently, a number of groups have
utilized Au salts in these transformations to great effect.1b,5
Recently, we reported6 the first intermolecular hydroaryla-
tion of allenamides7 with electron-rich aromatics using
an Au(I) catalyst to give the corresponding enamides. Ena-
mides8 are a class substrates that have become particularly
topical due to their use in the construction of heterocycles
and chiral amines and their presence in a number of natural
product frameworks.9 This transformation was high yielding
for most substrates and gave exclusively the E-enamide.
An intermolecular hydroamination of allenamides with aryl-
amines has been achieved under mild Au(I) catalysis con-
ditions delivering allylamino E-enamides stereoselectively
and in high yield. The reaction is made possible via a con-
venient method for conjugated N-acyliminium formation.
(4) For reviews on Au catalysis, see: (a) Arcadi, A. Chem. Rev. 2008, 108,
3266. (b) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180. (c) Hashmi, A. S. K.;
Rudolph, M. Chem. Soc. Rev. 2008, 37, 1766.
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R. E.; Zhang, Z.; Widenhoefer, R. A. Org. Lett. 2008, 10, 3157. (c) Duncan,
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Chem., Int. Ed. 2006, 45, 3314.
The addition of the N-H bond over alkene and alkyne
π-systems, the hydroamination transformation, represents a
powerful method for the introduction of the amine function-
ality.1 Such transformations give access to a range of valuable
nitrogen-containing building blocks such as amines, imines, and
enamines. Within this group of reactions the intermolecular
hydroamination of allenes has become increasingly important
due to the regiochemical factors in such transformations. Allenes
(1) can undergo either Markovnikov or anti-Markovnikov addi-
tion, giving rise to allylamines (2a) or imines (2b) (Scheme 1).
This first group of substrates, allylamines, are vital synthetic
building blocks since they are contained within a number of
important biological systems and are key intermediates in
organic synthesis.2
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H.; Hsung, R. P. Acc. Chem. Res. 2003, 36. For the use of allenamides: (b)
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A number of transition metal approaches toward the hydro-
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5406 J. Org. Chem. 2010, 75, 5406–5409
Published on Web 07/12/2010
DOI: 10.1021/jo101035n
r
2010 American Chemical Society

Hill et al.
JOCNote
SCHEME 2. The Proposed Hydroamination of Allenamides
with Arylamines
SCHEME 4. Hydroamination of Allenamide 5a
SCHEME 3. Preparation of Allenamides 5a,b and 7
Au(I)PPh3OTf generated from AuClPPh3 and AgOTf at room
temperature (Scheme 4). To our delight the hydroaminated
product 9a was isolated in 86% yield after chromatography.
The enamide was obtained exclusively as the E-isomer, and
the addition of the N-H bond to the activated allenamide gave
the Markovnikov product.
A comparable yield of 84% was also obtained with 5 mol %
of the Au(I) complex, AuPPh3(NTf2). The stereochemistry of
the E-enamide double bond was supported by a combination of
1H and 13C NMR, IR spectroscopy (coupling constant 14.0 Hz
and 1673 cm-1), and single crystal X-ray analysis.13
Importantly, unlike many of the methods10 for enamide
preparation, the reaction required no exclusion of air and
moisture and the reaction was extremely facile.
The applicability of this protocol was then further explored,
the results of which are summarized in Table 1. Haloanilines
8b and 8f successfully added to the allenamide giving the
enamides 9b and 9f in good yield (entries 1 and 5). Introduc-
tion of an electron-withdrawing ethyl ester para to the NH2
gave the enamide 9c in a moderate 61% yield (entry 2), while a
nitro group at the ortho position was tolerated and gave
the enamide 9d quantitatively (entry 3). 2,5-Dimethylaniline
8e and 3-methoxyaniline 8g both successfully added to the
activated allenamide to give the hydroaminated products 9e
and 9g, respectively (entries 4 and 6). While 2-fluoroaniline 8h
participated in the reaction to give 9h (entry 7), unfortunately
pentafluoroaniline 8i failed to add to the activated allena-
mides, presumeably due to its low nucleophilicity (entry 8).
Finally, N-methylaniline 8j readily participated in the hydro-
amination reaction giving the N-methylenamide 9j in near-
quantitative yield (entry 9).
While there are reported methods for the Au-catalyzed inter-
molecular hydroamination of allenes to give allylamines5 and
intermolecular protocols for the hydroaminations of allena-
mides7i within the literature, methods for the intermolecular
hydroamination of allenamides remain untouched to our know-
ledge. Whereas the hydroamination of allenes delivers syntheti-
cally useful allylamines, the intermolecular Markovnikov addi-
tion of an N-H bond over an allenamide 3 would deliver an
allylamino enamide 4, a substrate that would contain both an
allyl amine and an enamide within the one synthetic framework
(Scheme 2). Therefore, in this communication we would like to
share our results of the first intermolecular hydroamination of
allenamides using arylamine derivatives under our Au(I) cata-
lytic conditions.
The allenamides used for this study are shown in Scheme 3.
Cyclic allenamides 5a and 5b were synthesized by using an
adapted method of Heaney,11 and the acyclic allenamide 7
was synthesized via initial amide formation followed by
base-catalyzed rearrangement.12
The hydroamination reaction was also performed with
chiral and acyclic allenamides and the results are shown in
Table 2.
Our starting point would be the conditions used for our
hydroarylation protocol.6 Therefore a solution of allena-
mide 5a (1.00 equiv) and aniline 8a (1.05 equiv) in CH2Cl2
was treated with a catalytic amount (5 mol %) of cationic
Chiral allenamide 5b successfully underwent hydroamination
with both aniline 8a and 2-iodoaniline 8f giving the chiral
enamides 10a and 10b, respectively (entries 1 and 2). The acyclic
allenamide 7 also underwent hydroamination with anilines 8f
1
and 8g. While a crude H NMR of enamides 11a and 11b
(10) Methods for enamide synthesis: (a) Organocuprate addition to isocya-
nates: Snider, B. B.; Song, F. Org. Lett. 2000, 2, 407. (b) Acylation of imines:
Boeckman, R. K.; Goldstein, S. W.; Walters, M. A. J. Am. Chem. Soc. 1988, 110,
indicated full conversion to their enamide products the isolated
yields were modest at best. Comparable yields for the formation
of 11a and 11b were obtained when the Au(I) catalyst AuPPh3-
(NTf2) was used. While enamides 9a-j and 10a,b showed good
stability, enamides 11a,b had to be stored under nitrogen to
prevent degradation. Additionally, 11a and 11b exhibited con-
€
8250. (c) N-acylation/Peterson elimination: Furstner, A.; Brehm, C.; Cancho-
Grande, Y. Org. Lett. 2001, 3, 3955. (d) Pd-Cu-catalyzed C-N bond forma-
tions: Klapars, A.; Campos, K. R.; Chen, C.; Volante, R. P. Org. Lett. 2005, 7,
1185. Brice, J. L.; Meerdink, J. E.; Stahl, S. S. Org. Lett. 2004, 6, 1845. Wallace,
D. J.; Klauber, D. J.; Chen, C.; Volante, R. P. Org. Lett. 2003, 5, 4749. Cesati,
R. R., III; Dwyer, G.; Jones, R. C.; Hayes, M. P.; Yalamanchili, P.; Casebier,
D. S. Org. Lett. 2007, 9, 5617. Bolshan, Y.; Batey, R. A. Angew. Chem. 2008, 120,
2139. Angew. Chem., Int. Ed. 2008, 57, 2109; Chemler, S. R.; Fuller, P. H. Chem.
Soc. Rev 2007, 36, 1153. (e) Fe, Ru, Ru methods see: Stille, J. K.; Becker, Y. J.
Org. Chem. 1980, 45, 2139. (f) For organometallic additions to ynamides:
Gourdet, B.; Rudkin, M. E.; Watts, C. A.; Lam, H. W. J. Org. Chem. 2009,
74, 7849. (g) For Heck coupling strategies: Vallin, K. S. A.; Zhang, Q.; Larhed,
M.;Curran, D. P.;Hallberg, A.J. Org. Chem. 2003, 68, 6639. Harrison, P.; Meek,
G. Tetrahedron Lett. 2004, 45, 9277. (h) For co-oligomerization of N-vinyl amides
see: Tsujita, H.; Ura, Y.; Matsuki, S.; Wada, K.; Mitsudo, T.; Kondo, T. Angew.
Chem. 2007, 119, 5252. Angew. Chem., Int. Ed. 2007, 46, 5160. (i) For oxidative
Pd-catalyzed conjugate addition: Lee, J. M.; Ahn, D.-S.; Jung, D. Y.; Lee, J.; Do,
Y.; Kim, S. K.; Chang, S. J. Am. Chem. Soc. 2006, 128, 12954.
1
siderable broadening in their H NMR spectra and the 13C
NMR spectra for each showed two distinct conformers suggest-
ing the presence of rotamers; however, these rotamers could be
equilibrated at 373 K in d6-DMSO.
A mechanistic rationale for this transformation is outlined
in Scheme 5.
(13) Crystal data for 9a: C12H14N2O2, M = 218.25, monoclinic, I2/a; a =
16.015(3) A, b = 5.4583(9) A, c = 25.143(5) A, β = 99.848(2)°, V = 2165.5(7)
A3;Dcalc = 1.339 g/cm-3;μ(MoKR) = 0.093 mm-1;λ=, 0.71073 A, T= 150(2)
K; 12077 total reflections, 3283 unique data (Rint = 0.0277); solved by direct
methods and refined on F2 values to give R1 = 0.0428. See the Supporting
Information for further details.
(11) Heaney, H.; Ley, S. V. J. Chem. Soc., Perkin Trans. 1 1973, 499.
(12) Wei, L.-L.; Mulder, J. A.; Xiong, H.; Zificsak, C. A.; Douglas, C. J.;
Hsung, R. P. Tetrahedron 2001, 57, 459.
J. Org. Chem. Vol. 75, No. 15, 2010 5407

Hill et al.
JOCNote
TABLE 1. Substrate Scope for the Au(I)-Catalyzed Hydroamination of
SCHEME 5. Mechanistic Rationale for the Hydroamination
Reaction
Allenamide 5aa
undergo 1,2-addition giving 15, which can then undergo
protodemetalation to yield the observed E-enamide 16.
In summary, we have disclosed an Au(I)-catalyzed protocol
for the intermolecular hydroamination of allenamides with
arylamines. The reaction is facile, high yielding, and stereo-
selectively gives the E-enamide products. The products of this
reaction, allylamino enamides, have the potential to be valuable
building blocks in organic synthesis since they contain two vital
functionalities, allyl amines and enamides, within one frame-
work. The chemistry of this building block, mechanistic in-
sights, and the addition of alkylamines to the Au-activated
conjugated N-acyliminium species are currently being studied
in our group and will be reported on in due course.
Experimental Section14
Representative Hydroamination Method with Allenamide 5a.
To a solution of the allenamide 5a (63 mg, 1.05 equiv, 0.50 mmol)
in dichloromethane (3.00 mL) at room temperature was added the
aniline derivative (1.05 equiv) followed by AuPPh3OTf (from
AuClPPh3 [12.40 mg, 5.00 mol %, 0.025 mmol] and AgOTf [6.60
mg, 5 mol %, 0.025 mmol]) and the resulting solution was stirred
for up to 1 h at room temperature (monitored by tlc). The resulting
reaction mixture was then filtered through a plug of Celite and the
crude mixture purified by column chromatography (ethyl acetate/
petroleum ether mixture as indicated). 9a was obtained as a pale
yellow solid (Rf 0.42) (93 mg, 86%, mp 89-91 °C, CH2Cl2/ petro-
leum ether) [found (ES): MNaþ, C12H14N2O2, 241.0944, requires
MNaþ 241.0953]; IR (solution, CHCl3) 3441, 3012, 1759, 1673,
1602, 1504, 1482, 1417, 1250 cm-1; 1H NMR (400 MHz; CDCl3) δ
7.18 (t, J = 8.4 Hz, 2H), 6.91 (d, J = 14.0 Hz, 1H), 6.72 (t, J = 7.6
Hz, 1H), 6.62 (d, J = 7.6 Hz, 2H), 4.96 (dt, J = 6.4, 14.0 Hz, 1H),
4.42 (dd, J = 8.0, 9.2 Hz, 2H), 3.80 (d, J = 6.4 Hz, 2H), 3.73 (br s,
1H), 3.69 (t, J = 8.0 Hz, 2H); 13C NMR (100 MHz; CDCl3) δ
155.3 (C), 147.8 (C), 129.3 (CH), 126.4 (CH), 117.8 (CH), 113.1
(CH), 107.7 (CH), 62.2 (CH2), 43.8 (CH2), 42.5 (CH).
aReaction conditions: AuPPh3OTf (5 mol %), CH2Cl2, rt. bData for
all new compounds are contained within the Supporting Information.
cIsolated yields.
TABLE 2. Variation of the Allenamide in the Au(I)-Catalyzed Hydro-
amination Reactiona
Representative Hydroamination Method with Allenamide 5b.
To a solution of the allenamide 5b (50 mg, 1.00 equiv, 0.232 mmol)
in dichloromethane (2.00 mL) at room temperature was added the
aniline derivative (1.05 equiv) followed by AuPPh3OTf (from
AuClPPh3 [5.80 mg, 5.00 mol %, 0.012 mmol] and AgOTf [3.00
mg, 5 mol %, 0.012 mmol]) and the resulting solution was stirred
for up to 1 h at room temperature (monitored by tlc). The resulting
reaction mixture was then filtered through a plug of Celite and the
crude mixture purified by column chromatography (ethyl acetate/
petroleum ether mixture as indicated). 10a was obtained as a color-
less oil (Rf 0.5) (48 mg, 68%); [R]20D 26.0 (c 1.00, CHCl3) [found
(ES): MNaþ, C19H20N2O2, found 331.1413, requires MNaþ
331.1422]; IR (solution, CHCl3) 3014, 2926, 1755, 1670, 1503,
aReaction conditions: AuPPh3OTf (5 mol %), CH2Cl2, rt. bData for
all new compounds are contained within the Supporting Information.
cIsolated yields.
1
1413, 1310, 1238, 1206 cm-1; H NMR (400 MHz; CDCl3) δ
We believe that the cationic Au(I) salt activates the allen-
amide 12 to give a conjugated N-acyliminium intermediate
species 14. This can undergo either 1,2- or 2,3-addition by a
suitable nucleophile. In the case at hand the aniline derivatives
(14) Full experimental details and compound data are available in the
Supporting Information
5408 J. Org. Chem. Vol. 75, No. 15, 2010

Hill et al.
JOCNote
7.35-7.26 (m, 3H), 7.20 (t, J = 8.0 Hz, 2H), 7.15-7.13 (m, 2H),
6.84 (d, J = 14.4 Hz, 1H), 6.74 (t, J = 7.6 Hz, 1H), 6.66 (d, J = 8.4
Hz, 2H), 5.22 (dt, J = 6.4, 14.4 Hz, 1H), 4.27-4.17 (m, 3H), 3.87-
3.83 (m, 2H), 3.21-3.18 (m, 1H), 2.81-2.76 (m, 1H); 13C NMR
(100 MHz; CDCl3) δ 155.8 (C), 147.8 (C), 135.2 (C), 129.3
(CH), 129.3 (CH), 129.0 (CH), 127.4 (CH), 125.3 (CH), 117.9
(CH), 113.2 (CH), 108.3 (CH), 66.7 (CH2), 55.0 (CH), 44.2
(CH2), 36.3 (CH2).
Representative Hydroamination Method with Allenamide 7.
To a solution of the allenamide 7 (50 mg, 1.00 equiv, 0.289
mmol) in dichloromethane (3.00 mL) at room temperature was
added the aniline derivative (1.05 equiv) followed by AuP-
Ph3OTf (from AuClPPh3 [7.20 mg, 5.00 mol %, 0.014 mmol]
and AgOTf [3.70 mg, 5 mol %, 0.014 mmol]) and the resulting
solution was stirred for up to 1 h at room temperature (moni-
tored by tlc). The resulting reaction mixture was then filtered
through a plug of Celite and the crude mixture purified by
column chromatography (ethyl acetate/petroleum ether mixture
as indicated). 11a was obtained as a yellow oil (Rf 0.35) (52 mg,
46%) [found (ES): MNaþ, C17H17IN2O, found 331.415.0271,
requires MNaþ 415.0283]; IR (solution, CHCl3) 3441, 3011,
1638, 1590, 1506, 1389, 1069 cm-1; 1H NMR (400 MHz; CDCl3)
(mixture of rotamers) δ 7.64 (d, J=7.6 Hz, 1H), 7.49-7.40 (m,
5H), 7.20 (t, J=7.2 Hz, 1H), 6.78 (br s, 1H), 6.50 (br s, 1H),
6.48-6.44 (m, 1H), 5.16 (br s, 1H), 4.17 (br s, 1H), 3.77-3.75 (m,
2H), 3.27 (br s, 3H); 13C NMR (100 MHz; CDCl3) (mixture of
rotamers) δ 170.9 (C), 146.4 (C), 139.1 (CH), 135.7 (C), 133.0
(CH), 129.4 (CH), 128.7 (CH), 128.0 (CH), 119.0 (CH), 110.8
(CH), 105.7 (CH), 85.7 (C), 44.1 (CH2), 30.4 (CH3).
Acknowledgment. The authors thank Loughborough
University for financial support.
Supporting Information Available: Experimental proce-
dures, 1H and 13C NMR spectra, characterization for 9a-j,
10a,b, and 11a,b, and the crystallographic data for 9a. This
material is available free of charge via the Internet at http://
pubs.acs.org.
J. Org. Chem. Vol. 75, No. 15, 2010 5409