Gold(I)-catalyzed stereoconvergent, intermolecular enantioselective hydroamination of allenes

Article abstract of DOI:10.1002/anie.201201584

Gold and silver: A 1:2 mixture of [{(S)-1}(AuCl)₂] and AgBF ₄ catalyzes the enantioselective hydroamination of chiral, racemic 1,3-disubstituted allenes with N-unsubstituted carbamates to form N-allylic carbamates in good yield, with high regio- and diastereoselectivity, and up to 92% ee (see scheme, Cbz=benzyloxycarbonyl). Copyright

Full text of DOI:10.1002/anie.201201584

Angewandte
Chemie
DOI: 10.1002/anie.201201584
Asymmetric Catalysis
Gold(I)-Catalyzed Stereoconvergent, Intermolecular Enantioselective
Hydroamination of Allenes**
Kristina L. Butler, Michele Tragni, and Ross A. Widenhoefer*
ꢀ
The intermolecular, enantioselective addition of the N H
of (S)-2 under Table 1) and AgOTf (OTf = trifluoromethane-
sulfonate) in dioxane at 248C for 24 h led to isolation of N-
allylic carbamate (R)-3a in 35% yield as a single diastereo-
ꢀ
bond of an amine or carboxamide derivative across a C C
multiple bond (hydroamination) represents an attractive,
atom-economical approach to the synthesis of chiral, non-
racemic amines and amine derivatives.[1] Within this family of
transformations, the intermolecular enantioselective hydro-
amination (EHA) of allenes is of interest as a potentially
expedient route to enantiomerically enriched a-chiral allylic
amines, which are important chiral building blocks that are
utilized in the synthesis of complex nitrogen-containing
molecules.[2] However, despite considerable efforts in this
area,[1] effective intermolecular EHA processes are scarce,[3]
and the intermolecular EHA of allenes remains unknown.[4]
One of the challenges associated with the intermolecular
EHA of allenes is the regioselectivity of extant hydroamina-
tion catalysts, which form predominantly achiral products
from electronically unbiased monosubstituted allenes.[4,5] To
circumvent this regiochemical bias, we envisioned the stereo-
convergent, intermolecular EHA of chiral, racemic 1,3-
disubstutited allenes catalyzed by chiral bis(gold) phosphine
complexes. This approach builds upon our previous efforts in
the area of gold-catalyzed allene hydroamination.[6,7] In
particular, we have shown that achiral gold(I) N-heterocyclic
carbene (NHC) complexes catalyze the regio- and diastereo-
selective hydroamination of chiral 1,3-disubstituted allenes
with carbamates, and that allene racemization was rapid
under reaction conditions.[6] Furthermore, both we[7] and
Toste and co-workers[8] have demonstrated the enantioselec-
tive intramolecular hydroamination of allenes catalyzed by
chiral bis(gold) phosphine complexes.[9] Herein we describe
the stereoconvergent, enantioselective, intermolecular hydro-
amination of chiral, racemic 1,3-disubstituted allenes with
carbamates catalyzed by chiral bis(gold) phosphine com-
plexes.[10]
Table 1: Effect of supporting ligand, solvent, silver salt, and allene
concentration on the gold(I)-catalyzed enantioselective hydroamination
of 1 with benzyl carbamate.
Entry
L
AgX
Solvent
Yield [%][a]
ee [%]
Config.
1
2
3
4
5
6
7
8
(R)-2
(R)-4
(S)-2
(S)-2
(S)-2
(S)-2
(S)-2
(S)-2
(S)-2
(S)-2
AgOTf
AgOTf
AgSbF6
AgPF6
AgClO4
AgNTf2
AgBF4
AgBF4
AgBF4
AgBF4
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
toluene
THF
35
15
20
12
49
20
71
65
36
89
50
44
60
66
63
45
72
50
61
72[d]
R
R
S
S
S
S
S
S
S
S
9[b]
10[c]
dioxane
[a] Yield of isolated product; d.r. ꢁ25:1. Cbz=benzyloxycarbonyl.
[b] [1]=[H2NCbz]=0.4m, 48 h, yield determined by GC. [c] [1]=1.1m.
[d] 96% ee after recrystallization.
mer (d.r. ꢁ 25:1) with 50% ee (Table 1, entry 1).[11,12] Sub-
stitution of ligand (R)-2 with the SEGPHOS ligand (R)-4 led
to deterioration in both the yield and enantioselectivity of
hydroamination (Table 1, entry 2). Conversely, optimization
with respect to silver salt revealed that the use of AgBF4
instead of AgOTf led to marked improvement in both yield
and enantioselectivity of gold-catalyzed allene hydroamina-
tion (Table 1, entry 7). The reaction yield was further
improved through employment of a slight excess of allene
relative to benzyl carbamate and, in an optimized procedure,
treatment of benzyl carbamate (0.72m) with 1 (1.5 equiv) and
a catalytic 1:2 mixture of [{(S)-2}(AuCl)2] and AgBF4 in
dioxane at 248C led to isolation of (S)-3a in 89% yield with
72% ee (Table 1, entry 10). A single recrystallization from
warm hexanes increased the enantiopurity of (S)-3a to 96%
ee.
Initial experiments directed toward the intermolecular
EHA of allenes were only modestly encouraging. The
reaction of benzyl carbamate (0.72m) with 1-phenyl-1,2-
butadiene (1; 1 equiv) catalyzed by a 1:2 mixture of [{(R)-
2}(AuCl)2] ((R)-2 = (R)-DTBM-MeOBIPHEP, see structure
[*] K. L. Butler, M. Tragni, Prof. R. A. Widenhoefer
French Family Science Center, Duke University
Durham, NC 27708–0346 (USA)
E-mail: rwidenho@chem.duke.edu
[**] We thank the NIH (R01 GM080422-01) for support of this research.
K.L.B. was supported in part through a Charles Bradsher Fellowship
(Duke University) and M.T. was supported in part through a Marco
Polo Fellowship (Alma Mater Studiorum—University of Bologna).
In addition to benzyl carbamate, methyl carbamate; 9-
fluorenylmethyl carbamate; and trichloroethyl carbamate
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201584.
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underwent gold-catalyzed reaction with 1 to form N-allylic
carbamates 3b–3d with enantiopurities comparable to that of
3a (Table 2, entries 1–3).[13] Likewise, gold-catalyzed inter-
molecular EHA was effective for a number of 1-aryl-1,2-
butadienes (Table 2, entries 4–13). For example, gold(I)-
catalyzed intermolecular EHA was not restricted to 1-aryl-
1,2-butadienes, and gold-catalyzed reaction of 1,3-dialkyl-
substituted allene 7 with benzyl carbamate led to isolation of
N-allylic carbamate 8 in 94% yield with 68% ee [Eq. (1)].
Conversely, gold-catalyzed intermolecular EHA was not
effective for 1,3-disubstituted allenes lacking a methyl sub-
stituent.[14]
Table 2: Enantioselective hydroamination of allenes (1.1m) with carba-
mates (0.71m) catalyzed by a mixture of [{(S)-2}(AuCl)2] (2.5 mol%) and
AgBF4 (5 mol%) in dioxane at room temperature.
Entry
Ar
R[a]
Product
Yield [%][b]
ee [%][c]
Congruent with our expectations, allene racemization
occurred rapidly under reaction conditions. For example,
a solution of enantiomerically enriched (S)-5b (89% ee),
benzyl carbamate, and a catalytic mixture of [{(S)-2}(AuCl)2]
and AgBF4 was periodically analyzed by HPLC using a chiral
stationary phase; the analysis revealed complete racemization
of (S)-5b prior to any detectable formation of 6b (ꢂ 5 min;
Table 3). Interestingly, continued analysis of the reaction
mixture showed that the enantiopurity of 6b decreased from
69% ee at 21% conversion to a terminal value of 59% ee at
ꢁ 74% conversion (Table 3). This phenomenon was not
1
2
3
Ph (1)
Ph (1)
Ph (1)
CO2Me
Fmoc
Troc
3b
3c
3d
81
79
43
69
71
73
4
5
6
7
X=Me (5a)
X=OMe (5b)
X=CF3 (5c)
X=Br (5d)
Cbz
Cbz
Cbz
Cbz
6a
6b
6c
6d
97
85
82
99
72
60
75
69[d]
8
X=Me (5e)
X=i-Pr (5 f)
X=Ph (5g)
X=Br (5h)
Cbz
Cbz
Cbz
Cbz
6e
6 f
6g
6h
82
80
86
69
81
81
86
80
Table 3: Enantiopurity of allene and N-allylic carbamate as a function of
conversion for the gold-catalyzed hydroamination of (S)-5b (89% ee).
9[e]
10
11
12[e]
Cbz
6i
42
92
t [h]
Conv. [%]
5b ee [%]
6b ee [%]
(5i)
(5j)
ꢂ0.08
1.0
1.5
2.0
4.0
ꢂ2
21
39
44
74
ꢂ2
ꢂ2
ꢂ2
ꢂ2
ꢂ2
ꢂ2
ꢂ2
–
69
65
63
59
58
59
13[e]
Cbz
6j
44
72
5.0
6.0
81
>90
[a] Troc=2,2,2-trichloroethoxycarbonyl, Fmoc=fluorenylmethyloxy-
carbonyl. [b] Yield of isolated product; d.r.ꢁ25:1. [c] Enantiomeric
excess determined by HPLC analysis using chiral support. Absolute
configuration assigned as S based on analogy to (S)-3a. [d] 99% ee after
recrystallization. [e] Reaction run for 48 h.
restricted to the formation of 6b, and the enantiopurity of
3a formed in the gold-catalyzed reaction of 1 with benzyl
carbamate likewise decreased from 78% ee at 22% con-
version to a terminal value of 72% ee at ꢁ 83% conversion.
Although we initially considered that racemization of 3a
or 6b under the reaction conditions was responsible for the
conversion-dependent enantioselectivity displayed by the
gold-catalyzed intermolecular EHA of 1 or 5b, respectively,
this possibility was excluded. For example, stirring a solution
of (S)-3a (72% ee) and a catalytic 1:2 mixture of [{(S)-
2}(AuCl)2] and AgBF4 at room temperature for eight hours
either in the presence (1 equiv) or absence of benzyl
carbamate led to no detectable decrease in the enantiopurity
of (S)-3a [Eq. (2)]. Rather, a pair of experiments pointed to
the potentially deleterious effect of the N-allylic carbamate
product on the enantioselectivity of intermolecular EHA. For
example, treatment of a 1.5:1:1 mixture of 1, benzyl carba-
mate, and (S)-6c with a catalytic mixture of [{(S)-2}(AuCl)2]/
AgBF4 at 248C for 24 h led to isolation of (S)-3a in 75% yield
catalyzed reactions of benzyl carbamate with p-substituted
1-aryl-1,2-butadienes 5a–5d led to isolation of N-allylic
carbamates 6a–6d in 82–99% yield with 60–76% ee, albeit
without a clear relationship between the electron-donating or
-withdrawing properties of the arenes and the enantioselec-
tivity (Table 2, entries 4–7). Also worth noting is that the
enantiopurity of 6d increased from 69 to 99% ee after a single
recrystallization from warm hexanes (Table 2, entry 7). In
comparison, gold-catalyzed hydroamination of the ortho-
substituted 1-aryl-1,2-butadienes 5e–5h formed N-allylic
carbamates 6e–6h with 80–86% ee (Table 2, entries 8–11).
Gold(I)-catalyzed intermolecular hydroamination of the
more sterically hindered o,o-disubstituted 1-aryl-1,2-buta-
diene 5i occurred with even higher enantioselectivity (92%
ee) but with diminished yield (Table 2, entry 12). Gold-
2
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version of intermolecular EHA coupled with independent
analysis of the effect of N-allylic carbamate on the enantio-
selectivity of these transformations suggests that the nature of
the catalytically active species changes with increasing con-
centration of N-allylic carbamate. We continue to work
toward the identification of more selective and more general
catalytic systems for intermolecular EHA and toward the
development of ligand-modulated enantioselective gold cat-
alysis.
with 74% ee (Scheme 1), which is a nominally higher
enantioselectivity than was realized in the absence of (S)-6c
(Table 1, entry 10). However, the corresponding reaction of 1,
benzyl carbamate, and (S)-6c catalyzed by [{(R)-2}(AuCl)2]/
AgBF4 led to isolation of (R)-3a in 71% yield with 62% ee
(Scheme 1), which is a considerably lower enantioselectivity
than was observed in the absence of (S)-6c.
Received: February 28, 2012
Published online: && &&, &&&&
Keywords: allenes · asymmetric catalysis ·
.
asymmetric synthesis · gold · hydroamination
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Scheme 1. Effect of N-allylic carbamate (S)-6c on the hydroamination
of 1 with benzyl carbamate.
Rapid allene racemization precluded stereochemical
analysis of the gold-catalyzed intermolecular EHA of (S)-
5b (Table 3). However, we have previously established the
anti stereochemistry of gold(I)-catalyzed intramolecular
enantioselective allene hydrofunctionalization,[7,15] which is
consistent with the outer-sphere addition of the nucleophile
to a gold(I) p-allene complex. Although such a mechanism
requires that one of the two gold centers of a bis(gold) catalyst
ꢀ
participates in the allene activation/C N bond formation
process, there is no firm evidence that supports the direct
participation of the second gold center in these steps.[10,16]
There is, however, evidence that the ligation state of this
spectator gold center can affect the enantioselectivity of gold-
catalyzed hydrofunctionalization.[8] Given the noncoordinat-
ꢀ
ing nature of the BF4 counterion employed in the gold-
catalyzed intermolecular EHA of allenes, the spectator gold
center is presumably ligated with an allene, benzyl carbamate,
or either enantiomer of the N-allylic carbamate product.[17]
Therefore, the decreasing enantioselectivity of the gold-
catalyzed intermolecular EHA of allenes with increasing
conversion presumably reflects the increasing concentration
of a less selective N-allylic carbamate ligated catalyst species.
In summary, we have developed a gold(I)-catalyzed
protocol for the stereoconvergent, intermolecular enantiose-
lective hydroamination of chiral, racemic 1,3-disubstituted
allenes with N-unsubstituted carbamates. Furthermore, the
observed decrease in enantioselectivity with increasing con-
[4] Yamamoto has reported the stereospecific, intermolecular
hydroamination of chiral allenes: N. Nishina, Y. Yamamoto,
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[11] a) The absolute configuration of (S)-3a was determined by
25
optical rotation {½aꢃD ¼ꢀ39.4 (c = 0.19, CH2Cl2); lit. [for (S)-3a]
23
½aꢃD ¼ꢀ55.4 (c = 1.10, CH2Cl2)}.[11b] The absolute configurations
of all remaining N-allylic carbamates were assigned by analogy;
b) J. C. A. Hunt, P. Laurent, C. J. Moody, J. Chem. Soc. Perkin
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[13] Sulfonamides were less effective than were carbamates. For
example, gold-catalyzed reaction of 1 with p-toluenesulfonamide
at room temperature for 48 h formed (E)-4-methyl-N-(4-phenyl-
but-3-en-2-yl)benzenesulfonamide in 57% yield with 28% ee.
[14] For example, gold-catalyzed reaction of 1-phenyl-1,2-pentadiene
with benzyl carbamate at room temperature for 48 h formed (E)-
benzyl (1-phenylpent-1-en-3-yl)carbamate in 13% yield with
23% ee.
[15] Z. Zhang, R. A. Widenhoefer, Angew. Chem. 2007, 119, 287 –
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Asymmetric Catalysis
K. L. Butler, M. Tragni,
R. A. Widenhoefer*
&&&& — &&&&
Gold(I)-Catalyzed Stereoconvergent,
Intermolecular Enantioselective
Hydroamination of Allenes
Gold and silver: A 1:2 mixture of [{(S)-
1}(AuCl)2] and AgBF4 catalyzes the enan-
tioselective hydroamination of chiral,
racemic 1,3-disubstituted allenes with N-
unsubstituted carbamates to form N-
allylic carbamates in good yield, with high
regio- and diastereoselectivity, and up to
92% ee (see scheme, Cbz=benzyloxy-
carbonyl).
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