Gold(I)-catalyzed intramolecular amination of allylic alcohols with alkylamines

Article abstract of DOI:10.1021/ol103175w

A 1:1 mixture of (1)AuCl [1 = P(t-Bu)₂o-biphenyl] and AgSbF ₆ catalyzes the intramolecular amination of allylic alcohols with alkylamines to form substituted pyrrolidine and piperidine derivatives. Gold(I)-catalyzed cyclization of (R,Z)-8-(N-benzylamino)-3-octen-2-ol (96% ee, 95% de) led to isolation of (R,E)-1-benzyl-2-(1-propenyl)piperidine in 99% yield with 96% ee, consistent with the net syn addition of the amine relative to the departing hydroxyl group.

Full text of DOI:10.1021/ol103175w

ORGANIC
LETTERS
2011
Vol. 13, No. 6
1334–1337
Gold(I)-Catalyzed Intramolecular
Amination of Allylic Alcohols With
Alkylamines
Paramita Mukherjee and Ross A. Widenhoefer*
Duke University, Department of Chemistry, French Family Science Center, Durham,
North Carolina 27708-0346, United States
rwidenho@chem.duke.edu
Received December 30, 2010
ABSTRACT
A 1:1 mixture of (1)AuCl [1 = P(t-Bu)2o-biphenyl] and AgSbF6 catalyzes the intramolecular amination of allylic alcohols with alkylamines to form
substituted pyrrolidine and piperidine derivatives. Gold(I)-catalyzed cyclization of (R,Z)-8-(N-benzylamino)-3-octen-2-ol (96% ee, 95% de) led to
isolation of (R,E)-1-benzyl-2-(1-propenyl)piperidine in 99% yield with 96% ee, consistent with the net syn addition of the amine relative to the
departing hydroxyl group.
In 2002, Ozawa first demonstrated the intermolecular
amination of underivatized allylic alcohols without the
need for a Lewis acid cocatalyst through employment of
cationic palladium(II) bis(phosphine) complexes.1 Since
this time, a number of transition metal complexes
including Pd(0),2,3 Pt(II),4 Mo(VI),5 Bi(III),6 Au(I),7 and
Au(III)7 have been shown to catalyze the intermolecular
amination of allylic alcohols in the absence of Lewis acid
activators.8 Included in this family of transformations is a
subset that employs simple alkylamines as nucleophiles.2,4
The intramolecular amination of underivatized allylic
alcohols has also attracted attention as a route to functio-
nalized nitrogen heterocycles, and methods employing
Au(III),9 Bi(III),10 and Pd(II)11-13 catalysts in combina-
tion with carbamate or sulfonamide nucleophiles have
been documented. Of these, the Pd(II)-catalyzed methods
(1) Ozawa, F.; Okamoto, H.; Kawagishi, S.; Yamamoto, S.; Minami,
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€
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r
10.1021/ol103175w
Published on Web 02/11/2011
2011 American Chemical Society

suggested that alkylamines might also be suitable nucleo-
philes for gold(I)-catalyzed intramolecular allylic amina-
tion.18,19 Indeed, here we report the regio- and stereoselec-
tive gold(I)-catalyzed intramolecular amination of under-
ivatized allylic alcohols with alkylamines.
Table 1. Effect of Nucleophile on the Gold(I)-Catalyzed Intra-
molecular Amination of Allylic Alcohols 2
Toward the development of a gold(I)-catalyzed protocol
for the intramolecular amination of underivatized allylic
alcohols with alkylamines, we targeted 6-(N-benzylamino)-
5,5-diphenyl-2-hexenol (2a) in combination with the cata-
lyst system employed for intermolecular amination. This
combination proved highly efficient and treatment
of 2a with a catalytic 1:1 mixture of (1)AuCl and AgSbF6
(5 mol %) in dioxane at room temperature for 2 h led to
isolation of 2-vinyl pyrrolidine 3a in 97% yield (Table 1,
entry 1).20 In addition to N-benzylamine, an n-butyla-
mine (2b), tert-butylamine (2c), or primary amine (2d)
entry
R
substrate
time (h)
pyrrolidine
yield (%)a
1
Bn
2a
2b
2c
2d
2e
2e
2
2
3a
3b
3c
3d
3e
3e
97
96
87
91
48
97
2
n-Bu
t-Bu
H
3
2
4
48
72
48
5
6b
Cbz
Cbz
a Isolated yield in >95% purity. b AgClO4 used in place of AgSbF6.
are most highly developed and have been applied to the
synthesis of a number of naturally occurring nitrogen
heterocycles.12,13 However, in contrast to the intermolecu-
lar amination of allylic alcohols, metal-catalyzed intramo-
lecular amination of allylic alcohols with simple
alkylamines has not been demonstrated.14
Table 2. Substrate Scope of the Intramolecular Amination of
Allylic Alcohols Catalyzed by a 1:1 Mixture of (1)AuCl and
AgSbF6 (5 mol %) in Dioxane
We have recently reported the intermolecular amination
of underivatized allylic alcohols with imidazolidin-2-ones
catalyzed by a mixture of (1)AuCl [1 = P(t-Bu)2o-
biphenyl] and AgSbF6, a transformation that was distin-
guished by the high γ-regioselectivity of addition in the
case of unhindered allylic alcohols.15 Concurrent with our
efforts, Bandini and Aponick have developed effective
gold(I)-catalyzed methods for the intramolecular aryla-
tion16 and alkoxylation17 of allylic alcohols, and these
results pointed to the feasibility of gold(I)-catalyzed in-
tramolecular allylic amination. Furthermore, Aponick’s
demonstration of gold(I)-catalyzed intramolecular amina-
tion of propargylic alcohols with secondary alkylamines
(14) A single example of the acid-catalyzed intramolecular amination
of a tertiary allylic alcohol with an alkylamine has been documented.
Yokoyama, Y.; Hikawa, H.; Mitsuhashi, M.; Uyama, A.; Murakami, Y.
Tetrahedron Lett. 1999, 40, 7803.
(15) Mukherjee, P.; Widenhoefer, R. A. Org. Lett. 2010, 12, 1184.
(16) (a) Bandini, M.; Eichholzer, A. Angew. Chem., Int. Ed. 2009, 48,
9533. (b) Bandini, M.; Eichholzer, A.; Gualandi, A.; Quinto, T.; Savoia,
D. ChemCatChem 2010, 2, 661.
(17) (a) Aponick, A.; Biannic, B. Synthesis 2008, 3356. (b) Aponick,
A; Li, C.-Y.; Palmes, J. A. Org. Lett. 2009, 11, 121. (c) Aponick, A.; Li,
C.-Y.; Biannic, B. Org. Lett. 2008, 10, 669. (d) Aponick, A.; Biannic, B.;
Jong, M. R. Chem. Commun. 2010, 46, 6849. (e) Bandini, M.; Monari,
M.; Romaniello, A.; Tragni, M. Chem.;Eur. J. 2010, 16, 14272.
(18) Aponick, A.; Li, C.-Y.; Malinge, J.; Marques, E. F. Org. Lett.
2009, 11, 4624.
(19) For other examples of gold-catalyzed amination of benzylic or
propargylic alcohols, see: (a) Georgy, M.; Boucard, V.; Debleds, O.; Dal
Zotto, C.; Campagne, J.-M. Tetrahedron 2009, 65, 1758. (b) Terrasson,
V.; Marque, S.; Georgy, M.; Campagne, J.-M.; Prim, D. Adv. Synth.
Catal. 2006, 348, 2063.
(20) Control experiments established that both (1)AuCl and AgSbF6
are required for efficient conversion of 2a to 3a and provided strong
evidence against a Brønsted acid catalyzed pathway: Treatment of 2a
with either (1)AuCl (10 mol %) or HOTf (10 mol %) in dioxane at 23 °C
for 24 h led to no detectable consumption of 2a and no detectable
formation of 3a; treatment of 2a with AgSbF6 (10 mol %) in dioxane at
23 °C for 24 h led to <10% conversion to 3a.
a Isolated yield in >95% purity. b Diastereomeric ratio determined
by 1H NMR analysis of the crude reaction mixture.
Org. Lett., Vol. 13, No. 6, 2011
1335

moiety also functioned as an effective nucleophile for
gold(I)-catalyzed intramolecular allylic amination,
although the primary amine required 48 h to reach com-
pletion (Table 1, entries 2-4). In comparison, gold(I)-
catalyzed cyclization of benzyl carbamate 2e at room
temperature for 72 h formed pyrrolidine 3e in only 48%
yield (Table 1, entry 5). Substitution of AgClO4 for
AgSbF6 led to a marked increase in the efficiency of the
conversion of 2e to 3e (Table 1, entry 6), although this
transformation was still considerably slower than was the
gold(I)-catalyzed cyclization of substrates 2a-2c.
Scheme 1
We next investigated the effects of substitution, ring size,
and alkene configuration on the efficiency and diastereos-
electivity of gold(I)-catalyzed intramolecular allylic ami-
nation (Table 2). Cyclohexyl-substituted allylic alcohols4a
and 4b that possessed a benzylamine or R-methylbenzyl
amine nucleophile underwent intramolecular allylic ami-
nation to form pyrrolidines 5a and 5b, respectively, in high
yield and in the case of 4b with modest (2:1) diastereos-
electivity (Table 2, entries 1 and 2). gem-Dialkyl substitu-
tion along the alkyl chain that tethered the amino group to
the allylic alcohol moiety facilitated intramolecular allylic
amination but was not required (Table 2, entries 3-5). For
example, 6-amino-2-hexenol derivative 6a that possessed a
C5 phenyl group underwent cyclization at 60 °C to form a
separable 1.3:1 mixture of cis-7a and trans-7a in 95%
combined yield (Table 2, entry 3). Similarly, unsubstituted
6-(N-benzylamino)-2-hexenol (8; Z/E = 7.7:1) underwent
gold(I)-catalyzed cyclization at 60 °C to form 1-benzyl-2-
vinylpyrrolidine (9) in 86% yield (Table 2, entry 5).
Periodic analysisofthe conversion of 8 to 9 by GCrevealed
no significant difference in the rates of cyclization of (E)-8
and (Z)-8.
Gold(I)-catalyzed intramolecular allylic amination was
also effective for the formation of piperidine derivatives
(Table 2, entries 6-8). For example, gold(I)-catalyzed
cyclization of (Z)-6-(N-benzylamino)-2-heptenol (10; Z/E
g50:1) at 60 °C for 16 h led to isolation of 1-benzyl-2-vinyl
piperidine (11) in 91% yield (Table 2, entry 6). Similarly,
gold(I)-catalyzed cyclization of (Z)-8-(N-benzylamino)-3-
octen-2-ol (12) at 100 °C led to isolation of 2-(1-propenyl)
piperidine 13 in 99% yield with g50:1 diastereoselectivity
(Table2, entry 7). Gold(I)-catalyzed cyclizationofdithiane
derivative 14 that possessed a stereogenic center R to the
amino group at 100 °C for 48 h formed cis-2,6-disubsituted
piperidine 15 in 91% yield with 25:1 diastereoselectivity
(Table 2, entry 8). Piperidine 15 represents an advanced
intermediate in the synthesis of a range of naturally
occurring piperidine alkaloids.21
1). Piperidine (R,E)-13 represents a direct precursor to the
naturally occurring alkaloid (S)-(þ)-coniine, and this con-
nection was exploited to determine both the enantiopurity
and absolute configuration of (R,E)-13 generated via gold-
(I)-catalyzed cyclization of (R,Z)-12. To this end, one-pot
hydrogenative cleavage of the benzyl group and reduction
of the exocyclic CdC bond (Pd/C, MeOH, HCOOH)
followed by acidification with aqueous HCl gave (S)-(þ)-
coniine hydrochloridein 90% isolated yield asa white solid
{[R]20 þ6.9° (c 0.1, EtOH); lit. [R]21 þ7.1° (c 1.0,
D
D
EtOH)}.13,22 Chiral phase GC analysis of the correspond-
ing (S)-(þ)-coniine trifluoroacetamide (TFAA, Et3N,
85%) revealed an enantiomeric purity of 96% ee.
Together, these results established the complete transfer
of chirality in the conversion of (R,Z)-12 to (R,E)-13 and
net syn addition of the amine relative to the departing
hydroxyl group.
Although the syn substitution observed in the conver-
sion of (R,Z)-12 to (R,E)-13 is consistent with a concerted
SN20 pathway,23 a mechanism involving σ-activation of the
hydroxyl group appears unlikely given the low oxophilicity
of gold(I). Rather, the pronounced π-acidity of cationic
gold(I) complexes points to a mechanism involving nu-
cleophilic addition/OH-elimination initiated by π-activa-
tion of the allylic CdC bond.24,25 Within this framework,
the net syn-substitution in the conversion of (R,Z)-12 to
(22) For recent examples of the enantioselective synthesis of (S)-(þ)-
coniine, see: (a) Lebrun, S.; Couture, A.; Deniau, E.; Grandclaudon, P.
Org. Lett. 2007, 9, 2473. (b) Girard, N.; Pouchain, L.; Hurvois, J.-P.;
Moinet, C. Synlett 2006, 1679. (c) Nagata, K.; Nishimura, K.; Yokoya,
M.; Itoh, T. Heterocycles 2006, 70, 335. (d) Itoh, T.; Nishimura, K.;
Nagata, K.; Yokoya, M. Synlett 2006, 2207. (e) Xu, X.; Lu, J.; Li, R.; Ge,
Z.; Dong, Y.; Hu, Y. Synlett 2004, 122. (f) Gommermann, N.; Knochel,
P. Chem. Commun. 2004, 2324. (g) Passarella, D.; Barilli, A.; Belinghieri,
F.; Fassi, P.; Riva, S.; Sacchetti, A.; Silvani, A.; Danieli, B. Tetrahedron:
Asymmetry 2005, 16, 2225.
(23) (a) Magid, R. M. Tetrahedron 1980, 36, 1901. (b) Paquette, L. A.;
Stirling, C. J. M. Tetrahedron 1992, 48, 7383.
(24) For a recent review on the mechanisms of gold(I)-catalyzed
transformations, see: Hashmi, A. S. K. Angew. Chem., Int. Ed. 2010, 49,
5232.
To evaluate the selectivity of 1,3-chirality transfer in the
gold(I)-catalyzed intramolecular amination of allylic alco-
hols with secondary alkylamines, we employed enantio-
merically and diastereomerically enriched (R,Z)-12 (96%
ee, 95% de). Treatment of (R,Z)-12 with a catalytic 1:1
mixture of (1)AuCl and AgSbF6 led to isolation of (R,E)-
13 in 99% yield as a single diastereomer (g50:1) (Scheme
(25) For recent examples of cationic, two-coordinate gold π-alkene
complexes, see: (a) Brown, T. J.; Dickens, M. G.; Widenhoefer, R. A. J.
Am. Chem. Soc. 2009, 131, 6350. (b) Brown, T. J.; Dickens, M. G.;
Widenhoefer, R. A. Chem. Commun 2009, 6451. (c) Hooper, T. N.;
Green, M.; McGrady, J. E.; Patel, J. R.; Russell, C. A. Chem. Commun
2009, 3877. (d) Zuccaccia, D.; Belpassi, L.; Tarantelli, F.; Macchioni, A.
ꢀ
(21) Ying, Y.; Kim, H.; Hong, J. Org. Lett. 2011, ASAP; DOI:
10.1020/ol103064f.
J. Am. Chem. Soc. 2009, 131, 3170. (e) de Fremont, P.; Marion, N.;
Nolan, S. P. J. Organomet. Chem. 2009, 694, 551.
1336
Org. Lett., Vol. 13, No. 6, 2011

(R,E)-13 is consistent with either a syn-addition/syn-elim-
ination or an anti-addition/anti-elimination sequence.
Uenishi and Kawai have proposed π-activation mechanisms
involving hydroxyl group-directed syn-addition/syn-elim-
ination to account for the syn-substitution observed for the
Pd(II)-catalyzed alkoxylation26 and amination13 of allylic
alcohols. However, owing tothe strong preferenceof catio-
nic gold(I) complexes to form two-coordinate, as opposed
to three-coordinate, complexes,27 a mechanism involv-
ing hydroxyl group directed syn-addition/syn-elimination
appears unlikely. Alternatively, Maseras has provided
computational evidence for an anti-addition/anti-elimina-
tion sequence for the gold(I)-catalyzed isomerization of
allylic ethers in the presence of alcohol.28 A key component
of this pathway was the formation of a six-membered
chelate structure involving hydrogen bonding between
the nucleophile and allylic alkoxy group that both facil-
itates nucleophilic addition and stabilizes the initially
formed cationic intermediate.29
Scheme 2
Guided by the hypotheses of Maseras, we propose a
mechanism involving anti-addition/anti-elimination to ac-
count for the gold(I)-catalyzed conversion of (R,Z)-12 to
(R,E)-13. Within this construct, two diastereomeric reac-
tion manifolds are feasible resulting from cyclization of the
diasteromeric gold(I) π-alkene complexes si-I and re-I
(Scheme 2). In the former manifold, outer-sphere addition
of the pendant amine to the complexed CdC bond of si-I
would form the cyclic, hydrogen-bonded gold alkyl inter-
mediate (R,S,R)-II. Proton transfer to the hydroxyl group
followed by anti-elimination of water and displacement of
gold would form (R,E)-13. In the latter reaction manifold,
outer-sphere addition of the pendant amine to the com-
plexed CdC bond of re-I to form (S,R,R)-II followed by
proton transfer, anti-elimination, and alkene displacement
would form (S,Z)-13 (Scheme 2). A rationale for selective
formation of (R,E)-13 in preference to (S,Z)-13 can be
found in the consideration of the potential chairlike con-
formations of the initially formed hydrogen-bonded bicyc-
lic structures (R,S,R)-II and (S,R,R)-II. Whereas inter-
mediate(R,S,R)-IIcan adopt afullyequatorial-substituted
cis-decalin type conformation, intermediate (S,R,R)-II
possesses a pseudoaxial methyl substituent. This latter
species should experience unfavorable 1,3-diaxial interac-
tion with the piperidine methylene group, thereby disfa-
voring cyclization of gold π-alkene complex re-I leading to
selective formation of (R,E)-13.
In summary, we have developed a gold(I)-catalyzed
protocol for the intramolecular amination of allylic
alcohols with secondary alkylamines. The procedure
tolerated a number of alkylamines and substitution
patterns, was effective for the synthesis of both pyrro-
lidine and piperidine derivatives, and occurred with
selective 1,3-chirality transfer in the conversion of
(R,Z)-12 to (R,E)-13. This latter observation estab-
lished the net syn addition of the amine relative to the
departing hydroxyl group.
Acknowledgment. We thank the NIH (GM-080422) for
support of this research and Prof. J. Hong (Duke
University) for enlightening discussions and for disclosing
to us his efforts in the area of stereoselective piperidine
formation prior to publication.
(26) (a) Kawai, N.; Lagrange, J.-M.; Ohmi, M.; Uenishi, J. J. Org.
Chem. 2006, 71, 4530. (b) Kawai, N.; Lagrange, J.-M.; Uenishi, J. Eur. J.
Org. Chem. 2007, 2808. (c) Uenishi, J.; Vikhe, Y. S.; Kawai, N. Chem.;
Asian J. 2008, 3, 473.
(27) (a) Gimeno, M. C.; Laguna, A. Chem. Rev. 1997, 97, 511. (b)
Carvajal, M. A.; Novoa, J. J.; Alvarez, S. J. Am. Chem. Soc. 2004, 126,
1465. (c) Schwerdtfeger, P.; Hermann, H. L.; Schmidbaur, H. Inorg.
Chem. 2003, 42, 1334.
(28) Paton, R. S.; Maseras, F. Org. Lett. 2009, 11, 2237.
(29) Hashmi has recently highlighted the importance of hydrogen-
bonding interactions in the proton transfer steps in gold(I)-catalyzed
hydration of alkynes. Krauter, C. M.; Hashmi, A. S. K.; Pernpointner,
M. ChemCatChem 2010, 2, 1226.
Supporting Information Available. Experimental pro-
cedures, analytical and spectroscopic data, and copies of
HPLC traces and NMR spectra for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Org. Lett., Vol. 13, No. 6, 2011
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