Counterion-mediated hydrogen-bonding effects: Mechanistic study of gold(I)-catalyzed enantioselective hydroamination of allenes

Article abstract of DOI:10.1002/asia.201100135

Two of a kind: A new type of counterion-mediated syn-addition mechanism has been presented in gold-catalyzed enantioselective hydroamination reactions with [(R)-binap(AuOPNB)₂] (OPNB=para-nitrobenzoate). A combination of both hydrogen bonding and the N-Au interaction efficiently controlled the challenging enantioselectivity of allenes.

Full text of DOI:10.1002/asia.201100135

COMMUNICATION
DOI: 10.1002/asia.201100135
Counterion-Mediated Hydrogen-Bonding Effects: Mechanistic Study of
Gold(I)-Catalyzed Enantioselective Hydroamination of Allenes
Ji Hye Kim, Sung-Woo Park, Sae Rom Park, Sungyul Lee,* and Eun Joo Kang*[a]
Dedicated to Professor Eun Lee on the occasion of his retirement and 65th birthday
À
The use of transition metals as unsaturated C C bond ac-
tivators toward nucleophilic attack continues to grow expo-
nentially for efficient and atom-economic organic transfor-
mations. The majority of nucleophilic additions proceed
through the outer-sphere mechanism, that is, anti addition to
been selected as the form of chiral bisACTHNGUTERNNU(G gold)-phosphine com-
À
plexes, which could lead to formation of aurophilic Au Au
interactions.[7] Another intriguing phenomenon is the pro-
nounced counterion effect that points to the importance of
nonbonding interactions between the auxiliary (AuX) group
and the reactive gold center. Herein we report a new type
of the counterion-directed syn-addition pathway in gold-cat-
alyzed hydroamination reactions. In parallel, we have found
that the nonbonding interaction between the nucleophile
and gold in the pre-reaction complex is the origin of the
enantioselectivity of complex allene substrates.
À
the metal-coordinated C C multiple bond complexes. How-
ever, few alternative inner-sphere mechanisms[1] have been
proposed, in which the coordination of the nucleophile to
À
the metal is followed by insertion of a C C multiple bond
[2a,b]
À
into the M Nu bond, such as the Ir,
Ln,[2c] Pd,[2d] and
Au-catalyzed[2e–k] hydrofunctionalization reactions. While
the detailed variation of the inner-sphere mechanism highly
depends on the electronic and redox character of transition
metals, more importantly, these reactions are distinguished
by syn-stereochemical pathways from the outer-sphere
mechanism.
To obtain insight into the possible origins of enantioselec-
tivity in the gold-catalyzed hydroamination reactions, we
have carried out both experiments and quantum chemical
studies, employing the density functional theory method
B3LYP[9] with the 6-31G basis set and the effective core po-
tential for Au (Hay–Wadt VDZ),[10] as implemented in the
Gaussian 09[11] set program. Stationary structures are con-
firmed by ascertaining that all the harmonic frequencies are
real. The structure of the transition state is obtained by veri-
fying that one and only one of the harmonic frequencies is
imaginary, and also by carrying out the intrinsic reaction co-
ordinate (IRC) analysis along the reaction pathway. Zero
point energies (ZPE) are taken into account, and default
criteria are used for all optimizations.
Based on the previous observations in the gold-catalyzed
enantioselective hydroamination (Scheme 1),[5b] the enantio-
selectivity was concluded to be strongly affected by the re-
maining counterion coordinated to gold, and the para-nitro-
benzoate (OPNB) counterion proved to be an ideal one to
transfer the chiral information of the 2,2’-bis(diphenylphos-
phino)-1,1’-binaphthyl (binap) ligand to the relatively distant
reaction center. Furthermore, this counterion effect in gold-
catalyzed hydroamination was developed and applied by
using a chiral 2,2’-dihydroxy-1,1’-binaphthyl (binol)-derived
phosphate anion, thus rendering the chiral counterion-medi-
ated transition metal catalysis powerful.[12] To check the role
of the counterion in the catalytically active species, we
While intensive stereochemical investigations into various
kinds of gold(I) catalysis[3] suggested the anti-addition mech-
anism, our specific interest began from the significant enan-
tioselectivity in the gold(I)-catalyzed transformations of al-
lenes. Despite a number of reports on transition metal cata-
lyzed addition of heteroatom nucleophiles to allenes,[4,5] the
asymmetric variants are reported exclusively with the gold
catalyst.[6,7] This fact stimulates ideas for the perfect role of
a gold catalyst in the challenging control of stereoselectivi-
ties, especially when substituted allenes are used as sub-
strates. Enantioselective gold(I) catalysis is not restricted to
classic p-activation processes. The efficient gold catalyst has
[a] J. H. Kim,+ S.-W. Park,+ S. R. Park, Prof. S. Lee, Prof. E. J. Kang
Department of Applied Chemistry
Kyung Hee University
Yongin-si, Gyeonggi-do, 446-701 (Korea)
Fax : (+82)31-202-7337
E-mail: sylee@khu.ac.kr
ejkang24@khu.ac.kr
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100135.
1982
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 1982 – 1986

N-tosyl protecting group, after confirming that the omission
slightly alters the energetics of profiles,[14] although the path-
ways could vary according to the electronic character of the
amine.[3g] In the global minimum energy structure, the N Au
À
distance is quite small (RNÀAu =2.14 ꢁ), thus indicating
strong interactions between the amino and the gold atom.[15]
This complex is, however, not amenable to the hydroamina-
tion reaction, because the allene moiety is located far from
the amino group. The hydroamination reaction begins with
the coordination of the allene to the cationic gold catalyst,
[(R)-binapACTHNUTRGNEUNG
(AuOPNB)]AuI+, thus forming the h2 allene-gold
complexes described in the pre-reaction complexes. The
most characteristic feature of the optimized geometries of
Scheme 1. Effect of counterion in the gold(I)-catalyzed enantioselective
hydroamination.
À
pre-reaction complexes is the presence of N H···O hydro-
gen bonding between the amino alkyl chain and the ben-
zoate anion coordinated to gold; this allows nucleophilic
attack to occur at the cis position to gold, and not from the
less hindered opposite side. Despite extensive computational
searching, any attempt to find the latter type of pre-reaction
complexes failed. We extensively searched over the poten-
tial-energy landscape of the present system by designating
the initial conformation of the catalyst and the substrate for
anti addition and letting the system relax to any stationary
structure with all real frequencies by the Gaussian optimiza-
tion algorithm (Figure 2). We have found that no stationary
structures are obtained for the anti-addition pathway, and
that the initial structure drifts down to lower energy towards
a post-reaction complex (S), as shown in Figure 1.[16]
looked more closely to the isolated monocationic gold cata-
lyst with the OPNB counterion. In the calculated structure,
the RCO2 anion coordinates in a bidentate fashion to the
À
À
bis
N
and 2.10 ꢁ, respectively; this favors the equilibrium to the
monocationic complex by the counterion dissociation.[6b] Re-
markably, the prominent metallophilic interactions known
as aurophilic attraction, are observed between Au…Au cen-
ters (3.09 ꢁ) and this organized monocationic gold complex
is considered as the efficient active catalyst to provide enan-
tioselective reactions.[13]
Figure 1 presents the structures and the energy profiles of
the pathways towards the S and R isomers of the hydroami-
nation product. Here, we employ a model reaction without
Figure 1. Energy profile for two pathways in the syn-addition mechanism. Distances in ꢁ, and relative energies in kcalmolÀ1. pre-RC (S)=pre-reaction
complex towards the (S) isomer, etc., TS=transition state, post-RC=post-reaction complex, X=para-nitrobenzoate. Other protons on the amine and
phosphine groups are omitted to simplify the figure.
Chem. Asian J. 2011, 6, 1982 – 1986
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1983

E. J. Kang et al.
COMMUNICATION
Table 1. Gold(I)-catalyzed enantioselective hydroamination of unsymmetric
allenes.[a]
E/Z=2.2:1
E/Z=1.5:1
E/Z=2.0:1[b]
E/Z=2.6:1[b]
83% yield
97% yield
98% yield
89% yield
93(E), 80(Z)% ee 93(E), 93(Z)% ee 79(E), 89(Z)% ee 77(E), 93(Z)% ee
[a] 5 mol% [(R)-binap(AuOPNB)2], MeNO2 (0.3m), 508C, 15 h; Yield of
isolated products (E+Z) after column chromatography. [b] 3 mol% [(S)-
binap(AuOPNB)2].
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Figure 2. Snapshots from a conformation in which the amino group is lo-
cated in an anti position to the allene.
Although we expected that the Z isomers might predomi-
nate over E isomers, hypothesizing that the gold catalyst
would approach from the face of the less hindered methyl
substituent rather than the ethyl or isopropyl substituted
side, E isomers were obtained as the major products. The
1:1 mixture of E and Z isomers was treated with gold cata-
lyst to check the possibility of cyclization and subsequent
isomerization, however, the ratio did not change. Thus, we
theorized that the reaction would not be effectively con-
trolled by the steric differences of the allene substituents in
the coordinating process by the gold catalyst. High enantio-
meric excess (around 90%) was obtained for each E or
Z isomer, in accordance with the result of a nonbonding in-
teraction between the amine nucleophile and gold catalyst.
To investigate the coordination geometry of the gold cata-
lyst producing the unexpected E/Z ratio, the enantio-rich
allene substrate 3c (50% ee) was treated with the [(R)-
This structure of the pre-reaction complex may well de-
scribe the origin of the observed enantioselectivity. Of the
two pre-reaction complexes corresponding to two different
coordinating faces of allene, the pre-reaction complex to-
wards the S isomer (pre-RC (S)) exhibits a much shorter
N…Au distance (2.44 ꢁ) than that of pre-RC (R) (4.18 ꢁ),
because the interactions between the amino alkyl chain and
gold are not interfered by the allenic proton in the former
structure. The preorganized structure of pre-RC (S), calcu-
lated to be more stable (energy is lower by 4.8 kcalmolÀ1)
than pre-RC (R), might result in a lower activation barrier
to S isomers than the isomeric pathway (2.5 vs 4.8 kcalmolÀ1
from the pre-RCs, 23.6 vs 30.7 kcalmolÀ1 from the global
minimum energy structure). It seems that significant weak-
ening of hydrogen bonding (N H O distance increases
from 2.10 to 4.10 ꢁ) in the TS (R) renders it much less
stable than TS (S) in which the N H···O distance remains
…
À
binapACTHNGUTERN(UNG AuOPNB)2] and [(S)-binapACHTUNGRTNEUN(NG AuOPNB)2] catalysts
(Table 2). Both reactions predominantly afforded the
E isomer as a major product in the ratio of approximately
2:1, which is an identical result to that of racemic allene as a
starting material. This verifies that the two enantiomeric
species might interconvert reversibly under the gold(I)-cata-
lyzed reaction conditions.[6d,20]
À
essentially the same.[17] Therefore, the reaction pathway to-
wards the S isomer is much more feasible both in the ther-
modynamic and kinetic sense, in excellent agreement with
the experimentally observed enantioselectivity in the hydro-
amination of allenes. Deprotonation of the post-reaction
complex followed by protonolysis of the vinyl gold bond
(protodeauration), which is considered to proceed with re-
tention of configuration, is ruled out as the origin of enan-
tioselectivity.[18]
Table 2. Gold(I)-catalyzed enantioselective hydroamination of enantio-
rich allenes.[a]
To further provide the evidence for this syn mechanism,
we prepared a number of alkyl-substituted unsymmetric al-
lenes, and applied them to the gold-catalyzed enantioselec-
tive hydroamination reactions (Table 1). We envisioned that
this difference in spacial environments of allenes would give
different effects on the coordinating direction of the gold
catalyst. Treating various tosylamides with the binuclear
Precursor
L*
Product
Yield
E/Z
ee (E)
ee (Z)
rac-3c
(R)-binap
(R)-binap
(S)-binap
4c
4c
ent-4c
95
97
97
2.0:1
2.0:1
2.0:1
78
81
À82
88
90
À82
(S)-3c[b]
(S)-3c[b]
gold catalyst, [(R)-binapACHTNUTRGENNG(U AuOPNB)2], at 508C produced 2-
[a] Reaction conditions: 3 mol% L*ACHTUNTRGNEUNG(AuOPNB)2, MeNO2 (0.3m), 508C.
Yield (%) of isolated products (E+Z) after column chromatography.
[b] 50% ee of (S)-3c.
vinyl pyrrolidines with excellent yields, but showed low dia-
stereoselectivity of 2:1 to 3:1 in favor of the E isomers.[19]
1984
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Chem. Asian J. 2011, 6, 1982 – 1986

Gold(I)-Catalyzed Enantioselective Hydroamination of Allenes
Calculations were carried out for the intermediates of
(R)-4b/(S)-4b and (E)-4b/(Z)-4b to elucidate the observed
stereoselectivity in favor of the S,E isomer. We found that
the intermediates of the R,E and R,Z isomers are much less
favored than those of S isomers; this is consistent with our
previous discussions for Figure 1. The pre-reaction complex
towards S,Z isomer is calculated to be the most stable
(energy (Gibbs free energy) is 2.0 (2.9) kcalmolÀ1 lower
than S,E isomer; Figure 3). However, the post-reaction com-
plex towards S,E form is of slightly lower energy (by 0.8 kcal
molÀ1) and the resulting larger reaction Gibbs free energy
DG seems to make it more favored. This difference in
Acknowledgements
This research was supported by Basic Science Research Program through
the National Research Foundation of Korea (NRF) funded by the Minis-
try of Education, Science and Technology (2010-0013560) (E.J.K.), and
the Ministry of Health, Welfare and Family Affairs of Korea, and the
Ministry of Education, Science and Technology (Converging Research
Program, 2009-0081952) (S.L.).
Keywords: allenes · counterions · gold · hydrogen bonds ·
nucleophilic addition
Gibbs free energy could be understood by examining the
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1
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À
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.
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À
À
À
À
(S), aACTHNURGTENN(UG Au C2 C1)=98.18, aACHUTNGTREN(NGUN Au C2 C3)=123.08; for TS (R), a-
À
À
À
À
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by [(S)-3,5-tBu-4-MeO-MeObiphepACHTNUTRGNENG(U AuCl)2] (S)-2,2’-bis[di(3,5-di-
tert-butyl-4-methoxyphenyl)phosphino]-6,6’-dimethoxy-1,1’-biphenyl
(biphep) (2.5 mol%) and AgClO4 (5 mol%) in favor of the Z iso-
mers.[6d]
[20] a) B. Sherry, F. D. Toste, J. Am. Chem. Soc. 2004, 126, 15978–15979;
b) V. Gandon, G. Lemiꢈre, A. Hours, L. Fensterbank, M. Malacria,
Angew. Chem. 2008, 120, 7644–7648; Angew. Chem. Int. Ed. 2008,
47, 7534–7538.
[21] The NBO (natural bond orbital) charges of the allene-coordinated
Au atom and the other Au atom in the S,E isomer are 0.126 and
0.345 and in the S,Z isomer are 0.135 and 0.343, and the RAuÀAu dis-
tance in the S,E isomer is 3.853 ꢁ and in the S,Z isomer is 3.765 ꢁ.
À
The resulting Au Au Coulombic repulsion energy in the S,E isomer
is about 4% smaller than in the S,Z isomer, and this slightly smaller
[7] For the selected reviews of enantioselective gold(I) catalysis, see:
a) D. J. Gorin, B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108, 3351–
3378; b) R. A. Widenhoefer, Chem. Eur. J. 2008, 14, 5382–5391;
c) H. C. Shen, Tetrahedron 2008, 64, 3885–3903; d) Z. Li, C. Brouw-
er, C. He, Chem. Rev. 2008, 108, 3239–3265.
[8] a) H. Schmidbaur, W. Graf, G. Mꢅller, Angew. Chem. 1988, 100,
439–441; Angew. Chem. Int. Ed. Engl. 1988, 27, 417–419; b) P.
Pyykkç, Y. Zhao, Angew. Chem. 1991, 103, 622–623; Angew. Chem.
Int. Ed. Engl. 1991, 30, 604–605; c) P. Pyykkç, Angew. Chem. 2004,
116, 4512–4557; Angew. Chem. Int. Ed. 2004, 43, 4412–4456; d) H.
Schmidbaur, A. Schier, Chem. Soc. Rev. 2008, 37, 1931–1951;
e) R. J. Puddephatt, Chem. Soc. Rev. 2008, 37, 2012–2027.
À
Au Au repulsion seems to account for the enhanced stability of the
S,E isomer.
[22] For some investigations on vinyl gold(I) compounds, see: a) L. P.
Liu, G. B. Hammond, Chem. Asian J. 2009, 4, 1230–1236; b) D.
Weber, M. A. Tarselli, M. R. Gagnꢂ, Angew. Chem. 2009, 121, 5843–
5846; Angew. Chem. Int. Ed. 2009, 48, 5733–5736; c) A. S. K.
Hashmi, A. M. Schuster, F. Rominger, Angew. Chem. 2009, 121,
8396–8398; Angew. Chem. Int. Ed. 2009, 48, 8247–8249; d) A. S. K.
Hashmi, T. D. Ramamurthi, F. Rominger, Adv. Synth. Catal. 2010,
352, 971–975.
Received: February 10, 2011
Published online: July 6, 2011
1986
www.chemasianj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 1982 – 1986