Chemo- and regioselectivity-tunable Pd-catalyzed allylic alkylation of imines

Article abstract of DOI:10.1021/ja2039503

α-Carbanions of cyclic and acyclic imines have been successfully applied as nucleophiles in the Pd-catalyzed allylic alkylation reaction. Tuning of chemo- and regioselectivity has been realized by using t-BuOK/THF and LDA/toluene to give branched and line

Full text of DOI:10.1021/ja2039503

COMMUNICATION
pubs.acs.org/JACS
Chemo- and Regioselectivity-Tunable Pd-Catalyzed Allylic Alkylation
of Imines
||
Jian-Ping Chen,‡ Qian Peng,‡ Bai-Lin Lei,† Xue-Long Hou,*,†,‡ and Yun-Dong Wu*,‡,§,
†State Key Laboratory of Organometallic Chemistry and ‡Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis,
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
§School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School, Shenzhen 518055, China
Department of Chemistry, The Hong Kong University of Science & Technology, Kowloon, Hong Kong, China
S Supporting Information
b
The experimental data imply that the base has a significant influence
ABSTRACT: R-Carbanions of cyclic and acyclic imines have
been successfully applied as nucleophiles in the Pd-catalyzed
allylic alkylation reaction. Tuning of chemo- and regio-
selectivity has been realized by using t-BuOK/THF and
LDA/toluene to give branched and linear products,
respectively, with high regio- and diastereoselectivities. A
plausible mechanism is proposed on the basis of the experi-
mental results and DFT calculations.
on the regiochemistry. The linear product 4a was the major product
using LiHMDS (entry 1). To our surprise, the branched product 3a
became predominant when NaHMDS or KHMDS was the base
(entries 2 and 3). Moreover, when t-BuONa and t-BuOK were used,
the 3a/4a ratio sharply increased to 98:2 and 99:1, respectively, with
sustained high diastereoselectivity of 3a (entries 4 and 5). Next, a
survey of different leaving groups revealed that Cl was the best
choice, as cinnamyl chloride (2e) gave good yield and diastereos-
electivity (entry 9 vs entries 5ꢀ8). The yield increased further when
(p-MeOC6H4)3P was the ligand and 1.5 equiv of imine 1a was used
(entry 10). A control experiment showed that no desired allylation
products were formed in the absence of Pd catalyst (data not shown).
he palladium-catalyzed allylic alkylation reaction is not only one
T
of the most thoroughly studied transition-metal-catalyzed reac-
tions but also one of the most useful reactions in organic synthesis.1
While great achievements have been made, some challenging issues
remain to be addressed. One is the regioselectivity of the reaction with
monosubstituted allyl substrates. In most cases, especially in the reac-
tions using carbon nucleophiles, linear products are obtained when Pd
catalysts are used.1 Branched products have been provided in only a
few examples, such as in alkylations using catalysts with well-designed
ligands2,3 or substrates with directing groups4 and in the decarbox-
ylative alkylation of heteroaromatic alkanes.5Branched products have
also been obtained in reactions using N-nucleophiles, including
unprotected aziridines,6aꢀc hydrazine, and hydroxylamine.6d Another
issue is the limitation of carbanion nucleophiles. For a long time,
carbon nucleophiles have been limited to “soft” or stabilized carb-
anions. Although some enolates from ketones and carboxylic acid
derivatives have been used successfully as nucleophiles,2c,7,8exploration
of new types of “hard” carbanions as nucleophiles is still one of the
main subjects in studies of Pd-catalyzed allylic alkylation reactions.7n
Recently, our group realized the regio- and enantioselective Pd-
catalyzed allylic alkylation of monosubstituted allyl substrates2 and
also developed acyclic ketone enolates as well as R-carbanions of
acyclic amides as nucleophiles.2c,8 In this communication, we report
that R-carbanions of cyclic and acyclic aliphatic imines are also
suitable nucleophiles for the reaction9 and that a dramatic tuning of
regioselectivity can be achieved. A plausible mechanism to rationalize
the experimental observations is proposed on the basis of density
functional theory (DFT) calculations and further experiments.
Initially, the reaction of imine 1a and cinnamyl tert-butyl carbonate
2a in the presence of [Pd(C3H5)Cl]2/PPh3 and LiHMDS was
examined (eq 1). Allylated products 3a and 4a were obtained in a
ratio of 36:64 with a total yield of 60% after the hydrolysis of allylated
imine. The dr of 3a was determined to be 6:1. The prototypical reac-
tion was investigated to optimize the reaction conditions (Table 1).
After obtaining good conditions for the branched product
(conditions A), we turned our attention to improving the selectivity
for the linear product. Solvent optimization experiments showed that
using toluene as the solvent significantly promoted the linear product
but decreased the yield with LiHMDS as the base [entry 1 vs entry
12; for more data, see Table 2 in the Supporting Information (SI)].
To improve the yield, other lithium bases were examined. The use of
lithium diisopropylamide (LDA) as the base appeared to give a good
yield (77%) while the selectivity for the linear product remained high
(8:92) (entry 14). Thus, the favorable conditions were identified for
the selective formation of the linear product (conditions B).
The scope of this regioselectivity-tunable reaction was then inves-
tigated using conditions A and B. As shown in Table 2, both cyclic
and acyclic aliphatic imines were suitable nucleophile precursors,
providing the corresponding products in high yields. Using condi-
tions A, excellent regio- and diastereoselectivities were realized for all
of the reactions of imines 1 with 2e and cinnamyl chlorides 2fꢀj
having an electron-withdrawing or electron-donating group on the
phenyl ring. Branched products 3 were accompanied by yields
of <2% for linear products 4 (entries 1ꢀ13). In addition, excellent
diastereoselectivities were obtained except for entry 2, where a lower
Received: April 29, 2011
Published: August 09, 2011
r
2011 American Chemical Society
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COMMUNICATION
Table 1. Optimization of the Conditions for the Pd-Cata-
lyzed Reaction of 1a with 2a
Table 2. Pd-Catalyzed Regioselective Allylic Alkylation of
Imines 1 with Allyl Reagents 2
entry
base
2
% yieldb
3a/4ac
3a anti/sync
1
2
LiHMDS
NaHMDS
KHMDS
t-BuONa
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
LiHMDS
s-BuLi
2a
2a
2a
2a
2a
2b
2c
2d
2e
2e
2a
2a
2a
2a
60
25
12
26
51
60
64
17
70
88
48
29
61
77
36/64
78/22
68/32
98/2
99/1
98/2
98/2
95/5
99/1
99/1
95/5
7/93
12/88
8/92
6/1
7/1
7/1
9/1
7/1
7/1
24/1
13/1
32/1
24/1
4/1
ꢀ
3
4
5
6
entry
1; R1, R2
2
conditionsa % yieldb
3/4c
3 anti/sync
7
8
1
2
1a; (CH2)4 2e
1b; (CH2)5 2e
A
A
A
A
A
A
A
A
A
A
A
A
A
B
B
B
B
88
61
51
75
74
60e
46
43f
82
68
55
79
69
77
64
88
90
>98/2
>98/2
>98/2
>98/2
>98/2
>98/2
>98/2
>98/2
>98/2
>98/2
>98/2
>98/2
>98/2
8/92
24/1
7/1
9
10d
11e
12e
13e
14e
3d 1c; Me, H
2e
ꢀ
4
5
6
1d; Et, Me 2e
11/1
11/1
13/1
ꢀ
1e; Pr, Et
2e
ꢀ
1f; Me, Me 2e
LDA
ꢀ
7d 1g; H, Me2 2e
8d 1h; H, Et
a 1a/[Pd(C3H5)Cl]2/PPh3/Base/2a molar ratio = 100/2.5/12/110/
110. b Isolated yields. c Determined by 1H NMR analysis. d P(p-
MeOC6H4)3 was the ligand; 1a/[Pd(C3H5)Cl]2/ligand/t-BuOK/
2e molar ratio = 150/2.5/12/150/100. e Toluene was used as the
solvent instead of THF.
2e
2f
>50/1
12/1
24/1
32/1
16/1
>50/1
ꢀ
9
10
11
12
13
14
15
16
17
1a
1a
1a
1a
1a
1a
1b
1d
1e
2g
2h
2i
2j
anti/syn ratio of 7/1 was found. When unsymmetrical imine 1f
derived from 2-butanone was used as the substrate, two branched
allylic products were obtained in yields of 32 and 28% (entry 6).
Notably, aldimines 1g and 1h were also suitable substrates, providing
the desired products with high selectivity (entries 7 and 8). However,
no desired products were obtained when alkyl-substituted allyl
substrates (e.g., crotyl chloride) were used with t-BuOK as the base
(data not shown). X-ray structure analysis of the allyl product from
the reaction of 1a with 2h confirmed its anti stereochemistry.
When conditions B were used, the reactions of imines 1 with
cinnamyl tert-butyl carbonate 2a all provided linear products 4
predominantly, with 4/3 ratios of 88ꢀ92:8ꢀ12 (entries 14ꢀ17).
The mechanism for the formation of linear products with Pd
catalysts has been well-documented.1Thus, our focus was to probe
the mechanistic features leading to the surprising branched product
using both experimental and theoretical studies. Imine 1i from cyclo-
hexanone with an N-n-butyl substituent was synthesized and tested.
The reaction using conditions A predominantly gave the branched
product 3a, with a 3a/4a ratio of 95:5 and dr of 5:1 for 3a. The linear
product 4a was the major product (3a/4a = 13:87) under conditions
B. These results indicate that the regiochemistry is not determined by
the methoxy group in imine 1a.4
2a
2a
2a
2a
8/92
ꢀ
12/88
ꢀ
9/91
ꢀ
a Conditions A: THF solvent at rt; 1/[Pd(C3H5)Cl]2/P(p-MeOC6H4)3/
t-BuOK/cinnamyl chloride molar ratio = 150/2.5/12/150/100. Conditions
B: toluene solvent at 0 °C; 1/[Pd(C3H5)Cl]2/PPh3/LDA/2a molar ratio =
120/2.5/12/120/100. b Isolated yields. c Determined by 1H NMR analysis.
d 2 equiv of imine and 2 equiv of t-BuONa were used. e The ratio of R- to
R0-allylated products was 1:1.1. f Yield after reduction with NaBH4.
Scheme 1. Plausible Mechanism for Tuning of the Regio-
selectivity between Conditions A and B
As shown in Scheme 1, it is possible that the reaction of the
imine with t-BuOK produces an aza-allyl anion, which can serve
as an N-nucleophile to produce an N-alkyl-N-allyl enamine
intermediate. This intermediate may undergo a [3,30] rearrange-
ment followed by hydrolysis to afford the branched product.5
With LDA as the base, the hard acid Li+ should bind tightly to the
N of the aza-allylic anion, promoting normal C-alkylation to
afford the linear product.10
In order to test the above hypothesis, N-alkyl-N-allyl enamine
5 was prepared and examined under various reaction conditions.
As shown in eq 3, no desired rearrangement product was detected.
Thus, a direct [3,30]-aza-Cope rearrangement of an N-alkyl-N-allyl
enamine to form the branched product is ruled out.
On the basis of recent literature on metal-mediated allylꢀallyl
coupling reactions,11 we propose a Pd-mediated [3,30]-reductive
elimination to rationalize the formation of the branched product
when Na/K bases are used. In this mechanism, there must be a
critical transmetalation between the Pdꢀallyl complex CP, which is
obtained in the last oxidative addition step, and the metalꢀimine
anion to generate Int-A or Int-B (see Scheme 2).
DFT calculations using the Gaussian 03 program were carried
out to explore the proposed mechanism.12 As shown in Scheme 2,
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Journal of the American Chemical Society
COMMUNICATION
Scheme 2. Calculated Reaction Free Energies (kcal/mol) for
the Transmetalation between CP and M-Im (M+ = Li+, K+)a
a Values in parentheses are relative free energies without solvent effects.
the CP + M-Im is set to be the free energy zero point. The
formation of Int-A and Int-B is accompanied by the formation of
LiCl and KCl for M=Li+ and K+, respectively. The transmetala-
tion reaction13 between CP and M-Im is favorable if the metal
ion is the soft acid K+. Int-A and Int-B have similar stabilities. On
the other hand, when the counterion is the hard acid Li+, the
reaction is unfavorable. As shown in parentheses, the calculated
reaction free energies are similar without solvent effects (see SI
Table 3 for details). Thus, the transmetalation reaction for M+ =
K+ is >7.0 kcal/mol more favorable than that for M+ = Li+, mainly
because of the stronger binding of Li+ in M-Im. Thus, it is
concluded that the transmetalation reaction can easily be
achieved for M+ = K+. For Li+, the equilibrium is less favorable.
Further calculations for the K+ counterion case (Figure 1)
indicated that the η3-intermediate Int-A can easily rearrange into
the more stable η1-intermediate CP-A, which can undergo a very
facile [3,30]-reductive elimination to give the branched product
Pro-A. On the other hand, the [3,30]-reductive elimination transi-
tion state (TS-B) that leads to the linear product Pro-B has a
much higher activation energy. Thus, this mechanism correctly
rationalizes the high selectivity for the formation of the branched
product with K+ as the counterion.
Figure 1. Predicted free-energy profiles for the mechanisms leading to
branched and linear products under conditions A. Values in parentheses
are relative free energies without solvent effects.
The structures of TS-A and TS-B are shown in Figure 2. Both
transition structures have a planar coordination at the Pd center.
Figure 2. Structures and relative free energies of the transition states for
the inner-sphere (TS-A and TS-B) and outer-sphere (TS-C and TS-D)
mechanisms. H atoms on phenyl groups have been omitted for clarity.
Values in parentheses are relative free energies without solvent effects.
TS-A has the C C bond formation at the reactive C1 center,
3 3 3
and the seven-membered-ring geometry has no serious distor-
tion. However, TS-B has a highly distorted geometry, as indi-
cated by the large NꢀPdꢀC angle of 142°. This occurs because
(Scheme 3). According to the widely accepted reaction mechan-
ism, (S,S)-(Z)-7 and/or (R,R)-(E)-7 should be produced if the
nucleophile attacks the palladium of the allyl complex (inner-
sphere attack mechanism), while (S,S)-(E)-7 and/or (R,R)-(Z)-
7 should be the products when the nucleophile attacks the carbon
of the allyl complex directly (outer-sphere mechanism)1,15
(Scheme 3). When deuterated (S)-(Z)-6 was reacted with 1a
under conditions A, “racemic” products 7 were obtained in 23%
yield as a 1:1 mixture of E and Z isomers with a B/L ratio of 98:2
and an anti/syn ratio of 10:1. Both enantiomers were obtained
by chiral HPLC separation, and their absolute configurations
were determined to be (S,S)-(Z)-7 and (R,R)-(E)-7 by comparison
of the signs of their optical rotation with that of nondeuterated (R,R)-
3a, whose absolute configuration was assigned by X-ray analysis of a
single crystal obtained from chiral HPLC separation, reduction, and
esterification of (+)-3a (Scheme 4; also see the SI). These results
confirm our Pd-mediated [3,30]-reductive elimination mechanism.
In summary, R-carbanions of imines have been successfully
applied as nucleophiles in the Pd-catalyzed allylic alkylation reaction.
Tuning of chemo- and regio-selectivity has been realized by using
bases with different counterions. When the soft-acid counterion Na+
or K+ is used, the branched product is obtained. If the hard-acid ion
the C3 center is less reactive in the C C bond formation, as
3 3 3
indicated by the relatively strong C3ꢀPd bond, and largely
contributes to the high instability of TS-B. In addition, TS-B
also suffers from steric interactions between the Ph group on the
allyl and the bulky phosphine ligand.
The above mechanism is called an inner-sphere attack mech-
anism. We also calculated an outer-sphere attack mechanism
(or ionic mechanism) for the K+ case.14 As shown in TS-C and
TS-D (Figure 2), the imine anion attacks the Pd-coordinated
allyl without coordinating to Pd. Both transition structures have a
concerted C N and C C bond formation. TS-C and TS-D
3 3 3
3 3 3
are calculated to be much less stable than TS-A. In addition, TS-D,
which leads to the formation of the linear product, is calculated to be
more stable than TS-C leading to the branched product by ∼1.8
kcal/mol (including solvent effects). The destabilization of TS-C is
mainly due to serious steric interactions between the phenyl sub-
stituent on the allyl and the incoming cyclohexene ring, as indicated
by two short H C distances (∼2.67 and 2.60 Å). Thus, the outer-
3 3 3
sphere mechanism not only has a higher activation energy than the
inner-spheremechanism but alsogives the wrong(linear) product.
Finally, the deuterated, optically active allyl compound
(S)-(Z)-6 was used to probe the mechanistic features further
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Scheme 3. Formation of Stereospecific Products in the
Reaction of Imine 1a with (S)-(Z)-6 via Outer- and
Inner-Sphere Attack Mechanisms
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using the 6-31G* basis set (the LAN2LDZ basis set for Pd) (BS1). Vibrational
frequency calculations were carried out at the same level to obtain free energy
corrections. The energies were further estimated using a larger 6-311+G** basis
set (BS2) by single point calculations. Solvent effect of THF was estimated by
the IEFPCM method. All presented values are relative free energies (kcal/mol)
based on the calculated BS2 energies with BS1 free energy corrections. For
full reference of Gaussian 03 and detailed calculation procedures, see the SI.
(13) We studied the transmetalation reactions in Scheme 2 using
different counteranions as well as other possible transmetalation processes.
The calculated equilibrium energies were similar to those with one PPh3
ligand and different counteranions (Clꢀ and OCO2Meꢀ) and solvents
(toluene and THF). For details, see section 6-1 in the SI.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, ad-
b
ditional experimental and computational results, and crystal-
lographic data (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
xlhou@sioc.ac.cn; chydwu@ust.hk
’ ACKNOWLEDGMENT
This paper is dedicated to Professor Christian Bruneau on the
occasion of his 60th birthday. This work was financially supported by
the Major Basic Research Development Program (2011CB808700),
the National Natural Science Foundation of China (20872161,
20821002, 21032007), CAS, the External Cooperation Program of
CAS (GJHZ200816), the Shanghai Committee of Science and
Technology, and the Croucher Foundation of Hong Kong. J.-P.C.
and Q.P. thank the Croucher Foundation for studentships. We also
thank Professor Huaping Mo for valuable discussions.
(14) The inner-sphere and outer-sphere attacks involving two PPh3
ligands were also explored through calculations. These possible TSs were less
stable than that with one PPh3 ligand. For details, see section 6-1 in the SI.
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