Asymmetric allylic alkylation in supercritical carbon dioxide using P*-chiral diamidophosphite ligands

Article abstract of DOI:10.1016/j.mencom.2010.05.006

P*-Chiral diamidophosphite ligands in the Pd-catalyzed allylic alkylation of (E)-1,3-diphenylallyl acetate in supercritical carbon dioxide provide enantioselectivities up to 90% ee. The catalytic performance is affected greatly by temperature, CO₂ pressures and structure of catalysts.

Full text of DOI:10.1016/j.mencom.2010.05.006

Mendeleev
Communications
Mendeleev Commun., 2010, 20, 143–144
Asymmetric allylic alkylation in supercritical
carbon dioxide using P*-chiral diamidophosphite ligands
Sergey E. Lyubimov,*a Ilya V. Kuchurov,b Andrei A. Vasil’ev,b
Sergei G. Zlotinb and Vadim A. Davankova
a A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 499 135 2548; e-mail: lssp452@mail.ru
b N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2010.05.006
P*-Chiral diamidophosphite ligands in the Pd-catalyzed allylic alkylation of (E)-1,3-diphenylallyl acetate in supercritical carbon
dioxide provide enantioselectivities up to 90% ee. The catalytic performance is affected greatly by temperature, CO2 pressures and
structure of catalysts.
Palladium-catalyzed allylic substitution (the Tsuji–Trost reac-
tion) is a useful synthetic method for the formation of carbon–
3 L =
4 L =
N
Ph
N
Ph
carbon and carbon–heteroatom bonds.1 Application of chiral
ligands on prochiral substrates provides good and even excellent
asymmetric inductions.1–3 One of the most promising family
of such ligands is P*-chiral diamidophosphites prepared from
cheap S-glutamic acid and proved to be very effective in the
Pd-catalyzed allylic substitution reactions (up to 99% ee).4,5
Another significant challenge in metal complex asymmetric
catalysis is the use of alternative solvents often called as ‘green
media’ for chemical transformations.6 Supercritical carbon dioxide
(scCO2) presents a viable substitute for organic solvents, as
it is non-toxic, inflammable, and relatively cheap, has a low
critical temperature (31.1 °C) and an acceptable critical pressure
(72.9 atm).6,7 Recently, we showed that inexpensive phosphite-
type ligands are highly effective in the Rh-catalyzed asymmetric
hydrogenation of prochiral olefins (up to 99% ee) in scCO2.8
The number of publications on classical C–C cross-coupling
in scCO2 is scarce and is limited to Heck, Suzuki, Sonogashira
and Stille reactions (representative examples see in ref. 9). To
the best of our knowledge, the Tsuji–Trost reaction in scCO2
was not documented except for our recent single episode of
alkylation of (E)-1,3-diphenylallyl acetate 1 with dimethyl
malonate (up to 81% ee with 60% conversion) with the use
of sophisticated carborane-tethered P*-chiral diamidophosphite
palladium complex.10 In the present work we report on the
broader investigations of the same reaction (Scheme 1) using
the series of simpler palladium cationic and neutral complexes
with diamidophosphite ligands (Figure 1). Note that traditional
versions of the Tsuji–Trost reaction involve use of fully depro-
tonated carbanion of CH-acid or combination of BSA–AcOK
as a deprotonating system which seem inconsistent with electro-
philic character of CO2. Therefore, it seemed reasonable to
use metal carbonates which provide sufficient quasi-stationary
concentration of the required carbanion, although not fully
deprotonating CH-acid (cf. ref. 11).
*
*
N
N
P
P
OMe
OPh
Ph
N
H
O
O
P
N
5 L =
N
P
O
O
N
H
Ph
P
P
P
Pd
Pd
Pd
BF4
Cl
a
b
P
P
Pd
Cl
BF4
P
P
c
d
Figure 1 Cationic (a, c, d) and neutral (b) Pd-complexes with diamido-
phosphite ligands.
The use of the cationic palladium tetrafluoroborate complex
3a (Figure 1) and Cs2CO3 at a low temperature (40 °C) and
170 atm CO2 pressure yielded product 2 with 60% ee, though a
moderate conversion of 1 was observed (Table 1, entry 1). To
evaluate the effect of temperature with the aim of increasing
conversion, temperature was raised from 40 to 60 °C. In this
case, the same level of enantioselectivity was held but the
conversion did increase (entry 2). Changing of CO2 pressure
to lower or higher values (from 170 to 110 atm or from 170 to
240 atm) at 60 °C resulted in a decrease in conversion, but
the ee values remained the same (entries 2–4). Interestingly, the
neutral complex 3b, which can be prepared by simple mixing of
ligand 3 and [Pd(allyl)Cl]2 in CHCl3 and evaporating the solvent,
also gives 60% ee and an acceptable conversion at 60 °C and
170 atm pressure (entries 5 and 2).
MeO2C
CO2Me
Ph
+ CH2(CO2Me)2,
Cs2CO3, Pd/P*
OAc
Ph
Cationic complex 4a† afforded product 2 with better enantio-
selectivity, as compared to 3a, probably because of the higher
steric bulkiness of the ligand 4 (Table 1, entries 2–4 and 6–7).
Moreover, complete conversion was observed at 75 and 60 °C
using 170 atm CO2 pressure. The reaction at 240 atm pressure
*
Ph
scCO2, 18 h
Ph
1
2
Scheme 1 Pd-catalyzed allylic alkylation of (E)-1,3-diphenylallylic acetate.
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© 2010 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2010, 20, 143–144
with active substrate 1 is a challenge to evaluate scope and
limitations of this procedure, which is definitely associated with
solubility and solvation of the reaction components with scCO2.
Our preliminary experiment to react diethyl malonate with allyl
acetate (less active than 1) in scCO2 under conditions of entry 2
(Table 1) gave 88% GC yield of the target diethyl allylmalonate,
while diallylation did not occur. Attempted prenylation with
3-methylbut-1-en-3-yl acetate afforded only 4% of the product.
Therefore, further screening of substrates, catalysts and reaction
conditions is needed to promote using of scCO2 as a cheap and
inflammable medium in valuable Tsuji–Trost reaction.
Table 1 Pd-catalyzed enantioselective allylic alkylation of 1 with dimethyl
malonate in scCO2.
Pressure/
atm
HPLC
yield (%)
Entry
Catalyst
T/°C
ee (%)
1
2
3
4
5
6
7
8
9
3a
3a
3a
3a
3b
4a
4a
4a
5c
5d
5d
170
170
240
110
170
170
170
240
170
170
170
40
60
60
60
60
75
60
75
60
60
75
60
85
25
51
70
100
100
80
100
75
60 (S)
60 (S)
58 (S)
60 (S)
60 (S)
76 (S)
76 (S)
67 (S)
90 (S)
61 (S)
61 (S)
10
11
This work was supported by the Russian Foundation for
Basic Research (grant no. 09-03-12104-ofi_m).
84
using the same catalyst (4a) leads to a decrease in both the
conversion and enantioselectivity (entries 6, 8).
References
1
(a) B. M. Trost, Acc. Chem. Res., 1980, 13, 385; (b) B. M. Trost and
D. L. Van Vranken, Chem. Rev., 1996, 96, 395; (c) Z. Lu and S. Ma,
Angew. Chem. Int. Ed., 2008, 47, 258.
Alkylation of 1 using complexes 5c and 5d with a bidentate
ligand showed an unexpected result. In this case, the anion
had a marked influence on enantioselectivity and reaction rate.
Cationic complex 5c† with chloride anion gave complete con-
version of 1 in 18 h and high enantioselectivity (90% ee)
was observed (Table 1, entry 9). The tetrafluoroborate cationic
complex 5d exhibited 61% ee and from moderate to good
conversion (Table 1, entries 10, 11), with the conversion being
higher at the temperature growth. Note that in the last cases the
enantioselectivity remains independent of the temperature used,
but raising temperature leads to improvement of conversion.
The results obtained are somewhat advantageous to the data
of using the same catalysts under standard conditions5(b),12 (THF
or CH2Cl2, BSA–AcOK), when along with 90–98% ee values
the conversion remained moderate on 48 h of the reaction time.
In conclusion, we have performed the Tsuji–Trost reaction in
scCO2 using P*-chiral diamidophosphite Pd complexes as the
catalysts and Cs2CO3 as the base, providing thus satisfactory
conversions and ee values of the product 2. The results obtained
2
3
4
5
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†
Complexes 3a, 3b and 5d were prepared according to the published
6
(a) D. J. Cole-Hamilton, Adv. Synth. Catal., 2006, 348, 1341; (b) E. J.
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and M. Gomez, C. R. Chimie, 2007, 10, 152.
procedures.5(a),(c),12
For 4a: a solution of the ligand 4 (59 mg, 0.2 mmol) in CH2Cl2 (10 ml)
was added dropwise over 15 min to a vigorously stirred solution of 18 mg
(0.05 mmol) [Pd(allyl)Cl]2 in CH2Cl2 (5 ml). The mixture was stirred for
10 min, followed by dropwise addition of AgBF4 (20 mg, 0.1 mmol) in
THF (5 ml) over 5 min. The mixture was stirred for additional 30 min, and
the precipitate of AgCl was filtered off. The filtrate was concentrated at
reduced pressure and dried in vacuo (1 Torr) for 30 min. Yield, 71 mg
(87%). 31P{1H} NMR (CDCl3), d: 115.57. Found (%): C, 53.60; H, 5.30;
N, 6.65. Calc. for C37H43BF4N4O2P2Pd (%): C, 53.48; H, 5.22; N, 6.74.
MS (ESI), m/z (%): 744 [M – BF4]+ (100), 703 [M – BF4 – allyl]+ (10).
For 5c: a solution of the ligand 5 (55 mg, 0.1 mmol) in CH2Cl2 (10 ml)
was added dropwise over 10 min to a vigorously stirred solution of
[Pd(allyl)Cl]2 (18 mg, 0.05 mmol) in CH2Cl2 (5 ml). The mixture was
stirred for additional 10 min after which the solvent was evaporated under
reduced pressure (40 Torr). The residue was washed with diethyl ether
(5 ml) and dried in a vacuum (1 Torr, 30 min). Yield, 67 mg (92%).
31P{1H} NMR (CDCl3), d: 115.56. Found (%): C, 50.56; H, 5.65; N, 7.51.
Calc. for C31H41ClN4O4P2Pd (%): C, 50.49; H, 5.60; N, 7.60. MS (ESI),
m/z (%): 702 [M – Cl]+ (20), 661 [M – Cl – allyl]+ (100), 554 [L]+ (3).
Pd-catalyzed allylic alkylation of (E)-1,3-diphenylallyl acetate with
dimethyl malonate in scCO2. Pd-catalyst (0.02 mmol), (E)-1,3-diphenyl-
allyl acetate (0.1 ml, 0.5 mmol), dimethyl malonate (0.1 ml, 0.87 mmol)
and Cs2CO3 (244 mg, 0.75 mmol) were placed open to air into a 10 ml
autoclave. The vessel was filled with scCO2 by means of a syringe-press
to a total pressure of 60 atm. The mixture was allowed to equilibrate to
the reaction temperature (30 min) and then additional CO2 was added up
to the required pressure. After this, the mixture was stirred for 18 h and the
vessel was slowly depressurized. The resulting residue was stirred up with
CH2Cl2 and the mixture was filtered through silica gel. The filtrate was
concentrated in a vacuum to give the product 2 as yellow oil. Conversion
of substrate 1 and enantiomeric excess (ee) of product 2 were determined
using HPLC (Daicel Chiralcel OD-H column) as described previously.13
7
8
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Received: 7th December 2009; Com. 09/3431
– 144 –