Asymmetric synthesis of cyanohydrins catalysed by a potassium Δ-bis[N-salicylidene-(R)-tryptophanato]cobaltate complex

Article abstract of DOI:10.1070/MC2004v014n06ABEH002029

A chiral cobalt(III) complex of a Schiff base derived from (R)-tryptophan and salicylaldehyde catalysed asymmetric trimethylsilylcyanation of benzaldehyde at ambient temperature and a 20:1 substrate/catalyst ratio with the formation of enantiomerically en

Full text of DOI:10.1070/MC2004v014n06ABEH002029

, 2004, 14(6), 249–250
Asymmetric synthesis of cyanohydrins catalysed by a potassium
∆-bis[N-salicylidene-(R)-tryptophanato]cobaltate complex
Yuri N. Belokon,*a Alexander G. Bulychev,b Victor I. Maleev,a Michael North,c Ilja L. Malfanovb and
Nikolai S. Ikonnikova
a A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 095 135 5085; e-mail: yubel@mail.ru
b Department of Chemistry, Kaliningrad State University, 236000 Kaliningrad, Russian Federation
c School of Natural Sciences, University of Newcastle upon Tyne, Newcastle, NE1 7RU, UK
DOI: 10.1070/MC2004v014n06ABEH002029
A chiral cobalt(III) complex of a Schiff base derived from (R)-tryptophan and salicylaldehyde catalysed asymmetric trimethyl-
silylcyanation of benzaldehyde at ambient temperature and a 20:1 substrate/catalyst ratio with the formation of enantiomerically
enriched mandelonitrile (ee up to 77%).
We report the design and development of a new catalytic system
based on chiral cobalt(III) complex 1 and the use of this sys-
tem for asymmetric trimethylsilylcyanation of benzaldehyde.
Complex 1 was obtained from (R)-tryptophan, salicylaldehyde
and Na3[Co(CO3)3]1 with a total yield of 60% (Scheme 1) as a
mixture of diastereoisomers according to a procedure described
earlier.2
O
O
H
+ Na3[Co(CO3)3]
OH
H2N
H
OH
HN
i, ii, iii, iv
The resulting mixture of diastereomeric complexes, Λ-bis-
[N-salicylidene-(R)-tryptophanato]cobaltate sodium (Λ-1a) and
∆-bis[N-salicylidene-(R)-tryptophanato]cobaltate sodium (∆-1a),
was separated by flash column chromatography (Al2O3, EtOH).
The individual stereoisomers were additionally purified by chro-
matography on Sephadex LH-20 (EtOH/benzene, 1:3). Sodium
countercations in complexes ∆-1a and Λ-1a were exchanged
by other ions (Li+, K+, Cs+, Bu4N+ and H+) via ion-exchange
chromatography.†
O
O
H
M+
O
N
N
Ind
Co
O
O
Ind
H
O
All of the complexes were tested as catalysts for the tri-
methylsilylcyanation of benzaldehyde (Scheme 2).
The results of these experiments are presented in Table 1.
Although under conditions A complexes 1a–f were catalytically
active, some asymmetric induction (ee 19 and 6.5%, Table 1,
conditions A, entries 5 and 7) was provided only by complexes
∆-1c,d.
Triphenylphosphine, 1,2-bis(diphenylphosphinoethane), tert-
butanol, water, (R)-tryptophan and indole were tested as co-
catalysts in the reaction catalysed by 1a–f (Table 1, conditions
B). The reaction catalysed by ∆-1c was most sensitive to the
∆-1a–f
O
H
M+
a M = Na
b M = H
c M = K
d M = Li
e M = Cs
O
N
Ind
Co
O
N
O
H
O
†
All the complexes had satisfactory analytical data.
f
M = NBu4
O
Ind
Data for Λ-bis[N-salicyliden-(R)-tryptophanato]cobaltate potassium:
Λ-1a–f
25
25
[a]2D5 = +5281, [a] = +5506, [a] = +4893, (c 0.032, MeOH).
578
546
1H NMR (D2O) d: 3.1–3.2 (m, 2H), 3.4–3.5 (m, 2H), 4.8–4.9 (m, 2H),
6.5–7.3 (m, 18H), 7.7 (s, 2H). Found (%): C, 60.26; H, 4.31; N, 7.14.
Calc. for C36H28N4CoO6K·11/2H2O·1/2C6H6 (%): C, 60.31; H, 4.41;
N, 7.21.
Scheme 1 Reagents and conditions: i, EtOH, reflux, 3 h; ii, chromato-
graphy on Al2O3 (EtOH); iii, ion exchange on DOWEX-50Wx8; iv,
Sephadex LH-20 (C6H6/EtOH; 3:1).
Data for ∆-bis[N-salicyliden-(R)-tryptophanato]cobaltate potassium:
additives (Table 1, entry 5, conditions B). The most effective
co-catalyst was triphenylphosphine (ee 77%, Table 1, condi-
tions B, entry 5c). The chelating phosphine, DPPE, was less
effective than triphenylphosphine (ee 45%, Table 1, conditions B,
entry 5d). Indole was also effective in promoting the enantio-
25
25
[a]D25 = +3446, [a] = +3133, [a] = +473, (c 0.032, MeOH).
578
546
1H NMR (D2O) d: 3.1–3.2 (m, 2H), 3.4–3.5 (m, 2H), 4.8–4.9 (m, 2H),
6.5–7.3 (m, 18H), 7.7 (s, 2H). Found (%): C, 60.58; H, 4.81; N, 7.21.
Calc. for C36H28N4CoO6K·12/3H2O·2/3C6H6 (%): C, 60.60; H, 4.49;
N, 7.07.
Mendeleev Commun. 2004 249

, 2004, 14(6), 249–250
Table 2 Time dependence of the enantiomeric excess of mandelonitrile in
the ∆-1c catalysed reaction.a
O
H
O SiMe3
H
2 mol% catalyst
+
Me3SiCN
1 equiv.
CH2Cl2
(ambient
temperature)
eeb (%)†
CN
Entry
t/min
Yield (%)
1 equiv.
1
2
3
4
3
7
20
60
35
70
85
85
34 (R)
62 (R)
77 (R)
75 (R)
Scheme 2
selectivity of the reaction (ee 74%, Table 1, conditions B, entry
5e). The use of ButOH, H2O and (R)-tryptophane led to the com-
plete disappearance of asymmetric induction (ee 0%, Table 1,
conditions B, entry 5f). The change of the configuration of CoIII
stereogenic centre from ∆-1c to Λ-1c brought about loss of the
enantioselection (ee 0%, Table 1, conditions B, entry 6).
Evidently, both the configuration of the complex and the nature
of the countercation were the most important factors in deter-
mining the asymmetric efficiency of the catalysts, with the K+ ion
being much more efficient as compared to Li+ (Table 1, entries
7, 8), Na+ (Table 1, entries 1, 2) and H+ (Table 1, entries 3, 4).
Large lipophilic Cs+ (Table 1, entries 9, 10) and Bu4N+
(Table 1, entry 11) ions were also inefficient. Seemingly, the
stability and structure of the ion pairs formed in solution by the
aConditions: benzaldehyde (1 mmol), TMSCN (1.1 mmol), catalyst ∆-1c
(2 mol%), triphenylphosphine (0.1 mmol) added, CH2Cl2 (1 ml), stirring
under Ar at ambient temperature. bDetermined by chiral GLC.
complexes were of paramount importance in the reaction. This
concept was supported by the use of 18-crown-6 as an additive,
which resulted in the complete disappearance of the asymmetric
catalytic efficiency of ∆-1c.
The time dependence of the product ee in the asymmetric
trimethylcyanation of benzaldehyde catalysed by ∆-1c (condi-
tions B, c) is presented in Table 2. The steady increase of the
ee of the product with time indicates the formation of a real
catalytic species different from the original complex during the
reaction.
In summary, we identified a new multimetallic catalyst for
asymmetric C–C bond formation reactions. The results will
serve as an interesting addition to the rapidly growing field of
multimetallic catalysts in organic synthesis.4
Table 1 Asymmetric trimethylsilylcyanation of benzaldehyde.a
Conditions A
Yield eeb (%)
Conditions B
Entry Catalyst
Yield
(%)
eeb (%)
(configuration)
(%) (configuration)
1
2
3
4
5
∆-1a
Λ-1a
∆-1b
Λ-1b
∆-1c
80
85
70
72
90
0
0
0
0
80
80
70
76
6.5 (R)c
This study was supported by the Russian Foundation for
Basic Research (grant no. 02-03-32091).
0c
0c
0c
References
19 (R)
80–95 77 (R),c 45 (R),d
74 (R),e 0f
1 H. F. Bauer and W. C. Drinkard, J. Am. Chem. Soc., 1966, 88, 4150.
2 Yu. N. Belokon, V. M. Belikov, S. V. Vitt, T. F. Savel’eva, V. M. Burbelo,
V. I. Bakhmutov, G. G. Aleksandrov and Yu. T. Struchkov, Tetrahedron,
1977, 33, 2551.
3 Multimetallic Catalysts in Organic Synthesis, eds. M. Shibasaki and
Y. Yamamoto, Wiley-VCH, Weinheim, 2004.
6
7
8
9
10
11
∆-1c
∆-1d
Λ-1d
∆-1e
Λ-1e
∆-1f
85
80
80
70
75
90
0
80
83
85
70
75
92
0c
6.5 (S)
6.5 (S)c
0
0
0
0
0c
0c
0c
0c
aConditions A: benzaldehyde (1 mmol), TMSCN (1.1 mmol), catalyst 1
(2 mol%), CH2Cl2 (1 ml), stirring under Ar at ambient temperature; Con-
ditions B: the same as A with a co-catalyst (0.1 mmol) added. bDetermined
c
by chiral GLC. Co-catalyst is triphenylphosphine. dCo-catalyst is DPPE
f
[1,2-bis(diphenylphosphinoethane)]. eCo-catalyst is indole. Co-catalysts
are ButOH, H2O and (R)-tryptophane.
Received: 13th September 2004; Com. 04/2354
250 Mendeleev Commun. 2004