2174
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 10, October, 2004
Starodubtseva et al.
nation cycle was carried out as described above. The extract
obtained after each cycle was concentrated on a rotary evaporaꢀ
tor and the residue was dried in vacuo and analyzed by 1H NMR
spectroscopy and GLC. The results obtained are given in
Tables 1 and 2.
addition of both anhydrous and aqueous EtOH. The presꢀ
ence of water inhibits the formation of ketal as a byꢀ
product from compound 1 but favors more rapid deactiꢀ
vation of the catalyst and reduction in the degree of conꢀ
version of the substrate (see Table 1, entries III and IV).
Thus, the catalyst RuꢀBINAP used in the asymmetric
hydrogenation of practically important structural block 1
in solutions of tetraethylammonium or 1ꢀbutylꢀ3ꢀmethylꢀ
imidazolium salts can be recycled. For the reaction in the
presence of Et4N+Br–, the same high level of asymmetric
induction is retained in at least three successive cycles.
This work was financially supported by the Program of
the Presidium of the Russian Academy of Sciences 9P
"Directed Synthesis of Compounds with Desired Properꢀ
ties" (Project No. 1.7: "Development of Novel Methodꢀ
ological Techniques for Organic Synthesis").
References
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Ethyl 4ꢀchloroꢀ3ꢀoxobutyrate, (R)ꢀ and (S)ꢀBINAP,
[((S)ꢀBINAP)RuCl(pꢀMeC6H4Pri)]Cl (Fluka), 1ꢀbutylꢀ3ꢀ
methylimidazolium hexafluorophosphate, and 1ꢀbutylꢀ3ꢀ
methylimidazolium trifluoromethanesulfonate (Acros) were
used. The complex [RuCl2(C6H6)]2 was synthesized according
to a known procedure.19 Et4N+Br– was purified by recrystallizaꢀ
tion from EtOH and dried in vacuo (1—2 Torr) at 50 °C for 24 h.
Before use, 95% EtOH was purged with argon; other solvents
were dried and distilled in a flow of argon. Argon was purified by
passing through columns packed with a nickel—chromium cataꢀ
lyst, copper on Kieselguhr (80 °C), and molecular sieves (4A).
Hydrogen was purified by passing through columns packed with
a nickel—chromium catalyst and molecular sieves. All manipuꢀ
lations with catalyst recycling, including workꢀup of the reacꢀ
tion mixture between cycles, were carried out under purified
argon.
The degree of conversion of the starting oxo ester and the
selectivity of its hydrogenation (relative to diethyl ketal as a
byꢀproduct) were determined by 1H NMR spectroscopy on a
Bruker AMꢀ300 instrument; the ee of the hydrogenation prodꢀ
uct 2 were determined by GLC as described earlier.5
Catalyzed asymmetric hydrogenation (general procedure).
A catalyst (or its components for the in situ formation) and IL or
Et4N+Br– were placed in a glass tube, which was thrice evacuꢀ
ated and filled with argon. A solution of the substrate in an
appropriate solvent was degassed by three freezing (in liquid
nitrogen)—evacuation—thawing cycles under argon. Then the
solution of the substrate was added to the solution of the catalyst
in a tube with a magnetic stirring bar and the tube was placed in
a 30ꢀmL stainlessꢀsteel autoclave filled with argon. The autoꢀ
clave was purged with purified hydrogen. After the pressure of
hydrogen reached 12 atm, the reaction mixture was kept at a
specified temperature. After the first cycle of hydrogenation was
terminated, organic material was extracted three times with a
triple volume of degassed hexane. The layer of the ionic liquid
containing the catalyst or the solid residue, which represents a
mixture of the tetraethylammonium salt and the catalyst, was
evacuated, the tube was filled with argon, and the next hydrogeꢀ
19. R. A. Zelonka and M. C. Baird, Can. J. Chem., 1972,
50, 3063.
Received April 13, 2004;
in revised form June 9, 2004