Asymmetric Glyoxylate-Ene Reactions Catalyzed by Chiral Pd(II) Complexes in the Ionic Liquid [bmim][PF6].

The room temperature ionic liquid [bmim][PF6] was employed as the reaction medium in the asymmetric glyoxylate-ene reaction of α-methyl styrene (4a) with ethyl glyoxylate using chiral palladium(II) complexes as the catalysts. [Pd(S-BINAP)(3,5-CF3-PhCN)2](SbF6)2 (1b) showed the highest catalytic activity. Under the reaction conditions of 40 °C, 0.5 h, and 1b/4a molar ratio of 0.05, ethyl α-hydroxy-4-phenyl-4-pentenoate was obtained in excellent chemical yield (94 %) with high enantioselectivity (70 %). Other α-hydroxy esters can also be obtained in high chemical yields and enantioselectities through the glyoxylate-ene reactions of alkenes with glyoxylates catalyzed by 1b in [bmim][PF6]. Moreover, the ionic liquid [bmim][PF6] which contained the palladium(II) complex could be recycled and reused several times without significant loss of the catalytic activity.


Introduction
Organic reactions catalyzed by chiral transition metal complexes are important tools in modern asymmetric synthesis. In most cases, chiral catalysts used in asymmetric catalytic reactions are expensive, so recovery and reuse of catalysts has attracted great interest to reduce the environmental and economic costs. Immobilization of chiral catalysts with solid supports or polymers is a representative method to recover and reuse catalysts [1][2][3]. However, sometimes the catalytic activities of supported chiral catalysts will be lower than that under homogenous conditions and sometimes the enantioselectivity of products will decrease with each reuse of the catalysts. Room temperature ionic liquids are composed entirely of ions and have attracted extensive interest as excellent alternatives to conventional organic solvents due to their unique advantages of high thermal stability, nonflammability, low toxicity, negligible vapour pressure, high loading capacity, tunable polarity and interesting intrinsic physicochemical characteristics [4][5][6][7][8][9][10][11]. In addition, they possess the capacity to dissolve various organic, inorganic, and organometallic compounds. In ionic liquids, chiral transition metal complexes can be immobilized and organic products can be extracted with hexanes, toluene or ether. Thus, a new methodology of chiral catalysts separation and recycling is achieved. In fact, ionic liquids recently have been investigated as the reaction media for a variety of asymmetric catalytic reactions such as Diels-Alder, allylic amination, hydrogenation, Michael, fluorination, epoxidation and Aldol reactions [12][13][14][15][16][17][18]. In these reactions, the chiral catalysts used are recyclable at least several times.
As a part of our interest in performing glyoxylate-ene and other organic reactions in ionic liquids [38,39], we report herein a mild and effective procedure for the asymmetric glyoxylate-ene reaction catalyzed by chiral palladium(II) catalysts using the ionic liquid [bmim][PF 6 ] as the reaction medium. Furthermore, we demonstrate that the catalytic system can be recycled and reused for several runs without any significant loss of catalytic activity.

Effect of solvents and palladium(II) complexes
Palladium(II) complexes (Scheme 1) derived from (S)-BINAP, (S)-MeO-Biphep [40], and (S)-C3-TunePhos [41] were used as the chiral catalysts in the glyoxylate-ene reaction of α-methyl styrene (4a) with ethyl glyoxylate (5a). The reaction (0.5 mmol of 4a, 1.5 mmol of 5a, 0.025 mmol of catalyst) was performed in different organic solvents and two ionic liquids (Scheme 2), and the results were listed in Table 1   However, 1b and 1d, which contained SbF 6 -, were found to give higher enantioselectivity (59 and 63 % ee, respectively) than 1a and 1c which contained BF 4 -(35 and 28 % ee). The good catalytic activities of 1b and 1d were partially due to the higher group electronegativity of SbF 6 compared with that of BF 4 -. In addition, the less coordinating ability of SbF 6 towards palladium(II) was beneficial to the stability of the bidentate intermediate 7 which was formed through the complexation of the carbonyl groups of 5a with palladium(II) complex and could react with 4a to form (R)-6aa. When the anion was BF 4 -, intermediate 7 probably had more chance to convert to intermediate 8, which resulted in the observed decrease in product enantioselectivity [26,42].  The coordinated nitrile ligands of palladium(II) dication also showed some effect on enantioselectivity of the products, but the effect was slight. Considering that 1d showed the highest catalytic activity in 1,2-dichloroethane, we synthesized the similar catalysts [Pd(S-MeO-Biphep) ( In the presence of 1d, the reaction of 4a with 5a in chlorobenzene and 1,2-dichloroethane/toluene (v/v, 1/1) proceeded smoothly and similar enantioselectivity was obtained (ca. 65 % ee, entries 7, 8). Toluene was found as an excellent organic solvent for the reaction of 4a with 5a, and the highest ee value (70 %) was achieved (entry 9). However, the recycle and reuse of 1d was difficult when toluene was used as the reaction solvent. Thus, the ionic liquids 1-n-butyl-3-methylimidazolium tetrafluoroborate ( 6 ] were also investigated, and ee values were found to be improved compared with those obtained in 1,2-dichloroethane (entries [12][13][14][15][16]. Especially for 1a and 1c, ee values increased from 35 to 60 % and 28 to 65 %, respectively. A longer reaction time was needed for complete conversion of 4a using 1b as the catalyst due to its low solubility in [bmim][PF 6 ]. These experimental results suggested that ionic liquid [bmim][PF 6 ] could act as an appropriate medium in the palladium(II) complexes-catalyzed glyoxylate-ene reaction of 4a with 5a to produce (R)-6aa. Table 2 shows the effect of reaction temperature on the glyoxylate-ene reaction of 4a with 5a. At 0 o C, the conversion of 4a was 62 % after 24 h. The chemical yield and ee value of the product were 58 and 70 %, respectively. Obviously, when the reaction temperature increased from 0 to 50 o C, the reaction rate increased. The optimum reaction temperature was 40 o C, at which 94 % chemical yield and 70 % ee of the product were obtained only after 0.5 h. At a reaction temperature of 50 o C, the reaction was complete in 0.3 h, but the chemical yield decreased to 87 % because of more side reactions, such as acidcatalyzed dimerization of 4a, etc. It was noteworthy that the ee value of the product was always 70 % regardless of any variation of the reaction temperature. These results indicated that the enantioselectivity of (R)-6aa in the reaction of 4a with 5a catalyzed by 1d using [bmim][PF 6 ] as the reaction medium was independent of the reaction temperature.

Effect of the molar ratio of 1b/4a
The effect of the molar ratio of 1b/4a on the glyoxylate-ene reaction of 4a with 5a was examined using [bmim]PF 6 as the reaction medium at 40 o C, and the results are listed in Table 3. With a molar ratio of 0.01 between 1b and 4a, the latter was completely converted in 6 h, and the chemical yield and ee value were 89 and 69 %, respectively. Increasing the molar ratio of 1b/4a from 0.01 to 0.05, the reaction time was shortened to 0.5 h, and the chemical yield slightly increased to 94 %. When the molar ratio of 1b/4a further increased to 0.07, the reaction result did not obtain significant improvement. Thus, the molar ratio of 1b/4a should not be more than 0.05 from practical opinion.

Glyoxylate-ene reactions between different substrates
Glyoxylate-ene reactions between alkenes and glyoxylates catalyzed by 1d in [bmim][PF 6 ] were carried out at 40 o C and the results are summarized in Table 4. In all cases, the reactions proceeded smoothly to give the corresponding desired α-hydroxy esters in good chemical yields and enantioselectivities. The electron-withdrawing substituent group on the phenyl ring had an obvious influence on the reactivity of α-methyl styrenes. When 4-chloro-α-methyl styrene (4b) reacted with 5a or methyl glyoxylate (5b), reaction time of 2 h was needed which was longer than that of 4a reacted with 5a or 5b, though the ee values of α-hydroxy esters in these four reactions were very close (69-70%, entries 1-4). The highest ee value of α-hydroxy ester (82 %) was obtained in the glyoxylate-ene reaction of methylenecyclohexane (4c) with 5a. When 5a was replaced by 5b in the glyoxylate-ene reaction of 4c, the ee value of α-hydroxy ester decreased to 76 %.

Reuse of the catalytic systems
One of the main aims of using [bmim][PF 6 ] as the reaction medium in the glyoxylate-ene reaction was to study the possibility of recycling and reuse of catalytic systems. In the glyoxylate-ene reaction of 4a with 5a，the products could be separated by extracted with ether and the ionic liquid phase containing palladium(II) complexes could be reused several times. The results were listed in Table 5. Although the chemical yield of the product slightly decreased to 90 % in run 5 in the 1d-catalyzed glyoxylate-ene reaction of 4a with 5a, the ee value was maintained at 70 %. The slight decrease of chemical yield might be due to slight leaching of 1d. Similarly, palladium(II) complex 3 provided unchanged enantioselectivity in [bmim][PF 6 ] during three consecutive catalytic cycles.  In conclusion, we have demonstrated that 1b is an effective chiral palladium catalyst to catalyze the asymmetric glyoxylate-ene reaction in the ionic liquid [bmim] [PF 6 ]. Under the reaction conditions of 40 o C, 0.5 h, and 1b/4a molar ratio of 0.05, the glyoxylate-ene reaction of 4a with 5a afforded the product in 94 % chemical yield with 70 % ee. Other α-hydroxy esters can also be obtained in high chemical yields and enantioselectivities through the glyoxylate-ene reactions of alkenes with glyoxylates catalyzed by 1b in [bmim][PF 6 ]. In addition, the catalytic systems could be recycled and reused.

General methods
All reactions were carried out under a nitrogen atmosphere by using standard Schlenk line techniques in dried glassware. Solvents were treated prior to use according to the standard methods. The ionic liquids [bmim] [PF 6 ] and [bmim][BF 4 ] were synthesized according to the literature procedures [43]. Palladium(II) complexes 1a-1d, 2 and 3 were also prepared according to the procedures described in the literature [44]. The glyoxylate-ene reaction was monitored by HPLC (Varian Prostar) equipped with a C 18 column or GC (9790) equipped with a FFAP capillary column. The ee value was determined by HPLC (Agilent 1100) analysis on a Chialpak AD-H column with isopropanol/hexane as eluent or by GC (9790) using BETA DEX TM 325 column. The products obtained were purified and identified by 1 H NMR spectroscopy (Varian PLUS-400) and GC-MS (Finnigan Trace GC Ultrar-Finnigan Trace DSQ).

General procedure
To a stirred solution of a palladium(II) complex (0.025 mmol) in a reaction medium (organic solvent 5 ml or [bmim][PF 6 ] 1.5 g) under a nitrogen atmosphere was added glyoxylate (1.5 mmol) and alkene (0.5 mmol). When the reaction was complete, the mixture was extracted with ether. After ether was evaporated in vacuo, the product was further purified by column chromatography and identified by GC-MS and 1 H-NMR. The MS and 1 H-NMR data of the α-hydroxy esters are in good agreement with literature data [24,25,37]. The ionic liquid phase containing palladium(II) complexes could be directly used in the next run without further treatment.