Article and insecticides. drugs some of syntheses the in as precursors, synthetic derivatives. carbohydrate some including diols, and aminoalcohols as substances chiral such of amounts substoichiometric biologically active compounds active biologically of itsfaces. on the prochiral carbonyl compound, preferentially on one compound, which facilitates attack by the alkyl group (R-) formed between Zn(II), the chiral ligand, and the carbonyl *e-mail: [email protected] slow. extremely is alcohols, chiral obtain to procedures or ketones is currently one of the most important synthetic compounds (asymmetric alkylation) to prochiral aldehydes Introduction 0103 -5053$6.00+0.00 Printed inBrazil-©2009SociedadeBrasileiradeQuímica J. Braz. Chem.Soc., Vol. 20,No.1,74-79,2009. 1,2 addition of dialkylzinc to aldehydes and ketones and aldehydes to dialkylzinc of addition 1,2 organometallic of addition enantioselective 1,2 Departamento deQuímicaInorgánica y Analítica, Facultad deCienciasQuímicasyFarmacéuticas, 8,9 Enantioselective Addition ofDiethylzinc toBenzaldehyde Catalyzedbyan In these conditions, a mixed catalyst is is catalyst mixed a conditions, these In 5,6 of thereaction. catalyst of the Ti(IV)-sugar type that ensures the good-yield conversion and the enantioselectivity e aenantiosseletividade dareação. um catalisador quiral do tipo Ti(IV)-açúcar, o que assegura uma conversão com bom rendimento to benzaldehyde in CH in benzaldehyde to by formed system catalytic the for enantioselectivity) and (conversion conditions Optimum 1-phenyl-1-propanol. conversion)enantioselectivity(90% (45% moderate production and high-yield the for como catalisador chiral na alquilação assimétrica do benzaldeído com dietilzinco (Et dietilzinco com benzaldeído do assimétrica alquilação na chiral catalisador como used as a chiral catalyst in the asymmetric alkylation of benzaldehyde with diethylzinc (Et diethylzinc with benzaldehyde of alkylation asymmetric the in catalyst chiral a as used benzaldeído com Et com benzaldeído benzaldehyde with Et ao benzaldeído em CH catalítico formado por formado catalítico (45% moderada enantiosseletividade e 90%) de (conversão rendimento alto em 1-fenil-1-propanol de produção Keywords: diethylzinc, xylosederivative, Ti(IV), titanium ), with Ti(O with (1), 1,2-O-isopropylidene-α-D-xylofuranose D-xylose, of derivative A Um derivado da D-xylose, 1,2-O-isopropilideno-α 7 However, it is accelerated by by accelerated is it However, Organometallic Ti(IV) CompoundandaXyloseDerivative 4 and are also very useful very also are and ee ( Universidad deChile, Casilla233,Santiago 1, Chile and Ti(IV) were 10.0 mol % of % mol 10.0 were Ti(IV) and 1 2 José Parada,* Juan Herreraand A. Pedraza )). As condições ótimas (conversão e enantiosseletividade) para o sistema o para enantiosseletividade) (conversãoe ótimas condições S)). As Zn, o composto o Zn, 1-3 2 Zn compound 2 e Ti(IV) foram 10.0 mol % de % mol 10.0 Ti(IV)e foram 1 Cl which are natural are which 2 Cl 2 como solvente, a temperatura ambiente. Na alquilação assimétrica do 2 as a solvent, at room temperature. In the asymmetric alkylation of alkylation asymmetric the In temperature. room at solvent, a as 1 in substoichiometric amount with Ti(O em quantidade sub-estequiométrica com Ti(Ocom sub-estequiométrica quantidade em 1 enantioselectivity. is a chiral diol with a Ti(IV) compound (for example example (for Ti(O compound Ti(IV) a with diol chiral a is one configuration(RorS). moderate enantioselectivity. acid hydrolysis, 1-phenyl-1-propanol in high yield and with alkylation of benzaldehyde with diethylzinc to obtain, after optimally in the mixture of 0.1 mL (1 mmol) of anhydrous and derivative 1 compound Ti(IV) organometallic an by catalyzed 1,2 Enantioselective addition of diethylzinc to benzaldehyde Results andDiscussion derivative of D-xylose derivativeof -D-xilofuranose (1), com Ti(O and 1 equivalent of Ti(IV) with respect with Ti(IV) of equivalent 1 and 1 Recent studies have shown that when the ligand ligand the when that shown have studies Recent in enriched obtained, is alcohol the hydrolysis After We found that the that Wefound The purpose of this paper was to study catalysis by the e 1 equivalenterelação Ti(IV)1 de com e 1 i Pr) 4 ), the reaction occurs in high yield and with high 7-9

and Ti(IV) catalytic system acts system catalytic Ti(IV) and 1 i and Ti(Oand 1 Pr) 4 forms a chiral i Pr) i Pr) 4 2 foi usado ee ( i Zn) para Zn) Pr) 4 forma i )) of S)) 4 Pr) was 2 Zn) 4 in the asymmetric the in Vol. 20, No. 1, 2009 Parada et al. 75

benzaldehyde and 3 mL (3 mmol) of Et2Zn in 2.5 mL 1-phenyl-1-propanol preferably has the S configuration 13 of anhydrous CH2Cl2 in a Schlenk tube under nitrogen (Tables 1 and 2). at room temperature with constant stirring. Under these Enantioselectivity (45% ee) was achieved after a 6 h conditions, a sufficient amount of 1-phenyl-1-propanol reaction with 10.0 mol% of 1 (entry 8 in Table 1). A larger was obtained for its characterization and quantification amount of 1 (20%) did not increase enantioselectivity by gas chromatography (GC). From the area under significantly (entry 9 in Table 1) probably because the the chromatogram peaks we determined the yield sugar derivative 1 acts with Ti(IV) forming a chiral catalyst (conversion% of benzaldehyde into 1-phenyl-1-propanol) of the Ti(IV)-sugar type with an optimum amount of 1, and the enantioselectivity (ee%) of the reactions (Tables 1 and 2) and the products were identified by 1H-NMR. Table 1. Asymmetric alkylation of benzaldehyde with Et Zn catalyzed The predominant configuration was determined for each 2 by carbohydrate derivative 1 and Ti(OiPr) a catalytic system.11,12 4

To optimize the concentration conditions of the catalytic Entry Percentage %eed mol% Reaction system of 1 and Ti(IV), several reactions were carried Conversionc (Config.)e of 1b time / h out by using mixtures of 1 in variable substoichiometric 1 15 2(S) 0 3 amounts (2.5, 5.0, 10.0, and 20.0 mol%) with respect to 2 55 17(S) 2.5 3 benzaldehyde with 1 mmol of Ti(OiPr) , because with 4 3 60 20(S) 5.0 3 concentrations of that order other authors6,12 had achieved 4 80 33(S) 10.0 3 good results with other chiral catalysts and we had earlier 5 83 31(S) 20.0 3 obtained promising results with them.14,15 A high conversion of benzaldehyde into 1-phenyl-1- 6 53 20(S) 2.5 6 propanol was obtained after a 6 h reaction, with Ti(IV) 7 63 30(S) 5.0 6 and 10.0 mol% of 1 with respect to benzaldehyde at room 8 85 45(S) 10.0 6 temperature (entry 8 in Table 1), indicating that the catalyst 9 80 48(S) 20.0 6 formed in situ in this reaction is effective with small 10 50 10(S) 2.5 18 amounts of 1 (Table 1). After an 18 h reaction, maximum 11 45 20(S) 5.0 18 conversion of benzaldehyde into 1-phenyl-1-propanol 12 90 17(S) 10.0 18 was obtained with 10.0 mol% of 1 (entry 12 in Table 1). 13 29 26(S) 20.0 18 After a 24 h reaction, there was decreased conversion of 14 63 40(S) 10.0 24 benzaldehyde into 1-phenyl-1-propanol (entries 14 and 15 15 70 33(S) 20.0 24 in Table 1), because of decomposition of 1-phenyl-propanol a 11 Reaction with 1 mmol of benzaldehyde and 3 mmol of Et2Zn in after 18 hours. b the presence of 1 mmol of Ti(OiPr)4. Mol percentages referred to The optical rotation (αº) of all the asymmetric alkylation benzaldehyde. cDetermined by GC. dDetermined by GC with β-DEX 120 products with 1 had a negative sign, (−), indicating that the column. eDetermined from optical rotation.

a,b Table 2. Results of the asymmetric alkylation of benzaldehyde with Et2Zn catalyzed by carbohydrate derivative 1 and a metal ion after 6 h of reaction

Entry Porcentage conversionc ee%d (config.)e Metal center Solvent T / ºC

a a 1 43 15(S) Ti(IV) CH2Cl2 –20 a a 2 35 40(S) Ti(IV) CH2Cl2 0 a a 3 63 30(S) Ti(IV) CH2Cl2 20 b b 1 43 50(S) Ti(IV) CH2Cl2 –20 b b 2 70 40(S) Ti(IV) CH2Cl2 0 b b 3 85 45(S) Ti(IV) CH2Cl2 20 4b 66 35(S) Ti(IV)b THF 20 5b 90 50(S) Ti(IV)b Toluene 20

b 6 70 17(S) Co(II) CH2Cl2 20 b 7 13 3(S) Cu(II) CH2Cl2 20 a b c Reaction with 5.0 and 10.0 mol% of 1 with respect to 1 mmol de benzaldehyde and 3 mmol Et2Zn in the presence of 1 mmol of metal ion determined by GC. dDetermined by GC with β-DEX 120 column. eDetermined from optical rotation. 76 Enantioselective Addition of Diethylzinc to Benzaldehyde Catalyzed J. Braz. Chem. Soc.

R O RO O OR O in this case 10.0 mol% of that derivative with respect to 1/2 * Ti Ti * benzaldehyde. O O O H R Lowering the temperature from room temperature to Et 2 (OR)3TiO 0 °C and –20 °C decreased conversion of benzaldehyde into Ph Ti(OR) 4 1-phenyl-1-propanol (Table 2), but the lower temperature H Et did not significantly increase the enantioselectivity R O O O R Ph OR (ee%) of the reaction (Table 2).This is probably because O Ti O OR * Ti OR the determining step in the asymmetric reaction is * Ti Ti O OR O OR OR R controlled by the structure of the metal-sugar type O OR O R chiral catalyst, which should not depend very much on 6 3 temperature.7,8,11,12 Et2Zn or The use of other solvents such as toluene or THF EtTi(OiPr) 3 rather than CH2Cl2 produced no significant improvement H in conversion or enantioselectivity (Table 2). Thus, most O reactions were carried out in CH Cl 11 Ph 2 2. R R Et PhCHO O Et O O OR O In order to test the catalytic capacity of 1 with other Ti OR * Ti Ti * Ti metal ions, alkylation of benzaldehyde with ZnEt was OR 2 O O OR O OR OR R O R studied by using Co(II) or Cu(II). The reaction was carried

out with Co(II) or Cu(II) acetylacetonate (Co(acac)2 or 4 i 5 Cu(acac)2) (Table 2) instead of Ti(O Pr)4. Conditions for the reactions with these ions were the same as used with the Ti(IV) and 1 system. The catalytic system of Co(II) and 1 HO O R= iPr O O = OH gave lower conversion and enantioselectivity of 1-phenyl- * O O 1-propanol than those achieved with Ti(IV) and 1 (Table 2). 1 The Cu(II) and 1 system did not catalyze the reaction. These results indicate that the metal-sugar type catalyst in Scheme 1. the asymmetric alkylation must be an octahedral complex

Figure 1. Chromatogram in column methyl silicone gum 5m × 0.53mm × 2.65 μm of the asymmetric alkylation of benzaldehyde with Et2Zn in the presence of 10.0 mol% of 1. Entry 5, Table 1. Conditions in which the chromatogram was performed: Injector temperature, 250 ºC; detector temperature, 250 ºC; pressure, 10 psi; initial temperature, 100 ºC; initial time, 5 min; rate, 20 ºC min-1; final temperature, 180 ºC; final time, 18 min (top). Retention times (min) and peak areas of benzaldehyde and S- and R-1-phenyl-1-propanol present in the chromatogram are tabulated. Calculation of conversion % from these areas is shown at the bottom. Vol. 20, No. 1, 2009 Parada et al. 77

Figure 2. Chromatogram in column Supelco β-Dex 120 de 30 m × 0.25 mm × 0.25 μm of the asymmetric alkylation of benzaldehyde with Et2Zn in the presence of 10.0 mol% of 1. Entry 5, Table 1. Conditions in which the chromatogram was performed: Injector temperature, 250 ºC; detector temperature, 250 ºC; pressure, 10 psi; initial temperature, 100 ºC; initial time, 10 min; rate, 1 ºC min-1; final temperature, 140 ºC; final time, 4 min (top). Retention times (min) and peak areas of benzaldehyde and S- and R-1-phenyl-1-propanol present in the chromatogram are tabulated. Calculation of ee% from these areas is shown at the bottom. as obtained with Ti(IV) and Co(II), and not a square planar 5. To achieve the S configuration of the chiral alcohol, one formed mainly by Cu(II). the attached Et- moves to the carbonyl carbon and the Therefore, optimum catalytic conditions in asymmetric benzaldehyde oxygen probably moves simultaneously alkylation are with 10 mol% of 1 with respect to toward the second titanium center with the attached alkyl benzaldehyde, and Ti(IV) as the metal reaction center in group, giving complex 6. Complex 6 gives complex 2. 11 CH2Cl2 (entry 8 in Table 1). Regeneration of the starting complex 2 completes the The conversion and enantioselectivity of the reaction in catalytic cycle. i the presence of 1 and Ti(O Pr)4 probably involve formation of a complex of the “Ti(IV)-sugar” type, allowing ethyl Conclusions group (Et-) transfer preferentially to one of the faces of benzaldehyde, and favoring formation of the enantiomer The presence of derivatives of D-xylose (1) in a i with the S configuration of the product. substoichiometric amount with Ti(O Pr)4 in the asymmetric

The possible mechanism for asymmetric alkyl addition alkylation of benzaldehyde with Et2Zn forms a chiral is given in Scheme 1. The reaction of the xylose derivative catalyst of dimeric complexes 6 ensuring the conversion i 1 with 1 molar equiv. of Ti(O Pr)4 involves the dimeric and enantioselectivity of the reaction. The best catalytic i complex 2, because Ti(O Pr)4 reacts with 1 (1:1) in CH2Cl2, condition (conversion and enantioselectivity) was achieved i giving {[Ti(IV)(1)(O Pr)2]·2(CH2Cl2)}2 (C 36.26 (36.80); with 10.0 mol% of 1 with respect to benzaldehyde. H 5.48 (5.37); %found (%calculated)). Complex 2 further The conversion and enantioselectivity achieved in the i reacts with 1 molar equiv. of Ti(O Pr)4, giving another synthesis of the alcohol with 1 is due to the formation dimeric complex, 3. These dimeric complexes have within the reaction system of dimeric complexes Ti(IV)- been postulated in the literature on titanium catalysis of sugar (Scheme 1), facilitating transfer of the ethyl 16-22,25 asymmetric alkylation. Complex 3 reacts with Et2Zn group to one face of benzaldehyde. The intrinsic chiral i i or with EtTi(O Pr)3, giving complex 4. EtTi(O Pr)3 can be properties of carbohydrate 1 are transmitted through the i generated from reaction of excess Ti(O Pr)4 with Et2Zn, dimeric Ti(IV)-sugar type complexes formed during the as described for similar catalytic systems.7,25 Complex 4 synthesis of 1-phenyl-1-propanol, yielding preferably further reacts with 1 mol of benzaldehyde, giving complex its S enantiomer. The catalytic efficiency of the dimeric 78 Enantioselective Addition of Diethylzinc to Benzaldehyde Catalyzed J. Braz. Chem. Soc.

Ti(IV)-sugar complexes in the asymmetric alkylation run on a Bruker DRX-300 spectrometer at 300 MHz. of benzaldehyde with diethylzinc is determined by their Calculation of conversion% was made from the areas of stability and rigidity. the Ph-CH(Et)-OH proton signal of 1-phenyl-1-propanol located at 4.5 ppm and that of the CHO of benzaldehyde Experimental located at 10.0 ppm with respect to TMS. The enantiomeric excess (ee%) was estimated on an All reagents and solvents were analytical grade. HP 5890 series II gas chromatograph with an Allchrom plus program and a Supelco β-Dex 120 30 m × 0.25 mm Enantioselective 1,2-addition of diethylzinc to benzaldehyde × 0.25 μm chiral capillary column at initial temperature, catalyzed by a Ti(IV) organometallic compound and a 100 °C; initial time, 10 min; rate, 1 °C min-1; pressure, xylose derivative 10 psi. Retention times were the following: tR (S-1-phenyl-

1-propanol) = 28.7 min; tR (R-1-phenyl-1-propanol) = Compound 1 (19 mg, 10.0 mol% with respect to 29.7 min. benzaldehyde) was placed in a dry 50 mL Schlenk tube, The ee% was calculated from the corresponding areas closed with a silicone stopper, and air was removed of the signals of the S and R enantiomers of 1-phenyl- by purging three times with nitrogen and vacuum. The 1-propanol: ee% = ‌| S-1-phenyl-1-propanol area – R-1- following were then added successively: 0.1 mL (1 mmol) phenyl-1-propanol area | / (S-1-phenyl-1-propanol area + benzaldehyde, 2.5 mL dichoromethane, 0.3 mL (1 mmol) R-1-phenyl-1-propanol area) × 100. 97% titanium(IV) isopropoxide, and finally 3 mL (3 mmol) The predominant configuration of the products was -1 of a 1 mol L solution of Et2Zn in hexane. given by the sign of the optical rotation measured at 20 ºC The reaction proceeded with stirring for 3 hours at on a Perkin Elmer PE 241 polarimeter with the literature room temperature, and was stopped by adding a saturated data:10 o solution of ammonium chloride (releasing ethane and [α ]D (S-1-phenyl-1-propanol) = +48° at 20 ºC and o forming a white precipitate of zinc oxide). The mixture [α ]D (R-1-phenyl-1-propanol) = –48° at 20 °C. was transferred to a separatory funnel, 10 mL of 2 mol L-1 HCl were added, and the product was extracted with three Acknowledgments

10 mL portions of ethyl ether, dried with anhydrous MgSO4 and the ether was evaporated, yielding crude 1-phenyl-1- The authors acknowledge the financial support propanol. of FONDECYT under initiation project 2006 No. This general procedure was applied to all the catalytic 11060463. reactions with different concentrations, solvents and reaction temperatures as shown in Tables 1 and 2 under References Results and Discussion. 1. Noyori, R.; Asymmetric Catalysis in Organic Synthesis, Wiley: Product analysis New York, 1994. 2. Pu, L.; Yu, H-B.; Chem. Rev. 2001, 101, 757. To analyze the products and determine percentage 3. Paixão, M. W.; Braga, A. L.; Lüdtke; J. Braz. Chem. Soc. 2008, conversion, the sample, 0.4 μL, was injected into an HP 19, 813. 5890 series II gas chromatograph equipped with an Allchrom 4. Soai, K.; Niwa, S.; Chem. Rev. 1992, 92, 833. plus program and a methylsilicone-gum-type 5 m × 5. Hübsher, J.; Barner R.; Helv. Chim. Acta 1990, 73, 1061. 0.53 mm × 2.65 μm column. Working conditions were: 6. Jacobbsen, E. N.; Pfaltz, A.; Yamamoto, H.; Comprehensive Initial temperature, 100 °C; initial time, 5 min; rate, Asymmetric Catalysis, Springer: New York, 1999. 20 °C min-1; final time, 18 min; pressure, 10 psi. 7. You, J.-S.; Shao, M.-S.; Gau, H.-M.; Organometallics 2000,

Retention times were: tR(benzaldehyde) = 7.8 min 19, 3368. and tR(1-phenyl-1-propanol) = 11.9 min. The percentage 8. González-Sabín, J.; Gotor, V.; Rebolledo, F.; Tetrahedron: conversion was determined from the peak areas of Asymmetry 2006, 17, 449. products and unreacted benzaldehyde by: % conversion = 9. Parada, J.; Aguirre, P.; Herrera, J.; Pôster, XXVI Jornadas [(1-phenyl-1-propanol area) / (1-phenyl-1propanol area + Chilenas de Química, Sociedad Chilena de Química, benzaldehyde area)] x 100. Concepción, Chile, 2006. Conversion percentages were confirmed by 1H-NMR 10. Morrison, J. D.; Mosher, H. S.; Asymmetric Organic Reactions; spectroscopy. The products were dissolved in CDCl3 and American Chemical Society Books: Washington, DC, 1976. Vol. 20, No. 1, 2009 Parada et al. 79

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