J. Phys. Chem. 1994, 98, 12776-12781
12776
Enantiomer Recognition of Asymmetric Catalysts. Thermodynamic Properties of Homochiral and Heterochiral Dimers of the Methylzinc Alkoxide Formed from Dimethylzinc and Enantiomeric 3-exo-(Dimethylamino)isoborneol Masato KitamuraJ Seiji Suga,t Makoto NiwaJ Ryoji Noyori,**tZong-Xi Zhai$,* and Hiroshi Sugatp Department of Chemistry, Nagoya University, Chikusa, Nagoya 464-01, Japan, and Department of Chemistry and Microcalorimetry Research Center, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan Received: July 13, I994@
Reaction of dimethylzinc and (2s)- or (2R)-3-exo-(dimethylamino)isoborneol forms methylzinc alkoxide 1, which forms a dimeric structure 2 in a reversible manner. Depending on the chirality of the monomer 1, three stereoisomeric dinuclear complexes are formed; the homochiral dimerization of 1 leads to (S,s)- or (R,R)-2, whereas the heterochiral interaction forms (S,R)-2. The heterochiral dimer is thermodynamically favored over the homochiral dimer in both toluene solution and crystalline states. The vapor pressure osmometry method has revealed that the stereoisomeric dimers 2 in toluene are in equilibrium with the monomer at 40 "C, respectively. 1 with dissociation constants &om0 = (3.0f 1.0) x and &em =1x Calorimetric determination of the enthalpies of solution of the crystalline compounds indicates that the S,R heterochiral dimer is more stable than the S,Sor R,R homochiral dimer by 3.4f 0.8 kJ mol-' in the solid state.
Introduction Enantioselective addition of a dialkylzinc to a prochiral aldehyde catalyzed by 3-em-(dimethylamino)isobomeol(DAB)' exhibits an enormous nonlinear relation between the enantiomeric purity of DAIB and the enantiomeric excess of the alkylation product, as illustrated in Figure 1.2 Under certain selected conditions, (29-DAIB in 20% ee3 (2S:2R = 60:40) affects the methylation of benzaldehyde in toluene to give, after aqueous workup, (9-1-phenylethanol in 88% ee, a value close to the 92% ee achieved with enantiomerically pure (29-DAIB. This striking chirality amplification has been explained in terms of the enantiomer recognition of asymmetric catalysts outlined in Figure 2.4-6 The actual catalyst promoting the reaction of dimethylzinc and benzaldehyde is the chiral alkylzinc alkoxide 1 formed by the reaction of dimethylzinc and DAIB by the elimination of methane. The tricoordinate Zn catalyst, however, forms reversibly the dimeric, tetracoordinate compound 2 in hydrocarbon solvents. The homochiral dimerization of (9-or (R)-1 results in (S,S)-or (R,R)-2, respectively, while the heterochiral combination leads to (S,R)-2. The chirality of DAIB determines the diastereoselection in the dimerization as well. Thus, for steric reasons, (S,9-2 and (R,R)-2possess the S,Sand R,R configuration, respectively, at the two stereogenic Zn centers, and (S,R)-2 has the S,R structure.' Nonbonded repulsion caused by the bomane skeletons does not allow the formation of other stereoisomers. The heterochiral dimer is much more stable than the homochiral dimer. Therefore, when partially resolved (2S)-DAIB is used, the minor isomer, (R)-1, is mostly converted to (S,R)-2 by taking the same amount of the enantiomer, ( 9 - 1 . and the remaining major S isomer forms t Nagoya University. t Osaka University. P Permanent address: Qinghai Institute of Salt Lakes, Academia Sinica,
Qinghai, China. # Present address: Research Institute for Science and Technology, Kinki University, Kowakae, Higashi-Osaka 577, Japan. Abstract published in Advance ACS Abstracts, November 1, 1994. @
n
($9-2. The homochiral S,S dimer participates in the alkylation to a greater extent than the heterochiral S,R complex, since the former dissociates more easily into the real monomeric catalyst ( 9 - 1 (&om0 > in Figure 2), thereby exhibiting a notable chirality amplification. We disclose here the quantitative
0022-365419412098- 12776$04.5010 0 1994 American Chemical Society
Properties of Dimers of a Methylzinc Alkoxide
% ee 01 (Pq-DAIB
Figure 1. Chiral amplification in the catalytic enantioselective methylation of benzaldehyde in the presence of DAIB. Concentrations: 8 mM DAIB; 84 mM CsHsCHO; 84 mM Zn(CH&..
Khom
1/
(S4-2
Figure 2. Equilibrium between the homochiral dimers, (S,S)-2 and (R,R)-2, and the heterochiral dimer (S,R)-2.
difference in the thermodynamic stability of the homochiral and heterochiral dimer 2 in solution and crystalline states.
Experimental Section General Methods. 'H- and 13C-NMR spectra were measured in CDCl3 containing 0.03% Si(CH3)4 as internal standard or C,&CD3 on a JEOL JNM-GX270 and JNM-GSX270 spectrometer. In measurements in Ca5CD3, the methyl proton
J. Phys. Chem., Vol. 98, No. 48, 1994 12777 signal at 6 2.31 was used as a standard. Chemical shifts are reported in ppm (6) downfield from the methyl signal of Si(CH3)4, and the coupling constants (J) were expressed in Hz. The signal pattems of singlet, doublet, triplet, quartet, and multiplet, and broad signals were abbreviated to s, d, t, q, m, and br, respectively. Optical rotation was measured on a JASCO DIP-181 digital polarimeter with a 5 mm x 10 cm cell. Melting points were determined on a YANAKO micro melting point apparatus and were uncorrected. Elemental analyses were performed on a Perkin Elmer Model 240C at the Faculty of Agriculture, Nagoya University. Liquid chromatographic analyses were conducted on a Shimadzu LC-6A instrument equipped with a RHEODYNE 7125 injector and a Shimadzu SPDdA UV detector. Chromatography was done on a column of silica gel (Merck 7734 and 9385). All operations with the organozinc compounds were performed under an argon atmosphere using standard Schlenk techniques. Graphical expression of the mathematical equations was aided by the Mathematica program on an Apple Macintosh computer. Materials. The solvents, CH30H and CH2C12, for purification of DAIB were freshly distilled from Mg and CaH2, respectively. Purification of C&CH3 for the molecular weight measurement was done by distillation from a Na-K alloy. Commercial Zn(CH3)2 (Toyo Stauffer Chemical Co., Lot No. DMZ 812) was purified by vacuum distillation. All the stock solutions of organozinc compounds and the distilled C6HsCH3 were kept in Schlenk tubes equipped with Young's tap. Preparation of the Homochiral and Heterochiral Dimers 2. Purification of 3-exo-(Dimethylamino)isoborneol (DAIB). The crude (2S)-, (2R)-,and (f)-DAIB were prepared from (1R)(+)-camphor ([aI2O~ +45.2" (c 9.7, ethanol)), (18-(-)-camphor ([aI2O~-42.9" (c 10.0,ethanol)), and (&)-camphor, respectively, according to the known method.8 The optically and chemically pure (2s)- and (2R)-DAIB were obtained by the previously reported method.lb (f)-DAIB was purified via its benzoate by the following procedure. The crude (&)-DAIB (13.3 g, 0.0674 mol) containing some (f)-3-endo-(dimethylamino)bomeolwas dissolved in CH2C12 (200 mL) containing N(CzH5)3 (14.0 mL, 0.101 mol). Benzoyl chloride (11.7 mL, 0.101 mol) was added to the solution at 0 "C, and the mixture was stirred at room temperature for 18 h. After all volatiles were evaporated under reduced pressure, the resulting solid was partitioned between ether (300 mL) and 1 M aqueous NaOH solution (200 mL). The organic layer was extracted with 3 M aqueous HC1 (100 mL). The aqueous layer was made pH 11 by addition of NaOH pellets at 0 "C and extracted with ether (300 mL). Drying of the organic layer over anhydrous NazS04 and evaporation of the solvent afforded a brown oil, which was chromatographed on a silica gel column using a 7:1 - 1 :3 hexane-ether mixture as eluent to give the benzoate of (f)-DAIB (12.2 g, 40.5 "01) and (f)-3-endo-(dimethylamino)bomylbenzoate (1.7 g, 5.6 "01) as a waxy solid. Benzoate of (f)-DAIB: 'H NMR (CDC13) 6 0.81 (s, 3, CH3), 0.82 (s, 3, CH3), 1.08-1.16 (m, 1, CHH), 1.25-1.35 (m, 1, CHH), 1.42 (s, 3, CH3), 1.50-1.62 (m, 1, CHH), 1.71-1.83 (m, 1, CHH), 2.06 (d, 1, J = 4.5 Hz, CH), 2.13 (s, 6, N(CH3)2), 2.27 (d, 1, J = 6.9 Hz, NCH), 5.23 (d, 1, J = 6.9 Hz, CHOCOC6H5), 7.41-7.48 (m, 2, aromatic protons), 7.52-7.58 (m, 1, aromatic protons), 8.08-8.13 (m, 2, aromatic protons). Anal. Calcd for C19H27N02: C, 75.71; H, 9.03; N, 4.65. Found: C, 75.71; H, 9.21; N, 4.63. (f)-3endo-(Dimethy1amino)bomyl benzoate: 'H NMR (CDC13) 6 0.82 (s, 3, CH3), 0.94 (s, 3, CH3), 1.04 (s, 3, CH3), 1.23-1.36 (m, 1, CHH), 1.52-1.62 (m, 1, CHH), 1.75-1.84 (m, 1, CHH), 1.90-2.11 (m, 2, CH and CHH), 2.09 (s, 6, N(CH3)2), 2.54 (ddd, 1, J = 1.7, 4.0, 8.9 Hz, NCH), 5.45 (dd, 1, J = 2.0, 8.9
12778 J. Phys. Chem., Vol. 98, No. 48, 1994
Hz, CHOCOGH5), 7.43-7.65 (m, 3, aromatic protons), 8.148.18 (m, 2, aromatic protons). Anal. Calcd for C19H27N02: C, 75.71; H, 9.03; N, 4.65. Found: C, 75.68; H, 9.15; N, 4.58. The benzoate of (f)-DAIB was dissolved in a 0.346 M CH3OH solution of NaOCH3 (260 mL, 90 mmol), and the mixture was heated at reflux for 17 h. After removal of the solvent under reduced pressure, the residue was partitioned between 1.2 M aqueous HC1 (250 mL) and CH2Clz (250 mL) at 0 "C. Neutralization of the aqueous layer by addition of 6 M NaOH solution (60 mL) and extraction with CH2Cl2 (200 mL) and the usual workup afforded (f)-DAIB as an oil (7.2 g, 54% yield). This was mixed with an equimolar amount of (&)-tartaric acid (5.5 g, 37 mmol) in hot C2H50H (48 mL) and allowed to stand at 4 "C for 12 h. The resulting crystals (10.2 g) were separated and partitioned between CH2C12 and an ice-cooled 3 M NaOH solution. The organic layer was washed with water and dried over anhydrous NazS04. Evaporation of the solvent and bulbto-bulb distillation (bp 157- 165 "C/21 mmHg) afforded pure (f)-DAIB, [a]27D +0.046" ( c 2.23, ethanol). The optical purities of ( 2 9 - and (f)-DAIB were confirmed by HPLC analysis of the N-(3,5-dinitrophenyl)carbamates (column, Sumitom0 Chemical Co. SUMIPAX OA-4000; eluent, 99.5:0.5 hexane-ethanol mixture; flow rate, 1.0 mL min-'; detection, 254-nm light; tR, 17.4 min (carbamate from (29-DAIB) and 22.5 min (carbamate from (2R)-DAIB)). Molecular Weight Measurement by Vapor Pressure Osmometry. A Corona 114 apparatus was used for measurement of the molecular weight based on the differential vapor pressure technique. On account of the air- and moisture-sensitivenature of the organozinc compound^,^ the commercial apparatus was extensively modified before use. The vapor equilibration aluminum chamber was coated with poly(tetrafluoroethy1ene) (PTFE), and the inner and outer filter papers were exchanged from cellulose to PTFE fiber and glass fiber, respectively, to keep the vapor pressure constant. The instrument was connected to a recording voltmeter with a full scale of 10 mV (1000 DIGIT). The system was placed in an argon bag. Sample preparation was done in an argon-filled glovebox with a dew point of -90 "C. Sample Preparation. The toluene solutions of a standard sample of triphenylmethane (TPM) (1.19,2.31, and 4.66 g kg-I), ( S , 9 - 2 (1.38-3.97 g kg-'), and (S,R)-2 (1.29-4.08 g kg-') were prepared in the 0.1-0.8 mmol range in an argon-filled glovebox, and all the samples were kept there in Schlenk tubes equipped with a Young's tap. The samples were directly loaded on the introduction port via cannula under a slightly positive argon pressure. General Procedure for Measurement. At the start of the experiment the chamber was heated at 110 "C for 6 h under an argon stream, and C6H5CH3 (30 mL) was introduced at room temperature. The vapor pressure of C&CH3 in the system was equilibrated for 1 h at the measuring temperature fO.01 "C. The measurement routine was composed of three stages: (1) zero correction with C&CH3 solutions, (2) calibration using the C&CH3 solutions of TPM at three different concentrations to determine the equipment constant, and (3) measurement of the sample. The thermistor for the samples was washed with about 0.4 mL of the solutions for the subsequent measurement, and the identical sample was measured three times to minimize uncertainty. The zero point was adjusted before each measurement. Reestablishmentof the vapor equilibration after addition of one drop of sample required 10-20 min at 40 "C. Calculation of Molecular Weight at 40 "C. The numberaverage molecular weight (Mobs) was calculated from the equation
Kitamura et al. =-e'quip,'
M Ob'
AV
where Kquip = equipment constant, Cm= mass (g) of solute in 1 kg of solvent, and AV = differential voltage as DIGIT generated by differential vapor pressure between CsH5CH3 as a reference and the C&CH3 solution of a sample. KeqUip was estimated by substitution of the molecular weight of TPM (244.34) and the AV/Cm ( C , 0 ) value at 40 "C into eq 1. The observed AV values and molecular weights of (S,S)-2and (S,R)-2with the given constants C, and Kequip are listed below. (S,S)-2: Kequip = 2.97 X lo4, Cm = 1.38, AV = 133.5, Mobs = 307; 2.97 x lo4, 1.38, 132.5, 309; 2.97 x lo4, 2.88, 263.5, 325; 2.97 x lo4, 2.88, 266.5, 321; 2.97 x lo4, 3.97, 345.5, 341; 2.97 x lo4, 3.97, 352.0, 335. (S,R)-2: 2.97 x lo4, 1.29, 75.0, 511; 2.97 x lo4, 1.29,79.0,485; 2.97 x lo4, 2.77, 156.5, 526; 2.97 x lo4, 2.77, 155.0, 531; 2.97 x lo4, 4.08, 224.5, 540; 2.97 x lo4, 4.08, 233.5, 519.
-
Induction of Equations for Determination of &om0 and In the equilibrium involving 1 mol of the homochiral dimer ( S , 9 - 2 dissociating into 2 mol of the monomer (9-1, the number-average molecular weight Mobs is given by
Kheterom
where [S,Sl, [SI,Mdimer, and Mmonomer express the molar concentrations of (S,S)-2 and (9-1, and the molecular weights of the dimer and the monomer, respectively. The initial concentration of the homochiral dimer ( C , M) is calculated as (3) Considering the two equations, Khomo = [S12/[S,Sl and Mamer 2Mmonomer, eq 4 is deduced.
Since the heterochiral dimer (S,R)-2 is in equilibrium with ( S , 9 - 2 and (R,R)-2through the monomer ( 9 - 1 and (R)-l, Mobs of the heterochiral dimer can be related with c,Khomo, and Khekm as follows:
(7)
Here, since the concentrations of ( $ 9 - 2 and ( 9 - 1 are equal to those of (R,R)-2 and (R)-1, respectively, eqs 5-8 are transformed to
Properties of Dimers of a Methylzinc Alkoxide
J. Phys. Chem., Vol. 98, No. 48, 1994 12779 TABLE 1: Number-Average Molecular Weight Determined by Vapor Pressure Osmometw concentration,b homochiral dimer heterochiral dimer mM
(S,S)-2
2.02 2.16 4.34 4.5 1 6.21 6.39
(SA-2
485-511 307-309 526-531 321-325 335-341 519-540
Molecular weights were measured at 40 "C in toluene using the equipment constant Kquip of 2.97 x 104 which was determined with triphenylmethane as a standard. The molecular weight of the dimer is 553.5. As a pure dimer. (I
Calorimetric Measurement. The enthalpies of solution of the homochiral and heterochiral crystalline dimers were measured by using an LKB 8700-1 Precision Solution Calorimeter with a 25-mL reaction vessel. In order to test the reliability of the calorimeter, a standard sample of tris(hydroxymethy1)aminomethane (THAM) donated by the National Bureau of Standards (now NIST) through the courtesy of Dr. M. V. Kilday was used for the determination of accuracy and precision. About 1.4 mmol of THAM was dissolved into a 0.1 M HC1 aqueous solution at 25 "C. The average value of five determinations of the enthalpy of solution , As01H(THAM) = -(29.773 f 0.015) M mol-', was in good agreement with the literature values, -(29.740 f O.OIO),lo -(29.771 f 0.005)," and -(29.794 f 0.006),'* and the recommended value -(29.771 f 0.032) kJ m01-l.'~ Three kinds of stereoisomeric dimer, (S,S)-2, (R,R)-2,and (S,R)-2, were used for the measurement of the enthalpy of solution into C&CH3 at 25 "C. Each specimen was put into a thin glass-ampule under an argon atmosphere. The inlet tube of the ampule was plugged by a stopper, and the ampule was sealed off by using a pair of special bumers with fine nozzles. The amount of sample was in the range 0.014-0.019 g. The ampule was set into an ampule holder which acts as a stirrer and as part of the ampule breaker at the same time. The sealing part of the rotating shaft of the stirrer was coated with silicon grease to avoid the possible penetration of moisture during the measurement. Filling of the vessel with dry C a 5 CH3 and assembling of the calorimetric systems were carried out in a polyethylene bag filled with argon gas. The calorimeter was immersed in a water bath kept at 25.000 f 0.001 "C. Results Preparation of Dimeric Alkylzinc Alkoxides. The three stereoisomeric dinuclear complexes, (S,S)-, (R,R)-, and (S,R)2, were prepared by reaction of stoichiometric amounts of dimethylzinc and enantiomerically pure or racemic DAIB in toluene. The chemically pure samples were obtained by recrystallization from toluene. The single-crystal X-ray structures of ($57-2 and (S,R)-2 have been eitabli~hed.~The cryoscopic molecular-weight determination of (S,S)-2 in 23 mM benzene solution indicated the dimeric structure at the freezing tem~erature.~ The 'H NMR spectra of (S,S)-2and (S,R)-2 taken in toluene-& at -80 to 80 "C gave different sets of signals without change of the area ratios. Monomeric species 1 does exist in solution but is undetectable as an independent entity. The spectra reveal that the equilibrium of Figure 2 is readily established. The composition of stereoisomeric 2 is determined
-002 0
2
4
6
concentration, mM Khomo= (3.0 f 1 .O)X lo-' bGhom0= 9.3 f 0.9 kJ mol-'
400
0.001
300 Kh-
.
200 0
'
3
2
z 0.03
'
-
6 concentration,mM Khetero
4
1X
lo3
AGhetero= 29 f 3 kJ mol-'
Figure 3. Determination of Khomo (a) and (b) values at 40 OC. Observed molecular weight: (0) homochiral dimer (S,S)-2; (m) heterchiral dimer (S,R)-2. The K value is based on the unit of mol L-' for concentration. solely by the mole ratios of enantiomers of 1 and is unaffected by the way of mixing. Vapor Pressure Osmometry. Table 1 lists the numberaverage molecular weights of the dimers determined by vapor pressure osmometry in toluene at 40 "C as a function of the concentration ranging from 2 to 6 mh4 where the concentration refers to the pure dimer. The results of the triplicate experiments were reproducible. Thus the homochiral dimer (S,S)-2 and heterochiral dimer (S,R)-2 are indeed in equilibrium with the monomer 1 and, as expected, the molecular weight decreases as the concentration is lowered. The dissociation constant, Khomo,for the homochiral dimer 2 can be correlated by eq 4 with the observed molecular weight, Mobs, the molecular weight of the monomer, Mmonomer (276.7), and the initial molar concentration of 2, C (M). Graphical expression of eq 4 affords frame a of Figure 3. Its comparison with Mobs of (S,S)-2 gave Khomo of (3.0 f 1.0) x lo-' (based on the unit of mol L-' for
12780 J. Phys. Chem., Vol. 98, No. 48, 1994
Kitamura et al.
(S,s)-2 (cryst) + ( R m - 2 (cryst)
.) 2 x (-3.40
kJ mol-')
(S,8-2(cryst)
- 51.47 kJ
Aw,lHss = -51.47 kJ mol-'
(s.qfl-2 (soln) (f).R)-2(cryst)
Figure 4. Enthalpy diagram of homochiral and heterochiral 2 in crystalline and solution states. Dimer 2 in toluene solution (1.23 mM, 25 "C) contains its monomer 1.
TABLE 2: Experimental Data for the Enthalpy of Solutiona sample
mass, g Experiment 1 (S81-2 0.016 60 0.019 21 0.028 83 Experiment 2 (S,S)-2 0.015 20 0.016 01 0.015 81 0.019 20 (R,R)-2 0.015 11 0.016 86 0.015 78 0.019 35 Measured with toluene at 25 "C.
A s d , J R-'
-133.55 -132.95 - 131.91 -94.96 -92.55 -92.75 -91.83 -189.64 -187.09 -187.17 -185.74
concentration) in toluene at 40 "C. The Khetemvalue is a function of Mobs for the heterochiral dimer and Mmonomer as Well as Khomo, as represented by eq 13. The Khetem-parametricplotting of eq 13 with the average Khomovalue, 3.0 x lo-*, and Mmonomer, 276.7, led to graph b of Figure 3. Kheteroat 40 "C in toluene has been determined to be about 1 x by fitting Mobs on the simulated curves.14 According to the Gibbs equation, AG = -RT In K, the free energy differences between the dimer and monomer are calculated to be A G o m o = 9.3 f 0.9 kI mol-' and A G I , , ~=~ ~29 f 3 kJ mol-',14 respectively. Thus, under such conditions, the homochiral dimer is more prone to dissociate into the monomer than the heterochiral dimer by a factor of about 3000. Calorimetric Analysis. The experimental data for the enthalpy of solution of the dinuclear complexes are summarized in Table 2. The upper part of the table (Experiment 1) shows the results for the crystalline heterochiral dimer (S3)-2 dissolved into toluene at 25 "C. The enthalpy of solution A,,lH increases as the concentration is decreased. The same solution system could be produced from the homochiral dimers by dissolving crystalline ($57-2 into toluene followed by addition of the same amount of crystalline (R,R)-2. The results are given under Experiment 2. The large difference between the enthalpies of solution for the first and second steps in Experiment 2 should be noticed. The enthalpy change observed in the second step amounts to twice the quantity of that in the first step. Small deviations of the sample amount used in the second step from the equimolar composition were corrected for by using the data of the enthalpy of solution. The results are depicted in Figure
4 as the enthalpy diagram of the whole system in the crystalline and solution states. Since the enthalpy of solution depends on the concentration, the diagram refers to the fixed concentration of 1.23 mM, where all the A s o l H data are available by interpolation of small intervals in concentration. A direct consequence of this experiment is that the heterochiral dimer is more stable than that of the homochiral dimer by 3.4 f 0.8 kJ mol-' in the crystalline state. The enthalpy difference in solution, 25.88 f 0.8 kJ mol-', will be considered in the Discussion section. The reliability of the data is inferior to that for THAM. The data is probably affected by a minute amount of moisture that could not be removed thoroughly during a series of experimental operations.
Discussion As expected, the heterochiral dinuclear complex (S,R)-2 has proved to be more stable than the homochiral dimers, (S,S)and (R3)-2,in both crystalline and solution states. The enthalpy difference in the crystalline state is 3.4 W mol-'. This is most simply interpreted in terms of the difference in the geometry of each dimer as revealed by the single-crystal X-ray a n a l ~ s i s . ~ The homochiral dimers have a syn geometry with respect to the central 5/4/5 tricyclic system which suffers steric congestion. On the other hand, the heterochiral dimer possesses an anti 5/4/5 ring system which is more favored energetically. The situation is similar in solution. As described earlier, the Zn complexes in toluene at 40 "C establish the dimer-monomer equilibrium (Figure 2). Here, a complication arising from the dynamic equilibria emerges in the solution owing to solvation effects, which may differ energetically and entropically for the monomer and both types of dimer species. The relative stabilities of the stereoisomers in toluene are reflected in the magnitudes of the equilibrium constants, Khomo 3 x lo-* and Kheterozz 1 x at 40 "c. In the concentration range 2-6 mM, about 70% of the homochiral dimer dissociates into the monomer 1,whereas the heterochiral dimer does not dissociate to any considerable extent. The broken line in Figure 4 represents a hypothetical binary system of (R,R)-2 solution separated by ($57-2solution of the same concentration without contact. Since the enthalpy of mixing of enantiomers is known to be on the order of several joules per we can safely neglect this enthalpy effect in considering the ingredient of the enthalpy difference denoted by the thick arrow in the figure. As described above, however, the quantity AH zs -25.9 kJ mol-' is not the exact enthalpy
Properties of Dimers of a Methylzinc Alkoxide difference between the homo- and heterochiral dimers in solution. As has been clarified by the vapor pressure osmometric measurement at 40 "C, the dissociation equilibrium behavior is quite different between the homochiral and heterochiral dimers. The equilibrium constants will change with temperature through changes in the enthalpy and entropy of solvation for each species existing in the solution. This effect can be quantified simply by measuring the values of Khetero and at 25 "C. These data combined with those at 40 "C lead directly to the calculation of enthalpy difference between the dimers in solution. Unfortunately, however, the present apparatus of the vapor pressure osmometer is too insensitive to obtain reliable data for the equilibrium constant at 25 "C. Thus it is essential to develop a more sensitive method for detecting colligative properties of the solution or to find a better solvent with a vapor pressure higher than that of toluene in order to derive the complete thermodynamic properties of respective species in solution.
Acknowledgment. We acknowledge Mr. K. Masumoto, Corona Electric Co., for his valuable contribution to the improvement of the Corona 114 molecular-weight-measuring apparatus. This work was aided by the Ministry of Education, Science and Culture, Japan (Grant No. 05554016). References and Notes (1) (a) Kitamura, M.; Suga, S.; Kawai, K.; Noyori, R. J . Am. Chem. SOC. 1986, 108, 6071. (b) Noyori, R.; Suga, S.; Kawai, K.; Okada, S.;
Kitamura, M.; Oguni, N.; Hayashi, M.; Kaneko, T.; Matsuda, Y. J. Organomet. Chem. 1990, 19, 382. (2) Noyori, R.; Suga, S.; Kawai, K.; Okada, S.; Kitamura, M. Pure Appl. Chem. 1988, 60, 1597. See also: Oguni, N.; Matsuda, Y.; Kaneko, T. J. Am. Chem. SOC.1988, 110, 7877.
J. Phys. Chem., Vol. 98, No. 48, 1994 12781 (3) Enantiomeric excess (ee) is expressed by the following equation:
ee=
S-R lS+RI
where S and R indicate the amounts of the S and R enantiomers. (4) Kitamura, M.; Okada, S.; Suga, S.; Noyori, R. J. Am. Chem. SOC. 1989, 111, 4028. ( 5 ) Noyori, R.; Kitamura, M. Angew. Chem., Int. Ed. Engl. 1991, 30, 49. (6) Noyori, R. Asymmetric Catalysis in Organic Synthesis;Wiley: New York, 1994; Chapter 5 . (7) Cahn, R. S.; Ingold, C.; Prelog, V. Angew. Chem., Znt. Ed. Engl. 1966,5385. We appreciate the valuable advice of Professors Kazuo Hata and Yujiro Nomura about the notation of the absolute configuration of the dmuclear Zn complexes. (8) Beckett, A. H.; Lan, N. T.; McDonough, G. R. Tetrahedron 1969, 25, 5689. Beckett, A. H.; Lan, N. T.; McDonough, G. R. Tetrahedron 1969, 25, 5693. Chittenden, R. A.; Cooper, G. H. J . Chem. SOC. C 1970, 49. Daniel, A.; Pavia, A. A. Bull. SOC. Chim. Fr. 1971, 1060. Reetz, M. T.; Zierke, T. Chem. Ind. (London) 1988, 17, 663. (9) For determination of molecular weights of the organolithium compounds by vapor pressure osmometry, see: West, P.; Waack, R. J . Am. Chem. SOC. 1967, 89, 4395. Arnett, E. M.; Fisher, J. F.; Nichols, M. A,; Ribeiro, A. A. J . Am. Chem. SOC.1990,112,801. Arnett, E. M.; Moe, K. D. J . Am. Chem. SOC. 1991,113,7288. For barometry, see: Fraenkel, G.; Beckenbaugh, W. E.; Yang, P. P. J . Am. Chem. SOC. 1976, 98, 6878. Jackman, L. M.; DeBrosse, C. W. J . Am. Chem. SOC. 1983, 105, 4177. Jackman, L. M.; Scarmoutzos, L. M. J . Am. Chem. SOC. 1987, 109, 5348. (10) Wadso, I. Sci. Tools 1966, 13, 33. (11) Kilday, M. V.; Prosen, E. J. NBS Report No. 10621; National Bureau of Standards: Washington, D.C., 1971. (12) Kimura, T.; Takagi, S. J . Chem. Thennodyn. 1979, 11, 47. (13) Marsh, K. N. Recommended Reference Materials for the Realization of Physicochemical Properties; Blackwell Scientific Publishers: Oxford, U.K., 1987; p 291. (14) Experimental uncertainty in the determination of equilibrium constants by vapor pressure osmometry increases in the case where the equilibration state is extremely one-sided. Although the measured values of Khetero with the biggest amount of scatter are 5 x 10-5 to 5 x the simulation curve obtained with Khetem= 1.0 x and Khomo= 3 x is best fitted to the observed values. (15) Takagi, S.; Fujishiro, R.; Amaya, K. J . Chem. Soc., Chem. Commun. 1968, 480.
