Anal. Chem. 1990, 62. 539-541
Table 111. Accuracy of Isotopic Analyses of Chromatographically Separated Volatile Fatty Acids" STCb sodium formate sodium acetate (std I) sodium propionate
-32.0 f 0.3 (2) -29.5 f 0.1 ( 5 ) -32.2 f 0.1 (2)
this technique -32.3 -29.4 -32.3
f 0.5 (3) f 0.1 (3) f 0.1 (3)
Numbers in parentheses indicate number of measurements; uncertainties are 95% confidence limits. Sealed-tubecombustion. Recovery of acetate as acetic acid is not quantitative, but there is no isotopic fractionation associated with incomplete yield. The primary advantage of this technique is its applicability to samples containing micromolar quantities of acetate. Large volumes of liquid containing low concentrations of acetate can be filtered and dried, with the acetate concentrated in the soIid residue. This technique has been successfully applied to isotopic analysis of acetate in anaerobic, freshwater environments where concentrations of acetate are 20 p M . ACKNOWLEDGMENT We thank S. A. Studley for technical assistance. (1)
LITERATURE C I T E D Martens, C. S.;Blair, N. E.; Green, C. D.; DesMarais, D. 1986, 233, 1300-1303.
539
Whiticar, M. J.; Faber,
E.; Schoell, M. Geochim. Cosmochim. Acta 1986, 50, 693-709. Edwards, G. B.: McManus. W. R.: Biaham, M. L. J . Chromatoar. 1971, 63,397-40 1. Bethge, P. 0.; Lindstrom, K. Analysf 1974, 99, 137-142. Gardner, J. W.; Thompson, 0. E. Analyst 1974, 99, 328-329. Ansbaek, J.; Blackburn, T. H. Microb. Ecol: 1980, 5 , 253-264. Barcelona, M. J.; Liljestrand, H. M.; Morgan, J. J. Anal. Chem. 1980, 52. 321-325. ._. Christensen, D.; Blackburn, T. H. Mar. Bioi. 1982, 7 1 , 113-1 19. Parkes, R. J.; Taylor, J. Mar. Biol. 1983, 77, 113-118. Meinschein, W. G.; Rlnaldi, G. G. L.; Hayes, J. M.; Schoeller, D. A. Biomed. Mass Spectrom. 1974, 1 , 172-174. Blair, N. E.; Leu, A.; Munoz, E.; Olsen, J.; Kwong, E.; DesMarais. D. E. Appl. Environ. Microbiol. 1985, 50, 996-1001. Blair, N. E.; Martens, C. S.;DesMarais, D. J. Science 1987, 236, 66-68. Tyler, J. E.; Dibdin. G. H. J . Chromafogr. 1975. 105, 71-77. Molongoski, J. J.; Taylor, C. D. Appl. Environ. Microbiol. 1985, 50, 1112-1 114. Molongoski, J. J.; Kiug, M. J. Freshwater Biol. 1980, IO. 507-518. Jones, J. G.; Simon, B. M. Appl. Environ. Microbiol. 1985, 49,
.-.
- -
PA7-PAR .. .-.
Klrsten, W. Anal. Chem. 1954, 26, 1097. Buchanan, D. L.; Corcoran, B. J. Anal. Chem. 1959, 31, 1635-1638. Oakwood, T. S.;Mlller, M. R . J . Am. Chem. SOC. 1950, 72, 1849- 1855.
Hayes. J. M.; DesMarais, D. M.; Peterson, D.; Schoelbr, D.; and Taylor, S.P. Adv. Mass Specfrom. 1977, 7 , 475-480.
RECEIVED for review July 28,1989. Accepted December 12, J. Science
1989. This work has been supported by the National Aeronautics and Space Administration (NGR 15-003-118).
CORRESPONDENCE Direct Resolution of Enantiomeric Diols by Capillary Gas Chromatography on a Chiral Polysiloxane Derived from ( R ,R)-Tartramide Sir; Despite the great success in using a chiral stationary phase for affecting gas chromatographic enantiomer resolution, many problems still remain. Chiral diols, in particular, are important building blocks for the synthesis of natural products (1,2),and thus, various gas chromatographic phases for determining enantio-excess in regard to asymmetric synthesis have been extensively studied (3-7). However, diols should be derivatized for application to gas chromatographic systems. For example, the enantiomer resolution of aliphatic diols as boronates and acetals has been carried out by complexation gas chromatography using optically active metal chelates by Schurig et al. (3). Aliphatic diol enantiomers have been resolved on XE-60-~-valine-(R)-cu-phenylethylamide following conversion to cyclic carbonate using phosgen or by conversion to monoester with N-trifluoroacetylglycyl chloride on Chirasil-Val ( 4 , 5 ) .Konig et al. conducted the enantiomer resolution of aliphatic diols as cyclic carbonates on a hexakis(3-0acetyl-2,6-di-O-pentyl)-cu-cyclodextrin chiral stationary phase and the enantiomer resolution of alicyclic 1,2- and 1,3-diols following trifluoroacetylation on heptakis(3-0-acetyl-2,6-di0-pentyl)-P-cyclodextrin (6, 7). These achiral derivatizations render the analytical procedure more complex. In the present study, the direct enantiomer resolution of aliphatic and alicyclic diols without derivatization on a new chiral polysiloxane derived from (R,R)-tartramide was successfully conducted.
EXPERIMENTAL SECTION Apparatus. The gas chromatograph was comprised of a Shimadzu GC-14A equipped with a split injector and flame ionization detector. Helium served as the carrier gas. The inlet pressure of the capillary column was adjusted to 0.9 kg/cm2 and the split ratio was set at 1:60. The temperature of the injection port was maintained at 200 O C . Chromatographic signals were recorded and processed by a Shimadzu C-R4AX integrator. Reagents. 3,3-Dimethyl-l,2-butanediol, 1,2-pentanediol,2,4pentanediol, 1,3-~yclopentanediol(mixture of cis and trans), and trans-1,2-cycloheptanediolwere purchased from Aldrich. 1,2Butanediol, 2,3-butanediol, 1,2-hexanediol, 2,5-hexanediol, 2methyl-2,4-pentanediol, trans-1,2-cyclohexanediol,and 1,3cyclohexanediol (mixture of cis and trans) were obtained from Tokyo Kasei and 1,2-propanedioland 1,3-butanediol,from Wako. trans-1,2-Cyclopentanediolwas synthesized according to the procedure of Owen et al. (8). threo- and erythro-2,3-Pentanediol were synthesized from trans- and cis-2-pentene, respectively, according to the literature (9). Synthesis of Chiral Polysiloxane. Poly(hydromethy1siloxane) was synthesized according to the procedure of Bradshaw et al. ( I 0). (R&)-N-(lO-Undecenyl)-N'-isopropyldiacetyl"mide was synthesized according to our own method (11). Hydrosilylation was conducted in the presence of chloroplatinic acid. The resulting products were purified by using two Shodex GPC-A-80M columns in series with THF as the eluent. The pure acetyl polysiloxane thus obtained was hydrolyzed to give the gumlike product. IR (neat): 3600-3100,2960,2920, 1650, 1540,
0003-2700/90/0362-0539$02.50/00 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 5, MARCH 1, 1990
Scheme I. Synthetic Pathway of the Chiral Polysiloxane Derived from (R,R)-Tartramide
H3C
-$ i -CHI CHS
\
H ~ -$ C i -cH~, 1
n
?
HsC - S i -CHI C Hs
Table I. Gas Chromatographic Resolution of Enantiomeric Diols column compound
temp, "C
kl"
k2"
a
R,
Aliphatic Diols 1,2-propanediol 1,2-butanediol 1,2-pentanediol 1,2-hexanediol 1,3-butanediol 2,3-butanediol 2,4-pentanediol
65 65
65 75
65 65 65
2,5-hexanediol
65
2,3-pentanediol (threo) (erythro)
65 65
3,3-dimethyl-1,2-butanediol 2-methyl-2,4-pentanediol
65
65
Alicyclic Diols trans-1,2-cyclopentanediol 90 trans-1,2-cyclohexanediol 90 trans-1,2-cycloheptanediol 90 trans-1,3-cyclopentanediol 90 trans-1,3-cyclohexanediol 90
8.00 8.67 1.084 1.73 17.31 18.21 1.052 1.89 37.08 38.19 1.030 0.92 41.38 42.02 1.015 0.71 18.39 18.69 1.016 0.62 7.32 7.71 1.053 1.25 20.90 21.85 1.045 54.52 55.49 1.018 13.40 13.72 1.024 15.88 1.000 33.32 35.44 1.064 17.05
1.000
18.37 21.98 22.89 44.69 45.50 12.39 12.58 30.06 30.93
1.000
1.87 0.86 0.78 2.34
1.041 1.80
1.018 0.96 1.015 0.62 1.029 1.15
Capacity factors were calculated according to (retention time of solute - retention time of methane gas)/retention time of methane gas.
1405,1260,1120-1ooO,800cm-I. Anal. Found: C, 38.86; N, 1.89; H, 8.36. Column Preparation and Efficiency. A fused-silica capillary column (25 m X 0.25 mm i.d.) was washed with dichloromethane, dried through helium gas, and coated statically with a 0.30% solution of the chiral polysiloxane in dichloromethane. The column was subsequently conditioned with helium gas at a temperature range from 40 to 150 "C at 0.5 OC min-'. The McReynolds constants were determined at 50 "C. AI of each probe was obtained as 25 (benzene),238 (1-butanol), 101 (2-pentanone),121 (1-nitropropane), and 168 (pyridine), respectively. A TZ value of 35 (between n-nonane and n-decane) was obtained at the same temperature.
RESULTS AND DISCUSSION In our approach to the direct resolution of diol enantiomers by gas chromatography, the enantioselectivity of (R,R)-N,-
N'-dialkyltartramide is applied. This derivative shows enantioselectivity toward a broad range of enantiomers through hydrogen bond association in solution (12). We have already developed a chiral stationary phase derived from (R,R)-tartramide for liquid chromatography and found it capable of directly resolving 1,2-diols (11). On the basis of the results of a NMR study and X-ray crystal analysis, the dual hydrogen bonds between amide carbonyls of tartramide and hydroxyls of 1,2-diols were considered to the association mode responsible for the observed enantioselection in solution (13). Recently, novel chiral polysiloxanes derived from (R,R)tartramide for gas chromatographic enantiomer resolution were prepared at our laboratory (14). Although its scope of application is relatively restricted, the resolution observed on these phases indicated the enantioselectivity of (R,R)-tartramide exerted in solution to also be applicable to the gas chromatographic systems. To improve the performance of the chiral stationary phase derived from (R,R)-tartramide, the structure and preparation of the chiral polysiloxane were examined. In a newly designed chiral polysiloxane, the tartramide moiety was attached to the polysiloxane backbone through the long alkyl chain consisting of 11 methylene units. Increase in the distance between the polysiloxane backbone and tartramide moiety may enhance the accessibility of the solute enantiomers to tartramide moiety. The synthetic pathway presented in Scheme I involves the hydrosilylation of polyhydromethylsiloxane with alkenyl tartramide derivative. By this method of preparation, there can be no residual hydrogen bond sites such as amino and/or cyano functions which interact with solute enantiomers without enantioselectivity. The chiral polysiloxane obtained provided greater thermal stability than the previously developed chiral polysiloxane whose four methylene units served as a linkage of tartramide moiety to polysiloxane and underwent no loss of weight up to 240 "C, according to thermogravimetric analysis. The new chiral polysiloxane was found to provide a greater scope of chiral recognition. As expected from the enantioselectivity of the (R,R)-tartramide in solution, the direct enantiomer resolution of diols by gas chromatography was possible without the need for derivatization on a new chiral polysiloxane derived from (R,R)-tartramide. The chromato-
ANALYTICAL CHEMISTRY, VOL.
62,NO. 5, MARCH 1, 1990
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I
!
0
15
30
min
Flgure 1. Enantiomer resolution of 1,2-propanediol and 1,2+utanediol: column temperature, 65 OC isothermal; carrier gas, 0.9 kg/cm2 He.
0
15
30 m l n
Flgure 2. Separation of three isomers of 2,4-pentanedioi: column temperature, 65 OC isothermal: carrier gas, 0.9 kg/cm2 He.
graphic data are summarized in Table I. In the case of aliphatic 1,2-diols, the highest separation factor cy of 1.084 was obtained from 1,2-propanediol, and the separation factors of aliphatic 1,2-diols decreased with the length of the main chain. The elution order was determined with (S)-(+)-1,2-propanediol (obtained from Aldrich). The S enantiomer was eluted prior to the R enantiomer. The absolute configuration is generally related to the elution order on chiral stationary phases. The S enantiomers of other aliphatic 1,2-diols may thus possibly be consistently eluted prior to R enantiomers. Figure 1shows the enantiomer resolution of 1,2-propanediol and 1,2-butanediol. Enantiomer resolution of aliphatic diols with C2 symmetry was also successfully conducted. Figure 2 shows the enantiomer resolution of 2,4-pentanediol. The complete separation of three stereoisomers of 2,4-pentanediol was successfully carried out. As shown in Table I, the separation factors of aliphatic diols with C2 symmetry varied according to the distance separating the two hydroxyls, and it is significant that the enantiomers of 2,5-hexanediol were separated when the separation factor was 1.018. Although a number of aliphatic diols was separated, this was not possible for the enantiomers of 2-methyl-2,4-pentanediol. For 2,3-pentanediol containing threo and erythro isomers, the enantiomer resolution of threo-2,3-pentanediol was possible with a separation factor cy of 1.024 but that of the erythro isomer could not be done owing to anti conformation of the two hydroxyls (12). Although the enantiomer resolution of trans-1,2-cyclohexanediol was previously achieved without derivatization on Chirasil-Val, it could not be done in the case of other alicyclic diols ( 5 , 1 5 ) . That of a series of alicyclic diols was achieved on a chiral polysiloxane derived from (R,R)-tartramide as shown in Table I. Figure 3 shows the direct enantiomer
0
20
40 mln
F b w e 3. Enantiomer resolution of trans-l,2cyclohexanediol: column temperature, 90 OC isothermal; carrier gas, 0.9 kg/cm2 He.
resolution of trans-1,2-cyclohexanediol.Although trans-1,2cyclopentanediol was not resolved, trans-l,3-cyclopentanediol was so with a separation factor a of 1.015. The separation exceeded that of transfactor of trans-1,2-cyclohexanediol 1,&cyclohexanediol. The present new chiral polysiloxane was shown to function as a highly efficient stationary phase for the gas chromatographic resolution of diol enantiomers with small molecular weight. The enantioselectivity of (R,R)-tartramide based on hydrogen bond association is regenerated in the gas chromatographic system which permits highly sensitive detection and highly efficient resolution. In this paper, our discussion has been limited to the resolution of diol enantiomers. However, this chiral polysiloxane has a broad scope of application for enantiomer resolution. The extended application of polysiloxane derived from (R,R)-tartramide will be reported in the near future.
LITERATURE CITED (1) Mori, K.; Sasaki, M.; Suguro, T.; Masuda, S. Tetrahedron 1979, 35, 1601-1605. (2) Asami, M.; Mukaiyama, T. Chem. Lett. 1983, 93-96. (3) Schurig, V.; Wistuba, D. Tetrahedron Lett. 1984, 25, 5633-5636. (4) Konig, W. A.; Steinback, E.; Ermst, K. Angew. Chem., Jnt. Ed. Engl. 1984, 23, 527-528. (5) Koppenhoefer, B.; Walser, M.; Bayer, E., Abdalla. S. J. Chromatogr. 1988, 358, 159-168. (6) Konig, W. A.; Lutz. S.; Colberg, C.; Schmidt, N.; Wenz, G.; Bey, E.; Mosandl, A.; Gunther, C.; Kustermann, A. HRC CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1988, 1 1 , 621-625. (7) Konig, W. A.; Lutz, S.; Wenz, G.; Bey, E. HRC CC, J. High Resolut. Chromatogr . Chromatogr. Common. 1988, 1 1 , 506-509. (8) Owen, L. N.; Smith, P. N. J. Chem. SOC.1952, 4026-4035. (9) Miias, N. A.; Sussmann, S. J. Am. Chem. SOC. 1936, 5 8 , 1302-1304. (10) Bradshaw, J.; Aggarwai, S. K.; Rouse, C. A.; Tarbet, B. J.; Markieds, K. E.; Lee, M. L. J. Chromatogr. 1987, 405, 169-177. (11) Dobashi, Y.; Hara. S. J. Org. Chem. 1987, 52, 2490-2496. (12) Dobashi, Y.; Hara, S. J. Am. Chem. SOC. 1985, 107, 3406-3411. (13) Dobashi, Y.; Hara, S.; Iitaka, Y. J. Org. Chem. 1988, 53, 3894-3896. (14) Nakamura, K.; Hara, S . ; Dobashi, Y. Anal. Chem. 1989, 61, 2121-21 24. (15) Koppenhoefer, B.; Allmendinger, H.; Nicholson, G. Angew. Chem.. Int. Ed. Engl. 1985, 24, 48-49.
Kouji Nakamura* Takafumi Saeki Masaaki Matsuo Analytical Chemistry Research Laboratory Tanabe Seiyaku 3-16-89 Kashima, Yodogawa-ku, Osaka 532 Japan Shoji Hara Yasuo Dobashi Tokyo College of Pharmacy 1432-1 Horinouchi, Hachioji, Tokyo 192-03 Japan RECEIVED for review September 19,1989. Accepted November 28, 1989.
