Characterization of Vinyl-Substituted, Carbon−Carbon Double Bonds

absorptions at 998, 949, and 902 cm-1) and the 1590-. 1650 cm-1 region (the CdC stretch bands are observed at 1595 and 1642 cm-1 for cis compounds and...
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Anal. Chem. 1997, 69, 1827-1836

Characterization of Vinyl-Substituted, Carbon-Carbon Double Bonds by GC/FT-IR Analysis Alesˇ Svatosˇ† and Athula B. Attygalle*

Baker Laboratory, Department of Chemistry, Cornell University, Ithaca, New York 14853

Vapor-phase infrared spectra allow the determination of the stereochemistry of carbon-carbon double bonds conjugated with a vinyl group. Cis and trans isomers of unsubstituted 1,3-alkadienes can be differentiated on the basis of the differences observed in the 900-1000 cm-1 region (spectra of cis isomers show two bands at 993 and 906 cm-1, while those of trans compounds show three absorptions at 998, 949, and 902 cm-1) and the 15901650 cm-1 region (the CdC stretch bands are observed at 1595 and 1642 cm-1 for cis compounds and at 1604 and 1650 cm-1 for trans compounds). Compounds bearing CH2dCHC(CH3)dCHCH2- and CH2dCHC(dCH2)CH2- structural moieties, referred to as r- and β-type compounds, are frequently encountered as natural products. For compounds bearing r-type groups, the cis/trans configuration of the trisubstituted double bond can be determined unambiguously. An absorption at 30953091 cm-1, for the dCH2 stretch vibration, is common to both of these groups; however, due to the presence of two dCH2 groups, the relative intensity of the band is much higher for β-type compounds. For r-type compounds, a cis configuration at the C-3 carbon atom is characterized by a dCH2 wag absorption at 907-906 cm-1. For β-type compounds and 3E-r-type compounds, this band appears at 899-897 cm-1. In addition, a wavy “fingerprint” pattern with two minima at 1632 (low intensity) and 1595-1594 cm-1 (high intensity) is characteristic for β-type compounds. Our generalizations are based on spectra of cis and trans ocimene, myrcene, and dehydration products of many 3-methyl-1-alken-3-ols. Six isomers of farnesene can be characterized by GC/FT-IR. Furthermore, gas-phase IR allows the determination of the configuration of the trisubstituted double bond at C-3 in r-type farnesene congeners. For example, the homoand bishomofarnesene isomers from Myrmica ants were shown to include a 3Z bond. Many natural products, particularly terpenes, bear vinyl groups conjugated to carbon-carbon double bonds. A number of these vinyl compounds are either disubstituted 1,3-butadienes with a methyl group (designated as R-type, 1, 2) or monosubstituted 1,3-butadienes (β-type, 3) with a large substituent group attached † Permanent address: Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo na´m. 2, 166 10 Prague 6, Czech Republic.

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to the C-3 atom.1 Although 1H NMR spectroscopy can be used to characterize these carbon skeletons, relatively large sample amounts (10-100 µg) in nearly pure form are required. Frequently, natural product chemists are frustrated by the unavailability of samples that large. The farnesenes (4-9) are a group of common natural products which bear the aforementioned R- and β-type carbon skeletons.2-12 Although GC/MS is the most frequently used technique for the identification of farnesene isomers, characterization of all naturally occurring isomers using mass spectral data alone is not straightforward. Particularly, the mass spectra of (E)-β- and (Z)-βfarnesene are very similar. Consequently, many farnesenes isomers recognized by GC/MS have been reported without complete stereochemistry.13 Moreover, many reports on the identification of homo-, bishomo-, and trishomofarnesenes failed to discuss the stereochemistry because of the absence of a sensitive and reliable method for configuration determination of small amounts of compounds found as complex mixtures.14-16 (1) Connoly, J. D.; Hill, R. A. Dictionary of Terpenoids, Vol. 1, Mono- and sesquiterpenoids; Chapman and Hall: London, 1991. (2) Bowers, W. S.; Nault, L. R.; Webb, R. E.; Dutky, S. R. Science 1972, 177, 1121-1122. (3) Nault, L. R.; Phelan, P. L. In Chemical Ecology of Insects; Bell, W. J., Carde, R. T., Eds.; Chapman and Hall: London, 1984; pp 238-256. (4) Sakai, T.; Hirose, Y. Bull. Chem. Soc. Jpn. 1969, 42, 3615-3616. (5) Detrain, C.; Pasteels, J. M.; Braekman, J. C.; Daloze, D. Experientia 1981, 43, 345-346. (6) Attygalle, A. B.; Morgan, E. D. Chem. Soc. Rev. 1984, 245-278. (7) Knight, D. W.; Rossiter, M.; Staddon, B. W. J. Chem. Ecol. 1984, 10, 641649. (8) Naya, Y.; Prestwitch, G. D. Tetrahedron Lett. 1982, 23, 3047-3050. (9) Vander Meer, R. K.; Williams, F. D.; Lofgren, C. S. Tetrahedron Lett. 1981, 1651-1654. (10) Cavill, G. W. K.; Williams, P. J.; Whitefield, F. B. Tetrahedron Lett. 1967, 2201-2205. (11) Fletcher, M. T.; Kitching, W. Chem. Rev. 1995, 95, 789-828. (12) Murray, K. E. Aust J. Chem. 1969, 22, 197-204. (13) Bergstro ¨m, G.; Tengo¨, J. Chem. Scr. 1974, 5, 28-38. (14) Ollett, D. G.; Morgan, E. D.; Attygalle, A. B.; Billen, J. P. J. Z. Naturforsch. 1987, 42c, 141-146. (15) Jackson, B. D.; Cammaerts, M.-C.; Morgan, E. D.; Attygalle, A. B. J. Chem. Ecol. 1990, 16, 827-840.

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In the present study, we recorded vapor-phase infrared spectra of many conjugated vinyl compounds. The generalizations made from these spectra allowed not only the characterization of all isomers of farnesene but also the determination of the configuration of the trisubstituted double bond at the C-3 atom of R-type homo- and bishomofarnesene isomers from Myrmica ants. EXPERIMENTAL SECTION Instrumentation. Vapor-phase GC/FT-IR spectra (resolution, 8 cm-1) were recorded using a Hewlett-Packard (HP) 5890 gas chromatograph coupled to a HP 5965A IRD instrument equipped with a narrow-band (4000-750 cm-1) infrared detector (mercurycadmium-telluride) as described previously.17 Purity and identity of each sample were checked by GC/MS performed on a HP 5890 gas chromatograph coupled to a HP 5870B mass selective detector using a 25 m × 0.22 mm fused silica column coated with DB-5 stationary phase (J & W Scientific; 0.25 µm film thickness). For GC/MS analyses, the oven temperature was held at 60 °C for 3 min and increased to 250 °C at 10 °C/min. NMR spectra were recorded on a Unity-400 (1H NMR, 400 MHz; 13C NMR, 100.6 MHz; Varian) instrument for samples as CDCl3 solutions. Chemical shifts, given in ppm, are expressed as δ values measured from the residual CHCl3 signal (7.26 ppm) (tentative NMR assignments are denoted by asterisks). Column chromatography was performed on silica gel (Merck, H 60), and reactions were monitored by TLC on Baker-flex silica gel IB2-F plates (J.T. Baker). UV spectra were obtained using a diode array detector installed in a HP 1090 instrument. Chemicals. 1-Nonene (10), 2-nonanone, (()-nerolidol, (()(E)-nerolidol, vinylmagnesium bromide, and allyltriphenylphosphonium bromide were purchased from Aldrich Chemical Co. (Milwaukee, WI), and (11Z)-15-methyl-1,11-hexadecadiene (11) was a gift of Prof. H. J. Bestmann. All other chemicals were purchased from commercial sources and used as obtained. (3Z)- and (3E)-1,3-Tridecadienes. Allyltriphenylphosphonium bromide (0.70 g, 1.83 mmol) was dried at 100 °C and 0.1 mmHg for 3 h, suspended in dry THF (4 mL), and treated with a sodium hexamethyldisilazide18 (3.2 mL, 1.90 mmol) solution in toluene at room temperature for 0.3 h. The orange suspension formed was stirred for 3.3 h, cooled in an ice bath, and treated with distilled decanal (0.28 g, 1.8 mmol) in dry THF (0.4 mL). The reaction mixture was stirred for 14 h and then quenched with ice and HCl (2.5 mL of 35%). The product, a mixture of (3Z)- and (3E)-1,3-tridecadienes (48:52 by 1H NMR), was extracted into hexane and purified by silica gel column chromatography (0.25 g, 76% yield). An analytical sample was obtained by distillation at 150-180 °C/20 mmHg. (3Z)-1,3-Tridecadiene (12). The mixture of (3Z)- and (3E)1,3-tridecadienes (23 mg, 0.13 mmol) in CDCl3 (0.6 mL) was treated with tetracyanoethylene (TCNE, 32 mg, 0.25 mmol) in an NMR tube, and the progress of the reaction was monitored by 1H NMR spectroscopy.19,20 After 5 h, when signals for the 3E (16) Attygalle, A. B.; Morgan, E. D. J. Chem. Soc., Perkin Trans. 1 1982, 949951. (17) Attygalle, A. B.; Svatosˇ, A.; Wilcox, C.; Voerman, S. Anal. Chem. 1994, 66, 1696-1703. (18) Bestmann, H. J.; Stransky, W.; Vostrowsky, O. Chem. Ber. 1976, 109, 16941700. (19) Nesbitt, B. F.; Beevor, P. S.; Cole, R. A.; Lester, R.; Poppi, R. G. Tetrahedron Lett. 1973, 4669-4670 (20) Bestmann, H. J.; Attygalle, A. B.; Schwarz, J.; Garbe, W.; Vostrowsky, O.; Tomida, I. Tetrahedron Lett. 1989, 30, 2911-2914.

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isomer were completely absent, the solvent was evaporated and the residue dissolved in hexane. After purification on a Florisil column, 7.9 mg of pure (3Z)-1,3-tridecadiene was obtained. 1H NMR (400 MHz, CDCl3): δ 6.64 (1H, dddd, J ) 17.2, 11.2, 10.2, 1.5 Hz, dCH-2), 5.99 (1H, dd, J ) 10.3, 10.2 Hz, dCH-3), 5.45 (1H, dd, J ) 10.3, 7.8 Hz, dCH-4), 5.18 (1H, dd, J ) 17.1, 2.0 Hz, dCHRHβ), 5.08 (1H, d, J ) 11.3 Hz, dCHRHβ), 2.17 (2H, dd, J ) 7.8, 7.6 Hz, CH2-5), 1.37 (2H, m, CH2-6), 1.30-1.25 (12H, m), 0.87 (3H, t, J ) 6.7, 6.9 Hz, CH3). 13C NMR (100.3 MHz, CDCl3): δ 133.1 (dCH-2), 132.3 (dCH-3), 129.1 (dCH-4), 116.6 (dCH2-1), 31.9 (CH2-11), 29.6 (CH2-8*), 29.5 (CH2-9*), 29.3 (CH2-10), 29.2 (CH2-6*), 29.2 (CH2-7*), 27.7 (CH2-5), 22.7 (CH2-12), 14.1 (CH313). EI-MS: m/z (relative intensity) 180 (22) M+, 109 (10), 96 (25), 95(26), 82 (50), 81 (59), 79 (27), 69 (26), 68 (68), 67 (86), 57 (17), 55 (45), 54 (90), 43 (48), 41 (100). UV: λmax ) 227 nm. (3E)-1,3-Tridecadiene (13). A mixture of 1,3-tridecadiene stereoisomers (56 mg), hydroquinone (5 mg), and liquid sulfur dioxide (condensed at -70 °C, ∼1.5 mL) was kept for 2 days at -20 °C.19 The unreacted diene was separated from the more polar SO2 adduct (25 mg) by chromatography on Florisil using a gradient of ether in hexane. The 3E isomer was isolated by thermal decomposition of the sulfone. In fact, the sulfone decomposes stereospecifically in the GC injector (280 °C), releasing pure (3E)-1,3-tridecadiene. (3E)-1,3-Tridecadiene-SO2 Adduct. 1H NMR (400 MHz, CDCl3): δ 6.02 (2H, m, HCdCH), 3.74 (2H, q, J ) 16.1, 15.6, 15.1 Hz, SO2CH2), 3.65 (1H, dd, J ) 7.3, 7.3 Hz, SO2CH), 1.94 (1H, m, CH2-5a), 1.62 (1H, m, CH2-5b), 1.48 (2H, m, CH2-6), 1.4-1.2 (12H, m, CH2-7-12), 0.87 (3H, t, J ) 6.8, 6.8 Hz, CH3). 13C NMR (100.3 MHz, CDCl3): δ 130.4 (dCH-2*), 122.9 (dCH-3*), 64.5 (SO2CH4), 55.7 (SO2CH2), 31.8 (CH2-11), 29.4 (CH2-7*), 29.4 (CH2-8*), 29.3 (CH2-9*), 29.2 (CH2-10*), 28.6 (CH2-6), 27.0 (CH2-5), 22.6 (CH2-12), 14.1 (CH3-13). (3E)-1,3-Tridecadiene (13). 1H NMR (400 MHz, CDCl3): δ 6.31 (1H, ddd, J ) 17.1, 10.4, 10.3 Hz, dCH-2), 6.04 (1H, dd, J ) 15.0, 10.4 Hz, dCH-3), 5.71 (1H, ddd, J ) 15.3, 7.5, 7.2 Hz, dCH4), 5.08 (1H, d, J ) 16.9 Hz, dCHRHβ), 4.95 (1H, d, J ) 10.1 Hz, dCHRHβ), 2.07 (2H, dd, J ) 7.0, 7.0 Hz, CH2-5), 1.30-1.25 (12H, m), 0.87 (3H, t, J ) 6.7, 6.9 Hz, CH3). 13C NMR (100.3 MHz, CDCl3): δ 137.3 (dCH-2), 135.7 (dCH-3), 130.8 (dCH-4), 114.5 (dCH2-1), 32.6 (CH2-5), 31.9 (CH2-11), 29.6 (CH2-7*), 29.6 (CH28*), 29.5 (CH2-9*), 29.3 (CH2-10), 29.2 (CH2-6*), 22.7 (CH2-12), 14.1 (CH3-13). EI-MS: m/z (relative intensity) 180 (26) M+, 109 (10), 96 (25), 95(26), 82 (50), 81 (58), 79 (26), 69 (24), 68 (68), 67 (86), 57 (17), 55 (45), 54 (90), 43 (48), 41 (100). UV: λmax ) 227 nm. 3-Methyl-1-decen-3-ol. A solution of distilled 2-nonanone (282 mg, 2 mmol) in dry THF (5 mL) was treated with 1 M vinylmagnesium bromide (Aldrich) in THF (2.2 mL) at 0 °C. After 0.5 h, the reaction was quenched with saturated NH4Cl solution, and the mixture was extracted with ether. Flash chromatography of the crude material afforded 167 mg (50%) of the desired alcohol. 1H NMR (400 MHz, CDCl ): δ 5.90 (1H, dd, J ) 10.7, 17.4 Hz, 3 -CHd), 5.19 (1H, dd, J ) 1.2, 17.4 Hz, dCHRHβ), 5.03 (1H, dd, J ) 1.2, 10.7 Hz, dCHRHβ), 1.40 (2H, m, CH2C(OH)), 1.29 (1H, s, OH), 1.27 (3H, s, C(CH3)), 1.3-1.1 (8H, m, -CH2-), 0.88 (3H, t, J ) 6.9 Hz, CH3). 3-Methyl-1-dodecen-3-ol and 3-methyl-1-tridecen-3-ol were synthesized similarly, starting from 2-undecanone and 2-dodecanone, respectively.

Dehydration of 3-Methyl-1-alken-3-ols. A mixture of (3Z)3-methyl-1,3-decadiene (14), (3E)-3-methyl-1,3-decadiene (15), and 3-heptyl-1,3-butadiene (16) was prepared from 3-methyl-1decen-3-ol, using POCl3 as the dehydrating agent (see the section on preparation of farnesenes from nerolidol for experimental details). Similarly, 3-methyl-1-dodecen-3-ol and 3-methyl-1-tridecen-3-ol were dehydrated by the same procedure. 4-Methyl-1,3-undecadienes. Allyltriphenylphosphonium bromide (460 mg, 1.20 mmol) was suspended in dry ether (9 mL) and treated with n-butyllithium in hexane (0.75 mL, 1.2 mmol) over a peroid of 10 min at 0 °C. The mixture was stirred for 55 min 0 °C, and 2-nonanone (140 mg, 1 mmol) in dry THF (3 mL) was added. After the mixture was stirred for 10 h at room temperature, ice and HCl (2 mL) were added. Products were extracted with hexane and purified from the unreacted ketone by silica gel column chromatography. This afforded a 35:65 mixture (15 mg, 9% yield) of (3E)- and (3Z)-4-methyl-1,3-undecadiene (ratios determined by GC and 1H NMR). Although the twoisomer mixture was not separated, the NMR spectrum of the mixture allowed the assignment of most signals corresponding to each isomer. (3E)-4-Methyl-1,3-undecadiene (17). GC: tR ) 14.15 min (DB5). 1H NMR (400 MHz, CDCl3): δ 6.68 (1H, ddd, J ) 17.2, 10.7, 10.4 Hz, dCH-2), 5.84 (1H, db, J ) 10.7 Hz, dCH-3), 5.08 (1H, dd, J ) 16.8, 2.0 Hz, dCHRHβ), 4.97 (1H, dd, J ) 9.2, 1.8 Hz, dCHRHβ), 2.04 (2H, t, J ) 7.6 Hz, CH2-5), 1.75 (3H, bs, CH3-CHd), 1.50-1.25 (10H, m, CH2-6-10), 0.88 (3H, t, J ) 6.9 Hz, CH3). EI-MS: m/z (relative intensity) 166 (44), 137 (5), 123 (13), 109 (5), 107 (5), 96 (15), 95 (90), 93 (90), 93 (15), 91 (13), 83 (14), 82 (89), 81 (69), 80 (12), 79 (53), 78 (5), 77 (18), 69 (22), 68 (55), 67 (100), 66 (10), 65 (12), 57 (8), 56 (7), 55 (42), 54 (9), 53 (32), 51 (9), 43 (34), 42 (8), 41 (79). (3Z)-4-Methyl-1,3-undecadienes (18). GC: tR ) 13.85 min (DB5). 1H NMR (400 MHz, CDCl3): δ 6.66 (1H, ddd, J ) 17.1, 10.7, 10.4 Hz, dCH-2), 5.84 (1H, db, J ) 10.7 Hz, dCH-3), 5.06 (1H, dd, J )16.8, 2.0 Hz, dCHRHβ), 4.94 (1H, dd, J ) 9.5, 1.8 Hz, dCHRHβ), 2.15 (2H, t, J ) 7.6 Hz, CH2-5), 1.77 (3H, bs, CH3-CHd), 1.50-1.25 (10H, m, CH2-6-10), 0.88 (3H, t, J ) 6.9 Hz, CH3). EIMS: m/z (relative intensity) 166 (49), 137 (50), 123 (12), 109 (12), 96 (13), 95 (94), 93 (12), 83 (15), 82 (87), 81 (76), 80 (9), 79 (47), 77 (21), 69 (23), 68 (60), 67 (100), 66 (7), 65 (11), 57 (5), 56 (7), 55 (35), 54 (7), 53 (28), 51 (7), 43 (31), 42 (8), 41 (73). 4-Methyl-1,3-tridecadiene and 4-Methyl-1,3-tetradecadiene. Using 2-undecanone and 2-dodecanone as starting materials, cis and trans mixtures of 4-methyl-1,3-tridecadiene and 4-methyl1,3-tetradecadiene were synthesized by procedures similar to that described in the preceding section. Spectral data obtained were congruent with the expected structures. Mixture of Farnesene Isomers (4-9). Nerolidol (a mixture of E/Z isomers (1:1), 40 µL, 0.158 mmol) was dissolved in dry pyridine (1 mL) and cooled in an ice bath. Under an argon atmosphere, POCl3 (40 µL, 0.44 mmol) was added dropwise during a period of 5 min. The resulting mixture was stirred on the bath for 1 h and for 12 h at room temperature. The reaction mixture was poured into ice with hexane (1 mL), and the organic layer was separated. The aqueous layer was extracted twice with hexane (0.5 mL). Combined organic layers were washed with saturated NaCl solution and dried over MgSO4. A mixture of (3Z,6E)-R-farnesene (7), (3E,6E)-R-farnesene (9), and (6E)-β-farnesene (5) was prepared similarly, using (6E)-

nerolidol (96.7% E isomer, 40 µL) as the starting material. Natural Farnesenes. Five termite soldiers (Prorhinotermes simplex), from a laboratory colony maintained in the Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, were soaked in hexane (100 µL). About 20 gasters of Myrmica sabuleti ants were crushed in hexane (200 µL) and concentrated to 20 µL. Freshly peeled skins (4 g) from Granny Smith apples (Wegmans Food, Ithaca, NY) in a glass distillation apparatus were gently heated with a flame. The collected aqueous condensate was extracted with hexane (0.5 mL), and the extract was purified using a short column of Florisil (5 mm) by eluting with a mixture of hexane/ether (95:5 v/v). Pea aphids (Acyrthosiphon pissum, 3.6 g) were extracted with hexane (5 mL), and the extract was purified by a procedure similar to that used for the apple sample. RESULTS AND DISCUSSION Vapor-phase infrared spectra allow unambiguous recognition of vinyl groups in volatile molecules by diagnostic bands observed at 3085-3083 (out-of-phase dCH2 stretch), 1641 (carbon-carbon double bond), 993-992 (“trans” CH wag), and 914 cm-1 (dCH2 wag).21,22 In addition, spectra of low-molecular-weight vinylic compounds show a band at 1828 cm-1 which is an overtone of the 914 cm-1 band. For example, the spectrum of 1-nonene (10, Figure 1A) shows all the typical bands expected from a 1-alkene. Our previous reports show that infrared bands for vinyl groups can be recognized unambiguously even in the presence of absorptions for other functional groups, although sometimes a few of the vinyl bands may be obscured.17,23 Furthermore, we have established vapor-phase infrared correlations for the recognition of cis/trans configuration of CHdCH-type double bonds on the basis of 3036-3011 and 982-947 cm-1 absorptions.17,24 From the spectrum of (11Z)-15-methyl-1,11-hexadecadiene (11) depicted in Figure 1B, it is evident that an isolated cis double bond can be recognized by the 3012 cm-1 band, even in the presence of a 3085 cm-1 band for the vinylic dCH2 stretch. 1,3-Alkadienes. At a very early stage in the history of the analytical use of infrared spectroscopy, Rasmussen and Brattain25 recognized that cis and trans 1,3-pentadiene isomers can be differentiated by their vapor-phase infrared spectra. However, general correlation data essential for the comprehensive interpretation of vapor spectra of 1,3-alkadiene systems have not been compiled. To investigate spectra of 1,3-alkadiene systems, we synthesized a mixture of 3Z and 3E isomers of 1,3-tridecadiene. Addition of tetracyanoethylene (TCNE)19,20 or SO219 selectively to the trans isomer followed by column chromatography afforded pure samples of each isomer. The spectra of (3Z)-1,3-tridecadiene (12) and (3E)-1,3-tridecadiene (13) (Figure 1C,D) show the vinyl stretch band at 30933092 cm-1, denoting that the conjugation of the vinylic group to a carbon-carbon double bond shifts the 3085 cm-1 dCH2 stretch band to higher frequencies. A hypsochromic shift due to conjugation appears to be valid for 1,3-alkadienes in general, since the (21) Nyquist, R. A. The Interpretation of Vapor-Phase Infrared Spectra: Group Frequency Data; Sadtler Research Laboratories: Philadelphia, PA, 1984; Vol. 1. (22) Pouchert, C. J. The Aldrich Library of FT-IR Spectra, Vapor Phase, 1st ed.; Aldrich Chemical Co.: Milwaukee, WI, 1989; Vol. 3. (23) Attygalle, A. B. Pure Appl. Chem. 1994, 66, 2323-2326. (24) Attygalle, A. B.; Svatosˇ, A.; Wilcox, C.; Voerman, S. Anal. Chem. 1995, 67, 1558-1567. (25) Rasmussen, R. S.; Brattain, R. R. J. Chem. Phys. 1947, 15, 131-135.

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Figure 1. Gas-phase infrared spectra of 1-nonene (A), (11Z)-15methyl-1,11-hexadecadiene (B), (3Z)-1,3-tridecadiene (C), and (3E)1,3-tridecadiene (D) (resolution, 8 cm-1).

spectra reported for 1,3-hexadiene and 1,3-octadiene also show this band at 3090 and 3091 cm-1, respectively.26 In a previous study,24 we noted similar hypsochromic shifts for the dCH stretch bands corresponding to nonterminal-type conjugated CdC double bonds. In addition to the vinyl stretch band, spectra of cis and trans 1,3-tridecadienes show an additional absorption above 3000 cm-1. For the cis compounds, this dCH stretch band appears at 3016 cm-1, while that of the trans compounds occurs at 3010 cm-1. Although it is not common for trans compounds to show absorptions above 3000 cm-1, apparently the effect of conjugation shifts the trans dCH band sufficiently to higher frequencies to resolve the band from the more intense CH2 absorptions at 2934 cm-1, as we have noted previously.24 Although compounds with internal double bonds exhibit no significant bands for CdC stretch modes, presumably because the stretching affords little or no dipole moment change, the spectra of vinyl compounds show a notable peak at 1642-1641 cm-1 for the CdC stretch.17,21,23 From generalizations for condensed-phase spectra, it is known that conjugation lowers the (26) The National Institute of Standards and Technology (NIST) and Environmental Protection Agency (EPA) Gas Phase Infrared Database, version 1.0, 1992; U.S. Department of Commerce, Gaithersburg, MD 20899.

1830 Analytical Chemistry, Vol. 69, No. 10, May 15, 1997

frequency. In addition, this interaction splits the band to a complex pattern of two or more absorptions, with the lowest frequency peak as the most intense.25,27-29 Similarly, in gas-phase spectra, the conjugation of a vinyl group to a carbon-carbon double bond leads to the appearance of two CdC stretch bands. In the spectrum of (3Z)-1,3-tridecadiene, the two bands for inphase and out-of-phase stretch modes occur at 1642 and 1595 cm-1, respectively (Figure 1C). However, for (3E)-1,3-tridecadiene, the bands are shifted to higher frequencies and occur at 1650 and 1604 cm-1 (Figure 1D). Moreover, the relative intensity of the two bands is higher for the trans compounds, and the 1650 cm-1 band is slightly more intense than the 1604 cm-1 band. Interestingly, the difference of the frequencies (∆ν) for the two CdC bands is about 46-47 cm-1 for both isomers of 1,3tridecadiene. Conjugation also affects the 993-992 (“trans” CH wag) and 914 cm-1 (dCH2 wag) bands. For example, in the spectrum of (3Z)-1,3- tridecadiene, the dCH2 wag band is lowered to 906 cm-1, while that for the “trans” CH wag mode appears to be unmodified (Figure 1C). On the other hand, the spectrum of the (3E)-1,3tridecadiene shows a characteristic three-peak pattern (Figure 1D). For isolated vinyl groups, the intensity of the 992 cm-1 band is lower than that of the 914 cm-1 band. However, in the trans 1,3-diene spectrum, the intensity of the 998 cm-1 band is higher than that at 902 cm-1. Furthermore, a weak absorption is noted at 949 cm-1 in the spectrum of (3E)-1,3-tridecadiene. Previously, we noted that, when a trans CHdCH bond and a cis CHdCH bond are conjugated, the trans wag band splits into two absorptions which appear at 978-976 and 949-946 cm-1.24 For isolated trans HCdCH groups, the in-phase wag band occurs near 965 cm-1. For cis moieties, the vibration in which the HCdCH bonds wag in opposite directions is infrared inactive; however, its frequency remains near 965 cm-1.42,43 When cis and trans groups are conjugated, they interact to give two bands near 977 and 948 cm-1, where both groups vibrate in-phase and out-of-phase with each other.24 The nearly equal intensities result from the fact that the trans group is wagging for both vibrations, while the cis waggings do not contribute to the intensities.42,43 For alkyl substituted vinyl groups, the “trans in-phase” CH wag band appears near 992 cm-1 (Figure 1A). When a vinyl group is conjugated to a trans HCdCH group, they interact,42,43 and the resulting bands appear near 998 and 949 cm-1, which involve both vinyl and trans HCdCH group CH wags, vibrating in-phase and out-of-phase with each other (Figure 1D). The in-phase band near 998 cm-1 is the more intense because all four “trans” hydrogen atoms move in the same direction. On the other hand, when there is interaction between the vinyl 992 cm-1 vibration and the cis 965 cm-1 infrared inactive vibration, only the vinyl contributes to the conjugated group intensity observed at 993 cm-1 (Figure 1C). The determination of the purity of cis or trans isomers of 1,3dienes by gas chromatography alone is not straightforward, since both isomers coelute on most stationary phases such as DB-1, DB-5, and Carbowax.19,30 The present data demonstrate that cis and trans isomers of 1,3-alkadienes can be distinguished unam(27) Blout, E. R.; Fields, M.; Karplus, R. J. Am. Chem. Soc. 1948, 70, 194-198. (28) Mitzner, B. M.; Theimer, E. T.; Freeman, S. K. Appl. Spectrosc. 1965, 19, 169-185 (29) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman & Hall: London, 1975; p 45. (30) To´th, M.; So ¨zcs, G.; Van Nieukerken, E. J.; Phillip, P.; Schmidt, F.; Francke, W. J. Chem. Ecol. 1995, 21, 13-27.

899 (97.15) 899 (98.19) 899 (96.29) 989 (99.19) 989 (99.47) 989 (99.02)

897 (97.91) 897 (98.76) 897 (96.30) 987 (99.98) 987 (99.44) 987 (98.21)

Overtone of the 906/899 cm-1 band. a

3091 (98.53) 3091 (99.12) 3091 (98.05) 2-heptyl-1,3-butadiene (16) 2-nonyl-1,3-butadiene 2-decyl-1,3-butadiene

2936 (90.56) 2935 (91.87) 2934 (80.72)

2868 (96.37) 2866 (96.89) 2865 (92.26)

1799 (99.79) 1799 (99.83) 1798 (99.77)

1632 (99.62) 1632 (99.72) 1632 (99.54)

1595 (99.12) 1595 (99.43) 1595 (98.87)

1462 (99.03) 1462 (99.22) 1462 (98.29)

1385 (99.60) 1385 (99.70) 1385 (99.50)

1083 (99.35) 1083 (99.71) 1083 (98.94) 1386 (99.28) 1386 (99.59) 1385 (98.67) 1460 (98.85) 1461 (99.13) 1461 (97.12) 1608 (98.98) 1608 (99.40) 1608 (98.21) 1640 (99.29) 1640 (99.59) 1639 (98.74) 1797 (99.71) 1797 (99.85) 1797 (99.51) 2869 (95.03) 2866 (95.58) 2866 (86.61) 3094 (98.82) 3094 (99.27) 3094 (97.92) (3E)-3-methyl-1,3-decadiene (15) (3E)-3-methyl-1,3-dodecadiene (3E)-3-methyl-1,3-tridecadiene

2934 (87.94) 2934 (88.88) 2934 (68.59)

dCH2 wag

906 (98.88) 906 (99.38) 906 (98.62) 987 (99.49) 987 (99.75) 987 (99.44)

“trans” dCH wag -C(CH3)dCH-

1082 (99.70) 1088 (99.88) 1088 (99.76) 1383 (99.58) 1382 (99.81) 1382 (99.52)

CH3 def CH2 def

1457 (99.26) 1460 (99.50) 1460 (98.73) 1596 (99.67) 1596 (99.87) 1596 (99.66)

CdC str CdC str

1643 (99.83) 1643 (99.91) 1647 (99.82) 1814 (99.82) 1813 (99.96) 1814 (99.85) 2869 (97.15) 2866 (97.80) 2866 (94.10) 3095 (99.37) 3095 (99.67) 3095 (99.22) (3Z)-3-methyl-1,3-decadiene (14) (3Z)-3-methyl-1,3-dodecadiene (3Z)-3-methyl-1,3-tridecadiene

2934 (93.27) 2934 (94.41) 2933 (85.43)

overtonea CH2 sym + CH3 sym str CH2 asym + CH3 asym str dCH2 str compound

(31) Mu ¨ ller, D. G.; Gassmann, G.; Boland, W.; Marner, F.-J.; Jaenicke, L. Naturwissenschaften 1982, 69, 290-291. (32) Spande, T. F.; Garraffo, H. M.; Daly, J. W.; Tokuyama, T.; Shimada, A. Tetrahedron 1992, 48, 1823-1836.

band position, cm-1 (percent transmission)

biguously by GC/FT-IR. A composite infrared spectrum obtained from a 60:40 mixture of trans and cis isomers of 1,3-tridecadiene, in fact, showed the CdC bands at 1648 and 1603 cm-1. Thus, the positions of these bands are useful for the determination of isomeric purity of 1,3-alkadienes. In fact, monosubstituted 1,3dienes moieties are found in several natural products. For example, a novel moth pheromone,30 the algal pheromone desmarestene,31 and some alkaloids from poisonous frogs32 bear 1,3diene side chains. Although vapor-phase spectra of these frog alkaloids have been recorded, no attempts have been made to distinguish the cis and trans isomers on the basis of their CdC or other absorptions.32 Substituted 1,3-Alkadienes. To study the spectra of compounds bearing 2-methyl-1,3-butadienyl (1, 2) and 2-methylene3-butenyl moieties (3), the dehydration products of three alcohols, 3-methyl-1-decen-3-ol, 3-methyl-1-dodecen-3-ol, and 3-methyl-1tridecen-3-ol, were prepared. The infrared spectra of the products of 3-methyl-1-decen-3-ol, i.e., (3Z)-3-methyl-1,3-decadiene (14), (3E)-3-methyl-1,3-decadiene (15), and 3-heptyl-1,3-butadiene (16), are depicted in Figure 2. These spectra revealed many significant bands which allow unambiguous characterization of the 2-methyl1,3-butadienyl and 2-methylene-3-butenyl moieties in volatile compounds. Incidentally, GC/MS is not helpful for this purpose, since the mass spectra of each set of isomeric dienes are virtually identical. To show how reliable structural predictions can be made on the basis of very small differences in frequencies, data obtained from the three sets of isomeric dienes are presented in Table 1. As a rule, the precision and reproducibility of band positions in vapor-phase infrared spectrometry are better than (1 cm-1.

Table 1. Gas-Phase Infrared Spectra of Some 3-Methyl-1,3-alkadienes and 2-Alkyl-1,3-butadienes (Resolution, 8 cm-1)

Figure 2. Gas-phase infrared spectra of (3Z)-3-methyl-1,3-decadiene (A), (3E)-3-methyl-1,3-decadiene (B), and 3-heptyl-1,3-butadiene (C) (resolution, 8 cm-1).

Analytical Chemistry, Vol. 69, No. 10, May 15, 1997

1831

Figure 4. Gas-phase infrared spectra of (Z)-β-farnesene (A) and (E)-β-farnesene (B) (resolution, 8 cm-1).

Figure 3. Gas-phase infrared spectra of (Z)-ocimene (A), (E)ocimene (B), and myrcene (C) (resolution, 8 cm-1).

The absorptions for out-of-phase dCH2 stretch vibrations of dienes listed in Table 1 are shifted to higher frequencies, similar to those observed for linear 1,3-alkadienes. In fact, for (3Z)-3methyl-1,3-alkadienes, the dCH2 stretch band occurs at 3095 cm-1, while that for (3E)-3-methyl-1,3-alkadienes appears at 3094 cm-1. Lowest values are always observed for β-type isomers, for which the band occurs at 3091 cm-1. Based on these observations, we can generalize that the dCH2 stretch band occurs at a slightly higher frequency for CH2dCHC(CH3)dCH- groups than for CH2dCHC(dCH2)- groups. Moreover, the relative intensity of the dCH2 stretch band is significantly higher for the β-type, since these compounds bear two dCH2 groups, compared to the R-type with only one (Figure 2). Similar differences are evident in the spectra of isomeric monoterpene hydrocarbons (Z)-ocimene (19), (E)-ocimene (20), and myrcene (21) (Figure 3). For the ocimenes, both of which bear a CH2dCHC(CH3)dCH- group, the dCH2 band is observed at 3094-3093 cm-1, whereas for myrcene a relatively more intense dCH2 band is seen at 3091 cm-1. As observed for the linear 1,3-dienes, the band positions and intensity profiles of the two CdC stretch bands in the 1600 cm-1 region are diagnostically very useful. When the configuration at the C-3 position is cis, the two bands are observed at 1643 and 1596-1595 cm-1. These band positions are very similar to those observed for unsubstituted cis 1,3-dienes; however, the relative intensity of the higher frequency band is weaker. On the other hand, the trans isomers show two much more well-defined, bands at 1640 and 1608 cm-1, which, in fact, are different in both intensity and position from those of linear trans 1,3-dienes. In the spectra of linear trans compounds, the 1650 cm-1 band is stronger than the 1604 cm-1 band (Figure 1D), whereas for compounds bearing trans-2-methyl-1,3-butadienyl moieties, the 1640 band is weaker than that at 1608 cm-1. In fact, the spectrum of (E)-ocimene (20) shows similar profiles for the respective bands at 1638 and 1607 1832 Analytical Chemistry, Vol. 69, No. 10, May 15, 1997

cm-1 (Figure 3). The β-type compounds also show a profile with two minima at 1632-1630, and 1595-1594 cm-1 (Figure 2C); however, the higher frequency band is much less pronounced. This pattern is, in fact, visible even in the spectrum of myrcene (21, Figure 3C). For myrcene, an additional band is observed at 1660 cm-1; however, this represents the CdC stretch of the isolated trisubstituted double bond (Figure 3C). We found that the frequency differences of the two CdC bands (∆ν ) ν1 - ν2) is highly characteristic of the nature of the substituted 1,3-diene moiety. From the data presented in Table 1, we conclude that ∆ν values for cis-R-type (1), trans-R-type (2), and β-type (3) moieties are 47-51, 32-31, and 37 cm-1, respectively. These values agree well with vapor-phase data obtained for (Z)-ocimene (19, ∆ν ) 48 cm-1), (E)-ocimene (20, ∆ν ) 31 cm-1), and myrcene (21, ∆ν ) 35 cm-1). Interestingly, similar frequency differences can be observed in data reported from liquid-phase spectra as well. From liquid-phase data reported by Ohloff et al.,33 we calculated ∆ν values for (Z)-ocimene, (E)-ocimene, and myrcene to be 48, 32, and 37 cm-1, respectively. Although Murray12 and others35 have noted from liquid-phase data that the 1600 cm-1 absorption bands can be used to characterize the nature of this diene system, the validity of ∆ν values as a diagnostic marker has not been recognized previously. Farnesenes. All the isomers of farnesene bear an isopropylidene moiety, an internal trisubstituted double bond of cis or trans configuration, and a 1,3-diene system (4-9). From vaporphase infrared spectra presented in Figures 4 and 5, it is clear that band positions and intensities of each isomer are sufficiently unique to allow unambiguous identification of the isomer. In this way, three farnesenes obtained from a termite (P. simplex), an apple (Granny Smith), and an aphid (A. pissum) were identified unambiguously as the Z,E-R, E,E-R-, and E-β isomers, respectively (Table 2). In addition, generalizations we made for the substituted 1,3-alkadienes can be applied to derive structural information for each farnesene isomer. The dCH2 stretch band of farnesene spectra also shows a shift to higher frequencies. Compared to that of R-farnesenes, the (33) Ohloff, G.; Seibl, J.; Kovats, E. Liebigs Ann. Chem. 1964, 83-101. (34) Brieger, G.; Nestrick, T. J.; McKenna, C. J. Org. Chem. 1969, 34, 37893791. (35) Anet, E. F. L. J. Aust. J. Chem. 1970, 23, 2101-2108.

Figure 5. Gas-phase infrared spectra of (Z,Z)-R-farnesene (A), (Z,E)-R-farnesene (B), (E,Z)-R-farnesene (C), and (E,E)-R-farnesene (D) (resolution, 8 cm-1).

intensity of this band in β-farnesenes is higher due to the presence of two dCH2 groups in the latter (Table 2). As expected, the frequency of the dCH2 band is consistently lower for β-farnesenes (3090 cm-1) than that for the R-farnesenes (3093-3091 cm-1). Absorptions for the C-H stretches of other C-H bonds on carbon-carbon double bonds can be expected between 3050 and 2990 cm-1. However, these bands are usually of lower intensity, and in our spectra they all form a broad, unresolved hump on the high-frequency edge of the band for the asymmetric C-H stretch of the CH3 groups, which occur at 2975-2970 cm-1 for all farnesene isomers. Interestingly, the intensity of this CH3 band depends on whether the isomer bears a triply substituted 6Z or a 6E double bond. In the spectra of three farnesene isomers with 6Z configuration, compared to those bearing 6E configuration, this band is significantly stronger in intensity (Figures 4 and 5). The other absorptions, such as the CH2 asymmetric stretch band (2930-2928 cm-1) and the combined CH2 symmetric stretch and CH3 symmetric stretch band (2871-2869 cm-1), are similar in intensities for all isomers. As expected, the 1660-1590 cm-1 region of farnesene spectra is complex. Nevertheless, valuable information about the nature of the C-3 bond can be derived from these bands for CdC stretching vibrations. For example, this region of (Z)-β and (E)-

β-farnesene spectra, which show absorptions at 1632-1630, and 1595 cm-1, is indistinguishable for the two spectra since the diene part of both molecules is identical. Moreover, this region is virtually identical to that of myrcene. Similarly, the corresponding regions are very similar in the spectra of the two 3E-isomers of R-farnesene, which show absorptions at 1639/1638 and 1608/1607 cm-1 and resemble the corresponding regions of the (E)-ocimene spectrum. From data presented in Table 2, it is evident that, in farnesenes, when the double bond at the C-3 position bears a Z configuration, the dCH2 wag band shifts characteristically to 907-906 cm-1, while an E configuration, or a dCH2 group at the C-3 position, shifts it further to 899-897 cm-1. Furthermore, in the spectra of β-farnesenes, the intensities of the 899-898 cm-1 absorption is essentially double compared to those of R-fanesenes since the β-farnesenes bear two dCH2 groups. For 1-alkenes, the 914 cm-1 band for the dCH2 wag vibration is usually accompanied by a weak overtone at 1828 cm-1. The analogous overtone band for the 907 cm-1 absorption of (Z,Z)-Rfarnesene and (Z,E)-R-farnesene is observed at 1814-1813 cm-1, while the spectra of other four isomers show the overtone, arising from the 898-896 cm-1 band, at 1798-1796 cm-1. Moreover, the gas chromatographic elution order of compounds bearing the aforementioned three types structural moieties on DB-1 and DB-5 stationary phases is consistent.35 The β-type compounds always elute first, followed by the 3Z and 3E. The elution pattern of myrcene, (Z)-ocimene, and (E)-ocimene follows the same trend. Interestingly, the elution order of (6Z)-farnesene and (6E)-farnesene follow the same behavior [6Z-β first, followed by 3Z,6Z-R and 3E,6Z-R], indicating that, in these compounds, retention behavior is determined essentially by the nature of the conjugated diene moiety. In fact, the major differences in infrared spectra are also governed by this diene moiety. Hence, it is not surprising that Nishino and Bowers36 observed no basic differences among the spectra of four isomers of norfarnesene, which lacks this conjugated diene moiety. Several farnesene isomers are known from ants and other insects.37 Often, the structures are reported without complete configurational information.13 This is particularly true for farnesene homologs. For example, the homofarnesene and bishomofarnesene isomers from Myrmica ants were reported as 7-ethyl3,11-dimethyldodeca-1,3,6,10-tetraene (22) and 7-ethyl-3,11-dimethyltrideca-1,3,6,10-tetraene (23), respectively.16 We recorded the spectra of these two farnesene homologs and the (Z,E)-Rfarnesene obtained from the ant M. sabuleti (Figure 6). Except for the 2975-2973 cm-1 absorption, the three spectra appeared very similar. The presence of 906, 1596-1594, and 3094-3093 cm-1 bands in all three spectra disclosed that the configuration at the C-3 must be cis in all three compounds. In addition, the ∆ν values for the two CdC stretch bands were 47, 49, and 47 cm-1 for farnesene, bishomofarnesene, and bishomofarnesene, respectively, confirming the stereochemistry of the double bond at the C-3 atom as cis. (Unfortunately, the stereochemistry at the C-6 position of the homofarnesene (22) and at C-6 and C-10 of the bishomofarnesene (23) could not be assigned with the available data.) Interestingly, the 2742-2739 cm-1 band, which is apparent in the spectra of compounds bearing an isopropylidene moiety, is absent in the spectrum of the bishomofarnesene (23). (36) Nishino, C.; Bowers, W. S. Tetrahedron 1978, 32, 2875-2877. (37) Morgan, E. D.; Parry, K.; Tyler, R. C. Insect Biochem. 1979, 9, 117-121.

Analytical Chemistry, Vol. 69, No. 10, May 15, 1997

1833

1834

Analytical Chemistry, Vol. 69, No. 10, May 15, 1997

1234 (99.90)

1107 (99.80) 1080 (99.82) 988 (99.50)

899 (98.48) 834 (99.65)

(weak band) (weak band) (weak band) (weak band) “trans” dCH wag

dCH2 wag C-H wagd

906 (99.66) 834 (99.80)

987 (99.81)

988 (99.29) 898 (97.88) 835 (99.60)

1094 (99.75)

1228 (99.90)

3094 (99.73) 2973 (98.60) 2929 (98.29) 2871 (98.98) 2735 (99.87) 1814 (99.92) 1644 (99.87) 1594 (99.86) 1449 (99.61) 1381 (99.61) 1315 (99.88)

Z,Z-R (6), [20.6]

1217 (99.87) 1154 (99.86) 1105 (99.68)

3090 (98.82) 2970 (96.94) 2930 (95.11) 2870 (97.36) 2739 (99.85) 1798 (99.86) 1632 (99.65) 1595 (99.34) 1447 (99.02) 1384 (99.12) 1332 (99.74)

E-β (5), [20.2]

897 (99.21) 833 (99.62)

987 (99.58)

1088 (99.56)

1229 (99.90)

3093 (99.54) 2973 (97.46) 2928 (96.94) 2871 (98.25) 2740 (99.94) 1796 (99.90) 1638 (99.67) 1607 (99.63) 1450 (99.38) 1382 (99.31) 1316 (99.86)

E,Z-R (7), [20.9]

986 (99.71) 957 (99.76) 906 (99.50) 833 (99.73)

1209 (99.85) 1157 (99.87) 1097 (99.65)

3094 (99.71) 2975 (98.47) 2929 (97.77) 2871 (98.77) 2742 (99.91) 1813 (99.91) 1642 (99.84) 1595 (99.82) 1446 (99.50) 1383 (99.52) 1323 (99.83)

Z,E-R (8), [21.3]

896 (98.84) 833 (99.50)

986 (99.43)

1281 (99.75) 1215 (99.81) 1153 (99.78) 1091 (99.40)

3091 (99.33) 2973 (97.08) 2928 (95.42) 2871 (97.54) 2740 (99.87) 1796 (99.84) 1639 (99.48) 1608 (99.51) 1446 (99.11) 1385 (99.09)

E,E-R (9), [21.5]

986 (99.41) 957 (99.63) 906 (98.85) 834 (99.46)

1219 (99.83) 1154 (99.82) 1096 (99.22)

3093 (99.36) 2976 (96.60) 2927 (94.97) 2871 (97.24) 2741 (99.87) 1814 (99.88) 1642 (99.75) 1594 (99.68) 1445 (98.95) 1383 (98.98) 1327 (99.75)

termite [21.3]

898 (98.82) 836 (99.50)

986 (99.36)

1796 (99.93)

1796 (99.85) 1638 (99.52) 1607 (99.45) 1445 (99.16) 1385 (99.05) 1329 (99.79) 1280 (99.78) 1213 (99.88) 1153 (99.84) 1091 (99.29)

898 (98.46) 834 (99.71)

988 (99.50)

1105 (99.79)

1595 (99.54) 1446 (99.35) 1384 (99.41)

3090 (99.17) 2974 (98.01) 2930 (96.71) 2869 (98.27)

aphid [20.2] 3093 (99.29) 2976 (97.08) 2927 (95.48) 2870 (97.58)

apple [21.5]

source of natural farnesene isomera

a The gas chromatographic retention time in minutes is given in brackets. A 25 m × 0.32 mm fused silica column coated with DB-5 stationary phase (0.25 mm film thickness) was used. The oven temperature was held at 60 °C for 3 min and increased to 250 °C at 10 °C/min. b This band is apparent in spectra of compounds bearing an isopropylidene moiety. c Overtone of the 906/898 cm-1 dCH2 wag band. d Out-of-plane C-H wag vibration of the trisubstituted double bond.

3090 (99.13) 2971 (97.28) 2930 (96.66) 2870 (98.08) 2739 (99.87) 1798 (99.90) 1630 (99.75) 1595 (99.53) 1451 (99.29) 1381 (99.34) 1317 (99.85)

)CH2 str CH3 asym str CH2 asym str CH2 sym str (weak band)b overtonec CdC-CdC CdC-CdC CH2 def CH def

Z-β (4), [19.5]

synthetic farnesene isomera

band position, cm-1 (percent transmission)

Table 2. Band Positions (cm-1) and Intensities of Gas-Phase FT-IR Spectra of Synthetic and Some Natural Farnesene Isomers

Figure 7. Gas-phase infrared spectra of (3E)-4-methyl-1,3-undecadiene (A) and (3Z)-4-methyl-1,3-undecadiene (B) (resolution, 8 cm-1).

The absorption bands present in the 1150-1080 cm-1 region represents the -C(CH3)dCH- moieties. As expected, no bands are visible in this region in the spectrum of 3-heptyl-1,3-butadiene (16, Figure 2C). In general, the intensity of bands in this region is weaker for β-farnesenes than for R-farnesenes. Similar observation were made with the ocimenes (19, 20), for which the 11041103 cm-1 band is much stronger than that at 1106 cm-1 for myrcene (21, Figure 3). This region is particularly useful to distinguish between (E)- and (Z)-β-farnesenes. The (Z)-β-farnesenes clearly shows two absorption minima at 1107 and 1080 cm-1, while the (E) isomer show only one minimum at 1105 cm-1 (Figure 4).

Overtone of the 902/900 cm-1 band.

3090 (99.22) 3090 (99.74) 3090 (99.81) 3091 (99.26) 3091 (99.63) 3091 (99.30)

(3Z)-4-methyl-1,3-undecadiene (18) (3Z)-4-methyl-1,3-tridecadiene (3Z)-4-methyl-1,3-teradecadiene (3E)-4-methyl-1,3-undecadiene (17) (3E)-4-methyl-1,3-tridecadiene (3E)-4-methyl-1,3-tetradecadiene

a

dCH2 str

compound 2935 (91.03) 2934 (94.83) 2934 (95.49) 2935 (92.12) 2934 (92.32) 2935 (92.49)

CH2 asym + CH3 asym str 2868 (96.17) 2866 (97.93) 2866 (98.22) 2868 (96.91) 2865 (97.07) 2868 (97.06)

CH2 sym + CH3 sym str

1804 (99.78) 1803 (99.95) 1804 (99.95) 1805 (99.82) 1805 (99.93) 1805 (99.84)

overtonea

1646 (99.44) 1646 (99.81) 1646 (99.85) 1648 (99.50) 1648 (99.75) 1648 (99.53)

CdC str

1603 (99.82) 1603 (99.99) 1603 (99.96) 1603 (99.77) 1603 (99.88) 1603 (99.80)

CdC str

CH2 def 1460 (99.05) 1460 (99.53) 1461 (99.61) 1460 (99.25) 1460 (99.34) 1460 (99.30)

band position, cm-1 (percent transmission)

Table 3. Gas-Phase Infrared Spectra of Some 4-Methyl-1,3-alkadienes (Resolution, 8 cm-1)

Figure 6. Gas-phase infrared spectra of (Z,E)-R-farnesene (A), a homofarnesene (B), and a bishomofarnesene (C), from Myrmica sabuleti ants (resolution, 8 cm-1).

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1418 (99.47) 1418 (99.80) 1416 (99.85)

1382 (99.38) 1382 (99.76) 1381 (99.80 1384 (99.44) 1383 (99.63) 1384 (99.48)

CH3 def

984 (99.18) 983 (99.70) 983 (99.78) 984 (99.31) 983 (99.60) 984 (99.35)

“trans” dCH wag

901 (98.20) 900 (99.38) 900 (99.50) 902 (98.70) 901 (99.27) 902 (98.76)

dCH2 wag

4-Methyl-1,3-undecadienes. Another structural moiety of considerable interest is the 4-methyl-1,3-dienyl group. For example, the natural products (3E)-4,8-dimethyl-1,3,7-nonatriene and (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene,38 which were later shown to act as important chemical messengers in a tritrophic level interaction,39,40 bear this moiety. The determination of the configuration of the trisubstituted double bond in this moiety has been considered a difficult analytical problem. To determine the stereochemistry of the natural compounds, often both cis and isomers had to be synthesized.38,40 To establish the gas-phase infrared correlations for the determination of the configuration of this group, we prepared three 4-methyl-1,3-alkadienes, as a mixture of both geometric isomers, from the reaction of allylidene ylide with 2-alkanones. For example, a 35:65 mixture of 4-methyl-1,3-undecadiene isomers (17, 18) was obtained when the ylide from allyltriphenylphosphonium bromide was allowed to react with 2-nonanone. The geometry of each isomer in the mixture was determined from their 1H NMR spectra, which show notable differences in the chemical shifts of the methylene and methyl groups connected to the sp2 carbon. Using the published values for (3E)- and (3Z)4-methyl-1,3-nonadienes,41 the geometry of the less abundant isomer 18 (eluted earlier on DB-5 and DB-1 GC phases) was assigned to be 3Z. The chemical shifts values for the major isomer 17 corresponded to the published values for (3E)-4-methyl-1,3nonadiene. From the spectra of cis and trans isomers of 4-methyl-1,3undecadiene (Figure 7), weak but diagnostic differences were observed in the 1480-1360 cm-1 region. The cis isomer showed a band at 1418 cm-1 located between the bands at 1460 (CH2 def) and 1382 cm-1 (CH3 sym, Figure 7B). In contrast, the spectrum of the trans isomer showed this band only as a weak shoulder at 1420 cm-1 (Figure 7A). The significance of the 1418 cm-1 band was confirmed by the presence of a 1418-1416 cm-1 band in the (38) Maurer, B.; Hauser, A.; Froidevaux, J.-C. Tetrahedron Lett. 1986, 27, 21112112. (39) Turlings, T. C.; Tumlinson, J. H.; Lewis, J. W. Science 1990, 250, 12511253. (40) Dicke, M.; Van Beek, T. A.; Posthumas, M. A.; Ben Dom, N.; Van Bokhoven, H.; De Groot, AE. J. Chem. Ecol. 1990, 16, 381-396. (41) Underiner, T. L.; Goering H. L. J. Org. Chem. 1991, 56, 2563-2572. (42) Colthup, N. B. Appl. Spectrosc. 1971, 25, 368-371. (43) Colthup, N. B.; Daly, L. H.; Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, 1969; p 256. (44) Reedy, G. T.; Ettinger, D. G.; Schneider, J. F.; Bourne, S. Anal. Chem. 1986, 57, 1602-1609.

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spectra of cis isomers of 4-methyl-1,3-tridecadiene and 4-methyl1,3-tetradecadiene (Table 3). The 1420 cm-1 band was insignificantly weak in the spectra of corresponding trans isomers. In summary, we have demonstrated that GC/FT-IR data allow definitive deductions to be made about the stereochemistry of carbon-carbon double bonds conjugated to a vinyl group. Since such moieties are common in many natural compounds, we hope that many chemists will realize the value of the GC/FT-IR technique, since NMR is not the method of choice for samples available in minute amounts and in impure form. Another GC/IR technique that has been used recently is the matrix isolation (MI) method.44 In this technique, the GC effluent is mixed with argon and frozen into a solid matrix. Argon dilutes minute amounts of analytes; therefore, in the condensed material, analyte molecules are held almost individually. In this way, IR band broadening due to intermolecular interactions between analyte molecules and molecular rotations is reduced to a bare minimum. As a result, the infrared bands are much narrower and more intense than those observed in the vapor phase. For the compounds discussed in this paper, more useful results could have been obtained if the GC/MI-IR method were applied. However, MI is the most expensive GC/IR procedure, since the system has to operate under cryogenic temperatures and highvacuum conditions, and only a very few laboratories are fortunate to have one. Since there are no commercial vendors currently, it is unlikely that GC/MI-IR will become a routine technique in the near future. ACKNOWLEDGMENT We are greatly indebted to Prof. J. Meinwald and Dr. N. B. Colthup for offering many helpful suggestions and critically reading the manuscript and Prof. H. J. Bestmann (University of Erlangen, Germany) for sending us chemical samples. We are grateful to the Hewlett Packard Co. (Palo Alto, CA) for the donation of the GC/FT-IR equipment used in this study. The partial support of this research by NIH Grant GM 53830 is acknowledged with pleasure.

Received for review September 5, 1996. Accepted March 3, 1997.X AC960890U X

Abstract published in Advance ACS Abstracts, April 15, 1997.