Characterization of Vinyl-Substituted, Carbon−Carbon Double Bonds by GC/FT-IR Analysis

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Abstract
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 1590−1650 cm-1 region (the CC 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 CH2CHC(CH3)CHCH2- and CH2CHC(CH2)CH2- structural moieties, referred to as α- and β-type compounds, are frequently encountered as natural products. For compounds bearing α-type groups, the cis/trans configuration of the trisubstituted double bond can be determined unambiguously. An absorption at 3095−3091 cm-1, for the CH2 stretch vibration, is common to both of these groups; however, due to the presence of two CH2 groups, the relative intensity of the band is much higher for β-type compounds. For α-type compounds, a cis configuration at the C-3 carbon atom is characterized by a CH2 wag absorption at 907−906 cm-1. For β-type compounds and 3E-α-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 α-type farnesene congeners. For example, the homo- and bishomofarnesene isomers from Myrmica ants were shown to include a 3Z bond.