The Infra-red Spectra of Complex Molecules

Abstract

2-Acetoxy-írans-3-hexene and 4-acetoxy-inros-2-hexene have been pyrolyzed under conditions that gave partial pyrolysis. Both esters gave rise to a mixture of 1,3-and 2,4-hexadiene, the 2-acetoxy compound giving somewhat more of the 1,3-diene and the 4-acetoxy compound giving more of the 2,4-diene. The 1,3-diene was predominantly the trans isomer, and about half of the 2,4-diene was the trans, trans isomer. The unpyrolyzed ester recovered in each case was the same mixture of 2-acetoxy and 4-acetoxy compounds. Clearly, the esters underwent isomerization during the pyrolysis, and in this isomeriza-tion the irans-alkene configuration was essentially retained. It was the report of van Pelt and Wibaut1 that 4-acetoxy-2-hexene could be pyrolyzed to a 66% yield of 2,4-hexadiene which led us to adapt this procedure for the preparation of 2,4-heptadiene. The results2 of the pyrolysis of 2-acetoxy-frans-3-heptene and 4-acetoxy-¿rans-2-heptene were that both esters gave rise to 1,3-and 2,4-diene, the former ester producing slightly more of the 1,3-diene and the latter producing somewhat more of the 2,4-diene. The recovered, unpyrolyzed ester in each case was found to be the same mixture of the 2-acetoxy and 4-acetoxy compounds, and the data indicated that when the one ester isomerized to the other the trans configuration of the alkene linkage was essentially retained. These results would make one expect that the acetates of the corresponding six-carbon alcohols would behave in a similar fashion , and that one should not anticipate a good yield of 2,4-hexadiene from the pyrolysis of 4-acetoxy-2-hexene. The literature relative to the pyrolysis of allylic esters has been cited previously.2 Additional interesting examples are those reported by Alder and von Brachel3 where acetates of 3-hexene-2,5-diols were pyrolyzed to yield 1,3-cyclohexadienes. These would appear to be instances of the elimination of two molecules of acetic acid followed by a Cope rearrangement4 to give, finally, the cyclic 1,3-dienes. The allylic esters, 2-acetoxy-¿rons-3-hexene and 4-acetoxy-fraros-2-hexene, have now been prepared and pyrolyzed. ¿rans-2-Pentenal was prepared by the method of Hoaglin and Hirsch.5 The trans configuration of this aldehyde was confirmed by silver oxide oxidation of the aldehyde to trans-2-pentenoic acid, and comparison of this acid with a sample of írans-2-pentenoic acid which was prepared by the Doebner reaction. The infrared absorption curves of the two samples of írans-2-pen-tenoic acid were identical. It is rather well estab-lished6 that the Doebner reaction gives rise to a trans alkene linkage. The aldehyde was converted (1) A. to the allylic alcohol by reaction with the methyl Grignard reagent, and the alcohol esterified with acetic anhydride-pyridine to give the 2-acetoxy-írans-3-hexene. The infrared absorption curves of the aldehyde, the alcohol, and the acetate had very strong absorption at or near 965 cm.-1 characteristic of a trans alkene linkage, and no absorption which could be attributed to a cis alkene linkage.7 Gas chromatography of this ester showed it to contain 93% of the trans ester, 2% of the cis ester, and 5% of unidentified material. The 4-acetoxy-irans-2-hexene was obtained from the corresponding alcohol which was prepared from crotonaldehyde and the ethyl Grignard reagent. Crotonaldehyde has a trans alkene linkage.8 The infrared absorption curves of this acetate and the corresponding alcohol had very strong absorption at 965 cm.-1, and no absorption which could be attributed to a cis double bond. By gas chromatography this ester was found to contain 97% of the trans ester and 3% of the cis ester. On pyrolysis these esters gave the same type of data as was obtained previously2 with the acetates of the seven-carbon allylic alcohols. A temperature of 440-450° was required for pyrolysis of the acetates , and under the conditions employed the ester had a contact time of 3.5 sec. About an 80% yield of diene was obtained from each ester. The diene obtained from 2-acetoxy-irans-3-hexene was 54% I, 3-hexadiene and 46% 2,4-hexadiene, whereas that obtained from the 4-acetoxy-