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The Thermal Rearrangement of 1-Alken-5-ynes and 1,2,5-Alkatrienes1 William D. Huntsman, Jacob A. De Boer, and Marcia H. Woosley
Contributionfrom the Department of Chemistry, Ohio University, Athens, Ohio 45701. Received September 1,1966 Abstract: 1-Alken-5-ynesundergo a reversible Cope-type rearrangement at 340' to give 1,2,5-alkatrieneswhich, in turn, undergo cyclization to 3- and 4-methylenecyclopentenes. The effect of methyl substituents is reported, and possible mechanisms are discussed.
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ecent studies have revealed interesting thermal rearrangements of certain alkenynes and alkadiynes. For example, 1-methylene-2-vinylcyclopentane (2) arises from rearrangement of 6-octen-1-yne (1) at 400 O, and 3,4-dimethylenecyclobutene(4a) is formed from 1,5-hexadiyne (3a) at 335 O.2 Methyl-substituted dimethylenecyclobutenes 4b and 4c are formed from 1,5-heptadiyne (3b) and 2,6-octadiyne (3c), respectively. The methyl groups retard the cyclization reactions significantly. Stereospecificity in the rotational motion involved in the formation of the exocyclic groups was demonstrated by the conversion of meso-3,4-dimethyl1,5-hexadiyne (5) to 6 and rac-3,4-dimethyl-1,5-hexadiyne (7) to 8.
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It was of interest to extend these studies to the analogous I-alken-5-ynes. At the outset, it was anticipated that a simple Cope rearrangement might occur because models indicated a suitable geometry for this type of intramolecular reorganization. The infrared spectrum of the material obtained by heating 1-hexen-5-yne (9) at 340" did indeed exhibit absorption characteristic of a terminal allene, but vpc analysis showed the presence of three compounds in addition to starting material. One of these was shown to be the allene 10 by evidence presented below, The proportion of the other compounds in the reaction mixture increased when the
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temperature was raised to 385". At the same time, the fraction of 10 passed through a maximum and declined with increasingly drastic conditions. Separation of the three products was achieved by preparative vpc, and the fractions (i, ii, and iii) were shown to be homogeneous by analytical vpc. The infrared spectrum of i agreed with that reported for 1,2,5-hexatriene and the structure was confirmed by the nmr spectrum, which consisted of a twoproton multiplet at 7 7.25 (H-4), a two-proton multiplet at 5.36 (H-I), a three-proton multiplet at 4.95 (H-3 and H-6), and a one-proton multiplet at 4.12 (H-5). The multiplet at 7 5.36 resembled very closely the signal assigned to the terminal allenic protons in 1,2-pentadiene.5 Coupling constants obtained by a first-order treatment of the multiplet at 7 4.12 are: J4,5= 6.4 cps, J5,6(cis) = 9.5 cps, andJ5,6(lvans) = 17.5 cps. Fraction ii was shown to be 3-methylenecyclopentene (11) by comparison of spectral properties with those reported for this compound.6 Intense absorption occurred in the ultraviolet spectrum at 233 mp ( E 15,600), and characteristic infrared bands occurred at 6.18, 6.25 (conjugated double bond), and 11.70 p (RzC=CHz). In the nmr spectrum multiplets appeared at 7 7.52 (H-4, H-5), 3.90 (H-1, H-2), and 5.24 (H-6), with intensities in the ratio 2 : 1 : 1. Hydrogenation over platinum oxide gave methylcyclopentane, identified by comparison with an authentic sample. Fraction iii was identified as 4-methylenecyclopentene (12) on the basis of spectroscopic properties and the formation of methylcyclopentane upon hydrogenation. The ultraviolet spectrum showed only end absorption, whereas in the infrared spectrum characteristic bands appeared at 6.04 (>C=C
