Thermal Isomerisations, XXIV. Gas Phase Kinetics of the Pyrolysis of Some 3,3‐Dimethyl‐1‐alkyl‐cyclopropenes: Some Surprising Substituent Activation Effects and the Intramolecular Trapping of Vinylidene Intermediates

Abstract

The gas phase pyrolyses of four 3,3-dimethyl-l-alkylcyclopropenes were studied for which the 1-alkyl substituents are ethyl-(6), isopropyl-( 7 ) , 1'-butyl-(8) and 1'-isoamyl-(9). Rate data over a 50°C temperature range were obtained and Arrhenius parameters are reported both for overall reactions and individual pathways for all compounds. Tests confirm all reactions to be unimolecular and homogeneous. For 8 and 9, noteworthy products are cyclopentenes, whose presence demonstrates the involvement of vinylidene intermediates. The substantial product yields of 2,4-alkadienes together with dramatic rate enhancement effects of these 1-alkyl substituents (relative to 1-methyl) are explained by the involvement of vinylcarbene intermediates in this pathway. Minor, transient products from 6, 8 and 9 are probably isomeric cyclopropenes. This suggests further pathways for vinylidene intermediates, whose reactivity pattern is briefly reviewed.

Alkylidenecarbenes, Alkylidenecarbenoids, and Competing Species: Which Is Responsible for Vinylic Nucleophilic Substitution, [1 + 2] Cycloadditions, 1,5-CH Insertions, and the Fritsch−Buttenberg−Wiechell Rearrangement?

“…However, isopropyl migrated only 1.4-times as fast as methyl under these conditions, 41a, and this suggests that additional factors may also be important 41b , as exemplified further below.

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“…The active species arising from primary alkenyl triflates (section 2.2), from diazoalkenes (section 2.3.1), and from iodine(III) compounds with certain reservations (section 2.4.3) can be considered to be alkylidenecarbenes, responsible for insertion, nucleophilic addition, [1 + 2] cycloaddition, and FBW reactions. This tentative rule was fairly well substantiated only for bimolecular O-H insertion (45), for the addition of tetrahydrothiophene (130/131), and for [1 + 2] cycloaddition (207/208, and 211a/b in Scheme 1); it remains to be confirmed by additional examples from the various reaction modes.…”

“…However, isopropyl migrated only 1.4-times as fast as methyl under these conditions, 41a,42 and this suggests that additional factors may also be important. 41b Homologues of 19 with longer alkyl chains would very much prefer to form cyclopentenes by intramolecular 1,5-CH insertion, 35,45 as exemplified further below.…”

Thermochemical kinetics: does it still give insights?

“…While the full details are given in the original paper, 36 selected information on relative rates (selectivities) for intramolecular C-H insertion is shown in Table 9. This table also includes analogous data for other carbenes 47,48 obtained by both thermal and photochemical means. The similarity of the selectivities found for different carbenes suggests that their actual rate constants may not be too dissimilar.…”