Thermal isomerization of cyclobutenes. Part 8.—Cis- and trans-1,2,3,4-tetramethylcyclobutene and bicyclo[4.2.0]oct-7-ene

Abstract

The thermal isomerizations of cis and trans-l,2,3,4-tetramethylcyclobutene and bicyclo[4.2.0]oct-7-ene have been studied in the gas phase. In the temperature range 157-2Ol0C, cis-tetramethylcyclobutene isomerizes by a homogeneous first-order process to yield cisYtrans-3,4-dimethylhexa-2,4diene and the rate constants fit the Arrhenius equation kl = 1014.10 exp (-37,360IRT) sec-1.In the temperature range 125-167°C the trans tetrametliylcyclobutene isomerizes to trans,trans-3,4dimethylhexa-2,4-diene, but under the conditions employed this product undergoes surface-catalyzed polymerization. The rate of disappearance of the reactant obeyed first-order kinetics and the rate constants obtained fitted the Arrhenius equation, k2 = 10*3.85 exp (-33,59O/RT) sec-1. The isomerization of bicyclo[4.2.0]oct-7-ene yields cycloocta-l,3-diene by a homogeneous first-order process, and in the temperature range 236-285" the rate constants fit the Arrhenius equation, k3 = 1014.13 exp (-43,18O/RT) sec-1.Above 1 mm the rate constants of the three isomerizations are independent of pressure. The results obtained are compared with those for other cyclobutenes with special reference to the mechanism of the transformation.

History of the Woodward‐Hoffmann Rules. The No‐Mechanism Puzzle**

“…One such example involves the thermal chemistry of substituted cyclobutenes reported by Henry Montague (H. M.) Frey in the early and mid‐1960s [70–79] . Only when he studied 3,4‐disubstituted cyclobutenes in 1966 [78] could Frey comment on the relative energetics of concerted versus nonconcerted and conrotatory versus disrotatory motions in these cycloreversions.…”

“…Thus, Frey's pre‐1965 results do not provide insights into alternating chemistries, a necessity for anticipating the W‐H rules. Indeed, that Frey did study 3,4‐disubstituted cyclobuenes [77,79] but only after the publication of the Woodward‐Hoffmann paper on electrocyclizations, [10] indicative that solving the no‐mechanism problem was not within Frey's pre‐1965 radar screen. Indeed, in a 1969 letter from Frey to Woodward, [80] Frey indicates that only the reading of the Woodward and Hoffmann treatise [19] interested him in the mechanistic problems of organic chemistry.…”

The ‘<i>Yard Stick</i>’ to Interpret the Entropy of Activation in Chemical Kinetics: A Physical-Organic Chemistry Exercise

“…Example 1: Unimolecular gas-phase isomerization of bicycle[4.2.0]oct-7-ene [24] to give cyclooct-1,3-diene in the temperature range 508-558 K. The Intercept from the Arrhenius plot was found to be 14.1s -1 . At first sight this value is greater than the value of 13.2 obtained from the Universal Factor.…”

The Many Chemists Who Could Have Proposed the Woodward‐Hoffmann Rules (Including Roald Hoffmann) But Didn't: The Theoretical and Physical Chemists†**

“…In particular, Frey studied what he called "thermal unimolecular isomerizations" of a series of cyclobutenes, namely: 1-methylcyclobutene (11 a, Part 1, 1962); [153] [154] ; 1,3-dimethylcyclobutene and 1,4-dimethylcyclobutene (13 and 14, respectively; Part 4, 1965) [155] ; 3-methylcyclobutene (15, Part 5, 1965); 1-ethylcyclobutene (11 b, Part 6, 1965); [156] bicyclo[3.2.0]hept-6-ene (16, Part 7, 1966) [157] ; and cis-and trans-1,2,3,4-tetramethylcyclobutene and bicyclo[4.2.0]oct-7ene (3, 15 and 18, respectively; Part 8, 1966). [158] Frey was comprehensive, to be sure.…”