Thermal Reactions of 8-Methylbicyclo[4.2.0]oct-2-enes:  Competitive Diradical-Mediated [1,3] Sigmatropic, Stereomutation, and Fragmentation Processes

Abstract

[reaction: see text] At 275 degrees C, 8-exo-methylbicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,3] sigmatropic rearrangement to 5-methylbicyclo[2.2.2]oct-2-enes, of which the orbital symmetry-allowed si product is only marginally favored over the forbidden sr product; that is, si/sr is 2.4. Accompanying the [1,3] shift are significant amounts of epimerization and fragmentation. The 8-endo epimer 1b, which yields no [1,3] product, experiences primarily direct fragmentation and secondarily epimerization. A diradical intermediate can account for all such observations.

Palladium(II)-Catalyzed Rearrangement and Oligomerization Reactions of cis-Bicyclo[4.2.0]oct-7-ene

“…Significant quantities of both 7- and 8- exo -methylbicyclo­[4.2.0]­oct-2-ene ( exo -8a and exo -8b ) are obtained when the bicyclic 7-ene 4a is reacted with ( cis,cis -1,5-cyclooctadiene)­(methyl)­palladium chloride ( 9 ) using a low [ 4a ]/[ 9 ] ratio of 0.5/1. The stereochemistry of exo -8a , b suggests that also oligomers 5a – d are predominantly exo-linked. , The main bicyclic olefin product formed is the unsubstituted cis -bicyclo­[4.2.0]­oct-2-ene ( 4d ). The relative ratio 4d : exo -8a : exo -8b is 58:24:18 (via 1 H NMR).…”

Thermal Isomerization of cis,anti,cis-Tricyclo[6.3.0.02,7]undec-3-ene to endo-Tricyclo[5.2.2.02,6]undec-8-ene

“…In 1973, Berson and Holder reported that 7- endo -acetoxy-8- exo -methylbicyclo[4.2.0]oct-2-ene isomerized to substituted bicyclo[2.2.2]octenes with only modest selectivity favoring the symmetry-allowed si pathway ( si/sr = 2.2) . We have recently confirmed that 8- exo -methylbicyclo[4.2.0]oct-2-ene gives isomeric [1,3] carbon shift products reflecting a similar si/sr ratio, 2.4 (Table , entry 3).…”

“…The simplest mechanistic explanation for the thermal behavior of 1 is homolytic cleavage of the C1−C2 bond to generate an alkyl, allylic diradical intermediate that partitions itself between potential reformation of 1 , formation of the [1,3] sr isomerization product 2 , and fragmentation. The [1,3] shift in compound 1 is more competitive with fragmentation than in 8- exo -methylbicyclo[4.2.0]oct-2-ene (Table , entry 3), which at 315 °C affords predominantly fragmentation with lesser amounts of the [1,3] shift products and epimerization in a ratio of 74:15:11, respectively; the k f /( k sr + k si ) ratio is 4.9 …”

Molecular rearrangements through thermal [1,3] carbon shifts

“…The 8-methylbicyclo[4.2.0]oct-2-enes were prepared and gas phase thermal reactions at 275 • C were followed for each isomer (Scheme 29). 50 The 8-exo-methyl isomer isomerized to give 5-exomethylbicyclo[2.2.2]oct-2-ene and the 5-endo-methyl epimer in a 71 : 29 ratio. The 8-endo-methylbicyclo[4.2.0]oct-2-ene did not form a [1,3] shift product: it gave its epimeric 8-exo-methyl isomer and fragmented to yield 1,3-cyclohexadiene and propylene.…”