Abstract:'The racemization of (+)-2,3-divinyloxirane is facile at lMa, log (k,/s-l) = (11.3 + 0.5) -(30.1 +_ 0.4)/0 where 0 = 2.303RT in kcal mol-'. A slower but competitive rearrangement yields racemic 2-vinyl-2,3-dihydrofuran. The mechanisms of these reactions are discussed in terms of electrocyclic processes.-- Can. J. Chem. 51, 3718 (1973)
“…During the thermal isomerization the intermediates vinylcarbonyl ylides 16 ( 17 ) and 18 ( 19 ) are less reactive toward maleic anhydride and convert to the products without capturing maleic anhydride. It is well-known that sterically stable endo -vinylcarbonyl ylides such as 17 undergo disrotatory cyclization to give sterically unstable cis -dihydrofurans such as 5 . − 10 …”
Section: Discussionmentioning
confidence: 99%
“…During thermal isomerization of 3 and 4 to 5 , there was no observation of mutual isomerization between 3 and 4 . Crawford et al studied thermal isomerization in the gas phase of the parent vinyloxirane and calculated Δ H ‡ and Δ S ‡ for racemization of vinyloxirane and its isomerization to dihydrofuran as shown in Scheme 10a…”
Section: Discussionmentioning
confidence: 99%
“…Thermal or photochemical ring opening of oxiranes , or Δ 3 -1,3,4-oxadiazoline 3 and addition of a carbene/carbenoid onto the oxygen atom of a carbonyl group 4-7 have been known as traditional methods of generation of carbonyl ylides. Recently, intermolecular 4 or intramolecular reaction 5-7 of carbenes and metallocarbenoids with carbonyl compounds have been widely studied, and intermolecular or intramolecular 1,3-dipolar cycloaddition of the resultant carbonyl ylides have been utilized as a synthetic method of a oxygenated five-membered heterocycle or a complex polycyclic oxygen heterocycle. − Despite enormous studies of carbonyl ylides, some studies on vinylcarbonyl ylides via thermal ring opening of vinyloxiranes have been reported. − The thermal ring opening of oxiranes to carbonyl ylides requires high temperature, whereas the generation of carbonyl ylides via the carbenoid−carbonyl reaction can be performed under very mild conditions. A few examples of the generation of vinylcarbonyl ylides, which cyclized to dihydrofurans, from the reaction of carbenes with α,β-unsaturated ketones have been reported .…”
Rh(2)(OAc)(4)-catalyzed reaction of vinyldiazo compound 1a in the presence of p-methoxybenz- 2a, mesit- 2b, and p-chlorobenzaldehyde 2c gave a mixture of isomeric vinyloxiranes 3a-c and 4a-c, and sterically unstable (E)-dihydrofurans 5a-c, but not stable (Z)-dihydrofurans 6. However, the reactions with p-nitro- 2d and 2,4-dinitrobenzaldehyde 2e gave (Z)-dihydrofurans 6d,e along with 3d, 4d, and 5d,e. The reaction in the presence of maleic anhydride and dimethyl fumarate gave single 1,3-dipolar cycloadducts 11 and 13, respectively, indicating that a single conformer, the sterically unstable endo-aryl-endo-cyanostyryl carbonyl ylide 14 (15), is initially formed in the reaction of 1 with 2. It was concluded that the endo-vinyl-exo-aryl vinylcarbonyl ylides 17a-c arising from 2a-c undergo disrotatory cylization to exclusively produce 5, whereas the ylides 17d,e arising from 2d,e undergo partly symmetry-forbidden conrotaory cyclization to sterically stable trans-diaryldihydrofurans 6d,e as well as the symmetry-allowed process to 5d,e.
“…During the thermal isomerization the intermediates vinylcarbonyl ylides 16 ( 17 ) and 18 ( 19 ) are less reactive toward maleic anhydride and convert to the products without capturing maleic anhydride. It is well-known that sterically stable endo -vinylcarbonyl ylides such as 17 undergo disrotatory cyclization to give sterically unstable cis -dihydrofurans such as 5 . − 10 …”
Section: Discussionmentioning
confidence: 99%
“…During thermal isomerization of 3 and 4 to 5 , there was no observation of mutual isomerization between 3 and 4 . Crawford et al studied thermal isomerization in the gas phase of the parent vinyloxirane and calculated Δ H ‡ and Δ S ‡ for racemization of vinyloxirane and its isomerization to dihydrofuran as shown in Scheme 10a…”
Section: Discussionmentioning
confidence: 99%
“…Thermal or photochemical ring opening of oxiranes , or Δ 3 -1,3,4-oxadiazoline 3 and addition of a carbene/carbenoid onto the oxygen atom of a carbonyl group 4-7 have been known as traditional methods of generation of carbonyl ylides. Recently, intermolecular 4 or intramolecular reaction 5-7 of carbenes and metallocarbenoids with carbonyl compounds have been widely studied, and intermolecular or intramolecular 1,3-dipolar cycloaddition of the resultant carbonyl ylides have been utilized as a synthetic method of a oxygenated five-membered heterocycle or a complex polycyclic oxygen heterocycle. − Despite enormous studies of carbonyl ylides, some studies on vinylcarbonyl ylides via thermal ring opening of vinyloxiranes have been reported. − The thermal ring opening of oxiranes to carbonyl ylides requires high temperature, whereas the generation of carbonyl ylides via the carbenoid−carbonyl reaction can be performed under very mild conditions. A few examples of the generation of vinylcarbonyl ylides, which cyclized to dihydrofurans, from the reaction of carbenes with α,β-unsaturated ketones have been reported .…”
Rh(2)(OAc)(4)-catalyzed reaction of vinyldiazo compound 1a in the presence of p-methoxybenz- 2a, mesit- 2b, and p-chlorobenzaldehyde 2c gave a mixture of isomeric vinyloxiranes 3a-c and 4a-c, and sterically unstable (E)-dihydrofurans 5a-c, but not stable (Z)-dihydrofurans 6. However, the reactions with p-nitro- 2d and 2,4-dinitrobenzaldehyde 2e gave (Z)-dihydrofurans 6d,e along with 3d, 4d, and 5d,e. The reaction in the presence of maleic anhydride and dimethyl fumarate gave single 1,3-dipolar cycloadducts 11 and 13, respectively, indicating that a single conformer, the sterically unstable endo-aryl-endo-cyanostyryl carbonyl ylide 14 (15), is initially formed in the reaction of 1 with 2. It was concluded that the endo-vinyl-exo-aryl vinylcarbonyl ylides 17a-c arising from 2a-c undergo disrotatory cylization to exclusively produce 5, whereas the ylides 17d,e arising from 2d,e undergo partly symmetry-forbidden conrotaory cyclization to sterically stable trans-diaryldihydrofurans 6d,e as well as the symmetry-allowed process to 5d,e.
“…Scheme 1 has been suggested (1,2) to explain the racemization of (+)-2,3-divinyloxirane (1) along with its isomerization to 3-vinyl-2,3-dihydrofuran (6) and 4,5-dihydrooxepine (5). A sigmatropic (02a + n2s) process for the production of 6 was ruled out on the basis of the negligible rotation of 6 produced from (+)-I.…”
Section: Introductionmentioning
confidence: 99%
“…The carbonyl ylide 3 can be produced in more than one way in that it may be the result of a nonallowed disrotatory ring opening of 1, the racemization being favored by 5.2 kcal mol-' over the rearrangement process (1). It may also be produced by an isomerization of the carbonyl ylide 2 into the carbonyl ylide 3 by rotation about the carbon-oxygen bond of the ylide as shown in Scheme 1.…”
VESNA VUKOV and ROBERT J. CRAWFORD. Can. J. Chem. 53, 1367 (1975.The formation of vinyldihydrofuran (6) and dihydrooxepine (5) is sensitive to deuterium substitution at the terminal methylene of 2,3-divinyloxirane. Intra-and intermolecular isotope effects have been examined and suggest that 5 is produced on a competitive basis with 6 from a common intermediate.VESNA VUKOV et ROBERT J. CRAWFORD. Can. J. Chem. 53,1367 (1975.La formation du vinyldihydrofurane (6) et de la dihydrooxkpine (5) est sensible a l'introduction de deuterium dans le mkthylene terminal du vinyl-2,3-oxirane. On a determink les effets isotopiques intra-et intermolkulaires et ceux-ci suggerent que 5 est produit en competition avec 6 ii partir d'un intermediaire commun.
The first documented report of a possible Cope rearrangement was probably that of Baeyer, who prepared eucarvone by hydrobromination of carvone in 1894. Although the transformation was briefly studied at that time, it was not until the 1950s that this and other Cope‐type rearrangements received detailed attention. The thermal isomerization of
cis
‐divinylcyclopropane to cycloheptan‐1,4‐diene was reported by Vogel in 1960 during his studies of the Cope rearrangement of 1,5‐hexadienes annulated by a homologous series of carbocyclic rings. Scores of mechanistic studies followed this discovery upon the realization that the rearrangement could be related to the conceptually similar vinylcyclopropane–cyclopentene isomerization discovered a year earlier. It was also recognized that this rearrangement might be operating in the formation of cycloheptatriene from norcaradiene during photolysis of diazomethane in benzene. The topic received considerable attention in the 1960s, an era of mechanistic investigations of various concerted transformations. During the 1970s it enjoyed exploitation in many synthetic strategies, and the following decade the elements of this rearrangement were incorporated into tandem or multistep procedures in a preconceived manner. Many aspects of the various permutations of the Cope rearrangement have been previously reviewed.
The purpose of this review is to summarize the mechanistic, stereochemical, and practical results in this area in the context of the evolution of synthetic achievements during the last 40 years. Also described in this chapter are the transformations of several of the simple heteroatom permutations of this rearrangement in order to render appropriate comparisons of various systems. A discussion addresses those rearrangements of cyclopropanes, oxiranes, thiiranes, and aziridine rings substituted with vicinal vinyl groups. Excluded from this review are rearrangements of those divinylsubstituted three‐membered rings that contain more than a single heteroatom within the reacting manifold. Brief mention of these systems along with a guide to the literature is found in the last section of this chapter.
The literature is covered through December 1990. Many of the principal researchers in this field have been contacted and many unpublished transformations have been included in the tables.
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