Probing the Nature and Extent of Stabilization within Foiled Carbenes: Homoallylic Participation by a Neighboring Cyclopropane Ring

Abstract

Oxadiazoline 6 was synthesized to generate endo-tricyclo[3.2.1.0(2,4)]octan-8-ylidene (3) by either photolysis or thermolysis. Diastereomer 6a thermally decomposed twice as fast as 6b. Carbene 3 was trapped stereoselectively by acrylonitrile and diethylamine in high yields. It behaved as a nucleophilic carbene with electron-poor alkenes, like acrylonitrile, but as an electrophile with very electron-rich species, such as diethylamine. However, when the reactions were performed in cyclohexane and cyclohexene, isomerization of 3 was favored. Replacement of the double bond in 7-norbornenylidene (1) by the single bond in the endo-fused cyclopropane unit of carbene 3 led to similar outcomes. Carbene 3 rightfully belongs to the family of foiled carbenes.

Intermolecular Reactions of a Foiled Carbene with Carbonyl Compounds: The Effects of Trishomocyclopropyl Stabilization

“…[18][19][20] Computational chemistry is an indispensable method to study shortlived reaction intermediates. 21 It has provided insight into the nature of foiled carbenes, [22][23][24] such 4 as endo-tricyclo[3.2.1.0 2,4 ]oct-8-ylidene (1) (Figure 2a) [25][26][27][28] and bicyclo[2.2.1]hept-2-en-7-ylidene (2) (Figure 2b). 29,30 Intramolecular carbene cycloaddition is foiled because the product would be greatly strained by its inverted C atom.…”

“…32,33 A similar situation exists in carbene 1 (Figure 2a). 25 It is a homologue of carbene 2 wherein the vinylene group of 2 has been replaced by a fused endocyclic cyclopropane (cf. 1 in Scheme 1b), but not an exocyclic one (cf.…”

“…45 Again, carbenoid chemistry is suspected because the product had a syn-Br configuration. More definitive studies involving the intermolecular reactions of foiled carbenes with alkenes 25,48-50 and alcohols 25,51,52 indicate that they undoubtedly exhibit nucleophilic behavior. 53 Carbene nucleophilicity is achieved when an adjacent π-donor atom infuses electron density into the bicoordinate C atom's p-orbital.…”

“…The diastereomeric precursors r-10 and s-10 were used separately herein to generate carbene 1(Figure 4). They were dissolved in either PhC(O)H or PhC(O)Me and heated to T = 165 °C until fully decomposed (Scheme 3) 25. …”

“…Isotherm: 270 °C for 3 min.Materials. Oxadiazolines r-10 and s-10 were prepared and identified according to published methods 25. Freshly distilled benzaldehyde (PhC(O)H) and acetophenone (PhC(O)Me) were dried over active 4-Å molecular sieves prior to use.…”