Intramolecular [4+2] Cycloaddition Reactions of Conjugated Enynes

Development of New Cycloaddition Reactions Based on the Unique Reactivity of Unsaturated Hydrocarbons

“…19) A recent interesting study reported by Nakao, Shirakawa, Hiyama, and coworkers showed that a stannylative benzannulation occurred by the Pd-catalyzed reaction of enynes (or enynes and diynes) with hexabutyldistannoxane (Chart 11). 20) Since it has been postulated that the cyclic allene (1,2,4-cyclohexatriene) was converted to the benzene ring or reacted as a nucleophile, 21,22) their results (stannylative benzannulation), combined with our results (benzannulation), might indicate the intermediacy of a strained allene such as 20 (Chart 12). The reaction was further expanded by Gevorgyan, Yamamoto, and co-workers to the corss-benzannulation reaction between conjugated enynes and diynes.…”

Ab initio study of the mechanisms of intermolecular and intramolecular [4?+?2] cycloaddition reactions of conjugated enynes

“…According to experimental data, non-catalyzed thermal [4 2] enyne cycloaddition reaction is usually carried out in toluene solution. 1,3 The PCM approximation [18][19][20] was used to model toluene surrounding and, in addition to the former, the calculations for three other common solvents were performed [chloroform, dimethyl sulfoxide (DMSO) and water]. This may allow one not only to simulate exact experimental conditions, but also to estimate effect of the medium on the mechanism of the reaction in more detail and to make some predictions.…”

“…Recently, a wide class of [4 2] cycloaddition reactions was enriched with a new example, where enynes and diynes may be used as four-electron donors [1][2][3] . The simplest reactions of this type are depicted in Scheme 1.…”

“…Various substituted enynes were shown to undergo intramolecular cycloaddition reactions providing a new pathway to bicyclic products. [1][2][3][4] The proposed mechanism of the reaction is interesting. The six-membered allene (1,2,4-cyclohexatriene and its derivatives) were suggested as reaction intermediates (Scheme 1).…”

“…The energetic plausibility of cycloaddition reactions leading to allene intermediates was investigated at the MP4//MP2 level of theory, 2 and particularly for 1,2,4-cyclohexatriene the reaction was found to be exothermic, DG = À13.4 kcal mol À1 ; this value is half that À29.7 kcal mol À1 , estimated earlier. 1 …”