1,2,4-Cyclohexatrienes. cycloaddition

Formation of 7-hetera- and 7,8-diheterabicyclo[2.2.2]octa-2,5-dienes using synthetic equivalents of cyclohexatriene in [4- -2] cycloaddition reactions 97SF1327. 

Cossu S., Fabris F., De Lucchi O. Synthetic Equivalents of Cyclohexatriene in [4 + 2 Cycloaddition Reactions. Methods for Preparing Cycloaddncts to Benzene. Synlett W1 1327-1334... 

Danheiser et al. [133] reported a variety of intramolecular [4 + 2]-cycloadditions of a butenyne subunit with a remote acetylene moiety by thermolysis of the substrates, with the best yields being obtained in the presence of phenolic additives. Two examples are presented in Scheme 6.49. Of particular significance with regard to synthetic utility is the observation that protic and Lewis acids were powerful promoters of these reactions. The intermediacy of 1,2,4-cyclohexatriene derivatives, as shown in Scheme 6.49, is highly likely, at least in the non-catalyzed cases. 

Intermolecular [4+2]-cycloaddition of vinylallenes with alkynes is efficiently mediated by means of an electronically tuned rhodium catalyst (Scheme 16.81) [91]. A five-membered rhodacycle is formed from the vinylallene. Coordination followed by insertion of an alkyne to the rhodacycle generates a seven-membered rhodacycle, from which rhodium(I) is eliminated reductively to produce a cyclohexatriene, leading to the aromatic compound. 

Cyclic allenes have improved reactivity due to ring strain. The cycloaddition of 1,2,4-cyclohexatriene (28) with styrene (29), for example, afforded exclusively cyclobutane 31 (equation 9)18. Semi-empirical calculations (AMI) determined the diradical intermediate 30 to be at an energy minimum19. 

The thermal reaction of 3,5-bis(methoxycarbonyl)-4-oxo-4/f-pyrazole 1,2-dioxide (5) with 1,3,5-cyclohexatriene (6) yielded a mixture of endo-4 + 6-cycloadduct (7), an anti-exo-4-I-2-cycloadduct (8), and a yn-heterocage (10), derived from the intramolecular 1,3-dipolar cycloaddition reaction of the syn-endo-4 + 2-cycloadduct (9) (Scheme 3). ... 

In vinylallenes with a pendant phenyl or vinyl substituent eight-membered ring carbo-cyclic byproducts are formed by a head-to-head [4+4] cycloaddition From 1,2,4-cyclohexatriene and 2,5-dimethylfuran or fiiran, [4+2] cycloadducts are also obtained. Similarly, trapping of the strained allene 366 with diphenylbenzofuran affords the two isomeric [4+2] cycloadducts 367 and 368 in low yields... 

Benzene is not a cyclohexatriene but a delocalized cyclic system of six it electrons. It is a regular hexagon of six sp -hybridized carbons. All six p orbitals overlap equally with their neighbors. Its unusually low heat of hydrogenation indicates a resonance energy or aromaticity of about 30 kcal mol (126 kJ mol ). The stability imparted by aromatic delocalization is also evident in the transition state of some reactions, such as the Diels-Alder cycloaddition and ozonolysis. 

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