Bicyclopropenyl Systems

Bicyclopropenyl Systems. Hexakis(trifluoromethyl)bicyclopropenyl has remarkable thermal stability (r 2 h at 360°Q and it is worthy of mention that this thermodynamically least stable benzene isomer is kinetically the most stable compound. On photolysis, however, a mixture of all five valence-bond isomers is obtained. 

The bicyclic sultines depicted by (274) undergo easy ring cleavage with loss of sulphur dioxide to yield 1,4-dienes quantitatively and in 99.5% isomeric purity. A concerted [ 2, -I- + 2J cycloreversion has been invoked. 1,6-Homodiademane 

Photochemical Reactions.—The structural and geometrical isomerizations of 1,2-diphenylcyclopropane have been subjected to detailed analysis and the photochemical (and thermal) decomposition of cyclopropanone has been examined in the vapour phase.  

The details concerning the epimerization of 6-hydroxymethylbenzobicyclo[3,l,0]-hex-2-ene have been published. Although the photoextrusion of arylcarbene is a general reaction of arylcyclopropanes, the precise structure of the substrate is clearly of importance as demonstrated by (286 X = CH2) and (286 X = CO). The Berson-Willcott rearrangement of (287) to (288) has been shown to proceed with inversion of configuration at C-7. The photochemical behaviour of azaspirene 

Photorearrangement of bridge-labelled benzvalene gives rise to scrambled dideu-teriobenzenes, which is consistent with a radical pathway. The bicyclobutane tricyclo[4,l,0,0 ]hept-4-en-3-one collapses to tropone on irradiation.  

At higher temperatures both dl- and meso-(307 R = H) are produced. On photolysis, (306 R = Ph) gives (307 R = Ph) (95%) together with l,2-dimethyl-3,4,5,6-tetraphenylbenzene, and an analogous conversion of (308) into the 1-methyl-2,3,4,5-tetraphenyl-isomer occurs in 65% yield. The photochemically generated bicyclo- 

The photoconversions of bicyclopropenyls to benzenes have also been examined in detail, and by employing (308) the intermediacy of prismanes has been discounted, leaving the precise route as a matter of speculation.  

Cyclopropanes. A new theory for high-temperature ( 1200K) decomposition of cyclopropane has been enunciated. The enthalpies of 2- and 3-carene have been determined, as has that for quadricyclane and its isomerization process to nor-bornadiene.  

The thermolysis of halogenocyclopropanes can result in ring cleavage with con- 

Kozina, V. A. Aleshina, G. L. Gal chenko, E. F. Buinova, and 1.1. Bardyshev, Vesti Akad. Navuk. 

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Bromination of bicyclopropenyl system 369 at ambient temperature in absolute CHCI3 leads either to diene 372 (15%) and trienes 374-376 (15%, 35% and 10%, respectively) when R = H, or to the stable cyclopropenium salt 371 (95%) when R = Ph (equation 134)188. The electrophilic attack of bromine on compounds 369 creates the cationoid intermediates 370 which undergo either fragmentation to salt 371 (path a) or an electrocyclic ring opening (path b). When diene 372 is heated at about 150 °C in the solid state it rearranges to 1,2,3,5-tetraphenylbenzene 373 with concomitant loss of bromine. 

Bicyclopropenyl Systems. The first examples of thermal and photochemical Cope rearrangement in the bicyclopropenyl manifold have been discovered. Mild thermolysis of (306 R = H) results in partial conversion into (307 R = H), presumed to be the d/-isomer resulting from the favoured chair transition state (Scheme 44). 

Kinetic and thermodynamic data for valence isomers have been compared [249, 260]. The bicyclopropenyl isomer (Figure 9.105) has been prepared by a carbene addition process therefore, this system is quite unique in that all of the valence isomers are known and fully characterised. 

The Ag(I)-catalysed rearrangement of 3,3 -bicyclopropenyls to the Dewar benzene system is well established For example, reaction of the annelated system 287 affords ring-fused Dewar benzenes with catalytic amounts of Ag(I). 

The silver(i)-catalysed rearrangements of the bicyclopropenyls (234) and (235) have been the subject of mechanistic studies. This rearrangement affords a useful synthesis of bridged Dewar-benzenes (236) and (237). The trimethylene-bridged Dewar-benzene (236) is stable to thermolysis at 300°C in a flow system. The penta-methylene-bridged Dewar-benzene (237) rearranges at 282 °C to benzocycloheptene and the spirocyclic compound (238). The strained benzene (239) and the biradical (240) are suggested as intermediates in the formation of benzocycloheptene and (238) from (237). 

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