3,3 -Bicyclopropenyl rearrangement

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. 

The isomerisation of 3,3 -bicyclopropenyls to benzene derivatives is highly exothermic and can be brought about under thermal, photochemical or metal catalysed conditions. The bicyclopropenyl (353, R = H) rearranges in the presence of silver ion to produce one regioisomer of the Dewar benzene (354) 279) ... 

In agreement.with this (353, R = CN) does not rearrange under either thermal or silver catalysed conditions, the substituent apparently stopping either step depending on the ring which is initially attacked 280). The thermal reactions are complicated by the fact that, e.g., 3,3 -dimethyl-3,3 -bicyclopropenyl is converted to two diastereo-... 

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). 

Interconversion of 3,3 -bicyclopropenyl derivatives 1 and 3 is achieved either by photochemical or thermal Cope rearrangement956,957. It is thought that these rearrangements proceed through an a h-l,4-tricyclohexylene diradical intermediate 2. Prolonged heating of 3 results in the formation of benzene derivatives 5 and 6 via the Dewar benzene 4958. 

Another useful rearrangement is that of bicyclopropenyl compounds, which provides an attractive route to Dewar benzene derivatives as formulated in equations III and IV. The ratio of the products (9) and (10) formed in the latter reaction is highly dependent on the polarity of the solvent. 

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). 

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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