3,3 -Bicyclopropenyl

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. 

Recently, selective electrochemical trimethylsilylation of tetrachlorocyclopropene has been achieved to provide l-(trimethylsilyl)trichlorocyclopropene, which has been converted into hexakis(trimethylsilyl)-3,3 -bicyclopropenyl upon successive electrolysis (equation 86)110-112. 

The addition of a second equivalent of cyclopropene can then also occur 224). In the case of 3,3 -bicyclopropenyls (288), this provides a ready route to semibullvalenes, the epimeric intermediate cyclopropenes equilibrating and allowing intramolecular cycloaddition to occur 225). Addition also occurs to oxadiazin-6-ones 226), to 5-phenyl-... 

Similar ring expansions are observed when 3,3 -bicyclopropenyls are treated with silver ion (see section 9). Reaction of the propanoic acid derivative (352) with oxalyl chloride also leads to a three carbon ring expansion, although the detailed mechanism of the reaction is not clear 212) ... 

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-... 

A cycloaddition-cycloelimination sequence transformed 1,2,4,5-tetrazines 50 on reaction with 3,3 -bicyclopropenyl (51) into 3,4-diazanorcaradienes, which, on gentle heating in inert solvents, were transformed into semi-bullvalenes 52 <02EJO791>. 

Bicyclopropenyl (2,2) = (3 -cycloprop-l-enyl)-3-cycloprop-l-en, featuring two pairs of equivalent protons (1H one singlet and one doublet). Time-average symmetry D2h 93... 

J Bicyclopropenyl (2,2) (3 -cycloprop-1-enyl)-3-cycloprop-1-ene Billups and Haley93 Stable below 10°C, above that, decomposes to an unknown solid. 

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. 

Bullvalene and hypostrophene are members of a group of compounds all of whose formulas can be expressed by the symbol (CH)iq. " Many other members of this group are known. Similar groups of (CH) compounds exist for other even-numbered values of n . " For example, there are 20 possible (CH)g compounds, and hve possible (CH)g compounds, all of which are known benzene, prismane (p. 220), Dewar benzene (p. 1641), bicyclopropenyl, ° and benzvalene. An interesting example of a valence tautomerism is the case of 1,2,3-tri-tert-butylcyclobutadiene (p. 74). There are two isomers, both rectangular, and NMR spectra show that they exist in a dynamic equilibrium, even at — 185°C. ... 

A similar metal-complexed carbene was also involved for the isomerization and cleavage of a series of cyclopropenes in the presence of Ag(I) and of thallic and mercuric acetates . A series of Ag(I)-catalysed isomerizations of bicyclopropenyl compounds has been investigated For instance, on treatment with AgC104 in different solvents at — 20 °C, l,r-tetramethylenebicyclopropenyl (77) yielded two Dewar benzenes (78 and 79) (equation 49) . 

We conclude this section by briefly noting the immense thermal stabilization conferred upon strained molecules substituted by perfluoroalkyl groups. For example hexakis(trifluoromethyl)-3,3 -bicyclopropenyl (10) has a half-life of over 2 hours at... 

One may alternatively remove an electron from one of the n bonds of 3,3 -bicyclopropenyl, although in fact this process produces the same radical cation as before because of extensive through-bond interactions. Solution phase, photoexcited quinone-induced chemi-ionization of 3,3 -dimethyl-3,3 -bicyclopropenyl (22) results in the... 

The l,3 -bicyclopropenyl 119 affords a mixture of diphenylacetylene and enynes 218 on thermolysis (equation 74) Two pathways can account for the products. Path a involves... 

The photochemistry of l,3 -bicyclopropenyls is in marked contrast to their thermal chemistry (Section IV.B.l) 251 gives naphthalene 252 in 70% yield by a complex pathway. 

The aromatization of 3,3 bicyclopropenyls can be effected by amide ion as illustrated below ... 

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). 

Reduction of the triphenylcyclopropenium cation with zinc, magnesium and chromium salts, or by electrolysis affords hexaphenyl 3,3 -bicyclopropenyl via the triphenylcyclopro-penyl radical With lithium aluminium hydride the triarylcyclopropenium ions are reduced to the corresponding triarylcyclopropene. 

Trimethylcyclopropenyl radical, when generated by pulse radiolysis from the correspondinng cyclopropenium cation, complexes with the cation and also dimerizes to hexamethylbenzene, possibly via the 3,3 -bicyclopropenyl. 

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