Cyclopropenium cations complexes

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. 

Electrophilic attack on cyclopropenones takes place at carbonyl oxygen, as indicated by the formation of hydroxy cyclopropenium cations on protonation (see p. 28). Hydrogen-bonded complexation between the carbonyl oxygen of diphenyl cyclopropenone and the O-H hydrogen in water212 and substituted acetic acids213 is reported to give rise to well-defined 1 1-adducts (296). 

Notably, early attempts to similarly prepare cyclopropenyl complexes of group 6 molybdenum and tungsten, using [CpM(CO),] anions (M = Mo, W) and [C,(Bu-/),]BF4, resulted in the electrophilic attack of the cyclopropenium cation on the peripheral cyclopentadienyl ligand, to give hydride complexes (equation 196)270. These air-sensitive hydride complexes readily react with CC14, to afford the corresponding air-stable chloro complexes. 

The interaction of transition metal complexes with cyclopropenium cations 2C has been widely studied [48-50]. Besides the expected if (an example of if ligation has also been observed by Nixon et al. with phosphirenylium H [24 c])... 

Both the carbanion and carbocations are stable provided they contain [An+ 2) K electrons. For example, cyclopentadienyl anion, cyclopropenium cation, and tropyhum cation exhibit unusual stability. Stable carbanions do, however, exist. In 1984 Ohnstead presented the lithium crown ether salt of the diphenylmethyl carbanion from diphenylmethane, butyllithium, and 12-crown-4 at low temperatures. Addition of n-butyUithium to triphenyhnethane in THF at low temperatures followed by 12-crown-4 resulted in a red solution and the salt complex precipitated at —20°C. The central C-C bond lengths are 145 pm with the phenyl ring propelled at an average angle of 31.2° (Scheme 3.11). 

The special stability of compounds containing such bonds can be attributed to their being isoconjugate with the cyclopropenium cation and consequently aromatic. Likewise, the failure of radicals and carbanions to undergo Wagner-Meerwein rearrangements could be attributed to the fact that the intermediate n complexes would be isoconjugate with the nonaromatic cyclopropenium radical or the antiaromatic cyclopropenium anion. 

Alcarazo and coworkers prepared cyclopropenyhdenimines (azatriafulvene) 82 from cyclopropenium cation 83. The exocychc nitrogen atom in 82 exhibits all attributes of a nitrogen atom, namely, the avaUabihty of two lone pairs essentially localized on that atom. The azatriafulvene 82 reacts with [RhCl(CO)2] to form a complex 84 with CS2, it yields thioketone 85 (Scheme 6.19) [48]. 

The reaction of alkenes with Fischer carbene complexes most typically leads to cyclopropane products however, the formation of a three-membered ring product from a reaction with an alkyne has been observed on only one occasion. The reaction of the cationic iron-carbene complex (199) with 2-butyne presumably leads to the formation of the cyclopropene (200), which was unstable with respect to hydride abstraction by the starting carbene complex and the ultimate product isolated from this reaction was the cyclopropenium salt (201) and the benzyl-iron complex (202). Cyclopropene products have never been observed from Group 6 carbene complexes despite the extensive investigations of these complexes with alkynes that have been carried out since the mid 1970s. 

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