Cyclopropenium complexes

A carbon-iron bond is also formed by the reaction of the cyclopropenium salt 185 with dicarbonyl(i/5-cyclopentadienyl)(trimethylsilyl)iron [92], (Scheme 69) In the reaction with benzocyclobutenylidene- 5-cyclopentadienyliron(II) hexafluorophosphate 186, CpFe(CO)2R (R=cyelo-C3H5, CH2-cyclo-C3H5) is converted to the allene and butadiene complexes, 187 and 188, respectively [93]. (Scheme 70)... 

The reaction mechanisms have been clarified in some detail7In method (a) a complex sequence starts with the acetoxy cyclopropenium ion 126 and the cyclo-propenyl acetate 127 and finally leads to adducts 128 containing two moles of arylmalononitrile, which were isolated and shown to be the preferential precursors of quinocyclopropenes. In method (b) the ambivalent arylmalononitrile anion102) is reversibly attacked at the benzylic position at low temperatures, whilst at higher temperature (after dissociation of 114) attack at the o- and p-positions of the... 

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. 

A M-N M = Ti, Zr, Hf Formed from bis-alkynyl complexes, products contain peripheral cyclopropenium rings as well as heterorings 1999JOM115... 

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. 

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. 

Bis-(dialkylamino)cyclopropenylidenes, prepared by deprotonation of the corresponding cyclopropenium ions, have been studied spectroscopically and complexed with metal atoms. (For indirect methods of preparation of cyclopropenylidene metal complexes, see Section V.A). 

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). 

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