Cyclopropenium salt

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 inverse functionalization of the two components for a Wittig reaction has been described by Russian authors70 who combined the cyclopropenylide 70 with aldehydes to give the unstable methylene cyclopropenes 71 characterized by protonation as cyclopropenium salts 72 ... 

Cyclopropenones can be isolated from the organic phase either by extraction or by precipitation with strong acid42,43) in the form of stable, often well-defined proto-nation products, the hydroxy cyclopropenium salts 179 ... 

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. 

Reaction of [Pd2 (PhCH=CHCOCH=CHPh)3 ] CHC1, and the cyclopropenium salt [C3Ph(p-MeOPh)2]Br produces a product which, when treated with thallium(I) acetylacetonate, yields Pd3 C3Ph(p-MeOPh)2 2(MeCOCHCOMe)2. This material, shown in Figure 16, is composed... 

Dimethyl ditellurium reacted with trichlorocyclopropenium salts3 and with tetrabromocy-clopropene4 to produce tris[methyltelluro]cyclopropenium salts. 

If the reaction temperature is kept at — 70°C (or lower), a secondary amine other than the two mentioned above affords the diaminochloro-cyclopropenium salt in the following yield (%) dimethylamine 10, diethylamine 56, di-M-propylamine 56, di-f-propylamine 69. 

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. 

Treatment of 3-chlorocyclopropenes with nucleophiles produces the substituted cyclopropenes presumably via successive addition-elimination steps . Perchlorocyclopropene gives the trisubstituted cyclopropenium salts (equation 134). 

The nucleophilic capture of the trichlorocyclopropenium ion is not restricted to aromatics. In fact treatment of tetrachlorocyclopropene with secondary amines and thiols results in tris-amino- and thio- cyclopropenium ions, e.g. 76. One advantage of such species is in their reaction with nucleophiles, e.g. HO", which deliver the corresponding sp -hybridized derivative, e.g. enone 77, by an addition-elimination sequence The transammination reaction, initially performed in good-to-excellent yields with secondary amines, often employs a trimethylsilylamine which has allowed for the synthesis of the tris-(mono-organylamino)cyclopropenium salt (76a) in almost quantitative yield. 

In accord with the relatively high basicity of cyclopropenone (pXb = - 5.2 0.3) and its derivatives (substituents at C(l) and C(2) increase the basicity alkyl more so than phenyl), these compounds are protonated in strong acids to give the corresponding hydroxy-cyclopropenium salts e.g. 321 (R = The salts generated in this manner can... 

Cyclopropenyl cation The aromatic cyclic cation C3H3+. The salts are called cyclopropenium salts. 

Fig. 1. Cyclopropenes, cyclopropenium salts, cyclopropenyl radicals, and their dinitrogen- and diphosphorus-containing analogues...

Since compounds 2Pa, 2 Pa, and 2"Pa have only been recently discovered, little is known about their reactivity. However, some interesting features are emerging, which corroborate the fact that the diphosphirenylium salts 2P are more closely related to cyclopropenium salt 2C than diphosphirenium 2 P and of course 2"P. 

While diphosphirenium 2 Pa and also cyclopropenium 2C exist as borate salts, attempts to abstract chloride from diphosphirene l Pg with sodium tetraphenylborate led to the phenyl-substituted lH-diphosphirene l Ph [25] (Scheme 15). These results clearly demonstrate that diphosphirenylium 2P are much more electrophilic than either diphosphirenium 2 P or the cyclopropenium salts 2C. 

Cyclopropenium salts could be prepared in situ from tetrachlorocyclopropene and aluminum chloride. Reaction with a 1,2-diol or 1,2-dithiol rather than a simple alcohol led to acetals or dithioacetals. 

The benzopyrilium salt 28 yields a mixture of the esters 29 and 30, and 31 (Z = and S ) are convertible into the corresponding 32. Finally, It has been shown that cyclopropenium salts 33 also react and may form mixtures of the isomeric products 34 and 35. 

A more direct route to dialkylamino-l,2,3-triazines (293) is the thermolysis of the stabilized cyclopropenium azides (292) (Scheme 115). When all the groups on the cyclopropenium salt are dialkylamino and they differ, the largest ends up at the 5-position of the resultant triazine after thermolysis the mechan-... 

Factor (c) Due to the presence of an accessible a orbital at phosphorus, compound 38 can benefit from a -aromaticity, [56] although this type of stabilization is far weaker than that resulting from 7i-aromaticity, which is present in the well known cyclopropenium salts. 

The reaction of cyclopropenium salts with sulfur ylides leads to thiophenes in moderate yields. 8) The simplest rationale for this most interesting reac-... 

It followed from Hiickel s rule that the cyclopropenium cation, derived from cyclopropene by loss of a hydride anion, should be a stabilised system, having 2 ir-electrons delocalised over three ring atoms. Realisation of this led to atteiipts to prepare examples of cyclopropenium salts and this was achieved as follows in 1957 [1] ... 

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