Carbenium species

Forty years after the initial proposal, Sweet and Fissekis proposed a more detailed pathway involving a carbenium ion species. According to these authors the first step involved an aldol condensation between ethyl acetoacetate (6) and benzaldehyde (5) to deliver the aldol adduct 11. Subsequent dehydration of 11 furnished the key carbenium ion 12 which was in equilibrium with enone 13. Nucleophilic attack of 12 by urea then delivered ureide 14. Intramolecular cyclization produced a hemiaminal which underwent dehydration to afford dihydropyrimidinone 15. These authors demonstrated that the carbenium species was viable through synthesis. After enone 13 was synthesized, it was allowed to react with N-methyl urea to deliver the mono-N-methylated derivative of DHPM 15. 

Drawbacks as known from the Friedel-Crafts alkylation are not found for the Friedel-Crafts acylation. In some cases a decarbonylation may be observed as a side-reaction, e.g. if loss of CO from the acylium ion will lead to a stable carbenium species 8. The reaction product of the attempted acylation will then be rather an alkylated aromatic compound 9 ... 

When both 68263 (or Zn) and H are present within the catalyst, one can assume that the polar propyl carbenium species produced by reaction (3) over 63263 phase will possibly exchange with a zeolite proton through an alkyl surface migration... 

Glycosyl fluorides are excellent precursors for glycosylations. Lewis acids promote the defluorination leaving a stabilized carbenium species which is able to react with nucleophiles, e.g. N-nucleophiles. The configuration at the newly formed C-N bond is not only influenced by the sugar configuration but also by the utilized Lewis acid. This means that this is not a clear SNl-type reaction SN2-type pathways also have to be considered.15 18... 

There is considerable controversy with regard to the formation of persistent (long-lived) carbenium ions in zeolite solid acids (8). The conventional view has been that stable carbenium and even carbonium ions are readily formed from even weakly basic molecules such a propene. However, there is increasing evidence that the formation of persistent species requires a strongly basic precursor, and is thus only possible in select cases (25). Consequently, we feel that any positive claims of persistent carbenium species in zeolites warrant exceptional proof. 

In order to increase the stability of the reactive carbenium species and modify their reactivity, the effects of attachment of sulfide, pyridine, and phosphine groups on the alpha carbon have been studied (see Section II,D,3). Treatment of [Co2(/l,t, tj -HC2CH2)(CO)6]+ with the nucleophiles (nuc = SR2, PR3, and Py) affords the cationic complexes [Co2(p,rj1,ri2-HC2CH2nuc)(CO)6]+ in high yields, as shown in Eq. (9).79... 

Finally, during the mH optimization for Bl, a species of much lower energy ( 25.7 kJ mol 1 compared to 2 B0) was observed, which is the 1,6-anhydro dioxo-carbenium species 5 (see Scheme 4). The extra stability arises from the new C-O covalent bond. Neutral 1,6-anhydro species are often isolated as byproducts in... 

The carbenium ion intermediate can be stabilised with transition metals. However the resulting complex must be unstable enough to allow further reaction. This approach has important implications for the Ritter reaction, allowing the conversion of primary and secondary alcohols to stabilised carbenium species, a transformation that would otherwise be impossible. Several examples of this type of modification of the Ritter reaction exist, and notably in these complexed systems the reaction takes place with complete stereocontrol. 

Carbenium species 75 and phosphonium alkylidene 76 were obtained from the reaction mixture rather than the expected species when G2 was exposed to vinyl chlorides in benzene at room temperature, 77 (Scheme 2.31). 

Conversion of carbenium species into metallacyclobutane and similar compounds (structure A, reactions 39) are more facile [77,781 than the recombination of car-bene species as shown in reactions (38) ... 

Probably, hydride migration reactions in the intermediate propagating carbenium species occurred under these conditions. 

A novel route to tertiary alkyl iodides in good yields involves reaction of iodide ion with the tertiary chlorides under the influence of Lewis acid catalysts (such as ZnCl2), and seems to be dependent on electrophilic catalysis with in situ trapping of a carbenium species. 

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