Cations, propargyl

Some of the species most closely related to vinyl cations will be briefly considered. These are the propargyl cations and some isoelectronic analogues of vinyl cations, such as acyl cations, nitrilium cations and the imminium ions. 

Aryl cations are related to vinyl cations insofar as they are disubsti-tuted carbonium ions, although their chemistry is more related to the aromatic properties of the nucleus. This subject has recently been reviewed (Richey and Richey, 1970). 

Other disubstituted carbonium ions such as R2C+ and R2C2+ as well as monosubstituted cations, R2C==C+ and RC+ have not been suggested as relevant reaction intermediates and only for some of them mass spectral evidence has been reported. Although acetylium ions RC=C+ may be reasonably thought to be formed in the decomposition of dia-zonium precursors (Robson and Tedder, 1963) no clear evidence of their existence is as yet available. 

Propargyl cations (alkynylcarbonium ions) (172) may be viewed as the unsaturated analogues of allyl cations (173). The close relationship with vinyl cations is shown by the vinyl cation-type structure 172a 

In spite of the favourable steric arrangement, propargyl cations, although more stable than vinyl and ethyl cations, are less stable than allyl cations. This is indicated by the ionization potentials of the simple parent radicals, 8-25 and 8-16 eV (Lossing, 1963) respectively, and by the lower (by a factor of ca. 104) rate of unimolecular solvolysis in ethanol-water 4 1 of propargyl (174) than of allyl halides (175) (Burawoy and 

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Preformed Carbocationic Intermediates. Propargyl cations stabilized by hexacarbonyl dicobalt have been used to effect Friedel-Crafts alkylation of electron-rich aromatics, such as anisole, /V, /V- dim ethyl a n il in e and 1,2,4,-trimethoxybenzene (24). Intramolecular reactions have been found to be regio and stereo-selective, and have been used ia the preparatioa of derivatives of 9JT- uoreaes and dibenzofurans (25). 

In another conceptually novel [5 + 2]-process, Tanino and co-workers synthesized cycloheptene derivatives by stereoselective [5 + 2]-cycloadditions involving hexacarbonyldicobalt-acetylene complexes as the five-carbon component and enol ethers as the two-carbon component (Schemes 22 and 23).60 61 The role of the dicobalthexacarbonyl complex is to facilitate formation and reaction of the propargyl cation putatively involved as an intermediate in this reaction. The dicobalthexacarbonyl moiety can be removed using various conditions (Scheme 24) to provide alkane 60, alkene 62, and anhydride 63. 

The ratios of nucleophilic substitution versus [3,3]-sigmatropic rearrangement for the collapse of allenyl(aryl)iodine(III), generated from the reaction of aryliodanes with propargylsilanes in the presence of Bp3.0Et2 in alcohols, have been determined. The mechanism proposed by the authors ° involves the generation of propargyl cations from the allenyliodine (III) via a unimolecular pathway. 

Hexacarbonyldicobalt-stabilized propargyl cation is captured by a nucleophile. Subsequent oxidative demetallation then gives propargylated product. 

Since the first discovery of transition metal allenylidene complexes (M=G=C=C<) in 1976, " these complexes have attracted a great deal of attention as a new type of organometallic intermediates. Among a variety of such complexes, cationic ruthenium allenylidene complexes Ru =C=C=GR R, readily available by dehydration of propargylic alcohols coordinated to an unsaturated metal center, can be regarded as stabilized propargylic cation equivalents because of the extensive contribution of the ruthenium-alkynyl resonance form... 

Both cyclic and acyclic allylic cations have been produced in this way. Stable allylic cations have also been obtained by the reaction between alkyl halides, alcohols, or olefins (by hydride extraction) and SbF5 in SO2 or SOjCIF.26 Divinylmethyl cations27 are more stable than the simple allylic type, and some of these have been prepared in concentrated sulfuric acid.2 Arenium ions (p. 502) are important examples of this type. Propargyl cations (RCssCCR2 ) have also been prepared.29... 

In contrast to vinyl cations, propargyl cations 117a exist in mesomeric allenyl forms 117b, which can serve as models for vinyl cations. 

Extensive work has been carried out on these propargyl cations with a wide variety of substituents.214,315-317 Olah et al.316 found that in cation 118 the positive charge is extensively delocalized that is, the contribution of the 117b resonance form is significant. Komatsu et al.318 showed that both the a and y carbons become more shielded in ions bearing two or three phenylethynyl groups (cation 119) due to the more extended charge dispersion. 

Propargylic alcohols bearing a terminal triple bond react with electron-rich aromatic compounds in the presence of thiolate-bridged diruthenium complexes to give the propargylated aromatic compounds.30 l-Phenylprop-2-yn-l-ol, for example, reacts with 2-methylfuran to form (15). Intramolecular examples of the reaction were also reported. The process is believed to involve electrophilic attack by the ruthenium-stabilized propargyl cation. 

The carbonyl Co2(CO)6 forms stable 7i-complexes of alkynes 0/2 complexes). Four effects on alkyne reactivity are expected from this coordination (i) protection of the triple bond (ii) stabilization of the carbonium ion on the a-carbon (or propargylic cations (iii) syntheses of common and medium-size cycloalkynes and (iv) steric effects. 

The a-oriented alkynyl sugar 166, formed originally, is transformed to the /1-isomer 169 by forming the Co complex 167. Due to the stable propargylic cation of the Co complex, facile opening and closing of the dihydropyran ring in 167 yield the thermodynamically stable //-isomer 168 [43]. 

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