3- Propynyl cation

Silver trifluoroacetate is a suitable catalyst for various cationic rearrangements involving multiple carbon-carbon bonds [49 5(1] In the presence of silver trifluoroacetate, 2 propynyl acetates rearrange to the butadienyl acetates to give dienes that are useful in Diels-Alder reactions [49] (equation 22)... 

Butatrienyl cations 61 were also implied in the solvolysis of the w-propynyl vinyl triflate 6243. The vinyl triflate solvolyzes 35-70 times faster than the corresponding simple vinyl analogues and gives products resulting mainly from the a-alkynyl vinyl cation 63. A small amount of the allenic ketone 64 derived from the butatrienyl cation is also detected (equation 22). 

Alkynyliodonium triflates exhibit two distinct modes of reactivity with bis(triph-enylphosphine)ethyleneplatinum(O)113. When (terf-butylethynyl)phenyliodonium triflate is mixed with the Pt(0) complex in degassed dichloromethane, ethylene insertion occurs, and a cationic 3-propargylplatinum(II) triflate is obtained (equation 147). Employment of l-propynyl(phenyl)iodonium triflate in degassed toluene, on the other hand, leads to a square planar cr-propynylplatinum(II) complex. 

An extension of this work has resulted in the synthesis of related 6,6-disubsti-tuted (bipyridyl) ligand derivatives containing pyromellitimide spacers connected with propynyl, butynyl and propynyloxy linkers.The first of these ligands was shown to yield a chiral di-zinc(II) cationic complex of type [Zn2L2] . This species is stabilised by the presence of an aromatic guest p- or o-dimethoxybenzene) in its cavity. Related dinuclear species were also shown to form with cadmium(II) and nickel(II). 

As with alkenes, alkynes show a strong peak for the molecular ion. Their fragmentation patterns are also similar to those of alkenes. One of the most prominent peaks in the mass spectrum of most alkynes is from the delocalization stabilized 3-propynyl (propargyl) cation miz 39) or a substituted propargyl cation. 

The Nicholas reaction [103] has been successfully exploited for a novel glyco-sylation reaction based on the internal delivery concept (Scheme 31a). The suitably O-protected glycop3rranoside derivative 111 bearing the cobalt-complexed 4-alkoxy-6-phenyl-5-hexynoate residue at the anomeric position affords the corresponding propynyl cation 112 under Lewis acid catalysis. The intermediate 112 collapses spontaneously to form the oxoniiun ion delivering the cobalt-complexed y-lactone 113. The oxocarbenium cation is then intramolecularly captured by its counter alkoxy anion leading to glycoside 114. 

MeAlCl2 has been used as the catalyst for epoxide-initiated cation-alkene cyclizations. A tertiary alcohol has been converted to a hydrazide by reaction with MeAlC in the presence of MesitylenesulfonyUiydroTide. a-Trimethylsilyl enones can be prepared by isomerization of l-(trimethylsilyl)-2-propynyl trimethylsilyl ethers with MeAICl2. ... 

Cobalt-Complexed Propargyl Cations and Their Reactions. The stability and synthetic utility of hexacarbonyldicobalt complexed cations was first reported in 1971. They are readily formed by protonation of propargyl alcohol complexes or addition of electrophiles (protons, or alkyl or acyl cations) to vinylacetylene complexes. They can be isolated in pure form, usually by precipitation as hexafluorophosphates or tetrafluoroborates, but are more commonly generated in situ and used directly. Most of the early work is due to Nicholas, who has comprehensively reviewed the work up to 1986. A detailed description of the procedure for generating the l-methyl-2-propynyl complex and for its reaction with trimethylsilyloxycyclohexene (eq 51) has been given. ... 

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