Allenylmethyl cations

Alkylideneallyl cations can be described as resonance hybrids of 1-vinyl-substituted vinyl cations and allenylmethyl cations, and thus contain two reactive sites (the sp- and sp2-hybridized carbons) for nucleophilic addition (Scheme 1) (7,2). Hybridization affects the electronic and steric character of these reaction sites. The electronic property was deduced from the l3C NMR chemical shifts of alkylideneallyl cations measured under superacidic conditions (3) and also from the charge distribution calculated (4). The charge distributions are affected by substituents on the cation the sp2 carbon is more positive than the sp carbon when two methyl groups are introduced at the sp2 carbon. 

Propargyl and Allenylmethyl Cations (Mesomeric Vinyl Cations)... 

In the early 1980s, Siehl and co-workers319,320 prepared two allenylmethyl cations 120 and 121 using the codeposition technique developed by Saunders et al.49 The cations thus prepared also exhibit extensive mesomeric vinyl cation character. 

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