Alkenoyl cations

The structure of alkenoyl cations (unsaturated acylium ions) was studied by Olah et al.658 by NMR spectroscopic methods. They found only a limited contribution from structure 343b and a substantial contribution of the delocalized ketene-like structure 343c, which is due to the ability of the Jt-electrons of the double bond to stabilize the positive charge. Substitution at the (3-carbon increases further the importance of 343c relative to 343a. Diprotonation of propenoyl and isopentenoyl cations studied theoretically [ab initio GIAO-CCSD(T) method]659 has been shown to result in the formation of dication 344 (tertiary carbenium-acylium dication) that is, the positive charges are localized primarily on CO and the (3-carbon. 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

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