Tetramethylenehalonium ions

Tetramethylenehalonium Ions. 1,4-Halogen participation was first postulated to occur in the acetolysis of 4-iodo- and 4-bromo-l-butyl tosylates.402 In subsequent studies, Peterson and coworkers found anomalous rates in the addition of trifluoroacetic acid to 5-halo-l-hexenes403,404 and 5-halo-1-pentynes.405 406 Such observations were recognized as due to 1,4-halogen participation.341 Also, a study of the solvolysis of 8-chloroalkyl tosylates indicated rate-accelerating 1,4-chlorine participation effects up to 99-fold.407... 

Direct experimental evidence for 1,4-halogen participation comes from the direct observation (by NMR spectroscopy) of five-membered-ring tetramethylenehalonium ions by Olah and Peterson408 and by Olah et al.400... 

Similarly, several 2- and 2,5-substituted tetramethylenehalonium ions (174-178) have been prepared and studied by both 1H and 13C NMR spectroscopy.99 410 411... 

Quantitative Scales of Solvent Nucleophilicity. Solvolytic studies in solvents of low nucleophilicity led to renewed interest in quantitative measures of solvent nucleophilicity. Peterson and Waller (44) derived a scale of solvent nucleophilicity (Npw) from the rates of displacement by solvent of tetramethylenehalonium ions (VI) in liquid sulfur dioxide. The reaction is approximately half-order in carboxylic acid, possibly because dimer-monomer preequilibrium occurs (44). More recently, hydrolysis of the iodonium salt (VIII) in competition with anionic or solvent nucleophiles was studied. A scale of nucleophilicity relative to water was obtained by quan-... 

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See also in sourсe #XX -- [ , ]

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