Stable carbonium ions. XXXIII. Primary alkoxycarbonium ions

First examples of stable polyfluorinated bis- and tris-(alkoxy)methyl cations

“…Despite the fact that a large number of stable, long-lived polyfluorinated cations have been observed and characterized [4], the known examples of stable fluorinated alkoxy cations are limited to CH 3 OCFH + (generated by ionization of CH 3 OCF 2 H by SbF 5 ) [5,6], CF 3 (CH 3 )COCH 3 + , CF 3 C(OCH 3 ) 2 + (prepared by methylation of the corresponding esters) [7] and R f CH 2 OCFX + (X = F, H, fluoroalkyl, generated by reaction of R f CH 2 OCF 2 X with SbF 5 ) [8].…”

“…The generation of fluorinated carbocations from a neutral precursor usually results in a substantial downfield shift of the resonance of fluorine substituent directly connected to the positively charged carbon (Dd F = 50-300 ppm, 2 depending on the type of carbocation and the stabilizing effect of the group connected to the carbocationic center) [1][2][3][4][5]12,13]. Although values of Dd observed for primary alkoxycarbonium ions ROCFX + are in general smaller than other fluorinated cations, due to the strong ''back-donation'' effect of oxygen, which effectively reduces the positive charge of the carbocationic center, they are still substantial.…”

Recent Advances in NMR Studies of Carbohydrates

“…Recently, Martin and coworkers [132] presented the low-temperature study on glycosyl cations obtained by methodology presented by Olah [133]. To avoided potential problems associated with the presence of a substituent at C2, they focused their efforts on peracetylated 2-deoxy-β-Dglucopyranose.…”

Acylation of aromatic substrates with ketenes. An example of vinyl oxocation reactivity

“…T h e R R F for tolyl ketonelphenyl ketone was 1.3250. All competitions were done by comparing the rates of reaction with toluene for experimental convenience (12). The rate constant ratios were determined by the formula In all cases the substrate ratios to ketene were large enough to ensure small errors in the rate ratios.…”

“…One might rationalize this greater substrate selectivity on the basis of structure 1 in which a mode of delocalization of charge is shown for the substituted ketene - aluminum chloride complex which is not available for the analogous vinyl cation. Analogous "resonance" arguments are advanced for the known stability of carbocations from a-haloethers (12).…”