Trityl cation, conjugation

Fig. 10 Plot of E parameters against open circles, benzhydryl cations fdled circles, trityl cations squares, organometallic cations fdled triangles, tropylium ions open triangles, flavylium, xanthylium and other O-, S-, or N-conjugated cations.

A master equation has been derived from both the pA R of trimethyl- and triphenylcyclo-propenylium ions, as well as of the trityl cation, and the pA of conjugate acids of various carbanions, so that the heat of reaction can be predicted for the covalent bond formation between carbocations and carbanions. ... 

An olefinic bonds conjugated with the 1 position of a diene complex is highly reactive toward electrophilic reagents because of the stability of the resulting complexed cation. These reagents include not only protons but the trityl cation and bromine. ... 

As predicted from the pKr+ values (Table 7.4) the S-di-p-methoxybenzhydryl group [18] is cleaved much more rapidly than other S-benzhydryl thioethers. Konig et al. [55] report complete removal in 2 hr with trifluoroacetic acid (70 ) or in 10 min with added anisole. Photaki et al. [58] report 80% removal in 2 hr (20-25°) with trifluoroacetic acid and 15% phenol. The pKr value of this cation predicts it to be more stable than the S-trityl cation which in turn su ests a more rapid conversion of the conjugate acid of the thioether to the carbonium ion and thiol and a selectivity comparable with trityl in the reaction with nucleophiles. This prediction is not entirely borne out by facts and indeed the S-di-p-methoxybenzhydryl group falls between the benzhydryl and trityl in reactivity. This lowered reactivity is probably due to the presence of significant concentrations of the less reactive dication. 

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