Arylmethyl cations

Hydride abstraction from tertiary amines by arylmethyl cations leads to iminium ions which can be hydrolyzed or trapped with nucleophiles. For example, r-butyldimethylamine reacts with triphenylmethyl perchlorate to give a 93% yield of the iminium salt. This can be trapped with acetophenone to give Ae Mannich product (Scheme 1). 

The photochemical generation of arylmethyl cations from arylmethanols has been the topic of several papers this year. For mono-aryImethanols in which the aromatic group is phenyl, naphthyl, pyrenyl or phenanthryl, Wan has reported that ultra-violet light irradiation in aqueous alcoholic sulphuric acid solution yields the corresponding ether,presumably by way of an intermediate arylmethyl carbocation. Photolysis of (414) in aqueous alcohol to give (415) is found to proceed even in the absence of acid, while the non-cyclic analogue (416) reacts much less efficiently, even at low values of pH. This reactivity difference has been observed previously for fluorenol and benzhydrol and is attributed to extra stabilisation of the intermediate carbocation in the fluorenyl system. 

The discussion so far has treated only those reaction intermediates that can be formed from a parent unsubstituted PAH. We also wish to present the treatment for cases in which a delocalized it intermediate is formed from methyl-substituted derivatives. For example, in considering the formation of an arylmethyl cation, thermocyclic arguments (5, 6) have to be implemented to estimate the difference in Ec>(0 between the arylmethyl cation and the fragments before the union, that is, the neutral even PAH and the CH2 + ion. The localized positive charge of the CH2+ ion and the condition qr = 0 in the neutral even PAH leads to the following ... 

Thus, the PMO Fw and PMO-w methods correlate the formation of an arylmethyl cation with the following ... 

Two major contributions to the 7t-energy change in the ring-cleavage reaction leading to a PAHTC need to be included in the model. In a simplified Pariser-Parr-Pople (PPP) method with Hiickel orbitals as the basis [33], the PAHTC is an arylmethyl cation, with resonance energy... 

Yrs are two-centre two-electron repulsion energies. For aromatic ions, the onsite electron pair densities, i.e., the diagonal elements of the spinless second order density matrix are lower than those of any classical structure. For radicals, however, the pair densities are increased relative to those in the corresponding classical structures. Thus the resonance energy of the ion exceeds that of the radical by 21 Ec I [33]. FIMO and PMO do not differentiate between the formation of an arylmethyl radical and its carbocation. Empirically, radicals are better described by these methods this has been related to the constant Coulson charge order Q = 1 for arylmethyl radicals as opposed to the variable rt-charge order 1 on arylmethyl cations [16,39]. The actual PPP values of Ec have been correlated to an excellent accuracy to their PMO-o) counterparts [16]. 

Zhang, J., K. A. Connery, J. F. Brennecke, and J. E. Chateauneuf 1996, Pulse radiolysis investigations of solvation effects on arylmethyl cation reactivity in supercritical fluids . J. Phys. Chem. 100, 12394. 

As a Carbon Nucleophile in Lewis Acid-Catalyzed Reactions. Allyltrimethylsilane is an alkene some 10 times more nucleophilic than propene, as judged by its reactions with di-arylmethyl cations. It reacts with a variety of cationic carbon electrophiles, usually prepared by coordination of a Lewis acid to a functional group, but also by chemical or electrochemical oxidation, or by irradiation in the presence of 9,10-dicyanoanthracene. The electrophile attacks the terminal alkenic... 

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