Competition of C–H and C–O fragmentation in substituted p-methoxybenzyl ether radical cations generated by photosensitized oxidation

Abstract

Laser and steady-state photolysis, sensitized by 2,4,6-triphenylpyrylium tetrafluoroborate (TPP⁺BF₄⁻), of 4-methoxybenzyl ethers [4-CH₃O-C₆H₄CH(OR¹)R, 1a: R = H, R¹ = CH₃; 1b: R = H, R¹ = C(CH₃)₃; 1c: R = CH₃, R¹ = C(CH₃)₃; 1d: R = 4-CH₃O-C₆H₄, R¹ = CH₃] was carried out in CH₃CN in the presence of oxygen. In particular, steady-state irradiation of 1a, b and d produced benzylic alcohols (together with a small amount of acetamide for 1a and 1b) and the oxidation carbonyl compounds (esters and ketones); 4-methoxy-α-methylbenzyl alcohol was the only product observed with 1c. Time-resolved investigations of 1c and 1d gave evidence of the intermediate benzylic carbocation, coming from the ether radical cation formed within the laser pulse by an electron transfer process from the ether to the TPP⁺ excited state. These results suggested that, besides the deprotonation of the benzyl carbon, the cleavage of the C–OR¹ bond is also operative in the reaction pathways of the ether radical cation. The comparison of these results with those obtained in the photolysis of 1c with the uncharged 9,10-dicyanoanthracene (DCA) upholds a particular behavior of TPP⁺BF₄⁻, probably due to the specific electrostatic interactions of BF₄⁻ with the radical cation. This is also supported by quantum mechanical calculation results performed at the B3LYP/6-31G(d) level to obtain the charge and spin density distributions of radical cations, free and complexed with BF₄⁻.