Alkylation of pyridinium acceptors via thermal and photoinduced electron transfer in charge-transfer salts with organoborates

Colored salts of pyridinium tetraalkylborates [Py+,BR4-] are readily isolat ed by the metathesis of LiBMe4, LiBMePh3, or NaBPh4 with a series of pyridi nium triflates in aqueous solution. The interionic charge-transfer (CT) int eraction between the organoborate anions and the pyridinium cations is esta blished by their characteristic charge-transfer absorption bands and X-ray crystal structures. Thermal and photochemical CT activations of the salts l ead to efficient methyl transfer from tetramethylborate (or methyltriphenyl borate) to the pyridinium cations to afford the various nucleophilic adduct s (Py-Me) in good yields. Contact charge-transfer ion pairs are identified as the critical intermediates in the formal nucleophilic addition and thus play important roles in determining the regiochemistry of the alkylation an d the solid-state reactivity of the [Py+,BR4-] salts.