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 isolated by the metathesis of LiBMe4, LiBMePh3, or NaBPh4 with a series of pyridinium triflates in aqueous solution. The interionic charge-transfer (CT) interaction between the organoborate anions and the pyridinium cations is established by their characteristic charge-transfer absorption bands and X-ray crystal structures. Thermal and photochemical CT activations of the salts lead to efficient methyl transfer from tetramethylborate (or methyltriphenylborate) to the pyridinium cations to afford the various nucleophilic adducts (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 and the solid-state reactivity of the [Py+,BR4-] salts.