Only water: Phenols have been shown to undergo a ruthenium-catalyzed ortho-alkylation reaction (see scheme; Cy=cyclohexyl). Unactivated alcohols are used as the alkylating agents and water is the only by-product that is generated, making this transformation more atom-economical and greener than traditional methods.
Walton, J. W., & Williams, J. M. J. (2012). Ruthenium-catalyzed ortho-alkylation of phenols with alcohols by dehydrative coupling. Angewandte Chemie-International Edition, 51(49), 12166-12168. https://doi.org/10.1002/anie.201206246