Catalytic Acceptorless Dehydrogenation of Amino Alcohols and 2-Hydroxybenzyl Alcohols for Annulation Reaction under Neutral Conditions.

A base-free and acceptorless Ru-catalyzed dehydrogenative approach has been developed for the synthesis of -heterocycles by using 1,3-dicarbonyls and amino alcohols through a domino sequential enamine formation and intramolecular oxidative cyclization strategy. This unified approach is also applicable for the synthesis of O-heterocycles involving 2-hydroxybenzyl alcohol as a coupling reactant via consecutive C-alkylation and intramolecular cyclization steps. The present protocol is general for the synthesis of varieties of biologically important scaffolds, such as tetrahydro-4-indol-4-one, 3,4-dihydroacridin-1(2)-one, and tetrahydro-1-xanthen-1-ones derivatives using a single catalytic system, viz. RuHCO(PPh). Environmentally benign HO and H are the only byproducts in this domino process. Moreover, RuHCO(PPh)-catalyzed C3-alkylation of tetrahydro-4-indol-4-one using alcohol as a alkylating partner is also described in this report. For the first time, a solvent-free gram-scale reaction for the acceptorless dehydrogenative annulation has been demonstrated. A plausible mechanism for the Ru-catalyzed base-free and acceptorless dehydrogenative annulation of amino alcohols or 2-hydroxybenzyl alcohols has been provided with several experimental investigations and spectroscopic evidence.