Hafnium Triflate as an Efficient Catalyst for Direct Friedel–Crafts Reactions of Chromene Hemiacetals

The direct Friedel–Crafts reaction of chromene hemiacetals with indoles, furans and sterically hindered anilines has been accomplished with high selectivity and excellent yields in the presence of a catalytic amount of hafnium triflate [Hf(OTf)₄, 0.1 mol%, 0–40 °C]. The mild conditions tolerate various sensitive functional and protecting groups, and the products were confirmed unambiguously from their spectra and by single‐crystal X‐ray analysis. This direct Friedel–Crafts reaction of chromene hemiacetals should inspire and encourage the consideration of hafnium triflate in the development of mild reaction conditions for the efficient derivatization of hemiacetal‐containing compounds.     

Concerted mechanisms of acid-base catalysis in coordination sphere of metal complexes

Kinetics and mechanisms of the following reactions in organic solvents have been studied: (i) formation of Schiff bases from aniline and derivatives of salicylic aldehyde coordinated to Cu(II) and Zn(II); (ii) substitution of one -diketonate ligand coordinated to Fe(III) by another -diketonate ligand; (iii) formation of coordinated hemiacetals from -diketonate ligands and alcohols in coordination sphere of Fe(III); and (iv) formation of urethanes in coordination sphere of Fe(III) (acac)₂ OR where acac and OR are acetylacetonate and alkoxide anions. Such compounds as aniline, alcohols and phenylacetylene serve as catalysts of these reactions. Their catalytic action is explained by concerted processes of hydrogen atom transfer in cyclic intermediate structures.Presented in part at the Royal Society-U.S.S.R. Academy Workshop on Catalysis, Oxford, April 1989.