Zinc tetrafluoroborate hydrate as a mild catalyst for epoxide ring opening with amines: scope and limitations of metal tetrafluoroborates and applications in the synthesis of antihypertensive drugs (RS)/(R)/(S)-metoprolols

Abstract

The scope and limitations of metal tetrafluoroborates have been studied for epoxide ring-opening reaction with amines, and Zn(BF4)2·xH2O has been found to be a mild and efficient catalyst affording high yields under solvent-free conditions at rt with excellent chemo-, regio-, and stereoselectivities. The catalytic efficiency followed the order Zn(BF₄)₂·xH₂O ≫ Cu(BF₄)₂·xH₂O > Co(BF₄)₂·6H₂O ≫ Fe(BF₄)₂·6H₂O > LiBF₄ for reactions with cyclohexene oxide and Zn(BF₄)₂·xH₂O ≫ Co(BF₄)₂·6H₂O ≫ Fe(BF₄)₂·6H₂O > Cu(BF₄)₂·xH₂O for stilbene oxide, but AgBF₄ was ineffective. For reaction of styrene oxide with aniline, the metal tetrafluoroborates exhibited comparable regioselectivity (1:99–7:93) with preferential reaction at the benzylic carbon of the epoxide ring. A reversal of regioselectivity (91:1–69:31) in favor of the reaction at the terminal carbon of the epoxide ring was observed for reaction with morpholine. The regioselectivity was dependent on the electronic and steric factors of the epoxide and the pKa of the amine and independent of amine nucleophilicity. The role of the metal tetrafluoroborates is envisaged as “electrophile nucleophile dual activation” through cooperativity of coordination, charge–charge interaction, and hydrogen-bond formation that rationalizes the catalytic efficiency, substrate reactivity, and regioselectivity. The methodology was used for synthesis of cardiovascular drug metoprolol as racemic and enriched enantiomeric forms.