4176-71-0Relevant articles and documents
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
Pujala, Brahmam,Rana, Shivani,Chakraborti, Asit K.
, p. 8768 - 8780 (2011)
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.
Oxidative photofragmentation of α,β-amino alcohols via single electron transfer: Cooperative reactivity of donor and acceptor ion radicals in photogenerated contact radical ion pairs
Ci, Xiaohong,Kellett, Matthew A.,Whitten, David G.
, p. 3893 - 3903 (2007/10/02)
The studies presented in this paper show that α,β-amino alcohols undergo a very clean C-C bond cleavage upon SET (single electron transfer) oxidation by photoexcited electron acceptors in a process which generally culminates in two-electron reduction of the acceptors. For a number of different α,β-amino alcohols, the oxidative fragmentation occurs in a high chemical yield (>90%), yet with low to medium quantum efficiencies (0.0001-0.1) which vary strongly depending on the properties of electron donor (D), acceptor (A), and solvent. The net quantum efficiency reflects the competition between back electron transfer and the chemical redox process. Detailed mechanistic studies were carried out to investigate the visible light induced oxidative fragmentation of α,β-amino alcohols in the presence of electron acceptors including, thioindigo (TI), 9,10-dicyanoanthracene (DCA), 2,6,9,10-tetracyanoanthracene (TCA), and 1,4-dicyanonaphthalene (DCN). Cosensitized (biphenyl) photoredox leads to free ions, A- and D+, and moderately efficient unassisted fragmentation of D+. Quenching of 1A* by electron donor (D) to give a solvent separated radical ion pair (SSRIP) leads to a very inefficient reaction. In contrast, quenching to give a contact radical ion pair (CRIP) gives a relatively efficient reaction. This reaction is sensitive to the stereochemistry of the amino alcohol, suggesting a preferred anticoplanar configuration during the C-C bond cleavage process. The critical matching of reactivity of acceptor and donor ion radicals allows a rapid reaction to occur in the relatively narrow time window between formation and decay of the contact radical ion pair.
