Isothiourea-catalyzed enantioselective addition of 4-nitrophenyl esters to iminium ions
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Isothioureas catalyze the enantioselective addition of 4-nitrophenyl esters to tetrahydroisoquinoline-derived iminium ions. 4-Nitrophenoxide, generated in situ from initial N-acylation of the isothiourea by the 4-nitrophenyl ester, is used to facilitate catalyst turnover in this reaction process. Optimization showed that 4-nitrophenyl esters give the best reactivity in this protocol over a range of alternative aryl esters, with the observed enantioselectivity markedly dependent upon the nature of the iminium counteri-on. Highest yields and enantioselectivity were obtained using iminium bromide ions generated in situ via photoredox catalysis using BrCCl3 and Ru(bpy)3Cl2 (0.5 mol%) and commercially available tetramisole (5 mol%) as the Lewis base catalyst. The scope and limitations of this procedure was developed, giving the desired β-amino amide products in up to 96% yield, 79:21 dr and ermajor (2R,1′S) 99.5:0.5.
Arokianathar , J N , Frost , A B , Slawin , A M Z , Stead , D & Smith , A D 2018 , ' Isothiourea-catalyzed enantioselective addition of 4-nitrophenyl esters to iminium ions ' , ACS Catalysis , vol. 8 , no. 2 , pp. 1153-1160 . https://doi.org/10.1021/acscatal.7b02697
Copyright © 2017 American Chemical Society. This work has been made available online in accordance with the publisher’s policies. This is the author created accepted version manuscript following peer review and as such may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1021/acscatal.7b02697
DescriptionThe authors thank AstraZeneca and the EPSRC (grant codes EP/M506631/1; J.N.A. and EP/J018139/1; A.B.F.) for funding. The European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013) ERC Grant Agreement No. 279850 is also acknowledged. A.D.S. thanks the Royal Society for a Wolfson Research Merit Award.
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