- Preparation of chiral 1,2,4-triazolium salts as new NHC precatalysts
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The preparation of new chiral N,N′-disubstituted 1,2,4-triazolium based NHC salt precursors, Ia′ and Ib, from phenylhydrazine and l-phenylalanine is reported. The 1,3,4-trisubstituted triazolium salt Ia′ was obtained by a stepwise ring construction from l-phenylalanine via the corresponding imino ether and acetohydrazonamide, while a heterocyclic O-/N-heteroatom exchange strategy, based on a ring-opening/ring-closure of the oxadiazolium precursor, afforded the 1,4-disubstituted 1,2,4-triazolium salt Ib. The need for two different synthetic strategies is discussed.
- Strand, Ragnhild B.,Solvang, Tina,Sperger, Christian A.,Fiksdahl, Anne
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experimental part
p. 838 - 842
(2012/09/22)
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- The enantioselective benzoin condensation promoted by chiral triazolium precatalysts: Stereochemical control via hydrogen bonding
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The design of a new class of triazolium ion precatalysts incorporating protic substituents is described. These materials promote the enantioselective benzoin condensation of a range of aromatic aldehydes (1-62% ee). Catalyst evaluation studies strongly support the involvement of hydrogen bond donation by the catalyst in the stereocentre-forming step of the catalytic cycle.
- O'Toole, Sarah E.,Connon, Stephen J.
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experimental part
p. 3584 - 3593
(2010/01/06)
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