d-Camphor-derived triazolium salts for catalytic intramolecular crossed aldehyde–ketone benzoin reactions

Abstract

A series of triazolium salts has been synthesized from d-camphor and found to be efficient catalysts for intramolecular crossed aldehyde-ketone benzoin reactions, affording alpha-ketols bearing a quaternary carbon center with up to 93% ee.

Asymmetric Organocatalysis

“…You and co-workers synthesized a new triazolium salt from d-camphor (Scheme 82). The N-heterocyclic carbene catalyst 245a afforded an intramolecular crossed aldehyde-ketone benzoin reaction to give a-ketols containing a quaternary stereogenic center in high yields and with up to 93% ee [181]. The N-heterocyclic carbene catalyst 245b showed high efficiency in catalyzing intramolecular Michael reaction [182].…”

An Iridium(I) N‐Heterocyclic Carbene Complex Catalyzes Asymmetric Intramolecular Allylic Amination Reactions

“…Ther esults of examining NHCs derived from different triazolium salts are summarized as shown. To our delight, in the presence of 5mol %o f[ {Ir(cod)Cl} 2 ], 10 mol %ofL1, [17] and 10 mol %ofDBU, thereaction of 1ain CH 2 Cl 2 at room temperature afforded the desired allylic amination product 2a in 75 %y ield and 92 % ee without the observation of Friedel-Crafts alkylation reaction at C3 of the indole.O nly trace amounts of 2a were formed with the amino-indanol-derived triazolium salt L2. [18] Ther eaction proceeded smoothly with the (1R,2R)-DPEN-derived triazolium salt L3, [19] however,t he asymmetric induction was negligible.T he ee value was increased to 76 %a nd 89 % with L4 [20] and L5, [21] respectively,a st he chiral ligand precursor.W hen the l-t-butylalaninol-derived triazolium salt L6,i ntroduced by the group of Enders, [22] was used, the reaction afforded 2a in 82 %y ield and with 99 % ee.N o deleterious effect of yield and enantioselectivity was observed with 2.5 mol %of[{Ir(cod)Cl} 2 ].…”

Direct Catalytic Addition of Alkylnitriles to Aldehydes by Transition‐Metal/NHC Complexes

“…Preliminary screening of chiral Rh I /NHC complex derived from [{Rh(OMe)-(cod)} 2 ], triazolium salts L3-11, and DBU is summarized in Scheme 1. [19] 10 mol % of Rh/L3 complex promoted the reaction of acetonitrile (1 a) and aldehyde 2 c at 0 8C to furnish enantioenriched product 3 ac in 38 % ee. Replacing the pendant phenyl substituent on the NHC with the 1-naphthyl group retarded the reaction, which proceeded slowly at room temperature with lower enantioselectivity (L4).…”