Development of a novel phosphoramidite-selenide ligand for Pd-catalyzed asymmetric allylic substitution

Reversibility and Enantioselectivity of Palladium-Catalyzed Allylic Aminations: Ligand, Base-Additive, and Solvent Effects

The enantioselective, palladium-catalyzed reaction of benzylamine with (E)-1,3-diphenylallyl ethyl carbonate was examined with 12 different chiral ligands across a range of scaffold types. In 8 out of 12 cases, the observed enantiomeric excess was 36-92% higher when DBU or Cs2CO3 was added. Nucleophile crossover experiments between the N-benzyl-1,3-diphenylallylamine product and 4-methoxybenzylamine mechanistically linked the changes in enantioselectivity to reformation of the η3-allylpalladium intermediate. In the crossover reactions with 9 out of 12 chiral ligands, 10-75% less elimination to 1-phenylbutadiene was observed with Cs2CO3 than with DBU. Analysis of percent crossover vs percent completion of the simultaneous reaction of 1-phenyl-3-methylallyl ethyl carbonate in the crossover experiment revealed that (1) the formation of the 1,3-diphenylallylpalladium intermediate frequently occurred before the reaction of 1-phenyl-3-methylallyl ethyl carbonate was complete, (2) the addition of DBU or Cs2CO3 suppressed formation of the 1,3-diphenylallylpalladium intermediate, and (3) the less polar toluene and THF solvents resulted in less or slower formation of the 1,3-diphenylallylpalladium intermediate than the more polar DCM and DMF solvents.

Regio- and Enantioconvergent Hydroallylation of Acrylates Enabled by γ-Silyl-Substituted Allyl Acetates

Transition-metal-catalyzed asymmetric allylic substitution provides an efficient route to chiral organic molecules featuring an allyl moiety, key intermediates in the synthesis of biologically active compounds. However, the use of unsymmetrical 1,3-disubstituted allyl electrophiles has been severely constrained by the challenges of achieving both regio- and stereoselectivity simultaneously. Herein, we present γ-silyl-substituted allyl acetates as highly effective electrophiles for a regio- and enantioconvergent hydroallylation, enabling the construction of vicinal stereogenic centers. This method delivers allylated products in 44%-93% yield with 79:21 to >95:5 dr and 88% to >99% ee. Additionally, the silyl group in the products can be readily converted into other functional groups, such as acyl and aryl groups, enhancing their synthetic utility.

Application of sSPhos as a Chiral Ligand for Palladium-Catalyzed Asymmetric Allylic Alkylation

Palladium-catalyzed asymmetric allylic alkylation is a versatile method for C-C bond formation. Many established classes of chiral ligands can perform allylic alkylation reactions enantioselectively, but identification of new ligand classes remains important for future development of the field. We demonstrate that enantiopure sSPhos, a bifunctional chiral monophosphine ligand, when used as its tetrabutyl ammonium salt, is a highly effective ligand for a benchmark Pd-catalyzed allylic alkylation reaction. We explore the scope and limitations and perform experiments to probe the origin of selectivity. In contrast with reactions previously explored using enantiopure sSPhos, it appears that steric bulk around the sulfonate group is responsible for the high enantioselectivity in this case, rather than attractive noncovalent interactions.

Axially chiral N-alkyl-N-cinnamoyl amide type P,olefin ligands for Pd-catalyzed reactions

We synthesized N-alkyl-N-cinnamoyl amide type phosphine-olefin compounds 1 and found axial chirality in a C(aryl)-N(amide) bond in compounds 1 by HPLC analysis using a chiral stationary phase column. We successfully obtained enantiomeric isomers of 1 and demonstrated the use of (-)-1 for chiral ligands in Pd-catalyzed asymmetric allylic substitution reactions of allylic esters with indoles (up to 97% ee).