Nishimura T, * Takigushi Y, Hayashi T. * Kyoto University, Japan
Effect of Chiral Diene Ligands in Rhodium-Catalyzed Asymmetric Addition of Arylboronic
Acids to α,β-Unsaturated Sulfonyl Compounds.
J. Am. Chem. Soc. 2012;
134: 9086-9089
Key words
rhodium - arylboronic acids - α,β-unsaturated sulfonyls
Significance
Rh-catalyzed asymmetric addition of organoboron reagents to electron-deficient alkenes
has become a very reliable method for the synthesis of chiral carbonyl compounds (see
Review below). However, α,β-unsaturated sulfonyl compounds are prone to the competing
cine-substitution and thus far, selective addition was only possible with 2-pyridyl sulfones.
The authors report that the use of a Rh/diene catalytic system allows for the selective
addition of arylboronic acids to α,β-unsaturated sulfonyls in high yields and enantioselectivities.
Review
: G. Berthon, T. Hayashi, In Catalytic Asymmetric Conjugate Reactions; A. Córdova, Ed.; Wiley-VCH: Weinheim, 2010, 1–70.
Comment
In this present reaction, the use of the chiral diene ligand Fc-tfb* allowed for the
asymmetric addition of arylboronic acids to α,β-unsaturated sulfonyl compounds to
selectively form the addition product. Though both catalysts promote the asymmetric
addition of the arylboronic acid, the chiral diene ligand allowed faster protonation
of the alkylrhodium intermediate, whereas with the more commonly used bisphosphine
ligand BINAP, the competitive β-hydrogen elimination pathway led to mainly the cine-substitution product. This new catalytic system allowed the synthesis of a variety
of chiral sulfonyl compounds with high yields and excellent enantioselectivity, including
precursors for the modified Julia olefination (Ar = 2-benzothiazolyl).
