Brønsted Acid Catalyzed α′-Functionalization of Silylenol Ethers with Indoles

Harnessing Protonated 2,2'-Bipyridinium Salts as Powerful Brønsted Acid Catalysts in Organic Reactions

It is the first time that the readily available protonated 2,2'-bipyridinium salts are used as Brønsted acid catalysts to accelerate a series of organic transformations that included the hydration of aromatic alkynes, etherification of alcohols, cyclotrimerization of aliphatic aldehydes, Ritter reaction, Mannich reaction, Biginelli reaction, preparation of substituted alkenes from alcohols, synthesis of spirooxindole, bisindolylmethane, and noncyclized tetraketone with good to excellent yields. These results strongly suggest that there exists enormous potentiality in the development of the protonated 2,2'-bipyridinium catalytic system.

Regioselective and Enantioselective Synthesis of β-Indolyl Cyclopentenamides by Chiral Anion Catalysis

The regioselective and enantioselective synthesis of β-indolyl cyclopentenamides, a versatile chiral building block, by asymmetric addition of indoles to α,β-unsaturated iminium intermediates has been achieved through chiral anion catalysis. Key to the success of this methodology is the generation of a chiral anion-paired ketone-type α,β-unsaturated iminium intermediate from α-hydroxy enamides. Preliminary mechanistic studies and DFT calculations are consistent with a mechanism involving multiple, concurrent pathways for isomerization of the initially formed azaallylcation into the key α,β-unsaturated iminium intermediate, all mediated by the phosphoric acid catalyst.

Origins of Stereoselectivity in Chiral Aminoalcohol Catalysis of Oxyallyl Cation-Indole Reactions

The enantioselective coupling of indoles with racemic α-tosyloxy ketones mediated by a chiral amino alcohol catalyst is studied with density functional theory (DFT) calculations. The addition of indole to an oxyallyl cation intrinsically favors the (S,S) and (R,R) stereoisomeric products through electrostatic interactions in the transition state. Our model shows that the enantioselectivity is controlled by the cyclohexane moiety of the catalyst; selectivity diminishes upon removal of the cyclohexane ring. Substitution to enhance the enantioselectivity of this reaction is proposed.