Toward Catalytic, Enantioselective Chlorolactonization of 1,2-Disubstituted Styrenyl Carboxylic Acids.

An investigation into the use of Lewis base catalysis for the enantioselective chlorolactonization of 1,2-disubstituted alkenoic acids is described. Two mechanistically distinct reaction pathways for catalytic chlorolactonization have been identified. Mechanistic investigation revealed that tertiary amines predominately operate as Brønsted rather than Lewis bases.

Enantioselective construction of quaternary stereogenic carbon atoms by the Lewis base catalyzed additions of silyl ketene imines to aldehydes.

Silyl ketene imines derived from a variety of α-branched nitriles have been developed as highly useful reagents for the construction of quaternary stereogenic centers via the aldol addition reaction. In the presence of SiCl4 and the catalytic action of a chiral phosphoramide, silyl ketene imines undergo extremely rapid and high yielding addition to a wide variety of aromatic aldehydes with excellent diastereo- and enantioselectivity. Of particular note are the high yields and selectivities obtained from electron-rich, electron-poor, and hindered aldehydes.

Development and mechanism of an enantioselective bromocycloetherification reaction via Lewis base/chiral Brønsted acid cooperative catalysis.

The development of a binary catalyst system for bromocycloetherification, consisting of an achiral Lewis base and a chiral Brønsted acid, is described in detail. The results of preliminary kinetic studies are also presented.

Catalytic, asymmetric halofunctionalization of alkenes--a critical perspective.

Despite the fact that halogenation of alkenes has been known for centuries, enantioselective variants of this reaction have only recently been developed. In the past three years, catalytic enantioselective versions of halofunctionalizations with the four common halogens have appeared and although important breakthroughs, they represent just the very beginnings of a nascent field. This Minireview provides a critical analysis of the challenges that accompany the development of general and highly enantioselective halofunctionalization reactions.

Enantioselective bromocycloetherification by Lewis base/chiral Brønsted acid cooperative catalysis.

A binary catalyst system for the enantioselective bromocycloetherification of 5-arylpentenols is described. The combination of an achiral Lewis base and a chiral Brønsted acid affords good enantioselectivities for the cyclization of Z configured 5-arylpentenols to form bromomethyltetrahydrofurans. The constitutional site selectivity is highly dependent upon the aromatic substituent and the configuration of the double bond.

Lewis base catalysis of bromo- and iodolactonization, and cycloetherification.

Lewis base catalyzed bromo- and iodolactonization reactions have been developed and the effects of catalyst structure on rate and cyclization selectivity have been systematically explored. The effects of substrate structure on halolactonization reactions and the interaction of those effects with the effects of catalyst structure have been investigated, leading to synthetically useful improvements in cyclization selectivity. The knowledge acquired was applied to the development of Lewis base catalyzed bromo- and iodocycloetherification reactions.

On the absolute configurational stability of bromonium and chloronium ions.

Halonium ions have long been established as the critical intermediates in halogenation and halofunctionalization of alkenes. Although these workhorse reactions have been extensively studied mechanistically and employed synthetically, the paucity of enantioselective variants is striking. A central problem in the development of catalytic enantioselective halofunctionalizations is the reversible formation of halonium ions and the facile olefin-to-olefin transfer.

Enantioselective construction of quaternary stereogenic carbons by the Lewis base catalyzed additions of silyl ketene imines to aldehydes.

Silyl ketene imines derived from a variety of alpha-branched nitriles have been developed as highly useful reagents for the construction of quaternary stereogenic centers via the aldol addition reaction. In the presence of SiCl4 and the catalytic action of chiral phosphoramide (R,R)-5, silyl ketene imines undergo extremely rapid and high yielding addition to a wide variety of aromatic aldehydes with excellent diastereo- and enantioselectivity. Of particular note is the high yields and selectivities obtained from electron-rich, electron-poor, and hindered aldehydes.