Mechanism of enantioselective Ti-catalyzed Strecker reaction: peptide-based metal complexes as bifunctional catalysts.

Kinetic, structural, and stereochemical data regarding the mechanism of Ti-catalyzed addition of cyanide to imines in the presence of Schiff base peptide ligands are disclosed. The reaction is first order in the Ti-ligand complex; kinetic studies reveal DeltaS(dagger) = -45.6 +/- 4.1 cal K(-1) mol(-1), indicating a highly organized transition structure for the turnover-limiting step of the catalytic cycle. A mechanistic model consistent with the kinetic and stereochemical data is presented, where the Ti center is coordinated to the Schiff base unit of the ligand and the AA2 moiety of the peptidic segment of the chiral ligand associates and delivers HNC to the activated bound substrate. Thus, these studies illustrate that these non-C2-symmetric catalysts likely operate in a bifunctional fashion.

Catalytic asymmetric ring-opening metathesis/cross metathesis (AROM/CM) reactions. Mechanism and application to enantioselective synthesis of functionalized cyclopentanes.

Studies regarding the first examples of catalytic asymmetric ring-opening metathesis (AROM) reactions are detailed. This enantioselective cleavage of norbornyl alkenes is followed by an intermolecular cross metathesis with a terminal olefin partner; judicious selection of olefin is required so that oligomerization and dimerization side products are avoided. Results outlined herein suggest that the presence of suitably positioned heteroatom substituents may be critical to reaction efficiency. Mo-catalyzed tandem AROM/CM affords functionalized cyclopentyl dienes in >98% ee and >98% trans olefin selectivity; both secondary and tertiary ether products can be obtained. The examples provided include the catalytic synthesis of an optically pure cyclopentyl epoxide and dimethyl acetal. Mechanistic studies suggest that it is the more substituted benzylidene or silylated alkylidenes that are involved in the catalytic process (vs the corresponding Mo-methylidenes). Although electron rich benzylidenes react more efficiently, the derived electron poor Mo complexes promote AROM/CM transformations as well; alkylidenes that bear a boron substituent are unreactive.

Zr-catalyzed electrophilic carbomagnesation of aryl olefins. Mechanism-based control of Zr-Mg ligand exchange.

[reaction: see structure] The first examples of efficient electrophilic Zr-catalyzed carbomagnesations are disclosed, where in contrast to previous catalytic carbomagnesations the alkyl moiety of the electrophile is transferred (vs that of the Grignard reagent). The identity of the Grignard reagent is manipulated so that Zr-Mg exchange is facilitated, leading to the formation of alkylmagnesium halide products.

Catalytic asymmetric olefin metathesis.

This paper provides a survey of the first examples of efficient catalytic enantioselective olefin metathesis reactions. Mo-catalyzed asymmetric ring-closing (ARCM) and ring-opening (AROM) reactions allow access to myriad optically enriched compounds that are otherwise difficult to access.

Combinatorial catalysis: identification of potent chiral catalysts through fluorescent bead signaling.

Simultaneous labeling of beads with a fluorescent sensor and a unique catalyst allows chemists to identify the most active catalyst within a mixture. This approach is a valuable addition to the protocols available in the growing field of combinatorial catalysis.

Combinatorial catalyst discovery.

There have been recent attempts to use the principles of combinatorial chemistry and high-throughput screening strategies for catalyst identification. With the technology available that allows the synthesis of large libraries, scientists of varied backgrounds have implemented screening efforts to identify active and selective catalysts. Within this context, several techniques have come to light in the past year: infrared thermography is used to identify optimal catalysts by monitoring the change in temperature for exothermic reactions; fluorescence and colored-dye assays, a familiar tool to biologists, is being applied to the identification of catalysts that exhibit the highest activity. Whereas none of these screening methods provide a general solution to the problem of screening large combinatorial libraries (there is likely to be no general solution), each advance represents an important intellectual and technological step forward.

Catalyst discovery through combinatorial chemistry.

Combinatorial chemistry and high-throughput strategies can be used to identify effective small-molecule chiral catalysts. Infrared thermography has been used recently as the means to identify the most active catalyst.