Eric M. Ferreira

Photooxidizing Chromium Catalysts for Promoting Radical Cation Cycloadditions

The photooxidizing capabilities of selected Cr(III) complexes for promoting radical cation cycloadditions are described. These complexes have sufficiently long-lived excited states to oxidize electron-rich alkenes, thereby initiating [4+2] processes. These metal species augment the spectrum of catalysts explored in photoredox systems, as they feature unique properties that can result in differential reactivity from the more commonly employed ruthenium or iridium catalysts.

Stereoselective Syntheses of Trisubstituted Olefins via Platinum Catalysis: α-Silylenones with Geometrical Complementarity

The stereoselective syntheses of alpha-silylenones using catalytic PtCl(2) are reported. Via alkyne activation, alpha-hydroxypropargylsilanes are converted to (Z)-silylenones through a highly selective silicon migration. The complementary (E)-silylenones are accessed by a regioselective hydrosilylation of the ynone precursor. The synthetic utility of these compounds is demonstrated in cross-coupling reactions, highlighting the potential of these protocols for the syntheses of geometrically defined trisubstituted olefins.

Generation of α,β-Unsaturated Platinum Carbenes from Homopropargylic Alcohols: Rearrangements to Polysubstituted Furans

A number of diversely substituted furans are synthesized via a cycloisomerization process that goes through a unique metal carbene species. Both ligand structure and the nature of the leaving group are evaluated. The characteristics of the carbene intermediate can be modulated, resulting in highly selective hydrogen or silicon group migrations.

Uncovering the Roles of Oxygen in Cr(III) Photoredox Catalysis

A combined experimental and theoretical investigation aims to elucidate the necessary roles of oxygen in photoredox catalysis of radical cation based Diels-Alder cycloadditions mediated by the first-row transition metal complex [Cr(Ph2phen)3](3+), where Ph2phen = bathophenanthroline. We employ a diverse array of techniques, including catalysis screening, electrochemistry, time-resolved spectroscopy, and computational analyses of reaction thermodynamics. Our key finding is that oxygen acts as a renewable energy and electron shuttle following photoexcitation of the Cr(III) catalyst. First, oxygen quenches the excited Cr(3+)* complex; this energy transfer process protects the catalyst from decomposition while preserving a synthetically useful 13 μs excited state and produces singlet oxygen. Second, singlet oxygen returns the reduced catalyst to the Cr(III) ground state, forming superoxide. Third, the superoxide species reduces the Diels-Alder cycloadduct radical cation to the final product and reforms oxygen. We compare the results of these studies with those from cycloadditions mediated by related Ru(II)-containing complexes and find that the distinct reaction pathways are likely part of a unified mechanistic framework where the photophysical and photochemical properties of the catalyst species lead to oxygen-mediated photocatalysis for the Cr-containing complex but radical chain initiation for the Ru congener. These results provide insight into how oxygen can participate as a sustainable reagent in photocatalysis.

Differential Reactivities of Enyne Substrates in Ruthenium- and Palladium-Catalyzed Cycloisomerizations

Complementary methods for the transition-metal-catalyzed enyne cycloisomerizations of cyclic olefins have been developed. By using distinct ruthenium and palladium catalysts, decalins and 7,6-bicycles can be obtained with dichotomous stereochemical outcomes. The change in mechanism that accompanies the change in metal affords trans-fused 1,4-dienes with ruthenium and their cis-fused diastereomers under palladium catalysis. In the reactions under ruthenium catalysis, a coordinating group is required and acts to direct the metal to the same side of the carbocycle, resulting in the observed trans diastereoselectivity. Subtle changes in the carbocyclic substrate led to the discovery of a heretofore-unobserved mechanistic pathway, providing bicyclic cycloisomerization products under palladium catalysis and tricyclic products under ruthenium catalysis in N,N-dimethylacetamide (DMA). The differential effect of DMA supports a mechanism in which the coordination requirements of the two paths differ, allowing for the reaction to be shuttled through the metallacycle pathway (generating tricyclic products) when DMA is used as a solvent.

Platinum-catalyzed cyclizations via carbene intermediates: syntheses of complementary positional isomers of isoxazoles

A novel synthesis of regioisomeric isoxazoles is described. Using catalytic platinum, both propargylic N-hydroxycarbamates and N-alkoxycarbonyl amino ethers can be cyclized to form differentially substituted isoxazoles. Reaction conditions are developed that address specific aspects of the catalytic manifold. A unique mechanism involving a Pt-carbene intermediate is proposed, and deuterium labeling studies corroborate this hypothesis. This regiocomplementary approach to isoxazoles is highlighted in the syntheses of antirhinovirus analogues, illustrating the relevance of this science to medicinal chemistry.

Stereoselective Synthesis of Tetrasubstituted Olefins through a Halogen‐Induced 1,2‐Silyl Migration

Migrating through Si valley: The highly stereoselective formation of α-silyl-β- haloenones by way of silicon group migration is described. Electrophilic activation of the alkyne by N-halosuccinimides induced an anti-selective migration to give highly substituted enones (see scheme). These enone products can be readily converted to the all-carbon tetrasubstituted alkenes while maintaining their geometry.

Vicinal bisheterocyclizations of alkynes via nucleophilic interception of a catalytic platinum carbene.

A novel platinum-catalyzed double heterocyclization of propargylic ethers is described. The transformation exploits the intermediacy of a key α,β-unsaturated carbene. The reactivity of this carbene is such that systems can be developed which avoid a complicating 1,2-hydrogen migration, allowing remarkable versatility in the selective syntheses of oxygen- and nitrogen-containing vicinal bis-heterocyclic compounds.

C–H Activation: A surrogate for selectivity

Metal-coordinating groups are widely used to direct C–H functionalization. Now the combination of an alkene hydrosilylation, C–H activation and C–Si oxidation has been used to achieve a formal 1,4-dioxygenation of alkenes.

Highly enantiospecific platinum-catalyzed cycloisomerizations: synthesis of enantioenriched oxabicycloheptene derivatives.

Enantiospecific cycloisomerizations of 1,6-enynes to form oxabicyclo[4.1.0]heptene derivatives are described. Enantiospecificity is consistently high regardless of alkene or alkyne substitution, providing a general approach to greatly enantioenriched cyclopropanes. Additionally, a model for stereochemical transfer is proposed.