Brandon L. Ashfeld

Titanocene-Catalyzed Conjugate Reduction of α,β-Unsaturated Carbonyl Derivatives

A titanocene-catalyzed conjugate reduction of alpha,beta-unsaturated carbonyl derivatives has been developed. A series of carbonyl compounds including aldehydes, ketones, esters, and amides proved viable in the reduction process providing an efficient, chemoselective method for the catalytic reduction of unsaturated carbonyl derivatives.

Phosphine‐Based Redox Catalysis in the Direct Traceless Staudinger Ligation of Carboxylic Acids and Azides

The synthesis of amide C N bonds through nucleophilic acyl substitutions constitutes one of the most fundamental transformations in chemical synthesis. Recently, the Staudingertype ligation of carboxylic acid derivatives (e.g., acid chlorides, anhydrides, acyl selenides, and thioesters) and azides has become a preeminent strategy for amide C N bond construction (Scheme 1a). However, the generation of

Hierarchically porous materials via assembly of nitrogen-rich polymer nanoparticles for efficient and selective CO2 capture

Hierarchically porous materials were developed using a two-step procedure by (1) the stacking of high-nitrogen-content polymer nanoparticles to produce inter-particle mesoporous space and (2) chemical activation to create intra-particle micropores. The materials exhibited efficient CO2 uptake and enhanced CO2/N2 selectivity, which is 40% higher than that of the control material without a hierarchical structure.

Organozinc Generation via the Titanium-Catalyzed Activation of Alkyl Halides

A protocol for the generation of organozinc reagents via catalytic activation of alkyl halides is described herein. Subsequent nucleophilic addition to carbonyl derivatives provided the desired products in good to excellent yields (76-99%). Evidence suggests that titanocene dichloride catalyzes the formation of an organozinc species. This discovery will have wide ranging applicability in the generation of highly reactive organometallic reagents.

Titanocene-Catalyzed Multicomponent Coupling Approach to Diarylethynyl Methanes

A titanocene-catalyzed multicomponent coupling to provide diarylethynyl methanes is described. By combining the multifunctionality of Cp(2)TiCl(2) with the traceless dielectrophilicity of aryl aldehydes, all-carbon tertiary centers are obtained in 55-99% yield.

Cooperative Titanocene and Phosphine Catalysis: Accelerated C–X Activation for the Generation of Reactive Organometallics

The study presented herein describes a reductive transmetalation approach toward the generation of Grignard and organozinc reagents mediated by a titanocene catalyst. This method enables the metalation of functionalized substrates without loss of functional group compatibility. Allyl zinc reagents and allyl, vinyl, and alkyl Grignard reagents were generated in situ and used in the addition to carbonyl substrates to provide the corresponding carbinols in yields up to 99%. It was discovered that phosphine ligands effectively accelerate the reductive transmetalation event to enable the metalation of C-X bonds at temperatures as low as -40 °C. Performing the reactions in the presence of chiral diamines and amino alcohols led to the enantioselective allylation of aldehydes.

Solid‐State Covalent Capture of CO2 by Using N‐Heterocyclic Carbenes

Capture me! The first report of an N-heterocyclic carbene (NHC) as a solid-state carbon capture reagent is presented. Experimental and theoretical measurements demonstrate the ability of the NHC to react rapidly and stoichiometrically with CO2 at low partial pressures.

Aryl aldehydes as traceless dielectrophiles in bifunctional titanocene-catalyzed propargylic C-X activations.

The titanocene-catalyzed construction of all-carbon substituted tertiary centers directly from aromatic aldehydes is described. The starting aldehyde behaves as a traceless functionality in the formation of multiple carbon-carbon bonds through consecutive carbon-heteroatom bond activations. The sequential addition of a metal acetylide and a second carbon nucleophile to the dielectrophilic aldehyde enables the construction of symmetrical and unsymmetrical 1,4-diynes in good yields.

Phosphorus(III)-Mediated Stereoconvergent Formal [4+1]-Cycloannulation of 1,2-Dicarbonyls and o-Quinone Methides: A Multicomponent Assembly of 2,3-Dihydrobenzofurans

A phosphorus(III)-mediated formal [4+1]-cycloaddition of 1,2-dicarbonyls and o-quinone methides to provide 2,3-dihydrobenzofurans is described. By exploiting the carbene-like nature of dioxyphospholenes, dihydrobenzofurans bearing a quaternary center at C2 are obtained in 30-92% yield with diastereoselectivities up to ≥20:1. This study highlights the subtle steric interactions involved in the [4+1]-cycloannulation and the impact they have on yield and stereoselectivity in dihydrobenzofuran formation.

Amine‐Free Approach toward N‐Toluenesulfonyl Amidine Construction: A Phosphite‐Mediated Beckmann‐Like Coupling of Oximes and p‐Toluenesulfonyl Azide

Atom hopping: A chlorophosphite-mediated Beckmann ligation of oximes and p-toluenesulfonyl azide gives access to N-sulfonyl phosphoramidines in good to excellent yields. The reaction proceeds under exceptionally mild conditions and constitutes a bioorthogonal approach toward amidines by avoiding the use of amines and transition-metal catalysts. dmp-ol=3,3-dimethylpropanediol.