Gallium Tribromide Catalyzed Coupling Reaction of Alkenyl Ethers with Ketene Silyl Acetals

Asymmetric Construction of Ethenyl-Substituted Acyclic Quaternary Stereocenters at the α-Position of Carbonyl Surrogates via Stereoselective Sulfonylvinylation-Reductive Desulfonylation

N-Sulfinyl metalloenamines, derived from geometry-defined β,β-disubstituted enesulfinamides, undergo conjugate addition-elimination reaction with β-tosyl nitroolefin to afford α-sulfonylvinylated ketimines with high stereocontrol. Further desulfonylation using sodium naphthalenide gives carbonyl surrogates bearing a less-accessible acyclic quaternary α-stereocenters substituted with an ethenyl group and two sterically and electronically similar groups (e.g., methyl and ethyl). Synthetic application of the described protocol was demonstrated by enantioselective synthesis of (S)-bakuchiol.

Vinyltriarylbismuthonium Salts: Powerful Vehicles for α-Vinylation of Carbonyl Compounds

α-Vinyl-carbonyl compounds are a class of orthogonally functionalized molecules, wherein the intrinsic C═O and C═C bonds can be used to unlock distinctly different reactivities. In this report, we present a simple method for the direct α-vinylation of carbonyl compounds utilizing vinyltriarylbismuthonium ("Vinyl-Bis") salts, which are stable and readily preparable on a decagram scale. This transformation is accomplished efficiently through the reaction of an in situ generated Li-enolate intermediate with a Vinyl-Bis reagent, leading to the formation of α-vinylated carbonyl compounds in good to excellent yields and with a remarkably broad substrate scope. Critically, this vinylation method is effective for enolates generated via numerous methods, enabling the sequencing of reactions that generate enolates with the vinylation step and the ready synthesis of diversely functionalized compounds, thereby underscoring the versatility and practicality of this method. Analogous reactions of discrete Li-enolates with other vinyl units and with aryl groups are also demonstrated.

Gallium Hydride-Catalyzed Selective Hydroboration of Unsaturated Organic Substrates

The first examples of tetrasubstituted conjugated bis-guanidinate (CBG) supported monomeric and thermally stable gallium dihalides [LGaX2], (X=Cl (Ga-Cl), I (Ga-I)) and dihydride (Ga-H) [LGaH2] (where L={(ArHN)(ArN)-C=N-C=(NAr)(NHAr)}; Ar=2,6-Et2-C6H3) compounds are reported. The reaction of in situ generated LLi with 1.0 equiv. GaX3 (X=Cl, I) afforded compounds Ga-Cl and Ga-I. The reaction between Ga-Cl and Li[HBEt3] in benzene yielded the dihydride compound Ga-H. All reported compounds (Ga-Cl, Ga-I, and Ga-H) were characterized by NMR, HRMS, and single-crystal X-ray diffraction studies. Ga-H was probed for the hydroboration of carbodiimides (CDI), isocyanates, and isothiocyanates with HBpin. Compound Ga-H was also found effective for the catalytic hydroboration of imines, nitriles, alkynes, esters, and formates, affording the corresponding products in quantitative yields. Stoichiometric reactions with a CDI were performed to establish the catalytic cycle.

Synthesis of 2-isoxazolyl-2,3-dihydrobenzofurans via palladium-catalyzed cascade cyclization of alkenyl ethers

An efficient palladium-catalyzed cascade cyclization reaction of alkenyl ethers with alkynyl oxime ethers for the construction of poly-heterocyclic scaffolds has been developed.

Carbometalation and Heterometalation of Carbon-Carbon Multiple-Bonds Using Group-13 Heavy Metals: Carbogallation, Carboindation, Heterogallation, and Heteroindation

Organogallium and -indium compounds are useful reagents in organic synthesis because of their moderate stability, efficient reactivity and high chemoselectivity. Carbogallation and -indation of a carbon-carbon multiple bond achieves the simultaneous formation of carbon-carbon and carbon-metal bonds. Heterogallation and -indation construct carbon-heteroatom and carbon-metal bonds. Therefore, these reaction systems represent a significant synthetic method for organogalliums and -indiums. Many chemists have attempted to apply various types of unsaturated compounds such as alkynes, alkenes, and allenes to these reaction systems. This minireview provides an overview of carboindation and -gallation as well as heteroindation and -gallation.

Silver-Catalyzed Olefination of Acetals and Ketals with Diazoesters to β-Alkoxyacrylates

The first silver-catalyzed reaction of acetals or ketals with diazoesters leading to trisubstituted or tetrasubstituted β-alkoxyacrylates is now reported. A broad range of acetals and ketals bearing different substituents is compatible with this protocol and thus provides an attractive approach for the synthesis of complex β-alkoxyacrylates. The power of this method was further demonstrated by the successful synthesis of picoxystrobin, which is one of the most popular agricultural fungicides commercialized by Dupont.

Synthesis of 2-aryloxy butenoates by copper-catalysed allylic C–H carboxylation of allyl aryl ethers with carbon dioxide

2-Aryloxy butenoates were efficiently synthesized by deprotonative alumination of ally aryl ethers and subsequent Cu-catalysed carboxylation with CO₂.

Indium Tribromide Catalyzed Coupling Reaction of Enol Ethers with Silyl Ketene Imines toward the Synthesis of β,γ-Unsaturated Nitriles

Herein, a coupling reaction of enol ethers with silyl ketene imines in the presence of catalytic amounts of InBr3 and Me3 SiBr is described. Kinetic studies have revealed that an indium catalyst and Me3 SiBr accelerated the coupling process and the regeneration of the catalyst, respectively. Various types of enol ethers and silyl ketene imines are applicable. In addition, a formal synthesis of verapamil was achieved by using this novel coupling reaction.

Palladium-Catalyzed Decarbonylative Dehydration for the Synthesis of α-Vinyl Carbonyl Compounds and Total Synthesis of (-)-Aspewentins A, B, and C

The direct α-vinylation of carbonyl compounds to form a quaternary stereocenter is a challenging transformation. It was discovered that δ-oxocarboxylic acids can serve as masked vinyl compounds and be unveiled by palladium-catalyzed decarbonylative dehydration. The carboxylic acids are readily available through enantioselective acrylate addition or asymmetric allylic alkylation. A variety of α-vinyl quaternary carbonyl compounds are obtained in good yields, and an application in the first enantioselective total synthesis of (-)-aspewentins A, B, and C is demonstrated.

Synthesis of ethers by GaBr3 -catalyzed reduction of carboxylic acid esters and lactones by siloxanes

Ethers were synthesized by reduction of the respective esters catalyzed by gallium bromide (GaBr3 ) and using siloxanes, preferentially 1,1,3,3-tetramethyldisiloxane, as reductant. Methyl oleate, triglycerides, that is, tributyrine and glyceryl triundec-10-enoate as well as γ- and δ-lactones were converted into the respective ethers in high to moderate yields. γ-Lactones were reduced with high selectivity in the presence of a methyl ester functionality. The reduction has been carried out at room temperature or moderately elevated temperature of up to 60 °C using stoichiometric amounts of the reductant and 0.005-0.01 equiv of GaBr3 as catalyst per ester functionality without any solvent added. After a reaction time of 1-4 h the conversion of the substrate was 100 %. The product was separated from polymeric siloxanes formed as coupled product by simple distillation.