ABSTRACT
Chiral Cu(II) bisoxazolines have been shown to catalyze the coupling of acetyl-protected carbohydrates with N-silylated indoles to give the corresponding N-glycosides. Preliminary mechanistic experiments indicated that catalysis occurs through formation of a Cu-indolide complex with concomitant formation of TMS-OTf which together activate the sugar and deliver the indole nucleophile.
ABSTRACT
A novel diastereoselective, Lewis acid catalyzed 1,6-difunctionalization of galactose and mannose derivatives has been developed in one pot, via sequential nucleophile additions. Our studies point to the formation of a 3,6-anhydrosugar intermediate as key to the 1,6-site-selectivity. Starting material-specific reactivity occurs when competitive ring-opening C-O cleavage is possible, owed to basicity and stereoelectronic stabilization differences. Lastly, Mayr nucleophilicity parameter values helped predict which reaction conditions would be most suitable for specific nucleophiles.
Subject(s)
Boranes , Carbohydrates , Catalysis , GalactoseABSTRACT
A variety of C-glycosides can be obtained from the fluoroarylborane (B(C6F5)3) or silylium (R3Si+) catalyzed functionalization of 1-MeO- and per-TMS-sugars with TMS-X reagents. A one-step functionalization with a change as simple as the addition order and/or Lewis acid and TMS-X enables one to afford chiral synthons that are common (C-pyranosides), have few viable synthetic methods (C-furanosides), or are virtually unknown (anhydro-C-pyranosides), which mechanistically arise from whether a direct substitution, isomerization/substitution, or substitution/isomerization occurs, respectively.
Subject(s)
Furans/chemical synthesis , Glycosides/chemistry , Lewis Acids/chemistry , Catalysis , Furans/chemistry , Isomerism , Molecular StructureABSTRACT
Here, we report the quantitative electroreduction of CO2 to CO by a PNP-pincer iridium(i) complex bearing amino linkers in DMF/water. The electrocatalytic properties greatly depend on the choice of linker within the ligand. The complex 3-N is far superior to the analogues with methylene and oxygen linkers, showing higher activity and better selectivity for CO2 over proton reduction.
ABSTRACT
The synthesis and characterization of the room-temperature and solution-stable silylpalladium cations (PCy3)2Pd-SiR3+(C6F5)4B- (SiR3 = SiMe2Et, SiHEt2) and (Xantphos)Pd-SiR3+(BArf4) (SiR3 = SiMe2Et, SiHEt2; Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; BArf4 = (3,5-(CF3)2C6H3)4B-) are reported. Spectroscopic and ligand addition experiments suggest that silylpalladium complexes of the type (PCy3)2Pd-SiR3+ are three-coordinate and T-shaped. Addition of dialkyl ethers to both the PCy3 and Xantphos-based silylpalladium cations resulted in the cleavage of C(sp3)-O bonds and the generation of cationic Pd-alkyl complexes. Mechanistically enabling is the ability of silylpalladium cations to behave as sources of both electrophilic silylium ions and nucleophilic LnPd(0).
