Enantioselective C?H Bond Functionalization Triggered by Radical Trifluoromethylation of Unactivated Alkene

An asymmetric unactivated alkene/C H bond difunctionalization reaction for the concomitant construction of C CF₃ and C O bonds was realized by using a Cu/Brønsted acid cooperative catalytic system, thus providing facile access to valuable chiral CF₃‐containing N,O‐aminals with excellent regio‐, chemo‐, and enantioselectivity. Mechanistic studies revealed that this reaction may proceed by an unprecedented 1,5‐hydride shift involving activation of unactivated alkenes and a radical trifluoromethylation to initiate subsequent enantioselective functionalization of C H bonds. Control experiments also suggested that chiral Brønsted acid plays multiple roles and not only controls the stereoselectivity but also increases the reaction rate through activation of Togni’s reagent.     

Nickel‐Catalyzed Enantioselective C?C Bond Formation through C?O Cleavage in Aryl Esters

We report the first enantioselective C C bond formation through C O bond cleavage using aryl ester counterparts. This method is characterized by its wide substrate scope and results in the formation of quaternary stereogenic centers with high yields and asymmetric induction.     

Innentitelbild: Catalytic Asymmetric Functionalization of Aromatic C?H Bonds by Electrophilic Trapping of Metal‐Carbene‐Induced Zwitterionic Intermediates (Angew. Chem. 48/2014)

Asymmetric functionalization of aromatic C H bonds of N,N‐disubstituted anilines with diazo compounds and imines is reported for the efficient construction of α,α‐diaryl benzylic quaternary stereocenters in good yields with high diastereoselectivities and excellent enantioselectivities. This Rh^II/chiral phosphoric acid cocatalyzed transformation is proposed to proceed through a metal‐carbene‐induced zwitterionic intermediate which undergoes electrophilic trapping. To the best of our knowledge, this is the first asymmetric example of metal carbene‐induced intermolecular functionalization of aryl C H bonds.     

Rhodium(I)‐Catalyzed Enantioselective C?C Bond Activation

Relieving the strain : The rhodium(I)‐catalyzed activation of C C bonds in functionalized cyclobutanes opens a novel route to highly substituted carbo‐ and heterocycles. Particularly intriguing is the differentiation of enantiotopic C C bonds, which leads to the formation of highly enantiomerically enriched lactones, cyclopentanones, and cyclohexenones (see scheme).

     

Copper‐Catalyzed Aerobic Oxidative C?C Bond Cleavage for C?N Bond Formation: From Ketones to Amides

A novel copper‐catalyzed aerobic oxidative C(CO) C(alkyl) bond cleavage reaction of aryl alkyl ketones for C N bond formation is described. A series of acetophenone derivatives as well as more challenging aryl ketones with long‐chain alkyl substituents could be selectively cleaved and converted into the corresponding amides, which are frequently found in biologically active compounds and pharmaceuticals.     

Titanium‐Catalyzed Vinylic and Allylic C?F Bond Activation—Scope,Limitations and Mechanistic Insight

The hydrodefluorination (HDF) of fluoroalkenes in the presence of a variety of titanium catalysts was studied with respect to scope, selectivity, and mechanism. Optimization revealed that the catalyst requires low steric bulk and high electron density; secondary silanes serve as the preferred hydride source. A broad range of substrates yield partially fluorinated alkenes, such as previously unknown (Z)‐1,2‐(difluorovinyl)ferrocene. Mechanistic studies indicate a titanium(III) hydride as the active species, which forms a titanium(III) fluoride by H/F exchange with the substrate. The HDF step can follow both an insertion/elimination and a σ‐bond metathesis mechanism; the E/Z selectivity is controlled by the substrate. The catalysts’ ineffieciency towards fluoroallenes was rationalized by studying their reactivity towards Group 6 hydride complexes.     

Ruthenium‐Catalyzed C?C Bond Cleavage in Lignin Model Substrates

Ruthenium–triphos complexes exhibited unprecedented catalytic activity and selectivity in the redox‐neutral C C bond cleavage of the β‐O‐4 lignin linkage of 1,3‐dilignol model compounds. A mechanistic pathway involving a dehydrogenation‐initiated retro‐aldol reaction for the C C bond cleavage was proposed in line with experimental data and DFT calculations.