Mild copper-catalyzed vinylation reactions of azoles and phenols with vinyl bromides

An efficient and straightforward copper-catalyzed method allowing vinylation of N- or O-nucleophiles with di- or trisubstituted vinyl bromides is reported. The procedure is applicable to a broad range of substrates since N-vinylation of mono-, di-, and triazoles as well as O-vinylation of phenol derivatives can be performed with catalytic amounts of copper iodide and inexpensive nitrogen ligands 3 or 8. In the case of more hindered vinyl bromides, the use of the original bidentate chelator 8 was shown to be more efficient to promote the coupling reactions than our key tetradentate ligand 3. The corresponding N-(1-alkenyl)azoles and alkenyl aryl ethers are obtained in high yields and selectivities under very mild temperature conditions (35-110 degrees C for N-vinylation reactions and 50-80 degrees C for O-vinylation reactions). Moreover, to our knowledge, this method is the first example of a copper-catalyzed vinylation of various azoles. Finally, this protocol, practical on a laboratory scale and easily adaptable to an industrial scale, is very competitive compared to the existing methods that allow the synthesis of such compounds.     

Enantioselective Construction of 2,3‐Dihydrofuro[2,3‐b]quinolines through Supramolecular Hydrogen Bonding Interactions

The first asymmetric synthesis of 2,3‐dihydrofuro[2,3‐b]quinolines has been achieved by a cascade asymmetric aziridination/intramolecular ring‐opening process of differently substituted 3‐alkenylquinolones. Good yields and high enantioselectivities (up to 78 % yield and 95 % ee) were recorded when employing 2,2,2‐trichloroethoxysulfonamide as the nitrene source, PhI(OCOtBu)₂ as the oxidant, and a chiral C₂‐symmetric Rh^II complex as the catalyst (1 mol %). The catalyst bears two lactam motifs, which serve as binding sites for substrate coordination through supramolecular hydrogen‐bonding interactions.     

Heteroatom-substituted secondary phosphine oxides (HASPOs) as decomposition products and preligands in rhodium-catalysed hydroformylation

O,O'-3,3'-Di-tert-butyl-5,5'-dimethoxy-1,1'-biphenyl-2,2'-diyl phosphonate (1) is the hydrolysis product of several mono- and bis-phosphites used as ligands in industrial hydroformylation and other catalytic reactions. As a result of a tautomeric equilibrium, this pentavalent heteroatom-substituted phosphine oxide (HASPO) can rearrange to the corresponding trivalent phosphorus compound. The latter is able to react with typical rhodium-containing precursors frequently used for the generation of catalysts. The resulting species were characterised by NMR spectroscopy and X-ray structure analysis. Proof is given that a rhodium complex of 1 forms an active hydroformylation catalyst. Moreover, 1 can add to aldehydes, which are generated as products in the hydroformylation. Thus a broad range of subsequent reactions can be associated with the degradation of the original phosphite ligands, which has a strong influence on the overall outcome of the hydroformylation reaction.     

Exploiting the Multidentate Nature of Chiral Disulfonimides in a Multicomponent Reaction for the Asymmetric Synthesis of Pyrrolo[1,2‐a]indoles: A Remarkable Case of Enantioinversion

A new multicomponent coupling reaction for the enantioselective synthesis of pyrrolo[1,2‐a]indoles under the catalysis of a chiral disulfonimide is described. The high specificity of the reaction is a consequence of the multidentate character of the Brønsted acid catalyst. Insights from DFT calculations helped explain the unexpected high enantioselectivity observed with the simplest 3,3′‐unsubstituted binaphthyl catalyst as a result of transition‐state stabilization by a network of cooperative noncovalent interactions. The remarkable enantioinversion resulting from the simple introduction of substituents at 3‐ and 3′‐positions, the first reported example of this phenomenon in the context of binaphthalene‐derived Brønsted acid catalysis, was instead attributed to destabilizing steric interactions.     

Spiroketal‐Based Phosphorus Ligands for Highly Regioselective Hydroformylation of Terminal and Internal Olefins

A new class of bidentate phosphoramidite ligands, based on a spiroketal backbone, has been developed for the rhodium‐catalyzed hydroformylation reactions. A range of short‐ and long‐chain olefins, were found amenable to the protocol, affording high catalytic activity and excellent regioselectivity for the linear aldehydes. Under the optimized reaction conditions, a turnover number (TON) of up to 2.3×10⁴ and linear to branched ratio (l/b) of up to 174.4 were obtained in the Rh^I‐catalyzed hydroformylation of terminal olefins. Remarkably, the catalysts were also found to be efficient in the isomerization–hydroformylation of some internal olefins, to regioselectively afford the linear aldehydes with TON values of up to 2.0×10⁴ and l/b ratios in the range of 23.4–30.6. X‐ray crystallographic analysis revealed the cis coordination of the ligand in the precatalyst [Rh( 3 d )(acac)], whereas NMR and IR studies on the catalytically active hydride complex [HRh(CO)₂( 3 d )] suggested an eq–eq coordination of the ligand in the species.     

N‐Heterocyclic Carbene–Ytterbium Amide as a Recyclable Homogeneous Precatalyst for Hydrophosphination of Alkenes and Alkynes

The N‐heterocyclic carbene–ytterbium(II) amides (NHC)₂Yb[N(SiMe₃)₂]₂ ( 1 : NHC: 1,3,4,5‐tetramethylimidazo‐2‐ylidene (IMe₄); 2 : NHC: 1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene (IiPr)) and the NHC‐stabilized rare‐earth phosphide (IMe₄)₃Yb(PPh₂)₂ ( 3 ) have been synthesized and fully characterized. Complexes 1 – 3 are active precatalysts for the hydrophosphination of alkenes, alkynes, and dienes and exhibited much superior catalytic activity to that of the NHC‐free amide (THF)₂Yb[N(SiMe)₂]₂. Complex 1 is the most active precursor among the three complexes. In particular, complex 1 can be recycled and recovered from the reaction media after the catalytic reactions. Furthermore, it was found that complex 3 could catalyze the polymerization of styrene to yield atactic polystyrenes with low molecular weights. To the best of our knowledge, complex 1 represents the first rare‐earth complex that can be recovered after catalytic reactions.     

Use of Formic Acid as a CO Surrogate for the Reduction of Nitroarenes in the Presence of Dienes: A Two-Step Synthesis of N-Arylpyrroles via 1,2-Oxazines

Formic acid, activated by acetic anhydride and a base, was employed as a CO surrogate to deoxygenate nitroarenes to nitrosoarenes, a reaction catalyzed by a palladium/phenanthroline complex in the homogeneous phase. Nitrosoarenes were trapped by conjugated dienes to give 3,6-dihydro-2H-[1,2]-oxazines. The latter were then transformed into N-arylpyrroles employing CuCl as the catalyst. The reaction was designed to give the best results for pyrroles lacking any substituent in the 2 and 5 positions, which are difficult to produce employing most pyrrole syntheses.     

Regio- and stereoselective ring-opening reactions of epoxides with indoles and pyrroles in 2,2,2-trifluoroethanol

Aliphatic and aromatic epoxides react regio- and stereoselectively with indoles and pyrroles in 2,2,2-trifluoroethanol without the use of a catalyst or any other additive. While aromatic epoxides are selectively attacked at the benzylic position, aliphatic epoxides react at the less-substituted position. Chiral epoxides react with >99 % ee (ee=enantiomeric excess).     

Rhodium(II)‐Catalyzed Cycloaddition Reactions of Non‐classical 1,5‐Dipoles for the Formation of Eight‐Membered Heterocycles

A new type of intermolecular rhodium(II)‐catalyzed [5+3] cycloaddition has been developed. This higher‐order cycloaddition between pyridinium zwitterion 1,5‐dipole equivalents and enol diazoacetates enables the formation of eight‐membered heterocyclic skeletons, which are otherwise difficult to construct. The optimized cycloaddition occurs efficiently under mild conditions with a wide range of pyridinium zwitterions and with high functional‐group tolerance.