Ring-Closing Olefin Metathesis and Radical Cyclization as Competing Pathways

Radical-Mediated Cascade Spirocyclization of N-Benzylacrylamides with Polyhaloalkanes: Access to Polyhalo-Substituted Azaspirocyclohexadienones

A novel and mild metal-free catalyzed radical-mediated cascade spirocyclization of N-benzylacrylamides with polyhaloalkanes is proposed for the preparation of polyhalo-substituted azaspirocyclohexadienones. Notably, polyhaloalkanes are employed as efficient alkyl radical sources via the cleavage of C(sp³)-H bonds. This protocol undergoes a cascade radical addition and intramolecular cyclization/dearomatization process, and enables the easy construction of multiple chemical bonds and a spiro ring in a single reaction.

Diastereoselective Synthesis of Polysubstituted Piperidines through Visible-Light-Driven Silylative Cyclization of Aza-1,6-Dienes: Experimental and DFT Studies

A visible-light-driven radical silylative cyclization of aza-1,6-dienes featuring an acrylonitrile or acrylate moiety and an electron-neutral olefin was developed, which allows for stereoselective synthesis of densely functionalized piperidines in a highly atom-economical manner. Depending on the substitution pattern of the electron-neutral olefin, poor-to-excellent diastereoselectivity was observed. It was suggested that the 6-exo-trig cyclization was initiated by a chemoselective addition of silyl radical toward electron-deficient olefin and the geometry of the remaining olefin is closely associated with the cis-stereoselectivity. DFT calculations supported that a transition state with a cyano group locating at the axial position of the forming piperidine ring might be involved, in which either the increase of 1,3-diaxial repulsion or the lack of hydrogen bonding interaction will diminish diastereoselectivity.

Atom transfer radical cyclization of trichloroacetamides to electron-rich acceptors using Grubbs' catalysts: synthesis of the tricyclic framework of FR901483

Intramolecular Kharasch-type additions of trichloroacetamides on anisole and enol acetates catalyzed by Grubbs' ruthenium carbenes are described. This protocol provides access to highly functionalized 2-azaspiro[4.5]decanes, morphan compounds, and the azatricyclic core of FR901483.

Formal alkylation of allenes through highly selective radical cyclizations of allene-enes

Something radical: the first example of alkene-to-allene radical cyclization of allene-enes is reported. The highly chemoselective intermolecular radical addition reaction of the alkene and subsequent, exclusive exo-radical addition to the allene was realized with perfluoroalkyl radicals. A subsequent TBAF-promoted dehydroiodination of the cyclization products forms cyclopentanes and regenerates an allene moiety (TBAF=tetra-n-butylammonium fluoride).

Radicals in transition metal catalyzed reactions? transition metal catalyzed radical reactions? - a fruitful interplay anyway: part 2. Radical catalysis by group 8 and 9 elements

This review summarizes the current status of transition metal catalyzed reactions involving radical intermediates in organic chemistry. This part focuses on radical-based methods catalyzed by group 8 and group 9 metal complexes. Reductive and redox-neutral coupling methods catalyzed by low-valent metal complexes as well as catalytic oxidative C-C bond formations are reviewed.

Diastereoselective Ru-catalyzed cross-metathesis-dihydroxylation sequence. an efficient approach toward enantiomerically enriched syn-diols

Sequential catalysis has evolved as a powerful concept within the past years and allows the more efficient use of catalytically active expensive transition metals in organic synthesis. In this paper we present the stereoselective cross-metathesis-dihydroxylation of various olefins with chiral auxiliary substituted acrylamides. The chiral information (i.e., the auxiliary) introduced in the metathesis reactions allows for a stereoselective subsequent RuO4-catalyzed dihydroxylation. The sequence is concluded by an unusual kinetic resolution of the diastereomeric diols obtained in the oxidation reaction. As a consequence a variety of structurally diverse enantiomerically enriched diols are obtained. To the best of our knowledge the results summarized in this paper represent the first highly efficient diastereoselective RuO4-catalyzed oxidation.

Atom-Transfer Radical Addition (ATRA) and Cyclization (ATRC) Reactions Catalyzed by a Mixture of [RuCl2Cp*(PPh3)] and Magnesium

A new catalytic procedure for atom-transfer radical addition (ATRA) and cyclization (ATRC) reactions is described. The combination of the ruthenium(III) complex [RuCl(2)Cp*(PPh3)] (Cp*: pentamethylcyclopentadienyl) with magnesium allows these reactions to be performed under mild conditions with high efficiency. In most cases, the catalyst concentrations required are significantly lower than those used in previously reported procedures. It is suggested that magnesium acts as a reducing agent that generates and regenerates the catalytically active ruthenium(II) species. The precatalyst [RuCl(2)Cp*(PPh3)] has been analyzed by X-ray crystallography.

Tandem Catalysis: Generating Multiple Contiguous Carbon−Carbon Bonds through a Ruthenium-Catalyzed Ring-Closing Metathesis/Kharasch Addition

Tandem catalysis can offer unique and powerful strategies for converting simple starting materials into more complex products in a single reaction vessel while generating less waste and minimizing handling. In this regard, Grubbs' ruthenium alkylidene (Cy3P)2Cl2Ru=CHPh is shown to catalyze two mechanistically distinct transformations to offer a unique protocol that effects multiple bond changes in a single operation. A tandem ruthenium-catalyzed olefin ring-closing metathesis (RCM)/Kharasch addition allows for the facile preparation of bicyclic [3.3.0], [4.3.0], and [5.3.0] ring systems in one step from the appropriately functionalized acyclic precursors. For substrates where the intramolecular Kharasch addition fails, an intermolecular Kharasch addition is possible. By combining the intra- and intermolecular Kharasch additions with RCM, three new contiguous carbon-carbon bonds with multiple stereocenters can be generated by the ruthenium catalyst in a controlled fashion in one operation through two mechanistically distinct pathways.

Highly Efficient Ru−Pseudohalide Catalysts for Olefin Metathesis

Ruthenium alkylidene complexes containing one aryloxide "pseudohalide" ligand catalyze ring-closing metathesis of diene and ene-yne substrates with exceptionally high efficiency. Chromatographic removal of Ru residues is unexpectedly facile, offering a convenient means of isolating pure organic products in high yields.