One-Pot Multicomponent Synthesis of Azaindoles and Pyrroles from One Molecule of a Silicon-Tethered Diyne and Three or Two Molecules of Organonitriles Mediated by Zirconocene

Zirconocene dichloride catalysed one-pot synthesis of pyrroles through nitroalkene-enamine assembly

Zirconocene dichloride was found to be a highly efficient catalyst for the synthesis of multi-substituted pyrroles by a three-component, one-pot reaction.

Zirconocene and Si-tethered diynes: a happy match directed toward organometallic chemistry and organic synthesis

Characterizing reactive organometallic intermediates is critical for understanding the mechanistic aspects of metal-mediated organic reactions. Moreover, the isolation of reactive organometallic intermediates can often result in the ability to design new synthetic methods. In this Account, we outline synthetic methods that we developed for a variety of diverse Zr/Si organo-bimetallic compounds and Si/N heteroatom-organic compounds through the detailed study of zirconacyclobutene-silacyclobutene fused compounds. Two basic components are involved in this chemistry. The first is the Si-tethered diyne, which owes its rich reactive palette to the combination of the Si-C bond and the C≡C triple bond. The second is the low-valent zirconocene species Cp(2)Zr(II), which has proven very useful in organic synthesis. The reaction of these two components affords the zirconacyclobutene-silacyclobutene fused compound, which is the key reactive Zr/Si organo-bimetallic intermediate discussed here. We discuss the three types of reactions that have been developed for the zirconacyclobutene-silacyclobutene fused intermediate. The reaction with nitriles (the C≡N triple bond) is introduced in the first section. In this one-pot reaction, up to four different components can be combined: the Si-tethered diyne can be reacted with three identical nitriles, with differing nitriles, or with a nitrile and other unsaturated organic substrates such as formamides, isocyanides, acid chlorides, aldehydes, carbodiimides, and azides. Several unexpected multiring, fused Zr/Si organo-bimetallic intermediates were isolated and characterized. A wide variety of N-heterocycles, such as 5-azaindole, pyrrole, and pyrroloazepine derivatives, were obtained. We then discuss the reaction with alkynes (the C≡C triple bond). A consecutive skeletal rearrangement, differing from that observed in the reactions with nitriles, takes place in this reaction. Finally, we discuss the reaction with the C═X substrates (where X is O or N), including ketones, aldehydes, and isocyanides. Oxa- and azazirconacycles are formed via a new skeletal rearrangement. Our results show that the zirconocene and the Si-tethered diyne cooperate as a "chemical transformer" after treatment with various substrates, leading to a diverse range of cyclic Zr/Si organo-bimetallic compounds. This mechanism-derived synthesis of organometallic and organic compounds demonstrates that the investigation of metal-mediated reactions and the isolation of reactive organometallic intermediates not only contribute to the understanding of complex reactions but can also lead to the discovery of synthetically useful methods.

Cleavage and reorganization of Zr-C/Si-C bonds leading to Zr/Si-N organometallic and heterocyclic compounds

The t-BuCN-stabilized zirconacyclopropene-azasilacyclopentadiene complexes 2 are generated in situ in high yields from the Si-tethered diynes, Cp(2)Zr(II) species, and 2 equiv of t-BuCN via a coordination-induced Zr-C/Si-C bond cleavage and reorganization. Complexes 2 have been demonstrated to be synthetically very useful. A variety of novel Zr/Si organo-bimetallic compounds and Si/N heterocyclic compounds, such as azasilacyclopentadienes, azasilacyclohexadienes, and allenylazazirconacycles, are obtained in high yields when complexes 2 are treated with ketones, carbodiimides, alkynes, elemental sulfur (S(8)), acid chlorides, or nitriles. In this chemistry, t-BuCN behaves as both an initiator and a brake/release handle to initiate and control the reaction process.

Efficient one-pot synthesis of N-containing heterocycles by multicomponent coupling of silicon-tethered diynes, nitriles, and isocyanides through intramolecular cyclization of iminoacyl-Zr intermediates

An efficient multicomponent synthesis of 5-azaindoles and dihydropyrrolo[3,2-c]azepines was achieved by zirconocene-mediated coupling of silicon-tethered diynes, nitriles, and isocyanides. The synthesis, structures, and intramolecular cyclization of mono- and bis(iminoacyl)--Zr intermediates were investigated to elucidate the reaction process. Upon hydrolysis, the isolated mono(iminoacyl)--Zr intermediates underwent intramolecular cyclization to afford tetrasubstituted 5-azaindoles, whereas intramolecular cyclization of bis(iminoacyl)--Zr intermediates led to the formation of dihydropyrrolo[3,2-c]azepines. The structure of a bis(iminoacyl)--Zr intermediate, formed through insertion of two molecules of CyNC into the Zr--C bond, and structures of two dihydropyrrolo[3,2-c]azepines were characterized by single-crystal X-ray structural analysis.

Multicomponent reactions for the synthesis of pyrroles

Multicomponent reactions are one of the most interesting concepts in modern synthetic chemistry and, as shown in this critical review, they provide an attractive entry into pyrrole derivatives, which are very important heterocycles from many points of view including medicinal and pharmaceutical chemistry and materials science (97 references).