One-Pot Synthesis of Pyrrolo[3,2-d]pyridazines and Pyrrole-2,3-diones via Zirconocene-Mediated Four-Component Coupling of Si-Tethered Diyne, Nitriles, and Azide

Synthesis of pyrrolo[2,3-d]pyridazines and pyrrolo[2,3-c]pyridines (6-azaindoles). Experimental and theoretical study

A new synthetic route towards 6-azaindoles (pyrrolo[2,3-c]pyridines) and pyrrolo[2,3-d]pyridazines starting from 4-aroyl pyrroles is described. This overall protocol involves: (i) the Vilsmeier-Haack reaction to obtain pyrrolo-2,3-dicarbonyles and (ii) condensation with hydrazines or glycine methyl ester. The reaction mechanism between pyrrolo-2,3-dicarbonyl with phenyl hydrazine and glycine methyl ester has been modelled using DFT calculations to prove the formation of one from two possible isomers of condensation.

Silver-catalyzed acylative annulation of N-propargylated indoles with α-keto acids: access to acylated pyrrolo[1,2-a]indoles

A novel carbon-centered radical cyclization of N-propargyl indoles with α-keto acids to form acylated pyrrolo[1,2-a]indoles involving difunctionalisation of the alkyne moiety with concomitant annulation is disclosed.

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.

Unusual nitrile-nitrile and nitrile-alkyne coupling of Fc-C≡N and FC-C≡C-C≡N

The reactions of the Group 4 metallocene alkyne complexes, [Cp*2M(η2-Me3SiC2SiMe3)] (1 a: M=Ti, 1 b: M=Zr, Cp*=η5-pentamethylcyclopentadienyl), with the ferrocenyl nitriles, Fc-CN and Fc-C≡C-C≡N (Fc=Fe(η5-C5H5)(η5-C5H4)), is described. In case of Fc-C≡N an unusual nitrile–nitrile C-C homocoupling was observed and 1-metalla-2,5-diaza-cyclopenta-2,4-dienes (3 a, b) were obtained. As the first step of the reaction with 1 b, the nitrile was coordinated to give [Cp*2Zr(η2-Me3SiC2SiMe3)(N≡C-Fc)] (2 b). The reactions with the 3-ferrocenyl-2-propyne-nitrile FcC≡C-C≡N lead to an alkyne–nitrile C-C coupling of two substrates and the formation of 1-metalla-2-aza-cyclopenta-2,4-dienes (4 a, b). For M=Zr, the compound is stabilized by dimerization as evidenced by single-crystal X-ray structure analysis. The electrochemical behavior of 3 a, b and 4 a, b was investigated, showing decomposition after oxidation, leading to different redox-active products.

Methyl 1-benzyl-5-methyl-2,4-diphenyl-1H-pyrrole-3-carboxyl-ate

In the title compound, C₂₆H₂₃NO₂, the dihedral angles between the pyrrole ring and the two phenyl rings are 58.1 (6) and 71.5 (5)°. The mean planes of the 5-methyl­benzene ring and the carboxyl group are twisted by 89.5 (3) and 22.1 (9)°, respectively, from the pyrrole ring. In the crystal, weak C—H⋯O inter­actions lead to supra­molecular layers in the ab plane.

Selective C(sp2)-C(sp) bond cleavage: the nitrogenation of alkynes to amides

Breakthrough: A novel catalyzed direct highly selective C(sp2)-C(sp) bond functionalization of alkynes to amides has been developed. Nitrogenation is achieved by the highly selective C(sp2)-C(sp) bond cleavage of aryl-substituted alkynes. The oxidant-free and mild conditions and wide substrate scope make this method very practical.

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.