One-Pot Selective Syntheses of 5-Azaindoles through Zirconocene-Mediated Multicomponent Reactions with Three Different Nitrile Components and One Alkyne Component

Two-pot synthesis and photophysical studies of 1,6-disubstituted 5-aza-indoles from succinaldehyde and N-aryl propargylic-imines

A two-pot synthesis of 5-aza-indoles has been developed from aqueous succinaldehyde and N-aryl propargylic-imines. This overall protocol involves: (i) the metal-free [3 + 2] annulation of aqueous succinaldehyde and N-aryl propargylic-imines to access 2-alkynyl-pyrrole-3-aldehydes and (ii) Ag-catalyzed 6-endo-dig-cyclization to obtain substituted 5-aza-indoles in the second pot. The 5-aza-indoles showed engaging photophysical activities, and the practicality of this pot-economic gram-scale synthesis has been demonstrated.

Insertion of nitriles into zirconocene 1-aza-1,3-diene complexes: chemoselective synthesis of N-H and N-substituted pyrroles

The direct insertion of nitriles into zirconocene-1-aza-1,3-diene complexes provides an efficient, chemoselective, and controllable synthesis of N-H and N-substituted pyrroles upon acidic aqueous work-up. The outcome of the reaction (that is, the formation of N-H or N-substituted pyrroles) results from the different cyclization patterns, which depend on the relative stability and reactivity of the enamine-imine tautomers formed by hydrolysis of the diazazirconacycles.

Titanium-mediated cross-coupling reactions of 1,3-butadiynes with α-iminonitriles to 3-aminopyrroles: observation of an imino aza-Nazarov cyclization

Ti(OⁱPr)₄/2 ⁿBuLi-mediated highly efficient cross-coupling reactions of 1,3-butadiynes with α-iminonitriles are described, which provide convenient access to functionalized 3-aminopyrroles.

Unexpected zirconium-mediated multicomponent reactions of conjugated 1,3-butadiynes and monoynes with acyl cyanide derivatives

Going ber(zirc): zirconium-butadiyne complexes react with acyl cyanide derivatives through diverse reaction pathways. The complexes react with two molecules of carbamoyl cyanide to form 1,4-benzodiazepin-2-one-derivatives containing azazirconacycles. Reactions with either aroyl cyanides or alkyl cyanoformates lead to azazirconacyclopentenes containing a quaternary carbon center.

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.