Monoazadien-Komplexe elektronenarmer Übergangsmetalle, I. Zirconocen(1-aza-1,3-dien)1-Komplexe: Synthese, Struktur und Reaktionen

Selective propargylic C(sp3)-H activation of methyl-substituted alkynes versus [2 + 2] cycloaddition at a titanium imido template

The reaction of the titanium imido complex 1b with 2-butyne leads to the formation of the titanium azadiene complex 2a at ambient temperature instead of yielding the archetypical [2 + 2] cycloaddition product (titanaazacyclobutene) which is usually obtained by combining titanium imido complexes and internal alkynes. The formation of 2a is presumably caused by an initial propargylic C(sp³)–H activation step and quantum chemical calculations suggest that the outcome of this unexpected reactivity is thermodynamically favored. The previously reported titanaazacyclobutene I (which is obtained by reacting 1b with 1-phenyl-1-propyne) undergoes a rearrangement reaction at elevated temperature to give the corresponding five-membered titanium azadiene complex 2b.

An unexpected reactivity between a titanium imido complex and internal alkynes was unveiled yielding titanaazacyclobutenes instead of the expected [2 + 2] cycloaddition products.

Multiple and Highly Selective Alkyne-Isonitrile C-C and C-N Couplings at Group 4 Metallocenes

Reactions of the group 4 metallocene alkyne complexes [Cp'2 M(η(2) -Me3 SiC2 SiMe3 )] [Cp'2 =rac-(ebthi)=rac-1,2-ethylene-1,1'-bis(η(5) -tetrahydroindenyl): M=Ti, Zr, Hf; Cp'2 =Cp*2 (Cp*=η(5) -pentamethylcyclopentadienyl): M=Zr] with 2,6-dimethylphenyl isocyanide (2-xylyl isonitrile, XyNC) were investigated. Depending on the metal, the Cp' ligand, and/or the stoichiometry, as well as the reaction temperature and time, different products were obtained. The products included simple end-on coordination compounds of XyNC in addition to those of the coupling of Me3 SiC2 SiMe3 with two, three, or four isonitriles to form enimine complexes, aza-metallacycloallenes, and fused heterocyclic systems, respectively. One example of the latter tricyclic compounds was subject to demetalation by using HCl. Molecular structures of the heterometallacycles were determined by X-ray crystallography. The interconversion of the products was investigated by (1) H NMR spectroscopy.

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.

Rearrangements and stereomutations of metallacycles derived from allenes and imidozirconium complexes

The mechanisms of the rearrangements and stereoinversion of azametallacyclobutenes generated via [2+2] cycloaddition of allenes and imidozirconium complexes have been studied. Metallacycles derived from allenes bearing beta-hydrogen atoms racemize at room temperature by reversible beta-hydride elimination, a process which is also responsible for their eventual conversion to monoazadiene complexes. Metallacycles derived from diarylallenes racemize by reversible thermal bond homolysis at 95 degrees C; racemization of these metallacycles is also catalyzed by mild oxidants.