Michael Popall

Metal carbene chelates: stable reactive intermediates in cycloaddition reactions

Carbonylcarbene complexes bearing either heteroatoms or CC bonds in a carbene γ-position are susceptible towards facile decarbonylation. The coordination of the additional donor group affords stable five-membered metal carbene chelates. Olefin-chelated metallacycles are obtained via aminolysis of alkoxy carbene complexes using allylamine. Heteroatom metal carbene chelates are accessible from ortho-lithiated methoxyarenes in a two-step process involving a sequential nucleophile/electrophile addition to metal carbonyls followed by thermal decarbonylation. Both types of compounds have been studied systematically by X-ray analysis. The characteristic feature of oxametallacycles is a long metal to oxygen bond. The ready ring-opening is exploited in the carbene annulation reaction in which now the ligand coupling can be separated from the generation of a vacant coordination site at the metal template. The scope of this strategy is demonstrated by two approaches to anthracyclinones: Either ring B or C can be formed from 1,4-dimethoxy-2-naphthylcarbene or anisylcarbene synthons.

Carbenliganden als anthracyclinon-synthone I. methoxyarylcarbene als chelatliganden

Abstract Pentacarbonyl[methoxy(1-methoxy-2-aryl)carbene]chromium complexes are obtained via successive addition of 2-lithio-1-methoxyarenes and trimethyloxonium-tetrafluoroborate to hexacarbonylchromium. The pentacarbonyl compounds undergo intramolecular CO-substitution to give cis -tetracarbonyl[methoxy(1-methoxy-2-aryl)carbene] complexes containing a five-membered oxygen-chromacycle. For the anisyl compound the coplanarity of the “chromabenzofuran” system was established by X-ray analysis.

Incorporation of chromium carbenes in a silica matrix by sol-gel processing: application to aminolysis of alkoxycarbene complexes

Chromium carbene complexes have been immobilized in a silica matrix by sol-gel processing based on the polycondensation of (trialkoxy)silylethylphosphane ligands and tetraalkoxysilanes. The microstructure of the material obtained depends on the gelation conditions. In situ gelation of alkoxy- or aminocarbene complexes with tetramethoxysilane (TMOS) affords mesoporous materials with a homogeneous distribution of the metal complex. The metal carbene moiety is accessible for small substrates as demonstrated for the aminolysis of the incorporated methoxy(phenyl)carbene complex which slows down with increasing bulk of the amine.

Carbene ligands as anthracyclinone synthons VII. Quinone ketal functionalized carbonyl carbene complexes: Synthesis, structure and carbene annulation

Abstract The addition of lithiated quinone bis-ketals obtained via anodic oxidation of 2-bromo-1,4-dimethoxy-benzene or -naphthalene and subsequent metal-halogen exchange to hexacarbonyl-chromium or -tungsten leads to pentacarbonyl quinone bis-ketal metallates. Alkylation of these compounds with [Et3O][BF4] in methylene chloride is accompanied by rearomatization to give 1,4-dimethoxyarylcarbene complexes. In protic media the naphthoquinone bis-ketal metallates undergo a novel rearrangement/decarbonylation/elimination reaction to give pentacarbonyl monoketal dihydronaphthalidene complexes, which represent a new type of low-valent transition metal carbene complexes in which the stabilizing heteroatom is not bound directly to the carbene carbon. The X-ray analysis of the chromium complex indicates a conjugation of the metal-carbene bond and the vinyl-like enol ether substituent. Alkylation of the benzoquinone bis-ketal metallate using [Me3O][BF4] in a two-phase system leads to a pentacarbonyl bis-ketal carbene complex, which combines the quinone oxidation state in the carbene ligand with a zerovalent chromium center. Decarbonylation affords a tetracarbonyl carbene chelate with an extremely long chromium-oxygen bond which is responsible for a ready ring-opening of the chelate structure upon reaction with alkynes. This feature is taken advantage of in a regionspecific carbene annulation by 2-pentyne leading to a bis-ketal hydroquinone complex.

Carbenliganden als anthracyclinon-synthone: III. Anellierung von naphthyl(methoxy)carben-liganden: Regiospezifische (B + C)-ringverknüpfung in der daunomycinon-synthese☆

Abstract The chromium-mediated annulation of 1,4-dimethoxy-2-naphthyl(methoxy)- carbene with methyl 3-carbomethoxy-5-hexynoate and carbon monoxide offers a novel regiospecific (B + CD) route relevant to the total synthesis of 4-demethoxydaunomycinone.