Unexpected Reaction Pathways in the Reaction of Alkoxyalkynylchromium(0) Carbenes with Aromatic Dinucleophiles

Abstract

Thermal- or SiO(2)-induced reactions of the Michael adducts of 1,2-aromatic dinucleophiles and alkynylchromium(0) carbene complexes, compounds 7-10, form different products in good yields depending on the nature of the aromatic dinucleophile used. Thus, 1,2-diaminobenzene derivatives 7 and 8 rearrange to pentacarbonylchromium(0) isocyanide complexes 11, 12, 14, and 15 in a process that occurs through bicyclic intermediates 24. Adducts 9 derived from o-aminophenol give 2,3-dihydro-1,5-benzoxazepine derivatives 17 by intramolecular 1,2-addition, followed by protonation at the chromium center and reductive elimination. In contrast, base-promoted addition of the phenolic hydroxy group in compound 9 a affords 3-ethoxy-5-phenyl-5,6-dihydro-2H-1,6-benzoxazocin-2-one (18), together with the expected adduct 17 a. Compound 18 is formed by a nucleophilic addition to a CO ligand in a preformed carbene complex. This is a new example of the rare attack of a nucleophile on a CO ligand in a Fischer carbene complex. Adducts 10 form seven-membered-ring carbene complexes 19 and 20 by intramolecular aminolysis. In contrast, reaction of alkynyl carbene complexes with 1,8-diaminonaphthalene under very mild conditions leads to 2-substituted perimidines 33 together with the corresponding ethoxymethylmetal carbene complex 32 through an unprecedented fragmentation process in a formal retro-Aumann reaction.

Transfer of heterocyclic carbene ligands from chromium to gold, palladium and platinum

“…The assumption is supported by the results of earlier investigations [23] and by the isolation of an addition product, the corresponding alkenyl(alkoxy)carbene complex 3 (99% yield), when 1,2-dimethylhydrazine was replaced by 1,2-di-iso-propylhydrazine in the reaction with 1a (Scheme 2).…”

“…This was also true when the reaction of 1a with 1,2-di-iso-propylhydrazine was carried out at À78°C. In contrast, the reaction of the tungsten analogue of 1a with 1,2-dimethylhydrazine at À78°C afforded 27% of an alkynyl(hydrazino)carbene [23].…”

“…Deprotonation of the terminal alkynes phenylacetylene, 4-dimethylaminophenylacetylene and ethinylferrocene with n-butyllithium at À78°C and subsequent reaction with hexacarbonyl chromium followed by alkylation of the resulting metallate with triethyloxonium tetrafluoroborate gave the alkynyl(ethoxy)carbene complexes 1a [22], 1b and 1c [23]. Addition of 1,2-dimethylhydrazine (in situ generated from commercially available 1,2-dimethylhydrazine dihydrochloride and concentrated NaOH) to solutions of 1a-c in THF at ambient temperature afforded the 1,2-dimethyl-pyrazolin-3-ylidene complexes 2a-c in 67-94% yield (Scheme 1).…”

Mononuclear Biscarbene Complexes by Direct Nucleophile Addition to a CO Ligand of Fischer Arylcarbene Complexes

“…[9] On the other hand, mononuclear biscarbene complexes of Group 6 have been proposed as intermediates in the thermal carbene-carbene coupling reactions of either Fischer mononuclear monocarbene complexes [10] or dinuclear biscarbene complexes. [11] These carbene ligand coupling processes, that produced alkenes, have also been reported to occur at room temperature in the presence of a palladium(0) [12] or a copper(I) [13] catalyst.…”

13 Chromium, molybdenum, and tungsten

“…Unexpected reaction pathways were also observed with alternative aromatic dinucleophiles, o-aminophenol, and o-aminothiophenol. 125 The first examples of a meta-benzannulation from the reaction of Fischer carbene complexes with alkynes tethered to the a-carbon gives 1,3-and 1,4-disubstituted paracyclophanes together with metacyclophanes and tricyclic enones. 126 The yields of these products were found to be dependent upon the length of the tether and on the nature of the solvent.…”