[4+1]/[2+1] Cycloaddition Reactions of Fischer Carbene Complexes with alpha,beta-Unsaturated Ketones and Aldehydes

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José Barluenga,* Hugo Fanlo, Salomé López, Josefa Flórez

Instituto Universitario de Química Organometálica, Enrique Moles, Unidad Asociada al CSIC, Universidad de Oviedo, Julián Clavería 8, 33006 Oviedo, Spain

Group 6 Fisher carbenes are know to transfer carbene to electro-deficient alkenes. However, reactions with alpha,beta-unsaturated ketones and aldehydes (enones and enals) are generally poor. The article presented a new development in a successful use of Chromium-based Fisher carbene as a carbene transfer reagent to enones and enals.

First, reaction was invetigated with carbenes 1a and 1b with enone 2a.Reactions were found to proceed to give dihydrofurans in a formal [4+1] cycloaddition reaction. Products 3a and 3b were found to easily aromatize to furans 4a and 4b.

Next, the effect of solvents was studied.THF was found to be optimal for the reaction either at 80 C or 100 C or especially efficient in microwave. The scope the reaction was then studied with a large variety of enones, enals with various Fisher carbenes. The table showed the scope of the reaction. This is a large table, so it may appear illegible. Look at the article for the actual table.In general, enones react faster than enals. Some of these dihydrofurans had to be purified by deactivated silica gel to prevent aromatization to form furans and ring-opening to form 1,4-dicarbonyl compounds.

Additionally, the authors also proved that the formal [4+1] to give dihydrofurans went through a [2+1] carbene transfer to yield cyclopropane follow by rearrangements.

Subsequently, the authors showed that simple treatment of resulting dihydrofurans with HBF4 in Et2O and then SiO2 can cleanly provide aromatized products.

Furthermore, dihydrofurans of type 3 when treated with HCl in THF could ring-open to give 1,4-diketones, which is a product equivalent to acyl anion undergoing 1,4-addition (umpolung reactivity). As for 3i where aromatization was not possible, simple treatment with SiO2 also led to 1,4-diketone 6i.