ChemInform Abstract: Unexpected Ti^III/Mn‐Promoted Pinacol Coupling of Ketones.

Abstract

Alcohols Q 0230Unexpected Ti III /Mn-Promoted Pinacol Coupling of Ketones. -A number of aromatic ketones as well as unsaturated ketones can be used as substrates. Inter-and intramolecular reactions are possible. Additionally, it is shown that trapping of the initially formed ketyl radical with α,β-unsaturated esters allows efficient access to lactones likeXIII). -(PARADAS, M.; CAMPANA, A. G.; ESTEVEZ, R. E.; ALVAREZ DE CIENFUEGOS, L.; JIMENEZ, T.; ROBLES, R.; CUERVA*, J. M.; OLTRA, J. E.; J.

The Electron-Way: Metal-Catalyzed Reductive Umpolung Reactions of Saturated and α,β-Unsaturated Carbonyl Derivatives

“…42 It was later discovered by Cuerva, Oltra and co-workers that the homo-coupling of aromatic or vinylic ketones could be achieved as well, with high yield and stereoselectivity, using a similar catalyst system (Scheme 8 and Table 4, entry 16). 43 For the stoichiometric reaction, an outer-sphere SET from the metallic terminal reductant was proposed (Scheme 8, bottom), which is in contrast to the previously discussed mechanisms where the titanocene acted as electron-transfer catalyst (see Schemes 4 and 7). It was assumed instead that the titanium(III) species acts as a Lewis acid and template for the carbon-carbon bond-forming step.…”

“…Such outer-sphere electron transfers were proposed for the indium-catalyzed reactions and the titanium(III)-catalyzed pinacol coupling of ketones discussed above (see sections 2.1 and 2.2). 30,43 In general, the mechanisms of the reductive imine coupling catalyses still require further investigation.…”

Cp2TiCl-catalyzed highly stereoselective intramolecular epoxide allylation using allyl carbonates

“…At this point, we expected that an oxygenated function in the β-position would act as a good leaving group, thus directing the final elimination towards IV assisted by Cp 2 TiCl as the Lewis acid. 27,28 In fact, we had previously observed the Cp 2 TiCl-mediated radical fragmentation of β-acetoxy alkyl radicals toward the corresponding alkenes. 21,29 In this alternative radical pathway, Cp 2 TiCl would play a crucial dual role in the intramolecular epoxide allylation with oxygenated-allyl groups: (i) starting the reaction by homolytic opening of the oxirane ring and (ii) controlling the final product obtained by radical fragmentation.…”

Reacciones de propargilación de compuestos carbonílicos catalizadas por titanoceno (III)

“…Sin embargo, bajo nuestras condiciones de reacción, no hemos observado la aparición de productos de pinacolización. Esto indica que el proceso de adición de la especie homopropargílica es más rápido que la posible reacción de acoplamiento entre grupos carbonilo, que conduciría al producto de pinacolización [27] . También resulta interesante desde un punto de vista sintético que la configuración de los dobles enlaces  2 en los sustratos de partida 8 y 9 se retenga después del proceso de acoplamiento.…”

“…. Para ello, sometimos una serie de cetonas(18)(19)(20)(21)(22)(23)(24)(25)(26)(27) a las mismas condiciones de reacción empleadas en el caso de la propargilación de aldehídos. Aunque en este caso, se empleó cloruro de propargilo(2), que da mejores resultados que el correspondiente…”