Biomimetic carbene catalyzed oxidative activation of aldehydes

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Carbenes have been intensively used as catalysts for the umpolung of aldehydes. The well investigated benzoin condensation and the Strecker reaction are prominent examples. In these processes the carbene catalyst is used to intermediately generate the so called Breslow intermediate which further reacts in an ionic process with an aldehyde, with an activated electron-poor double bond or with other electrophiles. More recently, leaving groups at the -position of the Breslow intermediate have been installed to transfer the enol ether into an activated acylcarbenium ion. Most of the carbene-catalyzed transformations of aldehydes deal with ionic transformations of the Breslow intermediate. In the proposal we use single electron transfer oxidation reagents for the formation of activated acylcarbenium ions in biomimetic processes. This type of carbene catalysis also used by nature is not well investigated so far in synthesis. Among dioxygen other environmentally benign single electron transfer oxidants in particular nitroxides will be tested. Metal based oxidants will be avoided in these organocatalyzed reactions. Activation of ,-unsaturated aldehydes will generate activated Michael acceptors which should undergo 1,4-addition reactions. We plan to study stereoselective intramolecular 1,4-additions of alcohols, protected amines and thiols via carbene catalyzed redox activations. Moreover, acylcarbenium ions will also be used in intramolecular transition metal free aldehyde C-H-activation reactions for the formation of cyclic ketones.