Highly chemoselective direct crossed aliphatic-aromatic acyloin condensations with triazolium-derived carbene catalysts.

It has been shown for the first time that triazolium precatalysts promote (in the presence of base) highly chemoselective crossed acyloin condensation reactions between aliphatic and ortho-substituted aromatic aldehydes. An o-bromine atom can serve as a temporary directing group to ensure high chemoselectivity (regardless of the nature of the other substituents on the aromatic ring) which then can be conveniently removed. The process is of broad scope and is operationally simple as it does not require the preactivation of any of the coupling partners to ensure selectivity.

Efficient catalytic, oxidative lactonization for the synthesis of benzodioxepinones using thiazolium-derived carbene catalysts.

An efficient, oxidative carbene-catalyzed lactonization reaction has been developed. Using thiazolium precatalysts, a variety of benzodioxepinone products are accessible in good to excellent yields under mild and operationally simple conditions. The reaction does not require high dilution conditions and proceeds via mild and selective oxidation with azobenzene, which can easily be recovered and reused applying inexpensive FeCl(3) as a formal terminal oxidant.

Highly enantioselective benzoin condensation reactions involving a bifunctional protic pentafluorophenyl-substituted triazolium precatalyst.

Improved catalyst design by incorporating a hydrogen bond donating substituent to improve enantiocontrol together with an acidifying pentafluorophenyl substituent to enhance catalyst efficiency results in a triazolium ion precatalyst that promotes the asymmetric archetypal benzoin condensation with excellent efficiency and unprecedented enantioselectivity.

An efficient carbene-catalyzed access to 3,4-dihydrocoumarins.

Dihydrocoumarins play an important role as flavor and fragrance compounds and can be prepared efficiently from o-hydroxycinnamaldehydes in a mild, atom-economic N-heterocyclic carbene-catalyzed redox lactonization. Corresponding coumarins are accessible via a one-pot domino oxidation lactonization procedure in the presence of oxidants.