A direct intermolecular cross-benzointype condensation catalyzed by an N-heterocyclic carbene has been developed. The cross-coupling of commercially available aromatic aldehydes and trifluoromethyl ketones results in a-hydroxy-a-trifluoromethyl ketones bearing a quaternary stereocenter with excellent chemoselectivity and good to excellent yields.Keywords: cross-benzoin reaction; N-heterocyclic carbenes; organocatalysis; trifluoromethyl substitution; umpolungIn their pioneering and historic paper on the benzoyl compounds of 1832 [1] Wçhler and Liebig not only started the chemistry of aromatic compounds, such as benzaldehyde, benzoic acid, benzoyl chloride and benzamide, but also reported the cyanide-catalyzed (bitter almond oil) formation of benzoin, which in a sense can be seen as the start of organocatalysis. Since these early days of organic chemistry, the cyanide-and later carbene-catalyzed coupling of two aldehyde molecules to form the corresponding a-hydroxy ketones (benzoin, acyloin) has been investigated extensively. [2] Although significant progress has been made with enantioselective and intramolecular versions in recent years, [3] there still are limitations in the intermolecular coupling of two different aldehydes and of aldehydes with ketones. In these cases the outcome is typically a mixture of all possible symmetrical and unsymmetrical acyloins, with the chemoselectivity strongly depending on the relative thermodynamic stability of the corresponding products.[4] In order to overcome this problem several research groups have made excellent contributions by developing enzymatic approaches [5] or utilizing acylsilanes [6] and acylphosphonates [7] as acyl anion equivalents in cyanide-catalyzed transformations. In addition, very recently Scheidt et al. reported intermolecular cross-acyloin reactions by fluoride-promoted addition of O-silylthiazolium salts.[8] However, to the best of our knowledge, there is no precedent for a direct organocatalytic intermolecular coupling of aldehydes with ketones, which we would like to report in this communication.Based on our mechanistic proposal for the Stetter reaction, which relies on a reversible formation of the benzoin product prior to the Stetter product formation, [9] we envisaged to use benzoins as masked aldehyde equivalents.[10] To our delight, benzoin (1) in combination with an excess of trifluoroacetophenone (2) as reactive electrophilic ketone component led chemoselectively to the crossed acyloin product 3 in high yield, employing 20 mol% of the bicyclic triazolium salt 4[11] as precatalyst in the presence of 20 mol% DBU in THF at room temperature (Scheme 1).Since the crossed acyloin product 3 containing a tertiary alcohol seemed to be energetically preferred relative to benzoin (1) a simpler and more practical version of this concept would be to employ the aldehyde component directly in this transformation (Scheme 2). Under the reaction conditions given (10 mol% carbene precursor 4 and 10 mol% DBU), it was indeed possible to obtain the cross-co...