α-Unsubstituted Pyrroles by NHC-Catalyzed Three-Component Coupling: Direct Synthesis of a Versatile Atorvastatin Derivative.

A practical one-pot cascade reaction protocol provides direct access to valuable 1,2,4-trisubstituted pyrroles. The process involves an N-heterocyclic carbene (NHC)-catalyzed Stetter-type hydroformylation using glycolaldehyde dimer as a novel C1 building-block, followed by a Paal-Knorr condensation with primary amines. The reaction makes use of simple and commercially available starting-materials and catalyst, an important feature regarding applicability and utility. Low catalyst loading under mild reaction conditions afforded a variety of 1,2,4-substituted pyrroles in a transition-metal-free reaction with high step economy and good yields. This methodology is applied in the synthesis of a versatile Atorvastatin precursor, in which a variety of modifications at the pyrrole core structure are possible.

Mechanistic Studies on a Cooperative NHC Organocatalysis/Palladium Catalysis System: Uncovering Significant Lessons for Mixed Chiral Pd(NHC)(PR3) Catalyst Design.

A comprehensive investigation of the mechanism of the highly enantioselective Pd(PPh3)4/NHC-catalyzed annulation of vinyl benzoxazinanones and enals has been conducted. A study of reaction orders supports the postulated cooperative catalysis. Interestingly, a detailed investigation of the catalytically active palladium species pointed toward a dual role of the NHC acting as an organocatalyst and forming a novel mixed ligand Pd/NHC/phosphine complex. The catalytically active Pd/NHC/phosphine complex represents a new class of chiral palladium catalyst. Remarkably, phosphine plays a crucial role in this transformation. These complexes could be characterized by X-ray crystallographic analysis and employed as catalysts for the enantioselective [4 + 1] annulation reaction of vinyl benzoxazinones and sulfur ylides in good yields and good enantioselectivities.

Lewis acid catalysis: catalytic hydroboration of alkynes initiated by Piers' borane.

Terminal and internal alkynes are efficiently hydroborated to (E)-alkenyl pinacol boronic esters with excellent yields and selectivities using a Lewis acid catalyst. In the case of Piers' borane (HB(C6F5)2) the borane acts as a pre-catalyst generating dissymmetrically gem-diborylated species of the form RCH2CR'(Bpin)(B(C6F5)2) which are the active catalysts.

Cooperative N-Heterocyclic Carbene/Palladium-Catalyzed Enantioselective Umpolung Annulations.

A combination of NHC organocatalysis and transition-metal catalysis gives rise to fundamentally new cooperative reactivity and enables the regio- and enantioselective annulation reaction between enals and vinyl benzoxazinanones. The cooperative umpolung annulation eliminates mutual deactivation and leads to a diverse set of benzazepine derivatives in good yields with excellent enantioselectivities (up to 99% ee). The development of such a cooperative catalytic system dramatically expands the scope of NHC organocatalysis by opening up new metal-catalyzed reaction pathways for homoenolate intermediates.

Switchable selectivity in an NHC-catalysed dearomatizing annulation reaction.

The development of general catalytic methods for the regio- and stereoselective construction of chiral N-heterocycles in a diversity-oriented fashion remains a formidable challenge in organic synthesis. N-heterocyclic carbene (NHC) catalysis has been shown to produce a variety of outcomes, but control of the reactivity has rarely been demonstrated. Here we report a switchable catalytic activation of enals with aromatic azomethine imines that provides high selectivity using NHC organocatalysts. The original selectivity corresponds to the acidity of the base used in the reaction. The catalytically generated chiral homoenolate or enol intermediate undergoes enantioselective annulation with electrophiles such as N-iminoquinolinium ylides, N-iminoisoquinolinium ylides and ß-N-iminocarboline ylides. The good-to-high overall yields, high regioselectivities and excellent enantioselectivities observed are controlled by the catalyst and reaction conditions.

Enantioselective intramolecular hydroacylation of unactivated alkenes: an NHC-catalyzed robust and versatile formation of cyclic chiral ketones.

A highly enantioselective intramolecular N-heterocyclic carbene (NHC)-catalyzed hydroacylation reaction gives access to a range of cyclic ketones from unactivated olefin-substituted aldehydes (up to 99 % ee). Remarkably, aliphatic aldehydes were also transformed efficiently in an NHC-catalyzed hydroacylation reaction for the first time.