Synthesis and Evaluation of Azolium-Based Halogen-Bond Donors.

A method for the synthesis of iodinated imidazolium and triazolium N-heterocyclic halogen-bond-donor catalysts has been developed. This approach was applied to the synthesis of a variety of 1,2,4-triazolium salts to prepare a series of novel chiral halogen-bond-donor catalysts. The counterions of the iodinated triazoliums can be readily exchanged with chiral and achiral non-coordinating counterions to produce unique scaffolds. Their ability to promote/catalyse a conjugate addition reaction with indole was investigated. Through these initial studies, a set of general guidelines and considerations for the application of these halogen-bond donors in organocatalysis have been established.

A Cooperative Hydrogen Bond Donor-Brønsted Acid System for the Enantioselective Synthesis of Tetrahydropyrans.

Carbocations stabilized by adjacent oxygen atoms are useful reactive intermediates involved in fundamental chemical transformations. These oxocarbenium ions typically lack sufficient electron density to engage established chiral Brønsted or Lewis acid catalysts, presenting a major challenge to their widespread application in asymmetric catalysis. Leading methods for selectivity operate primarily through electrostatic pairing between the oxocarbenium ion and a chiral counterion. A general approach to new enantioselective transformations of oxocarbenium ions requires novel strategies that address the weak binding capabilities of these intermediates. We demonstrate herein a novel cooperative catalysis system for selective reactions with oxocarbenium ions. This new strategy has been applied to a highly selective and rapid oxa-Pictet-Spengler reaction and highlights a powerful combination of an achiral hydrogen bond donor with a chiral Brønsted acid.

NHC-Catalyzed Formal [2+2] Annulations of Allenoates for the Synthesis of Substituted Oxetanes.

An N-heterocyclic carbene (NHC)-catalyzed reaction of γ-substituted allenoates for the synthesis of substituted oxetanes has been developed. The method provides an approach to access substituted oxetanes in a single step and is the first example of an NHC-catalyzed formal [2+2]-annulation employing γ-substituted allenoates with trifluoromethyl ketones. Mechanistic and modeling studies provide a rationale for the divergence in reactivity observed compared to the analogous reaction using unsubstituted allenoates and inform a hypothesis to explain the observed diastereoselectivity under different reaction conditions.

A continuing challenge: N-heterocyclic carbene-catalyzed syntheses of γ-butyrolactones.

Catalytic, stereoselective N-heterocyclic carbene-catalyzed reactions facilitate efficient construction of many different heterocyclic compounds, such as the enantioenriched 5-membered (γ) lactones highlighted in this tutorial review. Herein, various strategies to enable formal [3+2] type annulations between electrophilic carbonyl equivalents and homoenolate nucleophiles for the synthesis of γ-lactones are summarized.

Formation of Aza-ortho-quinone Methides Under Room Temperature Conditions: Cs2CO3 Effect.

Since the first report of a facile, room temperature process to access aza-ortho-quinone methides (aoQMs) by Corey in 1999, this chemistry has remained dormant until our report of an enantioselective catalytic example in 2014. We report a theoretical and experimental study of the key to success behind these successful examples to enable broader exploitation of this useful intermediate. We have discovered that transformations involving the aoQM are remarkably facile with barriers <17 kcal/mol. The main difficulty of exploiting aoQM in synthesis is that they are unstable (ΔG > 30 kcal/mol), precluding their formation under mild conditions. The use of Cs2CO3 as base is critical. It provides a thermodynamically and kinetically favorable means to form aoQMs, independent of the salt solubility and base strength. The exothermic formation of salt byproducts provides a driving force (average ΔG = -30.8 kcal/mol) compensating for the majority of the inherent unfavorable thermodynamics of aoQM formation.

Emerging Roles of in Situ Generated Quinone Methides in Metal-Free Catalysis.

ortho-Quinone methides have emerged recently as useful electrophiles in metal-free catalysis. New strategies to access these species in situ that are compatible with simultaneous nucleophile generation have provided a suite of innovative and selective transformations accessing heterocycles for use in organic synthesis.

Practical synthesis of fluorous oxazolidinone chiral auxiliaries from alpha-amino acids.

[reaction: see text] A series of new fluorous-supported oxazolidinone chiral auxiliaries has been prepared using a versatile and general five-step pathway, starting from readily available chiral alpha-amino acids. The key feature of this synthesis is the efficient generation of a suitably active perfluoroalkyllithium species. By use of this protocol, the auxiliaries can be obtained in high enantiomeric purity and on multigram scales from L-phenylalanine and L-valine with overall yields as high as 55%. The new methodology also incorporates fluorous solid-phase extraction on the large scale, allowing bulk quantities (up to 25 g) of fluorous compounds to be purified from the crude reaction mixture.