Developments in Chiral Binaphthyl‐Derived Brønsted/Lewis Acids and Hydrogen‐Bond‐Donor Organocatalysis

Catalytic, highly enantioselective, direct amination of enecarbamates

Highly enantioselective electrophilic amination reaction of enecarbamates with dibenzylazodicarboxylate and oxygenated nucleophiles catalyzed by chiral phosphoric acids is reported. Subsequent reduction or oxidation of the resulting precursors of α-hydrazinoimines leads to 1,2-diamines or α-amino acid precursors, respectively, in excellent yields and enantioselectivities.

Quinine catalysed asymmetric Michael additions in a sustainable solvent

Diethyl carbonate is shown to be a suitable, sustainable solvent in which to carry out quinine catalysed asymmetric Michael additions of malononitriles to enones.

Quantification of electrophilic activation by hydrogen-bonding organocatalysts

A spectrophotometric sensor is described that provides a useful assessment of the LUMO-lowering provided by catalysts in Diels–Alder and Friedel–Crafts reactions. A broad range of 33 hydrogen-bonding catalysts was assessed with the sensor, and the relative rates in the above reactions spanned 5 orders of magnitude as determined via ¹H- and ²H NMR spectroscopic measurements, respectively. The differences between the maximum wavelength shift of the sensor with and without catalyst (Δλ_max^–1) were found to correlate linearly with ln(k_rel) values for both reactions, even though the substrate feature that interacts with the catalyst differs significantly (ketone vs nitro). The sensor provides an assessment of both the inherent reactivity of a catalyst architecture as well as the sensitivity of the reaction to changes within an architecture. In contrast, catalyst pK_a values are a poor measure of reactivity, although correlations have been identified within catalyst classes.

The ever-expanding role of asymmetric covalent organocatalysis in scalable, natural product synthesis

Following the turn of the millennium, the role of asymmetric covalent organocatalysis has developed into a scalable, synthetic paradigm galvanizing the synthetic community toward utilization of these methods toward more practical, metal-free syntheses of natural products. A myriad of reports on asymmetric organocatalytic modes of substrate activation relying on small, exclusively organic molecules are delineating what has now become the multifaceted field of organocatalysis. In covalent catalysis, the catalyst and substrate combine to first form a covalent, activated intermediate that enters the catalytic cycle. Following asymmetric bond formation, the chiral catalyst is recycled through hydrolysis or displacement by a pendant group on the newly formed product. Amine- and phosphine-based organocatalysts are the most common examples that have led to a vast array of reaction types. This Highlight provides a brief overview of covalent modes of organocatalysis and applications of scalable versions of these methods applied to the total synthesis of natural products including examples from our own laboratory.

New hydrogen-bonding organocatalysts: Chiral cyclophosphazanes and phosphorus amides as catalysts for asymmetric Michael additions

Ten novel hydrogen-bonding catalysts based on open-chain P^V-amides of BINOL and chinchona alkaloids as well as three catalysts based on rigid cis-P^V-cyclodiphosphazane amides of N¹,N¹-dimethylcyclohexane-1,2-diamine have been developed. Employed in the asymmetric Michael addition of 2-hydroxynaphthoquinone to β-nitrostyrene, the open-chain 9-epi-aminochinchona-based phosphorus amides show a high catalytic activity with almost quantitative yields of up to 98% and enantiomeric excesses of up to 51%. The cyclodiphosphazane catalysts show the same high activity and give improved enantiomeric excesses of up to 75%, thus representing the first successful application of a cyclodiphosphazane in enantioselective organocatalysis. DFT computations reveal high hydrogen-bonding strengths of cyclodiphosphazane P^V-amides compared to urea-based catalysts. Experimental results and computations on the enantiodetermining step with cis-cyclodiphosphazane 14a suggest a strong bidentate H-bond activation of the nitrostyrene substrate by the catalyst.

Secondary stereocontrolling interactions in chiral Brønsted acid catalysis: study of a Petasis–Ferrier-type rearrangement catalyzed by chiral phosphoric acids

Guided by computational studies, the involvement of non-classical C–H⋯O hydrogen bonds and π–π stacking interactions were found to be crucial for high stereocontrol in a chiral phosphoric acid-catalyzed reaction.

Enantioselective Prins cyclization: BINOL-derived phosphoric acid and CuCl synergistic catalysis

The synergistic catalysis between a chiral phosphoric acid and CuCl allows the development of the first enantioselective Prins cyclization.

Bifunctional ferrocene-based squaramide-phosphine as an organocatalyst for highly enantioselective intramolecular Morita–Baylis–Hillman reaction

Ferrocene is an excellent scaffold for organocatalysts. Bifunctional ferrocene-based phosphine (R_C,S_Fc)-1 shows high enantioselectivity in the intramolecular Morita–Baylis–Hillman reaction of 7-aryl-7-oxo-5-heptenals.

Development and mechanism of an enantioselective bromocycloetherification reaction via Lewis base/chiral Brønsted acid cooperative catalysis

The development of a binary catalyst system for bromocycloetherification, consisting of an achiral Lewis base and a chiral Brønsted acid, is described in detail. The results of preliminary kinetic studies are also presented.

Catalytic, asymmetric halofunctionalization of alkenes--a critical perspective

Despite the fact that halogenation of alkenes has been known for centuries, enantioselective variants of this reaction have only recently been developed. In the past three years, catalytic enantioselective versions of halofunctionalizations with the four common halogens have appeared and although important breakthroughs, they represent just the very beginnings of a nascent field. This Minireview provides a critical analysis of the challenges that accompany the development of general and highly enantioselective halofunctionalization reactions. Moreover, the focus herein, diverges from previous reviews of the field by identifying the various modes of catalysis and the different strategies implemented for asymmetric induction.

Rapid quantification of the activating effects of hydrogen-bonding catalysts with a colorimetric sensor

A sensor has been developed to quickly and simply assess the relative reactivity of different hydrogen-bonding catalysts. Specifically, blue-shifts seen upon treatment of H-bonding catalysts with the colorimetric compound 7-methyl-2-phenylimidazo[1,2-a]pyrazin-3(7H)-one correlate well to the K(eq) of binding to the sensor. The blue-shifts also show a high degree of correlation with relative rates in Diels-Alder reactions of methyl vinyl ketone and cyclopentadiene employing the H-bonding catalysts. The relevance of the sensor blue-shifts to the LUMO-lowering abilities of the H-bonding catalysts is discussed.

Bifunctional catalyst promotes highly enantioselective bromolactonizations to generate stereogenic C-Br bonds

A novel bifunctional catalyst derived from BINOL has been developed that promotes the highly enantioselective bromolactonizations of a number of structurally distinct unsaturated acids. Like some known catalysts, this catalyst promotes highly enantioselective bromolactonizations of 4- and 5-aryl-4-pentenoic acids, but it also catalyzes the highly enantioselective bromolactonizations of 5-alkyl-4(Z)-pentenoic acids. These reactions represent the first catalytic bromolactonizations of alkyl-substituted olefinic acids that proceed via 5-exo mode cyclizations to give lactones in which new carbon-bromine bonds are formed at a stereogenic center with high enantioselectivity. We also disclose the first catalytic desymmetrization of a prochiral dienoic acid by enantioselective bromolactonization.