David Monge

Acyl Phosphonates: Good Hydrogen Bond Acceptors and Ester/Amide Equivalents in Asymmetric Organocatalysis

This study demonstrates that unsaturated acyl phosphonates are excellent hydrogen-bond acceptors in enantioselective organocatalysis. By employing chiral thioureas or squaramides as catalysts, the acyl phosphonates are effectively coordinated and activated by hydrogen bonding, thereby providing successful relay of the chirality from the catalyst to the substrate. A variety of highly stereoselective conjugate additions to alpha,beta-unsaturated acyl phosphonates were performed, using different carbon-based nucleophiles such as oxazolones, indoles, and 1,3-dicarbonyl compounds. The reaction concept has been developed to be a double nucleophilic reaction, and it is shown that the acyl phosphonates serve as masked ester or amide equivalents, which upon quenching with the second nucleophile generate the parent structures in situ. Accordingly, formal C-C bond formation reactions of ester and amide substrates are achieved, affording a broad spectrum of optically active conjugate adducts in good yields and excellent enantioselectivities. Based on the experimental results, the mechanisms for the different reactions are discussed, including the approach of the oxazolones, indoles, and 1,3-dicarbonyl compounds to the acyl phosphonate coordinated to the catalyst and the role of the catalyst for the reaction course of the nucleophiles.

Asymmetric One‐Pot Sequential Organo‐ and Gold Catalysis for the Enantioselective Synthesis of Dihydropyrrole Derivatives

A direct asymmetric one-pot synthesis of optically active 2,3-dihydropyrroles from propargylated malononitrile and N-Boc-protected (Boc = tert-butoxycarbonyl) imines is presented. The approach is based on a bifunctional organocatalytic Mannich-type reaction and a subsequent gold-catalyzed alkyne hydroamination and isomerization. The compatibility of both catalytic systems is presented and the overall transformation results in good yields (up to 70 %) with high selectivities (endo/exo > 10:1) and enantioselectivities (up to 88 % ee). The absolute configuration of the final products is unambiguously established by X-ray analysis. To highlight the synthetic potential of the accessed heterocyclic compounds, their transformation into 1-pyrrolines, which represent direct precursors of pyrrolidines, is presented.

Synthesis of 1,2,4-triazolines: base-catalyzed hydrazination/cyclization cascade of α-isocyano esters and amides.

A convenient, efficient synthesis of 1,2,4-triazolines from α-isocyano esters/amides and azodicarboxylates is presented. The developed reaction cascade is based on a base-catalyzed hydrazination-type reaction followed by a subsequent cyclization providing the triazolines in good to excellent yields (75-99%). Phosphine-catalyzed and preliminary asymmetric phase-transfer catalysis approaches have also been investigated.

Masked Unsaturated Esters/Amides in Asymmetric Organocatalysis

This Concept article summarizes strategies and developments regarding the use of masked unsaturated esters/amides in asymmetric organocatalysis. Useful substrates are categorized by the design of their inherent carboxylate template. This template group not only enables their functionality as ester surrogates, but also define their accessibilities, modes of interactions with catalysts and the simplicity with which they transform back to the parent carboxylates. Both covalent and noncovalent catalytic systems are discussed and examples showing the entire process (from substrates-to-functionalized ester/amides) are given.

Pyridine–Hydrazones as N,N′-Ligands in Asymmetric Catalysis: Pd(II)-Catalyzed Addition of Boronic Acids to Cyclic Sulfonylketimines

The design, synthesis, and coordination features of a novel class of chiral pyridine-hydrazone ligands are described. As a first application, L/Pd(TFA)2 complexes served as catalysts in the 1,2-addition of arylboronic acids to saccharin-derived cyclic ketimines, affording products in high yields and enantioselectivities. The method was also applied to more challenging 3,4-disubstituted 1,2,5-thiadiazole 1,1-dioxides, affording again high yields and enantioselectivities along with high regioselectivities for unsymmetrically substituted derivatives.

Hydrazones as Singular Reagents in Asymmetric Organocatalysis

This Minireview summarizes strategies and developments regarding the use of hydrazones as reagents in asymmetric organocatalysis, their distinct roles in nucleophile-electrophile, cycloaddition, and cyclization reactions. The key structural elements governing the reactivity of these reagents in a preferred pathway will be discussed, as well as their different interactions with organocatalysts, leading to diverse activation modes. Along these studies, the synthetic equivalence of N-monoalkyl, N,N-dialkyl, and N-acyl hydrazones with several synthons is also highlighted. Emphasis is also put on the mechanistic studies performed to understand the observed reactivities. Finally, the functional group transformations performed from the available products has also been analyzed, highlighting the synthetic value of these methodologies, which served to access numerous families of valuable multifunctional compounds and nitrogen-containing heterocycles.

Synthesis of enantioenriched azo compounds: organocatalytic Michael addition of formaldehyde N-tert-butyl hydrazone to nitroalkenes

The unprecedented diaza-ene reaction of formaldehyde N-tert-butyl hydrazone with nitroalkenes can be efficiently catalyzed by an axially chiral bis-thiourea to afford the corresponding diazenes in good to excellent yields (60-96%) and moderate enantioselectivities, up to 84 : 16 er; additional transformation of diazenes into their tautomeric hydrazones proved to be operationally simple and high-yielding, affording bifunctional compounds which represent useful intermediates for the synthesis of enantioenriched β-nitro-nitriles and derivatives thereof.

Asymmetric organocatalytic Strecker-type reactions of aliphatic N,N-dialkylhydrazones

The enantioselective organocatalytic Strecker-type reaction of aliphatic N,N-dialkylhydrazones is presented. Using trimethylsilyl cyanide (TMSCN) as the cyanide source, the reaction can be efficiently catalyzed by a tert-leucine-derived bifunctional thiourea to afford the corresponding hydrazino nitriles in good to excellent yields (50-96%) and moderate to good enantioselectivities, up to 86% ee. Further transformations of the nitrile functionality allow access to useful protected hydrazino acids and imidazolidinones. Interestingly, some of the hydrazino nitriles and their derivatives could be recrystallized in high recovery, yielding essentially pure enantiomers.

Solvent-free synthesis of quaternary α-hydroxy α-trifluoromethyl diazenes: the key step of a nucleophilic formylation strategy

An efficient, scalable and operationally simple one-pot, 2-step strategy for the nucleophilic formylation of trifluoromethyl ketones is presented. The key step is an unprecedented diaza-carbonyl-ene reaction of formaldehyde tert-butyl hydrazone and trifluoromethyl ketones under solvent-free conditions. This reaction proved to be very fast, clean and high-yielding, affording densely functionalised α-hydroxy α-trifluoromethyl diazenes. The ensuing diazene-to-aldehyde transformation, avoiding protection/deprotection reactions and chromatographic purifications, and subsequent derivatizations in a one-pot fashion provide a direct entry to a variety of useful trifluoromethylated building blocks.

Bifunctional Squaramide Organocatalysts for the Asymmetric Addition of Formaldehyde tert-Butylhydrazone to Simple Aldehydes

The nucleophilic addition of formaldehyde tert-butylhydrazone to simple aldehydes (a formal hetero-carbonyl-ene reaction) can be performed with good reactivity and excellent enantioselectivity by virtue of the dual hydrogen-bonding activation exerted by amide-squaramide organocatalysts. The resulting hydroxydiazenes (azo alcohols) were isolated in high yields as enantiomerically enriched azoxy compounds after a regioselective azo-to-azoxy transformation. Subsequent derivatization provides an entry to relevant amino alcohols, oxazolidinones, and derivatives thereof.