Direct organocatalytic transfer hydrogenation and C-H oxidation: high-yielding synthesis of 3-hydroxy-3-alkyloxindoles

Readily available 3-alkylideneoxindoles were effectively reduced to 3-alkyloxindoles through transfer hydrogenation using Hantzsch ester as a reducing agent at ambient temperature and the greenness/sustainability of this protocol was assessed by correlation with Pd/C-mediated hydrogenation with hydrogen gas. Furthermore, an organocatalytic method was developed to access drug-like 3-alkyl-3-hydroxyoxindoles by C-H oxidation of 3-alkyl-indolin-2-one, using a catalytic amount of 1,1,3,3-tetramethylguanidine (TMG) as an organic base and dissolved oxygen in THF as an oxidant at room temperature. Key reaction intermediates were observed by controlled on-line ESI-HRMS experiments and identified by their corresponding mass (m/z) analysis. This two-step high-yielding transfer hydrogenation/C-H oxidation protocol was used for the total synthesis of medicinally important 3-cyanomethyl-3-hydroxyoxindole and formal total synthesis of (±)-alline and (±)-CPC-I in very good overall yields compared to previous methods.

Ligand libraries for high throughput screening of homogeneous catalysts

This review describes different approaches to construct ligand libraries towards high throughput screening of homogeneous metal catalysts.

Photocatalytic Synthesis of γ-Lactones from Alkenes: High-Resolution Mass Spectrometry as a Tool To Study Photoredox Reactions

A mild photocatalytic manifold for the synthesis of γ-lactones has been developed. Utilizing Ru(bpy)₃Cl₂ as the photocatalyst, a cheap and reproducible synthetic protocol for γ-lactones has been introduced. Mechanistic studies revealed the successful monitoring of photocatalytic reactions and radical intermediates via high-resolution mass spectrometry.

Mass Spectrometric Back Reaction Screening of Quasi-Enantiomeric Products as a Mechanistic Tool

In this account, we discuss a mass spectrometric method that enables unambiguous identification of intermediates involved in the enantioselective step of a catalytic cycle. This method, which we originally developed for rapid evaluation of chiral catalysts, is based on monitoring the back reaction of mass-labeled quasi-enantiomeric products by ESI-MS. In this way, the intrinsic enantioselectivity of a chiral catalyst can be determined directly by quantification of catalytically relevant intermediates. By comparing the results from the forward and back reaction, direct evidence for the involvement of a catalytic intermediate in the enantioselective step can be obtained. In addition, insights about the energy profile of the catalytic cycle may be gained. The potential of back reaction screening as a mechanistic tool is demonstrated for organocatalytic aldol reactions, 1,4-additions of aldehydes to nitroolefins, Diels-Alder reactions, Michael additions, and Morita-Baylis-Hillman reactions.

Accessing Centnerszwer's quasiracemate – molecular shape controlled molecular recognition

Reinvestigation of M. Centnerszwer's 1899 quasiracemate using tailor-made additives offers important insight to molecular shape directed supramolecular assemblies.