LongLarf

Two tocopherols and one (or two) S atom(s): An easy access to all combinations of dicopheryl sulfides and disulfides from the same starting materials and under very mild conditions is reported. The role of sulfur atom(s) on the chain‐breaking antioxidant activity profile of this sulfur derivatives of vitamin E is disclosed.

Abstract

Symmetrical ditocopheryl disulfides (Toc) ₂ S₂ and symmetrical and unsymmetrical ditocopheryl sulfides (Toc)₂ S were simply prepared under remarkably mild conditions with complete control of the regiochemistry by using δ‐, γ‐, and β‐tocopheryl‐N‐thiophthalimides (Toc‐NSPht) as common starting materials. The roles of sulfur atom(s), H‐bond and aryl ring substitution pattern on the antioxidant profile of these new compounds, which were assembled by linking together two tocopheryl units, are also discussed.

A DAP hand at rearrangement: 1,3,2‐Diazaphospholenes (DAPs) are efficient catalysts for ambient‐temperature reductive Claisen rearrangements. This method tolerates a wide variety of functional groups and is enantiospecific for substrates with existing stereogenic centers. The diastereoselectivity can be controlled by the choice of solvent and DAP catalyst.

Abstract

1,3,2‐Diazaphospholenes (DAPs) are an emerging class of organic hydrides. In this work, we exploited them as efficient catalysts for very mild reductive Claisen rearrangements. The method is tolerant towards a wide variety of functional groups and operates at ambient temperature. Besides being enantiospecific for substrates with existing stereogenic centers, the diastereoselectivity can be switched by varying solvents and DAP catalysts. The reaction kinetics show direct rearrangements of O‐bound phospholene enolates and provide a proof‐of‐principle for catalytic enantioselective reactions.

Synlett
DOI: 10.1055/s-0037-1611856

A facile, rapid, metal-free regioselective halogenation and thiocyanation of imidazo[1,2-a]pyridine/pyrimidine heterocycles has been achieved under solvent-free reaction conditions. Halogenations and thiocyanation of the heterocycles could be accomplished by simple grinding of reactants and hypervalent iodine reagents with the corresponding alkali metal or ammonium salts. The method has been extrapolated to a cleaner synthesis of brominated imidazo[1,2-a]pyridine/pyrimidine derivatives, starting from the corresponding heterocyclic amines and substituted α-bromoketones, utilising HBr generated in situ as the source of bromine.
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© Georg Thieme Verlag Stuttgart · New York

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Multi‐task catalysts: Unique self‐assembled macrocyclic multinuclear Zn^II and Ni^II complexes with binaphthyl–bipyridyl ligands (L) were synthesized. These complexes consisted of an outer ring (Zn₃L₃ or Ni₃L₃) and an inner core (Zn₂ or Ni). These complexes acted as multitask catalysts for CO₂ fixations.

Abstract

Unique self‐assembled macrocyclic multinuclear Zn^II and Ni^II complexes with binaphthyl‐bipyridyl ligands (L) were synthesized. X‐ray analysis revealed that these complexes consisted of an outer ring (Zn₃L₃ or Ni₃L₃) and an inner core (Zn₂ or Ni). In the Zn^II complex, the inner Zn₂ part rotated rapidly inside the outer ring in solution on an NMR timescale. These complexes exhibited dual catalytic activities for CO₂ fixations: synthesis of cyclic carbonates from epoxides and CO₂ and temperature‐switched N‐formylation/N‐methylation of amines with CO₂ and hydrosilane.

Abstract

Aromatic N‐oxides (ArN−OX) are desirable biologically active compounds with a potential for application in pharmacy and agriculture industries. As biocatalysis is making a great impact in organic synthesis, there is still a lack of efficient and convenient enzyme‐based techniques for the production of aromatic N‐oxides. In this study, a recombinant soluble di‐iron monooxygenase (SDIMO) PmlABCDEF overexpressed in Escherichia coli was showed to produce various aromatic N‐oxides. Out of 98 tested N‐heterocycles, seventy were converted to the corresponding N‐oxides without any side oxidation products. This whole‐cell biocatalyst showed a high activity towards pyridines, pyrazines, and pyrimidines. It was also capable of oxidizing bulky N‐heterocycles with two or even three aromatic rings. Being entirely biocatalytic, our approach provides an environmentally friendly and mild method for the production of aromatic N‐oxides avoiding the use of strong oxidants, organometallic catalysts, undesirable solvents, or other environment unfriendly reagents.

The first quarter century: The Buchwald–Hartwig amination enables the formation of C(sp²)−N bonds through the Pd‐catalyzed coupling of (hetero)aryl halides and pseudohalides with amines. This Minireview discusses the development of this methodology over the past 25 years, including highlights of some of the most recent applications.

Abstract

The Pd‐catalyzed coupling of aryl (pseudo)halides and amines is one of the most powerful approaches for the formation of C(sp²)−N bonds. The pioneering reports from Migita and subsequently Buchwald and Hartwig on the coupling of aminostannanes and aryl bromides rapidly evolved into general and practical tin‐free protocols with broad substrate scope, which led to the establishment of what is now known as the Buchwald–Hartwig amination. This Minireview summarizes the evolution of this cross‐coupling reaction over the course of the past 25 years and illustrates some of the most recent applications of this well‐established methodology.

Abstract

Epoxides are highly useful synthetic intermediates and they can be synthesized stereo‐ and regioselectively. Due to the strained three‐membered ring, their synthetic utility is limited. In many cases, it would be better to transform epoxides to other stable functional groups which could be used further in the synthesis. This isomerization would offer an alternate pathway to utilize the synthetic potential of epoxides. Despite the potential of such isomerization reactions, they have not been widely explored by chemists. In this review, the methods to isomerize epoxides into stable functional groups such as alcohols, aldehydes, ketones as well as trans‐epoxides, both stereo‐ and regioselectively, are discussed.

Synlett
DOI: 10.1055/s-0037-1611849

The enantioselective protonation of silyl enol ethers was realized in the presence of a pentacarboxycyclopenta-1,3-diene-based chiral Brønsted acid catalyst with water as an achiral proton source to give the corresponding α-aryl ketones in good yields and up to 75% ee.
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© Georg Thieme Verlag Stuttgart · New York

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Bio‐hybrid products: A transition‐metal catalyst system for the selective synthesis of cyclic and linear acetals from the combined utilization of carbon dioxide, molecular hydrogen, and biomass‐derived diols is presented. This new synthesis option paves the way to novel fuels, solvents, or polymer building blocks by the recently established “bio‐hybrid” approach (see figure).

Abstract

Herein a transition‐metal catalyst system for the selective synthesis of cyclic and linear acetals from the combined utilization of carbon dioxide, molecular hydrogen, and biomass derived diols is presented. Detailed investigations on the substrate scope enabled the selectivity of the reaction to be largely guided and demonstrated the possibility of integrating a broad variety of substrate molecules. This approach allowed a change between the favored formation of cyclic acetals and linear acetals, originating from the bridging of two diols with a carbon‐dioxide based methylene unit. This new synthesis option paves the way to novel fuels, solvents, or polymer building blocks, by the recently established “bio‐hybrid” approach of integrating renewable energy, carbon dioxide, and biomass in a direct catalytic transformation.

Aerobic epoxidation of tertiary allylic alcohols remains a significant challenge. Herein, we report an efficient and highly chemoselective copper‐catalyzed epoxidation and semipinacol rearrangement reaction of tertiary allylic alcohols with molecular oxygen. The solvent 1,4‐dioxane activates dioxygen, thereby avoiding the addition of sacrificial reductant.

Cutting with air: C(sp³)−C(sp³) single bonds in amines can be cleaved using homogeneous copper‐based catalysts in the presence of air. The utility of this novel methodology is demonstrated for C_α−C_β bond scission in >70 amines with excellent functional group tolerance.

Abstract

The selective cleavage of thermodynamically stable C(sp³)−C(sp³) single bonds is rare compared to their ubiquitous formation. Herein, we describe a general methodology for such transformations using homogeneous copper‐based catalysts in the presence of air. The utility of this novel methodology is demonstrated for C_α−C_β bond scission in >70 amines with excellent functional group tolerance. This transformation establishes tertiary amines as a general synthon for amides and provides valuable possibilities for their scalable functionalization in, for example, natural products and bioactive molecules.