LongLarf

Nature Chemistry, Published online: 25 November 2019; doi:10.1038/s41557-019-0353-3

Despite the potential of fluorinated compounds in pharmaceuticals and agrochemicals, the formation of C–F bonds remains challenging. It has now been shown that aryl sulfonium salts, which can be made by site-selective C–H functionalization, have advantageous photoredox reactivity compared to conventional (pseudo)halides and can be used for late-stage C–H fluorination.

Aliphatic amines strongly coordinate, and therefore easily inhibit, the activity of transition-metal catalysts, posing a marked challenge to nitrogen-hydrogen (N–H) insertion reactions. Here, we report highly enantioselective carbene insertion into N–H bonds of aliphatic amines using two catalysts in tandem: an achiral copper complex and chiral amino-thiourea. Coordination by a homoscorpionate ligand protects the copper center that activates the carbene precursor. The chiral amino-thiourea catalyst then promotes enantioselective proton transfer to generate the stereocenter of the insertion product. This reaction couples a wide variety of diazo esters and amines to produce chiral α-alkyl α–amino acid derivatives.

The aim of this article is to be a useful tool for synthetic chemists, that are interested in the synthetic aspects of photocatalysis and looking for a specific retrosynthetic disconnection. Not only are informative schemes provided but also are the availability of the catalysts and reagents as well as the functional group tolerance of the respective method briefly discussed.

Visible‐light‐mediated chemical reactions have become very popular within the last 15 years. Many excellent reviews have summarized reaction principles and recent results in photochemistry and photocatalysis. However, despite their popularity photochemistry and photoredox catalysis are not regular parts of the bachelor and master curriculum of chemistry education. As a consequence, most chemists will use in synthesis planning classic retrosynthetic disconnections and only rarely consider photocatalytic steps. The aim of this article is to be a useful tool for synthetic chemists, that are interested in the synthetic aspects of photocatalysis and looking for a specific retrosynthetic disconnection. Not only are informative schemes provided but also are the availability of the catalysts and reagents as well as the functional group tolerance of the respective method briefly discussed.

An Mn‐based catalyst permits the efficient single methylation of secondary carbon atoms and the double methylation of primary carbon atoms of primary and secondary alcohols, including purely aliphatic examples. The catalyst tolerates many functional groups, operates at low catalyst loadings and short reaction times, and upscaling is easily achieved.

Abstract

We report an earth‐abundant‐metal‐catalyzed double and single methylation of alcohols. A manganese catalyst, which operates at low catalyst loadings and short reaction times, mediates these reactions efficiently. A broad scope of primary and secondary alcohols, including purely aliphatic examples, and 1,2‐aminoalcohols can be methylated. Furthermore, alcohol methylation for the synthesis of pharmaceuticals has been demonstrated. The catalyst system tolerates many functional groups among them hydrogenation‐sensitive examples and upscaling is easily achieved. Mechanistic investigations are indicative of a borrowing hydrogen or hydrogen autotransfer mechanism involving a bimetallic K‐Mn catalyst. The catalyst accepts hydrogen as a proton and a hydride from alcohols efficiently and reacts with a chalcone via hydride transfer.

Nature Catalysis, Published online: 18 November 2019; doi:10.1038/s41929-019-0378-4

The synthesis of organophosphorus compounds from elemental phosphorus is an inefficient process, using multiple steps, stoichiometric metal complexes and/or hazardous reagents such as chlorine gas. Here, a direct photocatalytic route to convert white phosphorus (P4) into phosphines and phosphonium salts is reported.

Nature Chemistry, Published online: 18 November 2019; doi:10.1038/s41557-019-0375-x

Despite the regular occurrence of high-profile accidents leading to serious injuries or deaths among lab personnel, the state of academic lab safety research has languished. Existing studies in this area are summarized and critiqued in this Review and suggestions are made for future research directions.

Labile chalconium ions of the type [(Me₃Si)₃E]⁺ (E=O, S) were generated by reacting [Me₃Si‐H‐SiMe₃][B(C₆F₅)₄] and Me₃Si[CB] (CB⁻=carborate=[CHB₁₁H₅Cl₆]⁻, [CHB₁₁Cl₁₁]⁻) with Me₃Si‐E‐SiMe₃. A ligand exchange in case of the oxonium species, leading to the surprising formation of the persilylated [(Me₃Si)₂(Me₂(H)Si)O]⁺ oxonium ion, was observed. With Me₃Si[CB] as silylating reagent, a cyclic dioxonium dication of the type [Me₃Si‐μO‐SiMe₂]₂[CB]₂ was isolated in an anion‐mediated reaction.

Abstract

Attempts have been made to prepare salts with the labile tris(trimethylsilyl)chalconium ions, [(Me₃Si)₃E]⁺ (E=O, S), by reacting [Me₃Si‐H‐SiMe₃][B(C₆F₅)₄] and Me₃Si[CB] (CB⁻=carborate=[CHB₁₁H₅Cl₆]⁻, [CHB₁₁Cl₁₁]⁻) with Me₃Si‐E‐SiMe₃. In the reaction of Me₃Si‐O‐SiMe₃ with [Me₃Si‐H‐SiMe₃][B(C₆F₅)₄], a ligand exchange was observed in the [Me₃Si‐H‐SiMe₃]⁺ cation leading to the surprising formation of the persilylated [(Me₃Si)₂(Me₂(H)Si)O]⁺ oxonium ion in a formal [Me₂(H)Si]⁺ instead of the desired [Me₃Si]⁺ transfer reaction. In contrast, the expected homoleptic persilylated [(Me₃Si)₃S]⁺ ion was formed and isolated as [B(C₆F₅)₄]⁻ and [CB]⁻ salt, when Me₃Si‐S‐SiMe₃ was treated with either [Me₃Si‐H‐SiMe₃][B(C₆F₅)₄] or Me₃Si[CB]. However, the addition of Me₃Si[CB] to Me₃Si‐O‐SiMe₃ unexpectedly led to the release of Me₄Si with simultaneous formation of a cyclic dioxonium dication of the type [Me₃Si‐μO‐SiMe₂]₂[CB]₂ in an anion‐mediated reaction. DFT studies on structure, bonding and thermodynamics of the [(Me₃Si)₃E]⁺ and [(Me₃Si)₂(Me₂(H)Si)E]⁺ ion formation are presented as well as mechanistic investigations on the template‐driven transformation of the [(Me₃Si)₃E]⁺ ion into a cyclic dichalconium dication [Me₃Si‐μE‐SiMe₂]₂ ²⁺.

Publication date: 27 December 2019

Source: Tetrahedron, Volume 75, Issue 52

Author(s): Naoto Aoyagi, Yoshio Furusho, Takeshi Endo

Graphical abstract

Publication date: 12 December 2019

Source: Chem, Volume 5, Issue 12

Author(s): Saravanan Subramanian, Julius Oppenheim, Doyun Kim, Thien S. Nguyen, Wahyu M.H. Silo, Byoungkook Kim, William A. Goddard, Cafer T. Yavuz

The Bigger Picture

Summary

Graphical Abstract

In good shape: A manganese(I) complex containing a chiral (NH)₂P₂ macrocycle catalyzes the asymmetric transfer hydrogenation of a broad scope of ketones with the highest enantioselectivities obtained with this metal thus far. Noncovalent interactions between the incoming substrate and the macrocyclic ligand account for the excellent enantioselection.

Abstract

The bis(carbonyl) manganese(I) complex [Mn(CO)₂(1)]Br (2) with a chiral (NH)₂P₂ macrocyclic ligand (1) catalyzes the asymmetric transfer hydrogenation of polar double bonds with 2‐propanol as the hydrogen source. Ketones (43 substrates) are reduced to alcohols in high yields (up to >99 %) and with excellent enantioselectivities (90–99 % ee). A stereochemical model based on attractive CH–π interactions is proposed.

Nature, Published online: 13 November 2019; doi:10.1038/d41586-019-03489-1

Anxiety and depression in graduate students is worsening. The health of the next generation of researchers needs systemic change to research cultures.