LongLarf

A procedure for the synthesis of N‐methyl‐arylamines directly from nitroarenes using methanol as green methylating agent was developed. Key to success is the use of a specific catalyst system consisting of palladium acetate and the ligand 1‐​[2,​6‐​bis(isopropyl)​phenyl]​‐​2‐​[tert‐butyl(2‐pyridinyl)phosphi​no]​‐1H‐​Imidazole (L1). The generality of this protocol is demonstrated in the synthesis of more than 20 N‐methyl‐arylamines under comparably mild conditions. Combining this novel methodology with subsequent coupling processes using the same catalyst allows for efficient diversification of aromatic nitro compounds to a broad variety of amines including drug molecules.

Reductive functionalization of CO2 combining both the formation of the new bonds and CO2 reduction in the presence of reductant, e.g. molecular hydrogen, hydrosilane or hydroborane has become increasingly attractive, which enlarges the spectra of compounds directly available from CO2 thus provides fresh idea for CO2 chemistry. This microreview briefly summarizes recent advances in new bond construction using CO2 as formyl, methylene and methyl source with transition‐metal catalyst, which are divided into sections according to C‐N, C‐C and C‐O bonds formation in the presence of nitrogen‐, carbon‐ and oxygen‐nucleophiles respectively. In the end, the challenges and opportunities with future trend of the reductive functionalization of CO2 are also discussed.

Unnatural photosynthesis: Carbon dioxide is an attractive reagent for organic synthesis from the standpoint of global sustainability. This Minireview describes recent advances in the area of photoredox catalysis as an enabling strategy for promoting carboxylations that generate carboxylic acid products in reactions akin to a synthetic variant of nature's route to carbohydrates.

Abstract

Carbon dioxide is an attractive reagent for organic synthesis from the standpoint of global sustainability. Its widespread use, however, is hampered by the fact that it is poorly reactive. New catalysts and technologies that enable C−C bond constructions are thus of high intrinsic value. This Minireview describes recent advances in the area of photoredox catalysis as an enabling strategy for promoting carboxylations.

Cycloaddition reactions provide direct and convergent routes to cycloalkanes, making them valuable targets for the development of synthetic methods. Whereas six-membered rings are readily accessible from Diels-Alder reactions, cycloadditions that generate five-membered rings are comparatively limited in scope. Here, we report that dinickel complexes catalyze [4 + 1]-cycloaddition reactions of 1,3-dienes. The C₁ partner is a vinylidene equivalent generated from the reductive activation of a 1,1-dichloroalkene in the presence of stoichiometric zinc. Intermolecular and intramolecular variants of the reaction are described, and high levels of asymmetric induction are achieved in the intramolecular cycloadditions using a C₂-symmetric chiral ligand that stabilizes a metal-metal bond.

Pinch me! Comparing PNP−Ru pincer complexes with N‐methylated‐ and NH‐moieties demonstrated increased activity of the former complex. Remarkably, the highest activities were observed in acidic environment.

Abstract

Complexes Ru(H)(Cl)(CO)(HN{CH₂CH₂P(iPr)₂}₂) 7 and Ru(H)(Cl)(CO)(CH₃N{CH₂CH₂P(iPr)₂}₂) 8 were compared for the selective dehydrogenation (DH) of formic acid (FA) at different pH values. Remarkably, highest activities were observed in acidic environment (pH 4.5). Under all investigated conditions, the N‐methylated complex 8 showed improved performance compared to 7. These observations can be mechanistically explained with protonation of 4H/3Me being the key step in formic acid DH for both complexes.

In the context of solar to chemical energy conversion inspired by natural photosynthesis, we present the synthesis, electrochemical properties and photoinduced electron transfer processes of three novel zinc(II)‐gold(III) bis(porphyrin) dyads [ZnII(P)‐AuIII(P)]+. Time‐resolved spectroscopic studies indicate ultrafast dynamics (kET1 > 10^10 s–1) after visible light excitation finally yielding a charge‐shifted state [ZnII(P•+)‐AuII(P)]+ featuring a gold(II) centre. The lifetime of this excited state is quite long thanks to a comparably slow charge recombination (kBET2 = 3x10^8 s–1). The [ZnII(P•+)‐AuII(P)]+ charge‐shifted state is reductively quenched by amines in bimolecular reactions yielding the neutral zinc(II)‐gold(II) bis(porphyrin) ZnII(P)‐AuII(P). The electronic nature of this key gold(II) intermediate, prepared by chemical or photochemical reduction, is elucidated by UV/Vis, X‐band EPR, gold L3‐edge XANES and paramagnetic 1H NMR spectroscopy as well as by quantum chemical calculations. Finally, the gold(II) site in ZnII(P)‐AuII(P) is thermodynamically and kinetically competent to reduce an aryl azide to the corresponding amine paving the way to catalytic applications of gold(III) porphyrins in photoredox catalysis involving the gold(III/II) redox couple.

NHC organocatalysis: A straightforward organocatalytic method for the direct carboxylation of terminal alkynes towards propargylic esters, is reported. A simple, widely‐available, stable, and cost‐efficient N‐heterocyclic carbene precursor salt was used as the (pre)catalyst.

Abstract

An efficient and straightforward organocatalytic method for the direct, multicomponent carboxylation of terminal alkynes with CO₂ and organochlorides, towards propargylic esters, is reported for the first time. 1,3‐Di‐tert‐butyl‐1H‐imidazol‐3‐ium chloride, a simple, widely‐available, stable, and cost‐efficient N‐heterocyclic carbene (NHC) precursor salt was used as the (pre)catalyst. A wide range of phenylacetylenes, bearing electron‐withdrawing or electron‐donating substituents, react with allyl‐chlorides, benzyl chlorides, or 2‐chloroacetates, providing the corresponding propargylic esters in low to excellent yields. DFT calculations on the mechanism of this transformation indicate that the reaction is initiated with the formation of an NHC‐carboxylate, by addition of the carbene to a molecule of CO₂. Then, the nucleophilic addition of this species to the corresponding chlorides has been computed to be the rate limiting step of the process.

Publication date: 15 March 2019

Source: Tetrahedron, Volume 75, Issue 11

Author(s): Michael L. McGraw, Eugene Y.-X. Chen

Graphical abstract

Publication date: 5 April 2019

Source: Tetrahedron, Volume 75, Issue 14

Author(s): Brena L. Thompson, Zachariah M. Heiden

Graphical abstract

Synlett
DOI: 10.1055/s-0037-1611720

A straightforward, site-selective arylation of the bis(triflate) of estrone by Suzuki–Miyaura reactions has been developed. Monoarylation occurs selectively at the D-ring with good to excellent yield. Such products were exemplarily employed for the synthesis of estrones containing two different aryl substituents.
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© Georg Thieme Verlag Stuttgart · New York

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Photo mediator: An organic photocatalyst combined with suitable base allow the employment of readily available versatile nitroalkenes for alkenylation of ethers. Good Z/E selectivity was obtained via an efficient proton coupled electron transfer process.

Abstract

C−H alkenylation of cyclic ethers (THF, 1,4‐dioxane) using the readily available nitroalkenes as the alkenylating reagents has been developed. It allows the rapid access to the α‐alkenyl ethers with high E‐selectivity. The previous inaccessible α‐dienyl ethers are successfully obtained. Acyclic ether can also participate in this alkenylation process. The mechanism study reveals that alkenylation proceeded through a proton coupled electron transfer (PCET) process with a de‐nitration.

Broad spectrum: Chiral PNN/Mn catalysts for the asymmetric hydrogenation of a broad spectrum of ketones have been developed. The utility of the protocol was demonstrated in the asymmetric synthesis of a variety of key intermediates for chiral drugs. Preliminary mechanistic investigations indicate that an outer‐sphere substrate–catalyst interaction dominates the catalysis.

Abstract

A series of Mn^I complexes containing lutidine‐based chiral pincer ligands with modular and tunable structures has been developed. The complex shows unprecedentedly high activities (up to 9800 TON; TON=turnover number), broad substrate scope (81 examples), good functional‐group tolerance, and excellent enantioselectivities (85–98 % ee) in the hydrogenation of various ketones. These aspects are rare in earth‐abundant metal catalyzed hydrogenations. The utility of the protocol have been demonstrated in the asymmetric synthesis of a variety of key intermediates for chiral drugs. Preliminary mechanistic investigations indicate that an outer‐sphere mode of substrate–catalyst interactions probably dominates the catalysis.

A new route to transform carboxylic acids directly into potent acyl triflate electrophiles is reported. This transformation exploits oxidative carbonylation of carboxylic acids with I₂, and is postulated to involve the interception of the Hunsdiecker reaction with CO. Coupling this chemistry with arene trapping offers a mild, room‐temperature approach to generate ketones from carboxylic acids using available reagents, and with only CO₂ and salts as by‐products.

Abstract

We report a new strategy for the conversion of carboxylic acids into potent acid triflate electrophiles. The reaction involves oxidative carbonylation of carboxylic acids with I₂ in the presence of AgOTf, and is postulated to proceed via acyl hypoiodites that react with CO to form acid triflates. Coupling this chemistry with subsequent trapping with arenes offers a mild, room temperature approach to generate ketones directly from broadly available carboxylic acids without the use of corrosive and reactive Lewis or Bronsted acid additives, and instead from compounds that are readily available, stable, and functional group compatible.

Synlett
DOI: 10.1055/s-0037-1612230

An efficient method has been developed for the 1,2-hydro(cyanomethylation) of alkenes, in which a cyanomethyl radical species is generated from a cyanomethylphosphonium ylide by irradiation with visible light in the presence of an iridium complex, a thiol, and ascorbic acid. The cyanomethyl radical species then adds across the C=C double bond of an alkene to form an elongated alkyl radical species that accepts a hydrogen atom from the thiol to produce an elongated aliphatic nitrile. The ascorbic acid acts as the reductant to complete the catalytic cycle.
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Georg Thieme Verlag Stuttgart · New York

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Publication date: 29 March 2019

Source: Tetrahedron, Volume 75, Issue 13

Author(s): Mauro Fianchini, Feliu Maseras

Graphical abstract

Synlett
DOI: 10.1055/s-0037-1612057

Structure elucidation using NMR spectroscopy has become a vital part of the toolkit of modern synthetic chemistry. Characterisation of final products, quality control of production, analysis of complex mixtures in synthetic method development, and structure elucidation of isolated natural products are examples where NMR spectroscopy is a part of daily routine. The two factors that usually limit the applicability of NMR are resolution and sensitivity. The experimental method described in this Account, real-time pure shift acquisition, yields heteronuclear correlation spectra such as HSQC that offer significant improvements in both resolution and sensitivity, at negligible cost to the analyst. The advantages that real-time pure shift acquisition enjoys over conventional experiments are discussed and illustrated with selected examples including carbohydrate and alkaloid mixtures. Advanced data acquisition and processing techniques that reduce experiment time and are easily combined with pure shift NMR methods are also described.1 Introduction2 Simultaneous Sensitivity and Resolution Enhancement Using Real-Time Acquisition in HSQC3 Processing Pure Shift Data4 Pulse Sequences for Real-Time Pure Shift HSQC5 Conclusions and Future Perspectives
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© Georg Thieme Verlag Stuttgart · New York

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