Facile Energy Release from Substituted Dewar Isomers of 1,2­Dihydro-1,2­azaborinines Catalyzed by Coinage Metal Lewis Acids.

Molecular solar thermal systems (MOST) represent an auspicious solution for the storage of solar energy. We report silver salts as a unique class of catalysts, capable of releasing the stored energy from the promising 1,2-dihydro-1,2-azaborinine based MOST system. Mechanistic investigations provided insights into the silver catalyzed thermal backreaction, concurrently unveiling the first crystal structure of a 2-aza-3-borabicyclo[2.2.0]hex-5-ene, the Dewar isomer of 1,2-dihydro-1,2-azaborinine. Quantification of activation energies by kinetic experiments has elucidated the advantageous energy outcomes associated with Lewis acid catalysts, a phenomenon corroborated through computational analysis. By means of low temperature NMR spectroscopy, mechanistic insights into the coordination of Ag+ to the 1,2-dihydro-1,2-azaborinine were gained.

Nickel Catalyzed Cross-Coupling of Aryl and Alkenyl Triflates with Alkyl Thiols.

We report a nickel catalyzed C-S cross-coupling of aryl and alkenyl triflates with alkyl thiols. A variety of the corresponding thioethers were synthesized using an air-stable nickel precatalyst under mild reaction conditions with short reaction times. A broad substrate scope, including pharmaceutically relevant compounds, could be demonstrated.

Iron-Catalyzed Cross-Coupling of Thioesters and Organomanganese Reagents.

We report a Fukuyama-type coupling of thioesters with aliphatic organomanganese reagents utilizing a cheap and easily available iron(III) precatalyst. The reactions exhibit a wide tolerance of solvents and functional groups, allowing for the conversion of thioesters derived from natural products and pharmaceutical compounds. A strong steric impact from each reaction component (carboxylic moiety, thiol substituent and manganese reagent) was displayed, which enabled regioselective transformation of dithioesters. Mechanistic investigations showed that the released thiolate does not act as a mere spectator ligand, but rather positively influences the stability of intermediate alkyl(II)ferrates.

Tandem Olefin Isomerization/Cyclization Catalyzed by Complex Nickel Hydride and Brønsted Acid.

We disclose a nickel/Brønsted acid-catalyzed tandem process consisting of double bond isomerization of allyl ethers and amines and subsequent intramolecular reaction with nucleophiles. The process is accomplished by [(Me3P)4NiH]N(SO2CF3)2 in the presence of triflic acid. The methodology provides rapid access to tetrahydropyran-fused indoles and other oxacyclic scaffolds under very low catalyst loadings.

Tandem Acid/Pd-Catalyzed Reductive Rearrangement of Glycol Derivatives.

Herein, we describe the acid/Pd-tandem-catalyzed transformation of glycol derivatives into terminal formic esters. Mechanistic investigations show that the substrate undergoes rearrangement to an aldehyde under [1,2] hydrogen migration and cleavage of an oxygen-based leaving group. The leaving group is trapped as its formic ester, and the aldehyde is reduced and subsequently esterified to a formate. Whereas the rearrangement to the aldehyde is catalyzed by sulfonic acids, the reduction step requires a unique catalyst system comprising a PdII or Pd0 precursor in loadings as low as 0.75 mol % and α,α'-bis(di-tert-butylphosphino)-o-xylene as ligand. The reduction step makes use of formic acid as an easy-to-handle transfer reductant. The substrate scope of the transformation encompasses both aromatic and aliphatic substrates and a variety of leaving groups.

Tandem Acyl Substitution/Michael Addition of Thioesters with Vinylmagnesium Bromide.

A tandem reaction of thioesters with vinylmagnesium bromide is reported. The initial acyl substitution provides an α,ß-unsaturated ketone which further reacts with the liberated thiolate. This transition-metal-free synthesis of ß-sulfanyl ketones takes place under mild reaction conditions, whereas the addition of a second Grignard molecule is almost completely suppressed. The carefully chosen parameters enabled the transformation of different substrates in moderate to good yields.

Palladium-Catalyzed Tandem Isomerization/Hydrothiolation of Allylarenes.

Herein we report a tandem olefin migration/hydrothiolation of allyl benzenes facilitated by an in situ generated palladium hydride. A catalyst system composed of palladium acetate and bidentate ligand dtbpx (1,2-bis(di- tert-butylphosphinomethyl)benzene in the presence of catalytic amounts of triflic acid led to the tandem transformation, which furnished benzylic thioethers. The reaction exhibits high regioselectivity and can be conducted under mild conditions. The robustness of the catalyst is displayed through reactions with coordinating thiols.

Cross-Coupling of Chloro(hetero)arenes with Thiolates Employing a Ni(0)-Precatalyst.

A general and efficient Ni-catalyzed coupling of challenging aryl chlorides and in situ generated aliphatic and aromatic thiolates is described. The employed on-cycle, air-stable defined Ni precatalysts allow for transformation of a broad scope of substrates. A variety of functional groups and heterocyclic motifs as well as structurally varied thiols are tolerated at unprecedented moderate catalyst loadings and reaction temperatures. Depending on reaction conditions, aryl thiols can selectively undergo C-S or C-C couplings.

Nickel-Catalyzed Coupling of Arylzinc Halides with Thioesters.

The Pd-catalyzed Fukuyama reaction of thioesters with organozinc reagents is a mild, functional-group-tolerant method for acylation chemistry. Its Ni-catalyzed variant might be a sustainable alternative to expensive catalytic Pd sources. We investigated the reaction of S-ethyl thioesters with aryl zinc halides with hetero- and homotopic Ni precatalysts and several ligands. The results show that both homo- and heterotopic species may contribute to catalysis. The substrate scope using an operationally homogeneous defined Ni complex was established. Acyl radicals are postulated as short-lived intermediates.

Homogeneous Palladium-Catalyzed Transfer Hydrogenolysis of Benzylic Alcohols Using Formic Acid as Reductant.

We report the first homogeneous palladium-based transfer hydrogenolysis of benzylic alcohols using an in situ formed palladium-phosphine complex and formic acid as reducing agent. The reaction requires a catalyst loading as low as only 1 mol % of palladium and just a slight excess of reductant to obtain the deoxygenated alkylarenes in good to excellent yields. Besides demonstrating the broad applicability for primary, secondary and tertiary benzylic alcohols, a reaction intermediate could be identified. Additionally, it could be shown that partial oxidation of the applied phosphine ligand was beneficial for the course of the reaction, presumably by stabilizing the active catalyst. Reaction profiles and catalyst poisoning experiments were used to characterize the catalyst, the results of which indicate a homogeneous metal complex as the active species.

Synthesis of Benzofuranones via Palladium-Catalyzed Intramolecular Alkoxycarbonylation of Alkenylphenols.

Herein, a new catalytic system to synthesize benzofuranones is reported. A palladium-catalyzed intramolecular alkoxycarbonylation is employed to generate 3-substituted-benzofuran-2(3H)-ones from alkenylphenols under mild reaction conditions, linked to an ex situ formation of CO from N-formylsaccharin. The carefully chosen catalytic system enables an efficient reaction with a novel functional group tolerance, despite the high polymerization tendency of the starting material.

Metal-Catalyzed Synthesis and Use of Thioesters: Recent Developments.

While thioesters are common intermediates in biochemical processes, they are much less appreciated in organic synthesis, also compared to other carboxylic acid derivatives. However, their chemistry and reactivity is intriguing and diversified, reaching much further than the acyl substitution and aldol chemistry. Herein, we focus on metal-catalyzed reactions for the synthesis of thioesters as well as their transformations. Reactions such as thiocarbonylation, cross-coupling, decarbonylation, allylic substitution or dual photoredox/metal catalysis are discussed. On one hand, new atom economic methods allow for convenient synthesis of thioesters from well available starting materials. On the other hand, various synthetically important compounds can by synthesized due to the multifaceted reactivity of thioesters that we aimed to depict.

Regioselective Thiocarbonylation of Vinyl Arenes.

A palladium-catalyzed thiocarbonylation of styrene derivatives is reported for the first time. The combination of thiols as nucleophiles and a bidentate ligand ensures a unique reaction outcome with high regioselectivity toward the more valuable branched isomer and new reactivity. The ambient reaction conditions (temperature, catalyst loading) and the use of a CO surrogate render this transformation a useful method for the synthesis of thioesters from available feedstock. Various functional groups on arene and thiol substituents are tolerated by the system. Notably, challenging ortho-substituted styrenes are converted with unprecedentedly high regioselectivity.

Structures, Reactions, and Mechanisms: Stereochemistry in the Broadest Sense at the 51st Bürgenstock Conference.

Tradition and innovation: The 51st Bürgenstock Conference on Stereochemistry took place from May 1-6, 2016 and offered its usual mixture of impressive science and superb discussions in a wonderful atmosphere. In the Meeting Review, Ivana Fleischer outlines the program.

Chiral Catalyst Design: Cyclopentadiene-Based Brønsted Acids.

The combination of -I and -M effects and the aromaticity of the conjugate base renders the chiral pentacarboxy-substituted cyclopentadienes (PCCPs) acidic. Their convenient synthesis together with substrates derived from the chiral pool and their performance in the tested reactions renders them prodigious candidates to become a new and highly competitive class of chiral Brønsted acid catalysts.

In Situ FTIR and NMR Spectroscopic Investigations on Ruthenium-Based Catalysts for Alkene Hydroformylation.

Homogeneous ruthenium complexes modified by imidazole-substituted monophosphines as catalysts for various highly efficient hydroformylation reactions were characterized by in situ IR spectroscopy under reaction conditions and NMR spectroscopy. A proper protocol for the preformation reaction from [Ru3 (CO)12] is decisive to prevent the formation of inactive ligand-modified polynuclear complexes. During catalysis, ligand-modified mononuclear ruthenium(0) carbonyls were detected as resting states. Changes in the ligand structure have a crucial impact on the coordination behavior of the ligand and consequently on the catalytic performance. The substitution of CO by a nitrogen atom of the imidazolyl moiety in the ligand is not a general feature, but it takes place when structural prerequisites of the ligand are fulfilled.

Using aqueous ammonia in hydroaminomethylation reactions: ruthenium-catalyzed synthesis of tertiary amines.

The direct synthesis of tertiary amines from ammonia and olefins is presented. Using a combination of Ru3 (CO)12 and 2-phosphino-substituted imidazole ligand as catalyst system allows for hydroaminomethylation reactions of bulk aliphatic and functionalized olefins. Tertiary amines are obtained in an atom-efficient domino process in moderate to good isolated yields (45-76%) with excellent regioselectivities (n/iso up to 99:1).

Novel ruthenium-catalyst for hydroesterification of olefins with formates.

An alternative ruthenium-based catalyst for the hydroesterification of olefins with formates is reported. The good activity of our system is ensured by the use of a bidentate P,N-ligand and ruthenium dodecacarbonyl. A range of formates can be used for selective alkoxycarbonylation of aromatic olefins. In addition, the synthesis of selected aliphatic esters is realized. The proposed active ruthenium complex has been isolated and characterized.

Development of a ruthenium/phosphite catalyst system for domino hydroformylation-reduction of olefins with carbon dioxide.

An efficient domino ruthenium-catalyzed reverse water-gas-shift (RWGS)-hydroformylation-reduction reaction of olefins to alcohols is reported. Key to success is the use of specific bulky phosphite ligands and triruthenium dodecacarbonyl as the catalyst. Compared to the known ruthenium/chloride system, the new catalyst allows for a more efficient hydrohydroxymethylation of terminal and internal olefins with carbon dioxide at lower temperature. Unwanted hydrogenation of the substrate is prevented. Preliminary mechanism investigations uncovered the homogeneous nature of the active catalyst and the influence of the ligand and additive in individual steps of the reaction sequence.