Rhenium-Catalyzed Coupling of Propargyl Alcohols and Allyl Silanes

Copper-Catalyzed Enantioselective 1,2-Allylation of Azadienes with Allylboronates

Catalytic asymmetric 1,2-allylation of aurone-derived azadienes is very difficult to achieve due to the driving force for aromatization and the greater steric hindrance of 1,2-addition compared with 1,4-addition. By taking advantage of the ability of nitrogen ligated metal complexes, we successfully demonstrated the first example of copper-catalyzed 1,2-allylation of azadienes with allylboronates for the highly enantioselective synthesis of homoallylic amines. Meanwhile, the enantioenriched 1,4-addition products could also be obtained through a subsequent 3,3-sigmatropic rearrangement of the 1,2-addition products. Extensive DFT calculations were carried out to elucidate the origins of high regioselectivity (1,2- vs 1,4-) and enantioselectivity.

Divergent Geminal Alkynylation-Allylation and Acylation-Allylation of Carbenes: Evolution and Roles of Two Transition-Metal Catalysts

Cooperative bimetallic catalysis to access novel reactivities is a powerful strategy for reaction development in transition-metal-catalyzed chemistry. Particularly, elucidation of the evolution of two transition-metal catalysts and understanding their roles in dual catalysis are among the most fundamental goals for bimetallic catalysis. Herein, a novel three-component reaction of a terminal alkyne, a diazo ester, and an allylic carbonate was successfully developed via cooperative Cu/Rh catalysis with Xantphos as the ligand, providing a highly efficient strategy to access 1,5-enynes with an all-carbon quaternary center that can be used as immediate synthetic precursors for complex cyclic molecules. Notably, a Meyer-Schuster rearrangement was involved in the reactions using propargylic alcohols, resulting in an unprecedented acylation-allylation of carbenes. Mechanistic studies suggested that in the course of the reaction Cu(I) species might aggregate to some types of Cu clusters and nanoparticles (NPs), while the Rh(II)2 precursor can dissociate to mono-Rh species, wherein Cu NPs are proposed to be responsible for the alkynylation of carbenes and work in cooperation with Xantphos-coordinated dirhodium(II) or Rh(I)-catalyzed allylic alkylation.

Rhenium-catalysed reactions in chemical synthesis: selected case studies

This Perspective presents and discusses a selection of examples that reinforce the enabling and distinctive reactivity provided by homogeneous rhenium catalysis in chemical synthesis. Specifically, the ability for lower oxidation...

A combined experimental and computational study of NHC-catalyzed allylation of allenoate with MBH esters: new regiospecific and stereoselective access to 1,5-enyne

An NHC-catalyzed regiospecific allylation of α-substituted allenoates with MBH carbonates derived from aryl aldehyde furnished highly functionalized 1,5-enynes.

Direct activation of alcohols via perrhenate ester formation for an intramolecular dehydrative Friedel–Crafts reaction

A general and highly efficient intramolecular dehydrative Friedel–Crafts reactions via Re2O7 mediated hydroxyl group activation is described for the syntheses of tetrahydronaphthalene, tetrahydroquinoline, tetrahydroisoquinoline, chromane, and isochromane derivatives.

Fe-Catalysed Coupling Reactions Between Alkynes and Alcohols

In this review, we discussed about the synthetic developments in the field of Fe-catalysis for the formation of - bonds through the coupling of alkynes and alcohols, for the period of 13 years (2008-2020). These strategies fulfil important Green Chemistry principles, as they are highly atom economic (up to 100 %), no toxic by-products (only water), employs highly abundant and low toxic alcohols (no need for any pre-functionalization hence step economy) and cheaper Fe-catalysts. Having these advantages, one can predict that in the coming years a large number of fascinating new iron-catalyzed reactions will be developed for the organic synthesis. We hope that this review article will be highly useful for the synthetic community to design and develop new Fe-catalyzed coupling reactions and keeps the content in the right prospect.

Lewis Acid-Driven Meyer-Schuster-Type Rearrangement of Yne-Dienone

Developed herein is a Cu(II)-catalyzed Meyer-Schuster-type rearrangement of alkyne-tethered cyclohexadienone for the construction of m-enone-substituted phenols. The reaction involves an uncommon 5-exo-trig 1,6-enyne cyclization of alkyne-tethered-cyclohexadienone, aromatization-triggered C-O bond cleavage, and an electrocyclic 4π-ring-opening of oxetene intermediate. This atom-efficient transformation provides access to a wide range of synthetically important α-(m-substituted phenol)-α,β-unsaturated ketones, featuring a broad scope with labile functional group tolerance. The gram-scale demonstration makes this transformation synthetically viable. The synthetic application of α,β-unsaturated ketones is also showcased.

Regio- and Diastereoselective Rhodium-Catalyzed Allylic Substitution with Unstabilized Benzyl Nucleophiles

We have developed a highly regio- and diastereoselective rhodium-catalyzed allylic substitution of challenging alkyl-substituted secondary allylic carbonates with benzylzinc reagents, which are prepared from widely available benzyl halides. This process utilizes rhodium(III) chloride as a commercially available, high-oxidation state and bench-stable pre-catalyst to provide a rare example of a regio- and diastereoselective allylic substitution in the absence of an exogenous ligand. This reaction tolerates electronically diverse benzylzinc nucleophiles and an array of functionalized and/or challenging aliphatic allylic electrophiles. Finally, the configurational fluxionality of the rhodium-allyl intermediate is exploited to develop a novel diastereoselective process for the construction of vicinal acyclic ternary/ternary stereogenic centers, in addition to a cyclic ternary/quaternary derivative.

Rhenium(V)-oxo complexes [ReOX2(N∩O)(EPh3)] (X=Cl, Br, I; E=P, As) – synthesis, structure, spectroscopy, and catalytic properties

The present review aims to give a comprehensive survey about the chemistry of rhenium(V)-oxo complexes of general formula [ReOX2(N∩O)(EPh3)], where X=Cl, Br, I, E=P, As, and N∩O stands for uninegative chelating N∩O-ligand, carried out within the last four decades. In addition to the synthesis aspects, the available structural data as well as the results issued from techniques such as infrared and ultraviolet-visible spectroscopies are collected and discussed. Furthermore, a brief description of the applications of these compounds in catalysis is included.

Synthesis and reactivity of nitridorhenium complexes incorporating the mercaptoethylsulfide (SSS) ligand

A method for the preparation of nitridorhenium(v) complexes of the form (SSS)Re(N)(L) (where SSS = 2-mercaptoethylsulfide and L = PPh₃ and t-BuNC) has been described. These complexes react with Lewis acids allowing for the isolation of adducts. The lack of a significant steric profile on the SSS ligand combined with enhanced nucleophilicity of the nitrido group does not allow for the effective formation of frustrated Lewis pairs with these complexes and as a result these species are poor catalysts for the hydrogenation of unactivated olefins.

Diarylmethane synthesis through Re2O7-catalyzed bimolecular dehydrative Friedel-Crafts reactions

This manuscript describes the application of Re2O7 to the syntheses of diarylmethanes from benzylic alcohols through solvolysis followed by Friedel-Crafts alkylation. The reactions are characterized by broad substrate scope, low catalyst loadings, high chemical yields, and minimal waste generation. The intermediate perrhenate esters are superior leaving groups to chlorides and bromides in these reactions. The polarity and water sequestering capacity of hexafluoroisopropyl alcohol are critical to the success of these processes. Re2O7 is a precatalyst for HOReO3, which serves as a less costly and easily handled promoter for these reactions. Oxorhenium catalysts selectively activate alcohols in the presence of similarly substituted acetates, indicating a unique chemoselectivity and mechanism in comparison to Brønsted acid catalysis.

Selective Cleavage of C-O Bonds in Lignin Catalyzed by Rhenium(VII) Oxide (Re2 O7 )

The selective cleavage of C-O bonds in typical model lignin β-O-4 compounds and deconstruction of a realistic lignin feedstock catalyzed by Re₂ O₇ is described. High yields of C-O cleavage products (up to 97.8 %) from model compounds and oils (76.3 %) from organosolv pinewood lignin were obtained under mild conditions. Evidence for the pathway of this catalytic process is also provided.

Scope and advances in the catalytic propargylic substitution reaction

Nucleophilic displacement of the propargylic alcohol is one of the sought-after methods in the current scenario. The highly nucleophilic alkyne functional moiety along with its considerably acidic terminal hydrogen atom allows the propargylic unit to play a crucial role in organic synthesis by offering a handle for further synthetic transformations. Until 2000, the most fundamental propargylic substitution reaction was the Nicolas reaction, a multi-step transformation, developed in 1972, which involved cobalt as a stoichiometric promoter. Therefore, the direct catalytic substitution of propargylic alcohols was a highly desirable method for development. The pioneering work on the Ru-catalyzed propargylic substitution reaction in 2000 encouraged many researchers to develop several novel catalytic propargylic substitution reactions, which have made rapid progress since then. The purpose of this review is to emphasise the involvement of diverse types of Lewis acid, transition metal and Brønsted acid catalysts in the propargylic substitution reaction and provide an updated summary of the recent developments in this field. The selected examples presented here are the most significant and relevant ones and we believe that this will help the readers to comprehend the scope of the propargylic substitution reaction with diverse types of catalysts and will envisage the scientific community for the future developments in this field.

Direct nucleophilic displacement of the alpha-hydroxy of the propargylic alcohol is one of the sought-after methods in the current scenario. An updated summary of the recent developments in this field is presented here.

Hydrogen autotransfer and related dehydrogenative coupling reactions using a rhenium(i) pincer catalyst

A novel rhenium complex bearing a non-innocent PNP pincer ligand was prepared.

AIE Based Coumarin Chromophore - Evaluation and Correlation Between Solvatochromism and Solvent Polarity Parameters

A new class of red emitting extensively conjugated donor-π-acceptor type dyes bearing coumarin units have been synthesized by condensation of 7-(diethylamino)-2-oxo-2 H-chromene-3-carbaldehyde with different active methylenes. All the dyes are characterized by (1)H NMR, (13)C NMR and HRMS spectroscopy. The photophysical behaviour and the relation between structure and properties of the coumarin "push-pull" derivatives were investigated experimentally. The dyes exhibited positive solvatochromism and solvatofluorism in solution of varying polarity. These coumarin dyes show aggregation induced emission properties with red emitting fluorescence. They show absorption in the range of 501-528 and emission in the range of 547-630 nm. We evaluated photophysical properties of coumarin dyes using solvotochromism and solvent dependent shift in the emission wavelength. All the synthesized coumarin dyes COS1-COS4 are showing very good solvatochromic properties.

New insights into hydrosilylation of unsaturated carbon-heteroatom (C═O, C═N) bonds by rhenium(V)-dioxo complexes

The hydrosilylation of unsaturated carbon-heteroatom (C═O, C═N) bonds catalyzed by high-valent rhenium(V)-dioxo complex ReO2I(PPh3)2 (1) were studied computationally to determine the underlying mechanism. Our calculations revealed that the ionic outer-sphere pathway in which the organic substrate attacks the Si center in an η(1)-silane rhenium adduct to prompt the heterolytic cleavage of the Si-H bond is the most energetically favorable process for rhenium(V)-dioxo complex 1 catalyzed hydrosilylation of imines. The activation energy of the turnover-limiting step was calculated to be 22.8 kcal/mol with phenylmethanimine. This value is energetically more favorable than the [2 + 2] addition pathway by as much as 10.0 kcal/mol. Moreover, the ionic outer-sphere pathway competes with the [2 + 2] addition mechanism for rhenium(V)-dioxo complex 1 catalyzing the hydrosilylation of carbonyl compounds. Furthermore, the electron-donating group on the organic substrates would induce a better activity favoring the ionic outer-sphere mechanistic pathway. These findings highlight the unique features of high-valent transition-metal complexes as Lewis acids in activating the Si-H bond and catalyzing the reduction reactions.

The unexpected mechanism underlying the high-valent mono-oxo-rhenium(V) hydride catalyzed hydrosilylation of C=N functionalities: insights from a DFT study

In this study, we theoretically investigated the mechanism underlying the high-valent mono-oxo-rhenium(V) hydride Re(O)HCl2(PPh3)2 (1) catalyzed hydrosilylation of C=N functionalities. Our results suggest that an ionic S(N)2-Si outer-sphere pathway involving the heterolytic cleavage of the Si-H bond competes with the hydride pathway involving the C=N bond inserted into the Re-H bond for the rhenium hydride (1) catalyzed hydrosilylation of the less steric C=N functionalities (phenylmethanimine, PhCH=NH, and N-phenylbenzylideneimine, PhCH=NPh). The rate-determining free-energy barriers for the ionic outer-sphere pathway are calculated to be ∼28.1 and 27.6 kcal mol(-1), respectively. These values are slightly more favorable than those obtained for the hydride pathway (by ∼1-3 kcal mol(-1)), whereas for the large steric C=N functionality of N,1,1-tri(phenyl)methanimine (PhCPh=NPh), the ionic outer-sphere pathway (33.1 kcal mol(-1)) is more favorable than the hydride pathway by as much as 11.5 kcal mol(-1). Along the ionic outer-sphere pathway, neither the multiply bonded oxo ligand nor the inherent hydride moiety participate in the activation of the Si-H bond.

Assembly of 3-allyl-3-ethynyl-oxindole motifs via palladium(ii)-catalyzed quaternary allylation of 3-ethynyl-3-OBoc-oxindoles

A simple and efficient strategy for the synthesis of 3-allyl-3-ethynyl-oxindoles using Pd(ii) catalyst has been developed. Applications of these intermediates were realized by synthesizing structurally diverse carbocyclic spirooxindoles.

Re2O7catalyzed dienone-phenol rearrangement

The dienone-phenol rearrangement of 4,4-disubstituted cyclohexadienones to multi-substituted phenols is achieved by employing Re₂O₇as the catalyst.

Mechanistic insights into B–H bond activation with the high-valent oxo-molybdenum complex MoO2Cl2

The ionic mechanistic model involving the heterolytic cleavage of the B–H bond is slightly energetically favorable than the [2+2] addition mechanism for the high-valent oxo-molybdenum complex MoO₂Cl₂activating the B–H bond.

Stereoselective direct reductive amination of ketones with electron-deficient amines using Re2O7/NaPF6 catalyst

The first example of direct reductive amination (DRA) of ketones with electron-deficient amines (EDA) such as Cbz-, Boc-, EtOCO-, Fmoc-, Bz-, ArSO2-, etc. protected amines have been achieved using catalytic Re2O7/NaPF6. Excellent chemoselectivities as well as diastereoselectivity (for 2-alkyl cyclohexanones) were obtained.