Catalytic Wittig Reactions of Semi‐ and Nonstabilized Ylides Enabled by Ylide Tuning

The first examples of catalytic Wittig reactions with semistabilized and nonstabilized ylides are reported. These reactions were enabled by utilization of a masked base, sodium tert‐butyl carbonate, and/or ylide tuning. The acidity of the ylide‐forming proton was tuned by varying the electron density at the phosphorus center in the precatalyst, thus facilitating the use of relatively mild bases. Steric modification of the precatalyst structure resulted in significant enhancement of E selectivity up to >95:5, E/Z.     

Extending the Substrate Scope of Bicyclic P‐Oxazoline/Thiazole Ligands for Ir‐Catalyzed Hydrogenation of Unfunctionalized Olefins by Introducing a Biaryl Phosphoroamidite Group

This study identifies a series of Ir‐bicyclic phosphoroamidite–oxazoline/thiazole catalytic systems that can hydrogenate a wide range of minimally functionalized olefins (including E‐ and Z‐tri‐ and disubstituted substrates, vinylsilanes, enol phosphinates, tri‐ and disubstituted alkenylboronic esters, and α,β‐unsaturated enones) in high enantioselectivities (ee values up to 99 %) and conversions. The design of the new phosphoroamidite–oxazoline/thiazole ligands derives from a previous successful generation of bicyclic N‐phosphane–oxazoline/thiazole ligands, by replacing the N‐phosphane group with a π‐acceptor biaryl phosphoroamidite moiety. A small but structurally important family of Ir‐phosphoroamidite–oxazoline/thiazole precatalysts has thus been synthesized by changing the nature of the N‐donor group (either oxazoline or thiazole) and the configuration at the biaryl phosphoroamidite moiety. The substitution of the N‐phosphane by a phosphoroamidite group in the bicyclic N‐phosphane–oxazoline/thiazole ligands extended the range of olefins that can be successfully hydrogenated.     

Direct Catalytic Enantio‐ and Diastereoselective Mannich Reaction of Isocyanoacetates and Ketimines

A catalytic asymmetric synthesis of imidazolines with a fully substituted β‐carbon atom by a Mannich‐type addition/cyclization reaction of isocyanoacetate pronucleophiles and N‐diphenylphosphinoyl ketimines has been developed. When a combination of a cinchona‐derived aminophosphine precatalyst and silver oxide was employed as a binary catalyst system, good reactivity, high diastereoselectivities (up to 99:1 d.r.), and excellent enantioselectivities (up to 99 % ee) were obtained for a range of substrates.     

One‐Pot Cooperation of Single‐Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization‐Hydrosilylation Process

Realizing the full potential of oxide‐supported single‐atom metal catalysts (SACs) is key to successfully bridge the gap between the fields of homogeneous and heterogeneous catalysis. Here we show that the one‐pot combination of Ru₁/CeO₂ and Rh₁/CeO₂ SACs enables a highly selective olefin isomerization‐hydrosilylation tandem process, hitherto restricted to molecular catalysts in solution. Individually, monoatomic Ru and Rh sites show a remarkable reaction specificity for olefin double‐bond migration and anti‐Markovnikov α‐olefin hydrosilylation, respectively. First‐principles DFT calculations ascribe such selectivity to differences in the binding strength of the olefin substrate to the monoatomic metal centers. The single‐pot cooperation of the two SACs allows the production of terminal organosilane compounds with high regio‐selectivity (>95 %) even from industrially‐relevant complex mixtures of terminal and internal olefins, alongside a straightforward catalyst recycling and reuse. These results demonstrate the significance of oxide‐supported single‐atom metal catalysts in tandem catalytic reactions, which are central for the intensification of chemical processes.     

Synthesis and anticancer activity of chalcogenide derivatives and platinum(II) and palladium(II) complexes derived from a polar ferrocene phosphanyl–carboxamide

The polar phosphanyl‐carboxamide, 1′‐(diphenylphosphanyl)‐1‐[N‐(2‐hydroxyethyl)carbamoyl]ferrocene ( 1 ), reacts readily with hydrogen peroxide and elemental sulfur to give the corresponding phosphane‐oxide and phosphane‐sulfide, respectively, and with platinum(II) and palladium(II) precursors to afford various bis(phosphane) complexes [MCl₂( 1 ‐κP)₂] (M = trans‐Pd, trans‐Pt and cis‐Pt). The anticancer activity of the compounds was evaluated in vitro with the complexes showing moderate cytotoxicities towards human ovarian cancer cells. Moreover, the biological activity was found to be strongly influenced by the stereochemistry, with trans‐[PtCl₂( 1 ‐κP)₂] being an order of magnitude more active than the corresponding cis isomer. Copyright © 2010 John Wiley & Sons, Ltd.     

A Molecular Approach to Self‐Supported Cobalt‐Substituted ZnO Materials as Remarkably Stable Electrocatalysts for Water Oxidation

In regard to earth‐abundant cobalt water oxidation catalysts, very recent findings show the reorganization of the materials to amorphous active phases under catalytic conditions. To further understand this concept, a unique cobalt‐substituted crystalline zinc oxide (Co:ZnO) precatalyst has been synthesized by low‐temperature solvolysis of molecular heterobimetallic Co_4?xZn_xO₄ (x=1–3) precursors in benzylamine. Its electrophoretic deposition onto fluorinated tin oxide electrodes leads after oxidative conditioning to an amorphous self‐supported water‐oxidation electrocatalyst, which was observed by HR‐TEM on FIB lamellas of the EPD layers. The Co‐rich hydroxide‐oxidic electrocatalyst performs at very low overpotentials (512 mV at pH 7; 330 mV at pH 12), while chronoamperometry shows a stable catalytic current over several hours.     

Mechanistic Exploration of Intramolecular Aminodiene Hydroamination/Cyclisation Mediated by Constrained Geometry Organoactinide Complexes: A DFT Study

The present computational mechanistic study explores comprehensively the organoactinide‐mediated intramolecular hydroamination/cyclisation (IHC) of aminodienes by employing a reliable DFT method. All the steps of a plausible catalytic reaction course have been scrutinised for the IHC of (4E,6)‐heptadienylamine 1 t by [(CGC)Th(NMe₂)₂] precatalyst 2 (CGC=[Me₂Si(η⁵‐Me₄C₅)(tBuN)]^2?). For each of the relevant elementary steps the most accessible pathway has been identified from a multitude of mechanistic possibilities. The operative mechanism involves rapid substrate association/dissociation equilibria for the 3 t ‐S resting state and also for azacyclic intermediates 4 a , 4 s , easily accessible and reversible exocyclic ring closure, supposedly facile isomerisation of the azacycle’s butenyl tether prior to turnover‐limiting protonolysis. The following aspects are in support of this scenario: 1) the derived rate law is consistent with the experimentally obtained empirical rate law; 2) the accessed barrier for turnover‐limiting protonolysis does agree remarkably well with observed performance data; 3) the ring‐tether double‐bond selectivity is consistently elucidated, which led to predict the product distribution correctly. This study provides a computationally substantiated rationale for observed activity and selectivity data. Steric demands at the CGC framework appear to be an efficient means for modulating both performance and ring‐tether double‐bond selectivity. The careful comparison of (CGC)4f‐element and (CGC)5f‐element catalysts revealed that aminodiene IHC mediated by organoactinides and organolanthanides proceeds through a similar mechanistic scenario. However, cyclisation and protonolysis steps, in particular, feature a markedly different reactivity pattern for the two catalyst classes, owing to enhanced bond covalency of early actinides when compared to lanthanides.     

Origins of the Enantio‐ and N/O Selectivity in the Primary‐Amine‐Catalyzed Hydroxyamination of 1,3‐Dicarbonyl Compounds with In‐Situ‐Formed Nitrosocarbonyl Compounds: A Theoretical Study

Chemoselective control over N/O selectivity is an intriguing issue in nitroso chemistry. Recently, we reported an unprecedented asymmetric α‐amination reaction of β‐ketocarbonyl compounds that proceeded through the catalytic coupling of enamine carbonyl groups with in‐situ‐generated carbonyl nitroso moieties. This process was facilitated by a simple chiral primary and tertiary diamine that was derived from tert‐leucine. This reaction featured high chemoselectivity and excellent enantioselectivity for a broad range of substrates. Herein, a computational study was performed to elucidate the origins of the enantioselectivity and N/O regioselectivity. We found that a bidentate hydrogen‐bonding interaction between the tertiary N⁺? H and nitrosocarbonyl groups accounted for the high N selectivity, whilst the enantioselectivity was determined by Si‐facial attack on the (E)‐ and (Z)‐enamines in a Curtin–Hammett‐type manner. The bidentate hydrogen‐bonding interaction with the nitrosocarbonyl moieties reinforced the facial selectivity in this process.     

Development of versatile and silver-free protocols for gold(I) catalysis

The use of a versatile N‐heterocyclic carbene (NHC) gold(I) hydroxide precatalyst, [Au(OH)(IPr)], (IPr=N,N′‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) permits the in situ generation of the [Au(IPr)]⁺ ion by simple addition of a Brønsted acid. This cationic entity is believed to be the active species in numerous catalytic reactions. ¹H NMR studies in several solvent media of the in situ generation of this [Au(IPr)]⁺ ion also reveal the formation of a dinuclear gold hydroxide intermediate [{Au(IPr)}₂(μ‐OH)], which is fully characterized and was tested in gold(I) catalysis.     

Copper(I) complex covalently anchored on graphene oxide as an efficient and recyclable catalyst for Sonogashira reaction

In this study, the organosilane‐functionalized graphene oxide as a stabilizer was prepared by a facile one‐step silylation approach. [Cu(PPh₃)₃Cl] complex was successfully immobilized onto the graphene oxide surface through coordination interaction with organosilane ligand spacers. The supported catalyst showed enhanced catalytic performance toward Sonogashira reaction of aryl halides with phenylacetylene in water solvent compared with the homogeneous analogues, and it could be readily recycled and reused several times without discernible loss of its activity.     

Asymmetric oxidation of sulfides catalyzed by chiral (salen)Mn(III) complexes with a pyrrolidine backbone

Catalytic properties of a series of chiral (pyrrolidine salen)Mn(III) complexes for asymmetric oxidation of aryl methyl sulfides were evaluated. Moderate activity, good chemical selectivity and low enantioselectivity were attained with iodosylbenzene as a terminal oxidant. Enantioselectivity of sulfide oxidation was affected slightly by polar solvent and the sulfoxidation carried out in THF for thioanisole and in CH₃CO₂Et for electron‐deficient sulfides gave better enatioselctivities. The addition of the donor ligand PPNO (4‐phenylpyridine N‐oxide) or MNO (trimethylamine N‐oxide) only has a minor positive effect on the enantioselectivity. Also explored was the steric effect of the N_aza‐substituent in the backbone of (pyrrolidine salen)Mn(III) complexes on the enantioselectivity of sulfide oxidation. The sulfides' access pathway is discussed based on the catalytic results. Copyright © 2006 John Wiley & Sons, Ltd.     

Inverting External Asymmetric Induction via Selective Energy Transfer Catalysis: A Strategy to β‐Chiral Phosphonate Antipodes

Enantiodivergent, catalytic reduction of activated alkenes relays stereochemical information encoded in the antipodal chiral catalysts to the pro‐chiral substrate. Although powerful, the strategy remains vulnerable to costs and availability of sourcing both catalyst enantiomers. Herein, a stereodivergent hydrogenation of α,β‐unsaturated phosphonates is disclosed using a single enantiomer of the catalyst. This enables generation of the R‐ or S‐configured β‐chiral phosphonate with equal and opposite selectivity. Enantiodivergence is regulated at the substrate level through the development of a facile E → Z isomerisation. This has been enabled for the first time by selective energy transfer catalysis using anthracene as an inexpensive organic photosensitiser. Synthetically valuable in its own right, this process enables subsequent Rh^I‐mediated stereospecific hydrogenation to generate both enantiomers of the product using only the S‐catalyst (up to 99:1 and 3:97 e.r.). This strategy out‐competes the selectivities observed with the E‐substrate and the R‐catalyst.     

Synthesis and Rearrangement of P‐Nitroxyl‐Substituted PIII and PV Phosphanes: A Combined Experimental and Theoretical Case Study

Low‐temperature generation of P‐nitroxyl phosphane 2 (Ph₂POTEMP), which was obtained by the reaction of Ph₂PH ( 1 ) with two equivalents of TEMPO, is presented. Upon warming, phosphane 2 decomposed to give P‐nitroxyl phosphane P‐oxide 3 (Ph₂P(O)OTEMP) as one of the final products. This facile synthetic protocol also enabled access to P‐sulfide and P‐borane derivatives 7 and 13 , respectively, by using Ph₂P(S)H ( 6 ) or Ph₂P(BH₃)H ( 11 ) and TEMPO. Phosphane sulfide 7 revealed a rearrangement to phosphane oxide 8 (Ph₂P(O)STEMP) in CDCl₃ at ambient temperature, whereas in THF, thermal decomposition of sulfide 7 yielded salt 10 ([TEMP‐H₂][Ph₂P(S)O]). As well as EPR and detailed NMR kinetic studies, indepth theoretical studies provided an insight into the reaction pathways and spin‐density distributions of the reactive intermediates.     

Copper(II) and Sodium(I) Complexes based on 3,7‐Diacetyl‐1,3,7‐triaza‐5‐phosphabicyclo[3.3.1]nonane‐5‐oxide: Synthesis,Characterization, and Catalytic Activity

The reaction of 3,7‐diacetyl‐1,3,7‐triaza‐5‐phosphabicyclo[3.3.1]nonane (DAPTA) with metal salts of Cu^II or Na^I/Ni^II under mild conditions led to the oxidized phosphane derivative 3,7‐diacetyl‐1,3,7‐triaza‐5‐phosphabicyclo[3.3.1]nonane‐5‐oxide (DAPTA=O) and to the first examples of metal complexes based on the DAPTA=O ligand, that is, [Cu^II(μ‐CH₃COO)₂(κO‐DAPTA=O)]₂ ( 1 ) and [Na(1κOO′;2κO‐DAPTA=O)(MeOH)]₂(BPh₄)₂ ( 2 ). The catalytic activity of 1 was tested in the Henry reaction and for the aerobic 2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO)‐mediated oxidation of benzyl alcohol. Compound 1 was also evaluated as a model system for the catechol oxidase enzyme by using 3,5‐di‐tert‐butylcatechol as the substrate. The kinetic data fitted the Michaelis–Menten equation and enabled the obtainment of a rate constant for the catalytic reaction; this rate constant is among the highest obtained for this substrate with the use of dinuclear Cu^II complexes. DFT calculations discarded a bridging mode binding type of the substrate and suggested a mixed‐valence Cu^II/Cu^I complex intermediate, in which the spin electron density is mostly concentrated at one of the Cu atoms and at the organic ligand.     

Catalytic behavior tuning via structural modifications of silylated-diphosphine Ni(II) complexes for ethylene selective dimerization

The methylene spacers and an uncoordinated diphenylphosphine moiety in the scaffold of the CH₃Si(CH₂)_n(PPh₂)₃ and Si(CH₂PPh₂)₄-type silylated diphosphine Ni(II) complex systems have a marked impact on their catalytic performance in selective ethylene dimerization. Ni(II)-based precatalyst 1 , bearing two methylene spacers in its framework, exhibited the highest catalytic activity of 1.29 × 10⁸ g (mol_Ni)^-1 h^-1, while precatalyst 3 , with three methylene spacers, affords the highest product selectivity (88%) toward the C₄ fraction. Crystallographic investigations revealed that the precatalyst 3 adopts the mononuclear bidentate binding mode and the steric constraints of its uncoordinated diphenylphosphine moiety may successfully tailor the catalytic environment of the catalyst. The precatalyst 4 may form a dinuclear complex and exhibits high catalytic activity by changing the ligand/Ni molar ratio. The high C₄ selectivity of precatalyst 3 has been rationalized by density functional theory (DFT) calculations and found to be consistent with the experimental results. The study also revealed that designing new systems of Ni(II)-based complexes and their systematic modifications may further provide potential and industrially viable catalyst systems for selective ethylene oligomerization.     

A Bis(silylene)‐Substituted ortho‐Carborane as a Superior Ligand in the Nickel‐Catalyzed Amination of Arenes

The synthesis and structure of the first 1,2‐bis(NHSi)‐substituted ortho‐carborane [(LSi:)C]₂B₁₀H₁₀ (termed SiCCSi) is reported (NHSi=N‐heterocyclic silylene; L=PhC(NtBu)₂). Its suitability to serve as a reliable bis(silylene) chelating ligand for transition metals is demonstrated by the formation of [SiCCSi]NiBr₂ and [SiCCSi]Ni(CO)₂ complexes. The CO stretching vibration modes of the latter indicate that the Si^II atoms in the SiCCSi ligand are even stronger σ donors than the P^III atoms in phosphines and C^II atoms in N‐heterocyclic carbene (NHC) ligands. Moreover, the strong donor character of the [SiCCSi] ligand enables [SiCCSi]NiBr₂ to act as an outstanding precatalyst (0.5 mol % loading) in the catalytic aminations of arenes, surpassing the activity of previously known molecular Ni‐based precatalysts (1–10 mol %).     

An Efficient Catalyst System at Mild Reaction Conditions Containing Rare Earth Metal Complexes

An efficient rare earth metal complex‐catalyzed cycloaddition reaction of CO₂ with propylene oxide using Hdpza (di(2‐pyrazyl)amine) as a N‐donor ligand has been accomplished in good to excellent yields with high selectivity. The effects of different rare earth metal salts, ligands and reaction conditions were examined. Catalytic reaction tests demonstrated that the incorporation of ErCl₃ and Hdpza can significantly enhance the catalytic reactivity of the TBAB (nBu₄NBr, tetra‐n‐butyl ammonium bromide) towards cycloaddition reaction of CO₂ and propylene oxide that produce cyclic carbonates under mild conditions without any co‐solvent.     

Pd‐PEPPSI‐IPent‐SiO2: A Supported Catalyst for Challenging Negishi Coupling Reactions in Flow

A silica‐supported precatalyst, Pd‐PEPPSI‐IPent‐SiO₂, has been prepared and evaluated for its proficiency in the Negishi cross‐coupling of hindered and electronically deactivated coupling partners. The precatalyst Pd‐PEPPSI‐IPent loaded onto packed bed columns shows high catalytic activity for the room‐temperature coupling of deactivated/hindered biaryl partners. Also for the first time, the flowed Csp³–Csp² coupling of secondary alkylzinc reagents to (hetero)aromatics has been achieved with high selectivity with Pd‐PEPPSI‐IPent‐SiO₂. These couplings required residence times as short as 3 minutes to effect completion of these challenging transformations with excellent selectivity for the nonrearranged product.     

Effect of triarylphosphane ligands on the rhodium‐catalyzed hydrosilylation of alkene

A series of triarylphosphanes ( 1a , 2a , 3a , 4a , 5a , 6a , 7a , 8a , 9a , 10a , 11a ) have been synthesized. An X‐ray crystal structure analysis of (2‐bromophenyl)diphenylphosphane ( 1a ) unambiguously confirmed the constitution of the functionalized phosphane. The hydrosilylation reaction of styrene with triethoxysilane catalyzed with RhCl₃/triarylphosphane was studied. In comparison with the classic Wilkinson's catalyst, rhodium complexes with functionalized triarylphosphane ligands are characterized by a very high catalytic effectiveness for the hydrosilylation of alkene. Among these catalysts tested, RhCl₃/diphenyl(2‐(trimethylsilyl)phenyl)phosphane ( 8a ) exhibited excellent catalytic properties. Using this silicon‐containing phosphane ligand for the rhodium‐catalyzed hydrosilylation of styrene, both higher conversion of alkene and higher β‐adduct selectivity were obtained than with Wilkinson's catalyst. Copyright © 2014 John Wiley & Sons, Ltd.     

Enantioselective Access to (−)‐Ambrox® Starting from β‐Farnesene

Starting from inexpensive (E)‐β‐farnesene ( 1 ), an eight‐step enantioselective synthesis of the olfactively precious Ambrox^® ((?)‐ 2a ) has been performed. The crucial step is the catalytic asymmetric isomerization of (2E,6E)‐N,N‐diethylfarnesylamine ( 3 ) to the corresponding enamine (?)‐(R,E)‐ 4a , applying Takasago's well‐known industrial methodology. The resulting dihydrofarnesal ((+)‐(R)‐ 5 ) (90% yield, 96% ee), obtained after in situ hydrolysis (AcOH, H₂O), was then cyclized under catalytic SnCl₄ conditions, via its corresponding unreported enol acetate (?)‐(R)‐ 4b , to afford trans‐decalenic aldehyde (+)‐ 6a . Subsequent transformations furnished bicyclic ketone (?)‐ 8a and unsaturated nitrile (+)‐ 11 , both reported as intermediates to access to (?)‐ 2a .