Phosphane Catalysts Research Articles

Recent advances in catalytic Wittig-type reactions based on P(III)/P(V) redox cycling

Abstract Numerous organic transformations are based on the use of stoichiometric amounts of phosphorus reagents. The formation of phosphane oxides from phosphanes is usually the thermodynamic driving force for these reactions. The stoichiometric amounts of phosphane oxide which are formed as by-products often significantly hamper the product purification. Organophosphorus catalysis based on P(III)/P(V) redox cycling aims to address these problems. Herein we present our recent advances in developing catalytic Wittig-type reactions. More specifically, we reported our results on catalytic Wittig reactions based on readily available Bu3P=O as pre-catalyst as well as the first microwave-assisted version of this reaction and the first enantioselective catalytic Wittig reaction utilizing chiral phosphane catalysts. Further developments led to the implementation of catalytic base-free Wittig reactions yielding highly functionalized alkylidene and arylidene succinates.

Pd-Catalyzed Suzuki-Miyaura Cross-Coupling of Pentafluorophenyl Esters.

Although the palladium-catalyzed Suzuki-Miyaura cross-coupling of aryl esters has received significant attention, there is a lack of methods that utilize cheap and readily accessible Pd-phosphane catalysts, and can be routinely carried out with high cross-coupling selectivity. Herein, we report the first general method for the cross-coupling of pentafluorophenyl esters (pentafluorophenyl = pfp) by selective C–O acyl cleavage. The reaction proceeds efficiently using Pd(0)/phosphane catalyst systems. The unique characteristics of pentafluorophenyl esters are reflected in the fully selective cross-coupling vs. phenolic esters. Of broad synthetic interest, this report establishes pentafluorophenyl esters as new, highly reactive, bench-stable, economical, ester-based, electrophilic acylative reagents via acyl-metal intermediates. Mechanistic studies strongly support a unified reactivity scale of acyl electrophiles by C(O)–X (X = N, O) activation. The reactivity of pfp esters can be correlated with barriers to isomerization around the C(acyl)–O bond.

ChemInform Abstract: Enantioselective Synthesis of Chromanones via a Peptidic Phosphane Catalyzed Rauhut—Currier Reaction.

AbstractBiologically active α‐methylene‐δ‐valerolactones are enantioselectively obtained by intramolecular Rauhut—Currier reaction using a new bifunctional dipeptidic phosphane catalyst which is accessible in only four steps from cheap commercially available amino acids.

Enantioselective Synthesis of Chromanones via a Peptidic Phosphane Catalyzed Rauhut-Currier Reaction.

The enantioselective intramolecular Rauhut-Currier reaction has been developed using a bifunctional dipeptidic phosphane catalyst, providing a direct access to biologically active α-methylene-δ-valerolactones in high yields and enantiomeric excesses. The novel catalyst is accessible in only four steps from commercial sources and exhibits unusual binding selectivities for a small molecule, suggesting the possibility for long-range interactions between the catalyst and the substrate.

Palladium-catalyzed chemoselective allylic substitution, Suzuki-Miyaura cross-coupling, and allene formation of bifunctional 2-B(pin)-substituted allylic acetate derivatives.

A formidable challenge at the forefront of organic synthesis is the control of chemoselectivity to enable the selective formation of diverse structural motifs from a readily available substrate class. Presented herein is a detailed study of chemoselectivity with palladium-based phosphane catalysts and readily available 2-B(pin)-substituted allylic acetates, benzoates, and carbonates. Depending on the choice of reagents, catalysts, and reaction conditions, 2-B(pin)-substituted allylic acetates and derivatives can be steered into one of three reaction manifolds: allylic substitution, Suzuki-Miyaura cross-coupling, or elimination to form allenes, all with excellent chemoselectivity. Studies on the chemoselectivity of Pd catalysts in their reactivity with boron-bearing allylic acetate derivatives led to the development of diverse and practical reactions with potential utility in synthetic organic chemistry.

The Rise of the Iron Age in Hydrogen Evolution?

Turning over a new catalyst: The concept of hydrogen evolution from formic acid has developed rapidly in recent years. The change from ruthenium to iron as catalyst metal marks a milestone in the field. Novel iron phosphane catalysts have made considerable progress towards an applicable hydrogen delivery system.

Preparation of Chiral Multifunctional Thiourea–Phosphanes and Synthesis of Chiral Allylic Phosphites and Phosphane Oxides through Asymmetric Allylic Substitution Reactions of Morita–Baylis–Hillman Carbonates

AbstractWe developed a synthetic method to prepare chiral multifunctional thiourea–phosphane catalysts for the asymmetric allylic substitution reaction of Morita–Baylis–Hillman carbonates with diphenyl phosphite or diphenylphosphane oxide to give allylic phosphites and allylic phosphane oxides in high yields with excellent enantioselectivity.

ChemInform Abstract: Mixed Phosphane η5‐CpRuCl(PR3)2 Complexes as Ambifunctional Catalysts for anti‐Markovnikov Hydration of Terminal Alkynes.

AbstractA new in situ mixed phosphane catalyst, is mixed in situ, used for preparative anti‐Markovnikov hydration of alkynes in water‐rich media.

Chiral Bifunctional Thiourea–Phosphane Organocatalysts in Asymmetric Allylic Amination of Morita–Baylis–Hillman Acetates

AbstractA series of new chiral bifunctional thiourea–phosphane catalysts was synthesized and successfully applied in the catalytic, asymmetric allylic amination of Morita–Baylis–Hillman (MBH) acetates derived from the methyl vinyl ketone (MVK) or ethyl vinyl ketone (EVK) system, with phthalimide, affording the amination products in up to over 99 % yield and 90 % ee for a wide range of substrates derived from different aromatic aldehydes.

ChemInform Abstract: Direct Method for Carbon—Carbon Bond Formation: The Functional Group Tolerant Cobalt‐Catalyzed Alkylation of Aryl Halides.

AbstractDirect alkylation of electron‐poor aryl bromides and chlorides with alkyl bromides is smoothly achieved using a Co(II)—phosphane catalyst.

A Versatile Palladium/Triphosphane System for Direct Arylation of Heteroarenes with Chloroarenes at Low Catalyst Loading

Put a ring on it: The use of an air-stable, robust palladium/tridentate phosphane catalyst in direct CH and CCl activation reactions is reported (see scheme; DMAc=N,N-dimethylacetamide, TBAB=tetra-n-butylammonium bromide). Electron-rich, electron-poor, and polysubstituted furans (X=O), thiophenes (X=S), pyrroles (X=NR5), and thiazoles were arylated with chloroarenes in the presence of the catalyst. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Asymmetric Aza‐Morita–Baylis–Hillman Reactions of Alkyl Vinyl Ketones with N‐Protected Imines or In Situ Generated N‐Protected Imines

AbstractDABCO‐catalyzed aza‐MBH reactions of N‐Boc imines with MVK and EVK have been thoroughly investigated in the paper. The asymmetric version of this aza‐MBH reaction was also systematically investigated by using a chiral amine or a chiral phosphane catalyst. It was found that most of the N‐protected imines are suitable substrates under the mild reaction conditions and are able to give the corresponding adducts in moderate yields with high ee values. The TQO‐ or LB1‐catalyzed aza‐MBH reactions of N‐protected α‐amidoalkyl phenyl sulfones or α‐amidoalkyl p‐tolyl sulfones with MVK could be well conducted, which provides a facile and direct route to obtain highly enantioselective aza‐MBH adducts. The Boc protecting group of the aza‐MBH product could be easily removed under acidic conditions to give the corresponding α‐methylene‐β‐amino ketone or α‐methylene‐β‐amino alcohol derivatives in good yields.

ChemInform Abstract: Allylation of N‐Heterocycles with Allylic Alcohols Employing Self‐Assembling Palladium Phosphane Catalysts.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Allylation of N-Heterocycles with Allylic Alcohols Employing Self-Assembling Palladium Phosphane Catalysts

The first palladium catalyst system that allows the direct allylation of indoles with allylic alcohols as substrates with water being the only byproduct is presented. The application of self-assembling ligands based on complementary hydrogen bonding was the key to success.

Enantioselective Hydrogenation with Self‐Assembling Rhodium Phosphane Catalysts: Influence of Ligand Structure and Solvent

Three sets of new and related chiral phospholane and phosphepine ligands have been prepared for Rh-catalyzed enantioselective hydrogenation. The size and substitution pattern of the cyclic monophosphanes were varied. More importantly, the ligands differ in the nature of the heterocyclic group linked to the trivalent phosphorus atom: 2-pyridone or 2-alkoxypyridine. In the corresponding Rh complexes, the pyridone units of two monodentate P ligands can assemble by hydrogen bonding and form chelates. In contrast, synthetic precursors bearing alkoxypyridine appendages are not able to aggregate via intramolecular hydrogen bonds. The nature of self-assembly is dependent on the nature of the P ligand and the solvent used for the hydrogenation (CH2Cl2 vs. MeOH). These features affect the rate of the reaction as well as the enantioselectivity, which varied in the range of 0-99 % ee Complexation studies and DFT calculations were performed to explain these differences.

Nickel Catalyzed Grob Fragmentation: ω‐Dienyl Aldehydes Synthesis.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Reliable Suzuki Chemistry For Functionalised Polythiophene Synthesis

AbstractUntil recently, the synthesis of polythiophenes using Suzuki chemistry has proven difficult because of the ready protodeborylation of thiophene boronates. However, we now report that the new generation of bulky, electron-rich Pd(0)-phosphane catalysts are effective and reliable for the preparation of regioregular polyalkylthiophenes using Suzuki coupling. Moreover, the monomers can be prepared in high yield by Ir-catalysed borylation, without the need for strong organolithium bases, making this potentially a highly functional group-tolerant approach to polyalkylthiophene derivatives. Perfluoroalkylthiophenes also undergo this reaction.

Palladium-Catalyzed Cross-Methylation of Haloarenes Possessing Active Hydrogen Atoms by Intramolecularly Stabilized Dimethylindium and Dimethylaluminum Reagents

While the intramolecularly stabilized aluminum complex [(CH3)2AlOCH2CH2N(CH3)2]2 (2a) reacts readily with 4-bromophenol to give methane and [(4-BrC6H4O)2AlOCH2CH2N(CH3)2]2 (7), the demethylation of the analogous indium complex [(CH3)2InOCH2CH2N(CH3)2]22c is very slow. This inertness of 2c enables it to cross-methylate bromophenols and other bromoarenes with active hydrogen atoms in the presence of soluble palladium phosphane catalysts. The cross-methylation by 2c can be carried out under an ambient atmosphere. The ring-bound bromine atoms in 7 are replaced by CH3 groups when treated with an excess of the methylation reagent 2a, and the resulting aluminum cresolate liberates 4-methylphenol upon hydrolysis. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

ChemInform Abstract: Fast and Selective Homogeneous Hydrogenation with Nickel(II) Phosphane Catalysts.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Fast and selective homogeneous hydrogenation with nickel(II) phosphane catalysts

Nickel(II) phosphane complexes are able to hydrogenate oct-1-ene to n-octane with high turnover numbers and high selectivities at low temperature and moderate pressure.