Volume 91, Issue 1

Over several years, our research team has contributed to ligand design for metal-catalyzed stereoselective transformations. This has been achieved with the synthesis and application to organic transformations of interest of an array of structurally diverse P-containing ligands. These range from highly modular enantiopure phosphine–phosphite ligands to supramolecularly regulated enantioselective phosphorus-based catalysts. Our research in supramolecular interactions has also led to the discovery of an unprecedented halogen-bonded rhodium-catalyst.

The chemistry of organoelemental compounds including carbon-phosphorus derivatives is now one of the most rapidly developing fields of research, regarding both fundamental science and solution of applied problems. Extensive opportunities for the synthesis of organophosphorus compounds are opened up by the use of unconventional methods, first of all, electrochemical ones, which combine the benefits of usual homogeneous chemistry in solution and electrochemistry, where reactants are generated at the electrodes directly in the reaction system. The interest in the organic electrosynthesis is caused by several factors, including mild conditions (room temperature, atmospheric pressure), the possibility of conducting reactions in a closed system with a low concentration of the catalyst, which is readily regenerated. This mini-review generalizes the achievements in the field of development of new electrochemical, efficient and atom-economical, catalytic methods for the formation of aromatic carbon – phosphorus bonds and some historical background of these approaches.

The readily accessible and inexpensive dialkyl H -phosphonates are important building blocks for organic synthesis. This review specifically covers our recent work on the application of H -phosphonates as reactants for C–P bond formation, and as promoters for quinoline N -oxides to synthesize 2-functionalized quinolines.

Nucleophilic and electrophilic substitutions are the most often applied reactions in organophosphorus chemistry. They are closely interrelated, because in a reacting pair always one reagent is an electrophile, and another nucleophile. The reactions of electrophilic and nucleophilic substitutions at the phosphorus center proceed via the formation of a pentacoordinated intermediate. The mechanism of nucleophilic substitution involves the exchange of ligands in the pentacoordinate phosphorane intermediate, leading to the more stable stereomer under the thermodynamic control. Electrophilic substitution proceeds with retention of absolute configuration, whereas nucleophilic substitution with inversion of configuration at the phosphorus center.

The beneficial combination of microwave (MW) and ionic liquid (IL) additives was exploited in the direct esterification of a series of acyclic phosphinic and phosphonic acids giving rise to phenyl- H -phosphinates/methyl-phenylphosphinates/diphenylphosphinates and phenylphosphonic mono- and diesters, respectively. The latter is the first example for the direct esterification of a phosphonic acid.

The synthesis of α-aryl-α-aminophosphonates was performed by the three-component Kabachnik-Fields reaction of primary amines, benzaldehyde derivatives and dialkyl phosphites in a continuous flow microwave reactor. The target compounds could be obtained in high (~90%) yields without any catalyst in simple alcohols as the solvent. The flow process elaborated required shorter reaction times and lower excess of the reagent, as compared to the “traditional” batch reactions, and allowed the synthesis of the α-aminophosphonates on a somewhat larger scale.

Utilizing 2,4,6-tri- tert -butylphenyl as a sterically demanding group (abbreviated as Mes*), we have been successful in isolation, characterization, physical and chemical property investigation of low-coordinate organophosphorus compounds such as those carrying P=P and P=C bonds. Diphosphene, 1-phosphaethene, 1,3-diphosphaallene, 1-phosphaallene, and 3,4-diphosphinidenecyclobutenes are discussed in terms of stereochemistry including E/Z isomerization around P=P or P=C bond and R/S resolution of rotational isomers around crowded P–C bonds with chiral HPLC column. (π-Allyl)palladium complexes ligated with 3,4-diphosphinidenecyclobutene are useful in such catalytic organic reactions as the Tsuji-Trost reaction by which from a chiral s -alcohol, the corresponding chiral amine is obtained with retention of chirality suggesting double inversion process.

Phosphorus-radical participated difunctionalization reactions with unsaturated compounds have been recognized as powerful method for organic synthesis. This review covers our recent work on the application of transition metal catalyzed P -radical promoted difunctionalization for synthesis of organophosphorus compounds.

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 Bu 3 P=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.

In contrast to their mostly unstable isolobal carbon counterparts, oligophosphanide anions, such as M( cyclo -P 5 t Bu 4 ) (M=Li, Na) and M 2 (P 4 R 4 ) [M=Na, K; R=Ph, t Bu, 2,4,6-Me 3 C 6 H 2 (Mes)], have unique features, depending on their composition and structure, and are highly suitable building blocks for the synthesis of phosphorus-rich metal compounds. However, alkali metal oligophosphanediides are highly reactive and highly reducing, and a major problem is their tendency for disproportionation in reactions with electrophiles. This, however, can also give rise to a fascinating chemistry of making and breaking of P–P bonds. On the other hand, neutral cyclooligophosphines, such as cyclo -(P 5 Ph 5 ), are suitable stable ligands for the formation of phosphorus-rich metal complexes.

Several major challenges still remain in organophosphorus chemistry. Organophosphorus compounds are currently synthesized from phosphorus trichloride (PCl 3 ), even though the final consumer products (such as pesticides, flame-retardants, extractants) do not contain reactive phosphorus-chlorine bonds. In order to bypass phosphorus trichloride, significant interest has been devoted to functionalizing elemental phosphorus (P 4 , the precursor to PCl 3 ), red phosphorus (P red ), or phosphine (PH 3 ). Yet, phosphinates (ROP(O)H 2 ) are already available on an industrial scale and are the most environmentally benign, but their use as phosphorus trichloride replacements has been completely overlooked until a few years ago. An overview of some of the methodologies developed in my laboratory for P–C and P–O bond-forming reactions through phosphinate chemistry, as well as some selected applications, are presented. Another significant challenge remains the synthesis of P -stereogenic compounds. My group’s recent progress in this area is also discussed. Based on menthol as an inexpensive chiral auxiliary, various menthyl phosphinates can be synthesized. These phosphinates are precursor to P -stereogenic phosphines through well-established literature transformations.

The microwave-assisted Hirao reaction of bromobenzene and diarylphosphine oxides was performed at 120 °C using triethylamine as the base, and 5% of palladium acetate as the catalyst in ethanol. 5% Excess of the >P(O)H reagent served as the reducing agent, while another 10% as the preligand (in the >POH tautomeric form). It was found that the P–C coupling reaction was significantly faster with (2-MeC 6 H 4 ) 2 P(O)H ( A ) and (3,5-diMeC 6 H 3 ) 2 P(O)H ( B ), than with Ph 2 P(O)H ( C ) and (4-MeC 6 H 4 ) 2 P(O)H ( D ). Moreover, species A and B could be applied as selective P-ligands in the reaction of bromobenzene with C or D . Dependence of the effectiveness of “PdP 2 ” catalysts with diarylphosphine oxide preligands on the methyl substituents followed a reversed order as the reactivity of the diarylphosphine oxide species in the P–C coupling itself. Formation of the “PdP 2 ” catalyst from palladium acetate and diarylphosphine oxide has never been studied, but now it was evaluated by us at the B3LYP level of theory applying 6-31G(d,p) for C,H,P,O and SDD/MW28 for Pd including the explicit-implicit solvent model. The novel mechanism requiring three equivalents of the >P(O)H species for each of the palladium acetate molecule was in agreement with the preparative experiments. The ligation of palladium(0) with different P(III) species comprising the >POH form of the >P(O)H reagent was also studied, and the critical role of the steric hindrance on the ligation, and hence on the activity of the “PdP 2 ” catalyst was substantiated. Last but not least, the influence of the Me substituents in the aromatic ring of the P-reagents on the energetics of the elemental steps of the Hirao reaction itself was also evaluated.

1,3-Azaphospholo[1,5- a ]pyridines named as 2-phosphaindolizines were obtained from [4+1] cyclocondensation and 1,5-electrocyclization methods. These compounds incorporate several reactive functionalities and undergo electrophilic substitution, Diels-Alder reaction and 1,2-addition with hydrogen sulfide and sulfur across the >C=P– functionality and form η 1 -P coordination compounds with metal carbonyls. Thus, 2-phosphaindolizines could be used as precursors for accessing a variety of organophosphorus compounds.

The usual advantage of microwave (MW) assistance is making organic reactions faster and more efficient. In this article we present reaction types from organophosphorus chemistry, when MW-assisted transformations (e.g. the direct esterification and alkylating esterification of phosphinic acids) may be promoted by suitable catalysts, or vice versa, when a catalytic reaction is enhanced by MW irradiation (e.g. the Arbuzov reaction of aryl halides), and when catalysts may be omitted or simplified under MW irradiation as shown by the alkylation of active methylene containing P=O substrates/the Kabachnik–Fields reaction/deoxygenation of phosphine oxides, and the Hirao reaction, respectively.

This paper presents the results of an investigation into possible liquid viscosity standards to meet an industrial requirement for a liquid with a nominal viscosity of 20 mPa s at a temperature of 473 K and pressure of 200 MPa with a relative expanded uncertainty of less than 5%. There are no commercially available certified viscosity reference liquids that meet this requirement. Four candidate fluids were examined: squalane, Krytox GPL102, tris(2-ethylhexyl) trimellitate (TOTM), and dipentaerythritol hexa(3,5,5-trimethylhexanoate) (DiPE i C 9 ). Although none of these fluids satisfies all of the criteria, two fluids were identified as being suitable as International Standards for viscosity at temperatures up to 473 K and pressures below 200 MPa. These fluids are squalane and tris(2-ethylhexyl) trimellitate (TOTM), which at T =473.15 K and p =200 MPa present viscosity values of 5 mPa s and 10 mPa s, respectively.