Secondary Phosphine Boranes Research Articles

Improved synthetic routes to methylene-bridged P-chiral diphosphine ligands via secondary phosphine–boranes

Improved methods for the preparation of methylene-bridged diphosphine ligands are described. Both enantiomers of the key intermediate tert-butylmethylphosphine–borane were prepared via resolution or by the conversion of one enantiomer into the opposite enantiomer. The precursor borane complexes of bis( tert-butylmethylphosphino)methane ( t-Bu-MiniPHOS), bis((1,1,3,3-tetramethylbutyl)methylphosphino)methane ( t-Oct-MiniPHOS), and ( tert-butylmethylphosphino)(di- tert-butylphosphino)methane (trichickenfootphos) were prepared in good yields and enantiopure form.

A straightforward synthesis of unsymmetrical secondary phosphine boranes

A one-pot procedure for the synthesis of unsymmetrical alkyl-substituted secondary phosphine oxides is described. The sequential addition of N-benzylaniline to a solution of dichlorophenylphosphine and 1-methylimidazole in methylcyclohexane, separating of the protic ionic liquid formed, addition of Grignard reagent followed by hydrolysis gave unsymmetrical secondary phosphine oxides (SPOs) in high yield. The use of ionic liquids in the first step is essential and streamlined the synthesis. Unsymmetrical SPOs could be quantitatively reduced to secondary phosphine using a catalytic amount of Ti(OiPr)4 and tetramethyldisiloxane (TMDS) under mild reaction conditions.

Kinetic resolution of P-stereogenic phosphine boranes via deprotonation using s-butyllithium/(−)-sparteine

The attempted kinetic resolution of racemic secondary phosphine boranes [ t-BuPhP(BH 3)H and t-BuMeP(BH 3)H] by P–H deprotonation using 0.5 equiv of s-BuLi and (−)-sparteine was unsuccessful and generated racemic benzyl bromide-trapped adducts in 42–49% yield. In contrast, an efficient kinetic resolution was observed with racemic tertiary phosphine boranes [ t-BuPhP(BH 3)Me and t-BuEtP(BH 3)Me] by C–H deprotonation on the P–Me group using 0.5 or 0.6 equiv of s-BuLi and (−)-sparteine. For example, the use of 0.6 equiv of s-BuLi/(−)-sparteine with t-BuEtP(BH 3)Me and trapping with DMF gave the ( R)-aldehyde adduct in 37% yield and 83:17 er together with recovered ( R)- t-BuEtP(BH 3)Me in 44% yield and 74:26 er. These are the first examples of kinetic resolution of P-stereogenic phosphine boranes via deprotonation using s-BuLi/(−)-sparteine.

Formation of 1,4-diphosphinobenzenes via tele-substitution on fluorobenzenechromium complexes

The meta–tele-substitution of 2-(boranatophosphino)fluorobenzenechromium complexes took place with various lithiated secondary phosphine–boranes as nucleophiles to give para-substituted bis(boranatophosphino)benzenechromiums. It was revealed that the yield of the tele-substitution product was strongly affected by the strength of a proton acid. Isotope labeling experiments indicated that 1,5-hydrogen migration was involved in this transformation.

Palladium-Catalyzed C−P Coupling Reactions between Vinyl Triflates and Phosphine−Boranes: Efficient Access to Vinylphosphine−Boranes

Vinylphosphine-borane complexes are easily synthesized by palladium-catalyzed C-P cross-coupling of vinyl triflates with secondary phosphine-boranes. This method allows the synthesis of phosphine derivatives not always easily accessible by other approaches. The vinylphosphine derivatives are purified by chromatography on silica gel. The versatility of the method is proved by using various triflates and diaryl-, dialkyl- and alkylarylphosphine-borane precursors. Chiral enantiopure phosphine-boranes are synthesized from chiral triflates.

Resolution and Stereochemistry of tert-Butylphenylphosphinous Acid−Borane

A combined use of ephedrine and cinchonine as resolving agents enabled facile resolution of racemic tert-butylphenylphosphinous acid-borane (1) into the two enantiomers in ca. 31-32% yield each. The resolved 1 served as a model substrate to study stereoselective synthetic transformations of phosphinous acid-boranes yielding optically active phosphinite-borane, boranatophosphinous-sulfonic anhydride, secondary phosphine-borane, tertiary phosphine-borane, secondary phosphine oxide, and phosphinic halides. By the judicious choice of the reaction paths, either enantiomer of tert-butylphenylphosphine-borane and of tert-butylmethylphenylphosphine-borane could be stereoselectively obtained from a single enantiomer of 1.

Nucleophilic Aromatic Substitution Reactions of Fluorobenzenechromium Complexes with P‐Chiral Secondary Phosphine—Boranes: Synthesis of Optically Pure P‐Chiral (Dialkyl)arylphosphine—Boranes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Mild and Simple Preparation of Ketophosphine-boranes through Uncatalysed Hydrophosphination of Enones

An efficient and simple method for the synthesis of γ-ketophosphines is reported. This method is based on the atom efficient hydrophosphination reaction using secondary phosphine-boranes as precursors. Different examples with various substrates and phosphine precursors are described. The reaction occurs without any ­catalyst upon simple thermal activation (50-80 °C). Reduction of a γ-ketophosphine yields in good yield and with an excellent dia­stereoselectivity a new hydroxyphosphine.

Rhodium‐Catalyzed Dehydrocoupling of Fluorinated Phosphine–Borane Adducts: Synthesis, Characterization, and Properties of Cyclic and Polymeric Phosphinoboranes with Electron‐Withdrawing Substituents at Phosphorus

The dehydrocoupling of the fluorinated secondary phosphine-borane adduct R2PH.BH3 (R = p-CF3C6H4) at 60 degrees C is catalyzed by the rhodium complex [{Rh(mu-Cl)(1,5-cod)}2] to give the four-membered chain R2PH-BH2-R2P-BH3. A mixture of the cyclic trimer [R2P-BH2]3 and tetramer [R2P-BH2]4 was obtained from the same reaction at a more elevated temperature of 100 degrees C. The analogous rhodium-catalyzed dehydrocoupling of the primary phosphine-borane adduct RPH2.BH3 at 60 degrees C gave the high molecular weight polyphosphinoborane polymer [RPH-BH2]n (Mw = 56,170, PDI = 1.67). The molecular weight was investigated by gel permeation chromatography and the compound characterized by multinuclear NMR spectroscopy. Interestingly, the electron-withdrawing fluorinated aryl substituents have an important influence on the reactivity as the dehydrocoupling process occurred efficiently at the mildest temperatures observed for phosphine-borane adducts to date. Thin films of polymeric [RPH-BH2]n (R = p-CF3C6H4) have also been shown to function as effective negative-tone resists towards electron beam (e-beam) lithography (EBL). The resultant patterned bars were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS).

Nucleophilic aromatic substitution reactions of fluorobenzenechromium complexes with P-chiral secondary phosphine–boranes: synthesis of optically pure P-chiral (dialkyl)arylphosphine–boranes

Optically pure P-chiral (dialkyl)arylphosphine–boranes having high structural diversity were prepared by two- or three-component coupling of fluorobenzenechromium complexes, P-chiral secondary phosphine–boranes, and other nucleophiles. The stereochemical integrity at the P-stereogenic center was completely retained during the S NAr process when the reaction was carried out in THF at low temperature.

Palladium catalysed enantioselective phosphination reactions using secondary phosphine-boranes and aryl iodide

Preliminary results dealing with the enantioselective version of the C-P cross-coupling reaction between dissymmetric secondary phosphine-borane complexes and aryl iodide derivatives are presented. To gain information on the enantiodiscriminating step, direct observation of an intermediate involved in the catalytic cycle has been achieved by (31)P NMR spectroscopy.

Chemistry of phosphine–borane adducts at platinum centers: dehydrocoupling reactivity of Pt(ii) dihydrides with P–H bonds

The reaction of the Pt(II) dihydride complex cis-[PtH2(dcype)](dcype = 1,2-bis(dicyclohexylphosphino)ethane) with the primary or secondary phosphine-borane adducts PhRPH x BH3(R = H, Ph) was found to exclusively afford the mono-substituted complexes cis-[PtH(PPhR x BH3)(dcype)](1: R = H; 2: R = Ph)via a dehydrocoupling reaction between Pt-H and P-H bonds. Similar reactivity was observed between the uncoordinated phosphines PhRPH (R = H, Ph) and cis-[PtH2(dcype)], which gave cis-[PtH(PPhR)(dcype)](4: R = H; 5: R = Ph). The complexes were characterized by 1H, 11B, 13C and 31P NMR spectroscopy, IR, MS and, in the case of 2, X-ray crystallography that confirmed the cis geometries. The di-substituted complex cis-[Pt(PhPH x BH3)2(dcype)](3) was prepared from the reaction of cis-[PtCl2(dcype)] with two equivalents of Li[PPhH x BH3]. This suggested that steric reasons alone cannot be used to explain the lack of reactivity with respect to a second dehydrocoupling reaction involving the remaining Pt-H bond in complexes 1, 2, 4 and 5.

Easy access to alkylphosphine boranes starting from unactivated alkenes

An efficient and simple method for the preparation of P-alkyl substituted phosphines is described. The desired phosphines are produced in good yields by hydrophosphination of unactivated alkenes using secondary phosphine boranes under both conventional and microwave conditions. No catalyst is required, the reaction simply occurs at room temperature or under a slight thermal activation (60 °C in oil bath or under microwave irradiation). This methodology provides a means for the preparation of a structurally diverse set of tertiary phosphines from simple low-cost commercially available alkenes. To cite this article: D. Mimeau et al., C. R. Chimie 7 (2004) .

Regio- and stereoselective hydrophosphination reactions of alkynes with phosphine-boranes: access to stereodefined vinylphosphine derivatives.

Vinylphosphine-borane complexes are easily synthesized by regio- and stereoselective hydrophosphination of terminal alkynes with use of secondary phosphine-boranes as hydrophosphinating agent. The regioselectivity of the reaction is efficiently controlled by the choice of the activation process: thermal or metal catalyst activation. The vinylphosphine derivatives are purified by chromatography on silica gel. Scalability of the process is demonstrated on a gram scale. This simple route should promote the use of vinylphosphines as building blocks for organic and organometallic chemistry.

Mild reduction of chlorophosphine boranes to secondary phosphine boranes

A number of reducing reagents were assessed in the transformation of chlorophosphine boranes to secondary phosphine boranes. The efficiency of the process requires judicious matching between steric and electronic requirements of reductant and the substrate. The stereochemistry of the reduction was investigated by using a chiral precursor.

An Expedient Reduction ofsec-Phosphine Oxides tosec-Phosphine-boranesby BH3˙SMe2

Secondary phosphine oxides can be expeditiously converted into secondary phosphine-boranes by treatment with an excess of BH3˙SMe2 at room temperature. Selectivity of the conversion towards the formation of secondary phosphine-boranes is greatly improved by the addition of a small amount of water to the reaction mixture.

Formation of a Stable Rhodium(I) Dihydride Complex and Its Reactions with Prochiral Substrates of Asymmetric Hydrogenation

A new P-chirogenic diphosphine, (S,S)-α,α‘-bis(tert-butylmethylphosphino)-o-xylene (1), was synthesized in two steps from optically active secondary phosphine-borane 4 via the corresponding diphosphine-borane 3. The Rh(I) complex (2) of 1 was characterized by X-ray study, which revealed a significantly distorted C2-symmetry. The catalytic hydrogenation of four representative prochiral substrates gave modest enantioselection. Unlike all previously studied Rh(I) complexes of chiral diphosphines, 2 reacts with dihydrogen at −70 °C, producing two diastereomers of a solvate dihydride, 5a,b, in a ratio 1:0.07. The formation of 5a,b is almost quantitative; the hydrogen is not eliminated from 5. At temperatures above 20 °C, further transformation of 5 takes place, eventually yielding a bridging binuclear complex 6. The reaction of 5 with methyl Z-(α)-acetamidocinnamate (7) is slow at −80 °C, and gradual accumulation of a mixture of monohydride intermediates 15a−d could be monitored by 1H NMR spectroscopy. When 5 was reacted with dimethyl 1-benzoyloxyethenephosphonate (10), two monohydride intermediates, 19a,b, formed cleanly in a ratio 1:0.04, which corresponded to the 92% ee observed after reductive elimination and quenching the reaction mixture.

ChemInform Abstract: Addition of Borane-Protected Secondary Phosphines to Imines. A Route to Protected Mono-N-Substituted-α-aminophosphines.

Abstract Addition of a secondary phosphine–borane adduct to imines was examined in order to improve the synthesis of mono- N -substituted-α-aminophosphines. The reaction demonstrates, for the first time, successful addition of a phosphine–borane to a multiple carbon–nitrogen bond. The process tolerates a range of substituents on the imine and results in an efficient formation of a borane-protected α-aminophosphine. Borane protects the aminophosphine and improves the reliability of the Mannich-like phosphine addition to imines.

ChemInform Abstract: Stereoselective, Oxidative One-Carbon Degradation of Alkyl(dimethyl)phosphine-Boranes. Synthesis of Enantiomerically Enriched Secondary Phosphine-Boranes.

ChemInform Abstract: Stereoselective, Oxidative One-Carbon Degradation of Alkyl(dimethyl)phosphine-Boranes. Synthesis of Enantiomerically Enriched Secondary Phosphine-Boranes.

Addition of borane-protected secondary phosphines to imines. A route to protected mono- N-substituted-α-aminophosphines

Addition of a secondary phosphine–borane adduct to imines was examined in order to improve the synthesis of mono- N-substituted-α-aminophosphines. The reaction demonstrates, for the first time, successful addition of a phosphine–borane to a multiple carbon–nitrogen bond. The process tolerates a range of substituents on the imine and results in an efficient formation of a borane-protected α-aminophosphine. Borane protects the aminophosphine and improves the reliability of the Mannich-like phosphine addition to imines.