Are Boryl Radicals from Amine–Boranes and Phosphine–Boranes the Most Stable Radicals?

The relative stability of the radicals that can be produced from amine–boranes and phosphine–boranes is investigated at the G3‐RAD level of theory. Aminyl ([RNH]^.:BH₃) and phosphinyl ([RPH]^.:BH₃) radicals are systematically more stable than the boryl analogues, [RNH₂]:BH₂^. and [RPH₂]:BH₂^.. Despite similar stability trends for [RNH]^.:BH₃ and [RPH]^.:BH₃ radicals with respect to boryl radicals, there are significant dissimilarities between amine– and phosphine–boranes. The homolytic bond dissociation energy of the N?H bond decreases upon association of the amines with BH₃, whereas that of the P?H bond for phosphines increases. The stabilization of the free amine is much smaller than that of the corresponding aminyl radical, whereas for phosphines this is the other way around. The homolytic bond dissociation energy of the B?H bond of borane decreases upon complexation with both amines and phosphines.     

Synthesis of new C60 based phosphines

Two series of new phosphine derivatives based on C₆₀ protected by borane have been synthesized and characterized. These phosphines were used for two preliminary complexation trials with [RhCl(COD)]₂ and [Re(S₃CPh)₂(S₂CPh)] to afford, respectively, the corresponding complexes [RhCl(COD)(PRPh₂)] and [Re(S₂CPh)₃(PRPh₂)].     

Reaction of Vinyl Selenides with Secondary Phosphines and Elemental Selenium: One‐Pot Selective Synthesis of a New Family of Diselenophosphinic Se‐Esters

Alkyl vinyl selenides react with diverse secondary phosphines and elemental selenium in a 1.1:1:2 molar ratio (120–124°C, 20–40 min, 1,4‐dioxane) to afford selectively earlier unknown diselenophosphinic Se‐esters, R₂P(Se)SeCH(Me)SeR´, in 82–99% yield. This three‐component atom‐economic reaction proceeds via intermediate formation of diselenophosphinic acid R₂P(Se)SeH (generated from secondary phosphine and selenium), which adds to the double bond of vinyl selenide in a Markovnikov manner to give the target products.     

Umpolung of Methylenephosphonium Ions in Their Manganese Half‐Sandwich Complexes and Application to the Synthesis of Chiral Phosphorus‐Containing Ligand Scaffolds

Half‐sandwich manganese methylenephosphonium complexes [Cp(CO)₂Mn(η²‐R₂P?C(H)Ph)]BF₄ were obtained in high yield through a straightforward reaction sequence involving a classical Fischer‐type manganese complex and a secondary phosphine as key starting materials. The addition of various nucleophiles (Nu) to these species took place regioselectively at the double‐bonded carbon center of the coordinated methylenephosphonium ligand R₂P⁺?C(H)Ph to produce the corresponding chiral phosphine complexes [Cp(CO)₂Mn(κ¹‐R₂P? C(H)(Ph)Nu)], from which the phosphines were ultimately recovered as free entities upon simple irradiation with visible light. The synthetic potential of this umpolung approach is illustrated herein by the preparation of novel chiral pincer‐type phosphine–NHC–phosphine ligand architectures.     

Synthesis,characterization and hydroformylation activity of 7‐azaindolate‐bridged dinuclear rhodium(I)phosphines with pendant polar‐groups

New dinuclear Rh(I)–Phosphines of the types [Rh(µ‐azi)(CO)(L)]₂ ( 1,3 – 7 ) and [Rh(µ‐azi)(L)]₂ ( 8 ) with pendant polar groups, and a chealated mononuclear compound [Rh(azi‐H)(CO)(L)] ( 2 ) (where azi = 7‐azaindolate, L = polar phosphine) were isolated from the reaction of [Rh(µ‐Cl)(CO)₂]₂ with 7‐azaindolate followed by some polar mono‐ and bis‐phosphines ( L ₁ – L ₈ ). A relationship between Δδ³¹P‐NMR and ν(CO) values was considered to define the impact of polar‐groups on σ‐donor properties of the phosphines. These compounds were evaluated as catalyst precursors in the hydroformylation of 1‐hexene and 1‐dodecene both in mono‐ and biphasic aqueous organic systems. While the biphasic hydroformylations (water + toluene) gave exclusively the aldehydes, the monophasic one (aqueous ethanol) showed propensity to form both aldehydes and alcohols. The influence of bimetallic cooperative effects, and σ‐donor and hydrophilic properties of the phosphines with pendant polar‐groups in enhancing the yields and selectivity of hydroformylation products was emphasized. In addition, when strong σ‐donor phosphine was used, the π‐acceptor nature of pyridine ring of 7‐azaindolate spacer was found to be a considerable factor in facilitating the facile cleavage of CO group during hydroformylation and in supplementing the cooperative effects. Copyright © 2009 John Wiley & Sons, Ltd.     

Addition of secondary phosphines to a vinyl ether of diacetone-d-glucose: a new approach to optically active phosphines and their derivatives

Secondary phosphines react readily with a vinyl ether of diacetone-d-glucose under radical initiation conditions to give, in high yield, anti-Markovnikov adducts, diorganyl{2-[3-O-(1,2:5,6-di-O-isopropylidene)-d-glucofuranosyloxy]ethyl}phosphines, which oxidize almost quantitatively upon reacting with air oxygen or elemental sulfur to form the corresponding optically active phosphine oxides or sulfides.     

Synthesis of tris(organylthioethyl)phosphines and their derivatives on the basis of the reaction of phosphine with vinyl sulfides

Phosphine generated from the red phosphorus in the system KOH-H₂O-toluene adds regio- and chemoselectively to vinyl sulfides under radical initiation conditions with the formation of tris(2-organylthioethyl)phosphines, which are easily oxidized by elemental sulfur and selenium to the corresponding phosphine sulfides and phosphine selenides in 54–78% yield. The obtained adducts are split when treated with sodium amide in THF to give trivinylphosphine and trivinylphosphine chalcogenides.     

Synthesis and evaluation of P-chirogenic monodentate binaphthyl phosphines

P-Chirogenic monodentate binaphthyl phosphines were prepared in five steps from enantiomerically pure BINOL. This approach supposes the utilization of two methods previously developed in our group, the formation of secondary phosphine oxide, and the reduction of tertiary phosphine oxide using the association of tetramethyldisiloxane and Ti(OⁱPr)₄. During the last reduction step, only the formation of the more stable diastereoisomer was observed. This product was employed as a ligand for the palladium catalyzed hydrosilylation of styrene to afford the corresponding alcohol with high yield and enantiomeric excess.     

Lignin-type, α,β-unsaturated aldehydes of lignin-type in organic solvents

A 1:1 reaction of [HO(CH₂)₃]₃P with 4-hydroxy-3-methoxy-cinnamaldehyde (coniferaldehyde) or 3,5-dimethoxy-4-hydroxycinnamaldehyde (sinapaldehyde) in acetone at room temperature affords phosphonium zwitterions of the type R₃P⁺CH(4-O^?-Ar)CH₂CHO; other phosphines [R = Et, n-Bu, (CH₂)₂CN, and p-Tol] do not react under the same conditions. In alcohols R??OH(D) [R?? = CD₃, Et, (CD₃)₂CD, s-Bu, HOCH₂CH₂], the above phosphines (except the cyano-derivative) and those where R = i-Pr, Cy, Me₂Ph, MePh₂ do react within an equilibrium established between the reactants and the zwitterion-hemiacetal products R₃P⁺CH(4-O^?-Ar)CH₂CH(OH)(OR??) that are formed as a mixture of two diastereomers. The nature of the phosphine and the alcohol affects the equilibrium and the diastereomeric ratio.     

Aminophosphines Derived From 1-Amino-4-Methylpiperazine: Synthesis,Oxidation and Complexation Reactions

Abstract

Functionalized mono(amino)phosphines of the type Ph₂PNHR (1) and bis (amino)phosphine of the type PhP(NHR)₂ (2) have been synthesized by treating Ph₂PCl or PhPCl₂ with 1-amino-4-methylpiperazine. Ligands react with aqueous hydrogen peroxide, elemental sulfur, or selenium to give the corresponding chalcogenides in good yields. The molybdenum complexes of the aminophosphines have been obtained. All of the new compounds were characterized by IR, ¹H, and ³¹P-NMR spectroscopy and elemental analysis.     

Effect of electrolysis conditions on the process of anodic oxidation of tertiary phosphines in the presence of camphene

Anodic oxidation of tertiary phosphines (Et₃P, Pr₃P, Bu₃P, i-Bu₃P and Am₃P) in the presence of camphene and heterogenic base (trisodium phosphate) on platinum anode in acetonitrile solution of sodium perchlorate was studied. It is established that trialkylphosphine radical cations react with camphene to give two types of products: Camphenylphosphonium salts formed by elimination of proton, and phosphiniminoterpenylphosphonium salts which are obtained due to the rearrangement of terpenyl skeleton. Conditions of electrosynthesis are found where the summary yield of terpenylphosphonium products increases. The effect of length and degree of branching of alkyl substituents in trialkylphosphines on the rate of the reaction of phosphine radical cations with camphene and starting phosphine is found.     

Substituierte halogencarbonylmetallate des chroms,Molybdäns und Wolframs : II. Photochemische phosphin-abspaltung,ein neuer weg zu metall (VI A) carbonylderivaten

Irradiation of bis(phosphine) tetracarbonyl complexes L₂M(CO)₄ (M = Cr, Mo, W) in the presence of donor ligands (amine, nitrile, halide ion) leads, via loss of one phosphine ligand, to neutral (LL′M(CO)₄) or ionic ([LM(CO)₄X]^?) metal carbonyl compounds. The use of this reaction as the first step in a general synthesis of unsymmetrically disubstituted derivatives of Group VIA hexacarbonyls is discussed.     

Studies on tetrahydrofuran solutions of manganese(II) dihalides and tertiary phosphines and their reactions with dioxygen

Solutions of Mn(THF)₂Br₂ and MnI₂ and tertiary phosphines in tetrahydrofuran at 0°C are oxidised by dioxygen or electrochemically giving deep blue-purple solutions which have identical electronic spectra. The principal band with λ_max at 570 nm has ?>9000 1 cm^?1 mol^?1. Tertiary phosphines have been shown to be oxidised to the corresponding tertiary phosphine oxides. No evidence was found for the reversible formation of dioxygen-manganese complexes.     

Mechanism of Phosphine Borane Deprotection with Amines: The Effects of Phosphine,Solvent and Amine on Rate and Efficiency

The kinetics of borane transfer from simple tertiary phosphine borane adducts to a wide range of amines have been determined. All data obtained, including second‐order kinetics, lack of cross‐over, and negative entropies of activation for reaction of triphenylphosphine borane with quinuclidine and triethylamine, are consistent with a direct (S_N2‐like) transfer process, rather than a dissociative (S_N1‐like) process. The identities of the amine, phosphine, and solvent all impact substantially on the rate (k) and equilibrium (K) of the transfer, which in some cases vary by many orders of magnitude. P‐to‐N transfer is more efficient with cyclic amines in apolar solvents due to reduced entropic costs and ground‐state destabilisation. Taken as a whole, the data allow informed optimisation of the deprotection step from the stand‐point of rate, or synthetic convenience. In all cases, both reactants should be present at high initial concentration to gain kinetic benefit from the bimolecularity of the process. Ultimately, the choice of amine is dictated by the identity of the phosphine borane complex. Aryl‐rich phosphine boranes are sufficiently reactive to allow use of diethylamine or pyrrolidine as a volatile low polarity solvent and reactant, whereas more alkyl‐rich phosphines benefit from the use of more reactive amines, such as 1,4‐diaza[2.2.2]bicyclooctane (DABCO), in apolar solvents at higher temperatures.     

Atom-Economic Synthesis of Tris[2-(organylthio)ethyl]phosphine Oxides from Phosphine and Vinyl Sulfides

Abstract

Phosphine, generated from elemental phosphorus in the system KOH-toluene-H₂O, reacts with vinyl sulfides under free radical conditions (AIBN, dioxane, 65–70°C, atmospheric pressure) to form regiospecifically tris[2-(organylthio)ethyl]phosphines, which are readily oxidized in air to corresponding tris[2-(organylthio)ethyl]phosphine oxides.     

Acyl- und Alkylidenphosphane. XVI. (Dimethylaminomethyliden)- und (Diphenylmethyliden)phosphane

Acyl- and Alkylidenephosphines. XVI. (Dimethylaminomethylidene)- and (Diphenylmethylidene) phosphines Alkyl- or arylbis(trimethylsilyl)phosphines 1 (R = mesityl, C₉H₁₁ a ; (CH₃)₃C b ; C₆H₅ c ; CH₃ d ) react only very slowly with dimethylformamide 2 and benzophenone 4. After repeated addition of small amounts of solid sodium hydroxide, however, the reaction-time is shortened from several months to a few days. The reactions between 1 a or 1 b and 2 yield the (dimethylaminomethylidene)phosphines 3 a and 3 b ; from 1 a and 4 mesityl-(diphenylmethylidene)phosphine 5 a is obtained. The formation of the thermally labile phosphines 3 d and 5 c is proved by NMR spectroscopy; these compounds dimerize very fast to give 2,4-bis(dimethylamino)-1,3-dimethyl- 10 d and 1,2,2,3,4, 4-hexaphenyl-1,3-diphosphetane 15 c. Similarly the lithium trimethylsilylphosphides 6 a , 6 c and 6 e (R = (CH₃)₃Si) react with 2 or 4 to form 3 a and 5 c as well as (diphenylmethylidene)-trimethyl-silylphosphine 5 e .     

A combined magnetic circular dichroism and density functional theory approach for the elucidation of electronic structure and bonding in three- and four-coordinate iron(ii)–N-heterocyclic carbene complexes

Iron salts and N-heterocyclic carbene (NHC) ligands is a highly effective combination in catalysis, with observed catalytic activities being highly dependent on the nature of the NHC ligand. Detailed spectroscopic and electronic structure studies have been performed on both three- and four-coordinate iron(ii)–NHC complexes using a combined magnetic circular dichroism (MCD) and density functional theory (DFT) approach that provide detailed insight into the relative ligation properties of NHCs compared to traditional phosphine and amine ligands as well as the effects of NHC backbone structural variations on iron(ii)–NHC bonding. Near-infrared MCD studies indicate that 10Dq(T _d) for (NHC)₂FeCl₂ complexes is intermediate between those for comparable amine and phosphine complexes, demonstrating that such iron(ii)–NHC and iron(ii)–phosphine complexes are not simply analogues of one another. Theoretical studies including charge decomposition analysis indicate that the NHC ligands are slightly stronger donor ligands than phosphines but also result in significant weakening of the Fe–Cl bonds compared to phosphine and amine ligands. The net result is significant differences in the d orbital energies in four-coordinate (NHC)₂FeCl₂ complexes relative to the comparable phosphine complexes, where such electronic structure differences are likely a significant contributing factor to the differing catalytic performances observed with these ligands. Furthermore, Mössbauer, MCD and DFT studies of the effects of NHC backbone structure variations (i.e. saturated, unsaturated, chlorinated) on iron–NHC bonding and electronic structure in both three- and four-coordinate iron(ii)–NHC complexes indicate only small differences as a function of backbone structure, that are likely amplified at lower oxidation states of iron due to the resulting decrease in the energy separation between the occupied iron d orbitals and the unoccupied NHC π* orbitals.     

First-principle predictions of basicity of organic amines and phosphines in acetonitrile

Amines and phosphines are widely utilized as bases and basic organocatalysts in organic chemistry. Thus it is highly valuable to develop a coherent theoretical method that can accurately predict the basicity of structurally unrelated amines and phosphines in organic solvents from the first principles. Herein we developed the first ab initio protocol that could predict the pK_a value of any protonated amine or phosphine in acetonitrile through systematic benchmarking. By comparing to a variety of available experimental data (total number=98), it was determined that the precision of the optimized method in basicity prediction was as low as 1.1 pK_a unit. With the powerful new method in hand, we subsequently conducted some systematic studies about the basicity of organic amines and in particular phosphines, for which very few experimental data were available. It was found that the solvent exerted profound effects on the basicity of amines and phosphines. Accordingly we concluded that it was not valid to use gas-phase data to interpret the solution-phase basicity of amines and phosphines. Next we reported the basicity of a number of synthetically important aliphatic and aromatic amines and phosphines in acetonitrile. We also compared, for the first time, the α-substituent effects on the basicity of aliphatic amines and phosphines and the remote substituent effects on the basicity of aromatic amines and phosphines. Finally, we studied for the first time the basicity of cyclic amines and phosphines. It was found that the ring strain exerted some interesting effects on the basicity of amines and phosphines.     

Synthesis of Glycosylphosphine Oxides and Related Compounds

The benzyl-protected glycosyl acetates 1 , 6 , 11 , and 15 react with MeOPPh₂ under catalysis by TMSOTf to yield diastereoselectively the glycosylphosphine oxides 2 , 3 , 8 , 12 , 13 , and 16 , with a strong preference for the 1,2-cis-configurated anomers. Hydrogenolysis of the major products gave the crystalline, unprotected phosphine oxides 4 , 9 , 14 , and 17 , of which 4 was transformed in to the acetate 5 , and 9 into the benzoate 10 . The benzylated phosphine oxides 4 , 8 , 12 , and 16 were reduced with Cl₃SiH in the presence of a tertiary amine to form the phosphines 18 , 21 , 24 , and 26 , which were transformed into the phosphine sulfides 19 , 22 , 25 , and 27 . Moreover, 18 and 21 , were characterized as the borane adducts 20 , and 23 . The structure of the (arabinofuranosyl)phosphine oxide 12 , the corresponding sulfide 25 , and of the borane complex 20 were established by X-ray analysis. According to NMR spectroscopy, the equatorial pyranosylphosphine oxide 8 , the sulfide 22 , and the borane complex 23 adopt a ⁴C₁ conformation. The axial phosphine oxide 2 is a flattened ⁴C₁, the sulfide 19 exists as a B_2,5, and the borane complex 20 is a flattened ⁴C₁ in the solid sate and a B_2,5 in solution. Thus, the conformational behavior of these α-D -glucopyranose derivatives reflects the steric requirement of the P-substituents.     

DFT characterization of the mechanism for Staudinger/aza-Wittig tandem organocatalysis

A computational simulation with DFT calculations and microkinetic modeling is carried out on a complete catalytic cycle, involving Staudinger ligation, aza-Wittig condensation and phosphine oxide recycle, for non-truncated substrates and catalyst. The Staudinger reaction produces phosphazenes (R₃P?=?NR), also known as iminophosphoranes, from phosphines and organic azides. Electrophilic carbonyl groups react with phospazenes to produce imines and phosphine oxides. Recently the Staudinger reaction and the aza-Wittig condensation have been combined to spawn intramolecular tandems producing cyclic molecules of great pharmaceutical interest (e.g. benzoxazoles). The release of diatomic nitrogen combined with the formation of phosphine oxide represents the driving force of the reaction. The implementation of in situ recycling of the exhausted phosphine oxide into the Staudinger/aza-Wittig tandem improves the scopes and the applicability of the reaction, transforming it into a powerful and versatile synthetic tool.