Characterization of a soluble library of the Pseudomonas aeruginosa PAO1 membrane proteome with emphasis on c-di-GMP turnover enzymes.

Studies of protein-protein interactions in membranes are very important to fully understand the biological function of a cell. The extraction of proteins from the native membrane environment is a critical step in the preparation of membrane proteins that might affect the stability of protein complexes. In this work, we used the amphiphilic diisobutylene/maleic acid copolymer to extract the membrane proteome of the opportunistic pathogen Pseudomonas aeruginosa, thereby creating a soluble membrane-protein library within a native-like lipid-bilayer environment. Size fractionation of nanodisc-embedded proteins and subsequent mass spectrometry enabled the identification of 3358 proteins. The native membrane-protein library showed a very good overall coverage compared to previous proteome data. The pattern of size fractionation indicated that protein complexes were preserved in the library. More than 20 previously described complexes, e.g. the SecYEG and Pili complexes, were identified and analyzed for coelution. Although the mass-spectrometric dataset alone did not reveal new protein complexes, combining pulldown assays with mass spectrometry was successful in identifying new protein interactions in the native membrane-protein library. Thus, we identified several candidate proteins for interactions with the membrane phosphodiesterase NbdA, a member of the c-di-GMP network. We confirmed the candidate proteins CzcR, PA4200, SadC, and PilB as novel interaction partners of NbdA using the bacterial adenylate cyclase two-hybrid assay. Taken together, this work demonstrates the usefulness of the native membrane-protein library of P. aeruginosa for the investigation of protein interactions and membrane-protein complexes. Data are available via ProteomeXchange with identifiers PXD039702 and PXD039700.

Protonation of P-Stereogenic Phosphiranes: Phospholane Formation via Ring Opening and C-H Activation.

Protonation of cyclopropanes and aziridines is well-studied, but reactions of phosphiranes with acids are rare and have not been reported to result in ring opening. Treatment of syn-Mes*PCH2CHR (Mes* = 2,4,6-(t-Bu)3C6H2, R = Me or Ph, syn-1-2) or anti-Mes*PCH2CHPh (anti-2) with triflic acid resulted in regiospecific anti-Markovnikov C-protonation with ring opening and cyclophosphination of a Mes* ortho-t-Bu group to yield the phospholanium cations [PH(CH2CH2R)(4,6-(t-Bu)2-2-CMe2CH2C6H2)][OTf] (R = Me or Ph, 3-4), which were deprotonated with NEt3 to give phospholanes 5-6. Enantioenriched or racemic syn-1 both gave racemic 3. The byproduct [Mes*PH(CH2CH2Me)(OH)][OTf] (7) was formed from syn-1 and HOTf in the presence of water. Density functional theory calculations suggested that P-protonation followed by ring opening and hydride migration to C yields the phosphenium ion, [Mes*P(CH2CH2Me)][OTf], which undergoes C-H oxidative addition of an o-t-Bu methyl group. This work established a new reactivity pattern for phosphiranes.

FULL-MDS: Fluorescent Universal Lipid Labeling for Microfluidic Diffusional Sizing.

Microfluidic diffusional sizing (MDS) is a recent and powerful method for determining the hydrodynamic sizes and interactions of biomolecules and nanoparticles. A major benefit of MDS is that it can report the size of a fluorescently labeled target even in mixtures with complex, unpurified samples. However, a limitation of MDS is that the target itself has to be purified and covalently labeled with a fluorescent dye. Such covalent labeling is not suitable for crude extracts such as native nanodiscs directly obtained from cellular membranes. In this study, we introduce fluorescent universal lipid labeling for MDS (FULL-MDS) as a sparse, noncovalent labeling method for determining particle size. We first demonstrate that the inexpensive and well-characterized fluorophore, Nile blue, spontaneously partitions into lipid nanoparticles without disrupting their structure. We then highlight the key advantage of FULL-MDS by showing that it yields robust size information on lipid nanoparticles in crude cell extracts that are not amenable to other sizing methods. Furthermore, even for synthetic nanodiscs, FULL-MDS is faster, cheaper, and simpler than existing labeling schemes.

Electroneutral Polymer Nanodiscs Enable Interference-Free Probing of Membrane Proteins in a Lipid-Bilayer Environment.

Membrane proteins can be examined in near-native lipid-bilayer environments with the advent of polymer-encapsulated nanodiscs. These nanodiscs self-assemble directly from cellular membranes, allowing in vitro probing of membrane proteins with techniques that have previously been restricted to soluble or detergent-solubilized proteins. Often, however, the high charge densities of existing polymers obstruct bioanalytical and preparative techniques. Thus, the authors aim to fabricate electroneutral-yet water-soluble-polymer nanodiscs. By attaching a sulfobetaine group to the commercial polymers DIBMA and SMA(2:1), these polyanionic polymers are converted to the electroneutral maleimide derivatives, Sulfo-DIBMA and Sulfo-SMA(2:1). Sulfo-DIBMA and Sulfo-SMA(2:1) readily extract proteins and phospholipids from artificial and cellular membranes to form nanodiscs. Crucially, the electroneutral nanodiscs avert unspecific interactions, thereby enabling new insights into protein-lipid interactions through lab-on-a-chip detection and in vitro translation of membrane proteins. Finally, the authors create a library comprising thousands of human membrane proteins and use proteome profiling by mass spectrometry to show that protein complexes are preserved in electroneutral nanodiscs.

Configurational Lability at Tetrahedral Phosphorus: syn/anti-Isomerization of a P-Stereogenic Phosphiranium Cation by Intramolecular Epimerization at Phosphorus.

Tetrahedral main-group compounds are normally configurationally stable, but P-epimerization of the chiral phosphiranium cations syn- or anti-[Mes*P(Me)CH2 CHPh][OTf] (Mes*=2,4,6-(t-Bu)3 C6 H2 ) occurred under mild conditions at 60 °C in CD2 Cl2 , resulting in isomerization to give a syn-enriched equilibrium mixture. Ion exchange with excess [NBu4 ][Δ-TRISPHAT] (Δ-TRISPHAT=Δ-P(o-C6 Cl4 O2 )3 ) followed by chromatography on silica removed [NBu4 ][OTf] and gave mixtures of syn- and anti-[Mes*P(Me)CH2 CHPh][Δ-TRISPHAT]⋅x[NBu4 ][Δ-TRISPHAT]. NMR spectroscopy showed that isomerization proceeded with epimerization at P and retention at C. DFT calculations are consistent with a mechanism involving P-C cleavage to yield a hyperconjugation-stabilized carbocation, pyramidal inversion promoted by σ-interaction of the P lone pair with the neighboring ß-carbocation, and ring closure with inversion of configuration at P.

Intramolecular attack on coordinated nitriles: metallacycle intermediates in catalytic hydration and beyond.

Hydration of nitriles is catalyzed by the enzyme nitrile hydratase, with iron or cobalt active sites, and by a variety of synthetic metal complexes. This Perspective focuses on parallels between the reaction mechanism of the enzyme and a class of particularly active catalysts bearing secondary phosphine oxide (SPO) ligands. In both cases, the key catalytic step was proposed to be intramolecular attack on a coordinated nitrile, with either an S-OH or S-O- (enzyme) or a P-OH (synthetic) nucleophile. Attack of water on the heteroatom (S or P) in the resulting metallacycle and proton transfer yields the amide and regenerates the catalyst. Evidence for this mechanism, its relevance to the formation of related metallacycles, and its potential for design of more active catalysts for nitrile hydration is summarized.

Diastereoselective Synthesis of P-Stereogenic Secondary Phosphine Oxides (SPOs) Bearing a Chiral Substituent by Ring Opening of (+)-Limonene Oxide with Primary Phosphido Nucleophiles.

Kinetic separation of the commercially available cis/trans-(+)-limonene oxide mixture by ring opening with primary phosphido nucleophiles LiPHR (R = ferrocenyl, Ph, Cy, t-Bu, Mes* (Mes* = 2,4,6-(t-Bu)3C6H2)), followed by treatment with aqueous NH4Cl and H2O2, gave unreacted cis-(+)-limonene oxide and diastereoenriched mixtures of the secondary phosphine oxides (SPOs) PHR(trans-(+)-Lim-OH)(O), which could be separated by chromatography and/or recrystallization. This one-pot synthesis uses a cheap chiral material and commercially available primary phosphines to control the configuration of the new P-stereogenic SPOs, which are potentially useful as ligands for metal complexes in asymmetric catalysis.

Metal-Catalyzed P-C Bond Formation via P-H Oxidative Addition: Fundamentals and Recent Advances.

Metal-catalyzed addition of P-H bonds to alkenes, alkynes, and other unsaturated substrates in hydrophosphination and related reactions is an atom-economical approach to valuable organophosphorus compounds. Understanding the mechanisms of these processes may enable synthetic improvements and development of new reactions. The first step in several catalytic cycles is P-H oxidative addition to yield intermediate metal hydride complexes bearing M-P bonds. P-C bond formation may occur via substrate insertion into the M-H bond, followed by P-C reductive elimination, or by insertion into the M-P bond and C-H reductive elimination. In an alternative outer-sphere process, nucleophilic attack of a metal-phosphido (M-PR2) group on an unsaturated substrate and proton transfer involving the metal hydride yields the product. This Perspective reviews the mechanistic possibilities, with a focus on the P-H activation step, and recent progress in developing novel catalytic transformations involving P-C bond formation.

Diastereoselective Coordination of P-Stereogenic Secondary Phosphines in Copper(I) Chiral Bis(phosphine) Complexes: Structure, Dynamics, and Generation of Phosphido Complexes.

Diastereoselective coordination of racemic secondary phosphines (PHRR') to Cu(I) precursors containing chiral bis(phosphines) (diphos*) was explored as a potential route to P-stereogenic phosphido complexes. Reaction of [Cu(NCMe)4][PF6] with chiral bis(phospholanes) gave [Cu(diphos*)2][PF6] (diphos* = ( R, R)-Me-DuPhos (1), ( R, R)-Et-DuPhos (2), or ( R, R)-Me-FerroLANE) (3)) or the mono(chelates) [Cu(diphos*)(NCMe) n][PF6] (diphos* = ( R, R)- i-Pr-DuPhos, n = 2 (4); diphos* = ( R, R)-Me-FerroLANE, n = 1 (5)). Treatment of [Cu(NCMe)4][PF6] with diphos* and PHMe(Is) (Is = 2,4,6-( i-Pr)3C6H2) gave mixtures of diastereomers of [Cu(( R, R)- i-Pr-DuPhos)(PHMe(Is))(NCMe)][PF6] (6) and [Cu(( R, R)-Me-FerroLANE)(PHMe(Is))][PF6] (7); two of the three expected isomers of the bis(secondary phosphine) complexes [Cu(( R, R)- i-Pr-DuPhos)(PhHP(CH2) nPHPh)][PF6] ( n = 2 (8); n = 3 (9)) were formed preferentially in related reactions. Reaction of the halide-bridged dimers [Cu(( R, R)- i-Pr-DuPhos)(X)]2 or [Cu(( R, R)-Me-FerroLANE)(I)]2 with PHMe(Is) gave the labile adducts Cu(( R, R)- i-Pr-DuPhos)(PHMe(Is))(X) (X = Cl (10), Br (11), I (12)) and Cu(( R, R)-Me-FerroLANE)(PHMe(Is))(I) (13). Complexes 1, 6, and 8-11 were structurally characterized by X-ray crystallography. Variable temperature NMR studies of 6 and 8 showed that the secondary phosphine ligands underwent reversible dissociation. Deprotonation of 6 or 7 generated the P-stereogenic phosphido complexes Cu(diphos*)(PMeIs) (diphos* = ( R, R)- i-Pr-DuPhos (14) or ( R, R)-Me-FerroLANE) (17)), observed by 31P NMR spectroscopy, but decomposition also occurred. Density functional theory calculations were used to characterize the diastereomers of thermally unstable 17 and the inversion barrier in a model copper-phosphido complex. These observations provided structure-property relationships which may be useful in developing catalytic asymmetric reactions involving secondary phosphines and P-stereogenic copper phosphido intermediates.

Role of Coulombic Repulsion in Collisional Lipid Transfer Among SMA(2:1)-Bounded Nanodiscs.

Styrene/maleic acid (SMA) and related copolymers are attracting great interest because they solubilise membrane proteins and lipids to form polymer-encapsulated nanodiscs. These nanodiscs retain a lipid-bilayer core surrounded by a polymer rim and can harbour a membrane protein or a membrane-protein complex. SMA exists in different styrene/maleic acid molar ratios, which results in differences in hydrophobicity and solubilisation properties. We have recently demonstrated fast collisional lipid transfer among nanodiscs encapsulated by the relatively hydrophobic copolymer SMA(3:1). Here, we used time-resolved Förster resonance energy transfer to quantify the lipid-transfer kinetics among nanodiscs bounded by SMA(2:1), a less hydrophobic copolymer that is superior in terms of lipid and membrane-protein solubilisation. Moreover, we assessed the effects of ionic strength and, thereby, the role of Coulombic repulsion in the exchange of lipid molecules among these polyanionic nanodiscs. Collisional lipid transfer was slower among SMA(2:1) nanodiscs (kcol = 5.9 M-1 s-1) than among SMA(3:1) nanodiscs (kcol = 222 M-1 s-1) but still two to three orders of magnitude faster than diffusional lipid exchange among protein-encapsulated nanodiscs or vesicles. Increasing ionic strength accelerated lipid transfer in a manner predicted by the Davies equation, an empirical extension of the Debye-Hückel limiting law, or an extended equation taking into account the finite size of the nanodiscs. Using the latter approach, quantitative agreement between experiment and theory was achieved for an effective nanodisc charge number of z ≈ -33, which is an order of magnitude less than their nominal overall charge.

Inversion of Configuration at the Phosphorus Nucleophile in the Diastereoselective and Enantioselective Synthesis of P-Stereogenic syn-Phosphiranes from Chiral Epoxides.

Nucleophilic substitution results in inversion of configuration at the electrophilic carbon center (SN 2) or racemization (SN 1). The stereochemistry of the nucleophile is rarely considered, but phosphines, which have a high barrier to pyramidal inversion, attack electrophiles with retention of configuration at P. Surprisingly, cyclization of bifunctional secondary phosphine alkyl tosylates proceeded under mild conditions with inversion of configuration at the nucleophile to yield P-stereogenic syn-phosphiranes. DFT studies suggested that the novel stereochemistry results from acid-promoted tosylate dissociation to yield an intermediate phosphenium-bridged cation, which undergoes syn-selective cyclization.

Synthesis, Structure, and Luminescence of Copper(I) Halide Complexes of Chiral Bis(phosphines).

For investigation of structure-property relationships in copper phosphine halide complexes, treatment of copper(I) halides with chiral bis(phosphines) gave dinuclear [Cu((R,R)-i-Pr-DuPhos)(µ-X)]2 [X = I (1), Br (2), Cl (3)], [Cu(µ-((R,R)-Me-FerroLANE)(µ-I)]2 (5), and [Cu((S,S)-Et-FerroTANE)(I)]2 (6), pentanuclear cluster Cu5I5((S,S)-Et-FerroTANE)3 (7), and the monomeric Josiphos complexes Cu((R,S)-CyPF-t-Bu)(I) (8) and Cu((R,S)-PPF-t-Bu)(I) (9); 1-3, 5, and 7-9 were structurally characterized by X-ray crystallography. Treatment of iodide 1 with AgF gave [Cu((R,R)-i-Pr-DuPhos)(µ-F)]2 (4). DuPhos complexes 1-4 emitted yellow-green light upon UV irradiation at room temperature in the solid state. This process was studied by low-temperature emission spectroscopy and density functional theory (DFT) calculations, which assigned the luminescence to (M + X)LCT (Cu2X2 to DuPhos aryl) excited states. Including Grimme's dispersion corrections in the DFT calculations (B3LYP-D3) gave significantly shorter Cu-Cu distances than those obtained using B3LYP, with the nondispersion-corrected calculations better matching the crystallographic data; other intramolecular metrics are better reproduced using B3LYP-D3. A discussion of the factors leading to this unusual observation is presented.

Synthesis of a phosphapyracene via metal-mediated cyclization: structural and reactivity effects of acenaphthene precursors.

Metal-mediated synthesis of a new heterocycle, 1-phenyl-phosphapyracene (Ph-4, Ph-PyraPhos), by tandem phosphination/cyclization of peri-substituted 5-bromo-6-chloromethylacenaphthene (3) was investigated for comparison to Pt-catalyzed formation of 1-phosphaacenaphthenes (2, AcePhos) from the analogous naphthalene precursor (1). Reaction of PH2Ph with , NaOSiMe3 and a Cu catalyst gave ; a Pt catalyst yielded PHPh(CH2Ar) (Ph-11, Ar = 5-Br-acenaphthyl). Deprotonation of a complex of this secondary phosphine, [Pt((R,R)-Me-DuPhos)(Ph)(PHPh(CH2Ar))][PF6] (17), generated the phosphido intermediate Pt((R,R)-Me-DuPhos)(Ph)(PPhCH2Ar) (Ph-8), which cyclized to give [Pt((R,R)-Me-DuPhos)(Ph)(Ph-PyraPhos)][PF6] (18). Treatment of P-8 with silver triflate gave 18 and the cyclometalated phosphine complex [Pt((R,R)-Me-DuPhos)(κ(2)-(P,C)-5-Ph2PCH2-6-C12H8)][PF6] (21), which might form via Pt(iv) intermediates. The effects of the added "ace" bridge on structure and reactivity are discussed.

Synthesis, reactivity, and resolution of a C2-symmetric, P-stereogenic benzodiphosphetane, a building block for chiral bis(phosphines).

Although the pyramidal inversion barriers in diphosphines (R(2)P-PR(2)) are similar to those in phosphines (PR(3)), P-stereogenic chiral diphosphines have rarely been exploited as building blocks in asymmetric synthesis. The synthesis, reactivity, and resolution of the benzodiphosphetane trans-1,2-(P(t-Bu))(2)C(6)H(4) are reported. Alkylation with MeOTf followed by addition of a nucleophile gave the useful C(2)-symmetric P-stereogenic ligand BenzP* and novel analogues.

Synthesis and structure of intermediates in copper-catalyzed alkylation of diphenylphosphine.

Cu(I) catalysts for alkylation of diphenylphosphine were developed. Treatment of [Cu(NCMe)(4)][PF(6)] (1) with chelating ligands gave [CuL(NCMe)][PF(6)] (2; L = MeC(CH(2)PPh(2))(3) (triphos), 3; L = 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene (XantPhos)). These complexes catalyzed the alkylation of PHPh(2) with PhCH(2)Br in the presence of the base NaOSiMe(3) to yield PPh(2)CH(2)Ph (4). The precursors Cu(dtbp)(X) (dtbp =2,9-di-t-butylphenanthroline, X = Cl (5) or OTf (6)), CuCl, and 1 also catalyzed this reaction, but dtbp dissociated from 5 and 6 during catalysis. Both 2 and 3 also catalyzed alkylation of PHPh(2) with PhCH(2)Cl/NaOSiMe(3), but XantPhos dissociation was observed when 3 was used. When CH(2)Cl(2) was used as the solvent for alkylation of PhCH(2)Cl with precursors 2 or 3, or of PhCH(Me)Br with 2, it was competitively alkylated to yield PPh(2)CH(2)Cl (7), which was formed exclusively using 2 in the absence of a benzyl halide. Cu(triphos)-catalyzed alkylation of PhCH(Me)Br gave mostly PPh(2)CHMePh (8), along with some Ph(2)P-PPh(2) (9), which was also formed in attempted alkylation of dibromoethane with this catalyst. The phosphine complexes [Cu(triphos)(L')][PF(6)] (L' = PH(2)Ph (10), PH(2)CH(2)Fc (Fc = C(5)H(4)FeC(5)H(5), 11), PHPh(2) (12), PHEt(2) (13), PHCy(2) (Cy = cyclo-C(6)H(11), 14), PHMe(Is) (Is = 2,4,6-(i-Pr)(3)C(6)H(2), 15), PPh(2)CH(2)Ph (16), PPh(2)CH(2)Cl (17)), and [Cu(XantPhos)(L')][PF(6)] (L' = PHPh(2) (18), PPh(2)CH(2)Ph (19)) were prepared by treatment of 2 and 3 with appropriate ligands. Similarly, treatment of dtbp complexes 5 or 6 with PHPh(2) gave [Cu(dtbp)(PHPh(2))(X)] (X = OTf (20a) or Cl (20b)), and reaction of PPh(2)CH(2)Ph (4) with 1 formed [Cu(PPh(2)CH(2)Ph)(3)][PF(6)] (21). Complexes 2, 3, 11-14, 16, 17, 19, and 21 were structurally characterized by X-ray crystallography. Deprotonation of diphenylphosphine complex 12 in the presence of benzyl bromide gave diphenylbenzylphosphine complex 16, while deprotonation of 12 in CD(2)Cl(2) gave 17 containing a PPh(2)CD(2)Cl ligand. Low-temperature deprotonation of the soluble salt 12-[B(Ar(F))(4)] (Ar(F) = 3,5-(CF(3))(2)C(6)H(3)) in THF-d(8) gave the phosphido complex Cu(triphos)(PPh(2)) (22). Thermally unstable 22 was characterized by NMR spectroscopy and, in comparison to 12, by density functional theory (DFT) calculations, which showed it contained a polarized Cu-P bond. The ligand substitution step required for catalytic turnover was observed on treatment of 16 or 17 with PHPh(2) to yield equilibrium mixtures containing 12 and the tertiary phosphines 4 or 7; equilibrium constants for these reactions were 8(2) and 7(2), favoring complexation of the smaller secondary phosphine in both cases. These observations are consistent with a proposed mechanism for catalytic P-C bond formation involving deprotonation of the cationic diphenylphosphine complex [Cu(triphos)(PHPh(2))][PF(6)] (12) by NaOSiMe(3) to yield the phosphido complex Cu(triphos)(PPh(2)) (22). Nucleophilic attack on the substrate (benzyl halide or CH(2)Cl(2)) then yields the tertiary phosphine complex [Cu(triphos)(PPh(2)CH(2)X)][PF(6)] (X = Ph (16) or Cl (17)), and ligand substitution with PHPh(2) regenerates 12.

Synthesis of gold phosphido complexes derived from bis(secondary) phosphines. structure of tetrameric [Au(MesP(CH(2))(3)PMes)Au](4).

Treatment of 2 equiv of Au(THT)Cl (THT = tetrahydrothiophene) with the bis(secondary) phosphines HP(R) approximately PH(R) (linker approximately = (CH(2))(3), R = Mes = 2,4,6-Me(3)C(6)H(2) (1), R = Is = 2,4,6-(i-Pr)(3)C(6)H(2) (2), R = Ph (4); approximately = (CH(2))(2), R = Is (3); HP(R) approximately PH(R) = 1,1'-(eta(5)-C(5)H(4)PHPh)(2)Fe (5)), gave the dinuclear complexes (AuCl)(2)(mu-HP(R) approximately PH(R)) (6-10). Dehydrohalogenation with aqueous ammonia gave the phosphido complexes [(Au)(2)(mu-P(R) approximately P(R))](n) (11-15). Ferrocenyl- and phenylphosphido derivatives 15 and 14 were insoluble; the latter was characterized by solid-state (31)P NMR spectroscopy. Isitylphosphido complexes 12 and 13 gave rise to broad, ill-defined NMR spectra. However, mesitylphosphido complex 11 was formed as a single product, which was characterized by multinuclear solution NMR spectroscopy, solid-state (31)P NMR spectroscopy, and elemental analyses. Mass spectrometry suggested that this material contained eight gold atoms (n = 4). A structure proposed on the basis of the (1)H NMR spectra, containing a distorted cube of phosphorus atoms, was confirmed by X-ray crystallographic structure determination. NMR spectroscopy, including measurement of the hydrodynamic radius of 11 by (1)H NMR DOSY, suggested that this structure was maintained in solution. Density functional theory (DFT) structural calculations on 11 were also in good agreement with the solid-state structure.

Metal-catalyzed nucleophilic carbon-heteroatom (C-X) bond formation: the role of M-X intermediates.

Many important reactions that lead to carbon-heteroatom bond formation involve attack of anionic heteroatom nucleophiles, such as hydroxides, alkoxides, amides, thiolates and phosphides, at carbon. Related catalytic transformations are mediated by late transition metal complexes of these groups, which remain nucleophilic on metal coordination as a result of repulsive filled-filled interactions between the heteroatom lone pairs and metal d-orbitals and/or of polarization of the bonds Mdelta+-Xdelta-. This Perspective presents examples of catalytic nucleophilic C-X bond formation in both biological and synthetic systems and describes how changes in the metal, ancillary ligands and X groups may be used to tune nucleophilic reactivity.

Platinum-catalyzed enantioselective tandem alkylation/arylation of primary phosphines. Asymmetric synthesis of P-stereogenic 1-phosphaacenaphthenes.

Enantioselective tandem alkylation/arylation of primary phosphines with 1-bromo-8-chloromethylnaphthalene catalyzed by Pt(DuPhos) complexes gave P-stereogenic 1-phosphaacenaphthenes (AcePhos) in up to 74% ee. Diastereoselective formation of four P-C bonds in one pot with bis(primary) phosphines gave C2-symmetric diphosphines, including the o-phenylene derivative DuAcePhos, for which the rac isomer was formed with high enantioselectivity. These reactions, which appear to proceed via an unusual metal-mediated nucleophilic aromatic substitution pathway, yield a new class of heterocycles with potential applications in asymmetric catalysis.

Catalytic asymmetric synthesis of chiral phosphanes.

Chiral phosphanes, important ligands for metal-catalyzed asymmetric syntheses, are often prepared with compounds from the chiral pool, by using stoichiometric chiral auxiliaries, or by resolution. In some cases, this class of valuable compounds can be prepared more efficiently by catalytic asymmetric synthesis. This Concepts article presents an overview of these synthetic methods, including recent advances in catalysis by metal complexes, biocatalysis, organocatalysis, and ligand-accelerated catalysis.

Palladium-catalyzed asymmetric phosphination. Scope, mechanism, and origin of enantioselectivity.

Asymmetric cross-coupling of aryl iodides (ArI) with secondary arylphosphines (PHMe(Ar'), Ar' = (2,4,6)-R3C6H2; R = i-Pr (Is), Me (Mes), Ph (Phes)) in the presence of the base NaOSiMe3 and a chiral Pd catalyst precursor, such as Pd((R,R)-Me-Duphos)(trans-stilbene), gave the tertiary phosphines PMe(Ar')(Ar) in enantioenriched form. Sterically demanding secondary phosphine substituents (Ar') and aryl iodides with electron-donating para substituents resulted in the highest enantiomeric excess, up to 88%. Phosphination of ortho-substituted aryl iodides required a Pd(Et-FerroTANE) catalyst but gave low enantioselectivity. Observations during catalysis and stoichiometric studies of the individual steps suggested a mechanism for the cross-coupling of PhI and PHMe(Is) (1) initiated by oxidative addition to Pd(0) yielding Pd((R,R)-Me-Duphos)(Ph)(I) (3). Reversible displacement of iodide by PHMe(Is) gave the cation [Pd((R,R)-Me-Duphos)(Ph)(PHMe(Is))][I] (4), which was isolated as the triflate salt and crystallographically characterized. Deprotonation of 4-OTf with NaOSiMe3 gave the phosphido complex Pd((R,R)-Me-Duphos)(Ph)(PMeIs) (5); an equilibrium between its diastereomers was observed by low-temperature NMR spectroscopy. Reductive elimination of 5 yielded different products depending on the conditions. In the absence of a trap, the unstable three-coordinate phosphine complex Pd((R,R)-Me-Duphos)(PMeIs(Ph)) (6) was formed. Decomposition of 5 in the presence of PhI gave PMeIs(Ph) (2) and regenerated 3, while trapping with phosphine 1 during catalysis gave Pd((R,R)-Me-Duphos)(PHMe(Is))2 (7), which reacted with PhI to give 3. Deprotonation of 1:1 or 1.4:1 mixtures of cations 4-OTf gave the same 6:1 ratio of enantiomers of PMeIs(Ph) (2), suggesting that the rate of P inversion in 5 was greater than or equal to the rate of reductive elimination. Kinetic studies of the first-order reductive elimination of 5 were consistent with a Curtin-Hammett-Winstein-Holness (CHWH) scheme, in which pyramidal inversion at the phosphido ligand was much faster than P-C bond formation. The absolute configuration of the phosphine (SP)-PMeIs(p-MeOC6H4) was determined crystallographically; NMR studies and comparison to the stable complex 5-Pt were consistent with an RP-phosphido ligand in the major diastereomer of the intermediate Pd((R,R)-Me-Duphos)(Ph)(PMeIs) (5). Therefore, the favored enantiomer of phosphine 2 appeared to be formed from the major diastereomer of phosphido intermediate 5, although the minor intermediate diastereomer underwent P-C bond formation about three times more rapidly. The effects of the diphosphine ligand, the phosphido substituents, and the aryl group on the ratio of diastereomers of the phosphido intermediates Pd(diphos*)(Ar)(PMeAr'), their rates of reductive elimination, and the formation of three-coordinate complexes were probed by low-temperature 31P NMR spectroscopy; the results were also consistent with the CHWH scheme.