Phosphane–boranes: synthesis, characterization and synthetic applications

Design, Synthesis, and Evaluation of B-(Trifluoromethyl)phenyl Phosphine-Borane Derivatives as Novel Progesterone Receptor Antagonists

We previously revealed that phosphine-boranes can function as molecular frameworks for biofunctional molecules. In the present study, we exploited the diversity of available phosphines to design and synthesize a series of B-(trifluoromethyl)phenyl phosphine-borane derivatives as novel progesterone receptor (PR) antagonists. We revealed that the synthesized phosphine-borane derivatives exhibited LogP values in a predictable manner and that the P-H group in the phosphine-borane was almost nonpolar. Among the synthesized phosphine-boranes, which exhibited PR antagonistic activity, B-(4-trifluoromethyl)phenyl tricyclopropylphosphine-borane was the most potent with an IC50 value of 0.54 μM. A docking simulation indicated that the tricyclopropylphosphine moiety plays an important role in ligand-receptor interactions. These results support the idea that phosphine-boranes are versatile structural options in drug discovery, and the developed compounds are promising lead compounds for further structural development of next-generation PR antagonists.

Physicochemical characterization of B-hydroxyphenyl phosphine borane derivatives and their evaluation as nuclear estrogen receptor ligands

Increasing the structural options in medicinal chemistry is a promising approach to develop new drug candidates. In this research, we designed and synthesized a series of B-hydroxyphenyl phosphine borane derivatives and investigated their structure-property and structure-activity relationships. The synthesized B-phenylphosphine borane derivatives exhibited sufficient stability in aqueous media, weaker hydrophobicity than the corresponding alkanes and silanes, and sufficient affinity for lipid membranes to enable permeability. Several B-hydroxyphenyl phosphine borane derivatives exhibited significant estrogen receptor (ER) agonistic activity with superior ligand-lipophilicity efficiency (LLE). The phosphine borane framework appears to be a promising option for structural development in drug discovery studies.

Building Blocks for Molecular Polygons Based on Platinum Vertices and Polyynediyl Edges

Reactions of Cl2P(CH2)3PCl2 and p-MgBrC6H4X (X = a/OMe, b/OtBu, c/tBu, d/SiMe3) give the diphosphines (p-XC6H4)2P(CH2)3P(p-C6H4X)2 (1a-d; 47-66%). Additions of 1a,d to (COD)PtCl2 yield (CH2(CH2P(p-C6H4X)2)2)PtCl2 (2a,d; 62-88%), which upon reaction with butadiyne (2 equiv; HNEt2/cat. CuI) give (CH2(CH2P(p-C6H4X)2)2)Pt((C≡C)2H)2 (3a,d; 34-76%). Alternatively, 3a-d can be accessed from trans-(p-tol3P)2Pt((C≡C)2H)2 and 1a-d (30-87%). Reactions of (p-tol3P)2PtCl2 and H(C≡C)2SiR3 (2 equiv, HNEt2/cat. CuI; R = Me/Et/iPr) give trans-(p-tol3P)2Pt((C≡C)2SiR3)2 (77-95%), and subsequent additions of 1a,b,d yield the corresponding adducts (CH2(CH2P(p-C6H4X)2)2)Pt((C≡C)2SiR3)2 (R/X = Me/OMe, 5a; iPr/OMe, 6a; iPr/OtBu, 6b; iPr/SiMe3, 6d; 52-95%) and (for 5a) a luminescent diplatinum byproduct with trans Pt((C≡C)2SiMe3)2 units. 5a and 6b hydrolyze in the presence of F- to 3a,b (92-93%). Reaction of 2a and 3a (HNEt2/cat. CuI) affords the Pt4C16 polygon ([(CH2(CH2P(p-C6H4OMe)2)2)Pt(C≡C)2]4 as an H2NEt2+ Cl- adduct (66%). The 13C{1H} NMR spectra of 3a-d, 5a, and 6a,b,d feature complex AMXX' (CPtPP') spin systems, and simulations allow J values to be extracted. The crystal structures of 2a, 3a,b,d, 5a, and 6a are determined and analyzed.

Synthesis, Structure, Reactivity, and Intramolecular Donor-Acceptor Interactions in a Phosphinoferrocene Stibine and Its Corresponding Phosphine Chalcogenides and Stiboranes

Ferrocene-based phosphines equipped with additional functional groups are versatile ligands for coordination chemistry and catalysis. This contribution describes a new compound of this type, combining phosphine and stibine groups at the ferrocene backbone, viz. 1-(diphenylphosphino)-1'-(diphenylstibino)ferrocene (1). Phosphinostibine 1 and the corresponding P-chalcogenide derivatives Ph2P(E)fcSbPh2 (1E, fc = ferrocene-1,1'-diyl, E = O, S, Se) were synthesized and further converted to the corresponding stiboranes Ph2P(E)fcSb(O2C6Cl4)Ph2 (6 and 6E) by oxidation with o-chloranil. All compounds were characterized by spectroscopic methods, X-ray diffraction analysis, cyclic voltammetry, and theoretical methods. Both NMR spectroscopy and DFT calculations confirmed the presence of P → Sb and P═O → Sb donor-acceptor interactions in 6 and 6O, triggered by the oxidation of the stibine moiety into Lewis acidic stiborane. The corresponding interactions in 6S and 6Se were of the same type but significantly weaker. A coordination study with AuCl as the model metal fragment revealed that the phosphine group acts as the "primary" coordination site, in line with its higher basicity. The obtained Au(I) complexes were applied as catalysts in the Au-catalyzed cyclization of N-propargylbenzamide and in the oxidative [2 + 2 + 1] cyclization of ethynylbenzene with acetonitrile and pyridine N-oxides. The catalytic results showed that the stibine complexes had worse catalytic performance than their phosphine counterparts, most likely due to the formation of weaker coordination bonds and hence poorer stabilization of the active metal species. Nevertheless, the stibine moiety could be used to fine-tune the properties of the ligated metal center by changing the oxidation state or substituents at the "remote" Sb atom.

Universal and divergent P-stereogenic building with camphor-derived 2,3-diols

The access to P-stereogenic motifs has always been considered a very challenging and high attractive mission in modern organic synthesis. While several chiral auxiliaries employed by the practical Jugé-Stephan-like methodology have been developed, new type of readily accessible bifunctional ligands toward P-stereogenic building still remain much desirable. Herein, we present a powerful chiral template, camphor-derived 2,3-diols named CAMDOL, which were designed and synthesized from the commercially cheap camphorquinone in high yields at 50 grams scale with a column-free purification. Diverse P(III)-chiral compounds and their borane forms including phosphinous acids, phosphinites, and phosphines, as well as the corresponding P(V)-chiral compounds including phosphinates, phosphine oxides, phosphinothioates, phosphine sulfides, and secondary phosphine oxides were afforded in high yields and ee values through the optimal 2,3-diphenyl CAMDOL platform. An unusual C³-OP bond cleavage following the first P-OC² bond breaking was observed during the ring-opening process when quenching by NH₄Cl solution, which generates a unique but valuable camphor-epoxide scaffold as by-product.

Borane-Protecting Strategy for Hydrosilylation of Phosphorus-Containing Olefins

Ir-catalyzed hydrosilylation of the alkenyl phosphine borane complex 1 was achieved to give the corresponding products 2. Because the phosphino group coordinates with metals and is unstable under aerobic conditions, the formation of the corresponding borane adduct was effective not only to promote the target hydrosilylation but also to keep 1 stable under aerobic conditions. The removal of coordinated borane from 2 was readily performed with the treatment by 1,4-diazabicyclo[2.2.2]octane to apply to further transformations. The immobilization and following deprotection of 2 on the surface of mesoporous silica were also examined.

Syntheses, homeomorphic and configurational isomerizations, and structures of macrocyclic aliphatic dibridgehead diphosphines; molecules that turn themselves inside out

The diphosphine complexes cis- or trans-[upper bond 1 start]PtCl₂(P((CH₂) _n )₃P[upper bond 1 end]) (n = b/12, c/14, d/16, e/18) are demetalated by MC[triple bond, length as m-dash]X nucleophiles to give the title compounds (P((CH₂) _n )₃)P (3b-e, 91-71%). These "empty cages" react with PdCl₂ or PtCl₂ sources to afford trans-[upper bond 1 start]MCl₂(P((CH₂) _n )₃P[upper bond 1 end]). Low temperature ³¹P NMR spectra of 3b and c show two rapidly equilibrating species (3b, 86 : 14; 3c, 97 : 3), assigned based upon computational data to in,in (major) and out,out isomers. These interconvert by homeomorphic isomerizations, akin to turning articles of clothing inside out (3b/c: ΔH ^‡ 7.3/8.2 kcal mol^-1, ΔS ^‡ -19.4/-11.8 eu, minor to major). At 150 °C, 3b, c, e epimerize to (60-51) : (40-49) mixtures of (in,in/out,out) : in,out isomers, which are separated via the bis(borane) adducts 3b, c, e·2BH₃. The configurational stabilities of in,out-3b, c, e preclude phosphorus inversion in the interconversion of in,in and out,out isomers. Low temperature ³¹P NMR spectra of in,out-3b, c reveal degenerate in,out/out,in homeomorphic isomerizations (ΔG ^‡ _Tc 12.1, 8.5 kcal mol^-1). When (in,in/out,out)-3b, c, e are crystallized, out,out isomers are obtained, despite the preference for in,in isomers in solution. The lattice structures are analyzed, and the D ₃ symmetry of out,out-3c enables a particularly favorable packing motif. Similarly, (in,in/out,out)-3c, e·2BH₃ crystallize in out,out conformations, the former with a cycloalkane solvent guest inside.

Synthesis and coordination of hybrid phosphinoferrocenes with extended donor pendants

Combining a phosphinoferrocene fragment with extended multidonor moieties affords novel, flexible multidonor pro-ligands. This contribution describes the synthesis of two structurally similar functional phosphines, Ph₂PfcNHC(O)CH₂PPh₂ (1) and Ph₂PfcNHCH₂CH₂PPh₂ (2, fc = ferrocene-1,1'-diyl), and their coordination behaviour towards Pd(II). The former amidophosphine reacts with [PdCl₂(MeCN)₂] to produce the chelate complex [PdCl₂(1-κ²P,P')] as a mixture of cis and trans isomers, which convert into bis-chelate [PdCl₂(Ph₂PfcNC(O)CH₂PPh₂-κ³P,P',N)] upon reacting with a strong base (KOt-Bu). In contrast, the more flexible and more basic phosphinoamine 2 directly forms the cationic bis-chelate complex [PdCl(2-κ³P,P',N)]Cl via spontaneous self-ionisation. Subsequent halogen abstraction with Ag[SbF₆] results either in counter ion exchange to give [PdCl(2-κ³P,P',N)][SbF₆] or in the formation of a structurally unique complex [PdCl(2-κ⁴Fe,P,P',N)][SbF₆]₂ with an Fe → Pd dative interaction, depending on the amount of silver(I) salt used (1 or 2 equiv.).

Nickel-catalysed diversification of phosphine ligands by formal substitution at phosphorus

We report a diversification strategy that enables the direct substituent exchange of tertiary phosphines. Alkylated phosphonium salts, prepared by standard alkylation of phosphines, are selectively dearylated in a nickel-catalysed process to access alkylphosphine products via a formal substitution at the phosphorus center. The reaction can be used to introduce a wide range of alkyl substituents into both mono- and bisphosphines. We also show that the alkylation and dearylation steps can be conducted in a one-pot sequence, enabling accelerated access to derivatives of the parent ligand. The phosphine products of the reaction are converted in situ to air-stable borane adducts for isolation, and versatile derivatisation reactions of these adducts are demonstrated.

Phosphine substituents can be exchanged by standard alkylation of a phosphine and a subsequent dearylation of the resulting phosphonium salt. A wide variety of alkyl groups can be introduced into both mono- and bidentate ligands using this method.

Synthesis, coordination behavior, and catalytic properties of dppf congeners with an inserted carbonyl moiety

Dppf-type ligands, desymmetrized by an inserted carbonyl group coordinate Pd(ii) and Pd(0). Among the air-stable Pd(0) complexes, the compound containing the least electron-donating ligand gives the best catalytic results in the Stille reaction.

Borane Adducts of Aromatic Phosphorus Heterocycles: Synthesis, Crystallographic Characterization and Reactivity of a Phosphinine-B(C6F5)3 Lewis Pair

A phosphinine‐borane adduct of a Me₃Si‐functionalized phosphinine and the Lewis acid B(C₆F₅)₃ has been synthesized and characterized crystallographically for the first time. The reaction strongly depends on the nature of the substituents in the α‐position of the phosphorus heterocycle. In contrast, the reaction of B₂H₆ with various substituted phosphinines leads to an equilibrium between the starting materials and the phosphinine–borane adducts that is determined by the Lewis basicity of the phosphinine. The novel phosphinine borane adduct (6‐B(C₆F₅)₃) shows rapid and facile insertion and [4+2] cycloaddition reactivity towards phenylacetylene. A hitherto unknown dihydro‐1‐phosphabarrelene is formed with styrene. The reaction with an ester provides a new, facile and selective route to 1‐R‐phosphininium salts. These salts then undergo a [4+2] cycloaddition in the presence of Me₃Si−C≡CH and styrene to cleanly form unprecedented derivatives of 1‐R‐phosphabarrelenium salts.

Palladium-Catalyzed C-P(III) Bond Formation by Coupling ArBr/ArOTf with Acylphosphines

Palladium-catalyzed C-P bond formation reaction of ArBr/ArOTf using acylphosphines as differential phosphination reagents is reported. The acylphosphines show practicable reactivity with ArBr and ArOTf as the phosphination reagents, though they are inert to the air and moisture. The reaction affords trivalent phosphines directly in good yields with a broad substrate scope and functional group tolerance. This reaction discloses the acylphosphines' capability as new phosphorus sources for the direct synthesis of trivalent phosphines.

Tuning the P-C dative/covalent bond formation in R3P-C60 complexes by changing the R group

The P-C dative/covalent bonds formed in R₃P-C₆₀ complexes (R = OCH₃, N(CH₃)₂, NC₄H₈) have been affected by the nature of the R group. The highest stabilisation (18.7 kcal mol^-1) has been found in the last system. The contribution of dispersion energies in the stabilisation also varies depending on the R group. The nature of the P→C bond has been characterised using state-of-the-art quantum-chemical techniques including NBO, AIM and ELF. The P→C dative bond is significantly different from the prototype dative bonds appearing in H₃N→BH₃ as well as in the fullerene - secondary-amine complexes previously studied by us. The findings obtained through electron structure theory have been supported by 10 ps DFT-D MD simulations.

Easy Access to Phosphine-Borane Building Blocks

In this paper, we highlight the synthesis of a variety of primary phosphine-boranes (RPH2 ⋅BH3 ) from the corresponding dichlorophosphines, simply by using Li[BH4 ] as reductant and provider of the BH3 protecting group. The method offers facile access not only to alkyl- and arylphosphine-boranes, but also to aminophosphine-boranes (R2 NPH2 ⋅BH3 ) that are convenient building blocks but without the protecting BH3 moiety thermally labile and notoriously difficult to handle. The borane-protected primary phosphines can be doubly deprotonated using n-butyllithium to provide soluble phosphanediides Li2 [RP⋅BH3 ] of which the phenyl-derivative Li2 [PhP⋅BH3 ] was structurally characterized in the solid state.

Synthesis and characterisation of Pd(ii) and Au(i) complexes with mesoionic carbene ligands bearing phosphinoferrocene substituents and isomeric carbene moieites

While numerous functional ligands combining phosphine and imidazole-2-ylidene (or imidazolin-2-ylidene) donor moieties have already been reported, the chemistry of the corresponding functional mesoionic carbenes (MIC) derived from 1,2,3-triazoles remains nearly untapped. This contribution describes the synthesis of two isomeric series of triazolium salts bearing 1'-(diphenylphosphino)ferrocenyl substituents by [3 + 2] cycloaddition of a P-protected phosphinoferrocene alkyne with azides, or alternatively of a P-protected phosphinoferrocene azide with terminal alkynes, and by subsequent methylation. These salts were used to synthesize structurally unique Pd(ii) complexes featuring a P,C-chelating triazolylidene carbene ligands and Au(i)-MIC complexes with free phosphine groups. The latter were further utilised to prepare Pd(ii)Au(i) heterometallic complexes containing bridging ferrocene phosphino-carbenes as structurally flexible, donor-unsymmetric metalloligands. In addition, the reactivity of the newly prepared, P-protected phosphinoferrocene alkyne Ph₂PfcC[triple bond, length as m-dash]CH·BH₃ (fc = ferrocene-1,1'-diyl) was investigated, and representatives from all reported compound classes were structurally characterised.

Investigation of interactions in Lewis pairs between phosphines and boranes by analyzing crystal structures from the Cambridge Structural Database

The interactions between phosphines and boranes in crystal structures have been investigated by analyzing data from the Cambridge Structural Database (CSD). The interactions between phosphines and boranes were classified into three types; two types depend on groups on the boron atom, whereas the third one involves frustrated Lewis pairs (FLPs). The data enabled geometric parameters in structures to be compared with phosphine-borane FLPs with classical Lewis pairs. Most of the crystal structures (78.1%) contain BH₃ as the borane group. In these systems, the boron-phosphorus distance is shorter than systems where the boron atom is surrounded by groups other than hydrogen atoms. The analysis of the CSD data has shown that FLPs have a tendency for the longest boron-phosphorus distance among all phosphine-borane pairs, as well as different other geometrical parameters. The results show that most of the frustrated phosphine-borane pairs found in crystal structures are bridged ones. The minority of non-bridged FLP structures contain, beside phosphorus and boron atoms, other heteroatoms (O, N, S for instance).

P-B Desulfurization: An Enabling Method for Protein Chemical Synthesis and Site-Specific Deuteration

Cysteine-mediated native chemical ligation is a powerful method for protein chemical synthesis. Herein, we report an unprecedentedly mild system (TCEP/NaBH₄ or TCEP/LiBEt₃ H; TCEP=tris(2-carboxyethyl)phosphine) for chemoselective peptide desulfurization to achieve effective protein synthesis via the native chemical ligation-desulfurization approach. This method, termed P-B desulfurization, features usage of common reagents, simplicity of operation, robustness, high yields, clean conversion, and versatile functionality compatibility with complex peptides/proteins. In addition, this method can be used for incorporating deuterium into the peptides after cysteine desulfurization by running the reaction in D₂ O buffer. Moreover, this method enables the clean desulfurization of peptides carrying post-translational modifications, such as phosphorylation and crotonylation. The effectiveness of this method has been demonstrated by the synthesis of the cyclic peptides dichotomin C and E and synthetic proteins, including ubiquitin, γ-synuclein, and histone H2A.

Nucleophilic addition and substitution at coordinatively saturated boron by facile 1,2-hydrogen shuttling onto a carbene donor

The reaction of [(cAAC^Me)BH₃] (cAAC^Me = 1-(2,6-ⁱPr₂C₆H₃)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) with a range of organolithium compounds led to the exclusive formation of the corresponding (dihydro)organoborates, Li⁺[(cAAC^MeH)BH₂R]^- (R = sp³-, sp²-, or sp-hybridised organic substituent), by migration of one boron-bound hydrogen atom to the adjacent carbene carbon of the cAAC ligand. A subsequent deprotonation/salt metathesis reaction with Me₃SiCl or spontaneous LiH elimination yielded the neutral cAAC-supported mono(organo)boranes, [(cAAC^Me)BH₂R]. Similarly the reaction of [(cAAC^Me)BH₃] with a neutral donor base L resulted in adduct formation by shuttling one boron-bound hydrogen to the cAAC ligand, to generate [(cAAC^MeH)BH₂L], either irreversibly (L = cAAC^Me) or reversibly (L = pyridine). Variable-temperature NMR data and DFT calculations on [(cAAC^MeH)BH₂(cAAC^Me)] show that the hydrogen on the former carbene carbon atom exchanges rapidly with the boron-bound hydrides.

A clean and simple method for deprotection of phosphines from borane complexes

Simple, efficient, clean, and stereospecific protocols of protection of phosphorus atom with borane and deprotection from the borane complexes of the tertiary phosphines in mild conditions are reported. The proposed protection/deprotection reactions tolerate a range of functional groups and lead to pure products with excellent yield with no need for application of chromatographic or crystallisation purification procedures. For the first time mechanisms of the reactions of phosphine protection and deprotection have been studied based on experimental kinetic data as well as quantumchemical calculations, which allows designing reaction conditions suitable for a given substrate.

Synthesis, Structural Characterization and Catalytic Evaluation of Anionic Phosphinoferrocene Amidosulfonate Ligands

Triethylammonium salts of phosphinoferrocene amidosulfonates with electron-rich dialkyphosphino substituents, R2PfcCONHCH2SO3(HNEt3) (4a–c), where fc = ferrocene-1,1′-diyl, and R = i-Pr (a), cyclohexyl (Cy; b), and t-butyl (c), were synthesized from the corresponding phosphinocarboxylic acids-borane adducts, R2PfcCO2H·BH3 (1a–c), via esters R2PfcCO2C6F5·BH3 (2a–c) and adducts R2PfcCONHCH2SO3(HNEt3)·BH3 (3a–c). Compound 4b was shown to react with [Pd(μ-Cl)(η-C3H5)]2 and AgClO4 to afford the zwitterionic complex [Pd(η3-C3H5)(Cy2PfcCONHCH2SO3-κ2O,P)] (5b), in which the amidosulfonate ligand coordinates as a chelating donor making use of its phosphine moiety and amide oxygen. The structures of 3b·CH2Cl2, 4b and 5b·CH2Cl2 were determined by single-crystal X-ray diffraction analysis. Compounds 4a–c and their known diphenylphosphino analogue, Ph2PfcCONHCH2SO3(HNEt3) (4d), were studied as supporting ligands in Pd-catalyzed cyanation of aryl bromides with K4[Fe(CN)6] and in Suzuki–Miyaura biaryl cross-coupling performed in aqueous reaction media under mild reaction conditions. In the former reaction, the best results were achieved with a catalyst generated from [PdCl2(cod)] (cod = η2:η2-cycloocta-1,5-diene) and 2 equiv. of the least electron-rich ligand 4d in dioxane–water as a solvent. In contrast, the biaryl coupling was advantageously performed with a catalyst resulting from palladium(II) acetate and ligand 4a (1 equiv.) in the same solvent.

Efficient access to a designed phosphapalladacycle catalyst via enantioselective catalytic asymmetric hydrophosphination

A chiral phosphine auxiliary was generated with excellent ee via catalytic asymmetric hydrophosphination of 3-(naphthalen-1-ylmethylene)pentane-2,4-dione. The subsequent metal complexation of the monophosphine yielded two different coordination complexes depending on the reaction conditions. The ortho-palladation of both coordination complexes resulted in the formation of a single dimeric phosphapalladacycle complex that could be further converted to the monomeric bisacetonitrile derivative. Moreover, the palladium complex exhibits interesting oxophilicity as the stable bisaquo derivative could be isolated and characterized crystallographically. The catalytic potential of the phosphapalladacycle was also demonstrated.

Transition metal-free stereospecific access to (E)-(1-fluoro-2-arylvinyl)phosphine borane complexes

Synthesis of (E)-(1-fluoro-2-arylvinyl)phosphine boranes has been achieved under metal-free conditions.

Ruthenium complexes of P-stereogenic phosphines with a heterocyclic substituent

Optically pure P-stereogenic monophosphorus ligands containing a heterocyclic substituent have been prepared. They have been coordinated to Ru-η⁶-arene moieties in which the ligands act as mono- or bidentate. The complexes catalyse asymmetric transfer hydrogenation reactions with up to 70% ee.