1. Introduction

2. Synthesis of triphenylphosphine and its substituted derivatives from elemental phosphorus and aryl halides

3. Phosphination of 2-chloropyridine with elemental phosphorus: synthesis of tris(2-pyridyl)phosphine

4. Micellar catalysis in the synthesis of phosphinic and phosphonic acids from elemental phosphorus and alkyl bromides

5. Synthesis of tertiary phosphines and their derivatives with pharmacophore imidazole, pyrazole, or pyridine groups on the basis of elemental phosphorus: study of antibacterial and antitumor activity

6. One-pot synthesis of trialkylphosphine oxides and trialkylphosphine sulfides from elemental phosphorus, sulfur, and alkyl bromides: new effective extractants of heavy and noble metals

7. Conclusions

1. INTRODUCTION

Research in the field of synthesis of organophosphorus compounds using elemental phosphorus instead of toxic and aggressive phosphorus halides keeps developing actively. This direction is systematically and productively developed by the scientific schools of Academician Trofimov Boris Alexandrovich [16] and Academician Sinyashin Oleg Gerol’dovich [715], as well as by Professor Perruzini with colleagues [1619] and in the groups of professors Wolf [2024] and Tang [2527]. The use of superbases generated in the systems of alkali metal hydroxide/polar nonhydroxylic solvent or under conditions of phase-transfer catalysis to activate elemental phosphorus (primarily non-toxic and non-flammable red phosphorus) opened the way to direct phosphorylation by elemental phosphorus of such accessible organic compounds as alkyl-, allyl- and benzyl halides, styrenes, vinylpyridines, allylbenzenes, acetylenes and to synthesis of unknown or previously inaccessible organic phosphines and phosphine oxides [46]. Now this reaction is quoted in the Great Russian Encyclopedia 2004–2017 as the Trofimov–Gusarova reaction. This review presents new data on the development of this reaction, obtained mainly over the past five years when studying the reactions of elemental phosphorus with organic halides and developing on this basis original and convenient methods for the synthesis of organic phosphines—triphenylphosphine and its substituted derivatives [28], tris(2-pyridyl)phosphine [29], phosphinic acids—alkyl-H-phosphinic [30] and benzyl-H-phosphinic[31], alkylphosphonic [32] acids, and tris(pyrazolyl)methylphosphine oxides [33].

2. SYNTHESIS OF TRIPHENYLPHOSPHINE AND ITS SUBSTITUTED DERIVATIVES FROM ELEMENTAL PHOSPHORUS AND ARYL HALIDES

Triphenylphosphine remains one of the most popular tertiary phosphines, widely used in catalysis, in organoelement synthesis, and also for the creation of modern innovative materials [3445]. The traditional method of obtaining triphenylphosphine is based on the use of aggressive and toxic phosphorus trichloride and organometallic compounds [4652]. The works of academician O.G. Sinyashin describe the synthesis of triphenylphosphine from white phosphorus and halobenzenes under conditions of electrochemical and electrocatalytic activation [7, 14, 5355]. Metal complexes, rare metal salts [20, 56], or special onium salts [57] are also used to activate elemental phosphorus in such reactions.

As for red phosphorus, it was practically not used as a starting platform for the synthesis of triphenylphosphine. In 1984, Bornansini obtained triphenylphosphine with a yield of 80% from phenyl iodide in the Pn/Na/t-BuOH/NH3(liq.) system using additional UV irradiation (350 nm) of the reaction mixture [58]. However, this reaction did not receive further development, apparently due to the need for special equipment (an irradiator to be placed inside the reaction flask). It should be noted that for a long time attempts to involve phenyl halides in reaction with red phosphorus [59] under the conditions of the Trofimov–Gusarova reaction remained unsuccessful. Nevertheless, this problem was recently solved [28], as a result of which an effective and technologically advanced method for the synthesis of triphenylphosphine by direct phosphination of halobenzenes PhX (X = F, Cl, and Br) with the red phosphorus/KOH/polar nonhydroxylic solvent system was developed [28]. This turned out to be possible due to the following two significant amendments to the previously developed methodology for phosphorylation of electrophiles with red phosphorus: i) increasing the basicity of the superbasic system in use by excluding water from its composition and ii) initial rapid heating of the reaction mixture (up to 100–115°C) to initiate a short exothermic process (a kind of “heat shock”), which allows synthesis to be completed within 0.5–1 h.

The syntheses were carried out by immersion of a reaction flask (Pn/PhX/KOH/polar nonhydroxylic solvent S) upon intensive stirring in a glycerol bath preheated to 115°C. After a short exothermic effect (2–5 min, 120–180°C, depending on the nature of the solvent S), the temperature of the reaction mixture decreased to ~100°C within 0.5–1 h, and the resulting triphenylphosphine was isolated by conventional treatment: dilution of the reaction mixture with water and extraction with ether (Scheme 1).

Scheme
scheme 1

1.

The yield of triphenylphosphine depends primarily on the nature of the halogen X in halobenzenes, as well as on the nature of the solvent S used (Table 1).

Table 1. Optimization of the conditions for phosphination of phenyl halides

Under the best conditions, the phosphination of fluorobenzene in the KOH/N-methylpyrrolidone (NMP) system, the yield of triphenylphosphine was 74% (Table 1, exp. 1).

The developed methodology was also extended to substituted halobenzenes and halonaphthalenes. Thus, (2-fluoro- (1), 3-fluoro- (2), 3-chloro- (3), 4-fluoro- (4), and 4-chloro- (5) toluenes, 1-fluoro- (6), 1-chloro- (7), and 2-fluoro- (8) naphthalenes), and also 4-fluorobenzamide (9) react with red phosphorus in the KOH/S system (100–180°C, 0.5–1 h) to form corresponding tertiary phosphines 10-15 with yields of up to 70% (Scheme 2) [28].

Scheme
scheme 2

2.

The formation of phosphines during the phosphination of aryl halides by the red phosphorus/superbase system indicates that in these processes, polyphosphide anions A, formed during the cleavage of the P–P bond by the hydroxide anion (Scheme 3), win competition from the oxidized part B of the phosphorus macromolecule.

Scheme
scheme 3

3.

It is also worth noting the selectivity of these reactions leading to tertiary phosphines: in most cases, neither primary nor secondary phosphines were identified in the reaction mixtures. This is obviously explained by the features of the mechanism of the process occurring in a multiphase superbasic system.

Thus, during the synthesis, three successive nucleophilic substitution reactions in the aromatic ring (SNAr) are realized with the participation of phosphor-centered anions A, C, and E of various natures (Scheme 4).

Scheme
scheme 4

4.

It is assumed that the polyphosphide anions A and C should have increased nucleophilicity, that is, they should be supernucleophiles due to the α-effect of neighboring phosphorus atoms, which is expressed in a lower ionization potential and higher polarizability. The possible existence of varieties of polyphosphides in the form of micro-, submicro-, and nanoscale particles with higher surface energy may also contribute to the ease of the nucleophilic substitution under consideration. In superbasic media with their lower proton activity, the above-mentioned supernucleophiles are weakly solvated and therefore should have even higher reactivity. All this facilitates to the nucleophilic substitution of halogen atoms for polyphosphide anions in aromatic rings.

The formation of phosphide anions already associated with the aryl group is more preferable when disassembling the phosphorus macromolecule. Indeed, a more advantageous way of cleavage of the P–P bond of a polyphosphide fragment F by a hydroxide anion is the formation of a C type anion, in which the negative charge can be distributed to an aromatic fragment (Scheme 5). Alternative cleavage of the P–P bond in the fragment F with the formation of the G anion looks less preferable due to the repulsion of lone electron pairs of neighboring phosphorus atoms.

Scheme
scheme 5

5.

As a result, the aryl halide molecule will interact with the intermediate F so that two aryl substituents will be bound to the same phosphorus atom. This is another factor explaining the high selectivity of the reaction.

Thus, a convenient, environmentally acceptable, selective synthesis of triphenylphosphine and some of its substituted derivatives by means of the direct reaction of red phosphorus with aryl halides in the presence of super-strong bases has been developed. It was found that aryl fluorides exhibit the best activity in the phosphination reaction, and N-methylpyrrolidone, which is environmentally safe and inexpensive, turned out to be the best solvent. The developed synthesis replaces the existing environmentally hazardous technologies for the production of triphenylphosphine based on the use of toxic phosphorus chlorides and fire-explosive organometallic compounds.

3. PHOSPHINATION OF 2-CHLOROPYRIDINE WITH ELEMENTAL PHOSPHORUS: SYNTHESIS OF TRIS(2-PYRIDYL)PHOSPHINE

The chemistry of tris(2-pyridyl)phosphine, an effective tripodal ligand for the directed synthesis of important scorpionate complexes of transition metals and clusters, continues to develop actively. This is greatly facilitated by the highly competitive method recently developed by B.A. Trofimov and his coworkers for obtaining tris(2-pyridyl)phosphine 16 (yield up to 62%) by phosphination of 2-bromopyridine with elemental phosphorus in a superbasic suspension of KOH/DMSO (3 h) [60, 61]. As a by-product, the formation of tris(2-pyridyl)phosphine oxide with a yield of 10% was detected.

Recently, in order to improve the known method of obtaining tris(2-pyridyl)phosphine [60], a reaction of the commercially more affordable 2-chloropyridine (17) with red phosphorus was implemented [29, 62]. Thus, phosphination of chloropyridine 17 by the red phosphorus/KOH/DMSO(H2O) system under heating (125°C, 1 h, argon) proceeds chemoselectively and allows phosphine 16 to be synthesized with a yield of 70% (Scheme 6).

Scheme
scheme 6

6.

As in the case of arylation/getarylation of red phosphorus (Schemes 1 and 2), the advantage and the most specific feature of the developed synthesis of phosphine 16 is the selectivity of the reaction: the corresponding primary and secondary phosphines are not formed under these conditions [29].

Thus, the direct formation of the Csp2–P bond from 2-chloropyridine and red phosphorus in the presence of the KOH/DMSO superbase formed the basis of the currently most convenient method for the synthesis of important multifunctional tertiary phosphine 16 (previously it was obtained from aggressive phosphorus trichloride and organometallic compounds [6366]) and opened up new opportunities for further dynamic development of the chemistry of functional pyridyl-containing phosphines. In particular, luminescent complexes of copper [6773] and silver [7476], silver complexes with pronounced cytotoxic activity [77], iron complexes with magnetic properties [7880], catalytically active transition metal complexes [8183], and hybrid bismuth complexes [84] have been synthesized on the basis of tris(2-pyridyl)phosphine 16.

4. MICELLAR CATALYSIS IN THE SYNTHESIS OF PHOSPHINIC AND PHOSPHONIC ACIDS FROM ELEMENTAL PHOSPHORUS AND ALKYL BROMIDES

Phosphorylation of alkyl bromides with elemental phosphorus in the KOH/water/dioxane system in the presence of a standard phase transfer catalyst, triethylbenzylammonium chloride (TEBAC), at 90–95°C is now the most convenient method for the synthesis of trialkyl phosphine oxides (yield up to 75% [59, 85]), which are effective extractants of lithium [86, 87] and rare earth elements [88]. Recently, this reaction was directed to the predominant formation of alkyl-H-phosphinic acids with a yield of 41–47% by heating red or white phosphorus with alkyl bromide at 60–62°C in the KOH/H2O/PhMe/Et3BnNCl system in the presence of a significant excess of elemental phosphorus relative to alkyl bromide [89]. The corresponding di- and trialkylphosphine oxides are formed as side products in the reaction.

It turned out to be possible to increase the efficiency and selectivity of this reaction by using, for the first time, micellar catalysis to form the carbon-phosphorus bond [30]. As a result, an original chemoselective synthesis of long-chain alkyl-H-phosphinic acids 18 was developed by direct combined alkylation and oxidation of elemental phosphorus (Pn, red modification) in the multiphase system Pn/alkyl bromide/KOH/H2O/toluene in the presence of micellar recyclable catalysts—alkyl ethers of polyethylene glycols (PEG) (Scheme 7).

Scheme
scheme 7

7.

Dialkylated PEGs with sufficiently long alkyl substituents (C6–C8) and a molecular weight of 600–1000 showed the best results in the synthesis of long-chain alkyl-H-phosphinic acids 18.

The reaction scheme includes two main processes: (1) disassembly of polymer Pn molecules in the aqueous phase under the action of activated (due to the complexation of potassium cations with PEG ligands) hydroxide anions (micelle A) and the formation of oligomeric phosphide anions B, which are transported to the organic phase in the form of reversed micelles C, and (2) subsequent alkylation of polyphosphide anions B in organic phase by long-chain alkyl bromides (Scheme 8). In the latter process, polyphosphide anions successfully compete with hydroxide anions, because they have increased nucleophilicity due to the α-effect of neighboring phosphorus atoms. In the alkylated polyphosphide fragment D, the hydroxide anion regioselectively attacks the atom associated with the alkyl group, since it has a greater positive charge (due to the greater electronegativity of the neighboring carbon atom). In addition, this atom is sterically more accessible than neighboring phosphorus atoms surrounded by branched phosphorus fragments. Intermediate E is also attacked by the hydroxide anion regioselectively at a more positively charged (oxidized) phosphorus atom with the formation of target phosphinic acids 18.

Scheme
scheme 8

8.

Trimethylcethylammonium bromide turned out to be somewhat less effective in phosphorylation of alkyl bromides with red phosphorus [32]: when using this catalyst the maximum yield of alkylphosphinic acids was 76%.

The technological advantages of the developed method are the availability and safety of starting materials and catalysts, chemoselectivity, recyclability of the catalytic system (the organic phase containing the catalyst can be used repeatedly), and the possibility of optimizing the catalyst activity by changing the molecular weight and structure of PEG ethers.

Synthesized long-chain n-alkyl-H-phosphinic acids are used to stabilize nanoparticles [90] and modify the surfaces of metals and their oxides, as co-catalysts in metal complex catalysis [91], and also as building blocks, for example, for the synthesis of alkyl-H-phosphinates by chemoselective alkylation of the corresponding phosphinic acids with alkyl bromides (60–65°C, Et3N) [92].

It should be noted that micellar catalysts successfully used to produce alkyl-H-phosphinic acids 18 (alkylated PEG, trimethylcethylammonium bromide) proved ineffective in the reaction of red phosphorus with arylmethyl halides [31]. Good activity in this reaction was demonstrated by Triton-X-100 (Fig. 1), 4-(tert-octyl)phenyl ether of polyethylene glycol. As a result, on the basis of direct phosphorylation of arylmethyl halides with red phosphorus in a multiphase KOH/H2O/toluene system in the presence of Triton-X-100, a method was developed [31] for the selective synthesis (yields up to 65%) of arylmethyl-H-phosphinic acids 19 (Scheme 9), which are demanded organophosphorus reagents for biomedical purposes [9398].

Fig. 1.
figure 1

Triton-X-100.

Scheme
scheme 9

9.

The synthesis of alkyl phosphonic acids was also based on the use of micellar catalysis. Thus, the reaction of red phosphorus with alkyl bromides in the KOH/H2O/toluene system in the presence of 5 mol % of trimethylcethylammonium bromide (CTAB) produced [32] acids 20 with a yield of up to 71% (up to 91% in the presence of 10 mol % of CTAB). The potassium phosphinate formed in this process is neutralized/oxidized in situ with nitric acid (Scheme 10).

Scheme
scheme 10

10.

Long-chain alkylphosphonic acids are widely used in engineering for modification of various surfaces (metals and metal oxides), creation of nanoparticles [99, 100], as stabilizing ligands for colloidal quantum dots [101] (used for biological sensors [102, 103]), for the development of visible and near-infrared emitters [104], and for the production of fire-resistant thermoplastic or thermosetting polymers [105, 106].

5. SYNTHESIS OF TERTIARY PHOSPHINES AND THEIR DERIVATIVES WITH PHARMACOPHORE IMIDAZOLE, PYRAZOLE, OR PYRIDINE GROUPS ON THE BASIS OF ELEMENTAL PHOSPHORUS: STUDY OF ANTIBACTERIAL AND ANTITUMOR ACTIVITY

In recent years, the development of convenient direct approaches to the synthesis of organophosphorus compounds containing nitrogen heterocycles (imidazoles, pyrazoles, and pyridines) has become an urgent task, since these fragments are part of numerous medicines [57, 59, 63, 65, 67, 78, 107109]. The solution to this problem is facilitated by the direct phosphorylation of organic compounds with nitrogen-containing heterocycles by elemental phosphorus under conditions of superbasic catalysis.

Thus, in [33] a number of new or inaccessible phosphorus-containing azoles and azines were synthesized, and their antibacterial and cytotoxic activity was studied.

Phosphorylation of the double bond in 1-(4-vinylbenzyl)-1H-imidazole and -benzimidazole with red phosphorus in the KOH/DMSO system led to the production of new tris[4-(1H-imidazole-1-ylmethyl)phenylethyl]- (21) and tris[4-(1H-benzimidazole-1-ylmethyl)phenylethyl]- (22) phosphine oxides (Scheme 11) [33].

Scheme
scheme 11

11.

Tris[(5-pyrazolyl)methyl]phosphine oxide 23 has been successfully synthesized in the system of superbasic KOH/H2O/PhMe/phase transfer catalyst (TEBAC) from red phosphorus and 1-methyl-5-(chloromethyl)pyrazole hydrochloride (Scheme 12) [33].

Scheme
scheme 12

12.

Under similar conditions, red phosphorus reacts with 2-(chloromethyl)pyridine hydrochloride to form tris(2-picolyl)phosphine oxide 24 (Scheme 12) [110].

Tris(2-pyridyl)phosphine oxide 25, -sulfide 26, and -selenide 27 were obtained from tris(2-pyridyl)phosphine 16 by treatment with aqueous hydrogen peroxide and elemental sulfur or selenium, respectively (Scheme 13) [33].

Scheme
scheme 13

13.

Similarly, tris[2-(2-(or 4)pyridyl)ethyl]phosphine oxides, sulfides and selenides 2833 (Scheme 14) were synthesized [33] using a modified procedure [111]. The corresponding tertiary phosphines were obtained on the basis of red phosphorus and vinylpyridines in the superbasic KOH/DMSO system [112].

Scheme
scheme 14

14.

The reaction of tris[2-(2-pyridyl)ethyl]phosphine [112] with alkyl halides produced previously unknown phosphonium salts 34 and 35 (Scheme 15) [33].

Scheme
scheme 15

15.

For all synthesized compounds, their antibacterial activity was evaluated (in relation to Enterococcus durans, Bacillus subtilis, Escherichia coli, and Pseudomonas aeruginosa), and for compounds 23, 34, and 35, cytotoxic activity was also evaluated. It has been shown that phosphine chalcogenides with imidazole (21), pyrazole (23) and pyridine (28 and 32) fragments, as well as phosphonium salts 34 and 35, can be considered as new promising antibacterial agents, whereas compounds 23, 34, and 35 showed high cytotoxic activity [33].

It is important to note that compounds 23, 34, and 35 are water-soluble, and their activity was evaluated, including activity in the aqueous medium.

The results obtained can make a significant contribution to the development of highly effective agents for the treatment and prevention of bacterial and oncological diseases.

6. ONE-POT SYNTHESIS OF TRIALKYLPHOSPHINE OXIDES AND TRIALKYLPHOSPHINE SULFIDES FROM ELEMENTAL PHOSPHORUS, SULFUR, AND ALKYL BROMIDES: NEW EFFECTIVE EXTRACTANTS OF HEAVY AND NOBLE METALS

Sulfur-containing organophosphorus compounds are widely used in engineering, agriculture, and medicine: for protection against corrosion and salt deposits in industrial waters, as stabilizers and plasticizers of polymers, as monomers for ion-exchange and thermostable polymers, additives to lubricating oils and hydraulic liquids [113116], and also as complexons and extractants of noble, non-ferrous, and heavy metals in hydrometallurgical processes [117120]. Among the compounds used as extractants, organic phosphine sulfides and phosphine oxides occupy a special place: they allow extraction processes to be carried out with good selectivity and efficiency [121123].

In order to obtain promising heavy metal extractants, the reactions of elemental phosphorus and sulfur with alkyl bromides under conditions of superbasic catalysis were studied in [124]. It has been shown that alkyl bromides react with red phosphorus when heated (90–95°C, 5–8 h) in the KOH/H2O/PhMe/TEBAC system to form a mixture of organophosphorus compounds, in which the main products are alkylphosphines 37 and alkylphosphine oxides 38 (Scheme 16). The introduction of elemental sulfur (solution in toluene) at the final stage of the process leads to the production of target phosphine sulfides 39 as a result of the reaction of phosphines 37 with sulfur.

Scheme
scheme 16

16.

The resulting mixture of alkylphosphine sulfides and phosphine oxides proved to be highly effective in the extraction of heavy metals without separation into individual components. The degree of extraction with respect to Ni, Co, Zn, and Pb varies in the range of 99.90–99.99%, and with respect to Ag from 99.56 to 99.59% [124].

7. CONCLUSIONS

Thus, the data presented in this review are indicative of the important new contribution to the development of original direct reactions of elemental phosphorus with electrophiles (in particular, with organic halides), occurring in the presence of superbasic and micellar catalysts and allowing easy formation of C–P bonds. These reactions open up new opportunities for further study of the synthetic and practical potential of such demanded (for example, as ligands [125132]) and now available organophosphorus compounds as triaryl (or getaryl)phosphines and phosphine oxides, as well as phosphinic and phosphonic acids.