Chalcogenisation of unsaturated organohalogen compounds by elemental chalcogens and their metal derivatives

The presented review addresses the prospects for application of organochalcogen compounds in various fields of technology, medicine, agriculture and organic synthesis. The unsaturated chalcogenisation products appear to be of much greater interest compared to their saturated analogues, especially in organic synthesis. This study observes halogen derivatives of ethene, propene, propyne and butene subjected to chalcogenisation as unsaturated substrates. The indicated reagents are related either to large-tonnage products of industrial organochlorine synthesis or waste products of organochlorine production with their disposal presenting an important environmental task. According to analysed publications, chalcogenisation processes are based on the application of elemental chalcogens (sulphur, selenium, and tellurium) or their available metal derivatives (Na2S, etc.). In the reactions of chalcogens with unsaturated halogen derivatives, the elements both in an accessible form and in a free state are subjected to reductive activation resulting in the formation of anionic nucleophilic reagents. Complex metal hydrides, chalcogenide anions and rongalite are exampled for application in terms of reducing agents. The review emphasises the prospects of basic reduction systems based on hydrazine hydrate in activation processes. Special aspects in the introduction of caustic alkalis and an monoethanolamine organic amine as bases in these systems are described. For the considered chalcogenisation processes, conditions are specified providing the most optimal yield of certain products. In some particular cases, the stereochemistry of the obtained compounds is presented considering the formation conditions for the stereoisomers of a certain configuration. For a number of the obtained compounds, the prospects of practical application are provided. In general, the current review is intended for specialists working in the field of organic synthesis and application of organochalcogen compounds.


INTRODUCTION
The scope of organic chalcogen compounds can be seen to be constantly expanding. Although sulphur and its organic derivatives have been applied by humans since ancient times, more recent uses include unique medicines created on their basis, including antibiotics, anticancer drugs, drugs for human immunodeficiency, as well as protector drugs against ionising radiation. The technological application of organosulphur compounds is associated with the development of rubber vulcanisation agents, dyes, photographic materials, complexing agents, flotation reagents and extractants, polymerisation process regulators, as well as with the creation of corrosion protection agents and new types of polymeric materials, in particular for new generation chemical current sources and others directions [1]. The use of selenium and tellurium compounds was clearly indicated only in the second half of the last century and was associated, first of all, with their technical application in obtaining ligands for complex formation, organic semiconductors and electrically conductive polymers [2,3]. However, in recent decades, numerous data have been obtained on the biological role of selenium and tellurium organic derivatives [4][5][6]. A detailed study for the structure of organochalcogen compounds (OCC) served as an impetus for the development of many theoretical concepts in organic chemistry [7][8][9]]. An important direction in the application of OCCs involves their use as precursors in contemporary organic synthesis [10][11][12]. By virtue of these precursors, new nanomaterials, analogues of natural compounds are obtained along with typically unstable, highly reactive compounds. Thus, the synthesis and use in modifying drugs of this class of compounds presents a specific task in modern organic chemistry.
The development and improvement of methods for OCC synthesis is constantly stimulated by the continuous expansion in the scope of their application. The most universal method of introducing a chalcogenyl substituent into the structure of an organic molecule involves the nucleophilic substitution of halogen. Chalcogen-containing anions manifest themselves as nucleophiles in these reactions (Y 2-, Y n 2-, RY with Y = S, Se, Te and R denoting an organic radical). The specified anionic forms are included in some commercial chemicals, mainly sodium sulphide, applied in the form of nanohydrate (Na 2 S · 9Н 2 О). The application of the considered chalcogenide reagents is directly related to the possibility of their generation from accessible, storage-stable elemental chalcogens. In order introduce them into a nucleophilic substitution reaction, simple substances are converted into an anionic form by chemical activation carried out due to the reductive splitting of Y-Y bonds in elemental ХИМИЧЕСКИЕ НАУКИ / CHEMICAL SCIENCES chalcogens. Currently, although a wide range of reducing systems is available for application, their use is typically associated with the involvement of explosive reagents (organic derivatives of metals, hydrides and complex hydrides), toxic and combustible solvents, compounds of heavy metals in lower oxidation states and other factors reducing the preparative value of the developed methods. The generation of chalcogen-containing nucleophiles is effectively carried out using the hydrazine hydrate-alkali basic reduction system [13] and can be represented by the following scheme: 2nY + N 2 H 4 ·H 2 O + 4KOH → 2K 2 Y n + N 2 + 5H 2 O n = [1][2][3][4] In this reaction, the value of n is determined by the Y: KOH ratio.
Here, the resulting chalcogenide and polychalcogenide anions are reacting in the synthesis directly in the hydrazine hydrate solution without being isolated in an individual state. Using this activation method, a large number of saturated OCCs have been synthesised, including chalcogen-containing polymers and heterocyclic systems, some of whose unique properties have been described (see, for example, [14]).
From a practical point of view, OCCs with multiple structural bonds are of particular interest due to their significant expansion of the synthetic potential and practical significance of the studied compounds. In order to synthesise them, reactions of chalcogen-containing nucleophiles with unsaturated organohalogen compounds can be applied. In the substrates under consideration, halogen atoms can be attached to carbon atoms in both the sp 3 hybridisation state, characterised by nucleophilic reactions to proceed fairly easy, and the sp 2 hybridisation state. In the latter case, nucleophilic substitution may be hindered [15]. When the conditions change, halogen vinyl fragments in the resulting products enter into further chemical transformations, ensuring cascade reactions allowing valuable OCCs to be obtained.
The present review provides the main research results with the objects of study consisting in organohalogen derivatives of ethylene, propylene, pro-pyne and butylene. In addition, such derivatives are to be highlighted as relating either to large-tonnage organochlorine products or substances generated from organochlorine production wastes.
Halcogenisation of ethylen chlorinated derivatives. Thiylation of vinyl halides or their derivatives, having a halogen atom attached to the sp 2 -hybridised carbon atom, is carried out either using photochemical and thermal assistance [16,17] or by means of aprotic highly polar solvents [17]. In nucleophilic reactions, only di-and polyhalogen derivatives of ethene were studied.
The work [18] describes the photochemical reaction of atomic sulphur with 1,2-dichloroethene (1b), resulting in a low yielded dichlorothiirane. However, when the Z-1b compound and sodium sulphide nonahydrate reacts in acetonitrile with the presence of 15-crown-5 interphase transfer catalyst (0.4 equiv.), a mixture of cyclic unsaturated sulphur-containing compoundsunsaturated thiacrown esters (2a-h)was obtained at room temperature in 45 h with a total yield of 40 % [19] (Scheme 1).
The dimensions of the cavities for the (2d-h) joints consistently increase from 1.76 to 5.36 Å.
As an example, the complexation of crown ether selenium with silver ions is presented in [20].
According to the work [21], in the product mixture, the absence of compounds with a geminal arrangement of sulphur atoms relative to the double bond carbons is accounted for mechanism involving the isomerisation of the geminal intermediate into a vicinal product. The considered scheme of the mechanism illustrates the formation of E-configura-ted cycles. However, according to the authors, the initial act of the process is presented by direct nucleophilic substitution of chlorine atoms in vinylidene chloride (1a), leading to contradiction in the inertness notion of halogen atoms attached to a double bond.

ХИМИЧЕСКИЕ НАУКИ / CHEMICAL SCIENCES
In the case of applying (1a) dichloride and (1b) compound, the molecular weight of the resulting (6) oligomeric product comprises 2260-6120 and 1750-2000 units, respectively. Based on the elemental analysis data, the following structure was assigned to the oligomer: For oligomers obtained using the (1a) compound, the value of x and y was equal to 30-88 and 6-15, respectively. In the case of using the (1b) dichloride, the above values were 22-24 and 1-6, respectively. According to IR spectroscopy and taking the results of the E-and Z-polyvinyl sulphide (PVS) studies into account, the vinylene units in the oligomer chain are exclusively Z-configurated [23]. In current study, PVSs were obtained by polycondensation of E-and Z-1b dichlorides with E-and Z-isomers of hardly accessible sodium 1,2-ethendithiolate (Scheme 5).
The electrical conductivity of obtained PVS is also examined by the authors of work [23], and E, E-PVSs are demonstrated to possess higher conductivity in comparison with Z,Z-polymer with the lgσ value ranging approximately from -4,5 to -6 S/cm).

E-1b
For 1,4-Ditiin (2a) in the reaction represented by Scheme 4, the substantially higher yield is observed in comparison with the data, obtained by the authors of works [19,21], and its separation from other reaction products proceeds quite easily. A slightly higher yield of 47 % was established in the study for the reaction of Z-1b dichloride with Z-1,2-sodium ethanedithiolate in dry DMSO [24] (Scheme 6).

Z-1b
The (2a) compound involves an important ob ject for structural studies, making the development of preparatively available methods for its preparation to be an important synthetic task.
After the separation of (2a) and (6) products, the formation of (7) polysulphide anions in the remaining aqueous hydrazine solution was confirmed by their alkylation by 1-bromopropane with the isolation and identification of dipropyl polysulphides.
The formation of the monotypic products presented in Scheme 4 under application of vinylidene chloride (1a) or both isomers of 1,2-dichloroethene (1b), as well as the participation of hydrazine in the formation of (6) oligomers, allowed the authors of [22] to propose a scheme for product formation having the primary key act consisting in the dehydrochlorination of (1) compounds with the formation of highly reactive chloroacetylene (8) (Scheme 7). The presence of excess hydrazine in the reaction medium prevents the formation of macrocyclic structures presented in [19,21] and, with the participation of (8) reagent and S 2anions, results in the formation of nitrogen containing oligomers.
During the reaction of selenium and tellurium with (1a) vinylidene chloride under conditions similar to sulphur application, no analogues of 1,4-dithiine were isolated [25]. In the case of tellurium, the presence of diethyl telluride (9) is convincingly proved by the authors of this work with the formation accounted for the reductive hydrogenation of multiple bonds with the participation of hydrazine and telluride anions (Scheme 8).

Scheme 8 Схема 8
Cl Cl Tetrachloroethylene, a derivative of ethylene having four chlorine atoms, is the subject of largescale industrial production. Having relatively stable characteristics, this compound is widely applied in terms of a solvent for dry cleaning of textile materials and degreasing metal surfaces. The works [26,27] describe only the interaction of tetrachlorethylene with sodium polysulphide Na 2 S x formed upon activation of sulphur with sodium sulphide Na 2 S · H 2 O (Scheme 9) [27].
Although the reaction of tetrachlorethylene with Na 2 S 4 and Na 2 S 5 in aqueous alkaline DMF also results in polymer products, the Na 2 S 3 trisulphide in this reaction forms an individual compound of dimethylthioacetamide (17) with the participation of dimethylamine resulting from the alkaline hydrolysis of DMF [28] (Scheme 11).
The simplest chlorine derivative of propene, 3-chloropropene (allyl chloride), present a multi-ton product of industrial organochlorine synthesis. The high mobility of halogen in the allyl position ensures the production of a wide range of allyl chalcogenides [see, for example, 29,30] studied in the chalcogen Claisen rearrangement of allylaryl (heteroaryl) chalcogenides [31,32], identification the chalcogen atom characteristics affecting the allyl rearrangement in the series of allylphenylchalcogenides [33], as well as in the synthesis of various heterocyclic structures [34].

Halogen derivatives of propene and propyne
By application of AllX (X = Cl, Br, I) and elemental chalcogens in the hydrazine hydrate/KOH system, all three diallyl chalcogenides were obtained [35]. As expected, allyl iodide is the most reactional, followed by bromide and chloride. When AllCl is introduced in the synthesis of diallyl selenide, the by-product of allyl propyl selenide is formed due to the participation of hydrazine and atmospheric oxygen, in other words, hydrogenation of one double bond is observed [35,36]. In the presence of atmospheric oxygen in an alkaline medium, hydrazine forms a highly reactive diimide hydrogenating the intermediate-generated potassium allyl selenolate contained in the aqueous hydrazine phase (Scheme 12).

Scheme 12
Схема 12 Х = Cl, Br, I In parallel, the formed diallyl selenide and the allyl propyl selenide obtained by hydrogenation transits into the organic phase, avoiding further reduction. In the case of using allyl bromide, the reaction of diallyl selenide formation proceeds quickly and potassium allyl selenate transits from the aqueous hydrazine phase without hydrogenation [35].

Scheme 13 Схема 13
Cl S The diselenide anion obtained both in the hydrazine hydrate-KOH system and in the hydrazine hydrate-monoethanolamine system forms a monoselenide derivative (21) in a reaction with dichloride (18a) (72-70 % yield). However, in the latter case, diselenide (22)  No tellurium-containing products are formed by tellurium in the K 2 Te or K 2 Te 2 with dichloride forms (18a). The products of the reaction turned out to be propadiene (23) (78 %) and regenerated elemental tellurium [40] (Scheme 16).

23
The proposed reaction can serve as a convenient preparative method for producing propadiene, which is widely used in various organic synthesis processes [41].
1,3-Dichloropropene (18b) presents an isomer of dichloride (18a) with the chlorine atoms located in the 1,3-positions. Commercial product (18b) consists of E-and Z-isomers mixture with a proportion of 1.1 : 1. According to the work [42], in a reaction with (sulphur-hydrazine hydrate-base) systems, with either KOH or monoethanolamine is used as the base, dichloropropene (18b) behaves similarly to the 18a compound. In the first system, monosulphide (24) (72 % yield, a mixture of three geometric isomers) is formed during the generation of S 2 2anions (Scheme 17). When monoethanolamine is used as the base (also generating anions S 2 2- [13]), the corresponding disulphide (25)

S Cl
When applying selenium, a complex mixture of products is formed in both systems with the possibility of isolating the corresponding selenide in a yield of up to 15 % [42].
The reaction of dichloropropene (18b) with tellurium in the hydrazine hydrate-KOH system appears to be completely different from the corresponding reaction of the isomer (18a) provided in Scheme 16. As a result of the interaction of dichloride (18b) with tellurium activated to K 2 Te, diallyl telluride (26)  The method of obtaining diallyltelluride (26), presented in Scheme 18, turns to be preparatively more effective than the synthesis of this compound by allyl iodide.
Among the halogen derivatives of propyne, propargyl bromide (27) (29) is formed with a 61-70 % yield in the reaction of two equivalents of propargyl bromide (27) with sodium selenide (Scheme 20), obtained during the reduction of selenium by NaBH 4 in ethanol [44].

Scheme 20
Схема 20 With the substitution of propargyl bromides, the corresponding substituted propargyl sulphide and selenide were obtained, some of which were successfully applied in the synthesis of heterocyclic compounds [45].
No dipropargyl telluride was obtained due to its extreme instability.
Chlorinated derivatives of butene 1,3-Dichlorobut-2-ene (30) was studied in the chalcogenisation reactions as a C 4 unsaturated halogenated hydrocarbon. Dichloride (30) presents a waste product of chloroprene production. Using the (sulphur-hydrazine hydrate-KOH) system (sulphur is activated to anions S 2 2-), the corresponding disulphide (31) was obtained from dichlorobutene (30) in 84 % yield and successfully used in obtaining heterocyclic compounds: 5-methyl-1,2-dithiol-3-thione (32) and 3-thiophentiol (33) (Scheme 21) [46]. So far from this example, despite the fact that the dichloride (30) is a homolog of 1,3-dichloropropene (18b), its behaviour in chalcogenisation reactions appears to be significantly different. to the availability of precursors, the ease of implementation of the proposed methods and the high practical significance of the obtained products, the considered methods are a powerful tool in the development of organochalcogen synthesis and the expanded scope of OCC applications. For preparative purposes, the prospects and advanta-ges of using basic reduction systems based on hydrazine hydrate for chalcogen activation are presented in a particularly informative form. The authors hope the presented results will attract the attention of researchers involved in the synthesis, structural study and practical application of OCC.