Diaryl diselenides and benzisoselenazol-3(2 H )-ones as oxygen-transfer agents

Diaryl diselenides and related benzisoselenazol-3(2H)-ones are presented as the catalyst for hydogen peroxide and t -butylhydroperoxide oxidation of various types of organic substrates. Most of the reactions studied in our laboratory have a practical value. The aromatic aldehydes and ketones are oxidized to arenecarboxylic acids or converted to the phenols while cycloalkanones gave cycloalkanecarboxylic acids with ring contraction. From the azomethine compounds, depending on their structure, nitriles, parent carbonyl compounds, carboxylic acids or their esters are produced. Primary benzylamines gave nitriles, while the secondary are oxidized to nitrones. Alkylarenes are oxidized to the alkylaryl ketones and alkenes to epoxides. By oxidation of 1,4-dimethoxysubstituted and some polycondensed arenes the quinones are produced. The postulated mechanisms are discussed and the methods for synthesis of the title compounds are described.


Introduction
][3][4] Diaryl diselenides and benzisoselenazol-3(2H)-ones have been in the centre of our interest, because in many cases they can act more efficiently and selectively as oxygen-transfer catalysts then other selenium compounds and they are particularly useful for hydrogen peroxide and tbutyl hydroperoxide (TBHP) oxidation of variety organic compounds.Use of hydrogen peroxide and TBHP as the oxidants corresponds to modern trends in organic synthesis since both of these reagents are environmentally friendly, easily available and can be used in a large scale synthesis.Nevertheless, both of them are low or only moderately active toward most organic substrates, thus various activators or catalysts, among them selenium compounds, must be used.This brief review covers our contribution to the knowledge of the synthetic approach, properties and application of organoselenium compounds as the promotors of hydroperoxide oxidation of various groups of organic compounds.Our particular interest is focused on the title compoundsdiaryl diselenides and benzisolenazolon-3(2H)-ones because of their usefulness in organic synthesis.

Oxidation of carbonyl compounds
In late eightees Ludwik Syper of our laboratory discovered that diselenides: diphenyl (3), di(2nitrophenyl) (4) and di(2,4-dinitrophenyl) (5) effectively catalyzed Baeyer-Villiger oxidation of aromatic aldehydes and ketones 6 with hydrogen peroxide (Scheme 2).As a result a convenient and cheap method for transformation of aromatic aldehydes having polycondensed ring systems or electron-donating substituents, and polymethoxy derivatives of acetophenone into phenols 7 was elaborated. 8In a similar reaction α,β-unsaturated aldehydes 8 gave the vinyl compounds 9 accompanied by the products of their subsequent transformations. 9In all these reaction use of the areneseleninic acids, instead of their precursors diaryl diselenides, gave similar results.

Scheme 2
We postulated that the active oxygen donor should be areneperoxyseleninic acid, formed in situ,.Since peroxyseleninic acids remained unknown we showed that benzeneperoxyseleninic acid (10) could be obtained in 85 % yield through oxidation of diselenide 3 with excess of 30 % aq.H 2 O 2 at 0-5 o C. In a similar way 2-nitrobenzeneperoxyseleninic acid (11) and its 2,4-dinitro analogue (12) were obtained from corresponding diselenides 4 and 5 by their treatment with 90 % hydrogen peroxide (Scheme 3).Investigation of the reactivity of areneperoxyseleninic acids has shown that the use of the compound 11 as a stoichiometric reagent for Bayer-Villiger oxidation gave results similar to those when hydrogen peroxide in the presence of a catalytic amount of its precursor diselenide 4 was an oxidant. 10Most recently it has been found that aromatic aldehydes having in the ortho or para position an electron-donating substituent, such as a methoxy group, can be oxidized with hydrogen peroxide in the presence of selenium dioxide as catalyst to give a mixture of carboxylic acids and phenols or just phenols.In other cases oxidation of aliphatic, aromatic and heteroaromatic aldehydes led to carboxylic acids almost exclusively. 11When 2-phenylbenzisoselenazol-3(2H)one (13), named ebselen, was used as a catalyst and TBHP was an oxidant even electron-rich aromatic aldehydes were exclusively oxidized to carboxylic acids. 12he role of ebselen as an oxygen transfer agent remained ambiguous.By analogy to benzeneperoxyseleninic acid (10) having a hydroperoxide group, it seemed to be possible that hydroperoxyselenurane 15 would be an active intermediate.When ebselen (13) was treated with hydrogen peroxide an unstable crystalline compound, most probably 9a (R=H), was produced but could not be identified because upon isolation, it immediately fragmented to ebselen selenoxide, water and molecular oxygen.More stable hydroperoxyselenurane 16 (R=H) was obtained when ebselen analog 14 was treated with hydrogen peroxide.Another hydroperoxyselenurane 16 (R=t-Bu) was also obtained but it quickly lost oxygen, producing 17 (Scheme 5).Nevertheless, the identification of 16 and 17 provides strong support for the hypothesis that in the TBHP-ebselen system the active oxidant is the hydroperoxyselenurane 15. 12  (16)   [O]

Scheme 5
The results presented above explain the role of TBHP and ebselen in the chemoselective oxidation of aromatic aldehydes to arenecarboxylic acids.Consistent with the widely accepted mechanism for reaction of carbonyl compounds with peroxyacids, the first step is addition of hydroperoxyselenurane 15 to the carbonyl compound 18 to form a tetrahedral intermediate 19 (Scheme 6).The subsequent step of the Baeyer-Villiger rearrangement, which is ratedetermining, should be migration of the aryl group to the electrophilic oxygen atom of the peroxide bridge and simultaneously, cleavage of the O-O bond along with release of hydroxyselenurane R 1 OH.The phenol 20 should be the final product.Nevertheless, two bulky groups R 1 and R 2 in the vicinity of the electrophilic oxygen atom in the peroxy bridge hinder aryl migration and competitive hydride ion migration predominates.This pathway leads to ester 21, and finally to the acid 22.
It is known that cycloalkanones (23), oxidized with hydrogen peroxide in the presence of selenium dioxide, undergo Favorski-type rearrangement involving ring contraction and formation of cycloalkanecarboxylic acids (25). 13,14 lthough yields of the acids were low and did not exceed 37 %, this method was applied to the synthesis of some natural products. 15We reinvestigated this reaction using different selenium compounds as the potential catalysts.Among them poly(bis-9,10-anthracenylene) diselenide, named PADS (24), was found as the most effective one (Scheme 7). 16For example, cyclohexanone oxidized in the presence of 24 was converted into cyclopentanecarboxylic acid in 78 % yield (GC analysis) and isolated in 60 % yield.Since the cycloalkanones are cheap and easily available substrates this method is suitable for synthesis of acids 25, particularly those having five, six and seven-membered rings.Thus, H 2 O 2 -PADS can be regarded as a potential reagent for obtaining of bicyclo [4,3,0]

Scheme 7
The results of more detailed studies on the chemo-and stereoselectivity of this reaction support the mechanism, presented for oxidation of cyclohexanone in Scheme 8. Most probably it involves addition of two bulky arylselenium cations to both α-positions of the ketone, elimination of diaryl diselenide from the adduct 26, and finally the rearragement of intermediate 27 into the cyclopentanecarboxylic acid.

Oxidation of azomethine compounds
Oxidation of azomethine-containing compounds is more complex than oxidation of carbonyl compounds and some unexpected results have been found.The azomethine C=N group occurs in many organic molecules of fundamental importance.It is in many respects, intermediate between the C=C and C=O groups, and certain analogies and differences are observed.Both these groups have two electrons in binding π-orbitals and this accounts for some of their most characteristic properties.While carbon atoms present in these groups have no lone pair of electrons, the nitrogen and oxygen atoms possess it; this explains the distinctive properties of azomethine and carbonyl group.For these purposes oxidation of azomethine compounds drew our attention, even more than oxidation of carbonyl compounds, and various types of these compounds, such as aldimines, ketimines, oximes, hydrazones and azines, have been taken as the model substrates. 18t was known that hydrogen peroxide oxidation of N,N-dimethylhydrazones (28) gave nitriles (29) accompanied by substantial amounts of the parent aldehydes and aldazines but the reaction was limited to N,N-dimethylhydrazones derived from benzaldehyde and its analogues having electron-donating substituents in the aromatic ring. 19We found that the reaction proceeded with high chemoselectivity giving aromatic nitriles in fair yields when selenium dioxide, 2nitrobenzeneseleninic acid or ebselen was used as the catalyst. 20,21The reaction involved Noxidation of dimethylhydrazones 28 and Cope reaction of N-oxide 30 to nitrile 29.Although dimethylhydroxylamine (31) itself never was isolated as a product of this reaction, we identified products of its oxidation such as nitrosomethane (32), its dimer 33 and 1,3,5trihydroxyhexahydro-1,3,5-triazine (35) being a product of formaldoxime ( 34) trimerization (Scheme 9). 2  Most recently we have extended this method to the conversion of aromatic, heteroaromatic, and aliphatic N,N-dimethylhydrazones to nitriles, and we have found PADS (24) more versatile as the catalyst.The oxidant was 30 % hydrogen peroxide.The catalyst was used in a 5 % mol amount (related to the substrate) and the reaction carried out in tert-butanol in 55 o C for a period 2.5-8 h to give nitriles in good to excellent yield. 22he oxidation with the H 2 O 2 -PADS system can be explained according to the mechanism postulated earlier for other diaryl diselenides. 2,23In this case the compound 24, oxidized with hydrogen peroxide, underwent depolymerization to produce areneseleninic acid 36 which was subsequently converted into peroxyseleninic acid 37 (Scheme 10).The similar catalyst, poly(bis-1,2-phenylene) diselenide o-[SeC 6 H 4 Se] n named PPDS was aplied to hydrogen peroxide oxidation of aromatic aldazines, aldoximes and tosylhydrazones having an alkyl group, electron withdrawing or 3-methoxy, and 3-phenoxy substituents attached to arenecarboxylic acids.Exceptionally, oxidation of the azomethine compounds derived from 1-naphthaldehyde lead to (1-oxo-1,3-dihydroisobenzofuran-1-yl)acetic acid resulted from oxidative transformation of the formed in situ 1-naphthol. 24 In other work, we provided evidence that hydrogen peroxide and TBHP in the presence of a catalytical amount of ebselen might be used as a reagent for oxidation of azomethine groups in azines and oximes. 25Azines and oximes 38 derived from benzaldehydes oxidized with hydrogen peroxide in methanol gave parent aldehydes 39, their dimethylacetals 40 and methyl benzoates 41.The oximes gave esters 41 as the main products.The extended experiments carried out on the azine 42 shows that the result of the reaction depended on the oxidant used and on the reaction conditions, particularly on the solvent since methanol could participate in the reaction.The results are shown in Scheme 11.Ketazines were oxidized with H 2 O 2 -ebselen system exclusively to parent ketones, while oxidation of azine 43 derived from 2-acetylpyridine gave the mixture of expected ketone 44 and condensed triazole 45.The same compounds were produced when cerium(IV) ammonium nitrate (CAN) was an oxidant.Since CAN is well known one-electron oxidant generating free radical, it seems possible that a free radical mechanism is involved in the hydroperoxide oxidation of azines catalyzed by ebselen.

Oxidation of benzylamines
Oxidation of amines is an useful way for synthesis of variety nitrogen-containing organic compounds.The reaction leads to different products depending on the nature of the oxidizing agents and a type of amine. 26In our work selenium compounds were used as the catalysts for hydroperoxide oxidation of primary and secondary benzylamines.Four primary benzylamines 46a-d and three N-substituted benzylamines: N-methylbenzylamine (46e), dibenzylamine (46f) 1,2,3,4-tetrahydroisoquinoline (46g), were taken as the model substrates.Among different selenium compounds tested as catalysts PPDS and particularly ebselen had appreciable activity.They were used in 5 % mol while the stoichiometric oxidants were 30% hydrogen peroxide and 80 % TBHP.Although the results, presesented in Scheme 12, varied depending on the substrate, oxidant and catalyst used they provided evidence that the most effective and selective catalyst was ebselen and from primary amines 46a-d corresponding nitriles 47 can be obtained in satisfactory to high preparative yields 66-82 % thus the elaborated method has a practical value. 27 The possible mechanism of the oxidation of primary amines with hydroperoxides in the presence of ebselen, shown in Scheme 13, involves formation of hydroperoxyselenurane 15 as an active intermediate.
The first step of the reaction leads to the oxime 50 via highly reactive hydroxylamine 52 and nitroso compound 53, analogously as it has been postulated when benzylamines were oxidized with hydrogen peroxide in the presence of sodium wolframate, 28 peroxotungstophosphate 29 or methyl trioxorhenium. 30According to known reactions the oximes 50 can be subsequently dehydrated to the nitriles 47 or oxidized to arenecarboxylic acids 51 via aldehyde 49 as an itermediate.In some cases condensation of 49 with starting amine 46 to aldimine 41 takes place.N-Methylbenzylamine (46e) and dibenzylamine (46f) gave nitrones 54 and 55 as the major products similarly as it was observed earlier when secondary amines were oxidized with H 2 O 2 -SeO 2 . 31,32Most probably the reaction proceeds via dehydrogenation of the amine and subsequent N-oxidation of intermediated imine.The main reaction is accompanied by cleavage of the C-N bond resulting in formation of the nitrile 47, aldehyde 49, benzaldoxime 50 and carboxylic acid 51 in a similar manner to that described earlier for primary benzylamines.Tetrahydroisoquinoline 46g was dehydrogenated to isoquinoline 56 accompanied by N-oxide 57, in contrast to hydroxyalkyflavine-catalyzed oxidation with hydrogen peroxide where 3,4dihydroisoquinoline N-oxide is a product. 33To our knowledge it is the first example of aromatization of tetrahydroisoquinoline by hydroperoxides.

Oxidation of alkylarenes and other substituted arenes
Toluene (57a) and five other alkylarenes 57b-f were oxidized with TBHP in the presence of ebselen to benzaldehyde (58a) and ketones (58b-f) respectively (Scheme 15).Although the methyl group in toluene remained resistant toward oxidation and benzaldehyde was formed only in low yield, the benzylic methylene group in alkylarenes 57b-f was oxidized to a carbonyl group more efficiently.The experiments with ethylbenzene (5fb) have shown that although during long period conversion of the substrate increased, side reaction took place and selectivity of the main reaction decreased.A similar effect was observed when the oxidant was used in a large excess.No reaction was observed when catalysts such as titanosilicate TS-1, selenium(IV) oxide or ebselen were used for hydrogen peroxide oxidation of toluene. 25

Scheme 15
Anthracene 59 was oxidized with TBHP-SeO 2 to anthraquinone 60 in almost quantitative yield.In a similar reaction when TBHP-ebselen was an oxidant the quinone 52 was produced only in 41 % and the rest of substrate 59 remained unreacted.Oxidation of 59 with H 2 O 2 -ebselen gave quinone 60 only in 23 % yield while the major product was a dimer 61 formed in 50 % yield.Other arenes such as 2-methylnaphthalene, phenanthrene and acenaphthylene were oxidized to corresponding quinones in the yields below 21 %.Electron-donating substituents such as hydroxy or methoxy group made the aromatic ring more susceptible towards oxidation with hydroperoxides.For example, 2-methylnaphthalene treated with TBHP-ebselen was converted into the products only in 15 % and suspected menadione (63) was formed only in a trace amount ( ca. 3 %) while 2-methyl-1,4-dimethoxynaphthalene (52), oxidized with the same reagent, gave 63 in 60 % yield (Scheme 16). 34The result was similar to that one when the reagent was a one-electron oxidant such as Ce(IV) or Ag(II) cation. 35It suggests that the reaction proceeds via radical or cation-radical intermediates.

Epoxidation of alkenes
Alkenes can be epoxidized with any of a number of peroxyacids.Nevertheless, the method is cheaper and more covenient when, instead of 3-chloroperoxybenzoic or other peroxycarboxylic acid used in a stoichiomeric amount, an oxidant is generated in situ and recyclized with peroxyseleninic acid.It has been found that, styrene (64), resistant toward epoxidation, can be converted into epoxide 65 in 57.5 % preparative yield on treatment with hydrogen peroxide in the presence of anhydrous magnesium sulfate and bis(2,4-dinitrophenyl) diselenide ( 5) as a catalyst (other organoselenium compounds and selenium(IV) oxide were less active).The unreacted styrene can be recovered in 35.5 % yield and reused.Other differently substituted vinylbenzenes were epoxidized in the same way. 36esult of the oxidation of alkenes strongly depends on the catalyst used.We found that cyclooctene (66) oxidized with TBHP-ebselen gave the epoxide 67 accompanied with trace of 3hydroxycyclooctene (68) being a product of α-hydroxylation. 37On the contrary, the same reaction catalyzed by selenium(IV) oxide afforded 68 as the major product (Scheme 17).

Oxidation of organosulfur compounds
Near two decades ago it was found that ebselen could act against oxidative stress in a similar way as common selenoenzyme -glutathione peroxidase (GSH-Px). 7,39,40Some years later it was found that other 2-substituted bezisoselenazol-3(2H)-ones, cyclic selenenamides, and bis(2carbamoyl)phenyl diselenides are able to deactivate oxygen species present in the living cell such as peroxides, hydroperoxides, hydroxyl radical and superoxide anion. 6The mode of the biological action of ebselen and its analogues has been postulated to be similar to that observed for GSH-Px and results in dehydrogenation of thiols to disulfides while hydrogen peroxide is reduced to water. 7,41Later it was found that ebselen did not promote hydrogen peroxide oxidation of thiols such as N-acetylcysteine, butanethiol and octanethiol in nonenzymic conditions. 42Nevertheless, the selenenamide derived from camphor effectively catalyzed oxidation of phenylmethanethiol to the disulfide. 43The postulated mechanism of the enzyme-like action of ebselen is presented in Scheme 18. 7,41,43,44 When concentration of hydroperoxide is high, ebselen ( 13) is oxidized to selenoxide 69 which reacting with one molecule of the thiol gives selenosulfide 70.The intermediate 70 and the second molecule of thiol produce disulfide while the selenenic acid 71 is converted back to the ebselen.In the biological systems, where concentration of hydroperoxide is low, ebselen and thiol give the selenosulfide 72 which disproportionates to the disulfide and diselenide 73 is subsequently oxidized to selenenic anhydride 74 and finally to ebselen (13).A broad spectrum of selenium compounds, among them diaryl diselenides and benzisoselenazol-3(2H)-ones has been tested as the catalysts for hydrogen peroxide and tertbutylhydroperoxide oxidation of sulfides (75) into sulfoxides (76).When ebselen (13) or its open-chain analogue bis(2-carbamoylphenyl)phenyl diselenide (77) was employed as a catalyst the reaction was highly chemoselective and sulfoxides (76) were formed exclusively or were accompanied only with minute amounts of sulfones (Scheme 19). 21,45SSN 1551-7004 Page 242 MeOH, 20 o C 75 76 78-99% cat.= ebselen (13), , 77 78

Synthesis of diaryl diselenides
In the late eightees we developed a convenient method for practical synthesis of dialkyl and diaryl diselenides (82) by selenenylation of corresponding halides (81), lactones, oxiranes and tosylates with dilithium diselenide in aprotic medium (THF or HMPT).Dilithium diselenide was generated in situ from elemental lithium and selenium in the presence of diphenylacetylene, or later of 4,4 ' -di(t-butyl)biphenyl, as a catalyst.The method has a general value, competitive formation of selenides is avoided, yields of diselenides are high, also PPDS and PADS (24) were obtained in this manner. 16,24,47,48The synthesis of diselenide 24 from 9,10-dibromoanthracene (83) presented in Scheme 20 is an example.

Synthesis of ebselen and other benzisoselenazol-3(2H)-ones
There are alternative ways to synthesize ebselen (13) and other 2-substituted benzisoselenazol-3(2H)-ones (90).The first of them starts from benzanilide (84) which is selenenylated to intermediate 85 and subsequently cyclized to ebselen (13) by treatment with copper(I) bromide. 49 more general method for the synthesis of ebselen and other 2-substituted benzisoselenazol-3(2H)-ones (90) as well as for synthesis of 2-carbamoylphenyl diselenides (83), presented in Scheme 21, was elaborated in our laboratory as a modification of the old procedure reported by Lesser and Weiss. 48,50,51 Te first two steps of the synthesis involve diazotization of anthranilic acid (86) and repleacement of the diazonium group by the diselenide group by treatment of a diazonium salt with disodium diselenide, generated in situ from elemental selenium and hydrazine hydrate in the presence of sodium hydroxide.The formed 2,2'-diselenobisbenzoic acid (87), treated with an excess of thionyl chloride in the presence of catalytical amounts of DMF, gives 2-(chloroseleno)benzoyl chloride (89), or when thionyl chloride is used a stoichiometric amount 2,2'-diselenobis benzoyl chloride (88) is produced.The dichloride 87 is a reagent for tandem acylation-selenylation of primary amino groups giving a variety of 2-substituted benzisoselenazol-3(2H)-ones (90), among them ebselen (13)

Synthesis organoselenium compounds immobilized on solid supports
3][54][55] The covalent binding of catalyst with polymeric support offers a number of advantages over traditional solution-phase chemistry.In an ideal case, the supported catalysts can be recovered from reaction mixtures by simple filtration, they do not contaminate the product solution, they may be recycled, and they can help increase selectivity.Moreover, the immobilization of homogenous catalysts potentially allows their adaptation to continuous flow type processes.The use of polymeric supports in organic synthesis has become common practice.7][58] Athough a variety of selenium compounds have been used during the last three decades as stoichiometric oxidants or oxygen transfer catalysts 59 to our knowledge only one of them (polyester resin containing oxyselenium groups) has been used as an oxidant for conversion of benzyl, allyl and active alkyl bromides to aldehydes 60 and a second one (polystyrene-supported benzeneseleninic acid) as a catalyst for TBHP oxidation of benzyl and allyl alcohols to aldehydes and phenols to quinones. 61n our work 2-(chloroseleno)benzoyl chloride (89) was used as a key substrate for synthesis of silica-supported selenenamide MONICAT A (95) and benzisoselenazol-3(2H)-one HALICAT (97) (Scheme 22).

Conclusions
In conclusion we wish to say that diaryl diselenides, among them polyaryldiselenides, and 2substituted benzisoselenazol-3(2H)-ones, particularly ebselen, are effective catalysts for hydrogen peroxide and tert-butyl hydroperoxide oxidation of organic compounds such as sulfides, amines, azomethine compounds, aldehydes, cycloalkanones, alkenes and alkylarenes.Most of these reactions have a synthetic value because of their selectivity and high yields of the products.Diselenides act via peroxyseleninic acids as active intermediates while in the case of benzisoselenazol-3(2H)-ones selenahydroperoxides (hydroperoxyselenuranes) are active oxygen donors.These intermediates are involved in an ionic mechanism although tert-butyl hydroperoxide oxidation of alkylarenes and some other reactions known as one-electron processes suggests that free-radical mechanisms are also possible.
A new generation of polymer and silica-supported organoselenium compounds has been designed as recyclizable oxygen-transfer catalysts and synthesized.Preliminary results show their appreciable catalytical activity.

Issue in Honor of Prof. Mieczysław Mąkosza ARKIVOC 2004 (iii) 226-248 ISSN 1551-7004 Page 236
© ARKAT USA, Inc . The Se-N bond in the heterocyclic ring of 90 can easily be cleavaged by reducing agents such as hydrazine or triphenylphosphine and 2-carbamoylphenyl diselenides (91), among them close analogues of ebselen (77), are formed.The same diselenides 91 can be obtained directly in the reaction of chloride 88 with primary amines.Their oxidation lead to benzisoselenazol-3(2H)-ones (90) as shown in Scheme 1.