Highly efficient regioselective synthesis of organotellurium compounds based on the reactions of tellurium tetrachloride with 1-alkenes

Efficient regioselective syntheses of trichloro-(2-chloroalkyl)- λ 4 -tellanes, trichloro-(2-alkoxyalkyl)- λ 4 -tellanes, bis-(2-alkoxyalkyl)ditellanes and dichlorobis-(2-chloroalkyl)- λ 4 -tellanes in quantitative yields were developed based on tellurium tetrachloride and 1-alkenes (1-hexene and 1-heptene). Favorable conditions for selective preparation of both mono-and bis-adducts of tellurium tetrachloride with 1-alkenes were established. Methoxytelluration was accomplished by the reaction of tellurium tetrachloride with 1-alkenes in CH 2 Cl 2 /MeOH at room temperature. Ethoxytelluration was carried out in CH 2 Cl 2 /EtOH at reflux. Trichloro-(2-alkoxyalkyl)- λ 4 -tellanes were also obtained by nucleophilic substitution of chlorine atom in trichloro-(2-chloroalkyl)- λ 4 -tellanes with alcohols under unusually mild conditions.


Introduction
Organotellurium compounds are used in modern organic synthesis as intermediates and synthons. 1,24][5][6][7][8][9] Bioincorporation of telluromethionine provides a new approach to add heavy atoms to selected sites in proteins. 7][5][6][7] The distinguishing property of tellurium reagents to react with high regio-and stereo-selectivity finds increasing application in organic synthesis. 1,2Adducts of tellurium tetrachloride with acetylenes were recognized as important precursors and synthons for organic synthesis and applied in many approaches for the preparation of various functionalized alkenes in a highly regio-and stereo-selective manner. 1,27][18][19][20][21][22][23][24][25] Recently we have found that methoxytelluration can be accomplished by the reaction of tellurium tetrabromide with 1-hexene in methanol. 17The reaction proceeded with high regioselectivity to afford Markovnikov's products, tribromo-(2-methoxyhexyl)-λ 4 -tellane.This result showed for the first time that methanol can be used as solvent in reactions of tellurium tetrabromide without methanolysis of the Te-Br bond and demonstrated an example of an efficient methoxytelluration reaction.Similar methoxytelluration reactions were accomplished with styrene and 1-octene. 18,19Tellurium tetrachloride cannot be used under similar conditions due to methanolysis of the Te-Cl bond.However, we found that methoxytelluration reaction of tellurium tetrachloride can be carried out in a mixture of chloroform and methanol. 19,20he present paper is devoted to regioselective synthesis of organotellurium compounds based on chlorotelluration and alkoxytelluration reactions of tellurium tetrachloride with 1-hexene and 1-heptene.Examples of nucleophilic substitution of chlorine atom in trichloro-(2-chloroalkyl)-λ 4 -tellanes with alcohols under unusually mild conditions are discussed.Highly efficient syntheses of 2-haloalkyl, 2-methoxyalkyl and 2ethoxyalkyl tellanes and ditellanes in quantitative yields have been accomplished based on these reactions.Prior to our studies, the addition products of tellurium tetrachloride to 1-hexene or 1-heptene were not described in the literature.
We have found that best solvent for chemo-and regio-selective synthesis of monoadducts by addition of tellurium tetrachloride to alkenes 1 and 2 is carbon tetrachloride.The reactions of tellurium tetrachloride with 1-alkenes 1 and 2 proceed in carbon tetrachloride at room temperature with an equimolar ratio of reagents affording trichloro-(2-chloroalkyl)-λ 4 -tellanes 3 and 4 in quantitative yield (Scheme 1).The addition can also be accomplished in dichloromethane or chloroform at room temperature.However, the highest purity of monoadducts 3 and 4 was observed when the reactions were carried out in carbon tetrachloride.
The system CH2Cl2-methanol (5-3 : 1, equimolar ratio of reagents) is found to be favorable for accomplishing the methoxytelluration reaction of tellurium tetrachloride with 1-alkenes 1 and 2. The methoxytelluration reaction in the system CH2Cl2-methanol proceeds at room temperature affording trichloro-(2-methoxyalkyl)-λ 4 -tellanes 5 and 6 in quantitative yield and with higher purity than in the chloroformmethanol mixture. 19,20e found that tellanes 5 and 6 can be also obtained in quantitative yield by nucleophilic substitution of chlorine atom in tellanes 3 and 4 with alcohols under unusually mild conditions.The reaction proceeded smoothly at room temperature in the systems CH2Cl2-methanol or chloroform-methanol giving tellanes 5 and 6 in quantitative yields.The nucleophilic substitution reaction proceeded more slowly in methanol or in carbon tetrachloride-methanol mixture then in the system CH2Cl2-methanol.In order to accomplish nucleophilic substitution reaction it is sufficient to add methanol to a solution of tellanes 3 or 4 in dichloromethane or chloroform and allowed to stand overnight.Methanol can be added to a solution of tellanes 3 or 4 in carbon tetrachloride after accomplishing the addition reaction; however, warming the reaction mixture at 50-60 o C is necessary in order to complete nucleophilic substitution reaction and to obtain tellanes 5 and 6 in quantitative yield.This case can be regarded as one-pot synthesis of tellanes 5 and 6 by addition of tellurium tetrachloride to alkenes 1 and 2 followed by nucleophilic substitution of in situ formed tellanes 3 or 4. It is noteworthy that carrying out the reaction under these conditions avoids the methanolysis of the Te-Cl bond.
The ease of the reaction of nucleophilic substitution of chlorine is apparently due to the high electronwithdrawing effect of the trichlorotellanyl group.This opens up new possibilities for the functionalization of organotellurium compounds obtained by the addition of tellurium tetrachloride to alkenes.Noteworthy, under similar conditions the chlorine atoms in the bis-adducts dichlorobis-(2-chloroalkyl)-λ 4 -tellanes 7 and 8 were not displaced by methanol.This may indicate that electron-withdrawing effect of the trichlorotellanyl group is considerably higher than that of the dichloro(organyl)tellanyl group.

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© ARKAT USA, Inc It was found that the optimal conditions for preparation of bis-adducts 7 and 8 are reflux of tellurium tetrachloride with a 2.5-3-fold excess of alkenes in benzene (Scheme 2).The reaction proceeded chemo-and regioselectively to afford compounds 7 and 8 in quantitative yield.The ethoxytelluration reaction of tellurium tetrachloride with 1-alkenes 1 and 2 can be accomplished in the systems CH2Cl2/EtOH, CHCl3/EtOH or CCl4/EtOH (6-3 : 1).However, the reaction was slow at room temperature and usually required heating under reflux for 6 h in order to complete the ethoxytelluration and to obtain trichloro-(2-ethoxyalkyl)-λ 4 -tellanes 9 and 10 in quantitative yields (Scheme 3).The purity of the products varied from 94 to 96% (NMR data).The ethoxytelluration products with higher purity (>96%) were obtained by one-pot procedure by addition of tellurium tetrachloride to alkenes 1 and 2 followed by nucleophilic substitution of formed in situ tellanes 3 or 4. Similarly to the ethoxytelluration reaction, the onepot synthesis of tellanes 9 and 10 can be realized in the systems CH2Cl2/EtOH, CHCl3/EtOH or CCl4/EtOH and requires heating in order to complete the process (Scheme 3).For example, heating at reflux (40 o C) for 6 h in the system CH2Cl2/EtOH is sufficient for completion of the reaction.It is possible to add ethanol to the reaction mixture after accomplishing the addition of tellurium tetrachloride to 1-alkenes 1 and 2 in carbon tetrachloride and after 6 h reflux tellanes 9 and 10 can be isolated in quantitative yield.Reduction of compounds 5, 6, 9 and 10 with sodium metabisulfite in a two-phase system benzene-water at room temperature leads to bis-(2-alkoxyalkyl)ditellanes 11-14 in 72-80% yields (Scheme 4).It was found that the system NaBH4/water/THF is more efficient and selective for the preparation of ditellanes 11-14. 26When this system was applied for the reduction of compounds 5, 6, 9 and 10 at -20 o C (molar ratio of tellane-NaBH4 2 : 3), the target ditellanes 11-14 were obtained in near quantitative yields (95-98%) (Scheme 4).© ARKAT USA, Inc Organic ditellanes are important reagents for preparation of many other tellurium organic derivatives.Halogenation of diorganyl ditellanes provides widely used electrophilic reagents of the type RTeX and RTeX3, and the reduction of diorganyl ditellanes leads to corresponding organyltellurolate anions which are applied as strong nucleophilic reagents. 1,2ttempts to carry out the reduction of compounds 3, 4, 7 and 8 with sodium metabisulfite in a two-phase system benzene-water at room temperature as well as in the system sodium borohydride/water/THF at -20 o C led to mixtures of products and tellurium precipitation.
The structural assignment of compounds 3-14 was made by 1 H and 13 C NMR and confirmed by elemental analysis.The spin-spin coupling constants of 125 Te with the carbon atom of the CH2 group were measured for some synthesized compounds.The obtained values (138 and 115 Hz for compounds 4 and 6, respectively) correspond to the direct coupling constants ( 1 JTe-C).This indicates the addition of tellurium to occur at the terminal carbon atom of 1-alkenes according to Markovnikov rule.Bis-adducts 7 and 8 and ditellanes 11-14 have two chiral carbon atoms and represent equimolar mixtures of two diastereomers (d,l-and meso-forms, SR/RS and RR/SS).The diastereomers of bis-adducts 7 and 8 exhibit different signals in NMR spectra (in some cases the signals of two diastereomers coincided).The difference in chemical shift values of two diastereomers was observed especially for carbon atoms of the ClCH-CH2Te group.For example, the CH2Te group manifests at 55.1 and 55.4 ppm and the ClCH fragment appears at 58.0 and 58.1 ppm in the 13 C NMR spectrum of compound 7.No significant difference in chemical shift was observed for diastereomers of ditellanes 11-14.

Conclusions
Highly efficient syntheses of 2-chloroalkyl, 2-methoxyalkyl and 2-ethoxyalkyl tellanes and ditellanes in quantitative yield have been developed based on reactions of tellurium tetrachloride with 1-alkenes.The high chemo-and regio-selectivity of the reactions is worth noting: formation of the addition products exclusively according to the Markovnikov rule was observed.
Interesting results were obtained on studying the nucleophilic substitution reaction of chlorine by a methoxy group, which proceeded under unusually mild conditions at room temperature in the systems CH2Cl2methanol or chloroform-methanol, giving tellanes 5 and 6 in quantitative yields.The ease of the nucleophilic substitution of chlorine disclosed new possibilities for the functionalization of organotellurium compounds obtained by the addition of tellurium tetrachloride to alkenes.
The products 3-14 are valuable starting material for preparation of novel organotellurium compounds and intermediates for organic synthesis.