Synthesis and Spectroscopic Investigations of Some New rganotelluronium alts Based on Dicyclohexyl Telluride

디시클로헥실 텔루르염에 기반한 유기텔루로늄염의 성과 분광학적 분석

Abstract

Dicyclohexyl telluride was obtained in a high yield by the reaction of cyclohexyl bromide with NaTeH(prepared in situ) in an aqueous ethanolic solution. A series of new organotelluronium salts of the general formula ${(cyclo-C_6H_11)}_2Te(R)X$ (where R = $CH_3$, X = I (1); R = $C_2H_5$, X = Br(2); R = $C_2H_5$, X = I (3); R = C_3H_5$, X = Br (4)) were prepared by the reaction of ${(cyclo-C_6H_11)}_2Te$ with the corresponding alkyl halide. Reaction of 1 with NaBPh4 gave compound 5 ( i.e. R = CH3, X = BPh4 ‒) in 78% yield. Reaction of ${(cyclo-C_6H_11)}_2Te$ with benzyl bromide and 4-bromophenacyl bromide gave unexpectedly dibenzylcyclohexyltelluronium bromide (6) and bis(4-bromophenacyl)cyclohexyltelluronium bromide (7), respectively. Reaction of 6 with NaBPh4 gave the corresponding tetraphenylborate derivative (8) in high yield. $^1H$ NMR studies revealed that in $CDCl_3$solution compound 1 eliminated alkyl halide. Conductivity, IR, $^1H\;and\;^{13}C$ NMR and thermal data for the new compounds are presented and discussed.

디시클로헥실 텔루르염은 에탄올 수용액에서 NaTeH와 디시클로헥실 브롬화물의 반응에 의해 높은 수율로 얻어진다. 일반식 ${(cyclo-C_6H_11)}_2Te(R)X$ (where R = $CH_3$, X = I (1); R = $C_2H_5$, X = Br(2); R = $C_2H_5$, X = I (3); R = C_3H_5$, X = Br (4)) 인 유기텔루로늄의 새로운 시리즈는 그에 상응하는 알킬 할로겐화물과 ${(cyclo-C_6H_11)}_2Te$의 반응에 의해 만들어진다. $NaBPh_4$와 1의 반응은 78% 수율로 화합물 5를 제공하였다. 벤질 브롬화물과 ${(cyclo-C_6H_11)}_2Te$, 4-bromophenacyl bromide의 반응에서는 뜻밖에 각각dibenzylcyclohexyltelluronium 브롬화물과 and bis(4-bromophenacyl)cyclohexyltelluronium bromide을 얻었다. 높은 수율의 tetraphenylborate 유도체는 $NaBPh_4$과 6의 반응으로 얻었다. $CDCl_3$용매에서 $^1H$ NMR분석을 통해 화합물 1 이 알킬 할로겐화물의 제거 반응을 일으킴을 확인할 수 있었다. 새로운 화합물은 전도성, IR, $^1H$와$^{13}C$ NMR, 열분석를 통해 규명되었다.

INTRODUCTION

Recently, the synthesis, structures and properties of triorganotelluronium salts have received considerable attention.1,2 Thus, new and general approaches to a variety of unsymmetrical telluronium salts have been developed.3,4 A special attention is also given to the applications of these compounds in preparative organic synthesis.5,6 Our previous work described the synthesis and solution properties of a series of cyclic and non cyclic telluronium salts.7,8 It was observed that some of these telluronium salts are unstable toward reductive elimination of alkyl halide in organic solvents,7,8 such a CDCl3, DMSO and DMF. Recently, we reported the synthesis and solution properties of some new telluronium salts based on bis(cyclohexylmethyl) telluride.7a

The synthesis of dicycyclohexyl telluride9,10 and its dihalo,11,12 pseudohalido11 and diazido13 derivatives have been reported previously. No work has been done on the reactions of dicycyclohexyl telluride with alkyl halides, as far as we aware. Thus, the present work describes the synthesis, solution properties and some solid state data of a new series of telluronium salts based on dicyclohexyl telluride. Some unexpected reactions are also reported.

 

EXPERIMENTAL

Physical measurements

1H and 13C NMR spectra were recorded on Jeol EX-90FT (90 MHz) spectrometers with TMS as an internal reference. Infrared spectra were recorded with KBr discs in the range of 4000-200 cm-1 on a Pye-Unicam SP-300s infrared spectrophotometer. Microanalysis for carbon, hydrogen and nitrogen was obtained on a Carlo-Ebra EA1-108 Elemental Analyzer. All melting points were determined by a Gallenkamp melting point apparatus and are uncorrected. Molar conductances at 10-3-10-5 dilution in DMSO were measured with a WTW D812 conductivity meter LBR, using a standard cell with cell constant of 0.81 cm-1. The thermal analyses were carried out using Du Pont thermoanalyser, model 990, which measure TGA and DTG simultaneously. All measurements were carried out under nitrogen atmosphere. The analyses were performed by heating the sample at a steady rate of 50 ℃ min-1, from ambient temperature to 600 ℃.

Synthesis

All reactions were carried out in a dry oxygenfree, nitrogen atmosphere. Solvents were dried and distilled before use, using standard procedures.

Dicyclohexyl telluride

NaBH4 (2.1 g; 59 mmol) was added under nitrogen atmosphere at room temperature to a suspension of tellurium powder (2.55 g; 20 mmol) in 70 ㎤ ethanol/water (6/1). The reaction mixture was then refluxed whereupon a vigorous evolution of H2 started. All the tellurium was consumed within 20 min, producing a faintly yellow solution. To cooled resulting solution was added a solution of cyclohexyl bromide(6.52 g; 40 mmol) in ethanol (40 ㎤). The resulting solution was refluxed for 1 h and then water bath was removed and the stirring continued for 2 h at room temperature. The solution was filtered, pored into 500 ㎤ of distilled water and extracted with chloroform (4 × 50 ㎤). The combined extracts were dried with anhydrous MgSO4. Evaporation of solvent by a rotary evaporator, gave heavy orange oil with a persistent odour in 86% yield (4.70 g).

1H NMR (CDCl3) δ: 3.35 (m, Te-CH, 2H); 0.80-2.26 (m, Cy-CH2, 20H).

Methyldicyclohexyltelluronium iodide (1).

A freshly distilled methyl iodide (2.13 g; 15 mmol) and a solution of (cyclo-C6H11)2Te (2.94 g, 10 mmol) in diethyl ether (10 ㎤) were placed in a flask flushed with dry nitrogen. The reaction mixture was stoppered and left overnight. A white precipitate was formed, recrystallized from ethanol and dried in vacuo to give 3.62 g (60%) of compound 1, mp 118-120 ℃.

Anal. Calc. for C13H25TeI: C, 35.82; H, 5.78. Found: C, 35.78; H, 5.99.

1H NMR (CDCl3) δ: 2.45 (s, CH3, 3H); 3.81 (m, Te-CH, 2H); 1.00-2.40 (m, Cy-CH2, 20H).

Ethyldicyclohexyltelluronium bromide (2).

This compound was obtained as white crystals in 68% yield and prepared by the same way as compound 1 using ethyl bromide, mp 205 ℃.

Anal. Calc. for C14H27TeBr: C, 41.74; H, 6.76. Found: C, 41.71; H, 6.69.

Ethyldicyclohexyltelluronium iodide (3).

This compound was prepared by the reaction of ethyl iodide with (cyclo-C6H11)2Te under conditions reported for compound 1. White crystals, mp > 300 ℃.

Anal. Calc. for C14H27TeI: C, 37.38; H, 6.05. Found: C, 36.96; H, 5.53.

Allyldicyclohexyltelluronium bromide (4).

Reaction of allyl bromide with dicyclohexyl telluride under conditions reported for compound 1, gave white crystals of compound 4, mp 158 ℃ (dec.). Yield: 65%.

Anal. Calc. for C15H27TeBr: C, 43.42; H, 6.56. Found: C, 43.22; H, 6.32.

1H NMR (CDCl3) δ: 1.02-2.38 (m, Cy-CH2, 20H); 2.72 (d, CH2-Te, 2H, J = 7.3 Hz); 3.81 (qui, Te-CH, 2H, J = 6.9 Hz); 5.11 (m, CH2 = CH-, 2H); 5.78 (m, CH2 = CH-CH2, 1H).

Methyldicyclohexyltelluronium tetraphenylborate (5).

Compound 1 (0.40 g; 0.92 mmol) dissolved in 30 ㎤ of hot ethanol/water (3/1) was added to a solution of sodium tetraphenylborate (0.34 g; 1 mmol) in hot ethanol (10 ㎤). A voluminous white precipitate was immediately formed. The reaction mixture was stirred at room temperature for 1 h. The white precipitate was collected and washed several times with hot ethanol. Recrystallization from DMF and water gave a white solid of compound 5 in 78% yield, mp 161-162 ℃.

Anal. Calc. for C37H45BTe: C, 70.75; H, 7.22. Found: C, 70.53; H, 7.08.

1H NMR (CDCl3) δ: 2.90 (s, CH3, 3H); 3.90 (m, Te-CH, 2H); 0.80-2.20 (m, Cy-CH2, 20H); 6.80-7.90 (m, Ar-H, 20H).

Dibenzylcyclohexyltelluronium bromide (6).

This compound was prepared by the oxidative addition of benzyl bromide (2.57 g; 15 mmol) to dicyclohexyl telluride (2.94 g; 10 mmol) under previous conditions reported for compound 1. White crystals, mp 106-108 ℃. Yield: 69%.

Anal. Calc. for C20H25TeBr: C, 50.79; H, 5.33. Found: C, 50.55; H, 5.11.

1H NMR (CDCl3) δ: 3.50 (m, Te-CH, 1H); 0.80-2.40 (m, Cy-CH2, 10H), 4.45 (q, Ph-CH2, 4H, J = 7.2 Hz); 6.70-7.80 (m, Ar-H, 10H).

13C NMR (CDCl3) δ: 25.1, 27.5, 29.1, 30.5, 45.0, 121.5, 126.8, 129.0, 136.7.

Bis(4-bromophenacyl)cyclohexyltelluronium bromide (7).

4-Bromophenacyl bromide (4.17 g; 15 mmol) in 20 ㎤ of diethyl ether was added to dicyclohexyl telluride (2.94 g; 10 mmol) in 30 ㎤ of diethyl ether. The flask was stoppered and left overnight. The deposited white precipitate was separated and washed with chloroform and ethanol, respectively. Recrystallization from ethanol gave a white solid, mp 99-101 ℃. Yield: 52%.

Anal. Calc. for C22H23 O2TeBr3: C, 38.82; H, 3.38. Found: C, 39.28; H, 3.27.

1H NMR (CDCl3) δ: 3.40 (m, Te-CH, 1H); 1.00-2.70 (m, Cy-CH2, 10H), 4.45 (q, Ph-CO-CH2, 4H, J = 7.4 Hz); 7.40-8.70 (m, Ar-H, 8H).

Dibenzylcyclohexyltelluronium tetraphenylborate (8).

This compound was prepared by the same above method for compound 5 and gave white crystals in 75% yield, mp 138-140 ℃.

Anal. Calc. for C44H45 BTe: C, 74.20; H, 6.37. Found: C, 74.03; H, 6.14.

1H NMR (CDCl3) δ: 3.35 (m, Te-CH, 1H); 1.00-2.10 (m, Cy-CH2, 10H), 3.50 (s, Ph- CH2, 4H); 6.90-7.78 (m, Ar-H, 30H).

13C NMR (CDCl3) δ: 25.0, 27.5, 29.1, 30.5, 45.1, 121.3, 125.3, 126.4, 128.6, 129.2, 132.3, 136.7, 164.5, 167.

 

RESULTS AND DISCUSSION

Reaction of NaTeH, prepared in situ from reaction of tellurium powder with NaBH4 in an aqueous ethanol, with cyclohexyl bromide gave dicyclohexyl telluride in 86% yield. This method gave dicyclohexyl telluride in a high yield compare with other reported methods.9,10 This compound starts to decompose if left under ambient conditions for more than a day and should be used immediately to prepare its derivatives.

Oxidative addition of methyl iodide, ethyl bromide, ethyl iodide and allyl bromide to dicyclohexyl telluride produce the corresponding telluronium salts (i.e. compounds 1, 2, 3 and 4) in good yields. Reaction of 1 with sodium tetraphenylborate gave compound 5 in 78% yield (Scheme 1).

Unexpectedly, addition of benzyl bromide and 4-bromophenacyl bromide to dicyclohexyl telluride gave dibenzylcyclohexyltelluronium bromide (6) and bis(4-bromophenacyl)cyclohexyltelluronium bromide (7), respectively as unexpected products. Thus it seems likely that in both reactions, telluronium salt species were initially formed and reductively eliminated cyclohexyl bromide to form the unsymmetrical telluride ((cyclo-C6H11)Te-CH2Ar)). These unsymmetrical tellurides react again with benzyl bromide or 4-bromophenacyl bromide to afford compounds 6 and 7, respectively as shown in Scheme 2.

Scheme 1.Methods of preparation of compounds 1-8. (i) Te/NaBH4/ EtOH-H2O. (ii) alkyl halides. (iii) benzyl bromide or 4-phenacyl bromide. (iv) NaBPh4.

Scheme 2

The proposed mechanism is not unique, since it is well established that oxidative addition of alkyl halides to diorganyl tellurides proceeded via radical pathways and the products depended on the stability of alkyl radicals.14,15 The reaction of compounds 6 with sodium tetraphenylborate gave compound 8 in good yield, which supports the formation of compound 6, Scheme 1. An authentic sample of 6 was prepared by reaction of dibenzyl telluride with cyclohexyl bromide and gave the same melting point and the same IR spectrum in comparison with compound 6. Compounds 2 and 3 are insoluble in most common organic solvents and have high melting points in compare with other derivatives.

The IR spectra of 1-8 display characteristic band at certain region. All compounds showed medium to strong bands due to C-H asymmetric and symmetric stretching (νas and νs) in the range 2960-2900 and 1260-1240 cm-1, respectively. Two weak to medium bands due to νas(C-Te) and νs(C-Te) generally appeared at 510-495 and 663-635 cm-1, respectively. Thus, it seems that energy differences between νas(C-Te) and νs(C-Te) for (cyclo-C6H11)2Te and its derivatives is increased significantly in compare with other alkyl and heterocyclic tellurium compounds.7-9

Fig. 1.Plots of molar conductance ((ΛM) ohm-1 ㎠ mol-1)) against (concentration)1/2 for compounds 4, 5, 6 and 8 in DMSO solution.

1H NMR spectra of compounds 1 and 5-8 show all the expected peaks in proper intensity ratio.

The 1H NMR spectrum of compound 1 in CDCl3 shows signals due to CH2-cyclo, CH-Te and CH3-Te at δ 1.00-2.40(m), 3.81(m) and 2.45(s), respectively, but as time passes (24 h) the spectrum shows two extra new signals at δ 2.16(m) and at δ 3.36(s), due to CH-Te and CH3I, respectively. This may due to methylation and demethylation equilibrium of compound 1 in CDCl3 solution and as shown in the following equation:

Such behavior was also observed with other telluronium salt.7,8,16,17

The molar conductivities compounds 4, 5, 6 and 8 in 10-3 M of DMSO were determined and their values approach those expected for 1:1 electrolytes (ΛM ≈ 30 ohm-1 ㎠ mol-1). Plots of molar conductance (ΛM) against (concentration)1/2 are typical of weak electrolytes (Fig. 1) which agree well with previous works.7,8,16,17

The thermogravimetric curve for compound 1 shows one decomposition peak at 190 ℃. This loss reaches value of 48.8% (calculated 48.2%), which points to loss C6H11I and formation of C6H11TeCH3 as a residual product. A loss at 210 ℃ is observed for compound 6. This effect corresponds to removal of C6H11Br (mass loss observed 43.8% compared with the calculated 43.4%). Thus, same decomposition pattern can be applied for both compounds. On the other hand, compound 6 was completely evaporated at 190 ℃. On heating compound 8, a massive loss occurs at 210 ℃, amounting to 82.5% (calculated 82.0%), which can be attributed to the thermal extrusion of tellurium and as residual product. Extrusion of tellurium may be promoted thermally18 or photochemically.19 Thus we believe that tellurium is thermally extruded from compound 8.

In conclusion, a series of new organotellurium compounds derived from dicyclohexyl telluride have been prepared in good yield and their solid decomposition and solution properties have been studied.