Ditelluroles

Perchlorates of the radical cations 38 (87MI5) and iodides of the radical cations 40 (88MI8) were prepared by electrochemical oxidation, whereas the salts of the tetratelluratetracene radical cation (TTeT)2X were formed through its oxidation by CuX2 (X = Cl,Br) in DMF solution (811ZV1432). The electric conductivity of the latter as well as that of their selenium analogs achieves values of 1-2 ohm em.  

The molecular geometry of the tetratelluratetracene 44 determined by an X-ray diffraction study (81CSC663 82MI2) is shown in Fig. 5. 

The planar molecules in the crystal are stacked in layers, the neighboring ones being connected by secondary Te. . . Te bonds (3.70 A). These distances are 0.7 A shorter than the sum of the tellurium Van der Waals radii. 

Based on the extremely high nucleophilicity of the tellurolate anions, a general approach to the synthesis of the 1,3-ditelluroles 50 was developed (82TL1531) that involves the following sequence of reactions. 

The key step is the intramolecular cyclization of the intermediate telluro-late anions 52 formed by treatment of compounds 51 with equimolar amounts of Li2Te. The by-products of the reactions are the tellurides (RC=CTe)2CH2. The yields of the ditelluroles 50 are strongly affected by the substituents at the triple bond and lie in the range 4-65%. The method is similar to that suggested earlier for the synthesis of the 1,3-thiatelluroles (81RTC10). 

However, a reinvestigation of this reaction resulted in the isolation of orange plates with a melting point of 216° (from acetone). This substance did not have the absorption maximum at 400 nm characteristic for the Te — Te chromophore. NMR-spectral data indicated that this substance was a 1 1 mixture of cis- and trans-2-benzylidene-5-phenyl-2H-l,3-ditellurole and not a 1,2-ditellurole. 

The treatment of 2/f-l,3-ditelluroles in acetonitrile at 30° with an equimolar amount of triphenylcarbenium tetrafluoroborate produced substances, the NMR spectra of which suggested that they were 1,2-ditellurolium salts.  

2-ditellurolium cations were quite unstable and rapidly deposited tellurium. 

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Ditellurole (122) differs from the compounds described in the previous two sections due to its small ring size, combined with the presence of two sp -hybridized carbons (Fig. 45). As a result, formation of the intramolecular dimer radical cation... 

Synthesis of compound A.A solution of 1,3-dibromopropane (10.1 g, 0.05 mol) in benzene (100 mL) was added to a solution of sodium telluride (NajTe 17.4 g, 0.1 mol) in ethanol (700 mL). After 3 h sodium borohydride (3.8 g, 0.1 mol) was further added to the mixture to produce sodium propane-1,3-ditellurolate, and then to the mixture was added a solution of the dibromopropane (10.1 g, 0.05 mol) in benzene (100 mL). The whole mixtnre was stirred at room temperature for 2 h. After usual work-up, the crude products were purified by silica gel column chromatography (eluent n-hexane/benzene) to afford compound A, which was further purified by preparative liquid chromatography. 

Of derivatives of the 1,3-ditelluroles, only the ditellurafulvenes 2 are known. Their first representatives, cis- and f rans-2,6-diphenyl-1,4-ditellur-afulvenes 2 were obtained in 12% total yield along with fran.s-2,4-benzyli-dene-l,3-ditelluretane la when sodium phenylethynyltellurolate was treated with trifluoroacetic acid in ether (81CC828) (see Section II). 

Benzo-1,3-ditellurole 54 was prepared in 40-47% yield by reacting dibro-momethane with disodium benzene-o-ditellurolate in ethanol, the latter generated in situ through reduction of poly(o-phenylene)ditelluride 55 with NaBH4 (88KGS1144 91 MI 1). The polymeric ditelluride 55, in turn, was obtained from a two-step procedure starting with bis-(o-trimethylsilyl)-benzene. 

Another approach to the benzo-1,3-ditellurole 54 is founded in the reaction of bis(o-trimethylsilyl)benzene with the easily accessible bis(trichloro-telluro)methane (85JA675). The l,l,3,3-tetrachlorobenzo-l,3-ditellurole 56 is reduced to 54, yield 18-20% (91MI1). 

The reactivity of 1,3-ditellurole has been inadequately investigated. Lithiation of the simplest 1,3-ditelluroles with lithium diisopropylamide (LDA) leads (depending on their structure) to either 2- or 4(5)-lithio derivatives (83TL237). Thus, 4-lithio-l, 3-ditellurole 57 is formed from 1,3-ditellurole, whereas lithiation of its 4-phenyl derivative produces 2-lithio-4-phenyl-l,3-ditellurole 58. The latter result is explained by the steric hindrance that the 4-phenyl substituent creates for the attack of LDA at position 5 of the five-membered ring. 

Electrochemical oxidation of 1,3-ditellurole leads to its stable radical cation 59 ( max = 560 nm), which is also formed when nitrosonium tetra-fluoroborate in CH2C12 is employed as an oxidant (85JA6298). Treatment with hydrazine hydrate smoothly restores the initial ditellurole. Similar to the radical cation of naphthof 1,8-cd][ 1,2]ditellurole (81CB2622), 59 does not display an ESR signal and dimerizes to dication 60 (Amax = 610 nm) on lowering the temperature of the solution to -60°C. The formation of 60 is facilitated by an increase in the concentration of the 1,3-ditellurole. 

Molecular and crystal structures of the parent 1,3-ditellurole were studied by use of an X-ray diffraction method (85JA6298). Figure 6 displays the molecular geometry of 1,3-ditellurole. 

Thus far no reports have appeared on the isolation of 1,3-ditellurolylium cation salts in a pure state. Attempts to prepare 1,3-ditellurolylium boron tetrafluoride and its derivatives via treatment of 1,3-ditelluroles with tri-phenylmethyl boron tetrafluoride in MeCN solution failed (82TL1531). However, formation of the 1,3-ditellurolylium cation 74 was revealed by the H NMR spectrum in which the 2-H proton was shown to give a very low-field triplet (8 15.0 ppm, 4/Hh = 1.2 Hz). Cation 74 is sufficiently stable in solution only at low temperature. When an acetonitrile solution of 74 obtained from 1,3-ditellurole was heated to 30°C, the initial H NMR spectrum drastically changed to the A2X spectral pattern (8 13.8 ppm, d and 10.3 ppm, /, iJ = 6.9 Hz) corresponding to the spectrum expected for the 1,2-ditellurolylium cation 75. A plausible reaction scheme is shown below. A further elevation of the temperature of the solution resulted in an unidentified destruction process accompanied by the extrusion of elemental tellurium. 

The Lewis acid triphenylmethyl fluoroborate (trityl fluoroborate) has been used to effect net hydride abstraction from 1,3-ditelluroles to give 1,3-ditellurolylium ions (Scheme 9) (82TL1531). There is some question as to whether the loss of hydride occurs by a single two-electron process or by two separate one-electron oxidations coupled with a proton loss. 

The direct metalation of 1,3-ditellurole (60 E = H) gave quite different results than the direct metalation of 4-phenyl-1,3-ditellurole (61 E = H) with lithium diisopropylamide (LDA) (83TL237). The parent system was lithiated at a vinylic position to give vinyl substituted products of structure (60) following electrophilic capture with benzaldehyde, methanol-O-d and methyl iodide. Identical results were obtained with 1,3-dithiole and 4-phenyl-l,3-dithiole with LDA. 

The additions of disodium telluride, dilithium telluride and disodium selenide to chloromethyl alkynyl sulfides and tellurides give a variety of 1,3-dichalcogenoles as shown in Scheme 15 (81RTC10,82TL1531>. The preparation of the 1,3-ditelluroles required the use of dilithium telluride under carefully controlled conditions. 

H-1,3-Ditelluroles treated in acetonitrile with triphenylcarbenium tetrafluoroborate were converted to 1,3-ditellurolium cations at — 30°. These cations rearranged to 1,2-ditcllurolium cations at + 30°2 (p. 796). 

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