Tellurophene ring

Ring closures which depend on the conversion of the heteroatom into an electrophile are mostly associated with the formation of thiophene, selenophene and tellurophene rings and some illustrative examples are shown in Scheme 17. The last example which concerns the conversion of reaction with isocyanides is of particular interest since it appears to entail the attack of an electrophilic nitrogen species on the aryl ring. 

A route to 2-ethynylphenyl alkyl telluriums is suggested by the observation that butyl lithium cleaves the tellurophene ring in 3-bromobenzo[/>]tellurophene at low temperature producing butyl 2-ethynylphenyl tellurium in good yield. Phenyl lithium did not cleave the tellurophene ring even at 20°3. 

When l-formyl-2-chlorocydoalk-l-enes were used as starting materials, polycyclic ring systems containing the 2-substituted tellurophene ring were obtained in yields not exceeding 30%... 

Grignard reagents remove tellurium from the tellurophene ring in refluxing benzene in the presence of nickel-phosphine complexes7. 

With Replacement of Hydrogen Atoms in the Tellurophene Ring... 

Bromobenzotellurophene did not react with phenyl lithium. Butyl lithium in diethyl ether/hexane cleaved the tellurophene ring, even at — 50°, yielding butyl 2-ethynylphenyl tellurium2. 

When 3-chloro-2-(2-phenyl-2-chloroethenyl)-benzotellurophene was treated with sodium ethoxide in refluxing ethanol, the chloro-substituent on the tellurophene ring was replaced by ethoxide and the chloroethenyl group was converted into an ethynyl group2. 

Table 2 contains data on bond lengths and valence angles of the tellurophene ring. 

Tellurophene aldehydes and ketones readily undergo reduction reactions typical of carbonyl compounds. The Wittig reaction of 2,5-bis(dodecyloxy)- -xylene-bis(triphenylphosphonium bromide) with tellurophene-2,5-dicarbaldehyde leads to poly(2,5-tellurophenediylvinylene) (MW 7000-29000 Da) in 65% yield <1995MM8363>. The condensation reactions of 2,5-diphenyl-3,4-diformyltellurophene 15 have been employed for the synthesis of a number of bi- and polycyclic heterocycles with a fused tellurophene ring (Scheme 8) <1976CB3886>. 

With cyclopentadiene ruthenium 7t-complex [C5Me5Ru(MeCN)3](OTf), tellurophene reacts to form a stable sandwich Jt-complex 22 in 82% yield (Equation 10) <1998BKC706>. The -coordination of the tellurophene ring is confirmed by the strong high field shift of the Te NMR signal (107 ppm to be compared with 782 ppm signal of tellurophene). 

The hydrolysis of telluropyrylium dyes under aerobic conditions leads to the contraction of the tellurapyrane ring affording merocyanines containing a tellurophene ring in 29-53% yield (Scheme 22) <1997JOC4692>. 

This section is confined to the synthesis of tellurophene and its derivatives through direct construction of the tellurophene ring the numerous interconversions yielding derivatives from the preformed tellurophene nucleus are considered in the subsequent section. 

Quinones containing the tellurophene ring were obtained by Muller et al.ss by the reaction of an appropriate rhodium complex with amorphous tellurium (e.g., Section VI,D). 

The reaction of dibenzotellurophene with butyllithium causes the opening of the tellurophene ring and gives 2,2 -diphenyllithium and dibutyl telluride the kinetics of the reaction has been discussed.93... 

---