Electrophilic tellurium

Organyltellurium trichlorides and diorganyltellurium dichlorides are prepared starting from the electrophilic tellurium tetrachloride (or arylteUurium trichlorides) by ... 

In this method, the electrophilic tellurium tetrachloride is employed as starting material to introduce tellurium in organic substrates, in contrast to the preceding methods using nucleophilic tellurium species. 

A different approach to unsymmetrical diorganyl tellurides, in which an electrophilic tellurium species is used, involves the nucleophilic attack of organomagnesium or organo-lithium reagents to diorganyl ditellurides. 

Reactions involving the attack of a nucleophilic carbon atom activated by an a-carbonyl group at the electrophilic tellurium atom of tellurium tetrachloride belong to the oldest methods of preparative organic tellurium chemistry. 

Like vinylic tellurides, acetylenic tellnrides can be prepared by two general approaches, starting respectively from nncleophilic or electrophilic tellurium species. 

Interaction of Electrophilic Tellurium Species with Organic Substrates 613... 

The classes of organic tellurium compounds mentioned above are prepared by interaction of nucleophilic or electrophilic tellurium species with different classes of organic compounds. Such species are generated from elemental tellurium or from diorganoditellurides 4, as will be discussed in the following sections. 

Preparation of Electrophilic Tellurium Species 9.13.4.1 Tellurium Tetrachloride... 

Tellurium tetrachloride 16 was widely used in the past to introduce tellurium into organic substrates. It reacts with several functional groups giving organotellurium trichlorides 5 (X = C1), which constitute an important class of tellurium electrophiles. Tellurium tetrachloride 16 is prepared by passing a stream of dry chlorine over elemental tellurium in an appropriate glass apparatus (Equation (3)). 

This class of organotellurium compounds is an important source of nucleophilic (Sections 9.13.3.1 and 9.13.3.2) or electrophilic (Sections 9.13.4.2.4 and 9.13.4.3) tellurium species. Diorganoditellurides can also behave as electrophiles, but in such cases normally one organotellurium group is lost. In this way, the corresponding organotellurium halides (Section 9.13.4.3) are prepared as source of electrophilic tellurium. In the past, the title compounds were prepared mainly by reduction of aryltellurium trihalides, in some cases under drastic reaction conditions5 9 11 12 (Scheme 11). 

In this section, the reactions of electrophilic tellurium species will be considered. Among these species are tellurium tetrachloride 16, organotellurium halides 7, organotellurium trihalides 5, and also diorganoditellurides 4, since these species can be attacked by carbanions. Diorganotellurium oxides 8, arenetellurinic acid anhydrides 9, and other oxidizing agents will also be discussed in Section 9.13.6.3, since in these species the tellurium atom is electron deficient. 

The main routes to symmetrical diorganyl tellurides involve the direct reaction of nucleophilic telluride dianions (usually as Nii Tc) with alkylating or arylating reagents. Otherwise the electrophilic tellurium tetrahalides react with aryhnagnesium reagents, giving diaryl tellurides. 

If the diaryl ditelluride is previously converted in situ into the corresponding tellurenyl halide, both the electrophilic tellurium moieties are used. (The stable crystaUine 2-naph-thylteUurenyl iodide (see Section 3.7) reacts similarly.) The reaction is of general application, giving high yields of the expected tellurides with aromatic, aliphatic, cycloaliphatic, vinylid and acetylenic Grignard reagents, under mild conditions. 

R = EtOCH2 Ar= P-CIC6H4 Ar = p-MeOC6H4 3.16.2 Starting from electrophilic tellurium... 

Engman describes a similar procedure for the synthesis of dibenzotellurophene but starting from 2,2 -dilithiobiphenyls (18). The cyclization is realized with elemental tellurium, tellurium tetrachloride, or the dichlorotellurolene (19) which acts as a source of electrophilic tellurium. 

---