Ylides tellurium

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

Stoichiometric ylide cyclopropanations have been known for some time, with asymmetric variants using aminosulfoxonium ylides having been reported as early as 1968 [27]. Since then, procedures using stoichiometric amounts of sulfur, nitrogen, and tellurium ylides to achieve asymmetric cyclopropanations have been reported [16, 22, 86, 92-94]. The catalytic analogues of these reactions are discussed in the following sections. 

Krief, A. In The Chemistry of Organic Selenium and Tellurium Compounds. Synthesis of Selenium and Tellurium Ylides and Carbanions Application to Organic Synthesis Patai, S., Ed. Wiley, 1987 Vol 2, chap 17, p 684. 

The smooth condensation of 2,2 -diacetylbiphenyl to dibenzotropone in the presence of bis[4-methoxyphenyl] tellurium oxide is remarkable in view of previous unsuccessful attempts to effect this cyclization with conventional aldol catalysts2. Diaryl telluroxides are poor catalysts for aldol condensations involving acidic methylene groups, because formation of tellurium ylides removes the catalyst2. 

Dialkyl allyl telluronium bromides and dibutyl ethoxycarbonylmethyl (cyanomethyl, benzoylmethyl) telluronium bromide generated dialkyl alkylidene telluriums on treatment with potassium ferf-butoxide in tetrahydrofuran at low temperatures (s. p.837). The tellurium ylides were not isolated but were trapped with aldehydes and ketones4-6. 

Treatment of the tellurium ylide with concentrated hydrofluoric acid yielded 3,7-dimethyl-10-ethylphenotellurazine 5,5-difluoride2. 

Dimethyl-10-ethylphenotellurazine 10,10-dibromide condensed with 5,5-dimethyl-l,3-dioxocyclohexane to form a tellurium ylide in 80% yield1. 

Sulfur ylides are most frequently employed in this methodology. The cyclopropanations of Michael acceptors with arsenic, selenium and tellurium ylides " have also been... 

Krief A (1987) Synthesis of selenium and tellurium ylides and carbanions application... 

Dialkyl, diaryl, and alkyl aryl tellurium dihalides combine with l,l-dimethyl-3,5-dioxocyclohexane (dimedone) to form tellurium ylides -... 

Diorgano tellurium oxides condense with compounds containing activated methylene groups to form diorgano tellurium ylides ... 

V. Compounds with four tellurium-carbon single bonds or two tellurium-carbon single bonds and one tellurium-carbon double bond. Almost all of the tetraorgano tellurium Compounds are aromatic derivatives although tetramethyl tellurium was recently isolated. Aliphatic and aromatic tellurium ylides are known. 

Selenium and tellurium ylides take part in chemistry which is analogous to the reactions discussed thus far, and the subject has been well reviewed. Both alkenes and epoxides are formed in their reactions. 

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