Selenium-silicon rings

Electrophilic attack at ring silicon, sulfur, selenium, and phosphorus Nucleophilic attack at ring silicon Nucleophilic attack at ring boron... 

Germylene insertion into germacyclobutanes to give 1,2-digermacyclopentanes has already been described (Scheme 88). However, the tetraethyl derivative (95) is better made via the mercury derivative (Scheme 162). Like its silicon counterpart, (95) is slowly oxidized at room temperature, and inserts sulfur, selenium and bromine in the Ge—Ge bond. While (95) is less stable than the 1,2-digermacyclohexane, dichlorocarbene inserts into the /3-C—H bond of both rings, and not the Ge—C bond (Scheme 163) (69JOM(16)227). 

Photolysis of cyclic silicon-selenium ring systems (analogs of D3, (Me2SiO)3) apparently generated compounds with silicon-selenium double bonds these were trapped with hexamethylcyclotrisiloxane to yield the two-atom insertion products158 (equation 100). 

The (5,5)-fused bicyclic ring system with one fusion nitrogen atom and three additional heteroatoms in a 2 1 distribution over both live-membered rings (2N1) allows for a large variety of conceivable structures. The additional heteroatoms are mainly nitrogen, oxygen, and sulfur a few representatives of (5,5)-fused ring system also contain phosphorus, selenium or silicon atoms. 

Since genuine [4-1-2] cycloaddition products had previously not been prepared, it was surprising to find that the action of sulfur on 24 resulted in a formal [4-I-1] cycloaddition to furnish the first ftve-membered ring with an endocyclic silicon-silicon double bond 25. The heavier chalcogens selenium and tellurium did not react with 24. However, in the presence of small amounts of triethylphosphane smooth reactions with these elements did occur to furnish two further ftve-membered ring compounds 26, 27, each with an endocyclic silicon-silicon double bond (Eq. 8) [14]. 

Ti(OfPr)4, and chiral l,l -binaphthol (BINOL) gave enantiomerically enriched cfs-cyclopropanes (Scheme 22) (110). The proposed reaction pathway was via silicon migration and ring closure with the aid of selenium coordination. 

The aim of this Report is to review new methodology for the synthesis of saturated heterocyclic compounds. Emphasis is placed on reactions which involve a cyclization step and few ring transformations are described. Heterocycles which contain phosphorus, selenium, and silicon as the heteroatom are omitted. A few key steps in complex natural product syntheses are mentioned, but carbohydrates are excluded. The rapidly increasing numbers of reports describing the synthesis of saturated and unsaturated lactones and of crown ethers are covered in Chapters 3 and 10. 

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