Boron-selenium rings

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

For a review of anions a lo a selenium atom on small rings, see Krief Top. Curr. Chem. 1987, 135. 1-75. For alkylation a to boron, see Pelter Smith Brown Borane Reagents, Academic Press New York, 1988, pp. 336-341. 

Alkyl stannyl selenides are also useful nucleophilic selenium reagents. Tri-methylstannyl or tributylstannyl methyl selenides 58 react with halides 59 to produce unsymmetrical selenides 60 in good yields in the presence of a fluoride ion or by treatment with n-butyllithium (Scheme 44) [84]. Tributylstannyl phenyl selenide (61) reacts smoothly with acetals to give monoselenoacetals 62 in the presence of boron trifluoride etherate (Scheme 45 a) [85]. Similar reaction conditions were applied to the regioselective ring opening of epoxides (Scheme 45b) [86]. 

Elements. Those elements that form extended covalent (as opposed to metallic) arrays are boron, all the Group IV elements except lead, also phosphorus, arsenic, selenium and tellurium. All other elements form either only metallic phases or only molecular ones. Some of the above elements, of course, have allotropes of metallic or molecular type in addition to the phase or phases that are extended covalent arrays. For example, tin has a metallic allotrope (white tin) in addition to that with the diamond structure (grey tin), and selenium forms two molecular allotropes containing Se8 rings, isostruc-... 

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