With Exocyclic Halogen Substituents

Sulfur or selenium homocycles containing one (or more) exocyclic halogen substituent are prepared by the oxidation of the element with arsenic or 

---

Depending on the nature of both the halogen and the metal, the group 14 metalloles can react with halogens in three different ways (i) addition of 1 or 2 equivalents of halogen to the diene system, with the cyclic structure preserved, (ii) cleavage of one or two endocyclic E-C bonds, with destruction of the cyclic structure, and (iii) substitution of one or two exocyclic substituents, with preservation of the metallole structure. 

In contrast to the parent cydopropa[a]naphthalene, its 1,1-dichloro- and 1.1-difluoro derivatives may be synthesized by a variation of the original Billups scheme. The respective dihalocar-benes were added to l-bromo-3,4-dihydronaphthalene. A bromo substituent was then introduced at C3 of 7 by benzylic bromination with 7V-bromosuccinimide, after which aromatization to the very labile cycloproparene products 8 was elTected under mild conditions. In this sequence, the halogens were situated in such a way that no exocyclic double bonds could be formed upon dehydrobromination. The problem of rearrangement of such a double bond into the six-membered ring is therefore circumvented. Both the dichloro and the difluoro derivatives 8 are very sensitive to moisture, and have only been observed in solution. 

The replacement of fluorine, chlorine or bromine from the corresponding halogenocyclophosphazenes by another substituent (a nucleophile) is an important aspect of the chemistry of these ring systems [2, 7, 12]. First, this route provides a way of preparing cyclophosphazenes with substituents involving exocyclic P-N, P-0, P-S, P-C, P-H and P-M linkages. Thus, in a way it may be said that the halogen replacement reactions are the most... 

---