Strained oxygen heterocycles, ring-opening

NITRATE ESTERS FROM THE RING-OPENING OF STRAINED OXYGEN HETEROCYCLES... 

Epoxides are particularly reactive because ring opening releases ring strain, driving the reaction forward. However, we can tell you about some chemistry of the most important simple oxygen heterocycle,... 

With this large group of heterocycles, ring strain is of little or no importance. Ring-opening reactions are, therefore, rarer than in three- and four-membered heterocycles. The crucial consideration is rather whether a compound can be regarded as a heteroarene or whether it has to be classified as a heterocycloalkane or heterocycloalkene (see p 2). Various aromaticity criteria apply to heteroarenes, and as a consequence, different opinions have been expressed on this matter [1]. As will be shown by means of examples of the various systems, the nature and number of heteroatoms are the critical factors. The parent compound of the five-membered heterocycles with one oxygen atom is furan. 

In the search of novel materials with low dielectric constants (e) and low water absorption for the electronics industry as well as transparent, high oxygen permeable and crosslinkable materials for the fast production of contact lenses, the Ring-Opening Metathesis Polymerization (ROMP) became an interesting opportunity for our company [1]. In 1988, it was shown, that strained (heterocyclic) monomers with or without pendant functionalities, especially 7-oxa-norbornene (7-oxabicyclo[2.2.1]hept-5-ene) type monomers, can be efficiently polymerized in aqueous solutions at moderate temperatures (Scheme 1) [2]. 

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