Heteroatoms, three-membered ring opening with

The effect of different heteroatoms on the ring opening of substituted three-membered rings was investigated at the B3LYP/TZV+P level of theory.115 Two specific water molecules were included in the calculations and the COSMO method was used to include the bulk solvent. With the methyl thiolate nucleophile, thiirane was... 

Onium ions of small and large heterocyclics are usually produced by electrophilic attack on a heteroatom. In three- and four-membered rings nucleophilic attack on an adjacent carbon follows immediately, in most cases, and ring opening stabilizes the molecule. In large rings the onium ion behaves as would its acyclic analog, except where aromaticity or transannular reactions come into play (each with its electronic and steric pre-conditions). A wide diversity of reactions is observed. 

The 1,2,3-thiadiazole 1 possesses three contiguous heteroatoms in a five-membered ring and exists as a remarkably stable neutral aromatic compound. It is isomeric with the ring-opened a-diazothioketone 2 (Equation 1) although there is evidence that it reacts through this intermediate, all structural methods, including X-ray diffraction, point to 1 as the structure for a 1,2,3-thiadiazole. 

There have been relatively few reports concerning the chemistry of fused 5-6 bicyclic systems with one heteroatom in the five-membered ring and three in the six-membered ring, so this is a field of chemistry which is very open to new research. The only classes of compounds which have been widely reported are the S -nucleoside cyclic phosphates (1) and their analogues. Many thousands of papers have been written concerning these molecules in contrast to the very small number... 

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. 

---