Exocyclic heteroatoms

Mesoionic compounds (Section 5.07.1.3) are fully aromatic and usually have an exocyclic heteroatom bearing a charge attached to the ring. A new one-step method for converting the exocyclic oxygen of 3-phenyl-l,2,3-thiadiazolium-5-olate 52 into the exocyclic sulfur of 3-phenyl-l,2,3-thiadiazolium-5-thiolate 53 makes use of Lawesson s reagent (Equation 14) <1988BCJ2977>. 

Substitution on N(l) and N(4) can produce mesomerically stabilised zwitterions when the negative charge can reside on an exocyclic heteroatom or carbanion. A large number of compounds of this type (e.g., (8a,b)) have been prepared (Equation (5)) <76AHC(19)3>. The zwitterion (9), in which the R group is a polymer (-(CH2CH2)-), has been reported <90ZC285>. 

Heterocyclic monomers containing both endocyclic and exocyclic heteroatoms such as cyclic esters (lactones, lactide, carbonates) and cyclic anhydrides undergo coordination polymerisation or copolymerisation involving complex formation between the metal atom and the exocyclic heteroatom [100,124]. Polymerisation of /1-lactones is representative of such coordination polymerisations with catalysts containing an Mt-X active bond the initiation and propagation steps are as follows ... 

Analogously to the mechanism of epoxide polymerisation, the mechanism of the polymerisation of -lactone and other heterocycles with both endocyclic and exocyclic heteroatoms involves multicentred transition states with the participation of at least two metal atoms. 

Table 9.2 Homopolymerisation of heterocyclic monomers, containing both endocyclic and exocyclic heteroatoms, in the presence of coordination catalysts 2...

Since oxiranes are representative heterocyclic monomers containing an endo-cyclic heteroatom, and the most commonly polymerised of such monomers, they have been subjected to copolymerisations with heterocyclic monomers containing both an endocyclic and an exocyclic heteroatom. Coordination copolymerisations of heterocyclic monomers with different functions are focused on oxirane copolymerisation with cyclic dicarboxylic acid anhydride and cyclic carbonate. However, the statistical copolymerisation of heterocyclic monomers with an endocyclic heteroatom and monomers with both endocyclic and exocyclic heteroatoms have only a limited importance. Also, the block copolymerisation of oxirane with lactone or cyclic dicarboxylic acid anhydride is of interest both from the synthetic and from the mechanistic point of view. Block copolymerisation deserves special interest in terms of the exceptionally wide potential utility of block copolymers obtained from comonomers with various functions. It should be noted, however, that the variety of comonomers that might be subjected to a random, alternating and block polymerisation involving a nucleophilic attack on the coordinating monomer is rather small. 

Few examples of the alkylation of 1,3,4-oxadiazoles have been reported. The chemical shifts of the 2-methyl protons in the salts formed by protonation or alkylation of 2-methyl-5-phenyl-l,3,4-oxadiazole indicate that reaction takes place at ring nitrogen in position 3 (70JCS(C)1397). Alkylation of oxadiazolines (12) and aminooxadiazole (13) generally results in substitution at ring nitrogen in position 4 or 3 respectively, particularly under neutral conditions. In alkaline media, alkylation at the exocyclic heteroatom may occur. 2-Aryl-A-... 

In 2-substituted benzimidazoles the chloromethyl and cyanomethyl derivatives have lowest energy transitions of the n-n type but those of 5-chloro-2-trichloromethyl-, 2-trifiuoromethyl-, and 2-chloro-benzimidazoles are of the charge-transfer type. In the corresponding monocations and monoanions the lowest energy transitions are charge transfer and n-n respectively. The presence of a methylene group between the heterocycle and the exocyclic heteroatom reduces direct interactions <87JCS(P2)1465>. 

The translocation of exocyclic and endocyclic heteroatoms on a single heterocycle is shown below in D. In this case, the rearrangement is generally promoted by an exocyclic heteroatom on the ring. This is the most common example of a Dimroth rearrangement. 

The rule is that each BRF extends over all carbon-to-carbon bonds, and cannot be subdivided by breaking a carbon-to-carbon connectivity. The boundary points for a BRF are exocyclic heteroatoms. (For a more detailed discussion, see reference 4 and references therein). 

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