SRN1 Mechanism in Heteroaromatic Nucleophilic Substitution. Reactions Involving Certain Dihalogenated ?-Deficient Nitrogen Heterocycles

Article abstract of DOI:10.1021/jo00156a007

Photostimulated reactions of 2,6-,2,3-, 3,5-, and 2,5-dihalopyridines (1a-e) with pinacolone potassium enolate (2) in liquid NH3 lead to facile replacement of both halogens via a modified S_RN1 mechanism, which does not involve intermediate formation of monosubstitution products.The potassium salts of phenylacetonitrile (12a) and α-ethylphenylacetonitrile (12b) react with 2,6-dibromopyridine (1a) under similar conditions to afford a mixture of mono- and disubstituted products via a related S_RN1 process. 4,7-Dichloroquinoline (16) undergoes radical-chain displacement of chloride from C4 with enolate 2. 2,6-Dichloropyrazine (18) and 2,3-dichloropyrazine (20) also undergo monosubstitution with 2 and diisopropyl ketone potassium enolate (22), but these reactions appear to be mainly addition-elimination (S_NAr) processes.Treatment of 3,6-dichloropyridazine (27) with 22 results in addition of 22 to C4 of the substrate.Photoassisted reaction of 2,4-dichloropyrimidine (31) with 12a leads to exclusive chloride displacement from C4 via the S_RN1 mechanism.