21979-34-0Relevant academic research and scientific papers
pH Dependence of the Elimination of Isoquinolines from N-(2-Cyanoethyl)isoquinolinium Cations
Bunting, John W.,Moors, Rodney G.
, p. 2258 - 2262 (1989)
The reactions of N-(2-cyanoethyl)isoquinolinium cations (1a (unsubstituted), 1b (4-bromo), 1c (4-aminocarbonyl), 1d (4-cyano), 1e (5-nitro)) have been investigated in basic aqueous solution (pH 9-13) at 25 deg C and ionic strength 0.1.In these solutions, these cations are rapidly equilibrated with their C-1 pseudobases, and pseudobase alkoxide ions, and pKR(+) and pKRO(-) have been evaluated.Subsequently, 1a-1c and 1e undergo hydroxide ion catalyzed eliminations to give the appropriately substituted isoquinoline and acrylonitrile.The pH rate profiles for thesereactions are very dependent upon pKR(+) and pKRO(-) for the isoquinolinium cation.It is shown that the nonreactivity of 1d under these conditions is readily rationalized in terms of the overwhelming predominance of the nonproductive pseudobase species (and/or its alkoxide ion) over the entire pH region under study.Second-order rate constants (kOH) for the elimination reaction correlate with the pKa of the isoquinolinium cation, with βlg = -0.43.Elimination in basic D2O resulted in no observable incorporation of deuterium into the acrylonitrile product.These observations are shown to be consistent with either an E2 mechanism or an E1cB mechanism involving a hydrogen-bonded carbanionic intermediate in which internal return of the proton and loss of the nucleofuge are both faster than exchange with solvent.
