Bimolecular nucleophilic displacement at tertiary carbon centers: Aminolyses of 2-cyano-2-propyl and 1-cyanocyclooctyl arenesulfonates

Article abstract of DOI:10.1002/(SICI)1099-1395(199610)9:10<683::AID-POC840>3.0.CO;2-K

Kinetic studies were carried out on the reactions of anilines with 2-cyano-2-propyl and 1-cyanocyclooctyl arenesulfonates in acetonitrile at 50·0°C. The second-order rate constants for the former are in general greater than those for the latter but the rates of the two become comparable for a strong nucleofuge. The cross-interaction constants, ρ_XZ (and β_XZ), are considerably smaller (ca -0·04) than those for the primary (ca 0·33) and secondary (ca 0·12) compounds. The negative sign and small magnitude are consistent with a dissociative S_N2 mechanism with a loose transition state structure. The ab initio MO theoretical results for Cl^- + RCl?ClR + Cl^- at the MP2 level (MP2/6-31 + G*//MP2/6-31 + G*) confirm the looseness of the transition state for the tertiary (R) alkyl compounds. The average r*_(Cl...Cl) value is 4·88 ± 0·03 A, which is larger than those for the reactions at primary (4·68± 0·02 A) and secondary (4·80 ± 0·02 A) carbon centers. Thus a looser transition state with a smaller magnitude of ρ_XZ for the tertiary carbon centers has a larger theoretical r*_(Cl-Cl) value.