Eliminations promoted by weak bases. Part 7. E2C and E2H Reactions of cyclohexyl toluene-p-sulphonate with triphenylphosphine and other neutral bases

Abstract

The competitive elimination (E2) and substitution (S_N2) reactions of cyclohexyl tosylate with Triphenylphosphine are examined. Triphenylphosphine is taken to be representative of neutral weak bases which have good nucleophilic affinity for carbon. But it is a poor reagent for elimination when compared with anionic weak bases which are also good carbon nucleophiles. The stronger neutral bases triethylamine and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) are more effective than the anions in inducing elimination. It is concluded that whilst an interaction between the base and C_α is an important structural component of the E2C transition state, the interaction is primarily electrostatic rather than covalent (S_N2-like) as has been previously supposed. The reaction of Ph₃P with cyclohexyl bromide, a more E2H-like substrate, is almost completely substitution, indicating that neutral weak bases are also ineffective when more E2H-like transition states are involved, as is to be expected.