ACID-CATALYZED DEHYDRATION OF HETEROCYCLIC RING HYDRATES OF BENZOFURAN, BENZOTHIOPHENE, CHROMENE AND THIOCHROMENE

Rate constants and equilibrium constants have been measured for acid-c atalysed dehydration of hydrates of benzo-fused five- and six-membered heterocyclic and carbocyclic rings. Substrates studied include 2- and 3-hydroxy-2,3-dihydrobenzofuran, 3-hydroxy-2,3-dihydrobenzothiophene, 1-indanol, 4-chromanol, 4-thiochromanol and 1-tetralol. For the benzo furan-3-hydrate (3-hydroxy-2,3-dihydrobenzofuran), 1-indanol and 1-tet ralol rate constants have also been measured for the acid-catalysed ra cemisation of optically active substrates. These are believed to repre sent carbocation formation, and this supposition is confirmed by measu rements of product partitioning between benzofuran and benzofuran hydr ate products from solvolysis of the chloroacetate ester of the 3-hydra te. A surprising observation is that, in the five-membered ring series , heterocyclic oxygen and sulphur atoms have very small accelerating e ffects on rates of carbocation formation (1.3 and 2.5-fold, respective ly, relative to indanol) despite the possibility of direct resonance b etween the heteroatom and carbocation centre. For dehydration of the c orresponding six-membered rings indeed the heteroatoms slow the reacti on:in the case of oxygen (4-chromanol) by 100-fold. The possibility th at this behaviour arises from adverse inductive or stereoelectronic ef fects is discussed.