Enol–keto tautomerism of 9-anthrol and hydrolysis of its methyl ether

摘要

The equilibrium constant for keto–phenol tautomerisation of anthrone to 9-anthrol (KE = [phenol]/[ketone]) hasnbeen determined as pKE (u0001log KE) = 2.10 from ratios of rate constants for ketonisation of anthrol and phenolisationnof anthrone in aqueous acetic acid buffers at 25 u0002C. Combining this value with pKa = 10.0 for the ionization ofnanthrone, measured spectrophotometrically in piperazine and borate buffers, gives pKa = 7.9 for the phenolic hydroxyngroup of anthrol. Measurements of rate constants for tautomerisation showed acid catalysis by H3Ou0003 in aqueousnHCl but by the base component only in buffer solutions of weaker acids. The H3Ou0003-catalysed reaction is subjectnto a solvent isotope effect kH3O/kD3O = 4.8, consistent with protonation of 9-anthrol at the 10-carbon atom of thenanthracene ring in the rate-determining step. Comparison with hydrolysis of the methyl ether of anthrol showed thatnketonisation is faster by a factor of 3000. This large rate difference is consistent with NMR measurements whichnshow that deuterium isotope exchange at the 10-position of the anthryl methyl ether occurs in competition withnhydrolysis. This accounts for a 60–70 fold of the rate difference. The residue is attributed to (a) a normal differencenof 16-fold in protonation rates of phenols and the corresponding methyl ethers and (b) a minor contribution fromnsteric hindrance to resonance stabilisation of the anthracen-9-onium ion intermediate in the hydrolysis reactionnfrom interaction of the conjugating methoxy group with the 1,8-hydrogen atoms of the adjacent phenyl rings.