MECHANISM OF ACID-CATALYSED REDUCTION OF AROMATIC ALDEHYDES AND p-BENZOQUINONE DERIVATIVES BY AN NADH MODEL COMPOUND

Article abstract of DOI:10.1016/S0040-4020(01)87507-4

A series of aromatic aldehydes have successfully been reduced by N-metylacridan (AcH2) in the presence of perchloric acid (HClO4) in a mixture of acetonitrile and acetic acid (MeCN/MeCOOH, 4:1 v/v) at 323 K.The effects of HClO4 as well as Mg(ClO4)2 on hydride-transfer reactions from AcH2 to a series of p-benzoquinone derivatives (Q) in MeCN have also been examined.Perchloric acid shows both accelerating and retarding effects on the hydride-transfer reactions depending on the p-benzoquinone derivative.It is shown that AcH2 forms a 1 : 1 complex with HClO4 and the protonated species AcH3⁺ is inactive for the reduction of p-benzoquinon derivatives.Thus, for p-benzoquinone drivatives being weak oxidants such as p-benzoquinone which have the redox potentials E⁰(Q/Q^-.) < 0, HClO4 shows an overall accelerating effect on the reduction which proceeds through the acid-catalysed reaction of free AcH2 with Q, while for those being the stronger oxidants such as 2,3-dichloro-5,6-dicyano-p-benzoqinone which have E⁰(Q/Q^-.) > 0, HClO4 shows an overall retarding effect on the reduction where HClO4 has hardly catalysed the reaction of free AcH2 with Q.A proposed reaction mechanism involving electron transfer from AcH2 to Q followed by proton transfer from AcH2⁺ to Q^-. in the rate-determining step of the hydride-transfer reactions provides a quantitative evaluation of the single and unified correlation of the logarithm of the rate constant for the hydride-transfer reactions from free AcH2 to Q with the redox potential of Q in the absence and presence of Mg(ClO4)2 or HClO4.The electronic substituent effects on the rate of acid-catalysed reduction of aromatic aldehydes by AcH2 are shown to be very small, as being compatible with those observed for liver alcohol dehydrogenase (LADH)-catalysed reduction of the corresponding aldehydes by NADH.Moreover, the absolute value of the rate constant for the hydride-transfer step from free AcH2 to the protonated benzaldehyde in our model system is also shown to be compatible with that in the LADH-enzyme system.