The catalyst takes part in all those side reactions and the actual concentration of catalyst that is available for the MBH reaction is lower than assumed. In Figure , one can see that both side reactions, the incorporation of deuterium into the α-position of MA and the transesterification with deuterated methanol, are considerably faster using M19 than using DABCO. However, this does not necessarily mean that the concentration of M19 that is available for the MBH reaction is lower than that of DABCO, since that depends on the relative life-times of the intermediates.