Unusual stability and carbon acidity of a dicationic carbon species

1,1′-Methylenebis-pyridinium dication (MDP) is an unusual salt with two formal positively charged substituents attached to a central carbon and yet is remarkably stable to hydrolysis at pH < 8. However above this pH it undergoes a biphasic reaction liberating two moles of pyridine sequentially. The rate of the first phase is second order in hydroxide-ion, whilst the second is pH independent. The first phase is also accompanied by the generation of a chromophore at 366 nm which is identified as a pyridine ring opened unsaturated imino-aldehyde, formed by an ANRORC-type mechanism. This intermediate then ring closes to give the second mole of pyridine and formaldehyde. Below pD 8 there is a very slow alternative pathway for degradation which is first order in hydroxide-ion, liberates only one mole of pyridine and forms N-hydroxymethyl pyridinium ion. Rates of deuterium exchange of the central methylene in D2O are faster than those of the breakdown of MDP and are predominantly OD- catalysed with a small amount of buffer catalysis. The estimated pKa of MDP dication in H2O is unexpectedly high at 21.2 at 25degC and I = 1.0 M (KCl), but is about 9 units lower than the mono-cationic N-methylpyridinium ion. Deuterium exchange also occurs in the 2 and 6 positions of the pyridinium rings, but at a slower rate which is first order in deuteroxide ion and only competitive with the breakdown of MDP below pD 11.