Unusual stability and carbon acidity of a dicationic carbon species

Dharmit Mistry, Nicholas Powles, Michael I. Page

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Abstract

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

Original languageEnglish
Pages (from-to)10732-10736
Number of pages5
JournalJournal of Organic Chemistry
Volume78
Issue number21
DOIs
Publication statusPublished - 1 Nov 2013

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