By what reaction does (–)-sparteine decompose in ambient conditions?

Question

In its pure form (–)-sparteine is an only-slightly-yellow clear, viscous liquid. Yet, after only a week of being kept in dry, normal atmosphere (in a round bottom flask covered by a septa), some of it reacts with something to form a sticky, orange-brown substance. I've had to distill the sparteine I have twice due to whatever reaction is occurring, and I would like to know exactly what the reaction is so I don't have to distill every time I need it. The literature I found mentioned that it readily absorbs carbon dioxide in a 1-1 molar ratio, and I know that tertiary amines do sometimes react with CO₂, but I haven't been able to piece it together as something that could happen in ambient conditions.

Answer 1

Amines left out in the atmosphere will air-oxidize to give N-oxides, which typically have a yellow color. These N-oxides can decompose to give further byproducts. I would store your sparteine under nitrogen or argon atmosphere. The reaction proceeds through a radical chain mechanism with molecular oxygen [1].

From Wikipedia article for triethylenetetramine:

This oily liquid is colourless but, like many amines, assumes a yellowish color due to impurities resulting from air-oxidation.

From [2, p. 99] (I realize that sparteine is not an aromatic amine, but the result is the same):

It can be observed that arylamines develop a yellow or black color if left exposed to air for prolonged periods.

If you have access to triethylamine or other amines in your lab, do a control experiment: leave one vial open to air, and leave the other under identical conditions except sealed under an inert atmosphere. A few days worth of observation will convince you of the role in oxygen in the yellowing of amines.

References

1. Aleksandrov, A. L. Oxidation of Amines by Molecular Oxygen: 12. Reactions of Peroxide Radicals of Triethylamine and N,N-Dimethylcyclohexylamine. Russ Chem Bull 1980, 29 (11), 1740–1744. DOI: 10.1007/BF00949211.
2. Lawrence, S. A. Amines: Synthesis, Properties and Applications; Cambridge University Press, 2004. ISBN 978-0-521-78284-5.

Answer 2

Tertiary amines are widely used as electron donors in photoinduced electron transfer (PET) reactions. Electron transfer to an electronically excited acceptor yields the radical anion of the acceptor and the radical cation of the amine. When the stereoelectronic requirements are met, the radical cation deprotonates to give an α-aminoalkyl radical.

Note that the deprotonation is observed for acyclic and thus flexible trialkylamines, but not for DABCO ([2.2.2]-diazabicyclooctane, lacking a sufficient orbital overlap between an α-C-H bond and the (half-filled) orbital on the nitrogen. I'd say that the situation in sparteine is pretty similar to that in DABCO.

With molecular oxygen, the formation of an α-aminoalkyl radical as the first intermediate is nevertheless conceivable. The mechanism, however, is different.

Taking the triplet state character of ground state oxygen into account, i'd say the most likely step is the abstraction of an axial hydrogen atom at one of the α-bridgehead carbon atoms next to a nitrogen atom.