# Removal of oxygen from pyridine N-oxide

Under what condition do the reaction proceed with sole removal of oxygen in contrast with the product also ending up with chlorine substituent?

I'm familiar with both reaction mechanism, but can't seem to figure out when will a certain pyridine end up with or without chlorine. I guess one factor would be electron richness of the ring system, chlorine being deactivating subsituent ends up only on the electron rich pyridines.

• POCl3 is phosphorus oxychloride. What is POCl? – Nilay Ghosh Feb 2 '16 at 18:41
• Indeed there's no such thing like POCl - second reaction looks bad – Mithoron Feb 2 '16 at 19:03
• So, the answer should be corrected..... – Nilay Ghosh Feb 2 '16 at 19:05
• @Mithoron You mean the whole reaction is not feasible? (omitting now proposed side product) – wuschi Feb 2 '16 at 19:27
• dx.doi.org/10.1016%2FB978-008096519-2.00027-8 looks possible but art. is behind paywall – Mithoron Feb 2 '16 at 19:57

I'm not sure whether $\ce{PCl3}$ can lead to the installation of a chlorine atom at C-2. I performed a cursory Reaxys search and in the first page saw several examples of deoxygenation without chlorination, but no examples of concomitant chlorination.

As far as I know, treatment of pyridine N-oxides with $\ce{PR3}$ simply leads to deoxygenation, with $\ce{P(III)}$ being oxidised to $\ce{P(V)}$.

However, if you were to treat pyridine N-oxide with phosphorus oxychloride $\ce{POCl3}$, then you would see chlorination at C-2.

If you have a source that says that $\ce{PCl3}$ can deoxygenate and chlorinate, please do inform us. Otherwise as far as I can tell, you need $\ce{POCl3}$ to effect a chlorination.

### References

1. Joule, J. A.; Mills, K. Heterocyclic Chemistry, 5th ed.; Wiley: Chichester, U.K., 2010; pp 121, 155.
2. Weickgenannt, A. Pyridine N-Oxides, Baran Group Meeting, June 9, 2012; p 3.

I would argue that electron withdrawing groups will favour the oxidation to POCl3 while electron donating groups will favour the POCl path.

$$\ce{HPyN+-O- + PCl3 <=> HPyN+-O-PCl2 + Cl- <=> Cl-HPyN-O-PCl2}$$

With electron withdrawing groups the oxygen atom is more positively polarized than with electron donating groups, so let the bonds flip to the left:

$$\ce{Cl-HPyN-O-PCl2 + Cl- -> Cl- + HPyN + POCl3}$$

In the opposite case let the bonds flip to the right:

$$\ce{H-ClPyN-O-PCl2 -> H+ + ClPyN + POCl + Cl-}$$

• That's what OP says - you should say sth more than he, shouldn't you? – Mithoron Feb 2 '16 at 19:07
• You're right. Now my answer looks pretty redundant to me. – aventurin Feb 2 '16 at 19:18
• What Mithoron said in the comment section above is that "there's no such thing like POCl". So, you should check your answer....... – Nilay Ghosh Feb 2 '16 at 19:54
• POCl does not exist. However, Chlorine in PCl3 can easily substituted by other groups. I guess "POCl" is used to represent such an intermediate product in an informal way. – aventurin Feb 2 '16 at 20:13
• Hope the edit clarifies what I tried to say. – aventurin Feb 2 '16 at 20:57