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

Deoxygenation of pyridine N-oxides with PCl3

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.

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    $\begingroup$ POCl3 is phosphorus oxychloride. What is POCl? $\endgroup$
    – Nilay Ghosh
    Commented Feb 2, 2016 at 18:41
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    $\begingroup$ Indeed there's no such thing like POCl - second reaction looks bad $\endgroup$
    – Mithoron
    Commented Feb 2, 2016 at 19:03
  • $\begingroup$ So, the answer should be corrected..... $\endgroup$
    – Nilay Ghosh
    Commented Feb 2, 2016 at 19:05
  • $\begingroup$ @Mithoron You mean the whole reaction is not feasible? (omitting now proposed side product) $\endgroup$
    – wuschi
    Commented Feb 2, 2016 at 19:27
  • $\begingroup$ dx.doi.org/10.1016%2FB978-008096519-2.00027-8 looks possible but art. is behind paywall $\endgroup$
    – Mithoron
    Commented Feb 2, 2016 at 19:57

2 Answers 2

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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 $\ce{P(V)}$.

Deoxygenation by PCl3 without further functionalisation

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

Conversion of pyridine N-oxide to 2-chloropyridine

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.
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  • $\begingroup$ In practice, phosphorus oxychloride is the byproduct after removing the oxygen from the N-oxide; therefore would the parent N-oxide be chlorinated and deoxygenated as in reaction #2 if removing the byproduct from the reaction mixture is not effective? In other words, can we use another deoxygenating agent? Off the top of head, I would imagine use a alkali metal sulfite $\ce{Na2SO3}$, can the byproduct alkali metal sulfate $\ce{Na2SO4}$ can be disposed off and does not halogenate. $\endgroup$
    – Kav
    Commented May 21, 2021 at 18:23
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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:

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

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  • $\begingroup$ That's what OP says - you should say sth more than he, shouldn't you? $\endgroup$
    – Mithoron
    Commented Feb 2, 2016 at 19:07
  • $\begingroup$ You're right. Now my answer looks pretty redundant to me. $\endgroup$
    – aventurin
    Commented Feb 2, 2016 at 19:18
  • $\begingroup$ What Mithoron said in the comment section above is that "there's no such thing like POCl". So, you should check your answer....... $\endgroup$
    – Nilay Ghosh
    Commented Feb 2, 2016 at 19:54
  • $\begingroup$ 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. $\endgroup$
    – aventurin
    Commented Feb 2, 2016 at 20:13
  • $\begingroup$ Hope the edit clarifies what I tried to say. $\endgroup$
    – aventurin
    Commented Feb 2, 2016 at 20:57

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