Elimination of alcohols to give alkenes using POCl3
Description: When treated with phosphorus oxychloride (POCl3) alcohols are converted to alkenes via an elimination reaction.
Notes: Note that the more substituted alkene is formed (Zaitsev product)
Notes: The purpose of pyridine is to perform the key elimination step. Note in the second and third examples that the most substituted alkene is formed (Zaitsev rule).
Mechanism: 1,2-addition of the alcohol oxygen to the phosphorus (Step 1, arrows A and B) is followed by 1,2-elimination of chloride (Step 2, arrows C and D). Finally, deprotonation of the carbon β to the OPOCl2 leads to an elimination (E2) to give the alkene (Step 3, arrows E, F, and G).
Notes: Pyridine is used to do the key elimination step. It would also be reasonable to show a proton transfer step from OH to O in between steps 1 and 2.
(Advanced) References and Further Reading
- The Effect of Structure on the Course of Phosphoryl Chloride-Pyridine Dehydration of Tertiary Alcohols
Ronald R. Sauers
Journal of the American Chemical Society 1959 81(18), 4873-4876
- Stereospecificity and regiospecificity of the phosphorus oxychloride dehydration of sterol side chain alcohols
Jose Luis Giner, Christian Margot, and Carl Djerassi
The Journal of Organic Chemistry 1989 54(2), 369-373
This article by the legendary chemist Carl Djerassi (who developed norethindrone, the first female contraceptive) describes the selectivity of POCl3-pyridine dehydration conditions in steroid synthesis. It also has a general procedure for POCl3-pyridine dehydration in the experimental section.
- A general approach to linearly fused triquinane natural products. Total syntheses of (.+-.)-hirsutene, (.+-.)-coriolin, and (.+-.)-capnellene
Goverdhan Mehta, A. Narayana. Murthy, D. Sivakumar. Reddy, and A. Veera. Reddy
Journal of the American Chemical Society 1986108(12), 3443-3452
This paper by Prof. Goverdhan Mehta demonstrates the applicability of the POCl3-pyridine dehydration in natural product total synthesis.
- The 3-methylcholestanols and their derivatives
D. H. R. Barton, A. da S. Campos-Neves and R. C. Cookson
J. Chem. Soc., 1956, 3500-3506
This paper by Nobel Laureate Prof. Derek H. R. Barton has a POCl3-pyridine dehydration (see p. 3504-3505 in the experimental section).