# Why can't we oxidise allylic alcohol to aldehyde using pyridinium chlorochromate? [closed]

I read in my book that we cannot use pyridinium chlorochromate (PCC). Why is that?

• Reflux over activated MnO2 in Chloroform is a good mild way of taking allylic alcohols to aldehyde. Otherwise look at hypervalent iodine compounds organic-chemistry.org/chemicals/oxidations/… Jun 5 '20 at 14:08
• Thanks.I actually edited my question as I got the answer for the reagent to be used.(Mno2 and Seo2)But can you explain why PCC wont work Jun 5 '20 at 14:11
• Which book says that? I am unaware of a difficulty and can find examples in the literature of the oxidation of allylic alcohols with PCC (such as here www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alcohol2.htm) . The main reason for not using it is that it is a carcinogen and Chromium residues are toxic Jun 5 '20 at 14:52

There are indeed examples of $$\mathrm{PCC}$$ oxidation of allylic alcohols to $$\alpha, \beta$$-unsaturated aldehydes. The first example is from the original Corey-Suggs paper while the other two cases are from the work of Dauben and Michno.

References:

1. E. J. Corey, J. William Suggs, “Pyridinium chlorochromate. An efficient reagent for oxidation of primary and secondary alcohols to carbonyl compounds,” Tetrahedron Letters 1975, 16(31), 2647-2650 (https://doi.org/10.1016/S0040-4039(00)75204-X).
2. William G. Dauben, Drake M. Michno, “Direct oxidation of tertiary allylic alcohols. A simple and effective method for alkylative carbonyl transposition,” J. Org. Chem. 1977, 42(4), 682–685 (https://doi.org/10.1021/jo00424a023).
• @Mathew: Thanks for catching the journal error. Jun 5 '20 at 17:32

The question:

Why can't we oxidise allylic alcohol to aldehyde using pyridinium chlorochromate?

The question did not specify what kind of allylic alcohol OP is talking about. It could be either a primary, or secondary, or tertiary allylic alcohol (didn't cite the book so we don't know for sure). Regardless, pyridinium chlorochromate $$(\mathrm{PCC})$$ has been used to oxidize tertiary allylic alcohols. However, in these reactions, the newly formed carbonyl group is at carbon $$\beta$$ to hydroxyl bearing carbon (original allylic carbon. For example, tertiary allylic alcohols generated by Grignard reaction of vinylmagnesium bromide with relevant saturated ketones can be oxidized to the corresponding $$\alpha,\beta$$-unsaturared aldehydes in good to excellent yields with $$\mathrm{PCC}$$ (Ref.1):

$$\ce{R2-C=O ->[a) CH2=CHMgBr/ether][b) H3O+/H2O] R2C(OH)CH=CH2 \\ ->[PCC/CH2Cl2] R2C=CH -CHO}$$

However, before these discoveries on tertiary allylic alcohols, J. R. Holum (1961; Ref.2) has studied thoroughly on chromium(IV) oxide-pyridine complex $$\ce{CrO3.2C5H5N}$$ (first synthesis of pyridinium complexes, Ref.3). The abstract of that study (Ref.2) states that:

The chromium(IV) oxide-pyridine complex was found to be a good oxidizing agent at room temperature for the conversion of primary benzylic and allylic alcohols to their corresponding aldehydes. Data for twenty-one compounds are presented. Simple aliphatic and aromatic secondary alcohols are converted to ketones in fair to good yields; data for six are presented.

Thus, it can be concluded that what OP's textbook said about $$\mathrm{PCC}$$ and allylic alcohols might not exactly be correct.

In different perspective, $$\mathrm{PCC}$$ has been shown to be of particular value in the allylic oxidation of compounds containing an activated methylene group, such as 2,6-dihydropyrans. Indeed, it has been claimed that $$\mathrm{PCC}$$ is the reagent of choice in the allylic oxidation of $$\Delta^5$$-steroids. For example, when the reaction of $$\Delta^5$$-cholestoryl benzoate ($$\bf{1c}$$, Figure $$\bf{A}$$) with $$\mathrm{PCC}$$ is carried out in refluxing benzene, the isolated yield of $$\bf{2c}$$ was 89% while it was 78% when the reaction was performed in $$\mathrm{DMSO}$$ at $$\ce{100 ^\circ C}$$. In contrast, if the condition was pyridinium dichromate $$(\mathrm{PDC})$$ in pyridine at $$\ce{100 ^\circ C}$$, the yield was only 64% (Ref.4). In addition, these solvent systems are claimed to be superior to the more usual methylene chloride $$(\ce{CH2Cl2})$$. The biggest drawback of this system, the requirement of large excess of oxidizing reagent, has been successfully solved by using t-buyl hydroperoxide-$$\mathrm{PDC}$$ $$(\ce{t-BuOOH}/\mathrm{PDC})$$ mixture as an oxidant (Figure $$\bf{B}$$; Ref.5). However, $$\ce{t-BuOOH}/\mathrm{PCC}$$ system is found to be as not efficient as $$\ce{t-BuOOH}/\mathrm{PDC}$$ system. A low conversion was obtained using $$\mathrm{PCC}$$ catalyst has claimed to be due to the fact that $$\mathrm{PCC}$$ having a mildly acidic character (Ref.6).

References:

1. William G. Dauben, Drake M. Michno, “Direct oxidation of tertiary allylic alcohols. A simple and effective method for alkylative carbonyl transposition,” J. Org. Chem. 1977, 42(4), 682–685 (https://doi.org/10.1021/jo00424a023).
2. John R. Holum, “Study of the Chromium(VI) Oxide-Pyridine Complex,” J. Org. Chem. 1961, 26(12), 4814–4816 (https://doi.org/10.1021/jo01070a009).
3. Harry H. Sisler, Jack D. Bush, Oliver E. Accountius, “Addition Compounds of Chromic Anhydride with Some Heterocyclic Nitrogen Bases,” J. Am. Chem. Soc. 1948, 70(11), 3827–3830 (https://doi.org/10.1021/ja01191a085).
4. S. V. Ley, A. Madin, “Chapter 2.7: Oxidation Adjacent to Oxygen of Alcohol by Chromium Reagent,” In Comprehensive Organic Synthesis: Selectivity, Strategy, & Efficiency in Modern Organic Chemistry – Volume 7: Oxudation; Steven V. Ley, Volume Editor; Barry M. Trost & Ian Fleming, Editors-in-Chief; Fifth impression, Pergamon Press: Oxford, United Kingdom, 2005, pp. 251-290 (ISBN: 0-08-040898-3).
5. Manolis A. Fousteris, Anna I. Koutsourea, Sotiris S. Nikolaropoulos, Abdelkhalek Riahi, Jacques Muzart, “Improved chromium-catalyzed allylic oxidation of $$\Delta^5$$-steroids with t-butyl hydroperoxide,” * Journal of Molecular Catalysis A: Chemical * 2006, 250(1–2), 70-74 (https://doi.org/10.1016/j.molcata.2006.01.031).
6. E. J. Corey, J. William Suggs, “Pyridinium chlorochromate. An efficient reagent for oxidation of primary and secondary alcohols to carbonyl compounds,” Tetrahedron Letters 1975, 16(31), 2647-2650 (https://doi.org/10.1016/S0040-4039(00)75204-X).