Barton-McCombie deoxygenation

The Barton-McCombie deoxygenation is an organic reaction in which an hydroxy functional group in an organic compound is replaced by a hydride to give an alkyl group ^[1]^[2]. It is named for the British chemists Sir Derek Harold Richard Barton (1918–1998) and Stuart W. McCombie.

This deoxygenation reaction is a radical substitution. In the related Barton decarboxylation the reactant is a carboxylic acid.

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1 Mechanism
2 Variations
- 2.1 Alternative hydride sources
- 2.2 Trialkyl boranes
3 Scope
4 References

Mechanism

The reaction mechanism consists of a catalytic radical initiation step and a propagation step.^[3] The alcohol (1) is first converted into a xanthate (2). The other reactant tributyltin hydride 3 is decomposed by AIBN 8 into a tributyltin radical 4. The tributyltin radical abstracts the xanthate group from 2 leaving an alkyl radical 5 and tributyltin xanthate (7). The sulfur tin bond in this compound is very stable and provides the driving force for this reaction. The alkyl radical in turn abstracts a proton from a new molecule of tributyltin hydride generating the desired deoxygenated product (6) and a new radical species ready for propagation.

Variations

Alternative hydride sources

Main disadvantage of this reaction is the use of the tin hydride which is toxic, expensive and difficult to remove from the reaction mixture. One alternative is the use of tributyltin anhydride as the radical source and poly(methylhydridesiloxane) (PMHS) as the hydride source ^[4]. Phenyl chlorothionoformate used as the starting material ultimately generates carbonyl sulfide.

Trialkyl boranes

An even more convenient proton donor is provided by trialkylborane-water complexes ^[5] such as trimethylborane contaminated with small amounts of water.

In this catalytic cycle the reaction is initiated by air oxidation of the trialkylborane 3 by air to the methyl radical 4. This radical reacts with the xanthate 2 to S-methyl-S-methyl dithiocarbonate 7 and the radical intermediate 5. The CH₃B.H₂O complex 3 provides a proton for recombining with this radical to the alkane 6 leaving behind diethyl borinic acid and a new methyl radical.

It is found by theoretical calculations that that a O-H homolysis reaction in the borane-water complex is endothermic with an energy similar to that of the homolysis reaction in tributyltin hydride but much lower than the homolysis reaction of pure water.

Scope

A variation of this reaction was used as one of the steps in the total synthesis of azadirachtin ^[6]:

In another variation the reagent is the imidazole 1,1'-thiocarbonyldiimidazole (TCDI), for example in the total synthesis of pallescensin B.^[7]. TCDI is especially good to primary alcohols because there is no resonance stabilization of the xanthate because the nitrogen lonepair is involved in the aromatic sextet.

The reaction also applies to S-alkylxanthates. With triethylborane as a novel metal-free reagent, the required protons are recruited from protic solvents, the reactor wall or even (in strictly anhydrous conditions) the borane itself. ^[8]

References

^ Barton, D. H. R.; McCombie, S. W. (1975). "A new method for the deoxygenation of secondary alcohols". J. Chem. Soc., Perkin Trans. 1 16: 1574-1585. doi:10.1039/P19750001574.
^ Crich, D.; Quintero, L. (1989). "Radical chemistry associated with the thiocarbonyl group". Chem. Rev. 89: 1413-1432. doi:10.1021/cr00097a001.
^ Forbes, J. E.; Zard, S. Z. Tetrahedron Lett. 1989, 30, 4367.
^ α-D-Ribo-hexofuranose, 3-deoxy-1,2:5,6-bis-O-(1-methylethylidene). Tormo, J.; Fu, G. C. Org. Syn., Coll. Vol. 10, p.240 (2004); Vol. 78, p.239 (2002). (Article)
^ Deoxygenation of Alcohols Employing Water as the Hydrogen Atom Source David A. Spiegel, Kenneth B. Wiberg, Laura N. Schacherer, Matthew R. Medeiros, and John L. Wood J. Am. Chem. Soc. 2005, 127, 12513-12515. (doi:10.1021/ja052185l)
^ Synthesis of Azadirachtin: A Long but Successful Journey Gemma E. Veitch, Edith Beckmann, Brenda J. Burke, Alistair Boyer, Sarah L. Maslen, and Steven V. Ley Angew. Chem. Int. Ed. 2007, doi:10.1002/anie.200703027 DOI: 10.1002/anie.200703027
^ The first total synthesis of (±)-pallescensin B Wen-Cheng Liu and Chun-Chen Liao Chem. Commun., 1999, 117–118 117 Article
^ Part 2. Mechanistic aspects of the reduction of S-alkyl-thionocarbonates in the presence of triethylborane and air Allais F, Boivin J, Nguyen V Beilstein Journal of Organic Chemistry, 2007 3:45 ( 12 December 2007 ) doi:10.1186/1860-5397-3-46

Categories: Free radical reactions | Organic redox reactions

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Barton-McCombie_deoxygenation". A list of authors is available in Wikipedia.