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 [cite journal
author = Barton, D. H. R.; McCombie, S. W.
journal = J. Chem. Soc., Perkin Trans. 1
title = A new method for the deoxygenation of secondary alcohols
year = 1975
volume = 16
pages = 1574–1585
doi = 10.1039/P19750001574] [cite journal
author = Crich, D.; Quintero, L.
journal = Chem. Rev.
title = Radical chemistry associated with the thiocarbonyl group
year = 1989
volume = 89
pages = 1413–1432
doi = 10.1021/cr00097a001] . 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.

Mechanism

The reaction mechanism consists of a catalytic radical initiation step and a propagation step. [Forbes, J. E.; Zard, S. Z. "Tetrahedron Lett." 1989, "30", 4367.] 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 hydrogen atom 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 [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). ( [http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=v78p0239 Article] )] . Phenyl chlorothionoformate used as the starting material ultimately generates carbonyl sulfide.

Trialkyl boranes

An even more convenient hydrogen donor is provided by trialkylborane-water complexes ["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)] 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 hydrogen 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.

cope

A variation of this reaction was used as one of the steps in the total synthesis of azadirachtin ["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|DOI: 10.1002/anie.200703027] :

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In another variation the reagent is the imidazole 1,1'-thiocarbonyldiimidazole (TCDI), for example in the total synthesis of pallescensin B. ["The first total synthesis of (±)-pallescensin B" Wen-Cheng Liu and Chun-Chen Liao Chem. Commun., 1999, 117–118 117 [http://rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=a808714h&JournalCode=CC Article] ] . 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.

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

References

External links

* [http://www.organic-chemistry.org/namedreactions/barton-mccombie-reaction.shtm Barton-McCombie @ organic-chemistry.org]
* Chemical & Engineering News [http://pubs.acs.org/cen/news/83/i35/8335notw1.html article on alkylborane reaction]