Barton–McCombie deoxygenation

Barton–McCombie deoxygenation
Named after	Derek Harold Richard Barton Stuart W. McCombie
Reaction type	Organic redox reaction
Identifiers
Organic Chemistry Portal	barton-mccombie-reaction
RSC ontology ID	RXNO:0000134

The Barton–McCombie deoxygenation is an

and Stuart W. McCombie.

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

Mechanism

The

for this reaction. The alkyl radical 5 then abstracts a hydrogen atom from a new molecule of tributylstannane generating the desired deoxygenated product (6) and a new radical species ready for propagation.

Variations

Alternative hydrogen sources

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

Trialkyl boranes

An even more convenient hydrogen donor is provided by

contaminated with small amounts of water.

In this

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 (CH₃)₃B.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

with an energy similar to that of the homolysis reaction in tributylstannane 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.^{[citation needed]}

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.^[8]

See also

• Chugaev elimination

References

External links

• Barton-McCombie @ organic-chemistry.org
• Chemical & Engineering News article on alkylborane reaction