Carbonates, Barton-McCombie deoxygenation

Ammonium formate hydrogenolysis of 7t-allyl-palladium(II) complexes formed via oxidative addition of Pd(0) to propargylic carbonate esters is a useful method for preparing disubstituted alkynes, as depicted below. This method of deoxygenation avoids the use of expensive and toxic reagents often associated with the free-radical Barton-McCombie deoxygenation. °... 

The other type of radical chemistry of importance in the carbohydrate field is one-electron reductions. A handful of these reactions (such as the metallic Zn reduction of acetobromoglucose to triacetylglucal) have been used in synthesis for decades, but, starting with the Barton-McCombie deoxygenation of sugars in the mid-1970s there has been an explosion of interest, as increasingly sophisticated cascades of elementary radical steps have been devised. Such reactions are driven by the homolysis of weak bonds such as Sn-H or N-O under conditions of photolysis or mild thermolysis. Nature uses a similar basic principle in Type II ribonucleotide reductases, where the weak bond in question is the cobalt-carbon a bond in the corrin cofactor. ... 

Addition of carbon radicals to carbon-carbon double bonds is an important reaction that can be carried out under hydrogen transfer conditions [27], Peroxides are usually used as radical precursors and an application of this chemistry is presented in Scheme 3 (Eq. 3.1). More recently, reduction of alkyl radical by C-H hydrogen donor has been examined in order to find an environmentally friendly alternative to tin hydride. Zard has reported a simple and cheap alternative to tin hydride for Barton-McCombie deoxygenation reactions [80]. Heating of xanthates derived from carbohydrates in 2-propanol in the presence of dilauroyl peroxide affords the de-oxygenated products in good yields (Scheme 17, Eq. 17.1). 2-Propanol functions as... 

Radical deoxygenation of alcohols is important, and the reduction of xanthates prepared from alcohols, with Bu3SnH in the presence of AIBN is called the Barton-McCombie reaction (eq. 2.13) [37-51]. The driving force for the reaction is the formation of a strong C=0 bond from the C=S bond, approximately 10 kcal/mol stronger. This reaction can be used for various types of substrates such as nucleosides and sugars. Though methyl xanthates, prepared from alcohols with carbon disulfide and methyl iodide under basic conditions are very frequently used, other thiocarbonates, as shown in eq. 2.14, can also be employed. 

Besides carbon-halogen bonds, Bu3SnH is used routinely for hydrogenolysis of various carbon-heteroatom bonds. The deoxygenation from alcohols has been performed by a Barton-McCombie reaction where the addition of tin hydride to the triple bond is suppressed (Equation (2)).29-31... 

The products were obtained in excellent yields after simple hexane extraction. The removal of bromine and iodine proceeded smoothly. The removal of PhSe afforded methylcyclohexanone, indicating that the decarbonylation of acyl radicals takes place. The efficiency of deoxygenation of alcohols (Barton-McCombie reaction) is independent of the type of thiocarbonyl derivative (i.e. 0-arylthio-carbonate, O-thioxocarbamate, thiocarbonylimidazole or xanthate), as previously reported for (TMS)3SiH in organic solvents.25... 

Since its introduction by Barton and McCombie, the deoxygenation of thionocarbonyl derivatives of alcohols has become an important synthetic reaction and a valuable method for the generation of carbon-centered radicals.3-4 Xanthates, thionobenzotes, thionocarbonyl imidazolides, aryloxy thionocarbonate, N-phenylthionocarbamates and oxalate esters are conveniently deoxygenated with tin or silicon hydrides in boiling benzene or toluene.4-5... 

In their seminal paper, Barton and McCombie describe a method to deoxygenate secondary alcohols through a radical chain mechanism. This process was presented as an alternative to the standard conditions of derivatization of an alcohol to a tosylate/mesylate followed by reduction. Such polar processes are problematic with hindered carbon centers and can lead to rearrangements and/or eliminations if carbocationic intermediates are involved. 

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