Alcohols reductive deoxygenation

Furfural. Furfural is readily obtainable from dehydration of pentoses. Reduction of furfural can lead to a variety of products that are more volatile, more stable and possibly also more useful than furfural itself. Selective reduction of the aldehyde moiety leads to furfuryl alcohol (Scheme 15), whereas further reduction of the furan core will lead to tetrahydrofurfuryl alcohol. Reductive deoxygenation can result in the formation of either 2-methylfuran or 2-methyltetrahydrofuran, which can be used as liquid fuels or solvents. 

Scheme 5.9 illustrates some of the conditions that have been developed for the reductive deoxygenation of alcohols. Entries 1 to 4 illustrate the most commonly used methods for generation of thiono esters and their reduction by tri-M-butylstannane. These include formation of thiono carbonates (Entry 1), xanthates (Entry 2), and thiono imidazolides (Entries 3 and 4). Entry 5 is an example of use of dimethyl phosphite as the hydrogen donor. Entry 6 uses r .s-(trimethylsilyl)silane as the hydrogen atom donor. 

There are also examples in which phosphate esters of saturated alcohols are reductively deoxygenated.229 Mechanistic studies of the cleavage of aryl dialkyl phosphates have indicated that the crucial C-O bond cleavage occurs after transfer of two electrons.230... 

Radical deoxygenation. Tosylates of primary alcohols undergo deoxygenation in 75-100% yield by treatment with Nal and Bu,SnH in DME at 80°. This radical reduction can be applied to tosylates of secondary alcohols, but the rate of reaction is slower and yields are only moderate.1... 

Tri-/ -butyltin hydride also serves as a hydrogen-atom donor in radical-mediated methods for reductive deoxygenation of alcohols.131 The alcohol is converted to a thiocarbonyl derivative. These thioesters undergo a radical reaction with tri-w-butyltin... 

This procedure gives good yields from secondary alcohols and, by appropriate adjustment of conditions, can also be adapted to primary alcohols.132 Scheme 5.7 illustrates some of the conditions which have been developed for the reductive deoxygenation of alcohols. 

REDUCTIVE DEOXYGENATION OF ALCOHOLS AND KETONES N,N,N ,N -Tctramethyldiamldophosphorochloridate. REFORMATSKY REACTION Ethyl Ui-chloroacetate. Lithium diisopropylamide. REVERSE WITTIG REACTION Triphenyl-aisine oxide. 

Two types of reductions of epoxides, reduction giving alcohols and deoxygenation to alkenes, are dealt with in this chapter, because of their wide applicability in organic synthesis. These reactions provide access to alcohols and to alkenes when combined with appropriate methods for the synthesis of the starting epoxides, but the two-step conversion of alkenes to alcohols via epoxides is the more important. 

Reductive deoxygenation-rearrangement of 2-yne-l,4-diols to 1,3-dienes is a useful synthetic procedure since a large variety of ynediols are available in a few steps by sequential reaction at both ends of acetylene with aldehydes. Acetylenic 1,4-diols can be deoxygenated reductively by lithium aluminium hydride to form conjugated dienes of high stereoisomeric purity (equation 9). A modification to this procedure is the use of acetylenic 1,4-diol mono-THP derivative. Allenic tertiary alcohols which are intermediates in the reaction can be separated and subjected to reductive elimination rearrangement... 

Hong, F.-T., Paquette, L. A. BusSnH-catalyzed Barton-McCombie deoxygenation of alcohols. Single-step process for the reductive deoxygenation of unhindered alcohols. Chemtracts 1998,11, ST-72. 

Protection or reductive deoxygenation of alcohols and ketones. Ireland et al.2 have found that N,N,N, N -tetramethylphosphoroiiamidates (TMPDA derivatives) of alcohols and of ketone enolates are reduced in high yield by lithium-ethylamine. They are readily prepared by phosphorylation of alcoholate or enolate anions. The complete sequence is as follows. The alcoholate anion is simply prepared by treatment of an alcohol with a slight molar excess of n-butyllitliium. The enolate anions of saturated ketones are prepared by treatment with lithium diisopropylamide. In the case of a,/J-unsaturated ketones, lithium-ammonia reduction or conjugate organometallic addition is suitable. For phosphorylation of the Jnion a fivefold excess of N,N,N, N -tetramethyldiamidophosphorochloridate in 4 ] dimethoxyethane (or THF)-N,N,-N. N -tetramethylethylenediamine (TMEDA) is used. The reaction is complete after... 

The two segments were joined by addition of the lithium reagent derived from halogen-metal exchange of iodide 395 to aldehyde 391 to produce a mixture of diastereomeric alcohols 396. As simple reductive deoxygenation proved... 

Deoxygenation of alcohols. Thionocarbonates or xanthates of primary or secondary alcohols are deoxygenated in 89-100% yield by radical reduction with (C2H,)jSiH and (ChH5C())202. 

Thus, as shown by the example in Scheme 42, sequential treatment of the trime-thylsilyl ethers of a variety of tertiary alcohols with oxalyl chloride and the parent thionohydroxamic acid furnishes mixed oxalate esters which undergo reductive deoxygenation on subsequent reaction with a tertiary thiol in refluxing benzene [46]. The selectivity of this method for tertiary alcohols arises as a consequence of the relativity slow rates of decarboxylation of primary and secondary alkoxycarbonyl radicals. 

Reductions. The InCIj-Bu SnH/PhjP combination (catalytic in indium) reduces acid chlorides to aldehydes." Aryl ketones and secondary benzyl alcohols are deoxygenated with hydrosilanes using InCl, as cataly.st." Deoxygenative allylation of aryl ketones occurs when allyltrimethylsilane is added. ... 

Reductive deoxygenation of esters. Under irradiation, esters of nonprimary aliphatic alcohols are reduced to ethers and hydrocarbons (equation I). The relative amounts of the two products are profoundly affected by the R group. ... 

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