Deoxygenation, reductive

Table 15.3 gives some examples of reductive deoxygenation of 2-acyl and 3-acyl indoles. 

The coupling product is converted to the allyl mesylate 6. This is reductively cyclized to yield bis(cyclopentano)annulated methylenecyclooctanol 7a, which furnishes the hydrocarbon 7b on reductive deoxygenation. 

A direct attack of nucleophiles on the sulfur atom of the sulfone or sulfoxide group in acyclic or large-ring sulfones and sulfoxides is rather rare, or unknown, excluding metal hydride reductions and/or reductive deoxygenations. The situation is completely different in the three-membered ring systems. 

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... 

Another particularly reactive form of titanium is generated by including 0.25 equivalent of I2. This reagent permits low-temperature reductive deoxygenation to alkenes.241... 

As vicinal dibromides are usually made by bromination of alkenes, their utility for synthesis is limited, except for temporary masking of a double bond. Much more frequently it is desirable to convert a diol to an alkene, and several useful procedures have been developed. The reductive deoxygenation of diols via thiono carbonates was... 

The two-electron reductive cleavage of C-O from 188 to 189 was followed by the reductive deoxygenation of 189 and the formation of a linear bridging... 

Reduction to Alkanes. Carbonyl groups can be reductively deoxygenated to methylene functions if both of the two steps represented by Eqs. 1 and 2 proceed to completion. With aldehydes, this process leads to the transformation of the CHO group into a CH3 group. 

TABLE 4. Dienes and polyenes through reductive deoxygenations... 

A new synthesis of olefins by the reductive deoxygenation of vicinal diols using tungsten(iv) compounds, e.g. K2[WCl6], has been described. The process appears to be the reverse of the reaction of OsO with olefins and to involve transition state (90) since, in corresponding reactions, M0CI4 only forms... 

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. 

Both unsymmetrical diols and alkenes can be prepared by applying these methods to mixtures of two different carbonyl compounds. An excess of one component can be used to achieve a high conversion of the more valuable reactant. A mixed reductive deoxygenation using TiCl4/Zn has been used to prepare 4-hydroxytamoxifen, the active antiestrogenic metabolite of tamoxifen. 

---