Deoxygenation of ketones

An interesting deoxygenation of ketones takes place on treatment with low valence state titanium. Reagents prepared by treatment of titanium trichloride in tetrahydrofuran with lithium aluminum hydride [205], with potassium [206], with magnesium [207], or in dimethoxyethane with lithium [206] or zinc-copper couple [206,209] convert ketones to alkenes formed by coupling of the ketone carbon skeleton at the carbonyl carbon. Diisopropyl ketone thus gave tetraisopropylethylene (yield 37%) [206], and cyclic and aromatic ketones afforded much better yields of symmetrical or mixed coupled products [206,207,209]. The formation of the alkene may be preceded by pinacol coupling. In some cases a pinacol was actually isolated and reduced by low valence state titanium to the alkene [206] (p. 118). 

Phenols are transformed into aryl chlorides by PhPCl4779. Aromatic and aliphatic aldehydes give, in high yields, gem dibromides with combination of triphenylphosphite and bromine780. Brominative deoxygenation of ketones is achieved by 2,2,2-tribromo-l,2,3-benzodioxaphosphone781. Allylic alcohols are transformed into allylic iodides by reaction with P2I4782. 

In actual use, H2 is not a good reagent for deoxygenation of ketones and aldehydes. Deoxygenation can be accomplished by either the Clemmensen reduction (under acidic conditions) or the Wolff-Kishner reduction (under basic conditions). 

Vinyl triflates are important intermediates, since they can be used as synthetic precursors for vinyl cations and alkylidene carbenes. and as substrates for regiospecific coupling reactions.2 3 Vinyl triflates are also valuable intermediates in a mild, two-step procedure for the deoxygenation of ketones.4 5 These new triflating reagents are highly reactive and easy to prepare and handle. When compared with... 

The direct deoxygenation of ketones via the Clemmensen reduction requires strong acidic conditions and high temperatures hence it is not suitable for acid-sensitive compounds. 

Treatment of enolates with (R0)2P(0)C1 also results in 0-trapping to yield the corresponding enol phosphates. Dissolving metal reduction of enol phosphates is a useful procedure for the deoxygenation of ketones with concomitant, regiospecific formation of the alkene. ... 

Deoxygenation of ketones. Reductive deoxygenation of ketones can be effected with an aluminum source and a Lewis acidic aluminum reagent. In the case of diaryl ketones i-BuAICh can be used as the hydride source as well as the Lewis acid. 

Deoxygenation of ketones. A modified Clemmensen reduction uses zinc and A1C1.1 6H2O in aqueous THF. Hydrochloric acid is generated in situ. On the other hand, ultrasound promotes the reduction with Zn-HOAc in a selective manner (of 3-oxosteroids in the presence of ketone group at C-17 or C-20). Transpositional reduction of steroidal 4-en-3-ones (to 3-enes) can be achieved in 15 minutes. 

Deoxygenation of ketosteroids. The deoxygenation of ketones with this combination has been extended to ketosteroids. -Cholestene can be obtained in about 70% yield from 5a-cholestane-3-one with trimethylchlorosilane and zinc dust in THE at reflux. The reaction fails with 6-, 7-, 12-, 17-, and 20-ketones. This result suggests that a very bulky intermediate is involved. Thus it is possible to selectively deoxygenate diketones. For example, A -Sa-cholestene-... 

Deoxygenation of ketones to tritiated alkenes via the Shapiro-reaction ... 

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