Deoxygenation aromatic 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. 

Deoxygenative allylation of aromatic ketones can be performed by the InCI (-catalyzcd reaction with HSiMe2Cl and 10 (Equation (17)).85... 

A combination of chlorodimethylsilane and allyltrimethylsilane effectively promotes the deoxygenative allylation of aromatic ketones to terminal alkenes in the presence of a catalytic amount of InCl3 (Equation (94)). The choice of solvent is important the reaction of acetophenone proceeds only in dichloromethane or 1,2-dichloroethane. Aldehydes and aliphatic ketones give complicated mixtures. 

In the presence of excess acetyl halide and a tin(II) catalyst aromatic acetals react with allyltrimethylsilane to give a-allylbenzyl halides in good yield by double substitution of the acetal alkoxy groups (Scheme 10.127) [365]. The indium-catalyzed tandem reaction using a hydrosilane-allylsilane system enables deoxygenative allylation of aromatic ketones [366]. 

Deoxygenation of carbonyl compounds. Hydrous Sn02 is prepared from SnCU by precipitation with aqueous ammonia then drying and calcination at 300°C for 5 h. The reduction of aliphatic carboxylic acids gives the corresponding alcohols, whereas aromatic acids are further reduced to the hydrocarbons. Aromatic ketones also give hydrocarbons. 

Deoxygenation of aromatic ketones. Borane-dimethylamine, together with TiCl4, is a mild and efficient reagent for achieving the reduction. 

Aromatic ketones are deoxygenated by reaction with EtjSiH-TiCl. ... 

Deoxygenation of carbonyl compounds. This system, which has been shown to reduce alcohols to hydrocarbons, also reduces aliphatic and aromatic ketones and aromatic aldehydes to hydrocarbons. Yields are higher when the BF3 is scrubbed by HF before use. The reaction proceeds stepwise, since intermediate alcohols can be isolated. A large excess of the silane is required. 

Clemmensen reduction involving reaction of ketones with amalgamated zinc in the presence of hydrochloric acid is one of the classical methods of deoxygenation . It works well with aromatic ketones but its applicability to alicyclic systems is severely limited. The strongly acidic conditions and high temperature make this procedure too stringent, especially if other functional groups are present. Quite often side reactions are observed and complex mixtures of products are obtained. Nonetheless, the method has found favour in many cases (equation 60) ... 

Two related procedures for reductive coupling of aromatic ketones with titanium-based reagents have been reported (Tyrlik and Wolochowicz, 1973 Mukaiyama et al., 1973). The deoxygenative dimerization of aromatic carbonyl compounds can be achieved with low-valent tungsten complexes formed from tungsten hexachloride and alkyllithium as well (Sharpless et al., 1972). 

Reaction with Allylic Halides, Alcohols, and their Derivatives. Allylation of allyl and propargyl trimethylsilyl ethers as well as benzyl and propargylic alcohol derivatives proceeds in the presence of a catalytic amount of Lewis acids, such as ZnC, TMS(OTf), and B(C6F5)3. Direct substitutions of the hydroxyl group of allylic, ben-zylic, and propargylic alcohols are catalyzed by HN(S02F)2, a rhenium-oxo complex, and InCls (eq 27). A combination of chlorodimethylsilane and allyltrimethylsilane effectively promotes the deoxygenative allylation of aromatic ketones in the presence of a catalytic amount of an indium compound, such as indium trihalide or metallic indium (eq 28). Allylation of cyclic allylic acetates with allyltrimethylsilane can be catalyzed by molecular iodine. ... 

Salicylaldehyde reacts with trimethylsilylketene dithioacetal in the presence of a Lewis acid to form the chroman 502, the product of a deoxygenative divinylation (Equation 208) <2001JOC3924>. This reaction can also be applied to salicylaldimines <2003JOC4947>. Treatment of 3,5-dibromosalicylaldehyde with methyl vinyl ketone (MVK) in the presence of DABCO leads to a chroman-4-ol as the major product <2002J(P1)1318>. A stereoselective one-pot synthesis of vy/z-fused chromans from salicylaldehydes, aromatic amines and cyclic enol ethers is carried out in the... 

Aromatic aldehydes and ketones can also be deoxygenated with hydrogen over a palladium charcoal catalyst. The reaction occurs because the aromatic ring activates the carbonyl group towards reduction. Aliphatic aldehydes and ketones are not reduced in this. 

Fig (4) The transformation of the ketone (24) to the cyclic ether (9) applying the standard organic reactions is described It wa subjected to three sequencial reactions with reagents mentioned for the conversion to cyclic ether (30). Isopropylation and by aromatization, it produces the phenol (31), which is converted to pisiferol (4). This on subjection to oxidation, esterification and deoxygenation respectively, furnish O-methyl pisiferate (5) and this is easily converted to pisiferic acid (1). 

Fig (14) Olefin (107) has been converted to cyclic ether (114) by standard reactions. Its transformation to enone (115) is accomplished by annelation with methyl vinyl ketone and heating the resulting diketone with sodium hydride in dimethoxyethane. The ketoester (116) is subjected to Grignard reaction with methyllithium, aromatization and methylation to obtain the cyclic ether (117). Its transformation to phenolic ester (119) has been achieved by reduction, oxidation and esterification and deoxygenation. 

Silyl ketones behave differently. While aliphatic silyl ketones are reduced to a-silyl alcohols, aromatic congeners give l,2-diarylalkynes. In the presence of MejSiCl, the purported oxaytterbiacyclopropane intermediate can be trapped. Yet another pathway appears when the reaction is conducted in the presence of another ketone. A tandem deoxygenation-acylation reaction occurs with the silyl ketones donating the acyl moieties. ... 

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