Metal hydrides enones

Reactions of highly electron-rich organometalate salts (organocuprates, orga-noborates, Grignard reagents, etc.) and metal hydrides (trialkyltin hydride, triethylsilane, borohydrides, etc.) with cyano-substituted olefins, enones, ketones, carbocations, pyridinium cations, etc. are conventionally formulated as nucleophilic addition reactions. We illustrate the utility of donor/acceptor association and electron-transfer below. 

Next to the cyclopropane formation, elimination represents the simplest type of a carbon-carbon bond formation in the homoenolates. Transition metal homoenolates readily eliminate a metal hydride unit to give a,p-unsaturated carbonyl compounds. Treatment of a mercurio ketone with palladium (II) chloride results in the formation of the enone presumably via a 3-palladio ketone (Eq. (24), Table 3) [8], The reaction can be carried out with catalytic amounts of palladium (II) by using CuCl2 as an oxidant. Isomerization of the initial exomethylene derivative to the more stable endo-olefin can efficiently be retarded by addition of triethylamine to the reaction mixture. 

Fp-substituted enones and enals undergo cyclocarbonylations on treatment with metal hydrides or metal alkyls to provide y-lactones (Scheme 1.15) [45], Similarly, electron-rich primary amines afford dihydropyrrolones with iron-substituted (Z)-enals in the presence of titanium tetrachloride and triethylamine [46],... 

A convenient method for the specific introduction of 2H or 3h (or both) into a molecule is by ketone reduction with labeled metal hydride. Beale and MacMillan (10) have utilized this method for the preparation of GAs labeled at the 1, 2 or 3 positions from GA3 or GA7 (Figure 12). One point of interest is the lithium borohydride reduction of the enone formed by manganese dioxide oxidation of GA3 or GA7. When the reaction is carried out in anhydrous tetrahydrofuran it proceeds in two steps. Initially the lithium enolate is formed which incorporates a proton at carbon-2 from the acid used in the work-up, forming the 3 ketone. This ketone is reduced to the 3 -alcohol by the borohydride which is decomposed more slowly than is the lithium enolate. Thus it is possible to introduce two different labels in a single reaction. 

Fig. (2). The cyclization of enone (9), gives origin of two Cyclized products (10) and (11). Ketone (10), Ketone (10) is converted to the saturated ketone (14)under standard organic reactions.Bromination and dehydrobromination of ketone (14) yields the a,P-unsaturated ketone (IS), which on subjection to catalytic hydrogenation affords (16) and this on reduction, produces alcohol (17). The compound (13) yields (18) by standard reactions that are used for the transformation of (12) to (16). Reduction with metal hydride followed by oxidation affords ketone (11), which is converted to alcohol (17)...

Stork and Danheiser have developed a highly useful procedure for the synthesis of 4-alkylcyclohex-2-enones, which involves a -alkylations of cross-conjugated lithium dienolates of 3-alkoxycyclohex-2-enones, followed by metal hydride reduction of the carbonyl group and hydrolysis (Scheme 30). Numerous applications of this procedure have been reported.Two different alkyl groups may be introduced at the 6-position of a cyclohex-2-enone derivative without difficulty. While dialkylation is generally not a problem in alkylations of cross-conjugated dienolates of cyclohex-2-enones, it was observed when relatively acidic 3-chlorocyclohex-2-enones were employed. ... 

The second strategy involves addition of a group 14 metal hydride to palladium. For example, a combination of either tributyltin hydride or various silicon hydrides and palladium catalyst are efficient systems that affect the conjugate reduction of enones with high chemoselectivity and regioselectivity. 

Reductive aldol reaction of a,(5-unsaturated esters and enones with aldehyde mediated by a transition metal hydride complex and a hydride source, such as hydrosilane, is a versatile process to produce p-hydroxy carbonyl compounds (Scheme 15a) [21]. This reaction is thought to be an alternative transformation of Lewis acid-catalyzed Mukaiyama-type aldol reaction with silyl enol ethers or silyl ketene acetals (Scheme 15b). 

Optimum conditions for the reduction of saturated ketones by the complex reducing agent formed from sodium hydride, sodium t-amylate, and Ni" acetate (NiCRA) have been delineated.Reoxidation of the secondary alcohol products is dramatically postponed by the addition of alkali- or alkaline-earth-metal salts, and catalytic ketone reductions are achieved with NiCRA-MgBr2 mixtures. Full details of the reducing properties of various complex metal hydrides (12) of copper, formed by reaction of UAIH4 with appropriate lithium methylcuprates [equation (1)], have been published for example enones are reduced pre-... 

Reduction of Ketones and Enones. Although the method has been supplanted for synthetic purposes by hydride donors, the reduction of ketones to alcohols in ammonia or alcohols provides mechanistic insight into dissolving-metal reductions. The outcome of the reaction of ketones with metal reductants is determined by the fate of the initial ketyl radical formed by a single-electron transfer. The radical intermediate, depending on its structure and the reaction medium, may be protonated, disproportionate, or dimerize.209 In hydroxylic solvents such as liquid ammonia or in the presence of an alcohol, the protonation process dominates over dimerization. Net reduction can also occur by a disproportionation process. As is discussed in Section 5.6.3, dimerization can become the dominant process under conditions in which protonation does not occur rapidly. 

Allylic alcohols are isomerized via direct interaction of the ruthenium atom with alcohol. /3-Elimination of ruthenium hydride from metal alkoxide yields a ruthe-nium-enone species C which undergoes insertion of the olefinic moiety into the Ru-H to form an oxyallylic intermediate D. As a result, the hydrogen atom shifts from the a- to y-position of the allylalcohol. Protonolysis of the oxyallylic species leads to a saturated carbonyl compound and cationic unsaturated species, [CpRu(PPh3)2] A. 

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