Transition metal hydrides unsaturated carbonyl compounds

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. 

Epoxides can isomerize under the influence of transition metal catalysts. This formal 1,2-hydride shift is a method to prepare unsaturated carbonyl compounds from epoxides (Equation 54) <1998T1361>. This method has been extended as a double epoxide isomerization-intramolecular aldol condensation (Equation 55) <1996JOC7656, 1998TL3107>. m-Epoxides are isomerized to /ra r-epoxides under ruthenium catalysis <2003TL3143>. 

The olefinic bond of a,g-unsaturated carbonyl compounds can also be reduced by two transition metal hydrides, NaHFe2(CO)881 and NaHCr2(CO)10 82 Neither reagent reduces nitriles, ketones, aldehydes or non-conjugated carbon-carbon double bonds. 

Nucleophiles used in the seminal papers by Tsuji and co-workers were mostly stabilized carbon nucleophiles, and the method found an early synthetic application in a preparation of steroids." It soon became evident that many other types of nucleophiles could be used. In particular, hydride ion equivalents led to l-olefinsf ° " (see Sect. V.2.3.1), Silyl and stannyl enolates of simple ketones and aldehydes and esters can be aUylated, as well as allyl enol carbonates (see Sect. V.2.1.4), This is an indirect a-aUylation of ketones, aldehydes, and esters. Enol derivatives can take another reaction course under Pd(0) catalysis (Scheme 2). Thus, oxidation to a,/3-unsaturated carbonyl compounds ensues if reactions are performed in acetonitrile under precise sources of catalyst precursor. "" "" A full discussion on the dichotomy of allylation-oxidation has been published, as well as a comparison of the usefulness of several transition metals as catalysts in allylation of nucleophiles. ... 

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). 

The aim of this chapter is to examine the application of well-defined N-heterocyclic carbene (NHC) complexes, as well as the systems prepared in situ which involve free NHCs or the precursor salt, for the reduction of unsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reactions are based on electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types according to the hydride source used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

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