Catalytic reactions involving ketones

This section will give an outline of catalytic organic transformations where lanthanide alkoxides are, in particular, used as precatalysts. It must be assumed that other precatalysts underlie in situ formation of catalytically active Ln-O(alkoxide) moieties [229]. For example, in reactions involving ketones or aldehydes as substrates, enolate intermediates often act as the real active catalyst component. Many reactions are conducted in alcoholic solutions like MeOH or tBuOH or ethylene glycol [5]. Alcohols are often needed as proton source and their steric bulk can influence the product selectivities [230]. 

The majority of catalytic enantioselective allylation reactions involve the chiral Lewis-acid-catalysed additions of allylsilanes or allylstannanes to carbonyl compounds. Monothiobinaphthol has been used by Woodward et al. as a chiral promoter in the enantioselective catalytic allylation of aryl ketones with impure Sn(allyl)4, prepared from allyl chloride, air-oxidised magnesium and SnCl4. Therefore, the allylation of arylketones in these conditions was achieved very efficiently, since the corresponding allylic alcohols were formed in... 

Coupling of organostannanes with halides in a carbon monoxide atmosphere leads to ketones by incorporation of a carbonylation step.249 The catalytic cycle is similar to that involved in the coupling of alkyl or aryl halides. These reactions involve Reactions involving a migration of one of the organic substituents to the carbonyl carbon, followed by... 

These ketones can also be used in kinetic resolutions.107 The carbohydrate-derived ketones have been used in conjunction with acetonitrile and H202. The reactions are believed to proceed through dioxiranes generated by a catalytic cycle involving a peroxyimidic acid.108... 

Early work on the asymmetric Darzens reaction involved the condensation of aromatic aldehydes with phenacyl halides in the presence of a catalytic amount of bovine serum albumin. The reaction gave the corresponding epoxyketone with up to 62% ee.67 Ohkata et al.68 reported the asymmetric Darzens reaction of symmetric and dissymmetric ketones with (-)-8-phenylmenthyl a-chloroacetate as examples of a reagent-controlled asymmetric reaction (Scheme 8-29). When this (-)-8-phenyl menthol derivative was employed as a chiral auxiliary, Darzens reactions of acetone, pentan-3-one, cyclopentanone, cyclohexanone, or benzophenone with 86 in the presence of t-BuOK provided dia-stereomers of (2J ,3J )-glycidic ester 87 with diastereoselectivity ranging from 77% to 96%. 

The involvement of transition-metal allenylidene complexes in homogeneous catalysis was reported for the first time by B. M. Trost and co-workers in 1992 (Scheme 35) [293-295]. The catalytic reactions allowed the preparation of a wide variety of tetrahydropyranyl and furanyl p,y-unsaturated ketones starting from hydroxy-functionalized alkynols and allylic alcohols, the key step in the catalytic... 

The reaction mechanism proposed for the addition of organostannanes [29] is similar to that for organoboronic acids. An example of the reaction of methyl vinyl ketone 42 is outlined in Scheme 3.15. The catalytic cycle involves a cationic rhodium complex G, phenylrhodium H, and oxa-n -allylrhodium I. Stannyl enol ether 44 is formed by the reaction of oxa-n -allylrhodium I with Me3SnBF4, which upon hydrolysis gives the ketone 43. The lower yields in the absence of water were explained by the further reaction of 44 with methyl vinyl ketone 42. The rapid hydrolysis with water may prevent such oligomerization. 

Trust s group has shown that another selective reaction involving C—O bond formation followed by rearrangement and C—C bond formation occurred when Cp-containing ruthenium complexes were used as catalytic precursors. With RuCl(Cp)(PPh3)2 in the presence of NH4PF6, an additive known to facilitate chloride abstraction from the metal center, the addition of allylic alcohols to terminal alkynes afforded unsaturated ketones [46, 47]. It has been shown that the key steps are the... 

Although the catalytic reactions described above involve mononuclear Rh and Rh complexes, dinuclear Rh compoimds have also been studied as catalyst precursors in oxygenation reactions. The system [Rh2(p.-OAc)4]/ f-BuOOH is effective in the oxidation of cyclic alkenes such as cyclopentene, cyclohexene and cycloheptene, mainly to o, /i-unsaturated ketones and allylic acetates, but with poor yields (Eq. 4) [30,31]. 

Promising examples of the catalytic asymmetric Darzens condensation, which yields an epoxide product via carbon-carbon and carbon-oxygen bond formation, have been reported recently by two groups (Scheme 10.11). Toke and co-workers used crown ether 24 in the reaction to form the a,P-unsaturated ketone 78 [38b] with 64% ee, whereas the Shioiri group used the cinchona-derived salt 3a [52], which resulted in 78 with 69% ee. The latter authors propose a catalytic cycle involving generation of a chiral enolate in situ from an achiral inorganic base... 

S,y-Unsaturated a-keto esters such as tnms -MeCH=CHCOCC>2Et undergo enan-tioselective reaction with nitromethane, using new catalytic auxiliaries based on cinchona alkaloids.145 Carried out at -20 °C in DCM, the organocatalysts give high conversion, predominantly reaction at ketone only (typically <5% of product involves simultaneous addition to the alkene), and up to 97% ee. 

The second and final example of a computational study of a reaction mechanism that will be considered here is drawn from work carried out by the author s group and serves to illustrate some of the points discussed in the previous section. The reaction in question is the catalytic hydrogenation of ketones by ruthenium(bisphosphine) (diamine) complexes. This reaction was developed by the group of Professor Ryoji Noyori20 and was also studied by the group of Professor Robert Morris. The initial computational work discussed here was a collaboration with Professor Morris. It was motivated by the desire to test the feasibility of a proposed mechanism, involving a key ruthenium dihydride complex, that would transfer a hydride (from Ru) and a proton (from N) in a concerted step to the ketone (Figure 10.9). 

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