Acyclic ketones, hydrogenation

A study of the alkylation of the trimethylsilyl enol ether of octahydro-1 (2//)-naphthalenone reveals that the diastereoselectivity of the reaction is similar to that of the methylation of the corresponding lithium enolate (see Section 1.1.1.3.1.1.2 1.)89. Lewis acid cataly2ed phenyl-thioalkylations of the type indicated (i.e., 3 -> 4) have been used for a-alkylations of several cyclic and acyclic ketones, as well as aldehydes89. The easy removal of the phenylthio group by catalytic hydrogenation completes this convenient procedure for a-alkylation of carbonyl compounds89. 

Iodine-catalysed hydroperoxidation of cyclic and acyclic ketones with aqueous hydrogen peroxide in acetonitrile is an efficient and eco-friendly method for the synthesis of gem -dihydroperoxides and the reaction is conducted in a neutral medium with a readily available low-cost oxidant and catalyst.218 Aryl benzyl selenoxides, particularly benzyl 3,5-bis(trifluoromethyl)phenyl selenoxide, are excellent catalysts for the epoxidation of alkenes and Baeyer-Villiger oxidation of aldehydes and ketones with hydrogen peroxide.219 Efficient, eco-friendly, and selective oxidation of secondary alcohols is achieved with hydrogen peroxide using aqueous hydrogen bromide as a catalyst. Other peroxides such as i-butyl hydroperoxide (TBHP), sodium... 

The alkylation of asymmetric acyclic ketones takes place regioselectively on the most-substituted carbon, thus affording the syn isomers as major products. a-Hydroxyketones showed anti selective additions similar to that observed in related aldol, and Mannich-type additions (Scheme 2.39). Such selectivity is due to the preferred formation of the Z-enamine intermediate, stabilized by intramolecular hydrogen bonding between the hydroxy group and the tertiary amine of the catalyst [23]. 

One of the first examples of 8-hydrogen abstraction in acyclic ketones was the photocyclization of P-alkoxy ketones, in particular of P-ethoxypropiophenone (34), to the corresponding furanol derivatives 37 (Scheme 8.10). It was revealed that formation of enol 36 as well as a reversion to the starting ketone occur by 1,4-hydrogen transfer from the 1,5-biradical 35 causing the lower quantum efficiency for cyclization [12]. 

Lewis has reported that y-hydrogen abstraction in polycyclic ketones is more rapid than in acyclic ketones 68> and concluded that the rate increases are due to the increased number of frozen C—C bonds in the reactant. His measured activation parameters certainly support this interpretation. 

The lifetimes t of unfavorable conformations are determined by whatever decay reactions — physical and chemical — are available to each conformer. The decay rates r/1 of favorable conformations include rates of hydrogen abstraction. Quantum yields for hydrogen abstraction are determined by the relative rates of hydrogen abstraction and of decay, no matter what conformers they arise from. The type II reaction of acyclic ketones falls into this general class. The unit quantum yield for triplet state y-hydrogen abstraction in many ketones indicates that conformational change is faster than hydrogen abstraction which, in turn, is faster than any other form of decay. 

The catalytic hydrogenation of various cyclopropenones gives acyclic ketones 318 via the corresponding cyclopropanone intermediate 317 3 - 2,444,445 contrast, Pd/C promoted reaction of di-n-propylcyclopropenone delivers the a,j5-unsaturated aldehyde... 

Enamines show an amplified preference for the less substituted double bond at first this seems to contradict what we have just said, but the effect is greatest in cyclic ketones, e.g. 17, with cyclic amines.41 It is steric in origin and arises from the eclipsing of hydrogen atoms (A13 strain) shown in the more substituted enamine 19. Enamines of acyclic ketones can be persuaded to give only the less substituted regio-isomer by equilibration of the immonium salt in weak base.5... 

The asymmetric counteranion directed organocatalysis has been also applied to the enantioselective transfer hydrogenation of a,f)-unsaturated ketones employing catalyst 11, which involves a chiral cation such as a valine ester anunonium salt and a chiral binaphthol derived phosphate [23]. This combination, in the presence of the Hantzsch ester 4, is a very active and enantioselective system for the transfer hydrogenation of a variety of cyclic a,P-unsaturated ketones (Scheme 2.5). Acyclic ketones are also reduced but with slightly lower enantioselectivities. 

This general procedure for the synthesis of alkenes from the enamines, when applied to the acyclic enamines derived from the acyclic ketones by modification of hydroboration-elimination procedure, permits a facile, diastereospecific conversion to either (Z)- or (E)-alkene at will (A) The hydroboration of enamine with 9-BBN, followed by treatment with methanol gives (Z)-alkenes of 99% isomeric purity. (B) The hydroboration of the same enamine with borane-methylsulfide, followed by methanolysis and oxidation with neutral hydrogen peroxide gives ( )-alkenes of 99% isomeric purity (Scheme 24.2) [13a]. 

Transition-metal-catalyzed Baeyer-Villiger (BV) oxidations, namely, the transformation of cyclic and acyclic ketones into lactones and esters, respectively, has also become an important research topic in the past years owing to the wide applications of the products [2]. Owing to economic and environmental reasons, a growing attention has been paid to the replacement of organic peroxy acids, traditionally used as stoichiometric oxidants in the BV oxidation, by more atom-efficient and environmentally friendly oxidants such as molecular oxygen [3] or hydrogen peroxide [4],... 

To enhance the selectivity of hydrogen-peroxide oxidation, the group of Ziqiang Lei reported the unusual utilisation of aluminium trichloride as catalyst in the Baeyer-Villiger oxidation of cyclic and acyclic ketones, as shown in Scheme 18.30. 

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