Enones catalysis

Although Lewis acid-catalyzed-Diels-Alder reacdons of enones are common, there are few repoiTS on the catalysis of Dieis-Alder reacdon of nltroalkenes The reacdon of nltroalkenes with alkenes in the presence of Lewis acids undergoes a different course of reacdon to give cyclic nltronates fsee Secdon 8 3 Knochei repotted an enhanced reacdvity and seiecdvity of the intramolecular Dieis-Alder reacdon using silica gei as Lewis acid in hexane fEq 8 19 ... 

The asymmetric epoxidation of enones with polyleucine as catalyst is called the Julia-Colonna epoxidation [27]. Although the reaction was originally performed in a triphasic solvent system [27], phase-transfer catalysis [28] or nonaqueous conditions [29] were found to increase the reaction rates considerably. The reaction can be applied to dienones, thus affording vinylepoxides with high regio- and enantio-selectivity (Scheme 9.7a) [29]. 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

The alternative 1,3-diX disconnection (29b) requires the addition of allyl alcohol to enone (32) and this worked well with BF catalysis. 

Further insight into the P-borylation reaction of a,P-enones (Scheme 2.32) showed that the reaction can be carried out in THF, and the catalyst generated in situ from CuCl (5 mol%), the imidazolium salt (5 mol%), and NaO Bu (10 mol%), to form the [Cu(O Bu) (NHC)] as the catalysis initiating species. In this case, stable boron enolate products are formed which need to be hydrolysed by methanol to the ketone products [114]. 

Whereas the mono- and the S/S-dithioether moieties have been used to date, the 1,3-dithianyl motif was used for the first time in 2005 by Ricci et al. as a new hybrid ligand in asymmetric catalysis. Hence, a series of new chiral oxazoline-1,3-dithianes have been successfully applied to the copper-catalysed conjugate addition of ZnEt2 to enones (Scheme 2.16). The expected products were obtained in almost quantitative yields and enantioselectivities of up to 69% ee. 

Cycloaddition of 2-cyanoalk-2-enones with several conjugated dienes proceeded under zinc chloride catalysis.636 Zinc halides have also shown reactivity with phenylacetylenes.637 Zinc chloride is an effective Lewis acid catalyst in the Diels Alder reactions of the keto esters and the effects on stereochemistry of catalysts used have been examined.638... 

In the next step of the sequence, the authors sought to introduce a hydroxy-methylene substituent at the unsubstituted 7-position of the enone. This bond construction can be carried out by conducting a Baylis-Hillman reaction with formaldehyde. In this instance, the authors used a modification of the Baylis-Hillman reaction which involves the use of a Lewis acid to activate the enone [26]. Under these conditions, the enone 42 is treated with excess paraformaldehyde in the presence of triethylphosphine (1 equiv), lanthanum triflate (5 mol%), and triethanolamine (50 mol%). It is proposed that the lanthanum triflate forms a complex with the triethanolamine. This complex is able to activate the enone toward 1,4-addition of the nucleophilic catalysts (here, triethylphosphine). In the absence of triethanolamine, the Lewis acid catalyst undergoes nonproductive complexation with the nucleophilic catalyst, leading to diminution of catalysis. Under these conditions, the hydroxymethylene derivative 37 was formed in 70 % yield. In the next step of the sequence, the authors sought to conduct a stereoselective epoxidation of the allylic... 

Certain catalysts promote the reduction of ketones with organosilanes. The reduction of acetophenone with Et3SiH is catalyzed by the diphosphine 65 and gives only a small amount of overreduction to ethylbenzene.377 Aryl alkyl enones and ynones are reduced to the corresponding alcohols with triethoxysilane and the titanium-based catalyst 66.378 Trichlorosilane reduces acetophenone in 90% yield with /V-formylpyrrolidinc catalysis.379... 

Dienes such as 90 can be accessed by a multi-component reaction under ruthenium catalysis involving an allene 88 and an enone (methyl vinyl ketone in this case), with cerium(m) chloride as an additive in DMF (Scheme 26).95,96 With an allene concentration of 0.25 M, yields are moderate to good. Different ruthenium catalysts and additives were tested in order to optimize this reaction. CpRu(COD)Cl 89 and CpRu(MeCN)3PF6 appeared to be more versatile ones. The mono-, di-, tri-, and tetrasubstituted allenes have been investigated with methyl vinyl... 

With conjugated enone substrates, the alkoxymetallation leads to the formation of a metal enolate that can undergo a facile protonation to accomplish the hydroalkoxylation. Following this mechanism, various /3-alkoxyketones were obtained in good yields by the addition of primary and secondary alcohols to methyl vinyl ketone under cationic Pd(n) catalysis.443 Similarly, [Rh(COD)(OMe)]2 was found to catalyze the hydroalkoxylation of both methyl vinyl ketone and phenyl vinyl ketone (Equation (121)).444... 

L. V. Dinh, J. Gladysz, Transition Metal Catalysis in Fluorous Media Extension of a New Immobilization Principle to Biphasic and Monophasic Rhodium-Catalyzed Hydrosilylations of Ketones and Enones , Tetrahedron Lett. 1999, 40,8995. 

E. J. Corey, F.-Y. Zhang, Mechanism and Conditions for Highly Enantioselective Epoxidation of a,p-Enones Using Charge-Accelerated Catalysis by a Rigid Quaternary Ammonium Salt , Org. Lett. 1999,1,1287-1290. 

Under the catalysis of MeONa, MeOH can react with 1,2-allenyl ketones to give /i-methoxy-/3,y-unsaturated enones 441, which undergo migration of the C=C bond to afford the more favorable /3-methoxy-a,/3-unsaturated enones [197-199]. 

In almost the same manner, tandem hydroformylation/aldol condensation aldol condensation of ketoolefins, such as p,y-unsaturated ketones, gives a single cyclization product under acid catalysis. Similar to the stepwise reaction, the in situ generated aldehyde preferentially acts as the electrophilic carbonyl component, while the ketone acts as the nucleophilic enol to form the five-membered ring product. Subsequent dehydration and hydrogenation of the resulting enone readily occurs under the reductive reaction conditions used (Scheme 30) [84],... 

For these and similar reactions recently a variety of Lewis acidic aluminium, rare earth metals, and titanium alkoxides have been applied. Alkoxides have the additional advantage that they can be made as enantiomers using asymmetric alcohols which opens the possibility of asymmetric catalysis. Examples of asymmetric alcohols are bis-naphtols, menthol, tartaric acid derivatives [28], Other reactions comprise activation of aldehydes towards a large number of nucleophiles, addition of nucleophiles to enones, ketones, etc. 

Protection of a carbonyl function as the 1,3-dioxolane can be efficiently carried out with 1,2-bistrimethylsiloxyethane and EGA catalysis. Scheme 14 [38]. Application to 1-menthone leads to 91% of the acetal without epimerization at C-2. The reaction is not efficient for conjugated enones [38]. 

Under the conditions of ruthenium catalysis, alcohols and allylic acetates couple to form enones,... 

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