Ketones aromatic, conjugate addition

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

The mechanism of the Fiesselmann reaction between methylthioglycolate and a,P-acetylenic esters proceeds via consecutive base-catalyzed 1,4-conjugate addition reactions to form thioacetal Enolate formation, as a result of treatment with a stronger base, causes a Dieckmann condensation to occur providing ketone 8. Elimination of methylthioglycolate and tautomerization driven by aromaticity provides the 3-hydroxy thiophene dicarboxylate 9. 

Several attempts to take advantage of the intermediate boron enolate to achieve tandem conjugate addition-aldol reaction have been proposed [71]. Recently, Chandrasekhar [72] reported the addition of triethylborane to methyl vinyl ketone followed by the in situ trapping of the enolate by aromatic aldehyde (Scheme 26). 

In basic aqueous solutions with a [Rh(COD)2]BF4 catalyst, phenyltin trichloride was found to react with aromatic aldehydes or unsaturated ketones. In the presence of a strong aqueous alkali (KOH) PhSnCU is readily hydrolysed and the products of this reaction, such as e.g. PhSn(OH)3 add to the carbonyl function of aldehydes or undergo conjugate addition to unsaturated ketones (Scheme 6.35). In the absence of KOH no reaction takes place at all. Yields are generally high [73]. 

Dicarbonyl donors are excellent Michael donors in asymmetric conjugate addition to a,p-nnsatnrated ketones. Wang and co-workers [79] applied chiral Cinchona-thiourea catalyst 131 to various carbon donors in the addition to aromatic enones. A diverse array of nucleophiles, mainly 1,3-dicarbonyls proceeded smoothly in the conjugate addition to a,p-unsaturated enone 132 (Scheme 29). 

Cho, C. S. Motoffisa, S.-I. Ohe, K. Uemura, S. Shim, S. C. A new catalytic activity of SbCl3 in Pd(0)-catalyzed conjugate addition of aromatics to a,/3-unsaturated ketones and aldehydes with NaBPh and arylboronic adds. 

Aluminum salen complexes have been identified as effective catalysts for asymmetric conjugate addition reactions of indoles [113-115]. The chiral Al(salen)Cl complex 128, which is commercially available, in the presence of additives such as aniline, pyridine and 2,6-lutidine, effectively catalyzed the enantioselective Michael-type addition of indoles to ( )-arylcrolyl ketones [115]. Interestingly, this catalyst system was used for the stereoselective Michael addition of indoles to aromatic nitroolefins in moderate enantiose-lectivity (Scheme 36). The Michael addition product 130 was easily reduced to the optically active tryptamine 131 with lithium aluminum hydride and without racemization during the process. This process provides a valuable protocol for the production of potential biologically active, enantiomerically enriched tryptamine precursors [116]. 

The photoaddition of alkanes onto electron-poor alkynes (e.g., propiolate or acetilendicarboxylate esters) can be accomplished by a radical conjugate addition reaction [7]. Radicals have been generated either via hydrogen abstraction from cycloalkanes or via electron transfer from 2-alkyl-2-phenyl-l,3-dioxolanes. In the first case, the irradiation was pursued on an alkane solution of an aromatic ketone (used as the photomediator) and the alkyne. Under these conditions, methyl propiolate was alkylated upon irradiation in the presence of 4-trifluoromethylacetophenone to form acrylate 48 in 97% yield (E/Z= 1.3 1 Scheme 3.31) [78]. 

A catalytic tandem conjugate addition-aldol cyclization950 allowed the formation of five- and six-membered ring products from aromatic and aliphatic ketone precursors (Equation (218)).974... 

The next step must be the restoration of the aromaticity of the ring by the removal of the proton at the site of attack. This gives the aluminium enolate of the ketone. There is a proton now available to convert the aluminium enolate to the ketone and this is the final product. This is a useful reaction because it has added a benzene ring to a quaternary carbon atom—conjugate addition has overcome steric hindrance. 

The first step is conjugate addition of the amine. Under acid catalysis the ketone now cyclizes in the way we have just described to give a dihydroquinoline after dehydration. Oxidation to the aromatic quinoline is an easy step accomplished by many possible oxidants. 

Potassium hydride and potassium hexamethyldisilazide are the most commonly used metal sources to generate the alkoxide . But recently, the indium(I)-mediated tandem carbonyl addition-oxy-Cope rearrangement of y-pentadienyl anions to cyclohexenones and conjugated aromatic ketones has been reported. For example, indium alkoxide 8.32, obtained after the addition of 5-bromopenta-1,3-diene (8.31) to the aromatic conjugated ketones 8.30, undergoes a spontaneous oxy-Cope rearrangement to give 8.33 in 55% yield (Scheme 8.9). 

This amphiphilic conjugate alkylation has been used successfully for nucleophilic alkylation of electron deficient arenes, on the basis of the unprecedented conjugate addition of organolithiums to aromatic aldehydes and ketones by complexation with ATPH [136], Thus, initial complexation of benzaldehyde or acetophenone with ATPH and subsequent addition of organolithiums affords 1,6 adducts with high selectivity, as illustrated in Sch. 99. 

Other aromatic and aliphatic acid chlorides give good to excellent yields of the desired ketones using this procedure. Hindered or a. -unsaturated acid chlorides also function effectively, the latter forms a, -unsaturated ketones without competing conjugate addition (equations 75 and 76). 

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