Solvent effect unsaturated carbonyl

Reduction of a., -unsaturated carbonyl compounds. Hydrosilanes, particularly (QH,)2SiH2, in the presence of Pd(0), and a Lewis acid, particularly ZnCl2, can effect selective conjugate reduction of unsaturated ketones, aldehydes, and carboxylic acid derivatives. Chloroform is the solvent of choice. In addition, 1 equiv. of water is required. Experiments with D,0 and (C6H,),SiD2 indicate that... 

As an example of application of the ASEP/MD method described in the previous section, in this section we proceed to the discussion of solvent effects on radiative and non-radiative processes in acrolein. Acrolein or propenal is the smallest a,(3-unsaturated carbonyl compound. The presence of the carbonyl group and the C=C double bond makes it a compound of marked interest from a spectroscopic and photochemical points of view. In solution, acrolein displays a strong absorption band corresponding to a transition and a weak band, at lower frequencies asso-... 

Selective hydrogenation ofx,p-nnsatwated carbonyl compounds. Japanese chemists have effected selective hydrogenation of ,/7-unsaturated carbonyl compounds by a wine-red hydridoiron carbonyl complex generated in situ from iron pentacarbonyl and a small amount of base in moist solvents. Three different procedures can be used ... 

The formation of cyclobutanes starting from a,P-unsaturated carbonyl compounds and olefins is referred to as the de Mayo reaction. The carbonyl component is excited upon irradiation by ti <—ti transition. The regioselectivity is determined by orbital coefficients and polarity effects that depend on the solvent, but the stereochemical information of the olefin is not preserved in intermolecular processes, indicating the non-concerted character of a triplet reaction. In this case 1,4-biradical intermediates are formed and the most stable one determines the stereochemistry of the main product. The cycloadducts of pentanones are most often cis-fused while hexanones preferentially give transcycloadducts. ... 

Analogous additions were later reported by Ito and co-workers (Scheme 9) (20). The enamines for these additions were prepared from the corresponding aminals using a mild base in the presence of trimethylsilyl chloride. In turn, the aminals used are available from (— )-ephedrine and (S)-prolinol. The byproduct amine hydrochloride was removed either by distillation or by precipitation from a benzene solution. Enamines prepared by this method were found to be unreactive toward unsaturated carbonyl compounds in a variety of solvents. Importantly, it was found that use of a mild Lewis acid such as anhydrous MgCl2 or ZnCl2 in THF promoted the reactions. Thus, the addition of enamines 9.1 and 9.2 to methyl acrylate is achieved. Of the two enamines, the proline-derived 9.1 is the more effective auxiliary. The... 

Conjugate addition of nitroalkanes to a,6-unsaturated carbonyl compounds has been achieved using basic alumina, in the absence of a solvent.36 3-Nitropropanal has been prepared by conjugate addition of nitrite anion to acrolein reduction to 3-nitropropanol was effected with borane-dimethyl sulphide.36 -Nitro-acids and u-nitro-esters have been prepared by ring cleavage of 2-nitrocycloalkanones (Scheme 163).362... 

In Section 4, it is described that chlorotris(triphenylphosphine)rhodium(I) (7) is quite an effective catalyst for the hydrosilylation of carbonyl compounds. For this reason, extensive studies on asymmetric hydrosilylation of prochiral ketones to date have been based on employing rhodium(I) complexes with chiral phosphine ligands. The catalysts all prepared in situ are rhodium(I) complexes of the type, (BMPP>2Rh(S)a (8) [40] and (DIOP)Rh(S)Cl (6) [41], and a cationic rhodium(III) complex, [(BMPP)2lUiH2(S)2] Q04 (5) [42], where S represents a solvent molecule. An interesting polymer-supported rhodium complex (V) [41], and several chiral ferrocenylphosphines [43], recently developed as chiral ligands, have also been employed for asymmetric hydrosilylation of ketones. Included in this section also are selective asymmetric hydrosilylation of a,0-unsaturated carbonyl compounds and of certain keto esters. 

Baylis-Hillman reaction of an aldehyde with an a.y -unsaturated carbonyl compound is catalysed by l,4-diazabicyclo[2.2.2]octane (DABCO) a zwitterionic intermediate (60) is proposed. Lithium perchlorate is found to accelerate the reaction further (in diethyl ether solvent), presumably through further stabilization of such a species. The effect is not seen with most other metal salts, presumably because they cannot act as efficient, independent co-catalysts in the presence of a tertiary amine. 

Regioselective addition to a,p-unsaturated carbonyl compounds is an age-old pursuit, and reactions selective for either 1,2- or 1,4-addition are ubiquitous in modern organic synthesis. Nucleophile character (hard versus soft) and solvent polarity (contact versus separated ion pairs),among other factors, contribute to the reaction outcome. The typical reaction of Grignard reagents with enones results in mixtures of 1,2- and 1,4-addition products. From their earliest inception, Cu(i) catalysts and reactants have been used to effect selective 1,4-addition in this archetypical transformation of organocuprates. ... 

Childs RF, MulhoHand DL, Nixon A. Lewis acid adducts of a,p-unsaturated carbonyl and nitrile compounds. A calorimetric study. Can J Chem. 1982 60 809-812. Gutmann V. Solvent effects on the reactivities of organometaUic compounds. Coord Chem Rev. 1976 18 225-255. 

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