A,/?-enones

Addition with carbonyl compoundsThese reagents form 1,2-adducts with carbonyl compounds at -65° in high yield without enolization or reduction. 1,2-Adducts are also formed from a,0-enones or -enals. Yields are generally over 90%. 

Reaction with ketones. The reagent reacts with ketones in ether to introduce a C6H5SeCl2 group into the a-position. The products undergo selenoxide elimination to give a,0-enones they are reduced by thiourea to a-phenylseleno ketones. 

Conjugate reduction of enones.1 Under phase-transfer conditions (Aliquat, NaHC03), a,0-enones undergo exclusive 1,4-reduction with Na2S204 (yields 70-85%). 

Reduction of carbonyl groups.1 This reagent effects highly selective 1,2-reduction of a,0-enones in THF. 

Conjugated enones are more stable than nonconjugated enones for the same reason that conjugated dienes are more stable than nonconjugated dienes (Section 14.1). Interaction between the tt electrons of the C=C bond and the tt electrons of the C=0 group leads to a molecular orbital description for a conjugated enone that shows an interaction of the tt electrons over all four atomic centers (Figure 23.3). 

Like unsaturated ketones, a,0-unsaturated carboxylic acid derivatives, e.g. lactones and anhydrides, undergo cycloadditions to alkenes. As for the preparative conditions (direct irradiation or sensitized experiments) these compounds are situated somewhere in between enones on the one side and olefins on the other. 

Seebach et al. (154) showed that a 0-isopropylidene-2,3-dihydroxy-l, 1,4,4-tetraphenyl-l,4-diol (TADDOL)-derived aminothiolate serves as a moderately effective ligand for the catalyzed conjugate addition of Grignard reagents to cyclic enones. The catalyst, synthesized from the lithiothiolate and CuCl, exhibits optimal selectivities under a slow addition protocol (2 h), Eq. 124. 

Manganese-copper-catalyzed conjugate addition of organomagnesium reagents to a,p-enones is a general synthetic method. Table I shows that the reaction can be successfully performed even with enones of low reactivity. 30% Manganese chloride is necessary to obtain good yields. In most cases, however, 0.1% cuprous chloride is sufficient.3... 

Catalytic enantioselective epoxidation of 0, -ENONES WITH A BiNOL-ziNC-coMPLEx. 249... 

OrPhenybelenenylation of a,f -unsaturated esters.3 (F)-3r,/MJnsaturated esters are converted to a-phenylseleno-a,0-unsaturated esters in 20-65% yield by reaction with LDA followed by C6H5SeBr. The reaction is related to the reaction of a,/ -enones with pyridine and C H5SeCl (9, 28-29), and is also believed to involve conjugate addition of the base followed by selenenylation of the enolate. 

With a,0-disubstituted enones an unexpected mode of stereocontrol was evidenced [365, 366], Syn products (7) were obtained by addition of the lithium enethiolate of methyl dithioacetate to an a,0-disubstituted enone via diastereoselective auto-protonation of the intermediate enolate generated in the 1,4 addition. 

Conjugate addition-aldol reactions. A novel synthesis of a-substituted a,(3-enones involves conjugate addition of 1 to an a,p-enone the resulting 0-phenyl-selenoboron enolate undergoes aldol condensation with aldehydes. The adduct on oxidative elimination furnishes unsaturated p-hydroxy ketones.1 Example ... 

Majetich reports that the fluoride ion catalyzed carbocyclization of ally anion equivalents to a, 3-enones is sensitive to the structure of the a,p-enone in which competing 1,2- and 1,4-addition processes occur.116 For example, a,(3-enones (135) afford predominantly the jyn-bicyclo[4.3.0]nonan-3-ones (136)... 

Alternatively, the ambident oi-hetero substituted allyl anions have been utilized as homoenolate equivalents. For example, in the presence of HMPA, allyl phenyl sulfides (251),192 allyl phenyl sulfones (252)192b c and allyl phenyl selenides (253)192d e add to a,(3-enones in a l,4(0)-mode, while allyl phenyl sulfoxides (254) and allyl phosphine oxides (255) afford 1 A j-addition exclusively, irrespective of solvent used.193 Hua has shown that additions of either chiral sulfoxide (254 R1 = R2 = R3 = R4 = H, R5 = p-tolyl) or allyl oxazaphospholidine oxide (256) occur with excellent enantioselectivity (>95% ee).194 Similarly, Ahlbrecht reports that the a-azaallyl (257) adds exclusively in a 1 A j-mode to acceptor (59) to afford 1,5-diketones (Scheme 86).195... 

Mechanistically, the addition of a nickel(I) species to an ot,0-enone generates a nickel(QI) enolate (137) which undergoes transmetallation with an alkenylzirconium (132) and reductive elimination of nickel(I) to afford a zirconium enolate (139).63b... 

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