4-Phenyl-2-butanone determination

Isomeric enolate ions can be formed from unsymmetric dialkyl ketones, and the distribution of the two possible arylation products is mainly determined by the equilibrium concentration of the various possible enolate ions. However, the selectivity also depends on the structure of the attacking radical. Reactions with the enolate ions from 2-butanone afford arylation preferentially at the more substituted a carbon to render about twice as much 3-phenyl-2-butanone as 1-phenyl-2-butanone [68,69] however, in the reaction with the anion derived from /-propyl methyl ketone, the 1-phenyl derivative is predominantly formed [68]. When there is a substituent ortho- to the leaving group, the attack at the primary a carbon is enhanced [69,70]. 

Radical reactions have some stereochemical features that can be compared directly with their ionic counterparts, especially when the radical centre is adjacent to an existing stereogenic centre. The tris(trimethylsilyl)silyl radical adds to chiral ketones like 3-phenyl-2-butanone 7.59 to give a radical 7.60 flanked by a stereogenic centre. The hydrogen atom abstraction from a thiol, determines the relative stereochemistry, and the products 7.61 and 7.62 are analogous to those from the hydride reduction of the ketone. They are formed in the same sense, and the stereochemistry is explained by the Felkin-Anh picture 7.60. 

There appears to be at least one factor, in addition to angular strain and resonance stabilization, which determines the relative importance of photoinduced decarbonylation, 6-cleavage, and ring expansion of cyclobutanones. It has already been demonstrated that increasing a-alkylation of cyclo-butanone results in an increase in the extent of cyclic acetal formation (i.e., ring expansion). Comparison of [64a] and [64b] reveals a similar trend. The 3-phenyl-cyclobutanones [141] and [144], however, provide the most spectacular effect of a-substitution (25). In both cases, a-cleavage occurs exclusively at the most highly sub-... 

When enantiomerically pure (either R or S) 3-phenyl-2-butanone is dissolved in ethanol, no change occurs in the optical activity of the solution over time. If, however, a trace of either acid (e.g., aqueous or gaseous HCl) or base (e.g., sodium ethoxide) is added, the optical activity of the solution begins to decrease gradually and eventually drops to zero. When 3-phenyl-2-butanone is isolated from this solution, it is found to be a racemic mixture. Furthermore, the rate of racemization is proportional to the concentration of acid or base. These observations can be explained by a rate-determining acid- or base-catalyzed formation of an achiral enol intermediate. Tautomer-ism of the achiral enol to the chiral keto form generates the R and S enantiomers with equal probability. 

The results indicate that the product ratio is determined by the competition between the various reaction steps. When catalyzed by base, 2-butanone reacts with benzal-dehyde at the methyl group to give 1-phenyl-l-penten-3-one. Under acid-catalyzed conditions, the product is the result of condensation at the methylene group, namely, 3-methyl-4-phenyl-3-buten-2-one. Under the reaction conditions used, it is not possible to isolate the intermediate ketols, because the addition step is rate-limiting. These intermediates can be prepared by alternative methods, and they behave as shown in the following equations ... 

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