Acetone, enolization photolysis

Photolysis is a particularly effective and convenient way to promote substitution. Arenes containing various leaving groups react with acetone enolate ion at —33° under the stimulation of Pyrex filtered light to give phenylacetone in yields which sometimes are nearly quantitative. The reactivity order in terms of leaving groups is... 

The yield is invariant with changes in pressure, temperature, intensity, and added inert gases.26 The mechanism favored involves the formation of ethylene and the enolic form of acetone by way of a six-membered ring intermediate,46 and has recently been substantiated by results of studies of the photolysis of deuterated ketones.112 The mechanism is given below... 

The silicon-carbon double-bonded intermediates generated photo-chemically from a-alkenyldisilane derivatives react with both enolizable and nonenolizable ketones to give olefins (98). For instance, the photolysis of a-styrylpentamethyldisilane (49) in the presence of one molar equivalent of acetone gives l-trimethylsilyl-2-phenyl-3-methyl-2-butene in 13% yield as a single product. No silyl enol ether to be expected from the reaction of the intermediate with the enol form of acetone is observed. Similar irradiation of 49 with acetophenone affords (E)- and (Z)-l-trimeth-... 

Figure 14. CIDNP spectra (a) during and (b) after the photolysis of 1,4-benzoquinone and 2-propanol in CD3CN. The enol is labeled 1 and acetone as 2.

Flash photolysis has contributed much to our understanding of enol chemistry in aqueous solution. The equilibrium constant for the enolization of simple ketones, = C enoi/c ketonc is usually too small permit a direct determination of enol concentrations cenoi at equilibrium. For example, the enolization constant of acetone is pk) 8.3 the relative amount of 2-hydroxypropene is thus only 5 ppb in aqueous solution. Nevertheless, most reactions of carbonyl compounds proceed via enol intermediates, which, once... 

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