Hydrogen abstraction, ketones basics

Hydrogen abstractions in ketone/amine and quinone/amine systems continue to attract the attention of CIDNP spectroscopists [94] despite the facts that the application of CIDNP to these reactions dates back to 1974 [95] and that the basic mechanism — electron transfer from the amine DH followed by deprotonation of the resulting aminium cation DH + to give an a-aminoalkyl radical D" has already been cleared up in those early investigations [46]. CIDNP spectroscopy is very well suited to probe the microscopic details of such reactions that involve more than one radical intermediate. Polarizations can arise in both DH + and D, but the spin density distributions of these two radicals differ strongly. Hence, the polar-... 

Now when the proton gets attached to the same carbon centre, it can attach itself from the front as well as from the back thereby losing optical activity. Hence racemizes. In the case of ketone (B) there is a group CH2 in between the chiral centre and the carbonyl group. Hence such racemization in basic solution is ruled out as the hydrogen atom at the chiral centre in (B) is not acidic enough to be abstracted. 

Spencer and co-workers (150) have provided the first evidence for stereo-electronic control in a-deprotonation of iminium ions. Treatment of the B-hydroxy-ketone 486 (R=H) and the B-acetoxy-ketone 486 (8=08300) with weakly basic non-tertiary amines (CF3CH2NH2 or i(H20H20N) gives the bicyclic enone 489 via the formation of imines 487 and 488. Using appropriate deuterium labeling, they found that the axial proton (Hi) is preferentially removed and the stereoelectronic factor was estimated to be 18 (when 0R=0H) and 110 (when OR=OH3COO) in 486. They also found that in the hydroxide ion catalyzed conversion of 486 (8=08300) into bicyclic enone 489, the axial hydrogen was preferentially abstracted by a factor of 130. 

The reaction proceeds by abstraction of a proton from the hydroxyl group, followed by formation of an alkoxy group on the alkali cation, hydride abstraction and hydrogen (H2) liberation and subsequent formation of the aldehyde or ketone. The reaction of 2-propanol is commonly used as a test reaction in this regard over acid catalysts propene is formed, over basic catalysts acetone is the product (Scheme 9.19). 

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