Hydrogen atom transfer reactions fluorenone

Lewin and Cohen (1967) determined the products of dediazoniation of ben-zophenone-2-diazonium salt (10.42, Scheme 10-77) in five different aqueous systems (Table 10-7). About one-third of the yield is 2-hydroxybenzophenone (10.46) and two-thirds is fluorenone (10.45, run 1) copper has no effect (run 2). On the other hand, addition of cuprous oxide (run 3) has a striking effect on product ratio and rate. The reaction occurs practically instantaneously and yields predominantly fluorenone. As shown in Scheme 10-77, the authors propose that, after primary dediazoniation and electron transfer from Cu1 to 10.43 the sigma-complex radical 10.44 yields fluorenone by retro-electron-transfer to Cu11 and deprotonation. In the presence of the external hydrogen atom source dioxane (run 12) the reaction yields benzophenone cleanly (10.47) after hydrogen atom abstraction from dioxane by the radical 10.43. 

In this experiment, you will examine the reduction of 9-fluorenone (10) using sodium borohydride to give 9-fluorenol (12), as shown in Equation 17.17. This reaction is mechanistically analogous to the reduction of imines with sodium borohydride (Sec. 17.3) and involves the transfer of hydride ion (H ) from borohydride ion, BH4, to the electrophilic carbonyl carbon with concomitant transfer of the electron-deficient boron atom to the carbonyl oxygen. Theoretically, all four of the hydrogen atoms attached to boron may be transferred in this way to produce the intermediate borate salt 11, which is decomposed upon addition of water and acid to yield 9-fluorenol (12). 

Experiments with deuterated optically active primary amyl alcohol, fluorenone, and sodium ethoxide, for example, showed a direct transfer of hydrogen from the hydroxyl carbon to the carbonyl carbon atom without intervention of the solvent.41 We should expect that, like all carbonyl additions, hydride addition reactions would be acid-catalyzed. Apparently, in aluminum isopropoxide and isopropyl alcohol the optimum acid strength of the solution has been reached. Some alkoxide ions... 

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