Benzopinacol from benzophenone

An unusual type of reaction is anodic reduction which can be performed at certain metal anodes. Thus, when magnesium is used as an anode in the electrolysis of benzophenone in pyridine/sodium iodide solution, the anode is consumed and benzopinacol can be isolated from the anolyte 15S). Here reduction by univalent magnesium ion is postulated ... 

In the presence of a moderate hydrogen donor such as isopropanol, the oxygen-centered radical of the biradical abstracts a hydrogen atom from the a-position of isopropanol to give pinacole. For example, the benzophenone biradical, generated from the irradiation of benzophenone, abstracts a hydrogen atom from isopropanol to form an a,a-diphenyl-a-hydroxymethyl radical, which is then coupled to give benzopinacol (12) (eq. 1.5). 

The proposed mechanism for this reaction included the formation—by a one-electron transfer —of a benzophenone ketyl and a neopentyl radical in a solvent cage, which then could either collapse to produce the normal product or from which the radicals could escape to form different products, such as benzopinacol and neopentane (Scheme 6) ... 

Uranium. Ephritikhine ha.s established that treatment of either cyclohexanone or benzophenone with UCI4 and Na/Hg affords the pinacol adduct in good yield (Eq. 3.26) [42]. The uranium benzopinacolate intermediate from the homocou-pling of benzophenone was characterized by X-ray crystallography. Mechanistic studies indicate that carbon-carbon bond formation proceeds via ketyl dimerization [43]. 

Benzophenone (1 part), zinc (2 parts), and 80 % acetic acid (10 parts) are boiled for 15 min with effective shaking. After cooling, the crystals that have separated are collected and the filtrate is boiled again several times with zinc, the crystals being collected as above each time. The benzopinacol produced is washed with dilute acetic acid and recrystallized from boiling glacial acetic acid. It then melts at 185-186° with decomposition to benzophenone and benz-hydrol (diphenylmethanol). 

Evidence for Radicals from Ketones and Aldehydes. In the reaction of Grignard reagents such as the bulky neopentylmagnesium bromide with benzophenone considerable amounts of benzopinacol can be formed [76] ... 

See [77] and [78] on the formation of pinacols from reactions of organometallic reagents with (substituted) benzophenones. Indeed benzopinacol was found [79] among the products of almost all the Barbier reactions listed in Tables 2.18 through 2.21 in Sect. 2.4.4. Radical formation on the surface of the metal is most likely the origin of such sideproducts. 

A different type of photoreaction in ILs has been studied by Jones and co-workers the photoreduction of benzophenones by primary amines. Prior work by Cohen demonstrated that photolysis of benzophe-none in benzene in the presence of sec-butylamine afforded benzopinacol (and an imine). This reaction proceeds through a radical pair formed by hydrogen-atom abstraction by the triplet excited state ben-zophenone from the amine. In the much more polar environment of an IL, the radical pair may instead undergo single electron transfer to form an iminium cation and a hydroxyl-substituted carbanion. Proton transfer from the cation to the anion will yield benzhydrol and an imine. 

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