Benzophenone dissolving metals

Dissolving metals initially convert aldehydes, ketones, and esters into radical anions. Subsequently, proton donors may react with the latter, which leads to neutral radicals. This mode of reaction is used, for example, in the drying of THF or ether with potassium in the presence of the indicator benzophenone. Potassium and benzophenone react to give the deep-blue potassium ketyl radical anion A (Figure 14.45). Water then protonates ketyl A to the hydroxylated radical B as long as traces of water remain. Further potassium reduces B via another electron transfer to the hydroxysubstituted organopotassium compound C. C immediately tautomerizes to the potassium alkox-ide D. Once all the water has been consumed, no newly formed ketyl A can be pro-tonated so that its blue color indicates that drying is complete. 

It was suggested in the 1950s that the reduction of aliphatic ketones by dissolving metals proceeded by two sequential one-electron additions to provide a dianion (equation 1). This mechanism was based on the observation that benzophenone affords a dianion on reaction with excess Na in liquid NH3, and it was inferred that aliphatic ketones would behave similarly. A number of workers presented mechanistic rationalizations for the stereochemical course of the dissolving metal reductions of cyclic aliphatic ketones based on this dianion concept. However, in a 1972 review, it was noted that the reduction potentials of alkali metals were not sufficient to effect the addition of two electrons to an aliphatic carbonyl group, and an alternative mechanism was suggested which with some modification is now generally accepted. ... 

A potentially useful chemoselective dissolving metal reagent for the reduction of aromatic ketones in the presence of other functional groups is the combination Zn-DMSO and aqueous potassium hydroxide. In three examples (benzophenone, fluorenone and 4-benzoylpyridine), the yields of secondary alcohols were over 90%. Two other ketones (xanthone and thioxanthone) gave mixtures of alcohol and the hydrocarbon obtained by hydrogenolysis of a carbon-oxygen bond. ... 

Radicals can also be generated via the fragmentation of radical anions. Single electron reduction of various alkyl or aryl halides will lead to bond cleavage, as shown in Eqs. 10.44 and 10.45. The reductions commonly occur from dissolving metals such as Na and K, sodium naphthalenide (Na Ar ), sodium benzophenone ketyl, or from pulse radiolysis (e ). 

By and large, a hnely divided precipitate of a metal is a very effective one-electron reducer. For example, a hnely divided precipitate of Zr(0) was obtained on mixing naphthalene sodium derivative in THF with ZrCl4. The Zr(0) precipitate dissolved on addition of anthracene or benzophenone to form the corresponding zirconium salts of the anion-radicals (Terekhova et al. 1996). 

A similar situation may be obtained when alkali metals are immersed in ultrapure ethers containing benzophenone [53], The metal thus dissolves via formation of stable ketal radical anions in solution (and metal ions as well). It should be emphasized that the above processes occur even when the active metal is initially introduced into the solution covered by surface films (due to reactions with atmospheric contaminants). We assume that electron tunneling through the films enables the initiation of the dissolution process. This process breaks the film on the metal (as metal is depleted beneath the rigid surface film), thus enabling solvent molecules to reach the active surface and solvate more electrons. This increases the metal solubilization and the further breakdown of the initial surface films. Hence, an equilibrium between a bare metal and the blue solution can finally be reached, as explained above (Eq. 13). 

Qulnlnone (2).2 Potassium metal (10 g, 0 25 at g) was dissolved in t-BuOH (200 mL). Ttie excess of t-BuOH was removed in vacuum until a dry mobile powder was obtained. A suspension of t-BuOK and quinine 1 (32.4 g, 0. t mol), benzophenone (91.0 g. 0 5 moQ in PtiH (500 mL) was refluxed for 15-18 h under Ng Tbe cooled mixture was poured Into ice and extractedwilh 10%HCIuntillheHCIwascoloitess. The aad extract was washed with BgO and dripped with stirring into NH4OH and Ice Extraction with ElgO, washing of the extract with brme and evaporation of the extract afforded a total of 30-32 g of 2 (95-98%). mp 106-108 C. 

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