Aromatic ketones, metallation

Diols (pinacols) can be synthesized by reduction of aldehydes and ketones with active metals such as sodium, magnesium, or aluminum. Aromatic ketones give better yields than aliphatic ones. The use of a Mg—Mgl2 mixture has been called the Gomberg-Bachmann pinacol synthesis. As with a number of other reactions involving sodium, there is a direct electron transfer here, converting the ketone or aldehyde to a ketyl, which dimerizes. 

Metal ketyls are ion-radicals analogous to semiquinone ion radicals and may be considered either oxygen or carbon free radicals. They are readily prepared by treating aromatic ketones with alkali metals in dry ether, benzene, or liquid ammonia under an inert atmosphere.124 125 Benzophenone potassium has been shown to be paramagnetic in the solid state.126... 

The titanocene alkylidene complex 187 may be regarded formally as a metal equivalent of a ketene it requires no further activation and reacts directly with aromatic ketones 186 to give the arylallenes 188 (Scheme 5.27) [72], the yields being excellent (80-90%). 

Reductions of certain aromatic ketones with metal hydrides have been shown to involve radical intermediates formed by an electron-transfer mechanism (25). For example, the reaction of aluminum hydride with dimesityl ketone in THF produced a violet solution that gave an EPR spectrum indicative of the presence of a paramagnetic species. The paramagnetic species is an intermediate in the reduction of the ketone, and is believed to be a radical cation-radical anion pair (25). 

Reduction of nonconjugated aromatic ketones gave at metal cathodes (e.g., tin, copper, silver, palladium, zinc) the cis isomers (ds-H/OH) of cycKzed products in high diastereoselectivity. The electroreduction of 5-phenylpentan-2-one led to 70% of an exclusively ds-hexahydronaphthalene... 

Commonly, transition metals on non-reducible supports are used for the selective hydrogenation of aromatic ketones, with most research to date having been done on Pt-based catalysts. In particular, we will discuss here some interesting results published on this subject. 

As shown in Figure 1.26, a chiral Sm(III) complex catalyzes asymmetric reduction of aromatic ketones in 2-propanol with high enantioselectivity. Unlike other late-transition-metal catalysis, the hydrogen at C2 of 2-propanol directly migrates onto the carbonyl carbon of substrate via a six-membered transition state 26A, as seen in the Meerwein-Ponndorf-Verley reduction. ... 

Catalytic hydrogenation of aromatic ketones was successful over a wide range of catalysts under mild conditions (25°, 1 atm). Of the noble metal family some metals more than others tend to hydrogenolyze the alcohol formed, and/or saturate the ring. Evaluation of some metal catalyst is based on hydrogenation of acetophenone as a representative [35,814]. 

TS-1 zeolites have been used in the presence of H2O2 to perform the BV reaction on cyclic and aromatic ketones . Cyclohexanone and acetophenone can be oxidized at 80 °C with selectivities lower that 60%, due to the formation of a-hydroxyketones and other undesired products. The observed modest results seem to be associated with the poor selectivity of the active Ti-peroxo species. In this respect, Corma and coworkers developed new Sn-based heterogeneous catalysts able to selectively activate the ketone instead of 11202 . Cyclic ketones are transformed into the corresponding lactones and unsaturated cyclic ketones like 110 are oxidized to unsaturated lactones in very high chemoselectivitjf , unusual for other oxidizing systems (equation 75). As expected, the authors failed to detect the presence of metal-peroxo derivatives in their systems . ... 

Reaction of metal amides of primary amines with aromatic ketones to give imines [47]. 

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