Pinacols from aliphatic ketones

Preparative electrolysis of cyclohexanone17 in solutions containing 0.1 M (C4H9)4NBF4 as the electrolyte were carried out at —2.95 V(SCE), more positive potentials resulted in negligible current. When 0.01 M (DMP)BF4 was added to the solution, electrolysis of cyclohexanone was possible at —2.70 V(SCE). Thus, DMP+ caused a 0.25 V positive shift in the preparative reduction potential of cyclohexanone. DMP + also altered the nature of the product. In the presence of DMP+, cyclohexanone formed only the corresponding pinacol, while in its absence cyclohexanol was the sole product. From this and experiments with other aliphatic ketones (that will be described later) it could be concluded that DMP+ catalyzes the reduction and redirects the... 

The reduction of dialkyl acetals of aromatic aldehydes or ketones with a titanium(II) reagent gives pinacol ethers or the corresponding alkene in high yield (46-100% equation 116). Acetals derived from aliphatic aldehydes or ketones are reduced to the corresponding ethers. 

The treatment of carbonyl compounds with Sml2 and methyl chloroformate in the presence of molecular sieves affords the cyclic carbonates or biscarbonates of pinacols. This one-pot reaction proceeds rapidly even with aliphatic ketones. The stereochemistry of the reaction run by this procedure is different from that of conventional pinacolic couplings. Trifluoromethylated five-membered cyclic carbonates were prepared through the palladium-promoted reaction of tertiary trifluoromethylated propargylic alcohols and sodium carbonate (Scheme 6). ... 

Termination is principally via radical coupling forming hexabutylditin, or to a lesser degree via the coupling of ketyl radicals. In the case of the mr ketones a different mechanism is proposed. The rate of abstraction of H from the tributyltinhydride by benzylic radicals is slower than the corresponding abstraction by alkyl radicals. Since the rate at which the tributyltin radical will add to aromatic carbonyls is similar to the addition rate to aliphatic carbonyls, the dominant radical species for the tttt systems is the ketyl radical. The primary termination process involves the coupling of the predominant radical species resulting in pinacol formation. 

Indium. Kim has developed an indium-mediated pinacol coupling of aromatic aldehydes that proceeds in water or in water/t-BuOH (Eq. 3.14) [33]. Sonication greatly enhances the rate of the reaction. The diastereoselectivity of the pinacol coupling is variable, with d,l meso ratios ranging from 6 1 to 1 2. Aliphatic aldehydes, as well as ketones, are inert to these conditions. 

Chromium. In 1998, Boland reported a chromium(n)-catalyzed pinacol coupling reaction of aromatic aldehydes and ketones (Eq. 3.28) [46-48J aliphatic aldehydes are inert to these conditions. The turnover step relies upon a combination of manganese and Me3SiCl to regenerate CrX2 from an intermediate chromi-um(III) species (Fig. 3-3). 

Ephritikhine has also explored alternatives to silyl chlorides for liberating the coupling product from the initial metal pinacolate. In 1997 he reported that AICI3 may be employed in a TiCU-catalyzed, Li/Hg-mediated pinacol coupling of aliphatic aldehydes and ketones (Eq. 3.42) [63] the diastereoselectivity of this process is modest ( 2 1 for valeraldehyde). The proposed catalytic cycle is illustrated in Fig. 3-7. 

In fact, diketones in a medium sized ring give several transannular reactions such as pinacol coupling [equation (i)] or aldolization [equation The later reaction also occurs during hydrogenation of aliphatic monoketones under favorable conditions (Pd-on-carbon, zeolite, 200°C, 4 X 10 kPa) to produce methyl isobutyl ketone from acetone (in 96%) -... 

A titanium(Il) species formed from titanium trichloride and lithium aluminum hydride is a useful reagent for the reductive coupling of carbonyl compounds to olefins (McMurry, 1974 McMurry and Fleming, 1974). Both aliphatic and aromatic ketones can be converted to tetrasubstituted olefins in excellent yields. Reductive dimerization of retinal (CCLXXFV) affords j6-carotene (CCLXXV) in 85% yield. The course of the reaction can be accounted for by assuming pinacol formation followed by loss of titanium dioxide. 

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