Ketones, aliphatic pinacols

Samarium diiodide is another powerful one-electron reducing agent that can effect carbon-carbon bond formation under appropriate conditions.257 Aromatic aldehydes and aliphatic aldehydes and ketones undergo pinacol-type coupling with Sml2 or SmBr2. 

REDUCTION WITH ALUMINUM AMALGAM Reduction of Aliphatic-Aromatic Ketones to Pinacols [144]... 

The hydrodimerization of aromatic or aliphatic aldehydes and ketones yields pinacols (.100) (Eq. (184) ). 

Newman used the reagent for the reduction of aryl alkyl ketones to pinacols (yields 30-60%) with aliphatic ketones yields were negligible. Thus a solution... 

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. 

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. 

Pinacol coupling. Yb(0) metal can effect pinacol reduction of aromatic, but not aliphatic ketones.1 However, it can effect cross-coupling of benzophenone with aliphatic ketones, often in high yield (equation I). 

Aldehydes and ketones can be hy-drodimerized to pinacols (Eq. 2) [34-37]. With aromatic carbonyl compounds, the yields and selectivities are mostly higher than with aliphatic ones. The reaction has been extended to imines (Eq. 2, X = NAr, N-Bn) [38-41] and to heterohydrodimerizations affording, for example, y-lactones (Eq. 3) [42-44]. 

Somewhat less frequent than the reductions of aliphatic ketones to secondary alcohols and to hydrocarbons are one-electron reductions to pinacols. These are accomplished by metals such as sodium, but better still by magnesium or aluminum. Acetone gave 43-50% yield of pinacol on refluxing with magnesium amalgam in benzene [140], and 45% and 51% yields on refluxing in methylene chloride or tetrahydrofuran, respectively [825. ... 

Some additives accelerate the pinacol coupling reactions. Addition of Me3SiCl to Sml2 also accelerates the pinacol coupling reactions of aliphatic ketones and aldehydes. Pinacol coupling reactions are also promoted with samarium metal and a Lewis acid such as Et2AlCl or MesSiCl. Coordination of such a Lewis acid to a carbonyl oxygen facilitates the one-electron reduction by samarium. 

Nil2 or Fe(acac)3 in a catalytic amount also accelerates the pinacol coupling in pivalonitrile. The system can be applied to the coupling reactions of aromatic and aliphatic ketones (Equation (33)). ... 

The pinacol-type coupling of aliphatic aldehydes, in the presence of niobium(IH) salts, occurs, with a high anti diastereoselectivity (equation 131)512. In the case of aromatic aldehydes and ketones the alkene product is sometimes formed513. In both cases the cyclic acetals may also be formed. 

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... 

Tetraalkyl- and tetraatyl-ethylene glycols (pinacols) are made by reduction of ketones with active metals such as sodium, magnesium, and aluminum. The reaction is only fair for aliphatic and alicyclic ketones. Acetone, methyl ethyl ketone, cyclopentanone, and cyclohexanone all give less than 0% yields of pinacols. Mixtures of ketones are reduced to unsymmetrical pinacols. An active zinc-copper couple has been employed in the reduction of several simple olefinic aldehydes to dieodiols, e.g., crotonaldehyde to dipropenyl glycol,... 

Ethereal solvents, principally THF, either with or without sonication, have been reported to give results similar to those obtained on reductions in NH3 with no added proton donor, and pinacol formation as a major reaction path. a potentially useful selective reduction of unhindered cyclohexanones in the presence of other ketones using A1 amalgam in aqueous THF has been described and will be discussed in detail subsequently (Section 1.4.3.3.2).2 in this procedure aliphatic ketones give no pinacols however, aromatic ketones give only the corresponding pinacol.2 ... 

Aromatic ketones represent a rather special case in dissolving metal reductions. Under many conditions pinacol formation is the predominent reaction path (see Volume 3, Chapter 2.6). Also, the reduction potentials of aromatic carbonyl compounds are approximately 1 V less negative than their aliphatic counterparts. The reductions of aromatic ketones by metals in ammonia are further complicated by the fact that hydrogenolysis of the carbon-oxygen bond can take place (Chapter 1.13, this volume) and Birch reduction may intervene (Chapter 3.4, this volume). 

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