Pinacol results

Scheme 7 Benzophenone-based gelators 26-28 and photochemical transformation of 26 by irradiation (400-W high-pressure Hg lamp at 15 °C) of 2-propanol gel into pinacol resulting in gradual decomposition of the gel into solution...

Electrochemical reduction of carbon-fliionne bonds occurs at high pH when a carbonyl group is adjacent Polaiographic reduction of a a,a-tnfluoroacetophe-none without loss of fluonne predominates in acidic media to give the alcohol and the corresponding pinacol, whereas reduction of the unprotonated ketone results in hydrogenolysis of the tnfluoromethyl group to form acetophenone as product Il] (equation 8)... 

The reaction of crotonaldehyde and methyl vinyl ketone with thiophenol in the presence of anhydrous hydrogen chloride effects conjugate addition of thiophenol as well as acetal formation. The resulting j3-phenylthio thioacetals are converted to 1-phenylthio-and 2-phenylthio-1,3-butadiene, respectively, upon reaction with 2 equivalents of copper(I) trifluoromethanesulfonate (Table I). The copper(I)-induced heterolysis of carbon-sulfur bonds has also been used to effect pinacol-type rearrangements of bis(phenyl-thio)methyl carbinols. Thus the addition of bis(phenyl-thio)methyllithium to ketones and aldehydes followed by copper(I)-induced rearrangement results in a one-carbon ring expansion or chain-insertion transformation which gives a-phenylthio ketones. Monothioketals of 1,4-diketones are cyclized to 2,5-disubstituted furans by the action of copper(I) trifluoromethanesulfonate. ... 

Purely aromatic ketones generally do not give satisfactory results pinacols and resinous products often predominate. The reduction of ketonic compounds of high molecular weight and very slight solubility is facilitated by the addition of a solvent, such as ethanol, acetic acid or dioxan, which is miscible with aqueous hydrochloric acid. With some carbonyl compounds, notably keto acids, poor yields are obtained even in the presence of ethanol, etc., and the difficulty has been ascribed to the formation of insoluble polymolecular reduction products, which coat the surface of the zinc. The adffition of a hydrocarbon solvent, such as toluene, is beneficial because it keeps most of the material out of contact with the zinc and the reduction occurs in the aqueous layer at such high dilution that polymolecular reactions are largdy inhibited (see Section IV,143). 

Scheme 10.3 gives some examples of pinacol and related rearrangements. Entry 1 is a rearrangement done under strongly acidic conditions. The selectivity leading to ring expansion results from the preferential ionization of the diphenylcarbinol group. Entry 2, a preparation of 2-indanone, involves selective ionization at the benzylic alcohol, followed by a hydride shift. 

The subjects of this section are two reactions that do not actually involve carbo-cation intermediates. They do, however, result in carbon to carbon rearrangements that are structurally similar to the pinacol rearrangement. In both reactions cyclic intermediates are formed, at least under some circumstances. In the Favorskii rearrangement, an a-halo ketone rearranges to a carboxylic acid or ester. In the Ramberg-Backlund reaction, an a-halo sulfone gives an alkene. 

The above-mentioned results indicate the additive effect of protons. Actually, a catalytic process is formed by protonation of the metal-oxygen bond instead of silylation. 2,6-Lutidine hydrochloride or 2,4,6-collidine hydrochloride serves as a proton source in the Cp2TiCl2-catalyzed pinacol coupling of aromatic aldehydes in the presence of Mn as the stoichiometric reduc-tant [30]. Considering the pKa values, pyridinium hydrochlorides are likely to be an appropriate proton source. Protonation of the titanium-bound oxygen atom permits regeneration of the active catalyst. High diastereoselectivity is attained by this fast protonation. Furthermore, pyridine derivatives can be recovered simply by acid-base extraction or distillation. 

The 1,5- and 1,6-dialdehydes 22 and 24 undergo the annulative pinacol coupling to give the cyclic vzc-diols 23 and 25, respectively (Scheme 13) [29]. The vanadium-catalyzed intramolecular coupling reaction of 1,5-diketone 26 also proceeds with excellent selectivity (Scheme 14) although the intermolecular coupling of ketones such as acetophenone results in low diastereoselectivity under these conditions [21]. 

Carbonyl compounds are reduced to alcohols, hydrocarbons or pinacols (cf., for example, Eq. 5.1.8), where the result of the electrode process depends on the electrode potential. 

Conversely, electrolysis of ketones, (35), results in their cathodic reduction to radical anions (36), which dimerise to the dianions of pinacols (37) ... 

In a similar manner, terminal alkynes such as 1-14 participate in a Prins/pinacol reaction, resulting in a ring-expanding cyclopentene annulation to give compounds such as 1-15 in high yield (Scheme 1.5) [5]. 

In order to further examine the role of a pinacolic intermediate, a crossover experiment wa conducted. In the reaction of a lsl mixture of TBP and benzopinacol with U, a statistical distribution of all 6 coupled products was seen. This surprising result shows that the carbon-carbon bond of the pinacol is broken before the products are formed. 

Subsequently, the rapid unimolecular (mesolytic) fragmentation of the resulting pinacol cation radical followed by proton (or trimethylsilyl cation) transfer to quinone anion radical (within the solvent cage) yields the retropinacol products in equations (55) and (56) (equation 58). 

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. 

Compound 51 was found to be unstable and difficult to purify, as described in the literature [93—95]. Therefore, 51 was not isolated, but was instead converted to the stable pinacol 1-acetamido-l-hexylboronate derivative 52. However, the acylated derivative 52 could not be purified by column chromatography as it was destroyed on silica gel and partially decomposed on alumina. Fortunately, we found that it dissolves in basic aqueous solution (pH > 11) and can then be extracted into diethyl ether when the pH of the aqueous layer is 5—6. Finally, pure 52 was obtained by repeated washing with weak acids and bases. It should be mentioned here that exposure to a strongly acidic solution, which also dissolves compound 51, results in its decomposition. Compared with other routes, the present two-step method involves mild reaction conditions (THF, ambient temperature) and a simple work-up procedure. It should prove very useful in providing an alternative access to a-aminoboronic esters, an important class of inhibitors of serine proteases. 

The effect of pH on the periodate oxidation of seven anilines has been investigated. " The kinetics of periodate oxidation of aromatic amines have been studied. " - " Periodate oxidation of oxalic acid is catalysed by Mn(II). " The reaction of ethane-1,2-diol with periodate has been investigated under a variety of conditions and the results compared with those of earlier work and analogous studies on pinacol. " The 104 ion is the primary reactant, with H5IO6 as a secondary reactant the reverse is true for pinacol. The complex observed in previous work is shown not to be an intermediate, but rather to deactivate the reactants. 

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