Formation of Pinacols

Pinacols can be obtained from ketones by using reducing agents of very varied type  

The preparation of benzopinacol may be given as example of pinacol formation by zinc in acid solution 148 

Benzophenone (1 part), zinc (2 parts), and 80 % acetic acid (10 parts) are boiled for 15 min with effective shaking. After cooling, the crystals that have separated are collected and the filtrate is boiled again several times with zinc, the crystals being collected as above each time. The benzopinacol produced is washed with dilute acetic acid and recrystallized from boiling glacial acetic acid. It then melts at 185-186° with decomposition to benzophenone and benz-hydrol (diphenylmethanol). 

Bachmann149 6 7c described the preparation of benzopinacol in 95% yield by photoreduction of benzophenone in 2-propanol in sunlight. 

Gomberg and Bachmann150151 obtained a large number of aromatic pinacols in good yield by reduction with the binary mixture magnesium-magnesium bromide or iodide. For example, /i-(2-naphthyl)-1,2-diphenyl-1,2-ethane-diol can be prepared as follows 151 

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Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

We have broadened the scope of this reverse addition protocol to prepare a variety of boronic acids bearing different functional groups for use in Suzuki coupling reactions. The yield and quality of the boronic acid prepared by this reverse addition protocol is usually better than the sequential approach. The boronic acids can be used without further purification (formation of pinacols) in Suzuki coupling reactions. 

The ESR detection of benzophenone-ketyl radical coupled with the formation of pinacols as byproducts (in Scheme 9) provides the basis for an electron-transfer mechanism between carbonyl acceptors and various Grignard reagents48 (equation 23). 

Sasidharan et al. (258) reported the formation of pinacols from alkenes catalyzed by various titanosilicates. Aluminum-free Ti-beta exhibited better activity and pinacol selectivity than TS-1, TS-2, Ti-MCM-22, or mesoporous Ti-MCM-41 (Table XLVIII). The side products included the pinacolone, alcohol, and oligomers. The epoxide was the initial product, which underwent acid-catalyzed nucleophilic ring-opening by H2Q molecules to give the pinacol (Scheme 25). 

Formation of pinacol over various titanium-silicates... 

In claret wine turned bitter, Voisenet observed a divinylglycol, CHj=CH-CHOH-CHOH CH=CHj. It is not known which isomer is involved and whether the compound is optically active. The glycol is considered to be derived from acrolein. It could originate either by a reduction comparable to the formation of pinacol from acetone or by bioreduction of an acyloin-like condensation product of acrolein. 

This review has been concerned with the photochemistry of heterocyclic systems photoreactions which are more correctly designated as reactions of functional groups have, in general, been omitted or not seriously considered. Thus, the formation of pinacols by the photoreduction of such heterocyclic ketones as 3-acetylpyridine 453 and the keto sulfone (422)454 has not been included, nor has cis-trans isomerization been reviewed. 

Several studies have been made of the effect of added metal ions on the pinacol/alcohol ratio. Addition of antimony(m) chloride in catalytic amounts changes the product of the electrochemical reduction of acetophenone in acidic alcohol at a lead electrode from the pinacol in the absence of added metal salt to the secondary alcohol in its presence53. Antimony metal was suspected to be an intermediate in the reduction. Conversely, addition of Sm(in) chloride to DMF solutions of aromatic aldehydes and ketones54 and manganese(II) chloride to DMF solutions of hindered aromatic ketones55 results in selective formation of pinacols in excellent yields. When considering these results one should keep in mind the fact that aromatic ketones tend to form pinacols in DMF even in the absence of added metal ions1,29,45. 

Photoinitiators are generally aryl alkyl ketones or diaryl ketones (Table 2.19). For aryl alkyl ketones two free radicals are produced by homolytic scission of a C-C bond (Eq. (2.96)). Diaryl ketones are usually mixed with a tertiary amine the mechanism of production of free radicals involves H abstraction from the tertiary amine by the excited state PI, via a charge-transfer stabilized exciplex (Eq. (2.97)). The a-amino alkyl radical formed is very reactive and is in fact the true initiator because the cetyl radical disappears rapidly through a coupling reaction (formation of pinacol). 

One typical radical reaction is a coupling reaction. Oxidative decarboxylation coupling reaction of carboxylic acids by electrolysis (Kolbe electrolysis), intramolecular coupling reaction of diesters with Na (acyloin condensation), formation of pinacols from ketones or aldehydes with Na or Mg are well known classical methods [1,2]. Recently, oxidative... 

Careful consideration of Scheme 3.9 allows one to make reasonable suppositions about how to control further reactions of 16. Once radical 16 is formed, the outcome of the reaction depends upon the relative stoichiometry of other components present in the reaction system. The use of a proton donor such as trifluoroethanol leads to rapid conversion of either dimeric or monomeric ketyl to intermediate 18. Rapid production of 18 leads to dimerisation and formation of pinacol 19. If protonation of the ketyl is slow, reduction of 18 by excess ketyl or Sml2 followed by protonation leads to alcohol 21 through a traditional House-type mechanism (Scheme 3.10).9... 

Two thiolate radical anions can couple to give the dithiolate precursor, 5-49, of the second product. This is analogous to the well-documented coupling of ketyls (radical anions derived from one-electron reduction of carbonyl compounds), which results in the formation of pinacols (1,2-diols). 

Uenishi, J., Masuda, S., Wakabayashi, S. Intramolecular Sm and Sm promoted reaction of y-oxy-5-keto aldehyde stereocontrolled formation of pinacol and lactone. Tetrahedron Lett. 1991, 32, 5097-5100. 

Formation of pinacols by a reductive radical-radical coupling of carbonyl compounds, especially ketones ... 

Dimerization resulting from bimolecular reaction among free radicals resulting from electronation of the protonated forms of the ketone (acetophenone) can occur according to the following stereochemical representations of the formation of pinacols ... 

See [77] and [78] on the formation of pinacols from reactions of organometallic reagents with (substituted) benzophenones. Indeed benzopinacol was found [79] among the products of almost all the Barbier reactions listed in Tables 2.18 through 2.21 in Sect. 2.4.4. Radical formation on the surface of the metal is most likely the origin of such sideproducts. 

Carbonyl compounds undergo similar reductions by electron transfer, the formation of pinacols (7.95) and the acyloin condensation (7.96) being typical examples ... 

Example 8.6 Dienestrol TM 8.6 exhibits estrogenic activity. Propose the ret-rosynthesis that in the key step conceives the formation of pinacol. 

The formation of pinacolates of lanthanoids is supposed in the reactions of elemental Nd, Sm, Yb or naphthaleneytterbium with benzophenone [40, 41]. 

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