Ketones pinacol reaction

The pinacol rearrangement is a dehydration reaction that converts a 1,2-diol into a ketone. The reaction involves two carbocation intermediates. 

Photochemical pinacolization reactions of non-enolizable ketones have been the subject of many investigators.248 Photolysis of benzophenone in... 

The same idea allows us to make symmetrical 1,2-diols 33 by the pinacol reaction. Again we avoid cations and anions by making both halves radicals. These are generated by addition of electrons from metals to aldehydes and ketones. So an electron from sodium adds to acetone to give a radical anion 35 that might dimerise to give 33. 

The classic precursors used for generating diradicals are cyclic, bicychc and polycyclic diazenes. However, diradicals have also been made by Norrish type I photochemical extrusion of CO from cychc ketones, by thermal cleavage of vinyl and divinylcyclopropanes, by pinacol reactions of diketones, by Bergman-type cyclisations of endiynes, by several types of photoelectron transfer reactions and in other ways. Most synthetic applications have started with a derivative of 2,3-diazabicyclo[2.2.1]hept-2-ene which on heat-... 

The earliest studies on this reaction began with an attempt to generate simple symmetrical pinacols. Reaction of 1 equiv. of ytterbium metal with 2 equiv. of a diaryl ketone in IllF/HMPA provided excellent yields of the corresponding symmetrical pinacols (equation 98). Interestingly, when equimolar quantities of ytterbium metal and benzophenone were employed, the sole product isolated after aqueous work-up was benzhydrol. When D2O was utilized to quench this reaction mixture, C-deuterated benzhy-drol was formed (equation 99). These latter results in cated that a discrete ketone dianionic intermediate was generated in the reaction between ytterbium metal and diaryl ketones. 

The example shows the dimerization of acetone to give a diol (2,3-dimethylbutane-2,3-diol) whose trivial name, pinacol, is used as a name for this type of reaction using any ketone. Sometimes pinacol reactions create new chiral centres in this example, the two diastereoisomeric diols are formed in a 60 40 mixture. If you want to make a single diastereoisomer of a diol, a pinacol reaction... 

Reichwein, J. F., Pagenkopf, B. L. New Mixed Phosphonate Esters by Transesterification of Pinacol Phosphonates and Their Use in Aldehyde and Ketone Coupling Reactions with Nonstabilized Phosphonates. J. Org. Chem. 2003, 68,1459-1463. 

The diol 20 (pinacol) rearranges to pinacolone 22 in the presence of acid in the second step the methyl group migrates with its bonding electrons to give the stabilized cation 21, which loses a proton to give the final ketone 22 (reaction 5.17). 

A route " in which the 3,4-bond is made by an intramolecular pinacol reaction is nicely illustrated by the formation of a tricyclic thiophene with two cyclobutane fused rings. In this example, it was necessary to force the double dehydration required for aromatisation, because of the strain in the system. Starting materials for this route are easily obtained from sodium sulfide and two mole equivalents of a 2-bromo-ketone. 

In 1983 we recognized that aprotic THF solutions of Sml2 gave pinacol formation from aldehydes or ketones the reaction being quite fast with aldehydes or aromatic ketones [117]. However, aromatic carbonyl compounds were coupled in THF by the samarium/I2/MeOH system, Sml2 being formed in situ [118]. 

One such reaction is the acid-catalyzed conversion of diols to ketones. This reaction is sometimes referred to as the pinacol rearrangement. The classic example of the reaction is the conversion of 2,3-dimethylbutane-2,3-diol (pinacol) to methyl t-butyl ketone (pinacolone) ... 

Reducing metals (such as Mg) can couple ketones to give 1,2-diols this is called the Pinacol reaction. 

Pinacolization sometimes occurs when undesired, but it is difficult or unsatisfactory when needed. A few attempts to improve this reaction by sonication (Fig. 18) have met with limited success, and a general solution to this problem remains to be found. Aromatic aldehydes and ketones pinacolize when sonicated with low-valent titanium in DME.57 The reaction is highly sensitive to the nature of the solvent. In apolar media, the reduction of titanium trichloride does not occur, and in THF, the McMurry coupling to olefinsHO is preferred. 

The product distribution from the reaction of 3 with pinacols shows a similar temperature dependence to that of the ketone reactions, although the pinacol reactions proceed at a somewhat lower temperature. The reaction of 3 with benzopinacol at 50°C gives only TPE, with no TPA observed. 

Sodium sulfide reacts with two equivalents of a 2-halo-ketone to generate an a,a -thiobisketone, e.g. 37, and these can be cyclised to give aromatic thiophenes [100] via an intramolecular pinacol reaction promoted by low valent titanium, generated in situ, followed by the elimination of two equivalents of water, for example, from 37 to 39 via 38 (Scheme 63) [101]. 

Scheme 63 The use of a 2-halo-ketone to make an a,of -thiobisketone, ring closure using a pinacoL reaction and then double dehydration [101]...

The acyloin condensation involves essentially the same chemistry as the pinacol reaction, but the substrates are esters, rather than aldehydes or ketones, and most reactions use an alkali metal as the electron source (Figure 20.10) 4 moles of sodium are needed to complete the reaction. The reaction is often modified by addition of trimethylchlorosilane, to capture the dianion. This is called the Riihlman modification and allows the reaction to proceed successfully for intramolecular reactions giving rise to both small and medium rings, mainly by suppressing potential side reactions. Examples are shown in Figure 20.11. 

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