Reactions pinacol

Tellurium powder/KOH is an efficient system for the pinacolization of aromatic carbonyl compounds.  

The method is advantageous over others since it is a very fast reaction. 

Typical procedure. Benzaldehyde (0.53 g, 5 mmol) was dissolved in methanol (15 mL), tellurium powder (1.276 g, 10 mmol) and KOH (1.40 g, 25 mmol) were added and the reaction mixture was stirred. The reaction became vigorous immediately after the addition of KOH. The reaction mixture was filtered to remove the tellurium powder and water (50 mL) was added to the filtrate. A solid precipitated out which was filtered off under pressure. Some of the diol was obtained by extracting the filtrate with CH2CI2 (3x20 mL), drying with anhydrous Na2S04 and then concentrated using a vacuum rotatory evaporator. 

4 Alkylidenation of aldehydes and cyclopropanation of o jS-unsatu-rated carbonyl compounds with dibromomalonic esters 

Dibromomalonates react with aldehydes under the assistance of dibutyl telluride (2 equiv) to afford alkylidene malonates.  

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It is possible to reduce thiazole carbonyl derivatives with aluminium isopropoxide (56, 57). For example, 4-methyl-5-acetylthiazole gives 4-methyl-5-(a-hydroxyethyl)thiazole. With aluminium amalgam one obtains a duplication like to the pinacol reaction, and the yield is genaally poor... 

Self-condensation of the substituted propiophenone, 15, by the pinacol reaction proceeds to give the glycol, 16, as the meso isomer. (If it is assumed that the transition state for this reaction resembles product, this stereoselectivity can be rationalized on the grounds of steric interaction compare A, which leads to the observed product, with B.) Dehydration under very specialized conditions (acetyl chloride, acetic anhydride) affords the bisstyrene-type diene (17). Removal of the acyl groups by means of base affords the synthetic estrogen, dien-... 

A rearrangement essentially akin to a reversal of the pinacol/ pinacolone change, a retro pinacol reaction, is the dienone/phenol rearrangement,... 

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]. 

Quite recently, a domino Diels-Alder/Prins/pinacol reaction was reported by Barriaulfs group [38]. This novel method is very reliable and efficient for the synthesis of highly functionalized bicyclo[m.n.l]alkanones. In addition, Aube and coworker [39] used a combination of a Diels-Alder and a Schmidt reaction within the total synthesis of the Stemona alkaloid stenine [40]. 

Aromatic aldehydes undergo a pinacol reaction when treated with hexamethyl-disilane and tetra-n-butylammonium fluoride [55] using procedure 3.1.14.D. 

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

Diols, via Pinacol reaction, 10, 529 a-Diones, from Grignard reagents, 9, 55 DIOP, and chiral bisphosphane ligands, 10, 7 Diorganoditellurides halogenolysis, 9, 595... 

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. 

But how else might they have done it Perhaps equally obvious is to use the pinacol reaction to make the diol 68 and oxidise both alcohols. We shall now broaden the discussion by writing Ar for the substituents as many of the reactions have not in fact been done with o-tolyl substituents. Two of the best yielding methods are Mg in the presence18 of Me3SiCl and samarium iodide where the colour change from blue Sm(III) to yellow Sm(IL) is characteristic.19... 

A nice synthesis of the bicyclic alkene on the right starts with a pinacol reaction. 

Semipinacol rearrangements are pinacol reactions with no choice about which way to go... 

The titanium metal that is the source of electrons is produced during the reaction by reduction of T1CI3 using a zinc-copper mixture. This reaction is, in fact, unusual because, as we shall see below, pinacol reactions using titanium do not normally stop at the diol, but give alkenes. 

Titanium can be used as the metal source of electrons in the pinacol reaction and, provided the reaction is kept cold and no tie ft for too long, diols can be isolated from the reaction (see the example at the end of the previous section). However, unlike magnesium or aluminium, titanium reacts... 

Eight-membered heterocyclic diols were prepared from acyclic dicarbonyls via pinacol reactions (Scheme 81 <2004TL253>). The cis- or trans-diol stereoselectivity was controlled by the low-valent metal catalyst used. For instance, when using Sml2, the cis-trans ratio 193 194 was 3 1 (Scheme 28 <2004TL579, 2004T10385>). This method is a viable alternative to the metathesis-dihydroxylation strategy presented earlier. 

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-... 

With most catalysts in the pinacol reaction the normal linear dependence of rate coefficient on catalyst concentration is observed, but for the relatively strong base ammonia the dependence is markedly non-linear, and at high ammonia concentrations the rate coefficient approaches a limit. The non-linear dependence has been treated quantitatively by assuming a transition from alternative (h) to (c). Because the latter alternative involves a rate limiting step which is not catalysed, the limiting rate approached at high ammonia concentrations is explained (Buist et... 

In the range pH 9-11 the pinacol reaction deviates from strict second-order kinetics the second-order rate coefficient decreases with increase of periodate... 

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