Of pinacol

Dimethylbutadiene is formed when the vapour of pinacol is passed over alumina at 400 C. 

Dissolve I g. of pinacol (preparation, p. 148) in 20 ml. of water, and add 20 ml. of the 5% aqueous sodium periodate solution. After 15 minutes, distil the clear solution, collecting the first 5 ml. of distillate. Treat this distillate with 2,4-dinitro-phenylhydrazine solution A (p. 263). Acetone 2,4-dinitrophenyl-hydrazone rapidly separates from the solution when filtered off, washed with a small quantity of ethanol, and dried, it has m.p. 126-127°, and after recrystallisation from ethanol it has m.p. 128°. 

Add cautiously 15 ml. of concentrated sulphuric acid to 50 ml. of water in a 100 ml. distilling-flask, and then add 10 g. of pinacol hydrate. Distil the solution slowly. When about 40 ml. of distillate (consisting of pinacolone and water) have been collected, and no more pinacolone comes over, extract the distillate with ether. Dry the extract over sodium sulphate. Distil the dry filtered extract carefully, with the normal precautions for ether distillation (p. 164). When the ether has been removed, continue the distillation slowly, rejecting any fraction coming over below 100 . Collect the pinacolone, b.p. 106 , as a colourless liquid having a peppermint odour. Yield, 4 5-5 o g. A small quantity of higher-boiling material remains in the flask. 

Pinacol (tetramethylethyleneglycol). Pinacol hydrate may be dehydrated in the following manner (compare Section 11,39). Mix 100 g. of pinacol hydrate with 200 ml. of benzene and distil a mixture of water and benzene passes over. Separate the lower layer and return the upper layer... 

Pinacolone. In a 500 ml. round-bottomed flask carrying a dropping funnel and a connection to a condenser set for distillation, place 50 g. of pinacol hydrate and 130 ml. of QN sulphuric acid. Distil the mixture until the upper layer of the distillate no longer increases in volume (15-20 minutes). Separate the pinacolone layer from the water and return the latter to the reaction flask. Then add 12 ml. of concentrated sulphuric acid to the water, followed by a second 50 g. portion of pinacol hydrate. Repeat the distillation. Repeat the process twice more until 200 g. of pinacol hydrate have been used. 

Boronic esters are easily prepared from a diol and the boronic acid with removal of water, either chemically or azeotropically. (See Chapter 2 on the protection of diols.) Sterically hindered boronic esters, such as those of pinacol, can be prepared in the presence of water. Boronic esters of simple unhindered diols are quite sensitive to water and hydrolyze readily. On the other hand, very hindered esters, such as the pinacol and pinanediol derivatives, are exceedingly difficult to hydrolyze and often require rather harsh conditions to achieve cleavage. 

Unsymmetrical diols typically give a mixture of pinacol products. For example, the diol shown below might give eight distinct products (counting cis and tmns diastereomers as distinct products). In fact, it gives only the two shown. 

A study of the rearrangement of pinacols of the general type (177) has elegantly demonstrated the capacity of the thienyl group to supply... 

Acetic anhydride (75 ml) and pulverized calcium carbide (15 g) are refluxed for 60 minutes in an apparatus as described above. Sufficient benzene is then added to reduce the boiling temperature to 110-112°. After cooling, 14.4 g of pinacol is introduced, and the mixture is refluxed with stirring for 30 hours. The cooled mixture is then stirred into ice, extracted into ether and washed with sodium carbonate solution. The solution is dried, then filtered, and theether is evaporated. The residue is clarified with Norit and recrystallized from benzene-petroleum ether or methanol, mp 67-68°. 

A solution of potassium naphthalenide is prepared from 2.0 g (50 mmol) of potassium and 6.4 g (50 mmol) of naphthalene in 40 mL ofTHF. After 1 h at r.t. this mixture is diluted with 10 mL of diethyl ether and 10 mL of petroleum ether (bp 40-60 °C) and cooled to — 120 °C. 4.5 g (25 mmol) of ( )-l-methoxy-3-phenylthio-1-propcne arc added followed by 3.36 g (25 mmol) of chlorobis(l-dimethylamino)borane. This mixture is allowed to warm to r.t. over 3 h the solvents are removed in vacuo and the residue is carefully distilled through a 5-cm column at 10 2 Torr. The distillate, containing also naphthalene, is dissolved in 30 mL of diethyl ether and treated with 2.95 g (25 mmol) of pinacol for 3 h. The crude product is chromatographed over 30 g of basic alumina (activity 1) using petroleum ether (bp 40 -60°C) giving 9.2 g of a mixture of product and naphthalene the yield of product (89% E) is determined to be 60% by H-NMR analysis. Similarly prepared is ... 

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. 

The acidity dependences of V(V) oxidations are significant. That of pinacol , which undergoes 100% C-C cleavage, is a+bh ). The first (acid-independent) term is rare in V(V) oxidations and implies that V02 is the active oxidant the second term implies, on the basis of the Zucker-Hammett hypothesis, that the transition state has the structure (J5), the mechanism being... 

A comparison of the V(V) oxidations of acetoin, CH3CH(OH)COCH3, and 3-hydroxy-3-methylbutan-2-one, (CH3)2C(OH)COCH3, shows that whilst both rate laws include first-order terms in substrate and oxidant, the acidity dependence for the former compound is purely ho but that for the latter is a+bho). The C-methyl compound consumes only 2 equivalents of V(V) to give acetone and a mechanism similar to that for the oxidation of pinacol is proposed , viz. 

Boronic Ester Relative Retention Minimum Detectable Quantity (pg of pinacol) Optimum Detector Temperature ( C)... 

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. 

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. 

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

A recently reported vinylation of nitrones was accomplished by using ZnMe2 and vinylboronic esters of pinacol 256. The optimal conditions for the addition were 3.5 h at 60 °C in DMF. Yields of the products, A -allylic hydro-xyamines 257, varied significantly from 14% to 92% (Scheme 137).359... 

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). 

The stereoselective 1,4-addition of lithium diorganocuprates (R2CuLi) to unsaturated carbonyl acceptors is a valuable synthetic tool for creating a new C—C bond.181 As early as in 1972, House and Umen noted that the reactivity of diorganocuprates directly correlates with the reduction potentials of a series of a,/ -unsaturated carbonyl compounds.182 Moreover, the ESR detection of 9-fluorenone anion radical in the reaction with Me2CuLi, coupled with the observation of pinacols as byproducts in equation (40) provides the experimental evidence for an electron-transfer mechanism of the reaction between carbonyl acceptors and organocuprates.183... 

Indeed, the (200-fs) laser excitation of the EDA complexes of various benz-pinacols with methyl viologen (MV2+) confirms the formation of all the transient species in equation (59). A careful kinetic analysis of the decay rates of pinacol cation radical and reduced methyl viologen leads to the conclusion that the ultrafast C—C bond cleavage (kc c = 1010 to 1011 s- ) of the various pinacol cation radicals competes effectively with the back electron transfer in the reactive ion pair. 

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