Pinacol-type reactions

PINACOL-TYPE REACTIONS OF P-HYDROXY SULRDES AND SELENIDES... 

Although hydroxylation of phenylalanine to tyrosine looks like a typical electrophilic aromatic substitution, scientists at the U.S. National Institutes of Health discovered that the biochemical pathway combines epoxidation of the benzene ring followed by epoxide ring-opening with rearrangement. This rearrangement, which is the biochemical analog of the pinacol-type reactions described earlier, is known as the NIH shift. ... 

Electrochemical Functional Transformation, Scheme 4 Pinacol type reactions... 

Similar stereoselectivities are achieved in the allylation of enantiomerically pure proline-derived a-oxoamides47. l-Bromo-3-methy]-2-butcne reacts with clean allylic inversion. Since pinacol-type coupling products are also produced under the reaction conditions, this was taken as evidence for a radical addition mechanism47. 

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. ... 

Samarium-mediated Barbier-type reactions of carbonyl compounds were reported in a similar reaction system (Equation (6)).22 THF is the key solvent to obtain the product 33. In MeOH, pinacol coupling-type reaction proceeded predominantly, while the reaction failed to produce any desired compound in CH3CN.23... 

Depending on the reaction conditions, either olefinic or pinacol-type products can be obtained. Thus, treatment of enantiopure aldehyde (R) -2 with TiCls (DME) 1.5 and Zn/Cu in DME at elevated temperature gives the l,2-bis(phosphaferrocenyl) ethene 18 as a mixture of the E- and Z isomers in a ratio of 8 1 (Scheme 1.5.7) they can be separated by chromatography. 

From a stereochemical point of view, the pinacol-type coupling is much more interesting, because two new stereogenic centers are established at the former carbonyl C atoms. When the coupling reactions described above were carried out with (R)-2 at 0 °C the pinacol product 19 (Fig. 1.5.4) was obtained as one out of three possible stereoisomers. 

Racemic aldehyde 2 under similar reaction conditions yielded two diastereo-meric pinacols the configuration of all the products was assigned by X-ray diffraction. On the basis of these results a mechanistic scheme could be proposed to explain the stereochemical course of the coupling reactions [19]. It is assumed that the reaction proceeds via an intermediate titanaoxirane by insertion of a molecule of aldehyde 2 into the Ti—C bond - a proposal that has been put forward recently on the basis of DFT calculations [20]. The pinacol-type coupling products could be employed as bidentate P,P chelate ligands to Mo(CO)4 fragments however analogous experiments carried out with the olefinic derivatives 18 were unsuccessful [19]. 

Moreover, no aldol or pinacol-type dimerization of the carbonyl compound was observed, even in the case of easily reducible benzophenone or benzaldehyde derivatives. As observed previously, the nature of the solvent is an important factor for the success of the reaction. Indeed, no homoallylic alcohols were formed in DMF. 

But now commonly associated with Tiffeneau-Demianov rearrangement The classification now covers many reactions of the "pinacol type", including ... 

Exercise 16-40 Write a sequence of reactions whereby 2-methylpropene may be converted to 2-methylpropanal by way of a pinacol-type rearrangement. Would you expect any concomitant formation of 2-butanone Explain. 

The diterpene derivative 1 was treated with BF3-OEt2 and AC2O in an attempt to induce a "pinacol-type rearrangement leading to a ring B-homo derivative. No such reaction was observed. Instead, the only product which could be isolated (33%) was shown to be 2, formed by "a profound backbone rearrangement" of 1. 

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