Pinacol reaction with acid

In a similar reaction, 2,3,6-trimethoxydibenz[6,/]oxepin gives 10,11-dihydro-2,3,6-trimeth-oxydibenz[/>,/]oxepin-cw-10,l 1-diol upon treatment with osmium(VIII) oxide in the presence of A-methylmorpholine A -oxide.262 When treated with acid the diol undergoes a pinacol rearrangement to the corresponding xanthene-9-carbaldehyde. 

The Grignard reagents prepared from the activated magnesium appear to react normally with electrophiles. Thus reactions with proton donors, ketones, and carbon dioxide afford hydrocarbons, alcohols, and carboxylic acids, respectively. The reductive coupling of ketones to pinacols had also been accomplished with the activated magnesium. ... 

Trimethylsilylation of enolizable carbonyl compounds and alcohols has also been accomplished by the fluoride ion promoted reaction with hexamethyldisilane and ethyl trimethylsilylacetate [48, 49], with high stereospecificity giving Z-enol ethers from ketones [50]. l-Trimethylsilyl-(l-trimethylsilyloxy)alkanes, produced from the reaction of aldehydes with hexamethyldisilane, undergo acid-catalysed hydrolysis during work up to yield the trimethylsilylcarbinols [51]. In the case of aryl aldehydes, the initially formed trimethylsiloxy carbanion produces the pinacol (Scheme 3.1). 

To elucidate the reaction pathway, deuterium-labeled allenyl pinacol boronate 10 was prepared, and the addition reaction with hydrazonoester 6 was conducted in the presence of Bi(OH)3 and Cu(OH)2 (Scheme 4). In both Bi- and Cu-catalyzed cases, the reactions proceeded smoothly (in quantitative yields in both cases). In the Bi(OH)3-catalyzed reaction, a major product was allenyl compound 11, in which the internal position was deuterized. It was assumed that a propargyl bismuth was formed via transmetalation from boron to bismuth, followed by addition to hydrazonoester via y-addition to afford allenyl compound 11. Thus, two y-additions could selectively provide a-addition products [75, 76, 105, 106]. It was confirmed that isomerization of 10 did not occur. Recently, we reported Ag20-catalyzed anti-selective a-addition of a-substituted allyltributyltin with aldehydes in aqueous media [107], On the other hand, in the Cu(OH)2-catalyzed reaction, a major product was propargyl compound 12, in which the terminal position was deuterized. A possible mechanism is that Cu(OH)2 worked as a Lewis acid catalyst to activate hydrazonoester 6 and that allenyl boronate 10 [83-85] reacted with activated 6 via y-addition to afford 12. 

The pinacol rearrangement is a useful reaction that proceeds via a carbocation rearrangement. Treatment of 2,3-dimethyl-2,3-butanediol, also known as pinacol, with acid results in the formation of a ketone, pinacolone ... 

The intermediate cation in a pinacol rearrangement can equally well be formed from an epoxide, and treating epoxides with acid, including Lewis acids such as MgBr2, promotes the same type of reaction. 

The reaction with zinc is most efficient in pyridine containing a trace of water, although piperidine, dimethyl-formamide, and other bases may replace pyridine, or ethane did may be used (zinc in acidic media gives bis-steroidal products resulting from a pinacolic reduction— see below). 

The reaction with pinacol is second order (first order with respect to each reactant) in the range pH 0-11 (Price ef a/. , Duke and Bulgrin Buist et al. ). In the same pH range there is no evidence for the formation of an intermediate in appreciable concentration. The dependence of the second-order rate coefficient on pH is complex (Fig. 3) and shows maxima at pH 1 and pH 9.5. In the range pH 2-10 the reaction is general acid-base catalysed (Zuman et al. , Buist et al. ) and for base catalysis the catalytic constants obey the Bronsted catalysis law with a slope of 0.69. Asnotedabove, thekineticsof pinacol oxidation resemble those... 

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