2.2- Diphenyl-1,4-butanediol

In a one-liter flask fitted with a reflux condenser 31.8 g (0.265 mol) of acetophenone is dissolved in 130 ml of absolute ethanol and 130 ml of dry sulfur-free benzene, and 0.5 g of mercuric chloride and 8 g (0.296 g-atom) of aluminum foil are added. Heating of the mixture initiates a vigorous reaction which is allowed to proceed without external heating until it moderates. Then the flask is heated to maintain reflux until all of the aluminum has dissolved (2 hours). After cooling the reaction mixture is treated with dilute hydrochloric acid and the product is extracted with benzene. The combined organic layers are washed with dilute acid, with a solution of sodium carbonate, and with a saturated solution of sodium chloride they are then dried with sodium sulfate and the solvent is evaporated under reduced pressure. A rapid vacuum distillation affords a fraction at 160-170° at 0.5 mm which, after dissolving in petroleum ether (b.p. 65-110°), gives 18.0 g (56%) of 2,3-diphenyl-2,3-butanediol, m.p. 100-123°. 

Problem 14.35 What compounds would you use to prepare 2,3-diphenyl-2,3-butanediol,... 

Problem 14.59 Outline the steps and give the product of pinacol rearrangement of (a) 3-phenyl-l,2-propanediol, (b) 2,3-diphenyl-2,3-butanediol. ... 

A conventional preparation of 2,3-diphenyl-1,3-butadiene involves dehydration of meso-2,3-diphenyl-2,3-butanediol by acidic reagents including acetic anhydride,3-5 acetyl bromide,5 sulfanilic acid,6 and potassium hydrogen sulfate.7 Other procedures have been summarized2 previously. 

Simple diols have been subjected to the pinacol rearrangement in the presence of Amberlyst 15, a polystyrene sulfonic resin with a high concentration of acidic centers. When excess catalyst is used (g catalyst/g diol = 1.25) pinacol gives 3,3-dimethyl-2-butanone in near quantitative yield (batch reactor, 373 K, 2 h) [33]. Of the two isomeric ketones 3,3-diphenyl-2-butanone (10), formed with phenyl migration, is the sole product when 2,3-diphenyl-2,3-butanediol (9) is reacted (Scheme 6). This feature is very similar to that observed under homogeneous conditions in the presence of sulfuric acid [2]. An arylsulfonic silica catalyst prepared by grafting and sulfonation proved to be less selective [33]. 

Pillared layered clays (PILC) containing different hydrated cations in the interlayer space are also selective catalysts. Pinacol and 2,3-diphenyl-2,3-butanediol have been studied in the presence of Al-PILC, Zr-PILC, and Cr-PILC [38], Zr-PILC, with the lowest acidity, gave rise to the largest amount of diene from pinacol (32 % and 22 %, depending on the method of preparation). The rearrangement was selective in the presence of a mixed-metal pillared layered clay (Fe Aln-PILC, sealed tube, 398 K, 1 h, catalyst/diol = 5) [40],... 

Changes in selectivity in the reaiTangement of 2,3-diphenyl-2,3-butanediol (Scheme 6 9) were similar to those observed with M " -montmorillonites [38], and selectivity for the isomeric ketone formed by methyl migration (11) could also be increased by prolonged reaction (selectivity for 11 as high as 68-70% could be reached at 433 K after 17 h). This phenomenon was explained by ketone-ketone rearrangement (interconversion of 10 and 11). 

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