1,4-Cyclohexanediols, dehydration

The type of anchimeric assistance encountered in the trans-1,4-cyclohexanediol dehydration had also been shown in solvolytic reactions of noncyclic diols (6, 70) and of 1,4-cyclohexanediol system 71). 

Cyclopentanecarboxaldehyde has been prepared by the procedure described above 2 3 by the reaction of aqueous nitric acid and mercuric nitrate with cyclohexene 6 by the action of magnesium bromide etherate 6 or thoria 7 on cyclohexene oxide by the dehydration of frarei-l, 2-cyclohexanediol over alumina mixed with glass helices 8 by the dehydration of divinyl glycol over alumina followed by reduction 9 by the reaction of cyclopentene with a solution of [HFe(CO)4] under a carbon monoxide atmosphere 10 and by the reaction of cyclopentadiene with dicobalt octacarbonyl under a hydrogen and carbon monoxide atmosphere.11... 

The geometry of the cis-alkylcyclohexanol is favorable for trans elimination since the hydroxyl and the neighboring trans hydrogen are coplanar, but this is not true for the l,i-trans isomer hence the molecular conformation has to flip over, to set the hydroxyl group in the axial position for the trans elimination to occur. This would require a few kilocalories of energy and for frans-lert-butylcyclohexanol it would be more difficult to achieve than for IroMs-methylcyclohexanol. It is, therefore, possible that the trans elimination from a boat conformation, or possibly even an epimerization from the trans to the cis isomer which then undergoes a trans elimination reaction. Such an epimerization was found to occur under conditions of dehydration of certain alcohols over alumina, as will be seen under 1,4-cyclohexanediol. The more facile elimination of the cis-i-tert-butylcyclohexanol system as compared with the trans system in solution was also reported in the literature 63). 

The reaction of cis- and another example of stereospecificity of the dehydration reaction. This reaction, which was first reported by Olberg et al. in 1944 (67) and studied in more detail by Manassen and Pines (68), can be presented schematically as follows ... 

The cis- and fmns-1,4-cyclohexanediols were dehydrated at about 250° over aluminas containing 0-2yo by weight of sodium ions, The trans isomer formed 1,4-epoxycyclohexane as the main product and... 

The rate of dehydration of the cis diol was about fifty times slower than that of the trans diol and the product of the reaction consisted mainly of cyclohexenol. The 1,4-epoxycyclohexane formed in the reaction was formed after a prior epimerization of the cis to the trans diol this was demonstrated by means of tritium tracer technique. When irons-1,4-cyclohexanediol was dissolved in ieri-butyl alcohol-T having the hydroxyl hydrogen marked with tritium (C4H,OT) the 1,4-epoxycyclohexane produced in this reaction had a very low tritium content. A similar reaction carried out with cis-1,4-cyclohexanediol produced a highly tritiated 1,4-epoxycyclohexane. The insertion of tritium in the 1,4-epoxycyclohexane produced from the cis diol can be explained as follows ... 

The basic and acid sites on alumina surfaces have been represented graphically (43, 72). In order for the acid and the basic sites of the alumina to attack tra s-l,4-cyclohexanediol from different planes of the catalyst surface it is necessary for the dehydration to be restricted to submicroscopical holes or crevices or to occur between channels of those particles. Since the basic and acid sites of the alumina have to surround the cyclohexanediol, as in the solvolytic reaction, the alumina therefore can be considered as a pseudosolvent for such dehydration reactions. 

The dehydration of 1,4-cyclohexane diols emphasises the critical importance of stereochemical arrangement of the hydroxy functions in a substrate . The m-diol eliminates to give the cyclohexenol as the major product but the trans isomer yields mainly 1,4-epoxycyclohexane. As elimination from trans-1,4-cyclohexanediol occurs more rapidly than from t-butyl alcohol, anchimeric assistance was suggested, the intramolecular concerted ring closure occurring through a boat conformation, viz. 

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