Cyclopentane-l,2-diols

For further contributions on the dia-stereoselectivity in electropinacolizations, see Ref. [286-295]. Reduction in DMF at a Fig cathode can lead to improved yield and selectivity upon addition of catalytic amounts of tetraalkylammonium salts to the electrolyte. On the basis of preparative scale electrolyses and cyclic voltammetry for that behavior, a mechanism is proposed that involves an initial reduction of the tetraalkylammonium cation with the participation of the electrode material to form a catalyst that favors le reduction routes [296, 297]. Stoichiometric amounts of ytterbium(II), generated by reduction of Yb(III), support the stereospecific coupling of 1,3-dibenzoylpropane to cis-cyclopentane-l,2-diol. However, Yb(III) remains bounded to the pinacol and cannot be released to act as a catalyst. This leads to a loss of stereoselectivity in the course of the reaction [298]. Also, with the addition of a Ce( IV)-complex the stereochemical course of the reduction can be altered [299]. In a weakly acidic solution, the meso/rac ratio in the EHD (electrohy-drodimerization) of acetophenone could be influenced by ultrasonication [300]. Besides phenyl ketone compounds, examples with other aromatic groups have also been published [294, 295, 301, 302]. 

Lead tetraacetate is very frequently used for cleavage of 1,2-diols and preparation of fhe resulting carbonyl compounds. The rate of reaction is highly dependent on the stereochemistry of the substrate. There is usually correlation between the rate of oxidation and the spatial proximity of the hydroxy groups. For example, the rate of the oxidative cleavage of cis-cyclopentane-l,2-diol is much faster than that of trans isomer. It is, however, possible to oxidize trans-l,2-diol to diketone (Scheme 13.51) [72 a]. 

The O18 exchanges accompanying the pinacol rearrangement in the cyclohexane-1,2-diol and cyclopentane-l,2-diol systems have also been investigated. The pinacolic rearrangement of both cis- (17) and trans-1,2-dimethylcyclohexane-l,2-diol (18) in aqueous acid gives 1-acetyl-methylcyclopentane and a small amount of 2,2-dimethylcyclohexanone... 

The situation is more complicated in the case of the pinacolic rearrangement of the isomeric cyclopentane-l,2-diols (Bunton and Carr, 1963b). In aqueous perchloric acid, cis-1, 2-dimethylcyclopentane-l,2-diol (19) is converted into a mixture of the IruTW-isomer, some 2,2-dimethyl-cyclopentanone and a large amoimt of a tarry by-product (possibly a polymeric cyclopentadiene). No 0 is found in the unrearranged cis-diol. The m s-l,2-dimethylcyclopentane-l,2-diol (20), on the other... 

Determination of Absolute Configuration and Solution Conformation. Abraham et have employed H and C lanthanide induced shifts produced by Yb(fod)3 as an experimental probe of the solution conformations of cyclopen-tanol and cis- and trans-cyclopentane-l,2-diol. Analysis of the LIS was undertaken in conjunction with molecular mechanics and ab initio calculations of structure. 

A very dilute solution of cis-cyclopentane-l,2-diol in CCI4 shows bands at 3620 and 3455 cm-i. Explain. 

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