Organic Chemistry by Morrison and Boyd,Seventh Edition says -

When cyclohexanones are used as substrates, the stereoselectivity of the reduction is found to be dependent on the acidity or the basicity of the medium. The observations about this stereoselectivity are summed up in the form of a few empirical rules, called Barton's rules, which are as follows:

(i) For unhindered cyclohexanones, in the catalytic reduction with Adam's catalyst($\ce{PtO2}$) in acidic medium, the less stable axial alcohol predominates in the product

(ii)In a basic or a neutral medium , however, the same catalyst gives the thermodynamic product called the equatorial alcohol as the major product

(iii) But for hindered cyclohexanones, the less stable axial alcohol is formed in all $\ce{pH}$

As far as I know, catalytic hydrogenation is a syn addition which is performed by delivering the hydrogen from the same side when the substrate is adsorbed on the surface of the catalyst. From what I know about the theory of heterogenous catalysis, it is the nature of catalyst which governs all aspects of the product formed here.So I feel that the medium's nature shouldn't have much effect here.

My question is: What is the basis/rationale for these "Barton's rules"?


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