text: Even remote substituents on the benzene ring can affect the electron density on the aromatic ring and in turn influence the selectivity. In the hydrogenation of ketones using CBS catalysts, the ketone coordinates to the boron atom with the lone pair on the oxygen atom. In the following example, the inductive influence of the substituents can lead to differentiation of the two sp lone pairs on the oxygen atom.
  The relevant stereoelectronic interaction in the starting material is the n → σ*(C–C) interaction. The electron-withdrawing substituent on the benzene ring depletes the electron density on the aromatic ring and thus makes the σ*(C–C) orbital a better acceptor than σ*(C–C). These two stereoelectronic interactions use different lone pairs on the oxygen atom (the one antiperiplanar to the σ* in question for each), leading to lone pairs with different electron densities.  In particular, the enhanced depletion of electron density from the lone pair antiperiplanar to the 4-nitrophenyl group leads to weakened ability for that lone pair to coordinate to boron. This in turn results in the lone pair antiperiplanar to the 4-methoxyphenyl binding preferentially to the catalyst, leading to well-defined facial selectivity.  Under optimized conditions, the product is formed with excellent levels of enantioselectivity (95% ee).