Aldol reactions counterion effects

Usually, (Z)-boron enolates can be prepared by treating /V-acyl oxazolidones with di-K-butylboron triflate and triethylamine in CH2CI2 at 78°C, and the enolate then prepared can easily undergo aldol reaction at this temperature to give a, vy -aldol product with more than 99% diastereoselectivity (Scheme 3-4). In this example, the boron counterion plays an important role in the stereoselective aldol reaction. Triethylamine is more effective than di-wo-propylethyl amine in the enolization step. Changing boron to lithium leads to a drop in stereoselectivity. 

Simple Mukaiyama aldol and Diels-Alder reactions catalysed by cationic silicon Lewis acids show significant counterion effects.142... 

Although a majority of the catalytic complexes employed in the aldol reaction are bidentate, Carreira and coworkers published the synthesis of a new chiral tridentate chelating ligand for the efficient asymmetric induction of stereochemistry in aldol adducts. The Ti(IV) complex 68, an analog of the BINOL catalyst previously mentioned, was further stabilized by 3,5-di-tert-butylsalicyclic acid as a counterion to increase the yields, selectivity, and efficiency of the asymmetric reaction. This new catalyst is particularly effective in the addition of either O-trimethylsilyl, or O-ethyl, or O-methyl ketene to both aliphatic and aromatic aldehydes enantioselectively to obtain the respective aldol adduct. For example, the reaction of the silylketene acetal 90 with the aromatic aldehyde 89 in the presence of 68 obtains the aldol adduct 91 in high yield (91%) and excellent enantioselectivity (97% ee). 

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