Crossed-aldol Reactant Control
The problem of "control" in the aldol addition is best demonstrated by an example. Consider the outcome of this hypothetical reaction:
In this reaction, two unsymmetrical ketones are being condensed using sodium ethoxide. The basicity of sodium ethoxide is such that it cannot fully deprotonate either of the ketones, but can produce small amounts of the sodium enolate of both ketones. This means that, in addition to being potential aldol electrophiles, both ketones may also act as nucleophiles via their sodium enolate. Two electrophiles and two nucleophiles, then, have potential to result in four possible products:
Thus, if one wishes to obtain only one of the cross-products, one must control which carbonyl becomes the nucleophilic enol/enolate and which remains in its electrophilic carbonyl form.
Read more about this topic: Aldol Reaction
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