Many students begin learning about the Dieckmann Condensation (cyclic version of the Claisen Condensation) around week 25-26 of Organic Chemistry. This is shortly after learning aromatic chemistry, featuring the SNAr reaction. The Dieckmann Condensation begins rather innocently as did the Aldol Condensation, however there are twists and turns introduced owing to the fact this is intramolecular carbonyl chemistry with leaving groups, and thermally induced decarboxylation soon removes traces of the Dieckmann Condensation ever having happened. A clever test for such chemistry might start with alkene 1 leading to the synthesis of alkane 2 (Scheme 1). Where was the Dieckmann Condensation?
One of the more interesting reactions introduced to Organic Chemistry students when studying addition reactions to alkenes and alkynes is the reduction of a disubstituted alkyne using sodium in liquid ammonia to afford a trans-alkene. When elemental sodium is added to liquid ammonia, the mixture takes upon a blue color indicative of a solvated electron.
Na(s) + NH3(l) → Na(NH3) + e-(NH3) (blue)
The first really interesting reaction seen by Organic Chemistry students is the Diels-Alder [4+2] cycloaddition, an example of a concerted process where all the bond making and breaking take place concomitantly. The SN2 is also a concerted process, however the Diels-Alder reaction is a gem because of the high density of relative stereochemistry that can be established in a single synthetic step. The most interesting and unique of all organic reactions learned fall within the narrow category of sigmatropic rearrangements, intramolecular pericyclic processes wherein one σ-bond is exchanged for another σ-bond. Example reactions are the Cope and Claisen rearrangements.
The mechanism of the Wittig reaction is not without controversy. Two principal researchers, Prof. Edwin Vedejs (a former personal mentor) from the University of Wisconsin – Madison (now at the University of Michigan) and Prof. Manfred Schlosser from the Université of Lausanne, had very different points of view. The entire “argument” was based upon whether betaines are observed in the absence of lithium salts, i.e. under “salt-free” conditions.