Best supporting actors

I will dedicate tonight’s post to the silent heroes of organic synthesis, namely some of the excipients we have grown to love. While the term “excipient” is most commonly associated with drug formulation, there is something to be said about an analogy to some key additives that serve their supporting, yet critical, roles in organic synthesis. Without these molecules, you would not see the light of day in many instances where a particularly reactive species needs to be formulated and delivered to a flask near you. In my personal experience (we are going back 15 years for that), urea-hydrogen peroxide was the first combination that taught me to appreciate the supporting role of certain chemicals. In this case – urea, when mixed with hydrogen peroxide, turns into a nice solid material that is commercially available in the form of pellets. I used to buy this stuff from Acros and employed it in reactions where excess water, that comes through the use of aquous hydrogen peroxide solutions, was detrimental to catalysis. Now – how about the iodonium-collidine perchlorate? This is a widely used electrophilic iodinating reagent that benefits from the presence of collidine…

DABSO is a notable recent addition to the list of molecules that deliver valuable and highly reactive species in synthesis. In this compound, developed by Mike Willis of Oxford, the molecule of DABCO is complexed with two molecules of sulfur dioxide. This association enables handling a nasty gas (SO2) in a very straightforward fashion. I am showing just two examples from Mike’s OrgLett paper. The diene case is especially notable as it represents a great way to employ the reversibility of Diels-Alder reaction to protect (and later release) dienes. I have to say: there is something quite special about DABCO in terms of its “supporting” role in many other reagents (Selectfluor comes to mind).


6 thoughts on “Best supporting actors

  1. One nice trick for purifying fancy chiral phosphine ligands is to make phosphine-borane complex. The parent phosphines are quite sensitive to oxidation, so recrystallization is an option if one is careful but column chromatography is difficult (there is always some air on silica or in the solvents). But PR3.BH3 complexes are perfectly air-stable on column, or in solution. The phosphine is liberated from borane complexes… by excess of DABCO.

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