From color purple to alkenes

The color of adamantane thione is deep purple, which makes it one of the most strikingly beautiful compounds I have had a chance to observe (I also like the rock band…). The first time I saw adamantane thione was in 1995, when Dr. Grzegorz Mloston, a visiting scientist in the Olah lab, showed me a column onto which he loaded this compound. The band’s movement was sharp and purposeful, as if the molecule was trying to assert itself and tell me that color is not always due to the presence of some aromatic donor/acceptor chromophore. Indeed, I had to do some reading in order to understand why this color was so intensely bright!

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It was at about that time that I got interested in thiones. They do carry a couple of unfortunate “side effects”. First, they are not particularly stable to hydrolysis. This brings about the next challenge as these compounds are malodorous due to the hydrolyzed hydrogen sulfide, which our olfactory system can detect in exceedingly small amounts. Nonetheless, the relatively weak conjugation between carbonyl carbon and sulfur atoms enables some absolutely fascinating applications and I will talk about one of them: the Barton-Kellogg method. I saw it in action some 20 years ago and I have since considered the reaction to be a very special, albeit less widely utilized, method for making alkenes. We typically think of Wittig reaction, metathesis, eliminations, etc. However, if you want to make a “sterically challenged” alkene, few methods come close to the Barton-Kellogg process. During this reaction, a five-membered intermediate is created as a result of a dipolar cycloaddition. Triphenylphosphine then triggers the decomposition of the five-membered ring, liberating alkene, gaseous nitrogen, and sulfur.

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Professor Huisgen, the father of dipolar cycloaddition reactions, is in such awe of thiones that he refers to them as superdipolarophiles. Take a look at the reference link below.

http://onlinelibrary.wiley.com/doi/10.1002/hc.20262/abstract

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