I know it seems like all I do here is obituaries; I’ve written many posts in tribute to giants of our time who left us all too early. But with the impending time crunch (I’ve finally scheduled my thesis defense for December 10), I can only talk about things that make me jump up and take notice, so to speak.
I was just informed by my boss that Prof. Paul v. R. Schleyer passed away earlier this morning; he was found in his bathroom by his wife. Prof. Schleyer was a giant of physical organic chemistry, and a contemporary and close friend of Prof. George Olah. He received his PhD at Harvard under Paul Bartlett and started his career at Princeton University. In the book Cage Hydrocarbons, Schleyer mentions in his autobiographical account that he had started his tenure-track position at Princeton before his thesis was formally completed! Things back then were much more leisurely and less competitive in comparison to today. He also mentioned that he did not manage to get any publications during his PhD research, and that his thesis was a thorough review of carbocationic hydrocarbon chemistry, which got a positive review from Prof. R. B. Woodward. Nowadays, it would be unthinkable for anyone to apply for a faculty position at any university if he or she did not publish anything during the course of their PhD research!
Nonetheless, Prof. Schleyer mentioned that the experience at Harvard had prepared his mind for the big discovery he was to make at Princeton. Prof. Schleyer discovered that heating tetrahydrodicyclopentadiene in the presence of AlBr3 resulted in the formation of numerous products due to skeletal carbocationic rearrangements, most of which were liquid. When all the liquid product was distilled, a solid began to condense on the distillation head from sublimation of the crude left in the distillation pot. This crystalline solid was found to have an extremely high melting point and symmetry, which was only consistent with the properties of adamantane!
The efficient chemical synthesis of adamantane was one of the holy grails of synthetic organic chemistry until that time. Adamantane had been prepared by total synthesis by a rather laborious, low-yielding route. It had also been isolated previously from naturally occurring Czech petroleum distillates, but the amount obtained was very miniscule, limiting opportunities to study its reactivity and chemistry. Thus, Prof. Schleyer’s single-author 1957 JACS communication opened the floodgates to the study of the chemistry of adamantane and related cage hydrocarbons.
The mechanism of the rearrangement is rather complex; several deep-seated alkyl and hydride migrations are involved.
This is a rather abbreviated view of “adamantaneland”, showing only a few of the migrations and rearrangements involved. Nonetheless, the 1-adamantyl cation is the thermodynamic sink of this system, and all the isomeric cations inevitably rearrange to the 1-adamantyl cation under thermodynamic conditions.
Prof. Schleyer also worked on many disparate areas, including carbonium and onium ion chemistry, lithium chemistry, hypervalent species and many other topics. He was also one of the few experimental organic chemists to embrace computational methods, and this prompted him in the 1970’s to move eastward (from the US to Germany) in contrast to the hundreds of other professors at the time. He was a professor at the University of Erlangen-Nurenberg for over two decades. During that time, his group managed to synthesize and characterize the 1,3-dehydro-5,7-adamantanediyl dication, which was one of the first examples of 3-D aromaticity. It was featured on the cover of that issue of Angewandte Chemie!
I mentioned last year that the structure of the long-controversial 2-norbornyl cation had at last been solved by X-ray crystallography. This effort was also assisted by Prof. Schleyer. I remember at Prof. Olah’s 85th birthday in 2012 he had mentioned preliminary results in this subject, and was much more excited about this than about any of the other work he was presenting!
However, Prof. Schleyer’s most highly-cited paper is related to computational chemistry. In the 1990’s, he described the NICS effect, which could be used as a quantitative measure of aromaticity in a chemical species. The definition of “aromaticity” is rather slippery; most undergraduates will learn the basic definition as a cyclic array with 4n + 2 π electrons. However, this is difficult to extend to other species, and degrees of aromaticity are notoriously difficult to quantify (e.g. how do you know if one molecule is more “aromatic” than another?). With the NICS effect, this could be done easily and reliably using existing quantum chemical computational methods. I think this paper had over 4000 citations the last time I checked!
Of course, Prof. Schleyer was an influential teacher as well. Two of his most prominent students include Prof. Peter Stang (Editor of JACS) and Prof. Herbert Mayr.
I managed to get one of my books (Stable Carbocation Chemistry) signed by Prof. Schleyer in 2012 when he came to USC for Prof. Olah’s 85th birthday. I only purchased my copy of Cage Hydrocarbons the following year. Alas, now I will not be able to get that signed…