musings on music and life

November 21, 2014

Prof. Paul Schleyer passes away

Filed under: Chemistry — Tags: — sankirnam @ 6:06 pm

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!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!adamantyl dication

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…

November 13, 2014

amphoteros

Filed under: Chemistry, Uncategorized — Tags: — sankirnam @ 8:19 pm

I’ve been following Prof. Andrei Yudin’s blog for a while, and wanted to mention it here since he recently discussed an article by Prof. Karl Christe (one of my committee members). Prof. Yudin got his PhD with my advisor approximately 20 years ago, and after completing a postdoc with Prof. K. Barry Sharpless (Scripps), became a very successful professor at the University of Toronto. He initially did some interesting work with fluorinated BINAP and BINOL complexes and investigated their catalytic activity. However, his real claim to fame is his discovery of the rich chemistry of aziridine aldehydes.

One of the long-standing challenges in organic chemistry is the synthesis of compounds that contain complementary reactive groups in the same molecule. One example of this type of molecule is an unprotected amino aldehyde. Amines and aldehydes readily condense to form imines and/or hemiaminals depending on the conditions, and that is why one will never see small molecules containing free -NH2 and -CHO groups in the same structure. Prof. Andy Myers (Harvard) has synthesized N- and C– protected amino aldehydes as precursors for the in situ generation of free amino aldehydes.

Prof. Yudin elegantly demonstrated that if the amine is in the form of an aziridine (3-membered ring), then there is a kinetic barrier for imine formation, and thus unprotected (-NH) amines can exist along with an aldehyde functionality in the same molecule. These aziridine aldehydes can be accessed from DIBAL-H reduction of aziridine esters. They exist as homodimers and it is possible to chemoselectively access amine or aldehyde reactivity with the appropriate reagents and conditions.

Prof. Yudin continues his work with other intriguing amphoteric molecules and has also demonstrated novel applications for these new compounds, including their use towards the synthesis of cyclic peptides.

November 11, 2014

Gems from 1992

Filed under: Carnatic Music, Uncategorized — sankirnam @ 7:54 pm

I’ve long been obsessed with the recordings of TNS’s North American concert tour in 1992. It should be no surprise; TNS was the reigning Carnatic vocalist at that time, and the concerts in that tour reflect his absolute command over swara and laya. His voice was also in full form in all of those concerts, and the brigha-laden sangathis he was was able to create on the spot just leave one shaking his or her head in silent awe.

There’s another reason for my obsession, which is probably the biggest factor here. It is that this was my guru’s first concert tour of the US; he had the privilege of accompanying the top vocalist at that time for over 20 concerts when he was in his early 20’s! Narayanan sir’s playing style back then was also fundamentally different from his style today. All of the recordings of these concerts demonstrate his mastery over the use of the kappi mrudangam, and are an amazing demonstration of the traditions and legacy of Thanjavur Upendran sir.

I think these tracks speak for themselves in the quality of the vocal and mrudangam artistry, as well as the chemistry that TNS and Narayanan sir had in those concerts.

 

 

 

 

 

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