Something important happened in chemistry for the first time about 100 years ago. A molecule was built (nowadays we would say synthesized) specifically for the purpose of investigating a theory. It was cyclo-octatetraene or (CH)8, and it was made by Willstätter and Waser[cite]10.1002/cber.191104403216[/cite] to try to find out if benzene, (CH)6, was an aromatic one-off or whether it might be a member of a series, envisaged as (CH)n. Of course, a hell of a surprise was in store for Willstätter and Waser[cite]10.1002/cber.191104403216[/cite]! Prior to this synthesis, (CH)8 had never existed; nature had not gotten there first. In that sense, chemistry became much like mathematics had before it; it was OK to make molecules because they might be interesting, and for the purpose of investigating possible patterns in nature. So it is in this spirit that I suggest an interesting molecule here. It is a molecular trefoil, constructed by joining 15 pyrrole units together into a ring with appropriate linkers and in effect tying a knot in that ring. A trefoil knot to be specific.
Anatomy of an asymmetric reaction. The Strecker synthesis, part 2.
May 26th, 2010In the first part of the post on this topic, I described how an asymmetric sulfoxide could be prepared as a pure enantiomer using a chiral oxygen transfer reagent. In the second part, we now need to deliver a different group, cyano, to a specific face of the previously prepared sulfoxide-imine. The sulfoxide is now acting as a chiral auxilliary, and helps direct the delivery of the cyanide group to specifically one face of the imine rather than the other. After removal of the aluminum carrier for the cyano group and hydrolysis of the cyano group to a carboxylic acid group, we end up with an enantiomerically pure amino acid.
Anatomy of an asymmetric reaction. The Strecker synthesis, part 1.
May 24th, 2010The assembly of a molecule for a purpose has developed into an art form, one arguably (chemists always argue) that is approaching its 100th birthday (DOI: 10.1002/cber.191104403216) celebrating Willstätter’s report of the synthesis of cyclo-octatetraene. Most would agree it reached its most famous achievement with Woodward’s synthesis of quinine (DOI: 10.1021/ja01221a051) in 1944. To start with, the art was in knowing how and in which order to join up all the bonds of a target. The first synthesis in which (relative) stereocontrol of those bonds was the primary objective was reported in 1951 (10.1021/ja01098a039). The art can be taken one step further. It involves control of the absolute stereochemistry, involving making one enantiomer specifically (rather than the mirror image, which of course has the same relative stereochemistry). Nowadays, a synthesis is considered flawed if the enantiomeric excess (of the desired vs the undesired isomer) of such a synthesis does not achieve at least ~98%. It is routine. But ask the people who design such syntheses if they know exactly the reasons why their reaction has succeeded, you may get a less precise answer (or just a lot of handwaving; chemists also like to wave their hands as well as argue).
A Digital chemical repository – is it being used?
May 4th, 2010In this previous blog post I wrote about one way in which we have enhanced the journal article. Associated with that enhancement, and also sprinkled liberally throughout this blog, are links to a Digital Repository (if you want to read all about it, see DOI: 10.1021/ci7004737). It is a fairly specific repository for chemistry, with about 5000 entries. These are mostly the results of quantum mechanical calculations on molecules (together with a much smaller number of spectra, crystal structure and general document depositions). Today, with some help (thanks Matt!), I decided to take a look at how much use the repository was receiving.
WebCite and Jmol
April 18th, 2010Since I have gotten into the habit of quoting some of my posts in other contexts, I have started to also archive them using WebCite. One can quote the resulting archive as:
Carbobenzene: benzene with a difference
April 16th, 2010Some molecules, when you first see them, just intrigue. So it was with carbobenzene, the synthesis of a derivative of which was recently achieved by Remi Chauvin and co-workers (DOI: 10.1002/chem.200601193). Two additional carbon atoms have been inserted into each of the six C-C bonds in benzene.
The conformation of 1,2-difluoroethane
April 6th, 2010Here I offer another spin-off from writing a lecture course on conformational analysis. This is the famous example of why 1,2-difluoroethane adopts a gauche rather than antiperiplanar conformation.
The gauche and antiperiplanar conformations of 1,2-difluoroethane