Sometimes, connections between different areas of chemistry just pop out (without the help of semantic web tools, this is called serendipity). So here, I will try to join up some threads which emerge from previous posts. I had noted that antiaromaticity in cyclopropenium anion is lessened by the system adopting gross geometric distortions, which take the anionic lone pair out of conjugation from the ring.
Back in 1994, we published the crystal structure of the molecule below (X=H), a putative anti-malarial drug called halofantrine . Little did we realise that a whole area of organo catalysis based on a thiourea catalyst with a similar motif would emerge a little later. Here is how the two are connected. In our original article we described how our interest was sparked by observing the following chiral HPLC behaviour.
The reaction between a carbene and an alkene to form a cyclopropane is about as simple a reaction as one can get. But I discussed before how simple little molecules (cyclopropenyl anion) can hold surprises. So consider this reaction: Transition state for reaction between ethene and dichlorocarbene.
This is a continuation of the previous post exploring the transition state geometries of various types of ring closure as predicted by Baldwin’s rules. I had dealt with bond formation to a trigonal (sp 2 ) carbon; now I add a digonal (sp) example (see an interesting literature variation).
The Baldwin rules for ring closure follow the earlier ones by Bürgi and Dunitz in stating the preferred angles of nucleophilic (and electrophilic) attack in bond forming reactions, and are as famous for the interest in their exceptions as for their adherence. Both sets of rules fundamentally explore the geometry of the transition states involved in the reaction, as reflected in the activation free energies.
Streptomycin is an antibiotic active against tuberculosis, and its discovery has become something of a cause célèbre. It was first isolated on October 19, 1943 by a graduate student Albert Schatz in the laboratory of Selman Waksman at Rutgers University. I want to concentrate in this post on its molecular structure.
If you have not previously visited, take a look at Nick Greeves’ ChemTube3D , an ever-expanding gallery of reactions and their mechanisms. The 3D is because all molecules are offered with X, Y and z coordinates. You also get arrow pushing ‡ in 3D. Here, I argue that we should adopt Einstein, and go to the space-time continuum!
HCl reacting with a carbonyl compound (say formaldehyde) sounds pretty simple. But often the simpler a thing looks, the more subtle it is under the skin. And this little reaction is actually my prelude to the next post. The mechanism is studied using ωB97XD/6-311G(d,p) with a simulated solvent (acetic acid) included (but not explicit solvent setting up any hydrogen bonds). Transition state HCl + H2C=O. Click for 3D animation.
Many reaction mechanisms involve a combination of bond formation/cleavage between two non-hydrogen atoms and those involving reorganisation of proximate hydrogens. The Baeyer-Villiger discussed previously illustrated a complex dance between the two types. Here I take a look at another such mechanism, the methylation of a carboxylic acid by diazomethane.
Text books (is this a misnomer, much like “papers” are in journals?) in a higher-educational chemistry environment, I feel, are at a cross-roads. What happens next? Faced with the ever-increasing costs of course texts, the department where I teach introduced a book-bundle about five years ago. The bundle included all the recommended texts for an appreciable discount over individual purchase.
