I was intrigued by one aspect of the calculated transition state for di-imide reduction of an alkene; the calculated NMR shieldings indicated an diatropic ring current at the centre of the ring, but very deshielded shifts for the hydrogen atoms being transferred. This indicated, like most thermal pericyclic reactions, an aromatic transition state. Well, one game one can play with this sort of reaction is to add a double bond.
More than 60 million molecules are known, and many are fascinating. But beauty is in the eye of the beholder. Thus it was that I came across the attached molecule[cite]10.1021/ja982065w[/cite]. It struck me immediately as, well, beautiful! GOCTOH. Click for 3D. This is one that comes to life in 3D and I strongly urge you to inspect it as such by clicking on the above. Why is it so interesting?
A game chemists often play is to guess the mechanism for any given reaction. I thought I would give it a go for the decomposition of the tris-peroxide shown below.
Thalidomide is a chiral molecule, which was sold in the 1960s as a sedative in its (S,R)-racemic form. The tragedy was that the (S)-isomer was tetragenic, and only the (R) enantiomer acts as a sedative. What was not appreciated at the time is that interconversion of the (S)- and (R) forms takes place quite quickly in aqueous media.
When methyl manganese pentacarbonyl is treated with carbon monoxide in e.g. di-n-butyl ether, acetyl manganese pentacarbonyl is formed. This classic experiment conducted by Cotton (of quadruple bond fame) and Calderazzo in 1962[cite]10.1021/ic50001a008[/cite] dates from an era when chemists conducted extensive kinetic analyses to back up any mechanistic speculations. Their suggested transition state is outlined below.
The reaction described in the previous post (below) is an unusual example of nucleophilic attack at an sp 2 -carbon centre, reportedly resulting in inversion of configuration[cite]10.1021/ja00765a062[/cite]. One can break it down to a sequence of up to eight individual steps, which makes teaching it far easier. But how real is that sequence?
Every once in a while, one encounters a molecule which instantly makes an interesting point.
There is often a disconnect between how a text-book (schematically) represents a reaction and a more quantitive “reality” revealed by quantum mechanics. Is the bromination of ethene to give 1,2-dibromoethane one such example? Text-books will show how ethene interacts with bromine to form a cyclic bromonium cation, which with the liberated bromide anion makes for an ion-pair.
Metathesis reactions are a series of catalysed transformations which transpose the atoms in alkenes or alkynes.
Alkene metathesis is part of a new generation of synthetic reaction in which a double C=C bond is formed from appropriate reactants where no bond initially exists (another example is the Wittig reaction), with the involvement † of a 4-membered-ring metallacyclobutane ring 1 (again, very similar to the Wittig). I thought it might make a good addition to my collection of reaction mechanisms and so as the first step
