In the preceding post, I introduced Dewar’s π-complex theory for alkene-metal compounds, outlining the molecular orbital analysis he presented, in which the filled π-MO of the alkene donates into a Ag+ empty metal orbital and back-donation occurs from a filled metal orbital into the alkene π* MO. Here I play a little “what if” game with this scenario to see what one can learn from doing so.
Archive for the ‘Historical’ Category
The concept of a shared electron bond and its property of an order is almost 100 years old in modern form, when G. N. Lewis suggested a model for single and double bonds that involved sharing either 2 or 4 electrons between a pair of atoms[cite]10.1021/ja02261a002[/cite]. We tend to think of such (even electron) bonds in terms of their formal bond order (an integer), recognising that the actual bond order (however defined) may not fulfil this value. I thought I would very (very) briefly review the history of such bonds.
William Henry Perkin is a local chemical hero of mine. The factory where he founded the British (nay, the World) fine organic chemicals industry is in Greenford, just up the road from where we live. The factory used to be close to the Black Horse pub (see below) on banks of the grand union canal. It is now commemorated merely by a blue plaque placed on the wall of the modern joinery building occupying the location.
Infra-red spectroscopy of molecules was introduced 110 years ago by Coblentz[cite]10.1103/PhysRevSeriesI.20.273[/cite] as the first functional group spectroscopic method (“ The structure of the compound has a great influence on the absorption spectra. In many cases it seems as though certain bonds are due to certain groups.“). It hangs on in laboratories to this day as a rapid and occasionally valuable diagnostic tool, taking just minutes to measure. Its modern utility rests on detecting common functional groups, mostly based around identifying the nature of double or triple bonds, and to a lesser extent in differentiating between different kinds of C-H stretches[cite]10.1002/chem.201200547[/cite] (and of course OH and NH). One common use is to identify the environment of carbonyl groups, C=O. These tend to come in the form of aldehydes and ketones, esters, amides, acyl halides, anhydrides and carbonyls which are part of small rings. The analysis is performed by assigning the value of the C=O stretching wavenumber to a particular range characteristic of each type of compound. Thus ketones are said to inhabit the range of ~1715-1740 cm-1 and simple esters come at ~1740-1760 cm-1, some 20-30 cm-1 higher. Here I try to analyse how this difference arises.
The benzidine rearrangement is claimed to be an example of the quite rare [5,5] sigmatropic migration[cite]10.1021/ja00335a035[/cite], which is a ten-electron homologation of the very common [3,3] sigmatropic reaction (e.g. the Cope or Claisen). Some benzidine rearrangements are indeed thought to go through the [3,3] route[cite]10.1021/ja00309a041[/cite]. The topic has been reviewed here[cite]10.1002/poc.610020702[/cite].
I have written earlier about dihydrocostunolide, and how in 1963 Corey missed spotting the electronic origins of a key step in its synthesis.[cite]10.1021/ja00952a037[/cite]. A nice juxtaposition to this failed opportunity relates to Woodward’s project at around the same time to synthesize vitamin B12. The step in the synthesis that caused him to ponder is 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. Nowadays, quantum modelling can provide good in-silico estimates of the (free) energy barriers for such processes, which in this case is a simple keto-enol tautomerism. In a recently published article[cite]10.1002/chem.201202651[/cite], just such a simulation is reported. By involving two explicit water molecules in the transition state, an (~enthalpic) barrier of 27.7 kcal/mol was obtained. The simulation was conducted just with two water molecules acting as solvent, and without any additional continuum solvation applied. So I thought I would re-evaluate this result by computing it at the ωB97XD/6-311G(d,p)/SCRF=water level (a triple-ζ basis set rather than the double-ζ used before[cite]10.1002/chem.201202651[/cite]), and employing a dispersion-corrected DFT method rather than B3LYP.
All organic chemists are familiar with the stereochemical notation for bonds, as shown below. But I had difficulty tracking down when it was introduced, and by whom. I offer a suggestion here, but if anyone reading this blog has got a better/earlier attribution, please let us know!