There are many treasures in Woodward and Hoffmann’s (WH) classic monograph. One such is acetolysis of the endo chloride (green), which is much much faster than that of the exo isomer (red). The explanation given in their article (p 805) confines itself to succinctly stating that only loss of the endo halogen can be concerted with a required disrotatory ring opening of the cyclopropane. Demonstrating the truth of this statement by computational modelling turns out to be an interesting challenge.
I should start by saying that back in 1970, modelling this process quantitatively using extended Hückel Theory (used by Hoffmann to quantify the orbital behaviours) was not an option. It involves the formation of an ion pair from an initial covalent reactant, and then studying the dynamics of whether or not this formation is concerted with the pericyclic ring opening. These two mechanisms may or may not be conflatable (SN1 + electrocyclic). A lot is going on, and it is only very recently that quantum mechanical procedures for combining two such different electronic processes on an equal footing have become available. I did this by using a ωB97XD/6-311G(d,p) method combined with two explicit solvent molecules for the ion-pair, augmented with a continuum solvent field (in this case ethanoic acid).
The characteristics of the reaction coordinate (IRC) for the endo isomer are shown below, followed by an animation of the geometries.
What about the exo isomer (red above)? The absence of detectable reaction implies the experimental free energy barrier must be ≥ 40 kcal/mol. Cleavage of the C-Cl bond is effectively orthogonal with the electrocyclic opening of the C-C cyclopropyl ring, which forces the two mechanisms to occur separately. The first process is an SN1 ionisation of the halide to form the trans ion-pair below, which has a free energy of 34.9 kcal/mol relative to the reactant, which is then followed by electrocyclic ring opening as a separate step. This would make a lower bound to its calculated reaction rate at least 10 million times slower than the endo isomer. You can see from the coordinates below that the allyl cation cannot become co-planar, which explains its high energy (formation of trans cycloalkenes is known to occur by such a reaction).
So we have here two quite different reactions. The first is an example of a mechanistic morpheme, a solvolytically assisted pericyclic reaction. It combines two different mechanisms from different areas of organic chemistry (areas which are most often taught as separate lecture courses, and where the holistic connection between the two is rarely made). The second keeps the two reactions quite separate and sequential. All because of an apparently innocent difference in their stereo chemistries.Henry Rzepa. URL:http://www.ch.imperial.ac.uk/rzepa/blog/?p=5888. Accessed: 2011-12-16. (Archived by WebCite® at http://www.webcitation.org/63yAww6SH)
In the mid to late 1990s as the Web developed, it was becoming more obvious…
I have written a few times about the so-called "anomeric effect", which relates to stereoelectronic…
The recent release of the DataCite Data Citation corpus, which has the stated aim of…
Following on from my template exploration of the Wilkinson hydrogenation catalyst, I now repeat this…
In the late 1980s, as I recollected here the equipment needed for real time molecular…
On 24th January 1984, the Macintosh computer was released, as all the media are informing…
View Comments
What a valuable addition to my Advanced Organic Chemistry class! Thank you very much for posting it!