Is science sometimes in danger of getting tunnel vision? Recently published ebook author, Ian Miller, looks at other possible theories arising from data that we think we understand. Can looking problems in a different light give scientists a different perspective?

Challenge Answer: W-H Rule Exception

First, the simple answer to my challenge, which was whether there were exceptions to the Woodward Hoffmann rules.  The exception I wish to discuss involves the cyclization of pentadienyl carbenium ions to cyclopentenyl carbenium ions [Campbell et al. J A C S 91 : 6404-6410 (1969)]. Part of the reason for the challenge was to illustrate a further point, namely that quite a bit of critical information is out there, but if it is not recognized quickly as being important, it gets lost. However, it can be found because there is usually someone out there that knows about it. Such information could be recovered more readily if there were a web facility for raising such questions. To succeed, it must be easy to use and abuse prevented so that scientists in general will cooperate.
 
A further point I was trying to make was to emphasise logic. This was a problem at the end of my ebook Elements of Theory and these problems had the structure that the reader had to try to come to an answer, then analyse my given answer, parts of which were not necessarily correct, the idea being to promote the concept of critical thinking. Part of the answer I gave included what I thought was an innocent enough deception, namely the Woodward Hoffmann rules were violated because of extra substitution. That, of course, has a logical flaw; without the substitution you cannot tell whether the rules are violated or not. The reason for mentioning this here is that in the abstract to the example that Chris Satterley found, the same statement is made.
 
So, for a further little challenge, what is the most illogical thing you can associate with standard quantum mechanics? My answer soon.
 
The cyclization of pentadienyl carbenium ions could be followed because there was a methyl group at each end, and the stereochemistry was known. The deviations from the W-H rules were explained in terms of the steric clashes that occurred  as the ends came together: H-H, H-Me and Me-Me. The H-H clash produced more or less complete agreement with the W-H rules, but the Me-Me clash led to an almost 50-50 product distribution between the two possible cyclopentenyl ions. There is little doubt that a steric clash occurred as the carbenium ion approached the geometry needed for cyclization to take place, however at least one further thought should be considered. The pentadienyl carbenium ion should be planar, so the methyl-methyl clash does not give a preference to any particular rotation. A 50-50 mix of products suggests that when the ends come together, random vibrations lead the methyl groups to slide one way or the other, and both ways cyclize. This is the important bit: if the W-H rules were absolute, what would happen is that the prohibited route would not react, the material would remain in the pentadienyl form, reopen, then come together again, and would react only if/when it accidentally got into the correct configuration. The steric clash would result in both reactions being slower. As it happened, in the actual experiments all cyclizations were too fast to know about that aspect.
 
However, while additional substitution was present in the ring opening reactions of benzocyclobutene, the strain issue is not quite relevant because by the time the substituents generate a good clash, the system has proceeded far enough along the reaction coordinate that extra strain would simply drive the ring opening faster.
 
Which brings me to the second little challenge: is there any chemical theory that offers a rational explanation for these exceptions to the W-H rule? I must confess that back in the late 1960s I simply assumed that this exception merely meant that the W-H rules were simply preferences, and with a little discouragement, they could be over-ruled, however I am now convinced that this is just lazy, and when nature provides exceptions like this, it is trying to tell us something.
Posted by Ian Miller on Aug 16, 2011 6:16 AM Europe/London

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