ORGLIST: Challenge: To find Reaction with most bond forms/cleaves

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From: Rzepa, Henry (h.rzepa$##$ic.ac.uk)
Date: Mon Nov 30 1998 - 04:16:52 EST


Can I set the following challenge?

Over the years, I and my students have studied many types of reactions
using Quantum chemistry modelling techniques. I was recently asked
to write a lecture on "transition states", and started to wonder about
examples of reactions classified by the total number of bonds actually
formed or broken in what we will presume is a concerted (but not
necessarily synchronous) transition state.

Thus examples of 0, 1- 2- and 3- bond reactions are trivial to
set down. 4-bond reactions are less common, and 5-bond reactions
I suspect fairly scarce (I give two such here, namely peroxidation
of an alkene by peracid; deoxygenation of the ozonlolysis intermediate
by eg Ph3P). Can anyone think of any 6-bond and higher order reactions?
I suspect some tandem radical reactions must be strong challengers,
although here issues of whether the reactions are really concerted, or
stepwise must come to the fore. If anyone knows of any calculations
showing synchronicity in such tandem reactions, I would be most
interested.

Excluded from this challenge are what I would describe as examples
involving concurrent lesser-bond examples, ie two concurrent
Diels Alder additions to make a 4-bond reaction, which I think theory
would almost certainly predict were stepwise (although see
http://www.ch.ic.ac.uk/rzepa/RSC/P2/cr2/ )

Dr Henry Rzepa, Dept. Chemistry, Imperial College, LONDON SW7 2AY;
mailto:rzepa$##$ic.ac.uk; Tel (44) 171 594 5774; Fax: (44) 171 594 5804.
URL: http://www.ch.ic.ac.uk/rzepa/
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