Subject: Largest No. of bonds formed/cleaved in Concerted TS?

From: "Rzepa, Henry" <[email-masked]>
Date: Mon, 30 Nov 1998 09:18:02 +0000

Whilst writing a lecture on the diversity of transition states, the thought 
struck me that the vast majority of "concerted" transition states involve
0 -4 bonds being formed or broken (I leave aside controversial
questions such as e.g are  2-bond Diels Alder Reactions concerted
or really 1-bond biradicaloid reactions). Two examples of 5-bond
reactions readily spring to mind (epoxidation by peracid of an alkene; Ph3P deoxygenation
of a molozonide) but  6-bond and higher seem much rarer.  I 
suspect tandem radical additions might be such, although would probably
work out stepwise using theory?

Has anyone come across candidates for the world record
 at eg RHF or MCSCF ab initio levels of theory ?

It is of course possible to create artificial multi-bond reactions by
requiring two or more lesser-bond reactions to occur concurrently.
For example, we once looked at   12-bond reaction, involving six
synchronous Diels Alder additions (dimerisation of  C30H12),
although unsurprisingly, this proved to be a sixth order 
stationary point and not a real transition state.  Therefore any multi-bond
reaction which is really two or more concurrent smaller reactions is excluded
from the above. 

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