Subject: Summary of sigma delocalization

From: "Varnai Peter" <[email-masked]>
Date: Fri, 27 Oct 1995 17:05:56 GMT+100

Dear CCLers,

I expected a bit more interest in sigma aromaticity, but anyway, here 
is my short summary of letters I got. My thanks to those who replied.
If anyone has comments, new thoughts please contact me.

Original:

I am involved in a pericyclic reaction mechanism...My question is:
What do YOU think about having a TS that is stabilized by sigma 
aromaticity? Are You aware of any references on this topic? Probably
in Prof. Dewar's paper in Mol. Struct. Energ. (1988,6,1-61) there is 
something about this. Unfortunately, I don't have access to this 
paper.
I hope that You share Your opinions, and I can summarize to all of 
You!

Answers:

1, from Andreas Goeller 

you asked about work on sigma-aromaticity? Did you get any 
references? I'm working on this effect in three membered rings

    Y   Y          Y   Y      Y=F, OH, Cl, NH2, Br, I, CH3, H
         \ /            \ /       X=P, Si
          X     and      X        charge=+1 for P; 0 for Si
         / \            / \
        C===C          C---C

There is a sigma* orbital on phosphorus which interacts with the 
double or 
single bond C=C /C-C. We call this sigma*-aromaticity. 
I would interested to get the references you got from CCl to get more
background on this topic, even if it's not exactly the same problem.
Thanks in advance
                Bye, AHG...

2, from Vernon G. Box

What is sigma aromaticity and which cyclic hydrocarbons would not 
have it?

VB

3, from George Ford

There is nothing mysterious about conjugative interactions involving 
sigma 
bonds. After all their designation as "sigma" is based purely on 
their local
symmetry. 
You can find Dewar's ideas in an older, but much more accessible 
paper: 
Dewar, M. J. S., J. Am. Chem. Soc. 1984, 106, 669-682.

Hope this helps,

George Ford

#################################################################

Peter Varnai
PhD student
[email-masked]