From: Sengen Sun (sengensun$##$yahoo.com)
Date: Sat Mar 18 2006 - 13:18:15 EST
Thanks to every one for patience. I have received
quite a number of e-mails of tremendous interests.
Several people have to wait for a few days for me to
get back to them. I have to continue the public
discussion as soon as there are outstanding issues to
be addressed. But I will try my best to limit my
posts. This post is a very important one.
Here are extremely important questions by Mateo:
--- Mateo Alajarin <alajarin$##$um.es> wrote:
> Dear Dr Sun
>
> You recently wrote (in a message to orglist):
>
> >I do not have a beter way to explain, but let me
> give
> >you recent one of particular examples of conceptual
> >confusion made by Hoffmann (See JACS, 2002, 124,
> 6836,
> >Scheme 1, I will forward this article to any one if
> >requested by e-mail). There, the intramolecular
> proton
> >transfer was mistakenly treated as hydride
> transfer.
> >That is, the arrows should be drawn in the exactly
> >opposite direction.
>
> This paragraph caught specially my attention. May I
> ask you why you believe
> that the arrows in that Scheme should be drawn in
> the opposite direction?
> As far as you know, is there a rule (or a way, or a
> parameter) for
> identifying if, in a sigmatropic H shift between two
> carbon atoms, the
> migrating atom is more or less proton-like or
> hydride-like?
>
> Thanks in advance for your answer,
>
> Mateo.-
>
> XXXXXXXXXXXXXXXXXXXX
> Mateo Alajarin
> Dep. Quimica Organica
> Fac. Quimica
> Universidad de Murcia
> Campus de Espinardo
> 30100 Murcia
> Spain
> Phone +34.968.367497
> Fax +34.968.364149
> email: alajarin$##$um.es
> XXXXXXXXXXXXXXXXXXXX
>
Mateo's questions are quite common in the community.
But these questions are both very important and very
weird! My question is why these questions should be
questions?
If you have good knowledge of general acid and general
base in organic chemistry, you shouldn't ask these
questions. Alkane C-H is a very weak hydrocarbonic
acid, while CC pi electrons are a base. When they
react, what definitely CANNOT happen is the hydride
transfer from C-H to a CC pi bond no matter
intermolecular or intramolecular with or without
aromatic transition state.
Are there cases that C-H bond reacts as a hydride
donor? Yes. But it requires that the other reacting
partner be much stronger general acid (or very
electron-deficient). In general, proton transfer or
hydride transfer really depends on a particular
dynamic environment between two reactants.
Why such basic knowledge has been made so messy? We
have been driven insane by orbitals for decades!
Another person asked a much harder question for the
parent Diels-Alder reaction in my ppt presentation.
In Organic Chemistry by Fox & Whitesell, it says that
"the progression of this reaction is indicated by the
full-headed curved arrows showing the motion of each
electron pair as the p electrons of each reactant are
delocalized in the cyclic, conjugated transition
state". When I saw such a statement at the University
of Chicago Bookstore in 1997, I bought the book
immediately. I had never owned an organic chemistry
textbook in English as I had not seen a good one.
How did I know that Fox & Whitesell are surely right?
Karadakov et al proposed a concerted spin-recoupling
(JACS 1998, 120, 3975), but as a consequence of
artificial effects of their computational approach!
The problem is that we have denied the underlying
physics of chemical reactions. As in my ppt
presentation (if you have not seen it yet,
http://www.ccl.net/cca/documents/ConcertedCycloadditons_2/),
when two neutral molecules collide, the first physical
event is electron-electron repulsion. When the e-e
repulsion develops, the electrons will seek a more
comfortable place to stay in order to avoid the
repulsion. That is why the electron-pushing mechanism
is justified. It is such a simple thing. We cheat
ourselves by looking at the HOMO-LUMO interaction
picture that does not mean anything.
But the electron-pushing mechanism for the parent D-A
reaction was NOT significantly recognised, because we
could not understand any fundamental reasons of the
regioselectivity in cycloadditions. That is why I have
made a breakthrough here.
For the parent D-A reaction, it is not possible to get
some data to prove the electron-pushing mechanism
based on our current computational and experimental
techniques. But my computational data on other
cycloadditions validate the e-e pushing mechanism
indirectly.
The intra- or inter-molecular proton transfer can be
easily proved computationally. In the last century, I
studied Alder-ene reactions such as that between
ethene and propene that involves a proton transfer via
an aromatic transition state. The data are still
stored in one of my antique computers. I could not get
anything published. The faults don't lie with any
referees or editors but with our fundamentally wrong
chemical reaction theories based on MOs. One referee
asked, how do you know the electron flow is caused by
electrostatic forces? I gave up with laughing -:)
That is long enough. Thanks.
Regards to all.
Sengen
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