From: Sengen Sun (sengensun$##$yahoo.com)
Date: Wed Mar 22 2006 - 14:25:47 EST
As requested by a few people, here are some highlights
of my new mechanistic discovery and its significance:
1.
If you accept that the mechanism is about how and why
electron density moves around, a general mechanism of
concerted cycloadditions was never established at the
theoretical level for 70 years after the discovery of
D-A reaction.
2.
We chemists had been puzzled by cyclic reactions, and
lost our direction along the rings for decades. Our
thinkings were confined to the minimun energy path (or
IRC) approach which was established by Fukui and
others. The IRC path is actually a path from a big
ring of two reactants to a small ring of the final
product. That is, for the parent D-A, the two forming
sigma bonds are kept exactly equal length along the
whole path. The natural mystery is actually burried
deeply insight the ring. No one can understand
anything about what electrons really do.
3.
Concerted formation of two bonds is too tricky to be
understood based on our conventional knowledge. Even
Woodward and Katz were puzzled about why acrolein
dimerization does not take place according to matching
the polarity of two acrolein molecules. The observed
product is solely formed by connecting the two beta
carbons and by connecting oxygen of heteo-diene and
alpha carbon of the dienophile. This kind of regio
selectivity is very common in 1,3-dipolar
cycloadditions, and is against any of your intuitions
no matter you are an experimentalist or a
theoretician. There was absolutely not a clue for me
to make any guess. Some people mistakenly claimed that
the "electronically favored product" cannot be formed
because of orbital control, or because of secondary
orbital interaction or secondary forces etc.
4.
In 1998, I tested a strategy to break the ring of the
transition state structure, keeping only one partial
sigma bond intact but forcing the other to stay far
about 3.5 A away. Such a strategy would allow the
electronic interaction at the shorter partial bond to
be examined in an isolated manner while the effect at
the longer partial bond can be neglected. Later, I
have called this strategy as arbitrary or unballanced
collision dynamics.
5.
When I used such a strategy for acrolein dimerization,
I found that the polarity of the dienophine-CC double
bond is reversed upon a collision between the
diene-oxygen and dienophile-alpha-carbon. Such a
polarity reversal is indicated by an electron-rich
beta-carbon that can donate electron to an
electron-acceptor in approximation. In the same time,
the diene-oxygen dumps a small part of electron to the
dienophile-alpha-carbon in the arbitrary collision.
Consequently, the electron-accepting ability of
diene-beta-carbon is enhanced. These findings led me
to propose the electron-flow mechanism as the new
knowledge that we had never learned before.
6.
In extended studies, I found that the polarity change
of a CC double bond upon a collision with
electron-rich oxygen is a general phenomenon. This
phenomenon does not have any noticeable significance
in non-cyclic reactions, but can account for the
mysteries of regioselectivity in many cycloadditions.
7.
My conclusion is that the easiness of electronic
migration decides the regioselectivity or any other
chemical reactivity. This is essentially evident to
any one, but has never been documented anywhere. What
we had in the literature is that orbital overlapping
decides. But orbital overlapping is just a kind of
mathematical operations and does not have any physical
meaning. Therefore, no one understands it. Good
science should be easily understood.
I really appeal for help from the community to openly
criticize my scientific arguments.
By the way, two people complained that my pdf file at
the CCL site has a very bad quality. I am trying to
replace it with much more readable MS-Word file. Sorry
and thanks for your patience. But now I urge you to
read the ppt file if you have not read it yet:
http://www.ccl.net/cca/documents/ConcertedCycloadditons_2/
Regards to every one.
Sengen
--- HERVE Gregoire <G.HERVE$##$snpe.com> wrote:
> Dear Colleague,
>
> I am really interested in your new and general
> mechanism regarding concerted cycloaddition. I read
> your message to orglist and understand your feeling
> about referees and difficulties to change their
> minds about something new. I am not aware at all
> about this discovery, but as a scientist, I always
> try to keep in touch with novelties in organic
> chemistry and general chemistry also. Could you tell
> me please in a few words which differences regarding
> concerted cycloaddition mechanism have you
> highlighted? Can you suggest me a characteristic
> paper explaining this?
>
> Best wishes,
> Congratulations for this work,
>
>
> Grégoire HERVE
> Responsable études synthèse
> SME Centre de Recherches du Bouchet
> Groupe SNPE
> +33 (0)1 64 99 11 81
> g.herve$##$snpe.com
>
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