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The mechanism of concerted cycloadditions a philosophical challenge of scientific explanation

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Title: The mechanism of concerted cycloadditions a philosophical challenge of scientific explanation


1
The mechanism of concerted cycloadditions - a
philosophical challenge of scientific
explanation 
  • Sengen SunDiscovery Partners International,
    Inc.9640 Towne Centre Drive
  • San Diego, California 92121

2
Chemical reactions are about migrations of
nuclei and electrons
- A self evident philosophical thinking.
  • Experimental results tell how nuclei migrate.
  • Chemists use arrows or semi arrows to indicate
    electron
  • migration for understanding.

Chemical reaction mechanism where comes the
electron resource and where does it go in the
bond formation and breaking?
3
Pattern and explanation in chemistry
1. A pattern is a collection of phenomena.
Pattern is more related to migrations of nuclei.
2. Explanation is an understanding of phenomena.
Explanation is more based on electron migration.
Electron migration plays a deciding role in the
nuclear migration and is the essence of chemical
reaction mechanism.
4
Concerted cycloadditions (CCs)
  • They are highly useful reactions in the
  • formation of ring structures.
  • 2. They are specially interesting, stimulating,
  • and obfuscating.

5
Patterns in CCs
Alders rules on Diels-Alder reactions 1. The
reactivity of a Diels-Alder reaction is increased
when the electron-donating ability of a
substituent(s) on the diene is increased and/or
the electron-accepting ability of a
substituent(s) on the dienophile is increased.
2. Diels-Alder reactions are endo-selective.
3. Diels-Alder reactions are cis-oriented.
See J. Sauer. Angew Chem Int Ed 6 16-33,
1967.
6
Frontier molecular orbital theory
  • Chemical reactivity is decided by the smaller
  • HOMO-LUMO energy gap between two reactants.
  • 1. It was founded on Alders rules empirical
    rules.
  • 2. It is more an empirical pattern than an
    explaining theory.
  • 3. It is a quantitative empirical rule?
  • 4. It has no generality.
  • (M. J. S. Dewar. A critique of frontier orbital
    theory.
  • THEOCHEM 200 301-323, 1989. ).

7
Explanation of CCs was a blank for 70 years!

Both of Alders rules and FMO theory are about
the same pattern concerning outcomes of nuclear
rearrangements in the 3-D space based on the
characteristics of reactants. They are not an
equivalence to an explanation. They have to be
explained for understanding.
8
A law in chemists minds
- the Coulombic law on electrostatic forces
When this law is applied in chemical
reactions A negatively charged center attacks
positively charged one. Or a positively charged
center attacks negatively charged one.
Or aligning regions of charge concentration
with regions of charge depletion (Bader
MacDougall J. Am. Chem. Soc. 1985, 107, 6788).
9
Do some CCs violate Coulombic Law?
10
Are there two cases of reactions?
The one was the frontier-controlled case in
which the reaction was controlled by the
particular orbital interaction, and the other
was the charge-controlled case, where it was
controlled by electrostatic interaction of
charges.
K. Fukui, Science, 1982, 218, 747-218
11
Philosophical analyses of orbitals
  • Recent related analyses of orbitals
  • Bader et al. Angew. Chem. Int. Ed. Engl. 1994,
    33, 620-631
  • Bader, Int. J. Quant. Chem. 2003, 94, 173-177.
  • Gillespie Popelier, From Lewis to Electron
    Densities
  • Ogilvie, J. Chem. Ed. 1990, 67, 280-289.
  • Scerri, J. Chem. Ed. 2000, 77, 1492-1494.
  • Scerri, Philosophy of Science 2001, 68
    (Proceedings)S76-S88

12
But continuesthe excessive use of orbitals
1. Orbital Interactions and Their Effects on 13C
NMR Chemical Shifts for 4,6-Disubstituted-2
,2-dimethyl-1,3-dioxanes. J. Phys. Chem. A.
(Article) 2005 2. Phase Coherent Electronics A
Molecular Switch Based on Quantum
Interference. JACS 2002, 124, 4200. 3. The Role
of Orbital Interactions in Determining the
Interlayer Spacing in Graphite Slabs. JACS
2000. 4. Sulfur-Gold Orbital Interactions which
Determine the Structure of
Alkanethiolate/Au(111) Self-Assembled Monolayer
Systems J. Phys. Chem. B. 2002 106(49)
12727-12736
13
Quantum Reality? And easy understanding?
Murray Gell-Mann has described quantum mechanics
as 'that mysterious, confusing discipline which
none of us really understands but which we know
how to use'. Richard Feynman I think I can
safely say that no-one understands quantum
mechanics Do not keep asking yourself, if you
can possibly avoid it,
14
Biradical intermediate theoryby Firestone Dewar
Dewar JACS 1984, 106, 209-219. Multibond
reactions cannot normally be synchronous
15
Exploring electronic migration for understanding
  • Need theoretical evidence how electrons migrate
    in CCs
  • Electron (or charge) densities are the only
    appropriate indicator.
  • Changes of orbitals of whatever kinds are
    irrelevant.
  • There is no information of the electron flow
    along the minimum
  • energy path (or IRC) of CCs.

16
IRC approach leads to confusion
1. Karadakov, et al. J Phys Chem A 105
10946-10946, 2001. 2. Nguyen, et al. J Phys Chem
A 105 10943-10945, 2001. 3. Harcourt, et al. J
Phys Chem A 105 10947-10948, 2001. 4. K.
Sakata. Electron Reorganization along the
Intrinsic Reaction Coordinate in 1,3-Dipolar
Cycloaddition. J Phys Chem A 104 10001-10008,
2000.
17
Arbitrary collisions may reveal some valuable
information
18
Electron transfer
19
Electron densities in arbitrary collisions
manifest the electron flow
S. Sun, Chemistry Preprint Archive, 2001, issue
7, 199-208 S. Sun, Chemistry Preprint Archive,
2002, , issue 1, 49-56
20
Electron-electron repulsion the key factor to
the easiness of electron flow
B3LYP/6-31G
  • Conjugation Hindrances
  • Heteroatomic hindrance
  • Interruption of a conjugate system.

21
1,3-Dipolar cycloadditions
22
The parent Diels-Alder reaction
A collision between two reactants creates two
spots of more positive Columbic potential at the
two bond forming locations, a consequence of the
force field overlapping of the carbon nuclei at
the two pairs of reaction centers. Four of the
six pi electrons are relocated to occupy these
two spots to form two new sigma bonds.
23
Conclusions part 1
1. Based on computational studies, I have
discovered an electron migration mechanism of
CCs that has never been documented in the
literature. 2. For the first time in more than
70 years after the discovery of D-A Reactions,
the electronic mechanism can be systematically
explained based on concerted electron-donor and
acceptor relationship at the 2 bond-forming
locations. 3. Chemical reactivity is
fundamentally decided by the easiness of
electronic migration, - an evident philosophical
point.
24
Conclusions part 2
4. For the first time, it is discovered that
electron-rich oxygen polarizes CC bond by
pushing away electron density to drive the
cyclic electron flow in CCs. 5. It is discovered
that the nucleophilicity or electrophicility of
a reaction center depends on a particular dynamic
reaction environment. 6. Demonstrated solid
examples that computational chemistry is useful
in the understanding of chemical phenomena that
cannot be understood otherwise.
25
Conclusions part 3
7. Orbitals cannot be understood beyond a
mathematical tool. And the IRC approach covers up
the information of electron flow in CCs. 8. All
natural puzzles are easy ones once you solve
them. In the other words, good science can be
easily understood. 9. A call to experts in the
related fields to evaluate the claims I
made reject them or confirm them!
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