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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.

1. Experimental results tell how nuclei migrate.

2. Chemists use arrows or semi arrows to indicate electronmigration for understanding.

Chemical reaction mechanism:where comes the electron resource and where doesit go in the bond formation and breaking?



3

Pattern and explanation in chemistry

Explanation is more based on electron migration.

Electron migration plays a deciding rolein the nuclear migration and is the essenceof chemical reaction mechanism.

1. A pattern is a collection of phenomena.

Pattern is more related to migrations of nuclei.

2. Explanation is an understanding of phenomena.



4

Concerted cycloadditions (CCs)

1. They are highly useful reactions in the

formation of ring structures.

2. They are specially interesting, stimulating,and obfuscating.



5

Patterns in CCsAlder¶s rules on Diels-Alder reactions:

1. The reactivity of a Diels-Alder reaction is increased when theelectron-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

1. It was founded on Alder¶s 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. ).

-Chemical reactivity is decided by the smaller HOMO-LUMO energy gap between two reactants.



7

Explanation of CCs was a blank

for ~70 years!

Both of Alder¶s rules and FMO theory are about the same

pattern concerning outcomes of nuclear rearrangementsin 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 ³aligning 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?

O O

O

O

O

2H

H

3 H

HO

H

H

H

H

+ F

E E' F'

1

N O+ -

N

O

O O

O N

O

X



10

Are there two cases of reactions?³The one was the frontier-controlled case in whichthe reaction was controlled by the particular orbital

interaction,

and the other was the charge-controlledcase, where it was controlled by electrostaticinteraction of charges.´

K. Fukui, Science, 1982, 218, 747-218



11

Philosophical analyses of orbitalsRecent related analyses of orbitals:

1. Bader et al. Angew. Chem. I nt. Ed . E ngl. 1994, 33, 620-631

2. Bader, I nt. J . Quant. Chem. 2003, 94, 173-177.

3. Gillespie & Popelier, From Lewis t o E lect ron Densities«

4. Ogilvie, J . Chem. Ed . 1990, 6 7 , 280-289.

5. Scerri, J . Chem. Ed . 2000, 77, 1492-1494.

6. Scerri, P hil o sophy of Science 2001, 6 8 (Proceedings)S76-S88



13

Quantum Reality?

And easy understanding?

Murray Gell-Mann has described quantum mechanics

as 'that mysterious, confusing discipline which noneof us really understands but which we know how to use'.

Richard Feynman: ³I think I can safely say that no-one

understands quantum mechanics« Do not keep askingyourself, if you can possibly avoid it, «´



14

Biradical intermediate theory

by Firestone & Dewar Dewar J AC S 1984, 106 , 209-219.³Multibond reactions cannot normally be synchronous´

?

?OO

O

2

H

H

H

HO

H

H

H

H

+

1

O

O



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 minimumenergy path (or IRC) of CCs.



16

IRC approach leads to confusion

N O

+ -

N O

+ -

N O

+ -

1. Karadakov, et al. J P hy s Chem A 105: 10946-10946, 2001.2. Nguyen, et al. J P hy s Chem A 105: 10943-10945, 2001.

3. Harcourt, et al. J P hy s Chem A 105: 10947-10948, 2001.

4. K. Sakata. Electron Reorganization along the IntrinsicReaction Coordinate in 1,3-Dipolar Cycloaddition. J P hy s Chem A 104: 10001-10008, 2000.



17

Arbitrary collisions may reveal some

valuable information

O

O

OO



18

OH

H H

-0. 8

0. 60

-0. 30.

0. 6

0. 08

-0.3 8

0. 83

E

F

H

F-CH2 +0.056

OH

H

H H

H

HH

O

H

-0.440

0.228 0.115

-0.083

0.148-0.343

0.203

0.172

-0.343

0.172

0.203

-0.083

0.148

0.228

0.115

-0.440

2.10 Å

0. 8

OH

H

H

O

HH

H

H

-0. 29

0.246

0.

010.1

39

-0.065

-0.332

0.162

0.138

-0.400

0.224

0.134

-0.119

-0.292

0.181

0.165

F-CH2 -0.032

0.040

2.10 ÅH

Electron transfer



19

Electron densities in arbitrary collisions

manifest the electron flow

S. Sun, Chemist r y P r e pr int Ar chive, 2001, issue 7, 199-208S. Sun, Chemist r y P r e pr int Ar chive, 2002, , issue 1, 49-56

O

O

O

O



20

Electron-electron repulsion: the key

factor to the easiness of electron flow

O

O

O

O

1.920 Å2.223 Å2.4 7 Å 1.890 Å

Conjugation Hindrances:1. Heteroatomic hindrance2. Interruption of a conjugate system.

B3LYP/6-31G*



21

1,3-Dipolar cycloadditions

N O+ -

N

O

O O

O N

O

X

N O

+ -

N

O



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 nucleiat the two pairs of reaction centers. Four of the six pi electrons arerelocated to occupy these two spots to form two new sigma bonds.



23

Conclusions ± part 11. Based on computational studies, I have discovered an electronmigration mechanism of CCs that has never been documentedin the literature.

2. For the first time in more than 70 years after the discovery of D-A Reactions, the electronic mechanism can be systematicallyexplained based on concerted electron-donor and acceptor relationship at the 2 bond-forming locations.

3. Chemical reactivity is fundamentally decided by the easinessof electronic migration, - an evident philosophical point.



24

Conclusions ± part 24. For the first time, it is discovered that electron-rich oxygen

polarizes C=C bond by pushing away electron density to drivethe cyclic electron flow in CCs.

5. It is discovered that the nucleophilicity or electrophicilityof a reaction center depends on a particular dynamic reactionenvironment.

6. Demonstrated solid examples that computational chemistryis useful in the understanding of chemical phenomena thatcannot be understood otherwise.

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