Chemistry 125: Lecture 50February 12, 2010

More Electrophilic Addition

to Alkenes withNucleophilic Participation This

For copyright notice see final page of this file

Although the above two-step mechanism with intermediate IZnCH2-CH2-CH2I is plausible,

addition of IZnCH2I to H2C=CH2 actually occurs in a single step,

according to quantum mechanical calculation*, with the bent

transition state shown below: * A DFT Study of the Simmons-Smith Cyclopropanation Reaction.

A. Bottoni, et al., J. Am. Chem. Soc, 1997, 119, 12300

ICH2ZnIZn

I

I

ICH2ZnI

(at TransitionState Geometry)

LUMO

Mixes with HOMO

HOMO-2

Mixes with * LUMO

Zn

I

I

Epoxidation by Peroxycarboxylic acids

(sec 10.4a 423-425)

LUMO

HOMO-3

200

0

-200

-400

Orb

ital E

nerg

y (k

cal/m

ole)

OMOs

etc.

Peroxyformic AcidDistorted to

Transition State for O Transfer

p(O

*O-O

“-allylic”

CH2

H2CCH2

H2C

O

OO C

H

H“SN2 at O”

“-allylic”

“SN2 at H”

p()O nucleophile

*O-O electrophile

All happening togetherto give

carboxylate “leaving group”

(but not strictly in parallel)

Transition State

GeometryO-O

Strongly Stretched(from ~1.5Å)

O-HHardly

Stretched(from ~1Å)

kH/kD ~ 1

“Butterfly” mechanism

(not spiro)

suggested by Paul D. Bartlett

(1950)

calculatedJ. Amer. Chem. Soc. (1991) pp. 2338-9

downhill motionafter TS

Only one TS :

“Concerted but not Synchronous”

“spiro” meanstwo perpendicular

rings sharing a common atom

(here O1)

Note that arrows were not used as carefully

in those days.

Bartlett 1950

Problem:How about now?

(compare arrows with mechanism on previous frames and try drawing

a more accurate diagram)

Concerted Syn Addition

CH2CHO

HCC

H

OH

CH2

H

OR

ORO

OEtO

O CO2Et

TiRO

O

Remember Sharpless Asymmetric Epoxidation

R

ROO ••

RO+Ti

O

O

O

R

OEtO

CO2Et

O

RO

Ti

O

O

O OEtO

CO2Et

*

RCH2

HC

CCH

H

allyl alcohol

(R)-“epoxide”

(S)-epoxide precursor

Chiral“Oxidizing Agent”

LUMO?

HOMO?

is diastereomeric!

( also pO + *C=C )

Cf. Sec. 10.4bp. 426

20,000,000 tons$20 billion

per year

OLDCAMPUS

H2C CH2

O H2C=CH2 + O2

Ag

250°C15 atm

ethylene oxide

(84%)

20,000,000 tons$20 billion

per year

H2C CH2

O

ethylene glycol(antifreeze)

polyethers(solvents)

Sec. 10.4cpp. 427-430

Regiospecificity

Protonated Isobutylene Oxide

1.47Å1.61Å

+195

+79

+141.5 +140.2

Cuprates (Carbon Nucleophiles)

Stereospecificity

More ImpressiveRegiospecificity

Ozonolysis by Cycloadditions

(sec 10.5a 436-439)

ConcertedTransition State

(calc by quantum mech)

H2C CH2

O O O

+_

Motion along Reaction Coordinate through Transition State

O3

C2H4

side view end view

HOMO

LUMO

HOMO

Transition StateOrbital Mixing

makes two new bonds O3

C2H4

HOMO

LUMO

HOMO-1

Cycloaddition ofAllylic 1,3-Dipoles to Alkenes

7. (5 min) Having learned that the allylic system of O3 forms two bonds at once to an alkene, as shown below, one might think to try the same thing with the apparently analogous boron compound (assuming it exists). Explain in terms of the orbitals involved why this might not be such a good idea.

O

O

OO

O

O

H2

C

B

C H2

B

C H3

C H3

?

7

Ozone (O3) from the “top” (rotate back to view the 3 MOs from the 3

“allylic” out-of-plane 2p orbitals of 3 O atoms)

1 No ABN (anti-bonding node)Middle AO is largest (it overlaps twice)

2 One ABN node. Middle AO is absent. No significant overlap, thus ~ same energy as isolated 2p AO.

3 Two ABNs - highest energy MO. (I’m not sure why the middle AO looks about the same size as the terminal ones, it must be larger to be orthogonal to 1.)

Another allylic systemCH2-BH-CH2 from “top” (rotate back to view 3 MOs)

1 (middle B AO about same

size as C AOs; overlaps twice, but has lower nuclear charge)

2 Note how C AOs look

larger than O AOs of O3, because C AO is less dense

near the nucleus)

3 Most of the lower-energy

C AOs were used up

in 1 and 2.

1 Partly C AO just

looks big (but also C=O is short,

which makes CO overlap important)

2node no longer in

exact center

3BIG C AO

for this high-energy MO

H2C=O O+ -

“carbonyl oxide”

+ pO

- pO

* - pO

pO

CO

*CO

Central O overlaps C better than O, so consider right O interacting weakly with C=O orbitals.

1

2

3

more mixing(better E-match)

less mixing

..

..

Number

of electrons

LUMO

HOMO

HOMO

LUMO

(ends match *alkene LUMO)

(ends match alkene HOMO) (ends match

alkene HOMO)

(No alkeneHOMO match)

(No alkeneLUMO match)

..

Can’t make two bonds simultaneously for

cycloaddition to alkene!

..

..HOMO

LUMO

(ends match *alkene LUMO)

*

* Don’t worry about apparent bad overlap with the blue lobe of the central oxygen. It is far enough away because of the bend in O3.

OO

O

4C

BCH

H H

H

H 2 4+

H

OO

HC

Makes two bonds Makes two bonds

CH2

H2C OO O

+

Ozonolysis (Text Section 10.5a, pp. 436-439)

O

CH2

H2C OO:

Undergoes a “reverse” of the

previous process.

“Molozonide” is rather unstable because of

-O-O-O- group’sHOMO-HOMO mix.

CH2

O

OH2C

O+

OCH2

Undergoes a “reverse” of the

previous process.

to give carbonyl oxide

and C=O

Re-adds after rotation (avoids -O-O-O-)

CH2

O-O

OH2C

Ozonide

a Double Acetal

Mechanism for Acid-Catalyzed Hydrolysis of Acetal

RO

ROCH2

+H

HOH

:

:

RO

ROCH2

+ H ROH

RO-CH2

+

HO

ROCH2+

H

First remove RO, and replace it by HO.

HO

ROCH2

Now remove second RO, then H (from HO) +H

:HO

ROCH2

+ H

RO=CH2

+

cation unusually stable;thus easily formed

ROH

H-O-CH2

+

O=CH2 O=CH2

ROH

ROH

RO

ROCH2 O

H

H

:Overall Transformation:

H2O + Acetal Carbonyl + 2 ROHH+

(pp. 785-787)

(hemiacetal)

End of Lecture 50Feb. 12, 2010

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