chemistry 125: lecture 52 february 16, 2011 transition metal catalysis: hydrogenation &...

Chemistry 125: Lecture 52February 16, 2011

Transition Metal Catalysis:Hydrogenation & Polymerization

Additions by Radicals & Electrophilic Carbon;

Isoprenoids;Tuning Polymer Properties This

For copyright notice see final page of this file

Other “Simultaneous” ReagentsCl2C: (Carbene)

R2BH (Hydroboration)

CH2I2 Zn/Cu (Carbenoid)

O3 (Ozonolysis)

H-metal (Catalytic Hydrogenation)

R-metal (Metathesis, Polymerization)

RC (Epoxidation)OOH

O

OsO4 or MnO4- (Dihydroxylation)

OsO4 and Permanganate

Os analogueof

cyclic acetal

H-O-H

OsO4 is poisonous and expen$ive!Use as a 1% catalyst by adding oxidant.

H2O2 (1936) “NMO” (1976 - Upjohn)

ChiralAmineLigand

e.g. J&F Sec. 10.5c p. 443

Osmate Ester

H

HC C

H3C

CH3

HH

C C

H3CCH3

O O

OO

Os

OO

Os

OOOH H

Sharpless Asymmetric Dihydroxylation(1988)

SAD

C C

K+

O-O

Mn

OO

97% ee

C C

O O

O-O

Mn

OH HH2O

K+

OH

OH

85%yield

KMnO4

MeOH / H2ONaOH, 20°C

all syn

(S,S) + (R,R)

syn additionto trans 2-butene

H H H H*LUMO HOMO

orthogonal

Catalytic Hydrogenation

HOMO/LUMO : Concerted

H HH H*LUMOHOMO

C C

H H

C C

H H

C CC C

*LUMOHOMO

(“works” with metal catalysts!)

HOMO-HOMO repulsive emptyPd

e.g. J&F Sections Sec 4.9A, 168ff., 10.2a (410-413), 10.10 (452)

Orbital Varietyfrom Metals

Pd HOMO (4d )

Ethylene LUMO ()HOMO ()

HOMO-4Ethylene-PdComplex

…(4d)10 (5s) 0 (5p)0

13%

40% 4dxy

47% C-H

x2-y2z2xyxzyz

HOMO ()

Pd HOMO (4d)

Ethylene

UMO (5s)UMO (5p)

(4d)10 (5s) 0 (5p)0

HOMOEthylene-Pd

Complex

+6% 5s5% 5p

15% 4dz2

67%

Sigma Bond Analogue“Oxidative” Addition (crummy PM3 calculation)

H-H+Pd

10

5

0

kcal/mole

H2 dissociates on bulk Pd surface, then hydrides move.(entropy help)

bondingH2 to Pd

splitting H2

kcal/mole

H

Catalytic Hydrogenation “oxidative addition” “oxidative addition”

Pd

C C

Pd

C C

Pd

H H

Pd

H H

“reductive elimination” “reductive elimination”

Pd

HCC

H Pd

HCC

H

Pd addition concerted (syn)

Pd

HCC

H Pd

HCC

Experts discuss the extent of bonding in this “-complex”

H atoms replace Pd frontside syn hydrogenation product

Catalytic HydrogenationStereochemistry

syn addition

e.g. J&F pp. 412

Stereochemistry

No yields

specified!

No literature

reference!

A general elementary texte.g. Loudon, Sec. 7.9 E p. 313

pp. 20-22 of H. O. HouseModern Synthetic Chemistry (1972)

(a graduate-level text)

J. Chem. Soc., 1354 (1948)

H2 / Pt

R’ = Ac R’ = Ac

allylic isomers

Suppose there is an allylic H in the alkene:can lead to allylic rearrangement

H

Catalytic Hydrogenation

Pd

H

Pd

HCC

Pd

HCC

H

Pd

HCC

H

CC CH

Pd

H

Pd

H CC CH

CC

alkene isomerized

symmetric

C C CH H H

10

12

34 5

6

7

89

10

12

34 5

6

7

89

??

VII VIII

Alkene Metathesis

metallacyclobutane

C

C

Grubbs Catalyst

Ru

C

C

C

Ru

CC

C

Ru

C

C

C

Ru

C

C

C

Ru

C

Nobel Prize 2005

a metal alkylidene complex

Tall Prof. F. Ziegler(not Prof. Karl Ziegler)

with Prof. R. Grubbs

TouristsZiegler Grubbs

HostProf. S.-I. Murahashi

ROMPRing-Opening Metathesis Polymerization

Ru

CRuC

RuCn

n

metathesis

metatheses

Catalytic Hydrogenation

Ti

RCC

Ti

RCC

Ti

RCC

H

Pd

HCC

Pd

HCC

Pd

HCC

H

Pd

HCC

H25 x 106 tons(2004)

-(CH2-CH2)n-

n = 800-250,000

Ziegler-Natta Polymerization

45 x 106 tons(2007)

-(CH2-CH)n-CH3

n up to 105

isotactic

Heterogeneous Catalysthard to study mechanism

RR R

Et3Al + TiCl4

Stereochemistry: Tacticity

All head-to-tail, but stereorandom (atactic)

All head-to-tail, and stereoregular (isotactic)

All head-to-tail, and stereoregular (syndiotactic)

How do you know which is which?

NMR(coming soon)

R2B-HC=C-CH3

R’

RC-C-CH3

Stereochemistry: Tacticity

All head-to-tail, but stereorandom (atactic)

All head-to-tail, and stereoregular (isotactic)

All head-to-tail, and stereoregular (syndiotactic)

axisHomogeneous“Kaminsky” catalysts

activated by MAO(“methyaluminoxane”)

homotopicfaces

mirror

enantiotopicfaces

achiralfaces

C=C-CH3

R’C-C-CH3

RR

C=C-CH3

R’

C-C-CH3+

Alkenes approachfrom alternate faces

Radical Polymerization(e.g. J&F Sec 11.5 pp.487-489)

R

H

Occasional butyl side-chains inhibit close packing.

Cl CCl3CCl3R Cl

Controlling Polymer Chain LengthCCl4 is a “Chain-Transfer Agent”

shortens polymer molecules without terminating chain reaction

Properties like viscosity and melting point depend on

chain length.etc.

Cl

ktransfer/kpolymerization ~ 0.01 for styrene polymerization

When other termination is negligible, molecular length

~ kp[styrene] / kt[CCl4]“dispersity”

Alkene/Diene Oligomerization and Polymerization Using

Carbon Electrophiles

R+ (SN1)

R-L*

(SN2)

(“oligo”, a few)

CH3

CH3

H2C C

R+ Electrophile in Formation of 2,2,4-Trimethylpentane, “Isooctane”

CH3

CH3

CH3 C H

CH3

CH3

H2C C

CH3

CH3

CH3 C CH2

CH3

CH3

HCH2SO4+

CH3

CH3

CH3 C+

CH3

CH3

CH3 C +

CH3

CH3

CH2 C

H+

(defined as “100 octane”)

inter molecular hydride shift(Bartlett, 1944)

chainpoly(isobutylene)

“butyl rubber” air-tight

+

CH3

CH3

CH2 C

CH3

CH3

CH3 C

CH3

CH3

CH2 Cetc. etc.

CH3

CH3

H2C C

H

CH3

CH3 C CH3

+

e.g. J&F Sec. 12.13 pp. 554-562

R-L and R+ Electrophiles in*

Terpene/Steroid Biogenesis

End of Lecture 52February 16, 2011

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