Jacobsen asymmetric epoxidation of olefins

R = aryl, alkenyl, alkynylR’ = bulky group

Metal complexes of porphyrins, salens, phthlocyanins catalyze the reaction with O2 or O–donors (H2O2, ROOH, PhIO, NaOCl, RCOOOH, py-O, etc).

Typically, the O-donor generates a high-valent metal-oxo complex and the electrophilic oxo-atom is transferred to the hydrocarbon substrate.

Although porphyrin complexes are not so readily available as salen complexes, they can be used in combination with O2.

Reference: Katsuki, T. Synlett. 2003, 3, 281

For the Jacobsen epoxidation, a “lock-and-key” mechanism operates: the transition statecomplex with the lowest energy is the one leading to the major product.

R R'

(S,S)-cat (4 mol%)NaOCl (aq), pH 11

CH2Cl2, 4 oC R R'

H HO

O

Mn

N

tBu

tBu O

N

tBu

tBuCl

(S,S)-catJacobsen's pre-catalyst

Mechanistic considerations

Reference: Katsuki, T. Adv. Synth. Catal. 2002, 344, 131

cis-disubstituted olefins are better substrates than trans-disubstituted olefins.Trisubstituted olefins are very good substrates.

The observed selectivity is based on a side-on approach of the olefin.

O

MnN

tBu

tBuO

N

tBu

tBuO

L

s

Mn

O

L

s

shallow stepped conformation

Mn

O

L

s

deeply stepped conformation

The olefin approaches such that its bulkier substituent (L) is away from the 3’-substituent to minimize repulsion. The substituents on the benzylic carbons in the 3 or 3’ positions are directed away from the incoming olefin and strong repulsion cannot be expected. In order to improve enantioselectivities, a binaphtyl unit as the chiral element was used.

Trans-olefins are not good substrates because the desired orientation of the incoming olefin is destabilized by the interaction of the downward substituent (S) with the salen ligand. Deeply-folded Mn(salen)s are expected to be the catalyst suitable for trans-olefins.

Sharpless asymmetric dihydroxylation of olefins

O

OsO

OO

O

OsO

OO

O

OsO

OO

NR3

R3N: +

fast

slow

H2Ooxidation

OsO4 + NR3 catalytic

HO OH

"Os" stoichiometric

+

+

Ligands used for AD

AD-mix- contains:K3[Fe(CN)6]

K2CO3

(DHQD)2-PHALK2OsO2(OH)4

Conditions: t-BuOH, H2O (1:1)0 oC, 6-24 h

N

OMe

N

H Os

O

O O

ONN

O O

N

MeO

H

N

N

MeO

N

Et

H OH

R

N N

Ph

Ph

DHQD-R

N

N N

O

R = PYR R = PHAL R = IND

Oligomerization and Polymerization of

Olefins

Textbook H: Chapter 22.1 – 22.4, 22.9.1.1

Textbook A: Chapter 15.1, 15.3

Outline

Oligomerization SHOP: Shell Higher Olefin Process

Polymerization Polymers: definitions, structural/property relationships Historical aspects of Ziegler/Natta polymerization Heterogenous catalysts Metallocene catalysts: co-catalysts, mechanism Living polymerization Late transition metal catalysts

Ni-catalyzed oligomerization

SHOP: commercialized in 1977; in 1993 global annual production capacity was 106 tons.

C6 – C18 olefins: commercial valueThe rest: isomerization and metathesis

Ph2P

NiOO

-

Ph2P

NiO

H

O CH2=CH2

Ph2P

NiOO

Ph2P

NiOO

CH2=CH2

Ph2P

NiOO

CH2=CH2

Ph2P

NiOO Rn

Rn

Polymers: Definitions• Monomer: Any substance that can be converted to polymers.

• Polymer: Macromolecule built up by linking together large numbers of smaller molecules.

• Copolymer: Macromolecule built up by linking together two different monomers.

Naturally occurring polymers:

Synthetic polymers:

R

-olefin polyolefin

R

n

• Polymer structure greatly affects polymer properties.

H2NOH

O

R

HN

O

R n

aminoacid peptide

Synthesis of polymers Condensation reactions: all molecules are involved in the steady growth of

species of higher molecular weight. Addition reactions: reaction of the initiating species with monomers; a

limited number of growing polymer molecules exists in excess of monomers. Radical (and living radical) polymerization

Anionic polymerization Cationic polymerization Coordination polymerization

ethylene LDPE

radical

initiatorn

n

ethylene HDPE

n

Polymer molecular weight

Mn = njMj

nj

Mw = Mj2nj

Mjnj

PDI = Mw

Mn Molecular WeightW e i g h t f r a c t i o n

narrow molecular weight distribution

broad molecular weight distribution

Weight fraction

• Many important mechanical properties of a polymer depend on and vary with molecular weight: melting point, Tm (crystalline part); glass transition temp., Tg; crystallinity; strength; modulus (the relation between stress and deformation); viscosity; morphology of the polymer particles.

Mn: number average molecular weight; Mw: weight average molecular weight

•Gel Permeation Chromatography (GPC) is a tool for polymer molecular weight determination.

• Molecular weight distribution gives information about the distribution of different molecular weight chains within a polymer sample.

Ziegler/Natta polymerization: introduction• Karl Ziegler: German chemist, Nobel prize 1963• 1953/54: oligomerization of ethylene by trialkylaluminum compounds (high pressure and temperature).• In the presence of trace transition metals, it was found that the reaction took place at a much lower temperature and pressure.• Polymer had a linear, unbranched structure with high molecular weight, i.e. HDPE

http://www.nobel.se/chemistry/laureates/1963/

R2Al-R +n

Transition Metal

Catalyst• Giulio Natta: Italian chemist, Nobel prize 1963• Learned of Ziegler’s research, and applied findings to other -olefins such as propylene and styrene.• Resulting polypropylene was made up of two fractions: amorphous (atactic) and crystalline (tactic). Polypropylene is not produced in radical initiated reactions.

propylene polypropylene

n

Metallocene catalysts• 1955: Natta reported that Cp2TiCl2 activated by AlEt3 could polymerize ethylene with low activities.

• 1981: Sinn and Kaminsky discovered a high-activity catalyst:

x AlMe3 + x H2O Al

Me

O x+ 2x CH4

• The key to reactivity was the co-catalyst generated from the adventitious water, “methylalumoxane(s)” or MAO:

ZrCH3

CH3

+ AlMe3 + "H2O"ethylene

Very active catalyst!

Role of MAO• Ziegler/Natta catalysts are water sensitive; excess MAO serves to dry the solvent and monomers.

• MAO can abstract alkyl groups from a complex and generate cations. In this case, MAO is transformed into a weakly-coordinating anion.

• MAO can alkylate metal halides.

Al

Me

O x+ H2O Al

OH

O x+ CH4

Al

Me

O x+ [M]-Cl Al

Cl

O x+ [M]-Me

Al

Me

O x+ [M]-Me Al

Me

O x+ [M]

Me

Reference: Eilertsen, J. L. et al. Inorg. Chem. 2005, 44, 4843

Activation of metallocenes and olefin insertion

• The alkyl resides in one of the equatorial sites and the olefin binds to the other.

• The active metallocene catalyst is a cationic alkyl.

ZrMe

ZrMe

ZrMe

ZrMe

ZrMe

Zr

Me

ZrMe

Me+ AgBPh4

- Ag- CH3CH3

Zr(IV) d0

16 e

ZrMe

[BPh4]

Zr(IV) d0

14 e

-Olefin insertion

• -olefins can insert from two positions:

1,2-insertionR

1-position

2-position

ZrMe

ZrMe

ZrMe

­R

RR

Zr

RMe

ZrMe

ZrMe

ZrMe

­R

Zr

MeRR

R

2,1-insertion

• 1,2-addition is the major mode of insertion; 2,1-insertion usually leads to chain termination.

Chain termination

• -Hydrogen elimination

• -Alkyl elimination

• Chain transfer to co-catalyst

(L) MR

+P

R

(L) MP

RHR

(L) MP

RHR

(L) M (L) MH

+R

P

R

P

RRH

(L) MP

RRH

(L) MR'

+ R'2Al

R

P(L) M

R'Al

R'2(L) M

P

RH

+ AlR'3

R

P

Living polymerization: A special case

(L) M

R'

R

n+1

(L) MR

R

activator

activation

initiation (ki)

R'

propagation (kp)

n(L) M

R'

RR'(L) M

R

ki = rate of initiationkp = rate of propagation

• Initiator and intermediates are stable under reaction conditions.• There is no chain termination.• ki ≥ kp

This means that the rate of initiation is greater than rate of propagation and that all the metal centers are initiated before propagation takes place.

• Polymers with narrow molecular weight distributions are obtained.

Ti-based heterogeneous Ziegler/Natta catalystsFirst generation (Solid solution)• Different crystalline modifications of Ti(III) chloride, (TiCl3), and Al(C2H5)2Cl

Second generation (Donor modified)• TiCl3/AlR3/Lewis Base (e.g. ethers, esters, ketones, amines and phosphines)• Certain Lewis bases increase the stereospecificity of polymerization and increase the activity of the catalyst.

Third generation (Supported)• TiCl4 + Al/MgCl2/Lewis Base/AlR3

• Increase in catalyst surface area greatly increases polymerization activity.

Ti

Cl

CH2

Cl

Cl

Cl

P

Al

EtEt

H

H

H

HParticle

Core

Cossée-Arlman mechanism

M-P +1

23

MP

12

3

1,2-insertion

M P

3

2

13,1-insertion

MP

12

3

2,1-insertion

Late transition metal catalysts

N N

M

Pol

+

ArAr N

N NM Ar

Cl Cl

MAO

EthenePropene

Polymers

M = Ni, Pd

N N

M

S Pol

+

ArAr

M = Ni, Pd

COOMe

COOMe

MeOOC

Brookhart M = Fe, Co

Ethylene polymerization catalysts

Ethylene-acrylate copolymerization catalysts