Alkenes: Reactions with carbenes (McM 7.6) cont.

Alkenes: Addition of radicals etc - Polymerisation (McM 7.10, 14.7)

Alkenes: Formation by elimination reactions (McM 11.12, 11.13)

Substitution reactions: Cleavage of ethers (McM 18.5)

Organometallic coupling react. (McM 10.9, lab ex 10)

Polymerization of dienes (McM 14.7)

•Radical polymerization•Cationic polymerization

Natural and synthetic rubber

Acid cat polymerization of 1,3-butadiene

H H H

Allylic cationResonans stab.

H H

and so on

1,4-addition

H H H

Nu

H

Nu

Nu

H

Nu

1,2 addition 1,4 addition

Isoprene

*

nGutta-percha

**n

Nattural rubber

resin from the Isonandra Gutta tree (South east Asia)

Less elastic than nat. rubberIsolation under-water cablesLittle use today

Hevea brasiliensis, tropical Americas.Cautchuc

Elimination reactions - Repetition

H

XR

R

R'R'

E2: mechanism

B

R' R'

R R+ BH + X

•One step•2. order•Stereospecific

E1: mechanism

H

XR

R

R'R' H

R

R

R'R'

B

+ X

R' R'

R R

+ BH + X

and / orR' R

R R'

•Two steps•1. order•1. step rate limiting•1.step = 1. step in SN1•Not stereospecific

Elimination in competition with substitution

E2 and stereochemistry

Base

H R

RH

H

X

ReactantAnti Periplanar conformation

H R

RH

H

X

Base#

Anti transition state

H

RR

H

Alkene

equatorialaxial

*

*equatorialaxial

Large subst in e pos. favored

Deuterium Isotope Effect (McM 11.14)(Kinetic Isotope Effect)

•Important in elucidation of reaction mechanisms•Cleavage of C-H and C-D requires different amount of energy

Hookes Law, Stretching frequencies, IR

ν =1

2πc

f(m1+m2)

m1m2 m1m2

ν C-H: ca 3000 cm-1

ν C-D: ca 2200 cm-1

Relationship between Streching frequency (ν and zero-point energy (E0)E0 = 1/2 hνE0 C-H 18 kJ/molE0 C-D 13 kJ/molHigher activation energy for cleavage of C-DCleavage of C-H will be faster

TS#

Substitution reactions - Repetition

•One step•2. order•Stereospecific - Inversion•Sterical hindrance

•Two steps•1. order•1. step rate limiting•1.step = 1. step in E1•Not stereospecific - Racimisation•Stabilisation of carbocation

SN2: mechanism

X

R1

R2

R3

Nu

Nu

R1

R2

R3

SN1: mechanism

X

R1

R2

R3

Nu

R1

R2

R3

R1

R2 R3

Nu Nu

Nu

R1

R2

R3

X: Leaving group, Godd leaving group - weak base

C-C Bond Formation - Organometallic Coupling Reactions(McM 10.9, Lab ex 10)

C + X C C C

No nu subst if C sp2 or spReagents often unstableDifficult to makeStrong bases

R HNaNH2

R

pKa ca 25

CH3IR CH3

Br

Na

R

H

H

CH

Br

Elimination

pKa ca 44

pKa ca 60

pKa NH3 35

Grignard reagents and Organolithium reagents

R-XMg

R-Mg-X

R: prim, sek, tert alkyl alkenyl aryl

NucleophileStrong base

O

R1 R2

R2R1

R OMgX Reacts with carbonylsStrong bases

R-X2 Li

R--Li + LiX

R: prim, sek, tert alkyl alkenyl aryl

NucleophileStrong base

O

R1 R2

R2R1

R OLi

BuLi used a lot as strong base

RMgX or RLi does not react well with alkyl halides in substitutions

Organocuprates (McM 10.9)

2 R-Li + CuI R2Cu Li + LiI

2 R-MgX + CuI R2CuMgX + IMgX

Gilman reagentDialkylcuprate

Participate in substitution with alkylhalides (Cl, Br, I)

(R: alkyl, alkenyl, alkynyl, aryl)

R2Cu Li + R-X R-R

alkylhalide

+ RCu + LiX

less reactiveAlkylcopper(i)

Also reaction with alkenyl halides and arylhalides (sp2)

R2Cu Li

I

R

I

R'

R

R'

Not SN1 or SN2

Palladium catalyzed coupling reactions (Lab ex. 10)

Pd(II) + 4 L PdL4 [Pd(0)]

I

PdL

LI

PdL2

Pd(0)

2L

PdL

L

L: Ligand, often PPh3

Generation of active catalyst(PdL2)from commercially available Pd(II) orPdL4 = Pd(0)

Pd(II)Pd(II)

MetMet-I

Oksidative addition[Pd oxs from Pd(0) tto Pd(II)]

Transmetallation(Ph transfered from Met Pd Pdno red ox)

Reduktive eliminationPd redused back to Pd(0)C-C between aryls formed

Met: -ZnX, -B(OH)2, -SnR3 (-MgX)

Alkenyl or aryl halidesPd complex from aklyl halides unstable

Alkynyl, alkenyl, aryl, alkyl reacts (depending on the metal used)

N

N N

N

X

OCH3

O

Formed by Pd-cat coupling

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