alkenes: reactions with carbenes (mcm 7.6) cont
DESCRIPTION
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). - PowerPoint PPT PresentationTRANSCRIPT
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
NuH
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
HR
R
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 = 12πc
f(m1+m2)m1m2 m1 m2
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
Highly active against Mycobacterium tuberculosis