Reaction chemistry of complexes 1. Reactions involving the...
Transcript of Reaction chemistry of complexes 1. Reactions involving the...
Reaction chemistry of complexesThree general forms:1. Reactions involving the gain and loss of ligands
a. Ligand Dissoc. and Assoc. (Bala)b. Oxidative Additionc. Reductive Eliminationd. Nucleophillic displacement
2. Reactions involving modifications of the liganda. Insertionb. Carbonyl insertion (alkyl migration)c. Hydride elimination (equilibrium)
3. Catalytic processes by the complexesWilkinson, MonsantoCarbon-carbon bond formation (Heck etc.)
a) Ligand dissociation/association (Bala)
• Electron count changes by -/+ 2
• No change in oxidation state
• Dissociation easiest if ligand stable on its own(CO, olefin, phosphine, Cl-, ...)
• Steric factors important
MBr
+ Br-M
b) Oxidative Addition
Basic reaction:
• Electron count changes by +/- 2(assuming the reactant was not yet coordinated)
• Oxidation state changes by +/- 2• Mechanism may be complicated The new M-X and M-Y bonds
are formed using:• the electron pair of the X-Y bond• one metal-centered lone pair
LnM +X
YLnM
X
Y
One reaction multiple mechanisms
Concerted addition, mostly with non-polar X-Y bondsH2, silanes, alkanes, O2, ...
Arene C-H bonds more reactive than alkane C-H bonds (!)
Intermediate A is a σ-complex
Reaction may stop here if metal-centered lone pairsare not readily available
Final product expected to have cis X,Y groups
X
YLnM
X
YLnM + LnM
X
YA
Stepwise addition, with polar X-Y bonds– HX, R3SnX, acyl and allyl halides, ...
– low-valent, electron-rich metal fragment (IrI, Pd(0), ...)
Metal initially acts as nucleophile
– Coordinative unsaturation less important
Ionic intermediate (B)
Final geometry (cis or trans) not easy to predict
Radical mechanism is also possible
X YLnM
B
LnM X Y LnMX
Y
OC Ir ClPEt3
Et3P
OC Ir H
PEt3
Et3P
H
Cl
OC Ir I
PEt3
Et3P
H
Cl
OC Ir Cl
PEt3
Et3P
CH3
Br
Ir(I)
Ir(III)
Ir(III)
Ir(III)
H2
cis
cis
trans
HI
CH3Br
Cis or trans products depends on the mechanism
c) Reductive elimination
This is the reverse of oxidative addition - Expect cis elimination
Rate depends strongly on types of groups to be eliminated.
Usually easy for:• H + alkyl / aryl / acyl
– H 1s orbital shape, c.f. insertion
• alkyl + acyl
– participation of acyl p-system• SiR3 + alkyl etc
Often slow for:• alkoxide + alkyl• halide + alkyl
– thermodynamic reasons?
We will do a number of examples of this reaction
Complex Rate Constant (s-1) T(oC)
PdCH3Ph3P
Ph3P CH3
PdCH3MePh2P
MePh2P CH3
PdCH3P
P CH3
PhPh
PhPh
1.04 x 10-3 60
60
80
9.62 x 10-5
4.78 x 10-7
Relative rates of reductive elimination
Most crowded is the fastest reaction
PdCH3L
L CH3
+ solv
-L
PdCH3L
solv CH3
RELPd(solv) + CH3 CH3
Special case:Nucleophilic Attack on a Coordinated CO acyl anion
Fisher carbene
This is Fischer carbene It has a metal carbon double bond
Such species can be made for relatively electronegativemetal centers N.B. mid to late TMs
Fischer carbenes are susceptible to nucleophilic attack atthe carbon
Fischer carbenes act effectively as σ donors and π acceptors
The empty antibonding M=C π orbital is primarily on the carbon making it susceptible to attack by nucleophiles
Other type is called a Shrock carbene (alkylidene)
Characteristic Fischer-type Schrock-typeTypical metal (Ox. State)
Middle to late T.M.Fe(0), Mo(0) Cr(0)
Early T.M.Ti(IV), Ta(V)
Substituents attached to carbene carbon
At least one highly electronegative heteroatom
H or alkyl
Typical other ligands
Good p acceptors Good s and p donors
Electron count 18 10-18
Nucleophilic displacement
Ligand displacement can be described as nucleophilic substitutions
O.M. complexes with negative charges can behave as nucleophilesin displacement reactions Iron tetracarbonyl (anion) is very useful
RX R[Fe(CO)4]2- [ Fe(CO)4]-
CO
H+
OX
R
[ Fe(CO)4]-RO
H+ OH
R
R H
O
XR
X2
O2
R'X O
OHR
O
R'R
Modifications of the ligand
a) Insertion reactions
Migratory insertion!
The ligands involved must be cis - Electron count changes by -/+ 2
No change in oxidation state
If at a metal centre you have a σ-bound group (hydride, alkyl, aryl)
a ligand containing a π-system (olefin, alkyne, CO) the σ-bound
group can migrate to the π-system
1. CO, RNC (isonitriles): 1,1-insertion
2. Olefins: 1,2-insertion, β-elimination
M
R
MR
MR
COM
O
R
1,1 1,2
1,1 Insertion
The σ-bound group migrates to the π-system
if you only see the result, it looks like the π-system has inserted into the M-X bond, hence the name insertion
To emphasize that it is actually (mostly) the X group that moves, we use the term migratory insertion (Both possible tutorial)
The reverse of insertion is called elimination
Insertion reduces the electron count, elimination increases it
Neither insertion nor elimination causes a change in oxidation state
α- elimination can release the “new” substrate or compound
In a 1,1-insertion, metal and X group "move" to the same atom of the inserting substrate.
The metal-bound substrate atom increases its valence
CO, isonitriles (RNC) and SO2 often undergo 1,1-insertion
1,2 insertion (olefins)
Insertion of an olefin in a metal-alkyl bond produces a new alkyl
Thus, the reaction leads to oligomers or polymers of the olefin
• polyethene (polythene)• polypropene
MMe
SO2
MS Me
O OM
Me
CO
MMe
O
MR
MR
M
R
MR
Standard Cossee mechanism
Why do olefins polymerise?
Driving force: conversion of a π-bond into a σ-bondOne C=C bond: 150 kcal/molTwo C-C bonds: 2´85 = 170 kcal/molEnergy release: about 20 kcal per mole of monomer(independent of mechanism)
Many polymerization mechanismsRadical (ethene, dienes, styrene, acrylates)Cationic (styrene, isobutene)Anionic (styrene, dienes, acrylates)Transition-metal catalyzed (a-olefins, dienes, styrene)
Two examples
β Hydride elimination (usually by β hydrogens)
Many transition metal alkyls are unstable (the reverse of insertion)the metal carbon bond is weak compared to a metal hydrogenBond Alkyl groups with β hydrogen tend to undergo β elimination
M -CH2-CH3 M - H + CH2=CH2
To prevent beta-elimination from taking place, one can use alkyls that:
Do not contain beta-hydrogensAre oriented so that the beta position can not access the metal centerWould give an unstable alkene as the product
A four-center transition state in which the hydride is transferred to the metal An important prerequisite for beta-hydride elimination is the presence of an open coordination site on the metal complex - no open site is available - displace a ligand metal complex will usually have less than 18 electrons, otherwise a 20 electron olefin-hydride would be the immediate product.
The Monsanto acetic acid process
Methanol - reacted with carbon monoxide in the presence of a catalyst to afford acetic acid
Insertion of carbon monoxide into the C-O bond of methanol
The catalyst system - iodide and rhodium
Iodide promotes the conversion of methanol to methyl iodide,
Methyl iodide - the catalytic cycle begins:
1. Oxidative addition of methyl iodide to [Rh(CO)2I2]-
2. Coordination and insertion of CO - intermediate 18-electron acylcomplex
3. Can then undergo reductive elimination to yield acetyl iodide and regenerate our catalyst
Homogeneous cross coupling reactions: Heck reaction
CH2=CH2 > CH2=CH-OAc > CH2=CH-Me > CH2=CH-Ph > CH2=C(Me)Ph
krel: 14,000 970 220 42 1
Pd(0)
Pd(II)
Y = H, R, Ph, CO2R, CN, OMe, OAc NHAc
R-Pd(II)-XR-Pd(II)-X
R-Pd(II)-X
R-X
Y
Y
R
Y
HHR
YH
Pd(II)-X
HX