Conventional Catalytic cycle for hydrogenation with Wilkinson’s catalyst
RhP
P P
Cl
H RhP
PH
Cl
RhP
H H
Cl
P
H2C RhP
P
H
Cl
Cl RhP
PH2 oxidative addition
1, 2 -migratory insertion
reductiveelimination
R
CH2R
RCH2CH3
PP
alkenecoordination
R
P = PPh3
14e
The first step of thiscatalytic cycle is thecleavage of a PPh3 togenerate the activeform of the catalystfollowed by oxidativeaddition of dihydrogen.
RhP
P P
ClRh
P
H P
Cl
H
P
H RhP
P
H
Cl
RhP
H H
Cl
P
H2C RhP
P
H
Cl
Cl RhP
P
H2 oxidative addition
H2 oxidative addition
1, 2 -migratory insertion
reductiveelimination
P (due to transeffect of H )
R
CH2R
RCH2CH3
PP
alkene
catalytic cycle for hydrogenation
Kinetic studies have shown that the dissociation of PPh3 from the distorted square planar complex RhCl(PPh3)3 in benzene occurs only to a very small extent (k = 2.3 × 10–7 M at 25°C), andunder an atmosphere of H2, a solution of RhCl(PPh3)3 becomes yellow as a result of the oxidative addition of H2 to give cis-H2RhCl(PPh3)3.
The trans effect is the labilization (making unstable) of ligands that are trans to certain other ligands, which can thus be regarded as trans-directing ligands. The intensity of the trans effect (as measured by the increase in rate of substitution of the trans ligand) follows this sequence:H2O, OH− < NH3 < py < Cl− < Br− < I−, < PR3, CH3− < H−, NO, CO
AJELIAS L7-S18
R> > > >
>
R
R
R
R
R
R
R
RR>
R
RR
R
• Cis alkenes undergo hydrogenation more readily than trans alkenes
•Internal and branched alkenes undergo hydrogenation more slowly than terminal ones, and
Relative reactivity of alkenes for homogenous catalytic hydrogenation
AJELIAS L7-S19
Catalyst25°C, 1 atm H2
Turnover frequency (TOF) in h–1 for hydrogenation of alkenes
Wilkinson’s catalyst 650 700 13 NA
Schrock–Osborncatalyst
4000 10 NA NA
Crabtree’s catalyst 6400 4500 3800 4000
RhPh3P
Ph3P PPh3
ClRh
PPh3
PPh3
+
PF6
Schrock-Osborn's catalyst
IrPCy3
N
+
PF6
Crabtree's catalystWilkinson's catalyst
Fine tuning of a catalyst:
hydrogenation catalysts which are more efficient than Wilkinsons catalyst
The cationic metal center is relatively more electrophilic than neutral metal center and thus favours alkene coordination.
AJELIAS L7-S20
Hydrogenation with Crabtree’s catalyst
The di-solvated form of the active catalyst generated by the removal of COD [after it gets hydrogenated and leaves] favors coordination of sterically bulky alkenes as well.
IrPCy3
NPF6 Ir
PCy3
N
PF6
H
H
16e 18e
IrPCy3
N
PF6
H16e
IrS
PCy3
NPF6
16e
Ir
S
S PCy3
N
PF6
16e
oxidativeaddition
migratoryinsertion
σ
π
reductiveeliminationsolventcoordination
repeat ofcycle withcyclooctene
di-solvatedactive formof catalyst
H2
AJELIAS L7-S21
This mechanism is only for understanding not for the exam
Factors which have been found to improve the efficiency (better TOF) of transition metal catalysts for hydrogenation
• Making a cationic metal center : makes catalyst electrophillic for alkenecoordination
• Use of ligands (eg. Cyclooctadiene) which will leave at the initial stages of the cycle generating a di-solvated active catalyst : facilitates binding of even sterically hindered alkenes
• Use of chelating biphosphines: Cis enforcing: reduces steric hindrance at the metal centre
AJELIAS L7-S22
Ir
S
S PCy3
N
PF6
16edi-solvatedactive formof catalyst
IrPCy3
NPF6
16e
RhP
P
+
PF6
Cis enforcing
Problem solving- fill in the blanks
1,2 Migr. Insertion
1,1 Migr. Insertion
Oxidative addition
Bio Inorganic chemistry
Study of Inorganic elements in the living systems
Na
11
22.98
K
19
39.09
Ca
20
40.08
Mg
12
24.31Sodium potassium pump
(1/5th of all the ATP used)
Hemoglobin Vit B12 Hemocyanin Carbonic anhydraseMyoglobin CarboxypeptidaseCytochromesFerredoxin
Cu29
63.55Zn
30
65.38
Fe26
55.85Co
27
58.94
1. Regulatory Action Sodium potassium channels and pump
Na, K Nerve signals and impulses, action potential muscle contraction
2. Structural Role Calcium in bones, teeth
Ca, Mg provide strength and rigidity
3. Electron transfer agents Cytochromes: redox intermediates
Fe2+/Fe3+ membrane-bound proteins that contain heme groups and carry out electron transport in Oxidative phosphorylation
4. Metalloenzymes Carbonic anhydrase, Carboxypeptidase
Zn biocatalysts, CO2 to HCO3−, protein digestion
5. Oxygen carriers and storage Hemoglobin, Myoglobin, HemocyaninFe, Cu 18 times more energy from glucose in
presence of O2
6. Metallo coenzymes Vitamin B 12Co biomethylation
Important roles metals play in biochemistry
Structure of a metallo-protein : A metal complex perspective
Spiral - α helix form of protein Tape - β Pleated sheet form of protein
Prosthetic groups – A metal complex positioned in a crevice. Some of the ligands for this complex or some times all of the ligands are provided by the side groups of the amino acid units.
The geometry around the metal and bond distances and angles are decided by the protein unit
Myoglobin Carbonic anhydrase
Hemocyanin
Cytochrome C Coenzyme B12
Myoglobin
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes. Cofactors are either organic or inorganic. They can also be classified depending on how tightly they bind to an enzyme, with loosely-bound or protein-free cofactors termed coenzymes and tightly-bound cofactors termed prosthetic groups.
Metalloenzymes and Oxygen carriers = Protein + Cofactor
Porphyrins with different metals at its centre are a common
prosthetic group in bioinorganic chemistry
Chlorophyll
Protoporphyrin IX and Heme
15 different ways to arrange the substituents around the porphyrin. Only one isomer protopophyrin IX is found in the living system. Porphyrins are planar and aromatic
Proteins –consists of different amino acids in a specific sequence connected by the peptide bond –
HISTDINE This amino acid has a pKa of 6.5. This means that, at physiologically relevant pH values, relatively small shifts in pH will change its average charge. Below a pH of 6, the imidazole ring is mostly protonated.
GLUTAMIC ACID has carboxylic acid functional group which is hydrophilic, has pKa of 4.1 and exists in its negatively charged deprotonated carboxylate form at physiological pH ranging from 7.35 to 7.45.
VALINE is a branched-chain amino acid having a hydrophobic isopropyl R group. In sickle-cell disease, valine substitutes for the hydrophilic amino acid glutamic acid in hemoglobin.Valine is hydrophobic
A few important amino acids relevant to the present course
SERINE Serine is an amino acid having a CH2OH side group. By virtue of the hydroxyl group, serine is classified as a polar amino acid.Serine was first obtained from silk protein, a particularly rich source, in 1865.
The primary structure of a protein
The four levels of protein structure
H bond between side chains, hydrophobic interactions, disulfur linkages, electrostatic interactions
See youtube video “protein structure” Univ of Surrey ’
Hemoglobin- a quaternary structure of a protein
4 units
Each unit has a prosthetic group (heme) embedded in a crevice and partly coordinated by histidine units
In molecular biology theactive site (prostheticgroup) is part of anenzyme where substratesbind and undergo achemical reaction. It canperform its function onlywhen it is associatedwith the protein unit
Inorganic Active site / Prosthetic group
Ferredoxin (e transfer)Heme in Myoglobin (O2storage)
Nitrogen FixationCarbonic anhydrase Enzyme)
Inorganic Prosthetic group of three well known oxygen carriers
Present in Vertebrates
Present in molluscs
Present in some sea worms
Can the prosthetic unit part of a metalloprotein perform its normal function without the protein unit around it ?
Fe2+
Free Heme
+ O2 Fe2+ OO
Fe2+ OO
Fe2++ 2 Fe4+ O
Fe4+ O Fe2++ Fe3+ O
Fe3+
Reversible binding of O2 is possible on when protein unit is present around the heme unit
Why do we need oxygen or why do we breathe?
What happens to oxygen in our body and where does it happen?
How exactly does oxygen change to water ?
What does this reaction produce and how?
How exactly is oxygen carried around and stored in the body?
How exactly is CO2 removed from the body?
Oxygen : A few Questions
Cytochromes are, in general, membrane-bound (i.e. inner mitochondrial membrane) heme proteins containing heme groups and are primarily responsible for the generation of ATP via electron transport.
They are found bound on the inner mitochondrial membrane either as monomeric proteins (e.g., cytochrome c) or as subunits of bigger enzymatic complexes that catalyze redox reactions. These heme proteins are classified on the basis of the position of their lowest energy absorption band in the reduced state, as cytochromes a (605 nm), b (~565 nm), and c (550 nm).
Electron transfer agents Cytochromes: redox intermediates
Fe2+/Fe3+ membrane-bound proteins that contain heme groups and carry out electron transport in Oxidative phosphorylation
Electron transfer agents; e.g. Cytochrome C
N
N N
N
FeN
SN
H
protein
CH3
methionine residue of protein
HO O
OHO
S(Cys) Protein
S(Cys) Protein
Mitochondria: The powerhouse of the Animal Cell
Bio-units of the electron transport chain are present on the inner walls of the mitochondrion.
Analogous powerhouses on the plant cells are chloroplasts
Glycolysis + Oxidative phosphorylation: How food is converted into energy
Glucose + 36 ADP + 36 Pi + 36 H+ + 6 O2 6 CO2 + 36 ATP + 42 H2OGlucose gives 18 times more energy when oxidized
ATP + H2O ADP + Pi + H+ + energy Δ G0 = - 7.3 kCal/mole
Different forms of Cytochromes (exceptCytochrome P-450) are involved in theelectron transfer process leading to ATPsynthesis and conversion of O2 to H2O
See youtube video ‘cellular respiration ( electron transfer chain)’
ATP : Universal currency for energy
in living systems
See youtube video ‘gotta get that ATP’ for fun and learning!
Cytochromes a and a3Cytochrome c oxidase with electrons delivered to complex by soluble cytochrome c (hence the name)
Cytochromes b and c1 Cytochrome c reductase
Actual structure of ATP synthase unit (a molecular machine!)
