Organometallics Flashcards
1
Q
What is the formal definition of oxidative addition?
A
Reaction in form: LnM + X-Y -> LnMYX
Increases oxn state by 2 & coord number by 2
d elec count decreases by 2
The # of VE increases by 2
2
Q
What is required from a metal for oxidative addition (OA)?
A
d2 or greater
Coord unsaturated
Must have both empty and vacant orbitals
3
Q
What is an example of a complex which undergoes OA?
A
Vaska’s compound is classic
4
Q
How does intramolec OA occur?
A
Requires PPh3, pre-organised, entropically favoured
5
Q
How can bimolec OA occur?
A
6
Q
How can metals activate a C-H bond?
A
Intial is 16VE, intermediate is 18VE
Benzene faster as forms benzene intermediate
Can also use Ir equivalent
7
Q
What form of alkane CH bond activation at different T?
A
Mix of primary and secondary C-H activation prod
Under higher T - primary observed as the thermo product (sterics)
8
Q
What occurs in methane activation?
A
Elim governed by inverse kinetic isotope effect
H scrambling faster than methane elim
9
Q
What is the kinetic rate law for OA of d8 square-planar complexes?
A
rate = k2[M][XY]
characteristic of coord unsaturated systems (e.g. vaskas)
10
Q
What is the rate law of OA to a d8 trig bipyramidal complex?
A
Reaction slowed by adding extra phosphine
Find by SSA
11
Q
What are the mechanisms of OA?
A
1) Concerted
2) Sn2
3) Ionic
4) Radical
Importance generally decreases
12
Q
What is involved in a concerted mech of OA?
A
3-centre TS
13
Q
What are the features of concerted OA?
A
Coord unsaturated metal
Common when X-Y group has low bond polarity
Cis-addition (shown by IR - 2 stretches instead of 1 in trans)
14
Q
What is the mechanism in concerted OA?
A
TS is trig bipyr, and π-acid ligands (L) are in eq in this config as withdraw e- better
π-acid ligands folded back preferentially
15
Q
How does rate of OA on Vaska’s complex change with halide?
A
e- density on M determines ΔG‡
decreased π-donation lower down group means less e- density so intermediate with donating group forms more readily
16
Q
When is OA via nucleophilic attack favoured?
A
X-Y bond polar (being added)
Increased ligand basicity
Decreased size of ligands
TS stabilised by polar solvents
17
Q
What are the features of nucleophilic attack for OA?
A
M is e- pair donor
Dipolar TS
Trans addition
18
Q
What is ΔS‡ for nucleophilic attack OA?
A
Large -ve
Due to solvent organisation around dipolar TS
19
Q
How does e- donating nature of ligands change rate of nucleophilic attack OA?
A
More e- donating then faster rate of addition as higher e- density on Pd
20
Q
What is the primary ionic mechanism for OA?
A
HX dissociated in solution so add separately
Two variants - protonation then anionation is primary mech
21
Q
What favours the ionic mech for OA?
A
Polar solvent
Basic ligands
low oxn state metal
22
Q
What is the rate of primary ionic mech for OA?
A
Rate = k[complex][H+]
23
Q
What is the secondary ionic mech for OA?
A
24
Q
What is the radical chain mech for OA?
A
25
When are features of the radical chain OA ?
Radical initiator may be required
Alkyl loses stereochem as planar at C
Chain if radical escapes solvent cage
26
Define migratory insertion
Insertion of an unsaturated ligand into an adjacent M-X bond
Groups must be cis
27
What are features of migratory insertion reactions?
* No change in oxn state (except inserting ligand is M=C or M≡C)
* Vacant coord site created
* Reaction proceeds with retention of config
* Eqm position depends on bond strengths
* 1 e- oxn speeds up reaction (as e- deficient metal centre)
28
How do metal to ligand hydride migrations occur?
29
How can alkene-hydride migratory insertion be stopped?
The C-H bond shares e- density so can be detected spectroscopically
30
What is a good hydride abstracting reagent?
Ph3C+
31
How does alkyl migration occur?
32
How can you combine OA & alkyl migration for alkyl to acyl functionality?
On metal centre with CO ligand
OA of RI
Then migration of R to CO to give acyl
33
How can you convert acyl to alkyl group?
OA of acyl
Then alkyl migration, which displaces another ligand
34
What are the possible mechanisms of carbonylation?
(OC)5MnCH3 + CO ->(OC)5MnCOH3
Direct insertion
Knock-on insertion
Alkyl migration
Can determine which one using 13CO
35
What is the mechanism of direct CO insertion?
36
What is the mechanism of knock-on insertion?
37
What is the mechanism of alkyl migration?
38
What does number of CO IR active mdoes suggests different modes of insertion mech?
if 13 in CO is ~1970 cm-1
if 13 in acyl ~ 1625 cm-1
these are different when 12C
39
What rates are involved in carbonylation?
40
What is the rate law for direct insertion carbonylation?
k1 is RDS
1st order wrt alkyl-M
0th order in incoming ligand
41
What is the rate law for knock-on insertion?
k2 >>> k-2
so follow SSA (pre-eqm)
when high [L] then 1st order in starting material
when low [L] then mixed order kinetics
42
What electronic effects drive migratory insertion?
Lewis acid - better it is can coord to e- rich CO and reduce e- density.
Li+ > Na+ > (Ph3)2N+
Incoming ligand - best "trapping ligand"
PMe3 > PPhMe2 > PPh2Me > CO
R - migrating alkyl group, most e- rich makes best nuc
n-alkyl- > PhCH2-
43
Why does an e- deficient metal favour migratory insertion?
e- deficient M makes C of CO more electrophilic and susceptible to nucleophilic attack from the more e- rich alkyl group
44
What is the solvent effect on migratory insertion?
HMPA - selectively solvates cations, CO inserts in alkyl & then new CO adds
Nitromethane - polar solvent, stabilises Et migration & then new CO adds
45
What are denotations to different H elimination reactions?
Protons are bonded to these C
46
What is an overview of α-H elim?
Was thought rare, is because M(carbene) hydrides are v reactive
47
What are the types of carbenes?
Schrock - no π-accepting ligands, high oxn state metals (early TMs), H and alkyl on carbenoid C
Fischer has e- donating ligands
48
What is an overview of β-H elim?
Prescence/absence of β-H is most important factor in stability of an alkl ligand coord to a TM
49
What is an overview of γ-H elim?
50
How do Pt(II) long-chain alkyls undergo γ-H elimination?
More likely to be OA mech
51
What is an overview of reductive elim?
Concerted mech
52
What promotes reductive elim?
e- poor metal centre
Bulkyl Ln ligands
e- rich X-Y bond
53
What are some examples of reductive elim?
54
What process is the last step in Sonogashira coupling?
RE of aryl to terminal alkyne
55
What is required for RE?
cis orientation of LG
High formal charge on metal (if too e- rich then RE inhibited)
C-H > H-H > C-C
Stereochem retained
56
How can thermolysis or electrochem oxn cause RE in Fe(bipy)Et complexes?
57
What causes differences in rate of RE for different bond energies?
C-H > H-H > C-C
Need for reorientation of highly directional sp3 hybrid orbital leads to barrier for CH3-CH3 coupling
58
What is a heterogeneous catalyst?
Different phase from the reactants
~95% of all chemical processes use them
59
When are homogeneous catalysts used?
Selectivity critical and product-catalyst separation problems can be solved
60
What are some reactions which use homogeneous cat?
Hydrogenation
Carbonylation
Polymerisations
Hydroformylation
Olefin metathesis
61
What are the advantages of homogeneous catalysts?
Sig more selective for a single prod
More active
More easy to study mech
More easily modified
62
What are the disadvantages of homogeneous catalysts?
More sensitive to permanenet deactivation
More difficult to separate from product
63
What does turnover (TO) refer to in catalysis?
1 loop through catalyst cycle
64
What is turnover freq (TOF) in catalysis?
Number of passes through catalytic cycle per unit time
65
What is turnover number (TON) in catalysis?
Absolute # of passes through catalytic cycle before catalyst deactivated
66
What homogeneous catalyst used for alkene hydrogenation and why?
Wilkinson's catalyst
Cationic metal centre more electrophilic and favours alkene coord (often RDS)
Can also coord to many groups
67
How do you synthesise Wilkinson's catalyst?
RhCl3H2O + XS PPh3 -> RhCl(PPh3)3 + Ph3P=O + oxidised solvent
68
What is the trend of hydrogenation of different functional groups using Rh catalysts?
Determined by rate of coord to the catalyst
Unsaturated alkenes with polar assists in binding to catalyst have faster hydrogenation rates
Terminal alkynes hydrogenated at a faster rate than terminal alkenes due to better binding and insertion
69
What is the selectivity of Wilkinson's catalyst?
Hydrogenates most reactive multiple bonds first
Sterics and elec effects could be involved too
70
What groups are not reduced by Wilkinson's catalyst?
* Aldehyde
* Ketone
* Nitrile
* Nitro group
* Benzene
71
How can you study Wilkinson's catalyst in NMR?
Different shift of 31P next to Cl, so can be tracked
72
How can 18e- hydride complexes be a catalyst for alkene hydrogenation?
73
What is the mechanism of 18VE hydride catalysis?
74
What is the rate of 18VE hydride catalysis?
Rate = kK`[Rh][alkene`][H2]/(1 + K[alkene])
75
What is the selectivity of 18VE hydride catalysts?
More selec for terminal and cis alkenes due to crowded 5-coord structure when olefin complex
Less avtive than Wilkinson's catalyst
76
What is asymmetric hydrogenation?
Chiral catalyst and alkene results in chiral carbon centre after hydrogrenation
77
What are some chiral metal catalysts?
78
Why is asymm hydrogenation selective?
Alkene complexation occurs before H2 OA (which is RDS)
Minor diastereomer reacts sig faster to give the final chiral product
79
What is hydrocyanation?
Uses Ni/Pd complexes
Mechanistically similar to hydrogenation
80
How are the Ni catalysts for hydrocyanation?
81
What is hydrosilylation?
H-SiR3 addition to alkene or alkyne
Uses Pt and Pa catalysts
82
What is the mech and selectivity of hydrosilylation?
Preferentially adds cis, but isomerises
83
What is the hydroformylation reaction?
(OXO)
C-C forming reaction
2x possible isomeric products
84
What classes of hydroformylation catalysts are there?
Hydrido-Cobalt-Carbonyl
Hydrido-Cobalt-Phosphines
Hydrido-Rhodium-Phosphine-Carbonyls
M(CO)X(PR3)4-xH
85
How do Hydrido-Cobalt-Carbonyl catalysts form active species?
All form active precursor HCo(CO)4
HCo(CO)4 <-> HCo(CO)3 + CO
of which HCo(CO)3 is the active species
86
What is the detailed catalytic cycle for Hydrido-Cobalt-Carbonyl catalysts?
87
What is an overview of the catalytic cycle for Hydrido-Cobalt-Carbonyl catalysts?
Hydroformylation: alkene to aldehyde
88
What are the kinetics for hydroformylation by Hydrido-Cobalt-Carbonyl?
Rate = k`[alkene][Co]`[H2[CO]-1
Inverse to CO as dissociation from coord sat 18e- species required
Precursor only stable at certain CO partial pressures
CO pressure increase means slower rate but more linear (less reverse β-elim)
89
What is the reactivity of hydrido-rhodium-carbonyl wrt Co equivalent?
Rh complex more active than Co
So lower reaction conditions
Gives more linear - as square planar intermediate more stable with them and bulky phosphines
90
What is the mech of formylation of alkenes using hydrido-rhodium-carbonyl?
Gives sig more linear than branched
91
What is the mech of formylation of alkenes using hydrido-rhodium-phosphine-carbonyl?
92
What is selectivity of different formylation catalysts?
93
What is the Monsanto process?
Production of acetic acid:
CH3OH + CO -> CH3COOH
Catalysed by RhI2(CO)2-
94
What is the mechanism of the Monsanto process?
95
What is the Wacker process?
Catalyst - PdCl42-/H2O and CuCl2/O2
96
What is the rate eqn of the Wacker process?
Rate = `[alkene]`[PdCl42-/[H+]`[Cl`-]2
97
What is the mech of the Wacker process?
98
What is the role of CuCl2 in the Wacker synthesis?
Regens PdCl2
99
How does olefin oxidation react if different solvents?
100
What is alkene isomerisation?
Metal complex accelerates rearrangment of double bonds in alkenes to thermo most stable alkene
terminal <-> internal
isolated <-> conjugated
101
What catalysts are used for alkene isomerisation?
(R3P)2NiCl2 + H2 -> (R3P)2NiH
RhH(CO)(PPh3)3 -> RhH(CO)(PPh3)2
2nd steps are the actual catalysts
102
What is the mechanism of alkene isomerisation?
Two different β-H so thermo eqm
103
What is oligomerisation?
Alkenes coupling together to make a short chain or ring
104
What is the catalyst used for alkene oligomerisation?
Complex + PR3
105
What is the mech for alkene oligomerisation?
106
What is alkene metathesis?
107
What are the types of metathesis?
Ring closing
Ring opening polymerisation
Acyclic diene metathesis polymerization
Cross metathesis
108
What occurs in RCM and ROMP?
109
What occurs in ROM?
110
What occurs in cross-metathesis (CM)?
111
What is the mechanism for cross metathesis?
112
What is the mechanism of ring-opening methathesis polymerization (ROMP)?
113
What are typical alkene metathesis catalysts?
LnM=CH2
Schrock catalysts - Mo/W
Grubbs Catalysts - Ru carbene complexes
114
What are Schrock catalysts in alkene metathesis?
115
What are the advantages/ disadvantages of Schrock catalysts in alkene metathesis?
Adv:
* High activity
* Well defined
* No activators
Disadv:
* Poor tolerance to functional groups
* Oxygen/moisture sensitive
116
What is the structure of Grubbs catalyst?
117
What are the advantages of grubbs catalyst?
* Moderate activity
* Tolerant of functional groups
* Doesnt require activator
* Selectivity to sterically unhindered alkenes
* Not air-sensitive
118
What is the process producing polyolefins?
Alkene polymerisation
119
What are the steps in alkene polymerisation?
Initiation
Propagation
Termination
120
What occurs in initiation of alkene polymerisation?
Generating active catalyst from precursor
121
What occurs in the propagation step in alkene polymerisation?
Polymer chain growth which can occur multiple times
Via Cossee or Green-Rooney mechanism
122
What is the Cossee mech for alkene polymerisation?
123
What is the Green-Rooney mech for alkene polymerisation?
124
What is the mechanism of termination in the alkene polymeriation process?
Reaction step that stops polymer chain growth
