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Year 3 - Chemistry > Organometallics > Flashcards

Organometallics Flashcards

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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

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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

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3
Q

What is an example of a complex which undergoes OA?

A

Vaska’s compound is classic

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4
Q

How does intramolec OA occur?

A

Requires PPh3, pre-organised, entropically favoured

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5
Q

How can bimolec OA occur?

A
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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

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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)

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8
Q

What occurs in methane activation?

A

Elim governed by inverse kinetic isotope effect

H scrambling faster than methane elim

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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)

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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

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11
Q

What are the mechanisms of OA?

A

1) Concerted
2) Sn2
3) Ionic
4) Radical

Importance generally decreases

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12
Q

What is involved in a concerted mech of OA?

A

3-centre TS

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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)

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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

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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

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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

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17
Q

What are the features of nucleophilic attack for OA?

A

M is e- pair donor
Dipolar TS
Trans addition

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18
Q

What is ΔS‡ for nucleophilic attack OA?

A

Large -ve
Due to solvent organisation around dipolar TS

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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

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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

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21
Q

What favours the ionic mech for OA?

A

Polar solvent
Basic ligands
low oxn state metal

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22
Q

What is the rate of primary ionic mech for OA?

A

Rate = k[complex][H+]

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23
Q

What is the secondary ionic mech for OA?

A
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24
Q

What is the radical chain mech for OA?

A
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25
Q

When are features of the radical chain OA ?

A

Radical initiator may be required
Alkyl loses stereochem as planar at C
Chain if radical escapes solvent cage

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26
Q

Define migratory insertion

A

Insertion of an unsaturated ligand into an adjacent M-X bond
Groups must be cis

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27
Q

What are features of migratory insertion reactions?

A
  • 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)
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28
Q

How do metal to ligand hydride migrations occur?

A
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29
Q

How can alkene-hydride migratory insertion be stopped?

A

The C-H bond shares e- density so can be detected spectroscopically

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30
Q

What is a good hydride abstracting reagent?

A

Ph3C+

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31
Q

How does alkyl migration occur?

A
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32
Q

How can you combine OA & alkyl migration for alkyl to acyl functionality?

A

On metal centre with CO ligand
OA of RI
Then migration of R to CO to give acyl

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33
Q

How can you convert acyl to alkyl group?

A

OA of acyl
Then alkyl migration, which displaces another ligand

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34
Q

What are the possible mechanisms of carbonylation?

A

(OC)5MnCH3 + CO ->(OC)5MnCOH3

Direct insertion
Knock-on insertion
Alkyl migration

Can determine which one using 13CO

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35
Q

What is the mechanism of direct CO insertion?

A
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36
Q

What is the mechanism of knock-on insertion?

A
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37
Q

What is the mechanism of alkyl migration?

A
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38
Q

What does number of CO IR active mdoes suggests different modes of insertion mech?

A

if 13 in CO is ~1970 cm-1

if 13 in acyl ~ 1625 cm-1

these are different when 12C

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39
Q

What rates are involved in carbonylation?

A
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40
Q

What is the rate law for direct insertion carbonylation?

A

k1 is RDS

1st order wrt alkyl-M
0th order in incoming ligand

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41
Q

What is the rate law for knock-on insertion?

A

k2&raquo_space;> k-2
so follow SSA (pre-eqm)

when high [L] then 1st order in starting material
when low [L] then mixed order kinetics

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42
Q

What electronic effects drive migratory insertion?

A

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-

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43
Q

Why does an e- deficient metal favour migratory insertion?

A

e- deficient M makes C of CO more electrophilic and susceptible to nucleophilic attack from the more e- rich alkyl group

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44
Q

What is the solvent effect on migratory insertion?

A

HMPA - selectively solvates cations, CO inserts in alkyl & then new CO adds

Nitromethane - polar solvent, stabilises Et migration & then new CO adds

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45
Q

What are denotations to different H elimination reactions?

A

Protons are bonded to these C

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46
Q

What is an overview of α-H elim?

A

Was thought rare, is because M(carbene) hydrides are v reactive

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47
Q

What are the types of carbenes?

A

Schrock - no π-accepting ligands, high oxn state metals (early TMs), H and alkyl on carbenoid C
Fischer has e- donating ligands

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48
Q

What is an overview of β-H elim?

A

Prescence/absence of β-H is most important factor in stability of an alkl ligand coord to a TM

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49
Q

What is an overview of γ-H elim?

A
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50
Q

How do Pt(II) long-chain alkyls undergo γ-H elimination?

A

More likely to be OA mech

51
Q

What is an overview of reductive elim?

A

Concerted mech

52
Q

What promotes reductive elim?

A

e- poor metal centre
Bulkyl Ln ligands
e- rich X-Y bond

53
Q

What are some examples of reductive elim?

A
54
Q

What process is the last step in Sonogashira coupling?

A

RE of aryl to terminal alkyne

55
Q

What is required for RE?

A

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
Q

How can thermolysis or electrochem oxn cause RE in Fe(bipy)Et complexes?

A
57
Q

What causes differences in rate of RE for different bond energies?

A

C-H > H-H > C-C

Need for reorientation of highly directional sp3 hybrid orbital leads to barrier for CH3-CH3 coupling

58
Q

What is a heterogeneous catalyst?

A

Different phase from the reactants
~95% of all chemical processes use them

59
Q

When are homogeneous catalysts used?

A

Selectivity critical and product-catalyst separation problems can be solved

60
Q

What are some reactions which use homogeneous cat?

A

Hydrogenation
Carbonylation
Polymerisations
Hydroformylation
Olefin metathesis

61
Q

What are the advantages of homogeneous catalysts?

A

Sig more selective for a single prod
More active
More easy to study mech
More easily modified

62
Q

What are the disadvantages of homogeneous catalysts?

A

More sensitive to permanenet deactivation
More difficult to separate from product

63
Q

What does turnover (TO) refer to in catalysis?

A

1 loop through catalyst cycle

64
Q

What is turnover freq (TOF) in catalysis?

A

Number of passes through catalytic cycle per unit time

65
Q

What is turnover number (TON) in catalysis?

A

Absolute # of passes through catalytic cycle before catalyst deactivated

66
Q

What homogeneous catalyst used for alkene hydrogenation and why?

A

Wilkinson’s catalyst

Cationic metal centre more electrophilic and favours alkene coord (often RDS)

Can also coord to many groups

67
Q

How do you synthesise Wilkinson’s catalyst?

A

RhCl3H2O + XS PPh3 -> RhCl(PPh3)3 + Ph3P=O + oxidised solvent

68
Q

What is the trend of hydrogenation of different functional groups using Rh catalysts?

A

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
Q

What is the selectivity of Wilkinson’s catalyst?

A

Hydrogenates most reactive multiple bonds first

Sterics and elec effects could be involved too

70
Q

What groups are not reduced by Wilkinson’s catalyst?

A
  • Aldehyde
  • Ketone
  • Nitrile
  • Nitro group
  • Benzene
71
Q

How can you study Wilkinson’s catalyst in NMR?

A

Different shift of 31P next to Cl, so can be tracked

72
Q

How can 18e- hydride complexes be a catalyst for alkene hydrogenation?

A
73
Q

What is the mechanism of 18VE hydride catalysis?

A
74
Q

What is the rate of 18VE hydride catalysis?

A

Rate = kK[Rh][alkene][H2]/(1 + K[alkene])

75
Q

What is the selectivity of 18VE hydride catalysts?

A

More selec for terminal and cis alkenes due to crowded 5-coord structure when olefin complex

Less avtive than Wilkinson’s catalyst

76
Q

What is asymmetric hydrogenation?

A

Chiral catalyst and alkene results in chiral carbon centre after hydrogrenation

77
Q

What are some chiral metal catalysts?

A
78
Q

Why is asymm hydrogenation selective?

A

Alkene complexation occurs before H2 OA (which is RDS)

Minor diastereomer reacts sig faster to give the final chiral product

79
Q

What is hydrocyanation?

A

Uses Ni/Pd complexes
Mechanistically similar to hydrogenation

80
Q

How are the Ni catalysts for hydrocyanation?

A
81
Q

What is hydrosilylation?

A

H-SiR3 addition to alkene or alkyne
Uses Pt and Pa catalysts

82
Q

What is the mech and selectivity of hydrosilylation?

A

Preferentially adds cis, but isomerises

83
Q

What is the hydroformylation reaction?
(OXO)

A

C-C forming reaction
2x possible isomeric products

84
Q

What classes of hydroformylation catalysts are there?

A

Hydrido-Cobalt-Carbonyl

Hydrido-Cobalt-Phosphines

Hydrido-Rhodium-Phosphine-Carbonyls

M(CO)X(PR3)4-xH

85
Q

How do Hydrido-Cobalt-Carbonyl catalysts form active species?

A

All form active precursor HCo(CO)4

HCo(CO)4 <-> HCo(CO)3 + CO
of which HCo(CO)3 is the active species

86
Q

What is the detailed catalytic cycle for Hydrido-Cobalt-Carbonyl catalysts?

A
87
Q

What is an overview of the catalytic cycle for Hydrido-Cobalt-Carbonyl catalysts?

A

Hydroformylation: alkene to aldehyde

88
Q

What are the kinetics for hydroformylation by Hydrido-Cobalt-Carbonyl?

A

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
Q

What is the reactivity of hydrido-rhodium-carbonyl wrt Co equivalent?

A

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
Q

What is the mech of formylation of alkenes using hydrido-rhodium-carbonyl?

A

Gives sig more linear than branched

91
Q

What is the mech of formylation of alkenes using hydrido-rhodium-phosphine-carbonyl?

A
92
Q

What is selectivity of different formylation catalysts?

A
93
Q

What is the Monsanto process?

A

Production of acetic acid:
CH3OH + CO -> CH3COOH

Catalysed by RhI2(CO)2-

94
Q

What is the mechanism of the Monsanto process?

A
95
Q

What is the Wacker process?

A

Catalyst - PdCl42-/H2O and CuCl2/O2

96
Q

What is the rate eqn of the Wacker process?

A

Rate = [alkene][PdCl42-/[H+][Cl-]2

97
Q

What is the mech of the Wacker process?

A
98
Q

What is the role of CuCl2 in the Wacker synthesis?

A

Regens PdCl2

99
Q

How does olefin oxidation react if different solvents?

A
100
Q

What is alkene isomerisation?

A

Metal complex accelerates rearrangment of double bonds in alkenes to thermo most stable alkene

terminal <-> internal
isolated <-> conjugated

101
Q

What catalysts are used for alkene isomerisation?

A

(R3P)2NiCl2 + H2 -> (R3P)2NiH

RhH(CO)(PPh3)3 -> RhH(CO)(PPh3)2

2nd steps are the actual catalysts

102
Q

What is the mechanism of alkene isomerisation?

A

Two different β-H so thermo eqm

103
Q

What is oligomerisation?

A

Alkenes coupling together to make a short chain or ring

104
Q

What is the catalyst used for alkene oligomerisation?

A

Complex + PR3

105
Q

What is the mech for alkene oligomerisation?

A
106
Q

What is alkene metathesis?

A
107
Q

What are the types of metathesis?

A

Ring closing
Ring opening polymerisation
Acyclic diene metathesis polymerization
Cross metathesis

108
Q

What occurs in RCM and ROMP?

A
109
Q

What occurs in ROM?

A
110
Q

What occurs in cross-metathesis (CM)?

A
111
Q

What is the mechanism for cross metathesis?

A
112
Q

What is the mechanism of ring-opening methathesis polymerization (ROMP)?

A
113
Q

What are typical alkene metathesis catalysts?

A

LnM=CH2

Schrock catalysts - Mo/W

Grubbs Catalysts - Ru carbene complexes

114
Q

What are Schrock catalysts in alkene metathesis?

A
115
Q

What are the advantages/ disadvantages of Schrock catalysts in alkene metathesis?

A

Adv:
* High activity
* Well defined
* No activators

Disadv:
* Poor tolerance to functional groups
* Oxygen/moisture sensitive

116
Q

What is the structure of Grubbs catalyst?

A
117
Q

What are the advantages of grubbs catalyst?

A
  • Moderate activity
  • Tolerant of functional groups
  • Doesnt require activator
  • Selectivity to sterically unhindered alkenes
  • Not air-sensitive
118
Q

What is the process producing polyolefins?

A

Alkene polymerisation

119
Q

What are the steps in alkene polymerisation?

A

Initiation
Propagation
Termination

120
Q

What occurs in initiation of alkene polymerisation?

A

Generating active catalyst from precursor

121
Q

What occurs in the propagation step in alkene polymerisation?

A

Polymer chain growth which can occur multiple times
Via Cossee or Green-Rooney mechanism

122
Q

What is the Cossee mech for alkene polymerisation?

A
123
Q

What is the Green-Rooney mech for alkene polymerisation?

A
124
Q

What is the mechanism of termination in the alkene polymeriation process?

A

Reaction step that stops polymer chain growth

Year 3 - Chemistry (12 decks)

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