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Organic > Transition Metals in Synthesis > Flashcards

Transition Metals in Synthesis Flashcards

1
Q

What enhances sigma donation from alkene pi system to M in organometallic bonding

A

electron donating groups on alkene (push) and electron poor metal (pull)

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2
Q

What enhances pi donation from metal to alkene pi* system

A

electron withdrawing group on alkene (pull) and electron rich metal (pull)

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3
Q

Characteristics of carbenes

A

Neutral, electron deficient, very reactive

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4
Q

Description of a Schrock carbene

A

Electrophilic M, nucleophilic C, M=high oxidation state early TM e.g. Ti(IV)

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5
Q

Description of a Fischer carbene

A

Nucleophilic M, electrophilic C, electron donating group on C, M= low ox state middle or late TM eg Fe(0)

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6
Q

What are the properties of Ru catalysts

A

Somewhere in between Fischer and Schrock

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7
Q

What does catalyst initiation involve

A

Activation by loss of one ligand; vacant coordination site on M required for catalysis

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8
Q

Advantages of using a chelating aryl carbene catalyst

A

off-cycle cat stabilised therefore large TO, possible catalyst recovery with column chromatography, relatively insensitive to oxygen

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9
Q

How do N-heterocyclic carbene (NHC) ligands interact

A

They are singlet carbenes. They are strong sigma donors, poor pi acceptors (p orbital not available for pi backdonation due to resonance from 2 Ns)

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10
Q

What biases the equilibrium in RCM

A

Entropy (1 molecule becoming 2)

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11
Q

Why do substrates with 1,1-disubstituted alkene sites react slowly

A

Steric clash at 1,1-disubstituted end due to the bulky Ru centre leads to slow 2+2

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12
Q

What happens if there is one 1,1-disubstituted end and one electron rich end

A

Only one accessible alkene required for cyclisation; one electron rich alkene required for initiation and then second 2+2 is easy as it is intramolecular

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13
Q

Why are substrates with basic amines not compatible with [Ru] catalysed RCM, and how would you overcome this

A

Amine is a good donor ligand due to the lp on N and blocks the site for catalyst initiation on [Ru]. Protect the amine (Boc, rip off with TFA)

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14
Q

How can temporary tethers help

A

Control alkene geometry

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15
Q

What is the driving force of ring opening RCM

A

Thermodynamics: relief of starting material ring strain

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16
Q

Describe conditions for RCM

A

G2 (5%), DCM, reflux

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17
Q

Describe conditions for ring opening RCM

A

G1 (3%) benzene (45%)

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18
Q

Describe the approach of avoiding a problem with 1,1-disubstituted alkenes

A

Relay approach: initiate on electron rich chain; second 2+2 easy as intramolecular

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19
Q

Why is RCM often run at high dilution

A

favours RCM vs CM and gives higher yields

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20
Q

How do you bias the system in cross metathesis

A

Using an excess of one alkene favours the cross product

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21
Q

Describe conditions for CM

A

GHII (5%), DCM, RT

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22
Q

Why does CM favour the cross product

A

Catalyst equilibrates most reactive alkene (mono-homodimer) until it meets next most reactive alkene to generate product which is inert to metathesis and provides a thermodynamic sink

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23
Q

In ene-yne metathesis where does initiation occur?

A

The alkene, NOT alkyne

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24
Q

Give conditions for ring closing ene-yne metathesis

A

GHII 1% RT DCM

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25
Q

What is the driving force for ene-yne methathesis

A

Thermodynamics: conjugated product

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26
Q

What is the Mori Modification

A

Use of ethylene atmosphere increases the rate of reaction and yield; ethylene cycle creates unhindered carbene (fast metathesis) while normal cycle creates hindered carbene (slow metathesis)

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27
Q

Considerations to make in intermolecular ene-yne metathesis

A

In formation of 4mr (includes [Ru]), two R groups will be on opposite corners to prevent strain. Also, CM (2 alkenes) is possible but reversible; the alkene is often used in excess

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28
Q

Describe conditions for particularly effective ene-yne metathesis

A

Ethylene as the alkene. G2 (5%), 1 atm, PhMe, 80*C; ethylene means that there are no regioselectivity considerations

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29
Q

What is alkyne metathesis useful for

A

Ring closing reactions, macrocyclisation

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30
Q

What catalyst do you use for alkyne metathesis

A

Metal alkylidene based on W or Mo; active form formed in situ [Mo] —(DCM)—> [Mo] (triple bond)–H

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31
Q

Carbenes are.. (2)

A

neutral and electron deficient

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32
Q

In Schrock carbenes what is the nature of the metal?

A

High oxidation state early TMs eg Ti(IV)

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33
Q

Label electrophile and nucleophile in Schrock carbenes

A

Electrophilic metal

Nucleophilic carbon

34
Q

In Fischer carbenes what is the nature of the metal?

A

Low oxidation steate late TMs eg Fe(0)

35
Q

Label electrophile and nucleophile in Fischer carbenes

A

Nucleophilic metal, electrophilic carbon

36
Q

Metallocarbenoids are…

because…

A

extremely reactive and non-isolable because they have a resonant structure where there is a carbocation next to an electron withdrawing group which is very reactive

37
Q

Singlet free carbenes correspond to

& describe

A

Fischer carbenes
They have an empty p orbital and 2 electrons in sp2
They have strong carbon –> metal sigma bonding and weak metal to carbon C=E+ donation

38
Q

Triplet free carbenes correspond to

& describe

A

Schrock carbenes
They have one electron in the p and sp2 orbitals and show covalent behaviour
They have strong carbon-metal sigma bonding and weak metal-p pi bonding

39
Q

How do we generate metallocarbenoids?

A

From diazocompounds

Diazo + MLn –> lose N2, get metallocarbenoid

40
Q

What is the most reactive type of diazo compound and why?

A

Acceptor is more reactive than acceptor/acceptor and acceptor/donor because there is increased conjugation in the a/a or a/d variants and therefore you need a more active catalyst to use them

41
Q

Why is there a different pathway to prepare mono and disubstituted diazo compounds

A

The N anion prefers to attack the aldehyde rather than get protonated but the aldehyde is not present for disubstituted compounds

42
Q

Describe the “Lantern” catalyst and how it works

A

Rh2(OAc)4
4 bridging acetates between the two Rh’s, up down out and back
16e per Rh
Forms metallocarbenoids by electron transfer between Rh centres
Acetates anchor them together

43
Q

Explain the stereoselectivity of carbenoid catalysed cyclopropanation

A

Carbenoid approaches less hindered allene face i.e. look out for a bulky group
The carbocation then forms on the more substituted carbon

44
Q

What reagent (catalyst) rpomotes asymmetric cyclopropanation

A

Rh2(s-DOSP)4

hexane, RT

45
Q

What is the outcome of Cu(I) or Rh2(OAc)4 and diazo and a double bond?

A

cyclopropanation

works on alkenes, allenes, arenes(!!!)

46
Q

How do metallocarbenoids react with OH bonds (alcohols, water)

A

formal insertion of metallocarbenoid into the O-H bond to give alkylated products
the enolate protonation is enantiodetermining

47
Q

What happens with an N-H bond reacts with a metallocarbenoid

A

formal insertion into N-H bond

amines, anilines, carbamates, amides, aromatic amines, any N basically

48
Q

What 3 types of bonds can both Cu(1) and Rh2(OAc)4 metallocarbenoid insert into

A

OH, NH, SH

Can be intermolecular or intramolecular

49
Q

What catalyst do you need to insert into an unactivated CH bond

A

Rh(II)

50
Q

How do you determine the CH bond reactivity for reaction with Rh(II)

A

CH bond reactivity correlates to ability to stabilise positive transition state: consider carbocations
1 < 2 < 3< allylic < benzylic < etherial ~ alpha amino (protecting group dependent)

51
Q

While CH functionalisation with metallocarbenoids is often paralleled to classical strategies, what are the 3 advantages it has?

A
  • asymmetry
  • diastereoselectivity
  • more synthetically accessible starting materials
52
Q

How do you use a metallocarbenoid to generate an ylide

A

reaction with heteroatoms
oxygen attacks carbocation once metal is coordinated then metal leaves so C- O+ ylide is left
have to trap in situ as the carbonyl ylide is very reactive

53
Q

What happens in a Cu(I) or Rh(II) catalysed rearrangement reaction

A

C- O+ ylide is formed and then if there is an R group attached to the oxygen it will jump onto the negative carbon

54
Q

What is required for oxidative coupling

A

A free coordination site

55
Q

Describe the stereospecifity of migratory insertion

A

Addition of R1 and M(n+2) across a double bond is ALWAYS syn

56
Q

What is required for migratory insertion

A

A vacant coordination site

57
Q

What is required, of two R groups, for reductive elimination with the two R groups leaving as a dimer

A

They must be cis at the metal centre

58
Q

Describe the uses and reagents for Voldhart Co-trimerisation

A

3 alkynes, CpCo(CO)2 OR CpRh(CO)2 OR CpIR(CO)2, heat

It is a powerful method for the synthesis of substituted benzenes

59
Q

What is the main problem for Voldhart Co-trimerisation and why are intermolecular reactions not useful

A

Selectivity is problematic for fully intermolecular reactions; there are so many possible products of the first step –> statistical mixture of homo and hetero dimeric regioisomers after coupling in non-biased systems
Subsequent alkyne insertion suffers analogous problems

60
Q

In a fully intramolecular Voldhart Co-trimerisation where does the Co(I) initially coordinate?

A

Least hindered alkyne; the monosubstituted one

61
Q

Define the selectivity of migratory insertion of nitriles,

R-M(n+2) + HCN

A

R –> C

M –> N

62
Q

Carbonylation is a type of migratory insertion which (3)

A
  • requires a vacant site
  • is very fast
  • is reversible and the equilibrium is influenced by concentration of CO (pressure)
63
Q

What is the Pauson-Khand reaction

A 
alkyne + CO + alkene + Co2(CO)8 (36e, sat) ---> cyclic pentenone 
powerful synthesis 
reaction is stoichiometric in Co
CO comes from the catalyst
other TMs can be used
64
Q

Give 1 possible difficulty of the Pauson-Khand reaction

A

Regioselectivity can be a problem if there is small steric difference between R groups

65
Q

What is the selectivity of the alkene coordination to Co

A

Alkene coordinates less hindered face of Co

Due to sterics, the R group on the alkene coordinates away from the R groups from the alkyne

66
Q

What are the reagents and conditions for the Pauson-Khand reaction

A

Co2(CO)8 (stoich.)
PhMe
100*C, 18hours

67
Q

What are the reagents for Voldhart Co-trimerisation

A

CpCo(CO)2 (~40%)
PhMe
110*C

68
Q

What would you do to try to prevent competitive Voldhart Co-trimerisation if you were trying to perform a Pauson-Khand reaction

A

start with the pre-formed Co-alkyne complex

69
Q

What are the reagents and conditions for Rh catalysed Pauson-Khand

A

[RhCl(CO)2]2 5%
CO (1 atm)
xylene
Room temperature

70
Q

What are the reagents for Rh catalysed Voldhart like trimerisation

A

[RhCl(cod)2]2 5%
BINAP 10% (binap displaces cod)
THF
60*C

71
Q

What happens in terms of electrons/orbitals in pi-allyl formation with Pd

A

Pd puts a pair of electrons into the sigma* orbital of C-X

72
Q

What is the Pd precatalyst used for allylic substitution

A

Pd tetrakis (PPh3)4Pd 18e, becomes L2Pd(0)
OR
Pd2(dba)3.X X=solvent of recrystallisation
also becomes L2Pd(0)

73
Q

What is the Tsuji-Trost reaction

A

Pd-catalysed substitution of an allylic leaving group

74
Q

In the Tsuji-Trost reaction, which product is favoured in linear systems?

A

The linear product and the branched one is not generally formed

75
Q

What catalysts can you use to form the branched Tsuji-Trost product of a linear system

A

Mo or Ir

76
Q

What is the requirement of the chiral ligand that we use to carry out asymmetric Tsuji-Trost

A

It must be able to reach around to the opposite side of the allyl fragment so that the nucleophile “feels” the chirality

77
Q

What ligand is used for asymmetric Tsuji-Trost

A

Trost modular ligand
its the one with the walls and flaps
Pd ligates two phosphines
Different enantiomers available of the N-C-C-N part of the molecule and it is this chirality that determines the relative orientation of the phenyl groups: chirality of diamine transferred to conformation of phenyl groups

78
Q

How can we favour the branched product

A

Switching to electronic control
Primary carbocationic resonance form is strongly disfavoured
Can increase M-allyl carbocationic character by
- moving to 3rd row TM (easier to accomodate additional negative charge)
- using electron deficient phosphine ligands

79
Q

What are the reagents for Ir catalysed allylic substitution

A

[Ir(cod)Cl]2 2%
PR3 (8%) THF Room temperature
nucleophile

80
Q

How does the Ir catalysed allylic substitution work

A

Ir(I) / Ir(III) cycle operative

81
Q

What increases the efficiency of the Ir-catalysed allylic substitution

A

electronegative groups on P ligand increase the electron withdrawing nature of ligand and decrease sigma donation and increase pi backbonding
6% 24:76 PPh3
44% 53:47 P(OiPr)3
89% 96:4 P(OPh)3

82
Q

What is the criteria for an asymmetric Ir catalysed allylic substitution

A

The ligand must either

  • favour formation of 1 Ir-pi-allyl over the other by biasing the equilibrium (thermodynamic control)
  • favour attack of nucleophile onto Ir-pi-allyl over the other (kinetic control, equilibrium independent)

Organic (4 decks)

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