Nucleophilic Attack on Coordinated π-Ligands

Question 1

What are features of π-ligands?

Answer 1

• simplest example of a p-ligand = ethene
• although bonding of the larger p-ligands is more complex than for ethene, the basic principles are the same
• the p-ligands may be classified as cyclic or acyclic
• in the cyclic systems the bonding in the ligand is often aromatic, usually with 6p-electrons
  i.e. iso-electronic with benzene
• most important ligand in this group is cyclopentadienyl, C5H5– (aka Cp–)
• in acyclic ligands there is conjugation if the sp2 carbon atoms form a contiguous chain
• in both cases the ligands may be formally
• neutral (e.g. benzene and buta-1,3-diene)
• anionic (e.g. allyl and cyclopentadienyl)
• cationic (e.g. cycloheptatrienyl)
  MPC_2024
• for all p-ligands the regions of conjugation are planar and the metal lies within bonding distance above or below the plane

Question 2

Describe the notation for the number carbon atoms that π-ligands use to bond to TM

Answer 2

Use ηn-notation
* h is the Greek letter ‘eta’ and n is the number of carbons in bonding distance of the metal
- if all carbon atoms are bonded or if the number bonded to the metal is obvious, n is sometimes omitted
e.g. the allyl group can be:
i) η3-allyl (or η-allyl) with all 3 carbons interacting,
ii) η1-allyl (or σ-allyl) where it is attached by a σ-bond
* sometimes necessary to indicate specifically which carbon atoms are associated with the metal, done with prefix numbers

NOTE: the superscript attached to h is now redundant and is omitted

e.g. 1-3,5,6-h-cyclo-C8H11 indicates the metal is bonded to C-atoms 1,2,3,5 and 6 of the 8-membered ring

Question 3

Draw three examples of ηn-notation

Answer 3

Question 4

Draw examples of positive, neutral and anionic 6 π-electron ligands

Answer 4

Question 5

Draw examples of neutral and anionic 4 π-electron ligands

No positive

Answer 5

Question 6

Draw examples of positive 2 π-electron ligands

No neutral or anionic

Answer 6

Question 7

What happens when a coordinated π-ligand is activated?

Answer 7

• under normal conditions of organic chemistry, compounds with CC multiple bonds tend
  to be more readily attacked by electrophilic reagents than by nucleophilic reagents
• due to the excess electron density in the p-bonds
  e.g. - the addition of halogens or hydrogen halides to alkenes - electrophilic substitution of arenes
• however, when a p-ligand is coordinated to an electron-poor transition metal
  (i.e. when the metal is in a moderate to high oxidation state and/or when the complex is cationic), it may become very susceptible to nucleophilic attack
• this modification of reactivity forms the basis of several useful synthetic applications
• the reactions are general for organometallic complexes across the transition series
• some of the most useful examples are in palladium chemistry

Question 8

What is the mechanism for nucleophilic attack on a coordinated alkene?

Answer 8

• usually involves a metal in a moderate oxidation state
• proceeds rapidly for cationic complexes

Question 9

What is the mechanism for nucleophilic attack on a palladium aklene complexes?

Answer 9

• palladium alkene complexes particularly susceptible to nucleophilic attack
• weak nucleophiles (e.g. acetate, ethanol) will attack: (first mechanism pic)
• synthetic utility limited by the fact that it is stoichiometric in palladium (a relatively expensive metal)

Question 10

What is the basis of the Wacker Process?

Answer 10

A clever application of the redox chemistry of copper complexes enables the Pd(0) to be oxidized back to Pd(II) using oxygen as the primary oxidant
* forms the basis of the Wacker process for the oxidation of:
- ethene to ethanal
- propene to acetone
* the process is very versatile and conditions and reactants can be selected to produce a range of organic molecules from alkene precursors

Question 11

What is the mechanism for nucleophilic attack on a ally ligands?

Answer 11

• similar to attack on alkene complexes, and once again Pd complexes are very reactive
• several routes whereby allyl complexes of Pd can be formed
• one of the most frequently employed is to react an allyl acetate with a Pd(0) complex
  e.g. Pd(PPh3)4
• acetate is a good leaving group in this situation
• in general the Pd(0) complex need only be present in catalytic quantities* useful nucleophiles for these reactions are stable carbanions derived from malonate esters and b-ketoesters

Question 12

What are Davies-Green Mingos rules?

Answer 12

Predicting the kinetically controlled products of nucleophilic attack:
1) Nucleophilic attack occurs preferentially at even polyenes
2) Nucleophilic attack occurs preferentially at open polyenes
3) For even, open polyenes, attack is always at the
terminal C atom

NOTE: Rules should be applied in sequence