30 Questions
What is the main purpose of insertion and elimination in organometallic catalysis?
To combine and transform ligands to expel them as useful products in a catalytic cycle
What is the product of a 1,1 insertion reaction?
M–(AB)–X, where AB has formed a new bond with both M and X
What type of insertion is observed with CO as the ligand?
1,1 insertion
What is the product of a 1,2 insertion reaction with ethylene as the ligand?
MCH2CH2X
What type of ligands typically undergo 1,1 insertion?
η1 ligands
What happens to the overall reaction rate if k−1 is very small relative to k2[L]?
The rate is governed by k1
What determines the type of insertion reaction that occurs?
The type of ligand involved in the reaction
What is the nature of the reaction kinetics if k−1 is very large relative to k2[L]?
Second-order
What is the purpose of plotting l/kobs versus 1/[L]?
To find 1/k1 from the intercept and k−1/(k1k2) from the slope
What is the term often applied to the reactions involving intramolecular nucleophilic attack on A=B?
Migratory insertion
What is the location of the labeled CO in the product when the incoming ligand is 13CO?
Cis to the newly formed acetyl
What is the fate of the stereochemistry of the migrating group R in the intramolecular nucleophilic attack on A=B?
It is retained
What is the purpose of NMR and IR spectroscopy in this reaction?
To locate the labeled CO in the product
What is kobs in the context of this reaction?
A new term defined by Eq. 7.12
What is the role of back donation in M–(A=B) bonding?
It is a partial transfer of an M dπ electron pair to the A=B π*
What is the product of the reaction of CO insertion into metal–alkyl bonds?
Metal acyls
What is the mechanism of the CO insertion into metal–alkyl bonds?
Migratory insertion
What is the rate of the reaction of CO insertion into metal–alkyl bonds dependent on?
The concentration of the alkyl group and the ligand
What is the likely reason why the M–C bond in 7.8 is stronger than expected?
Because of back donation from metal dπ orbitals into the C–Ar σ*
What happens to the stereochemistry at both carbons in the usual syn insertion?
It is retained
What is the result of the rearrangement of the cis-vinyl complex to the trans isomer?
A sterically less hindered complex is formed
What is the general principle that must be considered when interpreting reactivity trends?
Consider all the bonds broken and formed
What is the likely outcome of insertion with simple α-olefins?
A mixed regiochemistry with no bias
What is the role of steric effects in the insertion of simple α-olefins?
They can bias the outcome in suitable cases
What is a possible reason for M–alkyl bonds to be stronger than M–H bonds in f-block metals?
The presence of electronegative substituents on the alkene
What is the role of [Rh(OEP)]2 in radical photopolymerization of CH2=CHCOOR?
It acts as a initiator for the radical chain reaction
What is the mechanism proposed for the reaction of butadiene with HMn(CO)5?
H atom transfer to give a radical that is subsequently trapped by the metal
What is the role of benzylic resonance in the insertion reaction?
It stabilizes the metalloradical intermediate
What is the result of M–M bond homolysis of [Rh(OEP)]2?
Formation of a metalloradical
What is the reactivity of butadiene with hydrides?
It undergoes 1,4 insertion with some hydrides
Study Notes
Insertion and Elimination
- Insertion and elimination reactions allow for the combination and transformation of ligands in the coordination sphere of a metal, ultimately leading to the formation of useful products.
- In insertion, a π-bound 2e ligand (A=B) inserts into an M–X bond to give M–(AB)–X, where AB has formed a new bond with both M and X.
Types of Insertion
- There are two main types of insertion: 1,1 and 1,2.
- In 1,1 insertion, M and X end up bound to the same atom of AB.
- In 1,2 insertion, M and X end up on adjacent atoms of AB.
- The type of insertion depends on the nature of A=B.
Factors Affecting Insertion
- η1 ligands give 1,1 insertion, and η2 ligands give 1,2 insertion.
- CO gives only 1,1 insertion, where both M and X end up bound to the CO carbon.
- Ethylene gives only 1,2 insertion, where the product has M and X on adjacent atoms of the ligand.
Kinetics of Insertion
- The rate of insertion depends on the magnitude of k−1 relative to k2[L].
- If k−1 is very small, the rate of the overall reaction is governed by k1, and we have a first-order reaction.
- If k−1 is very large, the rate of the overall reaction is governed by k2, and we have a second-order reaction.
- If k−1 is comparable to k2[L], the situation is more complicated, and the equation is usually rewritten as Eq. 7.11.
CO Insertion
- CO shows a strong tendency to insert into metal–alkyl bonds to give metal acyls.
- The usual mechanism of migratory insertion is shown in Eq. 7.7, where the alkyl group in the reagent undergoes a migration to the CO to give an acyl intermediate.
- The kinetics are reminiscent of dissociative substitution, except that the 2e site is formed at the metal in the migratory step, not by loss of a ligand.
Syn and Anti Insertion
- In the usual syn insertion, the stereochemistry at both carbons is retained.
- In some cases, apparent anti-insertion can occur, where the initially formed cis-vinyl complex rearranges to the sterically less hindered trans isomer.
Radical Pathways
- Radical pathways can also occur in insertion reactions, such as in the reaction of styrene with [Rh(OEP)]2.
- The reaction involves initial M–M bond homolysis to give a 15e metalloradical, which adds to the alkene to give a stabilized intermediate.
Explore the concepts of oxidative addition, substitution, insertion, and elimination in organometallic catalysis. Learn how these processes enable the transformation of organic reagents into products with regeneration of the metal species.
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