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''Insertion reactions''' are a type of [[chemical reaction]] in which a small molecule “inserts” itself into a metal-[[ligand]] bond. These reactions are typically [[organometallic chemistry|organometallic]] in nature and involve a bond between a [[transition metal]] and a [[carbon]] or [[hydrogen]].<ref name=DMA /> The term only refers to the result of the reaction and does not suggest a mechanism. It is usually reserved for the case where the [[coordination number]] and [[oxidation state]] of the metal remain unchanged.<ref name=Brown /> When these reactions are reversible, the removal of the small molecule from the metal-ligand bond is called extrusion or elimination.
#REDIRECT [[Migratory insertion]]
[[File:Insertions_1'1&1'2.gif|thumb|center|300px|Examples of type 1,1 (a) and 1,2 (b) resulting geometries for insertion reactions]]

There are two common insertion geometries— 1,1 and 1,2 (pictured above). Additionally, the inserting molecule can act either as a [[nucleophile]] or as an [[electrophile]] to the metal complex.<ref name=Brown /> These behaviors will be discussed in more detail for [[Carbon monoxide|CO]], nucleophilic behavior, and [[Sulfur dioxide|SO<sub>2</sub>]], electrophilic behavior.
==CO Insertion==
The insertion of carbon monoxide across a metal-carbon site to form an [[acetyl]] group is the oldest-known and most-studied metal-ligand insertion reaction. It proceeds by a 1,1 reaction coordinate, attaching the [[carbonyl]] carbon to both the metal and the ligand. The first CO insertion was discovered in 1957 by the reaction of CO with MnCO<sub>5</sub>CH<sub>3</sub>, forming Mn(CO)<sub>5</sub>COCH<sub>3</sub>.

===Mechanism===
The mechanism for the apparent CO insertion into a metal-[[alkyl]] bond is actually a [[migratory insertion]], with a migration of the alkyl group to another bound CO, followed by addition of a free CO (see figure below). This can be demonstrated by [[Carbon-13|<sup>13</sup>C]]-labeling the incoming CO ligand, which results in 100% of the labeled CO residing [[Cis-trans isomerism|cis-]] to the acetyl group. [[File:Octahedral.png|thumb|center|606px|CO Insertion reaction pathway for an octahedral complex]]

The CO insertion mechanism is not always a migration. The reaction of CO with (Cp)MeLFeCO, where L is a nucleophilic group such as [[Triphenylphosphine|PPh<sub>3</sub>]], yields a mix of both alkyl migration products and products formed by true insertion of bound carbonyls into the [[methyl]] group, which is controllable by the choice of solvent.<ref name="ACR">{{cite journal|last=Anderson|first=Gordon|title=Carbonyl-Insertion Reactions of Square Planar Complexes|journal=Acc. Chem. Res|year=1984|volume=17|issue=17|pages=67-74}}</ref>

[[Square planar molecular geometry|Square planar complexes]] can also undergo CO insertions. Insertion reactions in square planar complexes are of particular interest because their structure allows additional reaction mechanisms to occur. While just like [[Octahedral molecular geometry|octahedral complexes]], square planar complexes can undergo in-plane migration, their lack of out-of-plane steric hindrance renders them much more open to nucleophilic attack of the metal by the CO. Since square planar groups usually form 16 electron species, the 5-coordinate intermediate that forms is stabilized by the [[18-Electron rule]], and undergoes migratory insertion readily. <ref name=ACR /> In most cases the in-plane migration pathway is preferred, but, unlike the nucleophilic pathway, it is inhibited by an excess of CO. <ref name="CCR">{{cite journal|last=Cavell|first=Kingsley|title=Recent Fundamental studies on migratory insertion into metal-carbon bonds|journal=Coordination Chemistry Reviews|year=1996|month=11|volume=155|pages=209-243}}</ref>

[[File:Square_Planar.png|thumb|center|800px|Nucleophilic insertion and rearrangement of a square planar complex]]

===Effects on reaction rates===

*[[Steric effects|Steric]] strain - Increasing the steric strain of the [[chelation|chelate]] backbone in square planar complexes pushes the carbonyl and methyl groups closer together, increasing the reactivity of insertion reactions. <ref name=CCR />

*Oxidation state - Oxidation of the metal tends to increase insertion reaction rates. As the main rate-limiting step in the reaction is the migration of CH<sub>3</sub>- to CO, oxidizing the metal gives a greater partial positive charge on the CO carbon, increasing the rate of reaction.<ref name=Brown />

*[[Lewis acids and bases| Lewis acids]] - Lewis acids also increase the reaction rates, for reasons similar to metal oxidation increasing the positive charge on the carbon. Lewis acids bind to the CO oxygen and remove charge, increasing the electrophilicity of the carbon. This can increase the reaction rate by a factor of up to 10<sup>8</sup>, and the complex formed is stable enough that the reaction proceeds even without additional CO to bind to the metal.<ref name=Brown />

*[[Electronegativity]] of the leaving group - Increasing the electronegativity of the leaving alkyl group stabilizes the metal-carbon bond interaction and thus increases the [[activation energy]] required for migration, decreasing the reaction rate.<ref name="Shusterman">{{cite journal|last=Shusterman|first=Alan|coauthors=Idan Tamir, Addy Pross|title=The mechanism of organometallic micration reactions. A configuration mixing approach|journal=Journal of Organometallic Chemistry|year=1988|volume=340|pages=203-222| doi=10.1016/0022-328X(88)80076-7 }}</ref>

*[[Trans effect|Trans-effect]] – Ligands in an octahedral or square planar complex are known to influence the reactivity of the group they are trans- to. This ligand influence is often referred to as the trans-influence, and it varies in intensity between ligands. A partial list of trans-influencing ligands is as follows, from highest trans-effect to lowest:<ref name=ACR /> – [[aryl]], alkyl > NR<sub>3</sub> > PR<sub>3</sub> > AsR<sub>3</sub> > CO > [[Chlorine|Cl]]. Ligands with a greater trans-influence impart greater electrophilicity to the active site. Increasing the electrophilicity of the CO group has been shown experimentally to greatly increase the reaction rate, while decreasing the electrophilicity of the methyl group slightly increases the reaction rate. This can be demonstrated by reacting a square planar PNMCOCH<sub>3</sub> complex with CO, where PN is a [[bidentate]] P/N ligand bound to the metal. This reaction proceeds in much greater yield when the methyl group is trans-P and the CO trans-N, owing to nitrogen's higher trans-influence.<ref name=CCR />

===Reverse reaction===
The reverse reaction of decarbonylation of [[aldehydes]] is much more difficult to perform than the associated carbonylation, as it requires both a cis- empty site and enough energy to break a carbon-carbon bond. Additionally, such reactions are usually not [[catalytic]], as the extruded CO binds too tightly to the metal to be freed even under the high heats required for this reaction.<ref>{{cite journal|last=Beck|doi=10.1021/om9905106}}</ref> Extrusion of CO from an organic aldehyde is most famously demonstrated using [[Wilkinson's catalyst|Wilkinson's catalyst]], which undergoes [[oxidative addition]] of the aldehyde C-H instead of H-H, and then performs the extrusion of CO to form an octahedral product.<ref name="OHNO">{{cite journal|last=Ohno|first=K|coauthors=J. Tsuji|journal=I. Am. Chem. Soc|year=1968|volume=90|pages=99|doi=10.1021/ja01003a018}}</ref>

===Industrial applications===
The most widely-known and widely-used application of migratory insertion of carbonyl groups is the [[Monsanto process|Monsanto acetic acid process]]. This reaction, through an iodine intermediate, transforms methanol into acetic acid under relatively mild, catalytic conditions. It is used industrially to create over a million tons of acetic acid every year.

==SO<sub>2</sub> Insertion==
SO<sub>2</sub> is an electrophilic species. When SO<sub>2</sub> encounters a transition metal with alkyl ligands, it acts like a Lewis acid towards the alkyl ligand.<ref name="DMA">{{cite book|last=Douglas, McDaniel, and Alexander|title=Concepts and Models of Inorganic Chemistry 3rd Ed|year=1994|publisher=John Wiley & Sons, Inc.|isbn=9780471629788}}</ref>
===Mechanism===
The mechanism of SO<sub>2</sub> insertion is shown in the figure below.<ref name=blue /> First, the sulfur coordinates to the ligand with the electronegative oxygens pointing away from the complex. Then, two different pathways can occur to “insert” the sulfur dioxide between the metal and the alkyl ligand. The top pathway shows the creation of an ionic complex, O,O’-sulphinate where the metal has a positive charge and the ligand has a negative charge. The bottom pathway shows the creation of an O-sulphinate, which can be a stable product. Also, either of these intermediates can form an S-sulphinate, the most common SO<sub>2</sub> insertion species and a 1,1 type geometry. [[IR spectroscopy]] and [[NMR]] can determine the type of compound.<ref name="blue">{{cite book|title=Advances in Organometallic Chemistry|year=1974|publisher=Academic Press|author=A. Wojcicki|authorlink=Sulfur Dioxide Insertion Reactions|editor=Stone and West}}</ref> S-sulphinate has sulfur-oxygen stretching frequencies from 1250-1000 cm<sup>-1</sup> and 1100-1000 cm<sup>-1</sup>. The O, O'-sulphinate and O-sulphinate are difficult to distinguish as they have stretching frequencies from 1085-1050 cm<sup>-1</sup> and 1000-820 cm<sup>-1</sup> or lower. The pathway involving the O, O' sulphinate can generally be ruled out if the original metal complex fulfilled the 18 electron rule because the two metal-oxygen bonds would exceed the 18 electron rule. <ref name="Brown">{{cite book|title=The chemistry of the metal-carbon bond, vol. 2|year=1985|publisher=John Wiley & Sons|author=J.J. Alexander|authorlink=Insertions into transition metal-carbon bonds|editor=Hartley and Patai}}</ref>
[[File:Mechanism SO2 insertion labels.jpg|thumb|center|600px|Mechanism of SO<sub>2</sub> insertion]]
When SO<sub>2</sub> is inserting into a square planar complex that does not have 18 electrons, it inserts slightly differently. Since the metal is [[saturation (chemistry)|coordinatively unsaturated]], the SO<sub>2</sub> can pre-coordinate with the metal. Then, it inserts itself between the metal and the alkyl group as shown in the example below.<ref name="Puddephatt">{{cite journal|last=Puddephatt|first=R.A.|coauthors=Stalteri, M.A.|title=Competition between Insertion of Sulfur Dioxide into the Methyl- or Phenyl- Transition Metal Bond|journal=Journal of Organometallic Chemistry|year=1980|volume=193|pages=C27|DOI=10.1016/S0022-328X(00)86091-X}}</ref>
[[File:SO2 Insertion into Square Planar.jpg|thumb|center|800px|Mechanism of SO<sub>2</sub> Insertion Into a Square Planar Complex]]

===Effects on reaction rates===
The ability of SO<sub>2</sub> to insert depends upon the electronic interactions between the metal and the ligand. If there is [[pi bond|pi bonding]] between the ligand and metal, the SO<sub>2</sub> will not insert. When steric effects are negligible, the ability of SO<sub>2</sub> to insert increases with an increase in the electron donating capacity of the ligand and decreases with the electron withdrawing capacity of the ligand. These trends highlight the electrophilic nature of SO<sub>2</sub> insertion.<ref name=blue />

===Reverse reaction===
The insertion of SO<sub>2</sub> is rarely reversible.<ref name=DMA /> Often, in trying to desulfinate a compound, the whole compound is destroyed.<ref name=blue /> Some exceptions include: thermal/photochemical extrusion in [CpFe(CO<sub>2</sub>{S(O)<sub>2</sub>C<sub>6</sub>F<sub>5</sub>}] in [[toluene]] and thermal extrusion from [CpFe(CO){P(OPh)<sub>3</sub>}{S(O)<sub>2</sub>CH(Ph)(SiMe<sub>3</sub>)}] under vacuum.<ref name=Brown />
The reason that desulfination does not easily occur may be that when there is ligand expulsion, instead of the alkyl group migrating as happens in the case of CO, the SO<sub>2</sub> bonds again with the metal to form an O,O’ sulphinate.<ref name=Brown />

===Industrial applications===
There are few examples of SO<sub>2</sub> insertion in industry due to the low reactivity of SO<sub>2</sub> insertions and the low stability of the created products, but SO<sub>2</sub> insertion can be a step in production of [[sulfolane]]s. <ref name="Kurosawa">{{cite book|title=Fundamentals of Molecular Catalysis, Volume 3 (Current Methods in Inorganic Chemistry)|year=2003|publisher=Elsevier|isbn=0444509216|editor=Kurosawa, H.; Yamamoto, A.}}</ref><ref name="Dzhemilev">{{cite journal|last=Dzhemilev|first=K|coauthors=Kunakova, R.V.|title=Metal complex catalysis in the synthesis of organic sulfur compounds|journal=Journal of Organometallic Chemistry|year=1993|volume=455|doi=10.1016/0022-328X(93)80375-L}}</ref>

==Olefin Insertion==
[[Olefin]] insertions are insertion reactions involving [[alkene]]s and [[alkyne]]s and they can insert into metal-carbon as well as metal-hydrogen bonds. Many of olefin insertions' features are similar to those of carbonyl insertion in terms of the mechanism, however key differences are present. One of these is the potential to perform multiple insertions in tandem providing a mechanism for [[polymerization]]<ref name="Sinn & Kaminsky">{{cite journal|last=Sinn|first=H.|coauthors=Kaminsky, W.|journal=Advances in Organometallic Chemistry|year=1980|volume=18|pages=99}}</ref>.
===Mechanism===
The mechanism for olefin insertions into metal-carbon or metal-hydrogen bonds converts an olefin complex into an alkyl complex, forming a bond between the carbon or hydrogen with the carbon at the β position. This insertion at the same time forms a bond between the metal and the carbon at the α position. When a metal has sufficient electrons for [[pi backbonding|pi-backbonding]], the olefin can be coordinated to the metal before insertion. Depending on the ligand density of the metal, ligand dissociation may be necessary to provide a coordination site for the olefin.
[[File:Olefin_Insertion_B-elimination)_revised.gif|thumb|center|600px|Olefin insertion (forward reaction) and beta-elimination (reverse reaction)]]

Fundamental features of olefin insertions include the mechanism's dependance on the electron density of the metal complex it is inserting into. For instance, a coordinatively saturated 18-electron metal complex will likely require ligand dissociation to provide a vacant coordination site for the insertion to occur (this is also true for the β-elimination<ref name="Reger & Culbertson">{{cite journal|last=Reger|first=D.|coauthors=Culbertson, E.|title=Mechanism of the thermal decomposition of carbonyl(.eta.5-cyclopentadienyl)triphenylphosphineiron(II) alkyl derivatives into carbonyl(.eta.5-cyclopentadienyl)hydrido(triphenylphosphine)iron(II) and olefin|journal=Journal of the American Chemical Society|year=1976|month=May|volume=98|pages=2789|doi=10.1021/ja00426a020}}</ref>). In the case of coordinatively unsaturated metal complexes with lower electron counts (such as the 14-electron Cp*<sub>2</sub>ScCH<sub>3</sub> complex), there are already vacant coordination sites available for olefin insertion without the need for ligand dissociation.

===Effects on reaction rates===
Factors affecting the rate of olefin insertions include the formation of the cyclic, planar, four-center transition state involving incipient formation of a bond between the metal and an olefin carbon.
[[File:Olefin_Insertion_Transition_State.gif|thumb|center|600px|Transition state of olefin insertion reaction]]

From this transition state, it can be seen that a partial positive charge forms on the β-position carbon with a partial negative charge formed on the carbon or hydrogen initially bonded to the metal. This explains the subsequently observed formation of the bond between the negatively-charged carbon/hydrogen and the positively-charged β-carbon as well as the simultaneously formation of the metal-α-carbon bond.


This transition state also allows for consideration of the two most contributing factors to the rate of olefin insertion reactions. These include the orbital overlap of the alkyl group initially attached to the metal and the strength of the metal-alkyl bond. With greater orbital overlap between the partially positive β-carbon and the partially negative hydrogen/alkyl group carbon, the formation of a bond between them is facilitated and the insertion reaction rate is increased. With increasing strength of the metal-alkyl bond, the breaking of the bond between the metal and the hydrogen/alkyl carbon bond to form the two new bonds with the α-carbon and β-carbon (respectively) is slower, thus decreasing the rate of the insertion reaction<ref name="Burger et al.">{{cite journal|last=Burger|first=Barbara|coauthors=Thompson, Mark; Cotter, W; Bercaw, John|title=Ethylene insertion and .beta.-hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene|journal=Journal of the American Chemical Society|year=1990|month=February|volume=112|pages=1566|doi=10.1021/ja00160a041}}</ref>.

===Reverse reaction===
The reverse mechanism for olefin insertion into a metal-hydrogen bond is [[Beta elimination|β-elimination]], which can only occur if a vacant orbital and a vacant coordination position are available in addition to the presence of a hydrogen at the β position. If these conditions exist, the decomposition of the metal alkyl complex can take place resulting in a hydride/olefin complex that can subsequently dissociate the olefin.

===Industrial applications===
As mentioned previously, a fundamental feature of olefin insertion is the opportunity for multiple insertions, providing a mechanism for polymerization. An industrial application of this is ethylene polymerization, which is catalyzed heterogeneously by titanium(III) salts and aluminum alkyls (also known as [[Ziegler-Natta catalyst]]s). In these reactions, ethylene is coordinated to a vacant site on the titanium metal followed by its insertion. This can be repeated multiple times, potentially leading to high molecular weight polymers.
[[File:OlefinInsertion_Polymerization.gif|thumb|center|600px|Synthetic pathway of ethylene polymerization via olefin insertion reaction]]

==Other Insertion reactions==
CO<sub>2</sub> insertion could be important for use in [[feedstock]]s. CO<sub>2</sub> prefers insertion into metal-nitrogen bonds rather than metal-carbon bonds. Most CO<sub>2</sub> extrusion reactions can occur thermally. [[Isocyanide]]s (RNC) and [[nitrosyl]]s (NO) insert similarly to CO in many ways including the same reaction mechanism for insertion into a square planar complex. Insertions of unsaturated species such as O<sub>2</sub>, S, [[carbene]]s, and N have also been observed.<ref name=Brown />

==References==
{{Reflist}}

Revision as of 18:49, 15 December 2010

Insertion reactions' are a type of chemical reaction in which a small molecule “inserts” itself into a metal-ligand bond. These reactions are typically organometallic in nature and involve a bond between a transition metal and a carbon or hydrogen.[1] The term only refers to the result of the reaction and does not suggest a mechanism. It is usually reserved for the case where the coordination number and oxidation state of the metal remain unchanged.[2] When these reactions are reversible, the removal of the small molecule from the metal-ligand bond is called extrusion or elimination.

Examples of type 1,1 (a) and 1,2 (b) resulting geometries for insertion reactions

There are two common insertion geometries— 1,1 and 1,2 (pictured above). Additionally, the inserting molecule can act either as a nucleophile or as an electrophile to the metal complex.[2] These behaviors will be discussed in more detail for CO, nucleophilic behavior, and SO2, electrophilic behavior.

CO Insertion

The insertion of carbon monoxide across a metal-carbon site to form an acetyl group is the oldest-known and most-studied metal-ligand insertion reaction. It proceeds by a 1,1 reaction coordinate, attaching the carbonyl carbon to both the metal and the ligand. The first CO insertion was discovered in 1957 by the reaction of CO with MnCO5CH3, forming Mn(CO)5COCH3.

Mechanism

The mechanism for the apparent CO insertion into a metal-alkyl bond is actually a migratory insertion, with a migration of the alkyl group to another bound CO, followed by addition of a free CO (see figure below). This can be demonstrated by 13C-labeling the incoming CO ligand, which results in 100% of the labeled CO residing cis- to the acetyl group.

CO Insertion reaction pathway for an octahedral complex

The CO insertion mechanism is not always a migration. The reaction of CO with (Cp)MeLFeCO, where L is a nucleophilic group such as PPh3, yields a mix of both alkyl migration products and products formed by true insertion of bound carbonyls into the methyl group, which is controllable by the choice of solvent.[3]

Square planar complexes can also undergo CO insertions. Insertion reactions in square planar complexes are of particular interest because their structure allows additional reaction mechanisms to occur. While just like octahedral complexes, square planar complexes can undergo in-plane migration, their lack of out-of-plane steric hindrance renders them much more open to nucleophilic attack of the metal by the CO. Since square planar groups usually form 16 electron species, the 5-coordinate intermediate that forms is stabilized by the 18-Electron rule, and undergoes migratory insertion readily. [3] In most cases the in-plane migration pathway is preferred, but, unlike the nucleophilic pathway, it is inhibited by an excess of CO. [4]

Nucleophilic insertion and rearrangement of a square planar complex

Effects on reaction rates

  • Steric strain - Increasing the steric strain of the chelate backbone in square planar complexes pushes the carbonyl and methyl groups closer together, increasing the reactivity of insertion reactions. [4]
  • Oxidation state - Oxidation of the metal tends to increase insertion reaction rates. As the main rate-limiting step in the reaction is the migration of CH3- to CO, oxidizing the metal gives a greater partial positive charge on the CO carbon, increasing the rate of reaction.[2]
  • Lewis acids - Lewis acids also increase the reaction rates, for reasons similar to metal oxidation increasing the positive charge on the carbon. Lewis acids bind to the CO oxygen and remove charge, increasing the electrophilicity of the carbon. This can increase the reaction rate by a factor of up to 108, and the complex formed is stable enough that the reaction proceeds even without additional CO to bind to the metal.[2]
  • Electronegativity of the leaving group - Increasing the electronegativity of the leaving alkyl group stabilizes the metal-carbon bond interaction and thus increases the activation energy required for migration, decreasing the reaction rate.[5]
  • Trans-effect – Ligands in an octahedral or square planar complex are known to influence the reactivity of the group they are trans- to. This ligand influence is often referred to as the trans-influence, and it varies in intensity between ligands. A partial list of trans-influencing ligands is as follows, from highest trans-effect to lowest:[3] – aryl, alkyl > NR3 > PR3 > AsR3 > CO > Cl. Ligands with a greater trans-influence impart greater electrophilicity to the active site. Increasing the electrophilicity of the CO group has been shown experimentally to greatly increase the reaction rate, while decreasing the electrophilicity of the methyl group slightly increases the reaction rate. This can be demonstrated by reacting a square planar PNMCOCH3 complex with CO, where PN is a bidentate P/N ligand bound to the metal. This reaction proceeds in much greater yield when the methyl group is trans-P and the CO trans-N, owing to nitrogen's higher trans-influence.[4]

Reverse reaction

The reverse reaction of decarbonylation of aldehydes is much more difficult to perform than the associated carbonylation, as it requires both a cis- empty site and enough energy to break a carbon-carbon bond. Additionally, such reactions are usually not catalytic, as the extruded CO binds too tightly to the metal to be freed even under the high heats required for this reaction.[6] Extrusion of CO from an organic aldehyde is most famously demonstrated using Wilkinson's catalyst, which undergoes oxidative addition of the aldehyde C-H instead of H-H, and then performs the extrusion of CO to form an octahedral product.[7]

Industrial applications

The most widely-known and widely-used application of migratory insertion of carbonyl groups is the Monsanto acetic acid process. This reaction, through an iodine intermediate, transforms methanol into acetic acid under relatively mild, catalytic conditions. It is used industrially to create over a million tons of acetic acid every year.

SO2 Insertion

SO2 is an electrophilic species. When SO2 encounters a transition metal with alkyl ligands, it acts like a Lewis acid towards the alkyl ligand.[1]

Mechanism

The mechanism of SO2 insertion is shown in the figure below.[8] First, the sulfur coordinates to the ligand with the electronegative oxygens pointing away from the complex. Then, two different pathways can occur to “insert” the sulfur dioxide between the metal and the alkyl ligand. The top pathway shows the creation of an ionic complex, O,O’-sulphinate where the metal has a positive charge and the ligand has a negative charge. The bottom pathway shows the creation of an O-sulphinate, which can be a stable product. Also, either of these intermediates can form an S-sulphinate, the most common SO2 insertion species and a 1,1 type geometry. IR spectroscopy and NMR can determine the type of compound.[8] S-sulphinate has sulfur-oxygen stretching frequencies from 1250-1000 cm-1 and 1100-1000 cm-1. The O, O'-sulphinate and O-sulphinate are difficult to distinguish as they have stretching frequencies from 1085-1050 cm-1 and 1000-820 cm-1 or lower. The pathway involving the O, O' sulphinate can generally be ruled out if the original metal complex fulfilled the 18 electron rule because the two metal-oxygen bonds would exceed the 18 electron rule. [2]

Mechanism of SO2 insertion

When SO2 is inserting into a square planar complex that does not have 18 electrons, it inserts slightly differently. Since the metal is coordinatively unsaturated, the SO2 can pre-coordinate with the metal. Then, it inserts itself between the metal and the alkyl group as shown in the example below.[9]

Mechanism of SO2 Insertion Into a Square Planar Complex

Effects on reaction rates

The ability of SO2 to insert depends upon the electronic interactions between the metal and the ligand. If there is pi bonding between the ligand and metal, the SO2 will not insert. When steric effects are negligible, the ability of SO2 to insert increases with an increase in the electron donating capacity of the ligand and decreases with the electron withdrawing capacity of the ligand. These trends highlight the electrophilic nature of SO2 insertion.[8]

Reverse reaction

The insertion of SO2 is rarely reversible.[1] Often, in trying to desulfinate a compound, the whole compound is destroyed.[8] Some exceptions include: thermal/photochemical extrusion in [CpFe(CO2{S(O)2C6F5}] in toluene and thermal extrusion from [CpFe(CO){P(OPh)3}{S(O)2CH(Ph)(SiMe3)}] under vacuum.[2] The reason that desulfination does not easily occur may be that when there is ligand expulsion, instead of the alkyl group migrating as happens in the case of CO, the SO2 bonds again with the metal to form an O,O’ sulphinate.[2]

Industrial applications

There are few examples of SO2 insertion in industry due to the low reactivity of SO2 insertions and the low stability of the created products, but SO2 insertion can be a step in production of sulfolanes. [10][11]

Olefin Insertion

Olefin insertions are insertion reactions involving alkenes and alkynes and they can insert into metal-carbon as well as metal-hydrogen bonds. Many of olefin insertions' features are similar to those of carbonyl insertion in terms of the mechanism, however key differences are present. One of these is the potential to perform multiple insertions in tandem providing a mechanism for polymerization[12].

Mechanism

The mechanism for olefin insertions into metal-carbon or metal-hydrogen bonds converts an olefin complex into an alkyl complex, forming a bond between the carbon or hydrogen with the carbon at the β position. This insertion at the same time forms a bond between the metal and the carbon at the α position. When a metal has sufficient electrons for pi-backbonding, the olefin can be coordinated to the metal before insertion. Depending on the ligand density of the metal, ligand dissociation may be necessary to provide a coordination site for the olefin.

Olefin insertion (forward reaction) and beta-elimination (reverse reaction)

Fundamental features of olefin insertions include the mechanism's dependance on the electron density of the metal complex it is inserting into. For instance, a coordinatively saturated 18-electron metal complex will likely require ligand dissociation to provide a vacant coordination site for the insertion to occur (this is also true for the β-elimination[13]). In the case of coordinatively unsaturated metal complexes with lower electron counts (such as the 14-electron Cp*2ScCH3 complex), there are already vacant coordination sites available for olefin insertion without the need for ligand dissociation.

Effects on reaction rates

Factors affecting the rate of olefin insertions include the formation of the cyclic, planar, four-center transition state involving incipient formation of a bond between the metal and an olefin carbon.

Transition state of olefin insertion reaction

From this transition state, it can be seen that a partial positive charge forms on the β-position carbon with a partial negative charge formed on the carbon or hydrogen initially bonded to the metal. This explains the subsequently observed formation of the bond between the negatively-charged carbon/hydrogen and the positively-charged β-carbon as well as the simultaneously formation of the metal-α-carbon bond.


This transition state also allows for consideration of the two most contributing factors to the rate of olefin insertion reactions. These include the orbital overlap of the alkyl group initially attached to the metal and the strength of the metal-alkyl bond. With greater orbital overlap between the partially positive β-carbon and the partially negative hydrogen/alkyl group carbon, the formation of a bond between them is facilitated and the insertion reaction rate is increased. With increasing strength of the metal-alkyl bond, the breaking of the bond between the metal and the hydrogen/alkyl carbon bond to form the two new bonds with the α-carbon and β-carbon (respectively) is slower, thus decreasing the rate of the insertion reaction[14].

Reverse reaction

The reverse mechanism for olefin insertion into a metal-hydrogen bond is β-elimination, which can only occur if a vacant orbital and a vacant coordination position are available in addition to the presence of a hydrogen at the β position. If these conditions exist, the decomposition of the metal alkyl complex can take place resulting in a hydride/olefin complex that can subsequently dissociate the olefin.

Industrial applications

As mentioned previously, a fundamental feature of olefin insertion is the opportunity for multiple insertions, providing a mechanism for polymerization. An industrial application of this is ethylene polymerization, which is catalyzed heterogeneously by titanium(III) salts and aluminum alkyls (also known as Ziegler-Natta catalysts). In these reactions, ethylene is coordinated to a vacant site on the titanium metal followed by its insertion. This can be repeated multiple times, potentially leading to high molecular weight polymers.

Synthetic pathway of ethylene polymerization via olefin insertion reaction

Other Insertion reactions

CO2 insertion could be important for use in feedstocks. CO2 prefers insertion into metal-nitrogen bonds rather than metal-carbon bonds. Most CO2 extrusion reactions can occur thermally. Isocyanides (RNC) and nitrosyls (NO) insert similarly to CO in many ways including the same reaction mechanism for insertion into a square planar complex. Insertions of unsaturated species such as O2, S, carbenes, and N have also been observed.[2]

References

  1. ^ a b c Douglas, McDaniel, and Alexander (1994). Concepts and Models of Inorganic Chemistry 3rd Ed. John Wiley & Sons, Inc. ISBN 9780471629788.{{cite book}}: CS1 maint: multiple names: authors list (link)
  2. ^ a b c d e f g h J.J. Alexander (1985). Hartley and Patai (ed.). The chemistry of the metal-carbon bond, vol. 2. John Wiley & Sons.
  3. ^ a b c Anderson, Gordon (1984). "Carbonyl-Insertion Reactions of Square Planar Complexes". Acc. Chem. Res. 17 (17): 67–74.
  4. ^ a b c Cavell, Kingsley (1996). "Recent Fundamental studies on migratory insertion into metal-carbon bonds". Coordination Chemistry Reviews. 155: 209–243. {{cite journal}}: Unknown parameter |month= ignored (help)
  5. ^ Shusterman, Alan (1988). "The mechanism of organometallic micration reactions. A configuration mixing approach". Journal of Organometallic Chemistry. 340: 203–222. doi:10.1016/0022-328X(88)80076-7. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ Beck. doi:10.1021/om9905106. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
  7. ^ Ohno, K (1968). I. Am. Chem. Soc. 90: 99. doi:10.1021/ja01003a018. {{cite journal}}: Missing or empty |title= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ a b c d A. Wojcicki (1974). Stone and West (ed.). Advances in Organometallic Chemistry. Academic Press.
  9. ^ Puddephatt, R.A. (1980). "Competition between Insertion of Sulfur Dioxide into the Methyl- or Phenyl- Transition Metal Bond". Journal of Organometallic Chemistry. 193: C27. doi:10.1016/S0022-328X(00)86091-X. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ Kurosawa, H.; Yamamoto, A., ed. (2003). Fundamentals of Molecular Catalysis, Volume 3 (Current Methods in Inorganic Chemistry). Elsevier. ISBN 0444509216.{{cite book}}: CS1 maint: multiple names: editors list (link)
  11. ^ Dzhemilev, K (1993). "Metal complex catalysis in the synthesis of organic sulfur compounds". Journal of Organometallic Chemistry. 455. doi:10.1016/0022-328X(93)80375-L. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. ^ Sinn, H. (1980). Advances in Organometallic Chemistry. 18: 99. {{cite journal}}: Missing or empty |title= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  13. ^ Reger, D. (1976). "Mechanism of the thermal decomposition of carbonyl(.eta.5-cyclopentadienyl)triphenylphosphineiron(II) alkyl derivatives into carbonyl(.eta.5-cyclopentadienyl)hydrido(triphenylphosphine)iron(II) and olefin". Journal of the American Chemical Society. 98: 2789. doi:10.1021/ja00426a020. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  14. ^ Burger, Barbara (1990). "Ethylene insertion and .beta.-hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene". Journal of the American Chemical Society. 112: 1566. doi:10.1021/ja00160a041. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
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