Organometallic Chemistry Reactions at Coordinated Ligands PDF

Organometallic Chemistry CHE Reactions at Coordinated Ligands Insertion and de-insertion Reactions ©2022 Halawisi,KFU 1 Organometallic Chemistry CHE Migratory Insertion The reaction that modifies...

Organometallic Chemistry CHE Reactions at Coordinated Ligands Insertion and de-insertion Reactions ©2022 Halawisi,KFU 1 Organometallic Chemistry CHE Migratory Insertion The reaction that modifies the ligand consider intramolecular because the coordinated nucleophile or electrophile migrates to the ligand in question within the same molecule. In general, we can define a class of reactions whereby an unsaturated ligand formally inserts into a M-X bond (where X is an X-type ligand). This class of reactions is known as migratory insertion, and is observed across the transition series for metals in varying oxidation states. ©2022 Halawisi,KFU 2 Organometallic Chemistry CHE Migratory Insertion There are two main types of insertion: 1,1 insertion (mainly CO) in which the metal and the X ligand end up bound to the same atom 2 Z 1 2 1 M Y Z insertion occurs to the 1 position M Y X X 1,2 insertion in which the metal and the X ligand end up bound to the adjacent atom 1 2 Y Z X M 1 Z 2 M Y insertion occurs across the 1,2 positions X ©2022 Halawisi,KFU 3 Organometallic Chemistry CHE Insertion and de-insertion Reactions oxidation state coordination number electron count no change C.N -1 T.E.C -2 2 Z 1 2 1 1,1 insertion M Y Z M Y X X 1 2 Y Z X 1,2 insertion M Z 1 2 M Y X oxidation state coordination number electron count no change C.N - 1 T.E.C -2 ©2022 Halawisi,KFU 4 Organometallic Chemistry CHE Example As an example of migratory insertion, consider the insertion of ethylene into a metal-hydride. H +L H no change in metal ox. state L nM H L nM −L L nM L As a result of migratory insertion, a vacant coordination site is created at the metal. This site is usually filled by an incoming ligand. The reverse of the migratory insertion reaction, de-insertion, is also frequently observed. De-insertion is called by several names depending upon the groups involved (e.g. decarbonylation for CO, hydride elimination for H, etc.) ©2022 Halawisi,KFU 5 Organometallic Chemistry CHE Properties of migratory insertion reactions Migratory insertion reactions display several important features. 1. The ligands undergoing migratory insertion must be oriented cis with respect to one another within the coordination sphere of the metal. 2. Migratory insertion occurs by a concerted process leading to retention of configuration at the migrating group, X. 3. The position of the insertion/de-insertion equilibrium is dictated by the strengths of the metal-ligand bonds. 4. A vacant coordination site is created by migratory insertion. ©2022 Halawisi,KFU 6 Organometallic Chemistry CHE 1,1 insertion (CO insertion) Migratory insertion reactions involving migration of an X-type ligand to the coordinating atom of an unsaturated ligand. The insertion of CO into metal-alkyl bonds is a prevalent example of migratory insertion. This type of insertion is referred to as a 1,1-insertion. 2 Z 1 2 1 M Y Z insertion occurs to the 1 position M Y X X O CO C L L nM L nM R R L Recall that the product of this reaction, a metal-acyl complex. ©2022 Halawisi,KFU 7 Organometallic Chemistry CHE M-R vs. M-H insertion of CO CO insertions into M-alkyl bonds are very common whereas insertions into M-hydride bonds are rare. This fact is due to the relative strengths of M-C versus M-H bonds. CH3 CO OC CO CO OC CO Mn Mn CH3 CO insertion OC CO OC C CO CO O The reverse of CO insertion, decarbonylation, is also quite common but requires an empty coordination site. Such reactions are commonly observed for metal-formyl complexes. Decarbonylation of a metal-formyl generates a M-H and a M-CO bond. decarbonylation O - CO O Fe C Fe C Fe H OC H H OC CO OC CO ©2022 Halawisi,KFU 8 Organometallic Chemistry CHE Mechanism of CO insertion The mechanism for CO insertion in 18-electron complexes can be described by the following elementary steps: CO O k1 18-electron L nM C 16-electron R k−1 L nM R O O k2 C C + L R 18-electron R L nM L nM L Insertion of CO into the M-R bond precedes addition of L consistent with the tendency for 18-electron complexes to avoid 20-electron intermediates. ©2022 Halawisi,KFU 9 Organometallic Chemistry CHE Stereochemistry of migratory insertion As mentioned, migratory insertion occurs with retention of configuration at the migrating group, X. What about the stereochemistry at M? Two possible situations can be envisioned for migratory insertion of CO: L L M CH3 L C CH3 migrates to CO CH3 L Alkyl migration O L L M L C L O O CH3 C L L In this case, migration of alkyl group rather than M the CO (in cis position). This would give rise to a 5-coordinate intermediate,Lwith a vacant site available for attachment of L an incoming L. ©2022 Halawisi,KFU 10 Organometallic Chemistry CHE Stereochemistry of migratory insertion As mentioned, migratory insertion occurs with retention of configuration at the migrating group, X. What about the stereochemistry at M? Two possible situations can be envisioned for migratory insertion of CO: L L M CH3 L C CH3 migrates to CO CH3 L Alkyl migration O L L M L C L O O CH3 C L L CO inserts into the M-CH3 bond M L L CO migration In this case, migration of CO to give intramolecular CO insertion. This would give rise to a 5-coordinate intermediate, with a vacant site available for attachment of an incoming L. ©2022 Halawisi,KFU 11 Organometallic Chemistry CHE Stereochemistry of migratory insertion As mentioned, migratory insertion occurs with retention of configuration at the migrating group, X. What about the stereochemistry at M? Two possible situations can be envisioned for migratory insertion of CO: L L M CH3 L C CH3 migrates to CO CH3 L Alkyl migration O L L M L C L O O CH3 C L L CO inserts into the M-CH3 bond M L L CO migration Experimental determination: Using 13C-labeled CO, are consistent with migration of the Me group to CO. ©2022 Halawisi,KFU 12 Organometallic Chemistry CHE Kinetic effects on CO insertion Electron-withdrawing groups on the X-ligand lead to slower migration. More sterically demanding substituents on the X-ligand lead to faster migration. Aryl ligands migrate slower than alkyl ligands. For isostructural compounds, migration is fastest for 3d metals, followed by 4d then 5d due to strength of the M−C bond. Most of these kinetic effects can be rationalized by considering the M-L bond strengths involved in the reaction. ©2022 Halawisi,KFU 13 Organometallic Chemistry CHE 1,2-Insertions Alkenes and alkynes can undergo 1,2-insertion into M–H bonds. Migratory insertion reactions take place with migration of the X-ligand to the β-atom of the unsaturated ligand. This type of insertion is referred to as a 1,2-insertion. 1 2 Y Z X insertion occurs across the 1,2 positions M 1 Z 2 M Y X ©2022 Halawisi,KFU 14 Organometallic Chemistry CHE Insertions into M-H bonds Several common unsaturated ligands are capable of insertion into metal- hydride bonds. Olefins, alkynes, ketones, imines, etc. Migratory insertion of polyhapto ligands into M-H bonds occurs to the same face of the π-system. Migratory insertion is a concerted process! ‡ RR RR M and H added to the planar four-membered same face of the olefin transition state L nM C L nM C C R H C R H RR ©2022 Halawisi,KFU 15 Organometallic Chemistry CHE Insertions into M-H bonds ©2022 Halawisi,KFU 16 Organometallic Chemistry CHE β-H elimination The reverse of migratory insertion into an M-H bond is referred to as β- hydride elimination. This reaction is a very important decomposition mode for metal-alkyl complexes. As dictated by microscopic reversibility, β-hydride elimination requires a vacant coordination site cis to the alkyl ligand. Furthermore, the alkyl, β-H and M must be able to adopt a coplanar arrangement. RR syn coplanar arrangement C R L nM C R Hβ vacant coordination site Alkyl complexes that cannot adopt the syn coplanar arrangement are stable towards β-hydride elimination. ©2022 Halawisi,KFU 17 Organometallic Chemistry CHE Regiochemistry of olefin insertion Two different regiochemical outcomes can occur upon insertion of substituted olefins into M-H bonds. H 2 CH3 1,2-insertion CH3 L nM 1 1 2 H L nM H 1 L nM 2,1-insertion 2 CH3 1,2-Insertions are more common because they produce a less substituted alkyl complex, which is more stable. Additionally, the products of 2,1-insertion are usually prone to β-hydride elimination resulting in regeneration of the metal-hydride. ©2022 Halawisi,KFU 18 Organometallic Chemistry CHE Insertion of other unsaturates into M-H bonds Alkyne insertion in to M-H bonds typically gives the cis product consistent with a concerted process. R cis vinyl ligand R R R H L nM H L nM ©2022 Halawisi,KFU 19 Organometallic Chemistry CHE Insertion into M-C bonds Insertion of polyhapto ligands into M-C bonds is a powerful means of creating new organic architectures. Migratory insertion into a M-C bond generates a new C-C bond! The migratory insertion of olefins into M-C bonds has a larger thermodynamic driving force than insertion into M-H bonds because M- C bonds are weaker than M-H bonds. Despite the larger thermodynamic driving force, insertion of olefins into M-C bonds is kinetically slower than insertion into M-H bonds. Orbital overlap & M-C bond strengths. ©2022 Halawisi,KFU 20 Organometallic Chemistry CHE Insertion into M-C bonds Migration of the alkyl group occurs with retention of stereochemistry to give products consistent with addition of the metal and alkyl group to the same face of the unsaturated ligand. ‡ RR RR L nM C L nM C C R H C C R C H RR ©2022 Halawisi,KFU 21