Organo metallic reaction mechanism, Slides of Organometallic Chemistry

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Organometallic Chemistry Unit II

Organometallic^ Reaction

Mechanisms

Nucleophilic and Electrophilic Attack on Coordinated Ligands Ligand activation by metal that leads to a direct external attack at the ligand.Nucleophilic addition and nucleophilic abstraction reactionsNucleophilic addition and nucleophilic abstraction reactions.Electrophilic addition and electrophilic abstraction reactionsexternal reagent to directly attack the metal activated ligand without requiring priorbinding of the external reagent to the metal.g g The attacking reagent may be a nucleophile or an electrophile.^ Nucleophilic attack

Electrophilic attack The nucleophilic attack of the external reagent

electrophilic attack is favored if the Ligand The^ nucleophilic attack of the external reagentis favored if the Ligand Metal fragment is apoor^ π−base and a good^ σ−

electrophilic attack is favored if the LigandMetal fragment is a goodacid

π−base and a poor σ−acid the complex is cationic and/or when the other

the complex is anionic with the metal center at the complex is cationic and/or when the othermetal bound ligands are electron withdrawing

the complex is anionic with the metal center atlow−oxidation state and/or when the othermetal bound ligands are electron donatin the ligand getting activated gets depleted ofelectron density

the ligand getting activated becomes electronrich from the^ π−back donation of the metal y can undergo an external attack by a−−nucleophile Nu, like LiMe or OH

centerand thus can undergo an external attack by an+^ +^ electrophile Elike Hand CH3I attack of the nucleophiles may result in the

The attack of the electrophiles may result in formation of a bond between the nucleophilesand the activated unsaturated substrate it iscalled nucleophilic addition

the formation of a bond between theelectrophile and the activated unsaturatedsubstrate, in which case it is calledelectrophilic addition an abstraction of a part or the whole of theactivated ligand, in which case it is called thenucleophilic abstraction.

an abstraction of a part or the whole of theactivated ligand, in which case it is called theelectrophilic abstraction

Electrophilic addition Electrophilic abstraction

Alkyl abstractions are often achieved by Hg

2+^ that can proceed in two ways,

(i) by an attack at the

α −carbon of a metal alkyl bond leading to an

inversion of configuration at the alkyl carbon and (ii) by an attack at themetal center leading to retention of configuration at the alkyl carbon.

Cyclometallated compounds are organometallic complexes which contain metallacycles with an E-M-C sequence of sigma

Cyclometallation reactions

bonds where E is usually a 15 or 16 group donor atom, M is a metallic centre and C is a sp2 or sp3 carbon atom.Different^ methods^ of preparation^ of^ cyclometallated

compounds^ are^ known

( cyclometallation ,^ oxidative

addition ,^ transmetallation

,^ transcyclometallation ,^ expansion of the cyclometallated ring

,^ external nucleophilic attack on a

coordinated ligand ) This reaction consists in the intramolecular activation of a C-X sigma bond, where X is an atom of a non-metallic element,This reaction consists in the intramolecular activation of a C X sigma bond, where X is an atom of a non metallic element,of a coordination compound and gives place to the formation of the M-C sigma bond of the metallacycle.Cyclometallation reactions, which allow the activation of C-H, C-C or C-F bonds, are noteworthy since these bonds presenta large energy of dissociation and therefore a very limited chemical reactivity.C-H, C-C and C-F intramolecular activations. The intramolecular C-H activation reactions at Pd(II) and Pt(II) complexeshave been extensively studiedhave been extensively studiedPd(II), the intramolecular C-H activation (

cyclopalladation reaction

) takes place by an agostic interaction and with the help of an internal base, usually an acetato ligand This is called "the electrophilic mechanism".Platinum(II), in function of its coligands, can activate C-H bons by the electrophilic mechanism or by an oxidative addition mechanism.cyclo-palladation and -platination reactions, which take place through the electrophilic mechanism or by the oxidative addition mechanism. i ) the cyclometallation reaction has been used to introduce metal centres in dendrimers ii ) the catalytic functionalization of C-H bonds based in an intramolecular C-H bond activation has been achieved using Fe(II), Ru(0),Ru(II), Co(I), Co(III), Rh(I), Rh(III), Ir(I), Ir(III), Ni(II) and Pd(II) compounds, within other transition metal compounds, as catalysts)^ l^ ll^ d^

d^ f h^ l^ i^

l^ ll^ l^

f^ i^ i^

h^ h^ k^ d

iii )^ some cyclometallated compounds of the platinum group metals are excellent catalysts for some organic reactions, such as the Heck andSuzuki reactions and the Aza-Claisen reaction in the case of those of Pd(II),within other organic reactions, and have been applied in optical resolution, in supramolecular chemistry, and in photophysics, in this lattercase as sensors for molecules or ions, as catalysts for photoredox reactions, or as phosphors in OLEDs or in photovoltaic solar cells.