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Reactions of Organometallic Complexes with Nucleophiles The scheme below shows common reactions between a metal complex containing an unsaturated ligand (an alkene) and a nucleophile. Since we have already examined the different types of reactivity which can occur from nucleophilic attack at the metal (substitution reactions, coordination in a vacant site, insertion reactions), we are mainly concerned with reactions involving direct nucleophilic attack on unsaturated ligands. DIRECT NUCLEOPHILIC ATTACK ON THE LIGAND X + LxM Nu - X LxM Nu NUCLEOPHILIC ATTACK AT THE METAL substitution by salt metathesis - X- substitution coordination - alkene X Nu LxM X LxM LxM Nu Nu 1,2-insertion 1,2-insertion X LxM L xM Nu Nu Direct nucleophilic addition to an unsaturated ligand (e.g. CO, alkene, allyl, benzene) • Because they are electron rich, molecules such as CO, alkenes, polyenes and arenes generally do not react with nucleophiles in the absence of a metal. • Once attached to a metal, these ligands give up some of their electron density and become susceptible to direct nucleophilic attack (umpolung). • Unsaturated ligands are more susceptible to direct nucleophilic attack when: - there is less electron density on the metal (e.g. – π-acceptor co-ligands, overall positive charge). - the metal is coordinatively saturated – this avoids nucleophilic attack at the metal centre. We will look at (A) nucleophilic addition to CO, and (B) nucleophilic addition at π-ligands: A. Direct nucleophilic addition at CO Me3N O Me3N O (OC)5Mo C (OC)5Mo O C O (OC)5Mo PPh3 + NMe3 + O • + PPh3 (OC)5Mo C O R3NO (a trialkylamine oxide; R = Me or Et) is commonly used instead of heat or UV-irradiation to remove CO ligands in order to speed up dissociative substitution reactions. NaBH4 Re OC Re CO ON • O OC ON H The above reaction involves direct nucleophilic attack of H- on a CO ligand. Note: It is possible to get similar reactions with CNR or carbenes. B. Nucleophilic addition at π-ligands • There are many examples of direct nucleophilic attack on coordinated π-ligands, and these transformations can be very useful in synthesis. For example: H n BuLi Mo PPh2 Ph2P • Mo PPh2 Ph2P The regiochemistry of such reactions is predicted by the Davies / Green / Mingos (DGM) Rules. DGM Rules (these rules must be followed in order) Rule 1 – Even before Odd: Nucleophilic attack occurs preferentially at even polyenes Rule 2 – Open before Closed: Nucleophilic addition occurs preferentially to open polyenes (not closed). OPEN CLOSED Rule 3 - For open polyenes: - If Even Æ attack occurs at a terminal position. - If Odd Æ attack is usually NOT at a terminal position Æ attack is only at a terminal position if the metal is very strongly electron withdrawing. Reasons for the three DGM rules (1) Even polyenes are formally neutral (alkene, benzene vs Cp-, allyl-, C7H7-). (2) Largely an empirical observation. The reasons behind it are not well understood. (3) This has to do with the relative size of the lobes in the LUMO (see below): NuNu- LUMO NuHOMO LUMO LUMO HOMO HOMO allyl- (allyl+) butadiene for only the most electron withdrawing metals, this orbital becomes sufficiently depleted of electron density that it becomes the preferred site for nucleophilic attack. Extensions to the DGM rules • Secret rule zero! – nucleophiles generally add once to a monocation, twice to a dication etc. • For neutral open polyenes: because • M For monoanionic open polyenes: > > M M vs M > because the negative charge is delocalized over more carbons in pentadienyl than allyl PRACTICE QUESTIONS FOR DGM RULES Me W Mo :PPhMe2 SMe CO ? H- Mo N O Mo - ? H- ? 2 Me- ? ? 2 Co CN- ? Fe