Transcript Slide 1
Lecture 36 Organometallic reactions and catalysis 1) Reactions involving modification of ligands • • We will consider: (a) migratory insertion; (b) b-hydride elimination; (c) a-abstraction. Insertion. An insertion reaction involves coupling of two ligands in cis-position, neutral unsaturated (X), and anionic (Y), to form another anionic ligand (XY) at the same metal: neutral ligand X M vacancy Y X = CO, olefin, alkyne, CR2, O2, CO2, N insertion deinsertion anionic ligand M X Y CR Y = H, alkyl, aryl, OR, NR2 new anionic ligand CO OC OC Mn Carbonyl insertion CO OC Me OC CO CO O C Me CO Mn -CO OC OC O C Mn Me CO Carbonyl deinsertion 18 e • • CO CO 16 e CO 18 e New X-Y bond forms and the coordination vacancy appears. The vacancy is necessary for the reversal reaction which is called deinsertion or elimination. To prevent the reversal reaction from happening one can add a ligand which will coordinate to the open coordination site at the metal. 2) Mechanisms of ligand migratory insertion • Two mechanisms are known for insertion reactions, which involve either neutral ligand X migration to M-Y or anionic group Y migration to M-X. The reaction below proceeds via Me migration (most typical). Products of CO migration 13 13 OC L Mn Me OC CO L Mn CO OC L Mn CO CO Me OC CO L Mn CO CO 13 13 CO CO Me OC CO L Me Mn CO CO CO O C CO CO 13 -CO or 13 - CO OC L A Mn COMe L B CO L Mn CO COMe OC L CO Mn Me Me L Mn CO COMe OC L CO OC CO L Mn Me Products of Me migration COMe CO CO Me CO CO Mn D 13 CO CO Mn C CO 13 CO OC CO CO 13 2:1 Mixture of cis- and transisomers is observed experimentally 13 13 CO Me CO OC CO L Mn CO CO CO 3) Olefin migratory insertion / b-hydride elimination • Another important type of insertion is olefin insertion (1,2 insertion). Olefin insertion into M-H bond is reversible. The microscopic reverse is called b-hydride elimination. vacancy Olef in insertion Nb H a C H2 Nb b-hydride elimination • CO CO Nb 2 C2 H5 b-Hydride elimination is responsible for the instability of great variety of transition metal alkyl complexes where alkyl possesses b-hydrogen. Ph3P CH2CH2CH2CH3 Pt Ph3P a b CH2CH2CH2CH3 Ph3P - PPh 3 vacancy uns table • H bCH Bu Pt a CH2 H Ph3P a Bu Pt H Ph3P CH2 CH3 Pt Ph3P b Et a CH3 s table Et Olefin insertion into M-C bond is usually irreversible. It forms a base for numerous systems for catalytic olefin polymerization: Olefin insertion Zr Me CH2=CH2 Zr Me Zr C H2 H2 C CH2=CH2 Me ... 4) Carbene migratory insertion / a-hydride abstraction • Carbene can also insert into a number of M-X bonds Carbene insertion Re Ph3P • Re Ph3P H2C CH3 CH2 Re Ph3P CH3 The reversal reaction is called a-hydride abstraction. Cp* Ta(CH2Ph)3Cl2 • Cp*Li Cp* Ta CH2Ph Cl CH2Ph CH2Ph -HCH2Ph a-abstraction 2 PMe3 Cp* Ta CH2Ph Ta PMe3 -HCH Ph 2 Cl Cl CPh CHPh a-abstraction Me3P a-Hydride abstraction can also result from intramolecular a-CH bond oxidative addition to a metal. LnWIV CHPh H • H LnWVI CHPh H This is the rare case when an elimination reaction causes metal formal oxidation state to change. CO or olefin insertion / elimination reactions do not involve change in metal oxidation state.