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Carbenes and Olefin Metathesis M C + C C M C M C C C + C C Peter H.M. Budzelaar Metal-carbon multiple bonds • Many transition metals form not only M-C single bonds but also M=C and (more rare) even M≡C bonds. • Complexes containing an M=C bond are called carbene complexes – The ligand without the metal would be a free carbene • Complexes with M≡C bonds are called carbyne complexes • Both types of complexes tend to be reactive, and useful in catalysis • We distinguish between Fischer-type and Schrock-type carbene (and carbyne) complexes 2 Carbenes and Olefin Metathesis Schrock-type carbene complexes • "Standard" M=C bond: normal s and p bonds, both polarized as in Md+-Cd• Carbene not particularly stabilized, has a triplet ground state • Early/middle transition metal, high oxidation state M s M p – count carbene as 2- • Carbene carbon is nucleophilic • Prototype: – formed by "a-elimination" Ta 3 Ta 5 Carbenes and Olefin Metathesis s and p bonds TaMe3(=CH2) s bond 4 p bond Carbenes and Olefin Metathesis Real M=C double bonds TaBz2(CHPh)(amide-carbene) TaBz5 1.94 2.25 2.17-2.22 Å 5 Carbenes and Olefin Metathesis Synthesis of Schrock carbenes Usually from decomposition of alkyls (a-elimination) Ta 5 Np3Ta Variations: + Cp2TaMe3 E Cp2TaMe2+ CH2 - B Cp2Ta Me H Cp2Ta Cp2Ta Me 6 Cp2Ta Me CH2 Carbenes and Olefin Metathesis Reactivity of Schrock carbenes • With ketones: "Wittig" M C M + O O C + C C • With electrophiles: attack on carbene carbon Unhindered carbenes dimerize: M C + C M • With olefins: metallacycles M C – Metathesis catalysis ! + C C M C C M M C C C – "intermediate" carbenes are better for this 7 Carbenes and Olefin Metathesis Fischer-type carbene complexes • Ligand best described as strong s-donor, medium-weak p-acceptor • Carbene has singlet ground state – because of p-donor substituents p M C s M s M p • Metal: late, low oxidation state – count carbene as neutral ligand • Carbene carbon is electrophilic • Prototype: – nucleophilic attack at coordinated CO OMe (CO)5W Me 8 1) MeLi 2) MeI W(CO)6 Carbenes and Olefin Metathesis s donation, p backdonation C-O p Mo→C p-backdonation 9 C→Mo s-donation LUMO Carbenes and Olefin Metathesis Good ligands, but no "real" M=C bond 2.19 2.00-2.03 10 Carbenes and Olefin Metathesis Synthesis of Fischer carbenes • Usually from CO or RNC complexes via 1) Nucleophilic attack on C MeLi W(CO) (CO)5W 6 2) Electrophilic attack op O/N MeI O Li Me OMe (CO)5W Me • 11 Also: spontaneous isomerization of vinyl, olefin of acetylene complexes (Ru) Carbenes and Olefin Metathesis Reactivity of Fischer carbenes • Decomposition: a) Dimer of carbene (C=C) b) Olefin, via isomerization of carbene • Oxidation to ketone • Nucleophilic substitution on carbene carbon 12 Carbenes and Olefin Metathesis Arduengo carbenes • Free, stable carbenes R N N R • Good ligands, strong s-donors • More reluctant to dissociate than phosphines • Shape very different from that of typical phosphines • Easy and convenient synthesis from imidazolium salts 13 Carbenes and Olefin Metathesis Olefin metathesis catalysis NAr Mo R'O CMe2Ph R'O R' = (CF3)2CMe PCy3 Ph Cl Ru Cl PCy3 Ar = Schrock catalyst. Highly active but sensitive to functional groups. 14 Grubbs catalyst. Stable, but PCy3 must dissociate for the reaction to start. Tolerates many functional groups. Carbenes and Olefin Metathesis Olefin metathesis catalysis M C + C C M C M C C C + C C Driving force: • Escape of gaseous products + "ADMET" • Decrease of ring strain "ROMP" 15 Carbenes and Olefin Metathesis Olefin metathesis catalysis For Schrock catalysts, the equilibrium between carbene and metallacyclobutane can be tuned by the choice of substituents (OR groups). 16 Carbenes and Olefin Metathesis Carbyne complexes • Fischer: OMe M BX3 + M Me Me • Schrock: synthesis mostly ad-hoc Cp*TaBr2 17 Na/Hg dmpe Cp*Ta P P Cp*Ta H P P Carbenes and Olefin Metathesis Alkyne metathesis catalysis e.g. with (tBuO)3W M C + C C M M M C + C C Very sensitive to functional groups. Not (yet) very useful in organic synthesis. 18 Carbenes and Olefin Metathesis Carbene and carbenoid chemistry • Both main group metals and transition metals can be used to transfer carbenes to e.g. double bonds ("cyclopropanation"). • Main group metals: carbenoid reagents – Simmons-Smith: IZnCH2I, EtZnCH2I • Transition metals: with diazo compounds – (RCOO)4Cu2 and (RCOO)4Rh2 – Mode of action obscure 19 Carbenes and Olefin Metathesis