6/1/2015Carbenes en Olefin Metathesis1 Carbenes en carbynes alkene en alkyne metathesis Schrock en Fischer carbene complexes Stable carbenes Cyclopropanation, methylenation, metathesis

6/1/2015Carbenes en Olefin Metathesis2 Schrock carbenes "Standard" M=C bond: normal  and  bonds, both polarized as in M  + -C  - Carbene has a triplet ground state Early/middle transition metal, high oxidation state count carbene as 2- Carbene carbon is nucleophilic

6/1/2015Carbenes en Olefin Metathesis3 Schrock carbenes Prototype:

6/1/2015Carbenes en Olefin Metathesis4 Fischer carbenes Carbene has singlet ground state because of  -donor substituents Metal: late, low oxidation state count carbene as neutral ligand Carbene carbon is electrophilic

6/1/2015Carbenes en Olefin Metathesis5 Fischer carbenes Prototype:

6/1/2015Carbenes en Olefin Metathesis6 Arduengo carbenes Free, stable carbenes Good ligands, strong  -donors More reluctant to dissociate than phosphines

6/1/2015Carbenes en Olefin Metathesis7 Schrock carbenes Usually from decomposition of alkyls (  -elimination) Variations:

6/1/2015Carbenes en Olefin Metathesis8 Schrock carbenes With ketones: "Wittig" With electrophiles: attack on carbene carbon Unhindered carbenes dimerize

6/1/2015Carbenes en Olefin Metathesis9 Schrock carbenes With olefins: metallacycles Metathesis catalysis ! (but "intermediate" carbenes are better for this)

6/1/2015Carbenes en Olefin Metathesis10 Fischer carbenes Usually from CO or RNC complexes via 1) Nucleophilic attack on C 2) Electrophilic attack op O/N Also: spontaneous isomerization of vinyl, olefin of acetylene complexes (Ru)

6/1/2015Carbenes en Olefin Metathesis11 Fischer carbenes Decomposition: a) Dimer of carbene (C=C) b) Olefin, via isomerization of carbene Oxidation to ketone Nucleophilic substitution on carbene carbon

6/1/2015Carbenes en Olefin Metathesis12 Catalytic olefin metathesis Schrock catalyst. Highly active but sensitive to functional groups. Grubbs catalyst. Stable, but PCy 3 must dissociate for the reaction to start. Tolerates many functional groups.

6/1/2015Carbenes en Olefin Metathesis13 Catalytic olefin metathesis Driving: Escape of gaseous products Decrease of ring strain

6/1/2015Carbenes en Olefin Metathesis14 Carbyne complexes Fischer: Schrock: synthesis mostly ad-hoc

6/1/2015Carbenes en Olefin Metathesis15 Catalytic alkyne metathesis e.g. with Very sensitive to functional groups