Carbenes and Olefin Metathesis

Transcription

1 arbenes and Olefin etathesis Peter H.. Budzelaar

2 etal-carbon multiple bonds any transition metals form not only - single bonds but also = and (more rare) even bonds. omplexes containing an = bond are called carbene complexes The ligand without the metal would be a free carbene omplexes with 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 arbenes and Olefin etathesis

3 Schrock-type carbene complexes "Standard" = bond: normal σ and π bonds, both polarized as in δ - δ- arbene not particularly stabilized, has a triplet ground state Early/middle transition metal, high oxidation state count carbene as 2- arbene carbon is nucleophilic Prototype: formed by "α-elimination" σ π Ta Ta 5 3 arbenes and Olefin etathesis

4 σ and π bonds Tae 3 (=H 2 ) σ bond π bond 4 arbenes and Olefin etathesis

5 Real = double bonds TaBz 2 (HPh)(amide-carbene) TaBz Å arbenes and Olefin etathesis

6 Synthesis of Schrock carbenes Usually from decomposition of alkyls (α-elimination) Variations: Ta 5 Np 3 Ta p 2 Tae 3 E p 2 Tae 2 B - p 2 Ta H 2 e H p 2 Ta p 2 Ta p 2 Ta e e H 2 6 arbenes and Olefin etathesis

7 Reactivity of Schrock carbenes With ketones: "Wittig" O O With electrophiles: attack on carbene carbon Unhindered carbenes dimerize: With olefins: metallacycles etathesis catalysis! "intermediate" carbenes are better for this 7 arbenes and Olefin etathesis

8 Fischer-type carbene complexes Ligand best described as strong σ-donor, medium-weak π-acceptor π σ arbene has singlet ground state because of π-donor substituents etal: late, low oxidation state σ count carbene as neutral ligand arbene carbon is electrophilic π Prototype: nucleophilic attack at coordinated O (O) 5 W Oe e 1) eli 2) ei W(O) 6 8 arbenes and Olefin etathesis

9 σ donation, π backdonation -O π o σ-donation o π-backdonation LUO 9 arbenes and Olefin etathesis

10 Good ligands, but no "real" = bond arbenes and Olefin etathesis

11 Synthesis of Fischer carbenes Usually from O or RN complexes via 1) Nucleophilic attack on 2) Electrophilic attack op O/N W(O) 6 eli ei (O) 5 W (O) 5 W O Li e Oe e Also: spontaneous isomerization of vinyl, olefin of acetylene complexes (Ru) 11 arbenes and Olefin etathesis

12 Reactivity of Fischer carbenes Decomposition: a) Dimer of carbene (=) b) Olefin, via isomerization of carbene Oxidation to ketone Nucleophilic substitution on carbene carbon 12 arbenes and Olefin etathesis

13 Arduengo carbenes Free, stable carbenes R N N R Good ligands, strong σ-donors ore reluctant to dissociate than phosphines Shape very different from that of typical phosphines Easy and convenient synthesis from imidazolium salts 13 arbenes and Olefin etathesis

14 Olefin metathesis catalysis R'O NAr R'O o e 2 Ph R' = (F 3 ) 2 e l l Py 3 Ru Ph Py 3 Ar = Schrock catalyst. Highly active but sensitive to functional groups. Grubbs catalyst. Stable, but Py 3 must dissociate for the reaction to start. Tolerates many functional groups. 14 arbenes and Olefin etathesis

15 Olefin metathesis catalysis Driving force: Escape of gaseous products "ADET" Decrease of ring strain "ROP" 15 arbenes and Olefin etathesis

16 Olefin metathesis catalysis For Schrock catalysts, the equilibrium between carbene and metallacyclobutane can be tuned by the choice of substituents (OR groups). 16 arbenes and Olefin etathesis

17 arbyne complexes Fischer: Oe BX 3 e e Schrock: synthesis mostly ad-hoc p*tabr 2 Na/Hg dmpe p*ta P P p*ta P H P 17 arbenes and Olefin etathesis

18 Alkyne metathesis catalysis e.g. with (tbuo) 3 W Very sensitive to functional groups. Not (yet) very useful in organic synthesis. 18 arbenes and Olefin etathesis

19 arbene and carbenoid chemistry Both main group metals and transition metals can be used to transfer carbenes to e.g. double bonds ("cyclopropanation"). ain group metals: carbenoid reagents Simmons-Smith: IZnH 2 I, EtZnH 2 I Transition metals: with diazo compounds (ROO) 4 u 2 and (ROO) 4 Rh 2 ode of action obscure 19 arbenes and Olefin etathesis