M.C. White. Chem 153 Mechanism -47 Week of Septem ber 24th, 2002 Brookhart polymerization catalysts (BAr'4) CH Polymer Mw=110,000 Polymer Mw=390,000 (BAr‘4) BAr‘4 CH3 CH Insertion elimination (BAr‘4) (BAr'4 人m三心 CHa displacement Low Mw Brookhart JACS 1995(117)6414
M.C. White, Chem 153 Mechanism -47- Week of September 24th, 2002 N N R R Pd CH3 N N R R Pd CH3 H N N R R Pd CH3 N N R R Pd CH3 H N N R R Pd H N N R R Pd H N N R R Pd CH3 H + (BAr'4)- + (BAr'4)- + (BAr'4)- polymer propagation + (BAr'4)- β-hydride elimination insertion High Mw polymers associative displacement + (BAr'4)- + (BAr'4)- Low Mw polymers + (BAr'4)- N N Pd CH3 N N Pd CH3 + (BAr'4) - Polymer Mw = 110,000 + (BAr'4) - Polymer Mw = 390,000 Brookhart JACS 1995 (117) 6414. Brookhart Polymerization Catalysts
M.C. White/QChen Chem 153 Mechanism -48- Week of Septem ber 24th, 2002 n to nl-Cp via Slipped n'-Cp Intermediate lipped n@CpI P(CH3)3 P(H3)3 Oc Re OC—ReP(CH3) P(CH3)3 apped n cp 2(18e-) 18e Based on the observation that the rate of reaction of 1 with P(CH3)3 /C1 to form 2 depends on both the concentration of 1 and P(CH3),an associative mechanism was proposed. To account for associative substition at a formally coordinatively and electronically saturated center, the authors propose an m"slipped"Cp intermediate that forms concurrently with phosphine attack. c10 Casey oM1983(2)535
M.C. White/Q.Chen Chem 153 Mechanism -48- Week of September 24th, 2002 Based on the observation that the rate of reaction of 1 with P(CH3)3 to form 2 depends on both the concentration of 1 and P(CH3)3, an associative mechanism was proposed. To account for associative substition at a formally coordinatively and electronically saturated center, the authors propose an η3 "slipped" Cp intermediate that forms concurrently with phosphine attack. Casey OM 1983 (2) 535. η5 to η1-Cp via Slipped η3-Cp Intermediate Re OC OC CO OC Re P(CH3)3 OC CO proposed "slipped" η3 Cp intermediate 1 (18 e-) P(CH3)3 OC Re OC P(CH3)3 P(CH3)3 CO H 2 (18 e-) 18 e- η1 η Cp 5 Cp slipped η3 Cp 64oC 2
M C. White/M. w. Kanan Chem 153 Mechanism -49- Week ofSeptember 24th, 2002 n5 to n3 ring Folding co 18 18 18 e-complexes with cyclopentadienyl ryl, indenyl ligands under associative"substitution avoiding an energetically unfavorable 20 e intermediate via ligand rearrangement from n5 to n3 (cyclopentadienyl and indenyl) Haptotropic rearrangement may take the form of ring"slippage"where the ring is 2.98A centrally bonded to the metal and its aromaticity is disrupt a non-bonding distance "bending"where the conjugation of th 228A π system is broker Huttner J Organometallic Chem. 1978(145)329
M.C. White/M.W. Kanan Chem 153 Mechanism -49- Week of September 24th, 2002 η5 to η3 Ring Folding 2.28Å WII CO 18 eCO WII CO CO 18 e- 2.98Å: a non-bonding distance Huttner J. Organometallic Chem. 1978 (145) 329. 18 e- complexes with cyclopentadienyl, aryl, indenyl ligands may undergo "associative" substitution avoiding an energetically unfavorable 20 eintermediate via ligand rearrangement from η5 to η3 or even η1 (cyclopentadienyl and indenyl). Haptotropic rearrangement may take the form of ring "slippage" where the ring is acentrally bonded to the metal and its aromaticity is disrupted or ring "bending" where the conjugation of the π system is broken
M C. White. Chem 153 Mechanism -50 Week of september 24th, 2002 Ligand exchange: Dissociative Mechanism The rate-determining step in a dissociative ligand substition pathway is breaking the M-L bond. Because of the late, product-like transition state for forming the coordinatively unsaturated intermediate in such a process, the M-L BDE is a good approximation of the activation energy(EA) 18e rate of ethylene exchange via rate of ethylene exchange via associative displacement at dissociative displacement at 25i~4x10-0sec4 e Q BDE =31 kcal/mol rXn coordinate PPh3 (mmol) kx 10 sec-I 0.20 PPh RhPPh3 1.23 1.73 a 6-fold increase in the concentration of nucleophile does 18e- 16e 18e affect the rxn rate. Results are consistent with a mechanism where the rate determining step is ethylene rate =d [rh(cH4h= k, [rh(chahl di d is not affected by the concentration of the Cramer JACS 1972(94)5681 dt nucleophile
M.C. White, Chem 153 Mechanism -50- Week of September 24th, 2002 Ligand Exchange: Dissociative Mechanism O RhI O 16 erate of ethylene exchange via associative displacement at 25oC is ~ 104 sec-1 RhI 18 erate of ethylene exchange via dissociative displacement at 25oC is ~ 4 x 10-10 sec-1 BDE = 31 kcal/mol The rate-determining step in a dissociative ligand substition pathway is breaking the M-L bond. Because of the late, product-like transition state for forming the coordinatively unsaturated intermediate in such a process, the M-L BDE is a good approximation of the activation energy(EA). 1014 slower at 25oC RhI RhI EA BDE E rxn coordinate RhI 18 eRhI 16 e- PPh3 RhI PPh3 18 erate = -d [LRh(C2H4)2] dt = k1 [LRh(C2H4)2] PPh3 (mmol) 0.20 1.65 1.23 1.73 k x 104 sec -1 a 6-fold increase in the concentration of nucleophile does not significantly affect the rxn rate. Results are consistent with a mechanism where the ratedetermining step is ethylene dissociation and is not affected by the concentration of the nucleophile. 118oC Cramer JACS 1972 (94) 5681
M C. White. Chem 153 Mechanism -51- Week of september 24th. 2002 igand dissociation: sterics t-Bu t-Bu t-B Pd Pd catalyst"resting state Bi-Bi ineffective catalyst +L‖ dissociative-L The steric bulk of the bidentate phosphine ligand is thought to weaken the Pd-p bond, thereby favoring ligand dissociation requ to form the cata lytically active species Pdo catalytically active HiN NaOt-Bu also aryl Br, I, OTs also aniline, piperidine Hartwig JACS 1998(120)7369
M.C. White, Chem 153 Mechanism -51- Week of September 24th, 2002 Ligand dissociation: sterics P P Pd0 P Fe P Fe "ineffective catalyst" Hartwig JACS 1998 (120) 7369. The steric bulk of the bidentate phosphine ligand is thought to weaken the Pd-P bond, thereby favoring ligand dissociation required to form the catalytically active species. P P Pd0 P t-Bu t-Bu t-But-Bu Fe t-Bu t-Bu P t-Bu t-Bu Fe Cl also aryl Br, I, OTs + H2N also aniline, piperidine N H catalyst "resting state" dissociative P P Pd0 t-Bu t-Bu t-But-Bu Fe catalytically active species NaOt-Bu +L -L