Elementary Organometallic Reactions

Transcription

1 Elementary eactions CE 966 (Tunge) Elementary rganometallic eactions All mechanisms are simply a combination of elementary reactions. 1) Coordination -- issociation 2) xidative Addition -- eductive Elimination 3) σ-bond metathesis 4) Atom abstraction 5) Addition -- Elimination 6) Alkyl-Acyl ntercoversion (C migratory insertion) igand Substitution eactions ' ' issociative (like S 1) elative rates of substitution at 25 o Cin 2 2 t 2 2 u rel rate pyr pyr 57 pyr 0.5 Trans effect : (kinetic) strong σ-donors or π-acceptors accelerate substitution of substituents that are trans to themselves. elative rates of substitution of chloride Et 3 t Et 3 rel rate 1 40 C Et ,500 (C) 6 Cr coordinatively saturated 130 o C (C)5 Cr slow 3 fast open coordination site (C) 5 Cr 3 General order:,, alkene, C >> 3 > > > pyr > 3 > - Example: elmchen G.; T Associative chanisms Very common for square planar complexes! Associative u u intermediate nterchange (like S 2) u u tr ansition state u u ac(c 2 ) 2 Ac i r 2 [r(c)] 2 TF 2 r u 95% (91% ee) 5% ucleophilic attack occurs trans to phosphorus Trans influence: (thermodynamic) strong σ-donors weaken the bonds to atoms that are trans to themselves. General order:, > 3 > () 3, 2 > pyr >C, olefin,

2 Elementary eactions CE 966 (Tunge) Example: ippard, S. J. JACS S -C 2 =2.08Å -C 2 =2.21Å 2 C C 2 C 3 nverse electron demand associative substitution Stahl, S. S. JACS ; Stahl, S. S.; andis, C.. JACS egenerate exchange Eyring equation: -ΔG k = k bt h e T To determine activation parameters ondegenerate exchange = -Δ ΔS k b T h e T e ln k T = -Δ T ln k b h ΔS slope intercept 2 2 k f k r onitor the rate of approach to equilibrium. 2 2 F 3 C F 3 C CF 3 CF 3 ρ = 2.2 (exchange is faster with electron deficient alkenes) 1 line-broadening measures rates from s -1 Exchange rate gives: Δ = 7.3 kcal/mol ΔS = -24 eu. so k(25 o C) = s -1 exchange rate = k = π(w obs -W nat ) W = peak width at ½ height. Coalescence of peaks occurs when k = π(δν)/1.414 where Δν is the peak separation in z. "ormal" associative substitution U U "nverse elctron demand" U U

3 Elementary eactions CE 966 (Tunge) ntramolecular ligand/substrate activation is common 2 2 ote: Substitutions are also possible via a radical chain mechanism, but these processes are not as prevalent in catalysis. xidative addition Concerted n Stepw ise n n F 3 C n n n Cooperative ( radical) 2 n n n 2 - U 3 3 r xidative addition to radicals Example: Wayland, B. B. JACS s s h s s 2(T)h C 4 h() (T)h r 2 (T)h C 3 (T)h h() h(t) xidative addition can be accelerated by ligand dissociation Example: artwig, J. F. JACS 1995, (o-tol) 3 (o-tol) 3 - k (o-tol) 3 (o-tol) 3 rate is inverse firstorder in (o-tol) 3! Concerted (cis) Stepwise (trans) (o-tol) 3-3 r C 3 Vaska's complex 3 C r 2 3 r cis-addition C 3 C r r 3 C 3 C 3 trans-addition cis-addition kinetic thermodynamic or ligand association ate complexes: Amatore, C. Acc. Chem. es. 2000, k app (20 o C) -1 s

4 Elementary eactions CE 966 (Tunge) lefin coordination: ovis, T. JACS i i i i Et 2 Zn Et 2 Zn Et 4% ee Et 65% ee i r 2 ca. 4 times faster eductive Elimination: the microscopic reverse of oxidative addition. igand association can induce reductive elimination 3 3 eductive elimination can be stimulated by oxidation. 3 3 via C 3 C C C 3 C 3 C C 3 C 3 rate = k[][c 3 ] C 3 3 Z Z 2 fastest reverse reaction is difficult (C- activation) slowest reverse is unusual igand dissociation can induce reductive elimination Stille, J. K. JACS C C 3 C 3 rel rate C 3 1 C 3 3 C C 3 C C 3 solv C 3 reductive elimination requires dissociation σ-bond tathesis : Very common for early transition metals Bercaw, J. E. JACS Sc 2-80 o C hexane Sc Sc t is often difficult to tell whether late transition metals undergo σ- bond metathesis or oxidative addition/reductive elimination, but generally the latter is favored.

5 Elementary eactions CE 966 (Tunge) igratory nsertion Alkyl-acyl interconversion (a 1,1-insertion) oes methyl migrate or C insert? (Flood, T. C. JACS ) C C * n C C C () 3 Stereochemistry - retention C Fe C 3 C () 3 C C * C C * n n C () 3 C C C thy migration (observed) Fe Whitesides G.. JACS tbu retention C insertion (T observed) not: sonitriles and carbenes are isoelectronic with C. Fe tbu ydrometallation (a 1,2-insertion, very common) Almost always syn-addition Zr 3 h 3 Zr 3 h 3 pre-coordination 3 3 h β-ydride elimination (reverse of hydrometallation) ydrometallation is often reversible. C C like C and C Zr Zr Example: Whitby,. J. T t Bu C 2 5mol% dppf 10 mol% atbu (1.3 eq) Bu toluene, 109 o C 60% See: Gibson, T. T 1982, 157. Zr Zr n t Bu n α-ydride elimination (reverse of migratory insertion) much more rare than β-ydride elimination 3 C 3 C 3 C Ta 2 i C 3 3 C 3 C 3 C Ta C 3 C 3 3 C 3 C 3 C Ta C 3

6 Elementary eactions CE 966 (Tunge) Carbometallation Slower than hydrometallation but still very facile for early transition metals (and too) Casey, C.. JACS 2003, o C eterometalation Similar to carbometalation, but often thought to be more difficult. eactions that proceed by aminopalladation have been the subject of much recent investigation. See: Wolfe, J.. JACS eview: uniz, K. Chem. Soc. ev n n Carbometallation with late transition metals is normally slower Example: Amatore, C. Chem. Eur. J C 2 Ac F 25 o C k obs ~1.5x10-6 s -1 eductive coupling of ligands. C 2 C 2 The reductive coupling of alkenes and alkynes to form metallacycles is a common theme in catalysis. t is a specific subset of carbometallation reactions. Ti( 3 ) 2 probably better described as a carbometallation ntramolecular bicyclization: see ugent, W. A. JACS Ti Ti Ti Cycloaddition is a common way for aminometalation. ote: [22] is thermally allowed. Bergman,. G. JACS TF Zr ucleophilic attack on coordinated ligands. Zr Very common: ewis acids promote nucleophilic attack of most π- electrophiles. Attack on cationic π-complexes is very common. ibeskind,. rgmet C o C 2 Cu(C)i 2 TF -78 o C 89% o C C avies-green-ingos ules: 1) ucleophilic attack at even polyenes is preferred 2) ucleophilic attack at open polyenes is preferred 3) Even-open polyenes attacked at terminal C

7 Elementary eactions CE 966 (Tunge) 4) dd-open attacked at terminal C if is very electrophilic Attack on C: a useful synthesis of Fischer carbenes n C C C i n C C 3 BF 4 - n C C An alternative open transition state explains cases where inversion of configuration is observed (i.e. with S 2-activated alkyl halides) Sn ' Sn ' ' Electrophilic soft metals activate alkynes. ozaki,. eterocycles 1987, 297. Au Au Au 3 irect observation of transmetalation. artwig, J. F. JACS Et 3 TS h Et 3 TS B() 2 Et 3 TF, rt Et 3 Et 3 h B Et 3 Et 3 (4 equiv) C o C Et 3 Et 3 h Et 3 Transmetalation Catalysis Transmetallation is likely not a unique class of elementary reactions. Such processes most likely occur by standard ligand substitutions sequences or by σ-bond metathesis. onetheless, transmetalation is extremely important in catalysis and it requires some unique considerations. any transmetalations exploit the transfer of C to softer less electrophilic metals. Thus, a wide variety of organometallics can be prepared by treatment of metal halides with i or g. Examples: Transmetalation in Stille couplings Espinet,. JACS 1998, 8978; JACS recoordination of the transmetalating agent is important. Sn 3 associative substitution Sn 3 Sn 3 cyclic TS The cyclic transition state gives retention of configuration at C. Catalysis is a wholly kinetic phenomenon. Stability is often inversely proportional to activity. ther important concepts 1) recatalyst vs. Catalyst 2) esting State 3) ate-limiting step Calibrations ates always increase with temperature but What activation energy does a given temperature supply? (Assume a reaction that is completed in 4 hours t 1/2 = 1 h; k = 1.9 x 10-4 s -1 ) What is the activation energy of that reaction? -78 o C -50 o C 0 o C 25 o C 100 o C 16.5 kcal/mol 18.6 kcal/mol 20.6 kcal/mol 22.5 kcal/mol 29.1 kcal/mol

8 Elementary eactions CE 966 (Tunge) What is the effect of temperature on a reaction with a free energy of activation of 25 kcal/mol? (Assuming the activation energy is T independent) T 0 o C 25 o C 35 o C 45 o C 55 o C 65 o C k obs 5.5 x 10-8 s x 10-6 s x 10-5 s x 10-5 s x 10-4 s x 10-4 s -1 rxn time 583 d 11 d 2.7 d 18 h 5.2 h 1.6 h ow does temperature affect competing reaction pathways? ΔS =15eu ΔS =-30eu T igher temps disfavor bimolecular reaction paths.