Activation of arbon-ydrogen Bonds at Transition Metal enters! Thanks to John Bercaw for many nice figures for this lecture! oxidative addition/reductive elimination [L n M q ] L n M q2 L n M q σ complex L's good σ donors; M q very reduced, often d 8 M q2 d 6 mediated by σ complex: (i) small, normal k / k D for oxidative addition (ii) inverse k / k D for reductive elimination, if right favored (iii) normal k / k D for reductive elimination, if left favored generally observe low kinetic selectivity for various types of - bonds, primary often favored for sp 3 -
Activation of arbon-ydrogen Bonds at Transition Metal enters! oxidative addition/reductive elimination [L n M q ] L n M q2 sigma bond metathesis (, ' =, sp 3, sp 2, sp ) homolytic cleavage by two metals 2 L L n M q M q n M q L n L n M q1 L n M q1 L n M q ' L n M q "1,2 addition/elimination" (X =, possibly O) ' "electrophilic displacement" of (X = halide, aquo, etc.) L n M q X L n M q X - L n M q X L n M q X
- bond activation without redox: σ bond metathesis! M q ' M q ' p* 2 M q 16 electron; 14 electron common, ' =, alkyl, alkenyl, aryl, alkynyl rate:, ' = > sp > sp 2 > sp 3 unfortunately, opposite metathetical process does not occur: M q ' M q ' "Selective ydrocarbpn Activation: Principles and Progress" V 1990, hapters 3 (othwell) and 4 (Watson). Watson, JAS 1983, 6491. Thompson, et al. Pure and Appl. hem. 1984, 56, 1. Thompson, et al. JAS 1987, 203 Marks, et al. JAS 1985, 8091
Use of pentamethylcyclopentadienyl ligands simplifies metallocene chemistry by disfavoring oligomers and adducts of these highly coordinatively unsaturated group 3 and lanthanide metallocene complexes: III l l III III 3 3 Al 3 3 III L 3 (L = py, TF, etc.) 2 O 3 1. 12 M l aq 2. SOl 2 3. TF l 3 (TF) 3 2 eq Lip* refluxing xylenes sublime residue l yellow crystals Li alkene or alkyne 2 - - ( = higher alkyl, alkenyl) ( = 3, 6 5, 2 6 5 )
p*: a life-long obsession for Bercaw Group at altech: p* = [η5-5(3)5] = Zr 3 3 Me2Si Me3 vanity license plate, presented by Bercaw research group, ca. 1983 3 PMe3 u Me3P
/D exchange: the p* 2 - system ' ' /D exchange between - bonds of many substrates and D 2 / 6 D 6 is promoted by p* 2 : p* 2 D 2 p* 2 D D very fast p* 2 6 D 6 p* 2 6 D 5 D fast p* 2 D p* 2 D slow p* 2 6 D 5 p* 2 6 D 5 very slow p* 2 D 2 p* 2 D D fast p* 2 6 D 6 p* 2 6 D 5 D slow net: D 2 / 6 D 6 [p* 2 ] D 4 3 2 3 2 2 2 5 ( 3 ) 5, P( 3 ) 3 Si( 3 ) 4, TF (α's faster) (benzylic - slightly slower than aromatic -) ( 3 D first product) (1 o - much faster than 2 o -) (/D exchange with no ring opening) (very slow)
ates of σ bond metathesis is relatively sensitive to hybridization of reacting bonds ate of σ bond metathesis reaction maps with... (a) steric factors (b) s character of reacting bonds s s-s p* 2-80 p* 2 k 2 > 10 3 M -1 s-1 sp 3 s-s p* 2 3-80 p* 2 3 k 2 10 M -1 s-1 s sp 2 -s p* 2 6 5 25 p* 2 6 5 k 2 > 10 M -1 s-1 sp 3 sp 2 -s p* 2 3 6 5 80 p* 2 6 5 3 k 2 3 x 10-5 M -1 s-1 sp 3 sp 2 -s 60 p* 2 3 6 5 = p* 2 = 6 5 3 k 2 1 x 10-3 M -1 s-1 sp 3 sp-s p* 2 3 3 0 p* 2 3 3 k 2 > 10 M -1 s-1 sp 3 sp 3 -s 80 p* 2 3 13 3 p* 2 13 3 3 k 2 1 x 10-5 M -1 s-1
Faster rates of σ bond metathesis for sp 2 and sp hybridized - bonds might suggest electrophilic p* 2 - attacks at π bonds of substrate: 3 3 3 3 3 3 OMO of arene attacked by electrophilic p* 2 sterically worst approach and TS - σ bond attacked (lower energy than π orbitals) by p* 2 as for 2, 4, etc. sterically best approach and TS
Experimental probes of rates and regioselectivity for reaction of p* 2 3 with arenes provides convincing evidence for mechanism of σ bond metathesis X k (M -1 s -1 ) (p*-d 15 ) 2-3 6 5 X k 80 cyclo- 6 D 12 (p*-d 15 ) 2-6 4 X 4 F 3 3 Me 2 1.4 x 10-5 3.3 x 10-5 (k /k D = 2.9) 3.4 x 10-5 3.2 x 10-5 early TS: lot s of - bond breaking p* 2 3 2 hrs 36 hrs 80 115 23% 26% - 4 p* 2-3 - 4 3-4 p* 2 3 57% 61% p* 2 p* 2 less than 5% isomerization 7.5 hr at 80-4 p* 2 6% 13% p* 2 3 115 15% at 30% conversion cf. nitration of toluene, which yields ca. 58% ortho, 38% para, 4% meta. conclusions: σ-bond metathesis of arene - bonds is not "electrophilic aromatic substitution"-like; attacks at - σ bond directly; normal k /k D
Experimental probes of rates and regioselectivity for reaction of p* 2 3 with arenes provides convincing evidence for mechanism of σ bond metathesis X k (M -1 s -1 ) (p*-d 15 ) 2-3 6 5 X k 80 cyclo- 6 D 12 (p*-d 15 ) 2-6 4 X 4 F 3 3 Me 2 1.4 x 10-5 3.3 x 10-5 (k /k D = 2.9) 3.4 x 10-5 3.2 x 10-5 early TS: lot s of - bond breaking p* 2 3 2 hrs 36 hrs 80 115 23% 26% - 4 p* 2-3 - 4 3-4 p* 2 3 57% 61% p* 2 p* 2 less than 5% isomerization 7.5 hr at 80-4 p* 2 6% 13% p* 2 3 115 15% at 30% conversion cf. nitration of toluene, which yields ca. 58% ortho, 38% para, 4% meta. conclusions: σ-bond metathesis of arene - bonds is not "electrophilic aromatic substitution"-like; attacks at - σ bond directly; normal k /k D
Bonding interactions for σ bond metathesis valence bond description molecular orbital description M M M M M A TS δ A' 2a 1 b 2 δ M δ δ δ =?? M rate decreases, = sp > sp 2 > sp 3 M 1a 1 b 2 orbital overlap in TS decreases a 1 TS
Better total overlap leads to preferred metathetical direction for σ bond metathesis: M q ' M q ' M q ' M q ' 3 b 2 a 1 much better total overlap than 3 TS TS
Steric interactions with the p* ligands place constraints on - bond activation: & & 1 o sp 3 - sp 2 - sp - all three are sterically much preferred over... ' ' " & 2 o sp 3 - one bad interaction 3 o sp 3 - two bad interactions
Theoretical studies of σ bond metathesis confirm these pictures of the bonding interactions in the transition state Goddard and Steigerwald JAS 1984, 308. Upton and appé JAS 1985, 1206. l l 62 o 149 o 75 o Ziegler et. al. JAS 1993, 636. p p 70 o 55 o ~180 o - l and p very similar in their demands for metal orbitals - esssentially no barrier for sbm - d character in - bond is an important feature - isoelectronic [l 2 Ti] even more reactive. 11 kcal/mol p 2 3 4 6 kcal/mol p 2 3 4 p p 3 3 p p 3 3
Activation of arbon-ydrogen Bonds at Transition Metal enters! sigma bond metathesis (, ' =, sp 3, sp 2, sp ) L n M q ' L n M q L n M q ' σ complex L n M q ' TS ' L n M q σ complex ' L n M q very electrophilic, coordinatively unsaturated proceeds via a σ complex, although weakly held as compared to oxidative/addition reductive elimination normal k / k D kinetic selectivity for - bonds (i) primary strongly favored for sp 3 - (ii) rate increases with increased s character of reacting bonds: - 2 > sp > sp 2 > sp 3 - orbital overlap and very open '-- angle in TS lead to strong preference for observed metathetical direction for σ bond metathesis
Activation of arbon-ydrogen Bonds at Transition Metal enters! sigma bond metathesis (, ' =, sp 3, sp 2, sp ) L n M q ' L n M q L n M q ' σ complex L n M q TS ' ' L n M q σ complex ' L n M q very electrophilic, coordinatively unsaturated proceeds via a σ complex, although weakly held as compared to oxidative/addition reductive elimination normal k / k D kinetic selectivity for - bonds (i) primary strongly favored for sp 3 - (ii) rate increases with increased s character of reacting bonds: - 2 > sp > sp 2 > sp 3 - orbital overlap and very open '-- angle in TS lead to strong preference for observed metathetical direction for σ bond metathesis
Activation of arbon-ydrogen Bonds at Transition Metal enters! oxidative addition/reductive elimination [L n M q ] L n M q2 sigma bond metathesis (, ' =, sp 3, sp 2, sp ) homolytic cleavage by two metals 2 L L n M q M q n M q L n L n M q1 L n M q1 L n M q ' L n M q "1,2 addition/elimination" (X =, possibly O) ' "electrophilic displacement" of (X = halide, aquo, etc.) L n M q X L n M q X - L n M q X L n M q X
Bergman and Wolczanski simultaneously discover 1,2-addition of - bonds to [Zr=] Bergman, et al. JAS 1988, 8729 Zr IV 3-4 Zr IV Zr IV 6 5 ( = Me 3, 2,6-Me 2 6 3 ) Zr IV Zr IV Wolczanski, et al. JAS 1988, 8731 Zr IV 3 ( = Si(Me 3 ) 3 ) - 4 Zr IV 4 2 4 Zr IV 6 5 Zr IV
Jordan Bennett and Peter Wolczanski take a careful and in-depth look at mechanism for 1,2 addition of - to [Ti=] (and by inference to his [Zr=], [V=] and [Ta=] systems): Wolczanski, et al. JAS 1997, 10696-10719 (yeah, 23 pages!) (Me 3 ) 3 Si (Me 3 ) 3 SiO (Me 3 ) 3 SiO Ti IV 3 (Me 3 ) 3 SiO - 4 (Me 3 ) 3 SiO Ti IV Si(Me 3 ) 3 (Me 4 3 ) 3 SiO (Me 3 ) 3 SiO (Me 3 ) 3 SiO (Me 3 ) 3 SiO L - L L 3 Ti IV Si(Me 3 ) 3 - ΔG, ΔG o ΔΔG 's Ti IV Si(Me 3 ) 3 (L = OEt 2, TF, Me 3, py, PMe 3 ) (Me 3 ) 3 Si (Me 3 ) 3 SiO (Me 3 ) 3 SiO k / k D = 13.7(9) at 24.8 Ti IV = methyl ethyl cyclopentyl cyclopropyl aryl benzyl vinyl etc. ΔG increases - < -= 2 < -cyclopropyl < -phenyl < - 3 < -cyclobutyl ~ -ethyl < -n-butyl < -cyclopentyl > > > > > > > rate of 1,2---addition decreases
Activation of arbon-ydrogen Bonds at Transition Metal enters! TS for 1,2-- addition and 1,2-- elimination is 4-centered with nearly linear -- angle activation parameters, large k / k D, and relative thermodynamics indicate a concerted process with balanced Ti-, -, -, and Ti= bonding making/breaking and little charge build up [(Me 3 ) 3 SiO] 2 Ti=Si(Me 3 ) 3 is a potent electrophile that attacks - σ bond ΔΔG 's for 1,2-- addition roughly follow < sp 2 - < 1 o sp 3 < 2 o sp 3 - and span > 9 kcal/mol "1,2 addition/elimination" (X =, possibly O) L n M q X L n M q X L n M q X
Activation of arbon-ydrogen Bonds at Transition Metal enters! TS for 1,2-- addition and 1,2-- elimination is 4-centered with nearly linear -- angle activation parameters, large k / k D, and relative thermodynamics indicate a concerted process with balanced Ti-, -, -, and Ti= bonding making/breaking and little charge build up [(Me 3 ) 3 SiO] 2 Ti=Si(Me 3 ) 3 is a potent electrophile that attacks - σ bond ΔΔG 's for 1,2-- addition roughly follow < sp 2 - < 1 o sp 3 < 2 o sp 3 - and span > 9 kcal/mol "1,2 addition/elimination" (X =, possibly O) L n M q X L n M q X L n M q X
Activation of arbon-ydrogen Bonds at Transition Metal enters! oxidative addition/reductive elimination [L n M q ] L n M q2 sigma bond metathesis (, ' =, sp 3, sp 2, sp ) homolytic cleavage by two metals 2 L L n M q M q n M q L n L n M q1 L n M q1 L n M q ' L n M q "1,2 addition/elimination" (X =, possibly O) ' "electrophilic displacement" of (X = halide, aquo, etc.) L n M q X L n M q X - L n M q X L n M q X
Brad Wayland developed the cleanest and most generally reactive metalloradical system based on (porphyrin)h II, a stable, low spin d 7, 15- electron complex: Wayland, et al. JAS 1991, 5035; Inorg hem 1992, 148; JAS 1994, 7897; h II (OEP)h II Δ = 18.5(8) kcal/mol 2 (OEP)h II (OEP)h II h II (OEP) Δ 0 kcal/mol -15.5(8) Xy h II Xy Xy Xy h II h II Xy Xy Xy (TXP)h II Xy Δ = 15(1) kcal/mol 2 (TXP)h II (TXP)h II h II (TXP) -12(1) h II h II O (TMP)h II h II 2 (TMP)h II (TMP)h II h II (TMP) ~ 0 O
(porphyrin)h II radicals are remarkably reactive for methane cleavage: [(TXP)h] 2 4 o 0 kcal/mol S o = -7(3) e.u. k 1 k -1 rate forward = k 1 [ 4 ][[(TXP)h] 2 ] = 17(3) kcal/mol S = -25(7) e.u. solvent, benzene, unreactive for weeks at 80 (mixtures of (TXP)h & (TXP)h 6 5 fail to eliminate benzene over months at 80 ) (TXP)h- 3 (TXP)h- 2 (TMP)h 4 k 1 (TMP)h- 3 (TMP)h- o = -13.0(1.5) kcal/mol S o = -19(5) e.u. k -1 rate forward = k 1 [ 4 ][(TMP)h] 2 = 7.1(1.0) kcal/mol S = -39(5) e.u. k / k D = 8.6 at 25 ; = 5.1 at 80 again, solvent, benzene is totally unreactive! -13 kcal/mol = BDE(h-h) BDE(- 3 ) - BDE(h-) - BDE(h- 3 ) 0 105 60 58 kcal/mol from reaction w/ 2
Mechanism deduced from rate laws, k /k D, small Δ, and Δ o : [(porphyrin)h II ] 2 2 (porphyrin)h II (not detectable by epr at 90 K) (porphyrin)h II 4 [(porphyrin)h II ( 4 )] or 2 (porphyrin)h II 4 (porphyrin)h II (porphyrin)h II (105) h II (porphyrin) concerted (58) (60) (porphyrin)h III 3 h III (porphyrin) Termolecular TS with backside attack restricts substrates to 2 and unhindered sp 3 - bonds
Tethered binuclear (porphyrin)h II expands substrate scope: W. ui and B. Wayland JAS 2004, 8266-8274. h II O h II (= " h(m-xylyl)h ) O h II (m-xylyl)h II k - -h III (m-xylyl)h III - ΔG, ΔG o ΔΔG 's, k /k D 's k (M -1 s -1 ) at 296 K BDE(-)) k /k D (296 K) 2 40 104 1.6 (TMP system) 4 8.3 x 10-2 105 10.8 3 3 3.3 x 10-4 100* 6 5 2-2.0 x 10-4 90 5.0 3 O 1.0 x 10-2 96 9.7 *oxidative addition to - bond thermodynamically favored by about 10 kcal/mol, but this alternative not observed.
Activation of arbon-ydrogen Bonds at Transition Metal enters! homolytic cleavage by two metals 2 L n M q L n M q M q L n L n M q1 L n M q1 L n M q M q L n TS is 4-centered, termolecular with linear M---M angles activation parameters, large k / k D, and relative thermodynamics indicate a concerted process with balanced M-, -, and M- bonding making/breaking substrate scope limited to 2 and relatively unhindered sp 3 - bonds ΔG 's for homolytic cleavage of 2 and - bonds decrease regularly as the reactions become more favorable (ΔG o 's more negative) rates follow order - > 3 - > O 2 - > 3 2 - ~ 6 5 2 - and span about 5 orders of magnitude at 296 K
Activation of arbon-ydrogen Bonds at Transition Metal enters! homolytic cleavage by two metals 2 L n M q L n M q M q L n L n M q1 L n M q1 L n M q M q L n TS is 4-centered, termolecular with linear M---M angles activation parameters, large k / k D, and relative thermodynamics indicate a concerted process with balanced M-, -, and M- bonding making/breaking substrate scope limited to 2 and relatively unhindered sp 3 - bonds ΔG 's for homolytic cleavage of 2 and - bonds decrease regularly as the reactions become more favorable (ΔG o 's more negative) rates follow order - > 3 - > O 2 - > 3 2 - ~ 6 5 2 - and span about 5 orders of magnitude at 296 K
There are many examples of - bond "activation" of alkanes (including methane) by organometallic complexes under mild conditions oxidative addition/reductive elimination [L n M q ] L n M q2 sigma bond metathesis (, ' =, sp 3, sp 2, sp ) homolytic cleavage by two metals 2 L L n M q M q n M q L n L n M q1 L n M q1 L n M q ' L n M q "1,2 addition/elimination" (X =, possibly O) ' "electrophilic displacement" of (X = halide, aquo, etc.) L n M q X L n M q X - L n M q X L n M q X Unfortunately, thus far most of the organometallic complexes that react with - bonds of alkanes are decomposed by O 2 and/or O with the exceptions of some of those complexes that react via electrophilic displacement of
Similarities and differences among various - activation mechanisms: in some cases there are rather subtle differences that are often difficult to distinguish sigma bond metathesis (, ' =, sp 3, sp 2, sp ), simplest in some ways L n M q ' L n M q oxidative addition/reductive elimination vs sigma bond metathesis.. L n M q ' L n M q2 ' '.. L n M q ' via via L n M q.. L n M q ' 4 c, 4 e - ' "4 c", 6 e - ; better: two 3 c, 4 e - "electrophilic displacement" of (X = halide, aquo, etc.) vs oxidative addition (or even sbm!) L n M q X.. L n M q : X via X.. L n M q 4 c, 6 e - ote: all mechanisms have - sigma interaction with metal center in TS