Activation of Carbon-Hydrogen Bonds at Transition Metal Centers!

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1 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 -

2 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

3 - 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, Thompson, et al. Pure and Appl. hem. 1984, 56, 1. Thompson, et al. JAS 1987, 203 Marks, et al. JAS 1985, 8091

4 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 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 ( = higher alkyl, alkenyl) ( = 3, 6 5, )

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 PMe3 u Me3P

6 /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 ( 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)

7 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* p* 2 3 k 2 10 M -1 s-1 s sp 2 -s p* p* k 2 > 10 M -1 s-1 sp 3 sp 2 -s p* p* k 2 3 x 10-5 M -1 s-1 sp 3 sp 2 -s 60 p* = p* 2 = k 2 1 x 10-3 M -1 s-1 sp 3 sp-s p* p* k 2 > 10 M -1 s-1 sp 3 sp 3 -s 80 p* p* k 2 1 x 10-5 M -1 s-1

8 Faster rates of σ bond metathesis for sp 2 and sp hybridized - bonds might suggest electrophilic p* 2 - attacks at π bonds of substrate: 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

9 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 ) X k 80 cyclo- 6 D 12 (p*-d 15 ) X 4 F 3 3 Me x x 10-5 (k /k D = 2.9) 3.4 x x 10-5 early TS: lot s of - bond breaking p* hrs 36 hrs % 26% - 4 p* p* % 61% p* 2 p* 2 less than 5% isomerization 7.5 hr at 80-4 p* 2 6% 13% p* % 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

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11 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 ) X k 80 cyclo- 6 D 12 (p*-d 15 ) X 4 F 3 3 Me x x 10-5 (k /k D = 2.9) 3.4 x x 10-5 early TS: lot s of - bond breaking p* hrs 36 hrs % 26% - 4 p* p* % 61% p* 2 p* 2 less than 5% isomerization 7.5 hr at 80-4 p* 2 6% 13% p* % 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

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13 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

14 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

15 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

16 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, 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 kcal/mol p p p 3 3 p p 3 3

17 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

18 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

20 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 ) Zr IV Zr IV Wolczanski, et al. JAS 1988, 8731 Zr IV 3 ( = Si(Me 3 ) 3 ) - 4 Zr IV Zr IV 6 5 Zr IV

21 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, (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

22 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

23 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

25 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

26 (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 ) kcal/mol from reaction w/ 2

27 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

28 Tethered binuclear (porphyrin)h II expands substrate scope: W. ui and B. Wayland JAS 2004, 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) (TMP system) x x * x O 1.0 x *oxidative addition to - bond thermodynamically favored by about 10 kcal/mol, but this alternative not observed.

29 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 - > ~ and span about 5 orders of magnitude at 296 K

30 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 - > ~ and span about 5 orders of magnitude at 296 K

31 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

32 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