Recapping where we are so far

Transcription

1 Recapping where we are so far Valence bond constructions, valence, valence electron counting, formal charges, etc Equivalent neutral classification and MLX plots Basic concepts for mechanism and kinetics General considerations for catalysis Homogeneous/heterogeneous First order kinetics Second order kinetics Associative ligand substitution reactions Dissociative ligand substitution reactions Trans Influence and Trans Effect Transition State Theory Arrhenius eqn and activation energy Eyring eqn and entropies and enthalpies of activation

2 E a RT k = Ae lnk = ln A E a RT H E a1 HO H C H Br E a2 OH - + CH 3 Br E Br - + CH 3 OH

3 A K A* k* products A* k 1 Eyring eqn: ln( k T ) = ΔH A RT + ln(k B h )+ ΔS R products ( S /R) x x ln( k T ) x x x x slope = -( H /R) 1 T

4 Next we survey the elementary rxn types of organometallic chemistry

5 Types of organometallic reaction transformations Crabtree Chapter 6 radical coupling L n M R + R' R R' + L n M -1-1

6 Oxidative additions and reductive eliminations LM q + X-Y LM q+2 Concerted mechanisms for oxidative addition are very common and are very useful in cross-coupling catalysis. A pi-complex typically precedes the oxidative addition itself. X Y here a concerted step is emphasized (no charge build-up in TS)

7 Sonogashira cross-coupling. Couples a terminal alkyne with an aryl/vinyl halide using Pd 0 nucleophile electrophile the oxidative addn is typically presumed to be rate-limiting based on specific model studies. for concerted RE, a cis arrangement is required. order of reavity depends on C-X bond strength

8 The addition of dihydrogen (H 2 ) is an important step in catalytic hydrogenation reactions. Readily viewed as a concerted addition step. Oxidative addition of dihydrogen to Vaska s complex. Note the cis arrangement of the hydride ligands.??

9 The addition of dihydrogen (H 2 ) is an important step in catalytic hydrogenation reactions. Readily viewed as a concerted addition step. Oxidative addition of dihydrogen to Vaska s complex. Note the cis arrangement of the hydride ligands.???

10 The addition of dihydrogen (H 2 ) is an important step in catalytic hydrogenation reactions. Readily views as a concerted addition step. L = PPh 3 Oxidative addition of dihydrogen to Vaska s complex. Note the cis arrangement of the hydride ligands. The transition state for folding back is trigonal bipyramidal. The π-acidic ligands prefer the equatorial sites of the TBP geometry, which are rich in electrons capable of π bonding (you can convince yourself of this). As a consequence, π-acidic ligands get folded back preferentially, and tend to end up cis to their trans partners in the starting complex, as shown. Steric considerations also contribute (and likely dominate) to this result since bulky L (PPh 3 cone angle 145 ) ligands would less readily fold to the equatorial positions.

11 Oxidative addition need not be concerted, and the relative polarity of the solvent can impact the rate, (and possibly) mechanism, (and possibly) stereochemistry of a rxn. For MeI at RT H = 6 kcal/mol S = -50 cal K -1 mol -1 Thermodynamics are always relevant to consider. Vaska s complex doesn t react with CH 4 even though based on mechanistic concepts it could: kcal/mol C-H +104 Ir-H -60 H-CH 3 + Ir(PPh 3 ) 2 (CO)Cl Ir(CH 3 )(H)(PPh 3 ) 2 (CO)Cl Ir-CH 3-35 entropy +9 G 18 too uphill!

12 Where the precursor is 18-electron only one of the groups from the X-Y substate will be bound to the product, unless subsequent ligand substitution occurs as shown.

13 In very general terms, the more electron-rich a metal center (by virtue of strong sigmadonor ligands and lack of pi-acidic ligands, and perhaps a more negative charge) the more likely it is to undergo forward oxidative addition.

14 In very general terms, the more electron-rich a metal center (by virtue of strong sigmadonor ligands and lack of pi-acidic ligands, and perhaps a more negative charge) the more likely it is to undergo forward oxidative addition. Also, if the mechanism is concerted inner-sphere (as opposed to radical electron-transfer (ET)), an available orbital at the metal for initial substrate binding is needed. Potentially oxidizing (or reducing) substrates might instead proceed via initial ET rather than a concerted process, and hence don t require an available orbital.

15 In very general terms, the more electron-rich a metal center (by virtue of strong sigmadonor ligands and lack of pi-acidic ligands, and perhaps a more negative charge) the more likely it is to undergo forward oxidative addition. Also, if the mechanism is concerted inner-sphere (as opposed to radical electron-transfer (ET)), an available orbital at the metal for initial substrate binding is needed. Potentially oxidizing (or reducing) substrates might instead proceed via initial ET rather than a concerted process, and hence don t require an available orbital. iodide is least electronegative and hence makes Ir most electron-rich in this series (fastest OA rate) steric factors likely impact rxn rate across this halide series Rh is slower than Ir due to weakened orbital overlap / covalency

16 Cl Ir I PPh 3 Ph 3 P CO + O 2 25 C Ph 3 P O Cl PPh 3 Ir III CO O peroxide: O 2 2- ΔΗ = 13 kcal/mol ΔS = 21 e.u.

17 Another class of oxidative additions: oxidative couplings Mechanistically important in ethylene trimerization to hexene. Philips Petroleum Company.

18 Si H bonds undergo oxidative addition to electron-rich metal complexes. Electron-poor complexes may stop at the σ complex stage.

19 Termolecular oxidative additions of H 2, in which the two H atoms find their way to two different metal centers, are also known but suffer from entropic barriers. (TMP)Rh R Rh C H Rh N N Rh N N R R Rh H H Rh Metalloradical reactions of hydrogen (and hydrocarbons) occur through a termolecular transition state that contains two metalloradicals and the substrate. (M H H M; rate f = k[m ] 2 [S]).

20 This rxn is remarkable because the radical can be dissolved in benzene between K and it proceeds forward under 1atm CH 4. Equilibrium can be approached from both directions, and NMR resonances enable measurement of K (k 1 /k -1 ). Linearity of kinetic plots of 1/[Rh] vs time for more than 3 half-lives establish 2 nd order behavior in [Rh]. K f (353 K) = 7300 K f (373 K) = 3300 K f (393K) = [A] = 1 + kt [A] 0 Van t Hoff Plot

21 This rxn is remarkable because the radical can be dissolved in benzene between K and it proceeds forward under 1atm CH 4. Plotting the rate of approach to equilibrium at variable temperatures and 1 atm CH 4 establishes 1 st order in CH 4. Note data curves as equilibrium is approached. H = 7.1 ± 1.0 kcal/mol S = -39 ± 5.0 cal K -1 mol -1 K f (353 K) = 7300 K f (373 K) = 3300 K f (393K) = 1100

22 Tethering the Rh(II) radicals in a cofacial arrangement can facilitate the rxn kinetics. Cui and Wayland J. Am. Chem. Soc., 2004, 126 (26), pp

23 Remember, the term oxidative addition does not imply mechanistic information. Rather, it refers to an overall rxn step. 1-electron transfer steps are probably more common than is often assumed. J. Halpern, Accts Chem Res, 3, 386, 1970 Co II (CN) RX X-Co III (CN) R Co II (CN) R R-Co III (CN) Co II (CN) RX R-Co III (CN) X-Co III (CN) 5 3- k obs (M -1 s -1 ) RX 0.01 CH 3 I 0.06 CH 3 CH 2 I 1.2 (CH 3 ) 2 CHI 9.2 (CH 3 ) 3 CI consistent with radical processes

24 Compare these data to RX oxidative addition to another Co-complex. N O N H O Co O H N N O RX S N 2 N O N R H O Co O H N O N + X - L "Co(dmg)" L RX k obs (M -1 s -1 ) MeBr 2200 EtBr 1.6 iprbr 0.1 sterics dominate S N 2

25 Reductive eliminations truly 3-coordinate intermediate is unlikely

26 Reductive eliminations: Platinum model studies. But an alternative (and highly likely!) possibility to consider is that I - dissociates from the Pt center and externally attacks at Pt-CH 3 in S N 2 type reaction, rather than a concerted elimination process. How can this be tested? credit: G. Stanley

27 Ph 2 CH Ph 3 2 P CH 3 Pt P CH 3 Ph 2 I CH Ph 3 2 P CH 2 Pt P CH 3 Ph 2 I CH Ph 3 2 P CH 3 Pt P C Ph 2 H 3 I P CH 3 Pt P CH 3 Ph 2 H 3 C I It remains difficult to predict the detailed mechanistic pathways of these reactions. Specifically, it is often unknown whether a particular reaction will proceed directly or will require ancillary ligand dissociation or even association prior to bond cleavage/formation. We look to model studies.

28 Pt IV -C bond strengths of ~ kcal/mol Elimination requires high T ( ) Radical needs to be considered! Paths A & B are kinetically indistinguishable and should yield first order rate laws

29 Clean 1 st order kinetics in: benzene-d 6 (k = 4.2 x 10-6 s -1 ) THF-d 8 (k = 3.2 x 10-6 s -1 ) acetone-d 6 (k = 4.9 x 10-6 s -1 ) Eyring Plot from rates in C 6 H 6 H = 43 ± 4.9 kcal/mol S = 15 ± 4.0 cal K -1 mol -1 not much solvent dependence Cross-over experiment: (dppe)pt(ch 3 ) 4 + (dppe)pt(cd 3 ) 4 ~ exclusively CH 3 -CH 3 + CD 3 -CD 3 should exclude free CH 3 and bimolecular reductive elimination

30 Pt IV -C bond strengths of ~ kcal/mol Elimination requires high T ( ) Radical needs to be considered! Paths A & B are kinetically indistinguishable and should yield first order rate laws

31 bite angle 86 1 J P-Pt 1145 Hz very similar bite angle 84 1 J P-Pt 1147 Hz but considerably more rigid backbone only 4% after 300 h at 165 C in THF The ca. 100-fold difference between the rates of ethane reductive elimination from dppe and dppbz (dppe is faster) correlates with phosphine chelate flexibility and provides strong support for a mechanism involving phosphine dissociation prior to C-C coupling for (dppe)ptme 4.

32 Evidence has also been obtained to show that mechanism C (earlier slide: dechelation/dissociation of a phosphine arm) is kinetically viable. That is, phosphine chelate opening does occur, and it does so on a time scale that would allow it to precede the C-C coupling reaction. The observation of beta-hydride elimination products upon thermolysis of the closely related tetraalkyl complex mer-(dppe)ptme 3 Et at 165 C establishes that the phosphine dissociation to generate an open site takes place readily at this temperature. Beta-hydride elimination requires an open coordination site cis to the ligand with beta-hydrogens. Stay tuned for more on beta-hydride insertions/eliminations!

33 Similar ideas are observed for systems other than Pt: Models w/rh. Figure clipped directly from Crabtree (6 th Ed) Chapter 6

34 Other examples: Binuclear oxidative additions/eliminations.

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37 What is the stereochemistry at carbon for oxidative additions? The answer will of course depend on the mechanism. Classic example from G. Whitesides: JACS, 96, 2814, Me 3 CHDCHDX + Fp - Cp(CO) 2 Fe(CHDCHDCMe 3 ) + X - CMe 3 Fp - CMe 3 SO 3 - D D X H H - X - D H Fp D H X = threo (small 3 J HH ) > 95% erythro (large 3 J HH ) Br Note: inversion of stereochemistry suggests a TS akin to an S N 2 reaction.

38 One can also attempt a radical clock experiment: Br -THF > 97% -Br- Fp concerted? Fp - + I -THF -I - Fp + not concerted? Fp 70% 30% 10 8 s C Fp - + I ET I - I s -1 Fp Fp Note: The possibility of in-cage rather than out-of-cage radical recombination can complicate this type of analysis. Fp

39 Note: The possibility of in-cage rather than out-of-cage radical recombination can complicate this type of analysis. If one only sees the non ring-opened product the analysis is inconclusive: olefin insertion by a concerted mechanism can get you to the same product! Fp Fp Fp So, a rapid in-cage or out-of-cage radical recombination, followed by intramolecular olefin insertion, can lead to the same result as concerted oxidative addition. Be careful. These clocks are widely used in the literature as evidence against radical steps!