The Elementary Steps in TM Catalysis

Transcription

1 The Elementary Steps in TM Catalysis + + ligand exchange A oxidative addition > n + A B n+2 reductive elimination < B n n+2 oxidative coupling X + M' + M' X transmetallation migratory insertion > (carbo-, hydro-metalation) -elimination < (decarbo-, dehydro-metalation)

2 The Elementary Steps in TM Catalysis C (C) insertion Nu Nu nucleophilic addition at coordinated ligand E E electrophilic addition at coordinated ligand

3 Associative igand Substitution It usually takes place on square planar d 8 16e - complexes. N 3 equatorial (incoming) equatorial ("trans ligand") t square planar 2-2K + N 3 2-2K + t t N 2 2-2K + trigonal bipyramid 16e - 18e - 18e - equatorial (leaving) t N 3 square planar K 16e - - K + 2-2K + t N 3 18e -

4 The Vibration of entacordinated Complexes The Berry pseudorotation is a type of vibration causing molecules to isomerize by exchanging the two axial ligands.for examlpe, it occurs in trigonal bipyramidal molecules, such as Fe(C) 5, due to rapid interchange of the C ligands. apical apical 5 C 4 C 2 C Fe C 3 1 C C 5 C Fe C 4 2 C 3 1 C apical C C 3 2 C 4 5 C Fe 1 C apical trigonal bipyramid square pyramid trigonal bipyramid

5 Trans Influence and Trans Effect Trans Influence: the extent to which a ligand weakens the bond trans to itself. The stronger the σ-donor, the stronger its trans influence. This is a thermodynamic consequence. Trans effect : the power of a ligand to activate the substitution of the ligand located trans to itself. This is a kinetic effect Trans effect is favored by strong σ-donor ligands ( 3 Si,, Alk ) (which feature trans influence, too) as well as by strong π-acceptor ligands (olefins, C) (which do not show trans influence).

6 The Trans Effect et us consider an associative ligand substitution on a d 8 square planar complex. Association of the incoming ligand generates a five-coordinated intermediate with the original trans ligand t in the equatorial plane. The incoming ligand (Y) will approach the metal occupying an equatorial position of the five-coordinated intermediate, and, due to the principle of microscopic reversibility, the departing ligand (X) will also leave from the remaining equatorial position. Both strong σ-donor and strong π-acceptor ligands favor formation of the trigonal bipyramidal intermediate, though, for different reasons. Indeed, the former ligands destabilize the initial square planar complex weakening the M-X bond, whereas the latter ones stabilize the trigonal bipyramidal intermediate (hence its transition state, too; see ammond s postulate). The net result is departure of the ligand trans to t. The trans effect has important implications in the mechanism of several catalytic processes such as hydrogenations or asymmetric d-catalyzed allylations.

7 The Trans Effect et us consider an associative ligand substitution on a d 8 square planar complex: t -acceptor effect t -donor effect TB S1 S2 2 nd order rate depending on : Incoming ligand Y : 3 > y > N 3, - > 2 > - eaving ligand X : N - 3 > 2 > - > Br - > I - > N - 3 > SCN - > N - 2 > CN - Trans ligand* : 3 Si- > - ~ C - 3 ~ CN- ~ olefins ~ C > 3 ~ N - 2 ~ I - ~ > Br - - > N 2 ~ N 3 > - > N - 3 ~ 2 * igand trans with respect to the one being displaced Frankcombe, K. E.; Cavell, K. J.; Yates, B. F.; Knott,. B. rganometallics, 1997, 16, 3199.

8 The Trans Effect Some well-know examples of the trans effect t 2 2K eyrone, M. Ann. 1845, 51, 1 N 3 - K more labile bond t N 3 1 K N 3 has a stronger trans effect than N 3 N 3 t N 3 - K cis platin (an anticancer agent) 3 N 3 N N 3 t N 3 2 2X - N 4 X Jorgensen, S. M. J. rakt. Chem. 1886, 33, 489 more labile bond 3 N 3 N t N 3 1 X has a stronger trans effect than N 3 - N 4 X 3 N t trans N 3 Quaglaino, J., Shubert,. Chem. ev. 1952, 50, 201 more labile bond t 1 M N 3 ethylene has a stronger trans eff ect than 3 N - K t trans

9 About cis-platin After administration of cis-platin, a chloride ligand is displaced by water, then by a guanine base of DNA to give [t(guanine-dna)(n 3 ) 2 ] +. Crosslinking then occurs via displacement of the other chloride ligand, usually, by another guanine of DNA. This event interferes with cell division by mitosis. The damaged DNA first elicits DNA repair mechanisms, then, activates apoptosis when repair proves impossible. lmage taken from: eft: the structure of cisplatin coordinated to a dinucleotide containing two guanines. Notice the destacking of guanine bases, which would normally be parallel to one another. ight: the structure of cisplatin coordinated to two guanines in a DNA duplex. il,., ippard, S. J. In Encyclopedia of Cancer, J.. Bertino, Ed. Academic ress: San Diego, CA, 1997, Vol. 1, pp

10 Dissociative igand Substitutions The rate of the ligand exchange can be accelerated by bulky ligands, since loss of one ligand leads to release of steric strain M 0, d 10 Ni(C) 4 tetrahedral labile slow fast Ni(C) 3 + C v = k [Ni(C) 4 ] = 1 st order rate Ni(C) 3 Ni K D benzene, 25 C Ni + igand (Et) 3 (-p-tol) 3 (-ir) 3 (-o-tol) 3 3 Cone Angle K D < x x x 10-2 No Ni 4 The ability to control the bulk of the ligand permits to tune the reactivity of the metal complex. For example, if the dissociation of the phosphine ligand is the first step in a reaction, the reaction can be accelerated by utilizing a larger phosphine ligand. ikewise, if dissociation is a problem, then a smaller phosphine can be used.

11 Dissociative igand Substitutions Mn N 3 Tol, rt N Mn 3 N Mn Mn N

12 Some Examples of igand Substitution

13 Transmetalation Transfer of an group from a main group organometallic compound to a TM complex. When combined with reactions which introduce an group into the d complex such as oxidative additions or nucleophilic attack on alkenes, efficient C-C bond forming reactions ensue: cross-coupling (Zn: Negishi-Baba, B: Suzuki-Miyaura, Sn: Migita-Kosugi-Stille, Si: iyama, ) M '[d]x [d]' + MX + red. elim. Zn, B, Sn, Si... -' The main group organometallic must be more electropositive than d. The nature of X is also important.

14 Anion Capture The nucleophilic addition at palladium is usually followed by the reductive elimination, and the combination of these two elementary steps is known as the anion capture. The transiently generated -alkylpalladium complexes can be alkoxycarbonylated, with concomitant regeneration of d(0), by treatment with carbon monoxide in the presence of an alcohol (usually methanol) or amines. Stille, J. K. in Comprehensive rganic Synthesis, Trost, B. M.; Fleming, I. Eds.; ergamon ress, 1991, vol. 4, p Copéret, C.; Sugihara, T.; Negishi, E. Tetrahedron ett., 1995, 36, 1771.

15 igand Exchange via Slippage C C Mn C C C Mn C C C Mn C C Mn C C Mn I ; d 6 ; 18 e - saturated Mn I ; d 6 ; 16 e - unsaturated Mn I ; d 6 ; 18 e - saturated (isolated) Mn I ; d 6 ; 18 e - saturated 3 - allyl (2 sites) 1 - allyl (1 site) 5 - Cp (3 sites) 3 - Cp (2 sites) 1 - Cp (1 site) 6 -arene (3 sites) 4 -arene (2 sites) 2 -arene (1 site)

16 Nucleophilic Attack on Coordinated igands Alkenes: direct attack at the ligand, anti to the metal, usually at the most substituted carbon atom X [d]y insertion syn X [d]y Nu anti X[d] Nu + Y Nu: -, Ac -,, N 2, i Allyles: stabilized nucleophiles attack the ligand, anti to the metal. egioselectivity depends on the nature of the nucleophile and of the ancillary ligands [d]x Nu anti addition Nu [d(0)] decoordination Nu + [d(0)] 3-allyl complex complex

17 Chemically romoted Substitution C Ni C C C tetrahedral C C Fe C C C trigonal bipyramidal C C C Cr C C C octahedral easy dissociation these complexes undergo only promoted ligand substitution Amine N-oxide promoted ligand dissociation C N 3 N 3 C 2 fast C N 3 osphine-oxide promoted ligand dissociation C C 3 3 fast strong bond 3

18 xidative Addition

19 xidative Addition d(0) is electron rich and has a nucleophilic character. This transformation often represents the first step of a catalytic cycle and its mechanism depends on the nature of the metal as well as on the substrate involved. An increase of 2 in the formal oxidation state of the metal and of the coordination number is observed. A n + A B n+2 B The most commonly encountered systems: (d 10 / d 8 ): Ni(0) / Ni(II), d(0) / d(ii) (d 8 / d 6 ): Co(I) / Co(III), h(i) / h(iii), Ir(I) / Ir(III)

20 omogeneous d(0): The Seminal aper J. Chem. Soc. 1957

21 xidative Addition: The Seminal aper

22 Non-polar Substrates xidative addition of 2 takes place via a 3-center mechanism (central to catalytic hydrogenation and related to C- bond activation). 2 (C) "agostic" 2 e -, 3-center bond cis- insertion (C) (C) The 3-center TS is involved and there is a synergic electron flow : roof of the mechanism : M -donation empty -donation C C W C roof of the structure : X 3 Ir 3 C 2 C W C C isolated 2 3 Ir X 3 C I: 2 M- stretching bands (symm and asymm). If trans, only asymm. is expected

23 By the Way: Agostic Agostic interactions : Ti C 3 Co From the Greek : to hold one to oneself. It refers to a C- bond on a ligand that undergoes an interaction with the metal complex.

24 xidative Addition of C or Bonds terminal alkynes [d(0)] [d(0)] [d(ii)] [d] active methylenes alcohols C 2 Et CN [d(0)] [d] C 2 Et CN - [d(0)] --[d]

25 xidative Addition of Vinyl and Aryl alides alides: If the halide carries a -, dehydropalladation ( - elimination) may be a facile, and normally undesired, process, thereby generating an alkene. Accordingly, the most widely used substrates are vinyl-, aryl or benzyl-halides, whose corresponding -alkyl d complexes have no -. The better the leaving group, the faster the oxidative addition: N 2 + >> Tf > I > Br > Ts > Jutand, A.; Négri, S. rganometallics, 2003, 22, 4229 vinyl halides: clean retention mechanism is observed d d 3 Br d 3 Br 2 d Br d 2 Br aryl halides: via an η 2 -arene pre-reaction complex d X d X X d d X Senn,. M.; Ziegler, T. rganometallics 2004, 23, 2980

26 ates of xidative Addition of Aryl alides -d- -d- associative slow Ar-I fast -d-(ari) - dissociative slow Ar-Br -d - fast Ar Ar I d Br d Tol Tol tbu tbu tbu tbu d Fe Tol Tol Tol Tol Tol (Qos Tol) 2 d Fe Tol Tol Tol -d- fast - dissociative -d slow Ar- Ar d 1/2 Ar d d Ar xidative addition of chloro-, bromo-, and iodoarenes to sterically hindered (Qostol) 2 d occurs through three different mechanisms. Addition of I occurs by associative displacement of a phosphine. Addition of Br occurs by rate-limiting dissociation of phosphine. Addition of occurs by reversible dissociation of phosphine, followed by rate-limiting oxidative addition. Barrios-anderos, F.; artwig, J. F. J. Am; Chem. Soc. 2005, 127, 6944 Barrios-anderos, F.; Carrow, B..; artwig, J. F.. J. Am. Chem. Soc. 2009, 131, 8141

27 Special osphines for Suzuki and ther Couplings t-bu Bu-t Bu-t Bu-n Fe Cy 2 tbu 2 Fe tbu 2 Bu-t 3 (Fu) CataCXium A (Beller) Josiphos (artwig) Qos (artwig) tbu 2 Cy 2 i-r Cy 2 i-r i-r Cy 2 i-r 2 N Cy 2 i-r Cy 2 r-i i-r i-r Johnos (Buchwald) Sos (Buchwald) Xos (Buchwald) Brettphos (Buchwald) Daveos (Buchwald) uos (Buchwald) The first hint: significant catalytic activity is found only with phosphines which are both strongly basic (pka 4 6.5) and with well defined steric volume, i.e. the cone angle theta must exceed ca M. user, M.-T. Youinou and J. A. sborn, Angew. Chem., Int. Ed. Engl., 1989, 28, 1386.

28 Some eferences About These Special igands Fleckenstein, C. A. lenio. Chem. Soc. ev., 2010, 39, 694 Martin,.; Buchwald, S.. Acc. Chem. es. 2008, 41, 1461 Zapf A., Beller, M. Chem. Commun., 2005, 431 Zapf, A.; Ehrentraut, A.; Beller, M. Angew. Chem., Int. Ed. 2000, 39, 4153 artwig, J.F., Acc. Chem. es. 2008, 41, 534. Fu, G. C. Acc. Chem. es. 2008, 41, 1555 Jana,.; athak, T..; Sigman, M. S. Chem. ev , Wasa, M.; Engle, K. M.; Yu, J. Q. Isr. J. Chem. 2010, 50, These specially designed phospines have been mainly developed to run difficult crosscoupling reactions (Suzuki coupling, Buchwald-artwig aromatic amination, etc.), see later. As we will see, they not only favor oxidative addition step, but may also facilitate other steps of the catalytic cycle.

29 Features of Buchwald Dialkyl ortho-biarylphosphines General synthesis air stable 2 fixes conformation of 2 over the bottom ring enhancing rate of reductive elimination electron-rich alkyl substituents increase oxidative addition rate large substituents enhance the rate of reductive elimination and large substituents favor monoligand complexation [ 1 d(0)] 1 prevent cyclometalation. If bulky favor monoligand complexation [ 1 d(0)] The o-substituted aromatic ring slows rate of oxidation by oxygen and stabilizes d via d-ar -interaction Adapted from: Martin,.; Buchwald, S.. Acc. Chem. es. 2008, 41, 1461

30 chanistic ypothesis for xid. Add. of Sphos-d Cy Cy Cy Cy d Cy Cy Cy Cy d Cy Cy d + Cy Cy Cy Cy d Cy Cy d Cy Cy d non reactive resting state of the catalyst? highly reactive monoligated d(0) Barder, T. E.; Walker, S. D.; Martinelli, J..; Buchwald, S.. J. Am. Chem. Soc. 2005, 127, 4685.

31 X-ay Structure Sphos-dba and xid. Add. Computations X-ay crystal structure of dba-sos Barder, T. E.; Walker, S. D.; Martinelli, J..; Buchwald, S.. J. Am. Chem. Soc. 2005, 127, Cy Ar Cy d E rel 0.0 kca/mol -d coordination Cy Cy d Ar E rel 0.8 kca/mol 1 -aryl-d coordination DFT Barder, T. E.; Biscoe, M..; Buchwald, S.. rganometallics 2007, 26, 2183.

32 Alkyl and Benzyl alides owever, in the presence of suitable electron rich bulky phosphines, or heterocyclic carbenes, oxidative addition to d(0) becomes so facile that alkyl halides having -hydrogens can afford the corresponding -alkyl complexes under very mild conditions and without subsequent dehydropalladation. Br (t-bu) [d(0)], (t-bu) 2 2 Et 2, 0 C (94%) d Br (t-bu) 2 Kirchhoff, J..; Netherton, M..; ills, I. D.; Fu, G. C. J. Am. Chem. Soc., 2002, 124, Alkyl halides oxidatively add to d(0) via a SN2 like process. Inversion mechanism is observed but sometimes racemisation via double inversion takes place d X inversion X[d] d [d]x Stille, J. K.; atai, S. Ed. The chemistry of the metal-carbon bond, vol. 2, Wiley, 1985, chap 9. Netherton, M..; Fu, G. C. Angew. Chem. Int. Ed., 2002, 41, 3910

33 Allyl and ropargyl Systems propargylic systems (X = leaving group) X [d(0)] [d(0)] X [d(ii)] X X[d] C allylic systems (X = leaving group) X [d(0)] [d(0)] [d]x X

34 From d(ii) to d(iv) Alkyl halides may also oxidatively add to -d(ii)- complexes to give a usually unstable d(iv) complex. if reductive elimination is not possible [d] (II) C C C-X C [d] X (IV) C C reductive elimination [d] (II) C X + C-C d N N I d I N N Canty, A.J., Acc. Chem. es. 1992, 25, 83-90

35 eductive Elimination The reverse of oxidation addition eductive elimination produces coordinatively unsaturated metal centers A [M n+2 ] B [M n ] + A B Thermodynamics may be different : fast + - usually reversible fast + - usually irreversible slow + - usually irreversible D M- = ~145 kj/mol, D - = ~ 420 kj/mol, D M- = ~230 kj/mol, D - = ~435 kj/mol, D - = ~375 kj/mol

36 eductive Elimination eductive elimination is favored by methods that reduce electron density on the metal oxidation of the metal NC CN C addition of strong -acceptor ligands* such as: NC CN -Bulky monodentate ligands (Buchwald) F 3 C eductive elimination is inhibited by excess phosphine The fragments to be eliminated must occupy cis position on the metal or must rearrange from trans to cis prior to the reductive elimination. This is why chelating phosphines with large bite angles are best to favor E. d fast + d no reaction! d (*) Knochel,. et al. Angew. Chem. Int. 1998, 32, 2387

37 eductive Elimination d EDG EWG EDG d EDG d EWG EWG ease of reductive elimination d Ar CF 3 very slow! Ar-CF 3 + -d(0) from Brettos Watson, D.A, Buchwald, S.. et al. Science, 2010, 328, 1679

38 eductive Elimination bite angle slowest ~85 ~90 fastest ~100 J. E. Marcone, K. G. Moloy, J. Am. Chem. Soc. 1998, 120, 8527

39 Migratory Insertion / Dehydrometalation agostic () () () migratory insertion > (carbo-, hydro-metalation) -elimination < (decarbo-, dehydro-metalation) C C (C) insertion Usually a reversible process, no variation in the oxidation state Both the metal and the migrating group add to the same face of the olefin (syn addition) Migratory insertion generates a vacant site. Conversely, dehydrometalation ( - elimination) requires a vacant cis site on the metal. When an alkyl group migrates, its stereochemistry is maintained.

40 Migratory Insertion: Some Examples [d]x [d] X [d]x alkene or alkyne insertion into dx = : hydropalladation; = C: carbopalladation [d]x alkene insertion into -allyl-dx X C [d] [d]x X C [d] ' [d]x ' allene insertion into -allyl-dx ( = C, ) allene insertion into dx (' = C, ) C [d] X [d]x [d] X [d]x C insertion (carbonylation) into dx ( = C, ) alkene insertion into acyl-dx