Chapter 2 The Elementary Steps in TM Catalysis

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

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

3 igand Substitution vancant site needed intermediate of increased coord. number ASSIATIVE (S N 2 IKE) M n + ' 'M n 'M n-1 + usually coordinatively satured intermediate of decreased coord. number DISSIATIVE (S N 1 IKE) M M n ' n + ' 'M n-1 + INTEANGE no evidence for an intermediate Similarities with organic nucleophilic displacement reactions Associative S N 2-like processes are the most popular : ligand exchanges of coordinatively unsaturated 16e - d 8 square planar complexes: [Ni (II), Pd (II), Pt (II), h (I), Ir (I)] Substitution reactions proceeding via single electron transfer also exist (S N 2, S N 1)

4 2e - Associative Processes t Y - Pd Pd t via apical attack t 16 e - 18 e - Y trigonal bipyramidal Pd 16 e - Y 2 nd order rate depending on : Incoming ligand Y : 3 P > Py > N 3, l - > 2 > - eaving ligand : N 3 - > 2 > l - > Br - > I - > N 3 - > SN - > N 2 - > N - Trans ligand* : 3 Si - > - ~ 3 - ~ N - ~ olefins ~ > P 3 ~ N 2 - ~ I - ~ > Br - > l - > N 2 ~ N 3 > - > N 3 - ~ 2 * igand trans with respect to the one beeing displaced Frankcombe, K. E.; avell, K. J.; Yates, B. F.; Knott,. B. rganometallics, 1997, 16, 3199.

5 2e - Dissociative Processes 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() 4 tetrahedral labile slow fast Ni() 3 + v = k [Ni() 4 ] = 1 st order rate Ni() 3 Ni K D benzene, 25 Ni + igand P(Et) 3 P(-p-Tol) 3 P(-iPr) 3 P(-o-Tol) 3 P 3 one 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.

6 Some Examples of igand Substitution

7 Anion apture 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 σ-alkylpalladium complexes can be alkoxycarbonylated, with concomitant regeneration of Pd(0), by treatment with carbon monoxide in the presence of an alcohol (usually methanol) or amines. npd npd npd Pdn NEt 3 / ' ' ' Pd -NEt 3 ' Et 3 N NEt 3 Pd Et 3 N Pd Stille, J. K. in omprehensive rganic Synthesis, Trost, B. M.; Fleming, I. Eds.; Pergamon Press, 1991, vol. 4, p opéret,.; Sugihara, T.; Negishi, E. Tetrahedron ett., 1995, 36, 1771.

8 igand Exchange via Slippage Mn Mn Mn Mn 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 η 3 - allyl (2 sites) η 1 - allyl (1 site) η 5 - p (3 sites) η 3 - p (2 sites) η 1 - p (1 site) η 6 -arene (3 sites) η 4 -arene (2 sites) η 2 -arene (1 site)

9 Nucleophilic Attack on oordinated igands Alkenes: direct attack at the ligand, anti to the metal, usually at the most substituted carbon atom [Pd]Y insertion syn [Pd]Y Nu anti [Pd] Nu + Y Nu: l -, 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 [Pd] Nu anti addition Nu [Pd(0)] decoordination Nu + [Pd(0)] η3-allyl complex η 2 complex

10 hemically Promoted Substitution Ni tetrahedral Fe trigonal bipyramidal r octahedral easy dissociation these complexes undergo only promoted ligand substitution Amine N-oxide promoted ligand dissociation N 3 N 3 2 fast N 3 osphine-oxide promoted ligand dissociation P 3 P 3 fast strong bond 3 P

11 xidative Addition

12 xidative Addition Pd(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), Pd(0) / Pd(II) (d 8 /d 6 ): o(i) / o(iii), h(i) / h(iii), Ir(I) / Ir(III)

13 omogeneous Pd(0): The Seminal Paper J. hem. Soc. 1957

14 xidative Addition: The Seminal Paper

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

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

17 Non-polar Substrates terminal alkynes [Pd(0)] [Pd(0)] [Pd(II)] [Pd] active methylenes 2 Et [Pd(0)] 2 Et N [Pd] N alcohols - [Pd(0)] --[Pd]

18 Polar Substrates alides: If the halide carries a β-, dehydropalladation (β - 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 σ-alkyl Pd complexes have no β-. The better the leaving group, the faster the oxidative addition: N 2+ >> Tf > I > Br > Ts > l Jutand, A.; Négri, S. rganometallics, 2003, 22, 4229 vinyl halides: clean retention mechanism is observed Pd Pd 3 Br Pd 3 Br 2 Pd Br Pd 2 Br aryl halides: the reaction reminds a nucleophilic aromatic substitution [Pd] [Pd] [Pd(0)] +

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

20 Polar Substrates propargylic systems ( = leaving group) [Pd(0)] [Pd(0)] [Pd(II)] [Pd] allylic systems ( = leaving group) [Pd(0)] [Pd(0)] [Pd]

21 From Pd(II) to Pd(IV) Alkyl halides may also oxidatively add to -Pd(II)- complexes to give a usually unstable Pd(IV) complex. reductive elimination [Pd] (II) - [Pd] (IV) reductive elimination [Pd] (II) + - Pd N N I Pd I N N anty, A.J., Acc. hem. es. 1992, 25, 83-90

22 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

23 eductive Elimination Anything that reduces the electron density at the metal facilitates the reductive elimination. thods to reduce electron density are : - oxidation of the metal - addition of strong π-acceptor ligands such as : N N N N The fragments to be eliminated must occupy cis position on the metal or must rearrange from trans to cis prior to the reductive elimination P Pd P fast + P Pd P no reaction! P Pd P

24 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 Pd complex such as oxidative additions or nucleophilic attack on alkenes, efficient - bond forming reactions ensue: cross-coupling (Zn: Negishi-Baba, B: Suzuki-Miyaura, Sn: Migita-Kosugi-Stille, Si: iyama, ) M '[Pd] [Pd]' + M + red. elim. Zn, B, Sn, Si... -' The main group organometallic must be more electropositive than Pd. The nature of is also important.

25 Migratory Insertion / Dehydrometalation agostic () () () migratory insertion > (carbo-, hydro-metalation) β-elimination < (decarbo-, dehydro-metalation) () 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. onversely, dehydrometalation (β- elimination) requires a vacant cis site on the metal. When an alkyl group migrates, its stereochemistry is maintained.

26 Migratory Insertion: Some Examples [Pd] [Pd] [Pd] alkene or alkyne insertion into Pd = : hydropalladation; = : carbopalladation [Pd] alkene insertion into π-allyl-pd [Pd] [Pd] [Pd] ' [Pd] ' allene insertion into π-allyl-pd ( =, ) allene insertion into Pd (' =, ) [Pd] [Pd] [Pd] [Pd] insertion (carbonylation) into Pd ( =, ) alkene insertion into acyl-pd