Oxidative Addition. Contents. Reporter : Shiliang Tian. Supervisor : Prof. Zhangjie Shi. Introduction to oxidative addition reactions ;

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1 Oxidative Addition eporter : Shiliang Tian Supervisor : Prof. Zhangjie Shi ontents ntroduction to oxidative addition reactions ; Mechanism of oxidative addition reactions ; Some examples in mechanism research ; Summary and outlook. Acknowledgement 2

2 ontents ntroduction to oxidative addition reactions ; Mechanism of oxidative addition reactions ; Some examples in mechanism research ; Summary and outlook. Acknowledgement 3 General information about oxidative addition The origin of modern organo transition metal chemistry can be traced to the discovery of ferrocene in 1951 Fe T. J. Kealy and P.. Pauson *, Nature, 1951, 168,1039 4

3 Vaska s complex PPh 3 Ph 3 P auri Vaska (+1) 16e-. Vaska*, J. Am. hcm. Soc., 1961, 83, PPh 3 Ph 3 P 3 PPh 3 Ph 3 P 3 3 O PPh 3 Ph 3 P 2 Sn 3 O PPh PPh 3 3 Sn 4 O PPh 2 3 Ph Ph 3 P 3 P Ph 3 P O O 2 O 2 3 SO 2 OO 2 PPh 3 PPh 3 SO 2 Ph 3 3 P PPh 3 Ph 3 P Ph 3 P. Voska*, J. Am. hcm. Soc., 1962, 84, 679 6

4 Most common cases General nformation n M A B oxidative addition reductive elimination n M A B 16e 18e O.S. = +2.N. = +2 These reaction are not limited to transition metals; perhaps the most familiar oxidative Addition is the formation of Grignard reagents, but it can also occur whenever an element as two accessible oxidative oxidation states two unit apart: Me Br Mg Me Mg Br P 3 P 5 7 Other types of oxidative addition reactions 1. Electrophilic attack of metals n M E + n M E Examples: O.S. = +2.N. = +1 Fe(O) 5 + [Fe(O) 5 ] + [Mn(O) 5 ] Mn(O) 5-8

5 2. Binuclear oxidation reactions n M M n A B n M A n M B n M M n A B A n M B M n 18e O.S. = +1.N. = +1 u-u = 2.82Å u O O u O O hν u O O u(+1) d 7 u(+2) d 6 O O u K. Peter. Vollhardt et al. J. Am. hem. Soc., 1983, 105(6), Oxidative oupling 3 N Ph 2 P P Ph 2 r 3 N Ph 2 P P Ph 2 r r(+3) d 3 15e- r(+5) d 1 13e- The r on the right now has two new anionic alkyl ligands forming a metallocyclopentane ring system. While this is an oxidative addition, there is a special term for this type of reaction called oxidative coupling. The driving force for this reaction is 1. formation of a new - s-bond (stronger than a p-bond) 2. creation of two new strongly donating anionic ligands that can better donate to the metal even though one has technically lowered the electron count. 10

6 Definition of Oxidative addition An oxidative addition reaction is one in which (usually) a neutral ligand adds to a metal center and in doing so oxidizes the metal, typically by 2eand new anionic ligands end up bonded to the metal center. The transferring of the two electrons from the metal to the incoming ligand breaks a bond in that ligand forming two new anionic ligands. At least one of these new anionic ligands ends up bonded to the metal center. Oxidizes the metal and coordinative number increases!!! 11 General Features of Oxidative Additions the more electron-rich the metal is the easier the oxidative addition to the metal center: strongest donor ligands fewest π-acceptor ligands most negative charge 12

7 Electronic effect Steric effect Metal center 13 The formal oxidative state change is always +2 What about the real charge on the metal changes??? The real charge on the metal changes much less than that because A and B do not end up with pure -1 charges in n M(A)(B). The change in real charge depends on the electronegativity of A and B. arbonyl Stretching Frequencies of the Oxidative Addition Products from Vaska's omplexs eagent v(o)(cm -1 ) v(o)(cm -1 ) None O D Me F

8 ontents ntroduction to oxidative addition reactions ; Mechanism of oxidative addition reactions ; Some examples in mechanism research ; Summary and outlook. Acknowledgement 15 Mechanism of oxidative addition reactions Oxidative addition Two-electron Mechanism Single-electron Mechanism S N 2 onic Three center is-addition adical mechanism hain non-chain 16

9 Two-Electron Mechanisms Oxidative Addition Three-enter is-addition (oncerted pathway ) n M A B n M A B n M A B 16e M(0) 18e M(0) 18e M() The characteristics of this reaction mode would be (a) etention of configuration (b) rate = k[ n M][AB] (c) Metal center must be coordinatively unsaturated with a dn configuration where n >2. (d) A and B must be cis 17 two new anionic hydride ligands PPh 3 Ph 3 P (+1) 16e- + 2 PPh 3 Ph 3 P (+1) 18e- PPh 3 Ph 3 P (+3) 18e- these molecules do NOT contain electro-negative atoms and/or are not good oxidizing agents. Aside from 2, they are often considered to be non-reactive substrates: 2, - bonds, Si- bonds, S- bonds,b- bonds, S-S bonds, - bonds, etc. 18

10 S N 2 Oxidative Mechanism # n M X n M X n M X - The haracteristics of this mechanism are (a) nversion of configuration of carbon (b) A cationic intermediate n M (c) rate = k[nm][ -X] (d) assical X structure/reactivity patterns Me>primary>secondary>tertiary and -OTs > >Br>>F (e) and X might be cis or trans after the recombination step 19 PPh 3 Ph 3 P δ + δ + 3 Br S N 2 nucleophillic attack Ph 3 P O PPh 3 Br (+1) 16e- (+1) 16e- (+3) 18etwo new anionic ligands two new anionic ligands 3 PPh 3 Ph 3 P Br +Br 3 PPh 3 Ph 3 P (+3) 16e- + Br f the starting metal complex is 16e (as shown above) both ligands will usually end up coordinated to the metal to make an 18e- complex. What about the starting metal complex is 18e??? 20

11 e O O e(-1) 18e- oxidative addition O e + e O -O O e(+1) 18e- 1) O ligand dissociation 2) η 1 - to η 3 -allyl hapticity change + two new anionic ligands e(+1) 18e- n the case of a starting 18e complex (shown below) only one of the two anionic ligands (usually the strongest binding) generated from the oxidative addition will end up coordinated to the metal unless a separate substitution reaction occurs. 21 Another example: Na 2 [Fe(O) 4 ] X Na[Fe(O) 4 ] NaX Fe(-2) 18e ate aw Fe(0) 18e rate=k[na 2 Fe(O) 4 ][X] Structure-eactivity elationship Steric factor: * *10-3 eaving group effect 1.4*10-5 < 1*10-5 > Br > OTs > k(relative) *

12 ow to determine stereochemistry at carbon? Ph Ph 3 Pd X 3 Pd + X - - O MeO Ph MeO O 2 XPd Ph O 2 XPd Ph The processes involving O and MeO are known not to change configuration at carbon J. K. Still et al, Acc. hem. es., 1977, 10, Anion Assisted S N 2 oxidative addition General: M (n) Y - Y M (n) - Y M (n) - X Y M X - O h O - h O Me K 1 h Me O Me K 2 K 2 > K 1 Maitlis *, hem. Soc. ommun., 1984,

13 onic Mechanism Some molecules (e.g., Br, ) dissociate into ions in polar solvents + - Therefore oxidative addition of these molecules in polar solvents must involve ionic species. n M n M X polar solvent n M X n XM 25 -PPh Pt(PPh 3 ) [Pt(PPh 3 ) 3 ] - - -PPh 3 [Pt(PPh 3 ) 2 ] 18e d 10 tetrahedral 16e d 8 square planar 16e d 8 square planar [(cod) 2 ] [(cod) 2 ] + [(cod) 2 ] 16e d 8 square planar 18e d 8 TBP 18e d 6 octahedral O 2, g 2 also react by the same mechanism W. J. uow et.al., J. hem. Soc., Dalton 1978, 340;.. rabtree et al., J. Organomet. hem. 1979,181,203,. 26

14 One-electron electron Mechanisms for Oxidative Addition 1. Atom Abstraction and combination of the esulting adical with a Second Metal rate determining step M (n) X X M (n+1) fast M (n) M (n+1) Net: 2M (n) X M (n+1) X M (n+1) This is not a chain-mechanism! haracteristics : (a) 2:1 stoichiometry (b) acemization of carbon (c) rate = k[m][x] (d) eaction sequence reverse of S N 2 3 > 2 > 1 >Me (e) eaction sequence with respect to X - > -Br >- >> -OTs Thus -OTs reacts fast in S N 2,slow in adical mechanism o(n) 5 X o(n) 5 X 3-3- o(n) 5 o(n) 5 3- (rate determining) Jack alpern et al. J. Am. hem. Soc., 1965, 87, o(dm) 2 PBu 3 BnBr obr(dm) 2 PBu 3 Bn o(dm) 2 PBu 3 Bn o(dm) 2 PBu 3 Bn O N N O DM = dimethylglyoxime Jack alpern et al. J. Am. hem. Soc., 1969, 91, Other examples: Mn(O) 5 ; M(p)(O) 3 (M=r,Mo,W) 17-electron complexes! 28

15 2. nner-sphere Electron transfer/aged adical-pair Mechanism General : M (n) X M (n) X (n+1) M X cage collapse (n+1) M X cage escape n+2 M X X M (n+1) This is the mechanism we shall study here - M (n) M (n+1) Propagation step in radical chain mechnism 29 M (n) X M (n) X (n+1) M X cage n+2 M X haracteristics : (a) The products are very similar to those of a S N 2 reaction (b) The reactivity order is - > -Br > - > -OTs (this order is determined by thermodynamics) Note: in S N 2 -OTs most reactive 3 > 2 >1 > Me (this order is determined by the relative order of the radicals) This order is opposite to S N 2 mechanism (c) This pathway requires a coordinatively unsaturated metal capable of undergoing a formal two-electron oxidation (d) ArO does not work as an inhibitor 30

16 Ni 0 (PEt 3 ) 4 ArX -PEt 3 XNi 1 -PEt (PEt 3 ) 3 Ar 3 XNiAr(PEt 3 ) 2 S Ar S f Ar = (2,4,6-tri-tert-butyl)bromobenzene, the aryl radical can be detected directly by ES in the reaction mixture. J. K. Kochi et al. J. Am. hem. Soc., 1979, 101(21), Pt 3 fast Pt 2 Pt 2 X slow Pt 2 X cage fast Pt 2 X Follows the order 3 > 2 > 1 The radical is not traped by ArO as does not leave the cage ArO ArO 31 3.Outer-sphere Electron-Transfer Mechanism M n X X - [M (n+1) ] [M (n+2) ] X - X - [M (n+1) ] Net X [M (n+2) ] + X - M n This reaction is associated with coordinatively saturated metal centers 32

17 X pfe(o) 2 Na TF, 0 o -NaX Fe(O) 2 p Fe(O) 2 p X = 70 : 30 X= Br >97 : <3 X pfe(o) 2 - X - pfe(o) 2 S N 2 Fe(O) 2 p X - X - pfe(o) 2 Paul J. Krusic et al., J. Am. hem. Soc., 1977, 99(1), The rearrangement is indicative of radical formation cyclopropyl carbinyl radical butenyl radical adical hain pathway n X n X M (n) M (n+1) M (n+1) X M (n+2) X Propagation M (n+1) side reaction Termination haracteristics : (a) nitiators = impurities,o 2,hv (b) racemization of (c) The reactivity order is - > -Br > - > -OTs Note: in S N 2 -OTs most reactive (d) 3 > 2 >1 > Me This order is opposite to S N 2 mechanism (e) ArO does work as an inhibitor 34

18 nitiation O O Br Br O O Propagation X X X X Br Br = (,S)-PhFBrO 2 Et The reaction can be stoped by ArO Jay A. abinger et al, norg. hem., 1980, 19(11), ontents ntroduction to oxidative addition reactions ; Mechanism of oxidative addition reactions ; Some examples in mechanism research ; Summary and outlook. Acknowledgement 36

19 Miyaura-Suzuki eaction Kosugi-Migita-Stille eaction Negishi eaction Sonogashira eaction Kumada-orriu-Tamao eaction iyama eaction Transmetalation Oxidative addition eductive elimination nitial and rate-determining step 37 The oxidative addition of (sp3)-x to Pd0 The oxidative addition of (sp3)-x to Pd0 complexs [Pd4] (=phosphane) is usually an associative bimolecular process (SN2 reaction) nversion at stereogenic center Ph Ph 3Pd + 3Pd X X- - O Ph MeO O MeO Ph O 2XPd Ph 2XPd J. K. Still et al, Acc. hem. es., 1977, 10,

20 O 2 Me O 2 Me O 2 Me Pd 2 (dba) 3 SnBu 3 benzene dichloromethane 94 6 TF 95 5 acetone DMF acetonitrile 5 95 DMSO 3 97 ideo Kurosawa et al., J. Am. hem. Soc., 1990, 112, [Pd] [Pd] ideo Kurosawa et al., J. Am. hem. Soc., 1992, 114, The oxidative addition of (sp 2 )-X X to Pd 0 1. The palladium species in the catalytic cycle assical Mechanism for ross-oupling eactions 40

21 Jutand and Amatore demonstrated that the three-coordinated anionic Pd(0) species [Pd(P 3 ) 2 OAc] - or [Pd(P 3 ) 2 ] - were formed, instead of the classical two coordinated complex, by the reaction of Pd() salts with phosphine and are involved in the oxidative addition step 41 Amatore,. et al., Organometallics, 1992, 11, 3009 Amatore,., Jutand, A., Suarez, A., J. Am. hem. Soc., 1993, 115, 9531 Jutand, A.; Mosleh, A., Organometallics, 1995, 14, 1810 Amatore,. et al., Organometallics, 1995, 14,

22 The oxidative addition of (sp 2 )-X X to Pd 0 2. cis complexes in the oxidative addition A concerted interaction of a reactive [Pd 2 ] or [Pd(-)] (-=diphosphane) species with X in a three-center transitionstate Pd X Pd X Pd X Pd X Pd X Pd X The intermediates observed in the catalytic cycle are trans-[pdx 2 ] 43 n fact cis complexes are observed to result from the oxidative reaction in a few cases with (sp 2 ) electrophiles Kinetic product Thermodynamic product. Urata, M. Tanaka, T. Fuchikami, hem. ett., 1987,

23 Model reaction for the cis trans isomerization: One of the two main pathways: Pd 1 k -1 k 1 -[Pd] [Pd] Pd 1 k 2 [Pd] Pd 1 fast + - -[Pd] mono-iodo-bridged intermediate Pd 1 A.. asado, P. Espinet, Organometallics, 1998, 17, ontents ntroduction to oxidative addition reactions ; Mechanism of oxidative addition reactions ; Some examples in mechanism research ; Summary and outlook ; Acknowledgement 46

24 Oxidative addition Two-electron Mechanism Single-electron Mechanism S N 2 onic Three center is-addition adical mechanism hain non-chain 47 Acknowledgement Prof. Zhangjie Shi All members in our group 48

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