Organometallic Chemistry and Homogeneous Catalysis Dr. Alexey Zazybin Lecture N8 Kashiwa Campus, December 11, 2009 Types of reactions in the coordination sphere of TMC 3. Reductive elimination X-ML n-Y à ML n + X-Y In the process of reductive elimination: 1) TM reduces its oxidation state by 2 points 2) TM reduces coordination number by 2 points Reductive elimination is a key transformation in transition metal mediated 2 catalysis, often representing the product forming step in a catalytic cycle. General trend for reductive elimination from d 8 square planar complexes: Energy Reductive elimination is a reverse transition state reaction to oxidative addition. If two Oxidative Reductive addition elimination opposite reactions can take place at one and the same reaction M, X-Y conditions – they usually proceed through the same transition state X-M-Y (microscopic reversibility principle) Reaction coordinate 3 Features of Reductive Elimination H L L -H2 H Ir CO Cl Ir CO L L Cl Reductive elimination is promoted by presence of: • larger ligands L such as PPh 3 since (i) steric crowding is reduced on elimination and (ii) the product, which has a lower coordination number, is stabilized • electron withdrawing ligands such as CO since these stabilize the low oxidation state (in the case of CO this is particularly effective because of π-backbonding). Similarly, if you want to design a complex that will not undergo oxidative addition readily, use big and/or electron withdrawing ligands RE can be promoted by: 4 - Increasing the bite angle of the ligand - Increasing electrophilicity of metal center ( e.g. π-acids) - cis -orientation of ligands (for concerted RE) is required Large bite angles of diphosphines have been shown to enhance the rates of reductive elimination from square planar complexes presumably by bringing the two departing ligands closer together. RE can be promoted by: 5 - Increasing the bite angle of the ligand - Increasing electrophilicity of metal center ( e.g. πππ-acids) - cis -orientation of ligands (for concerted RE) is required Coordination of π-acidic ligand CF 3-C6H4-CH=CH 2 to the Ni-catalyst increases the rate of reductive elimination and the yield of the coupling product. RE can be promoted by: 6 - Increasing the bite angle of the ligand - Increasing electrophilicity of metal center ( e.g. π-acids) - cis -orientation of ligands (for concerted RE) is required Cis -elimination 7 X X -XY R P Y R3P Y R3P PR3 3 Pt Pt Pt R3P PR3 R3P PR3 R3P PR3 PR3 PR3 • Most eliminations occur by this 3-centre process • Reverse of 3-centre OA • The two X-type ligands must be cis Me Me Ph Br PR3 RE Pt Ph Me retention D D H PR3 H Some examples of reductive elimination: 8 H M-H bond strength: 50-60 kcal/mole a) M M + H-H 3 H M-C(sp ) bond strength: 30-40 kcal/mole R b) M M + R-H H-H bond strength: 104 kcal/mole H 3 R C(sp )-H bond strength: 95 kcal/mole M + R-R 3 3 c) M C(sp )-C(sp ) bond strength: 83 kcal/mole R Thermodynamics: a) 2 bonds breaking: 50+50, 1 bond forming: 104, so ~ 4 kcal of profit b) 2 bonds breaking: 50+30, 1 bond forming: 95, so ~ 15 kcal of profit c) 2 bonds breaking: 30+30, 1 bond forming: 83, so ~ 23 kcal of profit a) à b) à c) – thermodynamic favorability increases 9 Kinetics: a) à b) à c) – reaction rate decreases H Why the a) process is thermo- H a) M M dynamically most unfavorable H H but it has the most high reaction rate? C C b) M M H To answer we should compare H transition states of the process: C C c) M M C C Computational studies (Goddard JACS 1984, Dedieu Chem. Rev. 2000 ): 0 the spherical symmetry of the s orbitals of H allows the simultaneous breaking of the M-L σ-bonds while making the new σ-bond of the product. Best overlap Worst overlap RE is usually easy for : RE is often slow for : • H + alkyl / aryl / acyl • alkoxide + alkyl • alkyl + acyl • halide + alkyl • SiR 3 + alkyl etc Mechanistic possibilities of RE: 1 No cross-coupling products could be usually found in the corresponding experiments: Radical mechanisms are also found in reductive elimination. An important 2 example is the radical-chain mechanism of the dehydrogenation of cobalt carbonyl hydride: Mechanistic scheme of the RE (carbonyls are omitted): Interesting case of RE : solvent effect 4. Insertion ( Migratory insertion ) 3 - Intermolecular insertion: - Intramolecular insertion: X M X + Y M Y X M M Y X Y Nucleophile Y attacks X Unsaturated ligand Y insert into M-X Examples: ̙ ̙ 13 CpMo(CO)3CH2Ph + C6H11NC MeMn(CO)5 + CO CpMo(CO) 13 3 Me-C-Mn(CO)=O 4( CO) C-CH2Ph N C6H11 X 4 M M Y X Y M X Y MH , CC CC , CC MC O , CN , CN M Hlg CO, CO2, SO2, CS2 M N CH2 , MO CO, CN General Features: 1) No change in formal oxidation state of the TM 2) The two groups that react must be cisoidal to one another 3) A vacant coordination site is generated by the migratory insertion. 4) Migratory insertions are usually favored on more electron-deficient metal centers. Depending on the nature of the inserting group it is possible to 5 recognize 2 types of intramolecular insertion: X X X 1,1-migratory insertion X A 1,2-migratory insertion M A B M A M B B B M A CH 3 CH3 H H CH 2 CH M C O M C M 2 CH2 O M CH2 X O X X O S M SO2 M S X M SO2 M O O Regioselectivity of Insertion : Typically steric factors will result in a thermodynamic preference for the sterically less hindered carbon to be bonded to the metal center. Hydrozirconation of alkenes with the Schwartz reagent is a good example of this thermodynamic preference: Ph 6 Stereochemistry: Me CO O Ph H Retention of configuration CO Pt Cl Me retention OC Pt Cl H CO CO For concerted migratory insertion or elimination, carbon stereochemistry is nearly always retained: 7 Some Electronic effects R O R Z Z CO CO OC Fe + L OC Fe CO THF L C C O O best Lewis acid - can coordinate to electron-rich CO ligands and drain off some e- density + + + + Z = Li > Na > (Ph3)2N strongest coordinating ligand - best trapping ligand L = PMe3 > PPhMe2 > PPh2Me > CO most electron-rich alkyl group makes the best nucleophile for migrating to the electron-deficient CO −−− −−− R = n-alkyl > PhCH2 Migration vs. Insertion 8 There are two different “ directions ” that a migratory insertion can occur: A migration is when the anionic An insertion is when the ligand moves and performs a X neutral ligand moves and nucleophillic-like intramolecular M Y “inserts” into the bond attack on the electrophillic between the metal and neutral ligand anionic ligand X MY Y X M MigrationCH3 Insertion M CO O H3C O MC C CH3 M Let’s have a look on the CO insertion into the Mn-CH 3 bond: 9 Me Me O CO CO CO CO CO Mn Mn CO ∆ CO CO CO CO CO 1. Question : What kind of insertion takes place – intermolecular or intramolecular? -Intermolecular insertion: -Intramolecular insertion: X M X + Y M Y X M M Y X Y Nucleophile Y attacks X Unsaturated ligand Y insert into M-X To answer this question 13 CO should be used: Me Me O O - In the case of inter- 13C -In the case of C 13CO molecular insertion CO CO intramolecular insertion CO 13 C will go to acyl Mn unlabeled C appears in Mn CO group CO CO acyl group CO CO CO Me O 0 It was found that C 13CO 13 C label exists only CO Mn - Intermolecular insertion as a cabonyl ligand: CO CO CO 2. Question : How does insertion take place? CH3 Migration of CH 3 to CO Insertion of CO into Mn-CH 3 M CO H C O 3 O MC C CH3 M Migration Insertion O O CH3 O C C OC CO OC CO OC CO OC CO Mn Mn Mn Mn O OC CH OC 3 OC CH3 OC C C C C CH OO OO 3 Mn(+1) Mn(+1) Mn(+1) Mn(+1) 18e- 16e- 18e- 16e- Just looking at the direct reaction you can not make a choice 1 Energy transition state If two opposite reactions can take Oxidative Reductive place at one and the same reaction addition elimination conditions – they usually proceed M, X-Y through the same transition state X-M-Y (microscopic reversibility principle) Reaction coordinate In the opposite direction first vacant 4 variants of CO dissociation place should be organized: are interesting for us: Me Me O O C C Me Me 13CO O O CO CO C C Mn Mn 13 13 CO CO -CO CO CO CO CO CO CO CO Mn Mn Me Me CO O O CO CO C C 13 13 CO CO CO CO CO Mn Mn CO CO CO CO CO a) Let’s imagine that CH 3 migration takes place 2 O CH3 O C OC CO OC CO Mn Mn OC OC CH3 C C O O non-labled Me O Me O cis C CO C CO 13 13 CO CO Me CO CO CO CO Mn Mn Mn Mn CO CO CO CO CO CO Me CO CO CO CO Me trans O Me O cis CO C C CO 13 13 CO CO CO 13 13 CO CO Me CO Mn Mn Mn Mn Me CO CO CO CO CO CO CO CO CO CO non-labled: 25%, labled cis: 50%, labled trans: 25% cis:trans = 2:1 b) Let’s think that CO insertion takes place 3 H3C O C CH3 OC CO OC CO Mn Mn OC OC CO C C O O Me O Me O C Me C Me 13 13 CO CO CO CO CO CO CO Mn Mn Mn Mn CO CO CO CO CO CO CO CO CO CO CO Me O Me O Me C C 13 Me 13CO CO CO 13 13 CO CO CO CO Mn Mn Mn Mn CO CO CO CO CO CO CO CO CO CO CO non-labled: 25%, labled cis: 75%, labled trans: 0% all cis It was found that cis and trans isomer are formed in 2:1 ratio, 4 so migration of CH 3 takes place While most systems studied have been shown to do migrations, both are possible.