M.C. White. Chem 153 C-H Activation -241- Week of november 4. 2002 The Holy grail of catalysis R CHeMI CH3 CHOR C-H activation: Process where a strong C-h bond (90-105 kcal/mol) undergoes substitution to produce a weaker C-M bond(50-80 kcal/mol) Functionalization: Metal-C bond is replaced by any bond except C-H Methods have been identified to regioselectivity effect C-H activation. Recall that there is both a kinetic and thermodynamic preference to form the less sterically hindered 1 C-M intermediate(see Structure Bonding, pg. 32). The challenge lies in finding ways to selectively form the C-M intermediate under synthetically useful, mild conditions that enable functionalization and catalyst renewal ARTHUR: Yes we seek the Holy grail(clears throat very quietly ) Our quest is to find the holy grail KNIGHTS: Yes it I ARTHUR: And so we're looking for it KNIGHTS: Yes we are BEDEVERE. We have been for some time KNIGHTS: Yes ROBIN: Months ARTHUR: Yes. and any help we get is. is very.. helpful Bergman Acc. Chem. Res. 1995(28)154. Exerpt from"Monty Python and the Holy Grail: 1974
The Holy Grail of Catalysis ARTHUR: Yes we seek the Holy Grail (clears throat very quietly). Our quest is to find the Holy Grail. KNIGHTS: Yes it is. ARTHUR: And so we’re looking for it. KNIGHTS: Yes we are. BEDEVERE: We have been for some time. KNIGHTS: Yes. ROBIN: Months. ARTHUR: Yes…and any help we get is…is very…helpful. Bergman Acc. Chem. Res. 1995 (28) 154. Exerpt from “Monty Python and the Holy Grail”; 1974. M.C. White, Chem 153 C-H Activation -241- Week of November 4, 2002 R CH3 R CH2[M] R CH2R' C-H activation: Process where a strong C-H bond (90-105 kcal/mol) undergoes substitution to produce a weaker C-M bond (50-80 kcal/mol). Functionalization: Metal-C bond is replaced by any bond except C-H. ? Methods have been identified to regioselectivity effect C-H activation. Recall that there is both a kinetic and thermodynamic preference to form the less sterically hindered 1o C-M intermediate (see Structure & Bonding; pg. 32). The challenge lies in finding ways to selectively form the C-M intermediate under synthetically useful, mild conditions that enable functionalization and catalyst renewal
M.C. White, Chem 153 C-H Activation -242 Week of November 4. 2002 Bergman C-H Activation via Late, Nucleophilic Complexes π- backbonding> Hydrido(alkyl)metal oxidative addition complex egioselectivity: sp C-H>10 sp'C-H> 2 C-H>>>30 sp'C-H. There is both a kinetic and thermodynamic preference to form the least sterically hindered C-M o bond. Kinetic preference: activation barrier to g-complex formation is lower for less sterically hindered C-H bonds and bonds with more s character. Thermodynamic preference: stronger C-M bonds are formed (see Structure and Bonding, pg. 32) prone to non-productive reductive elimination in the presence of oxidants and non-productive v or protonolysis in the presence of protic reagents H2 M=Ir. 1 代H ligand 16e- proposed coordinatively and Relative rate constants for attack at a single electronically unsaturated C-H bond by 1 and 2 at-60oC MLcO hvor△ C-H bond krel(rh, 2)krel (Ir, 1) low OS metals capable of cyclopropane 10.4 2.1 18e donating electrons in f-bond n-hexane( 19) 5.9 M=Ir, 3 formation. Highly prone to air with acyclic substrates n-hexane(2%) 0 Rh, oxidation the rh complex inserts ne(1°)26 JACS1982(104)352(Cmp.1) only into IC-H bonds nOM 1984(3)508(competition exp) 1.1 JACS1983(105)7190(Cmp.3) Bergman JACS 1994(116)9585(Cmp. 4) arbitrarily set at 1-+cyclohexane
M.C. White, Chem 153 C-H Activation -242- Week of November 4, 2002 Bergman:C-H Activation via Late, Nucleophilic Complexes Bergman JACS 1982 (104) 352 (Cmp. 1). Bergman OM 1984 (3) 508 (competition exp). Graham JACS 1983 (105) 7190 (Cmp. 3). Bergman JACS 1994 (116) 9585 (Cmp. 4). These hydrido(alkyl)metal complexes are prone to non-productive reductive elimination in the presence of oxidants and non-productive protonolysis in the presence of protic reagents Relative rate constants for attack at a single C-H bond by 1 and 2 at -60oC. C-H bond benzene cyclopropane n-hexane (1o) n-hexane (2o) propane (1o) propane (2o) cyclopentane cyclohexane krel (Rh, 2) 19.5 10.4 5.9 0 2.6 0 1.8 1.0 krel (Ir, 1) 3.9 2.1 2.7 0.2 1.5 0.3 1.1 1.0 arbitraril y set at 1 with acyclic substrates the Rh complex inserts only into 1o C-H bonds regioselectivity: sp2 C-H > 1o sp3C-H> 2o sp3 C-H >>> 3o sp3 C-H. There is both a kinetic and thermodynamic preference to form the least sterically hindered C-M σ bond. Kinetic preference: activation barrier to σ-complex formation is lower for less sterically hindered C-H bonds and bonds with more s character. Thermodynamic preference: stronger C-M bonds are formed (see Structure and Bonding, pg. 32). MI OC CO CO MI L MIII L M H I L H 18 ehv or ∆ 16 e- 18 e- proposed σ-complex intermediate ligand dissociation M = Ir, 3 Rh, 4 MIII Me3P H2 hv or ∆ M = Ir, 1 Rh, 2 oxidative addition coordinatively and electronically unsaturated intermediate H H π-donor low OS metals capable of donating electrons in σ-bond formation. Highly prone to air oxidation. H C M M H C π-backbonding>> σ-donation oxidative addition σ-complex Hydrido(alkyl)metal complex
gie Evidence for intermolecular o-complex formation Q Chen Chem 153 C-H Activation -243 Week of November 4. 2002 CD3 hv(flash), Kr(165K) Rh/ D roll.nD Rh一Co DC 18 0v(1946cm4) 0v(1947cm2) Bergman JACS 1994(116)958 CO v(2008 cm g-compler 0.020 1946m 1947cm 0.012 2008cm OC +(CD3)C 0.004 0.004+ Tm·(m) DyC CD3 The reaction of Cp*Rh(COh with neopentane-di2 was monitored using low-temperature IR nash kinetic spectroscopy. The CO stretch at 1946 cm" was assigned to the initial intermediate Cp"Rh(COXKr)complex, which after photolysis-mediated formation shows rapid decay. During this time, a second CO stretch at 1947 cm"grows in and decays; this absorption is assigned to a transient intermediate Rh--CD o-complex. The absorption at 2008 cm"is known to correspond to the product h(CODCs Du1), which increases steadily throughout the course of the reaction. Note that this entire process occurs in less than 1.5 ms
M.C. White, Q. Chen Chem 153 C-H Activation -243- Week of November 4, 2002 Evidence for intermolecular σ-complex formation CO D D2C CD3 D3C CD3 RhIII OC CD2 D CD3 D3C CD3 RhI [Kr] OC RhI OC CO RhI OC CD3 D3C CD3 CD3 18 ehv (flash), Kr (165K) CO v (1946 cm-1) CO v (1947 cm-1) σ-complex CO v (2008 cm-1) D D2C CD3 D3C CD3 RhI OC RhI [Kr] OC Rh OC CD2(C(CD3)3 D + (CD3)4C to products ∆G (kcal/mol) ‡ -3.2 kcal/mol + 6.9 kcal/mol The reaction of Cp*Rh(CO)2 with neopentane-d12 was monitored using low-temperature IR flash kinetic spectroscopy. The CO stretch at 1946 cm-1 was assigned to the initial intermediate Cp*Rh(CO)(Kr) complex, which after photolysis-mediated formation shows rapid decay. During this time, a second CO stretch at 1947 cm-1 grows in and then decays; this absorption is assigned to a transient intermediate Rh---CD σ-complex. The absorption at 2008 cm-1 is known to correspond to the product Cp*Rh(CO)(D)(C5D11), which increases steadily throughout the course of the reaction. Note that this entire process occurs in less than 1.5 ms. Bergman JACS 1994 (116) 9585
M.C. White, Chem 153 C-H Activation -244- Week of November 4. 2002 Evidence for concerted c-H oxidative addition crossover experiment: evidence in support of a concerted mechanism. Ir—co cO 18 Less than 7% of the crossover products were observed by HNMR. This may be indicative of a minor radical pathway C Bergman JACS 1983(105 )3929
M.C. White, Chem 153 C-H Activation -244- Week of November 4, 2002 Evidence for concerted C-H oxidative addition IrI OC CO CO IrI OC IrIII OC H Bergman JACS 1983 (105) 3929. IrI OC D crossover experiment: evidence in support of a concerted mechanism. 18 ehv σ-complexes D12 IrII OC H IrII OC D H2C D11 H3C + IrI OC H IrIII OC D Less than 7% of the crossover products were observed by 1HNMR. This may be indicative of a minor radical pathway. IrIII OC Ir D III OC H D11 D11 D11
M.C. White, Chem 153 C-H Activation -245 Week of November 4. 2002 Dehydrogenation of alkanes to alkenes H2 generation via olefin dissociation and elimination of H. H2 must be rapidly and irreversibly removed to avoid olefin hydrogenation and isomerization H H p-hydride r ML 1.3 MLn-2, elimination, 3 ligands from the substrate in its oxidative addition ordination sphere mid-cycle H metal and m The first report (coe) PPh3 10 eq -10°C>40%C PhaP H CD2Cl2-60°C Phah PPh observed to form NMR recall: intermediate in cation call hydrogenation catal hydrogenation cataly Crabtree JACS 1979(101)7738
M.C. White, Chem 153 C-H Activation -245- Week of November 4, 2002 Dehydrogenation of alkanes to alkenes R R H2 -H2 Catalyst requirements: MLnx-3 "14e-" R MLn x 18e- 3 L H MLn-2x-1 H R H MLn-2x R H H β-hydride elimination 16e- 18eoxidative addition H2, R metal capable of shuttling between Mn and Mn-2 oxidation states complex capable of accomodating 3 ligands from the substrate in its coordination sphere mid-cycle regeneration via olefin dissociation and elimination of H2. H2 must be rapidly and irreversibly removed to avoid olefin hydrogenation and isomerization Ph3P Ir(III) PPh3 H H O + (BF4 - ) recall: intermediate in cationic hydrogenation catalysts 10 eq O CD2Cl2, -60oC (coe) Ph3P Ir(III) PPh3 H H + (BF4 - ) observed to form quantitatively by NMR -10oC->40o C Ir(I) PPh3 PPh3 + (BF4 - ) 75% recall: hydrogenation catalyst The first report: Crabtree JACS 1979 (101) 7738