M.C. White, Chem 153 C-H Activation -246- Week of November 4. 2002 Crabtree thermal dehydrogenation of alkanes to alkenes (p-FC6H43 Product distributions of linear alkenes are thought to result from isomerization of the initial kinetic I-ene product via intermediate Ir hydride species conditions gives similar olefin P(pFC6H小37lnM 4%0(3%) yields based on 55 mM frans-3-hexene 14%(18.5%) sacrificial H2 acceptor with cis-3-hexene 8%(7.5 %) 2d. 1. 4 tn 2d. 3 tn 2d.9 tn hydro proposed Mechanisn FCH小3 p-FC6l小3 OC(O)CH hydrogenation -CF2 hydrogenation (p-FC6H4)3l V-FC6H小 P(p-FC6H4) P(pFC6H4小3 OC(O)CF3 OC(OJCF isommerT-cTtIon (p-FC6H4)3l IrwIn>Cf (p-FC6H)3l P(p-FC6H03 tail-biting P(p-FC6HA3 P(p-FC6H43 (C6H乎-FB l43(P-FCH4)3P only trifluoroacetate complexes OC(O)CF3 were active in alkene HI (C6Hap-F)3P dehydrogenations. Their greater oxidative lability with respect to acetate (p-FC6H4)3l may allow more facile Plp-FC6H小3 (C6Hap-F3P interconversion from n' to n In-OC(O)CF3 necessary to provide an open (C,Hp-I coordination site for H Crabtree JACS 1987 (109)8025
M.C. White, Chem 153 C-H Activation -246- Week of November 4, 2002 Crabtree:thermal dehydrogenation of alkanes to alkenes Crabtree JACS 1987 (109) 8025. solvent H Ir(III) H O O P(p-FC6H4)3 P(p-FC6H4)3 CF3 + t-Bu 7.1 nM 355 mM 150oC t-Bu 2d, 1.4 tn tn = turnover # 2d, 3 tn 2d, 9 tn 4% (3%) + 56% (54%) + 18% (17.5%) + trans-3-hexene 14% (18.5 %) cis-3-hexene 8% (7.5 %) yields based on catalyst. 14 days Product distributions of linear alkenes are thought to result from isomerization of the initial kinetic 1-ene product via intermediate Ir hydride species. Subjecting 1-hexene to the reaction conditions gives similar olefin distributions (in parentheses). sacrificial H2 acceptor with unusually high heat of hydrogenation H Ir(III) H O O P(p-FC6H4)3 P(p-FC6H4)3 CF3 H Ir(III) H OC(O)CF3 P(p-FC6H4)3 (p-FC6H4)3P t-Bu t-Bu Ir(III) H O O P(p-FC6H4)3 P(p-FC6H4)3 CF3 t-Bu (C6H4p-F)3P Ir(I) (C6H4p-F)3P O O CF3 (C6H4p-F)3P Ir(I) (C6H4p-F)3P OC(O)CF3 H Ir(III) H OC(O)CF3 P(p-FC6H4)3 (p-FC6H4)3P R H Ir(III) H OC(O)CF3 P(p-FC6H4)3 (p-FC6H4)3P R R t-Bu R H Ir(III) O O P(p-FC6H4)2 P(p-FC6H4)3 CF3 F R H Ir(III) H OC(O)CF3 P(p-FC6H4)3 (p-FC6H4)3P R 14 eoxidative addition β-hydride elimination "tail-biting" Ir(III) H OC(O)CF3 P(p-FC6H4)3 (p-FC6H4)3P H R isomerization pathway hydrogenation pathway R isomerization hydrogenation Proposed Mechanism: only trifluoroacetate complexes were active in alkene dehydrogenations. Their greater lability with respect to acetate may allow more facile interconversion from η3 to η1 necessary to provide an open coordination site for H2 acceptor binding
M.C. White, Chem 153 C-H Activation -247 Week of November 4. 2002 Crabtree photochemical dehydrogenation of alkanes to alkenes Under conditions of CF methylcyclohexane is product. This is thought to result from a kinetic preference to form the sterically less P(Cy)3 7. 1 nM Methylenecyclohexane subjected to the h(254mm) reaction conditions results in only 25% 7 days +1227m(6219m(38)085031103s1mn onversion to the thermodyn tn w/out tbe present(in parentheses) 355mM atios reflect more isomerization activity Proposed Mechanism P(Cy)3 nate wavelength promotes reductive elimination of the dihydride catalyst leading directly to the Isommterteanon catalytically active 14e-complex. It's interesting hy 254nm to note that no reaction takes place with tbe in (Cy)3 P(Cyl P(Cy)3 the absence of 254 nm light. This implies that acts as a H2 acceptor from a photochemically 1OC(O)CF3 . oC(O)CF3 excited intermediate (Cya bride P(Cyl (CyP in the presence (Cy)3P addition (Cy)3B, (OJCF3 Crabtree JACS 1987 (109)8025
M.C. White, Chem 153 C-H Activation -247- Week of November 4, 2002 Crabtree:photochemical dehydrogenation of alkanes to alkenes Proposed Mechanism: Crabtree JACS 1987 (109) 8025. Irradiation with light of the appropriate wavelength promotes reductive elimination of the dihydride catalyst leading directly to the catalytically active 14e- complex. It's interesting to note that no reaction takes place with tbe in the absence of 254 nm light. This implies that tbe acts as a H2 acceptor from a photochemically excited intermediate. H Ir(III) H O O P(Cy)3 P(Cy)3 CF3 (Cy)3P Ir(I) (Cy)3P O O CF3 (Cy)3P Ir(I) (Cy)3P OC(O)CF3 14 eH Ir(III) H OC(O)CF3 P(Cy)3 (Cy)3P R H Ir(III) H OC(O)CF3 P(Cy)3 (Cy)3P R R t-Bu R oxidative addition β-hydride elimination Ir(III) H OC(O)CF3 P(Cy)3 (Cy)3P H R isomerization pathway H Ir(III) H O O P(Cy)3 P(Cy)3 CF3 * hv, 254nm t-Bu H2 H2 Some free H2 is formed even in the presence of tbe. solvent H Ir(III) H O O P(Cy)3 P(Cy)3 CF3 + t-Bu 7.1 nM tbe 355 mM hv (254 nm) t-Bu 2.77tn (1.6) + 2.19 tn (3.84) + 7 days Under conditions of hv and tbe, methylcyclohexane is the preferred product. This is thought to result from a kinetic preference to form the sterically less hindered M-C bond. Methylenecyclohexane subjected to the reaction conditions results in only 25% conversion to the thermodynamically more stable 1-methylcyclohexene. Although the reaction proceeds w/out tbe, the product ratios reflect more isomerization activity. + + H2 0.85 tn (0.32) 1.26 tn (0.82) tn w/out tbe present (in parentheses)
M.C. White. Chem 153 C-H Activation -248 Week of November 4. 2002 Tanaka: photochemical dehydrogenation PMe3 (solvent) 27h, 155 tn 1:79:20 138 tn, 17h 930tn,69h H a theoretical amount N, stream of H, was det Added phosphine ligand decreases the efficiency of in the gas phase the regioselectivity When a N, stream PMe3/Rh time(h) 12.3 towards formation if 1-hexene. Within the same PMe3/Rh ratio, an erosion in regioselectivity is increased to 195 tn 54 observed upon prolonged reaction times. This is 1241 18.7 10 Could this ratio also be reflective of the rates of 10 103417.2 olefin hydrogenation? Exposure of 1-hexene to the reaction conditions results in 2-hexene (35%)and hexane(63%)after 22h Proposed mechani CI.aPMe3 Added phosphine ligand may take up a acant coordination site cis to the M-alkyl, preventing formation of the gastic interaction necessary to effect B-hydride elimination. a decrease in both alkane dehydrogenation and h ht-promoted reductive P-Rh@"PMe3 14 elimination PMe
M.C. White, Chem 153 C-H Activation -248- Week of November 4, 2002 Tanaka: photochemical dehydrogenation Proposed Mechanism: 0.7mM hv, rt, N2 138 tn, 17 h A theoretical amount of H2 was detected in the gas phase. When a N2 stream was used, tn increased to 195 tn. 930 tn, 69h N2 stream + 1:79:20 + 27 h, 155 tn Me3P Rh(I) OC PMe3 Cl (solvent) H2 + PMe3/Rh 2 5 5 10 10 time (h) 1 3 22 3 22 hexenes 1- 2- 3- 1 12 6 28 10 11 4 4 4 3.4 2 1 1 1 1 TN 5.4 4.0 18.7 0.6 7.2 Added phosphine ligand decreases the efficiency of the reaction but increases the regioselectivity towards formation if 1-hexene. Within the same PMe3/Rh ratio, an erosion in regioselectivity is observed upon prolonged reaction times. This is indicative of catalyst mediated alkene isomerization. Could this ratio also be reflective of the rates of olefin hydrogenation? Exposure of 1-hexene to the reaction conditions results in 2-hexene (35%) and hexane (63%) after 22 h. Me3P Rh(I) OC PMe3 Cl 16 ehv CO Me3P Rh(I) PMe3 Cl 14 eR Rh(III) PMe3 Cl H PMe3 R H Rh(III) PMe3 Cl H PMe3 R H intermediate in Wilkinson hydrogenation R H2 light-promoted reductive elimination of H2 ?? Rh(III) PMe3 Cl H PMe3 H R Added phosphine ligand may take up a vacant coordination site cis to the M-alkyl, preventing formation of the agostic interaction necessary to effect β-hydride elimination. A decrease in both alkane dehydrogenation and olefin isomerization results. β-hydride elimination reductive elimination
M.C. White. Chem 153 C-H Activation -249- Week of November 4. 2002 Goldman Wilkinson 's Catalyst Varient A variety of sacrificial alkenes work in the Me3P 59 tn 53 tn dehydrogenation of cyclooctane,an especially reactive substrate. Cyclooctene H2(0ps9.60°C has a very low heat of hydrogenation (solvent) probably resulting from transannular steric alkane 4 tn severe in cyclooctene sacrificial alkene n-hexane gave hexenes in mode tn(9.) with norbornene as the h2 acceptor. No mention was made to the isomer distributions 9492. Proposed mechanism A PMe H2 PMe of octahedral dihydride comple initiate ligand dissociation. Wilkinson,'s hydrogenation catalyst(see hydrogenation, pg. 142), known to dissociate PMe3 PPh3 upon H2 oxidative addition, is cited as precedent for HRE this. There is no evidence that Co dissociates preferentially over PMe,. The authors invoke this to arrive at the same 14 e- ntermediate proposed in Tanaka's photochemical system PM PMe Me P- R"PMe3
M.C. White, Chem 153 C-H Activation -249- Week of November 4, 2002 Goldman: Wilkinson’s Catalyst Varient Proposed Mechanism: Goldman JACS 1992 (114) 9492. Me3P Rh(I) OC PMe3 Cl 16 eH2 Me3P Rh(III) H PMe3 Cl H CO Rh(III) PMe3 Cl H PMe3 18eH CO 16 ePh3P Rh(III) H PPh3 Cl H Me3P Rh(I) PMe3 Cl Tanaka's 14 e- intermediate Rh(III) PMe3 Cl H PMe3 H 0.7mM H2 (1000 psi), 60oC 1.5 h, x tn Me3P Rh(I) OC PMe3 Cl sacrificial alkene + + alkane , 59 tn , 106 tn , 53 tn t-Bu , 4 tn sacrificial alkenes n-hexane gave hexenes in modest tn (9.6) with norbornene as the H2 acceptor. No mention was made to the isomer distributions. A variety of sacrificial alkenes work in the dehydrogenation of cyclooctane, an especially reactive substrate. Cyclooctene has a very low heat of hydrogenation probably resulting from transannular steric repulsions in cyclooctane which are less severe in cyclooctene. (solvent) Formation of octahedral dihydride complex is thought to initiate ligand dissociation. Wilkinson's hydrogenation catalyst (see hydrogenation, pg. 142), known to dissociate PPh3 upon H2 oxidative addition, is cited as precedent for this. There is no evidence that CO dissociates preferentially over PMe3. The authors invoke this to arrive at the same 14 eintermediate proposed in Tanaka's photochemical system
M.C. White. Chem 153 C-H Activation -250 Week of November 4. 2002 Substrate directed dehydrogenation via c-H activation Possible intermediates HaCO (OTT 0 (OTr) H3C00 (OTT) F3CH2OH 70°C,60h CH Sames constructs a ligand for the metal from the requisite functionality of the target that directs C-H activation towards only one of the 2 ethyl substituents. This results in selective dehydrogenation to give the platinum hydride in 产0 (OTr) >90% yield. The reaction is stiochiometric in HcQ 0 platinum and the metal must be removed via treatment with aqueous potassium cyanide Sames JACS2000(122)6321
M.C. White, Chem 153 C-H Activation -250- Week of November 4, 2002 Substrate directed dehydrogenation via C-H activation N N O H3CO N PtIICH3 N N O H3C O N Pt N H N O H3CO O H CF3CH2OH N N O H3CO N PtIVCH3 H N N O H3CO N PtII H (OTf- ) + (OTf- ) + 70oC, 60 h Rhazinilam (OTf- ) + (OTf- ) + CH4 Possible intermediates: Sames constructs a ligand for the metal from the requisite functionality of the target that directs C-H activation towards only one of the 2 ethyl susbstituents. This results in selective dehydrogenation to give the platinum hydride in >90% yield. The reaction is stiochiometric in platinum and the metal must be removed via treatment with aqueous potassium cyanide. Sames JACS 2000 (122) 6321