M.C. White, M.S. Taylor Chem 153 C-H Activation -251 Week of November 4. 2002 Dehydrogenation of n-alkanes to terminal olefins Longer Cp (0.5 mol%) R 150°C sacrificial hydrogen acceptor norbomene, t-butylethylene, or 1-decene) At low conversions, l-octene is the major product of the dehydrogenation reaction(90 to >95% selectivity at 5% conversion, depending upon the acceptor used) Ethylene was not a suitable acceptor, resulting in inhibition of catalysis due to formation of a stable Ir-ethylene complex. As the reaction proceeds, olefin isomerization via sequential hydrometallation and p-hydride elimination erodes the kinetic selectivity, resulting in a mixture of olefin isomers R R Although the nature and the concentration of the sacrificial hydrogen acceptor had little effect on the factors had a large effect on the observed distribution of double bond isomers in the product.The authors propose that R he observed isomer distribution is largely determined by the competition between the sacrificial acceptor and the product lefin for insertion into the Ir-h bond of the dihydride R R R R Goldman. A. JACS 1999.121.4086 R R
M.C. White, M.S. Taylor Chem 153 C-H Activation -251- Week of November 4, 2002 Dehydrogenation of n-alkanes to terminal olefins A (0.5 mol%) 150°C Longer reaction times (norbornene, t-butylethylene, or 1-decene) sacrificial hydrogen acceptor Ir P P R R R R H H R = t-Bu, i-Pr At low conversions, 1-octene is the major product of the dehydrogenation reaction (90 to >95% selectivity at 5% conversion, depending upon the acceptor used). Ethylene was not a suitable acceptor, resulting in inhibition of catalysis due to formation of a stable Ir-ethylene complex. As the reaction proceeds, olefin isomerization via sequential hydrometallation and β-hydride elimination erodes the kinetic selectivity, resulting in a mixture of olefin isomers. Although the nature and the concentration of the sacrificial hydrogen acceptor had little effect on the reaction rate, these factors had a large effect on the observed distribution of double bond isomers in the product. The authors propose that the observed isomer distribution is largely determined by the competition between the sacrificial acceptor and the product olefin for insertion into the Ir-H bond of the dihydride intermediate. A A Ir P P R R R R n-Oct H Ir P P R R R R H H Ir P P R R R R H A Ir P P R R Goldman, A. JACS 1999, 121, 4086. R R
M.C. White. Chem 153 C-H Activation -252- Week of November 4. 2002 Direct carbonylation of benzene M PPh3 A PM CI 7.2 M (solvent) 73 tn RhCI(CO)(PPh3h Eisenberg JACS 1986(108)535 a photochemical CO(em) TN PMe3 is thought to increase the decarbonylation effectiveness of the Rh catalyst PMe talyst at rt 19 density at the metal thereby PE P(i-Prb 1947 2 promote oxidative addition and 3 by decreasing tail-biting of the P(p-tolyl)3 1982 2 Tanaka Chem. Lett 1987249 PPh Postulated mechanism Mesh A PMe 14 18 Meap Meap HrH H H e-
M.C. White, Chem 153 C-H Activation -252- Week of November 4, 2002 Direct carbonylation of benzene Postulated mechanism: The first report: (solvent) + CO 1 atm Ph3P Rh(I) OC PPh3 Cl 7.2 mM hv (295-420), rt, 40h O H 3 tn RhCl(CO)(PPh3)2 is a photochemical decarbonylation catalyst at rt. Eisenberg JACS 1986 (108) 535. Soon afterwards: (solvent) + CO 1 atm Me3P Rh(I) OC PMe3 Cl 0.21 mM hv (295-420), rt, 33h O H 73 tn Phosphine PMe3 PBu3 PEt3 P(i-Pr)3 P(p-tolyl)3 PPh3 P(OMe)3 CO (cm-1) 1970 1955 1957 1947 1979 1982 2011 TN 73 19 17 2 3 2 2 PMe3 is thought to increase the effectiveness of the Rh catalyst both by increasing electron density at the metal thereby promote oxidative addition and by decreasing tail-biting of the complex. Tanaka Chem. Lett. 1987 249. Tanaka JACS 1990 (112) 7221. Me3P Rh(I) OC PMe3 Cl 16 eCl Rh(I) PMe3 PMe3 14 eCl Rh(III) H Me3P PMe3 16 eRh(III) H Me3P PMe3 OC Cl Cl Rh(III) H Me3P PMe3 O Ph 18 eO H hv CO CO OC CO 18 e-
M.C. White. Chem 153 C-H Activation -253- Week of November 4. 2002 Direct carbonylation of alkanes PMe Cl 0.21 mM H The carbonylation reaction is highly cO regioselective towards primary C-H h(295-420,t33h bonds to give linear aldehydes with (solvent) I atm 0.6tn high selectivities. Unfortunately, the ndary photochemical reaction to give a Effects of irradiation wavelength: Flash photolysis revealed loss of CO dehomologated terminal alkene and ought to lead to the catalytically active 14e-species for C-H oxidative acetaldehyde in large quantities addition)is the dominant photoreaction of RhCI(COyPPh )2 at >330 Photo-induced Norrish Type lI Chemistry Tanaka Chem. Comm. 1987758 nm. Metal-to-ligand charge transfer band of Rh-CO 365 nm. Ford JACS 1989(111)1932. Absorption of non-conjugated aldehydes appear chacHo at-285 nm. It was hypothesized by Tanaka that cutting of the short-wavelength region capable of aldehyde excitation would improve 92 tn yields of the desired aldehyde While Norrish Type ll reactions leading to dehomologated terminal alkenes aldehyde tn nonene tm were suppressed by going to a longer wavelength,carbonylation selectivity (1- decanal,2-,3,4) towards the 1 position of the alkane was lost and catalytic activity was 295-420 610(855:4:23) 319 diminished. These results imply that photo-induced CO dissociation may not 325 26(8:45:1715:16) be the major pathway in this system for generating the complex capable of C-H activation of linear aliphatic alkanes Tanaka JACS 1990 7221 PMe3 The of benzene carbonylation by RhCI(CO)(PMe3h Irradation of a solution of irradiated at >290 nm(ca 314 nm,a Cl…Ra OC wavelength where Rh-co does not RAO"PMe3RhCICOXPMe 3) in the absence of co at-40.C afforded absorb)is proportional to CO pressure H Goldman proposes PM alkylhydrido complexes which photoelectronically excited intermediate were fully characterized by nmr as the species effecting C-H activation. NMR). Fields JA Goldman JACS 1994 (116)9498 1994(116)9492
M.C. White, Chem 153 C-H Activation -253- Week of November 4, 2002 Direct carbonylation of alkanes Aliphatic hydrocarbons: (solvent) + CO 1 atm Me3P Rh(I) OC PMe3 Cl 0.21 mM hv (295-420), rt, 33h O H 27 tn + 0.6 tn O H The carbonylation reaction is highly regioselective towards primary C-H bonds to give linear aldehydes with high selectivities. Unfortunately, the aldehydes formed readily undergo a secondary photochemical reaction (Norrish Type II) to give a dehomologated terminal alkene and acetaldehyde in large quantities. hv 285 nm O H H + CH3CHO 92 tn Tanaka Chem. Comm. 1987 758. Effects of irradiation wavelength: Flash photolysis revealed loss of CO (thought to lead to the catalytically active 14e- species for C-H oxidative addition) is the dominant photoreaction of RhCl(CO)(PPh3)2 at >330 nm. Metal-to-ligand charge transfer band of Rh-CO @ 365 nm. Ford JACS 1989 (111) 1932. Absorption of non-conjugated aldehydes appear at ~285 nm. It was hypothesized by Tanaka that cutting of the short-wavelength region capable of aldehyde excitation would improve yields of the desired aldehyde. wavelength (nm) aldehyde tn (1-decanal, 2-, 3-, 4-) nonene tn 295-420 >325 610 (85:5:4:2:3) 126 (8:45:17:15:16) 319 0 While Norrish Type II reactions leading to dehomologated terminal alkenes were suppressed by going to a longer wavelength, carbonylation selectivity towards the 1o position of the alkane was lost and catalytic activity was diminished. These results imply that photo-induced CO dissociation may not be the major pathway in this system for generating the complex capable of C-H activation of linear aliphatic alkanes. Tanaka JACS 1990 7221. Photo-induced Norrish Type II Chemistry Irradation of a solution of RhCl(CO)(PMe3)2 /C6H6 in the absence of CO at -40o C afforded two isomers of the 18 ealkylhydrido complexes which were fully characterized by NMR ( 1H, 31P, 13C NMR). Fields JACS 1994 (116) 9492. The rate of benzene carbonylation catalyzed by RhCl(CO)(PMe3)2 irradiated at >290 nm (ca. 314 nm, a wavelength where Rh-CO does not absorb) is proportional to CO pressure. Goldman proposes a photoelectronically excited intermediate as the species effecting C-H activation. Goldman JACS 1994 (116) 9498. Me3P Rh(I) OC PMe3 Cl 16 eMe3P Rh(I) OC PMe3 Cl 16 e- * Rh(III) H PMe3 PMe3 OC Cl 18 eR Cl Rh(III) H Me3P PMe3 18 eO R R OC CO R H O Revised proposed catalytic cycle:
M.C. White. Chem 153 C-H Activation -254 Week of november 4. 2002 Direct formation of aldimines PMe OC MeP CyNC RNc Cl 0.7mM h, rt, 36h (solvent) 6%/Rh 55 mM (solvent) tn low conversions may be due in part to R= cyclohexyl, 5 tn the low solubility of the isocyanide Me. 38%/h under the rxn conditions selectivities f-Bu. 3%/Rh not reflective of c-h activation via an organometallic intermedaite 12%Rh12%/Rh Tanaka Chem. Left 19872 RNC PPh h Jones notes that this system(unlike the one reported by Tanaka) is completely hv, rt, 36h ineffective at aldimine formation from aliphatic hydrocarbons 0 mM 4 tn Proposed mechanism R= neopentyl RNC PPh3 -PM PPh3 16 CNR 16e 16 Jones OM 1990(9)7 CNr
M.C. White, Chem 153 C-H Activation -254- Week of November 4, 2002 Direct formation of aldimines 1.0 mM + N C Ph3P Rh(I) RNC PPh3 Cl 0.2 mM hv, rt, 36h N H R R= neopentyl 4 tn Jones notes that this system (unlike the one reported by Tanaka) is completely ineffective at aldimine formation from aliphatic hydrocarbons. Jones OM 1990 (9) 718. Ph3P Rh(I) RNC PPh3 Cl 16 e- -PMe3 + PMe3 Rh(I) RNC PPh3 Cl 14 eRh(III) RNC PPh3 16 eCl H Rh(III) CNR PPh3 16 eCl H N R N H R CNR Proposed mechanism: (solvent) + RNC 55 mM Me3P Rh(I) OC PMe3 Cl 0.7 mM hv, rt, 36h N H 3 tn R R = cyclohexyl, 5 tn Me, 38%/Rh t-Bu, 3%/Rh CyNC (solvent) 6.0 mM + low conversions may be due in part to the low solubility of the isocyanide under the rxn conditions. Selectivities not reflective of C-H activation via an organometallic intermedaite. Me3P Rh(I) OC PMe3 Cl 0.7 mM hv, rt, 17h N Cy 6%/Rh H N Cy 12%/Rh 12%/Rh N H Cy + + The first report: Tanaka Chem. Lett. 1987 2373