Abstract:The compounds cpFe(C0)2H and HMn(C0)5 (cp = cyclopentadienyl) react with 1,3-dienes to give the 2-enylmetal products of 1,4-addition of the metal hydrides to the dienes; CIDNP studies indicate that the process involves a radical pair mechanism.
“…In 1977, Sweany and Halpern reported strong evidence for H • transfer in the hydrogenation of α-methylstyrene by HMn(CO) 5 (Scheme ); they observed chemically induced dynamic nuclear polarization (CIDNP) and an inverse isotope effect for HMn(CO) 5 /DMn(CO) 5 . Since then, there have been numerous reports of hydrogenations that occur through H • transfer. , 1 …”
Section: Introductionmentioning
confidence: 99%
“…Hydroformylation reactions may also involve H • transfer. Evidence for such a step in the hydroformylation of styrene by HCo(CO) 4 is offered by an inverse H/D kinetic isotope effect ( k (HCo(CO) 4 )/ k (DCo(CO) 4 )) and by the absence of an inverse CO dependence in the rate law. ,4c …”
Section: Introductionmentioning
confidence: 99%
“…Despite the importance of metal-to-olefin H • transfer, few kinetic studies 4b,, have been reported. If the rate-determining step in a hydrogenation is the first H • transfer to the olefin, and the resulting organic radical receives the second H • (rate constant k ‘[M−H]) faster than it transfers the first H • back to M • (rate constant k tr [M • ]) (see Scheme ), it is simple to determine k H from the rate at which M−H disappears in the presence of excess olefin.…”
Treating (etha(5)-C(5)H(5))Cr(CO)3H (1) or (etha(5)-C(5)H(5))Cr(CO)3D (1-d(1)) with an excess of olefin containing the opposite isotope generally leads to H/D exchange, although hydrogenation is also observed in some cases. Application of an appropriate statistical correction to the observed exchange rate gives kH and kD, the rate constants for H* (D*) transfer from (etha(5)-C(5)H(5))Cr(CO)(3)H (D) to various olefins. The values of kH and kD vary appreciably with the substituents on the double bond. Phenyl-substituted olefins accept H* more readily than do carbomethoxy-substituted olefins, although the latter accept H* more readily than do alkyl-substituted olefins. A methyl substituent on the incipient radical site increases k(H) at 323 K by a factor between 5 and 50. A methyl substituent on the carbon to which the H* is being transferred decreases kH substantially. On the whole, the rate constants for H* transfer reflect steric effects as well as the stability of the resulting carbon-centered radicals.
“…In 1977, Sweany and Halpern reported strong evidence for H • transfer in the hydrogenation of α-methylstyrene by HMn(CO) 5 (Scheme ); they observed chemically induced dynamic nuclear polarization (CIDNP) and an inverse isotope effect for HMn(CO) 5 /DMn(CO) 5 . Since then, there have been numerous reports of hydrogenations that occur through H • transfer. , 1 …”
Section: Introductionmentioning
confidence: 99%
“…Hydroformylation reactions may also involve H • transfer. Evidence for such a step in the hydroformylation of styrene by HCo(CO) 4 is offered by an inverse H/D kinetic isotope effect ( k (HCo(CO) 4 )/ k (DCo(CO) 4 )) and by the absence of an inverse CO dependence in the rate law. ,4c …”
Section: Introductionmentioning
confidence: 99%
“…Despite the importance of metal-to-olefin H • transfer, few kinetic studies 4b,, have been reported. If the rate-determining step in a hydrogenation is the first H • transfer to the olefin, and the resulting organic radical receives the second H • (rate constant k ‘[M−H]) faster than it transfers the first H • back to M • (rate constant k tr [M • ]) (see Scheme ), it is simple to determine k H from the rate at which M−H disappears in the presence of excess olefin.…”
Treating (etha(5)-C(5)H(5))Cr(CO)3H (1) or (etha(5)-C(5)H(5))Cr(CO)3D (1-d(1)) with an excess of olefin containing the opposite isotope generally leads to H/D exchange, although hydrogenation is also observed in some cases. Application of an appropriate statistical correction to the observed exchange rate gives kH and kD, the rate constants for H* (D*) transfer from (etha(5)-C(5)H(5))Cr(CO)(3)H (D) to various olefins. The values of kH and kD vary appreciably with the substituents on the double bond. Phenyl-substituted olefins accept H* more readily than do carbomethoxy-substituted olefins, although the latter accept H* more readily than do alkyl-substituted olefins. A methyl substituent on the incipient radical site increases k(H) at 323 K by a factor between 5 and 50. A methyl substituent on the carbon to which the H* is being transferred decreases kH substantially. On the whole, the rate constants for H* transfer reflect steric effects as well as the stability of the resulting carbon-centered radicals.
“…1,2 Early reports employed stoichiometric amounts of metal hydrides and activated alkenes. [3][4][5][6][7][8][9][10][11][12][13] Recently a range of exceptionally useful alkene hydrofunctionalization reactions have been recorded by Mukaiyama, Carreira, Boger, Baran, and others using cobalt-, manganese-, and iron-based catalysts (Scheme 1a). Reports from our laboratory and Shenvi and co-workers detail methods for the reduction of alkenyl halides 43,44 and unactivated alkenes 44 by hydrogen atom transfer.…”
It is shown that the reduction of alkenes by hydrogen atom transfer provides selectivities that are distinct from classical hydrogenation catalysts. The first alkene hydrobromination, hydroiodination, and hydroselenylation reactions that proceed by hydrogen atom transfer processes are also reported.
“…A magnetic field effect on the ratio of the two products would be anticipated. A variety of other organometallic reactions which have been discussed in the recent literature [107][108][109][111][112][113] have similar mechanisms involving radical pairs.…”
Section: The Mechanistic Role Of Outer Sphere Complexesmentioning
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