Hydrogen atom transfer reactions of transition-metal hydrides. Kinetics and mechanism of the hydrogenation of .alpha.-cyclopropylstyrene by metal carbonyl hydrides

Transition-metal hydrides generate α-alkoxy radicals by H• transfer to enol ethers. We have measured the rate constant for transfer from CpCr-(CO) 3 H to n-butyl vinyl ether and have examined the chemistry of radicals generated by such transfers. Radicals from appropriate substrates undergo 5-exo cyclization, with higher diastereoselectivity than the analogous allcarbon radicals. From such radicals it is straightforward to make substituted tetrahydrofurans.

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“…4 This gives k D = 0.0152(1). We use the k H /k D = 0.45 reported by Bullock and coworkers 5 , resulting in k H = 0.00684 (5).…”

Section: S12 Nmr Kinetic Measurementsmentioning

confidence: 99%

Direct Generation of Oxygen-Stabilized Radicals by H• Transfer from Transition Metal Hydrides

et al. 2015

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“…Fortunately, as the reaction shown in equation (30) demonstrates, more crowded substrates do not give products containing palladium. The selectivity of this reaction is an improvement over the thermal rearrangement, which, carried out at 180 • C for 13 h, produces the (E)-isomer in 75% yield.…”

Section: Group Migrationsmentioning

confidence: 99%

“…The organic radicals can be trapped by [Co II (dmgH)(PBu 3 )], but more conventional radical scavengers react preferentially with the hydrido complexes present. 30 A second example of an undoubted radical mechanism occurs during the vitamin B 12s reduction of either (E)-or (Z)-butendioic acid in aqueous solution. However, the point is better illustrated by the reduction of butyndioic acid 31 to (E)-butendioic acid by two molecules of vitamin B 12s .…”

Section: Radical Hydrogenation Of Alkenesmentioning

confidence: 99%

“…Unfortunately, more heavily substituted hexadienes than those shown in equation(30) do not undergo the rearrangement, presumably because they have very low binding constants with catalytic palladium(II) species. Nevertheless, the Claisen rearrangement of allyl vinyl ethers can only take place provided palladium coordination to the vinyl group is prevented by alkyl substituents 25.…”

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confidence: 99%

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Hydrogenation & Isomerization of Alkenes

Jardine¹

2005

Encyclopedia of Inorganic and Bioinorganic Chemistry

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Alkenes are isomerized by complexes of late, second or third series transition metals. This occurs either by successive β‐hydride abstraction reactions or, more rarely, by a π–allylic mechanism. Both decyclization and annulation reactions of strained, cyclic hydrocarbons are catalyzed by these complexes. Palladium complexes are active catalysts for the Cope‐ and Claisen rearrangements. Homogeneous, catalytic hydrogenation can occur via three possible routes. Ruthenium complexes catalyze transfer hydrogenation. Catalyst modifications can be made to minimize by‐product formation during hydrogenation or deuteriation.

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“…To date, the only isotope effect reported for Cp(CO) 3 CrH, by Bullock et al . [58] , is k H / k D = 0.45 ± 0.10 for its reaction with 2 -cyclopropylpropene at 68 ° C. We have used 0.45 (the effect of the temperature difference is negligible compared to the uncertainty) to estimate the k H values for hydrogen atom transfer to olefi ns; we have then calculated the relative rates from these k H values to deduce the chemoselectivity of this step [61] . Some of the olefi ns in Table 1 5) .…”

Section: H • Transfer Between M -H and C -C Bondsmentioning

confidence: 99%