Hydrogenation of .alpha.-methylstyrene by hydridopentacarbonylmanganese (I). Evidence for a free-radical mechanism

Free radical mechanisms in coordination chemistry

“…The emphasis on such processes has been reinforced by the highly novel chemistry associated with them, as well as by important and distinctive applications, notably homogeneous catalytic processes of unusual selectivity such as asymmetric catalytic hydrogenation (S) . In view of this, it is noteworthy that some of the most recent developments in the fields of homogeneous catalysis and coordination chemistry have once again focussed attention on free radical mechanisms and suggest that such mechanisms are more important and widespread than previously suspected, not only in new contexts but also in the context of processes, for ex~ple certain hydrogenation and hydroformylation reactions, that have prev~ously been interpreted in terms of non-radical mechanisms (6)(7)(8)(9). The present paper describes and discusses some of these developments and their implications.…”

Kinetic Destabilization of Metal–Metal Single Bonds: Isolation of a Pentacoordinate Manganese(0) Monoradical

“…[1][2][3] Ap rototypical example of this class is the zerovalent manganese pentacarbonyl radical [Mn(CO) 5 ]which has been extensively investigated for its role in radical-type olefin hydrogenation, [4,5] [7][8][9][10] or at higher temperatures by using either ultrafast spectroscopic [11] or nitrosoarene spin-trapping techniques. [12][13][14] Importantly,efforts to prepare stabilized variants of [Mn(CO) 5 ]bysubstitution of one or more CO ligands with more encumbering ligands such as phosphines (PR 3 )h ave been shown to extend the lifetime of [MnL 5 ]-type radicals to the order of hours in solution.…”

“…[1][2][3] Ap rototypical example of this class is the zerovalent manganese pentacarbonyl radical [Mn(CO) 5 ]which has been extensively investigated for its role in radical-type olefin hydrogenation, [4,5] as well as in the photolytic cleavage of the Mn À Mn bond of [Mn 2 (CO) 10 ]. [6] Despite these studies,[ Mn-(CO) 5 ]iswell recognized as ahighly unstable,organometallic transient, which rapidly dimerizes to [Mn 2 (CO) 10 ]byMn ÀMn bond formation.…”

“…[1][2][3] Ap rototypical example of this class is the zerovalent manganese pentacarbonyl radical [Mn(CO) 5 ]which has been extensively investigated for its role in radical-type olefin hydrogenation, [4,5] as well as in the photolytic cleavage of the Mn À Mn bond of [Mn 2 (CO) 10 ]. [6] Despite these studies,[ Mn-(CO) 5 ]iswell recognized as ahighly unstable,organometallic transient, which rapidly dimerizes to [Mn 2 (CO) 10 ]byMn ÀMn bond formation. Accordingly,this instability has necessitated that [Mn(CO) 5 ]becharacterized and studied under cryogenic conditions, [7][8][9][10] or at higher temperatures by using either ultrafast spectroscopic [11] or nitrosoarene spin-trapping techniques.…”

“…[6] Despite these studies,[ Mn-(CO) 5 ]iswell recognized as ahighly unstable,organometallic transient, which rapidly dimerizes to [Mn 2 (CO) 10 ]byMn ÀMn bond formation. Accordingly,this instability has necessitated that [Mn(CO) 5 ]becharacterized and studied under cryogenic conditions, [7][8][9][10] or at higher temperatures by using either ultrafast spectroscopic [11] or nitrosoarene spin-trapping techniques. [12][13][14] Importantly,efforts to prepare stabilized variants of [Mn(CO) 5 ]bysubstitution of one or more CO ligands with more encumbering ligands such as phosphines (PR 3 )h ave been shown to extend the lifetime of [MnL 5 ]-type radicals to the order of hours in solution.…”