Cage effects in organometallic radical chemistry. Fractional cage-recombination efficiency for photochemical caged-pair intermediates of Cp'2M2(CO)6 (M = molybdenum and tungsten; Cp' = .eta.5-C5H4CH3)

Cycloisomerization of Olefins in Water

“…It is noteworthy that for change in ratio to occur upon dilution, escape from the initial Co +2 / alkyl‐radical cage should be faster than the rate of isomerization (i.e. fractional cage‐recombination efficiency is probably low) . In other words, the relevant pathway for isomerization is bimolecular, inconsistent with reaction from the initial cage pair, despite a required movement of only a few angstroms and H .…”

Catalytic hydrogen atom transfer to alkenes: a roadmap for metal hydrides and radicals

“…For metalloradicals with strong eld ligands, stabilizing interactions between the SOMO of the 17-electron metalloradical and the Lewis base lone pair have been suggested to form an energetically-stabilizing 2-center, 3-electron bond, sometimes referred to as a "19-electron" species. 77,78,83 Although transiently formed "19-electron" reactive intermediates are speculative in MHAT catalyst systems, the identication of a "19-electron" dimethylglyoximato cobalt complex (Co(dmg) 2 (pyr) 2 ) complex may be relevant, since the corresponding metal-hydride (or its ligand bound equivalent) undergoes MHAT to electronically-activated alkenes. 84 Less is known about stabilizing interactions between WF MHAT "metalloradicals" and solvent, although calculations on Fe 2+ (acac) 2 showed that binding of two molecules of ethanol had a net DG ¼ À0.6 kcal mol À1 .…”

“…Pioneering work by Koenig, Finke and Tyler established that “cage effects” for metallo/organic radical pairs required distinctive mechanistic considerations from their organic radical counterparts. 57 , 77 , 78 Similar to cage efficiencies described for organic radical pairs, fractional cage efficiency, F c , was defined as the ratio of rate constants for cage collapse ( k collapse ) to the sum of all pathways from the solvent cage ( k collapse and k diffusion ) (eqn (1)); a higher F c indicated a “stronger” cage effect. 59 In the presence of a suitable trapping reagent, even at modest concentrations (0.1 M), the observed rate of trapped product formation ( k obs ) would be decreased by F c , as not all radical cage pairs formed ( k 1 ) would proceed toward product due to competition with cage collapse pathways (eqn (2)), noting that F c for most metal–ligand homolytic bond dissociations had a value ranging from 0.1 < F c < 0.9.…”

“… 59 In the presence of a suitable trapping reagent, even at modest concentrations (0.1 M), the observed rate of trapped product formation ( k obs ) would be decreased by F c , as not all radical cage pairs formed ( k 1 ) would proceed toward product due to competition with cage collapse pathways (eqn (2)), noting that F c for most metal–ligand homolytic bond dissociations had a value ranging from 0.1 < F c < 0.9. 78 Of particular note are the incredibly high F c values measured for many organocobalt complexes; in the case of adocobinamide (coenzyme B 12 ) in ethylene glycol, the F c was measured to be greater than 0.94. 58 F c = k collapse /( k collapse + k diffusion ) k obs = [1 – F c ] × k 1 …”

Cycloisomerization of Olefins in Water

“…At a1 00 mm concentration, linear isomerization was favored 1.2:1, whereas at 10 mm and at 1mm cycloisomerization predominated in ratios of 3:1and greater than 10:1, respectively.Itisnoteworthy that for change in ratio to occur upon dilution, escape from the initial Co +2 /a lkyl-radical cage should be faster than the rate of isomerization (i.e.fractional cage-recombination efficiency is probably low). [27] In other words,t he relevant pathway for isomerization is bimolecular,i nconsistent with reaction from the initial cage pair, despite ar equired movement of only af ew angstroms and HC abstraction. Bimolecular catalyst death (hydrogen evolution) may produce aC o +2 species capable of back-HAT, which reinforces this correlation between linear isomerization and concentration.…”