T-BuOO

The effect of solvation on the cr-effect has been studied by Curci and Di Furia (1975) for the reactions of t-BuOK and t-BuOOK with p-nitrophenyl diphenylphosphinate [1141 and p-nitrophenyl benzoate [115]. In water the cr-effects (rate ratios for t-BuOO and CF5CH20-) were 6.6 for [ 114] and 5.5... 

Complex (1) is a catalyst for selective oxidation of benzylic, allylic alcohols to aldehydes, and secondary alcohols to ketones using r-butyl hydroperoxide. Primary aliphatic alcohol oxidation failed. The use of cumyl hydroperoxide as radical probe discounted the involvement of i-BuO /t-BuOO. Hammett studies p = -0.47) and kinetic isotope effects kn/ku = 4.8) have been interpreted as suggesting an Ru—OO—Bu-i intermediate oxidant. 

The proposed mechanism (Scheme 1) involves the mixed-valence compounds [Rh2" " ( Ji-cap)4(OH)] and [Rh2 (p.-cap)4(OOt-Bu)] formed from the homolytic cleavage of t-BuOOH. The t-BuOO radicals in the medium promote a selective hydrogen abstraction from the alkene to give the allylic alkenyl radical. This species traps the peroxide in [Rh2 (p.-cap)4 (OOt-Bu)] to produce the alkenyl hydroperoxide, which rapidly decomposes to the isolated products, thus regenerating the catalyst. 

The sterically crowded ligand (106) forms a very rare square-planar Co complex, as shown by an X-ray crystal structure, and a spin triplet (paramagnetic) ground state was also identified in the solid state. The Co complex of (107) catalyzes the epoxidation of norbomene with t-BuOOH or Phi as terminal oxidant, catalysis driven by formation of t-BuOO radicals employing a Co redox process. 

In the case of HOO (and t-BuOO ), the general leveling of nucleophilic reactivity by protic solvents (water and alcohols) enhances its lifetime such that the net reactions for HOO with electrophilic substrates usually are most efficient and complete in H2O or t-BuOOH. Almost all other solvents react with HOO or facilitate its decomposition. The relative lifetime of HOO in various solvents is in the order H2O MeOH > Eton diglyme > pyridine PEGM 350 ) > MeCN > DMF DMSO. ... 

The recently estimated rate constants (116,161) for the identity reactions. buo/ -buoh (3 x lO M s" ) and k<.BuOO/ -BuOOH (5x10 M s ) in non-aqueous solvents differ by a factor of 60, which will cause only a modest, 8-fold increase in the estimated kf.Buolkt-BuOO The reasonable closeness between /sj.BuO/i-BuOH and / t.BuOO/i-BuOOH strengthens the assumption made for the chromium couples, i.e., that ( 22/ 33)° 1 is probably also correct to within an order of... 

The lack of (r-BuO)2 establishes that eqs 9 and 10 are unimportant at the low [r-BuOOH] used. Although t-BuOO was detected by spin trapping, the bimolecular reaction of eq 11 is also of little consequence since it is not competitive with unimolecular fragmentation of r-BuO at low [r-BuOOH]. t-BuOO is likely to result from the reaction of eq 12. 

TABLE 9. Reactivity of carbon-hydrogen bonds towards t-BuOO at 30... 

The value for reaction of t-BuOO with 3-methylpentane is 1603 0.07 for 3°-attack and 17.4 + 0.8 kcal mol for 2°-attack. Surprisingly, the intermolecular hydrogen-deuterium primary isotope effect is 24-28 (as measured with methylcyclohexane-di4 or 3-methylpentane-di4) and is much greater than the theoretical limit of 10 for complete loss of the zero-point energies for carbon-hydrogen bonds in the transition state . 

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