Transition Metal Ion Initiated Chain Reactions between tert-Butyl Hydroperoxide and Rhodium Hydrides. Rapid Hydrogen Atom Abstraction from Rhodium Hydrides by Methyl Radicals

Abstract

In the absence of catalysts, there is no reaction between t-BuOOH and macrocyclic rhodium hydrides LRhH(2+) (L(1) = [14]aneN(4), L(2) = Me(6)-[14]aneN(4)). The addition of Fe(aq)(2+) to such solutions results in rapid and catalytic oxidation of rhodium hydrides (LRhH(2+) + (CH(3))(3)COOH + H(+) --> LRh(3+) + CH(4) + (CH(3))(2)CO). This reaction occurs by a chain mechanism featuring methyl radicals and LRh(2+) as reactive intermediates. The rates are independent of the absolute concentrations of iron but vary with the ratio of the two oxidation states, -d[LRhH(2+)]/dt = k(obs)[t-BuOOH](2)[Fe(2+)]/[Fe(3+)]. The rate constants k(obs) are directly proportional to [H(+)]. In 0.10 M aqueous HClO(4) at 25 degrees C, the values are 80.8 M(-)(1) s(-)(1) (L(1)RhH(2+)) and 7.0 M(-)(1) s(-)(1) (L(2)RhH(2+)). A macrocyclic cobalt(II) complex, L(2)Co(2+), is also an effective chain initiator. The reaction occurs according to the rate law -d[L(1)RhH(2+)]/dt = 254[t-BuOOH][L(1)RhH(2+)], independent of [H(+)]. This reaction is not catalytic in L(2)Co(2+), which is slowly consumed in the initiation and termination steps. Methyl radicals react rapidly with L(1)RhH(2+) (k = 1.0 x 10(9) M(-)(1) s(-)(1)), L(2)RhH(2+) (1.4 x 10(8)), and L(1)Rh(2+) (8.6 x 10(8)).

Kinetics and Mechanism of Hydrogen‐Atom Abstraction from Rhodium Hydrides by Alkyl Radicals in Aqueous Solutions

“…[10] We have now extended our study to benzyl radicals, which absorb light in the near-UV (l max 317 nm, e max 5500 m À1 cm…”

“…[10] We have now extended our investigation to the measurement of the kinetic isotope effect (KIE) for the reaction of…”

“…All the kinetic and product data support a hydrogen-atom transfer mechanism for reactions (1) and (2), as we proposed initially for methyl radicals. [10] The difference in reactivity between methyl and benzyl radicals toward the (Table 1). In the re-…”

“…2 + would seem sufficient to make reactions (10) and (11) competitive with the radical self-reaction, and to lead to an incorrect (underestimated) value for k 1 .…”

“…Kinetic data for hydrogen-atom abstraction from rhodium(I) hydrides by benzyl radicals, [9] and rhodiumA C H T U N G T R E N N U N G (III) hydrides by alkyl radicals, [9b, 10] superoxometal complexes, [11] and complexes of Cr IV and Cr V have also been reported. [12] Several years ago [10] we examined the reaction between methyl radicals and macrocyclic rhodiumA C …”