Oxidation paramagnetic intermediates, reductive

Early attempts to fathom organic reactions were based on their classification into ionic (heterolytic) or free-radical (homolytic) types.1 These were later subclassified in terms of either electrophilic or nucleophilic reactivity of both ionic and paramagnetic intermediates - but none of these classifications carries with it any quantitative mechanistic information. Alternatively, organic reactions have been described in terms of acids and bases in the restricted Bronsted sense, or more generally in terms of Lewis acids and bases to generate cations and anions. However, organic cations are subject to one-electron reduction (and anions to oxidation) to produce radicals, i.e.,... 

The reduction of neutral carbonyls with alkali metal hydroxides or carbonates is an important route to HNCC dianions. In a few instances reduction of rhodium clusters has been reported to proceed further with oxidation of another CO. Examples are the formation of [Rh6(CO)i4] from [Rh6(CO)i5] 170) (Scheme 16) and the reduction of the dicarbide [Rhi2(C)2(CO)24] to give [Rhi2(C)2C023] 213). In the latter case, the unstable paramagnetic intermediate [Rh 12(0)2(00)23] has also been isolated upon slight alteration of the reaction conditions [Eqs. (32) and (33)]. 

B. Paramagnetic Intermediates in Oxidatively Induced Reductive Eliminations / 291... 

The detailed mechanism of P aeruginosa CCP has been studied by a combination of stopped-flow spectroscopy (64, 65, 84, 85) and paramagnetic spectroscopies (51, 74). These data have been combined by Foote and colleagues (62) to yield a quantitative scheme that describes the activation process and reaction cycle. A version of this scheme, which involves four spectroscopically distinct intermediates, is shown in Fig. 10. In this scheme the resting oxidized enzyme (structure in Section III,B) reacts with 1 equiv of an electron donor (Cu(I) azurin) to yield the active mixed-valence (half-reduced) state. The active MV form reacts productively with substrate, hydrogen peroxide, to yield compound I. Compound I reacts sequentially with two further equivalents of Cu(I) azurin to complete the reduction of peroxide (compound II) before returning the enzyme to the MV state. A further state, compound 0, that has not been shown experimentally but would precede compound I formation is proposed in order to facilitate comparison with other peroxidases. 

Dicyclopentadienyl derivatives in a formal oxidation state of 3.5 appear to be relatively common. A purple, paramagnetic compound with one impaired electron (1.32 BM) for two Ta atoms was isolated and formulated as a dimer (65 equation 93). 729 A similar reaction did not yield the niobium analog, but violet or red-brown compounds were observed as intermediates in the reduction of [MX2(Cp)2] to [MX(Cp)2] (X = C1, Br). They could be isolated in good yields ( 60%, Scheme 11).730... 

In the past three decades, evidence for enzyme-based protein radicals as the catalytic driving force or the key transient intermediate in enzymatic reactions has strengthened (11). Spectroscopic data from EPR suggest that these highly reactive species can be stored with remarkable stability inside an enzyme either as a transient intermediate or as a paramagnetic center and catalyze a wide range of biological reactions in electron transfer and oxidation/reduction. 

Rh U) and Ir(II) Species. Quite a large number of mononuclear (f Rh(II) and Ir(II) species have been isolated. In fact, formation of these species by either oxidation of the M(I) precursors (Table XV) or reduction of the M(III) precursors (Table XVI) occurs at remarkably accessible redox potentials. This suggests that such species could well play an important role in catalytic reactions mediated by these metals. Regarding their high reactivity, formation of Rh(II) and Ir(II) species will easily lead to catalyst deactivation. The paramagnetic (f M(II) species could even be active intermediates, although proven examples of catalytically active M(ll) species are (stUl) quite limited. 

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