Oxidation steric protection

In the light of the electrochemical data it seems reasonable to propose that in cases where the substituent provides effective steric protection of the central metal, dioxygen can act as a simple (outer-sphere) M(I)-to-M(II) oxidizing agent (reversible one-electron oxidation). In contrast, in those cases where the substituent offers insufficient steric protection, dioxygen can coordinate to the metal by oxidative addition forming the peroxo compound M(III)-022 (irreversible two-electron oxidation). 

Arnold et alP reported the synthesis and structural characterization of an aryl-oxide-functionalized samarium silsesquioxane complex. Steric protection of 2 by one SiMe2tBu group generated a new disilanol ligand, (c-C5H9)7Si709(0H)2(0SiMe2tBu)... 

The next landmark was the synthesis of the germylium, 22, and the stannylium ion, 23, by one-electron oxidations from the corresponding stable radicals with trityl TPFPB by Sekiguchi and co-workers. As in the case of the allyl cleavage to generate the mesityl-substituted cations, the reaction, in this case the oxidation, occurs at the periphery of the molecule and gives the possibility for efficient steric protection of the incipient cation. Both trivalent cations were obtained as their TPFBP salts and the crystal structure show well separated anions and cations. 

Despite the radical character of these autoxidations, they may exhibit some selectivity. This can, for example, be seen in a recent report by Kim et al., which notes the selective transformation of a titaniiun(IV) diamido dimethyl complex into a mixed alkoxide alkyl (Eq. 15) and the complete lack of reactivity of the corresponding dibenzyl complex with O2 [35]. Both of these observations are probably the result of steric protection. When the metal complex becomes too hindered to allow close interaction with O2, the oxidation can no longer be initiated. 

Carbonyl oxides are usually highly unstable and can be detected only by matrix isolation spectroscopy at low temperature or by FFP techniques. However, they can be stabilized through steric protection. Dimesityl ketone oxide (80c, ),niax = 398 nm) generated by the reaction of triplet dimesitylcarbene (19c) with O2 at... 

Dramatic shape selectivities in competitive olefin epoxidation was observed with picnic basket metalloporphyrins312 313 designed to exclude bulky axial ligands on one sterically protected porphyrin face. When oxidized with PhIO in acetonitrile in the presence of the rigid p-xylyl-strapped porphyrin, cis-2-octene reacted selectively versus ds-cyclooctene or 2-methyl-2-pentene, giving >1000 reactivity ratios.313,314 Some immobilized manganese(III) porphyrins proved to be as efficient as their homogeneous equivalents in epoxidation with PhIO.151,315... 

The absence of a convenient method for the preparation of enethiolisab-le thioketones prompted us to fulfil this need. The development of aliphatic thioketones has so far been mostly restricted to examples with steric protection, such as thiocamphor and adamantanethione. Among the difficulties for a general synthesis of aliphatic and acyclic thioketones were easy enethiolisation (enethiols are stable tautomers with thermodynamic stabilities close to those of the thione form), oligomerisation, and susceptibility to air oxidation. They were solved by the adaptation of a former German reac-... 

The C2 - H oxidative addition of imidazolium salts to metal complexes was recently proved for metals other than low valent group 10. The reaction of a ferrocenyl-bisimidazolium salt to [IrCl(cod)]2 in the presence of NEt3 provided the first evidence of the preparation of a stable NHC-Ir(III)-H complex by direct oxidative addition of the imidazolium salt [96]. It was proposed that the ferrocenyl fragment may be sterically protecting the M - H from further reductive elimination, but later it was shown that this fragment was not necessary in order to obtain the desired NHC-Ir(III)-H complexes (Scheme 43) [159]. The role of the weak base (NEt3 in this case) had to be reconsidered in order to explain the overall metallation process, and it was proposed that a mechanism as that shown in Scheme 44 may better explain the process. The oxidative addition of the C2 - H bond of the imida-... 

A variety of fundamental reactions have been investigated for the M=C and M=M double-bond species of heavier group 14 elements. In spite of the steric protection by bulky substituents, stable unsaturated compounds of heavier group 14 elements described here are often the snbject of many reactions such as oxidation reactions, addition reactions, and so on. 

Enols sterically protected at the -carbon have also been investigated. In general, the cyclic voltammetric response exhibits only the already mentioned ECE process, while the reversible benzofuran oxidation is not detected. The pertinent electrode potentials are compiled in Table 2. In the case of the enolato species, the ECE process probably proceeds through separated one-electron oxidations as those of Table 1. 

Other groups, one of which being Traylor et al. [206], subsequently applied this new concept in the synthesis of so-called second-generation metalloporphyrins. The presence of a great number of chloro substituents on the meso aryl rings renders the oxometal active species much more reactive and provides steric protection of the vulnerable meso positions. Remarkable stabihty of these new catalysts was observed even in the presence of very strong oxidative... 

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