Studies on transition metal-catalysed oxidations with hydrogen peroxide as terminal oxidant

In the last few decades, hydrogen peroxide has emerged as an environmentally viable alternative in many oxidative processes. This oxidant does not require the use of a co-reductant and in the presence of non-toxic, inexpensive metals, such as iron, effects homogeneous liquid phase oxidations, producing water as the sole by-product. This "green" combination represents a highly desirable solution to the environmental issue. Following these guidelines, hydrogen peroxide was chosen as terminal oxidant for the oxidation of unreactive substrates and for the oxidative cleavage of double bonds. This thesis work was therefore divided in three sections.In the first part an iron-catalysed oxidation system for saturated hydrocarbons, using hydrogen peroxide as oxidant, has been developed, providing a promising tool for large-scale applications.The method exhibited great advantages: the reaction proceeds under truly catalytic conditions, the catalyst does not require any ligand, and water is the only by-product formed. The presence of an additive, i.e. a carboxylic acid, was found to enhance the system and the effect of different acids was studied. To extend the scope of the reaction, several substrates were tested, such as arylalkanes, cycloalkanes and alcohols. Arylalkanes were easily converted into the corresponding ketones with excellent selectivities and the less reactive and more challenging cycloalkanes gave the corresponding alcohols and ketones with yields comparable to the best reported in literature so far. The system presented peculiar selectivity parameters that are consistent with the involvement of both alkylhydroperoxide or of Fe(OOH) species. In situ IR studies were then conducted to elucidate some mechanistic aspects. In the second part, the applicability of the Fe(ClO4)2/CH3COOH/H2O2 system to the oxidative cleavage of double bonds was demonstrated. The reaction conditions were studied and optimised, showing that the presence of an additive, namely acetic acid, is essential to enhance the feature of the system. Substrates with various substitution pattern were considered, revealing a limitation of the applicability of the protocol to alpha,alpha-disubstituted styrenes, which were easily and selectively transformed into the corresponding acetophenone derivatives in good yields.The third part of this thesis was dedicated to the synthesis and application of new chiral C1-symmetric triazacyclononanes. Amino acids were chosen as readily available source of chirality and the Crab-like methodology proved to be the ideal synthetic approach, granting the desired flexibility. Transition metal complexes of triazazcyclononanes are known to be efficient and selective catalyst for the CH oxidation of unreactive substrate using hydrogen peroxide as oxidant. The new C1-symmetric TACNs were employed in the Mn- and Fe-catalysed oxidation of ethylbenzene with H2O2 as oxidant and acetic acid as additive. Although only low conversions were obtained when iron was used as metal source, the system proved to be able of transferring the chiral information into the catalytic process, thus representing the first example of enantioselective CH-activation catalysed by in situ formed iron-TACN complexes. The chiral C1-symmetric TACN demonstrated activity also in the enantioselective Mn- catalysed epoxidation of simple olefins, with moderate to good conversions and modest ee. A second approach towards the enantioselective CH oxidation of hydrocarbons was also evaluated, using an isolated Mn-TMTACN complex in combination with chiral carboxylic acids