Synthesis, characterication and catalytic applications of organo-ruthenium(III) and -iron(III) porphyrins in sp³ C-H bond oxidation and amination reactions

Abstract

Transition metal-catalyzed oxo- and nitrene-transfer reactions to hydrocarbons are highly valuable and attractive protocols for the synthesis of many different bioactive and pharmaceutically useful molecules by providing streamlined synthetic routes, minimizing wastage of reagents and sometimes imparting high stereo-, chemo- and regio-selectivities. Although the reports of such group transfer reactions are burgeoning in recent decades, use of large excess substrates and expensive transition metals to achieve high yield makes these methodologies impractical. Furthermore, mechanistic insights into metal-oxygen and -nitrogen multiple bonds are important to the development of group transfer reactions. Herein, organo-ruthenium(III) and -iron(III) porphyrinato complexes were employed as new and novel catalysts for oxo- and alkylnitrene-transfer reactions respectively, in which mechanistic experiments were also included. First, new organo-ruthenium(III) porphyrin complexes [RuIII(por)Ph] which were well-characterized by a series of experiments, including 1H NMR, UV-vis, ESI-MS+, CV and X-ray crystallography, were used for catalytic epoxidation and oxidation of C-H bonds. Gratifyingly, up to 99% yields were observed for the C-H oxidation (oxidant: m-CPBA or [nBuN4]IO4) and epoxidation reactions (oxidant: [nBuN4]IO4) using 1 equiv. of substrate at ambient conditions. More importantly, the elusive Ru(V)-oxo species was stabilized by the strongly σ-donating phenyl ligand and hence spectroscopically characterized by X-band EPR, rR, UV-vis, ESI-MS+ experiments and examined with DFT calculations and kinetic experiments. Kinetic experiments confirmed that [RuV(por)(O)(Ph)] oxidizes hydrocarbons much faster than [RuVI(por)(O)2]. Second, a new Fe(III) NHC porphyrinato complex ([FeIII(TDCPP)(IMe)2]I) outcompeted all other known Fe catalysts for intramolecular sp3 C-H bond amination using alkyl azides without the discrimination of any type of C-H bonds and degree of alkyl azides (1o - 3o) at 10 mol% catalyst loading. In this work, a good regio-selectivity (this work = 5:1 vs literature = 0.5:0.75) was found when the C4-H and C5-H bonds were secondary and tertiary C-H bonds respectively. Furthermore, this protocol allows the direct syntheses of some highly important alkaloids, such as tropane, nornicotine and their analogues via C-H amination with moderate yields (up to 70%). Besides, a N-Boc protected cis-octahydroindole product can also be exclusively obtained with 68% yield through exocyclic 1o C-H bond amination. Third, high-valent Ru(VI)-alkylimido porphyrin complexes namely [RuVI(TDFPP)(NAd)2] and [RuVI(TDFPP)(NAd)(O)], were synthesized based on the reaction between the highly reactive [RuII(TDFPP)(CH3CN)2] and AdN3 (Ad = adamantyl). This reaction is more favourable than the metathesis between [RuVI(por)(O)2] and RNH2 as no heating and hygroscopic reagent are needed. [RuVI(TDFPP)(NAd)(O)] which was the first X-ray crystallographically characterized Ru alkylimido complex, was the hydrolyzed product of [RuVI(TDFPP)(NAd)2]. Because of its extreme moisture sensitivity, no X-ray crystal structure but 1H NMR spectrum was available for [RuVI(TDFPP)(NAd)2]. Instead, [RuII(TDFPP)(NH2Ad)2] was obtained during the crystallization of [RuVI(TDFPP)(NAd)2] in anhydrous n-hexane. Neither [RuVI(TDFPP)(NAd)2] nor [RuVI(TDFPP)(NAd)(O)] was capable of transferring its alkylimido moiety (NAd) to styrene, but the latter can transfer its oxo ligand to styrene, affording styrene oxide in >99% yield. It was believed that imido with electron-withdrawing/π-conjugative groups like NTs and NAr were more electrophilic and hence more reactive towards aziridination reaction.