Trapping of an Organic Radical by an O=CrVI Function

Abstract

Not only capable of generating organic radicals by H abstraction, chromyl chloride can also trap them again (see scheme). This is confirmed by the crystal structure of a Cr(V) oxo alkoxide, which was formed in three successive steps when bisadamantylidene oxide was allowed to react with CrO(2)Cl(2).

Syntheses, Structures, and Properties of Oxovanadium(V) Complexes with Ketone and Ketolate Ligands

“…A comparison reveals the excellent agreement between experiment an theory here, confirming the validity of our theoretical approach in the previous question with regard to the corresponding chromium system [4], as outlined above. As already assumed for 1 on the basis of such DFT calculations, in the vanadium complex it now becomes obvious also experimentally, that the α-keto H atom is in fact located directly beside the VϭO bond (2.56 Å ), stressing our previous statements for 1 [4]. The corresponding C-H bond should thus analogously be activated for an H atom transfer reaction even though 3 is stable at room temperature.…”

“…This is remarkable since C-H activation by chromyl chloride normally proceeds only at temperatures around 70°C with sufficient rates and then not in a selective fashion [5]. To explain this finding, we concluded that initially the keto function of L 1 coordinates to the chromium centre thereby locating the α-keto C-H bond in close proximity to one metal oxo bond in the corresponding complex 1 [4]. The system should thus be positioned quite high on the C-H activation (or more correctly the H atom transfer) barrier, already, and consequently in analogy to the cytochromes P450 an increase in reactivity and selectivity is reached.…”

“…Letzterer stellt eine mononukleare Oxovanadium(V) Verbindung mit einer sterisch anspruchsvollen Ligandensphäre dar, in der eine Koordinationsstelle zugänglich ist (der Triflat-Ligand ist nur schwach gebunden), und da sie zudem in organischen Solventien löslich ist, handelt es sich um ein interessantes Ausgangsmaterial für zukünftige Experimente. the natural reaction principles are applied [4]. The reaction of chromyl chloride with the spiroketone L 1 (see scheme 1) leads to the selective cleavage of only the α-keto C-H bond finally yielding the Cr V complex 2 under very mild conditions (Ϫ50°C).…”

“…There have been many attempts to model the function of such enzymes by means of molecular Fe(porphyrin) complexes [3]. We have recently demonstrated that even in the absence of ligands occurring in biological systems selective oxygenations can be performed making use of highly reactive metal oxo compounds, if only the natural reaction principles are applied [4]. The reaction of chromyl chloride with the spiroketone L 1 (see scheme 1) leads to the selective cleavage of only the α-keto C-H bond finally yielding the Cr V complex 2 under very mild conditions (Ϫ50°C).…”

A Chromium(II) Tetracarbene Complex Allows Unprecedented Oxidative Group Transfer

“…While chromium oxo and imido complexes have been synthesized via oxidative group transfer, they are almost all high valent Cr­(V) and Cr­(VI) complexes supported with auxiliary ligands such as porphyrins, phthalocyanins, salens, or β-diketiminates. − Furthermore, these high valent chromium complexes often showed limited reactivity for reductive transfer of the oxo or imido. ,,,− While lower valent chromium complexes may have more facile group transfer of an imido − or oxo, − only a very limited number of these complexes have been synthesized (Figure ).…”