Carbene-transition metal complexes formed by double C-H bond activation

The activation of a single sp(3) C-H bond of alkanes and their derivatives by electron-rich transition metal complexes has been a topic of interest since the landmark work by Bergman and Graham in 1982. Ten years later, it was shown that compounds of 5d elements, such as osmium and iridium, even enable a double alpha-C-H bond activation of alkane or cycloalkane derivatives containing an OR or NR(2) functional group, thus opening up a new route to obtain Fischer-type transition metal carbene complexes. Subsequent work focused in particular on the conversion of methyl alkyl and methyl aryl ethers into bound oxocarbenes and also of dimethyl amines to bound aminocarbenes. In the context of this work, it was recently shown that square-planar oxocarbene-iridium(I) complexes prepared in this way exhibit an unusual mode of reactivity: They react with CO(2), CS(2), COS, PhNCO, and PhNCS by an atom- or group-transfer metathesis, which has no precedent. Organic azides RN(3) and N(2)O behave similarly. Recent results confirm that this novel type of metathesis can be made catalytic, thus offering a novel possibility for C-H bond functionalization.