Carbene–Transition Metal Complexes Formed by Double CH Bond Activation

Abstract

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.

Structures, relative energies, and ligand dissociation energies of iridium carbonyl phosphine clusters

“…If one bridging carbonyl is dissociated from a tetrairidium cluster that has a bridging PH 3 , a H atom transfer always takes place, as shown in Fig. 5(a) (this corresponds to reactions (14), (34), (60), and (79) in Fig. 4).…”

“…Iridium complexes play important roles in oxidation (e.g., of alcohols [4][5][6], phenols [7][8][9], and amines [10,11]), hydrogenation [12,13], C-H activation [14][15][16], cycloaddition (e.g., [2 + 2 + 2] [17][18][19], [2 + 2 + 1] [20,21], and [4 + 2] [22,23]), cycloisomerization [1,2], and ring-opening reactions [24]. The richness of this chemistry explains the broad interest in the properties of the family of iridium complexes and clusters [25].…”

Metal–ligand multiple bonds as frustrated Lewis pairs for C–H functionalization

“…Whited and Grubbs explored the reactivity of a related iridium(I) carbene system, supported by Ozerov's amidophosphine pincer ligand and generated by multiple C–H activations [18,68–70], with a number of heterocumulenes such as those described above. Oxygen-atom, sulfur-atom, and nitrene-group transfers to the carbene were observed when carbon dioxide, carbonyl sulfide, and isocyanates were utilized, cleanly generating the Ir(I) carbonyl as a byproduct (Scheme 11) [71].…”

“…An alternative route involves the generation of a M═E FLP species by multiple C−H (or E–H) activations [18,70,86]. As mentioned above, Whited and Grubbs showed that an iridium(I) carbene system that exhibited FLP reactivity could be generated by multiple C–H cleavage events at tert -butyl methyl ether (MTBE) (Scheme 14) [71,87].…”

Iridium-catalyzed intramolecular [4 + 2] cycloadditions of alkynyl halides

“…Iridium complexes have been used as catalysts for a wide variety of reactions, including homogeneous hydrogenation [1–2], C–H activation [3–5], asymmetric ring-opening reactions [6], and a variety of cycloisomerizations [7–10], and cycloadditions [11–20]. Traditionally these types of cycloisomerization and cycloaddition reactions are possible by making use of other metal complexes such as Pd [21–22] and Co [23–25], but recent advances in iridium chemistry have expanded the scope of the metal complexes that can be used.…”