Redox Noninnocence of Carbene Ligands: Carbene Radicals in (Catalytic) C−C Bond Formation

Abstract

In this Forum contribution, we highlight the radical-type reactivities of one-electron-reduced Fischer-type carbenes. Carbene complexes of group 6 transition metals (Cr, Mo, and W) can be relatively easily reduced by an external reducing agent, leading to one-electron reduction of the carbene ligand moiety. This leads to the formation of "carbene-radical" ligands, showing typical radical-type reactivities. Fischer-type carbene ligands are thus clearly redox-active and can behave as so-called "redox noninnocent ligands". The "redox noninnocence" of Fischer-type carbene ligands is most clearly illustrated at group 9 transition metals in the oxidation state II+ (Co(II), Rh(II), and Ir(II)). In such carbene complexes, the metal effectively reduces the carbene ligand by one electron in an intramolecular redox process. As a result, the thus formed "carbene radicals" undergo a variety of radical-type C-C and C-H bond formations. The redox noninnocence of Fischer-type carbene ligands is not just a chemical curiosity but, in fact, plays an essential role in catalytic cyclopropanation reactions by cobalt(II) porphyrins. This has led to the successful development of new chiral cobalt(II) porphyrins as highly effective catalysts for asymmetric cyclopropanation with unprecedented reactivity and stereocontrol. The redox noninnocence of the carbene intermediates results in the formation of carbene-radical ligands with nucleophilic character, which explains their effectiveness in the cyclopropanation of electron-deficient olefins and their reduced tendency to mediate carbene dimerization. To the best of our knowledge, this represents the first example in which the redox noninnocence of a reacting ligand plays a key role in a catalytic organometallic reaction. This Forum contribution ends with an outlook on further potential applications of one-electron-activated Fischer-type carbenes in new catalytic reactions.

Ring Opening Metathesis Polymerization of Norbornene and Derivatives by the Triply Bonded Ditungsten Complex Na[W2(µ-Cl)3Cl4(THF)2]·(THF)3

“…The multiplicity of the signals could be attributed either to formation of several active species deriving from the 1/NBD interaction, to fasbo/esbo equilibria and/or concurrent metathetical reactions. Their broad unresolved nature may be due to paramagnetism of the active species [50], restricted rotation of the alkylidene ligand(s) and/or the existence of several species with nominally similar structural features and slightly differing environments, possibly isomeric alkylidene THF adducts of the type {"W x "(=CHP)(THF) y } (P = polymer chain; x, y = 1 or 2). Quenching the reaction mixture with benzaldehyde (1/NBD/PhCHO: 1/20/100) causes the disappearance of the high-field peaks.…”

Radical ligand-containing single-molecule magnets

“…While not intended to provide a comprehensive review on other metal-ligand radical complexes that do not exhibit slow magnetic relaxation, this article will nevertheless briefly survey classes of molecules that could potentially be exploited to realize new, metal-radical ligand singlemolecule magnets. For a more comprehensive account of other metal-radical ligand complexes, the reader is referred to broader and more inclusive review articles [9][10][11][12][13][14][15][16][17][18][19][20][21]. For the latest advances on single-molecule magnets, the reader is referred to some recent review articles [22][23][24][25][26][27][28][29][30].…”

Fischer mono- and biscarbene complexes of tungsten with mono- and dimeric heteroaromatic substituents

“…The possibility of employing these complexes in C-C bond formation reactions is expected, as the LUMO of electrophilic Fischer carbene complexes is carbon-centred [2]. After one electron reduction, a carbene radical is formed on the carbene carbon atom [3]. This allows for possible catalytic synthetic organic transformations.…”

“…Y = 2-thienyl or 2-furyl [5], CH 3 [6], Ph [7], ferrocenyl [8]), (b) different heteroatom substituents (X = O vs N [5,6]) and (c) ligand-substitution on the metal (L = PPh 3 , P(OPh) 3 , dppe (1,2-bis(diphenylphosphino)ethane), the chelated product of ethylene diamine) [9] or  2 -N-allyl-Nallylamino [7]. In this study we focus on the influence of (d) [(CO) 5 W=C(OEt)Ar] with Ar = 2-thienyl (1) and (7); 2-furyl (2) and (8); 2-(N-methylpyrrolyl) (3); 2,2'-bithienyl (4), (10) and (11); 2,2'-thienylfuran (5); N-methyl-2-(2'-thienyl)pyrrole (6); N,N'-Dimethylpyrrolo [3,2-b]pyrrole (9) and terthienyl (12).…”