Mediators cyclometalated

Scheme 17. Designing cyclometalated Ru11 and Os11 mediators (55).

Scheme 18. Synthetic routes to cyclometalated Ru11 mediators (55). 2-Phenylimidazole reacts similar to 2-phenylpyridine (228).

Scheme 19. Synthetic routes to cyclometalated Os11 mediators.

The Murai reaction (Scheme 4), the replacement of an ortho-CH on an aromatic ketone by an alkyl group derived from a substrate olefin, is catalyzed by a variety of Ru complexes. This C bond formation occurs via chelate directed C-H bond activation (cyclometalation) in the first step, followed by alkene insertion into RuH and reductive elimination of the alkylated ketone. In a recent example of the use of a related cyclometalation in complex organic synthesis, Samos reports catalytic arylation (Suzuki reaction) and alkenylation (Heck reaction) of alkyl segments of a synthetic intermediate mediated by Pd(II). 

Chatt and Davidson, in combination with subsequent work by Cotton et al., demonstrated that Ru with two bisphosphine ligands, RuOv -F.F-MejPCHjCHjPMejH, reacts to break a phosphine methyl C H bond to give a dimeric Ru(II) cyclometalated product (Equation 11.1).18 In addition, the combination of Ru(fc2-P,P-Me2PCH2CH2PMe2)2 with naphthalene provides the Ru(II) naphthyl hydride complex.19 These reactions were among the first examples of well-defined metal-mediated C H bond cleavage. 

Cyclometalation reaction is the transition metal-mediated C—R bond activation of a wide range of organic molecules, substituted with donor atoms E (E = N, O,... 

STATE OF THE ART IN SELECTIVE HETERO- AND CARBOCYCLIC SYNTHESES MEDIATED BY CYCLOMETALLATED COMPLEXES... 

Metallacycles are also featured in Chapters 2 and 3 of this volume. Cobalt- and nickel-based metallacycles are key cataljrtic intermediates in Transition-Metal-Catalyzed Cycloaddition Reactions of Bicy-clo[2.2.1]heptadienes by Professor Mark Lautens and Dr. William Tam of the University of Toronto (Chapter 2). The high sttain energy of norbomadiene makes possible many of the interesting processes covered in this informative chapter. It is followed by a comprehensive review of the State of the Art in Selective Hetero- and Carbocyclic Synthesis Mediated by Cyclometallated Complexes ... 

The first example of Ru-mediated activation of C(sp )-H bonds of indenyl-functionalised NHCs was reported in 2009. ° Reaction between free carbene IndCH2CH2lmR (Ind = indenyl, Im = imidazol-2-ylidene, R = Mes/Dipp) and Ru3(CO)i2 in refluxing benzene led to four distinguishable multinuclear Ru-NHC C-H activated products. Each product was characterised using NMR and IR spectroscopy, combustion analysis and ultimately X-ray crystallography. The authors propose that the C-H activated products form via a cyclometalated triruthenium intermediate, in which an indenyl proton (Ha) is susceptible to further activation to deliver the dinuclear r] -coordinated indenyl complex, followed by elimination of H2, CO and a Ru(CO)4 unit (Fig. 43A). Alternatively, a side-chain proton (Hb) and vinylic proton (Ho) may be activated to afford the trinuclear r], ri -coordinated complex, with concomitant loss of H2 (Fig. 43B). 

Metal-mediated transformations ofpyridines are thus mainly the realm of main group highly electron-rich compounds such as alkyl derivatives of Uthium, magnesium and zinc, aluminum hydrides, etc. This chemistry will be very succincdy covered with an emphasis on recent developments, as it will the related alkyl migration to pyridines which have been found in a number of f-block derivatives, a topic which has been recently reviewed. In particular, rollover cyclometalations, which are important processes that can be operative in bipy complexes, will not be mentioned, and the interested reader is referred to a recent account of this chemistry. 

The most common process involving the N-substituents of NHCs is C-H activation to give cyclometalated species. Such processes may not necessarily be deleterious for catalysis, as they can be reversed by the addition of H2. Indeed, in some cases the cyclometalated species were directly involved in mediating H2 transfer. Other reactions involving C-C and C-N activation of the N-substituents tend to have more drastic implications for catalyst integrity. However, in some cases these reactions involve novel processes that are of interest in their own right. A number of studies considered the decomposition reactions of Grubbs alkene metathesis catalysts and these will be considered in more detail in Section 2.4. 

An unusual AgaO-mediated isomerization of mesitylene complex 266, containing a dianionic chelating ligand derived from 2-acetamidocinnamic acid, has been reported to yield the cyclometallated alkenyl derivative 267 (Equation (18)). Other alkenyl-cyclometallated derivatives 285 are also known (see Section 6.15.3.3). 

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