Crabtree-type complexes

Pyridines are often used as catalysts or reagents particular notice has been paid recently to how pyridine coordinates to metal centers enabling a wide range of valuable reactions. Chiral pyridine-iridinium complexes, where an N-heterocyclic carbene replaces the phosphinite in Crabtree-type catalysts, have been shown to provide efficient, enantioselective hydrogenation (13AG(1)7422). Chiral-2-pyridylsulfinamides induce excellent enantiose-lecivity in the addition of diethyl zinc to aryl and alkyl aldehydes (13T8422). 

Although fluorides play a crucial Role in some Ir-catalyzed reactions [9,41], experimental information regarding relevant Ir-fluoride complexes is scarce. Vaska-type fluoride complexes have been reported but are not well characterized [42], while structurally characterized Ir-fluorides are even rarer [43]. Crabtree and coworkers estimated the hydrogen bridge strength of the F. .. H interaction in complexes 62 and 63 (see below) to be approximately 5-7kcalmol [29]. On the other hand,... 

It is only a small extrapolation to move from side-on complexed H2 to side-on complexed X— H, and Crabtree has commented on how these interactions are related. The name agostic is often used for the case of X = a suitably substituted carbon atom. There are also a number of examples of X = a suitably substituted silicon atorn. " The agostic interaction of a C—bond, (25), results in a low-frequency shift of the proton resonance (due to the development of hydride-like character) and substantial reduction in the one-bond coupling constant, V( C, H). This reduction can be 50% or more. Similarly, for X = SiRs, the one-bond, /( Si, H) value decreases. In the solid state one finds the C—H bond as a donor to the metal. There are many examples of this type of interaction. ... 

Three of many examples of directed hydrogenation are shown in Equations 15.15-15.17. Equation 15.15 shows the reduction of a homoaUylic alcohol, which was one of the substrates first used to demonstrate this effect. Equation 15.16 shows a more complex substrate in which the diastereoselective reduction by Crabtree s catalyst is directed by the amide function as part of tlie synthesis of pulmitoxins. Equation 15.17 shows that the addition of hydrogen can be directed to a hindered face of a bicyclic system. In this case, the cationic rhodium system qf Brown, as well as Crabtree s catalyst, led to hi selectivity. Many other reactions occur with high selectivity in the presence of Brown s cationic rhodium system. Diastereoselective additions to acyclic systems, along with a rationalization for the selectivity in these types of substrates, can be found in the review by Evans. ... 

Chapter 18 closes with a brief description of perhaps the most common type of C-H activation process H/D exchange. Shilov reported H/D exchange almost 40 years ago. Far too many examples of this reaction are now known to summarize in this forum. However, this reaction has been a test-bed for the capability of transition metal complexes to cleave aliphatic and aromatic C-H bonds. H/D exchange reactions have been conducted with homogeneous catalysts and either deuterium gas or a relatively inexpensive source of deuterium, such as benzene-d or Seminal papers on the use of Crabtree s cata-... 

Crabtree et al. created a very different classification scheme in which he systematically classifies the linear semibridging carbonyls into four subclasses depending on M2 and structures [5]. Type I complexes consist of early transition metals (up to group 6 Certain complexes with group 6 metals may also be classified... 

Further, Fenske-Hall calculations were able to show that Crabtree s type 111 complexes (which contain unsaturated very late transition metals that traditionally are not expected to be good at donating backbonding electrons, e.g., Cu(NR3)2 or Au (PR3)) may be able to interact more strongly with the linear semibridging carbonyls [93, 94]. 

None of the examples presented above support the idea that the carbonyl can be a four-electTOTi draior, as Iji orbitals of CO are generally very low in energy and will be poor donors. The final example here discusses if any possibilities exist that a carbonyl or similar ligand may act as a four-electron donor and the best candidate is the Crabtree s type I complex. For comparison, the thiocarbonyl-bridged species is introduced here, as the thiocarbonyl is both a better Jt-donor and a better ji-acceptor hopefully, this difference may magnify the ligand-metal jt-donor interacticm (Fig. 25). 

Concerning ring size expansion, since the first anionic sbc-membered example derived from malonic acid described by Cesar et al. [8], other sophisticated five-[5a,9], sbc-[10], and seven-membered [11] NHCs have been described and coordinated to rhodium and iridium, including sbc-membered amino/amido carbenes [10b] and caffeine-based diamino carbenes [12]. Also related to this type of ligands, Conejero and coworkers described a general method for the preparation of 2-pyridylidene-based rhodium complexes by decarboxylation of pyridinium car-boxylates [13], a method previously described by Crabtree and coworkers [14]. 

In order to achieve selective deuterium incorporation in ortho positions to a directing gronp, chelate-assisted C-H deuterations can be performed with cationic Ir complexes such as Crabtree s catalyst 27 (Scheme 23.49) [155, 158, 159]. Moreover, Pd complexes catalyze the same type of reactivity, even with weakly directing groups (Scheme 23.50) [22]. The proposed mechanisms for... 

Both Ir and Ru NHC eomplexes were successfully used for transfer-hydrogenation reactions, wherein an alcohol or amine is oxidized, followed by reaction of the oxidized species and hydrogenation to give the final product. Both Crabtree and Peris reported the use of Ir carbene complexes as catalysts in reactions of this type, sueh as the A -alkylation by alcohols (Equations... 

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