TRAP ligand

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

A different variation on this theme has been developed by Ito, where the TRAP ligands (37) form a nine-membered metallocycle [157-162]. The ruthenium catalysts seem to function best at low pressures, but highly functionalized dehydroamino esters can be reduced with high degrees of asymmetric induction [157, 159-164], as well as indoles [165]. 

Table 25.7 Selected results for Rh- and Cu-catalyzed hydrogenation using (R,R )-walphos and R-trap ligands (for ligand and substrates structures, see Figs. 25.14 and 25.16, respectively).

Therefore better methods for the chiral reduction of indole-2-carboxylic acid derivatives would provide an elegant synthesis of this intermediate. A study by Kuwano and Kashiwabara of the reduction of indole derivatives into the corresponding indohnes found that a range of the more common ligand systems gave almost no enantioselectivity but the TRAP ligand gave the chiral indolines in up to 95 % ee for reduction of the methyl ester (B, R=Me, R =H). Further developments are awaited. 

Selected Results for Rh-Catalyzed Hydrogenation Using Trap Ligands (39)... 

Hute trapped ligands onto HFLC column... 

When perfluoroolefins are codeposited with Pd atoms followed by addition of a trapping ligand, a metallocyclopropane derivative is formed (as opposed to a n-complex with normal olefins) by oxidative addition (see 5.8.2.3.2 for further discussion of olefin reactions). 

The synthesis and application of chiral ferrocene derivatives has attracted much interest.358 Hence the ferrocenyldiphosphine (163) (Josiphos) can be prepared by direct HPR2 substitution of the dimethylamino group (Equation (40)).359 Various other ferrocene-based chiral ligands are known (e.g., the TRAP ligands (164)).360-364... 

In place of carbon monoxide, isocyanides are often used as the isoelectronic compound. In 1986, Jones et al. reported that the low-valent ruthenium phosphine complex catalyzed intramolecular insertion of isocyanide into the sp3 C-H bond under thermal conditions (Eq. 33) [60,61 ]. Their finding provided a new route for synthesis of indole. An interesting feature of their reaction is that C-H bond cleavage occurs even in the presence of an excess of the trapping ligand, i.e., isocyanide. 

Controlling enantioselectivity at the enol centre alone can be achieved with special reagents and a very complex catalyst. An allylic carbonate with a fluorinated esterifying group allylates a prochiral enolate derived from i-propyl cyanopropionate catalysed by Pd, Rh, and the chiral Fe TRAP ligand 257, gives excellent results. The explanations for these last two examples are complicated and you are referred to the papers if you want to know more.60... 

Fig. 2.123. Protonation of [PtMe(H)2(Tp)] in the presence or absence of trapping ligand L.

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