Ligand-controlled reactions

In recent years, much attention has been focused on rhodium-mediated carbenoid reactions. One goal has been to understand how the rhodium ligands control reactivity and selectivity, especially in cases in which both addition and insertion reactions are possible. These catalysts contain Rh—Rh bonds but function by mechanisms similar to other transition metal catalysts. 

The mechanism shown in Scheme 5 postulates the formation of a Fe(II)-semi-quinone intermediate. The attack of 02 on the substrate generates a peroxy radical which is reduced by the Fe(II) center to produce the Fe(III) peroxide complex. The semi-quinone character of the [FeL(DTBC)] complexes is clearly determined by the covalency of the iron(III)-catechol bond which is enhanced by increasing the Lewis acidity of the metal center. Thus, ultimately the non-participating ligand controls the extent of the Fe(II) - semi-quinone formation and the rate of the reaction provided that the rate-determining step is the reaction of 02 with the semiquinone intermediate. In the final stage, the substrate is oxygenated simultaneously with the release of the FemL complex. An alternative model, in which 02 attacks the Fe(II) center instead of the semi-quinone, cannot be excluded either. 

Steric hindrance is well known to slow down the rates of ligand substitution reactions in square-planar metal complexes. An example for which steric hindrance controls the aquation rate is complex 9. The effect of 2-picoline on the rate of hydrolysis of CP trans to NH3 (cis to 2-picoline) is dramatic, being about 5 times as slow as the analogous CP ligand in the nonsterically hindered 3-picoline complex (Table I) (44). 

Concurrent with studies on cyclometalation, studies on the effects of the structure of phosphoramidite ligand had been conducted. Several groups studied the effect of the stmcmre of ligand on the rate and selectivity of these iridium-catalyzed allylic substitutions. LI contains three separate chiral components - the two phenethyl moieties on the amine as well as the axially chiral BINOL backbone. These portions of the catalyst structure can control reaction rates by affecting the rate of cyclometalation, by inhibiting catalyst decomposition, or by forming a complex that reacts faster in the mmover-limiting step(s) of the catalytic cycle. 

LMCT) transition indicates that PtCll is sorbed within Gn-OH dendrimers. The spectroscopic data also indicate that the nature of the interaction between the dendrimer and Cu or Pt ions is quite different. As discussed earlier, Cu + interacts with particular tertiary amine groups by complexation, but PtCl undergoes a slow ligand-exchange reaction, which is consistent with previous observations for other Pt + complexes [119]. The absorbance at 250 nm is proportional to the number of PtCl ions in the dendrimer over the range 0-60 (G4-OH(Pb+)n, n = 0 - 60), which indicates that it is possible to control the G4-OH/Pb+ ratio. 

Complexes with cyclic polydentate ligands containing NHC donor groups like 92 and 93 have been obtained in metal template-controlled reactions [223-225]. Related complexes (94 and 95) have been synthesized directly from polyimidazo-lium salts [226, 227] (Fig. 31). 

Case I (selectivity yj = Pi/Sp. = f ([L]o/lM)o)[M]o=const.) describes the ligand control in catalytic systems using closed reactors (autoclaves, ampoules etc.) [MJo does not change during the reaction the product distribution is determined by the ligand to metal ratio. 

Scheme 3.5-3. Typed), and analysis of the property-specific ligand control comparing partial L -control maps of the cyclodimer distribution for three different P-ligands (cf. Scheme 3.51-1.) Reaction conditions see Fig. 3.2-2. 5 COD, 6 VCH...

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