Ligand Systems

Figu re 8.6 Chiral diene ligands for the conjugate arylation. 

With these hgands enantioselectivihes in excess of 95% were obtained in the rhodium-catalyzed addition of phenylboronic acid to cyclohexenone. Furthermore, the results of kinetic experiments suggested that catalyst turnover occurred much more rapidly with these mixed rhodium complexes of hybrid ligands compared to the corresponding rhodium complexes of BINAP or cod [76]. 

Conjugate Arylation with Diphosphine-Palladium(ll) Complexes 

Ohta reported that paUadium(O) complexes coordinated by phosphine hgands catalytically induce 1,2-additions of arylboronic acids to aldehydes [81] as weU as 

Figu re 8.9 Competing reaction pathways in arylations of a,P-unsaturated aldehydes. 

---

The avidin-biotin complex, known for its extremely high affinity (Green, 1975), has been studied experimentally more extensively than most other protein-ligand systems. The adhesion forces between avidin and biotin have been measured directly by AFM experiments (Florin et al., 1994 Moy et al., 1994b Moy et al., 1994a). SMD simulations were performed on the entire tetramer of avidin with four biotins bound to investigate the microscopic detail of nnbinding of biotin from avidin (Izrailev et al., 1997). 

The Fenske-Hall method is a modification of crystal held theory. This is done by using a population analysis scheme, then replacing orbital interactions with point charge interactions. This has been designed for the description of inorganic metal-ligand systems. There are both parameterized and unparameterized forms of this method. 

Hahde ligands are found in homoleptic complexes as well as in mixed ligands systems. HaUde complexes of Ir(IV) such as [IrCy [16918-91-5] are readily reduced to Ir(III) species, eg [IrCy [14648-50-1], in neutral or basic solution, or in the presence of reducing agents such as KI, oxalate, or photochemical activation (173). 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

This shows that the pM value of the solution is fixed by the value of K and the ratio of complex-ion concentration to that of the free ligand. If more of M is added to the solution, more complex will be formed and the value of pM will not change appreciably. Likewise, if M is removed from the solution by some reaction, some of the complex will dissociate to restore the value of pM. This recalls the behaviour of buffer solutions encountered with acids and bases (Section 2.20), and by analogy, the complex-ligand system may be termed a metal ion buffer. 

Oxidation reactions were used in the synthesis of porphyrin d, the metal-free ligand system of naturally occurring heme d,. In a total synthesis of porphyrin d,12d oxo functions were introduced into isobacteriochlorin 3 by selenium dioxide oxidation to yield 4. The selenium dioxide selectively attacks the 3- and 8-positions of the partially reduced pyrrole rings of the chromophore. In another synthesis23a c of porphyrin d, an isobacteriochlorin 5, derived by... 

For the quantitative description of the cooperative process in the macromolecule-low molecular weight ligand systems, Hill s equation is used. It expresses the dependence of the degree of macromolecule saturation with the ligand (Y) on the equilibrium concentration of the ligand in solution [67] ... 

The scope of the present paper is limited to those cyclopentadienyl ligands that contain more than two bulky substituents and transition metal complexes derived thereof in order to be able to focus on the specific effects of these ligand systems. A selection of some mono-substituted cyclopentadienyl ligands will be treated also. Among the numerous reviews highlighting special aspects of cyclopentadienyl... 

A promising recent extension of the aminopyridinato ligand concept involves the use of potentially tridentate amidinato-cyclopentadienyl ligands. Typical complexes with M = Ti, Zr are depicted in Scheme 171. Other heterocycles can be incorporated in the ligand system instead of the pyridine ring. ... 

Table 3. N-M-N Bite angle in di(pyridyl) ligand systems...

---