Variation of Ligands

Othmar Stelzer (1), Stefan Rossenbach, Dietmar Hoff 

Yields and selectivities of BINOL-derived ligands in additions of Et2Zn and Me2Zn to 2-cyclohexenone are compiled in Tab. 7.2. 

Bidentate phosphorus ligands based on BINOL, such as phosphonite 23, phosphites 24 and 25, and phosphoramidite 26 (Tab. 7.2), with various bridging units were introduced by the groups of Reetz, Chan, and Waldmann [48-50]. Excellent enantioselectivities - up to 96% for ligand 23, for instance - were found. 

Although the presence of BINOL in the ligands so far discussed has shown itself to be particular effective, modification of the diol moiety provides new classes of ligands for this addition reaction. Alexakis, screening a number of chiral phosphites in the Cu(OTf)2-catalyzed 1,4-addition, showed that an ee of 40% could be obtained for the addition of Et2Zn to 2-cyclohexenone and of 65% for addition to chalcone, by using cyclic phosphites derived from diethyl tartrate [51]. 

A remarkable number of new BINOL- and TADDOL-based diiral ligands for tlie copper-catalyzed conjugate addition of R Zn reagents have recently been introduced, witli botli monodentate and bidentate ligands having proven capable of inducing bigli enanboselectivities [6, 11, 12, 46]. 

Yidds and selectivities of BlNOL-derived ligands in addlbons of Et/Zn and Me Zn to 2-cydobexenone ate compiled in Tab. 7.2. 

7 3 Cofjfjet Cotolyzed 1,4-Addition iocelective 1-addition of R Zn to 2-o/clohorenone ncing 

CTihancCTnent of tlie stereoselectivity was observ ed in tlie presence of H2O, resulting in an ee of 6196. 

7 Copper-catalyzed Enantiosefective Conjcfgate Addition Reactions of Ofganozinc Reagents 

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One way that molecular mechanics methods have been adapted to transition metal applications is by including one orbital-based term in the force field to describe the metal center. These terms are typically based on semiempirical methods or even some variation of ligand field theory. 

In the light of these results, it becomes important to question whether a particular catalytic result obtained in a transition metal-catalyzed reaction in an imidazolium ionic liquid is caused by a metal carbene complex formed in situ. The following simple experiments can help to verify this in more detail a) variation of ligands in the catalytic system, b) application of independently prepared, defined metal carbene complexes, and c) investigation of the reaction in pyridinium-based ionic liquids. If the reaction shows significant sensitivity to the use of different ligands, if the application of the independently prepared, defined metal-carbene complex... 

Clearly, the simple variation of ligands and metals within [MP, Fe (L)P] allows a heretofore unparalleled view of the mechanistic aspects of long-range electron transfer between proteins. 

Both Table XIX and Table XX show that distribution coefficients cover a much larger range for complexes than for the parent ligands, and that variation of ligand substituent can have a very large effect on... 

Until now most of the data we have had on systematic variation of ligands, or the effect of these variations on rate, have been with carboxylatopentaammine complexes. One reason for this is that these are among the slower reactions and have been more readily susceptible to kinetic studies. 

In this respect, metallocomplex catalysis is most preferable. In this process, variations of ligand and diluter types allow control of the catalyst activity. When combined with soft conditions, this provides for high selectivity of the process. 

FIG. 8.5. Effect of variation of ligand concentration on complexation yield. 

The ability to produce structurally varied products, by selection of building blocks, without modification to the synthetic protocols described is of particular utility in the preparation of libraries of ligands. This has facilitated investigation of the effect of subtle variation of ligand structure on the properties of the complexes produced. 

The pentamethylcyclopentadienyl Rh and Ir systems have proved capable of activating not only aromatic but also aliphatic sp G—H bonds. Photolysis of the M(Gp -> =) (PMe3)H2 complexes (M = Rh e 1 32.33) uberate H2, leaving a very reactive metal center. Reactions with benzene give the hydridophenyl derivatives and reactions with p-xylene give the 2,5-dimethylphenyl derivatives. Other variations of ligands have been found to give similar products. Additional dialkylphenyl derivatives have been synthesized in the Rh system, but there is little additional information on those factors which determine selectivities. 

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