Stereoselectivity in Coordination

G. P. Chiusoli Stereoselectivity in organic synthesis involving nickel-coordinated intermediates, pp. 77-107. 

Presumably, the stereoselectivity in these cases is the result of coordination of iridium by the functional group. The crucial property required for a catalyst to be stereodirective is that it be able to coordinate with both the directive group and the double bond and still accommodate the metal hydride bonds necessary for hydrogenation. In the iridium catalyst illustrated above, the cyclooctadiene ligand (COD) in the catalysts is released by hydrogenation, permitting coordination of the reactant and reaction with hydrogen. 

Many of the compounds used have additional functional groups, including ester, amide, ether, and acetal. These groups may be involved in coordination to samarium and thereby influence the stereoselectivity of the reaction. 

Contents K. Bemauer Diastereoisomerism and Dia-stereoselectivity in Metal Complexes M. S. Wrighton Mechanistic Aspects of the Photochemical Reactions of Coordination Compounds. 

With regard to the origin of stereoselectivity in these reactions catalyzed by Rh/ (X)-binap, the stereo-determining step is the insertion of an enone to the carbon-rhodium species coordinated with (5)-binap. As shown in Eq. (3.10), for example, 2-cyclohexen-l-one should approach from its Isi face to avoid the steric hindrance of the phenyl groups of the ligand, leading to the 1,4-adduct in (5) configuration, which is consistent with the observed stereochemical outcome ... 

The proposed mechanism is consistent with the experimental results obtained so far. For instance the effects of cation size and -coordination are readily understood in terms of the chelated complex [10a] as is the absence of stereoselectivity in the case of the corresponding 4-pyridyl derivatives where coordination with cation in the manner shown above is impossible. The similarity between vinyl addition and methylation stereochemistry is likewise consistent with the proposed mechanism. Thus it is the equilibrium between [10] and [11] that is primarily responsible for the observed stereochemistry. Work on the stereochemistry of other electrophylic reactions of [2a] is in progress. 

Selectivity and Stereochemistry. An important property of transition-metal complexes is that they coordinate groups in a specific manner permitting high regio-and stereoselectivity in the catalytic reaction. The migratory insertion step is a highly stereospecific transformation. The four-center transition state 16 illustrated for the Wilkinson catalyst requires a coplanar arrangement of metal, hydride, and alkene n bond ... 

The stereoselectivities in nucleophilic additions to various hexopyran-osid-2-163, -3-164, and -4-ulosesI6S 167 were extensively examined (see Tables II and III). The results in the Grignard reactions in Table III were commonly explained by the approach of the reagent from the sterically favored direction to the magnesium-coordinated conformations (left-hand side in the equilibration formulas), but the concept for the methyl-... 

The use of a silver salt in the stationary phase for the GLC of alkenes was one of the earliest examples of exploiting stereoselective features in coordination,99 and dicarbonylrhodium(I)-3-tri-fluoroacetylcamphorate (66) has been shown to provide an excellent stationary phase for the GC of monoalkenes, for complex formation is stronger and the effects of structural differences between cis and trans, and between positional isomers, is more decisive. 

The stereoselectivity in reactions of P-alkylthio-a,(3-enones with organocuprates is dependent upon both the solvent and bulk of the ligand in the cuprate. Ether solutions favor inversion, while THF solutions favor retention (Scheme 19). It was proposed that in coordinating solvents such as THF, a more reactive enolate is formed, which eliminates before rotation occurs, thereby giving retention of configuration.108... 

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