Lewis acids coordinated ligand reaction

Scheme 6.7 shows some other examples of enantioselective catalysts. Entry 1 illustrates the use of a Co(III) complex, with the chirality derived from the diamine ligand. Entry 2 is a silver-catalyzed cycloaddition involving generation of an azome-thine ylide. The ferrocenylphosphine groups provide a chiral environment by coordination of the catalytic Ag+ ion. Entries 3 and 4 show typical Lewis acid catalysts in reactions in which nitrones are the electrophilic component. 

Attempts were then made to perform asymmetric catalytic reactions using chiral Lewis acid catalysts [59]. Reaction of the nitrone 73 and the oxazolidinone 76 with 10 mol % of the bis(oxazoline) 12-Mg(II) catalyst, prepared by Corey s method [13], in the presence of 4-A molecular sieves afforded the cycloadduct 77 in high yield (>95 %) and high (> 95 %) endo selectivity and 82 % ee (Sch. 33). The presence of activated powdered 4-A molecular sieves was essential to the endo and enantioselec-tivity of the reaction in their absence they were 65 % and < 2 %, respectively. The reaction proceeded via an intermediate XXIX, proposed by Corey [13], in which the bis(oxazoline) ligand 12 and the oxazolidinone 76 are both bidentately coordinated to the magnesium and addition to the re face is favored because the si face of the bound oxazolidinone is masked by one of the phenyl substituents on the oxazoline rings. 

The metal acts essentially as a Lewis acid in this reaction. The terminal O atom of t-BuOOH is made more electrophilic by coordination of both its O atoms to a tartrate-allylic alcohol-Ti complex, and the terminal O atom is then transferred selectively to one enantioface of the nucleophilic C=C 7r bond. No change in Ti oxidation state or ligand count occurs. After the O atom transfer, a series of ligand substitutions regenerates the active species. 

Reaction of Ru3(CO)i2 with excess AIH3 in THF followed by acidification with 1 M H2SO4 yields hydrocarbons (CH4, C2H6, etc.), corresponding to reduction of —30% of the CO ligands originally present d. Similar results are obtained with M(CO)g systems (M = Cr, Mo, W) . Acid decomposition of Mg[CpFe(CO)2]-4THF also yields small amounts of hydrocarbons . In both of these studies, Lewis acid coordination of CO as in 1 is believed to play an important role. 

Previously, examples of chiral Lewis acid coordination of either the radical or radical acceptor involved coordination to Lewis basic sites, typically oxygen or nitrogen. However, certain transition metals are also capable of coordination to al-kenes and, if complexed to a chiral ligand, can also afford chiral addition products. This has been illustrated in the enantioselective atom transfer additions of alkane and arene-sulfonyl chlorides and bromotrichloromethanes to olefins using chiral ruthenium complexes. These reactions are thought to follow a radical redox chain process detailed in Eq. (20). 

In 1923, Gilbert Newton Lewis defined an acid as an electron pair acceptor and a base as an electron pair donor. This definition is even more inclusive than the previous one because it includes all Bronsted-Lowry acids and bases as a subset and provides the foundation for the field of coordination chemistry. A coordination compound is the product of a Lewis acid-base reaction, such as the one shown in Equation (14.11) and Figure 14.5, in which the metal ion (Lewis acid) and ligand (Lewis base) are held together by a coordinate covalent bond. 

The dependence between the cluster structure and the number of its valence electrons shows that oxidation, reduction, or other reactions leading to changes in the valence electron number must cause rearrangement of the clusters skeleton, or that such reactions must either increase or decrease the multiplicity of the metal-metal bond involved. The structural changes of clusters may also be caused by association or dissociation of Lewis acid type ligands within the coordination sphere, that is, protonation or deprotonation. 

Lewis Acid Catalysis and the Reactions of Coordinated Ligands... 

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