Dipolar ligands

Note the differences between crystal field theory and valence bond theory. In crystal field theory, there are no covalent bonds, no shared electrons, and no hybrid orbitals—just electrostatic interactions within an array of ions. In complexes that contain neutral dipolar ligands, such as H20 or NH3, the electrostatic interactions are of the ion-dipole type (Section 10.2). For example, in [Ti(H20)g]3+, the Ti3+ ion attracts the negative end of the water dipoles. 

To the silica are bound linear chains bearing aminopropyl, cyanopropyl, benzyl groups or dipolar ligands (zwitterions) which confer an intermediate polarity to the stationary phase (Figure 3.10). An improvement is noted in the separation of very small polar molecules which require mobile phases rich in water (cf. paragraph 3.8). For example, sugars, peptides and other hydrophilic compounds become separable under these conditions (see, e.g. Figure 3.14). For these modified stationary phases, silica gel acts as a support. 

The deduced preselection of the dipolar species in the electric field of the receptor may be assumed to enormously accelerate the diffusional approach of dipolar ligands [153]. The resulting formation of a diffusional encounter complex Figure 4.8) represents the initial step in molecular recognition and selection which is followed by a sequence of consecutive steps, during which more and more substructures make contact with the subsites of the receptor cleft, which eventually may fully encompass an inhibitor molecule when being complementary. 

In crystal field theory a complex is regarded as consisting of a central metal cation surrounded by ionic or dipolar ligands which are electrostatically attracted to the cation. The bonding within the complex arises from the electrostatic attraction between the nucleus of the metal cation and the electrons of the ligands. The interaction between the electrons of the cation and those of the ligands is entirely repulsive. These repulsions will be central... 

A model for the mechanism of the highly enantioselective AlMe-BINOL-cata-lyzed 1,3-dipolar cycloaddition reaction was proposed as illustrated in Scheme 6.13. In the first step nitrone la coordinates to the catalyst 11b to form intermediate 12. In intermediate 13, which is proposed to account for the absolute stereoselectivity of this reaction, it is apparent that one of the faces of the nitrone, the si face, is shielded by the ligand whereas the re face remains available... 

A new type of rigid polymer of 1,1-binaphthols was developed recently [41-43]. The 3,3 -crosslinked polymeric binaphthol ligand 18 in combination with AlMe3 was applied as the catalyst for the 1,3-dipolar cydoaddition (Scheme 6.16) [44]. Very high selectivities were obtained when the aluminum catalyst of 18 (20 mol%) was applied to the 1,3-dipolar cydoaddition reaction between nitrone la and al-kene 8a. The only observable diastereomer resulting from the reactions was exo-9a... 

Several titanium(IV) complexes are efficient and reliable Lewis acid catalysts and they have been applied to numerous reactions, especially in combination with the so-called TADDOL (a, a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol) (22) ligands [53-55]. In the first study on normal electron-demand 1,3-dipolar cycloaddition reactions between nitrones and alkenes, which appeared in 1994, the catalytic reaction of a series of chiral TiCl2-TADDOLates on the reaction of nitrones 1 with al-kenoyloxazolidinones 19 was developed (Scheme 6.18) [56]. These substrates have turned out be the model system of choice for most studies on metal-catalyzed normal electron-demand 1,3-dipolar cycloaddition reactions of nitrones as it will appear from this chapter. When 10 mol% of the catalyst 23a was applied in the reaction depicted in Scheme 6.18 the reaction proceeded to give a yield of up to 94% ee after 20 h. The reaction led primarily to exo-21 and in the best case an endo/ exo ratio of 10 90 was obtained. The chiral information of the catalyst was transferred with a fair efficiency to the substrates as up to 60% ee of one of the isomers of exo3 was obtained [56]. 

A mechanism for this reaction has been proposed [75], The first key intermediate in the reaction is the copper(I) acetylide 42. The additional ligand may be solvent or H2O. The acetylene moiety in 42 is activated for a 1,3-dipolar cycloaddition with the nitrone to give intermediate 43, with introduction of chirality in the product. A possible route to ris/traws-41 might be via intermediate 44. Finally, the cis isomer is isomerized into the thermally more stable trans-41. It should be mentioned that the mechanism outlined in Scheme 6.32 was originally proposed for a racemic version of the reaction to which water was added. 

In a more recent publication the same group mentions that Ag(I) salts in combination with chiral phosphine ligands can catalyze the 1,3-dipolar cycloaddition involving the azomethine precursor 64b and methyl vinyl ketone (Scheme 6.43) [87]. The reaction, which presumably also required a stoichiometric amount of the catalyst, proceeds to give 65b in a good yield with 70% ee. 

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