Molecular complexes with Lewis acid

In conclusion, these studies further demonstrate that the carbohydrates are plentiful sources of inexpensive chiral auxiliaries. On one molecular unit they carry a high density of functional and chiral information which can be amplitied by complexation with Lewis acidic centers. Taking advantage of these properties the carbohydrates can efficiently be used as the chiral templates in [4+2] cycloaddition reactions and also in other types of chemical transformations. 

Amongst some specific reactions used in regioselective syntheses, we note the cyclometallation processes [Sec. 2.2.5.1, reaction (2.8) Sec. 3.1.1.2, reactions (3.40)-(3.44) Sec. 3.3.2.3, reactions (3.226)-(3.228)]. In this respect, we note that C,N-donor ligands form, depending on the nature of Lewis acids, two types of complexes. In the case of immediate interaction (4.35) of azomethines 859 with titanium or tin tetrachlorides (MC14), the molecular complexes with M—N bond 860 [101] were obtained (route A), while the cyclometallation reaction with the use of palladium diacetate leads to binuclear chelates 861, in which the Pd — N, C metal-cycles are formed (4.35) [11,114-116] ... 

The l,T-binaphthalene-2,2 -diol complexes of Lewis acids have received considerably more attention. Mikami hrst reported the apphcation of BtNOL/ri(IV) complexes for enantioselective allylations of glyoxylates.ii Keck" and Umani-Ronchi/Tagliavinin independently devised allylation procedures with (/ )-BtN0L/Ti(0-iPr)4 and (5)-BIN0L/Ti(0-iPr)2Cl2, respectively (Scheme 5.2.83). The use of molecular sieves is essential for high reactivity and stereoselectivity. 

Beyond the self-association aspect of bis(acetylacetonates) of the type M(acac)2, there has been interest in these complexes as Lewis acids, and a review has recently appeared concerning this subject 46). Complexes of the type M(acac)2 react with basic ligands such as p5uridine and water to become six-coordinate adducts. Indeed, the self-association of M(acac)2 molecules as Ni(acac)2 can be thought of in terms of inter-molecular Lewis type donor-acceptor interactions. 

When a prochiral ( )-enolate is selectively (Si)-facially protonated, the result is the (H)-enantiomer. (Jle)-Facial protonation leads to the (S)-enantiomer. From the (Z)-enolate, the direct opposite is obtained. If it is not possible to control the ( )/(Z)-configuration of the enolate, in order to obtain good selectivity, one needs then an enantiomericaUy pure acid, whose protonation preference is dependent on the enolate configuration, i.e. for example, it transfers a proton (Si)-facially to the ( )-enolate, but (Re)-fadally to the (Z)-enolate. In many successful cases the enantiomericaUy pure acid is bonded to the metal of the enolate therefore, at the same time it acts also as a Lewis base. In addition, at least from a theoretical point ofview, enantioselective inter- and intra-molecular protonations with achiral acids are conceivable, in which another ligand of the enolate complex is enantionmericaUypure. 

In a number of transition metal complexes, the coordination modes of dialkyl H-phospho-nates correspond to a, b, or e in the above scheme. The coordination mode c is characteristic for silver, alkaline, and earth-alkaline metals. Coordination of type d is encountered in molecular complexes of dialkyl H-phosphonates with Lewis acids as well as in some nontransition and early transition metals as discussed further in the text. 

Lewis acid catalyst gives a molecular complex with a positive charge on chlorine and a negative charge on iron. Redistribution of electrons in this complex generates a chloronium ion, C1+, a very strong electrophile, as part of an ion pair. 

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