Lewis Boron-Containing

Figure 36 Organoboron polymers of PS with well-defined boron-containing Lewis acids for use as a cocatalyst in metallocene-catalyzed olefin polymerizations. (Adapted from ref. 81.)... 

In comparison to their boron analogs, polydentate Lewis acids containing the heavier main group elements are generally scarce. This state of affairs certainly... 

Even before the discovery of the katapinands vide supra), work upon bidentate Lewis acid hosts suggested the possibility of chelation of anionic sjjecies by acyclic-boron-containing ligands such as BF2CH2CH2BF2. In many ways mul-... 

Scheme 4. Catalytic, asymmetric aldol reaction with boron-containing Lewis acids a) Masamune, b) Corey and c) Yamamoto.

The synthesis of boron-capped iron(II) tris-dioximates has also been implemented using two other methods the rearrangement of the Tchugaev type Fe(HD)2Am2 bis-dioximates in the presence of boron-containing Lewis acids ... 

An apical functionalization of clathrochelates enables one to obtain complexes with improved chemical, physicochemical, biomimetic, and bioactive properties and characteristics that are primary governed by functionalized groups [65]. The possibility of an apical modification of macrobicyclic a-dioximates and oximehydrazonates is indicated by the relative availability of functionalized boron-containing Lewis acids as efficient capping agents. 

Although technically a heteroelement rather than a metal, some interesting Lewis acidic receptors for anions have been based on boron-containing systems. The first evidence for this type of anion receptor was published in 1967, one of the earliest examples of anion binding. Compound 21, when compared with 22, exhibited a chelate effect in the binding of methoxide anions (3). 

Boron-containing Heterocycles.—The enthalpies of adduct formation between pyridine, 2-picoline, 4-picoline, or 2,4,6-collidine and a number of heterocyclic boron derivatives (2-Br- or 2-organo-l,3,2-dihetero-borolans, -borinans, and -boroles) have been measured.282 The presence of Br greatly increases the Lewis acidity of the B atom. 

Theoretically, any species that contains an unshared electron pair could act as a base. In fact, most ions and molecules that contain unshared electron pairs undergo some reactions by sharing their electron pairs. Conversely, many Lewis acids contain only six electrons in the highest occupied energy level of the central element. They react by accepting a share in an additional pair of electrons. These species are said to have an open sextet. Many compounds of the Group IIIA elements are Lewis acids, as illustrated by the reaction of boron trichloride with ammonia, presented earlier. 

Another Lewis acid containing boron is boric acid. Boric acid (a weak acid used in eyewash) is an oxoacid with the following stracture ... 

Interestingly, the difference in ligand orientation between Bpin and Beat systems is found to be mainly steric in origin. In the case of Bpin system 9.10 the perpendicular orientation allows for interaction of boryl and hydride ligands via the formally vacant BO2 7t orbital (to which the B 2pz orbital is a major contributor) in a manner reminiscent of conventional boron-containing Lewis acids. In the case of Beat system 9.9, however, the near coplanar arrangement of ClRhB and BO2 planes means that the B- -H interaction involves the perpendicular BO2 a as the acceptor orbital (Fig. 29) [154]. 

Lewis Acids with Electron-Deficient Atoms Some molecular Lewis acids contain a central atom that is electron deficient, one surrounded by fewer than eight valence electrons. The most important of these acids are covalent compounds of the Group 3A(I3) elements boron and aluminum. As noted in Chapters 10 and 14, these compounds react to complete their octet. For example, boron trifluoride accepts an electron pair from ammonia to form a covalent bond in a gaseous Lewis acid-base reaction ... 

Novel boron-containing dihydropyrimidinones 159 are synthesized using boronic acid substituted aldehydes 158, a ketoester and a urea in the traditional three-component Biginelli reaction in the absence of an additional Lewis acid, as the boronic acid group itself acts as a Lewis acid (Scheme 60) (05CJC2052). 

Chiral Catalysts Containing Group 3 (Sc and Y) Metals Including Lanthanides and Actinides. Although lanthanide compounds were expected to be Lewis acid reagents and efficient asymmetric Diels-Alder reactions catalyzed by Lewis acids (containing aluminum and boron) were reported, only few asymmetric Diels-Alder reactions are known. 

Coordination interaction is also a factor that needs to be considered. Because a trigonal uncharged boronate contains a boron atom with an empty orbital, this can serve as an electron receptor for a coordination interaction. In addition to the previous report that unprotonated amines and carboxyl groups can serve as electron donors and thus can coordinate with boronic acids, the coordination of Lewis bases [e.g., fluoride ion) with boron can also occur, which can enhance the complexation between cis-diol-containing compounds and boronic acids. However, if there is a hydroxy group adjacent to the amine [e.g., ethanolamine), this hydroxy group can also interact with the boronate, which would block esterification between the boronate ligand... 

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