Boron compounds examples listed

Figure 5.1 lists a number of auxiliaries for asymmetric allyl addition to aldehydes. Substituted allyl boron compounds have also been used in reactions with achiral aldehydes. Table 5.1 lists several examples of 2- and 3-substituted allyl boron compounds, and the products derived from their addition. Note that for the E- and Z-crotyl compounds, the enantioselectivity indicated is for the isomer illustrated. In some cases, there was more than one of the other three possible isomers found as well. 

Many other compounds have been added to Cr/silica and claimed to tailor some polymer property. The list includes compounds of gallium [594], boron [548], molybdenum [615], and many others. The response (if any) of Cr/silica catalysts to these other modifiers has sometimes been weak at best, or the claims simply could not be confirmed under industrial conditions. For example, claims made in the literature [615] for molybdenum as a modifier could not be confirmed in our laboratory. 

A large number of heterocyclic boronates and, to a lesser degree, stannanes are available conunercially. Included in the suppliers lists are many compounds, the preparations and properties of which do not appear in the literature. They are often noted in papers just as reagents for syntheses of libraries for biological testing and, although this may be a practical approach, it is scientifically unsatisfactory because there is no trail of characterization, particularly as some of these compounds, for example azine a-boronates, are significant from a theoretical viewpoint. 

Subsequently a number of other boranes have been discovered, so that today there are no less than 14 that are well-characterized chemically, and for most the structures are well-defined. A convenient nomenclature for the boranes specifies the number of hydrogen atoms with an arabic number in parentheses. For example B5H9 pentaborane(9), and B5Hn pentabor-ane(ll). The hydrogen number may be omitted where only one compound with the number of boron atoms in question is known, viz., B4H10 tetra-borane. A list of the better characterized boranes is given in Table 8-1. 

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