Epoxide boron Lewis acids catalyze

Boron trifluoride is a highly moisture-sensitive gas (31). It is utilized in esterification, ether formation, Friedel-Crafts alkylation and acylation, and Lewis acid-catalyzed Diels-Alder reactions. A more widely used, easy-to-handle and convenient liquid source of BF3 is boron trifluoride etherate [BF3-0(C2H5)2] (32). Its main usage as catalyst is in the direct esterification of all types of acids, rearrangements, aldol condensation, and Lewis acid-catalyzed Diels-Alder reactions. It is the most frequently used acid in epoxide ring opening and rearrangement (33). 

Several instructive illustrations taken from the field of natural products will serve to conclude the present discussion of acid-catalyzed epoxide isomerization. Much has been said about the occurrence of simple i,2-hydride transfers in boron trifhioiide-catajyzed epoxy steroid rearrangements. Although initiated by a mineral acid rather than a Lewis acid, an instance of traneatmular hydride transfer has boon reported far a 9/3,110-epoxy steroid,1824 as shown in Eq. (484). Such migrations can in principle occur elsewhere. 

Since the epoxidation step involves no formal change in the oxidation state of the metal catalyst, there is no reason why catalytic activity should be restricted to transition metal complexes. Compounds of nontransition elements which are Lewis acids should also be capable of catalyzing epoxidations. In fact, Se02, which is roughly as acidic as Mo03, catalyzes these reactions.433 It is, however, significantly less active than molybdenum, tungsten, and titanium catalysts. Similarly, boron compounds catalyze these reactions but they are much less effective than molybdenum catalysts 437,438 The low activity of other metal catalysts, such as Th(IV) and Zr(IV) (which are weak oxidants) is attributable to their weak Lewis acidity. 

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