Migration of Organic Groups from Zirconium to Boron

Trialkoxyboranes and trialkoxyboroxins are synthesized by a novel and efficient route of esterifying boric acid with various alcohols in the presence of calcium hydride. Some of these boranes were used as precursors for transmetallation reactions. Trialkoxyboroxins can be obtained using a 1:1 molar ratio of boric acid to alcohol in the presence of calcium hydride. A special feature of trialkoxyboranes and trialkoxyboroxins is their ability to be interconverted by the addition of the appropriate amounts of boric acid or the hydroxyl compound in the presence of calcium hydride. Biscyclopentadienylzirconium-alkenyl-chlorides, Cp2Zr(alkenyl)Cl, are prepared from the hydrozirconation of terminal and internal alkynes. A systematic investigation of the transmetallation of these alkenyl groups from zirconium to a variety of boron compounds is presented. The reaction of the organozirconium with a boron chloride or bromide results in the exchange of the halogen and alkenyl group, forming structurally different types of alkenylboranes in good to high yields. A general method for preparing pure organohaloboranes (RBX2 and R2BX) was developed with high isolated yields and product purity. The reaction proceeds via the exchange of the anhydrides of boronic or borinic acids, (RBO)3 or R2BOBR2 with trihaloboranes (BX3). The newly formed organohaloboranes can contain hydroboratable (primary or secondary), non-hydroboratable (tertiary or aryl) and chiral groups. To demonstrate the utility of the facile migration from zirconium to boron, the syntheses of trans, trans-1,3-dienes and trisubstituted olefins are examined. Generally the olefin synthesis, via organoboranes, has been limited to coupling of alkylvinylboranes that contain symmetrical vinyl groups. The high regioselectivity of the hydrozirconation reaction with unsymmetrical internal alkynes overcomes this limitation. Thus, trisubstituted olefins containing three distinct organic groups about the double bond were readily synthesized using established organoborane chemistry. A number of preparations for zirconacycles have appeared in the literature. Since borocycles, ring structures containing a boron atom, are not easily prepared via normal hydroboration reactions, the use of metallacycles to prepare borocycle derivatives was investigated.