ChemInform Abstract: Transition Metal Promoted Reactions of Polyhedral Boranes and Carboranes

Abstract

The use of transition—metal catalysts has revolutionalized the synthetic strategies of organic chemistry; however, it has been only recently that these reagents have been applied in the inorganic area. Our work is described which has demonstrated that such reagents can be used to catalyze or promote a variety of transformations involving polyhedral boranes and carboranes, including: borane—acetylene addition, acetylene—borane insertion, borane—olefin coupling, dehydrocoupling, dehydrocondensation and cage—growth reactions. Selected examples which illustrate the scope and possible mechanisms of these reactions are discussed. INTRODUCTION One of the major problems in polyhedral boron cage chemistry has been the lack of general synthetic routes for the construction of larger cage systems. This limitation has both hampered the potential commercial applications of these types of compounds and seriously retarded further exploratory research in the area. Our work is attempting to address this problem by examining the use of transition metal reagents, similar to those widely employed in organic and organometallic chemistry, to induce high yield, selective transformations in polyhedral boranes and carboranes. Transition—metal reagents are widely used in organic chemistry to catalyze or promote a variety of transformations which may have parallels in polyhedral borane chemistry. Some reaction types and examples of possible applications are summarized in the following table: Metal Promoted Reactions Organic Polyhedral Boranes 1. Acetylene Addition Synthesis of Olefins, Synthesis of Alkenyl— Hydrosilylation boranes and —Carboranes 2. Olefin Substitution Arene—Olefin Coupling, Synthesis of Alkenyl— Synthesis of Substituted boranes and —Carboranes Olef ins 3. Cyclo—Addition Synthesis of Alicyclic Synthesis of Two—, Four— Compounds or Higher Carbon Carbor— ane Cages 4. Dehydrocoupling Arene—Coupling Synthesis of Multi—Cage Compounds 5. Dehydrogenations Synthesis of Unsaturated Cage Closure Reactions; Organics Generation of Reactive Fragements It should be noted, however, that while there may indeed be close analogies between metal—promoted organic and boron hydride reactions, there are also significant differences. For example, a typical C—H bond energy is substantially greater than that for an analogous B—H bond. This difference generally means that metal—catalyzed reactions involving boron hydrides occur under much milder conditions than the analogous organic systems. Likewise, polyhedral boranes are both more reactive (particularly toward basic ligands) and have sites of different reactivity in the same molecule, leading to more complex (and as a result more intriguing) reaction mechanisms and selectivities. As a result of our work we have now developed six separate classes of transition metal promoted polyhedral borane reactions which are described, with selected examples, in the following sections. 837 Catalyst B5H9 + RCCR' OC PPh z1IIIEI7 Ph3P Ci Ph3P