Carbenes in Ruthenium Based Olefin Metathesis Catalysts and Stabilization of Low Coordinate Boron Species.

摘要

Since their discovery, carbenes have been widely used as organocatalysts and as superb ligands for transition metal-based catalysts. They have also, more recently, been shown to stabilize reactive and low-valent main group systems.;Catalytic olefin metathesis has proven to be a powerful tool in various chemical fields. Research in this area has received considerable attention specifically with the development of new catalysts. The vast majority of catalysts developed, thus far, have been modifications to the Grubbs catalyst architecture. The research presented herein focuses on the development of a new route for the synthesis of new olefin metathesis catalysts and testing their activity.;A new method of preparing ruthenium alkylidene complexes starting with bis-carbene RuHCl species and alkenyl sulfides is developed. This provides a route to bis-mixed carbene ruthenium alkylidene complexes with a hemilabile tridentate carbene and conveniently installs both an alkylidene fragment and a thiolate in one step. The resulting Ru-alkylidene species are either inactive or minimally active for the standard metathesis tests. The species generated by the addition of one equivalent of BCl3, however, show improved activity for ROMP, RCM and CM either at room temperature or at slightly elevated temperatures. Halide exchange for these systems results in enhanced metathesis activity for the standard tests where catalytic olefin metathesis was observed at room temperature.;Cyclic (alkylamino)carbenes are utilized to stabilize iminoboryl moieties which have only been previously stabilized in the coordination sphere of transition metals. Some of the species are also shown to undergo [2+2] cycloaddition with CO2. CAACs are also used for the synthesis of a boron derivative, which is isoelectronic with singlet carbenes, namely a borylene. This species is shown to react with CO and H2, but in contrast with carbenes, it acts as an electrophile and therefore mimics the behavior of metals.