The reaction between (Me3CCH2)3 Ta=CHCMe3 and butyllithium to give a lithium salt of [(Me3CCH2)3Ta [CCMe3]- further suggested that group 6 high-oxidation-state alkylidyne species could be made, as was illustrated four years later at MIT with the synthesis of (Me3CCH2)3 WCCMe3, another distillable, thermally stable compound. Deprotonation of [(5-C5H5)2TaMe2]+ to yield (5-C5H5)2Ta (CH2)Me, the first isolable methylene complex, further emphasized the relationship between tantalum and phosphorus. Furthermore, reactions of tantalum alkylidene complexes suggested that the metal is in its highest possible oxidation state and that the alkylidene carbon is a strong nucleophile. Therefore, they became known as "nucleophilic" alkylidene or carbene complexes. One of the most mysterious reactions in the 1970s that captured the interest of chemists was olefin metathesis, a catalytic reaction that involved unknown Mo, W, or Re catalysts. The reaction consists of "chopping up" carbon-carbon double bonds in olefins and redistributing the alkylidenes (usually =CHR moieties) to give a mixture containing all possible olefins formed in that manner. My question upon my move to MIT in 1975 was, "Do the high-oxidation-state species that have been discovered have anything to do with olefin metathesis?" It took five years to prove that they do. For example, we were able to show that (PMe3)(t-BuO)2ClTa=CH-t-Bu reacts with styrene in the presence of PMe3 to provide the isolable benzylidene complex, (PMe3)2(t-BuO)2ClTa=CHPh, and that species of this type would metathesize cis-2-pentene to 2-butenes and 3-hexenes in the presence of PMe3 to the extent of 25-30 turnovers. This was the first time that an alkylidene analogous to the initial alkylidene could be isolated upon reaction with an olefin and the species actually responsible for metathesis identified conclusively. An important message was that bulky alkoxide ligands are beneficial to sustained metathesis reactions involving tantalum or niobium alkylidenes. Although tantalum was a reluctant player in the metathesis game, the principles learned from studies of tantalum alkylidene complexes were key to the later development of well-defined tungsten, molybdenum, and rhenium alkylidene complexes for the metathesis of olefins. Although I maintain only a small interest in tantalum organometallic chemistry today, tantalum started virtually all of the areas in which I am active; it still tantalizes.
Chemical & Engineering News
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