Organotantalum chemistry is the chemistry of chemical compounds containing a carbon-to-tantalum chemical bond. A wide variety of compound have been reported, initially with cyclopentadienyl and CO ligands. Oxidation statesvary from Ta to Ta.
Classes of organotantalum compounds
Alkyl and aryl complexes
was reported by Richard Schrock in 1974. Salts of − are prepared by alkylation of TaF5 using methyl lithium:
Tantalum alkylidene complexes arise by treating trialkyltantalum dichloride with alkyl lithium reagents. This reaction initially forms a thermally unstable tetraalkyl-monochloro-tantalum complex, which undergoes α-hydrogen elimination, followed by alkylation of the remaining chloride. Tantalum alkylidene complexes are nucleophilic. They effect a number of reactions including: olefinations, olefin metathesis, hydroaminoalkylation of olefins, and conjugate allylation of enones. Ethylene, propylene, and styrene react with tantalum alkylidene complexes to yield olefin metathesis products.
Reduction of TaCl5 under an atmosphere of CO gives the salts of −. These same anions can be obtained by carbonylation of tantalum arene complexes.
Tantalum arenes and alkyne complexes
Treatment of tantalum pentachloride with hexamethylbenzene, aluminium, and aluminium trichloride gives MAlCl4]2. Uncharacterized tantalum-alkyne complexes were described in the 1970s. Some catalyze cyclotrimerizations. The synthetic utility of tantalum-alkyne complexes was later expanded by K. Utimoto in 1989 when he used tantalum-alkyne complexes to synthesize trisubstituted allylic alcohols from acetylenes and aldehydes. In this work, Utimoto first reduces tantalum chloride with zinc to produce a low-valent tantalum species, which readily reacts with acetylenes to yield a tantalum-cyclopropene. Treatment of the tantalum-cyclopropene with THF, pyridine, and an aldehyde results in the formation of an oxatantala-cyclopentene, which upon aqueous work-up affords -allylic alcohols exclusively. Utimoto also noted that treatment of the oxatantala-cyclopentene with iodine resulted in the corresponding iodo alcohol.
Applications
Organotantalum compounds are of academic interest, but few or no commercialapplications have been described.
Tantalum-amido complexes
Organotantalum compounds are invoked as intermediates in C-alkylation of secondary amines with 1-alkenes using Ta5. The chemistry developed by Maspero was later brought to fruition when Hartwig and Herzon reported the hydroaminoalkylation of olefins to form alkylamines: Mechanistically, the first step of the catalytic cycle is believed to be the C-H activation of the bisamide, which results in the formation of a metallaaziridine. Subsequent olefin insertion, protonolysis of the tantalum-carbon bond, and β-hydrogen abstraction affords the alkylamine product. Notable advancements in this area were also made by Doye and Schafer when they showed tantalum catalyzed hydroaminoalkylations were exclusively branch selective and highly diastereoselective.
Transmetalation
Organotantalum reagents arise via transmetalation of organotin compounds with tantalum chloride. These organotantalum reagents promote the conjugate allylation of enones. Although the direct allylation of carbonyl groups is prevalent throughout the literature, little has been reported on the conjugate allylation of enones.
