Synthesis and reactivity of tantalum and tungsten alkyne complexes: Models for alkyne cyclization.
AuthorWexler, Pamela Andrea.
AdvisorWigley, David E.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractIntermediates in the cyclization reaction of alkynes have been isolated using the group 5 tantalum phenoxide reagents, Ta(DIPP)₂Cl₃(OEt₂) and Ta(DIPP)₃Cl₂(OEt₂) (DIPP = O-2,6-C₆H₃-i-Pr₂). The extent of cyclization has been effected by controlling the sterics at the metal center or the alkyne itself. Reducing the less congested bis phenoxide complex, Ta(DIPP)₂Cl₃(OEt₂), by two electrons in the presence of 2-butyne or 3-hexyne allowed the isolation of an arene complex, (C₆R₆)Ta(DIPP)₂Cl (R = Me, Et), which is formally classified as a 7-metallanorbornadiene. This complex can also be reduced by one more electron to produce a tanatalum (II) species that readily undergoes a one-electron addition reaction with halogenated reagents. This complex also underwent an intramolecular C-H activation of one of the alkyl groups on the arene ring. Attempts were made to try and generalize this cyclization and C-H activation chemistry to the group 6 metals. Tungsten phenoxide and mixed phenylimido-phenoxide reagents were synthesized for use in subsequent cyclization reactions. Reducing the bis phenoxide complex, W(DIPP)₂Cl₄, by two electrons in the presence of a variety of alkynes afforded the alkyne complexes W(DIPP)₂Cl₂(RC≡CR') (R = R' = Me, Et, Ph; R = CMe₃, R' = H). The mixed phenylimido-phenoxide complexes, W(NAr)(DMP)ₓCl₃₋ₓ (x = 1 or 2; NAr = N-2,6-C₆H₃-i-Pr₂; DMP = O-2,6-C₆H₃Me₂), were also reduced by two electrons in the presence of alkynes to afford adducts (i.e. W(NAr)(DMP)₂(EtC≡CEt)). These alkyne adduct complexes failed to undergo any cycloaddition reactions. Reduction of the tantalum tris phenoxide complex, Ta(DIPP)₃Cl₂(OEt₂), by two electrons in the presence of the bulky alkynes diphenylacetylene or trimethylsilyl-1-propyne afforded the isolation of the alkyne adducts (DIPP)₃Ta(PhC≡CPh) and (DIPP)₃Ta(Me₃SiC≡CMe) respectively. The alkyne adduct (DIPP)₃Ta(Me₃SiC≡CMe) undergoes regioselective cross-coupling reactions with smaller alkynes to afford metallacyclopentadienes. Metallacyclopentadienes can be formed directly from the reduction of the tris phenoxide complex in the presence of smaller alkynes (i.e. (DIPP)₃Ta(CEt=CEtCEt=CEt)). The alkyne adduct undergoes cyclization reactions with nitriles that contain α-hydrogens to yield metallacycloenamine complexes (DIPP)₃Ta(CSiMe₃=CMeC(=CHR)NH). The adduct also reacts with ketones to produce metallacyclic complexes with the formulation (DIPP)₃Ta(CSiMe₃=CMeC(RR')O).