Isolation of reactive intermediates in the cycloaddition reactions of alkynes promoted by tantalum phenoxide complexes.

Abstract

Intermediates in the cyclotrimerization of alkynes have been isolated using the tantalum phenoxide reagents, Ta(DIPP)₂Cl₃(OEt₂) and Ta(DIPP)₃Cl₂(OEt₂) (DIPP = 2,6-diisopropylphenoxide). The degree of cyclization has been controlled by effecting either the sterics of the metal center or the alkyne itself. Reduction of the less congested bis-phenoxide complex, Ta(DIPP)₂Cl₃(OEt₂), by two electrons in the presence of progressively smaller alkynes allowed the selective synthesis of successively higher coordinated cyclooligomers (alkyne adducts, metalacyclopentadienes, and 7-metalanorbornadienes, respectively). This complex also catalytically cyclotrimerizes phenylacetylene upon reduction. Besides resembling proposed intermediates in the catalytic cyclotrimerization of alkynes by transition metals, the direct conversion of each of these cyclooligomers to the next higher or lower step in the proposed mechanism was demonstrated. The alkyne adduct reacted with additional alkyne to provide metallacyclopentadienes in a very regioselective fashion (i.e. (DIPP)₃Ta(PhC=CPh) reacted with Me₃CC=CH to provide (DIPP)₃Ta(CPh=CPhCH=CCME₃)). The metallacyclic complex, (DIPP)₂ClTa(CCMe₃=CHCH=CCMe₃) was shown to undergo an unprecendented dissociation into a bis-alkyne complex upon thermolysis before rearranging to the less-congested metallacycle, (DIPP)₂ClTa(CCMe₃=CHCCMe₃=CH). This complex then reacted with an additional equivalent of t-butylacetylene to provide the arene complex, (η⁶- C₆H₃ᵗBu₃)Ta(DIPP)₂Cl, or with Me₃CC=N to yield η²-(N,C)-NC₅H₂ᵗBu₃Ta(DIPP)₂Cl. The alkyne adduct, (DIPP)₃Ta(PhC=CPh), also undergoes regioselective cross-coupling reactions with benzaldehyde to provide (DIPP)₃Ta(CPh=CPhCH(Ph)O). This metallacyclic alkoxide reacted with an additional equivalent of benzaldehyde and then undergoes a hydride transfer to provide the Meerwein-Ponndorf-Verley/Oppenauer type redox intermediate, (DIPP)₃(PhCH₂O)Ta(η²-CPh=CPhCPh=O). Nitriles coordinate to (DIPP)₃Ta(PhC=CPh) to provide the complexes, (DIPP)₃Ta(PhC=CPh) (RC=N). Nitriles which contain α-hydrogen react further to provide the metallacycloenamine complexes (DIPP)₃Ta(CPh=CPhC(=CHR)NH).$ These complexes have been shown to arise through a metallacycloimine followed by an unusual intermolecular tautomerization, as inferred from deuterium labeling and crossover experiments.