PREPARATION AND CHARACTERIZATION OF TANTALUM(III) OLEFIN COMPLEXES AND TANTALUM(V) METALLACYCLOPENTANE COMPLEXES MADE FROM ACYCLIC ALPHA-OLEFINS

TaCp(CHCMe3)Cl2 (Cp = η5-C5H5) reacts rapidly with ethylene and propylene to give the ethylene and propylene metallacyclopentane complexes, [formula omitted] and [formula omitted], respectively. Analogous Cp″ metallacyclopentane complexes (Cp″ = η5-C5H5) can be formed from (and are in equilibrium with) TaCp″(olefin)Cl2 complexes and olefin (olefin = ethylene, propylene, 1-pentene, and neopentylethylene). The analogous styrene, cis-2-pentene, and cyclooctene complexes do not form observable metallacycles. A mixed ethylene/RCH̿CH2 metallacycle (R = e.g., methyl) can be observed on adding RCH̿CH2 to TaCp″(ethylene)Cl2 at low temperatures. The configuration of [formula omitted] is trans by 13C NMR while a mixed metallacycle has two β-substituted isomers. Cp″ metallacycles decompose irreversibly by β-hydride elimination followed by reductive elimination to give primarily the 2, 3-disubstituted 1-butene dimer. The dimer is displaced by the free olefin which is always present to give back the olefin complex quantitatively. The greater stability of the ethylene metallacycle compared to the propylene metallacycle toward β-elimination suggests that a β hydrogen bound to a tertiary carbon atom is abstracted more rapidly than one bound to a secondary carbon atom. A deuterium-labeling experiment has shown that Dα atoms do not scramble singly or pairwise in a hex-1-ene-I, I-d2/ethylene metallacycle before or during decomposition. This result has been used as evidence against degenerate olefin-metathesis-like methylene exchange in Ta metallacyclopentane complexes. © 1979, American Chemical Society. All rights reserved.