Heterobimetallic Rebound: A Mechanism for Diene-to-Alkyne Isomerization with M‑--Zr Hydride Complexes (M = Al, Zn, and Mg)

The reaction of a series of M·Zr heterobimetallic hydride complexes
with dienes and alkynes has been
investigated (M = Al, Zn, and Mg). Reaction of M·Zr with 1,5-cyclooctadiene led to diene isomerization to 1,3-cyclooctadiene,
but for M = Zn also result in an on-metal diene-to-alkyne isomerization.
The resulting cyclooctyne fragment is trapped between Zr and Zn metals
in a heterobimetallic species that does not form for M = Mg or Al.
The scope of diene isomerization and alkyne trapping has been explored
leading to the isolation of three new heterobimetallic slipped metallocyclopropene
complexes. The mechanism of diene-to-alkyne isomerization was investigated
through kinetics. While the reaction is first-order in Zn·Zr at high diene concentration and proceeds with ΔH‡ = +33.6 ± 0.7 kcal mol–1, ΔS‡ = +23.2 ± 1.7
cal mol–1 K–1, and ΔG⧧298 K = +26.7 ±
1.2 kcal mol–1, the rate is dependent on the nature
of the diene. The positive activation entropy is suggestive of involvement
of a dissociative step. On the basis of DFT calculations, a heterobimetallic
rebound mechanism for diene-to-alkyne isomerization has been proposed.
This mechanism explains the origin of heterobimetallic control over
selectivity: Mg---Zr complexes are too strongly bound to generate
reactive fragments, while Al---Zr complexes are too weakly bound to
compensate for the contrathermodynamic isomerization process. Zn---Zr
complexes have favorable energetics for both dissociation and trapping
steps.