Heterobimetallic rebound: a mechanism for diene-to-alkyne isomerization with M-Zr hydride complexes (M = Al, Zn, and Mg)

Abstract

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.