Reaction of the proligand H[PhB(OxMe2)C5H5] with homoleptic rare-earth (tris)tetramethylaluminates Ln(AlMe4)3 provides mixed-metal compounds of the type (AlMe2){(OxMe2)2PhBCp}Ln(AlMe4)2 (Ln = Y (1Y), La (1La), Nd (1Nd)). The crystal structure of 1Nd revealed a piano-stool geometry around the lanthanide center, a rare O-oxazoline-lanthanide coordination, and inequivalent organoaluminum sites. The zwitterionic κ2-{Ph(C5H5)B(OxMe2)2}AlR2, formed by the protolytic reaction of H[PhB(OxMe2)C5H5] and (AlR3)2 (R = Me, Et, iBu), provides a complete family of heterobimetallic compounds (AlR2){(OxMe2)2PhBCp}Ln(AlMe4)2 (2Y, La, Nd ; R = Et, 3Y, La, Nd; R= iBu) upon reaction with Ln(AlMe4)3. These complexes are highly efficient and selective precatalysts for the exclusive metalation of terminal alkynes with trialkylaluminums under mild conditions. These catalytic aluminations yield alkynyldialkylalanes of the type RC≡CAlR’2, that are valuable nucleophiles in preferential alkynyl transfer chemistry. Together, the lanthanide, the trialkylaluminum, and the ancillary ligand affect the reaction rates resulting in peculiar trends. The reaction mechanism is characterized by a ternary experimental rate law: rate ~ [catalyst]1[alkyne]1[trialkylaluminum]1. This rate law is consistent with a two-step mechanism, in which one substrate reversibly associates with the catalyst followed by an irreversible addition of the second substrate. While the trialkylaluminum/lanthanide aluminate exchange is rapid and occurs at room temperature, conversion to alkynylaluminum is observed only at elevated temperatures (60 o C), suggesting that alkyne metalation is the second and irreversible step in the mechanism. Interestingly and in contrast to the fast AlMe3/AlMe4 exchange, the aluminum methyl groups on the B(OxMe2)AlMe and the tetramethylaluminate group do not undergo detectable exchange. Potential reaction intermediates Ln{µ-C≡CR}(n)AlMe(4-n) (n ≤ 2) have been identified by 1H and 13C NMR, synthesized (in-situ) independently and tested for competent catalytic activity.

Kinetics And mechanism of catalytic alumination Of 1-alkynes by rare-earth aluminates

2019 Fall National Meeting

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