Catalytic Oligomerization of Terminal Alkynes by Lanthanide Carbyls (η5-C5Me5)2LnCH(SiMe3)2 (Ln = Y, La, Ce)

Lanthanide and group 3 carbyls Cp*2LnCH(SiMe3)2 (1, Ln = Y; 2, Ln = La; 3, Ln = Ce) are active catalyst precursors for the oligomerization of terminal alkynes HC≡CR (R = alkyl, aryl, SiMe3). The regioselectivity and the extent of oligomerization depend strongly on the lanthanide applied as well as on the alkyne substituent R. For yttrium, alkyl-substituted alkynes are dimerized selectively to 2,4-disubstituted 1-buten-3-ynes whereas mixtures of two enyne isomers, i.e. 2,4-disubstituted 1-buten-3-ynes and 1,4-disubstituted 1-buten-3-ynes, are found for phenylacetylene and (trimethylsilyl)acetylene. The reactions with lanthanum and cerium produce, besides dimers, higher oligomers (trimers, tetramers) of various sorts (allenes and diynes). NMR studies indicate that lanthanide acetylides [Cp*2LnC≡CR]n, formed by σ-bond metathesis between the carbyls and the alkyne CH bond, are the active species in the catalytic cycle. Two oligomeric cerium acetylides [Cp*2CeC≡CR], (7, R = t-Bu; 20, R = Me) have been synthesized on a preparative scale. Spectroscopic data for these compounds suggest a significant contribution of the CC triple bond in the bonding of the acetylide unit.