Oligomerization and Cross-Oligomerization of Terminal Alkynes Catalyzed by Organoactinide Complexes

Abstract

Various organoactinides of the type Cp*2An(C⋮CR)2 (Cp* = C5Me5; An = Th, U) have been synthesized from the corresponding Cp*2AnMe2 complexes by addition of an equimolar amount or an excess of the corresponding terminal alkyne. Attempts to trap the mono(acetylide) complexes Cp*2An(C⋮CR)(Me) were successful for only the transient species Cp*2U(C⋮C(i-Pr))(Me). The bis(acetylide) complexes are active catalysts for the linear oligomerization of terminal alkynes HC⋮CR. The regioselectivity and the extent of oligomerization depend strongly on the alkyne substituent R, whereas the catalytic reactivities are similar for both organoactinides. Reaction with tert-butylacetylene regioselectively yields the 2,4-disubstituted 1-butene-3-yne dimer, whereas (trimethylsilyl)acetylene is regioselectively trimerized to (E,E)-1,4,6-tris(trimethylsilyl)-1,3-hexadiene-5-yne, with small amounts (3−5%) of the corresponding 2,4-disubstituted 1-butene-3-yne dimer. Oligomerization with less bulky alkyl- and aryl-substituted alkynes produces a mixture of oligomers. Cross-oligomerizations reactions induce the formation of specific cross dimers and trimers. Mechanistic studies on the selective trimerization of HC⋮CSiMe3 show that the first step in the catalytic cycle is the C⋮C bond insertion of the terminal alkyne into the actinide−acetylide bond. The kinetic rate law is first order in organoactinide and in alkyne, with ΔH⧧ = 11.1(3) kcal mol-1 and ΔS⧧ = − 45.2(6) eu. The turnover-limiting step is the release of the organic oligomer from the alkenyl−actinide complex. The latter key organometallic intermediate has been characterized by spectroscopic and poisoning studies. A plausible mechanistic scenario is proposed for the oligomerization of terminal alkynes.