Recent Progress in Half-sandwich Rare-earth-catalyzed Organic Synthesis

Half-sandwich rare-earth metal alkyl complexes bearing the silylene-linked cyclopentadienyl-amido ligand can serve as efficient catalyst precursors for the catalytic (Z)-selective head-to-head dimerization of terminal alkynes to give the corresponding conjugated (Z)-enynes, which represents the first example of exclusive formation of a (Z)-enyne compound in the dimerization of an aromatic alkyne. They also work for the catalytic addition of terminal alkyne C-H, amine N-H, and phosphine P-H bonds to carbodiimides (R'N = C = NR'), to give a new family of propiolamidines, guanidines, and phosphaguanidines, respectively, some of which were difficult to prepare previously. The half-sandwich rare-earth-catalyzed cross-coupling of terminal alkynes with isocyanides affords selectively the corresponding (Z)-1-aza-1, 3-enyne products for the first time. Mechanistic aspects of these catalytic processes are also described.