The creation of new polymer materials with well-controlled microstructures and desired properties relies on the development of new generations of polymerization catalysts. cis-1,4-Regulated polyisoprene (PIP) and polybutadiene (PBD) are among the most important elastomers used for tires and other elastic materials. As the demand for high-performance synthetic rubbers has increased, the development of highquality elastomers by polymerization of isoprene and butadiene has grown in importance. In addition, the limited supply of natural rubber has promoted the need for improved synthetic polyisoprene. [1] To date a variety of catalyst systems have been reported for the polymerization of butadiene and isoprene, among which catalysts based on rare-earth metals have attracted special attention because of their high activity and high cis-1,4 selectivity.[2] The catalysts utilized industrially for the cis-1,4 polymerization of butadiene and isoprene are heterogeneous Ziegler-Natta-type multicomponent systems that consist typically of a rare-earth metal (e.g., Nd) carboxylate, ethylaluminum chloride, and isobutylaluminum hydride. [2a-c] To mimic and improve the industry catalyst systems, and to gain a better understanding of the mechanistic aspects of these heterogeneous catalysts, various discrete rare-earth metal carboxylate and alkoxide complexes as well as rareearth metal/aluminum heterometallic alkyl complexes have been investigated.[3] Some of these molecular systems show very high cis-1,4 selectivity (up to 99 %) when combined with a chloride additive, such as Et 2 AlCl. However, similar to the heterogeneous industrial catalysts, such binary or ternary catalyst systems generally lack "livingness" and yield polymers with rather broad molecular-weight distributions. On the other hand, lanthanide metallocene-based catalyst sys- [4b] have been found to afford polymers with both high cis-1,4 selectivity (up to 99 %) and a narrow molecularweight distribution, or "livingness" (M w /M n = 1.20-1.23), in the polymerization of butadiene under appropriate conditions, [4] however, the polymerization of isoprene has not been achieved in a "living" fashion with such catalyst systems.[4f]Very recently, the combination of a Nd/Mg heterotrimetallic allyl complex with methylaluminoxane (MAO) was reported to show both high cis-1,4 selectivity (95-99 %) and a relatively narrow molecular-weight distribution (M w /M n = 1.3-1.7) for the polymerization of isoprene. [5,6] A common feature in the reported catalyst systems is that they all require an aluminum additive, such as AlR 2 Cl, AlR 3 , or MAO, to show high activity and high cis-1,4 selectivity, which makes it difficult to identify the true catalytic species and to understand the mechanistic aspects of the polymerization process. [2][3][4][5][6][7][8] Recently, cationic methylyttrium species, such as [YMe 2Àn (solv) x ] n+1 (n = 0, 1; solv = solvent), have been reported to show activity for the polymerization of butadiene and isoprene in the absence of an aluminum additive, but ...

The mechanism of the selective conversion of 1-alkynes to aldehydes by hydration was investigated by isolating organic and organometallic byproducts, deuterium-labeling experiments, and DFT calculations. The D-labeled acetylenic hydrogen of 1-alkyne was found exclusively in the formyl group of the resulting aldehydes. After the reaction, the presence of metal-coordinated CO was confirmed. All of the experimental results strongly suggest the involvement of a metal-acyl intermediate with the original acetylenic hydrogen also bound to the metal center as a hydride, with the next step being release of aldehyde by reductive elimination. Theoretical analyses suggest that the first step of the catalytic cycle is not oxidative addition of acetylene C [bond] H or tautomerization of eta(2)-alkyne to a vinylidene complex, but rather protonation of the coordinated 1-alkyne at the substituted carbon to form a metal-vinyl intermediate. This cationic intermediate then isomerizes to Ru(IV)-hydride-vinylidene via alpha-hydride migration of the vinyl group to the metal center, followed by attack of the vinylidene alpha-carbon by OH(-) to give the metal-hydride-acyl intermediate.