Niobium complexes dienes

Karstedt s catalyst, nickel analog, 8, 138-139 Kedarcidin, via ring-closing diene metathesis, 11, 208-209 Kendomycin, via ring-closing metathesis, 11, 239 KERD, see Kinetic energy release distribution Ketenes, niobium complexes, 5, 84 Ketenimines... 

Table 10. Polymerization of ethylene catalyzed by niobium-diene and tantalum-diene complexes/ MAOa...

Yasuda and coworkers51 reported a route to niobocene-allyl compounds by hydrometa-lation of conjugated dienes with niobium hydrido-olefin complexes, NbH(C5H5)2(olefin),... 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of 1,2- or 1,3-dialkyl-substituted allylniobium compounds starting from butadiene, (E,E) and (/i,Z)-2,4-hcxadicnc, (E)- and (Z)-l,3-pentadiene (equation 1), 3-methyl-l,3-pentadiene and isoprene (equation 2). All the allyl niobium compounds synthesized were isolated as air-sensitive pale-yellow crystals by crystallization from hexane. 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of... 

Niobium diene complexes [Cp Nb(Ti -diene)Cl2] (diene = buta-1,3-diene, isoprene, 2,3-dimethylbuta-1,3-diene) and [Cp Nb( n -2,3-dimethylbuta-l,3-diene)2] in the presence of a large excess of methylaluminoxane were found to be active catalysts for the polymerisation of ethene to give polyethylene with extremely narrow polydispersities357. 

On the other hand, efficient catalytic dimerization of simple alkenes can be usually achieved by early transition metal alkene or diene complexes. For example the niobium-butadiene complexes 44 and 45 also showed good catalytic activity for dimerization of isoprene into a mixture of the head-to-tail and head-to-head dimers 46 and 47 (Scheme 20). The former catalyst gave products in 85 15 ratio and the latter one gave rise to 70 30 ratio [26]. 

The niobium-diene complexes 57 and 58 are also known to be good catalysts for cyclotrimerization of terminal alkynes [26]. Interestingly, the former complex shows preference for the formation of 1,2,4-substituted benzenes, whereas the latter exhibits selectivity for 1,3,5-substituted isomers. 

Trivalent lanthanocene complexes that are isoelectronic (14e, (P) to cationic group 4 metallocenes are also active for olefin polymerization, although their activity is much less than that of Ti and Zr [28,29]. Mashima et al. have reported monocyclopentadienyl rf-diene complexes of tantalum and niobium that are isoelectronic to group 4 metallocenes. These complexes catalyze ethylene polymerization, albeit with low activity [30]. 

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