Yttrocene catalyst

One example of asymmetric diene silylation was reported using the binaphthalenediol-based yttrocene catalyst (Fig. 3) [49]. A variety of 1,5- and 1,6-dienes were cyclized in 70-95% yields, but with < 5-50% ees. Due to the slower cyclization of 1,6-diene substrates, PhMeSiH2 was in place of PhSiH3 to prevent hydrosilylation of the olefins. 

Among early and group III transition metals, the yttrocene catalysts have been studied in greatest detail. However, related metallocenes show great promise as catalysts for reductive cyclization. Neodymocene-catalyzed cyclization of 1,5- and 1,6-dienes 14a and 15a proceeds readily in the presence of silane to afford cyclopentanes 14b and 15b.37 Lutetocenes and samarocenes also catalyze silane-mediated cyclization of 1,5-diene 14a to cyclopentane 14b.38 39 In the case of the samarium-based metallocenes, the feasibility of borane-mediated cyclization has been established, as demonstrated by the highly diastereoselective conversion of phenyl-substituted diene 16a to cyclopentane 16b (Scheme ll).40... 

Employment of the less sterically hindered yttrocene catalyst [(Cp )2YMe]2 or the more reactive zwitterionic zirconocene catalyst Cp 2ZrMe(/x-Me)B(C6E5)3 allowed cascade cyclization/hydrosilylation of trienes that possessed one or more 1,1-disubstituted alkene. As examples, reaction of 2-(3-butenyl)-l,6-hexadiene and phenylsilane catalyzed by [(Gp )2YMe]2 gave silylated spirocycle 74 in 88% yield. Likewise, the reaction of the dialkenyl alkylidene cyclopentane 75 gave silylated propellane 76 in good yield (Equations (50) and (51)). 

More substituted double bonds are less reactive, and therefore, regioselective hydrosilylations of dienes are possible (8) and (9) [45], Yttrocene catalysts are less reactive in case of 1,1-disubstituted alkenes however, the larger samarocene is better suitable to catalyze this transformation with excellent diastereoselectivity (10) [47],... 

So far, the only example of an asymmetric hydrosilylation/carbocyclization sequence of a,co-hexadienes and heptadienes utilize the (i )-BINOL-derived yttrocene catalyst 16 to produce cyclopentanes and cyclohexanes in high yields but only low to moderate enantioselectivities of up to 50% ee (25) [75]. 

Scheme 11.8 Improved enantioselectivities in the hydroamination/cyclization of aminohexenes using a chiral octahydrofluorenyl yttrocene catalyst [38].

Table 1). Bercaw and Yasuda have reported neutral yttrocene catalysts with a similar ligand design although activities and molecular weights are low, stereoselectivities for propylene, butene, pentene, and hexene are very high [mmmm] 0.97). ... 

Since group 3 metallocene alkyls are isoelectronic with the cationic alkyls of group 4 catalysts they may be used as olefin polymerization initiators without the need for cocatalysts. The neutral metal center typically results in much lower activities, and detailed mechanistic studies on the insertion process have therefore proved possible.216-220 Among the first group 3 catalysts reported to show moderate activities (42 gmmol-1 h-1bar-1) was the yttrocene complex (77).221... 

The readily available yttrocene derivative (C5Me5)2YMe(THF) has been shown to be an effective catalyst for the hydrosilylation of internal alkynes [85]. A single stereoisomer, i.e. the product of cis addition of phenylsilane to the alkyne, is formed in the reaction with symmetrically substituted alkynes. Comparable reactions with a variety of unsymmetrically substituted internal alkynes resulted in a regioselective hydrosilylation reaction in which the silane moiety is placed at the sterically less hindered carbon atom of the alkyne. Various functional groups such as halides, amines, protected alcohols, and trisubstituted... 

The C2-symmetric neutral yttrocene complex 17 was the first stereoselective single-component catalyst (the catalyst precursor is a dimeric hydride-bridged yttrocene that does not require a Lewis acid activator to insert propylene) reported for propylene polymerization. This catalyst produces highly isotactic polypropylene at ambient temperature (mmmm = 91%, = 157 C), albeit with... 

In addition to zirconocenes, C -symmetric scandocenes and yttrocenes with two dimethylsilyl interannular linkers (38a-c, 39, Figure 4.18) have been synthesized and tested as polymerization catalysts for propylene and 1-pentene." " Owing to the electronics of these group 3 catalysts (14-electron, d electronic configuration) no cocatalysts are required. However, metallocene chlorides cannot be used for polymerization metallocene alkyl or hydride compounds are necessary. 

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