Yttrocene

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. 

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... 

To date, a single study pertaining to enantioselective yttrocene-catalyzed reductive diene cyclization is reported.36 Using the -symmetric yttrocene [(R,Y)-BnBpY-H]2, a range of 1,5- and 1,6-dienes are transformed to the corresponding cyclopentanes and cyclohexanes. In terms of asymmetric induction, the formation of cyclopentane 13b in 50% ee from 1,5-diene 13a represents the most favorable result (Scheme 10). 

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... 

The asymmetric cyclization-hydrosilylation of 1,5-dienes has been also reported by use of a chiral yttrocene complex (Scheme 32).87 Highest enantioselectivity (50% ee) was observed for 3,3-dimethyl-l,5-hexadiene with diphenylsilane. 

Yttrocene complexes catalyze the cascade cyclization/hydrosilylation of trienes to form saturated silylated bicyclic compounds.For example, reaction of the 4-silyloxy-4-vinyl-l,6-hexadiene 69 and phenylsilane catalyzed by Gp 2YMe(THF) at room temperature for 1 h followed by oxidation of crude 70a gave [3.3.0]bicyclic diol 70b in 73% yield over two steps as a single diastereomer (Scheme 18). Selective conversion of 69 to 70a presumably requires initial 1,2-hydrometallation of one of the less-hindered G=G bonds to form alkylyttrium alkene complex II (Scheme 18). Selective S-exo carbometallation of II in preference to -exo carbometallation would form cyclopentyl-methylyttrium complex III (Scheme 18). Gyclization of III via a chairlike transition state would form the strained /r< /75 -fused alkylyttrium complex IIIl, which could undergo silylation to form 70a. 

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)). 

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... 

Recently, the first example of a structurally proven [allyl] -> [alkyl] exchange reaction was reported. The addition of excess trimethylaluminum to the cyclopentadienyl-functionalized yttrocene monoallyl complex [CsMe4 (SiMe2CH2CH=CI I2)]2Y(f73-C3I I5) quantitatively yielded the dimeric alumi-... 

Scheme 57 Structurally evidenced [allyl] —> [alkyl] transformation in yttrocene chemistry [246]...

Scheme 61 Synthesis of mixed-alkylated yttrocene(III) tetraalkylaluminate complexes (top) and reversibility phenomena in aluminate chemistry (bottom) [24]...

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). ... 

---