Silyl complexes with lanthanides

The electropositive lanthanide and actinide elements are expected to form silyl complexes with properties similar to those of the early transition metals. Recently the first... 

Wang et al. investigated the catalytic behavior of cation exchange resin supported lanthanide(III) salts of the general structure (31) (Scheme 4.15), prepared from Dowex, Amberlite, Amberlyst and other resins [99]. It turned out that Am-berlyst XN-1010 and Amberlyst 15 complexed best with lanthanides(III). Thus, among others, electrophilic substitution of indole with hexanal and Mukayiama-type aldol reaction of benzaldehyde with ketene silyl acetal proceeded in excellent yields under catalytic conditions (Scheme 4.16). 

Isobutyl complexes, with Zr(IV) and Hf(IV), 4, 903—904 Isocyanates, lanthanide-catalyzed polymerization, 4, 152 Isocyanides bis-silylation, 10, 747 chalcogen-chalcogen additions, 10, 752 with chromium... 

In conclusion, one can look at the silyl hydride complexes in a number of ways, either as coordinated silane with a very elongated Si- -H bond or as complexes of hypervalent silicon moieties with an agostic Si-H bond [109]. It should be pointed out that the ability of Si to become hypervalent in the vicinity of a transition metal fragment has some important consequence on the reactivity of silane derivative in metathesis reaction with lanthanide complexes [115]. 

The first neutral lanthanide(II) silyl complex (Ph3Si)2Yb(THF)4 was synthesized by the reaction of PhsSiCl with metallic ytterbium in THF. The solid complex has a cen-tros3munetrical octahedral structure with a Yb atom bonded to four oxygen atoms of the THF molecules in the equatorial positions and to two Si atoms of the SiPhs fragment in the axial positions. The Yb—Si distance of 3.158(2) A is 0.1 A larger than the Sm—Si distance in Cp2 SmSiH(SiMe3)2 which is in agreement with the difference in ionic radius between Ln(II) and Ln(III) complexes. ... 

Mukaiyama aldol reactions, whereby trimethylsilyl enol ethers react with aldehydes in aqueous solution to form -ketoalcohols, have been promoted by new chiral lanthanide-containing complexes and a chiral Fe(II)-bipyridine complex with 0 outstanding diastereo- and enantio-selectivities. Factors controlling the diastereoselec-tivity of Lewis-acid-catalysed Mukaiyama reactions have been studied using DFT to reveal the transition-state influences of substituents on the enol carbon, the a-carbon of the silyl ether, and the aldehyde. The relative steric effects of the Lewis acid and 0 trimethyl silyl groups and the influence of E/Z isomerism on the aldol transition state were explored. Catalytic asymmetric Mukaiyama aldol reaction of difluoroenoxysilanes with /-unsaturated a-ketoesters has been reported for the first time and studied extensively. ... 

Molander has developed effective protocols for the cyclization/hydrosilylation of 1,6-enynes catalyzed by lanthanide metallocene complexes/ For example, reaction of cyclohexyl-substituted 1,6-enyne 15a with phenylsilane catalyzed by Cp 2YMe(THF) in cyclohexane at room temperature for 2h gave silylated alkylidene cyclopentane 16a as a 6.5 1 mixture of trans. cis isomers (Table 5, entry 1). The diastereoselectivity of the reaction depended strongly on the nature of the allylic substituent. For example, yttrium-catalyzed cyclization/ hydrosilylation of the ethyl-substituted enyne 15b gave silylated cyclopentane 16b in 88% yield as a single diastereomer (Table 5, entry 2). 

Lanthanide-catalyzed enyne cyclization/hydrosilylation was also applied to the synthesis of silylated alkylidene cyclohexane derivatives. For example, reaction of the 3-silyloxy-l,7-enyne 17 with methylphenylsilane catalyzed by Gp 2YMe(THF) at 50°G for 8h gave 18 in quantitative yield as a 4 1 mixture of trans cis isomers (Equation (11)). Employment of methylphenylsilane in place of phenylsilane was required to inhibit silylation of the initially formed yttrium alkenyl complex, prior to intramolecular carbometallation (see Scheme 8). 

A rare example of isospecific 3,4-polymerization of isoprene mediated by a constrained-geometry rare-earth metal initiator was reported by Z. Hou [270]. Binuclear silyl-linked cyclopentadienyl phosphido lanthanide dialkyl complexes were synthesized in good yields and activated with an equimolar amount of [Ph3C] [B(C6Fs)4] (Scheme 68). Cationic alkyl species were proposed as intermediates and an activation scenario was presented based on DFT calculations [270]. 

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