Samarium catalysts

Various types of alkylpytroles are prepared under mild conditions by reacting nitroalkenes with Imines in the presence of SmfOi-Pri- fEq. 10.11. Thus, the Grob-Camenish type reacdon is accelerated by samarium catalysts. 

A 1990 patent reported the quantitative cydization of 1,5-hexadiene 66 to 67 with Cp 2LuCH3 (Cp = C5Mc5 anion) in the presence of PhSiH3 (Scheme 14) [39]. This result was followed by reports that Cp 2NdCH(SiMe3)2 reductively cyclized both 66 and 1,6-heptadiene (68) under similar conditions [40] and that Cp 2SmCH(SiMe3)2 also served as a precatalyst for the cydization of 1,5-hexadiene [41]. In the case of the neodymium and samarium catalysts competitive alkene hydrosilylation was observed. 

The next homologues are 1- and 2-butyne, where similar isomerizations have been observed [20] a recent report describes the reaction on a basic, alkali metal-exchanged zeolite [21]. As an unexpected product, an allene was obtained in reactions with hydrogen and a samarium catalyst [16, 22]. 

Efforts to apply lanthanide metal catalysts for diene/non-diene block copolymerization by sequential monomer addition were reported for samarium catalysts (comparable to Nd) [658] and Nd catalysts [659-661]. In both cases the efforts towards the preparation of block copolymers comprising a poly(diene) building block (BD or IP) and a poly-e-caprolactone build-... 

Scheme 77). The product 239 was converted to (+)-pyrrolidine-197B 240. The tandem hydroamination/ cyclization of the allenylalkenylamine 241 occurred in the presence of the samarium catalyst 201 to give the bicyclic pyrrolizidine 242 in a high yield (Scheme 78). Hydrogenation of 242 led to (+)-xenovenine 243. 

Since this initial finding there have been significant improvements in the degree of asymmetric induction observed in these reactions, most notably due to the use of chiral Lewis acids. In one example Evans and co-workers developed a chiral samarium catalyst (17) that was capable of reducing aryl ketones (18) in the presence of i-PrOH with high levels of enantioselectivity.19... 

Scheme 9 Synthesis of magnesium alkyl chain growth products with a samarium catalyst system.

Figure 5.12 Mechanism of hydroamination with half-metaUocene samarium catalyst.

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