Organolanthanide complexes alkynes

Because the 5d orbitals of lanthanide metals are shielded by 4f orbitals, the lanthanide metals cannot effectively back-bond like the transition metals, and the alkynes and the CO are expected to have no significant chemistry with organolanthanide complexes. However, the divalent samarium complex (C5Me5)2Sm(THF)2 can reduce diphenyl ethyne, and subsequently activate CO to generate a tetracyclic compound, as shown in Figure 8.33 [113]. 

The organolanthanide-catalyzed alkene hydroamination has been reported. With this approach, amino alkenes (not enamines) can be cyclized to form cyclic amines, and amino alkynes lead to cyclic imine. The use of synthesized C-1 and C-2 symmetric chiral organolanthanide complexes give the amino alcohol with good enantioselectivity. 

The first chiral rare earth metal-based hydroam-ination catalysts were reported in 1992 using chiral lanthanocene. Organolanthanide complexes catalyze regios-elective intermolecular hydroamination of alkenes, alkynes. 

Since (CsMe5)2Sm(thf)2 was a crystallographically characterized low-valent lanthanide complex with catsJytic activity in hydrogenation reactions, it was an ideal complex to study with respect to the catalyses which were initiated by metal vapor products as discussed earlier. liiis system was of further interest since it initiated catalytic hydrogenation of alkynes at rates 1000 times faster than the previously studied organolanthanide... 

As discussed in the previous sections, hydrosilylation and hydroamination reactions can be catalyzed by essentially the same catalysts under very similar reaction conditions due to the similarity in their reaction mechanisms. Hence, both reactions can be performed in one synthetic procedure as a one-pot sequence. Although less explored than hydrosilylation of C-C multiple bonds, organolanthanide-catalyzed hydrosilylation of imines is a facile straightforward process [172,173]. Imines, in particular cyclic imines, are readily available via organolanthanide-catalyzed hydroamination of alkynes. Roesky and coworkers have demonstrated that A-silylated saturated heterocycles can be smoothly obtained (38) and (39) utilizing the bis(phosphinoamide)methanide complex 12 (Fig. 8) [57,58]. The higher reactivity of aminoalkynes in the hydroamination process makes this method a valuable alternative to aminoalkene hydroamination. 

Organolanthanide-catalyzed intermolecular hydrophosphination is a more facile process than intermolecular hydroamination. The reaction of alkynes, dienes, and activated alkenes with diphenylphosphine was achieved utilizing the ytterbium imine complex 9 (Fig. 8) as catalyst [185-188]. Unsymmetric internal alkynes react regioselectively, presumably due to an aryl-directing effect (48) [186]. 

An interesting organolanthanide-catalyzed reaction which has been studied in recent years is the addition of terminal alkynes to carbodiimides leading to the novel class of ,A -disubstituted propiolamidines. It was found that half-sandwich rare earth metal complexes bearing silylene-linked cyclopentadienyl-amido ligands can act as excellent catalysts in this addition reaction. As illustrated in Scheme 58, a rare earth amidinate species has been confirmed to be a true catalytic species [68]. 

Organolanthanide(III) complexes with nf- coordinated alkene or alkyne ligands are very rare. For alkene complexes of Ln(II) compounds, see Section 11. The coordination of the... 

W.J.Evans (33,34) prepared some divalent organolanthanides by co-condensation at low temperature of lanthanide metal vapours with unsaturated hydrocarbons (cyclopentadienes, alkynes) containing acidic hydrogen. Some organolanthanides showed catalytic activity. Thus, Sm(C Me ) (THF) catalyzes hydrogenation of 3-hexyne into cis-hexene (cis trans > 99 1 under mild conditions 25°C, 1 atm of hydrogen. The reaction is believed to involve the addition of a hydride Ln-H to the triple bond followed by hydrogenolysis with H (35). The same complex polymerizes ethylene (35). /... 

Evans WJ, Meadows JH, Hunter WE, Atwood JL (1984) Organolanthanide and organoyttrium hydride chemistry. 5. Improved synthesis of [(C5H4R)2YH(THF)]2 complexes and their reactivity with alkenes, alkynes, 1,2-propadiene, nitriles, and pyridine, including structural characterization of an alkylideneamido product. J Am Chem Soc 106 1291-1300... 

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