Hydroamination organolanthanide catalyst

Intramolecular hydroamination/cyclization, the addition of an N-H bond across an intramolecular carbon-carbon unsaturated bond, offers an efficient, atom economical route to nitrogen-containing heterocyclic molecules (Equation 8.37). Numerous organolanthanide complexes were found to be efficient catalysts for this transformation [124, 125]. The real active intermediates are organolanthanide amides, which are formed by the rapid protonolysis reactions of precatalysts with amine substrates. The proposed catalytic cycle of hydroamination/cyclization of aminoalkenes is presented in Figure 8.37 [124]. 

A number of (R)- and (,S )-organolanthanide alkyl and amide complexes 1, bearing a homochi-ral substituent R on one cyclopentadienyl ring, were prepared and their catalytic activity in the enantioselective hydroamination-cyclization of 4-pentenylamines 3 was examined 11 113 These complexes are converted to the catalytically active species 2 in the presence of a large excess of the amine. Furthermore, catalyst epimerization (S)-2 to (/ )-2 or vice versa occurs and is complete in the early stages of preparative-scale reactions however, equilibrium homochiralities are frequently high, in some cases >95%. 

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]. 

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. 

Organolanthanide complexes are known to be highly active catalysts for a variety of organic transformations, which can be either intramolecular or intermolecular in character. Successful intramolecular transformations include hydroelementation processes, which is the addition of a H-E (E = N, O, P, Si, S, H) bond across unsaturated C-C bonds, such as hydroamination, hydroalkoxylation, and hydrophosphination. Intermolecular transformations include a series of asymmetric syntheses, the amidation of aldehydes with amines, Tishchenko reaction, addition of amines to nitriles, aUcyne dimerization, and guanylation of terminal aUcynes, amines, and phosphines with carbodiimides. 

The aminoaUene hydroamination/cyclization reactions are highly diastereoselective and can provide concise routes to synthesize some natural products (Scheme 3). Using chiral organolanthanide complexes as catalysts, enantioselec-tive hydroamination/cychzation reactions are achieved, which provide a convenient route for the synthesis of chiral amines from simple, readily available prochiral substrates in a single step. 

Marks showed in the 1990s that the cyclohydroamination of alkenes was catalyzed by organolanthanide complexes and, in 2000, Hartwig reported the enantioselective hydroamination of p-trifluromethylstyrene by aniline using Noyori s catalyst [R-BINAP-Pd(0S02CF3)2] (toluene, 45°C, 81% yield, 81% ee). 

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