Ytterbium-imine complex

Recently, intermolecular hydrophosphination of alkynes was also reported with ytterbium-imine complex catalyst precursors [20]. Aromatic alkynes react at room temperature to afford ( )-isomers, while aliphatic ones require heating at 80 °C and, quite surprisingly, (Z)-isomers (trans-addition products) are formed preferentially (Table 4). In this respect the ytterbium-catalyzed reactions are different from the radical process, in which the ( )-isomer formed initially isomerizes to the (Z)-isomer. 

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]. 

Dehydrogenative silylation. I-Alkynes and amines are converted to silyl derivatives by hydrosilanes with iridium carbonyl" and ytterbium-imine complexes as catalyst, respectively. 

Dehydrogenative Silylation. Ytterbium-imine complexes have catalyzed the dehydrogenative silylation of terminal alkynes... 

Ytterbium/imine complex [Yb( / -Ph2CNPh)(HMPA)6] was reported as a catalyst for hydrophosphination of alkynes (Scheme 8.55) [137]. From 52% to quantitative product yields were found in the reaction. Regio- and stereoselective transformation was observed in case of internal alkynes, but the reaction with terminal alkynes gave a mixture of a- and -adducts with the former being... 

Takaki K, Takeda M, Koshoji G, Shishido T, Takehira K (2001) Intermolecular hydrophosphination of alkynes and related carbon—carbon multiple bonds catalyzed by ytterbium-imine complexes. Tetrahedron Lett 42 6357-6360... 

A mechanism that involves ytterbium phosphide species has been proposed, similarly to the foregoing intramolecular hydrophosphination. Generation of the phosphide species is supported by the formation of Ph2CDNHPh (after aqueous quench) upon treatment of the imine complex with Ph2PD (Scheme 15). Lanthanide phosphide is known to react with THF, forming a 4-diphenylphosphino-l-butoxyl species [21], which was indeed found as a side product in the catalytic hydrophosphination of disubstituted aliphatic alkynes run in THF, supporting further the ytterbium-phosphide intermediate (Scheme 16). 

Lanthanide(ll)-imine complexes, obtained by reduction of aromatic ketimines with samarium and ytterbium metal, effectively catalyze the hydrosilylation of imines. The proposed catalytic cycle for the imine hydrosilylation is outlined in Scheme 279.961 1033... 

The first example for catalytic and enantioselective hydrophosphonylation of cyclic imines using cyclic phosphites is the lanthanide BINOL complex catalyzed hydrophosphonylation of 3-thiazolines. The pharmacologically interesting thiazolidinyl phosphonates are synthesized with excellent optical purities of up to 99% ee using the ytterbium-BINOL complex (116). 

Reductive coupling of benzalimines with a ytterbium metal occurs in a mixture of THF and HMPA at room temperature (Equation (66)). " The reaction proceeds via an azaytterbiacyclopropane complex. This kind of metallacycle is isolated in the case of a benzophenone-imine. 

It has been demonstrated625 that ytterbium-aromatic imine dianion complexes can act as effective catalysts for the isomerization of terminal alkynes to internal alk-2-ynes. Isomerization of acetylenic pentafluorophenyl esters in the presence of phosphines has been found to give rise to activated dienoic acids, which have been coupled directly with amines (and alcohols) in a simple one-pot procedure626 (see Scheme 124). 

Some of the metal-based catalysts used in the asymmetric hydrophosphonylation of aldehydes (see Section 6.4) can also be applied to the phosphonylation of imines. For instance, Shibasaki s heterobimetallic BINOL complexes work well for the catalytic asymmetric hydrophosphonylation of imines. In this case lanthanum-potassium-BINOL complexes (6.138) have been found to provide the highest enantioselectivities for the hydrophosphonylation of acyclic imines (6.139). The hydrophosphonylation of cyclic imines using heterobimetallic lanthanoid complexes has been reported. Ytterbium and samarium complexes in combination with cyclic phosphites have shown the best results in the cases investigated so far. For example, 3-thiazoline (6.140) is converted into the phosphonate (6.141) with 99% ee using ytterbium complex (6.142) and dimethyl phosphite (6.108). The aluminium(salalen) complex (6.110) developed by Katsuki and coworkers also functions as an effective catalyst for the hydrophosphonylation of both aromatic and aliphatic aldimines providing the resulting a-aminophosphonate with 81-91% ee. ... 

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