Unusual Complex Chemistry of Rare‐Earth Elements: Large Ionic Radii—Small Coordination Numbers

Dehnicke, Kurt; Greiner, Andreas

doi:10.1002/anie.200390346

Cited by 102 publications

(57 citation statements)

References 127 publications

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“…It can be seen that light rare earth compounds produce PDLLA with higher yields and molecular weights. The reason of the activity difference between various rare earth catalysts may result from the nature of the rare earth metal and the ancillary ligands present in the coordination sphere of the metal [25][26][27][28][29] .…”

Section: Infl Uence Of Substitution Of the Aryloxide Ligands On The Pmentioning

confidence: 99%

A series of rare earth phenolates substituted by alkyl groups for D,L-lactide ring-opening polymerization

Wang

Niu

Zhang

et al. 2011

J. Wuhan Univ. Technol.-Mat. Sci. Edit.

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Single component rare earth phenolates substituted by various alkyl groups have been prepared and the correlation between the aryloxides' structure and catalytic activity in the ring-opening polymerization of D, L-lactide has also been investigated. The catalytic activity of all rare earth aryloxides, characteristics of the ring-opening polymerization as well as polymerization kinetics and mechanism were investigated. The results showed that both phenolates' catalytic activities and polymerization characteristics changed regularly, keeping in good concordance with variations in substitutents' number on phenol and structure of aryloxide ligands. The stronger ability of electron-donation of alkyl groups, the higher catalytic activity. Moreover, the more numbers of substituted alkyl on phenyl ring, the higher catalytic activity. The analyses of polymer ends revealed that the polymerization proceeded via a coordination-acyl-oxygen bond cleavage-insertion mechanism.

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Section: Infl Uence Of Substitution Of the Aryloxide Ligands On The Pmentioning

confidence: 99%

A series of rare earth phenolates substituted by alkyl groups for D,L-lactide ring-opening polymerization

Wang

Niu

Zhang

et al. 2011

J. Wuhan Univ. Technol.-Mat. Sci. Edit.

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“…[4,50] In contrast, rare-earth element Lewis acids (such as CeCl 3 ) tend, in several circumstances, to be incapable of chelation because the resulting large coordination number leads to long and therefore unstable metalligand bonds. [51] Chelation and nonchelation control have been proposed by us to justify the diastereofacial selectivity obtained in the reduction of α-alkyl-β-functionalised carbonyl compounds. On the basis of this, we have studied the reduction of N-protected α-alkyl-β-amino ketones 18 leading to the formation of syn-and anti-1-amino-3-hydroxy compounds (syn-19 and anti-19; Table 4).…”

Section: Diastereoselective Reduction Of N-protected α-Alkyl-β-amino mentioning

confidence: 99%

Diastereoselective Lewis Acid Mediated Reductions of α‐Alkyl‐β‐Functionalized Carbonyl Compounds

et al. 2005

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A high stereochemical induction has often been observed in the reduction of ketones with stereogenic centres close to the carbonyl function. The level of stereoselectivity is particularly high when “hard” donor groups such as nitrogen or oxygen atoms are present in the molecule. These results have been commonly explained in terms of a chelate‐controlled process. However, this interpretation has recently been challenged, and in many cases an open‐chain mechanism can better account for the observed results. Thus, Lewis‐acid‐mediated chelation and nonchelation control is one of the most fundamental and practical concepts in transferring a nucleophile moiety to carbonyl compounds. In order to clarify the role of a hard Lewis acid in controlling the stereochemical outcome of a given reaction, the Lewis acid mediated reduction of a series of α‐alkyl‐β‐functionalized carbonyl compounds with metallic hydrides in various solvents was recently investigated. In particular, the results derived from the reductions in the presence of dry CeCl3 were compared to those obtained under the same experimental conditions but in the presence of TiCl4, whose ability to form chelation complexes is well established. These investigations show a stereochemical outcome that is fully consistent with a chelation‐controlled pathway in the case of titanium, and an open‐chain‐controlled pathway in the case of CeCl3. The strongly chelating TiCl4 led to the syn isomer in high diastereomeric excess in noncoordinating solvents at –78 °C with borane–Lewis base complex reducing agents, while nonchelating CeCl3 produced a high excess of the anti isomer in coordinating solvents at the same temperature with metal borohydrides as reducing agent. Therefore, our method based on the choice of CeCl3 or TiCl4 as a Lewis acid under the reported conditions represents a significant contribution to the development of new stereoselective reductions of α‐alkyl‐β‐functionalized ketones. These procedures are one of the best known ways to obtain an alcoholic moiety. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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“…[12] A chelation and non-chelation control model has been proposed by us to justify the diastereofacial selectivity obtained in reductions of α-substituted β-functionalized carbonyl compounds. On the basis of these results, we have examined the reduction of N-protected α-substituted β-amino ketones (1) in the expectation of the formation of syn-and anti-1-amino-3-hydroxy compounds 2.…”

Section: Resultsmentioning

confidence: 99%

Lewis Acid‐Mediated Diastereoselective Reduction ofN‐Protected β‐Amino Ketones: Influence of the Nature of the Metal Atom and of the Nitrogen Protecting Group

et al. 2004

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Lewis acid-mediated chelation and non-chelation control is one of the most fundamental and practical concepts in modern organic chemistry. Extension of these concepts to the reduction of different N-protected α-substituted β-amino ketones indicated that diastereoselectivity in the hydride addition step is strongly dependent on the nature of the metal atom and on the type of protecting group. Strongly chelating TiCl 4 and an electron-rich nitrogen protecting group pro-

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Unusual Complex Chemistry of Rare‐Earth Elements: Large Ionic Radii—Small Coordination Numbers

Cited by 102 publications

References 127 publications

A series of rare earth phenolates substituted by alkyl groups for D,L-lactide ring-opening polymerization

A series of rare earth phenolates substituted by alkyl groups for D,L-lactide ring-opening polymerization

Diastereoselective Lewis Acid Mediated Reductions of α‐Alkyl‐β‐Functionalized Carbonyl Compounds

Lewis Acid‐Mediated Diastereoselective Reduction ofN‐Protected β‐Amino Ketones: Influence of the Nature of the Metal Atom and of the Nitrogen Protecting Group

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