TY - JOUR
T1 - C2-Symmetric Bis(oxazolinato)lanthanide Catalysts for Enantioselective Intramolecular Hydroamination/Cyclization
AU - Hong, Sukwon
AU - Tian, Shun
AU - Metz, Matthew V.
AU - Marks, Tobin J.
PY - 2003/12/3
Y1 - 2003/12/3
N2 - C2-symmetric bis(oxazolinato)lanthanide complexes of the type [(4R,5S)-Ph2Box]La[N(TMS)2]2, [(4S,5R)-Ar 2Box]La[N(TMS)2]2, and [(4S)-Ph-5,5-Me 2Box]La[N(TMS)2]2 (Box = 2,2′ -bis(2-oxazoline)methylenyl; Ar = 4-tert-butylphenyl, 1-naphthyl; TMS = SiMe3) serve as precatalysts for the efficient enantioselective intramolecular hydroamination/cyclization of aminoalkenes and aminodienes. These new catalyst systems are conveniently generated in situ from the known metal precursors Ln[N(TMS)2]3 or Ln-[CH(TMS) 2]3 (Ln = La, Nd, Sm, Y, Lu) and 1.2 equiv of commercially available or readily prepared bis-(oxazoline) ligands such as (4R,5S)-Ph2BoxH, (4S,5R)-Ar2BoxH, and (4S)-Ph-5,5-Me 2BoxH. The X-ray crystal structure of [(4S)- tBuBox]Lu[CH(TMS)2]2 provides insight into the structure of the in situ generated precatalyst species. Lanthanides having the largest ionic radii exhibit the highest turnover frequencies as well as enantioselectivities. Reaction rates maximize near 1:1 BoxH:Ln ratio (ligand acceleration); however, increasing the ratio to 2:1 BoxH:Ln decreases the reaction rate, while affording enantiomeric excesses similar to the 1:1 BoxH:Ln case. A screening study of bis(oxazoline) ligands reveals that aryl stereodirecting groups at the oxazoline ring 4 position and additional substitution (geminal dimethyl or aryl) at the 5 position are crucial for high turnover frequencies and good enantioselectivities. The optimized precatalyst, in situ generated [(4R,5S)-Ph2Box]La[N(TMS)2] 2, exhibits good rates and enantioselectivities, comparable to or greater than those achieved with chiral C1-symmetric organolanthanocene catalysts, even for poorly responsive substrates (up to 67% ee at 23 °C). Kinetic studies reveal that hydroamination rates are zero order in lamine substrate] and first order in [catalyst], implicating the same general mechanism for organolanthanide-catalyzed hydroamination/cyclizations (intramolecular turnover-limiting olefin insertion followed by the rapid protonolysis of an Ln-C bond by amine substrate) and implying that the active catalytic species is monomeric.
AB - C2-symmetric bis(oxazolinato)lanthanide complexes of the type [(4R,5S)-Ph2Box]La[N(TMS)2]2, [(4S,5R)-Ar 2Box]La[N(TMS)2]2, and [(4S)-Ph-5,5-Me 2Box]La[N(TMS)2]2 (Box = 2,2′ -bis(2-oxazoline)methylenyl; Ar = 4-tert-butylphenyl, 1-naphthyl; TMS = SiMe3) serve as precatalysts for the efficient enantioselective intramolecular hydroamination/cyclization of aminoalkenes and aminodienes. These new catalyst systems are conveniently generated in situ from the known metal precursors Ln[N(TMS)2]3 or Ln-[CH(TMS) 2]3 (Ln = La, Nd, Sm, Y, Lu) and 1.2 equiv of commercially available or readily prepared bis-(oxazoline) ligands such as (4R,5S)-Ph2BoxH, (4S,5R)-Ar2BoxH, and (4S)-Ph-5,5-Me 2BoxH. The X-ray crystal structure of [(4S)- tBuBox]Lu[CH(TMS)2]2 provides insight into the structure of the in situ generated precatalyst species. Lanthanides having the largest ionic radii exhibit the highest turnover frequencies as well as enantioselectivities. Reaction rates maximize near 1:1 BoxH:Ln ratio (ligand acceleration); however, increasing the ratio to 2:1 BoxH:Ln decreases the reaction rate, while affording enantiomeric excesses similar to the 1:1 BoxH:Ln case. A screening study of bis(oxazoline) ligands reveals that aryl stereodirecting groups at the oxazoline ring 4 position and additional substitution (geminal dimethyl or aryl) at the 5 position are crucial for high turnover frequencies and good enantioselectivities. The optimized precatalyst, in situ generated [(4R,5S)-Ph2Box]La[N(TMS)2] 2, exhibits good rates and enantioselectivities, comparable to or greater than those achieved with chiral C1-symmetric organolanthanocene catalysts, even for poorly responsive substrates (up to 67% ee at 23 °C). Kinetic studies reveal that hydroamination rates are zero order in lamine substrate] and first order in [catalyst], implicating the same general mechanism for organolanthanide-catalyzed hydroamination/cyclizations (intramolecular turnover-limiting olefin insertion followed by the rapid protonolysis of an Ln-C bond by amine substrate) and implying that the active catalytic species is monomeric.
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U2 - 10.1021/ja0364672
DO - 10.1021/ja0364672
M3 - Article
C2 - 14640652
AN - SCOPUS:0345392827
VL - 125
SP - 14768
EP - 14783
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 48
ER -