Scandium triflate-PYBOX complex

Fukuzawa et al. [99] found analogous scandium(III)triflate/ Pr-PyBOx complex as efficient catalyst for the asymmetric Diels-Alder reaction between cyclopentadiene or acyclic dienes and acyl-l,3-oxazohdin-2-ones with up to 90% ee. They latter described the same reaction in super critical CO2 in the presence of MSdA [ 100] that proceeded more rapidly than in CH2CI2 leading to the expected product with analogous selectivity. 

Allenyltrimethylsilanes add to ethyl glyoxalate in the presence of a chiral pybox scandium triflate catalyst to afford highly enantioenriched homopropargylic alcohols or dihydrofurans, depending on the nature of the silyl substituent (Tables 9.39 and 9.40) [62]. The trimethylsilyl-substituted silanes give rise to the alcohol products whereas the bulkier t-butyldiphenylsilyl (DPS)-substituted silanes yield only the [3 + 2] cycloadducts. A bidentate complex of the glyoxalate with the scandium metal center in which the aldehyde carbonyl adopts an axial orientation accounts for the observed facial preference ofboth additions. 

PYBOX complexes of scandium(III) triflate act as enantio- and diastereo-selective catalysts of ene reactions, including examples with trisubstituted alkenes.207... 

Enantiopure amide derivatives (64) of -unsaturated a-hydroxy acids have been made by addition of a vinylsilane, R2R1C=(4ISiMe3, to /V-phcnylglyoxamide.181 The reaction is catalysed by scandium(III) triflate complexed to a C2-symmetric PYBOX ligand derived from (f )-norephedrine. 

The asymmetric conjugate addition of thiophenol to ( >3-crotonoyloxazolidin-2-one, catalysed by the scandium(III) triflate complex of Ph-PYBOX, gave the corre- sponding adduct in 66% ee. Lanthanoid triflates gave lower enantioselectivities (<28% ee).132... 

Chiral 2-(3-oxoalkyl)pyrroles and 3-(3-oxoalkyl)indoles can also be accessed by reaction in the presence of 10 mol% of chiral bis(oxazoline)/metal complexes in CH2C12 in very high yields and with ee values over 90% <2005JA4154>. Alkylation of pyrrole and of substituted indoles with, -unsaturated acyl phosphonates <2003JA10780> or 2-acyl N-methylimidazoles catalyzed by a chiral bis(oxazolinyl)pyridine (pybox)/scandium(III) triflate complex also exhibits good enantioselectivity over a broad range of substrates <2005JA8942>. 

The Friedel-Crafts alkylation of the parent pyrrole and of substituted indoles with a,P-unsaturated acyl phospho-nates 468 <2003JA10780> and 2-acyl iV-methylimidazoles 469 catalyzed by the chiral bis(oxazolinyl)pyridine (pybox)/scandium(lIl) triflate complex 467 exhibits good enantioselectivities over a broad range of substrates (Scheme 97, Equation 113) <2005JA8942>. The desired alkylation products 470-472 were formed in good yields and enantioselectivities. 

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