Imidazolidinone salts, chiral

General Procedure for the Intramolecular Michael Addition with Chiral Imidazolidinone Salt [33] (p. 90)... 

A review of recent developments in 1,3-dipolar cycloaddition of nitrones with sila-, thia-, phospha-, and halo-substituted alkenes has been reported. A DFT study of solvent effects on the intermolecular 3-l-2-cycloaddition reaction of norbornadiene with 3,4-dihydroisoquinoline A(-oxide at 398.15 K indicated that the reaction proceeds via a synchronous concerted mechanism. Chiral imidazolidinone salts, in the absence of water, promote the 1,3-dipolar cycloaddition reaction of alanine-derived ketonitrones... 

The first enantioselective organocatalytic 1,3-dipolar cycloaddition of acyclic nitrones with acrolein and crotonal-dehyde has been reported <2000JA9874>. In particular, the reversible formation of iminium ions from a,/3-unsatu-rated aldehydes and the enantiopure imidazolidinone 535 provided ( A-4-formylisoxazolidines in high yields and ees (Equation 86). A polymer-supported version of catalyst 535 was also prepared <2004EJ0567>. The catalytic performance of various chiral pyrrolidinium salts in the cycloaddition of 1-cycloalkene-l-carboxaldehydes was also evaluated <2003EJO2782>. 

Analogous alkylations with unsaturated ketones can also be effected with silica-supported benzenesul-fonic acid sodium salt or, with some stereoselectivity, using a chiral imidazolidinone organo-catalyst. Optical induction can also be achieved in the addition of indole to alkyhdene malonates using bisoxazoline copper(II) complexes. ... 

Some interesting organocatalyzed asymmetric double conjugate additions have been recently reported. For instance, the hydrochloride salt of chiral imidazolidinone... 

MacMillan et al. studied the reactivity of transient enamines formed with organocatalysts with hypervalent iodine reagents in the presence of metal catalysts (Scheme 26.5a). They found that Cu(l) salts can be used in combination with chiral imidazolidinones to perform a-electrophilic trifluoromethylation [47], arylation [48], and vinylation [49] of aldehydes. As suggested by the authors, the most likely mechanism involves a copper-mediated I-O bond cleavage furnishing a highly... 

Northrup and MacMillan extended the iminium-mediated Diels-Alder reactions to a,p-unsaturated ketones using a new chiral amine catalyst (Schane 1.43) [66]. They found that cycloaddition of a,P-unsaturated ketones was unsuccessful with the chiral amine salts previously identified as excellent catalysts for enal activation. In contrast, the 2-(5-methylfuryl)-derived imidazolidinone 118 afforded good levels of enantiofacial discrimination while maintaining high reaction efficiency (89% yield, 25 1 endo/exo, 90% ee). 

Several protocols for the asymmetric reduction of a,p-unsaturated carbonyl compounds rely on the use of add salts of chiral imidazolidinones 6, a very popular catalyst class for enantioselective conjugate additions [22] (Fig. 2.2). 

The MacMillan group has also shown that cycloaddition reactions (see also Chapter 8) can be performed highly diastereo- and enantioselectively. The [3+2]-cycloaddition of nitrones and a,/i-un saturated carbonyl compounds in the presence of 20 mol% of a phenylalanine-derived imidazolidinone acid salt led to products with 99% ee [32]. An example of an enantioselective rearrangement reaction (see also Section 13.6) with 99% ee has been reported by the Fu group [33], who used 2 mol% of a planar chiral DMAP derivative as catalyst. 

Copper(II) salts are efficient one-electron oxidants for the generation of radicals from lithium enolates [1]. This concept was successfully applied for the oxidative coupling of ketones or amides 36 to afford the corresponding 1,4-dicarbonyl compounds 37 in good yield [17]. If an optically active imidazolidinone is used as chiral auxiliary, the reaction exhibits an excellent simple and induced diastereoselectivity (Scheme 12). 

Two C-C Bond-Forming Events In 2008, Frechet and coworkers described an impressive asymmetric cascade reaction promoted by soluble star polymers with core-confined catalytic entities [10]. The encapsulation of catalysts into soluble star polymers allowed the use of incompatible catalysts and prevented undesired interactions between these catalytic systems. The organocascade corresponded to a nucleophilic addition of Af-methylindole to a,p-unsaturated aldehydes followed by a Michael addition of the adduct to methylvinylketone (MVK) in the presence of H-bonding additive (Scheme 12.5). Each catalyst - imidazolidinone 8 for the nucleophilic addition and diphenylprolinol methyl ether 9 for the Michael addition - or their combination cannot mediate both reaction steps. In particular, p-toluenesulfonic acid (p-TSA) diminished the ability of the chiral pyrrolidine 9 to effect enamine activation. Therefore, p-TSA and 9 were encapsulated in the core of star polymers, which cannot penetrate each other. Imidazolidone 8 was added to the acid star polymer and diffused to the core to form the salt, which allowed the iminium activation and catalyzed the first step. The second step was catalyzed by the pyrrolidine star polymer in presence of the H-bonding additive 10, which... 

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