Lewis acid-catalyzed Claisen rearrangements

Lewis Acid-Catalyzed Claisen Rearrangements in the Preparation of Chiral Products... 

Recently, Hiersemann reported the first catalytic enantioselective Claisen rearrangement (Scheme 2.4) [11]. The 2-alkoxycarbonyl-substituted allyl vinyl ethers 11 are reactive under the Lewis acid catalysis. Therefore, the Claisen rearrangements proceed catalytically [12]. Usually the Lewis-acid-catalyzed Claisen rearrangement does not proceed catalytically because of a higher affinity of the carbonyl product for the Lewis acids than the ether substrate. But this 2-alkoxycarbo-nyl-substituted substrate 11 can coordinate to metals in a bidentate fashion. This 2-alkoxycarbonyl substrate has higher affinity for Lewis acidic Cu complexes than the simple ether substrate. In this system, chiral copper (II) bisoxazoline Cu (box) complex 13 is effective for the enantioselective Claisen rearrangement. 

Late transition-metal-catalyzed asymmetric Claisen rearrangement takes place in a different mode from that of Lewis-acid-catalyzed Claisen rearrangement Late transition metal catalysis is based on affinity for the Claisen diene system. Among late transition metals, palladium complexes are the most useful and effective for the Claisen rearrangement. 

Examples of achiral Lewis acid catalyzed Claisen rearrangement (a) B. M. Trost,... 

MacMillan has developed a Lewis acid-catalyzed addition-rearrangement sequence which facilitates the conjugate addition of allylamine 408 to allenoate ester 409 to generate the zwitterionic intermediate for the Claisen rearrangement. The minimization of steric interactions between the ammonium species and the y-allenyl substituent led... 

This section deals with Bronsted acid and Lewis acid catalyzed reactions, excluding Friedel-Crafts reactions, but including reactions such as nitrations, halogenations, and Claisen rearrangements. Friedel-Crafts reactions are discussed in the subsequent Sections 5.1.2.2 and 5.1.2.3. 

Isolation of intermediates by treatment of 2 with 3 equiv. of I at room temperature lot 12 hours. Presumably (CH3)2A1C1 is liberated from 1 during methylenation and this Lewis acid catalyzes the Claisen rearrangement. Five other examples of this transformation are cited. 

Several chiral organoaluminum Lewis acids catalyze the Claisen rearrangement of achiral allyl vinyl ethers to furnish chiral (3, y-unsaturated aldehydes with good enantioselectivity. Among the most effective catalysts is ATBN-F, a chiral aluminum tris(P-naphthoxide) species prepared from enantiomerically pure binapthol. ... 

Non-chloroaluminate ILs, which are in general poor nucleophiles, have proven to be attractive alternative media for Lewis acid catalyzed reactions. ILs may have a reaction rate accelerating effect, and they may improve selectivity and facilitate catalyst recovery. This is the case for scandium triflate catalyzed Diels-Alder cycloaddition [8,9], three-component (aldehyde, aniline, triethylphosphite) synthesis of a-aminophosphonates [10], Claisen rearrangement and cyclization reactions [11], or Friedel-Crafts reactions [12, 13]. 

The first example of Lewis-acid catalyzed aromatic Claisen rearrangement was reported in 1941 [30], that is, the BF,-AcOHmild conditions (80 °C). Since this discovery of acceleration by Lewis acid, numerous examples such as BCl, [31], BF, [32], EtjAlQ [33], ZnCl2 [34], and TiCl [35] have been reported. For details regarding catalysts for the Claisen rearrangement an excellent review article by Lutz [Ih] is recommended. 

The first results were reported by Hill et al. investigating the Lewis-acid-catalyzed formation of imines 82 [16]. In the presence of TiCl4 3-butenyl-2-N-phenyl-amine 81, diverse aldehydes 80 were mixed resulting in a sequence of initial enamine 82 formation and a consecutive aza-Claisen rearrangement to give prod-... 

MacMillan has developed a Lewis acid-catalyzed Bellus-Claisen rearrangement in which a range of allylamines can add to ketenes via Lewis acids activation (253). This activation-addition pathway generates the zwitterionic allyl alkenyl ammonium complex as shovra for 250 which effectively rearranges to amide 251 with excellent syn selectivity. 

In 1990, Yamamoto and Maruoka reported the first example of chiral aluminum Lewis acid-catalyzed asymmetric Claisen rearrangement (Scheme 6.162) [192]. Since chiral Lewis acids (2a) and (2b) derived from 3,3 -substituted BINOL derivative (1) exists as a mononuclear complex, high catalytic activity was realized. In this reaction, simple allyl vinyl ethers gave only poor results, although silylated substrates resulted in good enantioselectivity. 

The only examples of asymmetric Claisen rearrangements catalyzed by a chiral aluminum catalyst are those published by Yamamoto and coworkers [24-27]. The Claisen rearrangement of allyl vinyl ethers of type 91 (Sch. 13) can proceed via either of the two enantiomeric chair transition states 92 or 94. If a chiral Lewis acid is used to activate the process, the transition states become diastereomeric and preferential formation of one enantiomer of the product is possible. This is complicated because coordination of a chiral Lewis acid to the ether oxygen of allyl vinyl ether produces a new chiral center as indicated in 96 and asymmetric induction might occur from a substituent on aluminum to the oxygen and then from the oxygen to the C-3 carbon in the product. In their initial report, Maruoka, Banno, and Yamamoto found that a catalyst prepared from the 3,3 -disilyl-substituted BINOL 97 and trimethylaluminum was effective in this transformation [24]. The catalyst 98 was shown to be monomeric by measurement of freezing point depression. 

Although cycloadditions and rearrangements often proceed without catalysts, the selectivity of the reaction and the reactivity of very reluctant molecules can be enhanced by a Lewis acid. The Diels-Alder reaction can be performed at enhanced reaction rate in ionic liquids with the addition of ZnCl2 (eq. (4)) [42]. The Claisen rearrangement was also reported to be superior if catalyzed by Sc(OTf)3 in ionic liquids [43]. 

The Claisen rearrangement can be effectively catalyzed by Lewis acids, Bronsted acids, bases, Rh(I) and Pt(0) complexes as well as by silica . Several reviews were published recently in which the application of zeolites and acid-treated clays as catalysts for the Claisen rearrangement was described Thus, it was shown that the rearrangement conditions for phenolic allyl ethers can be dramatically milder if this reaction is carried out by thermolysis of a substrate immobilized on the surface of previously annealed silica gel for chromatography. For example, the thermolysis of ether 159 on silica gel (in a 159 Si02 ratio of 1 10 w/w) at 70°C gives the phenol 160 in 95% yield after 3.5 hours (equation 70). An additional example is shown in equation 71. ... 

Rearrangements. Under microwave irradiation the Claisen rearrangement of N-allyl-anilines is effectively catalyzed by BF3 OEt2. Other Lewis acids are much inferior catalysts under the same conditions. ... 

The Claisen rearrangement can be catalyzed by Lewis acids, often with improved stereoselectivity. Acyclic allyl vinyl ethers give a Claisen rearrangement upon treatment with a catalytic amount of Pd(II). 5 a Ho(fod) catalyst has been used as well. An example of this type of reaction is the conversion of 592 to a 22 71 mixture of 593 and 594 in 99% yield. 47... 

The aza-Claisen rearrangement can be catalyzed by Lewis acids such as titanium tetrachloride (TiCU). When aldehyde 653 reacted with 3-(A -phenyl)-1 -butene (654) in the presence of TiCU, the initially formed enamine (655) rearranged to 656 and was hydrolyzed to aldehyde 657 in 61% overall yield. ... 

Bidentate Lewis acid. This useful catalyst (1) with a high propensity for double coordination of the carbonyl group is prepared from the corresponding phenol and two equivalents of McjAI in CH Clj at room temperature. It catalyzes the reduction of 5-nonanone by BujSnH at -78° in 86% yield, whereas a reaction in the presence of the monodentate 0-dimethylaluminum 2,6-xylenoxide affords 5-nonanol in only 6%.. Accordingly, different catalytic efficiencies are also found in the Mukaiyama aldol reaction (e.g., 87% vs. 0% in the reaction between 1-trimethylsiloxy-l-cyclohexene and benzaldehyde) and the Claisen rearrangement of (fil-cinnamyl vinyl ether (96% vs. 0%). The contrasting ( >Zi-selectivity of the Michael adducts also reflects the different coordination states. 

The Claisen rearrangement can be catalyzed by Lewis acid catalysts (Figure 11.54), and the use of chiral Lewis catalysts provides a path to diastereoselective and enantioselective Claisen rearrangements. ... 

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