Diels TADDOL-catalyzed

Seeeach et al. also investigated the Ti-TADDOlate-catalyzed Diels-Alder reaction, see D. Seeeach, R. Dahinden,... 

In most TiCl2-TADDOLate-catalyzed Diels-Alder and 1,3-dipolar cycloaddition reactions oxazolidinone derivatives are applied as auxiliaries for the alkenoyl moiety in order to obtain the favorable bidentate coordination of the substrate to the catalyst... 

An important catalyst-substrate intermediate that applies to both the TiCl2-TADDOLate catalyzed 1,3-dipolar cycloadditions and Diels-Alder reactions has been isolated and characterized (353). The crystalline compound 248 has been characterized by X-ray analysis, showing that the oxazolidinone is coordinated to the titanium center in a bidentate fashion (Scheme 12.75). The four oxygen atoms. 

Taddol-Catalyzed Hetero-Diels-Alder Reaction of l-Aldimine-3-Siloxy Dienes [87] (p. 236)... 

Taddol-Catalyzed Diels-Alder Reactions of l-Amino-3-Siloxydienes [88] (p. 242)... 

It has been observed that enantioselective polymer-bound catalysts prepared by copolymerization produce in some cases better asymmetric inductions than systems prepared by grafting [175]. After much optimization, a monolithic polymer catalyst 51 suitable for a titanium-TADDOLate catalyzed Diels-Alder reaction was developed (Scheme 4.77). The monolith was applied in a flow system both under one pass and 24 h recirculation conditions, the latter producing the best yield (55%) and ee (23%) however, this contrasts poorly with the homogeneous batch reaction although the ee is comparable with the heterogeneous batch process. The reversal of topicity was also... 

Mukaiyama T, Inubushi A, Suda S, Kara R, Kobayashi S (1990) Chem Lett 1015 Substituted aromatic solvents have been shown to have a dramamtic effect on the enan-tioselectivity of Ti TADDOL-catalyzed Diels Alder addition reactions, see Narasaka K, Iwasawa N, Inoue M, Yamada T, Nakashima M, Sugimori J (1989) J Am Chem Soc 111 5340... 

SCHEME 6.17. TADDOL catalyzed oxo-hetero-Diels-Alder reactions. 

Figure 10.2 Proposed model for the taddol-catalyzed hetero-Diels-Alder reaction.

The mechanism for the catalytic enantioselective carbo-Diels-Alder reaction of N-alkenoyl-l,3-oxazolidin-2-one 4 with, e.g., cyclopentadiene 2 catalyzed by chiral TADDOL-Ti(IV) complexes 6 has been the subject for several investigations and especially, the structure of the intermediate for the reaction has been subject to controversy. The coordination of 4 to 6 can give five diastereomeric complexes A, B], B2, C], and C2, as outlined in Fig. 8.8. 

A simple, commercially available chiral alcohol, a,a,a a -tetraaryl-l,3-dioxo-lane-4,5-dimethanol (TADDOL, 7a), catalyzes the hetero- and carbo-Diels-Alder reactions of aminosiloxydienes with aldehydes and a-substituted acroleins to afford the dihydropyrones and cyclohexenones, respectively, in good yields and high enan-tioselectivities. More recently, it was reported that axially chiral biaryl diols 7b and 7c were more highly effective catalysts for enantioselective hetero-Diels-Alder reactions (Scheme 12.5). ... 

The enantioselective hetero-Diels-Alder (HDA) reaction of carbonyl compounds with 1,3-dienes represents an elegant access to optically active six-membered oxo-heterocycles. Since the pioneering work of Rawal et al. in 2003 [55], the enantioselective HDA reaction catalyzed by diols (such as TADDOLs) has become a flourishing field of research [56]. 

Because a comprehensive discussion of the transition state of hydrogen-bond catalysis will be presented by Berkessel in Chapter 3, the hydrogen bond catalyzed hetero Diels-Alder reaction of butadiene with carbonyl compounds will be discussed briefly here. Huang and Rawal reported that the hetero Diels-Alder reaction of aminodiene with aldehyde exhibited significant solvent effects (Scheme 2.7) [15]. The reaction in CHCfi was accelerated 30 times in comparison with that in THF, while that in i-PrOH was accelerated 630 times. They proposed that the Diels-Alder reaction was promoted by the hydrogen-bond activation of aldehyde. This finding resulted in the development of TADDOL catalyst [3]. 

Several researchers have reported synthetic approaches based on asymmetric Diels-Alder reactions catalyzed by TADDOL-Ti complexes [117-120]. Dendritic [121] and polymer-supported TADDOL-Ti complexes [122] have also been employed as recoverable and reusable catalysts to give comparatively high enantioselectivity. Transition-state models have been proposed independently by several groups for TADDOL-type titanium catalysis [121,123]. 

Polymer-supported TADDOL-Ti catalyst 79 prepared by chemical modification was poorly active in the Diels-Alder reaction of 3-crotonoyloxazolidinone with cyclo-pentadiene (Eq. 24) whereas polymeric TADDOL-Ti 81 prepared by copolymerization of TADDOL monomer 80 with styrene and divinylbenzene had high activity similar to that of the soluble catalyst. In the presence of 0.2 equiv. 81 (R = H, Aryl = 2-naphthyl) the Diels-Alder adduct was obtained in 92 % yield with an endolexo ratio of 87 13. The enantioseleetivity of the endo product was 56 % ee. The stability and recyclability of the catalyst were tested in a batch system. The degree of conversion, the endolexo selectivity, and the enantioseleetivity hardly changed even after nine runs. Similar polymer-supported Ti-TADDOLate 82 was prepared by the chemical modification method [99]. Although this polymer efficiently catalyzed the same reaction to give the (2R,2S) adduct as a main product, asymmetric induction was less than that obtained by use of a with similar homogeneous species. 

SCHEME 6.13. TADDOL and chiral bis-phosphoric acid catalyzed Diels-Alder reactions of acrolin-derivatives with amidodienes. 

Scheme 10.5 Enantioselective hetero-Diels-Alder reaction of Brassard s diene catalyzed by taddol 1.

Scheme 10.1 Enantioselective Diels-Alder and hetero-Diels-Alder reactions catalyzed by taddol 1.

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