Diels-Alder reaction -salen catalyst

In most of the successful Diels-Alder reactions reported, dienes containing no heteroatom have been employed, and enantioselective Diels-Alder reactions of multiply heteroatom-substituted dienes, e.g. Danishefsky s diene, are rare, despite their tremendous potential usefulness in complex molecular synthesis. Rawal and coworkers have reported that the Cr(III)-salen complex 15 is a suitable catalyst for the reaction of a-substituted a,/ -unsubstituted aldehydes with l-amino-3-siloxy dienes [21] (Scheme 1.28, Table 1.12). The counter-ion of the catalyst is important and good results are obtained in the reaction using the catalyst paired with the SbFg anion. 

Li et al.39 reported the hetero Diels-Alder reaction of alkyl-3-(t-butyldimethylsilyl) oxy-1,3-butadiene 95 with ethyl glyoxylate 96 in the presence of a salen-Co(II) catalyst 94 (2 mol%). Product 97 was obtained in 75% isolated yield with an endo. exo ratio >99 1. The enantiomeric excess of the endo-iorm was up to 52% (Scheme 5-30). 

Jacobsen studied the efficiency of a series of chiral (salen)Cr(III)X complex (X = Cl, N 3, F, BF4) for the asymmetric hetero Diels-Alder reaction of Danishefsky s diene with aldehydes. The best result was obtained with the terafluoroborate catalyst [65] (Eq. 8A.41). 

Transition-metal complexes with salen ligands can serve as catalysts for a multitude of stereogenic reactions including Diels-Alder reactions. We recently studied the application of polyglycerol-supported salen systems 18, 19 and 20 as chiral catalysts in the hetero Diels-Alder reaction between... 

The readily prepared and air-stable complex trans-[Ru(salen)(N0)(H20)](SbF6), when added (1 mol %) to Diels-Alder reactions in nitromethane, leads to acceleration by as much as 10. The presence of water slows the reaction, but does not inhibit it. The Ru catalyst is 1 or 2 orders of magnitude more efficient in catalyzing these reactions than trifluoroacetic acid (Sch. 51) [190]. Asymmetric versions of the Ru methodology have not appeared in the literature but chiral oxo(salen)Mn(V) complexes show some promise the results of the best (of ten compoimds tested) are included in Sch. 51 [191]. 

Hu, Y-J, Huang, X-D, Yao, Z-J, Wu, Y-L, Formal synthesis of 3-deoxy-D-manno-2-octulosonic acid (KDO) via a highly double stereoselective hetero-Diels-Alder reaction directed by a (salen)Co catalyst and chiral diene, J. Org. Chem., 63, 2456-2461, 1998. 

This process (hetero Diels-Alder reaction leading to a dihydropyran system) may be also conducted in an asymmetric version application of chiral transition-metal catalysts based on BINOL, BDMAP, bisoxazolines, etc. provides adducts in very high optical purity (ee up to 99%) [1,6], In a series of papers Jurczak reported recently a highly enantioselective cycloaddition of 1-methoxy-1,3-butadiene and butyl glyoxylate catalyzed with chiral salen complexes [21],... 

Cp—Cp coupling occurs, probably via the first-formed palladium phenolate (315) to give the bisquinone methide (316), and the latter spontaneously undergoes intramolecular Diels-Alder reaction to the natural lignan carpanone (317) in 46% yield, with stereocontrol at five chiral centers. High yields, up to 94%, have been recorded using oxygen as oxidant with a metal(II)-salen complex as catalyst, e.g. cobalt(II) salen. A low yield of carpanone was also obtained in electrooxidation. 8... 

During recent years, the homogeneous Lewis acid-catalyzed asymmetric Diels-Alder reactions and hetero-Diels-Alder (HDA) reactions have each undergone extensive study. Various chiral Lewis acids including aluminum, titanium or boron, and chiral ligands such as chiral amino alcohols, diols, salen, bisoxazoline or N-sulfonylamino acids have been used as the catalysts [84]. Much efforts have also been made in the investigation of heterogeneous diastereoselective Diels-Alder reactions. 

By ensconcing an In(III) ion in the A-oxide 55B originated from two homochiral pipe-colinic amides a catalyst for hetero-Diels-Alder reaction involving the Danishefsky diene and aldehydes is obtained. Cr(salen) 149 appears to have similar capability. ... 

Interesting exploration of this methodology presented Wu [147], where hetero Diels-Alder reaction between an electron-rich diene 269 and ethyl glyoxylate was assisted with salen-Co(II) catalyst (Scheme 57). 

Recently, Schulz was carried out electropolymerization, an original methodology to prepare polymer supported salen 113 [109][110]. It consisted in the introduction of a thiophene moiety, an electropolymerizable functionality, on the salen backbone (Scheme 62) followed by electropolymerization imder cyclic voltammetry conditions on a platinum grid. These polymeric catalyst 120a and 120b were then evaluated in hetero Diels-Alder reaction (HTR) between several aldehydes and l-methoxy-3-[(methylsilyl)oxy]-l,3-butadiene... 

Tridentate bis(oxazolinyl)pyridinyl rhodium and ruthenium pincer complexes are useful as catalysts for hydrosilylations and cyclopropanations. These NNN-type inorganic pincer complexes are not as stable, however, as phosphine or salen-type pincer complexes. On the other hand, an organometallic tridentate bis(oxazolinyl) phenyl NCN-type complex is stable. These optically active NCN-type pincer complexes act as efficient catalysts for enantioselective hetero Diels-Alder reactions of Danishefsky s diene with glyoxylates [26]. 

A SALEN-containing catalyst, A,A-bis(3,5-di-tert-butylsalicylidene)-l,2-cyclohexanediamino-cobalt(II), proved to be an efficient catalyst for the hydrolytic kinetic resolution of terminal epoxides (187) and the enantioselective ring opening of meso epoxides (188). The same complex proved to be active in the asymmetric hetero-Diels-Alder reaction (189). 

Later, several bisimine and diol-based chiral ligands were examined as Sc(OTf)3 complexes in the asymmetric Diels-Alder reaction of cyclopentadiene with 3-acryloyloxazolidin-2-one in the presence of 2,6-lutidine [134]. The scandium-salen complex was revealed to be the most effective catalyst and gave the endo adduct in 81% yield with 85% ee (endo/exo = 89/11). 

In 1995, Katsuki and Yamashita [11a] found that Mg-salen complex and Co- or Cr-salen complexes served as Lewis acid catalysts for asymmetric Diels-Alder reactions and hetero Diels-Alder (HDA) reaction of Danishefsky s diene (l-methoxy-3-trimethylsilyloxy-l,3-butadiene) with aldehydes. Later on, they found that second-generation (R,S)-(ON )Ru-salen complex bearing chiral binaphthyl and ethylenediamine units also served as a catalyst for asymmetric HDA reaction between Danishefsky s diene and benzaldehyde [llbj. Recently, in 2001 they reported that (R,S)-Ru(II)-salen complex served as a good catalyst for... 

Katsuki and Shimada have already found that Cr complexes with chiral (salen) bearing a binaphthyl served as efficient Lewis acid catalysts for Mukaiyama aldol and hetero Diels-Alder reactions. Recently, they have reported that Cr(salen) complexes... 

Enantioselective imidation of alkyl aryl sulfides with A -alkoxycarbonyl azides as a nitrene precursor is effected by using (OC)Ru(salen) complex as catalyst. The steric and electronic nature of the Af-alkoxycarbonyl group strongly affect the enantioselectivity and the reaction rate. In a systematic and well-executed study of ligand effects on Lewis-acid-catalyzed Diels-Alder reaction, it has been shown that the attachment of aromatic a-amino acid ligands to copper(II) ions leads to an increase in the overall rate of the Diels-Alder reaction between 3-phenyl-l-(2-pyridyl)-2-propene-l-one (Din) and cyclopentadiene... 

Utilization of heteroatom-substituted dienes in enantioselective Diels-Alder reactions was investigated by Rawal, who reported that both chromium(III) and cobalt(III) salen complexes are effective catalysts [103, 104]. The use of chromium(III) salen complex 197 [105] in the cycloaddition between azadiene 195 and aldehyde 196 is shown in Scheme 17.27. This provided endo product 198 in 96% ee as a single diastereomer. Subsequently, cycloadduct 198 was transformed into the Aspidosperma alkaloid (-i-)-taberso-nine (199) [104]. 

Costantino et al. [103] reported the direct aza-Diels-Alder reaction between 2-cyclohexen-l-one and benzaldimines in water, catalyzed by layered a-zirconium hydrogen phosphate (a-ZrP) in the presence of sodium dodecyl sulfate (SDS) at 30 ° C (Table 6.2). The yield of the product was good, and the reaction was faster and the exo-diastereoselectivity was higher than when an organic solvent was used. A Cr(III)(salen)Cl catalyst embedded into a self-assembled supramolecular aggregate displays substrate selectivity with an up to 3.5 fold increase in activity in favor of longer over shorter dienophiles in the reaction with cyclopentadiene under aqueous micellar conditions [104]. 

Jacobsen reported in 1990 that Mnm complexes of chiral salen ligands (41) were the most efficient catalysts available for the enantioselective epoxidation of alkyl- and aryl-substituted olefins.118 This stimulated a rapid development in the chemistry and applications of chiral SB complexes, which offer promising catalytic applications to several organic reactions, such as enantioselective cyclopropanation of styrenes, asymmetric aziridination of olefins, asymmetric Diels-Alder cycloaddition, and enantioselective ring opening of epoxides.4,119... 

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