Aluminum catalyst Diels-Alder reaction

Wulff and coworkers have applied their aluminum catalyst 2 containing a vaulted biphenanthrol ligand (VAPOL, Section 2.1) to the Diels-Alder reaction between methyl acrylate and cyclopentadiene [25] (Scheme 1.32). In this Diels-Alder reaction auto-induction is observed, because of a cooperative interaction between the product... 

The Diels-Alder reaction was thought for many years to be only slightly influenced by catalysts. However, in 1960, Yates and Eaton (6) clearly demonstrated that with certain dienophiles, the presence of a molar equivalent of aluminum chloride can cause a remarkable acceleration of the reaction. Providing the diene is not polymerized (7) or otherwise destroyed by the catalyst, the modification can be fruitfully employed to carry out the reaction at lower temperature and for shorter times. 

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

Several aluminum- and titanium-based compounds have been supported on silica and alumina [53]. Although silica and alumina themselves catalyze cycloaddition reactions, their catalytic activity is greatly increased when they complex a Lewis acid. Some of these catalysts are among the most active described to date for heterogeneous catalysis of the Diels-Alder reactions of carbonyl-containing dienophiles. The Si02-Et2AlCl catalyst is the most efficient and can be... 

Chloroaluminate ionic liquids (typically a mixture of a quaternary ammonium salt with aluminum chloride see Table 6.9) exhibit at room temperature variable Lewis acidity and have been successfully used as solvent/catalyst for Diels-Alder reactions [57]. The composition of chloroaluminate ionic liquids can vary from basic ([FMIM]C1 or [BP]C1 in excess) to acidic (AICI3 in excess) and this fact can be used to affect the reactivity and selectivity of the reaction. The reaction of cyclopentadiene with methyl acrylate is an example (Scheme 6.31). 

This procedure describes the preparation and application of an effective chiral catalyst for the enantioselective Diels-Alder reaction.11 The catalyst is derived from optically active 1,2-diphenylethylenediamine, the preparation of which (either antipode) was described in the preceding procedure. The aluminum-based Lewis acid also catalyzes the cycloaddition of crotonoyl oxazolidinones with cyclopentadiene,11 and acryloyl derivatives with benzyloxymethylene-cyclopentadiene. The latter reaction leads to optically pure intermediates for synthesis of prostaglandins.11... 

In total syntheses where a homogeneously catalyzed transfer hydrogenation is applied, almost exclusively aluminum(III) isopropoxide is utilized as the catalyst. At an early stage in the total synthesis of (-)-reserpine (65) by Woodward [106], an intermediate with two ketone groups and two C-C double bonds is formed (66) by a Diels-Alder reaction of para-benzoquinone (67) and vinylacry-late (68). The two ketone groups were reduced with aluminum(III) isopropoxide... 

Perhaps the most attractive method of introducing enantioselectivity into the Diels-Alder reaction is to use a chiral catalyst in the form of a Lewis acidic metal complex. In recent years, this area has shown the greatest progress, with the introduction of many excellent catalytic processes. Quite a number of ligand-metal combinations have been evaluated for their potential as chiral catalysts in Diels-Alder reactions. The most commonly used metals are boron, titanium, and aluminum. Copper, magnesium, and lanthanides have also been used in asymmetric catalytic Diels-Alder reactions. 

Hubbard and Miller87 used a Lewis acid catalyzed Diels-Alder reaction between y.y-disubstituted o. /i-unsaluralcd esters and cyclopentadiene in their approach toward oligomeric cyclopentanoids. In order for the reaction to proceed, they needed to add trimethylaluminum as a desiccant prior to addition of the Lewis acid catalyst aluminum trichloride. The endo/exo selectivity of the reaction with 97, depicted in equation 29, increased from 98/99 = 75/25 to 88/12 when the reaction temperature was dropped from room temperature to —20 °C. 

Oi and coworkers88 employed a cationic palladium(II) complex to catalyze Diels-Alder reactions. The benefits of such a catalyst compared to traditional catalysts such as boron and aluminum halides were reported to possess better stability to air and moisture,... 

The chiral dialuminum Lewis acid 14, which is effective as an asymmetric Diels-Alder catalyst, has been prepared from DIBAH and BINOL derivatives (Scheme 12.12). " The catalytic activity of 14 is significantly greater than that of monoaluminum reagents. The catalyst achieves high reactivity and selectivity by an intramolecular interaction of two aluminum Lewis acids. Similarly, the chiral trialuminum Lewis acid 15 is quantitatively formed from optically pure 3-(2,4,6-triisopropylphenyl)binaphthol (2 equiv) and MeaAl (3 equiv) in CH2CI2 at room temperature (Scheme 12.12). " The novel structure of 15 has been ascertained by NMR spectroscopic analysis and measurement of the methane gas evolved. Trinuclear aluminum catalyst 15 is effective for the Diels-Alder reaction of methacrolein with cyclopentadiene. Diels-Alder adducts have been obtained in 99% yield with 92% exo selectivity. Under optimum reaction conditions, the... 

Lewis acids such as zinc chloride, boron trifluoride, aluminum chloride, and diethylaluminum chloride catalyze Diels-Alder reactions.8 The catalytic effect is the result of coordination of the Lewis acid with the dienophile. The complexed dienophile is more electrophilic and more reactive toward electron-rich dienes. The mechanism of the cycloaddition is still believed to be concerted, and high stereoselectivity is observed.9 10 Lewis acid catalysts also usually increase the regioselectivity of the reaction. 

With Chiral Al Complexes Chiral bis(silyl)binaphthol-modified aluminum catalyst, which is originally developed for asymmetric hetero-Diels-Alder reaction [50], is successfully applied to asymmetric Diels-Alder reaction of cyclopentadiene with methyl acrylate or methyl propio-late [51 ] (Eq. 8A.28). The latter is a rather rare example in the literatures. 

Enantioselective Diels-Alder catalyst.3 The aluminum reagent 2, prepared by reaction of (CH3)3A1 with the N,N-ditriflate of (S,S)-1, is an effective catalyst for asymmetric Diels-Alder reactions of acrylates with dienes. Particularly high enan-tioselectivity obtains in the reaction of 3-acrylyl-l,3-oxazolidinone-2 with cyclopen-tadiene. In this case, the endo-exo ratio is >50 1, and the optical yield is 90%. The stereoselectivity probably results from binding of the catalyst to the acrylyl carbon group. 

Diels-Alder catalyst. The Diels-Alder reaction of cyclopentadiene with acrolein (equation I) catalyzed by BF3 etherate gives a mixture of endo- and exoadducts in the ratio 9 1. The endo-selectivity is improved by use of trimethylalu-minum. Introduction of a more bulky group on aluminum by use of DAD results... 

Within the last few years, it has conclusively been demonstrated that the Diels-Alder reaction is susceptible to catalysis with Lewis acids or Friedel-Crafts catalysts such as aluminum chloride (15, 18, 36, 37, 38, 39, 40, 57, 67, 68, 69, 70, 71, 82, 86). As a result of catalysis, it was possible to change the following ... 

These complexes are the first examples of multifunctional catalysts and demonstrate impressively the opportunities that can reside with the as yet hardly investigated bimetallic catalysis. The concept described here is not limited to lanthanides but has been further extended to main group metals such as gallium [31] or aluminum [32]. In addition, this work should be an incentive for the investigation of other metal-binaphthyl complexes to find out whether polynuclear species play a role in catalytic processes there as well. For example, the preparation of ti-tanium-BINOL complexes takes place in the presence of alkali metals [molecular sieve ( )]. A leading contribution in this direction has been made by Kaufmann et al, as early as 1990 [33], It was proven that the reaction of (5)-la with monobromoborane dimethyl sulfide leads exclusively to a binuclear, propeller-like borate compound. This compound was found to catalyze the Diels-Alder reaction of cyclopentadiene and methacrolein with excellent exo-stereoselectivity and enantioselectivity in accordance with the empirical rule for carbonyl compounds which has been presented earlier. 

Cycloadditions. The asymmetric Diels-Alder reaction of phenylmenthyl acrylate with 5-benzyloxymethyl-cyclopentadiene in the presence of Aluminum Chloride produces an 89% yield of the endo cycloadduct (eq 1), accompanied by 7% of the exo adduct. This provides a useful intermediate for the preparation of various prostaglandins. The Tin(IV) Chloride and Titanium(IV) Chloride catalyzed reactions with Cyclopenta-diene deliver a mixture of endo and exo adducts in 89% de, and 90% de, respectively (eq 2). The TiCU reaction gives an 89 11 endo. exo ratio, while the SnCU reaction gives an 84 16 endoiexo ratio. From a practical point of view, the titanium and tin catalysts are the best of the various Lewis acids surveyed. The use of TiCl4 is also the most effective for the reaction of the acrylate... 

Aben and Scheeren discovered that bornyloxyaluminum dichloride, which can be easily prepared from bomeol, aluminum chloride, and lithium alanate, acts as an effective catalyst for the hetero Diels-Alder reaction of siloxy dienes and a variety of aldehydes, as illustrated in Sch. 18 [37]. 

The earliest report of a reaction mediated by a chiral three coordinate aluminum species describes an asymmetric Meerwein-Poimdorf-Verley reduction of ketones with chiral aluminum alkoxides which resulted in low induction in the alcohol products [1]. Subsequent developments in the area were sparse until over a decade later when chiral aluminum Lewis acids began to be explored in polymerization reactions, with the first report describing the polymerization of benzofuran with catalysts prepared from and ethylaluminum dichloride and a variety of chiral compounds including /5-phenylalanine [2]. Curiously, these reports did not precipitate further studies at the time because the next development in the field did not occur until nearly two decades later when Hashimoto, Komeshima and Koga reported that a catalyst derived from ethylaluminum dichloride and menthol catalyzed the asymmetric Diels-Alder reaction shown in Sch. 1 [3,4]. This is especially curious because the discovery that a Diels-Alder reaction could be accelerated by aluminum chloride was known at the time the polymerization work appeared [5], Perhaps it was because of this long delay, that the report of this asymmetric catalytic Diels-Alder reaction was to become the inspiration for the dramatic increase in activity in this field that we have witnessed in the twenty years since its appearance. It is the intent of this review to present the development of the field of asymmetric catalytic synthesis with chiral aluminum Lewis acids that includes those reports that have appeared in the literature up to the end of 1998. This review will not cover polymerization reactions or supported reactions. The latter will appear in a separate chapter in this handbook. 

Table 9. Diels-Alder reactions with chiral aluminum catalysts producing adduct 3.

Chiral Aluminum Lewis Acids in Organic Synthesis 307 Table 10. Diels-Alder reactions with chiral aluminum catalysts 178 producing adduct 3. ... 

Diels-Alder reactions of aldehydes are typically much faster than those of esters and thus reasonable rates were observed by Rebiere, Riant, and Kagan with chiral aluminum catalysts derived from chiral diols for the reaction of unsaturated aldehydes with cyclopentadiene [48]. A series of chiral diols was screened for the reaction of methacrolein with cyclopentadiene. The catalyst was prepared in-situ from a chiral diol and 1 equiv. ethylaluminum dichloride at room temperature for 3 h. This was... 

The observations made for the VAPOL-aluminum catalyst in Table 15 are suggestive of asymmetric auto-induction similar to that for catalysis of the same reaction by a chiral aluminum catalyst prepared from the diol 225 (Sch. 24 Fig. 1) [48]. Because the Diels-Alder reaction between methacrolein and cyclopentadiene was too fast, monitoring of the time course of asymmetric induction, thus this was done for the reaction between methyl acrylate and cyclopentadiene [53]. The VAPOL-aluminum catalyst catalyzes the reaction of methyl acrylate and cyclopentadiene with asymmetric auto-induction as indicated in Fig. 2. The first data point that was collected was after 20 % conversion, at which point the cycloadduct 141 was 47 % ee when the last data point was collected at the end of the reaction it was found that 141 was 82 % ee. 

An effective chiral aluminum catalyst prepared from the bis-sulfonamide 263 was reported by Corey, Imwinkelried, Ikul, and Xiang for the Diels-Alder reaction of N-acyloxazolidinones [55]. They found that 10 mol % catalyst 266 would effect the reaction of A-acrylyl derivative 261 in 10 min at -78 °C to give the endo adduct 262 in 92 % yield and 91 % ee. The reaction of the N-crotyl derivative 197 was slower but... 

The first report of a chiral aluminum Lewis acid employed in a heteroatom Diels-Alder reaction utilized Koga s mentholoxy dichloroaluminum catalyst 4 [75]. trans-Piperylene and 1-methoxybutadiene were reacted with n-butyl glyoxalate and diethyl mesoxalate the results are summarized in Sch. 46. The asymmetric induction and chemical yield in these reactions are quite poor but the authors did find that moderate asymmetric induction could be obtained from reactions catalyzed by Eu(hfc)3. 

It has been reported that several transition metal complexes catalyze the hetero-Diels-Alder reaction between a variety of aldehydes, in particular benzaldehyde, and Danishefsky s diene (Sch. 52). With the [CpRu(CHIRAPHOS)] complex the ee is modest (25 %) (entry 1) [192]. The chiral complex VO(HFBC)2 performs better in this reaction (entry 2) [193]. In experiments directed towards the synthesis of anthra-cyclones, this complex was used in cycloadditions between anthraquinone aldehydes with silyloxy dienes. One example is shown in Sch. 53 [194]. Compared with the chiral aluminum catalyst developed earlier by Yamamoto and co-workers [195], the vanadium catalyst results in lower enantioselectivity but has advantages such as ease of preparation, high solubility, stability towards air and moisture, and selective binding to an aldehyde carbonyl oxygen in the presence of others Lewis-basic coordination sites on the substrate. 

The first successful examples of enantioselective Diels-Alder reactions catalyzed by chirally modified Lewis acids were reported by Koga [85]. The catalysts were prepared from menthol and AlEt2Cl [86]. Alumina-supported chiral menthoxy aluminum derivatives (64, 65, 66, 67) have been prepared by simple mixing of (-)-menthol, AlEt2Cl, and alumina in toluene under reflux. The reaction of methacrolein with cyclopentadiene (Eq. 20) was conducted with 67 as catalyst at -50 °C and afforded 81 % conversion with 31 % ee [87] Koga reported 57 % ee at -78 °C by use of an homogeneous catalyst [85]. Solid catalyst 69, prepared from silica gel-supported proli-nol 68 and AlEt2Cl (Eq. 21) is also an active catalyst in the same reaction, but with low enantioselectivity [87]. When the same catalyst was attached to crosslinked polystyrene (70) the ee in the reaction was lower [88]. 

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