Diels Alder reactions molecular sieves

Sulfolane (tetramethylenesulfone) [126-33-0] M 120.2, m 28.5 , b 153-154 /18mm, 285 /760mm, d 1.263, n 1.4820. Prepared commercially by Diels-Alder reaction of 1,3-butadiene and sulfur dioxide, followed by Raney nickel hydrogenation. The principle impurities are water, 3-sulfolene, 2-sulfolene and 2-isopropyl sulfolanyl ether. It is dried by passage through a column of molecular sieves. Distd... 

Narasaka et al.16 reported that 53 catalyzes Diels-Alder reactions of 54-type substrates with diene in the presence of 4 A molecular sieves (Scheme 5-18). A remarkable solvent effect on the enantioselectivity is observed. High enantio-selectivity is attained using mesitylene as the solvent. As shown in Scheme 5-18, the reaction of 54a with isoprene proceeds smoothly in this solvent, affording product 55a with 92% ee. Other 3-(3-substituted acryloyl)-l,3-oxazolidin-2-ones 54b-d also give good results (75-91% ee) when reacted with cyclopentadiene. 

Bis(oxazoline)-type complexes, which have been found useful for asymmetric aldol reactions, Diels-Alder, and hetero Diels-Alder reactions can also be used for inducing 1,3-dipolar reactions. Chiral nickel complex 180, which can be prepared by reacting equimolar amounts of Ni(C10)4 6H20 and the corresponding (J ,J )-4,6-dibenzofurandiyl-2,2 -bis(4-phenyloxazoline) (DBFOX/Ph) in dichloromethane, can be used for highly endo-selective and enantioselective asymmetric nitrone cycloaddition. The presence of 4 A molecular sieves is essential to attain high selectivities.88 In the absence of molecular sieves, both the diastereoselectivity and enantioselectivity will be lower. Representative results are shown in Scheme 5-55. 

The hydrated complexes 266c and 265c offer the distinct advantage of stability and ease of use. Their effectiveness as catalyst precursors in the Diels-Alder reaction has been addressed (200). A comparison of the behavior of these catalysts in the presence and absence of sieves in the reaction of acryloylimide and piperylene has revealed that the hydrated catalysts are effective precursors for this reaction. The triflate-derived catalyst 266c is ineffective unless the reaction is conducted in the presence of molecular sieves, Table IV. On the other hand, hydration does not adversely impact the performance of the hexafluoroantimonate catalyst 265c. The presence of sieves has a deleterious effect on this catalyst, leading to greatly reduced reactivity for reasons that are unclear. 

The hetero-Diels-Alder reaction between a,p-unsaturatcd ketoesters and nucleophilic alkenes has been described in two concurrent and independent reports (220, 222). As with acylphosphonates, these proved to be excellent substrates for catalyst 269c. The reaction proceeds efficiently in THF at low temperatures providing the cycloadduct in >99% ee at -78°C. Indeed, the impressive selectivity exhibited under these conditions allows the reaction to be conducted at a convenient temperature of 0°C, using the hydrated catalyst 266c in the presence of molecular sieves, Eq. 181. Observed diastereoselectivities... 

The abovementioned rate acceleration and selectivity enhancement brought about by catalysts are particularly marked when unactivated dienes and dienophiles are involved. Two molecules of 1,3-butadiene can react in a Diels-Alder reaction, one acting as diene and the other as a dienophile to produce 4-vinylcyclohexene (in 0.1% yield at 250°C in the absence of a catalyst). Cs+, Cu,+ and trivalent transition-metal exchanged montmorillonites534 as well as large-pore sodium zeolites (Na ZSM-20, NaY) and carbon molecular sieves,535 result in 20-35% yields with 95% selectivity. Large rate enhancement was observed when 1,3-cyclohexadiene underwent a similar cycloaddition536 in the presence of K10 montmorillonite doped with Fe3+ ... 

Diels-Alder reactions are thermal reactions requiring no catalysts (120). However, over the years both acid- and metal-based homogeneous or heterogeneous catalysts have been developed (121—127). Some catalysts used in Diels-Alder catalyzed reactions of butadiene are Fe(NO)2Cl—(CH3CH2)2A1C1, Pd[P(C H5)3]4, Cu(I) exchanged silica—alumina (128,129), large pore zeolites (130), and carbon molecular sieves. An electrochemical process has also been used to catalyze the self-condensation to vinylcyclohexene (131). When the asymmetric Ni catalyst (4) was used, specificity to the enantomeric (5)-4-vinylcyclohexene (132,133) was observed (26% enantiomeric excess). 

Various a,a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanols have been prepared from (R,R)-tartrate, which are called TADDOLs by Seebach et al. They studied the influence of the Ti catalyst preparation methods, the presence of molecular sieves, and the TADDOL structure in the enantioselective Diels-Alder reaction of acryloyl oxazolidinones [41] (Eq. 8A.22). Seebach also prepared polymer- and dendrimer-bound Ti-TADDOLates and used in catalytic asymmetric cycloadditions [42],... 

Chromium complex 53 was also shown to efficiently catalyze the inverse electron-demand hetero Diels-Alder reaction of a,(3-unsaturated aldehydes with alkyl vinyl ethers (Scheme 17.19).26 Although the uncatalyzed process required elevated temperatures and pressures to give dihydropyrans in good yields but poor endo. exo selectivities, the reaction proceeded at room temperature in the presence of 5 mol% of ent-53 and 4A molecular sieves in dichloromethane of tert-butyl methyl ether with excellent diastereoselectivity (endo. exo >96 4) and promising enantioselectivities (72-78% ee). Optimal results were achieved using a solvent-free system and excess vinyl ether. 

Even in the absence of Lewis acid functions, zeolites can accelerate gas phase Diels-Alder reactions. This rate enhancement, for instance in the butadiene cyclodimerization, is attributed to a concentration effect inside the zeolite pores. The effect is however not zeolite-specific any adsorbent with affinity for dienes, such as a carbon molecular sieve, displays similar effects (5). 

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. 

Chiral Lewis Acid. These chiral titanium reagents are widely used as chiral Lewis acid catalysts. The Diels-Alder reaction of methyl acrylate and cyclopentadiene affords the endo adduct in moderate enantioselectivity when a stoichiometric amount of the chiral titanium reagent (5) is employed (eq 6). Use of 3-(2-alkenoyl)-l,3-oxazolidin-2-ones as dienophiles greatly improves the optical purity of the cycloadduct when the 2-phenyl-2-methyl-1,3-dioxolane derivative (6) is used as a chiral ligand. Most importantly, the reaction proceeds with the same high enantioselectivity for the combination of various dienophiles and dienes even when 5-10 mol % of the chiral titanium reagent is employed in the presence of molecular sieves 4A (eqs 7 and 8). ... 

The use of catalyst 187 or 188 (see Sch. 43) in cycloadditions requires anhydrous conditions. Recently, several practical alternatives for this requirement have been reported. Evans has shown that the easily manipulated aquo complex prepared from 187 and water can be dehydrated to the active catalyst in the reaction vessel by addition of molecular sieves, without any loss of reactivity or selectivity [87]. Copper(II) perchlorate is available commercially as a hexahydrate. Ghosh and co-workers have reported that a complex 207 prepared from an aminoindanol-derived bisoxazoline and Cu(C104)2 6H2O is an excellent Lewis acid in Diels-Alder reactions (Sch. 46). It is interesting to note that the generally sluggish reactions with oxazolidinone croto-nates proceed with very high selectivity at room temperature [88]. 

The chiral Ni(II) complex shown in Sch. 58 catalyzes the enantioselective hetero-Diels-Alder reactions of thiabutadienes with 3-(2-propenoyl)-2-oxazolidinone to afford optically active dihydrothiopyrans [204]. Similar results are obtained with analogous Cu(II) complexes where catalyst loading can be reduced when molecular sieves are added. 

The Diels-Alder reaction of methacrolein with 1,3-dienol derivatives can also be catalyzed by the BINOL-derived titanium complex, although the catalyst must be freed from molecular sieves (MS) to give the endo adduct with high enantioselectivity (Sch. 50) [131], because MS act as achiral catalysts in the Diels-Alder reaction. The asymmetric Diels-Alder reaction catalyzed by the MS-free (MS-(-)) BINOL-Ti complex (L) can be applied naphthoquinone derivatives as dienophiles to provide entry to the asymmetric synthesis of tetra- and anthracyclinone [132] aglycones (Sch. 51). The sense of asymmetric induction is exactly the same as that observed in the presence of MS in the asymmetric catalytic reactions described above. 

Kobayashi and co-workers exploited the use of lanthanide in a variety of achiral reactions and extended them into several catalytic asymmetric reactions. Their work commenced with catalytic asymmetric Diels-Alder reactions [32], The reaction was performed with a chiral ytterbium catalyst prepared from Yb(OTf)3, binaphthol and a tertiary amine. The amine significantly influenced reaction selectivity. When triethyl-amine was used in the preparation of the catalyst, the desired product was obtained in moderate ee (33%) (Table 8, entry 1). After screening several reaction conditions, they found that, in general, bulky amines gave better results (entries 2-6). They suggested interesting explanations of this experimental result on the basis of investigations into catalyst structure. Consequently, the use of cw-l,2,6-trimethylpiperidine combined with 4 A molecular sieves (4A MS) was found to produce the best result (yield 77%, endolexo = 89/11, endo = 95% ee) (entry 6). 

Tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) ligands synthesized from tartaric acid have been extensively employed by Narasaka as the chiral control element in selective Diels-Alder reactions. Initial experiments were conducted with simple dienes and a,P-unsaturated imides using complex 44 (Scheme 36) [104,105]. Several rather subtle features have contributed to the success of these endeavors 1) the use of the acetophenone-derived dioxolane rather than the ac-etonide resulted in an increase of 20% ee 2) the use of alkyl-substituted benzenes as solvent augmented enantioselectivities relative to more common organic solvents e.g., CH2CI2, THF) [106] 3) use of 4 A molecular sieves was typically required to achieve maximum enantioselectivity. 

Mikami and coworkers conducted the Diels-Alder reaction with a catalyst prepared by mixing enantiomerically pure R)-56 and racemic 56 and observed a positive nonlinear effect however, they found no asymmetric amplification when they prepared the catalyst by mixing enantiomerically pure R)-56 and enantiomerically pure (S)-56 (i.e., linear correlation between catalyst and product ee). Introduction of molecular sieves restores the asymmetric amplification in the latter case, apparently by equilibration of R) R) and (S)(S) dimers into catalytically less active R) S) dimers. As expected, the reaction rate was faster for R)-56 than for ( )-56 derived from racemic binaphthol hgand ca. 5-fold faster). 

Amide formation. o-Halophenylboronic acids catalyze the Diels-Alder reaction of acrylic acid as well as condensation of carboxylic acids with amines at room temperature (in the presence of 4A-molecular sieves). ... 

Scheme 1. Synthesis of 1,2,3,4-tetrahydrocarbazoles by molecular sieve-catalyzed Diels-Alder reactions.

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