Methyl acrylate, Diels-Alder cycloaddition reaction

In all of these examples, the reaction products are under kinetic control. That is, the product mixture is determined by which product is made fastest rather than which is thermodynamically most stable. The Diels-Alder cycloaddition reaction gives another example of this phenomenon. The reaction of methyl acrylate with cyclopentadiene gives a mixture of two endo and exo products (see Scheme 2.5). 

The Diels-Alder cycloaddition reaction is a concerted reaction, with no change in the charge density upon going from reactants to the activated complex and finally to the products. Therefore, from the Hughes-Ingold approach, litde solvent effect is expected upon these reactions. In spite of this, dramatic solvent effects are often observed when a non-symmetric dienophile is used, and a mixture of products is formed. For instance, the reaction of methyl acrylate with cyclopentadiene gives a mixture of two products (see Scheme 10.61. 

Clays [2] and zeolites [3] have been reported as good catalysts for Diels-Alder synthesis. Recently, we have studied the Diels-Alder cycloaddition between methyl acrylate (1) and cyclopentadiene (2) (Figure 1) and have shown that the solvent [4], the calcination of the solid [5] and the exchanged cation [6] play a decisive role. We now report the results obtained from the reactions of cyclopentadiene with methyl and (-)-menthyl acrylates, catalysed by K10 montmorillonites exchanged with different cations and dried at 120°C or calcined at 550°C. 

Diels-Alder cycloadditions have greater synthetic flexibility when suitable substituents on the diene are available for elaboration to functionality which is otherwise difficult to build into the structure. To this end, several schemes to novel 1,3-dienes have been described this year. Hagemann s ester (79) is a useful intermediate in synthesis of terpenoids. The ester is now conveniently available by regioselective Diels-Alder reaction of 1-methyl-l,3-bis(trimethylsiloxy)buta-1,3-diene (80) with ethyl acrylate. The yield is 61% overall after 5 days heating in xylene at 170—180 C, followed by hydrolysis of the moisture-sensitive adduct (81). The diene (80) is readily prepared from acetylacetone and trimethylsilyl chloride in the presence of triethylamine-zinc chloride in an ether-benzene mixture. The related diene (82), similarly prepared from 2-formylbutan-3-one, is used to give the useful cyclic dienophile (83). These sequences are outlined in Scheme 18. ... 

The first step in a one-pot formation of a phenanthrene ring system from an enantiomericaUy pure rhenium-bound naphthalene is a TBDMS triflate-promoted Michael addition to 3-penten-2-one (eq 26). Electron-rich rhenium-bound naphthalenes also undergo TBDMS triflate-promoted conjugate addition reactions to less-activated Michael acceptors such as methyl acrylate, leading to the formal Diels-Alder cycloaddition product of naphthalene with this dienophile. ... 

Enamines have been observed to act both as dienophiles (46-48) and dienes (47,49) (dienamines in this case) in one-step, Diels-Alder type of 1,4 cycloadditions with acrylate esters and their vinylogs. This is illustrated by the reaction between l-(N-pyrrolidino)cyclohexene (34) and methyl t/-a i-2,4-pentadienoate (35), where the enamine acts as the dienophile to give the adduct 36 (47). In a competitive type of reaction, however, the... 

Evans s bis(oxazolinyl)pyridine (pybox) complex 17, which is effective for the Diels-Alder reaction of a-bromoacrolein and methacrolein (Section 2.1), is also a suitable catalyst for the Diels-Alder reaction of acrylate dienophiles [23] (Scheme 1.33). In the presence of 5 mol% of the Cu((l )-pybox)(SbF5)2 catalyst with a benzyl substituent, tert-butyl acrylate reacts with cyclopentadiene to give the adduct in good optical purity (92% ee). Methyl acrylate and phenyl acrylate underwent cycloadditions with lower selectivities. 

It has been established that alkoxy alkenylcarbene complexes participate as dienophiles in Diels-Alder reactions not only with higher rates but also with better regio- and stereoselectivities than the corresponding esters [95]. This is clearly illustrated in Scheme 51 for the reactions of an unsubstituted vinyl complex with isoprene. This complex reacts to completion at 25 °C in 3 h whereas the cycloaddition reaction of methyl acrylate with isoprene requires 7 months at the same temperature. The rate enhancement observed for this complex is comparable to that for the corresponding aluminium chloride-catalysed reactions of methyl acrylate and isoprene (Scheme 51). 

Good yields and high diastereoselectivities were obtained by using zeolites in combination with Lewis-acid catalyst [21]. Table 4.7 illustrates some examples of Diels-Alder reactions of cyclopentadiene, cyclohexadiene and furan with methyl acrylate. Na-Y and Ce-Y zeolites gave excellent results for the cycloadditions of carbocyclic dienes, and combining these zeolites with anhydrous ZnBr2 further enhanced the endo diastereoselectivity of the reaction. An exception is the cycloaddition of furan that occurred considerably faster and with better yield, in comparison with the classic procedure [22], when performed in the presence of sole zeolites. 

The aqueous medium also has beneficial effects on the diastereoselectivity of the Diels-Alder reactions. The endo addition that occurs in the classical cycloadditions of cyclopentadiene with methyl vinyl ketone and methyl acrylate is more favored when the reaction is carried out in aqueous medium than when it is performed in organic solvents (Table 6.4) [2b, c]. 

Indium trichloride [30] and methylrhenium trioxide [31] catalyze the aqueous Diels-Alder reaction of acrolein and acrylates with cyclic and open-chain dienes. Some examples of the cycloaddition of methyl vinyl ketone with 1,3-cyclohexadiene are reported in Scheme 6.18. MeReOs does not give satisfactory yields for acroleins and methyl vinyl ketones with substituents at the jS-position and favors the self-Diels-Alder reaction of diene. 

On the basis of available experimental data, it is impossible to choose a definite pathway of elimination of silanol. However, study of silylation of methyl P -nitropropionate (411) with BSA in the presence of trapping agents rigorously proved that silyl nitronate D is initially formed. This compound can be detected in the [3 + 2]-cycloaddition reaction with methyl acrylate product (413). If silylation of AN (411) is performed in the presence of ethyl vinyl ether, a-nitrosoalkene E can be successfully trapped in as heterodiene a Diels-Alder reaction. Dihydroox-azine (414) is formed, and its silylation affords isolable product (415). 

If X = AcO, intermediate SENA can be trapped by methyl acrylate in the [3+ 2]-cycloaddition reaction (isoxazolidine (416)). If X=C1, attempts to trap silyl nitronate failed however, nitroethylene was detected in a Diels-Alder reaction. By contrast, SENAs, in which X=OSiMe3 or NHPh, are quite stable. Therefore, the substituents X can be arranged in the following series of increasing elimination rates of SiX Cl > AcO > > PhNH. 

---