Diels-Alder reactions of isoprene

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

Zeolite and Lewis-acid catalysis in Diels-Alder reactions of isoprene [20b]... 

The combination of Lewis-acid catalysis and sc-COi has also been investigated. One of these studies involved the AlCls-catalyzed Diels Alder reaction of isoprene and maleic anhydride in sc-COi at 67 °C and at 74.5-78.5 bar [89]. The reaction rate was enhanced with respect to the uncatalyzed reaction and an unconcerted two-step mechanism was suggested [89]. 

Many Diels-Alder [4 + 2] cycloadditions show a powerful pressure-induced acceleration, which is often turned to good synthetic purpose as discussed in Section III.A.2. Table 1 illustrates the effect of pressure on the Diels-Alder reaction of isoprene with acrylonitrile as a representative example. This reaction is accelerated by a factor of 1650 by raising the pressure from 1 bar to 10 kbar28. 

TABLE 1. Pressure-induced rate acceleration of the Diels-Alder reaction of isoprene with acrylonitrile at 21 °C (AV = -35.4 cm3 mol-1, AV = -37.0 cm3 mol-1)28... 

Total synthesis of racemic limonene by Diels-Alder reaction of isoprene with methyl vinyl ketone and subsequent Wittig reaction of the resulting ketone with methylene triphenylphosphorane... 

The rate constant data for the Diels-Alder reaction of isoprene and maleic anhydride (23,24) may be correlated with IL of phenol blue at 35°C by... 

What has just been stated regarding the LCAO coefficients of the dienophile LUMO combined with the rules for the regioselectivity of any one-step cycloaddition leads to the following consequences for the Diels-Alder reactions of isoprene ... 

Table 2 Experimental and theoretical 2H and 13C KIEs for the Lewis acid catalyzed Diels-Alder reaction of isoprene with acrolein, methyl vinyl ketone and ethyl acrylate in toluene at 25°C...

Further calculations were performed on 2,3-diaza-1,3-butadiene with different heterodienophiles such as ethene, formaldehyde and formaldimine showing the same exo oxygen or nitrogen lone pair preference [49] as well as on the nitro-soethene/ethene system [50]. Recently, ab initio studies have also been performed for the Lewis acid catalysed hetero Diels-Alder reaction of isoprene and sulfur dioxide by Sordo [51]. 

With its concerted mechanism implying little charge distribution change along the pathway, the Diels-Alder reaction has been understood to have little rate dependence on solvent choice. For example, the relative rate of cyclopentadiene dimerization increases only by a factor of 3 when carried out in ethanol. The relative rate for the Diels-Alder reaction of isoprene with maleic anhydride (Table 7.1) varies by only a factor of 13 with solvents whose dielectric constants vary by almost a factor of 10, but the rate acceleration is not a simple function of the solvent polarity. Furthermore, the dimerizations of cyclopentadiene and 1,3-butadiene proceed at essentially identical rates in the gas and solution phases. ... 

Table 3 Diels-Alder Reactions of Isoprene, 2-Trimethylsilylmethyl- and 2-Ttimethylstannylmethyl-1,3-butadienes with Conjugated Enoyl Compounds (Scheme 43)...

It was shown that the Diels-Alder reaction of isoprene with methylvinylketone is catalyzed by zeolites Y, EMT, beta and ZSM-5. Significant activity can only be observed if the H-form of the zeolites is used and can be correlated with the Si/Al... 

TABLE 1. Pressure-induced rate acceleration of the Diels-Alder reaction of isoprene with acryloni-... 

Grieger and Eckert [21, 49] considered two explanations of the ratio O > 1 in the Diels-Alder reaction of isoprene with maleic anhydride a larger dipole moment of the transition state or secondary orbital interactions which can only occur in endo Diels-Alder reactions. The findings that the difference between the activation volumes of many endo and exo Diels-Alder reactions is small (AAV < 1-2 cm mol ) and that the activation volume of retro Diels-Alder reaction of the endo cycloadduct between dimethylfulvene and N-phenylmaleic imide is positive and that of the retro Diels-Alder reaction of the corresponding exo cycloadduct is negative (Scheme 2.6), rule out that secondary orbital interactions are important and induce a larger contraction of the volume of the endo transition state. 

Tab. 10.20. Effect of the medium on rate constants in the Diels-Alder reaction of isoprene and methyl vinyl ketone (T — 313.7 K, p = 0.1 MPa).

Fig. 10.10. Alternate transition structures for Diels-Alder reaction of isoprene with propenal (a) structure without formyl hydrogen bond (b) structure with formyl hydrogen bond. Dimensions are from B3LYP/6-31G( f) computations in the gas phase and in PMC with s = 4.335 (shown in parentheses). Adapted from Org. Lett., 5, 649 (2003), by permission of the American Chemical Society.

Scheme 5.1-6 Diels-Alder reactions of isoprene with methyl acrylate, acrylic acids, but-3-en-2-one and acrylonitrile in phosphonium tosylates.

It is finally worth noting that phosphonium tosylates [31], and more recently pyridinium-based ionic liquids [32], have also been used as solvents for the Diels-Alder reactions of isoprene with methyl acrylate, acrylic acids, but-3-en-2-one and acrylonitrile (Scheme 5.1-6). 

The use of molten salts based on phosphonium tosylates has also been reported for Diels-Alder reactions [175]. These salts have higher melting points than most ionic liquids in common use and hence the reactions were performed in a sealed tube. The authors claim very high selectivities in the reaction of isoprene with MVK or methyl acrylate. The effect of temperature on the selectivity in phosphonium tosylates gave reduced endoxxo ratios at higher temperatures [176]. The Diels-Alder reactions of isoprene with acrylonitrile, acrylic acid and methacrylic acid in pyridinium ionic liquids ([EtPy][BF4] or [EtPy][F3CC02]) were found to give the expected cyclohexene structures [177]. The authors show that... 

An interesting process for the manufacture of p-cresol, which shows that the build-up of aromatics from low molecular components should always be considered as an alternative, is the Diels-Alder reaction of isoprene and vinyl acetate. This process, however, has not been exploited on an industrial scale. 

Table III Aza-Diels-Alder Reactions of Isoprene with N-Galactosyl Imines 16 to give Chiral 3,4-Dehydropiperidines 17 (30)...

Furthermore, Fiirmeier and Metzger used the same procedure to study the electron transfer-initiated Diels-Alder reaction of isoprene and anethole, as well as the Diels-Alder dimerization of 1,3-cyclohexadiene. They unambiguously detected and characterized the respective transient radical cations of the dienophiles and of the Diels-Alder addition products. 

Next we have reactions where the reactants and products are neutral but polar. Here again solvents have no large effect but the reactions are faster in solution than in the gas phase and are faster, the more polar is the solvent. The reason for this is that the transition states of such reactions usually contain delocalized systems that are both more polar and more polarizable than the less delocalized systems in the reactants. A simple example is the Diels-Alder reaction of isoprene with maleic anhydride,... 

In the presence of molecular sieves 5A (MS 5A), scandium perfluoroalkanesul-fonate-catalyzed Diels-Alder reaction of isoprene and methyl vinyl ketone proceeded smoothly in dry dichloromethane (Scheme 12.14) [25]. It was found that water interfered with the reactions, contrary to most rare earth-catalyzed reactions that proceed smoothly in aqueous media. Among the scandium perfluoroalkanesul-fonates tested, Sc(OTf)3, scandium pentafluoroethanesulfonate (Sc(0S02C2Fs)3), and scandium nonafluorobutanesulfonate (Sc(0S02C4H9)3) gave the highest yields and selectivities. 

Isotope effects for the Lewis acid-catalysed Diels-Alder reactions of isoprene with methyl vinyl ketone, ethyl acrylate, and acrolein support a highly asynchronous 4-1-2-cycloaddition. The intramolecular cycloaddition of )3-arylsulfonyl vinyl ketones with 1,1-disubstituted dienes produce either 4 - - 2- or 2 -I- 2-cycloadducts depending... 

Examples of the use of activation volume for the reactions in supercritical fluids is the unimolecular decomposition of a-chlorobenzyl methyl ether in 1,1-difluoroethane Jc = 113.4°C), studied by Johnston and coworkers [54] and the Diels-Alder reaction of isoprene and maleic anhydride in supercritical carbon dioxide [56], described in section 3.1.2.1. In the latter study the large variation in the rate coefficient at 35°C and near-critical pressures was quantified as AV = - 1.39 X lO cm moF in the highly compressible region, as compared with - 38.4 cm mol at 200 bar. Paulaitis and Alexander interpreted their results as a solvent effect stemming from an induced quadrupole moment in the carbon dioxide molecule. 

The characterization and application of a series of highly acidic boronated aluminas, prepared by the reaction of BX3 with unactivated alumina, has been reported. The modified aluminas, BX / AI2O3, were found to catalyze the Diels-Alder reactions of isoprene with methyl acrylate, and methyl acrylate with cyclopentadiene (Scheme 23.29). Both reactions proceed in high yield and with high endo to exo selectivity (12-24 1). Recently, it was discovered that when microwave-activated alumina is employed, the endo to exo selectivity can reach up to 99 to 1. ... 

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