Transition state for the Diels-Alder reaction

The antibody-catalyzed Diels-Alder reaction developed by Schultz utilized a Diel-Alderase enzyme-like catalyst evolved from an antibody-combining site (Eq. 12.13). The idea is that the generation of antibodies to a structure that mimics the transition state for the Diels-Alder reaction should result in an antibody-combining site that lowers the entropy of activation by binding both the diene and dienophile in a reactive conformation. 

An ab initio MO calculation by Jorgensen revealed enhanced hydrogen bonding of a water molecule to the transition states for the Diels-Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile, which indicates that the observed rate accelerations for Diels-Alder reactions in aqueous solution arise from the hydrogenbonding effect in addition to a relatively constant hydrophobic term.7,76 Ab initio calculation using a self-consistent reaction field continuum model shows that electronic and nuclear polarization effects in solution are crucial to explain the stereoselectivity of nonsymmetrical... 

You want to find the transitions state for the Diels-Alder reaction ... 

Fig. 3.11 Using molecular mechanics to get the (approximate) transition state for the Diels-Alder reaction of butadiene with ethene. This procedure gives a structure with the desirable Cs, rather than a lower, symmetry...

Liquid polyethylene glycol in organic synthesis. Three main types of reaction have been studied substitution, oxidation and reduction (Figure 8.5). The Diels Alder reaction is frequently used as a benchmark for alternative reaction media. For the reaction of 2,3-dimethyl-1,3-butadiene with nitrosobenzene in PEG 300 or PPG 425, a 3.3-fold increase in rate was seen compared with dichloromethane and a 2.5-fold increase compared with ethanol. It has also been shown more recently that in comparison to conventional solvents, the transition state for the Diels-Alder reaction is stabilized in aqueous PEG solutions and this results in a lower activation energy. ... 

It is found that chiral dienes form better performing cationic Rh complexes than diphosphines, for use in catalyzing intramolecular Diels-Alder reaction of conjugate diene and alkyne units.A cationic Ru(I) catalyst 140 operates on the basis of one-point association of the dienophile prior to establishment of the transition state for the Diels-Alder reaction. The most effective case demonstrated thus far is an intramolecular process. ... 

The Diels-Alder reaction (Section 13.11) is also a pericyclic reaction. The transition state for the Diels-Alder reaction also involves six electrons ... 

FIGURE 5.38. (a) Interaction of hybrid AOs of (216) to form the correct AOs of (215), p being the resonance integral between the hybrid AOs (b) analogous interaction between 2p AOs of the two carbon atoms in ethylene to form bonding n ) and antibonding (n ) MOs (c) transition state for the Diels-Alder reaction between butadiene and ethylene (cf. Fig. 5.35) (d) transition state for the chelotropic reaction of equation (5.317). 

The rationale for the high enantioselectivity of the reaction when compared to 1,2-dihydropyridines bearing a chiral auxiliary on nitrogen is that the chiral center at the 2-position on the dihydropyridine 98 is positioned closer than an auxiliary to the reaction center in the transition state of the Diels-Alder reaction (Figure 4). 

The experimental observation is that maleic anhydride and cyclopentadiene give the endo adduct (174) faster than they give the exo adduct (176), even though the latter is thermodynamically the more stable. This is a general observation the transition state for most Diels-Alder reactions must be like 173 rather than like 175. In order to account for this, we examine the interaction of those parts of the frontier orbitals which are not directly involved in forming... 

Extended Hiickel Theory has been used to calculate an approximate energy surface for the [1,3] sigmatropic rearrangement connecting (171) to (172). The calculated relative ease of the various reaction paths is correlated with both the experimental data and the predictions of orbital symmetry theory. A non-coplanar transition state in the Diels-Alder reaction of cyclopentadiene with maleic anhydride, or with other five-membered cyclic dienophiles of C2 symmetry, is suggested by the relationship between the primary and secondary overlap integrals in the Salem-type analysis of the reaction path. ... 

Figure 1.22 The transition state for this Diels-Alder reaction is in sensible agreement with expectations.

Fig. 5.35 Geometry predicted by CASSCF ab initio calculations of the two possible transition structure geometries for the Diels-Alder reaction between ethene and butadiene. (Figure adapted from Houk KN, J Gonzalez and Y Li 1995. Pericyclic Reaction Transition States Passions and Punctilios 1935-1995. Accounts of Chemical Research 28 81-90.)...

Fig. 8.4 The four different transition-state structures considered for the Diels-Alder reaction of acrolein with a diene in the presence of a Lewis acid (BH3). The diene can add...

In the event, treatment of a rapidly stirred solution of 3 and sodium acetate in MeOH-tbO at 38 °C with PdCl2 results in the fomation of carpanone (1) in 46% yield. The ordered unimolecular transition state for the oxidative coupling reaction furnishes putative bis(quinodimethide) 2 stereoselectively. Once formed, 2 readily participates in an intramolecular Diels-Alder reaction4 to give carpanone (1). Two new rings and all five contiguous stereocenters are created in this spectacular sequential transformation.5... 

Fig. 10.11 The stepwise and concerted mechanisms for the Diels-Alder reaction between butadiene and ethylene. The reactants (lower left) proceed to the product, cyclohexene (lower right) either through a two step, two transition state mechanism involving the formation of a diradical intermediate (top center), or more directly through the symmetric synchronous transition state (bottom center) (Storer, J. W., Raimondi, L., and Houk, K. N., J. Am. Chem. Soc. 116, 9675 (1994))...

In the Diels-Alder reactions between ethene and butadiene and between acetylene and butadiene, respectively, the HOMOs are nearly isoenergetic and they are rather far away from the LUMOs (Figures 15.8 and 15.9). According to Equation 15.3, the transition states of these Diels-Alder reactions experience only a minor stabilization and, for this reason, these [4+2]-cycloadditions (Figure 15.24, column 3) are so much slower than the others (columns 2 and 4). 

One can compute, for example, the stabilizations AETS for the transition states of the para - and meta -selective cycloadditions, respectively, of acrylonitrile and isoprene according to Equation 15.2 with the data provided in Figure 15.26 (HOMO/LUMO gaps, LCAO coefficients at the centers that interact with each other). The result for the Diels-Alder reaction of Figure 15.25 is shown in Equations 15.4 and 15.5 ... 

The high simple diastereoselectivities seen in Figures 15.29 and 15.30 are due to the same preferred orientation of the ester group in the transition states. The stereostructure of the cycloadduct shows unequivocally that the ester group points underneath the diene plane in each of the transition states of both cycloadditions and not away from that plane. Figure 15.31 exemplifies this situation for two transition states of simple Diels-Alder reactions of 1,3-butadiene A shows a perspective drawing of the transition state of the acrylic acid ester addition, and B provides a side view of the addition of ethene, which will serve as an aid in the following discussion. Both structures were determined by computational chemistry. 

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