Journal of Molecular Structure (Theochem), 138 (1986) 117-120 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
RATIONALIZATION OF THE CATALYTIC EFFECT ON THE REACTIVITY-SELECTIVITY RELATIONSHIP IN DIELS-ALDER REACTIONS
V. BRANCHADELL,
A. OLIVA and J. BERTRAN
Lkpartament de Q&mica Ffsica, Universitat Barcelona (Spain)
Autbnoma
de Barcelona,
Bellaterra,
(Received 27 June 1985)
ABSTRACT The catalytic effect on the reactivityselectivity relationship in DieleAlder reactions is rationalized through the change of the type of energy profile. It is shown that this change depends not only on the ionic or non-ionic character of the uncatalyzed reaction, but also on the strength of the catalyst. INTRODUCTION
The catalytic effect of Lewis acids in Diels-Alder reactions has been known for many years [l-S]. In most cases, the increment of reaction rate is accompanied by an increase in selectivity [ 2-81. This fact apparently contradicts the accepted reactivity-selectivity relationship, according to which the more reactive reagents should be the less selective species. Epiotis and Shaik [9] have interpreted this anomalous behavior through the interaction between the no-bond configuration and the charge-transfer configuration. Depending on the smaller or greater energetic difference between both configurations, reactions can be classified as ionic and non-ionic, respectively. For the first ones, the stabilization of the charge-transfer configuration due to the catalyst causes a notable advance of the transition state towards the reactants on the reaction co-ordinate. Conversely, in non-ionic reactions, no advance of the transition state is produced, but the increase of interaction between both configurations causes the diminution of the potential barrier to occur with an increase of selectivity. According to Epiotis and Shaik, a Diels-Alder reaction is an example of non-ionic reaction. In previous papers [10-121 the present authors have studied the catalytic effect on the Diels-Alder reactions of acrolein with 1-hydroxybutadiene and 1-methylbutadiene by using the MIND0/3 method [13]. BFJ and NH: were used as catalysts. In all cases, the catalyst noticeably changes the reaction mechanism, increasing the two-step character of the processes. This change is more drastic in the NH;-catalyzed reactions, in which a total proton transfer between the catalyst and the substrate is involved [ 121. 0166-1280/86/$03.50
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For the reaction between 1-hydroxybutadiene and acrolein, both catalysts produce a diminution of selectivity [ 10, 121. The only way of interpreting this behavior in the framework of Epiotis and Shaik’s classification is to consider that this Diels-Alder reaction is ionic instead of non-ionic. In the reaction between 1-methylbutadiene and acrolein, the two catalysts give different results. When the reaction is catalyzed by BF3, the selectivity increases [ 111, this fact being in good agreement with experimental results [6] . On the contrary, NH; produces a diminution of selectivity [ 121. This dependence of the reactivityselectivity relationship on the catalyst has not been explicitly discussed in Epiotis and Shaik’s work, in which only the ionic or non-ionic character of the uncatalyzed reaction is considered. In the present paper we propose a classification of Diels-Alder reactions which will permit the reactivity-selectivity relationship to be rationalized taking into account the strength of the catalyst. ENERGY PROFILES AND REACTIVITY-SELECTIVITY
RELATIONSHIP
Depending on the shape of their energy profiles, Diels- Alder reactions can be classified into three different types, which are represented in Fig. 1. The shape of the energy profile is closely related to the charge transfer between the diene and the dienophile. The energy profile I corresponds to reactions with small charge transfer which take place in only one step. In the energy profile II an intermediate, situated in a well of very little depth appears, and the second transition state determines the reaction rate. This energy profile corresponds to Diels-Alder reactions with important charge transfer between the diene and the dienophile. Finally, a greater increment of the charge transfer leads to the energy
El
t
reaction
Fig. 1. Types of energy profiles for Diels-Alder
coordinate
reactions.
119
profile III, in which the two-step character of the process has been strengthened. The first transition state is now higher in energy than the second one and thus determines the reaction rate. The effect of the catalyst on the reactivity-selectivity relationship can now be easily interpreted. As a matter of fact, the role of the catalyst is to amplify the charge transfer between the diene and the dienophile. So, as was mentioned in the preceding paragraph, this increase of charge transfer can modify the type of energy profile of the reaction. If the energy profile for the catalyzed reaction is of type I or II, the position along the reaction coordinate of the transition state that determines the reaction rate hardly changes. In these cases, according to Epiotis and Shaik’s ideas, the increase of reactivity will be accompanied by an increase of selectivity. However, if the catalyst changes the energy profile to type III, the transition state that determines the reaction rate experiences an important advance towards the reactants and the selectivity diminishes. These considerations allow all the results mentioned in the introduction to be rationalized, including those which have not been discussed in the Epiotis and Shaik treatment. Table 1 presents the different types of energy profiles which were obtained [10-X2] for the reactions of acrolein with l-hydroxybutadiene and 1-methylbutadiene both uncatalyzed and catalyzed by BF3 and NH;. The energy profiles for the uncatalyzed reactions are of type I, except in the case of the reaction between l-hydroxybutadiene and acrolein corresponding to the formation of the ortho adduct, whose energy profile is of type II. When this reaction is catalyzed by BF3, the energy profile changes to type III and so the selectivity diminishes. In the reaction between l-methylbutadiene and acrolein, the BF3-catalyzed reactions are of type I and II, and an increase of selectivity is produced. Finally, the decrease of selectivity for both reactions when catalyzed by NH: can also be explained, since all the TABLE 1 Types of energy profilesa for the reactions of acrolein with 1 hydroxybutadiene l-methylbutadiene, uncatalyzed and catalyzed by BF, and NH: Reaction
Adduct
and
Catalyst
-
BF,
NH:
l-Hydroxybutadiene + acrolein
ortho
II
III
III
meta
I
II
III
1-Methylbutadiene + acrolein
ortho
I
II
III
meta
I
I
III
aRefs. 10-12.
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corresponding energy profiles are of type III. Therefore, while the reaction between 1-hydroxybutadiene and acrolein can be considered as ionic and the selectivity diminishes with both catalysts, the selectivity of the non-ionic reaction between l-methylbutadiene and acrolein increases or diminishes depending on the use of BF3 or NH;. CONCLUSION
The main conclusion of this paper is that the reactivity-selectivity relationship in catalyzed Diels-Alder reactions depends not only on the ionic or non-ionic character of the uncatalyzed reaction, but also on the strength of the catalyst. The increase of reaction rate produced by the catalyst will only be accompanied by an increase of selectivity in the case of Diels- Alder reactions with small charge transfer which are catalyzed by weak acids. REFERENCES 1 P. Yates and P. Eaton, J. Am. Chem. Sot., 82 (1960) 4436. 2 E. F. Lutz and G. M. Bayley, J. Am. Chem. Sot., 86 (1964) 3899. 3 J. Sauer and J. Kredel, Tetrahedron Lett., (1966) 731. 4 T. Inukai and T. Kojima, J. Org. Chem., 31 (1966) 1121. 5 T. Inukai and T. Kojima, J. Org. Chem., 31 (1966) 2032. 6 T. Inukai and T. Kojima, J. Org. Chem., 32 (1967) 869. 7 K. L. Williamson and Y. F. Li Hsu, J. Am. Chem. Sot., 92 (1970) 7385. 8 T. Cohen and Z. Kosarych, J. Org. Chem., 47 (1982) 4005. 9 N. D. Epiotis and S. Shaik, J. Am. Chem. Sot., 100 (1978) 1. 10 V. Branchadell, A. Oliva and J. Bertrln, Chem. Phys. Lett., 113 (1985) 197. 11 V. Branchadell, A. Oliva and J. Bertran, J. Mol. Struct. (Theochem), 120 (1985) 85. 12 V. Branchadell, A. Oliva and J. Bertran, J. Mol. Catal., in press. 13 R. C. Bingham, M. J. S. Dewar and D. H. Lo, J. Am. Chem. Sot., 97 (1975) 1285.