Butadiene reaction with acrolein

Other methods for the preparation of cyclohexanecarboxaldehyde include the catalytic hydrogenation of 3-cyclohexene-1-carboxaldehyde, available from the Diels-Alder reaction of butadiene and acrolein, the reduction of cyclohexanecarbonyl chloride by lithium tri-tcrt-butoxy-aluminum hydride,the reduction of iV,A -dimethylcyclohexane-carboxamide with lithium diethoxyaluminum hydride, and the oxidation of the methane-sulfonate of cyclohexylmethanol with dimethyl sulfoxide. The hydrolysis, with simultaneous decarboxylation and rearrangement, of glycidic esters derived from cyclohexanone gives cyclohexanecarboxaldehyde. 

The carbo-Diels-Alder reaction of acrolein with butadiene (Scheme 8.1) has been the standard reaction studied by theoretical calculations in order to investigate the influence of Lewis acids on the reaction course and several papers deal with this reaction. As an extension of an ab-initio study of the carbo-Diels-Alder reaction of butadiene with acrolein [5], Houk et al. investigated the transition-state structures and the origins of selectivity of Lewis acid-catalyzed carbo-Diels-Alder reactions [6]. Four different transition-state structures were considered (Fig. 8.4). Acrolein can add either endo (N) or exo (X), in either s-cis (C) or s-trans (T), and the Lewis acid coordinates to the carbonyl in the molecular plane, either syn or anti to the alkene. 

The mechanism of the carbo-Diels-Alder reaction has been a subject of controversy with respect to synchronicity or asynchronicity. With acrolein as the dieno-phile complexed to a Lewis acid, one would not expect a synchronous reaction. The C1-C6 and C4—C5 bond lengths in the NC-transition-state structure for the BF3-catalyzed reaction of acrolein with butadiene are calculated to be 2.96 A and 1.932 A, respectively [6]. The asynchronicity of the BF3-catalyzed carbo-Diels-Alder reaction is also apparent from the pyramidalization of the reacting centers C4 and C5 of NC (the short C-C bond) is pyramidalized by 11°, while Cl and C6 (the long C-C bond) are nearly planar. The lowest energy transition-state structure (NC) has the most pronounced asynchronicity, while the highest energy transition-state structure (XT) is more synchronous. 

Fig. 8.5 The calculated transition-state structure for the reaction of acrolein with butadiene leading to carbo-Diels-Alder adduct catalyzed by BH3 using a RHF/3-21G basis set [6]...

An important contribution for the endo selectivity in the carho-Diels-Alder reaction is the second-order orbital interaction [1], However, no bonds are formed in the product for this interaction. For the BF3-catalyzed reaction of acrolein with butadiene the overlap population between Cl and C6 is only 0.018 in the NC-transi-tion state [6], which is substantially smaller than the interaction between C3 and O (0.031). It is also notable that the C3-0 bond distance, 2.588 A, is significant shorter than the C1-C6 bond length (2.96 A), of which the latter is the one formed experimentally. The NC-transition-state structure can also lead to formation of vinyldihydropyran, i.e. a hetero-Diels-Alder reaction has proceeded. The potential energy surface at the NC-transition-state structure is extremely flat and structure NCA (Fig. 8.6) lies on the surface-separating reactants from product [6]. 

In an investigation by Yamabe et al. [9] of the fine tuning of the [4-1-2] and [2-1-4] cycloaddition reaction of acrolein with butadiene catalyzed by BF3 and AICI3 using a larger basis set and more sophisticated calculations, the different reaction paths were also studied. The activation energy for the uncatalyzed reaction were calculated to be 17.52 and 16.80 kcal mol for the exo and endo transition states, respectively, and is close to the experimental values for s-trans-acrolein. For the BF3-catalyzed reaction the transition-state energies were calculated to be 10.87 and 6.09 kcal mol , for the exo- and endo-reaction paths, respectively [9]. The calculated transition-state structures for this reaction are very asynchronous and similar to those obtained by Houk et al. The endo-reaction path for the BF3-catalyzed reaction indicates that an inverse electron-demand C3-0 bond formation (2.635 A... 

Further studies by Garcia, Mayoral et al. [10b] also included DFT calculations for the BF3-catalyzed reaction of acrolein with butadiene and it was found that the B3LYP transition state also gave the [4+2] cycloadduct, as happens for the MP2 calculations. The calculated activation energy for lowest transition-state energy was between 7.3 and 11.2 kcal mol depending on the basis set used. These values compare well with the activation enthalpies experimentally determined for the reaction of butadiene with methyl acrylate catalyzed by AIGI3 [4 a, 10]. 

The endo exo selectivity for the Lewis acid-catalyzed carbo-Diels-Alder reaction of butadiene and acrolein deserves a special attention. The relative stability of endo over exo in the transition state accounts for the selectivity in the Diels-Alder cycloadduct. The Lewis acid induces a strong polarization of the dienophile FMOs and change their energies (see Fig. 8.2) giving rise to better interactions with the diene, and for this reason, the role of the possible secondary-orbital interaction must be considered. Another possibility is the [4 + 3] interaction suggested by Singleton... 

C Primary kinetic isotope effects for the concerted reaction of butadiene with ethylene, for the stepwise reaction of butadiene with ethylene and for the concerted reaction of butadiene with acrolein, have also been calculated207. The experimental values of 1.0438 and 1.0474 found recently196 in the reaction of 2,3-dimethylbutadiene with [1-14C]- and [2-14C]-l-nitro-2-phenylethylene, respectively, similar at both reacting termini, are in accord with the calculated value of 1.046 for knc/ki4c (373.15 K) in a synchronous concerted reaction of butadiene with ethylene. The 14C KIE values predicted for the asynchronous acrolein reaction are 1.015 and 1.045 for the T and 2 isotopomer, respectively207. 

An extensive review of the hetero-Diels-Alder reactions of 1-oxabuta-1,3-dienes has been published. Ab initio calculations of the Diels-Alder reactions of prop-2-enethial with a number of dienophiles show that the transition states of all the reactions are similar and synchronous.Thio- and seleno-carbonyl compounds behave as superdienophiles in Diels-Alder reactions with cyclic and aryl-, methyl-, or methoxy-substituted open-chain buta-1,3-dienes.The intramolecular hetero-Diels-Alder reactions of 4-benzylidine-3-oxo[l,3]oxathiolan-5-ones (100) produce cycloadducts (101) and (102) in high yield and excellent endo/exo-selectivity (Scheme 39). A density functional theoretical study of the hetero-Diels-Alder reaction between butadiene and acrolein indicates that the endo s-cis is the most stable transition structure in both catalysed and uncatalysed reactions.The formation and use of amino acid-derived chiral acylnitroso hetero-Diels-Alder reactions in organic synthesis has been reviewed. The 4 + 2-cycloadditions of A-acylthioformamides as dienophiles have been reviewed. ... 

Thus, if you look at the different dienophiles in Fig. 2.1, the dimerization of butadiene 2.1 is slower than its reaction with acrolein 2.4. Methyl acrylate and methyl vinyl ketone have electron-withdrawing substituents Z of comparable power, and react at a similar rate, but cyclohexenone 2.5, which has a (5-alkyl substituent, is considerably less reactive. Nitroethylene has one of the most powerful electron-withdrawing groups, and is a very good... 

DFT has also proved useful for the calculation of substituent effects on rates. For example, studies of reactions of butadiene and cyclopentadiene with acrolein predict a large s-trans preference of the acrolein, and a 1 kcal/mol lower activation energy than with the ethylene reaction, but very similar energies of exo- and endo- transition states [38]. Jursic has also studied several heterocyclic cases [39]. 

Diels-Alder reactions of the type shown in Table 12.1, that is, Diels-Alder reactions between electron-poor dienophiles and electron-rich dienes, are referred to as Diels-Alder reactions with normal electron demand. The overwhelming majority of known Diels-Alder reactions exhibit such a normal electron demand. Typical dienophiles include acrolein, methyl vinyl ketone, acrylic acid esters, acrylonitrile, fumaric acid esters (fnms-butenedioic acid esters), maleic anhydride, and tetra-cyanoethene—all of which are acceptor-substituted alkenes. Typical dienes are cy-clopentadiene and acyclic 1,3-butadienes with alkyl-, aryl-, alkoxy-, and/or trimethyl-silyloxy substituents—all of which are dienes with a donor substituent. 

The Rates of Diels-Alder Reactions. Most Diels-Alder reactions require that the dienophile carries a Z-substituent before they take place at a reasonable rate. Butadiene 6.144 will react with ethylene, but it needs a temperature of 165 °C and high pressure, whereas the reaction with acrolein is faster, taking less time at a lower temperature. An X-substituent on the diene, on C-l or C-2, increases the rate further, with 1-methoxybutadiene 6.145 and 2-methoxybutadiene 6.146 reacting with acrolein at lower temperatures. Times and temperatures are not a reliable way of measuring relative rates, but these four reactions were taken to the point where the yields of isolated product are close to 80%. 

Bimey, D. M. Houk, K. N. Transition structures of the Lewis acid-catalyzed Diels-Alder reaction of butadiene with acrolein. The origins of selectivity, 7. Am. Chem. Soc. 1990,112, 4127-4133. 

TABLE 7.5 Activation Enthalpy (in kcal mol ) for the Reaction of Butadiene with Acrolein Computed for the Gas Phase, PCM, and Explicit Water Models"... 

The process is quite general for simple dienes and aldehydes. For example, the reaction of acrolein with cyclopentadiene, cyclohexadiene, or 2,3-dimethyl-l,3-butadiene gives cycloadducts with 8(F-84 % ee and exolendo = 12/88-< 1/99. The a-substituent on the dienophile increases the enantioselectivity (acrolein compared with methacro-lein). When there is /3-substitution in the dienophile, as in crotonaldehyde, the cycloadduct is almost racemic. On the other hand, for a substrate with substituents at both a and ji positions, high ee is observed, as for 2-methylcrotonaldehyde and cyclopentadiene (90 % ee, exolendo = 97/3). The active boron catalyst is beheved to have the structure shown in Eq. (8), with a five-membered ring and a free carboxyl group. The latter seems not to be crucial for the enantioselectivity because eomparable results are obtained when the carboxylic group is transformed into an ester. 

Ab initio calculations have been performed by Birney and Houk to define the transition state in Diels-Alder reactions catalyzed by boron derivatives [38]. As a model, the authors studied the reaction between butadiene and acrolein complexed with BH3. The preferred route is endo addition of the anti complex of s-cis acrolein. 

The polarizable continuum model (PCM) by Tomasi and coworkers [77-79] was selected to describe the effects of solvent, because it was used to successfully investigate the effect of solvent upon the energetics and equilibria of other small molecular systems. The PCM method has been described in detail [80]. The solvents and dielectric constants used were benzene (s = 2.25), methylene chloride (g = 8.93), methanol (g = 32.0), and water (g = 78.4). Full geometry optimizations were carried out for the discrete and PCM models. To simultaneously account for localized hydrogen bonding and bulk solvation effects, PCM single-point energy calculations have been conducted on stationary points of the acrolein and butadiene reaction with two waters explicitly... 

SULFUROUS OXIDE (7446-09-5) SO, Noncombustible liquefied gas under pressure or liquid. Contact with air forms hydrogen chloride fumes. Violent reaction with water or steam, forming sulfurous acid, a medium-strong acid and corrosion hazard. Reacts violently with acetylene, acrolein, alcohols, aluminum powder alkali metals (i.e., potassium, sodium) amines, ammonia, bromine pentafluoride butadiene caustics, cesium acetylene carbide chlorates, chlorine trifluoride chromium powder copper or copper alloy powders chlorine, diethylzinc, fluorine, ethylene oxide lead dioxide lithium acetylene carbide diamino-, metal powders monolithium acetylide-ammonia nitryl chloride potassium acetylene carbide potassium acetylide, potassium chlorate rubidium carbide silver azide sodium acetylide staimous oxide. Decon oses in... 

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