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110 Rev. Soc. Quím. Méx. Vol. 47, Núm. 2 (2003) Jorge A. Mendoza et al. Table 2. Preparation of the β-sulfanyl substituted captodative olefins 10b-10h from olefin 7b. Entry a RSH 10 (%)b 1 9d (R = Et) 10b (74) 2 9e (R = i-Pr) 10c (77) 3 9f (R = t-Bu) 10d (70) 4 9g (R = Bn) 10e (87) 5 9h (R = C 6 H 4 p-Br) 10f (91) 6 9i (R = C 6 H 4 p-OMe) 10g (93) 7 9j (R = C 6 H 4 p-Me) 10h (90) a All under N 2 atmosphere, with 1.3 mol equiv. of RSH and with 1.3 mol equiv. of Et 3 N, in DMF as solvent. The reaction was succesively maintained at 0 °C for 1 h, 120 °C for 1 h, and at 20 °C for 1 h. b After column and radial chromatography. ylamino group was used, due to its known aptitude as a leaving group [12]. In these cases the optimum reaction conditions were found to be similar to those employed for olefin 3a, that is, reacting olefin 7b in the presence of thiols 9d-9j and triethylamine as base, and heating from 0 °C up to 120 °C for 3 h (Table 2). The aliphatic thiols 9d-9g provided compounds 10b-10e in quite lower yields than those obtained when olefin 3a was used (Table 2, entries 1-4 vs. Table 1, entries 4-7); however, the yields increased for the preparation of 10g-10h when 7b was treated with thiophenols 9i-9j (Table 2, entries 6-7 vs. Table 1, entries 9-10). For both olefins, 3a and 7b, there were no significant differences in reactivity or yields to furnish the β-substituted alkenes 10 with the para substituted thiophenols 9h-9j (Table 1, entries 8-10 and Table 2, entries 5-7), considering that the temperature and the reaction time were comparable, regardless of the substituent. It is interesting that sodium thiolates 9a-9c reacted with olefin 3a to give the 1,4-addition products 10a-10c in good yields, instead of providing the hydrolisis products by addition to the p-nitrobenzoyloxy group, as observed when the corresponding alcohols were used. As expected, this behavior illustrates again the known greater softness of the sulphur atom with respect to the oxygen atom. The Z configuration of the double bond is maintained in all of the new olefins 10a-10h, as observed for the β-amino alkene 7a, indicating that it is largely more stable than the E Fig. 3. X-ray structure of captodative olefin 10f (ellipsoids with 30 % probability). configuration [6]. It is likely that the higher stability of the Z configuration is associated to destabilizing steric interactions found between the acetyl group and the beta substituent in the opposite E configuration. Such interactions may inhibit the effective conjugation of the enone moiety, and the delocalization of the heteroatom lone electron pair through the π system [6, 13]. The configuration of the double bond and the planar conformation of the enone moiety was confirmed by single crystal X-ray crystallography of olefin 10f (Fig. 3). It shows that both conjugated moieties, the enone and p-nitrobenzoyl groups, are in a quasi-orthogonal conformation. This agrees with those structures obtained for other analogues [5, 6, 11], as well as in the s-trans conformation of the enone. Interestingly, the p-bromophenyl ring lies out of the plane formed by the enone π-system. FMO calculations of β-heterosubstituted captodative olefins Frontier molecular orbital (FMO) theory has proven to be a reliable model to predict reactivity and regioselectivity in Diels-Alder [14] and 1,3-dipolar cycloadditions [15]. It has been able to explain the behavior of substituted olefins as dienophiles in Diels-Alder additions. For instance, the rate increases when the dienophile bears electron-withdrawing groups, whereas it decreases with dienophiles bearing electron-donating groups [16]. FMO theory has also been useful in explaining the reactivity and regioselectivity of captodative olefins with dienes such as isoprene (5) [2a, 11], showing that the interaction was controlled by normal electron demand (NED), i.e. the HOMOdiene/LUMO-dienophile interaction was the energetically most favorable. Furthermore, these results were supported by experimental measurement of ionization energies (IEs) and vertical attachment energies (VAEs) of 1a, whose relative values fit well with the calculated HOMO and LUMO energies, respectively [11]. Table 3. Ab initio HF/6-31G* Frontier Molecular Orbitals energies, and energy gaps (eV) of olefins 3a, 7a, 7b, 10a, 10f, and 10g, and isoprene (5). Entry Compound a HOMO LUMO HOMO-LUMO b 1 1ac –11.0460 2.4588 11.0781 2 3a –10.4288 2.1015 10.7208 3 7a –8.4516 2.9375 11.5568 4 7b –8.8119 3.1767 11.7960 5 10a –9.1412 2.4681 11.0874 6 10f –9.2764 2.3162 10.9355 7 10g –10.2014 2.1900 10.8093 8 5c –8.6193 3.5337 a Of the non-planar s-trans conformation for olefins 1a, 3a, 7a, 7b, 10f, and 10g, and of the s-cis conformation for olefin 10a and for the diene. b Energy gaps for the energetically more favorable HOMO-diene/LUMO-dienophile interaction. c Ref. [11].
Synthesis, Structural, and Theoretical Study of New β-Heterosubstituted Captodative Olefins 1-Acetylvinyl Arenecarboxylates 111 Table 4. Comparison of X-ray selected bond distances (Å) (estimated standard deviations) of the crystal structure of 10f with those of olefins 1a, 3a, and 7b, and with the average lenghts of bonds of the enone, enamine, vinyl ester and bromo vinyl systems. Bond 10f 1a a 3a b 7b b Average Length c O(4)-C(3) 1.208 (4) 1.215 (5) 1.20 (3) 1.231 (7) 1.222 C(2)-C(3) 1.470 (5) 1.490 (5) 1.40 (3) 1.41 (1) 1.462 C(1)-C(2) 1.320 (4) 1.306 (6) 1.42 (3) 1.366 (9) 1.340 C(1)-Y(5) 1.725 (4) 1.79 (3) 1.32 (1) 1.712 (S); 1.881 (Br); 1.358 (Nsp 2 ); 1.418 (Nsp 3 ) C(2)-O(6) 1.412 (3) 1.398 (4) 1.41 (3) 1.423 (7) 1.353 O(6)-C(7) 1.355 (4) 1.353 (4) 1.38 (2) 1.354 (6) 1.359 C(7)-O(8) 1.195 (4) 1.200 (4) 1.21 (3) 1.188 (6) 1.201 C(7)-C(9) 1.486 (4) 1.488 (5) 1.50 (3) 1.481 (7) 1.481 a Ref. [11]. b Ref. [6]. c Ref. [19] Table 5. Ab initio HF/6-31G* calculations of coefficientes (C i ) of the frontier molecular orbitals for olefins 1a, 3a, 7a, 7b, 10a, 10f, and 10g, and diene 5. a HOMO LUMO Compd b C 1 C 2 C 3 C 4 ∆C i c C 1 C 2 C 3 C 4 ∆C ι c 3a –0.2456 –0.2839 0.0095 0.1277 –0.0383 0.3188 –0.2499 –0.2783 0.2759 0.0689 7a –0.1192 –0.3110 –0.0215 0.1414 –0.1918 0.2701 –0.1547 –0.2314 0.2143 0.1154 7b –0.1711 –0.3702 –0.0190 0.1718 –0.1991 0.2799 –0.1433 –0.2564 0.2316 0.1366 10a –0.1848 –0.3135 –0.0038 0.1475 –0.1287 0.3189 –0.2200 –0.2807 0.2501 0.0989 10f –0.1625 –0.2694 –0.0019 0.1277 –0.1069 0.2872 –0.2073 –0.2506 0.2263 0.0799 10g –0.2082 –0.2400 0.0042 0.1174 –0.0318 0.2843 –0.2404 –0.2579 0.2574 0.0439 1a d –0.3593 –0.3565 0.0236 0.1676 0.0028 0.2940 –0.2386 –0.2889 0.2800 0.0554 5 d 0.3247 0.2523 –0.2180 –0.2857 0.0390 0.2591 –0.2236 –0.2306 0.2793 –0.0202 a These are the values of the 2p z coefficients, the relative 2p z ’ contributions and their ∆C i are analogous. b The FMOs of the non-planar s-trans conformation for olefins 1a 3a, 7a, 7b, and 10g, and s-cis for 5, 10a, and 10f. c Carbon 1 - carbon 2 for the olefins; carbon 1 - carbon 4 for the diene. d Ref. [11] Therefore, the electronic effect produced by the substituent in the beta position of the captodative olefins 3, 7, and 10 on their behavior in Diels-Alder reactions could be evaluated by calculating the FMO energies of these molecules, and correlating them with those of the corresponding FMOs of a diene such as 5. Table 3 summarizes the calculated (HF/6-31G*) FMO energies of bromo and amino alkenes 3a, 7a, and 7b, as well as the thio alkenes 10a, 10f, and 10g. The geometries were optimized with the same basis set, showing that the most stable geometry for 3a, 7a, and 7b, corresponded to the s-trans conformation of the enone moiety, and the non-planar conformation of the p-nitrobenzoyloxy group. However, only for thioether 10g, the enone s-trans conformation was more stable, while olefins 10a and 10f were the exception, since the s- cis conformer for the enone conjugate system was slightly more stable (0.72 kcal/mol for 10a, and 0.24 kcal/mol for 10f), which is not in agreement with the X-ray structure of 10f. Even though these energy differences are negligible, and, practically, both conformations are isoenergetic, the FMO energy values listed in Table 3 corresponded to those obtained for the most stable confomation in every molecule. The co-
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Page 7 and 8: Editorial La investigación cientí
Page 9 and 10: Editorial 103 Jacobo Gómez Lara (1
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