Decomposition mechanism of Birch alkylation products of α-tetralones

Decomposition mechanism of Birch alkylation products of α-tetralones

Journal of Molecular Structure (Theochem) 635 (2003) 173–182 www.elsevier.com/locate/theochem Decomposition mechanism of Birch alkylation products of...

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Journal of Molecular Structure (Theochem) 635 (2003) 173–182 www.elsevier.com/locate/theochem

Decomposition mechanism of Birch alkylation products of a-tetralones Guillermo R. Labadiea,*,1, Guillermina L. Estiu´b,c, Raquel M. Craveroa, Manuel Gonzalez Sierraa,* a

IQUIOS, Departamento de Quı´mica Orga´nica, Facultad de Ciencias Bioquı´micas y Farmace´uticas, Universidad Nacional de Rosario, Suipacha 531, S2002LRK Rosario, Argentina b CEQUINOR, Departamento de Quı´mica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC 962, 1900 La Plata, Argentina c Departamento de Quı´mica Inorga´nica, Analı´tica y Quı´mica Fı´sica, Universidad de Buenos Aires, Pabello´n 2, Ciudad Universitaria, C1428EHA Buenos Aires, Argentina Received 13 March 2003; revised 13 March 2003; accepted 24 May 2003

Abstract The Birch alkylation reaction, used in several synthetic paths in organic chemistry, faces the complication of the lack of stability of its products. Different mechanisms have been suggested for the decomposition step, which render different decomposition products. The comparison of these mechanisms can help to understand the origin of this complication, and, at the same time, to overcame it. This study shows the results of theoretical calculations applied to their comparison. The conclusions are in agreement with our previous experimental results. q 2003 Elsevier B.V. All rights reserved. Keywords: 1,4 Dienones radicals; Decomposition pathways; b-Elimination; Oxygen addition

1. Introduction The Birch reaction has been and is widely used in organic synthesis [1 – 7]. However, the reductivealkylation reaction of aromatic rings (Birch alkylation reaction) has been of relative little use in this area; despite the fact that it has high synthetic potential due to its capability to generate high functionality products. * Corresponding authors. Tel./fax: þ 54-341-4370477. E-mail addresses: [email protected] (G.R. Labadie), [email protected] (M.G. Sierra). 1 Present address: Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, UT 84112, USA.

The chemistry based on the Birch alkylation reaction of benzoic acids has undergone an important progress, which has been manifested in the research of Shultz [8 –12] and Subba Rao [13,14], among others. The reaction has not been as thoroughly applied to benzylic ketones, as the reduction of the carbonyl group competes, in this case, with the reduction of the aromatic ring. Because of this, no progress in the field has been reported until 1973, when Narisada and Watanabe [15] reported a procedure that allows the reduction of the aromatic ring without affecting the carbonyl group. An exhaustive study of the Birch reduction of the a-tetralone has been published afterwards [16]. Surprisingly this reaction has not

0166-1280/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0166-1280(03)00407-X

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been as extensively applied in organic synthesis as it could have been predicted from its success. Only few applications have appeared in the related literature since that time [17 – 19]. Our research group has been dealing with Birch alkylation reactions of benzylic ketones from some years now [20 – 23]. In this line, we have explored their application to a-tetralones [24] (Scheme 1) as well as the utilization of their products, as starting materials, in the synthesis of natural products [25]. The 8a-alkyl-1,2,3,4,6,8a-hexahydro-naphthalen-1ones (2), which are products of this reaction, show a high functionality, but also an important instability. In the synthesis of 2 (Scheme 1), we have found its stability related to the reaction conditions. When the benzyl ketones are the starting material, the product 2 decomposes in a short time when it is kept at room temperature. Decomposition occurs gradually, on the other hand, when it is stored at 2 20 8C, or in nitrogen atmosphere. Under these conditions, the product lasts for several weeks. The decomposition of 2 has not been previously discussed in the literature, but several, closely related studies point to it as radical mediated [26,27]. The reactivity of 1,4-dienes systems as 2 (Scheme 1) has been studied since 1950s. In his studies of cyclohexa-1,4-diene, Russell [28] suggested that the decomposition follows two steps of propagation (Scheme 2), where the formation of aromatic structures appears as the driving force for completion. According to this author, the diallylic radical A reacts with oxygen in a first step, generating benzene B and the hydroperoxide radical. This radical

reacts with the cyclohexa-1,4-diene C producing hydrogen peroxide and rendering again the radical A. The mechanism has been further confirmed by Howard and Ingold [29], who found that 1,4dihydronaphthalene also manifests the same behavior. The studies by Hendry [30] for the reaction conducted in liquid phase, also rendered coincident results. Baker [31] reported the decomposition by rearomatization of an intermediate of the synthesis of Giggerellin acid, previously obtained by means of a Birch alkylation reaction over its substituted benzoic acid portion. In 1986, Beckwith [32] found that the decomposition of methyl-1-alkylcyclohexa-2,5diene-1-carboxylate 3 (Scheme 3), produces the dienone 7 as main product, together with a lower proportion of the methyl benzoate 8. In agreement with Russell, Beckwith proposed a mechanism that leads to the generation of these two products, which begins by the formation of the diallylic radical 4. This radical is capable to react with an oxygen molecule to generate the hydroperoxide radical 5, which finally produce the dienone 7. The radical 4 can also follow a b-elimination reaction giving the methyl benzoate 8, predecessor of the 1,4-diene 3, and the radical R. In the research reported in Ref. [32], the dienone 10, which is a reorganization product of radical 5, was not isolated. Dealing with a system closely related to the previously mentioned ones (Scheme 1), and also characterized by its instability, we have followed the evolution of the decomposition products 2 by means of 1H NMR spectroscopy. We have examined several samples of 2a after being storaged, and

Scheme 1.

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175

Scheme 2.

found the starting material a-tetralone 1a as the main decomposition product. The stability of 2a has been studied in different conditions. One of the conditions was generated by means of a continuous bubbling of O2 to a 1,4-dienone dichloromethane solution at room temperature. After several days of bubbling, the products analyzed showed that the decomposition has not increased significantly, and no signals assignable to enones were found in the 1 H NMR spectrum. When a similar dichloromethane solution of 2a was irradiated with visible light for 2 h, at room temperature, and over a nitrogen atmosphere, complete decomposition was almost achieved, being a-tetralone 1a the main component. On the basis of the mechanism proposed by Beckwith, together with the results of our research, we postulate that the dienones initially react to give the corresponding diallylic radical 11 (Scheme 4), whose further b-elimination give the a-tetralone 1a. According to our experimental evidence, the alternative path, which involves reaction with O2, seems to be highly disfavored.

In order to understand the previously described behavior, which favors a b-elimination mechanism, we have performed semiempirical quantum chemical calculations, modeling the reactions of the 1,4dienones according to the mechanism shown in Scheme 4. To this end, we have studied the energy associated with each step of the suggested mechanism, using an AM1 methodology, which has been successful in describing several reactions involving radical formation as part of their mechanisms [33 –37].

2. Computational procedure We have applied semiempirical AM1 calculations to study the energy involved in each step of the reaction. In order to discern the influence of the different substituents (R) in the reaction mechanisms, different substrates have been modeled according to the R assignment shown in Scheme 4. Focusing on intermediates 11, 12 and 1a, which are the ones that can account for differences in the mechanism, we

Scheme 3.

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Scheme 4.

have followed the energy associated with the reactions through the theoretical identification of the transition intermediates. We have employed systematic methods to search for the most stable conformations of reactants (11) and products (12, 1a), fully optimizing the geometry without any restrictions by gradient-based methods (full optimization). These conformations have been used as starting points in a further search for the transition states, using a quadratic synchronous transit algorithm. Due to the size, and the number of structures that are necessary for an appropriate evaluation of the influence of R in the mechanism, together with the computational cost associated with the calculation of second derivates, semiempirical methods (AM1, [38]) were used. Calculations have been done at an UHF level due to the open shell character of the reactive radicals 11, the radical leavings groups (methyl, methoxymethyl and allyl) and the peroxyradicals 12, and the fact that we are dealing with bond-breaking in the reaction paths. Different initial conditions have been considered, defined by the distances between reactants (11-O2, ˚ ) and products (1a – R, 2.6– 3.2 A ˚ ), respect2.6 –3.2 A ively. The nature of the stationary points has been confirmed by means of second derivative calculations. For the reaction of radicals 11a– c with oxygen, the formation of a-hydroperoxide radicals and b-hydroperoxide radicals 12a – c has been comparatively evaluated, modeling the oxygen approach through the a- and b-face, respectively. b-Elimination

reactions of radicals 11a – c, which produces atetralone 1a together with methyl, methoxymethyl and allyl radicals, respectively, were also studied. As the reactions are known to occur in the surface of the pure compounds (oil), no solvent has been modeled. The compounds that are involved in this theoretical research have been previously prepared in our laboratory and their decomposition behavior is already known [24]. Our concern is centered in their decomposition mechanism, which has not been discern among the different possibilities. This computational research is aimed to compare the possible decomposition paths, analyzing the influence of the leaving radical in the b-elimination reaction. Activation energies and reaction energies are compared for each case. Whereas the first refers to the difference between the calculated energies of the transition structure and the reactant molecules, the second considers the calculated energies of products and reactants. The analysis has been mainly centered in the activation energies because the products, in their radical nature, do not model the actual final step of the reaction. Dehydration in the O2 addition, or recombination with a molecule of the media in the belimination reaction, should be included in order to evaluate the overall energy change. The reactant, also a radical, is stabilized by delocalization of the unpaired electron. Semiempirical procedures partially correct for electron correlation through the inclusion of

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experimental data as external parameters. This is an important consideration when activation energies are calculated. The values can be, however, overestimated, but they allow their evaluation in a comparative manner.

3. Results and discussion The calculated energies associated with the first reaction of the sequence shown in Scheme 4, which generates the diallylic radicals 11a –c, are shown in Table 1. The similarities of the data for the three radicals demonstrate the lack of dependence of the dissociation reaction on the nature of the axial substituent. In order to study the b-elimination decomposition reaction, we have searched for the transition states for the elimination reaction that produces a-tetralone 1a and the methyl, methoxymethyl and allyl radicals, respectively, starting from compounds 11a – c (Scheme 5). Similar transition states were found for the three reactions. Fig. 1 shows, as an example, Table 1 Heat of formation of radicals Compounds

DH (kcal/mol)

11a 11b 11c

11.98 12.01 11.98

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the calculated transition state for the reaction that, starting from 11a renders 1a as product. Table 2 shows the most relevant bond distances that describe the evolution from the initial to the final state for these reactions (cycle A), reporting their values for the most important points of the reaction coordinate. In the calculated transition states, the bond lengths are closer to those of the products for C4a – C5, C5 –C6, C6 –C7 and C7 – C8. This can be easily justified on the basis of the diallylic radical nature of the reactants, and the consequent higher stability of the a-tetralone 1a, mainly in relation to this moiety. C4a– C8a and C8 – C8a bond lengths have intermediate values between reactants and products. ˚ for the The distance between C8a and R is 2.098 A ˚ for 11b to 1a and 2.001 A ˚ reaction 11a to 1a, 2.036 A for 11c to 1a. This result can be compared with the one previously reported by Beckwith [39] for the C – C bond cleavage in the b-scission of the cyclopentyloxy radical. The author reported a transition state geometry also based on AM1-UHF calculations, where the C – C ˚ . Followundergoing cleavage has stretched to 1.92 A ing the same line, Wilsey et al. [40] have recently studied the cleavage reactions of alxoxy radicals using computational calculations of different levels of complexity. When studying the cleavage of different acyclic and cyclic systems with diverse leaving ˚ radicals, they found C – C distances close to 2.04 A for the transition states. These values, very close to ours, belongs to CASSCF calculations, with basis sets

Scheme 5.

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Fig. 1. Transition state of b-elimination of 11a.

ranging from STO-3G to 6-31G*, depending on the size of the molecule. The results of our calculations for the energies involved in the b-elimination reactions of compounds 11a –c are shown in Table 3. In spite of the structural similarity of the transition states, the associated activation energies differ, in some cases, in almost 10 kcal/mol. The comparative analysis of the energies of the leaving radicals (methyl, methoxymethyl, allyl for a, b, and c, respectively) shows that a higher stability of the latter correlates with a decrease in the AE. Beckwith [39] has also suggested a correlation between the structure of the leaving radical and the rate of the reaction, on the basis of electron spin resonance determinations. More recently, Walton [41] reported a relationship between the size of the leaving radical and the dissociation constant ðKd Þ to the b-elimination reaction of diallyl radical

of 1-carboxymethyl-1-alkyl-cyclohexa-2,4-diene that produced benzoic acid and the radical R, concluding that the rate of fragmentation increases with the thermodynamic stabilization of the released alkyl radical. A similar tendency is shown by our results (Fig. 2). At the same time, loss of radical with a low ionization potential (IP) occurs with a lower AE than formation of radicals with a higher IP, which, according to Benson [42], is consistent with a polarized TS with the negative charge centered on the leaving radical (Fig. 2). The inspection of the values of the calculated energies that are associated with ‘products’ (Table 3), in comparison with those of reactants, suggests no progress of the reaction for this the last step of the reaction path. The radicals should coordinate to other species of the media in order to attain stability. This step has not been considered in the calculations in order to avoid inferences that have not been experimentally demonstrated at the present moment. Scheme 5 shows an alternative path associated with the reaction of the radicals 11a– c with oxygen. In the same way previously described for the belimination reaction, similar transition states were found for the three compounds studied. However, two different possible mechanisms should be differentiated for these reactions, corresponding to an O2 approach facing either the a-face or the b-face of cycle A (Scheme 5). For each of these reactions, two different TS have been derived from the calculations, which are shown in Figs. 3 and 4 for the reaction of radical 11a with oxygen that renders the peroxyradical 12ab.

Table 2 ˚) Bond distance for b-elimination reactions of 11a– c (A Compounds

C4a –C5

C5 –C6

C6 –C7

C7 –C8

C8 –C8a

C4a–C8a

C8a –R

11a

Initial TS Final

1.384 1.399 1.400

1.406 1.400 1.393

1.412 1.404 1.395

1.373 1.390 1.392

1.496 1.429 1.402

1.500 1.432 1.405

1.537 2.098

11b

Initial TS Final

1.385 1.398 1.400

1.406 1.400 1.393

1.412 1.406 1.395

1.372 1.387 1.392

1.493 1.440 1.402

1.497 1.440 1.405

1.556 2.036

11c

Initial TS Final

1.384 1.396 1.400

1.406 1.402 1.393

1.402 1.406 1.395

1.373 1.386 1.392

1.495 1.441 1.402

1.499 1.444 1.405

1.547 2.001

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Table 3 b-Elimination reaction of dienones 11a–c Compound

Reactant

TS

Products

DH –

11a 11b 11c

214.26 252.59 5.80

12.41 234.68 25.49

5.41 247.36 5.60

26.67 17.91 19.69

Energies (heats of formation) of reactants, TS and products, and activation energies for the reactions DH – ; in kcal/mol.

Focusing first in the b-side approach for the R ¼ methyl case, the first calculated transition state (TS1) has an associated heat of formation of ˚ 2 9.980 kcal/mol, a C5 – O bond distance of 1.811 A ˚ and a O – O bond of 1.118 A. The second calculated transition state (TS2) has a heat of formation of 2 14.756 kcal/mol, with a C5– O bond distance of ˚ and an O – O bond distance of 1.123 A ˚ . This 1.745 A reaction seems to be non-concerted because of the existence of two transition states. A mechanism can be suggested consisting of an initial attack of the radical to the O – O double bond, which evolves through a first transition state where the carbon– oxygen bond is longer and the O2 bond partially retains its double bond character. In the second transition state the C5 –O distance has decreased, and the same has happened to the double character of the O – O bond. One imaginary frequency is calculated for each of the structures, at 871 and 939 cm21 for the first and second transition states, respectively, both corresponding to C –O stretching modes. The results found for the a-oxygen approach are similar, and two transition states can also be

Fig. 2. Correlation between heats of formation of leaving radicals (right) and ionization potentials of the substituents (left), vs. the activation energy for the elimination reaction of dienons 11a –c.

Fig. 3. First transition state for the reaction of 11a with bapproaching oxygen.

differentiated. The different TS are shown in Figs. 5 and 6 for the reaction of radical 11a with oxygen that renders the peroxyradical 12aa. The first transition state (TS1) has a heat of formation of 2 9.246 kcal/ ˚ and an O – O mol, a C5 – O bond distance of 1.813 A ˚ bond of 1.118 A. For the second transition state (TS2), the heat of formation is 2 14.063 kcal/mol, the C5 – O ˚ and the O – O distance 1.122 A ˚ . In distance 1.752 A this case one imaginary frequency has been found for each of the structures, at 929 and 1073 cm21 for the first and second transition states, respectively, corresponding both to C – O stretching modes. Therefore, on the basis of the similar results found in both cases, the same mechanism can be suggested for this a approach. The calculated activation energies of 33.439 and 31.225 kcal/mol for a and b approximation, respectively, favor, in this case, the formation of the bperoxyradical. This theoretical prediction is in agreement with our experimental findings when compound 11b is

Fig. 4. Second transition state for the reaction of 11a with bapproaching oxygen.

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Fig. 5. First transition state for the reaction of 11a with aapproaching oxygen.

considered. It is at variance, however, with our experimental results for the epoxidation of dienones, which gives a 2.3:1 (a:b) ratio, for both a and b derivatives, when it occurs on the most substituted double-bond; and renders only the a radical for c [43]. A similar ratio has been found in photooxygenation reactions, where a 1.85:1 a:b ration has been determined for allylic alcohol [44]. The topology of these molecules offers an steric hindrance to O2 bapproach, an effect whose importance depends on the characteristics of the substituent (R). For R ¼ methoxy, this effect is reflected in the relative stability of the TS, and may determine the evolution of the reaction.

Compounds

Reactives

TS1

TS2

Products

DH –

11aa-attack 11ab-attack 11ba-attack 11bb-attack 11ca-attack 11cb-attack

241.21 241.21 279.54 279.54 221.15 221.15

29.25 29.98 252.65 248.87 10.78 9.97

214.06 214.76 253.50 253.15 7.43 5.16

219.82 219.50 257.92 257.91 0.50 0.38

31.96 31.26 26.89 30.67 31.93 31.13

Table 4 shows the energies involved in the reactions of radicals 11a –c with oxygen when either a or b faces of the radicals are involved. The comparative analysis of the activation energies show the trends previously discussed. As it has been already mentioned in relation to the energy values reported in Table 3, ‘products’ should be considered as the last step calculated for the reaction, and not the final step of the experimental procedure. Re-aromatization (Scheme 4) is necessary to attain stability. The calculated activation energies are similar, with values around 30 kcal/mol in all cases. Tables 5 and 6 show the most relevant calculated bond distances as they evolve during the reactions of the radicals 11a –c with oxygen, when it approaches either the a or b face, respectively. No dependence on the radical, or in the a- or b-approaching scheme becomes evident from their analysis. The oxygen – oxygen bond is already polarized in the first TS, where a negative density Table 5 Bond distances for reactions of 11a–c with oxygen through the ˚) alpha face (A Compounds C4a –C5 C5–C6 C6– C7 C7 –C8 C –O

Fig. 6. Second transition state for the reaction of 11a with aapproaching oxygen.

O–O

11a Initial TS1 TS2 Final

1.384 1.360 1.356 1.342

1.406 1.448 1.454 1.484

1.412 1.450 1.457 1.485

1.373 1.350 1.347 1.335

1.085 1.813 1.118 1.752 1.122 1.517 1.162

11b Initial TS1 TS2 Final

1.385 1.361 1.357 1.343

1.406 1.447 1.454 1.484

1.412 1.451 1.457 1.485

1.372 1.350 1.347 1.335

1.085 1.814 1.118 1.753 1.122 1.516 1.162

11c Initial TS1 TS2 Final

1.384 1.360 1.357 1.342

1.406 1.448 1.454 1.483

1.402 1.450 1.456 1.484

1.373 1.350 1.347 1.335

1.085 1.813 1.118 1.751 1.122 1.515 1.162

G.R. Labadie et al. / Journal of Molecular Structure (Theochem) 635 (2003) 173–182 Table 6 Bond distances for reactions of 11a–c with oxygen through the beta ˚) face (A Compounds C4a–C5 C5 –C6 C6 –C7 C7–C8 C– O

O –O

11a Initial TS1 TS2 Final

1.384 1.361 1.358 1.341

1.406 1.447 1.454 1.483

1.412 1.450 1.457 1.484

1.373 1.351 1.348 1.335

1.085 1.811 1.118 1.745 1.123 1.521 1.161

11b Initial TS1 TS2 Final

1.385 1.362 1.359 1.343

1.406 1.447 1.454 1.483

1.412 1.451 1.457 1.484

1.372 1.352 1.348 1.335

1.085 1.811 1.119 1.745 1.124 1.523 1.161

11c Initial TS1 TS2 Final

1.384 1.361 1.358 1.342

1.406 1.447 1.454 1.483

1.402 1.450 1.456 1.484

1.373 1.352 1.348 1.335

1.085 1.813 1.118 1.747 1.123 1.523 1.161

charge of 2 0.14 a.u. is centered on the external O atom, and a positive 0.06 a.u. one on the internal one, which is obviously diminished through delocalization in the adjacent ring. The further increasing trend (2 0.17 a.u.) in the second TS, supports the previously proposed mechanism. It has been reported that the reaction of diallylic radicals with oxygen is reversible [45]. Our calculations give a difference between direct and inverse activation energies close to 10 kcal/mol. Considering the calculated activation energies for both decomposition pathways, we found that the b-elimination reaction is favored over either the formation of the a- or the b-peroxyradicals. Focusing on compound 11a, it is favored by 6.78 kcal/mol in the first case and by 4.59 kcal/mol in the second. b-Elimination is also favored for compound 11b. This time the energy difference is 8.980 kcal/mol when the a oxygen approach is considered, and 12.764 kcal/mol the comparison involves b approach. In the case of 11c, the calculated values are 12.241 and 11.436 kcal/mol for the a and b cases, respectively, always favoring the b-elimination. These results clearly show that the decomposition behavior of dienones 2a –c evolves though a belimination mechanism. The differences in the activation energy for the two reactions are in agreement with our experimental results, supporting the b-elimination product as the main result of decomposition.

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4. Summary and conclusions We have studied the decomposition behavior of Birch alkylation products of a-tetralones. Following the mechanism proposed by Beckwith, we found a preference for the b-elimination reaction over the reaction with oxygen for all the compounds that have been studied, in complete agreement with our experimental results. The transition states for the b-elimination reactions are similar for the different substituted structures, and also resemble those previously reported for other radical elimination reactions. The correlation between the activation energy calculated for the b-elimination reaction and the heat of formation of the leaving group, shows that the nature of the leaving group has an influence in the reaction energy profile. Two different transition states have been found for the reactions with oxygen. Their structure is again similar for the three cases under study. According to our predictions alpha/beta selectivity is dependent on the characteristics of the substituent R. This theoretical study has been capable of explaining our previous experimental results, related to the preferential decomposition of a-tetralones through a b-elimination reaction, which, according to our calculations, is energetically favored over the reaction with oxygen.

Acknowledgements We thank CONICET Argentina, Fundacio´n Antorchas and ANPCYT-Argentina for financial support. GRL thanks CONICET and FOMEC program for fellowships. GLE is a member of the scientific staff of Conicet, Argentina.

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