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Journal of Molecular Catalysis A: Chemical 371 (2013) 63– 69

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Catalysis A: Chemical

j our na l ho me p age: www.elsev ier .com/ locate /molcata

nusual reactivities of acridine derivatives in catalytic hydrogenation. combined experimental and theoretical study

ierre Mignona, Martin Tianob, Philippe Belmontb,c, Alain Favre-Réguillond,enry Chermettea, Fabienne Fachee,∗

Université de Lyon – Université Lyon1, Laboratoire des Sciences Analytiques, UMR5180, Equipe Chimiométrie et Modélisation, 43 boulevard du 11 novembre 1918, Villeurbanne-69622, FranceUniversité de Lyon – Université Lyon1, Laboratoire de Synthèse et Méthodologie Organiques (LSMO), ICBMS, Institut de Chimie et Biochimie Moléculaires et Supramoléculaires, 43oulevard du 11 novembre 1918, Villeurbanne F-69622, FranceInstitut Curie et CNRS, UMR 176, Equipe Chimie Organométallique, Hétérocycles et Cibles Biologiques (COHCB), Institut Curie, 26 rue d’Ulm, 75248 Paris cedex 05, FranceLaboratoire des Transformations Chimiques et Pharmaceutiques (LTCP), Conservatoire National des Arts et Métiers, 2 rue Conté, 75003 Paris, FranceUniversité de Lyon – Université Lyon1, UMR 5246 CNRS, Institut de Chimie et Biochimie Moléculaires et Supramoléculaires, Bât. Raulin, 43 boulevard du 11 novembre 1918,illeurbanne F-69622, France

r t i c l e i n f o

rticle history:eceived 23 October 2012eceived in revised form 8 January 2013ccepted 19 January 2013vailable online 9 February 2013

a b s t r a c t

Hydrogenation of acridine derivatives over Rh/Al2O3, has been studied. Strong influence of thepyrrolidino-substituent on the reduction pathway and reductions products was found. When, the reac-tion was performed on the unsubstituted acridine nucleus full conversion was obtained in 8 h. Under thesame conditions, when a pyrrolidine substituent was settled on the central ring of acridine (9-position),a pure product was obtained with the two lateral carbocycles reduced whereas the central heterocyclic

eywords:romatic hydrogenationcridine derivativesydrogenation pathwayFT calculations

ring was not reduced. When the pyrrolidine substituent was at the 1-position, pure partially reduced cen-tral heterocyclic ring was obtained, but the compound was rapidly re-oxidized by air. In order to clarifysuch substituent effect, theoretical calculations were performed. Considering the energies and thermo-dynamic values of each intermediate and product as well as the interaction with the catalyst surface, theselectivity and diversity in the reduced product formation were partially explained and different reactionpathways were drawn according to the substitution pattern on the acridine scaffold.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Acridine derivatives are compounds of interest due to theirroad biological activities against bacteria, parasites or tumours1–7]. Their properties depend mainly on the nature and the posi-ion of the substituents on the acridine nucleus [8].

In the course of our studies on hydrogenation of aromatic com-ounds [9–11], we found of interest to study the reduction ofcridine derivatives with the aim of controlling the chemioselectiv-ty of the reaction which should lead to new families of potentiallyiologically active molecules. In particular, hydrogenation of-amino acridine gave access to tacrine (1,2,3,4-tetrahydro-9-mino-acridine), a reversible acetylcholine inhibitor, which has

een the first drug approved for the treatment of Alzheimer dis-ase [12] in 1993/1994 and new derivatives are still sought after,lthough tacrine is now out of the market [13–19].

∗ Corresponding author.E-mail address: fache@univ-lyon1.fr (F. Fache).

381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.molcata.2013.01.015

Furthermore, the hydrodenitrogenation reaction (HDN), wherenitrogen is removed from the heteroaromatic nitrogen ring of com-pounds found in fossil fuels, is one of the most important and highlystudied reaction [20–22]. HDN is a complex process that involvesa variety a chemical sequences and among then, reduction of thearomatic carbocycles [22]. Numerous studies have been publishedon the acridine as a model of polynuclear heteroaromatic fuel prod-ucts. The reduction pathway depends on the nature of the metal,on the support and on reaction conditions. Two different compet-ing pathways for the reduction of acridine have been suggested,differing by the fact that the heterocyclic ring of acridine is beingreduced first, before the lateral carbocycles, or the inverse [23–28].In homogeneous catalysis, the reduction catalyzed by rutheniumor boron complexes [29,30] also showed that the pathway can bedifferent depending on the catalysts. The reduction by H2 overmetal/oxide surfaces has been the subject of several theoretical

works, studying the selectivity brought by the metal nature and/orthe oxide support [31–36]. All the calculations indicate that hydro-genation proceeds through dissociated hydrogen (i.e. atomic H·)which attacks the chemisorbed substrate over the surface allowing

6 ar Cat

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2

2

R

2

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2

(sgarop

4 P. Mignon et al. / Journal of Molecul

favourable interaction with the atomic hydrogen bound to theetalIn this article, we present our work on the catalytic hydrogena-

ion of pyrrolidine-substituted acridine derivatives, synthesizedccording to a method published by some of us where pyrroli-ine serves as a trigger for a cyclo-isomerization reaction leadingo pyrrolidino-aryl moieties [37,38]. However the reactivity of theubstrates over Rh/Al2O3 catalyst was found to be strongly depend-nt of the substituant position and could not be easily explained.hus, the hydrogenation of the unsubstituted acridine was carriedut, as a model compound, under the same experimental conditionso study the reduction pathway with Rh/Al2O3 catalyst. Experimen-al data combined with a theoretical approach based on gas phasealculation of the energies and thermodynamic values of the inter-ediate products allow us to propose a reaction pathway for the

eduction of the acridine derivatives. Thus, most of the unusualeactivity of pyrrolidine-substituted acridines in catalytic hydro-enation over Rh/Al2O3 catalyst could be explained.

. Experimental part

.1. Chemicals

All commercially available reagents were used as received.h/Al2O3 (5%) and acridine were purchased from Acros.

.2. Instrumentation

Hydrogenation experiments were performed in a stainless steelutoclave. NMR spectra were recorded with a Bruker AMX 300pectrometer. Mass data were also acquired on a Shimazu QP2010CMS apparatus, with injection on a SLB-5ms column lined with aass EI detection system.

.3. General procedure for the hydrogenation under H2 pressure

The acridine derivative 1, 3 or 5 (0.046 mmol) and Rh/Al2O32 mg, 0.0008 mmol) in 0.4 mL of MeOH were put in a stainlessteel autoclave. The solution was degased 3 times under hydro-en. The reaction mixture was then stirred at 40 bar H2 and heated

t 100 ◦C, for requested time (the final pressure was 45 bar). Theeaction mixture was filtrated through celite, the solvent evap-rated and the crude product analyzed by NMR without furtherurification.

N

N

N

NH2 40 ba r

Rh/Al2O3

MeOH, 100 °C, 8h

con v. 7 selecti v

H2 40 ba r

Rh/Al2O3

MeOH, 100 °C, 8h1

3

Scheme 1. Reduction outcome of 9- or 1-py

alysis A: Chemical 371 (2013) 63– 69

2.4. Computational details

Density Functional Theory calculations were performed withthe ADF [39] code using the PBE functional [40] and Triple Zeta pluspolarization basis sets. For all given compounds, various stereoiso-mers were calculated. In each case, frequencies were computed andwe always considered the stationary points with the lowest energy.The corresponding electronic energy and zero point energy wereused to compute the relative changes in electronic energy, enthalpyand free energy (�E, �H and �G) against the acridine and H2isolated molecules. The thermodynamic functions, including freeenergies, were calculated at 298.15 K and 1 atm.

3. Results and discussion

3.1. Reduction of substituted acridine derivatives

Hydrogenation of acridine derivatives over Rh/Al2O3, has beenstudied. For the reduction of compound 1, only compound 2 wasisolated (Scheme 1). However in the case of product 3, only com-pound 4 was obtained and underwent a fast re-aromatisation whenleft in the NMR tube (CDCl3). The re-aromatisation of 4 was slowerin C6D6.

These observed selectivities for compounds 1 and 3 are quitesurprising and the role of the pyrrolidine substituent uncertain. Toclarify the unexpected reactivity of acridine derivatives 1 and 3,the reduction of the unsubstituted acridine 5 was reinvestigatedexperimentally and theoritically.

3.2. Reduction of unsubstituted acridine 5

Standard conditions were applied to acridine 5 (Rh/Al2O3 with40 bar H2, 100 ◦C, methanol as solvent). Product distribution as thefunction of time is given in Fig. 1. After 30 min, the reaction mix-ture was analyzed by both 1H NMR and mass spectrometry. Theconversion was already high (50%) and the major product was 6,with only the central ring being reduced (compound 7 being theminor product). After 1 h, the conversion was slightly higher (60%),but the distribution of the products was different. The major reduc-tion product was this time 8 with the external rings reduced. After8 h of reaction, the conversion was total. We obtained a mixture ofthe fully reduced product 9 (80%) and the partially reduced prod-

uct 8. From these data, it appears that products 6 and 7 are rapidlyformed and that after 1 h they are converted into 8. An accumu-lation of 8 before the formation of 9 can also be noticed. Finallythese results show that the first reduction occurs on the central ring.

N

N

N

N

H

N

N

r.t .

p.atm

100%

0%it y 100 %

2

34

t1/2= 4h

rrolidino-acridine derivatives (1 or 3).

P. Mignon et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 63– 69 65

FR

Tpm

3

psigahbt

Table 1Energies and thermodynamic values of reduction products of compound 5(kcal/mol).

From compound 5 �E �H �G

6 −23.1 −16.7 −8.47 −44.2 −31.8 −14.9

Fh

ig. 1. Product distribution as the function of time for the reduction of acridine byh/Al2O3 with H2.

his confirms one of the two suggested mechanisms that were pro-osed in earlier works [23,27] for catalysis on Pd, Pt or Rh supportedaterials.

.3. Theoretical calculations

In order to understand the reduction mechanism of com-ounds 1 and 3 we have performed DFT calculations from theimplest model, the unsubstituted acridine 5 (Fig. 2). For eachntermediate, different isomers can be formed through the hydro-enation/adsorption process. For the observed compounds 5–9,

ll possible structures were computed. In the present work weave considered that the dominant role of the surface, as discussedelow, consists in a geometrical constraint towards the hydrogena-ion mechanism by hydrogenation on the same side of the aromatic

ig. 2. Gas-phase reaction profiles of the reduction of unsubstituted acridine 5. (Gibbs

ydrogen atoms above/below the plane of the aromatic ring.

8 −84.7 −60.0 −25.99 −122.8 −79.3 −18.2

ring. The reduction by H2 over metal/oxide surfaces has beenthe subject of several theoretical works, studying the selectivitybrought by the metal nature and/or the oxide support [31–36]. Allthe calculations indicate that hydrogenation proceeds through dis-sociated hydrogen (i.e. atomic H·) which attacks the chemisorbedsubstrate over the surface allowing a favourable interaction withthe atomic hydrogen bound to the metal. As a result for compound9, hydrogen atoms were added in the cis position on the centralring. The corresponding electronic energies and thermodynamicvalues are given in Table 1 and free energy changes are shown inFig. 2.

From 5 to 9, the hydrogenation of aromatic molecules leadsto more and more stable compounds (see �E and �H values,Table 1). From �G values one can see that the formation of eachintermediate is favourable, except for the final product 9. Indeedfrom 5 to 8, the change in free energy is negative in the range of−6 to −10 kcal/mol for each step. From 8, the positive change infree energy (+7.7 kcal/mol) shows that the formation of 9 is notfavoured, although it is observed experimentally. According to theexperimental conditions, H2 is present at high pressure (40 bar)which can influence drastically the equilibrium towards 9. More-over the positive change in free energy suggests that the reduction

from 8 to 9 is less easy than the previous ones. This may explainwhy the reaction is delayed and why accumulation of 8 (54% after1 h, see Fig. 1) is observed before the formation of 9.

free energies, in kcal/mol) and distances, in Å, of axial hydrogen and pyrrolidine

66 P. Mignon et al. / Journal of Molecular Cat

Table 2Energies and thermodynamic values of reduction products of compound 1(kcal/mol).

From compound 1 �E �H �G

10 −23.5 −16.9 −8.311 −43.5 −31.5 −13.0

3c

pb

cnbdeT

s

Fp

2 −84.0 −59.1 −24.812 −111.5 −67.9 −7.3

.4. Reduction of 1-pyrrolidino substituted acridine 1: theoreticalalculation

The same reduction pathway was considered for the substitutedroduct 1 even if only intermediates 11 and 2 have been detectedy NMR (Scheme 2).

The reduction reactions are still favourable and lead to stableompound 11, up to molecule 2. From 2, the formation of 12 isot favourable at all, the difference between the free energy valueseing of +17.5 kcal/mol. This value is 10 kcal/mol higher than theifference between 8 and 9. Moreover, the electronic energy and

nthalpy values of 12 are about 10 kcal/mol less than those of 9 (seeables 1 and 2).

Furthermore, steric interactions between the pyrrolidine sub-tituent and the external cycles could prevent the adsorption of

N

N

N

N

H

N

N

1 10

11

Scheme 2. Reduction

ig. 3. Gas-phase reaction profiles of the reduction of 9-pyrrolidino substituted acridinyrrolidine hydrogen atoms above/below the plane of the aromatic ring.

alysis A: Chemical 371 (2013) 63– 69

the intermediate 2 on the catalyst surface and thus prevent thehydrogenation of the central ring to give product 12 (Fig. 3).

3.5. Reduction of substituted acridine derivative 3

For compound 3 (Scheme 3) two routes were computed for thereduction of 4, the only detected intermediate.

We distinguish the formation of 13 and 14, depending onwhether the reduction occurs on the substituted aromatic cycleor not. Energies and thermodynamic values show that after theformation of compound 4 the reduction would rather lead to13 than to 14. Thus, the reduction is easier when it occurs onan unsubstituted aromatic cycle. From 13, the formation of 15would be favoured, if we considered the ��G (6.5 kcal/mol),but the difference is rather low compared to the ��G for 7–8(11 kcal/mol) and 11–2 (11.8 kcal/mol) (see Tables). Nevertheless,these data could not explain why products 15 or 16 were notformed.

In previous theoretical studies on the hydrogenation of aro-matic molecules the loss of aromaticity or resonance energy,

measured as the change in enthalpy upon addition of H2 ondouble bonds, was found to be a measure of the reactivity of thesubstrate [41,42]. In the present work the loss in aromaticity,going from one intermediate to the following one, is quite similar

N

N

N

N

H2 12

of compound 1.

e 1. (Gibbs free energies, in kcal/mol) and distances, in Å, of axial hydrogen and

P. Mignon et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 63– 69 67

N

N N

N

H

N

N

N

N

N

N

N

H3 4 13 15

14

16

ction

ff8(mismaatDtacmbbttsfi

Fp

N

Scheme 3. Redu

or the unsubstituted acridine or the substituted ones, exceptor reduction of 2–12. In that case, the resonance energy is only.8 kcal/mol (�H, Table 2), as compared to the other moleculesabout 20 kcal/mol; 8–9 and 15–16). Thus molecule 2 is less aro-

atic and less reactive, which can explain why the reduction of 2nto 12 is not observed (Scheme 2). Numerous previous theoreticaltudies on the hydrogenation of unsaturated cyclic and non-cyclicolecules on metal surfaces have been investigated [31–36]. In

ll cases on Pt, Pd, Ru–Zn and Ni surfaces the cyclic molecule isdsorbed on the surface before the hydrogen is being transferredo the aromatic carbon. Although calculations are performed at theFT/GGA level, without a specific exchange-correlation functional

o better describe dispersion forces, it is well established that theromatic molecules chemisorb the surface metal atoms via severalarbon atoms. According to the reported mechanisms, the aromaticolecule is more or less coplanar to the surface and carbon atoms

ind to surface metal atoms before the dissociated hydrogen iseing transferred to the aromatic carbon via cis addition throughhe same side of the aromatic ring. The effect of the surface on

he hydrogenation mechanism was respected in our moleculartructure searches. Thus, the unfavourable free energy changeor the hydrogenation from 2 to 12 could be explained by stericnteraction between pyrrolidine and the external cycles (Fig. 3). It

ig. 4. Gas-phase reaction profiles of the reduction of 1-pyrrolidino substituted acridinyrrolidine hydrogen atoms above/below the plane of the aromatic ring.

of compound 3.

should be noticed that the free energy change is also the highestobserved in our examples (17.5 kcal/mol, Table 2)

On the other hand, the selectivity observed for compound 2 can-not be simply explained by energy calculations. Thus, we have tofocus on the steric hindrance induced by the pyrrolidine and howthis substituent could modify the chemisorption of the substrates(Fig. 3).

During the reduction of the unsubtituted acridine 5, hydrogenatoms lie by about 1.08–1.18 A above the aromatic plane (Fig. 2).On the contrary, for substituted acridines, the pyrrolidine bringsan important steric hindrance, which prohibits the binding of thearomatic carbon atoms to the surface and thus the hydrogenationof the ring. This appears to be clear for compound 1. The pyrroli-dine prevents the hydrogenation of the central ring and the externalrings of 1 can chemisorb on the surface, leading to 11 and 2 (Fig. 3).Steric hindrance rather than energy explains that 2 is not reducedinto 12. The same behaviour may be observed for compound 3.The opposite external ring can be reduced but not the ring carry-ing the pyrrolidine. Thus in the case of compound 3 the reaction

was stopped to 13 the first intermediate being compound 4 withthe reduced central ring, as expected from Gibbs energy calcula-tions (Table 3; Fig. 4). From these remarks one may re-examine thereduction mechanism for compounds 1 and 3.

e 3. (Gibbs free energies, in kcal/mol) and distances, in Å, of axial hydrogen and

68 P. Mignon et al. / Journal of Molecular Cat

Table 3Energies and thermodynamic values of reduction products of compound 3(kcal/mol).

From compound 3 �E �H �G

4 −21.7 −15.3 −7.314 −37.2 −25.1 −8.513 −42.7 −30.4 −13.715 −78.3 −54.0 −20.2

bpmt

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16 −119.6 −76.4 −14.4

For compound 1, the reduction of the central ring is excludedecause of the presence of pyrrolidine substituent. As such, com-ound 10 should not be formed. In that case the hydrogenationechanism occurs directly and only on the external rings, following

he pathway described in Scheme 2.For compound 3, only compound 4 is observed experimentally

hile 13 is not. However 4 is observed to be rapidly re-oxidizedo 3 at normal conditions. This shows that 4 is not stable undertmospheric conditions although it is showed theoretically to betable and its formation favourable. For 13, as compared to 4 it isn external ring which is hydrogenated. Although it is more stablend thermodynamically favourable, one might imagine that 13 islso rapidly rearranged to 4 under atmospheric conditions and thenack to 3. However these hypotheses cannot be answered rigor-usly from the present results. Additional work would be necessaryn order to assess whether short-living 13 is formed or not, and inhat case if it is very rapidly re-oxidized to 3.

. Conclusion

This study combining experimental and theoretical techniquesimed to understand the hydrogenation reduction pathway of acri-ine derivatives over an Rh/Al2O3 catalyst. In our conditions, thensubstituted acridine is totally reduced. As a function of time,wo intermediates have been identified. Reduction pathway cane explained by energy calculations and steric hindrance. In com-arison, for substituted acridines, the reduction is stopped beforehe hydrogenation of the aromatic cycle carrying the pyrrolidineubstituent. For the acridine molecule substituted at the centraling, the energetics indicate that the last hydrogenation of theentral ring is less favourable than for the free acridine, show-ng that the reaction is much more difficult. This is in line withhe experimental observations. For the acridine substituted at aateral ring the energetics, however, do not explain the experimen-al observations. From previous theoretical studies it is assumedhat the hydrogenation takes place when the aromatic ring ishemisorbed on the metal surface. In that sense we expect arobable hindrance due to the pyrrolidine substituent. For thecridine substituted at the central ring, it clearly appears thathe pyrrolidine prevents the aromatic carbon of the central ringo bind the surface. This confirms the experimental observationsnd shows that the reduction pathway is somehow different fromhat of the free acridine, lateral rings being hydrogenated whilehe central ring is not. For the acridine substituted at the lateraling, the hindrance due to pyrrolidine substituent only preventshe hydrogenation of this ring. This however does not explainhe experimental observation. One could argue that the moleculeormed is rapidly re-oxydized at standard conditions, as it isbserved for molecule 4, but further studies are needed to conclude

n that point.

Finally, the combination of an experimental and theoreticalpproach of the present study allowed us to understand the reduc-ion mechanism of acridine 5 and to show that the mechanism isifferent for derivative 1.

[[[

alysis A: Chemical 371 (2013) 63– 69

Acknowledgements

H.C. and P.M. thank the Centre Informatique National del’Enseignement Supérieur (CINES) and Grand Equipement Nationalde Calcul Intensif (GENCI) for computer resources allocation,project: cpt2130.

M.T. thanks the Ministère Franc ais de l’Enseignement Supérieuret de la Recherche and P.B. thanks, the “Association pour laRecherche sur le Cancer” and the “Cluster de Recherche Chimie dela Région Rhône-Alpes.”

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.molcata.2013.01.015.

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