Diels-Alder reaction between naphthalene and N -phenylmaleimide under ambient and high pressure conditions

The rate and equilibrium constants for the Diels-Alder reactions between benzene and naphthalene as dienes and tetracyanoethylene, maleic anhydride and N -phenylmaleimide as dienophiles at 25 ° С were estimated from empirical rule . The highest yield of the adduct was predicted for the reaction of naphthalene with N -phenylmaleimide. The time of adduct formation in 50% yield exceeds 30 years. The use of gallium chloride as a catalyst affords the exo -adduct for seven days at room temperature. The rate ((2±0.5)•10 -6 L mol -1 s -1 ) and equilibrium constants (5±2 L mol -1 ) of this reaction were determined. Under high pressure conditions (8 kbar) reaction occurs with formation of both stereo isomers at 100 ° С during 80 hours


Introduction
Many dienes, including substituted benzenes and naphthalenes, are known 1 to form molecular complexes of the π,π-type due to the interaction of the highest occupied π-orbital of a dienedonor with the lowest unoccupied π-orbital of a dienophile-acceptor.3][4] Benzene does not react with dienophiles in the Diels-Alder reaction because of the high conjugation energy.Only UV irradiation in the presence of maleic anhydride results in the unstable adduct of [2+2]-addition, which is stabilized by the subsequent [4+2]-addition of the second dienophile molecule.4   The Diels-Alder reaction with naphthalene does not either occur at room temperature and standard pressure.The adduct was obtained 5 in ~1% yield for 24 h with a great maleic anhydride excess at 100 °C.More drastic temperature conditions are unfavorable for the yield of the target adduct, because the process is reversible.Only elevated temperature (100 °C) and pressure (10-12 kbar) make it possible to achieve 90% yield of the product (adduct of naphthalene with maleic anhydride) 6,7 The purpose of this work was to estimate the reactivity of simple arenes as dienes in the Diels-Alder reaction, and to use different ways of activation of reactants in this reaction.

Results and Discussion
It has previously been shown 2,3 that the quantitative description of the rate of the Diels-Alder reaction between different reagents containing C=C bonds at 25 °С requires data on the ionization potential of the diene (IP D ), electron affinity energy of the dienophile (E A ), interatomic C(1)-C(4) distance in the diene (R 1-4 ), and balance of energies of the bond cleavage and formation (reaction enthalpy, ∆H react ), equation 1.

Diene
Dienophile ∆H react logk 2 τ 0.5 LogK eq α eq 1 4 +17±4 -10.8±0.9 9 .10 9 -10.6±1.0 1 .10 -(8±1) 1 5 +1±4 -13.9±0.9 1 .10 13 -7.9±1.0 6 .10 -(6±0. Since the absolute value of activation enthalpy is much higher than the reaction enthalpy, an increase in the reaction rate constant (k 2 ) with temperature is sharper than a decrease in the equilibrium constant.However, the logK eq value for benzene is very low even in the reaction with N-phenylmaleimide (-5.6±1.0).Thus, for the Diels-Alder reactions involving benzene and N-phenylmaleimide in the presence of aluminum or gallium chlorides at 10-12 kbar and 25 °C, one can expect an increase in k 2 from 2•10 -14 to 2•10 -6 L mol -1 s -1 and in K eq from 2.5•10 -6 to 2.5•10 -3 L mol -1 with the maximum conversion not higher than 1-2% for 20-30 h.When the pressure is discharged, the K eq value again becomes equal to 2.5•10 -6 L mol -1 but the rate of adduct decomposition is rather low (7•10 - 9 s -1 ), which may allow one to detect its formation.It is known 2,3 that the introduction of donating groups, for example, methyl groups, into positions 1 and 4 of diene and replacement of ethylenic dienophiles by acetylenic dienophiles exert a favorable effect on the kinetic and thermodynamic parameters of the Diels-Alder reaction.This was experimentally confirmed for polymethylnaphthalenes and even polymethylbenzenes. 4It is clear that several products of the reactions between benzene and dienophiles with the double bond can much more easily be obtained by the interaction of the corresponding cyclohexadienes with acetylenic dienophiles.However, the affinity of the adducts to decomposition with benzene formation accompanies both these processes.For naphthalene the highest rate is expected in the reaction with tetracyanoethylene (4) but the conversion under standard conditions is so low that adduct formation cannot be detected (see Table 1).The experimental data at elevated temperatures indicate a much higher yield of the products in the reaction of naphthalene with Nphenylmaleimide 11 than that with maleic anhydride, 5 which agrees with the results of calculations (see Table 1).However, all these reactions do not occur at standard temperature because of very low rates.In this work, we studied the reaction of naphthalene (2) with Nphenylmaleimide (6) at room temperature.According to the presented calculations (see Table 1), the half-life (more than 30 years) is too long for experiment. The electron affinity energy of Nphenylmaleimide (6) for its transformation into the n,v-complex (7) with gallium chloride (Scheme 1) increases from 0.89 to 1.93 eV. 12 The rate of the Diels-Alder reaction between substituted anthracenes and activated dienophile (7) increases by 5•10 4 times. 13It follows from these data that the rate constant of the reaction of naphthalene with N-phenylmaleimide can increase from 1.2•10 -10 to 6•10 -6 L mol -1 s -1 .The endo-(m.p. 161 °C, with decomp.)and exoadducts (m.p. 172 °C, with decomp.)have recently 7 been isolated in the reaction of naphthalene with maleic anhydride at 100 °C and 12 kbar in CHCl 3 .The chemical shifts in the 1 H and 13 C NMR spectra differ significantly for the endo-and exo-adducts. 7However, the NMR spectra of the adduct obtained in the catalyzed reaction 2 + 7 (Scheme 2) indicate the presence of the only one isomer (8 or 9).Exo-isomer 8 with m.p. 206 °C was obtained 11 with yield 10% in the reaction 2 + 6 in boiling xylene (140 °C), whereas the adduct obtained in our work had m.p. 206-208 °C (with decomp.)Earlier 14 it was by mistake described as endo-isomer on the basis of the incorrect melting point.X-Ray crystal structure investigation of the reaction product shows that this molecule has exo-configuration, corresponding to the isomer 8. Molecule 8 has step-like conformation; five-membered cycle is planar, and nearly parallel to the condensed benzene ring (dihedral angle between these planes is 6.1(2) o ).Central bicycle has local symmetry C 3 , 6-membered cycles in bicyclic system have boat conformation.Phenyl substituent at the N14 atom is non coplanar with the plane of 5-membered heterocycle, dihedral angle between these planes is 52.9(2) o .Tetracyclic system of molecule has the local symmetry C s , geometric parameters of corresponding fragments are equivalent in the range of experimental errors.In general main geometry parameters of the molecule (bond lengths and bond angles) are normal.Crystal packing of this compound may be considered as lamellar, consists of centrosymmetric dimers of molecules, dimers formation by the π-π interactions between 5membered heterocycles and condensed benzene ring (Fig. 2.).Dihedral angles between planes of these rings are 6.1 o , distance between centers of rings is 3.94 Å, interplanar distance is 3.44 Å.The experimental value of the rate constant of the catalyzed reaction at 22±2 °C is equal to (2.0±0.5)•10 - L mol -1 s -1 , which corresponds satisfactorily to the calculated value (6•10 -6 L mol -1 s -1 ).The change in the absorption of the π,π-complex for 24 days (from 0.954 to 0.320) made it possible to estimate the equilibrium constant of the catalyzed reaction (5±2 L mol -1 ).Under high pressure conditions (8 kbar, 100 °С, 80 hours) reaction occurs with formation of both stereo isomers.According to NMR spectra, ratio of endo:exo isomers in reaction mixture was 30:70.

Experimental Section
General Procedures. 1 H (300 MHz) and 13 C (75 MHz) NMR spectra were recorded on a Varian Unity-300 instrument in a CDCl 3 solution using the signal of the proton of CHCl 3 , whose traces were in the solution, as an internal standard.The δ values were calculated relatively to Me 4 Si.

Measurement of the reaction rate.
A benzene solution of reactants 6 (0.0458 mol L -1 ), GaCl 3 (0.040 mol L -1 ), and 2 (0.416 mol L -1 ) was prepared.The solution was placed in a quartz cell (l = 1 cm), whose upper part was welded (through transient alloys) to a molybdenum glass tube.Dry argon was passed through the cell, and the tube was sealed.At these concentrations and temperature of 22±2 °C, crystals of the adduct are not precipitated even for 24 days.At an excessive naphthalene concentration (C 2 ), the current concentration of the π,π-complex (C c ) between reactants 2 and 7 is proportional to the current concentration of activated dienophile 7 where K c is the equilibrium constant of the π,π-complex between reactants 2 and 7(equation 2).The change in the absorption of the π,π-complex (A c ) for 24 days (from 0.954 to 0.320) made it possible to determine the rate constant ((2.0±0.5)•10 - L mol -1 s -1 ) from the initial region of the lnA c -t, and then the equilibrium constant of the catalyzed reaction (5±2 L mol -1 ) was calculated.
High pressure experiment was carried out in 2 ml Teflon ampoule in a piston cylinder type barostat 15 .The solution of 1.76 M N-phenylmaleimide and 1.93 M naphthalene in chloroform was placed in Teflon ampoule and kept at 8 kbar at 100 °С during 80 hours.Under high pressure conditions (8 kbar) reaction occurs with formation of the both stereo isomers at 100 °С during 80 hours.X-Ray structure determination.The X-Ray diffraction data for the crystal of ( 8 The stability of crystal and experimental conditions was checked every 2 hours using three control reflections, while the orientation was monitored every 200 reflections by centering two standards.No significant decay was observed.Corrections for Lorentz and polarization effects were applied.Absorption correction was not applied.The structures were solved by direct methods using SIR 16 and MolEN 17 packages.For the crystals all non-hydrogen atoms were refined anisotropically.H-atoms, located in ∆F maps, were refined isotropically at the last cycles.Analysis of intermolecular contacts and drawing of molecular and crystal structures were performed by program PLATON 18 .Compound characterization.Commercial N-phenylmaleimide (6) (Aldrich), benzene (reagent grade), and naphthalene (2) (reagent grade) were used.Chloroform (reagent grade) was purified by known methods 19 .exo-N-Phenyl 2,3-Benzobicyclo[2.2.2]octa-2,5-diene-7,8-dicarboximide (8).Gallium chloride (1.01 g, 0.1 mol*L -1 ) was dissolved in anhydrous benzene (reagent grade) containing Nphenylmaleimide (1.15 g, 0.115 mol*L -1 ), and then naphthalene (12.8 g, 1.76 mol*L -1 ) was added.Argon was passed through the resulting solution, and the reactor was tightly closed and left for 7 days at ~20 °C.Already two days after, a precipitate of the adduct complex with gallium chloride (8•GaCl 3 ) began to form.After 7 days, the precipitate was filtered off, powdered, washed with benzene and hexane, and dissolved in CH 2 Cl 2 .Gallium chloride was removed from adduct 8•GaCl 3 with aqueous ammonia, and the solution was washed with water and dried with MgSO 4 .The volume of the solution was brought to 20 mL, and the adduct was precipitated with hexane and dried in vacuo.The yield of pure adduct 8 was 0.6 g (30% calculated per taken imide 6), m.p. 206-208°С (with decomp.From the reaction mixture after high pressure synthesis, using the column chromatography (eluent petroleum ether:ethylacetate 1:6) exo-N-phenyl 2,3-Benzobicyclo[2.2.2]octa-2,5-diene-7,8-dicarboximide was obtained, with melting point and the other properties the same as described for the catalytic reaction product.Similarly to the adduct of naphthalene with maleic anhydride 7 , exo-isomer was eluted first, and endo-isomer was eluted the last and very hardly.
Endo-isomer was not separated as pure compound.

Figure 2 .
Figure 2. PLUTO drawing of the crystal structure of the compound 8.View along XO axe.