Synthesis, dynamic 1 H NMR and theoretical study of aryl-nitrogen single bond rotational energy barriers in highly functionalized 4 H -chromenes

The reactive intermediate was generated by reaction between 2,6-dimethylphenyl isocyanide and dialkyl acetylenedicarboxylates to react with β -diketones such as 1,3-cyclohexanedione or 5,5-dimethyl-1,3-cyclohexanedione to produce the dialkyl 2-(2,6-dimethylphenylamino)-5-oxo- 5,6,7,8-tetrahydro-4 H -chromene-3,4-dicarboxylate in fairly high yields 2a - c . The 1 H NMR spectra of these compounds exhibited dynamic effects that are attributed to restricted rotation around the aryl-nitrogen single bond. The calculated rotational energy barrier ( ∆ G ≠ ) for their interconversion of these compounds equals (57.2, 54.0 and 55.7)±2 kJ.mol -1 , respectively. In addition, theoretical study on the basis of rotation around the aryl-nitrogen single bond was investigated using ab intio method at HF/6-31G level theory. The theoretical rotational energy barrier for these interconversion were in a good agreement with the experimental rotational energy emerged from dynamic 1 H NMR


Introduction
Multicomponent reactions (MCRs), defined as one-pot reactions in which at least three functional groups join through covalent bonds, have been steadily gaining importance in synthetic organic chemistry. 1,2 romenes as a result of MCRs, have been the subject of the considerable chemical interest in the past decades because of their usefulness as biologically active agents. 3,4 ubstituted 4H-chromens are a new class of anti-cancer compounds. 5Many studies have been reported on the synthesis of the chromene ring system. 6,7][10][11][12][13] Herein, we wish to report synthesis, dynamic 1 H NMR and also theoretical study of highly functionalized 4H-cromenes as a complementary investigation of rotation around the arylnitrogen single bond in compounds 2a-c.
The structures of 2a-c were deduced from their elemental analysis, IR, 1 H NMR and 13 C NMR spectra.The mass spectra of these compounds displayed molecular ion peaks at appropriate m/z values, any initial fragmentation involve the loss of the ester moieties.The 1 H and 13 C NMR data for compounds 2a-c are given in the experimental part.The 1 H NMR spectrum of 2a exhibits five singlets arising from Ar-Me 2 group (δ= 2.27), methoxy protons (δ= 3.67 and 3.72), NH (δ= 9.79) and methine protons (δ= 4.56) and multiplet for diastereotopic protons of 3 methylene (δ= 2.29-2.52)and Ar-H (δ= 7.11).The 13 C NMR spectrum of 2a showed eighteen distinct resonances in agreement with 4H-chromene structure.The presence of one broad signal for the Ar-Me 2 group in the both 1 H and 13 C NMR spectra of 2 is relevant to dynamic effect, as a result of restricted rotation around the N-aryl bond in 25 ˚C.
Although we have not yet established the mechanism of the reaction between 2,6dimethylphenyl isocyanide and dialkyl acetylenedicarboxylate in the presence of 1,3cyclohexanedione or 5,5-dimethyl-1,3-cyclohexanedione, a possible explanation on the basis of the established chemistry 1,13 of isocyanides has been proposed in Figure 2. It is reasonable to assume that 2 result from an initial addition of the aryl isocyanide to the acetylenic ester and subsequent protonation of the 1:1 adduct by 1,3-cyclohexanedione.Then, the positively charged ion might be attacked by the enolate anion of the 1,3-dicarbonyl compounds in a Michael addition process to afford the keteneimine 3.Under the reaction condition, 3 could be isomerized for generation of fused heterocyclic compound 2 (see Figure 2).The 1 H NMR spectrum of 2 showed one single resonance arising from the Ar-Me 2 protons in CDCl 3 at 15 ºC.It is appreciably broadened with respect to the two corresponding signals measured at ambient temperature, whereas the two single resonances of methoxy protons remain unchanged.The Ar-Me 2 protons coalesce at approximetely -10 ºC.Investigation of the 1 H NMR spectra of 2 at variable temperatures allowed us to calculate the Gibbs free-energy barrier for the band rotation process. 14Using the expression k = π∆υ/√2, first order rate constant (k = 22.11s -1 ) calculated for the N-aryl bond rotation in 2a at -10 ˚C (see Table 1).Application of the absolute rate theory with a transmission coefficient (K) of one, gave Gibss free-energy barrier (∆G ≠ ) of 57.2±2 kJ.mol -1 .All known sources of errors were estimated and included in employed equation. 15The available data were not suitable for obtaining meaningful values of ∆H ≠ and ∆S ≠ , even though the errors in ∆G ≠ were not large. 16Effect of temperature on the rate constant was investigated on the basis of measurement of different chemical shift in a series of 1 H NMR spectra at variable temperature.The result was too small so that changes in first order rate constant and also the Gibbs free-energy of barrier are negligible in comparison with the results obtained previously at -10 ºC. 17In addition, the Gibbs free energies barrier equal 54.0 and 55.7 ±2 kJ.mol -1 were also calculated for 2b and 2c respectively.Rotational barrier of aryl-nitrogen single bond has also been calculated by ab initio method at HF/6-31G level of theory.All calculations have been performed by Gaussian 98 program package. 18Relative energy versus C 1 C 2 N 3 C 4 (see Figure 3) as a dihedral angle is plotted in Figure 4 and energy Profile is also shown in Figure 5.The corresponding structures, with respect to all points (a-h) in Figure 4 were drawn in Figure 6.The high jumps between abc and fgh points are corresponding to N-inversion.As can be seen, two weak intramolecular hydrogen bonds O•••HC could be formed between both CO 2 Me groups and the hydrogen atoms of CH 3 groups of aryl ring in structure d.Only one hydrogen bond O•••HC can be seen in structure f, because of the CO 2 Me groups is approximately orthogonal to the phenyl ring, and there is not any possibility for intramolecular hydrogen bonding in this structure.On the basis of emerged results, from both Figures 4 and 5, the rotational energy barrier calculated around aryl-nitrogen single bond (∆G # ≅ 60 kJ mol -1 ).The result is in a good agreement with experimental rotational energy barrier which was obtained by dynamic 1 H NMR data (∆E # ≅ 57.2±2 kJ mol -1 ).In conclusion, the reaction between 2,6-dimethylphenyl isocyanide and electron deficient acetylenic esters, such as dimethyl acetylenedicarboxylate (DMAD), in the presence of 1,3cyclohexanedione or 5,5-dimethyl-1,3-cyclohexanedione provides a simple one-pot entry into the synthesis of polyfunctional 4H-chromene derivatives of potential interest.Dynamic 1 H NMR study of compounds 2a-c confirmed a restricted rotation around the aryl-nitrogen single bond.In compound 2 with a non planar structure, rotational energy barrier around the aryl-nitrogen single bond, is less than compound 1 involving a planar structure.Furthermore, the obtained results from ab intio method at HF/6-31G level theory are in a good consistent with experimental dynamic 1 H NMR data.

Experimental Section
General Procedures.Melting points and IR spectra were measured on an Electrothermal 9100 apparatus and on a shimadzu IR-460 spectrometer, respectively.Elemental analysis for C, H and N were performed using a Heraeus CHN-O-Rapid analyzer.The 1 H and 13 C NMR spectra were measured on a BRUKER DRX-500 AVANCE instrument with CDCl 3 as an solvent at 500.1 and 125.7 MHz, respectively.The Mass spectra were recorded on a Finnigan-Matt 8430 mass spectrometer operating at an ionization potential of 70 eV.2,6-Dimethylphenyl isocyanide, dialkyl acetylenedicarboxylates, 1,3-cyclohexanedione and 5,5-dimethyl-1,3-cyclohexanedione

General procedure (Exemplified by 2a)
To a stirred solution of (0.112 g, 1 mmol) 1,3-cyclohexadione and (0.15 g, 1mmol) dimethyl acetylendicarboxylate in 6 mL CH 2 Cl 2 , a mixture of (0.131 g, 1 mmol) 2,6-dimethylphenyl isocyanide in 2 mL CH 2 Cl 2 was added, dropwise, at -10 ˚C over 5 minutes.(The isocyanides are toxic compounds but the toxicity of them are less than cyanides, nevertheless the isocyanides take into the lungs by inhalation and contact with skin, therefore this work was carried out inside the polyethylene glove bags under completely air-cleaner condition).The reaction mixture was then allowed to warm up at room temperature and stand to rest on a base for 5 days.The solvent was then removed under reduced pressure and solid residue 2a was washed with 2×5 mL cold diethyl ether.

Figure 6 .
Figure 6.Structures corresponding to a-h points at energy diagram.

Table 1 .
Selected proton chemical shift and activation parameters for 2a-c