Mass spectral analysis of some allylic esters

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2015-00071 Chemical and Biological Sci. 2015, Vol. 60, No. 9, pp. 3-8 This paper is available online at Received August 25, 2015. Accepted November 30, 2015. Contact Duong Quoc Hoan, e-mail address: hoanqduong@gmail.com 3 MASS SPECTRAL ANALYSIS OF SOME ALLYLIC ESTERS Duong Quoc Hoan1, David P. Brown2, Nguyen Dang Dat1, Pham Thi Linh1 and Cao Xuan Dinh3 1 Faculty of Chemistry, Hanoi National University of Education, 2 Deparment of Chemistry, St. John's College of Liberal Arts and Sciences, St. John’s University 8000 Utopia Parkway, Queens, NY 1143, USA 3 Duy Tan High School, Kon Tum Province Abstract. EI MS spectra show that ester bond cleavage is predominant. Compounds 8, 9 and 10 have a molecular weight 400 g/mole or more and suffered fragmentation along the GC column, hence their molecular ions were not observed. The collected molecular ions [M]+ correspond to the expected structures and followed [M+1] +, nitrogen rule, and some first fragmentations. The ortho disubstituted benzene ring such as compound 1 gives peaks at m/z 120 and m/z 161. Compounds 3, 5, 7, 8, 9 and 11 have fragmentations controlled by the resonance effect to explain the existence of a peak at m/z 147. An MS analysis of the compounds 3-[2-(allyloxy)phenyl] propanoic acid - containing esters shows a protonation followed by loss of hydrogen molecules as well. The ortho effect allows us to state that compound 11 has a peak at m/z 307 as well. Keywords: Allylic esters, EIMS, ortho effect, resonance effect. 1. Introduction Electron impact (EI) is one of the most popular methods used for generating ions for mass spectrometry. High-energy electrons (ca. 70 eV) bombard vapor phase sample molecules to eject an electron from a sample molecule producing a radical ation that is called the molecular ion. EI has some disadvantages such as the large internal energy method, and the rearrangement process complicates spectra, but is has many advantages, for example a reproducible method, high ionization efficiency, libraries of EI spectra to help identification, interface to GC possible, all vaporized molecules can be ionized (non-polar and i soluble) and molecular structural information [1, 2]. Macrolactones are novel heterocylic compounds containing ester linkage. There are many natural products that are macrolactones, one of those being macrosphelides [3, 4]. One synthetic method is ring close metathesis (RCM) of alkene substrates using Grubb‟s catalyst and similarity. Hence, eleven allylic esters, substrates for RCM, were synthesized and characterized using IR, NMR, elemental analysis and MS [5]. The MS analysis of these Duong Quoc Hoan, David P. Brown, Nguyen Dang Dat, Pham Thi Linh and Cao Xuan Dinh 4 alkene substrates helps us understand their structures. In this paper, the analysis is focused on not only on molecular weights but also on primary fragmentations based on effects such as resonance and ortho effects to obtain more data for these kinds of compounds. 2. Content 2.1. Experiments 2.1.1. Synthesis of some allylic esters The synthesis of these allylic estes was reported by D. Brown and H.Q. Duong [5]. It is briefly described in Figure 1. OH (CH2)n O OH n = 0, 2 KOH R Br R = H, CH3 O (CH2)n O OH n = 0, 2 R SOCl2 DCM, DMAP X (CH2)n O O R III: X= OH; NH2 O (CH2)n O n = 0, 2R X (CH2)n O O R X = O, NHI II IV Figure 1. First of all, phenolic carboxylic acids (I) were converted to ether carboxylic acids (II) based on the Williamson‟s ether synthesis. In fact, these reactions gave a mixture of ether carboxylic acids (II) and ether estes; however, the ether esters were hydrolyzed to obtain ether carboxylic acids (II). Then the coupling reaction of the ether carboxylic acids was accomplished with thionyl chloride and phenolic or a iline derivatives (III) to yield diene IV which are allylic esters. 2.1.2. Method Gas Chromatography Mass Spectrometry (GC/MS) analyses were performed using the Shimadzu GCMS-QP5050A system. All compounds were in a liquid state. They were dissolved in methanol and injected into the GC/MS to perform the MS spectra. 2.2. Results and discussion 2.2.1. Recognition of the molecular peak One of the difficulties in using the EI method for MS is recognition of the molecular ion peak [M]+ [1, 2] because thpeak is either weak or cannot be found. The best solution in this case is to obtain an intense peak at [M+1]+, [M+2]+ plus small fragmentations. Structures and M+, [M+1]+, and related fragments are shown in Table 1. All allylic esters contain C, H and O atoms in which C and H atoms are contributing [M+1]+ and O atom contribute to [M+2]+. However, in our cases, no [M+2]+ peaks observed. While the MS of compound 5 gives [M+1]+, others might be too small to see on MS spectra. The value of the % (M+1) compared with [M]+ is calculated based on C22H22O5 of compound 5 using the formula %(M+1)  (1.1 x 22) = 24% [2]. This value is in agreement with the experimental result. In addition, all of the molecular ion peaks adhere to the nitrogen rule. The molecular ion peaks of compound 1 to 10 are even numbered because of the absence of nitrogen atoms, while compound 11 gives an odd number since it has a nitrogen atom in the structure (Table 1). Mass spectral analysis of some allylic esters 5 Table 1. Molecular ion [M] + and molecular ion plus one [M+1] + Comp. Formula M + (Calcd./found (%)) m/z (%) Entry Structure 1 O O O O O C20H18O5 338.35/338 (1) 338 (1), 281(1), 161 (100), 133(28), 121 (11), 105 (20), 92(17), 77(6), 55(3), 41(53). 2 O O O O O C20H18O5 338.35/338 (3) 338 (3), 281(2), 161(100), 133(41), 105(20), 92(4), 77(18), 55(4), 41(9). 3 O O O O O C22H22O5 366.41/366 (6) 366 (6), 189(35), 161(15), 133(8), 121(22), 91(32), 77(10), 65(14), 55(75), 41(100) 4 O O O O O C22H22O5 366.41/366 (2) 366 (2), 206(8), 161(100),148(15), 133(39), 105(19),77(16), 65(5), 55 (6), 41(12). 5 O O O O O C22H22O5 366.41/366 (6) 366(6), 307(1), 218(5), 189(34), 161(24), 147 (16), 133(5), 121(40), 91(30), 77(10), 65(18), 55(75), 41 (100) 6 O O O O O C22H22O5 366.41/366 (3) 366 (3), 189(22), 161 (13), 147 (19), 133(6), 121 (50), 91(37), 77(12), 65(16), 55(50), 41(100). 7 O O O O O C22H22O5 366.41/366 (3) 366 (3), 189(21), 161(23), 147 (20), 133(5),121(48), 91(36),77(14), 64(15), 55(50), 41(100). 8 O O O O O C24H26O5 394.46/- (-) 246(6), 206(4), 189(33), 161(7), 148(21), 133(4), 120(15), 91(35), 77(15), 65(7), 55(68), 41(100). 9 O O O O O C25H28O5 408.49/- (-) 189(7), 160(37), 148(61), 145 (100), 133(22), 120(41), 105(12), 91(94), 77(39), 65(49), 57(53), 51(34). 10 O O O O O C26H30O5 422.51/- (-) 202(77), 174(18), 159(100), 148(62), 120(46), 91(91), 78(56), 57(64), 51(37), 45(16). 11 N H O O OO C22H23NO4 365.42/365 (6) 365 (6), 307(20), 177(100), 161(14), 119(56), 91(32), 77(17), 65(20), 55(15), 41(68). Note: “-“ no data 2.2.2. Resonance effect in fragmentations Since each compound contains two aromatic rings, a resonance effect is considered for two substituents in place of orth or para in aromatic rings [1, 2]. Compound 1 and 2 have almost the same structure except for the ortho-substituted benzoate in compound 1 and the meta-substituted benzoate in compound 2. Primary fragmentations of compound 1 are shown in Scheme 1. Either pathway (1) or (2) has a loss of radical C3H5O  (57) to form either oxonium or acyl ions m/z 281. Interestingly, the fragmentations are identical in both structures but the peak of m/z 120 is not observed in the compound 2‟s spectrum. See Table 2. This issue Duong Quoc Hoan, David P. Brown, Nguyen Dang Dat, Pham Thi Linh and Cao Xuan Dinh 6 is explained by a „resonance effect‟ that gives a peak at m/z 120 (see (3), Scheme 1) [2]. The resonance effect can be observed in eaction (4), Scheme 1, yielding the fragment m/z 161 as a base peak and a peak at m/z 120 or 121 [6, 7]. O O O O O 1 -C3H5O (57) O O O O m/z 281C20H18O5 Mol. Wt.: 338 -C3H5O (57) O O O O O m/z 281 C O O m/z 161 C10H9O3 (177) (1) (2) (3) C O O m/z 120 -C3H5 • (41) -C10H9O3 •(177) (4) H C O O m/z 121 H Scheme 1. Primary fragmentations of 2-[(allyloxy)carbonyl]phenyl-2- (allyloxy)benzoate (1) In contrast, compound 2 has a base peak at m/z 161 as in compound 1; however it is supposed that there is a cleavage of an ester bond to give a quite stable fragment m/z 161, Scheme 2. O O O O O O+ O -C10H9O3 • (177) 2, C20H18O5 Mol. Wt.: 338 m/z 161 Scheme 2. Primary fragmentation of compound 2 In cases of compound 5 and 7, the resonance effect is not prominent since the lone pair of electrons on the oxygen atom is conjugated with a C=O bond upon the ester group. Consequently, the fragment of m/z 309 is not stabilized. Hence, ester bond cleavage is prominent in these cases, Scheme 3. O O O O O 5, C22H22O5 Mol. Wt.: 366 O O O C O O O -C3H5O • (57) m/z 309 C7H5O2 • (121) m/z 189 x O O O O O 7, C22H22O5 Mol. Wt.: 366 O O C7H5O2 • (121) m/z 189 O O O -C3H5O • (57) m/z 309 x C O a) b) + + Scheme 3. Primary fragmentations of compounds 5 and 7 Compound 3, 5, 7, 8, 9 and 11 have same structure with regards to carboxylic moiety as drawn in Scheme 4. Besides the cleavage of the ester bond to form fragment m/z 189, the resonance effect helps in the cleavage of CH2-CH2 bond to produce fragment /z 147, Scheme 4. Mass spectral analysis of some allylic esters 7 O O O O O C7H5O2 • (121) m/z 189 Ar 3, 5, 7, 8, 9, and 11 -Ar O m/z 147 Scheme 4. Resonance effect in the fragmentation of 3-[2-(allyloxy)phenyl]propanoic acid-containing esters 2.2.3. Ortho effect in fragmentations The ortho effect is observed in the MS spectrum of compound 11 that contain an amide group in the ortho position of carboxylate in the aromatic rings. Consequently, the radical ion of molecule 11 is easy to eliminate and a neutral molecule of allylic alcohol forms the radical ion m/z 307 while a cleavage of the amide bond does not occur, Scheme 5 [1, 2]. -C3H5OH (58) O O C7H5O2 • (121) m/z 189 N H O O OO 11, C22H23NO4 Mol. Wt.: 365 O O N m/z 307 C O x Scheme 5. Ortho effect in the fragmentation of compound 11 2.2.4. Loss of a hydrogen molecule in the fragmentation of 3-[2-(allyloxy)phenyl] propanoic acid-containing esters O OH O O O C26H29O5 + Mol. Wt.: 421 O O O O O 10, C26H30O5 Mol. Wt.: 422 + H -H2 -C13H15O3 • (219) O OH m/z 202 O O O Ar 3, 5, 7, 8, and 9 -H2 O OH m/z 188 (1) (2) O OH O Ar -Ar N H O O OO (3) + H -H2 N H O O OHO C3H5O • (57) N H O O OH m/z 307m/z 36411, C22H23NO4 Mol. Wt.: 365 + H Scheme 6. Loss of a hydrogen molecule in the fragmentation of 3-[2-(allyloxy)phenyl]propanoic acid-containing esters Surprisingly, compound 3, 5, 7, 8 and 9 have a peak m/z at 188 along with a peak m/z at 189, Table 1 and Scheme 6. Compound 9 has a peak at m/z 188 with abundant 50%, but the peak Duong Quoc Hoan, David P. Brown, Nguyen Dang Dat, Pham Thi Linh and Cao Xuan Dinh 8 at m/z 189 is 5% only. Similarly, compound 10 has a peak at m/z 202 and compound 11 has a peak m/z 307, Table 1. The formation of peak m/z 189 is explained clearly in Scheme 3 and 4. In contract, peaks at m/z 188 and 202 are not understandable. It is clear that ester groups are basic centers; therefore they can be protonated following hydrogen elimination, Scheme 6. Since compound 11 has an amide bond that is not easily broken, the ester cleavage gives a peak at m/z 307. The addition of a proton and elimination of a hydrogen molecule gives a longer conjugated system that stabilizes the ions throughout the GC column. 3. Conclusion The EI MS spectra of 11 allylic esters was analyzed carefully. A molecular ion [M]+ peak has been determined based on [M+1]+, the nitrogen rule, fragmentation, the resonance effect, and the ortho effect. All molecular ions [M]+ match with the nitrogen rule and [M+1]+ ions. The Molecular ion [M]+ peaks of compound 8, 9 and 10 are not shown in the EI MS spectra. An MS analysis of compounds 3, 5, 7, 8, 9, 10 and 11 shows a loss of hydrogen molecule. Fragmentations are effected by the ortho effect or the resonance effect if an aromatic ring has an ether, amide or a carboxylate group in ortho or para each other. REFERENCES [1] Tran Thi Da, Nguyen Huu Dinh, 1999. Application of some spectroscopic methods in studying on molecular structure. Vietnam Education Publishing House. [2] Silverstein, R. M., Webster, F. X., Kiemle, D. J., 2005. Spectrometric identification of organic compounds. John Wiley  Sons, Inc. [3] Nicolaou, K. C. 1977. Synthesis of macrolides. Tetrahedron, Vol. 33, pp. 683-710. [4] Kobayashi, Y., Kumar, B. G., Kurachi, T., 2000. Total synthesis of macrosphelides B and A. Tetrahedron Lett., Vol. 41, p. 1559. [5] Brown D. P., Duong, H. Q., 2008. Synthesis of Novel Aromatic Macrolactones via Ring Closing Metathesis of Substituted Phenylalkanoic Acid Allylic Esters. J. Heterocyclic Chem., Vol. 45, p. 435. [6] Emery, E. M., 1960. Mass spectra of aromatic esters. Anal. Chem., Vol. 32, No. 11, pp. 1495-1506. [7] McLafferty F. W., Gohike R. S. 1959. Mass Spectrometric Analysis. Aromatic Acids and Esters. Anal. Chem., Vol. 31, No. 12, pp. 2076-2082.