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J. CHEM.SOC. PERKIN TRANS. 2 1993 Claisen Rearrangements and Cyclisations in Phenyl Propargyl Ethers under Electron Impact Conditions Devalla V. Ramana" and Marimanikuppam S. Sudha Department of Chemistry, Indian Institute of Technology, Madras 600 036, India An interesting Claisen rearrangement has been observed in phenyl propargyl ether, yielding signifi- cant amounts of the [M -CO] ion under electron impact conditions. The other competing pathway is the loss of a hydrogen radical from M" furnishing the most abundant ion in its mass spectrum. Expulsion of a hydrogen radical from the propargylic carbon leads to the propargylic cation species, while ejection of the ortho hydrogen gives rise to a benzopyrylium cation structure. Substituents on the phenyl ring do not favour the Claisen rearrangement, whereas ejection of the substituents producing benzopyrylium cations gains importance.The mechanisms and ion structures proposed are supported by high-resolution data, B/E and B2/E linked-scan spectra, Collision Activated Decomposition (CAD) -B/E linked-scan spectra and deuterium isotope labelling. Claisen rearrangements in the gas phase have gained import- 1*+ance in recent years, as in solution.'S2 Phenyl allenyl ethers lose CO by a Claisen rearrangement pathway from their molecular ions giving rise to intense [M -CO] ions under electron Uimpact conditions. Such a rearrangement in phenyl allyl ether aand N-allylaniline under positive CI conditions has been well e, mlz 94 (3) A documented.A Claisen rearrangement has also been noticed 1, Me', m/z 132 (49) /\under EI conditions in N-allylaniline leading to the expulsion of HCN to the extent of 1%. The anionic Claisen rearrangement leading to interesting fragment ions has been reported under negative CI conditions in deprotonated allyl phenyl a~etate,~ allyl phenyl ether' and allyl vinyl ether.' The thermal lo and catalysed Claisen rearrangement of phenyl propargyl ether is d,m/z W(5) /well established and under positive CI conditions its [M + H] p'Y 0";'ion loses CO, for which a Claisen rearrangement is propo~ed.~ m/Z 131(100) -co -H' 't Hence, it was considered interesting to study the behaviour of CgHfl(l31.04835) *-\ H phenyl propargyl ethers (compounds 1-10) under positive ion c, m/z 103 (53) b,m/z 104 (29)EI conditions.C8H7 (103.05323) C8Hs (104.06196) Scheme 1 C R' R2 A H 103 1 H H 9 2 OCH3H 3 4 H CH3 OCH3 H pJOXr-cH3' 5 6 H CI CH3 H ::.. 7 H CI 8 H NO2 H 10 Results and Discussion The most interesting fragment ion in the mass spectrum of phenyl propargyl ether (1) is the ion b at m/z 104 (Scheme 1, Table 1 and Fig. 1). The exact mass of the ion b is found to be 104.061 96 which corresponds to an elemental composition of C'H,. This fact reveals that b is formed by the expulsion of CO from the molecular ion of 1. The proposed direct formation of the ion b from M" of 1 is supported by the B/E linked-scan spectrum of M" at m/z 132 (Table 2) and B2/E linked-scan 20 -0 55 66', " D&D 'Ol 81 I 82 109 107 98 99 I i I \ y2 F C 111 H I1O8 m /z l 1130 140 spectrum of ion b (Table 2). Fig.I EI mass spectra at 70 eV of (a)compound I (b)compound 9 676 J. CHEM. SOC. PERKIN TRANS. 2 1993 Table I Partial mass spectra of compounds 1-10 Relative abundances of Compound M" [M -HI+ [M -R]+ b C d e Other ions 1 49( I 32)" loo( 131) lOO(131) 29( 104) 2 60( 162) 2( 161) 3(131) 4( 134) 3 26( 162) -l(131) l(134) 4 94( 146) 85(145) 90( 131) 15(118)5 96( 146) 80(145) 86(131) 15(118) 6 37( 166) 10( 165) 100(I3 I ) 5(138)7 39( 166) I2( 165) 100( 13 I ) 4( 138) 8 46( 177) 28( 176) lOO(131) 2( 149) 9 53( 137) 100(136) 49( 135) 33( 109) 10 25( 146) 10( 145) 10( 145) 5( 118) " The figures in parentheses indicate the m/z values of the ions. Table 2 B/E and B2/E linked-scan spectral data of compound I B/E Parent m/z values of daughter ions 53( 103) 5(93) 3(94) -IOO( 123) 1O( 124) 74(95) -loo(123) 9( 124) 29(95) 59( 117) 65( 107) 9(108) 17( 103), 50(79), 1OO(77) 47( 1 17) 70( 107) 9( 108) 19( 103), 58(79), loo(77) 20( 137) 14( 127) 4( 128) 39( 103), 63(99) 1 l(137) 34( 127) I O( 128) 43( 103), 60(99) 10(148) -l(139) 9( 161 ), 38( 160), 88( I 30) 53( 108) 7(98) 6(99)13(117) 3(93) 100(94) 32(131),8( 103) B2/E m/zvalues of parent ions Daughter ion mi:ion m/z with abundances in parentheses M", 132 131(100), 104(31), 103(36), 94(4), 93(10) 104 130(IOO), 103( 54), 77(4) 103 104 103(22), 78( loo), 77(94) I 103 85 102 .-h u) c.::I 1, l18i ,Iso , , , .-E 50 60 70 80 90 100 110 50 60 70 80 90 100 110 mlz Fig.2 CAD-B/E linked-scan spectra of (a) m/z 104 of phenyl prop- argyl ether (1) and (b) M'+, mi: 104 of styrene A Claisen rearrangement in the molecular ion of 1 is envis- aged for the formation of ion b (Scheme 1). The M" of 1leads, after Claisen rearrangement, to the allenic species a which ejects a molecule of carbon monoxide followed by skeletal rearrange- ment yielding the ion b. The styrene radical cation structure, proposed for b, is confirmed with the help of Collision Activated Decomposition (CAD)-B/E linked-scan spectra of b and the M" of styrene, taken as the reference compound, which are found to be identical (Fig. 2). The abundant ion c at m/z 103 is formed by the loss of a hydrogen radical from b.Ion c is-also formed from the molecular ion by the direct expulsion of CHO (Scheme I). with abundances in parentheses I 32(100) 132(20), 131(100), 104(25) H h,mlr 131 1100) \ ce f CQHfl(l31.04835) I H 1, M *+, mlz 132 (49) CgHe0 (132.05642) + g, mlz 131 (100) CQHH-~(1 31.04835) Scheme 2 The base peak in the mass spectrum of 1 is at m/z 131 (Table 1 and Fig. 1) which corresponds to the expulsion of a hydrogen radical from M" as shown by its accurate mass measurement and linked-scan spectra (Table 2). The hydrogen radical lost from M" can originate from either the propargylic carbon or the ortho position of the ring (Scheme 2). The mass spectrum of the [2H,]-compound 9 (Fig.1) reveals that the loss of H:D from the M" is in the ratio 2: 1 after isotopic correction. This suggests that the loss of propargylic hydrogen is twice that of the ortho hydrogen. The expulsion of a hydrogen radical from the ortho position [path (a)] would lead to the ion g having a benzopyrylium cation structure, as has been observed in the case of phenyl allenyl ether^,^ while the ejection of H' from the propargylic carbon [path (b)]would lead to the ion h. The CAD-B/E linked-scan spectrum of the ion at m/z 131 from 1 is compared with those from phenyl a-methylpropargyl ether (lo), 2H-1-benzopyran (11) and compound 6 (Fig. 3). It J. CHEM.SOC. PERKIN TRANS. z 1993 - 100 100 7103 80 80 60 60 40 40 20 20 0 0 129 100 103 80 -(b) 80 -(f) 102 77 60 -60 -63 130 d40 -40 20 -51 li I,0 I Y 4100 100 103.-c u) 1022 80 80 (91 77.-c 60 63 .-? 60 c -a 1302 40 40 5120 20 ,o 0 I t ' 50 70 90 110 130 150 80 402ou0 150 70 90 110 130 150 1mtz .-* CAD-B/E linked-scan spectra of ions at (a)m/z 131 of 1, (b)m/z 131 of 10, (c) m/z of 11, (d)m/z 131 of 6, (e)m/z 131 of 5, (f)m/z 131 of 7Fig.3 and(g)m/z 131 of8 can be noticed that the CAD-B/E linked-scan spectrum of the ion at m/z 131 from 6 is exactly identical with that from 11, whereas the CAD spectrum of the ion at m/z 131 from 10 is distinctly different when compared to that from 11. Further-more, the CAD-B/E linked-scan spectrum of the ion at m/z 131 from 1 resembles those from compounds 10 and 11.It is clear from these observations that the ion at m/z 131 from 1 is a mixture of two species, namely, the ion g having a benzo- pyrylium cation structure and the ion h, a propargylic cation species. It can further be inferred from the CAD spectra of the ion at m/z 13 1 from 10 and 11that the propargylic cation species does not cyclise to the benzopyrylium cation structure. Hence it can be concluded that the cyclisation must be occurring in the M" of 1 yielding f which then loses H' [path (a),Scheme 21, rather than cyclisation after the expulsion of hydrogen radical from the propargylic carbon to yield g. This hypothesis can also be confirmed from the fact that the 2[H,]-compound 9 will not be losing D' from its M' + if the cyclisation occurs after the initial ejection of the propargylic hydrogen. Compounds 2 and 3 undergo simple cleavage to yield the ions at m/z 123 (Table 1) having quinonoid structures as the most abundant ions (Scheme 3) in their mass spectra.The Claisen 1*+ I H 2, M'*, mlz 162 (60) d, mlz 123 (100) C10H1002 (162.06636) C,H& (1 23.04290) l*+ 3, M *+, mlz 162 (26) d, m/z 123 (100) C10H1002 (162.06636) C7H7Q (1 23.04287) Scheme 3 rearrangement and cyclisation occur to a very minor extent in these compounds. In contrast, compounds 4 and 5 undergo Claisen rearrangement yielding ions at m/z 118 (Table 1) of reasonable abundance while loss of H'is a major fragmentation pathway.The hydrogen radical can be lost in these compounds either from the propargylic carbon or from the methyl sub- stituent to give ring expanded tropylium cation structures. Interestingly, compounds 6, 7 and 8 favour cyclisation by the expulsion of the substituents leading to fragment gat m/z 131 as the most intense ions in their mass spectra. The ejection of the substituents in the p-isomers (compounds 5, 7 and 8) for the formation of the cyclised ion g [confirmed by the CAD-B/E linked-scan spectra, Fig. 3 (e), (f)and (g)] may involve substituent scrambling. All the fragmentation pathways shown in Schemes 1 and 2 are also noticed in other compounds studied. These are supported by the accurate mass measurements, B/E and BZ/E linked-scan spectra.The mass spectra of 1 and 4 taken at 70 eV under cold conditions (inlet temperature 50 "C and source temperature 20 "C) were identical to those taken under normal conditions. Also, compound 1 was recovered unchanged after pyrolysis at 120 "C and at 20 mmHg pressure for 30 min. These experiments confirm that the loss of CO through the Claisen rearrangement in these compounds is only due to electron impact and is not a thermal process. Conclusion It can be seen from this study that the substituents on the phenyl ring tend to facilitate cyclisation reaction by expelling the substituents from the molecular ions, while the Claisen rearrangement is not a predominant process. Surprisingly, no ortho interaction is noticed in compounds 2 and 4 between the ortho substituent and the triple bond, whereas in the case of o-nitrophenyl propargyl ether,' the principal fragment ion is derived by the transfer of an oxygen from the nitro group to the acetylenic moiety.Experimental All the compounds studied in this work have been described in the literature. Compounds 1-10 were prepared according to the procedure of I. Iwai and J. Ide l2 but using DMF as the solvent at room temperature.I3 The compounds were purified by column chromatography with hexane as the eluent. The struc- J. CHEM. SOC. PERKIN TRANS. 2 1993 tures were confirmed by IR and 'H NMR data. Mass spectra were recorded on a Finnigan MAT 8230 double focussing mass spectrometer. The mass spectra were run at 70 eV with an emission current of 100 pA and an accelerating voltage of 3 kV.All the compounds were introduced into the mass spectrometer through either the reference inlet at ll0"C or the direct insertion probe at 25 "C. Accurate mass measurements were carried out at a resolution of 8000 (I 0% valley) and PFK was used as the reference. The CAD-B/E linked-scan spectra, in the first field free region were investigated using helium as the collision gas with a Finnigan MAT 8230 mass spectrometer at an ionisation energy of 70 eV and an accelerating voltage of 3 kV. Acknowledgements The authors thank RSIC, IIT Madras for mass spectral facili- ties. One of us (M. S. S.) thanks CSIR, India for a research fellowship. References 1 R.P. Lutz, Chem. Rev., 1984,84,204. 2 F. F. Ziegler, Chem. Rev., 1988,88, 1423. 3 D.V. Ramana and M. S. Sudha, Org. Mass. Spectrom., 1992,27,1121. 4 E. E. Kingston, J. H. Beynon, J. G.Liehr, P. Meyrant, R. Flammang and A. Maquestiau, Org. Mass. Spectrom., 1985, 20, 351. 5 E. E. Kingston, J. H. Beynon, J. G. Liehr, P. Meyrant, R. Flammang and A. Maquestiau, Org. Mass. Spectrom., 1988, 23,437. 6 A. Vandezonneville, R. Flammang, A. Maquestiau, E. E. Kingston and J. H. Beynon, Org. Mass. Spectrom., 1986,21,351. 7 P. C. H. Eichinger,J. H. Bowie and R. N. Hayes, J. Org. Chem., 1987, 52, 5224. 8 P. C. H. Eichinger and J. H. Bowie, J. Chem. SOC., Perkin Trans. 2, 1988,497. 9 P. C. H. Eichinger and J. H. Bowie, Am. J. Chem., 1990,43, 1479. 10 R. Usha and K. K. Balasubramanian, Tetrahedron Lett., 1983, 24, 5023. 1 1 D. V. Ramanaand N. V. S. Ramakrishna, Bull. Chem.SOC.Jpn., 1989, 62, 3349. 12 I. Iwai and J. Ide, Chem. Pharm. Bull. (Jpn.),1962, 10,926. 13 R. Usha and K. K. Balasubramanian, Heterocycles, 1984,22, 1351. Paper 2106343C Received 26th November 1992 Accepted 1st January 1993

ISSN:1472-779X    

DOI:10.1039/P29930000675

出版商:RSC    

年代:1993

数据来源: RSC