Mass spectra of new heterocycles: X. Fragmentation of the molecular ions of 1-alkyl(cycloalkyl, aryl)-3-alkoxy(aryl)-2-methylsulfanyl-1H-pyrroles

Article abstract of DOI:10.1134/S1070428010070134

The mass spectra of previously unknown 1-alkyl(cycloalkyl, aryl)-3-alkoxy(aryl)-2-methylsulfanyl-1H-pyrroles were studied. Fragmentation of all 3-alkoxy-substituted pyrroles under electron impact (70 eV) follow both ether and sulfide decomposition paths;

Full text of DOI:10.1134/S1070428010070134

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2010, Vol. 46, No. 7, pp. 1038–1048. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © L.V. Klyba, N.A. Nedolya, O.A. Tarasova, E.R. Zhanchipova, O.G. Volostnykh, 2010, published in Zhurnal Organicheskoi Khimii,
2010, Vol. 46, No. 7, pp. 1039–1049.
Mass Spectra of New Heterocycles: X.* Fragmentation
of the Molecular Ions of 1-Alkyl(cycloalkyl, aryl)-3-alkoxy(aryl)-
2-methylsulfanyl-1H-pyrroles
L. V. Klyba, N. A. Nedolya, O. A. Tarasova, E. R. Zhanchipova, and O. G. Volostnykh
Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: nina@irioch.irk.ru
Received June 1, 2009
Abstract—The mass spectra of previously unknown 1-alkyl(cycloalkyl, aryl)-3-alkoxy(aryl)-2-methylsulfanyl-
1H-pyrroles were studied. Fragmentation of all 3-alkoxy-substituted pyrroles under electron impact (70 eV)
follow both ether and sulfide decomposition paths; In particular, 1-R-substituted 3-methoxy-2-methylsulfanyl-
1H-pyrroles (R = Me, Et, i-Pr, s-Bu, cyclo-C5H9, cyclo-C6H11, Ph) lose methyl radical group from both methoxy
and methylsulfanyl groups. The mass spectra of 1-sec-butyl- and 1-cycloalkylpyrroles also contained a strong
peak (10–49%) from odd-electron [M – CnH2n]+· ion formed via cleavage of the N–R bond with synchronous
hydrogen transfer. Cleavage of the O–Alk bond in the fragmentation of 3-alkoxy-1-isopropyl-2-methylsulfanyl-
1H-pyrroles (Alk = Et, i-Pr, t-Bu) was accompanied by rearrangement process leading to the corresponding
alkene and odd-electron 1-isopropyl-2-methylsulfanyl-1H-pyrrol-3-ol ion. The main fragmentation path of
1-alkyl-2-methylsulfanyl-3-phenyl-1H-pyrroles (Alk = Me, i-Pr) under electron impact involves dissociation
of the S–Me bond with formation of rearrangement 1H-[1]benzothieno[2,3-b]pyrrol-8-ium ion.
DOI: 10.1134/S1070428010070134
The importance of pyrroles and pyrrole-containing
structures in biology and various practical applications
is difficult to overestimate. Pyrrole ring constitutes
a structural unit of such vitally important natural com-
pounds as chlorophyll, hemoglobin, vitamin B12, pro-
digiosins, bile pigments, etc., pharmaceutical sub-
stances, and useful intermediate products in synthetic
organic chemistry, materials science, nonlinear optics,
and supramolecular chemistry [2]. It was proved that
introduction of heteroatoms and heteroatom-containing
substituents into pyrrole ring could radically change
physical, chemical, and biological properties of pyrrole
compounds and materials based thereon [3]. Alkylsul-
fanyl-substituted pyrroles are especially promising for
the design of biologically active compounds and opto-
electronic materials [3, 4]. Therefore, synthesis of al-
kylsulfanyl-substituted pyrrole structures and detailed
study on their properties, including their behavior
under electron impact, seem to be important.
pathways of only pyrrole itself and its alkyl and car-
bonyl derivatives were studied in sufficient detail [6].
Mass spectra of pyrroles with heteroatom-containing
substituents, e.g., alkoxy and alkylsulfanyl groups,
were not reported prior to our studies [1, 7–9]. We
were the first to record the mass spectra of two series
of alkylsulfanyl-substituted pyrroles, 2-[alkyl(alkenyl,
alkynyl)sulfanyl]-1H-pyrroles I [7] and 1-alkyl-2-
alkylsulfanyl-1H-pyrroles II [8] and reveal general
relations holding in their decomposition under electron
impact.
SR
SR
N
H
N
R
Ia–If
IIa–IId
R = Me (a), Et (b), Pr (c), Bu (d), H2C=CHCH2 (e),
HC≡CCH2 (f).
Mass spectrometric fragmentation of pyrroles was
the subject of a few publications [5, 6]; decomposition
The nature of substituent R in 1,2-disubstituted
pyrroles II affects the fragmentation pattern of their
molecular ions to a much stronger extent than does the
* For communication IX, see [1].
1038

MASS SPECTRA OF NEW HETEROCYCLES: X.
1039
Scheme 1.
(1) BuLi, THF–hexane
(2) R2NCS
(3) MeI
CH2
R1
·
R1 = AlkO
R1
CH2
R1
·
CuBr
(1) BuLi, THF–hexane
(2) R2NCS
(3) t-BuOH
SMe
N
MeS
N
R2
R2
CH
(4) MeI
R1
IIIa–IIIl
R1 = Ph
R1 = OMe, R2 = Me (a), Et (b), i-Pr (c), s-Bu (d), cyclo-C5H9 (e), cyclo-C6H11 (f), Ph (g); R2 = i-Pr, R1 = OEt (h), OPr-i (i), OBu-t (j);
R1 = Ph, R2 = Me (k), i-Pr (l).
substituent in 2-alkylsulfanyl-1H-pyrroles Ia–Id. The
reason is that the first step in the decomposition of the
latter is elimination of the alkyl substituent, which
gives rise to common 1,3-thiazin-1-ium ion with
m/z 98. Further decomposition of that ion follows the
same scheme for all compounds. Analogous process
with compounds II leads to ions having different
substituents on the nitrogen atom, so that their further
decomposition paths are also different. Fragmentation
patterns of the molecular ions of 2-allylsulfanyl- and
2-(prop-2-yn-1-ylsulfanyl)-1H-pyrroles Ie and If in-
volve mainly isomeric bicyclic structures [7], whereas
common decomposition pathways of pyrroles IIb–IId
are elimination of the R radical from the alkylsulfanyl
group and elimination of alkene molecule [8].
isopropyl-2-methylsulfanyl-1H-pyrrole (IIIj), the rela-
tive intensity of the molecular ion peak did not exceed
12%. The complete mass spectra of pyrroles IIIa–IIIl
are given in Table 1.
Analysis of the mass spectra showed that the mo-
lecular ions of 3-alkoxy-substituted 2-methylsulfanyl-
1H-pyrroles IIIa–IIIj decompose along two concur-
rent paths, ether and sulfide (channels 1 and 2, respec-
tively; Schemes 2–7). The main fragmentation path of
the molecular ion of 3-methoxy derivatives IIIa–IIIf
involves elimination of methyl radical from either
methoxy (ion A) or methylsulfanyl group (ion A′).
Further decomposition of [M – Me]+ can also take two
pathways due to different fragmentation patterns of
ions A and A′ (Table 2; Schemes 2, 3). In particular,
the fragmentation of IIIa involves successive elimina-
tion of neutral CO species and SH radical from ion A
with formation of odd-electron ion with m/z 81 (D)
which may be assigned 1-methylpyrrole structure
(Scheme 2, channel 1). The same ion is formed in the
decomposition of ion A′ as a result of successive
elimination of CS molecule (ion C) and OH radical
(Scheme 2, channel 2). The assumed structure of ion D
is confirmed by the presence in the mass spectrum of
a series of ions typical of decomposition of 1-methyl-
pyrrole [13], m/z (Irel, %): 80 (9) [M – H]+, 57 (15)
[M – C2H2]+·, 54 (9) [M – HCN]+·, 53 (14) [M – H –
HCN]+, 42 (72) [C2H4N]+.
The present work continues our systematic studies
on the mass spectra of new heterocyclic compounds
[1, 7–10] obtained from isothiocyanates and allene-
and acetylene-based carbanions [11]. We were the first
to study the behavior of previously unknown 1-sub-
stituted 3-alkoxy- and 3-phenyl-2-methylsulfanyl-1H-
pyrroles IIIa–IIIl under electron impact (70 eV).
Compounds IIIa–IIIl were synthesized according to
Scheme 1 [12]. Very recently we reported [9] on the
mass spectrum of only one representative of this series,
1-isopropyl-3-methoxy-2-methylsulfanyl-1H-pyrrole
(IIIc), in comparison with its structural isomers, 5-me-
thoxy-2,2-dimethyl-6-methylsulfanyl-2,3-dihydropyr-
idine and 3-methoxy-7-methyl-2-methylsulfanyl-4,5-
dihydro-3H-azepine. Characteristic ions ensuring reli-
able identification of each isomer in reactions mixture
were found.
Scheme 3 shows the main fragmentation pathways
of 3-methoxypyrroles IIIb–IIIf in which the aliphatic
substituent on the nitrogen atom is different than meth-
yl group (R2 ≠ Me). Unlike pyrrole IIIa, fragmentation
of ions A and A′ derived from compounds IIIb–IIIf
begins with expulsion of alkene molecule from the R2
substituent, leading to [A (A′) – CnH2n]+ ion with
m/z 128 (E or/and E′). The abundance of the latter
sharply increases in going from pyrrole IIIb (R2 = Et;
Unlike pyrrole and its alkyl derivatives that are
moderately stable to ionization under electron impact
[6], almost all the examined pyrroles IIIa–IIIi, IIIk,
and IIIl gave rise to stable molecular ions (Irel 64–
100%). Only in the mass spectrum of 3-tert-butoxy-1-
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KLYBA et al.
1040
Table 1. Complete mass spectraa of 1-alkyl(cycloalkyl, aryl)-3-alkoxy(aryl)-2-methylsulfanyl)pyrroles IIIa–IIIl (electron
impact, 70 eV)
Compound
Formula
m/z (Irel, %)
no.
157 (100) [M]+·, 158 (13) [M + 1]+·, 159 (5) [M + 2]+·, 144 (5), 143 (10), 142 (62) [M –
Me]+, 127 (4), 126 (3), 124 (3), 115 (3), 114 (13), 113 (3), 112 (6), 110 (12), 109 (12),
101 (3), 100 (3), 98 (20), 97 (3), 96 (5), 94 (3), 88 (5), 86 (4), 85 (4), 84 (7), 83 (4), 82
(14), 81 (52), 80 (9), 75 (3), 73 (15), 72 (8), 71 (5), 70 (10), 69 (9), 68 (7), 67 (7), 66
(6), 59 (6), 58 (14), 57 (15), 56 (7), 55 (16), 54 (9), 53 (14), 52 (7), 51 (3), 49 (8), 48
(10), 47 (17), 46 (6), 45 (27), 44 (8), 43 (7), 42 (72), 41 (26) [C2H3N]+·, 40 (8), 39 (18)
[C3H3]+, 38 (4), 37 (3)
OMe
IIIa
SMe
N
Me
171 (77) [M]+·, 172 (13) [M + 1]+·, 173 (8) [M + 2]+·, 158 (5), 157 (9), 156 (100) [M –
Me]+, 143 (<1) [M – C2H4]+, 141 (11), 140 (4), 128 (32) [M – Me – C2H4]+, 127 (4),
124 (9), 123 (9), 112 (21), 110 (4), 108 (8), 101 (4), 100 (9), 99 (3), 98 (3), 97 (3), 96
(15), 95 (35), 94 (5), 86 (4), 85 (6), 84 (16), 83 (4), 82 (7), 81 (3), 80 (20), 74 (5), 73
(8), 72 (4), 71 (4), 70 (7), 69 (10), 68 (8), 67 (16) [C4H4N]+·, 66 (3), 59 (6), 56 (11), 55
(6), 54 (11), 53 (14), 52 (11), 47 (4), 46 (3), 45 (14), 44 (4), 43 (4), 42 (8), 41 (15)
[C2H3N]+·, 40 (4), 39 (11) [C3H3]+, 38 (3)
OMe
IIIb
SMe
N
Et
185 (100) [M]+·, 186 (11) [M + 1]+·, 187 (6) [M + 2]+·, 172 (3), 171 (4), 170 (68) [M –
Me]+, 155 (4), 143 (3) [M – C2H4]+, 142 (5), 138 (3), 130 (4), 129 (5), 128 (80) [M –
Me – C3H6]+, 126 (3), 112 (10), 110 (3), 101 (3), 100 (27), 99 (3), 98 (4), 97 (4), 96
(14), 95 (3), 94 (4), 86 (4), 85 (5), 84 (26), 83 (3), 82 (4), 80 (6), 77 (3), 74 (3), 73 (5),
71 (3), 70 (5), 69 (7), 68 (6), 67(27) [C4H4N]+·, 66 (3), 59 (4), 58 (4), 57 (4), 56 (4), 55
(5), 54 (8), 53 (11), 52 (8), 47 (3), 45 (9), 43 (21), 42 (8), 41 (32) [C2H3N]+·, 40 (4), 39
(15) [C3H3]+
199 (84) [M]+·, 200 (10) [M + 1]+·, 201 (5) [M + 2]+·, 185 (5), 184 (50) [M – Me]+, 170
(4), 156 (3), 155 (3), 152 (4), 143 (10) [M – C2H4]+, 142 (3), 130 (5), 129 (6), 128
(100) [M – Me – C4H8]+, 124 (4), 112 (5), 110 (3), 100 (18), 98 (4), 97 (3), 96 (10), 86
(3), 85 (4), 84 (19), 83 (3), 82 (4), 80 (5), 78 (3), 73 (4), 70 (4), 69 (4), 68 (5), 67 (25)
[C4H4N]+·, 59 (3), 58 (3), 57 (11), 56 (3), 55 (8), 54 (6), 53 (8), 52 (5), 45 (7), 43 (3),
42 (6), 41 (30) [C2H3N]+·, 40 (2), 39 (13) [C3H3]+
OMe
IIIc
IIId
SMe
N
Pr-i
OMe
SMe
N
Bu-s
211 (86) [M]+·, 212 (13) [M + 1]+·, 213 (5) [M + 2]+·, 198 (4), 197 (8), 196 (69) [M –
Me]+, 181 (4), 179 (4), 178 (3), 164 (5), 163 (3), 146 (4), 144 (4), 143 (15), 142 (3),
138 (4), 136 (3), 132 (5), 130 (10), 129 (8), 128 (100) [M – Me – C5H8]+, 127 (3), 120
(3), 112 (6), 110 (3), 101 (3), 100 (21), 99 (4), 98 (7), 97 (7), 96 (8), 95 (4), 94 (3), 91
(3), 86 (3), 85 (6), 84 (19), 83 (4), 82 (6), 81 (3), 80 (6), 79 (5), 74 (4), 73 (5), 71 (8),
70 (6), 69 (15), 68 (9), 67 (36) [C4H4N]+·, 66 (4), 65 (7), 59 (4), 58 (4), 57 (10), 56 (6),
55 (10), 54 (8), 53 (10), 52 (7), 51 (4), 47 (4), 45 (10), 44 (3), 43 (10), 42 (7), 41 (62)
[C2H3N]+·, 40 (6), 39 (24) [C3H3]+
OMe
SMe
IIIe
N
225 (81) [M]+·, 226 (15) [M + 1]+·, 227 (5) [M + 2]+·, 212 (4), 211 (9), 210 (67) [M –
Me]+, 179 (3), 178 (11), 145 (3), 144 (5), 143 (49), 142 (6), 130 (9), 129 (13), 128
(100) [M – Me – C6H10]+, 126 (3), 112 (8), 110 (4), 100 (20), 99 (3), 98 (10), 97 (7), 96
(9), 94 (3), 86 (3), 85 (4), 84 (19), 83 (13), 82 (8), 81(11), 80 (7), 79 (7), 78 (3), 77 (4),
74 (3), 73 (5), 71 (3), 70 (6), 69 (6), 68 (7), 67 (28) [C4H4N]+·, 66 (3), 65 (4), 59 (4),
58 (3), 57 (3), 56 (7), 55 (65), 54 (13), 53 (20), 52 (8), 51 (4), 47 (4), 45 (10), 44 (4),
43 (7), 42 (7), 41 (77) [C2H3N]+·, 40 (4), 39 (32)
OMe
SMe
N
IIIf
OMe
219 (100) [M]+·, 220 (3) [M + 1]+·, 221 (6) [M + 2]+·, 206 (6), 205 (10), 204 (72) [M –
Me]+, 190 (4), 189 (7), 188 (9), 186 (10), 176 (12), 173 (7), 172 (7), 171 (10), 170
(22), 162 (4), 161 (5), 160 (8), 151 (6), 143 (21), 135 (9), 129 (4), 117 (8), 111 (4), 104
(14), 91 (8), 77 (38) [Ph]+, 69 (5), 58 (3), 51 (14), 44 (4), 39 (4) [C3H3]+
IIIg
SMe
N
Ph
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MASS SPECTRA OF NEW HETEROCYCLES: X.
1041
Table 1. (Contd.)
Compound
no.
Formula
m/z (Irel, %)
199 (100) [M]+·, 200 (15) [M + 1]+·, 201 (7) [M + 2]+·, 184 (11) [M – Me]+, 182 (3), 172
(5), 171 (7) [M – C2H4]+·, 170 (85) [M – Et]+, 167 (5), 156 (24) [M – C2H4 – Me]+, 155
(4), 154 (3), 153 (4), 142 (11), 140 (4), 138 (4), 130 (5), 129 (8), 128 (88) [M – Et –
C3H6]+, 127 (3), 126 (3), 124 (6), 122 (4), 115 (4), 114 (43) [M – C2H4 – Me – C3H6]+,
113 (4), 112 (28), 111 (4), 110 (6), 108 (3), 101 (5), 100 (18), 99 (3), 97 (7), 96 (13),
95 (4), 94 (9), 87 (3), 86 (13), 85 (7), 84 (29), 83 (6), 82 (11), 81 (7), 80 (13), 79 (4),
77 (3), 74 (6), 73 (7), 72 (5), 71 (6), 70 (8), 69 (17), 68 (8), 67 (14), 66 (3), 65 (4), 61
(3), 59 (11), 58 (7), 57 (12), 56 (7), 55 (20), 54 (11), 53 (20), 52 (12), 51 (5), 49 (9), 48
(3), 47 (6), 46 (3), 45 (15), 44 (22), 43 (56) [i-Pr]+, 42 (20), 41 (64) [C2H3N]+·, 40 (11),
39 (26) [C3H3]+
OEt
IIIh
SMe
N
Pr-i
213 (64) [M]+·, 214 (8) [M + 1]+·, 215 (3) [M + 2]+·, 198 (1) [M – Me]+, 172 (5), 171
(25) [M – C3H6]+·, 170 (51) [M – i-Pr]+, 158 (5), 157 (9), 156 (100) [M – C3H6 – Me]+,
130 (3), 129 (12), 128 (58) [M – i-Pr – C3H6]+, 122 (3), 116 (3), 115 (5), 114 (76) [M –
C3H6 – Me – C3H6]+, 112 (20), 110 (4), 101 (3), 100 (17), 96 (12), 95 (3), 94 (5), 87
(3), 86 (13), 85 (5), 84 (22), 83 (5), 82 (10), 81 (3), 80 (14), 74 (5), 73 (8), 71 (3), 70
(6), 69 (6), 68 (8), 67 (13), 60 (3), 59 (8), 58 (5), 57 (5), 56 (4), 55 (9), 54 (13), 53
(15), 52 (14), 47 (5), 46 (3), 45 (15), 44 (7), 43 (92) [i-Pr]+, 42 (25), 41 (96) [C2H3N]+·,
40 (9), 39 (39) [C3H3]+
227 (12) [M]+·, 228 (2) [M + 1]+·, 229 (1) [M + 2]+·, 212 (2) [M – Me]+, 173 (4), 172 (7),
171 (49) [M – C4H8]+·, 170 (8) [M – t-Bu]+, 158 (5), 157 (9), 156 (100) [M – C4H8 –
Me]+, 129 (9), 128 (11) [M – t-Bu – C3H6]+, 116 (3), 115 (5), 114 (54) [M – C4H8 –
Me – C3H6]+, 112 (11), 100 (5), 96 (6), 94 (3), 86 (8), 85 (3), 84 (9), 83 (3), 82 (7), 80
(8), 74 (3), 73 (4), 70 (3), 69 (3), 68 (4), 67 (7), 59 (5), 58 (4), 57 (30), 56 (6), 55 (8),
54 (7), 53 (9), 52 (8), 47 (3), 45 (9), 44 (4), 43 (38) [i-Pr]+, 42 (15), 41 (83) [C2H3N]+·,
40 (8), 39 (30) [C3H3]+
203 (100) [M]+·, 204 (19) [M + 1]+·, 205 (7) [M + 2]+·, 190 (7), 189 (17), 188 (85) [M –
Me]+, 187 (13), 186 (7), 175 (3), 174 (7), 173 (47) [M – Me – Me]+·, 172 (15), 171 (6),
161 (10), 155 (7), 154 (4), 149 (3), 148 (6), 147 (52) [M – Me – MeCN]+, 146 (7), 145
(6), 144 (3), 140 (4), 129 (4), 128 (14), 127 (5), 121 (3), 116 (3), 115 (20), 114 (4), 113
(4), 103 (8), 101 (37), 94 (41), 89 (11), 88 (5), 87 (4), 86 (12), 84 (12), 80 (4), 79 (5),
78 (4), 77 (17) [Ph]+, 76 (4), 75 (6), 74 (9), 71 (8), 69 (10), 65 (5), 64 (4), 63 (15), 62
(6), 54 (3), 52 (3), 51 (14), 50 (7), 45 (21), 42 (23), 41 (5) [C2H3N]+·, 40 (2), 39 (16)
[C3H3]+, 38 (3)
231 (77) [M]+·, 232 (12) [M + 1]+·, 233 (5) [M + 2]+·, 218 (3), 217 (8), 216 (50) [M –
Me]+, 189 (5), 176 (5), 175 (13), 174 (100) [M – Me – C3H6]+, 173 (14), 172 (4), 155
(5), 154 (5), 147 (22) [M – Me – C3H6 – HCN]+, 146 (3), 128 (3), 116 (3), 115 (15),
103 (3), 102 (4), 89 (5), 86 (3), 77 (5) [Ph]+, 69 (3), 65 (3), 63 (5), 51 (4), 45 (5), 43
(10), 41 (14) [C2H3N]+·, 39 (8) [C3H3]+
OPr-i
IIIi
IIIj
IIIk
SMe
N
Pr-i
OBu-t
SMe
N
Pr-i
Ph
Ph
SMe
N
Me
IIIl
SMe
N
Pr-i
a
Ion peaks with a relative intensity lower than 3% are not given.
(Irel 16–36%) odd-electron ion with m/z 67 (H) is
formed, which is likely to have the structure of unsub-
stituted pyrrole. The mass spectra also contained ion
peaks typical of fragmentation of pyrrole, m/z (Irel, %):
41 (15–77) [M – C2H2]+·, 40 (2–6) [M – HCN]+·, 39
(11–32) [M – CH2N]+.
Irel 32%) to pyrroles IIIc–IIIe having isopropyl
(Irel 80%), sec-butyl (Irel 100%), cyclopentyl
(Irel 100%), and cyclohexyl (Irel 100%) substituents on
the nitrogen atom (Table 2, Scheme 3). Further decom-
position of ions E and E′ (m/z 128) is similar to that of
ions A and A′ (m/z 142) proposed for pyrrole IIIa
(Scheme 2); it involves successive elimination of CO
(F) and HS· (Scheme 3, channel 1) or CS (G) and HO·
(Scheme 3, channel 2). In both cases, fairly abundant
Apart from channels 1 and 2 corresponding to
cleavage of the O–CH3 and S–CH3 bonds in the molec-
ular ions (Scheme 3), 3-methoxypyrroles IIIb–IIIf
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KLYBA et al.
1042
Table 2. Characteristic ions in the mass spectra of 1-alkyl(cycloalkyl)-3-methoxy-2-methylsulfanyl)-1H-pyrroles IIIa–IIIf
m/z (Irel, %)
Ion
IIIa
IIIb
IIIc
IIId
IIIe
IIIf
[M]+·
157 (100)
158 (13)0
159 (5)00
171 (77)
172 (13)
173 (8)0
185 (100)
186 (11)0
187 (6)00
199 (84)
200 (10)
201 (5)0
184 (50)
–
211 (86)
212 (13)
213 (5)0
196 (69)
–
225 (81)
226 (15)
227 (5)0
210 (67)
–
[M + 1]+·
[M + 2]+·
A, A′
B
[M – Me]+
142 (62)0 0156 (100) 170 (68)0
[A – CO]+
114 (13)0
–
–
C
[A′ – CS]+
98 (20)
–
–
–
–
–
D
[B – SH]+, [B – OH]+
[A, A′ – CnH2n]+, m/z 128
[E – CO]+, m/z 100
[E′ – CS]+, m/z 84
[C4H5N]+·, m/z 67
81 (52)
–
–
–
–
–
E, E′
F
–
–
–
–
(32)
0(9)
(16)
(16)
(80)
(27)
(26)
(27)
(100)
0(18)
0(19)
0(25)
(100)
0(21)
0(19)
0(36)
(100)
0 (20)
0(19)
0(28)
G
H
might be expected to give rise to one more decom-
position pathway involving synchronous dissociation
of the N–R2 bond and hydrogen transfer with elimina-
tion of alkene molecule and formation of odd-electron
ion with m/z 143 (J). This ion was assigned the struc-
ture of 3-methoxy-2-methylsulfanylpyrrole (Scheme 4,
channel 3). In fact, the mass spectra of 1-sec-butyl- 1-
cyclopentyl-, and 1-cyclohexylpyrroles IIId–IIIf con-
tained [M – CnH2n]+· ion peak with relative intensities
of 10, 15, and 49%, respectively. Radical cation I
decomposes in a way similar to decomposition of the
molecular ion of pyrrole IIIa, leading to ions with
m/z 128 (E, E′), 100 (F), 84 (G), and 67 (H).
bond with formation of ion A′) on the basis of the
available experimental data. Taking into account the
intensity of peaks from secondary ions B, C, F, and G
(Table 2), it may be presumed that the contributions to
the total ion current of ions A and A′ and those
resulting from their further decomposition products are
comparable, although the C–S bond in molecular ions
is known to be considerably stronger (102 kcal/mol)
than the C–O bond (74 kcal/mol) [14]. One should also
keep in mind that fragmentation of ion J (Scheme 4,
channel 3) also contributes somewhat to the intensity
of both E/E′ and F and G ion peaks.
Replacement of the methyl group on the nitrogen
atom (pyrrole IIIa) by phenyl (IIIg) does not change
the fragmentation pattern to a significant extent. As in
the mass spectrum of pyrrole IIIa (Scheme 2), the
main peaks in spectrum of IIIg were those correspond-
ing to [M – Me]+ (A, A′), [A – CO]+ (B), [A′ – CS]+
(C), and odd-electron G ion (m/z 143); the latter is
Insofar as the substituents on the oxygen (alkoxy
group) and sulfur atom (alkylsulfanyl group) in pyr-
roles IIIa–IIIf are similar (Alk = Me), it is difficult to
estimate the relative contributions of two possible
channels to the formation of [M – Me]+ ion (cleavage
of the C–O bond with formation of ion A or of the C–S
Scheme 2.
O
SMe
·
SMe
1
–CO
–SH
N
N
+·
OMe
Me
+·
Me
A, m/z 142 (62)
B, m/z 114 (13)
·
SMe
–Me
N
N
OMe
Me
OMe
Me
IIIa, M+·, m/z 157 (100)
D, m/z 81 (52)
·
–OH
S
N
2
–CS
N
Me
Me
A', m/z 142 (62)
C, m/z 98 (20)
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MASS SPECTRA OF NEW HETEROCYCLES: X.
1043
Scheme 3.
O
O
SMe
H
·
–SH
SMe
SMe
1
–[R2 – H]
–CO
N
N
N
H
R2
+·
OMe
A: IIIb, m/z 156 (100)
IIIc, m/z 170 (68)
IIId, m/z 184 (50)
IIIe, m/z 196 (69)
IIIf, m/z 210 (67)
E, m/z 128 (32,
80, 100, 100, 100)
F, m/z 100
(9, 27, 18, 21, 20)
+·
·
SMe
–Me
N
N
H
R2
IIIb, M+·, m/z 171 (77)
IIIc, M+·, m/z 185 (100)
IIId, M+·, m/z 199 (84)
IIIe, M+·, m/z 211 (86)
IIIf, M+·, m/z 225 (81)
H, m/z 67
(16, 27, 25,
36, 28)
OMe
S
OMe
OMe
–[R2 – H]
·
–OH
S
N
2
–CS
N
N
H
R2
H
A'
E', m/z 128 (32,
80, 100, 100, 100)
G, m/z 84 (16,
26, 19, 19, 19)
Scheme 4.
O
SMe
·
–SH
SMe
–CO
N
H
N
H
+·
+·
OMe
OMe
+·
3
E, m/z 128
F, m/z 100
·
–[R2 – H]
SMe
SMe
–Me
N
N
H
N
H
OMe
R2
OMe
IIIb–IIIf, M+·
J, m/z 143
(<1, 3,10, 15, 49)
H, m/z 67
·
–OH
S
N
–CS
N
H
H
E', m/z 128
G, m/z 84
Scheme 5.
O
SMe
·
SMe
1
–CO
–SH
N
N
+·
OMe
Ph
+·
Ph
A, m/z 204 (72)
B, m/z 176 (12)
·
SMe
–Me
N
N
OMe
OMe
Ph
·
–OH
S
N
2
–CS
N
Ph
IIIg, M+·, m/z 219 (100)
D, m/z 143 (21)
Ph
A', m/z 204 (72)
C, m/z 160 (8)
most likely to have the structure of 1-phenylpyrrole
(Scheme 5). In addition, peaks belonging to three more
series of ions were observed in the mass spectrum of
IIIg. One of these series results from elimination of
sulfur-containing species to produce fragment ions
with m/z (Irel, %): 186 (10) [M – HS]+, 170 (22) [A –
H2S]+, 171 (10) [A – HS]+·. The other series corre-
sponds to decomposition of the pyrrole ring with
charge localization on the phenyl-containing fragment
(ions K–N in Scheme 6). The same ions (K–N) could
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 7 2010

KLYBA et al.
1044
Scheme 6.
+·
+·
N
+·
N
OMe
Ph
N
CH
H
K'
K, m/z 117 (8)
L, m/z 104 (14)
+·
SMe
N
Ph
[Ph]+
IIIg, M+·, m/z 219 (100)
H, m/z 77 (38)
N
M, m/z 91 (8)
Scheme 7.
+·
+·
+·
+·
N
–C2H2
N
H
+·
K, m/z 117 (8)
K'
+·
N
[C6H5]+
·
–C4H4N
–HCN
m/z 116 (5)
H, m/z 77 (38)
D, m/z 143 (21)
·
–CH2N
m/z 115 (5)
be formed via analogous fragmentation of odd-electron
ion D. The third ion series (Scheme 7) is typical of
decomposition of 1-phenylpyrrole (ion D) with loss of
acetylene and hydrogen cyanide molecules or CH2N·
and C4H4N· radicals [6, 13].
Comparison of the mass spectra of pyrroles IIIh–
IIIj clearly shows that α-decomposition of the alkoxy
group with formation of [M – Alk]+ ion (A) and prod-
ucts of its subsequent fragmentation (ions E, F, and H;
Scheme 8) predominates in the fragmentation of
3-ethoxy and 3-isopropoxy derivatives IIIh and IIIi.
By contrast, the main fragmentation path of 3-tert-but-
oxypyrrole IIIj is rearrangement leading to [M –
CnH2n]+· ion with m/z 171 (O), (Table 1; Schemes 8, 9).
Further decomposition of ion O suggests that it has the
structure of 1-isopropyl-2-(methylsulfanyl)pyrrol-3-ol
(Scheme 9) [1].
Using 1-isopropyl-2-methylsulfanyl-1H-pyrroles
IIIc and IIIh–IIIj as examples we examined the effect
of stereoelectronic parameters of the 3-alkoxy substit-
uent. Increase in length and size of the alkyl group
(Me < Et < i-Pr < t-Bu) favors decomposition of the
C–O bond, so that the intensity of the molecular ion
peak sharply decreases. For example, the molecular
ions of pyrroles IIIc and IIIh are the most abundant
(Irel 100%) in their mass spectra, while the relative
intensities of the molecular ion peaks in the spectra of
IIIi and IIIj are 64 and 12%, respectively (Table 1,
Schemes 8, 9). Unlike 3-methoxy-2-methylsulfanyl-
1H-pyrroles IIIa–IIIg, the substituents on the oxygen
and sulfur atoms in compounds IIIh–IIIj are different,
so that the relative contributions of the corresponding
fragmentation channels of the molecular ions can be
estimated, though fairly arbitrarily (Scheme 8).
Judging by the intensity of ion peak A′ (1–11%),
the contribution of the sulfide path (channel 2) to the
total ion current in the mass spectra of pyrroles IIIh–
IIIj seems to be relatively small, whereas the intensity
of peaks belonging to fragment ion peaks P (m/z 156),
Q (m/z 128) and R (m/z 114) formed via decomposi-
tion of ion A′ is quite significant (24–100, 11–88, and
43–76%, respectively). Therefore, we must speak
about low stability of [M – Me]+ ion (A′) rather than
about minor contribution of channel 2. Ions with
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 7 2010

MASS SPECTRA OF NEW HETEROCYCLES: X.
1045
Scheme 8.
+·
O
OAlk
OAlk
S
OAlk
S
1
2
·
·
SMe
SMe
–Alk
–Me
–C3H6
N
N
N
N
H
CH(CH3)2
CH(CH3)2
CH(CH3)2
A, m/z 170
(85, 51, 8)
IIIh, M+·, m/z 199 (100)
IIIi, M+·, m/z 213 (64)
IIIj, M+·, m/z 227 (12)
A': IIIh, m/z 184 (11)
IIIi, m/z 198 (1)
E', m/z 142
(11, 2, 1)
IIIj, m/z 212 (2)
–C3H6
–CnH2n
OH
–CnH2n
OH
O
S
SMe
S
N
S
N
–CO
–C3H6
N
N
H
CH(CH3)2
CH(CH3)2
H
E, m/z 128
(88, 58, 11)
Q, m/z 128
(88, 58, 11)
P, m/z 156
(24, 100, 100)
R, m/z 114
(43, 76, 54)
–CO
SMe
–CS
–CS
OH
+·
OH
·
–SH
–C3H6
N
H
N
N
H
N
H
CH(CH3)2
F, m/z 100
(18, 17, 5)
H, m/z 67
(14, 13, 7)
m/z 112
(28, 20, 11)
m/z 70
(6, 8, 3)
·
–OH
+·
N
m/z 53
(20, 15, 9)
Scheme 9.
+·
+·
OAlk
OH
OH
OH
3
·
SMe
CH(CH3)2
SMe
S
N
S
N
–CnH2n
–Me
–C3H6
N
N
CH(CH3)2
CH(CH3)2
H
IIIh, M+·, m/z 199 (100)
IIIi, M+·, m/z 213 (64)
IIIj, M+·, m/z 227 (12)
O, m/z 171
(7, 25, 50)
P, m/z 156
(24, 100, 100)
R, m/z 114
(43, 76, 54)
+·
OH
·
–Me
SMe
–C3H6
N
H
S, m/z 129
(8, 12, 10)
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 7 2010

KLYBA et al.
1046
IIIk and IIIl. This path involves initial formation of
3-phenyl-2-thioxo-2H-pyrrolium ion A′ (Scheme 10,
Table 1), which is likely to be stabilized via rearrange-
ment into 1H-[1]benzothieno[2,3-b]pyrrol-8-ium A″.
The subsequent decomposition pattern of the latter
depends on the substituent on the nitrogen atom. Frag-
mentation of ions A′ and A″ derived from 1-methyl-
pyrrole IIIk follows two main pathways: (1) expulsion
of methyl radical with formation of odd-electron ion
with m/z 173 (T) which is likely to have 1H-[1]benzo-
thieno[2,3-b]pyrrole structure and (2) elimination of
acetonitrile molecule with formation of ion U with
m/z 147 (presumably with thiochromenium structure);
the corresponding ion peaks are characterized by com-
parable intensities. The main fragmentation path of
ions A′ and A″ generated from 1-isopropylpyrrole IIIl
is rearrangement through elimination of cyclopropene
molecule. Ion V thus formed, m/z 174, is the most
m/z 156 and 114 can also be formed according to chan-
nel 3, i.e., as a result of fragmentation of ion O
(Scheme 9), though this path is not determining, at
least for pyrroles IIIh and IIIi. Decomposition of ion
A could also give rise to ion E with m/z 128 (D)
(Scheme 8, channel 1).
Thus it is obvious that the diversity of possible
paths for formation of key fragment ions in the decom-
position of 3-alkoxypyrroles under electron impact
does not allow us to draw unambiguous conclusion on
the relative contributions of the main fragmentation
channels to the total ion current.
Unlike 1-phenylpyrrole IIIg, introduction of
a phenyl group into position 3 of the pyrrole ring
(instead of alkoxy group) makes the sulfide path the
main process in the fragmentation of 1-methyl- and
1-isopropyl-2-methylsulfanyl-3-phenyl-1H-pyrroles
Scheme 10.
+·
[C9H6N]+
·
–HCS
N
S
H
T, m/z 173 (47)
m/z 128 (14)
R2 = Me –Me
·
+·
Ph
Ph
R2 = Me
–MeCN
·
N
–Me
SMe
S
N
N
S
R2
R2
R2
H
IIIk, M+·, m/z 203 (100)
A': IIIk, m/z 188 (85)
IIIl, m/z 216 (50)
A"
IIIl, M+·, m/z 231 (77)
[C9H7]+
m/z 115 (20, 15)
–C2H2
–S
R2 = i-Pr –C3H6
S
U, m/z 147 (52, 22)
N
–HCN
S
[C7H5]+
H
H
V, m/z 174 (100)
m/z 89 (11, 5)
Scheme 11.
+·
Ph
+·
MeS
MeS
N
SMe
N
MeS
Ph
Me
Me
m/z 101 (37)
IIIk, M+·, m/z 203 (100)
W, m/z 161 (10)
W'
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 7 2010

MASS SPECTRA OF NEW HETEROCYCLES: X.
1047
abundant in the spectrum (100%). Elimination of hy-
drogen cyanide molecule from ion V could also give
rise to ion U (Scheme 10).
decomposition of the primary fragment ions. sec-Bu-
tyl, cycloalkyl, and phenyl substituents on the nitrogen
atom (IIId–IIIg) give rise to additional fragmentation
channels of the molecular ion.
In addition, the mass spectrum of pyrrole IIIk con-
tains ion peaks indicating decomposition of the pyrrole
ring. Presumably, phenyl-containing fragment ion with
m/z 161 has the structure of 1-methylsulfanyl-2-phen-
ylcyclopropenium (W) or 3-(methylsulfanyl)-1H-
indenium (W′) where the positive charge is stabilized
by the benzene ring. Nitrogen-containing fragment
with m/z 101 is likely to be a radical cation species
(Scheme 11).
EXPERIMENTAL
Compounds IIIa–IIIl were synthesized according
to the procedures reported in [12]. The mass spectra
(electron impact, 70 eV) were recorded on a Shimadzu
GCMS-QP5050A instrument (quadrupole mass ana-
lyzer, a.m.u. range 34–650; SPB-5 capillary column,
60 m×0.25 mm×0.25 μm; carrier gas helium, flow
rate 0.7 ml/min; injector and ion source temperature
250°C; gas inlet pressure 250 kPa; oven temperature
programming from 60 to 250°C at 10 deg/min).
Comparison of the spectra of two pyrrole series,
i.e., those containing similar substituents on the nitro-
gen atom (R2 = Me, i-Pr) but different substituents in
position 3 of the pyrrole ring (OMe, Ph) revealed
unusual effect of all substituents on both stability of
the molecular and/or fragment ions and selectivity of
their fragmentation. Replacement of the 3-methoxy
group in N-isopropylpyrrole IIIc by phenyl (IIIl) de-
stabilizes the molecular and fragment ions, and the
relative intensities of the [M]+· and [M – Me]+ ion
peaks decrease, respectively, from 100 to 77% and
from 68 to 50%. Analogous replacement in N-methyl-
pyrrole IIIa exerts no effect (pyrrole IIIk) or the effect
is the opposite. In both cases, the molecular ion peak
[M]+· is the base peak in the spectrum 100%, and
[M – Me]+ ion derived from IIIk (m/z 188, 85%) is
even more stable than analogous ion (m/z 142, 62%) in
the spectrum of IIIa. On the other hand, comparison of
the spectra of 3-methoxypyrroles IIIa and IIIc with
those of 3-phenylpyrroles IIIk and IIIl shows that the
spectra of N-methyl derivatives IIIa and IIIk are
richer and more complex than the spectra of N-iso-
propyl-substituted analogs IIIc and IIIl, though the
reverse pattern could be expected taking into account
the ability of long and/or branched alkyl chains to un-
dergo various rearrangements and diversity of possible
bond cleavage modes therein.
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 09-03-00890a) and by the Siberian Division of the
Russian Academy of Sciences (Program for Basic
Research of the Presidium of the Russian Academy of
Sciences, project no. 18.20).
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