Electron ionization-induced fragmentation of N- and O-alkoxymethylated carbostyril and phenanthridinone

Article abstract of DOI:10.1002/jms.652

Unimolecular fragmentation patterns of N-alkoxymethylated carbostyril and phenanthridinone and their O-alkoxymethyl isomers were studied. The main fragmentation reaction observed for the studied compounds is the elimination of an aldehyde molecule. The main products of this reaction are the appropriate N-methyl derivatives, but ions with other structures are also formed. This reaction is supposed to proceed via 1,3-H shift in the alkoxymethyl group in the case of the N-alkoxymethyl derivatives and by a multi-step mechanism for O-alkoxymethylated compounds. Another important fragmentation common for all studied compounds is the loss of an alkyl radical from N- and O-alkoxymethyl groups, yielding the appropriate stable isomeric cations, which, according to the results of the further fragmentation, undergo fast equilibration reaction via an ion-neutral complex. This process is accompanied by the unusually high kinetic energy release value. Copyright

Full text of DOI:10.1002/jms.652

JOURNAL OF MASS SPECTROMETRY
J. Mass Spectrom. 2004; 39: 781–790
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jms.652
Electron ionization-induced fragmentation of N- and
O-alkoxymethylated carbostyril and phenanthridinone†
Rafał Szmigielski and Witold Danikiewicz∗
Institute of Organic Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, P.O. Box 58, 01-224 Warsaw 42, Poland
Received 15 January 2004; Accepted 14 April 2004
Unimolecular fragmentation patterns of N-alkoxymethylated carbostyril and phenanthridinone and their
O-alkoxymethyl isomers were studied. The main fragmentation reaction observed for the studied
compounds is the elimination of an aldehyde molecule. The main products of this reaction are the
appropriate N-methyl derivatives, but ions with other structures are also formed. This reaction is supposed
to proceed via 1,3-H shift in the alkoxymethyl group in the case of the N-alkoxymethyl derivatives and by
a multi-step mechanism for O-alkoxymethylated compounds. Another important fragmentation common
for all studied compounds is the loss of an alkyl radical from N- and O-alkoxymethyl groups, yielding the
appropriate stable isomeric cations, which, according to the results of the further fragmentation, undergo
fast equilibration reaction via an ion–neutral complex. This process is accompanied by the unusually high
kinetic energy release value. Copyright  2004 John Wiley & Sons, Ltd.
KEYWORDS: fragmentation mechanisms; ion–neutral complex; kinetic energy release; N-alkoxymethyl derivatives;
carbostyril; phenanthridinone
INTRODUCTION
.
B
S
+
OR
A
.
N
In a previous paper we have described fragmentation
patterns of N-alkoxymethyl derivatives of benzosultams and
methanesulfonanilides (1 and 2).1
O
O
N
+
+
R
O
CH2
SO2
ion-neutral complex
O
O
R
R
B
CH2O
A
N
CH2O
N
SO2CH3
+
.
R
SO2
+
.
O
N
O
R
N
1
2
S
S
R = CH3, C2H5
O
O
Scheme 1. Elimination of CH2O molecule from the molecular
ions of N-(alkoxymethyl)benzosultams involving an ion–neutral
complex.
It was demonstrated that the molecular ions of these com-
pounds undergo a unique rearrangement reaction resulting
in the loss of a formaldehyde molecule independently on
the substituent R. This fragmentation has been rationalized
by the mechanism involving an ion–neutral complex (INC)
(Scheme 1).
The crucial step in this process consists in an alkyl group
transfer from oxonium cation to one of the nucleophilic cen-
ters of the radical within INC. The ability of oxonium ions
to alkylate the appropriate nucleophilic reagents in a gas
phase is well known;2,3 however, only in a paper by Tu and
Holmes4 was a reaction mechanism similar to that proposed
by us described. Such fragmentation reactions proceeding
via INC are still weakly recognized although several exam-
ples of them were reviewed in the early 1990s.5–8 Since that
time, many articles dealing with INC have been published
but no more recent review is available. Constant interest
in INC prompted us to search for other compounds with
N-alkoxymethyl groups which should undergo fragmenta-
tion according to this mechanism. It should be noted also
that many compounds with N-alkoxymethyl groups in their
molecules are biologically active products, mainly herbi-
cides, and are manufactured on a large scale. Better under-
standing of the fragmentation patterns of these compounds
ŁCorrespondence to: Witold Danikiewicz, Institute of Organic
Chemistry, Polish Academy of Sciences, ul Kasprzaka 44/52, P.O.
Box 58, 01-224 Warsaw 42, Poland. E-mail: witold@icho.edu.pl
†Part of this work was presented during the XXI Informal Meeting
in Mass Spectrometry, Antwerp, 2003.
Contract/grant sponsor: State Committee for Scientific Research;
Contract/grant number: 4 T09A 094 22.
Copyright  2004 John Wiley & Sons, Ltd.

782
R. Szmigielski and W. Danikiewicz
will help to develop methods for their detection and quan-
titation in environmental samples. Moreover, alkoxymethyl
groups are readily applied as O- and N-protective groups in
organic synthesis,9 so information about their fragmentation
patterns is still of interest.
OCH3
N
O
N
O
N
O
OCH2CH3
OCH3
3a
3b
3c
In a previous paper,10 we presented our results of the
study of electron ionization (EI)-induced fragmentation of
N-alkoxymethyl derivatives of anilides which are analogues
of 2. It turned out that in the case when the sulfonyl
group has been replaced by the carbonyl moiety, practically
no elimination of formaldehyde took place. Instead, the
elimination of an alkyl radical was the dominant process.
In this paper, we describe our results concerning another
class of compounds bearing an N-alkoxymethyl substituent:
N-alkoxymethyl derivatives of carbostyril (1H-quinolin-2-
one) and phenanthridinone (5H-phenanthridin-6-one). The
latter are compounds of considerable interest in organic
chemistry owing to their biological activity.11,12
N
O
OCH3
N
O
O
N
OCH3
OCH2CH3
4a
4c
4b
N
O
OCH3
N
CH3
5
6
Preliminary examination of the EI mass spectra of
N-(methoxymethyl)carbostyril (3a) and N-(methoxymethyl)
phenanthridinone (4a) (see Fig. 1) revealed that the elimi-
nation of a CH2O molecule is an important fragmentation
channel. This observation prompted us to study the mech-
anism of this process in detail, particularly in terms of the
possible participation of the ion–neutral complexes. Addi-
tionally, molecular ions of 3a and 4a undergo another
competitive reaction: elimination of an alkyl radical. This
seemingly trivial process appeared also to be interesting,
because (i) it was not observed for compounds 1 and 2
(R CH3), (ii) it was accompanied by an unusually high
kinetic energy release compared with typical heterolytic
bond cleavage reactions, as proved by the mass analyzed ion
kinetic energy (MIKE) spectrum, and (iii) it proceeds in the
same manner also for O-methoxymethylated derivatives 3c
and 4c.
In order to explain this and other features of the frag-
mentation mechanisms of 3a and 4a, we decided to study
also their analogues with a different alkoxymethyl group, i.e.
N-(ethoxymethyl)carbostyril (3b) and N-(ethoxymethyl)phe-
nanthridinone (4b), and their O-methoxymethylated iso-
mers, i.e. O-(methoxymethyl)carbostyril (3c) and O-(metho-
xymethyl)phenanthridinone (4c) (Scheme 2). During the
course of this work, it was found necessary also to syn-
thesize N-methylcarbostyril (5) and 2-methoxyquinoline (6)
as model compounds. Fragmentation of compounds 5 and 6
upon EI conditions has been already described by Clugston
and MacLean,13 but we decided to study these two com-
pounds again using more modern methods.
Scheme 2. Compounds used in the study.
The purity of all compounds under study was tested by
gas chromatography/mass spectrometry (GC/MS) and their
structures were confirmed by the 1H NMR and 13C NMR
spectra. Compounds 5 and 6 were prepared according to
known methods.
N-(Methoxymethyl)carbostyril (3a)
1H NMR (500 MHz, CDCl3), υ: 3.41 (s, 3H, OCH3); 5.71 (s,
3
2H, NCH2O); 6.63 (d, 1H, J D 9.5 Hz, —CH ); 7.15–7.60
3
(m, 4H, ArH); 7.64 (d, 1H, J D 9.5 Hz, —CH ). 13C NMR
(125 MHz, CDCl3), υ: 56.2 (OCH3); 72.8 (NCH2O); 114.9 (Ar);
120.4 (Ar); 121.1 (—CH ); 122.4 (Ar); 128.3 (Ar); 130.4 (Ar);
138.8 (Ar); 139.9 (—CH ); 162.4 (CO). MS, m/z (%): 189
(MCž, 58); 174 (63); 159 (36); 158 (28); 147 (10); 146 (100);
131 (7); 130 (22); 129 (34); 128 (86); 117 (6); 116 (8); 103 (11);
102 (11); 90 (7); 89 (15); 77 (22); 76 (6); 75 (8); 63 (11); 51
(16); 50 (7); 45 (63); 39 (7). HRMS: calculated for C11H11NO2,
°
189.0767; found, 189.0789. M.p. 60–61 C (white crystals from
hexane–ethyl acetate).
N-(ethoxymethyl)carbostyril (3b)
1H NMR (400 MHz, CDCl3), υ: 1.19 (t, 3H, 3J D 6.8 Hz,
OCH2CH3); 3.67 (q, 2H, 3J D 6.8 Hz, OCH2CH3); 5.78 (s,
3
2H, NCH2O); 6.65 (d, 1H, J D 9.6 Hz, —CH ); 7.23–7.55
3
(m, 4H, ArH); 7.68 (d, 1H, J D 9.2 Hz, —CH ). 13C NMR
(100 MHz, CDCl3), υ: 15.1 (OCH2CH3); 64.4 (OCH2CH3); 71.7
(NCH2O); 115.4 (Ar); 120.6 (Ar); 121.4 (—CH ); 122.6 (Ar);
128.5 (Ar); 130.6 (Ar); 139.1 (Ar); 140.1 (—CH ); 162.7 (CO).
MS, m/z (%): 203 (MCž, 25); 174 (32); 159 (100); 158 (33); 146
(92); 145 (34); 129 (28); 128 (61); 117 (24); 103 (8); 91 (2); 90 (10);
79 (13); 77 (16); 63 (5); 59 (12); 51 (10). Elemental analysis:
calculated for C12H13NO2, C 70.93, H 6.40, N 6.89; found, C
EXPERIMENTAL
Compounds
Compounds 3a, 3c, 4a and 4c were obtained by the alkyla-
tion of carbostyril and phenanthridinone with chloromethyl
methyl ether in NaH–THF and separation of the prod-
ucts using column chromatography. N-Ethoxymethylated
derivatives 3b and 4b were prepared in the same way, using
CH3CH2OCH2Cl as an alkylating agent. Surprisingly, in
this reaction no O-ethoxymethylated isomers were formed.
°
71.11, H 6.50, N 6.81%. M.p. 34–35 C (white crystals from
hexane–ethyl acetate).
O-(Methoxymethyl)carbostyril (3c)
1H NMR (500 MHz, CDCl3), υ: 3.58 (s, 3H, OCH3); 5.70 (s, 2H,
3
OCH2O); 6.94 (d, J D 8.8 Hz, —CH ); 7.32–7.88 (m, 4H,
Copyright  2004 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2004; 39: 781–790

Fragmentation of alkoxymethylated carbostyril and phenanthridinone
783
ArH); 8.00 (d, 1H, 3J D 8.8 Hz, —CH ). 13C NMR (125 MHz,
CDCl3), υ: 57.4 (OCH3); 92.0 (OCH2O); 112.9 (—CH ); 124.3
(Ar); 125.3 (Ar); 127.3 (Ar); 127.5 (Ar); 129.5 (—CH ); 139.1
°
to 100 C. Liquid matrix secondary ion mass spectrometry
(LSIMS) was performed using a 10 keV CsC gun as a primary
ion beam source and m-nitrobenzyl alcohol (NBA) as a
matrix.
(Ar); 146.3 (Ar); 160.9 (—N
C ). IR (KBr), ꢀ: 3052, 2954,
Accurate mass measurements for all significant peaks
were performed by the narrow-range high-voltage scanning
technique at a 10 000 resolving power (10% valley definition)
using perfluorokerosene (PFK) as the reference compound.
The accuracy of mass measurements was better than 10 ppm
with the exception of a few poorly resolved peaks resulting
from ions with the same nominal mass.
2828, 1619, 1609, 1574, 1508, 1429, 1384, 1315, 1238, 1157, 1088,
981, 945, 823, 757 cmꢀ1. MS, m/z (%): 189 (MCž, 13); 174 (40);
159 (39); 158 (22); 147 (8); 146 (83); 145 (6); 131 (5); 130 (13);
129 (30); 128 (62); 117 (7); 116 (9); 102 (8); 101 (12); 89 (18); 77
(12); 76 (5); 63 (13); 62 (6); 51 (10); 50 (9); 45 (100); 39 (13). M.p.
14
°
62–63 C (white crystals from hexane–ethyl acetate) (lit.
°
66–67 C).
Fragmentation pathways were confirmed by MIKE and
B/E D constant linked scan fragment ion spectra recorded for
metastable decomposition and, when required, also for the
collision-induced dissociation (CID) products. Both MIKE
and B/E linked scan spectra were recorded using a 30 s scan
time. Eight consecutive spectra were averaged to improve
the signal-to-noise ratio. In the CID experiments helium
was used as the collision gas. The pressure in the collision
chamber was set to reduce the parent ion abundance by 50%.
N-(Methoxymethyl)phenanthridinone (4a)
1H NMR (500 MHz, CDCl3), υ: 3.44 (s, 3H, OCH3); 5.76 (s,
2H, NCH2O); 7.20–8.54 (m, 8H, ArH). 13C NMR (125 MHz,
CDCl3), υ: 56.3 (OCH3); 73.5 (NCH2O); 115.9 (Ar); 118.9 (Ar);
121.4 (Ar); 122.7 (Ar); 122.8 (Ar); 124.9 (Ar); 127.7 (Ar); 128.9
(Ar); 129.3 (Ar); 132.6 (Ar); 133.8 (Ar); 136.7 (Ar); 162.1 (CO).
MS, m/z (%): 239 (MCž, 85); 224 (85); 209 (26); 208 (22); 197
(13); 196 (100); 180 (13); 179 (31); 178 (66); 166 (17); 152 (16);
°
151 (9); 140 (8); 139 (9); 76 (6); 45 (37). M.p. 87–88 C (white
crystals from hexane–ethyl acetate) (lit.15 93–94 C).
°
Calculations
Kinetic energy release, KER(T0.5), values were calculated for
a peak width at 50% of its height and were corrected for the
width of the parent ion peak in accordance to the equation
given elsewhere.16,17 Series of three consecutive measure-
ments for two selected ions show that the reproducibility of
KER(T0.5) values is better than 4%.
Density functional theory (DFT) calculations were per-
formed using the Gaussian 98 program package.18 The DFT
B3LYP/6–31G(d) method was used for geometry optimiza-
tion and frequency calculations. ZPE corrections were scaled
by the usual factor of 0.9804. Final energy was calculated at
the B3LYP/6–311 C G(d,p) level.
N-(Ethoxymethyl)phenanthridinone (4b)
1H NMR (500 MHz, CDCl3), υ: 1.87(t, 3H, 3J D 6.8 Hz,
OCH2CH3); 3.69 (q, 2H, 3J D 6.8 Hz, OCH2CH3); 5.79 (s,
2H, NCH2O); 7.20–8.49 (m, 8H, ArH). 13C NMR (125 MHz,
CDCl3), υ: 14.9 (OCH2CH3); 64.2 (OCH2CH3); 72.1 (s, 2H,
NCH2O); 116.1 (Ar); 118.9 (Ar); 121.4 (Ar); 122.6 (Ar); 122.7
(Ar); 124.9 (Ar); 127.6 (Ar); 128.8 (Ar); 129.3 (Ar); 132.6 (Ar);
133.8 (Ar); 136.7 (Ar); 161.9 (CO). MS, m/z (%): 253 (MCž
,
25); 224 (46); 209 (85); 197 (16); 196 (100); 195 (57); 180 (9);
179 (31); 178 (88); 167 (46); 166 (37); 152 (28); 151 (23); 140
(16); 139 (17); 89 (11); 76 (14); 59 (13). Elemental analysis:
calculated for C16H15NO2, C 75.88, H 5.92, N 5.53; found, C
°
75.66, H 5.98, N 5.59%. M.p. 88–89 C (white crystals from
hexane–ethyl acetate).
RESULTS AND DISCUSSION
EI mass spectra of 3a, 3c, 4a and 4c are presented in Fig. 1.
All fragmentation reactions described in this paper were
confirmed by the fragment ion spectra, both MIKE and B/E
linked scan (metastable and, if necessary, also CID), and by
the results of accurate mass measurements of the relevant
product ions.
The fragmentation pattern of N-(methoxymethyl)carbos-
tyril (3a) is representative for all compounds under study.
The main fragmentation pathways of the molecular ion of 3a
are presented in Scheme 3.
O-(Methoxymethyl)phenanthridinone (4c)
1H NMR (500 MHz, CDCl3), υ: 3.65 (s, 3H, OCH3); 5.89 (s,
2H, OCH2O); 7.44–8.55 (m, 8H, ArH). 13C NMR (125 MHz,
CDCl3), υ: 57.6 (OCH3); 92.3 (OCH2O); 119.8 (Ar); 121.8
(Ar); 122.0 (Ar); 122.6 (Ar); 124.6 (Ar); 124.9 (Ar); 127.2 (Ar);
128.1 (Ar); 128.7 (Ar); 130.9 (Ar); 135.0 (Ar); 142.9 (Ar); 157.6
(—N
C
). MS, m/z (%): 239 (MCž, 45); 224 (61); 209 (32);
208 (20); 197 (13); 196 (100); 195 (10); 181 (5); 180 (20); 179
(36); 177 (10); 167 (9); 166 (21); 152 (12); 151 (14); 140 (14);
139 (10); 75 (5); 45 (46). HRMS: calculated for C15H13NO2,
The molecular ion of 3a decomposes along three main
pathways giving rise to abundant ions in the spectrum. We
shall concentrate on two of them because the formation of
°
239.0914; found, 239.0946. M.p. 52–53 C (white crystals from
hexane–ethyl acetate).
C
CH3OCH2 cation is a fairly trivial process and does not
require any comments.
Mass spectra
All mass spectra were recorded on an AMD-604 double-
focusing mass spectrometer with BE geometry (AMD
Intectra, Germany). Standard EI spectra were obtained under
the following conditions: electron energy 70 eV, cathode
emission current 0.5 mA, acceleration voltage 8 kV, ion
Elimination of formaldehyde and acetaldehyde
The elimination of formaldehyde (from methoxymethyl
derivatives) and acetaldehyde (from ethoxymethyl deriva-
tives) molecules opens up one of the main fragmentation
channels of the compounds under study. Interestingly,
it takes place both for N-(methoxymethyl)carbostyril (3a)
°
source temperature 200 C. Samples were introduced using
a direct insertion probe heated, when required, from 30
Copyright  2004 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2004; 39: 781–790

784
R. Szmigielski and W. Danikiewicz
100
90
146
100
90
80
70
60
50
40
30
20
10
0
196
128
178
179
O
N
80
70
60
50
40
30
20
10
0
224
45
OCH3
45
N
O
174
3a
189
OCH3
4a
239
159
77
166
151
89
140
63
60
102
39
116
63
40
80
100 120 140 160 180 200
M/z
40 60 80 100 120 140 160 180 200 220 240
M/z
100
90
80
70
60
50
40
30
20
10
0
146
100
90
80
70
60
50
40
30
20
10
0
196
178
45
OCH3
128
O
N
OCH3
N
O
3b
224
4b
159
174
239
129
209
45
179
208
189
89
63
166
152
116
140
101
77
39
51
60
76
63
40
80
100 120 140 160 180 200
M/z
40 60 80 100 120 140 160 180 200 220 240
M/z
Figure 1. 70 eV EI spectra of the molecular ions of N-(methoxymethyl)carbostyril (3a), N-(methoxymethyl)phenanthridinone (4a) and
their O-methoxymethylated isomers 3c and 4c.
+
.
.
N
O
CH2O
CH3
OCH3
3a m/z = 189 (58%)
.
C
10H9NO+
m/z = 159 (36%)
.
H
N
O
H
+
+
CO
.
CH3O
CH2
C10H9NO+
m/z = 158 (28%)
O
H
m/z = 45 (63%)
CHO
CH2O
m/z = 174 (63%)
.
CH2O
H
(CO + H2O)
.
C9H9NO+
m/z = 131 (7%)
C9H8N+
m/z = 130 (22%)
CO
H2
.
H
.
H2O
C9H7N+
C10H9NO+
m/z = 146 (100%)
C9H6N+
m/z = 129 (34%)
m/z = 128 (86%)
HCN
HCN
.
+
+
C8H5
C6H5
m/z = 102 (11%)
m/z = 101 (2%)
Scheme 3. Fragmentation pathways of the molecular ion of N-(methoxymethyl)carbostyril (3a). Main pathways are marked with bold
arrows.
Copyright  2004 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2004; 39: 781–790

Fragmentation of alkoxymethylated carbostyril and phenanthridinone
785
m/z 130 and [M ꢀ CH2O ꢀ CO]Cž at m/z 131. Comparing
the relative intensities of the peaks with m/z 128 to 131 in the
CID B/E spectra, it can be seen that they are very close for 3b
and 3c and fairly similar for 3a, except for the intensity of the
m/z 129 peak. This result looks rather surprising because, if
the mechanism presented in Scheme 4 is correct, m/z 159 ions
derived from the molecular ions of 3a and 3b should have
the same structure. Nevertheless, this moderate difference
can be attributed to different energies of the ions formed
from the different precursors.
and its O-isomer (3c). Additional hints at the mecha-
nism of this process is provided by the mass spectrum
of N-(ethoxymethyl)carbostyril (3b). In this case, an elimi-
nation of a formaldehyde molecule was not observed. The
main fragment ion, giving the most intense signal in the
EI spectrum of 3b, corresponds to the loss of acetalde-
hyde instead. The same results were obtained also for
the phenanthridinone derivatives 4a–c. These results clearly
show that, in contrast to N-(alkoxymethyl)benzosultams and
N-(alkoxymethyl)methanesulfonanilides, N-(alkoxymethyl)
carbostyril and N-(alkoxymethyl)phenanthridinone do not
undergo fragmentation involving ion–neutral complexes
because, in the case of the former, source of the formalde-
hyde fragment was the N—CH2 —O moiety in the
N-alkoxymethyl group.1
The most likely product of the aldehyde molecule
loss from the molecular ions of 3a and 3b is
N-methylcarbostyril radical cation formed by a 1,3-H
shift in the N-alkoxymethyl group as presented in
Scheme 4. A similar reaction was described by Hettler
et al. for the N-propoxymethyl derivative of saccharin19
and by us for N-(alkoxymethyl)acetanilides.10 In an anal-
ogous way, O-(methoxymethyl)carbostyril (3c) should give
2-methoxyquinoline (6) radical cation.
Comparison of the spectra shown in Fig. 2 leads to some
important conclusions. CID B/E spectra of the molecular
ions of N-methylcarbostyril (5) and 2-methoxyquinoline (6)
are different with respect to the relative intensities of the
m/z 128–131 peaks. This result indicates that under CID
conditions N- and O-methyl derivatives of carbostyril do not
isomerize to each other or to the third, common product
before fragmentation. Consequently, it is not true that the
elimination of formaldehyde from the molecular ions of
3a and 3c leads only to the molecular ions of 5 and 6,
respectively. On the other hand, CID B/E spectra of m/z 159
ions derived from the molecular ions of 3a–c are all much
more similar to the spectrum of N-methylcarbostyril (5) than
the spectrum of 2-methoxyquinoline (6). One of the possible
rationalizations is that, indeed, loss of an aldehyde molecule
from the molecular ions of N-(alkoxymethyl)carbostyrils 3a
and 3b results in the formation of N-methylcarbostyril (5),
at least as the main product, according to the mechanism
shown in Scheme 4. Other isomeric ions with m/z 159 can
also be formed and are responsible for increased abundance
of the m/z 129 fragment compared with the spectrum of 5.
Because the CID B/E spectrum of the m/z 159 ion derived
from the molecular ion of O-(methoxymethyl)carbostyril (3c)
is also very similar to the spectrum of N-methylcarbostyril
(5), this compound, among others, can also be a product of
the CH2O elimination from 3c. Several mechanisms of this
reaction, each requiring at least two hydrogen shift processes,
can be proposed but on the present stage of the study they are
purely speculative. We have started a series of calculations
which should help to identify possible intermediates but the
results are far from completion.
.
+
.
+
– R-CHO
N
O
N
O
R
CH3
O
5
H
H
3a (R = H)
3b (R = CH3)
.
+
.
+
–CH2O
N
OCH3
N
O
6
3c
O
H
H
H
Concluding, both N- and O-alkoxymethyl derivatives of
carbostyril undergo elimination of an aldehyde molecule
from the terminal OR group resulting in the formation of N-
methylcarbostyril as the main product. CID B/E spectra
show that other isomeric product ions are also formed
because the relatively intense m/z 129 peak which is present
in these spectra is not produced in such yield by the
fragmentation of N-methylcarbostyril. The same results were
obtained for the analogous phenanthridinone derivatives.
Scheme 4. Mechanism of the formation of the radical cation of
N-methylcarbostyril (5) from the molecular ions of N-(alkoxyme-
thyl)carbostyrils 3a and 3b and formation of the radical
cation of 2-methoxyquinoline (6) from the molecular ion of
O-(methoxymethyl)carbostyril (3c).
Further fragmentation of the m/z 159 ions derived
from 3a–c and molecular ions of N-methylcarbostyril
(5) and 2-methoxyquinoline (6) shows that the situation
is not so simple. The CID B/E spectra of these ions
(Fig. 2) are similar to each other, but with some important
differences. In all cases the m/z 158 ion was the main
fragment but much more diagnostic are four ions between
m/z 128 and 131 (see insets in Fig. 2). According to the
accurate mass measurement results, these ions have the
following formulae: [M ꢀ CH2O ꢀ CH3O]C at m/z 128,
[M ꢀ CH2O ꢀ CH2O]C at m/z 129, [M ꢀ CH2O ꢀ CHO]C at
Elimination of an alkyl radical
For all compounds under study, elimination of an alkyl
radical is the dominant process, under both high-energy and
metastable conditions (Figs 1 and 3).
This process appeared to be interesting mainly because it
is accompanied by an unusually high KER(T0.5) value as for
simple heterolytic bond cleavage (Table 1).
For instance, the measured value of KER(T0.5) for
N-(methoxymethyl)carbostyril (3a) was 424 meV and for
Copyright  2004 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2004; 39: 781–790

786
R. Szmigielski and W. Danikiewicz
.
+
.
+
– CH2O
O
129
130
130
N
N
O
O
5
3a
158
158
129
128
130
131
2.5
2.0
1.5
1.0
0.5
0.0
0.25
0.20
0.15
0.10
0.05
0.00
131
128
129
132
127
127
126 128 130 132 134
126 128 130 132 134
102
103
141
89
146
116
77
77
89
124
116
64
62
60
70
80
90 100 110 120 130 140 150 160
60
70
80
90 100 110 120 130 140 150 160
.
+
.
+
– CH2O
O
129
130
128
130
128
O
O
N
N
131
6
158
3c
158
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
130
129
30
25
20
15
10
5
129
127
131
146
141
126 128 130 132 134
126 128 130 132 134
103
102
143
116
77
89
115
77
89
0
60
70
80
90 100 110 120 130 140 150 160
60
70
80
90 100 110 120 130 140 150 160
.
+
130
– CH3CHO
O
128
N
131
158
O
130
3b
129
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
132
127
126 128 130 132 134
103
115
77
89
141
124
63
60
70
80
90 100 110 120 130 140 150 160
Figure 2. CID B/E spectra of the m/z 159 ions derived from different sources.
O-(methoxymethyl)carbostyril (3c) 445 meV, whereas for
elimination of the same radical from the molecular ion of
acetophenone under the same conditions it is just 16 meV.
These findings indicate that the rearrangement of the
molecular ions of the presented compounds will to occur
just before the C—O bond cleavage. Moreover, comparison
of the CID-MIKE spectra of [M ꢀ CH3]C ions at m/z 174,
formed from 3a and 3c, showed that they are identical
within the experimental error. In particular, the half-widths
of the m/z 146 peaks resulting from the CO elimination
are almost identical, indicating the same KER values for
this process. This means that both [M ꢀ CH3]C ions are
likely to have the same structure. In order to rationalize the
observed results, we propose the fragmentation mechanism
in Scheme 5, which is analogous in part to one described in
a previous paper.10
After the 1,4-H shift, the heterolytic cleavage of the
O—CH3 bond has takes place for both molecular ions
Copyright  2004 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2004; 39: 781–790

Fragmentation of alkoxymethylated carbostyril and phenanthridinone
787
189
239
224
174
N
O
N
O
OCH3
OCH3
3a
4a
159
196
178
209
146
140
128
166
152
140
128
118
112
89 101
100
77
60
80
120
160
180
60
80
100 120 140 160 180 200 220 240
M/z
M/z
239
224
189
174
OCH3
N
O
N
O
OCH3
3b
4b
159
178
195
208
145
209
128
166
151
89
101
100
118
77
140
124
112
60
80
120
M/z
140
160
180
60
80
100 120 140 160 180 200 220 240
M/z
Figure 3. MIKE spectra of the molecular ions of N-(methoxymethyl)carbostyril (3a), N-(methoxymethyl)phenanthridinone (4a) and
their O-methoxymethylated isomers 3c and 4c.
Table 1. KER(T0.5) values for the alkyl radical elimination
under metastable fragmentation conditions from the
molecular ions of all compounds under study, with the
KER(T0.5) value for the methyl radical elimination from
the molecular ion of acetophenone recorded under the
same conditions added for comparison (see text)
reverse process, require [1,3] shifts of the formyl group and
a proton. Possible routes for these reactions are presented in
Scheme 6.
The first step involves the presence of an ion–neutral
complex of 2-hydroxyquinoline and [H—C O]C cation,
which is formed as the result of heterolytic cleavage of
the initial ion 3a. Within this complex on formylation
process can occur, yielding the appropriate cation of the
protonated 2-quinolyl formate. In the subsequent step, a
1,3-H shift occurs, leading directly to the cation 8. The
reverse sequence of reactions can lead from ion 8 to 7.
The same rationalization can be applied to explain the
elimination of the methyl radical from molecular ions of
N- and O-(methoxymethyl)phenanthridinones 4a and 4c.
We tried to select the most probable reaction path
by modeling ions 7 and 8 and possible transition states,
including ion–neutral complexes shown in Scheme 6. Using
the B3LYP/6–311CG(d,p)//B3LYP/6–31G(d) DFT method,
we found that ion 8 is about 9 kcal molꢀ1 (1 kcal D 4.184 kJ)
more stable than its N-formyl isomer 7. Unfortunately, our
attempts to model the transition states between the structures
7 and 8 have been unsuccessful so far, so we are not
able at present to estimate the activation energy required
for interconversion of these ions. Preliminary calculations
showed that it should be possible to model the respective
KER (T0.5
(meV)
)
ž
MCž ꢀ R ! (R D CH3, C2H5)
3a N-(methoxymethyl)carbostyril
3b N-(ethoxymethyl)carbostyril
3c O-(methoxymethyl)carbostyril
4a N-(methoxymethyl)phenanthridinone
4b N-(ethoxymethyl)phenanthridinone
4c O-(methoxymethyl)phenanthridinone
Acetophenone
424
411
445
450
427
386
16
of 3a and 3c to give highly stabilized cations 7 and 8
at m/z 174, respectively. According to what was written
above, these two ions must rearrange to one common
structure before further fragmentation. There are three
most likely possibilities: (i) structure 7, (ii) structure 8 or
(iii) another structure to which both ions, 7 and 8 ions
can rearrange. Transformation of 7 into 8, and also the
Copyright  2004 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2004; 39: 781–790

788
R. Szmigielski and W. Danikiewicz
.
CH3
+
.
N
+
1,4-H shift
O+
H
N
O
H
N
O
.
H
O
O
O
H
7
CH3
CH3
?
3a
.
CH3
+
.
N
+
1,4-H shift
OCH3
O
N
O
N
O
+
.
H
H
O
H
O
H
CH3
3c
8
Scheme 5. Postulated mechanism of the formation of the isomeric cations 7 and 8 by elimination of the methyl radical from N- and
O-methoxymethyl groups of the molecular ions of 3a and 3c.
O
+
+
N
O
H
O
N
O
H
..
N
O
H
..
H
+
H
C
H
O
7
Ion-neutral complex 1
1,3-H shift
1,3-H shift
1,3-H shift
+
+
O
N
H
N
H
O
N
O
H
H
O
H
+
O
H
C
O
8
Ion-neutral complex 2
Scheme 6. Possible mechanisms of the mutual transformation of the isomeric cations 7 and 8 by subsequent [1,3] shifts of the formyl
cation and a proton. Formyl cation shift is supposed to proceed through an ion–neutral complex.
– CO
H+
O
N
+
PathA
N
+
N
H
OH
+
+
H
O
H
OH
N
OH
– CO
H
O
?
Path B
7
– CO
H+
+
+
+
N
H
O
N
H
OH
O
N
H
O
H
10
8
Scheme 7. Possible competitive pathways of the extrusion of CO molecule from the [M ꢀ CH3]C ions of the general structure 7
and 8 derived from the molecular ions of isomeric N- and O-(methoxymethyl)carbostyrils (3a and 3b). The experiments with model
compounds 9 and 10 show that path A is not operative (see text).
Copyright  2004 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2004; 39: 781–790

Fragmentation of alkoxymethylated carbostyril and phenanthridinone
789
146
(a)
128
[M - R - CO]+
CID
m/z = 146
89
117
101
75
51
63
3a R = CH3
3b R = CH3CH2
3c R = CH3
40
50
60
70
80
90
100
110
120
130
140
150
M/z
(b)
(c)
146
146
+
+
128
CID
H
+
CID
+
H
N
145
N
H
O
118
O
H
m/z = 146
m/z = 146
117
89
89
73
101
90
63
75
63
51
101
50
40 50 60 70 80 90 100 110 120 130 140
M/z
40 50 60 70 80 90 100 110 120 130 140
M/z
150
150
Figure 4. (a) CID-MIKE spectra of [M ꢀ CH3 ꢀ CO]C ions at m/z 146, resulting from the molecular ions of N-(methoxymethyl)
carbostyril (3a), N-(ethoxymethyl)carbostyril (3b) and O-(methoxymethyl)carbostyril (3c). For comparison there are given
CID-MIKE-LSIMS(C) spectra of (b) the protonated carbostyril 10 and (c) protonated N-formylindole 9.
phenanthridinone), then [M ꢀ CH3 ꢀ CO]C cation should
transition states but the number of possible conformations to
be analyzed is very high. The results of this theoretical study
will be published upon completion.
have the structure of the protonated N-formylindole 9 (and
carbazole) (path A in Scheme 7).
Further decomposition of the [M ꢀ CH3]C ion consists
of the extrusion the CO molecule (Scheme 3). At this point,
the question concerning the structure of [M ꢀ CH3 ꢀ CH3 ꢀ
CO]C arises. It is well documented,4 that both unsubsti-
tuted molecules of carbostyril and phenanthridinone and
their N-alkylated derivatives readily expel a CO molecule
to yield indole and carbazole, respectively. If the same
process takes place for N-(methoxymethyl)carbostyril (and
On the other hand, if the same reaction takes place
on a carbonyl group attached to the nitrogen (ion 7) or
oxygen (ion 8) atom in the form of the formyl substituent
(path B in Scheme 7), then the protonated carbostyril cation
10 is expected. In order to confirm which hypothesis
is correct, we recorded CID-MIKE LSIMS(C) spectra of
the protonated carbostyril (10) and N-formylindole (9)
(Fig. 4).
Copyright  2004 John Wiley & Sons, Ltd.
J. Mass Spectrom. 2004; 39: 781–790

790
R. Szmigielski and W. Danikiewicz
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of onium ions and related species. Org. Mass Spectrom. 1993; 28:
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9. Greene TW, Wuts PGM. Protective Groups in Organic Synthesis
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13. Clugston DM, MacLean DB. Mass Spectra of oxygenated
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Comparison of these spectra with the shapes of the
respective CID-MIKE spectra recorded for [M ꢀ CH3 ꢀ CO]C
ions of 3a–c unambiguously shows that the latter are likely to
have the structure of the protonated carbostyril. This result,
however, does not give an answer to the question concerning
the structure(s) of the ion(s) resulting from an alkyl group
elimination from the molecular ions of compounds 3a–c.
Protonated carbostyril (10) can be formed from both ions 7
(along path B in Scheme 7) and 8, so the question of their
mutual interconversion is still open.
Acknowledgments
This work was financed by the State Committee for Scientific
Research, Grant No. 4 T09A 094 22. The authors would like to
express their thanks to Professor Dietmar Kuck of the University of
Bielefeld, Germany, for valuable discussions.
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K
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Abe H, Takeuchi Y. Synthesis of trisphaeridyne and
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