Chemistry and stereochemistry of benzyl-benzyl interactions in MH+ ions of dibenzyl esters upon chemical ionization and collision-induced dissociation conditions

Article abstract of DOI:10.1002/(SICI)1096-9888(199705)32:5<515::AID-JMS504>3.0.CO;2-B

Isobutane chemical ionization mass spectra of dibenzyl esters of a wide variety of aliphatic, olefinic, alicyclic and aromatic dicarboxylic acids exhibit abundant m/z 181 C₁₄H₁₃⁺ ions, indicating a highly general rearrangement process involving the formation of a new bond between the two benzyl groups. An extensive collision-induced dissociation and deuterium labeling study suggested that these ions are an almost equimolar mixture of isomeric α-o-tolylbenzyl, α-p-tolylbenzyl and p-benzylbenzyl cation structures, and this composition is identical for all the diesters examined. This structural assignment of the C₁₄H₁₃ ions suggests a mechanistic pathway for their generation, based on the formation of the new bond between the benzyl methylene group of the protonated benzoxycarbonyl and the phenyl ring of the other ester moiety via π- (and/or ion-neutral) and α-complexes. Stereoisomeric diesters show an unusual steric effect: trans-isomers give rise to much more abundant C₁₄H₁₃⁺ ions than the cis counterparts. This behavior is explained by stabilized proton-bridged structures of the MH⁺ ions of the cis-isomers.

Full text of DOI:10.1002/(SICI)1096-9888(199705)32:5<515::AID-JMS504>3.0.CO;2-B

JOURNAL OF MASS SPECTROMETRY, VOL. 32, 515È524 (1997)
Chemistry and Stereochemistry of Benzyl–Benzyl
Interactions in MH‘ Ions of Dibenzyl Esters upon
Chemical Ionization and Collision-induced
Dissociation Conditions
M. Edelson-Averbukh and A. Mandelbaum*
Department of Chemistry, TechnionÈIsrael Institute of Technology, Haifa, Israel
Isobutane chemical ionization mass spectra of dibenzyl esters of a wide variety of aliphatic, oleÐnic, alicyclic and
aromatic dicarboxylic acids exhibit abundant m/z 181 C H‘ ions, indicating a highly general rearrangement
1
4 13
process involving the formation of a new bond between the two benzyl groups. An extensive collision-induced
dissociation and deuterium labeling study suggested that these ions are an almost equimolar mixture of isomeric
a-o-tolylbenzyl, a-p-tolylbenzyl and p-benzylbenzyl cation structures, and this composition is identical for all the
diesters examined. This structural assignment of the C H‘ ions suggests a mechanistic pathway for their gener-
14 13
ation, based on the formation of the new bond between the benzyl methylene group of the protonated benzoxycarb-
onyl and the phenyl ring of the other ester moiety via p- (and/or ion–neutral) and r-complexes. Stereoisomeric
diesters show an unusual steric e†ect: trans-isomers give rise to much more abundant C H‘ ions than the cis
14 13
counterparts. This behavior is explained by stabilized proton-bridged structures of the MH‘ ions of the cis-isomers.
1997 by John Wiley & Sons, Ltd.
(
J. Mass Spectrom. 32, 515È524 (1997)
No. of Figures: 22 No. of Tables: 2 No. of References: 32
KEYWORDS: benzylÈbenzyl interactions; benzyl esters; rearrangement upon chemical ionization; stereochemical e†ects;
mechanism of fragmentation
step occurred in the [M [ H]` ion, resulting in a ben-
INTRODUCTION
zyltropylium structure.2
A similar rearrangement has been observed under
chemical ionization (CI) conditions in dibenzyl ether,
which gave rise to an abundant m/z 181 ion.4 The
observation of the m/z 181 ion in the high-pressure CI
mass spectral measurements of benzyl acetate has been
interpreted in terms of an ionÈmolecule reaction,
involving an initial carbonylÈcarbonyl interaction fol-
lowed by a rearrangement step, wherein a benzylÈ
benzyl CÈC bond formation took place.5,6
Skeletal rearrangements of gas-phase ions have been of
great interest since the early days of organic mass spec-
trometry. One of the early examples of such processes
was the formation of a C H` ion from the molecular
ion of tribenzylamine under electron impact (EI) ioniza-
tion, which must involve the formation of a bond
between two benzyl groups, which were not connected
in the original molecule (Scheme 1).1 The structure and
mechanism of formation of this ion have not been
explored in this study.
14 13
We have been interested for a long time in various
interactions between functional groups resulting in
stereochemical e†ects in gas-phase ions,7,8 and more
speciÐcally in proton bridging and proton transfer pro-
cesses between the protonated and free alkoxycarbonyl
groups in MH` ions of stereoisomeric diesters under CI
conditions.9h11 In this context, it was of interest to
examine the CI behavior of benzyl esters of dicarboxylic
acids, which could be expected to exhibit an analogous
benzylÈbenzyl interaction. Preliminary measurements
showed, that the above rearrangement indeed takes
place in such esters under CI conditions, a†ording
abundant m/z 181 ions. The objective of this work was
to explore the generality of this behavior, the steric
requirements of this rearrangement, the structure of the
resulting C H` ion(s) and the mechanism of this
EI
(
C H CH ) N ÈÈÈ
Õ
M`~ ÈÈÈ
Õ
C H`
14 13
6
5
2 3
m/z 181
Scheme 1
Analogous rearrangements leading to the m/z 181
C H` ion were reported later in the EI mass spectra
14 13
of N-nitrosodibenzylamine2 and o- and m-
dibenzoxybenzenes.3 In the former case it has been pro-
posed that the benzylÈbenzyl CÈC bond formation
*
Correspondence to: A. Mandelbaum.
Contract grant sponsor: Fund for Promotion of Research at the
14 13
Technion.
process.
CCC 1076È5174/97/050515È10 $17.50
Received 6 January 1997
Accepted 21 January 1997
(
1997 by John Wiley & Sons, Ltd.

5
16
M. EDELSON-AVERBUKH AND A. MANDELBAUM
ions in the CI mass spectra of benzyl diesters was to
RESULTS AND DISCUSSION
assure structural identity of these ions, when formed
from the variety of precursors 1È27. This goal was
achieved by measuring the CID spectra of these m/z 181
ions. The close similarity of the CID spectra indicate
identity of the C H` ions obtained from diesters 1È27.
Isobutane CI mass spectra of dibenzyl esters 1È27 of a
wide variety of aliphatic, oleÐnic, alicyclic and aromatic
dicarboxylic acids were measured, and the abundant
ions in these mass spectra are listed in Table 1. All the
mass spectra exhibit abundant m/z 181 ions, suggesting
a high degree of generality of the rearrangement process
resulting in the C H` ions. Collision-induced disso-
1
4 13
The C H` ion contains all the 14 C atoms of the
1
4 13
two benzyl groups, but only 13 H atoms. The question
of the origin of the missing fourteenth hydrogen atom
could be easily solved by deuterium labeling. The iso-
1
4 13
ciation (CID) spectra of the MH` ions of diesters 1È27
butane CI mass spectra of the bis(a,a-d -benzyl) ana-
2
logues of cis- and trans-19 (d -19) and the CID spectra
4
(
listed in Table 2) also exhibit the m/z 181 ion as the
major product ions. These ions are the most abundant
fragments in the majority of the examined compounds,
and in a number of cases (13È16) competing processes
are negligible or absent in the CI and CID mass spectra,
with the exception of the m/z 91 C H ` ion, which is
of their MH` ions clearly indicate exclusive abstraction
of a hydrogen atom from one of the phenyl rings in the
course of the rearrangement process (Scheme 2). The
resulting m/z 185 ions contain all the four benzylic deu-
terium atoms both under CI and CID conditions.
7
7
present in all the spectra. The results of both CI and
CID measurements indicate high generality for the
rearrangement process of dibenzyl esters leading to the
C H` ions.
The exclusively unimolecular nature of the rearrange-
ment under CI conditions was established by exami-
nation of the isobutane CI mass spectrum of the mixed
benzyl p-xylyl diester of adipic acid (28) (Scheme 3 and
Fig. 1). The exclusive formation of the m/z 195 C H`
1
4 13
A related competing rearrangement process results in
15 15
[
MH [ 180]` ions, which are of considerable abun-
ion (no m/z 181 ion was detected) clearly indicates the
absence of a contribution of an ionÈmolecule reaction
in this process.
dance in the CI and CID mass spectra of many of the
diesters examined. These protonated diacid cations are
formed by the loss of a neutral C H molecule from
Examination of the CID spectra of MD` ions,
obtained under CD CI and CD CN CI conditions,
1
4 12
the MH` ions, which also indicates formation of a new
bond between the two benzyl groups. Elimination of
benzyl alcohol and of dibenzyl ether are additional
competing reactions, observed in a large number of the
diesters that have been investigated.
4
3
Mechanism
The Ðrst point in the search for a mechanism of the
rearrangement leading to the formation of the C H`
Scheme 2
1
4 13
Scheme 3
(
1997 by John Wiley & Sons, Ltd.
JOURNAL OF MASS SPECTROMETRY, VOL. 32, 515È524 (1997)

BENZYLÈBENZYL INTERACTIONS IN DIBENZYL ESTERS
517
Table 1. Isobutane CI mass spectral data (relative abundance, %) of dibenzyl
esters 1–27
Sample
MH½
m/z 181
ÍMH É180˽
ÍMH ÉPhCH OH˽
2
ÍMH ÉBnOBn˽
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
2
47
50
70
56
13
21
5
100
100
100
100
100
100
100
100
100
100
100
100
29
—a
24
18
11
26
26
21
12
12
7
Ä0.1
Ä0.1
2
—a
—a
32
34
87
32
37
38
56
32
63
13
63
7
17
62
1
1
1
1
1
1
1
8
22
74
3
3
8
46
2
4
0.5
Ä0.1
Ä0.1
1
42
100
3
—a
—a
—a
—a
Ä0.1
3
—a
—a
—a
—a
62
100
23
100
4
100
21
Ä0.1
40
cis-17
trans-17
cis-18
100
84
100
13
70
6
63
38
100
56
20
Ä0.1
2
63
trans-18
cis-19
100
100
100
100
100
5
2
40
9
5
16
trans-19
cis-20
15
9
Ä0.1
40
Ä0.1
80
28
2
trans-20
cis-21
52
Ä0.1
50
5
Ä0.1
87
Ä0.1
100
Ä0.1
8
6
trans-21
Ä0.1
100
2
100
13
Ä0.1
8
22
23
24
25
26
27
100
100
100
100
100
28
1
5
2
2
6
1
Ä0.1
7
82
30
35
Ä0.1
6
22
5
3
2
—b
a The m/z values of these fragments are below the range of the CI measurement (¿m/z
00).
b The m/z value of this fragment (181) coincides with that of the C H½ ion (second
1
1
4 13
column in this Table).
showed that the external proton (or deuteron) is not
The distance between the two benzylic C atoms of the
benzoxycarbonyl groups in trans-19, trans-20, trans-21,
24, 25, 26 and 27 is also too large to allow bond forma-
tion between these two carbon atoms leading to ion a
via the mechanism shown in Scheme 6. Comparison of
the CID spectrum of the C H` ion obtained by the
transferred to the C H` ion in the course of the
14 13
rearrangement.
A simple mechanistic pathway that may be proposed
for the rearrangement, based on anchimerically assisted
transfer of a benzyl cation from the protonated ben-
zoxycarbonyl group to the carbonyl oxygen of the other
ester group, is shown in Scheme 4. The dibenzoxy car-
bocation A may be expected to a†ord the m/z 181 ion
based on precedents discussed in the Introduction. This
expectation is supported by the CID spectrum of the
m/z 241 1,1-dibenzoxyethyl carbocation B, obtained
from the dibenzyl ketal of acetone (2,2-dibenzoxypro-
pane) 29 under EI (Scheme 5). However, the high abun-
dances of the C H` ions in the CI mass spectra of
14 13
rearrangement from cis- and trans-19 (Fig. 2) with that
obtained from 1-bromo-1,2-diphenylethane (30) upon
isobutane CI (Fig. 3) is also incompatible with this
mechanism. These two CID spectra exhibit entirely dif-
ferent dissociation patterns, clearly showing that the
C H` ion, obtained from dibenzyl esters by the
1
4 13
rearrangement process, does not have the structure of
1,2-diphenylethyl carbocation (ion a), which is obtained
by CÈBr bond cleavage from 30 (see Scheme 7). This
mechanism is also inconsistent with the above results of
measurements of bis(a,a-d -benzyl) esters d -cis-19 and
14 13
trans-19, trans-20, trans-21, 24, 25, 26 and 27, where the
distances between the carbonylic oxygen of one ben-
zoxycarbonyl group and the benzylic C atom of the
other are large, are inconsistent with this mechanism.
Anchimeric assistance is sensitive to the distance
between the internal nucleophile and the positive center
of the substitution.7,12,13
2
4
d -trans-19.
4
Inspection of molecular models as well as semi-
empirical calculations14 show that interaction between
the two phenyl groups in the MH` ions of dibenzyl
esters is sterically possible even in 25, 26 and 27, where
(
1997 by John Wiley & Sons, Ltd.
JOURNAL OF MASS SPECTROMETRY VOL. 32, 515È524 (1997)

5
18
M. EDELSON-AVERBUKH AND A. MANDELBAUM
Table 2. CIDa mass spectral datab,c of MH‘ ions obtained from dibenzyl esters
upon CI
Sample
m/z 181
ÍMHÉ180˽
ÍMHÉPhCH OH˽
2
ÍMHÉBnOBn˽
m/z 91
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
38.3
42.1
54.3
38.5
53.1
58.7
74.5
52.7
45.1
57.6
53.6
62.1
31.7
55.7
35.3
55.6
35.0
38.0
10.3
66.5
59.8
81.5
36.1
41.9
Ä0.1
22.0
42.0
44.8
—d
3.8
Ä0.1
Ä0.1
2.1
Ä0.1
Ä0.1
Ä0.1
3.4
50.6
54.1
41.3
49.1
35.2
31.5
10.6
31.2
13.9
12.0
3.5
2.3
3.3
5.7
3.2
3.7
2.8
5.5
0.3
2.9
0.8
Ä0.1
Ä0.1
Ä0.1
4.0
5.7
1.5
8.1
21.7
6.9
14.7
8.9
1
1
1
1
1
1
1
Ä0.1
Ä0.1
3.8
Ä0.1
Ä0.1
Ä0.1
Ä0.1
1.9
36.1
14.4
Ä0.1
Ä0.1
1.6
6.8
64.5
33.9
53.9
28.6
40.9
23.2
32.6
19.9
29.2
Ä0.1
14.3
11.9
11.1
54.1
22.5
25.0
0.4
3.0
1.0
Ä0.1
35.0
26.8
55.5
1.6
Ä0.1
3.9
cis-17
0.7
trans-17
cis-18
2.9
9.1
0.1
1.5
trans-18
cis-19
1.4
10.6
5.9
1.5
3.7
trans-19
cis-20
8.7
Ä0.1
32.9
Ä0.1
88.9
16.4
6.2
Ä0.1
5.4
1.0
trans-20
cis-21
22.4
Ä0.1
2.1
Ä0.1
Ä0.1
2.6
2
2
2
2
6
7
10.4
10.0
1.2
1.8
—e
a 30 eV collision energy.
b The ion abundances are listed as percentages of the total product ion current (%S).
c CID spectra of trans-21, 23, 24 and 25 were not measured because of low abun-
dance or absence of MH½ ions in the isobutane CI mass spectra.
d The m/z value (91) of this fragment coincides with the C H ½ ion (last column in
this Table).
7
7
e The m/z value (181) of this fragment coincides with the C H½ ion.
1
4 13
the two benzoxycarbonyl groups are most distant. Such
an interaction could lead to formation of isomeric tolyl-
benzyl cations b (Scheme 8). Here again, CID study
involving the three isomeric a-bromoditolyls 31È33 dis-
proves this proposed pathway. The CID spectra of the
m/z 181 [M [ Br]` ions, obtained upon isobutane CI
from 31È33 (Fig. 4), show di†erent fragmentation pat-
terns than that of the C H` ion obtained by
1
4 13
rearrangement of dibenzyl esters (compare Fig. 2).
The failure of the above two attempts to propose a
mechanistic pathway for the rearrangement of dibenzyl
esters, based on interaction between the two benzylic
methylenes and between the two phenyl rings, leaves the
third possibility, i.e. interaction of the benzylic C atom
of one benzyl group with the phenyl ring of the other, as
the basis of the mechanism of formation of the C H`
1
4 13
ion from the diesters upon CI. Such a pathway would
result in one or more of the six possible isomeric a-
tolybenzyl and benzylbenzyl cation structures cÈh
shown in Scheme 9 for the m/z 181 C H` ion(s). CID
14 13
spectra were measured for the m/z 181 ions obtained by
isobutane CI-induced fragmentation of the six isomeric
benzylbenzyl alcohols 34È36 and a-tolylbenzyl alcohols
3
7È39 in order to solve this structural problem.
The CID spectra of the m/z 181 ions obtained from
Figure 1. Isobutane CI mass spectra of (a) dibenzyl adipate (5)
and (b) mixed benzyl p-xylyl diester of adipic acid (28).
the six model compounds 34È39 are shown in Fig. 5.
(
1997 by John Wiley & Sons, Ltd.
JOURNAL OF MASS SPECTROMETRY, VOL. 32, 515È524 (1997)

BENZYLÈBENZYL INTERACTIONS IN DIBENZYL ESTERS
519
Scheme 4
The CID spectra exhibit qualitative similarity, but they
show signiÐcant, although not large, di†erences in the
relative abundances of the product ions. The spectra of
the m/z 181 ions obtained from the two ortho-alcohols
Figure 3. CID spectrum (50 eV collision energy, relative ion
abundances (%) normalized to the most abundant product ion) of
the m/z 181 ÍM ÉBr˽ ion, obtained upon isobutane CI from 1-
bromo-1,2-diphenylethane (30).
3
4 and 37 are identical, suggesting an identical struc-
ture. The lower heat of formation of ion f [227.9 kcal
mol~1 (1 kcal \ 4.184 kJ), calculated by AM1] than
that of ion c (238.5 kcal mol~1) suggests structure f for
the ion obtained from both 34 and 37.
Visual comparison of the CID spectra of the C H`
14 13
ion obtained by rearrangement of the dibenzyl esters
e.g. of trans-19, see Fig. 2) with those of 34È39 shows
(
qualitative similarity, but no single spectrum of the six
model compounds indicated identity with that of the
ion in question. A computer-aided analysis has been
performed in order to determine an optimum com-
bination of CID data of the models that would Ðt the
CID spectrum of the rearrangement product. The latter
spectrum (see Fig. 2) is superimposable with that of the
calculated CID spectrum of a mixture of three C H`
1
4 13
ions e (35%), f (35%) and h (30%), shown in Fig. 6. The
structural conclusion of this analysis is shown in
Scheme 10.
Figure 2. CID spectrum (50 eV collision energy, relative ion
abundances (%) normalized to the most abundant product ion) of
the m/z 181 C14H½ ion, obtained by rearrangement from trans-19.
13
Scheme 5
Scheme 6
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1997 by John Wiley & Sons, Ltd.
JOURNAL OF MASS SPECTROMETRY VOL. 32, 515È524 (1997)

5
20
M. EDELSON-AVERBUKH AND A. MANDELBAUM
Scheme 7
Scheme 8
A plausible mechanism proposed for the rearrange-
ment process, based on the above analysis of the CID
spectra, is outlined in Scheme 11. The initial step in this
mechanism is the transfer of the benzyl group from the
protonated benzoxycarbonyl moiety to the phenyl ring
of the other ester group, resulting in the intermediate
benzyl n-complex C.15h20 The formation of the n-
complex C may be preceded by an ionÈneutral
complex21h24 shown in Scheme 12. The contribution of
the ionÈneutral complex may be of particular impor-
tance in cases where the distance between the two ben-
zoxycarbonyl groups is large. The n-complex C
rearranges to a mixture of two p-complexes D and D
1
2
with the benzyl substituent at the ortho and para posi-
tions, which in turn undergo proton transfer from the
benzenium moieties to the carbonyl oxygen atoms,
possibly with the intermediacy of the proton n-
complexes E and E (at least in D , where the hydro-
1
2
1
gen atom is at position 4 relative to the ester moiety).
The p-complexes D and D may be directly formed
1
2
from the above ionÈneutral complex.
An additional hydrogen migration is necessary to
a†ord the a-(o-tolyl)- and a-(p-tolyl)benzyl cations f and
h. A mechanistic pathway for the formation of ion f is
proposed in Scheme 13. Formation of ion h requires an
additional rearrangement step, probably involving
intermediacy of another proton n-complex.
Figure 4. CID spectra (50 eV collision energy, relative ion abun-
dances (%) normalized to the most abundant product ion) of the
m/z 181 C H½ ions obtained upon isobutane CI from (a) 31, (b)
1
4 13
3
2 and (c) 33.
Semi-empirical calculations (AM1) indicate signiÐ-
cantly di†erent enthalpies of formation for the three
product ions of the rearrangement: highest for p-
benzylbenzyl cation e (236.1 kcal mol~1), lower for a-(o-
tolyl)benzyl ion f (227.9 kcal mol~1) and lowest for a-(p-
tolyl)benzyl ion h (222.1 kcal mol~1). Formation of
mixture of these three ions under CI conditions indi-
cates the presence of an energy barrier, which does not
allow rearrangement to the most stable structure h in
the time regime of the mass spectral experiment.
The intermediacy of the benzyl n-complex C and/or
possibly the ionÈneutral complex in the mechanism of
formation of ions e, f and h from benzyl diesters
(Schemes 11 and 12) Ðnds support in the behavior of the
di-2,4,6-trimethylbenzyl esters cis- and trans-40. The
low thermal stability of these esters did not allow the
measurement of their CI mass spectra, and fast atom
bombardment (FAB) ionization was used to obtain
their MH` ions. The relatively low abundance m/z 265
C H` ion observed in the CID spectra of the MH`
20 25
Scheme 9
(
1997 by John Wiley & Sons, Ltd.
JOURNAL OF MASS SPECTROMETRY, VOL. 32, 515È524 (1997)

BENZYLÈBENZYL INTERACTIONS IN DIBENZYL ESTERS
521
Figure 5. CID spectra (50 eV collision energy, relative ion abundances (%) normalized to the most abundant product ion) of the m/z 181
H½ ions obtained upon isobutane CI from (a) 34, (b) 35, (c) 36, (d) 37, (e) 38 and (f) 39.
C
1
4 13
ions of 40 indicates the operation of a rearrangement
process analogous to that of dibenzyl esters, despite the
expected steric e†ect of the methyl groups at positions 2
and 6. The occurrence of the rearrangement in the
MH` ions of 40 suggests the intermediacy of an ionÈ
neutral complex or of n-complex F, which should be
undisturbed by the six methyl groups, and should result
in the p-complex G by substitution at the meta position,
leading to the m/z 265 C H` ion i (Scheme 14). The
preference for a n- over a p-complex in sterically hin-
dered systems has been proposed previously.15h17
Stereochemistry of the rearrangement
The C H` rearrangement ions are major fragment
14 13
20 25
ions under CI and CID conditions also in dibenzyl
esters with remote benzoxycarbonyl groups, such as
trans-19, trans-20, trans-21, 24, 25, 26 and 27 (see Tables
1
and 2). It has been stated above that interaction
between the two benzyl groups in the MH` ions of
Figure 6. Calculated CID spectrum of a mixture of the three
H½ ions e (35%), f (35%) and h (30%) (see text and Scheme
C
1
4 13
1
0).
Scheme 10
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1997 by John Wiley & Sons, Ltd.
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5
22
M. EDELSON-AVERBUKH AND A. MANDELBAUM
-
20 and -21 (in Tables 1 and 2) leads to a surprising
observation: the rearrangement process of the trans-
diesters with the remote benzoxycarbonyl groups results
in more abundant C H` ions than that of the cis-
1
4 13
isomers, where the distance between the two interacting
ester groups is small. This behavior di†ers from that of
alcohol elimination from MH` ions of stereoisomeric
methyl and ethyl diesters, which also requires inter-
action between the two alkoxycarbonyl groups (proton
transfer precedes the elimination step), and where only
the cis-isomers give rise to abundant [MH [ ROH]`
ions.9 It is also remarkable that the relative abundances
of the rearrangement ions increase with the length of
the chains of 1,u-alkanedioic esters 1È12 in their iso-
butane CI and CID mass spectra.
The relative abundances of the MH` ions in the CI
mass spectra of all cis-diesters are signiÐcantly higher
than those of the trans-isomers (see Table 1). This
observation indicates higher stability of the MH` ions
of the cis-isomers, which may result from the internal
proton bridged structures H and I, shown in Scheme 15.
In systems 18È21, other fragmentation processes, such
as elimination of benzyl alcohol and of dibenzyl ether
from the MH` ion, compete with the rearrangement
a†ording the C H` ion (see Tables 1 and 2). The
14 13
former two processes involve the formation of new
HÈO and benzylÈO bonds respectively, which also
require interaction between the two ester groups.
However, the spatial requirements of these two elimi-
nation processes di†er from that of the rearrangement
leading to the C H` ion: The elimination of alcohols
14 13
from MH` ions of diesters takes place via proton
bridged intermediates of the type of ion H, which may
be formed only when the inter-functional distance is
small,9 and it may be expected that a similar require-
ment controls the elimination of dibenzyl ether.
Scheme 11
dibenzyl esters is sterically possible even in 25, 26 and
2
7, where the two benzoxycarbonyl groups are most
The competition between the three fragmentation
processes in the cis-isomers might explain the unusual
steric e†ect in formation of the C H` ions in the ste-
distant. Therefore, the ready occurrence of the
rearrangement in these diesters is in keeping with the
mechanism proposed in Scheme 11.
1
4 13
reoisomeric pairs cis- and trans-18, -19, -20 and -21, but
not in 13È14 and 15È16, where the above processes are
unimportant or not observed at all (both under CI and
CID conditions, see Tables 1 and 2), and the rearrange-
ment a†ording the C H` ions is the only dissociation
Examination of the CI and CID data of the stereoiso-
meric pairs of esters 13È14, 15È16, cis- and trans-18, -19,
1
4 13
channel in the cis- and trans-isomers (except for forma-
tion of the m/z 91 C H ` ion, by a simple CÈO bond
7
7
cleavage). Stabilization of the MH` ions of the cis-
diesters by intramolecular proton bridging, shown in
Scheme 14, may be the cause of the suppressed
rearrangement in these isomers. The distance between
the benzyl methylene of one ester group and the phenyl
ring of the other may also play a role in lowering the
Scheme 12
Scheme 13
(
1997 by John Wiley & Sons, Ltd.
JOURNAL OF MASS SPECTROMETRY, VOL. 32, 515È524 (1997)

BENZYLÈBENZYL INTERACTIONS IN DIBENZYL ESTERS
523
Scheme 14
abundance of the C H` ions in the CI mass spectra of
the cis-diesters.
Finnigan TSQ-70B triple-stage quadrupole mass
spectrometer. The stereoisomeric pairs were introduced
as mixtures, and separations were performed on a DB-5
1
4 13
(
0.25 lm Ðlm thickness) 30 m ] 0.25 mm i.d. capillary
column at 110 ¡C (isothermal). The scan rate was 1 scan
s~1. The elution sequence for the stereoisomers in the
GC/MS analyses was as follows: 18t followed by 18c,
CONCLUSION
1
9c by 19t, 20t by 20c and 21c by 21t. CI measurements
The results of this study have established a highly
general rearrangement process of benzyl esters of dicar-
boxylic acids, giving rise to very abundant C H` ions
were performed at 150 ¡C ion source temperature and
0
.4 Torr (indicated) (1 Torr \ 133.3 Pa) reagent gas
14 13
pressure (isobutane, methane, d -acetonitrile). CID mea-
surements were performed with argon as the target gas
under CI and CID conditions. This rearrangement
involves an unusual long distance interaction between
the two reacting benzoxycarbonyl groups, a†ording
highly abundant m/z 181 ions in systems with remote
ester moieties, such as 24, 25, 26 and 27. The proposed
mechanism of this process explains this unusual feature.
It is noteworthy that the isomeric C H` ions aÈh
3
(
(
0.3 mTorr, indicated) at 30 and 50 eV collision energies
indicated). All CID data used for structural assign-
ments (presented in Figs 2È5) are average of Ðve mea-
surements, and were obtained on a single day under
identical conditions in order to assure reliable compari-
sons.
14 13
investigated a†ord distinguishable CID spectra (with
the exception of ions c and f), indicating the non-
occurrence of isomerization to common structure(s)
under isobutane CI conditions. This behavior suggests
retention of the benzylic nature of these ions rather than
transformation to substituted tropylium structures.
Materials
Benzyl diesters 1È19, 22È24 and 27 were prepared by
esteriÐcation of the corresponding dicarboxylic acids or
anhydrides with benzyl alcohol (neat or in toluene
solution) in the presence of catalytic amounts of p-
toluenesulfonic acid (PTSA). Commercial diacids and
anhydrides (except 1,4-bicyclo[2.2.2] octanedicarboxylic
acid, which was synthesized by a previously reported
procedure25) were used in the syntheses.
EXPERIMENTAL
Mass spectrometry
Deuterium-labeled diesters (d -cis-19 and d -trans-19)
4
4
The gas chromatographic/mass spectrometric (GC/MS)
analyses and CID measurements were carried out on a
were prepared by the above procedure, using cis- and
trans-cyclohexane-1,4-dicarboxylic acids and a-d -
2
Scheme 15
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1997 by John Wiley & Sons, Ltd.
JOURNAL OF MASS SPECTROMETRY VOL. 32, 515È524 (1997)

5
24
M. EDELSON-AVERBUKH AND A. MANDELBAUM
Scheme 16
benzyl alcohol, obtained by reduction of methyl ben-
Isomeric a-tolylbenzyl alcohols 37È39 were synthe-
sized by Grignard reaction of phenylmagnesium
bromide with the corresponding tolualdehydes.
Isomeric a-bromoditolyls 31È33 were prepared by
bromination of 4,4@-, 3,3@- and 2,2@-dimethylbiphenyls
with N-bromosuccinimide (NBS).32
zoate with LiAlD .
4
Dibenzyl esters 20, 21, 25 and 26 were obtained by
trans-esteriÐcation of the corresponding methyl diesters
with benzyl alcohol in the presence of PTSA (cis- and
trans-1,4- and 1,3-dimethyl-1,4- and 1,3-bis(methoxy-
carbonyl)cyclohexanes were prepared by methylation of
the methyl analogs of cis- and trans-18 and -19 with
iodomethane and lithium diisopropylamide in
tetrahydrofuran26).
1-Bromo-1,2-diphenylethane (30) was prepared from
1,2-diphenylethane by NBS bromination.32
Di-2,4,6-trimethylbenzyl esters cis- and trans-40 were
prepared by acylation of 2,4,6-trimethylbenzyl alcohol
with the corresponding acid dichlorides in the presence
of
4-N,N-dimethylaminopyridine
(DMAP)
alcohol
in
was
Acknowledgements
toluene.27,28
2,4,6-Trimethylbenzyl
obtained by reduction of mesitoic acid29 with LiAlH .
4
This work was supported by the Fund for Promotion of Research at
the Technion. We are grateful to Dr V. Ryaboy for his help with the
computer-aided analysis of the CID spectra and to Mr A. Etinger for
assistance with the mass spectral measurements.
The synthetic procedure30,31 used for preparation of
isomeric benzylbenzyl alcohols 35 and 36 is outlined in
Scheme 16.
REFERENCES
1
. K. Biemann, Abstracts of 13th Annual Conference on Mass
Spectrometry, p. 427, ASTM Committee E-14, St Louis, MO
17. M. Attina and F. Cacace, Int. J. Mass Spectrom. Ion Processes
120, R1 (1992).
(
1965).
18. D. Kuck and C. Matthias, J. Am. Chem. Soc. 114, 1901
(1992).
19. C. Matthias and D. Kuck Org. Mass Spectrom. 28, 1073
(1993).
2
3
4
. T. Axenrod and G. W. A. Milne, Chem. Commun., 67 (1968).
. A. V. Danks and R. Hodges, Aust. J. Chem. 25, 2721 (1972).
. E. E. Kingston, J. S. Shannon, V. Diakiw and M. J. Lacey, Org.
Mass Spectrom. 16, 428 (1981).
. M. Meot-Ner (Mautner), E. P. Hunter and F. H. Field, J. Am.
Chem. Soc. 99, 5576 (1977).
. D. Cameron and R. G. Cooks, J. Am. Chem. Soc. 101, 3152
20. G. Thielking, U. Filges and H.-F. GruŽ tzmacher, J. Am. Soc.
Mass Spectrom. 3, 416 (1992).
5
6
21. P. Longevialle, Mass Spectrom. Rev. 11, 157 (1992), and ref-
erences cited therein.
(
1979).
22. T. H. Morton, Org. Mass Spectrom. 27, 353 (1992), and refer-
ences cited therein.
23. D. Bowen, Org. Mass Spectrom. 28, 1577 (1993), and refer-
ences cited therein.
7
8
. A. Mandelbaum, Mass Spectrom. Rev. 2, 223 (1983).
. N. Morlender-Vais and A. Mandelbaum, Int. J. Mass
Spectrom. Ion Processes in press.
9
. A. Etinger, A. Idina and A. Mandelbaum, J. Am. Chem. Soc.
1
24. M. W. van Amsterdam, S. Ingemann and N. M. M. Nibbering,
J. Mass Spectrom. 30, 43 (1995).
25. E. W. Della and J. Tsanaktsidis, Aust. J. Chem. 38, 1705
(1985).
26. P. E. Pfeffer and L. S. Silbert, J. Org. Chem. 35, 262 (1970).
27. G. Hofle and W. Steglich, Synthesis, 619 (1972).
28. G. Hofle, W. Steglich and H. Vorbruggen, Angew. Chem., Int.
Ed. Engl. 17, 569 (1978).
15, 7397 (1993).
0. C. Denekamp and A. Mandelbaum, J. Mass Spectrom. 30,
421 (1995).
1. N. Khaselev and A. Mandelbaum, J. Mass Spectrom. 30, 1533
1995).
2. R. Shvily, T. Mu
1
1
1
1
1
(
Ž
ller, Y. Apeloig and A. Mandelbaum, J. Chem.
Soc., Perkin Trans. 2, 1997, in press.
3. F. W. McLafferty and F. Turecek, Interpretation of Mass
Spectra, 4th edn, Sections 8.3 and 8.11 University Science
Books, Mill Valley, CA (1993), and references cited therein.
4. AM1 and PM3 Methods, SPARTAN 2.1. Wavefunction, Inc.,
Irvine, CA, 1991–2.
29. P. E. Sokol, Org. Synth., Coll. Vol. 5, 706 (1973).
30. E. Wertheim, J. Am. Chem. Soc. 55, 2540 (1933).
31. E. L. Martin, Org. Synth., Coll. Vol. 2, 499 (1943).
32. M. D. Kellert and J. Sedlak, Org. Synth, Coll. Vol. 4, 108
(1963).
1
15. D. Kuck, Mass Spectrom. Rev. 9, 583 (1990).
16. H. H. Bueker and H.-F. GruŽ tzmacher, Int. J. Mass Spectrom.
Ion Processes 109, 95 (1991).
(
1997 by John Wiley & Sons, Ltd.
JOURNAL OF MASS SPECTROMETRY, VOL. 32, 515È524 (1997)