Thermal [2+2]-cycloadditions of diphenylketene with aryl- and hetaryl-substituted thioketones

Article abstract of DOI:10.3987/COM-14-S(K)56

The reaction of diphenylketene (1) with aryl- and hetaryl-substituted thioketones (2) gave the corresponding 3,3,4,4-tetraarylthietan-2-ones (3) in good yields. Remarkably, the reactions with bis-hetaryl-substituted thioketones occurred significantly faster compared with those involving the bis-aryl-substituted thioketones. The structure of compound 3c has been established by X-ray crystallography.

Full text of DOI:10.3987/COM-14-S(K)56

HETEROCYCLES, Vol. 90, No. 1, 2015
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HETEROCYCLES, Vol. 90, No. 1, 2015, pp. 529 - 539. © 2015 The Japan Institute of Heterocyclic Chemistry
Received, 29th June, 2014, Accepted, 29th July, 2014, Published online, 6th August, 2014
DOI: 10.3987/COM-14-S(K)56
THERMAL [2+2]-CYCLOADDITIONS OF DIPHENYLKETENE WITH
ARYL- AND HETARYL-SUBSTITUTED THIOKETONES
Grzegorz Mlosto!,a* Katarzyna Urbaniak,a Anna Szychowska,1,a Anthony
Linden,b and Heinz Heimgartnerb*
a: University of !ód", Department of Organic and Applied Chemistry, Tamka 12,
PL-91-403 !ód", Poland: e-mail: gmloston@uni.lodz.pl
b: University of Zurich, Department of Chemistry, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland; e-mail: heinz.heimgartner@chem.uzh.ch
Dedicated to Professor Dr. Isao Kuwajima on the occasion of his 77th birthday
Abstract – The reaction of diphenylketene (1) with aryl- and hetaryl-substituted
thioketones (2) gave the corresponding 3,3,4,4-tetraarylthietan-2-ones (3) in good
yields. Remarkably, the reactions with bis-hetaryl-substituted thioketones
occurred significantly faster compared with those involving the
bis-aryl-substituted thioketones. The structure of compound 3c has been
established by X-ray crystallography.
INTRODUCTION
Among heterocumulenes, ketenes are widely applied in cycloaddition reactions as highly reactive
dienophiles and dipolarophiles.2 In addition, they easily undergo [2+2] cycloadditions with diverse C=X
groups, yielding four-membered heterocycles (X = N, O)3 or cyclobutane derivatives (X = C).3a,4 Whereas
reactions of ketenes with imines are extensively studied as one of the most important methods for the
synthesis of !-lactams, similar [2+2] cycloadditions with thioketones are rarely reported and some
contradictions exist.5 The study performed with diphenylketene (1) and thiobenzophenone (2a) as well as
its para-substituted derivatives shows that thietan-2-ones of type 3a are the products of the reaction with
aromatic thioketones (Scheme 1).5c The analogous structure was attributed to the product obtained in the
reaction with methyl dithiobenzoate.6 A thietan-2-one was also formed via photocycloaddition of
9H-xanthene-9-thione with 1.7 However, the reaction of 1 with the in situ generated aryl cinnamoyl
thioketone follows the hetero-Diels-Alder pathway leading to 3,4-dihydro-2H-thiopyran-2-one
derivatives.8

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Reactions of diphenylketene (1) with aliphatic and cycloaliphatic thioketones are almost unknown. The
reaction with the enolizable diisopropyl thioketone (4a) is reported to yield the vinyl thioester (5) as the
exclusive product.9 The formation of products of type 3 is believed to occur via a concerted [2+2]
cycloaddition reaction. However, in a recent publication, the reaction of adamantanethione (4b) with 1 is
reported to give the 2:1-cycloadduct (6).10 On the other hand, the product formed in the reaction of 1 with
pivaloyl thioaldehyde (7) was identified as the 1:2-cycloadduct (8).11
Scheme 1
In general, thioketones are prone dienophiles and dipolarophiles, which react with some 1,3-dipoles, e.g.,
with gem-diaryl thiocarbonyl S-methanides (thiocarbonyl ylides), in a non-concerted manner via diradical
intermediates.12 Due to our ongoing interest in applications of thiocarbonyl compounds for the synthesis
of sulfur-containing heterocycles, and, on the other hand, in the mechanism of cycloadditions and the
reactivity of C=S compounds, we studied the reactions of 1 with a series of aryl- and hetaryl-substituted
thioketones.
RESULTS AND DISCUSSION
Whereas aryl-substituted thioketones such as thiobenzophenone (2a), thiofluorenone (2b),
dibenzosuberenethione (2c), and dibenzosuberanethione (2d) are well known and frequently applied as
model thioketones, their hetaryl analogues 2e – 2i are scarcely described. In a recent publication, we
reported a method for the synthesis of a series of symmetrical and non-symmetrical representatives,

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531
which were subsequently used for selected 1,3-dipolar cycloadditions and hetero-Diels-Alder reactions.13
The goal of the present study was to examine reactions of both aryl- and hetaryl-thioketones with
diphenylketene (1).
3
Ar1
Ph
Ar2
Ph
Reaction Time (h)
Yield [%]a)
a
ca. 48 h
ca. 48 h
62
69
b
c
ca. 48 h
72
d
e
f
ca. 48 h
ca. 24 h
ca. 24 h
67
68
65
Ph
Ph
g
h
i
ca. 0.5 h
ca. 0.5 h
ca. 0.5 h
70
69
62
a) Yields of isolated products.
Scheme 2
As a test experiment, freshly prepared 1 was reacted with 2a without any solvent at room temperature.
After two days, the blue color of 2a vanished, and the major product was isolated after chromatography

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HETEROCYCLES, Vol. 90, No. 1, 2015
13
(SiO2) and its structure 3a was established based on the spectroscopic data.5 In the C-NMR spectrum,
two signals of quaternary C-atoms absorbed at 63.5 and 93.2 ppm, proving the structure reported in the
literature. The minor product isolated as colorless crystals was identified as benzophenone (13C-NMR:
196.6 ppm (C=O)).
Similarly, reactions of 1 with 2b – 2d occurred smoothly at room temperature yielding the corresponding
thietan-2-one derivatives 3b – 3d as crystalline materials (Scheme 2). In all these cases, the reaction times
13
were comparable with the thiobenzophenone experiment (ca. 48 h), and the C-NMR analysis of the
crude reaction mixtures revealed the presence of benzophenone. The structure of 3c was unambiguously
established by X-ray crystallography (Figure).
The reactions of 1 with hetaryl-substituted thioketones (2e – 2i) were performed in an analogous manner.
Whereas the reactions with the non-symmetrical thioketones (2e, 2f) were completed after ca. 24 h, the
bis-hetaryl analogues (2g – 2i) reacted vigorously, and the color of the thioketone had already
disappeared after 0.5 h. Again, traces of benzophenone were identified in the crude mixtures obtained in
the reactions with 2e and 2f, but it could not be detected in reactions involving the most reactive
hetaryl-thioketones (2g – 2i). The corresponding thietan-2-ones (3e – 3i) were isolated by
chromatographic workup and were obtained as crystalline materials, which, upon heating to the melting
point temperature, colorized due to decomposition via release of the respective thioketone. In the case of
the Se-containing products (3f) and (3i), slow decomposition during storage at room temperature was
observed.
Figure. ORTEP plot14 of the molecular structure of 3c (arbitrary numbering of the atoms; 50%
probability ellipsoids)

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533
It is well established that sulfur heterocycles such as 1,2,4-trithiolanes can be selectively oxidized to give
the corresponding S-oxides.15 In the case of 3,3,4,4-tetraphenylthietan-2-one (3a), the attempted oxidation
with excess mCPBA in CH2Cl2 at 50 °C over 18 h was reported to yield a mixture of products, but the
expected S-oxide was not obtained.16 In our experiments, 3a and 3e were treated with ca. equimolar
amounts of mCPBA in CH2Cl2 solution at room temperature for 24 h. In both cases, no oxidation
occurred and the starting materials were recovered. In order to get the desired S-oxides of compounds 3,
we also tested the reaction of 1 with thiobenzophenone S-oxide17 using the same procedure as in the
reactions of 1 with 2a. The only product identified after separation of the mixture was benzophenone.
Whereas reactions of 1 with ethylene derivatives or with imines are believed to occur in a concerted
manner, the corresponding reactions with thioketones were suggested to proceed via a zwitterionic
intermediate.8 In the case of adamantanthione and pivaloyl thioaldehyde, the experimental proof for the
stepwise reaction mechanism is the formation of 1:210 or 2:111 adducts, respectively. The mechanism of
the reaction of 1 with some ",!-unsaturated aryl thioketones was also suggested to occur via a stepwise
mechanism.8 In the present study, the reactions with aryl- and hetaryl-thioketones (2) seem to follow a
concerted pathway, as no product resulting from the intermediacy of a zwitterionic or a diradical species
was observed. Apparently, the formation of benzophenone found in the mixtures after slow reactions of 1
with aromatic thioketones (2a – 2f) resulted from a side reaction of diphenylketene itself.20
CONCLUSIONS
The present study shows that aryl- and hetaryl-thioketones of type 2 react with diphenylketene (1)
analogously to the reaction with thiobenzophenone (2a), to give the expected thietan-2-ones (3). However,
bis-hetaryl thioketones (2g – 2i) are significantly more reactive, and their reactions with 1 occur without
the formation of benzophenone as a side product. Most likely, the reaction mechanism follows a
concerted [2+2] cycloaddition pathway. It is worth mentioning that the hetaryl-thioketones react with 1 to
give exclusively [2+2] cycloadducts. The products of the alternative hetero-Diels-Alder reactions with 1
acting as dienophile21a-21c and hetaryl-thioketones as heterodienes13 were not observed. Finally, the smooth
reaction of the non-symmetrical hetaryl-thioketones (2g – 2i) with 1 opens a new way for studying
hitherto unknown asymmetric [2+2] cycloadditions21d leading to thietan-2-ones.
EXPERIMENTAL
General remarks. Melting points were determined in a capillary using a Melt-Temp. II (Aldrich)
apparatus, and they are uncorrected. The IR spectra were recorded on a NEXUS FT-IR spectrophotometer
1
13
in KBr; absorptions in cm–1. The H- and C{1H}-NMR spectra were measured on a Bruker Avance III
instrument (600 and 150 MHz, resp.) using solvent signals as reference. Chemical shifts (#) are given in

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ppm and coupling constants J in Hz. HR-ESI-MS were recorded on a Bruker maXis spectrometer in the
Laboratory of Mass Spectrometry of the University of Zurich, and the TOF-MS for 3h was registered on
a SYNAPT G2-Si HMDS spectrometer (Institute of Organic Chemistry PAS, Warsaw). Elemental
analyses were performed in the Laboratory of the Polish Academy of Science (CBMiM) in #ód$.
Starting materials. All solvents and reagents are commercially available and were used as received.
Thioketones (2a – 2i) were obtained in a typical manner from the corresponding ketones and Lawesson’s
reagent in boiling toluene or benzene.13,22 Diphenylketene (1) was obtained from diphenylacetyl chloride
via reaction with triethylamine at 0 °C according to literature procedures.23
Reaction of thioketones (2) with diphenylketene (1). – General procedure. To a mixture of 1 mmol of
the corresponding thioketone (2) and a crystal of hydroquinone to inhibit polymerization, 1.1 mmol of the
yellow oil of freshly prepared diphenylketene (1) were added dropwise at room temperature under an
argon atmosphere. The resulting mixtures were kept at room temperature until the color of 2 disappeared
completely (ca. 48 h for 2a – 2d, ca. 24 h for 2e, 2f, and ca. 0.5 h for 2g – 2i). The crude mixtures were
purified chromatographically on silica gel using a mixture of hexane and CH2Cl2 (3.5:1.5) as eluent. In
the reactions with less reactive thioketones (2a – 2d), benzophenone was isolated as the more polar
fraction in variable amounts depending on the type of thioketone 2 used (traces – 20% yield). The
obtained products were additionally purified by crystallization from hexane or petroleum ether with a
small amount of CH2Cl2.
3,3,4,4-Tetraphenylthietan-2-one (3a).3c Yield: 242 mg (62%). Colorless crystals, mp 138 °C (dec.). IR
1
(KBr): 3059m, 3027m, 1732vs (C=O), 1540m, 1489s, 1443m, 1048s, 824s, 748s, 720s. H-NMR:
7.06–7.12 (m, 16 Harom); 7.13–7.16 (m, 4 Harom). 13C-NMR: 63.5, 93.2 (2 Cq); 126.9, 127.2, 127.3, 127.8,
129.2, 130.8 (20 CHarom); 138.4, 141.8 (4 Carom); 194.6 (C=O). HR-ESI-MS (MeOH/CHCl3+NaOH):
415.11271 (calcd. 415.11297 for C27H20NaOS [M+Na]+).
3',3'-Diphenylspiro[fluorene-9,4'-thietane]-2'-one (3b). Yield: 268 mg (69%). Colorless crystals, mp
168 °C (dec.). IR (KBr): 3060m, 3036m, 1753vs (C=O), 1636s, 1490m, 1447m, 1338m, 1091m, 1054s,
1
809m, 739s, 703m. H-NMR: 6.75 (d, JH,H = 7.6 Hz, 2 Harom); 6.96 (dt, JH,H = 7.6, 0.7 Hz, 2 Harom);
7.09–7.13 (m, 10 Harom); 7.31 (dt, JH,H = 7.5, 0.5 Hz, 2 Harom); 7.71 (d, JH,H = 7.6 Hz, 2 Harom). 13C-NMR:
59.0, 90.5 (2 Cq); 119.8, 127.0, 127.1, 127.5, 127.9, 128.8 (18 CHarom); 140.3, 140.4, 145.2 (6 Carom);
194.7 (C=O). HR-ESI-MS (MeOH/CHCl3+NaOH): 413.09706 (calcd. 413.09706 for C27H18NaOS
[M+Na]+).

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3',3'-Diphenylspiro[5H-dibenzo[a,d]cyclohepten-5,4'-thietane]-2-one (3c). Yield: 298 mg (72%).
Colorless crystals, mp 167 °C (dec.). IR (KBr): 3022m, 2956m, 1748vs (C=O), 1705m, 1596m, 1492m,
1478m, 1436m, 1327m, 1186m, 1049m, 831s, 765m, 727s, 697s, 645m. 1H-NMR: 6.54 (s, 2 CH=); 6.99 (t,
JH,H = 7.4 Hz, 4 Harom); 7.03–7.10 (m, 8 Harom); 7.21 (t, JH,H = 7.4 Hz, 2 Harom); 7.41 (dt, JH,H = 7.6, 1.1 Hz,
13
2 Harom); 8.02 (d, JH,H = 8.0 Hz, 2 Harom). C-NMR: 69.9, 93.3 (2 Cq); 126.8, 126.9, 127.3, 128.3, 128.4,
128.8, 130.8, 132.9 (18 CHarom, 2 CH=); 135.1, 136.4, 137.3 (4 Carom); 193.9 (C=O). Anal. Calcd for
C29H20OS (416.54): C 83.62; H 4.84; S 7.70. Found: C 83.75; H 4.91 S 7.82.
3',3'-Diphenylspiro[10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,4'-thietane]-2-one (3d). Yield: 279
mg (67%). Colorless crystals, mp 149 °C (dec.). IR (KBr): 3024m, 2981m, 1743vs (C=O), 1701m, 1597m,
1
1491m, 1478m, 1339m, 1162m, 1054m, 834s, 778m, 726s, 699s, 626m. H-NMR: 2.59–2.64 (m, 2 H,
CH2); 3.18–3.22 (m, 2 H, CH2); 6.87–6.89 (m, 2 Harom); 7.04–7.15 (m, 14 Harom); 7.98–8.00 (m, 2 Harom).
13C-NMR: 32.9 (2 CH2); 71.5, 94.2 (2 Cq); 126.1, 127.4, 127.6, 128.2, 129.9, 130.3, 130.4 (18 CHarom);
136.3, 139.0, 139.5 (6 Carom); 194.6 (C=O). Anal. Calcd for C29H22OS (418.65): C 83.22; H 5.30; S 7.66.
Found: C 83.35; H 5.27; S 7.71.
3,3,4-Triphenyl-4-(2-thienyl)thietan-2-one (3e). Yield: 270 mg (68%). Pale yellow crystals, mp 146 °C
(dec.). IR (KBr): 3059m, 3031m, 1735vs (C=O), 1598m, 1444s, 1236m, 1049m, 1000m, 827s, 753s, 699s.
1H-NMR: 6.59 (dd, JH,H = 3.8, 1.2 Hz, 1 Harom); 6.67 (dd, JH,H = 5.0, 3.8 Hz, 1 Harom); 7.04–7.10 (m, 6
13
Harom); 7.21–7.28 (m, 8 Harom); 7.43–7.46 (m, 2 Harom). C-NMR: 62.4, 94.2 (2 Cq); 126.3, 126.9, 127.2,
127.6, 127.7, 127.9, 128.6, 129.2, 130.0, 130.2 (18 CHarom); 137.2, 138.5, 140.1, 150.8 (4 Carom); 193.5
(C=O). HR-ESI-MS (MeOH/CHCl3+NaOH): 421.06913 (calcd. 421.06902 for C25H18NaOS2 [M+Na]+).
3,3,4-Triphenyl-(4-selenophen-2-yl)thietan-2-one (3f). Yield: 290 mg (65%). Pale brown crystals, mp
163 °C (dec.). IR (KBr): 3059m, 3029m, 1733vs (C=O), 1596m, 1489s, 1334m, 1282m, 1158m, 1118m,
1088m, 1048m, 1000m, 827s, 719s, 647m. 1H-NMR: 6.75 (d, JH,H = 3.7 Hz, 1 Harom); 6.91 (dd, JH,H = 5.6,
4.0 Hz, 1 Harom); 7.03–7.11 (m, 5 Harom); 7.21–7.29 (m, 8 Harom); 7.49–7.50 (m, 2 Harom); 7.79 (d, JH,H
=
13
5.6 Hz, 1 Harom). C-NMR: 64.3, 94.3 (2 Cq); 127.2, 127.3, 127.6, 127.7, 127.8, 127.9, 128.9, 129.0,
129.2, 130.0, 132.0, 133.1 (18 CHarom); 137.2, 138.6, 140.7, 159.1 (4 Carom); 193.6 (C=O). Anal. Calcd for
C25H18OSSe (445.44): C 67.41; H 4.07; S 7.20. Found: C 67.60; H 4.11; S 7.31.
4-(2-Furyl)-3,3-diphenyl-4-(2-thienyl)thietan-2-one (3g). Yield: 270 mg (70%). Yellow crystals, mp
136 °C (dec.). IR (KBr): 3039m, 3028m, 1746vs (C=O), 1599m, 1491s, 1375m, 1228m, 1156m, 1090m,
1044m, 1017m, 827s, 753s, 699s. 1H-NMR: 6.17 (dd, JH,H = 3.2, 1.8 Hz, 1 Harom); 6.24 (dd, JH,H = 3.2, 0.5
Hz, 1 Harom); 6.77 (dd, JH,H = 5.0, 3.7 Hz, 1 Harom); 6.88 (dd, JH,H = 3.6, 1.1 Hz, 1 Harom); 7.10–7.14 (m, 4

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Harom); 7.17–7.18 (m, 2 Harom); 7.27–7.28 (m, 4 Harom); 7.33–7.36 (m, 2 Harom). C-NMR: 53.8, 93.5 (2
Cq); 110.4, 112.1, 126.4, 126.8, 127.2, 127.4, 127.5, 127.7, 127.8, 128.6, 129.5, 143.0 (16 CHarom); 137.7,
138.0, 144.3, 153.6 (4 Carom); 192.6 (C=O). HR-ESI-MS (MeOH/CHCl3+NaOH): 411.04826 (calcd.
411.04839 for C23H16NaO2S2 [M+Na]+).
3,3-Diphenyl-4,4-bis(2-thienyl)thietan-2-one (3h): Yield: 278 mg (69%). Pale brown crystals, mp 130 °C
(dec.). IR (KBr): 3066m, 3026m, 1743vs (C=O), 1599m, 1449m, 1348m, 1076m, 1056m, 809m, 752m,
1
702s. H-NMR: 6.73–6.75 (m, 2 Harom); 6.77–6.78 (m, 2 Harom); 7.11–7.17 (m, 8 Harom); 7.26–7.28 (m, 4
Harom). 13C-NMR: 58.2, 95.0 (2 Cq); 126.4, 126.5, 127.5, 127.8, 128.7, 129.6 (16 CHarom); 137.6, 147.4 (4
Carom); 192.6 (C=O). TOF-MS-ES: 405.0438 (calcd. 405.0442 for C23H17OS3 [M+1]+, (56)), 427.0258
(calcd. 427.0261 for C23H16ONaS3 [M+Na]+, (100)).
3,3-Diphenyl-4,4-di(selenophen-2-yl)thietan-2-one (3i). Yield: 310 mg (62%). Pale brown crystals, mp
134 °C (dec.). IR (KBr): 3025w, 3001w, 1742vs (C=O), 1599m, 1492m, 1443m, 1383m, 1345m, 1229m,
1108m, 1029m, 858m, 694s. 1H-NMR: 6.98–7.02 (m, 4 Harom); 7.17–7.22 (m, 6 Harom); 7.35 (dd, JH,H = 7.6,
1.4 Hz, 4 Harom); 7.87 (dd, JH,H = 5.6, 1.1 Hz, 2 Harom). 13C-NMR: 61.9, 95.0 (2 Cq); 127.7, 127.8, 129.0,
131.7, 132.8 (16 CHarom); 137.7, 155.5 (4 Carom); 192.7 (C=O). HR-ESI-MS (MeOH/CHCl3+NaOH):
522.91445 (calcd. 522.91441 for C23H16NaOSSe2 [M+Na]+).
X-Ray Crystal-Structure Determination of 3c (Figure 1).24 All measurements were made on an Agilent
Technologies SuperNova area-detector diffractometer25 using MoK! radiation (" = 0.71073 Å) from a
micro-focus X-ray source and an Oxford Instruments Cryojet XL cooler. Data reduction was performed
with CrysAlisPro.25 The intensities were corrected for Lorentz and polarization effects, and an empirical
absorption correction using spherical harmonics25 was applied. Equivalent reflections were merged. The
data collection and refinement parameters are given below. A view of the molecule is shown in the Figure.
The structure was solved by direct methods using SHELXS-2013,26 which revealed the positions of all
non-H-atoms. The non-H-atoms were refined anisotropically. All of the H-atoms were placed in
geometrically calculated positions and refined by using a riding model where each H-atom was assigned a
fixed isotropic displacement parameter with a value equal to 1.2U of its parent C-atom. The refinement
eq
2
of the structure was carried out on F by using full-matrix least-squares procedures, which minimized the
function #w(Fo2–Fc2)2. A correction for secondary extinction was not applied. Neutral atom scattering
factors for non-H-atoms were taken from ref.27, and the scattering factors for H-atoms were taken from
ref.28 Anomalous dispersion effects were included in F ;29 the values for ƒ’ and ƒ” were those of ref.30 The
c
values of the mass attenuation coefficients are those of ref.31 All calculations were performed using the

HETEROCYCLES, Vol. 90, No. 1, 2015
537
SHELXL-2013 program.26
Crystal data for 3c: Crystallized from hexane/CH2Cl2, C29H20OS, M = 416.54, colorless, prism, crystal
_
dimensions 0.20 ! 0.22 ! 0.22 mm, triclinic, space group P1, Z = 2, reflections for unit cell determination
14381, 2$ range for cell determination 4–61°, a = 8.9451(2) Å, b = 10.4239(3) Å, c = 12.1195(3) Å, " =
93.516(2)°, # = 102.270(2)°, $ = 108.964(2)°, V = 1033.85(5) Å3, DX = 1.338 g%cm–3, µ(MoK!) = 0.176
mm–1, T = 160(1) K, % scans, 2$max = 61.0°, transmission factors (min; max) 0.935; 1.000, total reflections
measured 26089, symmetry independent reflections 5834, reflections with I > 2&(I) 5170, reflections used
in refinement 5834, parameters refined 280, final R(F) (I > 2&(I) reflections) = 0.0374, wR(F2) (all data) =
0.0978 (w = [&2(Fo2) + (0.0446P)2 + 0.3801P]–1 where P = (Fo2 + 2Fc2)/3, goodness of fit 1.054, final
'max/& = 0.001, '( (max; min) = 0.36; –0.24 e Å–3.
ACKNOWLEDGEMENTS
G. M., K.U., and H. H. acknowledge the National Science Center (PL-Cracow) for financial support
(Grant Maestro-3; Dec-2012/06/A/ST5/00219). We thank the analytical services of our institute for NMR
spectra and PD Dr. L. Bigler, University of Zurich, for HR-ESI-MS.
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