Attempts to prepare an all-carbon indigoid system

Article abstract of DOI:10.3762/bjoc.11.42

First attempts are described to prepare a precursor for an all-carbon analog of indigo, the tetracyclic triene 4. Starting from indan-2-one (9) the α-methylene ketone 13 was prepared. Upon subjecting this compound to a McMurry coupling reaction, it dimerized to the bis-indene derivative 17, rather than providing the tetramethyl derivative of 4, the hydrocarbon 14. In a second approach, indan-1-one (18) was dimerized to the conjugated enedione 21 through the bis-1-indene dimer 19. All attempts to methylenate 21 failed, however. When 19 was treated with the Tebbe reagent, the dimer 23 was produced, presumably through a Cope reaction of the intermediately generated isomer 22. The bis-indene derivative 23 can be alkylated with 1,2-dibromoethane to produce a 1:1 mixture of the spiro compounds 24 and 25. Although 9 could be reductively dimerized to 30, the conversion of this olefin to 14 failed.

Full text of DOI:10.3762/bjoc.11.42

Attempts to prepare an all-carbon indigoid system
Şeref Yildizhan1, Henning Hopf*1 and Peter G. Jones2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 363–372.
1Institut für Organische Chemie, Technische Universität
Braunschweig, Hagenring 30, D-38106 Braunschweig, Germany, Fax:
(+49)531-391-5388 and 2Institut für Anorganische und Analytische
Chemie, Technische Universität Braunschweig, Postfach 3329,
D-38106 Braunschweig, Germany, Fax: (+49)531-391-5387
doi:10.3762/bjoc.11.42
Received: 17 December 2014
Accepted: 03 March 2015
Published: 18 March 2015
Email:
Alicyclic compounds, part IX. [1]; for part VIII see [2]. Dedicated to
Professor Wolfgang Lüttke on the occasion of his 95th birthday.
Henning Hopf* - h.hopf@tu-bs.de
* Corresponding author
Associate Editor: B. Stoltz
Keywords:
© 2015 Yildizhan et al; licensee Beilstein-Institut.
License and terms: see end of document.
α-methylene ketones; Cope rearrangement; cross-conjugation; indigo;
McMurry coupling
Abstract
First attempts are described to prepare a precursor for an all-carbon analog of indigo, the tetracyclic triene 4. Starting from indan-2-
one (9) the α-methylene ketone 13 was prepared. Upon subjecting this compound to a McMurry coupling reaction, it dimerized to
the bis-indene derivative 17, rather than providing the tetramethyl derivative of 4, the hydrocarbon 14. In a second approach, indan-
1-one (18) was dimerized to the conjugated enedione 21 through the bis-1-indene dimer 19. All attempts to methylenate 21 failed,
however. When 19 was treated with the Tebbe reagent, the dimer 23 was produced, presumably through a Cope reaction of the
intermediately generated isomer 22. The bis-indene derivative 23 can be alkylated with 1,2-dibromoethane to produce a 1:1 mix-
ture of the spiro compounds 24 and 25. Although 9 could be reductively dimerized to 30, the conversion of this olefin to 14 failed.
Introduction
Cross-conjugated organic molecules are defined as unsaturated gated hydrocarbons are now known up to [13]dendralene, and
systems containing two π-electron systems (or lone pairs) that many of these potentially very valuable compounds are avail-
are in direct conjugation, whereby a third such system is able in gram quantities, allowing, often for the first time,
excluded from interaction [3]. Typical examples are 2-vinyl- comprehensive chemical studies [6].
buta-1,3-diene ([3]dendralene, 3-methylene-penta-1,4-diene),
benzophenone or urea.
Notwithstanding modern progress, the phenomenon of cross-
conjugation is as old as (scientific) organic chemistry. Many of
Whereas the hydrocarbon parent systems, the [n]dendralenes, the organic compounds that played an important role in the
have long been a neglected class of oligoenes [4], the recent dawn of (industrial) organic chemistry are cross-conjugated
preparative accomplishments of the Sherburn group have systems or were converted into cross-conjugated organic salts
changed the situation fundamentally [5,6]. These cross-conju- during the color-generating process.
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This is illustrated by indigo (1, Scheme 1) and its derivatives The methylation of 7 to 8 was achieved by the treatment with
(e.g., Tyrian purple) and the triphenylmethane-derived carboca- trimethylsulfonium iodide in the presence of n-butyllithium in
tions.
THF [16].
Starting from the generalized indigoid structure 2, the role of The spectroscopic data of these intermediates are incomplete in
the heteroatoms X and Y can be played not only by O and NH the chemical literature and are hence given in full detail in the
as in 1 itself, but by many other equivalent atoms and/or struc- experimental section (see Supporting Information File 1).
tural elements. In fact, the anellated benzene rings could be
omitted altogether, resulting in what Lüttke, a pioneer in this Unfortunately, all attempts to oxidize 8 to the α-methylene
area of dyestuff chemistry, has called urindigo (3, primordial ketone 12, failed (Dess–Martin reaction, IBX, Swern oxidation
indigo): representing the basic cross-conjugated π-electron etc.); the original plan was to dimerize this intermediate to 4 by,
system of the indigoid compounds [7-9].
e.g., a McMurry coupling reaction. Also the second route,
starting with the reaction of 2-indanone (9) with the Eschen-
Whereas the replacement of Y in 2 (X = O) by sulfur and sele- moser salt 10 according to [17] to give the iodide 11, was
nium [10,11] or even tellurium [12,13] has been known for unsuccessful, since the attempted Hofmann elimination to 12
some time, derivatives of “phosphaindigo” (Y = various substi- failed. Of course 12, if formed, might not have survived the
tuted phosphorus derivatives) have only been described more isolation process.
recently [14].
Since the target molecule 12 was expected to be a reactive com-
As far as we are aware, however, no attempt to prepare an all- pound, we decided to increase its stability by the introduction of
carbon equivalent of indigo has ever been described. A system two methyl substituents at its exocyclic double bond. Indeed,
that could qualify as such an all-carbon analog of 1 is the bis- when 9 was first metalated with LDA and the resulting enolate
anion 5, which itself should be obtainable by anionization of the quenched with acetone, the resulting ketol could be dehydrated
linearly conjugated triene 4.
in situ by treatment with diethyl chlorophosphate to yield 13
[18].
Although we have to accept for the time being that our different
approaches to preparing 4 (and 5) have so far been unsuc- Derivative 13 is a crystalline solid that can be kept in the refrig-
cessful (see below), we think that our initial efforts to attain this erator for longer periods of time without decomposition. Slow
goal are worth publication. Furthermore, we are convinced that evaporation of the solvent of a chloroform solution provided
5 will eventually become available.
single crystals of 13 that were suitable for X-ray structure
analysis. The resulting structure is shown in Figure 1.
Results and Discussion
Our first attempt to prepare hydrocarbon 4 started from indene Compound 13 crystallizes with two independent but closely
(6, Scheme 2). Epoxidation with m-chloroperbenzoic acid similar molecules (rmsd 0.05 Å) in the asymmetric unit; these
(MCPBA) according to a literature method [15] yielded the occupy independent alternating layers parallel to (
). The
epoxide 7 in meager yields (Scheme 2).
ring systems are almost exactly planar (mean deviations
Scheme 1: From indigo to heteroindigo derivatives and all-carbon-indigo.
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Beilstein J. Org. Chem. 2015, 11, 363–372.
Scheme 2: Attempts to prepare the α-methylene ketones 12 and 13.
<0.03 Å) with the atoms of the isopropylidene group lying To rationalize its formation we propose that the deoxygenation
approximately 0.2 Å to one side of the plane and the keto of 13 does indeed take place, but provides a carbenoid inter-
oxygen approximately 0.2 Å to the other side. The exocyclic mediate 15 rather than the vicinal diol complex that is usually
C=C bonds are slightly lengthened, at 1.352(3), 1.353(3) Å.
postulated to be formed during the McMurry dimerization.
Possibly the dimerization of the substrate molecule cannot take
Subjecting 13 to a McMurry coupling reaction (TiCl4/HgCl2 in place because of steric hindrance by the gem-dimethyl group.
THF) did not result in the formation of the expected dimer 14, a Intermediate 15 is a vinylcarbene that, in principle, has several
tetramethyl derivative of 4, but furnished dimer 17 in low yield options to react further. It could dimerize to the intended
(10%, Scheme 3).
product 14 (or its diastereomer), cyclize to a cyclopropene
Scheme 3: Dimerization of 13 under McMurry conditions.
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Beilstein J. Org. Chem. 2015, 11, 363–372.
Figure 1: a) Both independent molecules of compound 13 in the crystal; ellipsoids represent 50% probability levels. b) Packing diagram of compound
13 (first independent molecule only) viewed perpendicular to (
). Dotted lines indicate the weak hydrogen bond H7···O, 2.50 Å, which links the
molecules to form chains parallel to [101]. Other H atoms are omitted for clarity. The layers of the second molecule (not shown) are not topologically
equivalent; they do not contain an equivalent interaction.
derivative or react via its resonance structure 16 to the isolated in layers perpendicular to the a-axis. There are no noticeably
dimer 17. Clearly, among these alternatives, the last route is short intermolecular interactions.
preferred. In 17 the two benzene rings are anellated in anti-
orientation, i.e., as far apart as possible. There exists an alter- Our next approach towards 4 (or a derivative) started from
native structure, however, in which the two aromatic rings point indan-1-one (18, Scheme 4).
in the same direction. To distinguish between these two possi-
bilities based on spectroscopic evidence alone would not be Enolization of 18 was carried out with sodium hydride in THF
easy. Fortunately, however, single crystals of the isolated dimer, and on subsequent oxidative dimerization the expected bis-
suitable for X-ray analysis, were obtained, showing unambigu- ketone 19 was obtained. This compound is produced as a mix-
ously that the anti-orientation is preferred (Figure 2).
ture of two diastereomers in roughly 1:1 ratio (NMR analysis),
which can be separated by column chromatography. Since,
The molecule of 17 exhibits crystallographic inversion however, in the next step the connecting single bond is trans-
symmetry, but the true symmetry is close to C2h (rmsd 0.02 Å). formed into a double bond, we used the diastereomeric mixture
The ring system is planar, with a mean deviation of only for the subsequent oxidation. This was carried out by meta-
0.02 Å. The molecular packing involves a herringbone pattern lating 19 with sodium hydride again, and then oxidizing the
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Beilstein J. Org. Chem. 2015, 11, 363–372.
Figure 2: a) The molecule of compound 17 in the crystal; ellipsoids represent 50% probability levels. Only the asymmetric unit is numbered.
b) Herringbone packing of compound 17 viewed parallel to the a-axis. H atoms are omitted.
Scheme 4: Dimerization of indan-1-one (18) by a stepwise approach.
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Beilstein J. Org. Chem. 2015, 11, 363–372.
presumably resulting carbanion to 21 (a known compound [19]) Tebbe’s reagent provided a product. This, however, was not the
with Cu(OTf)2, which gave cleaner results than the chloride hoped-for diene 22, but an isomer, the hydrocarbon 23
employed previously.
(Scheme 5).
Unfortunately, all efforts to convert 21 into 4 failed. Thus The structure of hydrocarbon 23 was established by the usual
neither the Wittig reaction of 21 (MeP(Ph)3Br/n-BuLi, THF) spectroscopic data (see experimental section) and also by an
nor its treatment with Oshima–Lombardo reagent (Zn, CH2Br2, X-ray crystal structure determination (Figure 3).
TiCl4) [20,21] yielded a trace of 4.
Likewise, the exposure of 21 to Tebbe’s reagent (trimethylalu-
minum with titanocene dichloride, pyridine, toluene) led to no
sign of reaction in the desired sense. Nor was the tetramethyl
derivative of 4, hydrocarbon 14, obtained when 21 was
subjected to a crossed McMurry coupling with excess acetone.
In a stepwise approach, 21 was treated with either methyl-
lithium in ether or methylmagnesium bromide in the hope of
either preparing a mono- or the bis-tertiary alcohol derivative
20, which subsequently could be subjected to a dehydration
reaction. In both cases only minute amounts of products could
be obtained. When trying to purify these by column chromatog-
raphy on silica gel, the stationary phase turned blue, but no
defined products could be isolated.
Figure 3: The molecule of compound 23 in the crystal. Ellipsoids
represent 50% probability levels. Only the asymmetric unit is
numbered.
The molecule of 23 is planar (mean deviation 0.03 Å) and
Since it might have been the central conjugated butenedione displays crystallographic inversion symmetry, although the true
core that caused all these preparative difficulties, we next symmetry is close to C2h (rmsd 0.03 Å). The central C–C bond
decided to investigate the behavior of diketone 19, in which this seems slightly short at 1.520(2) Å, but a similar value of
conjugation is interrupted. Although its methylenation under 1.523(5) Å was observed in the only other known system with
Wittig conditions was again unsuccessful, the reaction with the two analogous five-membered rings joined by a –CH2–CH2–
Scheme 5: Methylenation of 19 and bisalkylation of the product 23 with 1,2-dibromoethane.
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Beilstein J. Org. Chem. 2015, 11, 363–372.
moiety [22,23]. The molecular packing is devoid of striking because of lack of material, no high quality NMR spectra of
features.
these two hydrocarbons could be obtained.
As far as the mode of formation of 23 is concerned, the isomer- The molecule 24 displays crystallographic inversion symmetry,
ization formally is a [3.3]sigmatropic rearrangement (Cope although the true symmetry is close to C2h (rmsd 0.02 Å). The
rearrangement). Since the rearrangement of structurally similar molecular packing involves herringbone layers, parallel to the
compounds [24], including the parent system hexa-1,5-diene, bc plane, in which two C–H···π contacts to the centroid of the
requires much higher temperatures than those given in aromatic six-membered ring are observed (H5···π 2.76 Å,
Scheme 5, we assume that the metalorganic reagent (or prod- H9A···π 2.83 Å). The structure determination of 25 was of
ucts derived therefrom) play a role in the isomerization.
limited accuracy because of twinning and disorder problems
(indeed, there may be a small amount of contamination by 24)
Hydrocarbon 23 is, in fact, a known compound. It has been and we therefore do not discuss it in detail. Both independent
prepared previously from indene (6) by other routes [25,26], but molecules display non-crystallographic mirror symmetry (rmsd
always in the form of mixtures also containing other isomers. 0.04 Å).
As far as we are aware, the above route is the only one that
provides isomerically pure 23, not surprising in view of its route Our final attempt for the present to prepare a precursor hydro-
of formation. These isomeric hydrocarbons are useful ligands carbon for an all-carbon indigoid system rests on the observa-
for the preparation of bridged Ziegler–Natta catalysts employed tion that the Thiele condensation of indene (6) with acetone in
for olefin polymerization [26,27].
pyrrolidine/methanol yields the benzofulvene 29 (Scheme 6).
Since compound 23 contains doubly activated methylene posi-
tions, it should be easy to alkylate or bis-alkylate it. Further-
more, use of a bis-electrophile such as 1,2-dibromoethane could
lead to the [2.2]indenophane 26 or its isomer 27, both potential
ligands for the preparation of novel metallocene derivatives and
also so far unknown.
The other conceivable spiro isomer 28 seems to be a less likely
product since its E-configurated double bond (within a seven-
membered ring) should result in considerable strain (anti-Bredt
hydrocarbon).
When 23 was treated with tert-BuLi in THF and the presum-
ably resulting bis-anion quenched with 1,2-dibromoethane, a
complex product mixture consisting largely of polymeric ma-
terial was formed. From this mixture, however, trace amounts
of two hydrocarbons (total yield 5%) could be separated by thin
layer chromatography. Further (column) chromatographic
attempts to obtain preparative amounts of the two isomers in
analytically pure form failed, however. The spectroscopic and
analytical data of this hydrocarbon mixture (see experimental
part) agreed with the two structural proposals 24 and 25 shown
in Scheme 5. An unambiguous structure determination had to
await X-ray crystallographic analysis. The required single crys-
tals were obtained from the different chromatographic fractions
(which contained either isomer in enriched form only) by
recrystallization from chloroform and dichloromethane/chloro-
form, respectively. As can be seen from the X-ray structures,
the former is the anti-spiro compound 24 (Figure 4) and the
latter its syn-isomer 25 (Figure 5), with the former possessing
the longer chromatographic retention time. Unfortunately,
Scheme 6: Cross-conjugated hydrocarbons by Thiele condensation.
A suitable methylene component for this type of condensation
would be the tetra-substituted olefin 2-(2-indanylidene)indane
(30), in which each methylene group is activated both by a
benzene ring and a double bond. We prepared this known olefin
[28] by a McMurry-type dimerization of 2-indanone (9). The
structure determination by the usual spectroscopic methods was
straightforward and the obtained spectra agreed with the
reported data [28]. The assignment was corroborated by a
single-crystal X-ray structure determination (Figure 6); single
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Beilstein J. Org. Chem. 2015, 11, 363–372.
Figure 4: a) The molecule of compound 24 in the crystal. Ellipsoids represent 50% probability levels. Only the asymmetric unit is numbered.
b) Packing diagram of compound 24 viewed perpendicular to the bc plane. C–H···π contacts to the centroid of the aromatic six-membered ring (seen
side-on) can be recognized, but are not drawn explicitly. H atoms not involved in these contacts are omitted.
crystals were obtained by slow evaporation of a pentane solu-
tion.
The molecule of 30 is planar (mean deviation 0.02 Å), but the
true symmetry is close to D2h (rmsd 0.02 Å). The central C=C
bond length is 1.327(2) Å. The molecular packing involves
herringbone layers, parallel to the plane (102), in which C-H···π
contacts from a methylene hydrogen to the centroid of the six-
membered ring are observed (H1B···π 2.63 Å).
Unfortunately, all our condensation experiments with 30, under
conditions that were successful for 6, failed and neither did we
Figure 5: One of the two independent molecules of compound 25 in
the crystal. Ellipsoids represent 50% probability levels.
obtain the mono- (31) nor the (desired) bis-condensation
product 14.
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Beilstein J. Org. Chem. 2015, 11, 363–372.
Figure 6: a) The molecule of compound 30 in the crystal. Ellipsoids represent 50% probability levels. Only the asymmetric unit is numbered.
b) Packing diagram of compound 30 viewed perpendicular to (102). C–H···π contacts to the centroid of the six-membered ring can be recognized, but
are not drawn explicitly. H atoms not involved in these contacts are omitted.
Conclusion
Supporting Information
The preparation of a suitable precursor for the first all-carbon
analog of an indigoid dyestuff has failed so far. It is conceiv-
Supporting Information File 1
able that unsubstituted α-methylene ketones such as 12 are too
reactive to be isolated under the reaction conditions. On the
other hand, if the methylene group is protected by two methyl
groups, the corresponding ketone, 13, becomes isolable, but
fails to participate in a McMurry dimerization, possibly because
of steric hindrance. If the double bond between the indane-
derived “halves” in, e.g., 4 is replaced by a single bond (as in
22), a Cope reaction is the most favorable process yielding the
ethano-bridged bis-indenyl derivative 23, which can be
employed for the introduction of additional linker units,
but is of no use for the preparation of the intended target
molecule 4.
Characterization data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-42-S1.pdf]
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