Sulfuration of organoborates, an underexploited method

Article abstract of DOI:10.1016/S0040-4039(00)00945-X

The combination of sulfurating agents with organoboranes is an underexploited synthetic transformation. The reactivity of borate complexes was investigated with several electrophilic sulfur species. Simple and practical methods for making carbon-sulfur bonds were created under almost neutral conditions. These enjoy a heavy metal-free environment and use not so toxic boron compounds. This is in contrast to the manipulation of poisonous H₂S or the use of sulfur with Grignard reagents, under highly basic and moisture-sensitive conditions. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00945-X

Tetrahedron Letters 41 (2000) 6053±6057
Sulfuration of organoborates, an underexploited method
Sebastien Kerverdo and Marc Gingras*
Â
Department of Chemistry, Faculty of Sciences, University of Nice-Sophia Antipolis, 28 Avenue Parc Valrose,
6108 Nice Cedex 2, France
0
Received 20 March 2000; accepted 7 June 2000
Abstract
The combination of sulfurating agents with organoboranes is an underexploited synthetic transformation.
The reactivity of borate complexes was investigated with several electrophilic sulfur species. Simple and
practical methods for making carbon±sulfur bonds were created under almost neutral conditions. These
enjoy a heavy metal-free environment and use not so toxic boron compounds. This is in contrast to the
manipulation of poisonous H S or the use of sulfur with Grignard reagents, under highly basic and
2
moisture-sensitive conditions. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: sulfuration; hypervalent elements; boron and compounds; disul®des; sul®des.
Although several methods exist for introducing sulfur into organic molecules, there is a de®ciency
1
of studies involving organoborates. We intend to describe here the sulfuration of these species in
a heavy-metal free environment. These salts have launched new organic methodologies, due to
the activation of some carbon±boron bonds. This has been exploited in many Pd-catalyzed
2
3
procedures with boronic acids (Suzuki couplings), with aryltri¯uoroborates or with tetra-
4
phenylborate. Classic examples also embrace organoborates in the sequence hydroboration±
oxidation, which has become a cornerstone in synthesis. As a comparison, parallel sulfur
reactions are still in their infancy.
Trialkyl- or triarylboranes were sporadically used in the early sulfurations with elemental sul-
fur. In 1961, Mikhailov and co-workers were among the pioneers to introduce sulfuration of
5
organoboranes with elemental sulfur. In 1970, other preliminary results were obtained for the
6
direct preparation of disul®des. The study was con®ned to four substrates and the yields varied
from 2±48%, with only one case at 64%. A speculative mechanism was proposed: the insertion of
a sulfur atom into a carbon±boron bond, followed by the hydrolysis of an arylthioborane to a
thiol intermediate; the latter being oxidized in situ to a disul®de. The transfer of only one boron
ligand was suggested.
*
Corresponding author: Fax: +33 4 93 44 04 25; e-mail: marc.gingras@wanadoo.fr
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00945-X

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From these mechanistic speculations, charged electron-rich species with a higher valence
number would react even better with sulfur electrophiles. Our investigations started with some
crystalline organoborate salts, listed in Fig. 1. Compound 1 is commercial, but we postulated that
salt 2 could be generated in situ. Reagent 3 was synthesized on a multiple gram-scale, according
to Vedejs and co-workers.7
Figure 1. List of organoborates used of generated
8
Other reactions with organoboranes, less related to this work, were achieved by transfer of
9
10
alkylthio groups from organic disul®des or by alkyl transfer from boranes to sulfenyl chlorides.
These lead to sul®des.
The next step was to test the reactivity of electrophilic sulfur reagents, like those presented in
Fig. 2. Many of them are commercial crystalline solids. The abundance of sulfur and its almost
non-odorous nature, is highly attractive. As a substitute, sulfur monochloride is cheaply
produced in bulk amounts. The rewards for developing new sulfuration procedures might
eventually be the replacement of poisonous, smelly and gaseous H S.
2
Figure 2. List of electrophilic sulfur reagents
Classic preparations of thiols and disul®des are reported in Eq. 1 (Scheme 1), from the reaction
of sulfur powder with highly basic Grignard reagents. Our procedures, with inexpensive S Cl ,
2
2
avoid strong bases (Eq. 2). It provides an expeditive synthesis of disul®des, without an additional
oxidation of thiols. In the same way, a combination of triphenylborane, CsF and elemental sulfur
gives rise to disul®des (unoptimized, Eq. 3). In contrast to our previous work on the sulfuration
of hypervalent organotins, reagent 6 selectively produces monosul®des (Eq. 4).11
Manipulation of air- and moisture-sensitive carbanions, such as Grignard reagents, is sometimes
annoying. Undesirable anhydrous titrations for a standardization is also tedious. Because borate
salts in Fig. 1 are either commercial, crystalline and non-hygroscopic, they are interesting
synthetic equivalents to these carbanions and all procedures are operating under almost neutral
conditions.
Table 1 is a compilation of assays and optimization, where we varied the following parameters:
organoboron used, sulfurating agent, ¯uoride source, temperature and reaction time. In a general
manner, tetrabutylammonium tetraphenylborate did not provide satisfactorily results either with
sulfur (Entry 4: 30% yield) or with sulfur monochloride (Entry 6: 11% yield). In spite of several
ꢀ
assays with 1, changes of molar ratios sulfur/boron complex and prolonged heating at 130 C did

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Scheme 1. Reactions of nucleophiles with sulfurating agents
not yield signi®cant amounts of phenyl disul®de. Phenyl boronic acid was useless under similar
conditions.
On the other hand, the reactivity of triphenylborane/CsF 2 and potassium phenyltri-
¯uoroborate 3 provided the best overall results. The sulfur was somewhat troublesome, because it
failed to react with 3 but gave decent yields with 2. Similarly, sulfur monochloride gave the best
match with potassium phenyltri¯uoroborate (Entries 11 and 12: 85 and 82%, respectively) but a
poor yield with triphenylborane/CsF 2 (Entry 3: 20%). Finally, sulfurating agent 6 produced a
reasonable yield of monosul®de with potassium phenyltri¯uoroborate 3, but a minor
contamination by disul®des was observed (Entries 14 and 15).
Overall, the reactions of organoborate compounds can selectively produce organic
monosul®des or disul®des, depending on the electrophilic sulfur source. The eciency varies
according to the boron salts, 3 being the most reactive. Interestingly, some reactions proceed even
ꢀ
ꢀ
at 50 C, instead of 130 C, which seems encouraging for future investigations and optimization
Entry 13). Finally, an excess of S Cl is advisable for better yields (Entries 11 and 12) and our
data suggest at this time that only one boron ligand can be transferred from 2.
(
2
2
At this stage, it would not be wise to delineate a precise mechanism for these sulfurations.
Some divergence in the behavior of organoborates leads to postulate distinct mechanisms. One of
these seems to operate in a carbanion-like manner and suggests an ionic mechanism with 3; the
production of monosul®des is in¯uencing us in this direction.12 The other mechanism is with
triphenylborane complex 2 and questions the possibility of generating radicals through an SH2
.
reaction, leading to a ®nal dimerization of RS . The by-product biphenyl directed us in this
thought. Radical pathways with boron are known, especially in the presence of radical initiators
such as oxygen.13
In summary, sulfuration of organoborates led to the controlled synthesis of sul®des or
disul®des. This demonstrated the relative reactivity of tetraphenylborate, ¯uorotriphenylborate
or phenyltri¯uoroborate anions, the latter being the most reactive. Additionally, these simple
methods were e€ected in a heavy metal-free environment, with not so toxic boron compounds.

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Table 1
Sulfuration of organoborates and boron compoundsa
Acknowledgements
We thank the University of Nice-Sophia Antipolis for start-up funds. S.K. is grateful to the
Government of France (MENRT), for a doctoral scholarship. We thank Professor Roland
Fellous for allowing us to share the laboratory and for his scienti®c enthusiasm.
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