Investigation into the Organobismuth Dismutation and Its Use for Rational Synthesis of Heteroleptic Triarylbismuthanes, Ar12Ar2Bi

Article abstract of DOI:10.1021/acs.organomet.9b00777

Organobismuthanes undergo dismutation, a substituent scrambling process, complicating the synthesis of unsymmetrically trisubstituted bismuthanes of the general formula Ar¹₂Ar²Bi. Although the dismutation is a mechanistically diverse phenomenon, at ambient or lower temperatures, dismutation is triggered mainly by an electrophilic bismuth source. Therefore, the selection of the electrophile, Ar¹₂BiX (X = tosylate or iodide if Ar¹ = mesityl) or Ar¹BiX₂ (X = tosylate), and its use in low concentration during the reaction is key to suppressing the dismutation, leading to new, streamlined protocols utilizing direct arylations of Ar¹₂BiX (X = OTs or I) or Ar¹Bi(OTs)₂ with organozincs affording heteroleptic triarylbismuthanes Ar¹₂Ar²Bi.

Full text of DOI:10.1021/acs.organomet.9b00777

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Investigation into the Organobismuth Dismutation and Its Use for
Rational Synthesis of Heteroleptic Triarylbismuthanes, Ar1 Ar2Bi
2
Thomas Louis-Goff, Arnold L. Rheingold, and Jakub Hyvl*
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ABSTRACT: Organobismuthanes undergo dismutation, a substituent scram-
bling process, complicating the synthesis of unsymmetrically trisubstituted
bismuthanes of the general formula Ar1 Ar2Bi. Although the dismutation is a
2
mechanistically diverse phenomenon, at ambient or lower temperatures,
dismutation is triggered mainly by an electrophilic bismuth source. Therefore,
the selection of the electrophile, Ar1 BiX (X = tosylate or iodide if Ar1 = mesityl)
2
or Ar1BiX2 (X = tosylate), and its use in low concentration during the reaction is
key to suppressing the dismutation, leading to new, streamlined protocols utilizing
direct arylations of Ar1 BiX (X = OTs or I) or Ar1Bi(OTs)2 with organozincs
2
affording heteroleptic triarylbismuthanes Ar1 Ar2Bi.
2
Ar12Ar2Bi F Ar1Ar22Bi F Ar13Bi+Ar32Bi
he growing interest in organobismuth chemistry is driven
(3)
T
by applications in synthetic chemistry,1−4 catalysis,5−7
medicinal chemistry,8 and material science9 without the
toxicity and environmental risks10 typically associated with its
heavy-atom congeners. Despite its potential, organobismuth
chemistry is far less developed in comparison with other main-
group organometallic chemistry due to two obstacles. First, the
C−Bi bond is weak (bond dissociation energy (BDE) = 46
kcal/mol11), making it prone to homolytic cleavage. Second,
dismutation, a substituent scrambling process first reported by
Challenger12 and further studied by Gilman, revealed that
tri(p-tolyl)bismuthane exchanged substituents with n-butyl-
lithium (eq 1).13 This complicates a selective synthesis of
unsymmetrical organobismuthanes of general formula Ar12Ar2Bi
(Ar1 ≠ Ar2).14
Besides the ate-complex-promoted mechanism of dismuta-
tion, other mechanistic modes have been proposed. For the
monoaryldialkylbismuthanes, a light-induced dismutation was
observed,18 suggestive of a radical mechanism, and in the
presence of bismuth trihalides an electrophilic mode likely
operates.17 Moreover, other factors are also involved; for
example, steric congestion around the bismuth center can
suppress the dismutation process completely.19 This inves-
tigation focuses on comparison of the ate-complex-promoted
and the electrophilic modes of dismutation, how to suppress
them, and then uses the information to optimize Bi−C bond
formation leading to an efficient and streamlined synthesis of
electronically diverse unsymmetrical organobismuthanes
Ar12Ar2Bi (1). These compounds (1) can now be prepared
through a simple two-step procedure that is no longer limited
by reagent scope20 or multistep processes.21
First, a systematic study of the effect of the type of
nucleophile and electrophile on the formation of the
unsymmetrical bismuthane 1 (Ar12Ar2Bi) and the related
dismutated products (Ar22Ar1Bi, Ar31Bi, and Ar32Bi) was
investigated. Our reaction of choice utilized diphenylbismuth
halide or tosylate, Ph2BiX, as the electrophilic source and an
organometallic nucleophile prepared from 4-bromobenzalde-
hyde dimethyl acetal affording easily separable diphenyl-
(p‐Tol)3Bi + 3n‐BuLi → 3p‐TolLi + (n‐Bu)3Bi
(1)
In 1967, Wittig observed the same exchange process for all
triaryl pnictogens, except nitrogen, and proposed a mechanism
involving an ate-complex.15 This ate-complex was suggested to
be an intermediate responsible for the formation of side
product, PhBi(n-Bu)2, in addition to the main product,
Ph2Bi(n-Bu), generated from reaction of Ph2BiCl and n-BuLi
(eq 2).16
+R − Li
−Ph − Li
Ph2Bi − R XoooooooooooY [Ph2BiR2]−Li+ XoooooooooooooY Ph − BiR2
ate‐complex
(2)
Received: November 14, 2019
Furthermore, Barton observed that the unsymmetrical
triarylbismuthanes Ar21Ar2Bi, where Ar1 ≠ Ar2, are also prone
to dismutation (eq 3).17 On the basis of this report,
unsymmetrical triarylbismuthanes were viewed as an unstable
and elusive species.
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(dimethoxymethylphenyl)bismuthane (1a) from the main
dismutated side product, triphenylbismuthane (Ph3Bi). The
ratio of products served as a measure of the dismutation
process (Scheme 1). Another advantage of this system is the
respectively. Surprisingly, the cyanocuprate formed from the
Grignard reagent (entry 5) showed a poor yield of 1a (19%),
and increased the amount of triphenylbismuthane (23%). The
highest yield was achieved with an organozinc (entry 6),
affording 1a in 74% yield with a somewhat elevated amount of
the Ph3Bi (12%). In entries 7−16, we varied the electrophile.
In comparison with diphenylbismuth chloride, diphenylbis-
muth iodide (entries 7−11) showed a somewhat similar trend,
with the best result achieved by the organozinc nucleophile
(entry 11) reaching the same yield of 1a (74%) as with
Ph2BiCl, reducing formation of triphenylbismuth to 7%. The
best-performing electrophile turned out to be diphenylbismuth
tosylate, Ph2BiOTs (2a), which afforded poor and moderate
yields with organolithium (entry 13) and lithium diorgano-
cuprates (entry 15), respectively, but excellent yields (exceed-
ing 90%) with Grignard (entry 12, 92%), magnesium
diorganocuprate (entry 14, 95%), and organozinc (entry 16,
94%) reagents. For further experiments, we selected organo-
zinc reagents, which demonstrated virtually no dismutation
and, moreover, displayed better functional group tolerance in
comparison with Grignard reagents, consuming less starting
material in comparison with diorganocuprate reagents that
utilize only one out of two aryl groups. These initial results
suggest that the selection of the electrophile is more significant
for a selective synthesis of heteroleptic triarylbismuthanes than
the selection of the nucleophile.
a
Scheme 1. A Model Reaction
a
A model reaction for testing various nucleophiles and electrophiles in
Bi−C bond-forming reactions.
relatively similar electronic properties of these two aryl groups
(Ar1 = Ph and Ar2 = C6H4CH(OCH3)2), limiting the
electronic influence on Bi−C bond formation process.
Dismutation was evaluated by varying the nucleophilic
sources (Table 1, entries 1−6) from hard nucleophiles,
Next, a comparison of nucleophilic and electrophilic modes
of dismutation by the addition of either a sub-stoichiometric
amount of PhMgBr, PhZnX (X = Br or Cl), MgCl2, ZnCl2,
Ph2BiCl, Ph2BiI, or Ph2BiOTs to diphenyl(4-methoxyphenyl)-
bismuthane 1b was explored (Scheme 2). We chose
bismuthane 1b, since an electron-rich group is a better leaving
a
Table 1. Isolated Yields
entry
reagent
ArMgBr
ArLi
[Ar2Cu]MgX
[Ar2Cu]Li
[ArCuCN]MgX
ArZnX
ArMgBr
ArLi
[Ar2Cu]MgX
[Ar2Cu]Li
ArZnX
ArMgBr
ArLi
[Ar2Cu]MgX
[Ar2Cu]Li
ArZnX
Ph2BiX
1a
BiPh3
1
2
3
4
5
6
7
8
Ph2BiCl
Ph2BiCl
Ph2BiCl
Ph2BiCl
Ph2BiCl
Ph2BiCl
Ph2BiI
Ph2BiI
Ph2BiI
Ph2BiI
Ph2BiI
Ph2BiOTs
Ph2BiOTs
Ph2BiOTs
Ph2BiOTs
Ph2BiOTs
30%
40%
64%
59%
19%
74%
25%
4%
32%
64%
74%
92%
21%
95%
40%
94%
30%
9%
4%
Scheme 2. Dismutation of Bismuthane 1b Triggered by
Addition of Nucleophilic and Electrophilic Reagents
7%
23%
12%
15%
13%
14%
17%
7%
<1%
15%
1%
9
10
11
12
13
14
15
16
4%
0%
a
Isolated yields of triarylbismuthane 1a and Ph3Bi from the model
reaction in Scheme 1. Typical reaction conditions: THF, −10 °C, 1 h
40 min. For more details please see the Supporting Information.
operating often through a single electron transfer mechanism
to soft nucleophiles operating by an ionic mechanism.22 All the
nucleophiles were compared to a single electrophile,
diphenylbismuth chloride, Ph2BiCl. Grignard reagents (entry
1) showed a rather poor yield of the product 1a (30%)
accompanied by a relatively high amount of the Ph3Bi (30%).
Organolithiums (entry 2) only slightly increased the yield of 1a
to 40% while significantly decreasing the amount of Ph3Bi to
9%. Organocuprates (entries 3−5), prepared from a Grignard
reagent (entry 3) or an organolithium (entry 4), showed
improved yields of 1a 64% and 59% and decreased the
formation of the triphenylbismuthane to 4% and 7%,
a
b
Recovered material. Molar ratio.
B
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Scheme 3. Preparation of Triarylbismuthanes 1 utilizing Diarylbismuth Precursors 2 and Arylbismuth Ditosylates 3
group than an electron-poor one,23 making 1b an ideal
substrate for studying dismutation of triarylbismuthanes. To
our surprise, the Grignard reagent (entry 1), organozinc (entry
2), MgCl2 (entry 3), and ZnCl2 (entry 4) had no effect within
the time period, and 1b was recovered in essentially
quantitative yields. However, the bismuth-based electrophiles,
Ph2BiCl (entry 5), Ph2BiI (entry 6), and Ph2BiOTs (entry 7)
decreased the amount of recovered 1b to 30%, 43%, and 33%
and afforded a mixture of three additional dismutated products
A, B, and C in a molar ratios of 11.3:9.2:5.7:1, 17.5:15.1:5.6:1,
and 18.6:15.1:3.7:1 of 1b/A/B/C after 1 h at room
temperature in tetrahydrofuran (THF), respectively. Subse-
quent experiments with Ph2BiI demonstrated that the time
frame for equilibration is variable, sometimes taking up to 6 h.
This induction period may indicate that a secondary reagent or
process derived from the reaction or decay of Ph2BiI might be
responsible for the dismutation. Nevertheless, these results
indicate that the ate-complex-intermediate mechanism is
accessible when the nucleophile is in excess but only at higher
temperatures (reflux in diethyl ether) and elongated reaction
times (20 h).13
achieved by slow addition of the electrophile into a solution of
nucleophile, rather than the previously reported reversed order
of the reagents, is a logical step in obtaining high yields of
heteroleptic triarylbismuthanes.16 To support this statement,
we ran two experiments differing in the order of the reagent
addition. In the first experiment, 1 equiv of nucleophile p-
CH3OC6H4ZnX was added in four portions into 1 equiv of
electrophile Ph2BiI over 2 h at room temperature affording the
isolated yield of 1b in 31% with the ratio of 1b/A/B being
2.8:2.1:1. In the second experiment, four portions of the
electrophile were added into the nucleophile under the same
conditions providing an isolated yield of 1b in 55% with the
ratio of 1b/A/B being 13.9:0.85:1 (see Supporting Informa-
tion for more details). This result supports our conclusion and
is in agreement with our previous results (Scheme 2).
To demonstrate the generality and applicability of our new
findings, a set of diverse unsymmetrical triarylbismuthines,
Ar12Ar2Bi, 1a−1h, was targeted through a two-step process
(Scheme 3, Methods A and B). Overall, bismuthanes 1 were
formed in good to high yields (83−99%) regardless of the
electronic properties of their substituents. In the first step,
previously reported diphenylbismuth tosylate (2a) (Method
A) was prepared from a simple combination of triphenylbis-
muthane with a slight excess of p-toluenesulfonic acid (p-
TsOH·H2O) in diethyl ether.24 We attempted to synthesize
In general, the dismutation is far more likely when triggered
by the electrophilic Ph2BiX, and it occurs at ambient
temperatures and shorter reaction times (1−6 h). Therefore,
keeping the concentration of the electrophile to a minimum,
C
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other diarylbismuth tosylates from the corresponding triar-
ylbismuthanes, such as Bi(p-CH3OC6H4)3, Bi(p-NCC6H4)3,
and trimesitylbismuthane, but were unable to isolate them in
sufficient purity, often obtaining inseparable mixtures of the
desired products Ar2BiOTs, starting materials Ar3Bi, and
arylbismuth ditosylates ArBi(OTs)2. Although these impure
species could be used in the following step, the yield was
diminished, and the corresponding product needed to be
isolated from a complicated reaction mixture (see Supporting
Information for more details). As stated previously, increased
steric congestion retards the dismutation process; therefore,
dimesitylbismuth iodide,19 Mes2BiI (2b), prepared by treat-
ment of trimesitylbismuthane with BiCl3 followed by halide
exchange with sodium iodide (Method B), afforded com-
parable results to the monotosylate 2a with minimal formation
of the dismutated byproducts (vide infra). In the second step,
diarylbismuth tosylate or iodide 2 was added to 1.2 equiv of
organozinc reagent, Ar2ZnX, at −10 °C, and the resulting
reaction mixtures were stirred for 100 min, affording the target
set of products 1 in favorable yields without formation of the
dismutated byproducts.
We also explored an alternative approach utilizing
monoarylbismuth ditosylates 3, prepared from the correspond-
ing homoleptic triarylbismuthane and 2 equiv of p-TsOH·H2O
under reflux in high yields (Scheme 3, Method C). In the
following step, compound 3 was treated with the organozinc
reagent yielding the corresponding heteroleptic triarylbismu-
thanes 1i−1l. However, in comparison with the previous
protocols, the yields obtained for 1i−1l were fairly low (52−
91%) relative to the first two synthetic protocols accessed
through diarylbismuth tosylates 2. To unambiguously confirm
the structure of our heteroleptic triarylbismuthanes 1, we
obtained an X-ray of 1k. The bond lengths and angles closely
match the parent triphenylbismuthane (see Supporting
Information for more details).25
In conclusion, we investigated the critical factors responsible
for the dismutation of heteroleptic triarylbismuthanes that had
previously obscured their syntheses. Our discovery is that
selection of the electrophile is crucial, improving efficiency by
minimizing dismutation, which otherwise diminishes the yield
and purity of the product. When the bismuth electrophiles
were compared, the best results were obtained with
diphenylbismuth tosylate (2a), which afforded the highest
yields of product 1 with virtually no formation of byproducts.
The superior performance of 2a over diphenylbismuth halides
may result from the chelate-like coordination of the tosylate
ligand to the organobismuth counterpart, or its inability to
form a bridged species, thus increasing its stability to
dismutation. Alternatively, it may reduce solubility, effectively
shortening the time the reactive species exists in solution. The
soft or hard nature of the nucleophile does not seem to have
much of an effect, since the Grignard and organozinc reagents
give comparable yields and amounts of dismutated byproducts.
Steric congestion around the bismuth center significantly
reduces dismutation, which can be demonstrated by better
performance in yield and selectivity of dimesitylbismuth iodide
(2b) versus Ph2BiI. A second crucial observation according to
our study (Scheme 2) is maintaining a minimal concentration
of the bismuth electrophile during the course of the reaction.
This was accomplished by adding the electrophile to a solution
of the nucleophile. These findings permit an efficient two-step
synthetic protocol utilizing either diarylbismuth or monoaryl
precursors and afford an electronically diverse set of
heteroleptic triarylbismuthanes 1, Ar21Ar2Bi, without the
formation of dismutated contaminants.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00777.
■
sı
Complete experimental details with NMR spectra
(PDF)
Accession Codes
CCDC 1984672 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Jakub Hyvl − Department of Chemistry, University of Hawai‘i at
Manoa, Honolulu, Hawaii 96822, United States; orcid.org/
̅
0000-0002-5509-5567; Email: hyvl@hawaii.edu
Authors
Thomas Louis-Goff − Department of Chemistry, University of
Hawai‘i at Manoa, Honolulu, Hawaii 96822, United States
̅
Arnold L. Rheingold − Department of Chemistry, University of
California, San Diego, La Jolla, California 92093, United
States; orcid.org/0000-0003-4472-8127
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.9b00777
Notes
The authors declare the following competing financial
interest(s): A provisional patent application was filed by
University of Hawai‘i, which is not yet published.
ACKNOWLEDGMENTS
J.H. is grateful for start-up funds and laboratory space provided
by the Univ. of Hawai‘i at Manoa. J.H. thanks M. Cain and J.
Romine for feedback on the manuscript draft, W. Brennessel
(Univ. of Rochester) for conducting elemental analysis, and W.
Y. Yoshida for NMR spectra acquisition.
■
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