The bis(allyl)bismuth cation: A reagent for direct allyl transfer by lewis acid activation and controlled radical polymerization

Article abstract of DOI:10.1002/anie.201206782

Cationic organometallic compounds of Group 3 (including the lanthanoids), 4, or 13 elements show increased Lewis acidity and electrophilicity compared to their neutral parent compounds. They serve as efficient polymerization catalysts and as unique organometallic reagents for stoichiometric transformations.[1-3] The Lewis acidity of neutral bismuth(III) compounds has been recognized and exploited in bismuthcatalyzed (co)polymerizations and organic transformations.[4] However, the number of isolated and characterized cationic organobismuth(III) compounds is still limited, and detailed investigations concerning their reactivity have not been reported to date.[5, 6] Only two systems with chelating di(aryl) ligands have been applied as Lewis acid-base catalysts in organic transformations.[6] Herein, we present the isolation and characterization of the bis(allyl)bismuth cation and its application as an efficient allyl transfer reagent and as initiator for the controlled radical polymerization of styrene.

Full text of DOI:10.1002/anie.201206782

Angewandte
Chemie
DOI: 10.1002/anie.201206782
Organobismuth Chemistry
The Bis(allyl)bismuth Cation: A Reagent for Direct Allyl Transfer by
Lewis Acid Activation and Controlled Radical Polymerization**
Crispin Lichtenberg, Fangfang Pan, Thomas P. Spaniol, Ulli Englert, and Jun Okuda*
Cationic organometallic compounds of Group 3 (including
the lanthanoids), 4, or 13 elements show increased Lewis
acidity and electrophilicity compared to their neutral parent
compounds. They serve as efficient polymerization catalysts
and as unique organometallic reagents for stoichiometric
transformations.[1–3] The Lewis acidity of neutral bismuth(III)
compounds has been recognized and exploited in bismuth-
catalyzed (co)polymerizations and organic transformations.[4]
However, the number of isolated and characterized cationic
organobismuth(III) compounds is still limited, and detailed
investigations concerning their reactivity have not been
reported to date.[5,6] Only two systems with chelating di(aryl)
ligands have been applied as Lewis acid–base catalysts in
organic transformations.[6] Herein, we present the isolation
and characterization of the bis(allyl)bismuth cation and its
application as an efficient allyl transfer reagent and as
initiator for the controlled radical polymerization of styrene.
Tris(allyl)bismuth (1) was first synthesized about 50 years
ago and has later been used as a bismuth source for
metalorganic chemical vapor deposition.[7] The bonding
mode of the allyl ligands has not been investigated to date.
Compound 1 was synthesized according to a simplified
literature procedure. The yellow liquid is temperature, light,
air, and vacuum sensitive and slowly decomposes (over
months) when stored under argon at À308C in the dark.
The main decomposition pathway is radical coupling of allyl
ligands with concomitant formation of a bismuth mirror.[8]
Attempts to crystallize 1 by cooling a sample in a sealed
capillary on the diffractometer remained unsuccessful. Tris
(methallyl)bismuth (2), which has been elusive to date, was
isolated as a yellow liquid (m.p. = 176 K) in low yield (see the
Supporting Information), and an in situ crystallization experi-
ment afforded its solid-state structure (Scheme 1).[9,37] The
methallyl ligands coordinate to the metal center in an h1
Scheme 1. Synthesis of bis(allyl)bismuth cation 3; [A]=[B(C6H3Cl2)4].
Molecular structure of 2 and cationic part of 3. Ellipsoids are set at
50% probability; hydrogen atoms are omitted for clarity. For 3, only
one of the two crystallographically independent cations is shown and
discussed, as the structural parameters are highly similar. Selected
bond lengths [ꢀ] and angles [8]: 2: Bi1–C1 2.324(16), Bi1–C5
2.310(17), Bi1–C9 2.309(16), C1–C2 1.44(2), C2–C3 1.32(2), C2–C4
1.51(3); C1-Bi1-C5 93.9(9). 3: Bi1–C1 2.273(10), Bi1–C4 2.242(11),
Bi1–O1 2.404(7), Bi1–O2 2.418(7), C1–C2 1.489(13), C2–C3 1.325(14);
C1-Bi1-C4 92.2(4), C1-Bi1-O1 91.5(3), O1-Bi1-O2 171.9(3).
2.331(2) ꢀ).[5g] The coordination geometry around the bis-
muth center in 2 is trigonal pyramidal with small C-Bi-C
angles of about 948.[10] Compound 2 is a typical molecular
crystal without short directional interactions between neigh-
boring molecules. Shortest intermolecular contacts are asso-
ciated with Bi···H(methyl) distances of ca. 3.2 ꢀ and owing to
stacking along the crystallographic b axis (see Supporting
Information). In comparison, intermolecular Bi···C interac-
tions have been revealed for [Bi(C5H5)3] in the solid state.[11]
In solution, the allyl ligands of 1 and 2 show h1 bonding modes
without fluxionality at ambient temperature in contrast to
[Bi(C5H5)3] in which the h1-bonded cyclopentadienyl ligands
show fluxional behavior.[11]
Organobismuth cations have so far been prepared by salt
elimination reactions.[5,12] However, this approach was not
feasible using [Bi(C3H5)2X] (X = halide) owing to their low
stability (see the Supporting Information). A protonolysis
reaction of 1 with the sufficiently strong Brønsted acid
[PhNMe2H][B(C6H3Cl2)4] allowed the isolation of the bis-
(allyl)bismuth cation 3 as a yellow solid (Scheme 1). Com-
pound 3 can be stored over months under inert gas
atmosphere in the dark without decomposition. The solid-
state structure of 3 was investigated by single-crystal X-ray
analysis. Compound 3 crystallizes in the orthorhombic space
group Pbca with Z = 16 (Scheme 1).[37] The bismuth center
adopts a bisphenoidal coordination geometry with the THF
ligands occupying the axial positions. The allyl ligands in 3
À
fashion and show localized single and double C C bonds. The
À
average Bi C bond lengths (2.32(2) ꢀ) are similar to the
corresponding value reported for the only other fully
characterized bismuth compound containing an allyl ligand,
allyl
[Bi(h1-C3H3){2,6-(Me2NCH2)2C6H3}2]
(Bi C
À
[*] C. Lichtenberg, F. Pan, Dr. T. P. Spaniol, Prof. Dr. U. Englert,
Prof. Dr. J. Okuda
Institut fꢁr Anorganische Chemie, RWTH Aachen
Landoltweg 1, 52056 Aachen (Germany)
E-mail: jun.okuda@ac.rwth-aachen.de
[**] Generous financial support by Fonds der Chemischen Industrie
(Kekulꢂ Scholarship to C.L.) is gratefully acknowledged. The authors
thank Dr. Klaus Beckerle for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206782.
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Communications
coordinate to the metal center in an h1 fashion. Thus, the
coordination mode of allyl ligands in allyl bismuth com-
pounds is retained upon cationization (s vs. p bonding), as
observed for allyl complexes of other Lewis acidic metals
(La,[13a] Nd,[13a] Zn,[13b] Al,[3c] Ga[3e]).[14] Steric factors can be
ruled out as a reason for the lack of p-coordination, as
bismuth cations with coordination numbers of up to eight
À43) JmolÀ1 KÀ1 determined for these compounds were
significantly smaller than that of 3. For [Zn(C3H5)2], the
negative activation entropy has been shown to correspond to
an intermolecular allyl exchange mechanism.[18a,19,20] In turn,
the positive DS° value of 3 suggests an intramolecular allyl
exchange mechanism for this compound.
The THF ligands in 3 are labile and can be substituted for
stronger donors, as demonstrated by the synthesis of [Bi-
(C3H5)2(HMPA)2][B(C6H3Cl2)4] (4; HMPA = hexamethyl-
phosphoramide). Single-crystal X-ray analysis of 4 revealed
a molecular structure similar to that of 3 (see the Supporting
Information).
Bismuth(III) catalysts have been used for the allylation of
aldehydes.[21,22] In these reactions, addition of a second
component, such as an allyltin compound, an allyl silane
(Sakurai reaction), or the combination of an allyl halide and
Fe0, Zn0 or Sn0, was required.[23] The direct carbometalation of
a carbonyl substrate by a well-defined organobismuth(III)
compound alone (such as [BiMe3] or [BiPh3]) has not been
observed to date.[24–26] In agreement with these earlier reports,
tris(allyl)bismuth (1) did not react with benzaldehyde in THF
at ambient temperature. Unexpectedly, 1 slowly reacted with
benzaldehyde under carbometalation when the Lewis acid
[BPh3] was added, as detected by NMR spectroscopy (26 h,
238C, 81% yield).[27] These results prompted us to investigate
the Lewis acidic bis(allyl)bismuth cation 3 as a direct allyl
transfer reagent. Indeed, 3 quantitatively reacted with two
equivalents of benzaldehyde in THF at ambient temperature
to give the carbometalation product in a rapid reaction
(Table 1, entry 1). Notably, coordination of aldehydes to an
organobismuth cation with a chelating diaryl ligand has
previously been reported.[5d] Thus, we suggest, that not only is
the Lewis acidity of the cationic bismuth center responsible
for the carbometalation reaction to occur, but also the lability
of the allyl ligands. From a mechanistic viewpoint, two
different roles have been discussed for the bismuth center in
have been reported.[15] The Bi C bond lengths are signifi-
À
cantly shorter compared to 2, and this shortening is ascribed
to the cationic charge in 3. Although similarities in the
coordination chemistry of bismuth and the lighter lanthanides
can be expected on the basis of similar charge-to-radius ratio,
the higher electronegativity and the stereochemically active
lone pair of bismuth can cause marked differences between
lanthanide and bismuth compounds.[16] In the case of allyl
complexes, the coordination chemistry of 3 clearly differs
from that of the bis(allyl)lanthanide cations [Ln(h3-C3H5)2-
(thf)4]+ (Ln = La, Nd; p coordination of (C3H5)À, C.N.(Ln) =
8).[13a] In contrast, close structural similarities of 3 with the
related Group 13 cations [E(h1-C3H5)2(thf)3]+ become appar-
ent when taking into account the equal contribution of lone
pairs and donor ligands to the steric number of a molecule
(E = Al, Ga; s coordination of (C3H5)À, C.N.(E) = 5).[3c,e]
Compound 3 is soluble in THF, dichloromethane, and
pyridine, but insoluble in diethyl ether and hydrocarbons. It
shows an increased stability in solution compared to 1.
Decomposition also occurs via radical pathways, as evident
from the formation of 1,5-hexadiene as a degradation prod-
uct.[17] In the 1H NMR spectrum of 3 in [D8]THF at 238C, the
allyl ligands cause an A4X pattern, indicating a fluxional
behavior (Figure 1, top). Cooling this sample to À958C
showed an A2MNX pattern for the allyl ligands, revealing
the h1 bonding mode as the ground state of allyl coordination
for compound 3 in solution (Figure 1, bottom). The allyl
exchange was investigated by means of lineshape analysis (see
the Supporting Information). Values of DG°,298 K = (40.9 Æ
5.0) kJmolÀ1, DH° = (42.4 Æ 2.2) kJmolÀ1
,
DS° = (5.0 Æ
9.3) JmolÀ1 KÀ1 and an allyl exchange rate of k = 4.3 ꢁ
105 Hz at 298 K were determined. The allyl exchange in
THF of a few other allyl metal compounds, namely [Mg-
(C3H5)Hal], [Ca(C3H5)([18]crown-6)][Zn(C3H5)3], and [Zn-
(C3H5)2], has been investigated by lineshape analysis (Hal =
Cl, Br) revealing exchange rates at 298 K of 1.0 ꢁ 104 Hz to
2.8 ꢁ 107 Hz.[18] Notably, the DS° values of (À16 to
Table 1: Reaction of 3 with aldehydes, imines, and ketones.
Entry
n
R1
R2
X
Product
Yield[a]
1
2
3
4
5
6
7
8
2
2
2
2
2
2
2
2
2
2
2
2
10
50
Ph
H
H
H
H
H
H
H
H
H
H
H
Ph
Ph
Ph
O
O
O
O
O
O
O
O
O
O
NTos
O
O
6a
6b
6c
6d
6e
6 f
6g
6h
6i
6j/k
6l
>99
>99
>99
>99
>99
>99
>99
>99
>99
4-FC6H4
4-ClC6H4
4-BrC6H4
4-CF3C6H4
4-PhC6H4
1-napthyl
4-CNC6H4
4-NO2C6H4
4-Me/4-MeOC6H4
Ph
9
10
11[b]
12
13
14
traces
54
Ph
Ph
Ph
6m
6m
6m
29[c]
64[c]
97[c]
O
1
Figure 1. Excerpts of H NMR spectra of 3 in [D8]THF at 238C (top)
and À958C (bottom); : resonances that are due to THF ligands and
[a] Determined by NMR spectroscopy. [b] Reaction time: 3 h. [c] Yield
based on monoinsertion product 6m (see main text).
*
[D8]THF.
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allylations of aldehydes with bismuth catalysts: 1) Lewis acid
activation of the substrate;[6c] and 2) transfer of the allyl
ligand to the substrate (with possible simultaneous Lewis acid
activation of the substrate).[21i] Our results provide the first
evidence for the direct carbometalation of a carbonyl com-
pound by a well-defined organobismuth reagent.[26] Further-
more, they show that Lewis acid activation of the substrate
induced by cationization of the organobismuth reagent is
essential.
were obtained (Table 2, entry 3; Figure 2).[32] End group
analysis revealed an allyl moiety to be covalently bonded to
the polymer chain. Reaction of 3 with [Sn(nBu)3H] in C6D6 at
We set out to investigate the electronic demand and
functional-group tolerance of the carbometalation of alde-
hydes by 3. Electron-poor aldehydes 5b–i were quantitatively
converted into the carbometalation products 6b–i (Table 1,
entries 2–9). When electron-rich substrates were used, only
traces of the carbometalation products were detected, major
side reactions being polymerization of the substrate and the
solvent.[28] Reactions of 3 with aldehydes show an excellent
functional group tolerance for electron poor substrates as
they are compatible with halide, nitrile, and even nitro
substituents (Table 1, entries 2–4,8,9). Substitution of the
aromatic substrate in the ortho- and meta-position did not
sterically inhibit the reaction (Table 1, entry 7). In all of these
reactions, two sets of signals with a relative intensity of 1:1
were observed for the carbometalation products and ascribed
to the R,R/S,S enantiomers and the R,S meso-compound.
Substrates other than aldehydes were also studied. The N-
protected aldimine 5l gave the carbometalation product 6l in
moderate yield (Table 1, entry 11).[29] In reactions of 3 with
benzophenone, the monoinsertion product [Bi(C3H5)(OCPh2-
(C3H5))(thf)2][A] (6m) was formed selectively (Table 1,
entry 12). A yield of 29% was observed initially (determined
spectroscopically), which did not increase after prolonged
reaction times although unconverted starting materials
remained unchanged. This indicates equilibrium conditions
for this reaction. Accordingly, excess benzophenone shifted
the equilibrium to the product side with formation or up 97%
of monoinsertion product 6m (Table 1, entries 13,14).[30]
Neutral organobismuth compounds have recently been
reported to be unique initiators for the controlled living
radical polymerizations of activated olefins, such as styrene.[31]
Neutral 1 and cationic species 3 were tested as initiators in
polymerization reactions, as these compounds undergo
Figure 2. Plot of Mn and Mw/Mn versus conversion for bulk polymeri-
zation of styrene at 1208C using 3 as an initiator.
ambient temperature gave propene as a main product,
suggesting that the allyl ligands of 3 are susceptible to
homolysis. Thus, the polymerization of styrene initiated by 3 is
likely to proceed by a controlled radical mechanism. A linear
correlation between the polymer yield and the number
average molecular weight revealed the living character of
the polymerization (Figure 2). The bulk polymerization of
styrene initiated by 3 was shown to follow first-order kinetics
in the range of 80–1208C with rate constants ranging from
(2.3 Æ 0.1) ꢁ 10À5 sÀ1 to (2.5 Æ 0.5) ꢁ 10À4 sÀ1.[33] Activation
parameters of DH° = (À64.9 Æ 0.8) kJmolÀ1 and DS° =
(150 Æ 18) JmolÀ1 KÀ1 were determined from an Eyring plot
in the same temperature range (see the Supporting Informa-
tion).[34] In preliminary experiments with 1 and 3 as initiators,
similar trends have been observed when other activated
olefins, such as methyl methacrylate, were used as monomers
(see the Supporting Information). These results provide the
first example of a cationic main group organometallic
compound to act as an initiator for controlled living radical
polymerizations.[35,36]
In conclusion, we have presented the structurally charac-
terized bis(allyl)bismuth cation, which is present in [Bi-
(C3H5)2(thf)2][B(C6H3Cl2)4] (3), that acts as a stoichiometric
allyl transfer reagent and as an initiator for the controlled
radical polymerization.
À
homolysis of the Bi C bond (see above). Compound 1 was
active as an initiator in the bulk polymerization of styrene to
give an atactic polymer with a broad molecular weight
distribution (polydispersity index (PDI) = 4.62; Table 2,
entry 1). In contrast, under identical conditions cationic 3 as
an initiator gave atactic polystyrene (PS) with a lower PDI of
1.62 (Table 2, entry 2). Under optimized reaction conditions,
high yields of PS with consistently low PDI values of 1.3–1.4
Received: August 21, 2012
Revised: September 21, 2012
Published online: November 14, 2012
Keywords: allyl ligands · allylation · bismuth · cationic species ·
radical polymerization
.
Table 2: Bulk polymerization of styrene.[a]
Entry
Initiator
T
t
Yield
[%]
Mn
Mw/Mn
[8C]
[h]
[gmolÀ1
]
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4.5
2
58
67
90
8400
9400
14900
4.62
1.62
1.38
[a] Monomer/initiator=100:1 (molar ratio).
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at RT in THF.
[9] The solid-state structure of the dibismuthine [Bi2Et4] has
recently been established using a similar method: A. Kuczkow-
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[23] Especially in the case of toxic allyltin, the need for co-reagents in
stoichiometric amounts diminishes any possible environmental
benefits owing to the use of a bismuth catalyst.
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13014 www.angewandte.org
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Angew. Chem. Int. Ed. 2012, 51, 13011 –13015

Angewandte
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Mn = 2.56.
[25] Pentaorganobismuth(V) compounds do not undergo carbome-
talation with carbonyl compounds. [BiPh5] reacts with enolizable
carbonyl compounds in a two-step sequence of deprotonation
followed by electrophilic arylation of the a-carbon, releasing
benzene and [BiPh3] as by-products; for example: D. H. R.
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ambient temperature, as detected by NMR spectroscopy.
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[29] A circa 60% conversion of 3 was observed; side products were
presumably due to reaction of 3 with the tosyl group. Degrada-
tion of 6l was observed at prolonged reaction times of t > 6 h
with the beginning of concomitant polymerization of the solvent.
[33] a) Apparent rate constants kapp of 1 ꢁ 10À5 sÀ1 to 4 ꢁ 10À5 sÀ1 at
908C and 1208C, respectively, and an activation energy EA of
55 kJmolÀ1 have been reported for the nitroxide-catalyzed
polymerization of styrene: C. R. Becer, R. M. Paulus, R.
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times have been reported for the polymerization of styrene with
[BiMe2(CMe2CO2Me)], but the apparent rate of reaction was
not explicitly mentioned: see Ref. [31a].
[34] An activation energy of EA = 67.9 kJmolÀ1 has been determined
from an Arrhenius plot; compare Ref. [33a].
[35] Neutral main-group organometallic complexes and metal-free
reagents have been reported as initiators or mediators in
controlled living radical polymerizations, for example: a) Hand-
book of Radical Polymerization (Eds.: K. Matyjaszewski, T. P.
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[37] CCDC 897779 (2) and CCDC 897780 (3) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[30] Compound
1 did not react with benzophenone (3 equiv)
regardless of the presence of [BPh3] (3 equiv).
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Angew. Chem. Int. Ed. 2012, 51, 13011 –13015
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 13015