On the nature of organoindium intermediates: The formation of readily isolable difluoropropargylindium reagents and their regioselectivity towards electrophilic substitutions

Article abstract of DOI:10.1002/chem.200801367

The structure and reactivity of intermediate propargylindium complexes have been investigated. Their reaction with electrophiles produced a difluoroalkyne or -allene, depending on the nature of the electrophiles. A mechanism based on the Curtin-Hammett principle was invoked to explain this phenomenon. A newly proposed mechanism on the formation of indium(III) complexes, through the intermediacy of indium(I) species, could help to explain the reaction of indium with 1,1,1-difluorobromo-2-alkynes in the presence of aldehydes.

Full text of DOI:10.1002/chem.200801367

FULL PAPER
DOI: 10.1002/chem.200801367
On the Nature of Organoindium Intermediates: the Formation of Readily
Isolable Difluoropropargylindium Reagents and their Regioselectivity
Towards Electrophilic Substitutions
Bo Xu and Gerald B. Hammond[a]
Abstract: The structure and reactivity of intermediate propargylindium complexes
have been investigated. Their reaction with electrophiles produced a difluoro-
A
Keywords:
Hammett principle
NMR spectroscopy · propargyls
allenyls
·
Curtin–
·
indium
·
AHCTREUNG
the intermediacy of indium(I) species, could help to explain the reaction of indium
with 1,1,1-difluorobromo-2-alkynes in the presence of aldehydes.
Introduction
During the last two decades, the use of indium in organic
synthesis has increased to the extent that nowadays this ele-
Scheme 1. Allylindium complexes proposed by Chan et al.
ment is regarded as an important tool in the arsenal of envi-
ronmentally friendly, metal-mediated organic reactions.[1–4]
The interaction of allylic or propargylic halides with indium
metal—through metal–halogen exchange/insertion or trans-
metalation—and subsequent addition to carbonyl com-
pounds, olefins, imines, or alkynes has been actively pur-
sued, but the mechanism of these reactions has eluded many
investigators due to the fleeting nature of the organoindium
intermediates.[5,6] Early on, it was suggested that a reactive
indium intermediate might possess a sesquihalide [In2R3X3]
structure.[7] In 1999, Chan proposed the intermediacy of an
indium(I) complex in a Barbier-type reaction of indium
metal with allyl bromide in aqueous media, ruling out both
the presence and participation of bromine or the indium sur-
face (Scheme 1).[8] In the reaction of propargyl bromide
with indium in THF, Chan and co-workers proposed the in-
Scheme 2. Propargylindium complex proposed by Chan et al.
ent in the indium(III) complex. According to the authors,
A
the competition for predominance by either InI or InIII spe-
cies in an allenyl–propargyl metaloprotic rearrangement de-
pended on steric factors and the solvent system utilized.[9] In
aqueous media, only an organoindium(I) discrete species
was observed. Although an allenylindium is favored at equi-
librium, methyl substitution shifts the equilibrium toward
termediacy of both indium(I) and indium(III) species, based
G
on NMR evidence (Scheme 2).[9] However, NMR alone
could not determine if one or two bromine atoms were pres-
propargylindium(I) and -indium(III) species. In all cases, the
A
[a] Dr. B. Xu,+ Prof. Dr. G. B. Hammond+
Department of Chemistry, University of Louisville
Louisville, Kentucky, 40292 (USA)
Fax : (+1)502-852-3899
E-mail: GB.Hammond@louisville.edu
[+] These authors share senior authorship of this paper.
products of their reaction with aldehydes have the regiose-
lectivity expected from an SE2’ pathway. Lohꢀs approach to
the subject consisted in examining the indium-mediated re-
action of trialkylsilyl propargyl bromide with aldehydes in
an effort to tune the regioselectivity toward the synthesis of
allenic alcohols or homopropargylic alcohols.[10] Chanꢀs earli-
er report produced allenic alcohols with moderate to good
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801367.
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1
regiocontrol; Loh utilized a combination of solvent (THF or
THF/water), the steric and chelation effects of the silicon
group on the g-carbon, and a combination of indium(0) and
monitored by H NMR spectroscopy. We surmised that the
substitution of CH2 by CF2 in a propargyl system not only
would stabilize a nascent organoindium complex without in-
troducing unwanted steric modifications due to fluorineꢀs
small size, but also it would facilitate the study of their reac-
tions by 19F NMR spectroscopy without interference of sol-
vent or reagents. We now report the results of our investiga-
tions on the mechanism of formation of organofluoroindium
reagents and their regioselectivity toward electrophiles.
indium(III), to obtain either allenic alcohols or homopropar-
G
gylic alcohols with high regioselectivities. As in Chanꢀs
mechanistic analysis, Loh also invoked a SE2’ pathway to ex-
plain the regiochemical outcome of his experiments, but he
did not comment on the nature of his indium complex(es).
In other developments, Lee used a palladium-catalyzed
cross-coupling reaction mediated by In0 to exclusively syn-
thesize aryl-substituted allenes starting from propargyl bro-
mides.[11] The existence of organoindium(I) and -indium
A
Results and Discussion
transient intermediates was also postulated by Baba and co-
workers in their indium(0)-mediated Reformatsky-type re-
Synthesis and identification of difluoropropargylindium
complexes: For some time now we have been interested in
the reaction of indium with difluorobromopropargyl sub-
strates as a way to incorporate fluorine selectively in organic
molecules. Our earlier work on the reaction of difluoropro-
pargyl bromide 1a (R=TIPS=triisopropylsilyl) with indium
in predominantly aqueous media, after extraction with di-
ethyl ether, produced a stable crude complex, loosely repre-
sented as 2 (Scheme 6).[15] This crude organoindium reagent
2 yielded allenylic product 3 or propargylic product 4 de-
pending on the nature of the electrophile (E+) used
(Scheme 6).[15,16]
action of bromoacetates with ketones
(Scheme 3)).[6,12] The
G
Scheme 3. Formation of low-valent indium species in the Reformatsky
reaction.
ACHTREUNG
interconversion of indium(I) to indium(III) complexes has
G
been proposed when an ionic liquid was used as solvent
(Scheme 4).[13] In addition to an indium(I) mechanism, in-
Scheme 4. Allylindium complexes formation mechanism in ionic liquids.
Scheme 6. Initial postulate on the nature of organofluoroindium species.
dium(II) species have also been reported to be products of
reaction of indium with perfluorinatedalkyl halide; these in-
dium(II) species exist in the form of dimers, and they tend
With the purpose of elucidating the structure of the
indium complex 2, a 1:1 mixture of 1a and indium metal in
predominantly aqueous media was sonicated at 5–108C
(bath) and monitored by 19F NMR spectroscopy. Two slowly
increasing resonance signals, centered at d=À89 ppm, were
detected after 0.5 h, and all the starting material was con-
sumed after 5–6 h (Figure 1a). Clearly, the 19F NMR spec-
trum of crude indium complex 2 has two peaks, which
means it contains at least two indium species. To get pure
indium complexes for further analysis, we isolated each
indium species using SiO2 flash chromatography, resulting in
two fractions, their 19F and 13C NMR spectra are shown in
Figure 1. We found that DMSO stabilized the indium com-
plexes to give white solids. These solids could be kept at
room temperature for several weeks without decomposition,
and stored for months in the refrigerator. Satisfactory
carbon and hydrogen analyses were also obtained. Their
to disproportionate to indium
A
(Scheme 5).[14]
From the reports summarized above, it is apparent that
the nature, mechanism of formation, and regiochemistry of
indium-mediated propargylation or allenylation are still the
subject of debate. This is due partially to the fact that orga-
noindium complexes are transient and short-lived intermedi-
ates, and that indium-mediated reactions could only be
Scheme 5. Mechanism invoking indium(II) species.
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Organoindium Chemistry
FULL PAPER
formation). The indium complexes also exhibit very strong
UV absorption, generally their lmax are around 250–300 nm,
and their emax are around 5000 dm3 molÀ1 cmÀ1.
Reaction of difluoropropargylindium(III) complexes with
G
electrophiles: The reaction of indium complex 5a and 6a
with relatively reactive electrophiles like CHOCOOH
(glyoxylic acid) and NCS (N-chlorosuccinimide) was first
considered. Because both starting material and product con-
tain fluorine, it should be relatively easy to follow the reac-
tion process by 19F NMR spectroscopy. Both difluoropropar-
gylindium complexes 5 and 6 react with reactive electro-
philes (CHOCOOH, NCS, and bromine) to give difluoro-
allene (Figures 2 and 3). This kind of reaction is usually
completed in less than 0.5 h at 0–58C; the [InRBr2]-type
indium complex 6 is slightly more reactive than [InR2Br]-
type indium complex 5. This may be due to steric effects.
The reaction of alkyl-substituted propargylindium com-
plexes 5c and 6c with glyoxylic acid and NCS also gave the
corresponding allenyl product 4 (see Supporting Informa-
tion).
Figure 1. NMR of indium complexes. a) 19F NMR spectrum of crude
indium complex 2; b) 19F NMR spectrum of the first fraction 6a; c)
19F NMR spectrum of the second fraction 5a; d) 13C NMR spectrum of
6a (only three carbon atoms in propargyl system are shown).
19F NMR chemical shifts (d=À88.8 and À89.1 ppm) and
13C NMR signals seemed to support a propargyl structure. If
present, an allenyl indium complex would have shown a
large downfield shift for the central sp carbon (around d=
200 ppm in 13C NMR spectrum). In addition, IR showed a
strong signal at 2172 cmÀ1; this is further evidence support-
ing the propargylic nature of the complex. Based on that in-
formation, the structures of those indium complexes were
assigned as 5a and 6a (Scheme 7).
Scheme 7. Preparation and separation of crude difluoropropargylindium
complex.
Since attempts to obtain suitable single crystals of 5a or
6a failed, we prepared a number of organoindium com-
plexes (Scheme 7). To our satisfaction, the triphenylsilyl-
substituted indium complex 5d crystallized by slow evapora-
tion of a saturated solution in mixture of DMSO and di-
chloromethane at room temperature. Its structure was deter-
mined by single-crystal X-ray techniques.[17] The other physi-
cal properties (melting points and UV absorption) of the
indium complexes were also examined (see Supporting In-
Figure 2. Reaction of indium
C
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G. B. Hammond and B. Xu
Figure 4. Reaction of 5 with p-nitrobenzaldehyde monitored by 19F NMR
spectroscopy.
Figure 3. Reaction of indium
19F NMR spectroscopy.
A
We also investigated the reaction of indium complex 5
and 6 with relatively unreactive electrophiles like aromatic
aldehydes. In contrast to the reactive electrophiles, such
electrophiles (e.g., benzaldehyde, p-nitrobenzaldehyde) give
only the propargyl product 3; this reaction usually proceeds
very slowly at room temperature and needs more than one
day to complete even at high temperatures (Figure 4).
From the above results, we can conclude the following: 1)
difluoropropargylindium complexes 5 and 6 (Scheme 8)
have similar regioselectivity towards electrophiles; 2) reac-
tion of 5 and 6 with reactive electrophiles give allenyl prod-
ucts 4; on the other hand reaction of 5 and 6 with relative
Scheme 8. Reaction pattern of propargyl indium complexes with electro-
philes.
product ratio will depend only on the difference in the free
energy of the transition state going to each product (DDG),
and not on the equilibrium constant between the intermedi-
ates. Taking indium complex 6a as an example, we proposed
that there is an equilibrium between propargylindium(III)
A
6a and its allenylic isomer 7a (Scheme 9). The interconver-
unreactive electrophiles yields propargyl products
(Scheme 8).
3
The reaction of propargylindium(III) with reactive elec-
G
trophiles to give difluoroallene through a SE2’ path can be
explained invoking an extension of electronic effects
through the triple bond. On the other hand, the reaction
with weak electrophiles that furnishes the difluoropropargyl
isomer could be explained using the Curtin–Hammett prin-
ciple.[18] This principle states that, for a reaction that has a
pair of reactive intermediates or reactants that interconvert
rapidly, each going irreversibly to a different product, the
Scheme 9. Pseudo-SE2 reaction of 6a with weak electrophiles.
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Organoindium Chemistry
FULL PAPER
sion of propargyl metal species and allenyl metal species has
been well documented in literature.[19–21] A potential energy
diagram for reaction of 6a with weak electrophiles is shown
in the Supporting Information. In this case, the energy of ac-
tivation for the reaction of indium complex with electro-
philes may be greater than the equilibrium of the two
indium complexes 6a and 7a. When the electrophile is weak
or unreactive, the reaction of 6a with this electrophile to
give difluoroallene 4 is very slow; 6a may isomerize to 7a
and then 7a react with weak electrophile to give propargyl
product 3. Because the overall reaction goes from a prop-
argyl starting material to a propargyl product, we may call
this reaction a pseudo-SE2 reaction (Scheme 9).
Formation of propargylindium(III) complexes 5 and 6: The
G
indium complexes 5 and 6 were capable of reacting with
electrophiles yielding allenes 3 or alkynes 4. However, this
result does not necessarily imply that other reactive indium
species cannot be formed during the reaction; it only means
Scheme 10. Equilibrium between propargylindium and allenylindium
complexes.
that the indium
isolable, and capable of reacting with electrophiles. Indium-
(III) complexes 5 and 6 may be just final products of the re-
action of indium with propargyl bromide 1, and perhaps
there could be other short-lived transient indium species
produced during the reaction.
To investigate if there was any intermediate indium spe-
cies produced during the reaction, we monitored the reac-
tion of 1c with indium using 19F NMR spectroscopy. The re-
sults are shown in Figure 5.
A
plexes could be indium(I) or indium(II) complexes. Indeed,
indium(II) complexes have been reported as intermediates
(Scheme 5),[14] but we believe that the formation of indiu-
m(II) is unlikely in our case, because a [InIIRBr]-type com-
plex would be paramagnetic, a property that would have
been readily apparent in our NMR experiments. We also
conducted EPR experiments on the intermediate indium
complex, and found no signs of the presence of paramagnet-
ic species. Thus, the low-valence indium complex peaks
were regarded as indium(I) complexes.
ACHTREUNG
We then checked the reactivity of these low-valence in-
dium(I) complexes. These are much more reactive than the
corresponding indium(III) complexes. For example, when p-
G
chlorobenzaldehyde was added to the indium complexes
shown in Figure 5a, after only 5 min at room temperature,
the two low-valence indium complexes had disappeared,
and the signal corresponding to propargyl alcohol 3c had
formed. On the other hand, the signals corresponding to
indium(III) 5c and 6c remained unchanged even after
U
24 h.[22] The reaction of a low-valence indium complex with
aldehyde also yields propargyl product 3c (Scheme 11); this
could be explained by the Curtin–Hammett principle in a
similar way as the reaction of indium
with aldehyde.
A
Figure 5. Monitor of the indium complex formation form 1c by 19F NMR
spectroscopy.
Since we observed the existence of low-valence indium
complexes, one important question remains: do these com-
plexes convert to more stable indium(III) complexes or do
E
According to the results shown in Figure 5, after 6 h, two
they decompose gradually? To answer this question, we fol-
propargylindium(III) complexes 5c and 6c were the domi-
A
lowed the progress of the reaction by 19F NMR spectroscopy
nant species after 6 h (Figure 5c). However, at the very be-
ginning of the reaction, we observed another pair of signals
(d=À88 and À112 ppm, Figure 5a), the intensities of which
decreased with time and disappeared after 6 h (Figure 5c).
These intermediates could be low-valence indium complexes
composed of a mixture of propargylic (À88 ppm) and alle-
nylic (À112 ppm) species. This is in contrast with indium-
ACHTREUNG
Scheme 11. Reactivity of low-valence indium species with aldehyde.
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G. B. Hammond and B. Xu
just after all the starting material 1c had been consumed.
For this purpose, we added an internal standard and sealed
the NMR tube. We did observe a signal increase corre-
react with InBr3 to give propargylindium(III) dibromide 6c.
This step is reasonable because it is similar to the reaction
of lithium reagents with InCl3 or InBr3, which is a common
G
sponding to the indium
A
way to make organoindium
complexes 7c or 8c also could react with propargylindium-
(III) dibromide 6c to give bis(difluoropropargyl)indium(III)
bromide 5c.
A
comitant decrease in the intensity of the signals correspond-
ing to indium(I) complexes; this result is shown in Figure 6.
This phenomenon was confirmed by repeated experi-
ments.[22]
A
ACHTREUNG
We were able to prepare complex 6c through an inde-
pendent route, by reacting 1c with InBr in THF. This is an-
other support for our mechanism (Scheme 13).
Scheme 13. Synthesis of complex 6c from InBr.
The in situ generation of an indium complex in the presence
of aldehyde: Unlike Grignard reagents many metal com-
plexes cannot be prepared in preformed fashion due to their
low thermal stability; the easiest way to overcome the stabil-
ity problem of a metal complex is its genesis in the presence
of electrophiles. So it is very common for the metal complex
to be generated in situ and then react with electrophiles in
real time before it undergoes decomposition. Our group has
reported several indium- or magnesium-mediated Barbier
type reactions of 1 with aldehyde, imine, and other electro-
philes.[15,27–29] From the above discussion, the formation of
Figure 6. Increase of indium(III) complex intensity.
A
How could an indium(I) complex convert to indium
species? Although it is known that indium(I) species tend to
disproportionate to indium
(III) and indium(0),[23] in our
case the indium(I) species contains no bromide, and hence it
could not disproportionate to propargylindium(III) bromide
5 or 6. To explain the conversion of low-valence indium to
indium(III) species, we have proposed a new mechanism
G
AHCTREUNG
AHCTREUNG
G
indium(III) complexes may involve the intermediacy of low-
A
(Scheme 12). First, difluoropropargyl bromide 1c reacts with
indium to give an equilibrating mixture of propargyl and al-
lenyl indium(I) (7c and 8c) and InIBr (see also Scheme 3).
Difluoropropargyl bromide 1c could react with the InBr
valence indium(I) complexes. Since a low-valence indium(I)
complex is much more reactive than corresponding indium-
A
1 in the presence of an electrophile like an aldehyde may
produced to give propargylindium
C
proceed through a different mechanism compared to the re-
action of preformed indium(III) complex with aldehydes. To
A
elucidate this question, we examined the reaction of 1c with
indium in the presence of an aldehydes (mole ratio of 1c/
indium/aldehyde=1:1:1); the reaction proceeded smoothly
at 0–58C, and after 2–3 h all the starting material 1c had dis-
appeared leaving behind two new signals in 19F NMR spec-
trum (Figure 7a), one of which corresponded to product 3c,
the other signal belonged to indium complex 6c (by compar-
ison with the isolated indium complex 6c, Figure 7b). This
phenomenon can be explained easily by the mechanism
shown in Scheme 14, similar to Scheme 12. First, difluoro-
propargyl bromide 1c reacts with indium to give an equili-
brating mixture of propargyl- and allenylindium(I) (7c and
8c) and InIBr. Difluoropropargyl bromide 1c could then
react with InBr formed to give propargylindium(III) dibro-
G
mide 6c; InBr also could disproportionate to InBr3 and
In0.[24] However, in this case the reaction took place in the
presence of an aldehyde, which we knew was very reactive
to indium(I) complex (Scheme 11). Hence, the indium(I)
species 7c and 8c are no longer able to react with InBr3 or
Scheme 12. Mechanism for indium
G
6c to give propargylindium(III) dibromide 5c as they did in
N
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Organoindium Chemistry
FULL PAPER
ExperimentalSection
Generalprocedure for monitoring of reaction by 19F NMR: The monitor-
ing of reactions was conducted in a NMR tube with screw cap. Before re-
action, the NMR tube was flushed with argon. A sealed capillary filled
with [D6]benzene was used to facilitate lock and shimming when non-
deuterated solvent was used. Variable-temperature control (Varian Inova
500) was used to control temperature of reaction during 19F NMR experi-
ment. The percentage yield was obtained using a,a,a-trifluoromethylben-
zene as internal reference.
For the experimental data for the synthesis and characterization of the
compounds described here, see the Supporting Information.
Figure 7. Reaction of 1c and indium in the presence of aldehyde. a)
19F NMR spectrum of reaction mixture after 3 h. b) 19F NMR spectrum of
isolated 6c.
Acknowledgement
The authors are grateful to the National Science Foundation for financial
support (CHE-0809683).
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The structure and reactivity of the difluoropropargyl indium
complex has been investigated. Their reaction with electro-
philes produced a difluoroalkyne or -allene, depending on
the nature of the electrophiles. We have invoked a mecha-
nism based on the Curtin–Hammett principle to explain this
phenomenon. And we also have postulated a mechanism for
the formation of indium(III) complexes through the inter-
A
mediacy of indium(I) species, which can also explain the re-
action of indium with 1 in the presence of aldehydes.
[29] M. Mae, J. A. Hong, G. B. Hammond, K. Uneyama, Tetrahedron
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Received: July 6, 2008
Published online: October 1, 2008
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