Crystallographic characterization of difluoropropargyl indium bromide, a reactive fluoroorganometallic reagent

Article abstract of DOI:10.1002/anie.200602823

Elusive intermediate trapped: γ-Substituted difluoropropargyl groups can stabilize an organoindium(III) complex. The resulting structure is the only known example of an indium atom bonded to a propargylic carbon atom (see picture; F blue, S yellow, C white, H green). Reactions with electrophiles produced a difluoroalkyne or -allene, depending on the type of electrophile. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200602823

Angewandte
Chemie
Organoindium Reagents
DOI: 10.1002/anie.200602823
Crystallographic Characterization of
Difluoropropargyl Indium Bromide, a Reactive
Fluoroorganometallic Reagent**
Bo Xu, Mark S. Mashuta, and Gerald B. Hammond*
Dedicated to Professor Raymond G. Plevey
on the occasion of his 66th birthday
Indium-promoted reactions and their application to synthetic
methodologies based on green chemistry have grown expo-
nentially in recent years.[1] The interaction of allylic or
propargylic halides with indium metal—through metal/halo-
[*] B. Xu, Dr. M. S. Mashuta, Prof. G. B. Hammond
Department ofChemistry
University ofLouisville
Louisville, KY 40292 (USA)
Fax: (+1)508-852-3899
E-mail: gb.hammond@louisville.edu
[**] Financial support by the National Science Foundation (CHE-
0513483) is gratefully acknowledged. We are grateful to Professor
Craig A. Grapperhaus (University ofLouisville) for his collaboration.
M.S.M. thanks the Kentucky Research Challenge Trust Fund for an
upgrade ofthe X-ray af cilities.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 7265 –7267
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Communications
gen exchange or transmetalation—and their subsequent
were very close to 1:4:1 (1.03:4:1.11) and 1:2:2 (0.98:2.02:2)
for the two compounds; satisfactory carbon and hydrogen
analyses were also obtained. IR absorption as well as 19F and
13C NMR signals supported the presence of a propargyl
species in solution. On the basis of this evidence, we proposed
structures 5a and 6a. In a similar fashion, we isolated 5b–d
and 6b–d.
Although attempts to obtain suitable single crystals of 5a–
c and 6a–c failed, we succeeded in obtaining X-ray quality
crystals of 5d by slow evaporation of a saturated solution in
DMSO and dichloromethane at room temperature. This
compound crystallized as colorless prisms in the C2/c space
group. Its ORTEP[9a] view (Figure 1) illustrates the first
reaction with electrophiles continue to be actively pursued.[2]
Initially, an indium sesquihalide (R3In2X3) was regarded as a
synthetic intermediate.[3] On the basis of recent NMR data,
researchers have proposed that indium(I) and indium(III)
complexes, or a combination thereof, could mediate allyla-
tion[4] and propargylation reactions.[5] Owing to their short-
lived nature, the structure of the transient organoindium
species is still a matter of debate. By incorporating fluorine on
the propargylic carbon atom of 1 prior to its reaction with
indium, we were able to stabilize the resulting organoindium
complex. Herein we report the first crystallographic charac-
terization of a reactive propargyl indium species.
Earlier, we had reported that the reaction of a silylfluoro-
propargyl bromide 1 with indium in predominantly aqueous
media produced a stable complex, which is loosely repre-
sented as 2 (Scheme 1).[6] The organoindium compound 2
yielded 3 or 4, depending on the nature of the electrophile (E)
used.[7,8]
Figure 1. ORTEP showing 45% displacement ellipsoids. H atoms are
shown as spheres ofarbitrary radii. Selected bond lengths [Š] and
angles [8]: In1-C1 2.217(2), In1-Br1 2.5363(5), In1-O1 2.2432(18), F1-
C1 1.398(3), F2-C1 1.392(3), C1-C2 1.456(3), C2-C3 1.201(4); C1-In1-
C1 129.12(13), C1-In1-Br1 115.44(6), O1-In1-Br1 91.76(5), C2-C3-Si1
176.0(2), C3-C2-C1 174.3(3).
Scheme 1. Indium-mediated difluoroallenylation and -propargylation.
With the purpose of elucidating the structure of 2, a 1:1
mixture of 1a (R = Si(iPr)3) and indium metal in predom-
inantly aqueous media was sonicated at 5–108C and moni-
tored by 19F NMR spectroscopy (Scheme 2). Two slowly
ꢀ
example of a crystallographically characterized In-C-C C
complex. Indeed, no crystal structures have been reported to
date in the Cambridge Structural Database for an indium
atom that is bonded to sp3 allylic or propargylic carbon
atoms.[10] Compound 5d adopts a trigonal-bipyramidal geom-
etry about the central indium atom with the two 1,1-difluoro-
3-triphenylsilylprop-2-ynyl ligands and the Br atom coordi-
nating in the trigonal plane while two dmso ligands occupy
À
axial positions. The In C bond length (2.217(2) Š) is typical
À
of trigonal-bipyramidal CH3In complexes, and the In Br
À
(2.5363(5) Š) and In O bond lengths (2.2432(18) Š) are also
standard.[11] The C1-In-C1’ bond angle (129.12(13)8) is
significantly greater than the ideal 1208 while the Br1-In-C1
and Br1-In-C1’ angles are contracted (115.44(6)8) to accom-
modate the large C1-In-C1’ bond angle. To complete the
trigonal bipyramid, the coordination of the axial dmso ligands
is distorted slightly from linearity (O1-In-O1’ 176.48(10)8).
Scheme 2. Synthesis ofdilfuoropropargyl indium bromides.
increasing resonance signals, centered at d = À89 ppm, were
detected after 0.5 hours, and all the starting material was
consumed after five to six hours. After ether extraction, the
resulting solution was relatively stable toward treatment with
a mild acid, but the complex decomposed if concentrated to
dryness. We isolated each species by using SiO2 flash
chromatography and found that DMSO stabilized the result-
ing white solids. These compounds could be kept at room
temperature for several weeks without decomposition, and
could be stored for months in the refrigerator. Elemental
analyses indicated that the indium/fluorine/bromine ratios
ꢀ
Most significantly the CF2C C group shows the unprece-
À
dented presence of sp hybridization (C2 C3 1.201(4) Š;
nearly linear C2-C3-Si1 (176.0(2)8) and C1-C2-C3 (174.3(3)8)
angles) located in a position alpha to the CF2 group.
To demonstrate the reactive nature of these fluoroorga-
nometallic complexes, we treated the obtained mixture of 5a
and 6a (Scheme 2) with different electrophiles. With a less
reactive electrophile (benzaldehyde), the reaction produced
3a in 51% yield after two hours in refluxing ether. With a
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Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
more reactive electrophile such as bromine, 4a was obtained
in 76% yield under very mild conditions (À208C, 0.5 hours).
In conclusion, we have synthesized and structurally
characterized a hitherto unknown propargyl indium reagent,
and we have used it to prepare a difluoroalkyne or -allene.
The mechanism of formation of fluoroorganoindium com-
plexes, their regiochemistry, and synthetic applications are
currently under investigation.
Received: July 15, 2006
Revised: August 22, 2006
Published online: October 6, 2006
Keywords: difluoropropargyl groups · fluorine · indium ·
.
metalation · structure elucidation
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Experimental Section
Indium powder (1 equiv) was added to a 0.15m solution of 1a
(5 mmol) in a mixture of water and THF (4:1). This mixture was
sonicated at 5–108C for 6–8 h. The temperature in the ultrasound
bath was adjusted by addition of ice, and the reaction was periodically
monitored by 19F NMR. After the starting material was consumed,
the reaction mixture was extracted by ether, and the organic layer was
washed with brine, dried over MgSO4, and concentrated. The crude
indium complex was dissolved in ether (5 mL) and separated by flash
chromatography (80 g SiO2). The column was eluted with ether
(300 mL), then by 5% methanol in ether (300 mL). DMSO (1 mL)
was added to each of the two fractions before complete solvent
removal. The mixtures were filtered, and the solvent removed from
the filtrate by rotary evaporation. The two white solids were washed
with hexane and dried in vacuum to give 5a (750 mg, 37%) and 6a
(400 mg, 13%).
[6] S. Arimitsu, B. Xu, T. Kishbaugh, L. Griffin, G. B. Hammond, J.
Fluorine Chem. 2004, 125, 641.
[7] Z. Wang, G. B. Hammond, J. Org. Chem. 2000, 65, 6547.
[8] B. Xu, G. B. Hammond, Angew. Chem. 2005, 117, 7570; Angew.
Chem. Int. Ed. 2005, 44, 7404.
5a·3dmso: M.p. 53–558C; 1H NMR (500 MHz, CDCl3, 258C,
TMS): d = 1.00–1.04 (m, 42H, CH(CH3)2), 2.73 ppm (s, 18H, dmso);
13C NMR (126 MHz, CDCl3, 258C, TMS): d = 11.3, 18.8, 39.5 (dmso),
92.5, 104.0 (t, J = 24.4 Hz), 131.0 ppm (t, J = 285 Hz); 19F NMR
(470 MHz, CDCl3, 258C, CF3Cl): d = À88.8 ppm (s, 2F). Elemental
analysis (%) calcd for C30H60BrF4InO3S3Si2: C 40.40, H 6.78; found:
C 40.59, H 6.52.
[9] a) ORTEP-3 for Windows, L. J. Farrugia, J. Appl. Crystallogr.
1997, 30, 565; b) SMART, V.5.625, Bruker Advanced X-ray
Solutions, Inc., Madison, WI, 2001; c) SAINT, V.6.22, Bruker
Advanced X-ray Solutions, Inc., Madison, WI, 2001; d) G. M.
Sheldrick, SADABS, V.2.02, Area Detector Absorption Cor-
rection, University of Gꢀttingen, Gꢀttingen, Germany, 1997;
e) SHELXS-90 G. M. Sheldrick, Acta Crystallogr. Sect. A 1990,
46, 467; f) G. M. Sheldrick, SHELXL-99, Program for the
Refinement of Crystal Structures, University of Gꢀttingen,
Gꢀttingen, Germany, 1997; g) SHELXTL 6.12, Program Library
for Structure Solution and Molecular graphics, Bruker
Advanced X-ray Solutions, Madison, WI, 2001.
[10] F. H. Allen, Acta Crystallogr. Sect. B 2002, 58, 380.
[11] a) R. J. Baker, M. L. Cole, C. Jones, M. F. Mahon, J. Chem. Soc.
Dalton Trans. 2002, 1992; b) C. Peppe, J. A. Nobrega, M. Z.
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626, 68; c) G. R. Willey, D. R. Aris, J. V. Haslop, W. Errington,
Polyhedron 2001, 20, 423.
6a·3dmso: M.p. 90–928C; 1H NMR (500 MHz, CDCl3, 258C,
TMS): d = 1.00–1.07 (m, 21H, CH(CH3)2), 2.73 ppm (s, 18H, dmso);
13C NMR (126 MHz, CDCl3, 258C, TMS): d = 11.3, 18.8, 39.5 (dmso),
93.5, 103.0 (m), 130.0 ppm (m); 19F NMR (470 MHz, CDCl3, 258C,
CF3Cl): d = À89.1 ppm (s, 2F). Elemental analysis (%) calcd for
C18H39Br2F2InO3S3Si: C 29.20, H 5.31; found: C 29.43, H 5.23.
A colorless crystal of 5d (dimensions 0.21 ” 0.19 ” 0.16 mm3) was
mounted on a glass fiber for collection of X-ray data on a Bruker
SMART APEX CCD diffractometer. The SMART[9b] software pack-
age (version 5.628) was used to acquire a total of 1868 thirty-second-
frame w-scan exposures of data at 100 K (2qmax = 56.188) using
monochromated MoKa radiation (0.71073 Š) from a sealed tube and a
monocapillary. Frame data were processed using SAINT[9c] (ver-
sion 6.36) to determine the final unit cell parameters (a =
34.025(3) Š, b = 10.1163(9) Š, c = 13.1193(11) Š, b = 95.925(2)8,
V= 4491.6(7) Š3, Z = 4, 1calcd = 1.505 MgmÀ3) to produce raw hkl
data that were then corrected for absorption (transmission min./
max. = 0.700/0.764, m = 1.615 mmÀ1) by using SADABS.[9d] The struc-
ture was solved by Patterson methods in the space group C2/c by
using SHELXS-90[9e] and refined by least-squares methods on F2 by
using SHELXL-97[9f] as part of the SHELXTL[9g] suite of programs.
All nonhydrogen atoms were refined anisotropically. Hydrogen
atoms were placed in their geometrically generated positions and
refined as a riding model. Methyl H atoms were included as fixed
contributions with U(H) = 1.5 ” Ueq (attached C atom) while the
torsion angle which defines its orientation was allowed to refine on
the attached C atom. Phenyl H atoms were assigned U(H) = 1.2 ” Ueq.
For all 5225 unique reflections (R(int) = 0.0381) the final anisotropic
full-matrix least-squares refinement on F2 for 265 variables converged
at R1 = 0.0363 and wR2 = 0.0896 with a GOF of 1.05.
CCDC-614782 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Angew. Chem. Int. Ed. 2006, 45, 7265 –7267
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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