Transition-metal-promoted reactions of boron hydrides. 16. Platinum- and palladium-catalyzed olefin addition and olefin dehydrogenative borylation reactions of arachno-6,8-C2B7H13: Syntheses and structural characterizations of 7-R-arachno-6,8-C2B7H12 and 7-(trans-R-CH=CH)-arachno-6,8-C2B7H12

Article abstract of DOI:10.1021/ja002178r

The chloroplatinic acid or platinum(II) bromide-catalyzed reaction of arachno-6,8-C₂B₇H₁₃ with ethylene results in hydroboration to yield the single product 7-(C₂H₅)-arachno-6,8-C₂B₇H ₁₂. Analogous reactions with 1-pentene or styrene yield a mixture of both hydroboration, 7-R-arachno-6,S-C₂B₇H₁₂, and dehydrogenative borylation, 7-(trans-R-CH=CH)-arachno-6,8-C₂B₇H₁₂, products. On the other hand, the palladium(II) bromide-catalyzed reaction of arachno-6,8-C₂B₇H₁₃ with ethylene yields predominantly the dehydrogenative borylation product (7-(CH₂=CH)-arachno-6,8-C₂B₇H₁₂ along with smaller amounts of 7-(C₂H₅)-arachno-6,8-C₂B₇H ₁₂. Palladium(II) bromide-catalyzed reactions with 1-pentene or styrene result in only dehydrogenative borylation to produce 7-(trans-R-CH=CH)-arachno-6,8-C₂B₇H₁₂. The hydroboration and dehydrogenative borylation products observed in the platinum- and palladium-catalyzed reactions are related to those that have been observed in recently reported metal-catalyzed hydrosilations and catecholborane or pinacolborane hydroborations and are consistent with a reaction mechanism involving competitive hydride-migration/reductive-elimination and boryl-migration/β-hydride-elimination steps. Crystallographic and DFT computational studies of the alkenyl-carbaboranes have also revealed unusual cage-bonding features that suggest π-bonding interactions of the B7 cage boron with its olefinic substituent.

Full text of DOI:10.1021/ja002178r

10868
J. Am. Chem. Soc. 2000, 122, 10868-10877
Transition-Metal-Promoted Reactions of Boron Hydrides. 16.
Platinum- and Palladium-Catalyzed Olefin Addition and Olefin
Dehydrogenative Borylation Reactions of arachno-6,8-C2B7H13:
Syntheses and Structural Characterizations of
7-R-arachno-6,8-C2B7H12 and
7-(trans-R-CHdCH)-arachno-6,8-C2B7H12
Daniel E. Kadlecek, Patrick J. Carroll, and Larry G. Sneddon*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
ReceiVed June 19, 2000
Abstract: The chloroplatinic acid or platinum(II) bromide-catalyzed reaction of arachno-6,8-C2B7H13 with
ethylene results in hydroboration to yield the single product 7-(C2H5)-arachno-6,8-C2B7H12. Analogous reactions
with 1-pentene or styrene yield a mixture of both hydroboration, 7-R-arachno-6,8-C2B7H12, and dehydrogenative
borylation, 7-(trans-R-CHdCH)-arachno-6,8-C2B7H12, products. On the other hand, the palladium(II) bromide-
catalyzed reaction of arachno-6,8-C2B7H13 with ethylene yields predominantly the dehydrogenative borylation
product (7-(CH2dCH)-arachno-6,8-C2B7H12 along with smaller amounts of 7-(C2H5)-arachno-6,8-C2B7H12.
Palladium(II) bromide-catalyzed reactions with 1-pentene or styrene result in only dehydrogenative borylation
to produce 7-(trans-R-CHdCH)-arachno-6,8-C2B7H12. The hydroboration and dehydrogenative borylation
products observed in the platinum- and palladium-catalyzed reactions are related to those that have been observed
in recently reported metal-catalyzed hydrosilations and catecholborane or pinacolborane hydroborations and
are consistent with a reaction mechanism involving competitive hydride-migration/reductive-elimination and
boryl-migration/â-hydride-elimination steps. Crystallographic and DFT computational studies of the alkenyl-
carbaboranes have also revealed unusual cage-bonding features that suggest π-bonding interactions of the B7
cage boron with its olefinic substituent.
Introduction
the reaction conditions, metal reagent, and olefin, 7-alkyl- or
7-alkenyl-arachno-6,8-C2B7H12 derivatives. We also report
crystallographic and DFT computational studies of these
products that reveal unusual cage-bonding features associated
with the alkenyl substituents.
Because of the emerging uses1 of organopolyboranes in such
diverse applications as boron neutron capture agents, noncoor-
dinating anions, extractants for radioactive wastes, and inorganic
polymer components, the development of new systematic ways
to functionalize polyboranes with organic substituents has been
of great recent interest. We have previously shown that both
platinum and palladium reagents can be used to catalyze the
reactions of certain polyboranes with terminal olefins and thus
provide selective, high-yield routes to their organo-substituted
derivatives. For example, either chloroplatinic acid or platinum-
(II) bromide can be used to catalytically hydroborate terminal
olefins with decaborane(14) to yield 6,9-R2B10H12 derivatives
in high yield.2 Likewise, palladium(II) bromide catalyzes both
olefin addition and olefin substitution reactions with pentabo-
rane(9)3,4 and borazine.5 In this paper, we report that these metal
reagents also catalyze the reactions of the arachno-6,8-C2B7H13
dicarbaborane with terminal olefins to yield, depending upon
Experimental Section
All manipulations were carried out using standard high-vacuum or
inert-atmosphere techniques as described by Shriver.6
Materials. Ethylene was obtained from MG Industries. Platinum-
(II) bromide, palladium(II) bromide, 1-pentene, and styrene were
purchased from Aldrich and used as received. Chloroplatinic acid
hexahydrate (H2PtCl6‚6H2O) (38-40% Pt) was purchased from Strem.
The arachno-6,8-C2B7H13 was prepared according to literature methods.7
Toluene was purchased from Fisher and distilled from sodium/
benzophenone ketyl prior to use. Hexanes and Celite 545 were
purchased from Fisher and used as received. Activated charcoal was
purchased from J. T. Baker and used as received.
Physical Measurements. 1H NMR spectra at 500.4 MHz, 11B NMR
spectra at 160.5 MHz, and 13C NMR at 125.8 MHz were obtained on
a Bruker AM-500 spectrometer equipped with the appropriate decou-
pling accessories. All 11B chemical shifts are referenced to external
BF3‚O(C2H5)2 (0.00 ppm), with a negative sign indicating an upfield
(1) Plesˇek, J. Chem. ReV. 1992, 92, 269.
(2) Mazighi, K.; Carroll, P. J.; Sneddon, L. G. Inorg. Chem. 1993, 32,
1963.
(3) Davan, T.; Corcoran, E. W., Jr.; Sneddon, L. G. Organometallics
1983, 2, 1693.
(4) Corcoran, E. W., Jr.; Sneddon, L. G. In AdVances in Boron and the
Boranes; Liebman, J. F., Greenberg, A., Williams, R. E., Eds.; VCR: New
York, 1988; p 71.
(6) Shriver, D. F.; Drezdzon, M. A. Manipulation of Air-SensitiVe
Compounds, 2nd ed.; Wiley: New York, 1986.
(7) Garrett, P. M.; George, T. A.; Hawthorne, M. F. Inorg. Chem. 1969,
8, 2008.
(5) Lynch, A. T.; Sneddon, L. G. J. Am. Chem. Soc. 1989, 111, 6201.
10.1021/ja002178r CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/21/2000

Transition-Metal-Promoted Reactions of Boron Hydrides
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10869
Table 1. Reaction Conditions and Product Ratios
yield
substrate/olefin/
catalyst ratios
T
(°C)
time
(h)
conversionc
(%)
alkyl/alkenyl
ratioc
(alkyl/alkenyl)
olefin
catalysta
solvent
TONd
(%)e
ethylene
ethylene
ethylene
ethylene
1-pentene
1-pentene
1-pentene
styrene
CPA
1.0/5.0/0.04
1.0/7.7/0.1
1.0/6.0/0.1
1.0/12.7/0.1
1.0/9.0/0.03
1.0/14.0/0.1
1.0/27.0/0.2
1.5/10.0/0.1
1.0/26.0/0.1
1.0/26.0/0.1
toluene
toluene
toluene
toluene
noneb
60
60
32
32
60
22
32
60
22
32
48
40
24
24
20
90
6
35
24
18
86
90
83
85
88
89
84
74
86
85
100/0
100/0
26/74
9/91
43/57
38/62
0/100
34/66
43/57
0/100
22
8
7
8
27
9
4
12
8
75/0
48/0
10/16
2/19
5/14
31/25
0/53
41/48
43/21
0/49
PtBr2
PdBr2
PdBr2
CPA
PtBr2
PdBr2
CPA
noneb
noneb
toluene
styrene
styrene
PtBr2
PdBr2
noneb
noneb
8
a CPA ) H2PtCl6‚6H2O. b These reactions were run in neat olefin. c Determined by 11B NMR. d Turnover number (TON) ) moles of product
divided by moles of catalyst, based on percent conversion by 11B NMR. e Unoptimized isolated yields relative to the amount of starting material
consumed.
Table 2. NMR Data
compound
nucleus
11Ba,b
δ (multiplicity, intensity, assignment, J (Hz))
7-(C2H5)-arachno-6,8-C2B7H12 (1)
15.3 (s, B7), -1.4 (d, B5,9, JBH 153), -18.3 (d, B2,3, JBH 161), -30.9
(d, B4, JBH 141), -50.2 (d, B1, JBH 152)
11B(calcd)c,d
11B-11Ba,e
1H{11B}a,f
16.6 (B7), -2.1 (B5,9), -16.0 (B2,3), -31.5 (B4), -53.8 (B1)
observed cross-peaks: B1-B2,3; B1-B4; B1-B5,9; B2,3-B5,9; B2,3-B7
3.23 (2, BH), 2.32 (2, BH), 2.00 (1, BH), 1.07 (m, 2, CH2CH3), 1.03
(m, 3, CH2CH3), 0.44 (1, BH), -0.39 (s, 2, cage-CH), -2.06
(s, 2, cage-CH), -2.65 (2, BHB)
13C{1H}a,g
13C(calcd)c,d
11Ba,b
12.65 (C11), 10.45 (C10, J
B 72), -11.57 (C6,8)
13
11
C-
15.48 (C10), 13.18 (C11), -9.09 (C6,8)
9.0 (s, B7), -1.3 (d, B5,9, JBH 153), -17.1 (d, B2,3, JBH 162), -29.8
(d, B4, JBH 161), -49.9 (d, B1, JBH 149)
7-(CH2dCH)-arachno-6,8-C2B7H12 (2)
11B(calcd)c,d
10.9 (B7), -2.4 (B5,9), -14.9 (B2,3), -30.8 (B4), -53.1 (B1)
6.46 (dd, 1, CHdCH2, 3JHH 18, 3JHH 13), 5.76 (d, 1, CHdCH2, 3JHH 13),
5.59 (d, 1, CHdCH2, 3JHH 18), 3.31 (2, BH), 2.55 (2, BH), 2.10
(1, BH), 0.60 (1, BH), -0.06 (s, 2, cage-CH), -1.71
(s, 2, cage-CH), -2.49 (2, BHB)
126.71, -11.32 (C6,8)
146.38 (C10), 129.72 (C11), -9.92 (C6,8)
10.3 (s, B7), -1.8 (d, B5,9, JBH 146), -17.7 (d, B2,3, JBH 161), -30.5
(d, B4, JBH 128), -49.5 (d, B1, JBH 148)
1H{11B}a,f
13C{1H}g,h
13C(calcd)c,d
11Ba,b
7-(CH3(CH2)2CHdCH)-arachno-6,8-C2B7H12 (3)
1H{11B}a,f
5.91 (m, 2, vinyl), 3.20 (2, BH), 2.44 (2, BH), 2.11 (m, 2, CH), 1.95
(1, BH), 1.43 (m, 2, CH), 0.93 (m, 3, CH), 0.54 (1, BH), -0.15
(s, 2, cage-CH), -1.82 (s, 2, cage-CH), -2.59 (2, BHB)
13C{1H}g,i
143.58, 128.48 (J
B 108), 37.75, 22.13, 13.73, -11.29 (C6,8)
13
11
C-
7-(C5H11)-arachno-6,8-C2B7H12 (4)
11Ba,b
14.8 (s, B7), -1.4 (d, B5,9, JBH 154), -18.2 (d, B2,3, JBH 162), -30.8
(d, B4, JBH 153), -50.2 (d, B1, JBH 147)
1H{11B}a,f
3.23 (2, BH), 2.32 (2, BH), 2.05 (1, BH), 1.38 (m, 6, CH), 1.10
(m, 2, CH), 0.98 (m, 3, CH), 0.50 (1, BH), -0.35 (s, 2, cage-CH),
-1.99 (s, 2, cage-CH), -2.63 (2, BHB)
9.6 (s, B7), -1.4 (d, B5,9, JBH 144), -17.1 (d, B2,3, JBH 157), -29.9
(d, B4, JBH 145), -49.6 (d, B1, JBH 150)
7-((C6H5)CHdCH)-arachno-6,8-C2B7H12 (5)
7-((C6H5)CH2CH2)-arachno-6,8-C2B7H12 (6)
11Ba,b
1H{11B}a,f
7.37-7.04 (m, 5, phenyl), 6.92-6.78 (dd, 2, vinyl), 3.36 (2, BH), 2.64
(2, BH), 2.13 (1, BH), 0.68 (1, BH), 0.05 (s, 2, cage-CH), -1.60
(s, 2, cage-CH), -2.41 (2, BHB)
11Ba,b
13.7 (s, B7), -1.1 (d, B5,9, JBH 152), -17.9 (d, B2,3, JBH 159), -30.5
(d, B4),j -50.4 (d, B1, JBH 149)
1H{11B}a,f
7.36-7.23 (m, 5, phenyl), 3.34 (2, BH), 2.77 (m, 2, CH), 2.46 (2, BH),
2.11 (1, BH), 1.56 (m, 2, CH), 0.56 (1, BH), -0.29
(m, 2, cage-CH), -1.87 (m, 2, cage-CH), -2.54 (2, BHB)
a C6D6. b 160.5 MHz. c B3LYP/6-311G*//B3LYP/6-311G*. d Although the calculated static structures have C1 symmetry, free rotation of the
alkyl/alkenyl substituent in solution would generate Cs symmetric structures and B2/B3, B5/B9, and C6/C8 would become equivalent. Thus, the
calculated chemical shift values for these atoms in this table are the average of the calculated static values (ppm): (1) B2 (-15.0), B3 (-16.9); B5
f
(-2.1), B9 (-2.2); C6 (-7.65), C8 (-10.52); (2) B2 (-15.9), B3 (-13.9); B5 (-3.1), B9 (-1.7); C6 (-6.88), C8 (-12.96). e 64.2 MHz. 500.4
j
MHz. g 125.8 MHz. h CD2Cl2. i CDCl3. JBH not resolved.
shift. All 1H and 13C chemical shifts were measured relative to internal
residual protons or carbons in the lock solvent and are referenced to
Me4Si (0.00 ppm). Gas chromatography/mass spectrometry (GC/MS)
was performed on a Hewlett-Packard 5890A gas chromatograph
(equipped with a cross-linked methylsilicone column) interfaced to a
Hewlett-Packard 5970 mass selective detector. Infrared spectra were
obtained on a Perkin-Elmer 1430 spectrophotometer. Flash column
chromatography was performed using silica gel (230-400 mesh,
Merck). Elemental analyses were performed at the University of
Pennsylvania microanalysis facility. Melting points were obtained on
a standard melting point apparatus and are uncorrected.
H2PtCl6‚6H2O- and PtBr2-Catalyzed Reactions of Ethylene. As
given in Table 1, the arachno-6,8-C2B7H13 and either H2PtCl6‚6H2O
or PtBr2 were charged into an 88 mL Andrews Glass pressure reaction
vessel (part no. 110-207-0003) equipped with a Whitey needle valve
and a magnetic stirbar. Toluene (10 mL) and ethylene (measured by
expansion of the gas into a known volume) were vacuum transferred
into the vessel, and then the mixtures were immersed and stirred in an

10870 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Kadlecek et al.
oil bath. Following termination of the reaction, isolation and purification
of the products by filtering the product mixture through a 1/1 mixture
of Celite 545/activated charcoal followed by flash column chromatog-
raphy (hexanes) yielded pure 7-(C2H5)-arachno-6,8-C2B7H12 (1) (Table
1): white solid; Rf ) 0.18; mp 40.5-41.5 °C; IR (NaCl, cm-1) 3060
(w), 3000 (w), 2950 (m), 2800 (w), 2760 (w), 2580 (vs), 2010 (w),
1450 (w), 1410 (w), 1380 (w), 1290 (w), 1160 (m), 1080 (m), 1060
(m), 1030 (m), 980 (w), 890 (m), 840 (w), 670 (w). Anal. Calcd: C,
34.11; H, 12.17. Found: C, 34.56; H, 12.42.
7-(trans-CH3(CH2)2CHdCH)-arachno-6,8-C2B7H12 (3) (Rf ) 0.61) and
7-(trans-(C6H5)CHdCH)-arachno-6,8-C2B7H12 (5), respectively (Table
1).
Crystallographic Data for Compounds 1, 3, and 5. Colorless
crystals of 7-(C2H5)-arachno-6,8-C2B7H12 (1) were grown by slow
evaporation of a hexanes solution at -25 °C in a drybox. Colorless
crystals of 7-(trans-CH3(CH2)2CHdCH)-arachno-6,8-C2B7H12 (3) were
grown by sublimation in a sealed NMR tube placed 3 in. above a hot
plate held at 60 °C. Colorless crystals of 7-(trans-(C6H5)CHdCH)-
arachno-6,8-C2B7H12 (5) were grown by slow evaporation of a CH2-
Cl2/hexanes solution at -25 °C in a drybox.
PtBr2-catalyzed reactions run at room temperature resulted in very
low conversions to a mixture of products corresponding to 1 and
7-(CH2dCH)-arachno-6,8-C2B7H12 (2) (vide infra).
Collection and Reduction of the Data. X-ray intensity data were
collected on a Rigaku R-AXIS IIc area detector employing graphite-
monochromated Mo KR radiation (λ ) 0.710 69 Å) at a temperature
of 198 K. Indexing was performed from a series of 1° oscillation images
with exposures of 15 min/frame. A hemisphere of data was collected
using 10° oscillation angles with exposures of 20 min/frame and a
crystal-to-detector distance of 82 mm. Oscillation images were pro-
cessed using bioteX,8 producing a listing of unaveraged F2 and σ(F2)
values which were then passed to the teXsan9 program package for
further processing and structure solution on a Silicon Graphics Indigo
R4000 computer. The intensity data were corrected for Lorentz and
polarization effects but not for absorption.
PdBr2-Catalyzed Reaction of Ethylene. As outlined in Table 1,
arachno-6,8-C2B7H13 and PdBr2 were reacted in toluene solution
according to the procedures described above. Isolation and purification
of the products by filtering the product mixture through a 1/1 Celite
545/activated charcoal plug followed by flash column chromatography
(2/1 hexanes/toluene) yielded pure 7-(C2H5)-arachno-6,8-C2B7H12 (1)
(Rf ) 0.56) and 7-(CH2dCH)-arachno-6,8-C2B7H12 (2) (Rf ) 0.48)
(Table 1). Data for 2: clear solid; mp < room temperature; IR (NaCl,
cm-1) 3075 (m), 2970 (s), 2930 (m), 2590 (vs), 2340 (m), 2320 (m),
1620 (m), 1545 (m), 1460 (w), 1420 (w), 1380 (m), 1260 (s), 1090
(m), 1060 (m), 1040 (m), 1010 (m), 890 (m). HRMS (m/e) calcd for
12C411B7 H14 (M - H) 139.1747, found 139.1784.
1
H2PtCl6‚6H2O- and PtBr2-Catalyzed Reactions of 1-Pentene and
Styrene. As given in Table 1, the arachno-6,8-C2B7H13 and either H2-
PtCl6‚6H2O or PtBr2 were added to a 100 mL Schlenk tube, equipped
with a magnetic stirbar, under a N2 purge. Either 1-pentene or styrene
was added via syringe, and then the reaction vessel was degassed on
the high-vacuum line. The chloroplatinic acid-catalyzed reactions were
stirred in a 60 °C oil bath, while the PtBr2-catalyzed reactions were
stirred at room temperature. In the chloroplatinic acid-catalyzed reaction
with styrene, toluene (3 mL) was added to prevent gelling of the
solution.
Solution and Refinement of the Structures. The structures were
solved by direct methods (SIR92).10 Refinement was by full-matrix
least-squares based on F2 using SHELXL-93.11 All reflections were
used during refinement (F2 values that were experimentally negative
were replaced by F2 ) 0). The weighting scheme used was w ) 1/[σ2-
2
(Fo ) + 0.0321P2 + 0.4162P], where P ) (Fo2 + 2Fc2)/3. Non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were refined
isotropically. Crystal and refinement data are given in Table 3. Refined
positional parameters are given in the Supporting Information. Selected
intramolecular bond distances and angles are given in Tables 4 and 5.
The reaction products were isolated and purified by passing the
product mixture through a 1/1 mixture of Celite 545/activated charcoal
followed by flash column chromatography to yield analytically pure
samples (Table 1). For 1-pentene: 7-(trans-CH3(CH2)2CHdCH)-
arachno-6,8-C2B7H12 (3) and 7-(C5H11)-arachno-6,8-C2B7H12 (4). Data
for 3: white solid; Rf(hexanes) ) 0.13; mp 48.5-49.5 °C; IR (NaCl,
cm-1) 3060 (w), 2980 (w), 2940 (m), 2920 (m), 2860 (w), 2560 (vs),
2000 (w), 1620 (s), 1450 (w), 1420 (w), 1370 (w), 1170 (m), 1080
(m), 1060 (m), 1020 (w), 970 (m), 890 (m), 660 (w). Anal. Calcd: C,
46.47; H, 11.70. Found: C, 46.67; H, 11.84. Data for 4: pale yellow
oil; Rf(hexanes) ) 0.21; IR (NaCl, cm-1) 3050 (m), 3000 (m), 2940
(s), 2910 (s), 2860 (m), 2840 (m), 2560 (vs), 2010 (w), 1710 (w), 1630
(w), 1470 (m), 1460 (m), 1410 (w), 1370 (w), 1220 (w), 1180 (w),
1160 (m), 1080 (m), 1060 (m), 1030 (m), 1020 (m), 980 (s), 890 (s),
680 (m). Anal. Calcd: C, 45.96; H, 12.67. Found: C, 46.60; H, 11.84.
For styrene: 7-(trans-(C6H5)CHdCH)-arachno-6,8-C2B7H12 (5) and
7-((C6H5)CH2CH2)-arachno-6,8-C2B7H12 (6). Data for 5: white solid;
Computational Studies. The DFT/GIAO/NMR method,12 using the
Gaussian 94 program,13 was used in a manner similar to that previously
described.14-16 The geometries were fully optimized at the B3LYP/6-
311G* level within the specified symmetry constraints (using the
standard basis sets included) on a (2)-processor Origin 2000 computer
running IRIX 6.5.5 or a (6)-processor Power Challenge XL computer
running IRIX 6.5.6. A vibrational frequency analysis was carried out
on each optimized geometry at the B3LYP/6-311G* level with a true
minimum found for each structure (i.e., possessing no imaginary
frequencies). The NMR chemical shifts were calculated at the B3LYP/
6-311G* level using the GIAO option within Gaussian 94. 11B NMR
GIAO chemical shifts are referenced to BF3‚O(C2H5)2 using an absolute
shielding constant of 102.24.15,17 The 13C NMR GIAO chemical shifts
(8) bioteX: A suite of programs for the Collection, Reduction and
Interpretation of Imaging Plate Data, Molecular Structure Corp., 1995.
(9) teXsan: Crystal Structure Analysis Program, Molecular Structure
Corp., 1985, 1992.
(10) SIR92: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidoro, G. J. Appl. Crystallogr. 1994,
27, 435.
(11) SHELXL-93: Program for the Refinement of Crystal Structures,
G. M. Sheldrick, University of Go¨ttingen, Germany, 1993.
(12) Yang, X.; Jiao, H.; Schleyer, P. v. R. Inorg. Chem. 1997, 36, 4897
and references therein.
(13) Gaussian 94, Revision E.2: Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T.; Peterson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham,
M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.;
Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. T.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart,
J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian, Inc., Pittsburgh,
PA, 1995.
Rf(2/1 hexanes/toluene) ) 0.35; mp 101.0-102.5 °C; IR (NaCl, cm-1
)
3060 (w), 3020 (w), 2990 (w), 2920 (w), 2560 (s), 1600 (m), 1480 (s),
1440 (s), 1410 (s), 1390 (s), 1380 (s), 1290 (w), 1250 (w), 1190 (m),
1180 (m), 1080 (w), 1060 (m), 1020 (m), 980 (w), 890 (m), 690 (m),
12
1
670 (m); HRMS (m/e) calcd for
C
11B7 H19 216.2138, found
10
216.2146. Data for 6: white solid; Rf(2/1 hexanes/toluene) ) 0.51;
mp 61.5-62.5 °C; IR (NaCl, cm-1) 3040 (w), 3000 (w), 2980 (w),
2930 (w), 2910 (w), 2560 (s), 2500 (m), 1480 (w), 1440 (w), 1400
(w), 1370 (w), 1250 (m), 1150 (w), 1080 (m), 1060 (m), 1020 (m),
990 (w), 890 (m), 690 (w), 660 (w); HRMS (m/e) calcd for 12C1011B71H20
(M - H) 217.2216, found 217.2225.
PdBr2-Catalyzed Reactions of 1-Pentene and Styrene. As given
in Table 1, the arachno-6,8-C2B7H13 and PdBr2 were added to a N2-
flushed 100 mL Schlenk tube equipped with a magnetic stirbar. The
1-pentene or styrene was added via syringe through the neck of the
reaction vessel, and then the mixture was degassed on the high-vacuum
line. The mixture was immersed in an oil bath with stirring. Flash
column chromatography (2/1 hexanes/toluene) of the 1-pentene and
styrene product mixtures with subsequent concentration of appropriate
fractions resulted in the isolation of analytically pure samples of
(14) Keller, W.; Barnum, B. A.; Bausch, J. W.; Sneddon, L. G. Inorg.
Chem. 1993, 32, 5058.
(15) Bausch, J. W.; Rizzo, R. C.; Sneddon, L. G.; Wille, A. E.; Williams,
R. E. Inorg. Chem. 1996, 35, 131.
(16) Tebben, A. J. M.S. Thesis, Villanova University, 1997.
(17) Tebben, A. J.; Bausch, J. W. Private communication.

Transition-Metal-Promoted Reactions of Boron Hydrides
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10871
Table 3. Crystallographic Data Collection and Structure Refinement Information
1
3
5
empirical formula
formula weight
crystal class
space group
Z
C4B7H17
140.85
monoclinic
P21/c (no. 14)
4
C7B7H21
180.91
monoclinic
C2/c (no. 15)
8
C10B7H19
214.92
monoclinic
P21/n (no. 14)
4
cell constants
a (Å)
b (Å)
13.8757(11)
6.8831(6)
10.1842(10)
101.813(4)
952.1(2)
20.8838(4)
5.5168(2)
20.8943(8)
90.056(2)
2407.27(14)
0.46
11.4347(4)
12.0143(5)
10.5920(3)
114.156(2)
1327.71(8)
0.52
c (Å)
â (deg))
V (Å3)
µ (cm-1
)
0.43
crystal size (mm)
0.35 × 0.12 × 0.04
0.38 × 0.22 × 0.04
0.48 × 0.40 × 0.36
D
calcd (g/cm3)
0.983
0.998
1.075
F(000)
radiation
2θ range (deg)
hkl collected
304
784
456
Mo KR (λ ) 0.710 69 Å)
6.00-50.68
-16 e h e +16
-8 e k e +8
-12 e l e +11
5251
1697 (Rint ) 0.0456)
1456 (F > 4σ)
1697
Mo KR (λ ) 0.710 69 Å)
5.52-50.7
-24 e h e +24
-6 e k e +6
-25 e l e +25
8846
2064 (Rint ) 0.0422)
1880 (F > 4σ)
2064
Mo KR (λ ) 0.710 69 Å)
5.18-50.74
-13 e h e +13
-14 e k e +14
-12 e l e +12
9941
2398 (Rint ) 0.0332)
2182 (F > 4σ)
2398
no. of reflns measd
no. of unique reflns
no. of obsd reflns
no. of reflns used in refinement
no. of params
169
211
231
Ra indices (F > 4σ)
R1 ) 0.0775
wR2 ) 0.1362
R1 ) 0.0927
wR2 ) 0.1416
1.297
R1 ) 0.0532
wR2 ) 0.1132
R1 ) 0.0603
wR2 ) 0.1165
1.149
R1 ) 0.0537
wR2 ) 0.1168
R1 ) 0.0605
wR2 ) 0.1205
1.154
Ra indices (all data)
GOF
final difference peaks (e/Å3)
+0.131, 0.115
+0.120, 0.099
+0.141, 0.127
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑w(Fo - Fc2)2/∑w(Fo )2}1/2
.
2
2
Table 4. Comparison of Selected Crystallographically Determined
and DFT-Calculated (B3LYP/6-311G* Level) Bond Lengths (Å)
and Angles (deg) for 1
of ethylene with arachno-6,8-C2B7H13 in toluene solution at 60
°C in the presence of either H2PtCl6‚6H2O (4.0 mol %, 48 h)
or PtBr2 (10.0 mol %, 40 h) (Table 1) resulted in ethylene
hydroboration to produce 7-(C2H5)-arachno-6,8-C2B7H12 (1) (eq
1) as the sole product.
X-ray Bond Lengths
B(7)-C(10)
C(10)-C(11)
B(7)-C(6)
B(7)-C(8)
B(2)-C(6)
1.562(3)
1.520(4)
1.723(3)
1.736(4)
1.667(3)
B(2)-B(7)
B(3)-B(7)
B(5)-C(6)
B(9)-C(8)
B(3)-C(8)
1.742(4)
1.740(3)
1.682(3)
1.671(3)
1.663(3)
H2PtCl6‚6H2O or PtBr2
arachno-6,8-C2B7H13 + xs H2CdCH2
8
60 °C, PhCH3
7-(C2H5)-arachno-6,8-C2B7H12
(1)
X-ray Bond Angles
C(11)-C(10)-B(7)
C(10)-B(7)-C(6)
C(10)-B(7)-C(8)
C(10)-B(7)-B(2)
C(10)-B(7)-B(3)
115.4(2)
120.8(2)
122.2(2)
128.1(2)
128.8(2)
C(6)-B(7)-C(8)
106.8(2)
114.5(2)
114.4(2)
57.2(1)
1
B(5)-C(6)-B(7)
B(9)-C(8)-B(7)
C(8)-B(7)-B(3)
C(6)-B(7)-B(2)
GC/MS and 11B NMR analyses of the product mixtures
resulting from the H2PtCl6‚6H2O- and PtBr2-catalyzed reactions
indicated 86% and 90% conversions, respectively, corresponding
to 22 and 8 turnovers for the two catalysts. Filtration of the
reaction mixtures and removal of the volatiles under reduced
pressure resulted in the isolation of an oily material that was
shown by GC/MS to be the single product 1, contaminated with
trace amounts of unreacted arachno-6,8-C2B7H13. Further
purification by flash column chromatography to remove the
unreacted arachno-6,8-C2B7H13 afforded analytically pure samples
of crystalline 7-(C2H5)-arachno-6,8-C2B7H12 (1) in 75% and
48% isolated yields for the H2PtCl6‚6H2O- and PtBr2-catalyzed
reactions, respectively.
57.5(1)
DFT Bond Lengths
B(7)-C(10)
C(10)-C(11)
B(7)-C(6)
B(7)-C(8)
B(2)-C(6)
1.583
1.537
1.738
1.742
1.679
B(2)-B(7)
1.743
1.741
1.695
1.691
1.678
B(3)-B(7)
B(5)-C(6)
B(9)-C(8)
B(3)-C(8)
DFT Bond Angles
C(11)-C(10)-B(7)
C(10)-B(7)-C(6)
C(10)-B(7)-C(8)
C(10)-B(7)-B(2)
C(10)-B(7)-B(3)
116.2
120.2
122.3
128.3
129.7
C(6)-B(7)-C(8)
106.2
113.9
113.8
57.6
B(5)-C(6)-B(7)
B(9)-C(8)-B(7)
C(8)-B(7)-B(3)
C(6)-B(7)-B(2)
57.7
Longer reaction times and higher temperatures resulted in
the production of more highly substituted products that could
not be easily separated by chromatographic methods. Lowering
the temperature of the chloroplatinic acid-catalyzed reactions
resulted in no reaction. Likewise, decreasing the temperature
of the platinum(II) bromide-catalyzed reaction decreased the
rate (∼5% conversion in 20 h), but also yielded a product
mixture that contained both 1 and small amounts of the
dehydrogenative borylation product 7-(CH2dCH)-arachno-6,8-
C2B7H12 (2).
are referenced to TMS using an absolute shielding constant of 184.38
and are corrected according to the method described by Schleyer.18
Results
Chloroplatinic Acid- and Platinum(II) Bromide-Catalyzed
Reaction of Ethylene with arachno-6,8-C2B7H13. The reaction
(18) Maerker, C.; Schleyer, P. v. R.; Salahub, D. R.; Malkina, O. L.;
Malkin, V. G. Private communication.

10872 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Kadlecek et al.
Table 5. DFT-Calculated (B3LYP/6-311G* Level) Bond Lengths
(Å) and Angles (deg) for 2 and Crystallographically Determined
Bond Lengths (Å) and Angles (deg) for 3 and 5
uct, while the H2PtCl6‚6H2O- or PtBr2-catalyzed reaction with
1-pentene or styrene (Table 1) resulted in both hydroboration
and dehydrogenative borylation to yield a mixture of the 7-alkyl-
and 7-alkenyl-arachno-6,8-C2B7H12 derivatives as given in eqs
3 and 4.
Bond Lengths for 2
B(7)-C(10)
C(10)-C(11)
B(7)-C(6)
B(7)-C(8)
B(2)-C(6)
1.555
1.336
1.747
1.715
1.676
B(2)-B(7)
B(3)-B(7)
B(5)-C(6)
B(9)-C(8)
B(3)-C(8)
1.744
1.749
1.691
1.711
1.683
arachno-6,8-C2B7H13 + xs H2CdCH(CH2)2CH3
H2PtCl6‚6H2O or PtBr2
8
Bond Angles for 2
7-(trans-CH3(CH2)2CHdCH)-arachno-6,8-C2B7H12
+
C(11)-C(10)-B(7)
C(10)-B(7)-C(6)
C(10)-B(7)-C(8)
C(10)-B(7)-B(2)
C(10)-B(7)-B(3)
126.4
119.7
123.4
126.0
128.9
C(6)-B(7)-C(8)
107.0
113.7
113.5
58.1
B(5)-C(6)-B(7)
B(9)-C(8)-B(7)
C(8)-B(7)-B(3)
C(6)-B(7)-B(2)
3
7-(C5H11)-arachno-6,8-C2B7H12
(3)
4
57.4
Bond Lengths for 3
arachno-6,8-C2B7H13 + xs H2CdCH(C6H5)
B(7)-C(10)
C(10)-C(11)
B(7)-C(6)
B(7)-C(8)
B(2)-C(6)
1.546(2)
1.322(2)
1.741(2)
1.713(2)
1.663(2)
B(2)-B(7)
1.745(3)
1.752(2)
1.669(2)
1.689(2)
1.669(2)
H2PtCl6‚6H2O or PtBr2
B(3)-B(7)
B(5)-C(6)
B(9)-C(8)
B(3)-C(8)
8
7-(trans-(C6H5)CHdCH)-arachno-6,8-C2B7H12
+
5
7-((C6H5)CH2CH2)-arachno-6,8-C2B7H12
(4)
Bond Angles for 3
C(11)-C(10)-B(7)
C(10)-B(7)-C(6)
C(10)-B(7)-C(8)
C(10)-B(7)-B(2)
C(10)-B(7)-B(3)
126.5(2)
119.9(1)
122.2(1)
127.5(1)
129.6(1)
C(6)-B(7)-C(8)
107.8(1)
114.6(1)
114.4(1)
57.5(1)
6
B(5)-C(6)-B(7)
B(9)-C(8)-B(7)
C(8)-B(7)-B(3)
C(6)-B(7)-B(2)
GC/MS and 11B NMR spectroscopic analyses of the H2PtCl6‚
6H2O (3 mol %, 20 h) and PtBr2 (10 mol %, 90 h) catalyzed
1-pentene reactions showed 88% and 89% conversions to the
two major products 3 and 4, corresponding to 27 and 9 catalyst
turnovers, respectively. The H2PtCl6‚6H2O (7 mol %, 35 h) and
PtBr2 (10 mol %, 24 h) catalyzed styrene reactions afforded
74% and 86% conversions to a mixture of 5 and 6, which
corresponds to 12 and 8 catalyst turnovers, respectively. Minor
amounts of higher substituted alkyl- and alkenyl-substituted
derivatives along with some unreacted arachno-6,8-C2B7H13
were also present in the reaction mixture.
The chloroplatinic acid-catalyzed reaction required elevated
temperatures (60 °C), while the platinum(II) bromide catalyst
was active at room temperature. Toluene was required as a
solvent in the chloroplatinic acid-catalyzed reaction of styrene
to prevent gelling of the reaction solution. Longer reaction times
resulted in increased amounts of di- and trialkyl- and -alkenyl-
substituted derivatives, but no change in the relative ratio of
the 7-alkyl and 7-alkenyl products. Separation and purification
of the products by flash column chromatography yielded
analytically pure samples in the yields given in Table 1.
Palladium(II) Bromide-Catalyzed Reaction of 1-Pentene
and Styrene with arachno-6,8-C2B7H13. Whereas the PdBr2-
catalyzed reactions with ethylene afforded a mixture of hy-
droboration and dehydrogenative borylation products, the PdBr2-
catalyzed reaction of arachno-6,8-C2B7H13 with 1-pentene (20
mol %, 6 h) or styrene (10 mol %, 18 h) resulted in exclusive
conversion to the 7-(trans-R-CHdCH)-arachno-6,8-C2B7H12
products 3 (84%) and 5 (85%) resulting from dehydrogenative
borylation (eqs 5 and 6).
57.0(1)
Bond Lengths for 5
B(7)-C(10)
C(10)-C(11)
B(7)-C(6)
B(7)-C(8)
B(2)-C(6)
1.548(2)
1.335(2)
1.722(2)
1.709(2)
1.663(3)
B(2)-B(7)
1.745(3)
1.752(3)
1.679(3)
1.695(2)
1.667(2)
B(3)-B(7)
B(5)-C(6)
B(9)-C(8)
B(3)-C(8)
Bond Angles for 5
C(11)-C(10)-B(7)
C(10)-B(7)-C(6)
C(10)-B(7)-C(8)
C(10)-B(7)-B(2)
C(10)-B(7)-B(3)
126.5(2)
119.9(1)
123.4(1)
126.3(1)
128.7(1)
C(6)-B(7)-C(8)
107.2(1)
114.1(1)
114.5(1)
57.6(1)
B(5)-C(6)-B(7)
B(9)-C(8)-B(7)
C(8)-B(7)-B(3)
C(6)-B(7)-B(2)
57.4(1)
Palladium(II) bromide-Catalyzed Reaction of Ethylene
with arachno-6,8-C2B7H13. In contrast to the platinum-catalyzed
reactions, the PdBr2 (10 mol %, 24 h) catalyzed reaction of
ethylene with arachno-6,8-C2B7H13 in toluene (Table 1) resulted
in mainly dehydrogenative borylation to produce 2 (74%) as
the major product, with the hydroboration product 1 (26%) being
produced in much smaller amounts (eq 2). Minor amounts of
higher substituted products and unreacted arachno-6,8-C2B7H13
were also present in the product mixture.
PdBr2
arachno-6,8-C2B7H13 + xs H2CdCH2 32 °C, PhCH 8
3
7-(C2H5)-arachno-6,8-C2B7H12
+
1
7-(CH2dCH)-arachno-6,8-C2B7H12
(2)
2
PdBr2
arachno-6,8-C2B7H13 + xs H2CdCH(CH2)2CH3 32 °C8
The 1/2 product ratio was found to be dependent on the
ethylene concentration, with an increase in the ethylene
concentration (Table 1) favoring the formation of 2 (91%)
relative to 1 (9%). Although the conversion rates and product
yields were high according to NMR analysis, the isolated yields
(Table 1) of pure materials were greatly reduced owing to
decomposition during the chromatographic separation.
7-(trans-CH3(CH2)2CHdCH)-arachno-6,8-C2B7H12
(5)
(6)
3
PdBr2
arachno-6,8-C2B7H13 + xs H2CdCH(C6H5) 32 °C8
7-(trans-(C6H5)CHdCH)-arachno-6,8-C2B7H12
Chloroplatinic Acid and Platinum(II) Bromide-Catalyzed
Reactions of 1-Pentene and Styrene with arachno-6,8-
C2B7H13. The platinum-catalyzed reactions of ethylene with
arachno-6,8-C2B7H13 gave exclusively the hydroboration prod-
5
Reactions run at room temperature showed no activity, while
those at temperatures above ∼40 °C gave no greater than 30%

Transition-Metal-Promoted Reactions of Boron Hydrides
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10873
Figure 2. 1H NMR (500.4 MHz) spectra of 7-(CH2dCH)-arachno-
6,8-C2B7H12 (2): (a) 11B-coupled and (b) 11B-decoupled.
the experimentally determined 11B chemical shifts and assign-
ments (from 2D 11B-11B COSY experiments) for 1, 4, and 6.
In addition to the proton resonances of their hydrocarbon
1
substituents, the 11B-decoupled H NMR spectra for 1, 4, and
6 each show the two cage-CH resonances of the endo- and exo-
hydrogens on the 6,8-CH2 groups, a bridge hydrogen resonance
of intensity 2 at high field, and resonances arising from the six
terminal BH groups in the expected ratios of 2/2/1/1. The 13C
NMR spectrum of 1 shows a resonance at -11.57 ppm
corresponding to the equivalent 6,8-cage carbons and the two
ethyl carbon resonances at 10.45 and 12.65 ppm. The peak at
10.45 ppm shows a multiplet coupling pattern characteristic of
Figure 1. 11B NMR (160.5 MHz) spectra of (a) 7-(C2H5)-arachno-
6,8-C2B7H12 (1) and (b) 7-(CH2dCH)-arachno-6,8-C2B7H12 (2).
13C-11
1
J
B, leading to its assignment to the boron-bonded methylene
conversion even when allowed to react for several days. H
13C-11B
carbon of the ethyl group. The observed J
of 72 Hz is
NMR analysis of the volatile components of the styrene reaction
revealed the formation of at least 1 equiv of ethylbenzene
relative to the amount of 7-(trans-(C6H5)CHdCH)-arachno-
6,8-C2B7H12 (5) formed during the reaction. Purification and
isolation of the products by flash column chromatography
afforded moderate yields of analytically pure 3 and 5 (Table
1).
Spectroscopic, Crystallographic, and Computational Char-
acterizations. Spectroscopic Analysis. The NMR data for all
compounds are given in Table 2. The 11B NMR spectra for the
7-alkyl products 1 (Figure 1a), 4, and 6 each showed a 1(s)/2
(d)/2(d)/1(d)/1(d) pattern, indicating Cs symmetry, with the low-
field singlet resonance consistent with substitution at that boron
position. Jel´ınek and co-workers19 have previously shown that
nucleophilic attack of Bu- ion on arachno-6,8-C2B7H13 results
in Bu substitution at the boron (B7) situated between the two
carbon vertices of the dicarbaborane. The 11B NMR chemical
shifts for 1, 4, and 6 are nearly identical to those reported for
7-Bu-arachno-6,8-C2B7H12 (14.94 (s), -1.95 (d), -19.00 (d),
-31.66 (d), and -51.26 (d) ppm), thus supporting their 7-R-
arachno-6,8-C2B7H12 formulations.
The NMR assignments discussed above were confirmed using
DFT/GIAO computations. These calculations use density func-
tional theory (DFT) to obtain an optimized molecular geometry,
followed by gauge-independent atomic orbital (GIAO) NMR
shift calculations to predict the chemical shifts and assignments
of each atom in the structure. Such computations have now been
shown to be amazingly reliable in correctly predicting experi-
mentally determined chemical shifts and assignments of po-
lyborane clusters and have thus provided a powerful new tool
for structure elucidation.14,15,20 As shown in Table 2, the DFT/
GIAO/NMR values calculated at the B3LYP/6-311G*//B3LYP/
6-311G* level for 1 are, in fact, in excellent agreement with
13C-11
B
consistent with the J
values that have been reported for
alkylated carbaboranes.20i,21
The 11B NMR spectra for the 7-alkenyl compounds 2 (Figure
1b), 3, and 5 show a pattern similar to that found for 1, 4, and
6, with the exception that the low-field singlet B7 resonance is
shifted upfield in 2 (9.0 ppm), 3 (10.3 ppm), and 5 (9.6 ppm)
relative to its shift in the alkyl derivatives 1 (15.3 ppm), 4 (14.8
ppm), and 6 (13.7 ppm). The DFT/GIAO calculations on 2 are
again in excellent agreement with the experimentally determined
values (Table 2), and correctly predict the upfield shift (calcd,
∆δ ) -5.7 ppm; exptl, ∆δ ) -6.3 ppm) of the B7 vertex in 2
relative to that found in 1.
1
The 11B-decoupled H NMR spectra of 2 (Figure 2), 3, and
5 are again similar to those observed for 1, 4, and 6 showing
the two cage-CH2 resonances, the upfield bridge hydrogen
resonance, and the 2/2/1/1 pattern of the remaining terminal
B-H vertexes. However, each compound also exhibits peaks
between 5.9 and 6.9 ppm arising from the olefinic protons. The
13C NMR spectra for both 2 and 3 contain the -11 ppm cage
carbon resonance. The spectrum for 3 also clearly resolves each
carbon of the alkenyl substituent, with two peaks in the olefinic
region at 128.48 and 143.58 ppm and three peaks in the alkyl
region at 37.75, 22.13, and 13.73 ppm. The peak at 128.48 ppm
(20) (a) Bu¨hl, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1992, 114, 477.
(b) Bu¨hl, M.; Gauss, J.; Hofmann, M.; Schleyer, P. v. R. J. Am. Chem.
Soc. 1993, 115, 12385. (c) Onak, T.; Tran, D.; Tseng, J.; Diaz, M.; Arias,
J.; Herrera, S. J. Am. Chem. Soc. 1993, 115, 9210. (d) Diaz, M.; Jaballas,
J.; Arias, J.; Lee, H.; Onak, T. J. Am. Chem. Soc. 1996, 118, 4405. (e)
Bausch, J. W.; Matoka, D. J.; Carroll, P. J.; Sneddon, L. G. J. Am. Chem.
Soc. 1996, 118, 11423. (f) Jaballas, J.; Onak, T. J. Organomet. Chem. 1998,
550, 101. (g) Cranson, S. J.; Fox, M. A.; Greatrex, R.; Greenwood, N. N.
J. Organomet. Chem. 1998, 550, 207. (h) Hofmann, M.; Fox, M. A.;
Greatrex, R.; Williams R. E.; Schleyer, P.v. R. J. Organomet. Chem. 1998,
550, 331. (i) Onak, T.; Jaballas, J.; Barfield, M. J. Am. Chem. Soc. 1999,
121, 2850. (j) Shedlow, A. M.; Sneddon, L. G. Inorg. Chem. 1998, 37,
5269.
(19) Jel´ınek, T.; Herˇma´nek, S.; Sˇt´ıbr, B.; Plesˇek, J. Polyhedron 1986, 5,
1303.
(21) Wrackmeyer, B. Prog. NMR Spectrosc. 1979, 12, 227.

10874 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Kadlecek et al.
Figure 4. DFT (B3LYP/6-311G* level) optimized geometry for
arachno-6,8-C2B7H13 (7). Selected bond distances (Å) and angles
(deg): B(7)-C(6) 1.705, B(7)-C(8) 1.704, B(2)-B(7) 1.741, B(3)-
B(7) 1.742, B(5)-C(6) 1.716, B(9)-C(8) 1.716; C(6)-B(7)-C(8)
108.3, B(5)-C(6)-B(7) 113.1, B(9)-C(8)-B(7) 113.0.
Figure 5. DFT (B3LYP/6-311G* level) optimized geometry for
7-(CH2dCH)-arachno-6,8-C2B7H12 (2).
Figure 3. (a) X-ray crystallographic and (b) DFT (B3LYP/6-311G*
level) optimized structures for 7-(C2H5)-arachno-6,8-C2B7H12 (1).
also shows a J13 -11 of 108 Hz which is 36 Hz greater than
C
B
that observed for the alkyl derivative 1, indicating, as expected,
a higher degree of s-character in the B7-C10 bond of the
alkenyl derivative 3 relative to 1.21
The IR spectra of 1, 2, 3, 4, 5, and 6 all show characteristic
B-H and C-H absorptions in the 2560-2590 and 2910-3060
cm-1 regions. The spectra of 2, 3, and 5 also contain a peak
between 1600 and 1620 cm-1 corresponding to the olefinic
double-bond stretch.
Crystallographic and Computational Analysis. Elucidation
of the detailed structure of the alkyl-substituted 7-(C2H5)-
arachno-6,8-C2B7H12 (1) was obtained by both X-ray crystal-
lographic and DFT computational studies. The crystallograph-
ically determined structure of 1 is shown in the ORTEP diagram
in Figure 3a, while its DFT-optimized geometry at the B3LYP/
6-311G* level is depicted in Figure 3b. These structural studies
confirm both the Cs symmetry indicated by the NMR studies
and that hydroboration has occurred at the B7 vertex of the
arachno-6,8-C2B7H13 carbaborane.
Figure 6. X-ray crystallographic structures of (a) 7-(trans-CH3(CH2)2-
CHdCH)-arachno-6,8-C2B7H12 (3) and (b) 7-(trans-(C6H5)-CHdCH)-
arachno-6,8-C2B7H12 (5).
There is excellent agreement between the trends observed in
the crystallographic and DFT-calculated values for the distances
and angles in 1 (Table 4). These values are also in the ranges
found in the crystallographically determined structure of
arachno-6,8-Me2C2B7H11,22 as well as the crystallographically
determined23 and DFT-calculated structures (Figure 4) of
arachno-6,8-C2B7H13 (7). As diagrammed in Figure 7, the only
significant difference between the structure of the parent
compound arachno-6,8-C2B7H13 (7) and 7-(C2H5)-arachno-6,8-
C2B7H12 (1) is that, in 1, the C6-B7 and C8-B7 lengths are
longer (∼0.03-0.04 Å) than in 7, perhaps reflecting an increased
bonding interaction of B7 with the alkyl carbon in 1 relative to
its hydrogen substituent in 7.
The DFT-optimized geometry for 7-(CH2dCH)-arachno-6,8-
C2B7H12 (2) is presented in Figure 5, and the X-ray crystallo-
graphically determined structures of 7-(trans-CH3(CH2)2CHd
CH)-arachno-6,8-C2B7H12 (3) and 7-(trans-(C6H5)CHdCH)-
arachno-6,8-C2B7H12 (5) are shown in the ORTEP drawings
in Figure 6. These structural studies establish that, in 2, 3, and
5, substitution has again occurred at the B7 position but, more
importantly, confirm that these compounds, unlike 1, 4, and 6,
have alkenyl substituents that have been produced by dehydro-
genative borylation of the olefins at the terminal olefinic carbon.
The C10-C11 double-bond lengths observed in 3 (1.322(2) Å)
(22) Voet, D.; Lipscomb, W. N. Inorg. Chem. 1967, 6, 113.
(23) Kang, S. O.; Sneddon, L. G. Unpublished results.

Transition-Metal-Promoted Reactions of Boron Hydrides
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10875
dihedral angles (C8-B7-C10-C11 axis) for 3 (-6.6(3)°) and
5 (11.9(2)°) compare well with the calculated dihedral angle
for 2 (-5.9°). This planarity contrasts with the structure of 1,
where the dihedral angle about the C8-B7-C10-C11 axis was
crystallographically determined to be 57.7(1)° (calculated 53.6°).
The crystallographic B7-C10 bond lengths in the 7-alkenyl-
substituted derivatives 3 (1.546(2) Å) and 5 (1.548(2) Å) are
also shorter than that of the 7-alkyl-substituted derivative 1
(1.562(3) Å). Likewise, the DFT-optimized B7-C10 bond
lengths for 2 (1.555 Å) and 1 (1.583 Å) show the same trend.
The shorter B7-C10 bond lengths for the alkenyl derivatives
are also consistent with the larger J13 -11 couplings, discussed
C
B
above, between these atoms in the NMR spectra of 3 relative
to 1. These trends in the B7-C10 bond lengths and coupling
constants and, in particular, the greatly shortened cage C8-B7
bond and cisoid planar orientation of the C8-B7 and C10-
C11 atoms strongly suggest that there is a π-bonding interaction
between the olefin and the B7 boron. This could involve
donation of π-electron density from the olefin into an orbital
on the B7 atom that is oriented perpendicular to the C8-B7-
C10-C11 plane. This increased electron density on B7 would
then enhance greater π-bonding interactions between the B7
and C8 cage atoms, thus increasing the C8-B7 bond order and
decreasing the C8-B7 distance. Indeed, both the cisoid planar
orientation of the C8-B7-C10-C11 atoms and the trends
observed in their bond lengths are reminiscent of the features
that appear in organic conjugated diolefins.24
Discussion
Studies of the detailed mechanisms of the platinum- and
palladium-catalyzed reactions were not undertaken; however,
the observed reactivities and product distributions in the
reactions of these catalysts with arachno-6,8-C2B7H13 are
consistent with previous results for their use in metal-catalyzed
olefin hydrosilations, as well as other polyborane olefin hy-
droborations. Thus, several general conclusions and reasonable
mechanistic pathways can be proposed.
A possible reaction sequence for the platinum-catalyzed
reactions of olefins with arachno-6,8-C2B7H13 is illustrated in
Figure 8. For those reactions employing chloroplatinic acid, an
induction period of several hours was observed during which
time the reaction solution underwent a slow change from a clear
solution with undissolved solid catalyst to a brown-black
solution with a suspended black solid. This induction period is
entirely consistent with the known chloroplatinic acid-catalyzed
olefin hydrosilations25 and decaborane olefin hydroborations,2
where the initial reduction of Pt(IV) to a catalytically active
Pt(II) species has been proposed. The platinum(II) bromide-
catalyzed reactions proceeded without any observable induction
period, again suggesting that Pt(II) is the active catalytic species.
We also found that sources of Pt(0) species, such as Karstedt’s
catalyst,26 were inactive, providing evidence that the Pt(II)/Pt-
(IV) cycle is the active catalyst system in the arachno-6,8-
C2B7H13 reactions. Thus, as diagrammed in the figure, following
olefin association, oxidative addition27 of the arachno-6,8-
C2B7H13 carbaborane to the Pt(II) center would result in the
formation of the Pt(IV) product A. The high product selectivity
Figure 7. (a) DFT and X-ray (with errors) bond lengths and angles
for arachno-6,8-C2B7H13 (7). (b) DFT and X-ray bond lengths and
angles for 7-(C2H5)-arachno-6,8-C2B7H12 (1) [dihedral angles (C8-
B7-C10-C11) (DFT) 53.4°, (X-ray) 57.7°]. (c) DFT and X-ray bond
lengths and angles for 7-(CH2dCH)-arachno-6,8-C2B7H12 (2; DFT),
7-(trans-CH3(CH2)2CHdCH)-arachno-6,8-C2B7H12 (3; X-ray), and
7-(trans-(C6H5)CHdCH)-arachno-6,8-C2B7H12 (5; X-ray) [dihedral
angles (C8-B7-C10-C11) (2) -5.9°, (3) -6.6°, (5) 11.9°].
and 5 (1.335(2) Å) are in excellent agreement with that
calculated for 2 (1.336 Å).
As noted above, the C6-B7 and C8-B7 distances in 1, 4,
and 6 are somewhat longer than those found in the parent
carbaborane 7, but within experimental error, they are equiva-
lent. However, in the alkenyl-substituted compounds 2, 3, and
5 there is a pronounced asymmetry. Thus, as diagrammed in
Figure 7, in both the DFT-optimized geometry of 2 and the
crystallographically determined structures of 3 and 5, the C8-
B7 bond lengths are substantially shorter (1.715 Å, 2; 1.713(2)
Å, 3; 1.709(2) Å, 5) than the C6-B7 bond lengths (1.747 Å, 2;
1.741(2) Å, 3; 1.722(2) Å, 5). Along with the shortening of the
C8-B7 bond, the C8-B9 bond in 2, 3, and 5 is lengthened
(1.711 Å, 2; 1.689 Å, 3; 1.695 Å, 5) relative to the B5-C6
bond (1.691 Å, 2; 1.669 Å, 3; 1.679 Å, 5). Additionally, in 2,
3, and 5 the C8-B7-C10-C11 atoms are all nearly coplanar
and the C10-C11 double bond is oriented in a cisoid fashion
relative to the C8-B7 bond. The crystallographically determined
(24) Benet-Buchholz, J.; Boese, R.; Haumann, T.; Traetteberg, M. In
The Chemistry of Dienes and Polyenes; Rappoport, Z., Ed.; Wiley:
Chichester, 1997; p 25.
(25) Speier, J. L. AdV. Organomet. Chem. 1979, 17, 407.
(26) Lewis, L. N.; Lewis, N.; Uriarte, R. J. In Homogeneous Transition
Metal Catalyzed Reactions; Moser, W. R., Slocum, D. W., Eds.; Advances
in Chemistry Series 230; American Chemical Society: Washington, DC,
1992; p 541.

10876 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Kadlecek et al.
hydride versus boryl (or silyl) migration and/or â-hydride
elimination versus reductive elimination rates. Previous studies
of related metal-catalyzed hydrosilations and hydroborations
have shown that these rates strongly depend on the reaction
conditions and the particular metal, olefin, and substrates.25,31
All products resulted from either addition or substitution of
the borane at the terminal olefinic carbon, with the dehydro-
genative borylation products also exhibiting exclusively trans-
stereochemistry. The platinum-catalyzed reaction of ethylene
affords exclusively the hydroboration product 1, whereas the
platinum-catalyzed reactions of 1-pentene and styrene yield
mixtures of hydroboration and dehydrogenative borylation
products. These observations are consistent with the previous
studies of transition-metal-catalyzed hydrosilations30 and cat-
echol borane hydroborations,29 where, in both cases, it was found
that sterically bulky olefins favor dehydrogenative substitution
relative to addition. This difference is proposed to result from
the greater repulsive interactions of bulkier metalloalkyls with
the metal center, borane or silane, and other coordinated ligands.
These repulsions then increase the rate of â-hydride elimination
relative to reductive elimination. The fact that both the platinum
and palladium reactions with 1-pentene and styrene afford the
7-trans-alkenyl products is also likely determined by the steric
interactions in complex C (Figure 8) between the metal center
and the borylalkyl. Thus, rotation about the carbon-carbon bond
of the borylalkyl complex to minimize steric interactions with
the boryl group would place a â-hydrogen in close proximity
to the metal center, thus facilitating â-H elimination with
retention of the trans-conformation.29
While platinum-catalyzed dehydrogenative borylation has
apparently not been observed before, we have already shown
that these types of reactions are favored for the palladium(II)-
catalyzed reactions of other polyboranes, such as pentaborane-
(9)3,4 and borazine.5 These prior results, plus the observation
that the palladium(II) bromide-catalyzed reactions of olefins with
arachno-6,8-C2B7H13 yield predominantly alkenyl products,
suggest that the boryl migration/â-hydride elimination pathway
is more favorable for Pd(II) than Pt(II) metal centers.
Figure 8. Possible reaction mechanism for the formation of both
hydroboration and dehydrogenative borylation products.
observed for functionalization at the B7 position indicates that
oxidative addition occurs exclusively at this vertex. Owing to
its location between the two carbon cage atoms, B7 is, in fact,
the most positive vertex in the arachno-6,8-C2B7H13 cage,28 and
catalyst attack at this position is consistent with the previously
observed reactivity patterns found in the platinum(II)-catalyzed
reactions of olefins and decaborane(14).2 Hydride migration to
the olefin in complex A to produce B, followed by reductive
elimination, provides a straightforward pathway to the 7-alkyl
products 1, 4, and 6. Alternatively, boryl migration to the olefin
in complex A could occur to produce complex C. If C then
undergoes reductive elimination, 7-alkyl products are again
produced. However, if instead of reductive elimination, â-hy-
dride elimination occurs to produce D, followed by dissociation
of the substituted olefin, then the 7-alkenyl products 2, 3, and
5 would result. Such dehydrogenative borylation29 and the
related dehydrogenative silylation30 reactions are well-known
and have been proposed to occur by such mechanisms. The
alkyl/alkenyl product ratio is then controlled by the relative
The optimum temperature for the palladium-catalyzed reac-
tions was found to be 32 °C. Reactions at room temperature
were very slow, while those run at higher than 40 °C appeared
(29) (a) Ma¨nnig, D.; No¨th, H. Angew. Chem., Int. Ed. Engl. 1985, 24,
878. (b) Evans, D. A.; Fu, G. C.; Anderson, B. A. J. Am. Chem. Soc. 1992,
114, 6679. (c) Burgess, K.; van der Donk, W. A.; Westcott, S. A.; Marder,
T. B.; Baker, R. T.; Calabrese, J. C. J. Am. Chem. Soc. 1992, 114, 9350.
(d) Baker, R. T.; Calabrese, J. C.; Westcott, S. A.; Nguyen, P.; Marder, T.
B. J. Am. Chem. Soc. 1993, 115, 4367. (e) Westcott, S. A.; Marder, T. B.;
Baker, R. T. Organometallics 1993, 12, 975. (f) Brown, J. M.; Lloyd-Jones,
G. C. J. Am. Chem. Soc. 1994, 116, 866. (g) Musaev, D. G.; Mebel, A. M.;
Morokuma, K. J. Am. Chem. Soc. 1994, 116, 10693. (h) Baker, R. T.;
Calabrese, J. C.; Westcott, S. A.; Marder, T. B. J. Am. Chem. Soc. 1995,
117, 8777. (i) Dorigo, A. E.; Schleyer, P.v. R. Angew. Chem., Int. Ed. Engl.
1995, 34, 115. (j) Marder, T. B.; Norman, N. C. Top. Catal. 1998, 5, 63.
(k) Murata, M.; Watanabe, S.; Masuda, Y. Tetrahedron Lett. 1999, 40, 2585.
(l) Vogels, C. M.; Hayes, P. G.; Shaver, M. P.; Westcott, S. A. Chem.
Commun. 2000, 51. (m) Widauer, C.; Gru¨tzmacher, H.; Ziegler, T.
Organometallics 2000, 19, 2097.
(30) (a) Onopchenko, A.; Sabourin, E. T.; Beach, D. L. J. Org. Chem.
1983, 48, 5101. (b) Ojima, I.; Fuchikami, T.; Yatabe, M. J. Organomet.
Chem. 1984, 260, 335. (c) Onopchenko, A.; Sabourin, E. T.; Beach, D. L.
J. Org. Chem. 1984, 49, 3389. (d) Randolph, C. L.; Wrighton, M. S. J.
Am. Chem. Soc. 1986, 108, 3366. (e) Ojima, I. In The Chemistry of Organic
Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989;
pp 1479-1526. (f) Tanke, R. S.; Crabtree, R. H. Organometallics 1991,
10, 415. (g) Brookhart, M.; Grant, B. E. J. Am. Chem. Soc. 1993, 115,
2151. (h) Christ, M. L.; Sabo-Etienne, S.; Chaudret, B. Organometallics
1995, 14, 1082. (i) Takeuchi, R.; Yasue, H. Organometallics 1996, 15, 2098.
(j) LaPointe, A. M.; Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1997,
119, 906. (k) Marciniec, B. New J. Chem. 1997, 21, 815.
(27) The mechanism presented in Figure 8 assumes that B-H oxidative
addition of arachno-6,8-C2B7H13 is a key step. However, others have
proposed that, in metal-catalyzed hydosilations and hydroborations, the
reaction of the metalloolefin with the silane or borane substrate occurs
instead by a σ-bond metathesis mechanism involving the direct addition of
a Si-H or B-H bond across the M-C bond of the coordinated olefin.
Thus, an alternate mechanism is also possible for the arachno-6,8-C2B7H13
reactions reported herein. See, for example: Hartwig, J. F.; Bhandari, S.;
Rablen, P. R. J. Am. Chem. Soc. 1994, 116, 1839.
(28) Dolansky, J.; Herma´nek, S.; Zahradn´ık Collect. Czech. Chem.
Commun. 1981, 46, 2479.
(31) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957.

Transition-Metal-Promoted Reactions of Boron Hydrides
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10877
to stop after only low conversions (∼30%), suggesting catalyst
deactivation. The 2/1 product ratios in the palladium-catalyzed
reactions with ethylene were found to depend on the ethylene
concentration. Thus, a reaction employing a 6-fold excess of
ethylene gave a 3/1 preference for the dehydrogenative bory-
lation product 2 over the hydroboration product 1, while a ∼13-
fold excess of ethylene resulted in a 10/1 preference for 2 over
1. These results are consistent with the fact that to have catalytic
dehydrogenative borylation, an additional equivalent of olefin,
either ethylene or 2, is required to react with the dihydrido
complex E in Figure 8 to regenerate the catalytically active Pd-
(II) species. Increasing the ethylene concentration would increase
the rate of reaction of complex E with ethylene relative to 2,
thus enhancing the rate of the dehydrogenative-borylation
reaction pathway. Since the 1-pentene and styrene reactions were
run in neat olefin, the concentration of olefin was high and only
dehydrogenative borylation products were observed. NMR
studies of the reactions with styrene in fact confirmed that
ethylbenzene and 5 were produced in equivalent amounts in
these reactions.
upon the reaction conditions, catalyst, and/or olefin, alkyl- or
alkenyldicarbaboranes resulting from either hydroboration or
dehydrogenative borylation reactions, respectively. The key
reaction steps for the dicarbaborane reactions appear to be
closely related to those recently proposed for both metal-
catalyzed olefin-hydrosilation/dehydrogenative-silylation and
monoborane-hydroboration/dehydrogenative-borylation reac-
tions. These results suggest that such catalysts may find
applications for the generation of a variety of important alkyl-
and alkenylpolyborane derivatives, and investigations to explore
the scope of these catalysts are currently in progress.
Acknowledgment. We thank the National Science Founda-
tion for support of this research.
Supporting Information Available: Tables listing refined
positional and thermal parameters, bond distances and bond
angles for compounds 1, 3, and 5 and Cartesian coordinates
and bond distances for the optimized geometries at the B3LYP/
6-311G* level for 1, 2, and 7 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
In summary, both platinum and palladium species have been
found to be active catalysts for the reactions of the arachno-
6,8-C2B7H13 dicarbaborane with olefins, and yield, depending
JA002178R