Reaction of aminodihydropentalenes with HB(C6F5) 2: The crucial role of dihydrogen elimination

Article abstract of DOI:10.1021/ja1092369

The aminodihydropentalene derivative 1a reacts with the Lewis acidic RB(C₆F₅)₂ boranes (2a-c) by C-C bond cleavage to yield the formal borylene insertion products 3. In contrast, 1a,b react with HB(C₆F₅

Full text of DOI:10.1021/ja1092369

ARTICLE
pubs.acs.org/JACS
Reaction of Aminodihydropentalenes with HB(C6F5)2: The Crucial Role
of Dihydrogen Elimination
Bao-Hua Xu, Gerald Kehr, Roland Fr€ohlich, Stefan Grimme, and Gerhard Erker*
Organisch-Chemisches Institut der Universit€at M€unster, Corrensstrasse 40, 48149 M€unster, Germany
S Supporting Information
b
ABSTRACT: The aminodihydropentalene derivative 1a reacts
with the Lewis acidic RB(C6F5)2 boranes (2a-c) by C-C bond
cleavage to yield the formal borylene insertion products 3. In
contrast, 1a,b react with HB(C6F5)2 at 55 °C by elimination of
dihydrogen to yield the iminium-stabilized zwitterionic hetero-
fulvenes 10a,b. The reaction pathways were studied by prepara-
tion of the kinetically controlled intermediates 7a,b and the
thermodynamically controlled products 9a,b, monitored by
variable-temperature NMR experiments, and supported by
DFT calculations. The trapping reactions of 9a with HCl and PhCHO, respectively, led to the addition products 13 and 14.
Compounds 3c, 7a,b, 10a,b, 11, 13, and 14 were characterized by X-ray diffraction.
’ INTRODUCTION
reversal of this reaction would then enable the system to undergo
ring expansion by alkyl shift with C-C bond rupture to yield the
observed final product 3.
The search for new methods and reagents to attack and activate
thermodynamicallystrongbonds hasseenmuch progress inrecent
years. The homolytic and heterolytic activation of dihydrogen is a
typical example.1-3 A good number of metal-induced methods are
known for H2 activation,1 along with their utilization for catalytic
hydrogenation reactions.2,3 In addition, we note some significant
recent progress in using metal-free reagents for the heterolytic
cleavage of dihydrogen, e.g. by frustrated Lewis pair chemistry.4
Activation of C-H bonds has almost become an established
area.5-7 Many methods have been developed in recent years that
provide useful synthetic protocols for functionalizing organic
frameworks by selective C-H bond cleavage.7 In contrast,
C-C bond activation and cleavage of nonactivated C-C bonds
are far less developed. In comparison to H-H or C-H activation
chemistry, C-C bond activation chemistry can still be regarded to
be in its infancy.8 Of course, there are a small number of “classical”
methods available (e.g., ozonolysis9 or the olefin metathesis
reaction10), but C-C bond activation is a field where significant
development and progress are still needed.
We recently observed a new reaction sequence that resulted in
the cleavage of an unactivated C-C bond by treatment of an
aminodihydropentalene substrate (1a) with the strongly electro-
philic borane reagents RB(C6F5)2 [R = CH3 (2a), CH2CH2Ph
(2b)] to yield the formal borylene insertion products 3.11 This
reaction (Scheme 1) results in the overall cleavage of a C-C
bond with concomitant formation of a pair of two new carbon-
boron bonds, and it requires some additional rearrangement
of substituents. A likely reaction sequence was proposed to
account for our observation that is initiated by borane addition
to generate 4. Subsequent 1,5-hydrogen shift would lead to 5,
which is electronically set up for intramolecular nucleophilic
attack of the [B]-R group at the adjacent electrophilic terminus
of an R,β/γ,δ-unsaturated iminium salt to give 6. Topological
Scheme 1
We have now reacted the aminodihydropentalene 1a and a
derivative (1b) with other boranes, notably the intramolecular
Lewis pair Mes2P-CH2-CH2-B(C6F5)2 (1c) and Piers's
borane (2d),12 and made some remarkable observations with
regard to this general reaction scheme, which may be regarded as
a sequence that formally involves an unprecedented variant of the
1,1-carboboration reaction.13-16
’ RESULTS AND DISCUSSION
Reaction of Dimethylaminodihydropentalene (1a) with
the Frustrated Lewis Pair Mes2P-CH2-CH2-B(C6F5)2
(2c). We recently described the synthesis of the 3-dimethylami-
nodihydropentalene derivative 1a by fulvene condensation of the
respective doubly methyl-substituted acrylamide.17 This was
Received: October 14, 2010
Published: February 18, 2011
r
2011 American Chemical Society
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Figure 1. Projection of the molecular geometry of the ring-expanded
product 3c. Selected bond lengths (Å) and angles (deg): C3-N1
1.320(3), C3-C4 1.427(3), C4-C9 1.373(3), C8-C9 1.436(3),
C7-C8 1.344(3), C7-B6 1.631(3), C5-B6 1.657(3), C5-B6-C7
109.8(2), C8-C7-B6 121.1(2), C4-C5-B6 112.5(2).
Figure 2. Molecular structure of the kinetic HB(C6F5)2 addition
product 7a.
2.81. The newly formed [B]CH2 group inside the boratacyclo-
hexadiene unit shows signals at δ 2.06 (1H) and δ 23.3 (13C,
broad). There is a single olefinic dCH- 1H NMR resonance (δ
6.61). Compound 3c shows a 11B NMR signal at δ -14.2 and a
trio of typical borate B(C6F5)2 19F NMR signals [δ -130.6 (o),
-162.3 (p), -165.2 (m)].
Scheme 2
Reactions of the Dimethylaminodihydropentalenes 1 with
Piers’s Borane. In order to learn more about the reactions of
the aminodihydropentalene substrates with reactive C6F5-sub-
stituted boranes, we treated the substrate 1a with HB(C6F5)2.
The borane reacted rapidly with 1a, even at -40 °C. From the
concentrated reaction mixture in dichloromethane, the addition
product 7a was obtained in 63% yield as a colorless solid by
crystallization at low temperature (-35 °C). Spectroscopic
analysis showed the presence of the [(C6F5)2BH]-borate sub-
carried out by dimethylsulfate activation followed by Cp-anion
addition and condensation, similar to the method described by
Hafner et al. for the synthesis of the parent aminopentaful-
venes.18 The frustrated Lewis pair (FLP) 2c was in situ generated
by HB(C6F5)2 hydroboration of dimesitylvinylphosphine.19 The
hydroboration reaction takes place regioselectively with anti-
Markovnikov orientation. Addition of 1a to the FLP in pentane
gave a precipitate that was isolated after 24 h at room tempera-
ture to furnish the product 3c in 54% yield as a yellow-red solid.
The product was characterized by elemental analysis, spectros-
copy, and X-ray diffraction (see Figure 1 and Scheme 2).
1
stituent at C5 (11B NMR: doublet at δ -19.4 with JBH ≈ 80
Hz). This was confirmed by X-ray crystal structure analysis of 7a,
which features the boryl substituent at C5 and the presence of a
doubly conjugated R,β/γ,δ-unsaturated iminium ion moiety (see
Figure 2 and Tables 1 and 2).
The product 7a is thermally labile. Warming it from -40 °C to
room temperature and keeping it at that temperature overnight
resulted in a complete conversion to the isomeric compound 9a.
Compound 9a was isolated as a yellow-red solid in 90% yield.
The product was identified spectroscopically (for details see
Table 2 and Experimental Section). It features a typical 11B NMR
doublet at δ -22.3 with a 1JBH coupling constant of 87 Hz.
The system 9a contains a conjugated N,N-dimethyliminium
ion unit with a boron hydride nucleophile attached at its terminus
C6. Therefore, it was tempting to see whether H-transfer would
occur upon heating to open a pathway analogous to that depicted
in Scheme 1 (see above) that would eventually lead to cleavage of
the C6-C5 bond. We heated 9a to 55 °C overnight and isolated
the product 10a as a red solid from the reaction mixture in 54%
yield. Structural analysis (see Scheme 3, Tables 1 and 2, and
Figure 3) revealed that here a different reaction pathway is
preferred, determined by the facile elimination of dihydrogen.
It is likely that the intermediate 8a is involved in the formation of
9a from 7a. Therefore, it may be assumed that 9a and 8a can also
be interconverted by a reversible, thermally induced 1,5-hydro-
gen shift and that subsequent loss of H2 may take place from the
intermediate 8a upon warming (see Scheme 3) to eventually
The X-ray crystal structure analysis of 3c shows that the
B(C6F5)2 unit of the reagent 2c has formally been inserted into
the CdC bond of the substrate 1a. To make that possible, the
Mes2P-CH2-CH2- group had to migrate from boron to
carbon. This group is consequently found removed from boron
and attached to the adjacent carbon atom C7. In order to make
room for the Mes2P-CH2-CH2- migration, the hydrogen
substituent at C7 had to move; it is now found at C5. In principle,
this is the result of an unusual variant of a 1,1-carboboration
sequence.13-16 The structural characterization shows that 3c
features a zwitterionic conjugated BC5N unit in the central
bicyclic framework. The heterocyclic boratacyclohexadiene
moiety shows the typical small deviation from planarity
(B6-C7-C8-C9 -1.9(3)°, C5-B6-C7-C8 25.3(3)°).
The Mes2P-CH2-CH2- unit that is attached at the ring
carbon atom C7 features an antiperiplanar chain conformation
(θ C7-C14-C15-P 176.6(2)°). The phosphorus coordina-
tion geometry is nonplanar (sum of the C-P-C angles: 316.2°).
1
In solution (C6D6), 3c shows the H NMR resonances of a
pair of methyl groups at the iminium nitrogen atom (δ 2.37,
1.54) and the [P]CH2CH2 methylene resonances at δ 2.94 and
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Table 1. Selected Structural Data of the Aminodihydropentalene/HB(C6F5)2 Reaction Productsa
1a
7a
anti-7bb
10a
10b
11
13
14c
C3-N1
1.316(3)
1.392(3)
1.434(3)
1.423(3)
1.359(3)
1.434(4)
1.374(4)
-
1.308(3)
1.420(3)
1.489(3)
1.372(3)
1.427(3)
1.356(3)
1.480(3)
1.712(3)
-
1.304(4)
1.417(4)
1.488(4)
1.364(4)
1.423(4)
1.351(4)
1.480(4)
1.720(4)
-
1.307(2)
1.399(2)
1.385(2)
1.432(2)
1.345(2)
1.452(2)
1.405(2)
-
1.308(4)
1.410(4)
1.399(4)
1.437(4)
1.355(4)
1.461(4)
1.420(4)
-
1.310(3)
1.426(3)
1.500(3)
1.374(3)
1.418(4)
1.349(3)
1.480(3)
1.741(3)
-
1.297(5)
1.435(5)
1.501(5)
1.345(5)
1.482(5)
1.559(5)
1.566(5)
-
1.309(7)
1.436(7)
1.509(7)
1.344(7)
1.491(7)
1.547(7)
1.565(7)
-
C3-C4
C4-C5
C4-C8
C7-C8
C6-C7
C5-C6
C5-B1
C6-B1
-
1.473(3)
121.1(2)
122.2(2)
116.7(2)
1.489(4)
121.9(3)
123.0(3)
115.1(3)
1.612(5)
108.3(3)
119.6(3)
107.8(3)
1.632(8)
111.9(4)
115.8(4)
108.3(4)
C5(C6)-B1-iPh(F)a
C5(C6)-B1- iPh(F)b
iPh(F)a-B1- iPh(F)b
-
113.6(2)
110.2(2)
112.1(2)
109.3(2)
113.4(2)
112.7(2)
111.0(2)
104.4(2)
112.0(2)
-
-
a Bond distances in angstroms and bond angles in degrees. See Figures 2-8 for the atom (non-systematic) numbering scheme used. b Values taken from
the major product. c Values taken from molecule A.
Table 2. Selected NMR Data of the Aminodihydropentalene/HB(C6F5)2 Addition Productsa
1ab
7ac
7bc,g
9ad
9bd
10ad
10bd
11e
13f
14f,g
1H NMR
3.36, 3.28
3.49, 2.80
3.53
NMe2
2-H2
5-H
3.26, 3.13
2.98
3.71, 3.32
2.98
3.71, 3.34
3.39, 2.66
4.32
3.35, 3.25
3.08
3.40 3.29
3.09
3.41 3.31
3.49, 2.77
6.80
3.37, 3.20
2.92, 2.85
4.69
3.53, 3.37
3.14, 3.12
2.92, 2.79
2.93
3.42, 3.27
2.91, 2.64
2.65, 2.59
3.50
6.01
4.30
3.54
6.77
6-H
6.47
7.97
7.97
-
-
-
-
7.73
7-H
5.74
6.15
6.17
6.43
6.45
6.10
6.13
6.11
2.74, 2.55
3.54
11B NMR
[B]
-
-19.4
-19.3
-22.3
-22.3
51.0
50.4
n.o.
-1.4
1.3
13C NMR
171.6
C3
C6
C7
C8
C4
161.6
128.7
105.1
157.7
120.6
174.3
169.3
120.5
193.1
142.6
175.2
169.0
121.7
189.3
144.5
170.5
212.8
128.7
205.2
135.3
170.8
146.2
113.0
158.0
126.5
171.7
n.o.
174.0
169.0
119.3
192.7
139.4
182.5
37.8
181.7
51.3
212.4
129.9
114.3
154.0
127.5
34.5
56.4
202.0
208.6
140.8
211.7
143.8
137.4
19F NMR
o-
-
-133.3
-134.2
-133.2
-134.2
-132.3
-132.3
-131.6
-131.8
-131.6
-131.7
-127.1
-130.7
-136.6
-137.5
-164.0
-164.7
-167.0
-167.4
-168.1
-168.7
-132.1
-133.1
-133.7
p-
-
-
-162.4
-163.2
-166.1
-166.3
-162.4
-163.2
-166.1
-166.3
-163.4
-166.6
-163.4
-166.6
-155.3
-155.9
-163.7
-155.3
-155.8
-163.7
-164.4
-167.8
-164.8
-165.4
-167.7
-168.0
m-
a See Figures 2-8 for the (non-systematic) atom numbering scheme used. b CD2Cl2 at 298 K (see ref 17 for C6D6). c CD2Cl2 at 233 K. d CD2Cl2 at
298 K. e CD2Cl2 at 193 K. f [D8]THF at 298 K. g Major isomer.
form 10a. We checked for the potential reversibility of the 9a f
10a þ H2 conversion. However, under our applied reaction
conditions, 10a did not react with H2 (2.5 or 60 bar) at room
temperature.
Compound 10a was characterized by X-ray diffraction. In
the crystal, the substituted dihydropentalene framework shows
the typical small out-of-plane torsion inside the “right” five-
membered ring system to avoid eclipsed substituent interaction
along the C1-C2 vector (C1-C2 1.546(3) Å, see Figure 3).
The remaining parts of the framework are arranged close to
coplanar. In principle, system 10a could be described by the
isomeric resonance forms [A]-[C] depicted in Scheme 3. X-ray
crystal structure analysis revealed that the actual structure
comes close to the iminium-stabilized zwitterionic heterofulvene
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Figure 3. Molecular structure of 10a.
Figure 4. Molecular structure of anti-7b.
B-C vector is “frozen” on the NMR time scale in solution even
at 298 K, which results in the observation of two sets of o-, p-, and
m-F resonances for the pair of E- and Z-C6F5 substituents.
We carried out the analogous reaction starting from the
monomethyl-substituted dimethylaminodihydropentalene sub-
strate 1b. Its treatment with Piers's borane [HB(C6F5)2]12 gave
similar products under comparable reaction conditions (see
Scheme 3 and Tables 1 and 2). The primary HB(C6F5)2 addition
product 7b in this series contains a pair of chirality centers. The
chiral center C1 was brought in from the (racemic) starting
material 1b. Addition of the HB(C6F5)2 electrophile to the “left”
cyclopentadienyl ring has created a new chirality center at C5
(see Scheme 3). Consequently, we observed the formation of a
pair of diastereoisomers (anti-7b, syn-7b) in a ratio of ca. 2:1.
Low-temperature crystallization from dichloromethane/pentane
by the diffusion method eventually gave single crystals of one
isomer that was characterized by X-ray diffraction.
Scheme 3
X-ray crystal structure analysis reveals the presence of anti-7b
(Figure 4). It has the single methyl substitutent at carbon atom
C1 and the [(C6F5)2BH]- substituent at C5 oriented trans to
each other at the connecting central bicyclic dihydropentalene
framework. The remaining structural parameters of anti-7b are
similar to those found for 7a (see above and Table 1).
description [A]. The C3-N bond length in 10a (1.307(2) Å) is
in the typical iminium range. The connected five-membered
carbocycle exhibits characteristic alternating C-C bond lengths,
with long C5-C6 (1.405(2) Å), C6-C7 (1.452(2) Å), and
C4-C8 (1.432(2) Å) bonds (C1-C8 1.495(2) Å) and short
C7-C8 (1.345(2) Å) and C4-C5 (1.385(2) Å) bonds. The
short B1-C6 bond, at 1.473(3) Å, is more than 0.1 Å shorter
than the adjacent B-C6F5 connections (B1-C21 1.584(3) Å,
B1-C31 1.583(3) Å). The boron center in 10a is planar
tricoordinated (sum of C-B-C bond angles: 360.0°), and the
(C6F5)2Bd unit is oriented coplanar with the adjacent un-
saturated five-membered ring of the dihydropentalene unit
(dihedral angles C7-C6-B1-C21 2.6(3)°, C7-C6-B1-C31
-177.3(2)°).
Storing 7b in dichloromethane at room temperature overnight
resulted in a clean formation of the single product 9b. It was
isolated as a yellow-red solid in 90% yield and characterized
spectroscopically. It features a 11B NMR doublet at δ -22.3 with
1
1
a typical JBH coupling constant of ca. 87 Hz and the 1-H H
NMR resonance at δ 3.29. Thermolysis of the solution of 9b
in dichloromethane (55 °C overnight) eventually generated a
reaction mixture from which crystals of 10b were obtained. The
compound was identified by X-ray crystal structure analysis (see
Figure 5 and Scheme 3; for further details see the Experimental
Section and the Supporting Information).
The 7f9 transformation formally can be described as a 1,5-
borane shift along the π-perimeter of the “left” five-membered
dihydropentalene ring. However, it remained to be determined
whether this isomerization reaction takes place intra- or inter-
molecularly. Having the pair of HB(C6F5)2 addition products 7a,
b available, we could perform the necessary experiments with
ease. Thus, in a typical experiment, we mixed the primary
addition product 7a, obtained by treatment of HB(C6F5)2 with
the 1,1-dimethyl-3-dimethylamino-1,2-dihydropentalene substrate
1
In solution, 10a features H NMR signals of the olefinic
hydrogens 5-H (δ 6.77) and 7-H (δ 6.10). We monitor a 11B
NMR resonance at δ 51.0, which indicates some transfer of π-
electron density to boron.20 The 19F NMR spectrum shows a
separation of the p- and m-fluorine resonances of the C6F5 ring
(Δδ 8.4 and 7.8), which presents an average value between
tricoordinated and tetracoordinated boron centers.21 Conse-
quently, the rotation of the (C6F5)2BdC unit around the
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Figure 6. Optimized structures (TPSS-D3/def2-TZVP) of the mini-
mum (a) and TS (b) for R = B(C6F5)3. The bond lengths are given in
angstroms.
Table 3. Reaction Barriers and Activation Enthalpies
(in Parentheses, All Values in kcal/mol) at the TPSS-D3/
def2-TZVP and B2PLYP-D3/def2-TZVP Levels for the
1,5-Shift in Monosubstituted Cyclopentadienes
Figure 5. Molecular structure of 10b.
R
TPSS
B2PLYP
Scheme 4
BF3
16.4
12.2
22.9
38.1
10.4
17.7 (17.1)
13.1 (12.8)
25.8 (24.1)
42.5 (41.7)
13.2 (12.5)
BH3
B(C6F5)3
CH3
SiH3
quantum chemical calculations based on Kohn-Sham density
functional theory (DFT24,25). These were carried out using the
TURBOMOLE quantum chemical program system.26 For the
corresponding minima and transition states (TSs), we performed
full structural optimizations assuming Cs symmetry for the TS.
Large triple-ζ Gaussian AO basis sets (def2-TZVP27), which
yielded almost convergent results for the investigated properties,
and the RI integral approximation28 were employed. As density
functional, the almost nonempirical meta-GGA (TPSS29) con-
structions were used for the geometry optimizations. In all DFT
treatments, we used the now well-established correction for long-
range London dispersion effects,30 in its third revised version
(DFT-D3 method31), that have more or less been absent in all
previous standard density functionals. Intramolecular dispersion
effects are of particular importance for large molecules when
bulky substituents (C6F5 in one of our cases) spatially come close
together (for a recent example, see ref 32). Single-point calcula-
tions were also performed using the B2PLYP double-hybrid
functional,33 which yields more accurate thermodynamic prop-
erties and reaction barriers. The stationary points were properly
characterized as either minima or TSs by computation of har-
monic vibration frequencies at the LDA/SV(P) level (which
yields structures similar to those obtained with TPSS-D3/
def2-TZVP).
The structures for R = B(C6F5)3 as an example, as obtained at
the DFT level, are show in Figure 6. The C-B bond length of
about 1.7 Å in the equilibrium structure increases to a typical
value of about 2.4 Å in the TS. The formal single C-C bonds in
the cyclopentadiene ring decrease by about 0.04 Å to a value of
1.45 Å, which indicates some π-delocalization in the TS. The two
hydrogen atoms bound to the carbon atoms above which the
group R moves are only slightly tilted out of the ring plane. The
cyclopentadiene ring in the TS is at least in part structurally
similar to the (planar) cyclopentadienyl anion. The Wiberg bond
(1a) with 1 molar equivalent of 1-methyl-3-dimethylamino-1,2-
dihydropentalene (1b) at -40 °C. Slowly warming the mixture
to 25 °C and keeping it at that temperature eventually resulted in
the observation of a ca. 1:1 mixture of the dehydrogenation
products 10a,b, along with ca. 50% of a ca. 1:1 mixture of the free
aminodihydropentalenes 1a,b (see Scheme 4; for further details,
including the intermediate observation of 7a,b and 9a,b, see the
Supporting Information). The outcome of this and related
experiments indicated that the 7af9a (and conversely the
7bf9b) isomerization takes place by an intermolecular route
involving reversal of the HB(C6F5)2-to-aminodihydropentalene
addition. Apparently, here intramolecular 1,5-HB(C6F5)2 migra-
tion along the π-system is not a facile process.22,23 This is a
surprising result. We tried to find some explanation for this
unexpected behavior by DFT analysis of the underlying 1,5-BX3
migration in the related substituted cyclopentadiene system.
Unfortunately, calculation of the B-C bond dissociation process
in the respective [R3BCp]- anion systems met with some dif-
ficulties. However, the theoretical description of the intramolec-
ular 1,5-boron shifts was possible. This gave a remarkable result.
The 1,5-shift of various functional groups (R = Me, BH3, SiH3,
B(C6F5)3) in the model R-C5H5 anions was investigated by
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Scheme 5
order for the formed/broken C-B bonds in the TS is 0.32, which
is a rather typical value for a concerted reaction and should be
compared to the value for the formal C-B single bond in the
minimum structure of 0.87.34 This means that the loss of covalent
bonding contribution of the broken C-B bond is, to a significant
degree, compensated by the forming bond.
Similar structures were obtained for the other considered
substituents. In addition to the experimentally investigated
B(C6F5)3 group, we conducted calculations for other boron-
containing groups (BH3 and BF3) and, for comparison, also
for methyl and silyl substituents.
As can be seen from the compilation of the obtained data
(Table 3), the calculated barriers are very strongly substituent
dependent, ranging from about 13 kcal/mol (for BH3 and SiH3)
to 42.5 kcal/mol (CH3). As expected, the values are slightly
underestimated by the semilocal TPSS density functional com-
pared to the more reliable B2PLYP results (which are discussed
in the following). The thermal and zero-point vibrational correc-
tions are also small (cf. values in parentheses). The value for the
experimentally studied B(C6F5)3 substituent, 25.8 kcal/mol, is
intermediate in our series but 8-12 kcal/mol larger than those
calculated for other boron-containing groups. This once again
shows how important it is to employ “real” substituents in
accurate quantum chemical studies. The computed activation
enthalpy of 24.1 kcal/mol is apparently too large for a sufficiently
fast chemical reaction at ambient temperatures to compete with
the observed alternative reaction pathway. This behavior marks a
pronounced substituent effect of the large C6F5 groups at boron.
Coming Back to the Carbon-Carbon Bond Activation
Scheme. Since we had shown that HB(C6F5)2 added to the
carbon atom C5 of the aminodihydropentalene substrates 1
under kinetic control to give the adducts 7 (see Scheme 3), it was
tempting to speculate that the alkyl-B(C6F5)2 reagents 2 might
behave similarly. That was indeed the case. We showed that the
reagent PhCH2CH2B(C6F5)2 (2b) reacted with substrate 1a in
dichloromethane at -80 °C to give the primary addition product
Figure 7. Molecular structure of 11.
The intermediate 5b was characterized spectroscopically [1H
NMR δ 6.77 (7-H), 2.26/1.30 (CH2-CH2-[B]), 3.45 (5-H2),
3.32/3.20 (NMe2); 13C δ 212.4 (br, dCB), 170.7 (CdNMe2),
127.8 (C7), 204.3 (C8), 135.1 (C4); 19F Δδ(p,m) = 2.4].11
In view of the course taken in the related reactions of 1a,b with
HB(C6F5)2, we assume that the 11f5b conversion may also
involve an intermolecular mechanism for the formal 1,5-borane
shift. We conclude that the observation of the preformation of
the addition product 11 under kinetic control leads to an
extension of the overall reaction scheme of the C-C bond
activation/cleavage of substrate 1 by the RB(C6F5)2 reagent (see
Scheme 1) to include the reaction sequence described in
Scheme 5.
In the C-C bond activation scheme (see Scheme 1), we
proposed 1,2-alkyl migration from boron to the adjacent con-
jugated dienyl iminium moiety as an essential step (5f6). In the
related HB(C6F5)2-derived series, a similar pathway to C-C
bond cleavage, which would be initiated by the 9f12 transfor-
mation, was apparently not opened (see Scheme 6). Instead, we
observed a more favorable H2 elimination, probably via 9h8
equilibration (see Scheme 3). The question remained whether
the 9f12 hydrogen shift was taking place at all or if it was just
unproductive in view of the preferred 9h8f10 reaction se-
quence.
1
11 (see Scheme 5). Compound 11 features the 5-H H NMR
signal at δ 4.69 as a broad singlet. It shows olefinic resonances at
δ 7.73 and 6.11 (6-H and 7-H).
Compound 11 was also characterized by X-ray diffraction (see
Figure 7). It shows the bulky PhCH2CH2B(C6F5)2-borane
moiety attached at carbon atom C5 of the dihydropentalene
core (B1-C13 1.644(3) Å, dihedral angle B1-C13-C14-C15
175.2(2)°). The iminium ion (C3-N1 1.310(3) Å) is doubly
conjugated to the diene unit inside the “left” cyclopentadiene
ring of the aminodihydropentalene system (C3-C4 1.426(3) Å,
C4-C8 1.374(3) Å, C8-C7 1.418(4) Å, C7-C6 1.349(3) Å).
Warming a sample of 11 in dichloromethane to room tem-
perature allowed us to observe the formation of the “rearrange-
ment” product 5b (see Schemes 1 and 5) before it eventually
reacted further by C-C bond cleavage to yield 3b (see Scheme 1).
In two experiments, we indirectly found out by trapping
reactions that the 9h12 equilibration can actually take place in
the aminodihydropentalene plus HB(C6F5)2 system.
An orange solution of 9a in dichloromethane was treated with
an equimolar amount of HCl (added as HCl solution in ether)
at room temperature. The solution became colorless after
some time, and after stirring the mixture overnight we isolated
product 13 in 73% yield. X-ray crystal structure analysis (see
Figure 8 and Table 1) revealed that actually this product must be
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Figure 8. Molecular structure of 13.
Scheme 6
Figure 9. Molecular structure of 14 (molecule A is shown).
center, and the dienamine subunit has added to the aldehyde
carbonyl carbon via its nucleophilic sp2 terminus, forming a new
stereogenic center at C13 (see Scheme 6). We found the product
that was formed along the sterically least hindered pathway.
In solution, a mixture of two diastereomers in a 1:6 ratio was
observed for 14, which we tentatively assigned as (6S*,7S*,13R*)
and (6S*,7S*,13S*), respectively. Consequently, we observed two
sets of three 19F NMR resonances for the diastereotopic C6F5
groups at boron for each diastereomer. The diastereotopic
hydrogens of the methylene carbon atoms C2 and C5 were
found in the 1H NMR spectrum at δ 2.91/2.64 (δ13C 53.2) and
δ 2.65/2.59 (δ13C 35.5), respectively, and the pair of C1 methyl
resonances at δ 1.11 and 0.05 (major isomer; for further details,
see the Supporting Information).
’ CONCLUSIONS
We have found a remarkable reaction system in which addition
of a borane of the type RB(C6F5)2 to the aminodihydropenta-
lenes 1 (see Scheme 1) results in activation and cleavage of a
strong, nonactivated CdC bond inside the “left” five-membered
carbocycle. This leads to a formal insertion of a borylene unit
[i.e., B(C6F5)2] into the CdC bond, with a concomitant series of
alkyl and hydrogen migration reactions. The overall reaction
sequence can formally be regarded as a unique 1,1-carboboration
reaction of an olefin.35
The present study has revealed a number of significant
mechanistic features of this remarkable reaction sequence. At
the same time, we have learned details about the scope and
limitations of the use of different boranes in this reaction. There
is a marked dependency on the nature of the “active” R-group of
the RB(C6F5)2 reagents: R = C6F5 marks one limiting situation,
where apparently the poor migratory aptitude of the electron-
poor C6F5 group is not sufficient to initiate the overall sequence
leading to CdC cleavage.11 R = H marks another limiting
situation, where at the stage of an essential intermediate (8, see
Scheme 3) a competing reaction pathway, namely facile,
thermally induced irreversible elimination of dihydrogen, takes
over and determines the overall reaction outcome. With such
rather detailed knowledge about this specific example of a
potentially important new reaction type, it is temping to search
for other and, it is hoped, more generally applicable examples
regarded to have been formed by HCl addition to the H-shifted
intermediate 12. The carbon atom C6 in 13 adjacent to boron is
sp3-hybridized and bears a hydrogen atom. Protonation took
place at the former dienamine terminus C7, and the chloride
anion was added to the borane. Consequently, 13 contains an
R,β-unsaturated iminium functionality (C4-C8 1.345(5) Å,
C4-C3 1.435(5) Å, C3-N1 1.297(5) Å).
In solution, 13 features a 11B NMR resonance at δ -1.4 and
the 1H NMR signal of the adjacent 6-H at δ 2.93. The 5-H/H0
hydrogens (δ 2.92/2.79) and the 7-H/H0 hydrogen atoms (δ
2.74/2.55) are pairwise diastereotopic (for further spectroscopic
details, see Table 2 and the Supporting Information).
The proposed intermediate 12 could also be trapped with
benzaldehyde. Treatment of 9a with 1 molar equivalent of
PhCHO in dichloromethane at room temperature gave a color-
less precipitate of the addition product 14 (see Scheme 6), which
we isolated in ca. 50% yield. Single crystals of 14 were obtained
from THF/pentane by the diffusion method. X-ray crystal
structure analysis (see Figure 9 and Table 1) revealed that a
stereoselective cycloaddition had taken place, leading to a system
of three annelated five-membered rings. The AB ring junction is
cis. The aldehyde oxygen atom has become attached to the boron
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Journal of the American Chemical Society
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of such specific Lewis acid-induced CdC bond activation
processes.
σ(I)], 546 refined parameters, R = 0.052, wR2 = 0.143, max (min) residual
electron density 0.43 (-0.31) e Å-3, hydrogen atoms calculated and
refined as riding atoms.
Preparation of 7a. Compound 1a (100.0 mg, 0.57 mmol) and
HB(C6F5)2 (198.0 mg, 0.57 mmol) were put together in a Schlenk flask
and cooled to -40 °C. Precooled dichloromethane (10 mL) was then
added. The reaction mixture was stirred at -40 °C for 15 min and
concentrated to 5 mL in vacuo. After crystallization at -35 °C, 7a was
obtained as a colorless solid (187.0 mg, 0.36 mmol, 63%). Crystals
suitable for X-ray crystal structure analysis were obtained by gas diffusion
of pentane into a solution of 7a in dichloromethane at -35 °C. Anal.
Calcd for C24H18BNF10: C, 55.31; H, 3.48; N, 2.69. Found: C, 54.85; H,
3.50; N, 3.01. 1H NMR (500 MHz, CD2Cl2, 233 K): δ = 7.97 (m, 1H,
6-H), 6.15 (m, 1H, 7-H), 4.30 (br s, 1H, BCH), 3.71 (s, 3H, NMe2Z),
’ EXPERIMENTAL SECTION
General Procedures. Reactions with air- and moisture-sensitive
compounds were carried out under argon using Schlenk-type glassware
or in a glovebox. Solvents were dried and distilled under argon prior to
use. The aminodihydropentalene derivatives 1a,b,17 dimesitylvinylpho-
sphine (Mes2PCHdCH2),19 and bis(2,3,4,5,6-pentafluorophenyl)borane
[HB(C6F5)2]12 were prepared as described in the literature. Styrene and
benzaldehyde were distilled using vacuum transfer prior to use. The
following instruments were used for physical characterization of the
compounds: elemental analyses, Foss-Heraeus CHNO-Rapid; ESI mass
spectra, Bruker Daltonics MicroTOF; NMR spectra, Varian UNITY
plus (1H, 599.9 MHz; 13C, 150.8 MHz; 19F, 564.4 MHz; 11B, 192.4
MHz) and Varian 500 MHz INOVA (1H, 499.9 MHz; 13C, 125.7 MHz;
19F, 470.3 MHz; 11B, 160.4 MHz). For 1H and 13C NMR data, chemical
shift δ is given relative to TMS and referenced to the solvent signal; for
19F NMR, δ is given relative to CFCl3 (external reference); for 11B
E
3.32 (s, 3H, NMe2 ), 3.27 (br, 1H, BH), 2.98 (s, 2H, 2-H), 1.23 (s, 3H,
Meb), 0.87 (s, 3H, Mea). 13C{1H} NMR (126 MHz, CD2Cl2, 233 K):
δ = 193.1 (C8), 174.3 (CdN), 169.3 (C6), 142.6 (C4), 120.5 (C7),
E
53.2 (C2), 53.0 (br, BCH), 44.2 (NMe2Z), 42.9 (NMe2 ), 38.7 (C1),
27.5 (Meb), 27.0 (Mea) [C6F5 not listed]. 11B NMR (160 MHz, CD2Cl2,
233 K): δ = -19.4 (1JBH ≈ 80 Hz). 19F NMR (470 MHz, CD2Cl2,
a
b
233 K): δ = -133.3 (br, 2F, o-C6F5 ), -134.2 (br, 2F, o-C6F5 ), -162.4
NMR, δ is given relative to BF3 Et2O (external reference). Assignments
3
a
b
b
(1F, p-C6F5 a), -163.2 (1F, p-C6F5 ), -166.1 (2F, m-C6F5 ), -166.3
of the resonances were supported by 2D experiments.
X-ray Crystal Structure Analyses. Data sets were collected with
a Nonius KappaCCD diffractometer. The following programs were
used: data collection, COLLECT (Nonius B.V., 1998); data reduction,
Denzo-SMN,36 absorption correction, Denzo;37 structure solution,
SHELXS-97;38 structure refinement, SHELXL-97;39 graphics, XP
(BrukerAXS, 2000). Graphics show the thermal ellipsoids at the 50%
probability level. R-values are given for observed reflections and wR2-
values for all reflections.
(2F, m-C6F5 ).
X-ray Crystal Structure Analysis of 7a. Formula C24H18BF10N, M =
521.20, colorless crystal 0.20 ꢀ 0.15 ꢀ 0.10 mm, a = 19.5277(8), b =
6.5056(3), and c = 18.7809(6) Å, β = 112.021(2)°, V = 2211.85(15) Å3,
F
calc = 1.565 g cm-3, μ = 1.332 mm-1, empirical absorption correction
(0.777 e T e 0.878), Z = 4, monoclinic, space group P21/c (No. 14), λ =
1.54178 Å, T = 223(2) K, ω and j scans, 13 524 reflections collected
((h,(k,(l), [(sin θ)/λ] = 0.60 Å-1, 3845 independent (Rint = 0.042)
and 3319 observed reflections [I g 2 σ(I)], 329 refined parameters, R =
0.045, wR2 = 0.121, max (min) residual electron density 0.28 (-0.26)
e Å-3, hydrogen atoms calculated and refined as riding atoms.
Synthesis of Compound 3c. A solution of Mes2PCHdCH2
(100 mg, 0.34 mmol) and HB(C6F5)2 (117 mg, 0.34 mmol) in pentane
(6 mL) was stirred for 15 min. A solution of 1a (65 mg, 0.37 mmol) in
pentane (3 mL) was then added at room temperature, and after 1 h a
yellow-red solid began to precipitate. After the reaction mixture had
been stirred for 24 h, the solid was isolated via cannula filtration and
washed twice with cold pentane (2.5 mL). Removal of all volatiles in
vacuo yielded 3c (150 mg, 54%) as a red powder. Crystals suitable for
X-ray crystal structure analysis were grown from a saturated pentane
solution of 3c at -30 °C. Anal. Calcd for C44H43BF10NP: C, 64.64; H,
5.30; N, 1.71. Found: C, 64.51; H, 5.36; N, 1.65. 1H NMR (500 MHz,
Preparation of 7b. Compound 1b (100 mg, 0.62 mmol) and
HB(C6F5)2 (216 mg, 0.62 mmol) were put together in a Schlenk flask
and cooled to -40 °C. Precooled dichloromethane (10 mL) was then
added. The reaction mixture was stirred at -40 °C for 15 min and
concentrated to 5 mL in vacuo. After crystallization at -35 °C, 7b was
obtained as a colorless mixture (204 mg, 0.40 mmol, 65%) containing a
major isomer (anti-7b, tentative assignment) and a minor isomer (syn-
7b) in a ratio of 2:1. Crystals of anti-7b suitable for X-ray crystal structure
analysis were obtained by gas diffusion of pentane into a solution of 7b in
dichloromethane at -35 °C. Anal. Calcd for C23H16BNF10: C, 54.47; H,
3.18; N, 2.76. Found: C, 54.04; H, 3.16; N, 3.07.
4
C6D6, 298 K): δ = 6.69 (d, JPH = 2.5 Hz, 4H, m-Mes), 6.61 (s,
1H, dCH), 2.94 (m, 2H, PCH2), 2.81 (m, 2H, BCH2), 2.41 (s, 12 H, o-
CH3Mes), 2.37 (br s, 3H, NMeZ), 2.11 (s, 6H, p-CH3Mes), 2.06 (br s, 2H,
B-CH2), 1.54 (br s, 3H, NMeE), 1.49 (br s, 2H, CH2), 0.89 (s, 6H, Me).
13C{1H} NMR (126 MHz, C6D6, 298 K): δ = 196.2 (br, dC-B), 189.0
NMR Analysis of the Major Isomer. 1H NMR (500 MHz, CD2Cl2,
233 K): δ = 7.97 (d, 3JHH = 3.9 Hz, 1H, 6-H), 6.17 (d, 3JHH = 3.9 Hz, 1H,
7-H), 4.32 (br, 1H, 5-H), 3.71 (s, 3H, NMe2Z), 3.39 (dd, 2JHH = 18.2 Hz,
(C9), 181.2 (CdN), 148.7 (dm, 1JFC ≈ 237 Hz, C6F5), 142.0 (d, 3JPC
=
1
E
13.3 Hz, o-Mes), 138.6 (dm, JFC ≈ 245 Hz, C6F5), 137.4 (p-Mes),
137.3 (dm, 1JFC ≈ 249 Hz, C6F5), 133.7 (d, 1JPC = 22.7 Hz, i-Mes), 130.3
(d, JPC = 2.8 Hz, m-Mes), 128.9 (C4), 121.0 (C8), 48.7 (C2), 43.2
(NMeE), 41.6 (C1), 41.2 (NMeZ), 36.5 (d, 2JPC = 20.9 Hz, BCH2), 27.0
3JHH = 5.6 Hz, 1H, 2-Hb), 3.34 (s, 3H, NMe2 ), 3.11 (m, 1H, 1-H), 2.66
(d, 2JHH = 18.2 Hz, 1H, 2-Ha), 0.83 (d, 3JHH = 7.2 Hz, 3H, Me). 13C{1H}
NMR (126 MHz, CD2Cl2, 233 K): δ = 189.3 (C8), 175.2 (CdN), 169.0
(C6), 144.5 (C4), 121.7 (C7), 53.0 (br, C5), 46.3 (C2), 44.1 (NMe2Z),
1
3
E
(d, JPC = 16.9 Hz, PCH2), 26.9 (Me), 23.3 (d, JPC = 13.3 Hz, o-
CH3Mes), 23.3 (br, B-CH2), 20.7 (p-CH3Mes). 11B{1H} NMR (160 MHz,
C6D6, 298 K): δ = -14.2 (ν1/2 = 90 Hz). 19F{1H} NMR (470 MHz, C6D6,
298 K): δ = -130.6 (2F, o-C6F5), -162.3 (1F, p-C6F5), -165.2 (2F, m-
C6F5). 31P{1H} NMR (202 MHz, 298 K, C6D6): δ = -20.6 (ν1/2 = 4 Hz).
42.9 (NMe2 ), 32.7 (C1), 19.0 (Me) [C6F5 not listed] . 11B NMR (160
MHz, CD2Cl2, 233 K): δ = -19.3 (1JBH ≈ 80 Hz). 19F NMR (470 MHz,
a
b
CD2Cl2, 233 K): δ = -133.2 (2F, o-C6F5 ), -134.2 (2F, o-C6F5b),
a
b
-162.4 (1F, p-C6F5a), -163.2 (1F, p-C6F5 ), -166.1 (2F, m-C6F5 ),
-166.3 (2F, m-C6F5 ).
X-ray Crystal Structure Analysis of 3c. Formula C44H43BF10NP 1/2-
NMR Analysis of the Minor Isomer. 1H NMR (500 MHz, CD2Cl2,
233 K): δ = 7.94 (d, 3J = 4.9 Hz, 1H, 6-H), 6.14 (d, 3JHH = 4.9 Hz, 1H,
7-H), 4.36 (br, 1H, 5-H), 3.72 (s, 3H, NMe2Z), 3.38 (m, 1H, 2-Hb), 3.33
3
C5H12, M = 853.65, colorless crystal 0.40 ꢀ 0.25 ꢀ 0.03 mm, a =
10.6333(3), b = 11.1091(5), and c = 19.3290(11) Å, R = 101.818(6), β =
98.509(2), and γ = 98.229(3)°, V = 2174.50(17) Å3, Fcalc = 1.304 g cm-3
,
(s, 3H, NMe2 ), 2.77 (m, 1H, 1-H), 2.70 (d, 2JHH = 18.4 Hz, 1H, 2-Ha),
E
μ = 1.222 mm-1, empirical absorption correction (0.641 e T e 0.964),
Z= 2, triclinic, space group P1 (No. 2), λ=1.54178Å,T=223(2) K, ωand
1.17 (d, 3JHH = 7.2 Hz, 3H, Me). 13C{1H} NMR (126 MHz, CD2Cl2,
233 K): δ = 188.6 (C8), 175.1 (CdN), 168.7 (C6), 144.4 (C4), 121.7
j scans, 26 347 reflections collected ((h,(k,(l), [(sin θ)/λ] = 0.60 Å-1
,
(C7), 53.5 (br, C5), 46.2 (C2), 44.1 (NMe2Z), 42.9 (NMe2 ), 32.5
E
7513 independent (Rint = 0.051) and 6425 observed reflections [I g 2
(C1), 18.8 (Me) [C6F5 not listed]. 11B NMR (160 MHz, CD2Cl2,
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Journal of the American Chemical Society
ARTICLE
233 K): δ = -19.3 (1JBH ≈ 80 Hz). 19F NMR (470 MHz, CD2Cl2,
X-ray Crystal Structure Analysis of 10a. Formula C24H16BF10N,
M = 519.19, yellow crystal 0.40 ꢀ 0.15 ꢀ 0.10 mm, a = 8.4497(1), b =
9.7617(1), and c = 14.2402(2) Å, R = 71.184(1), β = 84.869(1), and γ =
a
b
233 K): δ = -133.9 (2F, o-C6F5 ), -134.2 (2F, o-C6F5 ), -162.2 (1F,
a
b
b
p-C6F5 ), -163.1 (1F, p-C6F5 ), -166.2 (2F, m-C6F5 ), -166.7 (2F,
m-C6F5 ).
88.108(1)°, V = 1107.34(2) Å3, Fcalc = 1.557 g cm-3, μ = 1.330 mm-1
,
a
X-ray Crystal Structure Analysis of anti-7b. Formula C23H16BF10N,
M = 507.18, yellow crystal 0.30 ꢀ 0.15 ꢀ 0.07 mm, a = 19.0148(12), b =
6.3803(4), and c = 18.4629(10) Å, β = 109.883(4)°, V = 2106.4(2) Å3,
empirical absorption correction (0.618 e T e 0.879), Z = 2, triclinic,
space group P1 (No. 2), λ = 1.54178 Å, T = 223(2) K, ω and j scans,
14 219 reflections collected ((h,(k,(l), [(sin θ)/λ] = 0.60 Å-1, 3939
independent (Rint = 0.037) and 3658 observed reflections [I g 2 σ(I)],
330 refined parameters, R = 0.048, wR2 = 0.138, max (min) residual
electron density 0.40 (-0.39) e Å-3, hydrogen atoms calculated and
refined as riding atoms.
F
calc = 1.599 g cm-3, μ = 1.381 mm-1, empirical absorption correction
(0.682 e T e 0.910), Z = 4, monoclinic, space group P21/c (No. 14), λ =
1.54178 Å, T = 223(2) K, ω and j scans, 16 172 reflections collected
((h,(k,(l), [(sin θ)/λ] = 0.60 Å-1, 3693 independent (Rint = 0.047)
and 3398 observed reflections [I g 2σ(I)], 330 refined parameters, R =
0.059, wR2 = 0.140, max (min) residual electron density 0.28 (-0.26) e
Preparation of 10b. A solution of 9b (100 mg, 0.20 mmol) in
dichloromethane was warmed to 55 °C and stirred overnight. The
solvent was removed under vacuum, and the resulting solid was
suspended in cold pentane, filtered, and dried under vacuum. A mixture
of 10b together with 10b0 was obtained in a ratio of 1.5:1 as a red solid
(with traces of 1b) (50 mg, 0.10 mmol, 50%). Crystals suitable for X-ray
crystal structure analysis were obtained by gas diffusion of pentane into a
solution of 10b in benzene at room temperature. HRMS (ESI) exact
mass for [M þ H]þ (C23H15BNF10): calcd m/z 506.1132, found
Å
-3, hydrogen atoms calculated and refined as riding atoms, group
C2-C1-C9 refined with split positions using geometrical and thermal
restraints.
Preparation of 9a. A solution of 7a (100.0 mg, 0.19 mmol) in 10
mL of dichloromethane was stirred at room temperature overnight. The
solvent was removed under vacuum, and the resulting solid was
suspended in precooled pentane, filtered, and dried under vacuum.
Compound 9a was obtained as a yellow-red solid (89.0 mg, 0.17 mmol,
90%). 1H NMR (500 MHz, CD2Cl2, 298 K): δ = 6.43 (br, 1H, 7-H),
3.54 (s, 2H, 5-H), 3.35, 3.25 (each s, each 3H, NMe2), 3.08 (s, 2H, 2-H),
1.35 (s, 6H, Me). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K): δ = 212.8
1
506.1122. H NMR (500 MHz, CD2Cl2, 298 K): δ = 6.80 (s, 1H,
5-H), 6.13 (s, 1H, 7-H), 3.49 (dd, 2JHH = 18.4 Hz, 3JHH = 6.8 Hz, 1H,
E
2-Hb), 3.41 (s, 3H, NMe2Z), 3.32 (m, 1H, 1-H), 3.31 (s, 3H, NMe2 ),
2.77 (ddm, 2JHH = 18.4 Hz, 3JHH = 3.3 Hz, 1H, 2-Ha), 1.31 (d, 3JHH = 7.0
Hz, 3H, Me). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K): δ = 171.7
(CdN), 154.0 (C8), 128.5 (C5), 127.5 (C4), 114.3 (C7), 48.3 (C2),
1
(br, dCB), 205.2 (C8), 170.5 (CdN), 148.3 (dm, JFC ≈ 238 Hz,
C6F5), 138.4 (dm, 1JFC ≈ 244 Hz, C6F5), 137.1 (dm, 1JFC ≈ 246 Hz,
C6F5), 135.3 (C4), 128.7 (C7), 124.8 (br, i-C6F5), 53.6 (C2), 44.5
(C5), 42.7, 42.6 (NMe2), 40.0 (C1), 27.1 (Me). 11B NMR (192 MHz,
CD2Cl2, 298 K): δ = -22.3 (1JBH = 87 Hz). 19F NMR (564 MHz,
CD2Cl2, 298 K): δ = -132.3 (2F, o-C6F5), -163.4 (1F, p-C6F5), -166.6
(2F, m-C6F5).
E
43.5 (NMe2Z), 42.9 (NMe2 ), 30.7 (C1), 21.8 (Me), n.o. (C6) [C6F5
not listed]. 11B NMR (160 MHz, CD2Cl2, 298 K): δ = 50.4 (ν1/2 ≈ 700
Hz). 19F NMR (470 MHz, CD2Cl2, 298 K): δ = -131.6, -131.7 (br,
each 2F, o-C6F5), -155.3, -155.8 (br, each 1F, p-C6F5), -163.7 (br,
4F, m-C6F5).
Preparation of 9b. A solution of 7b (100 mg, 0.20 mmol) in 10
mL of dichloromethane was stirred at room temperature overnight. The
solvent was removed under vacuum, and the resulting solid was
suspended in precooled pentane, filtered, and dried under vacuum.
Compound 9b was obtained as a yellow-red solid (90 mg, 0.18 mmol,
90%). 1H NMR (500 MHz, CD2Cl2, 298 K): δ = 6.45 (s, 1H, 7-H), 3.53
X-ray Crystal Structure Analysis of 10b. Formula C23H14BF10N
3
CH2Cl2, M = 590.09, yellow crystal 0.35 ꢀ 0.30 ꢀ 0.20 mm, a =
7.9357(4), b = 11.1685(6), and c = 14.6427(9) Å, R = 94.818(2), β =
91.780(3), and γ = 103.119(4)°, V = 1257.76(12) Å3, Fcalc = 1.558 g
cm-3, μ = 3.155 mm-1, empirical absorption correction (0.405 e T e
0.571), Z = 2, triclinic, space group P1 (No. 2), λ = 1.54178 Å, T =
223(2) K, ω and j scans, 15 489 reflections collected ((h,(k,(l),
[(sin θ)/λ] = 0.60 Å-1, 4330 independent (Rint = 0.043) and 3651
observed reflections [I g 2 σ(I)], 368 refined parameters, R = 0.067,
2
(br s, 2H, 5-H), 3.49 (dd, JHH = 18.4 Hz, 3JHH = 5.8 Hz, 1H, 2-Hb),
E
3.36 (s, 3H, NMe2Z), 3.29 (m, 1H, 1-H), 3.28 (s, 3H, NMe2 ), 2.80 (d,
2JHH = 18.4 Hz, 1H, 2-Ha), 1.30 (d, 3JHH = 7.4 Hz, 3H, Me). 13C{1H}
NMR (126 MHz, CD2Cl2, 298 K): δ = 212.4 (br, dCB), 202.0 (C8),
171.6 (CdN), 148.3 (dm, 1JFC ≈ 238 Hz, C6F5), 138.5 (dm, 1JFC ≈ 244
Hz, C6F5), 137.4 (C4), 137.0 (dm, 1JFC ≈ 247 Hz, C6F5), 129.9 (C7),
125.0 (br, i-C6F5), 46.5 (C2), 44.4 (br, C5), 42.7 (br, NMe2), 33.9 (C1),
18.7 (Me). 11B NMR (192 MHz, CD2Cl2, 298 K): δ = -22.3 (1JBH ≈ 87
Hz). 19F NMR (564 MHz, CD2Cl2, 298 K): δ = -132.3
(2F, o-C6F5), -163.4 (1F, p-C6F5), -166.6 (2F, m-C6F5).
wR2 = 0.194, max (min) residual electron density 0.49 (-0.46) e Å-3
,
hydrogen atoms calculated and refined as riding atoms, solvent molecule
refined with split positions using geometrical and thermal restraints.
Preparation of 11 and 5b. Styrene (40 μL, 0.34 mmol) and
HB(C6F5)2 (117 mg, 0.34 mmol) were dissolved in pentane (6 mL)
and stirred for 15 min. A solution of 1a (65 mg, 0.37 mmol) in pentane
(3 mL) was then added at room temperature. Subsequently the reaction
mixture was stored at -36 °C. Crystals of 11 suitable for X-ray crystal
structure analysis came out overnight. Efforts to isolate 11 failed due to
its thermosensitivity.
Preparation of 10a. A solution of 9a (100.0 mg, 0.19 mmol) in
dichloromethane was warmed to 55 °C and stirred overnight. The
solvent was removed under vacuum, and the resulting solid was
suspended in cold pentane, filtered, and dried under vacuum. Com-
pound 10a was obtained as a red solid (53.0 mg, 0.10 mmol, 54%).
Crystals suitable for X-ray crystal structure analysis were obtained by gas
diffusion of pentane into a solution of 10a in benzene at room
temperature. HRMS (ESI) exact mass for [M þ H]þ (C24H16BNF10):
calcd m/z 520.1289, found 520.1278. 1H NMR (500 MHz, CD2Cl2, 298
K): δ = 6.77 (s, 1H, 5-H), 6.10 (s, 1H, 7-H), 3.40 (s, 3H, NMe2Z), 3.29
NMR Scale. An NMR tube containing 1a (5 mg, 0.03 mmol) and
2b (13 mg, 0.03 mmol) was cooled to -78 °C. Precooled CD2Cl2
(3 mL, -80 °C) was added, and then the NMR tube was sealed under
vacuum by using Schlenk techniques. Intermediate 11 was characterized
by NMR experiments at -80 °C. Afterward, the NMR sample was
stored at -30 °C overnight to give 5b (ca. 90% NMR yield at -20 °C).
NMR Analysis of 11. 1H NMR (500 MHz, CD2Cl2, 193 K): δ = 7.73
(br s, 1H, 6-H), 7.18 (m, 2H, m-Ph), 7.07 (m, 2H, o-Ph), 7.04 (m, 1H, p-
Ph), 6.11 (d, 1H, 3JHH = 4.6 Hz, 7-H), 4.69 (br s, 1H, BCH), 3.37 (s, 3H,
E
(s, 3H, NMe2 ), 3.09 (s, 2H, 2-H), 1.37 (s, 6H, Me). 13C{1H} NMR
(126 MHz, CD2Cl2, 298 K): δ = 170.8 (CdN), 158.0 (C8), 146.2 (C6),
E
b
a
128.8 (C5), 126.5 (C4), 116.8 (br, i-C6F5), 113.0 (C7), 55.5 (C2), 43.5
NMe2Z), 3.20 (s, 3H, NMe2 ), 2.92 (m, 1H, CH2 ), 2.85 (m, 1H, CH2 ),
2.17 (m, 2H, PhCH2), 1.42, 1.14 (each m, each 1H, BCH2), 1.16 (s, 3H,
Meb), 0.69 (s, 3H, Mea). 13C{1H} NMR (126 MHz, CD2Cl2, 193 K):
δ = 192.7 (C8), 174.0 (NdC), 169.0 (C6), 146.7 (i-Ph), 139.4 (C4),
127.6 (m-Ph), 127.3 (o-Ph), 124.4 (p-Ph), 119.3 (C7), 57.4 (br, BCH),
E
(NMe2Z), 42.9 (NMe2 ), 37.0 (C1), 30.5 (Me) [C6F5 not listed]. 11
B
NMR (160 MHz, CD2Cl2, 298 K): δ = 51.0 (ν1/2 ≈ 700 Hz). 19F NMR
(564 MHz, CD2Cl2, 298 K): δ = -131.6, -131.8 (each 2F, o-C6F5),
-155.3, -155.9 (each 1F, p-C6F5), -163.7 (4F, m-C6F5).
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ARTICLE
52.7 (CH2), 44.9 (NMe2Z), 42.6 (NMe2 ), 38.0 (C1), 33.3 (PhCH2),
Preparation of 14. A solution of 9a (100.0 mg, 0.19 mmol) in
dichloromethane (10 mL) was stirred for 5 min. Upon addition of
benzaldehyde (20.0 mg, 0.19 mmol) at room temperature, a white
precipitate formed gradually. After the reaction mixture was stirred
overnight, the precipitate was isolated via filtration. The residue was
then washed twice with a mixture (2 mL) of pentane and dichloro-
methane (1:1). All volatiles were removed in vacuo to yield minor
and major isomers in a ratio of 1:6 (60.0 mg, 50%), which were
tentative assignment as 14 and 140. Crystals of 14 suitable for X-ray
crystal structure analysis were obtained by gas diffusion of pentane
into a solution of the isomer mixture in tetrahydrofuran at -30 °C.
E
29.6 (br, CH2), 28.0 (Meb), 25.9 (Mea) [C6F5 not listed]. 19F{1H}
NMR (470 MHz, CD2Cl2, 193 K): δ = -127.1 (1F, o-C6F5 ), -130.7
(1F, o-C6F5 ), -136.6 (1F, o-C6F5 ), -137.5 (1F, o-C6F5 ), -164.0 (1F,
B
a
b
b
a
a
b
b
p-C6F5 ), -164.7 (1F, p-C6F5 a), -167.0 (1F, m-C6F5 ), -167.4 (1F,
b
a
m-C6F5 ), -168.1 (1F, m-C6F5 ), -168.7 (1F, m-C6F5 ).
X-ray Crystal Structure Analysis of 11. Formula C32H26BF10N
3
1/2C5H12, M = 661.42, colorless crystal 0.30 ꢀ 0.20 ꢀ 0.07 mm, a =
16.5180(8), b = 15.2872(6), and c = 25.8720(11) Å, β = 103.496(4)°,
V = 6352.6(5) Å3, Fcalc = 1.383 g cm-3, μ = 1.047 mm-1, empirical
absorption correction (0.744 e T e 0.930), Z = 8, monoclinic, space
group C2/c (No. 15), λ = 1.54178 Å, T = 223(2) K, ω and j scans,
24 171 reflections collected ((h,(k,(l), [(sin θ)/λ] = 0.60 Å-1, 5442
independent (Rint = 0.052) and 4167 observed reflections [I g 2 σ(I)],
448 refined parameters, R = 0.052, wR2 = 0.140, max (min) residual
electron density 0.28 (-0.37) e Å-3, hydrogen atoms calculated and
refined as riding atoms, solvent molecule refined using geometrical and
thermal restraints.
Anal. Calcd for C31H24BNOF10 0.25CH2Cl2: C, 57.87; H, 3.81; N, 2.16.
3
Found: C, 57.97; H, 3.61; N, 2.15. HRMS (ESI) exact mass for [M þ H]þ
(C38H31BNO2F10): calcd m/z 734.2283, found 734.2297.
NMR Analysis of the Minor Isomer. 1H NMR (500 MHz, TDF, 298
K): δ = 7.30 (m, 2H, o-Ph), 7.14 (m, 2H, m-Ph), 7.06 (m, 1H, p-Ph),
4.90 (d, 3JHH = 6.0 Hz, 1H, OCH), 3.54 (m, 1H, 7-H), n.o. (BCH), 3.39,
3.31 (each s, each 3H, NMe2), 3.09, 3.03 (each d, 2JHH = 18.0 Hz, each
1H, 2-H), 2.63, 2.44 (each m, each 1H, 5-H), 1.19, 1.00 (each s, each 3H,
Me). 13C{1H} NMR (126 MHz, TDF, 298 K): δ = 208.6 (C8), 182.8
(CdN), 148.1 (i-Ph), 141.6 (C4), 128.1 (m-Ph), 127.9 (o-Ph), 126.9
(p-Ph), 84.5 (OCH), 58.9 (C7), 53.7 (C2), n.o. (br, BCH), 43.7, 43.3
(NMe2), 42.5 (C1), 33.8 (C5), 27.0, 26.8 (Me). 11B NMR (160 MHz,
TDF, 298 K): δ = 2.4 (ν1/2 ≈ 50 Hz). 19F NMR (470 MHz, TDF, 298 K):
NMR Analysis of 5b. 1H NMR (500 MHz, CD2Cl2, 253 K): δ = 7.20
(m, 1H, m-Ph), 7.15 (m, 1H, o-Ph), 7.06 (m, 1H, p-Ph), 6.77 (s, 1H,
E
7-H), 3.45 (s, 2H, 5-H), 3.32 (s, 3H, NMe2Z), 3.20 (s, 3H, NMe2 ), 3.03
(s, 2H, CH2), 2.26 (m, 2H, PhCH2), 1.31 (s, 6H, Me), 1.30 (br, 2H,
BCH2). 13C{1H} NMR (126 MHz, CD2Cl2, 253 K): δ = 212.4 (br, C6),
1
204.3 (C8), 170.7 (CdN), 148.8 (i-Ph), 147.9 (dm, JFC ≈ 239 Hz,
a
b
b
C6F5), 137.5 (dm, 1JFC ≈ 245 Hz, C6F5), 136.5 (dm, 1JFC ≈ 251 Hz,
C6F5), 135.1 (C4), 128.1, 128.0 (o-, m-Ph), 127.8 (C7), 127.3 (br, i-C6F5),
124.5 (p-Ph), 53.0 (C2), 42.6, 42.5 (br, NMe2), 42.6 (C5), 39.7 (C1), 34.5
(PhCH2), 28.6 (br, BCH2), 26.8 (Me). 19F{1H} NMR (470 MHz, CD2Cl2,
253 K): δ = -133.5 (2F, o-C6F5), -165.5 (1F, p-C6F5), -167.9 (2F,
m-C6F5).
δ = -133.4 (2F, o-C6aF5 ), -135.2 (2F, o-C6F5 ), -165.58 (1F, p-C6F5 ),
b
a
-165.62 (1F, p-C6F5 ), -168.0 (2F, m-C6F5 ), -168.1 (2F, m-C6F5 ).
NMR Analysis of the Major Isomer. 1H NMR (500 MHz, TDF, 298
K): δ = 7.32 (m, 2H, o-Ph), 7.12 (m, 2H, m-Ph), 7.07 (m, 1H, p-Ph),
4.85 (d, 3JHH = 8.7 Hz, 1H, OCH), 3.54 (br t, 3JHH ≈ 8.5 Hz, 1H, 7-H),
3.50 (br quint, 3JHH ≈ 8.5 Hz, 1H, BCH), 3.42, 3.27 (each s, each 3H,
NMe2), 2.91, 2.64 (each d, 2JHH = 18.4 Hz, each 1H, 2-H), 2.65, 2.59
(each dd, 2JHH = 15.2 Hz, 3JHH = 8.5 Hz, each 1H, 5-H), 1.11, 0.05 (each
s, each 3H, Me). 13C{1H} NMR (126 MHz, TDF, 298 K): δ = 211.7
(C8), 181.7 (CdN), 146.6 (i-Ph), 143.8 (C4), 129.1 (o-Ph), 128.0 (m-Ph),
127.1 (p-Ph), 83.9 (OCH), 56.4 (C7), 53.2 (C2), 51.3 (br, BCH), 43.5, 43.4
(NMe2), 43.1 (C1), 35.5 (C5), 29.0, 24.1 (Me). 11BNMR(160 MHz, TDF,
298 K): δ = 1.3 (ν1/2 ≈ 80 Hz). 19F NMR (470 MHz, TDF, 298 K): δ =
-133.1 (2F, o-C6F5a),-133.7 (2F, o-C6F5b),-164.8 (1F, p-C6F5b),-165.4
(1F, p-C6F5a), -167.7 (2F, m-C6F5b), -168.0 (2F, m-C6F5a).
Preparation of 13. A solution of 9a (100.0 mg, 0.19 mmol) in
dichloromethane (10 mL) was stirred for 5 min. Upon addition of HCl
(2 M in Et2O, 0.1 mL, 0.20 mmol) at room temperature, a colorless
solution formed gradually. After the reaction mixture was stirred over-
night, the solvent were removed under vacuum. The residue was then
washed twice with a mixture (2 mL) of pentane and dichloromethane in
a 3:1 ratio. All volatiles were removed in vacuo to yield 13 (77.4 mg,
73%). Crystals suitable for X-ray crystal structure analysis were obtained
by gas diffusion of pentane into a solution of 6 in dichloromethane
at -30 °C. Anal. Calcd for C24H19BClF10N: C, 51.69; H, 3.43; N, 2.51.
X-ray Crystal Structure Analysis of 14. Formula C31H24BF10NO
3
Found: C, 51.19; H, 4.11; N, 2.11. 1H NMR (600 MHz, TDF, 298 K):
C4H8O, M = 699.43, colorless crystal 0.12 ꢀ 0.10 ꢀ 0.10 mm, a =
20.4261(7), b = 10.3146(3), and c = 31.7887(11) Å, β = 103.898(2)°,
V = 6501.4(4) Å3, Fcalc = 1.429 g cm-3, μ = 1.104 mm-1, empirical
absorption correction (0.879 e T e 0.898), Z = 8, monoclinic, space
group P21/n (No. 14), λ = 1.54178 Å, T = 223(2) K, ω and j scans,
55 470 reflections collected ((h,(k,(l), [(sin θ)/λ] = 0.60 Å-1, 10 652
independent (Rint = 0.117) and 5332 observed reflections [I g 2 σ(I)],
891 refined parameters, R = 0.080, wR2 = 0.265, max (min) residual
electron density 0.69 (-0.46) e Å-3, hydrogen atoms calculated and
refined as riding atoms, solvent molecules refined with split positions
using geometrical and thermal restraints. Due to crystal quality, the
accuracy of the analysis is poor.
δ = 3.53 (s, 3H, NMe2Z), 3.37 (s, 3H, NMe2 ), 3.14, 3.12 (AB, 2JHH
=
E
19.2 Hz, each 1H, 2-H), 2.93 (m, 1H, BCH), 2.92, 2.79 (each m, each
1H, 5-H), 2.74, 2.55 (each m, each 1H, 7-H), 1.23, 1.24 (each s, each 3H,
Me). 13C{1H} NMR (151 MHz, TDF, 298 K): δ = 208.6 (C8), 182.5
(CdN), 148.5 (dm, 1JFC ≈ 240 Hz, C6F5), 140.8 (C4), 139.0 (dm, 1JFC
≈ 245 Hz, C6F5), 137.3 (dm, 1JFC ≈ 246 Hz, C6F5), 126.4 (br, i-C6F5),
E
53.3 (C2), 43.6 (NMe2 ), 43.3 (NMe2Z), 41.7 (C1), 37.8 (br, C6), 34.7
(C5), 34.5 (C7), 26.14, 26.07 (Me). 11B{1H} NMR (192 MHz, TDF,
298 K): δ = -1.4 (ν1/2 ≈ 300 Hz). 19F{1H} NMR (470 MHz, TDF, 298
K): δ = 132.1 (2F, o-C6F5), 164.4 (1F, p-C6F5), 167.8 (2F, m-C6F5).
X-ray Crystal Structure Analysis of 13. Formula C24H19BClF10N
3
1/2CH2Cl2, M = 600.13, colorless crystal 0.50 ꢀ 0.10 ꢀ 0.07 mm, a =
9.4976(5), b = 12.4411(8), and c = 12.8064(8) Å, R = 88.834(4), β =
78.248(3), and γ = 74.602(5)°, V = 1427.40(15) Å3, Fcalc = 1.396 g
cm-3, μ = 2.787 mm-1, empirical absorption correction (0.336 e T e
0.829), Z = 2, triclinic, space group P1 (No. 2), λ = 1.54178 Å, T =
223(2) K, ω and j scans, 19 428 reflections collected ((h,(k,(l),
[(sin θ)/λ] = 0.60 Å-1, 4792 independent (Rint = 0.057) and 3882
observed reflections [I g 2 σ(I)], 376 refined parameters, R = 0.079,
wR2 = 0.261, max (min) residual electron density 1.14 (-0.24) e Å-3 in
the region of the disordered solvent molecule, hydrogen atoms calcu-
lated and refined as riding atoms, solvent molecule refined with split
positions using geometrical and thermal restraints.
’ ASSOCIATED CONTENT
S
Supporting Information. Text and figures giving further
b
experimental and spectroscopic details and crystallographic data
(CIF files) for 3c, 7a, 7b, 10a, 10b, 11, 13, and 14. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
erker@uni-muenster.de
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Journal of the American Chemical Society
ARTICLE
’ ACKNOWLEDGMENT
5233–5239. (g) Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2010, 132, 3301–3303. (h) Schwendemann, S.; Tumay, T. A.;
Axenov, K. V.; Peuser, I.; Kehr, G.; Fr€ohlich, R.; Erker, G. Organome-
tallics 2010, 29, 1067–1069.
Financial support from the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie is gratefully acknowl-
edged. We thank BASF for a gift of solvents.
(5) C-H activation: (a) Goldberg, K. I.; Goldman, A. S. ACS Symp.
Ser. 2004, 885, 440 pp. See also selected recent reviews on C-H
activation: (b) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102,
1731–1770; (c) Chen, X.; Engle, K. M.; Wang, D. H.; Yu, J. Q. Angew.
Chem., Int. Ed. 2009, 48, 5094–5115; Angew. Chem. 2009, 121, 5196–
5217. (d) Balcells, D.; Clot, E.; Eisenstein, O. Chem. Rev. 2010, 110,
749–823. (e) Jazzar, R.; Hitce, J.; Renaudat, A.; Sofack-Kreutzer, J.;
Baudoin, O. Chem. Eur. J. 2010, 16, 2654–2672. (f) Conejero, S.;
Paneque, M.; Poveda, M. L.; Santos, L. L.; Carmona, E. Acc. Chem.
Res. 2010, 43, 572–580. (g) Gunay, A.; Theopold, K. H. Chem. Rev.
2010, 110, 1060–1081.
(6) Some recent reports on C-H activation: (a) Ess, D. H.; Nielsen,
R. J.; Goddard, W. A., III; Periana, R. A. J. Am. Chem. Soc. 2009, 131,
11686–11688. (b) Vetter, A. J.; Rieth, R. D.; Brennessel, W. W.; Jones,
W. D. J. Am. Chem. Soc. 2009, 131, 10742–10752. (c) H€aller, L. J. L.;
Page, M. J.; Macgregor, S. A.; Mahon, M. F.; Whittlesey, M. K. J. Am.
Chem. Soc. 2009, 131, 4604–4605. (d) Lu, P.; Boorman, T. C.; Slawin,
A. M. Z.; Larrosa, I. J. Am. Chem. Soc. 2010, 132, 5580–5581.
(7) Some recent reports on C-H bond activation/C-C bond
formation: (a) Wu, J.; Cui, X.; Chen, L.; Jiang, G.; Wu, Y. J. Am. Chem.
Soc. 2009, 131, 13888–13889; (b) Zhou, H.; Xu, Y. H.; Chung, W. J.;
Loh, T. P. Angew. Chem., Int. Ed. 2009, 48, 5355–5357; Angew. Chem.
2009, 121, 5459–5461; (c) Nishikata, T.; Abela, A. R.; Huang, S.;
Lipshutz, B. H. J. Am. Chem. Soc. 2010, 132, 4978–4979; (d) Li, Q.; Yu,
Z. X. J. Am. Chem. Soc. 2010, 132, 4542–4543; (e) Nishikata, T.; Abela,
A. R.; Lipshutz, B. H. Angew. Chem., Int. Ed. 2010, 49, 781–784; Angew.
Chem. 2010, 122, 793–796. (f) de Petris, G.; Cartoni, A.; Troiani, A.;
Barone, V.; Cimino, P.; Angelini, G.; Ursini, O. Chem. Eur. J. 2010, 16,
6234–6242.
’ REFERENCES
(1) Review on metal-catalyzed hydrogen activation: (a) DuBois,
M. R. Chem. Rev. 1989, 89, 1–9; See also recent reports: (b) Sweeney,
Z. K.; Polse, J. L.; Bergman, R. G.; Andersen, R. A. Organometallics 1999,
18, 5502–5510; (c) Linck, R. C.; Pafford, R. J.; Rauchfuss, T. B. J. Am.
Chem. Soc. 2001, 123, 8856–8857; (d) Kato, H.; Seino, H.; Mizobe, Y.;
Hidai, M. J. Chem. Soc., Dalton Trans. 2002, 1494–1499; (e) Sellmann,
D.; Prakash, R.; Heinemann, F. W.; Moll, M.; Klimowicz, M. Angew.
Chem., Int. Ed. 2004, 43, 1877–1880; Angew. Chem. 2004, 116, 1913;
(f) Jalꢀon, F. A.; Manzano, B. R.; Caballero, A.; Carriꢀon, M. C.; Santos, L.;
Espino, G.; Moreno, M. J. Am. Chem. Soc. 2005, 127, 15364–15365;
(g) Wilson, A. D.; Newell, R. H.; McNevin, M. J.; Muckerman, J. T.;
Dubois, M. R.; Dubois, D. L. J. Am. Chem. Soc. 2006, 128, 358–366;
(h) Nagaraja, C. M.; Parameswaran, P.; Jemmis, E. D.; Jagirdar, B. R.
J. Am. Chem. Soc. 2007, 129, 5587–5596; (i) Ogo, S.; Kabe, R.; Uehara,
K.; Kure, B.; Nishimura, T.; Menon, S. C.; Harada, R.; Fukuzumi, S.;
Higuchi, Y.; Ohhara, T.; Tamada, T.; Kuroki, R. Science 2007, 316, 585–
587; (j) Matsumoto, T.; Nakaya, Y.; Tatsumi, K. Angew. Chem., Int. Ed.
2008, 47, 1913–1915; Angew. Chem. 2008, 120, 1939–1941; (k) Adams,
R. D.; Captain, B. Angew. Chem., Int. Ed. 2008, 47, 252–257; Angew.
Chem. 2008, 120, 258–263. (l) Armstrong, F. A.; Fontecilla-Camps, J. C.
Science 2008, 321, 498–499. (m) Misumi, Y.; Seino, H.; Mizobe, Y. J. Am.
Chem. Soc. 2009, 131, 14636–14637.
(2) Selected reviews on metal-catalyzed hydrogenation: (a) Bianchini,
C.; Meli, A. Acc. Chem. Res. 1998, 31, 109–116. (b) Bayoꢀn, J. C.; Claver,
C.; Masdeu-Bulto, A. M. Coord. Chem. Rev. 1999, 193-195, 73–145.
(c) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004,
248, 2201–2237. (d) Cui, X.; Burgess, K. Chem. Rev. 2005, 105, 3272–
3296. (e) Lau, C. P.; Ng, S. M.; Jia, G.; Lin, Z. Coord. Chem. Rev. 2007,
251, 2223–2237. (f) Minnaard, A. J.; Feringa, B. L.; Lefort, L.; De Vries,
J. G. Acc. Chem. Res. 2007, 40, 1267–1277. (g) Morris, R. H. Chem. Soc.
Rev. 2009, 38, 2282–2291. (h) Gridnev, I. D.; Imamoto, T. Chem.
Commun. 2009, 7447–7464. (i) Bartꢀok, M. Chem. Rev. 2010, 110,
1663–1705.
(8) Reviews on C-C bond activation: (a) Crabtree, R. H. Chem. Rev.
1985, 85, 245–269; (b) Rybtchinski, B.; Milstein, D. Angew. Chem., Int.
Ed. 1999, 38, 870–883; Angew. Chem. 1999, 111, 918–932. (c) Jun,
C.-H. Chem. Soc. Rev. 2004, 33, 610–618. (d) Park, Y. J.; Park, J. W.; Jun,
C.-H. Acc. Chem. Res. 2008, 41, 222–234. See also: (e) Li, T.; García, J. J.;
Brennessel, W.; Jones, W. D. Organometallics 2010, 29, 2430–2445.
(f) Nakao, Y.; Yada, A.; Hiyama, T. J. Am. Chem. Soc. 2010, 132, 10024–
10026 and references cited therein.
(3) Some recent reports on metal-catalyzed hydrogenation: (a) Zhu,
Y.; Fan, Y.; Burgess, K. J. Am. Chem. Soc. 2001, 132, 6249–6253. (b) Ito,
M.; Hirakawa, M.; Osaku, A.; Ikariya, M. Organometallics 2003, 22,
4190–4192. (c) Sandoval, C. A.; Ohkuma, T.; Mu~niz, K.; Noyori, R.
J. Am. Chem. Soc. 2003, 125, 13490–13503. (d) Bullock, R. M. Chem. Eur.
J. 2004, 10, 2366–2374. (e) Liu, X. Y.; Venkatesan, K.; Schmalle, H. W.;
Berke, H. Organometallics 2004, 23, 3153–3163. (f) Guan, H.; Iimura,
M.; Magee, M. P.; Norton, J. R.; Zhu, G. J. Am. Chem. Soc. 2005, 127,
7805–7814. (g) Casey, C. P.; Bikzhanova, G. A.; Cui, Q.; Guzei, I. A.
J. Am. Chem. Soc. 2005, 127, 14062–14071. (h) Mebi, C. A.; Frost, B. J.
Organometallics 2005, 24, 2339–2346. (i) Ito, M.; Sakaguchi, A.;
Kobayashi, C; Ikariya, T. J. Am. Chem. Soc. 2007, 129, 290–291.
(j) Sakamoto, M.; Ohki, Y.; Kehr, G.; Erker, G.; Tatsumi, K.
J. Organomet. Chem. 2009, 694, 2820–2824. (k) Maegawa, T.; Akashi,
A.; Yaguchi, K.; Iwasaki, Y.; Shigetsura, M.; Monguchi, Y.; Sajiki, H.
Chem. Eur. J. 2009, 15, 6953–6963. (l) Reguillo, R.; Grellier, M.;
Vautravers, N.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2010,
132, 7854–7855. (m) Seino, H.; Misumi, Y.; Hojo, Y.; Mizobe, Y. Dalton
Trans. 2010, 39, 3072–3082.
(4) Selected reviews on metal-free reagents for hydrogen activation:
(a) Stephan, D. W. Dalton Trans. 2009, 3129–3136; (b) Sumerin, V.;
Schulz, F.; Nieger, M.; Atsumi, M.; Wang, C.; Leskel€a, M.; Pyykk€o, P.;
Repo, T.; Rieger, B. J. Organomet. Chem. 2009, 694, 2654–2660;
(c) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46–76;
Angew. Chem. 2010, 122, 50–81; See also recent reports: (d) Kenward,
A. L.; Piers, W. E. Angew. Chem., Int. Ed. 2008, 47, 38–41; Angew. Chem.
2008, 120, 38–42. (e) Li, B.; Xu, Z. J. Am. Chem. Soc. 2009, 131, 16380–
16382. (f) Jiang, C.; Blacque, O.; Berke, H. Organometallics 2009, 28,
(9) Reviews: (a) Murray, R. W. Acc. Chem. Res. 1968, 1, 313–320.
(b) Hilt, G.; Weske, D. F. Chem. Soc. Rev. 2009, 38, 3082–3091. See also:
(c) Criegee, R. Justus Liebigs Ann. Chem. 1953, 583, 12.
(10) Selected recent reviews: (a) Hoveyda, A. H.; Zhugralin, A. R.
Nature 2007, 450, 243–251; (b) Vougioukalakis, G. C.; Grubbs, R. H.
Chem. Rev. 2010, 110, 1746–1787; (c) Basset, J. M.; Copꢀeret, C.;
Soulivong, D.; Taoufik, M.; Cazat, J. T. Acc. Chem. Soc. 2010, 43,
323–334; See also: (d) Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45,
3760–3765; Angew. Chem. 2006, 118, 3845–3850; (e) Schrock, R. R.
Angew. Chem., Int. Ed. 2006, 3748–3759; Angew. Chem. 2006, 118,
3832–3844.
(11) Xu, B.-H.; Kehr, G.; Fr€ohlich, R.; Erker, G. Chem. Eur. J. 2010,
16, 12538–12540.
(12) (a) Parks, D. J.; Spence, R. E. v. H.; Piers, W. E. Angew. Chem.,
Int. Ed. 1995, 34, 809–811; Angew. Chem. 1995, 107, 895–897;
(b) Spence, R. E. v. H.; Parks, D. J.; Piers, W. E.; MacDonald, M.-A.;
Zaworotko, M. J.; Rettig, S. J. Angew. Chem., Int. Ed. 1995, 34, 1230–
1233; Angew. Chem. 1995, 107, 1337–1340. (c) Spence, R. E. v. H.;
Piers, W. E.; Sun, Y.; Parvez, M.; MacGillivray, L. R.; Zaworotko, M. J.
Organometallics 1998, 17, 2459–2469. (d) Piers, W. E.; Chivers, T.
Chem. Soc. Rev. 1997, 26, 345–354. (e) Parks, D. J.; Piers, W. E.; Yap,
G. P. A. Organometallics 1998, 17, 5492–5503.
(13) Review: (a) Wrackmeyer, B. Coord. Chem. Rev. 1995, 145, 125–
156. See also: (b) K€oster, R.; Seidel, G.; S€uss, J.; Wrackmeyer, B. Chem.
Ber. 1993, 126, 1107–1114. (c) K€oster, R.; Seidel, G.; Klopp, I.; Kr€uger,
C.; Kehr, G.; S€ub, J.; Wrackmeyer, B. Chem. Ber. 1993, 126, 1385–1396.
(d) Wrackmeyer, B.; Maisel, H. E.; S€uss, J.; Milius, W. Z. Naturforsch.
3490
dx.doi.org/10.1021/ja1092369 |J. Am. Chem. Soc. 2011, 133, 3480–3491

Journal of the American Chemical Society
ARTICLE
1996, 51b, 1320–1324. (e) Wrackmeyer, B.; Bhatti, M. H.; Ali, S.; Tok,
O. L.; Bubnov, Y. N. J. Organomet. Chem. 2002, 657, 146–154.
(f) Wrackmeyer, B.; Tok, O. L.; Khan, A.; Badshah, A. Z. Naturforsch.
2005, 60b, 251–257. (g) Wrackmeyer, B. Heteroat. Chem. 2006, 17,
188–208.
(34) Wiberg, K. B. Tetrahedron 1968, 24, 1083–1096.
(35) Fan, C.; Piers, W. E.; Parvez, M.; McDonald, R. Organometallics
2010, 29, 5132–5139.
(36) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276,
302–326.
(14) (a) Meyer, F.; Schmidt, M. U.; Paetzold, P. Chem. Ber. 1995,
128, 947–951. (b) Paetzold, P.; Leuschner, E. Z. Anorg. Allg. Chem. 2002,
628, 658–660.
(37) Otwinowski, D.; Borek, D.; Majewski, W.; Minor, W. Acta
Crystallogr. 2003, A59, 228–234.
(38) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467–473.
(39) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(15) (a) Chen, C.; Fr€ohlich, R.; Kehr, G.; Erker, G. Chem. Commun.
2010, 46, 3580–3582. (b) Ugolotti, J.; Kehr, G.; Fr€ohlich, R.; Erker, G.
Chem. Commun. 2010, 46, 3016–3018. (c) Chen, C.; Kehr, G.; Fr€ohlich,
R.; Erker, G. J. Am. Chem. Soc. 2010, 132, 13594–13595. (d) Chen, C.;
Eweiner, F.; Wibbeling, B.; Fr€ohlich, R.; Senda, S.; Ohki, Y.; Tatsumi, K.;
Grimme, S.; Kehr, G.; Erker, G. Chem. Asian J. 2010, 5, 2199–2208.
(16) Jiang, C.; Blacque, O.; Berke, H. Organometallics 2010, 29, 125–133.
(17) Xu, B.-H.; Kehr, G.; Fr€ohlich, R.; Nauha, E.; Erker, G. Dalton
Trans. 2010, 39, 9973–9981.
(18) Hafner, K.; V€opel, K. H.; Ploss, G.; K€onig, C. Org. Synth. 1967,
47, 52–54.
(19) (a) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Fr€ohlich,
R.; Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072–5074.
(b) Spies, P.; Kehr, G.; Bergander, K.; Wibbeling, B.; Fr€ohlich, R.; Erker,
G. Dalton Trans 2009, 1534–1541.
(20) (a) Olmstead, M. M.; Power, P. P.; Weese, K. J. J. Am. Chem.
Soc. 1987, 109, 2541–2542; (b) Herberich, G. H.; Fischer, A. Organo-
metallics 1996, 15, 58–67; (c) Hoefelmeyer, J. D.; Solꢀe, S.; Gabbaï, F. P.
Dalton Trans. 2004, 1254–1258; (d) Chiu, C.-W.; Gabbaï, F. P. Angew.
Chem., Int. Ed. 2007, 46, 6878–6881; Angew. Chem. 2007, 116, 7002–
7005.
(21) (a) Piers, W. E. Adv. Organomet. Chem. 2005, 52, 1–76.
(b) Beringhelli, T.; Donghi, D.; Maggioni, D.; D’Alfonso, G. Coord.
Chem. Rev. 2008, 252, 2292–2313 and references cited therein.
(22) Review: (a) Spangler, C. W. Chem. Rev. 1976, 76, 187–217; See
also: (b) Gurskii, M. E.; Gridnev, I. D.; Il’ichev, Y. V.; Ignatenko, A. V.;
Bubnov, Y. N. Angew. Chem., Int. Ed. 1992, 31, 781–783; Angew. Chem.
1992, 104, 762–764. (c) Gurskii, M. E.; Gridnev, I. D.; Geiderikh, A. V.;
Ignatenko, A. V.; Bubnov, Y. N. Organometallics 1992, 11, 4056–4061.
(d) Bubnov, Y. N.; Gurskii, M. E.; Gridnev, I. D.; Ignatenko, A. V.
J. Organomet. Chem. 1992, 424, 127–132. (e) Gridnev, I. G.; Gurskii,
M. E.; Ignatenko, A. V.; Bubnov, Y. N. Organometallics 1993, 12, 2487–
2495. (f) Gurskii, M. E.; Gridnev, I. D.; Buevich, A. V.; Bubnov, Y. N.
Organometallics 1994, 13, 4658–4660. (g) Gridnev, I. D.; Schreiner,
P. R.; Gurskii, M. E.; Bubnov, Y. N.; Krasavin, A. O.; Mstislavski, V. I.
Chem. Commun. 1998, 2507–2508. (h) Tok, O. L.; Gridnev, I. D.;
Korobach, E. M.; Bubnov, Y. N. Chem. Commun. 2000, 311–312.
(23) (a) Gridnev, I. D.; Tok, O. L.; Gurskii, M. E.; Bubnov, Y. N.
Chem. Eur. J. 1996, 2, 1483–1488. (b) Gridnev, I. D.; Tok, O. L.;
Gridneva, N. A.; Bubnov, Y. N.; Schreiner, P. R. J. Am. Chem. Soc. 1998,
120, 1034–1043.
(24) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and
Molecules; Oxford University Press: Oxford, UK, 1989.
(25) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density
Functional Theory; Wiley-VCH: New York, 2001.
(26) Ahlrichs, R. et al. TURBOMOLE, version 5.9; Universit€at
Karlsruhe, 2006; http://www.turbomole.com.
(27) Weigend, F.; Ahlrichs, R. Phys. Chem., Chem. Phys. 2005, 7,
3297–3305.
€
(28) Eichkorn, K.; Treutler, O.; Ohm, H.; H€aser, M.; Ahlrichs, R.
Chem. Phys. Lett. 1995, 240, 283–289.
(29) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys.
Rev. Lett. 2003, 91, 146401.
(30) Grimme, S.; Antony, J.; Schwabe, T.; M€uck-Lichtenfeld, C. Org.
Biomol. Chem. 2007, 5, 741–758.
(31) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys.
2010, 132, 154104.
(32) Spies, P.; Fr€ohlich, R.; Kehr, G.; Erker, G.; Grimme, S. Chem.
Eur. J. 2008, 14, 333–343.
(33) Grimme, S. J. Chem. Phys. 2006, 124, 034108.
3491
dx.doi.org/10.1021/ja1092369 |J. Am. Chem. Soc. 2011, 133, 3480–3491