Alkenylborane-derived frustrated Lewis pairs: Metal-free catalytic hydrogenation reactions of electron-deficient alkenes

Article abstract of DOI:10.1021/om3006068

A series of alkenylboranes were prepared by 1,1-carboboration routes and used as Lewis acid components for the generation of frustrated Lewis pairs (FLPs). The reactions of 1-alkynes with B(C₆F₅) ₃ gave the RCH=C(C₆F₅)B(C₆F ₅)₂ systems 4a(R = n-C₃H₇), 4b (R = n-C₄H₉), 4c (R = Ph), and 4d (R = t-C₄H ₉), respectively. The alkenylborane/tBu₃P FLPs derived from compounds 4a-d reacted rapidly with dihydrogen (2.5 bar) at ambient temperature. The bulky system 4d left the C=C double bond of the alkenylborane unsaturated and gave the dihydrogen cleavage product [tBu₃PH][tBuCH= C(C₆F₅)BH(C₆F₅)₂] (10d). In contrast, the less bulky systems 4a/tBu₃P and 4b/tBu₃P split dihydrogen under these conditions and had their C=C double bonds cleanly reduced to yield the corresponding 1-pentafluorophenylalkyl hydridoborate salts [tBu₃PH][RCH₂CH(C₆F₅)BH(C ₆F₅)₂] 9a (R = n-C₃H₇) and 9b (R = n-C₄H₉), respectively. The 4c/tBu₃P FLP gave a mixture of both product types (9c/10c). 1,1-Carboboration of symmetrical internal alkynes gave the alkenylboranes R₂C=C(C ₆F₅)B(C₆F₅)₂4e (R = C₂H₅), 4f (R = n-C₃H₇), 4g (R = Ph), and 4h (R = p-MeC₆H₄), respectively. The 4e-h/tBu ₃P FLPs cleaved dihydrogen under mild conditions but retained their C=C double bonds to give the respective [tBu₃PH][R ₂C=C(C₆F₅)BH(C₆F₅) ₂] products (10e-h). Selected examples of these alkenylboranes undergo FLP reactions acting as catalysts for the hydrogenation of imines. Perhaps most remarkably, some of these alkenylboranes retain the C=C double bonds under FLP/H₂ reaction conditions and heterolytically split dihydrogen in the presence of the Lewis base DABCO and catalyze the hydrogenation of the electron-poor C=C double bonds of diaryl-substituted enones.

Full text of DOI:10.1021/om3006068

Article
pubs.acs.org/Organometallics
Alkenylborane-Derived Frustrated Lewis Pairs: Metal-Free Catalytic
Hydrogenation Reactions of Electron-Deficient Alkenes
J. Sreedhar Reddy,† Bao-Hua Xu,† Tayseer Mahdi,‡ Roland Frohlich,† Gerald Kehr,†
̈
Douglas W. Stephan,*,‡ and Gerhard Erker*,†
†Organisch-Chemisches Institut der Universitat Munster, Corrensstrasse 40, 48149 Munster, Germany
̈
̈
̈
‡Department of Chemistry, University of Toronto, 80 St. George Street,Toronto, Ontario, M5S3H6, Canada
S
* Supporting Information
ABSTRACT: A series of alkenylboranes were prepared by 1,1-carboboration routes and
used as Lewis acid components for the generation of frustrated Lewis pairs (FLPs).
The reactions of 1-alkynes with B(C6F5)3 gave the RCHC(C6F5)B(C6F5)2 systems 4a
(R = n-C3H7), 4b (R = n-C4H9), 4c (R = Ph), and 4d (R = t-C4H9), respectively. The
alkenylborane/tBu3P FLPs derived from compounds 4a−d reacted rapidly with
dihydrogen (2.5 bar) at ambient temperature. The bulky system 4d left the CC
double bond of the alkenylborane unsaturated and gave the dihydrogen cleavage product
[tBu3PH][tBuCHC(C6F5)BH(C6F5)2] (10d). In contrast, the less bulky systems 4a/
tBu3P and 4b/tBu3P split dihydrogen under these conditions and had their CC double
bonds cleanly reduced to yield the corresponding 1-pentafluorophenylalkyl hydridoborate
salts [tBu3PH][RCH2CH(C6F5)BH(C6F5)2] 9a (R = n-C3H7) and 9b (R = n-C4H9),
respectively. The 4c/tBu3P FLP gave a mixture of both product types (9c/10c). 1,1-
Carboboration of symmetrical internal alkynes gave the alkenylboranes R2C
C(C6F5)B(C6F5)2 4e (R = C2H5), 4f (R = n-C3H7), 4g (R = Ph), and 4h (R = p-MeC6H4), respectively. The 4e−h/
tBu3P FLPs cleaved dihydrogen under mild conditions but retained their CC double bonds to give the respective
[tBu3PH][R2CC(C6F5)BH(C6F5)2] products (10e−h). Selected examples of these alkenylboranes undergo FLP reactions
acting as catalysts for the hydrogenation of imines. Perhaps most remarkably, some of these alkenylboranes retain the CC
double bonds under FLP/H2 reaction conditions and heterolytically split dihydrogen in the presence of the Lewis base
DABCO and catalyze the hydrogenation of the electron-poor CC double bonds of diaryl-substituted enones.
Chart 1
INTRODUCTION
■
Frustrated Lewis pair (FLP) chemistry has seen numerous
advances in recent years, and the interest in and impact of the
field continue to grow.1 The inter- or intramolecular combina-
tion of sterically congested strong Lewis acids2 and Lewis bases
has allowed for the observation of a variety of unusual synergistic
reactions with a number of small molecules. To date, most, but
not all,3 Lewis acids employed have been boron-based. On the other
hand, a broader selection of phosphorus-,2a,4 nitrogen-,4o,5 or even
carbon-based6 Lewis bases have been employed. The inter- and
intramolecular FLPs typified by systems I and II (Chart 1)7 were
shown to react with alkenes8 or alkynes,8c,9 conjugated dienes,
diynes, and enynes,9c,10 and a great variety of carbonyl
compounds,3f,11 including carbon dioxide.12 Moreover, system
I was found to bind N2O,13 while the intramolecular system II
captures NO.14
Most remarkable is the ability of many FLPs to react with
dihydrogen7a,15 to effect heterolytic cleavage, affording the
respective phosphonium hydridoborate salt (V from I) or the
zwitterion (VI from II, Chart 1). Even more surprisingly, a
number of such systems may serve as metal-free catalysts for the
hydrogenation of some organic substrates.1a,f,4f,o,5l,16 To date, a
wide variety of systems able to heterolytically cleave dihydrogen
have been utilized to catalytically hydrogenate imines, dimines,1a
and N-heterocycles.1a,4e,n,o,5l−n,17 Moreover, reduction of elec-
tron-rich alkenes such as enamines, conjugated dienamines and
even silyl enol ethers has been reported.18 Most recently Stephan
and co-workers have reported the stoichiometric reduction of
Received: July 1, 2012
Published: July 17, 2012
© 2012 American Chemical Society
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aniline derivatives to the corresponding cyclohexylamines.19 For
all of these electron-rich substrates, the reductions are thought to
proceed via initial proton transfer to generate an activated
substrate (i.e., iminium ions or analogues) followed by hydride
delivery from the borohydride.
hydrogenation under our typical reaction conditions was
investigated. Herein, we report that selected examples of such
bulky alkenylborane systems are active catalysts for the
hydrogenation of imines and a few conjugated enone model
substrates. These results provide a broader scope of substrates
where FLP hydrogenations are useful. Moreover, these data
provide insight into the structure−activity relationship of
boranes in such metal-free hydrogenations.
Although the field of the FLPs is quite young, metal-free
catalyzed hydrogenation of these electron-rich substrates is
already becoming established. In contrast, both the stoichio-
metric and catalytic reduction chemistry of synthetically im-
portant electron-poor conjugated enones or ynones and related
organic substances by FLP/H2 systems is still in its infancy.
Indeed, only a few examples of such reductions have been
RESULTS AND DISCUSSION
■
Synthesis of Alkenylboranes. It has been previously
communicated that alkenylboranes are accessible from the
reaction of B(C6F5)3 with alkynes.8c,9a−f,22 The profound ability
of B(C6F5)3 to undergo a rapid 1,1-carboboration reaction with
many terminal alkynes affords a series of alkenylboranes of
formulas RCHC(C6F5)B(C6F5)2. Mechanistically, such re-
actions are thought to proceed by borane interaction with the
alkyne, prompting a 1,2-hydrogen migration along the alkyne
carbon framework and concomitant 1,2-C6F5 shift from boron to
the former terminal alkynyl carbon atom (C1), yielding the
alkenylborane. Initially, the 1,1-carboboration sequence is not
stereoselective, giving a mixture of Z- and E-alkenylborane
isomers. Subsequent photolysis (HPK 125, Pyrex) for many
systems results in efficient E- to Z-alkenylborane isomerization,
allowing Z-alkenylboranes to be obtained in high yields. A typical
example is the preparation of Z-CH3(CH2)2CHC(C6F5)B-
(C6F5)2 (4a). This species is obtained as a white solid in 90% yield
from a one-pot/two-step process of 1,1-carboboration and photolysis
(Scheme 1). In an analogous manner, the Z-alkenylboranes 4b
described. For example, Soos
́
et al.5g have described the catalytic
hydrogenation of the α,β-unsaturated terpenoid ketone carvone
by the mesityl-B(C6F5)2/DABCO frustrated Lewis pair. Erker
and co-workers recently reported20 that both the H2-activated
forms (Chart 1, V and VI) of the FLPs I and II, respectively, were
able to stoichiometrically hydrogenate the ynones 1a (R = Ph)
and 1b (R = tBu) to the corresponding cis-enones (cis-2a, 2b)
(Chart 2).
Chart 2. Hydrogenation of the Ynones to the Corresponding
cis-Enones
Scheme 1. Synthesis of Alkenylboranes
(R = n-butyl), 4c (R = Ph; E/Z = 30:1 mixture), and 4d (R = tBu)
were prepared.
Since it is likely that the FLP/H2 enone and ynone reactions
proceed by hydride addition followed by proton transfer, a
slightly less Lewis acidic borane that yields a more hydridic
borate should prove to be advantageous. Indeed we have
communicated that an alkenylborane (4d), derived from a 1,1-
carboboration reaction,21 is a Lewis acid20 that in combination
with the Lewis base tBu3P and dihydrogen cleanly effects the
stoichiometric cis-reduction of ynones. The use of 1,4-diaza-
bicyclo[2.2.2]octane (DABCO) as the Lewis base required
slightly more forcing conditions to hydrogenate the ynone 1b,
although this was achieved catalytically (Chart 2).
A second series of alkenylboranes (4e−h) were derived from
the analogous 1,1-carboboration reactions employing internal
alkynes. In these cases elevated temperatures were required to
yield the respective tetra-substituted alkenes bearing a geminal
pair of R groups at one end and a C6F5/B(C6F5)2 pair of
substituents at the other. Treatment of the respective symmetri-
cally substituted alkynes with B(C6F5)3 at 110 °C in toluene
consequently gave the alkenylboranes 4e−h in good yield
(Scheme 1). These 1,1-carboborations are further supported by
the crystallographic characterization of 4e (Figure 1).
In this paper, we report the use of such bulky alkenylboranes
for the hydrogenation reactions of α,β-unsaturated carbonyl
compounds. In addition, as these alkenylboranes are themselves
electron-poor olefins, the potential for “self” CC double-bond
Reactions of Alkenylboranes with Phosphanes. Efforts
to generate FLPs from alkenylboranes required combination
with sterically hindered phosphanes. In some cases these
mixtures were not inert. For example, a 1:1 mixture of the
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Scheme 3. Formation of 6e, 6g, 7c, 7e, 7g, and 8g
Figure 1. POV-ray depiction of the alkenylborane 4e. C: black, B:
yellow-green, F: pink. Hydrogen atoms are not shown.
Ph2CC(C6F5)BF(C6F5)(C6F4PCy2H) (8g) (Scheme 3). In some
cases these reactions were accelerated at elevated temperatures of
50−70 °C. These reactions are typified by the appearance of the
diagnostic broad 19F NMR signals at ca. −180 ppm, reflecting the
presence of a BF fragment in addition to the resonances
attributable to the C6F4 group. Three of the products (6e, 6g,
and 7g) were characterized by X-ray diffraction (6g: Figure 3;
6e, 7g: see Supporting Information).
phenyl-substituted alkenylboranes 4c and 4g and tBu3P react
upon heating for 24 h at 80 °C in toluene to form the salt 5g
(Scheme 2). An X-ray diffraction study of 5g (Figure 2)
Scheme 2. Formation of 5c and 5g
Figure 3. POV-ray depiction of the zwitterion 6g. C: black, B: yellow-
green, F: pink, P: orange. Hydrogen atoms are not shown.
Figure 2. POV-ray depiction of the anion of 5g. C: black, B: yellow-
green, F: pink, Hydrogen atoms are not shown.
Reactions of Alkenylborane-Derived FLPs with H2.
Despite the thermally driven formation of the boracycles 5 and
zwitterions 6−8, the alkenylboranes 4 form FLPs with bulky
phosphanes that are amenable to heterolytic H−H bond cleavage.
For example, the combination of dihydrogen (2.5 bar), the n-propyl-
substituted alkenylborane 4a, and tBu3P in toluene overnight at
ambient temperature results in the formation of the new species 9a,
which has taken up two equivalents of dihydrogen (Scheme 4). In
the 1H and 13C NMR spectra of compound 9a, signals at 3.23 (1H)
and 30.3 (br, 13C) ppm, respectively, are consistent with the presence
of a newly formed saturated C5 chain at boron (BCH). The [B]H
unit results in a 11B NMR doublet at δ −19.3 (1JBH ≈ 90 Hz), and the
31P NMR [P]H doublet resonance is seen at δ 59.2 (1JPH ≈ 433 Hz).
Compound 9a was characterized by X-ray diffraction (Figure 4),
revealing the hydrogenation of the olefinic fragment of 4a with the
formation of the phosphonium borate [tBu3PH] [CH3(CH2)2-
CH2CH(C6F5)BH(C6F5)2], 9a. The formerly olefinic bond has
identified it as the product of an internal electrophilic aromatic
substitution. In this case the borane is bound to the cis-oriented
vicinal phenyl substituent, while the phosphane is protonated,
affording the boracycle 5g. The product 5c was similarly obtained
from the E isomer of 4c alkenylborane and tBu3P at elevated
temperatures. The compound 5c was also characterized by X-ray
diffraction (see Supporting Information).
In contrast, the analogous reactions of the alkenylboranes 4c,
4e, and 4g with iPr3P, tBu2PH, or Cy2PH, resulted in para-attack
on a C6F5 group on boron, resulting in a nucleophilic aromatic
substitution reaction.23 The fluoride is trapped by the boron
electrophile, affording the zwitterionic salts R2CC(C6F5)BF-
(C6F5)(C6F4PiPr3) (R = Et 6e, Ph 6g), R2CC(C6F5)BF-
(C6F5)(C6F4PtBu2H) (R2 = Ph(H) 7c; R = Et 7e, Ph 7g), and
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Scheme 4. Formation of 9a, 9b, and 9a-D
Scheme 5. Formation of 9c and 10c
Figure 5. POV-ray depiction of the salt 9c. C: black, B: yellow-green, F:
pink, H: gray, P: orange. Only the PH, BH, CH, and CH2 hydrogen
atoms are shown.
Figure 4. POV-ray depiction of the zwitterion 9a. C: black, B: yellow-
green, F: pink, H: gray, P: orange. Hydrogen atoms except for BH and
PH are not shown.
butyl-substituted alkenylborane 4d, as treatment with tBu3P and
dihydrogen affords the phosphonium hydridoborate salt 10d, in
which the olefinic moiety is not hydrogenated (Scheme 6). In
been reduced, as evidenced by the C−C bond length of 1.512(8) Å,
while the corresponding B−C bond distance is 1.633(8) Å. The
boron center is tetra-coordinated with the C−B1−C angles
summing to 334.4°, while the geometry of the [tBu3PH] cation is
unexceptional.
Scheme 6. Formation of 10d−h
The analogous experiment was carried out with dideuterium,
4a, and tBu3P at 25 °C to afford 9a-D. The 2H NMR spectrum
displayed a pair of [sp3-C]-D resonances resulting from the
saturated hydrocarbon chain at δ 3.19 ([B]CD) and δ 1.85/1.62
(−CDH) as well as a broad [B]D deuteride resonance at δ 2.98
and a [P]D doublet at δ 5.23 (1JPD ≈ 66 Hz).24 The corre-
sponding reaction of the alkenylborane 4b with tBu3P and H2
yielded the related saturated product 9b in ca. 80% yield as a
white solid (Scheme 4).
The FLP derived from the phenyl-substituted alkenylborane
4c (E:Z 30:1) and tBu3P was less reactive. At 25 °C in pentane,
clean heterolytic cleavage of dihydrogen was achieved to yield ca.
85% of 10c after 3 days. In the absence of light, the hydrogenated
product was exclusively the E isomer, while an uncovered
reaction vessel yielded a ca. 1:1 isomeric E:Z mixture. Under
more forcing conditions, i.e., heating under H2 at 80 °C for 15 h
in toluene, a 9:2:4 mixture of salts 9c:10c:5c resulted (Scheme 5).
Nonetheless compound 9c has been successfully isolated and
fully characterized, including an X-ray structure confirming the
nature of the salt (Figure 5).
solution, compound 10d shows the typical 1H NMR features of
the intact alkenyl substituent at boron (1H: δ 5.40), the broad
1:1:1:1 intensity quartet at δ 3.00 (1JBH ≈ 90 Hz), and a doublet
at δ 5.17 (1JPH ≈ 431 Hz) attributable to [B]H and [P]H
fragments. The corresponding alkenyl-13C, 11B, and 31P NMR
signals were observed at δ 143.1, −18.8, and 59.6, respectively.
This phosphonium-alkenylhydridoborate salt 10d was also char-
acterized by X-ray diffraction (Figure 6). It features the intact
C1−C2 (1.329(5) Å) and CC double bond at boron (C1−B1
1.633(5) Å, B1−C1−C2 124.1(3)o). The trisubstituted alkene
also has the tert-butyl substituent at C2 (C1−C2−C3 135.0(3)o)
These data are consistent with a trend of decreased reactivity
with increasing steric bulk of the substituents at the end of the
alkenylborane CC double bond. This is also seen for the tert-
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Analogous reactions employing nitrogen-based Lewis bases
and dihydrogen were also briefly investigated. To this end,
compound 4d and either 1-azabicylo[2.2.2]octane (quinucli-
dine) or DABCO were combined under 60 bar of dihydrogen at
room temperature. This led to the rapid formation of the respec-
tive ammonium hydridoborate salts 11d and 12d (Scheme 7).
Scheme 7. Synthesis of 11d, 12d, and 12f
Figure 6. POV-ray depiction of the salt 10d. C: black, B: yellow-green,
F: pink, H: gray, P: orange. Only the PH and BH hydrogen atoms are
shown.
and the −C6F5 group in a Z-orientation. The plane of the penta-
fluorophenyl group at C1 is rotated markedly from the olefinic
substituent plane (θ C2−C1−C51−C52 96.2(4)o). The boron
and phosphorus atoms are typically pseudotetrahedral, with the
C−B−C and C−P−C bonding angles summing to 336.8° and
342.8°, respectively.
Following the same trend, the alkenylboranes 4e−h,
containing tetra-substituted CC double bonds, react similarly
with dihydrogen in the presence of tBu3P. In all these cases the
tetra-substituted CC double bonds of the boron Lewis acid
compounds remained intact, but the P/B pairs effected rapid
heterolytic dihydrogen splitting to afford the phosphonium-
hydridoborate salts 10e (R = C2H5), 10f (R = n-C3H7), 10g
(R = Ph), and 10h (R = p-MeC6H4) in good yields (Scheme 6).
Compounds 10e−g were characterized by X-ray diffraction
(10e: Figure 7; 10f, 10g: see the Supporting Information). The
Compound 11d was characterized by X-ray diffraction (see the
Supporting Information). In a similar fashion, the FLP derived
from the tetra-substituted alkenylborane 4f and the DABCO
Lewis base with dihydrogen under identical conditions gave the
corresponding ammonium hydridoborate salt 12f (Scheme 7).
Catalytic Hydrogenation of Imines. The ability of
electron-deficient boranes to effect catalytic hydrogenations of
imines has been previously demonstrated. This results in part
from the ability of the Lewis acid and substrate imine to effect
heterolytic cleavage of dihydrogen. As the alkenylboranes
described herein have been shown to be sufficiently electrophilic
to participate in such activation of dihydrogen, several of these
alkenylboranes were evaluated in catalytic hydrogenation of
imines. Compounds 4c, 4e, and 4g were employed in the
catalytic hydrogenations of several imine substrates (Table 1)
using a catalyst loading of 5 mol % at 120 °C. Reactions were
performed at dihydrogen pressures of 5 and 110 bar. In the case
of the prototypical sterically encumbered imine, Ph(H)C
NtBu, 4c and 4e proved to be effective catalysts, each resulting in
quantitative hydrogenations after 12 h. In contrast, species 4g
was ineffective, resulting in only 11% yield of the hydrogenated
amine after 24 h at 110 bar of dihydrogen at 120 °C. Using
the sterically bulky substrate Ph(H)CNCHPh2, of the three
catalysts only 4e was effective and only under high pressure
conditions, while for the substrate Ph(H)CNSO2Ph, the
alkenylboranes were ineffective at low pressure and displayed
marginal activity even at higher dihydrogen pressure. Collec-
tively, these data infer that increased steric congestion in either
these hydridoborate anions or substrates is problematic for
hydride delivery to transient iminium cations. Moreover, reduced
Lewis basicity at the nitrogen centers does not result in efficient
hydrogen activation.
Figure 7. POV-ray depiction of the salt 10e. C: black, B: yellow-green, F:
pink, H: gray, P: orange. Only the PH and BH hydrogen atoms are shown.
aryl-substituted alkenylborane (4g and 4h)/tBu3P FLPs were
found to be less reactive toward dihydrogen compared to alkyl-
substituted alkenylboranes/tBu3P FLPs toward dihydrogen
splitting under similar reaction conditions. Thus, more forcing
conditions (60 bar H2 and 80 °C) were used to generate the aryl-
substituted phosphonium alkenylhydridoborate salts, 10g and
10h, respectively (Scheme 6).
Catalytic Hydrogenation of Conjugated Enones. It had
also been shown that a conjugated ynone can be slowly
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a
4d/DABCO catalyst. In contrast, the respective p-methoxy
derivative 15c was not reduced under these conditions. These
results indicate that hydride transfer from the in situ formed
alkenylhydridoborate anion derived from the alkenylborane
under FLP conditions might be the decisive factor in achieving
the requisite reactivity for selective hydrogenation of electron-
deficient olefins. It seems that the alkenylboranes used here
display the appropriate balance between sufficient Lewis acidity
and steric bulk to allow for dihydrogen activation by the
respective FLPs. At the same time they feature sufficiently
reduced electrophilicity to make the resulting hydridoborate
nucleophilic enough to attack at the Michael position of the
enone. Whether this process is Lewis or Brønsted acid catalyzed
remains an open question; also the specific role of the DABCO
Lewis base or its conjugate DABCO-H+ Brønsted acid remains
unsolved.
Table 1. Imine Hydrogenations
imine substrate
cat.
T (h) yield (%) (5 bar) yield (%) (110 bar)
13a
13a
13a
13b
13b
13b
13c
13c
13c
4c
4e
4g
4c
4e
4g
4c
4e
4g
12
12
24
12
12
24
24
24
24
>99
>99
6
>99
>99
11
19
20
0
35
>99
trace
2
0
trace
0
11
CONCLUSIONS
■
18
We conclude that 1,1-carboboration reactions of alkynes with
B(C6F5)3 result in the formation of a series of useful alkenyl-
borane systems. These form reactive FLPs upon addition of a
suitable bulky Lewis base. Exposure of these FLPs to dihydrogen
in all cases studied here leads to rapid heterolytic cleavage of
dihydrogen. In the cases of sufficient steric bulk of the sub-
stituents at the alkenyl CC double bond this results in the
formation of the respective alkenylhydridoborate/phosphonium
salts. Remarkably, the less sterically crowded examples undergo a
concomitant efficient hydrogenation of the CC double bond,
giving the corresponding saturated 1-pentafluorophenylalkylhy-
dridoborate/phosphonium salts in high yield. In contrast, some
FLPs derived from the alkenylboranes proved resistant to
hydrogenation of the CC double bond under typical reaction
conditions. In addition, the combination of the alkenylborane 4d
and DABCO was shown to give an active catalyst for the
hydrogenation of enone substrates. This work illustrates that
judicious control of the nature of a FLP hydrogenation catalysts
can permit the reduction of a broadening scope of substrates.
a
Performed employing 5 mol % catalyst at 120 °C.
hydrogenated to the saturated ketone by a catalytic amount of
the FLP derived from the alkenylborane 4d and the amine Lewis
base DABCO.20 Herein, the hydrogenation of a small series of
enones with sufficiently bulky substituents at both the ketone and
alkene ends was studied (Table 2). Initially 5 mol % of the
a
Table 2. Catalytic Enone Hydrogenations
b
enone
alkenylborane
conversion (%)
yield (%)
15a
15a
15a
4a
4d
4f
2
81
74
47
92
EXPERIMENTAL SECTION
General Considerations. All reactions involving air- and/or
moisture-sensitive compounds were carried out under an inert gas
50
■
c
15a
4f
>99
trace
>99
40
15a
15b
15b
4h
4d
4f
atmosphere (Munster: argon purchased from Westfalen AG; Toronto:
̈
91
34
nitrogen) using Schlenk-type glassware and a glovebox (Munster:
̈
glovebox 150 B-G II from MBraun; Toronto: gloveboxes from
Innovative Technology, Vacuum Atmospheres, and MBraun). All
other manipulations were performed on a double-manifold N2 (H2)/
vacuum line with Schlenk-type glassware or in an N2-filled inert
atmosphere glovebox. The N2 and H2 gases were dried by passage
through a Dririte column. Solvents (Aldrich) were dried using an
Innovative Technologies solvent system (toluene, hexanes, pentane,
CH2Cl2). NMR spectra were obtained on a Bruker ARX 300 (1H: 300
MHz, 13C: 75 MHz), Bruker Avance 400 MHz, or Varian Inova 500 (1H:
500 MHz, 13C: 126 MHz, 19F: 470 MHz, 11B: 160 MHz) spectrometer.
a
5 mol % FLP catalyst, 10 bar of H2, 80 °C, 48 h in d6-benzene.
b
c
1
Determined by H NMR spectroscopy. 40 bar of H2.
respective FLP was employed, and the reactions were carried out
at 10 bar dihydrogen pressure for 48 h at 80 °C in benzene-d6.
The catalyst 4d/DABCO was the most effective catalyst of the
series. Under our standard conditions, monitoring by NMR
spectroscopy revealed 81% conversion of the enone 15a to the
saturated ketone 16a. On the other hand, the tetra-substituted
alkenylborane 4f gave a slightly inferior catalyst under the same
conditions; however, it led to a quantitative conversion of 15a to
16a under slightly more forcing conditions (40 bar H2). Both the
more bulky diaryl-substituted alkenylborane 4h and the much
less sterically congested system 4a made very poor catalysts. In
the latter case, the observed reactivity of 4a with dihydrogen
might be responsible for this unfavorable behavior (see above).
The introduction of a strongly electron-withdrawing substituent
at the distal aryl ring of the enone 15b accelerated the FLP
hydrogenation of the electron-poor CC double bond with the
1
For H NMR and 13C NMR chemical shifts (δ) are given relative to
TMS and referenced to the solvent signal (19F rel. to external CFCl3; 11
B
rel. to external BF3·Et2O). NMR assignments are supported by addi-
tional 1D and 2D NMR experiments. NMR spectra were recorded at 25
°C unless otherwise stated, and chemical shifts are reported in ppm.
NMR solvents were purchased from Cambridge Isotopes, dried over
CaH2 (CD2Cl2, C6D5Br, and CDCl3), vacuum distilled prior to use, and
stored over 4 Å molecular sieves in the glovebox. Elemental analyses
were performed on a Elementar Vario El III. IR spectra were recorded
on a Varian 3100 FT-IR (Excalibur Series). Melting points were
obtained with a DSC Q20 (TA Instruments). ESI mass spectra recorded
on a Bruker Daltonics MicroTof.
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Organometallics
Article
The imines25b,c 13 and the enones25a 15 were prepared by literature
methods. B(C6F5)3 was prepared according to a literature procedure2c,d
and purified by two successive sublimations (Toronto). The syntheses
of compounds 4a−h were described in our previously published
papers.22 Crystals of 4e suitable for X-ray diffraction were grown from a
concentrated pentane solution at −40 °C for one week.
p-B(C6F5)2), −167.9 (m, 2F, m-C6F5), −168.4 (m, 4F, m-B(C6F5)2). 31
P
NMR (162 MHz, CD2Cl2, 298 K): δ 58.3 (1JH−P = 427 Hz, PH). 11B
NMR (128 MHz, CD2Cl2, 298 K): δ −9.39. 13C{1H} NMR (101 MHz,
CD2Cl2, 298 K) partial: δ 130.0, 129.4 (Ph), 128.3 (Ph), 126.6, 124.6,
1
124.3, 120.0, 38.1 (d, JC−P = 26.6 Hz, tBu), 30.4 (tBu). X-ray data:
a = 11.2252(3) Å, b = 18.7357(5) Å, c = 19.1203(6) Å, V = 4021.2(2) Å3,
Z = 4, orthorhombic, space group P212121 7084 observed reflections
(I ≥ 2σ(I)), 559 refined parameters, R1 = 0.0459, wR2(all) = 0.1211,
GOF = 1.050.
Synthesis of [Et2CC(C6F5)B(C6F5)2], 4e. In a glovebox, a 50 mL
glass tube wrapped with aluminum foil, equipped with a small stir bar
and a Teflon screw tap, was charged with a solution of B(C6F5)3 (512
mg, 1.00 mmol) in toluene (15 mL). While mixing, 3-hexyne was added
via syringe (0.34 mL, 3.00 mmol), resulting in a light brown solution.
The solution was allowed to mix for 12 h at 120 °C. After cooling to
25 °C, all volatiles were removed and the crude product was recrys-
tallized from pentane at −40 °C to yield a pale brown powder in 78%
yield. Crystals suitable for X-ray diffraction were grown from a
concentrated pentane solution at −40 °C for one week. Anal. Calcd
(%) for C24H10BF15: C 48.52, H 1.70. Found: C 48.21, H 1.84. 1H NMR
(400 MHz, CD2Cl2, 298 K): δ 2.51 (m, 2H, 3JH−H = 7.43 Hz, CH2), 2.32
(m, 2H, 3JH−H = 7.4 Hz,CH2), 1.12 (t, 3H, 3JH−H = 7.4 Hz, CH3), 1.07 (t,
Synthesis of R2CC(C6F5)BF(C6F5)(C6F4PiPr3) (R = Et 6e, Ph
6g), R2CC(C6F5)BF(C6F5)(C6F4PtBu2H) (R2 = Ph(H) 7c; R = Et 7e,
Ph 7g), and Ph2CC(C6F5)BF(C6F5)(C6F4PCy2H), 8g. These
compounds were prepared in a similar fashion, and thus only one
preparation is detailed. In some cases elevated temperatures and
prolonged periods were employed. To a yellow solution of 4g (107 mg,
0.13 mmol) in toluene (1 mL) was added iPPr3 (25 mg, 0.16 mmol).
After 16 h at 25 °C, a white precipitate was observed. The reaction was
allowed to stir for 48 h at 25 °C, before adding pentane (3 × 2 mL). The
solvent was decanted, and the product was dried in vacuo to yield 6g
(82%) as a white solid.
3
3H, JH−H = 7.4 Hz, CH3). 19F NMR (377 MHz, CD2Cl2, 298 K): δ
6e: 25 °C for 16 h, 70% yield as a white solid. Anal. Calcd (%) for
C33H31BF15P: C 52.54, H 4.14. Found: C 52.55, H 4.44. 1H NMR (400
MHz, CD2Cl2, 298 K): δ 3.26 (m, 3H, iPr), 2.27 (m, 2H, CH2), 1.91 (m,
2H, CH2), 1.48 (dd, 3JH−P = 17.8 Hz, 3JH−H = 7.1 Hz, 18H, iPr), 0.89 (t,
3H, 3JH−H = 7.4 Hz, CH3), 0.74 (t, 3H, 3JH−H = 7.4 Hz, CH3). 19F NMR
(377 MHz, CD2Cl2, 298 K): δ −126.3 (br, 2F, C6F4), −131.6 (m, 2F, p-
C6F4), −133.0 (m, 2F, o-BC6F5), −139.4 (m, 1F, o-CC6F5), −140.2 (br,
−129.8 (m, 4F, o-B(C6F5)2), −139.3 (m, 2F, o-C6F5), −148.2 (t, 2F,
3JF−F = 19.9 Hz, p-B(C6F5)2), −156.0 (t, 1F, 3JF−F = 20.7 Hz, p-C6F5),
−161.3 (m, 4F, m-B(C6F5)2), −162.5 (m, 2F, m-C6F5). 11B NMR (128
MHz, CD2Cl2, 298 K): δ 60.9 (br s). 13C{1H} NMR (101 MHz, CD2Cl2,
298 K): δ 182.6 (Et2C), 146.9 (dm, 1JC−F = 248 Hz, CF), 143.8 (dm,
1JC−F = 244 Hz, CF), 143.4 (dm, 1JC−F = 258 Hz, CF), 140.4 (dm, 1JC−F
=
253 Hz, CF), 137.4 (dm, 1JC−F = 253 Hz, CF), 137.2 (dm, 1JC−F = 252
Hz, CF), 126.6 (br, CB), 116.9 (tm, 2JC−F = 20.7 Hz, ipso-C6F5), 114.6
(br, ipso-B(C6F5)2), 29.9 (CH2), 29.2 (CH2), 14.6 (CH3), 11.7 (CH3).
X-ray data: a = 10.9155(7) Å, b = 9.9660(7) Å, c = 11.6419(8) Å,
β = 115.278(2)°, V = 1145.18(13) Å3, Z = 2, monoclinic, space group:
P21, 3972 observed reflections (I ≥ 2σ(I)), 490 refined parameters,
R1 = 0.0294, wR2(all) = 0.688, GOF = 1.024.
3
1F, o-CC6F5), −162.5 (t, 1F, JF−F = 19.2 Hz, p-C6F5), −163.6 (t, 1F,
3JF−F = 20.8 Hz, p-C6F5), −166.8−167.0 (m, 4F, m-BC6F5, m-CC6F5),
−185.5 (br, 1F, BF). 31P{1H} NMR (400 MHz, CD2Cl2, 298 K): δ 52.1
(m). 11B NMR (128 MHz, CD2Cl2 298 K): δ 0.34 (d, 1JB−F = 48.0 Hz).
13C{1H} NMR (400 MHz, CD2Cl2, 298 K) partial: δ 150.5 (Et2C),
149.3 (dm, 1JC−F = 250 Hz, CF), 147.9 (dm, 1JC−F = 240 Hz, CF), 146.3
(dm, 1JC−F = 254 Hz, CF), 143.6 (dm, 1JC−F = 246 Hz, CF), 137.6 (dm,
1JC−F = 247 Hz, CF), 136.7 (dm, 1JC−F = 248 Hz, CF), 130.6 (br, CB),
Synthesis of [tBu3PH][HC(C6H4)C(C6F5)B(C6F5)2], 5c. In a
glovebox, a 50 mL glass tube equipped with a small stir bar and a Teflon
screw top was charged with a solution of (E)- and (Z)-4c (30:1) (77 mg,
0.12 mmol) and tBu3P (25 mg, 0.12 mmol) in toluene (3 mL). The
yellow solution was placed in an oil bath at 80 °C for 3 days. The solvent
was removed under vacuum, and the crude oil was washed with pentane
(3 × 5 mL) to yield 69% of the product as a white precipitate. Anal.
Calcd (%) for C38H33BF15P: C 55.90, H 4.07. Found: C 55.82, H 4.15.
1H NMR (400 MHz, CD2Cl2, 298 K): δ 7.27 (d, 1H, 3JH−H = 7.1 Hz,
C6H4), 7.19 (s, 1H, C(H)), 7.09 (d, 1H, 3JH−H = 7.1 Hz, C6H4), 6.91
(tm, 1H, 3JH−H = 7.10 Hz, C6H4), 6.81 (t, 1H, 3JH−H = 7.1 Hz, C6H4),
4.87 (d, 1H, 1JH−P = 429 Hz, PH), 1.52 (d, 27H, 3JH−P = 15.7 Hz, tBu).
19F NMR (377 MHz, CD2Cl2, 298 K): δ −131.2 (m, 4F, o-B(C6F5)2),
−140.0 (m, 2F, o-C6F5), −163.6 (t, 1F, 3JF−F = 21.1 Hz, p-C6F5), −165.4
(t, 2F, 3JF−F = 21.1 Hz, p-B(C6F5)2), −166.8 (m, 2F, m-C6F5), −167.5
(m, 4F, m-B(C6F5)2). 31P NMR (162 MHz, CD2Cl2, 298 K): δ 60.2
(1JH−P = 429 Hz, PH). 11B NMR (128 MHz, CD2Cl2, 298 K): δ −10.0;
13C{1H} NMR (101 MHz, CD2Cl2, 298 K) partial: δ 149.7
(PhCC(H)), 142.1 (C(H)), 128.5 (C6H4), 124.4 (C6H4), 124.3
(C6H4), 120.5 (C6H4), 37.7 (d, 1JC−P = 27.1 Hz, tBu), 30.0 (tBu). X-ray
data: a = 11.2573(6) Å, b = 11.8997(6) Å, c = 15.2899(8) Å,
α = 78.641(3)°, β = 71.732(3)°, γ = 67.422(2)°, V = 1789.05(16) Å3,
2
124.1 (br, ipso-BC6F5), 122.7 (tm, JC−F = 24.5 Hz, ipso-CC6F5), 87.8
1
2
(dt, JC−P = 70.7 Hz, JC−F = 18.0 Hz, ipso-PC6F4), 26.2 (CH2), 25.9
1
(CH2), 23.5 (dm, JC−P = 40.9 Hz, iPr), 16.8 (iPr), 12.0 (CH3),
11.8 (CH3). X-ray data: a =18.4883(10) Å, b =18.5351(11) Å, c =
19.2487(11) Å, V = 6596.2(7) Å3, Z = 8, orthorhombic, space group
Pbca, 5803 observed reflections (I ≥ 2σ(I)), 451 refined parameters,
R1 = 0.0516, wR2(all) = 0.1501, GOF = 1.039.
6g: Anal. Calcd (%) for C41H31BF15P: C 57.90, H 3.67. Found: C
1
58.21, H 4.14. H NMR (400 MHz, CD2Cl2, 298 K): δ 7.38 (d, 2H,
3JH−H = 7.4 Hz, o-Ph), 7.12−7.09 (m, 4H, o,m-Ph), 7.04−6.97 (m, 3H,
m-Ph, p-Ph), 6.92 (t, 1H, 3JH−H = 7.4 Hz, p-Ph), 3.07 (m, 3H, iPr), 1.33
(dd, 3JH−P = 17.6 Hz, 3JH−H = 6.4 Hz, 18H, iPr). 19F NMR (377 MHz,
CD2Cl2, 298 K): δ −124.7 (br, 2F, C6F4), −132.2 to 132.3 (br, 4F, o-
BC6F5, p-C6F4), −138.3 (m, 1F, o-CC6F5), −138.8 (br, 1F, o-CC6F5),
−162.1 (t, 1F, 3JF−F = 20.2 Hz, p-BC6F5), −162.3 (t, 1F, 3JF−F = 20.7 Hz,
p-CC6F5), −166.7 (m, 1F, m-CC6F5), −166.9 (m, 1F, m-CC6F5),
−167.1 (m, 2F, m-BC6F5), −180.1 (br, 1F, BF). 31P{1H} NMR (162
MHz, CD2Cl2, 298 K): δ 52.0 (m). 11B NMR (128 MHz, CD2Cl2, 298
K): δ 0.59 (d, 1JB−F = 52.5 Hz). 13C{1H} NMR (101 MHz, CD2Cl2, 298
K) partial: δ 149.6 (Ph2C), 149.4 (dm, 1JC−F = 251 Hz, CF), 147.8
(dm, 1JC−F = 242 Hz, CF), 145.9 (i-Ph), 143.9 (i-Ph), 143.3 (dm, 1JC−F
=
242 Hz, CF), 138.0 (dm, 1JC−F = 244 Hz, CF), 136.8 (dm, 1JC−F = 245
Hz, CF), 129.6 (o-Ph), 128.0 (o-Ph), 127.5 (m-Ph), 126.6 (m-Ph), 126.0
(p-Ph), 125.8 (p-Ph), 123.4 (br, ipso-BC6F5), 123.2 (tm, 2JC−F = 22.0 Hz,
ipso-CC6F5), 87.7 (dtm, 1JC−P = 72.0 Hz, 2JC−F = 18.2 Hz, ipso-PC6F4),
23.4 (dm, 1JC−P = 41.9 Hz, iPr), 16.9 (iPr). X-ray data: a = 11.0573(6) Å,
b = 13.0147(7) Å, c = 13.9942(7) Å, α = 81.889(2)°, β = 88.595(3)°, γ =
Z = 2, triclinic, space group P1, 6288 observed reflections (I ≥ 2σ(I)),
̅
505 refined parameters, R1 = 0.0566, wR2(all) = 0.1546, GOF = 1.018.
Synthesis of [tBu3PH][PhC(C6H4)C(C6F5)B(C6F5)2], 5g. Com-
pound 4g (28 mg, 0.041 mmol) and tBu3P (8 mg, 0.041 mmol) were
dissolved in toluene (0.5 mL) in a Teflon screw cap 50 mL glass bomb
with a small stir bar. The mixture was allowed to heat at 80 °C for 3 days.
The solvent was removed under vacuum, and the crude oil was washed
with pentane (3 × 5 mL) to yield the product as a white precipitate in
52% yield. Anal. Calcd (%) for C44H37BF15P: C 59.21, H 4.18. Found: C
58.69, H 4.38. 1H NMR (400 MHz, CD2Cl2, 298 K): δ 7.25−7.10 (m,
6H, Ph, C6H4), 6.94−6.85 (m, 3H, Ph, C6H4), 4.85 (d, 1H, 1JH−P = 427
85.825(2)°, V = 1988.23(18) Å3, Z = 2, triclinic, space group P1, 6992
̅
observed reflections (I ≥ 2σ(I)), 523 refined parameters, R1 = 0.0579,
wR2(all) = 0.1739, GOF = 1.045.
7c: 50 °C for 48 h, 66% yield. Anal. Calcd (%) for C34H25BF15P: C
53.71, H 3.31. Found: C 53.50, H 3.50. 1H NMR (400 MHz, CD2Cl2,
298 K): δ 7.17 (t, 2H, 3JH−H = 7.1 Hz, m-Ph), 7.13 (t, 1H, 3JH−H = 7.1 Hz,
p-Ph), 6.98 (d, 2H, 3JH−H = 7.1 Hz, o-Ph), 6.81 (s, 1H, C(H)), 6.33 (d,
1H, 1JH−P = 462 Hz, PH), 1.60 (d, 18H, 3JH−P = 19.3 Hz, tBu). 19F NMR
3
Hz, PH), 1.52 (d, 27H, JH−P = 15.7 Hz, tBu). 19F NMR (377 MHz,
CD2Cl2, 298 K): δ −132.2 (m, 4F, o-B(C6F5)2), −141.1 (m, 2F, o-C6F5),
−163.8 (t, 1F, 3JF−F = 20.7 Hz, p-C6F5), −166.2 (t, 2F, 3JF−F = 20.7 Hz,
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Organometallics
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(377 MHz, d8-THF, 298 K): δ −127.0 (br, 1F, C6F4), −127.3 (m, 1F, p-
C6F4), −127.7 (br, 1F, C6F4), −131.5 (m, 2F, o-BC6F5), −133.6 (m, 1F,
C6F4), −138.7 (m, 1F, o-CC6F5), −142.2 (m, 1F, o-CC6F5), −163.3
(t, 1F, JF−F = 20.3 Hz, p-BC6F5), −163.5 (t, 1F, JF−F = 20.7 Hz, p-
CC6F5), −166.8 (m, 1F, m-CC6F5), −167.6 to −167.7 (m, 3F, m-BC6F5,
m-CC6F5), −193.0 (br, 1F, BF). 31P{1H} NMR (162 MHz, CD2Cl2, 298
CD2Cl2, 298 K): δ 11.0 (br s). 11B NMR (128 MHz, CD2Cl2, 298K):δ 0.48
(br, BF). 13C{1H} NMR (101 MHz, d8-THF, 298 K) partial: δ 149.6
(Ph2C), 148.4 (dm, 1JC−F = 247 Hz, CF), 148.0 (dm, 1JC−F = 247 Hz,
CF), 146.1 (i-Ph), 145.6 (dm, 1JC−F = 252 Hz, CF), 145.4 (dm, 1JC−F
=
3
3
252 Hz, CF), 143.8 (ipso-Ph), 143.4 (dm, 1JC−F = 247 Hz, CF), 137.7
1
1
(dm, JC−F = 247 Hz, CF), 136.2 (dm, JC−F = 250 Hz, CF), 129.7
(o-Ph), 128.0 (o-Ph), 127.1 (m-Ph), 126.2 (m-Ph), 125.7 (p-Ph), 125.4
(p-Ph), 123.9 (tm, 2JC−F = 20.8 Hz, i-CC6F5), 87.5 (br, ipso-PC6F4), 28.6
K): δ 33.4 (br). 11B NMR (128 MHz, CD2Cl2, 298 K): δ 1.49 (d, 1JB−F
=
50.8 Hz, BF). 13C{1H} NMR (101 MHz, d8-THF, 298 K) partial: δ
148.9 (dm, 1JC−F = 244 Hz, CF), 148.4 (dm, 1JC−F = 244 Hz, CF), 145.0
(dm, 1JC−F = 248 Hz, CF), 143.7 (dm, 1JC−F = 249 Hz, CF), 142.8 (dm,
1JC−F = 240 Hz, CF), 139.5 (ipso-Ph), 138.0 (dm, 1JC−F = 247 Hz, CF),
137.0 (dm, 1JC−F = 247 Hz, CF), 136.6 (dm, 1JC−F = 246 Hz, CF), 136.3
(d, 1JC−P = 41.7 Hz, P-Cy), 26.3 (d, 2JC−P = 3.3 Hz, Cy), 25.5 (d, 3JC−P
=
15.6 Hz, Cy), 24.8 (Cy).
Synthesis of [tBu3PH][CH3(CH2)3CH(C6F5)BH(C6F5)2], 9a. A
solution of 4a (58 mg, 0.1 mmol) and tBu3P (20 mg, 0.1 mmol) in
toluene (2 mL) was degassed, and the reaction flask filled with H2 (2.5
bar). The reaction mixture was stirred overnight under H2 (2.5 bar), and
pentane (10 mL) was added, after which the supernatant was decanted.
Crystallizing the residue with CH2Cl2/pentane (v/v = 1:3, 4 mL)
at −35 °C and then drying in vacuo afforded 9a as a white powder (68 mg,
87%). Crystals suitable for X-ray crystal structure analysis were grown by
a CH2Cl2/pentane (v/v = 1:3) solution of 9a at −35 °C. Anal. Calcd for
1
(dm, JC−F = 247 Hz, CF), 134.8 (C(H)), 127.6 (o,m-Ph), 125.9
(p-Ph), 123.8 (br, i-BC6F5), 120.5 (tm, 2JC−F = 22.8 Hz, i-CC6F5), 90.1
(dtm, 1JC−P = 70.0 Hz, 2JC−F = 19.3 Hz, ipso-PC6F4), 35.5 (d, 1JC−P = 31.6
Hz, tBu), 26.6 (tBu).
7e: 50 °C, 48 h, 99% yield. Anal. Calcd (%) for C32H29BF15P: C 52.19,
H 3.95. Found: C 52.56, H 4.41. 1H NMR (400 MHz, CD2Cl2, 298 K): δ
1
6.33 (d, 1H, JH−P = 466 Hz, PH), 2.25 (m, 2H, CH2), 1.90 (m, 2H,
1
CH2), 1.59 (d, 18H, 3JH−P = 19.0 Hz, tBu), 0.88 (t, 3H, 3JH−H = 7.4 Hz,
CH3), 0.72 (t, 3H, 3JH−H = 7.4 Hz, CH3). 19F NMR (377 MHz, CD2Cl2,
298 K): δ −125.6 (br, 1F, C6F4), −126.2 (br, 1F, C6F4), −126.5 (m, 1F,
p-C6F4), −133.0 (m, 3F, o-BC6F5, C6F4), −139.4 (m, 1F, o-CC6F5),
C35H39BF15P: C, 53.45; H, 5.00. Found: C, 53.31; H, 5.10. H NMR
(600 MHz, CD2Cl2, 298 K): δ 5.29 (d, 1JPH = 433.5 Hz, 1H, PH), 3.23
(br t, 3JHH = 10.1 Hz, 1H, BCH), 2.94 (1:1:1:1 q, 1JBH ≈ 95 Hz, BH),
3
1.88/1.64 (each m, each 1H, CHCH2), 1.67 (d, JPH = 15.7 Hz, 27H,
tBu), 1.27/1.20 (each m, each 1H, MeCH2), 1.12 (m, 2H, CH2), 0.80
(t, 3JHH = 7.3 Hz, 3H, CH3). 13C{1H} NMR (151 MHz, CD2Cl2, 298 K):
3
−140.3 (br, 1F, o-CC6F5), −162.4 (t, 1F, JF−F = 20.8 Hz, p-C6F5),
3
−163.6 (t, 1F, JF−F = 20.8 Hz, p-C6F5), −166.7 to 166.9 (m, 4F, m-
1
δ 38.0 (d, JPC = 27.1 Hz, tBu), 33.5 (CH2), 33.0 (CHCH2), 30.3 (d,
BC6F5, m-CC6F5), −184.5 (br, 1F, BF). 31P{1H} NMR (162 MHz,
CD2Cl2, 298 K): δ 33.1 (br). 11B NMR (128 MHz, CD2Cl2, 298 K): δ
0.39 (d, JB−F = 40.8 Hz, BF). 13C{1H} NMR (101 MHz, CD2Cl2, 298 K)
partial: δ 150.6 (Et2C), 149.3 (dm, 1JC−F = 252 Hz, CF), 142.0 (dm,
2JPC = 5.9 Hz, tBu), 30.3 (br, BCH), 23.2 (MeCH2), 14.4 (CH3), [C6F5
not listed]. 19F NMR (564 MHz, CD2Cl2, 298 K): δ −132.6 (m, 4F),
−140.9 (br, 1F), −143.7 (br, 1F) (o-C6F5), −164.6 (t, 3JFF = 20.3 Hz),
−164.9 (t, 3JFF = 20.3 Hz), −166.2 (t, 3JFF = 21.3 Hz) (each 1F, p-C6F5),
−167.3 (m, 4F), −167.5 (m, 2F) (m-C6F5). 11B{1H} NMR (192 MHz,
CD2Cl2, 298 K): δ −19.3 (ν1/2 ≈ 60 Hz). 11B NMR (192 MHz, CD2Cl2,
298 K): δ −19.3 (d, 1JBH = 90.6 Hz). 31P{1H} NMR (202 MHz, CD2Cl2,
298 K): δ 59.2 (ν1/2 ≈ 3 Hz). 31P NMR (202 MHz, CD2Cl2, 298 K): δ
59.2 (dm, 1JPH ≈ 433 Hz). X-ray data: a = 9.9252(3) Å, b = 20.0004(11)
Å, c = 18.3802(7) Å, β = 93.002(3)°, V = 3643.6(3) Å3, Z = 4,
monoclinic, space group P21/n, 4875 observed reflections (I ≥ 2σ(I)),
490 refined parameters, R1 = 0.0889, wR2(all) = 0.2672, GOF = 1.067.
Synthesis of [tBu3PH][CH3(CH2)4CH(C6F5)BH(C6F5)2], 9b. A
similar procedure to that mentioned above for the preparation of
compound 9a was carried out by using starting material 4b to yield 9b as
a white powder (66 mg, 82%). Anal. Calcd for C36H41BF15P: C, 54.02;
H, 5.16. Found: C, 53.86; H, 5.25. 1H NMR (500 MHz, CD2Cl2, 298 K):
1JC−F = 240 Hz, CF), 143.7 (dm, 1JC−F = 240 Hz, CF), 138.5 (dm, 1JC−F
=
249 Hz, CF), 137.6 (dm, 1JC−F = 252 Hz, CF), 136.7 (dm, 1JC−F = 246
Hz, CF), 131.6 (br, CB), 123.9 (br, i-BC6F5), 122.6 (tm, 2JC−F = 22.0
Hz, ipso-CC6F5), 88.7 (m, ipso-PC6F4), 36.0 (d, 1JC−P = 30.9 Hz, tBu),
27.6 (tBu), 26.2 (CH2), 25.9 (CH2), 12.0 (CH3), 11.8 (CH3).
7g: 70 °C for 48 h, 42% yield off-white solid. Anal. Calcd (%) for
C40H29BF15P: C 57.44, H 3.49. Found: C 57.19, H 3.70. 1H NMR (400
MHz, CD2Cl2, 298 K): δ 7.40 (d, 2H, 3JH−H = 7.0 Hz, o-Ph), 7.30 (m,
1H, p-Ph), 7.13 (m, 4H, o,m-Ph), 7.02 (t, 2H, 3JH−H = 7.0 Hz, m-Ph),
1
6.95 (m, 1H, p-Ph), 6.27 (d, 1H, JH−P = 465 Hz, PH), 1.57 (d, 18H,
3JH−P = 19.2 Hz, tBu). 19F NMR (377 MHz, CD2Cl2, 298 K): δ −124.2
(br, 1F, C6F4), −124.5 (br, 1F, C6F4), −127.1 (m, 1F, p-C6F4), −132.4
(m, 2F, o-BC6F5), −133.5 (m, 1F, C6F4), −138.3 (m, 1F, o-CC6F5),
3
1
3
−139.0 (m, 1F, o-CC6F5), −161.9 (t, 1F, JF−F = 20.4 Hz, p-BC6F5),
δ 5.31 (d, JPH = 433.8 Hz, 1H, PH), 3.23 (br t, JHH = 10.1 Hz, 1H,
BCH), 2.93 (br, BH), 1.88/1.64 (each m, each 1H, CHCH2)1, 1.68 (d,
3JPH = 15.7 Hz, 27H, tBu), 1.21/1.18 (each m, each 1H, MeCH2)1, 1.23/
1.18 (each m, each 1H, 4-CH2)1, 1.14 (m, 2H, 3-CH2)1, 0.81 (m, 3H,
CH3) [1 from ghsqc experiment]. 13C{1H} NMR (126 MHz, CD2Cl2,
298 K): δ 38.0 (d, 1JPC = 27.1 Hz, tBu), 33.3 (CHCH2), 32.5 (C4), 30.9
(C3), 30.3 (tBu), 30.2 (br, B-CH), 23.2 (MeCH2), 14.3 (CH3) [C6F5 not
listed]. 19F NMR (470 MHz, CD2Cl2, 298 K): δ −132.6 (m, 4F), −140.9
(br, 1F), −143.7 (br, 1F) (o-C6F5), −164.6 (t, 3JFF = 20.3 Hz), −164.9
(t, 3JFF = 20.3 Hz), −166.2 (t, 3JFF = 21.3 Hz) (each 1F, p-C6F5), −167.3
(m, 4F), −167.5 (m, 2F) (m-C6F5). 11B{1H} NMR (160 MHz, CD2Cl2,
298 K): δ −19.3 (ν1/2 ≈ 70 Hz). 11B NMR (160 MHz, CD2Cl2, 298 K):
δ −19.3 (d, 1JBH = 93.1 Hz). 31P{1H} NMR (202 MHz, CD2Cl2, 298 K):
δ 59.1 (ν1/2 ≈ 3 Hz). 31P NMR (202 MHz, CD2Cl2, 298 K): δ 59.1 (dm,
1JPH = 433.9 Hz).
3
−162.2 (t, 1F, JF−F = 20.4 Hz, p-CC6F5), −166.6 (m, 1F, m-CC6F5),
−166.8 (m, 1F, m-CC6F5), −166.9 (m, 2F, m-BC6F5), −179.0 (br, 1F,
BF). 31P{1H} NMR (162 MHz, CD2Cl2, 298 K): δ 33.1 (br). 11B NMR
(128 MHz, CD2Cl2, 298 K): δ 0.68 (d, 1JB−F = 52.3 Hz, BF). 13C{1H}
NMR (101 MHz, CD2Cl2, 298 K): δ partial: 149.8 (Ph2C), 148.9
(dm, 1JC−F = 251 Hz, CF), 148.2 (dm, 1JC−F = 247 Hz, CF), 145.9 (dm,
1JC−F = 244 Hz, CF), 146.3 (i-Ph), 144.3 (ipso-Ph), 143.8 (dm, 1JC−F
=
244 Hz, CF), 138.1 (dm, 1JC−F = 249 Hz, CF), 136.7 (dm, 1JC−F = 244
Hz, CF), 129.9 (o-Ph), 128.3 (m-Ph), 127.9 (o-Ph), 127.0 (m-Ph), 126.4
(p-Ph), 126.2 (p-Ph), 123.6 (br, ipso-BC6F5), 123.5 (tm, 2JC−F = 21.2 Hz,
ipso-CC6F5), 89.1 (dtm, 1JC−P = 73.6 Hz, 2JC−F = 17.8 Hz, i-PC6F4), 36.1
(d, 1JC−P = 31.3 Hz, tBu), 36.0 (d, 1JC−P = 31.3 Hz, tBu), 27.7 (tBu), 27.6
(tBu). X-ray data: a = 9.8745(6) Å, b = 18.2413(10) Å, c = 23.7744(13)
Å, V = 4282.3(4) Å3, Z = 4, orthorhombic, space group P212121, 9855
observed reflections (I ≥ 2σ(I)), 565 refined parameters, R1 = 0.0728,
wR2(all) = 0.2161, GOF = 1.037. 8g: 70 °C for 24 h, 64% yield. Anal.
Calcd (%) for C44H33BF15P: C 59.48, H 3.74. Found: C 59.51, H 3.98.
1H NMR (400 MHz, d8-THF, 298 K): δ 7.41 (d, 2H, 3JH−H = 7.5 Hz,
o-Ph), 7.13 (d, 2H, 3JH−H = 7.6 Hz, o-Ph), 7.06 (t, 2H, 3JH−H = 7.6 Hz,
m-Ph), 7.03 (d, 1H, 1JH−P = 496 Hz, PH), 7.00−6.89 (m, 2H, p-Ph, p-Ph),
Synthesis of [tBu3PH][PhCH2CH(C6F5)BH(C6F5)2], 9c. In a
glovebox, a 50 mL glass tube equipped with a small stir bar and a
Teflon screw top was charged with a solution of (E)- and (Z)-4c (30:1)
(77 mg, 0.12 mmol) and tBu3P (25 mg, 0.12 mmol) in toluene (1 mL).
The yellow solution was degassed three times through a freeze−pump−
thaw cycle on the vacuum/H2 line and filled with H2 (4 bar) at −196 °C.
After the addition of H2 the reaction tube was allowed to mix at 80 °C for
15 h. The product was precipitated from solution by adding pentane
(20 mL) dropwise. The crude product was dried under vacuum and
shown to be a mixture of 5c, 9c, and 10c in a 4:9:2 ratio. Crystals of 9c
suitable for X-ray diffraction were grown from a layered C6D5Br/hexane
solution at 25 °C and isolated in ca. 30% yield. Anal. Calcd (%) for
3
6.97 (t, 2H, JH−H = 7.5 Hz, m-Ph), 3.00 (br, 2H, Cy), 2.17 − 1.34
(br m, 20H, Cy). 19F NMR (377 MHz, d8-THF, 298 K): δ −124.5 (br, 2F,
C6F4), −132.3 (br, 2F, o-BC6F5), −132.9 (br d, 2F, C6F4), −138.4 (m, 1F,
3
o-CC6F4), −139.0 (br, 1F, o-CC6F4), −162.0 (t, 1F, JF−F = 20.2 Hz,
p-C6F5), −162.3 (t, 1F, 3JF−F = 21.5 Hz, p-C6F5), −166.8 to −167.0 (m,
4F, m-BC6F5, m-CC6F5), −179.8 (br, 1F, BF). 31P{1H} NMR (162 MHz,
5645
dx.doi.org/10.1021/om3006068 | Organometallics 2012, 31, 5638−5649

Organometallics
Article
C38H37BF15P: C 55.63; H 4.55. Found: C 55.17, H 4.73. 1H NMR (400
MHz, C6D5Br, 298 K): δ 7.37 (d, 2H, 3JH−H = 7.6 Hz, o-Ph), 7.10 (t, 2H,
3JH−H = 7.6 Hz, m-Ph), 6.95 (t, 1H, 3JH−H = 7.6 Hz, p-Ph), 4.67 (d, 1H,
1JH−P = 434 Hz, PH), 4.14 (br m, 1H, CHB), 3.43 (m, 2H, CH2Ph), 3.42
(br, 1H, BH), 1.07 (d, 27H, 3JH−P = 15.7 Hz; tBu). 19F NMR (377 MHz,
C6D5Br, 298 K): −131.1 (m, 2F, o-B(C6F5)2), −131.2 (m, 2F,
o-B(C6F5)2), −139.5 (m, 1F, o-C6F5), −142.1 (m, 1F, o-C6F5),
13C{1H} NMR (151 MHz, CD2Cl2, 298 K): δ 143.1 (CH), n.o. (CB),
38.0 (d, 1JPC = 26.8 Hz, tBuP), 34.7 (tBu), 30.32 (tBu), 30.31 (tBuP)
[C6F5 not listed]. 19F NMR (564 MHz, CD2Cl2, 298 K): δ −130.9 (br m,
3
4F, o), −165.0 (t, JFF = 20.3 Hz, 2F, p), −168.0 (m, 4F, m) (BC6F5),
−140.9 (br m, 2F, o), −164.9 (t 3JFF = 21.4 Hz, 1F, p), −167.0 (m, 2F, m)
(C6F5). 11B{1H} NMR (192 MHz, CD2Cl2, 298 K): δ −18.8 (ν1/2 ≈ 50
Hz). 11B NMR (192 MHz, CD2Cl2, 298 K): δ −18.8 (d,
1JBH = 91.4 Hz). 31P{1H} NMR (202 MHz, CD2Cl2, 298 K): δ 59.6
3
3
−162.5 (t, 1F, JF−F = 20.9 Hz, p-B(C6F5)2), −162.7 (t, 1F, JF−F
=
(ν1/2 ≈ 3 Hz). 31P NMR (202 MHz, CD2Cl2, 298 K): δ 59.6 (dm, 1JPH
≈
21.1 Hz, p-B(C6F5)2), −163.7 (t, 1F, 3JF−F = 22.0 Hz, p-C6F5), −165.3
(m, 1F, m-B(C6F5)2), −165.5 (m, 1F, m-B(C6F5)2), −165.8 (m, 1F, m-
C6F5), −166.2 (m, 1F, m-BC6F5). 31P{1H} NMR (162 MHz, C6D5Br,
298 K): δ 58.5. 11B NMR (128 MHz, C6D5Br, 298 K): δ −18.7 (br, BH).
13C{1H} NMR (101 MHz, C6D5Br, 298 K) partial: δ 145.2 (ipso-Ph),
433 Hz). X-ray data: a = 10.2586(3) Å, b = 12.1826(5) Å, c = 16.1736(5)
Å, α = 89.894(3)°, β = 77.848(2)°, γ = 71.794(4)°, V = 1872.7(1) Å3,
Z = 2, triclinic, space group P1, 5818 observed reflections (I ≥ 2σ(I)),
̅
496 refined parameters, R1 = 0.0729, wR2(all) = 0.2201, GOF = 1.152.
Synthesis of [tBu3PH][Et2CC(C6F5)BH(C6F5)2], 10e. Com-
pound 4e (28 mg, 0.047 mmol) and tBu3P (9 mg, 0.047 mmol) were
dissolved in C6D5Br (0.5 mL) in a Teflon cap sealed J-Young tube. The
brown solution was degassed three times through a freeze−pump−thaw
cycle on the vacuum/H2 line and filled with H2 (4 bar) at −196 °C. After
the addition of H2 the reaction tube was allowed to sit in an 80 °C oil
bath for 16 h. To isolate the salt, the reaction mixture was layered with
hexane and placed in −40 °C freezer overnight to yield 98% of a white
precipitate. Crystals suitable for X-ray diffraction were grown from a
layered dichloromethane/hexane solution at −40 °C. Anal. Calcd (%)
for C32H29BF15P: C 54.15, H 4.92. Found: C 54.62, H 4.92. 1H NMR
(400 MHz, C6D5Br, 298 K): δ 4.73 (d, 1H, 1JH−P = 436 Hz, PH), 4.00
128.5 (o-Ph), 128.1 (m-Ph), 125.3 (p-Ph), 39.4 (CH2Ph), 36.7 (1JC−P
=
27.1 Hz, tBu), 33.4 (br, CHB), 29.3 (tBu). X-ray data: a = 37.440(3) Å,
b = 9.7274(6) Å, c = 24.1227(17) Å, β = 120.714(3)°, V = 7553.0(9) Å3,
Z = 8, monoclinic, space group C2/c, 6652 observed reflections (I ≥
2σ(I)), 509 refined parameters, R1 = 0.0528, wR2(all) = 0.1413, GOF =
1013.
Synthesis of [tBu3PH][Ph(H)CC(C6F5)BH(C6F5)2], 10c. In a
glovebox, a 50 mL glass tube equipped with a small stir bar and a Teflon
screw top was charged with a solution of (E)- and (Z)-4c (30:1) (77 mg,
0.12 mmol) and tBu3P (25 mg, 0.12 mmol) in pentane (40 mL). The
reaction flask was immediately covered with aluminum foil. The bright
yellow solution was degassed three times through a freeze−pump−thaw
cycle on the vacuum/H2 line and filled with H2 (4 bar) at −196 °C. After
the addition of H2 the reaction tube was allowed to mix at 25 °C for
3 days. A white precipitate was observed in the reaction tube as the
solution gradually became colorless. The E isomer of the product was
isolated in 86% yield. Anal. Calcd (%) for C38H37BF15P: C 55.63; H 4.55.
Found: C 55.17, H 4.73.
(br, 1H, BH), 2.48 (m, 2H, 3JH−H = 7.6 Hz, CH2), 2.03 (m, 2H, 3JH−H
=
7.3 Hz, CH2), 1.07 (d, 27H, 3JH−P = 16 Hz, tBu), 1.00 (t, 3H, 3JH−H = 7.6
3
Hz, CH3), 0.97 (t, 3H, JH−H = 7.3 Hz, CH3). 19F NMR (377 MHz,
C6D5Br, 298 K): δ −129.7 (br, 4F, o-B(C6F5)2, −139.2 (br, 2F, o-C6F5),
−163.4 (t, 1F, 3JF−F = 21.6 Hz, p-C6F5), −163.7 (t, 2F, 3JF−F = 20.5 Hz, p-
B(C6F5)2), −166.0 (m, 2F, m-C6F5), −166.4 (m, 4F, m-B(C6F5)2). 31
P
1
NMR (162 MHz, C6D5Br, 298 K): δ 57.7 (PH, JH−P = 436 Hz). 11B
NMR (128 MHz, C6D5Br, 298 K): δ −21.0 (br, BH). 13C {1H} NMR
(101 MHz, C6D5Br, 298 K) partial: δ 148.4 (dm, 1JC−F = 233 Hz, CF),
The same procedure above was repeated without covering the
reaction tube, yielding a white precipitate, which was isolated in 88%
yield and proved to be a mixture of the E and Z isomers.
1
1
148.3 (dm, JC−F = 236 Hz, CF), 146.2 (Et2C), 143.3 (dm, JC−F
=
Resonances common to both E and Z isomers: 1H NMR (400 MHz,
CD2Cl2, 298 K): δ 7.12 (m, 2H, m-Ph), 7.03 (m, 1H, p-Ph), 5.10 (d, 1H,
1JH−P = 431 Hz, PH), 1.67 (d, 27H, 3JH−P = 15.7 Hz, tBu). 31P{1H} NMR
(162 MHz, CD2Cl2, 298 K): δ 58.2. 13C{1H} NMR (101 MHz, CD2Cl2,
298 K): δ 148.2 (dm, 1JC−F = 234 Hz, CF), 147.7 (dm, 1JC−F = 234 Hz,
CF), 143.1 (dm, 1JC−F = 243 Hz, CF), 142.8 (dm, 1JC−F = 241 Hz, CF),
239 Hz, CF), 137.4 (dm, 1JC−F = 243 Hz, CF), 137.2 (dm, 1JC−F = 247
Hz, CF), 136.6 (dm, 1JC−F = 245 Hz, CF), 36.7 (d, 1JC−P = 27.1 Hz, tBu),
29.2 (tBu), 26.6 (CH2), 26.0 (CH2), 13.2 (CH3), 12.4 (CH3). X-ray
data: a = 8.8564(10) Å, b = 12.5788(15) Å, c = 17.423(2) Å, α =
81.339(5)°, β = 79.730(5)°, γ = 73.098(6)°, V = 1817.3(4) Å3, Z = 2,
triclinic, space group P1, 6402 observed reflections (I ≥ 2σ(I)), 489
̅
126.1 (ipso-B(C6F5)2), 123.8 (tm, 2JC−F = 23 Hz, i-C6F5), 37.6 (d, 1JC−P
26.8 Hz, tBu), 29.9 (tBu).
=
refined parameters, R1 = 0.0507, wR2(all) = 0.1169, GOF = 1.004.
Synthesis of [tBu3PH][(C3H7)2CC(C6F5)BH(C6F5)2], 10f. A
similar procedure to that mentioned above for the synthesis of
compound 9a was carried out by using the starting material 4f to
yield 10f as a white powder (63 mg, 76%). Crystals suitable for X-ray
crystal structure analysis were grown from a CH2Cl2/cyclopentane (v/v
= 1:5) solution of 10f at −35 °C. Anal. Calcd for C38H43BF15P: C, 55.22;
H, 5.24. Found: C, 54.76; H, 5.11. 1H NMR (600 MHz, CD2Cl2, 298 K):
δ 5.19 (d, 1JPH = 432.0 Hz, 1H, PH), 3.42 (br, 1:1:1:1 q, 1JBH ≈ 92 Hz,
E isomer: 1H NMR (400 MHz, CD2Cl2, 298 K): δ 7.49 (d, 2H, 3JH−H
=
7.74 Hz, o-Ph), 6.63 (s, 1H, C(H)), 3.90 (br q, 1H, 1JH−B = 90.8 Hz,
BH). 19F NMR (377 MHz, CD2Cl2, 298 K): δ −131.1 (m, 4F,
3
o-B(C6F5)2, −141.7 (m, 2F, o-C6F5), −163.7 (t, 1F, JF−F = 20.7 Hz,
3
p-C6F5), −164.5 (t, 2F, JF−F = 20.0 Hz, p-B(C6F5)2, −166.7 (m, 2F,
m-C6F5), −167.6 (m, 2F, m-B(C6F5)2. 11B NMR (128 MHz, CD2Cl2,
298 K): δ −21.9 (d, 1JB−H = 90.8 Hz, BH). 13C{1H} NMR (101 MHz,
CD2Cl2, 298 K): δ140.2 (ipso-Ph), 136.1 (Ph(H)C), 129.2 (o-Ph),
126.9 (m-Ph), 125.2 (p-Ph).
A
B
1H, BH), 2.15 (m, 2H, CH2 ), 1.76 (m, 2H, CH2 ), 1.65 (d, 3JPH
=
A
B
15.7 Hz, 27H, PtBu), 1.29 (m, 2H, CH2 ), 1.27 (m, 2H, CH2 ), 0.71 (t,
Z isomer: 1H NMR (400 MHz, CD2Cl2, 298 K): δ 6.92 (d, 2H, 3JH−H
= 7.74 Hz, o-Ph), 6.54 (s, 1H, C(H)), 3.35 (br q, 1H, 1JH−B = 90.8 Hz,
BH). 19F NMR (377 MHz, CD2Cl2, 298 K): δ −131.1 (m, 4F, o-
B(C6F5)2), −141.3 (m, 2F, o-C6F5), −163.3 (t, 1F, 3JF−F = 21.4 Hz, p-
C6F5), −164.3 (t, 2F, 3JF−F = 19.8 Hz, p-B(C6F5)2), −165.8 (m, 2F, m-
C6F5), −167.5 (m, 4F, m-B(C6F5)2). 11B NMR (128 MHz, CD2Cl2, 298
B
A
3JHH = 7.3 Hz, 3H, CH3 ), 0.70 (t, 3JHH = 7.3 Hz, 3H, CH3 ). 13C{1H}
NMR (151 MHz, CD2Cl2, 298 K): δ 144.2 (CPr), 132.6 (br, CB)1,
1
B
A
38.0 (d, JPC = 27.7 Hz, tBu), 36.5 (CH2 ), 36.0 (CH2 ), 30.3
B
A
A
B
(tBu), 21.6 (CH2 ), 21.3 (CH2 ), 14.7 (CH3 ), 14.5 (CH3 ) [C6F5 not
listed; 1 from GHMBC experiment]. 19F NMR (564 MHz, CD2Cl2, 298
K): δ −131.2 (br m, 4F, o), −165.4 (t, 3JFF = 20.3 Hz, 2F, p), −168.0 (m,
4F, m) (B C6F5), −140.5 (br m, 2F, o), −165.2 (t, 3JFF = 21.0 Hz, 1F, p),
−167.5 (m, 2F, m) (C6F5). 11B{1H} NMR (192 MHz, CD2Cl2, 298 K):
δ −21.6 (ν1/2 ≈ 50 Hz). 11B NMR (192 MHz, CD2Cl2, 298 K): δ −21.6
(d, 1JBH ≈ 91 Hz). 31P{1H} NMR (242 MHz, CD2Cl2, 298 K): δ 59.6
1
K): δ −19.4 (d, JB−H = 90.8 Hz, BH);. 13C{1H} NMR (101 MHz,
CD2Cl2, 298 K): δ 140.3 (ipso-Ph), 133.4 (Ph(H)C), 128.2 (m-Ph),
127.8 (o-Ph), 125.8 (p-Ph).
Synthesis of [tBu3PH][tBu(H)CC(C6F5)BH(C6F5)2], 10d. A
similar procedure to that mentioned above for compound 9a was
carried out by using starting material 4d to yield 10d as a white powder
(73 mg, 91%). Crystals suitable for X-ray crystal structure analysis were
grown from a CH2Cl2/pentane (v/v = 1:3) solution after 12 d at −35 °C.
Anal. Calcd for C36H39BF15P + 3/4CH2Cl2: C, 51.20; H, 4.73. Found: C,
(ν1/2 ≈ 3 Hz). 31P NMR (242 MHz, CD2Cl2, 298 K): δ 59.6 (dm, 1JPH
≈
432 Hz). X-ray data: a = 13.2049(3) Å, b = 16.7532(4) Å, c = 17.8062(6)
Å, β = 98.306(2)°, V = 3897.85(18) Å3, Z = 4, monoclinic, space group
P21/n, 6021 observed reflections (I ≥ 2σ(I)), 513 refined parameters,
R1 = 0.0415, wR2(all) = 0.1103, GOF = 1.047.
Synthesis of [tBu3PH][Ph2CC(C6F5)BH(C6F5)2], 10g. A so-
lution of 4g (69 mg, 0.1 mmol) and tBu3P (20 mg, 0.1 mmol) was
dissolved in toluene (1 mL), and the reaction mixture was kept under a
1
51.07; H, 4.47. H NMR (600 MHz, CD2Cl2, 298 K): δ 5.40 (br s,
1H, CH), 5.17 (d, 1JPH = 431.3 Hz, 1H, PH), 3.00 (1:1:1:1 q, 1JBH
=
93.1 Hz, 1H, BH), 1.63 (d, 3JPH = 15.7 Hz, 27H, tBuP), 0.80 (s, 9H, tBu).
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Organometallics
Article
hydrogen atmosphere of 60 bar (by using an autoclave system) and
heated at 80 °C for a period of 24 h. To isolate the salt, the reaction
mixture was layered with heptane and placed in a −35 °C freezer
overnight to yield a pale brown precipitate. The supernatant was
decanted, and washing the residue with pentane twice (2 × 2 mL) and
drying in vacuo afforded 10g as a white powder (65 mg, 72%). Crystals
suitable for X-ray crystal structure analysis were grown from a CH2Cl2/
heptane (v/v = 1:4) solution of 10g at room temperature. Anal. Calcd
for C44H39BF15P: C, 59.08; H, 4.39. Found: C, 58.81; H, 4.36. 1H NMR
(600 MHz, CD2Cl2, 298 K): δ 7.29 (m, 2H, o-PhB), 7.04 (m, 4H,
m.o-PhA), 6.97 (m, 2H, m-PhB), 6.96 (m, 1H, p-PhA), 6.89 (m, 1H,
Synthesis of [N(CH2CH2)3NH][tBu(H)CC(C6F5)BH(C6F5)2],
12d. A solution of 4d (59 mg, 0.1 mmol) and 1,4-diazabicyclo[2.2.2]-
octane (DABCO, 11 mg, 0.1 mmol) was dissolved in toluene (2 mL),
and the mixture was stirred 24 h under a hydrogen atmosphere of 60 bar
(by using the autoclave system). Pentane (10 mL) was added, after
which the supernatant was decanted. The residue was then dried in vacuo
1
to afford 12d as a white powder (61 mg, 86%). H NMR (500 MHz,
THF-d8, 298 K): δ 8.95 (s, 1H, NH), 5.43 (s, 1H, CH), n.o. (BH),
3.03 (s, 12H, CH2), 0.80 (s, 9H, tBu). 13C{1H} NMR (126 MHz, THF-
d8, 298 K): δ 143.1 (CH), n.o. (CB), 46.3 (CH2), 35.9 (tBu), 30.6
(tBu) [C6F5 not listed]. 19F NMR (470 MHz, THF-d8, 298 K): δ −130.5
3
p-PhB), 5.11 (d, 1JPH = 429.6 Hz, 1H, PH), 3.62 (br, 1:1:1:1 q, 1JHB
≈
(br m, 4F, o), −166.6 (t, JFF = 20.1 Hz, 2F, p), −169.3 (m, 4F, m)
(B C6F5), −140.6 (m, 2F, o), −166.4 (t, 3JFF = 20.8 Hz, 1F, p), −168.4
(m, 2F, m) (C6F5). 11B{1H} NMR (160 MHz, THF-d8, 298 K): δ −18.7
(ν1/2 ≈ 50 Hz). 11B NMR (160 MHz, THF-d8, 298 K): δ −18.7
(d, 1JBH ≈ 92 Hz).
93 Hz, 1H, BH), 1.65 (d, 3JPH = 15.9 Hz, 27H, tBu). 13C{1H} NMR (151
MHz, CD2Cl2, 298 K): n.o. (CB), 147.2 (i-PhA), 145.4 (CPh),
145.0 (i-PhB), 129.8 (o-PhB), 128.8 (o-PhA), 127.7 (m-PhA), 126.8
(m-PhB), 125.7 (p-PhA), 125.4 (p-PhB), 38.1 (d, JPC = 26.8 Hz, tBu),
1
30.4 (tBu) [C6F5 not listed]. 19F{1H} NMR (564 MHz, CD2Cl2, 298 K):
δ −130.7 (br m, 4F, o), −165.3 (t, 3JFF = 20.2 Hz, 2F, p), −168.3 (m, 4F,
m) (BC6F5), −140.0 (m, 2F, o), −163.8 (t, 3JFF = 21.1 Hz, 1F, p), −167.3
(m, 2F, m) (C6F5). 11B{1H} NMR (192 MHz, CD2Cl2, 298 K): δ −21.5
(ν1/2 ≈ 50 Hz). 11B NMR (192 MHz, CD2Cl2, 298 K): δ −21.5
(d, 1JBH ≈ 91 Hz). 31P{1H} NMR (243 MHz, CD2Cl2, 298 K): δ 60.0
Synthesis of [N(CH2CH2)3NH][(C3H7)2CC(C6F5)BH(C6F5)2],
12f. A solution of 4f (62 mg, 0.1 mmol) and 1,4-diazabicyclo[2.2.2]-
octane (DABCO, 11 mg, 0.1 mmol) was dissolved in toluene (2 mL),
and the mixture was stirred 24 h under a hydrogen atmosphere of 60 bar
(by using the autoclave system). Pentane (10 mL) was added, after
which the supernatant was decanted. The residue was then dried in vacuo
1
to afford 12f as a white powder (59 mg, 79%). H NMR (500 MHz,
(ν1/2 ≈ 4 Hz). 31P NMR (243 MHz, CD2Cl2, 298 K): δ 60.0 (dm, 1JPH
≈
CD2Cl2, 298 K): δ 10.75 (br, 1H, NH), n.o. (BH), 3.14 (s, 12H, CH2),
430 Hz). X-ray data: a = 13.3052(2) Å, b = 15.4057(4) Å, c = 20.3224(5)
Å, V = 4165.60(16) Å3, Z = 4, orthorhombic, space group P212121, 6975
observed reflections (I ≥ 2σ(I)), 567 refined parameters, R1 = 0.0352,
wR2(all) = 0.0858, GOF = 1.052.
A
B
A
B
2.09 (m, CH2 ), 1.78 (m, CH2 ), 1.29 (m, CH2 ), 1.25 (m, CH2 ),
B
A
0.71 (t, 3JHH = 7.4 Hz, 3H, CH3 ), 0.67 (t, 3JHH = 7.3 Hz, 3H, CH3 ). 13
C
NMR (126 MHz, CD2Cl2, 298 K): δ 144.4 (CPr), 132.1 (br, CB)1,
B
A
B
A
45.3 (CH2), 36.4 (CH2 ), 36.2 (CH2 ), 21.7 (CH2 ), 21.2 (CH2 ),
Synthesis of [tBu3PH][(MeC6H4)2CC(C6F5)BH(C6F5)2], 10h. A
similar procedure to that mentioned above for the preparation of
compound 10g was carried out using the starting material 4h to yield
10h as a white powder (76 mg, 81%). Anal. Calcd for C46H43BF15P: C,
59.88; H, 4.70. Found: C, 59.59; H, 4.41. 1H NMR (600 MHz, CD2Cl2,
298 K): δ 7.13 (d, 3JHH = 7.7 Hz, 2H, o-tolA), 6.90 (d, 3JHH = 8.0 Hz 2H,
o-tolB), 6.83 (d, 3JHH = 8.0 Hz, 2H, m-tolB), 6.77 (d, 3JHH = 7.7 Hz, 2H,
m-tolA), 5.15 (d, 1JPH = 430.7 Hz, 1H, PH), 3.57 (br, 1:1:1:1 q, 1JHB ≈ 92
Hz, 1H, BH), 2.17 (s, 3H, CH3 tolB), 2.16 (s, 3H, CH3 tolA), 1.65 (d,
3JPH = 15.8 Hz, 27H, tBu). 13C{1H} NMR (151 MHz, CD2Cl2, 298 K): δ
145.4 (Ctol), n.o. (CB), 144.4 (i-tolB), 142.4 (i-tolA), 135.1 (p-tolB),
134.9 (p-tolA), 129.7 (o-tolA), 128.7 (o-tolB), 128.3 (m-tolB), 127.3 (m-
A
B
1
14.8 (CH3 ), 14.5 (CH3 ) [C6F5 not listed;
from GHMBC
experiment]. 19F NMR (564 MHz, CD2Cl2, 298 K): δ −131.8 (br m,
4F, o), −164.4 (t, 3JFF = 20.2 Hz, 2F, p), −167.3 (m, 4F, m) (BC6F5),
−140.1 (br, 2F, o), −164.3 (t, 3JFF = 21.1 Hz, 1F, p), −167.0 (m, 2F, m)
(C6F5). 11B{1H} NMR (192 MHz, CD2Cl2, 298 K): δ −21.6 (ν1/2 ≈ 50
Hz). 11B NMR (192 MHz, CD2Cl2, 298 K): δ −21.6 (d, 1JBH ≈ 88 Hz).
General Procedure for Catalytic Hydrogenation Reactions.
Imines. (a) Reactions at 5 bar of H2: In a glovebox, a 100 mL glass bomb
equipped with a small stir bar and Teflon screw top was charged with
imine (1 mmol), catalyst (0.05 mmol), and dry toluene (2.5 mL). The
reaction bomb was degassed three times through a freeze−pump−thaw
cycle on the vacuum/H2 line and filled with H2 (4 bar) at −196 °C. The
flask was then sealed and warmed to room temperature. The reaction
was placed in a preheated oil bath set to 120 °C (H2 pressure: ca. 5 bar).
Aliquots were withdrawn at 12 h intervals, and the reaction was
monitored by NMR spectroscopy. (b) Reactions at 110 bar of H2: In a
glovebox, a series 4560 mini benchtop Parr Instrument reactors was
equipped with six 20 mL vials each containing a small stir bar. The vials
were charged with the imine, catalyst, and toluene and loosely capped.
The reactor was assembled in the glovebox. The reactor was taken out of
the box and connected to a hydrogen line. The line was purged five times
with hydrogen from a gas purifier cartridge (Matheson type 452), and
then the reactor was purged five times before filling with dihydrogen
(behind a blast shield). The reactor was heated with an oil bath to
120 °C for 20 min to equilibrate, and then the H2 pressure was increased
to 110 bar. Aliquots were taken in a glovebox after venting and rapid
cooling. Identification of the amine products was by comparison of
1H NMR spectra with literature values.
1
B
tolA), 38.1 (d, JPC = 25.2 Hz, tBu), 30.4 (tBu), 21.1 (CH3 ), 20.9
A
(CH3 ) [C6F5 not listed]. 19F{1H} NMR (564 MHz, CD2Cl2, 298 K): δ
−130.7 (br m, 4F, o), −165.6 (t, 3JFF = 20.3 Hz, 2F, p), −168.4 (m, 4F,
m) (BC6F5), −140.1 (m, 2F, o), −164.1 (t, 3JFF = 21.2 Hz, 1F, p), −167.4
(m, 2F, m) (C6F5). 11B{1H} NMR (192 MHz, CD2Cl2, 298 K): δ −21.5
(ν1/2 ≈ 60 Hz). 11B NMR (192 MHz, CD2Cl2, 298 K): δ −21.5 (d, 1JBH
≈ 89 Hz). 31P{1H} NMR (243 MHz, CD2Cl2, 298 K): δ 59.8 (ν1/2 ≈ 5
Hz). 31P NMR (243 MHz, CD2Cl2, 298 K): δ 59.8 (dm, 1JPH ≈ 430 Hz).
Synthesis of [HC(CH2CH2)3NH][tBu(H)CC(C6F5)BH(C6F5)2],
11d. A solution of 4d (59 mg, 0.1 mmol) and quinuclidine (11 mg,
0.1 mmol) was dissolved in toluene (2 mL), and the mixture was stirred
24 h under a hydrogen atmosphere of 60 bar (by using the autoclave
system). Pentane (10 mL) was added, after which the supernatant was
decanted. The residue was then dried in vacuo to afford 11d as a white
powder (64 mg, 91%). Crystals suitable for X-ray crystal structure
analysis were grown from a CH2Cl2/pentane (v/v = 1:3) solution of 11d
at −35 °C. 1H NMR (500 MHz, C6D6, 298 K): δ 6.82 (br, 1H, NH),
5.90 (s, 1H, CH), 2.75 (br, 1:1:1:1 q, 1JBH ≈ 82 Hz, 1H, BH), 2.19 (m,
6H, NCH2), 1.03 (s, 9H, tBu), 0.84 (m, 1H, CH), 0.60 (m, 6H, CH2).
13C{1H} NMR (126 MHz, C6D6, 298 K): δ 145.1 (CH), n.o. (CB),
Enones. Alkenylborane (4) (0.025 mmol) and 1,4-
diazabicyclo[2.2.2]octane (2.8 mg, 0.025 mmol) were dissolved in
C6D6 (2 mL), enone (15) (0.5 mmol) was added, and the light yellow
solution was stirred for 48 h under a hydrogen atmosphere of 10 bar at
80 °C by using an autoclave system. The resulting solution was analyzed
by NMR spectroscopy before and after completion of the reaction. The
sample contains a mixture of enone (15) and ketone (16). Afterward the
reaction was quenched with 2 mL of pentane (normal pentane with
moisture), and then all of the solvent was removed under reduced
pressure. Pure ketone was isolated by silica gel chromatography.
46.9 (NCH2), 35.0 (tBu), 30.4 (tBu), 22.0 (CH2), 18.1 (CH) [C6F5 not
listed]. 19F NMR (470 MHz, C6D6, 298 K): δ −131.2 (br m, 4F, o),
−161.2 (t, 3JFF = 20.5 Hz, 2F, p), −165.5 (m, 4F, m) (B C6F5), −140.8
3
(br m, 2F, o), −161.6 (t, JFF = 21.4 Hz, 1F, p), −165.3 (m, 2F, m)
(C6F5). 11B{1H} NMR (160 MHz, C6D6, 298 K): δ −17.8 (ν1/2 ≈ 40
Hz). 11B NMR (160 MHz, C6D6, 298 K): δ −17.8 (d, 1JBH ≈ 81 Hz).
X-ray data: a = 9.5401(3) Å, b = 19.9821(6) Å, c = 17.8109(5) Å,
β = 97.067(2)°, V = 3369.52(17) Å3, Z = 4, monoclinic, space group
P21/c, 4846 observed reflections (I ≥ 2σ(I)), 503 refined parameters,
R1 = 0.0576, wR2(all) = 0.1690, GOF = 1.109.
X-ray Data Collection and Reduction. (Munster) Crystals were
̈
coated in FOMBLIN Y oil, mounted on a glass fiber, and placed under
an N2 stream, thus maintaining a dry, O2-free environment for each
crystal. The data were collected on Nonius Kappa CCD diffractometers,
5647
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Organometallics
Article
both with APEXII detectors. In the case of Mo radiation a rotating anode
generator equipped with Montel mirrors was used. The frames were
integrated with the DENZO-SMN software package26 including
absorption corrections27 using the empirical multiscan method.
(b) Erker, G. Dalton Trans. 2011, 40, 7475−7483. (c) Erker, G.
Organometallics 2011, 30, 358−368. (d) Erker, G. C. R. Chim. 2011, 14,
831−841. (e) Stephan, D. W. Chem. Commun. 2010, 46, 8526−8533.
(f) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46−76.
(g) Stephan, D. W. Dalton Trans. 2009, 3129−3136. (h) Stephan, D. W.
Org. Biomol. Chem 2008, 6, 1535−1539.
(2) (a) Ullrich, M.; Lough, A. J.; Stephan, D. W. Organometallics 2010,
29, 3647−3654. (b) Erker, G. Dalton Trans. 2005, 1883−1890.
(c) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245−250.
(d) Massey, A. G.; Stone, F. G. A.; Park, A. J. P. Chem. Soc. London 1963,
212−213.
Structure Solution and Refinement. (Toronto) Non-hydrogen
atomic scattering factors were taken from the literature tabulations.28
The heavy atom positions were determined using direct methods em-
ploying the SHELXTL direct methods routine.29 The remaining non-
hydrogen atoms were located from successive difference Fourier map
calculations. The refinements were carried out by using full-matrix least-
squares techniques on F2, minimizing the function w(Fo − Fc)2 where
2
2
the weight w is defined as 4Fo /2σ(Fo ), where Fo and Fc are the ob-
served and calculated structure factor amplitudes, respectively. In the
final cycles of each refinement, all non-hydrogen atoms were assigned
anisotropic temperature factors in the absence of disorder or insufficient
data. In the latter cases atoms were treated isotropically. C−H atom
positions were calculated and allowed to ride on the carbon to which
they are bonded assuming a C−H bond length of 0.95 Å. H atom
temperature factors were fixed at 1.10 times the isotropic temperature
factor of the C atom to which they are bonded. The H atom
contributions were calculated, but not refined. The locations of the
largest peaks in the final difference Fourier map calculation as well as the
magnitude of the residual electron densities in each case were of no
chemical significance. Additional details are provided in the Supporting
Information.
(3) (a) Chapman, A. M.; Haddow, M. F.; Wass, D. F. J. Am. Chem. Soc.
2011, 133, 18463−18478. (b) Chapman, A. M.; Haddow, M. F.; Wass,
D. F. J. Am. Chem. Soc. 2011, 133, 8826−8829. (c) Appelt, C.;
Westenberg, H.; Bertini, F.; Ehlers, A. W.; Slootweg, J. C.; Lammertsma,
́
K.; Uhl, W. Angew. Chem., Int. Ed. 2011, 50, 3925−3928. (d) Menard,
G.; Stephan, D. W. Angew. Chem., Int. Ed. 2011, 50, 8396−8399.
(e) Schafer, A.; Reismann, M.; Schafer, A.; Saak, W.; Haase, D.; Muller,
́
T. Angew. Chem., Int. Ed. 2011, 50, 12636−12638. (f) Menard, G.;
Stephan, D. W. J. Am. Chem. Soc. 2010, 132, 1796−1797. (g) Frey, G. D.;
Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2007,
316, 439−441.
(4) (a) Harhausen, R.; Frohlich, R.; Kehr, G.; Erker, G. Organometallics
̈
2012, 31, 2810−2809. (b) Bertini, F.; Lyaskovskyy, V.; Timmer, B. J. J.;
De Kanter, F. J. J.; Lutz, M. E., A. W.; Slootweg, J. C.; Lammertsma, K.
Structure Solution and Refinement. (Munster). Non-hydrogen
̈
J. Am. Chem. Soc. 2012, 134, 201−204. (c) Stute, A.; Kehr, G.; Frohlich,
̈
atomic scattering factors were taken from the literature tabulations.28
The heavy atom positions were determined using direct or Patterson
methods employing the SHELXS routine.29 The remaining non-
hydrogen atoms were located from successive difference Fourier map
calculations. The refinements were carried out by using full-matrix least-
squares techniques on F2 employing the SHELXL routine. In the final
cycles of each refinement, all non-hydrogen atoms were assigned
anisotropic temperature factors in the absence of disorder or insufficient
data. In the latter cases atoms were treated isotropically. C−H atom
positions were calculated and allowed to ride on the carbon to which
they are bonded assuming C−H bond lengths between 0.94 and 0.99 Å
depending on the type of the carbon atom. H atom temperature factors
were fixed at 1.20 or 1.50 times the isotropic temperature factor of the C
atom to which they are bonded. The locations of the largest peaks in the
final difference Fourier map calculation as well as the magnitude of the
residual electron densities in each case were of no chemical significance.
Additional data for the structural studies are deposited.
R.; Erker, G. Chem. Commun. 2011, 47, 4288−4290. (d) Rosorius, C.;
Kehr, G.; Frohlich, R.; Grimme, S.; Erker, G. Organometallics 2011, 30,
̈
4211−4219. (e) Chen, D. J.; Wang, Y. T.; Klankermayer, J. Angew.
Chem., Int. Ed. 2010, 49, 9475−9478. (f) Schwendemann, S.; Tumay,
T. A.; Axenov, K. V.; Peuser, I.; Kehr, G.; Frohlich, R.; Erker, G.
̈
Organometallics 2010, 29, 1067−1069. (g) Neu, R. C.; Ouyang, E. Y.;
Geier, S. J.; Zhao, X. X.; Ramos, A.; Stephan, D. W. Dalton Trans. 2010,
39, 4285−4294. (h) Ramos, A.; Lough, A. J.; Stephan, D. W. Chem.
Commun. 2009, 1118−1120. (i) Ullrich, M.; Lough, A. J.; Stephan, D.
W. J. Am. Chem. Soc. 2009, 131, 52−53. (j) Jiang, C. F.; Blacque, O.;
Berke, H. Organometallics 2009, 28, 5233−5239. (k) Geier, S. J.; Gilbert,
T. M.; Stephan, D. W. J. Am. Chem. Soc. 2008, 130, 12632−12633.
(l) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Frohlich, R.; Erker,
̈
G. Angew. Chem., Int. Ed. 2008, 47, 7543−7546. (m) Huber, D. P.; Kehr,
G.; Bergander, K.; Frohlich, R.; Erker, G.; Tanino, S.; Ohki, Y.; Tatsumi,
̈
K. Organometallics 2008, 27, 5279−5284. (n) Chen, D. J.;
Klankermayer, J. Chem. Commun. 2008, 2130−2131. (o) Chase, P. A.;
Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46,
8050−8053.
ASSOCIATED CONTENT
* Supporting Information
■
S
(5) (a) Eros, G.; Nagy, K.; Mehdi, H.; Pap
Tarkanyi, G.; Soos
́ ́ ́
́
ai, I.; Nagy, P.; Kiral
, T. Chem.Eur. J. 2012, 18, 574−585. (b) Lu, Z.;
Cheng, Z.; Chen, Z.; Weng, L.; Li, Z. H.; Wang, H. Angew. Chem., Int. Ed.
́
y, P.;
̈
Synthetic, experimental, and crystallographic details are
deposited. This material is available free of charge via the
Internet at http://pubs.acs.org.
2011, 12227−12231. (c) Schwendemann, S.; Frohlich, R.; Kehr, G.;
̈
Erker, G. Chem. Sci. 2011, 2, 1842−1849. (d) Jiang, C. F.; Blacque, O.;
Fox, T.; Berke, H. Organometallics 2011, 30, 2117−2124. (e) Sumerin,
V.; Chernichenko, K.; Nieger, M.; Leskela, M.; Rieger, B.; Repo, T. Adv.
Synth. Catal. 2011, 353, 2093−2110. (f) Theuergarten, E.; Schluns, D.;
Grunenberg, J.; Daniliuc, C. G.; Jones, P. G.; Tamm, M. Chem. Commun.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dstephan@chem.utoronto.ca; erker@uni-muenster.de.
Notes
■
2010, 46, 8561−8563. (g) Eros, G.; Mehdi, H.; Pap
́
ai, I.; Rokob, T. A.;
Kiraly, P.; Tarkanyi, G.; Soos, T. Angew. Chem., Int. Ed. 2010, 49, 6559−
̈
The authors declare no competing financial interest.
́
́
́
́
6563. (h) Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476−
3477. (i) Geier, S. J.; Gille, A. L.; Gilbert, T. M.; Stephan, D. W. Inorg.
ACKNOWLEDGMENTS
■
Chem. 2009, 48, 10466−10474. (j) Axenov, K. V.; Kehr, G.; Frohlich, R.;
̈
Financial support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged (G.E.). J.S.R. thanks the Alexander von
Humboldt Foundation for a Fellowship. D.W.S. is grateful for the
financial support of NSERC of Canada and the award of a
Canada Research Chair.
Erker, G. Organometallics 2009, 28, 5148−5158. (k) Jiang, C. F.;
Blacque, O.; Berke, H. Chem. Commun. 2009, 5518−5520. (l) Chase, P.
A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008, 1701−1703.
(m) Sumerin, V.; Schulz, F.; Nieger, M.; Leskela, M.; Repo, T.; Rieger, B.
Angew. Chem., Int. Ed. 2008, 47, 6001−6003. (n) Sumerin, V.; Schulz, F.;
Atsumi, M.; Wang, C.; Nieger, M.; Leskela, M.; Repo, T.; Pyykko, P.;
Rieger, B. J. Am. Chem. Soc. 2008, 130, 14117−14118.
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