Asymmetric catalytic intramolecular hydroamination of aminoallenes by tantalum amidoalkoxide complexes

Article abstract of DOI:10.1021/om200446v

A series of tantalum complexes of chiral bidentate amidoalkoxide ligands was prepared. The crystal structure of one complex, Ta(NMe₂) ₃[-OCPh₂CH(CHMe₂)N(c-C₆H ₁₁)-] was obtained. Unlike previously described titanium complexes with these ligands, which are dimeric with bridging oxygen atoms, this tantalum complex is monomeric with an approximate trigonal-bipyramidal geometry. The resulting complexes were examined for their in situ activity for catalytic asymmetric hydroamination/cyclization of an aminoallene. Enantioselectivities up to 80% ee were observed for the cyclization of 6-methylhepta-4,5-dienylamine to 2-(2-methylpropenyl)pyrrolidine at 135 °C with 5 mol % catalyst loading.

Full text of DOI:10.1021/om200446v

ARTICLE
pubs.acs.org/Organometallics
Asymmetric Catalytic Intramolecular Hydroamination of
Aminoallenes by Tantalum Amidoalkoxide Complexes
Michelle C. Hansen,† Carolyn A. Heusser,† Tarun C. Narayan,† Kristine E. Fong,† Nagiko Hara,†
Alexander W. Kohn,† Alexander R. Venning,† Arnold L. Rheingold,‡ and Adam R. Johnson*,†
†Department of Chemistry, Harvey Mudd College, 301 Platt Boulevard, Claremont, California 91711, United States
‡Department of Chemistry, University of California, San Diego, 9500 Gilman Drive, MC 0358, La Jolla, California 92093, United States
S Supporting Information
b
ABSTRACT: A series of tantalum complexes of chiral bidentate ami-
doalkoxide ligands was prepared. The crystal structure of one complex,
Ta(NMe2)3[-OCPh2CH(CHMe2)N(c-C6H11)-] was obtained. Unlike
previously described titanium complexes with these ligands, which are
dimeric with bridging oxygen atoms, this tantalum complex is monomeric
with an approximate trigonal-bipyramidal geometry. The resulting com-
plexes were examined for their in situ activity for catalytic asymmetric
hydroamination/cyclization of an aminoallene. Enantioselectivities up to 80% ee were observed for the cyclization of 6-methyl-
hepta-4,5-dienylamine to 2-(2-methylpropenyl)pyrrolidine at 135 ꢀC with 5 mol % catalyst loading.
Scheme 1
’ INTRODUCTION
Hydroamination, the addition of an NꢀH bond across an
unsaturated CꢀC bond, is a highly atom economical method for
synthesizing nitrogen-containing compounds.1 Although the
reaction is generally thermoneutral or slightly exothermic, it
requires a catalyst due to the electronic repulsion between the
nitrogen and the unsaturated group. Both inter- and intramole-
cular variations of the reaction have been reported for the
hydroamination of olefins, alkynes, and allenes, and the field
has been extensively reviewed for a number of metal catalyst
systems including lanthanides,2 gold,3 early metals,4 and rhodium
and iridium.5 More recent reviews summarize the asymmetric
hydroamination reaction.6 There have also been several recent
reports of the hydroamination reaction (both inter- and intra-
molecular variants, with both alkyne and alkene substrates) in
protic media.7
Hydroamination of aminoallenes has been studied for almost
thirty years, and while initial reports used late metal catalysts such
as silver, mercury, and palladium,8 more recent work has focused
on titanium,9 lanthanide,10 and gold catalysts.11 The intramole-
cular hydroamination of 4,5-dienylamines (1, Scheme 1) can
proceed by two pathways. When the addition occurs at the
external double bond (path a), the reaction gives achiral tetra-
hydropyridine products (2). The corresponding addition to
the internal double bond (path b) yields nitrogen-containing
heterocycles with a pendant vinyl group that is available
for further substitution (3). The resulting pyrrolidine hetero-
cycles are chiral, and as such, there has been significant effort to
develop the asymmetric hydroamination reaction for both
aminoallene11b,c,12 and aminoalkene13 substrates.
in 2004 using chiral titanium amidoalkoxide complexes.14 The
dimeric complexes with bridging oxygen atoms15 can enter the
catalytic cycle proposed by Bergman16 by reacting with the
incoming substrate to form an imido complex (Scheme 2).
The imido can react via a [2+2] cycloaddition reaction to form
the corresponding azametallacyclobutane, and protonolysis by
additional substrate regenerates the catalytically active species.
The resulting pyrrolidine (3) was obtained in quantitative yield,
but the enantioselectivity of the reaction was low, up to 16% ee.
In an attempt to improve the enantioselectivity of the catalysis,
bulkier alkyl groups were added to the ligands, but these new
catalysts did not lead to higher enantioselectivity.17 We also
examined titanium sulfonamide alcohol complexes due to their
altered electronic properties, but again found no increase in
enantioselectivity.18
The first example of asymmetric intramolecular hydroamina-
tion of an allene by a titanium catalyst was reported by our group
Received: May 25, 2011
Published: August 08, 2011
r
2011 American Chemical Society
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|

Organometallics
ARTICLE
There have been some recent reports of using tantalum
catalysts for hydroamination19 and the closely related hydro-
aminoalkylation.20 Bergman initially studied stoichiometric
experiments with cationic tantalum complexes ([Bn2Tad
NCMe3]+), suggesting that the mechanism for intermolecular
alkyne hydroamination goes through the [2+2] cycloaddition
mechanism seen for Ti catalysts.19a A later, more thorough,
investigation showed that neither neutral nor cationic alkyl/
imido tantalum complexes were able to directly access a [2+2]
cycloaddition reaction manifold with alkynes.21 They hypothe-
sized that amido/imido tantalum complexes would be able to
access this pathway, however. To date, no detailed mechanistic
study for tantalum-catalyzed hydroamination has been carried
out. We investigated catalysis with tantalum complexes of our
sulfonamide alcohol ligands and found a significant increase in
enantioselectivity to a maximum of 34% ee.18 We recently
reported this as the first example of asymmetric hydroamination
using a group V metal catalyst. Two recent reports described the
use of V, Nb, and Ta complexes for the asymmetric hydroamina-
tion of alkenes.22 We sought to improve our results on the
asymmetric hydroamination of aminoallenes by investigating
catalysis using tantalum complexes of our previously reported
amidoalkoxide ligands.17
(isopropyl, cyclohexyl, or adamantyl), and the substituent in the
position R to oxygen (hydrogen, methyl, n-butyl, or phenyl)
(Chart 1). Of the 24 ligands in the series, we were originally
unable to synthesize one, L-H2VPP.17 However, during the
course of this study we found that it is possible to prepare this
ligand by using diethyl ether as solvent. The synthesis of
L-H2VPP was carried out according to the procedure outlined
in Scheme 3.
As reported previously,14,17 we have not seen evidence for the
racemization of the phenylalanine- or valine-derived ligands
during their synthesis. The addition of (S)-(+)-2,2,2-trifluoro-
1-(9-anthryl)ethanol to the ligands resulted in the shifting of the
ligand hydrogen signals due to the formation of transient
diastereomeric complexes, but we did not observe splitting of
the resonances into two sets of peaks, indicating that the ligands
were enantiomerically pure.23 However, when L-H2VPP was
examined by this procedure, it was found to be partially
racemized. Simulation using gNMR24 suggested that the ligand
was obtained in about a 75:25 ratio of enantiomers (50% ee). We
have had significant difficulties preparing this single ligand
derivative and include the results from the partially racemized
ligand in this report.
The opposite enantiomers of several ligands were also
prepared according to established procedures:17 D-H2PCP,
D-H2VCP, D-H2PPP, and D-H2PPM. Significant solvent effects
were observed when using phenyl Grignard reagents; in most
cases, yields improved when THF was replaced with diethyl ether
as the solvent, but in some cases, alkylation was achievable only
using THF. Details of the modified synthetic procedures are
included in the Experimental Section.
’ RESULTS AND DISCUSSION
We previously reported the synthesis of a library of bidentate
amino alcohols that could be readily prepared in a two-step
process from commercially available chiral amino acid deri-
vatives.17 The ligand abbreviation indicates, in order, the amino
acid derivative (valine or phenylalanine), the N-alkyl substituent
Tantalum complexes of the amidoalkoxide ligands were
synthesized by protonolysis reactions and liberation of HNMe2
(Scheme 4). 1H NMR spectroscopy (see Supporting Information
Scheme 2
Scheme 3a
a Reagents and conditions: (1) NaHCO3/acetone/NaBH(OAc)3, (2)
PhMgBr/ether.
Chart 1a
aLigands used in this study: iPr = isopropyl, c-C6H11 = cyclohexyl, 2-Ad = 2-adamantyl.
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Organometallics
ARTICLE
Scheme 4
Table 1. Crystal Data and Structure Refinement for
Complex 4
empirical formula
fw
C29H47N4OTa
648.66
temperature
wavelength
cryst syst
space group
a
123(2) K
Mo KR, 0.71073 Å
orthorhombic
P2(1)2(1)2(1)
9.2269(5) Å
16.2635(8) Å
19.5119(10) Å
2928.0(3) Å3
4
b
c
volume
Z
density (calcd)
absorp coeff
F(000)
1.471 g/cm3
3.781 mmꢀ1
1320
0.32 ꢁ 0.30 ꢁ 0.20 mm3
colorless block
cryst size
cryst color, habit
θ range for
data collection
index ranges
1.63ꢀ25.42ꢀ
ꢀ11 e h e 11, ꢀ19 e k e 17,
ꢀ23 e l e 13
reflns collected
indep reflns
14 424
5384 [R(int) = 0.0215]
100.0%
completeness
to theta = 25.00ꢀ
absorp corr
multiscan
refinement method
full-matrix
least-squares on F2
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
5384/0/324
0.949
R1 = 0.0156, wR2 = 0.0356
R1 = 0.0165, wR2 = 0.0358
0.507 and ꢀ0.260 e Åꢀ3
Figure 1. ORTEP of complex 4 showing 50% probability ellipsoids with
partial atom-numbering scheme. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Ta(1)ꢀN(1)
1.958(2), Ta(1)ꢀN(2) 1.991(2), Ta(1)ꢀN(3) 2.014(2), Ta(1)ꢀN(4)
2.026(2), Ta(1)ꢀO(1) 1.9963(19), N(1)ꢀTa(1)ꢀN(2) 96.20(10),
N(1)ꢀTa(1)ꢀN(3) 110.14(10), N(1)ꢀTa(1)ꢀN(4) 114.06(10), N-
(1)ꢀTa(1)ꢀO(1) 102.96(9), N(2)ꢀTa(1)ꢀN(3) 93.76(10), N(2)ꢀTa
(1)ꢀN(4) 90.70(9), N(2)ꢀTa(1)ꢀO(1) 159.99(9), N(3)ꢀTa(1)ꢀN-
(4) 134.76(9), N(3)ꢀTa(1)ꢀO(1) 85.11(8), N(4)ꢀTa(1)ꢀO(1)
76.33(8).
largest diff peak and hole
pyramidal with apical N(1), Rc(x) = 17.3%,25 as the basal ligands
exhibit significant distortions from the ideal geometry. The
N4ꢀTa1ꢀO1 angle exhibits a large deviation from the ideal
trigonal-bipyramidal geometry due to the reduced bite angle of
the chelate ring. The equatorial plane bond angles range from
110ꢀ to 134ꢀ. The sum of bond angles about each of the four
nitrogen atoms is just under 360ꢀ, as expected for a strongly
π-acidic metal center.
The TaꢀNMe2 bond distances are 1.958(2), 1.991(2), and
2.014(2) Å; the TaꢀN(ligand) distance is 2.026(2) Å, while the
TaꢀO distance is 1.9963(19) Å. These bond distances are nearly
identical to those found in Ta(NMe2)5,26 with TaꢀN distances
from 1.965(5) to 2.038(8) Å. The shortest bond distance in
Ta(NMe2)5 is that of the apical nitrogen in its distorted square-
pyramidal structure.
for spectra and assignments) supports formation of the com-
plexes due to shifted ligand resonances. Dimethylamine was
also observed, indicating that protonolysis of the starting
material by the incoming ligand took place. Almost all the
tantalum complexes were oils that could not be purified.
However, the reaction of Ta(NMe2)5 with L-H2VCP in diethyl
ether resulted in a solid compound, Ta(NMe2)3(VCP), 4.
Concentrating the solution by evaporation followed by storage
at ꢀ35 ꢀC resulted in the formation of crystals suitable for an
X-ray crystallographic study. An ORTEP diagram and partial
atom-numbering scheme of complex 4 is shown in Figure 1;
crystallographic data collection parameters and refinement
statistics are listed in Table 1.
The Cambridge Structural Database does not contain any
tantalum complexes with a bidentate amidoalkoxide ligand.27
Rothwell reported several square-pyramidal complexes of the
general form Ta(NMe2)4(OAr);28 each has a basal phenoxide
ligand. TaꢀN and TaꢀO bond distances for these complexes are
similar to those reported for complex 4.
The complex shown in Figure 1 adopts a slightly distorted
trigonal-bipyramid structure, Rc(x) = 9.8%,25 with O(1) and
N(2) in the axial positions and N(1), N(3), and N(4) in the
equatorial positions. The complex is less well described as square
The tantalum complexes of the bidentate amidoalkoxide
ligands were prepared in situ and examined for catalytic activity
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Organometallics
ARTICLE
Table 2. Hydroamination of 6-Methylhepta-4,5-dienylamine
at 135 ꢀC with Tantalum Amidoalkoxide Catalysts (5 mol %)
entry ligand ee/%a conv/%b t/h entry ligand ee/%a conv/%b t/h
entries 10, 11, 23, 24, 5, and 6; these contained phenyl groups Rto the
oxygen and an N-cyclohexyl substituent (entries 10, 11, 23, and 24) or
an N-isopropyl substituent (entries 5 and 6).
Since the enantioselectivity of the catalysis was consistently
high for the L-VCP ligand (entry 23), we sought to determine the
absolute stereochemistry of the pyrrolidine product. A reaction
using this ligand was quenched with benzyl bromide and
triethylamine to form the N-benzyl derivative; analysis of the
derivative by polarimetry showed it to also be the (ꢀ)-enantio-
mer. A number of closely related pyrrolidines consistently give a
negative optical rotation for the N-benzyl-(S)-enantiomer,29
which suggests that we are also favoring the formation of the
(S)-enantiomer of the pyrrolidine.
1
2
3
4
5
6
7
8
9
L-PPH
L-PPM
D-PPM 53c
13
46
24
100
100
100
100
100
35
69 16 L-VPH
15 17 L-VPM
20 18 L-VPB
10
2
78
100
39
162
23
24
98
L-PPB
1
23 19 L-VPPd 29
100
64
15
L-PPP
65
18 20 L-VCH
21 21 L-VCM
50 22 L-VCB
65 23 L-VCP
7
3
286
46
D-PPP 58c
L-PCH 13
L-PCM 35
90
8
100
100
100
65
43
100
100
100
100
33
74
18
L-PCB
32
80
23 24 D-VCP 79c
15
1H NMR spectroscopy of the precatalyst solutions showed
that there was usually either a slight excess of the free ligand or
Ta(NMe2)5. We carried out several experiments to determine
the effects (if any) of carrying out our catalytic reactions in situ.
First, a control experiment was carried out using Ta(NMe2)5
with no added chiral ligand (entry 29). Ta(NMe2)5 does catalyze
the reaction (albeit slowly, and with essentially no en-
antioselectivity), so it is possible that some of the reactions with
slow conversion or low enantioselectivity may in fact be due to
catalysis by the tantalum precursor complex instead of the
amidoalkoxide complex.
10 L-PCP
11 D-PCP 76c
16 25 L-VAH
15 26 L-VAM
116 27 L-VAB
24 28 L-VAP
115 29 none
230
2
3
336
334
134
42
44
12 L-PAH
13 L-PAM
14 L-PAB
15 L-PAP
6
2
6
100
91
100
100
28
37
3
24
100
525
25
a Determined by GC using Chiraldex B-DM ((5%). b Determined by
1H NMR spectroscopy. c (+)-Enantiomer was favored. d Ligand ob-
tained in 50% ee.
We then carried out thermal stability studies of the precatalyst
solutions to look for ligand redistribution or decomposition.
Approximately half of our precatalyst solutions were heated at
135 ꢀC for extended periods (>18 h), but no significant changes
in the hydroamination/cyclization of 6-methylhepta-4,5-dieny-
lamine to 2-(2-methylpropenyl)pyrrolidine. All catalytic reac-
tions were carried out at 135 ꢀC in C6D6 in J. Young NMR tubes
with 5 mol % catalyst loading. Reactions were monitored
periodically by 1H NMR spectroscopy and were quenched when
complete or when no further progress was observed after 24 h.
Approximately half of the reactions were complete in under 24 h,
although reaction times varied greatly (from 15 to more than
300 h), and not every reaction went to 100% conversion.
Enantiomeric excesses of the benzamide derivatives were deter-
mined by chiral GC-MS as reported previously.14 Results of the
catalytic study are given in Table 2.
Run-to-run repeatability of enantioselectivity values is (5%
ee, which is also approximately the error with repeat injections of
the same sample. A sample that had been stored for one year was
reinjected and found to have an enantioselectivity within error of
the previous value, indicating that the samples do not decompose
or racemize over time. Sample GC traces are included in the
Supporting Information.
Several trends can be observed from the data. Pairs of ligand
enantiomers give approximately equal and opposite stereoselec-
tivity (entries 2, 3, 5, 6, 10, 11, 23, and 24), with D-enantiomers of
the ligands favoring the formation of the (+)-enantiomer of the
pyrrolidine product. Ligands containing primary alcohol groups
tend to have extremely slow and/or low conversion to product
(entries 1, 7, 12, 16, 20, and 25). None of the catalysts with a
primary alcohol substituent achieved 100% conversion to
product, and in most cases, the reaction stopped near 50%
conversion. Ligands that had an N-adamantyl substituent tended
to give extremely poor enantioselectivity (<6% ee) of the
pyrrolidine (entries 12, 13, 25, 26, and 27), though several
(entries 14, 15, and 28) produced the pyrrolidine product with
moderate enantioselectivity (24ꢀ37% ee). Ligands that contain
phenyl substituents R to the oxygen tended to give high
enantioselectivity for the formation of the pyrrolidine product
(entries 5, 6, 10, 11, 15, 19, 23, 24, and 28). The six ligands that
gave the highest enantioselectivity were (in decreasing order of % ee)
1
to the H NMR spectra were observed over this time period;
importantly, we did not notice formation of either Ta(NMe2)5 or
free ligand.
Next, a more detailed study was carried out to determine the
effect of either a deficiency or excess of free ligand on the catalytic
reaction and enantioselectivity. About half of the ligands were
examined for catalysis with ligand to metal ratios that varied from
0.6:1 to 2:1. Changing the ratio of ligand to metal does not in
most cases significantly change the enantioselectivity of the
reaction (see Supporting Information for details). In two cases,
decreasing the ligand to metal ratio below 1:1 showed a change in
enantioselectivity on the order of 20% ee (L-PPB increased to
24% ee from 1% ee, and L-VCB increased to 20% ee from 8% ee);
the ligands L-PPP, D-PPP, L-PAH, L-VPB, L-VAM, and L-VAB all
showed slight or no decrease in enantioselectivity with reduced
L:M ratios. Increasing the L:M ratio above 1:1 significantly
changed the observed enantioselectivity for three ligands
(L-PPH decreased to 2% ee from 13% ee, L-PPP decreased to
21% ee from 65% ee, and L-PAP increased to 50% ee from 25%
ee); the ligands D-PPP, L-VPM, and L-VAH did not exhibit
significant changes in selectivity. No obvious trends are present
in these data.
Finally, experiments were carried out to investigate any
differences between isolated and in situ prepared complexes.
The complexes derived from L-VCP, L-PPP, L-VAM, and L-PPH
were prepared on a small scale (30ꢀ50 mg) in diethyl ether, and
residual solvent and dimethylamine were removed in vacuo.
These ligands were chosen to span a wide range of observed
enantioselectivities. As the complexes (even complex 4 on this
small scale) were isolated as oils, they were used as obtained in a
catalytic study; the results are presented in Table 3. The observed
enantioselectivities for the in situ and isolated complexes are
almost identical for L-VCP and L-PPP (entries 1ꢀ4), while for
L-VAM the isolated complex shows a slightly higher selectivity
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Organometallics
ARTICLE
Table 3. Comparison of Hydroamination with in Situ or
Isolated Tantalum Amidoalkoxide Catalysts
excesses than those we have previously observed with titanium,
there are reports of gold(I) phosphine complexes that catalyze
the intramolecular hydroamination of a number of N-tosylated
aminoallene substrates (including the N-tosylated derivative of
substrate 1) with enantioselectivity above 98% ee.11b,12 Similar
results have been seen for the asymmetric cyclization of other
aminoallenes with gem-dialkyl substituents.11c However, the
substrates for gold catalysis all contain protecting groups on
nitrogen, and no primary amine substrates were examined. Two
recent reports describe the tantalum-catalyzed intramolecular
hydroamination of closely related aminoalkene substrates that
led to cyclized pyrrolidine products with up to 80% ee.22 It is
worth noting, however, that these substrates include gem-
dialkyl substituents as opposed to the straight-chain aminoal-
lenes described here.
entry
ligand
L-VCP
ee/%a
conv/%b
t/h
1
2
3
4
5
6
7
8
isolated
in situc
isolated
in situc
isolated
in situc
isolated
in situc
68
74
63
65
12
3
100
100
38
16
18
L-PPP
L-VAM
L-PPH
39
100
83
18
133
335
135
69
44
8
28
13
24
a Determined by GC using Chiraldex B-DM ((5%). b Determined by
1H NMR spectroscopy. c Data from Table 2.
(entry 5) and for L-PPH the isolated complex shows a slightly
lower selectivity (entry 7). The results of this and the previous
experiments show that our system is robust to slight differences
in L:M ratio that are hard to avoid by our in situ method.
We then sought to determine why some catalytic reactions
’ CONCLUSIONS
Procedures for the synthesis of chiral bidentate amino alcohols
were optimized, allowing for the synthesis of a new sterically
bulky derivative of valine. A tantalum complex of one amido
alkoxide ligand was prepared and found to be monomeric by
X-ray crystallography. A series of tantalum complexes of these
ligands was investigated for the in situ catalytic asymmetric
hydroamination of aminoallenes. The L-enantiomers of the
ligands preferentially form (ꢀ)-2-(2-methylpropenyl)pyrroli-
dine, which we tentatively assign as the (S)-enantiomer. The
tantalum complexes catalyzed the reaction with significantly
higher enantioselectivity than their titanium counterparts. Li-
gands containing phenyl substituents R to the alcohol oxygen
tend to give the pyrrolidine product with high enantioselectivity.
Future work will be aimed at establishing the mechanism of this
transformation in order to better design our catalysts.
1
stalled or were more sluggish than others. There was some H
NMR evidence that suggested that trace amounts of THF might
be responsible for the reaction stalling, so we examined hydro-
amination with deliberately added solvents. Either THF or
diethyl ether were added to a catalytic reaction using the
tantalum complex of the D-PPP ligand. Addition of solvent either
at the start or at approximately 25% or 75% conversion showed
minimal changes in enantioselectivity and conversion and did not
noticeably slow the reaction progress.
We also examined the effect of temperature on enantioselec-
tivity by carrying out six reactions at 100 ꢀC. Catalysts derived
from L-PPH, L-PCH, and L-PAM did not react at this lower
temperature. Of the remaining three ligands, L-PPP, L-VPP, and
L-VPM, the time to reach 100% conversion was slower, but
enantioselectivities were within error except for the reaction
catalyzed by L-VPM, which gave a slightly higher selectivity of
14% ee (instead of 2% ee for the reaction at 135 ꢀC). Our
previous studies have also not shown a significant increase in
enantioselectivity of the reaction at reduced temperature.14,18
The tantalum catalysts with ligands derived from the L-amino
acids preferentially formed the (ꢀ)-enantiomer of the pyrroli-
dine, while titanium catalysts preferentially form the (+)-enan-
tiomer. The tantalum catalysts in almost all cases give the
pyrrolidine in higher enantiomeric excess than the corresponding
titanium catalysts;17 there are only two ligands that gave higher
enantioselectivity with titanium (L-PAH and L-VAB), though
even on titanium these two ligands gave low enantioselectivity
(<15% ee).
We considered that the monomeric structure of the tantalum
complexes, as opposed to the dimeric structure of the corre-
sponding titanium complexes,14 is important for the improved
enantioselectivity for hydroamination. We attempted solution
molecular weight determinations to verify the monomeric nature
of the tantalum complexes in solution,30 but the results have not
been reproducible due to precipitation of the complexes and
decomposition over the long time periods to reach equilibrium
for this measurement. We are continuing to examine the system
both experimentally and computationally in order to address this
hypothesis.
’ EXPERIMENTAL SECTION
General Procedures. All reagents were obtained from commercial
suppliers and were purified by standard methods31 or used as received.
Ligands were prepared by literature procedures.14,17 Solvents were
purified by vacuum transfer from sodium/benzophenone (C6D6) or
by passage through a column of activated alumina (Innovative Technol-
ogy PS-400-5-MD) and stored under nitrogen. Solutions of ligands
(ca. 0.05 M in C6D6) and 6-methylhepta-4,5-dienylamine (ca. 1.5 M in
C6D6) for catalysis were dried over molecular sieves overnight and
stored at ꢀ35 ꢀC. All air- and/or moisture-sensitive compounds were
manipulated under an atmosphere of nitrogen using standard Schlenk
techniques or in a glovebox (MBraun Unilab). 1H and 13C NMR spectra
were recorded at ambient temperature on a Br€uker Avance 400 spectro-
meter. Carbon assignments of complex 4 were made using DEPT
experiments. Polarimetry was carried out using a JASCO P1010 instru-
ment. GC analysis was carried out using a Hewlett-Packard 5890 Series
II gas chromatograph. Mass spectra were obtained at the Mass Spectro-
metry Laboratory, California Institute of Technology, Pasadena, CA.
Elemental analyses were performed by Columbia Analytical Services,
Tucson, AZ.
L-N-Isopropyldiphenylvalinol (L-H2VPP). L-N-Isopropylvaline methyl
ester (490 mg, 2.83 mmol) was dissolved in diethyl ether (20 mL) and
stirred under N2. The reaction was cooled to ꢀ78 ꢀC. Phenylmagnesium
bromide (3.8 mL, 3 M in diethyl ether, 11.4 mmol) was added dropwise.
The reaction was allowed to stir and warm to room temperature for
approximately 40 h. The reaction was then cooled to 0 ꢀC and quenched
by the addition of 1:1 saturated ammonium chloride/deionized
water (30 mL:30 mL), which gave a transparent, bright yellow solution.
Although the tantalum-catalyzed cyclizations reported here
result in pyrrolidine products with significantly higher enantiomeric
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Organometallics
ARTICLE
The aqueous layer was washed with diethyl ether (2 ꢁ 50 mL). The
combined organic layers were dried with anhydrous magnesium sulfate
and concentrated to yield a white powder. Some of the product was
purified by column chromatography (3% ethyl acetate, 0.5% triethyla-
mine, 96.5% hexanes). This was combined with microcrystals obtained
by slow cooling from 3:1 ethyl acetate/hexanes solution to yield a white,
Tris(dimethylamide)(L-VCP)tantalum (4). In the glovebox at room
temperature, tantalum pentakis(dimethylamide) (520 mg, 1.29 mmol)
and L-N-cyclohexyldiphenylvalinol (439 mg, 1.30 mmol) were dissolved
in diethyl ether (10 mL), which was allowed to slowly evaporate. When
approximately 2 mL of diethyl ether was remaining, the solution was
allowed to stand overnight and then stored at ꢀ35 ꢀC. The resulting
light yellow crystals were collected by vacuum filtration. The slow
evaporation procedure was repeated with the collected crystals to yield
1
microcrystalline powder (82.9 mg, 0.28 mmol, 10%). H NMR (400
MHz, CDCl3): δ 7.66 (dd, 2H, J = 1.2 Hz, 8.5 Hz, o-ArH), 7.55 (dd, 2H,
J = 1.2 Hz, 8.4 Hz, o-ArH), 7.25 (m, 4H, m-ArH), 7.17 (m, 2H, p-ArH),
5.75 (br s, 1H, NH), 3.64 (d, 1H, J = 1.8 Hz, CHiPr), 2.24 (m, 1H,
NCHMe2), 2.05 (m, 1H, CCHMe2), 1.03 (d, 3H, J = 7.0 Hz,
CCHMeMe0), 0.99 (d, 3H, J = 6.5 Hz, NCHMeMe0), 0.87 (d, 3H, J =
6.4 Hz, NCHMeMe0), 0.61 (d, 3H, J = 6.8 Hz, CCHMeMe0). 13C NMR
(100 MHz, CDCl3): δ 149.4, 146.1, 128.1, 126.5, 65.4, 47.6, 28.9, 24.2,
23.4, 16.2. HRMS (EI+): calcd for C20H28ON 298.2171, found
298.2144. [R]D = ꢀ37 (c 2.09 g/mL, EtOAc). Mp: 111ꢀ112 ꢀC.
Chiral Shift Study of L-H2VPP. A solution of L-H2VPP (0.050 M,
400 μL, 0.020 mmol) was examined by 1H NMR spectroscopy.
A solution of (S)-(+)-2,2,2-trifluoro-1-(9-anthryl)ethanol (0.18 M,
110 μL, 0.020 mmol) was added to the NMR tube, and the solution
was reexamined. Additional shift reagent (2 equiv total) was added,
and the solution was reexamined. A shoulder was observed on two of
the four isopropyl methyl doublets. Simulation of the methyl doub-
lets using gNMR suggested approximately 25% of the D-enantiomer
or an enantiomeric excess of 50% ee.
1
colorless crystals (466 mg, 0.72 mmol, 55%). H NMR (400 MHz,
C6D6): δ 7.70 (d, 2H, J = 7.2 Hz, ArH), 6.9ꢀ7.2 (m, 8H, ArH), 4.82 (m,
1H, NCHiPr), 3.23 (s, 18H, NMe2), 2.20 (m, 1H, CCHMe2), 1.90 (d,
1H, J = 11.0 Hz, CyH), 1.79 (d, 2H, J = 10.6 Hz, CyH), 1.61 (d, 1H, J =
12.3 Hz, CyH), 1.45 (m, 1H, CyH), 1.0ꢀ1.4 (m, 6H, CyH), 1.21 (d, 3H,
J = 7.2 Hz, CHMeMe0), 0.95 (d, 3H, J = 7.4 Hz, CHMeMe0). 13C NMR
(100 MHz, C6D6): δ 153.2 (4ꢀ, Ar), 149.9 (4ꢀ, Ar), 127.5 (CH, Ar),
126.8 (CH, Ar), 125.7 (CH, Ar), 125.69 (CH, Ar), 94.5 (4ꢀ), 78.1 (CH),
66.2 (CH), 46.0 (CH3, NMe2), 38.8 (cyclohexyl CH2), 35.8 (cyclohexyl
CH2), 34.0 (CH), 27.6 (cyclohexyl CH2), 27.5 (cyclohexyl CH2), 26.7
(cyclohexyl CH2), 22.0 (isopropyl CH3), 19.6 (isopropyl CH3) (two of
the eight aryl carbons overlap other signals). Anal. Calcd: C, 53.76; H,
7.30; N, 8.64. Found: C, 53.52; H, 7.64; N, 8.62. Mp: 138ꢀ141 ꢀC.
General Hydroamination Procedure. Hydroamination was carried
out with 5 mol % catalyst loading. Inside the glovebox, deuterated
benzene (175 μL), Ta(NMe2)5 (100 μL of a 0.0375 M solution, 3.75 ꢁ
10ꢀ3 mmol), ligand (75 μL of a 0.05 M solution, 3.75 ꢁ 10ꢀ3 mmol),
and 6-methylhepta-4,5-dienylamine (50 μL of a 1.5 M solution, 0.075
mmol, 20 equiv) were combined in a medium-walled J. Young NMR
D-N-Isopropyldimethylphenylalaninol (D-H2PPM). The D-enantiomer
of PPM was prepared analogously to the L-enantiomer, starting from D-
N-isopropylphenylalanine ethyl ester hydrochloride (2.06 g, 9.6 mmol)
dissolved in THF (75 mL) and methylmagnesium bromide (22 mL, 3 M
in ether) to yield a yellow oil (1.9 g, 8.5 mmol, 88% yield). The product
was purified by flash chromatography (5% ethyl acetate, 0.1% triethyla-
mine, 94.9% hexanes). NMR spectroscopy confirmed the formation of
the desired ligand with essentially equal and opposite optical rotation.17
HRMS (FAB+): calcd for C14H24ON 222.1858, found 222.1868
1
tube, and an H NMR spectrum was taken. The tube was placed in a
1
135 ( 2 ꢀC oil bath and monitored by H NMR until the reaction
reached completion or stopped converting. Percent conversion was
determined by NMR spectroscopy.
Benzamide Derivative. The reaction mixture was quenched with
2-propanol (1 mL) and added to a 20 mL vial along with diethyl ether
(3 mL), benzoyl chloride (9 μL, 0.08 mmol), and triethylamine (21 μL,
0.15 mmol). After stirring at room temperature for 2 h, water (1 mL) was
added, the layers were separated, and the organic layer was washed with
brine and dried with magnesium sulfate. The crude benzamide solution
(0.2ꢀ0.5 μL) was injected on the GC capillary column (Chiraldex
B-DM, 30 m ꢁ 0.25 μm, 100 ꢀC, 8 min, 1 ꢀC/min to 180 ꢀC, 180 ꢀC,
15 min). The two enantiomers were separated with retention times of
approximately 135 and 136 min. The latter time corresponds to the
enantiomer with negative optical rotation.
Benzyl Derivative. Benzyl bromide (9 μL, 0.08 mmol) and triethy-
lamine (21 μL, 0.15 mmol) were added to the J. Young NMR tube of a
completed hydroamination reaction (0.075 mmol). The tube was left to
sit for 18 h, and a crystalline solid precipitated out of solution.
2-Propanol (100 μL) was added to the solution, which was then filtered
through glass fibers in a pipet filter. The clear solution was diluted to a
total volume of 2 mL with benzene. The crude solution (0.2ꢀ0.5 μL)
was injected on the GC capillary column (Chiraldex B-DM, 30 m ꢁ
0.25 μm, 100 ꢀC, 8 min, 1 ꢀC/min to 180 ꢀC, 180 ꢀC, 15 min). The two
enantiomers were separated with retention times of approximately 100
and 101 min. The later time corresponds to the enantiomer with
negative optical rotation.
(M + H+). [R]D = +19.7 (c 2.18 ꢁ 10ꢀ2 g mLꢀ1, EtOAc).
3
D-N-Isopropyldiphenylphenylalaninol (D-H2PPP). The D-enantiomer
of PPP was prepared analogously to the L-enantiomer,17 starting from D-
N-isopropylphenylalanine methyl ester14 (0.632 g, 3.01 mmol) in
diethyl ether (35 mL) and phenylmagnesium bromide (4 mL, 3 M in
diethyl ether, 12 mmol) to yield pale yellow crystals that were recrys-
tallized from hexanes (0.2777 g, 0.81 mmol, 27%). NMR spectroscopy
confirmed the formation of the desired ligand with essentially equal and
opposite optical rotation.17 [R]D = ꢀ3.5 (c 9.75 ꢁ 10ꢀ3 g mLꢀ1
,
3
EtOAc).
D-N-Cyclohexyldiphenylvalinol (D-H2VCP). The D-enantiomer of VCP
was prepared analogously to the L-enantiomer,17 starting from D-N-
cyclohexylvaline methyl ester (0.878 g, 4.12 mmol) in diethyl ether
(35 mL) and phenylmagnesium bromide (5.5 mL, 3 M in diethyl ether,
16.5 mmol) to yield a white powder that was recrystallized from ethanol
(0.4373 g, 1.36 mmol, 33%). NMR spectroscopy confirmed the formation
of the desired ligand with essentially equal and opposite optical rotation.17
[R]D = +39.0 (c = 1.07 ꢁ 10ꢀ2 g mLꢀ1, EtOAc).
3
D-N-Cyclohexyldiphenylphenylalaninol (D-H2PCP). The D-enantio-
mer of PCP was prepared analogously to the L-enantiomer,17 starting
from D-N-cyclohexylphenylalanine methyl ester (288.6 mg, 1.34 mmol)
in diethyl ether (60 mL) and phenylmagnesium bromide (3 M in THF,
2.3 mL, 6.9 mmol) to yield an orange oil, which was purified by column
chromatography (20% ethyl acetate, 80% hexanes) to yield a white solid.
The solid was recrystallized from boiling ethanol to yield a white
microcrystalline powder (75.8 mg, 0.20 mmol, 15%). NMR spectros-
copy confirmed the formation of the desired ligand with essentially equal
Polarimetry of N-Benzyl Derivative. Benzyl bromide (9 μL, 0.08 mmol)
and triethyl amine (21 μL, 0.15 mmol) were added to the product of a
hydroamination catalyzed by Ta(NMe2)5 and L-VCP and left to stand
overnight. 2-Propanol (100 μL) was added, and the reaction mixture was
filtered through glass fibers and then diluted to a total volume of 2.0 mL with
benzene. The polarimetry reading (R) was ꢀ0.23ꢀ; thus the enantiomer
with the longer retention time is the (ꢀ)-enantiomer. The specific rotation
was calculated, considering the enantioselectivity of the reaction (53% ee for
and opposite optical rotation.17 [R]D= ꢀ6.85 (c 8.9 ꢁ 10ꢀ3 g mLꢀ1
,
this run, as determined by GC-MS), [R]D = ꢀ54 (c 8.08 ꢁ 10ꢀ3 g mLꢀ1
,
3
3
EtOAc).
benzene).
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Organometallics
ARTICLE
Synthesis and Isolation of Complexes for Hydroamination Study.
Ta(L-VCP)(NMe2)3 (4) was prepared by combining Ta(NMe2)5
(34 mg, 0.085 mmol) and L-H2VCP (30 mg, 0.089 mmol) in diethyl
ether (2 mL). The solvent was removed on the vacuum line to yield a
yellow oil, which was dissolved in C6D6 to make a hydroamination
solution (1.70 mL, 0.050 M). Ta(L-PPP)(NMe2)3 was prepared simi-
larly from Ta(NMe2)5 (39 mg, 0.097 mmol) and L-H2PPP (35 mg, 0.10
mmol) and dissolved in C6D6 (1.70 mL, 0.050 M). Ta(L-VAM)-
(NMe2)3 was prepared similarly from Ta(NMe2)5 (43 mg, 0.11 mmol)
and L-H2VAM (29 mg, 0.11 mmol) and dissolved in C6D6 (2.10 mL,
0.051 M). Ta(L-PPH)(NMe2)3 was prepared similarly from Ta(NMe2)5
(35 mg, 0.087 mmol) and L-H2PPH (17 mg, 0.088 mmol) and dissolved
in C6D6 (1.75 mL, 0.050 M). Hydroamination with the isolated complexes
was carried out by combining deuterated benzene (275 μL), the complex
solution (75 μL of a 0.05 M solution, 3.75 ꢁ 10ꢀ3 mmol), and
6-methylhepta-4,5-dienylamine (50 μL of a 1.5 M solution, 0.075 mmol,
20 equiv), and the reaction was monitored as described previously.
Hydroamination at 100 ꢀC. Hydroamination was carried out with
5 mol % catalyst loading. Inside the glovebox, deuterated benzene
(175 μL), Ta(NMe2)5 (100 μL of a 0.0375 M solution, 3.75 ꢁ 10ꢀ3
mmol), ligand (75 μL of a 0.05 M solution, 3.75 ꢁ 10ꢀ3 mmol), and
6-methylhepta-4,5-dienylamine (50 μL of a 1.5 M solution, 0.075 mmol,
20 equiv) were combined in a medium-walled J. Young NMR tube, and
an 1H NMR spectrum was taken. The tube was placed in a 100 ( 2 ꢀC oil
bath and monitored by 1H NMR until the reaction reached com-
pletion or stopped converting. Percent conversion was determined by
NMR spectroscopy. Enantioselectivity was determined as previously
described.
’ ACKNOWLEDGMENT
The authors gratefully acknowledge the financial support of
The John Stauffer Charitable Trust, The R.C. Baker Foundation,
and the National Science Foundation (REU-CHE-0648597,
REU-CHE-1005253, RUI-CHE-0615724, and RUI-CHE-1012445).
’ REFERENCES
(1) (a) M€uller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675–703.
(b) M€uller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M. Chem.
Rev. 2008, 108, 3795–3892. (c) Pohlki, F.; Doye, S. Chem. Soc. Rev. 2003,
32, 104–114.
(2) (a) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673–686.
(b) Zi, G. Dalton Trans. 2009, 9101–9109.
(3) (a) Widenhoefer, R. A. Chem.—Eur. J. 2008, 14, 5382–5391.
(b) Arcadi, A. Chem. Rev. 2008, 108, 3266–3325.
(4) (a) Hultzsch, K. C. Org. Biomol. Chem. 2005, 3, 1819–1824.
(b) Hultzsch, K. C. Adv. Synth. Catal. 2005, 347, 367–391. (c) Doye, S.
Synlett 2004, 1653–1672.
(5) Hesp, K. D.; Stradiotto, M. ChemCatChem 2010, 2, 1192–1207.
(6) (a) Chemler, S. R. Org. Biomol. Chem. 2009, 7, 3009–3019.
(b) Aillaud, I.; Collin, J.; Hannedouche, J.; Schulz, E. Dalton Trans.
2007, 5105–5118.
(7) (a) Ohmiya, H.; Moriya, T.; Sawamura, M. Org. Lett. 2009,
11, 2145–2147. (b) Zhang, X.; Zhou, Y.; Wang, H.; Guo, D.; Ye, D.; Xu,
Y.; Jiang, H.; Liu, H. Green Chem. 2011, 13, 397–405. (c) Bauer, E. B.;
Andavan, G. T. S.; Hollis, T. K.; Rubio, R. J.; Cho, J.; Kuchenbeiser,
G. R.; Helgert, T. R.; Letko, C. S.; Tham, F. S. Org. Lett. 2008, 10, 1175–
1178.
X-ray Crystal Structure of Tris(dimethylamide)(L-VCP)tantalum (4).
Suitable crystals were obtained by prolonged storage of an diethyl ether
solution of 4 at ꢀ35 ꢀC. A colorless block with approximate dimensions
of 0.32 ꢁ 0.30 ꢁ 0.20 mm was used for the X-ray crystallographic
analysis. The crystal was mounted on a Cryoloop with Paratone-N oil.
Diffraction intensity data were collected at 123(2) K using a Br€uker
Apex-II CCD diffractometer equipped with a graphite monochromator
and a Mo KR fine-focus sealed tube. Crystal data and refinement
parameters are summarized in Table 1. The systematic absences in the
diffraction data are consistent with the orthorhombic space group
P212121. The structure was solved using direct methods, completed by
subsequent difference Fourier syntheses, and refined by full-matrix least-
squares procedures. The correct absolute structure was unambiguously
determined; Flack parameter = 0.002(6).32 Data were corrected for
absorption effects using SADABS.33 All non-hydrogen atoms were
refined with anisotropic displacement coefficients, and hydrogen atoms
were treated as idealized contributions. All software and sources of
scattering factors are contained in the SHELXTL (6.10) program
package (G. Sheldrick, Br€uker XRD, Madison, WI).34 Images were
generated using CrystalMaker.35
(8) (a) Arseniyadis, S.; Gore, J. Tetrahedron Lett. 1983, 24, 3997–
4000. (b) Kinsman, R.; Lathbury, D.; Vernon, P.; Gallagher, T. J. Chem.
Soc., Chem. Commun. 1987, 243–244. (c) Fox, D. N. A.; Gallagher, T.
Tetrahedron 1990, 46, 4697–4710. (d) Meguro, M.; Yamamoto, Y.
Tetrahedron Lett. 1998, 39, 5421–5424.
(9) (a) Ackermann, L.; Bergman, R. G. Org. Lett. 2002,
4, 1475–1478. (b) Ackermann, L.; Bergman, R. G.; Loy, R. N. J. Am.
Chem. Soc. 2003, 125, 11956–11963.
(10) (a) Arredondo, V.; McDonald, F. E.; Marks, T. J. Organome-
tallics 1999, 18, 1949–1960. (b) Arredondo, V.; McDonald, F. E.; Marks,
T. J. J. Am. Chem. Soc. 1998, 120, 4871–4872. (c) Arredondo, V.; Tian,
S.; McDonald, F. E.; Marks, T. J. J. Am. Chem. Soc. 1999, 121, 3633–
3639.
(11) (a) Morita, N.; Krause, N. Eur. J. Org. Chem. 2006, 4634–4641.
(b) LaLonde, R. L.; Sherry, B. D.; Kang, E. J.; Toste, F. D. J. Am. Chem. Soc.
2007, 129, 2452–2453. (c) Zhang, Z. B.; Bender, C. F.; Widenhoefer, R. A. J.
Am. Chem. Soc. 2007, 129, 14148–14149.
(12) LaLonde, R. L.; Wang, Z. J.; Mba, M.; Lackner, A. D.; Toste,
F. D. Angew. Chem., Int. Ed. 2010, 49, 598–601.
(13) (a) Zi, G.; Zhang, F.; Xiang, L.; Chen, Y.; Fang, W.; Song, H.
Dalton Trans. 2010, 39, 4048. (b) Zi, G.; Zhang, F.; Liu, X.; Ai, L.; Song,
H. J. Organomet. Chem. 2010, 695, 730–739. (c) Reznichenko, A. L.;
Hultzsch, K. C. Organometallics 2010, 29, 24–27. (d) Xiang, L.; Zhang,
F.; Zhang, J.; Song, H.; Zi, G. Inorg. Chem. Commun. 2010, 13, 666–670.
(e) Xiang, L.; Song, H.; Zi, G. Eur. J. Inorg. Chem. 2008, 1135–1140.
(f) Zi, G.; Wang, Q.; Xiang, L.; Song, H. Dalton Trans. 2008, 5930–5944.
(g) Wood, M. C.; Leitch, D. C.; Yeung, C. S.; Kozak, J. A.; Schafer, L. L.
Angew. Chem., Int. Ed. 2007, 46, 354–358.
(14) Hoover, J. M.; Petersen, J. R.; Pikul, J. H.; Johnson, A. R.
Organometallics 2004, 23, 4614–4620.
(15) Petersen, J. R.; Hoover, J. M.; Kassel, W. S.; Rheingold, A. L.;
Johnson, A. R. Inorg. Chim. Acta 2005, 358, 687–694.
(16) Johnson, J. S.; Bergman, R. G. J. Am. Chem. Soc. 2001, 123, 2923–
2924.
’ ASSOCIATED CONTENT
S
Supporting Information. Additional experimental de-
b
tails, characterization data, and NMR spectra for the in situ
prepared complexes, sample GC traces, and complete crystal-
lographic data as a CIF file for compound 4 are available free of
charge via the Internet at http://pubs.acs.org. CCDC 817406
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by e-mailing data_request@ccdc.cam.ac.uk.
’ AUTHOR INFORMATION
(17) Hickman, A. J.; Hughs, L. D.; Jones, C. M.; Li, H.; Redford, J. E.;
Sobelman, S. J.; Kouzelos, J. A.; Johnson, A. R. Tetrahedron: Asymmetry
2009, 20, 1279–1285.
Corresponding Author
*E-mail: Adam_Johnson@hmc.edu.
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ARTICLE
(18) Near, K. E.; Chapin, B. M.; McAnnally-Linz, D. C.; Johnson,
A. R. J. Organomet. Chem. 2011, 696, 81–86.
(19) (a) Anderson, L. L.; Arnold, J.; Bergman, R. G. Org. Lett. 2004,
6, 2519–2522. (b) Lorber, C.; Choukroun, R.; Vendier, L. Organome-
tallics 2004, 23, 1845–1850.
(20) (a) Herzon, S. B.; Hartwig, J. F. J. Am. Chem. Soc. 2008,
130, 14940–14941. (b) Herzon, S. B.; Hartwig, J. F. J. Am. Chem. Soc.
2007, 129, 6690–6691. (c) Eisenberger, P.; Ayinla, R. O.; Lauzon, J.-M.
P.; Schafer, L. L. Angew. Chem., Int. Ed. 2009, 48, 8680–8689.
(21) Anderson, L.; Schmidt, J. A. R.; Arnold, J.; Bergman, R. G.
Organometallics 2006, 25, 3394–3406.
(22) (a) Reznichenko, A. L.; Emge, T. J.; Aud€orsch, S.; Klauber,
E. G.; Hultzsch, K. C.; Schmidt, B. Organometallics 2011, 921–924.
(b) Zhang, F.; Song, H.; Zi, G. Dalton Trans. 2011, 40, 1547–1566.
(23) Pirkle, W. H.; Sikkenga, D. L.; Pavlin, M. S. J. Org. Chem. 1977,
42, 384–387.
(24) Budzelaar, P. H. M. gNMR, 5.0.4; Adept Scientific: Letchworth,
United Kingdom, 2004.
(25) Yao, J. W.; Copley, R. C. B.; Howard, J. A. K.; Allen, F. H.;
Motherwell, W. D. S. Acta Crystallogr. 2001, B57, 251–260.
(26) Batsanov, A.; Churakov, A.; Howard, J.; Hughes, A.; Johnson,
A.; Kingsley, A.; Neretin, I.; Wade, K. J. Chem. Soc., Dalton Trans.
1999, 3867–3875.
(27) Allen, F. H. Acta Crystallogr. 2002, B58, 380–388.
(28) (a) Vilardo, J.; Salberg, M.; Parker, J.; Fanwick, P.; Rothwell, I.
Inorg. Chim. Acta 2000, 299, 135–141. (b) Thorn, ^M. G.; Parker, J. R.;
Fanwick, P. E.; Rothwell, I. P. Organometallics 2003, 22, 4658–4664.
(29) (a) Welter, C.; Dahnz, A.; Brunner, B.; Streiff, S.; Dubon, P.;
Helmchen, G. Org. Lett. 2005, 7, 1239–1242. (b) Seijas, J.; Vazqueztato,
M.; Castedo, L.; Estevez, R.; Onega, M.; Ruiz, M. Tetrahedron 1992,
48, 1637–1642. (c) Nagashima, H.; Gondo, M.; Masuda, S.; Kondo, H.;
Yamaguchi, Y.; Matsubara, K. Chem. Commun. 2003, 442–443.
(30) (a) Clark, E. Ind. Eng. Chem., Anal. Ed. 1941, 13, 820. (b) Burger,
B. J.; Bercaw, J. E. ACS Symp. Ser. 1987, 357, 79–98.
(31) Perrin, D. D.; Armarego, L. F. Purification of Laboratory
Chemicals; Pergamon Press: New York, 1988.
(32) Flack, H. D. Acta Crystallogr. 1983, A39, 876–881.
(33) Sheldrick, G. M. SADABS, Version 2.10; University of
G€ottingen: Germany, 2003.
(34) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(35) Crystalmaker; CrystalMaker Software Ltd: Oxford, England
(http://www.crystalmaker.com).
4623
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