Chiral macrocyclic N2P2 ligands and Iron(II): A marriage of interest

Article abstract of DOI:10.1021/om5005989

The N₂P₂ macrocyclic ligands (5S,8S,13E,14aS,18aS, 19E)-5,8-diphenyl-5,6,7,8,14a,15,16,17,18,18a-decahydrotribenzo[b,f,l][1,4,8,11] diazadiphosphacyclotetradecine ((1S,4S,9S,10S)-1a) and (5E,7R,8R,9E,15S,18S)-7, 8,15,18-tetraphenyl-7,8,15,16,17,18-hexahydrodibenzo[f,l][1,4,8,11] diazadiphosphacyclotetradecine ((1S,4S,9R,10R)-1b) were prepared by condensing the new, enantiomerically pure synthon 2,2′-((1S,1′S)-ethane-1,2- diylbis(phenylphosphinediyl))dibenzaldehyde ((S,S)-8), prepared in six steps from (2R,4S,5R)-3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaphospholidine-2-borane (3)), with (1S,2S)-cyclohexane-1,2-diamine and (1R,2R)-1,2-diphenylethane-1,2- diamine under high-dilution conditions. The opposite enantiomers of the diamines gave oligomeric products. The stereospecificity of the macrocyclization reaction is explained by conformational analysis based on the X-ray structures of (1S,4S,9S,10S)-1a and (1S,4S,9R,10R)-1b. The corresponding diamino macrocycles (1S,4S,9S,10S)-2a and (1S,4S,9R,10R)-2b were prepared by reduction of the imine moiety of (1S,4S,9S,10S)-1a and (1S,4S,9R,10R)-1b, respectively. Macrocycles (1S,4S,9S,10S)-1a, (1S,4S,9R,10R)-1b, and (1S,4S,9S,10S)-2a react with [Fe(OH₂)₆](BF₄)₂ in acetonitrile to give the corresponding stable, diamagnetic bis(acetonitrile) complexes [Fe(MeCN)₂(1)](BF₄)₂ (9a and 9b) and [Fe(MeCN)₂(2a)](BF₄)₂ (10a). Complex 9a exists as a 3:1 mixture of trans and λ-cis-β isomers, whereas 9b and 10a adopt the λ-cis-β configuration exclusively. The bis(acetonitrile) complexes are versatile precursors and were used to prepare the bromocarbonyl analogues [FeBr(CO)(1)]BPh₄ (11a and 11b).

Full text of DOI:10.1021/om5005989

Article
pubs.acs.org/Organometallics
Chiral Macrocyclic N2P2 Ligands and Iron(II): A Marriage of Interest
Raphael Bigler, Elisabeth Otth, and Antonio Mezzetti*
Department of Chemistry and Applied Biosciences, ETH Zurich, CH-8093 Zurich, Switzerland
̈
̈
S
* Supporting Information
ABSTRACT: The N2P2 macrocyclic ligands (5S,8S,13E,14a-
S,18aS,19E)-5,8-diphenyl-5,6,7,8,14a,15,16,17,18,18a-decahydro-
tribenzo[b,f,l][1,4,8,11]diazadiphosphacyclotetradecine
((1S,4S,9S,10S)-1a) and (5E,7R,8R,9E,15S,18S)-7,8,15,18-tetra-
phenyl-7,8,15,16,17,18-hexahydrodibenzo[f,l][1,4,8,11]diaza-
diphosphacyclotetradecine ((1S,4S,9R,10R)-1b) were prepared by
condensing the new, enantiomerically pure synthon 2,2′-
((1S,1′S)-ethane-1,2-diylbis(phenylphosphinediyl))-
dibenzaldehyde ((S,S)-8), prepared in six steps from (2R,4S,5R)-
3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaphospholidine-2-borane
(3)), with (1S,2S)-cyclohexane-1,2-diamine and (1R,2R)-1,2-
diphenylethane-1,2-diamine under high-dilution conditions. The
opposite enantiomers of the diamines gave oligomeric products.
The stereospecificity of the macrocyclization reaction is explained by conformational analysis based on the X-ray structures of
(1S,4S,9S,10S)-1a and (1S,4S,9R,10R)-1b. The corresponding diamino macrocycles (1S,4S,9S,10S)-2a and (1S,4S,9R,10R)-2b
were prepared by reduction of the imine moiety of (1S,4S,9S,10S)-1a and (1S,4S,9R,10R)-1b, respectively. Macrocycles
(1S,4S,9S,10S)-1a, (1S,4S,9R,10R)-1b, and (1S,4S,9S,10S)-2a react with [Fe(OH2)6](BF4)2 in acetonitrile to give the
corresponding stable, diamagnetic bis(acetonitrile) complexes [Fe(MeCN)2(1)](BF4)2 (9a and 9b) and [Fe(MeCN)2(2a)]-
(BF4)2 (10a). Complex 9a exists as a 3:1 mixture of trans and Λ-cis-β isomers, whereas 9b and 10a adopt the Λ-cis-β
configuration exclusively. The bis(acetonitrile) complexes are versatile precursors and were used to prepare the bromocarbonyl
analogues [FeBr(CO)(1)]BPh4 (11a and 11b).
Chart 1. Selected Well-Characterized Iron(II) Precatalysts
INTRODUCTION
■
Although iron chemistry has experienced a tremendous boost
over the past decade, well-defined, bench-stable, preferentially
diamagnetic iron(II) (pre)catalysts for enantioselective trans-
formations are still scarce, with systems containing multidentate
ligands clearly dominating.1 Besides the ubiquitous cyclo-
pentadienyl fragment,2 tetradentate ligands, in particular with
the N4 donor set, are the most prominent ones. However, such
N4 systems are generally achiral and give high-spin complexes.3
White and co-workers have prepared one of the few chiral
examples with a N4 donor set and used it in C−H oxidation
reactions (Chart 1).4
Also tetradentate ligands with a N2P2 or P4 donor set are
rare, and most of them are achiral. A N2P2 iminophosphorane
ligand has been used in the transfer hydrogenation of
acetophenone,5 and tripodal PP3 ligands have been used in
the semihydrogenation of alkynes and in the reduction of CO2,
bicarbonate, enals, and nitroarenes, respectively.6,7 A chiral, but
racemic open-chain P4 ligand and its bis(acetonitrile) Fe(II)
complex have recently been prepared, but no catalytic activity
has been reported.8
Rare examples of the use of chiral PNNP ligands in catalysis
with iron(II) are Morris’ complexes with ligands A and B
(Chart 2).9−12 Complex [Fe(MeCN)2(A)](BF4)2 hydrogenates
acetophenone to 1-phenylethanol with up to 39% ee and TOF
as high as 907 h−1.9 However, the complex is not stable under
transfer hydrogenation conditions and decomposes to iron(0)
nanoparticles doped with the chiral ligand.10 Changing the
phosphorus-imine bridge from 1,2-phenylene to methylene as
in trans-[FeBr(CO)(B)]BPh4 increases both the activity (TOF
= 2.0 × 104 h−1) and the selectivity (81% ee) in the transfer
Received: June 5, 2014
Published: July 23, 2014
© 2014 American Chemical Society
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double alkylation of deprotonated 1,2-bis(phenylphosphino)-
benzene under high-dilution conditions.23 In all cases, only the
meso isomer of the ligand was obtained. Smaller N2P2
macrocycles such as F and G cannot adopt a trans configuration
even for 3d transition metals because of the small ring size and
hence either give cis complexes (F)24 or bind metals in a
bidentate way with the phosphine donors (G).25,26 As before,
only the meso isomers are accessible. Recently, Gao has
prepared chiral, non-P-stereogenic N4P2 macrocycles and used
them in the iron(0)-catalyzed asymmetric H2-hydrogenation of
ketones with excellent enantioselectivity (up to 99% ee).27
Our research group has recently reported the first
enantiopure, C1-symmetric, macrocyclic N2P2 ligand
(1R,4S,9S,10S)-1a (Chart 2), its diamino analogue, and their
ruthenium(II) complexes.28,29 However, preliminary catalytic
tests in the transfer hydrogenation of acetophenone gave low
enantioselectivity, which we ascribed to the pseudo-meso
configuration of the PPh groups, leaving one side of the
complex almost unbiased. Therefore, also in view of the
successful application of ruthenium(II)/PNNP complexes in
enantioselective catalytic reactions such as asymmetric C−C
and C−heteroatom bond formation reactions,30,31 we have
pursued an alternative synthetic strategy that gives access to the
enantiomerically pure C2-symmetric diastereoisomer
(1S,4S,9S,10S)-1a, containing two stereogenic phosphorus
donors in the like configuration as described below.
Chart 2. Structural Comparison of Open-Chain PNNP and
Macrocyclic N2P2 Ligands
hydrogenation of acetophenone.11 Such second-generation
ligands form stable active catalysts that bear a monoreduced
amino/enamido ligand.12
Besides tetradentate ligands, tridentate PNP, PNN, and
NNN ligands have been prepared and used in catalysis in
combination with iron(II),13 but only a handful of well-defined,
chiral complexes have been prepared14−18 and used in the
asymmetric H2-hydrogenation (up to 85% ee)15 and in the
asymmetric hydrosilylation of ketones (up to 93% ee).16−18
To exploit iron as a cheap, low-toxic metal in catalysis with
high turnover numbers and low catalyst loading, robust iron
complexes are required.1 As summarized above, only few chiral
iron(II) complexes have been prepared, and besides Morris’
systems, most of them are paramagnetic, which complicates
their investigation and rational development. Furthermore,
their robustness poses a major challenge, such as in the case of
the open-chain PNNP complexes with ligand A (Chart 2),
which decompose under reaction conditions. As the bulkiness
of the mutually cis PPh2 groups in [Fe(MeCN)2(A)](BF4)2
may account for the instability of the iron catalyst under the
reaction conditions,10 we reasoned that enantiopure macro-
cyclic N2P2 ligands should reduce the steric bulk of the
tetradentate N2P2 ligand and stabilize the complex by virtue of
the macrocyclic effect.19 The synthesis of such macrocyclic
N2P2 ligands is challenging, though, because it requires the
control of the configuration at phosphorus. In fact, only a few
P-containing macrocycles have been prepared,20 and N2P2
macrocycles are even rarer and mostly achiral (Chart 3).21−26
The N2P2 macrocycles C and D were prepared by template
synthesis with nickel(II),21,22 whereas E was prepared by
RESULTS AND DISCUSSION
General Strategy. The retrosynthetic approach shown in
Scheme 1 indicates the enantiomerically pure bis-ortho-formyl-
■
Scheme 1. Retrosynthetic Analysis of Macrocycle 1a
substituted diphosphine (S,S)-8 as the synthon of choice to
prepare C2-symmetric N2P2 macrocycles by condensation with
a variety of chiral diamines. As the previous examples of N2P2
macrocycles have shown a strong preference for the meso
form,21−26,28 it is crucial to control the stereogenic center on
phosphorus at an early stage of the synthesis. To this goal, we
Chart 3. Previously Reported, Achiral N2P2 Macrocycles
́
have adapted Juge’s approach, which is excellently suitable to
control the stereochemistry on phosphorus by sequential and
selective nucleophilic cleavage of the P−O and P−N bonds of
(2R,4S,5R)-3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaphospholi-
dine-2-borane (3) (formed as a single diastereoisomer from
PhP(NEt2)2 and (−)-ephedrine).32 Starting from 3, we
prepared the enantiomerically enriched intermediate (S)-(2-
(methyl(phenyl)phosphino)phenyl)methanol borane (S)-6 in
three steps. Oxidative coupling, followed by oxidation and
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deprotection, gave the bis-ortho-formyl-substituted diphosphine
(S,S)-8 as a single enantiomer as described in detail below.
Synthesis of Dialdehyde (S,S)-8. The phosphorus−
oxygen bond of 3 was cleaved with lithium (2-(oxidomethyl)-
phenyl)lithium (prepared from (2-bromophenyl)methanol and
nBuLi) to give the phosphinamine borane 4 with high
diastereoselectivity (dr ≥ 95:5) and retention of configuration
at phosphorus (Scheme 2).32 The phosphorus−nitrogen bond
Scheme 3. Synthesis of Diphosphine (S,S)-8
Scheme 2. Synthesis of Intermediates (R)- and (S)-6
next step. We first studied the macrocyclization with both
enantiomers of (l)-cyclohexane-1,2-diamine and (l)-1,2-diphe-
nylethane-1,2-diamine under high dilution conditions (0.01 M)
in EtOH. The reaction with (1S,2S)-cyclohexane-1,2-diamine
gave a single species in the 31P{1H} NMR, which crystallized
from the solution after concentration. The macrocyclic N2P2
ligand (1S,4S,9S,10S)-1a was isolated in 67% yield by simple
filtration as a stable, off-white solid (Scheme 4).36
Scheme 4. Reaction of (S,S)-8 with (l)-Cyclohexane-1,2-
diamine
was subsequently cleaved in the presence of sulfuric acid in a 5-
exo-tet fashion to form the cyclic phosphinite borane (R)-5 with
the expected inversion of configuration (er = 95:5).32a
Treatment of (R)-5 (er = 95:5) with methyllithium or
methylmagnesium chloride in toluene at −78 °C afforded the
retention product (R)-(2-(methyl(phenyl)phosphino)phenyl)-
methanol borane, (R)-6 (Scheme 2), and is highly stereo-
selective (er = 90:10 and 91:9, respectively).
No epimerization at phosphorus was observed in the solid
state over several months, whereas heating in 1,2-dichloro-
ethane at 100 °C for 24 h gave a 0.20:0.79:0.01 mixture of
(SP,SP)-1a, (SP,RP)-1a, and (RP,RP)-1a. This indicates a strong
thermodynamic preference for the C1-symmetric isomers and
underlines the difficulties in the preparation of C2-symmetric
N2P2 macrocycles such as (1S,4S,9S,10S)-1a or (1S,4S,9R,10R)-
1b (see below). In contrast, the 31P{1H} NMR spectrum of the
reaction solution of (S,S)-8 with (1R,2R)-cyclohexane-1,2-
diamine shows several signals that are assigned to oligomeric
species. Several attempts to isolate and purify these products
failed. Similarly, only one enantiomer (in this case the (1R,2R)-
enantiomer) of 1,2-diphenylethane-1,2-diamine formed a
macrocycle with (S,S)-8, but the reaction is sluggish, and a
catalytic amount of HCl was added to accelerate it (Scheme 5).
As before, the macrocyclic ligand (1S,4S,9R,10R)-1b
precipitated upon concentrating the reaction solution and was
isolated by filtration in 69% yield.36 In the mismatched case
with (1S,2S)-1,2-diphenylethane-1,2-diamine, the 31P{1H}
NMR spectrum of the reaction shows several signals, which
were assigned to oligomeric products. Macrocyclization
attempts with both enantiomers of (l)-1,1′-binaphthyl-2,2′-
diamine and (l)-1,3-diphenylethane-1,3-diamine, as well as with
ethylene diamine and 2,3-dimethylbutane-2,3-diamine, gave no
characterizable products.
The formation of the retention product was unexpected, as
acyclic analogues undergo selective cleavage of the P−O bond
by SN2 attack with inversion at phosphorus.32 We attribute the
formation of the retention product (R)-6 to the coordination of
the organometallic species to oxygen, followed by intra-
molecular syn addition/elimination similarly to the opening of
3. Complete inversion was observed in the presence of TMEDA,
which probably hinders the formation of the MeLi-substrate
adduct and favors an SN2 reaction (er = 6:94; see also Table
S3). An analogous enantiodivergent synthesis of 6 from 3 has
been developed independently by Stephan and Mohar, who
also found that noncoordinating, apolar solvents are crucial for
the stereoselective opening of (R)-5.33 The phosphine borane
(S)-6 was then oxidatively coupled to build the protected 1,2-
diphosphine (S,S)-7,32a and the benzylic hydroxy groups were
converted to formyl groups by oxidation with MnO2 in EtOAc
(Scheme 3). Due to its low stability, the resulting diformyl
phosphine borane was deprotected without purification. Borane
removal with DABCO, followed by recrystallization from hot
methanol, gave diastereo-34 and enantiomerically35 pure (S,S)-8
as a yellow, air-stable solid.36 Overall, the protocol is highly
enantioselective, gives access to both enantiomers of dialdehyde
8 in six steps and 15% overall yield, and can be applied on a
multigram scale.
Conformational Analysis of 1a and 1b. The X-ray
structures of (1S,4S,9S,10S)-1a and (1S,4S,9R,10R)-1b indicate
that both macrocycles adopt conformations in which the allylic
Synthesis of the Macrocyclic Ligands. The condensation
of dialdehyde (S,S)-8 with chiral diamines was investigated in a
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(Chart 4, top), it is apparent that the most stable conformer of
the (SP,SP)/(SC,SC)-diastereoisomer is stereochemically pre-
Scheme 5. Reaction of (S,S)-8 with (l)-1,2-Diphenylethane-
1,2-diamine
Chart 4. Conformational Analysis of 1a (before Ring
Closure, Top) and of 1b (Bottom)
1,3-strain between the imine C−H bonds and the tertiary
carbon atoms of the diimino backbone is minimized (Figure
1).37 The lone pairs of adjacent P and N donors point in
pared for the second condensation step and thus for
macrocyclization, whereas the most stable conformer of the
(SP,SP)/(RC,RC)-diastereoisomer needs to rearrange to a less
stable structure to cyclize since the free amine is pointing away
from the aldehyde.
For (1S,4S,9R,10R)-1b, the analysis is carried out on the
macrocycle, and not the macrocyclization step, because the use
of HCl as catalyst yields the thermodynamic product. The
allylic 1,3-strain between the imine C−H bond and the tertiary
carbon centers of the diimino backbone carrying mutually trans
phenyl groups enforces a conformation of dialdehyde (S,S)-8
that can accommodate only the (1R,2R)-enantiomer of 1,2-
diphenylethane-1,2-diamine in the macrocycle (Chart 4,
bottom).
Reduction to Diamino Macrocycles. As in some cases
the diamino analogues of PNNP ligands have been more
successfully employed in asymmetric catalysis,38 we also
investigated the reduction of the imine moieties using different
hydride sources (Scheme 6). The imine reduction turned out to
be delicate, as reduction with NaBH4 in EtOH at elevated
temperatures led to epimerization at phosphorus.39 The
reaction with LiAlH4 in THF gave the diamino analogues
(1S,4S,9S,10S)-2a and (1S,4S,9R,10R)-2b as off-white solids in
modest yield. Therefore, the reactions were run on a small
Scheme 6. Synthesis of the Diamino Macrocycles
(1S,4S,9S,10S)-2a and (1S,4S,9R,10R)-2b
Figure 1. ORTEP drawing of (1S,4S,9S,10S)-1a (top) and
(1S,4S,9R,10R)-1b (bottom) (with ellipsoids at 30% probability).
opposite directions of the macrocycle (1S,4S,9S,10S)-1a, as
indicated by the N(1)−C(1)−C(2)−C(7) and N(2)−C(28)−
C(27)−C(22) torsion angles (−138.2(2)° and −158.0(2)°,
respectively). Assuming that the conformational preferences
observed for macrocycle (1S,4S,9S,19S)-1a also apply for the
intermediate in which the first imine bond has already formed
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scale, and (1S,4S,9S,10S)-2a and (1S,4S,9R,10R)-2b were not
purified further. We were not able to obtain X-ray-quality
crystals of the diamino ligands for comparison with 1a, 1b, and
the C1-symmetric analogue (1R,4S,9S,10S)-2a.28
Bis(acetonitrile) Complex [Fe(MeCN)2(1a)](BF4)2 (9a).
The cyclohexane-based macrocycle (1S,4S,9S,10S)-1a reacts
with [Fe(OH2)6](BF4)2 in CH2Cl2/MeCN at room temper-
ature to give the diamagnetic dicationic complex [Fe-
(MeCN)2((1S,4S,9S,10S)-1a)](BF4)2 (9a), whose formation
is indicated by an immediate color change to dark red. Complex
9a exists in solution as a ca. 3:1 mixture of trans and cis-β
isomers (Scheme 7), as indicated by the 31P{1H} spectrum of
the reaction solution, which shows a singlet for the trans isomer
at δ 93.4 (s) (78%).
Scheme 7. Synthesis of [Fe(MeCN)2((1S,4S,9S,10S)-
1a)](BF4)2 (9a)
Figure 2. ORTEP drawing of the complex cation of trans-9a(SbF6)2
(with ellipsoids at 30% probability).
Table 1. Selected Bond Distances (Å) and Angles (deg) in
9a
trans-9a(SbF6)2 and trans-[Fe(MeCN)2(A)](BF4)2
2
An additional AB system (δ 96.8 (d, JP,P′ = 41.3 Hz), 88.9
(d, 2JP,P′ = 41.3 Hz), 22%) is assigned to the minor cis-β isomer.
It should be noted that, of two possible cis-β isomers (Δ-cis-β
and Λ-cis-β), only the Λ-cis-β isomer can be formed due to the
(SP,SP)-configuration of the phosphorus stereocenters. The
31P{1H} NMR signals of 9a are significantly shifted to lower
field as compared to Morris’ iron(II) PNNP complexes
[Fe(MeCN)2(A)](BF4)2 (δ 53.4 (s))9a and [Fe(MeCN)2(B)]-
(BPh4)2 (δ 72.6 (s)),11b which we attribute to the ΔR
contribution of the five-membered chelate ring between the
two phosphines.40
Crystallization of the isomer mixture from MeCN/Et2O gave
pure trans-9a. Upon dissolving the crystals of the pure trans
isomer in CD2Cl2, the room-temperature 31P{1H} NMR
spectrum of the solution showed that a mixture of trans and
cis-β isomers is formed with the same 3:1 ratio obtained in the
synthesis described above. A 31P−31P EXSY experiment in
CD2Cl2 showed no cross-peaks between the two isomers. After
adding MeCN-d3, the 1H NMR signals of coordinated
acetonitrile disappeared, and free acetonitrile was observed.
We conclude that the observed isomers are in equilibrium with
each other and that ligand exchange is slow on the NMR time
scale.
The X-ray structure of trans-9a(SbF6)2 shows that the
macrocyclic ligand (1S,4S,9S,10S)-1a is large enough to
accommodate a low-spin iron(II) ion in the trans configuration
(Figure 2).41 The coordination to low-spin iron(II) forces the
P···N distances to shrink from 4.531(2) and 5.057(2) Å in the
free macrocycle (1S,4S,9S,10S)-1a to 4.123(6) and 4.118(5) Å
in trans-9a(SbF6)2 (Table 1). Upon coordination, the macro-
cycle slightly flattens, as indicated by the torsion angles (Table
S6), and adopts the typical “stepped” conformation observed
for open-chain PNNP complexes.9a,11a The two acetonitrile
ligands complete a slightly distorted octahedral coordination
sphere. The Fe−P and Fe−N bond distances involved in the
macrocyclic complex trans-9a(SbF6)2 are significantly shorter
than in the related open-chain complex [Fe(MeCN)2(A)]-
(BF4)2 (Table 1),9a which indicates that the compression
exerted by the macrocycle is significant.
Bis(acetonitrile) Complex [Fe(MeCN)2(1b)](BF4)2 (9b).
The reaction of (1S,4S,9R,10R)-1b with [Fe(OH2)6](BF4)2 in
CH2Cl2/MeCN at room temperature is accompanied by an
immediate color change to dark red and gives the diamagnetic,
dicationic complex [Fe(MeCN)2((1S,4S,9R,10R)-1b)](BF4)2
(9b) (Scheme 8). Complex 9b exists exclusively as the cis-β
isomer, as indicated by the AB system in the 31P{1H} NMR
2
2
spectrum (δ 103.7 (d, JP,P′ = 36.5 Hz), 91.4 (d, JP,P′ = 36.5
Hz)). The chemical shift values suggest that one of the
phosphines is trans to an imine and the other one trans to an
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products that we tentatively formulate as three isomers of
[Fe(MeCN)2((1S,4S,9R,10R)-2b)](BF4)2 (10b) (isomer A: δ
94.9 (d, 2JP,P′ = 25.5 Hz), 92.2 (d, 2JP,P′ = 25.5 Hz), 29%; isomer
B: δ 94.2 (s), 42%; isomer C: δ 93.3 (d, 2JP,P′ = 23.4 Hz), 78.5
Scheme 8. Synthesis of [Fe(MeCN)2((1S,4S,9R,10R)-
1b)](BF4)2 (9b)
2
(d, JP,P′ = 23.4 Hz), 30%). As all attempts to obtain a single
isomer by recrystallization failed, the complex was only
characterized in CD2Cl2 solution by 31P{1H} NMR spectros-
copy.
An X-ray analysis of crystals of 10a (obtained from MeCN/
Et2O) confirmed the cis-β configuration (Figure 3). The
acetonitrile ligand. Again, the 31P{1H} NMR signals for 9b are
significantly shifted to lower field compared to analogous open-
chain Fe(II)/PNNP complexes.40 Attempts to grow X-ray-
quality crystals of 9b were unsuccessful.
At difference with macrocycle (1S,4S,9S,10S)-1a, which
forms either trans or cis-β complexes, the 1,2-diphenyl-
ethylene-based ligand (1S,4S,9R,10R)-1b adopts the cis-β
configuration in all complexes prepared so far (vide infra). In
fact, the trans configuration of 9b would cause strong
nonbonded interactions of the acetonitrile ligands with the
PPh2 and with the axial phenyl groups of the diamine backbone.
Such interactions are responsible for the slight angular
distortions of trans-9a(SbF6)2 (Table 1) and would be more
severe in the trans configuration of 9b because of the pseudo-
axial phenyl groups of the diamine backbone.
Bis(acetonitrile) Complexes [Fe(MeCN)2(2)](BF4)2 (10).
The reaction of the diamino ligand (1S,4S,9S,10S)-2a with
[Fe(OH2)6](BF4)2 in CH2Cl2/MeCN gives the bis-
(acetonitrile) complex [Fe(MeCN)2((1S,4S,9S,10S)-2a)]-
(BF4)2 (10a) as an orange solid in 55% yield after
recrystallization from MeCN/Et2O (Scheme 9). The complex-
Figure 3. ORTEP drawing of the complex cation of 10a (with
ellipsoids at 30% probability).
Scheme 9. Synthesis of [Fe(MeCN)2((1S,4S,9S,10S)-
2a)](BF4)2 (10a)
absolute configuration of both nitrogen stereocenters was
unambiguously assigned as RN. Two acetonitrile ligands
complete the octahedral coordination sphere, in which the
largest deviations from the ideal octahedral geometry are
observed for N(1)−Fe−N(2), P(1)−Fe−P(2), and N(1)−Fe−
P(1) (85.26(8)°, 85.32(2)°, and 94.93(9)°, respectively).
Interestingly, the P(1)−Fe−N(2) angle (178.38(6)°) is close
to the ideal value, which shows that the ring size of the diamino
macrocycle (1S,4S,9S,10S)-2a is not the reason for the
preference of the cis-β configuration. In fact, for small N2P2
macrocycles (i.e., F), which favor the cis-β configuration, the
angle between the trans ligands of the macrocycle and the metal
is considerably reduced from the ideal value of 180°.24 We
assume that the change from sp2 nitrogen in the diimino ligand
(1S,4S,9S,10S)-1a to the sp3 nitrogen in the diamino ligand
(1S,4S,9S,10S)-2a increases the flexibility of the macrocycle and
favors its bent cis-β configuration.
Similarly to the diimino analogue trans-9a(SbF6)2, the Fe−P
and Fe−N bond distances to the macrocycle in 10a are
significantly shorter than in a related open-chain diamino
complex (Table 2).9c The acetonitrile ligand trans to
phosphorus is less tightly bound to iron (Fe−N(3), 1.962(2)
Å) than the one trans to the amine (Fe−N(4), 1.927(2) Å).
Also, the Fe−amine bond length trans to acetonitrile is shorter
(Fe−N(1), 2.020(2) Å) than the one trans to phosphorus (Fe−
N(2), 2.039(2) Å). Both findings reflect the higher trans
influence of phosphorus as compared to nitrogen.
ation of (1S,4S,9S,10S)-2a is much slower than with the
diimino analogue (1S,4S,9S,10S)-1a and requires 2 days at 50
°C for completion, which can be attributed to the additional
degrees of freedom caused by the reduction of the imine
moiety.
Due to the two new stereocenters on the nitrogen atoms, five
isomers (three trans and two Λ-cis-β isomers) are possible.9c
Nevertheless, complex 10a is formed as a single species, which
is formulated as a cis-β isomer on the basis of its 31P{1H} NMR
2
2
AB system (δ 103.3 (d, JP,P′ = 19.6 Hz), 92.4 (d, JP,P′ = 19.6
Hz)). The difference in chemical shift indicates that one
phosphine is trans to acetonitrile and one is trans to amine. In
contrast, (1S,4S,9R,10R)-2b reacts with [Fe(OH2)6](BF4)2 in
MeCN at 50 °C for 2 days to give a complex mixture of
Bromocarbonyl Complex [FeBr(CO)(1a)]BPh4 (11a).
When treated with KBr (2 equiv) in acetone under a CO
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Table 2. Coordination Bond Distances in 10a and trans-
9c
[Fe(MeCN)2(P(NH)(NH)P)](BF4)2
atmosphere at room temperature, the bis(acetonitrile) complex
9a gives the bromocarbonyl derivative [FeBr(CO)-
((1S,4S,9S,10S)-1a)]+, which was isolated as the tetraphenyl-
borate salt [FeBr(CO)((1S,4S,9S,10S)-1a)]BPh4 (11a)
(Scheme 10). The 31P{1H} NMR spectrum of 11a indicates
Figure 4. ORTEP drawing of the complex cation of (OC-6−34-A)-
11aSbF6 (with ellipsoids at 30% probability). Selected bond lengths
(Å): Fe−N(1) 1.928(5), Fe−N(2) 2.010(5), Fe−P(1) 2.1708(17),
Fe−P(2) 2.2413(16), Fe−Br(1) 2.437(1), Fe−C(35) 1.832(6),
C(35)−O(1) 1.135(8).
Scheme 10. Synthesis of [FeBr(CO)((1S,4S,9S,10S)-
1a)]BPh4 (11a)
This is in agreement with the 31P{1H} NMR spectrum, where
P(2) resonates at higher field compared to P(1) (δ 76.3 vs 84.8,
respectively).
The Fe−C(35) bond length (1.832(6) Å) is at the upper
limit of the range observed for related complexes, which we
attribute to the trans influence of the P(2) phosphine, whereas
the C(35)−O(1) bond distance (1.135(8) Å) is unexceptional
for Br/CO or MeCN/CO Fe(II) complexes (1.115(5)−
1.165(8) Å).9b,11b,f,h The largest deviation from the ideal
octahedral geometry involves the N(1)−Fe−P(2) angle, which
opens up to 102.70(15)°, possibly due to the strain of the cis-β
configuration. Hence, the C(35)−Fe−Br angle is compressed
to 80.5(2)°. The N(2)−Fe−P(1) angle of 165.48(15)° is far off
the ideal value of 180°, which suggests that the ring size of the
rigid diimino ligand (1S,4S,9S,10S)-1a is at the lower end of the
range required to accommodate the trans structure. In fact,
small macrocycles (such as ligand F in Chart 3) favor the cis-β
configuration.24
However, macrocycle (1S,4S,9S,10S)-1a must possess a
certain degree of flexibility since both configurations (trans
and cis-β) are accessible. The comparison of the geometry of
ligand (1S,4S,9S,10S)-1a in trans-9a(SbF6)2 and (OC-6−34-A)-
11aSbF6 shows that only a few torsion angles change drastically
(more than 15°; see Table 3). The distortions are localized in
the C(29)−N(2)−C(28)−C(27) pivot around the imine and
in the ethylene bridge between P(1) and P(2). Together, a
ring-flip of the P(1)−Fe−P(2) chelate ring and a rotation
around the N(2)−C(29) bond allow for the configuration
change from trans to cis-β and explain the flexibility of
(1S,4S,9S,10S)-1a (Figure 5).
the presence of three species in a ca. 1:1:1 ratio, which are
2
formulated as the trans isomer (AB pattern; δ 89.7 (d, JP,P′
=
2
46.9 Hz), 86.0 (d, JP,P′ = 46.9 Hz); 42%) and two Λ-cis-β
isomers. The two AX patterns are assigned to (OC-6−23-A)-
2
11a (δ 96.7 (d, JP,P′ = 37.4 Hz, P trans to Br), 91.2 (d, 2JP,P′
=
37.4 Hz, P trans to imine); 30%) and to (OC-6−34-A)-11a (δ
84.8 (d, 2JP,P′ = 31.7 Hz, P trans to imine), 76.3 (d, 2JP,P′ = 31.7
Hz, P trans to CO); 28%) on the basis of the trans influence of
the trans ligand.42
−
The SbF6 salt of one of the cis-β isomers of 11a was
Bromocarbonyl Complex [FeBr(CO)(1b)]BPh4 (11b).
The reaction of 9b with KBr (2 equiv) under CO gives a single
isomer, as judged by 31P{1H} NMR spectroscopy (δ 92.4 (d,
2JP,P′ = 43.8 Hz), 91.2 (d, 2JP,P′ = 43.8 Hz)), which was isolated
as the tetraphenylborate salt [FeBr(CO)((1S,4S,9R,10R)-
1b)]BPh4 (11b). On the basis of the large difference in the
2JP,C coupling of the phosphines to CO in the 13C{1H} NMR
spectrum (δ 212.7 (dd, 2JP,C = 58.9, 18.3 Hz)), the complex was
crystallized and structurally characterized as (OC-6−34-A)-
11aSbF6 (CO trans to phosphorus, Br trans to imine, Figure
4).41 As in 10a, the coordination bond distances reflect the
different trans influence of the ligands. The Fe−N(2) bond
trans to phosphorus is longer than Fe−N(1) trans to bromide
(2.010(5) vs 1.928(5) Å, respectively). Also, the Fe−P(1) bond
trans to imine is significantly shorter than the Fe−P(2) bond
trans to carbonyl (2.1708(17) vs 2.2413(16) Å, respectively).
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constants (due to the two phosphines being cis to CO),
resulting in a pseudo-triplet in the 13C{1H} NMR spectrum.
Unfortunately, all attempts to obtain crystals of 11b suitable for
X-ray analysis failed.
Table 3. Selected Torsion Angles of trans-9a(SbF6)2 and
(OC-6-34-A)-11aSbF6
The kinetic cis-β isomer (OC-6−34-A), which features
mutually trans CO and phosphine ligands, is energetically
unfavorable over the thermodynamic cis-β isomer with CO
trans to imine and bromide trans to phosphorus. We explain the
stereoselectivity by assuming that 11b is formed with the
intermediacy of [Fe(CO)(MeCN)((1S,4S,9R,10R)-1b)]2+,43
which then undergoes acetonitrile dissociation to give the 16-
electron fragment [Fe(CO)((1S,4S,9R,10R)-1b)]2+ (Scheme
12). We suggest that the latter five-coordinate species adopts a
Scheme 12. Stereochemistry of the Formation of 11b
distorted trigonal-bipyramidal structure with the π-accepting
carbonyl ligand in the equatorial position because of the
tendency of the (1S,4S,9R,10R)-1b macrocycle to adopt a cis-β
configuration as discussed above. The two phenyl substituents
of the diimino backbone, which are expected to be in a trans
conformation, disfavor two out of three possible directions of
bromide attack along the trigonal plane (in red in Scheme 12).
In particular, they block the attack between phosphine and
imine, which would result in a trans-complex, and also in
between imine and CO, which would give the other Λ-cis-β
isomer (bromide trans to phosphorus, CO trans to imine).
The attack along the sterically least congested direction (in
between CO and phosphine) is consistent with the observed
formation of the Λ-cis-β isomer with bromide trans to imine
and CO trans to phosphorus. In contrast, in the sterically less
demanding cyclohexane analogue (1S,4S,9S,10S)-1a, bromide
can attack from all three directions, which explains the
formation of a mixture of isomers for 11a.
Figure 5. Comparison of ligand (1S,4S,9S,10S)-1a in the trans
configuration (trans-9a(SbF6)2, top) and the cis-β configuration ((OC-
6−34-A)-11aSbF6, bottom).
CONCLUSION AND OUTLOOK
■
The first enantiomerically pure C2-symmetric, P-stereogenic,
macrocyclic N2P2 ligands (1S,4S,9S,10S)-1a and
(1S,4S,9R,10R)-1b were prepared by a straightforward,
enantiodivergent synthesis, and their X-ray structures gave
insight into the stereospecificity of the macrocyclization step.
The marriage of interest between macrocyclic N2P2 ligands and
iron(II) produces complexes that are monomeric, stable, and
diamagnetic and that can be used as fully characterized catalyst
precursors. We are currently exploring the potential of these
complexes in the asymmetric hydrogenation of ketones.
Depending on the diamine moiety and on the ancilliary
ligands, the Fe(II) complexes adopt either trans and/or Λ-cis-β
structures, which allows for rational design of the catalytic site.
As the formation of the cis-β isomers partially follows from the
relatively small size of the macrocycles, we are also preparing
larger analogues that are expected to favor the exclusive
formation of trans complexes.
formulated as the cis-β isomer with CO trans to phosphorus
and Br trans to imine (OC-6−34-A, Scheme 11). Both, the
other Λ-cis-β and the trans isomer should have similar coupling
Scheme 11. Synthesis of [FeBr(CO)((1S,4S,9R,10R)-
1b)]BPh4 (11b)
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MHz, CDCl3): δ 7.68−7.55 (m, 4H, Ar−H), 7.54−7.46 (m, 2H, Ar−
H), 7.45−7.38 (m, 3H, Ar−H), 5.62−5.48 (m, 2H, CH2), 1.90−0.30
(br m, 3H, BH3). 31P{1H} NMR (122 MHz, CDCl3): δ 125.1 (br m).
13C{1H} NMR (75 MHz, CDCl3): δ 142.8 (d, 2JP,C = 12.1 Hz, arom.),
EXPERIMENTAL SECTION
■
General Procedures. Reactions with air- or moisture-sensitive
materials were carried out under an argon atmosphere using Schlenk
techniques or in a glovebox under argon or purified nitrogen. All
solvents were distilled from an appropriate drying agent under argon
prior to use (CH2Cl2, CD2Cl2, MeOH, and MeCN from CaH2; Et2O,
hexane, and THF from Na/benzophenone; toluene and EtOH from
1
4
132.8 (d, JP,C = 47.6 Hz, arom.), 132.5 (d, JP,C = 2.3 Hz, arom.),
4
2
132.0 (d, JP,C = 2.4 Hz, arom.), 131.1 (d, JP,C = 11.9 Hz, arom.),
1
3
131.6 (d, JP,C = 59.0 Hz, arom.), 129.2 (d, JP,C = 9.9 Hz, arom.),
3
2
128.8 (d, JP,C = 10.1 Hz, arom.), 128.5 (d, JP,C = 13.6 Hz, arom.),
121.7 (d, 3JP,C = 8.6 Hz, arom.), 76.1 (d, 2JP,C = 9.5 Hz, CH2). 11B{1H}
NMR (96 MHz, CDCl3): δ −39.0 (m). Melting point: 97.5 °C. IR
(liquid film, cm−1): 3058 (C−H), 2938 (C−H), 2878 (C−H), 2375
(B−H), 2236, 1452, 1436, 1160, 1141, 1111, 1056, 982 (P−O−C).
HRMS (ESI): calcd for C13H14BNaOP m/z 251.0770, found m/z
251.0770 [M + Na]+. Anal. Calcd for C13H14BOP: C, 68.47; H, 6.19.
Found: C, 68.37; H, 6.14. HPLC: Chiralpak IC-3 (hexane/i-PrOH,
90:10, flow rate 1.0 mL/min, λ = 230 nm), retention times tR(minor)
= 8.2 min, tR(major) = 9.6 min; 90% ee. [α]20D: −20.3 (c 1.0, CH2Cl2).
Rf (EtOAc:hex = 3:7): 0.52.
1
Na). H and 13C positive chemical shifts in ppm are downfield from
tetramethylsilane. 31P{1H} NMR spectra are referenced to external
85% H3PO4. For most complexes and for some ligands, 13C{31P,1H}
spectra were measured with 31P (and H) decoupling to improve the
1
S/N ratio. Mass spectra were measured by the MS service of the
Laboratory of Organic Chemistry (ETH Zurich). Elemental analyses
were carried out by the Laboratory of Microelemental Analysis (ETH
Zurich). (1R,2R)-1,2-Diphenylethylene-1,2-diamine and (1S,2S)-1,2-
diphenylethylene-1,2-diamine were prepared by a literature proce-
dure.44 The preparation of N,N,N′,N′-tetraethyl-1-phenylphosphinedi-
amine and (2R,4S,5R)-3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaphospho-
lidine-2-borane (3) is described in the Supporting Information. All
commercially available materials were used as received.
̈
̈
(S)-(2-(Methyl(phenyl)phosphino)phenyl)methanol Borane,
(S)-6. The optimization of the nucleophilic opening of (R)-5 is
described in the Supporting Information. (R)-5 (22.8 g, 100 mmol,
90% ee) in toluene (0.25 L) was added to a solution of methyllithium
(156 mL, 1.6 M in Et2O, 250 mmol, 2.5 equiv) and N,N,N′,N′-
tetramethylethylenediamine (45.0 mL, 300 mmol, 3.0 equiv) in
toluene (1.1 L) at −78 °C over 5 h. After 1 h, 2-propanol (50 mL) and
1.5 M aqueous HCl solution (1.0 L) were added, and the aqueous
phase was extracted twice with toluene (2 × 0.4 L). The combined
organic phases were washed with saturated aqueous NaCl solution
(0.5 L) and dried over Na2SO4, and the solvent was removed under
reduced pressure. Flash column chromatography on silica gel
(EtOAc:hex = 2:8) afforded the product as a clear oil. Yield: 23.7 g
(1R,2S)-2-(((S)-(2-(Hydroxymethyl)phenyl)(phenyl)-
phosphino)(methyl)amino)-1-phenylpropan-1-ol Borane, 4. n-
Butyllithium (545 mL, 1.6 M in hexanes, 871 mmol, 2.58 equiv) was
added to a solution of 2-bromobenzyl alcohol (82.3 g, 440 mmol, 1.30
equiv) in tetrahydrofuran (1.3 L) at −78 °C over 0.5 h, and the
solution was stirred at room temperature for 1 h. A THF solution (0.3
L) of 3 (96.5 g, 338 mmol) was added at −78 °C, and the mixture was
warmed to room temperature over 6 h. Water (0.3 L) was added, and
the organic phase was removed under reduced pressure. Saturated
aqueous KH2PO4 solution (0.5 L) and aqueous 1 M HCl solution (0.5
L) were added, and the aqueous phase was extracted four times with
dichloromethane (4 × 0.5 L). The combined organic phases were
washed with saturated aqueous NaCl solution (1 L), dried over
Na2SO4, and filtered, and hexane (1 L) was added. Dichloromethane
was removed under reduced pressure to precipitate the product, which
was crystallized overnight at −20 °C. The white solid was filtered off,
dried at high vacuum, and used without further purification. Yield: 132
g (99%). 1H NMR (300 MHz, CDCl3): δ 7.68−7.62 (m, 1H, Ar−H),
7.61−7.23 (m, 13H, Ar−H), 4.95 (dd, 3JH,H′ = 4.0, 3.6 Hz, 1H, OCH),
1
(97%). H NMR (300 MHz, CDCl3): δ 7.68−7.39 (m, 9H, Ar−H),
2
2
4.71 (d, JH,H′ = 13.3 Hz, 1H, OCHH), 4.40 (d, JH,H′ = 13.3 Hz, 1H,
2
OCHH), 2.17 (br s, 1H, OH), 1.89 (d, JP,H = 9.9 Hz, 3H, CH3),
1.75−0.40 (br m, 3H, BH3). 31P{1H} NMR (122 MHz, CDCl3): δ 8.4
(br m). 13C{1H} NMR (75 MHz, CDCl3): δ 145.0 (d, 2JP,C = 10.1 Hz,
4
3
arom.), 132.1 (d, JP,C = 2.3 Hz, arom.), 131.9 (d, JP,C = 6.9 Hz,
arom.), 131.6 (d, JP,C = 9.7 Hz, arom.), 131.4 (d, JP,C = 2.5 Hz,
arom.), 131.3 (d, JP,C = 55.6 Hz, arom.), 130.7 (d, JP,C = 8.5 Hz,
arom.), 129.1 (d, JP,C = 10.1 Hz, arom.), 128.0 (d, JP,C = 8.7 Hz,
2
4
1
3
2
3
2
3
1
3
4.69 (dd, JH,H′ = 13.2 Hz, JH,H′ = 6.7 Hz, 1H, OCHH), 4.61 (dd,
arom.), 127.6 (d, JP,C = 52.9 Hz, arom.), 62.9 (d, JP,C = 6.6 Hz,
2JH,H′ = 13.2 Hz, 3JH,H′ = 6.9 Hz, 1H, OCHH), 4.30 (dqd, 3JP,H = 10.8
Hz, JH,H′ = 6.9, 4.0 Hz, 1H, NCH), 2.67 (d, JP,H = 7.6 Hz, 3H,
NCH3), 2.46 (dd, JH,H′ = 6.9, 6.7 Hz, 1H, CH2OH), 1.98 (d, JH,H′
OCH2), 13.6 (d, JP,C = 41.5 Hz, PCH3). 11B{1H} NMR (96 MHz,
1
3
3
CDCl3): δ −35.8 (m). IR (liquid film, cm−1): 3444 (O−H), 3058 (C−
H), 2922 (C−H), 2379 (B−H), 1437, 1418, 1133, 1035, 1000, 893.
HRMS (ESI): calcd for C14H18BNaOP m/z 267.1083, found m/z
267.1086 [M + Na]+. HPLC: Chiralpak IC-3 (hexane/i-PrOH, 90:10,
flow rate 1.0 mL/min, λ = 230 nm), retention times tR(minor) = 14.6
min, tR(major) = 18.2 min; 87% ee. [α]20D: +19.1 (c 1.0, CH2Cl2). Rf
(EtOAc:hex = 3:7): 0.34.
(((1S,1′S)-Ethane-1,2-diylbis(phenylphosphinediyl))bis(2,1-
phenylene))dimethanol Diborane, (S,S)-7. sec-Butyllithium (36.0
mL, 1.3 M in cyclohexane/hexane (92:8), 46.8 mmol, 2.1 equiv) was
added at 0 °C over 15 min to (S)-6 (5.44 g, 22.3 mmol, 87% ee) in
THF (110 mL) and stirred for 1 h. Copper(II) chloride (3.40 g, 24.5
mmol, 1.1 equiv) was added to the vigorously stirred solution at −78
°C, and the mixture was warmed overnight to room temperature.
Water (50 mL) was added and THF was removed under reduced
pressure. Aqueous 2 M HCl solution (50 mL) was added, and the
aqueous phase was extracted five times with dichloromethane (5 × 50
mL). The combined organic phases were washed with saturated
aqueous NaCl solution (100 mL) and dried over Na2SO4, and the
solvent was removed under reduced pressure. Flash column
chromatography on silica gel (EtOAc:hex = 4:6) afforded the product
as a white solid. Yield: 2.51 g (46%). NMR signals for the (R,S)-
diastereoisomer are overlapping with those of the like-product. 1H
NMR (300 MHz, CDCl3): δ 7.63−7.34 (m, 18H, Ar−H), 4.65 (d,
2JH,H′ = 13.2 Hz, 2H, OCHH), 4.36 (d, 2JH,H′ = 13.2 Hz, 2H, OCHH),
3
3
=
3
3.6 Hz, 1H, CHOH), 1.27 (d, JH,H′ = 6.9 Hz, 3H, CHCH3), 1.87−
0.42 (br m, 3H, BH3). 31P{1H} NMR (122 MHz, CDCl3): δ 68.4 (br
d, 1JP,B = 86 Hz). 13C{1H} NMR (75 MHz, CDCl3): δ 144.9 (d, JP,C
=
12.5 Hz, arom.), 142.5 (arom.), 132.8 (d, JP,C = 6.8 Hz, arom.), 132.2
(d, JP,C = 73.6 Hz, arom.), 131.8 (d, JP,C = 10.2 Hz, arom.), 131.7
1
(arom.), 131.11 (arom.), 131.07 (d, JP,C = 11.7 Hz, arom.), 128.7 (d,
1
JP,C = 10.2 Hz, arom.), 128.5 (d, JP,C = 61.2 Hz, arom.), 128.4
(arom.), 127.6 (arom.), 127.5 (d, JP,C = 8.8 Hz, arom.), 126.1 (arom.),
79.0 (d, 3JP,C = 2.6 Hz, OCH), 62.7 (d, 3JP,C = 4.9 Hz, OCH2), 58.2 (d,
2JP,C = 9.6 Hz, NCH3), 31.6 (d, 2JP,C = 2.9 Hz, NCH), 11.8 (d, 3JP,C
=
4.2 Hz, CHCH3). 11B{1H} NMR (96 MHz, CDCl3): δ −35.6 (m).
Melting point: 120.5 °C. IR (liquid film, cm−1): 3402 (O−H), 3060
(C−H), 2921 (C−H), 2386 (B−H), 2245, 1436, 1175, 1070, 1022,
1001, 951, 907. HRMS (ESI): calcd for C23H29BNNaO2P m/z
416.1925, found m/z 416.1926 [M + Na]+. Anal. Calcd for
C23H29BNO2P: C, 70.24; H, 7.43; N, 3.56. Found: C, 69.99; H,
7.33; N, 3.60. Rf (EtOAc:hex = 3:7): 0.28.
(R)-1-Phenyl-1,3-dihydrobenzo[c][1,2]oxaphosphole-1-bor-
ane, (R)-5. Sulfuric acid (17.6 mL, 316 mmol, 0.95 equiv) was added
to a solution of 4 (131 g, 333 mmol) in methanol (1.3 L) at 0 °C and
warmed to room temperature overnight. The solution was
concentrated to 0.3 L under reduced pressure and ethyl acetate (1.5
L) was added. The precipitated salts were filtered off, and the solvent
was removed under reduced pressure. Flash column chromatography
on silica gel (EtOAc:hex = 1:9) afforded the product as a colorless oil
2.70−2.50 (m, 2H, PCHH), 2.36−2.09 (m, 4H, PCHH + OH), 1.85−
0.30 (br m, 6H, BH3). 31P{1H} NMR (122 MHz, CDCl3): δ 16.5 (br
s). 13C{1H} NMR (75 MHz, CDCl3): 145.2 (m, arom.), 132.7 (m,
1
that solidified upon standing. Yield: 57.6 g (76%). H NMR (300
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arom.), 132.4 (br s, arom.), 132.0 (m, arom.), 131.8 (br s, arom.),
( 5 S , 8 S , 1 3 E , 1 4 a S , 1 8 a S , 1 9 E ) - 5 , 8 - D i p h e n y l -
5,6,7,8,14a,15,16,17,18,18a-decahydrotribenzo[b,f,l][1,4,8,11]-
diazadiphosphacyclotetradecine, (1S,4S,9S,10S)-1a. Diphos-
phine (S,S)-8 (600 mg, 1.32 mmol) and (1S,2S)-cyclohexane-1,2-
diamine (151 mg, 1.32 mmol, 1.0 equiv) were dissolved in ethanol
(130 mL), and the solution was stirred for 48 h at room temperature.
The solution was concentrated to 5 mL under reduced pressure, and
the resulting white solid was filtered off and washed with ice-cold
ethanol (10 mL). Yield: 470 mg (67%). 1H NMR (300 MHz,
CD2Cl2): δ 8.95 (s, 2H, NCH), 7.78−7.72 (m, 2H, Ar−H), 7.44−
7.24 (m, 16H, Ar−H), 3.51−3.39 (m, 2H, N−CH), 2.27−2.10 (m, 2H
PCHH), 1.98−1.78 (m, 8H, PCHH + CHH), 1.57−1.45 (m, 2H,
CHH). 31P{1H} NMR (122 MHz, CD2Cl2): δ −27.2 (s). 13C{1H}
NMR (75 MHz, CD2Cl2): δ 161.3 (t, 3JP,C = 11.5 Hz, NCH), 144.3
(m, arom.), 139.4 (m, arom.), 136.9 (m, arom.), 133.1 (arom.), 132.1
(m, arom.), 130.0 (arom.), 129.8 (arom.), 128.9 (m, arom.), 128.35
(m, arom.), 128.34 (arom.), 73.2 (N−CH), 33.5 (NCHCH2), 25.0
(NCHCH2CH2), 22.0 (m, PCH2). Melting point: 175.0 °C. IR (liquid
film, cm−1): 3052 (C−H), 2927 (C−H), 2855, 1636 (CN), 1585,
1481, 1448, 1432, 1079. HRMS (ESI): calcd for C34H35N2P2 m/z
533.2270, found m/z 533.2269 [M + H]+. Anal. Calcd for
C34H34N2P2: C, 76.67; H, 6.43; N, 5.26; P, 11.63. Found: C, 76.45;
H, 6.33; N, 5.11; P, 11.60.
131.1 (m, arom.), 129.3 (m, arom.), 128.8 (d, 1JP,C = 55.0 Hz, arom.),
1
128.2 (m, arom.), 125.8 (d, JP,C = 51.9 Hz, arom.), 63.0 (m, OCH2),
1
20.3 (d, JP,C = 38.1 Hz, PCH2). 11B{1H} NMR (96 MHz, CDCl3): δ
−37.5 (m). Melting point: 98.5 °C. IR (liquid film, cm−1): 3387 (O−
H), 3059 (C−H), 2380 (B−H), 2245 (B−H), 1437, 1107, 1059.
HRMS (ESI): calcd for C28H38B2NO2P2 m/z 504.2568, found m/z
504.2567 [M + NH4]+. Anal. Calcd for C28H34B2O2P2: C, 69.18; H,
7.05. Found: C, 68.57; H, 8.07. Samples from different experiments
gave similar results. HPLC: Chiralpak IC-3 (hexane/i-PrOH, 80:20,
flow rate 1.0 mL/min, λ = 230 nm), retention times tR((R,R)-7,
minor) = 8.6 min, tR((R,S)-7) = 9.6 min, tR((S,S)-7, major) = 12.7
min; 97% ee, dr = 1:18. Rf (EtOAc:hex = 3:7): 0.09.
2,2′-((1S,1′S)-Ethane-1,2-diylbis(phenylphosphinediyl))-
dibenzaldehyde Diborane, (S,S)-8·2BH3. Activated manganese-
(IV) oxide (207 g, 2.15 mol, 75 equiv) was added portionwise to
(S,S)-7 (13.9 g, 28.6 mmol) in ethyl acetate (700 mL), and the
mixture was stirred at room temperature in the dark for 4 h. The
solution was filtered, and the residue washed with ethyl acetate (1 L).
Careful removal of the solvent under reduced pressure afforded the
product as an off-white solid, which was used directly without
purification in the next step due to the low stability of the product.
1
Yield: 10.3 g (74%). H NMR (300 MHz, CDCl3): δ 10.11 (s, 2H,
( 5 E , 7 R , 8 R , 9E , 1 5 S , 1 8S ) - 7 , 8 , 1 5 , 1 8 - T e t r a p h e n y l -
7,8,15,16,17,18-hexahydrodibenzo[f,l][1,4,8,11]-
diazadiphosphacyclotetradecine, (1S,4S,9R,10R)-1b. Diphos-
phine (S,S)-8 (500 mg, 1.10 mmol), (1R,2R)-1,2-diphenylethane-1,2-
diamine (234 mg, 1.10 mmol, 1.0 equiv), and HCl (28 μL, 2 M in
Et2O, 56 μmol, 0.05 equiv) were dissolved in ethanol (110 mL), and
the solution was stirred at room temperature for 40 h. The solution
was concentrated to 20 mL under reduced pressure, and the resulting
white solid was filtered off and washed with ice-cold ethanol (10 mL).
Yield: 478 mg (69%). This material was used for complexation without
purification. Analytically pure samples were obtained by recrystalliza-
OCH), 8.12−7.86 (m, 4H, Ar−H), 7.81−7.66 (m, 4H, Ar−H),
7.63−7.28 (m, 10H, Ar−H), 2.86−2.58 (m, 2H, PCHH), 2.57−2.33
(m, 2H, PCHH), 1.90−0.30 (br m, 6H, BH3). 31P{1H} NMR (122
MHz, CDCl3): δ 20.8 (br s). IR (liquid film, cm−1): 3058 (C−H),
2920 (C−H), 2851 (C−H), 2383 (B−H), 1696 (CO), 1436, 1198,
1107, 1058. Rf(EtOAc:hex = 3:7): 0.31.
2,2′-((1S,1′S)-Ethane-1,2-diylbis(phenylphosphinediyl))-
dibenzaldehyde, (S,S)-8. 1,4-Diazabicyclo[2.2.2]octane (6.98 g, 62.2
mmol, 3.0 equiv) was added at 0 °C to (S,S)-8·2BH3 (10.3 g, 20.7
mmol) in toluene (250 mL) and warmed to room temperature
overnight. The solvent was removed under reduced pressure. Flash
column chromatography on silica gel (EtOAc:hex = 2:8) and
recrystallization from hot methanol afforded the product as a yellow
1
tion from dichloromethane/ethanol. H NMR (300 MHz, CD2Cl2): δ
8.17 (s, 2H, NCH), 7.67 (d, 2JH,H′ = 7.5 Hz, 4H, Ar−H), 7.56−7.49
(m, 4H, Ar−H), 7.47−7.10 (m, 18H, Ar−H), 7.02−6.95 (m, 2H, Ar−
H), 4.97 (s, 2H, N−CH), 2.74−2.58 (m, 2H, PCHH), 2.39−2.24 (m,
2H, PCHH). 31P{1H} NMR (122 MHz, CD2Cl2): δ −9.2 (s). 13C{1H}
NMR (75 MHz, CD2Cl2): δ 163.3 (NCH), 141.9 (arom.), 140.8
(m, arom.), 138.8 (m, arom.), 138.4 (m, arom.), 134.2 (m, arom.),
133.7 (m, arom.), 133.5 (arom.), 130.0 (m, arom.), 129.2 (m, arom.),
129.0 (arom.), 128.8 (m, arom.), 128.6 (arom.), 128.5 (arom.), 127.5
(arom.), 85.0 (N−CH), 26.2 (m, PCH2). Melting point: 205.0 °C
(dec). IR (liquid film, cm−1): 3046 (C−H), 2905 (C−H), 2842, 1631
(CN), 1463, 1449, 1390, 1028. HRMS (MALDI): calcd for
C42H37N2P2 m/z 631.2426, found m/z 631.2425 [M + H]+. The
crystals obtained by recrystallization contained CH2Cl2, which was not
fully removed under high vacuum (ca. 0.77 equiv), as indicated by the
integration of a one-pulse 1H NMR spectrum in CDCl3 (Figure S31).
Anal. Calcd for C42H36N2P2·0.77(CH2Cl2): C, 73.84; H, 5.44; N, 4.03.
Found: C, 73.53; H, 5.36; N, 3.81.
1
solid. Yield: 5.64 g (60%). H NMR (300 MHz, CD2Cl2): 10.54 (t,
4JP,H = 2.7 Hz, 2H, OCH), 7.92−7.85 (m, 2H, Ar−H), 7.55−7.44
(m, 4H, Ar−H), 7.37−7.25 (m, 12H, Ar−H), 2.28−2.01 (m, 4H,
PCHH). 31P{1H} NMR (122 MHz, CD2Cl2): δ −22.0 (s, (S,S)-8),
−22.2 (s, (R,S)-8), dr > 99:1. 13C{1H} NMR (75 MHz, CDCl3): 192.2
3
(t, JP,C = 10.0 Hz, OCH), 142.2 (m, arom.), 138.9 (m, arom.),
137.0 (m, arom.), 133.8 (arom.), 133.4 (m, arom.), 132.0 (arom.),
131.1 (m, arom.), 129.4 (arom.), 129.3 (arom.), 129.0 (m, arom.),
1
23.8 (d, JP,C = 3.9 Hz, PCH2). Melting point: 129.5 °C. IR (liquid
film, cm−1): 3052 (C−H), 2918 (C−H), 2823, 2742, 1693 (CO),
1584, 1482, 1433, 1198. HRMS (ESI): calcd for C28H25O2P2 m/z
455.1324, found m/z 455.1324 [M + H]+. Anal. Calcd for
C28H24O2P2: C, 74.00; H, 5.32; P, 13.63. Found: C, 73.81; H, 5.11;
P, 13.51. HPLC: no sufficient separation achieved with standard
columns. [α]20D: +35.9 (c 1.0, CH2Cl2). Rf(EtOAc:hex = 3:7) = 0.45.
For the determination of the enantiomeric purity, the product was
converted to (((1S,1′S)-ethane-1,2-diylbis(phenylphosphinediyl))bis-
(2,1-phenylene))dimethanol diborane, (S,S)-7. Sodium borohydride
(8.2 mg, 0.22 mmol, 5.0 equiv) was added to (S,S)-8 (19.8 mg, 43.6
μmol) in THF (1 mL) at 0 °C. After 1 h, borane dimethyl sulfide
complex (21 μL, 0.22 mol, 5.0 equiv) was added, and the solution was
stirred overnight at room temperature. Saturated aqueous NH4Cl
solution (20 mL) was added, and the aqueous phase was extracted
three times with EtOAc (2 × 20 mL). The combined organic phases
were washed with saturated aqueous NaCl solution (30 mL) and dried
over MgSO4, and the solvent was removed under reduced pressure.
Flash column chromatography on silica gel (EtOAc:hex = 1:2)
afforded the product as a white solid. Yield: 19.3 mg (91%). Analytical
data are in agreement with the above data for (S,S)-7. HPLC:
Chiralpak IC-3 (hexane/i-PrOH, 80:20, flow rate 1.0 mL/min, λ = 230
nm), retention times tR((R,R)-7, minor) = 7.8 min, tR((R,S)-7) = 9.7
min, tR((S,S)-7, major) = 13.1 min; >99% ee, dr = 33:1. The presence
of the (R,S)-isomer is attributed to racemization during the reduction.
( 5 S , 8 S , 1 4 a S , 1 8 a S ) - 5 , 8 - D i p h e n y l -
5,6,7,8,13,14,14a,15,16,17,18,18a,19,20-tetradecahydro-
tribenzo[b,f,l][1,4,8,11]diazadiphosphacyclotetradecine,
(1S,4S,9S,10S)-2a. (1S,4S,9S,10S)-1a (175 mg, 329 μmol) was added
to a solution of lithium aluminum hydride (125 mg, 3.29 mmol, 10
equiv) in THF (3.3 mL), and the mixture was stirred overnight at
room temperature. EtOAc (1 mL) was added followed, after 1 h, by a
1:1 mixture of sodium sulfate/Celite (1 g) and water (1 mL). After 2
h, the mixture was filtered with THF (50 mL) through a 1:1 pad of
sodium sulfate/Celite, and the solvent was removed under reduced
pressure. The crude product was dissolved in dichloromethane (4 mL)
and filtered, and hexane (15 mL) was added. The solution was
concentrated to 2 mL under reduced pressure, and the resulting white
solid was filtered off and washed with ice-cold hexane (4 mL). The
compound was used in the next step without further purification (95%
purity by integration of the 31P{1H} NMR spectrum). Yield: 121 mg
1
3
(69%). H NMR (300 MHz, CD2Cl2): δ 7.47 (ddd, JP,H = 8.9 Hz,
3JH,H′ = 7.2 Hz, 4JH,H′ = 1.9 Hz, 4H, Ar−H), 7.39 (ddd, 3JP,H = 7.4 Hz,
4095
dx.doi.org/10.1021/om5005989 | Organometallics 2014, 33, 4086−4099

Organometallics
Article
4
3JH,H′ = 7.4 Hz, JH,H′ = 1.5 Hz, 2H, Ar−H), 7.34−7.27 (m, 2H, Ar−
(m, arom., major), 133.1 (arom., major), 131.5 (m, arom., major),
131.3 (arom., major), 131.2 (arom., major), 130.7 (m, arom., major),
130.3 (m, arom., major), 127.9 (m, CH3CN, minor), 125.3 (m,
CH3CN, major), 121.4 (m, CH3CN, minor), 76.6 (N−CH, minor),
73.0 (N−CH, major), 72.4 (N−CH, minor), 31.1 (NCHCH2, minor),
30.4 (NCHCH2, major), 30.1 (NCHCH2, minor), 27.1 (m, PCH2,
minor), 25.7 (NCHCH2CH2, minor), 24.9 (NCHCH2CH2, minor),
24.7 (NCHCH2CH2, major), 24.4 (m, PCH2, major), 21.3 (m, PCH2,
minor), 4.1 (CH3CN, minor), 4.0 (CH3CN, minor), 3.2 (CH3CN,
major). Signals of aromatic carbon atoms of the minor isomer are not
resolved. IR (KBr, cm−1): 3056 (C−H), 2993 (C−H), 2929 (C−H),
2862, 2295 (MeCN), 2280 (MeCN), 2250 (MeCN), 1624 (CN),
1484, 1435, 1417, 1310. HRMS (MALDI): calcd for C34H34FFeN2P2
m/z 607.1531, found m/z 607.1524 [FeF((1S,4S,9S,10S)-1a)]+. Anal.
Calcd for C38H40N4B2F8FeP2: C, 54.07; H, 4.78; N, 6.64. Found: C,
53.91; H, 4.83; N, 6.72.
H), 7.24−7.18 (m, 6H, Ar−H), 7.15−7.08 (m, 4H, Ar−H), 4.12 (d,
2JH,H′ = 10.5 Hz, 2H, NCHH), 3.93 (d, 2JH,H′ = 10.5 Hz, 2H, NCHH),
3.27−2.92 (br s, 2H, NH), 2.69−2.58 (m, 2H, NCH), 2.57−2.49 (m,
2H, PCHH), 2.36−2.24 (m, 4H, PCHH + NCHCHH), 1.85−1.76
(m, 2H, NCHCHH), 1.37−1.29 (m, 4H, NCHCH2CHH). 31P{1H}
NMR (122 MHz, CD2Cl2): δ −30.9 (s). 13C{31P,1H} NMR (126
MHz, CD2Cl2): δ 146.8 (arom.), 141.2 (arom.), 136.1 (arom.), 133.5
(arom.), 131.8 (arom.), 131.4 (arom.), 130.0 (arom.), 128.8 (arom.),
128.2 (arom.), 128.1 (arom.), 63.0 (NCH2), 52.4 (NCH), 32.7
(PCH2), 25.9 (CH2), 24.5 (CH2). Melting point: 169 °C. IR (liquid
film, cm−1): 3249 (N−H), 3052 (C−H), 2927 (C−H), 2853, 1461,
1433, 1265. HRMS (ESI): calcd for C34H39N2P2 m/z 537.2583, found
m/z 537.2590 [M + H]+.
( 7 R , 8 R , 1 5 S , 1 8 S ) - 7 , 8 , 1 5 , 1 8 - T e t r a p h e n y l -
5,6,7,8,9,10,15,16,17,18-decahydrodibenzo[f,l][1,4,8,11]diaza-
diphosphacyclotetradecine, (1S,4S,9R,10R)-2b. (1S,4S,9R,10R)-
1b (90.0 mg, 143 μmol) was added to lithium aluminum hydride (1.8
mL, 2.4 M in THF, 4.3 mmol, 30 equiv), and the mixture was stirred
for 2 days at 45 °C. THF (5 mL) and EtOAc (1 mL) were added
followed, after 1 h, by a 1:1 mixture of sodium sulfate/Celite (1.5 g)
and water (1 mL). After 2 h, the mixture was filtered with THF (50
mL) through a 1:1 pad of sodium sulfate/Celite, and the solvent was
removed under reduced pressure. The crude product was dissolved in
dichloromethane (4 mL) and filtered, and hexane (15 mL) was added.
The solution was concentrated to 2 mL under reduced pressure, and
the resulting white solid was filtered off and washed with ice-cold
hexane (4 mL). The compound was used in the next step without
further purification. Yield: 45.6 mg (50%). 1H NMR (300 MHz,
CD2Cl2): δ 7.44−6.97 (m, 28H, arom.), 4.34 (d, 2JH,H′ = 12.9 Hz, 2H,
[Fe(MeCN)2((1S,4S,9R,10R)-1b)](BF4)2, 9b. A solution of [Fe-
(OH2)6](BF4)2 (78.8 mg, 227 μmol) in acetonitrile (6.0 mL) was
added dropwise to a solution of (1S,4S,9R,10R)-1b (150 mg, 238
μmol, 1.05 equiv) in dichloromethane (3.0 mL), and the mixture was
stirred for 2.5 h at room temperature. The solvent was removed under
reduced pressure. Recrystallization from acetonitrile/diethyl ether
afforded the product as a crystalline red solid. The product exists only
1
as the Λ-cis-β isomer. Yield: 160 mg (75%). H NMR (300 MHz,
CD2Cl2): δ 8.50−8.32 (m, 1H, NCH), 8.19 (br s, 1H, NCH),
8.08−7.23 (m, 23H, Ar−H), 6.81 (dd, J = 6.5, 3.0 Hz, 2H, Ar−H),
3
6.60−6.49 (m, 3H, Ar−H), 6.35 (d, JH,H′ = 12.5 Hz, 1H, N−CH),
3.79−3.46 (m, 2H, PCHH), 3.38−3.26 (m, 1H, PCHH), 3.30 (d,
3JH,H′ = 12.5 Hz, 1H, N−CH), 2.22−2.08 (m, 1H, PCHH), 2.15 (s,
3H, NCH3), 1.75 (s, 3H, NCH3). 31P{1H} NMR (122 MHz, CD2Cl2):
2
2
2
δ 103.7 (d, JP,P′ = 36.5 Hz), 91.4 (d, JP,P′ = 36.5 Hz). 13C{31P,1H}
NMR (126 MHz, CD2Cl2): 175.0 (NCH), 170.8 (NCH), 139.2
(arom.), 138.7 (arom.), 138.2 (arom.), 137.5 (arom.), 135.8 (arom.),
135.5 (arom.), 134.90 (arom.), 134.86 (arom.), 134.6 (arom.), 134.2
(arom.), 133.5 (arom.), 133.2 (arom.), 132.5 (arom.), 132.2 (arom.),
132.1 (arom.), 131.8 (arom.), 131.7 (arom.), 131.3 (arom.), 130.94
(arom.), 130.86 (arom.), 130.6 (arom.), 130.5 (arom.), 130.2 (arom.),
130.02 (arom.), 130.00 (arom.), 129.98 (arom.), 129.4 (arom.), 128.6
(arom.), 126.9 (CH3CN), 119.9 (CH3CN), 84.3 (N−CH), 78.7 (N−
CH), 28.6 (PCH2), 22.5 (PCH2), 4.4 (CH3CN), 4.1 (CH3CN). IR
(KBr, cm−1): 3059 (C−H), 2932 (C−H), 2317 (MeCN), 2283
(MeCN), 2250 (MeCN), 1620 (CN), 1603 (CN), 1562, 1484,
1435, 1416, 1305. HRMS (MALDI): calcd for C42H36FFeN2P2 m/z
705.1682, found m/z 705.1686 [FeF(((1S,4S,9R,10R)1b)]+. Anal.
Calcd for C46H42N4B2F8FeP2: C, 58.64; H, 4.49; N, 5.95. Found: C,
57.86; H, 4.60; N, 5.41. Samples from different experiments gave
similar results.
NCHH), 3.69 (s, 2H, NCH), 3.31 (d, JH,H′ = 12.9 Hz, 2H, NCHH),
2.69−2.59 (m, 4H, PCHH), 2.51−2.22 (br s, 2H, NH). 31P{1H} NMR
(122 MHz, CD2Cl2): δ −20.5 (s). 13C{31P,1H} NMR (126 MHz,
CD2Cl2): δ 146.1 (arom.), 141.9 (arom.), 140.3 (arom.), 139.2
(arom.), 135.5 (arom.), 132.4 (arom.), 131.1 (arom.), 129.3 (arom.),
129.2 (arom.), 129.0 (arom.), 128.43 (arom.), 128.38 (arom.), 128.0
(arom.), 127.3 (arom.), 67.8 (NCH2), 50.4 (NCH), 26.6 (PCH2).
Melting point: 145 °C (dec). IR (liquid film, cm−1): 3316 (N−H),
3054 (C−H), 3026 (C−H), 2922 (C−H), 2849, 1453, 1433, 1130.
HRMS (ESI): calcd for C42H41N2P2 m/z 635.2739, found m/z
635.2737 [M + H]+.
[Fe(MeCN)2((1S,4S,9S,10S)-1a)](BF4)2, 9a. A solution of [Fe-
(OH2)6](BF4)2 (218 mg, 626 μmol) in acetonitrile (16.7 mL) was
added dropwise to a solution of (1S,4S,9S,10S)-1a (350 mg, 657 μmol,
1.05 equiv) in dichloromethane (8.3 mL). After stirring for 2.5 h at
room temperature, the solvent was removed under reduced pressure.
Recrystallization from acetonitrile/diethyl ether afforded the product
as a crystalline orange solid. The product exists as a 3.5:1 mixture of
trans and Λ-cis-β isomers. Yield: 486 mg (92%). 1H NMR (700 MHz,
CD2Cl2): δ 9.33−9.28 (m, 1H (minor), NCH), 9.18 (d, 4JP,H = 3.7
Hz, 2H (major), NCH), 8.52 (br s, 1H (minor), NCH), 8.12−
7.63 (m, 10H (major) + 6H (minor), Ar−H), 7.52−7.45 (m, 4H
(minor), Ar−H), 7.42−7.26 (m, 8H (major) + 4H (minor), Ar−H),
7.14 (td, J = 7.9, 2.3 Hz, 2H (minor), Ar−H), 6.31−6.22 (m, 2H
(minor), Ar−H), 4.27−4.20 (m, 1H (minor), N−CH), 4.06 (m, 2H
(major), PCHH), 3.91−3.80 (m, 2H (major), N−CH), 3.61−3.28 (m,
2H (minor), PCHH), 3.14−3.08 (m, 1H (minor), N−CH), 2.90 (d, J
= 4.5 Hz, 2H (major) + 1H (minor), NCHCHH (major) + PCHH
(minor)), 2.69−2.50 (m, 2H (major) + 1H (minor), PCHH), 2.35−
2.12 (m, 4H (major) + 2H (minor), NCHCHH (major) +
NCHCH2CHH (major) + CHH (minor)), 2.09−1.99 (m, 2H
(minor), CHH), 1.96−1.92 (m, 1H (minor), CHH), 1.88 (s, 3H
(minor), NCH3), 1.84−1.80 (m, 1H (minor), CHH), 1.77 (s, 3H
(minor), NCH3), 1.71−1.60 (m, 2H (major), NCHCH2CHH), 1.59−
1.53 (m, 1H (minor), CHH), 1.39−1.21 (m, 1H (minor), CHH), 1.05
(s, 6H (major), NCH3). 31P{1H} NMR (122 MHz, CD2Cl2): δ 96.8
[Fe(MeCN)2((1S,4S,9S,10S)-2a)](BF4)2, 10a. A solution of [Fe-
(OH2)6](BF4)2 (34.0 mg, 98 μmol) in acetonitrile (2.6 mL) was added
dropwise to a solution of (1S,4S,9S,10S)-2a (55.0 mg, 102 μmol, 1.05
equiv) in dichloromethane (1.3 mL). After stirring for 2 d at 50 °C,
the solution was filtered. Layering with diethyl ether afforded the
product as a crystalline orange solid. The product exists as a single Λ-
1
cis-β isomer. Yield: 45.0 mg (55%). H NMR (300 MHz, CD2Cl2): δ
7.96 (t, J = 8.2 Hz, 1H, Ar−H), 7.76 (t, J = 7.5 Hz, 1H, Ar−H), 7.69−
7.43 (m, 8H, Ar−H), 7.39−7.23 (m, 3H, Ar−H), 7.13 (dd, J = 7.6, 4.4
Hz, 1H, Ar−H), 7.00 (t, J = 6.7 Hz, 2H, Ar−H), 6.28 (t, J = 8.8 Hz,
3
2
2H, Ar−H), 6.09 (br d, JH,H′ = 12.2 Hz, 1H, NH), 4.78 (d, JH,H′
=
2
17.3 Hz, 1H, NCHH), 4.60 (d, JH,H′ = 17.3 Hz, 1H, NCHH), 3.75−
2
3.63 (m, 1H, NCHH), 3.54 (d, JH,H′ = 17.6 Hz, 1H, NCHH), 3.53−
3.31 (m, 2H, PCHH), 3.14−2.93 (m, 2H, PCHH), 2.66 (br d, 3JH,H′
=
11.2 Hz, 1H, NH), 2.32−2.05 (m, 2H, CHH), 2.14 (s, 3H, NCH3),
2.11 (s, 3H, NCH3), 1.89−1.77 (m, 2H, NCH + CHH), 1.66−1.49
(m, 2H, NCH + CHH), 1.31−1.11 (m, 1H, CHH), 0.95−0.77 (m,
1H, CHH), 0.00 to −0.12 (m, 1H, CHH), −0.12 to −0.28 (m, 1H,
2
CHH). 31P{1H} NMR (122 MHz, CD2Cl2): δ 103.3 (d, JP,P′ = 19.6
2
2
2
(d, JP,P′ = 41.3 Hz, minor), 93.4 (s, major), 88.9 (d, JP,P′ = 41.3 Hz,
minor). 13C{1H} NMR (176 MHz, CD2Cl2): δ 172.6 (m, NCH,
minor), 170.3 (m, NCH, minor), 169.8 (NCH, major), 138.9 (m,
arom., major), 138.6 (m, arom., major), 134.2 (arom., major), 133.3
Hz), 92.4 (d, JP,P′ = 19.6 Hz). 13C{31P,1H} NMR (126 MHz,
CD2Cl2): δ 142.5 (arom.), 141.9 (arom.), 136.5 (arom), 133.5 (arom.)
133.43 (arom.), 133.36 (arom.), 132.9 (arom.), 132.4 (arom.), 132.0
(arom.), 131.8 (arom.), 131.7 (arom.), 131.6 (arom.), 131.33 (arom.),
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1
131.29 (arom.), 130.5 (arom.), 130.2 (arom.), 129.9 (arom.), 129.8
(arom.), 129.5 (arom.), 128.9 (arom.), 127.4 (CH3CN), 124.0
(CH3CN), 66.0 (NCH2), 65.4 (NCH2), 53.5 (NCH), 50.2 (NCH),
31.3 (PCH2), 30.2 (PCH2), 27.4 (NCHCH2), 27.2 (NCHCH2), 25.0
(NCHCH2CH2), 24.7 (NCHCH2CH2), 5.1 (CH3CN), 4.3 (CH3CN).
IR (liquid film, cm−1): 3251 (N−H), 3236 (N−H), 3058 (C−H),
2938 (C−H), 1481, 1436, 1263. IR (KBr, cm−1): 2170 (MeCN).
HRMS (MALDI): calcd for C34H38FFeN2P2 m/z 611.1839, found m/z
611.1840 [FeF((1S,4S,9S,10S)-2a)]+ . Anal. Calcd for
C38H44B2F8FeN4P2: C, 53.81; H, 5.23; N, 6.61. Found: C, 53.92; H,
5.13; N, 6.52.
(arom.), 127.3 (arom.), 126.2 (m, BPh4), 124.4 (d, JP,C = 47.7 Hz,
arom.), 122.3 (BPh4), 121.7 (d, 1JP,C = 32.0 Hz, arom.), 84.6 (N−CH),
84.1 (N−CH), 32.0 (dd, 1JP,C = 36.7 Hz, 2JP,C = 15.6 Hz, PCH2), 20.4
(dd, JP,C = 29.2 Hz, JP,C = 11.4 Hz, PCH2). IR (KBr, cm−1): 3157
(C−H), 3052 (C−H), 2997 (C−H), 2981 (C−H), 1988 (CO), 1617
(CN), 1579 (CN), 1478, 1434, 1426, 1402, 1304, 1265, 1137,
1099, 1066, 1030, 998. HRMS (MALDI): calcd for C42H36BrFeN2P2
m/z 765.0883, found m/z 765.0885 [FeBr((1S,4S,9R,10R)-1b)]+.
Anal. Calcd for C67H56BBrFeN2OP2: C, 72.26; H, 5.07; N, 2.52.
Found: C, 69.34; H, 4.92; N, 2.40. A possible explanation for the low
carbon content is combustion problems due to tetraphenylborate.45
1
2
[FeBr(CO)((1S,4S,9S,10S)-1a)]BPh4, 11a. A solution of 9a (455.5
mg, 540 μmol) and potassium bromide (128 mg, 1.08 mmol, 2.00
equiv) in acetone (13.8 mL) was stirred for 2 d under a CO
atmosphere (2.5 bar). The solvent was removed under reduced
pressure. The crude was dissolved in MeOH and filtered. Sodium
tetraphenylborate (185 mg, 0.540 μmol, 1.00 equiv) in MeOH was
added, and the orange precipitate was collected by filtration and
washed with diethyl ether. The product exists as a ca. 0.42:0.30:0.28
mixture of the trans and the two Λ-cis-β isomers. Yield: 463.2 mg
(85%). 1H NMR (300 MHz, CD2Cl2): δ 9.04−8.96 (m, 2H, NCH),
8.83−8.70 (m, 4H, NCH), 7.97−6.81 (m, 105H, arom.), 6.31−6.15
(m, 7H, arom.), 6.07−5.56 (m, 2H, arom.), 4.30−4.14 (m, 3H, N−
CH), 3.86−2.47 (m, 14H, N−CH + CHH), 2.37−1.72 (m, 15H,
CHH), 1.56−1.09 (m, 8H, CHH), 0.99−0.84 (m, 2H, CHH). 31P{1H}
NMR (122 MHz, CD2Cl2): δ 96.7 (d, 2JP,P′ = 37.4 Hz, OC-6−23-A, P
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, product characterization data, X-ray
1
structures, and H, 31P, and 13C NMR spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
REFERENCES
■
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2
trans to Br), 91.2 (d, JP,P′ = 37.4 Hz, OC-6−23-A, P trans to imine),
2
2
89.7 (d, JP,P′ = 46.9 Hz, OC-6−43), 86.0 (d, JP,P′ = 46.9 Hz, OC-6−
2
43), 84.8 (d, JP,P′ = 31.7 Hz, OC-6−34-A, P trans to imine), 76.3 (d,
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̈
̈
−
−6.6 (br s, BPh4 ). IR (KBr, cm−1): 3056 (C−H), 2983 (C−H), 2935
(C−H), 2859, 1974 (CO), 1610 (CN), 1580 (CN), 1561 (C
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calcd for C36H34BrFeN2OP2 m/z 695.0674, found m/z 695.0672
[FeBr(CO)((1S, 4S, 9S, 10S)-1a)]+ . Anal. Calcd for
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H, 5.24; N, 2.80. A possible explanation for the low carbon content is
combustion problems due to tetraphenylborate.45
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atmosphere (2.5 bar). The solvent was removed under reduced
pressure. The crude was dissolved in MeOH and filtered. Sodium
tetraphenylborate (36.3 mg, 106 μmol, 1.00 equiv) in MeOH was
added, and the orange precipitate was collected by filtration and
washed with diethyl ether. Only one Λ-cis-β isomer was observed.
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1
Yield: 111.0 mg (94%). H NMR (300 MHz, CD2Cl2): δ 8.37−8.31
(m, 1H, NCH), 7.88 (br s, 1H, NCH), 7.84−7.63 (m, 5H, Ar−
H), 7.58−7.39 (m, 8H, Ar−H), 7.37−7.23 (m, 15H, Ar−H), 7.17−
7.11 (m, 2H, Ar−H), 7.01−6.90 (m, 10H, Ar−H), 6.85−6.77 (m, 5H,
Ar−H), 6.74 (dd, J = 6.1, 2.9 Hz, 1H, Ar−H), 6.53 (t, J = 8.7 Hz, 2H,
3
Ar−H), 6.27 (d, JH,H′ = 11.4 Hz, 1H, N−CH), 3.61−3.47 (m, 1H,
3
PCHH), 3.38 (d, JH,H′ = 11.4 Hz, 1H, N−CH), 3.34−3.14 (m, 1H,
PCHH), 3.03−2.67 (m, 1H, PCHH), 2.56−2.38 (m, 1H, PCHH).
2
31P{1H} NMR (122 MHz, CD2Cl2): δ 92.4 (d, JP,P′ = 43.8 Hz), 91.2
(d, 2JP,P′ = 43.8 Hz). 13C{1H} NMR (126 MHz, CD2Cl2): δ 212.7 (dd,
2JP,C = 58.9, 18.3 Hz, CO), 173.7 (d, 3JP,C = 2.9 Hz, NCH), 171.5 (d,
1
3JP,C = 6.9 Hz, NCH), 164.6 (dd, JB,C = 98.6, 49.4 Hz, BC), 139.0
(d, JP,C = 11.7 Hz, arom.), 138.0 (d, JP,C = 13.3 Hz, arom.), 137.9 (d,
JP,C = 5.9 Hz, arom.), 136.5 (BPh4), 136.3 (arom.), 135.5 (m, arom.),
134.1 (d, JP,C = 7.4 Hz, arom.), 134.0 (arom.), 133.6 (m, arom.), 133.5
(m, arom.), 133.2 (d, JP,C = 5.9 Hz, arom.), 132.9 (arom.), 132.7 (d,
JP,C = 8.8 Hz, arom.), 132.4 (d, JP,C = 9.5 Hz, arom.), 132.3 (arom.),
131.7 (arom.), 131.5 (d, JP,C = 9.8 Hz, arom.), 131.3 (m, arom.), 131.1
(d, JP,C = 12.9 Hz, arom.), 131.0 (arom.), 130.8 (m, arom.), 130.3
(arom.), 129.8 (m, arom.), 129.7 (arom.), 129.3 (arom.), 129.2
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(18) Tondreau, A. M.; Darmon, J. M.; Wile, B. M.; Floyd, S. K.;
Lobkovsky, E.; Chirik, P. J. Organometallics 2009, 28, 3928−3940.
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Organometallics
Article
((1S,4S,9S,10S)-1a)]BPh4 (11a) with Et3OSbF6 (1.0 equiv) in CD2Cl2
at room temperature. After stirring overnight, two of the three isomers
(presumably (OC-6−43)-11a (trans-11a) and (OC-6−23-A)-11a (cis-
β-11a, Br trans to P)) had disappeared. Addition of MeCN (20 equiv)
and layering with Et2O gave red crystals of (OC-6−33)-[Fe(MeCN)2-
((1S,4S,9S,10S)-1a)](SbF6)2 (trans-9a(SbF6)2) and orange crystals of
(OC-6−34-A)-[FeBr(CO)((1S,4S,9S,10S)-1a)]SbF6 (cis-β-11aSbF6,
CO trans to P). Loss of CO in the presence of excess MeCN has
previously been observed; see ref 9b.
(42) For the stereochemical notation of coordination compounds,
see: Nomenclature of Inorganic Chemistry: IUPAC Recommendations
2005; Conelly, N. G.; Hartshorn, R. M.; Damhus, T.; Hutton, A. T.,
Eds.; RSC Publishing: Cambridge, 2005; pp 174−199.
(43) We assume that the reaction in Scheme 11 proceeds via
intermediate [Fe(CO)(MeCN)(1b)]2+ based on the observation that
[Fe(MeCN)2(1b)]2+ does not react with KBr unless in the presence of
CO; see also refs 11a and 11c.
(44) Pikul, S.; Corey, E. J. Org. Synth. 1993, 71, 22−29.
(45) Marco,
2003, 142, 13−19.
́ ́
A.; Compano, R.; Rubio, R.; Casals, I. Microchim. Acta
̃
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