Chromatographic resolution, solution and crystal phase conformations, and absolute configuration of tert-butyl(dimethylamino)phenylphosphine-borane complex

Article abstract of DOI:10.1021/jo0605647

The enantiomers of tert-butyl(dimethylamino)phenylphosphine-borane complex 2 have been separated by HPLC using cellulose tris-p-methylbenzoate as chiral stationary phase. The borane protection could be removed without racemization and the P-configuration

Full text of DOI:10.1021/jo0605647

Chromatographic Resolution, Solution and Crystal Phase
Conformations, and Absolute Configuration of
tert-Butyl(dimethylamino)phenylphosphine-Borane Complex
Jean-Vale`re Naubron,† Laurent Giordano,† Fre´de´ric Fotiadu,*,† Thomas Bu¨rgi,‡
Nicolas Vanthuyne,† Christian Roussel,† and Ge´rard Buono*,†
UMR 6180 CNRS: “Chirotechnologies: Catalyse et Biocatalyse”, UniVersite´ Paul Ce´zanne, EGIM,
AVenue Escadrille Normandie-Nie´men, 13397 Marseille Cedex 20, France, and UniVersite´ de Neuchaˆtel,
Institut de Chimie, AVenue BelleVaux 51, CH-2007 Neuchaˆtel, Switzerland
frederic.fotiadu@egim-mrs.fr; gerard.buono@egim-mrs.fr.
ReceiVed March 14, 2006
The enantiomers of tert-butyl(dimethylamino)phenylphosphine-borane complex 2 have been separated
by HPLC using cellulose tris-p-methylbenzoate as chiral stationary phase. The borane protection could
be removed without racemization and the P-configuration of the free aminophosphine 1 has shown to be
stable in solution. Infrared (IR) and vibrational circular dichroism (VCD) spectra have been measured in
CD2Cl2 solution for both enantiomers. B3LYP/6-31+G(d) DFT calculations allowed a prediction that
complex (S)-2 exists as three conformers in equilibrium and computed population-weighted IR and VCD
spectra. Predicted and experimental IR and VCD spectra compared very well and indicate that enantiomer
(+)-2 has the S absolute configuration. This assignment has been confirmed by an X-ray diffraction
study on a single crystal of (+)-2. The crystal structure of enantiomerically pure 2 appears to be very
close to the most stable computed conformer which proved to be predominant in solution.
Introduction
ligands are usually prepared by exchange reactions between a
chiral amine and a chlorophosphine or a dialkylaminophosphine
as a trivalent phosphorus precursor.2,3 Diastereomerically pure
aminophosphine derivatives bearing also chirality at the phos-
phorus atom have been obtained in a similar way via highly
diastereoselective exchange reactions, in which chirality is
transmitted from amine to phosphorus.4-6 However, literature
offers very rare examples of enantiomerically pure aminophos-
phines with the phosphorus atom as the single chirogenic center.
In their pioneering works, Horner and Jordan prepared enan-
tiomerically enriched diethylaminoethylphenylphosphine by
Chiral tricoordinated organophosphorus ligands play a central
role in the development of asymmetric homogeneous catalysis.1
Among these, a number of phosphine derivatives containing a
P-N bond have been described and successfully applied in
stereoselective catalysts.2 Most of these ligands bear chirality
on their organic moiety and not at the phosphorus center. Such
† CNRS.
‡ Institut de Chimie.
(1) (a) Brunner, H.; Zettlmeier, W. Handbook of EnantioselectiVe
Catalysis with Transition Metal Compounds, Ligands-References; VCH:
Basel, Switzerland, 1993; Vol. 2. (b) Noyori, R. Asymmetric Catalysis in
Organic Synthesis; Wiley & Sons: New York, 1994. (c) ComprehensiVe
Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
Springer-Verlag: Berlin, 1999. (d) Catalytic Asymmetric Synthesis; Ojima,
I., Ed.; Wiley-VCH: New York, 2000. For a pertinent review on
monophosphine ligands, see the following: (e) Lagasse, F.; Kagan, H. B.
Chem. Pharm. Bull. 2000, 48, 315-324. (f) Komarov, I. V.; Bo¨rner, A.
Angew. Chem., Int. Ed. 2001, 40, 1197-1200. (g) Tang, W.; Zhang, X.
Chem. ReV. 2003, 103, 3029-3069. (h) Valentine, D. H., Jr.; Hillhouse, J.
H. Synthesis 2003, 16, 2437-2460. (i) Jerphagnon, T.; Renaud, J.-L.;
Bruneau, C. Tetrahedron: Asymmetry 2004, 15, 2101-2111.
(2) See for instances: (a) Petit, M.; Mortreux, A.; Petit, F.; Buono, G.;
Peiffer, G. New J. Chem. 1983, 7, 593-596 (b) Buono, G.; Siv, C.; Peiffer,
G.; Triantaphylides, C.; Denis, P.; Mortreux, A.; Petit, F. J. Org. Chem.
1985, 50, 1781-1782. (c) Mortreux, A.; Petit, F.; Buono, G.; Peiffer, G.
Bull. Soc. Chim. Fr. 1987, 631-639. (d) Agbossou, F.; Carpentier, J. F.;
Hapiot, F.; Suisse, I.; Mortreux, A. Coord. Chem. ReV. 1998, 178-180,
1615-1645. (e) Dyer, P. W.; Fawcett, J.; Hanton, M. J.; Kemmitt, R. D.
W.; Padda, R.; Singh, N. J. Chem. Soc., Dalton Trans. 2003, 104-113.
(3) Quin, L. D. A Guide to Organophosphorus Chemistry; Wiley-
Interscience: New York, 2000.
10.1021/jo0605647 CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/20/2006
5586
J. Org. Chem. 2006, 71, 5586-5593

tert-Butyl(dimethylamino)phenylphosphine-Borane
electrochemical reduction or cyanolysis of the corresponding
resolved amidophosphonium salts.7 These phosphinous amides
proved to be configurationally stable but very sensitive to
oxidation. Our current research project required enantiopure
samples of both antipodes of a P-chirogenic aminophosphine
of known absolute configuration. We focused on tert-butyl-
(dimethylamino)phenylphosphine 1, for which racemic alkyl-
amino and dialkylamino analogues have been described.8 Several
diastereomeric alkylamino-tert-butylphenylphosphines related
to 1 deriving from a chiral amine moiety have also been
reported.4 Absolute configuration at the phosphorus atom has
been deduced from the known configuration of the amine moiety
and the relative stereochemistry that could be assigned either
from NMR studies or X-ray diffraction of the crystallized
phosphine or its borane adduct.4,5,9 In the case of P-chiral
compounds related to aminophosphine 1, absolute configurations
have been deduced from chemical correlation with phosphorus
derivatives of known stereochemistry.10 Recently, Polavarapu
et al. have used vibrational circular dichroism (VCD) spectros-
copy in conjunction with density functional theory (DFT)
calculations to determine the predominant conformation and
phosphorus absolute configuration of tert-butylphenylphosphine
oxide,11 tert-butyl-1-(2-methylnaphthyl)phosphine oxide,12 tert-
butylphenylphosphinothioic acid,13 and tert-butylphenylphos-
phinoselenoic acid,14 which are pentavalent phosphorus com-
pounds structurally close to 1. During the last years, VCD has
emerged as a technique of choice to assign absolute configu-
ration to chiral compounds and elucidate their conformational
equilibriums.15 The implementation of quantum mechanical
methods that provide reliable predictions of VCD spectra in
commercial quantum chemistry packages16 made application of
this technique more straightforward.17 General methodology
consists, first, to identify the stable conformations of a chiral
molecule, second, to compute theoretical spectra for each
individual conformer and deduce the population-averaged VCD
spectra for each enantiomer, and third, to compare these spectra
with the experimental record for an enantiopure or enantio-
merically enriched sample in solution.11-15 As far as we know,
this approach has not been previously applied to trivalent
phosphorus compounds.18
In this work, we apply such a combination of DFT calcula-
tions and VCD measurements to assign absolute configuration
to the enantiomers of borane adduct 2 of aminophosphine 1.
Borane protection prevents oxidation of trivalent phosphorus
species 1 and allows easier manipulation and storage.19 Chiral
liquid chromatography contribution to the determination of the
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in an equilibrium between pentavalent and trivalent tautomeric structures,
only one pentavalent (tetracoordinated P) form is present in solution.11,12
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Silicon Relat. Elem. 1996, 115, 273-300.
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J. Org. Chem, Vol. 71, No. 15, 2006 5587

Naubron et al.
SCHEME 1
absolute configuration of enantiomers have recently been
reviewed by Roussel et al.20 Preparative chiral HPLC combined
with physicochemical methods is a powerful tool to determine
absolute configuration: chiral chromatography provides pure
enantiomers and then the absolute configuration of an isolated
enantiomer can be assigned by X-ray analysis, VCD, NMR,
optical rotation, or CD. A systematic screening procedure of
commercially available chiral columns with an automated HPLC
system allows finding suitable conditions for resolution of
complex 2 on a semipreparative scale. From a comparison
between experimental infrared (IR) and VCD spectra and
theoretical predictions, the three conformers of 2 present in
solution are identified. The structure of (+)-tert-butyl(dimethy-
lamino)phenylphosphine-borane 2 in the solid phase is obtained
from single-crystal X-ray diffraction and corresponds to the most
stable conformation in solution. Finally, assignment of the
absolute configuration is confirmed by the X-ray studies.
chromatographic method developed was scaled-up with a
semipreparative Chiralcel OJ (250 × 10 mm) column. Iterative
injections afforded ca. 30 mg of each enantiomer with ee >
99% in 1 h.
Decomplexation of a sample of enantiopure dextrorotatory
borane complex 2 was achieved by heating the sample at 45
°C with DABCO; then the free aminophosphine 1 was purified
by filtration through activated neutral alumina and characterized
by 31P NMR spectroscopy. A new protection of this free
aminophosphine was achieved with BH3-SMe2. The enantio-
meric purity of the resulting complex (+)-2 was the same as
before de- and reprotection (>99% ee by chromatographic
analysis). This indicates that no epimerization occurs upon de-
and reprotection and that the P-configuration of the free
aminophosphine 1 is stable in solution under the chemical and
thermal conditions we used. This opens the way to applications
of enantiopure aminophosphine 1 in asymmetric synthesis or
catalysis. It is worth noting that borane complex (+)-2 might
also be used directly as a pro-ligand for transition metal
catalyzed reactions, according to a recently reported procedure.19f
DFT calculations and VCD spectroscopy were performed to
establish the absolute configuration at the phosphorus atom of
resolved (+) and (-)-2. Conformational analysis of (S)-2 at the
B3LYP/6-31+G(d) level was carried out by rotating simulta-
neously the phenyl and dimethylamino groups and allowing full
relaxation of the rest of the structure. Because of steric hindrance
around the tetracoordinated phosphorus center, one group could
not be rotated independently of the other and collapse of the
substituents occurred during conformational exploration. Careful
scanning of the potential energy surface by varying the C7C2-
PB and C14NPB dihedral angles (Figure 1) allowed us to find
three distinct minima.22 The corresponding fully optimized
geometries are depicted in Figure 1. The converged dihedral
angles, optimized electronic energies, Gibbs energies, and
relative populations are listed in Table 1. For the three structures,
the P-t-Bu and P-BH3 bonds exhibit slightly distorted stag-
gered conformations, as one can expect. In the highest energy
conformer a, the phenyl ring is nearly eclipsed with the P-B
bond, whereas it is almost perpendicular in the most stable form
c. In both cases, the dimethylamino group exhibits a synclinal
orientation with respect to the P-B bond. The pyramidalization
of the nitrogen atom, which is measured by the C14NPC13
dihedral angle (Figure 1), is affected by the rotation of the
phenyl ring. Probably because of strong steric repulsions with
the proximal ortho position of the phenyl ring, the NMe2 group
tends to adopt a more flattened geometry in conformation c
compared to a (C14NPC13 ) 158.6° instead of 143.8°).
Completely different orientations of the two rotatable substit-
uents were encountered in conformer b: the phenyl ring is
Results and discussion
Following a procedure similar to that used by Maier,8b
racemic phosphine-borane 2 was readily prepared from dichlo-
rophenylphosphine in a three steps one-pot reaction sequence
(Scheme 1). PhPCl2 was first allowed to react with 3 equiv of
tert-butylmagnesium chloride in ether, then with a large excess
of dimethylamine, to give tert-butyl(dimethylamino)phenylphos-
phine 1 that has not been isolated. Reactions were monitored
by 31P NMR spectroscopy, total conversion at each step being
confirmed by a single signal at 121.9 ppm for intermediate tert-
butylchlorophenylphosphine, then 85.4 ppm for aminophosphine
1. Further protection of 1 with BH3-SMe2 afforded rac-2 in
31% overall yield from starting material after chromatographic
purification (δ 31P ) 87.8 ppm). Complex 2 was air and
moisture stable, soluble and inert in common alkane, chloro-
alkane, methanol, and aromatic solvents. This prompted us to
intent to resolve rac-2 by liquid chromatography on chiral
stationary phase. The enantiomers of a number of racemic
organophosphorus compounds have been separated by chiral
HPLC. However, according to Chirbase database, no chiral
HPLC resolution of trivalent phosphorus compound including
a P-N bond, either free or as its borane adduct, has been
reported in the literature.21 Therefore, a screening procedure of
11 commercially available chiral stationary phases was applied
to find suitable analytical chromatographic conditions for
phosphine-borane complex 2. It resulted that only cellulose
tris-p-methylbenzoate (Chiralcel OJ-H) with hexane/propan-2-
ol 99/1 as the eluent was able to separate the enantiomers. The
(20) Roussel, C.; Del Rio, A.; Pierrot-Sanders, J.; Piras, P.; Vanthuyne,
N. J. Chromatogr. A 2004, 1037, 311-328.
(21) (a) Roussel, C.; Piras, P. Pure Appl. Chem. 1993, 65, 235-244. (b)
Roussel, C.; Pierrot-Sanders, J.; Hietmann, I.; Piras, P. CHIRBASE,
Database current status and derived research applications using molecular
similarity, decision tree and 3D , enantiophore . search. In Chiral
Separation Techniques - A Practical Approach, 2nd ed.; Subramanian,
G., Ed.; Wiley-VCH: Weinheim, Germany, 2001; pp 95-125.
(22) Because of the local C2 symmetry of the phenyl ring and the
possibility of inverting the nitrogen atom of the dimethylamino group during
potential energy surface exploration, redundant structures having the same
energy could be located. Only the three distinct stable conformations are
presented here.
5588 J. Org. Chem., Vol. 71, No. 15, 2006

tert-Butyl(dimethylamino)phenylphosphine-Borane
FIGURE 1. Optimized B3LYP/6-31+G(d) structures of the three stable conformations a, b, and c of (S)-tert-butyl(dimethylamino)phenylphosphine-
borane and atom numbering used to define characteristic dihedral angles.
TABLE 1. Conformations and B3LYP/6-31+G(d) Energies of (S)- tert-Butyl(dimethylamino)phenylphosphine-Borane
dihedral anglesa
energyb
∆Gc
populationd
(%)
conformera
C2C7PB
C14NPB
C14NPC13
electronic
Gibbs
(kcal mol-1
)
a
b
c
5.5
20.1
78.0
-14.7
59.8
-22.4
143.8
129.1
158.6
-892.09850
-892.09921
-892.09957
-891.81231
-891.81281
-891.81410
1.12
0.81
0.00
10.8
18.2
71.0
a See Figure 1 for labels, angles in degree. b In hartrees. c Relative Gibbs energy difference in kcal mol-1 d Population at 298 K based on Gibbs energies.
.
synclinal, having an intermediate torsion between the ones in a
and c, and the dimethylamino group adopts a local Cs symmetry
with respect to P-BH3. In this orientation, the nitrogen atom
presents a rigorously pyramidal arrangement, with 129.1° for
the C14NPC13 angle. Because of their antiperiplanar orientation,
a strong overlap between the nitrogen lone pair of electrons
and the P-B bond (i.e., negative hyperconjugation) should
explain this nitrogen sp3 type hydridization in conformer b.23
The two different orientations of the NMe2 group observed in
conformer a and c on one hand, and b on the other hand, are
reminiscent of previous findings concerning the geometry of
tris(dimethylamino)phosphoranes, (Me2N)3P-BH3,24 and more
generally P(NR2)3 skeletons and related fragments.25 In such
compounds, dialkylamino groups of two different coordination
geometries similar to what we found (either synclinal with a
flattened nitrogen environment or local Cs symmetry with a
pyramidal nitrogen environment) coexist in the same molecule
as the result of electronic effects. Contrary to previously studied
tetracoordinated phosphorus species bearing tert-butyl and aryl
substituents,11-14 the preferred conformation of complex 2 does
not correspond to a nearly perpendicular arrangement of the
phenyl ring with respect to the bulky tert-butyl group, as
observed in conformer b. Instead, conformer c in which the
phenyl ring is synclinal with respect to P-t-Bu and perpen-
dicular to P-BH3 is slightly more stable than b and a. However,
the energies of conformers a, b, and c are very similar, spanning
a range of 1.12 kcal mol-1. Furthermore, the computed barriers
to internal rotations are very low: for instance, the transition
state allowing the conversion of conformer a into b is only 2.45
kcal mol-1 (see Supporting Information). Therefore, (S)-2 is
predicted to exist as an equilibrium mixture of all three
conformations at 298 K. The equilibrium populations were
calculated from Gibbs energies for isolated molecules using
Boltzmann statistics (Table 1). Although conformer c is
predominant (71%), the alternative allowed conformations of
(S)-2 a and b, are predicted to be significantly populated.
The IR and VCD intensities were calculated for the three
conformers at the B3LYP/6-31+G(d) level. In each case, the
most intense bands correspond to B-H stretching in the 2300-
2400 cm-1 region. For instance, dissymmetric and symmetric
B-H stretching bands appear at 2415 (no. 1) and 2363 (no. 2)
cm-1, respectively, for conformer c. Calculations predict that
the corresponding VCD signals are relatively intense and present
characteristic positive-negative alternations which are isolated
from others bands of the spectra (see Supporting Information).
Therefore, these signals could have been of great interest to
assign absolute configuration to complex 2, and from a more
general point of view, to any phosphine-borane adduct that
should present similar bands. Unfortunately, we could not obtain
satisfactorily experimental spectra in this region because of
(23) Gilheany, D. G. Chem. ReV. 1994, 94, 1339-1374.
(24) Mitzel, N. W.; Lustig, C. J. Chem. Soc., Dalton Trans. 1999, 3177-
3183.
(25) (a) Mitzel, N. W.; Smart, B. A.; Dreiha¨upl, K.-H.; Rankin, D. W.
H.; Schmidbaur, H. J. Am. Chem. Soc. 1996, 118, 12673-12682. (b)
Belyakov, A. V.; Baskakova, P. E.; Haaland, A.; Swang, O.; Bilkov, L.
V.; Bolubinskii, A. V.; Bogoradovskii, E. T. Russ. J. Gen. Chem. 1998,
68, 244-250.
poorer signal-to-noise ratio above 1800 cm-1 26
,
overlap with
J. Org. Chem, Vol. 71, No. 15, 2006 5589

Naubron et al.
FIGURE 3. Comparison of the experimental VCD spectra of (-)-
tert-butyl(dimethylamino)phenylphosphine-borane (-)-2 and (+)-2 at
0.26 M in CD2Cl2 solution (top two traces) with predicted VCD spectra
(bottom three traces) of stable conformations of (S)-2 (structure and
conformation labels in Figure 1) obtained with the B3LYP/6-31+G(d)
level. For predicted spectra, wavenumbers are scaled by the factor
0.9614. The trace labeled “predicted” (third from top) is the population-
weighted spectrum with populations given in Table 1. Numbers indicate
corresponding features in the predicted spectrum of (S)-2 and the
experimental spectrum of (+)-2.
FIGURE 2. Comparison of the experimental IR spectrum of tert-butyl-
(dimethylamino)phenylphosphine-borane 2 at 0.06 M in CD2Cl2
solution (top trace) with predicted IR spectra (bottom three traces) of
stable conformations of the S configuration (structure and conformation
labels in Figure 1) obtained with the B3LYP/6-31+G(d) level. For
predicted spectra, wavenumbers are scaled by the factor 0.9614. The
trace labeled “predicted” (second from top) is the population-weighted
spectrum with populations given in Table 1. Numbers indicate
corresponding features in the predicted spectrum of (S)-2 and the
experimental spectrum of 2.
lower in intensity and broader than the corresponding band
(1052-1054 cm-1) in the computed spectrum.27 Detailed
comparison of the predicted frequencies to the experimental
values and vibrational assignments are given in Table 2S of
the Supporting Information. Most of the observed bands can
be assigned to vibration modes of conformer c and support the
conclusion that this species is predominant in solution. However,
since the experimental spectrum shows a low intensity band
(no. 14) at 1151 cm-1 corresponding to predicted absorption of
conformer b, it can be concluded that this species is also present
in solution in minor quantities, in agreement with theoretical
predictions (b: 18%, Table 1).
The calculated VCD spectra of (S)-2 are compared to the
experimental VCD spectra of (+) and (-)-2 in CD2Cl2 solutions
in Figure 3. Despite low intensities of the VCD signals, the
spectra of the enantiomers are nearly perfect mirror images, as
one could expect. Agreement between the predicted population-
weighted VCD spectrum of (S)-2 and the recorded spectrum of
(+)-2 is good enough to support unambiguous elucidation of
CO2 bands of the atmosphere, and relative weak intensities of
the VCD signals of our samples which are a consequence of
the conformational flexibility of the molecule (vide infra).
Because of interferences from CD2Cl2 absorption, the region
below 1000 cm-1 was not considered. According to previous
works,11-15 we thus focused our attention on the 1600-1000
cm-1 spectral range. The experimental IR spectrum in CD2Cl2
solution is shown in Figure 2, together with the spectrum of
the equilibrium mixture of conformers which was obtained by
weighting the spectrum of each conformer by its Boltzmann
population. As delineated in Figure 2, convincing assignment
of the fundamental transitions in the IR experimental spectrum
can be made. The agreement is excellent for most of the signals
(differences between predicted and observed frequencies are less
than 8 cm-1), with the exception of modes no. 7, 11, 17, and
18. The most notable differences concern the band (no. 17) at
1103 cm-1 in the experimental spectrum, which is shifted by
30 cm-1 and the band (no. 18) at 1068 cm-1, which is relatively
(26) Measurements above 1800 cm-1 required the removal of the low-
pass filter.
(27) These discrepancies should result from intrinsic limitations of the
methods used to predicted spectra.16,17
5590 J. Org. Chem., Vol. 71, No. 15, 2006

tert-Butyl(dimethylamino)phenylphosphine-Borane
the absolute configuration at the phosphorus atom. The assign-
ment of fundamental modes is straightforward and agrees with
the one done previously for IR spectra. The significant VCD
bands in the experimental spectrum of (+)-2 are a relatively
intense positive-negative-positive triplet with peak maxima
at 1476 (no. 3), 1462 (nos. 4,5), and 1441 cm-1 (no. 6),
respectively, a positive band at 1289 cm-1 (no. 11), a relatively
intense negative-negative-positive triplet at 1208 (no. 12),
1189 (no. 13), and 1181 cm-1 (no. 14), respectively, and finally,
a positive-positive-negative triplet at 1085 (no. 17), 1075 (no.
18), and 1067 cm-1 (no. 19), respectively. The same charac-
teristic features are clearly identified in the predicted VCD
spectrum of (S)-2, except that the bands (no. 4) at 1475 cm-1
and (no. 5) at 1467 cm-1 are better resolved compared to the
experimental spectra and that band no. 11 is shifted, as
previously observed with IR spectra. The similarity of the signal
shapes between the predicted and experimental spectra is also
worth noting. In the 1500-1400 cm-1 and 1200-1100 cm-1
regions, the pattern of the predicted population-weighted VCD
spectrum of (S)-2 is significantly distinct from the one of major
conformer c because of the superposition of intense bands from
the spectra of minor species a and b (Figure 3). Indeed, the
relative intensity of the positive peaks 3 and 6 is clearly affected
by the contributions from the spectra of conformers a and b.
Moreover, the positive-negative couplet 15-16 is absent from
the spectrum of c and results from the population-weighted
contribution of the spectrum of conformer b. Since the
experimental VCD spectrum of (+)-2 shows clearly these
features, this confirms that conformer b is present in CD2Cl2
solution at a concentration similar to the population predicted
by the DFT calculations. This is probably also the case for
conformer a. However, since the expected solution concentration
is lower, no definite conclusion can be derived from the spectra
for this minor conformer. It should be noted that conformational
flexibility of the molecule is detrimental to the signal intensity
in the experimental VCD spectrum. In fact, averaging of the
signals over the contributions of the three stable conformations
of (S)-2 leads to weaker band intensities in the population-
weighted VCD spectrum compared to single conformer spectra,
as shown in the 1500-1400 cm-1 and 1200-1100 cm-1 regions
(Figure 3).28
Complementary energy calculations at different theoretical
levels on the B3LYP/6-31+G(d) geometries were undertaken
to compare with the above results (see Supporting Information).
Extension of the basis set to 6-311+G(2d,2p) and use of a
different functional (PBE1PBE/6-311+G(2d,2p)) induced only
slight changes in the relative stabilities of the three conformers
and the resulting populations (Table 1S). As previously, the
predicted VCD spectra of (S)-tert-butyl(dimethylamino)phe-
nylphosphine borane adduct match well the experimental
spectrum of (+)-2 (Figure 4S). In contrast, MP2/6-311+G(2d,-
2p)//B3LYP/6-31+G(d) calculations predicted conformer b to
be predominant over c and a (64, 30, and 6%, respectively).
However, in this case, comparison between the experimental
and the population-weighted predicted VCD spectra led to a
much poorer agreement. This clearly indicates that the popula-
tions predicted using the MP2 energies do not reflect the
distribution of the three conformers in equilibrium in the sample
at 298 K, contrary to those calculated at the B3LYP level. Thus,
despite the intrinsic limitations of the computational methods,
FIGURE 4. Thermal ellipsoidal view of (+)-(S)-tert-butyl(dimethy-
lamino)phenylphosphine-borane. Ellipsoids are shown with 50%
probability.
the small energy differences between the three conformers, and
the low VCD signal intensities due to conformational flexibility,
B3LYP/6-31+G(d) calculations seem to agree reasonably well
with the experimental data and allow a conclusion on both the
predominant conformation and the absolute configuration of the
borane complex (+)-2.
The dextrorotatory enantiomer of tert-butyl(dimethylamino)-
phenylphosphine-borane was crystallized from hexane, afford-
ing colorless crystals with a melting point of 53 °C (uncor-
rected). The X-ray crystal structure determination was carried
out on a single crystal of (+)-2 by anomalous dispersion and
unequivocally revealed that the absolute configuration at the
chirogenic phosphorus atom is S, as shown in Figure 4. This
result fully confirms the assignment derived from VCD studies.
The unit cell consists of four molecules in the same conforma-
tion. The structure of (+)-(S)-2 in the crystalline phase exhibits
geometrical parameters and characteristic torsion angles which
are strikingly close to the values predicted by DFT calculations
for conformer c (Figure 1, Table 1): the tert-butyl group
presents a nearly staggered conformation, the phenyl ring is
almost perpendicular to the P-B bond (C7C2PB ) 82.6°)
whereas the dimethylamino group is synclinal (C14NPB )
-22.3°), and the nitrogen atom exhibits a flattened pyramidal
shape (C14NPC13 ) 161.1°). Thence, crystal packing does not
have any notable influence on the molecular conformation, and
the predominant conformer in solution and the structure in the
crystalline phase of (+)-(S)-2 are almost identical.29
Conclusions
Enantiomerically pure P-chirogenic aminophosphines are not
easily accessible compounds. The procedure reported here offers
straightforward access to the two enantiomers of tert-butyl-
(dimethylamino)phenylphosphine protected by borane via ex-
peditious resolution by semipreparative chiral HPLC. The
absolute configuration S was unequivocally assigned to the
dextrorotatory enantiomer (+)-2 by comparison between pre-
dicted and experimental VCD spectra and independently
confirmed by X-ray diffraction. The molecular structure of (+)-
(S)-2 in the crystal corresponds to the predominant conformer
c in CD2Cl2 solution. Indications of the presence of minor
proportions of less stable conformation b in the solution phase
are found in IR and VCD spectra. As far as we can know, this
is the first example of direct assignment of absolute configu-
ration to a P-chirogenic aminophosphine.
(28) Consequences of conformational flexibility for VCD studies in
combination with DFT calculations have been recently discussed.15j
(29) This result is in sharp contrast with recent findings by Polavarapu
et al. in the case of (-)-(S)-tert-butylphenylphosphinoselenoic acid.14
J. Org. Chem, Vol. 71, No. 15, 2006 5591

Naubron et al.
Spectroscopic Measurements. IR and VCD spectra were
recorded on a specific accessory coupled to a Fourier transform
infrared spectrometer. A photoelastic modulator set at l/4 retardation
was used to modulate the handedness of the circular polarized light.
Demodulation was performed by a lock-in amplifier. An optical
low-pass filter (<1800 cm-1) put before the photoelastic modulator
was used to enhance the signal/noise ratio. A transmission cell
equipped with KBr windows and a 0.1 mm Teflon spacer was used.
All solutions were prepared from a 4 mg sample in 70 µL of CD2-
Cl2. A VCD spectrum of the racemic mixture was subtracted from
the VCD spectrum of the pure enantiomers. For the individual
spectra of the enantiomers and racemic mixture, about 8800 scans
were averaged at 4 cm-1 resolution (corresponding to 2 h measure-
ment time). The spectra are presented without smoothing or further
data processing.
Calculations. The fully optimized geometries, vibrational fre-
quencies, IR, and VCD intensities for (S)-2 were calculated using
DFT with the B3LYP hybrid functional and the 6-31+G(d) basis
set, using the Gaussian 03 program.16 VCD intensities were
calculated using the procedure of Cheeseman et al. implemented
in the Gaussian 03 program.17 IR and VCD spectra were simulated
using Lorentzian band shapes, with full width at half-height γ )
5.0 cm-1. The wavenumbers of predicted spectra were scaled by
the factor 0.9614 suggested by Scott and Radom.30
Crystal and Molecular Structure of (+)-2. The X-ray data were
collected at 293 K with graphite-monochromatized Mo KR radia-
tion. The compound (+)-2 crystallizes in the orthorhombic system,
in space group P212121 with the unit cell consisting of four
molecules. The lattice constants were refined by a least-squares fit
of 3466 reflections in the θ range of 1.19-28.27° using standard
procedures.31 A total of 3436 observed reflections with I > 0σ(I)
were used to solve the structure by direct methods and to refine it
by full matrix least-squares using F2.32 The final refinement
converged to R ) 0.0389 for 137 refined parameters and 3189
observed reflections with I > 2σ(I). The absolute configuration at
the chiral phosphorus atom was established as SP. The absolute
structure was determined by the Flack method with the final
parameter ø ) -0.02(9).33 Cell refinement and data reduction were
carried out with the Denzo and Scalepack computing package,31
the structure solution SIR92,34 and the structure refinement
SHELXL-97.32b
Experimental Section
tert-Butyl(dimethylamino)phenylphosphine-Borane rac-2. A
solution of tert-butyl chloride (7.13 g, 8.45 mL, 77 mmol) in dry
Et2O (50 mL) was added slowly to Mg (1.82 g, 75 mmol) and
dibromoethane (1%) in dry Et2O (25 mL) under nitrogen. After
the addition, stirring was continued for 4 h under reflux. The
resulting gray suspension was then cooled in an ice bath, and a
solution of PPhCl2 (4.65 g, 3.50 mL, 26 mmol) in dry Et2O (35
mL) was added dropwise. After 18 h at room temperature, the
magnesium salts were filtered off under Ar to give a solution of
tert-butylchlorophenylphosphine. Complete and clean reaction was
observed by 31P NMR spectroscopy of the crude mixture. 31P NMR
(81 MHz, C6D6): δ 121.9 (s). The resulting mixture containing
tert-butylPhPCl was transferred by a syringe to a dropping funnel
and was slowly added to a stirred solution of dimethylamine (7.03
g, 156 mmol) in dry Et2O (150 mL) under Ar at -10 °C. After 12
h at room temperature, the solvent was partially evaporated under
reduced pressure, and the ammonium salts were filtered off through
Celite under Ar. The 31P NMR (81 MHz, C6D6) spectrum of the
resulting solution shows only one singlet at δ ) 85.4 ppm for the
tert-butyl(dimethylamino)phenylphosphine 1. BH3-SMe2 (2.27 g,
30 mmol) was added dropwise to a stirred and cooled solution at
0 °C of 1 in 130 mL of dry Et2O. After 12 h at room temperature,
evaporation of the solvent gave 3.9 g (67%) of the crude product.
Flash chromatography on deactivated silica gel (10% NEt3, AcOEt/
light petroleum 1:20) afforded 1.8 g (8.07 mmol, 31%) of tert-
butyl(dimethylamino)phenylphosphine-borane 2 as a colorless oil
that solidified on standing in freezer. 31P NMR (81 MHz, C6D6):
1
1
δ 87.8 (q, JPB ) 63.6 Hz). H NMR (200 MHz, C6D6): δ 0.81-
2.25 (q,1JBH ) 95.4 Hz, 3H), 1.12 (d, JPH ) 13.5 Hz, 9H), 2.44
3
3
(d, JPH ) 8.4 Hz, 6H), 7.04-7.08 (m, 3H), 7.62-7.71 (m, 2H).
13C NMR (50 MHz, C6D6): δ 27.3 (d, 2JPC ) 2.7 Hz, 3C), 34.8 (d,
1JPC ) 33.0 Hz, 1C), 40.7 (d, 2JPC ) 1.8 Hz, 2C), 128.3 (d, 2JPC
)
4
1
9.0 Hz, 2C), 130.5 (d, JPC ) 2.3 Hz, 1C), 132.2 (d, JPC ) 56.3
Hz, 1C), 132.4 (d, 3JPC ) 8.7 Hz, 2C). IR (c 0.06 in CD2Cl2) (cm-1):
3088, 3063, 2988-2808, 2391, 2349, 1487-1436, 1396, 1367,
1068, 989. TLC: Rf ) 0.22 (SiO2, Et2O). Anal. Calcd for C12H23-
BNP: C, 64.60; H, 10.39; N, 6.28. Found: C, 64.83; H, 10.31; N,
6.31.
Chromatographic Procedures. The analytical chiral HPLC
experiments were performed on a screening unit composed of eleven
chiral columns connected to a 12 positions valve, UV-detector, and
on-line polarimeter. Hexane and propan-2-ol, HPLC grade, were
degassed and filtered on a 0.45 µm membrane before use. The
different analytical columns (250 × 4.6 mm) tested were Chiralcel
OD-H, OJ-H, OC, OG, OB-H and Chiralpak AS and AD columns
from Chiral Technology Europa (Illkirch, France), Whelk-O1 (S,S)
and Ulmo (S,S) from Regis (Morton Grove, USA), Sumichiral OA-
2500 from Sumitomo Chemicals (Osaka, Japan) and Kromasil CHI-
TBB from Eka Nobel (Bohus, Sweden). Retention times Rt in
minutes, retention factors ki ) (Rti - Rt0)/Rt0 and enantioselectivity
factor R ) k2/k1 are reported. Rt0 was determined by injection of
tri-tert-butylbenzene. Analytical separation was performed on
Chiralcel OJ-H (250 × 4.6 mm, 5 µm) in hexane/propan-2-ol (99/
1) at 1 mL/min and 25 °C with UV detection 254 nm and
polarimeter, Rt1(-, R) ) 9.33, Rt2(+, S) ) 11.15, k1(-, R) ) 2.01,
k2(+, S) ) 2.60, R ) 1.29, Rs ) 2.15. Semipreparative separation
was performed on a typical HPLC unit including a valve with a
500 µL loop and a UV-detector. An amount of 500 µL of a 10
mg/mL solution of racemate was injected every 5 min on Chiralcel
OJ (250 × 10 mm, 10 µm): flow-rate ) 4.5 mL/min; mobile phase,
hexane/propan-2-ol (99/1); detection on UV 254 nm, Rt1(-, R) )
8.94, Rt2(+, S) ) 11. 56, k1(-, R) ) 1.80, k2(+, S) ) 2.62, R )
1.46, Rs ) 1.54. Twelve injections over 1 h afforded 30 mg of
each enantiomers. [R]20D +29.3 ((+)-2, c 1.0 in toluene, ee > 99%),
We have deposited all crystallographic data for this structure in
the Cambridge Crystallographic Data Centre under the number
273736.35
Acknowledgment. The financial supports by the CNRS and
the MENESR (including a fellowship for J.-V.N.) are gratefully
acknowledged. We are grateful to the CRC-MM (Fe´de´ration
de Recherche des Sciences Chimiques de Marseille FR1739)
and the National Computer Centre of Higher Education (CINES)
for computer time. We thank Dr. Michel Giorgi and the Service
Commun de Cristallochimie des Universite´s d′Aix-Marseille I
& III for the crystallographic studies.
(30) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502-16513.
(31) Otwinowski, Z.; Minor, W. Macromolecular Crystallography, part
A; Carter, C. W., Jr., Sweet, R. M., Eds.; Methods in Enzymology, Vol.
276; Academic Press: San Diego, CA, 1997; pp 307-326.
(32) (a) Sheldrick, G. M.; Kruger, G. M.; Goddard, R. Acta Crystallogr.,
Sect. A 1990, 46, 467-473. (b) Sheldrick, G. M. SHELXL97. Program for
the refinement of crystal structures; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(33) (a) Flack, H. D. Acta Crystallogr., Sect. A. 1983, 39, 876-881. (b)
Flack, H. D.; Bernardinelli, G. Acta Crystallogr., Sect. A. 1999, 55, 908-
915. (c) Flack, H. D.; Bernardinelli, G. J. Appl. Crystallogr. 2000, 33, 1143-
1148.
(34) Altamore, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435-436.
(35) University Chemical Laboratory, Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK.
[R]20 -30.1 ((-)-2, c 1.0 in toluene, ee > 99%). The NMR and
D
IR data of the two separated enantiomers were in perfect agreement
with the data of the racemic compound.
5592 J. Org. Chem., Vol. 71, No. 15, 2006

tert-Butyl(dimethylamino)phenylphosphine-Borane
Supporting Information Available: Computed energies and
populations at different theoretical levels; calculated IR and VCD
spectra for the three optimized conformations of (S)-tert-butyl-
(dimethylamino)phenylphosphine-borane complex (S)-2 and popu-
lation-weighted spectra; experimental IR spectra of rac-2 in CD2Cl2
solution; table with band positions and vibrational assignments;
Cartesian coordinates for the three optimized conformations of (S)-2
and two rotational transition states; table of crystal data, experi-
mental details, crystal packing diagram, and crystallographic
information file (CIF) for (+)-2. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO0605647
J. Org. Chem, Vol. 71, No. 15, 2006 5593