About the Inversion Barriers of P-Chirogenic Triaryl-Substituted Phosphanes

Article abstract of DOI:10.1002/ejoc.201701734

The racemization tendency of acyclic P-chirogenic phosphanes is investigated experimentally and theoretically. Results of these investigations are important for the construction and use of P-chirogenic ligands and organocatalysts frequently used in asymmetric syntheses. Proof is given that triaryl phosphanes undergo easier racemization than compounds in which the phosphorus atom is linked to alkyl substituents. Interestingly, bulky ortho aryl substituents have only a marginal effect, whereas P-naphthyl rings destabilize the compound.

Full text of DOI:10.1002/ejoc.201701734

DOI: 10.1002/ejoc.201701734
Full Paper
Chirogenic Phosphanes
About the Inversion Barriers of P-Chirogenic Triaryl-Substituted
Phosphanes
Jens Holz,*[a] Haijun Jiao,[a] Mark Gandelman,[b] and Armin Börner*[a,c]
Abstract: The racemization tendency of acyclic P-chirogenic phosphanes undergo easier racemization than compounds in
phosphanes is investigated experimentally and theoretically. which the phosphorus atom is linked to alkyl substituents. In-
Results of these investigations are important for the construc- terestingly, bulky ortho aryl substituents have only a marginal
tion and use of P-chirogenic ligands and organocatalysts fre- effect, whereas P-naphthyl rings destabilize the compound.
quently used in asymmetric syntheses. Proof is given that triaryl
Introduction
rhodium(I), for example in the asymmetric hydrogenation of
prochiral olefins.[9] In the following decades, due to the rather
sophisticated preparation methods P-chirogenic phosphanes
were somewhat neglected and phosphanes with chirality in the
backbone (stereogenic axes, planes or centers) became the fo-
cus as alternative ligands for asymmetric metal catalysis. Only
recently, ligands with stereogenic centers have once again at-
tracted more attention.[10] The inspiration for this enforcement
has been clearly summarized by Knowles who pointed out in
his Nobel-Prize lecture in 2001: “We felt strongly that if one
wanted to get high ee values, the asymmetry would have to be
directly on the phosphorus. That is where the action is.”[11]
Stereogenic centers in organic compounds emerge by the tetra-
hedral arrangement of four different substituents linked to the
atom in the center. This situation is well-known for elements of
the main group IV of the periodic Table (C, Si, Ge). However,
elements of the main group V are also able to form chiral com-
pounds when the central atom is surrounded by three different
substituents and a lone electron-pair. Usually N-chirogenic
amines cannot be isolated because of their low inversion
barriers,[1–3] whereas numerous enantiomers of analogous
P-chirogenic phosphanes display significantly higher configura-
tional stability.
In 1911, Meisenheimer isolated the first quaternary P-chiro-
genic compound (phosphane oxide) by means of kinetic resolu-
tion.[4] Fifteen years later, another compound of this type was
synthesized in optically pure form by the same research
group.[5] In the late 1950s, the first noncyclic quaternary phos-
phonium salts with stereogenic centers were separated into
enantiomers.[6] Based on this seminal work, trivalent P-chiro-
genic phosphanes (e.g., 1 and 2; Figure 1), were derived by
cathodic reduction of the corresponding benzyl-substituted
chiral phosphonium salts by Horner and co-workers.[7] These
pioneering investigations pathed the way for the first homoge-
neously catalyzed asymmetric reactions, where chiral phos-
phanes such as (R)-PAMP/(R,R)-DiPAMP (methyl[(o-methoxy-
phenyl)phenylphosphane]/1,2-ethanediylbis[(o-methoxy-
phenyl)phenylphosphane]),[8] were used as ancillary ligands for
[a] Leibniz-Institut für Katalyse e.V.,
A.-Einstein-Str. 29a, 18059 Rostock, Germany
E-mail: jens.holz@catalysis.de
armin.boerner@catalysis.de
Figure 1. Some examples of P-chirogenic phosphanes.
www.catalysis.de
[b] Schulich Faculty of Chemistry, Technion - Israel Institute of Technology,
Technion City,
Today, numerous P-stereogenic phosphanes are known that
can be used as ligands in several metal-catalyzed reactions to
give extremely high enantioselectivities. Typical examples are
Haifa 3200008, Israel
[c] Institut für Chemie, Universität Rostock,
A.-Einstein-Str. 3a, 18059 Rostock, Germany
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201701734.
QuinoxP*
(2-((R)-tert-butyl(methyl)phosphanyl)-3-(tert-butyl-
(methyl)phosphanyl)quinoxaline),[12] MiniPhos [bis(tert-butyl-
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(methyl)phosphanyl)methane],[13] and BisP* [1,2-bis(alkylmeth- stereochemical integrity of the compound, but it can be benefi-
ylphosphanyl)ethanes].[14]
Some of these chiral ligands are commercially available.
Quite recently, we reported on the synthesis and first catalytic
cially employed for the thermal epimerization of P-stereogenic
units to generate thermodynamically stable diastereomers.[27]
To our knowledge, no systematic investigations exist concern-
application of a broad number of chiral Xantphos [(9,9-di- ing the pyramidal inversion of P-chirogenic triaryl phosphanes.
methyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane)], DPE- Only some rough estimations can be found in the literature that
phos [(oxybis(2,1-phenylene))bis(diphenylphosphane)], and suggest that the racemization barrier for triaryl phosphanes
DBFphos (4,6-bis(diphenylphosphanyl)dibenzo[b,d]furan) type should be approximately 27 kcal mol–1 (113 kJ mol–1).[19]
ligands generally abbreviated as P*-Xantphos, P*-DPEphos or
P*-DBFphos, which form the most comprehensive libraries of
P-chirogenic phosphanes known to date (Figure 1).[15]
During our recent studies concerned with P*-Xantphos, P*-
DPEphos or P*-DBFphos, we were frequently faced by the race-
mization problem. For example, during the reduction of chiral
A sufficiently high configurational stability of P-chirogenic tertiary phosphane oxides, we observed that, after deprotection
phosphanes is the precondition for their use as ligands in asym- of the corresponding phosphane borane adducts and subse-
metric catalysis. In general and therefore in contrast to the quent oxidation to the corresponding phosphane oxides, the
above mentioned low pyramidal inversion barrier in amines enantiomeric purity dropped eventually from 98 % ee to ca.
with three different substituents, the hindrance for racemization 90 % ee.[28] Our first hypothesis that partial racemization occurs
in chiral trivalent (non-cyclic) phosphanes is significantly en- during oxidation could be excluded. Already during the depro-
hanced.[16,17] Usually, phosphanes have high, but not insur- tection of a chiral phosphane borane complex with 1,4-diazabi-
mountable, barriers to inversion. An accurate knowledge about cyclo[2.2.2]octane (DABCO) in toluene (40 °C, 20 hours) the
this property is necessary to assess the long-term stability of enantioselectivity was affected. This problem was overcome by
these compounds. The inversion barrier strongly depends on conducting the deprotection at room temperature. Moreover,
the substituents at the trivalent phosphorus atom. As a rule of sometimes we observed significant epimerization even during
thumb, calculations of Baechler and Mislow can be used, who the long-term storage at ambient temperature.[15]
estimated that the free energy of activation for the inversion of
trialkyl phosphanes (ΔG‡inv) is ca. 36 kcal mol–1 (151 kJ mol–1),
for dialkyl aryl phosphanes ca. 33 kcal mol–1 (138 kJ mol–1), and
for alkyl diaryl phosphanes ca. 30 kcal mol–1 (126 kJ mol–1).[18,19]
An overview of selected calculated and experimentally ob-
served inversion barrier energies is given in Table 1.
Results and Discussion
For a more detailed investigation of the racemization behavior,
P-chirogenic triaryl phosphanes 8a–n were synthesized
(Scheme 1). Following the well-known and established proce-
dure developed by Jugé and co-workers,[10c,29] the oxazaphos-
pholidine ring in 4 derived from the reaction of N,N,N′,N′-tetra-
ethyl-1-phenylphosphanediamine (3) with (–)-ephedrine was
opened selectively by reaction with substituted aryllithium. The
latter was generated in situ by the reaction of an aryl bromide
with sBuLi. The ephedrine moiety in 5 was cleaved by meth-
anolysis, which proceeds under inversion at the phosphorus
atom. Subsequently, the yielded phosphinite borane adduct 6
was treated with a second aryllithium reagent to yield the bor-
ane protected triaryl phosphane 7. Finally, the borane group
Table 1. Some inversion barrier energies of trivalent phosphanes.
PR1R2R3
R1
ΔG‡inv [kJ mol–1
]
calcd.
R2
R3
exp.
H
Me
cyclohexyl
Ph
Ph
Ph
Ph
Ph
H
Me
H
156[16]
199[21]
132[20]
149[16]
149[18]
134[22]
137[18]
129[18]
124[18]
121[23]
38[24]
Me
Me
Me
Me
Me
Me
Me
n-propyl
n-propyl
allyl
p-anisyl
2-naphthyl
1-naphthyl
2,6-iPr-Ph
Ph
2,6-iPr-phenyk 2,6-iPr-phenyl
Ph Ph
147[21] 112[25]
It becomes clear that, with increasing number of aromatic
substituents, the inversion barrier energy is diminished because
of the conjugation of the lone electron-pair at the P-atom with
π-orbital(s) of the adjacent aromatic ring(s). This leads to a flat-
tening of the pyramidal structure and a lowering of the energy
barrier. For the same reason, the pyramidal inversion at a P-
atom with adjacent double bonds as found in acyl phosphanes
and phospholes is facilitated.[26] In general, beside the effect of
diminishing the energy barrier due to (p-p)π-conjugation also a
steric influence of bulky substituents may force the pyramidal
inversion. Thus, for tris(2,6-diisopropylphenyl)phosphane an in-
version barrier energy of only 37.5 kJ mol–1 was found,[24]
whereas for triphenyl phosphane two larger values calculated
by different methods (see below) have been reported.[21,25] The
property of epimerization is not only a problem for the desired
Scheme 1. Synthesis of P-chirogenic phosphanes 8a–p.
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was removed under ambient conditions under the effect of corresponding borane adducts 7. Since the racemization/enan-
DABCO. For comparison, two P-stereogenic alkyl diaryl phos- tiomerization follows a kinetic of a first-order reaction, Equa-
phanes 8o,p were also synthesized by addition of methyllithium tion (1) holds. Given that the estimated portions of enantiomers
and n-butyllithium, respectively, to methylphosphinite 6a.
For the determination of the inversion barrier, we applied a
kinetic model for the estimation of the rate constant krac during
the thermal racemization at constant temperature. This process
consists of two single enantiomerization steps (with ken = k1 =
k–1, krac = k1 + k–1) and follows a reversible first-order kinetic. At
the end of the racemization, both enantiomers have the same
concentration (equilibrium constant K = 1). The inversion proc-
ess is demonstrated exemplarily for phosphane 8a in Scheme 2.
The activated molecule reverts via a planar transition state to
the parent (R)-enantiomer or switches to the (S)-enantiomer
with the same probability. In the same manner, the back-reac-
tion proceeds in parallel.
of 8 are comparable with the molar concentration in the solu-
tion, Equation (1) can be transformed into Equation (2) [herein
(S)-enantiomer means the major enantiomer].
For a graphical analysis, the natural logarithm value was plot-
ted against the time (t). The slope of the resulting regression
line affords the rate constants krac and ken for the racemization
process (Figure 2).
Scheme 2. Process of pyramidal inversion at the P-atom of 8a.
For our investigations the enantioenriched compounds of
8a–n were stirred in toluene at three (or sometimes two) con-
stant temperatures and the process of racemization was investi-
gated over a defined period of time. The progress of the reac-
tion was followed by HPLC thanks to a sufficient separation of
both enantiomers on a column with a chiral stationary phase.
In a first trial a sample was extracted from the solution and
quenched at 0 °C. After addition of an aliquot of BH3*DMS,
the parts of enantiomeric phosphanes were estimated as their
Figure 2. Plot of ln {[(%S)t – 50]/[(%S)0 – 50]} vs. time [t] with 8a (50 °C, krac
=
3.861×10–6 s–1).
Table 2. Calculated and experimental determined values for inversion barrier ΔG‡.
[a]
Aryl
R
ΔG‡
ΣCPC
(°)
ΔG‡
(50 °C)
]
ΔG‡
(70 °C)
]
ΔG‡
calcd.
(90 °C)
]
calcd.
calcd.
[kJ mol–1
calcd.
[kJ mol–1
[kJ mol–1
]
[kJ mol–1
8a
8b
8c
8d
8e
8f
8g
8h
8i
8j
8k
8l
8m
8n
8o
8p
2-naphthyl
2-naphthyl
2-naphthyl
2-naphthyl
2-naphthyl
1-naphthyl
1-naphthyl
3-tolyl
2-anisyl
3-anisyl
4-anisyl
3-Et-phenyl
4-DMX[b]
2-anisyl
4-anisyl
2-anisyl
2-anisyl
2-iPrO-phenyl
2-naphthyl
2-anisyl
4-anisyl
3-tolyl
121.8
119.9
113.7
108.7
118.9
118.8
119.4
119.1
113.6
110.9
118.7
113.0
107.6
123.2
126.9
126.5
301.2
302.2
302.4
302.6
301.0
302.1
303.0
302.3
300.3
299.7
301.2
300.7
302.0
302.6
300.4
300.0
114.7
114.3
115.9
113.7
115.5
115.4
116.5
117.1
117.4
117.2
115.9
116.5
118.9
116.7
124.3
120.9
115.2
115.3
117.0
115.1
114.6
114.4
116.1
116.8
117.4
117.7
115.6
117.5
118.0
116.8
124.5
121.1
114.9
114.7
116.4
118.0
118.1
117.0
115.6
117.2
119.8
4-DBF[c]
4-DBF[c]
4-DBF[c]
2-DPE[d]
2-DPE[d]
4-EtO-phenyl
2-naphthyl
2-naphthyl
methyl
n-butyl
126.0
121.6
[a] Obtained from DFT calculations with B3PW91-D3/TZVP including van der Waals dispersion effect. [b] 4-DMX = 9,9-dimethyl-9H-xanthen-4-yl. [c] 4-DBF =
Dibenzo[b,d]furan-4-yl. [d] 2-DPE = 2-phenoxyphenyl.
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After the experimental estimation of the rate constants ken para-substituted 4-ethoxy-group, we found almost the same in-
for the enantiomerization, the values of the activation energies version barrier. In contrast, the calculation predicts a significant
ΔG‡ were calculated by application of the Eyring equation difference (119.1 vs. 123.2 kJ mol–1). It seems that a single ortho
[Eq. (3)]. The results are summarized in Table 2.
substituent at one or two aromatic groups does not diminish
the energy barrier significantly (8a, 8e, 8i–m) and play therefore
a minor role. Thus, the larger and sterically more demanding
isopropoxy group also did not lead to a lowering of the activa-
tion energy (8i vs. 8j). Apparently, the substituents do not exert
additional “steric pressure” on the central phosphorus. However,
there are some arguments that a naphthyl group destabilizes
P-chirogenic phosphanes (8a–g, 8k). In all cases, these com-
pounds display the lowest energy barriers in relation with com-
parable congeners bearing a simple aromatic group (e.g., 8a vs.
8h, 8k vs. 8i,j). It seems that, in these phosphanes, the conjuga-
tion of the lone pair at the P-atom with the aromatic system of
the naphthyl group is more effective and therefore the energy
barrier is around 2–3 kJ mol–1 lower. On the other hand, it is
not unexpected that the phosphane with only a single alkyl
group (8o,p) shows the highest energy barrier for racemization.
The value for 8o is almost identical to those estimated by
Baechler and Mislow.[18] Phosphane 8p, with the n-butyl group,
is characterized by a significantly lower energy barrier in com-
parison to 8o.
For comparison, the calculated values for the activation en-
ergy ΔG‡
and the total of the three trigonal CPC-angles at
calc
the P-atom ΣCPC are also listed. The latter gives information
about the deviation from the planar activated state (ΣCPC =
360°). The larger the sum total the larger the planarity between
three substituents and the phosphorus atom. Thus, a smaller
activation energy and inversion barrier, respectively, can be as-
sumed if the sum total is closer to 360°. It is noted that there
are computational data available from the literature, but the
computed results, depending on the method and basis set, dif-
fer significantly. For example, the computed inversion barrier of
triphenyl phosphane is 147 kJ mol–1 (see Table 1) at the Har-
tree–Fock level with small basis set[21] and 112 kJ mol–1 at the
B3LYP level with moderate basis set.[25] To avoid such methodo-
logical shortcomings, we carried out high-level B3PW91 density
functional theory computation in conjugation with the large
TZVP basis set; and we also included the van der Waal disper-
sion correction in our calculations due to the large aromatic
substitutes at the P-center. To validate our computational meth-
odology, we computed the inversion free-energy barrier of
P(CH3)(Ph)(p-Tolyl) and P(CH3)(Ph)(p-Anisyl) as benchmark since
the experimental results are available.[18] It is noted that the
computed inversion free-energy barriers of P(Me)(Ph)(p-Tolyl)
and P(Me)(Ph)(p-Anisyl) is close to the experimental measured
values (129.5 vs. 126.8 as well as 132.6 vs. 128.9 kJ mol–1, re-
spectively), and the deviation is less than 3 % from the experi-
ment, which validates our method. In addition, the computed
inversion free-energy barrier of PPh3 is 116.6 kJ mol–1.
The estimation of the half-life period for the phosphanes at a
given temperature results from the application of Equation (4).
Although the differences between the estimated energy bar-
riers of triaryl phosphanes 8a–n are quite small, the associated
rates of racemization and resulting half-lives t1/2 deviate
strongly from each other. Thus, the half-life of 8d at 50 °C is
only 34 hours, whereas for 8m this period was increased to 10
days. At 70 °C the half-life periods amount to only some hours.
At 90 °C the values decrease to some minutes (see the Support-
ing Information). The estimated t1/2-values and rate constants
at 50 °C and 70 °C are illustrated in Table 3.
Considering our results in Table 2, the agreement between
theory and computation in inversion free-energy barriers is
generally within 98–95 %; and the worst agreement is found
for 8a (93.8 %) and 8m (90.5 %). The differences between the-
ory and experiment should be within the standard deviations
of the experiment and the error limitation of theory.
Table 3. Experimentally determined values of rate constants krac and half-life
periods t1/2
.
Clearly, besides the steric influence of the spatial arrange-
ment, the electronic properties of the aryl groups also play a
role. As already mentioned above, especially (p-p)π-interactions
of the lone p-orbital at the phosphorus atom with the aromatic
π-electron system can enhance the planarity of the trigonal
structure and lead therefore to a decrease of the inversion bar-
rier. However, the calculated values for the activation energy
(ΔG‡) sometimes are not in line with expectations. Thus, it
seems reasonable to expect that for compounds 8a–c the inver-
sion barrier for enantiomerization increases from the ortho-sub-
stituted derivative 8a to the para-substituted 8c due to the
decreasing steric demand of the P-substituent. But the calcu-
lated values predict that phosphane 8c possesses the smallest
inversion barrier and adopts a geometry with the largest pla-
narity. Our experimental data show contrastingly that phos-
phane 8c is slightly more stable than 8a,b. Also for phosphanes
8h and 8n, bearing an ortho-substituted anisyl group and a
Temperature
50 °C
70 °C
krac
t1/2
[h]
krac
t1/2
[h]
[s–1
]
[s–1
]
8a
8b
8c
8d
8e
8f
8g
8h
8i
8j
8k
8l
8m
8n
8o
8p
3.86×10–6
4.58×10–6
2.44×10–6
5.73×10–6
2.87×10–6
2.72×10–6
1.89×10–6
1.59×10–6
1.55×10–6
1.47×10–6
2.53×10–6
2.02×10–6
8.01×10–7
1.84×10–6
1.07×10–7
3.91×10–7
50
42
79
34
67
71
102
121
123
131
76
4.20×10–5
3.95×10–5
2.21×10–5
4.34×10–5
5.18×10–5
7.31×10–5
3.00×10–5
2.42×10–5
1.95×10–5
1.75×10–5
3.60×10–5
1.85×10–5
1.57×10–5
2.35×10–5
1.61×10–6
5.29×10–6
4.6
4.9
8.7
4.4
3.7
2.6
6.4
8.0
9.9
11.0
5.4
10.4
12.3
8.2
120
36.4
96
240
104
1803
493
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By application of the Arrhenius equation [Eq. (5)] it is possi- Experimental Section
ble to calculate the rate constants and corresponding half-life
General Information: All solvents were dried and freshly distilled
under argon before use. All reactions were performed under an
argon atmosphere by using standard Schlenk techniques. Thin-layer
chromatography was performed on pre-coated TLC plates (silica
gel). Flash chromatography was carried out with silica gel 60 (parti-
cle size 0.040–0.063 mm) with a CombiFlash RF system (Teledyne
ISCO). NMR spectra were recorded with a Bruker AV 400 spectrome-
ter at the following frequencies: 400.13 MHz (1H), 100.61 MHz (13C),
161.98 MHz (31P) or with a Bruker AV 300 [300.13 MHz (1H),
75.47 MHz (13C), 121.49 MHz (31P)]. Chemical shifts of 1H and 13C
NMR spectra are reported in ppm downfield from used solvent
CDCl3 as internal standard (δ =7.25 ppm/77.00 ppm). Chemical
shifts of 31P NMR spectra are referred to H3PO4 as external standard.
Signals are quoted as s (singlet), d (doublet), t (triplet), q (quartet),
m (multiplet) and br (broad). Mass spectra were recorded with a
Thermo Electron MAT 95-XP or an Agilent 1200/6210 Time-of-flight
LC-MS. For chiral HPLC analysis Agilent 1100 Series was used.
at ambient pressure and other temperature regimes. From
Equation (6) the activation energy can be extracted by graphi-
cal analysis (Figure 3). Therefore, the function of natural loga-
rithm of estimated rate constants for racemization is plotted vs.
the corresponding reciprocal of temperature. From the regres-
sion curve the activation energy and rate constant krac can be
derived.
Phosphanes 5a–f and 6a–f were synthesized according to reported
procedures (see reference[10c,29] and cited literature).
General Synthesis of 7a–n by Substitution of the Methoxy
Group in 6 by an Aryl Group: To a solution of aryl bromide
(2.5 mmol) in diethyl ether (10 mL), sBuLi solution (1.4 M, 1.79 mL
,
2.5 mmol) were added slowly at 0 °C. After one hour, the suspension
was cooled to –78 °C and a solution of the corresponding chiral
methylphosphinite 6a–f (2 mmol) in diethyl ether (10 mL) was
added over a period of 5–10 min. After 30 minutes, the mixture
was warmed to room temperature and then stirring was continued
overnight. For work up, the reaction was quenched with water
(10 mL) and the mixture was extracted with dichloromethane (3 ×
25 mL). After drying (Na2SO4) and evaporation of the solvent the
residue was purified by column chromatography.
Figure 3. Plot of ln krac vs. reciprocal of temperature (50, 70, and 90 °C) for
8j.
(S)-(+)-(2-Methoxyphenyl)(naphthalen-2-yl)(phenyl)phosphane-
borane Complex (7a): Yield: 86 %; white solid; m.p. 120–122 °C
(cyclohexane/AcOEt, 95:5); [α]2D4 = +2.8 (c = 1.0, CHCl3). 1H NMR: δ =
8.21 (br. d, J = 13.2 Hz, 1 H, ArH), 7.89–7.80 (m, 3 H, ArH), 7.70–7.36
(m, 10 H, ArH), 7.05 (ddt, J = 1.1, 2.0, 7.6 Hz, 1 H, ArH), 6.92 (ddd,
J = 0.8, 3.8, 8.3 Hz, 1 H, ArH), 3.52 (s, 3 H, OCH3), 2.1–0.6 (br. q, 3 H,
BH3) ppm. 13C NMR: δ = 161.1 (C-O), 135.7 (d, J = 12.2 Hz, CH),
134.0 (d, J = 11.0 Hz, CH), 133.8 (d, J = 2.0 Hz, C-C), 133.8 (d, J =
2.0 Hz, C-C), 132.5 (d, J = 10.3 Hz, 2 × CH), 132.4 (d, J = 12.4 Hz,
CH), 130.5 (d, J = 2.3 Hz, CH), 129.3 (d, J = 60.3 Hz, C-P), 128.3 (CH),
128.1 (d, J = 10.4 Hz, 2 × CH), 127.9 (d, J = 8.9 Hz, CH), 127.6 (d, J =
9.9 Hz, CH), 127.5 (CH), 127.4 (CH), 126.5 (d, J = 60.5 Hz, C-P), 126.4
(CH), 120.9 (d, J = 11.6 Hz, CH), 116.2 (d, J = 56.6 Hz, C-P), 111.5 (d,
J = 4.6 Hz, CH), 55.0 (OCH3) ppm. 31P NMR: δ = +19.4 (br. s) ppm.
MS (EI, 70 eV): m/z (%) = 355 (5) [M – H]+, 342 (100) [M – BH3]+,
311 (19) [M – BH3 – OCH3]+, 265 (3) [M – BH3 – Ph]+, 141 (74). HRMS
(ESI): m/z calcd. for C23H22BNaOP [M + Na]+: 379.13977; found:
379.13972. HPLC: >99 % ee, column: Reprosil 100 (150 × 4.6 mm);
hexane/iPrOH, 99.5:0.5; 1.75 mL/min; tR = 16.1 min (R)-enantiomer
and 19.1 min (S)-enantiomer.
In our example at 25 °C and 5 °C, the rate constants
for the racemization of 8j [krac = 5.55×10–8 s–1 (25 °C) and
2.11×10–9 s–1 (5 °C)] were calculated. The corresponding half-
life is 145 days and 10.4 years, respectively.
Conclusions
Our findings give clear evidence that the use of triaryl phosphanes
with stereogenic centers at elevated temperature should be re-
stricted. When such P-chirogenic phosphanes are used as ligands
or organocatalysts in asymmetric reactions at prolonged reaction
times the racemization process may lead to a diminishing of stereo-
control. Possibly, ligands in metal-catalyzed reactions may be pro- (S)-(–)-(3-Methoxyphenyl)(naphthalen-2-yl)(phenyl)phos-
tected towards epimerization/racemization by complexation to the phaneborane Complex (7b): Yield: 84 %; colorless oil (cyclohex-
1
metal center. Preliminary tests with some borane-protected chiral ane/AcOEt, 98:2 to 95:5); [α]2D3 = –9.7 (c = 0.5, CHCl3). H NMR: δ =
phosphanes of type 7 showed a significantly higher robustness.
However, it should be taken into consideration that ligand ex-
change processes, for example by use of an excess of the ligand in
relation to the metal, may facilitate such fatal racemization proc-
esses. Therefore, fast asymmetric reactions and moderate reaction
temperatures should always be targeted.
8.17 (br. d, J = 12.8 Hz, 1 H, ArH), 7.92–7.81 (m, 3 H, ArH), 7.70–7.32
(m, 9 H, ArH), 7.26–1.13 (m, 2 H, ArH), 7.05 (m, 1 H, ArH), 3.77 (s, 3
H, OCH3), 2.1–0.6 (br. q, 3 H, BH3) ppm. 13C NMR: δ = 159.6 (d, J =
12.9 Hz, C-O), 134.8 (d, J = 10.6 Hz, CH), 134.2 (d, J = 1.9 Hz, C-C),
133.2 (d, J = 9.6 Hz, 2 × CH), 132.6 (d, J = 11.6 Hz, C-C), 131.2 (d,
J = 2.3 Hz, CH), 130.5 (d, J = 57.5 Hz, C-P), 129.9 (d, J = 11.7 Hz, CH),
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129.0 (d, J = 58.0 Hz, C-P), 128.7 (d, J = 10.4 Hz, 2 × CH), 128.6 (CH),
128.5 (d, J = 9.9 Hz, CH), 128.0 (CH), 128.0 (d, J = 8.8 Hz, CH), 127.7
(CH), 126.1 (d, J = 58.3 Hz, C-P), 125.4 (d, J = 8.8 Hz, CH), 118.5 (d,
J = 11.8 Hz, CH), 116.9 (d, J = 2.6 Hz, CH), 55.2 (OCH3) ppm. 31P NMR:
δ = +22.0 (br. d) ppm. MS (EI, 70 eV): m/z (%) = 355 (4) [M – H]+,
342 (100) [M – BH3]+, 266 (49) [M – BH3 – Ph + H]+, 251 (34), 214
mixture). After 4 hours the solvent was evaporated and the residue
was purified by column chromatography. Yield: 46%; white solid;
m.p. 68–70 °C (cyclohexane/AcOEt, 99:1); [α]2D3 = +29.2 (c = 1.0,
CHCl3). 1H NMR: δ = 8.35 (br d, J = 13.1 Hz, 1 H, ArH), 7.89–7.81 (m,
3 H, ArH), 7.79–7.69 (m, 3 H, ArH), 7.63 (ddd, J = 7.6, 1.6, 0.8 Hz, 1
H, ArH), 7.59–7.38 (m, 6 H, ArH), 7.33 (ddd, J = 7.6, 1.6, 0.8 Hz, 1 H,
(53). HRMS (ESI): m/z calcd. for C23H22BNaOP [M + Na]+: 379.13977; ArH), 7.16 (dt, J = 7.7, 2.2 Hz, 1 H, ArH), 7.03–6.89 (m, 2 H, ArH), 6.29
found: 379.14023. HPLC: 99 % ee, column: Reprosil 100 (ddd, J = 8.0, 1.5, 0.4 Hz, 1 H, ArH), 1.61 (s, 3 H, CH3), 1.60 (s, 3 H,
(150 × 4.6 mm); hexane/iPrOH 99.75:0.25; 1.75 mL/min; tR = CH3), 2.1–0.7 (br. q, 3 H, BH3) ppm. 13C NMR: δ = 152.5 (C-O), 149.6
17.4 min (R)-enantiomer and 19.7 min (S)-enantiomer.
(C-O), 134.6 (d, J = 11.6 Hz, CH), 134.2 (d, J = 2.3 Hz, C-C), 133.8 (d,
J = 10.8 Hz, CH), 132.8 (d, J = 2.3 Hz, C-C), 132.8 (d, J = 9.8 Hz,
2 × CH), 131.4 (d, J = 4.3 Hz, C-C), 130.9 (d, J = 2.5 Hz, CH), 130.1
(d, J = 2.2 Hz, CH), 129.9 (C-C), 129.7 (d, J = 60.4 Hz, C-P), 128.7
(CH), 128.60 (d, J = 10.6 Hz, 2 × CH), 128.2 (d, J = 9.8 Hz, CH), 128.1
(d, J = 9.6 Hz, CH), 127.8 (CH), 127.7 (CH), 127.3 (CH), 126.7 (CH),
126.5 (d, J = 59.7 Hz, C-P), 125.3 (CH), 123.6 (CH), 123.3 (d, J =
11.6 Hz, CH), 116.1 (CH), 116.0 (d, J = 54.7 Hz, C-P), 34.4 (C), 31.7
(CH3), 31.5 (CH3) ppm. 31P NMR: δ = +18.7 (br. s) ppm. MS (EI, 70 eV):
(R)-(–)-(4-Methoxyphenyl)(naphthalen-2-yl)(phenyl)phos-
phaneborane Complex (7c): Yield: 86 %; white solid; m.p. 100–
102 °C (cyclohexane/AcOEt, 95:5); [α]2D2 = –1.7 (c = 1.0, CHCl3).
1H NMR: δ = 8.15 (br. d, J = 12.8 Hz, 1 H, ArH), 7.92–7.80 (m, 3 H,
ArH), 7.70–7.40 (m, 10 H, ArH), 6.99 (m, 2 H, ArH), 3.82 (s, 3 H, OCH3),
2.2–0.6 (br. q, 3 H, BH3) ppm. 13C NMR: δ = 162.0 (d, J = 2.6 Hz, C-
O), 134.9 (d, J = 10.9 Hz, 2 × CH), 134.5 (d, J = 10.9 Hz, CH), 134.1
(d, J = 1.9 Hz, C-C), 132.9 (d, J = 9.7 Hz, 2 × CH), 132.5 (d, J = 11.7 Hz,
C-C), 131.0 (d, J = 2.5 Hz, CH), 129.7 (d, J = 58.5 Hz, C-P), 128.6 (d,
J = 10.1 Hz, 2 × CH), 128.5 (CH), 128.4 (d, J = 9.7 Hz, CH), 127.9 (d,
J = 9.7 Hz, CH), 127.9 (CH), 127.7 (CH), 126.8 (d, J = 58.4 Hz, C-P),
126.8 (CH), 119.3 (d, J = 63.0 Hz, C-P), 114.4 (d, J = 10.7 Hz, 2 × CH),
55.2 (OCH3) ppm. 31P NMR: δ = +20.0 (br. s) ppm. MS (EI, 70 eV):
m/z (%) = 355 (1) [M – H]+, 342 (100) [M – BH3]+, 265 (8) [M – BH3
– Ph]+, 234 (29). HRMS (ESI): m/z calcd. for C23H22BNaOP [M + Na]+:
379.13977; found: 379.14003. HPLC: 99 % ee, column: Reprosil 100
(150 × 4.6 mm); hexane/iPrOH, 99.75:0.25; 2.00 mL/min;
tR = 20.6 min (S)-enantiomer and 22.4 min (R)-enantiomer.
m/z (%) = 458 (1) [M]+, 444 (73) [M – BH3]+, 429 (100) [M – BH3
–
CH3]+. HRMS (ESI): m/z calcd. for C31H28BNaOP [M + Na]+: 481.18685;
found: 481.18651. HPLC: 98 % ee, column: Lux 5u Cellulose-4
(150 × 4.6 mm); hexane/iPrOH, 99:1; 1.50 mL/min; tR = 11.9 min (R)-
enantiomer and 14.1 min (S)-enantiomer.
(S)-(+)-(2-Methoxyphenyl)(naphthalene-1-yl)(phenyl)phos-
phaneborane Complex (7f): Yield: 73 %; white solid; m.p. 143–
145 °C (cyclohexane/AcOEt, 95:5); [α]2D3 = +22.5 (c = 1.0, CHCl3).
1H NMR: δ = 8.14 (br d, J = 8.0 Hz, 1 H, ArH), 7.95–7.68 (m, 5 H,
ArH), 7.54–7.24 (m, 8 H, ArH), 7.04 (m, 1 H, ArH), 6.85 (m, 1 H, ArH),
3.34 (s, 3 H, OCH3), 2.2–0.7 (br. q, 3 H, BH3) ppm. 13C NMR: δ = 161.4
(C-O), 135.5 (d, J = 12.3 Hz, CH), 133.8 (d, J = 2.0 Hz, C-C), 133.7 (d,
J = 4.9 Hz, CH), 133.6 (d, J = 9.6 Hz, 2 × CH), 133.2 (d, J = 10.3 Hz,
C-C), 132.8 (d, J = 7.8 Hz, CH), 131.8 (d, J = 2.3 Hz, CH), 130.8 (d, J =
2.3 Hz, CH), 129.1 (d, J = 60.0 Hz, C-P), 128.7 (CH), 128.2 (d, J =
10.4 Hz, 2 × CH), 127.3 (d, J = 6.7 Hz, CH), 126.0 (d, J = 57.0 Hz, C-
P), 126.0 (d, J = 8.2 Hz, CH), 124.8 (CH), 124.6 (CH), 121.2 (d, J =
11.6 Hz, CH), 117.1 (d, J = 56.4 Hz, C-P), 111.8 (d, J = 4.3 Hz, CH),
55.2 (OCH3) ppm. 31P NMR: δ = +18.4 (br. s) ppm. MS (EI, 70 eV):
(R)-(–)-(3-Ethylphenyl)(naphthalen-2-yl)(phenyl)phosphane-
borane Complex (7d): Yield: 89 %; colorless oil (cyclohexane/
1
AcOEt, 97:3); [α]2D2 = –6.7 (c = 1.0, CHCl3). H NMR: δ = 8.15 (br. d,
J = 12.6 Hz, 1 H, ArH), 7.91–7.81 (m, 3 H, ArH), 7.68–7.31 (m, 12 H,
ArH), 2.65 (q, J = 7.6 Hz, 2 H, CH2), 1.20 (t, J = 7.6 Hz, 3 H, CH3), 2.0–
0.7 (br. q, 3 H, BH3) ppm. 13C NMR: δ = 144.9 (d, J = 10.1 Hz, C-C),
134.8 (d, J = 10.5 Hz, CH), 134.3 (d, J = 2.1 Hz, C-C), 133.2 (d, J =
9.6 Hz, 2 × CH), 132.7 (d, J = 11.9 Hz, C-C), 132.6 (d, J = 10.5 Hz,
CH), 131.2 (d, J = 2.3 Hz, CH), 130.9 (d, J = 2.3 Hz, CH), 130.6 (d, J =
8.7 Hz, CH), 129.4 (d, J = 58.1 Hz, C-P), 128.8 (d, J = 57.8 Hz, C-P),
128.7 (d, J = 9.6 Hz, 2 × CH), 128.7 (CH), 128.7 (d, J = 10.5 Hz, CH),
128.4 (d, J = 10.0 Hz, CH), 128.1 (d, J = 8.9 Hz, CH), 128.0 (CH), 127.7
(CH), 126.8 (CH), 126.5 (d, J = 58.0 Hz, C-P), 28.7 (CH2), 15.4
(CH3) ppm. 31P NMR: δ = +20.9 (br. d) ppm. MS (EI, 70 eV): m/z
(%) = 340 (100) [M – BH3]+, 318 (10), 299 (3), 254 (90). HRMS (ESI):
m/z calcd. for C24H24BNaP [M + Na]+: 377.16052; found: 377.16099.
HPLC: 99 % ee, column: Lux 5u Cellulose 4 (150 × 4.6 mm); hexane/
iPrOH, 99.5:0.5; 1.25 mL/min; tR = 12.4 min (S)-enantiomer and
13.4 min (R)-enantiomer.
m/z (%) = 356 (3) [M]+, 342 (100) [M – BH3]+, 311 (14) [M – BH3
–
OCH3]+, 141 (36). HRMS (EI): m/z calcd. for C23H19OP [M – BH3]+:
342.111680; found: 342.11607. HPLC: >99 % ee, column: Reprosil
100 (150 × 4.6 mm); hexane/iPrOH, 99.5:0.5; 1.50 mL/min; tR =
14.5 min (S)-enantiomer and 16.7 min (R)-enantiomer.
(R)-(–)-(4-Methoxyphenyl)(naphthalen-1-yl)(phenyl)phos-
phaneborane Complex (7g): Yield: 63 %; white solid; m.p. 167–
169 °C (cyclohexane/AcOEt, 95:5); [α]2D2 = –1.0 (c = 1.0, CHCl3).
1H NMR: δ = 8.15 (br. d, J = 8.5 Hz, 1 H, ArH), 7.97 (m, 1 H, ArH),
7.87 (m, 1 H, ArH), 7.68–7.22 (m, 11 H, ArH), 6.96 (m, 2 H, ArH), 3.83
(s, 3 H, OCH3), 2.2–0.6 (br. q, 3 H, BH3) ppm. 13C NMR: δ = 162.0 (d,
J = 2.2 Hz, C-O), 135.2 (d, J = 10.5 Hz, 2 × CH), 134.2 (d, J = 7.9 Hz,
CH), 133.9 (d, J = 7.7 Hz, C-C), 133.3 (d, J = 10.2 Hz, C-C), 133.2 (d,
J = 9.3 Hz, 2 × CH), 132.5 (d, J = 2.6 Hz, CH), 131.1 (d, J = 2.5 Hz,
CH), 129.7 (d, J = 58.0 Hz, C-P), 128.9 (d, J = 1.1 Hz, CH), 128.8 (d,
J = 10.2 Hz, 2 × CH), 128.8 (d, J = 6.7 Hz, CH), 126.5 (CH), 126.5 (d,
J = 54.5 Hz, C-P), 126.3 (CH), 124.7 (d, J = 10.2 Hz, CH), 119.4 (d, J =
63.1 Hz, C-P), 114.6 (d, J = 11.0 Hz, 2 × CH), 55.3 (OCH3) ppm. 31P
NMR: δ = +18.8 (br. s) ppm. MS (EI, 70 eV): m/z (%) = 342 (100) [M
– BH3]+, 341 (92) [M – BH3 – H]+, 233 (26). HRMS (EI): m/z calcd. for
C23H19OP [M – BH3]+: 342.111680; found: 342.11597. HPLC: 99 %
ee, column: Lux-5u Cellulose-4 (150 × 4.6 mm); hexane/iPrOH, 97:3;
1.50 mL/min; tR = 6.9 min (R)-enantiomer and 9.1 min (S)-enantio-
mer.
(S)-(+)-(9,9-Dimethyl-9H-xanthen-4-yl)(naphthalen-2-yl)-
(phenyl)phosphaneborane Complex (7e): 9,9-Dimethyl-9H-
xanthene (2 mmol, 420 mg) was treated with one equivalent of
nBuLi (1.25 mL of 1.6
M solution) in the presence of one equivalent
of TMEDA (232 mg) at 0 °C in diethyl ether (10 mL). After one hour
the reaction was continued at room temperature overnight. In a
second Schlenk-tube the chiral methylphosphinite 6a (1.9 mmol,
532 mg) was dissolved in hexane (5 mL) and DABCO (2 equiv.,
426 mg) was added. The solution was stirred at 40 °C for 20 hours,
then the white solid (DABCO*BH3) was filtered off. This solution was
added at –78 °C slowly to the lithiated xanthene and after one
hour the reaction was stirred at room temperature overnight. After
working up (see general procedure but under anaerobic conditions)
the raw product was dissolved in toluene (5 mL) and BH3*DMS
(6 mmol) was added (excess due to the remained DABCO in raw
(S)-(+)-(2-Methoxyphenyl)(phenyl)(3-tolyl)phosphaneborane
Complex (7h): Yield: 78 %; white solid; m.p. 110–111 °C (cyclohex-
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ane/AcOEt, 98:2). [α]D23 = +5.9 (c = 1.0, CHCl3). 1H NMR: δ = 7.65– 1.50 mL/min; tR = 9.8 min (R)-enantiomer and 12.4 min (S)-enantio-
7.30 (m, 9 H, ArH), 7.29–7.24 (m, 2 H, ArH), 7.03 (ddt, J = 7.5, 1.9,
1.0 Hz, 1 H, ArH), 6.92 (ddd, J = 8.3, 3.8, 0.9 Hz, 1 H, ArH), 3.53 (s, 3
H, OCH3), 2.34 (s, 3 H, CH3), 2.0–0.5 (br. q, 3 H, BH3) ppm. 13C NMR:
δ = 161.4 (C-O), 138.0 (d, J = 11.0 Hz, C-C), 136.0 (d, J = 11.9 Hz,
CH), 133.7 (d, J = 2.2 Hz, CH), 133.3 (d, J = 11.0 Hz, CH), 132.8 (d,
J = 9.9 Hz, 2 × CH), 131.5 (d, J = 2.5 Hz, CH), 130.5 (d, J = 2.5 Hz,
CH), 130.0 (d, J = 9.5 Hz, CH), 129.7 (d, J = 60.1 Hz, C-P), 129.2 (d,
J = 59.9 Hz, C-P), 128.2 (d, J = 10.6 Hz, 2 × CH), 128.1 (d, J = 10.6 Hz,
CH), 121.4 (d, J = 11.6 Hz, CH), 116.9 (d, J = 56.4 Hz, C-P), 111.7 (d,
J = 4.6 Hz, CH), 55.3 (OCH3), 21.4 (CH3) ppm. 31P NMR: δ = +18.3
(br. d) ppm. MS (EI, 70 eV): m/z (%) = 320 (6) [M]+, 319 (32) [M –
H]+, 306 (100) [M – BH3]+, 275 (26), [M – BH3 – OCH3], 215 (67)
[M – BH3 – C7H7]+, 197 (52), 183 (67). HRMS (ESI): m/z calcd. for
C20H22BNaOP [M + Na]+: 343.13972; found: 343.13997. HPLC: 98 %
ee, column: Lux 5u Cellulose-4 (150 × 4.6 mm); hexane/iPrOH, 99:1;
1.25 mL/min; tR = 12.6 min (R)-enantiomer and 14.2 min (S)-enantio-
mer.
mer.
(R)-(–)-Dibenzo[b,d]furan-4-yl(naphthalene-2-yl)(phenyl)phos-
phaneborane Complex (7k): Yield: 48 %; white solid; m.p. 81–84 °C
(cyclohexane/AcOEt, 98:2 to 96:4); [α]2D3 = –69.8 (c = 1.0, CHCl3).
1H NMR: δ = 8.33 (br. d, J = 13.3 Hz, 1 H, ArH), 8.15 (m, 1 H, ArH),
7.98–7.69 (m, 8 H, ArH), 7.60–7.27 (m, 9 H, ArH), 2.2–0.7 (br. q, 3 H,
BH3) ppm. 13C NMR: δ = 157.0 (C-O), 155.9 (C-O), 134.9 (d, J =
11.6 Hz, CH), 134.4 (d, J = 2.0 Hz, C-C), 133.1 (d, J = 10.2 Hz, 2 × CH,
CH), 132.7 (d, J = 12.1 Hz, C-C), 131.2 (d, J = 2.2 Hz, CH), 128.7 (CH),
128.6 (d, J = 10.6 Hz, 2 × CH), 128.6 (d, J = 60.4 Hz, C-P), 128.3 (d,
J = 10.0 Hz, CH), 128.1 (d, J = 9.3 Hz, CH), 128.0 (CH), 127.7 (CH),
127.6 (CH), 126.7 (CH), 125.7 (d, J = 59.4 Hz, C-P), 124.9 (d, J = 4.6 Hz,
C-C), 124.7 (d, J = 2.0 Hz, CH), 123.2 (C-C), 123.2 (d, J = 10.1 Hz, CH),
123.2 (CH), 120.7 (CH), 112.5 (d, J = 55.3 Hz, C-P), 111.9 (CH) ppm.
31P NMR: δ = +18.1 (br. d) ppm. MS (EI, 70 eV): m/z = 416 (1) [M]+,
402 (100) [M – BH3]+, 323 (18), 294 (23). HRMS (EI): m/z calcd. for
C28H21BOP [M – H]+: 415.14176; found: 415.14201. HPLC: 98 % ee,
column: Chiralpak AD-H (150 × 4.6 mm); hexane/iPrOH, 99:1;
1.25 mL/min; tR = 13.6 min (S)-enantiomer and 16.3 min (R)-enantio-
mer.
(R)-(+)-Dibenzo[b,d]furan-4-yl(2-methoxyphenyl)(phenyl)phos-
phaneborane Complex (7i): Yield: 71 %; white solid; m.p. 136–
137 °C (cyclohexane/AcOEt, 95:5); [α]2D3 = +40.1 (c = 1.0, CHCl3).
1H NMR: δ = 8.05 (dt, J = 7.7, 1.4 Hz, 1 H, ArH), 7.94–7.80 (m, 3 H,
ArH), 7.73 (ddd, J = 12.7, 7.7, 1.3 Hz, 1 H, ArH), 7.59 (ddd, J = 13.4,
7.7, 1.7 Hz, 1 H, ArH), 7.52–7.24 (m, 89 H, ArH), 7.01 (m, 1 H, ArH),
6.86 (ddd, J = 8.4, 4.2, 0.8 Hz, 1 H, ArH), 3.41 (s, 3 H, OCH3), 2.1–0.6
(br. q, 3 H, BH3) ppm. 13C NMR: δ = 161.6 (d, J = 1.0 Hz, C-OMe),
156.8 (C-O), 155.8 (C-O), 135.1 (d, J = 11.0 Hz, CH), 133.5 (d, J =
1.8 Hz, CH), 133.2 (d, J = 10.3 Hz, 2 × CH), 131.9 (d, J = 10.4 Hz, CH),
130.8 (d, J = 2.6 Hz, CH), 128.7 (d, J = 60.5 Hz, C-P), 128.1 (d, J =
10.8 Hz, 2 × CH), 127.3 (CH), 124.5 (d, J = 5.8 Hz, C-C), 123.6 (d, J =
2.5 Hz, CH), 123.3 (C-C), 122.9 (CH), 122.8 (d, J = 10.7 Hz, CH), 121.0
(d, J = 11.5 Hz, CH), 120.5 (CH), 116.3 (d, J = 58.9 Hz, C-P), 113.4 (d,
J = 58.3 Hz, C-P), 111.7 (CH), 111.5 (d, J = 4.9 Hz, CH), 55.3
(OCH3) ppm. 31P NMR: δ = +14.8 (br. s) ppm. MS (EI, 70 eV): m/z
(%) = 396 (1) [M]+, 382 (100) [M – BH3]+, 381 (19) [M – CH3]+, 365
(7) [M – OCH3], 351 (7) [M – BH3 – OCH3], 291 (23), 273 (27), 181 (44).
HRMS (EI): m/z calcd. for C25H21O2BP [M – H]+: 395.13667; found:
395.13718. HPLC: 98 % ee, column: Reprosil 100 (150 × 4.6 mm);
hexane/iPrOH, 99:1, 1.50 mL/min, tR = 13.6 min (R)-enantiomer and
15.3 min (S)-enantiomer.
(R)-(+)-(2-Methoxyphenyl)(2-phenoxyphenyl)(phenyl)phos-
phaneborane Complex (7l): Yield: 64 %; colorless oil (cyclohexane/
AcOEt, 98:2); [α]2D4 = +6.0 (c = 1.0, CHCl3). 1H NMR: δ = 7.81 (m, 2
H, ArH), 7.71 (ddd, J = 12.8, 7.8, 1.7 Hz, 1 H, ArH), 7.55 (ddd, J =
13.3, 7.6, 1.6 Hz, 1 H, ArH), 7.44–7.30 (m, 5 H, ArH), 7.21–7.08 (m, 3
H, ArH), 7.01 (m, 1 H, ArH), 6.94 (ddt, J = 7.5, 1.9, 0.9 Hz, 1 H, ArH),
7.86–7.76 (m, 2 H, ArH), 7.81 (m, 2 H, ArH), 3.57 (s, 3 H, OCH3), 2.1–
0.6 (br. q, 3 H, BH3) ppm. 13C NMR: δ = 161.1 (C-O), 159.1 (C-O),
155.2 (C-O), 135.0 (d, J = 10.8 Hz, CH), 134.8 (d, J = 10.4 Hz, CH),
133.3 (d, J = 10.4 Hz, 2 × CH), 132.9 (d, J = 2.0 Hz, CH), 132.5 (d, J =
1.9 Hz, CH), 130.4 (d, J = 2.6 Hz, CH), 129.4 (2 × CH), 129.0 (d, J =
60.6 Hz, C-P), 127.9 (d, J = 10.7 Hz, 2 × CH), 123.6 (CH), 122.8 (d, J =
10.9 Hz, CH), 120.8 (d, J = 11.0 Hz, CH), 119.9 (d, J = 57.8 Hz, C-P),
119.1 (2 × CH), 117.0 (d, J = 4.0 Hz, CH), 116.8 (d, J = 59.6 Hz, C-P),
111.3 (d, J = 4.8 Hz, CH), 55.3 (OCH3) ppm. 31P NMR: δ = +16.4 (br.
d) ppm. MS (EI, 70 eV): m/z (%) = 398 (1) [M]+, 384 (42) [M – BH3]+,
383 (49) [M – CH3]+, 367 (13) [M – OCH3]+, 353 (100) [M – BH3
–
OCH3]+. HRMS (EI): m/z calcd. for C25H23BO2P [M – H]+: 397.15232;
found: 397.15202. HPLC: 98 % ee, column: Chiralpak AD-H
(150 × 4.6 mm); hexane/iPrOH, 99:1; 1.50 mL/min; tR = 13.0 min (S)-
enantiomer and 17.5 min (R)-enantiomer.
(R)-(+)-Dibenzo[b,d]furan-4-yl(2-isopropoxyphenyl)(phenyl)-
phosphaneborane Complex (7j): Yield: 79 %; white solid; m.p. 65–
67 °C (cyclohexane/AcOEt, 98:2); [α]2D2 = +55.2 (c = 1.0, CHCl3). (R)-(–)-(4-Methoxyphenyl)(2-phenoxyphenyl)(phenyl)phos-
1H NMR: δ = 8.07 (dt, J = 7.7, 1.4 Hz, 2 H, ArH), 7.96–7.89 (m, 3 H,
ArH), 7.77 (ddd, J = 12.7, 7.6, 1.3 Hz, 1 H, ArH), 7.58 (ddd, J = 13.5,
7.6, 1.7 Hz, 1 H, ArH), 7.49–7.35 (m, 6 H, ArH), 7.34–7.28 (m, 2 H,
ArH), 6.96 (m, 1 H, ArH), 6.79 (dd, J = 8.3, 4.1 Hz, 1 H, ArH), 4.37
phaneborane Complex (7m): Yield: 60 %; colorless oil (cyclohex-
ane/AcOEt, 95:5); [α]D24 = –35.0 (c = 1.0, CHCl3). 1H NMR: δ = 7.82
(ddd, J = 13.0, 7.7, 1.7 Hz, 1 H, ArH), 7.73–7.61 (m, 4 H, ArH), 7.45–
7.30 (m, 4 H, ArH), 7.24–7.10 (m, 3 H, ArH), 7.05 (m, 1 H, ArH), 6.90
(sept, J = 6.0 Hz, 1 H, OCH), 1.8–0.6 (br. q, 3 H, BH3), 0.74 (d, J = (m, 2 H, ArH), 6.76 (ddd, J = 8.3, 3.6, 0.7 Hz, 1 H, ArH), 6.66–6.60 (m,
6.0 Hz, 3 H, CH3), 0.72 (d, J = 6.0 Hz, 3 H, CH3) ppm. 13C NMR: δ =
159.3 (C-OiPr), 156.8 (C-O), 155.8 (C-O), 135.3 (d, J = 11.0 Hz, CH),
133.6 (d, J = 10.4 Hz, 2 × CH), 133.2 (d, J = 2.0 Hz, CH), 132.1 (d, J =
10.6 Hz, CH), 130.8 (d, J = 2.3 Hz, CH), 128.8 (d, J = 60.4 Hz, C-P),
128.1 (d, J = 10.6 Hz, 2 × CH), 127.3 (CH), 124.4 (d, J = 4.5 Hz, C-C),
123.4 (d, J = 2.5 Hz, CH), 123.3 (C-C), 122.9 (CH), 122.8 (d, J = 10.5 Hz,
CH), 120.5 (CH), 120.1 (d, J = 11.7 Hz, CH), 116.2 (d, J = 60.1 Hz, C-
P), 114.0 (d, J = 58.3 Hz, C-P), 111.8 (CH), 111.5 (d, J = 4.8 Hz, CH),
69.1 (OCH), 20.8 (CH3), 20.8 (CH3) ppm. 31P NMR: δ = +14.1 (br.
2 H, ArH), 3.79 (s, 3 H, OCH3), 2.0–0.5 (br. q, 3 H, BH3) ppm. 13C NMR:
δ = 161.7 (d, J = 2.3 Hz, C-O), 159.7 (C-O), 154.8 (C-O), 136.2 (d, J =
12.6 Hz, CH), 134.9 (d, J = 11.0 Hz, 2 × CH), 133.4 (d, J = 1.9 Hz, CH),
132.6 (d, J = 9.8 Hz, 2 × CH), 130.5 (d, J = 2.3 Hz, CH), 129.7 (d, J =
61.1 Hz, C-P), 129.5 (2 × CH), 128.3 (d, J = 10.3 Hz, 2 × CH), 124.1
(CH), 122.8 (d, J = 11.1 Hz, CH), 119.7 (2 × CH), 119.2 (d, J = 65.2 Hz,
C-P), 119.2 (d, J = 54.9 Hz, C-P), 116.8 (d, J = 4.0 Hz, CH), 114.0 (d,
J = 11.6 Hz, 2 × CH), 55.2 (OCH3) ppm. 31P NMR: δ = +17.3
(br. d) ppm. MS (EI, 70 eV): m/z (%) = 398 (1) [M]+, 384 (53) [M –
d) ppm. MS (EI, 70 eV): m/z (%) = 410 (96) [M – BH3]+, 368 (100) [M BH3]+, 383 (100) [M – CH3]+, 367 (6) [M – OCH3]+, 353 (22) [M – BH3
– BH3 – C3H7 + H]+, 367 (45) [M – BH3 – C3H7]+, 351 (85) [M – BH3
– OCH3]+. HRMS (EI): m/z calcd. for C25H23BO2P [M – H]+: 397.15232;
– OC3H7]+, 289 (27), 273 (59), 199 (67). HRMS (ESI): m/z calcd. for found: 397.15282. HPLC: 98 % ee, column: Lux 5u Cellulose-1
C27H26BNaO2P [M + Na]+: 447.16605; found: 447.16659. HPLC: 99 % (150 × 4.6 mm); hexane/iPrOH, 99:1; 1.50 mL/min; tR = 8.0 min (R)-
ee, column: Reprosil 100 (150 × 4.6 mm); hexane/iPrOH, 99:1;
enantiomer and 9.3 min (S)-enantiomer.
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(S)-(–)-(4-Ethoxyphenyl)(phenyl)(3-tolyl)phosphaneborane
Complex (7n): Yield: 73 %; white solid; m.p. 70–71 °C (cyclohexane/
the corresponding borane adducts 7a–p (1.5 mmol) in hexane
(10 mL), DABCO (2 equiv., 336 mg) was added under argon atmos-
AcOEt, 95:5); [α]2D3 = –4.1 (c = 1.0, CHCl3). 1H NMR: δ = 7.62–7.27 phere. The solution was stirred at ambient temperature overnight
(m, 11 H, ArH), 6.95 (m, 2 H, ArH), 4.06 (d, J = 7.0 Hz, 2 H, OCH2),
2.34 (s, 3 H, CH3), 1.42 (d, J = 7.0 Hz, 3 H, CH3), 2.0–0.6 (br. q, 3 H,
and then the white solid (DABCO*BH3) was removed by filtration.
After washing once with hexane (5 mL) the solvent was evaporated
BH3) ppm. 13C NMR: δ = 161.1 (d, J = 2.4 Hz, C-OEt), 138.2 (d, J = under vacuum and the residue was purified by column chromatog-
10.3 Hz, C-C), 134.7 (d, J = 10.4 Hz, 2 × CH), 133.1 (d, J = 10.6 Hz,
CH), 132.7 (d, J = 9.6 Hz, 2 × CH), 131.7 (d, J = 2.5 Hz, CH), 130.7 (d,
J = 2.4 Hz, CH), 129.8 (d, J = 8.7 Hz, CH), 129.7 (d, J = 58.0 Hz, C-P),
129.3 (d, J = 58.3 Hz, C-P), 128.4 (d, J = 10.2 Hz, 2 × CH), 128.2 (d,
J = 10.9 Hz, CH), 119.0 (d, J = 62.9 Hz, C-P), 114.6 (d, J = 11.5 Hz,
2 × CH), 63.3 (OCH2), 21.1 (CH3), 14.4 (CH3) ppm. 31P NMR: δ = +19.4
(br. d) ppm. MS (EI, 70 eV): m/z (%) = 334 (1) [M]+, 333 (4) [M – H]+,
320 (100) [M – BH3]+, 291 (17) [M – C2H5]+, 243 (9) [M – Ph]+. HRMS
(EI): m/z calcd. for C21H24BNaOP [M – H]+: 357.15538; found:
357.15566. HPLC: 99 % ee, column: Chiralpak AD-H (250 × 4.6 mm);
hexane/iPrOH, 98:2; 1.25 mL/min; tR = 10.9 min (S)-enantiomer and
12.2 min (R)-enantiomer.
raphy. For the estimation of the enantioselectivity a small sample
was transferred back into its borane complex 7 and investigated by
HPLC.
(S)-(–)-(2-Methoxyphenyl)(naphthalen-2-yl)(phenyl)phosphane
(8a): Yield: 65 %; white solid; m.p. 55–56 °C (cyclohexane/AcOEt,
95:5); [α]2D3 = –21.6 (c = 0.5, CHCl3). 1H NMR: δ = 7.85 (br. d, J =
8.9 Hz, 1 H, ArH), 7.79–7.67 (m, 3 H, ArH), 7.46–7.26 (m, 9 H, ArH),
6.88–6.73 (m, 3 H, ArH), 3.67 (s, 3 H, OCH3) ppm. 13C NMR: δ = 161.0
(d, J = 15.2 Hz, C-O), 136.5 (d, J = 10.6 Hz, C-P), 134.1 (d, J = 11.0 Hz,
C-C), 134.0 (d, J = 23.2 Hz, CH), 133.8 (d, J = 20.2 Hz, 2 × CH), 133.6
(CH), 133.2 (C-C), 133.1 (d, J = 8.4 Hz, C-P), 130.3 (CH), 130.2 (d, J =
17.6 Hz, CH), 128.5 (CH), 128.3 (d, J = 7.1 Hz, 2 × CH), 127.9 (CH),
127.6 (d, J = 6.8 Hz, CH), 127.5 (CH), 126.4 (CH), 126.0 (CH), 125.3
(d, J = 12.6 Hz, C-P), 120.9 (CH), 110.1 (d, J = 1.2 Hz, CH), 55.4
(OCH3) ppm. 31P NMR: δ = –15.5 ppm. MS (EI, 70 eV): m/z (%) = 342
General Synthesis of 7o and 7p by Substitution of the Methoxy
Group in 6a by an Alkyl Group: Phosphinite 6a (3 mmol, 840 mg)
was dissolved in diethyl ether (20 mL) at –78 °C and methyllithium
or n-butyllithium (1.6
M
solution, 1.3 equiv, 2.44 mL) was added (100) [M]+, 327 (9) [M – CH3]+, 311 (11) [M – OCH3]+, 267 (8), 233
slowly. After one hour, stirring was continued at room temperature
overnight. The working up procedure was carried out as described
above.
(41), 141 (28). HRMS (EI): m/z calcd. for C23H19OP [M]+: 342.11680;
found: 342.11605. HPLC (as 7a): 98 % ee.
(S)-(–)-(3-Methoxyphenyl)(naphthalen-2-yl)(phenyl)phosphane
(S)-(–)-Methyl(naphthalen-2-yl)(phenyl)phosphaneborane Com- (8b): Yield: 87 %; white solid; m.p. 79–80 °C (cyclohexane/AcOEt,
1
plex (7m): Yield: 57 %; white solid; m.p. 68–69 °C (cyclohexane/
95:5); [α]2D2 = –6.0 (c = 1.0, CHCl3). H NMR: δ = 7.87–7.72 (m, 4 H,
AcOEt, 99:1 to 98:2); [α]D23 = –19.1 (c = 1.0, CHCl3). 1H NMR: δ = 8.29 ArH), 7.54–7.24 (m, 9 H, ArH), 7.00–7.87 (m, 3 H, ArH), 3.73 (s, 3 H,
(br. d, J = 12.7 Hz, 1 H, ArH), 7.93–7.82 (m, 3 H, ArH), 7.75–7.39 (m,
8 H, ArH), 1.95 (J = 10.1 Hz, 3 H, CH3), 1.9–0.4 (br. q, 3 H, BH3) ppm.
13C NMR: δ = 134.1 (d, J = 2.0 Hz, C-C), 133.3 (d, J = 10.9 Hz, CH),
132.6 (d, J = 11.4 Hz, C-C), 131.6 (d, J = 9.6 Hz, 2 × CH), 131.1 (d,
OCH3) ppm. 13C NMR: δ = 159.5 (d, J = 8.5 Hz, C-O), 138.5 (d, J =
11.0 Hz, C-P), 136.8 (d, J = 10.2 Hz, C-P), 134.3 (d, J = 10.4 Hz, C-C),
133.2 (d, J = 22.7 Hz, CH), 133.8 (d, J = 19.7 Hz, 2 × CH), 133.4 (C-
C), 133.2 (d, J = 8.4 Hz, C-P), 130.0 (d, J = 17.1 Hz, CH), 129.6 (d, J =
J = 2.3 Hz, CH), 130.5 (d, J = 56.5 Hz, C-P), 128.8 (d, J = 10.2 Hz, 7.8 Hz, CH), 128.9 (CH), 128.5 (d, J = 7.1 Hz, 2 × CH), 128.1 (CH),
2 × CH), 128.6 (d, J = 9.6 Hz, CH), 128.5 (CH), 127.9 (CH), 127.7 (CH), 127.9 (d, J = 6.7 Hz, CH), 127.7 (CH), 126.7 (CH), 126.2 (CH), 126.1
127.4 (d, J = 56.2 Hz, C-P), 126.9 (CH), 126.6 (d, J = 8.5 Hz, CH), 11.8
(d, J = 18.5 Hz, CH), 119.0 (d, J = 21.4 Hz, CH), 114.4 (CH), 55.1
(d, J = 40.2 Hz, CH3) ppm. 31P NMR: δ = +10.9 (br. d) ppm. MS (EI,
(OCH3) ppm. 31P NMR: δ = –3.6 ppm. MS (EI, 70 eV): m/z (%) = 342
70 eV): m/z (%) = 250 (100) [M – BH3]+, 235 (54) [M – BH3 – CH3]+, (100) [M]+, 311 (2) [M – OCH3]+, 263 (8), 233 (32), 158 (9) [C10H7P]+.
233 (67), 173 (7) [M – BH3 – Ph]+. HRMS (ESI): m/z calcd. for
C17H18BNaP [M + Na]+: 287.11345; found: 287.11338. HPLC: 99 % ee,
column: Reprosil 100 (150 × 4.6 mm); hexane/iPrOH, 99.5:0.5;
1.25 mL/min; tR = 12.6 min (R)-enantiomer and 14.3 min (S)-enantio-
mer.
HRMS (ESI): m/z calcd. for C23H20OP [M + H]+: 343.12463; found:
343.12509. HPLC (as 7b): 99 % ee.
(R)-(–)-(4-Methoxyphenyl)(naphthalen-2-yl)(phenyl)phos-
phaneborane Complex (8c): Yield: 57 %; white solid; m.p. 94–96 °C
(cyclohexane/AcOEt, 98:2); [α]2D3 = –3.8 (c = 0.5, CHCl3). 1H NMR: δ =
(S)-(–)-(n-Butyl)(naphthalen-2-yl)(phenyl)phosphaneborane 7.82–7.65 (m, 4 H, ArH), 7.47–7.26 (m, 10 H, ArH), 6.88 (m, 2 H, ArH),
Complex (7n): Yield: 76 %; white solid; m.p. 83–84 °C (cyclohexane/
3.72 (s, 3 H, OCH3) ppm. 13C NMR: δ = 160.4 (C-O), 137.8 (d, J =
AcOEt, 99:1); [α]2D3 = –19.4 (c = 1.0, CHCl3). H NMR: δ = 8.29 (br. d, 10.7 Hz, C-P), 135.6 (d, J = 21.6 Hz, 2 × CH), 135.4 (d, J = 11.4 Hz, C-
1
J = 12.1 Hz, 1 H, ArH), 7.93–7.82 (m, 3 H, ArH), 7.74–7.39 (m, 8 H,
ArH), 2.29 (m, 2 H, PCH2), 1.63–1.35 (m, 4 H, 2 × CH2), 0.89 (J =
7.2 Hz, 3 H, CH3), 1.8–0.4 (br. q, 3 H, BH3) ppm. 13C NMR: δ = 134.2
(d, J = 1.9 Hz, C-C), 134.0 (d, J = 10.8 Hz, CH), 132.7 (d, J = 11.4 Hz,
C-C), 132.0 (d, J = 9.0 Hz, 2 × CH), 131.0 (d, J = 2.6 Hz, CH), 129.9
(d, J = 55.0 Hz, C-P), 128.8 (d, J = 9.7 Hz, 2 × CH), 128.6 (CH), 128.5
(d, J = 9.7 Hz, CH), 127.9 (CH), 127.7 (CH), 127.0 (d, J = 7.6 Hz, CH),
126.9 (CH), 126.6 (d, J = 54.7 Hz, C-P), 25.3 (d, J = 35.7 Hz, PCH2),
C), 133.5 (d, J = 20.7 Hz, CH), 133.4 (d, J = 19.1 Hz, 2 × CH), 133.2
(d, J = 7.3 Hz, C-P), 133.1 (C-C), 129.8 (d, J = 18.1 Hz, CH), 128.4
(CH), 128.4 (d, J = 8.3 Hz, 2 × CH), 127.9 (CH), 127.8 (d, J = 6.9 Hz,
CH), 127.6 (CH), 127.4 (d, J = 8.2 Hz, C-P), 126.5 (CH), 126.1 (CH),
114.2 (d, J = 8.2 Hz, 2 × CH), 55.0 (OCH3) ppm. 31P NMR: δ =
–5.9 ppm. MS (EI, 70 eV): m/z (%) = 342 (100) [M]+, 327 (4) [M –
CH3]+, 311 (3) [M – OCH3]+, 265 (8) [M – Ph]+, 233 (29), 158 (16)
[C10H7P]+, 149 (43), 141 (14). HRMS (EI): m/z calcd. for C23H19OP
25.0 (CH2), 24.2 (d, J = 14.3 Hz, CH2), 13.5 (CH3) ppm. 31P NMR: δ = [M]+: 342.11680; found: 342.11656. HPLC (as 7c): 99 % ee.
+16.7 (br. d) ppm. MS (EI, 70 eV): m/z (%) = 306 (2) [M]+, 292 (99)
(R)-(–)-(3-Ethylphenyl)(naphthalen-2-yl)(phenyl)phosphane-
borane Complex (8d): Yield: 87 %; colorless oil (cyclohexane/
[M – BH3]+, 249 (46) (100) [M – BH3 – C3H7]+ 233. HRMS (ESI):
as oxide, m/z calcd. for C20H21NaOP [M + Na]+: 311.12222;
AcOEt, 95:5); [α]2D3 = –9.0 (c = 1.0, CHCl3). 1H NMR: δ = 7.85–7.70
found: 331.12209. HPLC: 99 % ee, column: Lux 5u Cellulose 1
(m, 4 H, ArH), 7.51–7.17 (m, 11 H, ArH), 7.13 (m, 1 H, ArH), 2.60 (q,
(150 × 4.6 mm); hexane/iPrOH, 99.5:0.5; 1.00 mL/min; tR = 10.0 min
(R)-enantiomer and 12.2 min (S)-enantiomer.
J = 7.6 Hz, 2 H, CH2), 1.18 (t, J = 7.6 Hz, 3 H, CH3) ppm. 13C NMR:
δ = 144.4 (d, J = 7.9 Hz, C-C), 137.1 (d, J = 10.4 Hz, C-P), 136.6 (d,
General Synthesis of Phosphanes 8a–p by Removing the BH3-
Group from Phosphane Borane Adducts 7a–p: To a solution of
J = 9.6 Hz, C-P), 134.7 (d, J = 10.4 Hz, C-P), 134.1 (d, J = 22.3 Hz,
CH), 133.8 (d, J = 19.3 Hz, 2 × CH), 133.5 (d, J = 23.0 Hz, CH), 133.3
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(C-C), 133.2 (d, J = 8.3 Hz, C-C), 131.0 (d, J = 8.2 Hz, CH), 130.0 (d,
J = 17.4 Hz, CH), 128.7 (CH), 128.5 (d, J = 6.3 Hz, CH), 128.5 (d, J =
6.9 Hz, 2 × CH), 128.5 (CH), 128.1 (CH), 127.9 (d, J = 6.9 Hz, CH), 127.7
(CH), 126.6 (CH), 126.2 (CH), 28.7 (CH2), 15.5 (CH3) ppm. 31P NMR:
δ = –4.1 ppm. MS (EI, 70 eV): m/z (%) = 340 (100) [M]+, 318 (10), 254
CH3) ppm. 13C NMR: δ = 161.2 (d, J = 15.0 Hz, C-O), 137.8 (d, J =
7.9 Hz, C-Me), 136.6 (d, J = 9.4 Hz, C-P), 136.2 (d, J = 8.9 Hz, C-P),
134.6 (d, J = 23.1 Hz, CH), 133.8 (d, J = 20.3 Hz, 2 × CH), 133.7 (CH),
130.8 (d, J = 17.2 Hz, CH), 130.3 (CH), 129.5 (CH), 128.5 (CH), 128.3
(d, J = 7.2 Hz, 2 × CH), 128.2 (d, J = 6.5 Hz, CH), 125.5 (d, J = 11.1 Hz,
C-P), 121.0 (d, J = 1.0 Hz, CH), 110.2 (d, J = 1.6 Hz, CH), 55.7 (OCH3),
21.4 (CH3) ppm. 31P NMR: δ = –16.8 ppm. MS (EI, 70 eV): m/z (%) =
306 (100) [M]+, 275 (21) [M – OCH3]+, 215 (33) [M – C7H7]+,197 (28),
183 (37). HRMS (ESI): m/z calcd. for C20H20OP [M + H]+: 307.12463;
found: 307.12448. HPLC (as 7h): 98 % ee.
(23), 233 (46), 127 (6) [C10H7]+. HRMS (ESI): m/z calcd. for C24H22
[M + H]+: 341.14536; found: 341.14576. HPLC (as 7d): 98 % ee.
P
(S)-(–)-(9,9-Dimethyl-9H-xanthen-4-yl)(naphthalen-2-yl)-
(phenyl)phosphane (8e): Yield: 87 %; white solid; m.p. 68–70 °C
(cyclohexane/AcOEt, 98:2); [α]2D3 = –41.6 (c = 1.0, CHCl3). 1H NMR:
δ = 7.93 (br. d, J = 9.1 Hz, 1 H, ArH), 7.84–7.72 (m, 3 H, ArH), 7.51–
7.31 (m, 10 H, ArH), 7.04–6.96 (m, 3 H, ArH), 6.67 (ddd, J = 7.5, 4.6,
1.6 Hz, 1 H, ArH), 6.60 (m, 1 H, ArH), 1.65 (s, 3 H, CH3), 1.63 (s, 3 H,
CH3) ppm. 13C NMR: δ = 152.3 (d, J = 14.1 Hz, C-O), 150.5 (C-O),
136.5 (d, J = 10.2 Hz, C-P), 134.5 (d, J = 24.8 Hz, CH), 134.0 (d, J =
10.4 Hz, C-C), 134.0 (d, J = 20.4 Hz, 2 × CH), 133.4 (C-C), 133.3 (d,
J = 9.1 Hz, C-P), 131.4 (d, J = 2.8 Hz, CH), 130.4 (d, J = 7.1 Hz, C-C),
130.4 (C-C), 130.3 (d, J = 16.6 Hz, CH), 128.7 (d, J = 2.2 Hz, CH),
128.5 (d, J = 7.3 Hz, 2 × CH), 128.1 (CH), 127.9 (d, J = 6.6 Hz, CH),
127.7 (CH), 127.2 (CH), 126.6 (2 × CH), 126.1 (CH), 125.3 (CH), 124.9
(d, J = 14.6 Hz, C-P), 123.2 (2 × CH), 116.5 (CH), 34.4 (C), 31.7 (CH3),
31.3 (CH3) ppm. 31P NMR: δ = –14.4 ppm. MS (EI, 70 eV): m/z (%) =
444 (52) [M]+, 429 (100) [M – CH3]+, 235 (8) [C16H12P]+, 214 (9).
HRMS (ESI): m/z calcd. for C31H26OP [M + H]+: 445.17158; found:
445.117110. HPLC (as 7e): 97 % ee.
(R)-(–)-Dibenzo[b,d]furan-4-yl(2-methoxyphenyl)(phenyl)phos-
phane (8i): Yield: 60 %; white solid; m.p. 150–152 °C (cyclohexane/
1
AcOEt, 95:5); [α]2D3 = –17.4 (c = 0.5, CHCl3). H NMR: δ = 7.95 (m, 2
H, ArH), 7.51 (m, 1 H, ArH), 7.45–7.23 (m, 9 H, ArH), 6.99–6.89 (m, 2
H, ArH), 6.84 (m, 1 H, ArH), 6.72 (m, 1 H, ArH), 3.75 (s, 3 H,
OCH3) ppm. 13C NMR: δ = 161.3 (d, J = 15.8 Hz, C-OMe), 156.8 (d,
J = 17.0 Hz, C-O), 156.1 (C-O), 135.0 (d, J = 9.3 Hz, C-P), 134.0 (d, J =
20.4 Hz, 2 × CH), 133.7 (CH), 131.7 (d, J = 1.9 Hz, CH), 130.5 (CH),
128.8 (CH), 128.5 (d, J = 7.0 Hz, 2 × CH), 127.1 (CH), 124.2 (C-C),
123.9 (d, J = 10.3 Hz, C-P), 123.6 (d, J = 3.0 Hz, C-C), 123.0 (CH),
122.6 (CH), 121.2 (CH), 121.0 (CH), 120.6 (CH), 120.1 (d, J = 16.2 Hz,
C-P), 112.1 (CH), 110.3 (d, J = 1.5 Hz, CH), 55.7 (OCH3) ppm. 31P NMR:
δ = –29.9 ppm. MS (EI, 70 eV): m/z (%) = 382 (100) [M]+, 351 (10)
[M – OCH3], 291 (23), 273 (28), 181 (48). HRMS (EI): m/z calcd. for
C25H19O2P [M]+: 382.11172; found: 372.11109. HPLC (as 7i): 96 % ee.
(S)-(–)-(2-Methoxyphenyl)(naphthalen-1-yl)(phenyl)phosphane
(8f): Yield: 56 %; white solid; m.p. 182–183 °C (cyclohexane/AcOEt,
(R)-(–)-Dibenzo[b,d]furan-4-yl(2-isopropoxyphenyl)(phenyl)-
phosphane Complex (8j): Yield: 83 %; white solid; m.p. 49–50 °C
(cyclohexane/AcOEt, 98:2); [α]2D3 = –2.9 (c = 1.00, CHCl3). 1H NMR:
δ = 7.98–7.89 (m, 2 H, ArH), 7.52 (ddd, J = 9.0, 0.8, 0.8, 1.3 Hz, 1 H,
ArH), 7.46–7.38 (m, 3 H, ArH), 7.37–7.23 (m, 6 H, ArH), 6.96 (ddd, J =
7.5, 5.2, 1.3 Hz, 1 H, ArH), 6.98 (dd, J = 8.3, 4.6 Hz, 1 H, ArH), 6.78
(m, 1 H, ArH), 6.72 (m, 1 H, ArH), 4.50 (sept, J = 6.0 Hz, 1 H, OCH),
1.06 (d, J = 6.1 Hz, 3 H, CH3), 1.02 (d, J = 6.0 Hz, 3 H, CH3) ppm.
13C NMR: δ = 159.2 (d, J = 14.4 Hz, C-OiPr), 158.6 (d, J = 17.1 Hz, C-
O), 156.1 (C-O), 135.4 (d, J = 9.6 Hz, C-P), 134.3 (d, J = 20.6 Hz,
2 × CH), 133.5 (d, J = 2.4 Hz, CH), 132.0 (d, J = 2.8 Hz, CH), 130.0
(CH), 128.7 (CH), 128.3 (d, J = 7.3 Hz, 2 × CH), 127.0 (CH), 125.3 (d,
J = 10.0 Hz, C-P), 124.1 (d, J = 1.8 Hz, C-C), 123.4 (d, J = 3.4 Hz, C-
C), 122.9 (CH), 122.6 (CH), 121.1 (CH), 120.5 (CH), 120.5 (d, J =
16.4 Hz, C-P), 120.4 (CH), 112.1 (CH), 111.8 (CH), 70.0 (OCH), 21.7
(CH3), 21. 5 (CH3) ppm. 31P NMR: δ = –28.8 ppm. MS (EI, 70 eV): m/z
(%) = 410 (88) [M]+, 368 (100) [M – C3H7 + H]+, 367 (46) [M – C3H7]+,
351 (91) [M – OC3H7]+, 289 (27), 273 (49), 199 (61). HRMS (ESI): m/z
calcd. for C27H24O2P [M + H]+: 411.15084; found: 411.15101. HPLC
(as 7j): 99 % ee.
1
98:2); [α]2D2 = –15.2 (c = 1.0, CHCl3). H NMR: δ = 8.45 (m, 1 H, ArH),
7.85 (m, 2 H, ArH), 7.50–7.29 (m, 9 H, ArH), 7.02 (ddd, J = 7.0, 5.0,
1.2 Hz, 1 H, ArH), 6.93 (m, 1 H, ArH), 6.81 (m, 1 H, ArH), 6.63 (m, 1
H, ArH), 3.75 (s, 3 H, OCH3) ppm. 13C NMR: δ = 161.3 (d, J = 15.6 Hz,
C-O), 135.7 (d, J = 8.1 Hz, C-C), 135.5 (d, J = 22.5 Hz, C-P), 134.3
(CH), 134.3 (d, J = 20.4 Hz, 2 × CH), 133.4 (d, J = 4.4 Hz, C-C), 133.8
(d, J = 12.8 Hz, C-P), 131.9 (CH), 130.5 (CH), 129.3 (CH), 128.8 (CH),
128.5 (d, J = 1.4 Hz, CH), 128.5 (d, J = 7.4 Hz, 2 × CH), 126.4 (d, J =
27.0 Hz, CH), 126.1 (d, J = 2.4 Hz, CH), 125.9 (d, J = 1.3 Hz, CH),
125.6 (d, J = 1.5 Hz, CH), 124.6 (d, J = 10.1 Hz, C-P), 121.1 (CH), 110.2
(d, J = 1.9 Hz, CH), 55.7 (OCH3) ppm. 31P NMR: δ = –25.6 ppm. MS
(EI, 70 eV): m/z (%) = 342 (100) [M]+, 327 (2) [M – CH3]+, 311 (18)
[M – OCH3]+, 267 (8), 233 (32), 141 (53). HRMS (EI): m/z calcd. for
C
23H19OP [M]+: 342.11680; found: 342.11607. HPLC (as 7f): 98 % ee.
(R)-(+)-(4-Methoxyphenyl)(naphthalen-1-yl)(phenyl)phos-
phaneborane Complex (8g): Yield: 81 %; colorless oil (cyclohex-
ane/AcOEt, 98:2); [α]D22 = +17.6 (c = 0.5, CHCl3). 1H NMR: δ = 8.43
(m, 1 H, ArH), 7.87 (m, 2 H, ArH), 7.54–7.25 (m, 10 H, ArH), 7.04 (m,
1 H, ArH), 6.91 (m, 2 H, ArH), 3.81 (s, 3 H, OCH3) ppm. 13C NMR: δ =
160.4 (C-O), 136.9 (d, J = 9.1 Hz, C-P), 135.9 (d, J = 21.8 Hz, 2 × CH),
135.0 (d, J = 21.8 Hz, C-P), 135.0 (d, J = 12.6 Hz, C-C), 133.8 (d, J =
19.5 Hz, 2 × CH), 133.3 (d, J = 4.4 Hz, C-C), 131.7 (CH), 129.3 (CH),
128.6 (CH), 128.6 (d, J = 1.6 Hz, CH), 128.4 (d, J = 7.2 Hz, 2 × CH),
126.5 (d, J = 6.3 Hz, C-P), 126.2 (d, J = 1.7 Hz, CH), 126.1 (d, J =
25.3 Hz, CH), 125.9 (CH), 125.5 (CH), 114.3 (d, J = 8.3 Hz, 2 × CH),
55.1 (OCH3) ppm. 31P NMR: δ = –15.6 ppm. MS (EI, 70 eV): m/z (%) =
342 (99) [M]+, 341 (100) [M – H]+, 327 (3) [M – CH3]+, 311 (1) [M –
OCH3]+, 265 (2) [M – Ph]+, 233 (20), 158 (4) [C10H7P]+. HRMS (EI):
m/z calcd. for C23H19OP [M]+: 342.11680; found: 342.11577. HPLC
(as 7g): 98 % ee.
(R)-(+)-Dibenzo[b,d]furan-4-yl(naphthalene-2-yl)(phenyl)phos-
phane (8k): Yield: 45 %; white solid; m.p. 65–67 °C (cyclohexane/
AcOEt, 95:5); [α]2D4 = +84.9 (c = 1.0, CHCl3). 1H NMR: δ = 7.97 (dd,
J = 7.6, 1.2 Hz, 1 H, ArH), 7.96 (m, 1 H, ArH), 7.89 (m, 1 H, ArH),
7.85–7.70 (m, 3 H, ArH), 7.54–7.24 (m, 12 H, ArH), 7.02 (ddd, J = 7.5,
2.0, 1.4 Hz, 1 H, ArH) ppm. 13C NMR: δ = 158.3 (d, J = 16.3 Hz, C-
O), 156.1 (C-O), 135.5 (d, J = 9.6 Hz, C-P), 134.4 (d, J = 23.4 Hz, CH),
133.9 (d, J = 20.0 Hz, 2 × CH), 133.5 (C-C), 133.2 (d, J = 8.6 Hz, C-C),
133.0 (d, J = 9.7 Hz, C-P), 131.7 (d, J = 2.2 Hz, CH), 130.1 (d, J =
17.6 Hz, CH), 128.9 (CH), 128.6 (d, J = 7.1 Hz, 2 × CH), 128.1 (CH),
128.0 (d, J = 7.0 Hz, CH), 127.7 (CH), 127.2 (CH), 126.8 (CH), 126.3
(CH), 123.8 (d, J = 2.9 Hz, C-C), 123.2 (CH), 123.2 (C-C), 122.8 (CH),
121.5 (CH), 120.6 (CH), 120.4 (d, J = 16.8 Hz, C-P), 112.1 (CH) ppm.
31P NMR: δ = –18.5 ppm. MS (EI, 70 eV): m/z (%) = 402 (1) [M]+, 325
(S)-(–)-(2-Methoxyphenyl)(phenyl)(3-tolyl)phosphane (8h): Yield:
81 %; white solid; m.p. 140–142 °C (cyclohexane/AcOEt, 98:2);
[α]2D3 = –2.0 (c = 1.0, CHCl3). 1H NMR: δ = 7.38–7.13 (m, 9 H, ArH), (100) [M – Ph]+, 323 (43), 275 (58) [M – C10H7]+, 273 (96), 235 (25)
7.05 (m, 1 H, ArH), 6.90 (m, 1 H, ArH), 6.92 (ddt, J = 7.4, 1.1, 0.7 Hz,
1 H, ArH), 6.71 (m, 1 H, ArH), 3.75 (s, 3 H, OCH3), 2.30 (s, 3 H,
[M – C12H8O], 233 (35). HRMS (EI): m/z calcd. for C28H19OP [M]+:
402.11680; found: 402.11612. HPLC (as 7k): 98 % ee.
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(R)-(–)-(2-Methoxyphenyl)(2-phenoxyphenyl)(phenyl)phos-
phane (8l): Yield: 80 %; colorless syrup (cyclohexane/AcOEt, 95:5);
7.0 Hz, 3 H, CH3) ppm. 13C NMR: δ = 138.8 (d, J = 11.9 Hz, C-P),
136.0 (d, J = 12.1 Hz, C-P), 133.3 (C-C), 133.2 (d, J = 23.4 Hz, CH),
133.2 (d, J = 8.7 Hz, C-C), 132.6 (d, J = 17.7 Hz, 2 × CH), 129.0 (d,
J = 14.0 Hz, CH), 128.4 (CH), 128.3 (d, J = 6.5 Hz, 2 × CH), 127.9 (CH),
127.9 (d, J = 6.0 Hz, CH), 127.7 (CH), 126.5 (CH), 126.2 (CH), 28.7 (d,
J = 15.4 Hz, PCH2), 27.5 (d, J = 10.6 Hz, CH2), 24.3 (d, J = 13.2 Hz,
CH2), 13.8 (CH3) ppm. 31P NMR: δ = –15.2 (br. d) ppm. MS (EI, 70 eV):
1
[α]2D3 = –12.5 (c = 1.0, CHCl3). H NMR: δ = 7.40–7.17 (m, 9 H, ArH),
7.05–6.94 (m, 2 H, ArH), 6.91–6.77 (m, 7 H, ArH), 3.71 (s, 3 H,
OCH3) ppm. 13C NMR: δ = 161.2 (d, J = 15.7 Hz, C-O), 159.5 (d, J =
17.3 Hz, C-O), 156.7 (C-O), 135.9 (d, J = 10.6 Hz, C-P), 134.2 (d, J =
21.2 Hz, 2 × CH), 134.0 (d, J = 1.2 Hz, CH), 133.8 (d, J = 1.2 Hz, CH),
130.2 (CH), 130.0 (CH), 129.4 (2 × CH), 128.6 (CH), 128.2 (d, J = m/z (%) = 292 (75) [M]+, 263 (6) [M – C2H5]+, 249 (100) [M – C3H7]+
7.4 Hz, 2 × CH), 128.2 (d, J = 13.9 Hz, C-P), 124.8 (d, J = 12.9 Hz, C-
P), 123.2 (CH), 123.2 (CH), 120.9 (CH), 119.2 (2 × CH), 117.4 (d, J =
1.8 Hz, CH), 110.2 (d, J = 1.3 Hz, CH), 55.6 (OCH3) ppm. 31P NMR:
δ = –26.7 ppm. MS (EI, 70 eV): m/z (%) = 384 (33) [M]+, 383 (45) [M
– H]+, 353 (100) [M – OCH3]+, 199 (17). HRMS (EI): m/z calcd. for
C25H20O2P [M – H]+: 383.11954; found: 383.11926. HPLC (as 7l): 99 %
ee.
233 (49), 158 (12) [C10H7P]+. HRMS (EI): m/z calcd. for C20H21P [M]+:
292.13754; found: 292.13696. HPLC (as 7p): 99 % ee.
General Method for the Thermal Racemization of Phosphanes
8a–p: Phosphanes 8a–m (0.20 mmol) were dissolved in toluene
(4 mL) in a Schlenk-tube and stirred under argon-atmosphere for
five minutes at ambient temperature. A sample (0.3 mL) was ex-
tracted, cooled to 0 °C in an ice bath and treated with borane-
(R)-(+)-(4-Methoxyphenyl)(2-phenoxyphenyl)(phenyl)phos-
phane (8m): Yield: 68 %; colorless oil (cyclohexane/AcOEt, 95:5);
dimethyl sulfide solution (2 M, 15 μL). The initial value of the enan-
tiomeric ratio was estimated by HPLC analysis. The Schlenk-tube
was transferred into an oil bath with a constant temperature (50 °C,
70 °C or 90 °C) and in chronological order further samples were
extracted. After quenching to 0 °C and addition of borane the
progress of racemization was determined by HPLC. Every sample
was analyzed at least twice. The integral values of every enantiomer
were received simultaneously at four different wavelengths be-
tween 225–280 nm and the average was calculated.
1
[α]2D3 = +6.6 (c = 0.5, CHCl3). H NMR: δ = 7.35–7.15 (m, 10 H, ArH),
6.99 (m, 1 H, ArH), 6.96 (m, 1 H, ArH), 6.89–6.75 (m, 6 H, ArH), 3.76
(s, 3 H, OCH3) ppm. 13C NMR: δ = 160.3 (C-OMe), 159.2 (d, J =
16.2 Hz, C-O), 156.6 (C-O), 137.0 (d, J = 10.4 Hz, C-P), 135.8 (d, J =
21.9 Hz, 2 × CH), 133.7 (CH), 133.6 (d, J = 20.0 Hz, 2 × CH), 130.0
(CH), 129.5 (2 × CH), 129.3 (d, J = 14.6 Hz, C-P), 128.4 (CH), 128.3 (d,
J = 6.8 Hz, 2 × CH), 126.7 (d, J = 8.0 Hz, C-P), 123.3 (CH), 123.2 (CH),
119.1 (2 × CH), 117.4 (d, J = 1.4 Hz, CH), 114.1 (d, J = 8.3 Hz, 2 × CH),
55.1 (OCH3) ppm. 31P NMR: δ = –17.9 ppm. MS (EI, 70 eV): m/z (%) =
384 (46) [M]+, 383 (100) [M – H]+, 307 (1) [M – Ph]+. HRMS (EI): m/z
calcd. for C25H20O2P [M – H]+: 383.11954; found: 383.11934. HPLC
(as 7m): 98 % ee.
The graphic evaluation and the calculation of the resulting rate con-
stants are illustrated in the Supporting Information.
Computational Details: Structure optimizations have been carried
out at the B3PW91[30] density functional level of theory under the
consideration of van der Waals dispersion correction (GD3BJ)[31]
with the all-electron TZVP basis set[32] by using the Gaussian 16
program package.[33] The optimized geometries were characterized
as energy minimums at the potential energy surface from frequency
calculations at the same level of theory; i.e., the energy minimum
structure has only real frequencies or authentic transition state have
only one imaginary vibration mode, which connects the reactant
and product. The Gibbs free energies that are used for discussion
and comparison are scaled with the thermal correction to Gibbs
free energies at 298 K (see the Supporting Information).
(S)-(–)-(4-Ethoxyphenyl)(phenyl)(3-tolyl)phosphane (8n): Yield:
84 %; colorless oil (cyclohexane/AcOEt, 98:2); [α]2D3 = –5.1 (c = 1.0,
CHCl3). 1H NMR: δ = 7.34–7.23 (m, 7 H, ArH), 7.21 (m, 1 H, ArH), 7.14
(m, 2 H, ArH), 7.3 (m, 1 H, ArH), 6.88 (m, 2 H, ArH), 4.02 (d, J =
7.0 Hz, 2 H, OCH2), 2.29 (s, 3 H, CH3), 1.41 (d, J = 7.0 Hz, 3 H,
CH3) ppm. 13C NMR: δ = 159.7 (d, J = 2.4 Hz, C-OEt), 138.0 (d, J =
10.6 Hz, C-C), 137.9 (d, J = 7.6 Hz, C-P), 137.6 (d, J = 9.9 Hz, C-P),
135.6 (d, J = 20.9 Hz, 2 × CH), 134.1 (d, J = 21.8 Hz, CH), 133.4 (d,
J = 19.3 Hz, 2 × CH), 130.4 (d, J = 16.9 Hz, CH), 129.3 (CH), 128.4
(CH), 128.4 (d, J = 6.3 Hz, 2 × CH), 128.3 (d, J = 6.3 Hz, CH), 127.4
(d, J = 7.7 Hz, C-P), 114.7 (d, J = 8.2 Hz, 2 × CH), 63.3 (OCH2), 21.4
(CH3), 14.8 (CH3) ppm. 31P NMR: δ = –6.5 ppm. MS (EI, 70 eV): m/z
(%) = 320 (100) [M]+, 291 (22) [M – C2H5]+, 243 (4) [M – Ph]+, 229
(22) [M – (2-EtO-Ph)]+, 152 (13) [C8H9OP]+. HRMS (EI): m/z calcd. for
C21H21OP [M]+: 320.13245; found: 320.13224. HPLC (as 7n): 99 % ee.
Acknowledgments
We thank Mrs. Gudrun Wenzel for very skilled technical assist-
ance. We acknowledge financial support by German-Israeli
Foundation for Scientific Research and Development (GIF, Re-
search Agreement Project no. I-1369-302.5/2016).
(S)-(+)-Methyl(naphthalen-2-yl)(phenyl)phosphane (8o): Yield:
64 %; white solid; m.p. 52–53 °C (cyclohexane/AcOEt, 99:1); [α]D23
=
+11.4 (c = 0.5, CHCl3). 1H NMR: δ = 8.03 ((br. d), J = 8.7 Hz, 1 H,
ArH), 7.90–7.79 (m, 3 H, ArH), 7.57–7.44 (m, 5 H, ArH), 7.42–7.33 (m,
3 H, ArH), 1.77 (J = 3.4 Hz, 3 H, CH3) ppm. 13C NMR: δ = 139.9 (d,
J = 11.3 Hz, C-P), 137.2 (d, J = 11.3 Hz, C-P), 133.2 (C-C), 133.2 (d,
J = 8.2 Hz, C-C), 132.2 (d, J = 22.2 Hz, CH), 132.1 (d, J = 18.2 Hz,
2 × CH), 128.7 (d, J = 15.2 Hz, CH), 128.4 (CH), 128.4 (d, J = 5.7 Hz,
2 × CH), 127.9 (d, J = 6.0 Hz, CH), 127.9 (CH), 127.6 (CH), 126.5 (CH),
126.2 (CH), 12.5 (d, J = 13.1 Hz, CH3) ppm. 31P NMR: δ = –26.0 ppm.
MS (EI, 70 eV): m/z (%) = 250 (100) [M]+, 235 (38) [M – CH3]+, 233
(64). HRMS (EI): m/z calcd. for C17H15P [M]+: 250.09059; found:
250.09013. HPLC (as 7o): 99 % ee.
Keywords: Phosphanes · Chirality · Chiral resolution ·
Isomerization · Racemization · P ligands · Inversion barrier
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Received: December 13, 2017
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Chirogenic Phosphanes
J. Holz,* H. Jiao, M. Gandelman,
A. Börner* .......................................... 1–12
About the Inversion Barriers of P-
Chirogenic Triaryl-Substituted Phos-
phanes
Racemization of P-chirogenic phos- than alkyl phosphanes. Bulky aryl sub-
phanes is strongly dependent on the stituents have only a marginal effect,
type of substituents. Triaryl phos- whereas naphthyl rings destabilize the
phanes undergo easier racemization compounds.
DOI: 10.1002/ejoc.201701734
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim