Bulky, optically active P-stereogenic phosphine-boranes from pure H-menthylphosphinates

Article abstract of DOI:10.1021/ja2034816

The transformation of readily available pure-H-menthylphosphinates into chiral phosphinous acid-boranes permits the elaboration of bulky P-stereogenic secondary phosphine-boranes. Taking advantage of the synthetic potential of these compounds, a broad range of hindered P-chiral tertiary phosphine-boranes has been prepared with excellent enantiomeric excesses. The utility of bulky o-tolylphosphines was illustrated by the synthesis of a rare enantiopure phosphapalladacycle (S_P,S_P)-12.

Full text of DOI:10.1021/ja2034816

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Bulky, Optically Active P-Stereogenic PhosphineÀBoranes from Pure
H-Menthylphosphinates
David Gatineau, Laurent Giordano,* and Gꢀerard Buono*
Equipe Chirosciences, UMR CNRS 6263 - ISM2 Universitꢀe Aix-Marseille III, Ecole Centrale de Marseille Av. Escadrille Normandie
Niemen, 13397 Marseille Cedex 20, France
S Supporting Information
b
Scheme 1. Synthesis of Diastereomerically Pure
ABSTRACT: The transformation of readily available pure-
H-menthylphosphinates into chiral phosphinous acid-bor-
anes permits the elaboration of bulky P-stereogenic second-
ary phosphineÀboranes. Taking advantage of the synthetic
potential of these compounds, a broad range of hindered
P-chiral tertiary phosphineÀboranes has been prepared
with excellent enantiomeric excesses. The utility of bulky
o-tolylphosphines was illustrated by the synthesis of a rare
enantiopure phosphapalladacycle (SP,SP)-12.
H-Menthylphosphinates 1a
a Reagents and conditions: a) NaH, rt, THF; b) PCl3, À50 °C, THF; c)
ArMgCl, À50 °C, THF; d) two recrystallizations from hexane at À20 °C
for 48 h. Tol = o-tolyl; Naph = 1-naphthyl.
he development of new metal-catalyzed pathways calls for
T
the design of new phosphine ligands with emphasis on the
electronic and steric parameters.1 For instance, electron-rich and
hindered-tertiary phosphines are of paramount importance in
CÀC bond-forming reactions.2 Although various methods to
prepare these ligands are available, very little attention has been
paid to enantiomerically pure counterparts useful in asymmetric
catalysis.3 Processes leading to enantiomerically pure phos-
phorus compounds for asymmetric catalysis still remain a
challenging topic.4 In this context, during the past decade, the
synthesis of P-stereogenic phosphines has been an intensive field
of research.5 P-stereogenic phosphines are very attractive since
the element of chirality is brought closer to the metal center in
organometallics or metal complexes.6 To date, the method of
Jugꢀe and co-workers for the preparation of optically active
tertiary phosphineÀboranes is the most convenient in terms of
versatility and stereoselectivity.7 However, the introduction of bulky
groups by this methodology remains difficult to achieve.8 Dynamic
resolution/alkylation of lithiated tert-butylarylphosphineÀboranes
with (À)-sparteine is useful procedure to alleviate these difficulties.9
Recently, Pietrusiewicz and Stankevic have also shown that the
resolution of tert-butylphenylphosphinous acidÀborane offers an
entry for the preparation of P-stereogenic compounds containing a
tert-butyl substituent.10 Our interest in enantioselective catalysis11
has prompted us to develop the synthesis of chiral secondary
phosphine oxides (SPOs) as chiral preligands.12 Recently, we
disclosed a straightforward route to optically active phosphinous
acidÀboranes from SPOs.13 In continuation of these studies, we
decided to examine the possibility to convert SPOs to optically
active hindered phosphineÀboranes. Hopefully this approach would
preclude the PdO bond reduction which is difficult to achieve when
stereochemical control is required.14 Herein, we report a convenient
procedure for the synthesis of optically active hindered tertiary
phosphineÀboranes and their utility for the enantioselective synth-
esis of chiral phosphapalladacycle.
Scheme 2. Preparation of Optically Pure
Phosphinous AcidÀBoranes 4a
a Reagents and conditions: a) t-BuLi (2.2 equiv), À78 °C, THF; b) H2O,
H+; c) recrystallization from hexane/Et2O; d) n-BuLi, À78 °C, THF; e)
TMSCl, À78 °Cfrt, THF; f) BH3 SMe2, THF; g) TBAF, THF.
3
First, we prepared various H-menthylphosphinates 1aÀc
from inexpensive PCl3 and (À)-menthol through addition of
arylmagnesium bromides to dichloro-((À)-menthyloxy)phos-
phine 2. Diastereomerically pure 1aÀc were obtained after two
recrystallizations on the multigram scale (up to 50 g).15 The
absolute configuration at the phosphorus atom was determined
by X-ray structural analysis for 1c and assigned by deduction for
1b (Scheme 1).
New enantiopure SPOs 3aÀc were prepared and converted to
chiral phosphinous acid-boranes 4aÀc according to our proce-
dures (Scheme 2).13
Second, we examined the reactivity of various chiral phosphi-
nous acidÀboranes 4 in stereoselective transformations. The
development of this method would be a straightforward
entry to optically pure hindered tertiary phosphineÀboranes.
Starting from 3b, we first focused on the formation of
Received: April 21, 2011
Published: June 16, 2011
r
2011 American Chemical Society
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|

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Scheme 3. Conversion of Optically Pure (SP)-4b into
Secondary PhosphineÀBorane (SP)-8ba
Table 1. Preparation of Optically Active Hindered Tertiary
PhosphineÀBoranes 9
a Reagents and conditions: a) MeSO2Cl, Et3N, 0 °C, CH2Cl2; b)
(MeSO2)2O, Et3N, À15 °C, CH2Cl2; c) NaBH4, 0 °Cfrt, EtOH.
Scheme 4. Preparation of Enantiopure Bulky Secondary
PhosphineÀBoranes 8
sulfonyloxyphosphineÀboranes 5 as key precursor of sec-phos-
phineÀboranes 8. Attempts to apply the Pietrusiewicz and
Stankevic procedure using 4b instead 4a proved to be unsuccess-
ful. Indeed, when enantiopure 4b was treated with mesyl chloride
in the presence of triethylamine in dichloromethane to form
mixed anhydride 5b, a complex mixture was obtained in which
the desired product was contaminated by products 6 and 7
(Scheme 3a).16
The formation of these could be explained by the presence
of chloride ions in the medium. A competitive nucleophilic
substitution takes place to form chlorophosphineÀborane 6,
which then reacts with deprotonated 4b to give phosphinous
acid anhydride bis-borane 7. Noteworthy in this case is that
the reaction carried out with mesyl anhydride instead of
mesyl chloride resulted in the total suppression of byproducts.
The resulting mixed anhydride (SP)-5b was then reduced by
sodium borohydride in ethanol to afford cleanly (SP)-8b in
90% yield after purification.17 The whole transformation can
be achieved in two successive steps with excellent yields
(Scheme 3b).18
These promising results prompted us to test other chiral
phosphinous acidÀboranes 4 using the optimized conditions
(See experimental section in SI). We were pleased to observe that
this one-pot procedure afforded the desired bulky P-stereogenic
sec-phosphineÀboranes 8 in satisfactory yields (Scheme 4). This
new procedure for the preparation of this class of compounds is
complementary to the related approaches.19 As such, taking
advantage of the reactivity of the PÀH bond, the synthetic potential
of these compounds opens new ways to a wide range of chiral
P-stereogenic phosphines.
Having in hand the sec-phosphineÀboranes 8, the synthesis of
optically pure tertiary hindered phosphineÀboranes 9 was
investigated. Alkylation of sec-phosphineÀboranes was per-
formed with various halide compounds to extend its applicability
(Table1).
a Yield after purification. b Enantiomeric excess was determined by
HPLC analysis (see Supporting Information). c Yield after recrystalliza-
tion. d Enantiomeric excess after recrystallization.
9l was established by comparison with known compounds,9c and
absolute configuration of 9i was established by X-ray structural
analysis.20 In accordance with Pietrusiewicz and Stankevic re-
sults, transformation of 4 into 9, resulted in inversion of con-
figuration at phosphorus atom with excellent stereoselectivity.21
The highest enantioselectivities (>98 ee) were observed for
compounds 9a, 9b, 9f, 9h and 9i (entries 1, 2, 6, 8, 9).
P-stereogenic,N-bidentate ligand 9f seems to be promising for
the design of new metal complexes and asymmetric catalysis.22
When allyl or propargyl bromide, (iodomethyl)trimethylsilane
and furfuryl chloride were used as alkylating agents, tertiary
phosphineÀboranes 9c, 9d, 9e, 9g, 9j, 9k were obtained with
fairly to good yields and satisfactory enantioselectivities (Entries
3, 4, 5, 7, 10, 11), which can be improve by a single crystallization
when crystals are obtained. The loss of enantioselectivity could
be imputed to the slow reaction at À78 °C. Thus, these results
suggest that, on warming up the reaction media, the conjugated
base of sec-phosphineÀboranes 8 racemize slowly.23
Ligand 9i containing o-tolyl substituent appears to be a good
candidate for the preparation of optically pure metallacyle.24 As a
proof of principle, phosphapalladacycle (()-11 was conveni-
ently formed from the racemic free tertiary phosphine 10 and
Pd(OAc)2. The structure was in full agreement with the NMR
data (Scheme 5a) and secured by a single-crystal X-ray analysis.
Unfortunately, when the phosphapalladacyle formation was
performed with chiral (RP)-9i, no suitable crystals of (SP,SP)-
11 were obtained for X-ray spectroscopy. Attempted recrystalli-
zation of (SP,SP)-11 resulted in partial degradation (black
palladium deposit). Acetato to chloro ligand exchange for the
phosphapalladacycle (()-11 (LiCl, acetone, rt)25 provided the
By using this procedure, the syntheses were achieved with high
levels of enantioselectivity. Absolute configuration of 9a, 9b, 9h,
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COMMUNICATION
Scheme 5. Preparation of Optically Active
Table 2. Asymmetric Addition of Alkynes 14 to 13
Phosphapalladacycles (SP,SP)-11 and (SP,SP)-12a
entry
alkyne
product
yielda
eeb
1
2
3
4
14a R = phenyl
(À)-15a
(À)-15b
(À)-15c
(À)-15d
98
90
91
82
27
29
24
36
14b R = p-MeOPh
14c R = 1-cyclohexen-1-yl
14d R = CH2OAc
a Yield after purification. b Enantiomeric excess was determined by
a Reagents and conditions: a) DABCO, 50 °C, toluene; b) Pd(OAc)2
(0.8 equiv); c) LiCl, rt, acetone/methanol; 3/1. DABCO = 1,4-
diazabicyclo[2.2.2]octane.
HPLC analysis (see Supporting Information).
products were obtained in excellent yields (Table 2, entries
1À3). These preliminary results are promising since an enantio-
meric excess of 36% was observed without optimization of the
design of palladacycle (Table 2, entry 4).
In conclusion, a route to optically active electron-rich bulky
P-stereogenic secondary and tertiary phosphineÀboranes has been
developed from (À)-menthol as convenient and friendly chiral
auxiliary to prevent the utilization of (À)-ephedrine and (À)-sparteine.
To make the most of the borane-free tolylphosphine 10 a rare
enantiopure palladacycle (SP,SP)-12 was prepared with retention of
configuration at the phosphorus atom. Asymmetric addition reac-
tions of alkynes to alkenes31 with this conformationaly restricted
chiral palladacycle as catalyst are in progress in our laboratories.
Figure 1. Crystal structure of complex trans-(SP,SP)-12 (ORTEP
drawing showing thermal ellipsoids at 40% probability). Hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and angles
(deg): C(1)ÀPd(1) 2.039(5); Cl(1)ÀPd(1) 2.460(1); Cl(2)ÀPd(1)
2.416(1); P(1)ÀPd(1) 2.203(1); C(1)ÀPd(1)ÀCl(2) 91.9(2); C(1)À
Pd(1)ÀP(1) 83.8(2); Cl(1)ÀPd(1)ÀCl(2) 85.89(4); Cl(1)ÀPd-
(1)ÀP(1) 98.39(4).
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures,
b
compound characterization data, and spectra for all new com-
pounds; CIF files for compounds 1c, 11, 9i, and 12. This material
is available free of charge via the Internet at http://pubs.acs.org.
chloropalladacycle (()-12 which exhibits complex NMR spec-
trum. In contrast, with chiral ligand (RP)-9i, NMR spectrum of
the phosphapalladacycle (SP,SP)-12 were dramatically simplified
compared to (()-12. Two diastereomeric complexes were
observed in 2:1 molar ratio and mass spectroscopy data are in
agreement with a dimeric structure, which suggested retention of
configuration at phosphorus atom (Scheme 5b).26,27
Ninety-five percent enantiomeric excess was determined by
31P NMR spectroscopy using known methods based on the
formation of two monomeric diastereomers with (S)-R-methyl-
benzylamine28 (see SI).
Finally, the optically pure trans-chloropalladacycle (SP,SP)-12
was slowly crystallized in EtOH, and suitable crystals for X-ray
analysis were isolated. The structure of (SP,SP)-12 was unambigu-
ously assigned and showed a square-planar geometry and retention
of configuration at the phosphorus atom (Figure 1). To the best of
our knowledge, structurally characterized enantiopure P-stereogenic
phosphapalladacycles are scarce, and only one example of a chiral
Pd(II) phosphapalladacycle has been reported so far.29
To test the activity of (SP,SP)-11, we briefly examined the
asymmetric version of addition reaction of alkynes 14 to nor-
bornadiene 13. Indeed, the HermannÀBeller phosphapallada-
cycle was known as the key catalyst for this interesting
transformation.30 Our results are listed in Table 2.
’ AUTHOR INFORMATION
Corresponding Author
laurent.giordano@centrale-marseille.fr; gerard.buono@
centrale-marseille.fr
’ ACKNOWLEDGMENT
We are grateful to the CNRS and the ANR (program
BLAN07-1_190839) for fundings. We also thank Dr. Nicolas
Vanthuyne for HPLC analyses, Dr. Michel Giorgi for X-ray
analyses, and Dr. Alphonse Tenaglia for useful suggestions. This
work is dedicated to Professor Alfredo Ricci on the occasion of
his retirement.
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