Modular synthesis of chiral phosphine-phosphite-ligands from phenolic precursors: A new approach to bidentate chelate ligands exploiting a P-O to P-C migration rearrangement

Article abstract of DOI:10.1002/adsc.200800146

An efficient and modular approach to bidentate phosphine-phosphite ligands formally derived from a 6-alkyl-2-phosphanylphenol, a chiral diol and phosphorus trichloride has been developed. In a key step, a borane-protected phosphinite, prepared from an o-b

Full text of DOI:10.1002/adsc.200800146

FULL PAPERS
DOI: 10.1002/adsc.200800146
Modular Synthesis of Chiral Phosphine-Phosphite-Ligands from
Phenolic Precursors: A New Approach to Bidentate Chelate
ꢀ
ꢀ
Ligands Exploiting a P O to P C Migration Rearrangement
Janna Velder,a Tobias Robert,a Ingo Weidner,a Jçrg-Martin Neudçrfl,a
Johann Lex,a and Hans-Günther Schmalza,*
a
Institut für Organische Chemie, Universität zu Kçln, Greinstrasse 4, 50939 Kçln, Germany
Fax : (+49)-221-470-3063; e-mail: schmalz@uni-koeln.de
Received: March 11, 2008; Revised: April 12, 2008; Published online: May 19, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: An efficient and modular approach to bi- contrast to an earlier approach, the new methodolo-
dentate phosphine-phosphite ligands formally de- gy is very general and tolerates bulky ortho-substitu-
rived from a 6-alkyl-2-phosphanylphenol, a chiral ents. The reliability of the operationally convenient
diol and phosphorus trichloride has been developed. protocol was demonstrated in the synthesis of a li-
In a key step, a borane-protected phosphinite, pre- brary of 16 new phosphine-phosphite ligands, starting
pared from an o-bromophenol by O-phosphanyla- from different ortho-alkylphenols. The modular con-
tion, is reacted with n-butyllithium to afford the cor- cept opens a rapid access to a broad variety of li-
responding ortho-phosphanylphenol (as the stable gands and might be useful in the search for and
borane adduct) through bromine-lithium exchange structural optimization of suitable ligands for specific
and anionic migration rearrangement. Treatment chirogenic transition metal-catalyzed transforma-
with phosphorus trichloride in the presence of a base tions.
and subsequent reaction of the in situ formed di-
chlorophosphite with
a
chiral diol (such as Keywords: chirality; diversity-oriented synthesis;
TADDOL or BINOL) affords the target P,P ligands ligand design; lithiation; metal catalysis; phosphite
in good overall yield (up to 60% over 4 steps). In ligands
Introduction
chiral ligands are, for instance, oxazoline-derived li-
gands[3] and ligands with a ferrocenyl[4] or a biaryl[5]
During the last decades, transition metal-catalyzed re- backbone.
actions have evolved as most powerful tools in asym-
As our own contribution to the field, we had previ-
metric organic synthesis and many classes of chiral li- ously disclosed a modular synthetic approach towards
gands were discovered which provide impressively ef- structurally diverse bidentate phenol-derived li-
ficient solutions for a growing number of chirogenic gands.[6] By screening a small library of these P,P, P,S-,
transformations.[1] However, the identification of suit- and P,N ligands in the Rh-catalyzed hydroboration of
able ligands for a specific metal-catalyzed process still styrene, the phosphine-phosphite ligand 1 was identi-
remains a challenge, as even potent catalysts often fied to be particularly selective (91% ee).[7] In fact,
show low activities for demanding substrates. Further- chiral phosphine-phosphite ligands[8] seem to possess
more, certain types of transformations cannot even be a significant (albeit little explored) potential for asym-
performed in a satisfying asymmetric manner at all. metric transition metal catalysis. For example, ligands
As rational ligand design is usually not possible based 2 (BINAPHOS), 3 and 4 (Figure 1) have been suc-
on our existing state of knowledge and theory, the dis- cessfully applied in Rh-catalyzed hydrogenation[9] and
covery of new potent structures is still a rather empir- hydroformylation[10] reactions as well as in Cu-cata-
ical enterprise. A successful approach is the screening lyzed asymmetric conjugate additions.[11]
of ligand libraries obtained by modular (diversity-ori-
In the course of our previous work (see above),[7] it
ented) synthesis based on “privileged” ligand archi- became apparent that a substituent (R’) in the ortho-
tectures.[2] Prominent classes of (bidentate) modular position to the phosphite moiety in P,P ligands of type
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Results and Discussion
Inspired by the work of Heinicke[12] and, in particular,
JugØ,[13] we envisioned exploiting a P O to P C mi-
gration rearrangement to introduce the phosphanyl
substituent in the ortho-position without the require-
ment to protect the phenol functionality (Scheme 2).
In such a process, an ortho-lithiated and borane-pro-
tected aryl phosphinite (11) would be supposed to re-
arrange to a more stable ortho-phosphanylphenolate
of type 12[13] from which the intermediates of type 8
would be obtained after work-up in a still borane-pro-
tected (thus easier to handle) form.
ꢀ
ꢀ
Much to our satisfaction, this consideration was
found to work out successfully. The overall synthetic
scheme, which finally allowed us to synthesize various
new ligands of type 10, even with bulky substituents
R’ (e.g., t-Bu, i-Pr, Ph), is summarized in Scheme 3
and Table 1. Starting from differently substituted phe-
nols (5’), ortho-bromination was routinely achieved
using NBS(cat. i-Pr2NH, DCM, reflux),[14] a method
Figure 1. Selected chiral phosphine-phosphite ligands.
10 seems to exhibit a positive influence on the perfor- which proved superior to others especially on a multi
mance of such ligands. However, in our initial at- 10 g scale. To suppress the formation of dibrominated
tempts to synthesize ligands of type 10 with R’¼H by-products and to circumvent the low solubility of
following the original approach (Scheme 1) we faced NBSin dichloromethane, a oSxhlet apparatus was
some unexpected problems. While the lithiation/phos- used to extract the NBSbit by bit from the thimble
phanylation of the THP-protected phenols (6) pro-
ceeded smoothly as before, the deprotection (THP
ether cleavage) of the resulting products (7) was not
achievable even under rather harsh conditions (e.g.,
1 equiv. p-TsOH, MeOH, reflux, 17 h). Also consider-
ing the highly air-sensitive nature of the expected
ortho-phosphanylphenol intermediates of type 8,[6,7,12]
we decided not to further pursue the original strategy.
We herein report a new synthetic protocol which
Scheme 2. Rearrangement of an ortho-lithiated and borane-
overcomes the above-mentioned problems and opens
protected aryl phosphinite (11) to an ortho-phosphanylphe-
a reliable, general and still highly modular access to
nolate of type 12.
chiral phosphine-phosphite ligands of type 10, requir-
ing only 4 steps starting from simple substituted phe-
nols.
Scheme 3. Improved and general synthesis of ligands of type
10. (a) NBS, cat. i-Pr2NH, CH2Cl2, reflux; (b) ClP(R5)2,
DABCO (1.1 equiv.), CH2Cl2, 0–258C, 2 h, then BH3·THF,
08C to r.t.; (c) n-BuLi (1.5 equiv.), THF, 08C; (d) PCl3
(1.0 equiv.), DABCO (10 equiv.), CH2Cl2, 3 h, rt, then chiral
diol (1.5 equiv.), 16 h, r.t.
Scheme 1. Original approach towards ligands of type 10; (a)
DHP, cat. PPTS, CH2Cl2, r.t.; (b) n-BuLi, THF, ꢀ788C to
r.t., then ClPR2, ꢀ788C to r.t.; (c) p-TsOH, MeOH, r.t.; (d)
THF, Et3N, ClP(OR)2.
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FULL PAPERS
Table 1. Synthesis of various ligands of type 10 according to Scheme 3.
Entry
R1
R2
R3
R4
Intermediate 13
(yield [%])[a]
R5
Intermediate 15
(yield [%])[a]
Chiral Diol
Final Ligand
N
G
A
1
2
3
4
5
6
7
8
Me
Me
Me
i-Pr
t-Bu
t-Bu
-(CH)4-
Ph
H
Me
H
H
H
H
H
H
H
H
t-Bu
H
H
H
H
t-Bu
H
H
H
H
H
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
13a (98)
13b (98)
13c (61)
13d (97)
13e (95)
13f (66)
13g (97)
13h (96)
13h
13b
13f
13h
13h
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
i-Pr
Ph
Ph
Ph
Ph
Ph
Ph
Ph
15a (92)
15b (71)
15c (83)
15d (85)
15e (91)
15f (81)
15g (91)
15h (91)
15i (70)
15b
15f
15h
15h
15b
TADDOL
TADDOL
TADDOL
TADDOL
TADDOL
TADDOL
TADDOL
TADDOL
TADDOL
BINOL
BINOL
BINOL
2-naphthyl-TADDOL
2-naphthyl-TADDOL
2-naphthyl-TADDOL
2-naphthyl-TADDOL
10a (79)
10b (79)
10c (68)
10d (75)
10e (86)
10f (94)
10g (74)
10h (79)
10i (35)[b]
10j (88)
10k (78)
10l (80)
10m (35)
10n (66)
10o (49)
10p (61)
H
H
H
Me
H
H
H
H
H
H
9
Ph
10
11
12
13
14
15
16
Me
t-Bu
Ph
Ph
i-Pr
t-Bu
t-Bu
13b
13e
13f
H
t-Bu
15e
15f
[a]
Isolated yield.
DIPEA was used instead of DABCO in the final step.
[b]
into the reaction mixture. The bromophenols 13 were the phenol with a chiral chlorophosphite (accessible
then converted into the O-phosphanylated and in one step from PCl3 and a chiral diol) resulted in
borane-protected derivatives 14 by reaction with a low yields. Obviously, the increased steric hindrance
chlorophosphine in the presence of DABCO in di- caused by the substituents R1 seems to impede the re-
chloromethane and subsequent addition of BH3 in action with the bulky chlorophosphites derived from
THF. Alternatively, the reaction could be performed chiral diols such as TADDOL[15] and BINOL.[16] Chal-
with equal success in THF using sodium hydride as a lenged by these circumstances, we devised a new
base. Without purification, the resulting phosphinites “one-pot” procedure in which the phosphanylphenol
of type 14, usually obtained as colourless solids, were (15) is first reacted with 1 equiv. of PCl3 in dichloro-
directly subjected to the key rearrangement methane at room temperature in the presence of an
(Scheme 2) by treatment of a solution in THF at with excess of DABCO as a base. The resulting dichloro-
an excess (1.5 equiv.) of n-BuLi to smoothly afford phosphite intermediate 16 is then converted to the
the corresponding (still BH3-protected) phosphines 15 corresponding phosphite by addition of the chiral diol
in good yields (Table 1). It is worthy of note that (Scheme 3). Using this method, various TADDOL-
n-BuLi (even at 08C) could be used instead of t-BuLi and BINOL-derived ligands of type 10 were reliably
at ꢀ788C (as described in the original protocol).[13] obtained in good yields (Table 1). With DABCO as a
Moreover, even if the starting material (14) was con- base, complete deprotection (removal of the BH3
taminated to some extent with the dibrominated spe- group) occurred under the reaction conditions. In the
cies, the reaction product (15) was obtained in very case of the more sensitive diisopropylphosphanyl
pure form, as a result of a process of “self-purifica- ligand 10i the less nucleophilic Hünigꢁs base (DIPEA)
tion” under the reaction conditions (due to non-pro- was used and the still protected product was isolated.
ductive Br/Li exchange followed by protonation Only moderate yields were observed when the more
during work-up). Both the borane-protected phos- bulky naphthyl-TADDOL[15a] was employed. The
phinites (14) and the corresponding rearranged phos- structures of the various chiral phosphine-phosphite
phines (15) proved to be air-stable and, in most cases, ligands of type 10 synthesized in the course of this
nicely crystalline compounds. The structures of select- study are shown in Figure 3.
ed compounds (14h, 14f, 15b, 15e, 15f and 15i) were
determined by X-ray crystallography (Figure 2) to un-
ambiguously probe the correct structural assignments. Conclusions
With the desired BH3-protected 6-alkyl-2-phospha-
nylphenols of type 15 in our hands, we next investi- Based on our earlier discovery that chiral phosphine-
gated the final introduction of the (chiral) phosphite phosphite ligands of type 10 possess a promising po-
unit. Initial attempts to achieve this goal as before tential for enantioselective transition metal catalysis,[7]
(compare Scheme 1)[6,7] via base-mediated reaction of the goal of the present study was to develop a general
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Figure 2. Structures of the borane-protected ortho-bromoarylphosphinites 14h and 14f (upper row) and of the borane-pro-
ꢀ
tected ortho-hydroxyarylphosphines 15i, 15e, 15f and 15b (middle and lower rows) in the crystalline state. Selected P B
bond lengths: 1.897 Š (14h), 1.907 Š (14f), 1.937 Š (15i), 1.931 Š (15e), 1.927 Š (15f), 1.926 Š (15b).
synthetic access to a broader variety of such com- migration rearrangement as a key step, this goal was
pounds. In particular, ligands with a sterically de- achieved. The new protocol disclosed herein indeed
manding substituent (R1 ¼H) in the ortho position to opens an efficient and reliable entry to the targeted
the phosphite moiety were not available through our ligand class in a diversity-oriented manner and has
ꢀ
ꢀ
previous synthesis. Exploiting an anionic P O to P C the following advantages: (1) circumvention of a pro-
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Figure 3. Chiral phosphine-phosphite ligands of type 10 synthesized in the course of the present study.
tecting group for the phenol function (shortening the ing a substructure of type 19 (Scheme 4).[18] Notewor-
overall sequence to four steps starting from simple thy is the circumstance that 17 represents a rather de-
substituted phenols); (2) enhanced operational sim- manding substrate which, in contrast to the parent un-
plicity through avoidance of sensitive intermediates substituted styrene, only gave unsatisfying enantiose-
(using BH3 adducts), and (3) toleration of bulky lectivities (ꢁ75 ee) under standard conditions when
ortho-substituents (e.g., tert-butyl). The practicability BINAP or our first generation ligand 1 was employed.
of the modular scheme was exemplified in the synthe- We observed a pronounced effect of the substitution
sis of a library of sixteen well-characterized ligands pattern at the ligand backbone and identified 10b as
using different phenol building blocks, chlorophos- the most effective ligand so far, giving intermediate
phanes and chiral diols (Figure 3).
18 with at least 93% ee.[18] Exploiting the efficient di-
As a first probe of its value, we recently screened versity-oriented synthetic scheme, further variation
this library in the Rh-catalyzed hydroboration[17] of (optimization) of the ligand structure should be possi-
the substituted styrene derivative 17, that is, the chiro- ble without unreasonable expenditure.
genic step in our synthesis of dihydroxycalamenenes
We are optimistic that by screening of libraries of
and other natural products (marine diterpenes) show- modular compounds of type 10 new useful ligands
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crude product (14a–i) which was used without further purifi-
cation.
General Procedure III: Rearrangement of BH3-
Protected Phosphinites to 2-Boranatodiphenyl-
phosphanylphenols (15a–h)
In a flame-dried Schlenk flask under argon a solution of a
BH3-protected phosphinite (14a–i, 1 equiv.) in THF was
cooled to 08C and treated with n-BuLi (1.5 equiv.). The mix-
ture was stirred for 2 h at this temperature, then quenched
with H2O, extracted with tert-butyl methyl ether and washed
with NH4Cl solution. The organic layers were dried
(MgSO4) and concentrated under reduced pressure to give
the crude product which was purified by flash chromatogra-
phy to provide the phosphines 15a–h as white solids.
Scheme 4. Application of ligand 10b in the enantioselective
synthesis of compounds of type 19. (a) 2 mol% [Rh-
A
DME, ꢀ458C; then pinacol (2.3 equiv.).
will be identified also for other types of enantioselec-
tive transition metal-catalyzed transformations. Corre-
sponding investigations are currently performed in
our laboratories and the results will be reported in
due course.
General Procedure IV: Synthesis of Chiral
Phosphine-Phosphite Ligands (10a–p)
A flame-dried Schlenk flask was put under argon and the
phosphine (15a–h; 1.0 equiv.) and DABCO (20 equiv.) were
dissolved in absolute CH2Cl2. The resulting solution was
stirred for 10 min at room temperature, then cooled to 08C
and a solution of PCl3 in CH2Cl2 (1.0 equiv,) was added
dropwise via syringe. The reaction mixture was stirred for
30 min at this temperature, warmed to room temperature
and stirred for 3 h. The milky suspension was cooled to 08C
and a solution of the chiral diol (1.5 equiv.) in CH2Cl2 was
added. After 30 min the resulting solution was allowed to
warm to room temperature and stirring was continued for
another 20 h. The solvent was evaporated to give the crude
product, which was purified by flash chromatography to
afford ligands 10a–p as white foams.
Experimental Section
For detailled (individual) procedures and characterization of
compounds, see the Supporting Information.
General Procedure I: ortho-Bromination to Bromides
13a–h
In a flame-dried flask equipped with a Soxhlet apparatus
and flushed with argon the substituted phenols 1a–h
(1 equiv.) and diisopropylamine (0.1 equiv.) were dissolved
in absolute CH2Cl2. The thimble was filled with NBS
(1 equiv.) and the system was heated to reflux for 16 h.
During this time, the NBSwas slowly consumed. After cool-
ing to room temperature the resulting mixture was treated
with 2M sulfuric acid. The layers were separated and the
aqueous layer was extracted with tert-butyl methyl ether.
The combined organic layers were washed with water and
brine, and dried (MgSO4). The solvent was removed and the
crude product was purified by flash chromatography.
Acknowledgements
This work was supported by the European Commission
(LIGBANK, FP6-NMP3-CT-2003–505267) and the Fonds
der Chemischen Industrie. The authors would like to thank
Chemetall GmbH for generous gifts of butyllithium. We
would like to thank Dr. Matthias Schäfer and Dr. Nils Schlçr-
er for competent support with mass spectrometric and NMR
measurements.
General Procedure II: Synthesis of BH3-Protected
Phosphinites (14a–i)
A flame-dried Schlenk flask was charged under argon with a References
substituted 2-bromophenol (13a–h, 1 equiv.) and DABCO
(1.1 equiv.) and dissolved in CH2Cl2. This solution was
stirred for 5 min at room temperature then cooled to 08C
and the chlorophosphine (1.1 equiv.) was added dropwise
via syringe. The resulting suspension was stirred 10 min at
this temperature, warmed to room temperatuere and stirred
for another 2 h. The reaction mixture was then cooled to
08C and a solution of BH3 in THF (2 equiv.) was added.
This resulting solution was stirred 15 min at 08C and 1 h at
room temperature before it was quenched with H2O (cau-
tion! strong H2 gas formation) and extracted with tert-butyl
methyl ether. The ethereal phase was washed with brine and
dried (MgSO4). The solvent was evaporated to give the
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