Enantioselective synthesis of cyanohydrin O-phosphates mediated by the bifunctional catalyst binolam-AlCl

Article abstract of DOI:10.1002/anie.200351552

(R)- and (S)-binolam-AICI complexes act as bifunctional catalysts to mediate the enantioselective cyanophosphorylation of aldehydes at room temperature. The resulting chiral cyanohydrin O-phosphates can be reduced to β-aminoalcohols or, when suitably substituted, can be transformed into γ-cyanoallylic alcohols through palladium-catalyzed allylic substitution, without loss of enantiomeric excess (see scheme).

Full text of DOI:10.1002/anie.200351552

Angewandte
Chemie
Owing to the fact that optically active a-hydroxynitriles
(cyanohydrins) are versatile building blocks for the synthesis
of biologically active compounds,[1] the search for simple
processes for the enantioselective hydrocyanation of carbonyl
compounds has occupied organic chemists for a number of
years. Enzymatic methods aside,[1b] until recently most of the
chiral catalysts designed and developed by chemists for these
endeavors have been monofunctional catalysts, such as
standard Lewis acids[2] or organocatalysts.[3] For reasons of
efficacy, organic chemists are interested in the design of
bifunctional and multifunctional catalysts. Recently, C2-sym-
metric bifunctional catalysts[4] based on a 2,2’-dihydroxy-1,1’-
binaphthalene–aluminum complex with either phosphinoyl or
amino arms at C3 and C3’ (1[5] and 2,[6] respectively) were
Asymmetric Cyanophosphorylation
Enantioselective Synthesis of Cyanohydrin O-
Phosphates Mediated by the Bifunctional Catalyst
Binolam–AlCl**
Alejandro Baeza, Jesffls Casas, Carmen Nµjera,*
JosØ M. Sansano, and JosØ M. Saµ*
shown to promote the highly enantioselective silylcyanation
of aldehydes. The asymmetric cyanation of ketones with
trimethylsilyl cyanide in the presence of a peptidic chiral
ligand is also promoted by an aluminum complex that acts as a
bifunctional catalyst.[7] However, the intrinsic instability of
cyanohydrins (and their silyl ethers) has moved researchers to
look for new routes to directly access robust, multipurpose O-
protected cyanohydrins. The asymmetric cyanoformylation of
ketones catalyzed by a chiral tertiary amine[8] and of
aldehydes by an yttrium–lithium–binol heterobimetallic com-
plex,[9] or by the monometallic aluminum complex 2,[10] are
just recent examples of the upsurge of activity in this area,
focused on the enantioselective synthesis of cyanohydrin O-
carbonates.
Dedicated to Professor Lutz F. Tietze
on the occasion of his 60th birthday
Modern organic chemistry is committed to a number of social
demands, including the pressure to develop catalytic, highly
efficient processes (in terms of both chemical yield and
stereochemical purity), which are not aggressive toward the
environment. With this in mind, we carried out a study on the
enantioselective synthesis of cyanohydrins and derivatives
thereof.
In continuation of our search for highly efficient catalytic
systems (perfect catalysts, as described by Noyori),[11] we now
report the first enantioselective cyanophosphorylation of
aldehydes catalyzed by the monometallic bifunctional system
2. We also describe herein a number of useful applications of
the resulting enantiomerically enriched cyanohydrin O-phos-
phates as valuable, previously unknown chiral building blocks
in standard modern organic synthesis.[12,13]
In our search for the best catalytic system for the
cyanophosphorylation of aldehydes, we examined the reac-
tion of p-chlorobenzaldehyde with commercially available
diethyl cyanophosphonate in the presence of a series of Lewis
acids under a variety of reaction conditions, as shown in
Table 1. All catalytic systems were freshly prepared by stirring
[*] Prof. Dr. C. Nµjera, Dipl.-Chem. A. Baeza, Dipl.-Chem. J. Casas,
Dr. J. M. Sansano
Departamento de Química Orgµnica, Universidad de Alicante
Apartado 99, 03080 Alicante (Spain)
Fax: (+34)96-590-3549
E-mail: cnajera@ua.es
Prof. Dr. J. M. Saµ
Departament de Química, Universitat de les Illes Balears
07071 Palma de Mallorca (Spain)
Fax: (+34)971-173-426
E-mail: jmsaa@uib.es
[**] This work was supportedby the DGES of the Spanish Ministerio de
Ciencia y Tecnología (PB97–0123 andBQU2001–0724-C02-) andby
Generalitat Valenciana (CTIOIB/2002/320). A.B. also thanks Gen-
eralitat Valenciana for a predoctoral fellowship.
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DOI: 10.1002/anie.200351552
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Table 1: Cyanophosphorylation of p-chlorobenzaldehyde catalyzed by
preformedLewis acid–( S)-3 complexes.[a]
Table 2: Enantioselective synthesis of (R)-cyanohydrin O-phosphates 4
catalyzedby complex ( S)-2.
Conversion [%][b]
ee [%][c]
Entry
Aldehyde
t [h]
4
Yield[%] [a]
ee [%][b]
Entry
Lewis acidSolvent
t [h]
1
2
3
4
PhCHO
4
20
10
50
a
b
c
89
88
87
87
98
96
98
26
1
2
3
4
5
6
7
8
Me2AlCl
Me2AlCl
CH2Cl2
THF
19
24
20
48
48
7
>98
96
>98
<15
–
>98
>98
>98
70
98
96
–
4-ClC6H4CHO
4-(MeO)C6H4CHO
4-NO2C6H4CHO
Me2AlCl
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
PhCH3
Me2AlCl[d]
Me2AlCl[e]
Et2AlCN
d
–
5
24
e
90
4
56
62
24
Me2AlOMe
Ti(OiPr)4
0.6
1
6
7
CH3CH=CHCHO
C5H11CH=CHCHO
2
6
f
g
89
90
88
94
[a] Reagents andconditions: catalyst (10 mol%) preformedfrom ( S)-3
andthe Lewis acid(1:1), CNPO(OEt) 2, solvent, room temperature, then
workup with 2m HCl. [b] Determinedby 1H NMR spectroscopic analysis
after acidic workup. [c] Determined by HPLC analysis on a chiral phase
(Daicel Chiralpak AD). [d] Molecular sieves (4 Š) were added (7.5%
water, 50 mg/0.25 mmol; see reference [6]). [e] Triphenylphosphane
oxide (20 mol%) was added.
8
4
h
89
90
9
10
C6H13CHO
PhCH2CH2CHO
3
2
i
j
90
90
98[c]
92
[a] Yields of the cyanohydrin O-phosphates after acidic hydrolysis and
column chromatography. [b] Enantiomeric excesses were determined by
HPLC analysis on a chiral phase (Daicel Chiralcel OD-H, andDaicel
Chiralpak AD andAS). [c] Determined by GC on a chiral phase ( g-
cyclodextrin).
a 1:1 mixture of the ligand (S)-binolam (3) and the desired
Lewis acid at room temperature in the appropriate solvent for
1 h. p-Chlorobenzaldehyde and diethyl cyanophosphonate
(3 equiv relative to aldehyde) were then added in one portion
at room temperature under an argon atmosphere, and the
resulting mixture was stirred for the time stated.
enantioselectivity was found to be poor (Table 2, entry 4),
which suggests the existence of a competing achiral route as a
result of coordination of the nitro group to the aluminum
center. It was thought that heteroaromatic aldehydes, or other
aldehydes with a basic site to compete with the amino-armed
catalyst, would be a challenging substrates. In fact, pyridine-3-
carboxaldehyde afforded almost racemic material (Table 2,
entry 5). However, the reaction of an aldehyde that bears a
thiazole moiety[14] led to 4h (a possible key intermediate in
the synthesis of epothilone A[14,15]) in excellent chemical yield
and stereochemical purity (Table 2, entry 8), in accordance
with the lower basicity of the thiazole nitrogen atom
(pKa 2.4). As expected, when the aluminum complex (R)-2
was used with the aldehydes in entries 1 and 2 of Table 2, the
enantiomeric products (S)-4 were obtained in reliable and
reproducible chemical yields and enantiomeric excesses. Most
importantly, the valuable chiral ligand 3 was readily extracted
from the reaction mixture (93% recovery) after acidic
workup, and could be reused without any significant loss of
activity. Thus, compound 4a was again obtained in 98% ee
(see Table 2, entry 1) with recovered ligand 3, as reported
previously.[6,11]
When we tried to synthesize compounds 4 from the
corresponding enantiomerically pure cyanohydrins, to deter-
mine the absolute configuration of our cyanophosphorylation
products, we observed that partial racemization occurred in
the presence of organic bases (triethylamine, pyridine, and N-
methylimidazole) and with both diethyl chlorophosphate[16]
and diethyl cyanophosphonate as phosphorylating agents.
Partial racemization was also observed when cyanophos-
phates 4a and 4j were synthesized by treating optically pure
cyanohydrins with diethyl chlorophosphite in pyridine, fol-
lowed by iodine-mediated oxidation.[17] In this reaction the
As shown (Table 1, entries 2 and 3), the best results were
obtained with (S)-binolam–AlCl 2 (10 mol%), generated in
situ by mixing (S)-binolam 3 and commercial diethylalumi-
num chloride in anhydrous toluene or THF. Although a very
high enantiomeric excess was observed when the reaction was
performed in THF (Table 1, entry 2), toluene was selected as
the solvent for further studies, as its use led to a cleaner
reaction (Table 1, entry 3). The reaction became impractically
slow at lower temperatures. No important effects analogous
to those observed with 4-Š molecular sieves and triphenyl-
phosphane oxide in the cyanosilylation of aldehydes in the
presence of complex 2[6] were observed in this cyanophos-
phorylation reaction (Table 1, entries 4 and 5). The reaction
was accelerated by using the corresponding chiral aluminum
methoxide complex, but a lower enantiomeric excess was
observed (Table 1, entry 7), even at lower temperatures.
Catalytic titanium aggregates also accelerated the addition
reaction but did not improve the enantioselectivity of the
cyanophosphorylation relative to the aluminum complexes
(Table 1, entry 8). A number of aldehydes were subjected to
the optimized reaction conditions to assess the scope and
limitations of the preparation of (R)-cyanohydrin O-phos-
phates 4 (Table 2).
Aromatic, aliphatic, and a,b-unsaturated aldehydes were
found to undergo highly efficient enantioselective cyano-
phosphorylation under these reaction conditions in high
yields (Table 2). Aromatic aldehydes with electron-withdraw-
ing substituents reacted somewhat slowly (Table 2, entries 2
and 4). Furthermore, in the case of p-nitrobenzaldehyde,
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Chemie
enantiomeric excess is lost in the first step as revealed by the
chiral HPLC data of the intermediate cyanohydrin O-
phosphite. These results clearly enhance the value of this
catalytic cyanophosphorylation of aldehydes, which ensures
access to enantiomerically enriched compounds 4.
To demonstrate interesting applications of the chiral
cyanohydrin O-phosphates 4 obtained, we investigated their
reduction to the corresponding aminoalcohols, as well as
stereospecific [3,3] sigmatropic chirality transfer. No loss of
enantiomeric excess was observed in the reduction of com-
pound (R)-4a with lithium aluminum hydride to give
enantiomerically enriched b-aminoalcohol (R)-5[18] and,
after protection of the amino group, its N-Boc derivative
6,[19] in good chemical yields (Scheme 1). These transforma-
tions also served to confirm the absolute configuration
already assigned to 4.
A strong nonlinear effect (NLE)[26] was apparent for the
enantioselective cyanophosphorylation of aldehydes.[27] This
unexpected behavior (to the best of our knowledge it is the
first example described in the literature for an aluminum-
based catalytic complex) can be assigned to the different
stability of dimeric species relative to monomers, according to
HF/6-31G* ab initio calculations.[28] Thus, whereas the combi-
nations R,R or S,S can form unstable dimeric species (relative
to monomers) with tetravalent aluminum atoms and non-
ligating aminomethyl arms, the corresponding R,S dimer
cannot form a similar species, and this combination gives rise
instead to much more stable, weakly bound monomers with
internal coordination by one aminomethyl arm.[29]
From a mechanistic viewpoint, we postulate that the
enantioselective cyanophosphorylation reaction described
above involves the intervention of a bifunctional catalyst
(either Lewis acid–Lewis base or Lewis acid–Brønsted base).
This is supported by the following observations. First, the
reaction promoted by the aluminum derivative of (S)-binol,
rather than that derived from (S)-binolam, led to the racemic
cyanohydrin O-phosphate in very low conversion (10%) after
48 h. Second, the addition of an external base (triethylamine,
20 mol%) under otherwise standard conditions led to the
acceleration of the reaction and induced a dramatic drop in
the enantiomeric excess of the product (38% ee).
Experimental Section
Dimethylaluminum chloride (1m solution in hexanes, 0.025 mmol,
25 mL) was added to a solution of enantiopure (S)-binolam 3[30]
(0.025 mmol, 11.4 mg) in dry toluene (1 mL) under argon, and the
resulting suspension was stirred at room temperature for 1 h. The
freshly distilled aldehyde (0.25 mmol) and diethyl cyanophosphonate
(0.75 mmol, 125 mL) were then added in one portion. The reaction
was monitored by 1H NMR spectroscopy and when it was judged
complete, aqueous hydrochloric acid (2m, 2 mL) and ethyl acetate
(2 mL) were added. The resulting mixture was stirred vigorously for
10 min, then the emulsion was filtered and the organic layer was
separated, dried (MgSO4), and concentrated. The residue was
purified by flash chromatography to give the pure cyanohydrin O-
phosphate 4.
Scheme 1. Reduction of chiral cyanohydrin O-phosphate 4a. Boc=tert-
butoxycarbonyl.
Palladium-catalyzed allylic displacement of allylic phos-
phates (R)-4 f and (R)-4g with sodium acetate as the
nucleophile gave the expected g-acetates 7 f and 7g as
0.25:1 and 0.2:1 Z/E mixtures, respectively. Basic hydrolysis
of the crude acetates 7, according to the reported proce-
dure,[20] led to the R-configured E cyanoallylic alcohols 8 f and
8g in 75 and 83% yield, respectively. No loss of enantiomeric
excess was observed (Scheme 2). These cyanoallylic alcohols
are important building blocks for organic synthesis.[20–24] The
Received: April 2, 2003 [Z51552]
Keywords: aluminum · asymmetric catalysis · homogeneous
.
catalysis · binaphthols · cyanohydrins
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