Novel chiral bis-phosphoramides as organocatalysts for tetrachlorosilane-mediated reactions

Article abstract of DOI:10.3390/molecules22122181

The formation of novel chiral bidentate phosphoroamides structures able to promote Lewis base-catalyzed Lewis acid-mediated reactions was investigated. Two different classes of phosphoroamides were synthetized: the first class presents a phthalic acid/pri

Full text of DOI:10.3390/molecules22122181

molecules
Article
Novel Chiral Bis-Phosphoramides as Organocatalysts
for Tetrachlorosilane-Mediated Reactions
Sergio Rossi * ID , Marco Ziliani, Rita Annunziata and Maurizio Benaglia *
ID
Dipartimento di Chimica, Università Degli Studi di Milano, via Golgi 19, 20133 Milan, Italy;
marco.ziliani@studenti.unimi.it (M.Z.); rita.annunziata@unimi.it (R.A.)
* Correspondence: maurizio.benaglia@unimi.it (M.B.); Sergio.rossi@unimi.it (S.R.);
Tel.: +39-02-5031-4171 (M.B.); +39-02-5031-4166 (S.R.); Fax: +39-02-5031-4159 (M.B.); +39-02-5031-4159 (S.R.)
Received: 5 October 2017; Accepted: 5 December 2017; Published: 8 December 2017
Abstract: The formation of novel chiral bidentate phosphoroamides structures able to promote
Lewis base-catalyzed Lewis acid-mediated reactions was investigated. Two different classes of
phosphoroamides were synthetized: the first class presents a phthalic acid/primary diamine
moiety, designed with the aim to perform a self-assembly recognition process through hydrogen
bonds; the second one is characterized by the presence of two phosphoroamides as side arms
connected to a central pyridine unit, able to chelate SiCl4 in a 2:1 adduct. These species were
tested as organocatalysts in the stereoselective allylation of benzaldehyde and a few other aromatic
aldehydes with allyl tributyltin in the presence of SiCl4 with good results. NMR studies confirm that
only pyridine-based phosphoroamides effectively coordinate tetrachlorosilane and may lead to the
generation of a self-assembled entity that would act as a promoter of the reaction. Although further
work is necessary to clarify and confirm the formation of the hypothesized adduct, the study lays the
foundation for the design and the synthesis of chiral supramolecular organocatalysts.
Keywords: phosphoroamides; metal-free catalysis; hypervalent silicon; allylation; self-assembly
1. Introduction
In recent years, chiral phosphoramides have attracted increasing attention due to their ability to
promote stereoselective reactions. The most important examples of mono- and bis-phosphoroamides
used in stereoselective processes were developed by Denmark [
extensive application as catalysts in the so called “Lewis base-catalyzed Lewis acid-mediated
reactions” [ ] where the combination of chlorosilanes with enantiomerically pure phosphoroamides or
phosphinoxides [ ] leads to an in situ formation of a new hypervalent (cationic) silicon species with
1]. These compounds have found
2
3
increased acidity at the silicon center that acts as chiral Lewis acid and efficiently promotes highly
stereoselective reactions (Figure 1) [4–6].
Cl
Si
Cl
Cl
Cl
Si
L
Cl
Cl
L3B*
L3B*
L3B*
L3B*
+
2
:
B*
Si Cl
L
Cl
Cl
Cl
L
Cl
Cl
Chiral
Lewis base
Increased Lewis acidity
Lewis acid
Figure 1. Formation of hypervalent silicon species.
In this process, a weak Lewis acidic species could be activated (converted into a stronger
acid) upon binding of a chiral Lewis base that is responsible for the control of the stereochemical
outcome of the reaction. C2 symmetric bis-phosphoroamides have been preferentially used, due to
Molecules 2017, 22, 2181; doi:10.3390/molecules22122181
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the possibility of generating a highly defined space around the silicon atom that can better guarantee
high stereocontrol. The conformational restriction provided by the linkage, if well optimized, favorably
influences the selectivity and the activity of the catalyst [2,7]. Generally, the structure of these bisdentate
phosphoroamides presents two distinct Lewis basic moieties connected through a covalent achiral
linker, and their synthesis requires a long and not trivial sequence of synthetic steps. In order to
overcome these limitations, a possible solution consists of the synthesis of more easily accessible chiral
bis-phosphoroamides, where the covalent tether between the two phosphoramide subunits could
be replaced with a non-covalent interaction. In this way, a new in situ bisdentate supramolecular
phosphoramide could be obtained and employed as an organocatalyst in Lewis base-catalyzed Lewis
acid-mediated reactions. In this work, we report the design and the synthesis of a small library of
new, potentially Self-Assembled PhosphorAmides derived Lewis bases (SAPAs) where the covalent
linker connecting the two phosphoramide units is replaced by a reversible linker, generated with
non-covalent interactions by hydrogen bonding (Figure 2).
Figure 2.
(a) Traditional approach for the activation of SiCl4 using bis-phosphoramides; (b) Design and
the synthesis of SAPAs Lewis bases.
There are several strategies to form bidentate ligands by self-assembly recognition of monodentate
ligands [ ], but the self-assembly between two homo-monodentate ligands through non-covalent
8
complementary interactions (one component assembly) represents the easier one since only
homo-complexes can be formed. The first example describing the selective and reversible assembly

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of a metal-coordinated two equal monodentate ligands was reported by Breit and Seiche in
2003 [ 10]. The known property of tautomeric pair 2-pyridone/2-hydroxypyridine to dimerize
9
,
through hydrogen bonds in aprotic solvents was exploited in the regioselective hydroformylation of
terminal alkenes to linear aldehydes. Following this strategy, Reek reported the use of MetamorPhos
as an adaptive supramolecular ligand for asymmetric hydrogenation [11]. In 2005, Love [12] described
the ability of urea-phosphine ligands to form well-defined chelates at palladium and rhodium
in the presence of a chloride ion, and few years later, van Leeuwen and Reek [13,14] reported a
systematic study of urea-functionalized phosphorous ligands (UREAphos) and palladium complexes
that self-associate by hydrogen bond formation. In 2006, Ding and co-workers reported the
application of a new class of phosphoramidite ligands for rhodium to promote the enantioselective
hydrogenation of several α,β-unsaturated esters [15], and two years later, Breit developed the use
of meta-carboxypeptidyl-substituted triarylphosphines and phosphites to construct PhanePhos-like
structures by means of an inter-ligand helical hydrogen-bonding network [16]. More recently, Gennari
reported two novel classes of chiral monodentate phosphite ligands, named PhthalaPhos [17,18] and
BenzaPhos [19], which contain, respectively, a phthalic acid primary diamine moiety and a benzamide
moiety (that presents both donor and acceptor hydrogen-bonding properties) connected to a chiral
1,10-binaphthol-derived phosphite moiety, responsible for the coordination of the supramolecular
ligand to the metal center. These ligands were satisfactorily employed in the rhodium-catalyzed
hydrogenation of acetamidoacrilates and acetamides.
Supramolecular constructs for the synthesis of ligands are well documented in transition-metal-
based catalysis [8,20–27] but only a few examples have been reported in an organocatalytic
approach [28].
For this reason, we decided to investigate the possibility of employing supramolecular
bisphosphoramide adducts in organocatalysis. In our hypothesis, the role of the metal as a coordinating
center for the bisdentate ligand could be replaced by the presence of a silicon atom, in its hypervalent
state (Figure 2).
2. Results and Discussion
Following this approach, we developed a strategy for the synthesis of a simple, low-weight
and relatively inexpensive phosphoramide-based catalyst, where the covalent tether was replaced
by non-covalent interactions, as H-bonds. The structure of the catalyst can be subdivided into three
different parts: a phosphorous-based catalytic center, responsible for the interaction with the silicon
atom; a self-recognition moiety, responsible of the aggregation of two catalyst units, and a linker to
distance these units, avoiding interferences between coordination and recognition processes.
Inspired by previously reported examples, we decided to start our investigation by employing the
phthalic acid primary diamide moiety as a recognition system [17–19]. Binaphthyl diamine-derived
phosphoramide was selected as the catalytic center, since this type of structure has proven to be an
efficient organocatalyst in many type of reactions involving the generation of hypervalent trichlorosilyl
species [
step consisted of the evaluation of the correct linker between these two functionalities. Based on
literature [29 30] and previous experiences in our group, the phosphorous atom needs to be connected
1,2]. As the Lewis base and the self-assembly recognition system are fixed, the subsequent
,
to three nitrogen atoms bearing an alkyl substituent to generate stable compounds. For these reasons,
different N-methylsubstituted anilines were selected as target linkers (Scheme 1).
The synthesis of the optimal linker necessary to connect the two subunits responsible for the
recognition process and the catalytic process requires particular attention. The geometry of the linkers
is extremely important because their conformational restriction influences the orientation of the two
subunits as well as the selectivity and reactivity of the final self-assembled catalyst. Since the linker
bears two amino-functionalities with similar reactivity, the primary amino group (employed for the
connection of the recognition system), was generated through the reduction of nitro or nitrile groups.

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Scheme 1. Retrosynthetic approach for the synthesis of SAPA derivates.
The final ligand could be then synthetized starting from the three general precursors
reported in Scheme 2. First, chloro-diamino-phosphine ( ) generated in situ by the reaction
of (S)-N,N0-dimethyl-[1,10-binaphthalene]-2,20-diamine (
) with PCl3 [31] was treated with aryl
methylamines ( ) to synthesize phosphoroamides ( ). At this point, the nitro- or the cyano-group
was reduced to amine, forming compounds ( ), which were converted in the desired ligands ( ) by
2
1
3
4
5
6
reaction with phthalic anhydrides. A small collection of SAPAs derivatives was then synthesized
with good yields. These compounds have been reported in Scheme 3 (see supporting information for
further details).
Scheme 2. General synthesis of SAPAs derivatives.
SAPAs 7a
–d derive from orto- meta- and para-substituted ((methylamino)methyl) anilines,
while compound 7e features 4-(aminomethyl)-N-methylaniline as a linker. In compounds 7f
–
h
the recognition part is separated from the phosphoroamide unit by the presence of a substituted
biphenyl moiety. Furthermore, for control experiments, compound 7h (no possibility of self-assembly
interaction) and compound 7i (no catalytic active site) were synthesized.
All the new ligands were employed as organocatalysts in the enantioselective allylation of
benzaldehyde with allyl tributyltin in the presence of SiCl4 (Table 1). One equivalent of benzaldehyde
was reacted with 1.2 equivalents of allyl tributyltin in the presence of 2 equivalents of SiCl4 in dry
DCM, using 10 mol % of the monodentate catalyst under a nitrogen atmosphere [32]. Results are
shown in Table 1.

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Scheme 3. Library of SAPAs derivatives.
Table 1. Enantioselective allylation of benzaldehyde.
Entry
Catalyst
Conc (M)
Yield (%)
ee (%) 1
1
2
3
4
5
6
7
8
9
7i
7a
7b
7c
7c
7d
7e
7f
0.5
0.5
0.5
0.5
0.05
0.5
0.5
0.5
0.5
0.5
0.5
/
/
83
58
73
28
78
90
45
80
79
98
13 (S)
60 (S)
70 (S)
73 (S)
68 (S)
Rac
44 (S)
34 (S)
60 (S)
65 (S)
7g
7h
7l
10
11
1
determined by HPLC in chiral stationary phase.
As expected, the phosphoramide unit is necessary to promote the reaction (entry 1).
Phosphoramide (7c) was able to catalyze the reaction with the formation of the product in 73%
yield and 70% enantiomeric excess. High dilution conditions were detrimental in terms of yield but did
not influence the stereochemical outcome of the reaction (entry 5). The butyl chain had no influence
on the reactivity or on the stereoselection (entry 6). Catalyst (7b) led to the formation of allyl alcohol
(8a) with lower yield than the catalyst (7a), but with better enantioselectivity (entry 2 vs. entry 3).
Catalyst (7e) was extremely reactive but did not exert any type of stereocontrol on the process (entry 7).
Catalysts (7g) and (7f) presented a good chemical efficiency but led to the formation of the product with
modest stereoselection (entries 8 and 9). By using catalyst 7l, unable to be involved in a self-assembly
recognition process, it was possible to observe the formation of the desired product with good yield
and stereoselectivity.

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On the basis of these results, and in order to understand if a real self-assembly recognition process
took place, we performed 1H-, 31P- and 29Si-NMR analysis on the catalysts in the presence and absence
of SiCl4. We first studied the dependence of the NH chemical shifts on the concentration of the ligand
7c in the 0.45–30 mM range; we determined that the self-aggregation of ligand 7c due to H-bonding
became not significant at 3.75 mM concentration. Thus, the NMR experiments on ligands
7 and SiCl4
were conducted at 3.75 mM in CD2Cl2.
◦
Different ratios of (SiCl4):(phosphoroamide ligand 7c) were investigated at
−
50 C by NMR.
Unfortunately, no chemical shift variation was observed in the 31P spectra of the chiral ligand after
the addition of SiCl4. 29Si spectra showed two signals only:
19 ppm, due to free SiCl4 and one at
46 ppm, corresponding to a tetravalent silicon atom bounded with oxygen atoms. No signals were
−
−
observed in the range from
−
200 to
−
210 ppm, expected for hexacoordinated silicon species [
2,
33
,34].
◦
Moreover, raising the temperature from
−
50 C to RT, it was possible to observe a partial degradation
of the ligand 7c corresponding to the elimination of 4-nBu-aniline 10 with consequent formation
of a phthalimide derivative . This could be explained by the coordination of SiCl4 to the amide
9
functionality that becomes sensitive to the nucleophilic attack from a nitrogen atom (Scheme 4).
Scheme 4. Degradation of chiral ligand 7c.
Since these results clearly indicated that no supramolecular structure was formed and that the
catalytic activity was formally expressed by a single monomeric ligand, we started to investigate
different recognition systems. Since the use of metal as an aggregation center is not compatible
with LB-catalyzed LA-mediated reactions, we focused our attention on the use of SiCl4 itself as
an aggregating agent for the formation of the supramolecular structure.
It is known that SiX4 compounds can generate hexacoordinated complexes in the presence of
1,1-phenanthroline, 2,20-bipyridine and pyridines [35
–42]. Among them, only pyridines are able to
generate trans-complexes with a 2:1 pyridine:Si ratio; therefore, we decided to synthesize a recognition
system (Scheme 5a). The capability of SiCl4 to form a Py2:SiCl4 adduct was also confirmed by our
preliminary 29Si-NMR experiments, where one single signal was observed at
CD2Cl2 (compatible with the bis-N-coordinated Si(Halogen)4 species).
−
178 ppm at 210 K in
New catalysts were then designed and synthesized in three steps only according to the
general procedure reported in Scheme 5b. The synthesis involved a Suzuki coupling between
3,5-dibromopyridine (11) and carboxyphenylboronic acid 12, leading to the formation of compound 12
This compound was then subjected to reductive amination, yielding diamine (14), which, upon
treatment with (S)-1,10-Binaphthyl-2,20-diamine 1, allowed us to obtain C2 chiral ligands 15 with
.
good yields. With this protocol, two new catalysts were synthesized (15a,b), in 52% and 56% yield
respectively, and they were investigated as organocatalysts in the enantioselective allyltributyltin
addition to aromatic aldehydes promoted by SiCl4 (Table 2).

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Scheme 5. (a) Hypothetical supramolecular complex; (b) Synthesis of compounds 15a-b.
Table 2. Enantioselective allylation of aldehydes.
Entry
Catalyst
R
Product
Yield (%)
ee (%) 1
1
2
3
4
15a
15b
15a
15a
1
H
H
Cl
8a
8a
8b
8c
71
30
83
31
61 (S)
29 (S)
62 (S)
rac
OMe
determined by HPLC on chiral stationary phase.
Catalyst 15a allowed us to obtain the desired product 8a in 71% yield and 61% ee, while
catalyst 15b generated allylic alcohol in lower yield and poor enantiomeric excess, indicating that the
meta substitution resulted in an unfavorable conformation adopted by the ligand when coordinated
with tetrachlorosilane. Product 8b derived from p-chlorobenzaldeyde was obtained in 83% yield and
a comparable level of enantioselection, but when electron-donating substituents were present (entry 4),
the yield and the stereoselection decreased considerably.
Although further work is necessary to clarify and to confirm the formation of the hypothesized
self-assembled adduct, these preliminary results are encouraging and lay the foundation for the study
of chiral supramolecular organocatalysts as efficient promoters of stereoselective reactions.

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3. Materials and Methods
1H-NMR, 13C-NMR and 29Si-NMR spectra were recorded with instruments at 300 MHz
(Bruker F300, Billerica, MA, USA) or 500 MHz (Bruker ADVANCE 500 or 600). Proton chemical
shifts are reported in ppm (
as the internal standard (CDCl3 =
125 MHz or 192.5 MHz, with complete proton decoupling. Carbon chemical shifts are reported in ppm
) relative to TMS with the respective solvent resonance as the internal standard (CDCl3, δ = 77.0 ppm).
29Si-NMR spectra were recorded operating at 99 MHz; chemical shifts are reported in ppm (
) relative
δ
) with the solvent reference relative to tetramethylsilane (TMS) employed
δ
7.26 ppm). 13C-NMR spectra were recorded operating at 75 MHz,
(δ
δ
to TMS. 31P spectra were recorded at 121.4 or 202.4 MHz and were referenced to phosphoric acid
(H3PO4) at 0.0 ppm. HPLC analysis was performed on an Agilent Instrument Series 1100 or 1200 series
on chiral stationary phase. Purification of the products was performed by column chromatography on
silica gel (230–400 mesh ASTM, Merck, Kenilworth, NJ, USA). All the solvents used are commercially
available ( 99%, chromatographic grade, purchased from Sigma Aldrich, St. Louis, MO, USA) and
≥
stored under nitrogen over molecular sieves (bottles with crown caps). Reactions were monitored
by analytical thin-layer chromatography (TLC) using silica gel 60 F254 pre-coated glass plates and
visualized using UV light.
3.1. General Procedure for the Synthesis of Nitro-Phosphoroamides (4)
0
0
0
N,N -Dimethyl-1,1 -binaphthyl-2,2 -diamine (
1
) (1 eq., 3.20 mmol, 1.0 g) and Et3N (3 eq., 9.6 mmol,
◦
1.33 mL) were dissolved in dry THF (32 mL). The homogeneous mixture was cooled to 0 C then
PCl3 (3 eq., 9.60 mmol, 0.84 mL) was added dropwise via a syringe whereupon a colorless precipitate
formed immediately. The reaction mixture was stirred at 0 C for 1.5 h and was then allowed to warm
◦
to room temperature and stirred for another 3 h. The volatiles were removed under high vacuum
(room temperature, 0.5 mmHg) and Et2O (30.0 mL) was added via syringe, then the mixture was
stirred for 5 min. Subsequently, the supernatant was canula-filtered into another round bottom flask.
The remaining precipitate in the reaction flask was washed again with Et2O (30 mL) and filtered (twice).
The volatiles were removed under high vacuum (room temperature, 0.5 mmHg) to yield a light yellow
solid. The solid was then dried for 12 h at reduced pressure (room temperature, 0.5 mmHg) to yield
a white solid foam (
2). Dry CH2Cl2 (40 mL) was added via syringe and the mixture was cooled to
0 ◦C. To this solution, a mixture of Et3N (2 eq., 6.40 mmol, 0.98 mL) and the desired methylamine (
3)
(1.2 eq., 3.84 mmol) dissolved in dry CH2Cl2 (4 mL) were added. The reaction mixture was allowed
to warm to room temperature and stirred for 20 h. A solution of mCPBA (70%) (1.5 eq., 4.80 mmol,
1.18 g) dissolved in 2 mL of THF was then added and the mixture was stirred for 20 h. After quenching
with 15 mL of NH4Cl saturated aqueous solution, the phases were separated and the aqueous layer
was washed with CH2Cl2 (10 mL). The combined organic extracts were washed with brine, dried
over Na2SO4, filtered and concentrated by rotary evaporation. The crude residue was purified by
silica gel flash chromatography using ethyl acetate (100%) as an eluent to yield phosphoroamides with
different yields.
3.2. General Procedure for the Synthesis of SAPAs Catalyst (7)
The desired phosphoroamide (5) (1 eq., 0.1 mmol) and the desired phthalisoimide (6)
(3 eq., 0.3 mmol) were dissolved in dry THF (2 mL). The homogeneous mixture was stirred at RT for
48 h, then quenched with 5.0 mL of HCl 5%. The phases, diluted with ethyl acetate were separated and
the obtained aqueous layer was washed with ethyl acetate (5.0 mL). The combined organic extracts
were washed with brine, dried over Na2SO4, filtered and the filtrate was concentrated by rotary
evaporation. The residue was purified by silica gel flash chromatography using different mixtures
furnishing the desired product.

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3.3. General Procedure for Allylation of Benzaldehyde
Phosphoramide catalyst (0.1 eq. or 0.05 eq.) was dissolved in CH2Cl2 (1.5 mL) under N2.
Allyltributyltin (1.2 eq., 0.54 mmol, 169 L) was added to this solution and the resulting mixture
was cooled to
78 ◦C (bath temperature). Then, freshly distilled SiCl4 (2 eq., 0.9 mmol, 104
added followed by the aldehyde (1 eq., 0.45 mmol). The resulting mixture was stirred at
µ
−
µ
L) was
78 C
◦
−
(bath temperature) for 6 h whereupon the cold reaction mixture was rapidly poured into a stirring
solution of 1/1 sat. aq. KF/1.0 M KH2PO4 (5 mL). This biphasic mixture was stirred vigorously for
12 h, then, diluted with CH2Cl2 (10 mL); the layers were separated and the aqueous one was washed
with CH2Cl2 (3
×
10 mL). The combined organic extracts were dried over Na2SO4, filtered and the
filtrate was concentrated by rotary evaporation under vacuum. The residue was purified by silica gel
flash chromatography using hexane/ethyl acetate (9:1, v/v) as an eluent with a plug of solid anhydrous
KF (15 mm) on the top of the column . The product-containing fractions were combined, and the
solvent was removed by rotary evaporation under vacuum to yield the desired allylic alcohol.
4. Conclusions
In conclusion, in this work, we investigated the synthesis of new chiral bidentate
phosphoroamides as promoters of Lewis base-catalyzed Lewis acid mediated reactions. A library
of different mono- and bis-phosphoramides was synthetized in good yields. These molecules were
characterized by the presence of three different designed subunits (a recognition moiety, a linker
and a catalytic center) in order to favor a recognition process through reversible interactions for the
synthesis of a new bisphosphoroamides adduct. While the use of a phthalic acid primary diamine
moiety for a self-assembly recognition process through hydrogen bonds in the presence of SiCl4
was unsuccessful, the use of pyridine as a chelating unit in combination with tetrachlorosilane gave
promising results.
These species were tested as organocatalysts in the Lewis base-catalyzed Lewis acid-mediated
addition of allyl tributyltin to benzaldehyde and other aromatic aldehydes, generating homoallylic
alcohol with good yields and enantioselectivities. Further investigations to study the self-assembly
recognition processes as well as the use of these chiral ligands in other silicon-based transformations
are under investigation in our laboratories and will be reported in due course.
Supplementary Materials: Supplementary Materials are available online.
Acknowledgments: S.R. and M.B. thank L. Pignataro for valuable discussions.
Author Contributions: S.R. and M.Z. performed the synthetic works, R.A. performed the NMR studies. M.B. and
S.R. designed the experiments of the project and supervised all studies reported in the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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