Chiral phosphine ligand libraries based on the Bull–James three-component supramolecular assembly

Article abstract of DOI:10.1080/10610278.2018.1564829

An approach to the synthesis of libraries of chiral phosphine ligands is described, using condensations of 2-formylarylboronic acids, diols or related compounds, and aminophosphines. The three-component nature of this condensation, along with the ready av

Full text of DOI:10.1080/10610278.2018.1564829

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Chiral phosphine ligand libraries based on the
Bull–James three-component supramolecular
assembly
Hsin Y. Su, Daniel Gorelik & Mark S. Taylor
To cite this article: Hsin Y. Su, Daniel Gorelik & Mark S. Taylor (2019): Chiral phosphine ligand
libraries based on the Bull–James three-component supramolecular assembly, Supramolecular
Chemistry, DOI: 10.1080/10610278.2018.1564829
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SUPRAMOLECULAR CHEMISTRY
https://doi.org/10.1080/10610278.2018.1564829
ARTICLE
Chiral phosphine ligand libraries based on the Bull–James three-component
supramolecular assembly
Hsin Y. Su, Daniel Gorelik and Mark S. Taylor
Department of Chemistry, University of Toronto, Toronto, Canada
ABSTRACT
ARTICLE HISTORY
Received 26 September 2018
Accepted 27 December 2018
An approach to the synthesis of libraries of chiral phosphine ligands is described, using
condensations of 2-formylarylboronic acids, diols or related compounds, and aminophosphines.
The three-component nature of this condensation, along with the ready availability of the
building blocks, enables the rapid generation of diverse structures. From a library of iminobor-
onate-derived phosphines, three ligands that gave 90% ee or greater in a benchmark palla-
dium-catalyzed allylic substitution reaction were identified. Significant variation of selectivity as
a function of the structure of each component was observed. 11B NMR spectroscopy was used
to evaluate the existence of B–N interactions in the free ligands as well as their Pd-derived
complexes. A bidentate P,N-coordination mode was inferred for ligands that gave high enan-
tioselectivity in the allylic substitution reaction.
KEYWORDS
Boronic acids; combinatorial
chemistry; enantioselective
catalysis; phosphine ligands
Abbreviation: BINOL: 1,1'-bi-2-naphthol; BSA: N,O-bis(trimethylsilyl)acetamide; DIOP: (2,3-O-isopro-
pylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane; HPLC: high-performance liquid chroma-
tography; NMR: nuclear magnetic resonance; THF: tetrahydrofuran
R3
CHO
R3
R1
HO
HO
N
R2
R1
+
+
PAr2
B
PAr2
O
H2N
3 H2O
O
B(OH)2
R2
Diverse chiral phosphines
t-Bu
N
B
PPh2
O
O
93% ee in a benchmark Pd-catalyzed
allylic substitution reaction
Me
Me
Me
Me
1. Introduction
coordination (6–9), hydrogen bonding (10–14), ion-
pairing (15, 16) and the formation of interlocked struc-
tures (17) – have been employed to build new chiral
phosphorus-centered ligands (18–20).
Methods that enable the rapid discovery of new chiral
ligand structures have an important role to play in the
discovery and optimization of enantioselective, transi-
tion metal-catalyzed reactions. Modular ligands that can
be assembled by high-yielding coupling reactions of
readily available, chiral components such as diols or
amino acids have been useful in this regard (1–5).
More recently, an alternative approach that relies on
supramolecular chemistry and self-assembly has
emerged: ligands are constructed by simple mixing of
two or more building blocks that interact through non-
covalent or reversible covalent interactions. A range of
supramolecular interactions – including metal-ligand
Reversible covalent interactions of boronic acids
have been applied extensively in chemical sensing
and molecular recognition (21, 22). Their advantages
include favourable thermodynamics (which can be
varied substantially by changing the structure of the
boronic acid and diol or related partner), equilibration
under relatively mild conditions, and compatibility
with a range of functional groups and reaction
media. As part of a research program aimed at devel-
oping new applications of such interactions in catalysis
CONTACT Mark S. Taylor
mtaylor@chem.utoronto.ca
Department of Chemistry, University of Toronto, 80 St. George St., Toronto M5S 3H6, Canada
Supplemental data for this article can be accessed here.
© 2019 Informa UK Limited, trading as Taylor & Francis Group

2
H. Y. SU ET AL.
(23), we considered whether condensations of boronic
acids could be employed as the basis for self-assembly
of libraries of phosphine ligands. Precedent for this
idea dates back to 1993, when the groups of Kagan
(24) and Jacobsen (25) reported the synthesis, com-
plexation behaviour and catalytic applications of boro-
nic ester-bearing analogs of the chiral bis(phosphine)
DIOP (1, Figure 1). More recently, the group of Gudat
has explored the assembly of bis(phosphines) through
complexation of main group Lewis acids to catechol-
bearing phosphine subunits (26, 27). Condensation
with boric acid was employed to generate
a zwitterionic Ag(I) complex (2, Figure 1). Self-
assembly of chiral organocatalysts through boronic
ester formation has also been studied. In these sys-
tems, the boronic ester group is often proposed to
play a functional role, either by direct complexation
to the substrate or by intramolecular coordination to
a catalyst-derived functional group such as a Brønsted
acid moiety (28–31).
The design concept for the self-assembled chiral phos-
phine ligands explored in this study is based on the Bull–
James three-component condensation of formylphenyl-
boronic acids, diols and primary amines (32, 33). This
process, in which the three components are linked by
Schiff base and boronic ester formation (Figure 2), has
been employed in the development of NMR- and fluor-
escence-based methods for determination of enantio-
meric excess (34–37), and in the self-assembly of
complex architectures such as macrocycles and cages
(38, 39). Applications in asymmetric synthesis or catalysis
are less well explored, with the use of a chiral iminobor-
onate as a chiral auxiliary for an enantioselective azide-
alkyne cycloaddition being a notable recent example
(40). Several features of the Bull–James assembly
appeared to offer unique advantages in the context of
using self-assembly to generate libraries of chiral phos-
phines. Firstly, it is a relatively rare example of a
supramolecular interaction that engages three different
components: the ability to efficiently generate condensa-
tion product from an equimolar mixture of the three
components stems, at least to some extent, from the
positive cooperativity between imine and boronic ester
formation (41). The three-component nature of the
assembly offers the prospect of particularly rapid genera-
tion of structural variants, as demonstrated by the utility
Figure 1. Previously reported examples of boronic ester-
containing phosphine derivatives.
Figure 2. The Bull–James three-component condensation.
Figure 3. Design for a library of chiral phosphines based on the Bull-James assembly. Potential coordination modes to a metal MLn
are shown below the reaction.

SUPRAMOLECULAR CHEMISTRY
3
of multicomponent reactions in diversity-oriented synth-
esis (42). Secondly, it is clear that the Bull-James assembly
is well suited to applications of chiral components, as
demonstrated by the numerous reported applications
involving enantioenriched amines and/or diols. Thirdly,
and related to this previous point, chiral phosphines
having free diol (24–26) or primary amine groups (43)
are readily accessible building blocks that could be
employed in such an application.
2. Results and discussion
2.1. Iminoboronate ligand building blocks
The components employed in the construction of chiral
iminoboronate-based phosphines explored in this study
are depicted in Figure 4. Formylarylboronic acids 3a–3c,
diols 4a–4i’ and β-aminophosphine 5d were purchased
from chemical suppliers. Aminophosphines 5a (44), 5b
(45) and 5c (46) were synthesized according to previously
reported protocols.
The assembly process employed to construct libraries
of chiral phosphine ligands in this study is shown in
Figure 3. It combines
a chiral aminophosphine,
2.2. Ligand assembly by three-component
condensation, and preliminary evaluation in an
enantioselective allylic substitution reaction
a formylarylboronic acid derivative, and a (chiral) diol or
related compound capable of two-point covalent inter-
action with a boronic acid. Both monodentate and
bidentate (P,N-) coordination modes – the latter being
possible only at the expense of the B–N interaction – can
be envisioned for this type of ligand framework. We
show that this three-component condensation can be
employed to generate a library of 100 chiral phosphine
ligands, differing both in structure and in enantioselec-
tivity for a representative palladium-catalyzed allylic sub-
stitution reaction. With the optimal combination of
aminophosphine, diol and formylarylboronic acid, the
allylic substitution product can be obtained in greater
than 90% enantiomeric excess (ee).
The assembly protocol developed for the determination of
enantiopurity of diols or amines by 1H NMR spectroscopy
(22) was adapted for condensation of 3a, 4b and 5a.
A solution of equimolar quantities of the three components
in degassed chloroform was heated to 50°C and stirred for
30 minutes (Figure 5). The assembly reaction proceeded to
completion, as judged by the disappearance of the signal
1
in the H NMR spectrum corresponding to the formyl
hydrogen of 3a (9.93 ppm in CDCl3) and the appearance
of a new signal corresponding to the imine hydrogen of
6aba (8.31 ppm). (The letters in the notation 6aba
Figure 4. Components of chiral iminoboronate-based phosphine library.

4
H. Y. SU ET AL.
Figure 5. Protocol for ligand assembly by three-component condensation. 11B NMR chemical shifts of 3a, 6aaa, 6ada and 6aha are
reported (CDCl3 solvent, with BF3•OEt2 (δ = 0 ppm), as an external reference standard).
correspond to those of the individual components 3a, 4b
and 5a from which it was constructed.) The condensation
took place more rapidly in the presence of molecular sieves
(data not shown), but it was more convenient to omit this
component when carrying out multiple, small-scale ligand
preparations in parallel. 11B NMR spectroscopy was used to
assess the coordination mode of the boron center in con-
structs 6aaa, 6ada and 6aha. The 11B NMR chemical shifts
for 6ada and 6aha (21 ppm and 15 ppm, respectively,
relative to BF3•OEt2 (δ = 0 ppm)) were in keeping with
reported data for uncharged, tetracoordinate boronic
esters having a B–N interaction (41, 47, 48). The lower
chemical shift of 6aha is consistent with a stronger B–N
interaction due to the increased Lewis acidity of the boron
center upon coordination to catechol (38, 47). In contrast,
the 11B NMR chemical shift of neopentyl glycol-derived
6aaa (28 ppm) was identical to that of formylphenylboro-
nic acid 3a, suggesting that a B–N interaction was absent in
this case. It should be noted that the structures of all
iminoboronates 6 were drawn in a consistent format (hav-
ing a B–N bond), even in cases such as 6aaa where the
11B NMR chemical shift was not consistent with such an
interaction being present.
The utility of the iminoboronate ligand design in
a benchmark enantioselective transformation was assessed
using the palladium-catalyzed allylic substitution of sub-
strate 7 with diethyl malonate (Table 1) (49). Under stan-
dard conditions for this type of transformation ([Pd(allyl)Cl]2
Table 1. Evaluation of chiral iminoboronates as ligands for enantioselective, palladium-catalyzed allylic alkylation.a.
Entry
Ligand (mol %)
MOAc
Conversion (%)b
ee (%)b
1
2
3
4
5
6
7
6aba (20)
6aba (20)
5a (20)
9aa (20)
10 (20)
6aaa (20)
6aaa (10)
KOAc
NaOAc
KOAc
KOAc
KOAc
KOAc
KOAc
>97
86
>97
51
66
>97
>97
88
83
67
65
61
91
83
aBSA denotes N,O-bis(trimethylsilyl)acetamide.
bConversion and ee were determined by HPLC analysis using a chiral stationary phase.

SUPRAMOLECULAR CHEMISTRY
5
pre-catalyst, N,O-bis(trimethylsilyl)acetamide (BSA), potas-
sium acetate, chloroform, 23°C) (50, 51), ligand 6aba led to
the formation of product (R)-8 in 88% enantiomeric excess
(ee) (>97% conversion of 7, as judged by high-performance
liquid chromatography (HPLC) analysis prior to purification:
entry 1). The ligand employed in this experiment was gen-
erated by assembly of the three components in CHCl3 at 50°
C for 30 minutes as described above, followed by removal
of the solvent in vacuo. The reagents needed for the Pd-
catalyzed allylic substitution were added to the resulting
material without further purification of the iminoboronate
ligand. A similar protocol was followed for all evaluations of
the iminoboronate ligands described below. Under pre-
parative conditions on 0.25 mmol scale, product (R)-8 was
isolated in 94% yield and 90% enantiomeric excess. Using
NaOAc in place of KOAc led to diminished conversion and
enantioselectivity (entry 2). To assess whether the Bull–
James assembly remained intact under the conditions of
palladium-catalyzed allylic substitution, aminophosphine
component 5a was evaluated as a chiral ligand for the
reaction (entry 3). The enantiomeric excess of 8 was sig-
nificantly lower than that obtained using the ligand con-
struct 6aba. The iminoboronic acid ligand 9aa obtained in
the absence of a diol component gave rise to lower catalytic
activity and enantioselectivity than 6aba (entry 4). The
same was true of iminophosphine 10 lacking a boronic
acid group (entry 5). Taken together, these control experi-
ments indicated that the three-component iminoboronate
assembly was indeed responsible for enantioselectivity
under the conditions of Table 1, entry 1. Furthermore, the
observation of lower enantioselectivity and reactivity using
iminophosphine 10 suggests that the boronate group does
not simply act as a sterically hindered substituent. The
effect of ligand:Pd ratio was evaluated using construct
6aaa (entries 6 and 7). The enantioselectivity was appreci-
ably lower using a 1:1 phosphine:Pd ratio rather than the
2:1 ratio employed for the remaining entries in Table 1.
6aha) gave lower enantioselectivities than those
derived from aliphatic diols. Among the aliphatic diols,
those having substituents capable of steric shielding of
the boron center upon condensation gave rise to the
most enantioselective catalysts (compare 6aaa and
6aba to 6aca, 6aea and 6afa). The overall trend
appears to be that the most selective ligands are
derived from diol components that would result in
a less Lewis acid boron center, and thus a weaker B–N
interaction. This observation may represent indirect evi-
dence for a P,N-coordination mode, since chelation to
palladium in this way would require the loss of the B–N
bond.
To probe the issue of monodentate versus bidentate P,
N-coordination further, ligands 6aaa and 6aha were trea-
ted with allylpalladium(II) chloride dimer in CDCl3 and the
resulting solutions were analyzed by 1H, 11B and 31P NMR
spectroscopy (see the Supplemental Material). In the case
of neopentyl glycol-derived ligand 6aaa, a single major
complex having a 31P NMR chemical shift of 16.6 ppm
(relative to 85% phosphoric acid in H2O, δ = 0.0 ppm) was
observed. As anticipated, no appreciable change in the
11B NMR chemical shift (δ = 28 ppm, characteristic of
a boronic ester without B–N coordination – see Section
2.2 above) was observed upon formation of this complex.
These observations suggested a P,N-coordination mode
for ligand 6aaa, consistent with the numerous successful
applications of this general class of ligands for Pd-
catalyzed allylic substitutions (43), and in particular the
results obtained using chiral β-iminophosphines (52, 53).
For catechol-derived adduct 6aha, addition of
[Pd(allyl)Cl]2 resulted in two predominant signals
(δ = 20.0 and 19.4 ppm, 2.4:1 ratio) in the 31P NMR spec-
trum. The 11B NMR chemical shift (δ = 15 ppm) did not
change to an appreciable extent from that of free 6aha,
suggesting that the B–N bond was maintained upon
1
complexation. The H NMR spectra were consistent with
the conclusion of a P,N-coordination mode for 6aaa but
not for 6aha: the signal corresponding to the azomethine
hydrogen of 6aaa underwent a significant downfield shift
upon complexation with Pd(II) (δ = 9.52 ppm for the
complex versus 8.32 ppm for the free ligand), whereas
a smaller change in the corresponding signal for 6aha
was observed upon complexation.
Returning to the data from Figure 6, an appreci-
able difference in selectivity between methyl
L-rhamnopyranoside-derived ligand 6aea and methyl
L-fucopyranoside-derived 6afa was observed. At this
stage, it is not clear whether this difference reflects
a difference in the steric environments of the cis-diol
groups of 4e and 4f (the latter being more hindered
due to the cis relationship between the 3-OH and
4-methyl groups), or a matching/mismatching effect
2.3. Effects of variation of diol and
aminophosphine component for ligands
constructed from 2-formylphenylboronic acid
Prior to generating a full library by variation of all three
components of the phosphine-functionalized Bull–
James assembly, we sought to determine whether
changing the structure of the diol and aminophosphine
components would have an appreciable effect on the
enantioselectivity of the benchmark reaction. Eight
diols were employed in condensations with 3a and
5a, and the resulting ligands were tested in the palla-
dium-catalyzed allylic substitution reaction (Figure 6).
Iminoboronates having coordinated phenol ligands
(BINOL derivatives 6aga, 6aga’ and catechol ester

6
H. Y. SU ET AL.
Figure 6. Effect of diol structure on enantioselectivity obtained with ligands derived from arylboronic acid 3a and aminophosphine
5a. BSA denotes N,O-bis(trimethylsilyl)acetamide. Conversion and ee were determined by HPLC analysis using a chiral stationary
phase.
with the chirality of the aminophosphine ligand (the
cis-diol groups have a pseudo-enantiomeric relation-
ship in 4e and 4f). This type of matching/mismatch-
ing effect was not evident in the enantioselectivities
of ligands 6aga and 6aga’, which differ in the con-
figuration of the BINOL component.
In a similar way, a sub-library was constructed by
keeping the boronic acid and diol components con-
stant (3a and 4a) while varying the identity of the
aminophosphine (Figure 7). Whereas the assemblies
derived from benzylic aminophosphine 5a and tert-
leucinol-derived 5c gave high enantioselectivities (with
the latter favouring the formation of the S-configured
product), inferior results were obtained for ligands
derived from 5b and 5d. For each assembly that gave
appreciable enantioselectivity (>10% ee), the enantio-
meric excess obtained with the corresponding free
aminophosphine is provided for comparison. The
results provide strong evidence that the intact imino-
boronates are responsible for the enantioselectivities
observed using the ligand self-assembly protocol.
2.4. Assembly and evaluation of a library of 100
iminoboronate-based chiral phosphines
Having demonstrated that variations of the structures
of the diol and aminophosphine components gave
rise to significant changes in the enantioselectivity

SUPRAMOLECULAR CHEMISTRY
7
Figure 7. Effect of aminophosphine structure on enantioselectivity obtained with ligands derived from arylboronic acid 3a and diol
4a. BSA denotes N,O-bis(trimethylsilyl)acetamide. The ee values obtained using aminophosphines 5a–5c as ligands are provided for
comparison. Conversion and ee were determined by HPLC analysis using a chiral stationary phase.
of the test reaction, we undertook the synthesis of
a 100-ligand library. Forty ligands derived from 2-for-
mylphenylboronic acid 3a were generated, using ten
OH-containing compounds (4a–4c, 4e–4i) and four
aminophosphines (5a–5d). Because ligands based on
5d generally showed relatively low levels of enantios-
electivity, this component was omitted from the sub-
libraries generated from boronic acids 3b and 3c.
Enantioselectivity data for all of the ligands are pro-
vided in Table 2, and are summarized in graphical
form in Figure 8. Some variations in catalyst activity
were also evident across the ligand library: assemblies
derived from catechol, BINOL and mandelic acid gen-
erally gave rise to lower substrate conversions than
those derived from aliphatic alcohols. The data pro-
vided in Figure 6 are representative of this effect, and
conversions for the 100-ligand library are not
included in the Table
Several overall trends can be inferred from Table 2
and Figure 8. Ligands derived from 2-formylphenyl-
boronic acid 3a were generally more selective than
those derived from thiopheneboronic acid 3c or
3-fluoro-substituted 3b, with the latter giving particu-
larly poor enantioselectivities (ee < 60% in all cases). Of
the aminophosphine components tested, 5a and 5c
gave rise to higher enantioselectivities than 5b and
5d. Interplay between the components is also evident
from the data. In particular, the trend of aliphatic,
sterically hindered diol components leading to higher
enantioselectivities than electron-deficient and/or steri-
cally unhindered ligands for boron is clear in several
cases, including for the assemblies derived from 3a and
5a or 5c, 3b and 5a, and 3c and 5a. However, this trend
did not hold for the series derived from boronic acid 3c
and aminophosphine 5c, or from boronic acid 3a and
aminophosphine 5d. In these cases, ligands derived
from catechol and mandelic acid were more selective
than those derived from the aliphatic alcohols. Ligands
based on aminophosphine 5c appeared to be more
sensitive to variations of the diol component than
those derived from 5a. Overall, of the 100 ligands
tested, three gave enantioselectivities of 90% ee or
higher, and another ten gave >80% ee. Limits to the
scope of application of these newly discovered ligands
were encountered upon attempted allylic substitutions
of (E)-4-phenylbut-3-en-2-ol and cyclohex-2-en-1-ol
derivatives, both of which resulted in lower reactivity
and enantioselectivity than substrate 7 (data not
shown).
2.5. Extension to decarboxylative allylic
substitutions: a cautionary tale
With the aim of applying this ligand library design to
a more challenging enantioselective transformation,
the palladium-catalyzed decarboxylative allylic

8
H. Y. SU ET AL.
Table 2. Enantioselectivity data for evaluation of a library of 100 chiral phosphine ligands in the Pd-catalyzed allylic substitution of
rac-7.a.
Aminophosphine 5a
Ligand % ee
Ligands derived from boronic acid 3a
Aminophosphine 5b
Ligand % ee
Aminophosphine 5c
Ligand % ee
Aminophosphine 5d
Ligand % ee
6aaa
6aba
6aca
6aea
6afa
90 (R)
88 (R)
81 (R)
81 (R)
87 (R)
76 (R)
74 (R)
70 (R)
66 (R)
69 (R)
6aab
37 (R)
60 (R)
14 (S)
22 (R)
34 (R)
8 (R)
14 (S)
7 (R)
13 (R)
27 (R)
6aac
92 (S)
93 (S)
81 (S)
83 (S)
88 (S)
83 (S)
51 (S)
75 (S)
73 (S)
0
6aad
7 (R)
4 (R)
9 (R)
13 (R)
33 (R)
43 (R)
23 (R)
69 (R)
67 (R)
52 (R)
6abb
6acb
6aeb
6afb
6agb
6abg’
6ahb
6aib
6abc
6acc
6aec
6afc
6agc
6agc’
6ahc
6aic
6abd
6acd
6aed
6afd
6agd
6agd’
6ahd
6aid
6aga
6aga’
6aha’
6aia
6aia’
6aib’
6aic’
6aid’
Ligands derived from boronic acid 3b
6baa
6bba
6bca
6bea
6bfa
6bga
6bga’
6bha
6bia
52 (R)
57 (R)
58 (R)
51 (R)
65 (R)
27 (R)
23 (R)
21 (R)
41 (R)
30 (R)
6ab
35 (S)
51 (S)
21 (S)
39 (S)
45 (S)
21 (S)
20 (S)
11 (S)
2 (S)
6bac
6bbc
6bcc
6bec
6bfc
6bgc
6bgc’
6bhc
6bic
4 (S)
6bbb
6bcb
6beb
6bfb
6bgb
6bgb’
6bhb
6bib
6bib’
46 (S)
26 (S)
30 (S)
29 (S)
47 (S)
14 (S)
52 (S)
33 (S)
41 (S)
6bia’
0
6bic’
Ligands derived from boronic acid 3c
6caa
6cba
6cca
6cea
6cfa
6cga
6cga’
6cha
6cia
83 (R)
82 (R)
72 (R)
61 (R)
66 (R)
26 (R)
43 (R)
34 (R)
36 (R)
38 (R)
6cab
6cbb
6ccb
6ceb
6cfb
6cgb
6cgb’
6chb
6cib
24 (S)
2 (R)
1 (S)
36 (R)
15 (R)
8 (S)
6cac
6cbc
6ccc
6cec
6cfc
6cgc
6cgc’
6chc
6cic
72 (S)
67 (S)
74 (S)
65 (S)
64 (S)
67 (S)
75 (S)
74 (S)
75 (S)
77 (S)
5 (S)
13 (R)
5 (R)
3 (R)
6cia’
6cib’
6cic’
a
BSA denotes N,O-bis(trimethylsilyl)acetamide. ee was determined by HPLC analysis using a chiral stationary phase.
alkylation of enol carbonate 11 (54–56) was investi-
gated (Figure 9). The stereochemical issues involved
with this transformation are distinct from those of
the benchmark allylic substitution described above,
and relatively few chiral ligands capable of delivering
product 12 in high enantiomeric excess – either by
decarboxylative allylic alkylation or by direct allyla-
tion of the β-ketoester – have been identified (57–
59). In light of previous work by Stoltz and co-work-
ers showing that phosphinooxazolines bearing elec-
tron-deficient aryl groups at phosphorus provided
highest selectivity in such decarboxylative allylic alky-
lations of enol carbonates (58, 59), we evaluated
ligand 13. The requisite bis(trifluoromethyl)phenyl-
substitued β-aminophosphine 5e was synthesized
according to a protocol developed in our laboratory
(60). While not synthetically useful, the 48% ee
obtained with ligand 13 was somewhat encouraging,
given that an extensive screen of ligands and
conditions was needed before a catalyst system
that gave product 12 in 70% ee was identified (58).
However, repeating this reaction using aminopho-
sphine component 5e gave an identical result to
that obtained using the ligand assembly, suggesting
that cleavage of the Bull–James assembly took place
under the conditions of catalysis, and that the ami-
nophosphine component was the active ligand. At
this stage, it is not clear whether a difference in
the reaction conditions or the relative activities of
the iminophosphine versus aminophosphine-based
complexes was responsible for this unsuccessful
application. In any case, the construction of
a library of electron-deficient phosphine iminoboro-
nates for decarboxylative allylic alkylation of 11 was
not pursued further in light of this result.
Another observation highlighting the potential for
disassembly of the iminoboronates under conditions
relevant to transition metal catalysis emerged from our

SUPRAMOLECULAR CHEMISTRY
9
Figure 8. (colour online) Graphs of enantioselectivity data for the library of iminoboronate-based chiral phosphines. The lighter
shading of the bars is used to denote congeners that gave (S)-8 as the major enantiomer in the allylic substitution reaction.
attempts to generate and structurally characterize
a nickel(II) complex (2, 61) of ligand 13 (Figure 10).
After treatment with nickel(II) tetrafluoroborate hexahy-
drate in ethanol, a crystal suitable for X-ray diffraction
analysis was obtained by slow diffusion of diethyl ether
into a solution of the complex in acetonitrile. The crystal-
line material was found to be the 2:1 complex of free
aminophosphine 5e and nickel(II), indicating that ligand
disassembly occurred under the conditions of complexa-
tion. In an attempt to reduce the likelihood of solvolysis

10
H. Y. SU ET AL.
activity and selectivity of complexes generated from the
free aminophosphine when using self-assembled ligands
of the type described here.
3. Conclusion
In conclusion, we have shown that the use of amino-
phosphines as components in the Bull–James supramo-
lecular assembly permits the rapid construction of chiral
phosphine ligands. This three-component condensation
offers several unique advantages for such applications,
including the ability to rapidly generate structural diver-
sity, the ready availability of each of the three compo-
nents, and the relatively high stability of the obtained
assemblies. The structure of each of the three compo-
nents was found to have a significant influence of the
enantiomeric excess obtained using the resulting
ligands in a palladium-catalyzed allylic substitution
reaction. This approach was used to synthesize
a library of 100 chiral phosphine ligands that ranged
in performance from <10% to >90% ee in the bench-
mark reaction. Although some general trends could be
inferred – for example, assemblies derived from alipha-
tic diols gave higher enantioselectivities than those
derived from phenols or hydroxy acids – the data also
revealed examples of ‘interplay’ between the compo-
nents that might not have been evident from
a conventional ligand optimization process. Control
experiments showed that the iminoboronate assem-
blies, and not the free aminophosphines, were respon-
sible for the observed enantioselectivities. However, an
attempted extension to palladium-catalyzed decarbox-
ylative allylic alkylation illustrated the potential for dis-
assembly of the constructs under catalytically relevant
conditions. Further opportunities exist to apply this
approach to the synthesis of chiral ligands or
organocatalysts.
Figure 9. Enantioselective, decarboxylative allylic alkylation of
enol carbonate 11 using assembly 13 and component 5e.
Conversion and ee were determined by HPLC analysis using
a chiral stationary phase.
of the iminoboronate, the complexation experiment was
repeated using acetonitrile as the solvent. Under these
conditions, an iminophosphine oxide resulting from pro-
todeboronation of 13 was obtained (results not
depicted). In any case, both the complexation experi-
ment as well as the results depicted in Figure 9 indicate
that disassembly of the phosphine-bearing iminoboro-
nates can occur in the presence of certain metal com-
plexes or under particular reaction conditions. The
relative activities of the complexes generated from the
aminophosphine and iminophosphine ligands will also
be important in catalytic applications. These results high-
light the importance of control experiments to assess the
CF3
2+
–
Ar Ar
2 BF4
Ar
P
Ni(BF4)2•6H2O
Ar
P
P
(0.5 equiv)
N
Ni
B
O t-Bu
EtOH, 80 °C
N
N
O
t-Bu
t-Bu
H2 H2
CF3
Me Me
13
14
Figure 10. (colour online) Attempted preparation of a Ni(II) complex of ligand 13. The structure of 14 determined by single crystal
X-ray diffraction analysis, with displacement ellipsoids drawn at the 30% probability level, is depicted on the right (tetrafluoroborate
counterions omitted for clarity).

SUPRAMOLECULAR CHEMISTRY
11
Purification by flash chromatography on silica gel (4%
EtOAc/hexanes) gave the product as a colorless oil in
94% yield and 90% ee. Characterization data were con-
3.1. Experimental section
Unless otherwise stated, all reactions and purifications
were carried out under argon atmosphere using
Schlenk, vacuum line, or glovebox techniques in dry,
oxygen-free solvents. Flash column chromatography
was carried out using 35–75 μm particle size silica gel.
Tetrahydrofuran (THF), diethyl ether, dichloromethane,
toluene and hexanes were dried using a solvent purifi-
cation system and degassed through three freeze-
pump-thaw cycles. Deuterated solvents were degassed
through three freeze-pump-thaw cycles. All commer-
cially available reagents and chemicals were used as
received without purification unless noted otherwise.
Allylic acetate rac-7 (62), enol carbonate 11 (63) and
aminophosphines 5a (44), 5b (45), 5c (46), and 5e (60)
were synthesized according to published protocols.
A 400 MHz spectrometer was employed for record-
ing 1H (400 MHz), 11B (128 MHz) and 31P (162 MHz) NMR
1
sistent with reported values (64). H NMR (400 MHz,
CDCl3): δ 7.31–7.12 (m, 10H), 6.44 (d, J = 15.7 Hz, 1H),
6.30 (dd, J = 15.7, 8.5 Hz, 1H), 4.23 (ddd, J = 11.0, 8.5,
0.8 Hz, 1H), 4.13 (q, J = 7.1 Hz, 2H), 3.97–3.90 (m, 2H),
3.88 (d, J = 11.0 Hz, 1H), 1.17 (t, J = 7.1 Hz, 3H), 0.97 (t,
J = 7.1 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ 168.0,
167.5, 140.4, 137.0, 131.8, 129.5, 128.8, 128.6, 128.1,
127.6, 127.2, 126.5, 61.7, 61.5, 57.9, 49.4, 14.3, 13.9.
Optical rotation: [α]D = +10.7 (c = 0.13 g/mL, CHCl3).
3.1.3. Procedure for decarboxylative allylic
alkylation of 11
A 0.5-dram vial equipped with a magnetic stirring bar was
charged with stock solutions of 2-formylphenylboronic
acid (0.10 M, 200 μL, 0.02 mmol), pinacol (0.10 M,
200 μL, 0.02 mmol) and 5e (0.25 M, 80 μL, 0.02 mmol).
The solvents were removed in vacuo. CHCl3 (50 μL) was
added under a positive pressure of argon. The reaction
was stirred at 50°C for 30 min. The volatiles were then
removed under high vacuum. Pd2(dba)3 (4.6 mg,
5.0 μmol) was added in a nitrogen-filled glovebox. 2:1
Pentane/toluene (50 μL) was added, and the reaction
stirred at 23°C for 30 min. Allylic carbonate 11 (25.0 mg,
0.1 mmol) was added under a positive pressure of argon.
The reaction was stirred at 23°C for 24 hours. The crude
reaction mixture was then filtered through a short plug of
Celite and rinsed three times with CH2Cl2. An aliquot of
the reaction mixture (5.0 μL) was used for conversion and
ee analyses using HPLC. HPLC conditions: Chiralpak
IA, AD-H and IB (three columns in series), 0.5% isopropa-
nol/hexanes, 210 nm, 0.5 mL/min, tr: 38.7 and 40.6 min.
(S)-12: Purification by flash chromatography on silica
gel (5% Et2O/hexanes) gave the product in 48% ee as
a colorless oil. Characterization data were consistent with
reported values (57). 1H NMR (400 MHz, CDCl3): δ
5.83–5.65 (m, 1H), 5.11–4.98 (m, 2H), 4.18 (q, J = 7.1 Hz,
2H), 2.60 (ddt, J = 14.0, 7.0, 1.2 Hz, 1H), 2.55–2.40 (m, 3H),
2.33 (ddt, J = 13.9, 7.8, 1.1 Hz, 1H), 2.08–1.93 (m, 1H),
1.82–1.57 (m, 3H), 1.45 (m, 1H), 1.24 (t, J = 7.1 Hz, 3H).
13C NMR (100 MHz, CDCl3): δ 207.6, 171.5, 133.3, 118.3,
61.2, 60.9, 41.1, 39.3, 35.8, 27.5, 22.5, 14.2. Optical rota-
tion: [α]D = – 75.1 (c = 0.20 g/mL, CHCl3).
1
spectra at ambient temperature. H chemical shifts are
reported in ppm relative to tetramethylsilane, and were
measured by referencing the spectra to residual pro-
tium in the solvent. 11B NMR spectra were referenced to
BF3•OEt2 (δ = 0 ppm), and 31P NMR spectra were refer-
enced to 85% H3PO4 in H2O (δ = 0 ppm).
3.1.1. General procedure for assembly of ligands
and evaluation in the allylic substitution of rac-7
A 0.5-dram vial equipped with a magnetic stirring bar was
charged with stock solutions of 2-formylphenylboronic
acid (0.10 M, 200 μL, 0.02 mmol), pinacol (0.10 M, 200 μL,
0.02 mmol) and 5c (0.25 M, 80 μL, 0.02 mmol). The solvents
were removed in vacuo. CHCl3 (50 μL) was added under
a positive pressure of argon. The reaction was stirred at 50°
C for 30 min. The volatiles were then removed under high
vacuum. [Pd(allyl)Cl]2 (0.13 M in CHCl3, 40 μL, 5.0 μmol)
was added under a positive pressure of argon and the
reaction stirred at 23°C for 15 min. Allyl acetate rac-7
(0.01 M in CHCl3, 10 μL, 0.10 mmol), bis(trimethylsilyl)
acetamide (49 μL, 0.20 mmol), diethyl malonate (30 μL,
0.20 mmol) and potassium acetate (0.50 mg, 5.0 μmol)
were added sequentially. The reaction was stirred at 23°C
for 24 hours. The crude reaction mixture was then filtered
through a short plug of Celite and rinsed three times with
CH2Cl2. An aliquot of the reaction mixture (5.0 μL) was
used for conversion and ee analyses using HPLC. HPLC
conditions: Chiralpak IA column, 2.0% isopropanol/hex-
anes, 0.5 mL/min, 254 nm, tr: 22.9 and 29.2 min.
3.1.4. Synthesis and recrystallization of complex 14
A 2-dram vial equipped with a magnetic stirring bar
was charged with 2-formylphenylboronic acid (30.0 mg,
0.2 mmol), 5e (84.0 mg, 0.2 mmol) and neopentyl glycol
(21.0 mg, 0.2 mmol). CHCl3 (50 μL) was added under
a positive pressure of argon. The reaction was stirred at
50°C for 30 min. The volatiles were then removed under
3.1.2. Diethyl (R)-2-(1,3-diphenylallyl)malonate
((R)-8)
The protocol described above was carried out on
0.25 mmol scale of rac-7, using ligand 6aba.

12
H. Y. SU ET AL.
high vacuum. Ni(BF4)2•6H2O (34.0 mg, 0.1 mmol) was
added in a nitrogen-filled glovebox. EtOH (2.5 mL) was
added under an inert atmosphere. The reaction was
sealed and stirred at 80°C for 30 min. The volatiles
were removed under high vacuum. A crystal suitable
for X-ray diffractometry (m.p. 143–147°C) was obtained
by diffusion of diethyl ether vapor into a solution of 14
in acetonitrile.
Single-crystal X-ray diffraction data were collected at
147 K with a Bruker Apex-II CCD diffractometer with
Mo-Kα radiation (λ = 0.71073 Å). The structure was
solved and refined using SHELXL2013 (65). Refinement
was by full-matrix least-squares on F2 using all data. The
structure has been deposited to the Cambridge
Crystallographic Database (CCDC 1869791).
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Dr. Alan J. Lough (University of Toronto) is gratefully acknowl-
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experiment and solving the structure of 14.
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Disclosure statement
No potential conflict of interest was reported by the authors.
(21) Wang, W.; Gao, X.; Wang, B. Curr. Org. Chem. 2002, 6,
1285–1317. DOI: 10.2174/1385272023373446
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Funding
Acknowledgment is made to the donors of the American
Chemical Society Petroleum Research Fund for partial support
of this research. Funding was also provided by NSERC
(Discovery Grants and Canada Research Chairs Programs)
and the Canada Foundation for Innovation (Project nos.
17545 and 19119).
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