Influence of phosphoramidites in copper-catalyzed conjugate borylation reaction

Article abstract of DOI:10.1021/om300194k

Copper(I) has become the preferred metal to catalyze the β-boration of α,β-unsaturated carbonyl compounds, and now we demonstrate that easily accessible monodentate chiral ligands, such as phosphoramidites and phosphites, can be convenient alternative ligands to induce asymmetry in the enantioselective version of this reaction, particularly in the β-boration of α,β-unsaturated imines.

Full text of DOI:10.1021/om300194k

Article
pubs.acs.org/Organometallics
Influence of Phosphoramidites in Copper-Catalyzed Conjugate
Borylation Reaction
Cristina Sole,† Amadeu Bonet,† Andre H. M. de Vries,‡ Johannes G. de Vries,‡ Laurent Lefort,*,‡
́
Henrik Gulyas,*,† and Elena Fernandez*,†
́
́
†Departamento de Química Física i Inorgan
̀
ica, University Rovira i Virgili, C/Marcel·lí Domingo s/n, 43007 Tarragona, Spain
‡DSM Innovative Synthesis B.V., P.O. Box 18, 6160 MD Geleen, The Netherlands
S
* Supporting Information
ABSTRACT: Copper(I) has become the preferred metal to
catalyze the β-boration of α,β-unsaturated carbonyl compounds,
and now we demonstrate that easily accessible monodentate chiral
ligands, such as phosphoramidites and phosphites, can be
convenient alternative ligands to induce asymmetry in the
enantioselective version of this reaction, particularly in the β-
boration of α,β-unsaturated imines.
Scheme 1. Representative Examples of Copper-Mediated
INTRODUCTION
■
Enantioselective β-Boration of α,β-Unsaturated Esters and
The progress in the direct enantioselective construction of α-
chiral boranes through the asymmetric conjugate borylation
reaction is motivated by the increased interest in difuntional-
ized organoboron compounds in medicine and organic
synthesis.1 Several transition metals catalyze the β-boration of
α,β-unsaturated carbonyl compounds, (Pt,2 Rh,3 and Ni4), but
copper has emerged as the most convenient and efficient metal
to mediate the chemoselective formation of β-borated esters,
ketones, nitriles, and amides,5a,b from previous work by
Miyaura and Hosomi.5c−e Importantly, the first attempt to
induce asymmetry in the β-boration of α,β-unsaturated
carbonyl compounds was made by Yun and co-workers using
a copper(I) salt modified with chiral diphosphines.6 The results
suggested that ligands with combined planar and central
chirality such as Josiphos or Mandyphos performed better than
axially chiral ligands based on biaryl backbones (Scheme 1a).
The enantiofacial differentiation in the conjugate borylation of
cyclic β,β-disubstituted unsaturated ketones was achieved by
Shibasaki and co-workers using copper(I) complexes modified
with diphosphines with stereogenic phosphorus.7 The same
authors found that chiral diamine ligands could also efficiently
transfer the chiral information in the asymmetric conjugate
borylation of acyclic α,β-unsaturated acceptors (Scheme 1b).8
Our research group has successfully applied copper complexes
of hemilabile P−N ligands (Scheme 1c)9 and copper complexes
of chiral N-heterocyclic carbene (NHC) ligands10 in the
enantioselective β-boration of α,β-unsaturated esters. Further
examples of chiral NHC ligands also delivered excellent results
in the copper(I)-mediated boron addition reactions of a variety
of α,β-unsaturated carbonyl compounds (Scheme 1d).11
Despite the fact that other metals modified with chiral
diphosphines have been postulated to be efficient catalytic
systems for the enantioselective β-boration reaction (Pd, Ni,12
a
Ketones in the Presence of Chiral Ligands
a
B(pin) = pinacolboryl unit.
and Rh13), the copper(I) complexes are still the most attractive
and economic enantioselective metal catalysts.
Our group has also identified several chiral phosphorus
ligands, such as Taniaphos and Josiphos, which induced
exceptional enantioselectivities in the β-boration of α,β-
unsaturated imines, establishing a simple one-pot three-step
synthetic route toward chiral γ-amino alcohols (Scheme 2).14
In our quest to develop both efficient and cost-effective
catalytic systems for the asymmetric conjugate borylation
reaction, we decided to investigate the possible application of
very cheap phosphoramidite ligands in this copper-mediated
Special Issue: Copper Organometallic Chemistry
Received: March 9, 2012
Published: April 26, 2012
© 2012 American Chemical Society
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any systematic study carried out from this point of view in the
area of catalytic boron conjugate addition reaction. Since the
beginning of the century, there has been a revival of the
applications of chiral monodentate phosphorus ligands in
homogeneous catalysis.15 Among all classes of chiral mono-
phosphorus ligands, phosphoramidites stand out as very cheap,
easy-to-synthesize, structurally highly diverse, and chemically
resistant compounds. Yun and co-workers briefly mentioned
that in the β-boration of α,β-unsaturated esters and nitriles they
had obtained incomplete conversions and very low enantio-
meric excesses (<7%) with copper(I) complexes modified with
binaphthol-derived phosphoramidites.6 Nevertheless, we rea-
soned that using a high-throughput experimentation, combined
with the large set of diverse chiral phosphoramidites and
phosphites accessible by DSM,16 a wider ligand screening could
Scheme 2. One-Pot Synthesis of γ-Amino Alcohols by
Copper-Mediated Enantioselective β-Boration of α,β-
Unsaturated Imines Followed by Diastereoselective CN
a
Reduction and C−B Oxidation
a
B(pin) = pinacolboryl unit.
reaction. Although the price of the chiral ligand is obviously an
important factor in the cost of the catalyst, we are not aware of
Chart 1. Small Library of Chiral Phosphoramidite and Phosphite Ligands for the Copper-Mediated Enantioselective β-Boration
of α,β-Unsaturated Esters, Aldehydes, and Nitriles
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Scheme 3. Parallel Ligand Synthesis and Their High Throughput Screening in the Copper-Mediated β-Boration Reaction Using
a Zinsser Lissy Robot
Table 1. Thirty-Two Parallel Copper(I)-Mediated β-Borations of Isboutyl Crotonate
be carried out, hopefully leading to a more enantioselective
copper catalytic system. Additionally, this effort could provide
useful information on ligand structure−catalytic performance
relationships.
respectively. The reacting amine or triethylamine is used as acid
scavenger, and, as the only purification step, the formed
hydrochloride is removed by filtration. Up to 32 chiral ligands
were tested in parallel in the asymmetric copper-catalyzed β-
boration of three different substrates: isobutyl crotonate,
cinnamaldehyde, and cinnamonitrile as representative sub-
strates for α,β-unsaturated ester, aldehyde, and nitrile,
respectively. Through this set of 96 experiments, we expected
to identify the structural features of the ligands that are the
most relevant to achieve high enantioselectivities in the β-
boration of the different activated olefins. The phosphorami-
dites and phosphites included in this study can be divided in
several groups according to their structure (Chart 1).
RESULTS AND DISCUSSION
■
The initial screening was carried out at DSM Innovative
Synthesis B.V. (Geleen, The Netherlands), and all chiral ligands
included in this study were prepared according to the novel
concept developed by DSM for parallel synthesis of ligand
libraries.16 This concept involves the synthesis of chiral
phosphoramidites17 and phosphites in one step from
chlorophosphites and the corresponding amine or alcohol,
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Table 2. Thirty-Two Parallel Copper(I)-Mediated β-Borations of Cinnamaldehyde
The ligands prepared were mainly phosphoramidites having a
binaphthyl or a partially hydrogenated H8-binaphthyl chiral
core (C2−C6). Most were prepared from secondary amines,
but primary amine derivatives (C3, C7, C8, D1) were also
included in the study. Two of these phosphoramidites, C1 and
C7, were prepared from aminophosphines and, thus, contain
diphenylphosphanyl functionalities. They were considered to
be chelating ligands. The remainder, despite the presence of
hard donor atoms in some cases, were regarded as
monodentate ligands. Some of the amines were chiral; thus,
the corresponding phosphoramidites have both axial and
central chirality (C8, D1−D4). Since taddol-derived mono-
dentate phosphorus ligands had been successfully used in
asymmetric Rh-hydroboration of styrene derivatives18 and Pd,
Pt enantioselective diboration of allenes and alkenes,19 we also
included the TADDOL-derived phosphoramidites D5 and D6
in the study. Although, in general, phosphites are more sensitive
to protic solvents than phosphoramidites, we decided to test
two binaphthol-derived phosphites (D7 and D8) in order to
compare P−N versus P−O linkages in the monodentate
ligands.
The first 96 reactions were performed in parallel.
CuOTf·4CH3CN was used as a copper(I) source instead of
CuCl due to its complete solubility in THF (Scheme 3). Stock
solutions of all reagents were prepared in THF, and an aliquot
of a stock solution of CuOTf·4CH3CN was dispensed into 96
(5 mL) vials with a liquid handling robot. Two equivalents of
ligands relative to Cu were used in the case of monodentate
phosphoramidites and phosphites, while one equivalent was
used in the case of the two bidentate phosphoramidite-
phosphine ligands C1 and C7. After 20 min of stirring, a freshly
prepared stock solution of a mixture of 3 mol % NaOtBu
relative to substrate, 1.1 equiv of bis(pinacolato)diboron
(B2pin2), and 2 equiv of methanol was dispensed into all 96
vials, followed by the stock solutions of the three substrates
(isobutyl crotonate, cinnamaldehyde, and cinnamonitrile) with
a substrate/catalyst ratio of 50. The β-boration reaction of
isobutyl crotonate and cinnamonitrile was carried out at room
temperature, and the reaction of the aldehyde was carried out at
70 °C, to favor the β-boration versus the 1,2-diboron
addition.10,20
Despite the relatively short reaction times (4 h), most of the
copper(I) catalytic systems modified with the phosphorami-
dites provided good conversions in the β-boration of isobutyl
crotonate (Table 1). Copper(I) modified with phosphoramidite
ligands derived from primary amines (C3, C7, C8, and D1)
provided only moderate conversions. Surprisingly, copper(I)
modified with bidentate ligands, phosphoramidites-phosphines
(C1, C7), gave similar but lower conversions (60−67%). The
substituents on the binol backbone of the phosphoramidites
exerted a subtle influence on the activity of the copper-
mediated β-boration of the α,β-unsaturated ester. Among the
3,3′-substituted ligands (B4, B5, and B8) the methyl- and
methoxycarbonyl-substituted derivatives allowed obtaining
higher conversions (98% and 96%) than the bromo-substituted
analogue (86%). Interestingly, 4,4′-bromo-substitution in ligand
B6 resulted in an even more dramatic decrease of the activity
(46%).
The enantioselectivity provided by the set of ligands was low
in general. Nevertheless, three results should be highlighted, as
they are well above the rest. Moderate enantioselectivies
(around 50% ee) were achieved when the copper(I) precursor
was modified with phosphoramidites with a taddol backbone
(5D, 6D). Ligand C3, based on 3,3′-dimethyl binol and amino-
pyridine, also provided a higher than average enantioselectivity
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Table 3. Thirty-Two Parallel Copper(I)-Mediated β-Borations of Cinnamonitrile
(41%). Enantiomeric excesses observed for the rest of the
ligands did not exceed 30%.
Reproducibility of these reactions was checked at slightly
higher scale with Schlenck-type techniques, obtaining com-
parable results in conversions and enantioselectivities. We also
explored the influence of the ligand to metal ratio, and we
observed that upon changing the ligand:copper ratio from 2:1
to 1:1, the enantioselectivities decreased in most cases. For
example, in the β-boration of isobutyl crotonate with
copper(I)-D6 the enantiomeric excess decreased from 51%
(when L:Cu = 2:1) to 40% (when L:Cu = 1:1). Also we
became interested in knowing more about the influence of
solvent on the reaction outcome, and we observed that upon
replacing THF with toluene, the enantiomeric excesses in the
β-boration of cinnamonitrile could be slightly improved (from
28% to 43% with Cu(I)-C8 and from 31% to 44% with Cu(I)-
D1).
In the case of the β-boration of α,β-unsaturated aldehydes
the issue of chemoselectivity must be taken into consideration,
since 1,2-addition of B2pin2 to the carbonyl group could take
place under the reaction conditions.20,21 Nonetheless, more
than half of the copper(I)-mediated β-borations of cinnamal-
dehyde (20/36) gave conversions higher than 80% (Table 2).
We were pleased to see that two of the catalysts containing
ligands A2 and B4 provided around 90−93% of conversion.
Since the reactions were carried out at 70 °C, all the
experiments showed total chemoselectivities toward the 1,4-
addition. Enantioselectivites were poor in most cases, but
interestingly, the best enantiomeric excesses were obtained
again with copper(I) complexes modified with taddol-derived
phosphoramidites (35% and 37% ee), as observed in the
asymmetric β-boration of isobutyl crotonate (Table 2). These
values are similar to the only example described so far in the
literature for the β-boration of cinnamaldehyde, whereby 40%
ee was obtained using copper(I) complexes modified with
chiral N-heterocyclic carbene ligands.10
We next focused on the β-boration of α,β-unsaturated imines
with CuOTf·4CH3CN modified with some of the best
performing phophoramidite ligands in the presence of B2pin2,
encouraged by the results of earlier studies carried out in our
group (Scheme 4).14a Table 4 shows that copper(I) complexes
The copper(I)-catalyzed asymmetric β-boration of α,β-
unsaturated nitriles was also studied using cinnamonitrile as
the model substrate (Table 3). In general, the average
conversion into the corresponding β-borated product was
lower than in the case of isobutyl crotonate and cinnamalde-
hyde. Nevertheless, copper(I) complexes modified with
phosphoramidites A1, A2, A3, A5, B4, C2, and D2 provided
>90% conversions. The asymmetric induction in the β-boration
of cinnamonitrile was also moderate. The best performing
ligands were phosphoramidites with an H8-binaphthyl chiral
core (C3: 54% ee, C4: 41% ee, and C5: 45% ee) and the
monophosphite prepared from binaphthol and phenol (D7:
44% ee).
Scheme 4. Copper(I)-Mediated β-Boration of α,β-
Unsaturated Imines and Further Functionalization
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to be able to compare them directly with those obtained in the
high-throughput experimentation previously described, we
found that in most of the experiments the catalytic activity
was maintained, although in the case of the β-boration of 9b the
conversions decreased significantly (Table 4, entries 12−16).
However, the enantiomeric excesses remained similar to what
was found for the β-boration of 9b with Cu(I)-E1 (92% ee,
Table 4, entry 15). It is interesting to note that while the
copper(I)-E1 catalytic system provided high enantiomeric
excesses, the analogous copper(I)-E2 catalytic system induced
much lower enantioselectivity (Table 4, entries 11, 16, 21).
However, the catalytic system based on copper(I)-B3 was both
very active and also very enantioselective, with ee values up to
90% in the β-boration of 9b and 9c (Table 4, entries 14, 19).
Table 4. Asymmetric Copper(I)−L* (L* = D5, D6, B3, E1,
and E2)-Mediated β-Boration of α,β-Unsaturated Imines
with B2pin2
CONCLUSIONS
■
A library of monodentate phosphoramidite ligands has been
screened in copper(I)-catalyzed β-boration of isobutyl
crotonate, cinnamaldehyde, and cinnamonitrile. Many of the
ligands form highly active catalysts. The enantioselectivities
significantly exceed the results previously reported in the
literature for phosphoramidites, but do not reach values needed
for industrial applications. However, the screening of this large
library of ligands allowed us to identify certain structural
elements consistently present in the best performing ligands,
such as the taddol backbone in the case of the ester and the
aldehyde. The best performance of the phosphoramidites has
been observed in the copper(I)-catalyzed β-boration of α,β-
unsaturated imines. In addition to very high conversions into
the desired β-borated products, the enantioselectivites also
exceeded 90% when phosphoramidites D5, B3, and E1 were
used to modify the copper catalyst. The β-boration of α,β-
unsaturated imines is the key step in a new methodology for the
synthesis of enantio- and diastereopure γ-amino alcohols. The
development of efficient and cost-effective copper catalysts
might be a great step toward the application of this method on
an industrial scale.
a
β-Boration conditions: 0.2 mmol of substrate, 2 mol %
CuOTf·4CH3CN, 4 mol % ligand, B2pin2 (1.1 equiv), NaOtBu (9
b
mol %), MeOH (2 equiv), THF (1 mL), 25 °C, 6 h. Reference 14a.
ASSOCIATED CONTENT
■
c
Conversion and ee given on the γ-amino alcohol, reduction/
oxidation: 3.0 equiv of reducing agent DIBAL-H, followed by the
addition of NaOH/H2O2 (aqueous) in excess, syn/anti ratio = 99:1.
β-Boration conditions: 0.25 mmol of substrate, 2 mol %
S
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
d
CuOTf·4CH3CN, 4 mol % ligand, B2pin2 (1.1 equiv), NaOtBu (3
mol %), MeOH (2 equiv), THF (1 mL), 25 °C, 4 h.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mariaelena.fernandez@urv.cat.
Notes
■
modified with ligands D6 and E1 were able to efficiently β-
borate the imines (E)-1-phenyl-N-(4-phenylbut-3-en-2-
ylidene)methanamine (9a) (Table 4, entries 1, 2) and (E)-N-
(4-phenylbut-3-en-2-ylidene)aniline (9b) (Table 4, entries 3,
4), with good to excellent enantioselectivities, within 6 h at
room temperature. The conversion and the enantioselectivity of
the α,β-unsaturated imine (E)-N-(4-phenylbut-3-en-2-ylidene)-
butan-1-amine (9c) were determined by the analysis of the
corresponding γ-amino alcohol14a (Table 4, entries 5, 6). It is
worth mentioning that, in general, quantitative conversions and
high enantioselectivities were observed. A particularly interest-
ing example is the β-boration of imine 9b with copper(I)
modified with the phosphoramidite E1 (Table 4, entry 4),
generating the β-borated product in quantitative conversion
and up to 95% ee.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank MEC for funding (CTQ2010-16226). C.S. acknowl-
edges the FPU grant; A. Bonet, an FPI grant. We thank
AllyChem for the gift of bis(pinacolato)diboron.
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