Nonracemic allylic boronates through enantiotopic-group-selective cross-coupling of geminal bis(boronates) and vinyl halides

Article abstract of DOI:10.1021/ja510266x

Under the influence of a chiral palladium catalyst, 1,1-bis(pinacolboronate) esters undergo asymmetric cross-coupling with bromoalkenes to generate nonracemic allyl boronates with high levels of enantioselectivity. The so-formed allyl boronates may be oxidized with hydrogen peroxide to provide secondary allylic alcohols or with nitrosobenzene to furnish nonracemic tertiary allylic alcohols. Mechanistic experiments suggest the operation of a pathway involving outer-sphere stereoinvertive transmetalation.

Full text of DOI:10.1021/ja510266x

Communication
pubs.acs.org/JACS
Nonracemic Allylic Boronates through Enantiotopic-Group-Selective
Cross-Coupling of Geminal Bis(boronates) and Vinyl Halides
Bowman Potter, Adam A. Szymaniak,† Emma K. Edelstein,† and James P. Morken*
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States
S
* Supporting Information
strategy that provides α-chiral allylic boronates, but this method
ABSTRACT: Under the influence of a chiral palladium
catalyst, 1,1-bis(pinacolboronate) esters undergo asym-
metric cross-coupling with bromoalkenes to generate
nonracemic allyl boronates with high levels of enantiose-
lectivity. The so-formed allyl boronates may be oxidized
with hydrogen peroxide to provide secondary allylic
alcohols or with nitrosobenzene to furnish nonracemic
tertiary allylic alcohols. Mechanistic experiments suggest
the operation of a pathway involving outer-sphere
stereoinvertive transmetalation.
can furnish γ,γ-disubstituted boronates with regio- and
stereocontrol only when cyclic substrates are employed (eq
2).3,4 Indeed, only the Aggarwal homologation reaction
furnishes chiral γ,γ-disubstituted allylic boronates for a range
of substrates in an asymmetric fashion.5 To address this gap in
catalytic synthesis technology, we considered cross-coupling of
vinyl electrophiles and readily available achiral geminal
bis(boronates) (eq 3).6−8 We recently reported that this
transformation can be accomplished with aryl electrophiles in
the presence of a Pd catalyst and a chiral monodentate
phosphoramidite ligand: upon enantiotopic-group-selective
cross-coupling, chiral benzylic boronates are generated.9 Herein
we report a new catalyst system that enables the reaction
between vinyl electrophiles and geminal boronates and allows
rapid access to enantiomerically enriched allylic boronates.
Additionally, we provide insight into the mechanistic features
that govern this transformation and delineate strategically
useful applications of the allylic boronates for the construction
of structurally challenging chiral motifs.
Our initial investigations focused on ligand selection for the
enantiotopic-group-selective cross-coupling of geminal bis-
(boronate) 1 and 1-bromo-2-methylpropene. Unlike the
cross-coupling of aryl electrophiles, the background reaction
in the absence of ligand was sluggish (Table 1, entry 1), and the
monodentate phosphoramidite ligand L1 that was effective in
aryl halide couplings failed to convert 1 to allylic boronate 3
(entry 2). Fortunately, JosiPhos ligand L2 afforded the desired
product, albeit in low yield, low enantioselection, and
contaminated by significant amounts of 1,4-diene 4 that
appeared to arise from coupling of boronate 3 and the vinyl
halide. A survey of related Josiphos-type ligands revealed that
alkyl substitution on the Cp-bound phosphine (R2) minimized
the yield of biscoupling product 4 and increased the
enantioselectivity (entry 3 vs 4). Further enhancing the size
of the alkyl groups from Cy to tert-Bu (L4) had a profound
impact on the reaction (entry 5): not only were the biscoupling
byproducts substantially minimized, but the enantioselectivity
of the reaction was enhanced from 65:35 to 90:10 (cf. entries 4
and 5). The nature of the benzylic phosphine substituents (R1)
also influenced the outcome of the reaction: increasing the size
from phenyl to o-tolyl resulted in lower yield and selectivity
(entries 5 and 6), whereas p-trifluoromethylphenyl groups
provided a higher-yielding transformation with a slight increase
in selectivity (entry 7; for an exhaustive ligand optimization, see
onracemic allyl boronates are valuable intermediates in
Norganic synthesis. In particular, α-chiral γ,γ-disubstituted
allylic boronates are attractive reagents: not only can they be
directly converted to chiral γ,γ-disubstituted allylic alcohols and
amines by oxidation and amination, but they can also engage in
carbonyl allylations that establish chiral all-carbon quaternary
centers.1 However, in spite of their ability to address important
synthesis problems, there remain few catalytic enantioselective
methods for the construction of α-chiral allylic boronates that
are doubly substituted at the γ position. Indeed, only two
methods have been developed to construct this important
motif, and they both result in specialized product functionality.
In a strategy developed by our lab,2 enantioselective 1,2-
diboration of 4,4-disubstituted-1,3-dienes (Scheme 1, eq 1)
results in a vicinal diboronate in which the allyl boronate unit
bears two γ substituents. Allylic borylation is an alternate
Scheme 1. Catalytic Enantioselective Routes to γ,γ-
Disubstituted Allyl Boronates
Received: October 6, 2014
© XXXX American Chemical Society
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Table 1. Survey of Ligands in the Cross-Coupling of
Bis(boronate) 1 and 1-Bromo-2-methylpropene
Table 2. Substrate Scope in the Asymmetric Cross-Coupling
of Vinylic Halides and Geminal Bis(boronates)
a
a
b
b
c
entry
ligand
2 (% yield)
3 (% yield)
er 3
1
2
3
4
5
6
7
none
L1
<2
<2
27
13
<2
<2
<2
<2
<2
−
−
57:43
65:35
90:10
63:37
91:9
<2
L2
21
L3
49
L4
55
L5
14
L6
95
d
8
L6
95 (86)
93:7
a
b
See the Supporting Information for the reaction conditions. Yields
were determined by NMR analysis in comparison to an internal
standard and may include small amounts of the regioisomeric 1,4-
c
dienes. Enantiomer ratios (er) were determined by chiral SFC
d
analysis of the corresponding secondary alcohols. With 1 mol % L6·
PdCl2.
the Supporting Information). To simplify the experimental
setup, the palladium dichloride complex with ligand L6 was
prepared, and a crystal structure is depicted in Figure 1.
a
See the Supporting Information for the reaction conditions. Yields
are isolated yields of purified materials and are averages of two
experiments. er values were determined by chiral SFC analysis of the
b
corresponding secondary allylic or homoallylic alcohols. Yield
1
determined by H NMR analysis versus an internal standard.
compounds 6−8. Of note, the reaction tolerated the presence
of a silyl ether (7), aryl chloride (9), and amide (10).
Moreover, even though Pd catalysis of alkyl bromide
dehydrohalogenation can be efficient,10 the cross-coupling
process described here is quite tolerant of an alkyl bromide
substituent (compound 8). With regard to the electrophile,
vinyl bromides with differentiated β substituents underwent
smooth coupling (compounds 11, 12, and 14−16), and a cyclic
vinyl bromide could also be employed effectively (product 13).
In the case of 2-monosubstitued vinyl bromides, only a
substrate with a bulky tert-butyl group afforded useful levels of
monoaddition (product 16); with the current catalyst system,
smaller monosubstituted vinyl bromides afford significant
amounts of biscoupling products (data not shown).
As mentioned above, chiral allyl boronates participate in a
range of useful reactions. To probe whether these apply to
unpurified products that arise from enantiotopic-group-
selective cross-couplings, the transformations in Scheme 2
were conducted. In the first experiment, geminal bis(boronate)
1 and 1-bromo-2-methyloctene were linked by cross-coupling
in the presence of 1 mol % L6·PdCl2 complex. After filtration
and a solvent swap, oxidation with NaOH/H2O2 furnished
allylic alcohol 17 in excellent yield and enantiomeric purity. In a
second experiment, addition to benzaldehyde was examined:
Figure 1. Crystal structure of L6·PdCl2. The structure was obtained
from a crystal of the racemic complex in the presence of CH2Cl2.
Coordinates have been deposited at the Cambridge Crystallographic
Data Centre under CCDC 1035172.
Importantly, when air-stable L6·PdCl2 was used as the catalyst,
lower catalyst loadings could be employed and the reaction
exhibited a slight improvement in selectivity (entry 8). Of
paramount importance, with L6·PdCl2, the reactions could be set
up without the aid of a drybox and required only 1 mol % catalyst
for effective reaction.
The scope of the enantiotopic-group-selective cross-coupling
in the presence of L6·PdCl2 was examined with a variety of
vinyl bromides and geminal bis(boronates) (Table 2). While
phenyl-containing substrates were processed effectively and
provided reaction products 3, 4, and 5, aromatic substituents
were not required, as demonstrated by the production of
B
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Journal of the American Chemical Society
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Scheme 2. Tandem Enantioselective Cross-Coupling/
Functionalization Cascades
Scheme 4. Analysis of the Stereochemistry of
Transmetalation in Cross-Couplings with L6·PdCl2
following cross-coupling, filtration, and a solvent swap into
toluene, benzaldehyde was added. As shown in Scheme 2, this
procedure provided homoallylic alcohol 18 bearing an adjacent
all-carbon quaternary center with complete enantiospecificity.11
Lastly, it was found that oxidation with nitrosobenzene, a
reaction that occurs with allyl migration,12 directly furnished
tertiary allylic alcohol 19 with excellent enantiomeric purity.
To study the ability of this catalytic cross-coupling to deliver
synthetically useful quantities of material, the reaction in
Scheme 3 was undertaken. In this experiment, the alkylation of
occur with inversion.14 To learn more about boron species
involved in this transmetalation, the pinacol ligands in 1 were
replaced with neopentylglycol or 2,4-dimethylpentane-2,4-diol
ligands. With these alternate boron ligands, lower reactivity and
lower selectivity were observed in the cross-coupling with 1-
bromo-2-methylpropene (18% 1H NMR yield, 85:15 er for the
1
former; 13% H NMR yield, 89:11 er for the latter). These
experiments suggest that the active participant in the cross-
coupling is less likely to be the bis(boronic acid)-derived from 1
but is more likely to be the mono- or bis(boronic ester)
derivative.
In summary, we have described a catalytic enantioselective
cross-coupling that delivers chiral γ,γ-disubstituted allylic
boronates from simple substrates under mild reaction
conditions. The reaction products can be used to address
important problems in asymmetric synthesis, and further
studies in this regard are in progress.
Scheme 3. Gram-Scale Enantioselective Cross-Coupling To
Furnish Allyl Boronate 3
ASSOCIATED CONTENT
* Supporting Information
Procedures, characterization, and spectral data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
(pin)BCH2B(pin) was conducted on a 5 mmol scale by
deprotonation with LiTMP and subsequent treatment with (2-
bromoethyl)benzene. This delivered 1 in good yield and
provided ample material to examine larger-scale cross-
couplings. As depicted, the coupling with 1-bromo-2-methyl-
propene occurred effectively on a larger scale and furnished α-
chiral allyl boronate 3 in 71% yield with 93:7 er. It should be
noted that the product contained material derived from
protodeboronation of 1, and while this is difficult to remove
on large scale, it does not interfere with subsequent allylation or
oxidation reactions.
S
AUTHOR INFORMATION
Corresponding Author
morken@bc.edu
■
Author Contributions
†A.A.S. and E.K.E. contributed equally.
Notes
The authors declare no competing financial interest.
Our previous studies of enantiotopic-group-selective cross-
coupling of aryl electrophiles employed a monodentate chiral
phosphoramidite ligand, and it was determined that these
reactions occurred with inversion of configuration at carbon
during the cross-coupling process. Considering that L6·PdCl2
contains a bidentate ligand, it was of interest to determine
whether the reaction in the presence of this complex occurs
with the same stereochemical outcome. To study this aspect,
(S)-10B-20 was prepared13 and subjected to cross-coupling with
1-bromo-2-methylpropene in the presence of both (S,R)-L6·
PdCl2 and (R,S)-L6·PdCl2 (Scheme 4). Mass spectral analysis
of the product from the reaction with the (S,R) complex was
consistent with a process in which the 10B-labeled B(pin) group
remained in the product, whereas the natural-abundance B(pin)
remained in the product when the (R,S) complex was
employed. If it is assumed that the reductive elimination
occurs with retention of configuration at carbon, the trans-
metalation reaction in the presence of L6·PdCl2 appears to
ACKNOWLEDGMENTS
■
We thank Dr. Bo Li for assistance with crystal structure
analysis. The NIH (GM-59471) is acknowledged for financial
support; AllyChem is acknowledged for a donation of B2(pin)2.
REFERENCES
■
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