Dual-Stereocontrol Asymmetric Cobalt-Catalyzed Hydroboration of Sterically Hindered Styrenes

Article abstract of DOI:10.1021/acscatal.6b02278

An oxazoline aminoisopropylpyridine (OAP) was designed and synthesized for cobalt-catalyzed asymmetric hydroboration of sterically hindered styrenes. A unique dual-stereocontrol phenomenon was observed using a rigid OIP·CoCl₂ complex or a flexible OAP with CoCl₂ as precatalysts, respectively. The reaction could be easily carried out on a gram scale to afford chiral alkylboronic esters which could be converted into diverse C-X (X = C, N, O) bond cross-coupling products. The mechanistically distinct pathways were proposed on the basis of deuterium experiments.

Full text of DOI:10.1021/acscatal.6b02278

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Article
Dual Stereocontrol Asymmetric Cobalt-Catalyzed
Hydroboration of Sterically Hindered Styrenes
Heyi Zhang, and Zhan Lu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02278 • Publication Date (Web): 29 Aug 2016
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Dual Stereocontrol Asymmetric Cobalt-Catalyzed Hydrobora-
tion of Sterically Hindered Styrenes
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Heyi Zhang and Zhan Lu*
Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310058, China
ABSTRACT: An oxazoline aminoisopropyl pyridine (OAP) was designed and synthesized for cobalt-catalyzed asymmetric
hydroboration of sterically hindered styrenes. A unique dual stereocontrol phenomenon was observed using rigid OIP˙CoCl2
complex or flexible OAP with CoCl2 as precatalysts, respectively. The reaction could be easily carried out with gram scales
to afford chiral alkylboronic esters which could be converted into diverse C-X (C, N and O) bond cross-coupling products.
The mechanistically distinct pathways were proposed based on deuterium experiments.
KEYWORDS: ligand design, dual stereocontrol, hydroboration, cobalt-catalyzed, alkenes
Chirality as an important and widely existing phenomenon
in nature has been utilized in pharmaceutical and material
science, as well as in organic synthesis, particularly in cata-
Scheme 1. Synthesis of Chiral Oxazoline Aminoisopropyl
Pyridine (OAP)
lytical asymmetric reactions.1 For traditional strategies, two
enantiomers could be obtained with high enantioselectivities
using two opposite chrial sources, however, many natural
chiral sources are available in only one absolute configuration
or unnatural chiral sources are very expensive. The ideal strat-
egy is to obtain both enantiomers of the product by using a
single chiral source.2 Asymmetric reactions to afford both
enantiomers with high enantioselectivities (>90% ee) are in-
teresting, useful and challenging. Using effects of metals,3
solvents,4 additives and counteranions,5 dual stereocontrol
highly enantioselectivities in various types of reactions were
observed. Change in catalyst substitutents is also a major ef-
fect which has been proven by Hou and Wu,6 Takacs,7 Ma,8
and Liao.9 However, changing functionalization groups in
catalyst to achieve both high enantioselectivities has not been
well explored.
Catatylical asymmetric hydroboration of alkenes is a direct,
efficient, and atom-economic method for synthesis of chiral
alkylboronic acid derivatives which are widely used in organic
synthesis capable of being transferred to numerous functional
groups via consecutive carbon-carbon and carbon-heteroatom
bonds formation reactions.10 Due to the difficulty in differenti-
ating two enantiotopic faces in prochiral substrates, the
asymmetric hydroboration of 1,1-disubstituted alkenes is still
a challenge.11 Since the pioneering works by Suzuki,11a Bur-
gess,11b,c Ito and Hayashi11d on Rh-catalyzed asymmetric hy-
droboration of α-methylstyrene, Mazet,11e Hoveyda,11f
Huang,11g Diéguez,11j and we,11h,i independently, developed
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Table 1. Optimization Studies for Asymmetric Hydrobora-
could be synthesized from commercially available 2,6-
tion of Styrenesa
dibromopyridine. Acylation of 2,6-dibromopyridine followed
by condensation with aniline or benzylic amine afforded 2-
bromo-6-iminopyridine (S1) which underwent methylation
with trimethylaluminum to give 2-aminoisopropyl-6-
bromopyridine (S2). Using our previously reported palladium-
catalyzed cross-coupling reactions of heteroaryl halide with
oxazoline,11i,14 the desired ligands (L*) could be obtained up
to 83% yields.
The first test reaction of 2-methyl(2’-methyl)styrene 1a
with HBpin in the presence of ligand (LAa) and CoCl2 using
NaBH(s-Bu)3 as a reductant in a solution of THF offorded the
hydroboration product 3a in 10% NMR yield with 58% ee
(entry 1, Table 1). Unexpectedly, the absolute configuration of
major products is opposite to one obtained by using
OIP˙CoCl2 precatalyst. This unique phenomenon is beyond
our design.
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entry ligand or complex yield of 3a (%)b ee of 3a (%)c
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LAa
LBa
LCa
LDa
LEa
LBb
LCb
LBc
LCc
LCc1
LCc2
L1
L2
(10)
82
78
(42)
(36)
60
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63
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85
(71)
(<5)
(10)
(<5)
80
58
86
92
79
56
75
72
84
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84
71
-
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The 2,6-diethyl and 2,6-dimethyl aniline-derived ligands
were quite suitable to afford 3a up to 82% yield with up to 92%
ee (entries 2 and 3). Further decreasing the steric hindrance on
phenyl ring or using benzyl amine, either reactivities or enan-
tioselectivities were diminished (entries 4 and 5). The valine-
derived oxazoline with less sterically bulky anilines were able
to promote hydroboration with moderate ee (entries 6 and 7).
The phenylalamine-derived oxazolines with various anilines
were investigated that LCc1 (2,6-dimethylaniline with 5,5-
dimethyl-4-benzyl oxazoline) was better to give 3a in 85%
yield with 84% ee (entries 8-11). Other OAP ligands, various
solvents and reductants were also examined; however no fur-
ther improvement was observed (SI). To identify the unique
property of newly designed ligands, classic PyOx and PyBox
ligands were used instead of OAP, either reactivity or enanti-
oselectivity were deminished dramatically (entries 12 and 13).
Simple aminoisopropyl pyridine was not efficient to inhabit
the background reaction (entry 14). We also roughly reinvesti-
gated the reactions using OIP˙CoCl2 complexes and found
complex C to provide 3a in 81% yield with reversed 83% ee
(entries 15-18). Based on above ligand screen, the combina-
tion of LCa with CoCl2 could efficiently catalyze the hydrobo-
ration of sterically bulky α-methyl styrene (conditions A). On
the other hand, another enantiomer could be obtained by using
OIP˙CoCl2 complexes from one single chiral source (condi-
tions B).
27
-
L3
complex A
complex B
complex C
complex D
-66
-82
-83
-59
76
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86
a1a (1 mmol), HBpin (1 mmol), ligand (8 mol%), CoCl2 (5 mol%),
NaBH(s-Bu)3 (15 mol%) in a solution of THF (0.5 M) at 25 C
o
for 5 hours. Or using cobalt complex A-D (5 mol%) instead of
ligand and CoCl2 (entries 15-18). bIsolated yields. NMR yields are
in the parenthesis. cEnantioselectivities were determined by chiral
HPLC analysis.
With standard conditions in hands, a variety of sterically
bulky α-substituted styrenes were investigated in Table 2. α-
Methyl(2’-methyl)styrenes with electron-rich groups, such as
methyl, methoxy, TBS-protected phenyl, dimethylamino
groups were suitiable for both conditions to produce the chiral
boronates 3b-f in 69-77% yields and 90-95% ee (R), 69-79%
yields and 85-93% ee (S), respectively. With electron-poor
subsitituent (Cl), the reaction under OAP conditions afforded
(R)-3g with 84% ee. On the contrary, (S)-3g was obtained
under OIP conditions with a slightly low ee (68%). With bro-
mide on phenyl ring, moderate enantioselectivities were ob-
served in both conditions (3h). Using more sterically bulky
groups such as ethyl, n-butyl, iso-propyl instead of methyl
group, OAP conditions showed better enantioselectivities than
OIP conditions (3i-k). Interestingly, 2’-phenyl α-
methylstyrene could participate both transformations to give 3l
in 92% and 95% ee, respectively, which is a suitable model to
potentially construct axial chirality. With ortho-methoxy or
ortho-benzyloxy group, (R)-3m-o were obtained under OAP
conditions in 58-81% yields with moderate ee, relatively, en-
asymmetric transition-metal-catalyzed hydroboration of α-
substituted styrenes. However, there is still a puzzle that the
more sterically hindered substrate such as α-methyl (ortho-
methyl)styrene in which two substitutions are quite easily
differentiated12 showed poor or moderate enantioselectivity in
the asymmetric hydroboration. Otherwise, the very expensive
unnatural D-valine should be used to give the opposite enanti-
omers. Herein, we designed and synthesized a novel oxazoline
aminoisopropyl pyridine (OAP) ligand for highly enantiose-
lective cobalt-catalyzed hydroboration of sterically bulky sty-
renes with an unexpected reversed absolute configuration.
We proposed that sterically bulky styrenes could not easily
coordinate the active metal center to set the right position due
to steric hindrance of the oxazoline iminopyridine (OIP).
Inspired by side arm strategy by Tang,13 we designed to use a
more flexible amine group instead of a rigid imino group for
modification of oxazoline pyridine. The novel oxazoline ami-
noisopropyl pyridine ligand (OAP) (Scheme 1, also see SI)
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Table 2. Substrate Scope for Dual Stereocontrol Asymmetric Hydroboration of Styrenesa
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aStandard conditions A: Unless otherwise noted, 1 (1.0 mmol), HBpin (1.0 mmol), LCa (0.08 mmol), CoCl2 (0.05 mmol), NaBH(s-Bu)3 (0.15
mmol) in 2 mL of THF at 25 °C under nitrogen for 5 h. Standard conditions B: similar with conditions A, using complex C (0.05 mmol) in-
stead of LCa and CoCl2. bUsing LCc1 instead of LCa. c12 h. d10 h.
antioselectivities under OIP conditions were up to 90% ee.
Scheme 3. Further Derivatizations
Although ortho-trifluoromethyl group decreased the reactivi-
ties (3p), the enantioselectivities were still good (88% ee un-
der conditions A). Increasing the α-substituent did affect the
reactivity and selectivity of OAP-type reactions, however,
high ees were observed under conditions B (3q-s). Sterically
bulky 1-naphthyl derived (R)-3t was obtained with 84% ee
which was better than that observed under conditions B. The
reactions of 1,2-disubstituted styrene 1u could be carried out
to give exclusively regioselective 3u in moderate yields and
ees.
Gram scale reactions could be carried out smoothly to
afford corresponding hydroboration products with good yields
and high enantioselectivities (Scheme 2).
Scheme 2. Gram Scale Reactions
To demonstrate the possibility of coordination with cobalt
salts, the ligand and CoCl2 were mixed in a solution of Et2O to
precipitate a dark blue solid whose structure was analyzed by
single crystal X-ray diffraction (Scheme 4).17 Using the
LCc˙CoCl2 as the precatalyst, the reaction could afford 3a in a
slightly low yield with good enantioselectivitiy.
The chiral boronates could be converted to diverse useful
chiral compounds through oxidation, amination, dichlorome-
thyllithium homologation and Suzuki-cross-coupling reactions
(Scheme 3). The absolute configuration of hydroboration
products was determined by the single crystal X-ray structural
analysis of the derivative (S)-8c (Scheme 3).15 The chiral al-
cohols 4 are the crucial precursors in synthesis of chiral
Thespesone (Scheme 3),16 which showed potential anticancer
activities.
Using methyl-protected fully substituted amine ligand L4,
almost no reactivity was observed for hydroboration, which
suggested that deprotonation of amine on the ligand might
occur during activating the precatalyst (Scheme 5). Interest-
ingly, removing the gem-dimethyl group on the link, the reac-
tivity and enantioselectivity of the hydroboration reaction
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Scheme 4. Cobalt Complex and Its Catalytic Reaction
Scheme 6. Possible Mechanism
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were also diminished dramatically which was consistent with
the Thrope-Ingold effect18 (Scheme 5).
To explain the details of transformations, the deuterium ex-
periments using DBpin were conducted (Scheme 5). Under
conditions A, the deuterium atom was only observed at the
benzyl position. Dissimilarly, under conditions B, 13% of
deuterium atoms was observed at the benzyl position and also
found on the borated carbon atom which indicated that the
alkene insertion to a cobalt-hydride bond was a fast equilib-
rium.
In summary, we reported a dual stereocontrol asymmetric
cobalt-catalyzed hydroboration of sterically hindered styrenes
by ultilizing a novel flexible oxazoline aminoisopropyl pyri-
dine ligand and a rigid oxazoline iminopyridine ligand, respec-
tively. The more challenging styenes could easily undergo the
enantioselective hydroboration to afford the chiral alkyl boro-
nates in a gram-scale which are benefit for further derivatiza-
tions, such as oxidation, amination, lithium carbonation and
Suzuki-cross-coupling reactions, to deliver diverse functional
products. The primary mechanisms based on control and deu-
terium experiments were proposed that two reactions under-
went mechanistically distinct pathways. Further studies on
chiral ligand design is undergoing in our laboratory.
Scheme 5. Control and Deuterium Experiments
ASSOCIATED CONTENT
Supporting Information. This material is available free of
charge via the Internet at http://pubs.acs.org. Experimental proce-
dures for the experiments, synthesis and isolation of the hydrobo-
ration products, NMR and HPLC data and spectra of products
(PDF). The cif documents of X-ray diffractions of compounds
(S)-8c and LCc˙CoCl2.
AUTHOR INFORMATION
Corresponding Author
* E-mail: luzhan@zju.edu.cn
Notes
The authors declare no competing financial interest.
Using amine as a ligand (cycle I in Scheme 6), the ligand
could undergo a deprotonation pathway to generate the active
OAP cobalt species. The coordination of cobalt species with
alkene might occur followed by oxidative addition of HBPin,
insertion of alkene to cobalt-boron bond and reductive elimi-
nation to regenerate the active OAP cobalt species and give
the hydroboration product. Based on the mechanism of the
bis(imino)pyridine cobalt-catalyzed hydroboration of alkene
with HBPin proposed by Chirik,19 the OIP cobalt-catalyzed
alkene hydroboration was proposed as cycle II in Scheme 6.
An OIP cobalt hydride species might be the active catalyst
which could coordinate with alkene followed by the insertion
and ligand exchange to regenerate the catalyst and deliver the
product. The stereochemistry might be determined at the coor-
dination step. The primary models to predict the stereochemi-
cal outcome of products were shown in Scheme S2 in SI,
however, further studies should be carried out to prove the
nature of the mechanism.
ACKNOWLEDGMENT
We are grateful for financial support from the National 973 Pro-
gram (2015CB856600), NSFC (21472162) and the “Thousand
Youth Talents Plan”.
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