Versatile Cobalt-Catalyzed Enantioselective Entry to Boryl-Functionalized All-Carbon Quaternary Stereogenic Centers

Article abstract of DOI:10.1021/jacs.8b06814

We report an asymmetric synthesis of chiral boryl-functionalized γ-lactams containing all-carbon quaternary stereocenters via a Co-catalyzed enantioselective hydroboration/cyclization of amide-tethered 1,6-enynes. These enantio-enriched γ-lactam products can be readily converted to a variety of cyclic and acyclic other chiral γ-lactams, pyrrolidin-2,3-diones, β-amino acid N-carboxyanhydrides, and β-amino carboxylic amides.

Full text of DOI:10.1021/jacs.8b06814

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Communication
A Versatile Cobalt-Catalyzed Enantioselective Entry to Boryl-
Functionalized All-Carbon Quaternary Stereogenic Centers
Chao Wang, and Shaozhong Ge
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06814 • Publication Date (Web): 16 Aug 2018
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A Versatile Cobalt-Catalyzed Enantioselective Entry to Boryl-
Functionalized All-Carbon Quaternary Stereogenic Centers
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Chao Wang and Shaozhong Ge*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
Supporting Information Placeholder
To develop asymmetric synthesis of chiral boryl-
functionalized -lactams containing all-carbon stereocen-
ters, we envisioned that amide-tethered 1,6-enynes contain-
ing an internal alkyne and a terminal gem-disubstituted
alkene unit could undergo Co-catalyzed hydrobora-
ABSTRACT: We report an asymmetric synthesis of chiral
boryl-functionalized -lactams containing all-carbon qua-
γ
γ
ternary stereocenters via a Co-catalyzed enantioselective
hydroboration/cyclization of amide-tethered 1,6-enynes.
These enantio-enriched
converted to a variety of cyclic and acyclic other chiral
lactams, pyrrolidin-2,3-diones, -amino acid N-
carboxyanhydrides, and -amino carboxylic amides.
γ-lactam products can be readily
tion/cyclization to form the desired borylated
products (eq 1).11 Once achieved,
-lactam products from
these reactions should possess versatile chemical reactivity
because they have several reactive groups, such as the α,β-
γ-lactam
γ
-
γ
β
β
unsaturation, amide, and boryl functionalities. These am-
ide-tethered 1,6-enynes have been recently employed for a
Pd-catalyzed non-asymmetric hydrohalogenation to prepare
All-carbon quaternary stereocenters are present in many
biologically active natural products and synthetic organic
compounds.1 The synthetic approaches to access these qua-
ternary carbon centers include chiral synthesis from natu-
rally occurring chiral pool,2 desymmetrization of symmet-
rical quaternary-carbon-containing molecules,3 and enanti-
oselective C-C bond-forming reactions.4 Among these ap-
proaches, catalytic enantioselective construction of all-
carbon quaternary carbons through C-C bond formation
reactions is particularly difficult because of their congested
nature. Accordingly, it is still a challenging, but highly de-
sirable task to develop enantioselective protocols for con-
struction of all-carbon quaternary stereocenters, especially
functionalized stereocenters that allow versatile derivations
to access a series of structurally diverse compounds.5
racemic halogenated
catalytic enantioselective synthesis of chiral boryl-
functionalized -lactams containing all-carbon quaternary
stereocenters. We also show that chiral -lactam products
δ
-lactams.12 Herein, we report the first
γ
γ
can be converted to a series of cyclic and acyclic small
molecules bearing all-carbon quaternary stereocenters by
standard functional group interconversions.
We initiated this study by identifying selective cobalt
catalysts and reliable conditions for the reaction of the am-
ide-tethered 1,6-enyne 1a with HBpin. We evaluated sever-
al cobalt catalysts generated in situ from Co(acac)2 and
chiral bisphosphine ligands for this asymmetric transfor-
mation. In general, these reactions were conducted with 3
mol% cobalt catalysts with 1a as a limiting reagent in the
presence of 1.3 equiv of HBpin, and the results of selected
experiments are listed in Table 1. The reaction catalyzed by
the combination of Co(acac)2 and (R,R)-quinoxP* produced
The chiral
γ
-lactam scaffold is part of the core structures
-lactams also
of a variety of bioactive compounds, and
γ
serve as useful building blocks to prepare N-heterocyclic
compounds because of the versatile chemistry of the amide
functionality.6 Even though numerous enantioselective pro-
tocols have been developed to prepare chiral
approaches for enantioselective synthesis of -lactams con-
taining all-carbon quaternary stereocenters are very lim-
ited.7e Considering broad synthetic applications of organo-
boron compounds in chemical synthesis, we are interested
in developing an enantioselective method to prepare boryl-
γ
-lactams,7
γ
the borylated γ-lactam 2a in a high yield, but with only
modest enantioselectivity (65% ee, entry 1 in Table 1). The
reactions conducted with catalysts generated from
Co(acac)2 and other bisphosphines, such as (R)-C3-
tunephos, (R)-segphos, (S,S)-chiraphos, (R,R)-BDPP, and
(R,R)-Me-ferrocelane, proceeded with low to modest enan-
tioselectivities (entries 2−6 in Table 1). To our delight, the
reactions catalyzed by Co(acac)2 and (R,R,S,S)-duanphos or
(S,S)-Ph-BPE occurred to full conversions of 1a and af-
forded 2a in high yields with high enantioselectivity (en-
tries 7 and 8 in Table 1). Further studies on the reaction
catalyzed by Co(acac)2 and (R,R,S,S)-duanphos in various
functionalized chiral
ternary stereocenters.
γ-lactams containing all-carbon qua-
Enantioselective cycloisomerization and reductive cy-
clization reactions of 1,6-enynes are widely conducted to
prepare chiral five-membered cyclic compounds.8 The ma-
jority of these reactions are used to construct tertiary stere-
ocenters,9 and only a limited number of reactions afford
products containing all-carbon quaternary stereocenters.10
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tion, the identified catalyst and conditions are also effective
to prepare -lactam with a tertiary stereogenic center (2l).
However, secondary N-allyl 3-phenylpropiolamides do not
undergo this Co-catalyzed cyclization reaction.
Table 2. Scope of 1,6-Enynes.a
solvents (entries 8−11 in Table 1) showed that the reaction
1
2
3
4
conducted in CH3CN afforded 2a in 88% yield and with
γ
92% ee (entry 11 in Table 1).
Table 1. Evaluation of Conditions for the Reaction of 1a.a
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aConditions: 1a (0.200 mmol), HBpin (0.260 mmol),
Co(acac)2 (6.0 ꢀmol), ligand (6.6 ꢀmol), solvent (1 mL), rt, 12
h; bThe conversion of 1a was determined by GC analysis with
aConditions: see conditions for entry 11 in Table 1; bisolated
as a Z/E mixture with Z:E = 2:1; c(R)-C3-tunephos (6.6 ꢀmol),
THF (1 mL); d4 mol % catalyst.
c
d
dodecane as an internal standard; isolated yields; ee was de-
e
Subsequently we studied the substrates containing vari-
ous aryl groups on the alkyne moiety for this catalytic
asymmetric hydroboration/cyclization reaction and the re-
sults are listed in Table 3. In general, a range of amide-
tethered 1,6-enynes containing para-, meta-, or ortho-
substituted aryl groups at acetylic position reacted to af-
termined by chiral HPLC analysis; The configuration of 2a
was determined by single-crystal X-ray analysis on the corre-
sponding alcohol obtained by the oxidation of 2a.
With an effective catalyst and identified conditions in
hand (entry 11 in Table 1), we studied the scope of 1,6-
enynes derived from propiolamides that undergo this co-
balt-catalyzed asymmetric hydroboration/cyclization and
the results are listed in Table 2. In general, a variety of N-
allyl 3-phenylpropiolamides containing various aliphatic or
aromatic substituents on the nitrogen (1a−1h) or in the allyl
groups (1i−1m) smoothly reacted with HBpin in the pres-
ence of 3 mol % Co(acac)2 and (R,R,S,S)-duanphos at room
termperature, and the corresponding enantiomerically en-
forded the desired chiral γ-lactams (2q−2u) in high yields
with high enantioselectivity. This asymmetric cyclization
shows good functional group tolerance, and a variety of
reactive groups, such as fluoro (2w), chloro (2v and 2x),
bromo (2y), acetal (2aa), amide (2ab), carboxylic ester
(2ac and 2ad), cyano (2ae), and pinacol boronic ester (2af),
are compatible with the identified reaction conditions. In
addition, 1,6-enynes with nitrogen- and sulfur-containing
heteroaryl groups also reacted with high enantioselectivity
(2ag and 2ah). However, N-allyl propiolamides containing
alkyl groups bound to the alkynyl unit do not undergo this
Co-catalyzed hydroboration/cyclization reaction.
riched
γ-lactams (2a−2m) are produced in high yields
(81−92%) with high enantioselectivity (85−97% ee). 1,6-
Enynes containing secondary amides did not undergo this
Co-catalyzed reaction. Furthermore, we showed that O- or
N-tethered 1,6-enynes also reacted to yield the correspond-
ing cyclic products (2n−2p) with high enantioselectivity.
The data in Table 2 indicate the steric property of the
substituent on the nitrogen of 1,6-enynes has a noticeable
influence on the enantioselectivity of this reaction. For ex-
ample, the substrates with increased steric hindrance
around the nitrogen atom reacted with slightly decreased
enantioselectivity (2a−2d). Substrates containing electroni-
cally and sterically varied aryl groups on the alkene moiety
also reacted with high enantioselectivity (2i−2k). In addi-
Scheme 1 summarizes the synthetic utility of this Co-
catalyzed entry to all-carbon stereogenic centers. The enan-
tioselective synthesis of 2a on a gram-scale was achieved
in 80% yield with 92% ee (Scheme 1A). Chiral alkyl-
boronate 2a underwent a series of stereospecific transfor-
mations without loss of enantiopurity. For example, 2a
could be oxidized by H2O2 to form chiral alcohol 3 in 86%
yield (Scheme 1B). Homologation of 2a with LiCH2Cl
produced chiral alkylboronate 4 in 45% yield (Scheme 1C).
Alkenylation of 2a with vinylmagnesium bromide afforded
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alkene 5 in 82% yield (Scheme 1D). Alkylboronate 2a also
se experiments suggested the chelation of enyne 1a to the
cobalt catalyst.14
1
2
3
4
5
6
underwent Pd-catalyzed Suzuki-Miyaura cross-coupling
with bromobenzene to give product 6 in 76% yield
(Scheme 1E). Furthermore, compound 6 could be convert-
ed to chiral pyrolidin-2,3-dione 7 in 80% yield with 92% ee
(Scheme 1F). Compound 7 readily underwent Baeyer-
Villiger oxidation with m-chloroperbenzoic acid to produce
Scheme 1. Derivatization of Chiral γ
-Lactam 2a.a
β
-amino acid N-carboxyanhydride 8 in almost quantitative
7
8
9
yield (Scheme 1G).13
β
-Amino acid N-carboxyanhydride is
a versatile electrophile and reacts with a variety of amine
nucleophiles. For example, compound 8 readily reacted
10
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with benzylamine and afforded acyclic β-amino carboxylic
amide 9 in 87% yield with 92% ee (Scheme 1H).
Table 3. Scope of Amide-Tethered 1,6-Enynes.a
aConditions: (B) H2O2 (30%), NaOH (3 M aq), THF, 0 oC, 3
h; (C) CH2BrCl (2.5 equiv), n-BuLi (2.5 equiv), THF, -78 C,
o
6 h; (D) vinylmagnesium bromide (4 equiv), I2 (4 equiv), THF,
-78 oC, 6 h; (E) Pd(OAc)2 (15 mol%), rac-BINAP (15 mol %),
o
KOH (5 equiv), PhBr (2 equiv), dioxane/H2O , 100 C, 24 h;
(F) RuCl3.xH2O (2 mol %), NaIO4 (3 equiv),
CH2Cl2/CH3CN/H2O, rt, 6 h; (G) mCPBA (1.2 equiv), CH2Cl2,
rt, 6 h; (H) BnNH2 (1.2 equiv), CH2Cl2, rt, 12 h.
Scheme 2. Deuterium-labeling Experiment, Control
Experiments and the Proposed Catalytic Pathway
b
aConditions: see conditions for entry 11 in Table 1. (S,S)-
Ph-BPE (6.6 ꢀmol) instead of (R,R,S,S)-duanphos.
To provide some insights for this Co-catalyzed construc-
tion of all-carbon stereocenters, we conducted some deuter-
ium-labeling and control experiments. The reaction of 1a
with DBpin under standard conditions afforded 2a-d1 in
84% yield and the deuterium atom in 2a-d1 is located on
the vinyl-carbon (Scheme 2A). To test the chelating effect
of enyne to the cobalt catalyst, we conducted the hydrobo-
ration of an amide-containing internal alkyne 10 and gem-
substituted alkene 11 (Scheme 2B and 2C). The reaction of
alkyne 10 under standard conditions provided the vinyl-
boronate 12 in 71% yield, and the other regioisomer 12’
was formed in <5% yield (Scheme 2B). The regioselectivi-
ty for hydroboration of the alkyne 10 is opposite to the re-
gioselectivity for hydroboration of the alkyne unit in 2a. In
addition, the hydroboration of the gem-substituted alkene
11 was very sluggish and gave product 13 in <5% yield
under standard conditions (Scheme 2C). The results of the-
Based on these experimental results, we proposed a
pathway for this Co-catalyzed production of chiral boryl-
containing
γ
-lactams (Scheme 2D).15 The activation of
Co(acac)2 with HBpin in the presence of chiral ligand L*
generates a chiral Co(I)-H species (L*)Co-H.9g,16 The che-
lation of enyne 1a with this (L*)Co-H forms the intermedi-
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(d) Holmes, M.; Nguyen, K. D.; Schwartz, L. A.; Luong, T.; Krische,
ate I-A, and the insertion of the alkyne unit of the chelated
enyne to Co-H produces a vinylcobalt species I-B. The
subsequent intramolecular, enantioselective migratory in-
sertion of the alkene unit of I-B generates an alkylcobalt
intermediate I-C, which then reacts with HBpin to release
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1
2
3
4
5
6
7
8
chiral γ-lactam product and regenerate the chrial Co(I)-H
species (L*)Co-H. The vinylboronate 14 from the hydrobo-
ration of the alkyne unit in 1a is not formed, and this indi-
cates that the rate of intramolecular insertion of alkene unit
in I-B is significantly higher than that of the reaction of I-B
with HBpin.
9
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In summary, we have developed an effective and enanti-
oselective protocol to prepare chiral boryl-functionalized γ-
lactams containing an all-carbon quaternary stereocenter
via Co-catalyzed hydroboration/cyclization of amide-
tethered 1,6-enynes. These chiral boryl-functionalized
lactams can be converted readily to a variety of cyclic and
acyclic chiral molecules, such as chiral -lactams, pyrroli-
din-2,3-diones, -amino acid N-carboxyanhydrides, and
-amino carboxylic amides. Therefore, this Co-catalyzed
γ-
γ
β
β
enantioselective entry provides a general method to prepare
a variety of building blocks containing all-carbon quater-
nary stereogenic carbons for chemical synthesis.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI:
Experimental/DFT details and characterization data (PDF)
Crystallographic data for 3 (CIF)
AUTHOR INFORMATION
Corresponding Author
* chmgsh@nus.edu.sg
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the Ministry of Education
(MOE) of Singapore (No. R-143-000-A07-112).
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(5) For selected recent examples on synthesis of functionalized all-
carbon stereocenters, see: (a) Krautwald, S.; Sarlah, D.; Schafroth, M.
A.; Carreira, E. M. Science 2013, 340, 1065. (b) Turnbull, B. W. H.;
Evans, P. A. J. Am. Chem. Soc. 2015, 137, 6156. (c) Nguyen, K. D.;
Herkommer, D.; Krische, M. J. J. Am. Chem. Soc. 2016, 138, 14210.
(15) See the Supporting Information for discussions on an alterna-
tive pathway involving a cobaltobicyclo[3,3,0] ring intermediate gen-
erated through oxidative cyclometalation of enynes.
(16) Teo, W. J.; Ge, S. Angew. Chem., Int. Ed. 2018, 57, 1654.
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