Cobalt-Catalyzed Asymmetric Hydroboration/Cyclization of 1,6-Enynes with Pinacolborane

Article abstract of DOI:10.1021/jacs.7b01708

We report a cobalt-catalyzed asymmetric hydroboration/cyclization of 1,6-enynes with catalysts generated from Co(acac)₂ and chiral bisphosphine ligands and activated in situ by reaction with pinacolborane (HBpin). A variety of oxygen-, nitrogen

Full text of DOI:10.1021/jacs.7b01708

Subscriber access provided by University of Colorado Boulder
Communication
Cobalt-Catalyzed Asymmetric Hydroboration/
Cyclization of 1,6-Enynes with Pinacolborane
Songjie Yu, Caizhi Wu, and Shaozhong Ge
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017
Downloaded from http://pubs.acs.org on April 28, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 5
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Cobalt-Catalyzed Asymmetric Hydroboration/Cyclization of 1,6-
Enynes with Pinacolborane
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Songjie Yu, Caizhi Wu, and Shaozhong Ge*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Supporting Information Placeholder
the propargylic ether unit with the copper catalyst is crucial
to achieve the borylative cyclization. Carbon- and nitrogen-
tethered 1,6-enynes do not undergo this copper-catalyzed
asymmetric transformation.
ABSTRACT: We report the a cobalt-catalyzed asymmetric
hydroboration/cyclization of 1,6-enynes with catalysts gen-
erated from Co(acac)2 and chiral bisphosphine ligands and
activated in situ by the reaction with pinacolborane
(HBpin). A variety of oxygen-, nitrogen-, and carbon-
tethered 1,6-enynes underwent this asymmetric transfor-
mation, yielding both alkyl- and vinyl-substituted boronate
esters containing chiral tetrahydrofuran, cyclopentane, and
pyrrolidine moieties with high to excellent enantioselectivi-
ties (86%–99% ee).
The hydroboration/cyclization of 1,6-enynes with HBpin,
a more atom-economical approach than borylative cycliza-
tion with B2Pin2, could potentially produce both vinyl and
alkyl boronate products (eq 1). This reaction has been stud-
ied with a chiral rhodium catalyst and affords only vinyl
boronate products.8 This rhodium system requires the use
of catecholborane (HBcat) to achieve good yields of vinyl
boronate products and generally shows low to modest en-
antioselectivities. In addition, the use of HBcat limits its
applications because of the poor stability of catechol boro-
nate products. In view of broad synthetic applications of
pinacol boronate esters9 and importance of chiral carbocy-
clic and heterocyclic structural motifs, we are interested in
developing a base metal-catalyzed asymmetric hydrobora-
tion/cyclization of 1,6-enynes with HBpin to prepare chiral
cyclic boronate esters. Here we report the first asymmetric
cobalt-catalyzed hydroboration/cyclization of 1,6-enynes
that produces both alkyl and vinyl boronate esters (eq 1).10
Chiral five-membered carbocyclic and heterocyclic struc-
tural motifs are present in a variety of bioactive natural
products (Figure 1).1 The transition metal-catalyzed enanti-
oselective cycloisomerizaiton of 1,6-enynes2 and related
reductive cyclization3 represent powerful approaches to
such chiral cyclic structural units.4 However, borylative
cyclization5 is arguably more important for synthetic appli-
cations because of the versatile chemistry of the boryl func-
tionality.6 Several catalysts have been reported for asym-
metric borylative cyclization of specific classes of alkene-
containing
multiply
unsaturated
substrates
with
bis(pinacolato)diboron (B2pin2).7
Recently, cobalt complexes have been emerging as active
catalysts for hydrogenation,11 hydroboration,12 and hydrosi-
lylation13 of alkenes and alkynes.14 During our effort in
developing cobalt catalysis, we have found that cobalt
complexes with bisphosphine ligands are effective for al-
kene hydrosilylation.15 In this study, we targeted cobalt
complexes with chiral bisphosphine ligands for asymmetric
hydroboration/cyclization of 1,6-enynes because the acces-
sibility of a wide range of chiral bisphosphine ligands pro-
vides a possibility to control both regio- and enantioselec-
tivity of this transformation.
Figure 1. Examples of related nature products
We initiated our studies of Co-catalyzed hydrobora-
tion/cyclization by evaluating the reaction between (3-
(allyloxy)prop-1-yn-1-yl)benzene (1a) and HBpin. We
tested various cobalt precursors, bisphosphine ligands and
solvents for this reaction (see SI for the detailed evaluation)
In 2013, Lin, Tian, and coworkers reported a Cu-
catalyzed asymmetric borylative cyclization of cyclohexa-
dienone-containing oxygen-tethered enynes with B2pin2 in
the presence of methanol as a proton source.7c Mechanistic
studies reveal that the coordination of the oxygen atom in
ACS Paragon Plus Environment

Journal of the American Chemical Society
and the selected examples are summarized in Table 1. The
Page 2 of 5
This asymmetric transformation shows high functional
group tolerance and a wide range of reactive groups, such
as trifluoromethyl (2b), silyl (2f), ether (2c and 2u), fluoro
(2h), chloro (2i), bromo (2j), ketone (2k), aldehyde (2l),
amide (2m), ester (2n and 2t), cyano (2o), and silylether
(2p) moieties, are compatible with the reaction conditions.
In addition, enynes containing pyridine, thiophene, or in-
dole moieties also reacted, affording the corresponding
vinyl boronate esters (2q–2s) with high yields and excellent
enantioselectivities. However, the reactions of enynes con-
taining internal alkenes, terminal alkynes, nitro, and unpro-
tected hydroxyl and amino groups did not occur. The abso-
lute configuration of 2b and 2n was determined to be (R)
by single-crystal X-ray diffraction.
1
2
3
4
5
6
7
8
reaction conducted with Co(acac)2 and (R,R)-QuinoxP*
(L1) in THF afforded the vinyl boronate product 2a as ma-
jor product with high enantioselectivity (98% ee) (entry 1).
Next, we tested this reaction in various solvents (entries 2–
4) and we found that the reaction conducted in toluene gave
2a in 85% isolated yield with excellent enantioselectivity
(99% ee) (entry 4). Other chiral bisphosphine ligands L2–
L6 were tested and these reactions showed lower selectivi-
ty for 2a over 3a (entries 5–9). In addition, these reactions
afforded 2a with lower enantioselectivities.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Evaluation of Conditions for Co-Catalyzed Hy-
droboration/Cyclization of Enyne 1aa
Table 2. Scope of 1,6-Enynes for Vinyl Boronate Estersa
aConditions: 1a (0.200 mmol), HBpin (0.300 mmol), Co(acac)2
b
(6.0 ꢀmol), ligand (8.0 ꢀmol), solvent (1 mL), rt, 12 h, conver-
sions of 1a and ratios of 2a:3a were determined by GC analysis
on the crude reaction mixture; cisolated yields; dee was deter-
mined by chiral HPLC analysis using the isolated compound 2a.
The scope of 1,6-enynes for this cobalt-catalyzed asym-
metric hydroboration/cyclization to produce vinyl boronate
esters is summarized in Table 2. In general, a wide range of
oxygen-, nitrogen-, or carbon-tethered 1,6-enynes reacted
in the presence of 3 mol % of Co(acac)2 and 4 mol % of
(R,R)-quinoxP* to provide vinyl boronate esters (2a–2x)
with modest to high isolated yields (59%–87%) and high
enantioselectivities (94%–99%).
aConditions: enyne (0.200 mmol), HBpin (0.300 mmol),
Co(acac)2 (6.0 ꢀmol), (R,R)-quinoxP* (8.0 ꢀmol), toluene (1
mL), rt, 12 h, isolated yields, and ee was determined by chiral
HPLC analysis; b60 °C; cTHF (1 ml).
The 1,6-enynes containing a series of electronically var-
ied aryl groups (2a–2c) reacted smoothly with high yields
and excellent enantioselectivities. Aliphatic 1,6-enynes also
reacted to give the corresponding vinyl boronate esters (2s–
2u) with high enantioselectivities. Nitrogen- and carbon-
tethered 1,6-enynes underwent hydroboration/cyclization at
60 °C to afford the corresponding vinyl boronate esters
(2v–2x) with modest yields and excellent enantioselectivi-
ties (99%).16
The data in Table 2 show that the enynes containing pa-
ra- (2b–2f) or the meta-substituted (2g) arenes reacted to
afford vinyl boronate esters as major products. However,
the enynes containing ortho-substituted aryl groups reacted
to provide alkyl boronate esters (3b and 3c in Table 3) with
good isolated yields and high enantioselectivities. Subse-
quently, we tested the reactions of oxygen-tethered 1,6-
enynes with two substituents at the propargalic position in
the presence of Co(acac)2 and (R,R)-QuinoxP*. These reac-
tions occurred smoothly at room temperature and afforded
ACS Paragon Plus Environment

Page 3 of 5
Journal of the American Chemical Society
(dppbz)2CoH (4).17 Importantly, this Co-H complex was
alkyl boronate esters (3d−3l in Table 3) with high isolated
1
2
3
4
5
6
7
8
active for the hydroboration/cylcization of enyne 1a (eq 3).
The results of these experiments suggest that Co(acac)2 can
be reduced by HBpin to generate a catalytically active CoI-
H species (see SI for the proposed mechanism involving
migratory insertion of 1,6-enyne into a CoI-H bond for this
hydroboration/cyclization of 1,6-enynes).
yields (69%–91%) and high ee (86%–92%).
When catalyzed by Co(acac)2 and (R,R)-quinoxP* at
room temperature, nitrogen- and carbon-tethered 1,6-
enynes reacted to give the products from anti-Markovnikov
hydroboration of the alkene without cyclization (see SI for
the details). However, under modified conditions with
(R,R)-L2 (see Table 1 for its structure), nitrogen- and car-
bon-tethered enynes underwent hydroboration/cyclization
smoothly at room temperature, affording the corresponding
alkyl boronate esters (3m−3s in Table 3) with high enanti-
oselectivities, albeit with modest isolated yields.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 3. Scope of 1,6-Enynes for Alkyl Boronate Estersa
Scheme 1. Transformations of Hydroboration/Cyclization
Products and Total Synthesis of (-)-Magnofargesin
aConditions: enyne (0.200 mmol), HBpin (0.300 mmol),
Co(acac)2 (6.0 ꢀmol), (R,R)-quinoxP* (8.0 ꢀmol), THF (1 mL),
rt, 12 h, isolated yields, and ee was determined by chiral HPLC
b
c
The synthetic utility of this cobalt-catalyzed transfor-
mation is shown in Scheme 1. The vinyl boronate ester 2q
reacted with 1-bromo-4-iodobenzene in the presence of
Pd(PPh3)4 to produce a tetra-substituted alkene 5 in high
yield (Scheme 1A). Vinyl boronate ester 2a underwent a
diastereoselective oxidative hydrolysis with NaBO3, afford-
ing the ketone 6 in 81% isolated yield with dr of 5:1
(Scheme 1A). Furthermore, we showed that an enantiomet-
rically pure 1,6-enyne 7 underwent a diastereoselective
hydroboration/cyclization, yielding an alkyl boronate ester
8, isolated as a single diastereomer in 76% yield (Scheme
1B). Finally, the synthesis of (-)-magnofargesin (10)
(Scheme 1C) was achieved in 33% yield with 94% ee via
the reaction sequence of the diastereo-divergent hydrobora-
tion/cyclization of enyne rac-9 and oxidation of the result-
ing alkyl boronate ester with H2O2 in the presence of aque-
ous NaOH solution.18
analysis; toluene (1 mL); The absolute configuration of 3l was
determined by single-crystal X-ray analysis on the corresponding
alcohol obtained by the oxidation of 3l. d(R,R)-L2 (8.0 ꢀmol).
To provide insight into the nature of the cobalt interme-
diate for this cobalt-catalyzed transformation, stoichio-
metric reactions with Co(acac)2, bisphosphine ligand, and
HBpin were conducted. As the hydroboration/cyclization of
enyne 1a catalyzed by the combination of Co(acac)2 and
dppbz occurred in a yield and a regioselectivity similar to
the corresponding reaction conducted with Co(acac)2 and
(R,R)-QuinoxP, we chose to study the activation of
Co(acac)2 with HBpin in the presence of non-chiral dppbz
ligand due to its accessibility. An NMR-scale reaction of
Co(acac)2 with 6 equivalents of HBpin in the presence of 1
equivalent of dppbz in THF-d8 revealed the formation of a
cobalt complex with a characteristic Co-H resonance (1H
NMR) at -14.4 ppm. The same cobalt species was formed
for the reaction conducted with 2 equivalent of dppbz (eq
2), and the X-ray analysis on the isolated material (Figure
S1) confirmed the identity of a CoI hydride complex
In summary, we have developed the first cobalt-
catalyzed asymmetric hydroboration/cyclization of 1,6-
enynes. The cobalt catalysts were generated in situ from
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 5
Kim, N. D.; Hah, J.-M.; Selim, K. B.; Choi, H. G.; Sim, T. Tetrahe-
dron 2012, 68, 1918.
Co(acac)2 and chiral bisphosphine ligands and activated by
1
2
3
4
5
6
7
8
the reaction with HBpin in the absence of additional activa-
tors. A range of oxygen-, nitrogen-, and carbon-tethered
1,6-enynes reacted to give both alkyl and vinyl boronate
esters containing chiral tetrahydrofuran, pyrrolidine, and
cyclopentane rings in modest to high yields and excellent
enantioselectivities. Further studies to reveal the detailed
mechanism of this transformation and to develop cobalt-
catalyzed asymmetric hydrofuncationalization/cyclization
of other multiply unsaturated substrates will be the subjects
of further work.
(4) For reviews on reductive cyclization and cycloisomerization of
enynes, see: (a) Trost, B. M.; Krische, M. J. Synlett. 1998, 1998, 1. (b)
Aubert, C.; Buisine, O.; Malacria, M. Chem. Rev. 2002, 102, 813. (c)
Deiters, A.; Martin, S. F. Chem. Rev. 2004, 104, 2199. (d) Villar, H.;
Frings, M.; Bolm, C. Chem. Soc. Rev. 2007, 36, 55. (e) Watson, I. D.
G.; Toste, F. D. Chem. Sci. 2012, 3, 2899. (e) Chen, W.-W.; Xu, M.-H.
Org. Biomol. Chem. 2017, 15, 1029.
(5) Buñuel, E.; Cárdenas, D. J. Eur. J. Org. Chem. 2016, 2016,
5446.
9
(6) For recent examples, see: (a) Iwai, Y.; Gligorich, K. M.; Sig-
man, M. S. Angew. Chem., Int. Ed. 2008, 47, 3219. (b) Zultanski, S.
L.; Fu, G. C. J. Am. Chem. Soc. 2011, 133, 15362. (c) Mlynarski, S.
N.; Karns, A. S.; Morken, J. P. J. Am. Chem. Soc. 2012, 134, 16449.
(d) Wang, C.; Wu, C.; Ge, S. ACS Catal. 2016, 6, 7585.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ASSOCIATED CONTENT
(7) (a) Burns, A. R.; González, J. S.; Lam, H. W. Angew. Chem.,
Int. Ed. 2012, 51, 10827. (b) Jiang, T.; Bartholomeyzik, T.; Mazuela,
J.; Willersinn, J.; Bäckvall, J.-E. Angew. Chem., Int. Ed. 2015, 54,
6024. (c) Liu, P.; Fukui, Y.; Tian, P.; He, Z.-T.; Sun, C.-Y.; Wu, N.-
Y.; Lin, G.-Q. J. Am. Chem. Soc. 2013, 135, 11700.
(8) Kinder, R. E.; Widenhoefer, R. A. Org. Lett. 2006, 8, 1967.
(9) For recent examples, see: (a) Iwai, Y.; Gligorich, K. M.; Sig-
man, M. S. Angew. Chem., Int. Ed. 2008, 47, 3219. (b) Shade, R. E.;
Hyde, A. M.; Olsen, J.-C.; Merlic, C. A. J. Am. Chem. Soc. 2010, 132,
1202. (c) Zultanski, S. L.; Fu, G. C. J. Am. Chem. Soc. 2011, 133,
15362. (d) Mlynarski, S. N.; Karns, A. S.; Morken, J. P. J. Am. Chem.
Soc. 2012, 134, 16449. (e) Wang, C.; Wu, C.; Ge, S. ACS Catal. 2016,
6, 7585.
(10) During the review process, Lu reported a Co-catalyzed non-
asymmetric hydroboration/cyclization of enynes and enantioselective
reactions were not achieved when chiral oxazoline iminopyridine
ligands were used. See: Xi, T.; Lu, Z. ACS Catal. 2017, 7, 1181.
(11) For recent examples, see: (a) Monfette, S.; Turner, Z. R.;
Semproni, S. P.; Chirik, P. J. J. Am. Chem. Soc. 2012, 134, 4561. (b)
Friedfeld, M. R.; Margulieux, G. W.; Schaefer, B. A.; Chirik, P. J. J.
Am. Chem. Soc. 2014, 136, 13178. (c) Fu, S.; Chen, N. Y.; Liu, X.;
Shao, Z.; Luo, S. P.; Liu, Q. J. Am. Chem. Soc. 2016, 138, 8588. (d)
Tokmic, K.; Fout, A. R. J. Am. Chem. Soc. 2016, 138, 13700. (e)
Tokmic, K.; Markus, C. R.; Zhu, L.; Fout, A. R. J. Am. Chem. Soc.
2016, 138, 11907. (f) Raya, B.; Biswas, S.; RajanBabu, T. V. ACS
Catal. 2016, 6, 6318.
(12) For recent examples, see: (a) Obligacion, J. V.; Chirik, P. J. J.
Am. Chem. Soc. 2013, 135, 19107. (b) Zhang, L.; Zuo, Z.; Wan, X.;
Huang, Z. J. Am. Chem. Soc. 2014, 136, 15501. (c) Zhang, L.; Zuo,
Z.; Leng, X.; Huang, Z. Angew. Chem., Int. Ed. 2014, 53, 2696. (d)
Obligacion, J. V.; Neely, J. M.; Yazdani, A. N.; Pappas, I.; Chirik, P.
J. J. Am. Chem. Soc. 2015, 137, 5855. (e) Palmer, W. N.; Diao, T.;
Pappas, I.; Chirik, P. J. ACS Catal. 2015, 5, 622. (f) Scheuermann, M.
L.; Johnson, E. J.; Chirik, P. J. Org Lett 2015, 17, 2716. (g) Zhang,
H.; Lu, Z. ACS Catal. 2016, 6, 6596.
The Supporting Information is available free of charge on
the ACS Publications website at DOI:
Experimental procedures, characterization of products, and
spectroscopic data (PDF)
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
chmgsh@nus.edu.sg
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the National University of
Singapore (No. R-143-000-614-133) and the Ministry of
Education (MOE) of Singapore (No. R-143-000-635-112).
The authors thank Dr. Yanxia Zhao at Northwest University
for solving the structure of the CoI-H complex 4.
REFERENCES
(1) (a) Bianchi, E.; Caldwell, M. E.; Cole, J. R. J. Pharm. Sci.
1968, 57, 696. (b) Miyazawa, M.; Hiroyuki, K.; Kameoka, H. Phyto-
chemistry 1996, 42, 531. (c) Huo, J.; Yang, S.-P.; Ding, J.; Yue, J.-M.
J. Nat. Prod. 2004, 67, 1470. (d) Cho, J. Y.; Williams, P. G.; Kwon,
H. C.; Jensen, P. R.; Fenical, W. J. Nat. Prod. 2007, 70, 1321.
(2) For recent examples of enantioselective cycloisomerization re-
actions of enynes, see: (a) Lei, A.; He, M.; Wu, S.; Zhang, X. Angew.
Chem., Int. Ed. 2002, 41, 3457. (b) Furstner, A.; Martin, R.; Majima,
K. J. Am. Chem. Soc. 2005, 127, 12236. (c) Nicolaou, K. C.; Li, A.;
Ellery, S. P.; Edmonds, D. J. Angew. Chem., Int. Ed. 2009, 48, 6293.
(d) Nishimura, T.; Kawamoto, T.; Nagaosa, M.; Kumamoto, H.;
Hayashi, T. Angew. Chem., Int. Ed. 2010, 49, 1638. (e) Teller, H.;
Furstner, A. Chem. Eur. J. 2011, 17, 7764. (f) Trost, B. M.; Ryan, M.
C.; Rao, M.; Markovic, T. Z. J. Am. Chem. Soc. 2014, 136, 17422. (g)
Masutomi, K.; Noguchi, K.; Tanaka, K. J. Am. Chem. Soc. 2014, 136,
7627. (h) Dieckmann, M.; Jang, Y.-S.; Cramer, N. Angew. Chem., Int.
Ed. 2015, 54, 12149. (i) Deng, X.; Ni, S.-F.; Han, Z.-Y.; Guan, Y.-Q.;
Lv, H.; Dang, L.; Zhang, X.-M. Angew. Chem., Int. Ed. 2016, 55,
6295.
(13) For recent examples, see: (a) Mo, Z.; Xiao, J.; Gao, Y.; Deng,
L. J. Am. Chem. Soc. 2014, 136, 17414. (b) Chen, C.; Hecht, M. B.;
Kavara, A.; Brennessel, W. W.; Mercado, B. Q.; Weix, D. J.; Holland,
P. L. J. Am. Chem. Soc. 2015, 137, 13244. (c) Du, X.; Zhang, Y;
Peng, D.; Huang, Z. Angew. Chem., Int. Ed. 2016, 55, 6671. (d) Guo,
J.; Lu, Z. Angew. Chem., Int. Ed. 2016, 55, 10835. (e) Teo, W. J.;
Wang, C.; Tan, Y. W.; Ge, S. Angew. Chem., Int. Ed. 2017, 56, 4328.
(14) For reviews, see: (a) Pellissier, H.; Clavier, H. Chem. Rev.
2014, 114, 2775. (b) Chirik, P. J. Acc. Chem. Res. 2015, 48, 1687. (c)
Gandeepan, P.; Cheng, C. H. Acc. Chem. Res. 2015, 48, 1194. (d)
Sun, J.; Deng, L. ACS Catal. 2016, 6, 290.
(15) Wang, C.; Teo, W.; Ge, S. ACS Catal. 2017, 7, 855.
(16) Reactions of these substrates at room temperature afforded the
products from anti-Markovnikov hydroboration of the terminal alkene
in these enynes and no cyclization was observed. See SI for the details.
(17) For a related Co(I)-H species (dppe)2CoH, see: Ciancanelli,
R.; Noll, B. C.; DuBois, D. L.; DuBois, M. R. J. Am. Chem. Soc.
2002, 124, 2984.
(3) For recent examples of enantioselective reductive cyclization
reactions of enynes, see: (a) Chakrapani, H.; Liu, C.; Widenhoefer, R.
A. Org. Lett. 2003, 5, 157. (b) Jang, H.-Y.; Hughes, F. W.; Gong, H.;
Zhang, J.; Brodbelt, J. S.; Krische, M. J. J. Am. Chem. Soc. 2005, 127,
6174. (c) Fan, B.-M.; Xie, J.-H.; Li, S.; Wang, L.-X.; Zhou, Q.-L.
Angew. Chem., Int. Ed. 2007, 46, 1275. (d) Park, J. H.; Kim, S. M.;
Chung, Y. K. Chem. –Eur. J. 2011, 17, 10852. (e) Ham, Y. J.; Yu, H.;
(18) For examples of synthesis of magnofargesin, see: (a) Wardrop,
D. J.; Fritz, J. Org. Lett. 2006, 8, 3659. (b) Chakraborty, P.; Jana, S.;
ACS Paragon Plus Environment

Page 5 of 5
Journal of the American Chemical Society
Saha, S.; Roy, S. C. Tetrahedron Lett. 2012, 53, 6584. (c)
1067.
Chakraborty, P.; Mandal, S. K.; Roy, S. C. J. Chem. Sci. 2016, 128,
1
2
3
Table of Contents
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment