Cobalt-Catalyzed Intermolecular Hydrofunctionalization of Alkenes: Evidence for a Bimetallic Pathway

Article abstract of DOI:10.1021/jacs.9b01857

A functional group tolerant cobalt-catalyzed method for the intermolecular hydrofunctionalization of alkenes with oxygen- and nitrogen-based nucleophiles is reported. This protocol features a strategic use of hypervalent iodine(III) reagents that enables a mechanistic shift from conventional cobalt-hydride catalysis. Key evidence was found supporting a unique bimetallic-mediated rate-limiting step involving two distinct cobalt(III) species, from which a new carbon-heteroatom bond is formed.

Full text of DOI:10.1021/jacs.9b01857

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Communication
Cobalt-Catalyzed Intermolecular Hydrofunctionalization
of Alkenes: Evidence for a Bimetallic Pathway
Xiao-Le Zhou, Fan Yang, Han-Li Sun, Yun-Nian Yin, Wei-Ting Ye, and Rong Zhu
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Apr 2019
Downloaded from http://pubs.acs.org on April 23, 2019
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Cobalt-Catalyzed Intermolecular Hydrofunctionalization of Alkenes:
Evidence for a Bimetallic Pathway
§
§
†
Xiao-Le Zhou, Fan Yang, Han-Li Sun, Yun-Nian Yin, Wei-Ting Ye, and Rong Zhu*
8
9
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engi-
neering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
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Supporting Information Placeholder
Here, we report the realization of this design with both preformed
ABSTRACT:
A functional group tolerant cobalt-catalyzed
and in situ generated I(III) reagents (Scheme 1c). In addition to a
kinetic dependence on the catalyst’s enantiopurity, we observed
that the hydrofunctionalization is second-order with respect to the
catalyst and zero-order regarding the stoichiometric reactants.
These findings are consistent with a mechanism involving a unique
C–Nu coupling process mediated by two Co complexes.
method for the intermolecular hydrofunctionalization of alkenes
with oxygen- and nitrogen-based nucleophiles is reported. This
protocol features a strategic use of hypervalent iodine(III) reagents
that enables a mechanistic shift from conventional cobalt–hydride
catalysis. Key evidence was found supporting a unique bimetal-
lic-mediated rate-limiting step involving two distinct cobalt(III)
species, from which a new carbon–heteroatom bond is formed.
Scheme 1. Intermolecular hydrofunctionalization via bi-
metallic-mediated coupling.
Catalytic carbon–heteroatom bond formation is of central im-
portance in modern organic chemistry practice.1 To this, inter-
cepting a carbon-centered radical or an equivalent intermediate
represents a useful strategy that is complementary to conventional
polar reactions.2 Such transformation is typically achieved via re-
acting with a radicalophile.3 Alternatively, coupling with a het-
eroatom-based nucleophile (NuH) can take place accompanied by
a one-electron oxidation process.4 Kochi pioneered study on the
latter in metal-mediated oxidations, which provides key insights
for developing catalytic transformations.5
Such divergent pathways can be found in the cobalt-catalyzed
Markovnikov-selective hydrofunctionalizations. These reactions
feature a hydrogen atom transfer (HAT) process, which generates
an alkyl radical that can be reversibly captured to form an organo-
cobalt species (Scheme 1a).6 Originated from Mukaiyama’s dis-
covery, elegant chemistry was developed by Carreira through in-
tercepting the HAT intermediate 1 with heteroatom-based radi-
calophiles.7 More recently, work from various groups has reshaped
this field.8 On the other hand, the mechanistically distinct oxida-
tive functionalization has historically received less attention in the
context of catalysis.9 A breakthrough was made by Shigehisa that
N-fluoropyridinium salt as a unique oxidant was found to effect
oxidative trapping process.10 Guided by this discovery, intramo-
lecular transformations were developed including cyclizations and
rearrangements, where crucial observations for carbocation-like
reactivity and catalyst-controlled selectivity were recently noted by
Shigehisa, Diaconescu and Pronin.11 Nevertheless, unlike the reac-
tions with radicalophiles, intermolecular oxidative functionaliza-
tion so far has achieved limited success other than solvolysis in
alcohols.12
We commenced our study by examining the hydroacyloxylation
reaction of 1-dodecene (4a) with a well-defined I(III) reagent 3 in
the presence of a silane and a catalytic amount of Co(II)-salen
complex at ambient temperature (Table 1). A set of optimal condi-
tions was identified as shown in entry 1. A combination of chiral
Co catalyst 7 and tetramethyldisiloxane (TMDSO) afforded the
desired benzoate product (5a) in 84% yield. The rest of the mass
balance was identified as isomerized alkenes (6a), which are likely
produced by the competing reverse HAT.8a Minimal stereocontrol
by this particular catalyst was noticed as 5a was produced in nearly
racemic form (ca. 5% ee).17 Alternative solvents were employed
and found to be inferior (entry 2). The choice of silane relies on a
balance between the stability and reactivity (entry 3). For example,
phenylsilane reacts instantly with 3 upon mixing, while tri-
ethylsilane is too stable. This protocol proved reasonably tolerant
toward moisture and air inclusion (entries 4 and 5).
A new system enabling efficient intermolecular coupling of 1 with
common oxygen- and nitrogen-based nucleophiles would offer
great synthetic potential as a milder and more selective alternative
to the acid- or heavy-metal-catalyzed Markovnikov-addition
methods.13 To this end, we sought to take advantage of cobalt’s
dual role (Scheme 1b). We envisioned that the addition of NuH to
iodosobenzene would furnish a group-transferring two-electron
oxidant 2.14 In the presence of a hydrosilane, the reaction of 2 with
two molecules of Co(II) complex affords a Co(III)–H and a
Co(III)–Nu intermediate, which are expected to promote the HAT
and oxidative C–Nu bond formation processes, respectively.15
Challenges are posed by numerous potential side reactions. These
comprise not only radical-type reactions via 1, but also competing
processes outside the metal hydride cycle.16
Key observations were made by evaluating the reaction
concentration and catalyst. First, diluting the reaction was found to
not only result in sluggish conversion, but also an increased share
of 6a (entry 6). Second, while identical yields were obtained using
either 7 or its enantiomer as expected (entry 7 vs 1), their racemic
mixture, however, displayed substantially attenuated catalytic
activity toward hydroacyloxylation (entry 8). Nearly 50% of 4a
remained after 18 h. Interestingly, the isomerization process seems
not affected. These findings prompted us to consider the potential
involvement of a higher-order pathway, which is supported by
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subsequent kinetic studies as discussed vide infra. In addition to 7,
Table 1. Evaluation of reaction conditions.a
several achiral Co complexes were tested (entries 9-11). 8 is
electronically similar to 7 and possesses additional steric bulk. This
complex gave slower conversion, and a slight increase in 6a was
detected. Finally, Co complexes with either a less electron-rich (9)
or more flexible (10) backbone did not promote the desired
reaction.
1
2
3
4
5
6
7
8
Entry
Changes from “Standard Conditions”
5a (%)b 6a (%)b
With the optimized protocol in hand, we first explored the reac-
tion scope with respect to the alkenes by evaluating their reactions
with 3 (Scheme 2a). A range of mono-substituted aliphatic alkenes
and styrenes were found to undergo hydroacyloxylation smoothly
at r.t., furnishing the corresponding benzoates 5 in medium to
good yields. It is worth noting that the desired products were ob-
tained with substrates that are incompatible with Brønsted or
Lewis acid-catalyzed methods (4e-g). For an -unsaturated car-
boxylic acid (4h), intramolecular trapping was not detected. De-
cent yields were obtained for electron-rich and cyclic styrenes (4i-l)
without modifying the protocol. For less electron-rich styrenes,
substantial dimerization was observed, which is attributable to
slower oxidative trapping and elevated free radical concentration.
Shifting to catalyst 8 and increasing the loading was found to miti-
gate this problem (4m-o). Additionally, we demonstrate propar-
gylic functionalization of a 1,3-enyne under similar conditions (4p).
However, 1,1-disubtituted alkenes and 1,3-dienes are not viable
substrates (4q-s).18
1
None
84
16
In THF, EtOAc, EtOH or toluene instead
of CH2Cl2
PhSiH3 or Et3SiH instead of TMDSO
With H2O (1.0 equiv.)
Open to air
2
< 10
8 ~ 21
3
4
< 5
69
63
24
81
36
67
< 5
5
< 5
21
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14
5
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5
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0.1 M instead of 0.3 M
ent-7 instead of 7
rac-7 instead of 7
8 instead of 7
7
8
9
10
11
9 instead of 7
10 instead of 7
a
Standard conditions: 4a (0.3 mmol, 1.0 equiv.), 3 (2.0 equiv.), TMDSO (2.0
b
equiv.), 7 (3.3 mol%) in 1.0 mL dichloromethane at r.t. for 18 h. Determined
by 1H NMR analysis of the crude reaction mixture.
Scheme 2. Evaluation of the reaction scope.a
a
b
Unless noted otherwise, yields correspond to isolated, analytically pure material. 4 (0.30 mmol, 1.0 equiv.), 3 (2.0 equiv.), TMDSO (2.0 equiv.), 7 (3.3 mol%) in 1.0 mL
dichloromethane at r.t. c 4 (0.30 mmol, 1.0 equiv.), NuH (4.0 equiv.), PhIO (2.0 equiv.), TMDSO (2.0 equiv.), 8 (3.3 mol%) in 1.0 mL ethyl acetate at r.t. d With additional
e
f
2,6-lutidine (1.0 equiv.). Determined by 1H NMR analysis of the crude reaction mixture. With 8 (10 mol%) in 3.0 mL solvent. Obtained as inseparable isomers (termi-
g
nal/internal = 7/3). h With 2-furoic acid (1.0 equiv.) and PhIO (1.0 equiv.).
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Next, the scope of nucleophiles was studied using in situ formed
involving a key interplay between two Co(III) species (Scheme 3).
The catalytic cycle begins with the formation of a Co–Nu (12) and
a Co–H (13) complexes, followed by HAT that generates a
secondary alkyl radical 15 in equilibrium with an organocobalt
complex 14. The reverse HAT would give either the starting
alkene or its isomers.22 Most importantly, our kinetics data
suggests that the r.d.s. for the desired transformation is
attributable to a bimetallic process, presumbly between 12 and 14,
which leads to final C–Nu bond formation.23 Such pathway has
few precedents in catalytic oxidative carbon–heteroatom forming
reactions.24 Nonetheless, it is worth noting that the concept of
bimetallic synergy has achieved phenomenal success in the
metal-salen-catalyzed epoxide opening reactions.25 Based on this
hypothesis, the discrepancy between the activities of rac-7 and
those of 7/ent-7 (Table 1) could imply higher barriers via
statistically formed heterochiral transition-state complexes.26 The
stereoselectivity obtained with 7/ent-7, though very moderate, also
strongly supports an inner-sphere mechanism.27
I(III) reagents from PhIO (Scheme 2b). Both 7 and 8 catalyzed the
transformation, and slightly better yields were obtained with the
latter. We were particularly interested in testing the compatibility
of our system with potential hydride-acceptors.19 It was found that
carboxylic acids and phenols containing ketones (11a), nitro
groups (11d), or Michael-acceptors (11b, c) were efficiently added
to alkenes without noticeable reduction. The chemoselectivity was
further illustrated by the selective functionalization of styrene in
the presence of an aliphatic alkene (11e). Increasing steric bulk in
the nucleophile attenuates the yield (11g-i). When the reaction was
carried out using stoichiometric amounts of PhIO and the carbox-
ylic acid, a useful yield (50%) was still obtained (11f). The mild
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conditions preserve acid-sensitive groups such as
a
tert-butyldimethylsilyl protected phenol and a tert-butyloxycarbonyl
protected secondary amine (11j). Additional examples involving
-substituted alkenes were shown (11k, l). Finally, we demonstrate
that amine derivatives could be accessed from a sulfonimide (11m,
n).
Scheme 3. Proposed catalytic cycle.
Scheme 4. Interrupting putative organocobalt(III).
Figure 1. Kinetic studies. a) b) Reaction profiles of 4a under
standard conditions (see Table 1) obtained by 1H NMR and
UV-Vis spectroscopy; c) Steady state rate of 5a formation as a
function of [Co] (lines added for a visual aid) and [Co]2 (inset); d)
Steady state rate of 6a formation as a function of [Co].
To shed light on the mechanistic origin of the concentration and
catalyst effects, we monitored the reaction of 4a and 3 under
standard conditions by 1H NMR and UV-Vis spectroscopy (Figure
1a, b). No characteristic absorption bands of Co(II) were
observable in the UV-Vis spectrum after a few minutes upon
mixing.20 Following an induction period that is typical for
Co-catalyzed HAT-type reactions, it appears that the formation of
both 5a and 6a followed zero-order kinetics until high conversion,
indicating that the stoichiometric reactants are likely not involved
in the rate-determining step (r.d.s.). Next, we performed the
reaction with various concentrations of 7 ([Co]) and measured the
steady state rates respectively (Figure 1c, d). It was found that the
rate at which 5a is produced is proportional to the square of [Co],
while that of the competing isomerization is proportional to
[Co].21 Therefore, the observed decrease in the ratio of 5a/6a
upon dilution is attributable to the different reaction orders.
To gain additional insight into the proposed intervention of the
organocobalt intermediate 14 in the C–Nu coupling process, we
performed experiments where the formation of 14 was interrupted
(Scheme 4). First, elevated temperature, which promotes the
homolysis of the organocobalt intermediate, led to significant
increase in isomerization and decrease in hydrofunctionalization
(Scheme 4a). Second, we tested a substrate (16) that upon HAT
would cyclize and produce a hindered secondary radical, which
sterically discourages a Co–C bond (Scheme 4b).8a As a result, the
isomerized alkene was obtained as the sole product. This
corroborates the observation that 1,1-disubstituted alkenes (4q, r)
failed in the hydroacyloxyaltion reaction despite the lower
oxidation potential of a tertiary alkyl radical than that of a
secondary one, since
a
tertiary alkylcobalt is disfavored.28
Collectively, these results are in accordance with our hypothesis
that the two competing pathways, namely oxidative
functionalization and isomerization, are operating in parallel from
14 and 15, respectively.
The rate law, in conjuction with the kinetic dependence on the
catalyst’s enantiopurity, points to
a mechanistic hypothesis
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substitution reactions: a radical alternative to SN1 and SN2 processes.
ACS Cent. Sci. 2017, 3, 692.
The mechanistic detail of the intriguing bimetallic-mediated
coupling remains unclear at this stage and will be a topic of future
investigation. Nevertheless, C–Nu bond formation accompanied
by a single-electron transfer (SET) between two Co(III) centers
1
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3. For examples: (a) Muñoz-Molina, J. M.; Belderrain, T. R.; Pérez, P. J.,
Atom transfer radical reactions as a tool for olefin functionalization - on
the way to practical applications. Eur. J. Inorg. Chem. 2011, 2011, 3155;
(b) Meyer, D.; Renaud, P., Enantioselective hydroazidation of
trisubstituted non-activated alkenes. Angew. Chem. Int. Ed. 2017, 56,
10858.
4. Selected reviews: (a) Murphy, J. A., The radical-polar crossover
reaction. In Radicals in organic synthesis, 2001; pp 298; (b) Zhang, N.;
Samanta, S. R.; Rosen, B. M.; Percec, V., Single electron transfer in
radical ion and radical-mediated organic, materials and polymer
synthesis. Chem. Rev. 2014, 114, 5848; (c) Zhu, X.; Chiba, S.,
Copper-catalyzed oxidative carbon-heteroatom bond formation: a recent
update. Chem. Soc. Rev. 2016, 45, 4504; For an excellent example of
C–C bond formation: (d) Kischkewitz, M.; Okamoto, K.;
Muck-Lichtenfeld, C.; Studer, A., Radical-polar crossover reactions of
vinylboron ate complexes. Science 2017, 355, 936.
could be possibly involved based on reports of electro- and
11f
chemical oxidations.9c,
In addition, a Co/Ni transmetalation
process via a Co(IV) intermediate was proposed by Shenvi for
arylative HAT-type reaction.29,30
In conclusion, we have developed a Co-catalyzed intermolecular
hydrofunctionalization reaction through the oxidative coupling
between a non-solvent oxygen- or nitrogen-based nucleophile and
an HAT intermediate derived from either an unactived alkene,
styrene or 1,3-enyne. Synthesis of a broad spectrum of esters,
aromatic ethers, and sulfonimides are achieved. Notably,
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experimental
evidence
for
a
unique
rate-limiting
bimetallic-mediated coupling process was obtained. Specifically,
C–Nu coupling via an organocobalt(III) and a Co(III)–Nu
complex is proposed to account for the observed rate law and
heterochiral effect. The identification of such pathway provides a
potentially valuable handle for expanding the scope of Co–H
chemistry through catalyst tuning. In a broader sense, these results
represent an emerging mechanistically distinct mode for
carbon–heteroatom bond formation in catalytic reactions. We
anticipate new design principles stem from this finding, for
instance, dimeric catalysts, would likely offer further increased
efficiency and potentially stereoselectivity, as seen in the classic
Jacobsen epoxide-opening reactions.25b These possibilities are
currently under investigation in our laboratory.
5. Kochi, J. K., Electron-transfer mechanisms for organometallic
intermediates in catalytic reactions. Acc. Chem. Res. 1974, 7, 351.
6. For excellent reviews, see: (a) Crossley, S. W.; Obradors, C.;
Martinez, R. M.; Shenvi, R. A., Mn-, Fe-, and Co-Catalyzed radical
hydrofunctionalizations of olefins. Chem. Rev. 2016, 116, 8912; (b)
Hoffmann, R. W., Markovnikov free radical addition reactions, a
sleeping beauty kissed to life. Chem. Soc. Rev. 2016, 45, 577; (c)
Demarteau, J.; Debuigne, A.; Detrembleur, C., Organocobalt complexes
as sources of carbon-centered radicals for organic and polymer
chemistries. Chem. Rev. 2019. Doi: 10.1021/acs.chemrev.8b00715.
7.(a) Isayama, S.; Mukaiyama, T., A new method for preparation of
alcohols from olefins with molecular oxygen and phenylsilane by the
use of bis(acetylacetonato)cobalt(ii). Chem. Lett. 1989, 18, 1071; (b)
Kato, K.; Mukaiyama, T., Nitrosation of α,β-unsaturated carboxamide
with nitric oxide and triethylsilane catalyzed by cobalt(II) complex.
Chem. Lett. 1990, 19, 1395; (c) Waser, J.; Nambu, H.; Carreira, E. M.,
Cobalt-catalyzed hydroazidation of olefins: convenient access to alkyl
azides. J. Am. Chem. Soc. 2005, 127, 8294; (d) Waser, J.; Gaspar, B.;
Nambu, H.; Carreira, E. M., Hydrazines and azides via the
metal-catalyzed hydrohydrazination and hydroazidation of olefins. J.
Am. Chem. Soc. 2006, 128, 11693; (e) Gaspar, B.; Carreira, E. M.,
ASSOCIATED CONTENT
Supporting Information. Experimental procedures, characteriza-
tion, spectra and kinetic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
Catalytic
hydrochlorination
of
unactivated
olefins
with
AUTHOR INFORMATION
para-toluenesulfonyl chloride. Angew. Chem. Int. Ed. 2008, 47, 5758.
8. Selected recent examples of Co–H catalysis: (a) Crossley, S. W.;
Barabe, F.; Shenvi, R. A., Simple, chemoselective, catalytic olefin
isomerization. J. Am. Chem. Soc. 2014, 136, 16788; (b) Green, S. A.;
Matos, J. L.; Yagi, A.; Shenvi, R. A., Branch-Selective hydroarylation:
iodoarene-olefin cross-coupling. J. Am. Chem. Soc. 2016, 138, 12779; (c)
Green, S. A.; Vasquez-Cespedes, S.; Shenvi, R. A., Iron-Nickel
§ These authors contributed equally.
† Current address: Department of Applied Biology and Chemical
Technology (ABCT), The Hong Kong Polytechnic University,
China.
dual-catalysis:
a new engine for olefin functionalization and the
Corresponding Author
* rongzhu@pku.edu.cn
formation of quaternary centers. J. Am. Chem. Soc. 2018, 140, 11317; (d)
Matos, J. L. M.; Vasquez-Cespedes, S.; Gu, J.; Oguma, T.; Shenvi, R.
A., Branch-Selective addition of unactivated olefins into imines and
aldehydes. J. Am. Chem. Soc. 2018, 140, 16976; (e) King, S. M.; Ma,
X.; Herzon, S. B., A method for the selective hydrogenation of alkenyl
halides to alkyl halides. J. Am. Chem. Soc. 2014, 136, 6884; (f) Ma, X.;
Dang, H.; Rose, J. A.; Rablen, P.; Herzon, S. B., Hydroheteroarylation
of unactivated alkenes using N-methoxyheteroarenium salts. J. Am.
Chem. Soc. 2017, 139, 5998; (g) Ma, X.; Herzon, S. B., Cobalt
bis(acetylacetonate)-tert-butyl hydroperoxide-triethylsilane: a general
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by startup fund from College of Chem-
istry and Molecular Engineering, Peking University and BNLMS.
The authors would like to thank Profs. Jianbo Wang and Tuoping
Luo (PKU) for insightful discussions and access to their instru-
mentation. Dedicated to Prof. Zhen Yang on the occasion of his
60th birthday.
reagent
combination
for
the
Markovnikov-selective
hydrofunctionalization of alkenes by hydrogen atom transfer. Beilstein J.
Org. Chem. 2018, 14, 2259; (h) Girijavallabhan, V.; Alvarez, C.;
Njoroge, F. G., Regioselective cobalt-catalyzed addition of sulfides to
unactivated alkenes. J. Org. Chem. 2011, 76, 6442; (i) Shigehisa, H.;
Nishi, E.; Fujisawa, M.; Hiroya, K., Cobalt-catalyzed hydrofluorination
of unactivated olefins: a radical approach of fluorine transfer. Org. Lett.
2013, 15, 5158; For recent work on Fe–H catalyzed C–heteroatom bond
forming reactions: (j) Gui, J.; Pan, C. M.; Jin, Y.; Qin, T.; Lo, J. C.;
Lee, B. J.; Spergel, S. H.; Mertzman, M. E.; Pitts, W. J.; La Cruz, T. E.;
Schmidt, M. A.; Darvatkar, N.; Natarajan, S. R.; Baran, P. S., Practical
olefin hydroamination with nitroarenes. Science 2015, 348, 886; (k)
Barker, T. J.; Boger, D. L., Fe(III)/NaBH4-mediated free radical
hydrofluorination of unactivated alkenes. J. Am. Chem. Soc. 2012, 134,
13588; (l) Leggans, E. K.; Barker, T. J.; Duncan, K. K.; Boger, D. L.,
Iron(III)/NaBH4-mediated additions to unactivated alkenes: synthesis of
novel 20'-vinblastine analogues. Org. Lett. 2012, 14, 1428; (m) Zheng,
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2
3
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6
7
8
9
9. Organocobalt(III) complexes are known to be oxidized to Co(IV)
species that is susceptible to nucleophilic attack: (a) Vol'pin, M. E.;
Levitin, I. Y.; Sigan, A. L.; Halpern, J.; Tom, G. M., Reactivity of
organocobalt(IV) chelate complexes toward nucleophiles: diversity of
mechanisms. Inorg. Chim. Acta 1980, 41, 271; (b) Vol'pin, M. E.;
Levitin, I. Y.; Sigan, A. L.; Nikitaev, A. T., Current state of
organocobalt(IV) and organorhodium(IV) chemistry. J. Organomet.
Chem. 1985, 279, 263; (c) Halpern, J., Oxidation of Organometallic
Compounds. Angew. Chem. Int. Ed. 1985, 24, 274. For a C–C bond
forming reaction that possibly proceed through Co(IV), see references
8b and 28a.
20. (a) Liao, C.-M.; Hsu, C.-C.; Wang, F.-S.; Wayland, B. B.; Peng,
C.-H., Living radical polymerization of vinyl acetate and methyl
acrylate mediated by Co(Salen*) complexes. Polym. Chem. 2013, 4,
3098; (b) Clarke, R. M.; Herasymchuk, K.; Storr, T., Electronic
structure elucidation in oxidized metal–salen complexes. Coord. Chem.
Rev. 2017, 352, 67. For a discussion, see supporting information (S22).
21. For data from individual experiments, see supporting information.
22. Organocobalt has been proposed to be an off-cycle species in
catalytic isomerization. See reference 8a.
23. The absence of substantial enantiomeric excess prevented detection
or exclusion of a potential non-linear effect at this point.
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10. Shigehisa, H., Studies on catalytic activation of olefins using cobalt
complex. Chem. Pharm. Bull. 2018, 66, 339.
11. (a) Shigehisa, H.; Koseki, N.; Shimizu, N.; Fujisawa, M.; Niitsu, M.;
Hiroya, K., Catalytic hydroamination of unactivated olefins using a Co
catalyst for complex molecule synthesis. J. Am. Chem. Soc. 2014, 136,
24. For important precedents of binuclear Pd intermediates in oxidative
catalysis, see: (a) Powers, D. C.; Ritter, T., Bimetallic redox synergy in
oxidative palladium catalysis. Acc. Chem. Res. 2012, 45, 840; (b)
Powers, D. C.; Ritter, T., Bimetallic Pd(III) complexes in
palladium-catalysed carbon–heteroatom bond formation. Nat. Chem.
2009, 1, 302.; For a recent review: (c) Pye, D. R.; Mankad, N. P.,
Bimetallic catalysis for C-C and C-X coupling reactions. Chem. Sci.
2017, 8, 1705.
25. (a) Hansen, K. B.; Leighton, J. L.; Jacobsen, E. N., On the
mechanism of asymmetric nucleophilic ring-opening of epoxides
catalyzed by (salen)CrIIIComplexes. J. Am. Chem. Soc. 1996, 118,
10924; (b) Ford, D. D.; Nielsen, L. P.; Zuend, S. J.; Musgrave, C. B.;
Jacobsen, E. N., Mechanistic basis for high stereoselectivity and broad
substrate scope in the (salen)Co(III)-catalyzed hydrolytic kinetic
resolution. J. Am. Chem. Soc. 2013, 135, 15595; (c) Mulzer, M.;
Whiting, B. T.; Coates, G. W., Regioselective carbonylation of
13534;
(b)
Shigehisa,
H.,
Functional
group
tolerant
markovnikov-selective hydrofunctionalization of unactivated olefins
using a cobalt complex as catalyst. Synlett 2015, 26, 2479; (c) Shigehisa,
H.; Hayashi, M.; Ohkawa, H.; Suzuki, T.; Okayasu, H.; Mukai, M.;
Yamazaki, A.; Kawai, R.; Kikuchi, H.; Satoh, Y.; Fukuyama, A.; Hiroya,
K., Catalytic synthesis of saturated oxygen heterocycles by
hydrofunctionalization of unactivated olefins: unprotected and protected
strategies. J. Am. Chem. Soc. 2016, 138, 10597; (d) Shigehisa, H.; Ano,
T.; Honma, H.; Ebisawa, K.; Hiroya, K., Co-Catalyzed hydroarylation
of unactivated olefins. Org. Lett. 2016, 18, 3622; (e) Shepard, S. M.;
Diaconescu, P. L., Redox-Switchable hydroelementation of a cobalt
complex supported by a ferrocene-based ligand. Organometallics 2016,
35, 2446; (f) A recent report on possible involvement of alkylCo(IV) as
an eletrophilic intermediate: Touney, E. E.; Foy, N. J.; Pronin, S. V.,
Catalytic radical-polar crossover reactions of allylic alcohols. J. Am.
Chem. Soc. 2018, 140, 16982.
trans-disubstituted epoxides to -lactones:
a viable entry into
syn-aldol-type products. J. Am. Chem. Soc. 2013, 135, 10930.
26. The stereogenic carbon atom bound to Co in 14 complicates a
strictly accurate description. In spite of this, considering the fast
equilibrium between 14 and 15, such kinetic effect, if not predominantly,
could be at least partially derived from the stereochemical heterogeneity
in the salen ligand. It should be also noted that the current data does not
exclude the effect of potential persistent off-cycle multinuclear species.
12.(a) Shigehisa, H.; Aoki, T.; Yamaguchi, S.; Shimizu, N.; Hiroya, K.,
Hydroalkoxylation of unactivated olefins with carbon radicals and
carbocation species as key intermediates. J. Am. Chem. Soc. 2013, 135,
10306;
(b)
Shigehisa,
H.;
Kikuchi,
H.;
Hiroya,
K.,
27.
A few alternative monometallic pathways were considered,
Markovnikov-Selective addition of fluorous solvents to unactivated
olefins using a Co catalyst. Chem. Pharm. Bull. 2016, 64, 371.
13. (a) Li, Z.; Zhang, J.; Brouwer, C.; Yang, C. G.; Reich, N. W.; He, C.,
Brønsted acid catalyzed addition of phenols, carboxylic acids, and
tosylamides to simple olefins. Org. Lett. 2006, 8, 4175; (b) Yang, C. G.;
He, C., Gold(I)-catalyzed intermolecular addition of phenols and
carboxylic acids to olefins. J. Am. Chem. Soc. 2005, 127, 6966; (c) Oe,
Y.; Ohta, T.; Ito, Y., Ruthenium catalyzed addition reaction of
carboxylic acid across olefins without -hydride elimination. Chem.
Commun. 2004, 1620; (d) Rosenfeld, D. C.; Shekhar, S.; Takemiya, A.;
Utsunomiya, M.; Hartwig, J. F., Hydroamination and hydroalkoxylation
catalyzed by triflic acid. Parallels to reactions initiated with metal
triflates. Org. Lett. 2006, 8, 4179.
14. Yoshimura, A.; Zhdankin, V. V., Advances in Synthetic
Applications of Hypervalent Iodine Compounds. Chem. Rev. 2016, 116,
3328. Other I(III) species containing Nu as ligands might also be
produced in addition to 2, which would lead to similar reactions
affording Co–H and Co–Nu complexes. It is worth mentioning that
using I(III) reagents can avoid the undesired fluorination side reaction
observed with N-fluoropyridinium salt. See reference 8i.
15. For Co(III) carboxylate-mediated oxidative trapping: Lande, S. S.;
Kochi, J. K., Formation and oxidation of alkyl radicals by cobalt(III)
complexes. J. Am. Chem. Soc. 1968, 90, 5196.
16. For instance, direct oxidation of the silane, decarboxylation (when
Nu is a carboxylate), and epoxidation. (a) Xu, K.; Wang, Z.; Zhang, J.;
Yu, L.; Tan, J., Cobalt-Catalyzed decarboxylative acetoxylation of
amino acids and arylacetic acids. Org. Lett. 2015, 17, 4476; (b) Koola, J.
D.; Kochi, J. K., Cobalt-catalyzed epoxidation of olefins. Dual pathways
for oxygen-atom transfer. J. Org. Chem. 1987, 52, 4545.
including: 1) oxidation of 15 by 12 or 2; 2) oxidation of 14 by 2.
However, they are ruled out due to inconsistent kinetics and failure to
account for the enantiopurity effect.
28. Wayner, D. D. M.; McPhee, D. J.; Griller, D., Oxidation and
reduction potentials of transient free radicals. J. Am. Chem. Soc. 1988,
110, 132. Our observations underline the mechanistic difference
between intra- and intermolecular reactions. While mono-cobalt
mediated radical oxidation is proposed in the former (reference 10), the
latter seems to necessitate a bimetallic pathway.
29. (a) For an excellent discussion on bimetallic-mediated C–C bond
formation process, see: Shevick, S. L.; Obradors, C.; Shenvi, R. A.,
Mechanistic interrogation of Co/Ni-dual catalyzed hydroarylation. J. Am.
Chem. Soc. 2018, 140, 12056. (b) For a dimeric iron species in
FeH-catalyzed olefin coupling reactions, see: Lo, J. C.; Kim, D.; Pan, C.
M.; Edwards, J. T.; Yabe, Y.; Gui, J.; Qin, T.; Gutierrez, S.; Giacoboni,
J.; Smith, M. W.; Holland, P. L.; Baran, P. S., Fe-Catalyzed C-C bond
construction from olefins via radicals. J. Am. Chem. Soc. 2017, 139,
2484.
30. Although the oxidation potential of a secondary alkyl Co(III) salen
complex is estimated to be slightly higher than the reduction potential of
a Co(III)–X complex by ca. 0.1 V, this gap is small and subjected to
change as the coordination environment varies, thereby possibly
allowing SET during catalytic reactions. See supporting information
(S25) for further discussion. (a) Chiang, L.; Allan, L. E.; Alcantara, J.;
Wang, M. C.; Storr, T.; Shaver, M. P., Tuning ligand electronics and
peripheral substitution on cobalt salen complexes: structure and
polymerisation activity. Dalton Trans 2014, 43, 4295; (b) Kurahashi, T.;
Fujii, H., Unique ligand-radical character of an activated cobalt salen
catalyst that is generated by aerobic oxidation of a cobalt(II) salen
complex. Inorg. Chem. 2013, 52, 3908; (c) Levitin, I.; Sigan, A. L.;
Vol'pin, M. E., Electrochemical generation and reactivity of
organo-cobalt(IV) and -rhodium(IV) chelates. J. Chem. Soc., Chem.
Commun. 1975, 469.
17. For comparison, -7% ee was obtained using ent-7 (entry 7). See
Table S1 for details.
18. See supporting information (S14) for details.
19. (a) Tsubo, T.; Chen, H.-H.; Yokomori, M.; Fukui, K.; Kikuchi, S.;
Yamada, T., Enantioselective borohydride reduction of aliphatic ketones
catalyzed by ketoiminatocobalt(III) complex with 1-chlorovinyl axial
ligand. Chem. Lett. 2012, 41, 780; (b) Inagaki, T.; Phong le, T.; Furuta,
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