Remote, Diastereoselective Cobalt-Catalyzed Alkene Isomerization-Hydroboration: Access to Stereodefined 1,3-Difunctionalized Indanes

Article abstract of DOI:10.1021/acscatal.9b03444

The remote, diastereoselective hydroboration of 2- and 3-substituted indenes with a 2,2′:6′,2″-terpyridine cobalt alkyl precatalyst is described that maintains high regio- and stereoselectivity independent of the starting position of the alkene. Several 1,2- and 1,3-disubstituted indanyl boronate esters were obtained with exclusive (>20:1 dr) selectivity for the trans diastereomer including synthetically versatile, stereodefined diboron derivatives. Alkene isomerization by a putative cobalt hydride intermediate precedes carbon-boron bond formation, leading to the observed regioselectivity for boron incorporation at the unsubstituted C(sp³)-H benzylic site. The regio- and diastereoselectivity of the transformation were maintained independent of the starting position of the alkene, as demonstrated by hydroboration of three isomers of methyl-substituted indene. Deuterium-labeling experiments support rapid and reversible insertion and β-hydride elimination to isomerize 3-methylindene and 1-exo-methylene-indane, accounting for the isotopic distribution observed in the products. Mechanistic studies, including stoichiometric experiments, density functional theory calculations, and kinetic analysis, support a mechanism in which 2,3-alkene insertion into a cobalt hydride intermediate determines both the regio- and diastereoselectivity of the catalytic reaction. Synthetic applications of the indanyl boronate esters were demonstrated through the elaboration of the products to several examples of 1,3-disubstituted indanes, important carbocyclic structural motifs in both pharmacological and bioactive molecules.

Full text of DOI:10.1021/acscatal.9b03444

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Article
Remote, Diastereoselective Cobalt-Catalyzed Alkene Isomerization-
Hydroboration: Access to Stereodefined 1,3-Difunctionalized Indanes
Nadia G. Leonard, W. Neil Palmer, Max R. Friedfeld, Máté J. Bezdek, and Paul J. Chirik
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b03444 • Publication Date (Web): 20 Aug 2019
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ACS Catalysis
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Remote, Diastereoselective Cobalt-Catalyzed Alkene Isomerization-
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Nadia G. Léonard, W. Neil Palmer, Max R. Friedfeld, Máté J. Bezdek, Paul J. Chirik*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
ABSTRACT: The remote, diastereoselective hydroboration of 2- and 3-substituted indenes with a 2,2’:6’,2”-terpyridine
cobalt alkyl precatalyst is described that maintains high regio- and stereoselectivity selectivity independent of the
starting position of the alkene. Several 1,2- and 1,3-disubstituted indanyl boronate esters were obtained with exclusive
(>20:1) selectivity for the trans diastereomer including synthetically versatile, stereodefined diboron derivatives. Alkene
isomerization by a putative cobalt hydride intermediate precedes carbonboron bond formation, leading to the
observed regioselectivity for boron incorporation at the unsubstituted C(sp3)-H benzylic site. The regio- and
diastereoselectivity of the transformation was maintained independent of the starting position of the alkene, as
demonstrated by hydroboration of three isomers of methyl-substituted indene. Deuterium labeling experiments
support rapid and reversible insertion and -hydride elimination to isomerize 3-methylindene and 1-exo-methylene-
indane, accounting for the isotopic distribution observed in the products. Mechanistic studies, including stoichiometric
experiments, DFT calculations, and kinetic analysis, support a mechanism in which 2,3-alkene insertion into a cobalt
hydride intermediate determines both the regio- and diastereoselectivity of the catalytic reaction. Synthetic applications
of the indanyl boronate esters were demonstrated through elaboration of the products to several examples of 1,3-
disubstituted indanes, important carbocyclic structural motifs in both pharmacological and bioactive molecules.
KEYWORDS: Diasteroselective, hydroboration, cobalt catalysis, chain-walking, indanes
INTRODUCTION
Demonstration
of
stereo-
and
regioselective
isomerization-hydroboration of alkenes is valuable for
broadening the applicability of this method toward the
synthesis of new organoboron compounds.
The synthetic utility of organoboron compounds
continues to motivate the development of methods for
the selective incorporation of boron into unique
molecular scaffolds.1 Transition metal-catalyzed alkene
hydroboration reactions that exhibit high chemo-, regio-
and stereocontrol are particularly valuable for accessing
synthetically valuable and modular targets.2 In particular,
catalysts that promote alkene isomerization coupled to a
subsequent carbon-element bond-forming sequence are
enabling tools in synthesis for the functionalization of
remote CH bonds that are distal to the reactive C=C
In 2013, our group reported pyridine(diimine) cobalt
alkyl
complexes
that
promote
isomerization-
hydroboration of tri- and tetrasubstituted alkenes with
pinacolborane (HBPin) under neat conditions to generate
terminal boronate esters (Scheme 1a).8 Isotopic labeling
experiments supported rapid migration of the cobalt
center along the alkyl chain with CB bond formation
occurring exclusively at primary carbon positions. -
Diimine-ligated cobalt precatalysts were later developed
and exhibited unprecedented activity for tetrasubstituted
alkenes, however sluggish reactivity was observed with
styrene likely due to the formation of inactive 3-allyl
complexes.9
While isomerization-hydroboration was initially
observed in rhodium-catalyzed alkene hydroboration,10,11
contemporary interest in the development of Earth-
abundant metal catalysts12 has demonstrated that this
tandem process is frequently observed with first-row
transition-metals to form terminal alkyl boronate esters
3
double bond This tandem reaction pathway is well
established in important catalytic processes such as
alkene
hydroformylation4
and
polymerization.5,6
Application of this strategy to the construction of bonds
in fine chemicals and more complex molecules is an
emerging frontier in catalysis directed toward synthesis.7
Through insertion of an alkene into a transition-metal
hydride bond followed by successive -H elimination-
reinsertion events, a remote carbonelement bond can
be constructed site-selectively in the molecule without
the need for
a
preinstalled directing group.
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from internal alkenes. Recently, Stradiotto, Turculet and
coworkers reported a comprehensive experimental and
computational study of (N-phosphinoamidinate)
achieving Markovnikov selectivity or more generally
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secondary C–B bonds is largely substrate dependent and
typically limited to styrene derivatives (Scheme 1b, path
B).18,19 Our laboratory has reported the synthesis of a
2,2’:6’,2”-terpyridine cobalt alkyl complex that promotes
the selective hydroboration of styrene with 25:1
regioselectivity for the Markovnikov product.9 The
regioselective hydroboration of 1,1-diborylalkenes was
also effectively catalyzed, resulting in formation of the
1,1,1-triboronate ester, likely due to a stabilizing -boryl
effect on the cobalt alkyl intermediate.20 Utilization of
tandem isomerization-hydroboration to yield branched
alkyl boronate esters is rarer, however and cyclic alkenes
are an unexplored yet important class of substrates. Our
group reported the isomerization-hydroboration of
linear terminal alkenes to yield branched products with
boron incorporation adjacent to -systems.21 Employing
a pre-isomerization step prior to HBPin addition, high
regioselecivities were achieved. Lu and coworkers later
expanded this mode of reactivity and developed a
general method for the enantioselective synthesis of
benzyl boronate esters with excellent enantioselectivity.22
complexes based upon Mn, Fe, Co, and Ni for
isomerization-hydroboration of octene isomers.13 Facile
isomerization as well as significantly lower barriers for
boron transfer to the terminal metal alkyl complex versus
isomeric secondary metal alkyl intermediates are the
origin of the terminal selectivity observed in the final n-
octylboronate ester. This conclusion is further supported
by the several well-established examples of cobalt-
catalyzed anti-Markovnikov selective hydroborations of
acyclic alkenes (Scheme 1b, path A)14 including the
enantioselective hydroboration of 1,1-disubstituted aryl
alkenes independently developed by Huang15 and Lu.16
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Extending
catalytic
alkene
isomerization-
Scheme 1. Strategies for Selectivity Control in Alkene
Hydroboration.
hydroboration to the formation of secondary CB bonds
in cyclic alkenes is attractive for developing remote
regio- and stereoselective reactions. Given the observed
preference for 2,2’:6’,2”-terpyridine cobalt-derived
catalysts to yield branched alkyl boronate esters,
application of this catalyst to the isomerization-
hydroboration of 3-substituted indenes was explored
(Scheme 1c). In addition to presenting a sterically
challenging and unexplored carbocyclic core for cobalt-
catalyzed hydrofunctionalization reactions, the indanyl
unit is an important, recurring substructure in a variety of
pharmaceuticals, bioactive compounds and natural
products (Figure 1).23,24,25,26 Common methods to prepare
indanes
include
Friedel-Crafts
or
Michael-type
cyclizations, ring-expansion or contraction methods, and
direct C-H activation.27 However, competing side
reactions can complicate these methods and prohibit
modular incorporation of substituents around the
indanyl ring.28 Metal-catalyzed hydroboration of readily
synthesized 3-substituted indenes is attractive as a
complementary method owing to the versatility of
boronate esters to be readily transformed to other
groups with control of stereochemistry.29
The preference of transition metal catalysts to favor
terminally selective C–B bond formation at primary sites
has presented a challenge for the development methods
for branched (Markovnikov) selective hydroboration.17
This particularly pronounced in cyclic alkenes where the
absence of primary sites to drive C–B bond formation
translate on to limitations with scope. In cobalt catalysis,
Figure 1. Examples of 1,3-disubstituted indanes in
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ACS Catalysis
a
b
pharmaceuticals and bioactive molecules. Ref. 23. Ref. 24.
cRef. 25. dRef. 26.
1
2
Here we describe the remote, diastereoselective
cobalt-catalyzed hydroboration of 3-substituted indenes
to selectively yield trans-1,3-disubstituted indanes.
Comprehensive investigations into the mode of action
for alkene isomerization and subsequent hydroboration
are presented. Deuterium labeling studies provide
insight into the regio- and facial preferences for alkene
insertion with the cobalt catalyst. The synthetic utility of
the trans-1,3-disubstituted indanyl boronate esters was
demonstrated through a host of bond constructions
from the boron group including vinylation and arylation.
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RESULTS AND DISCUSSION
Initial method development employed 0.10 mmol of
the indene substrate, and 1.05 equivalents of HBPin in
the presence of 5 mol% of cobalt precatalyst at 23 ºC for
20 h in 0.10 M toluene. Several cobalt alkyl complexes
were examined (Scheme 2, see Supplemental
Information for full details) with 2,2’:6’,2”-terpyridine
cobalt alkyl ([Co]-1) exhibiting the highest yield of
boronate ester for both the methyl and phenyl
substituted indenes (entries 1 and 2, respectively), as
determined by gas chromatograpy. Analysis of the
products by 1H, 13C and 1H correlation NMR experiments
established exclusive formation of 1,3-substituted
products with an anti disposition of the boron
substituent to the R group. Isolated yields of 96% and
90% were obtained for the phenyl and methyl
substituted indanes, respectively. Complexes [Co]-3 and
[Co]-4 exhibited similar regio- and diastereoselectivities
(entries 7-10) however, a significant side reaction
yielding alkane was observed with these precatalysts. The
diimine cobalt 3-allyl complex ([Co]-5) yielded no
hydroborated product, likely due to formation of a
stabilized -indanyl complex.9 By contrast, uncatalyzed
hydroboration with 9-borabicyclo[3.3.1]nonane (9-BBN)
resulted in 1,2-substituted products (entries 3 and 4),
further highlighting the unique regioselectivity enabled
by the cobalt-catalyzed method.
a
Unless otherwise noted, reactions conducted with 0.10
mmol of indene, 0.105 mmol HBPin, 0.005 mmol precatalyst,
in 1.0 mL toluene at 23 ºC for 20 h. Reactions conducted
with 0.10 mmol of indene, 0.50 mmol 9-BBN at 75 ºC for 48
h. Reported NMR yield of alcohol resulting from oxidation
b
c
of trialkyl borane. See SI for full details.
Reactions
d
conducted for 48 h. Yield and dr was determined from the
calibrated relative integrals of product and cyclooctane
signals measured by gas chromatographic analysis of the
unpurified reaction mixture.
The scope of the isomerization-hydroboration reaction
was investigated and the results of these experiments are
reported in Scheme 3. Standard conditions employed a
0.1 M solution of indene in toluene with 5 mol% of [Co]-
1 and 1.05 equiv of HBPin. The cobalt-catalyzed
isomerization-hydroboration of alkyl-substituted indenes
(1a-1d)
diastereoselectivities,
proceeded
with
even
high
for bulky
yields
and
isopropyl
substituted 1d, to the desired indanyl boronate esters
with complete selectivity for the trans isomer in all cases.
X-ray crystallography of the phenyl-substituted product
2e provided further structural confirmation for the
selective anti arrangement of the boron. Aryl
substituents containing electron-withdrawing (1g-1h) or
electron-donating (1i and 1j) groups at the 4-position
were also compatible with the catalytic protocol. For the
Scheme 2. Precatalyst Evaluation Studies.a
3,4-dichloroaryl
proceeded accompanied by significant competing
protodeborylation, resulting in the desired indanyl
substituted
1k,
hydroboration
boronate ester being isolated as
a
mixture.
Trifluoromethyl substituted indene (1l) was tolerated,
yielding 46% of the desired hydroborated product.
Additionally, substrates containing methoxy-group
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substitution at the 6-position of the indene (1b and 1f) Scheme 3. Diastereoselective Hydroboration of
1
2
3
4
5
6
7
8
underwent hydroboration with no loss in isolated yield of
the desired indanes. Heteroatom-substituted indenes
(1m-1o) were also tolerated, however in the latter case a
1:1 mixture of diastereomers was obtained, possibly due
to a directing effect of the methoxy group overcoming
the inherent selectivity of the reaction.
Substituted
Indenes.a
R1
R1
N
N
+
HBPin
Co
SiMe3
R2
R2
N
1a - r
2a - r
BPin
5 mol% [Co]-1
0.1 M PhMe, 23 ºC, 20 h
iPr
Me
Et
Boron-substituted indenes (1p-1s) were also explored.
Given the synthetic versatility of the boron substituent,
1,3-disubstituted indanyl diboron compounds are
interesting building blocks for the rapid construction of
complex molecular structures. Examples with two
different organoboron groups are particularly attractive
for selective, sequential functionalization reactions made
possible by the chemically differentiated boron groups.30
Boron substituted indenes 1o and 1p underwent
hydroboration to yield the corresponding 1,3-diboronate
esters. However, for the neopentyl glycol boronate ester
(BNeop, 1p), significant exchange of the pinacolato and
glycolato groups was observed, resulting in a 1:1 mixture
of 2o and 2p. To suppress this process, the N-
methyliminodiacetic acid (BMida, 1q) and 1,8-
diaminonaphthalene (BDan, 1r) derivatives were
9
R
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BPin
BPin
BPin
2d 87% (>20:1)
2a R = H
90% (>20:1)
91%b (>20:1)
2c 91% (>20:1)
R
2b R = OMe 97% (>20:1)
R
BPin
96% (>20:1) 2g R = F
86%b (>20:1) 2h R = Cl
BPin
2e R = H
91% (>20:1)
85% (>20:1)
Cl
2f R = OMe 97% (>20:1)
R = OMe 89% (>20:1)
R = NMe2 66% (>20:1)
2i
2j
Cl
CF3
SiMe2Ph
OMe
BPin
BPin
BPin
BPin
2k 23%c (>20:1)
2l 46%c (>20:1)
2m 74% (>20:1)
2n 30%c (1:1)
hydroborated.
Interestingly, hydroboration of 1r
BPin
BNeop
BMida
resulted in a 1:1 mixture of regioisomers identified as the
desired trans-1,3-diboron product and the 1,1-diboron
indane. It is possible that the geminal regioisomer results
from a combined electronic and steric effect of the BDan
group in which the naphthyl ring can stabilize cobalt at
the substituted position.31 To highlight the practicality of
the method, products 2a and 2e were prepared on gram
scale in 91% and 86% isolated yields, respectively (see SI
for details).
O
Me
BNeop =
B
Me
O
MeN
BPin
BPin
BPin
2od90% (>20:1)
2p 33%c (>20:1)
2qd 73% (>20:1)
BMida =
O
B
O
O
O
DanB
BDan
BPin
HN
B
2rd 63% (1:1)
:
BDan =
HN
(>20:1)
BPin
aUnless indicated otherwise, reactions were run on 0.1 mmol
scale with 5 mol% [Co] precatalyst. Percentages are isolated
yields unless otherwise noted. Diastereomeric ratio as
determined by GC and quantitative 13C{1H} NMR analysis
reported in parentheses. See SI for full experimental details.
bYield for gram scale reaction. See SI for details. cYield
determined by quantitative 13C{1H} NMR spectroscopy due
1
d
to overlapping signals in the H NMR. Reaction conducted
at 50 ºC for 20 h in THF.
Regioisomeric 1,2-disubstituted boronate esters were
prepared by hydroboration of symmetrical 2-substituted
indenes (Scheme 4). n-Butyl (1s), phenyl (1t), and BPin
(1u) variants proceeded with high yields and
diastereoselectivities to yield the trans-1,2-disubstituted
indane regioisomers, as confirmed by analysis of the
products by 1H, 13C and 1H correlation NMR experiments.
Scheme 4. Access to trans-1,2-Disubstituted Indanes
through Hydroboration.a
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ACS Catalysis
R
BPin
1.0 equiv. NaOH
3a R = Me 95%(>20:1)
3e R = Ph 97%(>20:1)
10 mol% [Co]-1
1
2
3
4
5
6
7
R
+
HBPin
BPin
R
5.0 equiv. H2O2
THF:H2O, 23 ºC, 30 min
1.0 M PhMe
50 ºC, 24 h
OH
R
1s - u
2s - u
BPin
Ph
BPin
nBu
BPin
4.0 equiv.
BrMg
4a R = Me 45% (>20:1)
4e R = Ph 31% (>20:1)
THF, 23 ºC, 30 min
then
I2, NaOMe
MeOH, -78 ºC to 23 ºC
2s 98% (>20:1)
2t 84% (>20:1)
2ub 95% (>20:1)
R = Ph or Me
R
8
9
a
Unless indicated otherwise, reactions were run on 0.1
R
mmol scale with 10 mol% [Co] precatalyst. Percentages are
isolated yields. Diastereomeric ratio as determined by GC
and quantitative 13C{1H} NMR analysis reported in
parentheses. See SI for full experimental details. Reaction
conducted in THF.
>20:1 dr
BPin
10 mol% Pd2(dba)3,
20 mol% PPh3
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5a R = Me 76% (>20:1)
3-iodotoluene, Ag2O
THF, 70 ºC, 48 h
5e R = Ph
90% (>20:1)
b
Me
Ph
4.5 M aq. KHF2
Synthetic Applications of Indanyl Boronate Esters
6e
92% (>20:1)
MeOH, 23 ºC, 1 h
The synthetic utility of selected examples of indanyl
boronate esters prepared in this study was investigated
and the results of these studies are presented in Scheme
5. A host of stereospecific transformations are available
to convert secondary boronate esters to other functional
groups.32 The sterodefined indanyl boron compounds
prepared by the cobalt-catalyzed method are attractive
and versatile synthons for additional elaboration.
Oxidation33 of 2a and 2e was achieved using a
NaOH/H2O2 solution to yield the trans alcohols, 3a and
3e, in 95% and 97% yield, respectively. Vinylation of 2a
and 2e, using Zweifel olefination conditions,34 afforded
4a and 4e in 45% and 31% yield, respectively. Using
conditions for the palladium-catalyzed cross coupling of
secondary organoboronate esters developed by Crudden
and coworkers,35 cross-coupled products 5a and 5e were
isolated in 76% and 90% yields with retention of
configuration. The trifluoroborate salt36 6e was also
isolated in 92% yield. These derivatization reactions
demonstrate the high synthetic utility of the indanyl
BF3K
aPercentages are isolated yields unless otherwise noted.
Diastereomeric ratio as determined by quantitative 13C{1H}
NMR analysis reported in parentheses. See SI for full
experimental details. dba = dibenzylideneacetone.
Mechanistic Studies
To gain insight into the mechanism of the catalytic
hydroboration reaction and importantly, the origins of
the regio- and diastereoselectivity, a series of additional
studies were conducted. The hydroboration of 3-methyl-
1H-indene (1a) was selected as a representative reaction
1
and monitored by H NMR spectroscopy in benzene-d6.
Immediately upon addition of HBPin to a 1.0 M solution
of 1a in benzene-d6 with 5 mol% [Co]-1, complete
consumption of [Co]-1 was observed. Additional
1
monitoring over the course of the reaction by H NMR
spectroscopy demonstrated clean conversion of 1a to
product 2a. No peaks corresponding to a new cobalt
species were observed over the course of the
transformation. An additional set of stoichiometric
experiments was conducted in an attempt to determine
the identity of the catalytically active cobalt intermediate
or catalyst resting state (Scheme 6). First, addition of one
equiv. HBPin to a 1:1 mixture of [Co]-1 and 1a in
benzene-d6 resulted in the immediate formation of a
dark solution (Scheme 6a). The 1H NMR spectrum
exhibited broad peaks corresponding to 1a with no
observable signals corresponding to [Co]-1. Vacuum
transfer of the volatiles from this reaction and analysis by
1H NMR spectroscopy established the generation of
boronate
esters
formed
from
cobalt-catalyzed
isomerization-hydroboration. Importantly, compounds
3a-6e are not accessible directly via existing cyclization
methods selectively,28 further highlighting the utility of
the products formed to yield disubstituted indanes with
a high degree of stereocontrol.
(trimethylsilyl)methylpinacolborane.
Continued
monitoring of the non-volatile components of the
reaction revealed formation of small amounts of 2a,
implicating formation of a cobalthydride following
activation of the corresponding alkyl with HBPin. In
related cobalt-catalyzed asymmetric hydrogenation
reactions of substituted indenes,37 alkene insertion is
Scheme 5. Representative Synthetic Applications of
Indanyl Boronate Esters. a
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a. Single turnover experiment
c. Possible Intermediates (not
observed)
slow and the resting state of the catalyst is the cobalt-
exclusive product
1
2
3
4
5
6
7
8
Me
hydride rather than a cobalt alkyl. In the present case,
attempts to observe the cobalthydride by NMR
spectroscopy have been unsuccessful. DFT calculations38
(B3LYP) on the putative [(Terpy)CoH] (Terpy = 2,2’:6’,2”-
terpyridine) were conducted for both the spin restricted
(RKS, Co(I), S = 0) and spin unrestricted (UKS, Co(II), S =
0 or S = 1) electronic structures.39 The UKS solution for
the S = 1 case was the lowest in energy and is best
described as a low spin Co(II) with a ferromagnetically
coupled ligand centered radical. Attempts to calculate a
high-spin solution converged to the low-spin
description. Nearly isoenergetic is the broken symmetry
(1,1) electronic structure, which is 1 kcal/mol higher in
energy. The small relative difference between the
energies of the singlet and triplet states may contribute
to the difficulty in spectroscopic observation of
[(Terpy)CoH]. However, additional experimental studies
are required to distinguish these possibilities. The RKS
solution was significantly higher in energy (+10 kcal/mol)
than the spin unrestricted solutions consistent with
ligand-centered radicals as important in potential
catalytic intermediates.
[Co]-1
1 equiv. HBPin
N
+
Me
N
1.0 M C6D6
23 ºC, 30 min
- PinBCH2SiMe3
Co
N
H
>20:1 dr
BPin
2a 29% yield
[(Terpy)CoH]
1a
-OR-
b. Activation of [Co]-1 under H2 followed by hydroboration
0.1 atm H2
exclusive product
1.0 M C6D6
23 ºC, 5 min
- SiMe4
N
Me
N
Co
N
[Co]-1
Me
H
+
9
Me
then
1 equiv. HBPin
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
>20:1 dr
2a quantitative
BPin
1 h
[(Terpy)Co(indanyl)]
1a
a. Single turnover experiment with [Co]-1, HBPin, and 1a. b.
Activation of precatalyst [Co]-1 under H2 (0.1 atm) followed
by hydroboration of 1a. c. Possible catalytic intermediates.
Kinetic analysis using the method of variable time
normalization developed by Burés41 was also conducted.
Given the limited spectroscopic evidence for the cobalt
intermediates formed under catalytic conditions, these
experiments were used to provide information on the
turnover-limiting step for the cobalt-catalyzed
hydroboration-isomerization.
conducted in a J-Young NMR tube and the reaction of
1b with HBPin was monitored by H NMR spectroscopy
at 23 ºC with [Co]-1 as the precatalyst. Full experimental
details are presented in the Supporting Information.
These experiments established an overall second order
rate law that is first order with respect to both alkene
and [Co]-1 but zero order with respect to HBPin. These
findings are consistent with alkene insertion into a
cobalt-hydride as turnover limiting step followed by fast
CB bond formation.42 This is distinct from turnover-
limiting C–B bond formation commonly observed for
terminal-selective hydroboration with acyclic alkenes8,12
and as calculated by Stradiotto, Turculet and Sydora.13a
Instead, the kinetic analysis supports a regime that is
similar to that proposed for cobalt-catalyzed
hydrogenation.37,40
Kinetic
trials
were
The computational results support a similar electronic
structure to that assigned to the cobalt alkyl derivatives,
which are best described as a cobalt(II) center engaged
in antiferromagnetic coupling with a terpyridine radical
1
anion.9
This electronic structure parallels that of
bis(imino)pyridine cobalt alkyl complexes.37 Detailed
computational studies by Hopmann on the mechanism
of bis(imino)pyridine cobalt-catalyzed hydrogenation
support broken symmetry electronic configurations
throughout the catalytic cycle40 and it is likely that a
similar feature is conserved here.
To further support the intermediacy of a cobalt-
hydride, 0.1 atm of H2 was added to a J-Young NMR
tube containing a benzene-d6 solution of an equimolar
mixture of [Co]-1 and 1a (Scheme 6b). Again, a dark
1
solution immediately formed and broad H NMR signals
Deuterium Labeling Experiments
were observed. Subsequent addition of HBPin by
vacuum transfer resulted in formation of the boronate
The regio- and diastereoselectivity of the cobalt-
catalyzed hydroboration was investigated through a
series of deuterium labeling experiments. Pinacolborane-
d1 (DBPin) was prepared by cobalt-mediated isotopic
exchange of HBPin with D2 gas (see SI for full details) and
generated DBPin in 85% isotopic purity. Deuteroboration
of 1a was achieved under standard catalytic conditions
and produced the deuterated boronate ester in 69%
isolated yield (Scheme 7a). For clarity, the numbering
convention of the indenyl ring systems presented below
and used throughout is based on placement of the
substituents in the final product, regardless of the
1
ester product 2a after 1 hour as judged by H NMR
spectroscopy, providing addition evidence for
a
catalytically competent cobalt hydride that likely forms
but is unobservable by 1H NMR spectroscopy.
Scheme 6. Stoichiometric Studies Probing the
Catalyst Resting State.
1
position of the starting alkene. Analysis of 2a-D by H,
2H, and 13C NMR spectroscopies established
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2
carbon positions determined by H and quantitative 13C{1H}
incorporation of deuterium at the 2-, 3-, and methyl
1
2
3
4
5
6
7
8
b
NMR spectroscopy included in brackets. Recovered alkene
from aliquot taken at partial conversion. Relative deuterium
positions. The total deuterium incorporation in the
product is consistent with the isotopic purity of DBPin.
Importantly, selective deuterium incorporation syn to the
methyl group at the diastereotopic 2-position was
observed. No deuterium was detected at the 1-position.
Analysis of the boronate ester product by GC-MS
revealed the formation of multiple deuterated
isotopologues, with the d1 derivative identified as the
major product. Analysis of recovered alkene after 10 h of
reaction time was consistent with preferential deuterium
incorporation at the methyl position, with a 5:1 relative
deuterium labeling observed for the methyl and 2-
position, respectively. Spectroscopic data and complete
assignments are reported in the Supporting Information.
Scheme 7. Deuterioboration of 1a and Isotopic
Incorporation.a
2
incorporation at methyl and 2-position determined by H
and quantitative 13C{1H} NMR spectroscopy. cRelative
percentages of isotopologues determined by GC-MS. See SI
for full details.
To rationalize the observation of the majority of the
deuterium at the methyl position and to gain insight into
the role of isomerization during catalysis, the
deuteroboration procedure was conducted with 1-exo-
methylene-indane, 1aa. The boronate ester was obtained
in 66% isolated yield with similar isotopic incorporation
to that observed with 1a (Scheme 7b). Analysis of an
aliquot of the reaction mixture after 5 min revealed full
conversion of 1aa to 1a-D with a 2:1 relative deuterium
incorporation at the methyl position versus the 2-
position of the alkene. These experiments support fast
and reversible isomerization between 1a and 1aa, likely
proceeding by preferential formation of a tertiary cobalt
alkyl intermediate (Scheme 7c). Subsequent -H
elimination followed by alkene dissociation and
reinsertion into a cobalt deuteride results in the
observed deuterated alkene, 1a-D. This isomerization
pathway is ultimately unproductive for hydroboration
and instead, insertion at the 2-position followed by -
hydride elimination from the 1-position is necessary
before hydroboration to the final observed product
(Scheme 7d).
To test this hypothesis and probe the pathway
depicted in Scheme 7d, 1ab was independently
evaluated for hydroboration (Scheme 8). Full conversion
of indene to the labeled boronate ester 2ab-D (78%
isolated yield) was observed after 5 min as a single
isotopomer and isotopologue, as determined by H, H,
quantitative 13C{1H} NMR spectroscopy and GC-MS
analysis. This labeling study supports the straightforward
syn 1,2-addition of the cobalt-deuteride to 1ab with no
evidence for isomerization prior to carbon-boron bond
formation. Further, observed boron incorporation on the
face anti to the methyl substituent is consistent with
alkene coordination and subsequent insertion being
selective for a single face of the alkene, accounting for
the diastereotopically pure deuterium incorporation and
trans selective arrangement of the final boronate ester
product.
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
a.
D [47%]
Me
[13%]
D
3
3
5 mol% [Co]-1
2
2
D [13%]
+
DBPin
0.10 M PhMe
23 ºC, 20 h
1
2a-D
1
1a
69% yield
BPin
Recovered alkeneb
Relative isotopologue %c
D1
47
relative incorporation
3
D1:D2 = 5:1
24
19
2
D2
10
(10 h)
1
1a-D
D0
D1
D2
D3
b.
D [51%]
CH2
2
[11%]
D
3
3
5 mol% [Co]-1
2
D [17%]
+
DBPin
0.10 M PhMe
23 ºC, 20 h
2aa-D
1
1
1aa
66% yield
BPin
Recovered alkeneb
Relative isotopologue %c
D1
45
relative incorporation
3
D1:D2 = 2:1
22
22
2
D2
11
(5 min)
1
D0
D1
D2
D3
1a-D
c.
1
2
Me
2
Me
3
CH2
2
[Co]
3
3
[Co]
D
D
-H elim
-H elim
2
D
D
D
1
1
1
3
1a
2
D
D
D
CH2
2
[Co]
2
3
1
3
3
1a-D
[Co]
2
1
1
1
1aa
d. Productive Isomerization-Hydroboration Pathway (SLOW)
D
D
D
D
D
i.
3
3
3
[Co]
D
-H elim
2
[Co]
2
2
D
D
D
[Co]
H
1
1
1
1a-D
ii. reinsertion
i. cobalt−deuteride insertion at
2-position
D
D
D
D
iii.
3
3
ii. fast reinsertion of
cobalt-hydride
2
2
H
H
DBPin
D
D
1
1
iii. DBPin releases
product
BPin
[Co]
aReactions conducted with 0.10 mmol of substrate in 1.0 mL
toluene at 23 ºC for 20 h. Percentage yield of products are
isolated yields. Percentage isotopic incorporation at the
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Scheme
8.
Deuterioboration
of
1ab.a
Scheme 9. Deuterium Labeling of a Selection of
Single isotopomer
Single isotopologue
1
2
3
4
5
6
7
8
Substituted
Indenes.a
Me
Me
Entry
Substrate
Productb
[44%]
Relative isotopologue %c
1
5 mol% [Co]-1
2
D
+
DBPin
D
0.10 M PhMe
23 ºC, 5 min
Me
49
Et
3
2ab-D1
[16%]
D
1ab
3
3
78% yield
BPin
29
2
16
5
2
D [20%]
1
Favored face of insertion
Disfavored face of insertion
1
2c-D
1
1c
D0
D1
D2
D3
BPin
N
N
N
N
[45%]
Co
Co
D
54
Me
D
D
Me
N
N
[16%]
D
9
3
3
27
H
Me
2
[18%]
D
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
2
2
11
9
1
Me
H
2ca-D
1
1ca
D0
D1
D2
D3
BPin
72
aReactions conducted with 0.10 mmol of 1ab in 1.0 mL
toluene at 23 ºC. Percentage yield of product is isolated
yield. Determination of isotopomer and isotopologue
determined by 2H and quantitative 13C{1H} NMR
spectroscopy included in brackets. See Supporting
Information for full spectroscopic data and complete
assignments.
Ph
3
Ph
[61%]
D
3
2
[16%]
15
D
3
2
12
0
1
2e-D
1
1e
1n
D0
D1
D2
D3
BPin
PinB
3
BPin
2
[24%]
D
36
3
32
32
2
4d
D
[50%]
1
0
1
2n-D
The cobalt-catalyzed deuteroboration-isomerization of
additional alkenes was also investigated, the results of
which are summarized in Scheme 9. Under standard
catalytic conditions, reaction of 1c or its exo-methylene
isomer, 1ca, with DBPin resulted in 78% and 92%
isolated yield of the deuterated product, respectively.
Analogous to the labeling observed with the methyl
substituted indenes, deuterium was incorporated at the
2-, 3-, and methylene position of the ethyl group in the
indane products 2c-D and 2ca-D. Again, selective
deuterium incorporation at the 2-position syn to the
ethyl substituent was observed, indicating a similar
insertion pathway as depicted in Scheme 7c and d.
Substrates 1e and 1n, selected specifically to confine
chain-walking during catalysis to the indenyl ring, were
also investigated through labeling experiments. In
contrast to the alkyl substituted examples explored,
analysis of the labeled boronate ester products 2e-D by
1H, 2H, and 13C NMR spectroscopies established
incorporation of deuterium at the 3- and 2-positions in a
syn relationship. This is consistent with the pathways
outlined in Scheme 7. The cobalt-deuteride can insert
into the 3- or 2-position. However, in the absence of an
alkyl substituent, alkene isomerization and subsequent
hydroboration can only occur in the latter case.
D0
D1
D2
D3
BPin
aReactions conducted with 0.10 mmol of indene, 0.105
mmol DBPin, and 5 mol% [Co] in 1.0 mL toluene at 23 ºC for
20 h. bPercentage isotopic incorporation at the carbon
position determined by 2H and quantitative 13C{1H} NMR
spectroscopies. cRelative percentages of isotopologues
determined by GC-MS. dReaction conducted with 0.10 mmol
of indene, 0.105 mmol DBPin, and 5 mol% [Co] in 2.0 mL
THF at 50 ºC for 20 h.
Having established a model for isomerization of the 5-
membered
ring
system,
the
cobalt-catalyzed
hydroboration analogous 6-membered ring substrates
(dialins) were explored (Scheme 10). A 0.10 M solution of
1-methyl-3,4-dihydronaphthalene (7a) in toluene was
subjected to the standard catalytic conditions. The
reaction was stirred at 23 ºC and monitoring the reaction
1
by H NMR spectroscopy over 24 h showed no evidence
for the hydroborated product. Other 1-substituted
dialins explored, phenyl (7b) and benzyl (7c) substituted
substrates, were similarly unreactive (entry 1). Increasing
the temperature and reaction time to 80 ºC and 72 h,
respectively, produced no change in reactivity in all
cases. The exocyclic-methylene substrate (7d) under
standard catalytic conditions at 23 ºC converted
quantitatively to the endocyclic alkene (7a) in <5 min, as
observed by 1H NMR (entry 2). This is consistent with fast
isomerization between exo- and endocyclic alkene
isomers as observed with the 5-membered ring
substrates. However, no evidence for further
isomerization into the ring was observed for the 6-
membered substrates. To rule out the possibility that 6-
membered ring alkenes are inactive for the
hydroboration catalyzed by [Co]-1, the hydroboration of
isomer 7e was carried out and proceeded to full
conversion of the starting alkene in <5 min at 23 ºC to
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ACS Catalysis
furnish a 2:1 mixture of regio- and diastereoisomers conditions. With the indene, an additional isomerization
1
(entry 3). Cooling the reaction to 0 ºC did not change the
regioselectivity, and the reaction still reached completion
in <5 min. In contrast to the 5-membered ring analog,
the methyl substituent for the 6-membered ring alkene
had little to no influence on the diastereoselectivity of
the reaction possibly due to smaller energy differences
between different facially bound cobalt hydride(alkene)
intermediates. In the absence of the methyl group,
hydroboration of 7f resulted in a slight increase in
regioselectivity (2.8:1, entry 4). The marked difference in
reactivity between the 5- and 6-membered ring
substrates may be a consequence of the lower degree of
ring strain in the 6-membered ring systems.43 Ring strain
release may drive alkene insertion for trisubstituted
indenes, whereas for the dialins, insertion is not as
favorable. This may explain why reactivity is only
observed for the disubstituted dialins 7e and 7f.
event is accessible, albeit slow, to furnish 1ab (Scheme
11, top). The accessibility of this pathway can be
explained by the relative planarity of the indenyl
substrate, placing the ring -hydrogens in the necessary
syn-periplanar arrangement required for transfer to the
cobalt center. In contrast, the 6-membered substrate
only has one kinetically accessible site for -hydrogen
elimination from the methyl group. The necessary
conformational inversion required to access the ring
methylene hydrogens may be kinetically disfavored.
Additionally, performing a mixing experiment in which a
1:1 mixture of 1a and 7a were subjected to the standard
hydroboration conditions in the presence of 5 mol% of
[Co]-1 showed full conversion of 1a to 2a in 20 h and no
conversion of 7a, indicating that while the 6-membered
ring is not suitable for isomerization-hydroboration, it
does not inhibit the reaction overall.
2
3
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
Scheme 11. Isomerization of 5- versus 6-Membered
Rings.
Scheme 10. Effect of Ring Substitution on the Cobalt-
Catalyzed
Hydroboration
of
Dialins.
BPin
1
1
2
3
2
3
5 mol% [Co]-1
R
R
+
HBPin
0.10 M PhMe
4
4
Entry
Substrate
Product
Yieldc
R
7a R = Me
1a
7b R = Ph
No reaction
Starting material
recovered
7c R = CH2Ph
CH2
Me
2a
>98%
The stoichiometric experiments, kinetic analysis, and
isotopic labeling studies support the mechanistic picture
for the cobalt-catalyzed diastereoselective hydroboration
of substituted indenes presented in Scheme 12.
Although the reactivity of the proposed cobalt
intermediates has hindered their spectroscopic
observation, insights have been gained on alkene
insertion preferences and competitive isomerization
pathways that are key to rationalizing the unique
regioselectivity of this transformation. Alkene 2,3-
insertion is the turnover-limiting step. For alkyl-
substituted indenes, competitive 3,2-insertion is fast and
reversible and ultimately unproductive for CB bond
formation. This presents a distinct mechanistic picture to
7d
7a
CH3
Me
Me
3b
:
70% (2:1)
BPin
7e
8e
2.6:1 drd
1.2:1 drd
BPin
BPin
BPin
4b
:
97% (2.8:1)
7f
8f
a Reaction conducted with 0.10 mmol of dialin, 0.105 mmol
HBPin, and 5 mol% [Co] in 1.0 mL toluene at 23 ºC for 24 h.
Heating at 80 ºC for an additional 48 h produced no
b
observable hydroborated product.
Reaction conducted
that
proposed
for
anti-Markovnikov
selective
with 0.50 mmol of dialin, 0.505 mmol HBPin, and 1 mol%
[Co] in 1.0 mL toluene at 0 ºC for 5 min. c Percentage yield
are isolated yields. d Diastereomeric ratio as determined by
GC and quantitative 13C{1H} NMR analysis. See SI for details.
hydroboration catalysts in which selectivity is controlled
by favorable boron transfer to a terminal metal alkyl
intermediate. The relatively open coordination
environment imparted to the cobalt center by the
terpyridine ligand may lower the barrier for secondary
CB bond formation, resulting in the observed
regioselectivity.
Scheme 11 presents further insights for the differing
reactivity between the 5- and 6-membered ring
substrates. In both cases, isomerization from the exo- to
the endocyclic isomers is facile under standard
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Scheme 12. Proposed catalytic cycle for the
diastereoselective isomerization-hydroboration of 1a
Paul J. Chirik: 0000-0001-8473-2898
Nadia G. Léonard: 0000-0002-0949-5471
Max R. Friedfeld: 0000-0001-5693-3705
Máté J. Bezdek: 0000-0001-7860-2894
1
2
3
4
5
6
7
8
with
[Co]-1.
[Co]-1
HBPin
3,2-insertion
fast
Me
Me3SiCH2BPin
ACKNOWLEDGMENT
3
1a
2
Me
Financial support was provided by the National Science
Foundation (NSF) Grant Opportunities for Academic Liaison
with Industry (GOALI) grant (CHE-1564379). NGL thanks the
National Science Foundation (NSF) for a Graduate Research
Fellowship (DGE-1148900).
3
1
Me
[Co]
N
H
H
2
N
Co
1
9
N
BPin
Selective
Secondary
1a
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
2,3-insertion
C−B Formation
turnover limiting
HBPin
fast

-H elim
CH2
reinsertion
fast
REFERENCES
(1) (a) Namirembe, S.; Morken, J. P. Reactions of
organoboron compounds enabled by catalyst-promoted
metalate shifts. Chem. Soc. Rev. 2019, Advance Article.
DOI: 10.1039/C9CS00180H. (b) Fyfe, J. W. B.; Watson, A.
J. B. Recent developments in organoboron chemistry:
old dog, new tricks. Chem 2017, 3, 31-55. (c) Collins, B.
S. L.; Wilson, C. M.; Myers, E. L.; Aggarwal, V. K.
Asymmetric synthesis of secondary and tertiary boronic
esters. Angew. Chem. Int. Ed. 2017, 56, 11700-11733.
(2) (a) Geier, S. J.; Vogels, C. M.; Westcott, S. A. In Boron
Reagents in Synthesis; Coca, A., Ed.; ACS Symposium
Series, Vol. 1236; American Chemical Society:
Washington, DC, 2016; pp 209– 225. (b) Vogels, C. M.;
Westcott, S. A. Recent advances in organic synthesis
using transition metal-catalyzed hydroborations. Curr.
Org. Chem. 2005, 9, 687-699. (c) Crudden, C. M.;
Edwards, D. Catalytic asymmetric hydroboration: recent
advances and applications in carbon-carbon bond-
forming reactions. European J. Org. Chem. 2003, 2003,
4695-4712.
(3) (a) Vasseur, A.; Bruffaerts, J.; Marek, I. Remote
functionalization through alkene isomerization. Nat.
Chem. 2016, 8, 209-219. (b) Sommer, H.; Juliá-
Hernández, F.; Martin, R.; Marek, I. Walking metals for
remote functionalization. ACS Cent. Sci. 2018, 4, 153-165
.
(4) Vilches-Herrera, M.; Domke, L.; Börner, A. Isomerization-
hydroformylation tandem reactions. ACS. Catal. 2014, 4,
1706-1724.
N
N
N
N
N
Co
Co
N
Me
Me
Chain-walking
CONCLUDING REMARKS
The regio- and
hydroboration of sterically hindered 2- and 3-
substittued indenes by a terpyridine cobalt complex
has been developed. This method provides
remarkably selective access to trans-1,2- and 1,3-
disubstituted indanyl boronate esters, the utility of
diastereoselective
remote
which was demonstrated through
a
host of
representative C-C, and C-O bond-forming reactions.
Deuterium labeling studies allowed for rationalization
of the observed regioselectivity of the isomerization-
hydroboration and provide insight on how alkene
insertion preferences dictate chain walking for remote
functionalization in ring systems and extend the
synthetic utility of the transformation.
(5) (a) Guo, L.; Dai, S.; Sui, X.; Chen, C. Palladium and nickel
catalyzed chain walking olefin polymerization and
copolymerization. ACS Catal. 2016, 6, 428-441.
(6) Seminal reports on chain-walking/polymerization (a)
Johnson, L. K.; Killian, C. M.; Brookhart, M. New Pd(II)-
and Ni(II)-based catalysts for polymerization of ethylene
and -olefins. J. Am. Chem. Soc. 1995, 117, 6414-6415.
(b) Möhring, V. K.; Fink, G. Novel polymerization of -
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on
the ACS Publications website.
Crystallographic data for 2e (CIF).
Additional experimental procedures, including general
considerations, additional experimental details, full
characterization data and complete NMR spectroscopic data
(PDF).
olefins
with
the
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AUTHOR INFORMATION
Corresponding Author
* E-mail: pchirik@princeton.edu
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TOC Graphic
Me
CH2
Selective access to
1,3-trans-disubstitued indanes
Ar
or
or
Me
Me
Me
Me
[Co]
+
HBPin
OH
Me
BPin
>20:1 dr
Chain-walking enables
remote functionalization
CH2
14
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