Cobalt Catalyzed Z -Selective Hydroboration of Terminal Alkynes and Elucidation of the Origin of Selectivity

Article abstract of DOI:10.1021/jacs.5b00936

A bis(imino)pyridine cobalt-catalyzed hydroboration of terminal alkynes with HBPin (Pin = pinacolate) with high yield and (Z)-selectivity for synthetically valuable vinylboronate esters is described. Deuterium labeling studies, stoichiometric experiments, and isolation of catalytically relevant intermediates support a mechanism involving selective insertion of an alkynylboronate ester into a Co-H bond, a pathway distinct from known precious metal catalysts where metal vinylidene intermediates have been proposed to account for the observed (Z) selectivity. The identity of the imine substituents dictates the relative rates of activation of the cobalt precatalyst with HBPin or the terminal alkyne and, as a consequence, is responsible for the stereochemical outcome of the catalytic reaction.

Full text of DOI:10.1021/jacs.5b00936

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Communication
Cobalt Catalyzed Z-Selective Hydroboration of Terminal
Alkynes and Elucidation of the Origin of Selectivity.
Jennifer V. Obligacion, Jamie M. Neely, Aliza N. Yazdani, Iraklis Pappas, and Paul J. Chirik
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b00936 • Publication Date (Web): 17 Apr 2015
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Cobalt Catalyzed Z-Selective Hydroboration of Terminal Alkynes and Elucidation of
the Origin of Selectivity.
9
Jennifer V. Obligacion, Jamie M. Neely, Aliza N. Yazdani, Iraklis Pappas and Paul J. Chirik*
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Department of Chemistry, Princeton University, Princeton, New Jersey 08544.
pchirik@princeton.edu
RECEIVED DATE (automatically inserted by publisher)
Scheme 1. Hydroboration of 1ꢀoctyne with 1 and 2 along with
solid state structure of 2 at 30% probability ellipsoids.
ABSTRACT:
A
bis(imino)pyridine
cobaltꢀcatalyzed
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hydroboration of terminal alkynes with HBPin (Pin =
pinacolate) with high yield and (Z)ꢀselectivity for synthetically
valuable vinylboronates is described. Deuterium labeling
studies, stoichiometric experiments and isolation of
catalytically relevant intermediates support a mechanism
involving selective insertion of an alkynylboronate ester into a
CoꢀH bond, a pathway distinct from known precious metal
catalysts where metal vinylidene intermediates have been
proposed to account for the observed (Z)ꢀselectivity. The
identity of the imine substituents dictates the relative rates of
activation of the cobalt precatalyst with HBPin or the terminal
alkyne, and as a consequence, is responsible for the
stereochemical outcome of the catalytic reaction.
BPin
(1.2 equiv)
+
HBPin
3 mol% 1
3 mol% 2
BPin
(E)-isomer (exclusive)
0.50 M THF
0.50 M THF
>98%
>98%
(Z)-isomer (major, 92:8)
Cy
CH3
Ar
N
N
N
N
Co
Co
iPr
N
N
CH3
iPr
1
2
hydroboration where hydrofunctionalization occurs selectively
at remote, terminal CꢀH positions. Modification of the ligand
architecture to 2,2’ꢀ6’,2’ꢀterpyridine or arylꢀsubstituted αꢀ
diimines produced catalysts with unprecedented activity and
selectivity for hindered triꢀ and tetrasubstituted alkenes.13
Application of C1ꢀsymmetric tridentate nitrogenꢀbased ligands,
a strategy pioneered for asymmetric hydrogenation,14 has been
applied to the enantioselective hydroboration of 1,1ꢀ
disubstituted alkenes.15 By contrast, the base metal catalyzed
hydroboration of alkynes has been much less thoroughly
explored and is limited to a select few iron examples.10b,d Here
we describe high activity bis(imino)pyridine cobalt catalysts
for the selective antiꢀMarkovnikov hydroboration of terminal
alkynes. Modification of the ligand substituents or the
conditions of the catalytic reactions allows manipulation of the
stereochemical outcome and a straightforward method for the
preparation of (Z)ꢀvinylboronates. Mechanistic studies are
consistent with (Z)ꢀstereoselectivity arising from synꢀ
hydrometallation of alkynylboronates arising from cobalt
acetylides, a pathway distinct from previous mechanistic
proposals invoking vinylidene compounds.
Initial catalytic experiments were conducted with
(iPrPDI)CoCH3 (1) given the demonstrated utility of this
compound in alkene hydrogenation,16 hydroboration11 and
dehydrogenative silylation.17 Stirring a 0.5 M THF solution
containing a 1.2:1 mixture of 1ꢀoctyne and HBPin in the
presence of 3 mol% of (iPrPDI)CoCH3 at 23 ºC for 6 hours
produced >98% conversion (GCꢀMS) to the (E)ꢀvinylboronate
product arising from antiꢀMarkovnikov addition of the borane.
Repeating the catalytic hydroboration reaction with 2, the
bis(imino)pyridine cobalt methyl complex where the 2,6ꢀ
diisopropyl substituents have been replaced with cyclohexyl
groups,18 yielded the (Z)ꢀvinylboronate in 76% isolated yield
with 92:8 stereoselectivity (Scheme 1).
Vinylboronates are versatile reagents in organic synthesis
owing to their use as nucleophilic partners in CꢀC bond
forming reactions.1,2 Hydroboration of terminal alkynes is a
straightforward and atom economical method for the
preparation of vinyl boronates and commonly yields products
with (E) stereochemistry resulting from antiꢀMarkovnikov, syn
addition of the BꢀH group to the C≡C bond.3,4 Methods for the
synthesis of the corresponding (Z) isomers are more
challenging and typically require multistep routes that are
potentially complicated by competing isomerization to the
more thermodynamically favored (E) isomer.5,6a
Examples of precious metalꢀcatalyzed alkyne hydroboration
to yield (Z)ꢀvinylboronates are rare.6b,7ꢀ9 Miyaura and
coworkers reported that rhodium and iridium precursors in
combination with PiPr3 and PCy3 are selective for (Z)ꢀ
vinylboronates in the presence of NEt3.7 The use of HBCat
(Cat = catechol) was required to achieve optimal selectivities
as direct hydroboration with HBPin resulted in lower (Z):(E)
ratios. Ruthenium pincer catalysts have recently been reported
by Leitner and coworkers that promote the (Z)ꢀselective
hydroboration of terminal alkynes at ꢀ15 ºC.8 Fürstner and
coworkers have also described a rare example of (Z)ꢀselective
hydroboration
of
internal
alkynes
using
[(η5ꢀ
C5Me5)Ru(CH3CN)3]PF6.9 In each of these examples,
deuterium labeling experiments supported formation of
intermediates with metalꢀcarbon double bonds that are
responsible for the observed selectivity.
There has recently been intense interests in developing base
metal catalysts for alkene and diene hydroboration.10 Cobalt
catalysts bearing redoxꢀactive bis(imino)pyridine11 and related
chelates12 have proven effective for alkene isomerizationꢀ
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Scheme 2. Deuterium labeling experiments with 2 and 1.
A
BPin
3 mol% 2
R
+
HBPin
R
THF, 23 ϒC
BPin
Z isomer
1.2 equiv
1.0 equiv
73%
27%
D
3 mol% 2
+
HBPin
+
D
0.50 M THF
23 ºC, 6 hours
72% conversion
D
BPin
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BPin
BPin
BPin
BPin
B
76% (92 : 8)
79% (97 : 3)
91% (97 : 3)
2a
2b
2c
D
t
3 mol% 2
93%
PhO
BuPh2SiO
PhthN
+
+
DBPin
+
0.50 M THF
23 ºC, 6 hours
>98% conversion
BPin
BPin
72% (98 : 2)
BPin
BPin
51% (>98 : 2)
50%a (97 : 3)
7%
D
2d
2e
2f
BPin
3 mol% 2
Ph
Ph
DBPin
DBPin
D
0.50 M THF
23 ºC, 6 hours
>98% conversion
BPin
BPin
BPin
>20:1
59%b (92 : 8)
61%b (84 : 16)
56%b (56 : 44)
2g
2h
2i
C
3 mol% 1
+
0.50 M THF
23 ºC, 6 hours
>98% conversion
BPin
X
BPin
BPin
BPin
D
>99%
83% (95 : 5)
76% (95 : 5)
p-OMe: 41% (87 : 13) 2l
m-F: 69% (>98 : 2) 2m
experiments for the catalytic deuterioboration of 1ꢀoctyne and
phenylacetylene produced the opposite result; using DBPin
placed the deuterium geminal to the [BPin] group in the (Z)
vinylboronates while the (E) isomer (for 1ꢀoctyne) contained
the isotopic label exclusively at the 2ꢀposition (Schemes 2b
and 2c).
Isotopic purity >98% was observed in all cases in the
position of deuteration. With 1, the same deuterium labeling
experiment exclusively furnished the (E) isomer where the
deuterium is at the 2ꢀposition. The combination of deuterium
labeling experiments coupled with the observation of a rapid
cobaltꢀhydride formation upon treatment of (iPrPDI)CoCH3
with HBPin11 supports a conventional mechanism for (E)
selectivity involving terminal alkyne insertion to a metal
hydride and product release from interaction of an
intermediate cobaltꢀvinyl compound with HBPin (see Figure
S14).4f Importantly, reaction of 1 with terminal alkynes was
sluggish and produced intractable mixtures of products over
the course of hours at 23 ºC.
For precious metal catalysts, the generally accepted
mechanism for (Z)ꢀselective alkyne hydroboration invokes
vinylidene intermediates and metalꢀboryl bond formation
(Scheme 3). Stoichiometric experiments with 2 eliminate this
possibility. Addition of one equivalent of either 1ꢀoctyne or
phenylacetylene resulted in immediate liberation of one
equivalent of methane along with the diamagnetic C2v
symmetric bis(imino)pyridine cobalt acetylide complexes,
(CyAPDI)CoC≡CR (R = Ph, 3; nHex, 4). Conducting the
analogous experiment whereby HBPin was added to a
benzeneꢀd6 solution of 2 resulted in generation of CH3BPin
and an unidentified paramagnetic cobalt compound.
Importantly, the observation of CH3BPin establishes that 2, in
analogy to 1, forms a cobaltꢀhydride upon treatment with
borane and it is this compound that converts to an unidentified
product. We note that hydrogenation of 2 in the absence of
boron also yielded the same low symmetry cobalt compound,
definitively eliminating a boronꢀcontaining product. Reactions
of 2 with alkynes are more facile than with HBPin.
2j
2k
Figure 1. Hydroboration of terminal alkynes with (CyAPDI)CoCH3.
Reaction Conditions: 3 mol% (CyAPDI)CoCH3, 23 °C, 0.50 M in
THF, 6 hours. Reported numbers are isolated yields obtained after
column chromatography. Numbers in parentheses are Z:E selectivities
determined by NMR spectroscopy. aNMR yield. bReaction run in neat
substrate for 24 hours.
The highest (Z) selectivity for 1ꢀoctyne was observed when
a slight excess (1.2 equiv) of the alkyne relative to HBPin was
used. In the absence of excess alkyne, postꢀcatalytic
isomerization of the (Z) product to the thermodynamically
preferred (E) isomer becomes competitive, likely promoted by
a residual cobaltꢀhydride (see Figures S4ꢀ5). Figure 1 reports
the results of hydroboration of a family of terminal alkynes
with HBPin at 23 ºC using these optimized conditions. High
yield and (Z) selectivity were observed for protected propargyl
ethers, propargyl phthalamide, arylꢀ and alkylꢀsubstituted
alkynes. The origin of erosion of selectivity observed with
cyclopropylacetylene is not currently understood. More
electron rich alkynes with secondary alkyl substitution
underwent slower hydroboration, requiring neat conditions
and 24 hours of reaction time. Aryl substituted alkynes with
electron donating (4ꢀOMe) and electron withdrawing (3ꢀF)
groups maintained high (Z) selectivity. The use of deactivated
silica was required for product isolation to prevent
isomerization and to remove the cobalt catalyst.
Deuterium labeling experiments were conducted to gain
insight into the origin of (Z)ꢀselectivity (Scheme 2).
Hydroboration of 1ꢀd1ꢀ1ꢀoctyne with HBPin under standard
conditions furnished
a
73:27 (Z):(E) mixture of
vinylboronates. The (Z) product contained deuterium
exclusively at the 2ꢀposition (2H, 13C NMR) while the (E)
isomer of the 1ꢀoctyne had the isotopic label exclusively at the
terminal position geminal to the boron substituent (Scheme
2a). Introduction of the isotopic label decreased the (Z):(E)
ratio, as well as the rate of the reaction (~72% vs. >98%
conversion with the natural abundance alkyne).19 The converse
labeling
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Scheme 3. Stoichiometric experiments exploring the activation
modes of 2 in catalytic (Z)ꢀselective alkyne hydroboration and
solid state structure of 5-Pro-(E).
Scheme 4. Interconversion of 5-Pro-(Z) and 5-Pro-(E) and
product distribution upon treatment with terminal of alkynes.
H
PinB
δ
-
δ+
BPin
H
k1
R
H
Ph
CyAPDI)Co
(
CyAPDI)Co
(
energy of TS:
R = nHex > R = Ph
Ph
k-1
[Co]
Ph
CyAPDI)Co
Ph
(
CyAPDI)Co
R
R
δ+
δ-
BPin
H
23 C
HBPin
Ph
5-Pro-(Z)
5-Pro-(E)
2
(
CyAPDI)Co
(
BPin
-CH4
H
K = 5.7
BPin
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3
R
k2
k3
R
5-Pro-(E)
major (85%)
5-Pro-(Z)
R
%Z %E rel. b/w k1, k-1, k2, k3
minor(15%)
BPin
Metal Vinylidene Pathway
(not operative)
Ph
n-hex
15 85
100
k2, k3 >> k1, k-1
Ph
0
k1, k-1 >> k2 >> k3
Ph
BPin
Z:E (15:85)
R
BPin
R
BPin
Ph
(Z)
(E)
(
CyAPDI)Co
H
nHex
Experimental support for isomer interconversion was
(
CyAPDI)Co
BPin
CyAPDI)Co
not observed
Ph
obtained by treatment of the 85:15 mixture of the isomers of 5
with 1 equiv of 1ꢀoctyne (Scheme 4). Only (Z)ꢀvinylboronate
was observed with the cobalt quantitatively converted to 4.
Repeating the experiment with 0.5 equiv of PhC≡CH, the
nHex
4
(
Ph
BPin
H
Z:E (>98:2)
isomers of
5 yielded an 85:15 mixture of (E):(Z)ꢀ
To
assess
the
catalytic
competency
of
the
vinylboronates while the remaining vinyl compound
maintained the 85:15 isomeric composition. The more acidic
PhC≡CH, like DCl, promotes fast protonation on the vinyl
ligand relative to isomerization (Scheme 4). With nHexC≡CH,
protonolysis is slower allowing equilibration of the proꢀ(E)
and proꢀ(Z) vinyl compounds likely due to a higher energy
transition state. Under catalytic conditions, the large excess of
alkyne facilitates product release such that formation of 5-
Pro-(E) does not occur during turnover. It is likely that
isomerization of 5 occurs through rotation of the single bond
in the zwitterionic carbene intermediate, although additional
support is required to verify this assertion.21 Crossover
experiments with 5 and free alkynylboronate establish no
exchange.
bis(imino)pyridine cobalt acetylides, a benzeneꢀd6 solution of
3 was treated with one equivalent of HBPin. An 85:15 mixture
of two isomeric cobalt vinyl complexes (5) was observed
1
immediately upon addition of HBPin. A combination of H
and 13C NMR spectroscopies and reactivity studies established
the same regiochemistry in the isomers of 5; the carbon bound
to the cobalt is phenyl substituted while the distal carbon is
bound to the [BPin] group. For example, treatment of the
isomers of 5 with excess DCl (1.0 M in Et2O) yielded a 75:25
mixture of (E):(Z)ꢀvinylboronates where deuterium was
located exclusively in the position geminal to the phenyl group
establishing only this carbon was bound to cobalt.20 The
difference in isomers of 5 arises from the stereochemistry
about the C=C bond where the major isomer is designated 5-
Pro-(E) while the minor is 5-Pro-(Z). Definitive assignment
of the stereochemistry was accomplished by measuring
coupling constants from an HMBC NMR experiment (Figure
S8). The nomenclature refers to the stereochemistry of the
cobalt vinyl complex that would give rise to the corresponding
stereochemistry of the vinlboronate product upon protonation.
Formation of bis(imino)pyridine cobalt vinyl compounds with
the [BPin] group located exclusively in the βꢀposition
definitively eliminates the traditionally proposed vinylidene
pathway as responsible for formation of the (Z)ꢀvinylboronate
(Scheme 3).
A
mechanism for the cobalt catalyzed (Z)ꢀselective
hydroboration of terminal alkynes is presented in Scheme 5.
Following formation of the cobalt acetylide, oxidative addition
of HBPin followed by reductive elimination of the
alkynylboronate and synꢀhydrometallation generates the proꢀ
(Z) cobalt vinyl intermediate. Reaction with additional alkyne,
likely the turnoverꢀlimiting step, liberates the observed (Z)ꢀ
vinylboronate. In the absence of alkyne, isomerization of the
proꢀ(Z) cobalt vinyl complex to the thermodynamically
preferred proꢀ(E) isomer occurs. The mechanism in Scheme 5
accounts for the optimized catalytic conditions and observed
ligand effects. With 2, reaction with alkyne to form the metalꢀ
acetylide is faster than hydride formation upon addition of
HBPin. With 1, the opposite trend is observed where HBPin is
more reactive, the cobaltꢀhydride is kinetically preferred and
(E)ꢀselective catalysis results. The slight excess of alkyne
(1.2:1) used for optimal (Z) selectivity suppresses CoꢀH
formation at the end of the catalytic reaction, avoiding postꢀ
catalytic product isomerization. Allowing the (Z)ꢀ
vinylboronate from DBPin addition to phenylacetylene to
stand for 30 minutes after completion of the reaction yielded
the (E) isomer with deuterium in the position geminal to the
[BPin] substituent.
Recrystallization of the mixture of 5-Pro-(E):5-Pro-(Z)
from pentane at ꢀ35 ºC produced crystals suitable for Xꢀray
diffraction identified as 5-Pro-(E) (Scheme 3). The cobaltꢀ
carbon bond is lifted out of the idealized metalꢀchelate plane
with the phenyl substituent directed away from the cyclohexyl
imine substituents. Short CoꢀO contacts of 2.430(9), 2.478(6)
and 2.928(9) Å were observed between the metal and the BPin
ligand in the three independent molecules in the asymmetric
unit and is likely, along with the transꢀdisposition of the large
alkene substituents, is responsible for the thermodynamic
preference for this isomer. Reꢀdissolving the crystals in
benzeneꢀd6 and analysis by NMR spectroscopy revealed an
85:15 mixture of isomers, suggesting either fortuitous crystal
selection or rapid equilibrium of the isomers at 23 ºC.
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(b) Negishi, E.; Williams, R. M.; Lew, G.; Yoshida, T. J. Organomet.
Scheme 5. Proposed mechanism for the formation of (Z)ꢀ
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Organomet. Chem. 1978, 156, 71. (d) Matteson, D. S.; Majumdar, D.
Organometallics 1983, 2, 230. (e) Brown, H. C.; Srebnik, M.; Bhat, N.
G. Tetrahedron Lett. 1988, 29, 2635. (f) Gano, J. E.; Srebnik, M.
Tetrahedron Lett. 1993, 34, 4889. (g) Deloux, L.; Srebnik, M.; Bhat, N.
G. J. Org. Chem. 1994, 59, 6871. (h) Soderquist, J. A.; Rane, A. M.;
Matos, K.; Ramos, J. Tetrahedron Lett. 1995, 36, 6847. (i) Molander,
G. A.; Ellis, N. M. J. Org. Chem. 2008, 73, 6841.
vinylboronates with 2.
2
H
R
CH4
R
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R
BPin
H
HBPin
H
(
CyAPDI)Co
(6) (a) Molybdenumꢀcatalyzed alkene cross metathesis has recently been
described as a direct route to (Z)ꢀvinylboronates although excess
vinylBPin and partial vacuum were required for high selectivity, see:
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H
R
(
CyAPDI)Co BPin
BPin
(
CyAPDI)Co
R
R
pro-E isomer
(
CyAPDI)Co
R
H
(8) Gunanathan, C.; Hölscher, M.; Pan, F.; Leitner, W. J. Am. Chem. Soc.
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(14) Monfette, S.; Turner, Z. R.; Semproni, S. P.; Chirik, P. J. J. Am. Chem.
Soc. 2012, 134, 4561.
(15) (a) Zhang, L.; Zuo, Z.; Wan, X.; Huang, Z. J. Am. Chem. Soc. 2014,
136, 15501. (b) Chen, J.; Xi, T.; Ren, X.; Cheng, B.; Guo, J.; Zhan, L.
Org. Chem. Front. 2014, 20, 13918.
(16) (a) Gibson, V. C.; Humphries, M. J.; Tellmann, K. P.; Wass, D. F.;
White, A. J. P.; Williams, D. J. Chem. Commun. 2001, 2252. (b)
Knijnenburg, Q.; Horton, A. D.; van der Heijden, H.; Smits, J. M. M.;
de Bruin, B.; Budzelaar, P. H. M.; Gal, A. W. J. Mol. Catal. A: Chem.
2005, 232, 151ꢀ159.
(17) Atienza, C. C. H.; Diao, T. N.; Weller, K. J.; Nye, S. A.; Lewis, K. M.;
Delis, J. G. P.; Boyer, J. L.; Roy, A. K.; Chirik, P. J. J. Am. Chem. Soc.
2014, 136, 12108.
An additional stoichiometric experiment was performed to
support the mechanism in Scheme 5. Addition of PhC≡CBPin
to 2 in the presence of HBPin at 23 ºC furnished CH3BPin and
an 85:15 mixture of 5-Pro-(E) and 5-Pro-(Z), clearly
supporting the competency of alkynylboronates for
regioselective insertion into CoꢀH bonds as well as rapid
isomerization between cobalt vinyl compounds in the absence
of excess alkyne.
In summary, a cobalt catalyzed method for the synthesis of
(Z)ꢀvinylboronates
by
regioꢀ
and
strereoselective
hydroboration of terminal alkynes has been developed.
Isotopic labeling and stoichiometric studies eliminate the
possibility of metal vinylidene intermediates and support a
previously unconsidered
hydrometallation reactions.
pathway for
(Z)ꢀselective
Acknowledgment. We thank Princeton University for financial
support. JVO acknowledges the Paul Maeder ’75 Fund for Energy
and the Environment through the Andlinger Center for Energy and the
Environment. We also thank W. Neil Palmer for useful discussions
and Dr. Istvan Pelczer for assistance with NMR spectroscopy.
Supporting Information Available: Complete experimental details,
representative NMR spectra and crystallographic data for 2 and 5-
(Pro)-E in cif format. This material may be accessed, free of charge,
via the internet at http//:pubs.acs.org.
(18) Bowman, A. C.; Milsmann, C.; Bill, E.; Lobkovsky, E.; Weyhermüller,
T.; Wieghardt, Chirik, P. J. Inorg. Chem. 2010, 49, 6110.
(19) The lower Z:E selectivity results from slow catalyst initiation to form
compound 3 and generates a small amount of a CoꢀH leading to the the
(E) isomer. The position of the deuterium in the (E)ꢀvinylboronate
product implies that the (E) isomer resulted from a CoꢀH rather than the
isomerization of 5-Pro-(Z) to 5ꢀPro-(E).
References
(1) (a) Suzuki, A. Angew. Chem. Int. Ed. 2011, 50, 6722 (b) Suzuki, A. J.
Organomet. Chem. 1999, 576, 147. (c) Miyaura, N.; Suzuki, A.; Chem.
Rev. 1995, 95, 2457.
(2) (a) Cordova, A. Ed. Catalytic Asymmetric Conjugate Reactions; Wileyꢀ
VCH: Weinheim, 2010. (b) Mahrwald, R., Ed. Modern Aldol Reactions;
WileyꢀVCH: Weinheim, 2010.
(3) (a) Brown, H. C.; Gupta, S. K. J. Am. Chem. Soc. 1972, 94, 4370. (b)
Brown, H. C.; Gupta, S. K. J. Am. Chem. Soc. 1975, 97, 5249. (c)
Tucker, C. E.; Davidson, J.; Knochel, P. J. Org. Chem. 1992, 57, 3482.
(4) For Eꢀselective hydroboration of terminal alkynes (metalꢀcatalyzed):
[Rh]: (a) Pereira, S.; Srebnik, M. Tetrahedron Lett. 1996, 37, 3283. (b)
Lee, T.; Baik, C.; Jung, I.; Song, K.ꢀH., Kim, S.; Kim, D.; Kang, S.ꢀO.;
Ko, J. Organometallics 2004, 23, 4569. (c) Neilson, B. M.; Bielawski,
C. W. Organometallics 2013, 32, 3121. [Zr]: (d) Pereira, S.; Srebnik,
M. Organometallics 1995, 14, 3127. [Ti]: (e) He, X.; Hartwig, J. F. J.
Am. Chem. Soc. 1996, 118, 1696. (f) [Cu]: Semba, K.; Fujihara, T.;
Terao, J.; Tsuji, Y. Chem. Eur. J. 2012, 18, 4179.
(20) The 85:15 ratio was determined from the 1H NMR spectrum of the
benzeneꢀd6 solution of the isomers of 5. Upon quenching with DCl, the
(Z):(E) ratio of the resulting organic product was measured by GC (the
small GC response factor variations in the (Z) and (E) vinylboronates
were not corrected) to be 80:20. Analyzing the organic product by
quantitative 13C NMR spectroscopy revealed a 75:25 ratio. We believe
that this slight discrepancy in the isomeric ratio is not due to
isomerization but rather due to the use of different analytical methods to
determine the ratio.
(21) Ojima, I.; Clos, N.; Donovan, R. J.; Ingallina, P. Organometallics 1990,
9, 3127.
(5) For synthesis of ZꢀalkenylꢀBR2 or ꢀB(OR)2 compounds (stoichiometric
methods): (a) Dieck, H. A.; Heck, R. F. J. Org. Chem. 1975, 40, 1083.
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