Regioselective Formation of (E)-β-Vinylstannanes with a Topologically Controlled Molybdenum-Based Alkyne Hydrostannation Catalyst

Article abstract of DOI:10.1002/anie.201802397

The regioselective formation of (E)-β-vinylstannanes has been a long-standing challenge in transition-metal-catalyzed alkyne hydrostannation. Herein, we report a well-defined molybdenum-based system featuring two encumbering m-terphenyl isocyanides that reliably and efficiently delivers (E)-β-vinylstannanes from a range of terminal and internal alkynes with high regioselectivity. The system is particularly effective for aryl alkynes and can discriminate between alkyl chains of low steric hindrance in unsymmetrically substituted dialkyl alkynes. Catalytic hydrostannation with this system is also characterized by an electronic effect that leads to a decrease in regioselectivity when electron-withdrawing groups are present on the alkyne substrate.

Full text of DOI:10.1002/anie.201802397

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Angewandte
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International Edition
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Accepted Article
Title: Regioselective Formation of (E)-β-Vinylstannanes by
a Topologically-Controlled Molybdenum-Based Alkyne
Hydrostannation Catalyst
Authors: Kyle Mandla, Curtis E. Moore, Arnold L. Rheingold, and
Joshua S. Figueroa
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802397
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b
Regioselective Formation of (E)- -Vinylstannanes by a
Topologically-Controlled
Hydrostannation Catalyst
Molybdenum-Based
Alkyne
Kyle A. Mandla, Curtis E. Moore, Arnold L. Rheingold and Joshua S. Figueroa*
Dedication ((optional))
Abstract: Regioselective formation of (E)-b-vinylstannanes has been
a
long-standing challenge in transition-metal-catalyzed alkyne
hydrostannation. Here we report a well-defined molybdenum-based
system featuring two encumbering m-terphenyl isocyanides that
reliably and efficiently delivers high regioselectivity for (E)-b-
vinylstannanes from a range of terminal and internal alkynes. The
system is particularly effective for aryl alkynes and can discriminate
between alkyl chains of low steric encumbrance in unsymmetrically-
substituted dialkyl alkynes. Catalytic hydrostannation in this system is
also characterized by an electronic effect that degrades
regioselectivity when electron-withdrawing groups are present on the
alkyne substrate.
Vinylstannanes are well recognized as versatile synthetic
precursors for C-C bond formation reactions.[1-2] Accordingly,
transition-metal-catalyzed alkyne hydrostannation has been
extensively investigated as an efficient, atom-economical route to
vinylstannanes.[3-4] However, despite substantial progress in the
development of alkyne hydrostannation catalysts, precise control
over hydrometallation regioselectivity continues to be an
important challenge. Several catalysts have now been reported
that reliably provide a-vinnylstannane products via catalyst-
controlled hydrometallation (Scheme 1, top).[3-11] In contrast,
catalysts that deliver (E)-b-vinylstannanes by regioselective syn
Sn-H addition have been particularly difficult to develop for alkyne
substrates that lack steric or electronic directing groups (Scheme
1, top).[4,12-16] Indeed, the only systems known to provide
reasonable regioselectivity for (E)-b-vinylstannanes are highly-
encumbered palladium/phosphine catalysts that operate through
Scheme 1. (top) Generalized reaction scheme for the production of
a
formal Pd(0)/Pd(II) couple.[3,4,12,14,15] However, the
vinylstannane
regio-isomers
from
metal-catalyzed
syn-
hydrostannation of terminal alkynes. (middle) The key square-planar,
C-alkenyl intermediates proposed to be responsible for
regioselectivity in Pd-catalyzed alkyne hydrostannation. (bottom)
Coneptual framework for enhancement of (E)-b-regioselectivity by
application of additional steric pressures in a six-coordinate Mo-based
alkyne-hydrostannation catalyst system. LBulk = encumbering ligand.
regioselectivity of these systems is not general over a wide scope
of substituted alkynes,[4,15] and the systems lack the predictability
associated with the most highly-regioselective catalysts for the
production of a-vinylstannanes.
A commonality among the Pd catalysts that show selectivity
for (E)-b-vinylstannanes is the formation of a square-planar, cis-
Pd(C-alkenyl)(SnR3)L2 intermediate during the hydrometallation
process (Scheme 1 middle). The square-planar coordination
geometry of this intermediate positions ancillary ligands both cis
and trans to the key C-alkenyl fragment that is generated upon
initial hydride transfer to the alkyne substrate. When these
catalyst systems are sufficiently encumbered, it has been argued
that regioselectivity for (E)-b-vinylstannanes originates from steric
pressure between
the cis-ancillary ligand and the a-
vinylstannane-producing C-alkenyl unit (Intermediate APd
,
Scheme 1).[15] These unfavorable interactions encourage the
formation of the (E)-b-vinylstannane-producing C-alkenyl
intermediate, where steric interference from the cis-ancillary
ligand is minimized (BPd, Scheme 1). Based on this postulate, we
reasoned that a modified catalyst architecture, where two
encumbering ligands are positioned cis to the C-alkenyl unit,
could further destabilized the formation of a-vinylstannane-
producing intermediates and lead to increased regioselectivity for
the formation of (E)-b-vinylstannanes via syn-hydrometallation.
[*]
K. A. Mandla, Dr. C. E. Moore, Prof. Dr. A. L. Rheingold, Prof. Dr. J.
S. Figueroa
Department of Chemistry and Biochemistry
Section of Synthesis, Materials and Chemical Biology
University of California, San Diego
9500 Gilman Drive MC 0358, La Jolla, CA 92093 (USA)
E-mail: jsfig@ucsd.edu
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Accordingly, we sought to develop an
encumbered alkyne-hydrostannation system
where an overall octahedral metal coordination
geometry was conserved throughout the
hydrometallation process. Such a modification
would allow two additional axial ligands to exert
mutually-cis steric pressures on a C-alkenyl unit
in a manner not achivable for systems that
generate square-planar intermediates (Scheme
1, bottom).[3,4,12,14,15]
Previously, we reported the six-coordinate
complex, MoI2(CO)2(CNArDipp2)2 (1; ArDipp2 = 2,6-
(2,6-(i-Pr)2C6H3)2C6H3; Figure 1),[17] which
features two sterically encumbering m-terphenyl
isocyanide ligands in a trans orientation.[18-20]
Notably, diiodide
coordinate,
1
is related to the six-
cis-Mo(C-alkenyl)(SnR3)L4
intermediates proposed by Kazmaier for alkyne
hydrostannation by the zero-valent, tris-
isocyanide tricarbonyl catalyst precursor,
Mo(CN-t-Bu)3(CO)3).[6,21-25] Indeed, Mo(CN-t-
Bu)3(CO)3 possesses relatively unencumbered
ligands and, correspondingly, its use as an
alkyne hydrostannation pre-catalyst results in
excellent regioselectivity for a-vinylstannanes.[6,21-25] Here we
show that the more sterically encumbered complex 1 also serves
as a highly-efficient alkyne-hydrostannation catalyst precursor.
Figure 1. (top) Syn-hydrostannation of diphenylacetylene using
molybdenum pre-catalysts 1 and 3. (bottom) Synthesis, proposed
formation and molecular structure of the zero-valent amine/C,H-
agostic complex 3 from divalent diiodide 1. Selected bond distances
(Å) and angles (˚) for 3: Mo-H1 = 2.126(5), Mo-C5 = 2.806(2), Mo-N3
= 2.327(3), Mo-H1-C5 = 125.11(30).[36]
However, due to its increased steric profile,
1 affords
regioselective formation of (E)-b-vinylstannanes for a wide scope
of terminal and unsymmetrically-substituted internal alkynes
without the need for sterically-biased substrates. Pre-catalyst 1
can be activated readily under mild conditions to a well-defined
and stabilized 14e– zero-valent active species, in which the rigid
and encumbering trans topology of the m-terphenyl isocyanide
ligands is maintained. In addition, we show that this ligand
orientation is responsible for the system’s high regioselectivity for
(E)-b-vinylstannanes, which can be further modulated by
substrate-dependent electronic perturbation of key intermediates
during hydrometallation.
In an initial screen of 1 for catalytic hydrostannation,
diphenyl acetylene was chosen as a test substrate. Using 2 mol%
1 and 1.05 equiv of HSnBu3, the syn-hydrostannation product (E)-
tributyl(1,2-diphenylvinyl)stannane was produced in 98% isolated
yield after 20 min of reaction at room temperature in C6D6 solution
(Figure 1, top). Notably, the efficiency of this reaction at room
temperature indicates that diiodide 1 is rapidly converted to a
catalytically-active species. Indeed, treatment of diiodide 1 with
2.0 equiv of HSnBu3 in the absence of alkyne substrate leads
Most notably, when diiodide 1 is treated with stoichiometric
HSnBu3 in the presence of diisopropylamine (HN(i-Pr)2), the
Mo(0) amine/C-H-agostic complex 3 can be isolated in ca. 80%
yield (Figure 1). Structural characterization of complex 3 reveals
a trans-orientation of CNArDipp2 ligands similar to that found in
diiodide 1 and an HN(i-Pr)2 ligand bound through an unusual k1-
N/h2-H,C chelate (Figure 1). The isolation of complex 3 strongly
suggests that the four-coordinate, zero-valent species
[Mo(CO)2(CNArMes2)2], which is analogous to the intermediate
proposed for hydrostannation by Mo(CN-t-Bu)3(CO)3,[21] is likely
produced in this reaction sequence and is sufficiently reactive to
be trapped with a weakly-coordinating substrate such as HN(i-
Pr)2. Consistent with this notion, 3 is also highly active for
diphenylacetylene hydrostannation, producing the corresponding
vinylstannane in 96% isolated yield under the same reaction
conditions as those employed for diiodide 1 (Figure 1, top).
Whereas complex 3 yields an active system for catalysis,
the operational simplicity of in situ activation of diiodide 1 with only
a slight excess HSnBu3 rendered it our preferred precursor for
additional hydrostannation studies. Accordingly, at 2.0 mol%
loading, complex 1 is effective for the hydrostannation of a range
of terminal and internal alkynes to (E)-b-vinylstannanes with
exceptional regioselectivity. In addition, catalytic hydrostannation
mediated by 1 proceeds efficiently at room temperature using only
1.05 equiv of HSnBu3 per alkyne substrate and without need for
additives or radical scavengers (Table 1).[6,24] For example, under
rapidly to the Mo(0) tetracarbonyl complex Mo(CO)4(CNArDipp2
)
2
(2)[18] through an apparent reduction/ligand redistribution process,
1
along with H2 and ISnBu3, as assayed by H NMR spectroscopy
and GCMS. Hydride-for-halide exchange has been observed
previously in reactions between HSnBu3 and metal-halide
complexes and likely proceeds by a radical-chain mechanism.[26-
27]
Accordingly, for this system, we propose that a double H/I
exchange process produces an intermediate Mo(0) ML4 species
that is active for alkyne hydrostannation, but redistributes when
substrate is not present (Figure 1).
these conditions,
corresponding (E)-b-vinylstannane
regioselectivity after 20 min in C6D6 solution (entry 1). The (Z)-b-
1
converts phenylacetylene to the
with 13:87 a:(E)-b
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10.1002/anie.201802397
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sterically demanding (entry 2) and electron-rich (entries 3-5)
terminal
aryl
alkynes
to
(E)-b-vinylstannanes
with
Table 1. Hydrostannation of Aryl and Alkyl Alkynes using
MoI2(CO)2(CNArDipp2)2 (1).a
regioselectivities greater than 98%. Similarly, unsymmetrically-
substituted aryl-alkyl internal alkynes can be converted to
vinylstannanes with a high degree of preferential placement of the
[SnBu3] unit proximal to the alkyl group (i.e. formal a-H addition;
entries 7 and 8).[28] These results indicate that the catalytic
intermediates generated by complex 1 likely impose strong steric
control over alkyne orientation during hydrostannation. However,
it is critical to note that the presence of strongly electron-
withdrawing substituents, such as in the case of p-nitrophenyl
acetylene, significantly degrades the regioselectivity displayed by
the system (entry 6). In fact, for this substrate, the a-vinylstannane
is marginally favored.
Select.b
(% (E)-b)
Entry
Yieldc
(%)
Major Product
1
2
87
98
95
97
3
4
91
98
92
94
Scheme 2. Hydrostannation of mifepristone using pre-catalyst 1.
5
6
7
8
99
45
92
80
93
86
94
96
Remarkably,
aliphatic
alkynes
also
undergo
9
91
91
94
93
hydrostannation to (E)-b-vinylstannanes with high regioselectivity
using complex 1 (entries 9-13). Of these latter examples, the
conversion of 2-hexyne to the corresponding (E)-b-vinylstannane
with 9:91 a:(E)-b regioselectivity is particularly noteworthy (entry
12), as this result indicates that the molecular topology accessible
from 1 can finely discriminate between methyl and propyl groups
on opposing ends of an unsymmetrically-substituted dialkyl
alkyne. However, when electron-withdrawing substituents are
present, as in the case of 4-cyano-1-butyne (entry 13), the
regioselectivity of the hydrostannation process is again
diminished. This observation illustrates that the electronic
properties of the alkyne substrate can alter the regioselectivity
governed by the inherent topological features of the catalyst
system. It is also important to note that sterically-biased propargyl
alcohols can be converted to b-(E)-vinylstannanes with high
regioselectivity (entries 14-16), as has been reported previously
for other hydrostannation catalysts.[4] Accordingly, the scope of
regioselective alkyne hydrostannation to (E)-b-vinylstannanes
using complex 1 is expanded, rather than compromised, relative
to other catalyst systems. To this end, it is noteworthy that 1
produces the (E)-b-vinylstannane isomer of mifepristone
(Scheme 2) with 2:98 a:(E)-b regioselectivity and in 98% yield,
thereby demonstrating that a fair degree of molecular complexity
can be accommodated by the system without loss of selectivity or
activity. Catalysis can be scaled without significant loss of
selectivity or activity as well. This was demonstrated by the
hydrostannation of 1.0 g of the internal alkyne 1-phenyl-1-
propyne, which proceeds in 90% yield and 91% regioselectivity
for the (E)-b-vinylstannane isomer at room temperature in 2
hours. Nevertheless, use of 3-butyne-2-one results in a non-
negligible degradation of regioselectivity (entry 17), further
illustrating that electron-withdrawing groups predictably alter the
regiochemical outcome of hydrostannation in this system.[29]
10
11
92
94
12
13
89
61
94
84
14
15
16
90
97
83
92
97
92
17
78
95
a
Reaction conditions: 2.0 mol % 1; 1.05 equiv HSnBu3; C6D6; room
temperature; 30 min. b Selectivity determined by 1H NMR analysis of the
reaction mixture. c Isolated as a mixture of a and b-(E) isomers. All runs
showed complete conversion to vinylstannanes in crude reaction
mixtures.
vinylstannane isomer, which is known to form in some metal-
catalyzed hydrostannation processes,[4] is not produced in this
reaction. Notably, this level of a:(E)-b regioselectivity for
phenylacetylene hydrostannation is markedly superior to that of
the well-utilized catalyst precursor PdCl2(PPh3)2 (i.e. a:(E)-
b = 46:54).[4-5] Complex 1 also out-performs the more encumbered
catalyst system Pd2(dba)3/[HPCy3]BF4/NEt(i-Pr)2, which has
previously yielded the greatest level of a:(E)-b regioselectivity for
phenylacetylene hydrostannation (i.e. a:(E)-b = 19:81).[15]
A
preliminary assessment of the hydrostannation
Other terminal aryl alkynes can also be converted with high
regioselectivity. As shown in Table 1, complex 1 transforms
mechanism by both experimental and computational studies
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10.1002/anie.201802397
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between the two C-alkenyl insertion products to the configuration
favoring production of the (E)-b-vinylstannane isomer (Figure 2).
This argument is analogous to that made for regioselective (E)-b-
vinylstannane formation using Pd and encumbering phosphine
ligands,[15] with the added feature that two cis-oriented ligands
provide steric pressures in this six-coordinate Mo system.
Notably, DFT calculations on the truncated, C-alkenyl
intermediates,
[Mo(k1–C–(C(H)=C(H)C6H4-p-
X))(SnMe3)(CNPh)2(CO)2], which would give rise to (E)-b-
vinylstannanes after C-Sn bond reductive elimination, optimize to
stable minima and have unremarkable structural features for X =
H, Me, OMe and NMe2 (Figure 2 and S5.1). However, for X = NO2,
the calculations reveal a pronounced H,C-agostic interaction
between the Mo center and a methyl group of trimethylstannyl unit
(Figure 2). This result indicates that the Lewis acidity of the Mo
center is augmented when electron-withdrawing substituents are
present on the (E)-b-producing C-alkenyl intermediate. Inclusion
of the full SnBu3 ligand in the calculations also results in a stable
minimum featuring an agostic interaction between an a-
methylene group and the Mo center (Figure S5.2). We therefore
tentatively propose that this increase in metal Lewis acidity may
produce a favorable pathway for b-hydride elimination/alkyne-
Figure 2. (top) Proposed interconversion of Mo-based (E)-b- and a-
reinsertion
to
the a-vinylstannane-producing C-alkenyl
producing
C-alkenyl
intermediates
via
reversible
b-H
intermediate,[35] despite the steric preference of the catalyst.
Lending credence to this notion is finding that hydrostannation of
p-nitrophenylacetylene with Mo(CN-t-Bu)3(CO)3 produces the a-
vinylstannane with >98% regioselectivity (see the ESI), thereby
indicating that in the absence of encumbering ligands, the
electronic profile of the alkyne dominates the regiochemical
outcome. While we are further evaluating the validity of this
mechanistic proposal, the combined observations suggest that
regioselective production of (E)-b-vinylstannanes by complex 1 is
sterically driven, but can be predictably modulated by the
electronic properties of the alkyne substrate or a reduction in the
steric profile of the Mo catalyst. Nevertheless, we anticipate that
this catalytic alkyne-hydrostannation system may be particularly
useful in applications when (E)-b-vinylstannanes are desired and
strongly electron-withdrawing substituents are not in close
proximity to either end of the alkyne substrate.
elimination/migratory insertion. (bottom) DFT-calculated structures for
(E)-b-producing, aryl-substituted Mo-based C-alkenyl intermediates
for X = NO2 (left) and H (right), with closest Mo×××H contacts between
the SnMe3 group indicated.
suggests that the encumbering steric profile of the CNArDipp2
ligands is primarily responsible for the regiochemical preference
of hydrostannation in post-insertion steps. However, the
electronic properties of the alkyne substrates also exert an
important influence over the properties of certain intermediates
along the hydrostannation pathway. Hydrostannation of
phenylpropyne using complex
1
and
a
1:1 mixture of
HSnBu3/DSnBu3 (3:1 [Sn]tot:[alkyne])
resulted in an
intermolecular-competition H/D isotope effect of 1.11(7), thereby
indicating that neither Sn-H bond cleavage nor C-H bond
formation, through either migratory-insertion or reductive
elimination, are the rate limiting aspects of catalysis in this
system.[30-31] Importantly, a plot of log(a:(E)-b) vinylstannane
isomeric ratio vs Hammett s+-parameter (Figure S4.4) for the p-
substituted terminal aryl alkynes listed in Table 1 reveals a distinct
positive correlation between selectivity for the (E)-b-vinylstannane
isomer and electron-releasing capacity of the para substituent.
We believe these observations reflect that the electronic
properties of the alkyne substrate affect the stability of post-
insertion intermediates, which are ulitimately responsible for
dictating the regiochemical outcome of the hydrostannation
processs prior to rate-limiting Sn-C bond reductive elimination.
It has been well established that Chalk-Harrod-type
hydrometallation processes are governed by interconversion of
C-alkenyl metal intermediates via reversible hydride migratory
insertion/b-H elimination (Figure 2).[32-34] In addition, in our studies
on Group 6 metal complexes featuring two CNArDipp2 ligands, we
have previously shown that the steric encumbrance of these
isocyanides enforce a mutually-trans orientation through multiple
redox transformations of the metal center.[18-19] We believe that to
relieve steric pressures, this constrained, trans-isocyanide
environment around the metal center shifts the equilibrium
Acknowledgements
We are grateful to the U.S. National Science Foundation (CHE-
1464978) for support. Professors Valerie A. Schmidt and Charles
L. Perrin are thanked for helpful discussions.
Keywords: Catalysis • Hydrostannaton • Alkynes • Molybdenum
• Isocyanides
[1]
[2]
[3]
J. K. Stille, Angew. Chem. Int. Ed. 1986, 25, 508-524.
J. K. Stille, B. L. Groh, J. Am. Chem. Soc. 1987, 109, 813-817.
N. D. Smith, J. Mancuso, M. Lautens, Chem. Rev. 2000, 100, 3257-
3282.
[4]
[5]
[6]
[7]
[8]
[9]
B. M. Trost, Z. T. Ball, Synthesis 2005, 853-887.
K. Kiyoshi, U. Hideto, W. Fumio, Chem. Lett. 1988, 17, 881-884.
U. Kazmaier, D. Schauss, M. Pohlman, Org. Lett. 1999, 1, 1017-1019.
L. T. Leung, S. K. Leung, P. Chiu, Org. Lett. 2005, 7, 5249-5252.
B. Ghosh, R. E. Maleczka, Tet. Lett. 2011, 52, 5285-2587.
S. M. Rummelt, A. Fürstner, Angew. Chem. Int. Ed. 2014, 53, 3626-
3630.
[10]
[11]
[12]
[13]
S. M. Rummelt, R. Radkowski, D. A. Rosca, A. Fürstner, J. Am. Chem.
Soc. 2015, 137, 5506-5519.
D.-A. Roşca, K. Radkowski, L. M. Wolf, M. Wagh, R. Goddard, W. Thiel,
A. Fürstner, J. Am. Chem. Soc. 2017, 139, 2443-2455.
Y. Ichinose, H. Oda, K. Oshima, K. Utimoto, Bull. Chem. Soc. Jap. 1987,
60, 3468-3470.
H. X. Zhang, F. Guibé, G. Balavoine, Tet. Lett. 1988, 29, 619-622.
This article is protected by copyright. All rights reserved.

10.1002/anie.201802397
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COMMUNICATION
[14]
H. X. Zhang, F. Guibe, G. Balavoine, J. Org. Chem. 1990, 55, 1857-
1867.
[15]
[16]
A. Darwish, A. Lang, T. Kim, J. M. Chong, Org. Lett. 2008, 10, 861-864.
S. Gupta, Y. Do, J. H. Lee, M. Lee, J. Han, Y. H. Rhee, J. Park, Chem.
Eur. J. 2014, 20, 1267-1271.
[17]
[18]
[19]
T. B. Ditri, C. E. Moore, A. L. Rheingold, J. S. Figueroa, Inorg. Chem.
2011, 50, 10448-10459.
T. Ditri, B. Fox, C. E. Moore, A. L. Rheingold, J. S. Figueroa, Inorg.
Chem. 2009, 48, 8362-8375.
A. E. Carpenter, C. C. Mokhtarzadeh, D. S. Ripatti, I. Havrylyuk, R.
Kamezawa, C. E. Moore, A. L. Rheingold, J. S. Figueroa, Inorg. Chem.
2015, 54, 2936-2944.
[20]
[21]
B. R. Barnett, L. A.; Labios, J. M. Stauber, C. E. Moore, A. L. Rheingold,
J. S. Figueroa, Organometallics 2017, 36, 944-954.
U. Kazmaier, M. Pohlman, D. Schauss, Eur. J. Org. Chem. 2000, 3,
2761-2766.
[22]
[23]
[24]
[25]
[26]
S. Braune, U. Kazmaier, J. Organomet. Chem. 2002, 641, 26-29.
H. Lin, U. Kazmaier, Eur. J. Org. Chem. 2007, 2839-2843.
A. O. Wesquet, U. Kazmaier, Adv. Synth. Cat. 2009, 351, 1395-1404.
P. Maity, M. R. Klos, U. Kazmaier, Org. Lett. 2013, 15, 6246-6249.
C. E. Kriley, C. J. Woolley, M. K. Krepps, E. M. Popa, P. E. Fanwick,I. P.
Rothwell, Inorg. Chim. Acta 2000, 300–302, 200-205.
W. S. Tyree, D. A. Vicic, P. M. B. Piccoli, A. J. Schultz, Inorg. Chem.
2006, 45, 8853-8855.
For an example of exclusive preferential placement of the SnBu3 unit
proximal to the aryl group using PdCl2(PPh3)2, see Ref. 13.
While the degradation of (E)-b-regioselectivity for 4-cyano-1-butyne
(entry 13) may potentially be due to a coordination effect of the 'flexible'
nitrile unit, we note that the substrates containing Lewis basic malimide
(entry 11) and propargyl alcohol groups to not lead to a signifinant
dimunation of (E)-b-regioselectivity.
[27]
[28]
[29]
[30]
M. Gómez-Gallego, M. A. Sierra Chem. Rev. 2011, 111, 4857-4963.
[31] The theoretical maximum kH/kD isotope effect for Sn-H bond cleavage
has been estimated as ~3.6. See: A. Vlcek, H. B. Gray J. Am. Chem.
Soc. 1987, 109, 286-287.
[32]
[33]
[34]
A. J. Chalk, J. F. Harrod, J. Am. Chem. Soc. 1965, 87, 16-21.
J. L. Speier, Adv. Organomet. Chem. 1979, 17, 407-447.
R. Trebbe, F. Schager, R. Goddard, K. R. Poerschke, Organometallics
2000, 19, 521-526.
[35]
N. M. Doherty, J. E. Bercaw, J. Am. Chem. Soc. 1985, 107, 2670-2682.
[36] CCDC 1589325 (3) contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre.
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10.1002/anie.201802397
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Opposite side of the issue:
K. A. Mandla, C. E. Moore, A.
L. Rheingold, J. S. Figueroa *
Alkyne
hydrostannation
catalysts are traditionally
effective at providing 1,1-
vinylstannanes (a-products),
with the selective delivery of
E-1,2- vinylstannanes ((E)-b-
products) being much more
challenging. Reported here is
Page No. – Page No.
Regioselective Formation of
(E)- -Vinylstannanes by a
b
Topologically-Controlled
a
highly
regioselective
Mo/isocyanide-based catalyst
that overcomes this difficulty.
Molybdenum-Based Alkyne
Hydrostannation Catalyst
This article is protected by copyright. All rights reserved.