Regioselective allene hydroarylation via one-pot allene hydrosilylation/Pd-catalyzed cross-coupling

Article abstract of DOI:10.1021/ol502766q

Advances in hydroarylation have been achieved by the development of a one-pot regioselective allene hydrosilylation/Pd(0)-catalyzed cross-coupling protocol. The regioselectivity is primarily governed by N-heterocyclic carbene (NHC) ligand identity in the hydrosilylation step and is preserved in the subsequent cross-coupling reaction. This methodology affords streamlined access to functionalized 1,1-disubstituted alkenes with excellent regiocontrol. (Chemical Equation Presented).

Full text of DOI:10.1021/ol502766q

Letter
pubs.acs.org/OrgLett
Regioselective Allene Hydroarylation via One-Pot Allene
Hydrosilylation/Pd-Catalyzed Cross-Coupling
Zachary D. Miller and John Montgomery*
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109-1055, United States
S
* Supporting Information
ABSTRACT: Advances in hydroarylation have been achieved by the
development of a one-pot regioselective allene hydrosilylation/Pd(0)-
catalyzed cross-coupling protocol. The regioselectivity is primarily
governed by N-heterocyclic carbene (NHC) ligand identity in the
hydrosilylation step and is preserved in the subsequent cross-coupling
reaction. This methodology affords streamlined access to functionalized
1,1-disubstituted alkenes with excellent regiocontrol.
Scheme 1. Goals of the Current Study
lkenylsilanes are versatile nucleophiles in metal-catalyzed
A
cross-coupling reactions. Numerous carbon−carbon bond-
forming reactions with alkenylsilanes have been well developed,1
including the powerful cross-coupling methodology extensively
developed by Hiyama and Denmark.2−4 Currently, the most
direct routes to alkenylsilanes are via metal-catalyzed hydro-
silylations of alkynes.5 However, subtle alterations in the π-
component, silane coupling partner, and metal catalyst often
have substantial effects on the reaction outcome, leading to
challenges in regiocontrol.1a Significant progress has been made
in accessing alkenylsilane structures through the development of
alternative methods, including reactions that introduce the silane
moiety through carbon framework rearrangements,6a,b Heck6c
and silyl-Heck reactions,6d,e or by nucleophilic additions to
electrophilic halosilanes.6f However, the regio- and stereo-
selective addition of silanes to simple π-components remains
an especially attractive point of entry to this structure class.
As a part of our laboratory’s interests in developing
regioselective catalytic processes,7 we set out to explore catalytic
allene hydrosilylation with the goal to discover conditions that
would provide highly selective access to either allylsilanes or
alkenylsilanes depending on the metal−ligand combination
employed. Our previous work in regioselective allene hydro-
silylations demonstrated that the selection of metal is an effective
tool for gaining access to either allylsilanes with palladium NHC
complexes or alkenylsilanes with nickel NHC complexes
(Scheme 1).7e While the nickel-based procedure provided useful
access to alkenylsilanes, the scope was somewhat limited, and
access to the classes of alkenylsilanes that are most useful in
cross-coupling methodology was not obtained. This inspired us
to search for conditions with palladium catalysis where the
inherent preference for allylsilane production could be reversed
to allow alkenylsilane synthesis by simple alteration of the N-
heterocyclic carbene ligand employed.
catalyzed cross-coupling event to provide a net hydroarylation
process. Although significant progress has been achieved in one-
pot alkyne hydroarylations with zinc8a−c and tin,8d one-pot
hydroarylations involving hydrosilylation/cross-coupling se-
quences typically provide 1,2-disubstituted alkenes and often
require purification of the alkenyl silane.9 To address the above
challenges, we report herein a general one-pot hydroarylation
procedure that utilizes a catalyst-controlled regioselective allene
hydrosilylation reaction in tandem with Pd-catalyzed cross-
couplings to provide functionalized 1,1-disubstituted alkenes
with exceptional regiocontrol (Scheme 1).
Our efforts to identify the conditions that favor alkenylsilanes
in palladium-catalyzed allene hydrosilylation began with an
evaluation of the effects of structural modification of NHC
ligands on reaction outcome. To facilitate rapid screening, we
utilized a protocol where the active catalyst is formed by the
deprotonation of the NHC hydrochloride salt with an equivalent
of KO-t-Bu base and Pd2dba3 precatalyst (Table 1). Our
preliminary experiments resulted in the observation that methyl
substitution of the backbone of N-mesityl ligands resulted in a
As additional motivation, identification of a palladium-
catalyzed entry to alkenyl silanes from allenes would potentially
allow a one-pot procedure to be developed involving
regioselective allene hydrosilylation followed directly by a Pd-
Received: September 18, 2014
Published: October 2, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
regioselectivity. Given the superior regioselection of alkenylsi-
lane production with IPr*OMe (5b) across the two different
silane structures, this procedure was adopted as the standard
procedure for alkenylsilane synthesis for the remainder of this
study.
Table 1. Ligand Effects in Allene Hydrosilylation
Variations in the silane coupling partner and allene
substitution were then explored with the optimized procedure
using ligand IPr*OMe (5b) as the free carbene (conditions A).
Hydrosilylations occurred in 1−2 h at rt with palladium catalysis
to afford a range of alkenylsilanes with a high degree of
regiocontrol (Table 2). With the allene variations depicted,
regioselectivity
entry
silane
L·HX major product (% yield)
(1:2)
1
2
Et3SiH
3a
3b
4a
4b
5a
5b
4a
4b
5a
5b
2a (80)
2a (85)
2a (75)
1a (83)
1a (50)
1a (90)
2b (77)
1b (73)
1b (62)
1b (79)
<2:>98
2:98
Et3SiH
3
Et3SiH
12:88
>98:2
>98:2
>98:2
3:97
4
Et3SiH
5
Et3SiH
Table 2. Silane and Allene Variation
a
a
6
Et3SiH
7
BnMe2SiH
BnMe2SiH
BnMe2SiH
BnMe2SiH
b
8
75:25
9
81:19
95:5
10
a
b
Reaction conducted in the absence of KO-t-Bu. Reaction conducted
with 2.5 mol % Pd2dba3 and 5.0 mol % ligand.
product
regioselectivity
entry
R1
Cy
conditions
silane
(% yield)
(1:2)
1
A
A
A
A
A
A
A
A
A
A
A
B
B
B
B
Me2PhSiH
Me2EtOSiH
Me2PhSiH
Me2EtOSiH
BnMe2SiH
Me2PhSiH
Me2EtOSiH
BnMe2SiH
Me2PhSiH
Me2EtOSiH
BnMe2SiH
BnMe2SiH
Me2EtOSiH
Me2EtOSiH
BnMe2SiH
1c (90)
1d (81)
6a (83)
6b (88)
6c (90)
>98:2
96:4
2
Cy
3
Ph
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
98:2
4
Ph
5
Ph
slight erosion of regioselectivity, with allylsilanes strongly
preferred in analogy with our prior findings.7e For example, the
hydrosilylation of cyclohexylallene with triethylsilane using IMes
(3a) as a ligand resulted in excellent regioselectivities (>98:2)
favoring allylsilane 2, while use of IMesMe (3b) as a ligand
proceeded with slightly decreased regioselectivity (98:2) in an
excellent chemical yield of 85% (Table 1, entries 1 and 2).
The effects of NHC ligand variation were further explored
with bulkier ligands (Table 1, entries 3−5). The reaction with the
comparatively large IPr (4a) as the ligand still resulted in the
allylsilane 2 as the major product with triethylsilane in 88:12
regioselectivity (Table 1, entry 3). However, use of IPrMe (4b) as
the ligand, modified from its IPr variant by simple dimethyl
substitution of the backbone, afforded alkenylsilane 1 with
triethylsilane in >98:2 regioselectivity in 83% yield. While the
steric differences of IPr and IPrMe have been previously
recognized including a description of %Vbur for silver and gold
complexes,10a this dramatic regiochemical reversal is nonetheless
surprising.10b Additionally, variation of N-aryl rather than the
backbone substituent also allows alkenyl silanes to be produced
with exceptional regiocontrol. Specifically, the use of commer-
cially available NHC carbene ligand IPr* (5a) or IPr*OMe
(5b)11 also provided a highly selective entry to alkenyl silanes,
with product 1 being obtained again in >98:2 regioselectivity in
both instances.
Given that an important objective of the current study is the
utilization of organosilicon structures that perform well in cross-
couplings, related hydrosilylations using BnMe2SiH were next
examined (Table 1, entries 6−8). The comparison of IPr (4a),
IPrMe (4b), IPr* (5a), and IPr*OMe (5b) exhibited similar
trends, with ligand 4a providing allylsilane 2 with 97:3
regioselectivity, ligand 4b providing a 75:25 mixture favoring
alkenylsilane 1, ligand 5a favoring alkenylsilane 1 with 81:19
selectivity, and ligand 5b favoring alkenylsilane 1 with optimum
95:5 regiocontrol. The 5a:5b comparison illustrates that both the
steric and electronic environment play an important role in
a
6
BnO
BnO
BnO
n-Oct
n-Oct
n-Oct
Cy
7a (95)
7
7b (84)
7c (85)
8a (83)
8b (87)
8c (91)
1b (94)
1d (99)
6b (94)
6c (92)
8
9
10
11
12
13
14
15
96:4
93:7
98:2
Cy
98:2
Ph
>98:2
>98:2
Ph
a
Reaction was conducted on a 6.8 mmol scale.
Me2PhSiH12a and BnMe2SiH12b were explored due to their
previous applications in cross-couplings, and Me2EtOSiH was
also examined, as recent studies have demonstrated that this
motif is effective in both fluoride-promoted and fluoride-free
cross-couplings.12c The hydrosilylation of cyclohexylallene with
Me2PhSiH afforded product 1c in 90% yield with Me2PhSiH
(Table 2, entry 1) while the reaction with Me2EtOSiH afforded
alkenylsilane 1d in 81% yield with high regioselectivity (96:4)
(Table 2, entry 2). Aromatic allene substituents were well-
tolerated with high regioselectivity (>98:<2) for each of the three
silanes to afford 6a−6c in excellent yields (Table 2, entries 3−5).
Similarly, a benzyloxy-substituted allene was also well-tolerated
with each of the silanes screened to yield alkenylsilanes 7a−7c in
excellent yields and regioselectivities (Table 2, entries 6−8). The
ease of scale-up was examined for product 7a, which was afforded
in 95% yield in 6.8 mmol scale (Table 2, entry 6). Simple
unhindered aliphatics were also well-tolerated with a slight
erosion of regioselectivity to form products 8a−8c with excellent
yields and regioselectivities (Table 2, entries 9−11).
Improved reaction conditions were optimized to include
standard benchtop assembly, without the use of air-free
technology such as use of a glovebox (conditions B, Table 2).
This protocol employs IPr*OMe·HBF4 salt 5b, KO-t-Bu base,
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Pd2dba3 precatalyst, and more concentrated reaction conditions
(0.5 M in THF). Reactions performed under these conditions
were faster and higher yielding than with conditions A (Table 2).
For example, four cases examined resulted in higher yields and
improved regioselectivities compared with the analogous
condition A experiments (Table 2, entries 12−15).
In parallel with our goal to develop a user-friendly hydro-
arylation protocol, we explored the use of optimum reaction
conditions B in tandem with a cross-coupling reaction with
electrophilic iodides (Table 3). In this procedure, the hydro-
coupled in excellent yields. In addition, a variety of heterocyclic-
containing iodides functioned under the reaction conditions, as
reactions with uracil-containing (Table 3, entry 6) and thienyl-
containing (Table 3, entry 7) iodides were tolerated with
excellent yields. More hindered substrates such as 2-iodotoluene
provided the expected product 15 in excellent yield (Table 3,
entry 8). Alterations in allene substitution had insignificant
impact on the outcome of the reaction, as aromatic allenes were
coupled with PhI (Table 3, entry 9, 16) and oxacyclic moieties
(Table 3, entry 10, 17) in excellent yields of 76% and 83%.
Additionally, a benzyloxy-substituted allene was successfully
coupled with PhI in high yield (Table 3, entry 11, 18). A
phthalimido-containing allene was coupled with pyrrole-
containing iodide to furnish hydroarylation product 19 in an
excellent yield of 89% (Table 3, entry 12). While yields were
typically optimum with an additional charge of palladium prior to
the cross-coupling step, this was not required in a larger scale
illustration using undeca-1,2-diene and phenyl iodide (Table 3,
entry 13).
Table 3. Regioselective Allene Hydroarylations
The mechanism and origin of regiochemistry reversal likely
derives from changing from a hydrometalation to a silylmetala-
tion pathway (Scheme 2). Our prior studies illustrated that the
Scheme 2. Origin of Regioselectivity Governed by Ligand Size
typical mode of addition in Pd-catalyzed hydrosilylations of
allenes favors allylsilane production, consistent with delivery of
the metal hydride to the allene central carbon. Typical NHC
ligands such as IMes (3a) and IPr (4a) favor the production of
allylsilanes by this route (21 to 23). In contrast, less common
bulkier ligands such as IPrMe (4b) or IPr*OMe (5b) more
effectively introduce steric repulsion between the organosilane
and ligand and reverse the pathway to favor alkenylsilane
production through silyl addition to the central allene carbon (24
to 26). Significant insights in the mechanism and regiochemistry
of related catalyzed additions to allenes come from the work of
Morken and Suginome involving additions of diboranes or
silylboranes.13 A recent computational study of allene hydro-
silylations proposed that cleavage of the Si−H bond and addition
to the allene central carbon proceed in a single step.14
In summary, careful selection of NHC ligand structure allows
excellent regiocontrol in allene hydrosilylations with palladium
catalysis, favoring either allylsilane or alkenylsilane products.
While common NHC ligands favor allylsilane production, a
variety of alkenylsilanes may be produced with the extremely
bulky IPr*OMe ligand. This methodology utilizes standard
benchtop assembly and achieves exceptional regiocontrol
without the use of directing groups or electronic substrate
biases. Highlights of this approach have been applied to the
development of a one-pot hydroarylation reaction that combines
the regioselective hydrosilylation strategy with a Pd(0)-catalyzed
cross-coupling reaction of aryl and hetereocyclic iodides. This
a
Reaction was conducted on a 4.5 mmol scale without the second
charge of Pd2(dba)3.
silylation reaction is monitored, and upon complete con-
sumption of allene, tetra-n-butylammonium fluoride (TBAF),
an iodide electrophile, and an additional 2.5 mol % Pd2dba3
precatalyst are added to the reaction mixture. The subsequent
cross-coupling reaction occurs within 30 min, affording hydro-
arylation products 9−18 without detection of alkene isomers
(Table 3). Although in very few cases there is slight formation of
regioisomeric allylsilanes, these isomers do not afford cross-
coupled minor isomers in the hydroarylation manifold likely due
to competing protodesilylation.
The scope of the one-pot hydroarylation reaction is general
and provides access to a variety of 1,1-disubstituted alkenes. This
method tolerates variation in silane structure, as BnMe2SiH and
Me2EtOSiH were successfully coupled with PhI in high yields of
95% and 85% respectively (Table 3, entries 1 and 2). Aryl iodides
with electron-withdrawing substituents (Table 3, entries 3 and 4)
and electron-rich groups (Table 3, entry 5) were successfully
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hydroarylation method provides streamlined access to branched
isomers in excellent yields without the requirement of isolation of
alkenylsilane intermediates.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and copies of NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jmontg@umich.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Institutes of Health
(GM57014). The laboratory of J. P. Wolfe (University of
Michigan, Ann Arbor, MI) is thanked for the gift of aryl iodides
used in preliminary optimization studies.
(10) (a) Gaillard, S.; Bantreil, X.; Slawin, A. M. Z.; Nolan, S. P. Dalton
Trans. 2009, 6967−6971. (b) For a description of the role of backbone
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Glass, J. L.; Wiggins, K. M.; Aarif, A. M.; Louie, J. J. Org. Chem. 2009, 74,
7935−7942.
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S. P. Organometallics 2013, 32, 330−339. (b) Nelson, D. J.; Collado, A.;
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