Nickel-catalyzed cross-coupling of styrenyl epoxides with boronic acids

Article abstract of DOI:10.1002/anie.201101191

Let's get multicatalytic! A Ni⁰ catalyst complexed with a biaryldialkyl monophosphine ligand facilitates C-C bond formation between styrenyl epoxides and aryl boronic acids (see scheme). X-ray analysis of a catalytically active nickel/ligand complex supports a redox pathway involving C sp 3-O bond activation. A variety of α-substituted alcohols were generated with good reaction efficiency by a multicatalytic sequence. Copyright

Full text of DOI:10.1002/anie.201101191

Communications
DOI: 10.1002/anie.201101191
Cross-Coupling
Nickel-Catalyzed Cross-Coupling of Styrenyl Epoxides with Boronic
Acids**
Daniel K. Nielsen and Abigail G. Doyle*
Epoxides are invaluable substrates for complex molecule
synthesis owing to their ready accessibility from alkene- and
carbonyl-containing chemical feedstocks and their proclivity
toward ring opening by a variety of nucleophiles.[1] Despite
recent advances, reactions with carbon-centered nucleophiles
are typically limited to the use of stabilized carbanions, strong
organometallic reagents, or p-rich arenes.[2] The identification
of a catalyst system capable of engaging epoxides and boronic
catalyst precursor for the coupling of p-tol styrene oxide and
PhB(OH)2; gratifyingly, the cross-coupled product was
observed, albeit in low yield (Table 1, entry 1). To assess the
role of various phosphine ligands, [Ni(cod)2] was next
evaluated as a metal source. With PCy3, reaction efficiencies
remained poor (Table 1, entry 2). However, catalyst com-
plexes arising from [Ni(cod)2] and dialkylbiaryl monophos-
phine ligands led to improved reaction efficiency (Table 1,
entries 3–5), consistent with the ability of sterically hindered,
electron-rich ligands to facilitate difficult oxidative inser-
tions.[13] Among the ligands tested, BrettPhos proved most
effective, and afforded the coupled product in 88% yield
(Table 1, entry 5).[14] To our knowledge, this reaction repre-
sents the first application of this ligand to a nickel-catalyzed
process.
À
acids in C C bond formation would be an attractive
alternative because of the low toxicity, high functional
group tolerance, and commercial availability of boron-based
nucleophiles. While palladium-catalyzed allylic substitutions
of vinyl epoxides and boronic acids have been docu-
mented,[3,4] reactions with simple aromatic or aliphatic
epoxides have not been reported. Nonetheless, insertion of
À
transition metals into the Csp3 O bonds of unactivated
A survey of other reaction parameters indicated that the
use of K3PO4 as base and toluene/water (1:0.16) as solvent was
optimal. Particularly noteworthy was the dependence of the
reaction outcome on the identity of the boronate nucleophile;
phenyl boronic acid was considerably more effective than
phenyl boroxine, whereas phenyl trifluoroborate was inactive
(compare Table 1, entries 5–7). To account for these data, we
propose that coordination or protonation by the boronic acid
epoxides has been realized in stoichiometric studies[5] and
proposed in a few catalytic transformations, including epoxide
carbonylation,[6] carboxylation,[7] halogenation,[8] isomeriza-
tion,[9] and reductive coupling.[10] Herein, we describe our
initial studies directed at the development of cross-coupling
reactions between epoxides and boronic acids by nickel(0)-
À
catalyzed C O activation. We report that 10 mol% of a
biarylmonophosphine-ligated nickel complex affords good
efficiency for reactions between styrenyl epoxides and a
variety of aryl boronic acids. Unlike ring opening with most
organometallic reagents, this transformation provides access
to a-substituted rather than b-substituted alcohol products.
Mechanistic studies support a multicatalytic sequence involv-
ing epoxide isomerization to an h2-oxanickellacycle by
Table 1: Optimization studies for the coupling of p-tol styrene oxide and
phenyl boronic acid.[a]
À
benzylic Csp3 O bond oxidative addition, followed by nickel-
catalyzed 1,2-arylation.
Styrenyl epoxide compounds were chosen as substrates
for initial studies because of their commercial availability and
potential bias toward oxidative addition at the activated
benzylic position.[11] On the basis of recent studies on C O
À
bond activation,[12] we first examined [NiCl2(PCy3)2] as a
Entry
Catalyst
Ligand
Nucleophile
Yield [%][b]
[*] D. K. Nielsen, Prof. A. G. Doyle
1
2
3
4
5
6
7
[NiCl2(PCy3)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
[Ni(cod)2]
None
PCy3
PhB(OH)2
PhB(OH)2
PhB(OH)2
PhB(OH)2
PhB(OH)2
(PhBO)3
13
21
67
68
88
20
0
Department of Chemistry, Princeton University
Washington Rd, Princeton, NJ 08544-1009 (USA)
Fax: (+1)609-258-2558
E-mail: agdoyle@princeton.edu
Homepage: http://www.princeton.edu/~doylegrp
JohnPhos
XPhos
BrettPhos
BrettPhos
BrettPhos
[**] We thank Dr. Pat Carroll of the University of Pennsylvania for X-ray
crystallographic analysis. Financial support was provided by
Princeton University and kind gifts from Eli Lilly and Sanofi-Aventis.
We also thank the MacMillan and Sorensen research groups for use
of their chemicals and instruments.
PhBF3K
[a] Reaction conditions: epoxide (1.0 equiv), PhB(OH)2 (4.0 equiv), base
(4.5 equiv), H2O (30 equiv). For further evaluation of reaction parame-
ters, see the Supporting Information. [b] Yields determined by HPLC
using methyl benzoate as a quantitative internal standard; average of
two runs. Cy=cyclohexyl, cod=cycloocta-1,5-diene, p-tol=p-tolyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101191.
6056
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Angew. Chem. Int. Ed. 2011, 50, 6056 –6059

facilitates oxidative addition of the epoxide to the nickel
catalyst.[15] Boron to nickel transmetalation may also be
accelerated through formation of a reactive “ate” complex.
This proposal is in agreement with recent studies on the
stoichiometric reaction of aryl boronic acids with a rhoda-
oxetane complex.[16]
Additional support for the redox role of nickel in epoxide
activation was derived from red crystals of a catalytically
active NiII complex obtained from a catalytic reaction. The
structure of this air-stable complex was elucidated by X-ray
crystallographic analysis, and shows a remarkable 1:1:1:1
combination of NiII, BrettPhos, styrene oxide, and phenyl
boroxine, wherein the biaryl phosphine ligand has formally
undergone cyclometalation with the epoxide and Ni0
(Figure 1).[25,26] Observation of an induction period when
Observation of a-substituted alcohols as the major
products in these reactions can be explained by initial
oxidative addition to access metallaoxetane 1[17,18] or its
ring-opened form, followed by b-hydride elimination and
reinsertion to afford h2-oxanickellacycle 2 (Scheme 1).[19]
Kulasegaram and Kulawiec[20a,b] have disclosed studies on
palladium-catalyzed epoxide isomerization that lend support
to this proposal.[20c–e,21] Subsequent nickel-catalyzed 1,2-
arylation with boronic acids, a transformation demonstrated
recently by the research groups of Itami and Aoyama,[22a,b]
would deliver the observed product.[22c–g] While the rearrange-
ment and arylation reactions are known separately, a simple
catalyst capable of effecting both elementary steps has not
been identified. Indeed, development of a catalyst system that
performs both operations in a single pot from easily accessed
and relatively stable epoxides avoids the synthesis and
handling of aryl acetaldehydes, which are inconvenient
substrates owing to their significant instability on the bench.
Although formation of the aldehyde intermediate could
proceed by thermal or Lewis acid catalyzed rearrangement of
the epoxide, the following data favor a redox pathway
involving oxidative addition. First, preliminary initial rate
studies indicate that the reaction displays a first order
dependence on [Ni(cod)2]. In addition, we find that phenyl-
acetaldehyde undergoes efficient 1,2-arylation at lower tem-
peratures than styrene oxide and we detect no buildup of
aldehyde during the reaction course. Taken together, these
data suggest that initial isomerization to phenylacetaldehyde
is nickel catalyzed and rate determining. While this does not
preclude rate-limiting Lewis acid catalyzed rearrangement,
NiII complexes are generally more effective Lewis acids than
Ni0 complexes, especially those ligated with electron-releasing
ligands such as BrettPhos.[23] Notably, use of catalysts that are
more Lewis acidic than Ni0/BrettPhos, such as Ni0/PPh3 or NiII
salts, afforded decreased yields (see the Supporting Informa-
tion for details).[24]
Figure 1. ORTEP plot of complex 3. Thermal ellipsoids are drawn at
30% probability and selected hydrogen atoms have been removed for
clarity. Selected bond lengths [ꢀ] and angles [8]: Ni–P 2.1986(5), Ni–C6
1.9698(15), Ni–C1 1.9789(15), Ni–C2 2.0459(15), Ni–O3 1.9172(11),
P-Ni-C6 87.72(5), C6-Ni-C2 72.67(6), C2-Ni-O3 91.50(5), O3-Ni-P
110.40(4).
10 mol% of 3 was used as a catalyst suggests that 3 is not
directly in the catalytic cycle (Figure 2, see the Supporting
Information for details).[27] However, use of 3 as the catalyst
for the reaction of p-fluoro styrene oxide with phenyl boronic
acid afforded good yields of products derived from both
styrene oxide and p-fluoro styrene oxide, thus indicating that
the epoxide incorporated into the metallacycle undergoes
productive reaction (for clarity, the yield of 5 in Figure 2 is
normalized out of a theoretical maximum of 0.0075 mmol).
Clearly, these data suggest that 3 accesses the catalytic cycle
À
and supports nickel oxidative addition into a Csp3 O bond as a
viable catalytic step.[28]
Based on these data, we suggest that 3 arises from ring
expansion of nickellaoxetane 1. Alternatively, oxidative
cyclization of the ligand, epoxide, and Ni0 could directly
access 3, which would then undergo a 1,2-hydride shift to
generate h2-oxanickellacycle 2.[29] Additional studies are in
progress to further clarify the mechanism of this process and
to determine if conditions can be identified that disfavor b-
hydride elimination, and therefore resulting in a direct cross-
coupling reaction between epoxides and organometallic
reagents.
With insight gained from mechanistic analysis, we inves-
tigated the scope of the reaction (Table 2). Generally, styrenyl
epoxides containing p- and m-substituents performed well
under the standard reaction conditions. More hindered
epoxides, including o-tolyl styrene oxide and o-fluoro styrene
oxide, also underwent coupling with phenyl boronic acid,
albeit with slightly depressed yields. Substrates bearing
strongly electron-releasing groups, however, fared poorly,
likely as a result of the instability of the oxirane to prolonged
heating at 1008C. To date, reactions with non-styrenyl
Scheme 1. Proposed catalytic cycle.
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Communications
Table 3: Coupling of p-tol styrene oxide with various boronic acids.[a]
Entry Product
1
Yield [%][b] Entry Product
Yield [%][b]
57
59
55
68
49
50
6
2
3
4
5
7
77
59
58
60
8
&
^
Figure 2. Yield of 4 ( ) and 5 ( ) (calculated from a theoretical
maximum of 0.0075 mmol) for a reaction catalyzed by 3 compared to
~
a [Ni(cod)2]/BrettPhos-catalyzed reaction ( ). Standard reaction con-
9
ditions: [Ni(cod)2] (10 mol%), BrettPhos (20 mol%), epoxide
(1.0 equiv), PhB(OH)2 (4.3 equiv), base (4.8 equiv), H2O (30 equiv), on
a 0.3 mmol reaction scale. Conditions using 3 as a catalyst: 3
(10 mol%), BrettPhos (10 mol%), epoxide (1.0 equiv), PhB(OH)2
(4.0 equiv), base (4.8 equiv), H2O (30 equiv), on a 0.075 mmol reaction
scale. Yields determined by HPLC using methyl benzoate as a
quantitative internal standard.
10
[a] Reaction conditions: epoxide (1.0 equiv), PhB(OH)2 (4.0 equiv), K3PO4
(4.5 equiv), H2O (30 equiv). [b] Yield of isolated product on a 0.3 mmol
reaction scale; average of two runs.
terminal epoxides have proven unsuccessful, which is in
agreement with the high activation barrier for oxidative
À
addition to aliphatic Csp3 O bonds.
Evaluation of boronic acid coupling partners has revealed
that a variety of nucleophiles are amenable to the reaction
(Table 3). Both electron-rich and electron-poor aryl boronic
acids are well tolerated. Particularly notable in this regard is
the use of p-methoxy phenyl boronic acid, as aryl methyl
ethers have been reported to undergo Suzuki–Miyaura
coupling reactions catalyzed by related nickel complexes.[30]
Unfortunately, heteroaromatic and carbonyl-containing aryl
boronic acids afford minimal product.[31] Despite this limi-
tation, the transformation provides a method for performing a
tandem isomerization/arylation reaction in one pot, thus
allowing access to synthetically valuable secondary alcohols
from readily available and inexpensive terminal epoxides.
À
In summary, we have developed a new benzylic Csp3
O
activation protocol that enables the catalytic functionalization
of epoxides with unstabilized carbon-centered nucleophiles.
Evidence suggests that the reaction proceeds through a
nickellaoxetane, which has promise as a valuable synthetic
À
intermediate in a variety of C C bond-forming manifolds. To
this end, studies with alternative nucleophiles are underway,
and work toward expanding the substrate scope to include a
wider variety of epoxide substrates will be reported in due
course.
Table 2: Coupling of substituted styrene oxides with PhB(OH)2.[a]
Received: February 16, 2011
Revised: April 8, 2011
Published online: May 13, 2011
Entry Product
1
Yield [%][b] Entry Product
Yield [%][b]
73
77
50
6
7
Keywords: cross-coupling · epoxides · multicatalytic reactions ·
.
nickel · Suzuki–Miyaura reaction
2
3
46
60
66
8
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À
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