Base metal catalysts for photochemical C-H borylation that utilize metal-metal cooperativity

Article abstract of DOI:10.1021/ja408861p

Heterobimetallic Cu-Fe and Zn-Fe complexes catalyze C-H borylation, a transformation that previously required noble metal catalysts. The optimal catalyst, (IPr)Cu-FeCp(CO)₂, exhibits efficient activity at 5 mol% loading under photochemical conditions, shows only minimal decrease in activity upon reuse, and is able to catalyze borylation of a variety of arene substrates. Stoichiometric reactivity studies are consistent with a proposed mechanism that exploits metal-metal cooperativity and showcases bimetallic versions of the classical organometallic processes, oxidative addition and reductive elimination.

Full text of DOI:10.1021/ja408861p

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Communication
Base Metal Catalysts For Photochemical C-
H Borylation That Utilize Metal-Metal Cooperativity
Thomas J Mazzacano, and Neal P Mankad
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 27 Sep 2013
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Base Metal Catalysts For Photochemical C-H Borylation That
Utilize Metal-Metal Cooperativity
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Thomas J. Mazzacano, Neal P. Mankad*
Department of Chemistry, University of Illinois at Chicago, 845 W. Taylor St., Chicago, IL 60607, USA
Supporting Information Placeholder
11
mation of established importance to organic synthesis
that previously required use of noble metals such as Rh
or Ir.
ABSTRACT: Heterobimetallic CuꢀFe and ZnꢀFe comꢀ
plexes catalyze CꢀH borylation, a transformation that
previously required noble metal catalysts. The optimal
catalyst, (IPr)CuꢀFeCp(CO) , exhibits efficient activity
2
at 5 mol% loading under photochemical conditions,
shows only minimal decrease in activity upon reꢀuse,
and is able to catalyze borylation of a variety of arene
substrates. Stoichiometric reactivity studies are conꢀ
sistent with a proposed mechanism that exploits metalꢀ
metal cooperativity and showcases bimetallic versions of
the classical organometallic processes, oxidative addiꢀ
tion and reductive elimination.
One major goal towards attaining a sustainable and
environmentally responsible paradigm for chemical synꢀ
thesis is to replace catalyst materials based on noble
metals (toxic, expensive, and scarce) with those based
on base metals (nonꢀtoxic, inexpensive, and earthꢀ
1 2
,
abundant). However, because many catalytic transforꢀ
mations rely on twoꢀelectron redox processes (e.g., oxiꢀ
dative addition and reductive elimination) inherent to
noble metal chemistry, the oneꢀelectron redox chemistry
of base metals tends to lack the predictability, efficiency,
and generality necessary for widespread use in homogeꢀ
3
neous catalysis by direct replacement.
Several strategies for achieving twoꢀelectron redox caꢀ
talysis with earthꢀabundant elements have been adꢀ
4
3,
5
vanced, including metalꢀligand cooperativity
and
6
Lewis acidꢀbase cooperativity, but their applications in
catalysis have been limited to relatively simple transꢀ
formations such as hydrogenation and dehydrogenaꢀ
tion reactions. An alternate strategy involves exploiting
metalꢀmetal cooperativity between two different base
Figure 1. A) Twoꢀelectron redox at singleꢀsite noble metal
6,
7
(
left) and heterobimetallic base metal (right) cores. B) Stoiꢀ
8
chiometric CꢀH borylation observed previously (see ref 12;
9
5
Fp = Fe(η ꢀC H )(CO) ; cat = catecholate;). C) Proposed
5
5
2
metals that individually prefer oneꢀelectron redox proꢀ
cesses in order to achieve net, twoꢀelectron redox chemꢀ
istry reminiscent of a singleꢀsite noble metal catalyst
mechanism for catalytic CꢀH borylation consisting of (i)
bimetallic oxidative addition, (ii) CꢀH functionalization,
and (iii) bimetallic reductive elimination.
10
(
Figure 1A). In this report, we show that this strategy
Prior to the development of noble metal catalysts for
can be used to develop heterobimetallic CuꢀFe and Znꢀ
Fe complexes as efficient, robust, and versatile catalysts
for CꢀH borylation, a sophisticated catalytic transforꢀ
11
CꢀH borylation, stoichiometric CꢀH borylation had
been demonstrated under photochemical conditions with
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base metal systems including the boryliron complex,
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FpBcat (Fp = Fe(η ꢀC H )(CO) , cat = catecholate).
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Catalytic turnover with this system was not achieved
because conversion of the immediate byproduct of CꢀH
borylation, FpH, to a boryliron species likely would inꢀ
volve twoꢀelectron redox pathways unavailable to a sinꢀ
gleꢀsite Fe system (Figure 1B). Instead, FpH rapidly
converted to inactive Fp by a oneꢀelectron (per Fe) reꢀ
2
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dox process under the reaction conditions. Although
previously it was thought necessary to utilize singleꢀsite
noble metal systems to achieve catalytic CꢀH borylaꢀ
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tion, our results show that catalysis can be achieved
with the Fpꢀbased boryliron system by exploiting a
mechanistic paradigm involving metalꢀmetal cooperativꢀ
ity (Figure 1C).
Using pinacolborane, HBpin (pin = pinacolate), as a
borylating reagent in neat C D , several CuꢀFp and Znꢀ
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14 15
,
Fp complexes synthesized in our laboratory
from
readily available building blocks were successful in genꢀ
erating C D Bpin catalytically (Figure 2 and Table
6
5
16 17
,
S1).
The optimal catalyst was (IPr)CuꢀFp (IPr =
N,N’ꢀbis(2,6ꢀdiisopropylphenyl)imidazolꢀ2ꢀylidene),
which at catalyst loadings as low as 5 mol% produced
C D Bpin in 71% yield after 24 h at room temperature
6
5
under UV irradiation. Successful borylation also was
achieved with catecholborane (HBcat) and
bis(pinacolato)diboron (B pin ) as boron sources, albeit
2
2
18
with lower product yields (Table S1). (IPr)Cu–Fp is a
highly robust catalyst under these conditions and can be
reused by adding additional substrate to a reaction soluꢀ
tion after a catalytic run without a significant decrease in
efficiency (Figure 2 and Table S2). On the other hand,
the homobimetallic species Fp and the boryliron comꢀ
2
plex FpBpin produced only trace amounts of borylated
product under catalytic conditions, and no product was
observed when the monometallic complexes (IPr)CuꢀCl
+
ꢀ
or K Fp were used as catalysts. Metalꢀmetal cooperativꢀ
ity inherent to heterobimetallic complexes such as
(IPr)CuꢀFp apparently is necessary to achieve this cataꢀ
lytic CꢀH functionalization. Although metalꢀmetal coopꢀ
erativity has been used to develop catalytic transforꢀ
9
,10
mations previously, it is rare to discover cases where
the catalytic reaction cannot also be achieved with the
individual monometallic components of the bimetallic
Figure 2. Selected catalyst optimization results for the phoꢀ
tochemical CꢀD borylation of benzeneꢀd . Catalyst loadings
were 10 mol% unless otherwise indicated. Full optimizaꢀ
6
15,19
system.
tion
results
are
presented
in
Table
S1.
#
= yield for second consecutive use.
In addition to benzene, other aromatic substrates also
were borylated using (IPr)CuꢀFp as a catalyst. The selecꢀ
2
tivity of C(sp )ꢀH borylation under these conditions is
dictated by steric factors, akin to Irꢀcatalyzed borylation
11
reactions. For example, borylation of metaꢀxylene with
HBpin proceeded selectively at the 5ꢀposition (Figure 3),
which under standard aromatic substitution conditions is
the least reactive position. Attempted borylation of paraꢀ
xylene produced only 6% conversion to borylated prodꢀ
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uct due to the lack of sterically accessible C(sp )ꢀH
faster than CꢀD borylation and was likely the result of
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bonds. Borylation of toluene proceeded in high yield,
with a ratio of ortho:meta:para functionalization of
the FpBpin intermediate reacting with hydrogenꢀd proꢀ
1
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duced from initial CꢀD borylation. Collectively (Figure
0
:0.62:0.38. Borylation of anisole and α,α,αꢀ
trifluorotoluene proceeded with lower product yields of
1% and 25%, respectively, and with regioselectivities
4A), these data indicate that bimetallic oxidative addiꢀ
tion of HBpin by (IPr)CuꢀFp is an equilibrium process
that lies to the Cu–Fe side, and that small concentrations
4
closely mirroring that of toluene (Figure 3). Only in the
case of anisole was orthoꢀborylation detected, likely due
to the coordinating ability of the methoxy substituent to
of [(IPr)CuH] and FpBpin account for the observed
2
reactivity. Consistent with these observations, (IPr)Cuꢀ
1
Fp was observed as the sole catalytic resting state by H
21 22
,
direct C–H activation.
NMR spectroscopy when conducting catalytic reactions
in C D .
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Figure 3. Substrate scope and regioselectivity of photoꢀ
chemical CꢀH borylation catalyzed by (IPr)CuꢀFp.
A plausible mechanism for heterobimetallic CꢀH
borylation, shown in Figure 1C, consists of three main
stages: (i) BꢀH bimetallic oxidative addition with metalꢀ
metal bond cleavage; (ii) photochemical CꢀH borylation
by the resulting boryliron intermediate; and (iii) HꢀH
bimetallic reductive elimination with metalꢀmetal bond
formation. Preliminary mechanistic studies with
Figure 4. Mechanistic studies for the proposed elementary
steps: A) reversible BꢀH activation, B) CꢀH functionalizaꢀ
tion, and C) H elimination.
2
Given these results regarding BꢀH activation, we proꢀ
pose that FpBpin is the active borylating reagent in soluꢀ
tion. Indeed, CꢀH borylation by the related species,
FpBcat, has been studied in detail by Hartwig and
(IPr)CuꢀFp, detailed below and summarized in Figure 4,
demonstrated the feasibility of each of these individual
reaction stages.
1
2
coworkers. As support for FpBpin being the active
borylating reagent present in our catalytic system, first
we confirmed that FpBpin is indeed capable of stoichiꢀ
ometric benzene borylation (86% yield). We also note
(Figure 4B) that a competition experiment using a 1:1
1
4
Consistent with our previous reactivity studies, we
expected that BꢀH activation of HBpin by (IPr)CuꢀFp
would yield [(IPr)CuH] , a known copper hydride diꢀ
mer, along with FpBpin. To our surprise, mixing toꢀ
gether (IPr)CuꢀFp and HBpin (10 equiv) in benzeneꢀd6
in the absence of UV irradiation produced no observable
reaction, even at elevated temperatures. Furthermore,
mixing together [(IPr)CuH] and FpBpin rapidly generꢀ
ated (IPr)CuꢀFp and HBpin. However, a rapid reaction
was observed when (IPr)CuꢀFp and HBpin were mixed
under a CO atmosphere. A roughly equimolar mixture
of O(Bpin) and HCO Bpin resulted, the latter of which
2
23
mixture of C H and C D indicated a kinetic isotope
6
6
6
6
effect (KIE) of 3.6 for catalytic CꢀH borylation by
IPr)CuꢀFp, which matches closely the KIEs of 3.3 and
.0 observed for stoichiometric CꢀH borylation by
(
3
2
1
2b
FpBcat and FpBpin, respectively, of the same mixture.
Assuming CꢀH borylation by FpBpin produces FpH as
the immediate byproduct, it is further reasonable to asꢀ
2
25
23,24
sume that protic FpH would combine with hydridic
[(IPr)CuH] to release H via bimetallic reductive elimiꢀ
2
2
2
2
is the known product of CO2 hydroboration by
24
nation. To confirm the feasibility of this elementary step
Figure 4C), we mixed together in situꢀgenerated FpH
and [(IPr)CuH] and observed rapid formation of
[(IPr)CuH]2. We also note that when monitoring the
11
(
photochemical borylation of C D by B NMR specꢀ
troscopy, rapid H/D scrambling was observed at the Hꢀ
Bpin position (Figure S2). This process occurred much
6
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2
1
(IPr)CuꢀFp by H NMR spectroscopy.
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Together, our data indicate that catalytic CꢀH borylaꢀ
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tion, a transformation that previously required noble
metal catalysts, can be achieved with base metal cataꢀ
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lysts by using a mechanistic paradigm that depends on
intimate cooperativity between two closely associated
yet distinct base metal sites. Because the proposed
mechanism for this transformation involves bimetallic
versions of classical organometallic reaction steps such
as oxidative addition and reductive elimination, we are
confident that this catalyst design strategy can be generꢀ
alized to approach other important catalytic reactions
that typically require noble metals to proceed.
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the Internet at http://pubs.acs.org.
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Corresponding Author
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Eꢀmail: npm@uic.edu
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J.ꢀY.; Tse, M. K.; Holmes, D.; Maleczka, R. E.; Smith, M. R. III,
ACKNOWLEDGMENT
Science 2002, 295, 305–308.
1
4
Financial support was provided by startꢀup funds from the
UIC Department of Chemistry and by a Pilot Research
Grant from the UIC Campus Research Board. Dan McIlꢀ
heny and Ben Ramirez assisted with NMR spectroscopy.
John (Art) Anderson assisted with GCꢀMS measurements.
Jayarathne, U.; Mazzacano, T. J.; Bagherzadeh, S.; Mankad, N.
P. Organometallics 2013, 32, 3986–3992.
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For a review of MꢀFp complexes, see: Gade, L. H. Angew.
Chem. Int. Ed. Engl. 2000, 39, 2658–2678.
16
For a review of C–H functionalization using Fe catalysis, see:
Sun, C.ꢀL.; Li, B.ꢀJ.; Shi, Z.ꢀJ. Chem. Rev. 2011, 111, 1293–1314. See
5
Upul Jayarathne synthesized (IPr)CuFe(η ꢀC Me )(CO)
used for catalyst screening.
also ref 17.
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b) Chen, M. S.; White, M. C. Science 2007, 318, 783–787. (c) Paꢀ
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B pin is converted to HBpin under the reaction conditions, likely
2 2
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4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACS Paragon Plus Environment