Visible Light Photocatalysis Using a Commercially Available Iron Compound

Article abstract of DOI:10.1021/acs.organomet.7b00499

[CpFe(CO)₂]₂ (1) (Cp = η⁵-C₅H₅) is an effective precatalyst for the hydrophosphination of alkenes with Ph₂PH under visible light irradiation, which appears to be a unique way to promote metal-catalyzed hydrophosphination. Additionally, 1 is a photocatalyst for the dehydrogenation of amine boranes and formation of siloxanes from tertiary silanes. These reactions have similar, if not improved, reactivity over the same transformations using 1 or related CpFeMe(CO)₂ under UV irradiation, consistent with the notion that hydrophosphination with 1 proceeds via formation of CpFe(CO)₂^?. These results demonstrate that catalyst selection can avail the use of commercially available LED bulbs as photon sources, potentially replacing mercury arc lamps or other energy intensive processes in known or new catalytic reactions.

Full text of DOI:10.1021/acs.organomet.7b00499

Article
pubs.acs.org/Organometallics
Visible Light Photocatalysis Using a Commercially Available Iron
Compound
†
Justin K. Pagano, Christine A. Bange, Sarah E. Farmiloe, and Rory Waterman*
Department of Chemistry, University of Vermont, Discovery Hall, Burlington, Vermont 05401, United States
*
S Supporting Information
5
ABSTRACT: [CpFe(CO) ] (1) (Cp = η -C H ) is an effective
2
2
5
5
precatalyst for the hydrophosphination of alkenes with Ph PH under
2
visible light irradiation, which appears to be a unique way to promote
metal-catalyzed hydrophosphination. Additionally, 1 is a photocatalyst
for the dehydrogenation of amine boranes and formation of siloxanes
from tertiary silanes. These reactions have similar, if not improved,
reactivity over the same transformations using 1 or related CpFeMe-
(
CO)2 under UV irradiation, consistent with the notion that
•
hydrophosphination with 1 proceeds via formation of CpFe(CO) . These results demonstrate that catalyst selection can
2
avail the use of commercially available LED bulbs as photon sources, potentially replacing mercury arc lamps or other energy
intensive processes in known or new catalytic reactions.
INTRODUCTION
catalytic hydrophosphination. This activity is likely the result of
■
a known photoactivation of 1, presumably generating CpFe-
Selective formation of P−C bonds is a continued synthetic
challenge despite the ubiquity of tertiary phosphines in modern
synthetic and catalytic protocols.
the catalytic synthesis of tertiary phosphines is metal-catalyzed
hydrophosphination, which imparts selectivity to the P−C
bond forming process and minimizes chemical waste.
variety of catalysts have been shown to engage in this reactivity
•
39−42
(
CO) .
This observation implied that visible light
2
1
−9
irradiation of 1 may promote additional reactions because
One attractive route for
•
CpFe(CO)2 is an intermediate in the UV activation of 1 as
39−42
well.
To test this hypothesis, catalytic reactions reported
with CpFe(CO) derivatives promoted by UV irradiation were
2
10−12
A
revisited using 1 under visible light irradiation. The visible light
promoted processes display similar, and in some cases
improved, reactivity over UV-activated processes involving 1
or derivatives. These observations support the idea that low-
energy activation pathways are available for commercial,
abundant catalysts.
1
3,14
15,16
ranging from the s-block
to lanthanides,
transition
1
0,11
17,18
metals,
and even main-group compounds.
Commercial, inexpensive catalysts are likely to be widely
1
9
adopted for any process. Despite the successes in catalytic
hydrophosphination, relatively rare metals are still at the fore of
2
0
21−23
24,25
research. Gaumont, Webster,
and Nakazawa
have
RESULTS AND DISCUSSION
each made critical advances in the utilization of earth-abundant
iron catalysts. At the same time, many hydrophosphination
reactions require significant heating.
reactions using visible light have grown in importance because
these reactions can be considered green due to the abundance
of visible photons from solar irradiation.
energy light sources such as mercury arc lamps that emit
photons in the UV (λirr < 300 nm) region remain common-
place for many photochemical/photocatalytic reactions. Iron-
based catalysis via visible light activation is known, with a
■
Hydrophosphination. Hydrophosphination of a variety of
alkenes with diphenylphosphine is possible with visible light
activation of 1 (Table 1). Catalytic reactions of alkenes and
diphenylphosphine with 5 mol % of 1 were run neat with
irradiation using a commercial LED bulb (λirr > 500 nm).
Reactions were shielded from ambient light and periodically
monitored to ensure the reaction temperature held between 25
2
1−25
Photocatalyzed
26−31
However, high-
4
3
and 28 °C, or slightly above ambient temperature.
Electron-deficient styrenes proved to be more readily
hydrophosphinated than unsubstituted or electron-rich styr-
enes. Notably, styrene itself was almost completely unreactive
in these systems despite the low, but reproducible, reactivity
observed for electron-rich substituents. Indeed, several samples
pioneering report by Nicholas in 2002 that used [Cp*Fe-
5
(
CO) ] (Cp* = η -C Me ) and near-IR irradiation to catalyze
2
2
5
5
32
the allylic amination of olefins with nitroarenes. Reports by
Sortais and Darcel that describe hydrosilylation of various C
O and CN containing molecules have followed. However,
these reactions often utilize more complex iron catalyst
1
of freshly distilled styrene were used with the same result. H
NMR spectra of these reactions reveal some unproductive
reaction of the styrene substrate, but the exact reaction (such as
3
3−38
precursors.
Herein, we report the hydrophosphination
5
activity of [CpFe(CO) ] (Cp = η -C H ) (1) with visible light
2
2
5
5
using commercial LED light, which, to the best of our
Received: June 30, 2017
knowledge, is the first example of visible light promoted
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00499
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Article
c
Table 1. Photocatalytic Hydrophosphination of Alkenes
of iron. This background reactivity is consistent with
observation of additional Markovnikov products for acrylamide
in the iron-catalyzed reaction.
Treatment of 1 with 1 equiv of Ph PH in benezene-d
2
6
solution, followed by irradiation in the visible for 30 min at
ambient temperature, resulted in significant formation of P−H
bond activation products as observed by multinuclear NMR
46,47
spectroscopy (eq 1).
Formation of CpFePPh (CO) and
2 2
CpFeH(CO) is consistent with a direct activation of a single
2
molecule of Ph PH by 1 equiv of photoactivated 1, or two Fp-
2
radicals. The observation of CpFeH(PHPh )(CO) appears to
2
47
be the reaction of Ph PH directly with CpFeH(CO) , but
2
2
39
that hydride is also known to decompose to 1. Additionally,
no P−H bond activation or any reaction was observed when 1
was treated with 1 equiv of Ph PH in the dark at ambient
2
temperature. These observations are consistent with the lack of
hydrophosphination for most substrates in the dark and implicit
of organometallic, rather than Lewis acidic, behavior from iron.
These data further imply that the catalysis is a closed shell
process rather than radical initiation by photogenerated
•
CpFe(CO)2 equivalents. Both phosphido compounds could
be competent for a nucleophilic delivery of diphenylphos-
48
phide, but we cannot discount insertion with our current data.
Photoactivation of 1. The photochemistry of 1 is a well-
49
studied process, with initial work dating back nearly 40 years.
Photoactivation of 1 and related CpFeX(CO) (X = halide,
2
pseudohalide, or methyl) derivatives occurs by several routes,
though some common products arise. For CpFeX(CO) ,
2
−
dissociation of either the X or carbonyl ligand depends on the
50−54
identity of X (Scheme 1A).
For 1, irradiation in the UV
commonly results in carbonyl ligand loss and ligand exchange.
•
However, such irradiation also affords 2 equiv of CpFe(CO)2
either directly or via recombination with dissociated CO
a
31
1
Conversion measured by inverse-gated P{ H} NMR spectrosco-
Scheme 1. (A) Primary UV-Irradiation Products from
2
2,44,45 b
py.
Products found in a 5:2 ratio of anti-Markovnikov to
Photoactivation of CpFeX(CO) Derivatives Used in
2
c
5
1−53,57
Markovnikov. Reaction conditions: 20.0 mg of 1 (0.056 mmol), 0.20
mL of Ph PH (1.13 mmol), alkene (1.13 mmol), ambient temperature,
visible light irradiation.
Catalysis.
(B) Visible Light Photoactivation of 1
41,42
2
Utilized in This Work
polymerization vs H-abstraction or other events) is unclear.
Michael acceptors such as methyl vinyl ester and acrylamide
were excellent substrates for this reaction, which is consistent
with previous studies of the hydrophosphination of these
1
1,18,21,23,44
species.
Neither reactions with secondary alkyl
t
phosphines, Cy PH and Bu PH, nor unactivated alkenes
2
2
(
e.g., ethyl vinyl ether, Table 1) provide any significant
formation of hydrophosphination products.
Control reactions provided some critical understanding of
the catalysis. In the absence of iron, most of these reactions
afford no new products. Similarly, without irradiation by visible
light, there was no appreciable conversion, even in the presence
of iron. For some Michael acceptors, there was competitive
spontaneous hydrophosphination. For example, there is
approximately 24% conversion of acrylamide to hydrophosphi-
nation products in the dark and 11% conversion in the absence
B
DOI: 10.1021/acs.organomet.7b00499
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Organometallics
Article
3
9−41
Likewise, some formation of CpFe(CO)2•
(Scheme 1A).
dehydrocoupling to occur. A control reaction of Me NHBH
2
3
can be observed for any CpFeX(CO) derivative irradiated in
the UV, even if primary products are CO or X dissociation
with 5 mol % of 1 at ambient temperature in the dark for 3 days
failed to afford any dehydrocoupling products. Additionally,
visible light irradiation of a THF solution of Me NHBH or
2
−
(
Scheme 1A). It is also known that irradiation in the visible
2
3
•
affords 2 equiv of CpFe(CO)2 directly, and the individual
NH BH at ambient temperature using only ambient lighting
3
3
42
steps have been probed in detail (Scheme 1B). This final
activation route is attractive for providing primarily, if not
exclusively, CpFe(CO)2• equivalents, which are doubtlessly
for 7 days failed to give any dehydrocoupling products,
confirming the need for both 1 and irradiation to achieve
dehydrocoupling in this system.
40,42,55,56
important in photocatalytic reactions involving 1.
Similarly, reaction of a THF solution of NH BH with 5 mol
Because CpFe(CO)2• is a common product in the high-
3
3
56
% of 1 under visible light irradiation gave a mixture of known
products, though the ratio of products differed from those
energy degradation of 1 (Scheme 1), it was hypothesized that
CpFe(CO) , generated from visible light irradiated 1, could
•
2
59
obtained under UV irradiation (Scheme 2). Indeed, the
engage in the rich catalytic chemistry already demonstrated for
product distribution is consistent with greater hydrogen loss
1
activated with UV irradiation. Evidence in support of this
(
i.e., activity) under these conditions. Nevertheless, the
hypothesis would indicate if the visible light prompted
hydrophosphination catalysis (vide supra) may be the basis
for farther reaching photocatalysis. Therefore, a study of
previously demonstrated catalytic dehydrocoupling of amine
boranes using 1 with UV light was carried out with visible
products observed are consistent with Manners’s proposal of
β-hydride elimination to afford H NBH for the UV-
2
2
57
activated system. In contrast to Me
NHBH
2
, when a mixture
3
of 5 mol % of 1 and NH BH in THF solution was irradiated
3 3
5
7−59
light.
Such a study was undertaken to understand the
with LED light for 1 h at ambient temperature and then the
reaction was then was allowed to stir without irradiation for an
additional 3 h at ambient temperature, there was no conversion
broader applicability of 1 to promote additional, chemically
important reactions under visible light irradiation.
1
1
Amine Borane Dehydrocoupling. Manners reported that
the UV irradiation from a medium pressure Hg lamp of a THF
solution of R NHBH (R = H, Me) with 5 mol % of 1 resulted
to dehydrocoupling products as observed by B NMR
spectroscopy. This is also in contrast to a similar experiment
57
2
3
using UV light, which suggests that the different photo-
57,59
in the dehydrocoupling of these compounds.
When a THF
activation pathways for 1 can cause substantial changes in
solution of R NHBH with 5 mol % of 1 at ambient
41,42,49
2
3
reactivity in specific cases.
Like the MeNH BH
2 3
temperature was irradiated with visible light (λirr > 500 nm,
−15 W, 450−800 lm) open to a N atmosphere for 4 h, a
reactions, both 1 and visible irradiation are required to release
hydrogen from ammonia borane.
6
2
color change from dark red to green was observed with gas
1
1
1
Going beyond [CpFe(CO) ] . The success of replicating or
2 2
evolution. Analysis of the solutions by B{ H} NMR
even increasing the activity of 1 in the dehydrocoupling of
amine boranes demonstrates that visible light activation of 1 has
broader reaching potential in catalysis than hydrophosphina-
tion. The specific photochemistry of 1 and that of its derivatives
spectroscopy revealed conversions to products akin to what
57,59
was observed by Manners and co-workers.
The catalytic
dehydrocoupling of Me NHBH gave the cyclic dimer
2
3
11
1
(
Me NBH ) in 94% conversion as evidenced by B{ H}
2 2 2
•
are different (Scheme 1), but the possibility that CpFe(CO)2
NMR spectroscopy (δ = 5.7) as well as (Me N) BH (δ = 29,
2
2
5
7
Scheme 2). Hydrogen evolution was measured using a gas
buret, which showed 0.89 ± 0.02 equiv of hydrogen was
produced over an average of three trials.
may be common or play a key role in these reactions is
tantalizing. To further probe the potential value of visible light
photocatalysis with 1, a UV-catalyzed reaction with a derivative
of 1, CpFeMe(CO) (2), was tested using visible irradiation of
2
Scheme 2. Visible-Light-Catalyzed Dehydrocoupling of
1.
a
57,59,60
Me NHBH (Top) and NH BH (Bottom)
Pannell reported that UV irradiation of a DMF solution of
2
3
3
3
PhMe SiH in the presence of 5 mol % of 2 for 4 h at ambient
2
temperature resulted in the quantitative formation of
54
(
PhMe Si) O. Under LED irradiation at ambient temperature
2 2
for 1 h, PhMe SiH was quantitatively converted to
2
(
PhMe Si) O by 4 mol % of 1 in DMF, as evidenced by
2 2
29
1
mass spectrometry and Si{ H} NMR spectroscopy (δ = 0.6)
eq 2).
(
4
mol % 1, LED
PhMe SiH ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ ⎯→ (PhMe Si) O + H
2
2
2
2
DMF, 4 h
1
00%
(2)
a
Percent conversions for the products are listed below, with literature
values obtained by irradiation from a Hg lamp in parentheses.
Visible light irradiation was required to achieve the observed
reactivity. Reactions performed in the dark utilizing otherwise
identical conditions resulted in no conversion. This example
illustrates that 1 can engage in the same catalytic reactivity
under LED irradiation as that of 2 under UV irradiation. This is
a noteworthy feature because 2 is a derivative of 1, which is a
commercial product. In a broader sense, this example
demonstrates that visible light activation of 1 may enable a
yet broader scope of catalytic reactivity.
Similar to Manners’s system, irradiation throughout the
reaction is not required despite the potentially different routes
57,59
by which 1 may be of photoactivated.
When a reaction
mixture as described above was only irradiated for 1 h, and the
reaction was then allowed to stir at ambient temperature for an
additional 3 h, similar conversions to dehydrocoupled products
were observed. Both 1 and photoirradiation are necessary for
C
DOI: 10.1021/acs.organomet.7b00499
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Article
Dehydrocoupling of NH BH . A 50 mL Schlenk flask was
CONCLUSIONS
3
3
■
charged with a stir bar, NH BH (15.7 mg, 0.50 mmol), 1 (9.0 mg,
3
3
Compound 1 promotes the hydrophosphination of styrenes
and terminal alkenes under mild conditions with visible light
irradiation, which represents the first example of visible light
promoted metal-catalyzed hydrophosphination. Additionally, 1
is a competent visible light activated precatalyst for other main-
group bond forming reactions. These reactions were previously
catalyzed by 1 using UV light from a mercury arc lamp. By
utilizing the low-energy activation pathway of 1 from
commercially available LED bulbs, comparable reactivity is
observed at lower energetic cost. These examples underscore
that additional photocatalysis may be possible with 1 under
visible irradiation.
0
.025 mmol), and THF (5 mL). The reaction was placed under a
slight positive pressure of N and was irradiated using an LED light
2
source for 4 h at ambient temperature, after which an aliquot was
taken and was analyzed by 11B{ H} NMR spectroscopy.
1
Dehydrocoupling of NH
Schlenk flask was charged with a stir bar, NH
BH
3
, Timed Irradiation. A 50 mL
BH (15.7 mg, 0.50
3
3
3
mmol), 1 (9.0 mg, 0.025 mmol), and THF (5 mL). The reaction was
placed under a slight positive pressure of N and was irradiated using
2
an LED light source for 1 h at ambient temperature, then was covered
with aluminum foil in the dark for 3 h to exclude light. At this time, an
11
aliquot was taken for B NMR spectroscopic analysis.
Generation of (PhMe Si) O from PhMe SiH. A PTFE-sealed
2
2
2
reaction vessel was charged with 1 (70.8 mg, 0.20 mmol), PhMe SiH
2
(
0.77 mL, 5.0 mmol), N,N-dimethylformamide (4.6 mL), and a stir
EXPERIMENTAL SECTION
General Considerations. All manipulations were performed
bar. The reaction was irradiated using an LED light source for 1 h,
after which an aliquot was taken for 29Si{ H} and MS analyses, which
revealed a complete reaction.
■
1
under an inert atmosphere of N using Schlenk line or glovebox
2
techniques using oxygen-free, anhydrous solvents. NMR spectra were
recorded using a Bruker AXR 500 MHz spectrometer, using external
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00499.
■
1
1
references of 10% BF ·Et O in CDCl for B NMR experiments,
3
2
3
*
S
29
31
SiMe in CDCl for Si experiments, and 85% H PO in H O for
P
4
3
3
4
2
1
NMR experiments (all references correspond to δ = 0). H NMR
spectra were recorded using a Bruker AXR 500 MHz spectrometer and
were referenced to residual solvent impurities (δ = 7.16 for benzene-
d6). GC spectra were collected using a Varian Saturn 2100T gas
chromatograph, and mass spectra were collected on an Applied
Biosystems 4000QTrap Pro. Ammonia borane was purchased from
Sigma-Aldrich and opened August 2014, after which it was stored at
6
1
A photograph of the experimental setup, UV/vis−NIR
spectra of LED bulbs used, and relevant spectra of
reactions (PDF)
62
−
(
35 °C under an inert atmosphere of N . Ph PH and CpFeMe-
2 2
63
AUTHOR INFORMATION
Corresponding Author
E-mail: rory.waterman@uvm.edu. Phone: 802 656-0178. Fax:
02 656-8705.
CO) (2) were prepared by modified literature procedures and
2
■
were stored under an inert atmosphere of N prior to use. All other
2
reagents were obtained from commercial suppliers and dried by
conventional means as necessary. Three different commercially
available LED light bulbs were used over the course of trials. Their
UV/vis−NIR spectra were measured using a Princeton Instruments
Acton 2300 spectrometer and are reported in the Supporting
Information. The temperature in the experimental setup was measured
to be slightly above room temperature (25−28 °C) and was checked
routinely to ensure minimal fluctuations in temperature.
*
8
ORCID
Rory Waterman: 0000-0001-8761-8759
Present Address
Chemistry Division, Los Alamos National Laboratory, Mail
†
Stop J-514, Los Alamos, NM 87545.
Hydrophosphination of Alkenes with Diphenylphosphine. A
screw-top NMR tube was charged with 1 (20.0 mg, 0.057 mmol), an
unsaturated organic (1.13 mmol), and diphenylphosphine (0.20 mL,
Notes
The authors declare no competing financial interest.
1
.13 mmol) and was irradiated with visible light at ambient
temperature for 6−72 h depending on substrate. Reactions were
monitored by 31P{ H} NMR spectroscopy approximately every 1 h to
ensure a complete reaction. Products were identified by their literature
ACKNOWLEDGMENTS
1
■
This work was supported by the U.S. National Science
Foundation (CHE-1265608 and CHE-1565658). We thank
Dr. Lane Manning (UVM) for assistance in measuring the
spectra of the LED bulbs used and Dr. Jaqueline Kiplinger
(LANL) for computational resources for completion of this
manuscript.
3
1
21
P NMR chemical shifts.
Dehydrocoupling of Me NHBH . A 50 mL Schlenk flask was
2
3
charged with a stir bar, Me NHBH (30.0 mg, 0.51 mmol), 1 (9.0 mg,
2
3
0
.025 mmol), and THF (5 mL). The reaction was placed under a
slight positive pressure of N and was irradiated using an LED light
2
source for 4 h at ambient temperature, after which an aliquot was
taken and was analyzed by 11B{ H} NMR spectroscopy.
1
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DOI: 10.1021/acs.organomet.7b00499
Organometallics XXXX, XXX, XXX−XXX