Visible-light and thermal driven double hydrophosphination of terminal alkynes using a commercially available iron compound

Article abstract of DOI:10.1039/c8cc00847g

A commercially available iron compound, [CpFe(CO)₂]₂ (1) (Cp = η⁵-C₅H₅), is an efficient catalyst for the double hydrophosphination of terminal aryl alkynes with diphenylphosphine under visible light irradiation or thermal conditions with a reduction of reaction times of up to two orders of magnitude for some substrates over literature reports. The 1,2-bis(diphenylphosphino)ethane products generated in these reactions are readily isolated in high yields.

Full text of DOI:10.1039/c8cc00847g

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Visible-Light and Thermal Driven Double Hydrophosphination of
Terminal Alkynes Using a Commercially Available Iron
Compound†
Received 00th January 20xx,
Accepted 00th January 20xx
Brandon J. Ackley, Justin K. Pagano,‡ and Rory Waterman*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A commercially available iron compound, [CpFe(CO)2]2 (1) (Cp = η5-
C5H5), is an efficient catalyst for the double hydrophosphination of
terminal aryl alkynes with diphenylphosphine under visible light
irradiation or thermal conditions with a reduction of reaction
times of up to two orders of magnitude for some substrates over
literature reports. The 1,2-bis(diphenylphosphino)ethane products
generated in these reactions are readily isolated in high yields.
attractive low-energy light source is visible light generated from LED
bulbs, which provide a high photon density at low cost. Visible light
promoted iron catalysis is known, with numerous reports by Sortais
and Darcel detailing a variety of transformations.21-23 We recently
reported the visible-light promoted hydrophosphination of styrenes
and terminal alkenes using a commercially available, air-stable iron
compound [CpFe(CO)2]2 (1),24 the photochemistry of which has
been well-studied.25 It was hypothesized that phosphido
compounds deriving from 1 would give improved rates of double
hydrophosphination based on Nakazawa’s mechanistic proposal.15
This hypothesis was borne out: 1 is an effective catalyst for the
regioselective double hydrophosphination of terminal aryl alkynes
under visible light photolysis conditions, in some cases reducing
reaction times by 48 hours, which also represents the first example
of photo-driven double hydrophosphination. Employing thermal
conditions comparable to Nakazawa, Di Giuseppe and Oro, and Cui
results in further improved reactivity, giving double
hydrophosphination products in excellent yields with minimal
reaction times.
Reaction of a 1:2 mixture of alkyne and diphenylphosphine
mixture with 5 mol % of 1 under visible-light photolysis at ambient
temperature gave double hydrophosphination products as
measured by 31P NMR spectroscopy (eq. 1).
A variety of para-substituted terminal aryl alkynes were tested
under these reaction conditions, all of which gave excellent
conversion to the double hydrophosphination products (Table 1).
The conversions observed are comparable to those reported by
Nakazawa15 and Cui17 under thermal conditions in most cases.
Additionally, the substrate scope is similar to that of all three
previous studies, displaying that this method is comparably
versatile to the established protocols. Employing a solvent in these
reactions
Bidentate phosphine ligands have enabled a wealth of catalytic
reactions, with significant advances in many C–X bond forming
reactions.1 Many P–C bond forming reactions necessary to form
these ligands are stoichiometric and generate salt waste, thus,
“greener” catalytic methods to achieve P–C bond formation are
desirable.2-7 Hydrophosphination is an attractive catalytic route to
forming P–C bonds, and despite recent advances in this reaction,5, 8-
18
significant challenges remain.5 In 2012, Nakazawa and coworkers
reported the first example of the double hydrophosphination of
terminal aryl acetylenes with secondary phosphines using the
ubiquitous iron compound CpFeMe(CO)2 (Cp = η5-C5H5) and
followed on with Cp*FeMe(CO)2 (Cp* = η5-C5Me5).15, 16 In 2016, Di
Giuseppe and Oro and reported a rhodium catalyst for the double
hydrophosphination of terminal alkynes with secondary
phosphines.19 Most recently, Cui and coworkers outlined highly
efficient double hydrophosphination of terminal alkynes using a
simple copper N-heterocyclic carbene catalyst.17 We have
demonstrated that triamidoamine-supported zirconium is effective
for the double hydrophosphination of internal alkynes with primary
phosphines.14 While this field has accelerated recently, there are
limited examples of catalysts and, with the exception of Cui’s,
reactions are lengthy.
Improving on iron-catalyzed processes is desirable in general
due to the relative abundance of iron. Indeed, highlighting catalytic
processes that utilize high-abundance metal compounds and low-
energy input are important to green chemistry principles.20 An
Table
1
Visible-light promoted double hydrophosphination
reactions.a
(1)
address: Chemistry Division, Los Alamos National Laboratory, Mail Stop J-514,
Los Alamos, NM, 87545
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Table 2 Isolated yields for thermal double hydrophosphination
reactions catalyzed by 1.
% diphosphinoethane
% E-vinylphosphine
Ar
b
C6H5
88
92
89
82
85
<1
1
<1
<1
<1
b
p-Me-C6H4
p-tBu-C6H4
c
% diposphinoethane
Ar
time/h
c
a
p–OMe-C6H4
C6H5
2
2
3
4
4
16
16
16
16
4
96
93
94
89
99
93
84
86
66
99
72
95
94
0
c
p-F-C6H4
a
a
p-Me-C6H4
Percent conversions as determined by 31P NMR spectroscopy. b Reaction conditions:
p-tBu-C6H4
a
0.056 mmol 1, 1.12 mmol alkyne, 2.24 mmol Ph2PH, neat, ambient temperature, LED
a
p–OMe-C6H4
p-F-C6H4
c
irradiation, 20 h. Reaction conditions 1) 0.056 mmol 1, 1.12 mmol alkyne, 2.24 mmol
a
Ph2PH, neat, ambient temperature, LED irradiation, 20 h 2) 1 mL THF, LED irradiation,
12–36 h.
a
p-NMe2-C6H4
Mesityla
2-pyridyla
3-pyridyla
results in no reactivity, highlighting the need for concentrated
conditions to engage in productive chemistry, which is also
consistent with observations by Nakazawa and coworkers in 2012.15
This strong dependence on concentration prevents quantitative
conversion to double hydrophosphination products, as these
products are solids and sufficiently disrupt reactivity upon
precipitation from the neat solution. Attempts to continue reaction
after precipitation by addition of solvent followed by extended (>
72 h) irradiation at ambient temperature did not result in additional
reactivity. Control reactions highlight the need for both 1 and
3-thiophenyla
a
p-CF3-C6H4
C6H5
8
7
48
b
c
C6H5
Alkyld
Control reaction
e
C6H5
48
0e
a Reaction conditions: 0.056 mmol 1, 1.12 mmol alkyne, 2.24 mmol Ph2PH, neat, 110 °C,
b
1–6 h. Reaction conditions: 0.056 mmol 1, 5.56 mmol phenylacetylene, 11.12 mmol
visible-light in this system. Excluding 1, visible-light from
a
c
Ph2PH, neat, 110 °C. Reaction conditions: 0.010 mmol 1, 10.0 mmol phenylacetylene,
d
e
commercially available LED bulb (λirr > 500 nm, 6–15 W, 450–800
lumens), or both results in no reactivity (Table 1). Reactions
20 mmol Ph2PH, neat, 110 °C. alkyl = hexyl, octyl. 1.12 mmol phenylacetylene, 2.24
mmol Ph2PH, no 1, neat, 110 °C.
conducted on the benchtop in
a well-lit laboratory or in a
These conditions do not allow for double hydrophosphination
with alkyl-substituted phosphines (Table 2), but there are new
substrates compared to previous reports, namely 4-ethynyl-N,N-
dimethylaniline and 2-ethynyl-1,3,5-trimethylbenzene. Under both
the thermal and photo-induced conditions, 1:1 reactions of alkyne
and phosphine resulted in a mixture of vinyl phosphine and double
hydrophosphination products, indicating limited selectivity for the
single hydrophosphination event. Monitoring reactions by 31P NMR
spectroscopy does suggests that these are sequential reactions, but
the rate of the second hydrophosphination event is apparently
comparable to that of the first.
Phosphorus containing byproducts are easily removed by silica
gel chromatography, and iron-containing products can be removed
by washing the chromatographed products with hexanes, as
described by Nakazawa.15 In some instances, purification could be
achieved by recrystallization from a saturated pentane solution (see
SI for details). Scaling the reaction of Ph2PH with phenylacetylene
from a ~1 mmol scale to a 50 mmol scale resulted in no depreciable
decrease in isolated yield, highlighting the use of 1 as an affordable
windowsill did not result in any conversion after seven days,
demonstrating that the high photon density from a LED light is
necessary for reactivity. The results from the visible-light promoted
process were encouraging, which led to the hypothesis that
employing thermal conditions using 1 as a catalyst would further
improve reactivity.
The reaction of a 1:2 mixture of alkyne and diphenylphosphine
with 5 mol % of 1 at 110 °C resulted in the rapid double
hydrophosphination of a variety of terminal aryl alkynes (eq 2).
In all cases, reaction times were significantly improved from the
visible-light conditions with modestly increased yields (Table 2),
representing some of the most efficient examples of double
hydrophosphination catalysis to date. Lowering the catalyst loading
to 0.1 mol
bis(diphenylphosphino)-1-phenylethane over 48 hours,
comparable yield to Nakazawa’s system (72 h, 5 mol
% resulted in 94% isolated yield of 1,2-
a
%
of
CpFe(CO)2Me, 110 °C)15 and comparable conditions to Cui’s (8 h,
1.25 mol % of CuCl2 and 5 mol % NHC, 110 °C).17 Like the visible-
light photolysis conditions utilizing 1, a variety of both electron-
donating and withdrawing substrates were amenable to this
system. In all reactions, small (< 5%) conversion to the E-
vinylphosphine (i.e., single hydrophosphination) was measured by
and easy-to-handle system to access
diphosphine ligands.
a variety of chelating
While alteration of the terminal alkyne did not result in much
variation of yield, substitution of the secondary aryl phosphines
greatly affected reactivity. Reaction of bis(p-tolyl)phosphine and
phenylacetylene with 5 mol % of 1 under either thermal or
photolysis resulted in meager conversions (20–30%) to the desired
double hydrophosphination product. However, 11% conversion to
tris(4-methylphenyl)phosphine was observed, as well as 57%
conversion to other unknown products (eq 3; see SI Figure 29 for
details).
(2)
Attempted purification including column chromatography,
recrystallization from various solvents, and preparatory thin-layer
chromatography failed to afford pure product. In all cases,
31P NMR spectroscopy.
2 | Chem. Commun.
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Conflicts of interest
There are no conflicts to declare.
(3)
Notes and references
1.
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separation from iron-containing byproducts was successful, though
phosphorus-containing byproducts, including one consistent with
the vinylphosphine, remained. One byproduct, tris(4-
methylphenyl)phosphine, was consistently formed in about 10% in
both thermal and photo-driven reactions. This observation parallels
α-phosphinidene elimination that has recently been reported for
2.
3.
4.
5.
C. A. Bange and R. Waterman, Chem. Eur. J., 2016, 22,
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J. K. Pagano, C. A. Bange, S. E. Farmiloe and R. Waterman,
Organometallics, 2017.
CpFeMe(CO)2.26
methylphenyl)phosphine produce
Control
reactions
of
1
and
bis(4-
6.
a
similar quantity of tris(4-
methylphenyl)phosphine among other unidentified products.
Efforts to replicate the PR group transfer chemistry reported for
CpFeMe(CO)2 using 1 were unsuccessful,26 indicating that 1 has
limited efficacy for α-phosphinidene elimination. More germane,
these observations suggest that modification of the phosphine
under these reaction conditions has a powerful effect on the
resulting yield of double hydrophosphination products.
Hydrophosphination with substrates other than diphenylphosphine
has been limited,5 which argues for greater exploration in this area,
especially for double hydrophosphination.
7.
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8.
9.
10.
11.
From prior study,27 1 dissociates into two equivalents of
12.
13.
•
CpFe(CO)2
under thermal or photochemical conditions, and
conducting this dissociation in the presence of diphenylphosphine
results in formation of an equivalent of CpFe(PPh2)(CO)2 and
CpFeH(CO)2,24 the latter of which is known to decompose to 1.28
While side reactions may occur, it is reasonable to presume that the
key iron-phosphido intermediate in these reactions is
CpFe(PPh2)(CO)2. Nakazawa has proposed Cp′Fe(PPh2)(CO) (Cp′ = Cp
or Cp*) as the key intermediate in double hydrophosphination
14.
15.
16.
17.
18.
19.
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16
catalyzed by Cp′FeMe(CO)2 derivatives.15, It was suggested that
sluggish catalysis in those systems resulted from coordination of the
phosphine prior to product displacement. The additional carbonyl
ligand appears to be pivotal in accelerating this catalysis, and the
validation of our hypothesis that 1 would afford faster catalysis
provides additional support to Nakazawa’s proposed catalytic
cycle.3f There are still peculiarities to the system such as the poor
reactivity of unactivated alkynes and limitation to internal alkynes
for both copper and iron catalysts that prompts further study of
these systems.
20.
21.
22.
In conclusion, commercially available iron compound
1 is
an efficient catalyst for the double hydrophosphination of
terminal aryl alkynes with diphenylphosphine employing either
visible-light or thermal conditions. This study represents the
first reported double hydrophosphination reaction promoted
by visible-light and provides a complement to the previous
work by Nakazawa and Cui utilizing first-row transition metals
to catalyze this reactivity through thermal methods. Both sets
of conditions (visible light or heat) gave diphosphine products
in excellent yields, were tolerant both electron-withdrawing
and donating substituents on the alkyne, and isolation of these
products without oxidation is shown to be simple.
23.
24.
25.
26.
27.
J. K. Pagano, C. A. Bange, S. E. Farmiloe and R. Waterman,
Organometallics, 2017, 36, 3891-3895.
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This work was supported by the U.S. National Science
Foundation (CHE-1265608 and CHE-1565658).
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This journal is © The Royal Society of Chemistry 20xx
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28.
T. A. Shackleton, S. C. Mackie, S. B. Fergusson, L. J.
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TOC graphic
Visible-Light and Thermal Driven Double Hydrophosphination of Terminal Alkynes Using a
Commercially Available Iron Compound
Brandon J. Ackley, Justin K. Pagano, and Rory Waterman*
An air-stable, commercial available iron catalyst provides fast efficient double
hydrophosphination of alkynes under thermal or photochemical conditions.