Markovnikov versus anti-Markovnikov Hydrophosphination: Divergent Reactivity Using an Iron(II) β-Diketiminate Pre-Catalyst

Article abstract of DOI:10.1002/chem.201702374

The ability to tune between different regioselectivities using a common pre-catalyst is an unusual yet highly desirable process. Here, we report the use of an iron(II) pre-catalyst that can be used to synthesize vinyl phosphines in a Markovnikov-selective

Full text of DOI:10.1002/chem.201702374

A Journal of
Accepted Article
Title: Markovnikov versus anti-Markovnikov hydrophosphination:
Divergent reactivity using an iron(II) β-diketiminate pre-catalyst
Authors: Andrew King, Kimberley Gallagher, Mary F. Mahon, and Ruth
L. Webster
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To be cited as: Chem. Eur. J. 10.1002/chem.201702374
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Markovnikov versus anti-Markovnikov hydrophosphination:
Divergent reactivity using an iron(II) β-diketiminate pre-catalyst
Andrew K. King, Kimberley J. Gallagher, Mary F. Mahon and Ruth L. Webster*[a]
Abstract: The ability to tune between different regioselectivities using
Remarkably, we also report an unprecedented solvent-mediated
shift in regioselectivity from Markovnikov to Z-selective anti-
a common pre-catalyst is an unusual yet highly desirable process.
Herein, we report the use of an iron(II) pre-catalyst that can be used
Markovnikov hydrophosphination.
to synthesize vinyl phosphines in a Markovnikov selective manner in
benzene, whereas a simple change to dichloromethane as the
reaction solvent leads to the Z-selective anti-Markovnikov
functionalization. Preliminary mechanistic that studies suggest
Markovnikov selectivity is a radical mediated process whereas the
anti-Markovnikov selectivity is not radical in nature, and is due to a
change in oxidation state, are reported.
The preparation of functionalized phosphorus-containing
compounds is an important and necessary undertaking, leading
to high value products for use in the chemical industries[1]
including catalysis,[2] chemical biology,[3] polymer[4] and
agrochemical sectors.[5] However, traditional methods for the
preparation of functionalized phosphines can be limiting in terms
of functional group tolerance and the quantity of waste
generated.[6] For example, very often organometallic reagents
such as Grignards are used to introduce functionality and many
reactions release stoichiometric amounts of salt waste, which can
lead to protracted purification processes, or they use P(V)
reagents which need to be reduced to P(III). This limits both the
usefulness and accessibility of such transformations. In contrast
catalytic hydrophosphination displays excellent functional group
tolerance and has the potential to be 100% atom economic.[7] In
Scheme 1. a) Previous work in the field of iron catalyzed hydrophosphination of
alkynes has allowed the development of double hydrophosphination and anti-
terms of alkyne hydrophosphination, examples of double
Markovnikov Z-selective hydrophosphination. b) This work provides
a
hydrophosphination to form 1,2-diphos products and anti-
Markovnikov mono-hydrophosphination catalysed by transition
metals,[8] lanthanides[9] and main group[9d, 10] pre-catalysts exist.
To the best of our knowledge, only two such examples have been
reported using iron, both by Nakazawa and co-workers and both
involve heating to 110 °C for several days (Scheme 1a).[11]
However, a challenge that remains in the HP of alkynes is being
able to selectively mono-hydrophosphinate in a Markovnikov
fashion.[12] In terms of the HP of alkynes only one such example
exists, which uses nickel catalysis, but with no analytical data,
harsh reaction conditions and with moderate selectivity[8a] it
seems fitting that a robust route to these challenging to prepare
phosphines is sought. We herein report for the first time iron(II)
catalyzed Markovnikov-selective hydrophosphination of alkynes
under rapid and mild reaction conditions (Scheme 1b).
comparatively mild route for Markovnikov and anti-Markovnikov
hydrophosphination of alkynes.
Reacting HPPh2 with phenyl acetylene in the presence of a
catalytic amount of 1 in C6H6 leads to the formation of the
Markovnikov product 2a in
a
highly selective manner
(Markovnikov:anti-Markovnikov 9:1, Table 1, Entry 1). After a
short optimization process it was found that the reaction can be
undertaken with 5 mol% 1, 1 mmol alkyne, 0.5 mmol HPPh2 in
C6H6 in 3 h with a modest reaction temperature of 50 °C. It is worth
noting that the reaction also works well at room temperature,
whereby complete conversion to 2a is obtained after 48 h with 5
mol% 1. Use of a 2:1 ratio of phosphine:phenylacetylene does not
promote the formation of the 1,2-diphosphosphane product (cf.
Nakazawa),[11a] but instead leads to
a
mixture of
tetraphenyldiphosphane (Ph2P–PPh2) and 2a. Use of FeCl2 does
not lead to the formation of 2a. With optimized conditions in hand
we proceeded to explore the substrate scope for this
transformation (Table 1).
[a]
A. K. King, K. J. Gallagher, Dr. M. F. Mahon, Dr. R. L. Webster
Department of Chemistry
University of Bath
Calverton Down, Bath, UK.
E-mail: r.l.webster@bath.ac.uk
Supporting information for this article is given via a link at the end of
the document.
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Table 1. Markovnikov-selective hydrophosphination substrate scope.
12
C6H4N
2l
< 20
Alkyne (RCCH)
R =
Yield
(%)[a]
Entry
Product
13[g]
2a
100 (82)
C6H5
2m 100 (81)
1
C6H5
2a’ 100 (89)[b]
14
HCC(C6H4)2
2n
100
2
4-tBu-C6H4
4-MeO-C6H4
4-H2N-C6H4
4-F-C6H4
2b
2c
2d
2e
100 (75)
100 (74)
78
Reaction conditions: alkyne (1 mmol), HPPh2 (87 μL, 0.5 mmol), 1 (14 mg, 5
mol%), C6H6 (350 μL), Ar, 50 °C, 3 h. 9:1 Markovnikov:anti-Markovnikov. [a]
Spectroscopic yield calculated from the consumption of HPPh2 against 1,3,5-
trimethoxybenzene as an internal standard. All products isolated as the
phosphine oxide unless stated otherwise (see supporting information), isolated
yield shown in brackets. [b] Phenyl acetylene (1.1 mL, 10 mmol), HPPh2 (870
μL, 5 mmol), 1 (140 mg, 5 mol%), C6H6 (3.5 mL), Ar, 50 °C, 24 h. Only trace
anti-Markovnikov observed. Isolated as the P(III) product. [c] Isolated as the
P(III) product. [d] Mixture of Markovnikov and anti-Markovnikov products. [e]
70 °C, 24 h. [f] 90 °C, 24 h. [g] Mixture of mono:di alkenyl product (85:15).
3[c]
4
Aryl acetylenes with a range of electronic properties are tolerated
in the reaction, giving a high spectroscopic and isolated yield of
product. The reaction also responds well to scale-up. When the
reaction is performed using 5 mmol HPPh2 (0.93 g), complete
conversion to the Markovnikov product is obtained in 24 h and
can be isolated cleanly as the P(III) compound in high yield (1.29
g, 89%, 2a’, Table 1, Entry 1). Strongly electron donating (2c,
Entry 3) and electron withdrawing (2e, Entry 5) groups can be
used and, unusually, 4-aminophenyl acetylene with its free
functionality primed for further transformations, also generates a
high yield of the desired Markovnikov product (2d, Entry 4). In
contrast, substitution in the ortho-position is not suitable for this
transformation (compare Entries 4 to 7 and 5 to 8). In the case of
both 2-amino-phenylacetylenes and 2-chloro-phenylacetylenes,
no unwanted side-reactions take place and therefore it can be
assumed that steric bulk and/or competing heteroatom
coordination inhibits reactivity. Silylacetylenes give high yield of
the desired product (Entries 9 and 10), but unfortunately meta-
and ortho-pyridyl substrates are not good reagents for this
transformation. Again this is presumably due to deactivation of the
alkyne (meta) or competing coordination (ortho). We are pleased
to report that H2PPh can be used to functionalize phenylacetylene
with a high level of mono-hydrophosphination selectivity (85:15
mono:di, Entry 13) whilst use of a diyne can lead to selective
single-hydrophosphination in the presence of diphenylphosphine
(Entry 14).
5
100 (80)
6
3-Me-C6H4
2f
88 (66)
7[d]
2-H2N-C6H4
2g
2h
33
no
reaction
8
2-Cl-C6H4
9[c],[e] (CH3)3Si
2i
2j
85 (60)
100
10[f]
(iPr3)3Si
We have previously reported a change in chemoselectivity with 1
when the solvent is changed.[13] Interested in the potential of
exploiting this further, we undertook the same alkyne
hydrophosphination transformation in CH2Cl2. Although more
11[e] C6H4N
2k
< 20
forcing conditions are necessary,
a complete switch in
regioselectivity is observed and the anti-Markovnikov product
forms as, predominantly, the Z-isomer. Again, a wide variety of
alkynes can be used in this transformation with the vast majority
of products obtained in excellent yield (Table 2).[14] It is worth
noting the higher yields obtained with the 2-chloro substrate
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(product 3e, Entry 5) and pyridylacetylenes (Entries 8 and 9).
These results might indicate that the potential for competing
coordination to iron and catalyst inhibition/deactivation is not a
limiting factor in this reaction mechanism.[15]
similar to those observed for a standard sp hybridized carbon-
carbon bond, although the C30-C31-C32 bond angle has
distorted from linearity to 159.7(2) °.
By using a pure sample of 4 in catalysis under standard
Markovnikov hydrophosphination conditions and comparing it to
catalysis performed by 1, only 33% 2a is obtained (Figure 2, 
(catalyzed by 1) and  (catalyzed by 4)). This would suggest that
4 is not an on-cycle intermediate. Leaving the reaction mediated
by 4 for an extended period of time (to allow for the splitting of the
dimer which may be slow) does not lead to a considerably greater
amount of 2a to be formed.
Table 2. Markovnikov-selective hydrophosphination substrate scope.
Alkyne (RCCH)
R =
Entry
Product
Yield (%)[a]
1
C6H5
3a
3b
100 (86)
2
3
4
4-tBu-C6H4
4-MeO-C6H4
3-Me-C6H4
100 (68)
66
3c
3d
72 (56)
5
2-Cl-C6H4
3e
56 (49)
6[b]
7[b]
8
(CH3)3Si
iPr3Si
3f
100 (55)
100 (88)
100 (77)
3g
3h
Figure 1. Molecular structure of complex 4. Ellipsoids are represented at 50%.
Hydrogen atoms omitted for clarity.
C6H4N
9
C6H4N
3i
26
Reaction conditions: alkyne (1 mmol), HPPh2 (87 μL, 0.5 mmol), 1 (14 mg, 5
mol%), CH2Cl2 (350 μL), Ar, 70 °C, 24 h. 95:5 Z:E, except Entries 1, 6, 7 (9:1
Z:E). [a] Spectroscopic yield calculated from the consumption of HPPh2 against
1,3,5-trimethoxybenzene as an internal standard. All products isolated as the
phosphine oxide unless stated otherwise (see supporting information), isolated
yield shown in brackets. [b] 90 °C, 24 h.
Intrigued by the contrasting selectivities and the different
mechanisms that must be at play, we sought to explore the
mechanism through stoichiometric reactions and simple reaction
monitoring studies. The Markovnikov catalysis undergoes a
striking color change, where the pre-catalyst is yellow in color, but
the reaction mixture on the addition of the reagents turns red, then
darkens to brown over the course of the reaction. A stoichiometric
reaction of 1 with phenylacetylene in C6H6 results in a solution
which can be crystallized by slow evaporation to give dark orange
plate-like crystals. Single crystal X-ray diffraction reveals an iron
acetylide dimer (4, Figure 1).[16] The end-on bound acetylene units
have carbon-carbon triple bond lengths of 1.223(3) Å (C30-C31),
Figure 2. Reaction monitoring for the formation of 2a using 5 mol% 1 () and
2.5 mol% 4 ().
We tentatively postulate a catalytic cycle which could involve 4 as
an off-cycle species which only forms in minor quantities (Scheme
2). The productive catalytic pathway involves the formation of an
iron phosphido intermediate (5), which then reacts with alkyne to
form an iron alkenyl[17] intermediate (6), where regioselectivity is
driven by sterics. Protonolysis releases the product and
regenerates 5. The iron phosphido has not been isolated from
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stoichiometric reactions because competitive and rapid
dehydrocoupling takes place within minutes at room temperature
to form Ph2P–PPh2.[13] However, it is evident that under catalytic
conditions C–P bond formation is more favorable than P–P bond
formation. When the concentration of alkyne is increased the rate
of reaction is suppressed, suggesting that the off-cycle formation
of 4 starts to dominate. No catalysis takes place with Ph2P–PPh2
as the phosphorus source, but the reaction is quenched by the
addition of a radical clock suggesting radicals play a role, but their
function is as yet undetermined. In this regard, it is important to
note that use of AIBN (20 mol%) in the absence of 1 under
Markovnikov reaction conditions gives 98% anti-Markovnikov
product (3a) with several other species formed in trace amounts,
including 2a,[18] suggesting that 1 is inherently involved in bond
formation and does not merely act to generate phosphinyl radicals
in solution.
Scheme 3. Formation of 7 is possible using dichloromethane as the halide
source and the oxidant.
Figure 3. Reaction monitoring for the formation of 3a using 5 mol% 1 () and
5 mol% 7 ().
In conclusion we have shown that from one common pre-catalyst
we can undertake divergent reactivity which is dictated by the
choice of solvent. The highly unusual Markovnikov
hydrophosphination product is formed in benzene and the anti-
Markovnikov product forms in dichloromethane. Preliminary
mechanistic studies would suggest that the source of these
different modes of reactivity are linked to oxidation state and the
mode of the C–P bond forming process. Fe(II) and radicals are
implicated in Markovnikov selective reactions, whereas for anti-
Markovnikov selectivity Fe(III) is generated and radicals are not
involved in C–P bond formation. The fact that such a vast change
in regioselectivity is observed with just a simple change in solvent
raises the question of what other iron catalyzed transformations
can be attenuated in this way.
Scheme 2. Tentative mechanism for Markovnikov selective hydrophosphination
of alkynes.
The change in selectivity observed in CH2Cl2, we believe, can be
attributed to a change in oxidation state of the metal complex.
Hessen has previously shown that Fe(III) complex 7 can be
alkylated with concomitant reduction using LiCH2TMS.[19] It is
therefore not surprising that the reverse reaction, whereby halide
abstraction from the reaction solvent with oxidation of 1,[20] leads
to the formation of 7 (Scheme 3). Reaction of 1 in CH2Cl2 at 50 °C
leads to a color change from yellow to green, synonymous with
the formation of 7. The new complex is also NMR silent, indicative
of a highly paramagnetic Fe(III) species. We do not believe that
Experimental Section
radicals
are
involved
in
catalytic
anti-Markovnikov
General reaction procedure Pre-catalyst 1 (14 mg (0.025 mmol, 5 mol%)
was added to a Schlenk tube under an atmosphere of argon along with
0.35 mL of C6H6 or CH2Cl2. Alkyne (1 mmol) and HPPh2 (87 μL, 0.5 mmol)
were then added to the reaction vessel and the corresponding solution was
stirred at 50 °C for 3 h (C6H6) or at 70 °C for 24 h (CH2Cl2) (unless stated
otherwise). Crude reaction mixtures were exposed to air and worked up
on the bench. In reactions where phosphine substrates are not fully
consumed the crude mixture was first eluted on silica gel (petroleum ether)
to remove the unreacted phosphine, a second fraction was then taken
using diethyl ether (Et2O) as the eluent. Hydrogen peroxide (30% in H2O)
was added to the Et2O phase and the solution stirred for 5-10 minutes.
Stirring was then ceased and the solution quenched with de-ionised water.
The layers were then separated and the aqueous layer was washed with
diethyl ether (2 x 20mL) and the organic layers combined, dried over
hydrophosphination because once the reaction has been initiated,
where 7 has formed in situ and 3a has started to be generated
during catalysis, addition of (chloromethyl)cyclopropane as a
radical clock has no effect on the reaction.
Use of an independently synthesized[21] sample of 7 in a reaction
monitoring study shows that catalysis performed by 7 closely
matches the catalytic competency of 1 in CH2Cl2 (Figure 3). We
believe that this is a case of Lewis acid-type activation of the
alkyne followed by nucleophilic attack by the phosphine. FeCl3 as
the pre-catalyst leads to 56% 3a and although lower than that
obtained with 1, solubility is likely to be limiting in this case.
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MgSO4 and filtered. Volatiles were then removed in vacuo and products
were isolated by flash chromatography on silica gel (Et2O).
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Acknowledgements
The University of Bath and the EPSRC is thanked for funding. The
EPSRC UK National Mass Spectrometry Facility at Swansea
University is thanked for mass spectrometry analyses.
[12] Markovnikov and anti-Markovnikov hydrophosphination of alkenes has
been achieved with iron pre-catalysts. FeCl2 gives Markovnikov product
and FeCl3 gives anti-Markovnikov product: L. Routaboul, F. Toulgoat, J.
Gatignol, J.-F. Lohier, B. Norah, O. Delacroix, C. Alayrac, M. Taillefer
and A.-C. Gaumont, Chem. Eur. J. 2013, 19, 8760-8764.
Keywords: homogeneous catalysis • iron • phosphanes •
phosphaalkenes • hydrophosphination
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70 °C (neat reaction conditions, overnight reaction). See: a) F. Alonso, Y.
Moglie, G. Radivoy and M. Yus, Green Chem. 2012, 14, 2699-2702; b)
Y. Moglie, M. J. Gonzalez-Soria, I. Martin-Garcia, G. Radivoy and F.
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under the optimized Markovnikov or anti-Markovnikov conditions.
[16] Crystal data for 4 (C74H92Fe2N4, CCDC 1521799). M = 1149.21, λ =
1.54184, monoclinic, space group P 1 21/n 1, a = 13.9894(4), b =
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Entry for the Table of Contents
COMMUNICATION
Highly selective Markovnikov
alkyne hydrophosphination is
reported in benzene, whereas on
changing the reaction solvent to
dichloromethane the selectivity
switches to give the Z anti-
Markovnikov product. Preliminary
mechanistic insight reveals that a
change in metal oxidation state
may be responsible for the
Andrew K. King, Kimberley J.
Gallagher, Mary F. Mahon, Ruth L.
Webster*
Page No. – Page No.
Markovnikov versus anti-
Markovnikov hydrophosphination:
Divergent reactivity using an
iron(II) β-diketiminate pre-catalyst
divergent reactivity observed.
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