Facile, Catalytic Dehydrocoupling of Phosphines Using β-Diketiminate Iron(II) Complexes

Article abstract of DOI:10.1002/chem.201503399

Catalytic dehydrocoupling of primary and secondary phosphines has been achieved for the first time using an iron pre-catalyst. The reaction proceeds under mild reaction conditions and is successful with a range of diarylphosphines. A proton acceptor is not needed for the transformation to take place, but addition of 1-hexene does allow for turnover at 50°C. The catalytic system developed also facilitates the dehydrocoupling of phenylphosphane and dicyclohexylphosphane. A change in solvent switches off dehydrocoupling to allow hydrophosphination of alkenes.

Full text of DOI:10.1002/chem.201503399

DOI: 10.1002/chem.201503399
Communication
&
Homogeneous Catalysis
Facile, Catalytic Dehydrocoupling of Phosphines Using
b-Diketiminate Iron(II) Complexes
Andrew K. King, Antoine Buchard, Mary F. Mahon, and Ruth L. Webster*[a]
Following the work of Hessen and co-workers,[13] we found
that sterically varied b-diketiminate iron(II) (trimethylsilyl)-
methylene complexes could be easily synthesized in one pot
and isolated as red (1) or yellow solids (2) in good yield
(Scheme 1). The 2,6-dimethyl complex (1) forms the formal
Abstract: Catalytic dehydrocoupling of primary and sec-
ondary phosphines has been achieved for the first time
using an iron pre-catalyst. The reaction proceeds under
mild reaction conditions and is successful with a range of
diarylphosphines. A proton acceptor is not needed for the
transformation to take place, but addition of 1-hexene
does allow for turnover at 508C. The catalytic system de-
veloped also facilitates the dehydrocoupling of phenyl-
phosphane and dicyclohexylphosphane. A change in sol-
vent switches off dehydrocoupling to allow hydrophosphi-
nation of alkenes.
Catalytic dehydrocoupling (DHC) is an attractive and efficient
method to synthesize PÀP bonds,[1] evolving H2 as the only by-
product. The resulting substrates are of great synthetic interest
and have found a wide range of applications in coordination
chemistry[2] and through further functionalization to produce
a diverse range of PÀC[3] and P-element bonds.[4] Although
DHC to make PÀP bonds is less well explored than the PÀB
and NÀB counterparts, a handful of elegant examples mediat-
ed by a small selection of platinum group,[5] group 4,[6] and
main group catalysts[7] along with Lewis acid catalysts[8] and
carbenes[9] exist. Given the ubiquity of the b-diketiminate FeII
motif, it is surprising to note that, although well explored in
terms of coordination chemistry and stoichiometric reactivi-
ty,[10] only a handful of examples of catalysis have been report-
ed using these iron complexes.[11] However, these complexes
evidently lend themselves to very important and challenging
catalytic transformations including CÀF bond activation[11b] and
hydroamination.[11c] An ideal scenario would be to use syntheti-
cally simple and inexpensive iron pre-catalysts to develop de-
sirable transformations, in this case the preparation of key
phosphorus motifs, without the need for expensive ligands,
exogenous reductants, or additives. We herein report the first
example of catalytic homo-DHC of phosphines using iron.[12]
DHC of a range of activated and deactivated phosphines pro-
ceeds under mild conditions with simple b-diketiminate FeII
complexes.
Scheme 1. Synthesis of b-diketiminate complexes 1 (thermal ellipsoids set at
50% probability) and 2 used to catalyze dehydrocoupling.
four-coordinate adduct with THF,[14] adopting a distorted tetra-
hedral geometry around the iron center, whereas the 2,6-diiso-
propyl complex (2) is three-coordinate.[13,15]
Our initial studies show that complexes 1 and 2 are excep-
tional stoichiometric reagents for the dehydrocoupling of
HPPh2 at RT with complete conversion after only 30 mins
(Table 1, entries 1 and 4). Pleasingly, both complexes also show
excellent catalytic activity upon heating to 708C in C6H6 (en-
tries 3, 6, and 7). The increase in steric bulk around the metal
center, changing from complex 1 to 2, delivers an improve-
ment in reactivity (compare entries 3 and 7).
An exogenous hydrogen acceptor is not needed to facilitate
the transformation but when 1-hexene is added to the reac-
tion mixture, there is further improvement in reactivity, with
100% conversion to the DHC product after 24 h at 508C
(entry 8). This is in line with observations made by Stephan
and co-workers,[16] where an equilibrium is believed to exist;
addition of an alkene removes hydrogen from the reaction
mixture and pushes the equilibrium in favor of DHC. Only
trace amounts of anti-Markovnikov hydrophosphination prod-
uct are observed when the reaction is performed in the pres-
ence of styrene where the spectroscopic yield of 3a is 100%
after 24 h at 708C. However, this latter transformation is highly
[a] A. K. King, Dr. A. Buchard, Dr. M. F. Mahon, Dr. R. L. Webster
Department of Chemistry
University of Bath
Claverton Down, Bath, BA2 7AY (UK)
E-mail: r.l.webster@bath.ac.uk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503399.
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Communication
Table 1. Optimization of DHC of HPPh2 using 1 and 2.[a]
Table 2. Extent of DHC reactivity of secondary phosphines using pre-cat-
alyst 2.[a]
Entry
[Fe]
Loading
[mol%]
Conditions
HPPh2
Entry
R2PH
Product
Substrate consumption[b]
[Isolated yield of product]
[%]
consumption [%][b]
1
2
3
4
5
6
7
1
1
1
2
2
2
2
2
100
5
5
100
5
5
5
5
5
RT, 30 min
508C, 24 h
708C, 24 h
RT, 30 min
508C, 24 h
708C, 18 h
708C, 24 h
508C, 24 h
708C, 24 h
100
22
71
100
48
68
100
100
trace
1
3a
3b
3c
3d
3e
3 f
100 [85]
95 [76]
33 [–]
2
3[c]
4[c]
5
8[c]
9
FeCl2·THF1.5
100 [60]
90 [82]
100 [90]
72 [57]
[a] Conditions: HPPh2 (0.5 mmol), C6H6 (0.35 mL). [b] Based on loss of PÀH
signal from 1H NMR, using 1,3,5-trimethoxybenzene as an analytical stan-
dard and evidenced by 31P{1H} NMR spectroscopy.[15] [c] 1 mmol 1-hexene
added.
6
7
3g
solvent dependent and a change to CH2Cl2 results in hydro-
phosphination (see below). Only a trace amount of 3a is ob-
served when FeCl2·THF1.5 is used (Table 1, entry 9).
8
3h
36 [–]
We proceeded to test the scope and extent of reaction
using pre-catalyst 2. We were pleased to observe that a range
of electronically activated, deactivated and sterically demand-
ing secondary phosphines dehydrocouple (Table 2). Phos-
phines bearing a halogen are excellent substrates for DHC,
with no evidence for dehalogenation. The presence of an N- or
O-substituent reduces reactivity and higher reaction tempera-
tures are necessary (Table 2, entries 3, 4, and 9), whereas in the
case of the ortho-methoxy substrate (entry 9), this combination
of steric bulk and heteroatom gives very little turnover even
with heating at high temperature. Under more forcing reaction
conditions, HPCy2 proceeds cleanly through to the DHC prod-
uct in modest yield (entry 10).
9[c]
3i
3j
10 [–]
28 [–]
10[d]
[a] Conditions: Phosphine (0.5 mmol), 2 (5 mol%), C6H6 (0.35 mL), 708C,
1
24 h. [b] Based on loss of PÀH signal from H NMR, using 1,3,5-trimethoxy-
benzene as an analytical standard.[15] [c] 1008C, 24 h. [d] 10 mol%, 1208C,
72 h.
We then investigated the proficiency of pre-catalyst 2 in the
DHC of primary phosphines; H2PCy and H2PPh (Scheme 2).
H2PPh undergoes DHC to form the PÀP dimer 4a cleanly in
60% spectroscopic yield (catalyzed by 5 mol% 2 at 1008C for
24 h), with no evidence for the formation of cyclic species. The
product is observed as a 1:1 ratio of the meso and rac isomers
(Scheme 2a). When more forcing conditions are applied
(10 mol% 2, 1208C, 72 h, Scheme 2b), although more of the
H2PPh starting material is consumed (89%), a mixture of prod-
ucts form. 31P{1H} NMR analysis of the reaction shows a mixture
of 4a, the cyclic pentamer and an unknown species in approxi-
mately 2:4:1 ratio (using inverse gated 31P{1H} NMR calibrated
against a PPh3 standard). In comparison, after 24 h at 1008C
using 5 mol% 2 only a trace amount of 1,2-dicyclohexyldiphos-
phane (4b) is observed. Although an increase in loading of 2,
reaction temperature and time (10 mol%, 1208C, 72 h) allows
the DHC of H2PCy, the reaction mixture is complex and con-
sists of an intractable mixture of products.
Scheme 2. DHC of primary phosphines.
can be isolated from their reaction with HPPh2 in C6H6. At-
tempts to isolate an iron phosphido complex from a salt meta-
thesis route (using either the iron m-chloroate complex[17] or di-
nuclear m-bromo complex[18] in the presence of LiPPh2 or
KPPh2) did not proceed cleanly through to the desired product.
We presume that catalysis is initiated by loss of Si(CH3)4 on for-
mation of an iron-phosphido intermediate. Indeed, 31P{1H} NMR
analysis of the catalytic mixture shows the formation of a new
species at 110 ppm,[15] this downfield signal is consistent with
The extreme reactivity when employing a stoichiometric
amount of either 1 or 2 is such that no catalytic intermediates
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an FeÀPR2 bond, in line with seminal independent reports
from Carty, Seyferth, and Wojcicki on studies of phosphido-
bridged iron carbonyl complexes,[19] but in this case the com-
plex is anticipated to be mononuclear due to steric hin-
drance.[20] This is substantiated by DFT calculations[15] along
with inert atmosphere ESI mass spectrometry of the crude cat-
alytic mixture, which shows the presence of a monomeric iron-
phosphido complex (m/z 659.3161, consistent with M+H
where M=C41H51FePN2), with no evidence for the presence of
a dimer. Following phosphido formation, reaction with phos-
phine could allow elimination of the DHC product along with
the potential formation of an iron hydride,[21] which could then
react with another equivalent of phosphine releasing H2 and
regenerating the iron phosphido, similar to previous reports
with Zr catalysts.[6b,22]
anti-Markovnikov hydrophosphination product, 5a, is ob-
tained.[24] Indeed, these hydrophosphination conditions[25] can
be applied to a range of activated alkenes, furnishing the
phosphine products in good to excellent yields (Scheme 4).
Scheme 4. Hydrophosphination of alkenes using 2 as the pre-catalyst.
Addition of 10 mol% TEMPO to the catalytic reaction mix-
ture inhibits reactivity (20% 3a), suggesting that these steps
may be radical mediated. Reaction of a stoichiometric amount
of radical clock ((iodomethyl)cyclopropane) with 2 results in
the quantitative formation of a bright orange iron–iodide com-
plex and 5-(trimethylsilyl)pent-1-ene (Scheme 3a). Addition of
Unfortunately, less active alkenes, such as allyl benzene and 1-
hexene yield only trace amounts of product under these condi-
tions, likewise reaction of styrene with HPCy2 and H2PPh is
poor.
It is not surprising that anti-Markovnikov hydrophosphina-
tion of activated substrates can be achieved under these cata-
lytic conditions, but this dramatic change in reactivity on
change in solvent is fascinating; moreover, addition of
10 mol% TEMPO is not detrimental to reactivity with no reduc-
tion in spectroscopic yield.
In summary, iron(II) b-diketiminate complexes possessing
a labile co-ligand prove to be excellent pre-catalysts for the de-
hydrocoupling of a range of primary and secondary phos-
phines. No additives are necessary for the transformation to
take place and an exogenous proton acceptor is not needed
to sequester the H2 released. Although synthetically simple,
the reaction mechanism is not trivial but appears to proceed
via a radical pathway. A detailed spectroscopic and electronic
study of the mechanism, along with investigations into the
substantial change in reactivity on change in solvent, is under-
way and will be reported in due course.
Scheme 3. Reactions with a radical clock.
Experimental Section
General method for the dehydrocoupling of phosphines: all
steps were performed under an inert atmosphere. To a Schlenk
tube 5 mol% (0.025 mmol) of pre-catalyst 1 or 2 was added in
0.35 mL of benzene. Phosphine (0.5 mmol) was then added to the
reaction vessel and the corresponding solution was stirred at 708C
for 24 h (or otherwise stated). To obtain spectroscopic yield and/or
isolated yield the reaction mixture was passed through a short
silica plug, eluting with CH2Cl2. To obtain spectroscopic yield, the
solution was charged with a known quantity of 1,3,5-trimethoxy-
benzene, concentrated, then an NMR sample prepared using C6D6.
This isolation method was necessary to remove the paramagnetic
component from the reaction mixture, allowing analysis by
1H NMR spectroscopy.
the radical clock to a catalytic reaction of 2 and HPPh2 results
in the formation of 26% 3a, diphenylphosphinous iodide and
1-butene (Scheme 3b). This would suggest that, during the
catalytic reaction, both off-cycle catalyst activation and on-
cycle DHC processes are radical mediated, with evidence for
this provided by addition of (iodomethyl)cyclopropane to the
catalytic reaction after 18 h where no further reactivity is ob-
served and 68% 3a is obtained along with diphenylphosphi-
nous iodide and 1-butene (Scheme 3c). HPPh2 does not react
with (iodomethyl)cyclopropane in the absence of 2. Based on
the radical nature of the reaction, ligand non-innocence
cannot be ruled out.[15,23]
Acknowledgements
As previously mentioned, a change in solvent results in
a substantial change in reactivity. Under standard DHC condi-
tions (cf. Table 1, entry 7) only a trace amount of 3a is ob-
served when CH2Cl2 is employed as the solvent, but in the
presence of 1.8 equiv styrene 75% spectroscopic yield of the
The University of Bath is thanked for a DTA studentship
(A.K.K.). Dr. Anneke Luben is thanked for assistance with air-
sensitive mass spectrometry. The EPSRC is acknowledged for
Chem. Eur. J. 2015, 21, 15960 – 15963
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Communication
[13] T. J. J. Sciarone, A. Meetsma, B. Hessen, Inorg. Chim. Acta 2006, 359,
1815–1825.
funding (EP/M019810/1) along with the NSCCS for computa-
tional support (Chem 852).
[14] Crystal data for C29H44FeN2OSi, 1: M=520.60, l=0.71073 Š, monoclinic,
space group C2/c, a=29.1850(4), b=11.0540(1), c=18.3670(3) Š, b=
98.750(1)8,
U=5856.43(14) Š3,
Z=8,
1calcd =1.181 gcmÀ3
,
m=
Keywords: homogeneous catalysis · iron · phosphanes ·
phosphorus · radicals
0.578 mmÀ1, F(000)=2240. Crystal size=0.25”0.25”0.20 mm, unique
reflections=6693 [R(int)=0.0599], observed reflections [I>2s(I)]=
4995, data/restraints/parameters=6693/0/335. Observed data; R1=
0.0376, wR2=0.0853. All data; R1=0.0612, wR2=0.0957. Max peak/
hole=0.435 and À0.373 eŠÀ3, respectively. CCDC 1060002 contains the
supplementary crystallographic data for this paper. These data are pro-
vided free of charge by The Cambridge Crystallographic Data Centre.
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Received: August 27, 2015
Published online on September 25, 2015
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