Mechanistic Investigation of Well-Defined Cobalt Catalyzed Formal E-Selective Hydrophosphination of Alkynes

Article abstract of DOI:10.1021/acs.organomet.8b00281

A formal E-selective hydrophosphination of terminal and internal alkynes catalyzed by a well-defined [Co(PMe₃)₄] (A) complex is achieved under mild conditions in good-to-excellent yield. The reaction does not require any additives and/or external base for an efficient hydrophosphination reaction. The reaction provided excellent scope and good functional tolerance. Detailed spectroscopic analysis (NMR, EPR, and UV-vis) revealed that the low valent cobalt(0) complex undergoes oxidative addition with diphenylphosphine, followed by hydrometalation with alkyne, and subsequent reductive elimination led to the expected product. The detailed spectroscopic analyses along with the isotopic labeled experiments facilitate to intercept the active intermediates that are involved in the catalytic cycle, which are detailed. It was revealed that the suprafacial (vide infra) delivery of H and phosphorus to π-alkynes in a syn-fashion led to formal E-vinyl phosphine.

Full text of DOI:10.1021/acs.organomet.8b00281

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Mechanistic Investigation of Well-Defined Cobalt Catalyzed Formal
E‑Selective Hydrophosphination of Alkynes
Jitendrasingh Rajpurohit,† Pardeep Kumar,† Pragya Shukla,† Muralidharan Shanmugam,*,‡
and Maheswaran Shanmugam*,†
†Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, Maharashtra, India
‡Manchester Institute of Biotechnology (MIB), The University of Manchester, 131 Princess Street, Manchester M1 7DN, U.K.
S
* Supporting Information
ABSTRACT: A formal E-selective hydrophosphination of
terminal and internal alkynes catalyzed by a well-defined
[Co(PMe3)4] (A) complex is achieved under mild conditions
in good-to-excellent yield. The reaction does not require any
additives and/or external base for an efficient hydrophosphi-
nation reaction. The reaction provided excellent scope and
good functional tolerance. Detailed spectroscopic analysis
(NMR, EPR, and UV−vis) revealed that the low valent
cobalt(0) complex undergoes oxidative addition with
diphenylphosphine, followed by hydrometalation with alkyne,
and subsequent reductive elimination led to the expected
product. The detailed spectroscopic analyses along with the
isotopic labeled experiments facilitate to intercept the active
intermediates that are involved in the catalytic cycle, which are
detailed. It was revealed that the suprafacial (vide infra) delivery of H and phosphorus to π-alkynes in a syn-fashion led to formal
E-vinyl phosphine.
Noble metals such as [Pt], [Pd], and [Ru] based catalysts
were predominantly employed to promote the hydrophosphi-
nation of unsaturated C−X (where X  C, O, S) bonds.41−43
Although conscious efforts were made by several research
groups to develop cheap, environmentally friendly, less toxic
metal catalysts (such as alkaline metal complexes), the reac-
tions either require high catalyst loading or suffer from poor
regio- and stereoselectivity of the products.14,44−48 Transition
metal catalysts developed for the hydrophosphination reaction,
on the other hand, often require other additives, in particular
with the late transition metal catalysts.12,49−52 For example,
Oshima and co-workers elegantly demonstrated the use of an
inexpensive copper and [Co(acac)2] catalyst for provoking
hydrophosphination of alkynes, but it requires additional strong
base Cs2CO3 and a reactive n-BuLi, respectively.53
Therefore, we sought to unveil a robust, cheap, and abun-
dant transition metal catalyst for hydrophosphination reaction.
We, herein, report an efficient, stereoselective hydrophosphi-
nation of alkynes catalyzed by the well-defined cobalt catalyst
[Co(PMe3)4] (A) and investigated the mechanism of the reac-
tion to intercept the various intermediates involved in the
catalytic cycle. Such mechanistic investigations are extremely
scarce in the literature. In this process, we have identified
various intermediates involved in the catalytic cycle with the
INTRODUCTION
■
Addition of an X−H bond (where X is a heteroatom) to alkyne
is of significant importance due to atom and step economy.1,2
This waste free method was widely employed for various
hydro-functionalization of unsaturated systems, especially
C−C double and triple bonds. Among them, hydrophosphi-
nation holds an edge over the others, due to the importance of
organophosphorus compounds in antibiotic and antitumor
activity, metal catalysis, and organocatalysis as witnessed in
recent times.3,4 Hydrophosphination reaction, although, can be
promoted by acids, bases, radicals,5,6 high temperature, and
organometallic reagents; these reactions suffer drastically from
lower selectivity, poor functional groups tolerance, and uncon-
trolled reactivity. The pioneering works of Gleuck et al. and
Pringle et al. showed that these problems could be resolved by
employing a metal catalyst.7,8 The metal catalyst often tends to
provide better regio- and stereoselective control compared to
the uncontrollable radical initiated reaction.3 However, the
sluggish progress in developing new metal catalysts is perhaps
due to that the resultant trivalent vinyl phosphine is an excel-
lent coordinating ligand, which is likely to poison the catalyst.
Hence, the catalyst development for hydrophosphination reac-
tion of unsaturated C−X (where X  C, O, N, S) bonds was
not much explored9−18 compared to the other hydro-function-
alization reactions such as hydroboration, hydrosilylation,
hydroamination, etc.16,19−40
Received: May 2, 2018
© XXXX American Chemical Society
A
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a b
,
aid of multispectroscopic analysis, and isotopic labeling
experiments, which are described below.
Scheme 2. Scope of the Terminal Alkynes
RESULTS AND DISCUSSION
■
This well-defined catalyst A yields formal E-selective hydro-
phosphination of terminal and internal alkynes exclusively under
mild reaction conditions, and the reaction proceeds smoothly
via syn-addition of a P−H bond to the alkyne without any
additional bases or organometallic reagent (Scheme 1). We began
Scheme 1. Cobalt Catalyzed Hydrophosphination of
Alkynes
our investigation by screening various reaction parameters
using 0.9 mmol of phenylacetylene and diphenylphosphine
with 5 mol % of A. The catalytic reaction was optimized
to isolate the product with better yield/stereoselectivity by
screening solvent, temperature, and catalyst loading (Tables
S1−S4 of the SI). A better stereoselectivity (E) was obtained
in low polar solvent compared to the polar solvents presumably
due to the sluggish isomerization process of the product in the
low polar solvent compared to the polar solvent. Such a
scenario witnessed in the literature, for example, solvent
polarity dependent stereoselective product obtained for
hydrogenation of alkynes.54
It is noticed that a high yield of the product was obtained
while using toluene as solvent with the catalyst loading of
5 mol % at 80 °C for terminal alkyne (100 °C for internal alkyne;
Scheme 1).
We observed neither dimerization of alkyne nor poisoning of
catalyst by the product, which is an added advantage for the
efficient product formation. Slightly higher temperature is
required as the catalyst is latent and liberation of PMe3 (from A)
is effective at higher temperature to access to active catalyst
precursor (vide infra). Careful analysis of the product indicates
that the phosphine and H are added on the same side of
the alkyne, which led to formal E-vinyl phosphine.
With the best conditions in hand, we next examined the
scope of terminal alkynes (Scheme 2). An electron donating
group such as Me, n-pentyl, and t-Bu substituent at the
p-position provided good yield with excellent stereo- and regio-
selectivity. Further, the crystal structure of 2c unequivocally
supports the E-selective product formation (Figure S1, Table S5),
where the phosphine and phenyl rings are trans to each other.
The electronic effect of the substituent (i.e., either electron
withdrawing or electron donating groups) on the phenyl ring
does not appear to have influence on the progress of the cata-
lytic reaction and hence the yield of product. For example,
electron withdrawing groups such as −Br, −F, and acetyl
groups as well as electron releasing groups (such as −OMe,
−OEt, OC5H11) were well tolerated and provide excellent
product yield without much variation in yield (2e−2j). The
reaction can be further extended to hydrophosphination of a
fused aromatic ring such as naphthyl, anthracyl, and pyrenyl
substituted acetylene in good yield (2k−2n). To our delight,
even heterocycles can be tolerated, albeit in moderate yield (2o).
a
All reactions were carried out under an argon atmosphere, unless
otherwise stated, using 1/PPh2/[Co] in 0.9/0.9/0.045 mmol at 80 °C
in toluene (3 mL). Isolated yield after sulfidation.
b
Additionally, the reactivity of aliphatic alkynes, namely,
tetrahydrocyclopentyl propynyl acetylene and 1-pentyne, is
tested. The expected hydrophosphinated product was isolated
in moderate yield (2p−q), but formation of a small amount of
another regioisomer (E′) is witnessed with the E:E′ ratio = 3:1
(see Table S6 of the SI). We would like to emphasize that
certain transition metal catalysts do not promote hydrophos-
phination reaction of aliphatic alkyne;12,13 those which facilitate
smooth progress of the reaction often lose their regioselectivity,
a common, yet challenging, problem known in the literature.18,55
Although hydrophosphination of terminal alkynes has been
reported in the past, their corresponding extension to the
internal alkynes often failed to provide the vinyl phosphine or
it did not provide any control over stereoselectivity due to the
vinylidene pathway.10 Under the optimized experimental con-
ditions (Scheme 1), we further extent the scope of the internal
alkynes (Scheme 3). Biaryl acetylene gave moderate isolated
yield of the expected product, but the product yield is signi-
ficantly improved (93%; 3c) when 1-phenylpropyne is used with
one single regioisomer. The regio- and stereoselectivity is abso-
lutely preserved in 3a−c, whereas, in the case of electronically
biased alkynes 3d and 3e, the regioselectivity is compromised
(E:E′ ratio for 3d (55:45) and 3e (69:31); see Table S6 of the SI)
to some extent. It is witnessed, hitherto, from the literature that
promoting hydrophosphination reaction of internal alkynes
B
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a
Scheme 3. Scope of the Internal Alkynes
a
All reactions were carried under the same conditions as in Scheme 2
at 100 °C.
with a heteroaromatic ring substituent (or electronically biased
substrate) is a challenging task. In the majority of cases, reac-
tion does not proceed due to the catalyst poisoning, while, in
the vice versa scenario, controlling stereo- and regioselectivity
of the reaction is a demanding task and an uphill process. The
analyses of product (3c and 3e) revealed that the phosphorus
atom prefers to add to the sterically less hindered carbon site
of an alkyne.
Intrigued by this reactivity and stereoselectivity, we have
turned our attention to predict the nature of the intermediate
formed to better understand the mechanism of reaction.
We first carried out deuterium labeling experiments using PPh2D
(78% deuterium enriched) with 4-t-butylphenylacetylene to
understand the origin of E-selective product formation. Careful
Figure 1. 1H NMR (panel 1) and 31P NMR (panel 2) of [Co(PMe3)4]
treated with PPh2H (green trace) in C6D6. The magenta trace
corresponds to substrate added into the [Co(PMe3)4] + HPPh2
reaction mixture. In both cases, the spectra were recorded in a
400 MHz NMR instrument using C6D6 as solvent. Panel 1 inset: The
1
2
analysis of H and H NMR revealed that the intensity of the
doublet of doublet observed at 7.58 ppm drastically reduced in
1
magnified region of H NMR spectra observed between −16 and
−18 ppm ($silicon grease). Panel 2 (*PPh3, PPh2-PPh2).
2
#
1H NMR with the concomitant appearance of an H signal in
2H NMR at the same ppm, which implies that the deuterium
atom (of the PPh2D) is bound to the internal carbon of the
alkyne (Figure S2 of the SI). The coupling constant observed
heterolytic cleavage via a 2c-3e− transition state to provide a
pentacoordinated [CoII(H)(PPh2)(PMe3)3] oxidative addition
product B, which is also consistent with EPR, UV−vis spec-
troscopy (vide infra).
1
3
in the H NMR signal (observed at 6.91 ppm (dd) JH‑H
=
16.3 Hz, 3JP‑H = 21 Hz)) and the peak observed at −11.2 ppm
in 31P NMR of the product confirms the syn-addition results in
E-selective product formation.
1
Formation of B was confirmed further by H NMR where
the methyl protons of −PMe3 groups (coupled with phos-
phorus of PPh2 group) were observed at 1.16 ppm as a doublet
and the aromatic protons of the −PPh2 group as multiplets
between 6.8 and 7.8 ppm which are coordinated to the Co(II)
ion in B (Figure 1; also see inset of panel 1)). Its corre-
sponding 31P signals are observed at 2.68 (−PMe3) and
32.5 (−PPh2) ppm (green trace in Figure 1 (panel 2)). The
We next examined the reaction by following the stoi-
chiometric experiment between A and PPh2H through various
spectroscopic techniques. Prior to the experiment, we have
1
characterized the catalyst A through H and 31P NMR spectra
in C6D6 at room temperature. The characteristic peaks for methyl
protons and the 31P signal are observed at 1.26 and −0.14 ppm,
respectively. This indicates that all the four −PMe3 groups
bound to the cobalt ion are chemically equivalent (Figure S3
of the SI).
An equimolar amount of the A and PPh2H were mixed, and
the reaction was monitored by the 1H NMR spectrum. A broad
signal observed at 0.89 ppm is attributed to a metal free −
PMe3 group in solution, which is likely due to ligand (PMe3)
dissociation from A, followed by σ-coordination of PPh2H.
This free PMe3 group 31P signal was noted at −62.5 ppm (see
green trace in Figure 1; assignment of this signal was based on
1H and 31P NMR recorded in C6D6 for commercially available
1 M solution of PMe3; Figure S4). This further undergoes
1
weak H NMR signal observed in −16.1 (doublet of quartet)
to −17.6 ppm (quintet) is ascribed to the hydride ion bound
to the Co(II) ion (intermediate B and C (vide infra)), which
is consistent with the chemical shift value reported in the
literature for a Rh-H complex.56 Due to highly reactive nature
of B, B presumably reacts with another equivalent of HPPh2
(likely via σ-bond metathesis), results in the formation of coupled
product PPh2-PPh2 and [CoII(H)2(PMe3)4] (C, Figure 1; see also
Scheme 4) complex. The catalytic reaction was carefully
monitored again through GC−MS analysis, which clearly reveals
that neither H2 gas evolution nor hydrogenated products (such as
alkenes and alkanes) were observed in the presence of an alkyne
C
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chemical shift value observed for the hydride bound to Co(II)
ion in B, in 1H NMR (Figure S7 of the SI). On the other hand,
we could not observe deuterium incorporation in species C.
The isotopic labeled experiments and detailed NMR analysis
unambiguously prove that the oxidative addition is the initial
step of the catalytic cycle.
Scheme 4. Proposed Mechanism
Next, we added the alkyne to the in situ generated species
(B and C); subsequent changes were monitored again through
1H and 31P NMR spectra (Figure 1, magenta trace). The 31P
signal (2.68 and 32.5 ppm) and 1H NMR signal (−16.35 ppm)
observed for species B have totally disappeared. Further, the
1
intense doublet observed in H NMR at 1.16 ppm for the
−PMe3 group coordinated to Co(II) in B, the signal intensity
drastically decreased. This entire scenario firmly advocates that
the alkyne inserted between Co-H, followed by reductive
elimination, led to styrenyldiphenylphosphine. A new intense
31P NMR signal at −11.2 ppm is the characteristic signature of
an E-selective styrenyldiphenylphosphine derivative, which is
consistent with the other literature reports.18 The unchanged
1H NMR signal intensity at 0.86 ppm (which was attributed to
the free −PMe3 that originated from A upon oxidative
addition) even after the reductive elimination indicates that
Co(0) regenerated after the reductive elimination is not the
same as A. This regenerated Co(0) is referred to as A′
hereafter. This designates that a concurrent ligand dissociation
and oxidative addition of PPh2H is the first step of the catalytic
cycle. Formation of A′ further strongly corroborated with UV−
vis and EPR measurements (vide infra). Further, C remains as
the resting state and is not catalytically active under the
reaction conditions. This fact is indeed verified from the
substrate (data not shown). This reveals that C is robust and
does not undergo any further reaction, which is consistent with
NMR observation that the peaks that correspond to C (both
1H and 31P) remain unchanged even after the addition of an
alkyne substrate (vide infra). The other alternative, reductive
elimination pathway for the formation of coupled product
(PPh2-PPh2) from B is omitted, because this will result in
Co(0) formation which is an EPR active species. However,
experimentally, the products obtained upon addition of PPh2H
with A were found to be EPR silent (vide infra), which implies
that cobalt remains in a +2 state. The quintet signature
observed at −17.4 ppm for the hydride ion bound to Co(II)
(see inset of Figure 1 (panel 1)) is due to the splitting of its
1H NMR signal by four chemically equivalent PMe3 groups
bound to Co(II) ion in C. The 1H NMR and 31P NMR signals
of the PMe3 group coordinated with the Co(II) ion in spe-
cies C were observed at 1.25 ppm and −4.4 ppm (Figure 1),
respectively. The existence of both B and C species (in the
stoichiometric reaction) was further indirectly confirmed by
the presence of a 31P signal at −14.9 ppm, which is attributed
to the formation of the Ph2P-PPh2 coupled product. The
observed chemical shift value of this coupled product is consis-
tent with the literature reports.57 Not only in a stoichiometric
reaction, but also in the presence of excess PPh2H (A:PPh2H =
1:20 (as per in the catalytic reaction condition, but in the
absence of alkyne substrate)), formation of a small amount of
Ph2P-PPh2 (∼5%) was observed along with B, which was con-
firmed through 31P NMR (Figure S5 of the SI). Nevertheless,
we would like to point out that the coupled product (Ph2P-PPh2)
formation is prevented during the catalytic reaction, as confirmed
through 31P NMR, in the presence an alkyne substrate (Figure S6
of the SI), which reveals the highly reactive nature of B.
To prove the origin of hydride in B and C from PPh2H upon
oxidative addition, we have performed the stoichiometric reac-
tion of deuterated diphenylphosphine (PPh2D (60% enriched))
1
unchanged H and 31P signal even after the addition of an
alkyne substrate (see magenta trace in Figure 1).
To provide additional support for the oxidative addition
of PPh2H with [Co(PMe3)4] (A), we have recorded a UV−
visible spectrum for both A and in situ generated products
(stoichiometric amount of A and PPh2H) in a sealed cuvette
(Figure 2). The spectral features of both A and in situ
Figure 2. UV−vis spectra of A (red trace), in situ generated
intermediate B upon PPh2H addition to A (blue trace), and magenta
trace represents the alkyne added to B measured in toluene (2.8 ×
10−8 mmol) at room temperature.
generated complexes are distinctly different. For A, there is no
significant absorption feature, whereas, for the in situ generated
product, two distinct d−d transitions centered at 404 and
473 nm are observed,58,59 for intermediate species B and C.
Addition of 4-bromophenylacetylene to the reaction mix-
ture results in no absorption features as like for A, with the
2
with A in C6H6. The H NMR spectrum of this benzene
solution shows a peak at −16.3 ppm, which is exactly the same
D
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synchronous disappearance of the absorption bands observed
earlier. The UV−vis spectrum upon addition of alkyne into the
in situ generated species, however, does not exactly resemble
that of A. This suggests that A is not a catalytically active species,
but A′ which is confirmed by its spin Hamiltonian parameters
extracted from EPR measurement compared to A (vide infra).
In order to shed light on the electronic structure of the
various species involved in the hydrophosphination catalytic
cycle, low temperature, X-band EPR measurements were per-
formed on A by dissolving in toluene, as a frozen toluene glass
at 5 K. The EPR spectrum of A measured at 5.0 K shows nicely
resolved hyperfine fine structures due to the interaction of
unpaired electron spin on cobalt with the nuclear spin of 59Co
(I(59Co) = 7/2). The cobalt hyperfine couplings are clearly
visible in the gII region, whereas it is severely overlapped and
poorly resolved in the g⊥ regions. Simulation of the experi-
mental EPR spectrum of A gave principal values of the g-tensor
and components of hyperfine coupling for 59Co nucleus;
g = [2.1626, 2.1601, 1.9832], A(59Co) = [130 70 179] MHz
with isotropic hyperfine coupling, of aiso(59Co) = 126.3 MHz
(see black trace in Figure 3).60 The Gaussian line width of
Co(0) complexes with organophosphorous ligands in the coor-
dination sphere with distorted tetrahedral geometry.61−65
When diphenylphosphine (HPPh2) is added into the solu-
tion of A, the characteristic EPR features of A completely
disappeared (see red trace in Figure 3). A similar trend has
been observed previously when Co(0) complex [Co(C2H7O2)2−-
Al(C2H5)3] was treated with dihydrogen gas.64 This implies that
ligand exchange takes place (oxidative addition) at the expense
of the valence state of the cobalt center in A. However, litera-
ture precedents are known for some reactions that there is
no change in oxidation state of Co(0) upon ligand substitu-
tion.61,66−68 Oxidative addition leads to the formation of species
B and C, which are EPR silent, in contrast to A. This suggests
that the cobalt ions in both B and C (Scheme 4) exist in a high-
spin divalent oxidation state presumably. The rationale for the
EPR silent signature of Co(II) ion in a high-spin state (S = 3/2)
is well-established already in the literature.58,69−74
When an alkyne is added to the in situ generated species, the
EPR signal is regenerated, with characteristic features of an
S = 1/2 spin state, i.e., Co(0) (see blue trace in Figure 3). The
regeneration of the EPR signal due to Co(0) unambiguously
suggests that reductive elimination is the final step of the
hydrophosphination catalytic cycle. The EPR spectrum of the
final step was simulated by considering S = 1/2 spin state with
the spin-Hamiltonian (SH) parameters; g = [2.1622 2.1317
2.002], A(59Co) = [136 48 104] MHz, with isotropic hyper-
fine coupling, of aiso(59Co) = 96 MHz. The SH parameters
extracted for Co(0) regenerated from the final step (A′) are
slightly different from the SH parameters extracted for A. This
suggests that the regenerated catalyst precursor (A′) possesses
a slightly modified coordination environment than A. A similar
trend has been reported previously when one/two of the organ-
ophosphorous ligands in the coordination environment of Co(0)
complexes were replaced by other ligands/solvents.61,65 The
vacant coordination site formed after the reductive elimination,
conceivably, occupied by the solvent molecule with the tenta-
tive molecular formula of [Co(PMe3)3(L)] (where L = solvent).
The spin-Hamiltonian parameters extracted for A′ repro-
duce only part of the experimental spectrum, which leave out
some of the spectral features in the 320−350 mT range (see
Figure 3 marked with # symbol). The origin of these EPR
signals is due to an unknown radical impurity. The SH param-
eters extracted for this from simulation are described in the SI
(see Figure S8 and its related text). The entire experimental
spectral features in the final step were reproduced by consi-
dering two different S = 1/2 spin states with the weighted
factor of 0.96 (for A′):0.04 (radical S = 1/2) (blue trace in
Figure 3). Currently, we do not know the origin of the radical
impurity (<4%). Overall, the modified coordination environ-
ment for the regenerated catalytic precursor (compared to A)
is well supported by EPR, NMR, and UV−vis data
undisputedly.
Figure 3. X-band CW-EPR spectra of [Co(PMe3)4] (black trace; A),
in situ generated products upon PPh2H addition to A (red trace; B).
Alkyne treated with B (blue trace) measured at 5.0 K in toluene
solution. The solid magenta traces represent the simulations of the
experimental spectra using the parameters described in main text.
* denotes the signal from the resonator cavity. # represents the EPR
signals that originate from a radical impurity (<4%). See main text for
details.
On the basis of the various spectroscopic observations
(NMR, UV−vis, and EPR (vide supra)), we have proposed the
working mechanism as depicted in Scheme 4. Complex A
undergoes initial ligand dissociation, followed by oxidative
addition of P-H, which led to the intermediate B. Intermediate B
in the absence of alkynes reacts with HPPh2 and leads to the
formation of C along with the minor coupled product (PPh2)2.
Initial coordination of alkyne to the intermediate B, followed
by insertion between Co-H, led to intermediate D, which then
proceeded to reductive elimination to produce vinylphosphine
and regenerate an active species A′ for the next catalytic cycle.
3.89 mT was used to reproduce the experimental line shape. The
observed EPR line shape and the extracted spin-Hamiltonian
parameters are in good agreement with the previously reported
E
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acetylene was added into it. It was sealed and heated at 80 °C for
30 min. 1H, 31P NMR was recorded for the resulting solution. A similar
procedure was followed for the isotopic labeled experiments (reaction
of A with DPPh2) as well, but the NMR was recorded in C6H6.
Procedure for UV−vis Follow-Up. A similar synthetic method was
followed as in the NMR follow-up, but the reactions were performed
in a Schlenk tube fitted with septa in a 1.0 mL toluene solvent. With
the help of a syringe, 0.05 mL of aliquot was taken from the reaction
mixture which is then transferred to a sealed UV−vis cuvette
(containing 3.5 mL of dry toluene) under an inert atmosphere.
Procedure for EPR Follow-up. A similar synthetic method was
followed as in the NMR follow-up, but the reactions were performed
in a Schlenk tube fitted with septa in a 1.0 mL toluene solvent. With
the help of a syringe, 0.05 mL of aliquot was taken from the reaction
mixture which is then transferred to a sealed EPR tube (containing
0.15 mL of dry toluene) under an inert atmosphere.
CONCLUSIONS
■
In conclusion, we have shown hydrophosphination of internal
and terminal alkynes for the first time using a well-defined low
valent cobalt(0) complex. The suprafacial addition of P−H
bonds across the alkyne in syn-fashion results in regio- and stereo-
selective vinylphoshpine as product. Deuterated labeled experi-
ments undisputedly unveil the origin of regio- and stereo-
selectivity of the isolated products. The reaction is general and
applicable for both terminal and internal alkynes with good
functional groups tolerance. The detailed NMR, UV−vis, and
EPR spectroscopic methods apparently disclose the presence
of species B ([CoIIH(PMe3)3(PPh2)]) in solution upon oxi-
dative addition of PPh2H with A. Further, in the absence of
alkyne, B reacts with HPPh2 and leads to C ([CoII(H)2(PMe3)4]).
Addition of an alkyne substrate into the in situ generated species B
facilitates reductive elimination which concomitantly regenerates
presumably a tricoordinated naked Co(0) active species. This
extensive study allows us to further extend the catalytic system
to activate other X−H bonds including inert C−H bonds,
which is currently underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.8b00281.
■
S
Crystallographic data obtained for 2c and relevant NMR
of the isolated products (PDF)
EXPERIMENTAL SECTION
■
Accession Codes
General Remarks. All the reactions were carried out under an
argon atmosphere unless otherwise mentioned. All the alkyne
substrate was purchased from a commercially available source, except
(1j−1n), (1ab−1ad) are synthesized as per the literature reports.75−77
Toluene was purified by means of distillation under a dry nitrogen
atmosphere over benzophenone/sodium ketyl. [Co(PMe3)4] was
synthesized as per a literature procedure.14 Chromatography purifi-
cation was carried out using silica (mesh size 60−120) for isolation of
the products; additionally, thin layer chromatography was performed
using Merck TLC silica gel 60 F254 plates. All the NMR (1H, 13C, 31P)
were recorded on a Bruker 400 AVANCE or 500 AVANCE and
referenced to the CDCl3 or C6D6 residual peak. 85% H3PO4 was used
as external reference for recording 31P NMR. UV−vis spectra were
recorded on a Shimadzu (UV-NIR-3600) at room temperature. Variable
temperature EPR was recorded using a Bruker EMS plus series instru-
ment, and the EPR spectra were simulated using easy spin software.60
Synthetic Procedure. General Procedure for the Hydro-
phosphination of Terminal Alkyne. In an argon filled glovebox,
16.5 mg of [Co(PMe3)4], 0.16 mL (0.9 mmol) of HPPh2, and 3 mL
of toluene was added in an Schlenk tube, and the mixture was stirred
at room temperature for 10 min before adding various alkynes
(0.9 mmol) into the reaction mixture. The resultant mixture was
heated at 80 °C for 1−2 h. Reaction was followed by 31P NMR, and
consumption of HPPh2 (−40.5 ppm) was monitored. The reaction
tube was cooled and 60 mg of S8 was added into the reaction mixture.
This was heated at 80 °C for 15 min before the reaction was quenched
with water. The mixture was transferred into a separatory funnel, and
the compound of interest was extracted in ethyl acetate. The pure
product of interest was isolated through column chromatography
(pet. ether/ethyl acetate (ratio: 99:1). The purity of the resultant
CCDC 1509786 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: eswar@chem.iitb.ac.in (Maheswaran Shanmugam).
*E-mail: muralidharan.shanmugam@gmail.com (Muralidharan
Shanmugam).
ORCID
Maheswaran Shanmugam: 0000-0002-9012-743X
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Maheswaran Shanmugam acknowledges the funding agencies
DST (EMR/2015/000592), IRCC, IIT Bombay, INSA (SP/
YSP/119/2015/1264) for the financial assistance. Muralidhar-
an Shanmugam acknowledges Manchester Institute of Biotech-
nology (MIB) and University of Manchester for financial
support.
1
product was confirmed through the H and 31P NMR where we had
obtained only a single peak corresponding to product.
General Procedure for the Hydrophosphination of Internal
Alkyne. The same reaction procedure was followed as above, but the
reaction mixture was heated at 100 °C for 12 h.
ABBREVIATIONS
Procedure for the NMR Follow-Up. To follow the intermediates
generated in the catalytic cycle, we performed several NMR
experiments in a J-Young NMR tube. The general procedure followed
is described below. 10 mg (0.0275 mmol) of [Co(PMe3)4] was
dissolved in 0.5 mL of C6D6; in the resulting homogeneous solution,
4.7 μL of (0.0275 mmol) HPPh2 was added. Upon addition, the
reaction mixture turns into dark brown, and the tube was sealed and
heated for 15 min at 70 °C. The resultant mixture was cooled to room
■
NMR,nuclear magnetic resonance; EPR,electron paramagnetic
resonance; UV−vis,ultraviolet visible; SI,Supporting Informa-
tion
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1
temperature and H, 31P NMR was recorded. The tube was brought
inside the glovebox, and 5 mg (0.0275 mmol) of 4-bromophenyl
F
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