Hydrophosphination using [GeCl{N(SiMe3)2}3] as a pre-catalyst

Article abstract of DOI:10.1039/d0cc05792d

Transformations catalyzed by germanium are scarce, with examples mainly limited to widely catalyzed processes such as polymerisation of lactide and hydroboration of carbonyls. Reported is the first example of hydrophosphination using a germanium pre-catalyst, yielding anti-Markovnikov products when diphenylphosphine is reacted with styrenes or internal alkynes at room temperature. This journal is

Full text of DOI:10.1039/d0cc05792d

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Hydrophosphination using [GeCl{N(SiMe3)2}3] as a pre-catalyst
A. N. Barrett, H. J. Sanderson, M. F. Mahon and R. L. Webster*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Transformations catalyzed by germanium are scarce, with
examples mainly limited to widely catalyzed processes such as
polymerisation of lactide and hydroboration of carbonyls. Reported
is the first example of hydrophospination using a germanium pre-
a) Ge(IV) pre-catalysts for polymerization of rac-lactide5-7
O
n
O
·(
THF)2
O
]
[Ge{N(SiMe)2}4
tBu
O
O
Ge
O
O
O
O
catalyst,
yielding
anti-Markovnikov
products
when
Me
n
N
Ge
O
diphenylphosphine is reacted with styrenes or internal alkynes at
room temperature.
O
O
N(SiMe3)2
N(SiMe3)2
tBu
Me
Me
iPr
O
N
O
Me
Over the last two decades, the use of main group compounds
as catalysts for organic reactions has seen a vast increase in
focus.1-3 Amongst this work, significant progress has been made
in the field of germanium catalysis. A fundamental challenge lies
in unlocking the catalytic potential of this group 14 element,
which is far behind tin in terms of its exploitation in catalysis,
and such work may prove instrumental to furthering
understanding and harbouring new discoveries in main group
catalysis. It is not surprising, based on the ability of Sn(IV)
complexes to act as polymerization catalysts,4 that initial
disclosures in the literature demonstrated the ability of Ge(IV)
complexes to act as initiators for the ring-opening
polymerization of lactide (Scheme 1a).5-7 Subsequently, there
have been numerous reports on low-coordinate Ge(II) species
acting as efficient pre-catalysts for the hydroboration of
carbonyls (Scheme 1b).8-11 This reactivity has also been
extended to the catalytic cyanosilylation of aldehydes.12, 13
Despite this progress, lactide polymerization and carbonyl
hydrofunctionalization are commonly reported catalytic
transformations,14, 15 therefore a desire remains to uncover use
for germanium (both Ge(II) and Ge(IV)) in the catalysis of a
wider range of transformations.16 It is clear from the current
literature that, in the presence of at least one labile co-ligand,
Ge(IV) complexes can catalyze non-redox, σ-bond metathesis type
transformations such as lactide polymerization. This indicates that
opportunities exist for Ge(IV) catalysts, particularly in
hydrofunctionalization reactions with pertinent substrates.
O Ge
Me
Me
tBu
O
N
Me
tBu
Me
b) Ge(II) pre-catalysts for hydroboration of aldehydes and ketones8-11
H
Ph
Ph
iPr
Ph
Si
N
3
PPh2
iPr
iPr
Ge
H
N
Ge
N
Ge
iPr
iPr
iPr
iPr
Ph Ph
aldehydes
iPr
aldehydes
aldehydes
& ketones
Cl
c) This work
Ge
N(SiMe3)2
N(SiMe3)2
N
Si)2
(Me3
PPh2
Ar
R
+
(5 mol%)
Ph2PH
or
or
Ar
R
PPh2
RT, 18 h
Ar
Simple, well-defined Ge(IV) pre-catalyst
Room temperature reactions
Internal alkynes & styrenes



R
19 examples
Scheme 1 Examples of germanium catalysis reported in the literature to-date.
The hydrophosphination (HP) of unsaturated hydrocarbons is
an atom economical process of forming P–C bonds.17 Using a
main group element to transform another is an area of growing
interest, amongst which examples of main group catalyzed HP
have emerged.18 Reaction mechanisms for these
transformations involving main group, and lanthanide19-23
,
complexes are often postulated to proceed via σ-bond
metathesis type steps at the catalyst center,24 leading us form
Department of Chemistry, University of Bath, Claverton Down, Bath, UK. BA2 7AY
Electronic Supplementary Information (ESI) available: Analysis data. CCDC xxxxx for
the crystal structure of 1. See DOI: 10.1039/x0xx00000x
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the hypothesis that a Ge catalyst may have success in this area.
To the best of our knowledge, no examples of Ge-catalyzed HP
have been reported. However, the heavier group 14 element,
Sn, has been shown to be active catalytically in the HP of alkenes
and alkynes by Waterman and co-workers, who, in 2014,
reported a modest scope of HP reactions using PhPH2.25
Subsequent studies on the same Sn(IV) system using Ph2PH in
2016 expanded the scope to further substrates.26 However,
these transformations required an atmosphere of H2 due to the
catalyst’s propensity to undergo homodehydrocoupling of the
phosphine substrate over the desired HP reaction. This, along
with the elevated temperatures necessary (65 °C) and high
catalyst loading (10 mol%), makes the reaction only marginally
more competitive than the analogous catalyst-free, thermal
process.27, 28 We were therefore keen to investigate if a more
active catalyst could be found within group 14 in the form of a
Ge(IV) species. Rather than start our studies with an over-
engineered catalyst structure, we looked to initiate our studies
with an operationally simple motif; understand the catalytic
process and thus be in a strong position to build in ligand design
at a later stage, based on understanding. We herein present the
first HP of alkenes using a structurally simple Ge(IV) pre-catalyst
under mild conditions (Scheme 1c).
View Article Online
DOI: 10.1039/D0CC05792D
(10 eq.)
NaN(SiMe
THF
Reflux, 96 h
3
)
2
GeCl4
1, 63%
Scheme 2 Synthesis and single crystal X-ray structure of 1 displaying the inherent C3v
symmetry of the molecule. Ellipsoids are represented at 30% probability and hydrogen
atoms have been omitted for clarity. Symmetry operations: i 1 + y – x, 1 – x, z; ii 1 – y, x –
y, z. Bond distance (Å) and angles (o): Ge1-Cl1, 2.2154(11); Ge1-N1, 1.8542(16); Cl-Ge-N1
102.12(6); N1-Ge-N1’ 115.71(4).
Table 1 Reaction optimization for the hydrophosphination of styrene using pre-catalyst
1.a
PPh2
1
+ HPPh2
D6
C6
RT, 18 h
2a
Reaction of GeCl4 with 10 equiv. of Na(HMDS) yields
Entry
Catalyst Loading
Solvent
Spectroscopic
/ mol%
Yieldb / %
[GeCl{N(SiMe3)2}3],
1, in good yield (63%, Scheme 2), the
structure of which was confirmed by X-ray diffraction.29 In an
analogous reaction, Thomas and co-workers propose the
formation of tetra(amido) THF adduct [Ge(HMDS)4(THF)2]:7 in
1
2
3
4c
5c
5
2.5
1
5
5
C6D6
C6D6
C6D6
THF
91
81
38
99
94
1
our synthesis H DOSY analysis indicates lack of persistent THF
C6D6
coordination in solution.29
aConditions: styrene (0.5 mmol, 57 μL), Ph2PH (0.5 mmol, 87 μL), 1 (5 mol%, 0.025
mmol, 14.7 mg), C6D6 (0.6 mL). bSpectroscopic yield calculated via integration
against 1,3,5-trimethoxybenzene internal standard. c0.6 mmol styrene.
Pleasingly,
1 is an efficient pre-catalyst for the HP of styrene
with diphenylphosphine (Ph2PH) and provides 91% conversion
to the anti-Markovnikov HP product 2a after 18 hours at RT with
a catalyst loading of 5 mol%, using C6D6 as solvent (Table 1,
Entry 1). Only trace quantities of homodehydrocoupling by-
product P2Ph4 is formed, therefore any concerns over a
competing reaction of this type prove insignificant.
Markovnikov product is never observed. Catalyst loadings of 2.5
mol% and 1 mol% result in the reduction of spectroscopic yield
to 81% and 38% respectively (Entries 2 and 3). Notably, no
reaction is observed when GeCl4 is used as the catalyst. Change
of solvent to THF gives similar conversion compared to benzene
(Entry 4). Other solvents (MeCN (vide infra), DCM, CDCl3) show
Naphthyl substituted phosphine 2n is formed in excellent yield
(Entry 14) and 1,1-diphenylethylene performs comparably well,
albeit under slightly more forcing conditions (50 °C, 24 h, Entry
15). Phenyl acetylene does not react, but internal alkynes
(Entries 16 to 19) react at RT: conditions that are highly
competitive compared to the leading literature, for example
Schmidt’s RT, La-catalyzed process in pyridine solvent30 and
Cui’s Ca- and Y-mediated chemistry that uses temperatures
ranging from RT-75 °C.31 Unfortunately, reactions of 2- and 4-
vinylpyridine and acrylonitrile with
1 all result in the formation
reactivity with
1 and are therefore not suitable for the HP
of unidentified polymers irrespective of order of addition of
Ph2PH. Methyl acrylate, butyl acrylate, cis- and trans-stilbene,
along with β-methyl styrene do not react. HP reactions using
unactivated alkenes (e.g. cyclohexene, allyl benzene) results in
reaction.29 A modest excess of styrene (1.2 equiv.) gives 94%
conversion in an inexpensive NMR solvent (Entry 5).
With the optimized conditions in hand (Table 1, Entry 5) we then
set out to investigate the substrate scope for the reaction. The
reaction proceeds with a wide range of styrenes (Table 2).
Biphenyl-substituted phosphine 2b is formed in excellent yield
no conversion while no stoichiometric reaction of
1 and HPCy2,
or catalytic HP of styrene using
1
and HPCy2, is observed.29
However, scale-up synthesis of 2a is successful: using 5 mmol
Ph2PH (0.93 g/ 0.87 mL), 6 mmol styrene, 2.5 mol% Ge, at RT
gives 94% after 24 h. Catalyst robustness was tested through an
iterative addition protocol: 0.5 mmol Ph2PH, 0.6 mmol styrene,
5 mol% 1, were reacted at RT in C6D6 until complete conversion
of Ph2PH was observed. Further equivalents of Ph2PH and
styrene were added sequentially until six additions had been
(Entry 2). p-Me-, p-MeO- and p-CF3-substituted phosphines (2c
-
2e) are formed in reduced conversions (Entries 3 to 5). Halide
substituents are well tolerated, with phosphine products 2f to
2h formed in good to moderate conversion (Entries 6 to 8). m-
and o-substituted phosphines (2i to 2m) are also formed in good
yield (Entries 9 to 13).
2 | J. Name., 2012, 00, 1-3
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achieved (3 mmol Ph2PH added) giving 87% spectroscopic Table 2 Substrate scope for HP using pre-catalyst 1a
View Article Online
yield.29
DOI: 10.1039/D0CC05792D
Alkene
Product
Spec.
Entry
Yield
(%)b
Focus was then turned to investigating the mechanism of the
reaction. A kinetic study of the reaction to form 2a was
undertaken via Variable Time Normalization Analysis,32
implicates the reaction is approximately first order with respect
to
1 and Ph2PH and a more complex relationship with styrene
(≤1, ≥0.5), in-line with a complex reaction where multiple
species can propagate from one Ge-center.29 There is no
reaction when a stoichiometric quantity of styrene is added to
1
2
3
4
5
6
7
8
R= H
4-Ph
4-Me
4-OMe
4-CF3
4-F
2a
2b
2c
2d
2e
2f
2g
2h
2i
94
98
94c
56c
83c
76c
94
64
92
82
89
89
84
1. Conversely, reaction of Ph2PH with 1 results in the formation
of a bright yellow color, which is also observed during catalysis.
1H DOSY analysis of this species shows fast diffusing HN(SiMe3)2
peaks and slower diffusing aryl resonances, consistent with
protonation of Ge-bound N(SiMe3)2 by HPPh2.29 Monitoring
4-Cl
4-Br
9
3-Me
3-Br
2-Me
2-OMe
2-Br
catalysis (15 mol%
1
) as a function of time by 31P NMR
10
11
12
13
2j
2k
2l
spectroscopy shows the Ph2PH doublet decreasing in intensity
as product 2a, and a singlet at −36 ppm, increase. The signal at
−36 ppm reaches a maximum when all Ph2PH is consumed (~
60% 2a formed), and then shows a slow decrease of this signal
into 2a for the remainder of the reaction. There is also a very
small singlet at −27 ppm that decreases in intensity slowly over
the course of the reaction. Our data is consistent with bis- and
tris-phosphido complexes (−27 and −36 ppm respectively) due
to the loss of P–H, the fact that integration of the −36 ppm signal
is consistent with 3 equiv. Ph2PH being sequestered from
solution, alongside 31P NMR signals being more shielded than
[GeCl{N(SiMe3)2}2(PPh2)] (not observed).33 Noteworthy is that a
stabilized radical, (•Ge{N(SiMe3)2}3), from photochemical
disproportionation of [Ge{N(SiMe3)2}2] has been reported.
However, we have conclusively ruled-out radicals from this
reaction.29
2m
PPh2
14
15
16
2n 90
2o 70c
84d
2p 3.9 : 1,
E : Z
99c
2q 1 : 1,
E : Z
17
18
81e
2r 1.3 : 1,
Combining these experimental observations, we propose that
the tris-phosphido species (3tris) is the principal active catalyst
for this transformation, and therefore postulate a mechanism
E : Z
PPh2
CF3
41f
2s 2 : 1,
19
based upon this (Scheme 3). After the formation of on-cycle 3tris
,
E : Z
styrene may then insert into the Ge–P bond in a regioselective
F3C
CF3
F3C
manner to form an alkyl intermediate (4mono tris). We have
/
bis/
aConditions: styrene (0.6 mmol), Ph2PH (0.5 mmol, 87 μL), 1 (5 mol%, 0.025 mmol,
14.7 mg), C6D6 (0.6 mL), 18 h, RT. bSpectroscopic yield calculated via integration of
product peak against a trimethoxybenzene internal standard. c50 °C, 24 h. dRT, 96
h. eRT, 48 h. Spec yield and E : Z calculated via 31P inverse gated NMR spectroscopy
against a P(OEt)3 internal standard. fRT, 72 h. Spec yield and E : Z ratio calculated
via 31P inverse gated NMR spectroscopy against a P(OEt)3 internal standard.
discounted this step occurring via attack of the β-position of
styrene by the P-atom as no telomerization side products are
observed.24 We propose 1,2-insertion to take place in a non-
stepwise manner in accordance with the reaction being non-
integer order with respect to styrene. From short-lived alkyl
intermediates
approximate integer relationship in Ph2PH. This step liberates
the product and reforms active species
During solvent screening, we also observed that
catalytically induce the trimerization of MeCN to 2,6-
dimethylpyrimidin-4-amine (Scheme 4). Base-mediated
cascade reactions34, 35 and an iridium hydride catalyst36 have
previously been used to obtain the same product under more
forcing conditions (e.g. Murahashi’s Ir-catalyzed method uses
IrH5(PiPr3)2 at 140 °C for 12 h giving TON = 12). This reactivity
represents a new avenue of reactions this germanium species is
capable of catalyzing and will be explored in due course.
4, stepwise protonolysis accounts for the
3mono
not observed)
GeCl{N(SiMe3)2}2(PPh2)
GeCl{N(SiMe3)2}(PPh2)2
(
R
3bis
3.
(minor)
GeCl(PPh2 2
)
R
Ph2P
Ph2P
4mono
GeClPPh2
4bis
1
could
3tris
GeCl(PPh2 3
)
slow
R
R
Ph2P
2
GeCl
n HN(SiMe3 2
)
Ph2P
PhH
i) Ph2
4tris
R
3
then
n Ph
2PH
ii) HN(SiMe3 2
)
fast
1
Scheme 3 Proposed method by which
1
catalyzes the HP of styrenes. 3bis is only
J. Name., 2013, 00, 1-3 | 3
observed in trace quantities.
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In summary, we have disclosed a rare example of a Ge(IV)-
catalyzed (non-polymerization) reaction. An operationally
simple Ge(IV) tris(amido) complex has shown excellent ability as
a pre-catalyst for the hydrophosphination of a range of styrenes
and internal alkynes. The transformations proceed at room
temperature over a time period competitive with transition
metal catalyzed analogues. Preliminary mechanistic studies
implicate the formation of a Ge-(tris)phosphido complex as the
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