Radical-initiated P,P-metathesis reactions of diphosphanes: Evidence from experimental and computational studies

Article abstract of DOI:10.1039/d1dt01013a

By combining the diphosphanes Ar2P-PAr2, where Ar = C6H5, 4-C6H4Me, 4-C6H4OMe, 3,5-C6H3(CF3)2, it has been shown that P,P-metathesis generally occurs rapidly under ambient conditions. DFT calculations have shown that the stability of unsymmetrical diphosphanes Z2P-PZ′2 is a function of the difference between the Z and Z′ substituents in terms of size and electronegativity. Of the mechanisms that were calculated for the P,P-metathesis, the most likely was considered to be one involving Ar2P radicals. The observations that photolysis increases the rate of the P,P-metatheses and TEMPO inhibits it, are consistent with a radical chain process. The P,P-metathesis reactions that involve (o-Tol)2P-P(o-Tol)2 are anomalously slow and, in the absence of photolysis, were only observed to take place in CHCl3 and CH2Cl2. The role of the chlorinated solvent is ascribed to the formation of Ar2PCl which catalyses the P,P-metathesis. The slow kinetics observed with (o-Tol)2P-P(o-Tol)2 is tentatively attributed to the o-CH3 groups quenching the (o-Tol)2P radicals or inhibiting the metathesis reaction sterically.

Full text of DOI:10.1039/d1dt01013a

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Radical-initiated P,P-metathesis reactions of
diphosphanes: evidence from experimental and
Cite this: DOI: 10.1039/d1dt01013a
computational studies†
a
b
c
Callum Branfoot,
Paul G. Pringle
Tom A. Young,
Duncan F. Wass
and
a
*
By combining the diphosphanes Ar
has been shown that P,P-metathesis generally occurs rapidly under ambient conditions. DFT calculations
have shown that the stability of unsymmetrical diphosphanes Z P–PZ’ is a function of the difference
2 2 6 5 6 4 6 4 6 3 3 2
P–PAr , where Ar = C H , 4-C H Me, 4-C H OMe, 3,5-C H (CF ) , it
2
2
between the Z and Z’ substituents in terms of size and electronegativity. Of the mechanisms that were
•
calculated for the P,P-metathesis, the most likely was considered to be one involving Ar
2
P radicals. The
observations that photolysis increases the rate of the P,P-metatheses and TEMPO inhibits it, are consistent
with a radical chain process. The P,P-metathesis reactions that involve (o-Tol) P–P(o-Tol) are anoma-
lously slow and, in the absence of photolysis, were only observed to take place in CHCl and CH Cl . The
2
2
Received 27th March 2021,
Accepted 27th April 2021
3
2
2
2
role of the chlorinated solvent is ascribed to the formation of Ar PCl which catalyses the P,P-metathesis.
DOI: 10.1039/d1dt01013a
The slow kinetics observed with (o-Tol)
2
P–P(o-Tol)
2
is tentatively attributed to the o-CH groups
3
•
rsc.li/dalton
quenching the (o-Tol)
2
P radicals or inhibiting the metathesis reaction sterically.
2
4–31
thesis of tertiary phosphines.
Several methods to prepare
Introduction
diphosphanes are available, including: Wurtz-type reductive
32–34
Dynamic and reversible chemical reactions have garnered coupling of chlorophosphines (with Li, Na, K, Mg and Hg);
1
26,27
increasing attention in medicine and biology, supramolecular salt metathesis between chlorophosphine and LiPR
2
;
dehy-
2
3–11
35–39
40
chemistry and materials science,
rapidly growing field of dynamic covalent chemistry (DCC).
In DCC, disulfides play a leading role, by virtue of the weakness divided into two categories: reactions where the P–P bond
and they are key to the drocoupling of HPR ;
chlorosilane elimination; P–N/P–P
2
1,12,13
41
bond metathesis. The chemistry of diphosphanes can be
42
of the S–S bond and their ready availability via chemical syn- remains intact and reactions where the P–P bonds are cleaved.
12,14–16
thesis and their presence in natural products.
decade, several other main group element–element bonds described in this article and include additions of X
In the last Reactions involving P–P cleavage are pertinent to the chemistry
to the R P–
) have been investigated for PR2 to give monophos species R PX (e.g. X = H, Cl) and
2
2
17,18
19
9,20,21
(O–O,
N–N and Se–Se
2
2 2
their potential in dynamic chemistry with a view to their appli- additions of R P–PR to CvC or CuC bonds (diphosphination)
cation in the field of polymeric materials. A notable omission to give diphos species (Scheme 1). It has also been shown that
from this group is diphosphanes (P R ) which contain a P–P diphosphanes containing P–N bonds can add to CvS or CvO
2
4
−
1 22
43,44
bond whose strength (ca. 52 kcal mol
)
is intermediate bonds.
Mechanisms have been proposed for some of the
−
1 23
−1 22
43,45
between S–S (ca. 62 kcal mol
)
and Se–Se (ca. 40 kcal mol )
P–P cleavage reactions including radical chain processes.
and therefore diphosphanes should be candidates for DCC.
Diphosphane metathesis has not been systematically inves-
3
1
Diphosphanes have received considerable attention as tigated, but an early P NMR study by Harris et al. showed
targets themselves and also as undesired byproducts in the syn- that rapid metathesis occurred between Me P–PMe and
Gilheany
2
4
2
6,47
(
F C) P–P(CF ) in CH Cl to form Me P–P(CF ) .
3 2 3 2 2 2 2 3 2
4
8
et al. have shown that the calculated high energy barrier to
P,P-metathesis via a concerted mechanism is not compatible
with the observed rapid kinetics for the Me P–P(CF ) system
a
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK.
E-mail: paul.pringle@bristol.ac.uk
2
3 2
b
Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA, UK
and suggested that pathways involving impurities in the diphos-
c
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff CF10
26
phanes may facilitate the P,P-metathesis process. Grubba et al.
3AT, UK
have succeeded in the preparation of a variety of pure, unsymme-
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1dt01013a
trical diphosphanes containing P–C(alkyl) and P–N bonds. By
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42
Scheme 1 Some reactions involving P–P cleavage of tetraphenyldiphosphane.
Scheme 2
3
1
1
employing low temperatures, they avoided unwanted P,P-meta- the P{ H} NMR spectrum obtained after mixing diphos-
theses that would lead to symmetrical diphosphanes contami- phanes B and C (entry 4, Table 1) and shows the singlets for
nating the products. Moreover, they reported that R PCl or R PLi the reactants along with an AB pattern for the product BC; the
2
2
2
2
can catalyse diphosphane metathesis. We have reported that spectra for the other diphosphane combinations are given in
diphosphane metathesis was a side reaction in the attempted the ESI.†
2
7
synthesis of some unsymmetrical diphosphanes.
The pure heterodiphosphane AE was prepared in order to
and E
In this study, we have investigated the P,P-metathesis reac- establish the P,P-metathesis equilibrium with A
tions of tetra-aryldiphosphanes (Scheme 2) using P{ H} NMR (Scheme 2) from the opposite direction. Grubba et al. have
spectroscopy and computational methods, with a view to explor- prepared a range of heterodiphosphanes (Z P–PZ′ ) by the
2
2
3
1
1
26
2
2
2 2
ing the dynamic nature of these P–P bonds. This has provided addition of Z PLi to Z′ PCl at low temperatures. However, our
some new insights into the diphosphane metathesis reaction.
attempts to extend this route to the preparation of AE were
unsuccessful, even with the reaction mixture maintained at
2 2
−78 °C throughout; instead, an equilibrium mixture of A , E
and AE was obtained. This contrasting behaviour may be due
Results and discussion
Thermodynamics of diphosphane metathesis
to the lack of thermodynamic stability of heterodiphosphane
26
AE compared with the Grubba heterodiphosphanes (see
Experimental equilibrium studies. The tetra-aryldiphos- below). The desired AE was prepared in >96% purity (after
phanes A –E (Chart 1) used in this study, were chosen in recrystallisation from methanol) via the protection/de-
2
2
order to sample a range of steric and electronic effects. All protection method (Scheme 3) that we previously developed
2
7,49
have been previously reported and were prepared by modified for the synthesis of heterodiphosphanes.
literature procedures for the reductive coupling of the corres- AE in CDCl do indeed equilibrate to a mixture of A
ponding chlorophosphines by Mg in THF (see ESI† for AE (see below for further details).
Solutions of pure
, E and
3
2
2
2
6,34
details).
In each of the diphosphane samples, residual
The diphosphane equilibrium constants for the P,P-meta-
chlorophosphine was not detected by P NMR spectroscopy theses (Scheme 2) fall in the range 3–30 (see Table 1), which
3
1
−
1
and was therefore estimated to be less than 0.05% (see ESI†).
When two of the diphosphanes A –E were mixed in CDCl
at ambient temperature, a reaction ensued to give an equili- close in energy and insensitive to the steric and electronic
brium mixture of homodiphosphanes X and Y
, and the het- effects of the aryl substituents. The equilibria that do not
erodiphosphane XY (Scheme 2). The data for the diphosphane involve E were established rapidly (see Table 1) despite the
metathesis reactions are given in Table 1.
dilution (12.5 mM) which is a useful property for the potential
corresponds to ΔG values between −0.5 and −2 kcal mol
,
demonstrating that the diphosphanes in this study are very
2
2
3
2
2
2
3
1
1
1
The P{ H} NMR data for the XY species (Table 1) show application of diphosphanes in dynamic chemistry. The
that the chemical shifts for the two signals are close to the anomalously slow P,P-metatheses involving E
2
are discussed
1
respective X and Y with J ≃160 Hz. For example, Fig. 1 is later.
2
2
XY
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Chart 1
Table 1 Data for the P,P-metathesis equilibria shown in Scheme 2
a
31
Entry
X
2
Y
2
K
Time /min
P NMR data for XY
b
1
A
A
A
B
B
2
2
2
B
C
D
C
D
D
2
4
5
30
4
15
3
4
4
4
30
30
20
90
15
20
20
700
1000
2800
4000
−14.9 −16.4
−15.9 −18.4
−15.9 −8.5
−16.8 −18.8
−11.7 −15.7
−10.5 −15.4
−16.6 −35.3
−19.1 −36.2
−19.0 −36.9
−17.9 −29.5
J
J
J
J
J
J
J
J
J
J
PP = 162 Hz
c
2
2
PP = 159 Hz
PP = 177 Hz
PP = 156 Hz
PP = 165 Hz
PP = 173 Hz
PP = 152 Hz
PP = 149 Hz
PP = 148 Hz
PP = 156 Hz
d
e
3
2
d
4
2
2
b
5
2
2
2
b
6
C
2
c
7
A
2
E
E
E
E
2
c
8
B
C
D
2
2
c
9
2
2
2
c
10
2
a
31
Time by which equilibrium had been established according to
P
b
c
d
e
NMR spectroscopy. In THF. In CDCl
see details in ESI†).
3
.
6
In C D
6
.
Estimated value
(
3
1
1
Fig. 1 The
P{ H} NMR spectrum of the P,P-metathesis reaction
P–P(p-Tol) (B ) and (p-Anisyl) P–P(p-Anisyl) (C ).
between (p-Tol)
2
2
2
2
2
2
Computational equilibrium studies. To understand better
the factors that control the stability of heterodiphosphanes
with respect to the corresponding homodiphosphanes, DFT
calculations were carried out to determine the thermo-
dynamics of the hypothetical equilibria shown in Scheme 4 in
which the range of P-substituents Z and Z′ is greatly extended
from the substituted aryls in Scheme 2. The values for ΔG are
collected in Table 2, and for some of the combinations
(
3
Table 2, entries 1–3 and 7–10), the calculated |ΔG| is less than
kcal mol− and therefore, these particular P,P-metathesis
1
equilibria are predicted to be finely balanced. The calculated
Scheme 3
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The calculated ΔG values for some of the Grubba hetero-
26
diphosphanes (Table 2, entries 16 and 18) suggest that these
should have high thermodynamic stability with respect to the
corresponding
homodiphosphanes.
By
contrast,
the
heterodiphosphine AE, as reported above, is calculated to be of
similar energy to the homodiphosphanes A and E (Table 2,
2 2
Scheme 4
entry 22).
Table 2 Calculated ΔG values for the diphosphane metathesis equili-
bria Scheme 4
Computational mechanistic studies of diphosphane
metathesis
ΔG/kcal mol−
1
Entry
Z
Z′
The mechanism of the diphosphane metathesis shown in
Scheme 5 has been previously investigated computationally by
1
2
3
4
5
6
7
8
9
Me
Me
Me
Me
Me
Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
NH
OH
OMe
F
CF
H
Me
NH
NMe
OMe
F
CF
H
Me
Ph
−0.5
−2.9
−2.9
−3.7
−6.6
−10.5
0.8
2
4
8
Gilheany et al. The lowest energy route that they could find
involved the ionic intermediate shown in Scheme 5 but the cal-
−1
culated barrier of 36.5 kcal mol was too high to explain the
3
rapidity of the P,P-metathesis that is observed experimentally.
We have investigated the mechanism of diphosphane meta-
thesis computationally using the degenerate exchange reac-
−0.7
1.3
2
10
11
12
13
14
15
16
17
18
19
20
21
22
2
−0.8
−4.7
−6.1
−4.4
−13.4
−3.3
−4.0
−6.8
−6.8
−11.2
−15.4
−3.4
1.2
tions of Me P–PMe (Scheme 6). Tetramethyldiphosphane was
2
2
chosen in order to limit the number of rotamers and thus the
conformational noise in the calculations. It was hoped to dis-
tinguish between the three plausible pathways (i–iii) shown in
Scheme 7: (i) a [2 + 2] concerted process involving a 4-mem-
3
t
Bu
t
Bu
t
Bu
t
Bu
NH
2
t
Bu
NMe
OMe
F
CF
Ph
2
t
Bu
t
Bu
t
Bu
3
o-Tol
ΔG of 1.2 kcal mol− (Table 2, entry 22) for A /E is remarkably
1
2
2
−
1
close to the experimental value of −0.8 kcal mol obtained
Scheme 5 Gilheany mechanism for a P,P-heterometathesis.
from the experimentally determined value of K = 4 in CDCl
3
solution (Table 1, entry 7).
Two gross trends emerge from the data in Table 2: (1) a bias
in favour of the XY species is calculated when there is a large
electronegativity difference between the X and Y fragments;
Scheme 6
thus Me
2
P–P(CF
3
)
2
is predicted (and experimentally
5
0
observed ) to be highly favoured (Table 2, entry 6); (2) a strong
bias in favour of the XY species is calculated when there is a
large difference in steric bulk between the X and Y fragments;
t
thus Bu
2
P–PH
2
is predicted to be highly favoured (Table 2,
entry 13). The largest calculated ΔG in favour of the XY species
t
2 2
is for Bu P–PF (Table 2, entry 20), at an apparent apotheosis
of both trends. The observed trends in the calculated relative
stabilities of the XY species in Scheme 4 can be rationalised as
follows: (a) electronegative Z substituents (such as F) will lead
to repulsive electrostatic interactions between the δ+ charges
generated on the adjacent P atoms in diphosphane X ; (b)
2
t
bulky Z′ substituents (such as Bu) will lead to steric conges-
2
tion in diphosphane Y ; (c) in XY, both of the P–P bond-desta-
bilising effects identified in (a) and (b) will be reduced and, in
some cases, a destabilising, repulsive, electrostatic (δ+/δ+)
interaction is replaced by a stabilising, attractive, electrostatic
(
δ+/δ−) interaction.
Scheme 7
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Fig. 2 (a) Reaction free energy profile for the P,P-homometathesis of Me
2
P–PMe
2
. Calculations performed at the PBE0-D3BJ/def2-TZVPP/PBE0-
D3BJ/def2-SVP level of theory. (b) Calculated structures of ts1 and int1. Note: ts1 and ts2 are the same; key distances given in Å. (c) Resonance
forms of int1.
bered intermediate or transition state; (ii) nucleophilic attack order of 52 kcal mol− . However, DFT calculations for Ph
1
P–
2
by one diphosphane on another to give an ionic intermediate PPh
2
(A
2
) suggest that excitation from S
0
→ S
1
or S
0
→ S
2
(
similar to the one suggested in Scheme 5); (iii) a [2 + 1] would result in rapid homolysis of the P–P bond, yielding
•
radical chain process.
phosphanyl radicals (Ar
2
P ) via intersystem crossing from S
) of A ; the T
1
or
The overall reaction profile calculated for the thermal
S
2
to the first excited triplet state (T
1
2
1
state is dis-
rearrangement of Me P–PMe (shown in Fig. 2(a) proceeds via sociative, and would lead to spontaneous radical formation.
2
2
•
intermediate int1 depicted in its lowest energy conformation in From the Ar
2
P , the P,P-metathesis barrier generating XY
Fig. 2(b). The structure of int1 can be viewed in terms of the species is only ∼5 kcal mol− , such that once radicals are gen-
1
two resonance forms int1A and int1B shown in Fig. 2(c) erated an equilibrium would be expected to be established
(
further details are given in the ESI†). The ionic form int1A is rapidly (see ESI† for details).
•
analogous to the intermediate suggested for the P,P-heterometa- The DFT results suggested that Ar
2
P radicals (generated
thesis shown in Scheme 5 and the covalent int1B contains an photolytically) were viable intermediates in the P,P-metathesis
unusual central P(V) moiety. The profile shown in Fig. 2 corres- reactions and so this hypothesis was pursued experimentally.
ponds to pathway (ii) of Scheme 7, and notably, attempts to
locate a synchronous, σ-bond metathesis pathway (corres-
ponding to pathway (i) in Scheme 7) were unsuccessful; no ima-
ginary normal modes corresponding to the 2 bond-breaking/
Qualitative experimental kinetic studies of diphosphane
metathesis
forming vibrations are present along the relaxed, symmetric P
The aryl diphosphane metathesis reactions with A2–D2
ring expansion surface. The energy of int1 of 44.1 kcal mol− for occurred readily (typically <1 h) under ambient conditions in
the PMe species is lower for the analogous PPh intermediate common organic solvents (C D , THF, PhMe, PhCl, CDCl and
ΔG = 38.1 kcal mol−1, see ESI†) but this is calculated to be not CD Cl ). By contrast, when the diphosphane E , containing
4
1
2
2
6
6
3
(
2
2
2
lowered by solvation with CH
high barrier is incompatible with the rapidity of the Ar
metathesis reactions that is observed experimentally.
2 2
Cl . Thus, the calculated very o-tolyl substituents, was one of the reaction partners, P,P-meta-
2
P–PAr
2
thesis was initially only observed to take place in CDCl (typi-
3
cally over 6–12 h) and CD Cl (typically over more than 24 h);
2
2
The radical chain pathway (iii) in Scheme 7 requires homo- the anomalous behaviour observed for the P,P-metatheses
lysis of a P–P bond. Thermolysis is not viable under ambient involving E2 will be discussed after consideration of the
conditions because the calculated P–P bond enthalpy is of the P,P-metatheses involving A –D .
2
2
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2 2
When cold (−78 °C) THF solutions of B and C were mixed
and the P,P-metathesis reaction monitored by low-temperature
3
1
P NMR spectroscopy, it was found that only traces of BC were
detected at −80 °C after 1 h. By raising the temperature in
increments of 20 °C, it was established that the P,P-metathesis
at −20 °C progressed at a convenient rate to monitor the
approach to P,P-metathesis equilibrium over a period of 2 h by
Scheme 8
3
1
P NMR spectroscopy (see Fig. 3). Although a detailed kinetic
study was not attempted, the data do not appear to fit a reac-
tion order of 1, 2 or 0.5 (see ESI†).
equilibration to the same degree took ca. 150 min. This is con-
sistent with diphosphane photodissociation being a signifi-
cant component of the mechanism of P,P-metathesis.
Evidence for radicals in the general P,P-metathesis reactions
The UV–vis spectra of A , C and E are shown in Fig. 4.
2
2
2
Each spectrum has a major absorbance between 225 and
50 nm, and a shoulder at ca. 270 nm. TD-DFT calculations for
were carried out using a range of functionals (CAM-B3LYP,
In light of the computational results above suggesting that rad-
icals may be involved in the mechanism of P,P-metathesis,
experiments were carried out to determine if there was evi-
dence of their presence. Two pieces of experimental evidence
support the involvement of radicals in the P,P-metathesis
process.
2
A
2
PBE0 and M06-2X) and good agreement with experiment was
found with CAM-B3LYP. These calculations suggest that the
major peak in each spectrum corresponds to coincident aro-
matic π → π* and S → S (see Fig. 5(b)) transitions and the
0
2
(
1) When A
2
and C
2
were mixed in the presence of the
2,2,6,6-tetramethyl-1-piperidinyloxy
0 1
shoulder peak corresponds to S → S (see Fig. 5(a)). The tran-
radical scavengers
sition electron density plots (Fig. 5) indicate that both tran-
sitions result in a significant delocalisation of electron density
from the P lone pairs. The first triplet state, accessible from
(
TEMPO) or tri(t-butyl)phenol (TTBP), the progress of the P,P-
metathesis reactions was greatly inhibited. Thus, a mixture of
A and C proceeded smoothly to the metathesis equilibrium
2
2
over a period of 2 h in CDCl , at −20 °C (with 35% conversion
3
2 2
at 1 h); by contrast, when the same A /C mixture was prepared
in the presence of 5 equiv. of TEMPO, there was no formation
of AC detected after 1 h. Moreover, when the reaction mixture
containing TEMPO was allowed to warm to ambient tempera-
ture, <10% of AC was detected even after 16 h. Instead, in the
3
1
presence of TEMPO, two singlet P resonances at +30.5 and
31.5 ppm were observed (ca. 10% of the total signal intensity),
+
consistent with the formation of Ar P(O)-TEMPO (Ar = Ph,
2
p-anisyl) adducts (Scheme 8) which would be expected to be
•
formed by the interception of the phosphanyl radicals (Ar
2
P )
5
1
by TEMPO.
2) When A and B were mixed in a standard NMR tube in
(
2
2
THF at ambient temperatures, equilibration was essentially
complete within ca. 15 min. However, when the same A /B
2
2
mixture in THF was prepared in an amberised NMR tube, to
minimise the transmittance of UV and visible radiation, the Fig. 4 UV–vis spectra of A
2
2 2 2 2
, C and E in CH Cl (20 μM).
31
1
2 2
Fig. 3 (a) Plot of heterodiphosphane BC formation as a function of time over 2 h; (b) P{ H} NMR spectra of the reaction between B and C at
−
20 °C in THF, shown in 10 min increments over the course of 1 h (with inverse-gated decoupling). See Fig. 1 for the peak assignments.
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Fig. 5 Transition electron density difference plots with the CAM-B3LYP/def2-TZVPP//PBE0-D3BJ/def2-SVP level of theory. (a) S
0
→ S
. Red and blue correspond to areas of depleted and excess electron density, respectively. Isosurface plotted at a contour of
1
and (b) S
.003 au.
0
→ S
2
0
either S
1
or S
2
via intersystem crossing, is dissociative, leading would also be initiated by photodissociation of Ar
2 2
P–PAr′ . The
to spontaneous formation of phosphinyl radicals. There are P,P-metathesis reactions generally proceed with a high degree of
small differences in the three spectra shown in Fig. 4. As chemoselectivity but traces of Ar PH were often evident after
2
expected, the p-OMe groups in C
in the π → π* band and there is a long ‘tail’ that extends abstraction of an H radical, presumably from the solvent.
towards the visible region, which explains why C is pale yellow. The radical mechanism for diphosphane metathesis in
2
produce a bathochromic shift several days of reaction which would arise from termination via
2
The evidence in support of radicals being present led to the Scheme 9 is reminiscent of the radical mechanism for di-
radical chain mechanism proposal in Scheme 9 where homoly- sulfide metathesis (Scheme 10) arrived at from calculations by
•
52
sis of the Ar P–PAr is followed by attack of the Ar P on the Ruipérez et al. and from experimental observations by Asua
2
2
2
5
3
second diphosphane Ar′
then the radical couplings with P–P bond formation.
The photodissociation requires UV radiation and it appears thesis and a [2 + 1] radical chain reaction initiated by sulfanyl
2
P–PAr′
2
. The termination steps are et al. DFT and MD calculations were applied to two mecha-
nisms of diaryldisulfide metathesis: a [2 + 2] concerted meta-
•
47
that there is sufficient background UVA-UVB radiation radicals (ArS ). The transition state in the concerted process
(
280–400 nm) for the initiation step to proceed.
The two degenerate propagating steps, where Ar P and Ar′ P
was calculated to be too high in energy to be viable, whereas
the radical process was viable provided the ArS radicals could
•
•
•
2
2
react with their respective dimers Ar
2
P–PAr
2
2 2
and Ar′ P–PAr′ , are be accessed. This reasoning mirrors ours for the mechanisms
omitted from Scheme 9. The reverse P,P-metathesis process proposed for diphosphane metatheses shown in Scheme 7.
It has been reported that the diaryldisulfide metathesis
between diphenyldisulfide and di(4-aminophenyl)disulfide pro-
ceeds spontaneously to equilibrium in ca. 12 h; this was acceler-
ated by UV photolysis, and TEMPO completely inhibited the
5
3
equilibration. These observations made on the disulfide meta-
54
thesis reactions (and analogous diselenide studies) are very
similar to our observations on the diphosphane metatheses.
Anomalous kinetics with (o-Tol) P–P(o-Tol)
2
2
The four P,P-metathesis reactions between tetra(o-tolyl)diphos-
phane (E ) and A –D did not appear to proceed at all in THF,
2
2
2
PhMe, PhCl, PhF or C
6 6
D under ambient conditions after 72 h
but do proceed in the chloroalkanes CH Cl and CHCl , albeit
2
2
3
slowly (see Table 1, entries 7–10). To probe this anomalous
behaviour further, we have investigated the A /E system
Fig. 6) in more detail including approaching the equilibrium
2
2
(
starting from an A
2
/E
2
mixture and starting from isolated AE.
Scheme 9
Scheme 10
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(
2 2
1) The lack of any reaction between A and E in THF after
72 h was replaced by 30% AE formation when the solution was
irradiated with a UV lamp for 2 h. Similarly, no equilibration
was observed in a THF solution of pure AE over a period of
72 h but proceeded smoothly over 2 h upon irradiation of the
same solution with a UV lamp. This suggests that the photo-
dissociation mechanism shown in Scheme 6 for the general
P,P-metathesis (Table 1, entries 1–6) is viable for the A /E
2
2
metathesis but requires a more intense source of UV radiation
than background UV for the initiation step.
(
2) The slow A /E metathesis equilibration in CDCl which
2 2 3
takes place over a period of 12 h was extinguished in the pres-
ence of 4 equiv. of the radical scavenger TEMPO, as no AE was
detected after 16 h.
(
3) Pulse-sonication (20 kHz) at 0 °C of a CHCl
solution of A and E reduced the time to establish equili-
brium from 12 h to <1 h. However, sonolysis had no accelerat-
ing effects on the reaction of A with E in THF or toluene.
Fritze and von Delius reported the metathesis of disulfides
Scheme 10) in chloroform was accelerated by sonolysis and
3 2 2
or CH Cl
2
2
2
2
(
3
1
1
Fig. 6
2 2
P{ H} NMR spectra for the mixture of A and E : (a) in toluene
•
suggested that this proceeded via RS radicals formed from the
reaction of disulfides with Cl generated by sonolytic chloro-
3
after 48 h, showing no detectable P,P-metathesis; (b) in CDCl after 6 h,
•
showing equilibrium with AE has been established.
1
6
form degradation.
4) The onset of equilibration of solutions of AE in CDCl
(
3
was preceded by an induction period of up to 6 h; moreover,
the time required to establish the P,P-metathesis equilibrium
2 2 3
The progress of the A /E metathesis in CDCl was followed
by P NMR spectroscopy (see ESI† for details) with a spectrum
3
1
2 2 3
from mixtures of A /E in CDCl varied from 6 to 12 h. These
recorded every 10 min for 12 h (see Fig. 7). The approach to
31
observations of rate-inconsistency are typical of radical
equilibrium was measured from integration of the P NMR
signals to produce the plot shown in Fig. 7. Analysis of the
curve revealed that it was not consistent with the integrated
5
5
processes.
In addition to the evidence for a radical-promoted process,
it is shown below that there is evidence of chlorophosphine
1
2
equations for an order of reaction of 0, , 1 or 2; this indicates
involvement in the A /E metathesis.
that the kinetics are complex, which would be expected if
more than one mechanistic pathway was operating (see below).
2
2
Evidence for chlorophosphine intermediacy in the anomalous
Evidence for radicals in the anomalous (o-Tol) P–P(o-Tol)
(o-Tol) P–P(o-Tol) metathesis reactions
2
2
2
2
metathesis reactions
There are several pieces of evidence supporting a role for rad- would reach equilibrium with AE within 12 h, the presence of
icals in the P,P-metathesis equilibrium shown in Fig. 6: O (100 equiv.) suppressed the reaction to the extent that no
Under the conditions where a mixture of A and E in CDCl
3
2
2
H
2
31
1
2 2
Fig. 7 (a) Plot of the data obtained from integration of the P{ H} NMR signals for the AE formed in the reaction between diphosphanes A and E ;
31
1
(
3
b) P{ H} NMR spectra in CDCl obtained by sampling over 12 h. See Fig. 6 for the peak assignments.
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AE was detected even after 20 h (indicating that <1% AE had
been formed). Instead, several minor signals (ca. 20% in total)
were present and those at δ+ 23.5, +21.4 were tentatively
2
assigned to Ar P(O)H (Ar = Ph or o-Tol). These observations are
consistent with the water reacting rapidly with the Ar PCl com-
2
pounds that are formed. It is notable that the addition of
water to the P,P-metathesis reaction mixture of A
2 2
with C in
CDCl had no discernible effect on the rapid formation of AC
3
2
which indicates that Ar PCl is not involved in the P,P-meta-
thesis for this typical system.
Scheme 12
When a solution of A alone in CHCl was irradiated with
2
3
UV, some conversion to Ph
2
PCl was observed directly. The stoi-
chiometric reaction of A
ately produced a mixture of A , AE, and Ar PCl (Ar = Ph, o-Tol)
2 2 3
with (o-Tol) PCl in CHCl immedi-
•
Cl is twofold: (a) to generate Ar PCl which promotes the P,P-
2
2
2
•
metathesis process (Scheme 11); (b) to form Ar P and initiate
a radical chain P,P-metathesis process similar to Scheme 9.
DFT studies (see ESI†) show that initiation by attack of Cl on
(
Scheme 11) along with several unidentified P-containing
species. After 4 h, E was detected along with small amounts
of secondary phosphines Ar PH (Ar = Ph, o-Tol). These obser-
2
2
•
2
•
Ar P–PAr generating Ar P (Scheme 12) is feasible: ΔG =
vations confirm that Ar
ters of P,P-metathesis.
2
PCl compounds are plausible promo-
2
2
2
1
‡
−1
−
35.3 kcal mol− and ΔG = 3.4 kcal mol (from the Ph
2
P–
•
PPh ⋯Cl association complex).
To explore whether Ar PCl could catalyse P,P-metathesis, a
2
2
The slow rate of the P,P-metathesis reaction between the
o-tolyl diphosphane E and A contrasts with the rapid rate of
the P,P-metathesis reaction between the isomeric p-tolyl
diphosphane B2 and A . The explanation for the lower rates
sub-stoichiometric amount (5 mol%) of Ph
2
PCl was added to a
chloroform solution of pure AE. The equilibration (Scheme 11)
2
2
3
1
1
was then monitored by P{ H} spectroscopy and compared
with the same process carried out in the absence of Ph PCl. In
the absence of Ph PCl, equilibrium was established in ca.
0 h, whereas in the presence of 5 mol% of Ph PCl, equili-
2
2
3
therefore lies in the proximity of the o-CH group to the
2
•
P-reaction centre. We have proposed that photolytic Ph2P
7
2
radical formation initiates the P,P-metathesis reactions of A2
brium was established in ca. 7 h (see ESI†).
(
see above) and, since the UV spectrum of E
2
resembles that of
The key observations on the P,P-metathesis reaction
between E and A are that: (1) the solvent (CHCl or CH Cl )
•
•
A2 (see Fig. 4), both Ph P and (o-Tol) P radicals should be
2
2
2
2
3
2
2
similarly accessible. We propose two tentative explanations for
the role of the o-CH groups in E . (1) The bulky o-tolyl substi-
tuents may be shielding E from radical attack by Ph P rad-
is critical for the reaction to proceed under ambient con-
ditions; (2) the reaction has the characteristics of being
3
2
•
radical-initiated; (c) Ar PCl catalyses the P,P-metathesis.
2
2
2
•
•
icals or hindering radical attack by (o-Tol) P on A . Some
It is known that CHCl
3
is a source of Cl radicals (especially
or sonolysis
2
2
5
6–60
61–64
support for a steric effect of an ortho-alkyl group on the rate of
under photolysis
) and therefore the
radical-initiated P,P-metathesis comes from calculations of
process shown in Scheme 12 is proposed in which the role of
2
Scheme 11 P,P-Metathesis promoted by Ar PCl.
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The facility with which diphosphanes undergo P,P-meta-
thesis is similar to the S,S-metathesis with disulfides and Se,
Se-metathesis with diselenides. This augurs well for the appli-
cation of diphosphanes in dynamic covalent chemistry.
Scheme 13
Conflicts of interest
‡
•
ΔG for Ar
2
P attack on Ar
2
P–PAr
2
where Ar = Ph, o-C
6
H
4
Me or
There are no conflicts to declare.
t
−1
o-C
6
H
4
Bu which gave values of 5.5, 6.7 and 13.3 kcal mol
respectively (see ESI† for details). These show that, although the
‡
6 4
calculated ΔG values for Ph and o-C H Me are similar, for
o-C H Bu, the calculated value is significantly greater. (2)
6 4
Acknowledgements
t
Alternatively, the o-CH groups may provide a means of quench- This research was funded by the Engineering and Physical
3
•
ing the radical chain by H intramolecular abstraction from the Sciences Research Council through the EPSRC Centre for Doctoral
3
proximal CH group to produce a benzylic radical by a process Training in Advanced Composites for Innovation and Science
that is illustrated in Scheme 13. Calculations show that this (grant number EP/L0160208/1). We thank the Department of
−
1
would be highly unfavourable energetically (by 14.4 kcal mol
)
Chemistry at Oxford and the Centre for Computational Chemistry
•
for the ground state (o-Tol)
2
P radical but the initially formed in Bristol for access to computing facilities.
P-centred diphosphane radical would have sufficient energy for
H transfer to take place (see ESI† for details). The energetic
•
C-centred radical could then abstract H and the diphosphane References
reform thereby quenching the P,P-metathesis.
Both of these explanations are speculative and further work
would be required to establish their veracity.
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