Structure-property-reactivity studies on dithiaphospholes

Article abstract of DOI:10.1039/c9dt03577j

The reaction of either toluene-3,4-dithiol or benzene dithiol with phosphorus(iii) trihalides generates the corresponding benzo-fused 1,3,2-dithiaphospholes, RC₆H₃S₂PX (R = Me (1), R = H (2); X = Cl, Br, I). The P-chloro-dithiaphospholes undergo: (a) halogen abstraction reactions with Lewis acids forming phosphenium cations; (b) substitution with LiHMDS base and; (c) reduction chemistry with sodium metal to generate the P-P σ-bonded dimer, (RC₆H₃S₂P)₂. Reduction catalysis of aldehydes with pinacolborane using dithiaphospholes is compared with their dioxaphosphole and diazaphosphole counterparts as pre-catalysts, revealing interesting differences in the reactivity of this series of compounds.

Full text of DOI:10.1039/c9dt03577j

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Structure–property-reactivity studies on
dithiaphospholes†‡
Cite this: DOI: 10.1039/c9dt03577j
a
b
b
Darren M. C. Ould, Thao T. P. Tran, Jeremy M. Rawson * and
a
Rebecca L. Melen
*
The reaction of either toluene-3,4-dithiol or benzene dithiol with phosphorus(III) trihalides generates the
corresponding benzo-fused 1,3,2-dithiaphospholes, RC PX (R = Me (1), R = H (2); X = Cl, Br, I).
6
3 2
H S
The P-chloro-dithiaphospholes undergo: (a) halogen abstraction reactions with Lewis acids forming
Received 4th September 2019,
Accepted 24th September 2019
phosphenium cations; (b) substitution with LiHMDS base and; (c) reduction chemistry with sodium metal
6 3 2 2
to generate the P–P σ-bonded dimer, (RC H S P) . Reduction catalysis of aldehydes with pinacolborane
using dithiaphospholes is compared with their dioxaphosphole and diazaphosphole counterparts as pre-
DOI: 10.1039/c9dt03577j
rsc.li/dalton
catalysts, revealing interesting differences in the reactivity of this series of compounds.
activate a diverse range of small gas molecules including CO
and dihydrogen.
Among the many aspects of main group chemistry, there
2
Introduction
1
0
In the last few decades there has been a renaissance in main
group chemistry stemming from the emerging materials appli- has been increasing use of heterocyclic phosphorus com-
1
cations of main group compounds alongside increasing appli- pounds as catalysts for organic transformations, exemplified
2
cations as reagents and catalysts in organic synthesis. In all by work by Zhang et al. on the nucleophilicity of diazapho-
1
1
these areas, performance is being enhanced by the inclusion spholene hydrides. Gudat first showed the hydridic nature of
of main group elements or main-group functional groups to the P–H bond in 1,3,2-diazaphospholenes (Scheme 1 top,
1
2
tailor electron-donating or withdrawing properties, enhancing X = H), which have been used in stoichiometric reduction
1
3
donor or acceptor ability, redox behaviour or Lewis acidity inter reactions and more recently by Kinjo in the catalytic hydro-
1
4
alia. For example, in materials chemistry responsive organo- genation of the NvN double bond with ammonia borane as
3
boron based luminescent materials and main group perovs- well as the catalytic hydroboration of carbonyl compounds
4
kites for solar cell applications have attracted considerable
attention. For the later p-block elements the redox active
nature of many electron-rich S/N and Se/N-based heterocycles
have led to the discovery of some of the highest magnetic
5
ordering temperatures for organic magnets, as well as mole-
6
cule-based conductors and as redox active paramagnetic
7
ligands inter alia. Alongside this, the ability to tune the Lewis
acidity of early p-block elements has attracted attention in
8
,9
main group promoted or catalysed organic transformations
while frustrated Lewis pair (FLP) combinations continue to
a
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building,
Cardiff, CF10 3AT, Cymru/Wales, UK. E-mail: melenr@cardiff.ac.uk
b
Department of Chemistry and Biochemistry, University of Windsor,
401 Sunset Avenue, Windsor, Canada. E-mail: jmrawson@uwindsor.ca
†
Information about the data that underpins the results presented in this article,
including how to access them, can be found in the Cardiff University data catalo-
gue at http://doi.org/10.17035/d.2019.0084454664
‡
Electronic supplementary information (ESI) available: X-ray data, NMR spectra.
CCDC 1951113–1951115, 1951125–1951127, 1951132, 1430534 and 824860. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9dt03577j
Scheme 1 Previous use of diazaphospholenes for catalysis (Top) and
this work (Bottom).
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1
5
with pinacolborane. Speed has also used 1,2,4,3-triazaphos- dithiol or benzene dithiol to the appropriate phosphorus
pholenes for the catalytic hydroboration of various carbonyl trihalide, PX , under ambient conditions with the derivatives
3
1
6
compounds, and reported the first use of diazaphospholenes 1a–2c recovered in very good to excellent yields (73–96%).
as chiral catalysts by introducing chirality at the R group on
1
7
31
the nitrogen atoms. The Cramer group has also used 1,3,2-
P chemical shift
1
8
diazaphospholenes for chiral reductive chemistry.
The
31
The dithiaphospholes were initially characterised by in situ
{
P
related phosphenium cations have also been used as pre-cata-
lysts in the reduction of pyridines and imines with pinacolbor-
ane or silanes.
1
H} NMR spectroscopy, which revealed a singlet resonance
centred at δ = 160.4, 163.3, or 155.0 ppm for 1a–1c respectively
1
9,20
and complete consumption of the PX starting material (ca. δ =
3
The ability to tailor the reaction chemistry through appli-
cation of the isolobal analogy gives rise to a large number of
heterocycles closely related to the diazaphosphole unit. This
3 3 3
219, 227 and 178 ppm for PCl , PBr and PI respectively). The
shielding constant σ can be broken down into diamagnetic
shielding (σ ) and paramagnetic shielding (σ ) contributions.
d
p
2
1
includes replacement of P by heavier pnictogens such as As
1
For H NMR spectroscopy the chemical shift is dominated by
the diamagnetic shielding which is related to the electron
density and hence sensitive to the electron-withdrawing/releas-
and one of our groups have recently identified that benzo-
fused diaza-chloro-arsole is highly active for the reduction of
2
2
aldehydes with pinacolborane. Conversely replacement of
R–N by isolobal chalcogens leads to heterocycles such as the
dioxaphospholes and dithiaphospholes (Scheme 1, bottom).
In the current study we report the syntheses, structures and
reactivity patterns of benzo-fused 1,3,2-dithiaphospholes,
which are closely related to N-heterocyclic phospholenes
1
ing nature of substituents attached to the H nucleus.
Conversely, for other nuclei, paramagnetic shielding is signifi-
cant. The paramagnetic shielding arises from mixing of the
ground state wavefunction with excited state wavefunctions
and is therefore closely related to the energy of low-lying states
where oxidation state and coordination geometry play an
important role. For a series of structurally closely-related com-
plexes such as phosphines empirical correlations can be
(NHPs) by replacement of NR by S. Finally we provide a com-
parative study of these dithiaphospholes with their isolobal
dioxaphosphole and diazaphosphole counterparts as pre-cata-
lysts for the hydroboration of aldehydes with pinacolborane.
made. For tertiary phosphines, PR , the chemical shift follows
3
27
δ
P
R R
= −62 + ∑σ where σ is a constant for a specific R group.
The additive nature of the chemical shielding effects for
different groups can therefore be used in a chemically mean-
ingful way. For example, the P NMR data for 1a–1c (Table 1)
do not follow a simple trend based on electronegativity argu-
3
1
Results and discussion
Synthesis
ments but do correlate with a similar trend observed for PX .
2
3
3
P-Halo-benzodithiaphospholes were prepared by Baudler in
973 who showed that reduction led to P–P coupling to form
3
1
From the reported P NMR data for PX
3 2
(X = Cl, Br, I, NMe ,
1
OMe, SMe and Ph), σ values can be derived (Table 1) and
R
the σ-bonded dimers 4 and 5 (Scheme 2). Burford showed that
treatment of these P-halo dithiaphospholes with Lewis acids
chemical shifts estimated based on the chemical environment
of the phosphine (Table 1). While imperfect the correlation
coefficient (0.85) is good, especially given the approximation
that the RC H S unit can be approximated by two SMe groups.
(
AlCl or GaCl ) forms the corresponding dithiaphosphenium
3 3
2
4–26
salts (Scheme 2).
In our study, the syntheses of 1a–1c and
6
3 2
2
a–2c essentially followed the protocols first described by
Baudler (Scheme 2) via the addition of either toluene-3,4-
Crystal structures of 1a–1c and 2a–2c
Crystals of these compounds were obtained by reducing the
volume of the crude reaction mixture to form a (super)satu-
rated CH Cl solution and then cooling to −40 °C. Crystals
2 2
31
Table 1
P NMR spectroscopy analysis
X
F
Cl
Br
I
σ
R
53.3
116.3
—
94
157
161.4
96.3
159
163.6
80
143
155.4
3
3
1
P δcalc for (MeS)
P δobs for [1]X
2
PX
1
X
OMe
NMe
2
SMe
Ph
σ
p
67.3
130.3
124.5
61.5
62.5
18.7
81.7
3
3
1
P δcalc for [1]X
P δobs for [1]X
124.5
125.5
1
a
b
c
94.4
113
a
b
c
For [1]OCH
2
Ph (this work). For [1]N(SiMe
)
3 2
(this work). For
Scheme 2 Synthesis of dithiaphosphole derivatives.
C H
6
4
(S-1) this work.
2
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Table 2 DFT analysis of compounds 1a–1c and 3 (cation)
1a
1b
1c
3
P–X bond order
P partial charge
X partial charge
0.86
0.87
0.90
N/A
+0.53
N/A
+0.53
−0.32
+0.06
+0.46
−0.27
+0.07
+0.36
−0.17
+0.07
a
S partial charge
+0.30
All computations used the M06-2X functional and 6-311G+(2d,p) basis
set for all atoms except the I atom in 1c, for which the basis set
a
Def2TZVP was implemented. Average value of the two S atoms taken.
2
9
phosphole compounds. In addition, NBO analysis showed
significant polarisation in this bond, with a natural charge of
+0.53 on the P heteroatom, and −0.33 on the Cl atom; the
S atoms exhibit natural charges of +0.06. Comparing the
results of 1a to 1b and 1c, a reduction in the polarisation of
the P–X bond was found to be accompanied by a small
increase in bond order. A similar NBO analysis on the cation
in 3b (vide infra) was performed which showed a bond order of
Fig. 1 (Top) Solid-state structures of 1a–1c (top left to right respect-
ively), (Bottom) crystal structure of 3b. Thermal ellipsoids drawn at 50%
probability and H-atoms removed for clarity.
1.35 and 1.37 for the P–S bonds and partial charges of +0.53
and +0.30 at P and S respectively.
typically formed over 18–36 h. Compounds 1a–1c were found
to crystallise in the monoclinic space group P2 /c with one
1
Reaction with Lewis acids
molecule in the asymmetric unit. Compound 2a crystallises in Stoichiometric treatment of 1a with the Lewis acids AlCl
3
and
the monoclinic space group P2 /n while 2b and 2c crystallise GaCl generated the corresponding phosphenium salts 3a and
3
1
in the triclinic space group P ˉ1 with two molecules in the asym- 3b respectively in excellent yield (90–93%) as highly air-sensi-
metric unit. Structure determination of 1a–1c revealed the tive crystalline solids. The solid-state structure of 3a has pre-
3
0
expected three-coordinate phosphorus centre with an exocyclic viously been reported by Cameron and Linden. Compound
P–X (X = Cl, Br, or I) bond (Fig. 1). The dithiaphosphole ring is 3b was found to crystallise in the monoclinic space group P2 /c
1
non-planar with a distinct envelope geometry associated with a with one molecule in the asymmetric unit. As with the pre-
folding about the S⋯S vector. On descending group 17, there viously determined structure of 3a, the solid-state structure of
is a decrease in the fold angle θ (defined as the angle formed 3b exhibits a planar C S P ring and notably shorter P–S bonds
2 2
between S P and C S planes) and a move towards molecular (1.998(6) Å and 2.024(5) Å) in relation to 1a (2.0936(7) Å and
2
2 2
planarity. The fold angle (θ) varies from of 26.06° (X = Cl) to 2.0954(7) Å), consistent with 3p–3p π conjugation and some
4.27° (X = Br) and 19.62° (X = I). Within the series 1a–1c an delocalisation within the formally 10 π aromatic benzodithia-
2
elongation of the P–X bond is observed in relation to a conven- phosphenium cation. This π-conjugated perspective is sup-
tional P–X bond. The bond lengths are 2.1134(7) Å (1a), ported through the computational studies described above
2
.3153(9) Å (1b), and 2.5730(12) Å (1c), cf. standard P–X bond where the P–S bond orders are 1.33 and 1.35 in the cation.
lengths of 2.008 Å (P–Cl), 2.206 Å (P–Br) and 2.490–2.493 Å The closest cation–anion contact in 3b is a P⋯Cl contact at
28
(
P–I). Similar features were found for 2a–2c. The symmetry 3.317(5) Å, which is well within the sum of the van der Waals
breaking generated by the presence of the methyl group leads radii (3.55 Å).
to chirality at the P centre. However, the presence of an
inversion centre in all these structures leads to structures Reduction
containing 50 : 50 mixtures of both enantiomers.
Reduction of the chloride salt 1a has previously been achieved
using sodium metal to form 4. However we found that the
rate of reduction of both 1a and 2a was particularly sensitive to
2
3
Computational studies of 1a–1c
To understand better the structure and bonding in these the size of the sodium pieces employed. Initial studies
dithiaphosphole compounds, we turned to computational required two to four days to ensure complete reduction (moni-
3
1
methods. Natural Bond Order (NBO) analysis (M06-2X and tored by P NMR spectroscopy) but initial pre-treatment of a
-311+G(2d,p)) was performed on 1a–1c (Table 2 and ESI‡), suspension of small pieces of sodium metal in toluene
6
which for 1a revealed a Wiberg P–Cl bond order of 0.86. This (100 °C, 20 min) afforded a fine sodium dispersion which led
reduced bond order can be explained by hyperconjugation to shorter reaction times (ca. two days). The P–P coupled
3
1
between the π electrons in the C
2
S
2
unit and the σ*(P–X) product
5
exhibits
a
diagnostic
P chemical shift at
orbital, in turn weakening the P–X bond, which is consistent +40.6 ppm, significantly upfield in relation to the P(III) deriva-
with observations we have previously reported on related diaza- tives 1a and 2a and comparable with [(MeO) C H S P] (δ =
2
6
2
2
2
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3
1
5
0 ppm). The methyl group can give rise to different dia-
3
1
stereoisomers but a careful examination of the P NMR data
failed to provide unambiguous evidence for the presence of
both diastereomers. However the presence of two diastereo-
mers could explain the inability, despite multiple attempts, to
grow good quality crystals of 5, which is in contrast to 4.
During the reduction process an intermediate was observed
at approximately 113 ppm which is tentatively attributed to a
dithiolate bridged intermediate (Fig. 2). Evidence for the
nature of this intermediate comprises good agreement of the
observed chemical shift with the closely related dialkoxybenzo-
Fig. 3 Solid-state structure of 4. Thermal ellipsoids drawn at 50% prob-
ability and H-atoms removed for clarity.
1
5
derivatives (δ = 115–120 ppm),
as well as comparative
−
1
the P–P single bond (201 kJ mol ) in relation to the N–N bond
(167 kJ mol− ) coupled with stronger 2p–3p N–S π-bonding in
DTAs in relation to 3p–3p P–S π-bonding in 4, i.e. the loss in
π-bonding is more than compensated by formation of a loca-
reduction chemistry of P-chloro-diazaphospholes using
1
3
2–34
Na(K).
Compounds 4 and 5 were isolated by filtration to remove
insolubles and dried in vacuo to afford yellow solids which,
upon slow evaporation of a saturated toluene solution,
afforded a polycrystalline mass. In the case of 4 small crystals
suitable for X-ray diffraction were cut from the bulk sample.
While all samples studied exhibited persistent twinning, a sat-
isfactory solution could be determined in the monoclinic
space group P2 /n with half a molecule in the asymmetric unit
1
with the structure of 4 located about an inversion centre
midway along the P–P bond (Fig. 3). The P–P bond length
−
36
lised 2c,2e bond for 4 and 5. The structure of 4 also exhibits
a folding of the C S P heterocycle about the S⋯S vector with a
2
2
fold angle of 33.01°, slightly greater than in 1a–1c
(
26.06–19.62° vide supra) and 2a (28.35°, see ESI‡).
Variable temperature solution EPR studies on 4 and 5
showed no tendency for homolytic bond cleavage to form rad-
icals up to 110 °C. This is in contrast to the isolobal diaza-
phospholes, containing the C (NR) P ring system, which gene-
2
2
3
7–41
rate radicals upon warming through P–P bond cleavage.
(2.233(2) Å) is comparable with that observed in the two other
The difference is likely due to the steric demands of the N–R
group. On the other hand, the P–P bond could be oxidatively
crystallographically determined systems containing S P–PS
2
2
sub-units; the one previously structurally characterised dithia-
3
1
3
1
cleaved with SO Cl , Br and I reforming 1a–1c based on
P
phosphole,
[(MeO)
2
C
6
H
2
S
2
P]
2
at 2.2350(16)
reported
Woollins at 2.2306(13) Å. It should be noted that there
Å
and the
2
2
2
2
NMR spectroscopy studies and preparative scale reactions.
binaphtho[1,8-de][1,3,2]dithiaphosphinine
by
3
5
appears to be a strong preference for dimerisation via localised Substitution chemistry on 1a
P–P σ-bond formation in these systems whereas the lighter
N-atom dithiazolyl analogues dimerise via multi-centre π–π
interactions. This is likely a result of the superior strength of
Reaction of 1a with Li[N(SiMe ) ] in a 1 : 1 mole ratio in
3 2
toluene afforded 1d (Scheme 3) as a colourless oil which was
31
characterised by multinuclear NMR spectroscopy. The
P
NMR chemical shift for 1d (+94.9 ppm) is a little lower than
that predicted based on the additive shielding approach
(
Table 1) but may reflect the electronic and steric dissimilarity
between NMe and N(SiMe . Condensation of 1d with two
2
3 2
)
further equivalents of 1a proved sensitive to the solvent
employed but reflux in MeCN for 18 h afforded the tri-functio-
3
1
nalised product 1e ( P NMR, δ = 86.8 ppm) as a white solid
87%) which was recrystallised by cooling a saturated CH Cl
(
2
2
solution to −20 °C. Compound 1e crystallised in the rhombo-
3
1
1
Fig. 2 In situ P{ H} NMR of the reduction of 1a with Na metal in
toluene to form 5.
Scheme 3 Synthesis of 1d and 1e.
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species present in solution, it exhibits some Lewis acid charac-
3
1
ter, based on in situ P NMR data. For example addition of
1
Ph
ppm, JPH = 444 Hz) whereas addition of Ph
singlet at 47 ppm. When initially working with the cationic
3
P afforded two doublets (δ = 52 ppm, JPP = 444 Hz; δ =
1
8
3
As afforded a
31
phosphenium 3b in THF, a chemical shift in the P NMR
spectrum at δ = 124 ppm was observed, which more closely
3
1
resembles a three-coordinate P(III)-centre (cf. 1a
60 ppm). Indeed, the chemical shift is in agreement with an
O-bound S P–O centre (expected around 130 ppm, Table 1). It
P δ =
1
2
was also found that, while visually monitoring the reaction,
1
the THF solution became viscous over one hour and H NMR
spectra analysis showed the rapid formation of poly(THF) at
ambient temperature. Indeed, under more controlled con-
ditions using 0.15 mol% 3b, 80% conversion of THF into poly
(
THF) occurred in 24 hours, with the reaction rate slowing over
time due to the increasing viscosity from the formation of poly
THF). The polymerisation of THF is a facile process and
follows the well-known cationic ring opening polymerisation
CROP) mechanism, in which 3b acts as the cationic
(
(
4
3
initiator. It should be noted that stirred solutions of (i) GaCl
Fig. 4 Molecular structure of 1e (Top) and supramolecular dimer linked with THF and (ii) 1a with THF gave no polymer formation, con-
3
via S⋯S contacts (Bottom).
firming the role of the phosphenium cation in the polymeris-
ation process. An end group analysis (integral of the terminal
3 2
CH group of the cationic initiator to the CH group of the
hedral space group P ˉ3 (Fig. 4) with the N atom on the three-
fold axis and a third of a molecule in the asymmetric unit. The
poly(THF) permitted number average molecular weights (Mn)
to be determined which fell in the range 1897–3163, corres-
ponding to 23 < n < 41 THF units (see ESI‡). Interestingly,
five-membered C S P ring in 1e again adopts an envelope geo-
2
2
metry with a fold angle of 20.66°. The P–S bond lengths in 1e
are 2.105(2) and 2.125(2) Å and the exocyclic P–N bond length
when attempting to use 3a for CROP no formation of poly
31
(
THF) occurred. In this case P NMR studies revealed a singlet
(1.7274(2) Å) falls in the normal range for P–N single bonds
(
δ = +161 ppm) which is believed to be due to reformation of
(
1.70–1.77 Å). The PNP bond angle (119.8°) is very close to that
1
a and formation of the adduct THF·AlCl . Similar degradation
3
2
expected for an sp -N suggesting a lone pair of p-character.
of the counterion has also been observed for the tetraphenyl-
borate salt which was shown to form MeC P–Ph and
Ph B, reflecting the high Lewis acidity of the P centre.
The structure of 1e is analogous to the previously reported
6
3 2
H S
2
1
dithia-arsole, (MeC
through a series of six symmetry-equivalent S⋯S contacts at
.512 Å, marginally shorter than the sum of the van der Waals
6
H
3
S
2
As)
3
N. Molecules of 1e aggregate
+
26
3
Attempts to polymerise propylene oxide or 1,4-dioxane with 3b
3
under the same conditions afforded a similar downfield shift
radii (3.60 Å). These contacts generate a supramolecular “S
6
”
31
(
P δ = 127 and 125 ppm respectively) to that observed in THF,
chair motif (Fig. 4).
consistent with adduct formation but there was no evidence
for polymerisation of either of these substrates.
Lewis acidity studies on 3b
Reduction catalysis
The phosphenium salts 3a and 3b are extremely air sensitive
in solution and particularly prone to hydrolysis reflected in a In the last few years phospholes, diazaphospholenes and their
3
1
diagnostic doublet in the P NMR spectrum (δ = 57.2 ppm, derivatives have received attention in catalytic reduction chem-
1
31
1
J
PH = 699 Hz) which collapses to a singlet in the P{ H} NMR istry by
a
number of different groups, including
2
6
31
15,22,44,45
spectrum. Prior work by Burford reported the P chemical ourselves.
We therefore looked to see whether these
shift for these cations around 410 ppm but indicated it was dithiaphospholes and their derivatives would be active in the
sensitive to both temperature and concentration. They pro- reduction of aldehydes with pinacolborane (HBpin). To investi-
posed the presence of aggregated species such as gate this, we initially synthesised a library of potential pre-cata-
+
−
[
(C H S P) Cl] (formed by disproportionation of 2 MCl to lysts. Previously the groups of Kinjo and Speed have demon-
6
4
2
−
2
4
−
+
form Cl and M
2
Cl
7
) in equilibrium with [C
6
H
4
S
2
P ]:
strated that the use of benzyloxy (Bn) and neopentyloxy (Np)
substituted phosphorus systems are effective in reduction
2
½C6H4S2Pꢀ½MCl4ꢀ ⇄ ½ðC6H4S2PÞ Clꢀ½M2Cl7ꢀ:
ð1Þ
2
15,44,45
catalysis.
In this context, the alkoxy derivatives 6a and
In this context Russell’s work on the related phosphenium 6b were prepared by reaction of the dithiaphosphole 1a with
+
+
42
ion {(RC)
4
P} led to isolation of the [{(RC)
4
P}
2
Cl] cation.
either benzyl alcohol or neopentyl alcohol in the presence of
Despite the uncertainty in the exact nature of the active base, affording compounds 6a and 6b in moderate yield
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Table 3 Pre-catalyst screening results
Scheme 4 Synthesis of dithiaphosphole pre-catalysts bearing a benzyl-
oxy or neopentyloxy group.
a
Entry
Pre-catalyst
Time (h)
Conversion (%)
1
2
3
4
5
6
7
8
9
None
1a
1b
1c
3a
3b
6a
6b
7a
7b
8a
8b
9a
9b
10
TMSOTf
AlCl3
24
24
24
24
12
12
24
24
24
24
24
24
24
24
6
<5
<5
14
61
>95
>95
9
(
63–65%) (Scheme 4). These compounds show upfield signals
3
1
31
in the P NMR spectrum compared to precursor 1a, with
P
resonances at δ = 124.5 and 123.6 ppm for 6a and 6b corre-
spondingly, in agreement with the computed change in their
chemical shift (Table 1). This methodology was also extended
to synthesise dioxaphosphole and diazaphosphole pre-cata-
lysts, providing an opportunity to compare the relative catalytic
8
<5
<5
<5
<5
30
35
>95
56
68
1
1
1
0
1
2
2 2
activity of this isolobal series of C E P (E = O, NR, S) hetero-
cycles (Scheme 5). To synthesise these compounds, the 13
1
1
1
4
5
6
chloride and bromide pre-cursors were first produced. In the
case of the dioxaphospholes, catechol was reacted with phos-
24
24
phorus(III) chloride or phosphorus(III) bromide and triethyl 17
amine to yield 7a and 7b in 36% and 41% yield respectively,
a
1
Conversion measured by in situ H NMR spectroscopy.
3
1
whose corresponding P NMR chemical shifts are δ = 173.6
and 195.3 ppm. In analogous fashion, reaction of 7a with
either benzyl alcohol or neopentyl alcohol afforded 9a and 9b
3
1
HBpin in CDCl
3
. The conversion was then monitored by both
H and F NMR spectroscopy at 2-, 6-, 12- and 24-hour inter-
(
68% and 64% yield) with upfield P NMR chemical shifts of δ
1
19
=
126.9 and 127.4 ppm respectively. Although the same pro-
vals. Table 3 shows the results of the pre-catalyst screening.
The dithiaphosphole 1a was found to be catalytically inactive,
with <5% conversion observed in the H NMR spectrum after
2
cedure was attempted for the diazaphosphole compound 8a,
addition of benzyl alcohol or neopentyl alcohol gave a mixture
1
3
1
of three species in the P NMR spectrum, only one of which
was the desired product (the others are believed to arise from
hydrolysis). Nevertheless, abstraction of the chloride from 8a
with TMSOTf to produce the cationic phosphenium 10 was
performed successfully.
4 hours. A mild improvement was found using 1b, with 14%
conversion detected after 24 hours. More interestingly though
was the use of 1c, which gave 61% product conversion to 11a
after 24 hours. The differences in performance of 1a–1c can be
attributed to the varying electronic properties of the structures,
as found by NBO analysis (vide supra) and correlate with the
P–X bond energy. Noteworthy, when using the benzyloxy and
neopentyloxy derived dithiaphospholes 6a and 6b, the catalytic
performance fared little better than their chloride precursor,
with just 9% and 8% conversion to 11a after 24 hours respect-
ively (see ESI‡ for NBO analysis). In addition to this, the cata-
lytic ability of the phosphenium cations 3a and 3b were exam-
ined. Both 3a and 3b afforded quantitative conversion to the
hydroborated product 11a in 12 hours. However, knowing that
With a range of pre-catalysts in hand, an initial screening
test was conducted in order to see which of these compounds
was the most active in the reductive catalytic hydroboration of
aldehydes. Therefore, 10 mol% of compounds 1, 3, 6, 7, 8, 9
and 10 were added to 4-(trifluoromethyl)benzaldehyde and
it was possible for dynamic exchange to occur in solution
−
between one of the chlorides on AlCl
centre to reform 1a and give free AlCl
4
and the phosphorus
3
(vide supra), a control
reaction was undertaken in which 10 mol% AlCl was used as
3
1
3
the pre-catalyst for the hydroboration catalysis. Using AlCl , H
NMR spectroscopy showed that after 24 hours only 68% conver-
sion to 11a had occurred. This therefore showed that the phos-
phenium species were indeed responsible for attaining quanti-
tative conversion of 4-(trifluoromethyl)benzaldehyde to 11a.
The dioxaphosphole pre-catalysts, 7a and 7b showed little
conversion to the hydroborated product, with <5% conversion
after 24 hours. However, employing 9a and 9b showed a slight
improvement on these results in relation to the sulfur deriva-
Scheme 5 Synthesis
compounds.
of
dioxaphosphole
and
diazaphosphole
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Paper
tives, giving 30% and 35% conversion to 11a respectively after
2
4 hours. Given the low conversion results for these pre-cata-
lysts, alternative solvents were explored and the catalysis
experiments repeated in both toluene and acetonitrile. Neither
afforded appreciable improvements in conversion after
2
4 hours. In a similar theme, the diazaphosphole compounds
8
a and 8b were found to be inactive for the catalysis.
Significantly 10 mol% of the cationic phosphenium 10 was
found to be highly active, giving quantitative conversion to 11a
1
in 6 hours, as revealed by H NMR spectroscopy. As a control
reaction, the use of TMSOTf was used as a potential pre-cata-
1
19
lyst. Analysis of both the H and F NMR spectra revealed that
6% conversion was achieved after 24 hours; with multiple
5
products observed.
Having discovered that the phosphenium triflate 10 was
clearly the most active pre-catalyst, reaction conditions were
optimised by first assessing the performance of a range of
solvents, while maintaining the catalytic loading of 10 at
1
0 mol%. The data are summarised in Table 4. Both CH
2 2
Cl
and CDCl gave quantitative conversion to the hydroborated
3
Scheme 6 Substrate scope for the hydroboration of aldehydes using a
product 11a, with the latter occurring at a faster rate than the phosphenium triflate 10. 0.6 ml of CDCl solvent, NMR yield calculated
3
1
a
b
former. Use of either THF or MeCN failed to give quantitative from in situ H NMR spectroscopy. 10 mol% 10, 20 mol% 10, 2.0 equiv.
HBpin.
conversion after 24 hours. In addition, use of toluene as a
solvent was attempted but 10 proved sparingly soluble. As
CDCl proved to be the most effective solvent, the reaction
3
conditions were further optimised. Increasing the equiva-
3
lence of HBpin to two as opposed to one made little differ- CDCl with one equivalent of HBpin. Using these optimised
ence to the reaction, with both giving >95% conversion conditions, a substrate scope was performed with a variety of
within six hours. Moving from 10 mol% to 5 mol% gave aldehydes in order to evaluate the effectiveness of 10 as a pre-
quantitative yields although the rate of product conversion catalyst for hydroboration reduction (Scheme 6). As already
was reduced, with the reaction now requiring 12 hours to discussed, 4-(trifluoromethyl)benzaldehyde was hydroborated
reach completion. On the other hand, use of both 2 mol% within 6 hours using 10 mol% of 10 to give quantitative con-
and 1 mol% failed to generate quantitative conversion within version to the product 11a.
2
4 hours, with just 48% and 27% conversion observed
On a similar theme, use of the electron deficient fluori-
respectively. Consequently, it was decided that for the sub- nated aldehydes, 4-, 3- and 2-fluorobenzaldehyde were con-
strate scope, 10 mol% of pre-catalyst 10 would be used in verted to 11b, 11c and 11d respectively within 12 hours. In
addition to fluorinated aldehydes, other electron-withdrawing
aldehydes were probed. Use of both 4-bromobenzaldehyde and
4
-formylbenzonitrile gave >95% conversion to 11e and 11f
respectively within 12 hours. On the other hand, when using
-nitrobenzaldehyde, only 67% conversion to 11g was detected
Table 4 Optimisation for catalytic reactions using 10 as the pre-
catalyst
4
1
by H NMR spectroscopy. Benzaldehyde was slow to hydrobo-
rate, but nevertheless full conversion was observed by
24 hours, while terephthalaldehyde gave full conversion to 11i
after 6 hours, albeit 20 mol% of 10 was used given the pres-
ence of two aldehyde groups. The electron donating dimethyl-
amino functional group was also tolerated, with 11j found in
quantitative yield after 6 hours. 2-Naphthaldehyde was readily
reduced to 11k in 12 hours. Lastly, we also tried the aliphatic
pivalaldehyde, where 46% conversion to 11l was achieved after
Loading
(mol%)
HBpin
(equiv.)
Time
(h)
Conversion
(%)
Entry
Solvent
CDCl
a
1
2
3
4
5
6
7
8
10
10
10
10
10
5
1.0
1.0
1.0
1.0
2.0
1.0
1.0
1.0
3
6
>95
b
CH Cl
2
2
12
24
24
6
12
24
24
>95
b
THF
MeCN
76
b
39
24 hours, the approximate half product conversion we attri-
a
CDCl
CDCl
CDCl
CDCl
3
3
3
3
>95
a
bute partially to its high steric demand. During the course of
performing the substrate scope, 4-methoxybenzaldehyde,
>95
a
2
1
48
a
27
4
-methylbenzaldehyde and 2,4,6-trimethylbenzaldehyde sub-
1
a
1
b
strates were also examined. In these cases H NMR spectra
gave additional unidentified signals that did not correspond to
Conversion measured by in situ H NMR spectroscopy. Conversion
measured by in situ F NMR spectroscopy.
1
9
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Dalton Transactions
product formation and consequently although consumption of ium for three days and distilled over argon. Deuterated sol-
the starting aldehyde was observed, their results are not vents were distilled and dried over molecular sieves and stored
included in Scheme 6. The ketone 4′-fluoroacetophenone was in a glove box before use. Starting materials were purchased
1
13
1
also trialled in the substrate scope, giving 57% consumption from commercial suppliers and used as received. H, C{ H},
1
9
19 11
31
27
as detected by F NMR spectroscopy. However, additional
signals to the reactant/product were identified.
F, B, P, and Al NMR spectra were recorded on a Bruker
Avance 300, 400, or 500 MHz spectrometer. Chemical shifts
Mechanistic studies on phosphenium based catalysis have are expressed as parts per million (ppm, δ) and are referenced
been explored by Kinjo and colleagues on related hydrobora- to CDCl (7.26/77.16 ppm), C D (7.16/128.06 ppm), or C D Br
3
6
6
6 5
1
9
tion of pyridines with HBpin. Upon stoichiometric addition (7.28/122.4 ppm for the most downfield resonance) as internal
of the pre-catalyst 10 to 4-(trifluoromethyl)benzaldehyde, no standards. Multinuclear NMR spectra were referenced to
3
1
11
19
31
significant change in the P NMR spectrum was observed, BF ·Et O/CDCl ( B), CFCl ( F), H PO ( P), and Al(NO )
2 3
3
2
3
3
3
4
2
7
indicating that there is no strong interaction between pre-cata- ( Al). The description of signals includes s = singlet, d =
lyst and aldehyde substrate and consequently 10. doublet, t = triplet, q = quartet, sept = septet, and m = multiplet.
Mechanistically we therefore postulate that this catalysis All coupling constants are absolute values and are expressed in
1
9
proceeds in an analogous way to that reported by Kinjo.
Hertz (Hz). IR-Spectra were measured on a Shimadzu IR
When stoichiometric amounts of 10 were added to HBpin to Affinity-1 photospectrometer. The description of signals
3
1
observe the active catalyst, P NMR spectroscopy instead includes s = strong, m = medium, w = weak, sh = shoulder, and
revealed significant decomposition product in the form of br = broad. Mass spectra were measured on a Waters LCT
PH
3
, as identified by a quartet signal appearing at δ = Premier/XE or a Waters GCT Premier spectrometer.
1
−238.5 ppm, with a J = 189 Hz. Furthermore, the intermedi-
PH
Synthesis of phosphole compounds
ate en route to PH
3
was also evident as a primary phosphine in
3
1
the P NMR spectrum (low intensity triplet at δ = −79.5 ppm
General procedure 1. Phosphorus(III) chloride (1.2 equiv.) or
1
(
JPH = 199 Hz)), suggesting the formation of a primary phos- phosphorus(III) bromide (1.0 equiv.) was added dropwise to a
phine with two P–H bonds via endocyclic cleavage. Similar solution of toluene-3,4-dithiol (1.0 equiv.) or benzene dithiol
results have been observed previously by Speed for the catalytic (1.0 equiv.) in CH Cl (3 mL), with the evolution of gas
observed. The reaction was allowed to stir at ambient tempera-
2
2
4
4
reduction of imines using a diazaphospholene catalyst.
Lastly, we wished to address why the different phosphole ture for 24 hours, after which the solvent was removed in
derived pre-catalysts performed so differently, and specifically vacuo. To the resulting oil, pentane (2 mL) was added and
to address the poor activity of the dithiaphosphole and dioxa- cooled to −40 °C for 4 hours to give the product 2-chloro-5-
phosphole pre-catalysts 6a and 9a. To do this, stoichiometric methylbenzo-1,3,2-dithiaphosphole (1a), 2-bromo-5-methyl-
addition of the pre-catalyst to HBpin in CDCl
3
was performed benzo-1,3,2-dithiaphosphole (1b), 2-chlorobenzo-1,3,2-dithia-
in order to monitor how quickly the respective active hydride phosphole (2a) or 2-bromobenzo-1,3,2-dithiaphosphole (2b) as
2
2
species formed. In the case of 6a, after 24 hours, both the a white powder.
3
1
11
P NMR and B NMR spectra showed little appreciable
2-Chloro-5-methylbenzo-1,3,2-dithiaphosphole
Compound 1a was synthesised according to general procedure 1
NMR spectrum and only the doublet resonance of HBpin at using phosphorus(III) chloride (880 mg, 6.40 mmol, 1.2 equiv.)
(1a).
3
1
change, with no indication of a hydride species in the
P
1
1
2
8.2 ppm was observed in the B NMR spectrum. For 9a, and toluene-3,4-dithiol (1.00 g, 4.74 mmol, 1.0 equiv.). Yield:
again no pre-catalyst formation was observed after 24 hours, 1.363 g, 6.18 mmol, 96%. Crystals suitable for single crystal
with only a small quantity of hydrolysis product detected, as X-ray diffraction were grown from a saturated solution of
1
1
identified by a doublet signal centred at δ = 7.7 ppm ( JPH
7
=
CH
2 2
Cl with a few drops of pentane added. H NMR (500 MHz,
3
1
3
4
06 Hz) in the P NMR spectrum. These studies therefore 295 K, CDCl ): δ/ppm 7.55 (dd, J
show that the slow rate of formation of the active catalyst is the 1H, Ar–H), 7.50 (s, 1H, Ar–H), 7.12 (ddd, JHH = 8.1 Hz, JHH
cause for the poor activity of the dithiaphosphole and dioxa- 1.2 Hz, JPH = 0.6 Hz, 1H, Ar–H), 2.39 (s, 3H, CH
= 8.1 Hz, J = 1.2 Hz,
PH
3
HH
3
4
=
5
13
1
3
). C{ H}
3
phosphole pre-catalysts 6a, 6b, 9a and 9b.
NMR (126 MHz, 295 K, CDCl ): δ/ppm 137.9 (d, J = 3.3 Hz,
C, Ar), 137.3 (1C, Ar), 134.4 ( JPC = 3.6 Hz, 1C, Ar), 128.1 (1C,
Ar), 126.6 (d, JPC = 5.5 Hz, 1C, Ar), 125.8 (d, JPC = 5.5 Hz, 1C,
Ar), 21.1 (1C, Ar–CH3). P{ H} NMR (202 MHz, 295 K, CDCl ):
δ/ppm 160.4 (s, 1P). IR νmax (cm ): 1458 (m), 1375 (sh), 1258
w), 1146 (w), 1036 (w), 874 (w), 804 (s), 685 (w) and 635 (w).
3
3
PC
1
2
2
3
1
1
3
Experimental
General experimental
−
1
(
+
+
+
All reactions were carried out under an atmosphere of dinitro- HRMS (EI ) m/z calculated for [M] [C H ClPS ] : 219.9337,
7
6
2
gen using standard Schlenk and glove box techniques. With found: 219.9341. Melting point 38–41 °C.
the exception of THF and Et O, all solvents used were dried by 2-Bromo-5-methylbenzo-1,3,2-dithiaphosphole (1b). Compound
2
passing through an alumina column incorporated into an MB 1b was synthesised according to general procedure 1 using
SPS-800 solvent purification system, degassed and finally phosphorus(III) bromide (346 mg, 1.28 mmol, 1.0 equiv.) and
stored in an ampoule fitted with a Teflon valve under a dinitro- toluene dithiol (200 mg, 1.28 mmol, 1.0 equiv.). Yield: 288 mg,
1
gen atmosphere. THF and Et O were dried over molten potass- 1.09 mmol, 85%. H NMR (400 MHz, 295 K, CDCl ): δ/ppm
2
3
Dalton Trans.
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Paper
3
4
7
.57 (dd, JHH = 8.1 Hz, JPH = 1.1 Hz, 1H, Ar–H), 7.51 (s, 1H, was added dropwise to a solution of 2-chloro-5-methylbenzo-
3
4
Ar–H), 7.14 (dd, J
= 8.1 Hz, J
= 1.1 Hz, 1H, Ar–H), 2.40 1,3,2-dithiaphosphole (1a) (401 mg, 1.82 mmol, 2.0 equiv.) in
HH
HH
1
1
3
(
3 3
s, 3H, CH ). C{ H} NMR (101 MHz, 295 K, CDCl ): δ/ppm MeCN (10 mL) at 0 °C. The mixture was allowed to slowly
3
3
1
3
1
38.8 (d, JPC = 3.2 Hz, 1C, Ar), 137.4 (1C, Ar), 135.3 (d, JPC
=
warm to ambient temperature before being heated to reflux for
2
.4 Hz, 1C, Ar), 128.2 (1C, Ar), 126.7 (d, J = 5.6 Hz, 1C, Ar), 16 hours. The resulting solution was cooled in an ice bath at
PC
2
31
1
25.9 (d, JPC = 5.5 Hz, 1C, Ar), 21.1 (1C, Ar–CH
3
). P{ H} NMR 0 °C for three hours to give a white precipitate. The MeCN was
−
1
(
162 MHz, 295 K, CDCl
3
): δ/ppm 163.3 (s, 1P). IR νmax (cm ): removed via filter cannula and the white solid was washed
1
1
5
2
456 (m), 1440 (sh), 1375 (w), 1258 (w), 1142 (w), 1115 (w), with pentane (3 × 10 mL). After which the solid was dried in
036 (w), 999 (w), 947 (w), 876 (w), 817 (s), 687 (w), 635 (w) and vacuo to give the product tris (5-methylbenzo-1,3,2-dithiapho-
+
+
+
41 (w). HRMS (EI ) m/z calculated for [M] [C
63.8832, found: 263.8838. Melting point 60–64 °C.
7
H
6
BrPS
2
] : sphol-2-yl)amine (1e) as
a
white solid. Yield: 451 mg,
1
0.79 mmol, 87%. H NMR (500 MHz, 295 K, CDCl ): δ/ppm
3
3
2
-Iodo-5-methylbenzo-1,3,2-dithiaphosphole (1c). Phos- 7.42 (d, JHH = 8.1 Hz, 1H, Ar–H), 7.36 (s, 1H, Ar–H), 6.99 (d,
3
13
1
phorus(III) iodide (334 mg, 0.08 mmol, 1.0 equiv.) in CH
2
Cl
2
3
JHH = 8.1 Hz, 1H, Ar–H), 2.34 (s, 3H, CH ). C{ H} NMR
(
(
2 mL) was added dropwise to a solution of toluene-3,4-dithiol (126 MHz, 295 K, CDCl ): δ/ppm 139.7 (1C, Ar), 136.3 (1C, Ar),
3
2 2
127 mg, 0.08 mmol, 1.0 equiv.) in CH Cl (3 mL). The reaction 136.1 (1C, Ar), 127.3 (1C, Ar), 125.1 (1C, Ar), 124.3 (1C, Ar),
3
1
1
was allowed to stir at ambient temperature for 24 hours, after 21.1 (1C, Ar–CH
which time the solvent was removed in vacuo. The resulting δ/ppm 86.6 (s, 1P). Elemental analysis Found (Calc. for
orange solid was washed with pentane (3 × 2 mL) and again ): C = 43.94% (44.27); H = 3.00% (3.18) N = 2.73%
dried in vacuo, to give the product 2-iodo-5-methylbenzo-1,3,2- (2.46). IR νmax (cm ): 1456 (m), 1375 (w), 1258 (w), 1115 (w),
dithiaphosphole as an orange solid. Yield: 185 mg, 0.59 mmol, 775 (br, s) and 685 (w). HRMS (AP ) m/z calculated for [M + H]
3 3
). P{ H} NMR (202 MHz, 295 K, CDCl ):
21 3 6
C H18NP S
−
1
+
+
+
7
3%. Crystals suitable for single crystal X-ray diffraction were [C21
grown from a saturated solution of CH Cl with a few drops of 150–152 °C.
pentane added. H NMR (500 MHz, 295 K, CDCl ): δ/ppm 7.55 2-Chlorobenzo-1,3,2-dithiaphosphole (2a). Compound 2a
dd, JHH = 8.1 Hz, JPH = 1.3 Hz, 1H, Ar–H), 7.50 (s, 1H, Ar–H), was synthesised according to general procedure 1 using phos-
H
19NP
3
S
6
] : 569.9055, found: 569.9059. Melting point
2
2
1
3
3
4
(
3
4
5
7
.15 (ddd, JHH = 8.1 Hz, JHH = 1.6 Hz, JPH = 0.7 Hz 1H, phorus(III) chloride (89 mg, 0.65 mmol, 1.2 equiv.) and
1
3
1
Ar–H), 2.41 (s, 3H, CH ). C{ H} NMR (126 MHz, 295 K, benzene dithiol (77 mg, 0.54 mmol, 1.0 equiv.). Yield: 104 mg,
CDCl ): δ/ppm 140.2 (d, JPC = 3.1 Hz, 1C, Ar), 137.5 (1C, Ar), 0.50 mmol, 93%. Crystals suitable for single crystal X-ray diffr-
3
1
5
3
3
3
2
36.7 ( JPC = 3.1 Hz, 1C, Ar), 128.2 (1C, Ar), 127.0 (d, JPC
=
action were grown from a saturated solution of CH
2
Cl
2
with a
2
1
.6 Hz, 1C, Ar), 126.2 (d, JPC = 5.6 Hz, 1C, Ar), 21.1 (1C, few drops of pentane added. H NMR (400 MHz, 295 K,
3
1
1
3
4
4
3 3 3
Ar–CH ). P{ H} NMR (202 MHz, 295 K, CDCl ): δ/ppm 155.0 CDCl ): δ/ppm 7.69 (ddd, JHH = 5.9 Hz, JHH = 3.3 Hz, JPH =
−
1
3
4
(s, 1P). IR νmax (cm ): 1458 (w), 1379 (w), 1254 (w), 854 (w), 1.3 Hz, 2H, Ar–H), 7.32 (dd, JHH = 5.9 Hz, JHH = 3.3 Hz, 2H,
+
13
1
8
04 (s), 692 (w), 637 (w) and 538 (w). HRMS (EI ) m/z calculated Ar–H). C{ H} NMR (101 MHz, 295 K, CDCl ): δ/ppm 137.8 (d,
3
+
+
3
2
for [M] [C
8
7
H
6
IPS
2
] : 311.8693, found: 311.8687. Melting point
JPC = 3.3 Hz, 2C, Ar), 126.9 (2C, Ar), 126.1 (d, JPC = 5.6 Hz, 2C,
3
1
1
6–90 °C.
5
3
Ar). P{ H} NMR (162 MHz, 295 K, CDCl ): δ/ppm 158.3 (s,
−
1
-Methyl-N,N-bis(trimethylsilyl)benzo-1,3,2-dithiaphosphol- 1P). IR νmax (cm ): 1441 (m), 1429 (sh), 1252 (w), 1103 (w),
-amine (1d). 2-Chloro-5-methylbenzo-1,3,2-dithiaphosphole 937 (w), 743 (s) and 662 (w). HRMS (EI ) m/z calculated for
1a) (241 mg, 1.09 mmol, 1.0 equiv.) dissolved in toluene [M] [C
5 mL) was added dropwise to lithium bis(trimethylsilyl)amide 40–42 °C.
183 mg, 1.09 mmol, 1.0 equiv.) dissolved in toluene (5 mL) at 2-Bromobenzo-1,3,2-dithiaphosphole (2b). Compound 2b
°C, using an ice bath. The reaction was allowed to slowly was synthesised according to general procedure 1 using phos-
+
2
+
+
(
(
(
6 4 2
H ClPS ] : 205.9181, found: 205.9176. Melting point
0
warm to ambient temperature and left to stir for 18 hours. LiCl phorus(III) bromide (186 mg, 0.69 mmol, 1.0 equiv.) and
salt was removed via filtering the yellow solution, which the benzene dithiol (98 mg, 0.69 mmol, 1.0 equiv.). Yield: 157 mg
solvent was removed in vacuo to give the product as a yellow g, 0.62 mmol, 91%. Crystals suitable for single crystal X-ray
1
oil. Yield: 354 mg, 1.02 mmol, 94%. H NMR (500 MHz, 295 K, diffraction were grown from a saturated solution of CH Cl
2
2
3
1
CDCl
3
): δ/ppm 7.38 (d, JHH = 8.0 Hz, 1H, Ar–H), 7.33 (s, 1H, with a few drops of pentane added. H NMR (400 MHz, 295 K,
Ar–H), 6.96 (d, JHH = 8.0 Hz, 1H, Ar–H), 2.36 (s, 3H, CH
= 1.9 Hz, 18H, SiMe3). C{ H} NMR (126 MHz, 295 K, 1.3 Hz, 2H, Ar–H), 7.33 (dd, J
3 3
CDCl ): δ/ppm 141.1 (1C, Ar), 137.7 (1C, Ar), 134.9 (1C, Ar), Ar–H). C{ H} NMR (101 MHz, 295 K, CDCl ): δ/ppm 138.6 (d,
26.2 (1C, Ar), 124.7 (d, JPC = 8.3 Hz, 1C, Ar), 123.9 (d, JPC
.3 Hz, 1C, Ar), 20.9 (1C, Ar–CH ), 4.26 (d, J = 9.7 Hz, 6C, Ar). P{ H} NMR (162 MHz, 295 K, CDCl ): δ/ppm 160.9 (s,
3 SiC 3
3 3
SiMe ). P{ H} NMR (202 MHz, 295 K, CDCl ): δ/ppm 93.9 (s, 1P). IR νmax (cm ): 1441 (m), 1425 (sh), 1252 (w), 936 (w),
P). HRMS (ES ) m/z calculated for [M + H] [C13
46.0705, found: 346.0714.
tris(5-Methylbenzo-1,3,2-dithiaphosphol-2-yl)amine
-Methyl-N,N-bis(trimethylsilyl)benzo-1,3,2-dithiaphosphol-2-
3
3
4
4
3
), 0.27 CDCl
3
): δ/ppm 7.70 (ddd, JHH = 5.9 Hz, JHH = 3.3 Hz, JPH
=
2
13
1
3
4
(
s, J
= 5.9 Hz, J = 3.3 Hz, 2H,
HH
SiH
HH
1
3
1
2
2
3
2
1
8
=
JPC = 3.3 Hz, 2C, Ar), 127.0 (2C, Ar), 126.3 (d, JPC = 5.7 Hz, 2C,
1
31
1
3
1
1
−1
+
+
+
+
+
1
3
2 2
H25NSi PS ] : 741 (s) and 662 (w). HRMS (EI ) m/z calculated for [M]
+
[C H BrPS ] : 249.8675, found: 249.8682. Melting point
6
4
2
(1e). 62–64 °C.
5
2-Iodobenzo-1,3,2-dithiaphosphole (2c). Phosphorus(III)
amine (1d) (300 mg, 0.91 mmol, 1.0 equiv.) in MeCN (10 mL) iodide (517 mg, 1.26 mmol, 1.0 equiv.) in CH Cl (2 mL) was
2
2
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3
1
added dropwise to a solution of benzene dithiol (179 mg, stirred at 100 °C for 4 days and monitored by P NMR spec-
.26 mmol, 1.0 equiv.) in CH Cl (3 mL). The reaction was troscopy. After completion, the reaction mixture was filtered
1
2
2
allowed to stir at ambient temperature for 24 hours, after and the filtrate evaporated in vacuo to afford a yellow solid.
which time the solvent was removed in vacuo. The resulting Recrystallisation by slow evaporation of a toluene solution
red solid was washed with pentane (3 × 2 mL) and again dried afforded very pale-yellow crystals of the dimer product 4 or 5.
in vacuo, to give the product 2-iodobenzo-1,3,2-dithiaphosp-
1,3,2-Benzodithiaphosphoryl dimer (4). Compound 4 was
hole as a red solid. Yield: 312 mg, 1.0 mmol, 83%. Crystals synthesised according to general procedure 2 using 2a (1.00 g,
suitable for single crystal X-ray diffraction were grown from a 4.84 mmol, 1 equiv.) and sodium (300 mg, 13.0 mmol, 2.7
1
saturated solution of CH
2
Cl
2
with a few drops of pentane equiv.). Yield: 413 mg, 1.21 mmol, 50%. H NMR (500 MHz,
1
added. H NMR (500 MHz, 295 K, CDCl
3
): δ/ppm 7.68 (ddd, 295 K, CDCl
3
31
): δ/ppm 7.03–7.01 (m, 4H, Ar–H), 6.63–6.61 (m,
JHH = 5.9 Hz, J = 3.3 Hz, J = 1.3 Hz, 2H, Ar–H), 7.34 (dd, 4H, Ar–H). P{ H} NMR (202 MHz, 295 K, CDCl ): δ/ppm
3
4
4
1
HH
4
PH
3
3
13
1
J
HH = 5.9 Hz,
126 MHz, 295 K, CDCl
Ar), 127.0 (2C, Ar), 126.5 (d, JPC = 5.7 Hz, 2C, Ar). P{ H}
NMR (202 MHz, 295 K, CDCl ): δ/ppm 152.4 (s, 1P). IR νmax 5 was synthesised according to general procedure 2 using 1a
12 2 4
JHH = 3.3 Hz, 2H, Ar–H). C{ H} NMR 38.3 (s, 2P). Elemental analysis Found (Calc. for C14H P S ):
3
(
3
): δ/ppm: 140.0 (d, JPC = 3.3 Hz, 2C, C = 43.2% (42.1); H = 2.9% (2.4). Melting point 224–225 °C.
2
31
1
5-Methyl-1,3,2-benzodithiaphosphoryl dimer (5). Compound
3
−
1
+
(
cm ): 1439 (m), 1420 (sh), 784 (s) and 662 (w). HRMS (EI ) (1.07 g, 4.85 mmol, 1 equiv.) and sodium (300 mg, 13.0 mmol,
m/z calculated for [M] [C H IPS ] : 297.8537, found: 297.8537. 2.7 equiv.). Yield: 630 mg, 1.70 mmol, 70%. H NMR
+
+
1
6
4
2
3
Melting point 76–78 °C.
5
(300 MHz, 295 K, CD
2
Cl
2
): δ/ppm 7.35 (d, JHH = 8.1 Hz, 2H,
-Methylbenzo-1,3,2-dithiaphosphenium tetrachloroalumi- Ar–H), 7.31 (s, 2H, Ar–H), 6.98 (d, JHH = 8.1 Hz, 2H, Ar–H),
3
3
1
1
nate
(3a).
2-Chloro-5-methylbenzo-1,3,2-dithiaphosphole 2.31 (s, 6H, CH3). P{ H} NMR (121 MHz, 295 K, CDCl ):
3
(
103 mg, 0.47 mmol, 1.0 equiv.) in CH
aluminium(III) chloride (62 mg, 0.47 mmol, 1.0 equiv.) in
CH Cl (2 mL) and left to stir at ambient temperature for 221–222 °C.
2 2
Cl (3 mL) was added to δ/ppm 40.6 (s, 2P). Elemental analysis Found (Calc. for
C
14
H
12
P
2
S
4
): C = 45.5% (45.4); H = 3.3% (3.3). Melting point
2
2
6
hours. After this the solvent was removed in vacuo and
General procedure 3. To a solution of 2-chloro-5-methyl-
2 2
washed with pentane (3 × 2 mL) and dried again in vacuo, benzo-1,3,2-dithiaphosphole (1.0 equiv.) in CH Cl (5 mL),
giving the product 5-methylbenzo-1,3,2-dithiaphosphenium benzyl alcohol (1.0 equiv.) or neopentyl alcohol (1.0 equiv.)
tetrachloroaluminate as a highly air-sensitive orange powder. and triethyl amine (1.0 equiv.) were added dropwise. The reac-
2
7
Yield: 139 mg, 0.39 mmol, 83%. Al NMR (130 MHz, 295 K, tion was allowed to stir at ambient temperature for 24 hours,
−
+
C D Br): δ/ppm 103.9 (s, 1 Al, AlCl ). HRMS (EI ) m/z calcu- after which the solvent was removed in vacuo. Toluene (2 mL)
6
5
4
+
+
lated for [M] [C
7
H
6
PS
2
] : 184.9649, found: 184.9649. IR νmax was subsequently added and the resulting solution was filtered
cm ): 1582 (w), 1458 (m), 1381 (w), 1261 (w), 1168 (br, m), through a plug of Celite to remove traces of ammonium salt,
−
1
(
9
64 (br, m), and 802 (w). Melting Point 126–130 °C.
Note: Attempted recrystallisation of 3a from THF afforded product.
a and a set of colourless crystals which were identified as 2-(Benzyloxy)-5-methylbenzo-1,3,2-dithiaphosphole
after which the solvent was again removed in vacuo to give the
1
(6a).
AlCl ·2THF by X-ray crystallography (identical to that reported Compound 6a was synthesised according to general procedure 3
3
4
6
previously).
using 1a (199 mg, 0.91 mmol, 1.0 equiv.) and benzyl alcohol
5
-Methylbenzo-1,3,2-dithiaphosphenium tetrachloro gallate (97 mg, 0.91 mmol, 1.0 equiv.). Yield: 167 mg, 0.57 mmol,
1
3
(
1
3b). 2-Chloro-5-methylbenzo-1,3,2-dithiaphosphole (347 mg, 63%. H NMR (400 MHz, 295 K, CDCl ): δ/ppm 7.50 (d, J
.57 mmol, 1.0 equiv.) in CH
gallium(III) chloride (277 mg, 1.57 mmol, 1.0 equiv.) in CH
2 mL) and left to stir at ambient temperature for 12 hours. 1.7 Hz, J = 0.7 Hz, 1H, Ar–H), 4.22 (d, J = 6.5 Hz, 2H,
PH PH
After this the solvent was removed in vacuo and washed with OCH
pentane (3 × 2 mL) and dried again in vacuo, giving the CDCl
product 5-methylbenzo-1,3,2-dithiaphosphenium tetrachloro 2.5 Hz, 1C, Ar), 136.2 ( J = 3.2 Hz, 1C, Ar), 136.1 (1C, Ar),
=
=
3
HH
2
Cl
2
(3 mL) was added to 8.1 Hz, 1H, Ar–H), 7.44 (s, 1H, Ar–H), 7.28–7.25 (m, 3H, Ar–H),
3
4
2
Cl
2
7.16–7.14 (m, 2H, Ar–H), 7.02 (ddd, JHH = 8.1 Hz, J
HH
5
3
(
1
3
1
2
), 2.36 (s, 3H, Ar–CH
3
). C{ H} NMR (101 MHz, 295 K,
3
3
3
): δ/ppm: 139.7 (d, JPC = 3.0 Hz, 1C, Ar), 136.8 ( JPC
=
3
PC
gallate as a highly air-sensitive orange powder. Yield: 492 mg, 129.2 (1C, Ar), 128.5 (1C, Ar), 128.2 (1C, Ar), 128.0 (1C, Ar),
2
1
.24 mmol, 79%. Crystals suitable for single crystal X-ray 127.1 (1C, Ar), 125.4 (1C, Ar), 124.9 (d, JPC = 6.4 Hz, 1C, Ar),
2
2
diffraction were grown from a saturated solution of CH Cl
with a few drops of pentane added. H NMR (500 MHz, 295 K, Ar–CH2) 21.0 (1C, Ar–CH
124.1 (d, JPC = 6.3 Hz, 1C, Ar), 67.9 (d, JPC = 9.1 Hz, 1C,
). P NMR (162 MHz, 295 K, CDCl
6 5
C D Br): δ/ppm 7.64 (d, JHH = 8.3 Hz, 1H, Ar–H), 7.43 (s, 1H, δ/ppm 124.5 (t, JPH = 6.5 Hz, 1P). IR νmax (cm ): 1456 (m),
2
2
1
31
3
3
):
3
3
−1
3
Ar–H), 7.08 (d, JHH = 8.3 Hz, 1H, Ar–H), 2.16 (s, 3H, CH ). 1364 (sh), 1217 (w), 1115 (w), 955 (s), 910 (sh), 725 (m), 689 (m)
3
+
+
+
+
+
+
HRMS (EI ) m/z calculated for [M] [C
found: 184.9650. IR νmax (cm ): 1582 (w), 1462 (m), 1383 (w), 292.0145, found: 292.0148.
7
H
6
PS
2 2
] : 184.9649, and 586 (w). HRMS (EI ) m/z calculated for [M] [C15H13OPS ] :
−
1
1
312 (w), 1099 (br, s), 966 (br, m), 872 (w) and 810 (s). Melting
5-Methyl-2-(neopentyloxy)benzo-1,3,2-dithiaphosphole (6b).
Point 104–106 °C.
Compound 6b was synthesised according to general procedure
General procedure 2. A suspension of 1a or 2a (1 equiv.) and 3 using 1a (223 mg, 1.01 mmol, 1.0 equiv.) and neopentyl
finely chopped sodium (2.7 equiv.) in toluene (20 mL) was alcohol (89 mg, 1.01 mmol, 1.0 equiv.). Yield: 179 mg,
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1
0
.66 mmol, 65%. H NMR (400 MHz, 295 K, CDCl
3
): δ/ppm (40 mL) and cooled to
0
°C, phosphorus(III) chloride
3
7
.46 (d, J = 8.1 Hz, 1H, Ar–H), 7.40 (s, 1H, Ar–H), 6.99 (ddd, (1.2 equiv.) or phosphorus(III) bromide (1.2 equiv.) and triethyl-
HH
HH = 8.1 Hz, JHH = 1.2 Hz, JPH = 0.5 Hz, 1H, Ar–H), 2.85 (d, amine (2.4 equiv.) were added dropwise. The reaction turned
3
4
5
J
J
3
PH = 6.1 Hz, 2H, OCH
2
), 2.36 (s, 3H, Ar–CH
̲ ) ). C{ H} NMR (101 MHz, 295 K, CDCl ): δ/ppm: 24 hours. The solution was filtered via filter canula to a new
3
3
), 0.77 (s, 9H, yellow and was allowed to stir at ambient temperature for
1
3
1
CH (CH
2
3 3
3
3
1
1
1
39.9 (d, JPC = 2.9 Hz, 1C, Ar), 136.4 (d, JPC = 3.2 Hz, 1C, Ar), Schlenk tube to remove the ammonium salt generated, after
2
35.8 (1C, Ar), 126.9 (1C, Ar), 124.8 (d, JPC = 6.5 Hz, 1C, Ar), which the solvent was removed in vacuo to give a powder. The
23.9 (d, JPC = 6.4 Hz, 1C, Ar), 75.4 (d, JPC = 9.9 Hz, 1C, powder was washed with pentane (3 × 2 mL) and again dried
), 31.6 (d,
–CH ), 21.0 (1C, Ar–CH
95 K, CDCl ): δ/ppm 123.6 (t, JPH = 6.1 Hz, 1P). IR νmax synthesised according to general procedure 5 using phosphorus
cm ): 1456 (m), 1364 (sh), 1258 (w), 1217 (w), 1117 (w), 972 (III) chloride (0.35 mL, 4.0 mmol, 1.2 equiv.), N,N′-diisopropyl-
s), 789 (m), 727 (m) and 687 (sh). HRMS (EI ) m/z calculated benzene-1,2-diamine (640 mg, 3.33 mmol, 1.0 equiv.), and
for [M] [C H OPS ] : 272.0458, found: 272.0452.
General procedure 4. To a solution of catechol (1.0 equiv.) yellow powder. Yield: 692 mg, 2.70 mmol, 81%. H NMR
dissolved in toluene (40 mL) and cooled to 0 °C, phosphorus (400 MHz, 295 K, CDCl ): δ/ppm 7.08 (s, 4H, Ar–H), 4.32 (sept,
III) chloride (1.2 equiv.) or phosphorus(III) bromide (1.2 equiv.)
and triethylamine (2.4 equiv.) were added dropwise. The reac- 1.0 Hz, 12H, CH(CH
tion immediately turned yellow and was left to stir at ambient CDCl ): δ/ppm 136.9 (d, JPC = 10.5 Hz, 2C, Ar), 121.3 (2C, Ar),
temperature for 24 hours. The solution was filtered via filter 111.6 (d, J = 1.6 Hz, 2C, Ar), 48.1 (d, J = 12.7 Hz, 2C, C
canula to a new Schlenk tube to remove the ammonium salt (CH
generated, after which the solvent was removed in vacuo to give CDCl
2
2
3
C(CH
3
)–C
̲
H
2
J
PC = 2.3 Hz, 1C, C
̲
3
)
3
–CH
2
), 26.5 in vacuo to give the pure product as a solid powder.
3
1
(3C, C(C
̲H
3
)
3
2
3
). P NMR (162 MHz,
2-Chlorobenzo-1,3,2-diazaphosphole (8a). Compound 8a was
3
2
(
(
3
−
1
+
+
+
triethyl amine (1.11 mL, 8.0 mmol, 2.4 equiv.). Product is a
1
2
17
2
1
3
3
3
4
(
JHH = 6.6 Hz, 2H, CH
̲
= 6.6 Hz, J
=
3 2
HH
PH
1
3
1
3 2
̲ ) ). C{ H} NMR (101 MHz, 295 K,
2
3
3
2
̲H
PC
PC
3
1
1
3
)
2
), 22.3 (4C, CH(C
3 2
̲H ) ). P{ H} NMR (162 MHz, 295 K,
3
): δ/ppm 147.1 (s, 1P). Values in agreement with
3
1
29
a yellow oil. P NMR spectroscopy revealed that this oil con- literature.
tained a mixture of both product and an unidentified side
2-Bromobenzo-1,3,2-diazaphosphole (8b). Compound 8b
product. Therefore, the oil was subjected to an air sensitive was synthesised according to general procedure 5 using
distillation, in which the pure product distils with heating and phosphorus(III) bromide (0.11 mL, 1.19 mmol, 1.2 equiv.),
under vacuum (5 mbar) to give the product as a colourless oil.
-Chlorobenzo-1,3,2-dioxaphosphole (7a). Compound 7a was 1.0 equiv.), and triethyl amine (0.3 mL, 2.38 mmol, 2.4 equiv.).
synthesised according to general procedure 4 using phosphorus Product is an orange powder. Yield: 227 mg, 0.75 mmol, 76%.
N,N′-diisopropylbenzene-1,2-diamine (191 mg, 0.99 mmol,
2
1
(
III) chloride (3.0 mL, 34.9 mmol, 1.2 equiv.), catechol (3.20 g,
H NMR (400 MHz, 295 K, CDCl
3
): δ/ppm 7.18–7.17 (m, 4H,
HH = 6.6 Hz, 2H, CH(CH ), 1.76 (dd,
JHH = 6.6 Hz, J = 1.0 Hz, 12H, CH(CH̲ ) ). C{ H} NMR
3 2
3
2
2
1
9.1 mmol, 1.0 equiv.) and triethylamine (9.7 mL, 69.8 mmol, Ar–H), 4.44 (sept,
J
̲
3 2
)
3
4
13
1
.4 equiv.). Distils at 44–52 °C under vacuum (5 mbar). Yield:
PH
1
2
3 3
.84 g, 10.5 mmol, 36%. H NMR (500 MHz, 295 K, CDCl ): (101 MHz, 295 K, CDCl ): δ/ppm 137.2 (d, JPC = 10.2 Hz,
3
δ/ppm 7.19–7.17 (m, 2H, Ar–H), 7.07–7.05 (m, 2H, Ar–H). 2C, Ar), 122.3 (2C, Ar), 112.4 (d, JPC = 1.6 Hz, 2C, Ar), 49.0 (d,
1
3
1
2
2
3
C{ H} NMR (126 MHz, 295 K, CDCl ): δ/ppm 144.4 (d, J
=
JPC = 11.8 Hz, 2C, C ̲H (CH ) ), 21.8 (d, JPC = 1.6 Hz, 4C,
3 2
3
PC
3
31
1
7
.5 Hz, 2C, Ar), 124.5 (2C, Ar), 114.2 (d, JPC = 0.9 Hz, 2C, Ar). CH(C
): δ/ppm 173.6 (s, 1P). IR 169.2 (s, 1P). IR νmax (cm ): 1477 (m), 1369 (m), 1288 (w),
νmax (cm ): 1470 (s), 1327 (w), 1217 (s), 1092 (w), 1009 (w), 1159 (m), 1007 (m), 932 (m) and 752 (m). HRMS (EI ) m/z
̲
3
)
2
). P{ H} NMR (162 MHz, 295 K, CDCl
3
): δ/ppm
3
1
1
−1
P{ H} NMR (202 MHz, 295 K, CDCl
3
−
1
+
+
+
+
8
93 (s), 739 (s), 716 (sh) and 617 (m). HRMS (EI ) m/z calcu- calculated for [M] [C12
H
18BrN
Melting point 100–106 °C.
General procedure 6. To a solution of 2-chlorobenzo-1,3,2-
was synthesised according to general procedure 4 using phos- dioxaphosphole (1.0 equiv.) in CH Cl (5 mL), benzyl alcohol
phorus(III) bromide (1.30 mL, 13.9 mmol, 1.2 equiv.), catechol (1.0 equiv.) or neopentyl alcohol (1.0 equiv.) and triethyl
1.27 g, 11.6 mmol, 1.0 equiv.) and triethylamine (3.9 mL, amine (1.0 equiv.) were added dropwise. The reaction was
7.8 mmol, 2.4 equiv.). Distils at 60–62 °C under vacuum allowed to stir at ambient temperature for 24 hours, after
2
P] : 300.0391, found: 300.0384.
+
+
lated for [M] [C
6 4 2
H ClO P] : 173.9637, found: 173.9640.
2
-Bromobenzo-1,3,2-dioxaphosphole (7b). Compound 7b
2
2
(
2
(
1
5 mbar). Yield: 1.04 g, 4.76 mmol, 41%. H NMR (500 MHz, which the solvent was removed in vacuo. Toluene (2 mL) was
= 3.4 Hz, subsequently added and the resulting solution was filtered
PH = 0.8 Hz, 2H, Ar–H), 7.19 (dd, JHH = 5.9 Hz, JHH = 3.4 Hz, through a plug of Celite to remove traces of ammonium salt,
H, Ar–H). C{ H} NMR (126 MHz, 295 K, CDCl ): δ/ppm 144.8 after which the solvent was again removed in vacuo to give the
3
d, J = 7.5 Hz, 2C, Ar), 124.7 (2C, Ar), 114.4 (d, J = 1.0 Hz, product as an oil.
PC PC
3
4
2
95 K, CDCl ): δ/ppm 7.31 (ddd, J
= 5.9 Hz, J
3
HH
3
HH
4
4
J
1
3
1
2
(
2
1
9
2
3
3
1
1
C, Ar). P{ H} NMR (202 MHz, 295 K, CDCl
3
): δ/ppm 195.3 (s,
2-(Benzyloxy)benzo-1,3,2-dioxaphosphole (9a). Compound
−1
P). IR νmax (cm ): 1497 (s), 1422 (sh), 1169 (m, br), 1098 (m), 9a was synthesised according to general procedure 6 using
84 (m), 937 (m), 826 (m) and 748 (m). HRMS (EI ) m/z calcu- 2-chlorobenzo-1,3,2-dioxaphosphole (206 mg, 1.18 mmol,
+
+
+
lated for [M] [C
6
H
4
BrO
2
P] : 217.9132, found: 217.9135.
1.0 equiv.), benzyl alcohol (128 mg, 1.18 mmol, 1.0 equiv.),
General procedure 5. To a solution of N,N′-diisopropyl- and triethylamine (119 mg, 1.18 mmol, 1.0 equiv.). Product is
29
benzene-1,2-diamine
(1.0 equiv.) dissolved in toluene a dark yellow/orange oil. Yield: 198 mg, 0.80 mmol, 68%.
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1
H NMR (400 MHz, 295 K, CDCl
3
): δ/ppm 7.32–7.30 (m, 3H, in 5 mL CH
2
Cl
2
) was added dropwise to a solution of 3 (40 mg,
Ar–H), 7.21–7.19 (m, 2H, Ar–H), 7.12–7.10 (m, 2H, Ar–H), 0.11 mmol, 1.0 equiv.) in toluene (5 mL) at 0 °C. The reaction
3
7
.02–7.00 (m, 2H, Ar–H), 4.60 (d,
J
PH = 6.9 Hz, 2H, OCH
2
̲
). mixture was stirred at 0 °C for 2 h and the solvent removed
1
3
1
2
C{ H} NMR (101 MHz, 295 K, CDCl ): δ/ppm 146.0 (d, JPC = in vacuo. Storage of the resultant colourless oil at −20 °C
.6 Hz, 2C, Ar), 136.7 (d, J = 2.9 Hz, 2C, Ar), 128.7 (Ar), 128.4 afforded colourless crystals of 1a. Yield: 42 mg, 0.19 mmol,
Ar), 127.7 (Ar), 123.0 (Ar), 112.2 (Ar), 65.9 (d, JPC = 2.0 Hz, 1C, 88%. H NMR (300 MHz, 295 K, C
2). P{ H} NMR (162 MHz, 295 K, CDCl
26.9 (s, 1P). IR νmax (cm ): 1474 (s), 1373 (w), 1229 (s), 980 1H, Ar–H), 1.80 (s, 3H, CH3). P{ H} NMR (202 MHz, 295 K,
3
3
7
(
PC
2
1
3
6
D
6
): δ/ppm 6.98 (d, JHH
=
3
1
1
3
C(CH
1
(
3 3
)
–CH
̲
3
): δ/ppm 8.1 Hz, 1H, Ar–H), 6.82 (s, 1H, Ar–H), 6.48 (d, JHH = 8.1 Hz,
−1
31
1
+
m), 916 (w), 824 (s), 729 (s), 692 (s) and 625 (m). HRMS (EI )
C
6
D
6
): δ/ppm 161.4 (s, 1P).
Reaction of 5 with Br . 0.88 mL (0.17 mmol, 1.1 equiv.) of a
-(Neopentyloxy)benzo-1,3,2-dioxaphosphole (9b). Compound freshly-prepared bromine solution (153 mg Br2 in 5 mL
b was synthesised according to general procedure 6 using CH Cl ) was added dropwise to a solution of 3 (57 mg,
+
+
m/z calculated for [M] [C13
H O
11 3
P] : 246.0446, found: 246.0441.
2
2
9
2
1
2
2
-chlorobenzo-1,3,2-dioxaphosphole (203 mg, 1.16 mmol, 0.15 mmol, 1.0 equiv.) in toluene (5 mL) at 0 °C. The reaction
.0 equiv.), neopentyl alcohol (103 mg, 1.16 mmol, 1.0 equiv.) mixture was stirred at 0 °C for 2 h and the solvent removed in
2 2
and triethyl amine (117 mg, 1.16 mmol, 1.0 equiv.). Product is vacuo to afford a yellow solid. Recrystallisation from CH Cl
a faint orange/yellow coloured oil. Yield: 168 mg, 0.74 mmol, afforded yellow crystals of 1b. Yield: 58 mg, 0.22 mmol, 70%.
1
1
3
6
2
4%. H NMR (400 MHz, 295 K, CDCl ): δ/ppm 7.08–7.05 (m,
H NMR (500 MHz, 295 K, C D ): δ/ppm 7.03 (d, J = 8.0 Hz,
6 6 HH
3
3
3
H, Ar–H), 6.99–6.96 (m, 2H, Ar–H), 3.21 (d, JPH = 6.5 Hz, 2H, 1H, Ar–H), 6.87 (s, 1H, Ar–H), 6.55 (d, JHH = 8.0 Hz, 1H, Ar–
1
3
1
31
1
OCH
̲
2 2 3 3 6 6
), 0.83 (s, 9H, CH CH̲ ). C{ H} NMR (101 MHz, 295 K, H), 1.86 (s, 3H, CH ). P{ H} NMR (202 MHz, 295 K, C D ): δ/
2
CDCl ): δ/ppm 146.0 (d, J = 7.7 Hz, 2C, Ar), 122.7 (2C, Ar), ppm 163.6 (s, 1P). Elemental analysis Found (Calc. for
3
PC
2
1
11.9 (2C, Ar), 73.6 (d, JPC = 2.0 Hz, 1C, C(CH
3
)
3
–C
), 26.2 (3C, C(CH
P{ H} NMR (162 MHz, 295 K, CDCl ): δ/ppm 127.4 (s, 1P). equiv.) in toluene (2 mL) was added dropwise to a solution of
̲
2
), 31.9 (d,
C
7
H
6
BrPS
2
): C = 31.4% (31.7); H = 2.4% (2.3).
3
J
PC = 2.7 Hz, 1C, C
̲
3
)
3
–CH
2
̲
3
)
3
–CH ).
2
Reaction of 5 with I
2
. A solution of I (33 mg, 0.13 mmol, 1.0
2
3
1
1
3
−
1
IR νmax (cm ): 1476 (s), 1366 (w), 1333 (w), 1231 (s), 1003 (s), 3 (48 mg, 0.13 mmol, 1.0 equiv.) in toluene (3 mL) at 0 °C. The
24 (s), 739 (m), 698 (m), 623 (m) and 536 (w). HRMS (EI ) m/z reaction mixture was stirred at 0 °C for 2.5 hours and the
+
8
+
+
calculated for [M] [C H O P] : 226.0759, found: 226.0756.
solvent removed in vacuo to give
,3-Diisopropyl-benzodiphosphenium triflate (10). To a solu- Recrystallisation from CH Cl afforded yellow crystals of 1c.
tion of 2-chloro-1,3-diisopropyl-benzodiazaphosphole (70 mg, Yield: 55 mg, 0.18 mmol, 68%. H NMR (500 MHz, 295 K,
a
yellow solid.
1
1
15 3
1
2
2
1
3
0
.27 mmol, 1.0 equiv.) dissolved in CH Cl (5 mL), trimethyl- C D ): δ/ppm 6.95 (d, J
silyl trifluoromethanesulfonate (73 mg, 0.33 mmol, 1.2 equiv.) Ar–H), 6.48 (d,
was added dropwise. The solution immediately turned golden
yellow and was allowed to stir for two hours at ambient temp-
erature. Afterwards, the solvent was removed in vacuo and
= 8.0 Hz, 1H, Ar–H), 6.79 (s, 1H,
HH = 8.0 Hz, 1H, Ar–H), 1.79 (s, 3H, CH
P{ H} NMR (202 MHz, 295 K, C ): δ/ppm 155.4 (s, 1P).
2
2
6
6
HH
3
J
3
).
3
1
1
6 6
D
General experimental for hydroboration catalysis
washed with pentane (3 × 2 mL) and again dried in vacuo to In a glovebox under a dinitrogen atmosphere, three separate
give the pure product 1,3-diisopropyl-benzodiphosphenium tri- vials were charged with the phosphorus catalyst (10, 5, 2 or
flate as a yellow powder. Yield: 88 mg, 0.24 mmol, 87%. 1 mol%), aldehyde (0.1 mmol, 1 equiv.), and HBpin (12.8 mg,
1
3
H NMR (400 MHz, 295 K, CDCl ): δ/ppm 7.71–7.69 (m, 2H, 0.1 mmol, 1 equiv.). By syringe, solvent (0.6 mL) was added to
3
Ar–H), 7.63–7.60 (m, 2H, Ar–H), 4.98 (sept, J
= 6.6 Hz, 2H, the vial containing HBpin and then mixed between the three
PH = 1.3 Hz, 12H, CH vials at least twice. The solution was transferred to a J. Young
HH
3
4
CH
̲
(CH
3
)
2
), 1.87 (dd,
J
HH = 6.6 Hz,
). C{ H} NMR (101 MHz, 295 K, CDCl
d, J = 6.0 Hz, 2C, Ar), 127.2 (2C, Ar), 114.3 (2C, Ar), 52.6 (d, 2 h, 6 h, 12 h and 24 hours. Product conversion was calculated
J
1
3
1
(
(
CH
3
2
)
2
3
): δ/ppm 138.6 NMR tube and multinuclear NMR spectra were acquired at
PC
2
31
1
1
J
PC = 10.8 Hz, 2C, C
̲
3
)
2
), 23.9 (4C, CH(C
3 2
̲H ) ). P{ H} from the in situ H NMR spectrum by integrating the aldehyde
1
9
1
NMR (162 MHz, 295 K, CDCl
NMR (376 MHz, 295 K, CDCl ): δ/ppm −78.4 (s, 3F, O SCF ). HBpin.
Values in agreement with literature.
3
): δ/ppm 216.0 (s, 1P). F{ H} signal and new resonance resulting from the hydride from
−
3
3
3
2
9
Characterisation of hydroborated products
,4,5,5-Tetramethyl-2-((4-(trifluoromethyl)benzyl)oxy)-1,3,2-
Polymerisation of THF
4
1
3
In a typical reaction THF (50 mL, 616 mmol, 1369 equiv.) was dioxaborolane (11a). H NMR (500 MHz, 295 K, CDCl ): δ/ppm
added to a sample of 3b (0.018 g, 0.45 mmol, 1.0 equiv.) (equi- 7.57 (d, JHH = 8.3 Hz, 2H, Ar–H), 7.44 (dd, JHH = 8.3 Hz, JHF
valent to 9.06 mmol L or 1 : 2469 w/w). The solution immedi- 0.7 Hz, 2H, Ar–H), 4.97 (s, 2H, CH ), 1.25 (s, 12H, CH ).
ately turned colourless and the progress of the polymerisation
3
3
4
=
−
1
2
3
1
1
B NMR (160 MHz, 295 K, CDCl
3
): δ/ppm 22.4 (s, Bpin).
F{ H} NMR (471 MHz, 295 K, CDCl ): δ/ppm −62.5 (s, 3F,
Ar–CF ). Values in agreement with literature.
1
19
1
was followed by H NMR spectroscopy.
3
2
2
3
Oxidation reactions with dimer 5
2
-((4-Fluorobenzyl)oxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
1
Reaction of 5 with SO
2 2 3
Cl . 0.35 mL (0.12 mmol, 1.1 equiv.) (11b). H NMR (500 MHz, 295 K, CDCl ): δ/ppm 7.33–7.29 (m,
3
of a freshly-prepared sulfuryl chloride solution (227 mg SO Cl2 2H, Ar–H), 7.01 (ov dd, JHH, HF, 8.8 Hz, 2H, Ar–H), 4.87 (s, 2H,
2
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1
1
CH
2
), 1.26 (s, 12H, CH
3
9
). B NMR (128 MHz, 295 K, CDCl
3
):
4,4,5,5-Tetramethyl-2-(neopentyloxy)-1,3,2-dioxaborolane (11l).
1
1
1
δ/ppm 22.3 (s, Bpin). F{ H} NMR (376 MHz, 295 K, CDCl ):
H NMR (400 MHz, 295 K, CDCl ): δ/ppm 3.50 (s, 2H, CH ),
3
3
2
1
1
δ/ppm −115.3 (s, 1F, Ar–F). Values in agreement with 1.07 (s, 12H, CH
3
3
), 0.88 (s, 9H, CH ). B NMR (128 MHz,
2
2
literature.
2
(
3
295 K, CDCl ): δ/ppm 22.1 (s, BPin). Values in agreement with
4
8
-((3-Fluorobenzyl)oxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane literature.
11c). H NMR (400 MHz, 295 K, CDCl
1
3
): δ/ppm 7.31–7.27 (m,
2
4
2
2
H, Ar–H), 7.11–7.05 (m, 2H, Ar–H), 6.97–6.92 (m, 1H, Ar–H),
1
1
.91 (s, 2H, CH ), 1.27 (s, 12H, CH ). B NMR (128 MHz, Conclusions
2
3
1
9
1
95 K, CDCl
95 K, CDCl
3
): δ/ppm 22.3 (s, Bpin). F{ H} NMR (376 MHz,
): δ/ppm −113.4 (s, 1F, Ar–F). Values in agreement
This work has provided a series of crystallographic studies on
a range of dithiaphosphole compounds. The solid-state struc-
tures of the halogen-containing species has revealed elongation
in the P–X bond (where X = halogen), with computational sup-
porting extensive polarisation of the P–X bond. These P-halo
dithiaphospholes have then be used in a series of additional
reactions. Crystallographic studies were then performed on
these products. This was followed by a comparative study of the
catalytic activity of these dithiaphospholes in relation to the
corresponding dioxaphosphole and diazaphosphole com-
pounds. Among those examined as metal-free pre-catalysts for
the hydroboration of a series of aldehydes with HBpin, the
phosphenium cation 1,3-diisopropyl-benzodiphosphenium tri-
flate showed excellent conversion rates under mild conditions.
3
2
2
with literature.
-((2-Fluorobenzyl)oxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
2
1
(11d). H NMR (500 MHz, 295 K, CDCl
3
): δ/ppm 7.47–7.42 (m,
3
1
J
H, Ar–H), 7.25–7.22 (m, 1H, Ar–H), 7.12 (td, JHH = 7.5 Hz,
4
HH = 1.1 Hz, 1H, Ar–H), 7.04–6.96 (m, 1H, Ar–H), 5.00 (s, 2H,
11
CH
2 3 3
), 1.27 (s, 12H, CH ). B NMR (128 MHz, 295 K, CDCl ):
19
1
δ/ppm 22.3 (s, Bpin). F{ H} NMR (376 MHz, 295 K, CDCl ):
δ/ppm −119.2 (s, 1F, Ar–F). Values in agreement with literature.
-((4-Bromobenzyl)oxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
3
2
2
2
1
3
(11e). H NMR (400 MHz, 295 K, CDCl ): δ/ppm 7.45 (d, J
=
3
HH
3
8
(
.7 Hz, 2H, Ar–H), 7.22 (d,
s, 2H, CH ), 1.26 (s, 12H, CH
CDCl ): δ/ppm 22.3 (s, Bpin). Values in agreement with
JHH = 8.7 Hz, 2H, Ar–H), 4.86
3
). B NMR (128 MHz, 295 K,
1
1
2
3
2
2
literature.
-(((4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)oxy)methyl)
benzonitrile (11f). H NMR (400 MHz, 295 K, CDCl ): δ/ppm
4
1
Conflicts of interest
3
7
.64–7.60 (m, 2H, Ar–H), 7.45–7.43 (m, 2H, Ar–H), 4.97 (s, 2H,
There are no conflicts to declare.
1
1
2 3 3
CH ), 1.26 (s, 12H, CH ). B NMR (128 MHz, 295 K, CDCl ):
δ/ppm 22.4 (s, 1B, Bpin). Values in agreement with literature.
,4,5,5-Tetramethyl-2-((4-nitrobenzyl)oxy)-1,3,2-dioxaborolane
2
2
4
1
3
Acknowledgements
=
= 8.9 Hz, 2H, Ar–H), 5.01 (s,
(11g). H NMR (500 MHz, 295 K, CDCl
3
): δ/ppm 8.18 (d, JHH
3
8
2
.9 Hz, 2H, Ar–H), 7.49 (d, J
HH
H, CH
RLM would like to acknowledge the EPSRC for an Early Career
Fellowship (EP/R026912/1). JMR would like to thank NSERC
for an operating grant (NSERC Discovery Grant RGPIN-2010-
1
1
2 3
), 1.26 (s, 12H, CH ). B NMR (160 MHz, 295 K,
CDCl
literature.
-(Benzyloxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (11h).
H NMR (400 MHz, 295 K, CDCl ): δ/ppm 7.26–7.24 (m, 2H,
Ar–H), 7.20–7.18 (m, 2H, Ar–H), 4.85 (s, 2H, CH ), 1.18 (s, 12H,
3
): δ/ppm 21.1 (s, Bpin). Values in agreement with
4
7
05523). We thank Dr A. D. Bond (Cambridge University) for
2
assistance with the structure of compound 3.
1
3
2
1
1
CH
Values in agreement with literature.
,4-bis(((4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)oxy)methyl)
3 3
). B NMR (128 MHz, 295 K, CDCl ): δ/ppm 22.3 (s, Bpin).
Notes and references
2
2
1
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and K. Oshima, Wiley, 2006.
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1
3
benzene (11i). H NMR (400 MHz, 295 K, CDCl ): δ/ppm 7.30
1
1
(s, 4H, Ar–H), 4.90 (s, 4H, CH
2 3
), 1.25 (s, 24H, CH ). B NMR
(128 MHz, 295 K, CDCl ): δ/ppm 22.3 (s, Bpin). Values in agree-
3
2
2
ment with literature.
N,N-Dimethyl-4-(((4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
1
oxy)methyl)aniline (11j). H NMR (400 MHz, 295 K, CDCl ):
3
3
3
δ/ppm 7.25 (d, JHH = 8.8 Hz, 2H, Ar–H), 6.75 (d, JHH = 8.8 Hz,
H, Ar–H), 4.82 (s, 2H, CH ), 2.95 (s, 6H, N(CH ), 1.26
s, 12H, CH3). B NMR (128 MHz, 295 K, CDCl ): δ/ppm 22.3
2
2
3
)
2
1
1
(
(
3
4
7
s, Bpin). Values in agreement with literature.
,4,5,5-Tetramethyl-2-(naphthalen-2-ylmethoxy)-1,3,2-dioxaboro-
4
1
lane (11k). H NMR (400 MHz, 295 K, CDCl ): δ/ppm 7.84–7.81
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3
(
(
(
m, 4H, Ar–H), 7.48–7.44 (m, 3H, Ar–H), 5.11 (s, 2H, CH
s, 12H, CH
s, Bpin). Values in agreement with literature.
2
), 1.29
1
1
3
). B NMR (128 MHz, 295 K, CDCl ): δ/ppm 21.1
3
2
2
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Paper
Dalton Transactions
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1
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