Methylation of Some Cyclic Triphosphenium Ions

Article abstract of DOI:10.1002/hc.10228

Six cyclic triphosphenium ions, including examples of five-, six-, and seven-membered ring systems, have been successfully methylated by excess methyl triflate to form the corresponding di-cations. This has been unequivocally established by means of

Full text of DOI:10.1002/hc.10228

Heteroatom Chemistry
Volume 15, Number 2, 2004
Methylation of Some Cyclic Triphosphenium
Ions
Keith B. Dillon and Richard J. Olivey
Chemistry Department, University of Durham, South Road, Durham DH1 3LE, UK
Received 2 October 2003; revised 1 November 2003
strated that protonation of the central phosphorus
atom in 1 was possible by using AlCl3 and BuCl
ABSTRACT: Six cyclic triphosphenium ions, includ-
ing examples of five-, six-, and seven-membered ring
systems, have been successfully methylated by ex-
cess methyl triflate to form the corresponding di-
cations. This has been unequivocally established by
means of 31P NMR spectroscopy; results are in good
agreement with literature data for this type of com-
pound. 2004 Wiley Periodicals, Inc. Heteroatom Chem
15:150–154, 2004; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.10228
t
[4,13]; we have more recently extended this work
to a number of other cyclic systems [Burton, J. D.;
Deng, R. M. K.; Dillon, K. B.; Olivey, R. J. in prepa-
ration]. The only report of a successful alkylation
is for the dication 2 derived from tetraphos, which
was chloromethylated by a mixture of CH2Cl2 (2 mol)
and AlCl3 (2 mol) to form a tetracationic species 3,
Eq. (1) [4].
ꢀ
C
INTRODUCTION
The first cyclic triphosphenium ion 1 (Scheme 1)
was described and fully characterized, including
a single crystal X-ray molecular structure as its
hexachlorostannate(IV) salt, by Schmidpeter and
coworkers in 1982 [1]. Subsequently several other
examples have been reported, including ions with
a five-membered ring [1–6], a six-membered ring
[4–7], two linked six-membered rings [4,7], and
a seven-membered ring [5,6], as well as four- [8]
and eight-membered neutral species with either one
[9,10] or two [11,12] units of three linked phospho-
rus atoms. The ions are all characterized by a low-
frequency 31P NMR shift for the central phosphorus
atom and a large 1JPP (>400 Hz) value. Where molec-
ular structures have been ascertained, the P–P dis-
tances are intermediate between normal single and
double P–P bonds [1,5,6]. The chemistry of cyclic
triphosphenium ions has been comparatively little
investigated, however. Schmidpeter et al. demon-
(1)
Compound 3 gave rise to an [A2B]2 31P{ H} NMR
1
1
spectrum, with δ PA 8.7, δ PB −79.8 ppm, JPP 282.4
1
Hz, compared with δ PA 17.6, δ PB −249.6 ppm, JPP
440.5 Hz for 2. As expected, the central phosphorus
atom PB showed a far larger shift change on alky-
lation than the outer phosphorus atoms PA, and the
P–P coupling constant was substantially reduced.
Two similar dicationic derivatives 4 and 5 of single-
ring cyclic triphosphenium ions have been synthe-
sized from noncyclic precursors, according to Eq. (2)
[13].
Correspondence to: Keith B. Dillon; e-mail: k.b.dillon@
durham.ac.uk.
2004 Wiley Periodicals, Inc.
150
(2)
c
ꢀ

Methylation of Some Cyclic Triphosphenium Ions 151
tween cyclic triphosphenium ion 1 and iodomethane
was unsuccessful, however, providing no evidence
for alkylation [14; Dillon, K. B.; Wilkinson, J. J.
unpublished work].
EXPERIMENTAL
All manipulations, including NMR sample prepa-
ration, were carried out either under an inert at-
mosphere of dry nitrogen or in vacuo. Chemicals
of the best available commercial grade were used,
in general without further purification. The 31P
NMR spectra of the diphosphanes were recorded
before reaction, to verify that no major phosphorus-
containing impurities were present. Some small
impurity peaks were found in the spectrum of ꢀ,ꢀꢁ-
bis(diphenylphosphino)-o-xylene, the precursor of
10, but these did not affect the assignment of the
main resonances. 31P NMR spectra were recorded
on Varian Mercury 200, Varian Unity 300, or Var-
ian VXR 400 Fourier-transform spectrometers, at
80.96, 121.40, and 161.91 MHz respectively; chem-
ical shifts were measured relative to external 85%
H3PO4. The ring systems 1 and 6–10 were prepared
from 2:3 reactions between PCl3 and the diphos-
phane in dichloromethane as solvent, Eq. (4), as
described previously [5,6]. The mixtures were al-
lowed to stir at room temperature to complete
the reaction as far as possible. After confirming
that the cyclic triphosphenium ion had formed,
an equimolar quantity of methyl triflate to that
of the PCl3 used was added by syringe. Often
further aliquots of methyl triflate were required
(Section Results and Discussion) to complete the
methylation.
RESULTS AND DISCUSSION
SCHEME 1
As mentioned above, a stoichiometric (1:1) reaction
between cyclic triphosphenium ion 1 (as its chloride
salt) and MeI provided no evidence for methylation
of the ring, the only cyclic species detected being the
starting material 1 [14; Dillon, K. B.; Wilkinson, J. J.
unpublished work]. Since the ring appeared to sur-
vive these conditions, the more powerful methylat-
ing agent methyl triflate was utilized in the present
work. Six representative cyclic triphosphenium ions
1 and 6–10 (Scheme 1) were generated in situ from
PCl3 and the appropriate diphosphane, as described
in earlier papers [5,6]. As can be seen from Scheme 1,
these include examples of five-, six-, and seven-
membered ring species. The results will be described
1
The NMR data for 4 (δ PA 52, δ PB −78 ppm, JPP
1
282 Hz) and 5 (δ PA 52, δ PB −79 ppm, JPP 283 Hz)
were very similar to those for 3. In view of the ex-
istence of these species, there seemed no good rea-
son why direct alkylation of cyclic triphosphenium
ions should not be possible. In this work we report
on the successful methylation of a number of cyclic
triphosphenium ions, using methyl triflate as the
alkylating agent; an excess of the latter reagent was
usually required. A preliminary experiment involv-
ing an attempted stoichiometric (1:1) reaction be-

152 Dillon and Olivey
for each system individually. In each case the 31P
NMR spectrum of the solution was recorded to check
that the cyclic triphosphenium ion had formed, be-
fore addition of methyl triflate.
J. A.; Dillon, K. B.; Howard, J. A. K.; Olivey, R. J.;
Roden, M. D. in preparation].
11 + 2 HCl → dppeH22+ + MePCl2
(3)
When an equimolar quantity of methyl triflate
was added to 1 (as its chloride), no methylation of
the ring was observed in the NMR spectrum imme-
diately, or after 1 day. A second equimolar amount
was added, and a new weak doublet and triplet were
detected, ascribed to the methylated dication 11, al-
though the original ring was still present after a fur-
ther 3 days. Two more equivalents of methyl triflate
(all volumes of methyl triflate were measured quan-
titatively, by syringe) were added, and the intensity
of the new doublet and triplet increased (Table 1), al-
though a weak doublet from the original ring was still
apparent. Interestingly, a weak signal at 193.2 ppm
suggested that even some MePCl2 had formed [15].
After another 1 day, an off-white precipitate was ob-
served, and following separation of the solid the new
ring signals had disappeared from the 31P NMR spec-
trum of the filtrate. Elemental analysis and solid-
state 31P NMR of the solid showed that it was a
mixture of decomposition products, however, so the
stability of the methylated ring appeared to be lim-
ited under these conditions. Some small crystals
were isolated from the filtrate, but these were proved
The results for 11 are in very good agreement
with those for 3, 4, and 5, as shown in Table 1, which
lists the 31P NMR data for all the cyclic triphosphe-
nium ions and their alkylated dicationic derivatives.
(Dications 4 and 5 were not prepared directly from
the cyclic precursor, as indicated in the Section
Introduction [13]; nevertheless they may properly
be regarded as alkylated derivatives of 1.) The sig-
nal from the central phosphorus atom PB moves
1
to much higher frequency on alkylation, and JPP is
considerably reduced. The outer phosphorus atoms
PA show a small shift to lower frequency, reflecting
the change in electron distribution.
The cis-1,2-bis(diphenylphosphino)ethene (dppE)
derivative 6 was slow to form, the reaction not hav-
ing gone to completion even after 4 weeks, at which
point methylation was attempted. Addition of a sto-
ichiometric (1:1) amount of methyl triflate and stir-
ring overnight led to a small amount of precipitate,
with the ring 6 still present in solution, although
a new doublet assigned to the methylated product
12 was detected. Rather surprisingly, doublet and
triplet signals from the norbornane-like trication 17
obtained as a by-product of protonation of the dppE
ring were also observed (δ 31P 54.5 t, 30.7 d ppm,
2JPP 27.5 Hz) [Deng, R. M. K.; Dillon, K. B.; Goeta, A.
E.; Thompson, A. L. in preparation]. A second equiv-
alent of methyl triflate was added, and the doublet
and triplet from 12 were apparent (Table 1). The sig-
nals were quite weak, however, and several impurity
peaks were present, suggesting that this system, too,
is of limited stability and/or solubility. Signals for 17
had also disappeared by this stage.
2+
by single-crystal X-ray diffraction to be dppeH2
(CF3SO3−)2. A doublet signal at δ 31P 11.6 ppm, JPH
1
533 Hz, was observed in the proton-coupled spec-
trum of the solution, and probably arises from this
cation, either formed by protonation of some un-
reacted diphosphane (although none was detected),
or by break-up of the ring, such as in Eq. (3). A
full report of this and related crystal and molecu-
lar structures of some protonated diphosphane salts
will be published elsewhere [Batsanov, A. S.; Boon,
The third five-membered ring system 7 exam-
ined, with an aromatic backbone, also formed slowly,
reaction being complete after ca 4 weeks. No change
was seen on addition of 1 equivalent of methyl tri-
flate, but four more equal volume additions drove
the reaction to completion, with formation of the
methylated product 13, and none of the original ring
7 still present. The solution was left in the freezer to
see if crystals would grow, but unfortunately this did
not occur. As well as the similarity in shifts and cou-
pling constants to those of other alkylated species
(Table 1), the NMR data for the shift of the middle
1
TABLE 1
δ
31P (ppm) and JPP (Hz) for Some Cyclic
Triphosphenium Ions and Their Alkylated Derivativesa
Triphosphenium Ions
Triphosphanediium Ions
1
1
δPA
δPB
JPP Ref
δPA δPB
JPP Ref
2 17.6 −249.6 440.5 [4]
3
8.7 −79.8 282.4 [4]
1 65.3 −230.5 450
1 65.3 −230.5 450
1 65.3 −230.5 450
6 72.2 −249.1 472
7 57.6 −212.7 451
8 23.1 −209.9 424
9 34.1 −211.3 455
10 25.0 −216.0 439
[5] 4 52 −78 282
[5] 5 52 −79 283
[5] 11 54.8 −91.3 284
[5] 12 57.7 −96.5 311
[6] 13 45.4 −68.6 291
[5] 14 12.5 −89.4 262
[5] 15 31.0 −75.6 308
[6] 16 15.8 −73.4 296
[13]
[13]
1
PB and coupling constant JPP correspond well with
those for the neutral species 18 [16] (R = Ph, δ PB
−31.5, 1 JPP 254.1 Hz (C6H6) [16]; R = Ph, δ PB −39.4,
1
aThe data for 1 and 6–16 are those recorded in the present work.
1JPP 265 Hz (CHCl3) [17]; R = Me, δ PB −67.5, JPP

Methylation of Some Cyclic Triphosphenium Ions 153
252 Hz [18]; R = Me, δ PB −68.8 ppm, 1JPP 251.8 Hz
Section Experimental, there were also small
amounts of some phosphorus-containing impurities
in the starting material for this system.
1
(THF) [16]; R = Et, δ PB not recorded, JPP 255 Hz
[18]).
In all of the mixtures except the one derived from
dppE (6, 12), signals observed late in the reaction se-
quence between 193.3 and 193.0 ppm suggested the
formation of some MePCl2 [15]. It should be pointed
out that in all systems there was some PCl3 still
present even after disappearance of the resonance
from the diphosphane starting material. This arises
because, although the reactions were carried out sto-
ichiometrically in a 2:3 ratio, as exemplified for dppe
in Eq. (4) [5,6], this process is not a “clean” one; a 1:2
reaction is also possible, Eq. (5). It is not possible to
prevent this reaction, which consumes less PCl3 per
mole of diphosphane, from occurring spontaneously
with that shown in Eq. (4).
The six-membered ring 1,3-bis(diphenylphosph-
ino)propane (dppp) derivative 8 had formed com-
pletely after 1 day. Addition of an equivalent quan-
tity of methyl triflate and stirring overnight produced
a clear solution, the 31P NMR of which showed a
new doublet and triplet (Table 1) from the methyl
derivative 14, as well as those from the original ring.
The new triplet also showed evidence of quartet split-
ting of each component (2JPH 7.7 Hz) when recorded
proton-coupled, arising from coupling to the methyl
protons.
Similarly the 7-membered ring 1,4-bis(diphenyl-
phosphino)butane (dppb) derivative 9 was fully
formed after 3 days, with no unreacted diphos-
phane present. After addition of one equivalent of
methyl triflate, a new doublet and triplet assigned to
the methylated product 15 were apparent, although
lower in intensity than the signals from 9. Three fur-
ther equimolar portions of methyl triflate were re-
quired before these signals disappeared completely.
Attempts at growing crystals were again unsuccess-
ful. Several impurity and/or decomposition product
signals were now evident in the NMR spectrum, how-
ever, as well as the resonances from 15, suggesting
that 15 was not very stable in solution. These in-
cluded protonated species at δ 11.7 (1JPH 510), 10.9
(1JPH 515), and 10.1 (1JPH 505 Hz) ppm, possibly from
the protonation of dppb [19,20].
Seven-membered ring system 10 was also pre-
pared; its formation was complete after 1 day. The
initial addition of an equimolar amount of methyl
triflate followed by overnight stirring caused the ap-
pearance of a new doublet from 16, though two fur-
ther equivalents were required before the resonances
from 10 disappeared and the triplet as well as the
doublet from 16 could be observed. Again there were
several impurities and/or decomposition product
peaks present, including two from species with P–H
bonds (δ 31P 9.2 ppm, 1JPH 522 Hz and δ 31P 8.1 ppm,
1JPH 509 Hz), thought to arise from the protonated
diphosphane or its derivatives, and suggesting that
the methylated seven-membered ring is not very sta-
ble under these conditions. As mentioned in the
2PCl3 + 3Ph2P(CH2)2PPh2 → 2
+ [Ph2P(Cl)(CH2)2P(Cl)Ph2]2+(Cl−)2
+Cl−
(4)
PCl3 + 2Ph2P(CH2)2PPh2 →
+ [Ph2P(CH2)2P(Cl)Ph2]+Cl−
+Cl−
(5)
The results overall present clear evidence that
methylation of five-, six-, and seven-membered ring
cyclic triphosphenium ions by methyl triflate takes
place as shown by Table 1. The probable reasons
why excess of the reagent is required is that other
species present even at low concentration in the orig-
inal solution may be methylated preferentially, and
that there may be a kinetic barrier to methylation of
the cyclic cations. This could arise for both electronic
reasons (repulsion between positive charges), and
from steric hindrance by the bulky phenyl groups
to the approach of the methylating agent. In this
context it is interesting to note that an attempt to
react 1 with ethyl triflate was unsuccessful, the orig-
inal ring still being present even after the addition
of four equivalents of ethyl triflate. It is also proba-
ble that the kinetic barrier to methylation will vary
with ring size, with larger rings being methylated
more readily. Some support for this hypothesis is
provided by the observation that 31P NMR signals
from the methylated product were not detected for
two of the three five-membered ring systems after
addition of an equimolar quantity of methyl triflate,
whereas some methylation was apparent under simi-
lar conditions for all of the larger rings. Further work
is necessary to elucidate this point fully.
As mentioned earlier, all the NMR data are in
very good agreement with results for the known

154 Dillon and Olivey
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species 3–5, included in Table 1 [4,13], and (for δ PB
and JPP) with those for the neutral compounds 18
1
[16–18]. We therefore conclude that methylation of
cyclic triphosphenium ions 1 and 6–10 is possible
to form cyclic dications, although excess methyl
triflate is usually required to accomplish this. The
stability of the product appears to vary with the
system, however.
[10] Karsch, H. H.; Witt, E. J Organomet Chem 1997, 529,
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ACKNOWLEDGMENTS
We thank C. F. Heffernan and A. M. Kenwright for
assistance in recording the proton-coupled 31P NMR
spectra.
[14] Wilkinson, J. J. Project Report, University of Durham,
2001.
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