Preparation of dichloro(2,4,6-tribromophenoxy)(1,2-diphenoxy)phosphorane and its nonoxidative chlorination reactions with alkyl and aryl phosphonates

Article abstract of DOI:10.1016/S0020-1693(02)01045-9

The reaction of tetrachloro(2,4,6-tribromophenoxy)phosphorane, (TBPO)PCl₄, with o-catechol yields dichloro(2,4,6-tribromophenoxy)(1,2-diphenoxy)phosphorane, (TBPO)(DP)PCl₂. Quantitative ³¹P[¹H] NMR spectroscopic

Full text of DOI:10.1016/S0020-1693(02)01045-9

Inorganica Chimica Acta 338 (2002) 240Á244
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Note
Preparation of dichloro(2,4,6-tribromophenoxy)(1,2-
diphenoxy)phosphorane and its nonoxidative chlorination reactions
with alkyl and aryl phosphonates
Houston Byrd a,*, Prakash C. Bharara a, Tyler A. Sullens a, Jeremiah D. Harden a,
Gary M. Gray b,*
a Department of Biology, Chemistry and Mathematics, University of Montevallo, Montevallo, AL 35115, USA
b Department of Chemistry, University of Alabama at Birmingham, 201 Chemistry Building, 901 14th Street South, Birmingham, AL 35294-1240, USA
Received 25 January 2002; accepted 24 April 2002
Abstract
The reaction of tetrachloro(2,4,6-tribromophenoxy)phosphorane, (TBPO)PCl4, with o-catechol yields dichloro(2,4,6-tribromo-
phenoxy)(1,2-diphenoxy)phosphorane, (TBPO)(DP)PCl2. Quantitative 31P[1H] NMR spectroscopic studies demonstrate that this
compound quantitatively converts secondary phosphonates into chlorophosphites. The chlorophosphites have been characterized
by their reactions with tetracarbonylnorbornadienemolybdenum(0), Mo(CO)4(NBD), to form cis-Mo(CO)4(P(OR)2Cl)2. These
reactions are also quantitative. The (TBPO)(DP)PCl2 has major advantages over other nonoxidative chlorinating agents. The
compound can be prepared in quantitative yield from stoichiometric amounts of the starting materials, the synthesis of the
(TBPO)(DP)PCl2 and the subsequent nonoxidative chlorination reaction can be carried out in one pot, and the byproduct of the
reaction does not contain active chlorides.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Nonoxidative chlorination; Chlorophosphites; Chlorinating agents; Phosphorous
1. Introduction
and bridging phosphiteÁsalicylaldiminiato ligands [11]
/
that cannot be obtained by other means.
Compounds containing a bond between phosphorus
Chlorophosphites, (RO)2PCl, can be prepared by the
reactions of PCl3 with 2 equiv. of the appropriate
alcohols, but these reactions are not very clean because
a mixture of the various substitution products is
generally obtained. An alternate approach is to prepare
the chlorophosphite from the corresponding phospho-
nates, (RO)2P(O)H via nonoxidative chlorination. The
phosphonates are easily prepared from dimethylpho-
sphonate via transesterification with the appropriate
alcohol.
in the ꢀ3 oxidation state and chlorine react readily and
/
cleanly with alcohols, thiols and amines under mild
reaction conditions to give the corresponding phosphor-
us(III) esters, thioesters and amides [1]. Such reactions
have been used in the synthesis of oligonucleotides [2Á
and other biomolecules containing phosphorus(V)Á
oxygen and Ánitrogen bonds [7,8] because the phos-
/6]
/
/
phorus(III) chlorides are more reactive and react more
cleanly than do the corresponding phosphorus(V)
chlorides [9]. These reactions have also been used to
prepare a variety of phosphorus containing ligands for
use in homogeneous catalysts [10], and the reactions of
coordinated chlorophosphite ligands have been used to
prepare complexes with cyclic bis(phosphite) ligands [11]
The only reagents that have been reported to carry
out the nonoxidative chlorinations of secondary phos-
phonates are dichlorotris(2,4,6-tribromophenoxy)-phos-
phorane, (TBPO)3PCl2, trichlorobis(2,4,6-tribromo-
phenyl)phosphorane,
(TBPO)2PCl3,
and
tetra-
chloro(2,4,6-tribromophenoxy)phosphorane, (TBPO)-
PCl4 [3,4]. The (TBPO)3PCl2 is potentially the most
useful of these reagents because its byproduct, tris(2,4,6-
* Corresponding authors
E-mail address: gmgray@uab.edu (G.M. Gray).
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 0 4 5 - 9

H. Byrd et al. / Inorganica Chimica Acta 338 (2002) 240Á
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241
tribromophenyl)phosphate, (TBPO)3PO, has no active
chlorides. However, we have recently demonstrated that
the only one of these compounds that can be cleanly
prepared is (TBPO)PCl4 [12]. The byproduct of non-
oxidative chlorination reactions with (TBPO)PCl4 is
dichloro(2,4,6-tribromophenyl)phosphate,
(TBPO)P(O)Cl2, and this material has active chlorides
that can react with the alcohols, thiols or amines that are
used in subsequent reactions. This means that large
excesses of these reagents must be used, and this
complicates the workup.
In this paper, we report that the reaction of
(TBPO)PCl4 with o-catechol yields a phosphorane that
can carry out nonoxidative chlorinations of secondary
phosphonates and whose byproduct does not contain
active chlorines. The results of our 31P[1H] NMR studies
of synthesis of this new phosphorane and of its reactions
with secondary phosphonates are reported in this paper.
2.3. Synthesis of dichloro(2,4,6-tribromophenoxy)(1,2-
diphenoxy)phosphorane (TBPO)(DP)PCl2
Typically, 1.14 g (5.48 mmol) of PCl5 and 1.85 g (5.48
mmol) of 2,4,6 tribromophenol was added to a 100 ml
Schlenk flask. To this flask, canula transfer added 20 ml
of dry benzene and N2 was bubbled through the solution
for one hr to remove the HCl byproduct. A solution of
0.610 g (5.48 mmol) of catechol in 30 ml of dry benzene
was then added to the PCl5ÁTBPO mixture by canula
/
transfer. Dry N2 was blown through the solution for 1 h
to remove the HCl byproduct.
2.4. Synthesis of di-n-butyl chlorophosphite
(PCl(OBut)2)
A benzene solution of (TBPO)(DP)PCl2 was prepared
as described above from 9.79 g (47 mmol) of PCl5, 5.54 g
(47 mmol) of 2,4,6 tribromophenol and 5.22 g (47 mmol)
of catechol in 150 ml of dry benzene. To this solution,
8.1 ml (47 mmol) of di-n-butyl chlorophosphite was
added via a 10.0 ml syringe. This reaction gave a
quantitative yield of the di-n-butyl chlorophosphite by
31P[1H] NMR spectroscopy.
2. Experimental
The product was isolated from the reaction mixture as
follows. The reaction was first evaporated to dryness
under aspirator vacuum, and then treated with 100 ml
dry hexane. This mixture was stirred overnight and then
filtered under N2 to remove the insoluble phosphate
byproduct. The hexanes were removed from the filtrate
under aspirator vacuum and the residual liquid was
distilled (73 8C, 0.2 mm Hg) to yield 4.53 g (50%) of the
pure di-n-butyl chlorophosphite.
2.1. Materials and methods
Benzene was dried by refluxing over calcium hydride
for at least 12 h followed by distillation. Chloroform-d
was dried over activated molecular sieves. Very dry
2,4,6-tribromophenol was obtained by drying the mate-
rial under vacuum in a desiccator containing activated
CaSO4 and P2O5 for at least 5 days. Phosphorus
pentachloride was used as received and was handled
exclusively in a Vacuum Atmospheres glove box under a
dry nitrogen atmosphere. Catechol was recrystallized
from dry benzene and was stored and handled exclu-
sively in a Vacuum Atmospheres glove box under a dry
nitrogen atmosphere. The Mo(CO)4(NBD) complex was
prepared by literature methods [13]. All materials were
handled either in the glove box or by standard Schlenk
techniques under a nitrogen atmosphere.
2.5. In situ reaction of (TBPO)(DP)PCl2 with alkyl or
aryl-phosphites and Mo(CO)4(NBD)
A benzene solution of (TBPO)(DP)PCl2 was prepared
as described above in the first section. To this solution,
5.48 mmol of the appropriate phosphite was added via a
1.0 ml syringe. This reaction produces the chloropho-
sphite as observed by 31P[1H] NMR spectroscopy. After
24 h, 0.822 g (2.74 mmol) of Mo(CO)4(NBD) was added
to the Schlenk flask under a vigorous N2 flow. The
31P[1H] NMR spectrum of this mixture indicated the
reaction is quantitative and complete after 1 h.
2.2. Characterization methods
All 31P[1H] NMR spectra were recorded on a Bruker
ARX-300 NMR spectrometer. The 31P[1H] NMR spec-
tra were referenced to external H3PO4 in a coaxial tube
containing chloroform-d1, and chemical shifts downfield
of H3PO4 are reported as positive. A sweep width of
50 000 Hz was used, and 32 768 data points were taken
in this window. From 16 to 32 scans with 308 inverse-
gated decoupling pulse sequences and pulse delays of 30
s were used to obtain quantitative spectra. The resolu-
tion of each spectrum was 1.5 Hz or 0.01 ppm.
3. Results and discussion
3.1. 31P[1H] NMR studies of the reactions of phosphorus
pentachloride, 2,4,6-tribromophenol and catechol
We have demonstrated in a previous paper [12] that
the reactions of phosphorus pentachloride and 2,4,6-
tribromophenol at various stoichiometries produce

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H. Byrd et al. / Inorganica Chimica Acta 338 (2002) 240Á244
/
(TBPO)PCl4. Then a stoichiometric amount, with re-
spect to PCl5, of catechol is added to the reaction
mixture. According to NMR analysis, the reaction is
complete after 1 h. The 31P[1H] NMR data for this
reaction is given in Table 1 and Fig. 3. Before the
catechol is added to the reaction mixture, the major
peak at ꢁ
observed by the NMR. Upon the addition of catechol,
this peak disappears and a new major peak at ꢁ34.0
/
65.6 ppm corresponding to (TBPO)PCl4, is
/
Fig. 1. Reaction of phosphorus pentachloride and 2,4,6-tribromophe-
nol.
ppm is observed. We attribute this new peak to be the
(TBPO)(DP)PCl2 molecule (Fig. 2). The advantage that
this phosphorane has is that once it is used to chlorinate
other compounds no active chlorides will be left, which
is the only limitation on the use of (TBPO)PCl4 since the
phosphate byproduct (TBPO)P(O)Cl2 still contains
active chlorides and must be separated from the
chlorophosphite before any nucleophilic substitution
reactions of the chloride can be carried out [12]. There
are two other minor peaks observed in when catechol is
three species as shown in Fig. 1. These were followed
using quantitative 31P[1H] NMR spectroscopy. These
studies indicated that at room temperature a 1:1
stoichiometry gives 95% (TBPO)PCl4 and 5%
(TBPO)2PCl3 while
a
1:2 stoichiometry gives
(TBPO)2PCl3 and 8%
1:3 stoichiometry gives
34%(TBPO)PCl4,
(TBPO)3PCl2 and
58%
a
20%(TBPO)PCl4, 65% (TBPO)2PCl3 and 15%
(TBPO)3PCl2. The fact that less of the more highly
substituted products are obtained than are expected
based on reaction stoichiometries suggests that the
formation of these material is hindered by the steric
bulk of the 2,4,6-tribromophenoxy groups and seems to
explain low yields obtained by Hatta [1]. The problem
with these compounds is that the byproduct from the
reaction still contains active chlorides, therefore, we
have synthesized a new compound in quantitative yield
that only has two active chlorines.
The synthesis of dichloro(2,4,6-tribromophenoxy)-
(1,2-diphenoxy)phosphorane (TBPO)(DP)PCl2 follows
a similar procedure and is outlined in Fig. 2. A
stoichiometric amount of PCl5 and 2,4,6-tribromophe-
nol is reacted under inert conditions in dry benzene for 1
h. As indicated previously, this reaction produces 95%
reacted with (TBPO)PCl4. The peak at ꢁ32.5 ppm is the
/
disubstituted catechol product. We were able to estab-
lish this by reacting a 2:1 ratio of catechol with
(TBPO)PCl4 in which the only observable NMR peak
is ꢁ32.5 ppm. The other minor peak at 7.0 ppm is
/
assigned to the hydrolysis product from the reaction
(TBPO)(DP)P(O)H, which is consistent with the hydro-
lysis product observed in our previous study [12].
As previously noted, Hammer has reported that
dichlorotris(phenoxy)-phosphorane can also be used as
a nonoxidative chlorinating agent for dimethyl phos-
phonate [7]. However, the nonoxidative chlorination of
dimethyl phosphonate with dichlorotris(phenyloxy)pho-
Table 1
31P[1H] for starting materials, chlorinating agents and reaction
products
Compound
31P[1H] (ppm)
TBPOPCl4
(TBPO)2PCl3
ꢁ65.6
ꢁ57.2
ꢁ64.5
ꢁ34.0
ꢁ32.5
7.0
(TBPO)3PCl2
(TBPO)(DP)PCl2
(TBPO)(DP)2
(TBPO)(DP)P(O)
(CH3O)2P(O)H
(ButO)2P(O)H
a
10.02Á10.84
7.8
0.7
/
(PhenO)2P(O)H
(CH3O)2PCl
(ButO)2PCl
b
169.8
167.5
158.6
ꢁ80.6
179.0
173.0
163.7
(PhenO)2PCl
PCl5
cis-Mo(CO)4(P(OCH3)2Cl)2
cis-Mo(CO)4(P(OBut)2Cl)2
cis-Mo(CO)4(P(OPhen)2Cl)2
The 31P[1H] of (CH3O)2P(O)H is concentration dependant and is
very sensitive to the other components of the solution.
a
Fig. 2. Synthesis of dichloro(2,4,6-tribromophenoxy)(1,2-diphenox-
y)phosphorane (TBPO)(DP)PCl2 and the chlorination reaction with
dialkyl or diaryl phosphonates.
b
Lippman synthesized this compound by a different route and
reported its chemical shift as 169 ppm [14].

H. Byrd et al. / Inorganica Chimica Acta 338 (2002) 240Á
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243
in a common solvent, benzene. Secondly, the phosphate
by product (TBPO)(DP)P(O) contains no active chlor-
ines and can be separated from the chlorinated phos-
phites. We have been able to isolate the
dibutylchlorophosphite in a 50% yield by vacuum
distillation. The 31P[1H] NMR of pure di-n-butyl
chlorophosphite is shown in Fig. 4.
3.3. Synthesis of cis-Mo(CO)4(P(OBut)2Cl)2 via
nonoxidative chlorination
In order to demonstrate the usefulness of nonoxida-
tive chlorination using (TBPO)(DP)PCl2, we have
synthesized a series of molybdenumÁphosphorous com-
/
plexes starting from di-n-butyl phosphonate, dimethyl
phosphonate and diphenyl phosphonate. The reactions
can be carried out by adding the stoichiometric amount
Mo(CO)4NBD to the purified chlorophosphite or in
situ. Both lead to quantitative yields as observed by
NMR. Fig. 4 shows the NMR spectra for the reaction
when a stoichiometric amount Mo(CO)4NBD is added
to the purified di-n-butyl chlorophosphite. The resulting
solid product has only one peak as observed by 31P[1H]
NMR at 173 ppm, which is shifted down field from that
of di-n-butyl chlorophosphite. This is indicative of the
coordinating the Mo to the dibutylchlorophosphite.
Purification of this product was difficult due to the
polymerization of the 2,5-norbornadiene. The coordina-
tion reactions can also be carried out in a one-pot
reaction. The (TBPO)(DP)PCl2 species is first prepared
by the reaction of equimolar amounts of phosphorus
pentachloride and 2,4,6-tribromophenol then the addi-
tion of an equal molar amount of catechol in benzene.
An equimolar amount of dialkyl or aryl phosphonate
(based upon the PCl5) is then added. Finally, a half-
Fig. 3. 31P[1H] NMR for the synthesis of dibutylchlorophosphite form
dichloro(2,4,6-tribromophenoxy)(1,2-diphenoxy)phosphorane.
(A)
31P[1H] NMR of (TBPO)PCl4. (B) 31P[1H] NMR of (TBPO)(DP)PCl2.
(C) 31P[1H] NMR of (ButO)2PCl.
sphorane does not occur to a significant extent at
ambient temperature in the absence of a basic solvent
such as pyridine.
3.2. Reactions of (TBPO)(DP)PCl2 with alkyl and aryl
phosphonates
We have studied the reactions of (TBPO)(DP)PCl2
with dimethyl, dibutyl, and diphenyl phosphonate using
31P[1H] NMR spectroscopy and observed that it is an
excellent nonoxidative chlorinating agent (Table 1).
When a stoichiometric amount of dimethyl phosphonate
is added to a solution of (TBPO)(DP)PCl2, the NMR
peak at ꢁ34.0 ppm completely disappears and two new
/
peaks appear. The first peak appears at 169.8 ppm and
corresponds to dimethyl chlorophosphonate, which is
consistent with our previous study [12]. Similar results
are observed with all the phosphonates. For example,
Fig. 3 shows the NMR spectra associated with the
reaction
of
di-n-butyl
phosphonate
with
(TBPO)(DP)PCl2. The peak at 167.5 ppm is assigned
to di-n-butyl chlorophosphite. A second peak at 7.0
ppm is the hydrolyzed phosphate byproduct
(TBPO)(DP)P(O), which is consistent with our previous
work
[12].
This
study
demonstrates
that
(TBPO)(DP)PCl2 reacts quantitatively with a variety
phosphonates to form the chlorophosphite and the
corresponding phosphate in a 1:1 stoichiometry as
shown in Fig. 2. There are two advantages to this
chlorinating route. First, all of the quantitative yields of
the reactions, as observed by NMR, are obtained
without the use of a basic solvent such as pyridine and
all of the reactions are carried out at room temperature
Fig. 4. 31P[1H] NMR for the reaction of di-n-butyl chlorophosphite
with Mo(CO)4NBD. (A) 31P[1H] NMR of distilled (ButO)2PCl. (B)
31P[1H] NMR of cis-Mo(CO)4((ButO)2PCl)2.

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H. Byrd et al. / Inorganica Chimica Acta 338 (2002) 240Á244
/
molar amount of solid Mo(CO)4NBD is added to the
reaction mixture because the Mo-complex has two
coordination sites. The conversion of dialkyl or aryl
chlorophosphite to cis-Mo(CO)4(P(OR)2Cl)2 is quanti-
tative. We have investigated this reaction using di-
methylphosphonate, dibutylphosphonate and diphenyl-
phosphonate (Table 1). We believe this demonstrates
that (TBPO)(DP)PCl2 is an excellent nonoxidative
chlorinating agent for phosphonate compounds. Also,
it should be possible to readily prepare a number of
other metal complexes with unique chlorophosphite
ligands using these synthetic methods.
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The authors thank the Chemistry Department of the
University of Alabama at Birmingham for facilities and
support of this research. In addition, the authors thank
the Dean of the College of Arts & Sciences at the
University of Monetvallo and the NSF-REU grant
CHE-9820282 for funding of this project.
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