Synthesis of new organophosphorus compounds using the atherton-todd reaction as a versatile tool

Article abstract of DOI:10.1002/hc.21006

This article discusses the behavior of seven organophosphorus compounds under Atherton-Todd conditions. Therefore, the reactivity and selectivity of different (phen)oxaphosphinines, dioxaphosphinines, dioxaphosphinanes, and diphenylphosphine oxide with three nucleophiles were systematically studied. The results prove the versatility of the Atherton-Todd reaction to a broad range of organophosphorus compounds with different phosphorus environments and reactive Pi-H bonds. The nucleophiles studied in this article were chosen as model substrates for amines and alcohols. Because organophosphorus molecules are important and versatile compounds, for a broad field of applications, novel synthetic approaches are of interest to both academia and industry. As an example, the single-step synthesis of the bridged 1,3-phenylene bis(diphenylphosphinate) with potential flame-retardant properties was added to this study. In addition, the reaction is utilized for the synthesis of a novel organophosphorus anhydride.

Full text of DOI:10.1002/hc.21006

Heteroatom Chemistry
Volume 23, Number 2, 2012
Synthesis of New Organophosphorus
Compounds Using the Atherton–Todd Reaction
as a Versatile Tool
Sebastian Wagner,1 Muriel Rakotomalala,1 Yana Bykov,1
Olaf Walter,2 and Manfred Do¨ring1
1Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology, 76344
Eggenstein-Leopoldshafen, Germany
2European Commission Joint Research Centre, Institute for Transuranium Elements, 76344
Eggenstein-Leopoldshafen, Germany
Received 6 September 2011; revised 27 December 2011
INTRODUCTION
ABSTRACT: This article discusses the behavior of
seven organophosphorus compounds under Atherton–
Todd conditions. Therefore, the reactivity and selec-
tivity of different (phen)oxaphosphinines, dioxaphos-
phinines, dioxaphosphinanes, and diphenylphosphine
oxide with three nucleophiles were systematically stud-
ied. The results prove the versatility of the Atherton–
Todd reaction to a broad range of organophospho-
rus compounds with different phosphorus environ-
ments and reactive P H bonds. The nucleophiles stud-
ied in this article were chosen as model substrates
for amines and alcohols. Because organophosphorus
molecules are important and versatile compounds,
for a broad field of applications, novel synthetic ap-
proaches are of interest to both academia and industry.
As an example, the single-step synthesis of the bridged
1,3-phenylene bis(diphenylphosphinate) with poten-
tial flame-retardant properties was added to this study.
In addition, the reaction is utilized for the synthesis
The field of organophosphorus chemistry has been
an important subject of both academic and indus-
trial research for several decades [1–3]. A wide range
of applications has been reported in different ar-
eas of chemistry such as coordination chemistry,
material science, homogeneous catalysis, develop-
ment of biologically active compounds or pesticides,
and additives for polymers such as lubricants and
antioxidants [4–10]. Recently, there has also been
a growing interest from the flame-retardant com-
munity regarding phosphorus-containing molecules
(e.g., 6H-dibenzo[c, e] [1,2] oxaphosphinine 6-oxide
(DOPO; 3) for epoxy resins) as environmentally
friendly alternatives to the existing, and often harm-
ful, halogenated systems [11–14]. Several new phos-
phorus heterocycles with flame-retardant properties
have been reported to exhibit similar flame retar-
dancy with reduced toxic effects [15,16]. In our
group, we make use of the Atherton–Todd approach
as a versatile synthesis tool to access novel deriva-
tives [17,18]. The Atherton–Todd reaction is a clas-
sic reaction in organophosphorus chemistry [19–21].
Originally, Atherton and Todd reported the reaction
as a very effective single-step route to yield phospho-
ramidates from diphenyl and dialkyl phosphonates
(Scheme 1). Other phosphorus species and phospho-
rus heterocycles were not investigated under these
conditions.
ꢀ
C
of a novel organophosphorus anhydride.
2012 Wi-
ley Periodicals, Inc. Heteroatom Chem 23:216–222, 2012;
View this article online at wileyonlinelibrary.com. DOI
10.1002/hc.21006
Correspondence to: Sebastian Wagner; e-mail: Sebastian.wagner
@kit.edu.
Supporting Information is available in the online issue at wi-
leyonlinelibrary.com.
c
ꢀ
2012 Wiley Periodicals, Inc.
216

Synthesis of New Organophosphorus Compounds Using the Atherton–Todd Reaction as a Versatile Tool 217
SCHEME 1 The reaction reported by Atherton and Todd.
One of the main advantages of this method is
the application of phenoxaphosphinines, oxaphos-
phinines, dioxaphosphinines, dioxaphosphinanes,
or phosphine oxides as substrates instead of the
highly moisture-sensitive trivalent phosphorus chlo-
rides. Even though phosphorus trichloride remains
the main entry point into organophosphorus chem-
istry, many P H reactive compounds are commer-
cially available. In addition, the Atherton–Todd reac-
tion is a one-pot reaction and no separate oxidation
step is needed. The reaction of carbon tetrachloride,
triethylamine, and an organophosphorus compound
containing a P H bond results in the in situ forma-
tion of a P(O) Cl oxychloride species [22]. This inter-
mediate can be observed via 31P NMR spectroscopy.
However, the reaction is described to be inefficient
when alcohols are used as nucleophiles [19]. On the
other hand, the recent literature indicates that some
alcohols can react with specific phosphorus species
under Atherton–Todd conditions [21]. Moreover, the
highly carcinogenic carbon tetrachloride can be re-
placed by iodoform or bromotrichloromethane; the
latter is reported to be the most efficient reagent for
this reaction [23,24].
Scheme 2 demonstrates two reaction path-
ways to obtain the desired products: the nucle-
ophilic substitution of phosphorus chlorides (i) fol-
lowed by an oxidation step and the single-step
Atherton–Todd reaction (ii). The nucleophilic sub-
stitution yields trivalent phosphorus species, which
are prone to side reactions [25]. The oxidation of
such compounds is often accompanied by hydroly-
sis or transesterification, depending upon the type
of method applied [26,27]. The Atherton–Todd re-
action starts from a pentavalent, often commer-
cially available, phosphorus species and therefore no
additional oxidation step is necessary. To demon-
strate the scope of this versatile reaction, we sys-
tematically studied the reactivity and selectivity of
six different, mostly heterocyclic compounds in-
cluding: 2,8-dimethyl-10H-phenoxaphosphinine 10-
oxide (1), diphenylphosphine oxide (2), 6H-dibenzo
[c, e] [1,2] oxaphosphinine 6-oxide (3), 5,5-dimethyl-
1,3,2-dioxaphosphinane 2-oxide (5), 5,5-dimethyl-
1,3,2-dioxaphosphinane 2-sulfide (6), and naphtho
[1,8-de] [1,3,2] dioxaphosphinine 2-oxide (7) under
SCHEME 2
(i) Nucleophilic substitution reaction. (ii)
Atherton–Todd reaction. NuH represents a nucleophilic
species (e.g., alcohols and amines).
Atherton–Todd conditions (Fig. 1). One additional
heterocycle, 6H-dibenzo[c, e] [1,2] oxaphosphinine
6-sulfide (4), recently published by Rakotomalala et
al., was also added to this report [16]. To the best
of our knowledge, the reactivity of thiophosphorus
compounds (e.g., 4 and 6) under these conditions is
only scarcely discussed in the literature. Improved
synthetic routes for 1 and 7 are presented in the
Supporting Information.
The compounds shown in Fig. 1 represent
different phosphorus environments with a de-
creasing amount of phosphorus–carbon bonds
from compounds 1–7. All organophosphorus
molecules (P(X) H) mentioned above were reacted
Heteroatom Chemistry DOI 10.1002/hc

218 Wagner et al.
FIGURE 1 Organophosphorus compounds (and their acronyms) applied in this study.
with phenylethylamine (PhCH2CH2NH2), piperi-
dine (C5H10NH), as well as with 2-phenylethanol
(PhCH2CH2OH), which can be considered as model
substrates (NuH) for primary and secondary amines,
and primary aliphatic alcohols.
The formation of anhydrides via the Atherton–
Todd reaction was also performed. The results of
this study can be extrapolated to other nucleophiles,
thus allowing us to access a broad range of new
compounds. As a result, the one-step synthesis of
a bridged compound with a rod-like geometry and
potential flame-retardant properties was included in
this study [28].
SCHEME 3 Synthesis of NDPO (7). (a) PCl3, 70◦C, 20 h.
(b) DCM, t-BuOH, 5◦C, 2 h.
Phenoxaphosphinine 1, diphenylphosphine ox-
ide 2, and the oxaphosphinine 3 were generally
found to react easily with all three nucleophiles un-
der Atherton–Todd conditions, yielding molecules
8–16. This is a pleasant result because there is a
strong demand for novel derivatives of 3 by the
flame-retardant industry.
All studied phosphorus molecules 1–7 were suc-
cessfully reacted with PhCH2CH2NH2 and C5H10NH
(Table 1). When PhCH2CH2OH was used the yields
were lower, and in the case of the thiodioxaphosphi-
nane 6 and dioxaphosphinine 7 the reaction was not
successful. Even though 5 successfully reacted with
PhCH2CH2OH, dioxaphosphinane and dioxaphos-
phinine structures seem to be inefficient for the re-
action with alcohols.
This is supporting our assumption that the suc-
cessful application of alcohols strongly depends on
the phosphorus species. The formation of anhy-
drides was observed in all cases when traces of water
were present. Hence, dry solvents and substrates are
necessary.
For all substrates but 4, 6, and 7, the forma-
tion of a P(X) Cl intermediate was observed in-
stantly via 31P NMR spectroscopy when triethy-
lamine was added to the reaction mixture (Ta-
ble 1). In this context, it should be mentioned
that chlorothiophosphates and their analogues are
much more stable than its chlorophosphate equiv-
alents as a result of the low polarity of the
P S bond. The preparation and purification of
RESULTS AND DISCUSSION
2,8 - Dimethyl - 10H - phenoxaphosphinine 10 - oxide
(DPPO, 1) was already reported in the literature [17].
However, we have developed an improved synthetic
route and workup procedure. The molecule is highly
prone to oxidation (yielding the acid) and there-
fore must be handled in dry and degassed solvents.
Compounds 2 and 3 are commercially available, and
compounds 4–6 were already described in the liter-
ature as mentioned above [16,29,30].
In addition, we synthesized naphtho[1,8-de]
[1,3,2] dioxaphosphinine-2-oxide (NDPO, 7) using a
novel synthetic approach developed in our labora-
tory. The precursor (32) was obtained by a reaction
of 1,8-dihydroxynaphthaline in phosphorus trichlo-
ride as a solvent at 70◦C; upon completion, excess
PCl3 was removed and purified for reuse. The appli-
cation of additional solvents (chloroform) resulted
in reduced yields. Treatment of the obtained product
(32) with tert-butanol in dichloromethane generated
7 in 98% yield (Scheme 3).
Table 1 presents the results of the Atherton–Todd
reaction between the P(X) H reactive organophos-
phorus molecules 1 to 7 with each nucleophile (NuH:
PhCH2CH2NH2, PhCH2CH2OH, and C5H10NH). All
reactions were performed with carbon tetrachloride
as an oxidation agent.
Heteroatom Chemistry DOI 10.1002/hc

Synthesis of New Organophosphorus Compounds Using the Atherton–Todd Reaction as a Versatile Tool 219
TABLE 1 Products of the Atherton–Todd Reaction between Compounds 1–7 (P(X) H) and PhCH2CH2OH, PhCH2CH2NH2,
and C5H10NH (NuH). The intermediates Were Observed by 31P NMR (250 MHz) Spectroscopy in CDCl3 and Not Isolated
Organophosphorus
Compound (P(X) H)
#
Nucleophile (NuH)
Intermediate (ppm)
16.2
Yield (%)
8
9
1
PhCH2CH2OH
PhCH2CH2NH2
C5H10NH
PhCH2CH2OH
PhCH2CH2NH2
C5H10NH
PhCH2CH2OH
PhCH2CH2NH2
C5H10NH
PhCH2CH2OH
PhCH2CH2NH2
C5H10NH
PhCH2CH2OH
PhCH2CH2NH2
C5H10NH
PhCH2CH2OH
PhCH2CH2NH2
C5H10NH
PhCH2CH2OH
PhCH2CH2NH2
C5H10NH
56
96
86
59
73
51
85
80
87
60
74
87
76
85
74
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
2
3
4
5
6
7
30.1
21.3
–
−1.4
–
79
76
–
65
70
–
SCHEME 4 Formation of the intermediates 5a and 5b (observed via 31P NMR). CCl3Br increases the reaction rate. NuH:
C5H10NH, PhCH2CH2NH2 and PhCH2CH2OH.
these compounds were recently illustrated by Fraix
et al. [31]. For a better understanding of the mech-
anism, the intermediates were monitored via 31P
NMR spectroscopy but not isolated. In the case of
compound 5, the formed intermediate 5a only re-
acted with the nucleophilic species at higher tem-
peratures (up to 60◦C) and demanded longer re-
action time for all three reagents (up to 48 h,
Scheme 4). Thus, the reaction of the intermedi-
ate 5a with the nucleophile (amine or alcohol) is
the rate-limiting step. The reaction speed could
be increased when bromotrichloromethane was
applied. The formation of the P(O) Br species 5b
was observed in this case. The 31P NMR signal of 5b
was shifted to higher fields as a result of the shielding
effect of bromine. The relative bond strength, and
therefore reactivity, of 5a (P(O) Cl bond) compared
to 5b (P(O) Br bond) played a crucial role in reac-
tion times and selectivity. The weaker phosphorus–
bromine bond is readily replaced by a nucleophilic
species. Other reactive species were reported or pro-
posed in the literature [32].
As
mentioned
above,
C5H10NH
and
PhCH2CH2NH2 were effective reagents for all
organophosphorus substrates (1–7). Dioxaphosphi-
nanes and dioxaphosphinines, on the other hand,
seem to be less applicable for the Atherton–Todd
reaction, especially with aliphatic alcohols. This is
Heteroatom Chemistry DOI 10.1002/hc

220 Wagner et al.
SCHEME 5 Synthesis of the mixed anhydride 31. a: CCl4,
NEt3, CHCl3, 5◦C, 2 h.
FIGURE 2 Crystal structures of compounds 10, 15, 16,
and 28.
FIGURE 4 Crystal structure of compound 31.
(Scheme 5). In this reaction, the acid functionality
of 30 acts as a nucleophile.
A crystal structure of 31 was successfully ob-
tained, proving the ability to obtain anhydrides with
help of the Atherton–Todd reaction (Fig. 4). More-
over, the product was observed to be very stable
against moisture, which is not common for this type
of organophosphorus anhydride.
FIGURE 3 Organophosphorus compound 29.
in agreement with the results reported by Atherton
and Todd that the reaction is an efficient tool to
access phosphoramidates. Crystal structures of
selected compounds are included in Fig. 2.
To verify the versatility of the method pre-
sented in this paper, we have synthesized the bridged
molecule 29 from compound 2 in 95% yield (Fig. 3).
The aromatic diol resorcinol was used as a nu-
cleophile instead of the monofunctional, aliphatic
PhCH2CH2OH. Because of its bridged structure, 29
could be used as a polymer additive with a rod-like
geometry and therefore only have a negligible impact
on the material properties [28].
The Atherton–Todd reaction can also be ex-
ploited to access novel anhydride derivatives. The
formation of anhydrides is common when traces of
water cause hydrolysis of the P(X) Cl intermedi-
ate. Commercially available 30 readily reacted with
4 to form the mixed anhydride 31 in 64% yield
CONCLUSION
In this article, we further studied the scope of
the Atherton–Todd reaction. To this end, we have
systematically reacted seven organophosphorus
molecules, with different phosphorus environments,
with three model substrates, including two different
amines and an alcohol. The results demonstrate that
the Atherton–Todd reaction can be applied to almost
any P−H reactive compound and nucleophile. How-
ever, the successful reaction with aliphatic alcohols
strongly depends on the environment of phospho-
rus atom. Reaction times could be decreased by re-
placement of carbon tetrachloride with the less toxic
bromotrichloromethane, as a result of the formation
Heteroatom Chemistry DOI 10.1002/hc

Synthesis of New Organophosphorus Compounds Using the Atherton–Todd Reaction as a Versatile Tool 221
of the more reactive phosphorus–bromine bond. The
These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033; or deposit@ccdc.cam.ac.uk).
phenoxaphosphinine 1 and the oxaphosphinines 3,
4 and diphenylphosphine oxide reacted readily with
amines and alcohols in good yields. On the other
hand, dioxaphosphinanes 5–6 and dioxaphosphi-
nine 7 often required higher reaction temperatures
and time and did not react with alcohols in all pre-
sented cases except compound 5. Because oxaphos-
phinines such as DOPO (3) are industrially rele-
vant compounds with flame-retardant properties,
the Atherton–Todd reaction represents an easy, one-
step synthetic tool to access novel derivatives.
Syntheses
The Supporting Information can be found in the on-
line version of this article, including a general proce-
dure for the Atherton–Todd reaction, the synthesis of
all compounds and starting materials, and spectro-
scopic details. The synthesis and experimental data
of compound 14 is given as a general procedure for
an Atherton–Todd reaction involving a P H reactive
compound.
EXPERIMENTAL
Materials and Instruments
Unless stated otherwise, solvents and chemicals
were obtained from commercial sources and used as
such without further purification. Compound 2 was
supplied by BASF SE (Ludwigshafen, Germany),
and compounds 3 and 30 were supplied by Schill
+ Seilacher (Bo¨blingen, Germany). NMR spectra
were recorded with a Bruker-Analytical BZH 250/52
(250 MHz) and a Varian INOVA-400 (400 MHz).
Chemical shifts are reported as δ values relative to
the solvent peak. Tetramethylsilane was used as a
standard. All 31P NMR spectra were measured pro-
ton decoupled. All 13C NMR spectra were measured
6-Phenethoxy-6H -dibenzo[c,e][1,2]oxaphosphi-
nine 6-oxide (14). A flame-dried three-neck flask fit-
ted with a condenser, a thermometer, and an ad-
dition funnel was charged with DOPO (3) (5.00 g,
23.14 mmol), carbon tetrachloride (3.91 g, 25.44
mmol), and 30 mL of dry chloroform. The reaction
mixture was cooled with an ice bath to 5◦C. The
additional funnel was charged with triethylamine
(2.57 g, 25.44 mmol) and 2-phenyl ethanol (2.82 g,
23.14 mmol) dissolved in 20 mL of dry chloroform.
The triethylamine and alcohol mixture were added
dropwise under vigorous stirring. The reaction tem-
perature was not allowed to exceed 10◦C. After 1 h,
the addition was complete, and NMR analysis in-
dicated complete conversion of the starting mate-
rial. The reaction mixture was washed three times
with 50 mL water to remove the triethylamine hy-
drochloride. The organic phase was isolated, dried
over MgSO4, filtered, and the solvent was removed
in vacuo. The spectroscopically pure product was ob-
tained as a colorless oil. Yield: 6.57 g, 19.53 mmol,
85%. 31P NMR (101 MHz, CDCl3) δ 11.5 ppm (s, 1P);
13C NMR (101 MHz, CDCl3) δ 45.0 (d, J = 5.7 Hz,
2C), 135.1 (s, 2C), 127.9 (s, 2C), 124.2 (s, 2C), 119.7
1
proton decoupled and phosphorus coupled. H pro-
ton spectra were measured phosphorus coupled.
Melting points are uncorrected and measured with
a Bu¨chi B-545. High-resolution mass spectrometry
(HR-MS) analyses were performed on a MicroMass
GCT (time of flight (TOF); electron ionization (EI),
70 eV) and Bruker micrOTOF (Nano ESI Offline). IR
spectra were recorded with a Varian 660-IR (FT-IR).
Elemental analysis was performed using a Vario EL
III from Elementar Analysensysteme GmbH.
Crystallographic Data
1
(s, 2C), 113.4 ppm (d, J = 8.1 Hz, 2C); H NMR
X-ray diffraction measurements were performed on
a Siemens SMART CCD 1000 diffractometer with
monochromated MoKα-irradiation collecting a full
sphere of data in the θ—ranging from 1.57◦ to 28.34◦.
The data were corrected for Lorentz and polariza-
tion effects and an empirical absorption correction
with SADABS was applied [33]. The structures were
solved by direct methods and refined to an optimum
(250 MHz, CDCl3) δ 7.95–7.82 (m, 2H), 7.83 (ddd,
J = 14.6 Hz, J = 7.5 Hz, J = 1.25 Hz, 1H),
7.68 (d, J = 7.5 Hz, 1H), 7.45 (td, J = 7.5 Hz, J
= 3.6 Hz, 1H), 7.39–7.29 (m, 1H), 7.29–7.17 (m,
6H), 7.06–7.02 (m, 1H), 4.38–4.28 (m, 2H), 2.91
ppm (t, J = 7.2 Hz, 2H); IR (KBr) ν˜: 3061 (w,
CAryl H), 3028 (w, Caryl H), 2957 (w, Caryl H), 2918
(w, Calkyl H), 2895, 1597 (s, P Caryl), 1583, 1476
(m, O CH2), 1431 (m, CH2), 1384, 1300 (s, P O),
1155 (vs, C-O−P), 1087 (s, P O C), 868 (m, P O),
798, 702, 602, 543, 488 cm−1; MS (ESI) m/z [M
+ H]+ 337, [2M + H]+ 673, [2M + Na]+ 695;
HR-MS (EI) calcd. for [12C12H9PO3]+ 232.0289,
R value with SHELX-97 [34]. Visualization for eval-
1
uation was performed with XPMA and figures were
created with ORTEP [35,36].
CCDC numbers 842383–842387 contain the sup-
plementary crystallographic data for this paper.
Heteroatom Chemistry DOI 10.1002/hc

222 Wagner et al.
found 232.0228 [M C8H8]+, calcd for [12C8H8]+
104.0626, found 104.0550 [M C12H9PO3]+; HR-MS
(ESI) calcd for [12C20H17PO3]+ 337.09153, found
337.09886 [M + H]+.
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pp. 637–640.
Supporting information concerning the synthesis
and analytical data (including X-ray data) for all
compounds may be found in the online version of
this article.
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Heteroatom Chemistry DOI 10.1002/hc