Synthesis of 4-(N,N-dialkylamino)benzyltriphenylphosphonium iodides from hydroxymethyltriphenylphosphonium iodide and N,N-dialkylaniline

Article abstract of DOI:10.1080/00397910802238783

Reinvestigation of the one-pot synthesis of 4-(N,N-dialkylamino) benzyltriphenylphosphonium iodides has shown that the product forms after the initial condensation of triphenylphosphine upon formaldehyde. Copyright Taylor & Francis Group, LLC.

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Synthesis of 4-(N,N-
Dialkylamino)benzyltriphenylphosp
Iodides from
Hydroxymethyltriphenylphosphoni
Iodide and N,N-Dialkylaniline
Matthew C. Davis a & Damon A. Parrish b
a Chemistry and Materials Division, Michelson
Laboratory, Naval Air Warfare Center, China Lake,
California, USA
b Laboratory for the Structure of Matter, Naval
Research Laboratory, Washington, DC, USA
Version of record first published: 20 Oct 2008.
To cite this article: Matthew C. Davis & Damon A. Parrish (2008):
Synthesis of 4-(N,N-Dialkylamino)benzyltriphenylphosphonium Iodides from
Hydroxymethyltriphenylphosphonium Iodide and N,N-Dialkylaniline, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 38:22, 3909-3918
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Synthetic Communications1, 38: 3909–3918, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802238783
Synthesis of 4-(N,N-Dialkylamino)
benzyltriphenylphosphonium Iodides from
Hydroxymethyltriphenylphosphonium
Iodide and N,N-Dialkylaniline
Matthew C. Davis1 and Damon A. Parrish2
1Chemistry and Materials Division, Michelson Laboratory, Naval Air
Warfare Center, China Lake, California, USA
2Laboratory for the Structure of Matter, Naval Research Laboratory,
Washington, DC, USA
Abstract: Reinvestigation of the one-pot synthesis of 4-(N,N-dialkylamino)
benzyltriphenylphosphonium iodides has shown that the product forms after
the initial condensation of triphenylphosphine upon formaldehyde.
Keywords: Formaldehyde, N,N-dialkylaniline, triphenylphosphine, Wittig
Ongoing research efforts in nonlinear optical organic compounds made it
necessary to prepare Wittig salts such as 1 and 2, (Fig. 1). These com-
pounds containing 4-N,N-dialkylaminobenzyl groups are convenient
building blocks for the reaction with arylaldehydes to prepare stilbene
derivatives.[1–4]
[5]
Porres et al. recently prepared 4-(N,N-dialkylamino)benzyltriphe-
`
nylphosphonium iodides, including 1 and 2, using conditions similar to
Bredereck et al.[6] They found that refluxing a mixture of N,N-dialkylani-
line, triphenylphosphine, paraformaldehyde, sodium iodide, water, and
acetic acid in either chloroform or toluene gave the phosphonium iodides
(4) in greatly reduced reaction times. It was implied that under these
Received April 17, 2008.
Address correspondence to Matthew C. Davis, Chemistry and Materials
Division, Michelson Laboratory, Naval Air Warfare Center, China Lake,
CA 93555, USA. E-mail: matthew.davis@navy.mil
3909

3910
M. C. Davis and D. A. Parrish
Figure 1. Phosphonium salts.
conditions the N,N-dialkylaniline first reacts to generate an in situ
4-(N,N-dialkylamino)benzyl iodide,[7] which then condenses with triphe-
nylphosphine in the traditional method[8] of synthesizing benzyl salts of
the latter (Route A, Scheme 1).
A reaction to prepare 1 was set up using the same conditions as
Porres et al.[5] Reaction progress was monitored by NMR by simply dis-
`
solving an aliqout of the reaction mixture in deuteriochloroform.[9] But
1
rather than H, which would be complicated in the aromatic region,
the disappearance of triphenylphosphine was monitored by 31P NMR
Scheme 1. Synthetic routes.

4-(N,N-Dialkylamino)benzyltriphenylphosphonium Iodides
3911
to determine reaction completetion. Shortly after starting the reaction,
three peaks were seen in the 31P NMR relative to external 85% phos-
phoric acid: ꢀ5ppm for triphenylphosphine, ꢀ21 ppm for product and
an unknown peak at ꢀ18 ppm. The reaction continued overnight, after
which time the mixture only contained the product peak. The same
ꢀ18 ppm 31P resonance was observed in the preparation of 2 as well.
Clearly, an intermediate phosphorus-containing compound was gener-
ated during the reaction.
Bredereck et al.[6] noted their procedure for synthesizing 4-(N,N-
dimethylamino)benzyltriphenylphosphonium iodide was based on a pri-
vate communication of H. Hoffmann. A thorough literature search shed
light on what may be the probable mechanism for this reaction. Hoffmann
et al.[10] first described the preparation of hydroxymethyltriphenylpho-
sphonium iodide 3 (Fig. 1) by heating a mixture of triphenylphosphine,
formaldehyde, and hydrochloric acid then adding sodium iodide. Other
salts were prepared later.[11–14] This reaction is analogous to that of
phosphine (PH3) with formaldehyde in aqueous hydrochloric acid,
forming tetrakis(hydroxymethyl)phosphonium chloride (THPC) described
earlier by A. Hoffman.[15,16] Later, in a short description of some chemistry
of 3, H. Hoffmann[17] mentions that N,N-dimethylaniline reacts to form
the corresponding 4-(N,N-dimethylamino)benzyltriphenylphosphonium
iodide, although no experimental details were given.
The salt 3 was prepared, and 31P NMR showed the chemical shift of
the phosphorus was 18.65 ppm, close to the chloride (17.7 ppm)[18] and
nitrate (18.6 ppm)[12] analogs, explaining the mystery resonance observed
during the one-pot synthesis of 4. Reaction of 3 and N,N-dialkylaniline in
chloroform in the presence of acetic acid also gave the triphenylphospho-
nium salts 4. So, the more likely mechanism for the one-pot synthesis of
1, 2, and others by this method is shown in Route B (Scheme 1). In the
first step, triphenylphosphine combines with formaldehyde to form the
hydroxymethyltriphenylphosphonium acetate, which undergoes metath-
esis with sodium or potassium iodide to form 3. The in situ formed 3 then
reacts with the pi-electron-rich N,N-dialkylaniline in an electrophilic
aromatic substitution.
Scheme 2. New phosphonium salt.

3912
M. C. Davis and D. A. Parrish
The chemistry of unsubstituted aniline with 3 is worth noting. Both
[5]
Porres et al. and Hoffmann[17] indicate that the reactivity is similar to
N,N-dialkylanilines, yielding 4-aminobenzyltriphenylphosphonium salts
(4, R ¼ H), although spectroscopic data were not provided. It was found
that reaction of aniline with 3 in ethanol gave 5 in almost quantitative
yield (Scheme 2), analogous to the reaction of aniline with THPC.[19,20]
`
1
It appears that 5 is an unreported molecule and H NMR clearly shows
this structure, as all five aromatic protons of the aniline appear and there
is a single NH resonance. It remains for further study whether 5 can
undergo rearrangement to a structure such as 4 (R ¼ H).
Crystal structures were obtained for 1–3 because it appeared they
were not reported previously (Chart 1).
[21]
ꢀ
Recently, Hernandez et al.
showed another method to prepare
4-(N,N-dialkylamino)benzyltriphenylphosphonium salts but requires
transformation of the N,N-dialkylaniline into the corresponding benzyl
alcohol by Vilsmeier formylation, followed by aldehyde reduction.
EXPERIMENTAL
Melting points were collected on an electrothermal capillary melting-
point apparatus and are not corrected. All NMR data were collected
on a Bruker Avance II 300-MHz spectrometer (1H at 300 MHz, 13C at
75 MHz, 31P at 121 MHz). NMR data free induction decays (FIDs) were
processed using NUTS software from Acorn NMR (Livermore, CA). All
1H and 31C spectra are referenced to tetramethylsilane (TMS). For 31P
NMR, spectra were referenced to 85% H3PO4 (from a new bottle) con-
tained in a capillary tube suspended in the middle of the 5-mm NMR
tube. All reagents were purchase from Sigma-Aldrich (Milwaukee) and
Chart 1. Crystal structures 1, 2, and 3 (from left to right, respectively).

4-(N,N-Dialkylamino)benzyltriphenylphosphonium Iodides
3913
used as received. Inverse Dean-Stark distillation receiver (catalog
no. CG-1266) was purchased from ChemGlass Scientific Apparatus
(Vineland, NJ). Elemental analyses were made by Atlantic Microlab,
Inc. (Norcross, GA).
4-(N,N-Dibutylamino)benzyltriphenylphosphonium Iodide (1)
A 250-mL, round-bottomed flask equipped with magnetic stirbar was
charged with 4.2 g of 3 (0.01 mol), 2.05 g of N,N-di-n-butylaniline
(0.01 mol), 100 mL of CHCl3, and 600 mg of glacial HOAc (0.01 mol,
1 equiv). An inverse Dean–Stark trap was equipped, and the mixture
was refluxed for 12 h with removal of the generated by-product H2O.
The mixture was rotary evaporated, leaving a crude oil. Trituration of
the oil with acetone gave a crystalline solid. The solid was collected on
a coarse-porosity glass frit (4.7 g, 78%). Recrystallization from EtOH
gave the title compound as colorless rhombohedrons. Mp 211–215 ꢁC
(lit.[5] 206–209 ꢁC). dH (CDCl3): 7.73–7.63 (m, 3H), 7.59–7.46 (m, 12H),
6.68 (dd, J ¼ 9.0 and 2.3 Hz, 2H), 6.25 (d, J ¼ 8.9 Hz, 2H), 4.72 (d,
J ¼ 12.7 Hz, 2H), 3.06 (t, J ¼ 7.8 Hz, 4H), 1.37 (pent, J ¼ 6.8 Hz, 4H),
1.18 (hext, J ¼ 7.7 Hz, 4H), 0.81 (t, J ¼ 7.3 Hz, 6H); dC (CDCl3):
148.23, 135.12 (d, J ¼ 3.4 Hz), 134.38 (d, J ¼ 9.8 Hz), 132.18 (d,
J ¼ 5.3 Hz), 130.26 (d, J ¼ 12.4 Hz), 117.93 (d, J ¼ 84.9 Hz), 111.77,
110.78, 50.62, 30.78 (d, J ¼ 46.6 Hz), 29.23, 20.26, 14.01; dP (CDCl3):
20.95. Elemental analysis calculated for C33H39INP: C, 65.23; H, 6.43;
N, 2.30. Found: C, 65.28; H, 6.42; N, 2.43.
4-(N-Ethyl-N-(2-hydroxyethyl)amino)benzyltriphenylphosphonium
Iodide (2)
A 500-mL, round-bottomed flask equipped with magnetic stirbar was
charged with 57.7 g of PPh3 (0.22 mol), 8.25 g of paraformaldehyde
(molecular weight ¼ 90, 0.092 mol, 0.42 equiv), 45.4 g of N-ethyl-N-(2-
hydroxyethyl)aniline (0.275 mol, 1.25 equiv), and 300 mL of CHCl3. The
mixture was stirred while 41.25 g of NaI (0.275 mol, 1.25 equiv) and a
mixture of 19.5 mL of H2O (1.08 mol, 4.9 equiv) and 55.5 mL of HOAc
(0.97 mol, 4.4 equiv) were added. A condenser was attached, and the mix-
ture was heated to reflux. After 1 h, all the solids had dissolved. After
refluxing for 8 h, an inverse Dean–Stark trap was equipped, and the
H2O=CHCl3 azeotrope was collected until completion (ꢀ8 h). The
mixture was cooled to rt whereby solids precipitated. Chloroform
(250 mL) and 200 mL of H2O were added to dissolve all the solids. The

3914
M. C. Davis and D. A. Parrish
organic phase was separated and washed again with 200 mL of H2O,
followed by 200 mL of brine, and then dried over anhydrous MgSO4.
The solvent was stripped at the rotovap, leaving a thick oil that foams
under vacuum. The oil was dissolved in 250mL of acetone and cooled in
an ice bath. The product crystallizes out after a short time. The pale green
microcrystalline powder was collected on a medium-porosity glass frit and
dried under vacuum (107.4 g, 86%). The title compound was obtained as
colorless rhomobedrons by recrystallization from EtOH. Mp 174–177ꢁC
(lit.[5] 165–170 ꢁC). dH (CDCl3): 7.85 (m, 3H), 7.71–7.54 (m, 12H), 6.75
(dd, J ¼ 9.0 and 2.3 Hz, 2H), 6.46 (d, J ¼ 8.7 Hz, 2H), 4.75 (d, J ¼ 12.7 Hz,
Hz, 2H), 3.68 (t, J ¼ 6.6 Hz, 2H), 3.39 (t, J ¼ 6.1 Hz, 2H), 3.31 (q,
J ¼ 7.1 Hz, 2H), 2.96 (bs, OH), 1.04 (t, J ¼ 7.1 Hz, 3H); dC (CDCl3):
148.24 (d, J ¼ 2.8 Hz), 135.11 (d, J ¼ 2.7 Hz), 134.22 (d, J ¼ 9.4 Hz),
132.06 (d, J ¼ 5.3 Hz), 130.24 (d, J ¼ 12.0 Hz), 117.20 (d, J ¼ 85 Hz),
112.09, 111.13 (d, J ¼ 8.5 Hz), 59.05, 52.19, 45.16, 30.68 (d, J ¼ 45.5),
11.87; dP (CDCl3): 20.89 (s). Elemental analysis calculated for C29H31
INOP: C, 61.39; H, 5.51; N, 2.47. Found: C, 61.15; H, 5.58; N, 2.46.
Hydroxymethyltriphenylphosphonium Iodide (3)
A 100-mL, round-bottomed flask equipped with magnetic stirbar was
charged with 5.2 g of PPh3 (0.02 mol), 3.6 g of paraformaldehyde
(0.04 mol, 2 equiv), and 20 mL of H2O. The mixture was stirred at rt,
and 4 mL of conc. HCl (0.048 mol) was added in one portion. The mixture
was heated to ꢀ70 ꢁC until all the solids dissolved (ꢀ15 min). In one por-
tion, 6 g of NaI (0.04 mol, 2 equiv) were added, and heating was continued
for 5 min. The mixture was cooled to rt, and a syrup precipitated. After
about 10 min, the syrup solidified into a white solid. The solid was filtered
and air dried on the frit. The solid was recrystallized from EtOH to give the
title compound as colorless microcrystals (5.1 g, 60%). Mp 168–170 ꢁC
(lit.[10] 170–177 ꢁC). dH (DMSO-d6): 7.96–7.88 (m, 3H), 7.86–7.73 (m,
12H), 6.88 (bs, OH), 5.68 (s, 2H); dC (DMSO-d6): 134.91 (d, J ¼ 2.6 Hz),
133.88 (d, J ¼ 9.9 Hz), 130.05 (d, J ¼ 12.0 Hz), 117.17 (d, J ¼ 82.7 Hz),
56.19 (d, J ¼ 60.9 Hz); dP (DMSO-d6): 18.65 (s); dP (CDCl3): 17.81 (s).
Elemental analysis calculated for C19H18IOP: C, 54.28; H, 4.28. Found:
C, 54.46; H, 4.33.
N-Phenylaminomethyltriphenylphosphonium Iodide (5)
A 100-mL, round-bottomed flask equipped with magnetic stirbar was
charged with 2.1 g of 3 (5 mmol) and 20 mL of anhydrous EtOH.

4-(N,N-Dialkylamino)benzyltriphenylphosphonium Iodides
3915
The mixture was heated briefly to dissolve all the solids. In one
portion, 465 mg of aniline (5 mmol, 1 equiv) were added, and the mix-
ture was stirred at rt. After 20 min, copious white solids precipitated.
The precipitate was collected on a coarse-porosity glass frit and air-
dried several hours (2.31 g, 93%). The white crystalline solid was found
to be a 1:1 complex of 5 and EtOH. Mp 115–120 ꢁC. dH (CDCl3): 7.81–
7.65 (m, 9H), 7.63–7.50 (m, 6H), 6.85 (t, J ¼ 7.9 Hz, 2H), 6.57–6.45 (m,
3H), 5.94 (bs, NH), 5.36 (d, J ¼ 5.2 Hz, 2H); dC (CD3 CN): 147.26,
136.24, 135.35 (d, J ¼ 9.6 Hz), 131.15 (d, J ¼ 12.4 Hz), 129.92, 120.11,
118.24 (d, J ¼ 83.2 Hz), 114.76, 42.26 (d, J ¼ 60.7 Hz); dP (CD3CN):
16.61 (s). Elemental analysis calculated for C25H23INP ꢂ EtOH: C,
59.86; H, 5.36; N, 2.59. Found: C, 60.13; H, 5.36; N, 2.56.
X-Ray Structure Determination of 1–3
All three crystals were mounted on a MiteGen Micromount using a small
amount of Cargille immersion oil. Data were collected on a Bruker three-
circle platform diffractometer equipped with a Smart Apex II CCD detec-
tor. The crystals were irradiated using graphite monochromated MoKa
radiation (k ¼ 0.71073). A Bruker Kryo-Flex low-temperature device
was used to keep the crystals at a constant temperature during data col-
lection, (Table 1).
For all three compounds, data collection was performed and the
unit cell was initially refined using APEX2 [v2.1].[22] Data reduction
was performed using SAINT [v7.34A][23] and XPREP [v6.12].[24] Correc-
tions were applied for Lorentz, polarization, and absorption effects
using SADABS [v2.03].[25] The structure was solved and refined with
the aid of the programs in the SHELXTL-plus [v6.10] system of
programs.[26] The full-matrix least-squares refinement on F2 included
atomic coordinates and anisotropic thermal parameters for all non-H
atoms. The H atoms were included using a riding model. The butyl
groups and phenyl rings of compound 1 have some small directional
freedom, requiring constraints to be placed on the model to keep the
refinement stable. One of the hydroxyethyl groups of compound 2 is
disordered over two positions, with the relative population ratio of
73:27.
CCDC 678122, CCDC 678123, and CCDC 678124 contain the sup-
plementary crystallographic data for this article (compounds 1, 2, and 3,
respectively). These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk=data
request=cif.

3916
M. C. Davis and D. A. Parrish
Table 1. Single-crystal X-ray diffraction analysis of compounds 1–3
Compound
1
2
3
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C33H39INP
607.52
223(2)
Monoclinic
C2=c
C29H31INOP
567.42
173(2)
Triclinic
P-1
C19H18IOP
420.20
173(2)
Monoclinic
P21=c
Unit cell
dimensions (A, )
a ¼ 27.616(3)
a ¼ 11.302(3)
a ¼ 9.2389(7)
ꢁ
˚
b ¼ 37.221(3)
c ¼ 23.909(3)
a ¼ 90
b ¼ 12.208(3)
c ¼ 20.762(6)
a ¼ 76.024(4)
b ¼ 82.317(4)
c ¼ 71.979(3)
2638.0(12)
4
b ¼ 17.4224(13)
c ¼ 11.4574(9
a ¼ 90
b ¼ 90.356(2)
c ¼ 90
b ¼ 103.3130(10)
c ¼ 90
3
˚
Volume (A )
Z
24575(5)
32
1.314
1794.7(2)
4
1.555
0.52 ꢃ 0.50 ꢃ 0.08
Density (Mg=m3)
Crystal size (mm)
1.429
0.23 ꢃ 0.22 ꢃ
0.23 ꢃ 0.14 ꢃ
0.06
112720
0.03
27741
13110
Reflections collected
Independent reflections 25156
18638
4480
[Rint ¼ 0.0265]
[Rint ¼ 0.0376]
[Rint ¼ 0.0201]
Completeness (%)
Data=restraints=
parameters
99.7 97.8
25156=600=1112 13110=0=609
99.6
4480=0=201
Goodness of fit on F2
1.040
1.053
R1 ¼ 0.0291
1.026
R1 ¼ 0.0188
Final R [I > 2sigma(I)] R1 ¼ 0.0702
ACKNOWLEDGMENTS
The financial support from the Office of Naval Research in gratefully
acknowledged. Thanks to Andrew P. Chafin (NAWCWD) for comments
and suggestions about the manuscript. Thanks to Dr. Robert D.
Chapman (NAWCWD) for the generous loan of the inverse Dean–Stark
trap. Thanks to Drs. Brian Zentner and Hung Hoang (both of
NAWCWD) for supplying the triphenylphosphine.
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