Synthesis of air-stable zwitterionic 2-phosphiniminium-arenesulfonates

Article abstract of DOI:10.1016/j.tetlet.2012.06.112

Efficient synthetic methodology for preparation of 2-phosphiniminium-5- methylbenzenesulfonate zwitterions is reported. Staudinger reaction between phosphines and n-propyl 2-azido-5-methylbenzenesulfonates followed by sulfonate ester deprotection using py

Full text of DOI:10.1016/j.tetlet.2012.06.112

Tetrahedron Letters 53 (2012) 4832–4835
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of air-stable zwitterionic 2-phosphiniminium-arenesulfonates
⇑
Christopher T. Burns , Suisheng Shang, Rajesh Thapa, Mark S. Mashuta
Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Efficient synthetic methodology for preparation of 2-phosphiniminium-5-methylbenzenesulfonate zwit-
terions is reported. Staudinger reaction between phosphines and n-propyl 2-azido-5-methylbenzenesulf-
onates followed by sulfonate ester deprotection using pyridinium tetrafluoroborate/pyridine afforded the
zwitterions in excellent yields. This new route directly accesses ortho-substituted-arenesulfonate ligands
Received 21 May 2012
Revised 26 June 2012
Accepted 26 June 2012
Available online 2 July 2012
that incorporate a phosphinimine, a strong
r-donor.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Phosphinimine
Phosphiniminium
Staudinger reaction
Benezenesulfonate esters
2-Amino-5-methylbenzenesulfonyl chloride
Introduction
date, few examples exist where an exocyclic phosphinimine group
has been incorporated into a mixed donor ligand system for use in
the synthesis of late transition metal complexes.3c–e We present
here an efficient and modular synthesis for preparing 2-phosphin-
iminium-5-methylbenzenesulfonate zwitterions.
We have recently begun a program on the synthesis, study, and
application of ortho-phosphinimine-arenesulfonates (NPSO) in
organometallic and organic catalysis.1 Anionic phosphinimide do-
nors have been exploited extensively as ancillary ligands for both
early and late transition metals.2 Less extensively explored, the
coordination chemistry of neutral phosphinimines, both as mono-
dentate and bidentate ligands, is beginning to receive attention.3
These ligands act as good electron-donor ligands toward transition
metals and are able to form strong metal–nitrogen bonds. This abil-
ity is due to the fact that delocalization about the phosphinimine PN
moiety produces compounds with enhanced basicity and nucleo-
philicity.4 The excellent donor properties of neutral phosphinimine
ligands were elucidated by the synthesis of tungsten tetracarbonyl
complexes containing benzene bridged bidentate phosphinimine-
imine ligands.3e The mCO values for these tungsten complexes are
some of the lowest values reported for bidentate bis-nitrogen li-
Results and discussion
There are relatively few examples of ortho-substituted arene-
sulfonate bidentate ligands described. The first are the 2-phos-
phine-arenesulfonate ligands (PSO, A, Fig. 1) which have received
considerable attention in the last decade as ancillary ligands for
Pd(II) and Ni(II) olefin insertion polymerization catalysts.6 PSO li-
gands have also been used to stabilize ruthenium complexes that
catalyze allylic alkylations of heterocycles and amines.7 The sec-
ond, chiral ortho-NHC-benzenesulfonate ligands (B, Fig. 1), were re-
ported by Hoveyda and have been used in the copper-catalyzed
asymmetric conjugate addition, allylic alkylation, hydroboration,
and diboronation reactions.8
Based on the literature, we realized that the best route to our
NPSO ligands was to use commercially available 2-aminobenzene-
sulfonic acids. Protection of the sulfonic acid group as an alkyl ester
allowed for the clean conversion of the 2-amino group to a phos-
phinimine. Phosphinimines, while relatively air stable, decompose
under the relatively harsh deprotection conditions (acetic acid at
110 °C) used by Hoveyda in the synthesis of B to form Ph3P@O
and an aryl amine. Other deprotection routes were explored.9
The NPSO alkyl esters 4–9 were synthesized in good yields by a 4
step reaction sequence. Protected 2-amino-5-methylbenzenesulfo-
nate esters can be prepared on a multi gram scale in two steps as
shown in Scheme 1. We successfully converted 2-amino-5-methyl-
gands on tetracarbonyltungsten(0) groups. The low
cate that a phosphinimine is a strong -donor and a poor
m
CO values indi-
-acceptor
r
p
ligand versus comparable tetracarbonyltungsten(0) complexes that
contain benzene bridged bidentate iminophosphine ligands.5
As well as being strong
r-donor ligands, when neutral phos-
phinimines with the P atom exocyclic to the chelate ring coordi-
nate to transition metals via nitrogen the steric bulk is slightly
removed from the metal center.3b This confers a second coordina-
tion sphere environment that contains steric bulk to protect a me-
tal while leaving the first coordination sphere more open.2a To
⇑
Corresponding author. Tel.: +1 502 852 5977; fax: +1 502 852 8149.
E-mail address: ctburn02@louisville.edu (C.T. Burns).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.112

C. T. Burns et al. / Tetrahedron Letters 53 (2012) 4832–4835
4833
Ph
N
Ph
N
R
R
H
4–9. A common deprotection route is the SN2 reaction between io-
dide and an alkyl sulfonate ester in acetone.18 Acetone is not
acceptable as a solvent in these reactions due to the potential
Aza-Wittig reaction between phosphinimines 4–9 and acetone
forming the corresponding dimethylimine and Ph3P@O.12 In con-
trast to a literature report describing quantitative deprotection of
pentyl benzenesulfonate by [nBu4N][I] in refluxing C6H6 in
10 min,19a we found no reaction occurred when isobutyl ester 5
was refluxed for 12 h with 2 equiv of [nBu4N][I] in C6H6.19b
Widlanski reported that isobutyl benzenesulfate was deprotec-
ted by piperidine (1 equiv) in CDCl3 at 25 °C in 100% yield after
24 h.20 No deprotection of isobutyl ester 5 was observed in CDCl3
using piperidine (1 or 3 equiv) even when the reaction was re-
fluxed for 24 h. We believe the lack of reactivity is due to the pres-
ence of the bulky phosphinimine group ortho to the sulfonate ester
which prevents proper approach of piperidine to allow for dis-
placement of the isobutyl group. Ester 7 was reacted with piperi-
dine (5 equiv) in CDCl3 at 68 °C. After 96 h heating, we observed
80% conversion of 7 to N-propyl-piperidinium 2-triphenylphos-
phinimine-5-methylbenezenesulfonate (10) with only 3% Ph3P@O
present.21 While the deprotection of 7 with piperidine to form 10
was successful the reaction rate was less than ideal. Based on our
success using piperidine for the deprotection of 7, we turned our
attention to protonation of 7 prior to deprotection by a nucleo-
philic nitrogen base.
P
Mes
S
O
O
O
S
O
O
O
R = alkyl,aryl
B
A
Figure 1. ortho-Substituted benzenesulfonate bidentate zwitterions.
benzenesulfonic acid into 2-amino-5-methylbenzenesulfonyl chlo-
ride (1) in 80% yield.10 The 2-amino-5-methylbenzenesulfonyl
chloride was converted into one of the four alkyl-2-amino-5-meth-
ylbenzene sulfonate esters (2a–c) in 81–93% yield by the reaction of
1 with the corresponding alcohol in the presence of 1,4-diazabicy-
clo[2.2.2]octane (DABCO).9,11
The amino esters 2a–c were converted into their corresponding
alkyl 2-azido-5-methylbenzenesulfonate esters (3a–c) via diazoti-
zation of the amine followed by reaction with sodium azide in
water (Scheme 1). Azides 3a and 3b were isolated as solids while
3c was isolated as an oil, all in good yields (71–83%). The 2-phos-
phinimine-5-methyl-benzenesulfonate esters (4–9) were synthe-
sized by employing the Staudinger reaction between azides 3a–c
and PPh3, Ph2PMe, (m-tolyl)3P, and (p-tolyl)3P (Scheme 1).12 When
phosphines were added to toluene solutions of azides 3a–c rapid
nitrogen evolution was observed at 25 °C and all reactions were
complete in 4 h at 25 °C.13 Compounds 4–9 were characterized
by 1H, 13C, and 31P NMR. The 31P NMR chemical shifts for the iso-
lated phosphinimines fall in the range of 3.2–2.2 ppm which is
consistent with the values for aryl phosphinimines observed in
the literature (d = 10–0 ppm).14 Phosphinimine formation was not
observed between (o-tolyl)3P and azide 3b even when the reaction
was heated at 60 °C in toluene for 12 h.15 We believe the lack of
reactivity between 3b and (o-tolyl)3P can be explained by compar-
ing the steric bulk of (o-tolyl)3P and PPh3 using cone angles.16 The
We envisioned phosphinimine protonation and propyl sulfo-
nate ester deprotection by the liberated base could be carried out
in a single reaction vessel. In choosing an appropriate H+ source
to protonate the phosphinimine nitrogen of 7 we wanted an acid
whose conjugate base could not deprotonate the resulting phos-
phiniminium salt.22 We chose to use pyridinium tetrafluoroborate
as our H+ source (pKa = 5.2) and felt the free pyridine may be com-
petent for the deprotection reaction.22b Indeed, rapid and complete
protonation of 7 is achieved by using [HPy][BF4] at 25 °C in CDCl3
(Scheme 2).
smaller PPh3 (H = 145°) can approach and react with azide 3b to
The phosphiniminium N–H of 11 was not observed in the 1H
spectrum, but the 31P NMR spectrum confirmed 11 (d = 25 ppm).14
When this sample was allowed to sit for 24 h at 25 °C, 30% of 11
had reacted with the pyridine to form 2-triphenylphosphinimini-
um-5-methylbenzene- sulfonate (12) and N-propylpyridinium
tetrafluoroborate. We have found that the deprotection reaction of
11 with pyridine to form 12 proceeds at a faster rate when it is
heated.23 Combining [HPy][BF4] with 7–9 in CH2Cl2 in the presence
of excess pyridine and then refluxing the solution overnight forms
NPSO zwitterions 12–14 in good yields (Scheme 3). Zwitterions
12–14 are air stable and can be isolated using column
chromatography.24
To confirm the structure of 12, a crystal suitable for X-ray crys-
tallography was obtained by slow diffusion of benzene into a
CH2Cl2 solution of 12 at 23 °C. An ORTEP diagram of the corre-
sponding structure is shown in Figure 2 with selected bond dis-
tances and angles listed. The structure of 12 shows the presence
of the phosphiniminium group in which N(1) is protonated. The
P(1)–N(1) bond distance [1.6327(17) Å] suggests a high degree of
single bond character much like the P–N single bond found in
[Ph3PN(H)Ph][Cl2Pd(C8H11)] [1.624(3) Å] and [Ph3PN(H)Ph][BF4]
form a phosphazide intermediate which decomposes via loss of
N2 to form 5 even in the presence of the bulky ortho-alkyl sulfonate
ester. Steric repulsion between the much bulkier (o-tolyl)3P
H = 194°) and the ortho-alkyl sulfonate ester in 3b prevents the
formation of a phosphazide and thus the phosphinimine cannot
(
form.17
Synthesis of discrete metal complexes using our NPSO ligands
requires deprotection of 2-phosphinimine-arenesulfonate esters
1a. ClSO3H
0 °C, 1 h
b. 80 °C, 2 h
NH3
O
NH2
OR
t
2a, (R = CH2 Bu)
2. ROH, DABCO
CH2Cl2
2b, (R = i Bu)
2c, (R = nPr)
S
S
O
O
O
O
2a-c
25 °C, 16 h
3. HBF4,NaNO2
N3
P(Ar)2R1
H2O, 0 °C, 30 min
4. NaN3
toluene
25 °C, 4 h
S
OR
O
H2O, 0 °C, 1-6 h
O
3a-c
R1
t
Ar
1
4
5
6
, 95% (R = CH2 Bu; Ar = R = Ph)
Ar
PPh3
N
PPh3
H
P
i
1
[HPy][BF4]
, 89% (R = Bu; Ar = R = Ph)
N
N
i
1
, 22% (R = Bu; Ar = Ph; R = Me)
CDCl3, 25°C
- Py
BF4
7, 63% (R = nPr; Ar = R1 = Ph)
8, 60% (R = nPr; Ar = R1 = m-tolyl)
9, 72% (R = nPr; Ar = R1 = p-tolyl)
S
OnPr
S
OnPr
S OR
O
O
O
O
O
O
7
4-9
11
Scheme 1. Synthesis of 2-phosphinimine-5-methyl-benzenesulfonate esters.
Scheme 2. Protonation of NPSO ester 7.

4834
C. T. Burns et al. / Tetrahedron Letters 53 (2012) 4832–4835
Ar
Ar
Ar
Ar
Ar
Ar
P
P
1.5 Py
N
N
H
S O
[HPy][BF4]
CH2Cl2
Δ,12 h
S
O
OnPr
- [nPrPy][BF4]
O
O
O
7, (Ar = Ph)
8, (Ar = m-tolyl)
9, (Ar = p-tolyl)
12, 96% (Ar = Ph)
13, 70% (Ar = m-tolyl)
14, 70% (Ar = p-tolyl)
Scheme 3. Synthesis of NPSO zwitterions 12–14.
Figure 3. ORTEP-326 diagram of 15 showing 50% ellipsoids. H atoms are omitted for
clarity with the exception of H1n which shows a strong intramolecular interaction:
N1–H1n, 0.81(2) Å; O1—H1n, 2.12(3) Å; N1–H1n—O1, 140.7(3)°. Selected bond
lengths (Å): P1–N1, 1.6380(12); N1–C6, 1.4124(17); S1–C1, 1.7811(14).
solution which was refluxed overnight. Gratifyingly, after work-up
the Ph2PMe NPSO zwitterion 15 was isolated in 70% yield (Scheme
4). This is a marked improvement on the low yield obtained for
NPSO sulfonate ester 6. We have applied our ‘‘one pot’’ synthesis
to multiple commercially available phosphines and isolated zwit-
terions 12 and 15–18 in good yields. In the case of the more elec-
tron rich phosphines, PhPMe2 (16) and Bn3P (18), we found that
optimal yields were obtained by stirring the [HPy][BF4]/pyridine
CH2Cl2 solution at 25 °C for 48 and 72 h respectively instead of
refluxing for 12 h. The 31P NMR chemical shifts for 12 and 15–18
are consistent with values for phosphiniminium salts observed in
the literature.3h,14,27
The X-ray crystal structure of 15 is shown in Figure 3. Suitable
crystals were obtained by slow diffusion of pentane into a CH3OH
solution of 15 at 23 °C. Similar to the structure of 12, a phosphin-
iminium N–H is also present in 15. The P(1)–N(1) bond distance
[1.6380(12) Å] is statistically the same as the phosphiniminium
P–N bond in 12 again supporting single bond character.25 The
phosphiniminium Ph2PMe substituent in 15 has much less steric
effect than the Ph3P of 12. Again the phosphorous atom of the
phosphiniminium is situated below the plane of the aryl ring of
the 5-methylbenzenesulfonate fragment but the C5–C6–N1–P1
torsion angle in 15 is greatly reduced 14.9(2)°.
In summary, we have developed a reliable synthetic protocol for
preparing 2-phosphiniminium-5-methylbenzenesulfonates 12–18.
Their structures were unambiguously proven by multinuclear NMR
and X-ray crystallography. This method represents an optimized,
flexible synthetic route for directly accessing ortho-substituted-
arenesulfonate ligands that incorporate phosphinimine functional-
ity. The modular nature of this synthesis allows for the creation of
a diverse ligand library where steric and electronic properties
of the phosphinimine can be manipulated with ease. On-going
studies in our laboratory are directed at developing catalytic
organometallic late transition metal complexes derived from
2-phosphinimine-5-methylbenzenesulfonate ligands.
Figure 2. ORTEP-326 diagram of 12 showing 50% ellipsoids. H atoms are omitted for
clarity with the exception of H1n which shows a strong intramolecular interaction:
N1–H1n, 0.79(2) Å; O1—H1n, 2.07(3) Å; N1–H1n—O1, 143.6(3)°. Selected bond
lengths (Å) P1-N1, 1.6327(17); N1-C6, 1.422(3); S1–C1, 1.784(2).
[1.635(4) Å] as well as the P–N single bond in the endocyclic phos-
phiniminium group of [4-(2,6-iPr2C6H3)N(H)PPh2)C12H7O][SO3CF3]
[1.633(2) Å].25a,25b,3h The P(1)–N(1) bond distance in 12 is
significantly longer than that found in the free phosphinimines
Ph3P@NPh
[1.603(3) Å]
and
4-(2,6-iPr2C6H3)NPPh2)C12H7O
[1.559(2) Å].25c,3h In addition, the triphenylphosphine group on
the nitrogen atom of the phosphiniminium is situated below the
arene ring plane of the 5-methylbenzenesulfonate fragment with
a C5–C6–N1–P1 torsion angle of 39.2(3)°.
With zwitterions 12–14 in hand, we envisioned the possibility
to overcome the low yields of the NPSO sulfonate esters and to
shorten the synthesis. We combined the Staudinger reaction with
the [HPy][BF4]/pyridine deprotection sequence in a single reaction
flask, thus avoiding isolation of the NPSO sulfonate esters alto-
gether. We reacted azide 2c with Ph2PMe in toluene at 25 °C for
2 h, removed the toluene under vacuum, and dissolved the residue
in CH2Cl2. [HPy][BF4] and excess pyridine were added to the CH2Cl2
R3
R1
R2
P
PR1(R2)R3
C6H5Me
[HPy][BF4],Py
CH2Cl2
Temp.,Time
-[nPrPy][BF4]
N3
N
H
S OnPr
O
S O
O
O
O
Acknowledgments
2c
12, 70%, reflux/12 h (R1,R2,R3 = Ph)
15, 70%, reflux/12 h (R1 = Me; R2,R3 = Ph)
16, 70%, 25 °C/48 h (R1,R2 = Me ;R3 = Ph)
17, 65%, reflux/12 h (R1 = Bn ;R2,R3 = Ph)
18, 70%, 25 °C/72 h (R1,R2,R3 = Bn)
We thank the donors of the American Chemical Society Petro-
leum Research Fund (50401-DN13) and the University of Louisville
for generously supporting this research. R. T. acknowledges fellow-
ship support from Department of Chemistry. M. S. M thanks the
Department of Defense (W81XWH) from the Telemedicine and
Scheme 4. ‘One Pot’ synthesis of 2-phosphiniminium-arene-sulfonate zwitterions.

C. T. Burns et al. / Tetrahedron Letters 53 (2012) 4832–4835
4835
9. Miller, S. C. J. Org. Chem. 2010, 75, 4632–4635.
Advanced Technology Research Center of the US Army, the Depart-
ment of Energy (DEFG02-08CH11538), and the Kentucky Research
Challenge Trust Fund for the upgrades of our X-ray facilities.
10. Schweitzer, H.; Hentrich, W.; Burr, K. U.S. Patent 1911,719, May 30, 1933.
11. The synthesis of both the methyl- and allyl-2-amino-5-methyl-
benzenesulfonate esters was attempted but apparent thermal instability in
both of these alkyl esters led to decomposition during work-up and isolation.
12. Staudinger, H.; Meyer, J. Helv. Chim. Acta 1919, 2, 635–646. The poor isolated
Supplementary data
yield of NPSO ester
6 is believed to be due to reaction of 6 with H2O;
Ph2PMe = O and amino ester 2b were observed as side products..
13. Synthesis of NPSO ester 5 was attempted using amino ester 2b and Ph3PBr2 in
the presence of excess NEt3. After work-up, 2b was recovered along with
Ph3P@O as the only phosphorous containing product.
Supplementary data associated with this article can be found, in
theonlineversion, at http://dx.doi.org/10.1016/j.tetlet.2012.06.112.
14. Braun, T. P.; Gutsch, P. A.; Zimmer, H. Z. Naturforsch. 1999, 54b, 858–962.
15. 1H NMR of the reaction mixture showed unreacted 3b and (o-tolyl)3P and 31P
NMR confirmed the presence of free (o-tolyl)3P (d = À30.3 ppm).
16. Tolman, C. A. Chem. Rev. 1977, 77, 313–348.
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a
protonated phosphinimine is similar to that of
a
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27. 31P NMR shifts for zwitterions 12 and 15–18; 12 (d = 31.6 ppm), 13
(d = 31.6 ppm), 14 (d = 31.4 ppm), 15 (d = 39.0 ppm), 16 (d = 43.7 ppm), 17
(d = 36.1 ppm), 18 (d = 48.9 ppm). The N–H is visible in the downfield region of
the 1H NMR spectra of 12–18 as a sharp doublet (2JP–H = 7–10 Hz) except for 15
whose 1H NMR spectrum was recorded in CD3OD due to poor solubility of 15 in
CD2Cl2 or CDCl3.