Hydrolysis rates of alkyl and aryl sulfinamides: evidence of general acid catalysis

Article abstract of DOI:10.1016/j.tetlet.2007.08.081

Sulfinamides are important in enantioselective synthesis, as rare post-translational modifications of proteins and as isosteres of the amide bond. Little is known about the rates of hydrolysis for aliphatic sulfinamides or the mechanism of hydrolysis. In this Letter, we show that sulfinamides hydrolyse by predominantly a non-specific acid/base catalysis with phosphate buffer but by varying the buffer concentration, it was possible to determine the hydrolysis rates of a range of sulfinamides with water through non-linear least squares regression.

Full text of DOI:10.1016/j.tetlet.2007.08.081

Tetrahedron Letters 48 (2007) 7452–7455
Hydrolysis rates of alkyl and aryl sulfinamides: evidence
of general acid catalysis
Andrew M. Piggott and Peter Karuso*
Department of Chemistry and Biomolecular Sciences, Macquarie University, NSW 2109, Australia
Received 10 June 2007; revised 14 August 2007; accepted 21 August 2007
Available online 25 August 2007
Abstract—Sulfinamides are important in enantioselective synthesis, as rare post-translational modifications of proteins and as iso-
steres of the amide bond. Little is known about the rates of hydrolysis for aliphatic sulfinamides or the mechanism of hydrolysis. In
this Letter, we show that sulfinamides hydrolyse by predominantly a non-specific acid/base catalysis with phosphate buffer but by
varying the buffer concentration, it was possible to determine the hydrolysis rates of a range of sulfinamides with water through non-
linear least squares regression.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Sulfinamides are unique isosteres of the amide bond,1
closely mimicking the transition state of both the hydro-
lysis and cis–trans isomerism reactions of peptides, and
therefore show potential as both protease2–4 and PPI-
ase5 inhibitors. In fact they have been described both
as substrates for proteases6 and nanomolar inhibitors
of proteases.7 Sulfinamides have also recently found util-
ity in enantioselective synthesis as chiral auxillaries8,9
and highly stereoselective organocatalysts.10 Sulfina-
mides have also been shown to form naturally in pro-
teins between the e-amino group of lysine and the
sulfur of methionine and these adducts have been stud-
ied by mass spectrometry.11 However, despite their
chemical and biological importance and potential as
drug motifs, few experimental studies have been
performed on the hydrolysis of sulfinamides.12–14 In
addition, theoretical calculations15,16 of the relative
energies of potential reaction intermediates have proven
inconclusive. Consequently, the exact mechanism by
which sulfinamides undergo hydrolysis, and the rate-
limiting step of the reaction, are still unclear. More
importantly, in the design of enzyme inhibitors, the
structural features affecting the rate of hydrolysis are
unknown.
In order to understand the mechanism of sulfinamide
hydrolysis more fully, and thereby design more stable
sulfinamide-containing drugs or probes, we have for
the first time, investigated the hydrolysis rates of simple
aliphatic sulfinamides that incorporate different elec-
tronic and steric parameters. A small set of simple sulfi-
namides was synthesised from the corresponding sulfinyl
chloride and two equivalents of amine at À78 ꢁC in
anhydrous dichloromethane (Supplementary data). It
was essential to react the amines and sulfinyl chlorides
at low temperature as they react violently at room tem-
perature. The resulting amine hydrochloride was filtered
off and the crude sulfinamide was washed with water
and dried. This yielded pure sulfinamides in ꢀ70%
yields.
The acid-catalysed hydrolysis of sulfinamides results in
cleavage of the S–N bond and loss of an amine residue
as the leaving group. The formation of this amine spe-
cies causes an increase in the pH of the reaction mixture
as the hydrolysis reaction proceeds (Supplementary
Fig. S1). In order to simplify calculations, it is often con-
venient to use a buffer solution to maintain a constant
pH, and subsequently determine the pseudo-first-order
rate constants for the hydrolysis reactions. For this
À
study, a phosphate buffer system (H3PO4=H2PO4
pH 3.0) was employed to maintain a constant pH through-
out the hydrolysis reactions. Initially, it was found that
the rate of hydrolysis was linearly dependent on the
hydrogen ion concentration (Fig. 1). Unexpectedly, the
rates of hydrolysis of the seven aliphatic sulfinamides
Keywords: Sulfinamide; Hydrolysis; Non-specific acid/base catalysis;
Enzyme inhibitors.
*
Corresponding author. Tel.: +612 9850 8290; fax: +612 9850 8313;
e-mail: peter.karuso@mq.edu.au
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.08.081

A. M. Piggott, P. Karuso / Tetrahedron Letters 48 (2007) 7452–7455
7453
pH4
pH3
pH5
pH6
pH7
pH8
O
S
O
S
O
S
O
S
a
b
Ph
1.50
1.25
1.00
0.75
0.50
0.25
0.00
N
H
N
H
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
Ph
N
H
N
1
2
3
4
<
<
<
O
S
O
S
O
S
N
N
N
<
5
<
6
<
7
Figure 3. Order of pseudo-first-order rate constants for the specific
H3O+-catalysed hydrolysis of seven aliphatic sulfinamides at 25 ꢁC.
2
3
4
5
pH
6
7
8
0
50 100 150 200 250
Time (min)
Figure 1. Rates of hydrolysis in 50 mM phosphate buffer from pH 3–8
for N-(methanesulfinyl)pyrrolidine 5, fitted to a one-phase exponential
associate (a). A log/log plot of reaction rate versus pH (b) shows a
linear relationship, indicating that the observed reaction is acid
catalysed and first order.
spectroscopy (Fig. 2a). The initial rates were plotted
against the phosphate concentration to reveal that the
reaction was pseudo-first order with respect to phos-
phate concentration (Fig. 2b), and subsequently using
the Dynafit17 package to calculate the pseudo-first-order
rate constant for the specifically H3O+-catalysed and
phosphate catalysed reaction (Supplementary Figs. S3-
10). The Dynafit package uses a non-linear least-squares
regression algorithm to fit data to a set of arbitrary reac-
tion mechanisms. This allowed the rates of specific and
general acid catalysis to be separated, and the pseudo-
first-order rate constants determined individually. The
pseudo-first-order rate constants for the H3O+-catalysed
hydrolysis of the six sulfinamides studied are given in
Table 1.
studied were also strongly dependent on the concentra-
tion of phosphate buffer used (Fig. 2). This suggests that
general (non-specific) acid/base species can catalyse the
hydrolysis of sulfinamides (Fig. 3).
The rate of specific acid-catalysed hydrolysis for each
sulfinamide was determined by varying the concentra-
tion of phosphate buffer (pH 3.0) between 1 and
200 mM, following the hydrolysis reaction with UV
The phenomenon of general acid catalysis has been de-
scribed previously in other species such as imines,18,19
but has not been previously described in sulfinamides.
The existence of general acid catalysis in sulfinamides
has important implications for hydrolysis studies that
employ buffer systems to maintain a constant pH
throughout the reaction. However, as alluded to by Cor-
des and Jencks,18 the kinetic difference between general
acid catalysis and specific acid/general base catalysis is
ambiguous. Therefore, it is possible that the hydrolysis
of sulfinamides is catalysed by the general acid H3PO4,
which assists in protonaÀtion of the sulfinamide, or by
the general bases H2PO4 and HPO42À, which assist in
abstraction of a proton at some stage in the reaction.
Clearly, a more detailed examination of the mechanism
of sulfinamide hydrolysis, including the initial site of
protonation and the rate limiting step of the reaction,
is required to assist in distinguishing between these
two possibilities.
a
-2.5
b
-3.0
The majority of nucleophilic displacement reactions at a
chiral sulfinyl sulfur atom occur with inversion of stereo-
chemistry.20 This inversion can be accommodated
by either a two-step addition–elimination mechanism
(Scheme 1), involving a hypervalent trigonal bipyrami-
dal sulfurane intermediate, or by a concerted SN2
displacement mechanism (Scheme 2), involving a transi-
tion state and no sulfurane intermediate. Okuyama,
et al.21,22 found evidence of 18O exchange from 18O@S
to H318O+ during the acid catalysed hydrolysis of sulfi-
namides, which can only occur through a sulfurane
intermediate, thus supporting a two-step addition–elim-
ination mechanism. The two-step mechanism also
explains nucleophilic substitution reactions that do
not proceed with 100% inversion. In these cases, pseudo-
-3.5
1
2
3
4
5
6
7
-4.0
-4.5
-5.0
-5.5
-3.5
-3.0
-2.5
-2.0
log [phos]
-1.5
-1.0
-0.5
Figure 2. Effect of varying phosphate buffer concentration on the
hydrolysis rate of alkyl sulfinamides. (a) Progress curves in 1–200 mM
phosphate buffer (pH 3) of N-(methanesulfinyl)piperidine 6 and (b)
log/log plots of initial velocities (v0) versus phosphate concentration.

7454
A. M. Piggott, P. Karuso / Tetrahedron Letters 48 (2007) 7452–7455
Table 1. Rates of specific acid (kcat) and non-specific acid (kphos) catalysis of seven alkyl sulfinamides determined in phosphate buffer
Compound
MW
93.15
lM
Response
kphos
kcat (·10À6
)
CH3SONHCH3
PhSONHCH3
1
2
3
4
5
6
7
532
202
397
107
157
408
604
0.00194
À0.00486
0.00128
0.01355
0.00837
0.01439
0.03286
0.01170
0.01272
0.01547
5.49 0.08
6.55 0.1
10.5 0.3
18.4 0.1
19.7 0.3
32.9 0.5
55.7 0.6
155.22
163.28
155.22
133.21
147.24
107.18
CH3SON(CH(CH3)2)2
CH3SONHPh
CH3SONPyrrolidine
CH3SONPiperidine
CH3SON(CH3)2
À0.00748
0.00409
0.00168
0.001733
Comparison of compounds 1 and 2 shows that replace-
ment of a methyl group on the sulfur side with a more
electron-donating aromatic ring only slightly increases
the rate of hydrolysis. The presence of the aromatic ring
on the sulfur side should result in an increase in electron
density on the sulfinyl oxygen atom, thus facilitating
protonation. A far larger increase in hydrolysis rate
would be expected if initial protonation of the oxygen
were the rate limiting step. Conversely, comparison of
compounds 1 and 4 shows that replacement of a methyl
group on the nitrogen side with an electron-withdrawing
aromatic ring actually increases the rate of hydrolysis
threefold. The presence of the aromatic ring on the
nitrogen side should withdraw electrons away from the
nitrogen, making protonation less favourable, and sub-
sequently decreasing the rate of hydrolysis. Clearly, if
nitrogen protonation does occur, it is not the rate-limit-
ing step of the hydrolysis reaction.
sulfurane
intermediate
R
H OH2
O
OH
S
NHR'
H2O
S
OH
S
R
NHR'
R'HN
R
proton
transfer
H3O+
+
RNH2
O
S
R
S
H
+
HO
R'H2N
O
R
OH
H2O
Scheme 1. Two-step addition–elimination mechanism of sulfinamide
hydrolysis.
H2O
O
S
OH
S
O
S
NHR'
H2O
The most surprising results of the current work are that
the tertiary sulfinamides (5–7) had the fastest rates of
hydrolysis. For example, adding another methyl to the
nitrogen side of N-methylmethanesulfinamide 1 resulted
in over an order of magnitude increase in hydrolysis
rate. However, the cyclic tertiary sulfinamides were
more stable but still not as stable as any of the second-
ary sulfinamides. This may be accounted for in the rela-
tive rigidity of the 5- and 6-membered rings compared to
dimethyl 7 derivative and supports the hypothesis that
the rate limiting step is proton transfer from the oxygen
to the nitrogen, which would require more reorganisa-
tion from the cyclic sulfinamides. The rates of hydrolysis
for this small set of compounds could not be correlated
to any single factor such as pKa of the leaving group,
MW, volume etc. The slower rates of hydrolysis of the
sulfinamides containing a secondary nitrogen atom
may, however, be the result of stabilisation by intermo-
lecular hydrogen bonding. The large dipole moment of
the S–O group is conducive to the formation of particu-
larly strong hydrogen bonds, which would stabilise the
molecules and result in slower rates of hydrolysis.
R
R
NHR'
R
OH
Scheme 2. Concerted (SN2) mechanism of sulfinamide hydrolysis.
rotation of the trigonal bipyramidal sulfurane interme-
diate results in partial retention of stereochemistry.23,24
While available evidence25 supports a two-step addi-
tion–elimination mechanism for sulfinamide hydrolysis,
the initial site of protonation and the rate limiting step
of the reaction are still unclear, and are the subject of
conflicting arguments. Ab initio calculations by Bagno
et al.26 have shown the oxygen protonated form of
CH3SONH2 to be 10.6 kcal molÀ1 more stable than
the nitrogen protonated form. The oxygen protonation
model is also supported by 14N NMR studies of several
sulfinamides,26 which show no evidence of 14N line
narrowing that is characteristic of nitrogen protonation.
However, studies of the infrared spectra of sulfinamides
by Bujnicki et al.27 showed the S@O and N–CH2 IR
peaks to shift to higher wavenumbers on protonation.
The authors concluded that this must be the result of
nitrogen protonation as this is the only case in which
the strength of S@O and N–C bonds increase. It was
also found that 15N NMR signals of sulfinamides shift
upfield upon protonation, indicative of the nitrogen
atom becoming more positive, as would occur on
protonation.19
In conclusion, we have found that aliphatic secondary
sulfinamides are relatively stable at basic pH but can
be readily hydrolysed at pH 3 and that tertiary sulfina-
mides are considerably more reactive than secondary
sulfinamides. The rate of hydrolysis has been shown to
be strongly dependent on the buffer concentration and
that phosphate was a most effective buffer for promoting
hydrolysis suggesting a non-specific acid/base catalysis.
These results are important in the design of sulfina-
mide-based enzyme inhibitors and peptide isosteres.
Our results suggest that while initial protonation of the
oxygen is more likely than protonation of the nitrogen,
it is not the rate limiting step in the hydrolysis reaction.

A. M. Piggott, P. Karuso / Tetrahedron Letters 48 (2007) 7452–7455
7455
11. Raftery, M. J.; Geczy, C. L. J. Am. Soc. Mass Spec. 2002,
13, 709–718.
Supplementary data
12. Asefi, H.; Tillett, J. G. J. Chem. Soc., Perkin Trans. 2 1979,
1579–1582.
13. Wagner, B. J.; Doi, J. T.; Musker, W. K. J. Org. Chem.
1990, 55, 5940–5945.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.08.081.
14. Biasotti, J. B.; Andersen, K. K. J. Am. Chem. Soc. 1971,
93, 1178–1182.
References and notes
15. Bagno, A.; Bujnicki, B.; Bertrand, S.; Comuzzi, C.;
Dorigo, F.; Janvier, P.; Scorrano, G. Chem. Eur. J.
1999, 5, 523–536.
16. Kim, C. K.; Lee, I. Bull. Korean Chem. Soc. 1997, 18, 880–
885.
17. Kuzmic, P. Anal. Biochem. 1996, 237, 260–273.
18. Cordes, E. H.; Jencks, W. P. J. Am. Chem. Soc. 1963, 85,
2843–2848.
1. Basch, H. In The Chemistry of Sulphinic Acids; Patai, S.,
Ed.; Esters and their Derivatives; John Wiley & Sons Ltd:
New York, 1990; pp 9–34.
2. Moree, W. J.; van der Marel, G. A.; Liskamp, R. M. J.
Tetrahedron Lett. 1991, 32, 409–412.
3. Moree, W. J.; van der Marel, G. A.; Liskamp, R. M. J.
Tetrahedron Lett. 1992, 33, 6389–6392.
4. Moree, W. J.; van der Marel, G. A.; Liskamp, R. J. J. Org.
Chem. 1995, 60, 5157–5169.
19. Sayer, J. M.; Conlon, P. J. Am. Chem. Soc. 1980, 102,
3592–3600.
5. Karuso, P.; Moran, D.; Poudrel, J.; Zhang, Q.; Lindskog,
P.; Janssen, A.; Jung, E.; Piggott, A.; D’Jamirze, A.
‘‘Design and Modelling of Sulfoxide and Sulfinamide
Peptidyl-Prolyl Isomerase Inhibitors’’; 5th Australian
Molecular Modelling Conference, September 21–23,
1999, University of Queensland. p 19.
20. Mikolajczyk, M.; Drabowicz, J. Top. Stereochem. 1982,
13, 333.
21. Okuyama, T.; Lee, J. P.; Ohnishi, K. J. Am. Chem. Soc.
1994, 116, 6480–6481.
22. Okuyama, T. Phosphorus, Sulfur Silicon Relat. Elem. 1994,
95, 113–125.
6. Roenn, R.; Gossas, T.; Sabnis, Y. A.; Daoud, H.;
Aakerblom, E.; Danielson, U. H.; Sandstroem, A. Bioorg.
Med. Chem. 2007, 15, 4057–4068.
7. Mugford, P. F.; Magloire, V. P.; Kazlauskas, R. J. J. Am.
Chem. Soc. 2005, 127, 6536–6537.
23. Mikolajczyk, M.; Drabowicz, J.; Bujnicki, B. Tetrahedron
Lett. 1985, 26, 5699–5702.
24. Mikolajczyk, M. Phosphorus Sulfur 1986, 27, 31–42.
25. Lee, J. P.; Kim, W. R.; Lee, Y. H.; Koo, I. S. Bull. Korean
Chem. Soc. 2006, 27, 941–943.
8. Senanayake, C. H.; Krishnamurthy, D.; Lu, Z.-H.; Han,
Z.; Gallou, I. Aldrichim. Acta 2005, 38, 93–104.
9. Ellman, J. A. Pure Appl. Chem. 2003, 75, 39–46.
10. Pei, D.; Wang, Z.; Wei, S.; Zhang, Y.; Sun, J. Org. Lett.
2006, 8, 5913–5915.
26. Bagno, A.; Eustace, S. J.; Johansson, L.; Scorrano, G. J.
Org. Chem. 1994, 59, 232–233.
27. Bujnicki, B.; Drabowicz, J.; Mikolajczyk, M.; Kolbe, A.
Phosphorus, Sulfur Silicon Relat. Elem. 1994, 95, 419–
420.