Kinetics and Mechanisms of Hydrolysis and Aminolysis of Thioxocephalosporins

Article abstract of DOI:10.1021/jo035471t

The effect of replacing the β-lactam carbonyl oxygen in cephalosporins by sulfur on their reactivity has been investigated. The second-order rate constant for alkaline hydrolysis of the sulfur analogue is 2-fold less than that for the natural cephalosporin. The thioxo derivative of cephalexin, with an amino group in the C7 side chain, undergoes β-lactam ring opening with intramolecular aminolysis by a reaction similar to that for cephalexin itself. However, the rate of intramolecular aminolysis for the S-analogue is 3 orders of magnitude greater than that for cephalexin. Furthermore, unlike cephalexin, intramolecular aminolysis in the S-analogue occurs up to pH 14 with no competitive hydrolysis. The rate of intermolecular aminolysis of natural cephalosporins is dominated by a second-order dependence on amine concentration, whereas that for thioxocephalosporins shows only a first-order term in amine. The Bronsted β_nuc for the aminolysis of thioxo-cephalosporin is +0.39, indicative of rate-limiting formation of the tetrahedral intermediate with an early transition state with relatively little C-N bond formation.

Full text of DOI:10.1021/jo035471t

Kin etics a n d Mech a n ism s of Hyd r olysis a n d Am in olysis of
Th ioxocep h a losp or in s
Wing Y. Tsang,† Anupna Dhanda,‡ Christopher J . Schofield,‡ and Michael I. Page*,†
Department of Chemical and Biological Sciences, University of Huddersfield,
Queensgate, Huddersfield HD1 3DH, U.K., and The Dyson Perrins Laboratory and
The Oxford Centre for Molecular Sciences, South Parks Road, Oxford OX1 3QY, U.K.
m.i.page@hud.ac.uk
Received October 7, 2003
The effect of replacing the â-lactam carbonyl oxygen in cephalosporins by sulfur on their reactivity
has been investigated. The second-order rate constant for alkaline hydrolysis of the sulfur analogue
is 2-fold less than that for the natural cephalosporin. The thioxo derivative of cephalexin, with an
amino group in the C7 side chain, undergoes â-lactam ring opening with intramolecular aminolysis
by a reaction similar to that for cephalexin itself. However, the rate of intramolecular aminolysis
for the S-analogue is 3 orders of magnitude greater than that for cephalexin. Furthermore, unlike
cephalexin, intramolecular aminolysis in the S-analogue occurs up to pH 14 with no competitive
hydrolysis. The rate of intermolecular aminolysis of natural cephalosporins is dominated by a second-
order dependence on amine concentration, whereas that for thioxocephalosporins shows only a first-
order term in amine. The Bronsted ânuc for the aminolysis of thioxo-cephalosporin is +0.39, indicative
of rate-limiting formation of the tetrahedral intermediate with an early transition state with
relatively little C-N bond formation.
SCHEME 1
In tr od u ction
Amide resonance is usually described in terms of
charge transfer from N to O as shown in the traditional
resonance formulation (Scheme 1). This, in turn, is then
used to rationalize the large barrier to rotation in amides
and their low chemical reactivity toward nucleophilic
substitution at the acyl center. In thioamides it is
expected that there would be greater π charge transfer
from N to S due to the larger size of S and weaker π bond
of CdS, despite the smaller electronegativity of S com-
pared with O (Scheme 1). Hence, thioamides have a
larger barrier to C-N bond rotation1 and are more easily
protonated on sulfur.2 Nonetheless, the effect of substi-
tuting O for S on the chemical reactivity of amides is less
clear. This is of particular interest in the reactivity of
the cephalosporin (1) group of antibiotics and their thioxo
analogues, the thioxocephalosporins (2), because of the
alleged reduction of amide resonance in these bicyclic
systems as a result of the pyramidalization of N.3
However, the evidence for reduced amide resonance in
penicillins and cephalosporins is minimal, and most
experimental observations indicate that resonance sta-
bilization is “normal” in these derivatives.4,5 We were,
therefore, interested in comparing the chemical reactivity
of thioxo-â-lactams with their O-analogues.
Exp er im en ta l Section
Kin ectic P r oced u r es. Standard UV spectroscopy was
carried out on a Cary 1E UV-vis spectrophotometer (Varian,
Australia) equipped with a 12-compartment cell block. The
instrument was used in double beam mode, allowing six
† The University of Huddersfield.
‡ The Dyson Perrins Laboratory.
(1) Wiberg, K. B.; Rush, D. J . J . Am. Chem. Soc. 2001, 123, 2038-
2046.
(2) Min, B. K.; Lee, H.-J .; Choi, Y. S.; Park, J .; Yoon, C.-J .; Yu, J .-
A. J . Mol. Struct. 1998, 471, 283-288.
(3) Woodward, R. B. In The Chemistry of Penicillin; Clarke, H. T.,
J ohnson, J . R., Robinson, R., Eds.; Princeton University Press: Prin-
ceton, 1949; p 443.
(4) Page, M. I. In The Chemistry of â-Lactam Antibiotics; Page, M.
I., Ed.; Blackie: Glasgow, 1992; Chapter 2, pp 79-100.
(5) Page, M. I. Adv. Phys. Org. Chem. 1987, 23, 165.
10.1021/jo035471t CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/17/2003
J . Org. Chem. 2004, 69, 339-344
339

Tsang et al.
TABLE 1. Su m m a r y of Ra te Con sta n ts for Th ioxocep h a losp or in s a n d Cep h a losp or in s a t 30 °C a n d Ion ic Str en gth 1.0 M
(KCl)
a
b
d
cephalosporin
pKa
kOH (M- 1s-1
)
koc (s-1
)
kRNH (M-1 s-1
)
2
thioxocephalexin (7)
cephalexine (5)
thioxocephalosporin (3)
cephalosporin (4)
6.86 ( 0.07
(1.29 ( 0.42) × 10-2
(1.45 ( 0.05) × 10-2
2.90 × 10-2
(9.48 ( 0.78) × 10-3
7.02
5.67 × 10-2
5.83 × 10-6
1.47 × 10-1
1.13 × 10-4
The acidity of the 7-side chain amino group. The second-order rate constant for the hydroxide-ion catalyzed hydrolysis. c The first-
a
b
d
order rate constant for uncatalyzed intramolecular aminolysis. The second-order rate constant for uncatalyzed aminolysis with
propylamine. e At 35 °C from ref 10.
SCHEME 2
reaction cells to be followed in a single run. The cell block was
thermostated using a Peltier system.
pH measurements were made with a φ40 (Beckman, Ful-
lerton, USA) pH meter using a semi-micro calomel electrode
(Beckman). A calibration of the pH meter was carried out at
30 °C using pH 6.99 (0.01, pH 4.01 (0.02, or pH 9.95 (0.02
calibration buffers.
AnalaR grade reagents and deionized water were used
throughout. Organic solvents were glass distilled prior to use
and stored under nitrogen. For solution pHs g3 and e11 the
pH was controlled by the use of e0.2 M buffer solutions of
formate (pKa 3.75), ethanoate (pKa 4.72), MES (pKa 6.1), MOPS
(pKa 7.2), TAPS (pKa 8.4), CAPSO (pKa 9.6), and CAPS (pKa
10.4). For general pH work, buffers were prepared by partial
neutralization of solutions of their sodium salts to the required
pH. In all experiments, temperatures were maintained at 30
°C and ionic strength at 1.0 M with AnalaR grade KCl unless
otherwise stated. Reaction concentrations were generally
within the range of 2 × 10-5-2 × 10-4 M to ensure pseudo-
first-order conditions.
Hydroxide ion concentrations were calculated using pKw
(H2O) ) 13.83 at 30 °C.
F IGURE 1. Dependence of the pseudo-first-order rate con-
stant against pH for the degradation of thioxo- and oxocepha-
losporin derivatives in aqueous solution at 30 °C, I ) 1.0 M
(KCl): 9, thioxocephalexin (7); [, cephalexin (5); 4, thioxo-
cephalosporin (3).
Reactions studied by UV spectrophotometry were com-
menced by injections of dioxan stock solutions of the substrate
(5-50 µL) into the cells containing preincubated buffer (2.5
mL). Final reaction cells contained e5% dioxane v/v. The pH
of the reaction cells was measured before and after each kinetic
run at 30 °C; kinetic runs experiencing a change >0.05 units
were rejected. Reactant disappearance or product appearance
was followed at absorbance change maxima for individual
compounds. The solubility of compounds was ensured by
working within the linear range of absorbance in correspond-
ing Beer-Lambert plots. Pseudo-first-order rate constants
from exponential plots of absorbance against time or gradients
of initial slopes were obtained using the CaryBio software
(Varian). pH-rate profiles were modeled to theoretical equa-
tions using the Scientist program (V2.02, Micromath Software
Ltd., USA).
for the alkaline hydrolysis of the O-analogue (4) is given
in Table 1. The S-derivative (3) is just 2-fold less reactive
than the O-cephalosporin (4), and at first glance, there-
fore, it appears that there is little difference in reactivity
between the O- and S-analogues.
Previous studies of the hydrolysis of thioamides are
complicated by the slow reaction rates and S/O ex-
change.6,7 Nonetheless, these earlier studies also appear
to indicate that thioamides have a reactivity similar to
that of the corresponding amide. If the rate-limiting step
for alkaline hydrolysis is formation of the tetrahedral
intermediate then any significant differences in reso-
nance energies between amides and thioamides should
be reflected in different activation energies. A more
Resu lts a n d Discu ssion
1. Th ioxocep h a losp or in Hyd r olysis. The alkaline
hydrolysis of the 3-methyl-7â-(phenylacetamido)thioxo-
ceph-3-em-4-carboxylic acid (3) gives the ring-opened thio
acid (Scheme 2), and the rates of hydrolysis were studied
in water at 30 °C and an ionic strength of 1.0 M (KCl).
The pH-rate profile is shown in Figure 1, which dem-
onstrates that the pseudo first-order rate constant is
linearly dependent on hydroxide-ion concentration. For
comparison, the corresponding second-order rate constant
(6) Peeters, O. M.; DeRanter, C. J . J . Chem. Soc., Perkin Trans. 2
1976, 1062-1065. Peeters, O. M.; DeRanter, C. J . J . Chem. Soc., Perkin
Trans. 2 1974, 1832-1835. Rosenthal, D.; Taylor, T. I. J . Am. Chem.
Soc. 1957, 79, 2684-2690.
(7) Mollin, J .; Bouchalova, P. Collect. Czech. Chem. Commun. 1978,
43, 2283. Zavoianu, D.; Cavadia, E. Chem. Oil. Gas 1971, 7, 38-44.
340 J . Org. Chem., Vol. 69, No. 2, 2004

Hydrolysis and Aminolysis of Thioxocephalosporins
SCHEME 3
The pseudo first-order rate constant for the reaction
of (7) is satisfactorily described by eq 1 where kobs is the
observed first-order rate constant, Ka is the dissociation
constant of the side chain amino group, ko is the first-
order rate constant for intramolecular aminolysis, and
kOH is the apparent second-order rate constant for
hydroxide ion catalyzed hydrolysis or intramolecular
aminolysis (Scheme 3). The latter is subject to significant
error and possibly makes an only minor contribution to
the observed rate of reaction.
Ka
kobs
)
× ko + kOH[OH-]
(1)
Ka + [H+]
The ratio Ka/(Ka + [H+]) describes the fraction of the
substrate present in the free base form, and the rate of
intramolecular aminolysis is dependent on the concentra-
tion of the reactant with an unprotonated amino group.
Curve-fitting by SCIENTIST gives Ka ) 1.38 × 10-7 (i.e.,
pKa ) 6.86), ko ) 9.48 × 10-3 s-1 and kOH ) 1.29 × 10-2
M-1 s-1. For comparison, the pH-rate profiles for the
reaction of cephalexin (5) in water are also given in
Figure 1. A similar sigmoidal dependence of the rate upon
pH is seen between pH 4 and 9 for the oxygen derivative,
but above pH 9 the rate increases with pH as the
hydroxide-ion catalyzed hydrolysis and hydroxide-ion
catalyzed intramolecular aminolysis become important.
From the pH-rate profiles of the thioxo-â-lactam and its
oxygen analogue, it is apparent that: (i) the rate of
intramolecular aminolysis in the thioxo-â-lactam is about
103-fold faster than that in cephalexin; (ii) competitive
hydroxide-ion catalyzed hydrolysis is observed with ceph-
alexin at high pH10 but with the thioxo-â-lactam in-
tramolecular aminolysis occurs over the entire pH range
studied (Figure 1). The rate constants for these reactions
of thioxo-â-lactams and their oxygen analogues are
compared in Table 1.
detailed discussion of the energetics of acyl transfer
reactions of amides and thioamides is given later.
2. In tr a m olecu la r Am in olysis of Th ioxocep h a -
losp or in . Many cephalosporins (1) possess a leaving
group at C3′ which is generally expelled after â-lactam
ring opening.4,5,8,9 However, cephalexin (5) is a cepha-
losporin with a methyl group at C3 and with an amino
group in the side-chain at C7. It is known that the
degradation of cephalexin in water occurs with both
hydrolysis and intramolecular aminolysis, the relative
importance of which varies with pH.10 Nucleophilic attack
by the amino group in the C7 side chain on the â-lactam
carbonyl carbon causes ring opening and formation of a
piperazinedione derivative (6). The thioxo analogue of
cephalexin,
3-methyl-7â-(2-aminophenylacetamido)-
thioxoceph-3-em-4-carboxylic acid (7), also reacts in water
with intramolecular aminolysis by the 7â-amino side
chain to give (9) (Scheme 3). The rate of this reaction
was studied in water at 30 °C and an ionic strength of
1.0 M (KCl), and the pH-rate profile is shown in Figure
1. The pseudo first-order rate constant for the degrada-
tion of (7) is linearly dependent on hydroxide-ion con-
centration below pH 7 and is independent of pH above
pH 7. The break in the pH-rate profile indicates either a
change in rate-limiting step or the ionization of a group
in the substrate11 with a pKa of about 7. The latter seems
more likely as this corresponds to the expected pKa of
the amino group in the C7 side chain (7).
Extrapolation of the hydroxide-ion catalyzed hydrolysis
of (3) indicates that a similar rate of hydrolysis for (7)
would only be observed above pH 14 (Figure 1). This is
consistent with the observation that no significant hy-
droxide-ion catalyzed hydrolysis was found for (7) in the
pH range studied and that intramolecular aminolysis for
the thioxo analogue of cephalexin (7) remains pH-
independent even at high pH.
It is interesting to note that the thioxo-â-lactam and
its oxygen analogue react at a similar rate toward
hydroxide-ion catalyzed hydrolysis but the intramolecular
aminolysis is 103-fold faster with thioxo-â-lactam than
that of its oxygen analogue. This difference between O
and N nucleophiles reacting with thioamides encouraged
us to study the intermolecular aminolysis of thioxocepha-
losporins.
3. In ter m olecu la r Am in olysis of Th ioxocep h a -
losp or in . The rate of aminolysis of the thioxocepha-
losporins (3) was studied in water at 30 °C and ionic
strength of 1.0 M (KCl) (Scheme 4), using the amine both
as reactant and buffer. The dependence of the observed
(8) Page, M. I.; Proctor, P. J . Am. Chem. Soc. 1984, 106, 3820.
(9) Buckwell, S. C.; Page, M. I.; Longridge, J . L.; Waley, S. G. J .
Chem. Soc., Perkin Trans. 2 1988, 1823.
(10) Bundgaard, H. Arch. Pharm. Chem. Sci. Ed. 1976, 4, 25.
(11) Page, M. I.; Williams, A. Organic and Bioorganic Mechanisms;
Longman: Essex, U.K., 1997; pp 30-36.
J . Org. Chem, Vol. 69, No. 2, 2004 341

Tsang et al.
SCHEME 4
pseudo first-order rate constants for the reaction of (3)
in aqueous solution buffered with 2-cyanoethylamine on
total amine concentration is shown in Figure 2. The
observed pseudo first-order rate constants are linearly
dependent on the total concentration of amine. Unlike
the aminolysis of natural cephalosporins8 and peni-
cillins,12-14 there is no second-order dependence on amine
concentration. The zero intercepts indicate the dominance
of aminolysis at the expense of hydrolysis. The slopes of
lines in Figure 2 are defined as kcat, and increase with
increasing pH. A plot of kcat against the fraction of free
base of 2-cyanoethylamine, R, in the solution gives a
straight line (Figure 3). The linear dependence of kcat on
R indicates the insignificance of any hydroxide-ion cata-
lyzed aminolysis. The left and the right intercepts give
the catalytic constants for the acidic and the basic species
of the amine. Since the left intercept is indistinguishable
from zero, the deprotonated form of amine is the only
species responsible for the catalysis.
F IGURE 2. Dependence of observed pseudo-first-order rate
constants for the reaction of 3-methyl-7â-(phenylacetamido)-
thioxoceph-3-em-4-carboxylic acid (3) with 2-cyanoethylamine
against total amine concentration at the pH indicated, 30 °C,
I ) 1.0 M (KCl).
The rate law that adequately describes the reaction of
the thioxocephalosporin (3) in aqueous solutions of amine
buffer is given by eqs 2-4
kobs ) kOH[OH-] + kcat[RNH2]tot
(2)
kobs ) kOH[OH-] + kRNH R[RNH2]tot
+
2
kRNH + (1 - R)[RNH2]tot (3)
3
kobs ) kOH[OH-] + kRNH [RNH2]
(4)
2
where kobs is the observed pseudo first-order rate con-
stant, kOH is the second-order rate constant for hydroxide-
ion catalyzed hydrolysis, [RNH2]tot is the total concen-
tration of amine, R is the fraction of free base, unproto-
nated amine, and kRNH is the second-order rate constant
2
F IGURE 3. Dependence of the second-order rate constant,
for uncatalyzed aminolysis reaction. The aminolysis of
the O-analogue of (3), the cephalosporin (4), has previ-
ously been studied, the rate law is dominated by general
base- and hydroxide-ion catalyzed aminolysis,8 and it is
experimentally difficult to determine the rate constant
for the uncatalyzed aminolysis reaction. However, a
reasonable estimate of the second-order rate constant for
uncatalyzed aminolysis of (4) with propylamine is given8
in Table 1 and is about 103-fold less than that of (3). This
is consistent with the observation that the rate of
intramolecular aminolysis of the thioxo derivative (7) is
about 103-fold greater than that in (5). It is interesting
to note that the rate of aminolysis of the oxocephalosporin
(4) is always slower than that of the thioxocephalosporin
k
cat, on the fraction of free base, R, of amine for the aminolysis
of 3-methyl-7â-(phenylacetamido)thioxoceph-3-em-4-carboxy-
lic acid (3) with 2-cyanoethylamine in aqueous solution, 30 °C,
I ) 1.0 M (KCl).
(3) in up to 2 M concentration of amine despite the former
being second-order in amine because of general base
catalysis.
The mechanism of the aminolysis of oxocephalosporins
has been shown to involve the formation of a tetrahedral
intermediate followed by diffusion of a base into the same
solvent cage as the intermediate as shown in Scheme 5.8
The generalized rate law for aminolysis is given by eq 5
k1k2[RNH2][base]
kobs
)
(5)
(12) Morris, J . J .; Page, M. I. J . Chem. Soc., Perkin Trans. 2 1980,
212.
k-1 + k2[base]
(13) Morris, J . J .; Page, M. I. J . Chem. Soc., Perkin Trans. 2 1980,
220.
where k1 and k-1 are the rate constants for formation of
the tetrahedral intermediate and its reversal to reac-
tants, respectively, and k2 is the diffusion controlled rate
(14) Gensmantel, N. P.; Page, M. I. Chem. Commun. 1978, 374.
Gensmantel, N. P.; Page, M. I. J . Chem. Soc., Perkin Trans. 2 1979,
137.
342 J . Org. Chem., Vol. 69, No. 2, 2004

Hydrolysis and Aminolysis of Thioxocephalosporins
SCHEME 5
TABLE 2. Secon d -Or d er Ra te Con sta n ts for th e
Un ca ta lyzed Am in olysis, kRNH , of th e
2
Th ioxocep h a losp or in (3) a s a F u n ction of th e p Ka of th e
Con ju ga te Acid of th e Am in e a t 30 °C a n d Ion ic Str en gth
1.0 M (KCl)
amine
pKa
kRNH /M-1 s-1
2
2-cyanoethylamine
2-methoxyethylamine
propylamine
8.21
9.66
10.79
(1.50 ( 0.02) × 10-2
(5.93 ( 0.31) × 10-2
(1.47 ( 0.06) × 10-1
F IGURE 4. Bronsted plot for the second-order rate constants,
kRNH2, for the aminolysis of the thioxocephalosporin (3) as a
function of the pKa of the attacking amine.
SCHEME 6
constant for the encounter of the intermediate with a
base (Scheme 5). The rate-limiting step is determined by
the relative rates of partitioning of the tetrahedral
intermediate to reactants and products, k2[base]/k-1. For
oxocephalosporins, the term in the rate law which is
dependent on base concentration is dominant and the
rate-limiting step is the encounter controlled step, k2,
because k-1.k2[base]. The observed pseudo first-order
rate constant, kobs, for the reaction which is base cata-
lyzed is then given by eq 6. However, at high base
concentration of hydroxide ion it is observed that the rate
of aminolysis of oxo-cephalosporins becomes independent
of base concentration because k2[base].k-1, and k1
becomes the rate-limiting step, eq 7.
kRNH , for the aminolysis of (3) against the pKa of the
2
conjugate acid of the amine is given in Figure 4, the slope
of which gives a Bronsted ânuc of +0.39. The ânuc value
for the formation of the tetrahedral intermediate is
expected8,12 to be about +1.0, so the observed value
indicates a relatively early transition state with only a
small amount of C-N bond formation (Scheme 6).
There are two observed rate constants which are first-
order in amine concentration for the rate of aminolysis
of cephalosporins.8 Under most experimental conditions,
the uncatalyzed aminolysis represents a very minor
pathway and the observed rate constant corresponds to
breakdown of the zwitterionic tetrahedral intermediate
and shows a ânuc of ca. 1.0 indicative of a unit positive
charge on nitrogen in the transition state.8,12 The major
pathway for aminolysis is general base catalyzed, but in
strongly basic conditions the rate of deprotonation of the
tetrahedral intermediate by base becomes faster than
that of the breakdown to reactants and so another term
in the rate law becomes first-order in amine correspond-
ing to rate-limiting formation of the tetrahedral inter-
mediate. The ânuc for this step is +0.3, suggesting a
transition state with little charge development on the
attacking amine and a small amount of C-N bond
formation.8,14 It thus appears that the transition state
structures for nucleophilic attack of amines on cepha-
losporins and thioxocephalosporins are similar (Scheme
6).
k1k
kobs
)
2[RNH2][base]
(6)
(7)
k-1
kobs ) k1[RNH2]
For the aminolysis of the thioxocephalosporin (3),
however, no base catalysis is observed in the pH range
studied. According to the mechanism for the aminolysis
of cephalosporins described above, k-1 must then be
smaller than k2[base] and formation of the tetrahedral
intermediate must be rate-limiting. In summary, the rate
of aminolysis of cephalosporins is normally base catalyzed
because the tetrahedral intermediate reverts to reactants
faster than it reacts with base whereas for thioxo-
cephalosporins the reverse is true.
To determine whether this was a general phenomenon
and to elucidate the transition state structure, the
aminolysis of (3) was also studied with a variety of
amines of differing basicity. For all amines studied, there
was no term in the rate law which showed a significant
dependence on base concentration. Aminolysis of thioxo-
cephalosporins (3) occurs with only a first-order depen-
dence on amine concentration, and the overall second-
order rate constants for several amines are given in Table
2, along with the pKa of the conjugate acid of the amine.
A Bronsted-type plot of the second-order rate constants,
The observed rate constant for the uncatalyzed ami-
nolysis of the thioxocephalosporin (3) with 2-methoxyeth-
ylamine (5.93 × 10-2 dm3 mol-1 s-1) can be compared with
that for the formation of tetrahedral intermediate in the
aminolysis of the cephalosporin (4) with the same amine
(1.76 × 10-1 dm3 mol-1 s-1). The thioxo-â-lactam is only
3-fold less reactive than the oxo-â-lactam. The second-
order rate constants for the alkaline hydrolysis of (3) and
(4) show a similar difference in reactivity.
J . Org. Chem, Vol. 69, No. 2, 2004 343

Tsang et al.
Why does the rate of aminolysis of cephalosporins
change from a dominant second-order term in amine to
an exclusive first-order term as the â-lactam carbonyl
oxygen is changed to sulfur? For natural cephalosporins,
the rate-limiting step for aminolysis is breakdown of the
tetrahedral intermediate, whereas for the thioxo deriva-
tive it is formation of the intermediate. If it is assumed,
in the aminolysis of cephalosporin and analogous thioxo-
â-lactam, that the acidity of the attacking amine in both
tetrahedral intermediates are similar (Scheme 5) so that
the k2[base] term would not be significantly different, i.e.,
proton transfer to any catalyzing base is thermodynami-
cally favorable and hence at the diffusion-controlled rate.
The reason for different rate-limiting steps for the oxo
and thioxo derivatives must then be due to different rates
of reversion of the respective tetrahedral intermediates
to reactants. The k-1 term must be smaller with thioxo-
â-lactam. A smaller rate of C-N fission by expulsion of
the attacking amine results from a greater activation
energy which could indicate that the thioxo-â-lactam is
less stable than â-lactam and/or the zwitterionic tetra-
hedral intermediate formed from the thioxo-â-lactam and
amine is more stable than that formed with oxo-â-
lactams. It is likely that a dominant feature controlling
the relative stabilities of the zwitterionic tetrahedral
intermediates is the more favorable sulfur anion com-
pared with oxygen. A large contribution to the higher
acidities of thiols compared with alcohols is the greater
stability of sulfur anions compared with the heavily
solvated alkoxide ions. In conclusion, thioxo-â-lactams
and oxo-â-lactams show a similar reactivity toward
nucleophiles, however for reactions of oxo-â-lactams
which normally involve rate-limiting breakdown of the
tetrahedral intermediate thioxo-â-lactams may have an
earlier rate-limiting step because of the slower rate of
reversion of the intermediate back to reactants and so
occur at a faster overall rate.
Ack n ow led gm en t. We thank the University of
Huddersfield for financial support.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
1
tails of synthesis and H and 13C NMR spectra for (3) and (7).
This material is available free of charge via the Internet at
http://pubs.acs.org.
J O035471T
344 J . Org. Chem., Vol. 69, No. 2, 2004