Substituent effects upon rates of deamination and base strengths of substituted N-tritylamines

Article abstract of DOI:10.1039/b104916j

Substituted N-tritylamines undergo deamination in aqueous acidic solution containing organic cosolvents, and follow a pseudo first-order specific acid-catalysis rate law (k_obs = k₀ + k_H[H₃O⁺]); the products are the corresponding trityl alcohols in equilibrium with the trityl carbenium ions, and ammonium ions. The change from 4,4′-dimethoxy-to 4,4′,4″-trimethoxy-tritylamine increases reactivity by a factor of about 30 in both k₀ and k_H, but N-alkyl and N-aryl groups have much greater effects (ca. 10⁶ for both k₀ and k_H for 4,4′-dimethoxytritylamine) which are largely independent of the nature of the N-alkyl and N-aryl groups. The anions of the catalytic strong acids (perchlorate, chloride, bromide, and nitrate) have only small effects as also does the concentration of acetonitrile as cosolvent; ethanoic acid as cosolvent is mildly rate-enhancing. Substituents in the aniline residue of N-(4,4′-dimethoxytrityl)-anilines have virtually no effect upon either k₀ or k_H (ρ = 0 for both). The results are interpreted by a mechanism involving a pre-equilibrium heterolysis of the (substituted) tritylammonium ion to give an ion-molecule pair which may undergo diffusional or acid-catalysed dissociation before the (substituted) trityl cation undergoes equilibrium nucleophilic capture by water. Base strengths of some substituted N-tritylamines have been measured; the substituted N-trityl group causes all amines (aliphatic and aromatic) to be comparably strongly basic (pK_BH- ca. 9).

Full text of DOI:10.1039/b104916j

View Article Online / Journal Homepage / Table of Contents for this issue
Substituent effects upon rates of deamination and base strengths
of substituted N-tritylamines
Moisés Canle L.,a Ibrahim Demirtas†b and Howard Maskill*b
2
a Chemistry Department, University of La Coruña, 15071 La Coruña, Spain
b Chemistry Department, University of Newcastle, Newcastle upon Tyne, UK NE1 7RU
Received (in Cambridge, UK) 5th June 2001, Accepted 19th July 2001
First published as an Advance Article on the web 13th August 2001
Substituted N-tritylamines undergo deamination in aqueous acidic solution containing organic cosolvents, and
follow a pseudo first-order specific acid-catalysis rate law (kobs = k0 ϩ kH[H3Oϩ]); the products are the corresponding
trityl alcohols in equilibrium with the trityl carbenium ions, and ammonium ions. The change from 4,4Ј-dimethoxy-
to 4,4Ј,4Љ-trimethoxy-tritylamine increases reactivity by a factor of about 30 in both k0 and kH, but N-alkyl and N-
aryl groups have much greater effects (ca. 106 for both k0 and kH for 4,4Ј-dimethoxytritylamine) which are largely
independent of the nature of the N-alkyl and N-aryl groups. The anions of the catalytic strong acids (perchlorate,
chloride, bromide, and nitrate) have only small effects as also does the concentration of acetonitrile as cosolvent;
ethanoic acid as cosolvent is mildly rate-enhancing. Substituents in the aniline residue of N-(4,4Ј-dimethoxytrityl)-
anilines have virtually no effect upon either k0 or kH (ρ = 0 for both). The results are interpreted by a mechanism
involving a pre-equilibrium heterolysis of the (substituted) tritylammonium ion to give an ion–molecule pair which
may undergo diffusional or acid-catalysed dissociation before the (substituted) trityl cation undergoes equilibrium
nucleophilic capture by water. Base strengths of some substituted N-tritylamines have been measured; the substituted
ϩ
N-trityl group causes all amines (aliphatic and aromatic) to be comparably strongly basic (pKBH ca. 9).
decomposition of (substituted) trityl ethers, which (like the
Introduction
corresponding alcohols) are only weak bases in aqueous solu-
The substituted trityl group (triphenylmethyl) has been widely
tion, kobs = kH[H3Oϩ] as there is no detectable reaction at
used as a protecting group for hydroxy and amino functions.1
[H3Oϩ] = 0. A mechanism for the deprotection of a generic N-
The parent trityl group is most easily introduced (using trityl
tritylamine, Tr–NH–R, based upon analogy with the reactions
chloride) but the least easy to remove, requiring acidic con-
of trityl ethers, is shown in Scheme 2. In the pre-equilibrium,
ditions which, in some cases, compromise the integrity of
the rest of the molecule.2 p-Methoxy substituted analogues,
conveniently prepared using the tetrafluoroborates,3 are much
easier to deprotect. Early work indicated that cleavage of
substituted trityl ethers under acidic conditions occurs by pre-
equilibrium protonation followed by a simple SN1 cleavage,
Scheme 2 Deprotection of a generic N-tritylamine under acidic
conditions.
however, the initial substituted tritylamine is virtually wholly
protonated under the normal reaction conditions (dilute strong
acid containing a low concentration of cosolvent) since the
amines are strong bases and [H3Oϩ] >> [tritylamine]0. Also
in contrast to the corresponding deprotection of trityl ethers,
the departure of the nucleofuge is irreversible regardless of
aqueous dilution since the nucleofuge, being a strongly basic
amine, will be rapidly trapped by protonation in the acidic
medium. According to this mechanism, heterolysis of the pro-
tonated amine is the rate-limiting step of an SN1 reaction, and
the rate constant of this elementary step, k in Scheme 2, will
Scheme
1
Simplified mechanism for the specific acid-catalysed
deprotection of trityl ethers in aqueous solution.
Scheme 1,4 although later work has shown that ion–molecule
pairs are involved.5–7 The extent of return from the trityl cation
and the alcohol will be insignificant in dilute aqueous solution,
and k should be independent of [H3Oϩ] as has been observed
for the closely related cleavage of trityl alcohols;6,7 but the
overall reaction is specific acid-catalysed due to the dependence
of the concentration of the reactive intermediate (the pro-
tonated substrate) upon [H3Oϩ]. Thus, in the first-order
be the observed overall pseudo first-order rate constant, kobs
,
i.e. kobs = k and is independent of [H3Oϩ] as long as [H3Oϩ] >>
[TrNH2]0. The subsequent nucleophilic capture by water of the
substituted trityl carbenium ion and associated deprotonation
to give the corresponding substituted trityl alcohol constitute
a reversible post-equilibrium. This final post-equilibrium is, of
course, exactly the same as in the deprotection of trityl ethers,
† Present address: Department of Chemistry, University of Gazios-
manpasa, 60250, Tokat, Turkey.
1748
J. Chem. Soc., Perkin Trans. 2, 2001, 1748–1752
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104916j

View Article Online
and has been extensively investigated in the past starting from
the trityl alcohol. It is the basis of the well known HR acidity
function scale,8 and comprises steps which are all much more
rapid than the deaminations of the substituted primary
tritylamines.6,7,9,10
Our kinetics study of the deprotection of substituted N-
tritylamines was initiated to provide a basis in synthesis to allow
improved control in the selective removal of one substituted
trityl group in the presence of others that are differently sub-
stituted, or of one trityl group in the presence of an identical
one but on a different type of amino group in a multi-protected
polyamine. We have already reported kinetics results for the
deprotection of several substituted N-trityl-N-alkylamines,9
and a detailed mechanistic investigation of the deamination of
4,4Ј-dimethoxytritylamine under controlled acidic conditions.10
This earlier work showed that the simple mechanism of Scheme
2 is inadequate; this reaction, like the cleavage of trityl ethers
(but for a quite different reason), is also acid catalysed, i.e.
kobs = k0 ϩ kH[H3Oϩ]; these tritylammonium ions are perhaps
unique in having no basic site at the reaction centre, yet undergo
an acid-catalysed reaction. The simplest mechanism that can
accommodate this rate law is shown in Scheme 3. It involves the
Fig. 1 Structures (all arene substituents are para) of 1–4.
equilibrium constant for the reaction between carbenium
ϩ
ion, water, and hydronium ion (i.e. the pKR of the particular
carbocation), in addition to the initial concentration of the
substrate and the acidity of the medium.13 In a representative
case, we established that the final equilibrium under the same
experimental conditions was the same starting from equal con-
centrations of the trityl alcohol and the tritylamine (although
the former reacted much faster).
Reactions with half-lives greater than a few minutes were
investigated by a conventional UV spectrophotometric method
as previously described;6,7,9,10 faster reactions were investigated
using multi-mixing stopped-flow spectrophotometers.14 All
reactions were reproducible and experimental pseudo first-
order rate constants, kobs, are believed to be reliable to better
than 5%. It was generally observed that the experimental
pseudo first-order rate constants, kobs, were linearly dependent
upon the hydronium ion concentration (although upward
curvature at acidities greater than about 1 mol dmϪ3 was
evident). The linear relationship may be expressed as eqn. (1);
kobs = k0 ϩ kH[H3Oϩ]
(1)
Scheme 3 Mechanism for deprotection of N-tritylamines under acidic
conditions.
kH (the second-order rate constant for the acid-catalysed reac-
tion) and k0 (the first-order rate constant of the uncatalysed
reaction) were obtained either graphically or by linear
regression.
intermediate formation of an ion–molecule pair which reacts
further with and without acid catalysis.9–11 We have also
reported base strengths and crystal structure determinations
of several N-trityl-N-alkylamines.12 We now report results of
further pKa determinations and a kinetics study of deamin-
ations of additional substituted N-trityl-N-alkylamines shown
in Fig. 1.
Values of k0 and kH (reliable, we believe, to about 20%)
are shown in Table 1 along with the pKa values of the corre-
sponding tritylammonium ions (i.e. the pKBH of the amine)15
ϩ
measured in water at constant ionic strength containing
sufficient acetonitrile to maintain a homogeneous solution
under the reaction conditions. We have already established in
Methods and results
representative cases that substituting Liϩ or NH4 for Naϩ at
ϩ
Our substrates were made from the substituted trityl chloride or
tetrafluoroborate with amines as appropriate using established
methods, and have already been described.9,12 In all cases,
our samples had spectroscopic and other properties in accord
with literature values and ascribed structures.
constant perchlorate and hydronium ion concentrations has no
appreciable kinetic effect.9 Rate constants of 4,4Ј-dimethoxy-
and 4,4Ј,4Љ-trimethoxy-tritylamines (DMTrNH2 and TMTr-
NH2) were also measured at other temperatures between 288
and 328 K which allowed determination of the activation
parameters shown as footnotes to Table 1. Replacing perchloric
acid with other strong acids (hydrochloric, hydrobromic, and
nitric) is shown in Table 2 to have little effect upon catalysed
and uncatalysed reactions of TMTrNH2 under the conditions
employed. The effect of the composition of the aqueous
acetonitrile is negligible, and ethanoic acid as cosolvent is
slightly rate-enhancing (Table 2); additionally, tetrahydrofuran
was tried as a cosolvent but was unsuitable as it did not give a
single phase with high concentrations of aqueous electrolytes.
The effects of the nature of the cosolvent and its composition
with water are similar for DMTrNH2 (Table 3).
Reaction rates of (N-substituted) monomethoxy-, di-
methoxy-, and trimethoxy-tritylamines were measured at 25 ЊC
in water containing a small proportion of acetonitrile for solu-
bility reasons, and concentrations of up to about 1 mol dmϪ3 in
perchloric acid; the ionic strength was maintained constant
using sodium perchlorate. Initial concentrations of substrates
were typically 1–6 × 10Ϫ5 mol dmϪ3. Some measurements were
made with the parent tritylamine, but a higher temperature and
higher acidities (hence a higher ionic strength) had to be used in
order to get measurable rate constants. In all cases, rates were
measured by monitoring the increase in UV absorbance due
to the formation of the (substituted) trityl carbenium ion. In
all of these reactions, the terminal absorbance depended upon
the concentration of the (substituted) trityl carbenium ion in
the final equilibrium mixture which, in turn, depended upon the
A routine pH titration technique was used for determining
the pKa’s of the substituted N-tritylalkyl- and N-tritylaryl-
ammonium ions.12,17 Acetonitrile was used as cosolvent to over-
come solubility difficulties in water alone and, although this is
J. Chem. Soc., Perkin Trans. 2, 2001, 1748–1752
1749

View Article Online
Table 1 Acidity constants and rate constants for uncatalysed and acid-catalysed reactions of the conjugate acids of substituted tritylamines in
aqueous perchloric acid containing 1–2% acetonitrile; ionic strength (NaClO4) = 1 mol dmϪ3, 25 ЊCa
k0/sϪ1
kH/dm3 molϪ1 sϪ1
ϩ
Tritylamine
pKBH (% CH3CN)
TrNH2, 1a
MMTrNHCH2CO2Me, 2b
MMTrNHPh, 2c
DMTrNH2, 3a
DMTrNHPh, 3c,i
9.2(16)
—
—
6.9 × 10Ϫ5 b
34.0
3.8 × 10Ϫ5 b
59.1
9.25
145
9.5(28)c
8.9(39)e
9.2(17)e
9.5(53)e
8.8(17)e
9.3(87)e
—
3.49 × 10Ϫ5 d
1.2 × 10Ϫ5 d
41
37
45
43
38
9
37
72
10
56
23
142
33
DMTrNHC6H4Me, 3c,ii
DMTrNHC6H4OMe, 3c,iii
DMTrNHC6H4F, 3c,iv
DMTrNHC6H4NO2, 3c,v
DMTrNH(p-NO2Bn), 3d
DMTrNHC3H7, 3e
DMTrNHOMe, 3f
TMTrNH2, 4a
TMTrNH(p-NO2Bn), 4d
TMTrNHC3H7, 4e
TMTrNHBn, 4g
TMTrNHC4H9, 4h
TMTrN(Me)Bn, 4i
—
—
48
0.20f
1.3f
1.68 × 10Ϫ4g
118
9.3(26)c
—
3.81 × 10Ϫ4 g
5.8
4.75
4.9
5.3
5.7
9.7(26)
9.4(32)
9.7(36)
9.9(20)
146
138
143
128
a pKBH values of the bases, i.e. the pKa values of the corresponding alkylammonium ions,15 were measured in acetonitrile–water solution, ionic
ϩ
strength 1.0 mol dmϪ3 (NaClO4) at 21 ЊC; all arene substituents are at position 4. b 340 K, ionic strength (NaClO4) = 3.5 mol dmϪ3, 4% MeCN.
9
c Ref. 9. d k0 = 3.1 × 10Ϫ5 sϪ1 and kH = 1.6 × 10Ϫ5 dm3 molϪ1 sϪ1
;
from rate measurements made between 288 and 328 K, ∆H‡ = 105 5 and
126 8 kJ molϪ1, and ∆S‡ = 21 10 and 85 15 J KϪ1 molϪ1, for k0 and kH, respectively. e Ref. 12. f Ref. 16. g k0 = 89 × 10Ϫ5 sϪ1 and kH = 50 ×
10Ϫ5 dm3 molϪ1 Ϫ1;9 from rate measurements made between 288 and 328 K, ∆H‡ = 97 5 and 100 5 kJ molϪ1, and ∆S‡ = 15 10 and 19 10
s
J KϪ1 molϪ1, for k0 and kH, respectively.
Table 2 Effect of catalytic acids and solvent composition upon rate
constants for uncatalysed (k0) and acid-catalysed (kH) reactions of
4,4Ј,4Љ-trimethoxytritylammonium cation in aqueous solution at 298 K;
ionic strength = 1 mol dmϪ3a
unlikely to have had a significant effect, its proportion changed
slightly as the ammonium salts were titrated against standard
sodium hydroxide. The initial acidic pH of the solution for
titration was chosen to minimise solvolytic deamination of the
N-tritylalkyl- and N-tritylaryl-ammonium ions. Results are
included in Table 1.
Acid (% cosolvent)
104k0/sϪ1
104kH/dm3 molϪ1 sϪ1
HClO4 (1% MeCN)
HCl (1% MeCN)
3.81b
2.02
2.44
2.44
1.74
2c
1.68b
1.80
0.92
1.21
3.1
Discussion
HBr (1% MeCN)
HNO3 (1% MeCN)
HClO4 (51% MeCN)
HClO4 (78% MeCN)
HCl (60% AcOH)
HCl (95% AcOH)d
ϩ
It is evident from the pKBH values of the substituted trityl-
amines, that the substrates in these deamination reactions
under the acidic conditions of the kinetics experiments are
actually the corresponding substituted tritylammonium
cations.15 Consequently, although kH is the rate constant of the
5c
8.6
11
3.6
10
a The ionic strength was maintained constant with the sodium salt of
the catalytic acid. b From Table 1. c These results are less precise due to
the poorer quality of the linear correlation of kobs with [HClO4]. d Ionic
strength not kept constant.
ϩ
real acid-induced reaction of the (substituted) ions, Tr–NH3
(or Tr–NH2Rϩ), k0 corresponds to the hypothetical reaction
of these ions at [H3Oϩ] = 0. The insensitivity of the kinetics of
deamination of TMTrNH2 to the nature of the strong acid
catalyst (Table 2) is in accord with previous studies using
DMTrNH2, and is as expected for a specific acid-catalysed
mechanism. The absence of a major solvent effect upon the
rates of deaminations of both DMTrNH2 and TMTrNH2
shown in Tables 2 and 3 indicates that the choice of cosolvent
need be guided only by solubility issues in deprotection reac-
tions in synthesis.
Table 3 Effect of cosolvent upon rate constants for uncatalysed (k0)
and perchloric acid-catalysed (kH) reactions of 4,4Ј-dimethoxy-
tritylammonium cation in aqueous solution at 298 K; ionic strength
(NaClO4) = 1 mol dmϪ3
Cosolvent
105k0/sϪ1
105kH/dm3 molϪ1 sϪ1
Monomethoxytritylamine (MMTrNH2) is much less reactive
than the dimethoxy- analogue,18 and the parent tritylamine
(TrNH2) is even less reactive. Furthermore, trityl and mono-
methoxytrityl carbenium ions are much less stable in aqueous
solution than dimethoxy- and trimethoxy- analogues. Conse-
quently, it was not possible to obtain reliable rate measurements
on TrNH2 and MMTrNH2 under conditions (temperature,
acidity, and ionic strength) comparable with those employed
for dimethoxy- and trimethoxy- analogues. The similarity in
the rate-enhancing effects of the extra 4-methoxy substituent
between DMTrNH2 and TMTrNH2 upon both k0 and kH
(Table 1, a factor of about 30) suggests that the effect is upon
the initial pre-equilibrium ionization (k1/kϪ1) rather than the
subsequent kd and ka steps (Scheme 3). This is readily under-
stood as the resonance effect of the extra methoxy substituent
upon the stability of the dimethoxytrityl cation. The 4-methoxy
substituent effect is much smaller, however, than the effect of
1% MeCNa
60% AcOH
95% AcOHb
3.49
5.5
6
1.2
2.9
7
a It was shown that, under otherwise the same experimental conditions
at 298 K, kobs increased only from 4.6 to 6.5 × 10Ϫ5 sϪ1 as the proportion
of MeCN increased from 1 to 60%. b Ionic strength not kept constant.
an N-alkyl group (a factor of about 106 upon both k0 and kH
for DMTrNH2). Moreover, the rate-enhancements due to
resonance of the second and third methoxy substituents
virtually disappear in the N-alkyl derivatives, i.e. the N-alkyl
has a levelling effect upon the rate-enhancing resonance effect
of the second and third 4-MeO groups. Furthermore, com-
parisons of reactivities of mono-, di-, and tri-methoxy-
substituted analogues with a common substituent on the
1750
J. Chem. Soc., Perkin Trans. 2, 2001, 1748–1752

View Article Online
nitrogen, e.g. –NH–Ph, or –NH–CH2CO2Me,9 do not appear
to follow any readily explicable pattern. Within the dimethoxy-
trityl series, however, all carbon-bonded N-substituents lead
to similarly large rate enhancements (Table 1), and the alkoxy
group of N-(dimethoxytrityl)-O-methylhydroxylamine has an
effect smaller by a factor of only about 200 on k0 and 50 on
kH compared with the alkyl substituents. Replacement of an
alkyl on the nitrogen by an alkoxy yields an α-effect nucleofuge/
nucleophile,19 so there will be a higher proportion of internal
return for the hydroxylamine, i.e. k1/kϪ1 in Scheme 3 will be
smaller, with an associated reduction in the overall rate con-
stants, k0 and kH, compared with the N-alkyl analogues.
standard sodium hydroxide which had been previously stand-
ardised against potassium hydrogen phthalate.
Kinetics
The deamination reactions of the trityl-substituted ammonium
cations occur in aqueous perchloric acid (containing 1–2%
of acetonitrile to overcome solubility problems) to give
equilibrium mixtures of the trityl cations and trityl alcohols.
Trityl, 4,4Ј-dimethoxytrityl, and 4,4Ј,4Љ-trimethoxytrityl cations
absorb strongly at 432, 495, and 483 nm, respectively, and rates
were measured by monitoring increases in absorbances at
or close to these wavelengths. Conventional kinetics were
measured using a Cecil 5502 double beam spectrophotometer
controlled by an Elonex PC-433 and with the cell block
thermostatted by water circulating from a Grant W6 water
bath. The temperature was monitored with a platinum
resistance thermometer in the cell block. Stopped-flow kinetics
were carried out using an Applied Photophysics SX-17MV
stopped-flow spectrofluorimeter at 25 0.1 ЊC.14 Rates of
decomposition under acidic conditions were investigated under
the usual pseudo first-order conditions. The observed pseudo
first-order rate constants (averages of at least six runs) were
plotted against [H3Oϩ] to give the second-order catalytic
constant from the gradient and the first-order rate constant for
the uncatalysed reaction from the intercept.
We reported previously that the effect of substituents upon
the base strengths of anilines, ρ(σϪ) = Ϫ2.9,20 is absent in N-
tritylanilines;12 moreover, all the N-tritylanilines we investigated
ϩ
were comparable as bases with simple alkylamines, i.e. pKBH
ca. 9. We ascribed this to steric inhibition of resonance between
the lone pair on the nitrogen and the aryl residue and its
para-substituents (if present). We now observe that k0 values
for N-trityl(aryl)amines are generally higher than for N-
trityl(alkyl)amines, but independent of the substituent in the
arylamine (ρ = 0 for k0), and kH values are lower than for
N-alkyl analogues, but again independent of the substituent in
the arylamine (ρ = 0 for kH). On the basis that arylamines are
poorer nucleophiles and weaker bases than alkylamines, kϪ1
and ka should be smaller for arylamines than for alkylamines,
whereas k1 should be larger and kd much the same. It follows
(Scheme 3) that k0 should be larger for N-trityl(aryl)amines
than N-trityl(alkyl)amines but kH smaller as is observed (Table
1) but only by a factor of about 10 for k0 and even less for kH.
The absence of an effect by substituents in the aniline moiety
upon both k0 and kH is more intriguing and, like the origin of
the massive rate enhancements caused by N-alkyl and N-aryl
groups, is the subject of ongoing investigations.
pKa Determinations
A pH titration method was used and has already been
described.11 A combined glass electrode coupled to a Metrohm
716 DMS Titrino automatic titrator was used to record the pH
during the titration of the conjugated acid of the amine against
standard sodium hydroxide. The reported pKa values were
optimised by fitting the experimental results to the appropriate
sigmoidal equations derived from the Henderson–Hasselbach
equation using a non-linear optimisation algorithm.
Activation parameters for the forward reaction (K.k) in
Scheme 1 for R = H, have already been reported (∆H‡ = 63 kJ
molϪ1 and ∆S‡ = Ϫ19 J KϪ1 molϪ1),7 and are in accordance
with a fast second-order catalytic mechanism (low ∆H‡
and modestly negative ∆S‡). This acid-catalysed cleavage of
alcohols (and ethers) compares with the k0 reaction channel in
the present investigation (Scheme 3), but with an important
difference. In the k0 reaction channel of Scheme 3, the reactant
is already protonated, so the reaction is essentially a first-order
unimolecular reaction of an alkylammonium ion. Accordingly,
there is no adverse translational component to the entropy of
activation, as there is for the second-order bimolecular process,
and there is no change in charge type as the reactant proceeds
to the transition structure. In the absence of complicating
differential solvation effects, therefore, ∆S‡ for the dissociative
unimolecular mechanism should be modestly positive as is
observed: ∆S‡ values for k0 for DMTrNH2 and TMTrNH2 are
21 and 15 J KϪ1 molϪ1, respectively. This entropy effect alone,
of course, would lead to the deaminations being faster than
Acknowledgements
We thank the Turkish Government for a studentship (ID), and
Drs A. P. Henderson and Elke Stix for the preparation of some
compounds.
References
1 B. Helferich, Adv. Carbohydr. Chem., 1948, 3, 79; J. F. W. McOmie,
Protective Groups in Organic Chemistry, Plenum Press, London and
New York, 1973; T. W. Greene and P. G. M. Wuts, Protective Groups
in Organic Synthesis, Wiley-Interscience, 3rd edn., New York, 1999.
2 L. Zervas and D. M. Theodoropoulos, J. Am. Chem. Soc., 1956, 78,
1359; P. T. Gilman and H. G. Khorana, J. Am. Chem. Soc., 1959,
81, 4647; M. Smith, D. H. Rammler, I. H. Goldberg and
H. G. Khorana, J. Am. Chem. Soc., 1962, 84, 430; H. Schaller,
G. Wiemann, B. Lerch and H. G. Khorana, J. Am. Chem. Soc., 1963,
85, 3821.
3 C. Bleasdale, S. B. Ellwood and B. T. Golding, J. Chem. Soc., Perkin
Trans. 1, 1990, 803; A. P. Henderson, J. Riseborough, C. Bleasdale,
W. Clegg, M. R. J. Elsegood and B. T. Golding, J. Chem. Soc., Perkin
Trans. 1, 1997, 3407.
the cleavage of ethers and alcohols at [H3Oϩ] = 1 mol dmϪ3
.
The major cause, therefore, of the much faster cleavage of the
C–O bonds in the protonated alcohols and ethers compared
with cleavage of the C–N bond in the deamination of trityl-
ammonium ions is the difference in enthalpies of activation
(ca. 63 kJ molϪ1 compared with ca. 100 kJ molϪ1). There is
no reaction channel in the cleavage of the alcohols (or ethers)
corresponding to the acid-catalysed deamination. We see, how-
ever, that ∆H‡ values for this kH process for both DMTrNH2
and TMTrNH2 are virtually the same as for the uncatalysed k0
reactions.
4 H. Nicholson and P. A. H. Wyatt, J. Chem. Soc. (B), 1968, 198;
M. J. Postle and P. A. H. Wyatt, J. Chem. Soc., Perkin Trans. 2, 1972,
474.
5 A. Thibblin, J. Chem. Soc., Perkin Trans. 2, 1992, 1195.
6 J. Cugeiras and H. Maskill, Can. J. Chem., 1999, 77, 530.
7 J. Cugeiras and H. Maskill, J. Chem. Soc., Perkin Trans. 2, 2000, 441.
8 N. C. Deno, J. J. Jaruzelski and A. Schriesheim, J. Am. Chem. Soc.,
1955, 77, 3044; N. C. Deno and A. Schreisheim, J. Am. Chem.
Soc., 1955, 77, 3051; N. C. Deno, H. E. Berkheimer, W. L. Evans and
H. J. Peterson, J. Am. Chem. Soc., 1959, 81, 2344; E. M. Arnett
and R. D. Bushick, J. Am. Chem. Soc., 1964, 86, 1564; M. J. Cook,
N. L. Dassanayake, C. D. Johnson, A. R. Katritzky and T. W. Toone,
J. Am. Chem. Soc., 1975, 97, 760; R. A. Cox and K. Yates, Can. J.
Chem., 1983, 61, 2225.
Experimental
Preparations of compounds have already been described.9,12
Water was glass distilled and concentrations of stock solu-
tions of perchloric acid were determined by titration against
J. Chem. Soc., Perkin Trans. 2, 2001, 1748–1752
1751

View Article Online
9 M. C. Lopez, J. C. Martinez, I. Demirtas, H. Maskill and E. Stix,
Org. React., 1997, 30, 71.
10 C. Bleasdale, B. T. Golding, W. H. Lee, H. Maskill, J. Riseborough
and E. Smits, J. Chem. Soc., Chem. Commun., 1994, 93; J. Crugeiras
and H. Maskill, J. Chem. Soc., Perkin Trans. 2, 1998, 1901.
11 R. A. McClelland, V. M. Kanagasabapathy, N. S. Banait and
S. Steenken, J. Am. Chem. Soc., 1992, 114, 1816.
16 M. Canle L., W. Clegg, I. Demirtas, M. R. J. Elsegood, J. Haider,
H. Maskill and P. C. Miatt, J. Chem. Soc., Perkin Trans. 2, 2001,
preceding paper (DOI: 10.1039/b103569j).
17 A. Albert and E. P. Serjeant, Ionization Constants of Acids and
Bases. A Laboratory Manual, John Wiley and Sons, Inc., New York,
1962; E. J. King, Acid-Base Equilibria, Pergamon Press, New York,
1965.
12 M. Canle L., W. Clegg, I. Demirtas, M. R. J. Elsegood and
H. Maskill, J. Chem. Soc., Perkin Trans. 2, 2000, 85.
13 W. H. Lee and H. Maskill, J. Chem. Soc., Perkin Trans. 2, 1994,
1463.
14 X. L. Armesto, M. Canle L., M. I. Fernández, M. V. García and
J. A. Santaballa, Tetrahedron, 2000, 56, 1103.
18 M. Canle L. and H. Maskill, unpublished results.
19 N. S. Isaacs, Physical organic chemistry, Longman, London,
1987; K. R. Fountain, T. W. Dunkin and K. D. Patel, J. Org. Chem.,
1997, 62, 2738; I.-H. Um and E. Buncel, J. Org. Chem., 2000, 65,
577.
20 A. I. Biggs and R. A. Robinson, J. Chem. Soc., 1961, 388;
G. N. Okafo, R. Brown and P. Camilleri, J. Chem. Soc., Chem.
Commun., 1991, 864.
15 H. Maskill, The physical basis of organic chemistry, Oxford
University Press, Oxford, 1985.
1752
J. Chem. Soc., Perkin Trans. 2, 2001, 1748–1752