A R T I C L E S
Novak et al.
Scheme 8
in the Supporting Information, by reacting the hydroxylamines with 2
equiv of AcCl in THF. The hydroxamic acid 5a was obtained by basic
hydrolysis of 1a in THF/H2O as described in the Supporting Informa-
tion.
Kinetic and Product Studies. Purification of solvents and general
procedures for performing kinetic studies, product studies by HPLC
methods, and data handling have been described.19,28 All reactions were
performed in 5 vol % CH3CN-H2O at an ionic strength of 0.5
maintained with NaClO4 except for those reactions that occurred in
1.64 M HClO4. Buffers used to maintain pH were Na2HPO4/NaH2-
PO4, NaOAc/HOAc, or HCOONa/HCOOH. At pH e2.5 HClO4
on a heteroatom other than the nitrenium N.22c It was argued
that this would reduce the electrophilic reactivity at the nitrenium
N necessary for formation of the C-8 adduct. All seven of the
ions that fall below the correlation line do have a dominant
resonance structure in which the charge is localized onto a
heteroatom (O or N) with all heavy atoms maintaining an octet
of electrons, but so do two of the ions that fall on the line
(28a,b).
If kaz is diffusion limited for 2a-d, kd-G for all four cations
is in the range of (4-9) × 107 M-1 s-1. The relatively slow
trapping of these ions by d-G may play a role in the detection
of the N-2 adducts, 18c,d. If the reaction of 2c with d-G to
form the C-8 adduct did proceed at the diffusion-controlled limit,
while the rate constant for formation of the N-2 adduct remained
unchanged, the relative yield of 18c would be ca. 1% rather
than the observed 22%.
solutions were used. Initial concentrations of 1a-d of ca. 5 × 10-5
M
for kinetics and product studies were obtained by 15 µL injection of a
0.01 M stock solution in DMF into 3.0 mL of the solution that had
been incubated at 20 °C for 15 min. Repetitive wavelength scans were
collected between 200 and 400 nm for at least 5 half-lives of the
reaction. Absorbance vs time data for 1a were collected at 256 (pH
0.8-4.4) and 263 nm (pH 4.8-7.4). For 1b data were collected at 252
(pH 0.9-3.7) and 262 nm (pH 3.9-7.4). Kinetic data for 1c and 1d
were collected at 266 nm at all pH values. Data were fit to the first-
order rate equation to obtain kobs.
28 Rate constants for the disappearance
of 1a were also determined in acidic solutions by the initial rates
method, using HPLC with UV detection at 256 nm. Spectrophotometric
titrations were carried out at 257 nm for 1a, 255 nm for 1b, and 266
-
nm for 1c. Kinetics of decomposition of 1a-d in the presence of N3
and d-G were performed in 0.02 M 2/1 Na2HPO4/NaH2PO4 buffers at
pH 6.90. These reactions were followed at the following wavelengths:
1a, 323 (N3-) and 314 nm (d-G); 1b, 272 (N3-) and 316 nm (d-G);
and 1c and 1d, 266 (N3-) and 335 nm (d-G).
The results obtained for the heterocyclic nitrenium ions
2a-d, 27, 28a, and 28b show that these ions retain many of
the characteristic reactions of carbocyclic nitrenium ions:
Reaction products of 1a and 1b in phosphate and acetate buffers
and in HClO4 solutions were monitored by HPLC on a C8 reverse-
phase analytical column with UV detection at 320 nm, using 50/50
MeOH/H2O eluent buffered with 0.05 M 1:1 NaOAc/HOAc at a flow
rate of 1 mL/min. Product yields were determined by triplicate 20 µL
injections after 10 half-lives of the reaction. Identical conditions were
used for 1c and 1d except that products were monitored at 300 nm
and the eluting buffer was 40/60 MeOH/H2O. For the determination
of kphosT/ks selectivity ratios, 0.08 M phosphate buffers were made with
varying buffer ratios. These buffers were diluted to 0.04, 0.02, 0.01,
and 0.005 M while maintaining the ionic strength. After 10 half-lives
of the reaction, product yields were determined by HPLC methods.
Details of the isolation and characterization of the buffer and solvent-
derived products 4c,d, 7a,b, 8a,b, 9a,b, 10a,b, 11c,d, and 12c,d are
described in the Supporting Information.
Product Studies in the Presence of N3- and d-G. All buffers were
Na2HPO4/NaH2PO4 (B/A ) 2/1; pH 6.9), µ ) 0.5, 5 vol % CH3CN/
H2O. All reactions were carried out at 20 °C. The buffer solutions had
been incubated at 20 °C for 15 min prior to the addition of 1a-d. The
product yields were determined by HPLC (UV detection at 256, 300,
and 320 nm, using 50/50 or 40/60 MeOH/H2O eluent buffered as
described above).
-
selective reaction with N3 to generate substitution products,
reaction with d-G to form C-8 adducts, and efficient reduction
by I-. The more stable ions 2a-d exhibit unusual reactions
with HPO4 or AcO- to generate addition products in which
2-
the aromatic character of one or more rings is lost.
In carbocyclic ions typically only H2O reacts to form stable
addition products in which aromaticity is lost. This occurs
presumably because of the poor leaving group ability of OH-
in products such as 29.20b Attack of a second equivalent of H2O
(Scheme 8) is kinetically disfavored at moderate pH (except
for 25 and related stilbenyl ions) because such reactions appear
to require specific acid catalysis and do not lead to products
stabilized by recovery of aromatic character.20b Apparently the
aromatic character of the heterocyclic rings of 2a-d is
considerably weaker allowing better leaving groups such as
-
HPO4 and AcO- to form addition products (Scheme 4) that
do not revert to the nitrenium ion. Attack of H2O on the inital
addition product to form products such as 7, 8, 9, 10, and 12 is
more favorable in these cases because there is less driving force
for rearomatization.
The product yields for 1a in N3- solutions were determined in 0.16
M total buffer with N3- concentrations ranging from 3 × 10-5 to 5 ×
10-4 M. The initial concentration of 1a was ca. 5 × 10-6 M, which
was obtained by 15 µL injections of a 0.001 M stock solution in DMF
into 3.0 mL of the buffer solution. The product yields were determined
by HPLC as described above.
Experimental Section
Synthesis. The syntheses of Glu-P-1, Glu-P-2, MeIQx, IQx, and
the nitro derivatives NO2Glu-P-1, NO2Glu-P-2, NO2MeIQx, and
NO2IQx have been described in the literature.25,26 The hydroxylamines
NOHGlu-P-1, NOHGlu-P-2, NOHMeIQx, and NOHIQx were syn-
thesized by the method of Kazerani and Novak by N2H4 reduction of
the nitro compounds.27 The esters 1a-d were prepared, as described
-
The N3 selectivity ratio for 1b was determined by two methods:
(1) varying phosphate concentrations while maintaining constant azide
concentration and (2) varying azide concentrations while maintaining
constant phosphate concentration. (1) Buffers were prepared with 0.02-
0.16 M total phosphate, and constant 5 × 10-6 M N3-. The initial
(25) Takeda, K.; Shudo, K.; Okamoto, T.; Kosuge, T. Chem. Pharm. Bull. 1978,
26, 2924-2925. Hashimoto, Y.; Shudo, K.; Okamoto, T. J. Am. Chem.
Soc. 1982, 104, 7636-7640. Bristow, N. W.; Charlton, P. T.; Peak, D. A.;
Short, W. F. J. Chem. Soc. 1954, 616-629. Astik, R. R.; Thaker, K. A. J.
Ind. Chem. Soc. 1981, 58, 1013-1014. Saint-Ruf, G.; Loukakou, B.;
N’Zouzi, C. J. Heterocycl. Chem. 1981, 18, 1565-1570.
(27) Kazerani, S.; Novak, M. J. Org. Chem. 1998, 63, 895-897.
(28) Novak, M.; Pelecanou, M.; Roy, A. K.; Andronico, F. M.; Plourde, F. M.;
Olefirowicz, T. M.; Curtin, T. J. J. Am. Chem. Soc. 1984, 106, 5623-
5631.
(26) Grivas, S.; Olsson, K. Acta Chem. Scand. 1985, B39, 31-34. Grivas, S. J.
Chem. Res. 1988, 84.
9
7980 J. AM. CHEM. SOC. VOL. 124, NO. 27, 2002