Ambident Ethyl N-Nitrosocarbamate Anion
A R T I C L E S
NO2C6H4sNdNsOMe59 toward elimination of N2 and the
formation of Ar-OMe at low temperature can be explained by
the participation of the ipso carbon atom in the transition state
according to mechanism A in Figure 11.
tional minima were located at the B3LYP/6-31G(d) level of
theory by setting different geometry constraints or by perturbing
the geometry of the C1-symmetry structures. Transition struc-
tures were obtained using the QST2 keyword at the B3LYP/
6-31G(d) level of theory, which served as starting points for
calculations at the MP2(fc)/6-31G(d) level of theory (key-
words: TS, ReadFC). Vibrational frequencies were used to
characterize the nature of the stationary points and to obtain
thermodynamic parameters. Zero-point energy (ZPE) corrections
for the MP2 results were scaled by 0.9670.63 The WBOI were
obtained using the NBO64 algorithm supplied in the Gaussian
package.
Following general recommendations,65 energy changes and
differences were derived as the differences of SCF energies of
individual species computed using the diffuse function-
augmented 6-31+G(d) basis set at the geometries obtained with
the 6-31G(d) basis set (single-point calculations). Thermody-
namic corrections were obtained using the 6-31G(d) basis set.
All computational results discussed in the text were obtained
at the MP2(fc)/6-31+G(d)//MP2(fc)/6-31G(d) level of theory,
and the details are listed in Table S2 in the Supporting
Information.
The mechanism shown in Figure 11 represents a concerted
alternative to a nonconcerted heterolytic or homolytic decom-
position pathway for the considered compounds. It can be
speculated that the choice between these mechanisms depends
largely on (i) the ability of the molecule to adopt the cis
configuration (relative thermodynamic stability of the conform-
ers and cis-trans isomers43,60), (ii) the ability of groups Y and
R to support the positive charge, and (iii) the medium (polarity,
protic solvents, and pH61). For simple systems such as com-
pounds 2-5 in nonpolar and nonprotic solvents the concerted
pathway appears to be preferred.
Finally, the experimental and computational data allow us to
comment on a report describing the isolation of methoxydiazene
5a from a reaction of 2-Ag and MeI and its purification by
vacuum distillation at 84 °C.9 The preparation could not be
repeated in our lab, and the thermal stability of 5a required for
distillation is doubtful. On the basis of the decomposition rate
constant measured for 5b (k ) 1.77 × 10-5 s-1) at 35 °C and
the calculated activation energy for 14-Z (25.3 kcal/mol), the
estimated half-lifetime for 5a at 84 °C is about 2 min.
Experimental Section
1H and 13C NMR spectra were recorded at 400 and 100 MHz,
respectively, and referenced to the solvent: CD2Cl2 δ 5.32 ppm for 1H
and 54.0 ppm for 13C; CD3CN δ 1.93 ppm; DMSO δ 2.49 ppm for 1H
and 39.5 ppm for 13C relative to TMS. Elemental analysis was provided
by Atlantic Microlab, Norcross, GA.
Reaction of 2-Bu4N with Alkyl Halides. General Procedure. The
Bu4N salt of nitrosourethane 2-Bu4N (25 mg, 0.07 mmol) was dissolved
in NMR solvent (CD2Cl2 or CD3CN, 0.25 mL) and transferred into an
NMR tube. A solution of the alkylation agent (0.07 mmol) in the same
solvent (0.25 mL) was added at 0 °C (for MeI and PhCH2Br) or ambient
temperature (EtI and i-PrI), and the progress of the reaction was
monitored by 1H NMR. Characteristic chemical shifts are listed in Table
1.
Conclusions
Alkylation of an essentially free diazotate anion 2 in solution
gives three regioisomers in yields that are affected by the nature
of the electrophile. With more bulky electrophiles, the less
thermodynamically stable diazene 5 dominates among the
products, but its yield is compromised by the competing
decomposition of the diazotate anion 2. The observed low yield
of 5 in the reactions of 2-Bu4N can perhaps be increased to a
practical level by using stronger electrophiles and larger ester
groups. Diazene 5 shows moderate stability at ambient tem-
perature and low sensitivity to silica gel, which offers a method
for isolating the pure material for further studies.
The MP2 level calculations are in agreement with experi-
mental observations. They reveal the mechanistic complexity
and suggest a four-center transition state for the decomposition
of the nitrosourethane anion and its three alkylation regio-
isomers: N-nitroso, azoxy, and diazotate compounds. It is
expected that the postulated mechanism is general for relative
compounds in aprotic solvents.
Computational Details. Quantum-mechanical calculations
were carried out initially at the B3LYP/6-31G(d) and finally at
the MP2(fc)/6-31G(d) level of theory using the Linda-Gaussian
98 package62 on a Beowulf cluster of 16 processors. Geometry
optimizations were undertaken using appropriate symmetry
constraints and default convergence limits. Global conforma-
Thermolysis of the Anion 2 and Kinetic Measurements. Ni-
trosourethane salt 2-Bu4N (15 mg) was dissolved in CD2Cl2 (0.7 mL)
and transferred into an NMR tube. The sample was kept at a constant
temperature, and 1H NMR spectra were taken at regular time intervals
until almost complete disappearance of the starting material resulted.
The ratio of intensities of the signals of the ethyl group quartet in 2
(4.16 ppm) and the butyl group pseudotriplet in Bu4N+ (3.22 ppm)
was used to calculate the rate constant. The major products of the
decomposition of 2 were identified as monoethyl carbonate [9: δ 1.12
(t, J ) 7.1 Hz, 2H) and 3.79 (q, J ) 7.1 Hz, 2H)] and ethanol [δ 1.17
(t, J ) 7.1 Hz, 2H) and 3.62 (q, J ) 7.1 Hz, 2H)].
Thermal Stability of the O-Alkyl Derivatives. General Procedure
for Kinetic Measurements. The reaction mixtures obtained in CH2-
Cl2 were passed through a silica gel plug to remove the ammonium
salts, and the regioisomers were quickly eluted with methylene chloride
as one fraction. The solvent was carefully evaporated, and the resulting
oily residue of products was dissolved in CD2Cl2 (0.5 mL). The solution
was kept at 35 °C, and 1H NMR spectra were collected at regular
intervals. The ratio of intensities of the signals of the O-alkyl methylene
group in the starting material (5b: 4.56 ppm; 5d: 5.52 ppm) to that in
the N-alkyl derivative (5b: 3.77 ppm; 5d: 4.91 ppm) was used to
calculate the rate constant. The major products of the decomposition
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