A New Nitrogen Atom Transfer Reaction
A R T I C L E S
Synthesis of 3b. In a 50-mL round-bottom flask was prepared a
solution of 1-NCPh (600 mg 1, 100 mg PhCN, 0.96 mmol) in 20 mL
of Et2O. To the purple solution was added solid (C6F5S)2 (192 mg,
0.48 mmol), resulting in a rapid darkening to deep blue. The reaction
mixture was allowed to stir for 5 min, whereupon it was filtered and
concentrated to dryness. Recrystallization from Et2O furnished product
3b as a dark, microcrystalline solid. Yield: 503 mg, 0.54 mmol (3
Synthesis of 4d. A purple solution of 1 (200 mg, 0.32 mmol) and
PhCN (33 mg, 0.32 mmol) in 5 mL of Et2O was cooled until frozen
and then allowed to thaw. To the thawing solution was added a room-
temperature solution of benzoyl peroxide (39 mg, 0.16 mmol) in Et2O
(2 mL). As the reaction mixture was allowed to warm to room
temperature, the initial purple color was observed to become blue and
then purple again. Stirring was continued for an additional 5 min, at
which point the reaction mixture was filtered through Celite, and the
filtrate so obtained was concentrated to dryness. Recrystallization from
Et2O furnished 4d as large brown crystals. Yield: 169 mg, 0.22 mmol
(2 crops, 71%). 1H NMR (500 MHz, C6D6): δ 1.68 (s, 18H, C6H3Me2);
3.98 (br s, 27H, CMe3); 4.87 (s, 3H, Ar para); 5.64 (s, 6H, Ar meta);
6.20 (t, 1H, phenyl para); 7.27 (poorly resolved t, 2H, phenyl meta);
7.58 (poorly resolved d, 2H, phenyl ortho). Anal. Calcd for C43H59N3O2-
Mo: C, 69.24; H, 7.97; N, 5.63. Found: C, 68.95; H, 8.11; N, 5.55.
Kinetic Measurements. Kinetics of â-EC6F5 elimination from 3b
(E ) S) and 3c (E ) Se) were measured using UV-vis spectroscopy
over a 40 °C range (E ) S, 25-65 °C; E ) Se, 15-55 °C). For 3b,
ca. 0.1 mM solutions in THF were prepared from freshly crystallized
material and then transferred to a quartz UV cell containing a micro
stirbar. For 3c, an identical protocol was followed, except that the more
rapid kinetics in this system required that the starting ketimide be
generated in situ. In all cases, very good fits to the first-order kinetic
equation were obtained. In addition, rate constants obtained from traces
at three different wavelengths (400, 475, 585 nm) were within 10% of
each other. The kinetic data are summarized in Tables S1 and S2.
Stopped-Flow Kinetic Measurements. Toluene solutions of the
reagents were prepared in a Vacuum Atmospheres or an MBraun
glovebox filled with argon and placed in Hamilton gas-tight syringes.
Time-resolved spectra were acquired at temperatures from -80 to -40
°C using a Hi-Tech Scientific (Salisbury, Wiltshire, UK) SF-43 Multi-
Mixing CryoStopped-Flow Instrument in a diode array mode. Control
kinetic experiments in the systems that did not contain benzoyl peroxide
used a powerful xenon arc lamp. All kinetic experiments with benzoyl
peroxide were done with a low-power visible lamp to prevent
decomposition of peroxide induced by UV light.32
The stopped-flow instrument was equipped with stainless steel
plumbing, a 1.00-cm stainless steel mixing cell with sapphire windows,
and an anaerobic gas-flushing kit. The instrument was connected to an
IBM computer with IS-2 Rapid Kinetic software (Hi-Tech Scientific).
The temperature in the mixing cell was maintained to (0.1 K, and the
mixing time was 2 to 3 ms. The driving syringe compartment and the
cooling bath filled with heptane (Fisher) were flushed with argon before
and during the experiments, using anaerobic kit flush lines. All flow
lines of the SF-43 instrument were extensively washed with degassed,
anhydrous toluene before charging the driving syringes with reactant
solutions.
All of the experiments were performed in a single-mixing mode of
the instrument, with a 1:1 (v/v) ratio. The system was studied using
two mixing approaches: (1) first syringe, complex 1; second syringe,
premixed PhCN and benzoyl peroxide and (2) first syringe, premixed
complex 1 and PhCN; second syringe, benzoyl peroxide.
1
crops, 56%). H NMR (300 MHz, C6D6): δ 1.32 (br s, 27H, CMe3);
2.25 (s, 18H, C6H3Me2); 6.57 (s, 2H, phenyl ortho); 6.63 (t, 1H, phenyl
para); 6.69 (s, 3H, Ar para); 6.91 (s, 6H, Ar ortho); 7.01 (t, 2H, phenyl
meta). 19F NMR (282 MHz, C6D6): δ -135.43 (dd); -156.71 (t);
-163.02 (m). UV-vis (Et2O, 25 °C): λmax 318 (sh, ꢀ ) 19000
M-1cm-1); 577 (ꢀ ) 7800 M-1cm-1) nm. The thermal instability of
this compound precluded satisfactory elemental analysis and 13C NMR
data.
Generation and Spectroscopic Observation of 3c. A solution of
1 (17 mg, 0.027 mmol) and PhCN (ca. 5 mg, 0.49 mmol) was prepared
in 0.5 mL of C6D6. To this was added a solution of (C6F5Se)2 (7 mg,
0.014 mmol) in 0.5 mL of C6D6, resulting in an immediate color change
to deep blue. The mixture was rapidly transferred to a J. Young NMR
tube which was then removed from the glovebox and frozen in an ice
bath while being transported to the NMR spectrometer. The very first
1H NMR spectrum that is obtained shows a 10:1 mixture of 3c and 4c.
Over the course of 30 min, the signals corresponding to 3c are observed
to decay, and those corresponding to 4c grow in. 1H NMR (300 MHz,
C6D6): δ 1.29 (br s, 27H, CMe3); 2.27 (s, 18H, C6H3Me2); 6.73 (3H,
Ar para); 6.85 (6H, Ar ortho); 7.03 (t, 2H, phenyl meta) (other
resonances obscured). UV-vis (Et2O, 25 °C): λmax 579 (ꢀ ) 5500
M-1cm-1) nm.
Synthesis of 4b. To an Et2O solution of 1 (200 mg, 0.32 mmol)
was added an Et2O solution of (C6F5S)2 (64 mg, 0.16 mmol), resulting
in a color change to purple-brown upon mixing. The solution was
filtered through Celite, and the filtrate so obtained was concentrated
to dryness. Recrystallization from Et2O (-35 °C) furnished 150 mg
1
(0.18 mmol, 57%) of dark crystalline material. H NMR (300 MHz,
C6D6): δ 1.39 (s, 27H, CMe3); 2.05 (s, 18H, C6H3Me2); 6.25 (br s, 6H,
Ar ortho); 6.39 (s, 3H, Ar para). 19F NMR (282 MHz, C6D6): δ
-131.33 (dd); -159.66 (t); -163.78 (m). 13C NMR: δ 21.61
(C6H3Me2); 31.73 (CMe3); 63.31 (N-CMe3); 127.09; 129.03; 132.41
(m, C-F); 136.15; 136.68 (m, C-F); 140.19 (m, C-F); 143.63 (m,
C-F); 146.81 (m, C-F); 151.58. UV-vis (Et2O, 25 °C): λmax 389
(sh, ꢀ ) 4500 M-1cm-1); 563 (ꢀ ) 1900 M-1cm-1) nm. Anal. Calcd
for C42H54N3MoSF5: C, 61.23; H, 6.61; N, 5.10. Found: C, 60.95; H,
6.68; N, 5.04.
Synthesis of 4c. Synthesized analogously to 4b. Yield: 150 mg,
0.17 mmol, 54%. 1H NMR (300 MHz, C6D6): δ 1.40 (s, 27H, CMe3);
2.04 (s, 18H, C6H3Me2); 6.21 (br s, 6H, Ar ortho); 6.38 (s, 3H, Ar
para). UV-vis (Et2O, 25 °C): λmax 387 (sh, ꢀ ) 5500 M-1cm-1); 555
(ꢀ ) 1900 M-1cm-1) nm. Anal. Calcd for C42H54N3MoSeF5: C, 57.93;
H, 6.25; N, 4.83. Found: C, 57.79; H, 6.08; N, 4.78.
Spectroscopic Observation of 3d. A J. Young NMR tube was
charged with 1 (24 mg, 0.038 mmol), PhCN (9 mg, 0.087 mmol), and
0.4 mL of toluene-d8. The resulting purple solution was frozen in the
glovebox coldwell. To the frozen purple solution was then added
benzoyl peroxide (5 mg, 0.020 mmol) in 0.4 mL of toluene-d8, after
which the contents of the NMR tube were frozen completely. The NMR
tube was quickly removed from the glovebox and transferred to a dry
ice/acetone bath where it was allowed to thaw with gentle shaking to
ensure good mixing of the reagents. The NMR tube was maintained at
-78 °C before being transferred to an NMR probe precooled to -15
In both mixing conditions the concentration of the reagents were
varied: complex 1 (0.3-0.225 mM), PhCN (0.3-3 mM), benzoyl
peroxide (0.15-3.6 mM).
Under both mixing conditions, consistent and reproducible results
were obtained, and the formation of the ketimide complex intermediate
3d was observed.
Spectral changes and reaction rates in control experiments (mixing
of complex 1 with PhCN, and mixing of complex 1 with benzoyl
peroxide) were distinct from those observed in the ternary system (1
+ excess PhCN + benzoyl peroxide).
The reactions were monitored for 3-5 half-lives. A series of 4-6
measurements at each temperature gave an acceptable standard deviation
1
°C. A H NMR spectrum was taken as soon as thermal equilibration
was obtained. Subsequent spectra were taken in order to monitor the
smooth conversion of 3d to 4d and PhCN. 1H NMR (500 MHz, toluene,
-15 °C): δ 1.29 (s, 9H, CMe3); 1.33 (s, 18H, CMe3); 2.14 (s, 6H,
C6H3Me2); 2.39 (s, 12H, C6H3Me2); 8.31 (1H, d, phenyl para) (other
aryl resonances obscured).
(32) Bradley, J. D.; Roth, A. P. Tetrahedron Lett. 1971, 3907-10.
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J. AM. CHEM. SOC. VOL. 128, NO. 14, 2006 4883