Laser flash photolysis measurements of the kinetics of carbon-nitrogen bond rotations in α-amide radicals

Full text of DOI:10.1021/jo981538e

1022
J . Org. Chem. 1999, 64, 1022-1025
Sch em e 1
La ser F la sh P h otolysis Mea su r em en ts of
th e Kin etics of Ca r bon -Nitr ogen Bon d
Rota tion s in r-Am id e Ra d ica ls
Osama M. Musa, J ohn H. Horner,* and
Martin Newcomb*
Department of Chemistry, Wayne State University,
Detroit, Michigan 48202
Received J uly 31, 1998
R-Amide radicals are important for radical-based syn-
thetic methodology for several reasons. They are rela-
tively simple to generate by radical addition to an
acrylamide or by atom or group abstraction or homolytic
fragmentation from the R-position of an amide, and they
have proven to be especially useful in the rapidly evolving
area of acyclic stereochemical control of radical reac-
tions.1,2 The utility of R-amide radicals in diastereose-
lective reactions is related to the rate constants for
conformational equilibration of these species. Although
rapid on the laboratory time scale, the rate of rotation of
the C-C bond between the carbonyl and R-carbon atoms
of an R-amide radical is slow in comparison to that of
many radical reactions, with a rate constant of about 2
× 104 s-1 at ambient temperature.3 Thus, in many cases,
the populations of C-C conformational isomers of an
R-amide radical will be fixed in the initial reactions that
generate these species, and control of these populations
could have a profound effect on diastereoselectivity.
The rate constants for conformational interconversions
by rotations about the carbonyl carbon to nitrogen bonds
of R-amide radicals have not been determined previously,
but it is clear that C-N bond rotations are also impor-
tant. Relatively slow C-N bond rotation will affect the
diastereoselectivity of reactions of unsymmetrically N-
substituted R-amide radicals in a manner similar to that
of slow C-C bond rotation. Curran and Tamine demon-
strated that the production of lactams by cyclizations of
R-amide radicals containing allylic groups on nitrogen
can be partially limited by slow C-N bond rotation that
isolates conformer A which cannot cyclize from conformer
B which can.4 One can deduce from the results of Curran
and Tamine that C-N rotation rate constants are
somewhat larger than those for C-C bond rotation.4 We
report here the results of laser flash photolysis (LFP)
kinetic studies which confirm that deduction.
PTOC ester radical precursors 3 were prepared from the
corresponding malonate monoamides 2 by methods simi-
lar to those used for the synthesis of related R-amide
radical precursors.5 PTOC ester 3a was especially un-
stable, and crude samples were used in kinetic studies.
Laser irradiation (355 nm) of precursors 3 gave R-amide
radicals 4 by homolysis of the N-O bond of the PTOC
ester and instant (on the ns time scale) decarboxylation
of the acyloxyl radicals thus formed. Cyclizations of
radicals 4 gave diphenylalkyl radicals 5 that have long
wavelength λmax in the 330-334 nm range as expected6
and were readily monitored by UV spectroscopy. Figure
1 shows the time-resolved spectrum of 5b from reaction
of radical 4b.
The LFP experimental design was the same as that
recently described.7 Dilute solutions of PTOC esters 3
were thermally equilibrated, sparged with helium, and
allowed to flow through a flow cell in the kinetic
spectrometer. Temperatures were measured with a ther-
mocouple in the flowing stream. Rate constants for
formation of radicals 5 were determined in THF at
various temperatures. Because radicals 4 were sym-
metrically substituted at the R-carbon, there was no
possibility of C-C conformers that react with different
rate constants.
Resu lts a n d Discu ssion
5-exo cyclizations of radicals containing the diphen-
ylethenyl moiety are quite rapid, and radicals (E)-4 were
expected to cyclize with rate constants of about 1 × 108
The experimental concept (Scheme 1) is related to the
preparative studies reported by Curran and Tamine.4
(1) Porter, N. A.; Giese, B.; Curran, D. P. Acc. Chem. Res. 1991, 24,
296-304.
(4) Curran, D. P.; Tamine, J . J . Org. Chem. 1991, 56, 2746-2750.
(5) Musa, O. M.; Choi, S. Y.; Horner, J . H.; Newcomb, M. J . Org.
Chem. 1998, 63, 786-793.
(6) Chatgilialoglu, C. In Handbook of Organic Photochemistry;
Scaiano, J . C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 2; pp 3-11.
(7) Horner, J . H.; Tanaka, N.; Newcomb, M. J . Am. Chem. Soc. 1998,
120, 10379-10390.
(2) Curran, D. P.; Porter, N. A.; Giese, B. Stereochemistry of Radical
Reactions: Concepts, Guidelines, and Synthetic Applications; VCH:
Weinheim, 1995.
(3) Strub, W.; Roduner, E.; Fischer, H. J . Phys. Chem. 1987, 91,
4379-4383.
10.1021/jo981538e CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/05/1999

Notes
J . Org. Chem., Vol. 64, No. 3, 1999 1023
range of 105-106 s-1. LFP kinetic studies with a nano-
second-resolution apparatus are quite accurate in this
range. The response time of our apparatus introduces
insignificant errors when kobs < 5 × 107 s-1, and bimo-
lecular processes (radical-radical reactions and reactions
of radicals with residual oxygen) are not important
because they have pseudo-first-order rate constants of
< 1 × 104 s-1. The LFP rate constants for first-order
processes are also precise because hundreds of data
points are obtained in the LFP study.
The rate constants for the slow reactions of radicals 4
give the Arrhenius functions in eqs 1 and 2, where errors
are at 2σ and θ ) 2.3RT kcal/mol. The log A parameters
are those expected for rotational processes which have
little entropic demand (log A at 300 K is 13.1 for a process
with ∆Sq ) 0) as opposed to the organized transition
states of 5-exo cyclizations which result in log A values
of 9-10. Rate constants for C-N rotations at 20 °C
calculated from eqs 1 and 2 are 5.1 × 105 s-1 for 4a and
2.7 × 105 s-1 for 4b. The C-N bond rotations in R-amide
radicals are somewhat more than an order of magnitude
faster than the C-C bond rotations between the carbonyl
carbon and R-carbon.3
F igu r e 1. Time-resolved spectrum from reaction of radical
4b at 19.9 °C. The traces are at 1.0, 1.8, 2.6, and 4.2 µs after
laser irradiation with data at 0.2 µs subtracted to give a
baseline. Radical 4b is growing in with λmax at 332 nm, and
the byproduct radical from the photolysis, 2-pyridinethiyl with
λmax at 490 nm, is decaying. Symbols for the 4.2 µs data show
the wavelengths monitored. The inset shows the kinetic trace
at 330 nm; the x-axis is time in microseconds.
Ta ble 1. Obser ved Ra te Con sta n ts for th e Slow
Rea ction s of Ra d ica ls 4
b
10-5kobs (s-1
)
radical
tempa (°C)
set 1
set 2
4a
10.1
19.8
29.3
38.5
46.3
2.2
11.0
19.9
30.0
40.2
47.9
3.12 ( 0.10
5.00 ( 0.16
7.48 ( 0.49
12.5 ( 0.8
15.8 ( 1.2
0.89 ( 0.07
1.61 ( 0.07
2.51 ( 0.09
4.79 ( 0.13
8.35 ( 0.15
12.1 ( 0.5
3.11 ( 0.21
4.82 ( 0.12
8.13 ( 0.52
12.5 ( 0.8
17.4 ( 1.3
0.86 ( 0.10
1.58 ( 0.07
2.67 ( 0.05
4.69 ( 0.39
8.03 ( 0.29
11.9 ( 1.0
(for 4a ) log(k × s) )
(12.01 ( 0.26) - (8.44 ( 0.38)/θ (1)
(for 4b) log(k × s) )
4b
(12.94 ( 0.13) - (10.06 ( 0.17)/θ (2)
To demonstrate the significance of the C-N bond
rotation in synthesis, we conducted reactions with PTOC
ester 3b in benzene at 20 °C in the presence of Bu3SnH
at relatively high (1.2 M) and lower (0.6 M) concentra-
tions of tin hydride. The rate constant for tin hydride
reaction with a 2° R-amide radical at 20 °C is 3 × 106
a
b
(0.3 °C. Errors are 2σ.
s-1 at ambient temperature.5,8-10 Radicals (Z)-4, on the
other hand, cannot cyclize. If C-N bond rotation in
radicals 4 is slower than cyclization of (E)-4, then one
should observe rapid (possibly instant on the ns time
scale) signal growth from cyclization of (E)-4 followed by
slower growth resulting from rotation of conformer (Z)-4
to (E)-4 followed by cyclization.
The expected double signal growth was found for both
radicals 4. The fast reaction was too rapid to measure at
ambient temperature. The inset in Figure 1 shows a
kinetic trace at 19.9 °C taken near λmax of 5b; “instant”
signal growth from cyclization of (E)-4b in the first 100
ns is followed by slower growth over several microsec-
onds. At lower temperatures, the fast reaction could be
measured; for example, the rate constant for radical 4a
was 5 × 107 s-1 at -46 °C and that for 4b was 5 × 107
s-1 at -40 °C. The demonstration that the fast reactions
are more than 2 orders of magnitude faster than the slow
reactions (see below) is important for two reasons. It
shows that the kinetics of the slow processes will not be
convoluted with those of the fast processes and it requires
that rotations of the (Z)CN conformers to the (E)CN
conformers are effectively irreversible.
M-1 s-1 5
, but we assumed that the rate constant for
Bu3SnH reaction with a 3° R-amide radical will be
smaller than this on the basis of results with 2° and 3°
R-ester radicals which show such an effect.8 Accordingly,
Bu3SnH was expected to react with R-amide radical 4b
with a pseudo-first-order rate constant that is greater
than the first-order rate constant for C-N rotation in
the high Bu3SnH concentration experiment. Because the
cyclization of radical (E)-4b is much faster than both tin
hydride trapping and C-N rotation, essentially all (E)-
4b initially produced or formed by rotation of (Z)-4b will
cyclize. In practice, the reaction in the presence of 1.2 M
Bu3SnH gave an 80:20 ratio of lactam 6 to acyclic amide
7 as determined by GC, whereas at 0.6 M Bu3SnH the
ratio of 6 to 7 increased to 92:8.
Table 1 contains the kinetic results for the slow
processes for radicals 4. The rate constants are in the
The C-N bond rotation in 4b can be used as a radical
clock11,12 to calculate the rate constant for tin hydride
trapping from the above results. The initial populations
(8) Newcomb, M.; Horner, J . H.; Filipkowski, M. A.; Ha, C.; Park,
S. U. J . Am. Chem. Soc. 1995, 117, 3674-3684.
(9) J ohnson, C. C.; Horner, J . H.; Tronche, C.; Newcomb, M. J . Am.
Chem. Soc. 1995, 117, 1684-1687.
(10) Horner, J . H.; Martinez, F. N.; Musa, O. M.; Newcomb, M.;
Shahin, H. E. J . Am. Chem. Soc. 1995, 117, 11124-11133.
(11) Griller, D.; Ingold, K. U. Acc. Chem. Res. 1980, 13, 317-323.
(12) Newcomb, M. Tetrahedron 1993, 49, 1151-1176.

1024 J . Org. Chem., Vol. 64, No. 3, 1999
Notes
of (E)- and (Z)-4b produced in radical chain reactions of
the PTOC ester 3b are not known, but we assume that
they are about equal. For an initial isomer ratio of 1:1,
the product ratios indicate that tin hydride trapping was
0.66 times as fast as rotation at 1.2 M Bu3SnH and 0.19
times as fast as rotation at 0.6 M Bu3SnH. With an
adequate number of data points, one could determine
accurately both the initial populations of conformers and
the rate constant for Bu3SnH reaction with 4b, but an
approximate rate constant for tin hydride trapping of the
3° amide radical at 20 °C of 1 × 105 M-1 s-1 one calculates
from these results should be useful for synthetic planning
purposes.
The very fast 5-exo cyclizations of the (E)CN conformers
of radicals 4 also provide some useful kinetic information.
The reactions were too fast for measurements over a wide
temperature range, but one can assume that these
cyclization reactions will have log A parameters similar
to those of related radicals, or log A ≈ 9.5. Using this
value and measured rate constants for cyclizations of
radicals (E)-4 at low temperatures, one calculates ap-
proximate rate constants for cyclization at ambient
temperature of 2 × 108 and 1 × 108 s-1 for 4a and 4b,
respectively. The diphenylethenyl group typically ac-
celerates 5-exo radical cyclizations at ambient tempera-
ture by about a factor of 200,5 so one would estimate that
the simple analogues of radicals 4 with terminal vinyl
groups will cyclize at ambient temperature with rate
constants in the range of 0.5 × 106 to 1.0 × 106 s-1, or
about 1 order of magnitude larger than those for C-N
bond rotation. Thus, tin hydride in high concentrations
might trap considerable amounts of the (E)CN conformers
of simple terminal vinyl analogues of 4 in competition
with cyclization.
Exp er im en ta l Section
Gen er a l. NMR spectra were recorded at 300 or 500 MHz (1H)
and 75 or 125 MHz (13C). Radial chromatography was performed
on a model 7924T Chromatotron (Harrison Research). Tri-n-
butyltin hydride was prepared by the method of Hayashi et al.13
Tetrahydrofuran (THF) was distilled under a nitrogen atmo-
sphere from sodium benzophenone ketyl. Methylene chloride was
distilled under nitrogen from phosphorus pentoxide. Benzene
was distilled under nitrogen from calcium hydride. High-
resolution mass spectral analyses were conducted by the staff
of the Central Instrument Facility at Wayne State University.
N-Bu tyl-3,3-d ip h en yl-2-p r op en yla m in e (1). To butylamine
(27.14 mL, 274.5 mmol) in 15 mL of THF was added dropwise
3-bromo-1,1-diphenyl-1-propene14 (1.5 g, 5.49 mmol). The mix-
ture was allowed to stir overnight at room temperature. The
THF was removed under reduced pressure, and the residue was
partitioned between ether and water. The organic layer was
separated, dried over MgSO4, and filtered, and the solvent was
removed under reduced pressure to give 1.36 g of 1 as a yellow
1
oil (5.13 mmol, 94%). H NMR: δ 7.42-7.17 (m, 10 H), 6.20 (t,
1 H, J ) 6.9 Hz), 3.32 (d, 2 H, J ) 6.9 Hz), 2.58 (t, 2 H, J ) 6.9
Hz), 1.47-1.28 (m, 5 H), 0.90 (t, 3 H, J ) 7.5 Hz). 13C NMR: δ
143.08, 142.17, 139.71, 129.74, 128.39, 128.14, 128.09, 127.34,
127.24, 127.15, 49.26, 48.65, 32.25, 20.45, 13.98. HRMS: calcd
for C19H23N, 265.1831; found, 265.1837.
N-Bu tyl-N-(3,3-diph en yl-2-pr open yl)m alon am ic acid eth -
yl ester was prepared by slowly adding chlorocarbonylacetic acid
ethyl ester15 (0.22 g, 1.47 mmol) to a stirred solution of amide 1
(0.39 g, 1.47 mmol), Et3N (0.15 g, 1.47 mmol), and DMAP (0.018
g, 1.47 mmol) in CH2Cl2 (15 mL) at room temperature. The
mixture was stirred for 2 h. Water was added, and the resulting
mixture was extracted three times with ether. The combined
ethereal extracts were washed with a saturated aqueous NaCl
solution, dried over MgSO4, filtered, and concentrated under
reduced pressure. Column chromatography (hexanes-EtOAc,
3:1) gave 0.45 g of the title ester as a yellow oil (1.19 mmol, 81%).
Complex 1H and 13C NMR spectra due to slow conformer rotation
are provided in Supporting Information. HRMS: calcd for
C24H29NO3, 379.2147; found, 379.2144.
In comparison to other radical intermediates, the
R-amide radicals are among the better understood from
a kinetic perspective with rate constants for conformer
interconversions,3 cyclizations,5 and reactions with tin
hydride5 available. In addition, a simple R-amide radical
clock based on the 5-hexenyl radical cyclization has been
calibrated5 and can be used for timing bimolecular
reactions in indirect competition studies.12 In many cases,
one can take advantage of the known kinetics to predict
the outcomes of synthetic reactions.
N-Bu tyl-N-(3,3-diph en yl-2-pr open yl)m alon am ic Acid (2a).
The above ethyl ester (0.076 g, 0.20 mmol) and LiOH (0.033 g,
0.80 mmol) in 95% EtOH (10 mL) were heated at reflux for 5 h.
The mixture was cooled and concentrated. The residue was
dissolved in water, and the resulting solution was acidified with
HCl. The mixture was extracted three times with ether, and the
combined ethereal extracts were washed with
a saturated
aqueous NaCl solution, dried over MgSO4, and concentrated
under reduced pressure. Column chromatography (hexanes-
EtOAc, 1:1) gave 0.066 g of 2a as an oil (0.19 mmol, 94.3%).
Complex 1H and 13C NMR spectra due to slow conformer rotation
are provided in Supporting Information.
Even without a detailed prospective kinetic analysis,
one should appreciate two important consequences of the
rate constants for C-N bond rotations of R-amide radi-
cals. The first-order rate constants for these rotations are
quite similar to the pseudo-first-order rate constants one
will have in many bimolecular reactions of radicals;
therefore, it should often be possible to express or
suppress the rotational process as desired by changing
the concentration of a reagent, as we demonstrated in
the tin hydride reactions with radical 4b. In addition,
the low entropy demand for the C-N rotation is impor-
tant. Bimolecular reactions have log A parameters of
about 9 whereas the C-N rotations have log A of 12-
13. As reaction temperatures are increased, the rotations
will become increasingly competitive with bimolecular
reactions. Curran and Tamine clearly appreciated that
fact when they suggested that one should “heat it up” to
improve yields of lactams from 5-exo cyclizations.4 If
instead one wishes to prevent conformer equilibration in
order to improve diastereoselectivity, as in radical pro-
duction from a conformationally biased precursor, one
should “cool it down”.
N-Bu t yl-N-(3,3-d ip h en yl-2-p r op en yl)m a lon a m ic Acid
2-Th ioxo-2H-p yr id in e-1-yl Ester (3a ). To a solution of mal-
onamic acid 2a (0.062 g, 0.18 mmol) and 2,2′-dipyridyl disulfide
bis-N-oxide16 (0.05 g, 0.194 mmol) in dry CH2Cl2 (10 mL) in a
flask wrapped with aluminum foil was added tributylphosphine
(0.05 mL, 0.194 mmol) at 0 °C under N2. The reaction was stirred
at room temperature for 2 h before aqueous Na2CO3 (10%, 10
mL) was added. The organic layer was separated, and the
aqueous layer was extracted with CH2Cl2 three times. The
combined organic extracts were washed with brine, dried over
MgSO4, and concentrated under reduced pressure. The crude
samples appeared to contain approximately equal amounts of
the desired PTOC ester and N-hydroxypyridine-2-thione (from
hydrolysis of the PTOC ester) in addition to Bu3P. Attempts to
purify the crude material by chromatography resulted in exten-
sive decomposition. Kinetic studies were performed with crude
samples used immediately after preparation and NMR analysis.
(13) Hayashi, K.; Iyoda, J .; Shiihara, I. J . Organomet. Chem. 1967,
10, 81-94.
(14) Davis, M. A.; Herr, F.; Thomas, R. A.; Charest, M. P. J . Med.
Chem. 1967, 10, 627-636.
(15) Holmquist, B.; Bruice, T. C. J . Am. Chem. Soc. 1969, 91, 2993-
3002.
(16) Barton, D. H. R.; Samadi, M. Tetrahedron 1992, 48, 7083-7090.

Notes
N-Bu tyl-N-(3,3-d ip h en yl-2-p r op en yl)d im eth ylm a lon a m -
J . Org. Chem., Vol. 64, No. 3, 1999 1025
light for 1 h at room temperature. Radial chromatography
on silica gel (hexane-EtOAc, 3:1) gave 0.015 g of lactam
6 as an oil (0.045 mmol, 50%). 1H NMR: δ 7.42-7.13 (m,
10 H), 3.88 (d, 1 H, J ) 13 Hz), 3.20-3.36 (m, 1 H), 3.08-3.14
(m, 1 H), 2.92-3.00 (m, 1 H), 2.82 (t, 2 H, J ) 11 Hz),
1.16-1.44 (m, 4 H), 1.04 (s, 3 H), 0.84-0.94 (m, 3 H), 0.76
(s, 3 H). 13C NMR: δ 143.2, 142.9, 128.9, 128.6, 128.2, 127.6,
126.85, 126.78, 52.8, 49.4, 47.2, 43.9, 42.3, 29.3, 25.2, 20.0,
ic a cid eth yl ester was prepared by slowly adding 2-chloro-
carbonyl-2-methylpropanoic acid ethyl ester15 (0.48 g, 3.02 mmol)
to a stirred solution of amine 1 (0.80 g, 3.02 mmol), Et3N (0.31
g, 3.02 mmol), and DMAP (0.04 g, 0.30 mmol) in CH2Cl2 (15 mL)
at room temperature. The mixture was stirred for 2 h. Water
was added, and the resulting mixture was extracted three times
with ether. The combined ethereal extracts were washed with
a saturated aqueous NaCl solution, dried over MgSO4, filtered,
and concentrated under reduced pressure. Column chromatog-
raphy on silica gel (hexanes-EtOAc, 3:1) gave 0.51 g of the
desired ester as an oil (1.24 mmol, 71%). Complex 1H and 13C
NMR spectra due to slow conformer rotation are provided in
Supporting Information. HRMS: calcd for C26H33NO3, 407.2461;
found, 407.2466.
18.7, 13.8 (carbonyl
C not observed). HRMS: calcd for
C23H29NO, 335.2249; found, 335.2251.
N-Bu tyl-N-(3,3-d ip h en yl-2-p r op en yl)-2-m eth ylp r op a n a -
m id e (7). Amine 1 (1.03 g, 3.89 mmol) and Et3N (0.65 mL, 4.67
mmol) were added dropwise to a solution of isobutyryl chloride
(0.49 mL, 4.67 mmol) in diethyl ether (20 mL) at 0 °C. The
solution was stirred for 12 h. After filtration of the ammonium
salts, the filtrate was washed with water, saturated NaHCO3,
and saturated NaCl. The organic layer was dried over MgSO4,
filtered, and concentrated under reduced pressure. Column
chromatography on silica gel (hexanes-EtOAc, 3:1) gave 0.98 g
of acyclic amide 7 as an oil (2.93 mmol, 75%). Complex 1H and
13C NMR spectra due to slow conformer rotation are provided
in Supporting Information. HRMS: calcd for C23H29NO, 335.2249;
found, 335.2250.
Kin etic Stu d ies. LFP studies in THF were accomplished as
previously described;7 each “set” in Table 1 is an independent
value determined by summing 10-15 runs to improve the signal-
to-noise ratio. Indirect kinetic studies of the reaction of 3b in
the presence of Bu3SnH were performed as described;5 the
reaction mixtures were analyzed on a GC equipped with a TC
detector, and yields of 6 and 7 were calculated against an
internal standard of eicosane using response factors determined
with authentic samples.
N-Bu tyl-N-(3,3-d ip h en yl-2-p r op en yl)d im eth ylm a lon a m -
ic Acid (2b). The above ester (0.28 g, 0.69 mmol) and LiOH
(0.11 g, 2.74 mmol) in 95% EtOH (10 mL) were heated at reflux
for 5 h. The mixture was cooled and concentrated. The residue
was dissolved in water, and the resulting solution was acidified
with HCl. The mixture was extracted three times with ether,
and the combined ethereal extracts were washed with a satu-
rated aqueous NaCl solution, dried over MgSO4, and concen-
trated under reduced pressure. Column chromatography on
silica gel (hexanes-EtOAc, 1:1) gave 0.20 g of 2b as an oil (0.52
mmol, 95%). Complex 1H and 13C NMR spectra due to slow
conformer rotation are provided in Supporting Information.
N-Bu tyl-N-(3,3-d ip h en yl-2-p r op en yl)d im eth ylm a lon a m -
ic Acid 2-Th ioxo-2H-p yr id in e-1-yl Ester (3b). To a solution
of acid 2b (0.10 g, 0.264 mmol) and 2,2′-dipyridyl disulfide bis-
N-oxide16 (0.073 g, 0.29 mmol) in dry CH2Cl2 (10 mL) in a flask
wrapped with aluminum foil was added tributylphosphine (0.073
mL, 0.29 mmol) at 0 °C under N2. The reaction was stirred at
room temperature for 2 h before aqueous Na2CO3 (10%, 10 mL)
was added. The organic layer was separated, and the aqueous
layer was extracted with CH2Cl2 three times. The combined
organic extracts were washed with brine, dried over MgSO4, and
concentrated under reduced pressure. Column chromatography
on silica gel (hexanes-EtOAc, 1:1) gave 0.09 g of PTOC ester
Ack n ow led gm en t. This work was supported by a
grant from the National Science Foundation (CHE-
9614968). We thank Prof. D. P. Curran for suggesting
these experiments and Mr. N. Miranda, Mr. I. Samano,
Mr. R. Gappy, and Ms. M. Nguyen for experimental
assistance.
1
3b as a yellow oil (0.18 mmol, 68%). Complex H and 13C NMR
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra (16 pages). This material is contained in libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS; see
any current masthead page for ordering information.
spectra due to slow conformer rotation are provided in Support-
ing Information.
N-Bu t yl-3,3-d im et h yl-4-(d ip h en ylm et h yl)-2-p yr r olid i-
n on e (6) was isolated from the reaction of PTOC ester 3b
(0.044 g, 0.09 mmol) and Bu3SnH (0.052 g, 0.18 mmol) in
3.0 mL of dry THF. The reaction mixture was sealed with
a septum, flushed with nitrogen, and photolyzed with visible
J O981538E