Generation of Aryloxyl Radicals from Aryl Oxalates
J . Org. Chem., Vol. 61, No. 5, 1996 1737
(3-Meth oxyp h en oxy)oxa lyl ter t-Bu tyl P er oxid e (10). A
BOC solution in pentane was made as described above, using
3.79 g of oxalyl chloride (29.8 mmol) and 1.34 g (14.9 mmol)
of tert-butyl hydroperoxide. A solution of 3-methoxyphenol (0.9
g, 7.5 mmol) and pyridine (1.3 g, 15 mmol) in 15 mL of ether
was then slowly syringed in and the reaction stirred for 15
min. The reaction was worked up as described for 9 to give
10 as a clear oil (3.5 g, 57%), which was not further purified.
IR (Nujol, cm-1): 1791, 1762 (s, CdO). 1H-NMR (80 MHz,
CDCl3, δ): 1.42 (s, 9 H), 3.78 (s, 3 H), 6.7-6.9 (m, 4 H).
(2,4,6-Tr ich lor op h en oxy)oxa lyl ter t-Bu t yl P er oxid e
(11). A BOC solution in pentane was made as described for
9, using 10.4 g of oxalyl chloride (81.2 mmol) and 3.66 g (40.6
mmol) of tert-butyl hydroperoxide. A solution of 2,4,6-trichlo-
rophenol (4.00 g, 20.3 mmol) and pyridine (1.71 g, 20.3 mmol)
in 15 mL of ether was then slowly syringed in and stirred for
15 min. The reaction was worked up as described for 9 to give
18.0 g (64%) of yellow solid 11, which was not further purified.
IR (Nujol, cm-1): 1791, 1762 (s, CdO). 1H-NMR (80 MHz,
CDCl3, δ): 1.41 (s, 9 H), 7.53 (s, 2 H).
tr a n s-3-Stilben oxyoxalyl ter t-Bu tyl P er oxide (12). BOC
was made with oxalyl chloride (0.60 g, 4.7 mmol) and tert-
butyl hydroperoxide (0.21 g, 2.3 mmol) in pentane as described
above. The pentane and excess oxalyl chloride were evapo-
rated under vacuum at 0 °C, and the system was purged with
nitrogen, at which time 10 mL of anhydrous ether was added.
A solution of 3-hydroxystilbene (0.15 g, 0.75 mmol) and
pyridine (0.095 g, 1.1 mmol) in 5 mL of ether was then slowly
syringed in and stirred for 15 min. The reaction mixture was
then filtered and washed with aqueous solutions of 10% H2-
SO4, 10% NaHCO3, and water, before being dried over MgSO4
and filtered. The ether solution was then placed in a small
brown vial and fitted with a septum, and the ether was
evaporated by passing nitrogen over at 0 °C. The remaining
solvent was removed by vacuum to give 0.015 g (60%) of clear
oil 12, which was not further purified. IR (neat, cm-1): 1789,
1769 (s, CdO). 1H-NMR (80 MHz, CDCl3, δ): 1.43 (s, 9), 6.83-
7.03 (m, 2 H), 7.15-7.50 (m, 9 H).
tr a n s-4-Stilben oxyoxalyl ter t-Bu tyl P er oxide (13). BOC
was made with oxalyl chloride (1.33 g, 10.5 mmol) and tert-
butyl hydroperoxide (0.94 g, 10.5 mmol) as described above.
A solution of 4-hydroxystilbene (1.50 g, 5.23 mmol) and
pyridine (0.44 g, 5.23 mmol) in 5 mL ether was then slowly
syringed in and stirred for 15 min. The reaction mixture was
then worked up and solvent removed as described for 12 above
to yield white solid 13 (0.70 g, 40%, mp 70-71 °C). IR (Nujol,
cm-1): 1794, 1771 (s, CdO). 1H-NMR (80 MHz, CDCl3, δ):
1.34 (s, 9 H), 7.10-7.55 (m, 11 H).
IR (Nujol, cm-1): 1745 (s, CdO). 1H-NMR (80 MHz, CDCl3,
δ): 1.41 (s, 36 H), 3.78 (s, 6 H), 6.78 (s, 4 H).
Bis(2,6-d i-ter t-bu tyl-4-m eth ylp h en yl) Oxa la te (16). To
NaH (240 mg, 10 mmol) in 10 mL of dry diethyl ether at 0 °C
under nitrogen was added carefully dropwise 4-methyl-2,6-di-
tert-butylphenol (2.2 g, 10 mmol) in 7 mL of ether. After the
resultant white suspension was stirred for 30 min, oxalyl
chloride (430 µL, 5 mmol) in 5 mL of ether was added slowly
dropwise with the reaction still cooled in an ice bath. The
reaction was then stirred at 0 °C for 30 min. The final mix
was poured onto ice, washed with 3 N HCl, saturated sodium
bicarbonate, and water, and then dried briefly over MgSO4.
Evaporation under reduced pressure gave a gum, which was
recrystallized from pentane at -20 °C to give a pale yellow
powder of 16 (830 mg, 17%, mp 129-130 °C). Anal. Calcd
for C32H46O4: C 77.69; H 9.37; O 12.94. Found: C 77.41; H
9.19. IR (KBr, cm-1): 1763 (s, CdO). 1H-NMR (80 MHz,
CDCl3, δ): 1.43 (s, 36 H), 2.24 (s, 6 H), 7.22 (s, 4 H).
Bis(2,6-d i-ter t-bu tylbip h en ylyl) Oxa la te (17). 3,5-Di-
tert-butyl-4-hydroxybiphenyl (0.81 g, 2.9 mmol), n-butyllithium
(1.8 mL, 2.9 mmol, 1.3 M in hexanes), and oxalyl chloride (0.18
g, 1.4 mmol) were used in the same procedure described for
14. Recrystallization from hexane at -78 °C yielded 17 as a
fine white powder (0.11 g, 12%, mp 226-227 °C). Anal. Calcd
for C42H50O4: C 81.52; H 8.14; O, 10.34. Found: C 82.07; H
8.29. IR (Nujol, cm-1): 1758 (s, CdO). 1H-NMR (80 MHz,
CDCl3, δ): 1.44 (s, 36 H), 7.15-7.65 (m, 14 H).
Bis(3-m eth oxyp h en yl) Oxa la te (19). Oxalyl chloride (2.5
g, 20 mmol) was dissolved in 25 mL of anhydrous ether and
stirred under a nitrogen atmosphere at 0 °C for 15 min.
3-Methoxyphenol (4.9 g, 40 mmol) and pyridine (3.3 g, 40
mmol) in 15 mL of ether were added via an equilibrating
addition funnel, precipitating an off-white solid. After stirring
for 1 h, the ether solution was washed with aqueous solutions
of 10% H2SO4, saturated Na2CO3, and water. The solid was
filtered, dissolved in methylene chloride, and dried over
MgSO4. Evaporation under reduced pressure gave a white
powder which was recrystallized from 95% ethanol to give 19
(5.0 g, 83%, mp 145-147 °C). Anal. Calcd for C16H14O6: C
63.57; H, 4.67; O 31.76. Found: C 63.06; H 4.77. IR (Nujol,
cm-1): 1761 (s, CdO). 1H-NMR (80 MHz, CDCl3, δ): 3.75 (s,
6 H), 6.40-7.12 (m, 6 H).
Bis(4-m eth oxyp h en yl) Oxa la te (20). 4-Methoxyphenol
(2.0 g, 16.1 mmol), NaH (390 mg, 16 mmol), and oxalyl chloride
(700 mL, 8 mmol) were used in the same procedure described
for 16. Dissolution in 1:1 diethyl ether:THF at 40 °C and then
cooling to -78 °C yielded 20 as colorless needles (570 mg, 24%,
mp 155-156 °C). Anal. Calcd for C16H14O6: C 63.57; H 4.67;
O, 31.76. Found: C 63.82; H 4.82. IR (KBr, cm-1): 1763 (S,
CdO). 1H-NMR (80 MHz, CDCl3, δ): 3.83 (s, 6 H), 6.80-7.20
(AA′BB′ para Ar-H, J ) 11 Hz, 8 H).
Bis(2,4,6-Tr ich lor op h en yl) Oxa la te (21). 2,4,6-Trichlo-
rophenol (6.3 g, 32 mmol), oxalyl chloride (6.3 g, 32 mmol),
and 2.5 mL (32 mmol) of pyridine were used in the same
procedure described for 19. Recrystallization from ethyl
acetate yielded 3.5 g (50%) of colorless prisms of 21, mp 186-
188 °C. Anal. Calcd for C14H4O4Cl6: C, 37.46; H, 0.90; O,
15.06; Cl, 47.39. Found: C, 37.61; H,1.05; Cl, 47.17. IR (KBr,
cm-1): 1776 (S, CdO). 1H-NMR (80 MHz, CDCl3, δ): 7.53 (s,
4 H).
2,4,6-Tr i-ter t-bu tylp h en yl Oxa la te, P h en yl Ester (22).
AOC 1 (1.7 g, 4.9 mmol) was dissolved in 15 mL of anhydrous
ether and stirred under a nitrogen atmosphere at 0 °C for 15
min, and then a solution of the phenol (0.38 g, 4.1 mmol) and
pyridine (0.34 g, 4.1 mmol) in 30 mL of ether was added over
30 min via an equilibrating addition funnel. The solution was
stirred for 30 min more and then washed with aqueous
solutions of 10% H2SO4, saturated Na2CO3, and water. Drying
over MgSO4 and evaporating the ether under reduced pressure
afforded a brownish-white solid, which was recrystallized from
95% ethanol to yield white rodlike crystals of 22 (0.8 g, 49%,
mp 104-105 °C). Anal. Calcd for C34H40O4: C 76.06; H 8.35;
O 15.59. Found: C 76.29; H 8.49. IR (Nujol, cm-1): 1780,
1751 (s, CdO). 1H-NMR (80 MHz, CDCl3, δ): 1.34 (s, 9 H),
1.41 (s, 18 H), 7.20-7.50 (m, 7 H).
Bis(2,4,6-tr i-ter t-bu tylp h en yl) Oxa la te (14). 2,4,6-Tri-
tert-butylphenol (7.25 g, 27.6 mmol) was dissolved in 40 mL
of anhydrous ether and stirred under a nitrogen atmosphere
at 0 °C for 15 min. n-Butyllithium (17.8 mL, 27.6 mmol, 1.3
M in hexanes) was added slowly by syringe, precipitating a
white solid which was stirred for 10 min, before oxalyl chloride
(1.75 g, 13.8 mmol) in 25 mL of ether was added via an
equilibrating addition funnel, precipitating a pinkish-white
solid. After stirring for 1 h, the ether solution was washed
with aqueous solutions of 10% H2SO4, saturated Na2CO3, and
water. Drying over MgSO4 and evaporating the ether under
reduced pressure gave 7.30 g of a white powder which was
recrystallized from 95% ethanol to yield 14 (4.88 g, 61%, mp
240-141 °C). Anal. Calcd for C38H58O4: C 78.84; H 10.10; O
11.06. Found: C 79.12; H 9.93. IR (Nujol, cm-1): 1755 (s,
CdO). 1H-NMR (80 MHz, CDCl3, δ): 1.36 (s, 18 H), 1.48 (s,
36 H), 7.45 (s, 4 H).
Bis(2,6-d i-ter t-bu tyl-4-m eth oxyp h en yl) Oxa la te (15).
3,5-Di-tert-butyl-4-hydroxyanisole (2.0 g, 8.5 mmol), n-butyl-
lithium (3.7 mL, 8.5 mmol, 2.5 M in hexanes), and oxalyl
chloride (0.5 g, 4.2 mmol) were used in the same procedure
described for 14. Column chromatography (silica gel, eluent
9:1 v/v hexane:chloroform with increasing polarity up to 4:6
hexane chloroform) and recrystallization of chromatographed
product in absolute ethanol at -78 °C yields 0.21 g (9.5%) of
15 as pale yellow prisms, mp 199-202 °C. Anal. Calcd for
C
32H46O6: C 72.97; H 8.80; O 18.23. Found: C 73.17; H 8.75.