chloroformate in 50 mL of ether was slowly added 1.31 g
(15.4 mmol) of piperidine in 20 mL of ether, and the mixture
was stirred under cooling for 20 min and at room temperature
for 1.5 h. The solution was washed successively with water, a
5% hydrochloric acid solution and a saturated sodium chloride
solution, dried over anhydrous magnesium sulfate and evapor-
ated to dryness. A solid residue was recrystallised from ethanol
to give 0.98 g (41%) of colorless crystals that melt at 102–
Base proliferation in solution. The carbamates (1 and 2a),
mesitylene as an internal standard and a catalytic amount of
amine were dissolved in a solvent containing a small amount of
tetramethylsilane. The solutions placed in sealed NMR tubes
were heated at 100 uC in an oven and were subjected to
intermittent NMR measurements. The consumption of the
carbamate was followed by monitoring decreases of the proton
signals of the methine and methylene groups, while the
formation of dibenzofulvene 3 was monitored by observing
the proton signal of the vinyl group.
1
105 uC. H-NMR (90 MHz, CDCl3): d (ppm) 1.55 (br s, 6 H,
CH2), 3.3–3.6 (m, 4 H, CH2), 4.1–4.5 (m, 3 H, CH, CH2), 7.1–
7.7 (m, 8 H, Ar-H). IR (KBr, cm21): 2936, 1686, 1437, 1259,
1232, 1151, 1106, 762, 739. Anal. Found: C, 77.90; H, 6.72; N,
4.41. Calcd for C20H21NO2: C, 78.14; H, 6.89; N, 4.56%.
Sensitivity determination. Photoresist solutions were pre-
pared by dissolving PGMA (0.08 g mL21), the photobase
generator 4 (10 wt% relative to the polymer) and the base
amplifier 2a (10, 20 or 43 wt% relative to the polymer) in
cyclohexanone or the base amplifier 2b (2, 4 or 9 wt% relative to
the polymer) in 1,1,1,3,3,3-hexafluoro-2-propanol. The solu-
tions were spin-coated on silicon wafers and heated at 110 uC
for 60 s to give thin films of 0.37–0.49 mm and 1.1 mm thickness
for 2a and 2b systems respectively. The thin films were exposed
to UV light from an Hg–Xe lamp without a glass filter,
followed by heating at 110 uC on a hot stage for 15 min,
developing with 2-methoxyethyl acetate for 30 s and rinsing
with ethanol. The thicknesses of the residual films were
measured to evaluate photosensitivity.
1-(9-Fluorenylmethoxycarbonyl)-cis-2,6-dimethylpiperidine (1c).
This was prepared in a similar manner from cis-2,6-dimethyl-
piperidine to give an oily substance which was purified by
column chromatography on silica gel with a 20 : 1 (v/v) mixture
of hexane and ethyl acetate to afford an 86.9% yield of colorless
1
crystals that melt at 80–82 uC. H-NMR (90 MHz, CDCl3): d
(ppm) 1.10 (s, 3 H, CH3), 1.19 (s, 3 H, CH3), 1.3–1.9 (m, 6 H,
CH2), 4.0–4.6 (m, 5 H, CH, CH2), 7.1–7.9 (m, 8 H,
Ar-H). IR (KBr, cm21): 2944, 1703, 1447, 1409, 1307, 1085,
763, 738. Anal. Found: C, 78.84; H, 7.57; N, 4.14. Calcd for
C22H25NO2: C, 78.77; H, 7.51; N, 4.18%.
9-Fluorenylmethyl N,N-diethylcarbamate (1d). This was
synthesised in a similar way in 86.4% yield as a colorless oil.
1H-NMR (90 MHz, CDCl3): d (ppm) 1.04 (t, J ~ 7.2 Hz, 6 H,
CH3), 3.32 (q, J ~ 7.2 Hz, 4 H, CH2), 4.1–4.5 (m, 3 H, CH,
CH2), 7.1–7.9 (m, 8 H, Ar-H). IR (neat, cm21): 2974, 1696,
1478, 1451, 1426, 1273, 1168, 757, 739. Anal. Found: C, 76.92;
H, 7.18; N, 4.80. Calcd. for C19H21O2N: C, 77.25; H, 7.17; N,
4.74%.
Results
Molecular design and synthesis
Our efforts have been focused on the development of base
amplifiers, which should fulfill the following requirements.
First, the base amplifier should be readily decomposed by base-
catalysis to liberate a base which leads to autocatalytic
decomposition. Second, the base amplifier should be thermally
stable in the absence of base, at least under the reaction
conditions, to enhance the autocatalytic decomposition
initiated by the photogenerated base. Thirdly, the liberated
base is a strong enough base to catalyse subsequent chemical
reactions and so display a nonlinear chemical transformation.
Consequently, we designed 9-fluorenylmethyl carbamates, 1
and 2, taking note of the fact that the 9-fluorenylmethoxy-
carbonyl group is one of the most useful protecting groups for
amino residues in peptide synthesis allowing amino residues to
be generated under basic conditions.15 Note that no informa-
tion about the nonlinearity of base-catalysed kinetics of this
type of carbamate (1b) has so far been mentioned although
autocatalytic decomposition of a carbamate containing an
aliphatic amine moiety generates an aliphatic amine, carbon
dioxide and dibenzofulvene (3), as shown in Scheme 1.
In addition to the above requirements, an amine generated
from a base amplifier should have no or less nucleophilic
reactivity to suppress irreversible side reaction(s). Accordingly,
an amine generated from a base-amplifying carbamates should
exhibit an appropriate level of steric hindrance to reduce
nucleophilicity, while strong steric hindrance has to be avoided
in order to secure the role of the amine as a reasonable proton
acceptor. The carbamates 1c–1e were designed and synthesised
on the basis of such considerations. Besides, the dicarbamates,
2a and 2b, with relatively high molecular weights were prepared
to minimize the volatility of the deprotected amines because an
annealing process at elevated temperatures is necessary not
only for the base proliferation reaction, but also for subsequent
reactions initiated by the amines.
9-Fluorenylmethyl N,N-dicyclohexylcarbamate (1e). This
was prepared from dicyclohexylamine in 77.9% yield as
colorless crystals that melt at 190–194 uC. 1H-NMR
(90 MHz, CDCl3): d (ppm) 0.5–2.0 (m, 22 H, CH, CH2),
4.20 (t, J ~ 4.6 Hz, 1 H, CH), 4.62 (d, J ~ 4.6 Hz, 2 H, CH2O),
7.1–7.9 (m, 8 H, Ar-H). IR (KBr, cm21): 2926, 1674, 1441,
1290, 1238, 1167, 1119, 1046, 744. Anal. Found: C, 80.56; H,
8.32; N, 3.42. Calcd for C27H33NO2: C, 80.37; H, 8.24; N,
3.47%.
1,3-Bis[1-(9-Fluorenylmethoxycarbonyl)-4-piperidyl]propane (2a).
This was prepared in a similar manner in 36.0% yield as
colorless crystals that melt at 119–120 uC. 1H-NMR (90 MHz,
CDCl3): d (ppm) 0.8–1.9 (m, 16 H, CH, CH2), 2.5–3.0 (br, 4 H,
N-CH2), 3.9–4.5 (m, 10 H, CH, CH2O, N-CH2), 7.1–7.9 (m,
16 H, Ar-H). IR (KBr, cm21): 2918, 1695, 1432, 1251, 1217,
1146, 1103, 759, 741. Anal. Found: C, 78.93; H, 7.21; N, 4.20.
Calcd for C43H46N2O4: C, 78.86; H, 7.08; N, 4.28%.
1,6-Bis[(9-fluorenylmethoxy)carbonylamino]hexane (2b). To
a solution of 9-fluorenylmethanol (7.84 g, 40.0 mmol) in dry
benzene (60 mL) containing a catalytic amount of dibutyltin
dilaurate (0.1 g) was added under reflux a solution of
hexamethylene diisocyanate (3.36 g, 20.0 mmol) in dry benzene
(20 mL) under a nitrogen stream. After heating at reflux for 2 h
and cooling to room temperature, crystalline precipitates were
collected to be recrystallised from cyclohexanone and washed
with acetone to give 10.0 g (89.3%) of colorless crystals that
melt at 175 uC by decomposition. 1H NMR (90 MHz, CDCl3):
d (ppm) 1.0–1.7 (m, 8 H, CH2), 2.9–3.4 (m, 4 H, CH2NH), 4.0–
4.9 (m, 8 H, CH, CH2, NH), 7.1–7.9 (m, 16 H, Ar-H). IR (KBr,
cm21): 3336, 2943, 1685, 1529, 1450, 1259, 1139, 1004, 758, 737.
Anal. Found: C, 77.20; H, 6.44; N, 4.94. Calcd for
C36H36N2O4: C, 77.12; H, 6.47; N, 5.00%.
The carbamates, 1a–e and 2a, were synthesised by the reaction
of the corresponding amines with 9-fluorenylmethyl chloro-
formate, whereas the carbamate 2b was conveniently prepared
from 9-fluorenylmethanol and hexamethylene diisocyanate.
They are thermally stable crystals except for oily 1d. The
carbamates may undergo base-catalysed E1cB elimination to
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 3 3 6 – 3 4 3
3 3 7