Generation of Antiaromatic Cyclopentadienyl Cations
J. Am. Chem. Soc., Vol. 119, No. 10, 1997 2375
t-Bu), 2.07 (d, 3, J ) 1.9 Hz, CH3CdC), 3.50 (q, 2, JHF ) 9.9 Hz,
CH2CF3), 5.62-5.73 (m, 2, 2CH)C); 13C NMR (CDCl3) δ 17.1, 26.1,
16b. The presence of 3% 16b in the product mixture could not be
excluded on the basis of the H NMR. The products were identified
1
2
29.7, 33.2, 36.2, 61.4 (q, JCF ) 33.9 Hz, CH2CF3), 93.5 124.5, (q,
by comparison of their EIMS peaks to those for the corresponding
products from trifluoroethanolysis, as follows: (16a) m/z 241 (M+, 19),
226 (M+ - CH3, 22), 185 (M+ - C4H8, 48), 170 (M+ - C5H11, 100),
152 (90), 57 (C4H9+, 92); (16b) m/z 241 (M+, 13), 226 (4), 185 (49),
170 (100), 152 (64), 57 (55); (17) m/z 192 (M+ + 2, 5), 191 (M+ +1,
42), 190 (M+, 11), 177 (M+ - CH3 +2), 176 (M+ - CH3 +1, 99),
175 (M+ - CH3, 59); 135 (M+ - C4H7, 45), 134 (M+ - C4H8, 100),
133 (M+ - C4H9, 66), 57 (C4H9+, 66); (18) m/z 134, 119, 92 (C2D5-
1JCF ) 278 Hz), 128.6, 134.0, 144.9, 157.7; 19F NMR (CDCl3) δ -75.37
(t, JHF ) 9 Hz); UV λhexane 271 nm (ꢀ ) 930); EIMS m/z 290 (M+, 7),
max
275 (M+ - CH3, 4), 234 (M+ - C4H8, 33), 219 (M+ - C5H11, 73),
178 (22), 57 (C4H9+, 100); HRMS m/z calcd for C16H25OF3 290.1858,
obsd 290.1862.
1,3-Di-tert-butyl-5-methylene-1,3-cyclopentadiene (17): 1H NMR
(CDCl3) δ 1.13 (s, 9, t-Bu), 1.15 (s, 9, t-Bu), 5.61 (d, J ) 1.8 Hz,
C4H), 5.65 (br s, 1, CHH), 5.86 (br s, 1, CHH), 6.13 (t, 1, C2H),
OCMe2+), 57 (t-Bu+); (19) m/z 241 (M+, 16), 226 (M+ - CH3, 26),
+
185 (M+ - C4H8, 49), 170 (M+ - C5H11, 81), 152 (63), 57 (C4H9
100).
,
1
assignments confirmed by the H,13C-heteronuclear coupled spectrum
(2D HMQC); 13C NMR (CDCl3) δ 29.2, 31.96, 32.02, 33.09, 116.6 (d,
1JCH ) 161.8 Hz, C4), 119.6 (t, 1JCH ) 158.1 Hz, CH2), 128.1 (d, J )
1,3-Di-tert-butyl-5-hydroxy-5-phenylcyclopenta-1,3-diene (14b).
As described for 14a the reaction of 13 (0.279 g, 1.45 mmol) with
PhLi (0.9 mL, 1.8 M in cyclohexane/ether, 1.62 mmol) gave 0.290 g
161.8 Hz, C2), 146.6, 151.0, 155.9; UV λhexane 246 (ꢀ ) 15 000), 370
max
(ꢀ ) 420) nm; EIMS m/z 190 (M+, 40), 175 (M+ - CH3, 100), 133
(M+ - C4H9, 87), 119 (M+ - C5H11, 50), 57 (C4H9+, 52); HRMS m/z
calcd for C14H22 190.1722, obsd 190.1725.
1
of an oil which by H NMR contained 50% of the desired product.
Chromatography (5:95 EtOAc/hexane, Rf ) 0.25) gave 14b (0.080 g,
0.03 mmol, 20%) as a pale yellow oil: IR (CDCl3) 3601 cm-1 (OH);
1H NMR (CDCl3) δ 0.98 (s, 9, t-Bu), 1.13 (s, 9, t-Bu), 1.95 (brd s, 1,
OH), 5.62 (d, 1, J ) 1.9 Hz, CHdC), 6.05 (d, 1, J ) 2.1 Hz, CHdC),
7.1-7.4 (m, 5, Ph); 13C NMR (CDCl3) δ 28.7, 30.8, 32.0, 34.4, 89.4,
124.8, 125.2, 126.2, 127.9, 133.0, 140.4, 153.0, 163.8; EIMS m/z 270
3,5-Dimethyl-2-tert-butyl-5-[2′-(2′′,2′′,2′′-trifluoroethoxy)-2′-pro-
pyl]-1,3-cyclopentadiene (18): 1H NMR (CDCl3) δ 1.106 (s, 3, CH3),
1.114 (s, 6, CMe2), 1.178 (s, 9, t-Bu), 2.05 (d, 3, J ) 1.5 Hz, CH3CdC),
3.73 (q, 2, JHF ) 8.7 Hz, CH2CF3), 5.87 (d, 1, J ) 1.4 Hz, CHdC),
5.92 (m, 1, CdCH); 13C NMR (CDCl3) δ 16.4, 17.2, 21.2, 29.7, 29.8,
(M+, 50), 255 (M+ - CH3, 91), 214 (M+ - C4H8, 56), 199 (M+
-
2
1
32.8, 59.2, 60.7 (q, JCF ) 33.7 Hz), 79.1, 124.4 (q, JCF ) 278 Hz),
C5H11, 94), 158 (48) , 57 (C4H9+, 100); HRM S m/z calcd for C19H26O
270.1984, obsd 270.1991.
135.4, 140.5, 140.8, 153.6; 19F NMR (CDCl3) δ -75.1 (t, JHF ) 8.5
hexane
Hz); UV λ
246 nm (ꢀ ) 2000); EIMS m/z 290 (M+, 3), 219 (7),
max
150 (5), 141 (CF3CH2OCMe2+, 100); HRMS m/z calcd for C16H25F3O
290.1858, obsd 290.1847.
1,3-Di-tert-butyl-5-phenyl-5-cyclopenta-1,3-dienyl Trifluoro-
acetate (15b). As described for 15a the reaction of 14b (22 mg, 0.082
mmol) with pyridine (8 µL, 0.1 mmol) and (CF3CO)2O (44 µL, 0.31
mmol) gave after chromatography (10:90 EtOAc/hexane, Rf ) 0.6) 15b
(20 mg, 0.053 mmol, 65%) as a clear liquid: IR (CDCl3) 1789 cm-1
1,3-Di-tert-butyl-5-methyl-5-cyclopenta-1,3-dienyl 2,2,2-Trifluo-
roethyl Ether (19): 1H NMR (CDCl3) δ 1.09 (s, 9, t-Bu), 1.21 (s, 9,
t-Bu), 1.55 (s, 3, Me), 3.50 and 3.51 (ea q, 1, JHF ) 8.9 Hz, CH2CF3),
1
5.35 (bd, 1, J ) 1.6 Hz, CdCH), 6.05 (d, 1, J ) 2.0 Hz, CdCH); 13
C
(CdO); H NMR (CDCl3) δ 0.92 (s, 9, t-Bu), 1.17 (s, 9, t-Bu), 5.78
2
(d, 1, J ) 2.0 Hz, CHdC), 6.24 (d, 1, J ) 2.0 Hz, CHdC), 7.2-7.4
(m, 5, Ph); 13C NMR (CDCl3) δ 28.5, 30.3, 32.2, 34.3, 97.2, 114.3 (q,
1JCF ) 287 Hz), 124.4, 127.2, 127.4, 128.37, 128.44, 135.4, 154.3 (q,
2JCF ) 42 Hz), 155.9, 158.7; 19F NMR (CDCl3) δ -75.3; EIMS m/z
366 (M+, 10), 310 (M+ - C4H8, 40), 57 (C4H9+, 100); HRMS m/z
calcd for C21H25F3O2 366.1807, obsd 366.1818.
NMR (CDCl3) δ 22.7, 28.7, 30.6, 31.9, 33.8, 61.8 (q, JCF ) 34 Hz),
1
90.8, 124.3 (q, JCF ) 278 Hz), 126.8, 127.4, 154.5, 157.9; 19F NMR
(CDCl3) -74.6 (t, J ) 8.9 Hz); UV λhexane 273 nm (ꢀ ) 2000); EIMS
max
m/z 290 (M+, 10), 275 (M+ - CH3, 22), 234 (M+ - C4H8, 43), 219
(M+ - C5H11, 58), 141 (34), 57 (C4H9+, 100); HRMS m/z calcd for
C16H25F3O 290.1858, obsd 290.1860.
Trifluoroethanolysis of 15b. As for 15a a solution of 15b (12 mg,
0.032 mmol) in 0.6 mL of CF3CD2OD was heated to 61 °C for 2 h.
After workup 1.9 mg of material was obtained which by mass spectral
analysis contained unreacted 15b or an isomer (EIMS m/z 366 (M+))
and substitution product derived from 15b (R1 ) CF3CH2) (EIMS m/z
354 (M+), 298 (M+ - C4H8)).
Kinetic Measurements. Kinetics were measured using Perkin-
Elmer Lambda 12 and Varian 210 spectrophotometers using the general
procedures and solvent preparation as reported previously.5 In a typical
procedure solutions of 15a (2 µL, 0.118 M in CH3CN) and 2,6-lutidine
(4 µL, 0.118 M in CH3CN) were added to 1.2 mL of 97% TFE, and
the decrease in absorbance (∆A ) 0.1) was monitored at 300 nm. Stable
end points were observed, with an isobestic point at 335 nm and a
final λmax at 375 nm, primarily due to fulvene 17. In the absence of
buffer the end point was not stable, and the rates were measured using
0.1-0.25 times the concentration of 15a used in the presence of 2,6-
lutidine. For measurements of 15b a solution of the substrate (4 µL,
0.01 M in CH3CN) was added to 1.2 mL of 97% TFE, and the increase
in absorbance was monitored at 272 nm (∆A ) 0.05). For reactions
in HFIP the concentrations of 15b were 2-5 times larger.
In a separate experiment a solution of 15a (3.5 mg, 0.0115 mmol)
and 2,6-lutidine (2.7 µL, 0.023 mmol) in 1 g of CF3CD2OD in an NMR
tube was monitored by 1H NMR and showed a half-life for reaction of
about 47 min, and after 247 min showed no residual 15a. The reaction
mixture was poured into water and extracted with ether which was
dried and evaporated, and the product was chromatographed (5:95
EtOAc/hexane) and analyzed by GC/MS to give peaks with the
following retention times (min), with identification as shown (Vide infra)
based on their MS patterns: 7.9 (unidentified, but probably 16b, as
+
ions at m/z 236 (M+ - C4H8), 221 (M+, - C5H11), and 57 (C4H9
were present), 7.97 (19), 8.03 (17), 8.13 (16a), and 8.75 (18).
)
The alcohol 14a was not observed in any of the product studies,
and was shown to be stable under the reaction conditions.
Ethanolysis of 15a. A solution of 15a (6.4 mg, 0.0211 mmol) in
0.65 mL of EtOH-d6 with 2,6-lutidine (5 µL, 0.043 mmol) in an NMR
1
tube was heated at 100 °C and monitored by H NMR. During the
reaction the ratio of the products formed showed some variation with
time, but after 6.5 h 15a had disappeared and the ratio of products
appeared constant. The reaction mixture was poured into H2O and
pentane, the aqueous layer was extracted five times with pentane, and
the combined organic layers were dried and evaporated, and by analogy
with the product from trifluoroethanolysis of 15a appeared by 1H NMR
to contain the ether 16, fulvene 17, and the ether 19 in a ratio of 42:
40:18. No signals corresponding to those of 18 were detected. Analysis
of the products by GC/MS showed 5% unreacted starting material and
the products 16a, 17, 18, and 19 (R1 ) C2D5) in relative percentages
of 43, 32, <1, and 17%, respectively, in good agreement with the NMR
results. In addition a signal corresponding to 3% of the total, and with
a fragmentation pattern similar to 16a, was observed, which may be
Acknowledgment. Financial support by the Natural Sciences
and Engineering Research Council of Canada is gratefully
acknowledged.
Supporting Information Available: 1H NMR spectra (8
pages). See any current masthead page for ordering and Internet
access instructions.
JA962494Z