dition of 9-fluorenone (Scheme 1). Terphenyl alcohol 5c was
prepared similarly from 4,4′-dibromoterphenyl. Diyne 6b was
prepared by an Eglinton homocoupling of 9-ethynylfluorenol
precursors to 4b and 4c were to be prepared similarly and
would also be predicted to be elusive via elimination of the
monolithiated species, so no further attempts were made at
synthesizing these species.
The neutral precursors 5b,c and 6b were then converted
to their respective bisfluorenyl dications 3b,c and 2b via
6
followed by methylation of the alcohols. Triyne 6c could
7
not be prepared from KO-t-Bu and 1,6-dichlorohexadiyne
or n-BuLi and the ditosyl derivative of 2,4-hexadiyne-1,6-
8
diol and subsequent addition to 9-fluorenone due to
ionization with Magic acid in SO
2). The samples were quickly analyzed by H and C NMR.
2
ClF at -78 °C (Scheme
1
13
decomposition of the dianion intermediate. In both attempts,
9
-fluorenone was recovered quantitatively.
Scheme 2. Generation of Spacer-Separated Dications
Scheme 1. Synthesis of the Dication Precursors
The experimental and calculated shifts for 2b and 3b,c
are given in Table 1. The agreement between experimental
Table 1. Experimental and Calculated 1H Chemical Shifts for
a
2
b, 3b, and 3c
1H
2b(exp) 2b(calc) 3b(exp) 3b(calc) 3c(exp) 3c(calc)
1
2
3
4
,8
,7
,6
,5
5.712
5.357
5.891
5.244
7.034
6.728
7.291
6.688
6.377
5.971
6.377
5.971
7.141
7.220
7.532
7.003
7.497
7.097
8.251
8.432
6.665
6.338
6.749
6.414
7.192
7.333
7.368
7.456
7.192
7.627
7.317
8.252
8.378
8.382
ring 1 o′
ring 1 o′′
ring 2 o′
a
Shifts were calculated with the GIAO method and basis set B3LYP/
-311 g(d,p) on geometries optimized at basis set B3LYP/6-31 g(d).
6
The dibromo precursor to 4a, 1,4-dibromobicyclo-
and calculated shifts (shown in Figure 1 for 1, 2a,b, and
3a-c) is good, correlation coefficient of 0.974, and supports
the use of this basis set for the calculation of NICS values.
The paratropic chemical shifts of the protons of the fluorenyl
systems clearly demonstrate their antiaromaticity. The aver-
9
[
2.2.2]octane, was synthesized from the analagous dicar-
1
0
boxylic acid via a double Cristol-Firth-modified Huns-
1
1
diecker reaction. However, 9-fluorenone was recovered in
near-quantitative yields (along with small amounts of 9-fluo-
renol) after attempts at dilithiation of this dibromo species
and subsequent addition using methods identical to the
synthesis of 3a-c. It is known that the intermediate
1
age values of the experimental H shifts for the fluorenyl
5
system in 3a-c are 5.865 ppm, 6.184 ppm, and 6.542 ppm,
1
respectively. The average values of the experimental H shifts
5
(
4-bromobicyclo[2.2.2]octan-1-yl)lithium is unstable and will
for the fluorenyl system in 2a,b are 5.520 ppm and 5.551
12
readily eliminate; thus, the dilithiation could not occur. The
ppm, respectively. This data indicates experimentally that
the antiaromaticity of similar bisfluorenyl dicationic systems
decreases as the distance between the individual fluorenyl
cations increases.
(
(
(
6) Eglinton, G.; Galbraith, A. R. Chem. Ind. 1956, 73, 7–8.
7) Hlavaty, J.; Kavan, L.; Sticha, M. Perkin 1 2002, 705–706.
8) Rubin, Y.; Lin, S. S.; Knobler, C. B.; Anthony, J.; Boldi, A. M.;
1
3,14
Diederich, F. J. Am. Chem. Soc. 1991, 113, 6943–9.
The NICS(1)zz
values for the fluorenyl system of 1,
(9) Dewar, M. J. S.; Goldberg, R. S. J. Am. Chem. Soc. 1970, 92, 1582–
2
a-c, 3a-c, and 4a-c along with the distance between the
6
.
(
10) Chang, H. X.; Kiesman, W. F.; Petter, R. C. Synth. Commun. 2007,
3
7, 1267–1272.
(13) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes,
(
11) Cristol, S. J.; Firth, W. C., Jr. J. Org. Chem. 1961, 26, 280.
N. J. v. E. J. Am. Chem. Soc. 1996, 118, 6317–6318
(14) Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.;
Puchta, R.; Schleyer, P. v. R. Org. Lett. 2006, 8, 863–866
.
(12) Wiberg, K. B.; Pratt, W. E.; Bailey, W. F. J. Am. Chem. Soc. 1977,
9
9, n/a.
.
5606
Org. Lett., Vol. 10, No. 24, 2008