mixtures, indicating that IPP dimerizes rapidly in dilute acidic
solution. Phenol was absent in the product also. Instead, a
7
sulfonated phenol 2 could be discerned in NMR (Figure 2)
and thus 2 stayed in the water layer. The IPP dimer 3a (mp
6
)
128 °C), the principle linear dimer, could be isolated by
crystallization in yields of about 80% from the quenched
6
product. The other two dimers, 3b-c, isolated as a viscous
oil (5-15%) seemed to be the thermodynamic products, since
the later yields could be enhanced at the expense of 3a either
under prolonged heating of 3a or exposure to acidic media
(see Figure 1).
IPP dimers, 3a-c, were found to be excellent precursors
for syntheses of IPP and indane-ring-containing bisphenols
such as 4 and 5. Conveniently, all transformations were done
in one-step simply by treatment of 3 with specific reagents.
For instance, IPP cation 1 could be regenerated in the absence
2 4
of 2 by addition of 3a-c in concentrated H SO (Scheme
2).
Figure 2. 1H NMR of IPP cation 1.
1
structure that was evident by its H NMR absorptions located
Scheme 2. Syntheses of 1, IPP, and Polyphenols from 3
at δ 3.44 (s, 6H), 7.75 (d, 2H), and 8.92 (d, 2H) (Figure 2)
and 13C NMR at 28.310 (2C), 120.815 (2C), 145.367 (2C),
158.735 (1C), and 180.455 (1C). These absorptions are
almost identical to the corresponding protons of 2-isopro-
5
pylidene-(4-methoxy-phenyl) carbocation reported earlier.
2 4
IPP cation 1 is remarkably stable in concentrated H SO for
days, reflecting its resistance to sulfonation reaction, possibly
because of two deactivated cationic groups connected at 1-
and 4-positions of its benzene structure simultaneously
(Scheme 1).
Scheme 1. Generation and Quenching of IPP Cation 1
1
As shown in Figure 2b, H NMR of IPP cation 1 that was
2 4
generated from 3 and concentrated H SO was simpler and
cleaner than those from BPA because there are no spectral
complications from compound 2 and only absorptions
corresponding to protons from cation 1 were present. Heating
3a-c under reduced pressure with trace NaOH as the catalyst
rapidly produced pure IPP as a distillate in 86% (bp ) 125-
1
35 °C).
IPP dimers 3a-c were converted into indane-bisphenol
(mp ) 192 °C) by the action of strong carboxylic acids.
48
Although formic acid seemed satisfactory for the purpose,
trifluoroacetic acid (TFA) has been identified to be the most
efficient acid media, since the solution afforded the highest
yield of 4 (90%) at 31 °C (Table 1).
When the acid solution of IPP cation 1 was poured slowly
into excess ice-water, solid precipitates formed instantly.
(
4) (a) Schnell, H.; Krimm, H. Angew. Chem., Int. Ed. Engl. 1963, 2,
3
73. (b) Krimm, H.; Schnell, H. DE 1235894, 1967. (c) Mimaki, K.; Takese,
T.; Iwasa, M.; Morimoto, T. U.S. Patent 4 054 611, 1977.
(5) Jost, R.; Sommer, J.; Engdahl, C.; Ahlberg, P. J. Am. Chem. Soc.
980, 102, 7663.
The precipitates were identified to be a mixture of three linear
6
IPP dimers, 3a-c. No IPP could be detected in the product
1
(
6) Complete name of 3a is 4-methyl-2,4-bis(p-hydroxyphenyl)-pent-1-
(
3) (a) Crivello, J. V.; Lai, Y. L. J. Polym. Sci., Part A: Polym. Chem.
995, 33, 653. (b) Crivello, J. V.; Ramdas, A. J. Macromol. Sci., Pure
Appl. Chem. 1992, A29 (9), 753. (c) Yamazaki, N.; Morimoto, Y. GB
2
4
ene and 3b-c are 4-methyl-2,4-bis(p-hydroxyphenyl)-pent-2-enes (Webb,
R. F.; Hinton, I. G. U.S. Patent 3 264 358, 1966).
(7) Pouchert, C. J. Aldrich library of NMR spectra, 1983, 1599A.
(8) Complete name of 4 is 1-(4-hydroylphenyl)-1,3,3-trimethyl indan-
6-ol (Farnham, A. G. U.S. Patent 3 288 864, 1966).
1
031898, 1980. (d) Fujiwara, H.; Takahashi, A.; Miyamoto, M. U.S. Patent
032 513, 1977.
2342
Org. Lett., Vol. 6, No. 14, 2004