4904
J . Org. Chem. 1997, 62, 4904-4905
Ta ble 1. Sp ecific Ra tes of Solvolysis of
4-Ch lor o-2,2,4,6,6-p en ta m eth ylh ep ta n e (4) a n d
3,3-Dim eth yl-1-n eop en tylbu tyl Mesyla te (5) a t 25.0 °C
Gr u n w a ld -Win stein Rela tion s in th e
Solvolyses of High ly Con gested Sim p le
Secon d a r y a n d Ter tia r y Alk yl System s.
Evid en ce for th e Br øn sted Ba se-Typ e
Solva tion in th e Sta n d a r d 1- a n d
2-Ad a m a n tyl System s
ka (s-1
)
solventb
4
5
100E
90E
80E
70E
60E
100M
70A
50A
100T
70T
50T
2.17 × 10-4 c
1.56 × 10-3 f
5.27 × 10-3 f,g
1.50 × 10-2 f
4.28 × 10-2 f
2.02 × 10-3 f
5.96 × 10-3 f
6.14 × 10-2 f
2.8d,i
7.96 × 10-7 d,e
3.24 × 10-6 c
8.93 × 10-6 c
1.72 × 10-5 c
4.03 × 10-5 c
3.66 × 10-6 c,h
5.11 × 10-6 c
5.68 × 10-5 c
3.37 × 10-4 c
2.70 × 10-4 c
3.15 × 10-4 c
1.51 × 10-2 f,k
4.09 × 10-6 c,l
Ken’ichi Takeuchi,* Yasushi Ohga,
Takuhiro Ushino, and Masaaki Takasuka
Department of Energy and Hydrocarbon Chemistry,
Graduate School of Engineering, Kyoto University,
Sakyo-ku, Kyoto 606-01, J apan
2.3d,j
Received April 21, 1997
97HFIP
AcOH
The improved Grunwald-Winstein relation (eq 1) is a
linear free energy relationship between solvolysis rates
and an empirical parameter YX related to solvent ionizing
power. The YX scales are defined by log(k/ k0) based on
1- or 2-adamantyl compounds (1 and 2, respectively)
having the leaving group X.1,2 In both eq 1 and YX, k
and k0 are specific rates in a given solvent and 80%
aqueous ethanol, respectively, at 25 °C.
1.34 × 10-3 c,l
a
Determined by duplicate runs for 4 and a single run for 5,
their initial concentrations being 0.02 mol L-1 in titrimetric runs
and (1-2) × 10-4 mol L-1 in conductimetric runs. Buffered with
b
0.025 mol L-1 2,6-lutidine unless otherwise noted. E, M, A, and
T denote ethanol, methanol, acetone, and 2,2,2-trifluoroethanol,
respectively, and the numbers preceding E, M, and A indicate
volume % of the organic components in aqueous mixtures at 25
°C and those for T and HFIP denote weight % of T and HFIP.
c Determined titrimetrically within an experimental error (2%.
log(k/ k0) ) mYX + c
(1)
Extrapolated from data at other temperatures. e The titrimetric
d
specific rates were 2.16 × 10-5 s-1 at 50.0 °C and 3.65 × 10-4 s-1
at 75.0 °C. f Determined conductimetrically within an experimen-
The rear side of the standard 1- and 2-adamantyl
systems is blocked to nucleophilic attack by solvent.
Therefore, any downward dispersions of the log(k/ k0)
points in the plot against YX for a substrate in less
nucleophilic solvents such as 2,2,2-trifluoroethanol (TFE),
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), and trifluoro-
acetic acid (TFA) have been interpreted to indicate that
the rates in more nucleophilic solvents such as aqueous
ethanol and aqueous acetone are enhanced by nucleo-
philic assistance to ionization by solvent.1,2
Recently, we reported that 2-chloro-2,4,4-trimethyl-
pentane (3) exhibits an excellent linear correlation with
YCl for 17 solvents.3 More recently, we found that the
rate in 70% HFIP is also well accommodated to the linear
relation.4 The results were interpreted to show that the
neopentyl group in 3 effectively shields the rear side in
the manner to hinder the nucleophilic solvent interven-
tion involving both of direct nucleophilic attack and
nucleophilic solvation toward the carbocationic center.3,4
On the other hand, in the long history of the study of
solvolysis reactions, there have not been reported any
simple secondary and tertiary alkyl substrates whose
mYOTs or mYCl relations (eq 1) exhibit upward dispersion
of the points for the fluorinated solvents. We now report
that such behavior is observed in the solvolysis of heavily
congested tertiary and secondary substrates, 4-chloro-
2,2,4,6,6-pentamethylheptane (4) and 3,3-dimethyl-1-
neopentylbutyl mesylate (5). We interpret the results to
tal error (0.5%. g A reported value is 5.25 × 10-3 s-1 (ref 6). The
h
specific rate at 50.0 °C was 9.17 × 10-5 s-1
.
The specific rates
i
determined in the presence of 0.0012 mol L-1 2,6-lutidine were
0.0547 s-1 at -20.0 °C, 0.124 s-1 at -12.0 °C, and 0.351 s-1 at
j
-0.6 °C. The specific rates determined in the presence of 0.0012
mol L-1 2,6-lutidine were 0.0575 s-1 at -10.5 °C, 0.108 s-1 at -5.0
°C, 0.180 s-1 at -0.6 °C, and 0.177 s-1 at -0.4 °C. No buffer
k
was added. Buffered with 0.025 mol L-1 NaOAc.
l
suggest stronger Brønsted base-type solvation5 (solvation
by hydrogen-bonding) toward the cationic moiety of
standard systems 1 and 2 than in substrates 4 and 5.
The known chloride 46 and new mesylate 57 were
prepared from the corresponding known alcohols. The
solvolysis rates were determined under buffered condi-
tions (except aqueous HFIP): the data at 25 °C are listed
in Table 1. Figure 1 shows the plots of log k values for
1b
8,9
4 and 5 against YCl and YOTs
,
respectively.
The straight lines in Figure 1 have been drawn for
ethanol and aqueous ethanol points to give mEW for 4 and
5 of 0.58 ( 0.02 (r 0.998) and 0.63 ( 0.03 (r 0.998),
respectively. The points of methanol and aqueous ac-
etone for 4 and those of these solvents and acetic acid
* To whom correspondence should be addressed. Tel.: +81 75 753
5689. Fax: +81 75 761 0056. E-mail: ktake@scl.kyoto-u.ac.jp.
(1) Reviews: (a) Bentley, T. W.; Schleyer, P. v. R. Adv. Phys. Org.
Chem. 1977, 14, 1. (b) Bentley, T. W.; Llewellyn, G. Prog. Phys. Org.
Chem. 1990, 17, 121. (c) Kevill, D. N.; D’Souza, M. J . J . Phys. Org.
Chem. 1992, 5, 287. (d) Liu, K. T. J . Chinese Chem. Soc. 1995, 42,
607. (e) Kevill, D. N. In Advances in Quantitative Structure-Property
Relationships; Charton, M., Ed.; J AI Press: Greenwich, CT, 1996; Vol.
1, pp 81-115.
(2) (a) Schadt, F. L.; Bentley, T. W.; Schleyer, P. v. R. J . Am. Chem.
Soc. 1976, 98, 7667. (b) Bentley, T. W.; Carter, G. E. J . Am. Chem.
Soc. 1982, 104, 5741.
(3) Takeuchi, K.; Ohga, Y.; Ushino, T.; Takasuka, M. J . Phys. Org.
Chem. 1996, 9, 777.
(5) For a recent leading article on Brønsted base-type solvation in
solvolytic reactions, see: Richard, J . P.; J agannadham, V.; Amyes, T.
L.; Mishima, M.; Tsuno, Y. J . Am. Chem. Soc. 1994, 116, 6706.
(6) Brown, H. C.; Berneis, H. L. J . Am. Chem. Soc. 1953, 75, 10.
(7) 5: mp 7-8 °C. Spectral and microanalytical data were consistent
with the structure. For the precursor alcohol, see: Herberg, C.;
Verevkin, S. P.; No¨lke, M.; Beckhaus, H.-D.; Ru¨chardt, C. Liebigs Ann.
1995, 515.
(8) Fujio, M.; Saeki, Y.; Nakamoto, K.; Yatsugi, K.; Goto, N.; Kim,
S.-H.; Tsuji, Y.; Rappoport, Z.; Tsuno, Y. Bull. Chem. Soc. J pn. 1995,
68, 2603.
(4) Takeuchi, K.; Ohga, Y.; Ushino, T.; Takasuka, M. J . Phys. Org.
Chem., in press.
(9) In this paper, we employ the YOTs values revised by Fujio et al.8
Previous values reported by Bentley et al. are compiled in ref 1b.
S0022-3263(97)00709-3 CCC: $14.00 © 1997 American Chemical Society