Electronic effects of icosahedral carboranes. Retentive solvolysis of (1,2-dicarba-closo-dodecaboran-1-yl)benzyl p-toluenesuffonates

Full text of DOI:10.1021/ja993517n

180
J. Am. Chem. Soc. 2000, 122, 180-181
Electronic Effects of Icosahedral Carboranes.
Retentive Solvolysis of
(1,2-Dicarba-closo-dodecaboran-1-yl)benzyl
p-Toluenesulfonates
Yasuyuki Endo,* Takehiko Sawabe, and Yoshiyuki Taoda
Graduate School of Pharmaceutical Sciences
UniVersity of Tokyo, 7-3-1, Hongo, Bunkyo-ku
Tokyo 113-0033, Japan
Figure 1. In the icosahedral cage structure, b represents a carbon atom
and other vertexes represent BH units.
ReceiVed September 30, 1999
Table 1. Pseudo-First-Order Rate Constants (k1 s-1) and
Activation Parameters for the Hydrolysis of 1-4 in 70%
Dioxane-d12-D2O
The icosahedral closo carboranes (dicarba-closo-dodecaboranes)
have been described as three-dimensional aromatic systems, by
analogy with the two-dimensional benzene ring,1 and the implica-
tions of this for electronic interaction with substituents have been
of particular interest since the first studies on these compounds
some 30 years ago. Concerning the electronic effect of icosahedral
carboranes on a substituent outside the cage, investigations of
pKa values of carboranylbenzoic acids and carboranylanilinium
ions,2 and of 19F NMR chemical shifts of carboranylfluoroben-
zenes,3 showed that the icosahedral carboranes behave as strongly
electron-withdrawing groups in the sequence ortho . meta >
para toward carbon substituents. These investigations have also
shown that the electron-withdrawing inductive effect of the
carborane cage is similar to that of halogens, and that ground-
state cage-ring-π interaction is not important. Nevertheless, the
electron-delocalizing effect of the icosahedral carboranes, espe-
cially the electronic bonding structure of the two cage carbons in
o-carboranes, remains ambiguous. Although the electron-delocal-
izing effect in the static situation has been evaluated by
spectroscopic methods,1a,4 the effect in the dynamic situation (e.g.,
kinetics) has not been examined. In this paper, we report the first
example of kinetic investigation of a carbocation adjacent to an
o-, m-, or p-carboranyl cage, and the discovery of a unique
character of the o-carboranyl moiety.
To evaluate the electronic effects of the icosahedral carboranes,
we focused on the hydrolysis of (1,2-, 1,7-, and 1,12-dicarba-
closo-dodecaboran-1-yl)benzyl p-toluenesulfonates (1, 2, 3). The
(o-carboranyl)benzyl tosylate 1 was prepared from 1,2-dicarba-
closo-dodecaborane by employing tetrabutylammonium fluoride
(TBAF)-promoted addition5 with benzaldehyde followed by
reaction with p-toluenesulfonyl chloride. The benzyl tosylates, 2
and 3, were prepared by the reaction of corresponding lithiates
of 1,7- and 1,12-dicarba-closo-dodecaboranes with benzaldehyde
followed by reaction with p-toluenesulfonyl chloride.6 (2-Methyl-
1,2-dicarba-closo-dodecaboran-1-yl)benzyl p-toluenesulfonate (4)
was prepared from 1-methyl-1,2-dicarba-closo-dodecaborane7 by
a similar procedure to that used for 2.
1
2
3
4
temp (°C)
50.7
60.7
70.7
80.7
90.7
100.7
rel rate (calcd) 1840
5.87 × 10-5
1.21 × 10-4
2.71 × 10-4
6.37 × 10-5
5.24 × 10-4 5.56 × 10-6 6.28 × 10-6 1.35 × 10-4
1.80 × 10-5 1.76 × 10-5 2.61 × 10-4
4.62 × 10-5 4.82 × 10-5
1.0
1.4
356
at 25.0 °C
∆Hq (kcal/mol) 16.1
27.1
-6.1
26.0
-9.0
16.9
-28.8
∆Sq (eu)
-28.3
the corresponding alcohols 6-10 quantitatively. The rate constants
and activation parameters for the hydrolysis of 1-4 are sum-
marized in Table 1. The m- (2) and (p-carboranyl)benzyl tosylates
(3) showed almost the same values of rate constant and activation
parameters. The k1 values of approximately 6 × 10-6 s-1 at 80
°C for 2 and 3 were remarkably smaller than the k1 value of 1.03
× 10-3 s-1 at 50 °C for (1-adamantyl)benzyl tosylate (5), in which
the substituent resembles the carborane cage in molecular size
and shape. This result is consistent with a strongly electron-
withdrawing effect by the icosahedral carboranes in 2 and 3,
compared to the electron-donating adamantyl group in 5. How-
ever, the hydrolysis of (o-carboranyl)benzyl tosylate (1), bearing
what is thought to be the most electron-withdrawing group among
the icosahedral carboranes, was 100-1000 times faster than that
of 2 and 3. The activation parameters of the hydrolysis of the
o-carboranyl derivative were ∆Hq ) 16.1 kcal/mol, ∆Sq ) -28.3
eu. These values differ significantly from those of 2 (∆Hq ) 27.1
kcal/mol, ∆Sq ) -6.1 eu) and 3 (∆Hq ) 26.0 kcal/mol, ∆Sq )
-9.0 eu), suggesting that the processes involved are more
“dissociative” than that of 2. It appears that the mechanism of
hydrolysis of 1 is distinct from that of 2, and that the transition
state of hydrolysis of 1 is stabilized.
The kinetic experiments of the hydrolysis on the (carboranyl)-
benzyl tosylates (1-4) and (1-adamantyl)benzyl tosylate (5) in
70% dioxane-d12-D2O were performed by NMR measurement
of the decrease of starting materials. The hydrolyses of 1-5 gave
A stereochemical examination of the hydrolysis of optically
active (carboranyl)benzyl tosylates (1-3) provided more persua-
sive evidence of a distinction between the o-carboranyl group
and the others. The optically active (carboranyl)benzyl tosylates
were prepared from corresponding alcohols, which were obtained
by optical resolution of the racemic alcohols (6-8) as
(-)-camphonic acid esters, followed by alkaline hydrolysis. The
optical purity of the optically active (carboranyl)benzyl derivatives
(1) (a) Wu, S.-H.; Jones, M., Jr. Inorg. Chem. 1988, 27, 2005-2008. (b)
Fox, M. A.; MacBride, J. A. H.; Peace, R. J.; Wade, K. J. Chem. Soc., Dalton
Trans. 1998, 401-411.
(2) Hawthorne, M. F.; Berry, T. E.; Wegner, P. A. J. Am. Chem. Soc. 1965,
87, 4746-4750.
(3) Zakharkin, L. I.; Kalinin, V, N.; Rys, E. G. IzV. Akad. Nauk SSSR, Ser.
Khim. 1974, 2632-2635.
1
was determined by H NMR measurement in the presence of a
(4) Harmon, K. M.; Harmon, A. B.; Thompson, B. C. J. Am. Chem. Soc.
1967, 89, 5309-5313.
chiral shift reagent: tris[3-(heptafluoropropylhydroxymethylene)-
d-camphorato]europium(III). Hydrolysis of (+)-R-m- ((+)-R-2)
and ((+)-R-p-carboranyl)benzyl tosylate ((+)-R-3) afforded ra-
cemic 7 and 8 under the same conditions as used for the kinetic
examination. The results indicated that the hydrolyses of 2 and 3
proceed through typical SN1 processes. However, hydrolysis of
(5) Nakamura, H.; Aoyagi, K.; Yamamoto, Y. J. Am. Chem. Soc. 1998,
120, 1167-1171.
(6) Attempts to prepare (1,7-dicarba-closo-dodecaboran-1-yl)benzyl alcohol
by the TBAF-promoted addition procedure were unsuccessful, affording benzyl
alcohol and the corresponding nido anion.
(7) Phadke, A. S.; Morgan, A. R. Tetrahedron Lett. 1993, 34, 1725-1728.
10.1021/ja993517n CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/18/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 1, 2000 181
Table 2. Pseudo-First-Order Rate Constants (k1 s-1) and
Activation Parameters for the Acetolysis of 1-4 in CD3COOD
Scheme 1
1
2
3
4
temp (°C)
100.6
108.0
115.6
rel rate (obsd) 0.45
1.32 × 10-5 3.06 × 10-5 3.92 × 10-5 1.67 × 10-5
2.82 × 10-5 6.23 × 10-5 8.36 × 10-5 3.28 × 10-5
5.80 × 10-5 1.13 × 10-4 1.39 × 10-4 6.72 × 10-5
1.0
1.3
0.53
at 108.0 °C
∆Hq (kcal/mol) 28.5
24.4
-14.2
23.6
-15.8
27.8
-8.5
∆Sq (eu.)
-7.0
(+)-R-(o-carboranyl) benzyl tosylate ((+)-R-1, 100% ee.) afforded
(+)-R-6 in a 71% ee.8 Similarly, methanolysis of (+)-1 proceeded
with similar rate constants (at 60 °C) to those of the hydrolysis
to afford (+)-R-(o-carboranyl)benzyl methyl ether ((+)-R-11) in
a 73% ee.9
When the rate of reaction is greater than expected, and the
configuration at a chiral carbon is retained and not inverted or
racemized, mechanisms such as intramolecular neighboring-group
effects or an intermolecular SNi mechanism can be anticipated.
To examine the behavior of these solvolyses, we performed
acetolysis of the (carboranyl)benzyl tosylates (1-4) in CD3COOD.
The results were distinct from those in the case of hydrolysis
and methanolysis. The rate constants and activation parameters
for the acetolysis are summarized in Table 2. The m- (2) and
p-carboranylbenzyl tosylates (3) showed almost the same rate
constants as in the case of the hydrolysis. However, the acetolysis
of o-carboranylbenzyl tosylate (1) was slower than that of 2 and
3. The activation parameters of the acetolysis of the o-carboranyl
derivative (∆Hq ) 28.5 kcal/mol, ∆Sq ) -7.0 eu) were quite
different from those in the case of hydrolysis (∆Hq ) 16.1 kcal/
mol, ∆Sq ) -28.3 eu). Further, acetolysis of the optically active
carboranyl tosylates (1-3) afforded racemic products (12-14).
The results indicated that the acetolyses of 1-3 proceed through
typical SN1 processes. The significant mechanistic change upon
changing the solvent from D2O to CD3COOD suggested that
neighboring-group participation by the electron-delocalized C-C
bond in o-carboranes, such as formation of a nonclassical
carbocation 15, could be excluded.
the nido-7,8-C2B9H12-. The nucleophilic attack occurs at the 3-
or 6-position boron atom, since these are the most electron-
deficient borons in the carborane cage. Although the deboronation
proceeds in the case of m-carborane, the reaction rate is much
lower than that in the case of o-carborane.11,13 Recently, the
hydrogen-bonding character and CH-π interaction of C-H on
the o-carborane cage have been discussed in the field of
supramolecular chemistry.14 Participation of 2-C-H, however,
should be excluded in this case, because the hydrolysis rate of
(2-methyl-o-carboranyl)benzyl tosylate (4) was similar to, but
somewhat slower than that of 1 and the activation parameters
were similar to those of 1. These results can be interpreted in
terms of steric hindrance of the 2-methyl group to formation of
the transition state. The structure 17 would afford the six-
membered cyclic transition state 16, in which the oxygen on the
side of sulfonate may be placed on the opposite side (6-position).
Thus, the transition state may be highly stabilized, and a
considerable loss of entropy is involved, i.e. ∆Sq is negative. Then
elimination of sulfonic acid would give the retentive product. This
process is consistent with the alteration of the mechanism to an
SN1 process in the case of acetolysis of 1, because acetic acid is
too weak a nucleophile to interact with boron in the carborane
cage. Partial recemization in the hydrolysis can be interpreted in
terms of a contribution of the usual SN1 process via the
carbocation 18, as in the hydrolysis of 2 and 3.
We propose a mechanism involving a six-membered cyclic
structure 16 (intermolecular SNi mechanism) as shown in Scheme
1. The first step of the reaction is interaction between the oxygen
atom of nucleophile, which is hydrogen-bonded to sulfonate, and
the 3-position boron atom in the carborane cage (such as 17). It
is well-known that deboranation of o-carborane by nucleophiles
such as alkoxides,10 aliphatic amines,11 and fluoride anion12 affords
(8) The absolute configuration of 1 was determined by X-ray crystal-
lography of (+)-R-(1,2-dicarba-closo-dodecaboran-1-yl) p-bromobenzene-
sulfonate.
(9) Methanolysis of 2 and 3 at 60 °C was too slow to allow determination
of the kinetic parameters.
The exceptional kinetic features and retentive nucleophilic
substitution of (o-carboranyl)benzyl tosylate compared with other
carboranyl derivatives is noteworthy, and should be helpful for
analyzing the electronic bonding structure of the two cage carbons
in o-carboranes and for investigating fundamental theoretical
features of carboranes.
(10) Wiesboeck, R. A.; Hawthorne, M. F. J. Am. Chem. Soc. 1964, 86,
1642-1643.
(11) Hawthorne, M. F.; Wegner, P. A.; Stafford, R. C. Inorg. Chem. 1965,
4, 1675. Hawthorne, M. F.; Young, D. C.; Garrett, P. M.; Owen, D. A.;
Schwerin, S. G.; Tebbe, F. N.; Wegner, P. A. J. Am. Chem. Soc. 1968, 90,
862-868.
Supporting Information Available: Details of synthesis, spectral
data for compounds 1-14, and experimental procedures of the kinetic
examination (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
(12) Tomita, H.; Luu, H.; Onak, T. Inorg. Chem. 1991, 30, 812-815.
(13) Fox, M. A.; MacBride, J. A. H.; Wade, K. Polyhedron 1997, 16, 2499-
2507.
(14) Hardie, M. J.; Raston, C. L. J. Chem. Soc., Chem. Commun. 1999,
1153-1163.
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