Propeller rotation of aryl groups 717
in trimesityl-element systems. Whereas trimesitylmethane
exhibits a barrier of 21.9 kcal molꢀ1,1 the barrier for trime-
sitylsilane is ¾12 kcal molꢀ1 and that for trimesitylstannane
is below our range of observation (<¾8 kcal molꢀ1). As orig-
inally pointed out by Finocchiaro et al.,1 this reduction is
probably the result of the longer M—aryl bond lengths in
the order C—C < C—Si < C—Sn (Finocchiaro et al.1 did
not discuss tin). As the aryl rings become more distant from
the central atom, the methyl substituents at the ortho posi-
tions are further from the analogous groups on the adjacent
aryl rings.
It is interesting that the first alkyl group to be examined
in this context, allyl, has a lower barrier than chloro for both
mesityl and duryl systems. Both coalescence temperature
and barrier are slightly lower. Certainly the vinyl portion of
the allyl group can be positioned away from the other groups
attached to the central atom. Apparently the C—H bonds
that then of necessity must point towards the respective
groups on either side must not increase congestion around
the central atom, compared with chloro.
The effect of duryl compared with mesityl has been
discussed by Pinkus and Custard.5 The meta methyls serve to
enhance steric crowding around the ortho positions through
buttressing. Although direct meta–meta interactions between
adjacent rings should be small and not contribute to the
aryl—M barrier, the presence of the meta methyls eliminates
or diminishes angle distortions by the ortho methyl groups
that might relieve ortho–ortho interactions. With enhanced
crowding, the aryl—M barrier increases by ¾2 kcal molꢀ1
for the chlorosilanes (X D Cl) and the allylsilanes (X D allyl).
The buttressing effect does not, however, operate in the
same way for the hydrogen cases (X D H). One possible
explanation is that the small size of X allows the ortho
methyl group to distort towards H. The meta methyl pushes
the ortho methyl towards H, thereby reducing inter-ring
ortho–ortho interactions and the barrier to aryl rotation. Since
this deformation does not occur for mesityl, the ortho–ortho
interaction is higher.
There are not many systems in which the barrier can
be measured at two different temperatures. The clear
differences in coalescence temperatures (Table 1) translate
into small differences in G‡, modulated by the values
of ꢀ. These differences imply that S‡ is non-zero for
these systems. Unfortunately, two-point kinetics yield very
poor results. It is possible to approximate S‡ to the range
ꢀ15 to ꢀ35 cal molꢀ1 Kꢀ1. Aside from being negative, these
numbers have errors that are too large to permit further
interpretation.
and duryl, suggesting that this spherical group offers no
opportunity for steric reduction through M—X rotation. The
anisotropic allyl group has several different faces that can
allow the group to alter its steric interactions.
EXPERIMENTAL
Variable-temperature NMR experiments were carried out on
a Varian INOVA spectrometer for which the frequency of 1H
was 500 MHz. The temperature of the probe was calibrated
by measurement of peak positions of the methanol and
ethylene glycol standards.
Tridurylsilane (4)
A 200 ml, three-necked, round-bottomed flask, equipped
with a rubber septum, a condenser and a glass stopper, was
charged with pieces of Na (2.4 g, 0.105 mol), bromodurene
(6.39 g, 0.03 mol), dry benzene (75 ml) and a stirring bar.
The flask was given an N2 atmosphere and HSiCl3 (1 ml,
0.01 mol) was added from a syringe. The mixture was heated
to reflux and stirred overnight. The dark blue mixture was
cooled and filtered through a Celite pad. The resulting yellow
solution was concentrated by rotary evaporation. The dark
yellow residue was crystallized from hexane to give white
crystals: 1.2 g, 28%; 1H NMR (CDCl3), υ 2.24 (s, 18H), 2.37 (s,
18H), 5.93 (s, 1H), 7.18 (s, 3H); 13C NMR (CDCl3), υ 20.4, 21.3,
133.2, 134.0, 137.9, 140.8; 29Si NMR (CDCl3), υꢀ40.3.
Chlorotridurylsilane (5)
A 100 ml, round-bottomed flask was charged with tridurylsi-
lane (1.49 g, 3.5 mmol), PCl5 (1.12 g, 5.4 mmol), CCl4 (30 ml)
and a magnetic stirring bar. The mixture was heated at
reflux under N2 for 36 h. The resulting yellow solution was
concentrated by rotary evaporation, and the residue was
dissolved in 50 ml of hexane. Methanol (15 ml) was added
slowly to decompose the unreacted PCl5. The organic layer
was separated, dried (MgSO4) and concentrated by rotary
evaporation to give a yellow solid. The solid was crystallized
from hexane to produce a white powder: 1.25 g, 77%; 1H
NMR (CDCl3), υ 2.18 (s, 18H), 2.22 (s, 18H), 7.05 (s, 3H); 13
C
NMR (CDCl3), υ 20.7, 22.4, 133.5, 133.7, 134.7, 139.3, 139.9,
141.l; 29Si NMR, υ ꢀ 2.7. Anal. Calcd for C30H39SiCl, C 77.80,
H 8.49; found, C 77.85, H 8.45%.
Allyltridurylsilane (6)
A 100 ml, round-bottomed flask fitted with a rubber septum
was charged with allyltriphenylstannane (4.27 g, 11.0 mmol)
and a stirring bar. Anhydrous tetrahydrofuran (25 ml) and
(quickly) phenyllithium (1.8 M, 6.1 ml, 11.0 mmol) in diethyl
ether–cyclohexane were added via a syringe. After 30 min,
the suspension was transferred under N2 through a wide-
bore cannula to an enclosed glass frit and filtered into a 100 ml
flask containing 1.50 g (3.2 mmol) of chlorotridurylsilane.
The resulting dark red solution was stirred at room
temperature for 3 days. The then yellow reaction mixture
was quenched with H2O and extracted twice with hexane.
The organic portion was dried (MgSO4) and concentrated by
rotary evaporation. The residue was chromatographed over
neutral alumina with hexane as eluent to give a white solid:
In summary, barriers to aryl—M rotation in Ar3MX
systems decrease in the order C > Si > Sn for the central
atom M. When X is large (allyl or Cl), buttressing from the
meta methyls raises the barrier for duryl in comparison with
mesityl. When X is small (H), the reverse occurs, whereby
the duryl barrier is lower than the mesityl barrier. For H,
buttressing reduces the barrier through angle deformation
that lowers ortho–ortho interactions. Because buttressing
varies with the substituent X, the barriers depend in
a complex fashion on the substituent. Nonetheless, the
chloro systems exhibit the highest barriers for both mesityl
Copyright 2001 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2001; 39: 714–718