Conformational studies by dynamic NMR. 71. Stereodynamics of triisopropyl(aryl)silanes in solution and in the solid state

Article abstract of DOI:10.1021/jo991580p

A study has been carried out of the conformations of triisopropyl(aryl)silanes (i-Pr)₃SiAr, (Ar = phenyl, 1-naphthyl, and 2- naphthyl) both as to the orientation of the three isopropyl groups and the conformation about the silicon-aromatic bond

Full text of DOI:10.1021/jo991580p

J . Org. Chem. 2000, 65, 1729-1737
1729
Con for m a tion a l Stu d ies by Dyn a m ic NMR. 71.1 Ster eod yn a m ics of
Tr iisop r op yl(a r yl)sila n es in Solu tion a n d in th e Solid Sta te
J . Edgar Anderson*
Chemistry Department, University College, Gower Street, London WC1E 6BT, UK
Daniele Casarini
Chemistry Department, University of Basilicata, Potenza, Italy
Lodovico Lunazzi* and Andrea Mazzanti
Department of Organic Chemistry “A.Mangini”, University of Bologna, Risorgimento, 4,
Bologna 40136, Italy
Received October 11, 1999
A study has been carried out of the conformations of triisopropyl(aryl)silanes (i-Pr)3SiAr, (Ar )
phenyl, 1-naphthyl, and 2-naphthyl) both as to the orientation of the three isopropyl groups and
the conformation about the silicon-aromatic bonds. The report comprises dynamic NMR studies
of conformational interconversions in solution and in the solid state as well as molecular mechanics
calculations. The barriers for the stereomutation processes measured in the crystalline state were
found significantly higher than those in solution.
Sch em e 1. Con for m a tion of a n Isop r op yl Gr ou p
in (i-P r )3X-Y. Tor sion An gle H-C-X-Y
Deter m in es th e La bel
In tr od u ction
When three secondary groups, such as isopropyl, are
attached to the same tetrahedral central atom, their
interaction leads to interesting ranges of conformations
and interconversion processes.2 Each isopropyl group is
sterically anisotropic, resembling a methyl group on
approach from the methine side and a tert-butyl group
on approach from the side of the geminal methyl groups.
In compounds such as (i-Pr)3X-Y, the interactions of
isopropyl groups with each other and with X and Y, which
largely determine the energy of conformations, should
depend on the length of C-X and X-Y bonds (which can
be quite different as X and Y change), on the C-X-C
and C-X-Y bond angles, and on the conformations along
individual i-Pr-X bonds, which determine the confronta-
tion between isopropyl groups. The steric demands of
group Y, if it is large, have a significant effect on
conformations.
Sch em e 2. Seven Con for m a tion s of (i-P r )3X-Y
Th a t Ar e Not Degen er a te n or En a n tiom er ic
The conformation of any one isopropyl group with
respect to the rest of the molecule may be one of the three
kinds conveniently defined by the torsion angle of the
methine proton with respect to the X-Y bond. These are
anti “a”, +gauche “g”, and -gauche “-g” and are shown
in Scheme 1. Torsion angles may often be quite different
from 60° and 180°, if such a distortion reduces the
interactions of groups. The overall molecule can then be
represented as in the idealized structures I-VII (Scheme
2) which make no attempt to indicate possible distortions.
The labels beneath I-VII are self-explicatory, the iso-
propyl group conformations being reported in the order
left, central, right.
With three staggered conformations for each of three
X-CH bonds there are 33 ) 27 conformation types for
(1) (a) Part 70: Casarini, D.; Lunazzi, L.; Mazzanti, A.; De Lucchi,
O. Fabris, F. J . Org. Chem. 2000, 65, 883-888. (b) Part 69: Anderson,
J . E.; Casarini, D.; Lunazzi, L.; Mazzanti, A. Eur. J . Org. Chem. 2000,
479.
(2) Anderson, J . E.; Casarini, D.; Lunazzi, L. J . Org. Chem. 1996,
61, 1290.
the overall molecule, but because of degenerate and
enantiomeric forms, structures I-VII represent all pos-
sibilities. The (a, g, g) conformation I, for example, is
found as degenerate forms (g, a, g) and (g, g, a), and also
10.1021/jo991580p CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/01/2000

1730 J . Org. Chem., Vol. 65, No. 6, 2000
Anderson et al.
Sch em e 3. Exa m p le of a Con for m a tion III
(a , g, -g) Ha vin g th e P la n a r Su bstitu en t P la ced
in a Ch a n n el F or m ed by F ou r Isop r op yl Meth yl
Gr ou p s
as three enantiomeric versions (a, -g, -g), (-g, a, -g)
and (-g, -g, a), that is six in all, so when talking of the
(a, g, g) conformation we include these other five versions
if that is appropriate. Even if such conformations were
so stable as to be the only kind populated, there would
still be a complex dynamic equilibrium within each
degenerate set or between two enantiomeric sets of
conformations. Structure I, V, and VI each exist as two
enantiomeric sets, each containing three degenerate
forms. Structures II and III both exist as three degener-
ate forms; there are two enantiomeric forms of IV,
whereas structure VII is unique: thus the 27 expected
conformations.
It will be noted that anti conformations maximize
parallel 1,3-interactions particularly of methyl groups,
round the middle of the molecule near Y (see conforma-
tion VII) while gauche conformations balance such com-
pression in the middle, and remote from Y at the bottom
of the molecule (see conformations IV and V). The size
of Y might therefore affect the population of such
conformations for individual isopropyl groups, and of
course rotation away from perfect staggering of i-Pr-X
bonds may reduce interactions.
Triisopropylmethane (i-Pr)3CH may be taken as a
model for such compounds, and it exists3 as a rapid
equilibrium between (a, g, g) and (g, g, g) types of
conformation, viz. I and IV. Vicinal coupling constants
suggest that there is about three times as much of the
former conformation, so allowing for its higher degen-
eracy, the two conformations are of very similar energy.
Molecular mechanics calculations favor the former by
only 0.21 (MM2) or 1.07 (MM3) kcal mol-1. There is a
barrier calculated to be about 3 kcal mol-1 to the
interconversion of I and IV and thus, because of the high
symmetry of the latter, to interconversion of degenerate
forms of I. There is an experimentally measured (NMR)
barrier of 6.6 kcal mol-1 to interconversion of I and IV
with the [(a, -g, -g) + (-g, -g, -g)] enantiomeric set.
Similar results have been reported4 for tricyclohexyl-
methane for which a crystal structure confirms that (a,
g, g) conformation and its enantiomer are preferred in
the solid state.
We now report investigations of three triisopropyl-
(aryl)silane compounds:
(i-Pr)3Si-Ar
Ar ) Ph (1); Ar ) 2-naphthyl (2); Ar ) 1-naphthyl (3)
All four carbon-silicon bonds are markedly longer than
the corresponding carbon-carbon bonds, so the interac-
tions of isopropyl groups with Y and with each other
should be reduced. We have already reported1b a study
of aliphatic triisopropylsilyl compounds (i-Pr)3Si-Y, where
Y ) H, Cl, t-Bu and found that the preferred conforma-
tion is (a, g, -g). Barriers to isopropyl rotation are indeed
low in these compounds so that in the case of (i-Pr)3Si-
Cl, for instance, isochronous NMR signals were observed
even at -165 °C in solution. Only in the low temperature
CP-MAS NMR solid state spectra could this rotation be
rendered sufficiently slow for an experimental observa-
tion.
As an additional piece of information we refer to the
Cambridge Crystallographic Data Base9 where nine
structures with a triisopropylsilyl group attached to an
sp2-hybridized atom were retrieved. Eight of these adopt
the (a, g, -g) conformation III, with the planar system
placed in a channel formed by four of the methyl groups
(as shown diagramatically in Scheme 3), with only one
example preferring the (a, a, g) conformation VI.
Triisopropylethane (i-Pr)3C-CH3 shows behavior par-
ticularly relevant to the present work, having a group Y
that is more sterically demanding.5 The preferred con-
formation still appears to be (a, g, g), but now the (g, g,
g) conformation, which places three methyl groups in the
proximity of the substituent Y, is much less stable. As
the temperature is lowered, two sets of changes were seen
in the NMR spectrum, and at the lowest temperature six
separate signals were detected for isopropyl methyl
groups5 in contrast to two signals for (i-Pr)3CH.3 The two
sets of changes correspond first to slowing the intercon-
version of degenerate conformations [(a, g, g) to (g, a, g)
to (g, g, a)] via the (g, g, g) conformation, and second to
slowing the interconversion of enantiomeric conforma-
tions [(a, g, g) to (a, -g, -g) to etc.] via the (a, a, g) and
(a, a, -g) conformations, with barriers of 6.8 and 6.4 kcal
mol-1, respectively.5 Other compounds that were studied
are triisopropylamino,2,6,7,8 and tris(diethylamino)methane.2
Resu lts a n d Discu ssion
Tr iisop r op yl(p h en yl)sila n e (1) a n d Tr iisop r op yl-
(2-n a p h th yl)sila n e (2). The 75.5 MHz 13C NMR spec-
trum of PhSi(Pri)3 (1) in CF2Cl2/CF3Br displays two sharp
lines for the Me and CH carbons down to -130 °C. Below
-150 °C these lines broaden and eventually split, at -166
°C (Figure 1), with the methyl carbons yielding three
equally intense lines, and the aliphatic methine carbons
yielding a pair of lines, with a 2:1 intensity ratio. The
lines of the phenyl carbons, on the contrary, always
remain sharp.
Computer line shape simulation of both the Me and
aliphatic CH signals yields essentially the same rate
constant (72 s-1 at -158 °C), which corresponds to a ∆Gq
value of 5.5 kcal mol-1
.
(7) Boese, R.; Bla¨ser, D.; Antipin, M. Y.; Chaplinski, V.; de Meijere,
A. Chem. Commun. 1998, 781.
(8) Bock, H.; Go¨bel, I.; Bensch, W.; Solouki B. Chem. Ber. 1994, 127,
347.
(9) Allen, F. H.; Davies, J . E.; Galloy, J . J .; J ohnson, O.; Kennard,
O.; Maccrae, C. F.; Mitchell, E. M.; Smith, J . M.; Watson, D. G. J .
Chem. Inf. Comput. Sci. 1991, 31, 187 (“J uly 1997 CSD Release”).
(3) Anderson, J . E.; Koon, K. H.; Parkin, J . E. Tetrahedron 1985,
41, 561.
(4) Columbus, I.; Biali, S. E. J . Org. Chem. 1993, 58, 7020.
(5) Anderson, J . E.; Bettels, B. R. Tetrahedron 1990, 46, 5353.
(6) Bock, H.; Go¨bel, I.; Havlas, Z.; Liedle, S.; Oberhammer Angew.
Chem., Int. Ed. Engl. 1991, 30, 187.

Stereodynamics of Triisopropyl(aryl)silanes
J . Org. Chem., Vol. 65, No. 6, 2000 1731
Sch em e 4. Com p u ted MM3 Str u ctu r es of th e
P r efer r ed Con for m er s of 1 (left) a n d 2 (r igh t)
(i.e., the one having the two gauche CH bonds pointing
toward the anti substituent as shown in Scheme 4, left)
is indeed the most stable form and also PM3 and AM1
calculations12 confirm that. The corresponding three ϑ
values in Table 1 represent the three dihedral angles
C1-Si-C-H made by each of the CH bonds of the
isopropyl moieties with the Si-Ph bond. In this confor-
mation two CH carbons are in practice enantiotopic,
owing to the low-energy libration processes, having an
averaged angle of |58 ( 7°|, i.e., the mean between |+64|
and |-51°| (Table 1). This conformation also accounts for
the existence of three pairs of methyl groups, as experi-
mentally observed, because the methyl carbons of the
isopropyl group having the CH bond in the position anti
are obviously enantiotopic. The two methyl carbons of
the isopropyl group having the C-H bond in the position
+gauche are diastereotopic, but each of them is sym-
metry-related to the corresponding methyl of the isopro-
pyl group having the CH bond in the position -gauche.
The calculations also indicate that this (a, g, -g) confor-
mation has the plane of the phenyl ring perpendicular
to the molecular plane of symmetry (i.e., the dihedral
angle C2-C1-Si-CHanti is about 90°, as shown in
Scheme 4, left). Thus, even if the Ph-Si rotation is slow
on the NMR time scale at low temperature, the ortho and
meta carbons will remain enantiotopic. The other com-
puted energy minima of Table 1 do not bear the sym-
metry corresponding to the experimental spectrum, with
the exception of the other (a, -g, g) conformation (i.e.,
the one with the gauche C-H bonds pointing away from
CH bond anti, as in conformer II of Scheme 2) which,
however, has a relative energy much too high (1.8 kcal
mol-1) to be appreciably populated (in this conformation
the phenyl ring is coplanar, rather than orthogonal with
respect to the molecular plane of symmetry).
F igu r e 1. Temperature dependence of the 13C (75.5 MHz)
aliphatic signals of 1.
Ta ble 1. MM3 Rela tive En er gies (E, in k ca l m ol-1) of
P h -Si(i-P r )3, 1. th e Dih ed r a l An gles (T) Ar e Th ose
betw een th e C-H Isop r op yl a n d th e P h en yl-Si Bon d s
conformation
E
ϑ1
ϑ2
ϑ3
(a, g, -g)
(a, a, g)
(g, -g, -g)
(a, -g, g)
(a, g, g)
0
179
-166
76
64
-51
58
-48
76
90
51
-155
1.23
1.67
1.82
1.95
2.81
6.01
-153
-46
-77
51
53
146
180
- 179
(g, g, g)
(a, a, a)
41
-175
To explain the spectral multiplicity of the 13C spectrum
at low temperature the molecule must adopt a conforma-
tion where one methine carbon is diastereotopic with
respect to the remaining two enantiotopic methine car-
bons. A possible conformation of this type is that having
one C-H bond anti to the Si-Ph bond, with the other
two C-H bonds gauche, their corresponding H-C-Si-
Ph dihedral angles being opposite in sign, as in confor-
mations (a, g, -g) or (a, -g, g).
The absence of line broadening for the lines of the
phenyl ring carbons can be explained in two possible
ways: either the Ph-Si rotation is still fast at -166 °C,
or the phenyl group is locked in a position where the
ortho and meta carbons remain equivalent.
Two rotation pathways about the (Me)2CH-Si bond are
available for the two types of isopropyl groups in the most
stable (a, g, -g) conformation, and the corresponding
barriers were computed (in the MMX framework) by
driving the C1-Si-C-H angles ϑ in steps of 10°, whereas
allowing all the other parameters to relax. Two maxima
of energy were computed for the rotation of the isopropyl
having the CH bond in the anti position, their values
being 4.0 (when ϑ ) 0°) and 5.6 kcal mol-1 (when ϑ )
115°). The rotation process for the other two isopropyl
Molecular mechanics calculations (MMX10 and MM311
force field) found the seven expected conformational
types, which with their degenerate or enantiomeric
versions make up the 27 possible staggered arrange-
ments. As shown in Table 1 the (a, g, -g) conformation
(10) Program PC Model, Serena Software, Bloomington, IN.
(11) Allinger, N. L.; Yu, Y. H.; Lii, J .-H. J . Am. Chem. Soc. 1989,
111, 8551.
(12) MOPAC package computer program.

1732 J . Org. Chem., Vol. 65, No. 6, 2000
Anderson et al.
groups (having the C-H bonds in the gauche positions)
also yielded two barriers: 5.4 and 6.0 kcal mol-1, when
ϑ ) 0° and ) 115°, respectively. To achieve a complete
rotation, the isopropyl groups have thus to overcome a
barrier of 5.6 (anti) and of 6.0 (gauche) kcal mol-1; these
values are so close as to be indistinguishable, and their
average (5.8 ( 0.2 kcal mol-1) matches very well the
experimental result (5.5 ( 0.2 kcal mol-1).
As mentioned above, it is impossible to know whether
the phenyl ring is still in rapid rotation or whether its
motion is slow on the NMR time scale since, owing to
the symmetry, the spectral multiplicity is the same in
both cases. To decide between these two situations, we
investigated the triisopropyl(2-naphthyl)silane, 2, where
the degeneracy has been eliminated by substituting the
phenyl with the 2-naphthyl moiety, which has essentially
the same steric requirements. MMX and MM3 calcula-
tions confirmed that the most stable form of 2 is still the
(a, g, -g) conformation and that the plane of the naphthyl
ring is also orthogonal to the Si-C-Hanti plane. Thus, if
the rotation of the 2-naphthyl ring becomes slow on the
NMR time scale, the two gauche isopropyl groups would
display two different signals, yielding, as a whole, three
methine carbon lines. Likewise, the three methyl carbons
signals of derivative 1 will become six in the case of 2
(Scheme 4, right). The aliphatic signals of the spectrum
of 2 at -165 °C were essentially equal to those 1 at the
same temperature, a clear indication that the aryl-Si
rotations are still rapid when the i-Pr-Si rotation has
become slow. Also the barrier for the process, which
makes equivalent the isopropyl signals of 2 (5.5 ( 0.2
kcal mol-1), is the same as in 1.
F igu r e 2. Aliphatic region of the solid-state CP-MAS spec-
trum (75.5 MHz) of 2 as function of temperature. The Me
signal, which a single line at +24 °C, splits into a 1:2 pair of
lines at -63 °C.
1-naphthyl group [(i-Pr)3Si-Ar, with Ar ) 1-naphthyl,
3], the spatial requirements are greatly modified, and
different conformations undergoing different stereody-
namic processes are expected to occur.
The solid-state CP-MAS spectrum of crystalline 2 at
room-temperature yields a single line for both the methyl
and the methine carbons. On lowering the temperature
the methyl line broadens and splits, at -65 °C, into two
signals with an integrated intensity of 1:2 (Figure 2). As
the width of the more intense signal is markedly broader
than the other, the spectrum must be interpreted as due
to three lines, two of which are not resolved, indicating
that the preferred conformer has the same symmetry (a,
g, -g) as in solution. Contrary to the methyl, the methine
carbons signal remains a single line at low temperature
because the expected 2:1 doublet, which was observed in
solution, has, in the solid, a shift difference smaller than
the line width. The absence of additional splitting with
respect to the solution spectrum indicates that also in
the solid state the naphthyl-Si bond rotation is still rapid
in the NMR time scale, at least at -65 °C. Computer
simulation of the methyl signals at about -15 °C yields
Dyn a m ic NMR in Solu tion . All the 13C lines of 3
broaden on lowering the temperature below -120 °C and
subsequently sharpen, eventually displaying two groups
of signals at -165 °C, corresponding to a pair of conform-
ers. At this temperature the isopropyl methine carbons
yield a pair of 2:1 signals in the range 12-14 ppm and
three 1:1:1 signals in the range 16-18 ppm (Figure 3,
bottom left trace), their relative proportion being about
0.9:1, respectively.14 The single methyl line is likewise
split into a number of overlapping peaks, that appear as
six signals, with a 1:1:2:3:3:2 intensity ratio as suggested
by computer simulation. The presence of two almost
equally populated conformers is confirmed by the spec-
trum of the aromatic region, where three lines are also
split into pairs of nearly equally intense peaks at -165
°C.
q
a ∆G value of 13.0 ( 0.3 kcal mol-1. The dynamic
The patterns of the aliphatic CH signals at -165 °C
indicate that one of the two conformers is wholly asym-
metric, thus chiral, having three equally intense lines,
whereas the other conformer comprises a plane of sym-
metry, since it displays two lines with a 2:1 intensity
ratio. Molecular mechanics calculations predicted a very
large number of energy minima for 3, due to the presence
of the 1-naphthyl moiety, which allows many more
possible dispositions for the isopropyl groups, with re-
spect to the aromatic substituent, than those available
in the case of 1 or 2. A number of these forms, including
all the most stable, are reported in Table 2, where they
are labeled according to Scheme 5.
process of 2 in the solid state has therefore a barrier
much higher than that (5.5 kcal mol-1) measured in
solution, this being a consequence of the resistance to
molecular motions occurring in the crystal lattice, as
observed in a number of other cases.13
Tr iisop r op yl(1-n a p h th yl)sila n e (3). When the 2-
naphthyl is replaced by the more sterically demanding
(13) Miller, R. D.; Yannoni, C. S. J . Am. Chem. Soc. 1980, 102, 7396.
Elguero, J .; Fruchier, A.; Pellegrin, V. J . Chem. Soc., Chem. Commun.
1981, 1207. Riddell, F. G.; Arumagam, S.; Anderson, J . E. J . Chem.
Soc., Chem. Commun. 1991, 1525. Lambert, J . B.; Xue, L.; Howton, S.
C. J . Am. Chem. Soc. 1991, 113, 8958. Barrie, P. J .; Anderson, J . E.;
J . Chem. Soc. Perkin Trans. 2 1992, 2031. Riddell, F. G.; Arumagam,
S.; Harris, K. D. M.; Rogerson, M.; Strange, J . H. J . Am. Chem. Soc.
1993, 115, 1881. Riddell, F. G.: Cameron, K. S.; Holmes, S. A.; Strange,
J . H. J . Am. Chem. Soc. 1997, 119, 7555. Casarini, D.; Lunazzi, L.;
Mazzanti, A. J . Org. Chem. 1998, 63, 9125.
(14) The ratio changes slightly at higher temperatures, eventually
becoming 1.1:1 at -143 °C.

Stereodynamics of Triisopropyl(aryl)silanes
J . Org. Chem., Vol. 65, No. 6, 2000 1733
Ta ble 2. MM3 Rela tive En er gies (E, in k ca l m ol-1) for
th e Eigh t Mor e-Sta ble Con for m er s of Ar -Si(i-P r )3 (Ar )
1-n a p h th yl, 3). Con for m er s w ith En er gies High er th a n
1.9 k ca l m ol-1 a r e n ot Rep or ted , Excep t (g, g, g) a n d (a , a ,
a ). Th e Nu m ber in g is th a t of Sch em e 5. Th e Dih ed r a l
An gles (T) Ar e Th ose betw een th e C-H Isop r op yl a n d th e
Na p h th yl-Si Bon d s
region). The corresponding methyl signals should appear
as a 2:2:2 triplet, and this too is compatible with the
spectrum observed at -165 °C (Figure 3). Conformers 3a
and 3b are both asymmetric and either should give rise
to a 1:1:1 CH triplet, as observed at -165 °C (Figure 2,
16-18 ppm region), or to six equally intense methyl
signals, which are again compatible with the methyl
signals observed at the same temperature. It is notable
that 3a and 3b differ only in the θ3 torsion angles (their
values being -176° and 44°, respectively, as in Table 2).
As a consequence their interconversion requires rotation
of only isopropyl no. 3 (Scheme 5) with no significant
adjustment of the other torsion angles. Indeed MM3
calculations suggest a barrier of only 4.8 kcal mol-1 for
this process. Therefore, it is conceivable to propose that
the above-mentioned aliphatic lines corresponds to the
weighted average of the signals of 3a and 3b.15
conformation
E
Ra
ϑ1
ϑ2
ϑ3
(g, -g, a) 3a
(g, -g, g) 3b
(a, -g, g) 3c
0
0.23
0.36
-15
-5
0
49
49
180
-75
-79
-80
-176
44
80
(a, -g, a) 3d
(g, a, a) 3e
(a, -g, -g) 3f
(a, -g, a) 3g
(g, -g, g) 3h
1.35
1.56
1.57
1.69
1.86
-8
-14
-16
-22
-12
-175
54
-169
-163
50
-79
136
-83
-74
-80
-145
177
-76
-155
-91
(g, g, g)
(a, a, a)
4.2
6.0
15
-45
35
-131
66
172
63
-155
a
For the meaning of the angle R see Scheme 5.
The dynamic process appearing in the spectrum at
temperatures higher than -165 °C shows a quite unex-
pected feature. The three methine carbon lines at 16-
18 ppm broaden and coalesce (at -149 °C) into a single
signal (17.0 ppm), and the two lines at 12-14 ppm do
likewise at about the same temperature, to give a single
line at 13.4 ppm. On further raising the temperature,
these two single signals begin to broaden again and only
above -135 °C coalesce into an unique, sharp CH line at
15.1 ppm. These features point out the presence of
independent dynamic processes, each with its own energy
of activation which, although quite similar, are undoubt-
edly distinct. One such process exchanges the isopropyl
groups within the structure which gives rise to a triplet
(rate constant ka), and a second process exchanges the
isopropyl groups within the structure which gives rise
to a 2:1 doublet (rate constant ka′), whereas the third
process reflects exchange between those two types of
structures (rate constant kb), i.e., the triplet as a whole
and the doublet as a whole. In principle ka values are
not equal to ka′ values, and indeed slightly different sets
for such values were employed in the simulation (Figure
3, right). From this interpretation a barrier with a ∆Gq
of 5.7 kcal mol-1 corresponds to the rate constants ka, a
∆Gq ) 5.9 kcal mol-1 corresponds to ka′, and the highest
barrier (∆Gq ) 6.25 kcal mol-1) corresponds to the rate
constants kb.
Sch em e 5. Sch em a tic View of Ar -Si(i-P r )3 (Ar )
1-n a p h th yl, 3) a lon g th e Ar -Si Bon d . C1, C2, C3
Id en tify th e CH Ca r bon s (see Ta ble 2). P 1, P 2, P 3
Cor r esp on d to th e Sp a tia l P osition s w ith Resp ect
to a F ixed Na p h th a len e Rin g
Sch em e 6. Com p u ted MM3 Str u ctu r es of th e
Th r ee Mor e Sta ble Con for m er s of 3
In the aromatic region each conformer only has a single
line for each naphthyl carbon, so that the two nearly
equally populated conformers can yield only the single
barrier for their mutual interconversion, rather than the
three values afforded by the much more complex aliphatic
region. Of the 10 aromatic signals only three turned out
to have shift differences large enough to display the line-
broadening effects due to a dynamic process. The tem-
perature dependence observed for the line of the quater-
nary carbon at the furthest downfield field is reported
in Figure 4. It is quite evident, however, that such a
signal splits in an asymmetric manner (the upfield line
at 138.8 ppm is much broader than the downfield one at
140.05 ppm), yet at the lowest temperature, where the
exchange process has become slow, the two lines are
equally sharp. This is typical of a major species exchang-
ing with a substantially shifted minor one and indeed in
There are three conformations (3a , 3b, 3c) with very
similar energies (they lie within 0.36 kcal mol-1, as
reported in Table 2) that are much more stable than all
the others (even the fourth next stable conformation 3d
has an energy more than 1 kcal mol-1 higher).
As shown in Scheme 6, conformation 3c has a plane of
symmetry so that a 2:1 doublet is expected for the
corresponding methine carbons, in agreement with the
signals observed at -165 °C (Figure 3, 12-14 ppm
(15) Unless the exchanging lines have an extremely high shift
difference, exchange broadening cannot be detected when the barrier
is as low as 5 kcal mol-1. As we shall discuss later, such a large
separation only occurred for
a single aromatic line, so that this
assumption eventually found experimental support.

1734 J . Org. Chem., Vol. 65, No. 6, 2000
Anderson et al.
F igu r e 3. Left: Temperature dependence of the 13C (75.5 MHz) aliphatic signals of 3. Right: computer simulation of the methine
carbon signals obtained with the rate constants (in s-1) reported (see text for the meaning of ka, ka′, and kb).
the spectrum at -165 °C an additional line at 133.95 ppm
(relative intensity about 15%) was detected. This line
disappears when the temperature is increased by a few
degrees. Computer line shape simulations (Figure 4) were
obtained with rate constants (ka) corresponding to a
barrier of 5.7 kcal mol-1 (a value which turns out to be
equal to the lowest barrier determined in the aliphatic
region) and with a second set of rate constants (kc)
ent, conformer 3c exists in three degenerate forms (a1,
-g2, g3), (g1, a2, -g3) and (-g1, g2, a3) which are labeled
3c, 3c′, and 3c′′, respectively. Likewise there are degen-
erate forms 3a ′ and 3a ′′ for 3a and 3b′ and 3b′′ for 3b. 16
Scheme 7 shows how scrambling may be achieved in
the case of 3c. Rotation of each isopropyl group in turn,
accompanied at some point by a naphthyl-Si rotation,
interconverts 3c into 3c′, and an analogous set of
rotations leads either 3c or 3c′ to 3c′′. Even the most
stable of the intermediate conformations (i.e., 3e) is too
high in energy (see Table 2) to be significantly populated;
thus all of them are experimentally invisible. When such
a process if fast in the NMR time scale, each carbon
would spend an equal time in the anti, +gauche, -gauche
environments, so that a single averaged signal will be
seen. On this pathway, conformers 3a or 3b have never
been visited.
corresponding to an even lower barrier of 5.4 kcal mol-1
.
The latter process (which is that due to the exchange of
the small line at 133.95 ppm with its major companion
at 138.8 ppm) could be observed only in this case (and
not in the case of the aliphatic lines, nor of other aromatic
lines) owing to the fortunate occurrence of an unusually
large shift separation (i.e., 366 Hz at 75.5 MHz).
It is now necessary to account for the observations that,
on raising the temperature to -149 °C, the rapidly
exchanging set (3a + 3b) and conformer 3c are able to
make their own CH carbons dynamically equivalent (each
thus giving a single CH signal) with constants ka and ka′
lower than that (kb) for interchanging with each other.
This requires that both the pair (3a + 3b) and 3c each
scramble the CH lines without losing their conforma-
tional identity. Such a scrambling requires not only
appropriate rotation of the isopropyl groups but also a
change of their positions with respect to the naphthalene
ring, which implies some rotation about the naphthyl-
Si bond. Even in the presence of all these motions, the
two conformations, nonetheless, must not interchange.
In the presence of a rapidly rotating naphthyl substitu-
Sch em e 7. P ossible P a th w a y for th e
In ter con ver sion of th e Th r ee Degen er a te F or m s
3c, 3c′, 3c′′ (∆Gq ) 5.9 k ca l m ol-1).
Th e Con for m er s in Squ a r e Br a ck ets Ha ve Too
High a n En er gy To Be Ap p r ecia bly P op u la ted
3c, (a1, -g2, g3) f [a1, a2, g3] f [a1, a2, -g3] f (g1, a2, -g3), 3c′
3c′, (g1, a2, -g3) f [g1, a2, a3] f [-g1, a2, a3] f (-g1, g2, a3), 3c′′
Scheme 8 shows how an analogous scrambling may be
achieved in the case of 3b. Again only a high energy, thus
experimentally invisible, intermediate (g, g, g) is visited.

Stereodynamics of Triisopropyl(aryl)silanes
J . Org. Chem., Vol. 65, No. 6, 2000 1735
Since, as mentioned above, 3a exchanges with 3b,
scrambling with 3a ′ and 3a ′′ may also follow the same
pathway. A single averaged signal can thus be obtained,
without ever visiting conformer 3c. Schemes 7 and 8 thus
explain how two single signals are observed at -149 °C
for the methine carbons.
barriers of 5.7, 6.6, and 6.7 kcal mol-1 for 3c, 3b, and
3a , respectively. The value for 3c is indeed almost equal
to that experimentally measured for the symmetric
conformer (5.9 kcal mol-1), and also the other two differ
by only 1 kcal mol-1 from that determined for the
asymmetric conformer (5.7 kcal mol-1). Naphthalene
rotation has its high energy point when one isopropyl
passes through the plane of the naphthalene at the peri-
position, and after examining a model of the starting
conformation, the reported barriers were calculated by
ensuring, quite reasonably, that the isopropyl group
rotates in concert, in the sense that keeps its methine
hydrogen, rather than a methyl group, near the peri-
hydrogen. The interconversion proposed in Scheme 8 is
readily modeled since it centers around the highly
symmetrical (g, g, g) conformation and only two torsion
angles change significantly. The barrier here is computed
to be 6.8 kcal mol-1 without naphthyl rotation and 6.6
kcal mol-1 with accompanying naphthyl rotation. If a
total view of all four rotors could be theoretically modeled,
the calculated barrier for such interconversion could only
be lower than 6.6 kcal mol-1, thus becoming even closer
Sch em e 8. P ossible P a th w a y for th e
In ter con ver sion of th e Th r ee Degen er a te F or m s
3b, 3b′, 3b′′ (∆Gq ) 5.7 k ca l m ol-1).
Th e Con for m er in Squ a r e Br a ck et Ha s Too High
a n En er gy To Be Ap p r ecia bly P op u la ted
3b, (g1, -g2, g3) f [(g1, g2, g3,] f (-g1, g2, g3), 3b′
3b′, (-g1, g2, g3) f [(g1, g2, g3,] f (g1, g2, -g3), 3b′′
Further support to this interpretation also comes form
the spectrum of the methyl groups. According to the
model of Scheme 7, when the three CH carbons of the
conformer 3c become equivalent, the corresponding six
methyl groups must also become equivalent. In fact each
isopropyl group in turn adopts the anti conformation, and
when this motion is rapid the methyls within each
isopropyl group become enantiotopic.
to the experimental values of 5.7 kcal mol-1
.
Isopropyl rotation, without concomitant naphthyl group
rotation, can be modeled straightforwardly. In the case
of the 3a into 3b interconversion mentioned above, a
simple rotation of one group achieves this smoothly with
a barrier of 4.8 kcal mol-1. The third dynamic process of
5.4 kcal mol-1 observed in the aromatic region might thus
be attributed to the slowing down of this process, which
would lead to separate signals of different intensities for
these conformers at -165 °C. This interpretation stems
from the fact that the observed barrier not only is lower
On the other hand the processes of Scheme 8, while
they make the methine carbons equivalent, still leave the
two methyls within any isopropyl group distinct, so that
two methyl signals are expected. This is because the local
plane of symmetry (i.e., the one bisecting the isopropyl
group) never coincides, even with fast rotation, with the
dynamic plane of symmetry of the molecule as a whole,
contrary to the case of 3c. The complex pattern of the
signals, due to nine overlapping methyl lines observed
at -165 °C, broaden and coalesce, at -149 °C, into three
lines, with an integrated intensity (checked by computer
simulation) of about 3:6:3 as shown in Figure 2. Thus
the intense central line (indicated by a triangle) corre-
sponds to the six equivalent methyl groups predicted by
the motion of Scheme 7 for the set of conformers 3c, 3c′,
3c′′. The two equally intense lines (indicated by squares)
each corresponds to three methyl groups, as predicted
by the motion of Scheme 8 for the set of conformers 3a
+ 3b (the line on the left of the major signal, being
sharper, appears taller than its equally intense broader
companion on the right). Only above -149 °C the inter-
conversion of 3c with the pair 3a + 3b becomes rapid
(having a barrier of 6.25 kcal mol-1) eventually leading
to a single line for the CH as well as for the CH3 carbons
(Figure 3).
Molecu la r Mech a n ics Ca lcu la tion s. MM3 calcula-
tions of bond rotations help to elucidate the conforma-
tional processes that give rise to the spectral changes of
compound 3. With four asymmetric rotors the possibili-
ties are very complex, but a few useful conclusions
emerge from these computations. Independent rotation
of the naphthyl group in 3 appears to have a barrier
much higher than any experimentally observed, the
lowest we could find being as high as 8.6 kcal mol-1 in
the case of conformer 3a . We were able, however, to
model a concerted rotation of the naphthyl and of one
isopropyl group from various conformational minima with
than the previous ones (i.e., 5.7, 5.9, and 6.25 kcal mol-1
)
but is also similar to the theoretically calculated value
(4.8 kcal mol-1) for the exchange of 3a with 3b. Any
alternative explanation would involve conformers calcu-
lated to be much less stable.
This interpretation also requires that the highest
barrier of 6.25 kcal mol-1 corresponds to the interchange
of the pair of conformers 3a + 3b with conformer 3c, with
the naphthyl group still rapidly rotating above -150 °C.
Accordingly, this same barrier of 6.25 kcal mol-1 should
be also obtained when monitoring the aromatic signals
in the same temperature range, because, in principle,
they too should display different shifts, even in the
presence of fast naphthyl rotation. On the other hand,
the highest barrier determined from the aromatic signals
could be detected only below -150 °C, and its value
turned out to be equal to the lowest of the barriers (5.7
kcal mol-1) determined from the aliphatic signals.17 This
apparent discrepancy is due to the fact that the shift
difference of the aromatic signals in conformer 3c and
in the set (3a + 3b) is initially quite small, being partly
averaged by the fast rotation of the naphthyl group. Only
when, on further cooling, the mentioned concerted pro-
cess renders the naphthyl rotation sufficiently slow does
the separation of some aromatic signals become large
(17) The aliphatic and aromatic spectral regions were simulta-
neously acquired at the same identical temperature. The differences
in the free energies of activation cannot be thus attributed to
differences in the determination of the relative temperatures. The only
uncertainities are those on the rate constants derived from the
theoretical fitting of the line shape which, given the quality of the
experimental spectra, was about (20%. Such uncertainity about the
rate constants leads to an error of (0.05 kcal mol-1 in the measurement
of the relative ∆Gq values at these temperatures.
(16) The degenerate forms are 3a ) (g1, -g2, a3), 3a ′ ) (a1, g2, -g3),
3a ′′ ) (-g1, a2, g3), and 3b ) (g1, -g2, g3), 3b′ ) (-g1, g2, g3), 3b′′ ) (g1,
g2, -g3). Of course also the enantiomers 3a * ) [-(g1), -(-g2), -(a3)]
and 3b* ) [-(g1), -(-g2), -(g3)] have three degenerate forms but in a
nonchiral medium this distinction is immaterial.

1736 J . Org. Chem., Vol. 65, No. 6, 2000
Anderson et al.
F igu r e 4. Left: Temperature dependence of the lowest field
quaternary carbon signal of 3. Right: computer simulation
obtained with the rate constants (in s-1) reported (see text for
the meaning of ka and kc).
F igu r e 5. Experimental 13C solid-state CP-MAS spectrum
(75.5 MHz) of the aliphatic region of 3 at -95 °C (bottom),
with the arrows identifying the CH lines. On the top is
reported the computer simulation obtained with asymmetric
and symmetric conformers in a 63:37 proportion (see text).
enough to display observable line-broadening effects
(even in these conditions, however, only three, out of ten
aromatic signals, have different shifts). This explains,
therefore, why the higher of the two barriers measured
in the aromatic region apparently corresponds to the
lowest, rather than to the highest, barrier determined
in the aliphatic region.
The line broadening due to the exchange process occurs
at much higher temperature (range -60 °C to 0 °C) than
in solution, indicating that, as for 2, the barrier for 3 has
substantially increased in the solid. The complexity of
the spectral patterns did not allow us to obtain a very
accurate simulation, so that it was impossible to ascertain
whether the two processes described in Schemes 7 and 8
also occur in the crystal. Nonetheless at -30 °C a ∆Gq
value of about 10.8 ( 0.5 kcal mol-1 could be estimated
for the interconversion barrier of the two conformers in
the solid state. The difference of about 5 kcal mol-1 with
respect to solution is the consequence of the crystal lattice
effects, as observed in many other instances. 13 Although
quite unlikely, there is also a small possibility that lattice
effects in the solid might render relatively more stable
conformations calculated to be less stable according to
Table 2, yet having a symmetry compatible with the
multiplicity observed at low temperatures.
Solid -Sta te NMR. The ambient temperature CP-MAS
solid-state spectrum of 3 displays aliphatic carbon signals
that broaden on cooling and eventually split, at -95 °C,
into a number of peaks that, similarly to what observed
in solution, belong to a pair of distinct conformers. In
particular the methine carbons display a 1:1:1 triplet
(indicated by the three bent arrows in Figure 5) for one
conformer and a single line (at 11 ppm) for the second
one, all surely identified by their disappearance in the
nonquaternary suppression (NQS) sequence. The triplet
can be assigned to one or the other of the asymmetric
conformers 3a and 3b. A mixture of these two in rapid
exchange, as in solution, seems less likely to occur in the
crystalline state and indeed the relative shift of the three
lines are different. The more intense upfield single line
is assigned to conformer 3c; the 2:1 doublet splitting
observed in solution (36 Hz at 75.5 MHz) is not resolved
in the solid, due to a broader (50 Hz) line width. Whereas
in solution the two types of conformers were almost
equally populated, in the solid state the intensity of the
asymmetric conformer has increased, the ratio now being
63:37. The slightly greater dispersion of the methyl
signals allowed to resolve seven out of the nine lines
expected for one asymmetric and one symmetric con-
former. The simulation of Figure 5 was in fact obtained
by superimposing on the four mentioned CH signals the
six Me peaks (one carbon each) of an asymmetric
conformer and the three methyl peaks (two carbons each)
of the symmetric conformer 3c, in the mentioned propor-
tion. The symmetry assigned to these conformers, on the
basis of the multiplicity of the methine carbon signals in
the solution spectrum, thus agrees with that derived from
the multiplicity of the methyl carbon signals in the solid
state.
Whereas a X-ray determination would have been most
helpful, it was impossible to obtain a structure for 3.
Despite the fact that good diffraction patterns were
observed, a unit cell could not be determined. This failure
is almost certainly due to the simultaneous presence of
two species in the crystal, as proved by the low-temper-
ature solid-state NMR spectrum.
Con clu sion s
Derivatives 1 and 2 adopt a single, symmetric confor-
mation (a, g, -g) where the aromatic rings still rotate
rapidly at any attainable temperature. The barrier for
the rotation of the isopropyl groups was found, in both
cases, to be 5.5 kcal mol-1 in solution, and 13.0 kcal mol-1
for 2 in the solid state. The more hindered derivative 3
displays two distinct 13C spectra at low temperature. The
symmetric is due to the (a, -g, g) conformation (3c), the
asymmetric to the rapidly interconverting pair of con-

Stereodynamics of Triisopropyl(aryl)silanes
J . Org. Chem., Vol. 65, No. 6, 2000 1737
Tr iisop r op yl(1-n a p h th yl)sila n e (3): 1H NMR (CDCl3,
300 MHz) δ 1.18 (d, 18H, J ) 7.6 Hz), 1.75 (septet, 3H, J )
7.6 Hz), 7.53 (m, 3H), 7.90 (m, 3H), 8.17 (m, 1H); 13C NMR
(CDCl3, 75.5 MHz) δ 13.93 (CH), 19.45 (Me), 125.85 (CH),
125.95 (CH), 126.19 (CH), 129.75 (CH), 129.82 (CH), 130.16
(CH), 134.15 (q), 135.06 (q), 136.34 (CH), 138.57 (q); MS (m/z)
284 (M+). Anal. Calcd for C19H28Si: C, 80.21; H, 9.92; Si, 9.87.
Found: C, 80.22; H, 9.90; Si, 9.84.
formers 3a and 3b, i.e., (g, -g, a) and (g, -g, g),
respectively. These assignments were based upon the
results of MM comparisons. Interconversion of the rapidly
exchanging pair 3a + 3b with 3c has a barrier of 6.25
kcal mol-1 in solution and 10.8 kcal mol-1 in the solid
state. Two other dynamic processes observed in solution
correspond to the concerted rotation of 1-naphthyl ring
with the isopropyl groups within conformer 3c (5.9 kcal
mol-1) and within the pair 3a + 3b (5.7 kcal mol-1). A
fourth dynamic process, due to the interconversion of 3a
with 3b, was also detected in the aromatic region, with
an even lower barrier of 5.4 kcal mol-1. Thus all the four
motions occurring in 3 have been experimentally detected
and their barriers determined.
NMR Mea su r em en ts. The samples for the low tempera-
ture determinations were prepared by connecting to a vacuum
line the NMR tubes containing the desired compounds and
condensing therein the gaseous CF2Cl2 and CBrF3 in about a
3:1 proportion. A few drops of TMS-d12 were added for
reference and lock. The tubes were subsequently sealed in
vacuo and introduced into the precooled probe of the 300 MHz
spectrometer operating at 75.5 MHz for 13C. The temperatures
were calibrated by substituting the sample with a precision
Cu/Ni thermocouple before the measurements. The high
resolution 13C NMR solid-state CP-MAS spectra were also
obtained at 75.5 MHz. The samples were packed in a zirconia
rotor spun at the magic angle with a speed in the range of
3-4 kHz. The chemical shifts were measured, by replacement,
with respect to the lower frequency signal of the adamantane
(29.4 ppm). The cooling was achieved by means of a flow of
dry nitrogen, precooled in a heat exchanger immersed in liquid
nitrogen. Line shape simulations of the solution and solid-state
spectra were obtained with a PC computer program based
upon the DNMR 6 routines.19
Exp er im en ta l Section
Ma ter ia l. Triisopropyl(aryl)silanes 1-3 were prepared
according to the following general procedure. To an aryllithium
solution, prepared by adding n-butyllithium (16 mmol) to a
solution of the appropriate aryl bromide (15 mmol in 40 mL
of anydrous THF) kept at -78° was added the appropriate silyl
chloride (20 mmol in 5 mL of THF) at -78 °C. The mixture
was allowed to reach ambient temperature and after 1-2 h
was quenched with aqueous NH4Cl. The organic layers were
extracted (Et2O) and dried (Na2SO4), and the solvent was
evaporated at reduced pressure. The crude product was
purified by preparative TLC (SiO2) with pentane as eluent.
Tr iisop r op yl(p h en yl)sila n e (1):18 1H NMR (CDCl3, 300
MHz) δ 1.08 (d, 18H, J ) 7.5 Hz), 1.41 (septet, 3H, J ) 7.5
Hz), 7.35 (m, 3H), 7.48 (m,2H); 13C NMR (CDCl3, 75.5 MHz) δ
10.77 (CH), 18.57 (Me), 127.44 (CH), 128.48 (CH), 133.72 (q),
135.27 (CH); MS (m/z) 234 (M+). Anal. Calcd for C15H26Si: C,
76.84; H, 11.18; Si, 11.98. Found: C, 76.79; H, 11.23; Si, 11.95.
Tr iisop r op yl(2-n a p h th yl)sila n e (2): 1H NMR (CDCl3, 300
MHz) δ 1.10 (d, 18H, J ) 7.4 Hz), 1.50 (septet, 3H, J ) 7.4
Hz), 7.47 (m, 2H), 7.57 (dd, 1H, J ) 11.5 Hz, J ) 1.8 Hz), 7.83
(m, 3H), 7.99 (bs, 1H);13C NMR (CDCl3, 75.5 MHz) δ 11.75
(CH), 19.30 (Me), 126.32 (CH), 126.81 (CH), 127.10 (CH),
128.30 (CH), 128.77 (CH), 132.40 (CH), 133.24 (q), 133.57 (q),
134.15 (q),136.60 (CH); MS (m/z) 284 (M+). Anal. Calcd for
Ack n ow led gm en t. Thanks are due to Dr. J . A.
Steed (King’s College, London, UK) for several attempts
to obtain a crystal structure of 3 and to Dr. A. E. Aliev
and Mr. J . G. Outeirinho (University College, London,
UK) for helping, respectively, with the low temperature
CP-MAS spectra and with retrieving structures from the
Cambridge Crystallographic Data Base. Financial sup-
port has been obtained from MURST, Rome (national
project “Stereoselection in Organic Synthesis”) and from
the University of Bologna (Finanziamento d′ Ateneo
1997-1999).
C
19H28Si: C, 80.21; H, 9.92; Si, 9.87. Found: C, 80.18; H, 9.93;
Si, 9.95.
J O991580P
(18) Schlosser, M.; Choi, J . H.; Takagishi, S. Tetrahedron 1990, 46,
5633.
(19) QCPE program no. 633, Indiana University.