Rotational barriers of biphenyls having heavy heteroatoms as ortho-substituents: Experimental and theoretical determination of steric effects

Article abstract of DOI:10.1039/c1ob06688a

The free energies of activation for the aryl-aryl rotation of 17 biphenyl derivatives, bearing a heavy heteroatom (S, Se, Te, P, Si, Sn) as ortho substituent, have been measured by variable temperature NMR. These numbers, so called B values, represent a meaningful measure of the steric hindrance exerted by the selected substituents. DFT computations match quite satisfactorily the experimental barriers and the ground state geometries as well (determined, in two cases, by X-ray diffraction). The present values extend the available list of B values and thus provide an enlarged basis for the compilation of the space requirements of standard substituents, based solely on experimental determinations.

Full text of DOI:10.1039/c1ob06688a

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PAPER
Rotational barriers of biphenyls having heavy heteroatoms as ortho-
substituents: experimental and theoretical determination of steric effects†
Lodovico Lunazzi,a Michele Mancinelli,a Andrea Mazzanti,*a Susan Lepri,b Renzo Ruzziconi*b and
Manfred Schlosser*c
Received 5th October 2011, Accepted 14th November 2011
DOI: 10.1039/c1ob06688a
The free energies of activation for the aryl–aryl rotation of 17 biphenyl derivatives, bearing a heavy
heteroatom (S, Se, Te, P, Si, Sn) as ortho substituent, have been measured by variable temperature NMR.
These numbers, so called B values, represent a meaningful measure of the steric hindrance exerted by the
selected substituents. DFT computations match quite satisfactorily the experimental barriers and the
ground state geometries as well (determined, in two cases, by X-ray diffraction). The present values
extend the available list of B values and thus provide an enlarged basis for the compilation of the space
requirements of standard substituents, based solely on experimental determinations.
(which is obviously bulkier than chlorine) has, nonetheless, a
Introduction
smaller Avalue (0.47–0.49).2
In a different approach, Sternhell et al.6 assessed the torsional
An indicator often used for estimating the steric hindrance of
various groups is based upon the free energy difference (ΔG° in
kcal mol−1) between equatorially- and axially-substituted confor-
mers of cyclohexane.1,2 These quantities are known as A values
and cover a range of between 0.28 (fluorine) and 4.9 kcal mol−1
(tert-butyl).1c,2 A very large A value indicates that the proportion
of the axial conformer is so small that it is impossible to
measure directly the conformer ratio and to obtain a reliable
determination solely on an experimental basis. The investigators
thus resorted to an indirect determination by means of the so
called counterpoised method using disubstituted cyclohexanes
and assuming additivity of the A values.2–4 There are also nega-
tive A values, that occur when the axial is more stable than the
equatorial conformer, as in the case of organomercuric deriva-
tives of cyclohexane.5 In addition, it has to be pointed out that
other effects, besides the size of the substituent, play a role in
determining the ratio of these conformers. For instance chlorine
is reported2 to have an A value of 0.51–0.53 whereas iodine
barriers of ortho-substituted biphenyls by variable temperature
NMR. However, in order to detect decoalescence of diastereo-
topic groups in a temperature range accessible to their instrumen-
tation, they introduced two ortho-substituents and thus had to
postulate additivity of the repulsion caused by the two substitu-
ents in order to extract the individual steric parameters. But this
assumption was unwarranted: indeed when the rotation barrier of
mono-substituted ortho-biphenyl derivatives could be sub-
sequently measured,7 the experimental barriers were found to be
significantly lower than those deduced on the basis of the addi-
tivity assumption. For this reason we undertook the task of
measuring a number of these barriers (called B values): we have
been able, so far, to measure 29 such values,7–10 ranging from
the 4.4 kcal mol−1 for fluorine9 to the 18.1 kcal mol−1 for the
Me3N+ group8 (Table 1). As an example, this scale correctly
indicates that the B value of chlorine (7.7) is actually smaller
than that of the bulkier iodine (9.9)7a in agreement with the trend
of the Taft-type E’s steric parameters,11 as reported by Dubois
et al.12 (0.02 for chlorine and 0.50 for iodine).
aDepartment of Organic Chemistry “A. Mangini”, University of
Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy. E-mail:
andrea.mazzanti@unibo.it; Fax: +39 051 2093654; Tel: +39 051
2093632
In order to get information about the steric requirements of
groups containing a heavy heteroatom bonded to the ortho pos-
ition of biphenyls, we determined in the present work the B
values of substituents like –SiR3, –SnR3, –PR2, –SR, –SeR,
–TeR (R = alkyl or phenyl).
bChemistry Department, University of Perugia, Via Elce di Sotto 10,
I-06100 Perugia, Italy. E-mail: ruzzchor@unipg.it; Fax: +39 075
5855262; Tel: +39 075 5855543
cInstitute of Chemical Sciences and Engineering, Ecole Polytechnique
Fédérale (EPFL–BCh), CH-1015 Lausanne, Switzerland. E-mail:
manfred.schlosser@epfl.ch; Tel: +41 021 6939351
Results and discussion
†Electronic supplementary information (ESI) available: DFT calculated
structures for compounds 3 and 9, X-ray structure of 9. CCDC reference
numbers 849731–849732. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1ob06688a
To measure by NMR spectroscopy the rotational barrier of
ortho-substituted biphenyls it is necessary to place in the meta’-
position two enantiotopic groups that turn into diastereotopic
This journal is © The Royal Society of Chemistry 2012
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Table 1 Experimental B values (ΔG≠rot in kcal mol−1) and computed rotational barriers (ΔE≠rot in kcal mol−1) to aryl–aryl rotation of biphenyl
derivatives bearing a single ortho substituent7–10
ortho substituent
ΔG≠rot exper.
ΔE≠rot calc.
ortho substituent
ΔG≠rot exper
ΔE≠rot calc.
Me
Et
i-Pr
t-Bu
7.4
8.7
11.1
15.4
10.5
16.5
7.9
7.5
7.7
8.1
7.6
7.1
8.6
11.1
15.3
9.2
16.9
7.8
7.4
8.1
8.4
7.8
Br
I
OH
OMe
OCF3
OCH2OCH3
CHvCH2
CuCH
CuN
CHO
COOH
COOMe
MeCO
t-BuCO
8.7
10.0
5.4
5.6
5.5
5.7
8.2
6.0
5.9
10.2
7.7
7.7
8.0
6.7
8.5
9.9
5.3
4.5
4.8
6.1
8.5
5.3
5.2
11.0
8.5
8.3
8.3
7.2
CF3
C(CF3)2OH
CH2OH
C6H5
C6F5
NH2
NO2
NMe2
+NMe3
F
6.9
18.1
4.4
6.8
18.2
4.3
Cl
7.7
7.3
Table 2 Experimental B values (ΔG≠rot in kcal mol−1) and DFT
computed rotational barriers (ΔE≠rot in kcal mol−1) for the aryl–aryl
rotation of compounds 1–17a
groups when, below a certain temperature, the interconversion
for the two conformational enantiomers (atropisomers) by aryl–
aryl rotation becomes slow enough.7 The interconversion barrier
between these two enantiomers corresponds to the rotational
barrier (i.e. the B value) of the substituted biphenyl. A number
of diasterotopic probes in the 3′-position of 2-substituted biphe-
nyls were tested for this purpose: Me2CH,7 (i-Pr)3Si,8
i-PrMe2Si,8,10 (CF3)2COH9 and it was demonstrated that the
rotational barrier was found to remain the same, within the
experimental uncertainty of 0.15 kcal mol−1, regardless of
the choice of probe. For instance the barriers determined using
the isopropyl group as a probe were found7 to be 15.4, 8.75 and
9.9 kcal mol−1 for biphenyls bearing, as an ortho substituent,
t-Bu, Br and I, respectively. When the i-PrMe2Si probe was
employed, these values were found to be 15.5, 8.7 and 10.0 kcal
mol−1, respectively.8
ΔG≠rot
exper.
ΔE≠rot
Compd
R
calc.b
|θ|c
1
2
3
4
5
6
7
8
SCH3
SPh
SOPh
SO2Ph
SePh
8.6
8.3
8.6
12.8
9.1
9.9
9.1
11.8
9.4
8.4
8.8
125
125
55
8.1
10.3d
9.2
59
116
117
125
97
TePh
10.2
10.0
13.3
9.2
P(CH3)2
PO(CH3)2
PPh2
9
66
10
11
12
13
14
POPh2
10.2
11.8
12.7
10.4
12.1
11.3
9.5
12.9
10.0
11.2
56
61
59
115
115
P(C6H11)2
PO(C6H11)2
Si(CH3)3
Si
The present study relied exclusively on the isopropyldimethyl-
silyl group as the diastereotopicity probe for compounds 1–15
(listed in Table 2). The isopropyldimethylsilyl substituent, in
fact, can be incorporated in the biphenyl moiety by Suzuki–
Miyaura coupling of dimethyl(3-iodophenyl)isopropylsilane
with 2-bromophenylboronic acid. The resulting 2-bromo-3′-
(dimethylisopropylsilyl)biphenyl is the common precursor to
products 1–15 that were obtained by lithium/bromine permu-
tation with tert-butyllithium followed by the reaction with the
suitable electrophile. Both 2-benzenesulfinyl and 2-benzenesol-
fonyl derivatives were prepared from the corresponding 2-phe-
nylthio derivative by selective oxidation procedures. If the
substituent in the 2-position bears two enantiotopic groups, as in
compounds 16 and 17, then we need a second substituent in the
3′-position (chlorine in our case) to generate conformational
enantiomers. The silylation at the 2-position of a biphenyl
moiety using bulky trialkylchlorosilane is anything but straight-
forward. Thus, the 2-silyl derivatives, 16 and 17, were satisfac-
torily prepared from 2-bromo-3′-chlorobiphenyl in a one-pot
procedure consisting in the lithium/bromine permutation with
tert-butyllithium, followed by reaction of the resulting 2-lithiobi-
phenyl with dichlorodimethylsilane and, finally, addition of 2-
ethylmagnesium chloride and phenyllithium, respectively, to the
(biphenyl-2-yl)chlorodimethylsilane intermediate.
[CH(CH3)2]3
Sn(CH3)3
15
9.1
8.8
120
16
17
Si(CH3)2C2H5 9.9
Si(CH3)2Ph 9.8
10.2
10.9
116
63
a X = Si(CH3)2[CH(CH3)2] for compounds 1–15 and X = Cl for
compounds 16, 17. b See experimental section for details. c Aryl–aryl
computed twist angle (absolute value).d 13.2 kcal mol−1 at the CISD/6-
31+G(d) level.
A typical example of the variable temperature NMR exper-
iment is shown in Fig. 1, where the temperature dependence of
the silicon bonded methyl signal is displayed in the case of the
ortho substituted PhS derivative 2. The single methyl line broad-
ens on cooling, decoalesces at about −113 °C and splits into two
sharp signals separated by 8.5 Hz (at 600 MHz) at −128 °C.
The computer line shape simulations reported on the right
provide the rate constants for the aryl–aryl rotation process, cor-
responding to a free energy of activation (ΔG≠) of 8.3 kcal
mol−1 (i.e., B value). Table 2 lists the values obtained for the
compounds 1–17 investigated.
In the case of the sulfoxide 3, the situation is different from
that of the other compounds since the sulfur atom is itself a
chiral center. For this reason the silicon bonded methyls are
1848 | Org. Biomol. Chem., 2012, 10, 1847–1855
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Fig. 2 Left: temperature dependence of the silicon bonded 1H NMR
methyl signals of ortho phenyl sulfoxide derivative 3 in CHF2Cl/
CHFCl2 at 600 MHz. Right: line shape simulation obtained with the
indicated rate constants.
Fig. 1 Left: temperature dependence of the silicon bonded 1H NMR
methyl signal of ortho phenyl sulfide derivative 2 in CHF2Cl/CHFCl2 at
600 MHz. Right: line shape simulation obtained with the indicated rate
constants.
6-31G(d) level] indicate that the two conformational diastereo-
isomers generated by the restricted aryl–aryl rotation have an
energy difference of only 0.1 kcal mol−1, corresponding to a
58 : 42 population ratio at −120 °C, the diastereoisomer (R*),
(P*) being more stable. This ratio is in good agreement with the
experimental value, thus confirming that the measured barrier is
due to the aryl–aryl rotation. Calculations also explain why the
barrier for the aryl–aryl rotation in the sulfoxide 3 (8.6 kcal
mol−1) is essentially equal, within the experimental uncertainty,
to that of the sulfide 2 (8.3 kcal mol−1). In the transition state for
the aryl–aryl rotation the oxygen of sulfoxide points away from
the biphenyl moiety (see Fig. S1 of ESI†) so that its steric hin-
drance is analogous to that of the sulfide. In the case of sulfone
4, the reverse is true in that there is always at least one oxygen
pointing toward the biphenyl moiety and this explains the sub-
stantially higher barrier observed (12.8 kcal mol−1). This
interpretation is confirmed by the trend of the computed barriers
diastereotopic even at ambient temperature and thus display two
equally intense signals. At low temperature the aryl–aryl axis
behaves as a stereogenic axis and therefore two conformational
diastereoisomers, with different populations, are generated. In
fact, as shown in Fig. 2, at −120 °C two pairs of lines emerge
with a 55 : 45 intensity ratio. At intermediate temperatures
(between −50 and −110 °C) these lines are broadened by the
exchange process and the line shape simulation provides the cor-
responding rate constants.13
In principle the two mentioned diastereoisomers could be
alternatively generated by the restricted rotation around the aryl–
SO bond rather than around the aryl–aryl bond. In fact, the
effects due to the restricted aryl–SO bond rotation have been
detected in the low temperature NMR spectra of a number of
sulfoxides.14,15
showing that the theoretical value for 4 (10.3 or 13.2 kcal mol−1
,
In the present case, such a rotation would generate two differ-
ently populated rotamers having the sulfoxide oxygen either syn
or anti to the phenyl bearing the –SiMe2 Pr group. However cal-
depending on calculations) is indeed significantly higher than
those of 2 and 3 (see Table 2).
i
In order to corroborate the reliability of our computational
approach it would be instructive to compare the calculated par-
ameters with their experimental counterpart. This was accom-
plished in the case of phosphine oxide 10 which is a solid
compound and yielded appropriate single crystals. In Fig. 3 its
X-ray structure is displayed, together with the two structures
having the lowest energies, as computed by the DFT approach.
The latter differ by the relative position of the silicon atom with
respect to the oxygen of the phosphine oxide (syn or anti), but
culations indicate that the anti rotamer, having the aryl and the
CSO planes coplanar, is 1.8 kcal mol−1 more stable than the syn
rotamer, where aryl and the CSO plane are not coplanar, the
latter accounting for a population of only 0.3% at −120 °C. This
ratio is at variance with the observation of the 55 : 45 ratio, so
that the observed process cannot be attributed to the aryl–SO
rotation as only one of these rotamers would be expected to be
significantly populated. On the other hand, calculations [B3LYP/
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the B values measured for various OR groups (R = H, Me, CF3,
CH2OMe) that all lie in the very restricted range of 5.4–5.7
(Table 1). Indeed our present calculations indicate that the DFT
computed barrier (B value) for the OPh substituent (4.1 kcal
mol−1) is, as conceived, definitely lower than that computed for
SPh (8.8 kcal mol−1, as in Table 2). As a further evidence, the B
value determined for OMe (5.6, as in Table 1) is much smaller
than that measured for SCH3 (8.6, as in Table 2), due to the
smaller van der Waals radius of oxygen (1.52 Å) with respect to
sulfur (1.80 Å).18 According to computations, the phenyl of the
X-Ph groups (and likewise the R substituents of the various X-R
groups) point away from the ortho-hydrogens of the 3′ substi-
tuted ring and thus should not contribute significantly to the
steric effect. This would explain why such an effect, in this
series, is essentially determined by the X atom size rather than
by the C–X bond length of the substituent.
With the addition of the present measurements, as many as 46
steric parameters of this type (B values) become available (col-
lected in Table 1 and 2): this represents a quite large body of
values useful for estimating the steric effect of substituents based
solely on experimental determinations.
Fig. 3 Top: Structures, according to DFT computation, [B3LYP/6-
31G(d) level] of the two most stable conformers of compound 10. Syn
and anti descriptions refer to the relative position of the oxygen with
respect to the i-PrMe2Si group and (M),(P) descriptions refer to the
aryl–aryl chirality axis. Bottom: experimental structure according to
X-ray diffraction.
Experimental
they have exactly the same free energy when the zero point cor-
rection16 is applied to the total energy. The presence in the solid
state of only one of these two structures, corresponding to the
anti form, is probably due to the crystal field effect.15 As it
appears in Fig. 3 (right), computations reproduce very well the
experimental structure. A good agreement was also observed
when the structure of the most stable computed conformer of
phosphine 9 was compared with its X-ray structure (Fig. S2 of
ESI†). This indicates that these computations are quite reliable.
As already observed,7–10 B3LYP calculations are also able to
reproduce, rather satisfactorily, the experimental barriers (B
values) of compounds 1–17. In fact, the average deviation
between the computed and experimental barriers is only 0.6 kcal
mol−1, a value which is in line with the deviation previously
observed for the derivatives reported in Table 1.
General
All commercial reagents were used without further purification.
Starting materials were purchased from Aldrich-Fluka (CH-9479
Buchs). Air- and moisture-sensitive compounds were stored in
Schlenk tubes or Schlenk burettes. They were protected and
handled under an atmosphere of 99.995% pure nitrogen, using
appropriate glassware. Tetrahydrofuran and diethyl ether were
stored over potassium hydroxide pellets in the presence of
cuprous chloride, from which they were distilled, before being
redistilled from sodium wire in the presence of benzophenone.
The constant temperature of −75 °C was maintained by using
liquid nitrogen/butyl acetate baths. Ice baths were used for reac-
tions carried out at 0 °C. Melting points were corrected after the
thermometer calibration by authentic standards. H, 13C and 31P
1
NMR spectra were recorded at 400, 100.6 and 161.9 MHz,
respectively, in deuterochloroform solutions. Chemical shifts (δ)
are given in ppm by using tetramethylsilane and phosphoric acid
as internal standards. Mass spectra were obtained by electron
impact fragmentation at 70 eV ionization potential. The purity of
all final products was testified by elemental analyses (performed
by Dr. E. Solari of the Analytical Services of EPFL-ISIC and
the REDOX Company in Monza) and by gas chromatography
using two capillary columns of different polarity (30 m ×
0.35 mm × 0.25 μm DB 5MS [5% phenylmethylpolysiloxane]
Conclusions
The data collected in Table 1 and 2 reveal the trimethylammonio
group to be the bulkiest (B = 18.1) whereas tert-butyl is some-
what smaller (B = 15.4) and trimethylsilyl (B = 10.4) and tri-
methylstannyl (B = 9.1) are considerable smaller. Obviously the
longer the carbon–heteroatom bond, the farther away are the
sterically interfering methyl groups. The X–C bond lengths are
1.50, 1.54, 1.89, 2.15 Å when X is N+, C, Si and Sn, respect-
ively.17 Remarkably this trend is at variance with Taft’s type par-
ameters that rank tert-butyl (E’s = 1.43) below trimethylsilyl
(E’s = 1.79).12
and 30
m
×
0.35 mm
×
0.25 μm DB23 [50%
cyanopropylmethylpolysiloxane]).
In the case of the model compounds, 2, 5, 6, bearing a phenyl
group, the steric effect is essentially determined by the size of
the atom X and thus increases with the van der Waals radius, as
shown by the 8.3, 9.1, 9.9 B values for X = S, Se, Te, respect-
ively (Table 2), the radii being 1.80, 1.90, and 2.06 Å, respect-
ively.18 Although the B value for OPh is not yet available, the
corresponding steric effect is certainly even lower, as shown by
Product preparation
2-Bromo-3′-(dimethylisopropylsilyl)biphenyl, precursor of
products 1–15, was available from previous work.8
2-Bromo-3′-chlorobiphenyl. The precursor to products 16 and
17, was prepared according to the Suzuki–Miyaura19 protocol.
1850 | Org. Biomol. Chem., 2012, 10, 1847–1855
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Ethanol (20 mL), benzene (40 mL), 1-chloro-3-iodobenzene
(2.0 g, 8.4 mmol), 2.0 M aqueous potassium carbonate (6.5 mL),
tetrakis[triphenylphosphine] palladium(0) (0.16 g, 0.14 mmol)
were added consecutively to 2-bromophenylboronic acid (1.9 g,
9.5 mmol). The mixture was kept at reflux for 3 h. After cooling
and addition of water, the mixture was extracted with diethyl
ether (80 mL) and the organic phase was dried with sodium
sulfate. After the solvent was evaporated, chromatography of the
residue on silica gel (0.2 L, eluent, petroleum ether) provided the
J = 7.8 and 1.2 Hz, 1 H), 7.5 (m, 2 H), 7.36 (t, J = 7.5 Hz, 1 H),
7.34 (s, 1 H), 7.3 (m, 2 H), 7.2 (m, 3 H), 7.02 (d, J = 7.3 Hz, 2
H), 0.97 (s, 7 H), 0.25 (s, 3 H), 0.21 (s, 3 H). 13C NMR: δ
144.8, 143.4, 140.9, 139.2, 137.1, 134.9, 133.6, 130.7, 130.6,
130.3, 129.8, 128.7 (2 C), 128.4, 127.6, 125.5 (2 C), 123.8, 17.5
(2 C), 13.6, –5.4 (2 C). MS: m/z (%) 378 (M+, 1), 363 (3), 335
(100), 319 (20), 319 (20), 261 (37), 184 (15), 75 (12), 43 (3).
Analysis: calcd for C23H26OSSi (378.60) C 72.96, H 6.92;
found C 72.63, H 7.07.
1
pure product (2.041 g, 91%) as a colorless oil; H NMR: δ 7.4
3-(Dimethylisopropylsilyl)-2′-(phenylsulfonyl)biphenyl
(4).
(m, 1 H), 7.3 (m, 6 H), 7.22 (ddd, J = 8.0, 7.3 and 1.9 Hz, 1 H).
13C NMR: δ 142.6, 141.1, 133.7, 133.2, 131.1, 129.4, 129.2,
129.1, 127.7, 127.6, 127.4, 122.3. MS: m/z (%) 270 (M+ + 3,
23), 269 (M+ + 2, 12), 268 (M+ + 1, 91), 266 (M+ − 1, 72), 186
(6), 152 (100), 126 (9), 75 (15). Analysis: calcd for C12H8BrCl
(267.55) : C 53.87, H 3.01; found C 53.59, H 3.18.
Carbon tetrachloride (2.0 mL), acetonitrile (2.0 mL), water
(4.0 mL) sodium periodate (0.53 g, 2.5 mmol) and ruthenium tri-
chloride hydrate (about 0.1 mg, 0.05 mol%) were added con-
secutively to sulfide 2 (0.30 g, 0.83 mmol) and the suspension
was stirred at +25 °C for 1 h while a red precipitate formed.
Water (30 mL) and ether (30 mL) were added, the organic layer
was separated, washed with saturated sodium bicarbonate
(30 mL), then with brine (30 mL), and dried with sodium
sulfate. The solvent was evaporated at reduced pressure, and the
crude product was passed on silica gel (30 mL, 1 : 1 (v/v) diethyl
ether–petroleum ether mixture as the eluent). After solvent evap-
oration, the residue was kept for 3 h at +40 °C per 0.05 mmHg
3-(Dimethyisopropylsilyl)-2′-(methylthio)biphenyl
(1). At
−75 °C, tert-butyllithium (1.5 mmol) in hexanes (0.8 mL) and,
30 min later, dimethyl disulfide (0.14 g, 1.5 mmol) were added
to 2-bromo-3′-(isopropyldimethylsilyl)biphenyl (0.50 g,
1.5 mmol) in diethyl ether (5.0 mL) while stirring. The tempera-
ture was allowed to rise to +25 °C and, after solvent evaporation
at reduced pressure, followed by chromatography of the residue
on silica gel (30 mL, eluent, petroleum ether), 0.364 g (81%) of
1
leaving a colorless very viscous oil (0.314 g, 96%). H NMR: δ
8.46 (dd, J = 7.0 and 2.0 Hz, 1 H), 7.6 (m, 2 H), 7.43 (dt, J =
7.4 and 1.2 Hz, 1 H), 7.35 (tt, J = 7.2 and 1.5 Hz, 1 H), 7.2 (m,
6 H), 7.1 (m, 1 H), 7.02 (ddd, J = 7.6, 1.9 and 1.3 Hz, 1 H),
0.96 (s, 7 H), 0.18 (s, 6 H). 13C NMR: δ 142.6, 140.8, 139.6,
137.6, 137.2, 135.3, 133.1, 132.8, 132.7, 132.3, 130.5, 128.5,
128.1 (2 C), 127.6 (2 C), 127.5, 126.4, 17.5 (2 C), 13.5, –5.4 (2
C). MS: m/z (%) 394 (M+, 1), 379 (8), 351 (100), 337 (5), 226
(23), 211 (36), 195 (27), 167 (70), 152 (22) 109 (10), 77 (34),
43 (15). Analysis: calcd for C23H26IO2SSi (394.61) C 70.01, H
6.64; found C 69.89, H 6.66.
The same procedure described above for the synthesis of
thioethers 1 and 2 was employed to prepare compounds 5, 6, 7,
9, 11, 13 and 14 from the 2-bromo-3′-(dimethylisopropylsilyl)
biphenyl and an equimolar amount of the appropriate electrophi-
lic reagent.
1
a colorless viscous oil was collected. H NMR: δ 7.55 (s, 1 H),
7.5 (m, 1 H), 7.4 (m, 2 H), 7.3 (m, 2 H), 7.2 (m, 2 H), 2.34 (s, 3
H), 0.98 (bs, 7 H), 0.26 (s, 6 H). 13C NMR: δ 141.3, 139.6,
138.3, 137.2, 134.9, 133.0, 130.0, 129.5, 127.8, 127.3, 125.4,
124.7, 17.6 (2 C), 16.0, 13.8, −5.3 (2 C). MS: m/z (%) 300 (M+,
42), 285 (1), 257 (100), 241 (23), 227 (10), 197 (26), 128 (15),
59 (21). Analysis: calcd for C18H24SSi (300.53) C 71.94, H
8.05; found C 72.13, H 7.98.
3-(Dimethylisopropylsilyl)-2′-(phenylsulfenyl)biphenyl (2)
was prepared from the 2-bromo-3′-(dimethylisopropylsilyl)
biphenyl (1.0 g, 3.0 mmol) and phenyl disulfide (0.66 g,
3.0 mmol) analogously as described in the preceding paragraph.
Chromatography of the crude product on silica gel (60 g, eluent,
petroleum ether) afforded the pure product (0.946 g, 87%) as a
1
viscous opalescent oil. H NMR: δ 7.52 (bs, 1 H), 7.5 (m, 1 H),
3-(Dimethylisopropylsilyl)-2′-(phenylseleno)biphenyl
(5).
7.42–7.14 (m, 11 H), 0.94 (s, 7 H), 0.21 (s, 6 H). 13C NMR: δ
143.6, 139.6, 138.0, 136.0, 134.8, 134.5, 132.9, 131.8, 131.2 (2
C), 130.6, 129.5, 129.0 (2 C), 127.9, 127.1, 126.9, 126.8, 17.5
(2 C), 13.7, −5.4 (2 C). MS: m/z (%) 362 (M+, 6), 347 (2), 319
(100), 303 (11), 287 (9), 195 (21), 165 (13), 135 (23), 77 (11),
43 (9). Analysis: calcd for C23H26SSi (362.60) C 76.18, H 7.23;
found C 76.20, H 7.39.
From 2-bromo-3′-(dimethylisopropylsilyl)biphenyl (0.50 g,
1.5 mmol) and diphenyl diselenide (0.47 g, 1.5 mmol) a slightly
yellow oil (0.430 g, 70%) was obtained after chromatography of
1
the crude product on silica gel. H NMR: δ 7.55 (s, 1 H), 7.51
(d, J = 6.2 Hz, 1 H), 7.5 (m, 2 H), 7.4 (m, 2 H), 7.3 (m, 5 H),
7.2 (m, 2 H), 0.97 (s, 7 H), 0.25 (s, 6 H). 13C NMR: δ 143.6,
140.6, 138.3, 134.8 (2 C), 134.5, 133.1, 132.4, 131.6, 130.4,
130.1, 129.3 (2 C), 129.2, 127.9, 127.8, 127.3, 126.6, 16.7 (2
C), 13.7, −5.4 (2 C). MS: m/z (%) 410 (M+, 30), 367 (25), 289
(6), 195 (16), 165 (13), 135 (100), 77 (5), 43 (4). Analysis: calcd
for C23H26SeSi (409.50) C 67.46, H 6.40; found C 67.17, H
6.64.
3-(Dimethylisopropylsilyl)-2′-(phenylsulfinyl)biphenyl (3). N-
Chlorosuccinimide (0.12 g, 0.90 mmol) was added to 3-(isopro-
pyldimethylsilyl)-2′-(phenylsulfenyl)biphenyl
(0.30
g,
0.83 mmol) in methanol (5.0 mL) and the mixture was stirred
for 1 h at 0 °C and then 30 min at +25 °C. The solvent was evap-
orated under reduced pressure, diethyl ether was added (3.0 mL)
and the mixture was filtered to remove the insoluble succinimide.
After the solvent evaporation and chromatography of the residue
on silica gel (25 mL, diethyl ether–petroleum ether 1 : 1 v/v)
0.290 g (89%) of product as a colorless viscous oil was col-
3-(Dimethylisopropylsilyl)-2′-(phenyltelluro)biphenyl (6). The
reaction of 2-bromo-3′-(dimethylisopropylsilyl)biphenyl (0.50 g,
1.5 mmol) with diphenyl ditelluride (0.61 g, 1.5 mmol) gave a
pale orange oil (0.420 g, 61%), after chromatography of the
1
crude reaction product on silica gel. H NMR: δ 7.82 (dd, J =
1
lected. H NMR: δ 8.19 (dd, J = 7.9 and 0.9 Hz, 1 H), 7.60 (td,
7.8 and 0.9 Hz, 2 H), 7.5 (m, 2 H), 7.43 (t, J = 7.5 Hz, 1 H), 7.4
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(m, 2 H), 7.3 (m, 5 H), 7.05 (bt, J = 6.7 Hz, 1 H), 0.99 (bs, 7 H),
0.29 (s, 6 H). 13C NMR: δ 146.4, 142.9, 140.6 (2 C), 138.9,
134.2, 133.6, 133.5, 129.6 (2 C), 129.3, 128.7, 128.5, 128.1,
127.7, 126.8, 119.6, 114.9, 17.7 (2 C), 13.8, −5.3 (2 C). MS:
m/z (%) 460 (M++2, 21), 458 (M+, 19), 339 (7), 337 (6), 195
(12), 165 (10), 77 (5), 53 (3). Analysis: calcd for C23H26TeSi
(458.14) C 60.30, H 5.72; found C 60.51, H 5.85.
biphenyl (0.5 g, 1.5 mmol) and chlorodiphenylphosphine
(0.33 g, 1.5 mmol). Half volume (5.0 mL) of the ethereal sol-
ution of the crude reaction product was concentrated at reduced
pressure and passed through argon purged silica gel (30 mL) by
eluting with degassed petroleum ether. The product (0.296 g,
90%) was collected as white rhombic crystals, m.p. 95–96 °C.
1
CCDC ref. number: 849731. H NMR: δ 7.57 (bt, J = 7.7, 1 H),
7.3 (m, 15 H), 7.0 (m, 2 H), 0.85 (bs, 7 H), 0.05 (s, 6 H). 13C
NMR: δ 148.6 (d, J = 29 Hz), 140.8 (d, J = 6.0 Hz), 137.8 (d, J
= 12 Hz, 2 C), 137.7, 135.6 (d, J = 14 Hz), 135.2 (d, J = 4.4 Hz,
2 C), 134.3, 133.8 (d, J = 20 Hz, 4 C), 132.8, 130.2 (d, J = 5.7
Hz), 129.8, 128.5 (d, J = 30 Hz, 2 C), 128.3 (d, J = 7.4 Hz, 4
C), 127.2, 126.9, 17.6 (2 C), 13.5, −5.6 (2 C). 31P NMR: δ
−12.2. MS: m/z (%) 438 (M+, 60), 437 (100), 395 (35), 337
(32), 183 (55), 135 (39), 73 (39), 43 (29). Analysis: calcd for
C29H31PSi (438.19) C 79.41, H 7.12; found C 79.09, H 7.03.
2-Dimethylphosphino-3′-(dimethylisopropylsilyl)biphenyl (7).
tert-Butyllithium (0.5 mL, 1 mmol) was added to 2-bromo-3′-
(dimethylisopropylsilyl)biphenyl (0.33 g, 1.0 mmol) in diethyl
ether at −75 °C under argon atmosphere. After 30 min, phos-
phorus trichloride (0.090 mL, 0.14 g, 1.02 mmol) was added
and the temperature was allowed to rise to −40 °C. A dense
white precipitate formed. The mixture was allowed to react at
that temperature for 1 h while stirring before it was cooled at
−75 °C and methyllithium (2.1 mmol) in dimethoxymethane
(0.7 mL) was added. The cold bath was removed, the tempera-
ture was allowed to rise to 0 °C. After the white precipitate had
settled the supernatant ethereal solution was transferred under
argon in a second Schlenk tube and the solvent was evaporated
by argon bubbling through. GC-MS analysis of the resulting
orange viscous oil (0.16 g) diluted in argon saturated diethyl
ether showed the presence of the expected biphenyldimethylpho-
sphine together with 3-(dimethylisopropylsilyl)biphenyl (about
30%) and traces of the starting material. An aliquot (0.020 g) of
this mixture was used as such in dynamic NMR experiments.
The remaining amount was rapidly passed through 1.0 g of
argon purged silica gel allowing the eluted solution to percolate
under argon atmosphere into a 30 mL Schlenk tube. The solvent
was evaporated under vacuum (0.1 mmHg) leaving a slightly
[3-(Dimethylisopropylsilyl)biphenyl-2′-yl]diphenylphosphine
oxide (10). The crude 9 (5.0 mL) was treated with 30% aqueous
hydrogen peroxide (0.2 mL, 1.6 mmol) at +25 °C and after
5 min, the solvent was evaporated at reduced pressure. Chrom-
atography of the residue on silica gel (30 mL) by eluting with
1 : 4 diethyl ether–petroleum ether mixture allowed to collect the
product (0.297 g, 87%) as white rhombic crystals, m.
p. 112–113 °C. CCDC ref. number: 849732. 1H NMR: δ 7.6 (m,
5 H), 7.46 (ddd, J = 13, 7.8 and 1.1 Hz, 1 H), 7.3 (m, 10 H),
7.20 (dt, J = 7.3 and 1.2 Hz, 1 H), 7.04 (t, J = 7.3 Hz, 1 H), 0.90
(s, 7 H), 0.14 (s, 6 H). 13C NMR: δ 148.0 (d, J = 4 Hz), 139.6
(d, J = 4.5 Hz, 2 C), 137.4, 135.1, 134.2 (d, J = 12 Hz), 133.3,
132.8, 132.3, 132.2 (d, J = 9.9 Hz), 131.8 (d, J = 3.5 Hz), 131.5
(d, J = 9.4 Hz, 4 C), 131.1 (d, J = 3.5 Hz, 2 C), 130.7, 128.0 (d,
J = 12 Hz, 4 C), 126.4 (d, J = 12 Hz), 126.4, 17.6 (2 C), 13.4,
−5.4 (2 C). 31P NMR: δ 29.4. MS: m/z (%) 454 (M+, 2), 439
(3), 411 (100), 337 (32), 195 (39), 77 (26), 43 (52). Analysis:
calcd for C29H31OPSi (454.62) C 76.62, H 6.87; found C 76.64,
H 6.84.
1
yellow oil behind (0.052 g, 32%). H NMR: δ 7.59 (ddd, J =
9.7, 8.4 and 1.3 Hz, 1 H), 7.53 (bs, 1 H), 7.50 (bd, J = 7.4 Hz, 1
H), 7.4 (m, 3 H), 7.3 (m, 2 H), 1.10 (d, J = 3.9 Hz, 6 H), 0.90
(bs, 7 H), 0.26 (s, 6 H). 13C NMR: δ 141.1 (d, J = 5.6 Hz),
138.3 (d, J = 13 Hz), 137.9 (d, J = 15 Hz), 133.0, 132.7, 130.1
(2 C), 129.2, 128.1 (d, J = 20 Hz), 127.5, 127.3, 127.0, 17.8 (2
C), 14.3 (d, J = 14 Hz, 2 C), 14.0, −5.1 (2 C). 31P NMR: δ
−50.0. MS: m/z (%) 314 (M+, 37), 313 (100), 299 (2), 271 (33),
213 (8), 183 (11), 135 (14), 73 (13). Analysis: calcd for
C19H27PSi (314.48) C 72.57, H 8.65; found C 72.74, H 8.83.
2-Dimethylphosphinoyl-3′-(dimethyisopropylsilyl)biphenyl
(8) was prepared by adding a slight excess of hydrogen peroxide
to an ethereal solution of the above biphenyldimethylphosphine
7 (0.050 g, 0.16 mmol). After the solvent evaporation at reduced
pressure, chromatography of the crude product on silica gel
(eluent diethyl ether) afforded a colorless viscous oil (0.033 g,
Dicyclohexyl[3-(dimethylisopropylsilyl)biphenyl-2′-yl]phosphine
(11) was analogously prepared from the 2-bromo-3′-(dimethyl-
isopropylsilyl)biphenyl (0.50 g, 1.5 mmol) and chlorodicyclo-
hexylphosphine (0.35 g, 1.5 mmol). Half of the ethereal solution
of the crude product (5.0 mL) was concentrated at reduced
pressure and passed through argon purged silica gel (30 mL) by
eluting with degassed petroleum ether to obtain a colorless
1
viscous oil (0.189 g, 60%). H NMR: δ 8.1 (m, 1 H) 7.5 (m, 4
H), 7.3 (m, 3 H), 1.7 (m, 10 H), 1.5 (m, 4 H), 1.2 (m, 8 H), 0.96
(bs, 7 H), 0.25 (s, 6 H).). 13C NMR: δ 142.8, 140.1, 138.2,
137.2 (d, J = 8.3 Hz), 134.9, 133.0 (2 C), 131.2, 129.5, 128.5,
127.2 (d, J = 18 Hz), 126.3, 38.0 (d, J = 31 Hz, 2 C), 28.1 (d, J
= 17 Hz, 2 C), 26.8, 26.0 (4 C), 25.9 (2 C), 25.6, 17.5 (2 C),
13.6, −5.3 (2 C). 31P NMR: δ 20.40. MS: m/z (%) 450 (M+, 54),
449 (100), 407 (8), 367 (30), 325 (11), 183 (32), 73 (22), 59
(21), 55 (15). Analysis: calcd for C29H43PSi (450.71) C 77.28,
H 9.62; found C 77.08, H 9.92.
1
63%). H NMR: δ 8.19 (ddd, J = 13, 6.7 and 2.3 Hz, 1 H), 7.5
(m, 3 H), 7.44 (bs, 1 H), 7.41 (t, J = 7.3 Hz, 1 H), 7.3 (m, 2 H),
1.36 (d, J = 13 Hz, 6 H), 0.95 (bs, 7 H), 0.26 (s, 6 H). 13C
NMR: δ 144.6, 144.5 (d, J = 31 Hz), 140.4 (d, J = 3.3 Hz),
138.8, 134.8, 133.6, 132.2 (d, J = 8.0 Hz), 131.1, 131.0 (d, J =
9.9 Hz), 129.9, 127.3 (d, J = 11), 127.2, 18.9 (d, J = 71 Hz, 2
C), 17.5 (2 C), 13.6, −5.4 (2 C). 31P NMR: δ 35.81. MS: m/z
(%) 330 (M+, 0.5), 329 (1), 315 (3), 287 (100), 271 (2), 213
(33), 183 (16), 165 (7), 73 (3). Analysis: calcd for C19H27OPSi
(330.48) C 69.05, H 8.23; found C 69.35, H 8.50.
Dicyclohexyl[3-(dimethylisopropylsilyl)biphenyl-2′-yl]phosphine
oxide (12). Aqueous (30%) hydrogen peroxide (0.10 mL,
0.88 mmol) was added to the second portion of the above ethe-
real solution of 11 (5.0 mL) and 5 min later the solvent was
evaporated. Chromatography of the crude product on silica gel
[3-(Dimethylisopropylsilyl)biphenyl-2′-yl]diphenylphosphine
(9) was prepared from 2-bromo-3′-(dimethylisopropylsilyl)
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(eluent, 1 : 4 (v/v) diethyl ether–petroleum ether mixture) gave
0.22 g (65%) of a colorless viscous compound. H NMR: δ 8.1
trimethyltin chloride (0.30 g, 1.5 mmol). A portion of the crude
1
product was purified by HPLC on a Kromasil C18 column (250 ×
1
(m, 1 H), 7.49 (bd, J = 7.3 Hz, 1 H), 7.4 (m, 2 H), 7.3 (m, 1 H),
7.25 (bs, 1 H), 7.1 (m, 2 H), 1.6 (m, 10 H), 1.3 (m, 6 H), 1.0 (m,
6 H), 0.92 (bs, 7 H), 0.20 (s, 6 H). 13C NMR: δ 147.9 (d, J = 7.0
Hz), 143.9 (d, J = 5.9 Hz), 141.5, 138.5, 134.3 (d, J = 5.5 Hz),
133.4 (d, J = 4.0 Hz, 2 C), 131.1 (d, J = 9.6 Hz), 130.4, 129.5
127.2 (d, J = 3.0 Hz), 127.0 (d, J = 15 Hz), 37.9 (d, J = 65 Hz,
2 C), 26.3 (2 C), 26.2 (6 C), 25.5 (2 C), 17.5 (2 C), 13.8, −5.3
(2 C). 31P NMR: δ 49.6. MS: m/z (%) 466 (M+, 11), 423 (100),
383 (81), 341 (26), 301 (4), 283 (8), 183 (62), 75 (26), 55 (23).
HRMS calcd for C29H43OPSi 466.2821; found 466.2825.
Analysis: calcd for C29H43OPSi (466.71) C 74.63, H 9.29;
found C 74.06, H 9.51.
10, 5 μm) using acetonitrile as mobile phase. H NMR: δ 7.5 (m,
1 H), 7.42 (dt, J = 7.3 and 2.9 Hz, 1 H), 7.36 (bs, 1 H), 7.3 (m, 4
H), 7.26 (dt, J = 7.7 and 1.6 Hz, 1 H), 0.9 (m, 7 H), 0.19 (s, 6 H),
−0.10 (s, 9 H). 13C NMR: δ 150.7, 144.5, 142.2, 138.7, 136.2,
134.1, 132.7, 129.3, 128.7, 128.2, 127.3, 126.3, 17.5 (2 C), 13.7,
−5.3 (2 C), −8.1 (3 C). MS: m/z (%) 418 (M+, 1), 403 (68), 287
(24), 195 (24), 180 (17), 165 (29), 73 (100), 59 (30), 43 (3).
HRMS: calcd for C20H30SiSn 418.1139; found 418.1143; calcd
for C19H27SiSn (M − Me) 403.0904; found 403.0905.
3-Chloro-2′-(dimethylethylsilyl)biphenyl (16). At −75 °C,
tert-butyllithium (1.9 mmol) in hexanes (1.1 mL) was added to
2-bromo-3′-chlorobiphenyl (0.50 g, 1.9 mmol) in diethyl ether
(5.0 mL). After 30 min, dichlorodimethylsilane (0.17 mL,
1.9 mmol) and ethylmagnesium chloride (0.94 mL, 1.9 mmol)
were added by a glass syringe and the mixture was allowed to
react 1 h before the cold bath was removed allowing the tempera-
ture to rise to +25 °C. Water (50 mL) was added and the mixture
was extracted with diethyl ether (3 × 20 mL). The collected
organic phases were dried with Na2SO4 and the solvent was
evaporated at reduced pressure. Chromatography of the residue
on silica gel (eluent, petroleum ether) allowed to recover 0.34 g
3-(Dimethylisopropylsilyl)-2′-(trimethylsilyl)biphenyl (13). At
−75 °C, tert-butyllithium (0.4 mL, 0.64 mmol) was added to 2-
bromo-3′-(isopropyldimethylsilyl)biphenyl (0.21 g, 0.63 mmol)
in THF (5 mL). After 5 min, chlorotrimethylsilane (0.12 mL,
0.96 mmol) was added with a glass syringe and the mixture was
kept 1 h before the cold bath was removed and the temperature
was allowed to rise to +25 °C. Water (20 mL) was added, the
mixture was extracted with diethyl ether (3 × 10 mL) and the
collected organic extracts were dried with Na2SO4. After solvent
evaporation at reduced pressure, chromatography of the residue
on silica gel (eluent, petroleum ether) allowed to collect 0.150 g
(73%) of the product as a colorless oil. 1H NMR: δ 7.63 (dd, J =
7.1 and 1.4 Hz, 1 H), 7.49 (dt, J = 7.3 and 1.2 Hz, 1 H), 7.41
(broad s, 1 H), 7.4 (m, 3 H), 7.3 (m, 2 H), 0.9 (m, 7 H), 0.24 (s,
6 H), −0.02 (s, 9 H); 13C NMR: δ 149.5, 143.6, 138.5, 138.0,
134.7, 134.5, 132.6, 129.7, 129.5, 128.4, 126.9, 126.2, 17.6 (2),
13.8, 0.51(3), −0.52 (2). MS: m/z (%) 326 (M+, 10), 311 (8),
283 (11), 195 (100), 126 (18), 73 (60). Analysis: calcd for
C20H30Si2 (326.62) C 73.54, H 9.26; found C 73.49, H 9.35.
1
(66%) of the expected product as a colourless oil. H NMR: δ
7.63 (dd, J = 6.4 and 1.3 Hz, 1 H), 7.3 (m, 5 H), 7.2 (m, 2 H),
0.86 (t, J = 7.9 Hz, 3 H), 0.52 (q, J = 7.9 Hz, 2 H), 0.01 (s, 6
H). 13C NMR: δ 147.7, 146.2, 137.3, 135.1, 133.4, 129.5, 129.2,
128.9, 128.5, 127.4, 127.1, 126.6, 8.4 (2), 7.4, −1.8. MS: m/z
(%) 259 (M+ − 15, 8), 245 (100), 229 (52), 209 (30), 165 (27).
Analysis: calcd for C16H19ClSi (274.86) C 69.92, H 6.97; found
C 69.90, H 6.86.
3-Chloro-2′-(dimethylphenylsilyl)biphenyl (17). At −75 °C,
tert-butyllithium (1.9 mmol) in hexanes (1.1 mL) was added to
2-bromo-3′-chlorobiphenyl (0.50 g, 1.9 mmol) in diethyl ether
(5 mL). After 30 min, dichlorodimethylsilane (0.17 mL,
1.9 mmol) and phenyllithium (1.0 mL, 1.9 mmol) were added
with a glass syringe and the mixture was allowed to react 1 h
before the cold bath was removed allowing the temperature to
rise to +25 °C. After the usual work up, chromatography of the
residue on silica gel (eluent, petroleum ether) gave the expected
product as colorless needle shaped crystals, 0.477 g (79%), m.
p. 77–78 °C (after crystallization from petroleum ether). 1H
NMR: δ 7.66 (dd, J = 6.9 and 1.4 Hz, 1 H), 7.3 (m, 8 H), 7.18
(dd, J = 6.9 and 1.2 Hz, 1 H), 7.13 (t, J = 7.7 Hz, 1 H), 7.02 (s,
1 H), 6.95 (d, J = 7.6 Hz, 1 H), 0.24 (s, 6 H). 13C NMR: δ
148.0, 145.6, 139.4, 136.5, 135.7, 133.7 (2 C), 133.4, 129.5,
129.4, 129.0, 128.8, 128.7, 127.7 (2 C), 127.5, 127.0, 126.7,
−1.2 (2 C). MS: m/z (%) 322 (M+, 3), 307 (58), 229 (100), 165
(2), 155 (9). Analysis: calcd for C20H19ClSi (322.90) C 74.39, H
5.93; found C 74.42, H 6.00.
3-(Dimethylisopropylsilyl)-2′-(triisopropylsilyl)biphenyl (14).
At −75 °C tert-butyllithium (0.4 mL, 0.64 mmol) was added to
a
solution of 2-bromo-3′-(isopropyldimethylsilyl)biphenyl
(0.21 g, 0.63 mmol) in diethyl ether saturated with lithium per-
chlorate (5 mL). After 5 min, chlorotriisopropylsilane (0.27 mL,
1.3 mmol) was added with a glass syringe and the mixture was
allowed to react 1 h before the cold bath was removed and the
temperature was allowed to rise to +25 °C. After 12 h, water was
added, the organic layer was separated and the aqueous layer
was extracted with diethyl ether (3 × 20 mL). The collected
organic phases were dried with sodium sulfate and the solvent
was evaporated at reduced pressure. Chromatography on silica
gel allowed to recover the main product 3-(dimethylisopropylsi-
lyl)biphenyl (0.122 g, 75%) and 0.042 g (11%) of the expected
1
product as a colorless oil. H NMR: δ 7.6 (m, 1 H), 7.4 (m, 6
H), 7.2 (m, 1 H), 0.96 (m, 28 H), 0.22 (s, 6 H). 13C NMR:
(CD3CN): δ 150.3, 144.4, 137.7, 136.8, 134.7, 133.9, 132.9,
131.0, 130.3, 128.3, 126.9, 126.0, 18.9 (6 C), 17.1 (3 C), 13.7,
12.6 (2 C), −6.0 (2 C). MS: m/z (%) 367 (M+ − C3H7, 38), 281
(4), 223 (13), 195 (26), 126 (25), 101 (100), 73 (76), 59 (30).
HRMS: calcd for C26H42Si2 410.2825; found 410.2821.
Variable temperature NMR measurements
Variable temperature NMR spectra were recorded with a Varian
INOVA spectrometer operating at a field of 14.4 Tesla (600 MHz
for 1H). Spectra of compound 4 were recorded in CDCl3, spectra
of 8 and 12 in CD2Cl2. When the temperature had to be
3-(Dimethylisopropylsilyl)-2′-(trimethylstannyl)biphenyl (15)
was prepared analogously as the disilylated biphenyl 14 from 2-
bromo-3′-(isopropyldimethylsilyl)biphenyl (0.50 g, 1.5 mmol) and
This journal is © The Royal Society of Chemistry 2012
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decreased below −100 °C, the NMR tubes containing the com-
pound were manipulated at a vacuum line. First a small amount
(approx. 0.05 mL) of hexadeuterobenzene (or acetone-d6 in the
case of 7) was introduced by means of a microsyringe for
locking purposes. The NMR tube was immersed in liquid nitro-
gen and evacuated in order to condense about 0.45 mL of chloro-
difluoromethane (Freon 22) and about 0.15 mL of
dichlorofluoromethane (Freon 21) transferred as gases from
lecture bottles (samples of compounds 9, 10, 11 and 17 were pre-
pared using CDFCl2 as a single solvent). The tubes were sub-
sequently sealed under reduced pressure (0.01 mbar) using a
methane/oxygen torch. After a few hours at ambient temperature,
the samples could be safely introduced into the probe head of
the spectrometer, pre-cooled to −30 °C. Low temperature
basis sets. Harmonic vibrational frequencies were calculated for
all stationary points. As revealed by the frequency analysis, ima-
ginary frequencies were absent in all ground states whereas just
one imaginary frequency was associated with each transition
state. Visual inspection of the corresponding normal mode30
validated the identification of the transition states.
The energy values listed in Table 2 represent total electronic
energies. In general, these give the best fit with experimental
DNMR data.31 Therefore, the computed value have not been cor-
rected for zero-point energy contributions or other thermodyn-
amic parameters. This avoids artifacts that might result from the
ambiguous choice of an adequate reference temperature, from
empirical scaling factors,32 and from the idealization of low-fre-
quency vibrators as harmonic oscillators.33
1
600 MHz H spectra (compounds 1–13, 15–17) were acquired
without spinning, using a 5 mm dual direct probe with a 9000
Hz spectral width, 2.0 μs (20° tip angle) pulse width, 3 s acqui-
sition time and 1 s delay time. A shifted sine bell weighting
function20 equal to the acquisition time (i.e., 3 s) was applied
before the Fourier transformation. Usually 32 to 64 scans were
collected. Low temperature 150.8 MHz 13C spectra (compounds
2, 3, 6, 11, 14, 16, 17) were acquired without spinning and under
proton decoupling conditions with a 38 000 Hz spectral width,
4.2 μs (60° tip angle) pulse width, 1 s acquisition time and 1 s
delay time. A line broadening function of 1–2 Hz was applied
before the Fourier transformation. Usually 128 to 512 scans were
collected.
Temperature calibrations were performed before the exper-
iments, using a digital thermometer and a Cu/Ni thermocouple
(models C9001 and KX2384, respectively, Comark Ltd., Hert-
fordshire, UK) placed in an NMR tube filled with isopentane.
The conditions were kept as equal as possible with all sub-
sequent work. The uncertainty in temperature measurements was
estimated as 2 °C. Line shape simulations were performed
using a PC version of the QCPE DNMR6 program.21 Electronic
superimposition of the original spectrum and of the simulated
one enabled the determination of the most reliable rate constant.
The rate constants obtained at various temperatures afforded the
free energy of activation ΔG‡ for bond rotation by applying the
Eyring equation.22 Although the transition states of the present
compounds are intrinsically more ordered than the ground states,
the experimental free activation energies do not display appreci-
able variations with temperature within the experimental uncer-
tainty. This implies a rather small activation entropy ΔS‡, as
observed in the majority of conformational NMR dynamic
processes.23,24
Acknowledgements
The authors are indebted to the University of Bologna (RFO
funds 2009), the Ministero dell’Università e Ricerca, Roma
(MUR PRIN 2008 prot. 2008LYSEBR004) and the Schweizer-
ische National-Fonds zur Förderung der wissenschaftlichen For-
schung, Bern (grant 20-100′336-02) for financial support.
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