494 J . Org. Chem., Vol. 66, No. 2, 2001
Grilli et al.
Ta ble 4. Com p a r ison of th e Mu ltip licity of th e 13C NMR
Sp ectr u m of 3 in Solu tion (CDCl3) a n d in th e Solid Sta te
a t Am bien t Tem p er a tu r ea
Dim esitylth iok eton e, 3.35 To a mixture of mesitylene (25
mL) and AlCl3 (4.3 g, 33 mmol) cooled to -20 °C, thiophosgene
(1 mL, 13 mmol, neat) was added dropwise and after 20 min
the mixture was slowly heated to 80-90 °C, monitoring the
formation of the thioketone by TLC. After 30 min the deep
red mixture was cooled to ambient temperature and then
poured onto 200 g of crusched ice. The product was extracted
with Et2O, and the organic layers were washed twice with
aqueous KOH and then dried over Na2SO4. The solvents were
removed at reduced pressure followed by distillation of the
residual mesitylene. The crude was purified on silica gel in
two steps, first on a column eluted with 10:1 v/v petroleum
ether/Et2O and then on preparative TLC (eluent: 10:1 v/v
pentane/toluene) to yield 0.40 g (11%) of pure dimesitylthioke-
tone. Blue solid, Mp 92-93 °C. 1H NMR (200 MHz, CDCl3, 22
°C, TMS): δ ) 2.18 (s, 12H, Me), 2.34 (s, 6H, CH3), 6.90 (s,
4H, CH). 13C NMR (50.3 MHz, CDCl3, 22 °C, TMS): δ ) 21.0
(CH3), 21.9 (CH3), 130.1 (CH), 135.3 (q), 138.6 (q), 147.6 (q),
244.8 (CS).
signal
CS
δ (ppm) in solution
δ (ppm) in solids
244.8
244.5
C(quat)
147.6 [2], 138.6 [2],
135.3 [4]
148.1, 146.8, 139.6 [2], 138.6,
137.8, 134.3, 132.6
CH
Me
130.1 [4]
21.9 [4], 21.0 [2]
132.0, 130.9, 129.1, 128.3
24.6, 23.6, 22.3 [2], 20.8, 19.3
aThe numbers of the corresponding carbons are indicated in
brackets.
the seven lines detected in solution (Table 4). In this case,
four lines are displayed by the four CH carbons, thus
unambiguously showing that the four meta positions are
diastereotopic in the crystalline state. As a consequence,
the lines with a double intensity observed for the
quaternary and for the methyl carbons (Table 4) are
undoubtedly due to accidental coincidences.
Cr ysta l Da ta of Dim esitylk eton e, 1. C19H22O (266.37),
monoclinic, space group P21/n, Z ) 4, a ) 10.3430(7) Å, b )
12.5006(8) Å, c ) 12.0976(8) Å, â ) 95.572(2)°, V ) 1556.75-
(18) Å3, Dc ) 1.136 g cm-3, F(000) ) 576, µMo ) 0.068 cm-1, T
) 293 K. Data were collected using a graphite monochromated
Mo KR X-radiation (λ ) 0.710 73 Å) range of 2.48° < θ <
25.00°. Of 13 722 reflections measured, 2730 were found to
be independent (Rint ) 0.0432), 2075 of which were considered
as observed [I > 2σ(I)], and were used in the refinement of
181 parameters leading to a final R1 of 0.0792 and an Rall of
0.0924. The structure was solved by direct method and refined
by full-matrix least squares on F2, using SHELXTL 97
program packages. In refinements, weights were used accord-
Con clu sion s
It has been demonstrated that two equivalent mesityl
rings bonded to a trigonal carbon adopt, in solution, a
chiral propeller-like conformation, equal to that deter-
mined by X-ray diffraction in the crystalline state.
According to MM calculations, the two helical enanti-
omers interconvert through a one-ring flip pathway in
the case of dimesitylketone but through a two-ring flip
pathway in the case of 1,1-dimesitylethylene, the com-
puted barriers (5.4 and 10.7 kcal mol-1) being in good
agreement with those experimentally determined (4.6
and 9.2 kcal mol-1, respectively). An intermediate value
(6.5 kcal mol-1) was measured for the enantiomerization
barrier of dimesitylthioketone. The solid-state NMR
spectra indicate that these barriers are much higher in
ing to the scheme w ) [σ2(Fo2) + (0.1891P)2 + 0.0000P]-1
,
2
where P ) (Fo + 2Fc2)/3. The hydrogen atoms, including the
OH hydrogen, were located by geometrical calculations and
refined using a “riding” method. wR2 was equal to 0.2833. The
goodness of fit parameter S was 1.238. The largest difference
peak and hole were 0.484 and -0.437 e Å-3. Crystallographic
data (excluding structure factors) have been deposited with
the Cambridge Crystallographic Data Center, CCDC-145898.
NMR Mea su r em en ts. The samples for the low-tempera-
ture measurements were prepared by connecting to a vacuum
line the NMR tubes containing the desired compounds dis-
solved in some C6D6 or CD2Cl2 for locking purpose and
condensing therein the gaseous solvents by means of liquid
nitrogen. The tubes were subsequently sealed in vacuo and
introduced into the precooled probe of the 300 MHz spectrom-
eter operating at 75.45 MHz for 13C or the 400 MHz spectrom-
eter operating at 100.6 MHz for 13C. The assignment of the
13C signals was obtained by means of DEPT sequences. The
temperatures were calibrated by substituting the sample with
a precision Cu/Ni thermocouple before the measurements.
Total line shape simulations were achieved by using a PC
version of the DNMR-6 program.36 Since at the low temper-
atures required to observe the exchange process the intrinsic
line width was significantly temperature dependent, the values
measured for the line of the para methyl group (which does
not experience exchange broadening) were assumed to be equal
to that of the two ortho signals. We also checked that errors
as large as 50% on this value affected the activation energy
by less than 0.05 kcal mol-1 in the temperature range
investigated.1a The high-resolution 13C NMR solid state CP-
MAS (cross polarization magic angle spinning) spectra were
obtained at 75.45 MHz. The compounds were introduced into
a tightly sealed 7 mm zirconia rotor, spun at the magic angle
with a speed of 3.0-3.5 kHz. The line assignment was obtained
by the “nonquaternary suppression” pulse sequence. The
cooling was achieved by means of a flow of dry nitrogen,
precooled in a heat exchanger immersed in liquid nitrogen.
the crystal, but only their lower limit (19 kcal mol-1
could be ascertained.
)
Exp er im en ta l Section
Ma ter ia ls. Dim esitylk eton e, 1.33 To a cooled (-78 °C)
solution of mesityllithium, prepared by addition of butyl-
lithium (11 mmol) to a solution of mesityl bromide (2.39 g, 12
mmol in 40 mL of THF), a solution of mesitylaldehyde (1.48
g, 10 mmol in 10 mL of THF) was added. After 30 min, the
solution was allowed to warm and was quenched with aqueous
NH4Cl. The product was extracted with Et2O and dried (Na2-
SO4), and the solvent was removed at reduced pressure. The
crude was washed with pentane to give 2.42 g (90%) of pure
dimesitylcarbinol. Oxidation with pyiridinium chlorochromate
(2.90 g, 13.5 mmol in 50 mol of CH2Cl2) at room temperature,
followed by filtration on silica, yielded 2.30 g (96%) of almost
pure dimesitylketone, purified by crystallization (methanol).
Crystals suitable for X-ray analyses were obtained by slow
crystallization in methanol. Mp 138-140 °C. 1H NMR (400
MHz, CDCl3, 22 °C, TMS): δ ) 2.13 (s, 12H, Me), 2.26 (s, 6H,
CH3), 6.82 (s, 4H, CH). 13C NMR (100.6 MHz, CDCl3, 22 °C,
TMS): δ ) 20.7 (CH3), 21.1 (CH3), 129.8 (CH), 136.6 (q), 138.4
(q), 140.0 (q), 202.5 (CO). The DSC trace of 1 was obtained
with a scanning rate of 5°/min, heating the sample from -70
to +160 °C; the only observed phase transition corresponded
to the melting point.
1,1-Dim esityleth ylen e, 2. Prepared as reported in the
literature.34
(34) Roberts, R. M.; El-Khawaga, A. M.; Roengsumram, S. J . Org.
Chem. 1984, 49, 3180.
(35) Kappert, W.; Sander, W.; Landgrafe, C. Liebigs Ann./ Recl.
1997, 2519.
(36) QCPE program nï. 633, Indiana University, Bloomington, IN.
(33) Fuson, R. C.; Southwick, P. L.; Rowland, S. P. J . Am. Chem.
Soc. 1944, 66, 1109.