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R.A. Petros et al. / Journal of Organometallic Chemistry 692 (2007) 4768–4773
THF were distilled from sodium/benzophenone under N2
prior to use. Toluene, Et2O, CH2Cl2, and hexanes were
purified by passage through a column of alumina as
described by Grubbs [15]. Benzene-d6 was degassed, and
which was again added to the solution containing the zirc-
onocene dichloride. The mixture was stirred overnight.
The solvent was removed under vacuum leaving a yellow
solid, which was dissolved in benzene (20 mL) and fil-
tered. The remaining solid was washed with benzene
(7 mL). The filtrates were combined and brought to dry-
ness under vacuum. The remaining yellow solid was
washed with hexanes and dried under vacuum (56 mg,
˚
then stored over 4 A molecular sieves under nitrogen.
(S,S)-(EBI)ZrCl2 (S,S-1a) was a gift from Prof. Richard
F. Jordan (University of Chicago, prepared by Matthew
LoCoco). Racemic (SBI)ZrMe2 was prepared by literature
methods [16], and converted to racemic 4b by known pro-
cedures [17].
1
62% yield). H NMR (500 MHz, C6D6): d 7.31 (d, C6H,
JHꢁH = 8.5 Hz, 2H), 7.06 (m, C6H, 4H), 6.89 (m, C6H,
2H), 6.41 (d, C5H, JHꢁH = 3.3 Hz, 2H), 5.65 (d, C5H,
JHꢁH = 3.3 Hz, 2H), 2.83–2.66 (m, CH2CH2, 4H), ꢁ0.97
(s, ZrMe2, 6H).
NMR spectra were recorded on a Bruker DMX-500
1
(500 MHz for H), and were referenced by assigning the
residual solvent peak (C6D5H) to 7.15 d. Chiral HPLC
was performed on a Varian instrument (model 9012 solvent
delivery system) with variable wavelength detector (model
9050) using a Chiralcel OD-H column (Daicel Industries)
with flow rate = 1 mL/min, 95:5 n-hexanes:iPrOH,
k = 220 nm.
3.4. Methylalumination reactions
Reactions catalyzed by 4 were carried out as in the fol-
lowing example (with S,S-4a). An NMR tube was charged
with (S,S)-(EBI)ZrMe2 (1.6 lmol) and [Ph3C][B(C6F5)4]
(1.6 lmol), 900 lL C6D6, and wrapped in aluminum foil
to exclude light. After 2 h, AlMe3 (400 lL, 29.2 mM,
C6D6) was added. After an additional 1 h, allylbenzene
(400 lL, 18.1 mM, C6D6) was added and the reaction
was allowed to sit for 3 h. The reaction was approximately
80% complete (as judged by a decrease in the signal for
PhCH2CHCH2, d 4.97) with the expected aluminum alkyl
>90% (by 1H NMR) of the product (5a). 1H NMR
(500 MHz, C6D6): d 2.56 (d, PhCH2CHCH3CH2AlMe2,
J = 8.3 Hz, 1H), 1.93 (bs, PhCH2CHCH3CH2AlMe2,
2H), 1.12 (d, PhCH2CHCH3CH2AlMe2, J = 5.3 Hz, 3H),
and from ꢁ0.22 to ꢁ0.08 (m, PhCH2CHCH3CH2AlMe2,
2H); a peak for PhCH2CHCH3CH2AlMe2 was not
observed due to overlap with AlMe3. These chemical shifts
are in agreement with those reported previously for the
same compound in CD2Cl2 [20]. O2 was bubbled through
the sample for 20 min; addition of 0.1 mL of water com-
pleted the conversion of the aluminum alkyl to the corre-
sponding alcohol, 2-methyl-3-phenyl-1-propanol (6a). The
ee of the product (6a) was 29%, determined by a direct
comparison of its chiral HPLC trace with that of racemic
6a.
The methylalumination of styrene was carried out by
substituting styrene for allylbenzene in the procedure
above. The % ee for the methylalumination of styrene
was determined by comparing the chiral HPLC trace of
enantioenriched 6b (prepared with enatiopure S,S-4a as
the catalyst) with that of racemic 6b (prepared with racemic
4a as the catalyst). Both enantiomers of 6b are commer-
cially available.
Methylalumination reactions catalyzed by S,S-1 were
carried out as in the following example (with S,S-1b). In
a glovebox, an NMR tube was charged with (S,S)-
(SBI)ZrCl2 (S,S-1b) (600 lL, 3.3 mM, C6D6), MAO
(178 lL, 0.9 M, C6D6), and then allylbenzene (55 lL,
98 mM, C6D6). The tube was then wrapped in aluminum
foil to exclude light. The sample was monitored for 6 h
by 1H NMR, during which time the reaction reached
3.1. Measurement of koff for racemic 4b1
Selective population inversion experiments were per-
formed at 300 K with a 180ꢃselective-s-90nꢃ onselective – acquire
1H NMR pulse sequence [18]. Spectra were taken at each
of 20 different mixing times (s). The peak areas were
recorded and the data fit using CIFIT [19]. The entire
experiment was repeated six times, giving koff values that
averaged 0.16(1) sꢁ1
.
3.2. 2-Me-3-Ph-1-propanol (6a) [20]
Racemic 6a was synthesized for comparison with the
enantioenriched products of asymmetric carboalumination
reactions. trans-2-Me-3-Ph-2-propen-1-ol (1.5 g, 9.5 mmol)
in EtOH (20 mL) was hydrogenated in a Fisher–Porter bot-
tle under 50 psi H2 using Adams’s catalyst (70 mg). Both
enantiomers are known compounds: (S)-6a, CAS 22436-
06-2; (R)-6a, CAS 77943-96-5. 1H NMR (500 MHz,
C6D6): d 7.18 (m, Ph, 2H), 7.06 (m, Ph, 4H), 3.18 (m,
PhCH2CHMeCH2OH, 2H), 2.64 (m, PhCH2CHMe-
CH2OH, 1H), 2.23 (m, PhCH2CHMeCH2OH, 1H), 1.70
(m, PhCH2CHMeCH2OH, 1H), 0.90 (br s, PhCH2CHMe-
CH2OH, 1H), 0.79 (d, JHꢁH = 6.8 Hz, PhCH2CHMe-
CH2OH, 3H).
3.3. (S,S)-(EBI)ZrMe2 [11]
Me2Mg was prepared by mixing a solution of MeMgBr
(240 lL, 3 M in Et2O) with dioxane (70 lL, 0.77 mmol) in
Et2O (30 mL) and stirring overnight. This solution was
added by cannula filter to a solution of (S,S)-(EBI)ZrCl2
(S,S-1a) (100 mg, 0.24 mmol) dissolved in Et2O (20 mL).
The remaining solid was washed with Et2O (10 mL),
1
For complete details for selective population inversion experiments see
[7].