Synthesis and optical activity analysis of chiral titanium(IV) sec-butoxide and its group IV analogues

Article abstract of DOI:10.1016/j.tetasy.2008.02.008

The complete series of chiral group IV sec-butoxides has been obtained by the treatment of M(NMe₂)₄ (M = Ti, Zr, Hf) with resolved (R)-(-)- or (S)-(+)-2-butanol. These complexes were analyzed by polarimetry, revealing a decreasing tr

Full text of DOI:10.1016/j.tetasy.2008.02.008

Available online at www.sciencedirect.com
Tetrahedron: Asymmetry 19 (2008) 543–548
Synthesis and optical activity analysis of chiral titanium(IV)
sec-butoxide and its group IV analogues
Samantha N. MacMillan, Kaysia T. Ludford and Joseph M. Tanski*
Department of Chemistry, Vassar College, Poughkeepsie, NY 12604, USA
Received 4 September 2007; accepted 4 February 2008
Abstract—The complete series of chiral group IV sec-butoxides has been obtained by the treatment of M(NMe2)4 (M = Ti, Zr, Hf) with
resolved (R)-(ꢀ)- or (S)-(+)-2-butanol. These complexes were analyzed by polarimetry, revealing a decreasing trend in molar rotation
down the group, from titanium to hafnium. Catalysis screenings showed that the resolved titanium sec-butoxides, in conjunction with
the resolved BINOL (BINOL = 1,10-bi-2-naphthol) of the opposite configuration designation, catalyzed the addition of dimethyl zinc to
benzaldehyde with higher enanteoselectivity than that observed for resolved BINOL with titanium(IV) isopropoxide or with the tita-
nium(IV) sec-butoxide of the same configuration designation.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
chiral group IV sec-butoxides, M(O2Bu)4 (M = Ti, Zr, Hf),
and the results of the stereoselective addition of dimethyl
zinc to benzaldehyde, mediated by the resolved Ti(O2Bu)4
species.
Group IV alkoxides are a topic of continued interest with
respect to their structure and varied chemistry.1–5 These
compounds act as important precursors in metal oxide
materials; high dielectric constant hafnium materials will
soon be used in sub-100 nm computer chip technology.5
Group IV alkoxides, most notably Ti(OiPr)4, also serve
as catalysts in numerous organic transformations. As
growth in the pharmaceutical and chemical industries has
increased the demand for the production of enantiomeri-
cally pure products, strategies employing chiral intermedi-
ates have been developed to carry out enantioselective
syntheses with high stereoselectivity.6–9 Increased enantio-
meric excess has been observed when bulky or coordinating
groups are added to chiral ligand frameworks, necessitat-
ing multi-step organic ligand synthesis prior to metal coor-
dination and catalyst screening. In the stereoselective
addition of alkyl groups to aromatic aldehydes, strategies
include the use of chelating chiral auxiliary ligands, such
as amino alcohols,10,11 BINOL,12–14 TADDOL,15,16
Salan,17 N-sulfonylated amino alcohols,18,19 and bis(sul-
fonamide)20,21 to afford high enantiomeric excess. Employ-
ing simple terminal chiral alkoxides is an alternative
approach.12,21–24 Herein, we report the synthesis and char-
acterization of the previously unreported complete series of
2. Results and discussion
2.1. Synthesis
Alcoholysis of tetrakis(dimethylamido)titanium with an
excess (6 equiv) of (R)-(ꢀ)- or (S)-(+)-2-butanol in benzene,
followed by vacuum distillation, provided the chiral tita-
nium(IV) alkoxides, Ti(OR-2Bu)4 and Ti(OS-2Bu)4, as color-
less liquids in high yield (Scheme 1). The procedure for the
synthesis of the zirconium and hafnium complexes was sim-
ilar; the treatment of tetrakis(dimethylamido)zirconium
and hafnium with a stoichiometric amount of (R)-(ꢀ)- or
(S)-(+)-2-butanol in benzene, followed by a high tempera-
ture vacuum distillation, gave the corresponding chiral
O
N
N
N
N
OH
*
O
M
4
M
4HNMe2
+
+
O
O
M=Ti,Zr,Hf
*
Corresponding author. Tel.: +1 845 437 7503; fax: +1 845 437 5732;
e-mail: jotanski@vassar.edu
Scheme 1. Synthesis of resolved group IV sec-butoxides (M = Ti, Zr, Hf).
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.02.008

544
S. N. MacMillan et al. / Tetrahedron: Asymmetry 19 (2008) 543–548
zirconium(IV) and hafnium(IV) alkoxides as colorless,
viscous oils.
ides have similar molar rotations and may reflect that these
complexes are similar in size due to the metals having
essentially equivalent covalent radii. In order to evaluate
the concentration dependence of the optical activity, polar-
imetry analyses were carried out for the (R)-isomers of the
group IV sec-butoxides at concentrations of 1, 5, and 10
(Fig. 1). The slope of each line of best fit is equal to the spe-
cific rotation of the corresponding complex. Over the range
of concentrations studied, the rotation for each compound
decreases linearly with sample concentration, indicating
that solution structure does not affect the magnitude of
rotation.
2.2. Optical activity analysis
Optical activity analyses were performed for each of the
alkoxides synthesized. Both the specific rotation (a) and
molar rotation (U) were determined (Table 1), and were
found to decrease down the group, from titanium to haf-
nium. Although not a complete series, a similar trend
was found for the group 10 complexes [(1R,10R)-2,6-bis-
[1-(diphenylphosphino)ethyl]phenyl]chloro-platinum and
-palladium, with specific rotations of ꢀ297.8 and ꢀ323.6,
respectively.25 To date, no other complete series of homo-
logues of a transition metal group with chiral ligands,
homoleptic or heteroleptic, characterized by optical activ-
ity has been reported.
2.3. Asymmetric methylation of benzaldehyde
To assess the potential of chiral group IV sec-butoxides as
asymmetric catalysts, the methylation of benzaldehyde to
give 1-phenylethanol in the presence of dimethyl zinc and
Ti(OR)4 (R = iPr, R-2Bu, or S-2Bu) was studied, employing
a procedure similar to that previously developed for diethyl
zinc (Scheme 2).27 Analogous studies employing diethyl
zinc and achrial Ti(OiPr)4 have already been reported.14,27
The alkylation of aldehydes with dimethyl zinc gives much
lower enantiomeric excesses than diethyl zinc,13,16,19,28
although it is known that for both the cases, the dialkyl
zinc reagent does not directly alkylate the aldehyde
substrate.13
When comparing the specific rotation of homologues, this
decreasing trend is not unexpected; if two substances have
unequal molecular weights, but similar rotatory strength,
or power to rotate polarized light, the substance with the
smaller molecular weight will have a larger rotation simply
because there are more molecules per unit weight.26 For the
group IV sec-butoxides, the decreasing trend in molar rota-
tion may be due to differences in both rotatory strength
and molecular size. The Zr(O2Bu)4 and Hf(O2Bu)4 alkox-
In the presence of Ti(OR-2Bu)4 and Ti(OS-2Bu)4, a 17%
excess of R- and 12% excess of S-1-phenylethanol were
observed, respectively (Table 2).29
Table 1. Specific and molar rotation values for M(O2Bu)4 (c 1, C6H6)
M(O2Bu)4
a (deg.)
[a]D
[U]
(10ꢀ1 deg cm2 gꢀ1
)
(10 deg cm2 molꢀ1
)
Ti(OR-2Bu)4
Ti(OS-2Bu)4
Zr(OR-2Bu)4
Zr(OS-2Bu)4
Hf(OR-2Bu)4
Hf(OS-2Bu)4
ꢀ0.906
+0.881
ꢀ0.230
+0.222
ꢀ0.121
+0.120
ꢀ83.1
+83.1
ꢀ20.9
+22.65
ꢀ11.75
+12.4
ꢀ282.9
+282.85
ꢀ80.2
+86.9
OH
*
O
Ti(OR)4,Me2Zn, BINOL
CH2Cl2, 3oC
CH3
H
ꢀ55.3
+58.3
Scheme 2. Dimethyl zinc addition to benzaldehyde.
Table 2. Data for the addition of dimethyl zinc to benzaldehydea
Entry
Ti(OR)4
OiPr
BINOL
% Convb
% eec
Config.d
1
2
3
4
5
6
7
8
9
None
None
None
Racemic
Racemic
Racemic
(R)
(S)
(R)
(S)
(S)
81
54
33
98
99
99
99
99
99
99
99
99
—
17
12
—
8
11
46
40
37
40
58
56
—
O
R-2Bu
(R)
(S)
—
OS-2Bu
OiPr
OR-2Bu
(S)
(R)
(R)
(S)
(R)
(S)
(S)
(R)
O
S-2Bu
OiPr
OiPr
O
R-2Bu
10
11
12
OS-2Bu
OR-2Bu
OS-2Bu
(R)
a Ti(OR)4 (1.4 mmol in 5 mL CH2Cl2), 0.21 mmol BINOL, 6.0 mmol
Me2Zn/toluene, 1.0 mmol benzaldehyde.
b After 24 h, as determined by 1H NMR.
c Determined by chiral GC, Alltech Chiraldex B-DM.
d As determined by the analysis of an authentic sample of (R)-(+)-1-
phenylethanol.
Figure 1. Concentration dependence of observed rotations for M(OR-2Bu)4
(M = Ti, Zr, Hf).

S. N. MacMillan et al. / Tetrahedron: Asymmetry 19 (2008) 543–548
545
Owing to the low percentage conversion observed, the reac-
tions were repeated, by adding a catalytic amount of race-
mic BINOL (BINOL = 1,10-bi-2-naphthol); the percentage
conversion for each titanium alkoxide increased to over
98%. Notably, for the reactions involving racemic BINOL
and chiral alkoxides, the enantiomeric excesses decreased
and were accompanied by a switch in the configuration
of the alcohol in excess. The change in the enantiomeric ex-
cess of the alcohol in excess (Dee) was observed to be 25%
and 23% for Ti(OR-2Bu)4 and Ti(OS-2Bu)4, respectively. In
the addition of diethylzinc to benzaldehyde, the addition of
chiral additives, including chiral alcohols, has been shown
to influence the selectivity of the reaction when a chiral tri-
dentate bis(sulfonamide) titanium catalyst was used.20 An
analogous observation has also been reported for the addi-
tion of diethyl zinc to benzaldehyde mediated by an achiral
bis(phenol) and the chiral monodentate alkoxide
Ti[OSCH(Et)(p-Tol)]4.23 These results prompted an investi-
gation employing resolved (R)- or (S)-BINOL to poten-
tially increase the enantiomeric excess of the product
alcohol.
dinuclear species.13,30 The chirality of both the terminal
alkoxide ligands and the BINOL presumably affects both
the stability of the dinuclear species and the facial selectiv-
ity of subsequent aldehyde binding.16
Different from the ‘chiral environment amplification’
approach,21 which uses an achiral chelating ligand to trans-
mit and amplify enantioselectivity from a single chiral com-
ponent, this dichiral approach uses complementary
matched pairs composed of two different chiral ligands to
form an activated complex that reinforces enantioselectivi-
ty.34 Interestingly, in a similar study of the addition of
diethyl zinc to 4-tolualdehyde using the chiral monodentate
alkoxide Ti[OSCH(Et)(Ph)]4 and a chiral bis(sulfonamide),
the magnitude of the product alcohol’s enantiomeric excess
was also observed to be different when chelating bis(sulfon-
amide) of the opposite configuration was used: (R,R)-
bis(sulfonamide) 84%, (S,S)-bis(sulfonamide) 81%.21
However, a similar enantiomeric excess was observed
when the achiral terminal alkoxide Ti(OiPr)4 was studied
with the resolved (R,R)-bis(sulfonamide) 79%. In this sys-
tem, the chiral alkoxide derived from HOSCH(Et)(Ph)
had little influence on the enantioselectivity, contrasting
the BINOL/Ti(O2Bu)4 system reported here, in which the
enantiomeric excess observed from the matched pair is sig-
nificantly larger than that observed from the mismatched
pair or achiral alkoxide with resolved BINOL. Ongoing
studies are currently focused on using resolved group IV
sec-butoxides in the dichiral asymmetric activation
approach to further increase enantioselectivity in reactions
such as the alkylation of aromatic aldehydes with dialkyl
zinc reagents.
In the presence of achiral Ti(OiPr)4 and (R)- or (S)-BI-
NOL, the enantiomeric excesses of 46% for (R)- and 40%
(S)-1-phenylethanol were observed, respectively, similar
to values reported in the literature.30 The addition of re-
solved BINOL to chiral titanium alkoxides of the same
configuration resulted in product configurations and enan-
tiomeric excesses similar to the addition of resolved BI-
NOL to achiral Ti(OiPr)4. However, the addition of
resolved BINOL to chiral titanium alkoxides of the oppo-
site configuration designation increased the enantiomeric
excess by at least 16% over the combinations with the same
configuration. In either case, the product alcohol obtained
was of the same configuration as the resolved BINOL used.
Both the (S)-BINOL/Ti(OR-2Bu)4 and the (R)-BINOL/
Ti(OS-2Bu)4 pairs produce higher enantiomeric excesses,
58% and 56% respectively, than the 50% excess previously
reported for the addition of dimethyl zinc to benzaldehyde
under similar catalytic conditions.30
3. Conclusions
In conclusion, a previously unreported complete series of
chiral group IV sec-butoxides was synthesized from
resolved (R)-(ꢀ)- and (S)-(+)-2-butanol and was analyzed
by polarimetry. A decreasing trend in the specific rotation
was observed down the group, from titanium to hafnium.
An example of asymmetric activation, the simple resolved
titanium sec-butoxide, combined with resolved BINOL of
the opposite configuration designation, yields a dichiral
matched pair that mediates the addition of dimethyl zinc
to benzaldehyde with higher enantioselectivity than
resolved BINOL with Ti(OiPr)4 or the mismatched dichiral
pair.
We propose that this observation is an example of a
matched pair asymmetric activation phenomena31 related
to ‘double diastereoselectivity’, as described by Masa-
mune,32 and ‘specific interactivity’, as described by Dani-
shefsky.33 According to Masamune, double asymmetric
synthesis resulting in higher enantiomeric excess, as well
as enantiomeric control, can be achieved by the interaction
of a matched pair of homochiral reactants. Danishefsky
adds that the substantial diastereomeric preference may
be achieved beyond that afforded by the chiral compo-
nents, based on a ‘specific interactivity’ between them. In
the matched pairs of (S)-BINOL/Ti(OR-2Bu)4 and (R)-BI-
NOL/Ti(OS-2Bu)4, the choice of BINOL in the matched
pair determines enantiomeric control; however, the chiral-
ity of the terminal alkoxide ligands and the BINOL influ-
ences the facial selectivity of the alkyl group as it is
transferred to the aldehyde, resulting in higher enantio-
meric excess than obtained either from chiral component
by itself or from a mismatched pair. In BINOL/Ti(OR)4
systems in particular, the proposed intermediate implicated
in the mechanism of the alkylation of arylaldehydes is a
4. Experimental
4.1. General experimental
Unless otherwise stated, all manipulations were carried out
at room temperature in an inert atmosphere glove box or
under a nitrogen atmosphere using standard Schlenk tech-
niques. Anhydrous solvents were purchased from Aldrich
and stored under nitrogen. Resolved (R)-(ꢀ)- and
(S)-(+)-sec-butanols were purchased from Aldrich and
degassed by the freeze–pump–thaw–degas method prior
to use. Tetrakis(dimethylamido)titanium was prepared by

546
S. N. MacMillan et al. / Tetrahedron: Asymmetry 19 (2008) 543–548
the literature method.35 Tetrakis(dimethylamido)zirconium
and hafnium were purchased from Strem. All other re-
agents were purchased from commercial sources and used
as received. All glasswares were oven dried at a tempera-
ture of 235 °C prior to use.
4.4. Synthesis of resolved Ti(OS-2Bu)4
A
solution of (S)-(+)-2-butanol (6 equiv, 4.67 g,
63.10 mmol) in benzene was treated with tetrakis(dimethyl-
amido)titanium (2.349 g, 10.48 mmol) in benzene at 0 °C.
The reaction mixture was allowed to return to room tem-
perature and was then heated to 40 °C over the course of
1 h, evacuating the headspace throughout. The solvent
was removed in vacuo and the residue purified by vacuum
distillation (95 °C, 10 mmHg) yielding a clear, colorless li-
quid (2.721 g, 76%). IR Data (KBr salt plate, cmꢀ1):
2967 (s), 2925 (s), 2876 (s), 2730 (w), 2717 (w), 2698 (w),
2657 (w), 2625 (w), 2580 (w), 1463 (s), 1369 (s), 1356 (m),
1345 (m), 1336 (s), 1296 (m), 1266 (m), 1161 (s), 1128 (s),
1036 (s), 1001 (s), 977 (s), 942 (s), 830 (s), 793 (m), 806
(m), 779 (m), 652 (s), 613 (s), 544 (w), 528 (w), 497 (w),
454 (m). 1H NMR (300 MHz, C6D6): d 4.25 (m, 1H,
–CH), 1.55 (m, 1H, –CH2), 1.44 (m, 1H, –CH2), 1.24 (d,
3H, J = 6.0 Hz, CH–CH3), 0.98 (t, 3H, J = 7.4 Hz, CH2–
CH3). 13C NMR (MHz, C6D6): d 82.50 (–CH), 34.20
(–CH2), 25.26 (CH–CH3), 11.21 (CH2–CH3). Anal. Calcd
1H and 13C{1H} NMR spectra were recorded at room tem-
perature using a Bruker Avance DPX 300 MHz spectro-
meter. 1H and 13C chemical shifts are reported and
referenced to the residual solvent resonances of 7.15 ppm
(1H) and 128.62 (t) (13C) for benzene-d6. Infrared spectra
were recorded on a Thermo Nicolet Nexus 670 FT-IR spec-
trometer and are reported in cmꢀ1. Optical rotation analy-
ses were recorded on a Rudolph Research Analytical
Autopol III polarimeter in anhydrous benzene solution.
Chiral GC (Alltech Chirladex B-DM column) analyses
were carried out on a HP 6890 Series GC system. Elemen-
tal analyses were carried out by Desert Analytics, Inc., AZ,
USA; samples were handled under an inert atmosphere.
4.2. General procedure for the asymmetric addition of
Me2Zn to benzaldehyde
for C16H36O4Ti: C, 56.47; H 10.66; N, 0.00. Found: C,
56.65; H, 10.67; N, 0.13. ½aꢂD ¼ þ83:1 (c 1.0, C6H6).
24
To a stirring solution of BINOL (0.21 mmol) in CH2Cl2
(5 mL) was added Ti(OR)4 (1.4 mmol). After 1 h, the reac-
tion mixture was cooled to 3 °C and 3 mL of dimethylzinc
solution (2 M in toluene, 6 mmol) was added. After stirring
for 30 min, benzaldehyde was added (1 mmol) and the mix-
ture was stirred at 3 °C for 24 h. The reaction was
quenched with 1 M HCl (20 mL), filtered, and extracted
with diethyl ether (3 ꢁ 20 mL). The combined organic lay-
ers were washed with brine, dried over anhydrous magne-
sium sulfate, filtered, and concentrated. The remaining oil
4.5. Synthesis of resolved Zr(OR-2Bu)4
To a benzene solution of tetrakis(dimethylamido)zirco-
nium (2.239 g, 8.37 mmol) at 17 °C was slowly added a
slight excess of (R)-(ꢀ)-2-butanol (2.543 g, 34.31 mmol)
in benzene. The reaction mixture was stirred under a posi-
tive nitrogen flow at 17 °C for 20 min. The solution was
allowed to return to room temperature and the headspace
evacuated several more times over a 2 h period. The reac-
tion was stirred overnight, after which time the volume
was reduced by half. The remaining mixture was trans-
ferred to a distillation apparatus and the solvent removed
in vacuo. The residue was purified by high temperature dis-
tillation (185 °C, 10 mmHg) yielding a clear, colorless vis-
cous oil (2.180 g, 66%). IR Data (KBr salt plate, cmꢀ1):
2965 (s), 2917 (s), 2875 (s), 2839 (s), 2644 (w), 2625 (w),
2573 (w), 1464 (s), 1367 (s), 1347 (s), 1316 (w), 1296 (w),
1263 (m), 1124 (s), 1048 (s), 1004 (s), 960 (m), 931 (s),
916 (s), 892 (s), 826 (s), 801 (m), 780 (m), 675 (w), 606
1
was analyzed by H NMR and chiral GC (Alltech Chiral-
dex B-DM) to determine the percent conversion of 1-phen-
ylethanol and its enantiomeric excess, respectively.
4.3. Synthesis of resolved Ti(OR-2Bu)4
A
solution of (R)-(ꢀ)-2-butanol (6 equiv, 4.920 g,
66.38 mmol) in benzene was treated with tetrakis(dimethyl-
amido)titanium (2.470 g, 11.02 mmol) in benzene at 0 °C.
The reaction mixture was allowed to come to room temper-
ature and then heated to 40 °C over the course of 1 h, evac-
uating the headspace throughout. The solvent was removed
in vacuo and the residue purified by vacuum distillation
1
(s), 555 (s), 463 (s). H NMR (300 MHz, C6D6): d 4.38
(m, 1H, –CH), 1.95 (m, 1H, –CH2), 1.78 (m, 1H, –CH2),
1.47 (d, 3H, J = 6.0 Hz, CH–CH3), 1.02 (t, 3H,
J = 7.4 Hz, CH2–CH3). 13C NMR (MHz, C6D6): d 77.80
(–CH), 33.90 (–CH2), 24.11 (CH–CH3), 11.59 (CH2–
(90 °C, 10 mmHg) yielding
a
clear, colorless liquid
CH3). Anal. Calcd for C16H36O4Zr: C, 50.09; H 9.46; N,
24
(3.41 g, 91%). IR Data (KBr salt plate, cmꢀ1): 2967 (s),
2925 (s), 2876 (s), 2732 (w), 2713 (w), 2698 (w), 2658 (w),
2626 (w), 2581 (w), 1463 (s), 1369 (s), 1356 (m), 1336 (s),
1296 (m), 1267 (m), 1161 (s), 1128 (s), 1037 (s), 1000 (s),
977 (s), 942 (s), 830 (s), 806 ( m), 793 (m), 781 (m), 652
(s), 613 (s), 544 (w), 525 (w), 544 (w), 497 (w), 454 (m).
1H NMR (300 MHz, C6D6): d 4.29 (m, 1H, –CH), 1.59
(m, 1H, –CH2), 1.46 (m, 1H, –CH2), 1.28 (d, 3H, J = 6.2,
CH–CH3), 1.01 (t, 3H, J = 7.4, CH2–CH3). 13C NMR
(MHz, C6D6): d 82.50 (–CH), 34.20 (–CH2), 25.26 (CH–
CH3), 11.21 (CH2–CH3). Anal. Calcd for C16H36O4Ti: C,
0.00. Found: C, 49.25; H, 10.23; N, 2.01. ½aꢂD ¼ ꢀ20:9 (c
1.0, C6H6).
4.6. Synthesis of resolved Zr(OS-2Bu)4
To a benzene solution of tetrakis(dimethylamido)zirco-
nium (2.500 g, 9.34 mmol) at 17 °C was slowly added a
slight excess of S-(+)-2-butanol (2.850 g, 38.5 mmol) in
benzene. The reaction mixture was stirred under a positive
nitrogen flow at 17 °C for 20 min. The solution was
allowed to return to room temperature and the headspace
evacuated several more times over a 2 h period. The reac-
tion was stirred overnight, after which time the volume
56.47; H 10.66; N, 0.00. Found: C, 56.23; H, 10.45; N,
24
<0.02. ½aꢂD ¼ ꢀ83:1 (c 1.0, C6H6).

S. N. MacMillan et al. / Tetrahedron: Asymmetry 19 (2008) 543–548
547
was reduced by half. The remaining mixture was trans-
ferred to a distillation apparatus and the solvent removed
in vacuo. The residue was purified by high temperature dis-
tillation (150 °C, 10 mmHg) yielding a clear, colorless vis-
cous oil (2.295 g, 62%). IR Data (KBr salt plate, cmꢀ1):
2963 (s), 2930 (s), 2875 (s), 2839 (s), 2657 (w), 2625 (w),
2580 (w), 1464 (s), 1367 (s), 1347 (m), 1317 (w), 1296 (w),
1267 (m), 1133 (s), 1046 (s), 1004 (s), 983 (s), 931 (s), 916
(s), 826 (m), 797 (w), 779 (m), 675 (m), 606 (s), 555 (s),
489 (s). 1H NMR (300 MHz, C6D6): d 4.37 (m, 1H,
–CH), 1.95 (m, 1H, –CH2), 1.76 (m, 1H, –CH2), 1.46 (d,
3H, J = 6.2 Hz, CH–CH3), 1.00 (t, 3H, J = 7.5 Hz, CH2–
CH3). 13C NMR (MHz, C6D6): d 77.79 (–CH), 33.87
(–CH2), 24.10 (CH–CH3), 11.58 (CH2–CH3). Anal. Calcd
(w), 2659 (w), 2630 (w), 2586 (w), 1463 (s), 1368 (s), 1350
(s), 1318 (w), 1297 (w), 1268 (m), 1134 (s), 1096 (s), 1049
(s), 1006 (s), 981 (s), 962 (m), 934 (s), 904 (s), 827 (s), 802
1
(w), 779 (m), 604 (s), 581 (s), 559 (s), 493 (s), 430 (m). H
NMR (300 MHz, C6D6): d 4.49 (m, 1H, –CH), 1.97 (m,
1H, –CH2), 1.80 (m, 1H, –CH2), 1.48 (d, 3H, J = 5.5 Hz,
CH–CH3), 1.01 (t, 3H, J = 7.5 Hz, CH2–CH3). 13C NMR
(MHz, C6D6): d 77.60 (–CH), 33.95 (–CH2), 24.23 (CH–
CH3), 11.54 (CH2–CH3). Anal. Calcd for C16H36O4Hf: C,
40.81; H 7.70; N, 0.00. Found: C, 40.75; H, 7.51; N,
24
<0.05. ½aꢂD ¼ þ12:4 (c 1.0, C6H6).
Acknowledgments
for C16H36O4Zr: C, 50.09; H 9.46; N, 0.00. Found: C,
24
49.39; H, 11.69; N, 1.75. ½aꢂD ¼ þ22:65 (c 1.0, C6H6).
We thank the Office of the Dean of the Faculty of Vassar
College and the Petroleum Research Fund of the American
Chemical Society (PRF No. 46030-GB3) for support.
4.7. Synthesis of resolved Hf(OR-2Bu)4
To a benzene solution of tetrakis(dimethylamido)hafnium
(2.204 g, 6.21 mmol) at 17 °C was slowly added a slight
excess of R-(ꢀ)-2-butanol (1.890 g, 25.50 mmol) in ben-
zene. The reaction mixture was stirred under a positive
nitrogen flow at 17 °C for about 30 min. The solution
was allowed to return to room temperature and the head-
space evacuated several more times over a 2 h period.
The reaction was stirred overnight, after which time the
volume was reduced by half. The remaining mixture was
transferred to a distillation apparatus and the solvent
removed in vacuo. The residue was purified by high tem-
perature distillation (180 °C, 10 mmHg) yielding a clear,
colorless viscous oil (2.313 g, 79%). IR Data (KBr salt
plate, cmꢀ1): 2970 (s), 2932 (s), 2876 (s), 2844 (s), 2786
(m), 2733 (w), 2659 (w), 2629 (w), 2586 (w), 1463 (s),
1367 (s), 1349 (s), 1318 (w), 1297 (w), 1268 (m), 1229 (w),
1139 (s), 1097 (s), 1050 (s), 1006 (s), 982 (s), 962 (s), 933
(s), 904 (s), 827 (s), 802 (w), 779 (m), 603 (s), 582 (s), 559
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1
(s), 493 (s), 433 (s). H NMR (300 MHz, C6D6): d 4.48
(m, 1H, –CH), 1.96 (m, 1H, –CH2), 1.79 (m, 1H, –CH2),
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CH3). Anal. Calcd for C16H36O4Hf: C, 40.81; H, 7.70; N,
24
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4.8. Synthesis of resolved Hf(OS-2Bu)4
To a benzene solution of tetrakis(dimethylamido)hafnium
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´
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