Optically active bis(β-diketonate) complexes of titanium

Article abstract of DOI:10.1039/c0dt00828a

The 2,2′-bis(methylene)biphenyl bridged bis(diketonate) (Tol ₂Bob)Ti^IV fragment is resolved by reaction of the isopropoxide complex with (R)-1,1′-bi-2-naphthol, which selectively forms (S,Δ)-(Tol₂Bob)Ti(R-BINOL). The binap

Full text of DOI:10.1039/c0dt00828a

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www.rsc.org/dalton | Dalton Transactions
Optically active bis(b-diketonate) complexes of titanium†
Natcharee Kongprakaiwoot, Jack B. Armstrong, Bruce C. Noll and Seth N. Brown*
Received 12th July 2010, Accepted 6th August 2010
DOI: 10.1039/c0dt00828a
The 2,2¢-bis(methylene)biphenyl bridged bis(diketonate) (Tol2Bob)TiIV fragment is resolved by
reaction of the isopropoxide complex with (R)-1,1¢-bi-2-naphthol, which selectively forms
(S,D)-(Tol2Bob)Ti(R-BINOL). The binaphtholate ligand can be cleaved from titanium by careful
treatment with trifluoromethanesulfonic acid to give (S,D)-(Tol2Bob)Ti(OTf)2, which has a half-life
toward racemization of at least 34 h at 51 ◦C. Racemization of the (Tol2Bob)TiIV fragment is strongly
accelerated under protic conditions, probably due to protonolysis of one of the diketonate ligands.
Analogous optically active titanium bis(diketonates) can also be prepared by using an optically active
2,2¢-bis(methylene)-1,1¢-binaphthyl bridged bis(diketonate) ligand, Tol2Bobbinap, prepared from
(R)-BINOLH2. Complexation of (R)-Tol2BobbinapH2 with Ti(OiPr)4 gives only a single diastereomer
with the K configuration at titanium. The bis(diketonate)titanium(IV) fragment gives rise to
characteristic signals in circular dichroism spectra which can be used to identify the configuration at the
metal centre.
(e.g., eqn (1)).5 Compared to titanium complexes of simple
Introduction
unlinked diketonates, these complexes show greatly enhanced
optical stability (they are not fluxional by NMR spectroscopy),
and there is a strong linkage between the configuration of the
biphenyl backbone and the configuration at titanium (only the
(S,D)/(R,K) diastereomer is observed). Analogous complexes with
hydroxamates replacing one or both of the diketonates appear to
share these stereochemical features.6
Optically active transition metal complexes are of central im-
portance in the development of enantioselective metal-catalyzed
reactions. In most cases, the dissymmetric environment is provided
by an optically active organic ligand. Optically active metal
complexes containing achiral ligands—in other words, complexes
where the metal itself is the only chiral center—have been known
for over a century and played a key role in the elucidation
of the octahedral geometry of six-coordinate complexes.1 Such
complexes have, however, attracted relatively little attention for
their potential use in asymmetric catalysis.2
One limitation of chiral-at-metal complexes in these applica-
tions is that while it is relatively easy to resolve kinetically inert
complexes, they tend to be unreactive in catalytic reactions, while
labile complexes that are potentially reactive are difficult to resolve.
A case in point involves octahedral cis-bis(diketonato)titanium(IV)
complexes. The bis(diketonate)titanium(IV) fragment has been
shown to be remarkably selective in its binding to 1,1¢-bi-2-
naphtholate (BINOL), and its selectivity has been attributed to
its dissymmetric electronic structure rather than to any significant
nonbonding interactions with the BINOL moiety.3 It would be
intriguing to apply this electronic stereochemical control element
in a catalytic reaction. Unfortunately, the well-known proclivity
of bis(diketonate)titanium(IV) compounds to undergo trigonal
twists, resulting in racemization of the metal center on the NMR
timescale,4 would seem to preclude any application of this metal-
based chirality in asymmetric synthesis.
(1)
Here we describe the resolution of the (Tol2Bob)TiIV fragment
by its complexation with optically active BINOL. Cleavage of
the binaphtholate results in production of a reactive titanium
fragment with moderate optical stability, although protic condi-
tions result in rather facile racemization. The successful imple-
mentation of an alternative strategy to form an optically active
bis(diketonate)titanium fragment, by using an optically active 2,2¢-
bis(methylene)binaphthyl (rather than -biphenyl) linker, is also
described.
Experimental
More recently, we reported that bis(diketonates) with a 2,2¢-
bis(methylene)biphenyl bridge between the diketonate ligands
(“R2Bob” ligands) readily form cis-a chelates with titanium(IV)
General procedures
Unless otherwise noted, all procedures were carried out under inert
atmosphere using standard vacuum line or drybox techniques.
Methylene chloride and chloroform were dried over 4 A molecular
sieves, then transferred over calcium hydride. Diethyl ether was
dried over sodium/benzophenone, and hydrocarbon solvents over
Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall,
University of Notre Dame, Notre Dame, IN, 46556-5670, USA. E-mail:
Seth.N.Brown.114@nd.edu; Fax: +1 574 631 6652; Tel: +1 574 631 4659
† CCDC reference numbers 784598–784601. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00828a
˚
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sodium. Deuterated solvents were purchased from Cambridge
Isotope Laboratories and were dried in the same way as their
protio analogues. Dry solvents were vacuum transferred from
their respective drying agents either directly into reaction flasks,
or were stored in the drybox before use. Titanium tetraiso-
propoxide (Aldrich), (R)- or (S)-1,1¢-bi-2-naphthol (BINOLH2,
TCI America) and (R,R)- and (S,S)-2,2-dimethyl-a,a,a¢,a¢-
tetraphenyl-1,3-dioxolane-4,5-dimethanol (TaddolH2, Strem or
Aldrich) were commercial products and were used without further
purification.
1424 (m), 1358 (w), 1321 (w), 1301 (m), 1285 (m), 1269 (m),
1240 (w), 1219 (m), 1184 (m), 1158 (m), 1120 (s), 1100 (s), 1071 (s),
1051 (s), 1015 (s), 984 (m), 953 (m), 887 (s), 867 (s), 856 (s), 845
(s), 835 (s), 818 (w), 794 (m), 769 (s), 750 (m), 738 (m), 724 (s), 714
(s), 690 (s), 679 (m), 669 (s), 639 (s), 622 (s), 603 (m), 590 (s), 568
(s), 540 (s), 536 (m), 524 (m), 510 (m), 494 (s), 474 (s). FAB-MS:
m/z = 833 (M + H+). UV-Vis (CH2Cl2): lmax 388 nm (e = 1.7 ¥
104 M-1 cm-1), 292 (3.4 ¥ 104). CD (CH2Cl2): 485 nm (De = -5 M-1
cm-1), 430 (-7), 370 (-38), 321 (+31), 288 (+28), 261 (-86). Anal.
Calcd for C54H40O6Ti: C, 77.88; H, 4.84. Found C, 78.05; H, 5.14.
NMR spectra were measured on a Varian VXR-300 or VXR-
500 FT-NMR spectrometer and are reported in ppm downfield of
TMS (1H, 13C) or CFCl3 (19F). Infrared spectra were recorded as
nujol mulls on KBr plates on a Perkin Elmer Paragon 1000 FT-
IR spectrometer. UV-visible spectra were collected on a Beckman
DU-7500 diode-array spectrophotometer with samples in 1 cm
quartz cuvettes whose temperature was maintained by circulating a
water–ethylene glycol mixture through the cell block. Mass spectra
were obtained on a JEOL JMS-AX 505HA mass spectrometer us-
ing the FAB ionization mode and 3-nitrobenzylalcohol as a matrix.
In all cases, observed intensities were in satisfactory agreement
with calculated isotopic distributions. Circular dichroism spectra
were obtained as CH2Cl2 solutions (~5 ¥ 10-5 M) on an Aviv 62D2
spectropolarimeter. Elemental analyses were performed by M-H-
W Laboratories (Phoenix, AZ).
(S,D)-(Tol2Bob)Ti(OTf)2. In the drybox, (S,D,R)-(Tol2Bob)-
Ti(BINOL) (0.2017 g, 0.24 mmol), a magnetic stirrer bar and
8 mL CH2Cl2 were added into a scintillation vial. The solution
was stirred until all the solid dissolved, then transferred into a
250 mL Erlenmeyer flask. Into another scintillation vial, triflic
acid (0.0727 g, 0.48 mmol) was weighed and dissolved in 3 mL
CH2Cl2. The triflic acid solution was added to the solution of the
binaphtholate complex with rapid stirring. Immediately upon the
completion of addition, 70 mL of hexane was added. The dark
brown precipitate was filtered, washed with 2 ¥ 3 mL of hexanes
and dried in vacuo for 1 h. Yield: 0.1533 g (75%; the product is
contaminated with a small amount of the starting binaphtholate).
NMR data were identical to those previously reported for the
racemic material.5
(S,D,R,R)-(Tol2Bob)Ti(Taddol). In the drybox, (S,D,R)-
(Tol2Bob)Ti(BINOL) (0.3063 g, 0.37 mmol), (R,R)-TaddolH2
(0.1907 g, 0.41 mmol), 12 mL of chloroform and a magnetic
stirrer bar were added to a 20 mL vial. The vial was capped
tightly and taken out of the drybox. The mixture was heated
for 5 d in a 50 ◦C oil bath. After evaporation of the solvent,
the residue was redissolved in 4 mL benzene in the air, and then
12 mL hexane was added. After standing at room temperature
overnight, the reddish brown precipitate was filtered and washed
with 2 ¥ 1 mL hexanes. The precipitate was then redissolved in
4 mL dichloromethane and layered with 8 mL of hexanes. After
standing at room temperature for a few hours, deep red crystals
of (S,D,R)-(Tol2Bob)Ti(BINOL) precipitated. The mixture was
allowed to stand until the first yellow crystals of the product
appeared (~5 h), at which point the solution was decanted away
from the red crystals, which were discarded. After standing at room
temperature overnight, the product precipitated out as yellow
crystals which were isolated by decantation and washing with 2 ¥
1 mL hexanes. The decanted solution and washes were collected
and allowed to stand at room temperature until another crop
of product formed, and the process repeated to give a total of
four crops of (S,D,R,R)-(Tol2Bob)Ti(Taddol), for a total combined
yield of 0.1083 g (29%). 1H NMR (CDCl3): d 0.66 (s, 6H, C(CH3)2),
2.41 (s, 6H, tolyl CH3), 3.15 (d, 14 Hz, 2H, CHH¢), 3.93 (d, 14 Hz,
2H, CHH¢), 5.25 (s, 2H, COCHCO), 5.30 (s, 2H, Taddol OCH),
6.99 (m, 16H, ArH), 7.30 (m, 12H, ArH), 7.33 (dd, 7, 1.5 Hz,
4H, Taddol o-C6H5), 7.80 (dd, 7, 1.5 Hz, 4H, Taddol o-C6H5).
Preparation of (Tol2Bob)Ti complexes
(S,D,R)-(Tol2Bob)Ti(BINOL). In the drybox, 1.0294
g
(Tol2Bob)Ti(OiPr)2 (1.5441 mmol) was placed in a scintillation
vial with 0.2230 g (R)-1,1¢-binaphthyl-2,2¢-diol (0.7788 mmol,
0.50 equiv). The contents of the vial were dissolved in dry benzene,
which immediately turned a deep red color. The vial was allowed
to stand for one week, during which time a mixture of desired
product and starting material precipitated. The supernatant was
pipetted out of the vial and transferred to a small round-bottom
flask. The solid product was washed with 7 mL dry benzene to
remove the diisopropoxide complex, and the wash added to the
supernatant. The contents of the flask were then stripped down on
a rotary evaporator (in the air) and were returned to the drybox
to be redissolved in a minimal amount of dry benzene. Crystals
were harvested from this solution twice more in the manner
described above for a final yield of 0.4504 grams of air-stable
(S,D,R)-(Tol2Bob)Ti(BINOL) (0.5408 mmol, 70% yield based on
BINOL). If crystallization does not take place, the compound can
be purified by chromatography on silica gel in the air, eluting with
CHCl3. The (R,K,S) isomer is prepared analogously using (S)-
BINOLH2. 1H NMR (CDCl3): d 2.42 (s, 6H, CH3), 3.34 (d, 14 Hz,
2H, CHH¢), 4.04 (d, 14 Hz, 2H, CHH¢), 5.54 (s, 2H, COCHCO),
7.14 (d, 9 Hz, 4H, Tol 3,5-H), 7.21 (m, 10H, ArH), 7.40 (m, 4H,
ArH), 7.42 (d, 9 Hz, 4H, Tol 2,6-H), 7.49 (td, 7, 1.5 Hz, 2H,
biphenyl 4- or 5-H), 7.90 (d, 8 Hz, 2H, BINOL 5-H), 7.94 (d,
5
1
1
9 Hz, 2H, BINOL 3-H). 13C{ H} NMR (C6D6): d 21.83 (CH3),
13C{ H} NMR (CDCl3): d 21.84 (C(CH3)2), 27.52 (tolyl CH3),
46.14 (CH2), 103.90 (COCHCO), 117.61, 121.25, 123.71, 126.30,
127.43, 128.45, 128.61, 129.08, 129.37, 129.46, 130.13, 131.08,
132.96, 134.11, 135.08, 137.05, 140.99, 143.49, 164.41 (BINOL
C2), 181.78 (CH2CO), 191.74 (TolCO); one carbon not found. IR
(KBr, cm-1): 3054 (m), 3030 (m), 3013 (m), 1606 (m), 1579 (m),
1544 (w), 1520 (w), 1499 (w), 1472 (m), 1460 (m), 1438 (m),
46.33 (CH2), 82.46 (Taddol OCH), 95.32 (Taddol Ph2COTi),
101.05 (COCHCO), 110.33 (Taddol OC(CH3)2O), 126.43, 126.77,
126.83, 126.87, 127.80, 127.84, 128.05, 128.26, 128.99, 129.35,
129.46, 133.24, 133.41, 137.01, 140.47, 142.02, 144.17, 148.12,
179.92 (CH2CO), 190.63 (TolCO). IR (evapd film, cm-1): 3058 (w),
3022 (w), 2989 (w), 2921 (w), 1594 (m), 1582 (m), 1560 (m),
10106 | Dalton Trans., 2010, 39, 10105–10115
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1547 (m), 1519 (s), 1496 (s), 1447 (w), 1425 (w), 1412 (w), 1368
(s), 1323 (m), 1299 (w), 1242 (w), 1215 (w), 1184 (m), 1163 (w),
1120 (m), 1097 (m), 1074 (m), 1020 (m), 983 (m), 889 (m), 824 (w),
796 (m), 776 (m), 728 (m), 697 (m), 640 (m). UV-Vis (CH2Cl2):
cantation and washing with hexane, and a second crop isolated
after the washes were allowed to stand at -35 ◦C overnight. (The
washes from the lithiation procedure after the crystallization of
the second crop of lithio compound were quenched with aqueous
NH4Cl to recover 1.3908 g dimethylbinaphthyl [15%].) Each crop
of the crystalline lithium compound was carboxylated by the
following procedure: the material was placed in a three-neck round
bottom flask with a Teflon needle valve and sealed with rubber
septa, which was taken out of the drybox and affixed to a vacuum
line. Ether (40 mL) was added by vacuum transfer. After that,
under nitrogen flow one of the rubber septa was replaced with
a one-arm tee vented to a bubbler. At -78 ◦C, with the needle
valve closed, the second rubber septum was taken out, about
2 g of dry ice were added, and the septum replaced. The dark
red suspension turned clear yellow, and a light yellow precipitate
l
max 380 nm (e = 2.3 ¥ 104 M-1 cm-1), 285 (3.6 ¥ 104). CD (CH2Cl2):
367 nm (De = -39 M-1 cm-1), 319 sh (+21), 294 (+34). FABMS:
m/z 1012 (M+). Anal. Calcd for C65H56O8Ti: C, 77.07; H, 5.57.
Found: C, 77.23; H, 5.34.
(S,D,S,S)-(Tol2Bob)Ti(Taddol). This compound was prepared
using (S,D,R)-(Tol2Bob)Ti(BINOL) (0.2684 g, 0.32 mmol) and
(S,S)-TaddolH2 (0.1667 g, 0.36 mmol) by the same procedure
described for (S,D,R,R)-(Tol2Bob)Ti(Taddol) up through the
point at which the crude reaction mixture was stripped down
on a rotary evaporator. At this point, the residue was dissolved
in 4 mL benzene and a reddish brown powder was precipitated
by addition of 12 mL hexanes. After filtration and washing with
2 ¥ 1 mL of hexanes, the precipitate was redissolved in 4 mL
of dichloromethane and layered with 8 mL of hexanes. After
standing at room temperature for a few hours, deep red crystals of
unreacted (S,D,R)-(Tol2Bob)Ti(BINOL) formed. After standing
at room temperature overnight, the supernatant was pipetted away
from the red crystals and stripped down on a rotary evaporator.
The orange residue was dissolved in 2 mL CH2Cl2 and precipitated
with 4 mL hexanes. The yellow precipitate was collected by
filtration and washed with 2 ¥ 1 mL hexanes. The filtrate was
collected, evaporated to dryness, and a second crystallization from
CH2Cl2/hexanes (1 mL : 2 mL) yielded a second crop of product,
for a combined yield of 0.1024 g (S,D,S,S)-(Tol2Bob)Ti(Taddol)
◦
formed. After stirring at -78 C for 10 min, the precipitate was
filtered through a glass frit in the open air, washed with 2 ¥ 10 mL
ice-cold ether and transferred into a 250 mL round bottom flask.
The solid was digested by stirring with methylene chloride (80 mL)
and 6 M HCl (40 mL) until it had completely dissolved. The
mixture was poured into a 500 mL separatory funnel, and the
top water layer was discarded. A clear light yellow organic layer
was collected, dried over MgSO4, filtered and stripped down on a
rotary evaporator. The yellow residue was stirred in 20 mL CHCl3
and the white precipitate isolated by filtration and washing with
2 ¥ 5 mL chloroform. A second crop of the product was obtained
by stripping down the filtrate, resuspending in chloroform, and
filtering. The combined yield of (R)-1,1¢-binaphthyl-2,2¢-diacetic
acid was 4.5890 g (39%; 46% based on recovered starting material).
1
1H NMR (acetone-d6): d 3.40 (AB quartet, DdAB = 0.03 ppm, JAB
=
(31%). H NMR (CDCl3): d 0.70 (s, 6H, C(CH3)2), 2.43 (s, 6H,
tolyl CH3), 3.22 (d, 14 Hz, 2H, CHH¢), 3.93 (d, 14 Hz, 2H, CHH¢),
5.17 (s, 2H, COCHCO), 5.19 (s, 2H, Taddol OCH), 6.91 (t, 7 Hz,
2H, Taddol p-C6H5), 7.01 (t, 7 Hz, 4H, Taddol m-C6H5), 7.10
(t, 8 Hz, 2H, biphenyl 4- or 5-H), 7.16 (m, 12H, ArH), 7.30
(d, 8 Hz, 4H, Tol 2,6-H), 7.33 (d, 8 Hz, 2H, biphenyl 3- or 6-
H), 7.40 (t, 7 Hz, 2H, biphenyl 4- or 5-H), 7.56 (d, 8 Hz, 4H,
16.5 Hz, 4H, CHAHB), 7.03 (d, 8 Hz, 2H, 8-H), 7.24 (td, 7, 1 Hz,
2H, 6- or 7-H), 7.47 (td, 7, 1 Hz, 2H, 6- or 7-H), 7.70 (d, 8 Hz,
2H, 5-H), 8.00 (d, 8 Hz, 2H, 3- or 4-H), 8.05 (d, 8 Hz, 2H, 3- or
1
4-H). 13C{ H} NMR (acetone-d6): d 39.08 (CH2), 126.72, 126.96,
127.23, 128.94, 128.96, 129.47, 133.25, 133.74 (2C), 135.86, 172.50
(CO). IR (evapd film, cm-1): 3056 (w), 1714 (s, nCO), 1509 (w),
1410 (m), 1228 (m), 820 (m), 767 (m), 746 (w). FABMS: m/z 370
(M+). Anal. Calcd for C24H18O4: C, 77.82; H, 4.90. Found: C,
77.74; H, 4.76.
1
Taddol o-C6H5), 7.71 (m, 4H, Taddol o-C6H5). 13C{ H} NMR
(CDCl3): d 21.82 (Taddol C(CH3)2), 27.46 (tolyl CH3), 46.12
(CH2), 83.36 (Taddol OCH), 95.98 (Taddol Ph2COTi), 101.64
(COCHCO), 110.45 (Taddol OC(CH3)2O), 126.48, 126.67, 126.96,
127.55, 127.76, 128.05, 128.81, 129.48, 129.86, 133.70, 134.26,
136.88, 140.53, 141.90, 143.62, 147.98, 180.98 (CH2CO), 191.17
(TolCO); two aromatic carbons overlapped with other signals. IR
(evapd film, cm-1): 3056 (w), 3022 (w), 2994 (w), 2924 (w), 1594 (s),
1561 (m), 1519 (s), 1497 (s), 1447 (m), 1423 (m), 1413 (m), 1367
(s), 1327 (m), 1302 (m), 1244 (w), 1213 (w), 1185 (m), 1163 (m),
1119 (m), 1097 (m), 1073 (s), 1020 (m), 983 (m), 887 (m), 825 (w),
779 (m), 748 (m), 719 (m), 697 (m), 680 (m), 640 (m). FABMS: m/z
1013 (M+H+). UV-Vis (CH2Cl2): lmax = 290 nm, e = 4.4 ¥ 104 M-1
cm-1; lmax = 385 nm, e = 2.6 ¥ 104 M-1 cm-1. CD (CH2Cl2): 372 nm
(De = -23 M-1 cm-1), 307 (+26). Anal. Calcd for C65H56O8Ti: C,
77.07; H, 5.57. Found: C, 77.10; H, 5.64.
(R)-1,1¢-Binaphthyl-2,2¢-diacetyl chloride, (2-ClCOCH2C10H6)2.
In the drybox, (R)-1,1¢-binaphthyl-2,2¢-diacetic acid (1.5940 g,
4.30 mmol), PCl5 (2.0622 g, 9.90 mmol) and a magnetic stirrer
bar were charged into a 100 mL two-neck round-bottom flask.
One neck was capped with a rubber septum and the other was
attached to a Teflon needle valve. The flask was taken out of the
drybox and toluene (25 mL) was added. The reaction mixture was
stirred at room temperature under a flow of N2 for 30 min to give
a clear yellow solution. The solvent was removed in vacuo, and
the oily residue was heated with a hot water bath (100 ◦C) for
1 h under dynamic vacuum to sublime away any unreacted PCl5,
1
yielding a thick brown oil (1.6228 g, 93%). H NMR (C6D6): d
3.49 (AB quartet, DdAB = 0.06 ppm, JAB = 18 Hz, 4H, CHAHB),
6.93 (td, 7, 1 Hz, 2H, 6- or 7-H), 7.05 (d, 9 Hz, 2H, 8-H), 7.11
(d, 9 Hz, 2H, 5-H), 7.16 (td, 7, 1 Hz, 2H, 6- or 7-H), 7.61 (d,
Preparation of BobbinapH2
1
(R)-1,1¢-Binaphthyl-2,2¢-diacetic acid, (R)-(C10H6-2-CH2CO2H)2.
(R)-2,2¢-Dimethyl-1,1¢-binaphthyl7 (9.0563 g, 32.10 mmol) was
lithiated using n-BuLi/TMEDA according to the published
procedures,8 with the crystalline organolithium isolated by de-
9 Hz, 2H, 3- or 4-H), 7.63 (d, 9 Hz, 2H, 3- or 4-H). 13C{ H} NMR
(C6D6): d 51.29 (CH2), 126.84, 127.33, 127.83, 128.68, 128.84,
129.71, 130.06, 133.37, 133.82, 135.69, 171.99 (CO). IR (evapd
film, cm-1): 3057 (w), 2916 (w), 1790 (s, nCO), 1595 (w), 1509 (w),
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1398 (w), 1361 (w), 1330 (w), 1308 (w), 1223 (w), 1187 (w),
1145 (w), 1023 (w), 980, (m), 959 (m), 927 (w), 878 (w), 834 (w),
803 (w), 773 (m), 742 (w), 714 (m), 665 (w). HRMS (FAB): Calcd.
for C24H16Cl2O2 (M+): 406.0527. Found: 406.0507.
the drybox and affixed to a vacuum line. The solvent was removed
in vacuo, yielding a yellow oil. The oil was taken into the drybox
and triturated with 20 mL of hot hexanes. After the suspension
cooled to room temperature, a light yellow solid was isolated by
filtration and washed with 2 ¥ 2 mL hexanes. Yield: 0.7902 g, 71%.
1H NMR (C6D6): d 1.40 (d, 6 Hz, 6H, CH(CH3)(CH¢3)), 1.47 (d,
6 Hz, 6H, CH(CH3)(CH¢3)), 2.10 (s, 6H, Tol CH3), 3.04 (d, 13 Hz,
2H, CHH¢), 3.41 (d, 13 Hz, 2H, CHH¢), 4.60 (s, 2H, COCHCO),
5.19 (sept, 6 Hz, 2H, CH(CH3)2), 6.99 (d, 8 Hz, 4H, Tol 3,5-H),
7.05 (d, 8 Hz, 2H, 8-H), 7.11 (td, 7, 1 Hz, 2H, 6- or 7-H), 7.33
(td, 7, 1 Hz, 2H, 6- or 7-H), 7.43 (d, 8 Hz, 2H, 3- or 4-H), 7.45
(d, 8 Hz, 2H, 3- or 4-H), 7.76 (d, 8 Hz, 4H, Tol 2,6-H), 7.82 (d,
(R)-1,1¢-Binaphthyl-2,2¢-bis-(4-p-tolylbutane-2,4-dione), (C10H6-
CH2COCH C(OH)C6H4-p-CH3)2,
(R)-Tol2BobbinapH2. In
the drybox, solution of (R)-1,1¢-binaphthyl-2,2¢-diacetyl
a
chloride (1.2487 g, 3.07 mmol) in 20 mL of ether was prepared in
a 100 mL round-bottom flask sealed with a rubber septum. Into a
125 mL Erlenmeyer flask, LiN(Si(CH3)3)2 (2.0692 g, 12.37 mmol,
Aldrich), a magnetic stirrer bar and 20 mL of ether were added.
4¢-Methylacetophenone (1.70 mL, 12.7 mmol, Aldrich) was added
dropwise to the solution of LiN(Si(CH3)3)2. The flask was capped
with a rubber septum. The two solutions were taken out of the
drybox. The enolate solution was cooled in an ice bath for 20 min.
The acyl chloride solution was added dropwise to the stirring
enolate solution via syringe, forming a cloudy yellow mixture.
After stirring for 5 min at 0 ◦C, the mixture was suction filtered in
the air to isolate a yellow precipitate, which was washed with 3 ¥
10 mL ice-cold ether and transferred into a 250 mL round-bottom
flask. The volume of the filtrate was reduced to about 5 mL,
cooled in an ice bath for 10 min, filtered and washed as described
above, and combined with the first crop of solid. The solid was
digested by stirring with 50 mL 18% HCl and 50 mL benzene
until it had completely dissolved. The mixture was transferred
into a separatory funnel, and the aqueous layer was discarded.
The clear orange organic layer was washed with 50 mL saturated
NaHCO3, dried over MgSO4, filtered and stripped down on a
rotary evaporator, yielding an orange residue. The residue was
redissolved in 30 mL of ether, and the product crystallized upon
standing at 10 ◦C overnight. After filtration and washing with 2 ¥
10 mL ice-cold ether, the product was obtained as a light beige
solid. The filtrate was stripped down on a rotary evaporator, and
the purification step was repeated until no more of the product
1
8 Hz, 2H, 5-H). 13C{ H} NMR (CDCl3): d 21.82 (Tol CH3), 25.43
(CH(CH3)(C¢H3)), 25.47 (CH(CH3)(C¢H3)), 46.86 (CH2), 78.46
(CH(CH3)2), 99.84 (COCHCO), 125.88, 125.97, 126.85, 127.96,
128.16, 128.60, 128.95, 131.70, 131.95, 133.25, 134.15, 135.25,
136.27, 142.12, 179.75 (CH2CO), 189.80 (TolCO). IR (nujol mull,
cm-1): 1598 (s), 1553 (s), 1519 (s), 1495 (s), 1318 (m), 1183 (m),
1162 (m), 1118 (s), 1017 (s), 985 (s), 850 (m), 778 (m), 759 (m).
FABMS (NPOE matrix): m/z 766 (M+), 707 (M+ - OiPr), 648 (M+
- 2OiPr). UV-Vis (CH2Cl2): lmax 378 nm (e = 3.3 ¥ 104 M-1 cm-1),
284 nm (5.4 ¥ 104). CD (CH2Cl2): 386 nm (De = +31 M-1 cm-1),
346 (-33), 317 (-14), 286 (+54). Anal. Calcd for C48H46O6Ti: C,
75.19; H, 6.05. Found C, 74.90; H, 5.82.
(R,K)-(Tol2Bobbinap)Ti(OTf)2
In the drybox, (R,K)-(Tol2Bobbinap)Ti(OiPr)2 (0.3590 g,
0.47 mmol), 10 mL of benzene and a magnetic stirrer bar were
added into a scintillation vial. Trimethylsilyl trifluoromethanesul-
fonate (0.25 mL, 1.38 mmol) was added, immediately forming a
red solution. The vial was capped tightly, taken out of the drybox
◦
and heated 1 d in a 50 C oil bath. The mixture was taken into
the drybox and layered with 10 mL hexanes. A red precipitate
formed after standing at room temperature overnight. The solid
was isolated by decantation, washed with 2 ¥ 2 mL hexanes and
dried 1 h under vacuum to yield 0.4283 g (97%) of the bis(triflate)
1
precipitated out to give a combined yield of 1.3034 g (70%). H
NMR (C6D6): d 1.98 (s, 6H, CH3), 3.43 (AB quartet, DdAB
=
0.04 ppm, JAB = 15 Hz, 4H, CHAHB), 5.49 (s, 2H, COCHCO),
6.85 (d, 9 Hz, 4H, Tol 3,5-H), 6.93 (td, 7, 1 Hz, 2H, 6- or 7-H),
7.16 (td, 7, 1 Hz, 2H, 6- or 7-H), 7.24 (d, 8 Hz, 2H, 8-H), 7.56
(d, 9 Hz, 4H, Tol 2,6-H), 7.67 (d, 8 Hz, 2H, 3- or 4-H), 7.68
(d, 8 Hz, 2H, 3- or 4-H), 7.77 (d, 8 Hz, 2H, 5-H), 16.75 (s, 2H,
1
complex. H NMR (CDCl3): d 2.49 (s, 6H, Tol CH3), 3.32 (d,
14 Hz, 2H, CHH¢), 3.61 (d, 14 Hz, 2H, CHH¢), 4.87 (s, 2H,
COCHCO), 7.26 (d, 8 Hz, 2H, 3- or 4-H), 7.31 (d, 8 Hz, 4H,
Tol 3,5-H), 7.51 (m, 4H, 7- and 8-H), 7.61 (d, 8 Hz, 4H, Tol
2,6-H), 7.65 (d, 8 Hz, 2H, 3- or 4-H), 7.67 (m, 2H, 6-H), 8.02
1
OH). 13C{ H} NMR (C6D6): d 21.71 (CH3), 44.41 (CH2), 97.32
1
(d, 8 Hz, 2H, 5-H). 13C{ H} NMR (CDCl3): d 22.33 (Tol CH3),
(COCHCO), 126.51, 127.20, 127.39, 127.84, 128.67, 129.02,
129.11, 129.69, 132.71, 133.57, 133.81, 134.03, 136.12, 143.13,
183.11 (CH2CO), 194.87 (TolCO). IR (cm-1): 3051 (w), 2919 (w),
1609 (s, nCO), 1568 (m), 1505 (m), 1296 (m), 1270 (m), 1184 (m),
1117 (m), 777 (w), 763 (m). FABMS: m/z 603 (M+). Anal. Calcd
for C42H34O4: C, 83.70; H, 5.69. Found: C, 83.63; H, 5.84.
44.60 (CH2), 107.52 (COCHCO), 125.72, 127.25, 128.06, 128.36,
128.49, 129.82, 130.03, 130.30, 131.11, 131.56, 133.58, 133.90,
135.47, 148.11, 184.68 (CO), 189.30 (CO); CF3 not observed. 19
F
NMR (CDCl3): d -77.28. IR (nujol mull, cm-1): 1607 (w), 1526 (s),
1365 (s), 1318 (m), 1284 (w), 1238 (w), 1186 (s), 982 (m), 810 (w),
765 (w), 736 (w).
Preparation of (Tol2Bobbinap)Ti complexes
(R,K)-(Tol2Bobbinap)Ti(OiPr)2. In the drybox, (R)-Tol2-
BobbinapH2 (0.8700 g, 1.44 mmol) and a magnetic stirrer bar
were added into a 100 mL two-neck round-bottom flask. The
solid was dissolved in 30 mL of benzene. To this solution was added
titanium(IV) isopropoxide (0.50 mL, 1.6 mmol). The reaction flask
was capped with rubber septa, and the mixture was stirred at room
temperature for 5 min. After that, one of the rubber septa was
replaced with a Teflon needle valve. The flask was taken out of
(R,K,R,R)-(Tol2Bobbinap)Ti(Taddol)
In the drybox, a scintillation vial was charged with (R,K)-
(Tol2Bobbinap)Ti(OiPr)2 (0.2238 g, 0.29 mmol), (R,R)-TaddolH2
(0.2061 g, 0.44 mmol), benzene (10 mL) and a magnetic stirrer
bar. The vial was capped tightly and heated in a 75 C oil bath
overnight. The clear brown solution was opened to the air and
the volatiles removed on a rotary evaporator. The yellow residue
◦
10108 | Dalton Trans., 2010, 39, 10105–10115
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was redissolved in 5 mL benzene and then diluted to three times
its initial volume with hexanes, while stirri◦ng, forming a yellow
precipitate. The suspension was kept at 10 C overnight, filtered
and washed with 2 ¥ 2 mL hexanes, giving a bright yellow solid
(0.1950 g, 60%). 1H NMR (C6D6): d 0.91 (s, 6H, C(CH3)2), 2.16 (s,
6H, Tol CH3), 3.02 (d, 13.5 Hz, 2H, CHH¢), 3.34 (d, 13.5 Hz, 2H,
CHH¢), 4.49 (s, 2H, COCHCO), 5.84 (s, 2H, Taddol OCH), 6.80
(td, 7, 1.5 Hz, 2H, 6- or 7-H), 6.99 (d, 7.5 Hz, 4H, Tol 3,5-H), 7.02
(d, 8 Hz, 2H, 3- or 4-H), 7.04 (d, 8 Hz, 2H, 3- or 4-H), 7.18 (m,
12H, Taddol m-, p-C6H5), 7.32 (td, 7, 1.5 Hz, 2H, 6- or 7-H), 7.45
(d, 8 Hz, 2H, 8-H), 7.52 (d, 7.5 Hz, 4H, Tol 2,6-H), 7.82 (d, 8 Hz,
2H, 5-H), 8.02 (d, 8 Hz, 4H, Taddol o-C6H5), 8.16 (d, 8 Hz, 4H,
X-Ray crystallography
Crystals of (S,D,R)-(Tol2Bob)Ti(BINOL) were grown by
slow evaporation from ether, while crystals of (R,K)-
(Bobbinap)Ti(OiPr)2 were obtained from hexanes and
those of (S,D,R,R)-(Tol2Bob)Ti(Taddol) were grown from
dichloromethane/hexane. Crystals of (Tol2Bob)Ti(OTf)2·2CDCl3
deposited from a reaction of (S,D,R)-(Tol2Bob)Ti(BINOL) with
HOTf in CDCl3. In each case, the crystal to be analyzed was
placed in inert oil and transferred to the tip of a glass fiber
in the cold N2 stream of a Bruker Apex CCD diffractometer
(T = 100 K). Data were reduced, correcting for absorption and
decay, using the program SADABS. The structures were solved
using direct methods, and all nonhydrogen atoms were refined
anisotropically. The proper absolute configurations of the three
chiral crystals were confirmed by refining the Flack x parameter9
and in all cases agreed with the absolute configurations of the
starting materials used in the preparations. Hydrogen atoms were
located on difference maps and refined isotropically. Calculations
used SHELXTL (Bruker AXS),10 with scattering factors and
anomalous dispersion terms taken from the literature.11 Further
details about the structures are in Table 1.
1
Taddol o-C6H5). 13C{ H} NMR (C6D6): d 21.86 (Taddol CH3),
28.13 (Tol CH3), 46.91 (CH2), 84.16 (Taddol OCH), 97.13 (Taddol
CPh2OTi), 101.29 (COCHCO), 111.44 (Taddol OC(CH3)2O),
126.41, 126.44, 127.10, 127.24, 127.37, 127.59, 127.81, 128.18,
128.48, 128.92, 128.96, 129.44, 130.63, 132.48, 132.64, 133.88,
134.73, 135.82, 136.68, 142.56, 144.43, 149.03, 181.33 (CO), 191.34
(CO). IR (evapd film, cm-1): 3055 (w), 3024 (w), 2983 (w), 2921 (w),
1593 (m), 1561 (m), 1549 (w), 1518 (s), 1495 (s), 1447 (w), 1429 (w),
1411 (w), 1367 (s), 1321 (m), 1306 (w), 1287 (w), 1241 (w), 1215 (w),
1186 (m), 1164 (m), 1118 (m), 1097 (m), 1082 (m), 1070 (m),
1020 (m), 984 (w), 886 (m), 823 (w), 796 (w), 782 (m), 762 (m),
729 (m), 699 (m). FABMS: m/z 1113 (M + H+). UV-Vis (CH2Cl2):
l
max 382 nm (e = 2.6 ¥ 104 M-1 cm-1), 290 (6.4 ¥ 104). CD (CH2Cl2):
Results and discussion
384 nm (De = +31 M-1 cm-1), 338 (-16), 312 (-17), 284 (+36).
Anal. Calcd for C73H60O8Ti: C, 78.77; H, 5.43. Found: C, 78.70;
H, 5.24.
Resolution of the (Tol2Bob)Ti fragment through formation of its
binaphtholate
The (dike)2TiIV fragment shows a very strong ability to discrimi-
nate between the enantiomers of 1,1¢-bi-2-naphtholate (BINOL2-),
with only the (D,R)/(K,S) diastereomer of (dike)2Ti(BINOL)
observable by NMR spectroscopy.3 The same diastereomer has
also been observed in the solid state in (dike)2Ti(2,2¢-biphenolate)
complexes.12 In simple (dike)2Ti(BINOL) complexes, the highly
fluxional (dike)2Ti fragment can respond to the configuration of
the bound binaphtholate to adopt the lower-energy (matched)
configuration. We hypothesized that the much more geometrically
constrained linked diketonate fragment (Tol2Bob)Ti would form
both a matched and a mismatched diastereomer with BINOL,
which could in principle be separated. Use of the commercially
available optically active BINOL would then result in resolution
of the (Tol2Bob)Ti fragment.
(R,K,S,S)-(Tol2Bobbinap)Ti(Taddol)
This compound was prepared by the same procedure de-
scribed for (R,K,R,R)-(Tol2Bobbinap)Ti(Taddol) using (R,K)-
(Tol2Bobbinap)Ti(OiPr)2 (0.4062 g, 0.53 mmol) and (S,S)-
TaddolH2 (0.3725 g, 0.79 mmol). Yield: 0.2547 g, 43%. 1H NMR
(C6D6): d 0.87 (s, 6H, C(CH3)2), 2.19 (s, 6H, Tol CH3), 3.06 (d,
13.5 Hz, 2H, CHH¢), 3.42 (d, 13.5 Hz, 2H, CHH¢), 4.68 (s, 2H,
COCHCO), 5.90 (s, 2H, Taddol OCH), 6.81 (td, 7, 1.5 Hz, 2H, 6-
or 7-H), 6.88 (d, 7.5 Hz, 4H, Tol 3,5-H), 6.89 (td, 7, 1.5 Hz, 2H, 6-
or 7-H), 6.97 (d, 8 Hz, 2H, 3- or 4-H), 7.00 (d, 8 Hz, 2H, 3- or 4-H),
7.21 (m, 12H, Taddol m-, p-C6H5), 7.23 (d, 7.5 Hz, 4H, Tol 2,6-H),
7.40 (d, 8 Hz, 2H, 8-H), 7.75 (d, 8 Hz, 2H, 5-H), 7.77 (d, 8 Hz,
4H, Taddol o-C6H5), 8.26 (d, 8 Hz, 4H, Taddol o-C6H5). 13C{ H}
1
NMR (C6D6): d 21.92 (Taddol CH3), 28.19 (Tol CH3), 47.08
(CH2), 83.19 (Taddol OCH), 96.73 (Taddol CPh2OTi), 101.07
(COCHCO), 111.20 (Taddol OC(CH3)2O), 126.27, 126.40, 127.13,
127.39, 127.42, 127.50, 128.55, 128.81, 128.87, 128.92, 129.71,
130.27, 132.18, 132.57, 133.85, 134.15, 135.88, 136.60, 142.60,
144.94, 149.08, 180.51 (CO), 191.23 (CO); one aromatic carbon is
not observed. IR (evapd film, cm-1): 3054 (w), 3034 (w), 2990 (w),
2919 (w), 1593 (m), 1561 (m), 1547 (w), 1519 (s), 1496 (s), 1446 (w),
1412 (w), 1368 (m), 1320 (m), 1306 (w), 1243 (w), 1215 (w),
1184 (m), 1164 (m), 1120 (m), 1098 (w), 1074 (m), 1021 (m),
986 (m), 889 (m), 822 (w), 799 (m), 782 (w), 762 (w), 728 (w),
697 (m), 650 (w), 640 (w), 628 (w). FABMS: m/z 1113 (M + H+).
UV-Vis (CH2Cl2): lmax 379 nm (e = 3.1 ¥ 104 M-1 cm-1), 287 (6.4 ¥
104). CD (CH2Cl2): 372 nm (De = +73 M-1 cm-1), 336 (-14), 303
(-26). Anal. Calcd for C73H60O8Ti: C, 78.77; H, 5.43. Found: C,
77.67; H, 5.17.
In fact, racemic (Tol2Bob)Ti(OiPr)2 reacts slowly (~3 d at
RT in C6D6) with optically active BINOLH2 to give a mix-
ture of products. The most abundant product is a single di-
astereomer of chelated (Tol2Bob)Ti(BINOL). The majority of
the remaining material appears to be monodentate compounds
(Tol2Bob)Ti(OiPr)(BINOL-H), as judged by the presence of both
isopropyl signals and signals due to unsymmetrical Tol2Bob and
BINOL groups in the in situ 1H NMR spectra of reaction mixtures.
Reaction with 0.5 equiv. optically active BINOLH2 results in
formation of almost 50% of the chelated BINOL complex, with
about 50% remaining diisopropoxide complex and only traces of
monodentate BINOL complexes by 1H NMR. The air-stable red
binaphtholate chelate can be separated from the other materials by
either column chromatography or fractional crystallization from
benzene to give a pure single isomer in 35% isolated yield (70%
based on BINOLH2) (eqn (2)).
This journal is
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Table 1 X-Ray crystallographic details for (S,D,R)-(Tol2Bob)Ti(BINOL), (Tol2Bob)Ti(OTf)2·2CDCl3, (S,D,R,R)-(Tol2Bob)Ti(Taddol) and (R,K)-
(Tol2Bobbinap)Ti(OiPr)2
(S,D,R)-(Tol2Bob)Ti(BINOL) (Tol2Bob)Ti(OTf)2·2CDCl3 (S,D,R,R)-(Tol2Bob)Ti(Taddol) (R,K)-(Tol2Bobbinap)Ti(OiPr)2
Molecular formula
Formula weight
T/K
Crystal system
Space group
Total data collected
No. of indep reflns.
Rint
C50H40O6Ti
832.76
100(2)
Tetragonal
P43212
117 265
6907
0.0424
C38H28D2Cl6F6O10S2Ti
1087.34
100(2)
C65H56O8Ti
1013.00
100(2)
Triclinic
P1
21 338
11 051
0.0226
10542
9.1257(3)
11.5661(4)
13.5882(5)
65.765(2)
74.173(2)
87.031(2)
1255.28(8)
1
C48H46O6Ti
766.75
100(2)
Monoclinic
P21
37 130
9848
0.0233
9182
11.6568(3)
7.8240(2)
21.7522(6)
90
92.5330(16)
90
1981.92(9)
2
0.266
Triclinic
¯
P1
93 398
14 774
0.0417
11437
Obsd refls [I > 2s(I)] 6167
˚
a/A
13.8132(3)
13.8132(3)
21.5907(9)
90
90
90
4119.6(2)
4
11.1948(5)
14.8069(7)
16.0008(8)
62.997(2)
71.233(3)
78.832(3)
2233.95(18)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
m/mm-1
0.262
0.49 ¥ 0.23 ¥ 0.20
356
0.724
0.28 ¥ 0.18 ¥ 0.05
698
0.231
0.25 ¥ 0.21 ¥ 0.11
891
Crystal size/mm
No. refined params
R1, wR2 [I > 2s(I)]
0.29 ¥ 0.14 ¥ 0.13
680
0.0271,0.0685
0.0361, 0.0939
0.0428, 0.1008
0.0283, 0.0731
(2)
Based on previous studies, it was expected that the matched
diastereomer of the compound formed from (R)-BINOL would
have the D configuration at titanium,3 and that the D configuration
at titanium would correspond to the (S) configuration of the
biphenyl backbone of the Tol2Bob ligand.5 X-Ray crystallography
confirms that the chelated compound is indeed the predicted
isomer, (S,D,R)-(Tol2Bob)Ti(BINOL) (Fig. 1, Tables 1 and 2). The
compound crystallizes in the chiral tetragonal space group P43212
and has crystallographically required C2 symmetry. Its titanium–
oxygen distances are essentially identical to those shown in the
unlinked diketonate-BINOL complexes (dbm)2Ti(BINOL) and
t
t
˚
( BuCOCHCO Bu)2Ti(BINOL) (average Ti–BINOL 1.863(11) A,
˚
Ti–dike cis to BINOL 1.962(13) A, Ti–dike trans to BINOL
2.015(16) A). The aryloxide–titanium distances in the chelated
3
˚
BINOL derivatives are perceptibly longer, and the trans effect
perceptibly smaller, than in analogous complexes of monodentate
aryloxides such as (acac)2Ti(O-2,6- Pr2C6H3)2 (Ti–OAr 1.834(5) A,
Fig. 1 Thermal ellipsoid plot of (S,D,R)-(Tol2Bob)Ti(BINOL). Hydro-
gen atoms are omitted for clarity.
i
˚
13
cis-Ti–O 1.985(5) A, trans-Ti–O 2.046(5) A) or (tBu2Bob)Ti(O-
˚
˚
2,6-iPr2C6H3)2 (Ti–OAr 1.8173(11) A, cis-Ti–O 1.9685(10) A,
121.82(8)◦) interfere significantly with the ability of the in-plane
oxygen lone pair to donate to the metal center.15
˚
˚
5
˚
trans-Ti–O 2.0808(11) A). These differences may be attributed
to the poorer p donation possible from the chelating BINOL
ligand compared to monodentate aryloxides. While Ti–OAr
distances in aryloxides are not correlated with Ti–O–C angles
for complexes that display the usual obtuse angles observed
in monodentate aryloxides (140–180◦),14 the very acute angles
imposed by the nonplanar seven-membered chelate (Ti–O–C =
The enhanced stability of the (S,D,R) diastereomer of
(Tol2Bob)Ti(BINOL) is consistent with literature precedent and
has been justified on electronic grounds.3 However, the absolute in-
ability to observe significant amounts of the (R,K,R) diastereomer
is puzzling. Apparently the poorer p bonding in this diastereomer
outweighs the entropic benefit of the chelate effect, making the
10110 | Dalton Trans., 2010, 39, 10105–10115
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◦
˚
Table 2 Selected bond distances (A) and angles ( ) in (S,D,R)-(Tol2Bob)Ti(BINOL), (Tol2Bob)Ti(OTf)2·2CDCl3, (S,D,R,R)-(Tol2Bob)Ti(Taddol) and
(R,K)-(Tol2Bobbinap)Ti(OiPr)
(S,D,R)-(Tol2Bob)Ti(BINOL)
(Tol2Bob)Ti(OTf)2·2CDCl3
(S,D,R,R)-(Tol2Bob)Ti(Taddol)
(R,K)-(Tol2Bobbinap)Ti(OiPr)2
Ti–O1
Ti–O2
Ti–O3
Ti–O4
Ti–O5
Ti–O6
2.0044(10)
1.9808(9)
1.8682(10)
1.9279(12)
1.9140(11)
1.9190(12)
1.9073(11)
1.9940(12)
1.9906(12)
2.0304(11)
2.0002(11)
2.0604(11)
1.9611(11)
1.8014(10)
1.7880(10)
2.0549(9)
2.0148(10)
2.1342(9)
1.9983(10)
1.7811(10)
1.7930(9)
O1–Ti–O1a
O1–Ti–O2
O1–Ti–O3
O1–Ti–O2a
O1–Ti–O3a
O1–Ti–O4
O1–Ti–O5
O1–Ti–O6
O2–Ti–O2a
O2–Ti–O3
O2–Ti–O3a
O2–Ti–O4
O2–Ti–O5
O2–Ti–O6
O3–Ti–O3a
O3–Ti–O4
O3–Ti–O5
O3–Ti–O6
O4–Ti–O5
O4–Ti–O6
O5–Ti–O6
Ti–O3–C22
Ti–O5–X
84.23(6)
82.49(4)
93.15(4)
89.72(4)
168.69(4)
85.43(5)
87.24(5)
81.71(4)
81.09(4)
83.08(4)
79.01(4)
89.23(5)
176.02(5)
91.05(5)
88.04(5)
166.98(4)
96.35(5)
84.42(4)
167.77(4)
93.83(4)
169.51(6)
100.87(4)
86.50(4)
87.14(5)
87.54(4)
81.23(4)
171.21(5)
90.85(5)
96.62(5)
167.27(4)
90.46(5)
95.80(5)
161.38(4)
92.75(4)
97.38(4)
91.47(6)
85.63(5)
94.01(5)
175.74(5)
94.64(5)
90.45(5)
87.95(5)
83.41(4)
88.24(4)
175.48(5)
98.17(5)
92.81(5)
94.77(5)
82.89(4)
89.01(4)
172.81(4)
96.63(4)
97.17(4)
98.12(4)
121.82(8)
137.17(7)a
139.20(8)a
145.37(9)b
148.53(9)c
137.12(9)c
Ti–O6–Y
148.65(10)b
a X = S1, Y = S2. b X = C51, Y = C54. c X = C52, Y = C62.
chelated compound unstable with respect to alcoholysis under the
reaction conditions used. A similar effect was seen in the reaction
of (Fc2Bob)Ti(OiPr)2 with catechol, where the limited p donation
from the catechol favors formation of the monodentate catechol
complex (Fc2Bob)Ti(OC6H4-2-OH)2, and the chelate complex is
again unobservable.16
The binaphtholate ligand can be cleaved from
(Tol2Bob)Ti(BINOL), for example by treatment with trifluoro-
methanesulfonic acid to give (Tol2Bob)Ti(OTf)2. However, if the
solution is allowed to stand for an extended period, particularly
in the presence of excess acid, then the product racemizes. For
example, crystals of racemic (Tol2Bob)Ti(OTf)2·2CDCl3 deposit
upon treatment of optically active (S,D,R)-(Tol2Bob)Ti(BINOL)
with HOTf in CDCl3 (Fig. 2), although of course the (S,D)/(R,K)
relative configuration of the biphenyl and titanium centres is
retained, as it is in all R2Bob and related compounds observed to
date.5,6,16
distances are typical of those seen in other titanium triflates in
oxygen-rich environments.18
If a slightly deficient amount of HOTf is used and the
product precipitated promptly with hexane, then optically pure
(S,D)-(Tol2Bob)Ti(OTf)2 can be prepared from the binaphtho-
late complex (eqn (3)), although the triflate is contaminated
by traces (~2%) of the binaphtholate. The optical purity can
be judged by reaction of the compound with (R,R)-TaddolH2
in the presence of Et3N, which gives diastereomerically pure
(S,D,R,R)-(Tol2Bob)Ti(Taddol) (see below). Once isolated, (S,D)-
(Tol2Bob)Ti(OTf)2 has modest optical stability. Upon heating for
2 d at 51 ◦C in CDCl3, the optical purity decreases to 38% ee
(as judged by quenching with (R,R)-TaddolH2/Et3N), giving an
approximate half-life for racemization of 34 h at 324 K. Since
this reaction may be catalyzed by traces of protic impurities in
the solvent, this represents a lower limit of the intrinsic optical
stability of (Tol2Bob)Ti(OTf)2.
The short Ti–diketonate bonds in (Tol2Bob)Ti(OTf)2 (Table 2)
suggest an extremely electron-poor titanium centre in this com-
pound. For example, the axial diketonate Ti–O bonds are shorter
than the corresponding bonds in most other bis(diketonate) com-
˚
pounds, with the only rivals being the 1.905–1.928 A bonds found
in bis(diketonate) titanium chlorides.17 The equatorial Ti–O bonds
trans to triflate are the shortest known in (dike)2Ti compounds,
˚
shorter even than the 1.944 A bond trans to ether in the cationic
etherate [(Tol2Bob)Ti(OiPr)(OEt2)]BArF.5 The titanium–triflate
(3)
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Fig. 3 Thermal ellipsoid plot of (S,D,R,R)-(Tol2Bob)Ti(Taddol). Hydro-
gens are omitted for clarity.
Fig. 2 Thermal ellipsoid plot of (Tol2Bob)Ti(OTf)2·2CDCl3. Hydrogens
and solvent of crystallization are omitted for clarity. The crystal is racemic,
with the (R,K) isomer shown.
more stable) original diastereomer. The sensitivity of the optical
(but not chemical) stability to protic reagents suggests that such
compounds reversibly protonolyze one arm of the bis(diketonate)
ligand, which readily dissociates from the titanium, rotates around
the biaryl bond with a much lower barrier, and recoordinates to
the titanium.
Given the facile racemization of the (Tol2Bob)Ti fragment
in the presence of alcohols, one would expect that reaction of
(Tol2Bob)Ti(OiPr)2 with 1 equiv. optically active BINOLH2 to give
high yields of the more stable (S,D,R)-(Tol2Bob)Ti(BINOL), since
epimerization of (R,K)-(Tol2Bob) would be driven by formation
of the more thermodynamically stable binaphtholate.20 Unfortu-
nately, such a thermodynamic resolution has not yet been achieved
in practice. Yields of (S,D,R)-(Tol2Bob)Ti(BINOL) do not exceed
50% even in the presence of excess (R)-BINOLH2 or at extended
reaction times.
The (Tol2Bob)Ti fragment can undergo alcohol exchange
reactions with retention of configuration at titanium. For
example, heating (S,D,R)-(Tol2Bob)Ti(BINOL) with (R,R)-
TaddolH2 produces principally one diastereomer, (S,D,R,R)-
(Tol2Bob)Ti(Taddol), which can be isolated in pure form af-
ter recrystallization and whose stereochemistry was verified by
X-ray crystallography (Fig. 3). The metrical data for the complex
(Table 2) are unexceptional, although there is a slight distortion
from ideal C2 symmetry, particularly in the titanium–diketonate
distances, probably caused by packing forces in the crystal. The
structure of the unlinked bis(diketonate)titanium(IV) complex
(acac)2Ti(Taddol) has been reported and appears to be the (K,R,R)
isomer in the solid, though no details were provided.19
Examination of NMR spectra of the crude reaction mixture
does reveal small amounts (~5%) of the other diastereomer, whose
NMR properties can be verified by preparation of its enantiomer
from (S,S)-TaddolH2 and (S,D,R)-(Tol2Bob)Ti(BINOL). Thus,
the (Tol2Bob)Ti fragment racemizes slightly under these condi-
tions. Other protic reagents also cause racemization; for example,
heating (S,D,R)-(Tol2Bob)Ti(BINOL) in neat 2-propanol (74 ◦C,
1 wk) produces (Tol2Bob)Ti(OiPr)2 in modest yield and in
completely racemic form.
Overall, (Tol2Bob)TiX2 complexes are vastly more stable to
racemization than unlinked (dike)2TiX2 complexes. For example,
(acac)2TiCl2 racemizes with a rate constant (extrapolated to 324 K)
of 1400 s-1,4a about 2 ¥ 108 times faster than (Tol2Bob)Ti(OTf)2.
The enhanced optical stability can be rationalized on the ba-
sis of the tight coupling between the biaryl and the metal-
centred chirality: even if epimerization at titanium takes place,
the configuration at the biphenyl group is retained, effectively
promoting re-epimerization at titanium to restore the (much
Optically active complexes of bis(methylene)binaphthyl bridged
bis(b-diketonates): the Bobbinap ligand
Because of the sensitivity of the optical stability of the (Tol2Bob)Ti
fragment to protic media, we sought an alternative strategy
to produce an enantiomerically pure bis(diketonate) fragment.
Given the tight coupling between the configuration of the
bis(methylene)biphenyl linker and the configuration at titanium
observed in all (R2Bob)Ti complexes, it seemed reasonable that
a bis(diketonate) ligand bridged by an optically stable 2,2¢-
bis(methylene)-1,1¢-binaphthyl fragment would provide a suit-
able entry to such compounds. The preparation of this “Bob-
binap” ligand is outlined in Scheme 1. Optically active (R)-2,2¢-
dimethylbinaphthyl (prepared in two steps from commercially
available (R)-BINOLH2 ) is lithiated with nBuLi/TMEDA as
7
initially described for the racemic compound by Raston and
coworkers8a and more recently applied to the optically active
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complex compared to the biphenyl complex (Dd = -0.35 and
-0.74 ppm, respectively, in C6D6), suggesting that these hydrogens
are in the shielding cone of the backbone naphthyl groups. NMR
spectroscopy shows that only one diastereomer of the compound
is present, and that isomer is confirmed to be the (R,K) isomer
by X-ray crystallography (Table 1, Fig. 4). Bond lengths and
angles in the compound are unexceptional (Table 2), although
the compound shares with the Taddol complex a slight distortion
from C2 symmetry. The presence of the binaphthyl group, rather
than biphenyl, in the backbone of the bridged diketonate has
little effect on its structure. Indeed, despite the additional steric
hindrance afforded by benzannulation, the (Bobbinap)Ti(OiPr)2
shows the smallest dihedral angle between the biaryl groups of
any of the compounds described here (105.3(2)◦, compared to
115.2(2)◦ in the BINOL complex, 110.4(2)◦ in the bis(triflate), or
114.0(2)◦ in the Taddol complex).
Scheme 1 Synthesis of Tol2BobbinapH2.
material.8b Carboxylation of the crystalline organolithium pro-
vides the diacid, which is converted to the acyl chloride using PCl5.
Formation of the bis(diketone) is achieved by Claisen condensa-
tion with 4 equiv. of the lithium enolate of 4¢-methylacetophenone
to give Tol2BobbinapH2. As we have previously observed,5,6,16 use
of two additional equivalents of the enolate as the sacrificial
base is critical in working with this substrate which contains
acidic a hydrogens, and precipitation of the lithium salt of the
diketonate prior to workup greatly simplifies purification of the
final bis(diketone).
The bis(diketone) ligand (R)-Tol2BobbinapH2 is readily met-
alated on treatment with Ti(OiPr)4 (Scheme 2). Analogous
metalation of the biphenyl-bridged Tol2BobH2 ligand at room
temperature produces a mixture of monomeric and oligomeric
compounds, which require heating in the presence of excess
Ti(OiPr)4 to convert to the desired monomer.5 In contrast, the
binaphthyl-bridged compound gives only monomeric product
(as judged by in situ 1H NMR spectroscopy), so metalation
can be carried out at room temperature and without excess
1
titanium. The H NMR spectrum of (Tol2Bobbinap)Ti(OiPr)2 is
very similar to that of its biphenyl-bridged analogue, except that
one of the diastereotopic methylene protons and the diketonate
methine proton are shifted markedly upfield in the binaphthyl
Fig. 4 Thermal ellipsoid plot of (R,K)-(Tol2Bobbinap)Ti(OiPr)2. Hydro-
gen atoms are omitted for clarity.
The alkoxide ligands in (R,K)-(Tol2Bobbinap)Ti(OiPr)2 are
readily replaced by alcohol exchange. For example, the com-
plex reacts with either (R,R)- or (S,S)-TaddolH2 to form the
diastereomeric chelated complexes (R,K,R,R)- or (R,K,S,S)-
(Tol2Bobbinap)Ti(Taddol) (Scheme 2). In each case, monitoring
the reactions in situ indicates that they proceed quantitatively to
give only a single diastereomer, with no trace of the alternative
diastereomer detectable by 1H NMR spectroscopy. This confirms
that the stereochemical integrity of the binaphthyl moiety has
been preserved throughout the ligand synthesis, metalation, and
alcohol exchange reaction. The isopropoxide groups can also be
removed by treatment with trimethylsilyl triflate to produce the
triflate complex (R,K)-(Tol2Bobbinap)Ti(OTf)2.
Scheme 2 Preparation and reactivity of (R,K)-(Tol2Bobbinap)Ti(OiPr)2.
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Circular dichroism spectroscopy of optically active
bis(diketonato)titanium(IV) complexes
The availability of optically active titanium(IV) bis(diketonate)
complexes allows for the first time an investigation of their
chiroptical properties. The circular dichroism spectrum of (S,D,R)-
(Tol2Bob)Ti(BINOL) displays a feature with a negative Cotton
effect at 370 nm associated with an optical band at 388 nm, and two
peaks with positive Cotton effects at 321 and 288 nm associated
with the UV feature at 292 nm (Fig. 5).
Fig. 6 Circular dichroism spectra (CH2Cl2) of (S,D,S,S) (solid line) and
(S,D,R,R) (dashed line) isomers of (Tol2Bob)Ti(Taddol).
Fig. 5 UV-visible (dotted line, right axis) and CD (solid line, left axis)
spectra of (S,D,R)-(Tol2Bob)Ti(BINOL) in CH2Cl2.
These two optical bands have been assigned as largely diketonate
p–p* in character, but with significant admixture of ligand to
titanium charge transfer character.21 The weaker negative Cotton
effects in the CD at long wavelength are presumably associated
with the low-energy tail of the 388 nm band discernible in the UV-
visible spectrum; similar splitting of this band has been observed
previously in halide complexes such as (Tol2Bob)TiCl216 and is not
observed in alkoxide complexes. There is also a short-wavelength
feature with strongly negative ellipticity at 261 nm.
The same general features are also apparent in the CD spectra
of (S,D)-(Tol2Bob)Ti(Taddol) complexes (Fig. 6), and in mirror
image in the (R,K)-(Tol2Bobbinap)Ti alkoxides (Fig. 7). Notably,
the signs of the CD features are unaffected by the configuration of
the Taddol ligand (and are similar in the isopropoxide complex as
well). There are noticeable differences among the sets of spectra.
For example, the splitting of the positive band at ~310 nm that
was evident in the spectrum of the binaphtholate complex is
much less apparent in the spectra of the (Tol2Bob)Ti(Taddol)
complexes, although the presence of two bands is discernible in the
unsymmetrical peak shape of the feature. Likewise, the negative
peak at high energy is not as distinct in the (Tol2Bob)Ti(Taddol)
complexes. Conversely, the splitting of the medium energy band
is very apparent in the Bobbinap complexes, and the high-energy
feature is also prominent (now at positive De, as expected for
these K-configured complexes), and are shifted ~20 nm to longer
wavelength. (Curiously, there is no corresponding shift in the UV-
visible spectra, which are very similar between the Tol2Bob and
Tol2Bobbinap complexes.) The intensities of the bands vary as
well; for example, the (S,D,R,R)/(R,K,S,S) complexes have long-
wavelength bands that are consistently about twice as intense (and
Fig. 7 Circular dichroism spectra (CH2Cl2) of (R,K,R,R)-(Tol2Bobbi-
nap)Ti(Taddol) (solid line), (R,K,S,S)-(Tol2Bobbinap)Ti(Taddol) (dashed
line), and (R,K)-(Tol2Bobbinap)Ti(OiPr)2 (dotted line).
peak at ~5 nm shorter wavelength) than their (S,D,S,S)/(R,K,R,R)
diastereomers. But the general features of the CD spectra are
sufficiently consistent to be reliable reporters on the configuration
of the bis(diketonate)titanium fragment, with a negative Cotton
effect at ~370 nm and a positive Cotton effect at ~320 nm indicating
a D configuration. This could be useful in probing the ability of
ligands to induce chirality on configurationally mobile (dike)2Ti
complexes in solution.
Conclusions
Two strategies have been implemented successfully to pro-
duce optically active bis(diketonate) complexes of titanium(IV).
Both strategies rely on the high configurational stability of
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2,2¢-bis(methylene)biaryl-linked bis(diketonato) (“Bob”) com-
plexes of titanium, which contrasts with the fluxionality of simple
bis(diketonate)titanium(IV) complexes. In the first strategy, the
racemic biphenyl-bridged complex can be resolved by selective
formation of the (S,D,R) diastereomer of (Tol2Bob)Ti(BINOL)
using optically active (R)-BINOL. While the binaphtholate can
be removed from titanium on careful treatment with triflic acid
to form optically active (Tol2Bob)Ti(OTf)2, the optical stability of
the compound is modest (t1/2 for racemization = 34 h at 51 ◦C)
and racemization is strongly accelerated in the presence of protic
reagents. The second strategy builds chirality into the backbone
using an optically active 2,2¢-bis(methylene)-1,1¢-binapthyl bridge;
strong geometric coupling between the biaryl bridge enforces a
K configuration at titanium using the (R)-Tol2Bobbinap ligand.
Regardless of ancillary ligation, the circular dichroism spectra
show a negative ellipticity at ~370 nm and positive ellipticity
at ~320 nm for the D isomers, with the K isomers showing the
reverse pattern. Application of these optically active complexes in
stereoselective synthesis is currently being explored.
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Acknowledgements
We gratefully acknowledge the US National Science Foundation
(CHE-0518243) for financial support of this work. Partial support
for the X-ray diffraction facility was also provided by the NSF
(CHE-0443233).
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©