Stereoselective dimerization of racemic C3-symmetric Ti(iv) amine triphenolate complexes

Article abstract of DOI:10.1039/b702958f

A novel, mononuclear Ti(iv) amine triphenolate complex obtained by reaction of Ti(OiPr)₄ with tris(2-hydroxy-3-phenylbenzyl)amine bearing phenyl ortho-substituents affords quantitatively and spontaneously the corresponding heterochiral -oxo din

Full text of DOI:10.1039/b702958f

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Stereoselective dimerization of racemic C3-symmetric Ti(IV) amine
triphenolate complexes†‡
Ge´rald Bernardinelli,a Thomas M. Seidel,b E. Peter Ku¨ndig,*b Leonard J. Prins,c Andrej Kolarovic,c
Miriam Mba,c Marta Pontinic and Giulia Licini*c
Received 26th February 2007, Accepted 1st March 2007
First published as an Advance Article on the web 15th March 2007
DOI: 10.1039/b702958f
A novel, mononuclear Ti(IV) amine triphenolate complex
obtained by reaction of Ti(OiPr)4 with tris(2-hydroxy-3-
phenylbenzyl)amine bearing phenyl ortho-substituents af-
fords quantitatively and spontaneously the corresponding
heterochiral l-oxo dinuclear compound whereas an analo-
gous chiral, enantiopure complex maintains its mononuclear
structure even in the presence of an excess of water.
stereoselective, since a similar chiral titanatrane 2b in enantiopure
form maintains its mononuclear structure and does not form
the corresponding l-oxo complex, not even in the presence
of large amounts of water and under much harsher reaction
conditions. Therefore a stereo-self-discrimination process between
two enantiomeric structures is at the origin of the l-oxo complex
formation.
The amine triphenol ligands 1 are readily available via an
efficient route involving reductive amination.6 Reaction of 1a with
one equivalent of Ti(OiPr)4 in dry CHCl3 yields the corresponding
Tripodal C3 symmetric amine triphenolate ligands 1 have attracted
attention in the last years due to their ability to form com-
plexes with a wide variety of transition metals and main group
elements.1–5 In particular, earlier reports on Ti(IV) complexes 2,
based on NMR data and supported by X-ray crystallography,
show that they are present as a racemic mixture of D and
K complexes, resulting from a helical wrapping of the ligand
1
mononuclear, C3 symmetric Ti(IV) complex 2a. The H NMR
spectrum of 2a in CDCl3 shows a single set of signals for the
aromatic protons and the six methylene protons appear as a single
broad singlet at 3.62 ppm. This is in agreement with the formation
of a C3 symmetric complex in which the two enantiomers racemise
at room temperature (Scheme 1).
around the metal ion, with racemisation barriers of DG‡
=
65.7–74.4 kJ mol−1.3 These reports also demonstrated that the
stability of titanatranes 2 in solution strongly depends on the
steric size of the peripheral substituents.3,4 Complexes bearing
small ortho substituents tend to give aggregates like l-oxo and
bis l-oxo complexes in the presence of traces of water. In contrast,
complexes bearing bulky groups like tert-butyl are highly stable
and resistant to hydrolysis. The corresponding l-oxo complex
cannot be obtained by direct hydrolysis of the alkoxide, which is
moisture stable, but only via evaporation to dryness of a solution of
the corresponding Ti(IV)–hydroxy complex. A bent l-oxo bridge
(155.53◦) was determined in the crystal structure and it was
observed that the l-oxo bridge was not stable in solution and
easily hydrolysed in the presence of traces of water.1
Here we report that a related titanatrane 2a, bearing bulky
phenyl substituents in ortho positions, spontaneously forms a
highly stable l-oxo dinuclear complex in the presence of only traces
of water, with rather different structural characteristics: a linear
Ti–O–Ti bond with the peripheral phenyl groups intercalating
and disposed in a propeller like fashion shielding the Ti–O–Ti
core and with the two chiral titanatrane groups having opposite
configuration (D and K). The dimerization process is highly
Scheme 1 Reagents and conditions: (i) Ti(O-i-Pr)4 (1 equiv.), CHCl3, rt;
(ii) CHCl3–H2O, rt.
However, upon standing in solution, complex 2a readily trans-
forms into a new highly symmetric species, whose 1H NMR
spectrum again shows a single set of signals for the ligand protons,
but also reveals that the apical i-PrOH ligand of 2a is released.
Interestingly, the methylene protons of the new species now appear
as an AB system with resonances at 4.36 and 3.28 ppm respectively,
indicating that the racemisation rate of the titanatrane moiety in
the new species is slower on the NMR time scale. The spontaneous
formation of the new species is quantitative in a few hours and
light yellow crystals precipitated out of the solution. ESI-MS
aLaboratory of X-ray Crystallography, University of Geneva, 24 Quai Ernest
Anserment, 1211, Geneva 4, Switzerland
bDepartment of Organic Chemistry, University of Geneva, 30 Quai
Ernest Anserment, 1211, Geneva 4, Switzerland. E-mail: Peter.Kundig@
chiorg.unige.ch; Fax: +41 223793215; Tel: +41 223796093
cDipartimento di Scienze Chimiche, via Marzolo I-35131, Padova, Italy.
E-mail: giulia.licini@unipd.it; Fax: +39 049 8275239; Tel: +39 0498275289
† The HTML version of this article has been enhanced with colour images.
‡ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b702958f
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experiments (1 lM, acetonitrile, HCOOH 0,1%, as mobile phase)
revealed that the new species is a dinuclear l-oxo complex 3.
Interestingly, 3 is stable under ESI experimental conditions being
the only species detectable in the spectrum as the H+ (m/z = 1233),
Na+ (m/z = 1255) and K+ (m/z = 1271) adducts.
˚
˚
of the phenyl ring are 2.74 A or 2.78 A which suggests participation
of CH–p interactions in the stabilisation of the complex.10 This
structural characteristic is reflected also in solution. The 1H NMR
spectrum of 3 exhibits significant upfield shifts for the signals of
the peripheral phenyls (ca. 1 ppm). Interestingly, only three sets
of signals are present for the ortho, meta and para protons, with
relative intensities in a 2 : 2 : 1 ratio, thus indicating fast rotation
of the phenyl rings on the NMR time scale.
The synthesis of the complex in a mixture of CHCl3–C6H6 al-
lowed us to obtain crystals suitable for X-ray analysis (Fig. 1). The
crystal structure of 37 confirms the formation of a dinuclear Ti(IV)
complex with a linear l-oxo bridge linking two titanatrane units
of opposite stereochemistry. The l-oxo bridge is located on a
crystallographic centre of inversion leading to a perfectly linear
Ti–O–Ti bond angle. This contrasts with the bent bridge found
in a corresponding l-oxo complex with t-Bu ortho- and para-
The two titanatrane units in 3 maintain the helical conformation
around the metal ion as in the monomeric complexes but,
remarkably, starting from a racemic mixture of complexes, D and
K, exclusively the heterochiral compound (D–K) is formed. No
traces of the diastereomeric homochiral complexes (D–D or K–K)
are present in the 1H NMR spectrum. VT NMR experiments up to
100 ◦C (300 MHz, toluene-d8) showed no significant modification
of the spectrum nor the appearance of another diastereomeric
substituents.1 The Ti–Ocentral distance (1.7983(7) A) is in the range
˚
observed in other similar complexes8,9 but it is shorter than the one
1
˚
found in complexes bearing a bent bridge (1.8251 and 1.8256 A).
◦
The Ti atom shows a trigonal bipyramidal geometry with Ti–N
species. The fact that even at 100 C the methylene signals of 3
˚
bonds of 2.327(3) A.
remain an AB system indicates that the new complex is much more
stable towards racemisation than the mononuclear complex 2.
In order to determine whether or not the formation of the l-
oxo complex 3 implies a stereo-self-discrimination process, we
decided to investigate the behaviour of an analogous, enan-
tiomerically pure complex. This requires complete control of
ligand helicity in the complex. Canary and co-workers had
reported that the helicity of Zn(II) and Cu(II) tripodal complexes
of tris(pyridylmethyl)amino ligands can be controlled by the
introduction of a single stereogenic center in one of the benzylic
positions.11 We speculated that this observation in 5-membered
metallacycles could also hold for 6-membered metallacycles
present in the enantiomerically pure Ti(IV) amine tri(phenolate)
complex 2b. The synthesis of ligand (S)-1b was undertaken to
confirm or refute this hypothesis. The route chosen was analogous
to that published recently by one of us.12 Conversion of hydrox-
yarylaldehyde 4 to its benzylether and imine formation with S-
(+)-2-amino-2-phenylethanol afforded (S)-5. MeLi addition gave
the expected, diastereoisomerically highly enriched (S,S)-6 which
was converted to the HCl salt (S)-7 of the primary amine (S)-7
via Pb(OAc)4 oxidation followed by hydrolysis. Reaction of (S)-
7 with two equivalents of 2-benzyloxy-3-bromomethyl-biphenyl,
followed by hydrogenolysis of the benzyl groups gave ligand (S)-1b
(Scheme 2).
Fig. 1 Top: ORTEP view of 3. Ellipsoids are represented at 40%
probability level. The central O atom is located on a centre of inversion.
◦
˚
Selected bond distances [A] and angle [ ]: Ti–Ocentral = 1.7983(7), Ti–N =
2.327(3), Ti–O–Ti = 180.0. Disordered solvent molecules are omitted for
clarity. Bottom : View along the N–Ti–O–Ti–N axis.
Scheme 2 Reagents and conditions: (i) K2CO3, BnBr (1.2 equiv),
CHCl3–MeOH; (ii) (S)-(+)-2-amino-2-phenylethanol (1.0 equiv), EtOH,
MS, rt; (iii) MeLi (4 equiv.), THF, −78 ◦C; (iv) Pb(OAc)4 (1.2 equiv.),
K2CO3, MeOH; (v) AcOH–H2O; (vi) Et2O–HCl; (vii) K2CO3, 2-benzy-
loxy-3-bromomethyl-biphenyl (2 equiv.), MeCN, reflux; (viii) H2, Pd/C.
The peripheral phenyl rings from the two ligands intercalate
around the l-oxo bridge: each internal ortho proton points toward
the p-cloud of the next ring, belonging to the other titanatrane
moiety. The perpendicular distances between Hortho and the centre
1574 | Dalton Trans., 2007, 1573–1576
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Ligand (S)-1b cleanly reacts with Ti(O-i-Pr)4 in dry CHCl3
yielding the complex (S)-2b (Scheme 1). The 1H NMR spectrum in
CDCl3 shows the quantitative formation of a single mononuclear
Ti(IV) complex. Due to the loss of C3 symmetry of the system,
We acknowledge with thanks the sponsorship of this project
by MIUR, FIRB-2003 CAMERE-RBNE03JCR5 project, the
University of Padova, CP0A054893 and Assegni di ricerca di
Ateneo 2006 (M.M. and M.P. fellowships), Givaudan S.A., Vernier
(T.S. fellowship) and the Swiss State Secretariat for Education
and Research (E.P.K, SER C02.0029/COSTD24). The project was
carried out in the framework of COST, Action D24 ‘Sustainable
Chemical Processes: Stereoselective Transition Metal-Catalysed
Reactions’ (WG D24/0005/2001).
1
the H NMR spectrum of complex (S)-2b is more complicated
then that of 2a, but it shows a single set of resonances for each of
the diastereotopic benzylic protons. VT NMR experiments from
−40 up to 60 ◦C gave no indication of another diastereomeric
species. This confirms that the presence of a remote stereogenic
centre is sufficient to control the helicity of the complex also for
the titanatrane 2b.13
The circular dichroism spectrum of the Ti(IV) complex (S)-2b
shows significantly enhanced signals compared with the spectrum
of the free ligand ((S)-1b) (Fig. 2).
Notes and references
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7 Crystal data for 3: {O[(TiN(C13H10O)3]2}(C6H6)2(CHCl3)0.79, M =
1483.4, monoclinic, space group P21/n, a = 13.2828(8), b = 15.6644(7),
◦
3
˚
˚
c = 18.5844(12) A, b = 105.269(7) , V = 3730.3(4) A , Z = 2, T =
Fig. 2 Circular dichroism spectra (CH2Cl2) for ligand (S)-1b and complex
(S)-2b.
200 K, l = 0.36 mm−1, Dc = 1.32 g cm−3, k(MoKa) = 0.71073 A,
˚
27925 measured reflections, 7259 unique (Rint = 0.084) from which
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(SIR-97) and refined by full matrix least-squares (XTAL 3.2) on F. R =
0.044, wR = 0.042, GOF = 1.33(2). CCDC reference number 629066.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b702958f.
This is consistent with the presence of a propeller-like confor-
mation for the mononuclear complex (S)-2b. Based on literature
precedent and on molecular model studies,11 the preferred di-
astereoisomer is expected to have the methyl group positioned
anti with respect to the phenyl moiety. On the basis of the
configurations present in the two heteronuclear moieties of the
l-oxo complex 3 this should generate a D (right-handed) propeller-
like twist.
Complex (S)-2b turned out to be stable towards reaction with
water. We were unable to detect a l-oxo complex even after the
addition of water (10 equiv.) and heating to 60 ◦C. Extensive
heating resulted in partial decomposition. ESI-MS experiments
(1 lM, acetonitrile, HCOOH 0.1% as mobile phase) did not show
the presence of any l-oxo species in solution, even after long
reaction times. The behaviour of the enantiopure complex (S)-
2b confirms that the formation of the l-oxo dimer 3 is driven by
a chiral self-discrimination process of two enantiomeric moieties.
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triphenolamine bearing ortho and para t-Bu groups has been published.
A similar synthetic sequence was used. In analogy with what we
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