Synthesis, structural characterization, and reactivity of Cp2- and (CpMe)2-ligated titanaaziridines and titanaoxiranes with fast enantiomer interconversion rates

Article abstract of DOI:10.1021/om300825n

A new synthetic route to (CpR)₂-ligated titanaaziridines and titanaoxiranes from stable Ti(II) precursors has been developed, and the enantiomer interconversion rate constants for chiral titanaaziridines and titanaoxiranes have been measured for the first time. The titanaaziridines (CpR)₂Ti(η²-N(R¹)CHPh)(L) (R = H (10), Me (12); R¹ = Ph (a), o-anisyl (b), SiMe₃ (c); L = PMe ₃ (a, c), -OMe (b)) and titanaoxiranes Cp₂Ti(η ²-Ph(R)CO)(L) (R = Ph (14), H (15); L = PMe₃) have been synthesized and characterized spectroscopically; titanaaziridine 10a and titanaoxirane 14 have been characterized by X-ray crystallography. The enantiomer interconversion rate constants for the chiral titanaaziridines and titanaoxiranes have been measured by variable-temperature NMR; k_inv for 10b is the fastest enantiomer interconversion rate constant reported for any metallaaziridine or metallaoxirane to date. Titanaaziridines 10 and 12 undergo exchange reactions with C=C and C=X bonds, whereas the titanaoxiranes 14 and 15 undergo insertions.

Full text of DOI:10.1021/om300825n

Article
pubs.acs.org/Organometallics
Synthesis, Structural Characterization, and Reactivity of Cp2- and
(CpMe)2‑Ligated Titanaaziridines and Titanaoxiranes with Fast
Enantiomer Interconversion Rates
Ling Li, Kathleen E. Kristian, Arthur Han, Jack R. Norton,* and Wesley Sattler
Department of Chemistry, Columbia University, New York, New York 10027, United States
S
* Supporting Information
ABSTRACT: A new synthetic route to (CpR)2-ligated
titanaaziridines and titanaoxiranes from stable Ti(II) pre-
cursors has been developed, and the enantiomer interconver-
sion rate constants for chiral titanaaziridines and titanaoxiranes
have been measured for the first time. The titanaaziridines
(CpR)2Ti(η2-N(R1)CHPh)(L) (R = H (10), Me (12); R1 =
Ph (a), o-anisyl (b), SiMe3 (c); L = PMe3 (a, c), −OMe (b))
and titanaoxiranes Cp2Ti(η2-Ph(R)CO)(L) (R = Ph (14), H (15); L = PMe3) have been synthesized and characterized
spectroscopically; titanaaziridine 10a and titanaoxirane 14 have been characterized by X-ray crystallography. The enantiomer
interconversion rate constants for the chiral titanaaziridines and titanaoxiranes have been measured by variable-temperature
NMR; kinv for 10b is the fastest enantiomer interconversion rate constant reported for any metallaaziridine or metallaoxirane to
date. Titanaaziridines 10 and 12 undergo exchange reactions with CC and CX bonds, whereas the titanaoxiranes 14 and 15
undergo insertions.
INTRODUCTION
■
Many η2-imine complexes of transition metals1 and lantha-
nides2 (“metallaaziridines’’ or “azametallacyclopropanes”) have
been synthesized and used for the synthesis of new organic and
organometallic compounds. In particular, the reactions of η2-
imine complexes of Nb, Ta, and Zr have produced (in some
cases, asymmetrically) pyrroles,3 α-amino alcohols,4 α-amino
titanaaziridines have been invoked as intermediates in Ti-
acid esters,5 α-amino amidines,6 ureas,4b 1,2-diamines,7 and
mediated cross-coupling reactions.18 The most synthetically
allylic amines,8 as well as structurally interesting organometallic
useful systems are those produced by the reaction of Sato’s
compounds.9 The abundance and low cost of early transition
reagent, Ti(OiPr)4/2iPrMgCl (eq 1), with imines;18a,19 treating
metals make their metallaaziridines especially attractive, as both
stoichiometric and catalytic reactions are practical.
Many of the closely related (and isoelectronic) η2-aldehyde
and η2-ketone complexes (“metallaoxiranes”) have also been
prepared, although these have been less used in organic
synthesis. The η2 and η1 complexes of aldehydes10 and
ketones11 with the middle transition-metal rhenium have
been thoroughly investigated by Gladysz and his co-workers.
Early transition-metal complexes of η2-aldehydes and η2-
ketones are common. Niobocene complexes of ketene12 and
formaldehyde13 have been prepared and structurally charac-
them with aqueous acid gives the corresponding amines (eq
terized by Bruno and his co-workers. Dimeric zirconaoxiranes
1),19h as expected. These systems can produce homoallylic and
with bridging ketone or aldehyde ligands, for example,
allylic amines,19h,20 γ-lactams,21 α-amino ketones and allenes,19g
[Cp2Zr(η2-Ph2CO)]2 (1), have been reported by the Erker
amino alcohols,22 and N-containing heterocycles, such as
pyrroles,23 piperidines,22b,24 and quinolizidines/indolizidines.25
Regio- and stereoselective versions of some of these reactions
have been developed,19g,h,20a,21b,22b,25,26 and their generation by
group,14 while a Zr2 complex with a bridging acetone has been
reported by the Norton group.15 Ketone ligands that bridge Zr
and Al have been reported by Waymouth, Clauser, and
Grubbs,16 while neutral and anionic η2 aldehyde complexes of a
single Zr have been made by Askham and his group.17
The isolation of titanaaziridines and titanaoxiranes has lagged
behind that of zirconaaziridines and zirconaoxiranes, though
Received: August 27, 2012
© XXXX American Chemical Society
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reducing agents other than RMgX has been investigated.27
Titanaaziridines have also been implicated as intermediates in
other synthetically attractive reactions.28
analogous reaction of Cp2TiMe(OTf)35 with LiN(Ph)CH2Ph
yields the comproportionation products Cp2TiMe2 and Cp2Ti-
(N(Ph)CH2Ph)2 rather than a titanaaziridine.36
A few titanaaziridines that have been isolated are shown in
Ti(II) Precursors. Sato’s Ti(II) reagent (eq 1) is produced
by reducing a Ti(IV) precursor with an organometallic reducing
agent.18a,37 Although this method is suitable for effecting
organic transformations, it has drawbacks as a method for
producing pure titanaaziridines. The organometallic reducing
agentsGrignards or organolithiumsproduce stoichiometric
quantities of metal salts that are difficult to remove, and can
undergo side reactions. Furthermore, the Sato reagent is
unstable at room temperature.
Chart 1. The Cp2-ligated titanaaziridine 229 was prepared by
Chart 1. Isolable Titanaaziridines and Titanaoxiranes
In contrast, the “masked titanocene” Cp2Ti(PMe3)2 (8),38
first prepared by Rausch, can be stored for months under Ar at
room temperature. Complex 8 is synthesized by the reduction
of Cp2TiCl2 with Mg0 in the presence of excess PMe3; in
solution, it is in equilibrium with Cp2Ti(PMe3) + PMe3, with 8
favored at room temperature.38 The reaction of PhCCPh
with 8 yields a stable η2-acetylene complex, Cp2Ti(η2-PhC
CPh)(PMe3) (9).39 We sought to use the stable Ti(II)
precursor 8 in analogous reactions with imines to form stable,
isolable η2-imine complexes of titanocene.
Cp2 Titanaaziridines. When 8 is treated with 1 equiv of N-
benzylidene aniline in C6D6, the color changes from dark
the reaction of Cp2TiMe with N-benzylidene aniline; methane
was evolved via intermolecular hydrogen atom abstraction from
a Cp ligand, reducing the maximum yield to 50%. The amidate-
ligated bridging titanaaziridine 3 was formed by C−H bond
activation, followed by amine elimination.30 The monomeric
bis(aryloxy)- and aminotroponiminate-ligated titanaaziridines
431 and 532 arose from alkyl migrations to bound isocyanides.
These syntheses have not proven general enough to produce a
variety of substituted titanaaziridines for synthetic applications.
Two titanaoxiranes that have been isolated and characterized
by X-ray crystallography are also shown in Chart 1. The dimer
[Cp2Ti(Ph2C2O)]2 6 was prepared by Floriani from Cp2Ti-
(CO)2 and diphenylketene.33 The monomer (Ar′O)2Ti(η2-
Ph2CO)(PMe3) (Ar′O = 2,6-diphenylphenoxide) 7 was
prepared by Rothwell by an indirect route from the
corresponding propylene complex and benzophenone.34
The reactivity of these isolable titanaaziridines and
titanaoxiranes in synthetically useful transformations has not
been thoroughly investigated. Several redox, substitution, and
decomposition reactions of titanaaziridines 2 and 4 have been
reported, but they do not produce useful organic products.
We have sought a general method for the preparation of Cp2-
ligated titanaaziridines and titanaoxiranes. We have structurally
characterized these complexes and investigated their ability to
undergo insertion reactions that lead to useful organic products.
Furthermore, because the configurational lability of chiral Cp2
zirconaaziridines has made them useful for dynamic kinetic
asymmetric transformations (DYKAT’s),5c,9 we have measured
the rates at which the enantiomers of Cp2-ligated titanaazir-
idines and titanaoxiranes interconvert.
brown to dark red over 3 h (eq 2). The Cp peak at δ 4.57 in the
1H NMR of 8 disappears, and two new Cp resonances
(assigned to 10a) grow in at δ 5.11 and δ 5.20. The PMe3
singlet remains broad, but shifts upfield from δ 0.84 in 8 to δ
0.74 in 10a. The singlet at δ 8.13 due to the imine proton of
free N-benzylidene aniline disappears, and a broad 1H singlet
(also assigned to 10a) grows in at δ 2.87 (near the chemical
shift, δ 2.47, of the methine proton in the analogous
zirconaaziridine, Cp2Zr(η2-N(Ph)CHPh)(PMe3)3b,8a). A new
peak appears in the 13C NMR at δ 52.5, near the peak (δ 46.8)
of the imine carbon in the known zirconaaziridine. Both the
inequivalence of the Cp peaks and the new N(Ph)CHPh peaks
indicate that the N-benzylidene aniline in 10a has an η2
coordination mode, with an accompanying reduction in its
C−N bond order.
The IR spectrum of a mixture of 8 and 1 equiv of N-
benzylidene aniline in benzene initially shows a band at 1627
cm−1, from the CN stretch of the free N-benzylidene aniline.
Over the course of 2 h, this band disappears as 10a forms; no
new CN band appears in this region, indicating a reduction
in the C−N bond order as the imine becomes bound.
At 298 K, the 31P{1H} NMR of 10a in THF-d8 shows a
broad singlet at δ 13.9, which sharpens and shifts slightly
downfield on cooling. The PMe3 peak in the 1H NMR of 10a in
THF-d8 at 298 K shifts from δ 1.29 to δ 1.00, toward that of
free PMe3 (δ 0.95), when 1 equiv of free PMe3 is added. These
results make it clear that the coordinated PMe3 in 10a
dissociates and thus exchanges with free PMe3.
RESULTS AND DISCUSSION
■
Preparation of Titanaaziridines. In the Buchwald syn-
thesis of Cp2 zirconaaziridines,3b,5b methyl amide complexes
Cp2Zr(Me)(N(CH2R2)R1) eliminate methane to form zirco-
naaziridines Cp2Zr(η2-N(R1)CHR2)(L). Unfortunately, the
B
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Complex 10a decomposes in solution without excess PMe3.
However, impure samples of 10a can be precipitated from the
reaction mixture and stored in the solid state under an inert
atmosphere at −20 °C. Single crystals suitable for X-ray analysis
were obtained by allowing a solution of 10a in benzene with 1
equiv of free PMe3 to stand at room temperature overnight.
The crystal structure of 10a is shown in Figure 1.
methoxy oxygen is coordinated in 10b and that the PMe3 is
not.
The analogous reaction with N-benzylidene trimethylsilyl-
amine (eq 2) appears to result in the formation of 10c
(although its formation is much slower12 h at room
temperaturethan that of 10a and 10b). The titanaaziridines
10b and 10c also decompose in solution without excess PMe3,
though they are stable in solution at room temperature for days
with excess PMe3.
(CpMe)2 and Cp*2 Titanaaziridines. In an effort to make
a more electron-rich Ti(II) precursor, we tried to prepare
(CpMe)2Ti(PMe3)2 (11) (eq 3). The reduction of
(CpMe)2TiCl2 with Mg0 in the presence of excess PMe3 over
3 days at room temperature appeared to give 11 as the only
1
product. The H NMR in C6D6 showed Cp resonances at δ
4.61 and 4.47, a PMe3 peak at δ 0.85, and a broad CpMe peak at
δ 1.64.
The reactions of 11 with 1 equiv of N-benzylidene aniline, N-
benzylidene anisidine, or N-benzylidene trimethylsilylamine
appeared to produce (eq 4) the titanaaziridines (CpMe)2Ti(η2-
Figure 1. ORTEP view of Cp2Ti(η2-N(Ph)CHPh)(PMe3) (10a).
Thermal ellipsoids drawn at 30% probability level. Select hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and bond
angles (deg): Ti(1)−N(1), 1.9923(19); Ti(1)−C(11), 2.302(2);
C(11)−N(1), 1.382(2); Ti(1)−P(1), 2.5759(8); C(11)−Ti(1)−
N(1), 36.65(8); C(11)−Ti(1)−P(1), 76.95(6); N(1)−Ti(1)−P(1),
113.55(6).
The significant elongation of the C(11)−N(1) bond
(1.382(2) Å) in 10a compared with that in a free imine
supports the description of 10a as a titanaaziridine. The
C(11)−N(1) bond length of 10a is comparable to the C−N
bond lengths in other isolated titanaaziridines, including 3
(1.368(4) Å), 4a−4d (1.410(3)−1.421(7) Å), and 5 (1.417(4)
Å). The Ti−N bond (1.9923(19) Å) in 10a is slightly longer
than the Ti−N bonds in 4a−4d (1.846(4)−1.855(2) Å) and 5
(1.883(2) Å), which contain oxygen and nitrogen donor
ligands; it is shorter than the analogous bond in the bridging,
dinuclear titanaaziridine 3 (2.046(2) Å). The bond angles
within the metallaaziridine ring of 10a are comparable to those
in Cp2-ligated zirconaaziridines and to those in titanaaziridines
4b and 4c.9
The Ti−P bond length (2.5759(8) Å) is similar to that in
Rothwell’s bis(aryloxy)-ligated titanaoxirane 734 (2.592(3) Å)
and shorter than that in the closely related zirconaaziridine
Cp2Zr(η2-N(Ph)CHPh)(PMe2Ph) (2.7227(3) Å).40 Interest-
ingly, the coordinated PMe3 is closer to the carbon of the
bound imine in 10a, in an “outside” configuration.9 Rothwell’s
bis(aryloxy) ligated titanaaziridines 4a−4d similarly exhibit
“outside” coordination of the stabilizing pyridine ligands.
Though most zirconaaziridines exhibit a thermodynamic
preference for “inside” coordination, with the stabilizing
donor ligand closer to the nitrogen of the zirconaaziridine
ring, the PMe2Ph in Cp2Zr(η2-N(Ph)CHPh)(PMe2Ph) like-
wise exhibits “outside” coordination.40
N(Ph)CHPh)(PMe3) (12a), (CpMe)2Ti(η2-N(o-anisyl)-
CHPh) (12b), and (CpMe)2Ti(η2-N(TMS)CHPh)(PMe3)
(12c). These titanaaziridines 12, although stable in solution
with excess PMe3 (like 10a−10c), decompose if the PMe3 is
removed (though more slowly than 10a−10c).
The “Cp*2Ti(II)” synthon is accessible in situ through the
reduction of Cp*2TiCl2 with 1% sodium amalgam.41 We tried
carrying out this reduction in the presence of N-benzylidene
aniline, but by NMR data, the initially formed titanaaziridine
Cp*2Ti(η2-N(Ph)CHPh) (13) decomposed within a few
hours.
Preparation of Titanaoxiranes from Ketones/Alde-
hydes. When 1 equiv of benzophenone is added to a solution
of Cp2Ti(PMe3)2 (8) at room temperature in C6D6, the color
changes quickly from dark brown to purple as the titanaoxirane
14 is formed (eq 5). The NMR spectra of 14 vary with
temperature in a complex way, suggesting (but not proving)
that 14 dimerizes at low temperatures. (Recall that 1, the Zr
analogue of 14, is a dimer.) Single crystals of 14 were grown
from a toluene solution at −20 °C, and its structure determined
by X-ray crystallography (Figure 2).
In efforts to produce a titanaaziridine that is stable without
excess phosphine ligand, we added an o-methoxy group (10b)
1
and replaced the NPh with an N(TMS) (10c). By H NMR,
the addition of N-benzylidene anisidine to 8 appeared to
produce 10b after 1 h (eq 2), with a methine resonance at δ
4.23 (near that in the Zr analogue, δ 3.825b) and a methoxy
It is instructive to compare the structure of 14 with that of
16
the known titanaoxirane dimer [Cp2Ti(PhC2O)]2 (6) and
1
resonance at δ 2.68 in the H NMR. The upfield shift of the
that of the titanaoxirane monomer (Ar′O)2Ti(η2-Ph2CO)-
(PMe3)34 (7). In 14, in contrast with 7, C(11), O(1), and P(1)
methoxy, from δ 3.37 in the free imine, implies that the
C
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known complex Cp2Ti(η2-PhCCPh)(PMe3) (9, eq 7a).39 In
contrast, the reaction of 10a with 1-octyne (eq 7b) proceeds to
1
completion (by H NMR) over 4 h at room temperature,
forming a mixture of insertion products that is predominantly
16. The reactions of 10a with ethylene carbonate, substituted
alkenes, isocyanates, carbodiimides, and ketones, however,
result in displacement of the imine rather than insertion. The
titanaaziridines appear to undergo insertion of some less-
hindered CC and terminal CC bonds, but, in general,
undergo exchange reactions with CX bonds and substituted
CC/internal CC bonds.
Figure 2. ORTEP view of Cp2Ti(η2-Ph2CO)(PMe3) (14). Thermal
ellipsoids drawn at 20% probability level. Selected bond lengths (Å)
and bond angles (deg): Ti(1)−O(1), 2.0057(14); Ti(1)−C(11),
2.2477(19); C(11)−O(1), 1.360(2); Ti(1)−P(1), 2.590(9); C(11)−
Ti(1)−O(1), 36.75(6); C(11)−Ti(1)−P(1), 107.39(6); O(1)−
Ti(1)−P(1), 70.63(5).
These results are in sharp contrast to those obtained with the
“(iPrO)2Ti(II) + imine” system, in which proposed, but not
isolated, titanaaziridine intermediates appear to undergo
insertion reactions. It is apparent that the nature of the metal
center in the (CpR)2M(II) system differs significantly from that
in the (iPrO)2Ti(II) system, though their ligand sets are
nominally isoelectronic.44 We have demonstrated that the
imines in Cp2-ligated titanaaziridines 10 and 12 are bound in
the η2 mode and that 10 and 12 are monomeric in solution (as
are the related zirconaaziridines), though they do not undergo
insertions with most unsaturated bonds. Hydrolysis experi-
ments imply that the (iPrO)2Ti(II) system also binds imines in
the η2 mode, but further information about the structure of the
resulting complexes is not available. The reactive intermediates
in this system are not necessarily monomeric; such structural
differences could have a significant role in determining
exchange versus insertion reactivity.
lie in a plane probably because of the greater bulk of the
phenylphenoxide ligands in 7. The coordinated PMe3 in 14 is
close to the oxygen of benzophenone, showing “inside”
coordination, as observed for most Cp2-ligated zirconaazir-
idines;5b the PMe3 in 7 is “outside”. The C−O distance in 14
(1.360(2) Å) is slightly shorter than that in 7 (1.397(8) Å), but
longer than that of 6 (1.311(4) Å), and is characteristic of a C−
O single bond. The Ti−C(11) distance (2.2477(19) Å) is
slightly longer than the values for 7 and 6 (2.150(7) and
2.099(3) Å, respectively). The Ti−O distance of 2.0057(14) Å
is slightly longer than the value in 7 (1.849(5) Å) and slightly
shorter than that in 6 (2.037(2) Å). The C(11)−Ti−O bond
angle of 36.75(6)° is comparable with the angle in 6 (36.9(1)°)
and 7 (40.0(2)°).
To prepare a titanaoxirane with a chiral center, we added an
equivalent of benzaldehyde in cold toluene-d8 to a solution of 8
in the same solvent at −78 °C and obtained a dark red solution
that appeared to contain the titanaoxirane Cp2Ti(η2-PhCHO)-
(PMe3), (15) (eq 6). The upfield 13C NMR chemical shift (δ
79.1, C6D6) and 1H NMR chemical shift (δ 4.15, tol-d8) of the
methine confirm the η2-coordination mode of benzaldehyde in
15.19h
Reactivity of Titanaoxiranes. The Ti−C bond of the
titanaoxirane 14 inserts CC and CX bonds (Scheme 1).
The addition of 1 equiv of benzaldehyde to 14 at RT results
in an immediate color change from purple to bright red and the
formation of the stable insertion product 18; acidolysis of 18
yields 1,2-triphenylethane-1,2-diol (>99%). The reaction of
methyl acrylate with 14 is complete after 5 min at room
temperature, forming 19 as the only regioisomer; acidolysis of
19 yields methyl 3-hydroxy-2-methyl-3,3-diphenylpropanoate
(99%). The reaction of 14 with excess ethylene carbonate
results in a color change from purple to brown within 5 min;
1H NMR suggests that conversion to the insertion product 17
is complete in 30 min. Unfortunately, 17 decomposes over
several hours at room temperature.
The reaction with ethylene, in contrast with the CO
insertions, is slow. When ethylene is in excess, formation of 20
is complete only after 4 h at 50 °C. Similar conditions are
required for the insertion of styrene into 14, which forms 21 as
a single regioisomer. The preference for the 2,1 insertion
product has precedent in the reaction of titanaoxirane 7 with
excess propylene, which yields an oxatitanacyclopentane with
the Me substituent on the carbon α to the Ti as the major
product.34,45 Earlier,46 Negishi and co-workers demonstrated
that the favored metallacycle from the reaction of Cp2ZrBu2
Reactivity of Titanaaziridines. Constrained geometry40
(cg) and Cp2 zirconaaziridines insert CC, CC, and CX
bonds,1a,7,8,42 forming zirconacycles that can be hydrolyzed to
isolate organic products. However, a few attempts to carry out
similar reactions with titanaaziridines have led to coordination
of the CC or CX in place of the coordinated imine (vide
infra).31,32,43
The titanaaziridines that we have prepared also undergo
exchange rather than insertion in most cases. The reaction of
10a with PhCCPh proceeds slowly at room temperature (or
over 3 h at 50 °C) to form free N-benzylidene aniline and the
D
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CpMe protons. However, at 298 K, there is a single Cp peak for
10b and a single CpMe peak for 12b, indicating that these
titanaaziridines have fast enantiomer interconversion rates at
that temperature.
Scheme 1. Reactivity of Titanaoxirane 14
At 234 K, 12b has two distinct C5H4Me peaks and eight
distinct C5H4Me peaks, showing that the two CpMe ligands are
inequivalent at that temperature and that enantiomer
interconversion is slow on the NMR time scale. The CpMe
peaks of 12b coalesce at approximately 257 K (Figure S1,
Supporting Information), implying a kinv of 51(1) s−1 and a
ΔG⧧ of 12.9 kcal mol−1 at that temperature.
We have similarly measured kinv for the titanaaziridines 10a−
10c and 12a,12c at the coalescence temperatures of their Cp
and CpMe peaks. These results are in Table 1, along with
previously reported5c values of kinv measured at 298 K for
comparable zirconaaziridines.
It seems clear that the o-anisyl titanaaziridines 10b and 12b
undergo enantiomer interconversion more rapidly than any
metallaaziridines examined to date. We have estimated kinv and
ΔG⧧ at 298 K for the titanaaziridine 10b by measuring the
widths of its C5H5 peaks as a function of temperature (see
Table S1 in the Supporting Information). The results, kinv,298
≈
4 × 104 s−1 and ΔG⧧ = 11 kcal mol−1, show that the
298
with excess styrene is the zirconacyclopentane Cp2Zr(κ2-
CH(Ph)CH2CH(Et)CH2), which has the phenyl substituent
α to the Zr.
Thus, Cp2-ligated titanaoxiranes, unlike the titanaaziridines
we have prepared herein from (CpR)2Ti(II), do readily insert
substituted CC and CX bonds.
enantiomers of 10b interconvert orders of magnitude faster than
those of the analogous zirconaaziridine, Cp2Zr(η2-N(o-anisyl)-
CHPh) (kinv = 5.3 × 10−6 s−1 and ΔG⧧ = 24.6 kcal mol−1 at 298
K5c).
Calculations suggest that the enantiomers of Cp2 zirconaazir-
idines interconvert via η1 intermediates (Scheme 3),5c
a
mechanism like that established for aromatic η2 aldehydes
coordinated to Re.47 The same mechanism is likely for
titanaaziridines like 10 and 12.
Rates of Interconversion of Titanaaziridine Enan-
tiomers. The η2 binding of an aldimine or an unsymmetrically
substituted ketimine to (CpR)2M(II) (M = Ti, Zr) produces
either of two enantiomeric metallaaziridines, depending upon
which face of the ligand is coordinated (Scheme 2). The
insertion of an enantiopure CX compound leads to a dynamic
kinetic asymmetric transformation, or DYKAT, if the
enantiomers of a metallaaziridine interconvert more rapidly
than they insert a CX, that is, if kinv > kR[CX] and kS[C
X] in a system like that in Scheme 2.5c,e,9 Faster enantiomer
interconversion enables us to add the insertion reagent more
quickly and still obtain the maximum diastereomer ratio (kR/
kS). For many zirconaaziridines, kinv have been measured and
used to effect DYKATs with diastereomer ratios up to 95:5.5c
To see how the enantiomer interconversion rates of Ti
complexes compare with those of their Zr analogues, we have
measured kinv for the titanaaziridines 10 and 12. In either
enantiomer of a chiral metallaaziridine, the two cyclo-
pentadienyl ligands are inequivalent. However, if the enan-
tiomers interconvert quickly on the NMR time scale, the
cyclopentadienyl ligands (and their NMR resonances) become
equivalent. In the room-temperature 1H NMR spectra of the Cp
titanaaziridines 10a and 10c, there are two peaks for the Cp
Rate of Interconversion of the Enantiomers of the
Titanaoxirane 15. At 298 K, the 1H NMR of the
benzaldehyde adduct 15 in toluene-d8 shows a single broad
Cp peak. At lower temperatures two distinct Cp peaks are
observed (Figure 3). The peaks coalesce at 295 K, giving a kinv
of 168(1) s−1 and a ΔG⧧ of 14.2 kcal mol−1 at that temperature.
This rate constant is faster than any measured for a
zirconaaziridine to date and within the range found for the
various titanaaziridines 10 and 12 (Table 1).
CONCLUSION
■
We have prepared Cp2- and (CpMe)2-ligated titanaaziridines
and titanaoxiranes from stable Ti(II) precursors and imines or
aldehydes/ketones; the reaction tolerates a variety of
substituents and proceeds at room temperature. The new
titanaaziridines and titanaoxiranes have faster enantiomer
interconversion rates than those previously reported for other
metallaaziridines or metallaoxiranes, and likely interconvert via
η1-coordinated intermediates. The titanaaziridines exchange
their imine ligands with most CC, CC, and CX,
whereas their zirconaaziridine and titanaoxirane analogues
insert such multiple bonds.
1
protons; in the room-temperature H NMR spectra of the
CpMe titanaaziridines 12a and 12c, there are two peaks for the
Scheme 2. Reaction of Enantiopure Substrate with a Racemic Zirconaaziridine
E
dx.doi.org/10.1021/om300825n | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 1. Enantiomer Interconversion Rate Constants of Metallaaziridines (C5H4R)2M(η2-N(R1)CHPh)(L)
a
a
R1
compd. or ref
L
R
M
kinv (s−1
)
ΔG⧧(kcal/mol)
T (K)
o-anisyl
ref 5c
10b
−OMe
−OMe
−OMe
THF
H
Zr
Ti
Ti
Zr
Ti
Ti
Zr
Ti
Ti
5.3(7) × 10−6
310(1)
24.6
11.5
12
298
260
257
298
364
372
298
371
357
H
12b
Me
H
51(1)
TMS
Ph
ref 5c
10c
9.1(1) × 10−3
40.5(5)
20.2
18.8
19.1
20.4
18.8
18.6
PMe3
PMe3
THF
H
12c
Me
H
378(3)
ref 5c
10a
7(2) × 10−3
59.7(3)
PMe3
PMe3
H
12a
Me
351(2)
a
Measured at temperature T.
Scheme 3. Proposed Enantiomer Interconversion
Mechanism
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ASSOCIATED CONTENT
* Supporting Information
Synthetic procedures, NMR spectra, crystallographic and
structural results, and details for the VT-NMR experiments.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by the National Science Foundation
(CHE-0749537), and by Boulder Scientific and OFS Fitel. A.H.
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gratefully acknowledges a summer fellowship from the Societe
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dx.doi.org/10.1021/om300825n | Organometallics XXXX, XXX, XXX−XXX