A new ONO3- trianionic pincer-type ligand for generating highly nucleophilic metal-carbon multiple bonds

Article abstract of DOI:10.1021/ja302222s

Appending an amine to a C=C double bond drastically increases the nucleophilicity of the β-carbon atom of the alkene to form an enamine. In this report, we present the synthesis and characterization of a novel CF ₃-ONO^3- trianionic pincer-type ligand, rationally designed to mimic enamines within a metal coordination sphere. Presented is a synthetic strategy to create enhanced nucleophilic tungsten-alkylidene and -alkylidyne complexes. Specifically, we present the synthesis and characterization of the new CF₃-ONO^3- trianionic pincer tungsten-alkylidene [CF₃-ONO]W=CH(Et)(O^tBu) (2) and -alkylidyne {MePPh ₃}{[CF₃-ONO]W≡C(Et)(O^tBu)} (3) complexes. Characterization involves a combination of multinuclear NMR spectroscopy, combustion analysis, DFT computations, and single crystal X-ray analysis for complexes 2 and 3. Exhibiting unique nucleophilic reactivity, 3 reacts with MeOTf to yield [CF₃-ONO]W=C(Me)(Et)(O^tBu) (4), but the bulkier Me₃SiOTf silylates the tert-butoxide, which subsequently undergoes isobutylene expulsion to form [CF₃-ONO]W=CH(Et)(OSiMe ₃) (5). A DFT calculation performed on a model complex of 3, namely, [CF₃-ONO]W≡C(Et)(O^tBu) (3′), reveals the amide participates in an enamine-type bonding combination. For complex 2, the Lewis acids MeOTf, Me₃SiOTf, and B(C₆F₅)₃ catalyze isobutylene expulsion to yield the tungsten-oxo complex [CF ₃-ONO]W(O)(ⁿPr) (6).

Full text of DOI:10.1021/ja302222s

Article
pubs.acs.org/JACS
A New ONO3‑ Trianionic Pincer-Type Ligand for Generating Highly
Nucleophilic Metal−Carbon Multiple Bonds
Matthew E. O’Reilly, Ion Ghiviriga, Khalil A. Abboud, and Adam S. Veige*
Department of Chemistry, Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, Florida 32611, United States
S
* Supporting Information
ABSTRACT: Appending an amine to a CC double bond
drastically increases the nucleophilicity of the β-carbon atom of
the alkene to form an enamine. In this report, we present the
synthesis and characterization of a novel CF3−ONO3‑
trianionic pincer-type ligand, rationally designed to mimic
enamines within a metal coordination sphere. Presented is a
synthetic strategy to create enhanced nucleophilic tungsten−
alkylidene and −alkylidyne complexes. Specifically, we present the synthesis and characterization of the new CF3−ONO3‑
trianionic pincer tungsten−alkylidene [CF3−ONO]WCH(Et)(OtBu) (2) and −alkylidyne {MePPh3}{[CF3−ONO]W
C(Et)(OtBu)} (3) complexes. Characterization involves a combination of multinuclear NMR spectroscopy, combustion analysis,
DFT computations, and single crystal X-ray analysis for complexes 2 and 3. Exhibiting unique nucleophilic reactivity, 3 reacts
with MeOTf to yield [CF3−ONO]WC(Me)(Et)(OtBu) (4), but the bulkier Me3SiOTf silylates the tert-butoxide, which
subsequently undergoes isobutylene expulsion to form [CF3−ONO]WCH(Et)(OSiMe3) (5). A DFT calculation performed
on a model complex of 3, namely, [CF3−ONO]WC(Et)(OtBu) (3′), reveals the amide participates in an enamine-type
bonding combination. For complex 2, the Lewis acids MeOTf, Me3SiOTf, and B(C6F5)3 catalyze isobutylene expulsion to yield
the tungsten−oxo complex [CF3−ONO]W(O)(nPr) (6).
Some of the most highly active alkene26,27 and alkyne28−33
metathesis catalysts employ a push−pull idea by including
electron-rich imido or amido ligands coupled with electronically
poor fluorinated alkoxides (−OC(CF3)2CH3). The fluorinated
alkoxides serve to weaken the alkoxide donor strength and
create an electrophilic metal, but the role of the imido or amido
is not as clear.31,34 For monodentate amido ligands, the lone
pair on the nitrogen typically orients perpendicular to the
metal−carbon multiple bond and donates into a low-lying
vacant dxy orbital (Figure 1, left).29,31,32,35−37 Donating electron
INTRODUCTION
■
Transition-metal multiple bonds of groups 4 and 5, in particular
the first row derivatives, have a significant ionic component1−5
and are highly nucleophilic.1,2,5−7 Superlative examples include
the alkylaluminum stabilized Tebbe’s reagents (Cp2Ti(μ-
CH2)(μ-X)Al(CH3)2 (X = Cl, Me)8−11 and recently Mindiola’s
titanium ([PNP]TiCtBu)12−21 and vanadium ([nacnac]V
CtBu) alkylidynes.22,23 In contrast, group 6 metal−carbon
multiple bonds24 of molybdenum and tungsten are more
covalent and correspondingly less nucleophilic.1,3−5 However,
appropriate ancillary ligand design provides the possibility to
fine-tune the electronic structure of molybdenum and tungsten
multiple bonds to create highly nucleophilic species. Method-
ologies that accentuate the nucleophilicity of M−C multiple
bonds are important for applications in 1,2-CH bond activation
and olefin and alkyne metathesis. Herein, we present a rational
approach to creating tungsten−carbon multiple bonds with
high nucleophilicity at the carbon by employing a push−pull
strategy that exploits an enamine bonding concept for the push.
To create a highly nucleophilic polarized metal−carbon
bond, the ancillary ligand must accentuate the electrophilicity of
the metal and impart nucleophilicity to the α-carbon, a so-
called push−pull effect.25 However, inducing an electronically
unsaturated metal center actually serves to diminish the
nucleophilicity at the α-carbon by forming a more covalent
M−C multiple bond. The challenge in polarizing the M−C
bond lies in accumulating electron density on the α-carbon
while removing it from the metal center.
Figure 1. Left: amido p-orbital aligned with dxy. Right: amido p-orbital
rotated out of alignment.
density into an empty dxy orbital stabilizes the N-atom lone-pair
but operates against the goal of creating an electrophilic metal
ion. A different strategy is to purposely orient the amido lone
pair collinear or closely aligned with the metal−carbon bond
axis (Figure 1, right). The result delocalizes the N-atom lone
Received: March 14, 2012
© XXXX American Chemical Society
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pair through the metal−carbon multiple bond, and instead, the
α-carbon atom receives additional electron density.
involving oxygen-atom transfer,58 nitrogen-atom transfer,59 and
dioxygen activation.60 In addition, Heyduk et al. introduced
redox active trianionic ONO3‑ and NNN3‑ pincer-type
ligands.61−64 Yet to be reported is an ONO3‑ pincer-type
ligand that incorporates a push−pull design and is redox
insulated.
Enamines, exploited originally by Stork and co-workers in
Stork-enamine alkylations, exhibit an analogous bonding
interaction.38−40 The practicality of enamine chemistry now
extends well beyond stoichiometric reactions to exciting
discoveries in organocatalysis.41−45 A simple resonance
structure depiction helps to explain the observed increased
nucleophilicity of the β-carbon of an enamine (Scheme 1)39,40
Figure 3 depicts a new CF3−ONO3‑ pincer-type ligand that
incorporates all the design features required to create high
Scheme 1. Two Possible Resonance Contributions for an
Enamine (Top) and Amidoalkylidene (Bottom)
Figure 3. Push−pull effect of the [CF3−ONO]3‑ pincer-type ligand.
oxidation state nucleophilic metal−carbon multiple bonds. In
contrast to Heyduk’s ONO3‑ amido-bisphenoxide ligand,61,64
which can easily access multiple redox states, the quaternary
carbon of the alkoxide acts as a redox insulator. The fluorinated
alkoxides induce an electrophilic metal center (pull) and the
constrained pincer framework prevents amido π-donation into
the dxy orbital, and instead, the lone pair interacts with the
metal−carbon bond (push). Accomplishing this objective, we
now report the two-step synthesis of the new trianionic pincer-
type ligand [CF3−ONO]H3 (1) and its corresponding
tungsten−alkylidene [CF3−ONO]WC(Et)(OtBu) (2) and
−alkylidyne {MePPh3}{[CF3−ONO]WC(Et)(OtBu)} (3)
complexes. Demonstrating high nucleophilicity, addition of a
Me3SiOTf to 3 expels isobutylene in an intramolecular C−H
bond activation pathway. Moreover, the reaction is catalytic for
2.
and, therefore, the potential application to metal−carbon
multiple bonds. For both examples, the lone pair on nitrogen
“pushes” electron density on the carbon atom two bonds away.
A somewhat more sophisticated description, albeit perhaps less
convincing, involves a delocalization of the N-atom lone pair
into the CC, or in the case of alkylidenes, into the MC
bond (Figure 2). The energetic consequence is the HOMO
RESULTS
■
Synthesis and Characterization of [CF3−ONO]H3 (1).
Preparing the ligand precursor [CF3−ONO]H3 (1) involves
treating bis(2-bromo-4-methylphenyl)amine65 with 3.1 equiv of
nBuLi in Et2O and then adding hexafluoroacetone at −78 °C
(eq 1). Warming to 25 °C and an acidic workup yields isolable
Figure 2. Truncated qualitative orbital diagram of the bonding analogy
between enamines40 and amidoalkylidenes.
proligand 1 in 35% yield. In the 19F{1H} NMR spectrum of 1
(CDCl3), two broad multiplets attributable to the fluorine
atoms appear at −76.3 and −74.9 ppm. The fact that two
signals appear indicates a slow rotation around the aryl−
C(CF3)2OH bond at 25 °C. Coalescence of the signals occurs
upon heating a sample of 1 to 45 °C in an NMR probe.
Routine 1H and 13C{1H} NMR spectroscopic techniques
corroborate the identity and purity of 1 (see Supporting
Information). Notable features in the 1H NMR spectrum
include a singlet at 2.36 ppm for the aryl−methyl protons and a
broad resonance spanning from 7.0 to 7.5 ppm that
corresponds to the protons of the amine and the two alcohol
groups.
orbital, consisting of significant electron density on the β-
carbon of the enamine or the α-carbon of an alkylidene, is
destabilized. The challenge for synthesizing an inorganic version
of an enamine is that unrestricted amido ligands preferentially
orient the lone pair perpendicular to the M−C bond to
maximize bonding with an unoccupied, often dxy, orbital
(Figure 1). A solution is to constrain the amido donor in a
multidentate ligand. Trianionic NCN46,47 and OCO48−51
pincer ligands provide such a rigid meridional environment
and have recently been exploited for catalytic aerobic
oxidation,52 alkene isomerization53 and polymerization,54,55
alkyne polymerization,56 and fundamental transformations57
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Synthesis and Characterization of [CF3−ONO]W
CH(Et)(OtBu) (2). In benzene, combining proligand 1 with
(tBuO)3WC(Et)66 results in the immediate formation of the
trianionic pincer alkylidene complex [CF3−ONO]WCH-
(Et)(OtBu) (2) according to eq 2. Isolation of reasonably pure
The alkylidene bond orients with the alkyl group pointing
away from the amido (anti-isomer) and does not rotate with an
appreciable rate even at 100 °C. No signals attributable to an
exchange with the syn-isomer appear in variable temperature
1H NMR spectra of 2 from −60 to 100 °C. In Schrock’s
tungsten imido alkylidene, the syn-isomer predominantly forms
(Ksyn/anti = 5000), but access to the anti-isomer is possible by
exposing a sample to UV-radiation at −85 °C for several
hours.68 The relaxation of the anti-isomer back to syn occurs
between −53 and −38 °C.68 Interestingly, the rate of relaxation
decreases as the alkoxide ligands become more fluorinated.69
Invoking similar conditions for complex 2, a toluene-d8 solution
of 2 exposed to 366 nm light for 4 h at −78 °C does not yield
1
any detectable syn-isomer, as determined by H NMR (500
2 only requires removal of all volatiles in vacuo; recrystallizing 2
in pentane provides analytically pure material. Single crystals
amenable to an X-ray diffraction experiment deposit upon
cooling a concentrated pentane solution of 2 to −35 °C.
Structure refinement of the diffraction data provides the
molecular structure of 2 presented in Figure 4.
MHz) spectroscopy.
Synthesis and Characterization of {[CF3−ONO]W
C(Et)(OtBu)}{MePPh3} (3). Deprotonating alkylidenes with
methylenetriphenylphosphorane (Ph3PCH2) is a convenient
method to access the corresponding alkylidyne anion.70
Treating alkylidene 2 with Ph3PCH2 in pentane at 25 °C
precipitates the W-alkylidyne anion 3 (eq 3). Complex 3 turns
to a bright red color upon dissolving in benzene or ether.
Removing the solvent by vacuum yields a red oil, but the
addition of cold pentane returns 3 to a yellow powder.
Multinuclear H−13C gHSQC and H−13C gHMBC NMR
spectroscopic experiments confirm the identity of 3 and permit
the absolute assignment of all resonances in the 1H and
13C{1H} NMR spectra (see Supporting Information). Most
pronounced is the downfield shift to 280.6 ppm for the WCα
1
1
Figure 4. Molecular structure of [CF3−ONO]WCH(Et)(OtBu)
(2) with ellipsoids drawn at the 50% probability level, with hydrogen
atoms removed for clarity.
1
alkylidyne carbon in the 13C{1H} NMR spectrum. In the H
NMR spectrum, a doublet (JHP = 13.31 Hz) at 2.44 ppm is
attributable to the methyl protons of the phosphonium
countercation, and the corresponding phosphorus resonates
at 21.8 ppm in the 31P{1H} NMR spectrum. Consistent again
with a C1-symmetric complex, the 19F NMR spectrum contains
four distinct quartets at −69.38, −71.24, −74.08, and −76.38
ppm.
Complex 2 is C1-symmetric, and occupying the basal plane of
the distorted square-pyramidal tungsten(VI) geometry are the
ONO3‑ trianionic pincer ligand and tert-butoxide. In the apical
position resides a propylidene ligand with a W−Cα bond length
of 1.882(4) Å, which is consistent with a double bond and
similar to two other OCO3‑ trianionic pincer W-alkylidenes that
have bond lengths of 1.887 and 1.913 Å.67 Consistent with a
WC double bond, the 13C{1H} NMR spectrum of 2 contains
a downfield resonance at 260.3 ppm [1J(13C,183W) = 173.1 Hz]
and the corresponding alkylidene proton (WCHR) resonates
as a triplet at 7.36 ppm. An interesting structural feature
appears in the trianionic pincer ligand framework. Unable to lie
coplanar, the N-aryl rings twist, thereby lowering the solid-state
symmetry from potentially Cs to C1. This twist and resulting
low symmetry persists in solution as four distinct quartets
appear in the 19F{1H} NMR spectrum of 2 at −71.2, −71.5,
−73.9, and −77.2 ppm. The low symmetry also results in
diastereotopic −CβH2− methylene protons for the propylidene
ligand that appear as two multiplets at 5.08 and 4.79 ppm. The
propylidene methyl appears as a triplet at 0.77 ppm (3J = 7.36
Hz). The nitrogen atom of the ONO ligand is trigonal planar
[sum of angles = 359.6(4)], consistent with sp2 hybridization.
Reactivity Studies, Nucleophilic at Carbon. Adding
methyl triflate to 3 in a sealable NMR tube results in alkylation
of the alkylidyne carbon to form [CF3−ONO]WC(Me)-
1
(Et)(OtBu) (4) (eq 4). Confirming the identity of 4, a H
NMR spectrum of the reaction mixture reveals a resonance
attributable to the newly formed methyl protons (W
C(CH3)Et) at 4.90 ppm (3H). The −CβH2− methylene
protons are diastereotopic, resonating as two sets of multiplets
C
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at 4.65 and 4.53 ppm, similar to complex 2. The −OtBu
protons resonate at 1.21 ppm (9H). The W−Cα resonates at
284.3 ppm, consistent with other reported tungsten dialkyl-
substituted alkylidenes.71−73 The 1H−13C gHMBC spectrum of
4 confirms the connectivity among the methyl protons at 4.90
ppm, the diastereotopic methylene protons at 4.65 and 4.53
ppm, and the alkylidene carbon. Unidentifiable and inseparable
minor impurities precluded the large-scale purification of 4.
Isobutylene Expulsion from 3. Adding the larger
electrophile Me3SiOTf to complex 3 in a sealable NMR tube
1
provides an interesting result. The products observed by H
NMR spectroscopy are isobutylene (4.71 ppm, 2H; and 1.56
ppm, 6 H) and the new alkylidene complex [CF3−ONO]W
CH(Et)OSiMe3 (5). Complex 5 exhibits a new set of
diastereotopic −CβH2− methylene protons at 5.28 and 4.91
ppm, and the trimethylsiloxide protons appear at 0.12 ppm.
Corroborating the identity of 5, the WCH proton resonates
at 7.20 ppm and the corresponding 13C{1H} signal appears at
262.1 ppm. Multinuclear 1H−13C gHSQC and 1H−13C
gHMBC NMR spectroscopic experiments confirm the identity
of 5 and permit the absolute assignment of all resonances in the
1H and 13C{1H} NMR spectra (see Supporting Information).
Complex 5 is unstable at ambient temperature and decomposes
to unidentifiable and intractable impurities.
n
Figure 5. Molecular structure of [CF3−ONO]W(O) Pr (6) with
ellipsoids drawn at the 50% probability level, with hydrogen atoms
removed for clarity.
−71.8, −75.6, and −76.4 ppm. The α-protons of the propyl
1
group {WCH2CH2CH3) resonate in the H NMR spectrum as
a multiplet at 3.00 ppm. The β-protons and γ-protons of the
propyl group appear as a multiplet at 2.65 ppm and a triplet at
0.92 ppm, respectively. The nitrogen atom is sp2 hybridized and
a vector perpendicular to the amido plane, representing the
lone pair on nitrogen, forms a 39.72° torsion angle with the
WO bond.
Catalytic Isobutylene Expulsion from 2. As mentioned
above, complex 2 contains a restricted amide rotation similar to
that of complex 3. The amide lone pair and alkylidene of 2 form
a torsion angle of 44.34°, potentially creating a nucleophilic
MC double bond.
COMPUTATIONAL RESULTS
■
Employing DFT calculations, the model complexes 2′, 2-Me′,
and 3′, representing 2, the intermediate 2-Me, and 3,
respectively, were geometry optimized. Figure 6 depicts the
computed structures and Table 1 lists pertinent bond lengths
and angles. Experimental bond lengths and angles for 2 serve to
calibrate the calculated structure of 2′. The tungsten
coordination sphere metric parameters are agreeable and it is
clear that the calculation reproduces the twist of the ONO
backbone observed experimentally. A vector perpendicular to
the C3−W1−C13 plane, representing the nitrogen lone pair,
creates a 41.7° dihedral angle with the W1−C1 bond axis (the
experimental value is 40.5°). Also, the alkylidene orients in the
same direction as in 2. The alkylidene ethyl group points away
from the N-atom. A quantifiable parameter confirming the
similar orientation is the dihedral angle C22−C21−W1−O3;
for 2 it is 10.2° and within 2′ it is 11.7°. In general, there is a
small overestimate of most of the bond lengths by 0.02 Å or
less. For example, the N1−W1 distance of 2.013 Å in 2′ is
slightly longer than in 2 [1.993(3) Å], and the computed
alkylidene W1−C21 bond length of 1.898 Å matches the
experimental value of 1.882(4) Å.
Indeed, treating complex 2 with Me3SiOTf also results in
isobutylene expulsion as well as the formation of [CF3−
ONO]W(O)nPr (6). The absence of Me3SiOTf in the product
suggests it acts as a catalyst in the expulsion of isobutylene from
2. Indeed, adding 5 mol % Me3SiOTf, MeOTf, or B(C6F5)3 to a
solution of 2 catalyzes isobutylene expulsion to form 6
1
quantitatively by H NMR spectroscopy (eq 6). Stripping the
solvent from 6 initially yields a thick blue oil, but crystals
suitable for single crystal X-ray analysis gradually form (isolated
yield = 73%). The solid state structure of 6 (Figure 5) contains
a tungsten(VI) ion in a trigonal bipyramidal geometry with
equatorial plane angles N1−W1−O3 = 129.69°, N1−W1−C21
= 121.46°, and C21−W1−O3 = 108.82(10)°. The W1−O3
bond is 1.704 Å, which is consistent with other reported neutral
WVIO complexes.74−93 The 19F{1H} NMR of 6 confirms a
C1 symmetric species in solution with four quartets at −71.2,
The computed structures of 2-Me′ and 3′ also reproduce the
ONO twist and some interesting trends emerge between the set
of three complexes. Most apparent is the W−N bond distance,
which increases with 2-Me′ (1.982 Å) < 2′ (2.013 Å) < 3′
(2.142 Å). The computation accurately predicts a trend
assignable to an increase in electronic saturation at the metal
ion. Complex 3′ is anionic, thus electron-rich, whereas 2-Me′ is
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Figure 6. Geometry-optimized structures for 2′, 2-Me′, and 3′.
of 3′. Considering the reasonable match in metric parameters,
single point calculations of each complex were performed and
the resulting electronic structures were evaluated (vide infra).
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the
Single Crystal X-ray Structure of 2 and DFT Geometry-
Optimized Structures of 2′, 2-Me′, and 3′
bond lengths
2
2′
2-Me′
3′
DISCUSSION
■
W1−O1
W1−O2
W1−O3
W1−N1
W1−C21
C21−C22
O3−C28
angles
1.953(2)
1.931(2)
1.819(2)
1.993(3)
1.882(4)
1.499(5)
1.982
1.953
1.836
2.013
1.898
1.519
1.884
1.880
2.136
1.982
1.986
1.507
1.498
2-Me′
2.001
1.991
1.917
2.142
1.769
1.493
MeOTf alkylation at the Cα atom of the alkylidyne in 3 has a
different reactivity pattern compared to previous examples.
Alkylating agents preferentially attack the ancillary ligands,
leaving both neutral and anionic tungsten alkylidynes intact.95
Similarly for anionic molybdenum imido alkylidyne complexes,
both small ([Me3O][BF4]) and large (Me3SiOTf) electrophiles
selectively attack the imido N-atom.70 The direct alkylation of
the WCα bond by MeOTf has no precedent in the literature.
Enamines react with electrophiles at the β-position, whereas
unfuctionalized alkenes do not. Electronically, complex 3 is
analogous to an enamine. Examining the electronic structure of
3 through single-point calculations provides insight into how
the CF3−ONO pincer ligand influences the tungsten alkylidyne
bond.
Figure 7 (left) depicts a truncated molecular orbital diagram
that illustrates the bonding combination between the amido
lone pair and the WC π-bond. The key feature is the forced
torsion angle between the alkylidyne bond and the amide lone
pair. This contrasts the typical arrangement in which the
nitrogen lone pair orients perpendicularly to the alkylidyne
bond to maximize π-donation into the empty dxy. The rigid
ONO pincer geometry within 3 prevents the amide from
orienting perpendicularly to the alkylidyne and instead the
LUMO of 3′, consisting of the dxy orbital, contains no orbital
interaction with the amido lone pair (Figure 7, left).
2
2′
3′
O1−W1−O2
O1−W1−C21
N1−W1−O3
N1−W1−C21
O2−W1−C21
O3−W1−C21
C22−C21−W1
C28−O3−W1
144.95(11)
104.44(13)
154.64(11)
99.81(14)
109.41(13)
105.50(13)
137.2(3)
145.68
156.17
99.44
146.18
103.39
153.28
98.06
103.65
155.42
97.98
151.23
101.36
104.08
107.40
134.18
108.38
106.48
137.44
106.26
108.65
176.20
111.50
electron-poor, due to the loss of π-donation upon methylating
the alkoxide O-atom. Correspondingly, the W−O bond length
of 2.136 Å for the bound tert-butyl methyl ether in 2-Me′ is
appropriately longer than the tert-butoxide of 3′, and only slight
shorter by ∼0.05 Å than the crystallographically characterized
diethyl ether W−O bond of 2.185(2) Å found in the related
OCO3‑ pincer alkylidyne [tBuOCO]WC(tBu)(Et2O).56
The most salient feature of complex 3′ is the alkylidyne W
C bond with a length of 1.769 Å that matches experimentally
determined values. Though there are no structurally charac-
terized trianionic pincer alkylidyne anions known, two neutral
OCO3‑ pincer complexes have WC bond lengths of 1.755(2)
and 1.759(4) Å. For additional comparison, Schrock’s
(ArO)2NpWC(tBu)94 complex contains a WC bond
length of 1.755(2) Å, a difference of only ∼0.01 Å with that
The alkylidyne π-bond facing the nitrogen [HOMO(−2)]
displays overlap with the nitrogen’s lone pair, lowering its
energy relative to the adjacent π-bond [HOMO(−1)] by
0.01139 au (Figure 7, left). A torsion angle of 42.8° between
the lone pair and the alkylidyne bond results in significant
overlap. The corresponding antibonding combination corre-
sponds to the HOMO orbital analogous to an enamine (Figure
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Figure 7. (Left) The HOMO, HOMO(−1), and HOMO(−2) orbitals of 3′. (Right) The HOMO, HOMO(−1), and HOMO(−2) orbitals of 7′
(isovalue = 0.051 687).
2). This bonding−antibonding interaction raises the energy of
the HOMO, thus increasing the nucleophilicity of the
alkylidyne α-carbon.
MC bond destabilizes the HOMO orbital and places
increased electron density on the α-carbon.
Isobutylene expulsion provides more evidence for the
nucleophilicity of 3. Scheme 2 illustrates a proposed
mechanism for isobutylene expulsion. In the first step,
Me3SiOTf attacks the tert-butoxide ligand to yield the
trimethylsilyl-tert-butyl ether adduct, 3-SiMe3. Then, acting as
a nucleophile, the W-alkylidyne deprotonates the tert-butyl
group to expel isobutylene and form 5. tert-Butoxide is a
common ligand, especially for tungsten complexes featuring
M−C multiple bonds, but this is the first occurrence of its
degradation via alkylidyne deprotonation and is clear evidence
of the highly nucleophilic character of the W−Cα atom.
Additional evidence that the N-atom plays an important role
comes from reactivity studies employing the related OCO3‑
pincer ligand. Without the N-atom a different reaction occurs;
addition of MeOTf to the analogous OCO trianionic pincer
alkylidyne anion {MePPh3}{[tBuOCO]WC(tBu)(OtBu)}
produces [tBuOCO]WC(tBu)(OEt2).56 The reaction pro-
ceeds via Me-alkylation of the tert-butoxide but deprotonation
In stark contrast is the single-point calculation from a
geometry-optimized structure performed on the model anionic
alkylidyne {(Ph2N)WC(Me)(OC(CF3)2Ph)2(OtBu)}− (7′).
The amido ligand orientation matches the analogous complex
{3,5-C6H3Me2)tBuN}WC(tBu){OC(CF3)2Me}2 calculated
by Tamm and co-workers.31 Depicted in Figure 7 (right) are
the single point calculations of 3′ and 7′, aligned for easy
comparison. The model complex 7′ features an unrestricted
amido ligand N(C6H5)2, yet retains the electron-withdrawing
OC(CF3)2C6H5 groups. In 7′ the amido ligand orients to
maximize π-donation into the empty dxy orbital, which serves to
stabilize the HOMO orbital comprised mostly of the N-atom
lone pair. However, the M−C π-bonding orbitals are
completely unaffected by the N-atom lone pair. By comparing
the electronic structures, it is evident that purposely
constraining the N-atom lone pair to be collinear with the
F
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Article
Scheme 2. Proposed Mechanism for Isobutylene Expulsion
from 3
Figure 8. Truncated X-ray structure of 2 (left) and geometry-
optimized structure 2-Me′ (right) illustrating the 77° rotation of the
WC bond.
Scheme 3. Proposed Mechanism for Isobutylene Expulsion
from 2 (LA = Me+, Me3Si+, and B(C6F5)3
does not occur; instead, MeOtBu forms. Notably, the Me+ adds
to the alkoxide and not the alkylidyne α-carbon as in 3.
Not all of the divergent chemistry between the CF3−ONO3‑
and the OCO3‑ ligands are attributable to the N-atom alone. An
additional significant difference is the fluorinated alkoxides on
CF3−ONO3‑, which create an electrophilic tungsten ion.
Silylating the −OtBu of 3 removes π-donation from the
alkoxide and leaves only a weakly σ-donating ether. The result
is an even more electrophilic tungsten metal. Yet, other
electrophilic tungsten alkylidynes supported by three −OC-
(CF3)Me are known metathesis catalysts that show stability
toward substrates containing ether, ester, ketone, aldehyde,
acetal, and thioether moieties96 and are only protonated via
hydrohalic acids.97,98 The combination of the N-atom and the
electron-withdrawing CF3 groups must lead to deprotonation
of the tert-butoxide.
Figure 9. The HOMO, HOMO(−1), and HOMO(−5) orbitals of 2-
Me′ (isovalue = 0.051 687).
ligands are also highly nucleophilic. Scheme 3 depicts the
proposed mechanism for the Lewis acid-catalyzed isobutylene
expulsion from 2. Two plausible sites for electrophilic (LA)
attack on 2 are the amido N-atom and the tert-butoxide O-
atom. The N-atom is too sterically crowded, especially for large
electrophiles such as B(C6F5)3 and Me3SiOTf, which catalyze
the reaction too. Thus, initial attack must occur at the tert-
butoxide to form 2-LA. Proceeding from 2-LA, the alkylidene
deprotonates the tBu group to form isobutylene and 6-LA. The
Lewis acid catalyst is then released to provide 6.
The Lewis acid-catalyzed expulsion of isobutylene from 2
An interesting question arises regarding the deprotonation
event. Structural characterization and subsequent variable-
demonstrates that W−C double bonds with the CF3−ONO3‑
G
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distilled or vacuum transferred, and stored over 4 Å molecular sieves.
Bis(2-bromo-4-methylphenyl)amine,65 PPh3CH2,70 and (tBuO)3W
C(Et)66 were prepared according to published literature procedures.
All other reagents were purchased from commercial vendors and used
without further purification. NMR spectra were obtained on Varian
Gemini 300 MHz, Varian Mercury Broad Band 300 MHz, or Varian
Mercury 300 MHz spectrometers. Chemical shifts are reported in δ
(ppm). For 1H and 13C{1H} NMR spectra, the solvent peak was
referenced as an internal reference. Infrared spectra were obtained on a
Thermo Scientific Nicolet 6700 FT-IR.
DFT Calculations. Spin-restricted density functional theory
calculations, including geometry optimization and single point analysis,
were performed for 2′, 2-Me′, 3′, and 7′ using a hybrid functional (the
three parameter exchange functional of Becke (B3)99 and the
correlation functional of Lee, Yang, and Parr (LYP)100 (B3LYP) as
implemented in the Gaussian 03 program suite.101 The LANL2DZ
basis set were used for all atoms within 2′, 2-Me′, 3′, and 7′.102 The
geometry was optimized using atomic coordinates from the crystal
structure as an initial input and calculations for the vibrational
frequencies were performed alongside the geometry optimization to
ensure the stability of the ground state as denoted by the absence of
imaginary frequencies. Molecular orbital pictures were generated from
Gabedit at their reported isovalues.
temperature NMR experiments indicate that the alkylidene
WC bond in 2 is perpendicular to the tert-butoxide ligand
(see Figure 8 for an illustration). To complete the
deprotonation the π-bond needs to approach the proton of
the tert-butoxide. Curious as to the orientation of the alkylidene
in 2-LA, we performed a geometry optimization calculation of
2-Me′, in which Me+ serves as the Lewis acid (see Figure 6
above). The electrophile accepts a pair of electrons from the
oxygen atom that normally π-donate into the W-dxy orbital.
Most pronounced and interestingly, the alkylidene in 2-Me′
rotates ∼77° from that of the experimentally determined
structure of 2. Illustrated in Figure 8 is a truncated X-ray
structure of 2 and the computed structure 2-Me′. The double
bond clearly orients toward the MeOtBu. A single-point
calculation of 2-Me′ again reveals that the HOMO and
HOMO(−5) are the amide/alkylidene π-antibonding and π-
bonding orbitals, respectively, analogous to that calculated for 3
and a prototypical enamine (Figure 9). The LUMO of 2-Me′
(not depicted) consists of the dxy orbital and contains only a
small component from the N-atom lone pair and ether ligand.
Synthesis of 2,2′-(Azanediylbis(3-methyl-6,1-phenylene))-
bis(1,1,1,3,3,3-hexafluoropropan-2-ol) (1). Inside a nitrogen-filled
glovebox, an n-butyllithium solution (10.9 mL, 2.5 M, 27.3 mmol) was
added dropwise to a Schlenk-flask containing a solution of bis(2-
bromo-4-methylphenyl)amine (3.103 g, 8.79 mmol) in diethyl ether
(30 mL) at −35 °C. The reaction mixture was stirred for 2 h while
warming to room temperature. The reaction flask was fitted with a dry
ice condenser before exiting the box. The reaction flask was connected
to a Schlenk line through the port on the dry ice condenser. The
reaction solution was cooled to −78 °C, and dry ice and acetone were
added to the condenser. Hexafluoroacetone was first measured into a
graduated glass pressure flask by condensing at −78 °C (5 mL, 6.6 g,
39 mmol). The pressure flask was then connected to the reaction flask
via a side arm. The pressure flask was allowed to slowly warm to room
temperature, causing the hexafluoroacetone to evaporate and condense
into the reaction flask. After complete transfer, the pressure flask was
removed. The reaction mixture was allowed to warm to room
temperature while the dry ice/acetone condenser was kept filled (the
hexafluoroacetone will condense on the coldfinger and drip back into
the solution). The reaction mixture was stirred for at least 3 h before
allowing the dry ice/acetone to expire and the excess hexafluor-
oacetone to leave through the Schlenk manifold. To the resulting red
solution was added HCl in Et2O (27.3 mL, 1 M). A colorless
precipitate formed (LiCl) and the mixture was filtered. The filtrate was
reduced under vacuum to a thick oil followed by adding hexanes to
precipitate the product as a light-pink powder, which was filtered (1.66
g, 35% yield). 1H NMR (CDCl3, 300 MHz, 25 °C): δ = 7.5−7.0 (b, 3
H, NH and 2 OH), 7.37 (s, 2H, Ar-H), 7.17 (d, 2H, 3J = 8.35 Hz, Ar-
CONCLUSION
■
Presented above is evidence for an inorganic enamine: a unique
strategy for creating nucleophilic W−C multiple bonds by
constraining an amide ligand lone pair orientation. Tradition-
ally, tungsten alkylidenes/alkylidynes are weakly nucleophilic.
Thus, a straightforward synthetic strategy to create nucleophilic
alkylidene and alkylidyne complexes is to purposely orient an
amido ligand lone pair toward the alkylidyne/alkylidene bond.
To accomplish this feat we report the convenient two-step
synthesis of a new CF3−ONO3‑ trianionic pincer-type ligand.
The “push” via alignment of an amido lone pair, coupled to the
“pull” of the CF3 groups, serve to polarize the M−C bond and
create a nucleophilic α-carbon. Evidence is the direct alkylation
of the alkylidyne in 3 with MeOTf; and isobutylene expulsion
upon addition of the larger Me3SiOTf. Complementing these
results is the Lewis acid-catalyzed expulsion of isobutylene from
2, proposed to occur via tert-butyl deprotonation by the
nucleophilic α-carbon of the WC bond. The reaction works
with several Lewis acids [MeOTf, Me3SiOTf, and B(C6F5)3]
and insight into their role comes from a geometry optimization
of the proposed Lewis acid adduct 2-Me′. The calculation
reveals a critical reorientation of the alkylidene double bond
that places it according to an enamine alignment and in
position to deprotonate the tert-butyl group.
Using a trianionic pincer framework to constrain amide
orientation provides a new approach to increasing nucleophil-
icity of M−C multiple bonds. Another example of a constrained
amido adjacent to M−C multiple bonds is Mindiola’s PNP1‑
titanium alkylidyne,12−21 though being a first row metal and
already highly nucleophilic, the amide contribution may be
difficult to assess. The ease of synthesis of proligand 1 relative
to other trianionic pincer ligands will facilitate exploration of
this ligand with many other metal ions as well as other M−X
multiple bonds (where, X = O, N, and P).
3
H), 8.83 (d, 2H, J = 8.35 Hz, Ar-H), and 2.36 (s, 3H, CH3) ppm.
19F{1H} NMR (CDCl3, 282 MHz, 25 °C): δ = −74.9 (b) and −76.3
(b) ppm. 13C{1H} NMR (CDCl3, 75 MHz, 25 °C): δ = 142.8 (s, Ar
C), 134.3 (s, Ar C), 132.1 (s, Ar C), 128.5 (s, Ar C), 126.0 (s, Ar C),
120.78 9 (s, Ar C), and 21.0 (s, CH3) ppm. 13C{19F} NMR (CDCl3): δ
= 122.8 (s, CF3) and 80.3 (s, Ar(CF3)2COH) ppm. ESI-MS: 530.0984
[1 + H]+, 552.0803 [1 + Na]+, and 574.0623 [1 − H + 2Na]+.
Synthesis of [CF3−ONO]WCH(Et)(OtBu) (2). A benzene
solution (2 mL) of 1 (376.4 mg, 0.711 mmol) and WC(Et)(OtBu)3
(315.9 mg, 0.711 mmol) were combined and stirred for 0.5 h. The
solvent was evaporated and the residual solid was placed under
vacuum for 4 h. Single crystals of 2 were grown from a pentane
EXPERIMENTAL SECTION
1
solution at −35 °C (0.352 g, 60%). H NMR (C6D6, 300 MHz, 25
■
°C): δ = 7.72 (s, 2H, Ar-H), 7.36 (t, 1H, 3J = 7.64 Hz,
General Procedure. Unless specified otherwise, all manipulations
were performed under an inert atmosphere using standard Schlenk or
glovebox techniques. Pentane, hexanes, toluene, diethyl ether (Et2O),
dichloromethane, tetrahydrofuran (THF), and 1,2-dimethoxyethane
(DME) were dried using a GlassContour drying column. Benzene-d6
(Cambridge Isotopes) was dried over sodium−benzophenone ketyl,
3
3
WCHCH2CH3), 6.81 (d, 1H, J = 8.49 Hz, Ar-H), 6.66 (d, 1H, J =
3
3
8.21 Hz, Ar-H), 6.60 (d, 1H, J = 9.06 Hz, Ar-H), 6.59 (d, 1H, J =
2
3
3
8.49 Hz, Ar-H), 5.08 (ddq, 1H, J = 15.0 Hz, J = 7.36 Hz, J = 7.36
2
3
Hz, WCHC(H′)(H)CH3), 4.79 (ddq, 1H, J = 15.0 Hz, J = 7.36 Hz,
3J = 7.36 Hz, WCHC(H′)(H)CH3), 2.00 (s, 3H, CH3′), 1.96 (s, 3H,
H
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Journal of the American Chemical Society
Article
CH3), 1.21 (s, 9H, OC(CH3)3, and 0.77 (t, 3J = 7.36 Hz,
WCHCH2CH3) ppm. 19F{1H} NMR (C6D6, 282 MHz, 25 °C): δ =
−71.2 (q, 3F, 4J = 9.61 Hz), −71.5 (q, 3F, 4J = 12.0 Hz), −73.9 (q, 3F,
3J = 7.04 Hz), 6.71 (d, 1H, Ar-H, 3J = 8.21 Hz), 6.61 (d, 1H, Ar-H, 3J =
3
3
8.21 Hz), 6.53 (d, 1H, Ar-H, J = 8.21 Hz), 6.51 (d, 1H, Ar-H, J =
8.21 Hz), 5.28 (m, 1H, WCHC(H′)(H)CH3), 4.91 (m, 1H,
WCHC(H′)(H)CH3), 1.97 (s, 3H, Ar−CH3), 1.93 (s, 3H, Ar−
4
4J = 9.60 Hz), and −77.2 (q, 3F, J = 9.61 Hz) ppm. 13C{1H} NMR
3
(C6D6, 126 MHz, 25 °C): δ = 260.3 (s, WCHCH2CH3, with satellites
1J(13C,183W) = 173.1 Hz), 146.8 (s, Ar C), 145.8 (s, Ar C), 135.1 (s, Ar
C), 134.6 (s, Ar C), 133.0 (s, Ar C), 131.7 (s, Ar C), 127.5 (s, Ar C),
124.7 (s, Ar C), 124.5 (s, Ar C), 124.3 (s, Ar C), 90.3 (s, OCMe3), 33.6
(s, WCHCH2CH3), 29.8 (s, OC(CH3)3), 21.4 (s, WCHCH2CH3),
21.0 (s, Ar-CH3′), and 20.8 (s, Ar-CH3) ppm. Anal. Calcd for
C27H27F12NO3W (825.33 g/mol): C, 39.29; H, 3.30; N, 1.70. Found:
C, 39.25; H, 3.37; N, 1.58.
CH3′), 0.67 (t, 3H, WCHCH2CH3, J = 7.33 Hz), and 0.12 (s, 9H,
OSi(CH3)3) ppm. 19F{1H} NMR (C6D6, 470 MHz, 25 °C): −69.38
(q, 3F, 4J = 9.61 Hz), −71.24 (q, 3F, 4J = 9.61 Hz), −74.08 (q, 3F, 4J =
4
9.61 Hz), and −76.38 (q, 3F, J = 9.61 Hz) ppm. 13C{1H} NMR
(C6D6, 126 MHz, 25 °C): δ = 262.1 (s, WCHCH2CH3), 145.8 (s, Ar
C), 145.0 (s, Ar C), 134.5 (s, Ar C), 134.0 (s, Ar C), 132.0 (s, Ar C),
131.0 (s, Ar C), 127.2 (s, Ar C), 126.9 (s, Ar C), 126.4 (s, Ar C), 123.7
(s, Ar C), 123.6 (s, Ar C), 123.1 (s, Ar C), 31.3 (s, WCHCH2CH3),
20.6 (s, WCHCH2CH3), 20.2 (s, Ar-CH3′), 20.0 (s, Ar-CH3), and −0.1
(s, OSi(CH3)3) ppm.
Synthesis of {[CF3−ONO]WC(Et)(OtBu)}{MePPh3} (3). A
pentane solution of CH2PPh3 (131.5 mg, 0.476 mmol, 1.6 equiv)
was filtered prior to dropwise addition to a stirring pentane solution of
2 (243.4 mg, 0.295 mmol, 1 equiv). Red oil formed upon complete
addition and the reaction mixture was triturated in the pentane
solution for 6 h to yield a yellow powder. The powder was filtered,
Synthesis of [CF3−ONO]W(O)(nPr) (6). To a 2.0 mL benzene
solution of 2 (398 mg, 4.82 × 10−4 mol) was added trimethylsilyl
triflate (1 mg, 4.5 × 10−6 mol) in benzene (1 mL). The solution
changed from reddish-brown to aquamarine blue over the period of 2
h. The solution was evaporated in vacuo to remove solvent and
trimethylsilyl triflate, providing an oil. Crystals of 6 formed from the
oil after 3 h. The crystals were removed by spatula (0.274 g, 74.6%).
1H NMR (C6D6, 300 MHz, 25 °C): δ = 7.52 (s, 2H, Ar-H), 6.73 (d,
1H, Ar-H, 3J = 8.21 Hz), 6.68 (d, 1H, Ar-H, 3J = 8.80 Hz), 6.61 (d, 1H,
1
washed with pentane, and dried overnight (0.170 g, 83%). H NMR
(C6D6, 300 MHz, 25 °C): δ = 7.81 (s, 1H, Ar-H), 7.66 (s, 1H, Ar-H),
3
3
7.51 (d, 1H, Ar-H, J = 8.35 Hz), 7.20 (d, 1H, Ar-H, J = 8.95 Hz),
7.01 (b, (C6H5)3PCH3), 6.98 (b, (C6H5)3PCH3) 6.92 (dd, 1H, Ar-H,
3J = 8.65 Hz, 4J = 1.64 Hz), 6.80 (dd, 1H, Ar-H, 3J = 8.35 Hz, 4J = 1.64
Hz), 4.35 (q, 2H, WCCH2CH3, 3J = 7.61 Hz), 2.44 (d, 3H,
3
Ar-H, J = 8.21 Hz), 6.52 (d, 1H, Ar-H, 3J = 8.80 Hz), 3.03−2.97 (m,
2
2H, WCH2CH2CH3), 2.81−2.52 (m, 2H, WCH2CH2CH3), 1.92 (s,
3H, Ar−CH3), 1.88 (s, 3H, Ar−CH3), and 0.92 (t, 3H,
WCH2CH2CH3, 3J = 7.33 Hz) ppm. 19F{1H} NMR (C6D6, 282
(C6H5)3PCH3, J = 13.31 Hz), 2.16 (s, 3H, Ar−CH3), 2.09 (s, 3H,
Ar−CH3′), 1.63 (s, 9H, OC(CH3)3), and 0.80 (t, 3H, WCCH2CH3, 3J
= 7.64 Hz) ppm. 31P{1H} NMR (C6D6, 121 MHz, 25 °C): δ = 21.8
4
4
4
ppm. 19F{1H} NMR (C6D6, 282 MHz, 25 °C): −69.38 (q, 3F, J =
MHz, 25 °C): −71.17 (q, 3F, J = 8.48 Hz), −71.77 (q, 3F, J = 9.69
4
4
4
4
Hz), −75.55 (q, 3F, J = 9.69 Hz), and −76.38 (q, 3F, J = 8.48 Hz)
ppm. 13C{1H} NMR (C6D6, 126 MHz, 25 °C): δ 146.5 (s, Ar), 145.5
(s, Ar), 135.5 (s, Ar), 133.9 (s, Ar), 132.8 (s, Ar−H), 127.5 (s, Ar−H),
125.4 (s, Ar), 123.0 (s, Ar−H), 82.7 (s, WCH2CH2CH3, with satellites
1J(13C,183W) = 109.1 Hz), 26.9 (s, WCH2CH2CH3), 21.0 (Ar-−CH3),
20.7 (s, WCHCH2CH3), and 19.3 (s, Ar-CH3′) ppm. Anal. Calcd for
C24H22F12NO3W (769.22 g/mol): C, 36.76; H, 2.83; N, 1.79. Found:
C, 36.67; H, 2.87; N, 1.79.
8.30 Hz), −71.24 (q, 3F, J = 10.38 Hz), −74.08 (q, 3F, J = 10.38
Hz), and −76.38 (q, 3F, 4J = 8.30 Hz) ppm. Anal. Calcd for
C46H44F12NO3PW (1096.64 g/mol): C, 50.15; H, 4.03; N, 1.27.
Found: C, 50.15; H, 3.96; N, 1.32.
[CF3−ONO]WC(CH3)(Et)(OtBu) (4). To a benzene (1 mL)
solution of 3 (0.024 g, 2.1 × 10−5 mol) was added MeOTf (0.004 mg,
2.1 × 10−5 mol). The reaction solution turned immediately from red
to brown and the solvent was removed in vacuo. The residue was
dissolved in Et2O and filtered to remove a colorless precipitate
(MePPh3OTf). The solvent was removed again in vacuo and the
residue was dissolved in pentane and filtered. Removal off all volatiles
from the filtrate provides 4 along with intractable impurities (<10%),
thus precluding combustion analysis. Multinuclear and 2D NMR
techniques provide the unambiguous characterization of 4 and the
absolute assignment of all 1H and 13C{1H} NMR resonances. 1H
NMR (C6D6, 500 MHz, 25 °C): δ = 7.67 (s, 1H, Ar-H), 7.65 (s, 1H,
Ar-H), 6.76 (d, 1H, 3J = 8.50 Hz, Ar-H), 6.71 (d, 1H, 3J = 8.50 Hz, Ar-
H), 6.61 (d, 1H, 3J = 8.80 Hz, Ar-H), 6.51 (d, 1H, 3J = 8.80 Hz, Ar-H),
4.87 (s, 3H, WC(CH3)CH2CH3), 4.62 (m, 1H, WCHC(H′)(H)CH3),
4.50 (m, 1H, WCHC(H′)(H)CH3), 1.93 (s, 6H, CH3), 1.19 (s, 9H,
ASSOCIATED CONTENT
■
S
* Supporting Information
1H, 13C{1H}, 31P{1H}, 2-D NMR spectra, IR, and crystallo-
graphic data. This material is available free of charge via the
Internet at http://pubs.acs.org/.
AUTHOR INFORMATION
■
Corresponding Author
veige@chem.ufl.edu
3
OC(CH3)3, and 0.70 (t, J = 7.33 Hz, WCHCH2CH3) ppm. 19F{1H}
4
Notes
NMR (C6D6, 470 MHz, 25 °C): δ = −76.3 (q, 3F, J = 8.48 Hz),
−75.6 (q, 3F, 4J = 9.69 Hz), −71.1 (q, 3F, 4J = 9.69 Hz), and −70.8 (q,
3F, 4J = 8.48 Hz) ppm. 13C{1H} NMR (C6D6, 126 MHz, 25 °C): δ =
284.3 (s, WC(CH3)CH2CH3), 145.3 (s, Ar C), 142.8 (s, Ar C), 134.2
(s, Ar C), 133.2 (s, Ar C), 131.9 (s, Ar C), 131.6 (s, Ar C), 127.3 (s, Ar
C), 126.9 (s, Ar C), 126.5 (s, Ar C), 124.0 (s, Ar C), 122.8 (s, Ar C),
89.9 (s, -OC(CH3)), 34.0 (s, WC(CH3)CH2CH3), 29.1 (s, OC-
(CH3)3), 23.5 (s, WC(CH3)CH2CH3), 20.4 (s, Ar-CH3′), 20.2 (s, Ar-
CH3), and 18.4 (s, WC(CH3)CH2CH3) ppm.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by UF, the Alfred P. Sloan
Foundation, and the NSF in the form a CAREER award to
A.S.V. (CHE-0748408). K.A.A. thanks UF and the NSF for
funds to purchase X-ray equipment (CHE-0821346).
[CF3−ONO]WCH(Et)(OSiMe3) (5). To a diethyl ether (1 mL)
solution of 3 (0.143 g, 1.30 × 10−4 mol) was added Me3SiOTf (0.029
mg, 1.31 × 10−4 mol). The reaction solution turned immediately from
red to brown and a colorless precipitate formed (MePPh3OTf). The
solid was removed by filtration and the filtrate was reduced to provide
a brown oil. The product was immediately redissolved in C6D6 to
prevent decomposition. The product was found to decompose when
left as an oil for several hours, thus precluding combustion analysis.
However, the complex was stable long enough in solution to be
characterized by multinuclear and 2D NMR techniques, thus providing
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■
1
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7.68 (s, 1H, Ar-H), 7.66 (s, 1H, Ar-H), 7.20 (t, 1H, WCHCH2CH3,
I
dx.doi.org/10.1021/ja302222s | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
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