Fast "Wittig-Like" Reactions As a Consequence of the Inorganic Enamine Effect

Article abstract of DOI:10.1021/jacs.5b01599

(Chemical Equation Presented). The tungsten alkylidyne [CF₃-ONO]W≡CC(CH₃)₃(THF)₂ (3) {where CF₃-ONO = (MeC₆H₃[C(CF₃)₂O])₂N^3-} supported by a trianionic pincer-type ligand demonstrates enhanced nucleophilicity in unusually fast "Wittig-like" reactions. Experiments are designed to provide support for an inorganic enamine effect that is the origin of the enhanced nucleophilicity. Treating complex 3 with various carbonyl-containing substrates provides tungsten-oxo-vinyl complexes upon oxygen atom transfer. The rates of reactivity of 3 are compared with the known alkylidyne (DIPP)₃W≡CC(CH₃)₃ (DIPP = 2,6-diisopropylphenoxide). In all cases (except acetone), complex 3 exhibits significantly faster overall rates than (DIPP)₃W≡CC(CH₃)₃. New oxo-vinyl complexes are characterized by NMR, combustion analysis and single crystal X-ray diffraction. Treating 3 with acid chlorides provides the tungsten oxo chloride species [CF₃-ONO]W(O)Cl (4) and disubstituted alkynes. In the case of acetone the oxo-vinyl complex results in two rotational isomers 10_syn and 10_anti. The rate of isomerization was determined for the forward and reverse directions and was complimented with DFT calculations.

Full text of DOI:10.1021/jacs.5b01599

Article
pubs.acs.org/JACS
Fast “Wittig-Like” Reactions As a Consequence of the Inorganic
Enamine Effect
Stella A. Gonsales, Matias E. Pascualini, 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: The tungsten alkylidyne [CF3−ONO]WCC(CH3)3(THF)2 (3) {where CF3−ONO = (MeC6H3[C-
(CF3)2O])2N3−} supported by a trianionic pincer-type ligand demonstrates enhanced nucleophilicity in unusually fast
“Wittig-like” reactions. Experiments are designed to provide support for an inorganic enamine effect that is the origin of the
enhanced nucleophilicity. Treating complex 3 with various carbonyl-containing substrates provides tungsten-oxo-vinyl complexes
upon oxygen atom transfer. The rates of reactivity of 3 are compared with the known alkylidyne (DIPP)3WCC(CH3)3 (DIPP
= 2,6-diisopropylphenoxide). In all cases (except acetone), complex 3 exhibits significantly faster overall rates than (DIPP)3W
CC(CH3)3. New oxo-vinyl complexes are characterized by NMR, combustion analysis and single crystal X-ray diffraction.
Treating 3 with acid chlorides provides the tungsten oxo chloride species [CF3−ONO]W(O)Cl (4) and disubstituted alkynes. In
the case of acetone the oxo-vinyl complex results in two rotational isomers 10syn and 10anti. The rate of isomerization was
determined for the forward and reverse directions and was complimented with DFT calculations.
pair electron density across the unsaturated bond to the carbon
two bonds away.
INTRODUCTION
■
Creating highly nucleophilic metal−carbon multiple bonds is an
important synthetic challenge and can result in new and
unusual reactivity. For example, the trianionic ONO3− pincer
alkylidene [CF3−ONO]WCHCH2CH3(OtBu) deprotonates
its own tert-butoxide ancillary ligand to release isobutylene
producing the tungsten oxo-alkyl complex [CF3−ONO]W-
(O)CH2CH2CH3.1 From Mindiola’s laboratories, the pincer
supported titanium alkylidyne [(PNP)TiCC(CH3)3]2−4
(where PNP− = N[2-PiPr2-4-methylphenyl]2) can activate C−
H bonds from benzene, methane, ethane, propane, linear
alkanes C4−C8, and some cyclic alkanes.5−7 A method to
elevate the nucleophilicity of metal−carbon multiple bonds is
to apply the “inorganic enamine” effect.1 Figure 1 depicts the
relationship between organic enamines8 and an inorganic
enamine via a straightforward resonance depiction. Constrain-
ing an N atom lone pair to be collinear with a metal−carbon
multiple bond is the basis for the interaction. In both cases, the
resonance contributions effectively delocalize the N atom lone
Figure 2 depicts a truncated molecular orbital diagram that
summarizes previous work on the electronic consequences of
the inorganic enamine effect.1 In Figure 2A, the amido ligand
rotates freely, thus the lone pair on nitrogen preferentially
orients to maximize overlap with the empty dxy orbital on the
metal. The interaction stabilizes the N atom lone pair in a
bonding molecular orbital as the HOMO. It is clear that no
electron density resides on the α-carbon of the alkylidyne. In
contrast, a trianionic ONO3− pincer ligand constrains the N
atom lone pair in Figure 2B to be coplanar with one of the
metal−carbon π-bonds. The forced interaction has three
important effects: (1) the interaction splits the energies of M-
C π-bonding orbitals; (2) the HOMO orbital is π* in character,
and thus overall is destabilized; and (3) the HOMO orbital
contains significant electron density on the α-carbon, akin to an
enamine. Taken together the inorganic enamine effect
destabilizes the complex and accentuates the nucleophilicity
of the α-carbon.
Positive support that the inorganic enamine effect is indeed
real requires the α-carbon to not only react with protons, but
other electrophiles as well, and the reactivity should be
accelerated, i.e., demonstrably more nucleophilic. Herein, we
Figure 1. Resonance contributions for an enamine (left) and the
analogous inorganic enamine (right).
Received: February 12, 2015
Published: March 20, 2015
© 2015 American Chemical Society
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pylphenoxide)17 cleaves CO bonds to form oxo-vinyl
complexes according to Scheme 1C. In an interesting example,
acid chlorides react with (DIPP)3WCC(CH3)3 to produce
disubstituted acetylenes in a single step,18 an overall three bond
group transfer. In this work, we use Wittig reactions to evaluate
the relative nucleophilicity of metal−carbon multiple bonds.
Reported previously, treating the proligand [CF3−ONO]H3
(1) with (tBuO)3WCC(CH3)3 in Et2O leads to the trianionic
pincer alkylidyne complex [CF3−ONO]WCC(CH3)3
(Et2O) (2) (Scheme 2).19 The Et2O complex can be
Scheme 2. Improved Synthesis of the Tungsten Alkylidyne
[CF3−ONO]WCC(CH3)3(THF)2 (3)
Figure 2. Truncated molecular orbital diagram illustrating the
electronic consequences of an inorganic enamine. (A) Amido ligand
freely rotates and preferentially orients to overlap with the N atom
lone pair with the unoccupied dxy orbital and (B) constrained within a
pincer ligand the N atom lone pair is forced to overlap with one of the
W−C π-bonds.1
present extraordinarily rapid Wittig-like reactions with the
trianionic pincer alkylidyne complex [CF3−ONO]WCC-
(CH3)3(THF)2. A combined, synthetic, kinetic, and theoretical
analysis provides conclusive evidence for the accentuated
nucleophilicity of metal−carbon multiple bonds within an
inorganic enamine.
RESULTS AND DISCUSSION
■
Predicting chemical outcomes based solely on the electronic
structure of molecules is critical to discovering new reactivity. A
classic case is the juxtaposition of Schrock alkylidenes/ynes9,10
and Fischer carbenes/ynes.11 As predicted by their electronic
structure, high-oxidation-state metal carbon multiple bonds are
nucleophilic and exhibit Wittig chemistry (Scheme 1). Tebbe’s
reagents Cp2Ti(μ-CH2)(μ-X)Al(CH3)2 (X = Cl, Me)12−15 and
Schrock’s (Me3CCH2)3M(CHCMe3) (M = Nb and Ta)
alkylidenes16 react with aldehydes, ketones, esters, and amides
(Scheme 1B). Alkylidynes can also participate in Wittig-like
chemistry.17,18 (DIPP)3WCC(CH3)3 (DIPP = 2,6-diisopro-
challenging to prepare in pure form. To improve the
preparation, treating complex 2 with a few drops of THF
produces the bis-THF complex [CF3−ONO]WCC(CH3)3
(THF)2 (3), which is more amenable to purification and a
significant improvement to the synthesis.
Scheme 3 depicts the room-temperature reactivity of
complex 3 with various carbonyl-containing substrates. The
reaction between 3 and p-methoxybenzoyl chloride results in an
instantaneous color change from blue to green, to form the
oxo-chloride complex [CF3−ONO]W(O)Cl (4) and 1-(3,3-
dimethylbut-1-yn-1-yl)-4-methoxybenzene (5), in 91% yield.
The net reaction is a peculiar three bond swap, CO and C−
Cl for WO and W−Cl. The reaction presumably proceeds
via first formation of a metallaoxetane18,20 followed by β-
chloride elimination.21 N−atom transfers to acid chlorides
proceed via a similar mechanism.22,23 Complete conversion and
confirmation of the identity of the alkyne was confirmed by 1H
and 13C{1H} NMR spectroscopy. Complex 3 also reacts with
pivaloyl chloride to produce 4 and 2,2,5,5-tetramethylhex-3-yne
(6) in 76% yield.
Scheme 1. Wittig and “Wittig-Like” Reactions with
Alkylidenes and Alkylidynes
Following the same procedure, the reactivity of 3 with esters,
ketones, and amides, was also explored (Scheme 3). An
instantaneous color change from blue to red occurs upon
adding 1 equiv of methyl phenylpropiolate to 3. The red color
signals the production of the oxo-vinyl complex [CF3−
ONO]W(O){κ-C−O−(CH3)3CCC(OCH3)(CCC6H5)}
(7), in 82% yield. Interestingly, the alkylidyne attacks the
carbonyl group of phenylpropiolate rather than the alkyne.
Attack at the carbonyl must occur first because attack at the
alkyne would form a metallacyclobutadiene complex, which is
irreversible.19 Evidence for the identity of complex 7 comes
from a combination of solution phase NMR studies and solid-
state structural characterization. The 19F{1H} NMR spectrum
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Scheme 3. Reactivity of 3 with Various Carbonyl Compounds
of 7 exhibits four quartets at −70.28, −70.56, −75.51, and
−75.94 ppm, corresponding to each CF3 group in a C1-
symmetric compound. The 1H NMR spectrum exhibits a
singlet at 3.23 ppm attributable to the methyl group and the tBu
protons resonate at 1.63 ppm.
An X-ray diffraction experiment performed on single crystals
of 7 provides its unambiguous assignment (Figure. 3).
and is consistent with related tungsten-oxo complexes.23−27
The C28−C29 bond distance is 1.194(6) Å, indicating that the
CC triple bond remains intact. A bond length of 1.348(6) Å
between C21 and C22 confirms that a newly formed double
bond is present in 7.
Complex 3 also reacts immediately in benzene with 1 equiv
of ethyl acetate to produce the green oxo-vinyl complex [CF3−
ONO]W(O){κ−C-O-(CH3)3CCC(OCH2CH3)CH3} (8),
1
in 65% yield. H NMR spectroscopic characterization indicates
8 is also C1-symmetric and exhibits diastereotopic protons
resonating at 3.40 and 3.80 ppm for the −OCH2CH3 protons.
The methyl protons (−OCH2CH3) appear as a doublet of
doublets centered at 0.33 ppm and the tBu protons resonate at
1.43 ppm. Four quartet resonances at −69.74, −70.59, −75.12,
and −76.09 ppm in the 19F{1H} spectrum of 8 offer additional
support for a C1 complex.
Upon treating a benzene solution of 3 with dimethylforma-
mide (DMF), an instantaneous color change from blue to red
occurs. Removing the solvent under reduced pressure provides
the red oxo-vinyl complex [CF3−ONO]W(O){(CH3)3CC
CH(N(CH3)2)} (9) in 81% yield. Crystals suitable for X-ray
diffraction deposit from a slowly evaporating solution of 9 in
Figure 3. Molecular structure of 7. Ellipsoids are drawn at the 50%
probability level, and hydrogen and disordered fluorine atoms are
removed for clarity.
1
diethyl ether. A combination of H, 19F{1H}, and 13C{1H}
NMR spectroscopy, combustion analysis, and X-ray diffraction
studies confirm the identity of 9 (Figure. 4). Clearly different
from complex 7, the N atom of the vinyl group in 9 does not
coordinate to form a chelate. The metal center adopts a
distorted square-pyramidal geometry (τ = 0.32)28 with basal
plane angles of ∠O1−W1−O2 = 156.46(9)° and ∠C21−W1−
N1 = 137.13(11)°. The W1−O3 distance of 1.699(2) Å is
statistically identical to 7. Without a chelate such as in 7, the
tBu group in 9 orients toward open space on the opposite side
of the oxo ligand.
Importantly, the molecular structure reveals that the OMe
group coordinates to the tungsten ion trans to the oxo ligand
rendering the complex with a distorted octahedral geometry
(∠O4−W1−O3 = 163.27(12)°, ∠O1−W1−O2 = 159.42(12)°,
and ∠N1−W1−C22 = 146.04(15)°). A long W1−O3 bond
length of 2.413(3) Å indicates the OMe is weakly bound. For
comparison, complex 2 contains a W-OEt2 bond length that is
shorter (2.1144(19) Å).19 The W1−O4 distance is 1.691(3) Å
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Figure 5. PES scan for the highlighted (black) OWCC dihedral angle
of 10.
Figure 4. Molecular structure of 9. Ellipsoids are drawn at the 50%
probability level, and hydrogen and disordered arene atoms are
removed for clarity.
the vinyl group. The calculated rotational barriers of 23.04 kcal·
mol−1 for the forward reaction and 21.38 kcal·mol−1 for the
reverse process are in good agreement with the experimental
values.
Complex 3 also deoxygenates acetone to yield the green oxo-
vinyl complex [CF3−ONO]W(O){(CH3)3CCC(CH3)2}
(10) in 73% yield. Confirmation for the identity of 10 comes
from 1H, 19F{1H}, and 13C{1H} spectroscopy, elemental
analysis, and solid-state characterization. NMR studies indicate
that 10 exists as two rotational isomers due to restricted
rotation about the W−C bond as 10syn and 10anti in a 2:1 ratio
(25 °C); where syn and anti refer to the orientation of the vinyl
double bond relative to the oxo ligand (Scheme 4). Similar
rotation isomers were assigned to products that form upon
reacting (DIPP)3WCC(CH3)3 with acetone, but no X-ray
structural data were provided.17
Green crystals suitable for X-ray diffraction studies grow
upon slow diffusion of pentane into a concentrated diethyl
ether solution of 10syn at −23 °C. The solid-state structure
(Figure 6) confirms the syn orientation. Redissolving the
Scheme 4. Rotational Isomers 10syn and 10anti
Figure 6. Molecular structure of 10syn. Ellipsoids are drawn at the 50%
probability level, and hydrogen atoms are removed for clarity.
Complex 10syn forms initially as the kinetic product, and then
gradual isomerization to 10anti establishes the equilibrium. The
isomerization occurs at room temperature over a period of 20 h
and is easily monitored by 19F{1H} NMR spectroscopy. For the
forward reaction of 10syn to 10anti a barrier of 23.7 0.4 kcal·
mol−1 was determined. In the reverse direction (10anti to 10syn),
a barrier of 23.3 0.4 kcal·mol−1 was found. The difference of
0.4 kcal·mol−1 between the two isomers is in agreement with
the NMR determined equilibrium distribution of 2:1 for
1
crystals in C6D6 and obtaining a H NMR spectrum reveals
only resonances attributable to 10syn, though as before 10anti
slowly forms to establish equilibrium once dissolved. The
crystallographic data indicate the tungsten ion adopts a
distorted square-pyramidal geometry (τ = 0.37)28 with basal
plane angles of ∠O1−W1−O2 = 159.52(7)° and ∠C21−W1−
N1 = 137.46(9)°. The bond length of 1.365(4) Å between C21
and C22 is appropriate for a CC double bond, and again the
tungsten-oxo bond is similar to 7 and 9 with a bond length of
1.6967(16) Å.
Rates of Reactions: (DIPP)3WCC(CH3)3 vs 3. Table 1
lists the results from the kinetic analysis for the reaction of
various CO-containing substrates with 3 and (DIPP)3W
CC(CH3)3. Impressively, complex 3 exhibits extraordinarily fast
overall reaction rates. Schrock reported “Wittig-like” reactivity
for the tungsten alkylidyne (DIPP)3WCC(CH3)3,17 however
reaction times of hours were observed for most substrates. For
complex 3 each reaction end point occurs within 10 s.
10syn:10anti
.
Insight into the rotation about the W−C bond comes also
from DFT calculations at the B3LYP level of theory,29−31
where the OWCC dihedral angle was scanned in 10°
increments. Figure 5 depicts an asymmetric potential energy
surface (PES) as expected for a C1-symmetric complex. The
PES displays two minima corresponding to 10syn and 10anti and
two high-energy barriers that prevent free rotation about the
W−C bond. These barriers result from the large steric
hindrance between the flanking CF3 groups of the pincer and
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complex 3 reacts up to 4 orders of magnitude faster than
(DIPP)3WCC(CH3)3. An important result discounts the
notion that 3 could be less sterically encumbered than
(DIPP)3WCC(CH3)3. DMF binds to (DIPP)3WCC-
(CH3)3 instantaneously but takes hours to complete the C
O bond cleavage. The instantaneous binding indicates steric
impedance to the metal is not a problem, yet the reaction
requires heating to 50 °C for hours to complete. In contrast,
complex 3 binds and cleaves the CO bond in DMF within 10
s. Other Schrock alkylidynes are slow to react; the alkylidyne
(tBuO)3WCC(CH3)3 requires 16 h and 60 °C to react with
acetone.17 Combined with the fact that the trianionic OCO3−
pincer alkylidyne complex [tBuOCO]WCC(CH3)3(THF)2
does not react with DMF, it is clear that an electronic factor
must be responsible for the increased rates observed for 3. The
diverse chemistry of metal−carbon multiple bonds is intimately
related to the nucleophilicity/electrophilicity of the α-carbon.
The “inorganic enamine” effect provides synthetic chemists a
new design tool to tailor catalysts.
Table 1. Reaction Times and Rates for “Wittig-Like”
Reactions Performed with Complexes 3 and (DIPP)3W
CC(CH3)3
3
(DIPP)3WCC(CH3)3
a
a
substrate
acetone
t
rate M−1·s−1
t
rate M−1·s−1
≤10 s
≤10 s
≤10 s
≤10 s
≤10 s
≥0.002
≥0.002
≥0.002
≥0.002
≥0.002
≤10 s
12 h
≥0.002
−
b
DMF
EtOAc
2 d
90 min
12 h
1.2 × 10−7
3.7 × 10−6
4.6 × 10−7
Ph2CO
p-MeOBzCO2Cl
a
Rate refers to the time frame to complete full conversion.
Temperature had to be increased to 50 °C.
b
Comparing these observations with the ones reported by
Schrock,17 the overall rates for 3 are surprisingly faster (>4
orders of magnitude).
(DIPP)3WCC(CH3)3, synthesized according to literature
precedent from {(CH3)3CH2}3WCC(CH3)3,32 was sub-
jected to the identical reaction conditions as 3. Monitored by
1H NMR spectroscopy and in sealed NMR tubes, the same
concentration of each alkylidyne was used in all cases (0.02 M
in C6D6). Complex 3 consumes all substrate within 10 s, thus
an estimated minimum overall reaction rate of 0.002 M−1·s−1
for acetone, ethyl acetate, DMF, p-methoxybenzoyl chloride,
and methyl phenylpropiolate was determined. In contrast,
changing from brown to deep red, the reaction of (DIPP)3W
CC(CH3)3 (0.02 M in C6D6) with acetone was the only
reaction with an instantaneous color change. More typical,
adding 1 equiv of ethyl acetate to (DIPP)3WCC(CH3)3
results in complete conversion after 2 days. For comparison,
complex 3 reacts within 10 s with ethyl acetate with a minimum
rate of 0.002 M−1 s−1, whereas (DIPP)3WCC(CH3)3
requires 2 days demonstrating a rate of approximately 1.2 ×
10−7 M−1·s−1. The acceleration is 4 orders of magnitude faster
for 3. Complex 3 also reacts within 10 s with the larger
substrate benzophenone, whereas (DIPP)3WCC(CH3)3
requires 90 min. Treating (DIPP)3WCC(CH3)3 with DMF
instantaneously forms the DMF adduct,17 however complete
conversion to the oxo-vinyl complex requires 12 h heating at 50
°C. For DMF, a direct comparison of reaction rates is not
possible since higher temperatures are needed for (DIPP)3W
CC(CH3)3 to react, but again 3 reacts instantaneously at 25 °C.
Formation of acetylene from (DIPP)3WCC(CH3)3 and p-
methoxybenzoyl chloride requires 12 hours to achieve complete
conversion resulting in a rate of approximately 4.6 × 10−7 M−1·
s−1, again 4 orders of magnitude slower than 3.
ASSOCIATED CONTENT
■
S
* Supporting Information
Full experimental procedures, NMR spectra, and X-ray
crystallographic details for CCDC 1048942−1048944. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*veige@chem.ufl.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.S.V. thanks UF and the National Science Foundation for
financial support of this project (CHE-1265993). K.A.A. thanks
UF and the NSF CHE-0821346 for funding the purchase of X-
ray equipment. Computational resources and support were
provided by the University of Florida High-Performance
Computing Center.
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■
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