Chain propagation and termination mechanisms for polymerization of conjugated polar alkenes by [Al]-based frustrated Lewis pairs

Article abstract of DOI:10.1021/ma5019389

A combined experimental and theoretical study on mechanistic aspects of polymerization of conjugated polar alkenes by frustrated Lewis pairs (FLPs) based on N-heterocyclic carbene (NHC) and Al(C₆F₅)₃ pairs is reported. Thi

Full text of DOI:10.1021/ma5019389

Article
pubs.acs.org/Macromolecules
Chain Propagation and Termination Mechanisms for Polymerization
of Conjugated Polar Alkenes by [Al]-Based Frustrated Lewis Pairs
Jianghua He,† Yuetao Zhang,†,‡ Laura Falivene,§,∥ Lucia Caporaso,∥ Luigi Cavallo,*,§,∥
and Eugene Y.-X. Chen*,†
†Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States
‡State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China
§Physical Sciences and Engineering Division, Kaust Catalysis Center, King Abdullah University of Science and Technology (KAUST),
Thuwal 23955-6900, Saudi Arabia
∥Dipartimento di Chimica e Biologia, Universita
̀
di Salerno, Via Papa Paolo Giovanni II, I-84084 Fisciano, Italy
S
* Supporting Information
ABSTRACT: A combined experimental and theoretical study on
mechanistic aspects of polymerization of conjugated polar alkenes by
frustrated Lewis pairs (FLPs) based on N-heterocyclic carbene
(NHC) and Al(C6F5)3 pairs is reported. This study consists of three
key parts: structural characterization of active propagating inter-
mediates, propagation kinetics, and chain-termination pathways.
Zwitterionic intermediates that simulate the active propagating species
in such polymerization have been generated or isolated from the FLP
activation of monomers such as 2-vinylpyridine and 2-isopropenyl-2-
oxazolineone of which, IMes+-CH2C(Me)(C3H2NO)Al(C6F5)3
−
(2), has been structurally characterized. Kinetics performed on the
polymerization of 2-vinylpyridine by ItBu/Al(C6F5)3 revealed that the
polymerization follows a zero-order dependence on monomer
concentration and a first-order dependence on initiator (ItBu) and
activator [Al(C6F5)3] concentrations, indicating a bimolecular,
activated monomer propagation mechanism. The Lewis pair polymer-
ization of conjugate polar alkenes such as methacrylates is
accompanied by competing chain-termination side reactions; between the two possible chain-termination pathways, the one
that proceeds via intramolecular backbiting cyclization involving nucleophilic attack of the activated ester group of the growing
polymer chain by the O-ester enolate active chain end to generate a six-membered lactone (δ-valerolactone)-terminated polymer
chain is kinetically favored, but thermodynamically disfavored, over the pathway leading to the β-ketoester-terminated chain, as
revealed by computational studies.
achieved remarkable successes in many areas of chemistry,
chiefly activation of small molecules,2 catalytic hydrogenation,3
and new reactivity/reaction development.4
INTRODUCTION
■
The chemistry of “frustrated Lewis pairs” (FLPs) has attracted
an explosive level of recent interest since the FLP concept was
uncovered through the seminal works of Stephan and Erker.1 A
FLP was initially described as a nonclassical Lewis pair
comprising a bulky Lewis acid (LA), such as E(C6F5)3 (E =
B, Al), and a bulky Lewis base (LB), such as PtBu3 and PMes3
(Mes = 2,4,6-Me3C6H2), that are sterically precluded from
forming stable classical LB → LA adducts (CLAs). Nowadays,
FLPs also include those Lewis pairs with a weak LB---LA bonds
due to electronic frustration. Such FLPs exhibit the largely
unquenched, orthogonal LA and LB reactivity that can promote
unusual reactions, or reactions that were previously known to
be possible only by transition-metal complexes, and display
FLP-induced or enhanced reactivity in activation of small
molecules, catalyzing the rapidly growing interest in the FLP
chemistry.1 In its relatively short history, the FLP chemistry has
Lewis pair (LP) polymerization,5 which utilizes either a CLA
or a FLP in which the LA and LB work cooperatively to activate
the monomer substrate and participate in chain initiation as
well as chain propagation of termination/transfer events, has
attracted recent interest in polymerization catalysis for the
synthesis of several different classes of polymers. FLPs
consisting of the bulky aluminum LA Al(C6F5)3 and bulky
phosphine or N-heterocyclic carbene (NHC) LBs have been
utilized to initiate rapid polymerization of conjugated polar
alkenes,6 including linear and cyclic acrylic monomers such as
Received: September 18, 2014
Revised: October 21, 2014
© XXXX American Chemical Society
A
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Scheme 1. Proposed Chain Initiation and Propagation Mechanism for Polymerization of Conjugated Polar Alkenes Carrying a
Functional Group (FG) by LPs5,6,9 and a List of the Monomers Examined in This Study
methyl methacrylate (MMA) as well as biorenewable α-
methylene-γ-butyrolactone (MBL) and γ-methyl-α-methylene-
γ-butyrolactone (MMBL), into high molecular weight (MW)
polymers.7 This polymerization was proposed to proceed
through a bimolecular, activated monomer propagation
mechanism via zwitterionic phosphonium or imidazolium
enolaluminate active species (AC*, Scheme 1), which have
been structurally characterized.5,6 CLP adducts of phosphines
(P) and boranes (B) have been found to exhibit unexpectedly
high activity for the polymerization of MMBL, while the P/B
FLPs exhibit negligible activity under the same conditions.8
Conjugate-addition polymerization of the monomers bearing
the CC−CN functionality, such as 2-vinylpyridine (2-VP)
and 2-isopropenyl-2-oxazoline (iPOx) (Scheme 1), is brought
about effectively by NHC/Al(C6F5)3 FLPs, but not by the
related P/Al(C6F5)3 FLPs.9 The ring-opening (co)-
polymerization of heterocyclic monomers such as lactide and
lactones proceeds in a controlled manner in the presence of
Zn(C6F5)2-based LPs,10 and the radical polymerization of
styrene is mediated by the persistent FLP-NO aminoxyl radical
derived from N,N-cycloaddition of a cyclohexylene-bridged
intramolecular P---B FLP to nitric oxide.11 LP cooperativity has
been exploited to provide a facile approach for controlling
regioselectivity of the polymerization of dissymmetric divinyl
polar monomers such as 4-vinylbenzyl methacrylate producing
soluble functional polymers.12 Most recently, LPs based on N-
heterocyclic olefins (NHO) as the LB and Al(C6F5)3 or AlCl3
as the LA have also been found to be highly active for the
polymerization of conjugated polar alkenes such as meth-
acrylates and acrylamides; deactivation of the active propagat-
ing species (the imidazolium enolaluminate) was proposed to
lead to the polymer chains end-capped by the six-membered
lactone, presumably resulted from nucleophilic backbiting of
the polymeric enolaluminate anion to the carboxyl carbon of
the adjacent unit, with concomitant release of the methoxyl
group.13
The studies overviewed above have resulted in significant
recent progress in polymerization catalysis by FLPs or CLAs.
However, although the chain initiation and propagation events
as well as the scope of monomer and LPs have been examined
in great detail,6,9 polymerization kinetics and chain-termination
mechanisms for the polymerization of conjugated polar alkenes,
such as MMA, MMBL, 2-VP, and iPOx, by FLPs based on
NHC bases and [Al] acids have not been reported. Such
fundamental insight will undoubtedly enhance our under-
standing of the polymerization by FLPs in terms of the
mechanism of chain growth vs termination and polymer chain
end structures, which will guide the design of more effective
LPs for such polymerization reactions. Hence, the central
objective of this work was to investigate the kinetics and chain-
termination mechanism of the polymerization of typical
conjugated polar alkenes by FLPs, using two prototype
systems: ItBu/Al(C6F5)3 and IMes/Al(C6F5)3.
EXPERIMENTAL SECTION
■
Materials, Reagents, and Methods. All syntheses and
manipulations of air- and moisture-sensitive materials were carried
out in flamed Schlenk-type glassware on a dual-manifold Schlenk line,
on a high-vacuum line, or in an inert gas-filled glovebox. NMR-scale
reactions were conducted in Teflon-valve-sealed J. Young-type NMR
tubes. HPLC-grade organic solvents were first sparged extensively with
nitrogen during filling 20 L solvent reservoirs and then dried by
passage through activated alumina (for Et2O, THF, and CH2Cl2)
followed by passage through Q-5 supported copper catalyst (for
toluene and hexanes) stainless steel columns. Benzene-d6 and toluene-
d8 were dried over sodium/potassium alloy and vacuum-distilled or
filtered, whereas CD2Cl2 and CDCl3 were dried over activated Davison
4 Å molecular sieves. HPLC-grade dimethylformamide (DMF) was
degassed and dried over CaH2 overnight, followed by vacuum
distillation (CaH2 was removed before distillation). NMR spectra
were recorded on Varian Inova 300 (300 MHz, 1H; 75 MHz, 13C; 282
MHz, 19F) or a Varian 400 MHz spectrometer. Chemical shifts for 1H
and 13C spectra were referenced to internal solvent resonances and are
reported as parts per million relative to SiMe4, whereas 19F NMR
spectra were referenced to external CFCl3.
Methyl methacrylate (MMA), 2-isopropenyl-2-oxazoline (iPOx), 2-
vinylpyridine (2-VP), and n-butyl methacrylate (BMA) were
purchased from Sigma-Aldrich Co., while γ-methyl-α-methylene-γ-
butyrolactone (MMBL) was purchased from TCI America. These
monomers were first degassed and dried over CaH2 overnight,
followed by vacuum distillation. Further purification of MMA involved
titration with neat tri(n-octyl)aluminum to a yellow end point,14
followed by distillation under reduced pressure. All purified monomers
were stored in brown bottles and stored inside a glovebox freezer at
−30 °C. N-Heterocyclic carbenes (NHCs), including 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene (IMes) and 1,3-di-tert-butylimida-
zol-2-ylidene (ItBu), were purchased from Strem Chemical Co.
Phosphines, including PMes3, PPh3, and PtBu3, as well as butylated
hydroxytoluene (BHT-H, 2,6-di-tert-butyl-4-methylphenol) were
purchased from Alfa Aesar Chemical Co. BHT-H was recrystallized
from hexanes prior to use. PtBu3 was first degassed and dried over
CaH2 overnight, followed by vacuum distillation. Tris-
(pentafluorophenyl)borane, B(C6F5)3, obtained as a research gift
from Boulder Scientific Company, was further purified by recrystalliza-
tion from hexanes at −30 °C. Al(C6F5)3, as a (toluene)0.5 adduct,
denoted as [Al], was prepared by reaction of B(C6F5)3 and AlMe3 in a
1:3 toluene/hexanes solvent mixture in quantitative yield;15 this is the
modified synthesis based on literature procedures.16 Although we have
experienced no incidents when handling this material, extra caution
should be exercised, especially when dealing with the unsolvated form
because of its thermal and shock sensitivity.
Isolation of Adduct iPOx·Al(C6F5)3 (1). Adduct 1 was isolated as
an off-white solid in quantitative yield using the same procedure as
described for the isolation of the adduct (2-VP)·Al(C6F5)3.9 1H NMR
(C6D6, 23 °C): δ 5.43 (br s, 1H, CH2=), 4.62 (br s, 1H, CH2=), 3.25
(t, J = 9.3 Hz, 2H, CH2−O), 3.07 (t, J = 9.3 Hz, 2H, CH2−N), 1.28 (s,
B
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3H, CH3−). 19F NMR (C6D6, 23 °C): δ −122.6 (d, J = 16.6 Hz, 6F, o-
F), −152.2 (t, J = 19.7 Hz, 3F, p-F), −161.1 (m, 6F, m-F).
General Polymerization Procedures. Polymerizations were
performed either in 25 mL flame-dried Schlenk flasks interfaced to
the dual-manifold Schlenk line for runs using external temperature
bath or in 30 mL glass reactors inside the glovebox for ambient
temperature (ca. 25 °C) runs. In a typical polymerization procedure, a
predetermined amount of a LA or its monomer adduct, such as [Al],
iPOx·Al(C6F5)3, (2-VP)·Al(C6F5)3, or B(C6F5)3, was first dissolved in
a monomer (0.5 mL for iPOx or 0.51 mL for 2-VP, 200 equiv relative
to the LB) and 3.1 mL of solvent (CH2Cl2 or toluene) inside a
glovebox. Benzene (0.369 g, 4.73 mmol) was added as an internal
standard to each reactor if needed. The polymerization was started by
rapid addition of a solution of a LB (1 equiv of a phosphine or an
NHC) in 1.0 mL of solvent (CH2Cl2 or toluene) via a gastight syringe
to the above mixture containing the LA and monomer under vigorous
stirring. The amount of the monomer was fixed for all polymerization.
After the measured time interval, a 0.2 mL aliquot was taken from the
reaction mixture via syringe and quickly quenched into a 4 mL vial
containing 0.6 mL of undried “wet” CDCl3 stabilized by 250 ppm of
In Situ Generation of Imidazol−ium Oxazolinylaluminate
IMes+-CH2C(Me)(C3H2NO)Al(C6F5)3 (2). A Teflon-valve-sealed
J. Young-type NMR tube was charged with IMes (17.4 mg, 57.0
mmol) and 0.3 mL of C7D8. A 0.3 mL C7D8 solution of iPOx·
Al(C6F5)3 (36.4 mg, 57.0 mmol) was added to this tube via pipet at
ambient temperature. The resulting colorless mixture was allowed to
react for 10 min before analysis by NMR, which showed the clean and
1
quantitative formation of zwitterionic species 2. H NMR (C7D8, 23
°C): δ 6.60 (s, 4H, Ph), 5.71 (s, 2H, NCH=), 3.38 (br s, 4H, CH2),
2.98 (s, 2H, CH2), 1.61 (s, 12H, Ph−Me), 1.20 (s, 3H, =CMe). 19F
NMR (C7D8, 23 °C): δ −121.9 (d, J = 18.0 Hz, 6F, o-F), −157.3 (t, J
= 19.6 Hz, 3F, p-F), −163.5 (m, 6F, m-F). 13C NMR (C7D8, 23 °C): δ
159.4, 151.8, 140.8, 133.8, 130.2, 129.4, 121.4, 65.0, 62.6, 49.7, 29.4,
20.3, 18.5, 16.6.
X-ray Crystallographic Analysis of IMes+-CH2C-
−
(Me)(C3H2NO)Al(C6F5)3 (2). The molecular structure of 2 has
1
BHT-H; the quenched aliquots were later analyzed by H NMR to
been confirmed by single crystal X-ray diffraction analysis. A 20 mL
glass vial was charged with iPOx·Al(C6F5)3 (0.16 mmol) and 4 mL of
CH2Cl2, while another vial was charged with IMes (0.16 mmol) and
10 mL of hexanes. The two vials were cooled to −30 °C, and the
solution of IMes was layered on the iPOx·Al(C6F5)3 solution via pipet
at low temperature. The vial was stored in the glovebox freezer at −30
°C for 1 week to afford single crystals of 2 suitable for X-ray diffraction
analysis. The crystals were quickly covered with a layer of Paratone-N
oil (Exxon, dried and degassed at 120 °C/10−6 Torr for 24 h) after
decanting the mother liquor. A crystal was then mounted on a thin
glass fiber and transferred into the cold nitrogen stream of a Bruker
SMART CCD diffractometer. The structure was solved by direct
methods and refined using the Bruker SHELXTL program library.17
The structure was refined by full-matrix least-squares on F2 for all
reflections. All non-hydrogen atoms were refined with anisotropic
displacement parameters, whereas hydrogen atoms were included in
the structure factor calculations at idealized positions. Selected
crystallographic data for 2: C45H33AlF15N3O, monoclinic, space
group P2(1)/c, a = 11.7821(14) Å, b = 18.266(2) Å, c = 19.231(2)
Å, β = 96.320(6)°, V = 4113.7(8) Å3, Z = 4, Dcalcd = 1.524 mg/m3,
GOF = 1.047, R1 = 0.0426 [I > 2(I)], wR2 = 0.11185 (all data).
CCDC-972836 contains the supplementary crystallographic data for
this structure. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
obtain the percent monomer conversion data. After the polymerization
was stirred for the stated reaction time, then the polymer was
immediately precipitated into 200 mL of hexane, stirred for 1 h,
filtered, washed with hexane, and dried in a vacuum oven at 50 °C
overnight to a constant weight.
Polymerization Kinetics. Kinetic experiments were carried out in
a stirred glass reactor at ambient temperature (ca. 25 °C) inside an
argon-filled glovebox using the polymerization procedure already
described above, with [Al]/[ItBu] ratios of 1.5:0.5, 2:1, 2.5:1.5, and
3:2, [2-VP]0 was fixed at 946 mM for all polymerizations, where ItBu =
2.38, 4.77, 7.16, and 9.54 mM and [Al] = 7.17, 9.54, 11.9, and 14.3
mM in 5 mL mixture solutions. Benzene was added an internal
standard to each reactor. At appropriate time intervals, 0.2 mL aliquots
were withdrawn from the reaction mixture using a syringe and quickly
quenched into 4 mL septum-sealed vials containing 0.6 mL of undried
“wet” CDCl3 mixed with 250 ppm BHT-H. The quenched aliquots
1
were analyzed by H NMR for determining the ratio of [2-VP]t at a
given time t to [2-VP]0, [2-VP]t:[2-VP]0. Apparent rate constants
(kapp) were extracted from the slopes of the best fit lines to the plots of
[2-VP]t:[2-VP]0 vs time. Another set of kinetic experiments were
carried out to determine the kinetic order with respect to [Al]. In these
experiments, the ratio of [2-VP]0:[4]0 was fixed at 200:1, with [2-VP]0
= 946 mM and [4]0 = 4.73 mM for all polymerizations. The [Al]0:[4]0
ratio was varied as 0.2:1, 0.4:1, 0.6:1, 0.8, and 1:1, where [4]0 = 4.73
mM and [Al]0 = 0.944, 1.89, 2.83, 3.78, and 4.73 mM in 5 mL of total
solutions. Benzene was added an internal standard to each reactor. The
rest of the procedure was the same as described above.
Polymer Characterizations. Polymer number-average molecular
weights (Mn) and molecular weight distributions (Đ = Mw/Mn) were
measured by gel permeation chromatography (GPC) analyses carried
out at 40 °C and a flow rate of 1.0 mL/min, with DMF as the eluent,
on a Waters University 1500 GPC instrument equipped with one
PLgel 5 μm guard and three PLgel 5 μm mixed-C columns (Polymer
Laboratories; linear range of molecular weight = 200−2 000 000). The
instrument was calibrated with 10 PMMA standards, and chromato-
grams were processed with Waters Empower software (version 2002).
The isolated low-MW polymer samples were analyzed by matrix-
assisted laser desorption/ionization time-of-flight mass spectroscopy
(MALDI-TOF MS); the experiment was performed on a Ultralex
MALDI-TOF mass spectrometer (Bruker Daltonics) operated in
positive ion, reflector mode using a Nd:YAG laser at 355 nm and 25
kV accelerating voltage. A thin layer of a 1% NaI solution was first
deposited on the target plate, followed by 0.6 μL of both sample and
matrix (dithranol, 10 mg/mL in 50% CAN, 0.1% TFA). External
calibration was done using a peptide calibration mixture (4−6
peptides) on a spot adjacent to the sample. The raw data were
processed in the FlexAnalysis software (version 2.4, Bruker Daltonics).
Computational Methods. All the density functional theory
(DFT) calculations were performed using the Gaussian09 package
and followed the procedures described in our prior publications.18
Geometry optimizations were performed with the BP86 GGA
In Situ Generation of Imidazolium Oxazolinylaluminate
−
ItBu+-CH2C(Me)(C3H2NO)Al(C6F5)3 (3). A Teflon-valve-sealed J.
Young-type NMR tube was charged with ItBu (8.5 mg, 47 mmol) and
0.3 mL of C6D6. A 0.3 mL C6D6 solution of iPOx·Al(C6F5)3 (30 mg,
47 mmol) was added to this tube via pipet at ambient temperature.
The resulting colorless mixture was allowed to react for 10 min before
analysis by NMR, which showed the clean and quantitative formation
of zwitterionic species 3 as two isomers A (major) and B (minor) in a
1
2:1 ratio. H NMR (C6D6, 23 °C) for 3A: δ 6.18 (s, 2H, NCH=),
4.05−3.26 (m, 6H, CH2), 1.64 (s, 3H, Me), 1.00 (s, 18H, tBu); 3B: δ
6.02 (s, 2H, NCH=), 4.05−3.26 (m, 6H, CH2), 1.24 (s, 3H, Me), 0.91
t
(s, 18H, Bu). 19F NMR (C6D6, 23 °C) for 3A: δ −122.0 (d, J = 16.9
Hz, 6F, o-F), −157.1 (t, J = 19.7 Hz, 3F, p-F), −163.4 (m, 6F, m-F);
3B: δ −121.4 (d, J = 18.6 Hz, 6F, o-F), −156.6 (t, J = 19.7 Hz, 3F, p-
F), −162.9 (m, 6F, m-F). 13C NMR (C6D6, 23 °C) for 3A: δ 157.6,
148.0, 116.8, 67.7, 65.6, 61.8, 49.1, 31.0, 28.8, 14.2; 3B: δ 159.7, 148.3,
117.0, 66.7, 65.6, 62.3, 55.0, 31.0, 28.6, 19.2.
Isolation of Imidazolium Pyridylaluminate ItBu+-
−
CH2CH(C5H4N)Al(C6F5)3 (4). A 20 mL glass vial was charged
with ItBu (0.17 g, 0.95 mmol) and 5 mL of toluene, while another vial
was charged with (2-VP)·Al(C6F5)3 (0.50 g, 0.79 mmol) and 10 mL of
toluene. The two vials were mixed via pipet at ambient temperature to
give a red suspension. The solvent was removed, and the solid residue
was washed by hexanes (3 × 5 mL) to give 4 as an orange-red solid in
quantitative yield after drying under vacuum. The spectral and X-ray
structural data were reported previously.9
C
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functional of Becke and Perdew,19,20 and the standard split-valence
basis set with a polarization function of Ahlrichs and co-workers (SVP
keyword in Gaussian) was used.21 The reported energies have been
obtained via single point energy calculations on the optimized
geometries with the M06 functional with the triple-ζ basis set of
Ahlrichs (TZVP keyword in Gaussian09). The solvent (toluene)
effects were included with the default Gaussian PCM implementa-
tion.22 Thermal corrections from gas-phase frequency analysis,
performed with the BP86 functional and the SVP basis set on the
BP86 optimized geometries, were added to this in solvent energy to
obtain the free energies.
RESULTS AND DISCUSSION
■
Generation and Structural Characterization of Zwit-
terionic Intermediate 2. NHC/Al(C6F5)3 FLPs have been
found to be active for polymerization of iPOx, producing
medium high molecular weight polymers with Mn = 7.37 × 104
g/mol (NHC = ItBu) and Mn = 1.50 × 104 g/mol (NHC =
IMes).9 The polymers PiPOx produced exhibited a broad
molecular weight distribution of Đ ∼ 3.0, showing that the
polymerization of iPOx by the Al-based FLPs is less controlled
than the group-transfer polymerization catalyzed by rare-earth
metal complexes.23 Nonetheless, it is of fundamental interest to
understand the active species responsible for this polymer-
ization by FLPs.
Figure 1. X-ray crystal structure of zwitterionic intermediate 2.
Hydrogen atoms have been omitted for clarity and ellipsoids drawn at
50% probability. Selected bond lengths [Å] and angles [deg]: Al(1)−
N(3) 1.8534(17), Al(1)−C(40) 2.022(2), Al(1)−C(28) 2.023(2),
Al(1)−C(34) 2.023(2), C(1)−C(4) 1.499(3), C(4)−C(5) 1.509(3),
C(5)−C(7) 1.343(3); N(3)−Al(1)−C(40) 118.36(8), N(3)−Al(1)−
C(28) 106.94(8), C(40)−Al(1)−C(28) 112.22(8), N(3)−Al(1)−
C(34) 108.04(8), C(40)−Al(1)−C(34) 97.92(8), C(28)−Al(1)−
C(34) 113.26(9).
Reaction of iPOx·Al(C6F5)3 (1) with IMes at RT in C7D8
cleanly generates the corresponding zwitterionic imidazolium
oxazolinylaluminate IMes+-CH2C(Me)(C3H2NO)Al-
[1.499(3) Å], for the newly formed σ-bond between IMes and
iPOx. This structure represents one of only a handful of
structurally characterized zwitterionic intermediates that
simulate the active propagating species for the Lewis pair
polymerization, including those zwitterionic intermediates
derived from FLP activation of monomers such as MMA,6
MBL,6 and 2-VP.9
−
(C6F5)3 (2, Scheme 2), as readily characterized by NMR
Scheme 2. Generation of Zwitterionic Intermediate 2 from
Activation of iPOx by IMes/Al(C6F5)3
Kinetics of Polymerization by ItBu/Al(C6F5)3. To
investigate the kinetics of the polymerization of conjugated
polar alkenes by FLPs, we initially examined several monomers
and FLPs for their suitability for kinetic studies and finally
arrived at the polymerization of 2-VP by ItBu/Al(C6F5)3
because its polymerization rate and ability to cleanly generate
the isolable, active zwitterionic intermediate (i.e., 4) were found
most suitable for the kinetic study using the method employed
in this study. Accordingly, we examined kinetics of the 2-VP
polymerization by ItBu/Al(C6F5)3 in toluene at RT with two
different sets of experiments. In the first set of kinetic
experiments, the alane [Al] was dissolved in a fixed amount
of 2-VP (946 mM for all polymerizations) and toluene, and the
polymerization was started by addition of ItBu. The [Al]/[ItBu]
ratio was varied at 1.5:0.5, 2:1, 2.5:1.5, and 3:2 such that at each
ratio there was 1 equiv of the LA [Al] left to activate the
monomer upon formation of 1 equiv of the zwitterionic species
that consumes equimolar LB and LA. As can be seen from a
representative kinetic plot for the 2-VP polymerization by [Al]/
ItBu in a 2.5/1.5 ratio, the polymerization clearly followed zero-
order kinetics with respect to [2-VP] concentration (Figure 2);
the same zero-order dependence was observed for all the [Al]/
ItBu ratios investigated in this study (Figure S1a−d). A double-
logarithm plot (Figure 3) of the apparent rate constants (kapp),
obtained from the slopes of the best-fit lines to the plots of [2-
VP]t/[2-VP]0 vs time, as a function of ln[ItBu] was fit to a
straight line (R2 = 0.991) with slope = 1.08, revealing that the
propagation is first order in the LB concentration, [ItBu].
In the second set of kinetic experiments, we employed the
pregenerated, isolated zwitterionic imidazolium pyridylalumi-
spectroscopy (see Experimental Section). Most notable spectral
features manifesting this transformation include (a) conversion
of the sp2-olefinic =CH2 in the monomer or its LA adduct 1 (δ
5.43, 4.62 ppm) to the sp3-methylene (−CH2−) group in 2 (δ
1
2.98 ppm) as shown by H NMR; (b) disappearance of the
characteristic NMR signal for the carbene moiety at C(2) (δ
219.7 ppm) in the NHC IMes upon its reaction with the [Al]
activated monomer as shown by 13C NMR; and (c) high-field
shifted para- and meta-fluorine resonances by 5.1 and 2.4 ppm
(to δ −157.3 and −163.5 ppm, respectively) in the 19F NMR
spectrum (relative to [Al] or its monomer adduct 1),
characteristic of the aluminate formation. Replacing IMes
with ItBu for the same reaction led to formation of the
analogous zwitterionic imidazolium oxazolinylaluminate ItBu+-
−
CH2C(Me)(C3H2NO)Al(C6F5)3 (3); however, this reac-
tion gave two double-bond (E and Z) isomers in a 2:1 ratio
(see Experimental Section).
The molecular structure of 2 (Figure 1) has been confirmed
by single-crystal X-ray diffraction analysis. It is clear from the
diffraction data that, as a consequence of conjugate addition of
IMes to adduct 1, a double bond is formed between C(5) and
C(7) in 2 with a bond length of 1.343 (3) Å, whereas a single
bond is formed between C(4) and C(5) [1.509(3) Å], from a
previous CC double bond, as well as between C(1) and C(4)
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Figure 2. Zero-order plot of [2-VP]t/[2-VP]0 vs time for the
polymerization of 2-VP by Al(C6F5)3/ItBu (2.5/1.5) in toluene at
RT. Conditions: [2-VP]0 = 946 mM, [Al]0 = 11.9 mM, [ItBu]0 = 7.16
mM in 5 mL solution. Similar plots with other [Al]/[ItBu] ratio runs
were depicted in Figure S1a−d.
Figure 4. Plot of ln(kapp) vs ln[Al] for polymerization of 2-VP by 4/
Al(C6F5)3.
VP]0 vs time, as a function of ln[Al] was fit to a straight line (R2
= 0.980) with a slope of 0.977. Thus, the kinetic order with
respect to [Al], given by the slope of ∼1, reveals that the
propagation is also first-order in the LA concentration, [Al].
Overall, the 2-VP polymerization by the ItBu/Al(C6F5)3 pair
follows a bimolecular, activated monomer propagation
mechanism, as previously predicted by a computational study
for the MMA polymerization by the LB/Al(C6F5)3.6a Note-
worthy is that such polymerization kinetics (i.e., zero-order
dependence on monomer concentration and first-order
dependence on initiator and LA activator concentrations)
were also observed in the group-transfer polymerization
catalyzed by zirconocenium cations25 and silylium ions.26
Such kinetics are consistent with the propagation mechanism in
that the C−C bond forming step via intermolecular Michael
addition of the propagating species to the LA-activated
monomer is the rate-limiting step and the release of the LA
catalyst from its coordinated last inserted monomer unit in the
growing polymer chain to the incoming monomer is relatively
fast.24
Chain Termination in the Polymerization by IMes/
Al(C6F5)3. The above 2-VP polymerization results reiterated the
common theme observed from the polymerization of other
conjugated polar alkenes such as MMA and MMBL by CLPs or
FLPs:6,9 such polymerization is hampered by chain-termination
side reactions, chiefly evidenced by the much higher observed
Mn than the calculated Mn (which results in low initiator
efficiencies of typically lower than 30%), broad molecular
weight distributions of the resulting polymers, and the inability
to produce well-defined block copolymers via sequential
copolymerization. The two possible chain-termination path-
ways that compete with chain propagation cycles are proposed
in Scheme 3. Pathway (a) is proposed to proceed via
intramolecular backbiting cyclization involving nucleophilic
attack of the activated antepenultimate ester group of the
growing polymer chain by the C-ester enolate active chain end
to generate a cyclic β-ketoester-terminated polymer chain. This
mode of backbiting cyclization was observed previously in the
polymerization of acrylates by a metallocenium catalyst27 and is
also ubiquitous in the anionic polymerization of acrylates.28
Pathway (b) is proposed to proceed via intramolecular
backbiting cyclization involving nucleophilic attack of the
activated adjacent ester group of the growing polymer chain by
the O-ester enolate active chain end to generate a six-
membered lactone (δ-valerolactone)-terminated polymer
chain. This possible mode of chain termination was recently
Figure 3. Plot of ln(kapp) vs ln[ItBu] for the 2-VP polymerization by
Al(C6F5)3/ItBu in toluene at RT.
nate ItBu-CH2CH(C5H4N)Al(C6F5)3 (4), in combination
with a varied amount of [Al] to examine the kinetic order with
respect to [Al]. Specifically, the [Al]/[4] ratio was varied from
0.2, 0.4, 0.6, 0.8, to 1.0, while keeping the [2-VP]:[4] ratio fixed
at 200:1. While the rate of the polymerization was significantly
enhanced with an increase in the [Al]/[4] ratio from 0.2 to 1.0,
the molecular weight of the isolated resulting polymer PVP
upon achieving quantitative monomer conversion did not
follow this trend: Mn = 174 kg/mol, Đ = 1.91 (ratio = 0.2); Mn
= 95.4 kg/mol, Đ = 1.45 (ratio = 0.4); Mn = 116 kg/mol, Đ =
1.40 (ratio = 0.6 ratio); Mn = 90.4 kg/mol, Đ = 1.44 (ratio =
0.8); Mn = 88.3 kg/mol, Đ = 1.44 (ratio = 1.0). It is clear that
the MW of the polymer obtained from all the ratio runs was
much higher than the calculated (Mn = 21.0 kg/mol), thus
giving rise to low initiator efficiencies from 12% to 24%;
however, the highest MW polymer (thus the lowest initiator
efficiency of 12%) achieved at the lowest [Al]/[4] ratio (0.2)
and the lowest MW polymer (thus the highest initiator
efficiency of 24%) obtained at the highest [Al]/[4] ratio (1.0)
are consistent with the reasoning that a higher amount of the
free [Al] in the solution promotes more efficient chain
initiation and faster chain propagation, characteristic of an
activated monomer polymerization process.24 Kinetic plots for
this set of kinetic experiments again showed that the
propagation is zero order in [2-VP] for all the [Al]/[4] ratios
investigated in this study (Figure S2a−e). A double-logarithm
plot (Figure 4) of the apparent rate constants (kapp), obtained
from the slopes of the best-fit lines to the plots of [2-VP]t/[2-
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Scheme 3. Proposed Two Possible Backbiting Chain-
Termination Pathways That Compete with Chain
Propagation Cycles in the Polymerization of Methacrylates
by the LB/LA Pair [LB = IMes, LA = Al(C6F5)3 in the
Following Examples]
Figure 6. Plot of m/z values from Figure 5 vs the number of BMA
repeat units (n) for A series.
intercept of 373.44 (Figure 7). A closer examination of these
two series of mass ions revealed that the mass difference
reported to be operative in the MMA polymerization by NHO/
Al(C6F5)3.13
To probe these chain-termination side reactions in the
conjugate-addition polymerization by LB/LA pairs, we analyzed
the chain-end groups of the low-MW polymers (PBMA and
PMMBL) produced by such pairs. As can be seen from Figure
5, the MALDI-TOF MS spectrum of the low-MW PBMA
sample consisted of two series of mass ions with rather similar
intensities, with the spacing of the mass ions within each series
being that of the value for PBMA repeat unit (m/z = 142). A
plot of m/z values for series A in the MS spectrum vs the
number of BMA repeat units (n) afforded a straight line with a
slope of 142.09 and an intercept of 305.21 (Figure 6); the slope
corresponds to the mass of the BMA monomer, whereas the
intercept is the sum of the masses of IMes (304) and H (1)
moieties, where IMes was derived from the chain initiation by
IMes and H was derived from the acidic work-up after the
polymerization. In short, the peaks in series A correspond to
the linear, living polymer chain produced by IMes/Al(C6F5)3.
A linear plot of m/z values for series B in the MS spectrum vs
the BMA repeat units (n) gave the same slope but a different
Figure 7. Plot of m/z values from Figure 5 vs the number of BMA
repeat units (n) for B series.
between the linear polymer chain structure mass ions (series A)
and the structure with the proposed cyclic chain end (series B)
n
is a mass equivalent of BuOH. Thus, the intercept of the B
series plot is the sum of the masses of IMes and the cyclic β-
ketoester moieties, where IMes was derived from the chain
initiation by IMes (304) and the cyclic β-ketoester [142 (BMA)
− 73 (loss of BuO) = 69] was derived from the backbiting
chain-termination process during the polymerization (Scheme
3). Another low-MW PBMA sample produced by IMes/
Figure 5. MALDI-TOF mass spectrum of the low-MW PBMA produced by IMes/Al(C6F5)3 (1/2) in toluene at room temperature.
F
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Figure 8. MALDI-TOF mass spectrum of the low-MW PMMBL produced by IMes/Al(C6F5)3 (1/2) in toluene at RT. Inset: plot of m/z values of
the main series vs the number of MMBL repeat units (n).
Al(C6F5)3 at 0 °C gave a similar MALDI-TOF mass spectrum
(Figure S3). It should be noted here that the MALDI-TOF MS
results cannot differentiate between the possible cyclic β-
ketoester and δ-valerolactone chain ends as the mass difference
is one monomer unit (Scheme 3). However, DFT calculations
detailed in the following indicated a clear kinetic preference for
the formation of the cyclic δ-valerolactone chain end.
chain by the O-ester enolate active chain end (pathway b in
Scheme 3). As differentiation of these two types of chains end
became impossible by our current experimental methods using
NMR and MS, we sought a solution to addressing this
fundamental issue by computation studies.
DFT calculations were used to compare the two possible
termination pathways with the chain growth pathway (Scheme
4). We started calculations on a model of the propagating
We reasoned that MMBL, being the cyclic analogue of the
linear methacylates, should be more resistant toward chain
termination via backbiting due to its robust, five-membered
cyclic structure. Indeed, the MALDI-TOF MS spectrum of the
low-MW PMMBL produced by IMes/Al(C6F5)3 (1/2) showed
one major series of mass ions. A plot of m/z values of this series
vs the number of MMBL repeat units (n) yielded a straight line
with a slope of 112.06 (mass of MMBL) and an intercept of
305.11 (Figure 8); again, the intercept is the sum of the masses
of IMes and H moieties, indicating that the majority of the
polymer chains produced in this polymerization is the linear,
living chain. However, two minor series of mass ions were also
present in the MS spectrum, indicative of side reactions such as
chain termination or transfer; one of such series gave a
structure without apparent chain ends (with an intercept being
the mass of Na+), which is consistent with a scenario that chain
transfer to monomer occurs through deprotonation of the
MMBL monomer by the propagating enolate anion, and the
resulting anionic monomer initiates new chains, as shown in the
related organocatalytic polymerization system by the NHC
alone.29
Scheme 4. Energetics (kcal/mol) of the β-Ketoester and δ-
Valerolactone Chain Termination Pathways and of the
Chain Growth (Numbers in Parentheses Are Free Energies)
Computational Study of Chain Termination. The above
experimental investigation into the possible chain termination
pathways involved in the polymerization of conjugate polar
alkenes such as MMA and BMA by FLPs such as IMes/
Al(C6F5)3 (1/2) led to two types of possible chain ends: cyclic
β-ketoester and δ-valerolactone (Scheme 3). The cyclic β-
ketoester chain end could be generated by intramolecular
backbiting cyclization involving nucleophilic attack of the
activated antepenultimate ester group of the growing chain by
the C-ester enolate active chain end (pathway a in Scheme 3),
while the δ-valerolactone chain end could be generated by
intramolecular backbiting cyclization involving nucleophilic
attack of the activated adjacent ester group of the growing
species where the polymer chain is simulated with three MMA
units terminated with a methyl group. In this model the LB is
not included since it is assumed to be far enough away from the
active center to have a noticeable influence on the mechanisms
considered, and the overall model is more representative of a
long PMMA chain. A comparison between the two possible
G
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chain termination pathways indicates that the δ-valerolactone
termination pathway is clearly favored over the β-ketoester
termination pathway, since transition state TSδ lies 12.5 kcal/
mol below transition state TSβ, despite the fact that the β-
ketoester termination product is thermodynamically favored by
20.1 kcal/mol. While the higher stability of the β-ketoester
reflects the relative stability of the functional groups in the two
termination products, the energy preference for TSδ over TSβ
can be ascribed to the different nucleophilicity of the enolate C
and O atoms, which attack the LA-activated CO group of a
previously added MMA molecule. Indeed, analysis of the
natural atomic charges in the growing chain end (i.e., the
starting structure at 0 kcal/mol) reveals a negative charge of
−0.83e on the enolate oxygen atom that will be involved in the
nucleophilic attack in TSδ and a negative charge of only −0.23e
on the enolate carbon atom that will be involved in the
nucleophilic attack in TSβ. This large difference in the atomic
charge indicates a much higher nucleophilicity of the LA-bound
oxygen relative to the carbon, which is consistent with the
clearly lower energy barrier for the δ-valerolactone termination
pathway. Incidentally, we also calculated the energy barrier for
both the β-ketoester and δ-valerolactone termination pathways
in a scenario where no LA is coordinated to the CO group
being attacked in backbiting; energy barriers at least 8.0 kcal/
mol higher were found in both termination cases, showing the
remarkable activating effect of the LA.
As for a comparison between the kinetically favored δ-
valerolactone chain-termination pathway and the chain growth
reaction, corresponding to the addition of a LA-activated MMA
molecule to the growing polymer chain, the latter is largely
favored in terms of potential energy, explaining why relatively
high-MW polymers can be achieved by the current catalyst
system. This conclusion was deduced from the results that the
potential energy barrier for the chain growth, calculated as the
energy difference between transition state TSp of Scheme 4 and
the growing chain plus a LA-MMA adduct, is 15.1 kcal/mol
lower than the potential energy barrier for the chain
termination, calculated as the energy difference between
transition state TSδ of Scheme 4 and a growing chain
presenting a second LA coordinated to the CO group of
the penultimate MMA molecule. Indeed, this growing chain is
the one present in the reaction media immediately after
addition of a new LA-MMA adduct. Furthermore, due to the
different molecularity of the addition (bimolecular) and of the
considered chain termination reactions (unimolecular), this
large difference in the potential energy barriers is reduced to
only 2.5 kcal/mol when entropy effects, which disfavor the
bimolecular chain growth transition state, are taken into
account and the comparison is performed in terms of free
energy barriers.30
A structural comparison of the TS geometry for the chain
growth and the chain termination pathway along the δ-
valerolactone pathway is depicted in Figure 9. In the chain
growth transition state TSp, the bond distance of the emerging
C−C bond, deriving from attack of the methylene C atom of
the LA-MMA adduct by the enolate C atom of the growing
chain, is 2.09 Å, while the Al−O bond involving the exiting LA
group, 1.87 Å, is already slightly longer than the Al−O bond of
the LA-MMA adduct that is going to be added to the growing
chain, 1.82 Å. Overall, the two bulky LA moieties are well
separated in space, minimizing steric repulsion. The δ-
valerolactone termination transition state TSδ, instead, presents
a highly concerted four-center geometry, with a very short
Figure 9. Geometries of the chain growth transition state TSp and the
δ-valerolactone chain termination transition state TSδ. Relevant
distances are reported in Å.
emerging C−O bond, 1.63 Å only, and the exiting LA molecule,
originally bonded to the enolate atom of the chain end, already
in the act of being transferred to the OMe group of the enolate
chain end. The role of the second LA molecule, attached to the
CO of the penultimate MMA unit, is activating the CO
bond. Steric interaction between the two LA molecules is
minimal, since they are oriented away from each other (cf.
Figure 9).
CONCLUSIONS
■
In summary, this contribution investigated mechanistic aspects
of conjugate-addition polymerization of conjugated polar
alkenes by the [Al]-based FLPs by focusing on active
propagating intermediate characterization, propagation kinetics,
and chain termination pathways. In this context, we successfully
isolated and structurally characterized the zwitterionic inter-
mediate IMes+-CH2C(Me)(C3H2NO)Al(C6F5)3 (2), de-
−
rived from FLP activation of iPOx by IMes/Al(C6F5)3, which
simulates the active propagating species in such polymerization.
Analogous intermediate ItBu+-CH2C(Me)(C3H2NO)Al-
−
(C6F5)3 (3) was also generated from activation of iPOx by
ItBu/Al(C6F5)3, and ItBu+-CH2CH(C5H4N)Al(C6F5)3 (4)
−
was isolated from activation of 2-VP by ItBu/Al(C6F5)3. The
structure of intermediate 2 is the first characterized active
intermediate involved in the polymerization of iPOx by FLPs
and represents one of only a handful of structurally
characterized zwitterionic intermediates that simulate the active
propagating species for the Lewis pair polymerization of
conjugate polar alkenes.
H
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5746. (e) Erker, G. Dalton Trans. 2011, 40, 7475−7483. (f) Stephan,
D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46−76. (g) Stephan,
D. W. Dalton Trans. 2009, 3129−3136. (h) Stephan, D. W. Org.
Biomol. Chem. 2008, 6, 1535−1539.
(2) Selected recent examples on activation of small molecules by
FLPs: (a) Lawrence, E. J.; Oganesyan, V. S.; Hughes, D. L.; Ashley, A.
E.; Wildgoose, G. G. J. Am. Chem. Soc. 2014, 136, 6031−6036.
(b) Dobrovetsky, R.; Stephan, D. W. J. Am. Chem. Soc. 2013, 135,
4974−4977. (c) Sajid, M.; Elmer, L.-M.; Rosorius, C.; Daniliuc, C. G.;
Grimme, S.; Kehr, G.; Erker, G. Angew. Chem., Int. Ed. 2013, 52,
2243−2246. (d) Appelt, C.; Slootweg, J. C.; Lammertsma, K.; Uhl, W.
Angew. Chem., Int. Ed. 2013, 52, 4256−4259. (e) Bertini, F.;
Lyaskovskyy, V.; Timmer, B. J. J.; de Kanter, F. J. J.; Lutz, M.;
Ehlers, A. W.; Slootweg, J. C.; Lammertsma, K. J. Am. Chem. Soc. 2012,
Polymerization kinetics were performed on a prototype
system most suitable for this study, the polymerization of 2-VP
by the ItBu/Al(C6F5)3 FLP. The kinetic data revealed that the
polymerization follows a zero-order dependence on monomer
concentration and a first-order dependence on initiator (LB)
and activator (LA) concentrations. Such kinetics imply a
bimolecular, activated monomer propagation mechanism in
that the C−C bond forming step via intermolecular Michael
addition of the propagating species to the LA-activated
monomer is the rate-limiting step, and the release of the LA
catalyst from its coordinated last inserted monomer unit in the
growing polymer chain to the incoming monomer is relatively
fast.
The Lewis pair polymerization of conjugated polar alkenes
by CLAs or FLPs is accompanied by competing chain-
termination side reactions. The two possible chain-termination
pathways were proposed: one that proceeds via intramolecular
backbiting cyclization involving nucleophilic attack of the
activated antepenultimate ester group of the growing chain by
the C-ester enolate active chain end to generate a cyclic β-
ketoester chain end and the other that proceeds via
intramolecular backbiting cyclization involving nucleophilic
attack of the activated adjacent ester group of the growing
chain by the O-ester enolate active chain end to generate a δ-
valerolactone chain end. Analyses of low molecular weight
polymer samples produced by IMes/Al(C6F5)3 with MALDI-
TOF MS spectroscopy provided evidence for such chain
termination side reactions but cannot conclusively state which
process is operative in this polymerization. On the other hand,
DFT calculations showed that the formation of cyclic δ-
valerolactone-terminated chain ends is kinetically favored
(lower energy barrier by 12.5 kcal/mol) but thermodynamically
disfavored (less stable by 20.1 kcal/mol), as compared to the
formation of β-ketoester-terminated chain ends.
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Haase, D.; Muller, T. Angew. Chem., Int. Ed. 2011, 50, 12636−12638.
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(g) Lu, Z.; Cheng, Z.; Chen, Z.; Weng, L.; Li, Z. H.; Wang, H. Angew.
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(3) Selected recent examples on catalytic hydrogenation by FLPs:
(a) Chernichenko, K.; Madarasz, A.; Papai, I.; Nieger, M.; Leskela, M.;
́ ́
̈
Repo, T. Nat. Chem. 2013, 5, 718−723. (b) Greb, L.; Daniliuc, C.-G.;
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ASSOCIATED CONTENT
* Supporting Information
Single-crystal X-ray diffraction data, computational details, and
additional figures. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
2010, 49, 9475−9478. (e) Ero
̋
s, G.; Mehdi, H.; Pap
́
ai, I.; Rokob, T. A.;
Kiraly, P.; Tarkanyi, G.; Soos
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6559−6563. (f) Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. J. Am.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: luigi.cavallo@kaust.edu.sa (L.C.).
(h) Axenov, K. V.; Kehr, G.; Frohlich, R.; Erker, G. J. Am. Chem.
̈
■
Soc. 2009, 131, 3454−3455.
(4) Selected recent examples on new reactivity/reaction development
with FLPs: (a) Wang, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. J. Am.
Chem. Soc. 2014, 136, 3293−3303. (b) Rocchigiani, L.; Giancaleoni,
G.; Zuccaccia, C.; Macchioni, A. J. Am. Chem. Soc. 2014, 136, 112−
*E-mail: eugene.chen@colostate.edu (E.Y.-X.C.).
Notes
The authors declare no competing financial interest.
́
115. (c) Menard, G.; Hatnean, J. A.; Cowley, H. J.; Lough, A. J.;
Rawson, J. M.; Stephan, D. W. J. Am. Chem. Soc. 2013, 135, 6446−
ACKNOWLEDGMENTS
6449. (d) Sajid, M.; Kehr, G.; Wiegand, T.; Eckert, H.; Schwickert, C.;
■
Pottgen, R.; Cardenas, A. J. P.; Warren, T. H.; Frohlich, R.; Daniliuc,
̈
̈
This work was supported by the National Science Foundation
(NSF-1150792) for the study carried out at Colorado State
University. L.C. thanks the HPC team of Enea for using the
ENEA-GRID and the HPC facilities CRESCO in Portici, Italy.
We thank Boulder Scientific Co. for the research gift of
B(C6F5)3 and Dr. Brian Newell for assistance on X-ray
structural analysis.
C. G.; Erker, G. J. Am. Chem. Soc. 2013, 135, 8882−8895.
(e) Cardenas, A. J. P.; Culotta, B. J.; Warren, T. H.; Grimme, S.;
Stute, A.; Frohlich, R.; Kehr, G.; Erker, G. Angew. Chem., Int. Ed. 2011,
̈
50, 7567−7571. (f) Chapman, A. M.; Haddow, M. F.; Wass, D. F. J.
́
Am. Chem. Soc. 2011, 133, 8826−8829. (g) Menard, G.; Stephan, D.
W. Angew. Chem., Int. Ed. 2011, 50, 8396−8399. (h) Zhao, X.;
Stephan, D. W. J. Am. Chem. Soc. 2011, 133, 12448−12450.
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(i) Menard, G.; Stephan, D. W. J. Am. Chem. Soc. 2010, 132, 1796−
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