Synthesis, Structure, and Reactivity of Palladium Proazaphosphatrane Complexes Invoked in C-N Cross-Coupling

Article abstract of DOI:10.1021/acs.organomet.8b00453

The preparation and reactivity of elusive palladium proazaphosphatrane complexes that represent putative intermediates in C-N cross-coupling reactions are described. Variable transannulation in these compounds, as determined by X-ray crystallography, vali

Full text of DOI:10.1021/acs.organomet.8b00453

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis, Structure, and Reactivity of Palladium
Proazaphosphatrane Complexes Invoked in C−N Cross-Coupling
Adrian D. Matthews,† Gregory M. Gravalis,† Nathan D. Schley,‡ and Miles W. Johnson*,†
†Department of Chemistry, University of Richmond, Richmond, Virginia 23173, United States
‡Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States
S
* Supporting Information
ABSTRACT: The preparation and reactivity of elusive
palladium proazaphosphatrane complexes that represent
putative intermediates in C−N cross-coupling reactions are
described. Variable transannulation in these compounds, as
determined by X-ray crystallography, validates the previously
untested hypothesis that proazaphosphatranes undergo con-
formational changes to stabilize catalytic intermediates. The
competence of these complexes as catalytic intermediates is
supported through stoichiometric and catalytic coupling
reactions, providing the first examples of discrete proazaphosphatrane complexes employed in cross-coupling.
between the initial examination of proazaphosphatranes as
ligands for transition metals7 and a recent re-examination of
this area. Yang and co-workers have determined the Tolman
electronic and steric parameters8,9 of nickel tricarbonyl
proazaphosphatrane complexes, demonstrating that proaza-
phosphatranes exhibit highly variable cone angles (152−207°)
depending on substitution at the equatorial nitrogen (Neq) and
are among the most electron-donating phosphine ligands
known.9 The Martinez group has provided evidence that these
ligands exhibit electronic characteristics akin to both tradi-
tional phosphines and N-heterocyclic carbenes, as demon-
strated in the synthesis of silver and gold complexes.10 Despite
these advances in understanding the steric and electronic
properties of proazaphosphatranes as ligands, the reactivity of
their complexes has only been examined with regard to
transmetalation.10 Surprisingly, palladium complexes of these
ligands remain unreported despite their invocation in catalysis.
Insight into the coordination chemistry of such complexes may
assist in harnessing proazaphosphatranes for new reactions and
in improving established ones.
INTRODUCTION
■
The use of conformationally flexible ligands has emerged as a
highly effective strategy for the cross-coupling of sterically
encumbered electrophile−nucleophile pairs.1 These supporting
ligands undergo structural changes in response to steric
crowding from reactive ligands, promoting cross-coupling
between sterically bulky coupling partners, including aryl
halides with two ortho substituents. Additionally, the dynamic
steric profiles of flexible ligands may promote both oxidative
addition and reductive elimination.1e Proazaphosphatranes,
commonly known as Verkade’s superbases,2 are perhaps the
most ubiquitous of these ligands. These caged, bicyclic
molecules undergo dramatic structural deformation due to
transannulation, the intramolecular interaction between the
axial nitrogen (Nax) and phosphorus in these molecules, upon
binding to an electrophile to form an azaphosphatrane (eq 1).3
The effectiveness of proazaphosphatranes as ligands in
catalysis is largely predicated on three hypotheses: (1)
oxidative addition is promoted due to electron donation to
the metal center from transannulation;5d (2) steric bulk at Neq
facilitates reductive elimination;5d and (3) transannulation
increases with electron deficiency at the metal center.11 To
evaluate these hypotheses, we have synthesized and examined
catalytic intermediates that may be encountered in Buchwald−
Hartwig amination (BHA) using the most prolifically
employed proazaphosphatrane, triisobutylphosphatrane (L).
This model system was chosen due to the thoroughly studied
The combination of a palladium source and one of these
strong bases4 generates a complex that can form C−N5 and
C−C1c,6 bonds from challenging substrates. Furthermore, the
facile and modular synthesis of proazaphosphatranes permits
systematic tuning of reactivity.5c
Although the application of proazaphosphatranes in catalysis
is well-studied, the coordination chemistry of these ligands
remains relatively unexplored. Over two decades elapsed
Received: July 2, 2018
© XXXX American Chemical Society
A
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mechanism of BHA (Scheme 1)12 and the multiple reports of
amination using L as a supporting ligand.5
Scheme 1. General Mechanism for C−N Cross-Coupling
dissociation of L from complex 1 may explain why oxidative
addition complexes are not directly accessible from complex 1.
Determination of kinetic or thermodynamic parameters for the
interconversion between complex 1 and oxidative addition
complexes 2 is precluded by the lack of formation of
arylpalladium(II) halide in the absence or presence of
exogenous L as well as the low solubility of 1.17
Electron-deficient complexes 2a and 2b were characterized
by X-ray crystallography as dimers (Figure 2). Though L has a
RESULTS AND DISCUSSION
■
A palladium bis(proazaphosphatrane) complex (1) is readily
prepared in high yield by treating Pd2dba3 with excess L (eq
2).13 Notably, the same palladium source and solvent are used
Figure 2. ORTEP diagrams of complex 2a (left) and 2b (right).
Ellipsoids are shown at 50% probability. Hydrogens and solvent
molecules are excluded for clarity.
in many catalytic reactions.15 Previous attempts in the
literature to access this complex were unsuccessful,5b
potentially due to the exceedingly low solubility of the
complex in common organic solvents. The solid-state structure
of complex 1 (Figure 1) shows a linear complex with a
transannular Nax−P distance of 3.482(2) Å, similar to the
distance observed in nickel(0) complexes.9
cone angle greater than that of P(t-Bu)3 (200° vs 182°), the
latter permits isolation of monomeric palladium(II) aryl halide
complexes.18 The favorability of a dimer in the solid state of
the proazaphosphatrane complexes may be attributed to the
flexibility of the isobutyl substituents, as has been observed by
the Shaughnessy group with neopentyl phosphines,1c,19 and a
lack of a stabilizing agostic interaction trans to the aryl ligand.18
Complexes similar to compounds 2a−d are excellent
synthons for amido complexes that are implicated in BHA.
Palladium amides are synthetically challenging but have
provided a wealth of information concerning C−N bond
formation.20 Consistent with observations that C−N cross-
coupling is most facile with an electron-rich amido ligand and
electron-deficient aryl ligand,20a treatment of trifluoromethyl-
substituted complex 2b with KNPh2 at low temperatures21
results in a red solution that subsequently darkens to form
palladium black and the corresponding triarylamine (eq 4). In
Figure 1. ORTEP diagram of complex 1. Ellipsoids are shown at 50%
probability. Hydrogens are excluded.
Oxidative addition by palladium bis(phosphine) complexes
is well-documented;14 however, treatment of 1 with a variety of
aryl halides to form an oxidative addition complex was
unsuccessful. An orthogonal, low-temperature approach using
Pd(COD)(CH2TMS)2 provides access to a series of
palladium(II) proazaphosphatrane complexes with varied aryl
ligands (eq 3). These complexes decompose in solution at
room temperature to form mixtures that include 1, biaryls,15
and palladium black.16 Their solution-state stability tracks with
the electron-withdrawing ability of the aryl substituent, with
half-lives varying from minutes in the case of electron-rich
complex 2d to days for electron-poor complexes 2a and 2b.
This lack of thermal stability and a potentially high barrier to
B
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a
contrast, reaction of methoxy-substituted complex 2d with the
electron-poor amide KN[3,5-(CF3)2C6H3]2 results in a similar
red solution, but the color persists even at room temperature
(eq 5). Compound 3 can be isolated from this solution and
characterized crystallographically as a three-coordinate amido
complex (Figure 3). Unlike other palladium phosphine
complexes featuring the same amido and aryl ligands, 3 does
not undergo reductive elimination to form the corresponding
triarylamine.20b
Table 1. Key Distances in Proazaphosphatrane L
A (Å)
B (Å)
C (Å)
D (Å)
1
2a
2b
3
3.482(2)
3.260(6)
3.265(7)
3.206(3)
0.714(1)
0.644(4)
0.628(4)
0.628(2)
2.563(2)
2.515(5)
2.522(5)
2.513(2)
0.205(2)
0.107(7)
0.122(8)
0.067(3)
b
a
b
One N(i-Bu)CH2CH2 group is excluded for clarity. Average of four
distances in the unit cell.
neighboring methylene groups). This is most dramatic when
comparing 1 and 3. The combined distances B and D are
approximately 0.276 Å shorter in 3 than 1. Whereas 1 clearly
possesses a proazaphosphatrane ligand with Nax protruding
away from phosphorus in an exo conformation,23 L on complex
3 is nearly planar at Nax, demonstrating quasi-atrane
character.2a Taken together, these comparisons offer the first
support for the hypothesis that proazaphosphatranes undergo
varying degrees of transannulation in catalytic intermediates as
a function of the electron density of the metal to which the
ligand is bound. Such interactions may prevent ligand
dissociation during catalysis and make palladium sufficiently
electron-rich to oxidatively add C−Cl bonds under mild
conditions.1c,5d,6a
Figure 3. ORTEP diagram of 3. Ellipsoids are shown at 50%
probability. Hydrogens are excluded for clarity.
The catalytic viability of proposed BHA intermediates 1, 2b,
and 3 was tested. Both 1 and 2b promote the coupling of 4-
bromobenzotrifluoride and diphenylamine in high yield under
literature conditions (eq 5).5b To our knowledge, these
reactions represent the only examples of well-defined
proazaphosphatrane complexes being used as precatalysts.
Furthermore, this reactivity shows their potential relevance in
the industrially essential BHA reaction. Complex 3 shows no
reactivity, consistent with the inability of this compound to
undergo C−N bond formation stoichiometrically; however,
this molecule is a stable model system for a key intermediate.
CONCLUSIONS
■
In summary, previously putative catalytic intermediates in the
palladium proazaphosphatrane-catalyzed BHA have now been
prepared for the first time. These complexes exhibit catalytic
and stoichiometric reactivity that is consistent with their
presence on a catalytic cycle. Importantly, the structure of the
proazaphosphatrane changes in response to the oxidation state
of palladium and the coordination sphere of the metal. These
results substantiate the long-held hypothesis that the extent of
transannulation in a proazaphosphatrane ligand changes
throughout a catalytic process. Synthetic and computational
studies are underway to further interrogate the role of
transannulation on catalysis and to harness this phenomenon
in developing new reactivity.
The palladium complexes that we have synthesized provide
new insight into the transannulation of proazaphosphatranes
and permit the first examination of transannular distance (A in
Table 1) as a function of the ligand set on a single metal.22 The
greatest transannular distance is observed in 1 (3.482(2) Å).
This length is likely due to the electron-rich palladium center
and not the mutually trans disposition of the two
proazaphosphatranes as a trans-platinum(II) dichloride bis-
(proazaphosphatrane) complex has a relatively short trans-
annular distance of 3.250 Å.7a Moreover, electron-poor
complexes 2a and 2b exhibit transannular distances shorter
than those of bis(proazaphosphatrane) complex 1. Amido
complex 3, which has a single L-type donor ligand and a highly
electron-deficient amido group trans to L, has the shortest
transannular distance of the series (3.206(3) Å). Notably, in all
four cases, the distance between the plane formed by the
equatorial nitrogens and the methylene groups neighboring Nax
(C) is nearly invariable. The change in transannular distance is
due to the shortening of distances B (the Neq plane to
phosphorus) and D (Nax to the plane formed by its
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out in a
nitrogen-filled MBraun LABstar Pro glovebox unless otherwise noted.
All glassware was oven-dried overnight at greater than 110 °C prior to
use. All liquid aryl halides were passed through activated alumina and
stored over 3 Å molecular sieves prior to use. 4-Bromobenzotri-
fluoride (Aldrich), bromobenzene (Aldrich), 4-bromobenzonitrile
(Oakwood), and 4-bromoanisole (Aldrich) were purchased from
commercial sources. Anhydrous pentane was purchased from Aldrich
and stored over 3 Å molecular sieves prior to use. All other solvents
were collected from a Glass Contour solvent purification system,
degassed, and stored over 3 Å molecular sieves. Anhydrous
deuterobenzene (C6D6) was purchased from Aldrich and stored
over 3 Å molecular sieves. Pd2(dba)3 (Aldrich), (COD)Pd-
(CH2TMS)2 (Strem), and 2,8,9-triisopropyl-2,5,8,9-tetraaza-1-
phosphabicyclo[3,3,3]undecane (L, Aldrich) were used as received
24
20b
from their respective vendors. KNPh2 and KN[3,5-CF3(C6H3)]2
were prepared by literature procedures. 1,3,5-Trimethoxybenzene was
purified by sublimation. Authentic samples of biaryl compounds were
C
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prepared by literature methods.25 Chromatography was performed
using SiliaFlash F60 silica. Elemental analyses were performed by
Midwest Microlab, LLC.
(ipso), 107.4 (ipso), 57.7 (br, i-PrCH2), 51.0 (ethylene CH2), 49.6
(ethylene CH2), 28.8 (d, J = 3.8 Hz, (CH3)2CH), 21.9 (CH3); note:
one aromatic carbon was not observed. 31P NMR (202 MHz, C6D6):
δ 94.8 ppm. IR (ATR, cm−1): 2954, 2220 (nitrile), 1570, 1470, 1390,
1174, 1031, 811. Anal. Calcd for C50H86Br2N10P2Pd2: C, 47.59; H,
6.87; N, 11.10 Found: C, 47.43; H, 6.73; N, 11.22.
1
Spectroscopy. H, 13C{1H}, 19F{1H}, and 31P{1H} spectra were
collected on Bruker AV-300 and AV-500 NMR spectrometers at
ambient temperature unless otherwise noted. 1H NMR chemical shifts
(δ) are reported in parts per million (ppm) relative to the solvent
(7.16 ppm for C6D5H). 13C NMR spectra were referenced relative to
the solvent signal (128.06 ppm for C6D6). 31P{H} and 19F{H} were
referenced using the absolute reference function of the MNova 9.0.1
NMR software package. Infrared spectra were recorded on a Nicolet
iS10 FT-IR spectrometer. In all dilute 1H NMR spectra, a small, broad
artifact is observed at 1.37 ppm; a spectrum of C6D6 is provided for
[(N(CH2CH2Ni-Bu)3PPd(C6H4-p-CF3)Br]2 (2b). This synthesis
was performed on a 0.10 mmol scale using the general procedure.
The product was isolated as a pale yellow solid (43.6 mg, 0.065 mmol,
65% yield). X-ray quality crystals were grown from a 2:1 pentane/
toluene mixture at −20 °C. 1H NMR (500 MHz, C6D6): δ 7.74 (dd, J
= 7.9 Hz, 5.4 Hz, 2H, H meta to Pd), 7.24 (d, J = 7.9 Hz, 2H, H ortho
to Pd), 3.04 (br s, 6H, i-PrCH2), 2.62 (br s, 6H, ethylene CH2), 2.39
(t, J = 5.0 Hz, 6H, ethylene CH2), 2.00 (br s, 3H, (CH3)2CH)), 0.98
(d, J = 6.6 Hz, 18 H, CH3). 13C NMR (126 MHz, C6D6): δ 137.9 (C
meta to Pd), 123.4 (C ortho to Pd), 57.8 (i-PrCH2), 51.1 (ethylene
CH2), 49.7 (ethylene CH2), 28.8 ((CH3)2CH), 22.0 (CH3); note: ipso
carbons were not observed. 31P NMR (202 MHz, C6D6): δ 95.8 ppm.
19F (282 MHz, C6D6): δ −61.6 ppm. IR (ATR, cm−1): 2955, 2927,
2872, 1585, 1390, 1318, 1160, 1037, 1009, 850. Combustion analysis
was not pursued because an impurity at 0.58 ppm (1H NMR) was
observed in varying quantities despite recrystallization.
1
reference in the Supporting Information. When possible, H and 13C
resonances were assigned using COSY, HSQC, and NOESY
experiments. Due to the low solubility and/or thermal stability of
some complexes, the signal-to-noise is low in many 13C NMR spectra.
An inseparable impurity observed at ca. 0.6 ppm by 1H NMR in some
spectra may be the product of cyclometalation, a decomposition
product suggested in the literature.5b
Pd(P(i-BuNCH2CH2)3N)2 (1). To a 20 mL scintillation vial in the
glovebox were added Pd2(dba)3 (27.4 mg, 0.030 mmol, 1.0 equiv)
and toluene (2 mL). P(i-BuNCH2CH2)3N (46.2 mg, 0.135 mmol, 4.5
equiv) in toluene (2 mL) was added to the stirred palladium solution.
Within 20 min, the solution changed from a dark wine color to a deep
burgundy. The reaction mixture was stirred for 16 h, during which
time the reaction mixture became green and heterogeneous. The
reaction mixture was diluted with toluene until homogeneous,
approximately 40 mL, and passed through a pad of Celite to remove
solid palladium. The resulting yellow solution was concentrated to a
brown solid. The crude solid was suspended in Et2O and cooled to
−35 °C, and the supernatant was then decanted. This suspension−
decantation process was repeated until the supernatant was colorless,
indicating the absence of dibenzylideneacetone. Residual solvent was
evaporated under vacuum to yield the desired complex as an
analytically pure, colorless solid (39.6 mg, 0.050 mmol, 83% yield). X-
ray quality crystals were grown from a 5:1 Et2O/toluene mixture at
[(N(CH2CH2Ni-Bu)3PPd(C6H5)Br]2 (2c). This synthesis was
performed on a 0.05 mmol scale using the general procedure. The
product was isolated as an off-white solid (21.7 mg, 0.0359 mmol,
1
72% yield). H NMR (500 MHz, C6D6): δ 7.71 (t, J = 6.5 Hz, 2H,
Ar), 7.03 (t, J = 7.4 Hz, 2H, Ar), 6.87 (t, J = 7.2 Hz, 1H, p-H), 3.17
(br s, 6H), 2.70 (br s, 6H), 2.46 (t, J = 5.1 Hz, 6H), 2.13 (s, 3H,
(CH3)2CH)), 1.04 (d, J = 6.6 Hz, 18H, CH3). 13C NMR (126 Hz,
C6D6): δ 137.8 (d, J = 5.0 Hz), 123.1, 58.1, 51.3, 49.6, 28.9, 22.0;
note: two signals were not observed. 31P NMR (202 MHz, C6D6): δ
97.6. IR (ATR, cm−1): 2955, 2860, 1561, 1396 1103, 1009, 805.
Combustion analysis was not pursued due to the thermal instability of
the complex.
[(N(CH2CH2Ni-Bu)3PPd(C6H4-p-OMe)Br]2 (2d). This synthesis
was performed on a 0.10 mmol scale using the general procedure. The
product was isolated as a pale yellow solid (35.6 mg, 0.0560 mmol,
1
1
−35 °C. H NMR (500 MHz, C6D6): δ 3.33 (ap q, J = 6.1 Hz, 5.64
56% yield). H NMR (500 MHz, C6D6): δ 7.56 (dd, J = 8.4, 5.5 Hz,
2H, Ar), 6.77 (d, J = 8.2 Hz, 2H, Ar), 3.38 (s, 3H, OMe), 3.20 (br s,
6H), 2.73 (br s, 6H), 2.47 (ap t, J = 5.0 Hz, 6H), 2.15 (br s, 3H,
(CH3)2CH)), 1.05 (d, J = 6.6 Hz, 18H, CH3). 31P NMR (202 MHz,
C6D6): δ 97.5. IR (ATR, cm−1): 2951, 2864, 1480, 1464, 1388, 1183,
1117, 1014, 836. Combustion analysis and 13C NMR data were not
collected due to the low thermal stability of the complex.
Hz, 12H, i-PrCH2), 2.87 (br s, 12H, CH2Nax), 2.75 (t, J = 5.0 Hz,
12H, CH2Neq), 2.27 (sept, J = 6.8 Hz, 6H, (CH3)2CH)), 1.14 (d, J =
6.6 Hz, 36H, CH3). 13C NMR (126 MHz, C6D6): δ 60.0 (t, JC−P
16.5 Hz, i-PrCH2), 52.0 (CH2Nax), 48.0 (CH2Neq), 29.5 (t, JC−P
=
=
3.00 Hz, Me2CH), 21.6 (CH3). 31P NMR (202 MHz, C6D6): δ 134.5.
IR (ATR, cm−1): 2949, 2926, 2823, 1117, 1014, 836, 701. Anal. Calcd
for C36H78N8P2Pd: C, 54.63; H, 9.93; N, 14.16. Found: C, 54.30; H,
9.71; N, 14.01.
(N(CH2CH2Ni-Bu)3PPd(C6H4-p-OMe)(N[3,5-CF3(C6H3)]2) (3).
Thawing THF (2 mL) was added to a vial containing complex 2d
(19.8 mg, 0.031 mmol, 1.0 equiv), and the suspension was frozen in
the glovebox cold well. KN[3,5-CF3(C6H3)]2 (15.5 mg, 0.031 mmol,
1.0 equiv) was dissolved in THF (4 mL) and layered onto the frozen
suspension of 2d. The suspension was permitted to thaw and mix with
the amido salt solution, changing within 30 min from a yellow
suspension to a homogeneous red solution. The reaction mixture was
concentrated, and the residue was dissolved in pentane. The pentane
solution was passed through a syringe filter, and the resulting solution
was concentrated to saturation. The pentane solution was stored at
−35 °C until crystals formed. The supernatant was decanted, and
residual solvent was removed in vacuo, yielding the desired
compound as orange crystals (15.5 mg, 0.016 mmol, 50% yield). X-
ray quality crystals were grown from a saturated pentane solution at
−35 °C. 1H NMR (500 MHz, C6D6): δ 7.91 (s, 4H, o-H 3,5-
(CF3)2C6H3), 7.31 (s, 2H, p-H 3,5-(CF3)2C6H3), 7.02 (dd, J = 8.7,
4.4 Hz, 2H, anisyl H ortho to Pd), 6.45 (d, J = 8.6 Hz, 2H, anisyl H
meta to Pd), 3.21 (s, 3H, OCH3), 2.66 (dd, J = 11.4, 7.2 Hz, 6 H, i-
PrCH2), 2.43−2.3 (m, 6H, ethlyene), 2.32−2.24 (m, 6H, ethylene),
1.42 (sept, J = 6.5 Hz, 3H, (CH3)2CH)), 0.80 (d, J = 6.5 Hz, 18H,
CH3). 13C NMR (126 Hz, C6D6): δ 158.6 (ipso), 156.1 (ipso) 134.6
(anisyl C ortho to Pd), 133.0 (ipso), 132.8 (ipso), 120.7 (o-C 3,5-
(CF3)2C6H3), 114.3 (d, J = 4.0 Hz, anisyl C meta to Pd), 110.9 (p-C
3,5-(CF3)2C6H3) 55.4 (d, J = 15.4 Hz, i-PrCH2) 54.9 (OCH3) 50.4
(ethylene CH2), 47.8 (ethylene CH2), 30.2 (i-PrCH2), 20.3 (CH3);
General Procedure for the Synthesis of Oxidative Addition
Complexes (Complexes 2a−2d). A 20 mL scintillation vial was
charged with (COD)Pd(CH2TMS)2 (38.9 mg, 0.10 mmol, 1.0 equiv)
and pentane chilled to −35 °C (4 mL). To this solution was added a
chilled solution of pentane (8 mL) containing proazaphosphatrane
(34.2 mg, 0.10 mmol, 1.0 equiv) and aryl halide (3.0 equiv). The
reaction mixture was warmed to room temperature and stirred for
16−18 h, at which point the precipitate was permitted to settle and
the supernatant was decanted. The resulting solid was again stirred in
pentane and the mother liquor decanted. Residual solvent was
removed in vacuo to yield the desired product. Reactions conducted
on a 0.05 mmol scale in (COD)Pd(CH2TMS)2 were conducted
identically but on half the scale. For all complexes, small quantities of
pentane remain despite prolonged exposure to vacuum.
[(N(CH2CH2Ni-Bu)3PPd(C6H4-p-CN)Br]2 (2a). This synthesis was
performed on a 0.05 mmol scale using the general procedure. The
product was isolated as a pale yellow solid (25.2 mg, 0.0399 mmol,
80% yield). X-ray quality crystals were grown from a saturated toluene
1
solution at −35 °C. H NMR (500 MHz, C6D6): δ 7.61 (dd, JC−P
=
8.0 Hz, 5.3 Hz, 2H, H meta to Pd), 6.96 (d, J = 7.8 Hz, 2H, H para to
Pd), 2.98 (br s, 6H, i-PrCH2), 2.60 (br s, 6H, ethylene CH2), 2.37 (t,
J = 4.8 Hz, 6H, ethylene CH2), 1.98 (br s, 3H, (CH3)2CH), 0.93 (d, J
= 6.6 Hz, 18H, CH3). 13C NMR (126 Hz, C6D6): δ 161.2 (d, JC−P
=
16.1 Hz, CN), 138.4 (d, J = 5.0 Hz, Csp2−H), 129.5 (Csp2−H) 119.9
D
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note: CF3 not observed. 31P NMR (202 MHz, C6D6): δ 101.8. 19F
(282 MHz, C6D6): δ −62.8. IR (ATR, cm−1): 2951, 1386, 1271,
1161, 1117. Anal. Calcd for C41H52F12N5OPPd: C, 49.93; H, 5.26; N,
7.03. Found: C, 49.09; H, 6.01; N, 6.82. Analytically pure material was
not obtained despite recrystallization; however, the structure can be
proposed with confidence based on NMR and crystallographic data.
Use of 2b as a Precatalyst. In a glovebox, 2b (0.0040 mmol, 2.7
mg, 2 mol %) was dissolved in toluene (1 mL) and added to a vial
containing NaOt-Bu (0.300 mmol, 28.8 mg). Additional toluene (1.5
mL) was used to transfer residual 2b. The reaction mixture
immediately became dark pink. To the mixture was added Ph2NH
in toluene (1.5 mL). To the vial was added a 0.015 M stock solution
of L (0.015 M in toluene, 0.003 mmol, 200 μL, 2 mol %) followed by
4-bromobenzotrifluoride (0.20 mmol, 28 μL). The vial was removed
from the glovebox and stirred at 80 °C for 16 h. The reaction mixture
became a dark green-black. The mixture was filtered through a
medium porosity frit with additional toluene and concentrated on a
rotary evaporator to yield a green-black solid. The crude material was
purified by column chromatography (2.5% ethyl acetate/97.5%
hexane) to yield a white solid (59.4 mg, 0.190 mmol, 95% yield).
1H NMR data for the product match that in the literature.26
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: miles.johnson@richmond.edu.
■
ORCID
Nathan D. Schley: 0000-0002-1539-6031
Miles W. Johnson: 0000-0002-7009-6138
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The University of Richmond is acknowledged for start-up
funds (M.W.J.) and for undergraduate summer fellowships
(A.D.M. and G.M.G.). Funding from Vanderbilt University is
gratefully acknowledged (N.D.S.). Miriam A. Bowring, C.
Wade Downey, Allegra L. Liberman-Martin, and Kristine A.
Nolin are thanked for helpful discussions.
Use of 1 as a Precatalyst. The above reaction was performed
identically but with 1 (3.2 mg, 0.0040 mmol, 2 mol %) in place of 2b,
and without the addition of L. The desired product was isolated as a
white solid (61.1 mg, 0.195 mmol, 98% yield).
REFERENCES
■
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rigid bodies using the method of Guzei.29 Their thermal parameters
were refined with similarity restraints. A disordered isopropyl group
was modeled as a 2-position disorder using similarity restraints placed
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ASSOCIATED CONTENT
■
S
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Tabulated X-ray crystallographic data and NMR spectra
(PDF)
Accession Codes
CCDC 1850176−1850179 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
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DOI: 10.1021/acs.organomet.8b00453
Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00453
Organometallics XXXX, XXX, XXX−XXX