Interception of intermediates in phosphine oxidation by mesityl nitrile-: N -oxide using frustrated Lewis pairs

Article abstract of DOI:10.1039/c8dt01717d

Phosphine oxidation by MesCNO is rapid; however, an FLP strategy intercepts the 1,3 addition products including [MesC(R₃P)NOB(C₆F₅)₃] (R = Ph 1, p-tol 2), [MesC(Mes₂PH)NOB(C₆F₅)₃] 3 (MesC(NOB(C₆F₅)₃)Ph₂P)₂(CH₂)_n (n = 2: 4, 3: 5) and [MesC(Ph₃P)NOB(C₆F₄H)₃] 6. These species are shown to react with tBuOK or [Bu₄N]F permitting the oxidation to proceed via a process involving borane dissociation. Similarly, the equilibrium established by 1 with B(C₆F₄H)₃ and 6 with B(C₆F₅)₃ provides experimental support for the "Cummins mechanism" for these phosphine oxidations.

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Interception of Intermediates in Phosphine Oxidation by
Mesityl Nitrile-N-oxide using Frustrated Lewis Pairs
Kevin M. Szkopŧ, Diya Zhuŧ, Lauren E. Longobardi, Julian Heck, Douglas W. Stephan*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Phosphine oxidation by MesCNO is rapid, however, an FLP strategy intercepts the 1,3 addition products including
[MesC(R3P)NOB(C6F5)3] (R = Ph 1, p-tol 2), [MesC(Mes2PH)NOB(C6F5)3] 3 (MesC(NOB(C6F5)3)Ph2P)2(CH2)n (n = 2: 4, 3: 5) and
[MesC(Ph3P)NOB(C6F4H)3] 6. These species are shown to react with tBuOK or [Bu4N]F permitting the oxidation to proceed
via a process involving borane dissociation. Similarly, the equilibrium established by 1 with B(C6F4H)3 and 6 with B(C6F5)3
provide experimental support for the “Cummins mechanism” for these phosphine oxidations.
www.rsc.org/
reagent mesityl nitrileꢀNꢀoxide (MesCNO, Mes
trimethylphenyl) to effect the oxidation of triarylphosphines, the
complexes (Ar(tBu)N)3V,24 (Ar(tBu)N)3MoP24ꢀ28 and the
=
2,4,6ꢀ
Introduction
The emergency of the concept of “frustrated Lewis pairs”
(FLPs) a decade ago, was predicated on the finding that
combinations of Lewis acids and bases could activate the
dihydrogen molecule in a heterolytic fashion.1ꢀ4 Nonetheless, this
field has broadened dramatically in recent years, as the chemistry
has been shown to extend to equilibria where free Lewis acids
and bases are accessible from the corresponding adducts.5 In
addition, recent work has provided evidence of radical pathways,
and thus homolytic cleavages in FLP chemistry.6ꢀ7 These
developments notwithstanding, FLP chemistry has also spawned
numerous advances in main group hydrogenation catalysis,
including elaborations to highly selective asymmetric reductions.
FLPs has also been exploited to capture small molecules.1ꢀ4 For
example, olefins, dienes, alkynes, cyclopropanes, CO2,8 CO,9ꢀ13
SO2,14ꢀ15 RNSO16 and NO17ꢀ20 have all been shown to react with
FLPs21 (Scheme 1) to given products derived from the three
components. Interestingly, in these cases, pairwise combinations
of the Lewis acid, the Lewis base ,and the substrate lead to no
reactions. On the other hand, FLPs can also be exploited to divert
reactions and intervene in cases where pairwise reactions are
known. For example, the reaction of N2O with phosphines results
in phosphine oxidation. However, the reaction of FLPs with N2O
prompted its capture as tBu3PN2OB(C6F5)3.22 In recent work,23 we
have shown that reactions of FLPs can intervene in the azide
oxidation of phosphines, or in the Lewis acid mediated cyclization
of isocyanates. In both of these cases, the presence of FLPs
rather than the individual reagents allows the interception of these
substrates.
diphosphane, (Me2C4H6)2P2.29 These oxygen atom transfer
reactions (Scheme 2) are reported to be facile and proceed
rapidly “within minutes” and this ease is attributable to the low NꢀO
bond
Scheme 1 Examples of FLP reactions where FLP intervenes in pairwise reaction.
dissociation enthalpy (BDE) in MesCNO (52 kcal/mol). In probing
the mechanism, Cummins et al.29 showed that reaction of carbene
and MesCNO afforded the adduct in which the carbene binds to
the electrophilic carbon atom of MesCNO. This led these authors
to propose that phosphines react in a similar fashion, forming
corresponding adducts with MesCNO which rearranges to a 4ꢀ
membered ring transition state which effects oxygen atom transfer
liberating phosphine oxide and MesCN. While this proposed
pathway was also supported by computational data, the
intermediate phosphineꢀMesCNO adducts were not isolable nor
To this end, we noted the Cummins group has exploited the
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spectroscopically observable. In this communication, we exploit parameters were similar to those seen for 1 and 2, suggesting the
FLPs to intervene in the oxidation of a series of phosphines by formulation of 3 as [MesC(Mes2PH)NOB(C6F5)3].
MesCNO, providing further experimental data in support of the
In contrast, Mes3P reacted very slowly with B(C6F5)3 and
“Cummins mechanism”.
MesCNO. After 24 h, while resonances attributable to free
phosphine and the 1,3ꢀaddition product were observed in the
31P{1H} NMR spectrum, this product was not isolable. Similarly,
reactions
of
Ph2P(C6F5)/B(C6F5)3/MesCNO
and
P(C6F4CF3)3/B(C6F5)3/MesCNO generated the 1,3ꢀaddition
products in situ in low yields but were not amenable to clean
isolation. No reaction was observed in the corresponding
reactions of PhP(C6F5)2 and P(C6F5)3. This latter observation is
presumably a result of the electronꢀdeficient nature of these
phosphines, making them both poor nucleophiles and difficult to
oxidize.
Scheme 2 Examples of oxygen atom transfer reactions using MesCNO.
Results and Discussion
The addition of one equivalent of MesCNO to a suspension of
freshly prepared [Ph3PB(C6F5)3] in CH2Cl2 led to the dissolution of
the insoluble adduct yielding a clear and colourless solution. After
20 minutes, analysis by multinuclear NMR spectroscopy revealed
complete consumption of the starting materials and formation of a
new species 1, which was isolated as a white powder in 88 %
yield. In the 31P{1H} NMR spectrum, 1 exhibited a sharp new
resonance at 5.4 ppm. The 19F{1H} NMR spectrum showed a
small chemical shift gap between the resonances attributable to
the meta and para fluorine atoms of the perfluorinated arene rings,
consistent with the presence of a fourꢀcoordinate borate centre.
This was further supported by the 11B{1H} NMR spectrum of 1,
which displayed a sharp resonance at 0.5 ppm. Together, these
data were consistent with 1 being derived from the combination of
the Lewis acidꢀbase adduct and MesCNO. 1,3 Addition to the
CNO fragment was proposed based on the analogy to reactions
with RNSO, prompting the formulation of 1 as the zwitterionic
product [MesC(Ph3P)NOB(C6F5)3] (Scheme 3). Dissolution of 1 in
benzene and layering with pentane at ambient temperature
yielded diffraction quality single crystals. The subsequent
crystallographic study confirmed the above formulation of 1. In the
solid state, the phosphine and borane fragments are disposed in a
pseudoꢀtransoid orientation with BꢀO, PꢀC and CꢀN distances of
Figure 1 POV-ray depiction of
blue; O: red.
1. C: black; F: pink; P: orange; B: yellow-green; N:
1.522(3) Å, 1.841(3)
Å and 1.284(3) Å, respectively. The
corresponding PꢀCꢀN and NꢀOꢀB were determined to be 123.8(2)°
and 116.3(2)°, respectively. This geometry results in a throughꢀ
space PꢀO distance of 2.874(2) Å. This geometry is reminiscent of
that seen in the species [tBu3PN2OB(C6F5)3].22
In a similar fashion, (p-tol)3P reacted with B(C6F5)3 and
Scheme 3 Synthesis of 1-6
.
MesCNO
to
give
the
product
formulated
as
[MesC(R3P)NOB(C6F5)3] (R = p-tol, 2), isolated in 41 % yield. The
corresponding reaction of Mes2PH, B(C6F5)3 and MesCNO in
CH2Cl2 gave the product 3 in 65 % yield (Scheme 3). The
diagnostic PꢀH resonance was identified at ꢀ39.1 ppm as a
doublet with a coupling constant of 518 Hz in the 31P{1H} NMR
The corresponding reactions of bidentate phosphines with two
equivalents of both B(C6F5)3 and MesCNO were probed.
(Ph2P)2(CH2)2 (dppe) was combined with B(C6F5)3 and MesCNO,
resulting in a 31P{1H} NMR spectrum which showed a new
resonance at 12.6 ppm attributable to the product 4. The
corresponding 19F{1H} NMR spectrum showed resonances at ꢀ
1
spectrum, typical for P(V) JPH constants.31 Other spectroscopic
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133.8, ꢀ160.0, and ꢀ165.7 ppm. The 11B{1H} NMR spectrum of 4 1.806(5) Å and 1.284(5) Å, respectively and the corresponding Pꢀ
displayed a sharp resonance at 0.1 ppm. These data supported CꢀN and NꢀOꢀB are 125.9(3)° and 114.8(3)°, respectively.
the assignment of 4 as a diꢀzwitterionic salt containing two
phosphonium and borate centres, (MesC(NOB(C6F5)3)Ph2PCH2)2
(Scheme 3). Crystallographic study of 4 was confirmed this
formulation (Figure 2). The crystallographic C2 symmetry places
the two PCNOB fragments on opposite sides of the diphosphine
unit. The BꢀO, PꢀC and CꢀN distances in 4 were found to be
1.528(3) Å, 1.844(3) Å and 1.283(3) Å. The corresponding PꢀCꢀN
and NꢀOꢀB were determined to be 124.0(2)° and 109.8(2)°,
respectively. In a similar fashion, reaction of (Ph2P)2(CH2)3 (dppp)
with two equivalents of B(C6F5)3 and MesCNO afforded a new
species 5 in 88% yield, which exhibited a singlet resonance at 9.8
ppm in the 31P{1H} NMR spectrum. This and the related NMR data
supported
the
formulation
of
5
as
(MesC(NOB(C6F5)3)Ph2PCH2)2CH2 (Scheme 3).
Figure 3 POV-ray depiction of
blue; O: red.
6. C: black; F: pink; P: orange; B: yellow-green; N:
The isolation of compounds 1-6 illustrate that the presence of
the Lewis acids precludes the normal course of phosphine
oxidation by MesCNO. The BꢀO bonds in these products
presumably provide both steric and electronic deterrents to the
molecular reorientation that is required for oxygen atom transfer.
This infers that removal of the borane from species 1-6 would
prompt phosphine oxidation. To confirm this premise, 1 was
treated with excess of potassium tertꢀbutoxide (KOtBu) or excess
tetrabutylammonium fluoride ([Bu4N]F) (Scheme 4). The
corresponding 31P{1H} NMR data after 30 minutes showed
complete conversion of 1 to Ph3PO with the liberation of nitrile.
Additionally, upon addition of excess KOtBu or [Bu4N]F to a
mixture of 1 and P(pꢀtol)3, 31P{1H} NMR spectroscopy revealed
consumption of 1 with the quantitative and exclusive formation of
Ph3PO together without oxidation of P(pꢀtol)3. Collectively, these
data are consistent with a dissociative equilibrium in which borane
is liberated from 1. This allows borane capture by t-butoxide or
fluoride generating [MesC(Ph3P)NO] which then undergoes
oxygen atom transfer effecting phosphine oxidation.
Scheme 4 Reactions of
1 with TBAF and KOtBu.
Figure 2 POV-ray depiction of
B: yellow-green; N: blue; O: red.
2 (top) and 4 (bottom) C: black; F: pink; P: orange;
Seeking further evidence for borane dissociation an EXSY
study of with equimolar amount of B(C6F5)3 was undertaken.
While this showed no exchange of the Lewis acid on the NMR
time scale, addition of one equivalent of B(C6F4H)3 to led to
slow chemical exchange. After 16 hours at ambient
temperature, this mixture equilibrated to give a mixture of
B(C6F5)3 and (Scheme 4) with a ratio of 1: 6 of a 70:30
(Scheme 5). The prevalence of in this equilibrium is
1
This synthetic protocol was amenable to alternative boranes.
For example, combination of PPh3, MesCNO and B(C6F4H)3
afforded compound 6. (Scheme 3). The formulation of 6 as
[MesC(Ph3P)NOB(C6F4H)3] was consistent with the spectroscopic
data and confirmed by crystallographic study (Figure 3). The
metric parameters of 6 were found to be very similar to those
described for 1 with BꢀO, PꢀC and CꢀN distances of 1.514(5) Å,
1
1,
6
1
consistent with the slightly lower Lewis acidity of B(C6F4H)3. In
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1
referenced to the residual solvent signals (CDCl3: H= 7.26 ppm
and 13C = 77.2 ppm). Chemical shifts (δ) are reported in ppm and
coupling constants (J) are listed as absolute values in Hz.
Multiplicities are reported as singlet (s), doublet (d), triplet (t),
multiplet (m), overlapping (ov), and broad (br). Highꢀresolution
mass spectra (HRMS) were obtained on a JMSꢀT100LC JOEL
DART mass spectrometer. Elemental analyses for C, H, and N
were performed by ANALEST (University of Toronto) employing a
Perkin Elmer 2400 Series II CHNS Analyser.
the converse experiment, addition of B(C6F5)3 to
6 resulted in
the same equilibrium mixture after standing for 16 hours.
X-ray Diffraction Studies: Single crystals were coated with
paratone oil, mounted on a cryoloop and frozen under a stream of
cold nitrogen. Data were collected on a Bruker Apex2 Xꢀray
diffractometer at 150(2)
K for all crystals using graphite
monochromated MoꢀKα radiation (0.71073 Å). Data were
collected using Bruker APEXꢀ2 software and processed using
SHELX and an absorption correction applied using multiꢀscan
within the APEXꢀ2 program. All structures were solved and refined
by direct methods within the SHELXTL package. These data can
be obtained free of charge from the Cambridge Crystallographic
Data Centre.
Scheme 5 Lewis acid exchange equilibrium.
Despite the rapid oxidation of phosphines by MesCNO, FLPs
intervene in the reactions of phosphine with MesCNO intercepting
the 1,3ꢀaddition products 1-6. This binding to the oxygen atom
precludes oxygen atom transfer from nitrogen to phosphorus.
Lewis acid dissociation from these species is documented and
this provides an avenue for Lewis acid exchange. In addition,
capture of liberated borane by KOtBu or [Bu4N]F permits the
oxygen atom transfer to occur effecting oxidation of the nitrileꢀ
bound phosphine.
Mass Spectrometry Studies: All attempts to observe the
products by highꢀresolution mass spectrometry failed due to the
instability of these compounds under mass spectrometry
conditions, by either ESI or DART methods. In positive mode, the
protonated phosphonium cation or protonated phosphine oxide
were observed. By negative mode, [HOꢀB(C6F5)3] or [H2NOꢀ
B(C6F5)3] were observed.
Conclusions
These data are consistent with the Cummins29proposition that Synthesis of [MesC(Ph3P)NOB(C6F5)3] (1) PPh3 (65.3 mg, 0.25
the direct oxidation of phosphine by MesCNO proceeds via initial mmol, 1 equiv.) and B(C6F5)3 (127.9 mg, 0.25 mmol, 1 equiv.)
interaction of phosphine with the electron deficient C atom of were combined in 2 mL CH2Cl2. To the resulting suspension, a
MesCNO, generating the species that is captured by borane in the solution of MesCNO (40.3 mg, 0.25 mmol, 1 equiv.) in 1 mL of
presence of an FLP. We are continuing to explore FLP systems to CH2Cl2 was added dropwise, yielding a clear and homogenous
develop applications for reactions of small molecules and solution. After 20 minutes, 5 mL of cold pentane was added with
catalysis and to garner insight into reaction mechanisms.
vigorous stirring which led to the precipitation of a white
precipitate. The solution was then decanted and dried in vacuo,
yielding the desired product as a white powder. Yield: 206.5 mg
(88% isolated yield). Diffraction quality single crystals were
obtained through slow diffusion of pentane into benzene at room
temperature. 1H NMR (600 MHz, CDCl3, 253 K): δ 7.80 (m, 4H,
Ar), 7.67 (m, 2H, Ar), 7.48 (m, 5H, Ar), 7.18 (m, 2H, Ar), 6.71 (m,
2H, Ar), 6.64 (s, 2H, mꢀH, Mes), 2.16 (s, 3H, pꢀCH3, Mes), 1.90 (s,
6H, oꢀCH3, Mes).19F{1H} NMR (377 MHz, CDCl3): δ ꢀ132.3 (d, 3JFF
Experimental Section
General Remarks All reactions and workꢀup procedures were
performed under an inert atmosphere of dry, oxygenꢀfree N2 by
means of standard Schlenk techniques or glovebox techniques
(VAC glovebox equipped with a ꢀ25 °C freezer) unless otherwise
specified. All glassware was ovenꢀdried and cooled under vacuum
before use. Dichloromethane (DCM) were distilled over CaH2.
Pentane and hexane were collected from a Grubbsꢀtype column
system manufactured by Innovative Technology and degassed.
Solvents were stored over activated 4 Å molecular sieves.
Molecular sieves, type 4 Å (pellets, 3.2 mm diameter) purchased
from Sigma Aldrich were activated prior to usage by iteratively
heating under vacuum for 24 hours. CDCl3 purchased from
Cambridge Isotope Laboratories was vacuum distilled, further
degassed, and stored over activated 4 Å molecular sieves in the
glovebox for at least 8 hours prior to use. Unless otherwise
mentioned, chemicals were purchased from Sigma Aldrich or TCI.
Mesityl nitrile Nꢀoxide (MesCNO) was prepared using literature
methods.32 B(C6F5)3 was purchased from Boulder Scientific and
sublimed under vacuum at 85 °C prior to use. NMR spectra were
recorded at room temperature (298K) unless otherwise mentioned
on a Bruker Avance III 400 MHz, an Agilent DD2 500, an Agilent
DD2 600, and an Agilent DD2 700 Spectrometers. Spectra were
3
= 22.6 Hz, 2F, oꢀC6F5), ꢀ160.0 (t, JFF = 20.8 Hz, 1F, pꢀC6F5), ꢀ
165.5 (m, 2F, mꢀC6F5). 31P {1H} NMR (162 MHz, CDCl3): δ 5.4 (s).
11B{1H} NMR (128 MHz, CDCl3): δ 0.5 (s).13C{1H} NMR (126 MHz,
CDCl3): δ 148.09 (dm, 239 Hz, C6F5), 140.28 (d, 3 Hz), 139.11
(dm, 252 Hz, C6F5), 139.08 (d, 4 Hz), 136.84 (dm, 232 Hz, C6F5),
134.49 (b), 133.92 (d, 11 Hz), 132.76, 132.48, 129.62 (b), 129.03,
128.74, 128.60, 21.14, 20.75. Elemental analysis: Calc.: C
59.06%, H 2.80%, N 1.51%. Exp.: C 58.76%, H 2.46%, N 1.43%
Synthesis of [MesC((p-tol)3P)NOB(C6F5)3] (2) Solutions of P(pꢀ
tol)3 (43.9 mg, 0.14 mmol, 1 equiv.) and B(C6F5)3 (73.8 mg, 0.14
mmol, 1 equiv.) were combined in 4 mL of CH2Cl2. The mixture
remained homogeneous. To this solution, 2 mL of CH2Cl2 were
carefully layered and the solution was left undisturbed for 5
minutes. A solution of MesCNO (23.2 mg, 0.14 mmol, 1 equiv.) in
2 mL CH2Cl2 was then carefully layered on top of the reaction vial.
This final 3ꢀlayer mixture was left undisturbed for 48 hours at
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ambient temperature. After this time, clear and colourless
diffractionꢀquality crystals precipitated from the yellow reaction
mixture. The solvent was decanted and the crystalline product
washed with pentane (3 x 5 mL), then dried in vacuo. Yield: 57.8
mg (41% isolated yield). Note: the yield can be improved by
cooling the filtrate to ꢀ35 °C over one week, yielding additional
product as an amorphous white precipitate. Once precipitated
from solution, compound 2 has very low solubility in common
organic solvents, preventing full characterization by multinuclear
Hz), 133.77 (d, 10Hz), 129.69, 129.44 (d, 13 Hz), 128.94, 127.80
(d, 16 Hz), 121.49 (b), 117.13 (d, 82 Hz), 29.91, 21.07, 20.59. The
resonance for the ipsoꢀB(C6F4H)3 carbon is likely not observed.
Elemental analysis: Calc.: C 56.45%, H 2.66%, N 1.61%. Exp.: C
55.64%, H 2.53%, N 1.60%
Synthesis of (MesC(NOB(C6F5)3)Ph2PCH2)2CH2 (5) A 1 mL
CH2Cl2 solution of dppp (16 mg, 0.039 mmol) was added to a 1
mL CH2Cl2 solution of B(C6F5)3 (40 mg, 0.078 mmol). To the
mixture, a solution of MesCNO (13 mg, 0.078 mmol) in 1 mL of
CH2Cl2 was added dropwise. After 20 mins, 5 mL of cold pentane
was added with vigorous stirring, yielding a white precipitate. The
solution was then decanted, yielding the desired product as a
1
NMR spectroscopy. H, 11B{1H}, 19F{1H} and 31P{1H} NMR spectra
are reported. Chemical connectivity and bulk purity are
unambiguously confirmed by singleꢀcrystal Xꢀray diffraction
1
studies and elemental analysis, respectively. H NMR (600 MHz,
1
white powder. Yield: 51.4 mg (88% isolated yield). H NMR (400
THFꢀd8, 313 K): δ 7.72ꢀ7.26 (br, 12 H, Ar), 6.67 (s, 2H, mꢀH, Mes),
2.38 (br, 9H, pꢀCH3, Ar), 2.15 (s, 3H, pꢀCH3, Mes), 1.95 (s, 6H, oꢀ
CH3, Mes). 19F{1H} NMR (377 MHz, THFꢀd8): δ ꢀ134.6 (m, 2F, oꢀ
B(C6F5)3), ꢀ164.4 (m, 1F, pꢀB(C6F5)3), ꢀ169.5 (m, 2F, mꢀB(C6F5)3);
31P{1H} NMR (162 MHz, THFꢀd8): δ 4.0 (s); 11B{1H} NMR (128
MHz, THFꢀd8): δ ꢀ2.4 (b); Elemental Analysis: Calc.: C 60.20%, H
3.30%, N 1.43%. Exp.: C 59.45%, H 3.29%, N 1.45%
MHz, CDCl3): δ 8.28ꢀ8.24 (m, 4H, Mes), 8.03ꢀ7.90 (ov, 15H,
P(C6H5)3), 4.02 (br, 4H, CH2), 2.79 (s, 6H, pꢀCH3, Mes), 2.55 (s,
12H, oꢀCH3, Mes), 1.95 (br, 2H, CH2). 19F{1H} NMR (377 MHz,
3
3
CDCl3): δ ꢀ133.7 (d, JFF = 22.6 Hz, 2F, oꢀC6F5), ꢀ160.3 (t, JFF
=
18.9 Hz, 1F, pꢀC6F5), ꢀ165.8 (m, 2F, mꢀC6F5). 31P{1H} NMR (162
MHz, CDCl3): δ 9.8 (s). 11B{1H} NMR (128 MHz, CDCl3): δ 0.3 (s).
13C{1H} NMR (176 MHz, CDCl3) δ 147.84 (dm, 241 Hz, C6F5),
140.08, 139.00 (dm, 250 Hz, C6F5), 137.61 (d, 3 Hz), 136.73 (dm,
236 Hz, C6F5), 134.68 (d, 3 Hz), 133.57 (d, 11 Hz), 130.44 (d, 11
Hz), 129.22 (d, 12 Hz), 128.72, 127.57 (d, 16 Hz), 116.90 (d, 80
Hz), 34.14, 31.61, 20.89, 20.41, 14.10 (d, 10 Hz). The resonance
for the ipsoꢀB(C6F4H)3 carbon is likely not observed. Elemental
analysis: Calc.: C 56.68%, H 2.75%, N 1.59%. Exp.: C 56.42%, H
2.65%, N 1.61%
Synthesis of [MesC(Mes2PH)NOB(C6F5)3] (3) PMes2H (31.8 mg,
0.12 mmol, 1 equiv.) and B(C6F5)3 (60.2 mg, 0.12 mmol, 1 equiv.)
were combined in 2 mL CH2Cl2. To the resulting suspension, a
solution of MesCNO (18.9 mg, 0.12 mmol, 1 equiv.) in 1 mL of
CH2Cl2 was added dropwise to the lightꢀyellow solution. After 20
minutes, the solvent was removed in vacuo to yield a faint yellow
gel. Yield: 72.1 mg (65% isolated yield). 1H NMR (400 MHz,
1
CDCl3): δ 9.10 (d, JPH = 510.8 Hz, 1H, PH), 7.46–7.45 (ov, 6H,
Synthesis of [MesC(Ph3P)NOB(C6F4H)3] (6) Solutions of PPh3
(22.0 mg, 0.084 mmol, 1 equiv.) and B(C6F4H)3 (38.5 mg, 0.084
mmol, 1 equiv.) were combined in 3 mL of CH2Cl2, yielding a white
precipitate. To this heterogenous mixture, a solution of MesCNO
(13.6 mg, 0.084 mmol, 1 equiv.) in 1 mL CH2Cl2 was added
dropwise. After complete addition of MesCNO, the reaction
mixture was clear and homogeneous. The solvent removed in
vacuo, and the white solid was recrystallized from
benzene/pentane at room temperature over 24 hours. Yield: 52.5
mg (71 % isolated yield). 1H NMR (400 MHz, CDCl3, 298 K): δ
8.0ꢀ7.1 (b, 15H, P(C6H5)3), 6.7 (m, 3H, B(C6F4H)3), 6.6 (s, 2H, CH,
Mes), 2.16 (s, 3H, pꢀCH3, Mes), 1.93 ppm (s, 6H, oꢀCH3, Mes).
19F{1H} NMR (376 MHz, CDCl3, 298 K): δ ꢀ132.9 (m, oꢀB(C6F4H)3),
ꢀ143.4 ppm (m, mꢀB(C6F4H)3). 31P{1H} NMR (162 MHz, CDCl3,
298 K): δ 5.5 ppm (s); 11B{1H} NMR (128 MHz, CDCl3, 298 K): δ
0.1 (br); 13C{1H} NMR (125 MHz, CDCl3, 298 K): δ 148.21 (dm,
248 Hz, C6F5), 145.34 (dm, 255 Hz, C6F5), 139.95, 139.15, 134.50
(d, 164 Hz), 131.83 (d, 35 Hz), 130.29 (b), 129.62, 128.74 (b),
128.08, 119.17 (dt, 84 Hz, 8 Hz), 102.57 (dt, 166 Hz, 22 Hz),
21.40, 20.14. The resonance for the ipsoꢀB(C6F4H)3 carbon is
likely not observed. Elemental Analysis: Calc.: C 62.75%, H
3.21%, N 1.59%. Exp.: C 62.52%, H 3.40%, N 1.62%.
P(Mes)2), 2.89ꢀ2.84 (ov, 18H, Mes), 2.74ꢀ2.68 (ov, 9H, Mes).
19F{1H} NMR (377 MHz, CDCl3): δ ꢀ133.7 (d, 3JFF = 23.4 Hz, 2F, oꢀ
3
C6F5), ꢀ161.5 (t, JFF = 20.4 Hz, 1F, pꢀC6F5), ꢀ166.4 (m, 2F, mꢀ
1
C6F5). 31P NMR (162 MHz, CDCl3): δ ꢀ39.1 (d, JPH = 518.4
Hz,).11B{1H} NMR (128 MHz, CDCl3): δ 0.1 (s).13C{1H} NMR (126
MHz, CDCl3): δ 148.02 (dm, 240 Hz, C6F5), 144.84 (d, 3 Hz),
143.40 (d, 10 Hz), 140.04, 139.12 (dm, 248 Hz, C6F5), 139.01 (d,
3 Hz), 138.19 (d, 13 Hz), 136.82 (dm, 247 Hz, C6F5), 131.33 (d,11
Hz), 129.28, 126.19(d, 13 Hz), 121.34 (b) 114.18 (d, 80 Hz),
22.59, 22.52, 21.89, 21.25, 21.24, 21.07. The resonance for the
ipsoꢀB(C6F4H)3 carbon is likely not observed. Elemental analysis:
Calc.: C 58.56%, H 3.63%, N 1.48%. Exp.: C 58.09%, H 3.41% N
1.50%.
Synthesis of (MesC(NOB(C6F5)3)Ph2PCH2)2 (4) A 1 mL CH2Cl2
solution of dppe (10.0 mg, 0.025 mmol) was combined with a 1
mL CH2Cl2 solution of B(C6F5)3 (25.0 mg, 0.050 mmol). A white
precipitate formed. To this suspension, a solution of MesCNO (8.0
mg, 0.050 mmol) in 1 mL of DCM was added dropwise. After 20
mins, 5 mL of cold pentane was added with vigorous stirring,
yielding a white precipitate. The product was washed with 10 mL
of pentane and collected by filtration. Yield: 38 mg (87% isolated
yield). Diffraction quality single crystals were grown by vapour
diffusion of hexane into a CH2Cl2 solution. 1H NMR (400 MHz,
CDCl3): δ 8.25 (m, 4H, Mes), 8.01ꢀ7.90 (ov, 20H, P(C6H5)3), 4.01
(br, 4H, CH2), 2.79 (s, 6H, pꢀCH3, Mes), 2.55 (s, 12H, oꢀCH3,
Mes).19F {1H} NMR (377 MHz, CDCl3): δ ꢀ133.8 (d, 3JFF = 24.9 Hz,
Conflicts of interest
There are no conflicts to declare.
3
2F, oꢀC6F5), ꢀ160.0 (t, JFF = 20.7 Hz, 1F, pꢀC6F5), ꢀ165.7 (m, 2F,
mꢀC6F5). 31P {1H} NMR (162 MHz, CDCl3): δ 12.6 (s). 11B {1H}
NMR (128 MHz, CDCl3): δ 0.1 (s). 13C {1H} NMR (126 MHz,
CDCl3): δ 148.07 (dm, 240 Hz, C6F5), 140.31, 139.23(dm, 248 Hz,
C6F5), 137.62 (d, 3 Hz), 137. 03 (dm, 244 Hz, C6F5), 134.90 (d, 3
Acknowledgements
D.W.S. gratefully acknowledges the financial support from
NSERC Canada and the award of Canada Research Chair.
D.W.S. is also grateful for the award of an Einstein Fellowship at
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TU Berlin. K.M.S. and L.E.L. are grateful for the awards of
NSERC of Canada postgraduate scholarships. Dr. Andrew R.
Jupp is gratefully acknowledged for the supply of B(C6F4H)3.
22. E. Otten, R. C. Neu and D. W. Stephan, J. Am. Chem. Soc.,
2009, 131, 9918-9919.
23. T. C. Johnstone, G. N. J. H. Wee and D. W. Stephan,
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24. T. D. Palluccio, E. V. Rybak-Akimova, S. Majumdar, X. Cai,
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6 | J. Name., 2012, 00, 1-3
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TOC
While phosphine oxidation by MesCNO is rapid, FLPs can
be used to intercept 1,3-addition intermediates products.
These species react with BuOK or [Bu4N]F permitting the
t
oxidation to proceed via a process involving borane
dissociation, providing experimental support for the
Cummins mechanism”
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