N,N-addition of frustrated lewis pairs to nitric oxide: An easy entry to a unique family of aminoxyl radicals

Article abstract of DOI:10.1021/ja302652a

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes₂P-C₆H₁₀-B(C₆F₅) ₂] 1b reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical 2b formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical 2b undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product 3b. FLP-NO 2b reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH 3b and FLP-NO-CH₂Ph 4b, both characterized by X-ray diffraction. Structurally related FLPs [Mes₂P-CHR¹-CHR ²-B(C₆F₅)₂] 1c, 1d, and 1e react analogously with NO to give the respective persistent FLP-NO radicals 2c, 2d, and 2e, respectively, which show similar HAA and O-functionalization reactions. The FLP-NO-CHMePh 6b derived from 1-bromoethylbenzene undergoes NO-C bond cleavage at 120 °C with an activation energy of E_a = 35(2) kcal/mol. Species 6b induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily available by N,N-cycloaddition of C ₂-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide.

Full text of DOI:10.1021/ja302652a

Article
pubs.acs.org/JACS
N,N-Addition of Frustrated Lewis Pairs to Nitric Oxide: An Easy Entry
to a Unique Family of Aminoxyl Radicals
Muhammad Sajid,† Annika Stute,† Allan Jay P. Cardenas,‡ Brooks J. Culotta,‡ Johannes A. M. Hepperle,†
Timothy H. Warren,*,‡ Birgitta Schirmer,† Stefan Grimme,*,§ Armido Studer,*,† Constantin G. Daniliuc,†
Roland Frohlich,† Jeffrey L. Petersen,∥ Gerald Kehr,† and Gerhard Erker*,†
̈
†Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat, Corrensstrasse 40, 48149 Munster, Germany
̈
̈
̈
‡Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, United States
§Mulliken Center for Theoretical Chemistry, Institut fur Physikalische und Theoretische Chemie, Universitat Bonn, Beringstr. 4,
̈
̈
D-53115 Bonn, Germany
∥Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045, United States
S
* Supporting Information
ABSTRACT: The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes2PC6H10B(C6F5)2] 1b reacts
rapidly with NO to give the persistent FLP-NO aminoxyl radical 2b formed by P/B addition to the nitrogen atom of NO. This
species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT
calculations. The reactive oxygen-centered radical 2b undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene
to give the diamagnetic FLP-NOH product 3b. FLP-NO 2b reacts with toluene at 70 °C in an HAA/radical capture sequence to
give a 1:1 mixture of FLP-NOH 3b and FLP-NOCH2Ph 4b, both characterized by X-ray diffraction. Structurally related FLPs
[Mes2PCHR1CHR2B(C6F5)2] 1c, 1d, and 1e react analogously with NO to give the respective persistent FLP-NO
radicals 2c, 2d, and 2e, respectively, which show similar HAA and O-functionalization reactions. The FLP-NOCHMePh 6b
derived from 1-bromoethylbenzene undergoes NOC bond cleavage at 120 °C with an activation energy of Ea = 35(2) kcal/
mol. Species 6b induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give
polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily
available by N,N-cycloaddition of C2-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide.
undergo cooperative reactions,5,8 which distinguishes them
from classical Lewis-acid- or Lewis-base-catalyzed reactions.
INTRODUCTION
The number of examples of sterically encumbered, persistent
■
The cooperative FLP reactions are then more favorable than
aminoxyl radicals (or nitroxyl radicals or nitroxides, as they are
the sum of their three (or more) hypothetical bilateral reaction
sometimes synonymously called) is steadily increasing, and they
are utilized for diverse applications.1,2 They have long been
parts (i.e., better than additive). It seems that cooperative FLP
behavior often becomes favored by (or may even require) weak
used as essential reagents for controlling free-radical polymer-
ization processes (nitroxide-mediated polymerization, NMP).3
interactions between the bulky Lewis acid/Lewis base
components. Reactive FLPs5,8 have been used extensively in
This leads to marked changes in the properties of the obtained
polymers principally because of a dramatic reduction of their
small-molecule activation or binding.
polydispersities.4 Frustrated Lewis pair (FLP) chemistry is a
Many FLP examples have been shown to react in sometimes
unique ways with dihydrogen,6,9−11 organic carbonyl com-
significant emerging field in the chemical sciences.5 It is
characterized by the modification of pairs of Lewis acids and
Lewis bases such that they become coexistent in solution,6
usually achieved by introducing appropriate steric bulk.7
Frustrated Lewis pairs can have their active Lewis acid/Lewis
base components react separately (i.e., conventionally) with
added substrates, but they typically have the potential to
pounds,12 alkenes,13 alkynes,14 conjugated π-systems,15 and
even CO2 or N2O.17 We recently combined the seemingly
16
unrelated fields of N-oxyl radicals and frustrated Lewis pair
Received: March 19, 2012
Published: May 1, 2012
© 2012 American Chemical Society
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chemistry through capture of NO by an FLP. The intra-
molecular ethylene-bridged frustrated Lewis pair 1a, one of the
most active hydrogen-activating FLPs,18 cleanly reacts with
nitric oxide. The phosphorus Lewis base and the boron Lewis
acid components of FLP 1a add to the nitrogen atom of NO to
give the new heterocyclic “FLP-NO” N-oxyl radical 2a (Scheme
1).19 The new persistent aminoxyl radical 2a was isolated as a
bar) at ambient temperature resulted in precipitation of the
FLP-NO adduct 2b as a pale turquoise solid in 81% yield.
Single crystals obtained by diffusion of pentane into a
dichloromethane solution of 2b allowed X-ray crystal structure
analysis, which confirmed the formation of the new five-
membered heterocycle by N,N-addition of the P/B Lewis pair
to NO.24 The resulting bicyclic framework is trans-fused at the
bridgehead carbon atoms C1 and C2, which is a consequence
of the hydroboration route25 to its precursor 1b (see above and
Scheme 2). Inside the five-membered core of 2b, we find
characteristic bond lengths of P1N1 = 1.719(2) Å and B1
N1 = 1.597(3) Å, along with a P1N1B1 angle of
115.9(1)o.26 The coordination geometry at the nitrogen atom
N1 is trigonal planar (sum of bond angles at N1 = 359.9°). The
N1O1 bond is still relatively short at 1.296(2) Å, which
indicates a substantial degree of NO multiple-bond character,
but it is, of course, much longer than the nitric oxide NO
distance of 1.151 Å.27 The N1O1 distance of 1.296(2) Å in
2b compares well with the NO bond lengths in 2a (see
Scheme 1 and Table 1) and in TEMPO [1.284(8) Å]28 or t-
Bu2NO [gas-phase value = 1.28(2) Å]29 (see Scheme 3).
Scheme 1
pale blue/turquoise solid in good yield and characterized by a
number of physical methods, including X-ray diffraction and
electron paramagnetic resonance (EPR) spectroscopy. Some-
what surprisingly, it is significantly more reactive than the
prototypical TEMPO radical20 in H-atom abstraction (HAA)/
O-functionalization21 reactions. Reaction with suitable organic
hydrocarbon substrates RH (cyclohexene, ethylbenzene)
gives 1:1 mixtures of FLP-NOH 3a and FLP-NO-R at room
temperature. We have now substantially extended these initial
findings by employing a variety of related intramolecular
phosphorus/boron (P/B) FLPs for the capture of nitric oxide,
which has led to a series of novel examples of this new family of
FLP-NO radicals. In this report, we illustrate their syntheses,
structures, and chemical reactivities, which include HAA and O-
functionalization pathways along with their use in NMP. We
also discuss their electronic structures, which rationalize the O-
centered reactivity of NO upon capture by these FLPs.
Table 1. Selected Structural Data for FLP-NO Radicals 2 and
Corresponding Diamagnetic FLP-NOH Compounds 3
a b
,
angle sum
at N
compound
NO
NP
NB
PNB
2a19
3a19
2b
1.296(2)
1.422(2)
1.296(2)
1.430(3)
1.304(2)
1.430(3)
1.325(3)
1.432(3)
1.713(1)
1.632(2)
1.719(2)
1.631(2)
1.707(2)
1.631(2)
1.686(3)
1.624(3)
1.592(2)
1.561(3)
1.597(3)
1.559(4)
1.581(2)
1.564(4)
1.589(5)
1.576(4)
114.3(1)
114.3(1)
115.9(1)
119.9(2)
116.2(1)
119.6(2)
116.1(2)
118.7(2)
360.019
359.819
359.9
360.0
360.1
359.8
359.9
3b
2c
3c
2d
3e
359.9
a
b
For the structural formulas, see Schemes 2−4 and Figure 1. Bond
lengths in angstroms, angles in degrees.
Scheme 3
RESULTS AND DISCUSSION
■
Formation and Characterization of the New FLP-NO
Radicals. Dimesitylcyclohexenylphosphane undergoes clean
hydroboration with Pier’s borane [HB(C6F5)2]22 to yield the
intramolecular P/B frustrated Lewis pair 1b, which is a
powerful H2-activating system (Scheme 2).23 It is also potent
toward the capture of NO. Stirring the in situ prepared
frustrated Lewis pair 1b in pentane under a NO atmosphere (2
Scheme 2
Compound 2b exhibits a multiline EPR spectrum at room
temperature in fluorobenzene solution that results from
coupling of the unpaired electron to 14N, 31P, and 11/10B nuclei
(see Table 2 and Figure 2). Simulation of the signal centered at
g = 2.0084 gives A(14N) = 20.4 MHz, A(31P) = 50.5 MHz, and
A(11B) = 8.9 MHz (coupling to 10B neglected). Although these
hyperfine couplings are closely related to those found for 2a
(Table 2),19 A(14N) in these P/B FLP-NO species is
significantly lower than found in the related aminoxyl radicals
TEMPO30 and t-Bu2NO31 (43.5 and 43.3 MHz, respectively, in
toluene).
FLP-NO radical 2b features a UV/vis spectrum characterized
by a long-wavelength absorption at λmax = 708 nm (5.0 M−1
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Figure 3. Vibrational fine structure present in the optical spectrum of
2b (fluorobenzene, room temperature). Numbers that appear as labels
indicate the peak positions in cm−1.
Figure 1. Molecular structure of compound 2b.
Table 2. Spectroscopic Parameters of FLP-NO Radicals 2 in
Table 3. Calculated Mulliken Spin-Density Populations of
FLP-NO Radicals 2 and Calculated Bond Dissociation
Enthalpies (ΔH) of Corresponding FLP-NOH Compounds
a
Fluorobenzene Solution at Room Temperature
bc
d
,
EPR
UV/vis
a
3
compound
g
A(14N)
A(31P)
A(11B)
λ (ε)
2a19
2b
2c
2.0089 18.5 [16.7] 48.5 [46.9] 9.1 [10]
2.0084 20.4 [17.4] 50.5 [46.7] 8.9 [10.1]
2.0085 19.5 [16.9] 50.5 [47.7] 9.0 [10.1]
705 (6.7)
708 (5.0)
712 (4.2)
708 (6.1)
754 (9.1)
2
3
b
b
compound
N•
O•
ΔH(OH) (kcal mol−1
)
a
0.33
0.33
0.32
0.33
0.34
0.44
0.54
0.54
0.54
0.54
0.54
0.50
75.8
74.8
76.0
75.5
73.9
65.0
2d
2e
2.0081 19.5 [17]
50.5 [46.4] 9.1 [10.1]
b
2.0083 20.7 [18.2] 51.0 [47.4] 9.0 [9.5]
c
a
b
For structures, see Schemes 2−4 and Figure 1. EPR parameters
d
c
obtained by simulation. A in megahertz, 10B contribution neglected,
e
d
DFT-calculated values in brackets. λ in nanometers, ε in M−1 cm−1.
TEMPO
a
All values taken from the TPSS-D3/def2-TZVP calculations.
Mulliken spin-density populations (electrons).
b
Consistent with its O-centered radical character, 2b readily
undergoes a hydrogen-atom abstraction (HAA) reaction with
CH bonds of modest strength such as 1,4-cyclohexadiene
[CH bond dissociation energy (BDE) ≈ 76 kcal/mol].39
Addition of this H-atom donor to 2b readily resulted in the
formation of the diamagnetic FLP-NOH compound 3b (see
Scheme 2). It shows typical borate (δ 11B = −4.6) and
phosphonium (δ 31P = +48.9) heteronuclear NMR resonances
and a 1H NMR OH signal at 4.36 (1H, d, 3JPH = 10 Hz). The
X-ray crystal structure analysis (see Figure 4 and Table 1)
Figure 2. X-band EPR spectrum and simulation for 2b (fluoroben-
zene, room temperature).
cm−1). Similarly to 2a, it features vibronic coupling with an
average spacing of 1097(24) cm−1 (see Figure 3). Because the
NO stretch in analogue 2a appears at 1474 cm−1 [ν(15NO) =
1457 cm−1], a value consistent with some NO multiple-bond
character, this lower energy vibration corresponds to the
electronic excited state of 2b expected to have a reduced NO
bond order. For instance, 2a exhibits a vibronic coupling of
1109(11) cm−1 in its optical spectrum, whereas 3a features a
NO single bond with a NO stretch at 1110 cm−1
[ν(15NO) = 1082 cm−1].19
The structural and electronic features of FLP-NO radical 2b
were manifested by DFT calculations (see Table 3). Calculated
Mulliken spin-density populations indicate that the FLP-NO
radical is slightly more oxygen-centered as compared to
TEMPO, with the unpaired electron density strongly biased
toward oxygen (O, 0.54 e−; N, 0.33 e−; see Scheme 3 and Table
3).19 Computed isotropic hyperfine couplings nicely support
the experimental results (Table 2; see the Supporting
Information for further details).
Figure 4. Molecular structure of FLP-NOH compound 3b.
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reveals the NOH unit. Its N1O1 bond is much longer
[1.430(3) Å] than that in the FLP-NO radical [2b, 1.296(3) Å]
from which it was derived. The nitrogen coordination of 3b in
the crystal is again trigonal-planar, but here the P1N1 bond
[1.631(2) Å] is markedly shorter than that in 2b [1.719(2) Å]
and even the B1N1 bond in the [N]OH compound 3b is
slightly shorter [1.559(4) Å] than the BN bond in the radical
2b [1.597(3) Å].
NO capture by C2-bridged P/B FLPs appears general. FLPs
1c and 1d,18 which have each a single aryl (1c) or alkyl (1d)
substituent attached at the C2 bridge, cleanly react with NO to
give the respective FLP-NO radicals (2c, 2d), isolated as
turquoise solids in good yields (2c, 69%; 2d, 63%) (Scheme 4).
All of the FLP-NO radicals 2c−2e readily undergo HAA
reactions with 1,4-cyclohexadiene at room temperature to form
the respective FLP-NOH compounds in good yields (see
Scheme 4; 3c, 63%; 3d, 57%; 3e, 62%). The diamagnetic
products 3c−3e were characterized by NMR spectroscopy. As a
typical example, the methyl-substituted system 3d shows
heteronuclear NMR resonances at δ = −6.3 (11B) and δ =
+52.4 (31P). It features 19F NMR signals of a pair of
diastereotopic C6F5 groups at boron and a pair of
diastereotopic mesityl substituents at phosphorus (for details,
see the Experimental Section and the Supporting Information).
In addition, the [P]CH(CH3)CH2[B] bridge of com-
1
pound 3d gives H NMR signals at δ = 3.39 (1H), δ = 1.17
3
3
(3H, dd, JPH = 19.2 Hz, JHH = 6.8 Hz, CH3), and δ = 1.78/
1.62 (CH2).
Scheme 4
We also characterized the diamagnetic FLP-NOH com-
pounds 3c and 3e by X-ray diffraction. The molecular structure
of compound 3e features the typical elongated NO bond
[1.432(3) Å] found in FLP-NOH species 3a and 3b, as well as
a nonplanar [P]CH2CH(SiMe3)[B] bridging unit (for
structural details of the compounds 3c and 3e, see Table 1 and
the Supporting Information). The [N]OH bond dissociation
energies of FLP-NOH compounds 3a−3e were also calculated
by DFT to provide a measure of the relative radical stabilization
energies of the corresponding FLP-NO radicals 2a−2e.
For this purpose, structural optimizations were performed at
the TPSS32 level using the large Gaussian-AO basis set def2-
TZVP33 and the resolution of identity (RI) approximation,34,35
followed by single-point calculations at the high B2PLYP
level.36 Both methods were enhanced with the recently
developed D3 dispersion correction.37,38 Together with
thermodynamic corrections, this yielded the enthalpy values
given in Table 3, which have an estimated accuracy of 1−2
kcal/mol (compare the Supporting Information for further
details). The obtained values (see Table 3) show that the FLP-
NO radicals are slightly less thermodynamically stabilized than
the TEMPO reference (see Scheme 3): For example, the O
H bond dissociation enthalpy (at 298 K) for 2b is 74.8 kcal/
mol, whereas it is only 65.0 kcal/mol in TEMPO-H. We
rationalize the higher [N]OH dissociation energies in FLP-
NOH species 3a−3e in terms of the higher Mulliken spin-
density populations at O of about 0.54 e− for 2a−2e compared
to 0.50 e− for TEMPO. These effects render the FLP-NO
radicals prepared in this study quite reactive toward H-atom
abstraction (HAA) reactions.
The closely related FLP 1e, which bears a SiMe3
substituent at the bridging carbon atom adjacent to boron,
also reacts readily to form the respective FLP-NO radical 2e,
which was isolated in 67% yield (Scheme 4).These new FLP-
NO species 2c−2e are similar in electronic structure to 2a and
2b, as judged by their EPR spectra as well as supporting DFT
calculations (for further details, see Tables 2 and 3 and the
Supporting Information). The X-ray structure of 2d shows the
typical twist-like conformation40 of the central five-membered
heterocycle featuring the planar tricoordinate coordination
environment at the central nitrogen atom and the nonplanar
arrangement of the CH(CH3)CH2 unit at the “back-
side” of the molecule bridging the Mes2P and B(C6F5)2 units.
The bonding parameters in 2d around nitrogen are similar to
those found for the other FLP-NO radicals (see Table 1): The
N1O1 bond is quite short [1.325(3) Å]. The X-ray structure
of phenyl-substituted FLP-NO radical 2c exhibits similar
metrical parameters (for details, see Table 1 and the Supporting
Information).
Robust FLP-NO Linkage. We see no evidence of exchange
of NO between different FLPs. For instance, stirring FLP-NO
species 2b with FLP 1a for 16 h at room temperature in
fluorobenzene, followed by quenching of any FLP-NO species
present with 1,4-cyclohexadiene, resulted in the exclusive
Scheme 5
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formation of FLP-NOH species 3b (Scheme 5). Similarly,
allowing FLP-NO species 2a to stand in the presence of FLP
1b and then quenching with 1,4-cyclohexadiene provided only
FLP-NOH species 3a. These findings are consistent with initial
calculations that capture of NO is thermodynamically strongly
favored [ΔE = −24.6 kcal/mol and ΔG(298) = −11.9 kcal/mol
for 1a]19 and irreversible under the typical conditions
employed.
FLP-NO O-Centered CH Functionalization Reactiv-
ity. FLP-NO radicals 2 functionalize relatively strong CH
bonds in substrates RH by an HAA/radical capture sequence
to initially form FLP-NOH 3, followed by capture of R• by
another equivalent of FLP-NO radical 2 to give FLP-NOR.
In reactions of 2a and 2c−2e with neat toluene (CH BDE
≈ 90 kcal/mol)39 at 80 °C for ca. 1 h, clean HAA at the
benzylic position took place to form the respective FLP-NOH
compounds 3a and 3c−3e, along with the FLP-NOCH2Ph
products 4a and 4c−4e. Compounds 3 and 4c−4e were
separated by column chromatography, and each compound was
isolated in good yield from these experiments (typically about
40%; maximum yield = 50% because 2 equiv of 2 are required)
(Scheme 6). Compounds 3a/4a have not been separated as yet
(present in a ratio of 1:0.8 as determined by 31P NMR
spectroscopy of the crude reaction mixture).
Figure 5. Molecular structure of FLP-NO-benzyl compound 4d.
the P1N1 and N1B1 bonds are rather short (see Table 4
and Figure 5). In compound 4d, the coordination geometry at
nitrogen deviates only marginally from trigonal-planar.
Compound 4a shows a very similar structure in the solid
state (for details, including a view of the molecular structure,
see the Supporting Information).
Scheme 6
We also reacted the bicyclic FLP-NO radical 2b with toluene.
H-abstraction took place at only 70 °C in toluene solution
(2 h) to give a near-quantitative yield of products 3b and 4b
1
(see Scheme 7). The H NMR spectrum of chiral compound
4b shows the AB spin system of the benzylic OCH2 group at
δ = 4.95/3.77 (2JHH = 9.6 Hz) and a 13C NMR resonance at δ =
76.2. Compound 4b shows the heteronuclear NMR resonances
of its core five-membered ring at δ = +48.7 (31P) and −4.5
(11B) and the signals of the diastereotopic pair of C6F5 rings at
boron, as well as the mesityl substituents at phosphorus. The X-
ray crystal structure analysis of 4b features the central
heterobicyclic core with a trans-fusion between the six- and
five-membered ring systems (see Figure 6). The benzyl group is
oriented away from the heterocyclic ring system [dihedral angle
N1O1C50C51 = (I) 177.0°, (II) 177.3°].
Reaction of FLP-NO radical 2b with excess cyclohexene in
benzene gave the H-abstraction product FLP-NOH 3b in 41%
yield, along with the formal allylic substitution product FLP-
NO(2-cyclohexenyl) 5b (see Scheme 8). The latter could have
been formed by addition of the FLP-NO radical to the
cyclohexene double bond, followed by intermolecular H-
abstraction or, because FLP-NO (2b) is a rather reactive
oxygen-centered radical, by direct allylic H-abstraction followed
by radical recombination.41 From the currently available
experimental data, we cannot distinguish between these two
mechanistic possibilities. Of interest is the generation of an
additional chiral center that results in the formation of a pair of
diastereoisomers (5bA and 5bB) observed in a close-to-
equimolar ratio (combined yield of 40%). The 5bA and 5bB
diastereoisomers were separated by column chromatography
and separately crystallized to allow the determination of their
relative configuration by X-ray crystal structure analysis. In
solution, compounds 5bA and 5bB show similar, yet distinct,
NMR spectra. Each compound features the 1H/13C NMR
signals of the cyclohexylene bridge and the O-bonded
cyclohexenyl substituent.
The reaction of phenyl-substituted FLP-NO 2c with toluene
serves as an illustrative example. After FLP-NO 2c was allowed
to react in neat toluene at 80 °C for 1 h, workup and
chromatographic separation gave FLP-NOH product 3c (42%)
and FLP-NOCH2Ph product 4c (40%) in good isolated
yields. Compound 4c was characterized by C,H,N elemental
analysis and by spectroscopy. It features a 31P NMR resonance
at δ = +50.5 and a 11B NMR signal at δ = −6.1. It shows the
typical sets of 19F NMR signals of a pair of diastereotopic 
1
C6F5 substituents at boron and the H/13C NMR signals of a
diastereotopic pair of mesityl substituents at phosphorus, and
1
we monitored the ABX H NMR spin system of the [P]
CHPhCH2[B] bridge at δ = 4.68, 2.53, and 1.66. The
products 4a, 4d, and 4e show similar spectroscopic features (for
details, see the Experimental Section and the Supporting
Information). The X-ray structure of compound 4d clearly
reveals attachment of the benzyl group at the [N]O oxygen
(see Figure 5). The central five-membered heterocyclic
framework features the typical twist-like conformation with
the staggered CH(CH3)CH2 bridge connecting the
planar PNB unit at the front side of the molecule. Both
Each exhibits the signals of the pairs of diastereotopic mesityl
substituents at phosphorus and C6F5 groups at boron.
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a b
,
Table 4. Selected Structural Data of FLP-NOR Compounds 4, 5, and 6bA
compound
NO
OC
PN
BN
PNB
NOC
angle sum at N
4a
1.431(5)
1.436(2)
1.439(2)
1.432(2)
1.440(3)
1.436(3)
1.440(3)
1.424(4)
1.476(6)
1.453(3)
1.454(3)
1.433(3)
1.469(4)
1.459(4)
1.464(4)
1.466(4)
1.638(4)
1.647(2)
1.645(2)
1.646(2)
1.656(3)
1.646(2)
1.651(2)
1.664(3)
1.580(7)
1.581(3)
1.583(3)
1.581(3)
1.593(4)
1.587(4)
1.590(4)
1.612(5)
116.4(3)
118.0(1)
117.9(1)
116.9(1)
116.1(2)
117.1(2)
116.3(2)
115.2(2)
111.2(4)
110.5(2)
110.6(2)
114.8(2)
113.9(2)
114.6(2)
113.1(2)
116.1(3)
357.1
358.4
358.1
355.5
354.4
357.1
356.6
353.6
4b (molecule A)
4b (molecule B)
4d
5bA
5bB (molecule A)
5bB (molecule B)
6bA
a
b
For the structural formulas, see Schemes 6−9 and Figure 5. Bond lengths in angstroms, angles in degrees.
Scheme 7
whereas the diastereoisomer 5bB shows the respective NMR
resonances at δ = −4.1 (11B) and δ = +51.2 (31P) (for further
details, see the Experimental Section and the Supporting
Information).
In the crystal, compound 5bA shows the typical
heterobicyclo[4.3.0]nonane-derived structural core with a
trans-fusion between the six- and five-membered rings. The
NB and NP bond lengths are in the typical range (see
Table 4). The NO bond is much longer than was found in
the corresponding radical, and the [N]O oxygen atom has the
2-cyclohexenyl group bonded to it. Attachment of the [N]O
moiety to an allylic position of the cyclohexene substituent
creates a new chiral center at the carbon.
The trans-FLP-NO framework features two chiral centers at
its bridgehead carbon atoms of defined relative configuration.
Overall the (racemic) product 5bA can, according to its X-ray
crystal structure analysis, be assigned the relative configuration
of R*,R*,S*-5bA. Compound 5bB contains two crystallo-
graphically independent molecules in the unit cell (see Table
4).
FLP-NO-Mediated Free-Radical Polymerization of
Styrene. Given the robust nature of the FLP-NO moiety, it
was tempting to examine the possibility of controlling radical
polymerization processes of alkenes by these novel types of
aminoxyl radicals derived from the capture of NO by frustrated
Lewis pairs. We initiated a preliminary study involving styrene
polymerization for which we would need a diamagnetic
benzylic FLP-NOCH(R)Ph-type precursor with a suffi-
ciently low OC bond dissociation energy. Because the rigid
nature of the cyclohexenyl P/B backbone could help ensure
stability at high temperatures, we chose the ethylbenzene
derivative 6b to serve as an initiator (see Scheme 9 and Figure
7). In principle, we could have prepared 6b by direct reaction of
2b with ethylbenzene, but this would have resulted in a loss of
50% of the FLP-NO starting material due to the “unproductive”
formation of the stoichiometric FLP-NOH byproduct 3b (see
Figure 6. View of the molecular structure of one of the two
independent molecules of 4b.
Compound 5bA shows the heteronuclear NMR features of the
five-membered core at δ = −4.1 (11B) and δ = +50.8 (31P),
Scheme 8
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Scheme 9
cleavage of the [N]OCHMePh oxygen−carbon bond,
resulting in the formation of the FLP-NO 2b and a benzylic
radical. Each of these radicals quickly abstracts hydrogen atoms
from thiophenol, giving ethylbenzene and FLP-NOH 3b (see
Scheme 10).
Scheme 10
Figure 7. Molecular structure of the diastereoisomer R*,R*,R*-6bA.
Kinetic data were plotted to determine the dissociation rate
constant, kd, of 1.7(5) × 10−4 s−1 at 120 °C. The activation
energy (Ea) for the thermally induced homolytic [N]O
CHMePh oxygen−carbon cleavage in compound 6bA was
calculated using the Arrhenius equation. Assuming an
Arrhenius A factor of 2.4 × 1014 s−1, which is typical for C
O bond homolysis of styryl-derived alkoxyamines,44 we found
an Ea value of 34.9 kcal/mol. This value is significantly larger
than Ea for the CO bond homolysis of styryl-TEMPO, which
lies at 31.8 kcal/mol.44 This observation is in line with the
generally higher reactivity of the FLP-NO-radicals compared to
TEMPO, as reflected by their higher spin density at oxygen and
their larger HO bond dissociation energies in the
corresponding hydroxylamines (see above). We then con-
ducted nitroxide-mediated polymerizations of styrene with
alkoxyamine 6bA as the initiator/regulator (Scheme 11).
Reactions were performed in sealed tubes in neat styrene at
above). Therefore, we chose to prepare 6b by a copper-
mediated redox route in analogy with the respective organic
alkoxy amine systems.42 We treated the FLP-NO radical 2b
with (racemic) 1-bromoethylbenzene, Cu powder, and Cu(II)
triflate in the presence of 4,4′-di-tert-butyl-2,2′-bipyridine (75
°C, 2 days). Workup gave a total yield of 6b of 73%. The new
product was admixed with some FLP-NOH (3b) contami-
nation. Chromatography then gave the samples of the pure 6bA
(R*,R*,R*) and 6bB (R*,R*,S*) diastereoisomers. The 6bA
compound showed characteristic NMR features at δ = −3.2
(11B), δ = +50.3 (31P), and δ = 4.38 (1H, 1H, q, 3JHH = 6.4 Hz,
PhCH), whereas the 6bB diastereoisomer featured the
analogous NMR signals at δ = −4.0 (11B), δ = +51.9 (31P),
3
and δ = 5.15 (1H, 1H, q, JHH = 6.6 Hz, PhCH). The X-ray
structure of compound 6bA features the typical heterobicyclic
framework with a trans-junction between the rings and the 1-
phenylethyl substituent bonded to the FLP-NO oxygen atom
(see Table 4 and Figure 7). The relative configuration of this
diastereoisomer is R*,R*,R*-6bA.
Scheme 11. NMP of Styrene with Alkoxyamine 6bA as the
Initiator/Regulator
Planning to use compound 6bA to initiate the radical
polymerization of styrene,3 we first determined the activation
barrier of the thermally induced homolytic [N]OCHMePh
oxygen−carbon bond cleavage.2c,43 Compound 6bA and 10
equiv of thiophenol were dissolved in deuterated toluene and
sealed in an NMR tube under Ar. At 120 °C, we monitored the
1
conversion of compound 6bA to compound 3b by H NMR
spectroscopy, observing an increase in the intensity of the
[N]OH signal and a decrease in the intensity of the benzylic
H in compound 6bA. This reaction takes place by homolytic
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Table 5. Autopolymerization of Neat Styrene and NMP of Styrene with 0.5% Initiator/Alkoxyamine 6bA: Variation of
Temperature and Polymerization Time
entry
initiator (mol %)
time (h)
temp (°C)
conversion (%)
Mn,th (g/mol)
Mn,exp (g/mol)
PDI
1
2
0.5
0.5
0.5
0.5
−
3
2
3
2
2
120
130
120
130
130
39
98
36
89
36
8200
20000
8800
18100
−
35100
34300
40500
29700
271700
1.35
1.30
1.35
1.34
1.61
a
3
a
4
5
a
Free aminoxyl radical 2b (0.025%) was added.
120 and 130 °C for 2−3 h under argon using 0.5 mol % 6bA.
Conversion to polystyrene was determined gravimetrically, and
the molecular weight and polydispersity index (PDI) of the
polymers were analyzed by size-exclusion chromatography
(SEC) (Table 5). All polymers obtained showed well-defined
molecular weights and PDIs below the theoretical limit of 1.5
that can be obtained for a noncontrolled radical polymerization.
For instance, at 120 °C, we isolated polystyrene (PS) in a
moderate yield (39%) with a number-average molecular weight
(Mn) of 35100 g/mol and a PDI of 1.35 (entry 1). We noted
that, at this temperature, a significant amount of alkoxyamine
6bA remained unreacted. This is in agreement with the high
activation energy measured for the CO bond homolysis in
6bA. Consequently, the experimentally determined Mn value
was far larger than the theoretical calculated Mn value in this
case. Therefore, we repeated polymerization at a higher
temperature (130 °C) for a decreased reaction time of 2 h
(entry 2). Pleasingly, under these conditions, polymerization
occurred with high conversion (98% yield), giving PS with a Mn
of 34300 g/mol and marginally narrower PDI (1.30).
Unreacted alkoxyamine 6bA was no longer identified (entry
2). Note that, at a polymerization temperature of 130 °C,
styrene autopolymerization already contributes to a significant
extent (entry 5). We decided to add free FLP-NO 2b to control
the growth of polymer chains started by the autoinitiation
process (entries 3 and 4). As expected, in the presence of free
aminoxyl radical, conversion slightly decreased. However, the
PDI and Mn values were not significantly altered.
by these readily prepared intramolecular frustrated Lewis pairs
(1) results in a dramatic activation of NO toward HAA and
CH functionalization reactions. Whereas NO is typically a
very poor partner in HAA reactions owing to the modest H
NO bond strength of 47 kcal/mol,39 capture and activation of
NO by the FLP provides a “spin-density Umpolung” toward
the O-atom, dramatically enhancing its reactivity. The resulting
FLP-NO radicals are highly valuable new members of the
important family of aminoxyl radicals. Their easy and
straightforward preparation from readily available FLPs through
simple reaction with NO, their thermodynamic stability, and
their persistence as sterically hindered oxygen-centered radicals
will probably offer many new and interesting uses, especially
because their structures can probably be specifically tailored to
suit the needs of a particular application.
EXPERIMENTAL SECTION
■
General Procedures. All syntheses involving air- and moisture-
sensitive compounds were carried out using standard Schlenk-type
glassware (or in a glovebox) in dry solvents under an atmosphere of
argon. NMR spectra were recorded on a Varian Inova 500
spectrometer (19F, 470 MHz; 11B, 160 MHz; 31P, 202 MHz) and on
a Varian UnityPlus 600 spectrometer (19F, 564 MHz; 11B, 192 MHz;
31P, 243 MHz). For 19F NMR spectra, chemical shifts δ are given
relative to CFCl3 (external reference); for 11B NMR spectra, chemical
shifts δ are given relative to BF3·Et2O (external reference); and for 31
P
NMR spectra, chemical shifts δ are given relative to H3PO4 (85% in
D2O) (external reference). NMR assignments were supported by
additional two-dimensional NMR experiments. Elemental analyses
were performed on an Elementar Vario El III instrument. IR spectra
were recorded on a Varian 3100 FT-IR spectrometer (Excalibur
Series). Melting points and decomposition points were obtained with a
DSC 2010 differential scanning calorimeter (TA Instruments). High-
resolution mass spectrometry (HRMS) was performed on a GTC
Waters Micromass instrument (Manchester, U.K.). Size-exclusion
chromatography (SEC) was carried out with degassed tetrahydrofuran
as the eluent at a flow rate of 1.0 mL/min at room temperature on a
system consisting of a Smartline Pump 1000 solvent delivery system
(Knauer), a set of two PLgel 5 μm MIXED-C columns (300 × 7.5
mm, Polymer Laboratories), and a Knauer refractive index (RI)
differential refractometer detector. Data were analyzed with PSS
WinGPC Compact V.7.20 software (Polymer Standards Service)
based on calibration curves built on polystyrene standards (Polymer
Laboratories Polystyrene Medium MW Calibration Kit S-M-10 to
determine the molecular weight of polystyrene) with peak molecular
weights ranging from 1530 to 1319000 g/mol. For X-ray diffraction,
data sets were collected with a Nonius KappaCCD diffractometer. The
programs used were COLLECT (Nonius B.V., 1998) for data
collection, Denzo-SMN46 for data reduction, Denzo47 for absorption
correction, SHELXS-9748 for structure solution, SHELXL-9749 for
structure refinement, and XP (BrukerAXS, 2000) for graphics.
Thermals ellipsoids are shown with 50% probability, R values are
given for observed reflections, and wR2 values are given for all
reflections.
CONCLUSIONS
■
The use of intramolecular P/B FLPs to capture NO to form the
unique family of versatile, nitroxide-like species 2 seems crucial.
We showed previously that the intermolecular frustrated Lewis
pair t-Bu3P/B(C6F5)3 disproportionates nitric oxide19 to give a
combination of the known t-Bu3POB(C6F5)3 and known
t-Bu3PNNOB(C6F5)3 addition product,17 closely
related to the NO reactivity of phosphanes in the absence of
a Lewis acid.45 By directly enabling the synergistic effects of the
phosphane Lewis base and borane Lewis acid upon initial
contact with NO in solution, our C2-linked intramolecular FLPs
(1) react differently with NO, delivering five-membered
heterocycles reminiscent of nitroxides through N,N-cyclo-
addition of NO.
A combination of experimental and theoretical studies points
to the considerable stability of the FLP-NO linkage that results
in a family of readily isolable, turquoise species 2 that bear
significant resemblance to organic aminoxyl radicals such as
TEMPO. The new FLP-NO radicals 2 exhibit greater oxygen-
centered HAA/O-atom functionalization chemistry as com-
pared to the ubiquitous TEMPO radical, owing to significant
polarization of the unpaired electron density toward the O
atom in this family of FLP-NO species 2. Thus, capture of NO
General Procedure for Synthesis of FLP-NO Radicals 2.
Bis(pentafluorophenyl)borane [HB(C6F5)2] (Pier’s borane) and the
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corresponding alkenylphosphane were dissolved in equivalent amounts
in n-pentane to produce a yellow solution of compound 1. The
solution was cooled to −78 °C, the cooling bath was removed, and
NO gas was added to the flask containing the solution (1.5−2.0 bar,
manometer pressure). The color of the solution changed to greenish,
and a precipitate was formed within a few minutes; after some time,
the color of the reaction mixture changed to brown. After the reaction
mixture had been stirred for 45 min at room temperature, the gas
pressure was released, and the solvent was removed by a filter canula.
The residue was washed with n-pentane and dried under a vacuum to
reveal a turquoise solid. Crystallization was carried out by slow
diffusion of n-pentane into a CH2Cl2 solution of 2 at −35 °C.
2b: Cyclohexenyldimesitylphosphane (643 mg, 1.83 mmol) and
HB(C6F5)2 (632 mg, 1.83 mmol) in n-pentane (50 mL) were stirred
for 1 h at room temperature to produce 1b. Reaction of 1b with NO
(2.0 bar, manometer pressure) gave 2b (1.057 g, 81%, turquoise
crystals). HRMS: calcd for C36H32NOPBF10Na, 749.20471; found,
749.20420. X-ray crystal structure analysis of 2b: formula
C36H32BF10NOP·1/2C5H12, M = 762.48, colorless crystal, 0.13 ×
0.10 × 0.07 mm, a = 8.6599(3) Å, b = 10.7452(8) Å, c = 21.1333(6) Å,
α = 84.775(4)°, β = 82.784(2)°, γ = 86.802(5)°, V = 1940.86(17) Å3,
ρcalc = 1.305 g cm−3, μ = 1.326 mm−1, empirical absorption correction
ρcalc = 1.420 g cm−3, μ = 0.197 mm−1, empirical absorption correction
(0.971 ≤ T ≤ 0.994), Z = 2, triclinic, space group P1 (No. 2), λ =
̅
0.71073 Å, T = 223(2) K, ω and φ scans, 8853 reflections collected
( h, k, l), [(sin θ)/λ] = 0.59 Å−1, 5962 independent (Rint = 0.043)
and 4428 observed reflections [I > 2σ(I)], 460 refined parameters, R =
0.079, wR2 = 0.170, max (min) residual electron density = 0.36
(−0.33) e Å−3, hydrogen atoms calculated and refined as riding atoms.
[For more details on the paramagnetic FLP-NO radicals, see the
Supporting Information.]
General Procedure for the Preparation of Compounds 3.
Compound 2 was dissolved in benzene to produce a green solution.
1,4-Cyclohexadiene (about 10 equiv) was added to the solution, and
the green color instantly disappeared. The solvent was removed under
a vacuum, the obtained residue was dissolved in CH2Cl2, and slow
diffusion of n-pentane into this solution at −35 °C gave compound 3
as a colorless crystalline solid.
3b: 1,4-Cyclohexadiene (0.20 mL, 2.11 mmol) and 2b (151 mg,
0.20 mmol) in benzene gave a colorless solution that was crystallized
to produce compound 3b (103 mg, 68%). The obtained crystals were
1
suitable for an X-ray crystal structure analysis. H NMR (600 MHz,
CDCl3, 298 K): δ = 7.03 (d, 4JPH = 4.0 Hz, 1H, m-MesA), 6.94 (s, 1H,
m-MesB), 6.86 (s, 1H, m′-MesA), 6.75 (d, 4JPH = 4.0 Hz, 1H, m′-MesB),
2
(0.846 ≤ T ≤ 0.913), Z = 2, triclinic, space group P1 (No. 2), λ =
4.34 (t, J = 8.5 Hz, 1H, OH), 3.06 (qm, JPH ≈ 3JHH ≈ 3JHH ≈ 12 Hz,
̅
1.54178 Å, T = 223(2) K, ω and φ scans, 20588 reflections collected
( h, k, l), [(sin θ)/λ] = 0.60 Å−1, 6626 independent (Rint = 0.050)
and 5437 observed reflections [I > 2σ(I)], 502 refined parameters, R =
0.049, wR2 = 0.133, max (min) residual electron density = 0.69
(−0.26) e Å−3, hydrogen atoms calculated and refined as riding atoms.
2c: 1-Phenylethenyldimesitylphosphane (500 mg, 1.34 mmol) and
HB(C6F5)2 (465 mg, 1.34 mmol) in n-pentane were stirred for 30 min
at room temperature to produce 1c. Reaction of 1c with NO (1.5 bar,
manometer pressure) gave 2c (687 mg, 69%, green crystals). HRMS:
calcd for C38H30NOPBF10Na, 771.18971; found, 771.18998. X-ray
crystal structure analysis of 2c: formula C38H30BF10NOP, M = 748.41,
colorless crystal, 0.42 × 0.25 × 0.17 mm, a = 11.6585(3) Å, b =
16.4256(5) Å, c = 18.7671(4) Å, β = 106.113(1)°, V = 3452.68(16)
Å3, ρcalc = 1.440 g cm−3, μ = 1.485 mm−1, empirical absorption
correction (0.574 ≤ T ≤ 0.786), Z = 4, monoclinic, space group P21/c
(No. 14), λ = 1.54178 Å, T = 223(2) K, ω and φ scans, 27234
reflections collected ( h, k, l), [(sin θ)/λ] = 0.60 Å−1, 5999
independent (Rint = 0.045) and 5419 observed reflections [I > 2σ(I)],
475 refined parameters, R = 0.043, wR2 = 0.116, max (min) residual
electron density = 0.29 (−0.30) e Å−3, hydrogen atoms calculated and
refined as riding atoms.
1H, 1-H), 2.65 (s, 3H, o-CH3MesA), 2.49 (s, 3H, o-CH3MesB), 2.33
(m, 1H, 5-H), 2.31 (s, 3H, p-CH3MesA), 2.27 (s, 3H, p-CH3MesB),
2.17 (dm, 3JHH ≈ 12 Hz, 1H, 2-H), 2.00 (s, 3H, o′-CH3MesA), 1.89 (br
t, J ≈ 12 Hz, 1H, 6-H), 1.74 (s, 3H, o′-CH3MesB), 1.69 (br m, 1H, 3-
H), 1.62 (m, 1H, 4-H), 1.19 (m, 1H, 3-H′), 1.15 (m, 1H, 4-H′), 1.09
(m, 1H, 2-H′), 0.86 (m, 1H, 5-H′). 13C{1H} NMR (126 MHz,
CDCl3,298 K): δ = 144.7 (d, 2JPC = 7.4 Hz, o′-MesA), 144.2 (d, 2JPC
=
6.8 Hz, o-MesB), 143.1 (d, 5JPC = 2.8 Hz, p-MesA), 142.4 (d, 5JPC = 2.9
2
2
Hz, p-MesB), 140.4 (d, JPC = 12.5 Hz, o-MesA), 139.6 (d, JPC = 14.2
Hz, o′-MesB), 132.5 (d, 3JPC = 10.2 Hz, m-MesB), 131.9 (d, 3JPC = 12.3
Hz, m-MesA), 131.6 (d, 3JPC = 11.0 Hz, m′-MesA), 130.7 (d, 3JPC = 11.6
Hz, m′-MesB), 122.6 (d, 1JPC = 78.2 Hz, i-MesB), 121.9 (d, 1JPC = 86.9
Hz, i-MesA), 46.6 (d, 1JPC = 58.8 Hz, C-1), 35.6 (br, C-6), 30.1 (d, 3JPC
= 12.2 Hz, C-5), 26.5 (C-2), 26.4 (br, C-4), 26.2 (d, 3JPC = 14.6 Hz, C-
3), 24.3 (br, o-CH3MesA), 24.0 (d, JPC = 3.9 Hz, o′-CH3MesA), 23.4
3
(d, 3JPC = 3.9 Hz, o′-CH3MesB), 21.02 (d, 5JPC = 1.4 Hz, p-CH3MesA),
20.97 (d, JPC = 1.4 Hz, p-CH3MesB), 20.6 (d, JPC = 4.5 Hz, o-
CH3MesB), (C6F5 not listed). 11B{1H} NMR (192 MHz, CDCl3, 298
K): δ = −4.6 (ν1/2 ≈ 150 Hz). 31P{1H} NMR (243 MHz, CDCl3, 298
K): δ = 48.9 (ν1/2 ≈ 20 Hz). 19F NMR (564 MHz, CDCl3, 273 K,): δ
5
3
A
= −127.0 (m, 1F, o-C6F5 ), −128.7 (m, 1F, o-C6F5B), −134.0 (m, 1F,
B
A
3
2d: Propenyldimesitylphosphane (140 mg, 0.45 mmol) and
HB(C6F5)2 (156 mg, 0.45 mmol) in n-pentane (30 mL) were stirred
at room temperature for 30 min to produce 1d. Reaction of 1d with
NO (2.0 bar, manometer pressure) gave 2d (205 mg, 63%, green
crystals). HRMS: calcd for C33H28NOPBF10H, 687.19147; found,
687.19091. X-ray crystal structure analysis of 2d: formula
C33H28BF10NOP, M = 686.34, colorless crystal, 0.17 × 0.07 × 0.02
mm, a = 7.9245(5) Å, b = 10.6961(5) Å, c = 18.9336(19) Å, α =
o′-C6BF5 ), −145.5 (m, 1F, o′-C6F5 ), −159.6 (t, JFF = 43.4 Hz, 1F, p-
3
A
C6F5A), −159.7 (t, JFF = 43.6 Hz, 1F, p-C6F5 ), −163.5 (m, 1F, m-
B
A
C6F5 ), −163.9 (m, 1F, m-C6F5 ), −164.7 (m, 1F, m′-C6F5 ), −165.0
B
(m, 1F, m′-C6F5 ). HRMS: calcd for C36H33NOPBF10H, 728.23122;
found, 728.23141. IR (KBr): υ′/cm−1 = 3522 (s, NO-H), 2936 (m),
1641 (s), 1605 (s), 1514 (m), 1382 (s), 1278 (m). X-ray crystal
structure analysis of 3b: formula C36H33BF10NOP·C5H12, M = 799.56,
colorless crystal, 0.35 × 0.18 × 0.08 mm, a = 17.4646(2) Å, b =
10.6452(1) Å, c = 20.7316(2) Å, β = 91.147(1)°, V = 3853.53(7) Å3,
ρcalc = 1.378 g cm−3, μ = 1.359 mm−1, empirical absorption correction
(0.648 ≤ T ≤ 0.899), Z = 4, monoclinic, space group P21/n (No. 14),
λ = 1.54178 Å, T = 223(2) K, ω and φ scans, 41168 reflections
collected ( h, k, l), [(sin θ)/λ] = 0.60 Å−1, 6761 independent (Rint
= 0.069) and 5252 observed reflections [I > 2σ(I)], 507 refined
parameters, R = 0.058, wR2 = 0.167, max (min) residual electron
density = 0.75 (−0.42) e Å−3, hydrogen atoms calculated and refined
as riding atoms.
79.222(4)°, β = 79.203(7)°, γ = 88.914(3)°, V = 1548.40(2) Å3, ρcalc
=
1.472 g cm−3, μ = 1.597 mm−1, empirical absorption correction (0.773
≤ T ≤ 0.969), Z = 2, triclinic, space group P1 (No. 2), λ = 1.54178 Å,
̅
T = 223(2) K, ω and φ scans, 15934 reflections collected ( h, k, l),
[(sin θ)/λ] = 0.60 Å−1, 5086 independent (Rint = 0.064) and 3488
observed reflections [I > 2σ(I)], 431 refined parameters, R = 0.054,
wR2 = 0.128, max (min) residual electron density = 0.22 (−0.24) e
Å−3, hydrogen atoms calculated and refined as riding atoms.
2e: E-(2-Trimethylsilyl)ethenyldimesitylphosphane (500 mg, 1.36
mmol) and HB(C6F5)2 (470 mg, 1.36 mmol) in toluene (25 mL) were
heated to 80 °C for 30 min. The solvent was removed under a vacuum
to produce 1e, which was dissolved in n-pentane (30 mL). Reaction of
1e with NO (1.5 bar, manometer pressure) gave 2e (670 mg, 67%,
turquoise crystals). HRMS: calcd for C35H34NOPBF10SiNa,
767.19729; found, 767.19913. X-ray crystal structure analysis of 2e:
formula C35H34BF10NOPSi, M = 744.50, colorless crystal, 0.15 × 0.09
× 0.03 mm, a = 10.5468(3) Å, b = 10.6122(3) Å, c = 17.4537(7) Å, α
= 86.333(1)°, β = 85.007(1)°, γ = 63.541(2)°, V = 1741.55(10) Å3,
3c: 1,4-Cyclohexadiene (0.1 mL, 1.06 mmol) and compound 2c (96
mg, 0.13 mmol) in benzene gave a colorless solution that was
crystallized to give compound 3c (61 mg, 63%). The obtained crystals
were suitable for X-ray crystal structure analysis. 1H NMR (500 MHz,
CD2Cl2, 183 K) (selected resonances): δ = 7.29 (d, 3JHH = 7.4 Hz, 1H,
o-Ph), 7.15 (t, 3JHH = 7.4 Hz, 1H, m-Ph), 7.09 (t, 3JHH = 7.4 Hz, 1H, p-
Ph), 6.83 (t, 3JHH = 7.4 Hz, 1H, m′-Ph), 6.11 (d, 3JHH = 7.4 Hz, 1H, o′-
Ph), 4.79 (br m, 1H, NOH), 4.56 (br m, 1H, 1-H), 2.27 (m, 1H, 2-H),
3
1.81 (br dm, JPH ≈ 44 Hz, 1H, 2-H′). 13C{1H} NMR (126 MHz,
10164
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Article
CD2Cl2, 183 K) (selected resonances): δ = 137.2 (i-Ph), 128.9 (d, 3JPC
= 7.1 Hz, o-Ph), 127.9 (o′-Ph), 127.4 (m′-Ph), 127.3 (m-Ph), 127.1 (p-
Ph), 48.4 (d, 1JPC = 56.2 Hz, C-1), 29.9 (br, C-2). 11B{1H} NMR (192
MHz, CD2Cl2, 253 K): δ = −4.6 (ν1/2 ≈ 250 Hz). 31P{1H} NMR (243
MHz, CD2Cl2, 253 K): δ = 45.0 (ν1/2 ≈ 80 Hz). 19F NMR (470 MHz,
4a: Compound 2a (40.0 mg, 0.059 mmol) was dissolved in toluene
(5 mL). After the solution had been stirred at room temperature for 4
days, the solvent of the colorless solution was removed in vacuo to give
a mixture of 3a (56%) and 4a (44%) (determined by 31P NMR).
Crystals of compound 4a suitable for X-ray crystal structure analysis
were obtained by slow diffusion of n-pentane into a solution of the
crude reaction mixture in CH2Cl2. HRMS: calcd for C39H33BF10NOP
A
CD2Cl2, 183 K): δ = −129.8, −132.7 (each m, each 1F, o′-, o-C6F5A),
B
−132.5, −143.5 (each br, each 1F, o′-, o-C6F5 ), −159.6 (1F, p-C6F5 ),
1
+ Na+, 786.21253 g/mol; found, 786.21373 g/mol. H NMR (500
B
−160.5 (1F, p-C6F5 ), −163.9, −164.2 (each br, each 1F, m-, m′-
B
A
MHz, CDCl3, 228 K) (selected resonances): δ = 5.10, 3.79 (each br,
C6F5 ), −164.3, −164.7 (each br, each 1F, m-, m′-C6F5 ). Elemental
analysis: calcd for C38H31NOPBF10, C, 59.53; H, 4.44; N, 1.93; found,
C, 60.19; H, 4.98; N, 1.69. IR (KBr): υ′/cm−1 = 3545 (s, NO-H), 1642
(s), 1605 (s), 1513 (s), 1458 (s), 1277 (s), 1101 (s). X-ray crystal
structure analysis of 3c: formula C38H31BF10NOP, M = 749.42,
colorless crystal, 0.33 × 0.22 × 0.02 mm, a = 10.8277(2) Å, b =
11.4950(3) Å, c = 14.8377(3) Å, α = 87.546(1)°, β = 73.836(1)°, γ =
82.013(2)°, V = 1756.54(7) Å3, ρcalc = 1.417 g cm−3, μ = 1.459 mm−1,
empirical absorption correction (0.644 ≤ T ≤ 0.971), Z = 2, triclinic,
P
each 1H, CH2O), 3.15, 2.43 (each br, each 1H, CH2 ), 1.85, 1.34 (each
B
br, each 1H, CH2 ). 13C{1H} NMR (126 MHz, CDCl3, 228 K)
1
(selected resonances): δ = 76.4 (br, CH2O), 30.5 (d, JPC = 66.5 Hz,
P
B
CH2 ), 19.7 (br, CH2 ). 11B{1H} NMR (160 MHz, CDCl3, 228 K): δ
= −6.1 (ν1/2 ≈ 1200 Hz). 31P{1H} NMR (202 MHz, CDCl3, 228 K): δ
= 46.4 (ν1/2 ≈ 10 Hz). 19F NMR (470 MHz, CDCl3, 298 K): δ =
−131.8 (br, 2F, o-C6F5), −160.4 (br, 1F, p-C6F5), −165.2 (br m, 2F,
m-C6F5). X-ray crystal structure analysis of 4a: formula
C39H33BF10NOP, M = 763.44, colorless crystal, 0.23 × 0.03 × 0.01
mm, a = 10.9350(12) Å, b = 12.8316(16) Å, c = 16.1354(10) Å, α =
space group P1 (No. 2), λ = 1.54178 Å, T = 223(2) K, ω and φ scans,
̅
22323 reflections collected ( h, k, l), [(sin θ)/λ] = 0.60 Å−1, 5834
independent (Rint = 0.066) and 4380 observed reflections [I > 2σ(I)],
476 refined parameters, R = 0.052, wR2 = 0.129, max (min) residual
electron density = 0.27 (−0.27) e Å−3, hydrogen atoms calculated and
refined as riding atoms.
68.007(4)°, β = 89.889(5)°, γ = 73.784(11)°, V = 2002.3(4) Å3, ρcalc
=
1.266 g cm−3, μ = 1.289 mm−1, empirical absorption correction (0.756
≤ T ≤ 0.987), Z = 2, triclinic, space group P1 (No. 2), λ = 1.54178 Å,
̅
T = 223(2) K, ω and φ scans, 26288 reflections collected ( h, k, l),
[(sin θ)/λ] = 0.60 Å−1, 6673 independent (Rint = 0.092) and 4090
observed reflections [I > 2σ(I)], 484 refined parameters, R = 0.090,
wR2 = 0.224, max (min) residual electron density = 0.27 (−0.39) e
Å−3, hydrogen atoms calculated and refined as riding atoms.
General Procedure for the Preparation of Compounds 4b−
e. Compound 2 was dissolved in toluene (green solution) and heated
to 70−80 °C for 1−2 h, which resulted in a light yellow solution. The
solvent was removed in a vacuum, and the residue was dissolved in a
minimum amount of CH2Cl2 and loaded onto a silica gel column.
Chromatography (CH2Cl2/n-pentane = 1:5) gave first compound 4
followed by compound 3.
3d: 1,4-Cyclohexadiene (0.1 mL, 1.06 mmol) and compound 2d
(92 mg, 0.13 mmol) in benzene gave a colorless solution that was
crystallized to give compound 3d (92 mg, 57%). 1H NMR (600 MHz,
CD2Cl2, 298 K) (selected resonances): δ = 4.73 (t, J = 7.3 Hz, 1H,
3
OH), 3.39 (m, 1H, H-1), 1.78 (m, 1H, H-2), 1.62 (ddd, JPH = 35.0
Hz, 2JHH = 13.9 Hz, 3JHH = 5.2 Hz, 1H, H-2′), 1.17 (dd, 3JPH = 19.2 Hz,
3JHH = 6.8 Hz, 3H, CH3). 13C{1H} NMR (151 MHz, CD2Cl2, 298 K)
1
(selected resonances): δ = 38.9 (d, JPC = 59.1 Hz, C-1), 30.4 (br, C-
2
2), 17.8 (d, JPC = 1.2 Hz, CH3). 11B{1H} NMR (192 MHz, CD2Cl2,
298 K): δ = −6.3 (ν1/2 ≈ 110 Hz). 31P{1H} NMR (243 MHz, CD2Cl2,
298 K): δ = 52.4 (ν1/2 ≈ 40 Hz). 19F NMR (564 MHz, CD2Cl2, 298
4b: 2b (80 mg, 0.11 mmol) in toluene (1.0 mL) at 70 °C for 2 h
gave the compounds 3b (38 mg, 47%) and 4b (41 mg, 45%). Crystals
of 4b suitable for X-ray crystal structure analysis were obtained by slow
A
B
K): δ = −131.3 (m, 2F, o-C6F5 ), −138.1 (m, 2F, o-C6F5 ), −160.7 (t,
A
3
B
3JFF = 20.3 Hz, 1F, p-C6F5 ), −161.5 (t, JFF = 20.2 Hz, 1F, p-C6F5 ),
B
A
1
−165.1 (m, 2F, m-C6F5 ), −165.7 (m, 2F, m-C6F5 ). HRMS: calcd for
C38H31NOPBF10Na, 772.19754; found, 772.19778. IR (KBr): υ′/cm−1
= 3537 (s, NO-H), 1642 (s), 1605 (s), 1513 (s), 1460 (s), 1277 (s),
1104 (s).
diffusion of n-pentane into a CH2Cl2 solution at −35 °C. H NMR
(500 MHz, CD2Cl2, 298 K) (selected resonances): δ = 7.21 (m, 3H,
m-, p-Ph), 6.98 (m, 2 H, o-Ph), 4.95 (dd, 2JHH = 9.6 Hz, 4JPH = 3.5 Hz,
2
2
1H, PhCH2), 3.77 (br d, JHH = 9.6 Hz, 1H, PhCH2), 3.18 (q, JPH
≈
3JHH ≈ 3JHH = 11.8 Hz, 1H, 1-H), 2.32 (m, 1H, 2-H), 2.11 (br m, 1H,
5-H), 1.95 (br m, 1H, 6-H), 1.72 (br, 1H, 3-H), 1.57 (br, 1H, 4-H),
1.24 (m, 1H, 3-H′), 1.16 (m, 1H, 2-H′), 1.13 (m, 1H, 4-H′), 0.72 (m,
1H, H-5′). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K) (selected
resonances): δ = 136.9 (br d, 4JPC = 1.3 Hz, i-Ph), 128.5 (d, 5JPC = 2.1
Hz, o-Ph), 128.3 (m-Ph), 128.0 (p-Ph), 76.2 (m, PhCH2), 46.2 (d, 1JPC
= 59.6 Hz, C-1), 35.8 (br, C-6), 29.9 (br d, 3JPC = 12.0 Hz, C-5), 27.3
3e: 1,4-Cyclohexadiene (0.1 mL, 1.06 mmol) and compound 2e (97
mg, 0.13 mmol) in benzene gave a colorless solution that was
crystallized to give compound 3e (61 mg, 62%). The obtained crystals
were suitable for X-ray crystal structure analysis. 1H NMR (500 MHz,
CD2Cl2, 193 K) (selected resonances): δ = 4.45 (d, J = 7.5 Hz, 1H,
OH), 3.03 (m, 1H, H-1), 2.35 (m, 1H, 1-H′), 1.26 (m, 1H, 2-H),
−0.27 [s, 9H, Si(CH3)3]. 13C{1H} NMR (126 MHz, CD2Cl2, 193 K)
1
(C-2), 26.7 (d, JPC = 14.7 Hz, C-3), 26.5 (br, C-4). 11B{1H} NMR
3
(selected resonances): δ = 31.9 (d, JPC = 62.0 Hz, C-1), 16.1 (br, C-
2), −2.1 [Si(CH3)3]. 11B{1H} NMR (160 MHz, CD2Cl2, 193 K): δ =
−5.2 (ν1/2 ≈ 1000 Hz). 31P{1H} NMR (202 MHz, CD2Cl2, 193 K): δ
= 45.0 (ν1/2 ≈ 15 Hz). 19F NMR (470 MHz, CD2Cl2, 193 K): δ =
(160 MHz, CD2Cl2, 298 K): δ = −4.5 (ν1/2 ≈ 150 Hz). 31P{1H} NMR
(202 MHz, CD2Cl2, 298 K): δ = 48.7 (ν1/2 ≈ 20 Hz). 19F NMR (470
A
MHz, CD2Cl2, 298 K): δ = −126.9 (m, 1F, o-C6F5 ), −127.6 (m, 1F,
A
B
B
B
A
−126.3 (m, 1F, o-C6F5 ), −128.3 (m, 1F, o-C6F5 ), −128.9 (m, 1F, o′-
o-C6F5 ), −133.4 (m, 1F, o′-C6F5 ), −134.1 (m, 1F, o′-C6F5 ), −160.9
B
A
3
3
A
3
C6F5B), −146.9 (m, 1F, o′-C6F5 ), −160.0 (t, JFF = 21.4 Hz, 1F, p-
(t, JFF = 20.4 Hz, 1F, p-C6F5 B), −161.8 (t, JFF = 20.4 Hz, 1F, p-
3
A
B
B
C6F5A), −160.2 (t, JFF = 21.3 Hz, 1F, p-C6F5 ), −163.9 (m, 1F, m-
C6F5 ), −165.6 (m, 1F, m-C6F5 ), −165.7 (m, 1F, m′-C6F5 ), −166.1
A
B
A
A
C6F5 ), −164.4 (m, 1F, m′-C6F5 ), −164.6 (m, 1F, m-C6F5 ), −164.9
(m, 1F, m′-C6F5 ), −166.3 (m, 1F, m-C6F5 ). HRMS: calcd for
C43H39NOPBF10NH4, 835.30409; found, 835.30573. X-ray crystal
structure analysis of 4b: formula C43H39BF10NOP, M = 817.53,
colorless crystal, 0.23 × 0.20 × 0.17 mm, a = 9.5024(1) Å, b =
19.8280(3) Å, c = 21.6374(2) Å, α = 74.247(1)°, β = 81.653(1)°, γ =
78.048(1)°, V = 3821.44(8) Å3, ρcalc = 1.421 g cm−3, μ = 0.157 mm−1,
empirical absorption correction (0.965 ≤ T ≤ 0.974), Z = 4, triclinic,
B
(m, 1F, m′-C6F5 ). HRMS: calcd for C35H35NOPBF10SiH, 746.22381;
found, 746.22491. IR (KBr): υ′/cm−1 = 3526 (s, NO-H), 2952 (br),
1644 (s), 1606 (s), 1515 (m), 1451 (s), 1249 (s), 1110 (m). X-ray
crystal structure analysis of 3e: formula C35H35BF10NOPSi, M =
745.51, colorless crystal, 0.27 × 0.12 × 0.06 mm, a = 9.4443(2) Å, b =
10.7197(2) Å, c = 19.2926(5) Å, α = 82.938(1)°, β = 86.248(1)°, γ =
82.508(1)°, V = 1919.39(7) Å3, ρcalc = 1.290 g cm−3, μ = 0.179 mm−1,
empirical absorption correction (0.953 ≤ T ≤ 0.989), Z = 2, triclinic,
space group P1 (No. 2), λ = 0.71073 Å, T = 223(2) K, ω and φ scans,
̅
37560 reflections collected ( h, k, l), [(sin θ)/λ] = 0.66 Å−1,
17591 independent (Rint = 0.046) and 14151 observed reflections [I >
2σ(I)], 1039 refined parameters, R = 0.059, wR2 = 0.160, max (min)
residual electron density = 0.37 (−0.36) e Å−3, hydrogen atoms
calculated and refined as riding atoms.
space group P1 (No. 2), λ = 0.71073 Å, T = 223(2) K, ω and φ scans,
̅
9146 reflections collected ( h, k, l), [(sin θ)/λ] = 0.59 Å−1, 6597
independent (Rint = 0.035) and 5357 observed reflections [I > 2σ(I)],
461 refined parameters, R = 0.066, wR2 = 0.159, max (min) residual
electron density = 0.30 (−0.28) e Å−3, hydrogen atoms calculated and
refined as riding atoms.
4c: 2c (67 mg, 0.09 mmol) in toluene (1.0 mL) at 80 °C for 1 h
1
gave compounds 3c (28 mg, 42%) and 4c (30 mg, 40%). H NMR
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(600 MHz, CD2Cl2, 298 K) (selected resonances): δ = 7.22 (m, 3H,
m-, p-PhCH2), 7.17 (m, 1H, p-Ph), 7.05 (br m, 2H, m-Ph), 6.95 (m,
2H, o-PhCH2), 6.77 (br, 2H, o-Ph), 5.19 (d, JHH = 9.4 Hz, 1H,
minimum amount of CH2Cl2 and loaded onto a silica gel column.
Column chromatography (CH2Cl2/n-pentane = 1:10) gave compound
5bA (*RF = 0.34, 16.0 mg), compound 5bB (*RF = 0.31, 16.0 mg), a
mixture of compounds 5bA and 5bB (12.0 mg), and compound 3b
(*RF = 33.0 mg). The combined yield for compound 5 was 40%, and
the yield for compound 3b was 41%.
2
2
3
3
PhCH2), 4.68 (td, JPH ≈ JHH = 14.8 Hz, JHH = 4.2 Hz, 1H, 1-H),
3.70 (d, 2JHH = 9.4 Hz, 1H, PhCH2), 2.53 (td, 2JHH ≈ 3JHH = 14.0 Hz,
3JPH = 8.5 Hz, 1H, 2-H), 1.66 (ddd, JPH = 39.2 Hz, JHH = 14.0 Hz,
3JHH = 4.2 Hz, 1H, 2-H′). 13C{1H} NMR (151 MHz, CD2Cl2, 298 K)
(selected resonances): δ = 138.7 (i-Ph), 136.9 (i-PhCH2), 129.5 (br, o-
Ph), 128.6 (o-PhCH2), 128.3 (m-PhCH2), 128.2 (d, 4JPC = 2.0 Hz, m-
3
2
5bA: 1H NMR (500 MHz, CD2Cl2, 298 K) (selected resonances): δ
3
3
= 5.02 (dt, JHH = 10.5 Hz, J = 3.5 Hz, =CHCH2), 4.73 (dm, JHH
=
10.5 Hz, 1H, =CH), 4.38 (br m, 1H, OCH), 3.11 (m, 1H, 1-H), 2.00
(br, 1H, 6-H). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K) (selected
resonances): δ = 129.8 (=CHCH2), 126.4 (d, J = 10.7 Hz, =CH), 75.1
(dd, 3JPC = 13.0 Hz, J = 2.2 Hz, OCH), 45.3 (d, 1JPC = 60.9 Hz, C-1),
34.9 (br, C-6). 11B{1H} NMR (160 MHz, CD2Cl2, 298 K): δ = −4.1
(ν1/2 ≈ 200 Hz). 31P{1H} NMR (202 MHz, CD2Cl2, 298 K): δ = 50.8
(ν1/2 ≈ 20 Hz). 19F NMR (470 MHz, CD2Cl2, 298 K): δ = −126.5 (m,
5
Ph), 128.1 (p-PhCH2), 128.0 (d, JPC = 2.8 Hz, p-Ph), 76.4 (m,
PhCH2), 49.8 (d, 1JPC = 56.2 Hz, C-1), 33.4 (br, C-2). 11B{1H} NMR
(192 MHz, CD2Cl2, 298 K): δ = −6.1 (ν1/2 ≈ 200 Hz). 31P{1H} NMR
(243 MHz, CD2Cl2, 298 K): δ = 50.5 (ν1/2 ≈ 20 Hz). 19F NMR (470
MHz, CD2Cl2, 183 K): δ = −127.9, −132.03 (each m, each 1F, o-
A
B
C6F5 ), −132.08, −134.8 (each m, each 1F, o-C6F5 ), −159.6 (t, 3JFFB
=
A
B
A
A
3
1F, o-C6F5 ), −126.8 (m, 1F, o-C6F5 ), −133.1 (m, 1F, o′-C6F5A),
21.0 Hz, 1F, p-C6F5 ), −161.4 (t, JFF = 21.0 Hz, 1F, p-C6F5 ),
B
3
A
−135.3 (m, 1F, o′-C6F5 ), −160.8 (t, JFF = 20.8 Hz, 1F, p-C6F5B),
−164.94, −165.5 (each m, each 1F, m-C6F5 ), −164.99 (m, 2F, m-
3
B
B
−162.1 (t, JFF = 20.8 Hz, 1F, p-C6F5 ), −165.6 (m, 1F, m′-C6F5 ),
C6F5 ). Elemental analysis. Calcd for C45H37NOPBF10: C, 64.38; H,
B
A
−165.8 (m, 1F, m-C6F5 ), −166.1 (m, 1F, m′-C6F5 ), −166.5 (m, 1F,
4.44; N, 1.67, Found: C, 63.91; H, 4.25; N, 1.58.
A
m-C6F5 ). Elemental analysis. Calcd for C44H41NOPBF10: C, 62.47; H,
4d: 2d (100 mg, 0.14 mmol) in toluene (5.5 mL) at 80 °C for 90
min gave compounds 3d (37 mg, 37%) and 4d (39 mg, 34%). Crystals
of 4d suitable for X-ray crystal structure analysis were obtained by slow
5.12, N, 1.73. Found: C, 62.22, H, 5.05, N, 1.62. X-ray crystal structure
analysis of 5bA: formula C42H41BF10NOP·C7H8, M = 899.67, colorless
crystal, 0.40 × 0.10 × 0.03 mm, a = 11.0700(1) Å, b = 21.3258(3) Å, c
= 18.9405(3) Å, β = 105.085(1)°, V = 4317.33(10) Å3, ρcalc = 1.384 g
cm−3, μ = 0.146 mm−1, empirical absorption correction (0.944 ≤ T ≤
0.996), Z = 4, monoclinic, space group P21/n (No. 14), λ = 0.71073 Å,
T = 223(2) K, ω and φ scans, 12743 reflections collected ( h, k, l),
[(sin θ)/λ] = 0.59 Å−1, 7454 independent (Rint = 0.030) and 6030
observed reflections [I > 2σ(I)], 573 refined parameters, R = 0.069,
wR2 = 0.194, max (min) residual electron density = 0.57 (−0.50) e
Å−3, hydrogen atoms calculated and refined as riding atoms.
1
diffusion of n-pentane into a CH2Cl2 solution at −35 °C. H NMR
(600 MHz, CD2Cl2, 300 K) (selected resonances): δ = 7.22 (m, 3H,
m-, p-Ph), 6.97 (m, 2H, o-Ph), 5.14, 3.75 (each d, 2JHH = 9.7 Hz, each
1H, PhCH2), 3.44 (m, 1H, 1-H), 1.89 (m, 1H, 2-H), 1.31 (ddd, 3JPH
=
=
2
3
3
37.2 Hz, JHH = 14.1 Hz, JHH = 4.1 Hz, 1H, 2-H′), 1.22 (dd, JPH
18.8 Hz, 3JHH = 6.7 Hz, 3H, CH3). 13C{1H} NMR (151 MHz, CD2Cl2,
298 K) (selected resonances): δ = 137.1 (i-Ph), 128.5 (o-Ph), 128.3
(m-Ph), 128.0 (p-Ph), 76.3 (m, PhCH2), 38.5 (d, 1JPC = 59.7 Hz, C-1),
2
32.4 (br, C-2), 18.4 (d, JPC = 2.2 Hz, CH3). 31P{1H} NMR (121
5bB: 1H NMR (500 MHz, CD2Cl2, 298 K) (selected resonances): δ
= 5.65 (dm, 3JHH = 10.2 Hz, 1H, =CHCH2), 5.35 (dm, 3JHH = 10.2 Hz,
1H, =CH), 4.49 (br, 1H, OCH), 3.11 (m, 1H, 1-H), 2.03 (br, 1H, 6-
MHz, CD2Cl2, 297 K): δ = 51.8 (ν1/2 ≈ 20 Hz). 19F NMR (470 MHz,
A
CD2Cl2, 183 K): δ = −128.1, −132.3 (each m, each 1F, o-C6F5A),
B
−132.4, −135.2 (each m, each 1F, o-C6F5 ), −159.8 (m, 1F, p-C6F5A),
H). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K) (selected resonances):
B
−161.6 (m, 1F, p-C6F5 ), −164.9, −165.5 (each m, each 1F, m-C6F5 ),
1
δ = 131.0 (=CHCH2), 129.0 (=CH), 78.1 (br, OCH), 45.4 (d, JPC
=
B
−165.1, −165.3 (each m, each 1F, m-C6F5 ). HRMS: calcd for
61.4 Hz, C-1), 34.5 (br, C-6). 11B{1H} NMR (160 MHz, CD2Cl2, 298
K): δ = −4.1 (ν1/2 ≈ 150 Hz). 31P{1H} NMR (202 MHz, CD2Cl2, 298
K): δ = 51.2 (ν1/2 ≈ 15 Hz). 19F NMR (470 MHz, CD2Cl2, 298 K): δ
C40H35NOPBF10NH4, 795.27279; found, 795.27519. X-ray crystal
structure analysis of 4d: formula C40H35BF10NOP, M = 777.47,
colorless crystal, 0.23 × 0.15 × 0.07 mm, a = 10.8645(7) Å, b =
12.6096(4) Å, c = 14.7239(7) Å, α = 79.426(2)°, β = 69.495(4)°, γ =
75.950(4)°, V = 1822.01(16) Å3, ρcalc = 1.417 g cm−3, μ = 0.427 mm−1,
empirical absorption correction (0.735 ≤ T ≤ 0.907), Z = 2, triclinic,
A
= −125.1 (m, 1F, o-C6F5 ), −126.6, −135.4 (each m, each 1F, o-
B
A
3
C6F5A), −132.9 (m, 1F, o′-C6F5 ), −160.6 (t, JFF = 20.0 Hz, 1F, p-
3
B
C6F5B), −162.0 (t, JFF = 20.3 Hz, 1F, p-C6F5 ), −165.8 (m, 2F, m-
A
A
C6F5 ), −166.0 (m, 1F, m′-C6F5 ), −166.3 (m, 1F, m-C6F5 ). HRMS:
calcd for C42H41NOPBF10Na, 830.27514; found, 830.27742. X-ray
crystal structure analysis of 5bB: formula C42H41BF10NOP, M =
807.54, colorless crystal, 0.20 × 0.17 × 0.05 mm, a = 11.8794(2) Å, b
= 16.4445(2) Å, c = 19.7784(3) Å, α = 97.514(1)°, β = 94.091(1)°, γ
= 92.643(1)°, V = 3814.78(10) Å3, ρcalc = 1.406 g cm−3, μ = 0.156
mm−1, empirical absorption correction (0.969 ≤ T ≤ 0.992), Z = 4,
space group P1 (No. 2), λ = 1.54178 Å, T = 223(2) K, ω and φ scans,
̅
21742 reflections collected ( h, k, l), [(sin θ)/λ] = 0.60 Å−1, 6246
independent (Rint = 0.043) and 5321 observed reflections [I > 2σ(I)],
494 refined parameters, R = 0.046, wR2 = 0.134, max (min) residual
electron density = 0.23 (−0.29) e Å−3, hydrogen atoms calculated and
refined as riding atoms.
4e: 2e (83 mg, 0.12 mmol) in toluene (1.0 mL) at 80 °C for 1 h
1
triclinic, space group P1 (No. 2), λ = 0.71073 Å, T = 223(2) K, ω and
gave compounds 3e (34 mg, 34%) and 4e (36 mg, 32%). H NMR
̅
φ scans, 18493 reflections collected ( h, k, l), [(sin θ)/λ] = 0.59
Å−1, 13109 independent (Rint = 0.033) and 10674 observed reflections
[I > 2σ(I)], 1031 refined parameters, R = 0.064, wR2 = 0.148, max
(min) residual electron density = 0.39 (−0.31) e Å−3, hydrogen atoms
calculated and refined as riding atoms.
(600 MHz, CD2Cl2, 253 K) (selected resonances): δ = 7.23 (m, 3H,
2
m-, p-Ph), 6.96 (m, 2H, o-Ph), 4.79, 3.78 (each dm, JHH = 8.7 Hz,
each 1H, PhCH2), 3.28 (m, 1H, 1-H), 2.41 (br m, 1H, 1-H′), 1.42 (br,
1H, 2-H), −0.33 [s, 9H, Si(CH3)3]. 13C{1H} NMR (151 MHz,
CD2Cl2, 253 K) (selected resonances): δ = 135.6 (i-Ph), 129.1 (o-Ph),
128.2 (m-, p-Ph), 76.6 (m, PhCH2), 31.8 (d, 1JPC = 63.1 Hz, C-1), 17.0
(br, C-2), −2.4 [Si(CH3)3]. 11B{1H} NMR (160 MHz, CD2Cl2, 298
K): δ = −4.7 (ν1/2 ≈ 150 Hz). 31P{1H} NMR (202 MHz, CD2Cl2, 298
K): δ = 44.9 (ν1/2 ≈ 50 Hz). 19F NMR (564 MHz, CD2Cl2, 253 K): δ
Synthesis of Compound 6. Compound 2b (109 mg, 0.150
mmol), Cu powder (10.0 mg, 0.167 mmol), 4,4′-di-tert-butyl-2,2′-
bipyridyl (2.0 mg, 0.004 mmol), and copper(II) trifluoromethanesul-
fonate (2.0 mg, 0.006 mmol) were suspended in benzene, and 1-
bromoethylbenzene was added. The resulting greenish reaction
mixture was degassed by freeze−pump−thaw cycles and heated to
75 °C under argon for 2 days. The rown liquid part of the reaction
mixture was taken up by glass pipet and loaded as such onto a silica gel
column. Column chromatography (CH2Cl2/n-pentane = 1:5) gave
compound 6bA (RF = 0.48, 35 mg), compound 6bB (RF = 0.41, 38
mg), a mixture of compounds 6bA and 6bB (18 mg), and compound
3c (RF = 0.22, 18 mg). The total yield for compound 6 was 73%.
6bA: 1H NMR (600 MHz, CD2Cl2, 298 K) (selected resonances): δ
= 4.38 (q, 3JHH = 6.4 Hz, 1H, PhCH), 3.22 (m, 1H, H-1), 1.94 (m, 1H,
6-H), 1.22 (br, 3H, CH3). 13C{1H} NMR (151 MHz, CD2Cl2, 298 K)
A
B
= −126.1 (m, 1F, o′-C6F5 ), −126.3 (m, 1F, o-C6F5 ), −130.8 (m, 1F,
A
B
3
o-C6F5 ), −132.7 (m, 1F, o′-C6F5 ), −160.6 (t, JFF = 20.6 Hz, 1F, p-
A
3
B
C6F5B), −161.1 (t, JFF = 20.7 Hz, 1F, p-C6F5 ), −164.9 (m, 1F, m-
B
A
C6F5 ), −165.1 (m, 1F, m′-C6F5 ), −165.7 (m, 1F, m-C6F5 ), −166.2
(m, 1F, m′-C6F5A). HRMS: calcd for C42H41NOPBF10SiNa,
858.25281; found, 858.25315.
Preparation of Compound 5. The aminoxyl radical 2b (80.0 mg,
0.11 mmol) was dissolved in benzene (1.5 mL), and cyclohexene (0.5
mL, 5.0 mmol) was added. The green color of the reaction mixture
disappeared on stirring at room temperature for 1.5 h. The volatiles
were removed in a vacuum, and the colorless residue was dissolved in a
10166
dx.doi.org/10.1021/ja302652a | J. Am. Chem. Soc. 2012, 134, 10156−10168

Journal of the American Chemical Society
Article
1
(selected resonances): δ = 145.0 (i-Ph), 85.0 (PhCH), 45.5 (d, JPC
=
Petroleum Research Fund (PRF#51971-ND3). G.E. thanks
Boulder Scientific Company and T.H.W. thanks W. R. Grace
for gifts of B(C6F5)3.
61.4 Hz, C-1), 34.4 (br, C-6), 25.8 (m, CH3). 11B{1H} NMR (192
MHz, CD2Cl2, 299 K): δ = −3.2 (ν1/2 ≈ 150 Hz). 31P{1H} NMR (243
MHz, CD2Cl2, 298 K): δ = 50.3 (ν1/2 ≈ 15 Hz). 19F NMR (564 MHz,
A
CD2Cl2, 298 K): δ = −121.0, −134.2 (each m, each 1F, o-C6F5 ),
B
−127.7, −132.6 (each m, each 1F, o-C6F5 ), −161.0 (t, 3JFF = 20.5 Hz,
REFERENCES
■
A
B
1F, p-C6F5 ), −161.6 (t, 3JFF = 20.5 Hz, 1F, p-C6F5 ), −165.5, −165.7
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B
A
(each br, each 1F, m-C6F5 ), −166.6 (m, 2F, m-C6F5 ). HRMS: calcd
for C44H41NOPBF10K, 870.24907; found, 870.24830. X-ray crystal
structure analysis of 6bA: formula C44H41BF10NOP, M = 831.56,
colorless crystal, 0.17 × 0.13 × 0.07 mm, a = 19.6305(5) Å, b =
11.3087(4) Å, c = 18.2740(5) Å, β = 105.963(2)°, V = 3900.30(2) Å3,
ρcalc = 1.416 g cm−3, μ = 0.155 mm−1, empirical absorption correction
(0.974 ≤ T ≤ 0.989), Z = 4, monoclinic, space group Cc (No. 9), λ =
0.71073 Å, T = 223(2) K, ω and φ scans, 14745 reflections collected
( h, k, l), [(sin θ)/λ] = 0.59 Å−1, 5737 independent (Rint = 0.045)
and 5263 observed reflections [I > 2σ(I)], 530 refined parameters, R =
0.051, wR2 = 0.108, max (min) residual electron density = 0.30
(−0.24) e Å−3, hydrogen atoms calculated and refined as riding atoms.
6bB: 1H NMR (600 MHz, CD2Cl2, 298 K) (selected resonances): δ
= 5.15 (q, 3JHH = 6.6 Hz, 1H, PhCH), 3.18 (m, 1H, H-1), 2.08 (br m,
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1H, 6-H), 0.60 (d, JHH = 6.6 Hz, 3H, CH3). 13C{1H} NMR (151
3
MHz, CD2Cl2, 298 K) (selected resonances): δ = 144.7 (d, 4JPC = 1.5
Hz, i-Ph), 80.6 (m, PhCH), 45.4 (d, 1JPC = 60.9 Hz, C-1), 34.8 (br, C-
6), 22.4 (d, JPC = 7.9 Hz, CH3). 11B{1H} NMR (192 MHz, CD2Cl2
4
298 K): δ = −4.0 (ν1/2 ≈ 200 Hz). 31P{1H} NMR (243 MHz, CD2Cl2
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A
K): δ = −122.8, −133.4 (each m, each 1F, o-C6F5 ), −126.6, −135.4
B
3
A
(each m, each 1F, o-C6F5 ), −160.8 (t, JFF = 20.2 Hz, 1F, p-C6F5 ),
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Elemental analysis. Calcd for C43H39NOPBF10: C, 63.17; H, 4.81, N,
1.58. Found: C, 62.64, H, 4.80, N, 1.58.
General Procedure for the Polymerization of Styrene. A heat-
gun-dried Schlenk tube was charged with initiator 6bA and styrene.
The tube was subjected to three freeze−thaw cycles and then sealed.
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ASSOCIATED CONTENT
* Supporting Information
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̈
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Stephan, D. W. Chem. Commun. 2007, 5072−5074. (b) Huber, D. P.;
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spectra, X-ray crystallographic details, H NMR and 13C NMR
1
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spectra, and two-dimensional NMR data of the compounds.
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AUTHOR INFORMATION
Corresponding Author
■
̈
Rieger, B. Angew. Chem. 2008, 120, 6090−6092; Angew. Chem., Int. Ed.
2008, 47, 6001−6003. (b) Chen, D.; Klankermayer, J. Chem. Commun.
2008, 2130−2131. (c) Holschumacher, D.; Bannenberg, T.; Hrib, C.
G.; Jones, P. G.; Tamm, M. Angew. Chem. 2008, 120, 7538−7542;
thw@georgetown.edu; grimme@thch.uni-bonn.de; studer@
uni-muenster.de; erker@uni-muenster.de
Notes
Angew. Chem., Int. Ed. 2008, 47, 7428−7432. (d) Eros, G.; Mehdi, H.;
̈
The authors declare no competing financial interest.
́ ́ ́ ́ ́
Papai, I.; Rokob, T. A.; Kiraly, P.; Tarakanyi, G.; Soos, T. Angew. Chem.
2010, 122, 6709−6713; Angew. Chem., Int. Ed. 2010, 49, 6559−6563.
ACKNOWLEDGMENTS
■
(e) Theuergarten, E.; Schluns, D.; Grunenberg, J.; Daniliuc, C. G.;
̈
Financial support from the Deutsche Forschungsgemeinschaft
(S.G., A.S., and G.E.) and the European Research Council
(G.E.) is gratefully acknowledged. T.H.W. acknowledges
support from the National Science Foundation (CHE-
0957606 to T.H.W., CHE-0840453 to Georgetown for an
EPR spectrometer), as well as the donors of the ACS
Jones, P. G.; Tamm, M. Chem. Commun. 2010, 46, 8561−8563.
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