P-C bond activation chemistry: Evidence for 1,1-carboboration reactions proceeding with phosphorus-carbon bond cleavage

Article abstract of DOI:10.1021/ja1110283

A series of diarylphosphinyl-substituted acetylenes of the type (aryl) ₂P-C≡C-R (aryl = phenyl or mesityl, R = Ph or n-propyl) react with the strongly Lewis acid reagent B(C₆F₅)₃ in toluene at elevated temperatures (70-105 °C) to give the 1,1-carboboration products 4. Treatment of bis(diphenylphosphinyl)acetylene with B(C₆F₅)₃ under analogous conditions proceeded with phosphinyl migration to yield the 1,1-carboboration product 4d, bearing a geminal pair of Ph₂P substituents at one former acetylene carbon atom and a C₆F₅ substituent and the remaining -B(C ₆F₅)₂ group at the other. Prolonged thermolysis of 4d resulted in an intramolecular aromatic substitution reaction by means of Ph₂P attack on the adjacent C₆F₅ ring to yield the zwitterionic phospha-indene derivative 7. The compounds 4a, 4c, 4d, and 7 were characterized by X-ray diffraction.

Full text of DOI:10.1021/ja1110283

ARTICLE
pubs.acs.org/JACS
P-C Bond Activation Chemistry: Evidence for 1,1-Carboboration
Reactions Proceeding with Phosphorus-Carbon Bond Cleavage
Olga Ekkert, Gerald Kehr, Roland Fr€ohlich, and Gerhard Erker*
Organisch-Chemisches Institut der Universit€at M€unster, Corrensstrasse 40, 48149 M€unster, Germany
S Supporting Information
b
ABSTRACT: A series of diarylphosphinyl-substituted acetylenes of the type
(aryl)2P-CtC-R (aryl = phenyl or mesityl, R = Ph or n-propyl) react with
the strongly Lewis acid reagent B(C6F5)3 in toluene at elevated temperatures
(70-105 °C) to give the 1,1-carboboration products 4. Treatment of
bis(diphenylphosphinyl)acetylene with B(C6F5)3 under analogous conditions proceeded with phosphinyl migration to yield the
1,1-carboboration product 4d, bearing a geminal pair of Ph2P substituents at one former acetylene carbon atom and a C6F5
substituent and the remaining -B(C6F5)2 group at the other. Prolonged thermolysis of 4d resulted in an intramolecular aromatic
substitution reaction by means of Ph2P attack on the adjacent C6F5 ring to yield the zwitterionic phospha-indene derivative 7. The
compounds 4a, 4c, 4d, and 7 were characterized by X-ray diffraction.
’ INTRODUCTION
Scheme 1
Being able to selectively attack or cleave unactivated strong
bonds to the element carbon has been of an increasing interest in
methodological developments in chemistry. Much progress has
been achieved in recent years in C-H activation,1 so we by now
have quite a spectrum of methods available to selectively attack
strong, otherwise unactivated carbon-hydrogen bonds to make
their respective positions in carbon-containing frameworks syn-
thetically available.2 Much less attention has been focused on the
chemistry of other nonactivated strong bonds to carbon.3 Quite
recently some interest has arisen in finding new methodologies
for selectively cleaving nonactivated carbon-carbon bonds.4 In
this context we have described some remarkable carbon-carbon
bond cleavage reactions5 induced by very reactive, strongly elec-
trophilic borane reagents.6-8 We had shown that some R-B-
(C6F5)2 reagents (R = CH3 or C6F5) react with internal alkynes
by means of selective 1,1-carboboration reactions9-11 to yield 2.5
In the course of this addition/skeletal rearrangement process, a
strong C(sp)-C(sp3) or C(sp)-C(sp2) linkage becomes cleaved
and an alkyl or aryl group undergoes a formal 1,2-shift to the
adjacent acetylene carbon atom (C2) in order to make room at
its origin (C1) for the boron attachment with concomitant alkyl
or aryl migration from boron to carbon. This remarkable,
thermally induced transformation yields novel alkenyl boranes
which have been employed as very reactive substrates5 in metal-
catalyzed cross-coupling reactions.
phosphinyl-substituted acetylenes with the strongly electrophilic
borane reagent 1 and found that this resulted in clean 1,1-
carboboration reactions with formation of the interesting alk-
enylene-bridged B/P products 4. We here describe these first
representative examples.
’ RESULTS AND DISCUSSION
The bulky P-substituted acetylene dimesityl(phenylethynyl)-
phosphine (3a) was prepared by treatment of lithiated phenyl-
acetylene with Mes2PCl.15 Compound 3a was mixed with the
strongly electrophilic borane B(C6F5)3 (1)6 in toluene solution
and then kept for 6 h at 105 °C. Workup gave the 1,1-car-
boboration product 4a in ca. 60% yield (see Scheme 2). The new
product was characterized spectroscopically, by C,H elemental
analysis, and by an X-ray crystal structure analysis. Suitable single
crystals were obtained at -36 °C from toluene/pentane by the
diffusion method.
We have now tried to extend this work to the activation of
strong carbon-phosphorus σ-bonds (see Scheme 1). There is a
very specific methodology available for P-C bond cleavage,12
but finding novel, rather generally applicable P-C bond activa-
tion proceedures is highly desirable, especially if they can be
coupled with C-C and B-C bond formation, as implied by the
1,1-carboboration methodology. 1-Bora-2-phospha-alkenes of
the type 4 and related systems have been of interest, e.g., to
material science.13,14 We have now reacted a selected series of
Received: December 8, 2010
Published: March 02, 2011
r
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Scheme 2
Scheme 3
Compound 4a features a 11B NMR resonance at ca. δ 1 and a
broad 31P NMR signal at δ 15.2. It shows a broad 13C NMR
signal of the [B]C(2)d carbon atom at δ 161.2 and the cor-
responding dC(1)[P] carbon resonance at δ 146.7 (1JPC
=
46.8 Hz). The 19F NMR resonances of the single carbon bound
C6F5 group occur at δ -136.0 (o), -154.1 (p), and -162.4 (m),
whereas the B(C6F5)2 unit shows a set of 19F NMR signals of
double intensity at δ -126.9 (o), -156.1 (p), and -164.0 (m),
with a chemical shift separation of the meta- and para-19F
resonances [Δδ(m,p)] of 7.9.17
The analogous reaction of B(C6F5)3 (1) with the diphenyl-
phosphinyl-substituted acetylene (3b, see Scheme 2) proceeded
at much lower temperature (6 h, 70 °C) to give the correspond-
ing 1,1-carboboration product 4b that was isolated in 77% yield
after the usual workup procedure. It shows a 31P NMR resonance
at δ þ13.8 and a 11B NMR signal at δ -6. The 19F NMR signals
of the -B(C6F5)2 group show a chemical shift difference of
Δδ(m,p) = 7.5, with the 19F NMR set of signals of the single
carbon bound C6F5 group at δ -138.3 (o), -154.0 (p), and
-161.7 (m). The 13C NMR resonances of the CdC backbone of
the four-membered heterocyclic product 4b occur at δ 161.3
(br, [B]Cd) and δ 143.3 (1JPC = 53.0 Hz, dC[P]), respectively.
We next reacted the alkyl P-substituted acetylene 1-diphenyl-
phosphinyl-1-pentyne (3c) with B(C6F5)3. Like in the 3b/
B(C6F5)3 case described above, the 3c/B(C6F5)3 reaction took
6 h at 70 °C in toluene solution to go to completion. We obtained
a mixture of two products in a ratio of ca. 10:1 that was isolated in
a combined yield of 56%. The major product was identified as the
1,1-carboboration product 4c. The analogous reaction of 1 and
3c in toluene-d8 at 105 °C for 6 h yielded a 5:1 product mixture of
4c and 6 (see Scheme 3).
We obtained single crystals of 4c by diffusion of pentane vapor
into a dichloromethane solution at -36 °C. In the crystal,
compound 4c shows the typical distorted four-membered het-
erocyclic structure (B1-P1 bond length: 2.038(3) Å).17,18 The
former acetylene carbon atom C1 now bears both the n-propyl
substituent and the bulky -PPh2 group, whereas C2 now has a -
C6F5 substituent and the remaining -B(C6F5)2 moiety attached
to it (Figure 2; bond distances C1-C2 1.345(4) Å, C1-P1
1.802(3) Å, C2-B1 1.651(4) Å, angles C2-C1-P1 96.1(2)°,
C1-C2-B1 107.1(2)°, C1-P1-B1 77.8(1)°, C2-B1-P1
78.8(2)°). The sum of C-B-C bonding angles at the boron atom
Figure 1. Molecular structure of the 1,1-carboboration product 4a.
X-ray crystal structure analysis revealed the presence of a
rearranged framework typically resulting from a 1,1-carbobora-
tion reaction. It shows that both of the substituents that were
originally 1,2-positioned in the alkyne C2 framework are now 1,1-
bonded: both the phenyl substituent and the bulky -PMes2
moiety are found bonded to the same carbon atom (C1) of the
product 4a (see Figure 1; bond distances C1-C31 1.479(3) Å,
C1-P1 1.833(2) Å, angles C31-C1-P1 134.5(2)°, C2-C1-
C31 128.0(2)°). The newly introduced components from the
B(C6F5)3 reagent are now found bonded to the adjacent former
acetylene carbon atom C2, namely the C6F5 substituent that was
shifted from boron to carbon and the remaining -B(C6F5)2 unit
(bond distances C2-C41 1.490(3) Å, C2-B1 1.637(3) Å,
angles C41-C2-B1 128.2(2)°, C1-C2-C41 123.5(2)°). The
bulky -B(C6F5)2 and -PMes2 antagonists are found Z-attached
at the remaining C(2)dC(1) double bond (C1-C2 1.349(3) Å).
The boron Lewis acid forms a weak adduct with the adjacent
phosphorus Lewis base (B1-P1 2.115(2) Å). The boron atom
features a slightly pyramidalized geometry of the C2-B(C6F5)2
unit with bond angles C2-B1-C51 114.0(2)°, C2-B1-C61
115.9(2)°, and C51-B1-C61 114.2(2)° (∑ = 344.1°). The
coordination sphere at the phosphorus atom P1 is characterized
by bond angles C1-P1-C21 113.8(1)°, C1-P1-C11 111.9(1)°,
and C11-P1-C21 109.5(1)° (∑ = 335.2°). Overall this has
resulted in the formation of a four-membered heterocyclic
structure16 of the 1,1-carboboration product 4a (angles inside
the four-membered ring: C1-C2-B1 108.1(2)°, C2-C1-P1
97.6(1)°, C1-P1-B1 75.3(1)°, C2-B1-P1 70.0(1)°).
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Figure 2. Molecular structure of compound 4c.
Scheme 4
Figure 3. Molecular structure of the 1,1-carboboration product 4d.
proven. Therefore, we decided to react B(C6F5)3 with bis-
(diphenylphosphinyl)acetylene (3d) to create a case where there
is no choice for the 1,1-carboboration reaction but to take place
with migration of a diphenylphosphinyl group.
Bis(diphenylphosphinyl)acetylene (3d) was synthesized from
trichloroethene, n-butyllithium (3 equiv), and chlorodiphenyl-
phosphine.20 The doubly phosphinyl-substituted alkyne 3d was
then reacted with B(C6F5)3 (1) in toluene at 80 °C. It took ca. 9 h
for the reaction to go to completion. The 1,1-carboboration
product 4d was isolated from the reaction mixture in 60% yield
(Scheme 4). The product features a pair of 31P NMR signals at
δ þ24.2 (br, [B]P) and -6.3 (JPF = 28 Hz, 2JPP = 10.3 Hz) and a
11B NMR resonance at δ -5. The 13C NMR carbon signals of
B1 amounts to 349.2°, and the sum of C-P-C angles at P1 is
332.8°. Compound 4c shows the typical spectroscopic features of
this class of compounds [e.g., 11B NMR δ -6; 31P NMR δ þ15.2;
13C NMR δ 161.5 [B]Cd, 146.7 (1JPC = 49.1 Hz) dC[P]; for
further details see the Experimental Section and the Supporting
Information].
the C2 bridge occur at δ 173.7 ([B]Cd) and 146.4 (dd, 1JPC
=
53.0 Hz, 32.7 Hz, dC[P]2), and the product also shows the
typical sets of 19F NMR signals of the geminal pair of C6F5
and -B(C6F5)2 groups at carbon atom C2.
We tentatively assign the minor product from this reaction the
structure of compound 6 (see Scheme 3). This is based on its
characteristic spectroscopic features, obtained from the 4c/6
mixture, and on a comparison with a closely related product
(7, see below) that was characterized by X-ray diffraction. We
assume that compound 6 is the product of an intramolecular
nucleophilic substitution reaction,19 formed subsequent to the
1,1-carboboration reaction by attack of the phosphorus nucleo-
phile on the adjacent C6F5 ring. This requires a Z-orientation
between the two groups, as found in the (not directly observed)
1,1-carboboration isomer 5 (see Scheme 3).
Compound 4d was characterized by X-ray diffraction (single
crystals were obtained from pentane/dichloromethane at -36 °C
by the diffusion method). X-ray crystal structure analysis shows
the presence of a geminal pair of diphenylphosphinyl substitu-
ents at the former acetylene carbon atom C1 (Figure 3; bond
lengths C1-P2 1.826(6) Å, C1-P1 1.807(5) Å, angle P1-C1-
P2 125.8(3)°). The phosphorus atom P1 weakly coordinates to
the adjacent boron Lewis acid (B1-P1 2.038(7) Å). The sum of
C-B1-C bond angles at boron is 348.1°, and the sum of C-
P1-C bond angles at its adjacent phosphorus atom P1 is 333.0°,
whereas the sum of C-P2-C angles is 307.0°. The (C6F5)2B
group is bonded to C2 (B1-C2 1.646(8) Å), which also bears a
C6F5 substituent (C2-C51 1.489(7) Å). The four-membered
heterocyclic framework of the 1,1-carboboration product 4d
shows typical bonding parameters of C2-C1 1.367(7) Å and
bond angles C2-C1-P1 95.1(4)°, C1-P1-B1 78.2(2)°, P1-
B1-C2 78.9(3)°, and B1-C2-C1 107.3(4)°.
Compound 6 shows the 19F NMR resonance of the B-F unit
at δ -184.6 (11B NMR signal at δ 0.6), a set of four 19F NMR
signals of the remaining C6F4 ring (δ -123.4, -129.7, -139.1,
-151.2), and a 31P NMR resonance at δ 33.9 (for further details
see the Experimental Section and the Supporting Information).
In the above-described reactions of B(C6F5)3 with the phos-
phinyl-substituted alkynes 3a-c, we assumed that the R2P group
migrated during the course of the 1,1-carboboration sequence.
This is likely from the characteristic overall thermal behavior of
these reactions, but the R2P migration had not strictly been
Compound 4d was shown to undergo a slow isomerization
reaction by means of an intramolecular nucleophilic attack of the
Z-PPh2 group at the adjacent C6F5 substituent. After 3 days at
105 °C in toluene, the product of this intramolecular nucleophilic
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Scheme 5
reactions,9 substrates such as R2P-CtC-H and R2P-CtC-
SiMe3 might be good candidates for future extension of this work.
’ EXPERIMENTAL SECTION
General Procedures. All syntheses involving air- and moisture-
sensitive compounds were carried out using standard Schlenk-type
glassware (or in a glovebox) under an atmosphere of argon. Solvents
were dried with the procedure reported by Grubbs24 or distilled from
appropriate drying agents and stored under an argon atmosphere. NMR
spectra were recorded on a Bruker AC 200 P (1H, 200 MHz; 31P, 81
MHz; 11B, 64 MHz), a Bruker AV 300 (1H, 300 MHz; 13C, 76 MHz; 31P,
122 MHz; 11B, 96 MHz; 19F, 282 MHz), a Bruker AV 400 (1H, 400
MHz; 13C, 101 MHz; 31P, 162 MHz), a Varian Inova 500 (1H, 500 MHz;
13C, 126 MHz; 19F, 470 MHz; 11B, 160 MHz; 31P, 202 MHz), and a
Varian UnityPlus 600 (1H, 600 MHz; 13C, 151 MHz; 19F, 564 MHz; 11B,
192 MHz; 31P, 243 MHz). For 1H and 13C NMR, chemical shifts δ are
given relative to TMS and referenced to the solvent signal. For 19F
NMR, chemical shifts δ are given relative to CFCl3 (external reference).
Figure 4. Projection of the product 7 formed by an intramolecular
nucleophilic substitution reaction of 4d.
aromatic substitution reaction, 7,19 was isolated as orange crystals
in 35% yield. The compound was characterized by X-ray diffrac-
tion. It features a phospha-indene-type framework with phos-
phorus atoms P1 and P2 bonded to the same carbon atom
(Figure 4; bond lengths P1-C2 1.794(2) Å, P2-C2 1.845(2) Å,
angle P1-C2-P2 124.3(1)°). Phosphorus atom P1 has attacked
the C6F5 group that is bonded to the ring carbon C3 (C3-C52
1.495(3) Å) and has replaced a fluorine atom (P1-C51 1.790(2)
Å). The single fluorine atom is found bonded to boron (B1-F1
1.424(3) Å). The resulting -B(F)(C6F5)2 substituent is bonded
to the ring carbon atom C3 (C3-B1 1.644(3) Å).
For 11B NMR, chemical shifts δ are given relative to BF3 Et2O (external
3
reference). For 31P NMR, chemical shifts δ are given relative to H3PO4
(85% in D2O) (external reference). NMR assignments were supported
by additional 2D NMR experiments. Elemental analyses were performed
on a Elementar Vario El III. IR spectra were recorded on a Varian 3100
FT-IR (Excalibur Series). Melting points were obtained with a DSC
2010 (TA Instruments). HRMS was recorded on GTC Waters Micro-
mass (Manchester, UK). For X-ray crystal structure analyses, data sets
were collected with a Nonius KappaCCD diffractometer. Programs
used: data collection, COLLECT (Nonius B.V., 1998); data reduction,
Denzo-SMN;25a absorption correction, Denzo;25b structure solution,
SHELXS-97;25c structure refinement, SHELXL-97;25d and graphics, XP
(BrukerAXS, 2000). Thermals ellipsoids are shown with 50% prob-
ability, R -values are given for the observed reflections, and wR2 values
are given for all reflections.
In solution, compound 7 shows a pair of 31P NMR signals at
δ 39.1 and -2.5 (2JPP = 7 Hz), a 11B NMR resonance at δ 0.3,
and the signal of the single fluorine atom at boron at δ -171.5
(for further details see the Experimental Section and Supporting
Information).
’ CONCLUSIONS
There is increasing evidence that the 1,1-carboboration reac-
tion is developing from a reaction that is amenable for some
special organometallic substrates to a rather general and useful
synthetic tool for generating novel carbon frameworks from
simple, nonactivated acetylenes that bear ordinary organic sub-
stituents.5,9-11 This development was made possible by using the
electrophilic C6F5 containing borane 1 and related reagents. We
have now shown that this remarkable reaction can even be
extended to phosphinyl-substituted alkynes. The reaction requires
somewhat forcing conditions, similar to those of the related 1,1-
carboboration reactions of simple internal alkynes that we had
recently reported, but it proceeds cleanly and gives the respective
phosphorus-containing 1,1-carboboration products 4 in good
yields (Scheme 5).21
Previous examples of the alkenylene-bridged P/B systems 4
had often been synthezised by boron-based nucleophilic sub-
stitution routes, employing specifically22,23 substituted alkynyl
borate reagents 8 (Scheme 5). Our new 1,1-carboboration route
to these interesting systems now provides a phosphorus based
principal synthetic alternative which will probably serve to
rapidly expand the scope of utilization these interesting com-
pounds in the material sciences and for synthetic purposes.5 In
view of previous results9 showing that 1-alkynes and, e.g.,
trimethylsilylacetylenes very easily undergo 1,1-carboboration
Materials. Dimesitylchlorophosphine,15 diphenyl(phenylethynyl)-
phosphine (3b)26 and B(C6F5)3 (1)6 were prepared according to
literature procedures. The compounds 3a,22a 3c27 and 3d20 were pre-
pared in a similar fashion to 3b.
Synthesis of Compound 4a. B(C6F5)3 (1) (0.400 g, 0.780 mmol)
and 3a (0.290 g, 0.780 mmol) were dissolved in toluene (20 mL) and stirred
for 6 h at 105 °C. While the reaction mixture was stirred overnight at room
temperature, a white solid precipitated. After isolation via cannula filtration,
the residue was washed twice with pentane (15 mL). Drying under vacuum
gave the product (0.406 g, 0.460 mmol, 59%) as a white solid. Crystals
suitable for X-ray crystal structure analysis were grown by slow diffusion of
pentane into a solution of 4a in toluene at -36 °C. Anal. Calcd for
C44H27BF15P: C, 59.89; H, 3.08. Found: C, 59.84; H, 3.14. IR (KBr):
~ν/cm-1 = 3406 (br m), 3026 (w), 2934 (w), 2359 (m), 1639 (s), 1518 (s),
1456 (s), 1287 (m), 1094 (s), 980 (s), 761 (m), 506 (m). Decomp (DSC):
231 °C. 1H NMR (500 MHz, 298 K, C6D6): δ = 7.04 (m, 2H, o-Ph), 6.82
4
(m, 3H, m-, p-Ph), 6.43 (d, JPH = 3.3 Hz, 4H, m-Mes), 2.13 (s, 12H,
o-CH3Mes), 1.89 (s, 6H, p-CH3Mes). 13C{1H} NMR (126 MHz, 298 K,
C6D6):δ=161.2(br, BCd), 146.7 (d, 1JPC =46.8Hz, dCP), 143.4 (d, 2JPC
=
8.8 Hz, o-Mes), 142.1 (d, 4JPC = 2.7 Hz, p-Mes), 137.9 (d, 2JPC = 1.9 Hz,
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3
i-Ph), 131.0 (d, JPC = 9.1 Hz, m-Mes), 129.0 (p-Ph), 128.8 (m-Ph),
14.0 (CH3) [C6F5 not listed]. 19F{1H} NMR (470 MHz, 298 K,
C6D6): δ = -129.7 (m, 4F, o-BC6F5), -139.6 (m, 2F, o-C6F5), -
154.5 (t, 3JFF = 21.6 Hz, 1F, p-C6F5), -156.4 (m, 2F, p-BC6F5), -161.9
(m, 2F, m-C6F5), -163.7 (m, 4F, m-BC6F5) [ΔδB(m,p) = 7.3]. 11B{1H}
NMR (96 MHz, 300 K, C6D6): δ = -6 (v1/2 ≈ 260 Hz). 31P{1H} NMR
(162 MHz, 300 K, C6D6): δ = 15.2 (v1/2 ≈ 80 Hz). For 6 [selected
resonances are listed]: 1H NMR (400 MHz, 300 K, C6D6): δ = 7.26 (m,
4H, o-Ph), 7.01 (m, 2H, p-Ph), 6.88 (m, 4H, m-Ph), 2.95
(m, 2H, dCH2), 1.34 (m, 2H, CH2), 0.56 (m, 3H, CH3) [assignment
by 2 D NMR experiments]. 13C{1H} NMR (101 MHz, 300 K, C6D6): δ
= 135.7 (p-Ph), 133.1 (d, 2JPC = 11.4 Hz, o-Ph), 130.6 (d, 3JPC =13.1 Hz,
m-Ph), 127.8 (d, 1JPC ≈ 55 Hz, dCP), 30.6 (dCH2), 24.6 (CH2), 21.2
(CH3) [assignment by 2 D NMR experiments]. 19F{1H} NMR
(470 MHz, 298 K, C6D6): δ = -123.4 (br s, 1F, C6F4), -129.7
(br s, 1F, C6F4), -132.9 (m, 4F, o-BC6F5), -139.1 (m, 1F, C6F4),
-151.2 (m, 1F, C6F4), -159.6 (t, 3JFF = 20.6 Hz, 2F, p-BC6F5), -165.1
(m, 4F, m-BC6F5), -184.6 (br m, 1F, B-F) [ΔδB(m,p) = 5.5]. 11B{1H}
NMR (96 MHz, 300 K, C6D6): δ = 0.6 (v1/2 ≈ 140 Hz). 31P{1H} NMR
(162 MHz, 300 K, C6D6): δ = 33.9 (v1/2 ≈ 20 Hz).
127.3 (d, 3JPC = 3.6 Hz, o-Ph), 123.3 (d, 1JPC = 34.6 Hz, i-Mes), 24.0 (d,
3JPC = 5.9 Hz, o-CH3Mes), 20.5 (p-CH3Mes) [C6F5 not listed]. 19F{1H}
NMR (470 MHz, 298 K, C6D6): δ = -126.9 (br, 4F, o-BC6F5), -136.0
(m, 2F, o-C6F5), -154.1 (t, 3JFF = 21.7 Hz, 1F, p-C6F5), -156.1 (t, 3JFF
=
21.1 Hz, 2F, p-BC6F5), -162.4 (m, 2F, m-C6F5), -164.0 (m, 4F, m-
BC6F5) [ΔδB(m,p) = 7.9]. 11B{1H} NMR (160 MHz, 298 K, C6D6): δ ≈
1 (v1/2 ≈ 700 Hz). 31P{1H} NMR (202 MHz, 298 K, C6D6): δ = 15.2
(v1/2 ≈ 40 Hz).
X-ray crystal structure analysis of 4a: formula C44H27BF15P, M =
882.44, colorless crystal 0.40 ꢀ 0.30 ꢀ 0.25 mm, a = 12.2935(3), b =
21.9361(7), and c = 14.0308(5) Å, β = 102.427(1)°, V = 3695.1(2) Å3,
F
calc = 1.586 g cm-3, μ= 1.663 mm-1, empirical absorption correction
(0.556 e T e 0.681), Z = 4, monoclinic, space group P21/c (No. 14), λ=
1.54178 Å, T = 223(2) K, ω and j scans, 28 710 reflections collected
((h,( k,( l), [(sin θ)/λ] = 0.60 Å-1, 6493 independent (Rint = 0.044)
and 5823 observed reflections [I g 2 σ(I)], 556 refined parameters, R =
0.044, wR2 = 0.124, max (min) residual electron density 0.32 (-0.28)
e Å-3, hydrogen atoms calculated and refined as riding atoms.
Synthesis of Compound 4b. B(C6F5)3 (1) (0.536 g, 1.05 mmol)
and 3b (0.300 g, 1.05 mmol) were dissolved in toluene (20 mL) and
stirred for 6 h at 70 °C. Subsequently the solvent was removed, the
residue was washed twice with pentane (15 mL), and all volatiles were
removed in vacuo to yield 4b (0.634 g, 0.803 mmol, 77%) as a yellow
solid. Anal. Calcd for C38H15BF15P: C, 57.17; H, 1.89. Found: C, 57.10;
H, 2.40. IR (KBr) ν~/cm-1 = 3406 (br m), 3064 (w), 2360 (m), 1646 (s),
1519 (s), 1463 (s), 1285 (m), 1096 (s), 966 (s), 693 (s), 518 (m). Mp
(DSC): 251 °C. Decomp (DSC): 272 °C. 1H NMR (500 MHz, 298 K,
C6D6): δ = 7.36 (m, 4H, o-PhP), 7.12 (m, 2H, o-Ph), 6.88 (m, 3H, p-Ph/
p-PhP), 6.84 (m, 2H, m-Ph), 6.76 (m, 4H, m-PhP). 13C{1H} NMR
(126 MHz, 298 K, C6D6): δ = 161.3 (br, BCd), 148.7 (dm, 1JFC ≈ 240
X-ray crystal structure analysis of 4c: formula C35H17BF15P, M =
764.27, colorless crystal 0.30 ꢀ 0.07 ꢀ 0.01 mm, a = 9.5993(5), b =
11.3438(8), and c = 15.4280(15) Å, R = 87.124(5), β = 89.616(4), and
γ = 70.239(3)°, V = 1579.0(2) Å3, Fcalc = 1.607 g cm-3, μ = 1.843 mm-
1, empirical absorption correction (0.608 e T e 0.982), Z = 2, triclinic,
space group P1 (No. 2), λ= 1.54178 Å, T = 223(2) K, ω and j scans,
23 382 reflections collected ((h,( k,(l), [(sin θ)/λ] = 0.60 Å-1, 5445
independent (Rint = 0.070) and 4230 observed reflections [I g 2 σ(I)],
470 refined parameters, R = 0.049, wR2 = 0.123, max (min) residual
electron density 0.25 (-0.33) e Å-3, hydrogen atoms calculated and
refined as riding atoms.
NMR-Scale Reactions. (a) Heating of B(C6F5)3 (1) (81.0 mg, 0.159
mmol) and 3c (40.2 mg, 0.159 mmol) in toluene-d8 (1 mL) for 2 h at
105 °C resulted in a reaction mixture of 4c and 6 in a 5:1 ratio
(monitored by 31P NMR). Continuing heating (105 °C) for an
additional 48 h did not change the 4c/6 ratio (5:1). (b) Heating of a
light-protected sample of B(C6F5)3 (1) (83.2 mg, 0.163 mmol) and 3c
(41.0 mg, 0.163 mmol) in toluene-d8 (1 mL) for 6 h at 105 °C resulted in
a reaction mixture of 4c and 6 in a 5:1 ratio (monitored by 31P NMR).
The control experiment without light protection, reacting B(C6F5)3 (1)
(82.2 mg, 0.161 mmol) and 3c (40.5 mg, 0.161 mmol) in toluene-d8
(1 mL) for 6 h at 105 °C also resulted in a reaction mixture of 4c and 6 in
a 5:1 ratio. For 6: 1H NMR (500 MHz, 298 K, C7D8): δ = 7.29 (m, 4H,
o-Ph), 7.07 (m, 2H, p-Ph), 6.94 (m, 4H, m-Ph), 2.87 (m, 2H, dCH2),
1.25 (m, 2H, CH2), 0.49 (t, 3JHH = 7.3 Hz 3H, CH3). 13C{1H} NMR
(126 MHz, 298 K, C7D8): δ = 179.4 (br, dCB), 135.9 (d, 4JPC = 3.3 Hz
p-Ph), 133.3 (d, 2JPC = 11.5 Hz, o-Ph), 130.7 (d, 3JPC =13.1 Hz, m-Ph),
1
1
Hz), 144.0 (dm, JFC ≈ 250 Hz), 141.0 (dm, JFC ≈ 250 Hz), 140.5
(dm, 1JFC ≈ 250 Hz), 138.1 (dm, 1JFC ≈ 250 Hz), 137.6 (dm, 1JFC
=
250 Hz, C6F5), 143.3 (d, 1JPC = 53.0 Hz, dCP), 135.2 (d, 2JPC = 1.9 Hz,
i-Ph), 132.5 (p-PhP), 132.1 (d, 2JPC = 9.3 Hz, o-PhP), 129.7 (p-Ph), 129.4
(m-Ph), 129.3 (d, 3JPC = 10.4 Hz, m-PhP), 127.0 (d, 3JPC = 3.2 Hz, o-Ph),
1
124.6 (d, JPC = 43.8 Hz, i-PhP), 115.8 (br, i-C6F5). 19F{1H} NMR
(470 MHz, 298 K, C6D6): δ = -129.9 (m, 4F, o-BC6F5), -138.3
(m, 2F, o-C6F5), -154.0 (t, 3JFF = 21.5 Hz, 1F, p-C6F5), -156.1 (m, 2F,
p-BC6F5), -161.7 (m, 2F, m-C6F5), -163.6 (m, 4F, m-BC6F5)
[ΔδB(m,p) = 7.5]. 11B{1H} NMR (160 MHz, 298 K, C6D6): δ = -6
(v1/2 ≈ 320 Hz). 31P{1H} NMR (202 MHz, 298 K, C6D6): δ = 13.8
(v1/2 ≈ 60 Hz).
Synthesis of Compounds 4c and 6. B(C6F5)3 (1) (0.400 g,
0.780 mmol) and 3c (0.197 g, 0.780 mmol) were dissolved in toluene
(20 mL) and stirred for 6 h at 70 °C. Subsequently the solvent was
removed, and the residue was washed twice with pentane. The solid was
suspended in toluene and filtered via cannula. After removal of toluene
in vacuo, a mixture of 4c and 6 (ratio 10:1) (0.333 g, 0.440 mmol, 56%)
was obtained as a white solid. Crystals of 4c suitable for X-ray crystal
structure analysis were grown by slow diffusion of pentane into a
solution of 4c/6 in dichloromethane at -36 °C. For the mixture 4c:6 =
10:1: Anal. Calcd for C35H17BF15P: C, 55.00; H, 2.24. Found: C, 54.51;
H, 1.95. IR (KBr): ν~/cm-1 = 3406 (br m), 3060 (w), 2973 (m), 2880
(w), 2357 (w), 1645 (s), 1518 (s), 1468 (s), 1383 (m), 1288 (m), 1110
(s), 971 (s), 922 (m), 744 (m), 506 (m). Mp (DSC) (4c): 214 °C. mp
(DSC) (6): 186 °C. Decomp (DSC): 270 °C [for further details see the
Supporting Information]. For 4c: 1H NMR (400 MHz, 300 K, C6D6): δ
= 7.27 (m, 4H, o-Ph), 6.93 (m, 2H, p-Ph), 6.86 (m, 4H, m-Ph), 2.19 (m,
2H, dCH2), 1.20 (m, 2H, CH2), 0.55 (m, 3H, CH3). 13C{1H} NMR
(101 MHz, 300 K, C6D6): δ = 161.5 (br, BCd), 146.7 (d, 1JPC = 49.1 Hz,
dCP), 132.38 (d, 2JPC = 8.4 Hz, o-Ph), 132.36 (d, 4JPC = 3.3 Hz, p-Ph),
129.2 (d, 3JPC = 10.6 Hz, m-Ph), 125.4 (d, 1JPC3 = 42.1 Hz, i-Ph), 115.8
(br s, i-BC6F5), 33.2 (dCH2), 21.3 (d, JPC = 1.6 Hz, CH2),
1
127.9 (d, JPC ≈ 65 Hz, dCP; from the ghmbc experiment), 115.4
(d, 1JPC = 83.0 Hz, i-Ph), 30.7 (t, J = 13.9 Hz, dCH2), 24.7 (t, J = 2.8 Hz,
CH2), 13.95 (CH3) [C6F5 not listed]. 19F{1H} NMR (470 MHz, 298 K,
C7D8): δ = -123.8 (br s, 1F, C6F4), -129.3 (m, 1F, C6F4), -132.8
(m, 4F, o-BC6F5), -139.9 (m, 1F, C6F4), -151.5 (m, 1F, C6F4),
3
-160.1 (t, JFF = 20.5 Hz, 2F, p-BC6F5), -165.4 (m, 4F, m-BC6F5),
-184.4 (br m, 1F, B-F) [ΔδB(m,p) = 5.3]. 11B{1H} NMR (160 MHz,
298 K, C7D8): δ = 0.6 (v1/2 ≈ 140 Hz). 31P{1H} NMR (202 MHz, 298 K,
C7D8): δ = 33.9 (v1/2 ≈ 20 Hz). (c) Heating of B(C6F5)3 (1)
(82.2 mg, 0.161 mmol) and 3c (40.5 mg, 0.161 mmol) in toluene-d8
(1 mL) for 3 h at 105 °C by simultaneous irradiation (Heraeus Nolelight
HPK 125 W, Pyrex filter) resulted in a reaction mixture of 4c and 6 in a
5:1 ratio (monitored by 31P NMR).
Synthesis of Compound 4d. B(C6F5)3 (1) and 3d (35.5 mg, 0.09
mmol) were dissolved in toluene (20 mL) and stirred for 9 h at 80 °C.
Subsequently toluene was removed, and the residue was washed twice
with pentane. The solid was dissolved in less toluene to let the impurities
precipitate overnight at room temperature. Afterward the toluene solution
4614
dx.doi.org/10.1021/ja1110283 |J. Am. Chem. Soc. 2011, 133, 4610–4616

Journal of the American Chemical Society
ARTICLE
was separated. Removal of all volatiles in vacuo yielded 4d (0.174 g, 0.19
mmol, 60%) as a white-yellow solid. Crystals suitable for X-ray crystal
structure analysis were grown by slow diffusion of pentane into a
solution of 4d in dichloromethane at -36 °C. HRMS: calcd for
parameters, R = 0.050, wR2 = 0.142, max (min) residual electron density
0.79 (-0.55) e Å-3, hydrogen atoms calculated and refined as riding atoms.
’ ASSOCIATED CONTENT
BP2H20C44F15H, 907.09740; found, 907.10066. IR (ATR): ν~/cm-1
=
S
2383 (br w), 2313 (w), 1646 (w), 1515 (m), 1464 (s), 1384 (w), 1092
(s), 970 (s), 901 (m), 740 (s), 690 (s). Mp (DSC): 258 °C. 1H NMR
(500 MHz, 298 K, C7D8): δ = 7.26 (m, 2H, o-PhP), 7.10 (m, 2H,
o-PhPþ), 6.83 (m, 1H, p-PhPþ), 6.71 (m, 3H, p-PhP, m-PhPþ), 6.66 (m,
2H, m-PhP). 13C{1H} NMR (126 MHz, 298 K, C7D8): δ = 173.7 (br,
Supporting Information. Additional experimental and
b
spectroscopic details and X-ray crystallographic data (CIF) for
4a, 4c, 4d, and 7. This material is available free of charge via the
Internet at http://pubs.acs.org.
BCd), 146.4 (dd, 1JPC = 53.0 Hz, 1JPC = 32.7 Hz, dCP), 135.0 (d, 2JPC
=
’ AUTHOR INFORMATION
22.8 Hz, o-PhPþ), 133.1 (d, 2JPC = 10 Hz, o-PhP), 132.5 (dd, 1JPC = 9.5
Hz, 3JPC = 3.3 Hz, i-PhPþ), 132.1 (d, 4JPC = 3.0 Hz, p-Ph), 129.9 (d, 4JPC
= 1.0 Hz, p-PhPþ), 128.8 (d, 3JPC = 11.3 Hz, m-PhP), 128.7 (d, 3JPC = 9.1
Corresponding Author
erker@uni-muenster.de
Hz, m-PhPþ), 124.8 (d, JPC = 44.3 Hz, i-PhP) [C6F5 not listed].
1
19F{1H} NMR (470 MHz, 298 K, C7D8): δ = -128.8 (m, 4F,
3
’ ACKNOWLEDGMENT
o-BC6F5), -138.0 (m, 2F, o-C6F5), -156.2 (t, JFF = 21.3 Hz, 1F,
3
p-C6F5), -156.4 (t, JFF = 21.3 Hz, 2F, p-BC6F5), -163.1 (m, 2F,
Financial support from the Deutsche Forschungsgemeinschaft
is gratefully acknowledged.
m-C6F5), -163.7 (m, 4F, m-BC6F5) [ΔδB(m,p) = 7.3]. 11B{1H} NMR
(160 MHz, 298 K, C7D8): δ = -5 (v1/2 ≈ 230 Hz). 31P{1H} NMR (202
MHz, 298 K, C7D8): δ = -6.3 (td, JPF = 28.4 Hz, 2JPPþ = 10.3 Hz, P),
24.2 (br, v1/2 ≈ 70 Hz, Pþ).
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X-ray crystal structure analysis of 4d: formula C44H20BF15P2 CH2Cl2,
3
M = 991.28, colorless crystal 0.10 ꢀ 0.10 ꢀ 0.04 mm, a = 9.7976(2), b =
10.8460(4), and c = 21.1870(7) Å, R = 95.231(2), β = 102.492(2), and
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mm-1, empirical absorption correction (0.752 e T e 0.889), Z = 2,
triclinic, space group P1 (No. 2), λ= 1.54178 Å, T = 223(2) K, ω and j
scans, 25 965 reflections collected ((h,(k,(l), [(sinθ)/λ] = 0.60 Å-1
,
6932 independent (Rint = 0.110) and 4337 observed reflections [I g 2
σ(I)], 596 refined parameters, R = 0.077, wR2 = 0.224, max (min)
residual electron density 0.70 (-0.82) e Å-3, hydrogen atoms calculated
and refined as riding atoms.
Synthesis of Compound 7. Heating of a solution of 4d (0.08
mmol, 72.4 mg) in toluene for 3d at 105 °C followed by cooling at room
temperature gave the precipitated product 7 (0.028 mmol, 25.1 mg,
35%) as orange crystals. These crystals were suitable for X-ray analysis,
after filtration via cannula and washing with very small amount of
toluene. HRMS: calcd for C44H20BF15P2H, 907.09665; found,
907.09354. IR (ATR): ν~/cm-1 = 1666 (w), 1588 (w), 1514 (m),
1493 (m), 1453 (s), 1379 (w), 1274 (m), 1091 (br s), 955 (m), 743 (s),
691 (s). Decomp (DSC): 276 °C. 1H NMR (500 MHz, 298 K, CD2Cl2):
δ = 7.82 (m, 2H, p-PhPþ), 7.64 (m, 4H, m-PhPþ), 7.62 (m, 4H, o-PhPþ),
7.19 (m, 4H, o-PhP), 7.11 (m, 2H, p-PhP), 6.92 (m, 4H, m-PhP).
13C{1H} NMR (126 MHz, 298 K, CD2Cl2): δ = 189.7 (br, dCB),
135.8 (d, 4JPC = 2.5 Hz, p-PhPþ), 135.7 (d, 2JPC = 23.2 Hz, o-PhP), 133.6
(d, 2JPC = 12.3 Hz, o-PhPþ), 130.8 (d, 3JPC = 13.3 Hz, m-PhPþ), 129.5
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(d, 4JPC = 0.8 Hz, p-PhP), 128.2 (d, 3JPC = 8.4 Hz, m-PhP), 115.5 (d, 1JPC
=
84.6 Hz, i-PhPþ) [dCP, i-PhP not observed; C6F4, C6F5 not listed].
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-129.5 (m, 1F, C6F4), -133.3 (m, 4F, o-BC6F5), -141.8 (m, 1F, C6F4),
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(dd, JPF = 177.8 Hz, 2JPPþ = 7.1 Hz, P).
X-ray crystal structure analysis of 7: formula C44H20BF15P2 C7H8,
3
M = 998.48, orange crystal 0.30 ꢀ 0.23 ꢀ 0.15 mm, a = 10.4606(4), b =
13.1363(5), and c = 16.4911(7) Å, R = 91.614(2), β = 103.553(2), and
γ= 95.609(3)°, V= 2189.39(15) Å3, Fcalc = 1.515 g cm-3, μ= 1.816 mm-1
,
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dent (Rint = 0.043) and 7024 observed reflections [I g 2 σ(I)], 610 refined
4615
dx.doi.org/10.1021/ja1110283 |J. Am. Chem. Soc. 2011, 133, 4610–4616

Journal of the American Chemical Society
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