Boronation of 1,8-bis(diphenylphosphino)naphthalene: Formation of cyclic boronium salts

Article abstract of DOI:10.1021/om900017n

Treatment of l,8-bis(diphenylphosphino)naphthalene (1) with borane-dimethyl sulfide complex in THF/Et₂O solution affords monoborane adduct 3 irrespective of the molar ratio of the reagents. The monoboronation product has been characterized by m

Full text of DOI:10.1021/om900017n

Organometallics 2009, 28, 4929–4937 4929
DOI: 10.1021/om900017n
Boronation of 1,8-Bis(diphenylphosphino)naphthalene: Formation of
Cyclic Boronium Salts^
†
,‡
§
§
ꢀ
Krzysztof Owsianik, Remi Chauvin,* Agnieszka Balinska, Michaz Wieczorek,
Marek Cypryk,† and Marian Mikozajczyk*,†
†Centre of Molecular and Macromolecular Studies, Department of Heteroorganic Chemistry, Polish
‡
ꢀ ꢀ
Academy of Sciences, 90-363 yodz, Sienkiewicza 112, Poland, Laboratoire de Chimie de Coordination
CNRS, 205 Route de Narbonne, 31077 Toulouse Cedex 4, France, and Institute of Chemistry and
Environmental Protection, Jan Dlugosz Academy, 42-200 Cze-stochowa, Al. Armii Krajowej 13/15, Poland
§
Received January 8, 2009
Treatment of 1,8-bis(diphenylphosphino)naphthalene (1) with borane-dimethyl sulfide complex
in THF/Et2O solution affords monoborane adduct 3 irrespective of the molar ratio of the reagents.
The monoboronation product has been characterized by multinuclear (1H, 31P, 13C, 11B) NMR
spectroscopy and MS-FAB measurements. Optimization of its structure with DFT methods at the
B3LYP/6-31þG(d) level of theory revealed that steric hindrance around the diphenylphosphino
group prevents formation of the bis-borane adduct. In dichloromethane or chloroform solution
boronation of bis-phosphine 1 with H3B Me2S results in the unexpected formation of a cyclic
dihydroboronium chloride, 4 Cl. The corresponding cyclic iodide 4 I and triflate 4 TfO are formed
3
3
3
3
by treatment of bis-phosphine 1 with H3B Me2S followed by methyl iodide and methyl triflate,
respectively, in THF/Et2O solution. The hexafluorophosphate 4 PF6 prepared by anion exchange
from the chloride 4 Cl has also been characterized by single-crystal X-ray diffraction. It has been
demonstrated that the cyclic boronium salts are formed in two steps. The first is the formation of
monoborane adduct 3, which in the second step undergoes cyclization with concomitant reduction of
the chlorohydrocarbon solvents, methyl iodide or triflate. Some aspects of this unprecedented
reductive cyclization have been elucidated by DFT calculations.
3
3
3
Introduction
The introduction of a second phosphinyl substituent at the
boron atom leads to a new class of boron-containing cations
of the type [(R3P)2BH2]þ. Of special interest are the cyclic
boronium salts obtained from bidentate diphosphine
ligands.3-5 The structures of these salts obtained thus far
are depicted in Chart 1.
Recently, Imamoto and co-workers prepared chiral dihydro-
boronium derivatives of (S,S)-1,2-bis(tert-butylmethylphos-
phino)ethane B and used them as convenient and efficient
chiral ligand precursors in rhodium-catalyzed enantioselec-
tive hydrogenation of olefins.4 Later, they were also able to
synthesize sterically congested dihydroboronium iodide C
derived from (R,R)-1,2-bis(tert-butylmethylphosphino)ben-
zene, however, according to a completely different strategy.5
Almost all the achiral and chiral cyclic boronium salts A
and B have been prepared in a conventional way from a
Phosphine-borane complexes, R3P BH3, represent an im-
3
portant class of organophosphorus compounds that have
attracted growing interest over the past decades.1 Due to
peculiar chemical and physical properties, these adducts
have found widespread use in synthetic and stereochemical
studies. In contrast to phosphines, which are sensitive to
oxidation and usually difficult to handle, phosphine-borane
complexes are air-stable compounds and can be easily con-
verted into the corresponding phosphines. With chiral phos-
phines as well as with other chiral tricoordinate phosphorus
compounds the process of boronation and removal of BH3
from the adducts formed occur with complete preservation
of stereochemical integrity at the phosphorus atom. There-
fore, the borane group became a very useful protecting group
in the synthesis of various chiral tricoordinate phosphorus
compounds.2
(3) (a) Sigl, M.; Schier, A.; Schmidbaur, H. Chem. Ber. Recueil 1997,
130, 1411. (b) Schmidbaur, H.; Wimmer, T.; Reber, G.; Muller, G. Angew.
Chem., Int. Ed. Engl. 1988, 27, 1071. (c) Martin, D. R.; Merkel, C. M.; Ruiz,
J. P. Inorg. Chim. Acta 1985, 100, 293. (d) Martin, D. R.; Merkel, C. M.;
Ruiz, J. P. Inorg. Chim. Acta 1986, 115, L29. (e) Miller, N. E.; Muetterties,
E. L. J. Am. Chem. Soc. 1964, 86, 1033. (f) Costa, T.; Schmidbaur, H. Chem.
Ber. 1982, 115, 1374. (g) Schmidbaur, H.; Gampes, S.; Paschalidis, Ch.;
Steigelmann, O.; M€uller, G. Chem. Ber. 1991, 124, 1525.
(4) Miyazaki, T.; Sugawara, M.; Danjo, H.; Imamoto, T. Tetrahe-
dron Lett. 2004, 45, 9341.
(5) Yamamoto, Y.; Koizumi, T.; Katagiri, K.; Furuya, Y.; Danjo, H.;
Imamoto, T.; Yamaguchi, K. Org. Lett. 2006, 8, 6103.
^ Dedicated to Prof. Marek Zaidlewicz on the occasion of his 70th birthday.
*To whom correspondence should be addressed. E-mail: marmikol@
bilbo.cbmm.lodz.pl (M.M.); chauvin@lcc-toulouse.fr (R.Ch.).
(1) (a) Brunel, J. M.; Faure, B.; Maffei, M. Coord. Chem. Rev. 1998,
178-180, 665. (b) Carboni, B.; Monnier, L. Tetrahedron 1999, 55, 1197. (c)
Schmidbaur, H. J. Organomet. Chem. 1980, 200, 287. (d) Flores-Parra, A.;
Contreras, R. Coord. Chem. Rev. 2000, 196, 85. (e) Lane, C. F. Chem. Rev.
1976, 76, 773.
€
(2) Ohff, M.; Holz, J.; Quirmbach, M.; Borner, A. Synthesis 1998,
1391.
r
2009 American Chemical Society
Published on Web 08/12/2009
pubs.acs.org/Organometallics

4930 Organometallics, Vol. 28, No. 17, 2009
Owsianik et al.
Chart 1
Scheme 2
base 1,8-bis(dimethylamino)naphthalene known as proton
sponge. Diphosphine 1 is a highly congested bidentate ligand
with a rigid C3-backbone and a very short nonbonding
7
˚
P
P distance of 3.052 A.
3 3 3
Scheme 1
Such sterically crowded molecules tend to exhibit unusual
physical and spectroscopic properties, and their chemical
reactivity differs markedly from that of unstrained anal-
ogues. In this paper we report our results on the boronation
of 1 as well as a completely unexpected mode of formation of
cyclic boronium salts.
diphosphine and a haloborane. The cyclization step in these
syntheses involves the replacement of a halogen on a boron
atom in a transient monoborane adduct by the second
phosphine group as depicted in Scheme 1.
The only exception is the formation of a cyclic boronium
salt derived from 1,8-bis(dimethylphosphino)naphthalene.
Costa and Schmidbaur3f found that in the reaction of the
Results and Discussion
Synthesis and Characterization of Monoborane Adduct. In
initial experiments, we tried to protect both phosphorus
atoms in 1 and obtain bis-borane adducts 2. Therefore, bis-
phosphine 1 was treated with an excess (2.2 mol) of borane-
dimethyl sulfide complex in Et2O solution. Removal of the
solvent afforded a homogeneous product in 90% yield as a
yellowish solid. Its spectroscopic properties ruled out the
formation of 2 and indicated that the monoadduct 3 was
produced (Scheme 2). This was supported by the 31P NMR
spectra. Thus, the 31P{1H} NMR spectrum of the reaction
product showed two resonances at 25.7 and -11.6 ppm, not
one as expected for two equivalent phosphorus nuclei in 2.
The latter signal assigned to PIII appeared as a doublet
with JPP=60.4 Hz and was in a region typical for triarylpho-
sphines. The observed coupling constant is larger than
above diphosphine with THF BH3 an equilibrium mixture
3
of the bisphosphine-diborane adduct and cyclic boronium
salt was formed (eq 1). However, mechanistic details of this
interesting formation of a cyclic boronium salt were not
elucidated.
4
normal JPP values (up to 10 Hz) but far from being as the
large as the typical 1JPP values for P-P bonded 1,8-naphthal-
ene systems (200-300 Hz). This is not consistent with the
isomeric P-P bonded structure 3a. Most probably this value
indicates considerable through-space coupling associated
with the close proximity of the phosphorus atoms.8
As part of our ongoing studies aimed at the synthesis of
new phosphorus ligands, especially chiral ones,6 we turned
our attention to 1,8-bis(diphenylphosphino)naphthalene
(dppn, 1), a phosphorus analogue of the non-nucleophilic
(7) Jackson, R. D; James, S.; Orpen, A. G.; Pringle, P. G. J.
Organomet. Chem. 1993, 458, C3–C4.
(8) For a discussion of coupling constants in the naphthalene system
€
see for example: (a) Karac-ar, A.; Freytag, M.; Thonnessen, P. G.;
ꢀ
(6) See for example: (a) Omelanczuk, J.; Karac-ar, A.; Freytag, M.;
Bartsch, R.; Schmutzler, R. J. Organomet. Chem. 2002, 643, 68. (b)
Kilian, P.; Slawin, A. M. Z.; Woollins, J. D. Dalton Trans. 2006, 2175. (c)
Kilian, P.; Slawin, M. Z.; Woollins, D. Phosphorus, Sulfur, Silicon 2004,
179, 999.
Jones, P. G.; Bartsch, R.; Mikozajczyk, M.; Schmutzler, R. Inorg. Chim.
Acta 2003, 350, 583. (b) Zurawinski, R.; Donnadieu, B.; Mikoajczyk, M.;
Chauvin, R. Organometallics 2003, 22, 4810.
_
ꢀ

Article
Organometallics, Vol. 28, No. 17, 2009 4931
˚
Figure 1. B3LYP/6-31þG(d)-optimized structure of adduct 3. The shortest H(B)-H(Ph) distances in A are shown.
As our various attempts to obtain by crystallization
Scheme 3
analytically pure sample as well as X-ray quality crystals of
3 failed, a geometry optimization of this structure with DFT
methods was carried out. Figure 1 shows the calculated
structure with nonbonding contacts between the borane
hydrogens and ortho-hydrogens of the P-phenyl rings.
˚
The calculated P-B bond length of 1.995 A was found to
be much shorter than the nonbonding distance between
˚
boron and the second phosphorus atom (3.427 A). Both
phosphorus atoms are close to each other and separated only
˚
by 3.286 A, which is slightly longer than in a free ligand 1
˚
(3.125 A by calculation, 3.052 A by X-ray) but is well within
˚
˚
the sum of the van der Waals radii (3.60 A). An inspection of
the structural details of 3 revealed that 2-fold boronation of 1
by such a small molecule as BH3 could not be achieved due to
steric hindrance around the diphenylphosphino groups.
Interestingly, bis-boronation occurs with the dimethylphos-
phino analogue of 1.3f The observed monoboronation of
bis-phosphine 1 is in accord with the previously described
monomethylation of 1, which occurs exclusively even with an
excess of a strong methylating agent such as methyl triflate.9
Synthesis and Characterization of Cyclic Boronium Salts.
Unexpectedly, when dichloromethane or chloroform was
phino)naphthalene boronium chloride 4 Cl (Scheme 3).
Moreover, the same cyclic salt was obtained in 93% yield
3
in a conventional way starting from 1 and H2ClB SMe2
3
used as solvent, the reaction of 1 with H3B SMe2 complex
complex. Additionally, the cyclic salt 5 Br was obtained in
3
3
resulted in high-yield formation of a new product that
exhibited physical and spectroscopic properties completely
different from those observed for the monoadduct 3
(Scheme 3).
92% yield by treatment of 1 with dibromoborane-dimethyl
sulfide (eq 2).
The 31P{1H} NMR spectrum of the white solid isolated in
92% yield showed a single broad resonance at 4.7 ppm,
indicating equivalent phosphorus nuclei. Similarly, a single
11B NMR signal at -33.6 ppm was observed. These and
other spectroscopic data (see Experimental Section) are
consistent with the structure of 1,8-bis(diphenylphos-
Metathesis of the salt 4 Cl with an excess of KPF6 in
3
methanol gave the corresponding hexafluorophosphate,
4 PF6, as a white solid. Crystallization from dichloro-
methane/toluene gave colorless crystals of the CH2Cl2 solvate
€
(9) Karac-ar, A.; Klaukien, V.; Freytag, M.; Thonnessen, H.;
Omelanczuk, J.; Jones, P. G.; Bartsch, R.; Schmutzler, R. Z. Anorg.
ꢀ
3
Allg. Chem. 2001, 627, 2589.

4932 Organometallics, Vol. 28, No. 17, 2009
Owsianik et al.
Table 2. Crystal Data and Structure Refinement for 4 PF6
3
formula
C34H28BP2 F6P CH2Cl2
3
3
Mr
T (K)
739.21
90(2)
colorless, parallelepiped
triclinic
P1
color, habit
cryst syst
space group
Z
2
˚
a (A)
11.964(3)
12.191(3)
12.717(3)
70.72(3)
85.66(3)
72.35(3)
1667.8(8)
1.472
0.40
Mo KR
none
0.925, 0.980
37 897
14 518
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Dx (Mg m-3
)
μ (mm-1
radiation
)
absorp corr
Tmin, Tmax
no. of measd reflns
no. of indep reflns
no. of reflns with I > 2σ(I)
Rint
Figure 2. ORTEP view of the X-ray crystal structure of 4 PF6
with 50% probability displacement ellipsoids for non-hydrogen
atoms. The PF6 anion and CH2Cl2 are omitted for clarity.
3
10 106
0.031
35.0
θ
max (deg)
˚
Table 1. Selected Bond Lengths (A) and Angles (deg) for 4 PF6
h, k, l ranges
R[F2 > 2σ(F2)]
wR(F2)
-19f19, -19f19, -18f20
0.045
0.119
3
Bond Lengths
S
no. of params
(4/σ)max
4Fmax, Fmin (e A
extinction corr
1.06
440
0.001
0.68, -0.40
none
P1-B1
P2-B1
P1-C9
P2-C1
C1-C10
C9-C10
1.9069(16)
1.9127(15)
1.8012(14)
1.8093(15)
1.4462(18)
1.4411(18)
P1-C17
P1-C11
P2-C23
P2-C29
B1-H37
B1-H38
1.8043(14)
1.8047(13)
1.8095(14)
1.8116(14)
1.061(17)
1.107(16)
-1
˚
)
around boron is tetrahedral. The bond angles H-B-H
and P1-B-P2 are 115.3(12)ꢀ and 104.14(7)ꢀ, respectively.
The latter value is the smallest among the ring angles in
bis(phosphino)boronium salts reported to date.
Bond Angles
P1-B1-P2
C1-P2-B1
104.14(7)
110.73(6)
124.37(9)
126.32(11)
122.61(9)
109.25(7)
115.3(12)
109.05(6)
106.77(6)
C17-P1-B1
C11-P1-B1
C23-P2-C29
C23-P2-B1
C29-P2-B1
B1-P2-C1-C10
B1-P1-C9-C10
P1-C9-C10-C1
P2-C1-C10-C9
114.35(7)
110.86(6)
108.22(6)
111.47(7)
110.36(6)
19.88(12)
-37.27(12)
8.50(17)
A geometry optimization of the cyclic cation [4]þ with
DFT methods [B3LYP/6-31G(d)] afforded the struc-
ture (Figure 2, see Supporting Information) that closely
matches the X-ray structure discussed above. Moreover,
the calculated geometry parameters of the ring are very
similar to those reported for 1λ5-phospha-3-phosphonia-2-
boratobenzene.3g
C10-C1-P2
C9-C10-C1
C10-C9-P1
C9-P1-B1
H37-B1-H38
C9-P1-C11
C17-P1-C11
1.02(18)
The structural parameters of the cation [4]þ are indicative
of a high thermodynamic stability of the six-membered
heterocyclic ring, as confirmed by the theoretical calcula-
(4 PF6 2/3CH2Cl2) suitable for X-ray analysis. The salt
3
3
crystallizedin thetriclinicspace groupP1 withthreemolecules
of 4 PF6 and two CH2Cl2 molecules in the unit cell. The
tions discussed below. The salt 4 Cl was found to be
3
3
structure is shown in Figure 2, and important distances and
anglesinthecycliccationofthesalt4 PF6 aregiveninTable1.
unreactive. It did not react with HCl or Br2 even at high
temperatures and was resistant to air and moisture. This is in
contrast with the lower chemical stability and hence higher
reactivity of monoadduct 3.
Mechanistic Aspects. The efficient formation of the cyclic
boronium salt 4 Cl in chlorohydrocarbon solvents provides
3
Crystal data, information related to data collection, and
refinement details are listed in Table 2 (see Experimental
Section).
The six-membered heterocyclic ring adopts a sofa con-
formation with the boron atom deviating most strongly from
the naphthalene plane (B=-0.7061 A, P1=0.2259 A, P2=
-0.0835 A). The naphthalene and P1B1P2 planes form an
3
the first, clear-cut evidence for their participation in the
reaction between 1 and H3B SMe2.10 As a starting point to
˚
˚
3
˚
study the nature of these interactions, the progress of the
reaction was followed by 31P NMR. After mixing both
reagents the resonance of 1 (δP=-14.9 ppm) was observed.
After 1 h it disappeared and two signals characteristic of the
monoborane adduct 3 (δP = -11.2 and 25.1 ppm) were
observed. Finally, after 12 h only a broad singlet due to the
angle of 42.09(8)ꢀ. Both phosphorus atoms P1 and P2 and
carbon atoms C1, C10, and C9 lie practically in one plane, as
the dihedral angle between the planes P1P2C1C9 and
C1C10C9 is only 4.23ꢀ. The two P-B distances of 1.906(9)
˚
˚
A for P1-B and 1.912(7) A for P2-B are equal within the
limit of standard deviations and are smaller than those
observed in the corresponding five-membered bis-phos-
formation of 4 Cl (δP=4.7 ppm) was visible in the spectrum.
3
3a,4
˚
phine boronium rings (1.92-1.97 A).
compared with the sum of the covalent radii of the two
˚
elements (1.88 A) suggest a strong coordination in the six-
membered ring. As expected, the coordination geometry
These values as
(10) Kanth and Brown observed unusual rapid hydroboration of
alkenes using diborane in chlorohydrocarbon solvents. However, the
role of such solvents in this process has not been clarified and is still
obscure: Kanth, J. V. B.; Brown, H. C. Tetrahedron Lett. 2000, 41, 9361.

Article
Organometallics, Vol. 28, No. 17, 2009 4933
Scheme 4
(a) hydride transfer from borane to methyl chloride as-
sisted by two molecules of PH3
Therefore, there is no doubt that the adduct 3 upon reaction
with CH2Cl2 or CHCl3 underwent cyclization to 4 Cl. This
3
was confirmed by independent experiments (Scheme 4).
Thus, keeping the solution of the preformed 3 in dichloro-
methane or chloroform for 12 h at room temperature
(b) reaction analogous to reaction 3 with 1,8-bis-
(phosphino)naphthalene 6 where two nucleophilic centers
exhibit reduced mobility
resulted in the formation of the salt 4 Cl in 95% and 92%
3
yield, respectively (Scheme 4, a, b). Moreover, when 3 was
treated with methyl iodide in THF solution for 4 h at room
temperature, the cyclic boronium iodide 4 I was obtained in
3
95% yield (Scheme 4, c). Similarly, the cyclization of 3 to the
corresponding triflate, 4 CF3SO3, was effected with methyl
triflate (Scheme 4, d).
3
The formation of methane as a second product in the last
reaction was confirmed by a simple NMR test. Equimolar
amounts of 3 and methyl triflate dissolved in C6D6 were
placed in the NMR tube, and the 1H and 13C NMR spectra
were recorded after 2 h. In addition to other signals, a high-
field signal at 0.16 ppm in the proton and at -3.77 ppm in the
carbon spectrum were observed. These signals are due to
methane formed in the cyclization reaction (compare for
methane 0.14 and -2.1 ppm for proton11 and carbon12
resonances, respectively). Hence, in all the reactions shown
above, cyclization of 3 to boronium cation [4]þ is accompanied
by reduction of chlorohydrocarbon solvents or methyl
iodide and methyl triflate. This new way of cyclization may
be called reductive cyclization. It is probable that the cyclic
boronium salt obtained from 1,8-bis(dimethylphosphino)-
naphthalene7 is formed by a similar reductive cyclization.
However, a piece of experimental evidence is required to
confirm this view.
(c) reaction 5 analogous to reaction 4 with sterically
congested 1,8-bis(diphenylphosphino)naphthalene
The B3LYP/6-31þG*-calculated structures of 1,8-diphos-
phinonaphthalene (6) and 1,8-bis(diphenylphosphino)-
naphthalene (1) are in good agreement with the crystal
structures reported in the literature,7,14 which supports the
reliability of the computational method applied for the
present problem.
In our opinion, a possible mechanism for the reductive
cyclization would involve two reactions occurring in a more
or less concerted way. The first would be nucleophilic attack
at boron in 3 by the neighboring diphenylphosphino group
and departure of a hydride ion. The latter, in turn, should
attack an electrophilic sp3-carbon atom bonded to a good
leaving group. The very efficient reduction achieved by 3 was
In all the above reactions the first reaction step is the
formation of the relatively strong BH3-PH2R complex
(complexation energy >20 kcal/mol, Figure 3), which is
probably the reactive intermediate in the hydride transfer
reaction. The energy profiles for reactions 3-5 are shown in
Figure 3. The energy barriers for these reactions decrease in
the order 3 > 4 > 5. The same order is observed for the
barriers of the reactions 3 and 4 in THF (Figure 3, values in
brackets), although the barriers are reduced by about 7 kcal/
mol compared to those in the gas phase. The Gibbs energy
barrier for reaction 3 is also significantly higher than that
of reaction 4. It should be noticed that in the lowest energy
transition state found for the hydride transfer (reaction 3)
the additional PH3 molecule forms a weak P-H-P
bond and not a P-B bond. Such interaction is not
possible for substituted phosphines. These results suggest
surprising in light of recent theoretical calculations by Pratt
~^
13
and Nguyen, who found that the reduction of chloro-
methane by the borane complexes of dimethyl ether and
dimethyl sulfide requires very high activation free energies,
which means that it practically does not occur.
Theoretical Calculations. To better understand the mecha-
nism of the discovered reductive cyclization and to evaluate
the role of the second diphenylphosphino group in this
process, DFT calculations for the following model reactions
3-5 were carried out:
(11) See for example: Kawano, Y.; Yasue, T.; Shimoi, M. J. Am.
Chem. Soc. 1999, 121, 11744.
(12) Kalinowsky, H.-O.; Berger, S.; Braun, S. 13CNMR Spektrosko-
pie; Georg Thieme Verlag, 1997.
(14) Reiter, S. A.; Nogai, S. D.; Karaghiosoff, K.; Schmidbaur, H. J.
Am. Chem. Soc. 2004, 126, 15833.
~^
(13) Pratt, L. M.; Nguyen, N. V. J. Phys. Chem. A. 2006, 110, 687.

4934 Organometallics, Vol. 28, No. 17, 2009
Owsianik et al.
Figure 3. Electronic energy (at 0 K), ΔE, and Gibbs free energy
(at 298 K), ΔG, profiles in kcal/mol for reactions 3-5; the
relative energies for TSs in THF are given in brackets.
Figure 4. Relative energy (at 0 K), ΔE0, and Gibbs free energy
(at 298 K), ΔG298, profiles in kcal/mol for reaction 4 with 1,8-
diphosphinonaphthalene (a), and for an analogous reaction
with 1,5-diphosphinonaphthalene (b); the energy differences in
kcal/mol are shown at the arrows.
that the assistance of the second PH3 molecule in the hydride
transfer is not effective. On the other hand, the low ΔGq
barrier for reaction 4 reflects the smaller entropy change
upon transition state formation, due to the limited degree of
freedom in 1,8-diphosphinonaphthalene. Experimental data
indicate strong repulsive interactions between the lone pairs
at phosphorus atoms in 6.14 Since we have not performed the
thermochemistry calculations for reaction 5, the Gibbs
energy barrier for this reaction is not known; however, it is
likely to be similar to that of reaction 4.
In order to examine possible interactions of boron with a
neighboring P atom, we have performed the natural bond
orbital (NBO) analysis of the complexes of 6 and 1 with BH3.
Calculations indicate that this interaction is insignificant.
Wiberg bond order between nonbonded phosphorus and
boron atoms and the nPfσ*B-H electron delocalization
energy are negligible. Moreover, all the B-H bond distances
are almost identical; thus there is no visible weakening of the
B-H bond opposite the neighboring phosphorus. No sig-
nificant P-P interaction in the ground state was detected,
either. The P-B interaction in the transition states of the
hydride transfer reactions 4 and 5 slightly increases. The
Wiberg P-B bond order in the TS of reaction 5 is 0.05, and
the nPfp*B delocalization energy is ca. 4 kcal/mol. Thus, the
nucleophilic assistance by the second phosphine group plays
only a minor role in stabilization of the transition state of the
hydride transfer reaction. The lower energy barriers for
reactions 4 and 5 in comparison with reaction 3 result mainly
from the higher energy of the diphosphinonaphthalene sub-
strates (due to steric congestion and to electrostatic repulsion
between phosphorus atoms) rather than from a nucleophilic
assistance of the second phosphine moiety. In order to verify
this conclusion, we performed calculations of the hydride
transfer reaction involving 1,5-diphosphinonaphthalene (7),
in which the second phosphine group is configurationally
unable to assist in the nucleophilic attack. This isomer is 5.9
kcal/mol lower in energy than the 1,8-isomer 6, which mainly
reflects the energy of steric hindrance in the latter. The
complex of 7 with BH3 is also lower in energy than the

Article
Organometallics, Vol. 28, No. 17, 2009 4935
Figure 5. B3LYP/6-31þG(d)-optimized structure of the transition state structures for model reaction 4 and for reaction 5 with 1,8-
bis(diphenylphosphino)naphthalene.
corresponding complex with 6, but the energies of transition
states are similar. Consequently, the energy barrier of the
hydride transfer involving 1,8-diphosphinonaphthalene (6)
(reaction 4) is 5 kcal/mol lower than that involving 1,5-isomer
7 (Figure 4a; obviously, the reaction of 7 does not lead to a
cyclic boronium cation; therefore, we cannot compare the
energies of the products). This result clearly demonstrates
that the main reason for the enhanced reacti-
vity of 6 is the high-energy-lying ground state due to steric
reasons. The steric congestion is predicted to be even larger in
1, which leads to the lowering of the energy barrier of reaction
5 as compared to reaction 4.
which reduces electrophilic sp3-carbon species. DFT calcula-
tions indicated that the transition state energy for reduction
of methyl chloride by the monoborane complex of 1,8-
bis(diphenylphosphino)naphthalene is much lower than that
for reduction of methyl chloride by borane itself. This
difference is due to high thermodynamic stability of the
cyclic boronium cation and steric congestion in the mono-
borane adduct.
Experimental Section
General Procedures. All reactions and manipulations were
carried out under an atmosphere of argon using standard
Schlenk and vacuum-line techniques. Common solvents were
dried, distilled under argon, and degassed before use. 1,8-Bis-
(diphenylphosphino)naphthalene (1) was prepared by literature
methods.15 H3B SMe2 (2
M
solution in diethyl ether),
3
H2ClB SMe2, and HBr2B SMe2 were purchased from Aldrich.
3
3
1H, 13C, 19F, and 31P NMR spectra were recorded on a Bruker
AC 200 spectrometer at 200.16, 50.33, 188.34, and 81.02 MHz,
respectively. 11B NMR spectra were recorded on a Bruker DRX
500 spectrometer at 160.46 MHz. IR spectra were measured on
an Ati Mattson Infinity FT IR 60. MS-FAB and spectra were
recorded on a Finnigan MAT 95 spectrometer. The melting
points were measured using a PHMK Boetius (VEB Analytik
Dresden) apparatus and were uncorrected.
Cyclization is thus predicted to take place in the late stage
of the reaction, when the hydride transfer is well advanced
and a positive charge is built on boron. The driving force for
the reaction is thermodynamic stability of the boronium
cation (Figure 3). The calculated transition state structures
for the model reaction 4 and for the investigated reaction 5
are depicted in Figure 5. They show close similarities, which
supports the reliability of the computational method applied
for the present problem.
1,8-Bis(diphenylphosphino)naphthalene-Borane Complex (3).
To a solution of 1 (0.338 g, 0.68 mmol) in THF (5 mL) was added
complex H3B SMe2 (0.41 mL, 0.82 mmol, 2 M solution in Et2O)
3
dropwise at room temperature. The solution was stirred for 1 h.
The solvent was removed under reduced pressure, and the
yellowish residue was washed with Et2O (2 ꢀ 10 mL) to give 3
1
(0.312 g, 90%). H NMR (C6D6): δ 7.95-7.30 (m, 8H, CH),
7.13-6.65 (m, 18H, CH), 3.50-2.00 (br m, 3H, BH). 13C{1H}
NMR (C6D6): δ 141.4 (s, C), 141.1 (d, JCP=4.8 Hz, CH), 140.5
Conclusions
(s, C), 138.7 (d, JCP=15.5 Hz, C), 137.9 (s, CH), 137.2 (d, JCP
20.1 Hz, C), 136.3 (dd, JCP=8.4, 8.4 Hz, C), 134.9 (dd, JPC
10.5, 10.5 Hz, CH), 134.4 (d, JPC=17.6 Hz, CH), 134.0 (d, JPC
8.5 Hz, CH), 130.5, 129.4, 129.2, 129.1, 129.0, 126.9 (s, CH),
125.1 (d, JPC = 9.2 Hz, CH). 31P{1H} NMR (C6D6): δ 25.7
(br s, PBH3), -11.6 (d, JPP = 60.4 Hz, PPh2). 11B NMR
(C6D6): δ -28.5 (h1/2 =540 Hz). MS-CI: m/z 511 [M þ H]þ.
The correct microanalysis data were not obtained due to very
easy oxidation
=
=
=
New cyclic bis(diphenylphosphino)boronium salts 4 X
3
(X=Cl, I, CF3SO3, PF6) with a 1,8-naphthalene backbone
have been obtained and characterized. They are formed from
the monoborane adduct of 1,8-bis(diphenylphosphino)na-
phthalene (3), which undergoes facile cyclization in the
presence of chlorohydrocarbon solvents, methyl iodide,
and triflate. The cyclization process is accompanied by
concomitant reduction of the above methyl derivatives and
chlorohydrocarbon solvents. Formally, this reductive cycli-
zation involves nucleophilic substitution at boron by the
phosphine group with the hydride anion as a leaving group,
(15) van Soolingen, J.; de Lang, R.-J.; den Besten, R.; Klusener, P. A.
A.; Veldman, N.; Spek, A. L.; Brandsma, L. Synth. Commun. 1995, 251,
741.

4936 Organometallics, Vol. 28, No. 17, 2009
Owsianik et al.
[1,8-Bis(diphenylphosphino)naphthalene]dihydroboron Chlori-
de (4 Cl). To a solution of 1 (0.300 g, 0.60 mmol) in CH2Cl2 (or
CHCl3) (5 mL) was added dropwise H3B SMe2 complex
(0.36 mL, 0.72 mmol, 2 M solution in Et2O) at room tempera-
ture. The solution was stirred for 12 h. The solvent was removed
under reduced pressure, and the white residue was washed with
129.9 (d, JCP=12.6 Hz, CH) 126.9 (d, JCP=11.1 Hz, CH), 123.3
(dd, JCP=6.0, 69.9 Hz, C), 117.3 (d, JCP=61.3 Hz, C). 31P{1H}
NMR (CD2Cl2): δ 5.2 (br s, PBH2), -144.4 (septet, JPF=710.6
Hz, PF6). 11B NMR (CD2Cl2): δ -37.2 (s). 19F{1H} NMR
(CD2Cl2): δ -73.0 (d, JPF=710.4 Hz). IR (KBr): 2479, 2410,
1438, 1102, 839 cm-1. FAB-MS: m/z 509 [M - PF6]þ. Anal.
3
3
Et2O (2 ꢀ 8 mL) to give 4 Cl (0.301 g, 92%); mp 231-232 ꢀC.
Calcd for C34H28P3BF6 2/3CH2Cl2 (710.96): C, 58.56; H, 4.17.
3
3
1H NMR (CDCl3): δ 8.62-8.45 (m, 2H, CH), 7.95-7.10 (m,
24H, CH), 3.75 (br s, 2H, H2O), 3.60 - 0.50 (br m, 2H, BH2)
=
Found: C, 58.87; H, 4.21.
[1,8-Bis(diphenylphosphino)naphthalene]bromohydroboron Bro-
13C{1H} NMR (CDCl3): δ 140.0, 137.6 (s, CH), 137.4 (d, JCP
mide (5 Br). To a solution of 1 (0.268 g, 0.54 mmol) in toluene
(5 mL) was added a solution of HBr2B SMe2 (0.152 g,
3
11.6 Hz, C), 135.4 (d, JCP=32.0 Hz, CH), 135.3 (d, JCP=6.1 Hz,
C), 133.0 (d, JCP=11.4 Hz, CH), 129.6 (d, JCP=11.8 Hz, CH),
128.0 (d, JCP=26.0 Hz, CH), 127.1 (d, JCP=8.1 Hz, CH), 123.0
(dd, JCP=70.5, 6.1 Hz, C), 115.8 (d, JCP=65.2 Hz, C). 31P{1H}
3
0.65 mmol) in toluene (2 mL) at room temperature. The solution
was stirred for 1 h; then the solvent was removed under reduced
pressure, and the obtained white residue was washed with Et2O
NMR (CDCl3): δ 4.7 (br s). 11B NMR (CDCl3): δ -33.6 (h1/2
=
(3 ꢀ 6 mL) to give 5 Br (0.332 g, 92%); mp 215-216 ꢀC.
3
430 Hz). IR (KBr): 2480, 2416 (νB-H), 1437, 1309, 1102, 714,
693 cm-1. FAB-MS: m/z 509 [M - Cl]þ. HRMS-(FABþ) m/z:
[M - Cl]þ. The product is hygroscopic (contains some water).
1H NMR (CD3OD): δ 8.67-8.45 (m, 2H, CH), 7.90-7.00 (m,
24H, CH). 13C{1H} NMR (CD3OD): δ 140.9, 138.2 (s, CH),
137.1 (d, JPC=9.5 Hz, C), 136.9 (d, JPC=33.8 Hz), 135.3 (d,
Alternative Method. Monoborane complex 4 Cl was also
J
PC=12.2 Hz, CH), 135.1 (d, JPC=12.1 Hz, CH), 134.8 (d, JPC
10.1 Hz, CH), 134.3 (d, JPC =4.7 Hz, CH), 131.4, 130.5 (dd,
PC=5.7, 5.7 Hz, CH), 128.2 (dd, JPC=5.2, 5.2 Hz, CH), 122.5
=
3
prepared from 1 (160 mg, 0.322 mmol) and H2ClB SMe2
3
(43 mg, 0.386 mmol) in a Schlenk flask in THF (5 mL) according
to the procedure described above (163 mg, 93%).
[1,8-Bis(diphenylphosphino)naphthalene]dihydroboron Iodide
J
(dd, JPC=77.6, 5.7 Hz, C), 120.9 (dd, JPC=76.1, 5.7 Hz, C),
117.4 (dd, JPC=70.3, 5.7 Hz, C). 31P{1H} NMR (CDCl3): δ -3.5
(br s). 11B NMR (CD3OD): δ -23.5 (h1/2=540 Hz). IR (KBr):
2424 (νB-H), 1436, 1099, 689, 570, 545, 529 cm-1. HRM-
S-(FABþ) m/z: [M - Br]þ calcd for C34H27BBrP2 587.08709;
found 587.08783. Anal. Calcd for C34H27P2BBr2 (668.16): C,
61.11; H, 4.08. Found: C, 60.89; H, 3.97.
(4 I). To a solution of 1 (0.308 g, 0.62 mmol) in THF (5 mL)
3
was added dropwise H3B SMe2 complex (0.37 mL, 0.74 mmol, 2
3
M solution in Et2O) at room temperature. The resultant solution
was stirred for 1 h, and then 1.2 equiv of MeI (0.105 g,
0.74 mmol) was added. The mixture was stirred for 4 h, and
the solvent was removed under reduced pressure to leave a
yellowish residue, which was washed with Et2O (2 ꢀ 5 mL) to
X-ray Structure Determination of Compound 4 PF6. Crystal
3
and molecular structure was determined using data collected at
low temperature on an Xcalibur PX CCD κ-geometry (Oxford
Diffraction) diffractometer16 with graphite -monochromatized
Mo KR radiation. The compound crystallizes in the triclinic
system, in space group P1 with the unit cell consisting of two
molecules of CH2Cl2. Crystal data and experimental details are
shown in Table 2. The lattice constants were refined by least-
squares fit of 18 111 reflections in the θ range 4.45-38.74ꢀ. A
total of 14 518 independent reflections with I > 0 were used to
solve the structure by direct methods and to refine it by full
matrix least-squares using F2.17,18 Anisotropic thermal para-
meters were refined for all non-hydrogen atoms. The final
refinement converged to R=0.0448 for 440 refined parameters
and 10 106 observed reflections with I g 2σ(I). Data processing
was carried out with CrysAlisCCD and CrysAlisRED (Oxford
Diffraction);16,19 structure solution with SHELXS;17 and struc-
ture refinement with SHELXL.18 Crystallographic data for the
give 4 I (0.371 g, 94%), mp 220-221 ꢀC. 1H NMR (CD3OD):
3
δ 8.60-8.45 (m, 2H, CH), 7.90-7.10 (m, 24H, CH), 3.30-1.75
(br m, 2H, BH). 13C{1H} NMR (CD3OD): δ 140.5 (s, CH), 137.8
(s, CH), 136.6 (dd, JCP=9.0, 9.0 Hz, C), 134.4 (d, JCP=11.1 Hz,
CH), 130.9 (d, JCP=12.1 Hz, CH), 130.8 (s, CH), 127.8 (d, JCP
10.8 Hz, CH), 124.6 (dd, JCP=6.0, 70.4 Hz, C), 117.9 (dd, JCP
=
=
3.7, 68.5 Hz, C). 31P{1H} NMR (CD3OD): δ 4.8 (br s). 11B NMR
(CD3OD): δ -37.2 (s). IR (KBr): 2481, 2417, 1435, 1102,
714, 692 cm-1. FAB-MS: m/z 509 [M - I]þ. Anal. Calcd for
C34H28P2BI (636.27): C, 64.18; H, 4.44. Found: C, 63.99;
H, 4.65.
[1,8-Bis(diphenylphosphino)naphthalene]dihydroboron Triflate
(4 CF3SO3). According to the above procedure starting from 1
3
(0.308 g, 0.62 mmol), H3B SMe2 complex (0.37 mL, 0.74 mmol,
3
2 M solution in Et2O), and TfOMe (0.121 g, 0.74 mmol), the salt
4 CF3SO3 (0.388 g, 95%) was obtained; mp 205-206 ꢀC.
3
1H NMR (CDCl3): δ 8.65-8.30 (m, 2H, CH), 7.90-7.00 (m,
24H, CH), 3.60-1.80 (br m, 4H, H2O, BH). 13C{1H} NMR
(CDCl3): δ 140.5, 137.8 (s, CH), 136.7 (dd, JCP=8.9, 8.9 Hz, C),
134.4 (d, JCP=11.1 Hz, CH), 134.3 (s, CH), 130.9 (d, JCP=12.1
Hz, CH), 130.8 (s, CH), 127.8 (d, JCP=10.6 Hz, CH), 124.6 (dd,
JCP=6.0, 70.3 Hz, C), 118.4 (dd, JCP=3.8, 68.6 Hz, C). 31P{1H}
structure 4 PF6 have been deposited with the Cambridge Crys-
3
tallographic Data Centre as Supplementary Publication No.
296784. Copies of the data can be obtained free of charge on
application to the CCDC, 12 Union Road, Cambridge CB21-
EZ, U.K. (fax, (þ44) 1223-336-033; e-mail, deposit@ccdc.com.
ac.uk).
NMR (CDCl3): δ 4.8 (br s). 11B NMR (CDCl3): δ -34.9 (h1/2
=
Theoretical Methods. All calculations were performed using
the density functional theory methods with the Gaussian 03
program.20 Equilibrium geometries were optimized with the
B3LYP/6-31þG(d) method. All potential energy minima and
transition states were identified by frequency analysis. Transi-
tion states, except for the one involving 1,8-bis(diphenyl)-
phosphinonaphthalene, were further examined by the IRC
calculations to verify the reaction pathways. Final electronic
energies for the stationary points were calculated at the B3LYP/
6-311þG(2d,p) level for the B3LYP/6-31þG(d) geometries (this
430 Hz). 19F{1H} NMR (CDCl3): δ -77.4 (s). IR (KBr): 2480,
2425, 1274, 1030, 636 cm-1. FAB-MS: m/z 509 [M - CF3SO3]þ.
HRMS-(FABþ) m/z: [M - CF3SO3]þ, calcd for C34H28BP2
509.17655; found 509.17631. The product is hygroscopic.
[1,8-Bis(diphenylphosphino)naphthalene]dihydroboron Hexa-
fluorophosphate (4 PF6). To a solution of 4 Cl (0.289 g,
3
3
0.53 mmol) in MeOH (5 mL) was added a solution of KPF6
(0.107 g, 0.58 mmol) in MeOH (5 mL). The solution was stirred
at room temperature for 1 h. Methanol was removed on a rotary
evaporator, and the residue was dissolved in CH2Cl2 and
purified by extraction with water to give a white solid of
(16) CrysAlisCCD Version 1.171, data collection program; Oxford
Diffraction Poland: Wroclaw, Poland, 2003.
4 PF6 (0.295 g, 85%). Crystallization of the product from
3
€
(17) Sheldrick, G. M. SHELXS97; University of Gottingen, Germany,
1997.
(18) Sheldrick, G. M. SHELXL97; University of Gottingen, Germany,
1997.
(19) CrysAlisRED Version 1.171, data reduction program;Oxford
Diffraction Poland: Wroclaw, Poland, 2003.
CH2Cl2/toluene (10:1) resulted in colorless crystals; mp 194-
1
195 ꢀC. H NMR (CD2Cl2): δ 8.50-8.30 (m, 2H, CH), 7.85-
€
7.10 (m, 24H, CH), 5.30 (s, 2H, CH2Cl2), 3.50-2.00 (br m, 2H,
BH). 13C{1H} NMR (CD2Cl2): δ 139.4, 136.8 (s, CH), 135.4 (m,
C), 133.4 (s, CH), 133.3 (d, JCP=12.1 Hz, CH), 130.0 (s, CH),

Article
Organometallics, Vol. 28, No. 17, 2009 4937
level of theory is denoted as B3LYP/6-311þG(2d,p)//B3LYP/6-
31þG(d)). Thermal corrections to the enthalpy and entropy at
298.15 K were scaled by 0.98. Calculations were performed for
the gas phase conditions. Atomic charges and Wiberg bond
orders were computed according to natural bond orbital theory
using the Gaussian built-in option.21 Solvation energies in THF
were calculated using a continuum solvation model and the
SCRF-PCM method22 as implemented in Gaussian 03 using gas
phase geometries. UAHF atomic radii have been used for cavity
definition.
(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
Acknowledgment. This work was performed with the
ꢀ
ꢀ
support of the Laboratoire Europeen Associe (LEA) financed
by the CNRS (France) and with the support of the European
Commission within the fifth Framework Programme
(contract ICA-CT-2000-70021-Center of Excellence).
Supporting Information Available: Crystallographic data as a
CIF file and unit cell for 4 PF6, B3LYP/6-31G(d)-optimized
3
1
structure of cyclic borane cation [4]þ, and H, 31P, 13C NMR
spectra of 3 and 4 Cl. This material is available free of charge via
the Internet at http://pubs.acs.org.
3
(21) Glendening, E. D.; Reed, A. E.; Weinhold F. NBO ver. 3.1.;
University of Wisconsin, Madison: 1998.
(22) Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027.