6-Phenyldibenzo[b,d]phosphinine

Article abstract of DOI:10.1515/znb-2003-0810

The title compound (2b) was obtained in 7 steps from 2-phenylbenzhydrol (5). The phenyl substituent of 2b confers thermal stability to the [5]phosphaphenanthrene system by steric protection; in addition, conjugative interaction is revealed by the UV spect

Full text of DOI:10.1515/znb-2003-0810

6-Phenyldibenzo[b,d]phosphinine
Pieter de Koe and Friedrich Bickelhaupt
Scheikundig Laboratorium, Vrije Universiteit,
De Boelelaan 1083, NL-1081 HV Amsterdam, The Netherlands
Reprint requests to Prof. Dr. F. Bickelhaupt. E-mail: bicklhpt@few.vu.nl
Z. Naturforsch. 58b, 782 – 786 (2003); received April 14, 2003
The title compound (2b) was obtained in 7 steps from 2-phenylbenzhydrol (5). The phenyl sub-
stituent of 2b confers thermal stability to the [5]phosphaphenanthrene system by steric protection; in
addition, conjugative interaction is revealed by the UV spectrum. The analogy to the corresponding
arenes is briefly discussed.
Key words: Aromatic Phosphorus Heterocycles, UV Spectroscopy
Introduction
In contrast to benzene or pyridine and their deriva-
tives, the phosphorus analogs, the phosphinines (or
phosphabenzenes) [1– 4] tend to be less stable both
thermally and towards oxygen, in particular if ad-
ditional aromatic rings are annelated to the cen-
tral heterocycle. Thus, in the series of tricyclic
phosphinines, both [9]phosphaanthracene 1a [5] and
[5]phosphaphenanthrene2a [6] were too unstable to be
isolated in pure form (Scheme 1). However, introduc-
tion of a phenyl substituent as in 1b increased the ther-
mal stability sufficiently to allow isolation [7]. For that
reason, we undertook the synthesis of 2b, the phenyl
derivative of the parent 2a, in order to probe the gen-
eral validity of this principle and to facilitate a compar-
ison between 1 and 2 and their carbon analogues 3 and
4, respectively.
Scheme 1.
Experimental details of our preparation of 2b have
so far not been published except in a thesis [2,8]. In the
meantime, the photoelectron spectrum of 2b [9] as well
as an alternative synthetic approach to 2b starting from
5-benzyldibenzophosphole [10] have been reported.
unstable product with a high tendency toward cycliza-
tion to 9-phenylfluorene.However, heating freshly pre-
pared 6 with triethyl phosphite furnished 7 in 58 %
yield. Hydrolysis of 7 by heating under reflux, first in
concentrated aqueous hydrogen bromide and then in
water, gave 8, which was cyclized to 9 by heating at
360 ◦C under vacuum. Reduction of 9 with trichlorosi-
lane furnished 10 which on treatment with phosgene
gave 11.
Results and Discussion
In analogy to the strategy which had proven success-
ful in the case of 1a,b and 2a, we performed the syn-
thesis of 2b by base catalyzed elimination of hydrogen
chloride from the immediate precursor 11 which was
obtained as outlined in Scheme 2.
The tendency to eliminate hydrogen chloride from
11 under formation of 2b was even higher than the
analogous reaction in the linear series leading to 1b
Passing gaseous hydrogen bromide into a benzene [7]. This follows e.g. from the observation that at-
solution of 2-phenylbenzhydrol (5) afforded 6 as an tempted vacuum distillation of 11 gave a mixture of
c
0932–0776 / 03 / 0800–0782 $ 06.00 ꢀ 2003 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/25/18 8:20 PM

P. de Koe – F. Bickelhaupt · 6-Phenyldibenzo[b,d]phosphinine
783
Table 1. UV spectra of tricyclic arenes 1 – 4.
Compound λmax
∆λmax
∆λmax
[nm][b]
50
Solvent[c]
[nm]
[nm][a]
1a [5]
426
432
431
372
339
376
384
386
343
299
toluene
diethyl ether
diethyl ether
diethyl ether
diethyl ether
hexane
ethanol
ethanol
hexane
95% ethanol
1b [7]
1c [15]
2a [6]
6
48
29
40
2b[d]
−33
3a [12]
3b [13]
3c [16]
4a [14]
4b [11]
8
−44
[a]
Shift of λmax on replacement of hydrogen by phenyl (a → b);
shift of λmax on replacement of carbon by phosphorus (3 → 1 or
[b]
4 → 2, respectively); [c] the solvent dependence of λmax is generally
small (λmax [polar] < λmax [apolar]) and in the order of 3 – 4 nm;
[d]
this work.
Scheme 2.
11 and 2b containing 85 – 90% of the latter. Pure
2b was prepared by treatment of 11 with DBU (1.5-
◦
diazabicyclo[5.4.0]undec-5-ene) in DMF at 20 C; af-
ter the usual workup followed by vacuum sublima-
tion, 2b was obtained as colorless crystals. The iden-
tity of 2b was established by elemental analysis and
spectral data. Besides the phosphorus chemical shift
(δ(31P) = 186 [10]), the most convincing and informa-
tive indicator is the UV spectrum (Fig. 1).
Fig. 1. UV Spectra of 2b (——–, in diethyl ether) and 4b
(– – –, in 95% ethanol).
Generally [2 – 4], the UV spectra of phosphaarenes
are rather similar in shape and extinction coefficient to
those of the parent arenes. However, the longest wave-
length absorption λmax is shifted even further when a
CH group is replaced by phosphorus; for the combi-
nations 3/1 and 4/2, this shift is ∆λmax = 30 – 50 nm
(Table 1).
phorus series (3a→3b and 1a→1b, respectively) leads
to a small bathochromic shift only (∆λmax = 6 –
8 nm) of the longest wavelength absorption. As in
the anthracene derivatives the influence of the phenyl
and the methyl substituent on the absorption spec-
tra is practically the same (1b: λmax = 432 nm, 1c:
Of special interest is a comparison between the λmax = 431 nm [15]; 3b: λmax = 384 nm, 3c: λmax
=
spectra of 1a/2a on the one hand and of 1b/2b on 386 nm [16]), it is evident that the phenyl group has
the other. In the anthracene series, introduction of a no conjugative interaction with the anthracene system,
phenyl substituent in both the carbon and the phos- because presumably for steric reasons, it is forced into
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/25/18 8:20 PM

784
P. de Koe – F. Bickelhaupt · 6-Phenyldibenzo[b,d]phosphinine
parison to the unsubstituted parent 2a, the phenyl sub-
stituent increases the thermal stability considerably,
but 2b remains sensitive towards oxygen. The UV
spectrum reveals conjugation between the [5]phos-
phaphenanthrene system and the phenyl group; it also
again confirms the similarity between phosphaarenes
(such as 2b) and their carbon analogues (such as 4b),
except for the well-known trend of phosphaarenes to
show a shift towards longer wavelength .
Scheme 3.
an orientation perpendicular to the anthracene plane by
the presence of two peri-hydrogen atoms at the adja-
cent benzene rings [13, 15]. By contrast, in the phenan-
threne series, and again both for carbon and phospho-
rus (4a→4b and 2a→2b, respectively), the shift on in-
troduction of the phenyl substituent is quite large and
hypsochromic (∆λmax = −33 to −44 nm). It is ob-
vious that the seeming hypsochromicity is in reality
due to the fact that in the parent and the phenyl sub-
stituted compounds, the longest wavelength absorp-
tions are due to different transitions (4a, 2a: α-band,
4b, 2b: p-band [17]). For both 4b and 2b, this means
that conjugation between the phenyl substituent and
the phenanthrene chromophore does occur because of
a higher degree of coplanarity than in 1b and 3b, which
is facilitated by reduced steric hindrance in the phenan-
threne series as it has only one peri-hydrogen atom at
the ring directly adjacent to the phenyl group.
Experimental Section
The synthesis of 2b, 10, and 11 was performed under
spectrally pur◦e nitrogen in glassware which had been oven-
dried at 150 C. Solid starting materials were dried under
vacuum, liquid starting materials were distilled under nitro-
gen. Solvents were distilled from LiAlH4 (pentane, diethyl
ether, cyclohexane, toluene) or from sodium/benzophenone
(THF). Air sensitive compounds were handled in a Schlenk
apparatus; melting points were determined in sealed glass
1
capillaries. H NMR spectra were obtained at 20 ◦C with a
Varian A-60 spectrometer at 60 MHz (TMS as external stan-
dard), UV spectra with a Perkin-Elmer 137 spectrophotome-
ter, IR spectra with a Perkin-Elmer 237 spectrometer, and
mass spectra with an AEI-MS9 mass spectrometer. Elemen-
tal analyses were performed in our laboratory.
6-Phenyldibenzo[b,d]phosphinine (2b)
With regard to thermal stability, the effect of the in-
troduction of a phenyl substituent as in 1b and 2b is
comparable. Contrary to the unsubstituted analogues
of the a-series, both possess a strongly increased ther-
mal stability which allows sublimation at 130– 150 ◦C.
However, the protective action of the phenyl group to-
wards oxygen is insufficient. While in the solid state
both 1b and 2b are reasonably stable towards air, they
are prone to oxidation in solution so that preparation
and handling had to be performed in a high vacuum
system. When a sealed vessel containing a solution of
2b in toluene accidentally broke and was kept standing
under air overnight, subsequent extraction with aque-
ous ammonia and TLC analysis of the mixture revealed
the formation of at least three (phosphorus)acids; crys-
tallization from ethanol gave a colorless product in low
yield to which on the basis of elemental analysis and
IR data the structure of 12 (Scheme 3) was tentatively
assigned.
The preparation and handling of 2b was carried out in
a closed glass vessel sealed under high vacuum [18,19].
A solution of 11 (1.50 g, 4.85 mmol) and DBU (0.84 g,
◦
5.54 mmol) in DMF (45 ml) was kept overnight at 20 C.
The reaction mixture was evaporated and the residue was ex-
tracted 4 times with cyclohexane (250 ml total). The cyclo-
hexane extract was evaporated and the residue crystallized
from toluene, followed by vacuum sublimation (bath temper-
ature of 130 – 140 ◦C). The yield could not be determined di-
rectly; titration according to Mohr of the DBU•HCl left after
cyclohexane extraction indicated that 79% of DBU•HCl had
been obtained. 2b: M.p. 124 – 131 ◦C [20]. – UV/vis (Et2O):
λ
max [nm] (lg ε) = 247 (4.53), 257.5 (4.50), 265.5 (sh, 4.47),
279 (sh, 4.29), 291 (4.23), 339 (4.08). – 1H NMR (CS2):
δ = 9.38 – 9.13 (m, 2 H), 9.05 – 8.6 (m, 1 H), 8.43 – 7.77
(m, 10 H). – MS (EI, 70 eV): m/z (%): = 272 (100) [M+],
244 (3), 242 (4), 241 (5), 240 (4), 239 (10), 165 (2), 152
(0.5), 136 (6) [M2+], 135 (6), 122 (8). – C19H13P: calcd. for
[M+] 272.0755; found 272.0749. – C19H13P (272.27): calcd.
C 83.81, H 4.81, P 11.38; found C 83.9, H 5.2, P 11.2.
Conclusion
2-Phenylbenzhydryl bromide (6)
At 20 ◦C, gaseous hydrogen bromide was introduced into
Starting from 2-phenylbenzhydrol(5), the title com-
pound 2b was prepared in a 7 step synthesis. In com- a solution of 5 [21] (5 g, 19.2 mmol) in benzene (25 ml) un-
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/25/18 8:20 PM

P. de Koe – F. Bickelhaupt · 6-Phenyldibenzo[b,d]phosphinine
785
til saturation was reached. The water formed was removed 5,6-Dihydro-6-phenyldibenzo[b,d]phosphinine (10)
by pipettation, CaCl2 was added, and hydrogen bromide
Trichlorosilane (46 g, 340 mmol) was gradually added
to a boiling suspension of 9 (20 g, 65.3 mmol) in benzene
(70 ml). The reaction mixture became homogeneous and was
heated under reflux for 24 h. Under cooling and stirring in an
ice bath, 20% aqueous NaOH (air-free, 100 ml) was added.
The water layer was pipetted off and the organic layer was
washed with 2N HCl (air-free) followed by water. The or-
ganic layer was dried (MgSO4, air-free) and filtered under
nitrogen pressure through a glass tube filled with glass wool
into a distillation flak. The solvent was evaporated and the
residue distilled to furnish 10 as a colorless viscous oil (8.5 g,
47%); according to the 1H NMR spectrum, it consisted of a
mixture (1.5 : 1) of the E/Z isomers 10A and 10B. 10: B.p.
was again introduced until saturation. After filtration (glass
wool) and evaporation of the filtrate to dryness, the oily
residue was crystallized from petroleum ether. The colorless
white crystals of 6 were pure according to bromine analy-
sis (boiling an aliquot of a solution in dioxane with a so-
lution of KOH in ethanol, followed by neutralization with
dilute H2SO4 and titration according to Mohr: C19H15Br
(323.23): calcd. Br 24.72; found Br 24.6). After standing
overnight in a desiccator, the material was largely converted
to 9-phenylfluorene [21].
Diethyl 2-phenylbenzhydrylphosphonate (7)
◦
170 C/13 mubar). – 1H NMR (CS2): δ = 8.3 – 7.25 (m,
A mixture of freshly prepared 6 (23.9 g, 74 mmol) and
1
3
triethyl phosphite (25 g, 150 mmol) was gradually heated to
13 H); 10A: 5.04 (dd, JP,H = 194 Hz, JH,H = 2.5 Hz,
PH), 4.80 (dd, 2JP,H = 3JH,H = 2.5 Hz, CH); 10B: 4.83 (dd,
1JP,H = 192 Hz, 3JH,H = 11 Hz, PH), 4.38 (dd, 2JP,H = 7 Hz
3JH,H = 11 Hz, CH). – C19H15P (274.29): calcd. C 83.19,
H 5.51, P 11.29; found C 83.5, H 5.2, P 11.6.
◦
50 C and maintained at this temperature for 0.5 h. Then
the reaction mixture was heated under reflux for 5 h. Af-
terwards, ethyl bromide and excess triethyl phosphite were
distilled off, and the residue was subjected to vacuum distil-
lation (1.3 mubar), yielding 9-phenylfluorene (0.7 g, 3.9%,
◦
5-Chloro-5,6-dihydro-6-phenyldibenzo[b,d]phos-
phinine (11)
b.p. 125–162 C) and 7 (16.2 g, 58%) as a colorless oil. 7:
◦
1
B.p. 163 C. – H NMR (CCl4): δ = 8.26 – 8.05 (m, 1 H),
2
7.5 – 7.0 (m, 13 H), 4.46 (d, JP,H = 25 Hz, 1 H), 4.14 –
A solution of phosgene in toluene was prepared by con-
densing phosgene (1 ml) into a calibrated vessel cooled to
−80 ◦C, followed by distillation into toluene (15 ml) cooled
3.39 (m, 4 H), 1.06 (dt, 3JH,H = 7 Hz, 4JP,H = 4 Hz, 6 H). –
C23H25O3P (380.41): calcd. C 72.61, H 6.62, P 8.14; found
C 72.0, H 6.7, P 8.2.
◦
to −80 C; the concentration (0.814 mmol/ ml) was deter-
mined after hydrolysis of an aliquot and titration of chlo-
ride according to Mohr. This phosgene solution (18.6 ml,
15.1 mmol) was added dropwise to a solution of 10 (3.95 g,
14.4 mmol) in toluene (25 ml) cooled to −80◦C. After warm-
ing to 20 ◦C, the colorless precipitate was removed by filtra-
tion under nitrogen pressure through a glass tube filled with
glass wool. The filtrate was evaporated to dryness and the
oily residue subjected to distillation (1.3 mubar, bath temper-
ature 170 – 180 ◦C) to furnish (rather) pure 11 (3.46 g, 78%)
as a slightly yellowish viscous oil. 11: 1H NMR (CS2): δ =
8.50 – 7.17 (m, 13 H), 5.22 (s, 1 H). – C19H14ClP (308.74):
calcd. C 73.91, H 4.57, P 10.03, Cl 11.49; found C 73.9,
H 4.8, P 10.2, Cl 10.2.
2-Phenylbenzhydrylphosphonic acid (8)
A mixture of 7 (65.0 g, 171 mmol) and concentrated hy-
drobromic acid (1 l) was heated under reflux for 20 h. After
cooling, the solid material was filtered off and boiled with
water for 5 h; the precipitate was filtered off and recrys-
tallized from ethanol (96%) to yield colorless crystals of 8
◦
(50.4 g, 91%). 8: M.p. 231.5 – 234 C. – 1H NMR ([D6]
DMSO): δ = 8.32 – 7.73 (m, 4 H), 7.43 – 6.8 (m, 12 H), 4.48
2
(d, JP,H = 24,5 Hz, 1 H). – C19H17O3P (324.31): calcd.
C 70.36, H 5.28, P 9.55; found C 70.7, H 5.8, P 9.6.
5,6-Dihydro-5-hydroxy-5-oxo-6-phenyldibenzo[b,d]phos-
phinine (9)
Reaction of 2b with air
Adapting a method described by Lynch [22], 8 (38.9 g,
120 mmol) was heated under vacuum at a bath temperature
At one occasion, an evacuated vessel containing a solution
of 350 ◦C for 6 h. After cooling, the glassy residue was bro- of 2b in toluene broke. After standing in air overnight, the so-
ken up and dissolved by boiling with NaOH (1N, 250 ml) for lution was evaporated to dryness to give as a residue a color-
10 min, followed by cooling and acidification with 2N HCl. less oil (1.24 g), 0.48 g of which dissolved in dilute ammonia.
The precipitate thus obtained was recrystallized from ethanol The ammonia extract contained at least three acids accord-
(96% ) to yie◦ld colorless crystals of 9 (28.5 g, 78%). 9: M.p. ing to TLC (silica gel, propanol/25% ammonia 7 : 3). After
1
244.5 – 251 C. – H NMR ([D6]DMSO): δ = 8.18 – 6.83 acidification and three crystallizations from 96% ethanol, 12
2
(m, 14 H), 4.59 (d, JP,H = 24.0 Hz, 1 H). – C19H15O2P was ◦obtained (10.4 mg) as colorless crystals. 12: M.p. 250 –
(306.29): calcd. C 74.50, H 4.94, P 10.11; found C 74.4, 251 C. – IR (KBr): 1185 cm−1 (P=O), 970 cm−1 (P-OH),
H 5.0, P 10.1.
800 – 600 cm−1 (substitution pattern identical to that of 9);
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/25/18 8:20 PM

786
P. de Koe – F. Bickelhaupt · 6-Phenyldibenzo[b,d]phosphinine
(CHCl3): 3660 cm−1 (C-OH); for similar characteristic data (322.29): calcd. C 70.80, H 4.69, P 9.61; found C 70.3, H 5.1,
on monocyclic analogues of 12 see ref. [23]. – C19H15O3P P 9.8.
[12] H. H. Perkampus, I. Sandeman, C. J. Timmons, UV At-
las of Organic Compounds, Vol. II, E2/1, Butterworths,
London, Verlag Chemie, Weinheim (1966).
[13] E. Clar, D. G. Stewart, J. Am. Chem. Soc. 74, 6235
(1952).
[1] K. Dimroth, Top. Curr. Chem. 38, chapter 3 (1973);
G. Ma¨rkl in Multiple Bonds and Low Coordination in
Phosphorus Chemistry (Ed.: M. Regitz, O. J. Scherer),
chapter 5, Thieme, Stuttgart (1990); Phosphorus: The
Carbon Copy (Ed.:
[14] H. H. Perkampus, I. Sandeman, C. J. Timmons, UV
Atlas of Organic Compounds, Butterworths, London,
Verlag Chemie, Weinheim, Vol. III, E3/1 (1966).
[15] C. Jongsma, H. Vermeer, F. Bickelhaupt, W. Scha¨fer,
A. Schweig, Tetrahedron 31, 2931 (1975).
K. B. Dillon, F. Mathey, J. F. Nixon), chapter 8.5, Wiley,
Chichester (1998).
[2] P. Le Floch in Phosphorus-Carbon Heterocyclic Chem-
istry: The Rise of a New Domain (Ed.: F. Mathey), Perg-
amon, Amsterdam, chapter 5.2 (2001).
[16] R. A. Friedel, M. Orchin, Ultraviolet Spectra of Aro-
matic Compounds, Wiley, New York, Chapman and
Hall, London, No. 389 (1951).
[3] L. Nyula´szi, Chem. Rev. 101, 1229 (2001).
[4] F. Mathey, Angew. Chem. 115, 1616 (2003); Angew.
Chem. Int. Ed. 42, 1578 (2003).
[17] M. Hesse, H. Meier, B. Zeeh, Spektroskopische Meth-
oden in der organischen Chemie, 4th ed., p. 16, Georg
Thieme Verlag, Stuttgart (1991).
[18] A. D. Vreugdenhil, C. Blomberg, Recl. Trav. Chim.
Pays-Bas 82, 453 (1963).
[5] P. de Koe, F. Bickelhaupt, Angew. Chem. 79, 533
(1967); Angew. Chem. Int. Ed. 6, 567 (1967).
[6] P. de Koe, R. van Veen, F. Bickelhaupt, Angew. Chem.
80, 486 (1968); Angew. Chem. Int. Ed. 7, 465 (1968).
[7] P. de Koe, F. Bickelhaupt, Angew. Chem 80, 912
(1968); Angew. Chem. Int. Ed. 7, 889 (1968).
[8] P. de Koe, Ph. D. Dissertation, chapter 4, Vrije Univer-
siteit, Amsterdam (1969).
[9] W. Scha¨fer, A. Schweig, F. Bickelhaupt, H. Vermeer,
Rec. Trav. Chim. Pays-Bas. 93, 17 (1974).
[10] F. Nief, C. Charrier, F. Mathey, M. Simalty, Tetrahedron
Lett. 21, 1441 (1980).
[19] J. W. F. L. Seetz, B. J. J. van de Heisteeg, H. J. R. de
Boer, G. Schat, O. S. Akkerman, F. Bickelhaupt in
Organometallic Syntheses (Ed.: R. B. King, J. J. Eisch),
Vol. 4, p. 392, Elsevier, Amsterdam (1988).
[20] The m.p. of 85 ◦C, reported for 2b in ref. [10] is proba-
bly too low as it was determined on a Kofler bank under
air: F. Nief, personal communication.
[21] C. F. Koelsch, J. Am. Chem. Soc. 56, 480 (1934).
[22] E. R. Lynch, J. Chem. Soc. 3729 (1962).
[23] K. Dimroth, K. Vogel, W. Mach, U. Schoeler, Angew.
Chem. 80, 359 (1968); Angew. Chem. Int. Ed. 7, 371
(1968).
[11] R. A. Friedel, M. Orchin, Ultraviolet Spectra of Aro-
matic Compounds, No. 383, Wiley, New York, Chap-
man and Hall, London (1951).
Brought to you by | University of Sussex Library
Authenticated
Download Date | 9/25/18 8:20 PM