Syntheses and characterization of bis(trifluoromethyl)phosphino naphthalenes and acenaphthenes

Article abstract of DOI:10.1039/b916425a

Syntheses of heteroleptic 1,8-bis(phosphino)naphthalenes and 5,6-bis(phosphino)acenaphthenes were attempted using several synthetic strategies. Reaction of aryllithium with triphenylphosphite gave ArP(OPh) ₂ (Ar = substituted naphthalene or acenaphthene), which was transformed into ArP(CF₃)₂, using a nucleophilic trifluoromethylation reaction with Me₃SiCF₃/CsF. The importance of the correct choice of solvent for the trifluoromethylation reactions is discussed. The incompatibility of ArP(CF₃)₂ with organolithium hampered the attachment of the second phosphine functionality to the organic backbone. Tetraphenoxyethylene was obtained in a small amount as a side product in the trifluoromethylation reaction. Selected new compounds were characterized by single-crystal X-ray diffraction. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b916425a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Syntheses and characterization of bis(trifluoromethyl)phosphino naphthalenes
and acenaphthenes†
Piotr Wawrzyniak, Alexandra M. Z. Slawin, J. Derek Woollins and Petr Kilian*
Received 10th August 2009, Accepted 8th October 2009
First published as an Advance Article on the web 5th November 2009
DOI: 10.1039/b916425a
Syntheses of heteroleptic 1,8-bis(phosphino)naphthalenes and 5,6-bis(phosphino)acenaphthenes were
attempted using several synthetic strategies. Reaction of aryllithium with triphenylphosphite gave
ArP(OPh)2 (Ar = substituted naphthalene or acenaphthene), which was transformed into ArP(CF3)2,
using a nucleophilic trifluoromethylation reaction with Me3SiCF3/CsF. The importance of the correct
choice of solvent for the trifluoromethylation reactions is discussed. The incompatibility of ArP(CF3)2
with organolithium hampered the attachment of the second phosphine functionality to the organic
backbone. Tetraphenoxyethylene was obtained in a small amount as a side product in the
trifluoromethylation reaction. Selected new compounds were characterized by single-crystal X-ray
diffraction.
helps to stabilize the negatively charged carbon atom, whilst the
Introduction
positive charge at phosphorus is delocalized into the dialkylamino
Substituent effects are a powerful tool in chemist’s hands, since
groups. This arrangement stabilizes the (normally highly reactive)
they allow modification of the reactivity and the structure of
carbenes so that they can be isolated and stored for weeks or longer
at ambient temperatures.3
molecules via electronic and steric effects. One interesting scenario
is the situation where two electronically disparate substituents (i.e.
electron acceptor A and electron donor Do) are present in one
molecule. The notation “captodative” or “push-pull” is used in
such situations. For example, the attachment of electron acceptor
and donor groups to an atom bearing an unpaired electron, results
in a stabilizing effect stemming from additional delocalization of
the spin population (Scheme 1, top).1,2 Situations where electron
accepting group(s) and electron donating group(s) are attached
to two atoms in a vicinal fashion are also of interest. Thus
Bertrand’s carbenes (Scheme 1, bottom) benefit from the presence
of electronically disparate substituents. This is best illustrated on
the most stable ylide form of these species, shown in the bottom
right of Scheme 1. The trimethylsilyl group on the carbon atom
Both the previous examples deal mainly with movements of
the p-electrons (delocalization of p-bonds) as a result of attaching
electron donating and accepting substituents. However, in certain
situations, even s-bond formation can be directed by substituent
effects. For example, in the phosphine-phosphine complex 1B,
the formation of the P–P bond is facilitated by the substituents
attached to its two phosphorus atoms.4 Isopropylphosphino
groups are sufficiently s-donating, and chloride groups sufficiently
s-accepting, to allow rehybridization of the two phosphine
environments (structure 1A in Scheme 2) into phosphonium-
phosphoranide (structure 1B) with the formation of a new s P–P
bond. Calculations showed 1B is the most stable structure (in
the gas phase),5 and structure 1B was found to be present also
in the crystal (by X-ray diffraction).4 The electronically disparate
substituents are essential for the formation of the P–P bond in 1B,
since in both 2 and 3 (which are homoleptic relatives to 1B) the
P ◊ ◊ ◊ P interaction is purely repulsive.
Scheme 1 Electron delocalization in captodative (push-pull) stabilized
radicals (top) and in Bertrand’s carbenes (bottom).
Scheme 2
School of Chemistry, EaStCHEM, University of St Andrews, St Andrews,
Fife, UK KY16 9ST. E-mail: pk7@st-andrews.ac.uk; Fax: +44 1334 463808;
Tel: +44 1334 467304
† Electronic supplementary information (ESI) available: X-ray structure
data of tetraphenoxyethene 16. CCDC reference numbers 743890, 743891,
743892 and 743893. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b916425a
Given that 1B is the first room temperature stable example of
the phosphine-phosphine complex, it was of interest to synthesize
some more examples of heteroleptic peri-substituted naphthalenes
and to study the interplay of the electronic nature of the
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substituents and structure in these molecules. The results of our
synthetic efforts towards this aim are the subject of this paper.
Furthermore, because compounds containing P–Cl bonds are
generally very reactive, their replacement with more air and
moisture stable alternatives is also of interest. Our attention
turned to trifluoromethyl groups, which are strongly electron
withdrawing groups, frequently used in transition metal ligand
design, but also in main group chemistry. Efficient syntheses of
phosphines with one or more perfluoroalkyl substituents (Rf)
are surprisingly rare, with only a few examples in the literature
of convenient reaction pathways which employ easily accessible
phosphorus precursors and use easy to handle reagents.6 In one
of the few exceptionally convenient syntheses, the commercially
available Ruppert’s reagent, Me3SiCF3, was successfully used to
perform nucleophilic trifluoromethylation reactions with a range
of phosphorus substrates.6,7 The family of these substrates was
extended recently by a range of phenoxy derivatives, i.e. P(OPh)3,
RP(OPh)2 and R2P(OPh) (R = alkyl and aryl,8 ferrocene-1,1¢-
diyl9). Very conveniently, the reported –OPh → –CF3 metatheses
proceeded at room temperature, in the presence of dry CsF as the
only catalyst, using common ether solvents. Our starting phenoxy
derivatives [ArP(OPh)2] (Ar = suitably substituted naphthalenes
and acenaphthenes) were accessible from the nucleophilic P–C
coupling reaction of aryllithiums with triphenylphosphite, using
the procedure reported recently.10
stirring at -78 ◦C for an additional 6 h, the mixture was left
to warm up to room temperature and was stirred overnight.
Chlorotrimethylsilane (1.1 cm3, 9 mmol) was added dropwise and
after 2 h of stirring the volatiles were evaporated in vacuo. The
residue was suspended in diethylether (60 cm3) and the insoluble
LiCl was filtered off. Evaporation of the volatiles in vacuo afforded
5 as colorless solid, yield 1.61 g, 81%. The purity of such obtained 5
was sufficient for further syntheses. dP (109.4 MHz, CDCl3) 148.9
(s); m/z (EI) 424.0 (M+).
NapBr[P(CF3)2] 6 (from 5). CsF (0.47 g, 3.11 mmol) was
suspended in a solution of 5 (0.53 g, 1.25 mmol) in THF (20 cm3).
Me3SiCF3 (0.46 cm3, 3.11 mmol) was added dropwise and the
reaction mixture was stirred for 4 h. The volatiles were evaporated
in vacuo, the residue was suspended in diethylether (10 cm3),
the resulting suspension was filtered and volatiles were removed
in vacuo. The residue was subjected to column chromatography
(silica, eluent hexane), which afforded 6 (0.32 g, 66%) as a colorless
solid. Very pure 6 was obtained by additional recrystallization
from hexane (Found: C, 38.50; H, 1.44. C12H6BrF6P requires
C, 38.43; H, 1.61; dH (300.1 MHz, CDCl3) 8.23 (dd, J = 7.4
and 3.2 Hz, 1H), 7.96 (dd, J = 8.2 and 1.0 Hz, 1H), 7.86 (dd,
J = 7.5 and 1.3 Hz, 1H), 7.78 (dt, J = 8.1 and 1.5 Hz, 1H),
7.49 (m, 1H), 7.27 (m, 1H); dC (75.5 MHz, CDCl3) 137.5 (s,
CH), 135.4 (d, J = 6.0 Hz, Cq), 133.9 (s, CH), 133.8 (s, CH),
1
Because of its convenience, we have adopted Caffyn’s strategy8
in our syntheses of bis(trifluoromethyl)phoshino peri-substituted
naphthalene (and acenaphthene) intermediates and we report
our synthetic and characterization data in this paper. However,
despite testing several strategies to attach the second phospho-
rus functionalities to the peri-positions, the synthesis of these
bis(phosphine) systems has not been achieved.
128.8 (d, J = 2.0 Hz, CH), 127.5 (m, JCF = 322 Hz, 2¥ CF3),
126.0 (s, CH), 124.7 (s, CH), 118.3 (d, J = 4.6 Hz, Cq); dP
(121.5 MHz, CDCl3) -7.2 (septet, 2JPF = 79.4 Hz); dF (282.3 MHz,
2
CDCl3) -52.5 (d, JFP = 79.3 Hz, 1H); m/z (CI) 374.9 (M+H+);
m/z (EI) 373.9 (M+), 304.9 (M-CF3), 254.9 (C10H6BrPF, base
peak), 226.0 (C10H6PCF3), 157.0 (C10H6P), 126.0 (C10H6); HRMS
(CI) m/z 374.9370, C12H779BrF6P requires 374.9373; 376.9349,
C12H781BrF6P requires 376.9352; correct isotopic patterns were
observed for all listed MS peaks.
Experimental
=
(PhO)2C C(OPh)2 16. Attempts were made to grow crystals
General procedures
of 6 by standing of a saturated solution of the crude material
from preparation above (before column chromatography) in hex-
ane/diethylether. Latter crystallization fractions afforded small
amount (ª 15 mg) of 16 in the form of several colorless platelets.
dH (270.2 MHz, CDCl3) 7.33-7.24 (m, 2H), 7.14-7.01 (m, 3H); dC
Abbreviations Nap (naphthalene-1,8-diyl) and Acenap
(acenaphthene-5,6-diyl) were used in the experimental part
to improve clarity. All experiments were carried out in standard
Schlenk glassware or in a glove box with strict exclusion of air and
moisture, using a nitrogen or argon atmosphere. Solvents were
dried on an MBraun solvent purification system. Compounds 4,11
812 and 154 were prepared according to the published procedures.
CsF was dried at 120 ◦C in vacuo. Column chromatography
was performed with non-degassed solvents, although the air
exposure of the fractions eluted from the column was kept to a
minimum. Unless stated otherwise, the new compounds were fully
=
(67.9 MHz, CDCl3) 155.7 (i-C), 137.4 (C C), 129.6 (m-CH), 123.4
(p-CH), 116.8 (o-CH); m/z (ES+) 419.1 (M+Na+); HRMS (ES+)
m/z 419.1259, C26H20O4Na requires 419.1259.
Attempted preparation of Nap[P(NMe2)2][P(CF3)2] 11. To a
solution of 6 (0.20 g, 0.53 mmol) in THF (10 cm3) cooled to -78 ◦C,
n-BuLi (0.22 cm3 of 2.5 M solution in hexanes, 0.55 mmol) was
added dropwise. The reaction mixture was stirred at -78 ◦C for 2 h,
then a solution of ClP(NMe2)2 (82 mg, 0.53 mmol) in THF (5 cm3)
was added dropwise. The mixture was let to warm to r.t. and was
stirred overnight. The volatiles were evaporated in vacuo and the
resulting viscous liquid was examined by 31P and 19F NMR, which
showed that a complex mixture was formed.
characterized by 31P{ H}, 19F, H and 13C{ H} nmr, including
1
1
1
1
31
measurement of H{ P}, 31P (1H coupled), H–H DQF COSY,
H–P HSQC and H–C HSQC experiments. Measurements were
performed at 25 ◦C; 85% H3PO4 was used as external standard in
31P, CFCl3 as external standard in 19F, TMS as internal standard
in 1H and 13C NMR.
NapBr[P(OPh)2] 5. To a solution of 1,8-dibromonaphthalene
4 (1.34 g, 4.7 mmol) in THF (40 cm3), n-BuLi (1.88 cm3 of 2.5 M
solution in hexanes, 4.7 mmol) was added dropwise at -78 ◦C.
The reaction mixture was stirred at -78 ◦C for additional 2h,
then triphenylphosphite (1.23 cm3, 4.7 mmol) was added. After
Attempted preparation of Nap[PiPr2][P(CF3)2] 12. A proce-
dure analogous to that of attempted preparation of 11 was used,
using 6 and ClPiPr2 as starting materials. 31P NMR showed
a complex mixture of phosphorus containing compounds was
formed.
86 | Dalton Trans., 2010, 39, 85–92
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NapBr[P(OEt)2] 7. To a solution of 4 (3.72 g, 13.1 mmol) in
THF (50 cm3), nBuLi (5.20 cm3 of 2.5 M solution in hexanes,
13.1 mmol) was added dropwise at -78 ◦C. After stirring at -78 ◦C
for an additional 3h, ClP(OEt)2 (1.9 cm3, 13.07 mmol), dissolved
in THF (5 cm3), was added dropwise and the reaction mixture
was allowed to warm to room temperature and stirred overnight.
The volatiles were evaporated and the residue was suspended in
Et2O (50 cm3). After 2 h of vigorous stirring, the suspension was
filtered using a sintered glass funnel. The solid on the sinter was
washed with Et2O (2¥ 15 cm3) and the washings were combined
with the filtrate. Evaporation of volatiles in vacuo gave 7 (3.73 g,
87%) as a viscous liquid, the purity of which was sufficient for
further syntheses. Crystals suitable for X-ray study were grown
from hexane. dH (270.2 MHz, CDCl3) 8.34 (m, 1H, ArH), 7.88-
7.78 (m, 3H, ArH), 7.52 [m (ªt), J = 7.6 Hz, 1H, ArH], 7.25 (dd,
J = 7.5 and 7.9 Hz, 1H), 3.97 (m, 4H, 2¥ CH2), 1.28 (t, J =
7.1 Hz, 6H, 2¥ CH3); dP (109.4 MHz, CDCl3) 152.3; m/z (EI)
326.0 (M+); HRMS (EI) m/z 326.0074, C14H1679BrO2P requires
326.0071; 328.0054, C14H1681BrO2P requires 328.0052.
for X-ray study were grown from hexane–Et2O (Found C, 41.74;
H, 2.09. C14H8BrF6P requires C, 41.92; H, 2.01); dH (300.1 MHz,
CDCl3) 8.15 (dd, J = 7.5 and 3.5 Hz, 1H, CH), 7.76 (d, J = 7.5 Hz,
1H, CH), 7.34 (m (ªdt), J = 7.5 and 1.2 Hz, 1H, CH), 7.13 (m (ªdt),
J = 7.5 and 1.4 Hz, 1H, CH), 3.30 (m, 4H, 2¥CH2); dC (75.5 MHz,
CDCl3) 154.2 (s, Cq), 148.1 (d, J = 2.4 Hz, Cq), 141.8 (d, J = 7.2 Hz,
Cq), 140.2 (s, CH), 136.6 (s, CH), 129.0 (m, 1JCF = 321 Hz, 2¥ CF3),
121.9 (s, CH), 120.8 (s, CH), 114.5 (d, J = 2.8 Hz, Cq), 31.1 (s, CH2),
30.1 (s, CH2); dP (1H decoupled, 121.5 MHz, CDCl3) -7.7 (septet,
JPF = 80.9 Hz); dP (1H coupled, 121.5 MHz, CDCl3) -7.7 (septet of
d, JPF = 80.9 Hz, 3JPH = 3.2 Hz); dF (282.3 MHZ, CDCl3) -52.8 (d,
+
2JFP = 81.0 Hz); m/z (CI, carrier gas methane) 429.0 (M+C2H5 ),
400.9 (M+H+), 380.9 (M-F, base peak), 330.9 (M-CF3), 322.0
(M-Br+H), 283.0, 234.0 (C12H9Br); HRMS (CI) m/z 400.9534,
C14H979BrF6P requires 400.9529; 402.9513, C14H981BrF6P requires
402.9509. Correct isotopic patterns were observed for all listed MS
peaks.
Attempted preparation of Acenap[P(NMe2)2][P(CF3)2] 13.
A
procedure analogous to that of the attempted preparation of 11
was used, using 10 and ClP(NMe2)2 as starting materials. 31P NMR
showed a complex mixture of phosphorus containing compounds
was formed, with no signal assignable to the desired product 13
observed. 19F NMR confirmed that a complex mixture of fluorine-
containing products was formed.
Attempted preparation of NapBr[P(CF3)2] 6 from 7. The trans-
formation of 7 to 6 was attempted using the same reaction
conditions as in the above preparation of 6 from 5. An equimolar
amount of CsF was used in the reaction. Both THF and di-
ethylether were tested as reaction solvents. 31P NMR spectroscopy
showed that no 6 was present in the reaction mixture 4 h after
addition of Me3SiCF3 (at r.t.).
Attempted preparation of Acenap[PiPr2][P(CF3)2] 14 from 10.
A procedure analogous to that of the attempted preparation of
11 was used, using 10 and ClPiPr2 as starting materials. 31P NMR
showed a complex mixture of phosphorus containing compounds
was formed.
AcenapBr[P(OPh)2] 9. To
a solution of 5,6-dibromo-
acenaphthene 8 (4.04 g, 12.95 mmol) in THF (50 cm3) cooled to
-78 ◦C, n-BuLi (5.2 cm3 of 2.5M solution, 13.0 mmol) was added
dropwise. The reaction mixture was stirred at this temperature
for an additional 2 h. To the resulting deep yellow solution,
triphenylphosphite (3.40 cm3, 12.95 mmol) was added. After
stirring at -78 ◦C for an additional 5h, the mixture was left
to warm up to room temperature and was stirred overnight.
Chlorotrimethylsilane (3.2 cm3, 26 mmol) was added, and after
2h of stirring the volatiles were evaporated in vacuo. The residue
was suspended in diethylether (60 cm3) and the insoluble LiCl was
filtered off. Following crystallization from THF–hexane 9 (2.5 g,
Attempted preparation of Acenap(PiPr2)[P(CF3)2] 14 from 15.
CsF (0.38 g, 2.5 mmol) was suspended in the solution of 15 (0.40 g,
1 mmol) in THF (15 cm3). Me3SiCF3 (0.37 cm3, 2.5 mmol) was
added and the reaction mixture was stirred overnight. 31P NMR
indicated no reaction took place. No reaction was also observed
when Et2O was used as solvent.
Attempted preparation of (PhO)P(CF3)2. To a suspension of
P(OPh)3 (0.52 cm3, 2.0 mmol) and CsF (0.906 g, 6.0 mmol) in
diethylether (20 cm3), Me3SiCF3 (0.65 cm3, 4.4 mmol) was added
dropwise at -78 ◦C. The reaction mixture was then left to warm to
r.t. overnight. 31P NMR spectrum of the mixture after the reaction
showed the presence of P(CF3)3 and unreacted P(OPh)3, with no
signal assignable to (PhO)P(CF3)2 present.
◦
43%) was afforded as a colorless solid, m.p. 102-104 C (Found
C, 63.72; H, 3.73. C24H18BrO2P requires C, 64.16; H, 4.04; dH
(300.1 MHz, CDCl3) 8.35 (dd, J = 7.3, 3.2 Hz, 1H), 7.69 (d, J =
7.4 Hz, 1H), 7.30 (d, J = 7.4 Hz, 1H), 7.23–7.13 (m, 4H), 7.12–6.92
(m, 7H), 3.34–3.18 (m, 4H, 2¥ CH2); dC (75.5 MHz, CDCl3) 156.0
(d, J = 9.1 Hz, Cq), 151.2 (s, Cq), 147.2 (s, Cq), 142.1 (s, Cq), 134.6
(s, CH), 133.4 (d, J = 7.2 Hz, CH), 133.0 (d, J = 16.8, Cq), 132.6
(d, J = 37.9, Cq), 130.0 (s, CH), 123.7 (s, CH), 121.2 (s, CH), 120.5
(s, CH), 120.4 (s, CH), 120.3 (s, CH), 115.1 (d, J = 2.0 Hz, Cq),
30.8 (s, CH2), 30.3 (s, CH2); dP (121.5 MHz, CDCl3) 152.7 (s); m/z
(EI) 448.0 (M+).
X-ray crystallography
Table 1 lists details of data collections and refinements for 7, 9
and 10. Data were collected at 93 K using a Rigaku MM007
generator with Mo-Ka radiation (l = 0.71073 A).13 Intensity data
˚
AcenapBr[P(CF3)2] 10. CsF (3.00 g, 19.83 mmol) was sus-
pended in a solution of 9 (2.97 g, 6.61 mmol) in THF (30 cm3).
Me3SiCF3 (1.52 cm3, 16.25 mmol) was added dropwise at -78 ◦C.
The reaction mixture was left to warm to r.t. and was stirred
for 3 h. The resulting suspension was filtered using a sinter and
the volatiles were removed in vacuo. The residue was subjected to
column chromatography (silica, eluent hexane), which afforded 10
(1.59 g, 60%) as a colorless solid, m.p. 132-135 ◦C. Crystals suitable
were collected using w and j steps for 7 and 16, and w steps for
9 and 10, accumulating area detector images spanning at least a
hemisphere of reciprocal space. All data were corrected for Lorentz
and polarization effects. Absorption effects were corrected on the
basis of multiple equivalent reflections. Structures were solved by
direct methods and refined by full-matrix least-squares against F2
(SHELXL).14 Non-hydrogen atoms were refined anisotropically.
The hydrogen atoms located on carbon atoms were assigned riding
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Table 1 Crystallographic data for 7, 9 and 10
chromatography performed in air (on silica, eluent hexane). Once
collected, the fractions containing 6 were promptly stripped off the
solvent and the resulting white solid 6 was stored under nitrogen.
6 was further characterized by multinuclear NMR (1H, 13C, 19F,
31P) and MS, including exact mass determination. Additional
recrystallization of the product after column chromatography gave
high purity material, as verified by microanalyses (C, H).
Since the OPh→CF3 metathesis reaction with Ruppert’s reagent
is of significant synthetic interest, we have investigated its appli-
cability with related alkoxy (rather than aryloxy) substrate 7. To
this end, 7 was synthesized from 4 by monolithiation, followed by
coupling reaction with ClP(OEt)2. 7 was obtained in good yield
(87%) and was fully characterized by 1H and 31P NMR, MS and
by single crystal X-ray diffraction.
The X-ray structure of 7 (Table 1 and Table 2, Fig. 1) reveals
a significantly twisted naphthalene ring with the two peri-bonds
(C–Br and C–P) being distorted from the parallel direction with
the pseudo-dihedral angle Br(9)-C(9) ◊ ◊ ◊ C(1)-P(1) being 16.9(2)◦.
The interaction of the two peri-substituents is repulsive, and a large
positive splay angle of 14.1(2)◦ clearly supports this observation
7
9
10
Chemical formula
Formula weight
Crystal system
Space group
C14H16BrO2P C24H18BrO2P
C14H8BrF6P
401.08
327.15
Monoclinic
P21/n
12.9724(19)
7.7244(9)
15.217(2)
113.655(3)
1396.7(3)
4
449.26
Orthorhombic Monoclinic
P212121
7.2718(9)
8.0587(9)
33.861(4)
—
P21/n
˚
a/A
7.097(5)
9.591(7)
20.681(10)
94.920(12)
1402.5(16)
4
˚
b/A
˚
c/A
b (◦)
V/A
Z
3
˚
1984.3(4)
4
Reflections collected
8680
16844
13150
2525 (0.1127)
Reflections independent 2454 (0.0221) 3572 (0.1543)
(Rint)
R (F2, all data)
Flack parameter
0.0315
n/a
0.1014
0.06(3)
0.1499
n/a
isotropic displacement parameters and constrained to idealized
geometries.
Results and discussion
˚
Table 2 Selected bond lengths (A), angles (deg), and dihedral angles (deg)
Monolithiation of 1,8-dibromonaphthalene 4, followed by re-
action with triphenylphosphite afforded 1-bromo-8-diphenoxy-
phosphino naphthalene 5 in a good yield (81%, Scheme 3). The
mixture after the reaction was treated with excess Me3SiCl, which
was added to quench the LiOPh by-product by formation of LiCl
and volatile Me3SiOPh. LiCl was removed by filtration of the ether
extract through a sinter, the following evaporation of the volatiles
giving 5 as a colorless solid. The crude product was characterised
by 31P NMR, which indicated that 5, with a highly characteristic dP
148.9, was the sole phosphorus-containing product of the reaction.
No extra purification of 5 was required prior to its reaction
with an excess of Ruppert’s reagent (CF3SiMe3) in the presence
of an equimolar amount of CsF. The desired nucleophile medi-
ated OPh→CF3 metathesis reaction proceeded cleanly, and was
completed within a few hours at room temperature as observed
for 7, 9 and 10
7
9
10
P(1)-C(1)
C(9)-Br(9)
P(1)-O(11)
P(1)-O(13) or equivalent
P(1)-C(11)
1.854(2)
1.913(2)
1.6434(16)
1.6392(16)
—
1.838(9)
1.922(10)
1.661(8)
1.666(8)b
—
1.823(15)
1.908(15)
—
—
1.906(18)
1.865(18)
132.0(14)
121.5(11)
121.4(12)
—
P(1)-C(12)
—
—
C(1)-C(10)-C(9)
P(1)-C(1)-C(10)
Br(9)-C(9)-C(10)
O(11)-P(1)-O(13) or
equivalent
127.0(2)
124.69(17)
122.37(17)
99.01(9)
130.1(9)
123.9(8)
123.1(7)
99.0(4)c
C(11)-P(1)-C(12)
Br(9)-C(9) ◊ ◊ ◊ C(1)-P(1)
Splay anglea
—
16.9(2)
14.1(2)
—
4(1)
17(1)
94.8(8)
14(1)
14.9(2)
2
by 31P NMR.15 6 (dP -7.2, septet, JPF = 79.4 Hz) was obtained
a Splay angle = P(1)-C(1)-C(10) + C(1)-C(10)-C(9) + Br(9)-C(9)-C(10) -
as the major phosphorus containing product. Despite 6 being
slightly oxygen sensitive, it was possible to purify it by column
360. b [P(1)-O(17)]. c [O(11)-P(1)-O(17)].
Scheme 3
88 | Dalton Trans., 2010, 39, 85–92
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Table 3 NMR parameters of the newly synthesized compounds
5
9
15
7
6
10
dP
148.9
152.7
132.8a
152.3
-7.2
-7.7
dF
-52.5
-52.8
2JPF
79.3
81.0
a
dPiPr2 9.1 ppm, JPP = 199.5 Hz.
Fig. 1 Crystal structure of 7. Two molecules forming a loose dimeric pair
are shown. Hydrogen atoms are omitted for clarity.
(see Table 2 footnote for splay angle definition). The (nonbonding)
˚
Fig. 2 Crystal structure of 9. Hydrogen atoms are omitted for clarity.
P(1) ◊ ◊ ◊ Br(9) distance is 3.10(1) A, which is significantly shorter
16
˚
than sum of the respective van der Waals radii (3.65 A), but
much longer than any P–Br covalent bond (2.15 0.10) A. The
17
˚
modification of the backbone allowed the growth of X-ray quality
crystals of both 9 and 10. They were also characterised by mult-
inuclear NMR and by MS, including exact mass determination.
Selected NMR parameters of all new compounds discussed in
this paper are shown in Table 3, and as expected the NMR
characteristics are very similar in the related pairs of naphthalene
and acenaphthene counterparts (i.e. 5 and 9 as well as 6 and 10).
The X-ray structures of 9 and 10 (Table 1 and Table 2, Fig. 2 and
Fig. 3) reveal repulsive interactions of the two peri-substituents,
with large positive splay angles of 17(1) and 14.9(2)◦, respectively.
The acenaphthene backbone in 9 is slightly less twisted than
that in 10, with Br(9)-C(9) ◊ ◊ ◊ C(1)-P(1) angles being 4(1)◦ (in 9)
and 14(1)◦ (in 10). As observed earlier in 7, the intramolecular
ethoxy groups are oriented so that one of them points away from
the peri-region, whilst the other one, together with the phosphorus
atom’s lone pair, remains in relative vicinity of the peri-region and
therefore close to the bromine atom. The molecules pack to form
dimers, with the two naphthalene units in the dimers having almost
co-planar orientation, and the peri-regions facing each other. The
intermolecular contacts are rather weak [both P(1) ◊ ◊ ◊ Br(9A) and
˚
Br(9) ◊ ◊ ◊ P(1A) are 3.50(1) A)] (Fig. 1).
The reaction of 7 with CF3SiMe3/CsF was attempted using
the same conditions as in the successful preparation of 6 from 5.
Using THF or Et2O as a solvent, no formation of 6 was observed
after several hours reaction at room temperature, as monitored
by 31P NMR. Clearly, the ethoxy group is not replaced as readily
as phenoxy group in the reactions with Ruppert’s reagent, which
might be rationalized by the differing ‘leaving group’ behaviour of
OEt- and OPh-.18
Since we were not successful in growing X-ray quality crystals of
5 and 6, several of the reactions described above were performed
with slightly modified substrates - 5,6-disubstituted acenaphthenes
(Scheme 4). No deviations from the reactivity of the related
1,8-disubstituted naphthalenes were observed, however the slight
˚
non-bonding P ◊ ◊ ◊ Br distances in both 9 [3.16(1) A] and 10
˚
[3.19(1) A] are shorter than the sum of their van der Waals radii
˚
(3.65 A). The phosphorus atoms have approximate tetrahedral
coordination, although with all the bond angles around it being
significantly smaller than ideal tetrahedral angles, and some
rather acute [(C(1)-P(1)-O(11) 93.7(4) in 9, C(11)-P(1)-C(12)
94.8(8)◦ in 10]. However, this is not unusual for phosphites [such
as P(OPh)3]19 and bis(trifluoromethyl)phosphines [such as 1,1¢-
bis(bis(trifluoromethyl)phosphino) ferrocene].9
Scheme 4
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trifluoromethyl group of trifluoromethylbenzene was used as a
directing group in ortholithiation reactions with n-butyllithium
(to obtain 1-lithio-2-trifluoromethyl benzene).21 On the other
hand, elimination of metal fluoride is a common problem in
chemistry of trifluoromethylphosphanides such as NaP(CF3)2.22
The compatibility of the trifluoromethyl group attached to a
phosphorus atom with respect to organolithiums is of relevance
to our studies since in the next step of our synthetic investigations,
we have attempted to attach a second phosphine group to a peri-
position, next to the bis(trifluoromethyl)phosphine group in 6 (as
shown in Scheme 3) and in 10 (as shown in Scheme 4). We have
utilized a standard P–C coupling strategy, reacting aryllithium
(obtained by a metal-halogen exchange of the remaining bromine
atom in 6 and 10, respectively) with chlorophosphine (both ClPiPr2
and ClP(NMe2)2 were tested). 31P and 19F NMR spectra of the
reaction mixture after standard work-up (see experimental for an
example) indicated that complex mixtures were formed in each
case. The reaction selectivity did not improve with change of
solvent (THF, Et2O and hexane were tested), nor with lowering
the reaction temperature to -100 ◦C. Detailed inspection of
Fig. 3 Crystal structure of 10. Hydrogen atoms are omitted for clarity.
As described above, the adoption of Caffyn’s strategy8 in our
peri-substituted systems was successful and 6 and 10 were prepared
in good yields. Notably the choice of solvent seemed to be rather
critical for these trifluoromethylating reactions. Diethylether was
used as a solvent in the reactions reported in Caffyn’s original
communication,8 as well as in a more recent paper.9 When
reactions of 5 and 9 with Ruppert’s reagent CF3SiMe3 were
attempted using Et2O as a solvent (at conditions identical to
those described in the paper), no reaction was observed. On
the other hand, replacing Et2O with THF afforded 6 and 10 in
good yields. We have extended the solvent study further, with the
reaction of P(OPh)3 with Ruppert’s reagent being tested in both
Et2O and THF. Whilst quantitative transformation to P(CF3)3 was
observed within several hours of mixing the reactants in Et2O (in
agreement with Caffyn’s paper),8 no reaction took place at the
same conditions with THF as a solvent. The correct choice of
the solvent is clearly critical for the trifluoromethylation reaction,
therefore it is important to test a range of ethereal solvents
with problematic substrates before discarding the strategy as not
suitable.
the 31P NMR spectra showed the presence of quartets (2JPF
ª
80 Hz), stemming from the single CF3 group attached to the
phosphorus atom, as well as septets, stemming from the presence
of P(CF3)2 motif. Unfortunately, no signals assignable to the
desired products 11, 12, 13 and 14 were observed in any of the
attempted preparations.
Failure of our attempts to synthesize heteroleptic peri-
bis(phosphino) naphthalenes and acenaphthenes 11, 12, 13 and
14, by the strategy shown in Schemes 3 and 4, prompted us
to investigate the alternative strategy shown in Scheme 5. This
strategy, which was hoped to afford 14, does not require the
lithiation step after the -CF3 group is introduced onto the
substrate, and therefore should eliminate problems caused by their
incompatibility (see above). 15 was obtained by the reaction of
corresponding organolithium with P(OPh)3, and was purified by
crystallization from hexane.4 In an attempt to trifluoromethylate
15, it was stirred with Ruppert’s reagent (with addition of
equimolar amount of CsF as catalyst) at r.t. for several hours,
i.e. at conditions comparable to those used in the transformation
of 9 to 10. Both Et2O and THF were tested as a solvent, however
15 did not undergo any reaction as observed by 31P and 19F NMR.
Interestingly,
the
intermediate
heteroleptic
species
(PhO)P(CF3)2 and (PhO)2PCF3 were not observed in the
reaction of P(OPh)3 with Ruppert’s reagent, even when the
appropriate molar ratio of reagents to support obtaining these
was used. Thus the 31P NMR spectra of the mixture after the
reaction of P(OPh)3 and CF3SiMe3 in the molar ratio 1 : 2
(reacted for several hours in Et2O at r.t., in the presence of
CsF catalyst) indicated that only completely trifluoromethylated
product P(CF3)3 and unreacted P(OPh)3 were present. No signals
assignable to intermediate mixed species (PhO)P(CF3)2 and
(PhO)2PCF3 were detected (eqn (1)) in 31P NMR spectra.20
Scheme 5
(1)
The trifluoromethyl groups attached to aromatic hydrocarbons
are generally not sensitive to alkyl- and aryllithiums. Indeed, the
Scheme 6 Isomeric structures of 15.
90 | Dalton Trans., 2010, 39, 85–92
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Scheme 7
A tentative explanation of the inferior reactivity of 15 in the
metathesis reaction could be the decreased electrophilicity of the
P(OPh)2 phosphorus atom in 15. This makes the initial attack of
nucleophile (presumed to be a key step in the metathesis reaction
with Ruppert’s Reagent) less favourable. The electrophilicity of
the P(OPh)2 phosphorus atom in 15 could be decreased by partial
donation of the lone pair of the PiPr2 phosphorus atom to the
phosphorus atom of the P(OPh)2 group, as depicted in limiting
isomeric structures shown in Scheme 6. The 31P NMR parameters
of 15 support this observation, with the P(OPh)2 phosphorus atom
of 15 (dP(OPh)2 132.8 ppm) being significantly more shielded than
the related P atom in 9 (dP(OPh)2 152.7 ppm). Unfortunately, we
have been unable to obtain crystals of 15 suitable for single crystal
determination.
was involved in the reaction. This is in contrast to the mentioned
literature example (where the key intermediate was :CCl2 carbene),
which required refluxing the reactants in monoglyme (b.p. 85 ◦C).
Conclusion
Although we have not achieved the synthesis of heteroleptic peri-
substituted naphthalenes and acenaphthenes as planned initially,
those synthetic steps that were successful gave some interesting
new insights. The crucial role of the solvent in the trifluoromethy-
lation reactions with Ruppert’s reagent was recognised; as was
the specificity of the reaction for the easy leaving phenoxy (OPh)
groups as opposed to alkoxy groups (as shown on example of
OEt group). Interestingly, the ArP(OPh)2→ArP(CF3)2 metathesis
reaction proceeded at a good rate when a bulky bromine atom was
in the peri position to the phosphorus atom. In contrast, when
a PiPr2 group was present in the peri-position, the metathesis
reaction was inhibited completely.
On a similar note, the P–C coupling reactions of ArLi with
P(OPh)3 gave the desired products in good yield and purity,
irrespective of the fact that the reaction centre was subjected
to significant crowding from a bulky bromine atom in the peri-
position.
Tetraphenoxyethene 16 was identified as a side-product in the
preparation of 6 from 5 (Scheme 3). It was obtained from the
crude mixture after the reaction in the form of several colorless
crystals, the isolated yield being very small. The crystals were
suitable for X-ray study, which confirmed their identity. 1H and 13
C
NMR of 16 were in very good agreement with literature values.23
16 was further characterized by ES MS, including exact mass
determination. Although 16 has been reported in the literature
earlier,23 the single crystal X-ray structure has not been reported
previously and is therefore attached as supplementary material.†
The closely related tetrakis(pentafluorophenoxy)ethane was
formed in the reaction of pentafluorophenol with sodium
trichloroacetate (which serves as the :CCl2 carbene precursor).24
Based on this literature precedent a plausible mechanism can be
drawn (Scheme 7). The mechanism includes the initial formation
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92 | Dalton Trans., 2010, 39, 85–92
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