Nickel- and Palladium-Catalyzed Homocoupling of Aryl Triflates. Scope, Limitation, and Mechanistic Aspects

Article abstract of DOI:10.1021/jo961464b

Whereas the direct reduction of aryl triflates affords mainly phenols and some arenes, the presence of a catalytic amount of palladium or nickel results in the formation of biaryls. The homocoupling is performed in the presence of an electron source, either a cathode or zinc powder. A judicious choice of the metal (nickel or palladium), the ligand (monodentate or bidentate phosphine), and the reduction process (electrochemical or chemical) allows the synthesis of functional symmetrical biaryls. Nickel and palladium complexes ligated by bidentate ligands such as NiCl₂(dppf) and Pd(OAc)₂ + 1 BINAP are very efficient for the homocoupling of 1-naphthyl triflate, since the dimer was obtained in almost quantitative yield. However, the homocoupling is sensitive to steric hindrance, excluding for the moment the synthesis of atropisomers. The homocoupling proceeds via an activation of the C - O bond of the aryl triflate by a palladium(0) (or a nickel(0)) complex, providing an intermediate arylpalladium(II) (or nickel(II)) complex that after activation by electron transfer affords a new complex able to undergo a second oxidative addition with the aryl triflates.

Full text of DOI:10.1021/jo961464b

J . Org. Chem. 1997, 62, 261-274
261
Nick el- a n d P a lla d iu m -Ca ta lyzed Hom ocou p lin g of Ar yl Tr ifla tes.
Scop e, Lim ita tion , a n d Mech a n istic Asp ects
Anny J utand* and Adil Mosleh
Ecole Normale Supe´rieure, De´partement de Chimie, CNRS URA 1679, 24 Rue Lhomond,
75231 Paris Cedex 5, France
Received J uly 31, 1996X
Whereas the direct reduction of aryl triflates affords mainly phenols and some arenes, the presence
of a catalytic amount of palladium or nickel results in the formation of biaryls. The homocoupling
is performed in the presence of an electron source, either a cathode or zinc powder. A judicious
choice of the metal (nickel or palladium), the ligand (monodentate or bidentate phosphine), and
the reduction process (electrochemical or chemical) allows the synthesis of functional symmetrical
biaryls. Nickel and palladium complexes ligated by bidentate ligands such as NiCl2(dppf) and
Pd(OAc)2 + 1 BINAP are very efficient for the homocoupling of 1-naphthyl triflate, since the dimer
was obtained in almost quantitative yield. However, the homocoupling is sensitive to steric
hindrance, excluding for the moment the synthesis of atropisomers. The homocoupling proceeds
via an activation of the C-O bond of the aryl triflate by a palladium(0) (or a nickel(0)) complex,
providing an intermediate arylpalladium(II) (or nickel(II)) complex that after activation by electron
transfer affords a new complex able to undergo a second oxidative addition with the aryl triflates.
In tr od u ction
coupling (eq 2) of aryl halides in the presence of a
chemical reductant12 or of the electrons provided by a
cathode.13
Biaryls exhibit a large variety of physical and chemical
properties.1 When unsymmetrically substituted by donor
and acceptor groups, they are used as materials for
nonlinear optics.2 They are also precursors of rigid liquid
crystals.3 Moreover, they can be used as redox media-
tors.4 When substituted in the ortho and ortho′ position,
they form atropisomers and therefore are employed as
chiral ligands for asymmetric syntheses.1,5 Some natural
biaryls have biological activity.1
ArX + Ar′m Pd or Ni8 Ar-Ar′ + mX
2 ArX + 2e (or Zn) Pd or Ni8 Ar-Ar + 2X- (or ZnX2)
(1)
(2)
The cross-coupling allows the synthesis of symmetrical
and unsymmetrical biaryls, whereas the homocoupling
is restricted to the synthesis of symmetrical ones. How-
ever, the homocoupling takes the advantageous of by-
Biaryls1,6 are usually synthesized from aryl halides,
either in the presence of a stoichiometric amount of
copper (Ullmann reaction)6a
(5) (a) Tamao, K.; Yamamoto, H.; Matsumodo, H.; Miyake, N.;
Hayashi, T.; Kumada, M. Tetrahedron Lett. 1977, 17, 1389. (b) Hayashi,
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101, 3129.(d) Miyashita, A.; Yasuda, A.; Takaya, H.; Toriumi, K.; Ito,
T.; Souchi, T.; Noyori, R. J . Org. Chem. 1980, 102, 7932. (e) Noyori,
R.; Tomino, I.; Yamada, M.; Nishizawa, M. J . Am. Chem. Soc. 1984,
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Morimoto, T.; Achiwa, K. Tetrahedron Asymmetry 1992, 3, 13. (j)
Rosini, C.; Franzini, L.; Raffaeli, A.; Salvadori, P. Synthesis 1992, 503.
(k) Schmid, R.; Cereghetti, M.; Heiser, B.; Scho¨nholzer, P.; Hansen,
H. J . Helv. Chim. Acta 1988, 71, 897. (l) Schmid, R.; Foricher, J .;
Cereghetti, M.; Scho¨nholzer, P. J . Helv. Chim. Acta 1991, 74, 370. (m)
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51.
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P. E. Synthesis 1974, 9. (b) Sainsbury, M. Tetrahedron 1980, 36, 3327.
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1972, 144. (b) Tamao, K.; Minato, A.; Miyake, N.; Matsuda, T. Chem.
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King, A. O.; Okumodo, N. J . Org. Chem. 1977, 42, 1821.
2ArX + Cu f Ar-Ar + CuX2
or via two reactions, catalyzed by a nickel or palladium
catalyst: cross-coupling (eq 1) between aryl halides and
aryl nucleophiles (Grignard,5a,b,7,8 organozinc,2c,e,f,8,9 alu-
minum,9 stannane,10 or borane11 derivatives) or homo-
* To whom correspondence should be addressed.
X Abstract published in Advance ACS Abstracts, December 15, 1996.
(1) Bringmann, G.; Walter, R.; Weirich, R. Angew. Chem., Int. Ed.
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S0022-3263(96)01464-8 CCC: $14.00 © 1997 American Chemical Society

262 J . Org. Chem., Vol. 62, No. 2, 1997
J utand and Mosleh
passing the synthesis of aryl nucleophiles, which some-
times requires two steps (e.g., synthesis of an aryl
Grignard or aryllithium reagent followed by exchange
with a zinc,2c,8 borane11 or stannane10 derivative). More-
over, the formation of arylmagnesium or lithium reagents
may be not compatible with functional groups substituted
on the aryl ring. Many syntheses of biaryls have been
achieved by the cross-coupling reaction, and since 1987,
this reaction was widely developed from aryl triflates.14
ArOTf + Ar′m Pd or Ni8 Ar-Ar′ + mOTf
(3)
Indeed, it has been shown that palladium(0) or nickel(0)
complexes activate the Ar-O bond of aryl triflates via
an oxidative addition, which produces arylpalladium14o,15
or arylnickel intermediates prone to react with nucleo-
philes. In 1992, we demonstrated that it was possible
to invert the reactivity of aryl triflates and make them
react with electrophiles such as carbon dioxide (synthesis
of aryl carboxylic acids)16 or the aryl triflate itself
(synthesis of biaryls),16 provided the presence of a pal-
ladium catalyst and a source of electrons supplied by a
cathode (eq 4).
F igu r e 1. (a) Cyclic voltammetry of 1-naphthyl triflate (2 mM)
in DMF (0.3 M n-Bu4NBF4) at a stationary gold disk electrode
(L ) 0.5 mm) with a scan rate of 0.2 V s-1, at 20 °C. (b) The
cyclic voltammetry was performed in the same conditions as
in (a) but the potential was reversed just after the reduction
peak R0.
on the scope, the limitation, and the mechanism of the
palladium- and nickel-catalyzed homocoupling of aryl
triflates.
2ArOTf + 2e
9
Pd8 Ar-Ar + 2TfO-
(4)
Resu lts a n d Discu ssion
In 1993, we extended the scope of the palladium-
catalyzed homocoupling of aryl triflates with zinc powder
as the electron source and with nickel catalysts associ-
ated to bidentate ligands (eq 5).
E lect r och em ica l P r op er t ies of Ar yl Tr ifla t es,
Alon e a n d in th e P r esen ce of P d Cl2(P P h 3)2 in DMF .
Aryl triflates are electroactive compounds.20 The cyclic
votammogram of aryl triflates, 2 mM in DMF containing
n-Bu4NBF4 (0.3 M) as supporting electrolyte, exhibited
an irreversible reduction peak. See, for example, the
reduction peak R0 of 1-naphthyl triflate in Figure 1a.
The electrochemical reduction of 1-naphthyl triflate at
2ArOTf + Zn Pd or Ni8 Ar-Ar + Zn(OTf)2
(5)
The great accessibility of aryl triflates from easily
available phenols19 makes them competitive with aryl
iodides or bromides. We wish now to report more details
Ep ) -1.95 V produced some naphthalene (formed by
R0
fast protonation of the 1-naphthyl anion), characterized
by its reversible reduction peak at EpR ) -2.51 V (Figure
1a) and 1-naphthoxide detected on the reverse scan, by
its irreversible oxidation peak21 at EpO ) -0.07 V (Figure
1a,b).
(13) (a) Troupel, M.; Rollin, Y.; Sibille, S.; Fauvarque, J . F.; Perichon,
J . J . Chem. Res., Synop. 1980, 26. (b) Fauvarque, J . F.; Petit, M. A.;
Pflu¨ger, F.; J utand, A.; Chevrot, C.; Troupel, M. Makromol. Chem.,
Rapid Commun. 1983, 4, 455. (c) Torii, S.; Tanaka, H.; Morizoki, K.
Tetrahedron Lett. 1985, 26, 1655. (d) Amatore, C.; J utand, A. Organo-
metallics 1988, 7, 2203. (e) Amatore, C.; Carre´, E.; J utand, A.; Tanaka,
I.; Ren, Q.; Torii, S. Chem. Eur. J . 1996, 2, 957.
ArOTf + 2e f Ar- + TfO- R0
Ar- + H+ f ArH
(6)
(7)
(14) (a) Echavarren, A. M.; Stille, J . K. J . Am. Chem. Soc. 1987,
109, 5478. (b) Echavarren, A. M.; Stille, J . K. J . Am. Chem. Soc. 1988,
110, 4051. (c) Oh-e, T.; Miyaura, N.; Suzuki, A. Synlett 1990, 221. (d)
Fu, J .-m.; Snieckus, V. Tetrahedron Lett. 1990, 31, 1671. (e) Robl, J .
A. Synthesis 1991, 56. (f) Roth, G. P.; Fuller, C. E. J . Org. Chem. 1991,
56, 3493. (g) Kelly, T. R.; J agoe, C. T.; Gu, Z. Tetrahedron Lett. 1991,
32, 4263. (h) Kiely, J . S.; Laborde, E.; Lesheski, L. E.; Bucsh, R. A. J .
Heterocycl. Chem. 1991, 28, 1581. (i) Godard, A.; Rovera, J . C.; Marsais,
F.; Ple´, N.; Que´guiner, G. Tetrahedron Lett. 1992, 48, 4123. (j) Saa, J .
M.; Martorell, G.; Garca-Rosa, A. J . Org. Chem. 1992, 57, 678. (k)
Watanabe, T.; Miyaura, N.; Susuki, A. Synlett 1992, 207. (l) Sengupta,
S.; Leite, M.; Soares Raslan, D.; Quesnelle, C.; Snieckus, V. J . Org.
Chem. 1992, 57, 4066. (m) Ritter, K. Synlett 1993, 735. (n) Oh-e, T.;
Miyaura, N.; Suzuki, A. J . Org. Chem. 1993, 58, 2201. (o) Farina, V.;
Krishnan, B.; Marshall, D. R.; Roth, G. P. J . Org. Chem. 1993, 58,
5434. (p) Ciattini, P. G.; Morera, E.; Ortar, G. Tetrahedron Lett. 1994,
35, 2405. (q) Quesnelle, C. A.; Familoni, O. B.; Snieckus, V. Synlett
1994, 349.
ArH + 1e f ArH•-
ArOTf + 2e f ArO- + Tf- R0
ArO- f ArO• + 1e
R
(8)
(9)
O
(10)
Reactions 6 and 9 proceed via a common intermediate
formed by the first electron transfer:
ArOTf + 1e f ArOTf•- R0
(11)
(15) J utand, A.; Mosleh, A. Organometallics 1995, 14, 1810.
(16) J utand, A.; Negri, S.; Mosleh, A. J . Chem. Soc., Chem. Commun.
1992, 1729.
This intermediate radical anion, ArOSO2CF3•-, evolves
by either cleavage of the C-O bond
(17) J utand, A.; Mosleh, A. Synlett 1993, 568.
(18) (a) A dimerization of aryl triflates was reported to be efficient
only under ultrasonication and limited to NiCl2(PPh3)2 as catalyst, used
with a large excess of PPh3, which makes the workup more complicated.
See: (b) Yamashita, J .; Inoue, Y.; Kondo, T.; Hashimoto, H. Chem.
Lett. 1986, 407. (c) A NiCl2(PPh3)2-catalyzed polymerization of aryl bis-
(triflates) was also reported. See: Persec, V.; Okita, S.; Weiss, R.
Macromolecules 1992, 25, 1816.
(20) To our knowledge the electrochemical reduction of aryl triflates
is not reported. The electrochemical reduction of aryl tosylates affords
mainly phenols. See: Civitello, E. R.; Rapoport, H. J . Org. Chem. 1992,
57, 834.
(21) For the electrochemical oxidation of phenoxides see: Hapiot,
P.; Pinson, J .; Yousfi, N. New J . Chem. 1992, 16, 877.
(19) Stang, P. J .; Hanack, M.; Subramanian, L. Synthesis 1982, 85.

Ni- and Pd-Catalyzed Homocoupling of Aryl Triflates
ArOTf•- f Ar• + TfO-
or cleavage of the O-S bond
J . Org. Chem., Vol. 62, No. 2, 1997 263
Ta ble 1. Red u ction P ea k P oten tia ls of Ar yl Tr ifla tes
a n d of th e Ar ylp a lla d iu m (II) Com p lexes Resu ltin g fr om
th e Oxid a tive Ad d ition
(12)
Pd0(PPh3)2Cl- + ArOTf f ArPdCl(PPh3)2 + TfO-
ArOTf•- f ArO• + Tf-
(13)
Red
Red
a
a
Ar-
ArOTf, Ep
ArPdCl(PPh3)2, Ep
p-MeCOC6H4-
p-CNC6H4-
TfOC6H4-
p-CF3C6H4-
p-ClC6H4-
o-ClC6H4-
p-FC6H4-
-1.68
-1.80
-2.24
-2.21
-2.50
-2.45
-2.54
-2.63
-2.70
-2.71
-2.67
-2.74
-1.95
-2.01
-2.12
-1.65
-1.75
-2.15
-2.00
-2.41
-2.20
-2.54
-2.20
-2.46
-2.53
b
These reactions are followed by the reduction of the
aryl or aryloxy radical at a potential less negative than
that of the starting compound ArOTf
Ar• + 1e f Ar-
ArO• + 1e f ArO- E′ < ER0
E < ER0
(14)
(15)
C6H5-
o-MeC6H4-
p-MeC6H4-
p-MeOC6H4-
p-tBuC6H4-
1-naphthyl-
2-naphthyl-
2-pyridyl-
and thus, the overall process is a two-electron transfer
occurring at the potential of R0 (eqs 6 and 9).
The reduction peak potentials of some aryl triflates,
determined vs SCE at a gold electrode, with a scan rate
of 0.2 V s-1, are collected in Table 1.
-2.68
-1.89, -2.35c
nd
-2.00
a
Volt vs SCE. Reduction peak potentials were determined at a
steady gold disk electrode at the scan rate of 0.2 V s-1, in DMF
As expected, aryl triflates are more easily reduced
when substituted by electron-withdrawing groups. They
are reduced at more negative potentials than the corre-
sponding aryl iodides but are more easily reduced than
containing n-Bu4NBF4 (0.3 M), 20 °C. No reaction c The ligand
b
on the palladium was n-Bu3P.
aryl bromides and chlorides: compare Ep
) -2.10,
Red
-2.63, -2.70, >-2.80 V for, respectively, PhI, PhOTf,
PhBr, and PhCl.
An exhaustive electrolysis conducted on a solution of
1 mmol of 1-naphthyl triflate in DMF (50 mL), at the
controlled potential of -2 V, afforded mainly the 1-naph-
thol (86%)16 and a small amount of naphthalene (13%),
demonstrating that the cleavage of the O-S bond in the
intermediate radical anion ArOTf•- (eq 13) is highly
favored compared to the cleavage of the C-O bond (eq
12), the corresponding ArO• radical being more stable
than Ar•. It is important to notice that the direct
reduction of aryl triflates does not produce any dimer,
i.e., 1,1′-binaphthyl. The desired dimerization should
thus be catalyzed by a transition metal complex able to
activate the C-O bond of the aryl triflate such as
palladium(0) or nickel(0) complexes.
The cyclic voltammogram of PdCl2(PPh3)2, 2 mM, in
DMF exhibited an irreversible reduction peak R1 at -0.91
V (Figure 2). As already reported, the oxidation peak
O1 detected on the reverse scan at -0.03 V characterizes
the electrogenerated anionic palladium(0) complex li-
gated by one chloride anion: Pd0(PPh3)2Cl-.22
F igu r e 2. Cyclic voltammetry of PdCl2(PPh3)2 (2 mM) in DMF
(0.3 M n-Bu4NBF4) at a stationary gold disk electrode (L )
0.5 mm) with a scan rate of 0.2 V s-1, at 20 °C. Variation of
the oxidation peak current O1 of the electrogenerated pal-
ladium(0) complex in the presence of 1-naphthyl triflate,
successively 0, 4, 10, and 20 mM.
presence of chloride anions.15 Therefore, in the present
case, the oxidative addition leads to a neutral aryl-
palladium complex ArPdCl(PPh3)2 (eq 17).
Pd0(PPh3)2Cl- + ArOTf f ArPdCl(PPh3)2 + TfO-
(17)
PdIICl2(PPh3)2 + 2e f Pd0(PPh3)2Cl- + Cl-
R1
(16)
It has been established that the oxidative addition of
aryl iodides with the electrogenerated Pd0(PPh3)2Cl-
complex afforded the trans-complex ArPdI(PPh3)2 via a
pentacoordinated anionic complex ArPdICl(PPh3)2-, de-
tected and characterized by its oxidation peak located at
a more positive potential than that of O1.23 Due to the
When the reduction of the bivalent palladium was
performed in the presence of increasing amounts of
1-naphthyl triflate, we observed that the magnitude of
the oxidation peak O1 of Pd0(PPh3)2Cl- progressively
decreased (Figure 2), demonstrating that the electro-
generated palladium(0) complex reacted with the aryl
triflate. We recently established that oxidative addition
of aryl triflates with a palladium(0) complex such as
Pd0(PPh3)4 did not afford a neutral complex ArPd(OTf)-
(PPh3)2 but a cationic arylpalladium(II) complex ArPd-
(PPh3)2+, with TfO- as the counteranion, due to the poor
affinity of the triflate anion for the palladium(II) center.15
However, a neutral complex trans-ArPdCl(PPh3)2 was
formed when the oxidative addition was performed in the
Pd0(PPh3)2Cl- + ArI f ArPdICl(PPh3)2- f ‚‚ f
trans-ArPdI(PPh3)2 + Cl- (18)
very low affinity of the triflate anion for the palladium(II)
center,15 the formation of a stable pentacoordinated
-
anionic intermediate complex such as ArPd(OTf)Cl(PPh3)2
is less probable, and actually no oxidation peak that could
have characterized such a pentacoordinated species was
(22) Amatore, C.; Azzabi, M.; J utand, A. J . Am. Chem. Soc. 1991,
113, 8375.
(23) Amatore, C.; J utand, A.; Suarez, A. J . Am. Chem. Soc. 1993,
115, 9531.

264 J . Org. Chem., Vol. 62, No. 2, 1997
J utand and Mosleh
Ta ble 2. Com p a r a tive Rea ctivity of P a lla d iu m (0)
Com p lexes in th eir Oxid a tive Ad d ition w ith
1-Na p h th yl-OTf in DMF a t 20 °C
Pd0
kapp (M-1 s-1
7.5 × 10-2
)
Pd0
kapp (M-1 s-1
)
Pd0(PPh3)4
5.5
Pd0(PPh3)2Cl–
[PdIICl2(PPh3)2 + 2e]
Pd0(PPh3)4 +
4.3 × 10-1
150Cl- a
a
Introduced as n-Bu4NCl.
detected on the reverse scan at the time scale of the cyclic
voltammetry. This means that were the complex
-
ArPd(OTf)Cl(PPh3)2 initially formed in the oxidative
addition, its half-life time would be considerably smaller
-
than that of ArPdICl(PPh3)2
.
F igu r e 3. (a) Cyclic voltammetry of PdCl2(PPh3)2 (2 mM) in
DMF (0.3 M n-Bu4NBF4) in the presence of p-CF3C6H4OTf (2
mM) at a stationary gold disk electrode (L ) 0.5 mm) with a
scan rate of 0.05 V s-1, at 20 °C. The current has been
multiplied by a factor 2 when compared to (b). (b) (- - -) Cyclic
voltammetry of p-CF3C6H4OTf (2 mM) alone. (s) Cyclic vol-
tammetry of a mixture of PdCl2(PPh3)2 (2 mM) and p-CF3C6H4-
OTf successively 2 and 4 mM.
The rate constant of the oxidative addition of
Pd0(PPh3)2Cl- with the 1-naphthyl triflate was estimated
by cyclic voltammetry24 and compared to that of Pd0(PPh3)4
alone15 or in the presence of chloride anions.15 The
results collected in Table 2 show that, as expected,22 the
palladium(0) complex Pd0(PPh3)2Cl-, generated by reduc-
tion of PdCl2(PPh3)2, is the most reactive complex because
it is ligated by one chloride and by only two phosphines.
Therefore, since the oxidative addition of aryl triflates
with palladium(0) complexes is slow compared to that of
aryl iodides,25 the use of PdCl2(PPh3)2 as a precursor of
a reactive palladium(0) complex able to activate aryl
triflates is highly recommended. But even with this very
reactive palladium(0) complex, its oxidative addition with
10 equiv of 1-naphthyl triflate was not complete at 20
°C, during the time scale of the cyclic voltammetry (9 s
at the scan rate of 0.20 V s-1) since the oxidation peak
O1 of Pd0(PPh3)2Cl- remained partially present, as shown
in Figure 2. To observe a faster oxidative addition, the
reduction of PdCl2(PPh3)2 was performed in the presence
of a more reactive compound: p-CF3C6H4OTf 15 and at a
smaller scan rate 0.05 V s-1, which means during a longer
time (38 s). Under these conditions, the oxidative addi-
tion of the electrogenerated Pd0(PPh3)2Cl- with 1 equiv
of p-CF3C6H4OTf was total, as attested by the absence
of the oxidation peak O1 on the reverse scan (Figure 3a).
When the reduction scan was performed at more
negative potential than that of PdCl2(PPh3)2 (R1), a new
reduction peak R2 appeared at -1.90 V, i.e. at a less
negative potential than that of p-CF3C6H4OTf, R0 at
-2.12 V (Figure 3b). This new reduction peak R2 is
assigned to the reduction of the arylpalladium(II) com-
plex p-CF3C6H4PdCl(PPh3)2 resulting from the oxidative
addition.
F igu r e 4. Cyclic voltammetry performed under the experi-
mental conditions of an electrolysis. (a) Cyclic voltammetry of
PdCl2(PPh3)2 (2 mM) in DMF (0.1 M n-Bu4NBF4) in the
presence of o-ClC6H4OTf (20 mM) at a stationary gold disk
electrode (L ) 0.5 mm) with a scan rate of 0.2 V s-1, at 20 °C.
The current has been multiplied by a factor 10 when compared
to (b). (b) (- - -) Cyclic voltammetry of o-ClC6H4OTf (20 mM)
alone. (s) Cyclic voltammetry of a mixture of PdCl2(PPh3)2 (2
mM) and o-ClC6H4OTf (20 mM).
erated by reaction of Pd0(PPh3)4 with p-CF3C6H4OTf in
the presence of 1 equiv of chloride ions15 and that of the
electrogenerated one, detected at R2 (-1.90 V).
The reduction peak current R2 grew when the concen-
tration of the aryl triflate increased, demonstrating that
peak R2 was “catalytic”. This demonstrates that the
reduction of the arylpalladium(II) complex performed in
the presence of the aryl triflate affords a new compound
and a palladium(0) complex able to activate again the
aryl triflate in a second catalytic cycle and so on. On
the same voltammogram, at more negative potential,
another reduction peak R3 was detected at -2.18 V. The
magnitude of this peak increased in the presence of an
authentic sample of the dimer 4,4′-CF3C6H4C6H4CF3,
demonstrating that R3 is the reduction peak of the dimer.
The same behavior was observed for another aryl triflate
o-ClC6H4OTf (less reactive than p-CF3C6H4OTf) as shown
in Figure 4.
PdIICl2(PPh3)2 + 2e f Pd0(PPh3)2Cl- + Cl-
R1
(16)
Pd0(PPh3)2Cl- + p-CF3C6H4OTf f
p-CF3C6H4PdCl(PPh3)2 + TfO-
V + e, R2
(19)
The formation of a neutral complex ArPdCl(PPh3)2 was
then definitively established by comparing the reduction
peak potential (-1.91 V) of p-CF3C6H4PdCl(PPh3)2, gen-
(24) For the determination of rate constants by cyclic voltammetry
see ref 22.
The reduction of PdCl2(PPh3)2 performed in the pres-
ence of 10 equiv of o-ClC6H4OTf also resulted in the
(25) PhOTf was found to be 104 times less reactive than PhI and
1.7 times more reactive that PhBr.15

Ni- and Pd-Catalyzed Homocoupling of Aryl Triflates
J . Org. Chem., Vol. 62, No. 2, 1997 265
Sch em e 1
formation of the catalytic peak R2 at -2.20 V while the
reduction peak R0 of the aryl triflate at -2.45 V had
almost completely disappeared.
Therefore, the electrogenerated palladium(0) complex
Pd0(PPh3)2Cl- activates aryl triflates to afford neutral
complexes ArPdIICl(PPh3)2 more easily reduced than the
starting aryl triflates. Dimerization occurs after activa-
tion of the arylpalladium(II) complex by electron transfer,
with regeneration of the initial palladium(0) complex. It
ArPdCl(PPh3)2 + 2e + ArOTf f
Ar-Ar + Pd0(PPh3)2Cl- + TfO- (20)
is reported that the reduction of arypalladium(II) com-
plexes affords an anionic ArPd0(PPh3)2- species according
to the following reactions:26
ArPdCl(PPh3)2 + 2e f ArPd0(PPh3)2- + Cl- (21)
ArPd0(PPh3)2- a Ar- + Pd0(PPh3)2
(22)
The mechanism of the palladium-catalyzed electrodimer-
ization of aryl iodides has been elucidated, and the key
step was found to be the oxidative addition of the anionic
arylpalladium(0) complex ArPd0(PPh3)2 with the aryl
-
iodide, affording a transient pentacoordinated species13e
i.e., 1-naphthyl-PdCl(n-Bu3P)2, was found to be reduced
at a more negative potential than that of the 1-naphthyl
triflate (Table 1), excluding the use of PdCl2(n-Bu3P)2 as
a catalyst for the homocoupling.
ArPd0(PPh3)2- + ArX f Ar2PdIIX(PPh3)2- f
Ar2PdII(PPh3)2 + X- (23)
P a lla d iu m -Ca ta lyzed Electr osyn th esis of Bia r yls
fr om Ar yl Tr ifla tes. The reaction was first tested on
the 1-naphthyl triflate:
followed by reductive elimination:
Ar2PdII(PPh3)2 f Ar-Ar + Pd0(PPh3)2 (24)
OTf
Since the intermediate complex ArPdCl(PPh3)2 was
identified when starting from aryl triflates, the pal-
ladium-catalyzed electrodimerization of aryl triflates
should proceed by a similar mechanism except that the
low affinity of triflate anion for palladium(II) center15 in
DMF excludes the formation of anionic palladium(II)
complexes ligated by a triflate anion. Therefore, the
mechanism of the palladium catalyzed electrodimeriza-
tion of aryl triflates can be described as shown in Scheme
1.
[Pd] 10%
2
+ 2e
+ 2TfO–
(25)
DMF
The electrolyses were carried out in DMF, in a divided
cell, at controlled potential, until the electrolysis current
dropped to 5% of its initial value. The results of the
electrolyses are collected in Table 3.
Whereas in the absence of any catalyst, no dimer was
formed (Table 3, entry 1), the addition of a catalytic
amount of a palladium catalyst resulted in the formation
of 1,1′-binaphthyl, at a less negative potential than the
reduction potential of the 1-naphthyl triflate. The dimer-
ization was very sensitive to the temperature, and by
comparing entries 2-5 (Table 3), we observed that the
best yield in dimer was obtained at 90 °C. A higher
temperature resulted in a lower yield probably due to
decomposition of the catalytic species. Some 1-naphthol
was formed as a byproduct, but it could be easily
converted back to the initial 1-naphthyl triflate. The
formation of 1-naphthol can be limited by decreasing the
value of the electrolysis potential (compare Table 3,
entries 4 and 6). The dimerization was not efficient at
low temperature, and a large amount of 1-naphthol was
thus produced. This shows that the oxidative addition
of the electrogenerated palladium(0) with the 1-naphthyl
triflate was too slow at room temperature and that under
these conditions the direct reduction of the 1-naphthyl
triflate to the 1-naphthol could not be avoided and thus
became preponderant.
This mechanistic approach of the palladium-catalyzed
electrodimerization of aryl triflates investigated by cyclic
voltammetry allows the determination of the reduction
potentials of the intermediate arylpalladium(II) com-
plexes, ArPdCl(PPh3)2, i.e., the potentials at which the
electrolyses should be conducted to produce biaryl. The
reduction potentials of some ArPdCl(PPh3)2 intermediates
formed via the oxidative addition are collected in Table
1. In every case these complexes are more easily reduced
than the precursor aryl triflate. This is a strict condition
for the formation of biaryl. In some cases, the difference
between the two potentials is small (Table 1) but we will
see later on that it is high enough to ensure the success
of the dimerization, provided the electrosyntheses were
carried out at controlled potentials.
In order to enhance the rate of the oxidative addition,
PdCl2(n-Bu3P)2 was used as a precursor of a more reactive
palladium(0) complex. However, the complex resulting
from its oxidative addition with the 1-naphthyl triflate,
(26) Amatore, C.; J utand, A.; Khalil, F.; Nielsen, M. F. J . Am. Chem.
Soc. 1992, 114, 7076.

266 J . Org. Chem., Vol. 62, No. 2, 1997
J utand and Mosleh
Ta ble 3. Electr osyn th esis of 1,1′-Bin a p h th yl fr om 1-Na p h th yl-OTf Ca ta lyzed by P a lla d iu m Com p lexes (Eq 25)
1-naphthyl-
n
OTf,a mmol
catalyst (10%)
Eb (V)
T, °C
ArAr,c %
ArH,c %
ArOH,c %
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
10
1
1
1
none
-2.0
-1.7
-1.7
-1.7
-1.7
-1.6
-1.7
-1.7
-1.7
-1.7
-1.7
-1.4
-1.7
20
20
60
90
120
90
90
90
90
90
90
90
90
0
7
13
33
33 (32)
36
42
34
33
36
33
43
46
50
45
86
60
28 (27)
13
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(MePPh2)2
PdCl2(dppm)
PdCl2(dppe)
PdCl2(dppp)
40 (34)
50
36
45
50
30
35
33
31
22
2d
13
28
26
18
21
11
0
10
11
1
35
40
12
1
PdCl2(dppb)
a
b
Solvent: DMF, 50 mL containing n-Bu4NBF4 (0.3 M) as supporting electrolyte. Electrolysis potential vs SCE. c Yields are relative
to the initial 1-naphthyl-OTf that was completely converted and determined on the crude mixture, after workup, by 1H NMR (250 MHz)
spectroscopy, using CHCl2CHCl2 as internal standard. Isolated yields of pure products are given in parentheses. 1-Naphthyl-OTf
d
recovered: 18%.
Ta ble 4. P r op er ties of F r ee a n d Liga ted P h osp h in es
Sch em e 2
phosphine
pKa
phosphine cone
angle,30 deg
P-Pd-P bite
angle,31 deg
28
PPh3
2.73
4.57
145
136
MePPh2
dppm
dppe
72.7
85.8
dppp
90.6
dppb
> 90
The formation of naphthalene, a second byproduct, is
more troublesome. The latter probably arose from pro-
tonation of the anion 1-naphthyl- formed by reduction
of the intermediate 1-naphthylpalladium(II) complex (see
reactions b and e in Scheme 1). However, the origin of
the proton is not clear.27 Decreasing the amount of the
supporting electrolyte n-Bu4NBF4, which might be a
source of proton via an Hoffmann elimination or using
LiBF4 as the supporting electrolyte, did not improve the
selectivity of the reaction, evidencing that the proton
source was either the solvent or residual water. The use
of TMU (tetramethylurea) did not improve the selectivity
neither.
considered30 (Table 4). A less bulky phosphine such as
MePPh2 should favor the oxidative addition but should
disfavor the reductive elimination.29d
To get a better selectivity, a series of catalysts were
tested, in which the palladium was ligated by either
mono- or bidentate phosphines. Two oxidative additions
(reactions a and c) and a reductive elimination (d) are
involved in the mechanism described in Scheme 1. We
do not know a priori what is the rate-determining step
of the catalytic cycle. By using a more basic phosphine
than PPh3 such as MePPh2 (see their respective pKa
values28 in Table 4), we expected faster oxidative
additions29a,b and a slower reductive elimination.29c The
fact that the yield in dimer was lower (compare Table 3,
entries 4 and 8) tends to prove that the reductive
elimination (d) is the rate-determining step of the cata-
lytic cycle. This hypothesis was confirmed when the
respective cone angles of these two phosphines were
When the palladium was ligated by bidentate ligands
(Table 3, entries 9-12), the yield in dimer was lower
compared to that obtained with PPh3 as ligand, and the
1-naphthol was formed in a higher amount due to a less
efficient catalytic cycle. Since this is the first report of
the dimerization of aryl derivatives catalyzed by pal-
ladium complexes ligated by bidentate ligands, the mech-
anism of the dimerization is not known. However, since
PhPdX(dppe) complexes were found to be reduced by 1
electron,26 the mechanism should be very similar to that
of the NiCl2(dppe)-catalyzed dimerization of aryl halides,
in which only monoelectronic transfers are involved.13d
Therefore, we propose the Scheme 2, in which L-L
symbolizes a bidentate ligand.
(27) The palladium-catalyzed electroreduction of aryl triflates to
arenes, in the presence of a proton source, has been recently reported.
See: Chiarotto, I.; Cacchi, S.; Pace, P.; Carelli, I. J . Electroanal. Chem.
1995, 385, 235.
(28) Rahman, M. M.; Liu, H. Y.; Prock, A.; Giering, W. P. Organo-
metallics 1987, 6, 650.
(29) (a) Stille, J . K.; Lau, K. S. Y. Acc. Chem. Res. 1977, 10, 434. (b)
Amatore, C.; Carre´, E.; J utand, A.; M’Barki, M. A. Organometallics
1995, 14, 1818. (c) Negishi, E. I.; Takahashi, T.; Akiyoshi, K. J .
Organomet. Chem. 1987, 334, 181. (d) Gillie, A.; Stille, J . K. J . Am.
Chem. Soc. 1980, 102, 4933. (e) Brown, J . M.; Guiry, P. J . Inorg. Chim.
Acta 1994, 220, 249.
Two oxidative additions (a′) and (c′) and a reductive
elimination (d′) are involved in Scheme 2. In the case of
bidentate ligands, we have to consider their P-Pd-P bite
angle31 (Table 4). Oxidative additions are supposed to
be disfavored for high P-Pd-P angles, whereas the
reductive eliminations are favored,29d,e the bulkiness of
the ligand making the two aryl groups closer. In the
(30) Tolman, C. A. Chem. Rev. 1977, 77, 313.

Ni- and Pd-Catalyzed Homocoupling of Aryl Triflates
J . Org. Chem., Vol. 62, No. 2, 1997 267
Ta ble 5. Electr osyn th esis of Bia r yls fr om Ar yl Tr ifla tes Ca ta lyzed by P d Cl2(P P h 3)2 (Eq 26)
entry
ArOTf a
E,b V
ArAr,c % (isolated)
ArH,c %
ArOH,c %
1
2
3
4
5
6
7
8
p-CNC6H4OTfd
p-CF3C6H4OTfd
p-ClC6H4OTfd
o-ClC6H4OTfd
C6H5OTfd
-1.5
-1.7
-1.8
-2.0
-2.0
-2.0
-2.0
-1.4
4,4′-CNC6H4C6H4CN 70 (55)
4,4′-CF3C6H4C6H4CF3 68 (45)
4,4′-ClC6H4C6H4Cl (57)
2,2′-ClC6H4C6H4Cl (34)
C6H5C6H5 30
C6H5C6H5 83 (76)
4,4′-MeOC6H4C6H4OMe (15)
2,2′-bipyridine (78)
0
0
0
0
nd
nd
0
15
5
8
26
30
10
56
0
C6H5OTfe
p-MeOC6H4OTfd
2-C5H4NOTfd
0
a
b
Solvent: DMF, 50 mL containing n-Bu4NBF4 (0.3 M) as supporting electrolyte. Electrolysis potential vs SCE. c Yields are relative
to the initial ArOTf that was completely converted and determined on the crude mixture, after workup, by 1H NMR (250 MHz) spectroscopy,
using CHCl2CHCl2 as internal standard. 1 mmol of ArOTf with 0.1 mmol of PdCl2(PPh3)2. e 10 mmol of ArOTf with 1 mmol of PdCl2(PPh3)2.
d
series of bidentate ligands tested (Table 3, entries 9-12),
the highest yield in dimer was obtained for dppb, which
possesses the highest P-Pd-P angle. This suggests that
the reductive elimination (d′) is the rate-determining step
of the catalytic cycle.32
Triphenylphosphine appears to be the best ligand for
the electrosynthesis of 1,1′-binaphthyl. This reaction was
scaled up from 1 to 10 mmol provided 10% of the catalyst
was used (Table 4, entry 7). Indeed, when the percentage
of the catalyst was decreased from 10 to 1%, the 1-naph-
thol was the major product (56%). In that case, the
magnitude of the reduction wave of the 1-naphthyl
triflate became much higher than that of the catalytic
wave, and their respective reduction potentials (Table 1)
are too close to avoid the direct reduction of the 1-naph-
thyl triflate to 1-naphthol.
the case of poorly reactive aryl triflates (those substituted
in the ortho position or those substituted in the para
position by electron-donor groups such as OMe). In those
cases, the catalytic efficiency was low, and it was not
possible to avoid the direct reduction of the aryl triflate
to ArOH. The fact that lower yields in dimer were
obtained from 1-naphthyl triflate or from phenyl triflates
substituted in the ortho position indicates that the
dimerization is sensitive to steric hindrance around the
palladium.
In all cases, the homocoupling was regiospecific. It was
compatible with functional groups substituted on the
phenyl ring (CN, CF3, Cl), except for NO2, which was
further reduced, leading to mixtures of biaryls containing
the nitro group and/or the NH2 group. The dimerization
was successfully extended to heteroaromatic ring such
as pyridyl.
P a lla d iu m - a n d Nick el-Ca ta lyzed Syn th esis of
Bia r yls fr om Ar yl Tr ifla tes in th e P r esen ce of Zin c
P ow d er a s Red u cta n t. We have established that the
key step of the palladium-catalyzed electrochemical ho-
mocoupling of aryl triflates was the activation of an
intermediate arylpalladium complex by electron transfer.
Since easily oxidizable metals can supply electrons, zinc
powder12 was tested in a chemical homocoupling of aryl
triflates, a process that is a priori more easily handled
than the electrochemical one. The chemical process was
first optimized on the 1-naphthyl triflate because its
electrochemical homocoupling was limited to 50% yield
and 1,1′-binaphthyls, when substituted in the 2,2′ posi-
tions, are good candidates for the formation of atropiso-
mers.5 Indeed, our purpose was to realize the enantio-
selective homocoupling of hindered aryl triflates, in the
presence of catalysts ligated by optically active phos-
phines, to get atropisomers.5 Thus, bidentate phosphine
ligands that are often optically active5 were tested.
The experimental conditions leading to the best yield
in 1,1′-binaphthyl were used to synthesize biaryls ac-
cording to reaction 26:
2ArOTf + 2e [PdCl2(PPh3)2], 10%8 Ar-Ar + 2TfO- (26)
DMF, 90 °C
The electrolyses were performed under controlled poten-
tials. The potential was always chosen slightly less
negative than the reduction peak potential of the aryl-
palladium(II) intermediate (Table 1) to avoid the direct
reduction of the aryl triflate. The results collected in
Table 5 indicate that good yields in dimer were generally
obtained when starting from phenyl triflates substituted
in the para position by electron-withdrawing groups.
An aryl triflate substituted by an electron-donor group
(OMe) was considerably less reactive. This proves that
when the phenyl triflates were substituted by an electron-
donor group, the oxidative addition of the palladium(0)
with the aryl triflate was the rate-determining step.
Indeed, we have already reported that the oxidative
addition of para-substituted phenyl triflates with
Pd0(PPh3)4 follows a Hammett correlation with a positive
value of F (+2.55), the oxidative addition being faster
when the substituent is an electron-withdrawing group.15
The selectivity of the homocoupling was higher than
in the case of the 1-naphthyl triflate, since no arene ArH
was detected. The corresponding phenol was the only
byproduct that was obtained in significant yield, only in
OTf
[Pd] 10%
2
+ Zn
+ Zn(OTf)2
(27)
DMF, 90 °C
From the results collected in Table 6, it is shown that
the homocoupling of 1-naphthyl triflate proceeds in the
presence of zinc powder as a chemical source of electrons
with better yields than in the electrochemical process.
This demonstrates that zinc was able to reduce the
arylpalladium(II) intermediate.
Various palladium(II) complexes, precursors of pal-
ladium(0) catalysts, have been tested. Mixtures of
Pd(OAc)2 and phosphines were good catalyst precursors.
The mixture Pd(OAc)2 + 2PPh3 afforded the dimer in a
(31) (a) Steffen, W. L.; Palenik, G. J . Inorg. Chem. 1976, 15, 2432.
(b) Hayashi, T.; Konishi, M.; Kobori, Y.; Kumada, M.; Higuchi, T.;
Hirotsu, K. J . Am. Chem. Soc. 1984, 106, 158. (c) Cabri, W.; De
Bernardinis, S.; Francalanci, F.; Penco, S. Santi, R. J . Org. Chem. 1990,
55, 350. (d) Ozawa, F.; Kubo, A.; Matsumoto, Y.; Hayashi, T.; Nishioka,
E.; Yanagi, K.; Moriguchi, K.-I, Organometallics 1993, 12, 4188.
(32) In the NiCl2(dppe)-catalyzed electrodimerization of PhBr, the
reductive elimination from the complex Ph2NiIIIBr(dppe) was found
to be the rate-determining step of the catalytic cycle when the PhBr
concentration was high.13d

268 J . Org. Chem., Vol. 62, No. 2, 1997
J utand and Mosleh
pare Table 6, entries 10 and 11)35c contrary to Pd(0)-
(dppe)2. Among the bidentate ligands tested, BINAP was
the most efficient one, affording the fastest homocoupling
with quantitative yield in dimer (Table 6, entry 10),
which is of interest since this ligand is optically active.
Whereas addition of a large excess of chloride ions
generally has a beneficial influence on palladium-
catalyzed cross-coupling reactions of aryl triflates with
nucleophiles,14a,b,j,o in our case, addition of chlorides
(Table 6, entry 2) slowed down the reaction and resulted
in a poor selectivity (formation of 1-naphthol). In the
presence of a large excess of chloride, a new complex was
Ta ble 6. Syn th esis of 1,1′-Bin a p h th yl fr om
1-Na p h th yl-OTf Ca ta lyzed by P a lla d iu m Com p lexes, in
th e P r esen ce of Zin c P ow d er (Eq 27)a
entry
catalyst (10%)
t, h
ArAr,b %
ArH,b %
1
2
3
4
5
6
7
8
9
PdCl2(PPh3)2
6
6
6
2.5
7
8
8
6
7
59
23
61
51
67
58
0
64
13
98
61
14
13
9
33
13
31
0
19
13
2
PdCl2(PPh3)2 + 40LiClc
PdCl2(dppf)
Pd(OAc)2 + 2PPh3
Pd(OAc)2 + 3PPh3
Pd(OAc)2 + 1dppe
Pd(OAc)2 + 2dppe
Pd(OAc)2 + 1dppf
Pd(OAc)2 + 1DIOP
Pd(OAc)2 + 1BINAP
Pd(OAc)2 + 2BINAP
10
11
2.5
2.5
formed Pd0(PPh3)2Cl2 known to be less reactive in
2-
39
oxidative additions22 than Pd0(PPh3)2Cl-, and conse-
quently the direct reduction of 1-naphthyl triflate to
1-naphthol became a competitive process.
a
1-Naphthyl-OTf: 3 mmol in 2 mL of DMF at 90 °C. Zn: 60
mmol. b Yields are relative to 1-naphthyl-OTf. c Recovered 1-naph-
thyl-OTf: 32%; 1-naphthol: 26%.
Since nickel(0) complexes are able to activate the C-O
bond of aryl triflates,14 l,q a series of nickel complexes were
also tested as catalysts for the dimerization of 1-naphthyl
triflate. By comparing Table 6 and Table 7, one observes
faster reaction but with a smaller yield than the mixture
Pd(OAc)2 + 3PPh3 (Table 6, entries 4 and 5). It is
reported that triphenylphosphine reduces Pd(OAc)2-
(PPh3)2 to a palladium(0) complex and is oxidized to
triphenylphosphine oxide.29b,33 In the present case, zinc
powder seems to be a better reductant of the palla-
dium(II) complex than PPh3. Indeed were the phosphine
the reductant, 1 equiv of PPh3 should be oxidized to
triphenylphosphine oxide; therefore, the mixture of
Pd(OAc)2 + 2PPh3 should produce a palladium(0) com-
plex, not very stable, because it is ligated by only 1
phosphine and therefore not efficient for the coupling.
Since the reverse was observed, we are inclined to
conclude that zinc powder reduced the mixture Pd(OAc)2
+ 2PPh3, yielding a palladium(0) complex ligated by
2PPh3 that was therefore more reactive than the pal-
ladium(0) ligated by 3PPh3, produced by reduction of
Pd(OAc)2 + 3PPh3 by zinc, but also less stable; that is
why the yield in dimer is lower (Table 6, entry 4). In
those cases, the arylpalladium(II) intermediate is ArPd-
(OAc)(PPh3)2. Indeed, in an independent experiment, we
established that addition of PhOTf to the palladium(0)
generated from the mixture Pd(OAc)2 + 3 PPh3 led to
the formation of ArPd(OAc)(PPh3)2 by comparison with
an authentic sample.33b
OY
[Ni] 10%
2
+ Zn
+ Zn(OY)2
(28)
KI
that nickel complexes were more efficient in the chemical
process than palladium complexes, since the homo-
coupling of the 1-naphthyl triflate was performed at lower
temperatures, in a mixture of THF and DMF, with higher
yields of dimer.
We found that NiCl2(PPh3)2 was an efficient catalyst
when used alone without an excess of ligand.36 Whereas
nickel complexes ligated by bidentate ligands were
reported to be inefficient for the dimerization of aryl
halides,12b we discovered that they were very efficient for
the dimerization of aryl triflates, and an almost quanti-
tative yield (93%, Table 7, entry 8) was obtained with
NiCl2(dppf) as catalyst. Whereas extra chloride had an
inhibited effect on the palladium-catalyzed homocoupling
of aryl triflates (see above), on the contrary, the nickel-
catalyzed homocoupling needed to be performed in the
presence of a large excess of iodide ions, and a decay of
the amount of potassium iodide caused a dramatic decay
of the yield (Table 7, entries 4 and 7). A large excess of
zinc powder was required, and by comparing entries 4-6
(Table 7), we observed that a decrease of the amount of
zinc powder resulted in a slower and slower reaction,
suggesting that the zinc powder was involved in the rate-
determining step of the catalytic cycle. Similar observa-
tions were reported for the nickel-catalyzed homocoupling
of aryl halides.12b A mechanism was proposed in which
the monoelectronic reduction of the intermediate
ArNiIIXL2 complex by zinc was the rate-determining step
of the overall process. Addition of iodides would favor
the reduction process by formation of a pentacoordinated
species ArNiIIXIL2-. In addition to these observations,
we propose that iodide ions play a second role. By
stabilizing the nickel(0) species, they slow down its
oxidative addition with the aryl triflates, avoiding the
accumulation (and probably the deactivation) of the
arylnickel(II) intermediate, whose reduction by zinc
powder is very slow. On the basis of the reported
The fact that a mixture of Pd(OAc)2 + 2dppe gave rise
34
to an inefficient catalyst (probably Pd(0)(dppe)2 ) also
seems to prove that zinc was a better reductant for the
palladium(II) complex than dppe (Table 6, entries 6 and
7). Similarly, we think that zinc was a better reductant
for the palladium(II) than BINAP35a since the dimer was
formed in almost quantitative yield (Table 6, entry 10)
from the mixture Pd(OAc)2 + 1BINAP.35b Under such
conditions, the reduction of Pd(OAc)2 + 2BINAP led to
Pd(0)(BINAP)2 that was, as expected, less efficient than
Pd(0)(BINAP) but still catalyzed the homocoupling (com-
(33) (a) Amatore, C.; J utand, A.; M’Barki, M. A. Organometallics
1992, 11, 3009. (b) Amatore, C.; Carre´, E.; J utand, A.; M’Barki, M. A.;
Meyer, G. Organometallics 1995, 14, 5605.
(34) Fitton, P.; Rick, E. A. J . Organomet. Chem. 1994, 28, 287.
(35) (a) It has been reported that mixture of Pd(OAc)2 and 3BINAP
resulted in the spontaneous formation of Pd(0)(BINAP)2, 1BINAP was
able to reduce the palladium(II) to palladium(0) and was oxidized to
BINAP oxide: BINAP(O). See: Osawa, F.; Kubo, A.; Hayashi, T. Chem.
Lett. 1992, 2177. (b) Were zinc a poor reductant for the mixture of
Pd(OAc)2 and 1BINAP, this system should spontaneously afford
BINAP(O) and a poorly reactive naked palladium(0) complex. Since
we get the dimer with a good yield, we assume that zinc is a better
reductant than binap and that the reduction results in the formation
of the very reactive 14e complex: Pd(0)(BINAP). (c) The complex
Pd(0)(BINAP)2, although formally an 18e complex, is still able to
catalyze Heck reaction. See ref 31d.
(36) This contrasts with a precedent reported paper.18a,b

Ni- and Pd-Catalyzed Homocoupling of Aryl Triflates
J . Org. Chem., Vol. 62, No. 2, 1997 269
Ta ble 7. Syn th esis of 1,1′-Bin a p h th yl fr om 1-Na p h th ol Der iva tives, Ca ta lyzed by Nick el Com p lexes, in th e P r esen ce of
Zin c P ow d er (Eq 28)
n
1-naphthyl-OY,a OY
OTf
catalyst (10%)
Znb
KIb
T, °C
t, h
solvent
THF
ArAr,c %c
ArH,c %
1
2
3
4
5
6
7
8
9
10
11
12
13
NiCl2(PPh3)2
NiCl2(dppe)
NiCl2(dppe)
NiCl2(dppe)
NiCl2(dppe)
NiCl2(dppe)
NiCl2(dppe)
NiCl2(dppf)
NiCl2(PPh3)2
NiCl2(dppe)
NiCl2(dppe)
NiCl2(dppe)
Ni(Acac)2
20
20
20
10
7.5
5
10
20
20
20
20
20
20
4
4
4
4
4
4
2
4
4
4
4
4
4
67
67
67
67
67
67
67
67
140
140
150
140
140
4
4
4
4
6
6
4
3.5
4
6
92
60
85
85
61
23
3
93
70
88
45
0
8
40
15
14
16e
9f
11
6
9
10
23
0h
0h
THF
DMF + THFd
DMF + THFd
DMF + THFd
DMF + THFd
DMF + THFd
DMF + THFd
DMF
OTs
DMF
DMF
DMF
DMF
OSO2PhFg
OCONEt2
6
16
8
0
ArOY: 3 mmol in 2 mL of solvent. Equivalent relative to 1-naphthyl-OY. c Yields are relative to 1-naphthyl-OY. DMF + THF (1
a
b
d
mL + 1 mL). e Recovered 1-naphthyl-OTf: 12%. f Recovered 1-naphthyl-OTf : 69%. Para isomer. Recovered 1-naphthyl-OCb: 100%.
g
h
Sch em e 3
optically active ligands. However, before the scope and
limitation of the homocoupling of hindered naphthyl and
aryl triflates can be defined, the generality of the homo-
coupling of aryl triflates was investigated with two
cheaper catalysts: NiCl2(dppe) and PdCl2(PPh3)2 (eq 29).
[Pd]
(29)
+ Zn(OTf)2
2
OTf + Zn
[Ni] + KI
Z
Z
Z
[Pd] = PdCl2(PPh3)2 and [Ni] = NiCl2(dppe)
From the results gathered in Table 8, it is shown that
the dimerization of substituted aryl triflates was achieved
in good yields with either PdCl2(PPh3)2 or NiCl2(dppe)
as catalysts that possessed complementary properties.
Indeed, the nickel catalyst was more efficient for the
dimerization of aryl triflates substituted by electron-
donor groups, while the palladium was found to be more
suitable for the dimerization of aryl triflates substituted
by electron-withdrawing groups. The nickel(0) complex
generated by reduction of NiCl2(dppe) by zinc was so
reactive that C-Cl and C-CN bonds were activated,
resulting in the formation of byproducts in particular
oligomers (Table 8, entries 2 and 8). The palladium(0)
complex generated by reduction of PdCl2(PPh3)2 by zinc
was less reactive, resulting in a good compatibility with
functional groups such as CN and Cl. But the pal-
ladium(0) could not catalyze the dimerization of unacti-
vated aryl triflates such as those substituted by electron-
donor groups. We have already reported that the oxidative
addition of palladium(0) complexes with aryl triflates was
accelerated by the presence of electron-withdrawing
groups (Hammett correlation with F ) +2.55).15 In the
case of poorly reactive aryl triflates, nickel complexes
were found to be efficient provided the amount of KI was
increased (Table 8, entry 17) or more reactive catalyst
such as the one ligated by dppf was used (Table 8, entry
18). In all cases, the homocoupling was regiospecific.
Com p a r ison betw een th e Electr och em ica l a n d
th e Ch em ica l P r ocess. The palladium-catalyzed elec-
trodimerization of aryl triflates (Scheme 1) and the nickel
catalyzed dimerization of aryl triflates in the presence
of zinc as a chemical reductant (Scheme 3) both proceed
by a similar route in the very first steps of the catalytic
cycle: formation of an arylpalladium(II) or arylnickel(II)
complex that requires further activation by transfer of
electrons, supplied by either a cathode or zinc powder.
In the case of the palladium-catalyzed dimerization of
very reactive aryl triflates such as those substituted by
mechanism of the nickel dimerization of aryl halides12b,13d
and on our own observations, we proposed Scheme 3 for
the nickel dimerization of aryl triflates with zinc as
electron supplier.
Since nickel complexes are efficient catalysts, the
expected less reactive 1-naphthol derivatives were sub-
mitted to the homocoupling (Table 7, entries 9-13). The
dimer was produced in high yield from the tosylate
derivative provided working in pure DMF at higher
temperature. The carbamate derivative was totally
unreactive even with Ni(Acac)2, yet reported to be ef-
ficient in cross-coupling reactions of aryl carbamates with
l,q
nucleophiles.14
For NiCl2(dppe) as a catalyst, the
following order of reactivity was established:
ArOSO2CF3 > ArOSO2C6H4CH3 >
ArOSO2C6H4F . ArOCONEt2
The NiCl2(PPh3)2-catalyzed dimerization of aryl mesyl-
ates, ArOSO2CH3, was recently reported,37 and they were
found to be less reactive than aryl triflates.
Thus, the dimerization of 1-naphthyl triflate in the
presence of zinc powder has been optimized with two
catalysts associated with bidentate ligands, NiCl2(dppf)
and Pd(OAc)2 + 1BINAP, opening the way toward
(37) Percec, V.; Bae, J . Y.; Zhao, M.; Hill, D. H. J . Org. Chem. 1995,
60, 176.

270 J . Org. Chem., Vol. 62, No. 2, 1997
J utand and Mosleh
Ta ble 8. P d Cl2(P P h 3)2- a n d NiCl2(d p p e)-Ca ta lyzed Syn th esis of Bia r yls fr om Ar yl Tr ifla tes in th e P r esen ce of Zin c
P ow d er (Eq 29)
4,4′-Z(C6H4)2Z,d
% (isolated)
ArOTf
recovered
n
ZC6H4OTf,a
Z
catalystb (10%)
solventc
DMF
DMF + THF
DMF
DMF + THF
DMF
DMF + THF
DMF
DMF + THF
DMF + THF
DMF
DMF + THF
DMF
DMF
DMF
DMF
DMF
t, h
T, °C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
p-CN
Pd
Ni
Pd
Ni
Pd
Ni
Pd
Ni
Ni
Pd
Ni
Pd
Ni
Pd
Ni
Pd
Nih
Nii
4
4
4
4
4
4
5
4
4
4
4
4
6
6
6
4
4
6
90
67
90
67
90
67
90
67
67
90
67
90
90
90
90
90
67
90
(85)
70 (45)
(76)
(93)
69
86 (63)
17
0
(85)
30
(99)
7
0
0e
0
0
0
p-CF3
p-CO2Et
p-Cl
0
47
0f
0
32
0
66
0
100
0
p-F
H
p-Me
(94)
0
o-Me
(53)g
0
p-MeO
p-t-Bu
100
0
55
DMF + THF
DMF
(98)
(36)
a
b
ZC6H4OTf: 3 mmol; Zn: 60 mmol. Pd ) PdCl2(PPh3)2: 0.3 mmol; Ni ) NiCl2(dppe): 0.3 mmol + KI (12 mmol). c The reactions were
d
performed in 2 mL of DMF or in a mixture of 1 mL of DMF and 1 mL of THF. Yields are based on the initial aryl triflate and were
determined on the crude mixture, after workup, by 1H NMR spectroscopy using CHCl2CHCl2 as internal standard. e Formation of 4,4′′-
CN(C6H4)3CN. f Formation of 42% (isolated yield) of 4,4′′′-Cl(C6H4)4Cl. 2,2′-Me(C6H4)2Me. 18 mmol of KI. NiCl2(dppf).
g
h
i
Ta ble 9: Com p a r ison betw een th e Electr och em ica l a n d
th e Ch em ica l P r ocess
mentioned that the nickel-catalyzed chemical dimeriza-
tion was achieved at lower temperatures because oxida-
tive additions of nickel(0) complexes are always faster
than that performed from palladium(0) complexes. The
palladium-catalyzed chemical or electrochemical homo-
coupling of 1-naphthyl triflate afforded comparable yields
(Table 9, entries 9 and 10). When performed with a
nickel catalyst, the chemical process was more efficient
than the electrochemical one, but the former should be
carried out in the presence of potassium iodide. This
means that the reductant power of the zinc powder was
too low to reduce the corresponding ArNiCl(dppe) com-
plex, which is formed in the absence of iodides, but that
it was high enough to reduce an ArNiI2(dppe)-, complex
formed when the oxidative addition was performed in the
presence of iodide ions (Scheme 3).
electron t,a T, ArAr,
ArOTf, Ar
catalyst (10%)
source
h
°C
%
1
2
3
4
5
6
7
8
9
p-CF3C6H4-
p-CF3C6H4-
C6H5-
PdCl2(PPh3)2 cathode
PdCl2(PPh3)2 Zn
4
4
4
4
4
4
4
4
4
6
3
4
90
90
90
90
67
90
90
67
90
90
90
67
68
76
83
30
99
15
0
98
51
59
56
82
PdCl2(PPh3)2 cathode
PdCl2(PPh3)2 Zn
C6H5-
C6H5-
NiCl2(dppe)b
Zn
p-MeOC6H4-
p-MeOC6H4-
p-MeOC6H4-
1-naphthyl-
PdCl2(PPh3)2 cathode
PdCl2(PPh3)2 Zn
NiCl2(dppe)b
Zn
PdCl2(PPh3)2 cathode
PdCl2(PPh3)2 Zn
10 1-naphthyl-
11 1-naphthyl-
l2 1-naphthyl-
NiCl2(dppe)c
NiCl2(dppe)b
cathode
Zn
a
b
Reaction times. In the presence of 4 equiv of KI compared
to the aryl triflate. c The electrosynthesis was performed in DMF
at -1.7 V vs SCE.
P a lla d iu m - or Nick el-Ca ta lyzed Hom ocou p lin g of
Hin d er ed Ar yl Tr ifla t es in t h e P r esen ce of Zin c
P ow d er . When substituted in the ortho and ortho′
positions, biaryls become optically active, and these
atropisomers may be used as optically active ligands5 or
be their precursors. Our purpose was to extend the
catalytic homocoupling of aryl triflates described above
to the synthesis of atropisomers and to use optically
active ligands on the metal in order to perform asym-
metric homocoupling. Indeed, to our knowledge, no
example of catalytic asymmetric homocoupling of aryl
triflates (or halides) has been reported.38 Since the
dimerization of 1-naphthyl triflate was found to be
quantitative with Pd(OAc)2 + 1BINAP or NiCl2(dppf) as
catalysts, the latter were tested in the homocoupling of
hindered aryl triflates (although dppf is not an optically
active ligand, it was, however, tested as a model since
some optically active ligands derive from substituted
ferrocene5b,39). The dimerization of compound 1, in which
the 1-naphthyl triflate was substituted by a methyl
group, in the ortho position, was achieved using NiCl2-
(dppf) as catalyst (Table 10).
electron-withdrawing groups (see, for example, entries
1 and 2 of Table 9), the efficiency of the electrochemical
and the chemical processes was comparable.
More interesting is the comparison of the two processes
for the dimerization of less reactive aryl triflates. Indeed,
the palladium-catalyzed dimerization of C6H5OTf and
p-MeOC6H4OTf was found to be more efficient when
performed electrochemically (compare Table 9, entries 3
and 4, 6 and 7). This is easily interpreted by the fact
that the intermediate arylpalladium(II) complexes, when
substituted by electron-donor groups, are reduced at very
negative potentials (see Table 1). In such cases, the zinc
powder was not able to reduce them, and thus, the
reaction was limited by the reducing power of the
chemical reductant, whereas very high negative poten-
tials were easily reached by simply tuning the potential
of the cathode. However, by changing the catalyst from
PdCl2(PPh3)2 to NiCl2(dppe), the chemical reduction by
zinc became more efficient than the electrochemical
process (compare Table 9, entries 3-5, 6-8). Aryl-
nickel(II) complexes are more easily reduced than the
corresponding arylpalladium(II) complexes and therefore
are easily reduced by zinc (e.g., the reduction peak
potential of PhNiCl(dppe) is -1.80 V vs SCE13d whereas
that of PhPdCl(PPh3)2 is -2.20 V26). It should also be
(38) Racemic diphosphines in the biphenyls series were synthesized
from aryl iodides by an Ullmann reaction in the presence of a
stoechiometric amount of copper(0). See: Schmid, R.; Foricher, J .;
Cereghetti, M.; Scho¨nholzer, P. Helv. Chim. Acta 1991, 74, 370.
(39) Zhang, W.; Hirao, T.; Ikeda, I. Tetrahedon Lett. 1996, 37, 4545.

Ni- and Pd-Catalyzed Homocoupling of Aryl Triflates
J . Org. Chem., Vol. 62, No. 2, 1997 271
Ta ble 10. P a lla d iu m or Nick el Ca ta lyzed Dim er iza tion
of Hin d er ed Ar yl Tr ifla tes in th e P r esen ce of Zin c
P ow d er
Surprisingly, the homocoupling did not work in the
presence of an optically active ligand such as (S,R)-
PPFOMe associated to NiBr2 (the arene was only formed
in 85% yield), although this ligand was very efficient in
the enantioselective cross-coupling reaction between
2-methyl-1-naphthyl bromide and (2-methyl-1-naphthyl)-
magnesium bromide.5b This suggests that the intermedi-
ates of the homocoupling and the cross-coupling are
different (see Scheme 3). The palladium(0) complex
generated from the mixture of Pd(OAc)2 + 1(S)-BINAP
catalyzed the homocoupling but with a very low yield
compared to that of the reported cross-coupling reaction,5b
so no effort was undertaken to determine whether the
resulting dimer was optically active or not. Nevertheless,
the homocoupling of hindered aryl triflates seemed to be
feasible, even from an aryl triflate deactivated in the
oxidative additions by the presence of the methyl group
as electron donor, in the ortho position.
Therefore, two unreported ortho-substituted aryl tri-
flates, compounds 2 and 3, were synthesized in four steps
according to Schemes 4 and 5, with an overall yield of
56% and 49%, respectively.
The homocoupling of compound 2 should produce the
diphosphine dioxide, a precursor40 of the BINAP ligand5c-h
(eq 30), whereas that of compound 3 should produce the
corresponding dimer, a precursor40 of the MeO-BIPHEP
ligand5k-m (eq 31).
t, ArAr, ArH,
ArOTf a
catalyst, (10%)
NiCl2(dppf)b
Pd(OAc)2 + 1(S)-BINAP
h
%
%
OTf
2
7
66
16
19
60
CH3
1
NiCl2(dppf)b
1
2
0
0
93
88
O
OTf
Pd(OAc)2 + 1(S)-BINAP
PPh2
2
NiCl2(dppf)b
Pd(OAc)2 + 1(S)-BINAP 18
18
0
0
24c
23d
O
OTf
MeO
PPh2
3
a
The reactions were performed at 100 °C. 1/Zn: 0.5 mmol/10
mmol in 0.5 mL of DMF. 2/Zn: 0.2 mmol/4 mmol in 0.2 mL of
DMF. 3/Zn: 0.2 mmol/4 mmol in 0.2 mL of DMF. b In the presence
of 4 equiv of KI relative to the aryl triflate. c Formation of ArOH
d
(15%). Recovered ArOTf (40%). Formation of ArOH (20%).
Recovered ArOTf (37%).
OTf
P(O)Ph2
Sch em e 4
HSiCl3
Pd or Ni
?
P(O)Ph2
P(O)Ph2
+ Zn
2
O
OH
OCb
1) sec-BuLi/TMEDA
THF/–78°C
2
Cl
NEt2
Pyridine
2) ClP(O)PPh2
94%
PPh2
2a
(30)
PPh2
OCb
P(O)Ph2
BINAP
OTf
84%
MeO
P(O)Ph2
2b
Pd or Ni
?
+ Zn
OH
P(O)Ph2
Tf2O, Et3N
KOH, EtOH
3
2b
91%
2c
MeO
MeO
P(O)Ph2
HSiCl3
MeO
MeO
PPh2
PPh2
(31)
P(O)Ph2
OTf
P(O)Ph2
MeO-BIPHEP
79%
P(O)Ph2 was chosen as a substituent in the ortho
position rather than PPh2 because it is an electron-
withdrawing group that should accelerate the rate of the
oxidative additions and the resulting dimer (phosphine
oxide) should not be a good ligand for palladium or nickel
complexes compared to the corresponding diphosphine.41
However, neither NiCl2(dppf) nor Pd(OAc)2 + 1(S)-
BINAP catalyzed the homocoupling of compound 2; the
2
corresponding arene was only formed by a fast reaction
(Table 10). These results suggest that, whatever the
catalyst, the first oxidative addition of the palladium(0)
(or nickel(0)) proceeds well (Schemes 2 and 3) leading to
the corresponding arylpalladium(II) (or arylnickel(II))
complex but that the activation of these intermediates
by electron transfer produced a complex unable to
undergo a second oxidative addition with the aryl triflate,
probably due to steric hindrance around the metal. A
similar behavior was observed for compound 3, except
that the formation of the arene was slower probably due
to the deactivating effect of the methoxy group, an
(40) For the reduction of phosphine oxide to the corresponding
phosphine see: (a) Naumann, K.; Zon, G.; Mislow, K. J . Am. Chem.
Soc. 1969, 91, 7012. (b) Uozomi, Y.; Tanahashi, A.; Lee, S.-Y.; Hayashi,
T. J . Org. Chem. 1993, 58, 1945.
(41) Indeed, we observed from Table 6 that using two bidentate
phosphine ligands per palladium resulted in either a nonreactive
catalyst (entry 7) or a less efficient one (entry 11).

272 J . Org. Chem., Vol. 62, No. 2, 1997
J utand and Mosleh
under reduced pressure. The solvents were degassed prior use.
Zinc powder (200 mesh) was commercial (Acros Organics). (S)-
BINAP, (S,R)-PPFOMe, DIOP, and dppf were commercial
(Aldrich). PdCl2(PPh3)2,42 PdCl2(MePPh2)2,42 PdCl2(dppm),31a
PdCl2(dppe),43 PdCl2(dppp),31a PdCl2(dppb),31a PdCl2(dppf),31b
NiCl2(dppe),44 and NiCl2(dppf)44 were synthesized according
to reported procedures. 1-Naphthyl triflate,45 and 2-methyl-
1-naphthyl triflate (1) were synthesized from 1-naphthol and
2-methyl-1-naphthol (commercial from Aldrich), respectively,
by reaction of the naphthol with trifluoromethanesulfonyl
anhydride in the presence of triethylamine in dichloromethane.
Para- and ortho-substituted phenyl triflates were synthesized
from the corresponding phenols (commercial from Aldrich)
according to the same procedure.45 They were characterized
by their 1H NMR and mass spectra, which were identical to
those of previously reported compounds.14a,19
2-Meth yl-1-n a p h th yl tr ifla te (1): 10.1 g (92% yield); oily
liquid. 1H NMR (200 MHz, CDCl3) δ 8.06 (d, J ) 8.5 Hz, 1H),
7.84 (d, J ) 8.5 Hz, 2H), 7.78 (d, J ) 8.5 Hz, 2H), 7.65-7.48
(m, 2H), 7.36 (d, J ) 8.5 Hz, 1H), 2.57 (s, 3H); 13C NMR (CDCl3)
δ 142.8 (-CO), 133.5, 128.9, 128.4, 128.2, 127.8, 127.6, 127.1,
126.3, 120.9, 118.7 (q, J CF ) 320 Hz, CF3), 17.2 (CH3); m/e (CI
NH3) 308 (M + 18), 290 (M+). Anal. Calcd for C12H9F3O3S:
C, 49.66; H, 3.13. Found: C, 49.74, H: 3.12.
Sch em e 5
O
OH
OCb
1) sec-BuLi/TMEDA
THF/–78°C
MeO
MeO
Cl
NEt2
Pyridine
2) ClP(O)PPh2
96%
3a
OCb
MeO
P(O)Ph2
80%
3b
OCb
MeO
P(O)Ph2
Tf2O, Et3N
KOH, EtOH
3b
90%
3c
OTf
MeO
P(O)Ph2
1-Na p h th yl tosyla te was synthesized using a procedure
similar to the synthesis of 1-naphthyl triflate.45 To a stirred
solution of 1-naphthol (4.32 g, 30 mmol) in 170 mL of distilled
dichloromethane and triethylamine (4.87 mL, 35 mmol) at -50
°C was added dropwise p-toluenesulfonyl chloride (6.67 g, 35
mmol) for 10 min. After being warmed overnight to room
temperature, the solution was hydrolyzed with aqueous HCl
(10%). The organic phase was washed with water and dried
over MgSO4. After evaporation of the solvent, the crude
mixture was crystallized from dichloromethane and petroleum
ether: white crystals; 7.6 g (85% yield); mp ) 92 °C; 1H NMR
(250 MHz, CDCl3) δ 7.93 (dd, J ) 7.5, 1.5 Hz, 1H), 7.84-7.73
(m, 2H), 7.79 (d, J ) 8 Hz, 2H), 7.52-7.43 (m, 2H), 7.38 (t, J
) 7.5 Hz, 1H), 7.28 (d, J ) 8 Hz, 2H), 7.21 (dd, J ) 7.5, 1.5
Hz, 1H), 2.42 (s, 3H); m/e (EI) 298 (M+), 155, 143, 115, 91.
1-Na p h th yl p-F lu or oben zen esu lfon a te. Same procedure
as for the synthesis of 1-naphthyl tosylate, from 1-naphthol
(4.32 g, 30 mmol) and p-fluorobenzenesulfonyl chloride (6.81
g, 35 mmol), was used: white crystals; 8.5 g (94% yield); mp
) 93 °C; 1H NMR (250 MHz, CDCl3) δ 7.91 (dd, J ) 8.5 Hz, J
) 5 Hz, 2H), 7.86-7.82 (m, 2H), 7.78 (d, J ) 8 Hz, 1H), 7.52-
7.44 (m, 2H), 7.39 (t, J ) 8 Hz, 2H), 7.17 (t, J ) 8.5 Hz, 2H);
m/e (EI) 302 (M+), 143, 115 (100), 95.
71%
3
electron donor, in the oxidative addition. Therefore,
steric hindrance appears to be a severe limitation for the
palladium- or nickel-catalyzed homocoupling of aryl
triflates.
Con clu sion
It is shown that phenols, which are commonly available
compounds, are converted to the corresponding biaryls
via the triflate derivatives, in the presence of a catalytic
amount of a palladium or a nickel complex and an
electron source, either a cathode or zinc powder. Best
ArOH (Tf) O8 ArOTf +e, [Pd] or [Ni]8 ArAr
2
yields in the binaphthyl series were obtained with nickel
and palladium complexes, when ligated by bidentate
ligands, such as dppf and BINAP, respectively. The
homocoupling proceeds by activation of the C-O bond of
the aryl triflate by a palladium(0) or a nickel(0) complex,
affording an arylpalladium(II) or nickel(II) complex
whose activation by electron transfer produces a new
complex able to undergo a second oxidative addition with
the aryl triflate followed by formation of the dimer.
Whereas aryl triflates are usually known to react with
nucleophiles in the presence of a palladium or a nickel
catalyst, electron transfer results in an inversion of their
reactivity since it was shown in this work that they react
with themselves, which means with electrophiles.
1-Na p h th yl N,N-Dieth ylca r ba m a te (2a ).46 To a stirred
solution of 1-naphthol (7.2 g, 50 mmol) in 250 mL of anhydrous
pyridine was slowly added N,N-diethylcarbamoyl chloride (6.77
g, 50 mmol). The solution was stirred under reflux for 6 h
and then poured over ice and extracted with ethyl ether. The
organic phase was washed with aqueous HCl 10% and then
with a NaHCO3 aqueous solution and dried on MgSO4. After
flash chromatography, 11.42 g of pure 2a was collected as pale
orange crystals (94% yield): mp ) 120 °C; 1H NMR (250 MHz
CDCl3) δ 7.96-7.86 (m, 2H), 7.72 (d, J ) 8 Hz, 1H), 7.55-
7.49 (m, 2H), 7.46 (d, J ) 8 Hz, 1H), 7.31 (d, J ) 7 Hz, 1H), (q,
J ) 7 Hz, 2H), 3.47 (q, J ) 7 Hz, 2H), 1.40 (t, J ) 7 Hz, 3H),
1.27 (t, J ) 7 Hz, 3H); 13C NMR (CDCl3) δ 154.2 (s, CdO),
147.4 (s, CO), 134.6 (s), 127.9 (d), 127.5 (s), 126.1 (d), 125.4
(d), 125.2 (d), 121.2 (d), 118 (d), 42.3 (t, CH2), 42 (t, CH2), 14.4
(q, CH3), 13.4 (q, CH3); IR (KBr pellet) 1730 (νCdO) cm-1; m/e
(CI NH3) 261 (M + 18), 244 (M + 1).
Exp er im en ta l Section
2-(Dip h en ylp h osp h in yl)-1-n a p h t h yl N,N-d iet h ylca r -
ba m a te (2b) was synthesized according to the procedure
Gen er a l Meth od s. 1H NMR spectra were recorded at 200
MHz or at 250 MHz (TMS as internal reference). 31P NMR
spectra were recorded at 101.2 MHz (H3PO4 as external
reference). 13C and 19F NMR spectra were recorded at 62.9
(TMS as internal reference) and at 235 MHz (CFCl3 as external
reference), respectively.
All reactions were performed under argon in vessel con-
nected to a Schlenk line. Flash chromatography was per-
formed with silica gel 60, 230-400 mesh (Merck).
Ch em ica ls. Tetrahydrofuran was distilled on sodium/
benzophenone and dimethylformamide on calcium hydride
reported for the ortho lithiation of O-aryl carbamates.47
A
solution of s-BuLi (35 mL of a solution 1.4 M, 48 mmol) was
(42) Hartley, F. R. Organomet. Chem. Rev., Sect. A. 1970, 6, 119.
(43) Westland, A. D. J . Chem. Soc. 1965, 3060.
(44) Chatt, J .; Both, G. J . Chem. Soc. 1965, 3238.
(45) Hirota, K.; Isobe, Y.; Maki, Y. J . Chem. Soc., Perkin Trans. 1
1989, 2513.
(46) Lustig, E.; Benson, W. R.; Dut, N. J . Org. Chem. 1967, 32, 851.
(47) (a) Beak, P.; Snieckus, V. Acc. Chem. Res. 1982, 15, 306. (b)
Snieckus, V. Chem. Rev. 1990, 90, 879.

Ni- and Pd-Catalyzed Homocoupling of Aryl Triflates
J . Org. Chem., Vol. 62, No. 2, 1997 273
added to 70 mL of THF at -78 °C, followed by TMEDA (7.25
mL, 48 mmol). A solution of 1-naphthyl N,N-diethylcarbamate
(9.72 g, 40 mmol) in 50 mL of THF was added. After 45 min,
the anion was quenched with ClP(O)Ph2 (9.2 mL, 48 mmol) at
-78 °C. After being warmed overnight to room temperature,
the solution was hydrolyzed with water and extracted with
diethyl ether. Flash chromatography performed on the crude
mixture (eluent: petroleum ether/ethylacetate, 8/2) afforded
7 Hz, 2H), 1.00 (t, J ) 7 Hz, 3H), 0.92 (t, J ) 7 Hz, 3H); 13C
NMR (CDCl3) δ 152.3, 151.61, 142.9, 131.8 (d, J ) 10.5 Hz),
131.6, 128.1 (d, J ) 12 Hz), 127.2, 125.8 (d, J ) 14 Hz), 125.2
(d, J ) 7.5 Hz), 117.0, 56.3 (OCH3), 42.0 (CH2), 41.7 (CH2),
13.7 (CH3), 13.0 (CH3); 31P NMR (CDCl3) δ 26.89; IR (KBr
pellet) 1717 (νCdO), 1214 (νPdO) cm-1; m/e (CI NH3) 424 (M +
1).
2-(Dip h en ylp h osp h in yl)-6-m eth oxyp h en ol (3c). The
Same procedure as for 2c was followed. The reaction was
performed from 3b (12.7 g, 30 mmol) with a reflux of 24 h. 3c
was isolated as pale pink crystals after chromatography
(eluent: petroleum ether/ethylacetate, 7/3): 8.74 g (90% yield);
mp ) 164 °C; 1H NMR (250 MHz, CDCl3) δ 11.04 (broad s,
1H, OH), 7.73-7.64 (m, 4H), 7.59-7.52 (m, 2H), 7.49-7.41 (m,
4H), 6.98 (broad d, J ) 7.9 Hz, 1H), 6.80 (td, J ) 7.9 Hz, J PH
) 4.2 Hz, 1H), 6.64 (ddd, J PH ) 12.5 Hz, J ) 7.9, 1.5 Hz, 1H),
3.87 (s, OCH3); 13C NMR (CDCl3) δ 153.5, 149.0 (d, J ) 11
Hz), 132.6 (d, J ) 2.8 Hz), 132.1 (d, J ) 10.5 Hz), 131.0, 128.7
(d, J ) 12.5 Hz), 123.3 (d, J ) 9.5 Hz), 119.2 (d, J ) 14.5 Hz),
115.3, 111.7 (d, J ) 103 Hz), 56.1 (OCH3); 31P NMR (CDCl3) δ
38.90; m/e (CI NH3) 325 (M + 1). Anal. Calcd for C19H17O3P:
C, 70.37; H, 5.28. Found: C, 70.28; H, 5.35.
1
14.9 g of 2b as white crystals (84% yield): mp ) 121 °C; H
NMR (250 MHz CDCl3) δ 7.87 (d, J ) 8 Hz, 1H), 7.81-7.67
(m, 6H), 7.58-7.46 (m, 8H), 7.31 (dd, J ) 12, 8 Hz, 1H), 3.50-
3.39 (m, 1H), 3.17-3.06 (m, 2H), 2.98-2.90 (m, 1H), 1.16 (t, J
) 7.2 Hz, 3H), 0.98 (t, J ) 7.2 Hz, 3H); 13C NMR (CDCl3) δ
152.4, 151.8, 136.6, 133.6 (d, J ) 80 Hz), 132.0, 131.9, 131.8
(d, J ) 6.5 Hz), 128.4, 128.1 (d, J ) 16 Hz), 127.8, 127.0, 125.5
(d, J ) 11 Hz), 122.8, 122.4 (d, J ) 102 Hz), 42.2, 41.8, 14.2,
13.1; 31P NMR (CDCl3) δ 27.45; IR (KBr pellet) 1731 (νCdO),
1272 (νPdO) cm-1; m/e (EI) 443 (100, M+), 344, 219, 201, 100.
2-(Dip h en ylp h osp h in yl)-1-n a p h th ol (2c). 2-(Diphenyl-
phosphinyl)-1-naphthyl N,N-diethylcarbamate (13.3 g, 30
mmol) and potassium hydroxide (8 g, 140 mmol) were stirred
in 200 mL of ethanol under reflux for 16 h.48 The solution
was neutralized with HCl (10%) and extracted with dichloro-
methane. The crude product was purified by chromatography
(eluent: petroleum ether/ethyl acetate, 7/3) followed by crys-
tallization from dichloromethane-petroleum ether, which
yields 2c, 9.35 g (91%), as pink crystals: mp ) 170 °C; 1H
NMR (200 MHz, CDCl3) δ 12.3 (s, 1H, OH), 8.42 (dd, J ) 8.5,
1.7 Hz, 1H), 7.8-7.69 (m, 5H), 7.62-7.44 (m, 8H), 7.27-7.23
(m, 1H), 7.04 (dd, J ) 12, 8.5 Hz, 1H); 13C NMR (CDCl3) δ
162.7 (d, J ) 2.7 Hz), 136.4, 132.5 (d, J ) 2.7 Hz), 132 (d, J )
10.5 Hz), 131.9 (d, J ) 105 Hz), 128.9, 128.6 (d, J ) 12.5 Hz),
127.3, 126 (d, J ) 10.5 Hz), 125.9, 125.7 (d, J ) 9 Hz), 123.3,
118.8 (d, J ) 12 Hz), 102.2 (d, J ) 106 Hz); 31P NMR (CDCl3)
δ 40.6; m/e (CI NH3) 345 (M + 1). Anal. Calcd for C22H17O2P:
C, 76.74; H, 4.98. Found: C, 76.83; H, 5.04. The spectra were
identical to those reported for the same compound prepared
according to another procedure.49
2-(Dip h en ylp h osp h in yl)-6-m eth oxyp h en yl Tr ifla te (3).
Same procedure as for 2. The reaction was performed from
3c (4.9 g, 15 mmol). The crude mixture was chromatographed
(eluent: petroleum ether/ethylacetate, 6/4) and afforded 4.8 g
1
of 3 as colorless crystals (71% yield): mp ) 164 °C; H NMR
(250 MHz, CDCl3) δ 7.76-7.67 (m, 4H), 7.57-7.44 (m, 6H),
7.33 (td, J ) 8 Hz, J PH ) 2.5 Hz, 1H), 7.23 (dd, J ) 8, 1.5 Hz,
1H) 6.94 (ddd, J PH ) 12.5 Hz, J ) 8, 1.5 Hz, 1H), 3.90 (s, 3H,
OCH3); 13C NMR (CDCl3) δ 151.3 (d, J ) 8.3 Hz), 140.7, 132.3
(d, J ) 3 Hz), 132.0 (d, J ) 10.1 Hz), 130.6, 128.6 (d, J ) 12.6
Hz), 128.2, 127.2, 126 (d, J ) 7.7 Hz), 118.7 (q, J CF ) 322 Hz,
CF3), 117.3 (d, J ) 2.2 Hz), 56.1 (OCH3); 31P NMR (CDCl3) δ
26.1; 19F NMR (CDCl3) δ -70.83; IR (KBr pellet) 3055, 2995,
1608, 1570, 1474, 1420, 1430, 1293 (νCOMe), 1210 (νPdO) cm-1
;
m/e (CI NH3) 457 (M + 1). Anal. Calcd for C20H16F3O5PS: C,
52.64; H, 3.53. Found: C, 52.74; H, 3.58.
Electr och em ica l Setu p a n d Electr och em ica l P r oce-
d u r e for Cyclic Volta m m etr y. Cyclic voltammetry was
performed with a homemade potentiostat50 and a waveform
generator, PAR Model 175. The cyclic voltammograms were
recorded with a Nicolet 3091 digital oscilloscope. Experiments
were carried out in a three-electrode cell connected to a
Schlenk line. The cyclic voltammetry was performed at a
stationary disk electrode (a gold disk made from cross section
of wire (L ) 0.5 mm) sealed into glass) with a scan rate of 0.2
V s-1 or 0.05 V s-1. The counterelectrode was a platinum wire
of ca. 1 cm2 apparent surface area; the reference was a
saturated calomel electrode (Tacussel) separated from the
solution by a bridge (3 mL) filled with a 0.3 M n-Bu4NBF4
solution in DMF. Twelve mL of DMF containing 0.3 M
n-Bu4NBF4 was poured into the cell.
Gen er a l P r oced u r e for Cyclic Volta m m etr y. A 16.8 mg
amount of PdCl2(PPh3)2 (0.024 mmol, 2 mM) was added to the
cell and the cyclic voltammetry performed. The 1-naphthyl
triflate (0.048 mmol, 4 mM and successively 10 and 20 mM)
was then added and cyclic voltammetry performed again. In
an other set of experiments corresponding to the experimental
conditions of the electrolyses, the cyclic voltammetry was
performed just before the electrolysis first on the aryl triflate
alone (1 mmol, 20 mM) in 50 mL of DMF and then in the
presence of a 70 mg amount of PdCl2(PPh3)2 (0.1 mmol, 2 mM).
Gen er a l P r oced u r e for P r ep a r a tive Electr olyses. Pre-
parative electrolyses were carried out at room temperature in
a two-compartment air-tight three-electrode cell. The two
compartments were separated by a sintered glass disk (poros-
ity 4). The cathode was a carbon cloth (ca. 10 cm2 surface
area). The anode was a magnesium rod. The reference was
a saturated calomel electrode (Tacussel) separated from the
solution by a bridge (3 mL) filled with a 0.3 M n-Bu4NBF4
solution in DMF. The cathodic and anodic compartments were
filled with 50 and 5 mL, respectively, of DMF containing
2-(Dip h en ylp h osp h in yl)-1-n a p h th yl tr ifla te (2). The
same procedure as for the synthesis of 1-naphthyl triflate45
was followed except that a large excess of NEt3 (4 equiv) and
trifluoromethanesulfonic anhydride (4 equiv) relative to 2c (8.6
g, 25 mmol) was necessary. Flash chromatography over
alumina and crystallization from petroleum ether and di-
chloromethane give 9.25 g of 2 as colorless crystals (78%
yield): mp ) 170 °C; 1H NMR (200 MHz, CDCl3) δ 8.35-8.20
(m, 1H), 7.93-7.88 (m, 1H), 7.78-7.67 (m, 7H), 7.57-7.46 (m,
6H), 7.27 (dd, J ) 12, 8.5 Hz, 1H). 13C NMR (CDCl3) δ 149.2,
136.2 (d, J ) 2 Hz), 132.6 (d, J ) 108 Hz), 132.0 (d, J ) 10
Hz), 129.2, 128.6 (d, J ) 12 Hz), 128.2, 127.8, 127.5 (d, J ) 11
Hz), 127.0 (d, J ) 6 Hz), 123.32 (d, J ) 97 Hz), 122.3, 118.7
(q, J CF ) 315 Hz, CF3); 31P NMR (CDCl3) δ 28.20; 19F NMR
(CDCl3) δ -72.15; IR (KBr pellet) 3056, 1628, 1592, 1563, 1433,
1216 (νPdO) cm-1; m/e (CI NH3) 477 (M + 1). Anal. Calcd for
C23H16F3O4PS: C, 57.99; H, 3.39. Found: C, 58.17; H, 3.42.
2-Met h oxyp h en yl N,N-Diet h ylca r b a m a t e (3a ). The
same procedure as for 2a from 2-methoxyphenol (6.2 g, 50
mmol)was followed. A total of 10.7 g of 3a was isolated as an
orange oil (96% yield): 1H NMR (200 MHz, CDCl3) δ 7.26-
7.06 (m, 2H), 6.96-6.88 (m, 2H), 3.82 (s, 3H), 3.42 (broad q,
4H), 1.23 (broad t, 6H); 13C NMR (CDCl3) δ 154.2 (CdO), 151.6
(CO), 140.6, 126.2, 123.4, 120.7, 112.5, 55.9 (OCH3), 42.3 (CH2),
42.0 (CH2), 14.1 (CH3), 13.5 (q, CH3); m/e (CI NH3) 224 (M +
1).
2-(Dip h en ylp h osp h in yl)-6-m et h oxyp h en yl
N,N-Di-
eth ylca r ba m a te (3b). The same procedure as for 2b was
followed. The reaction was performed from 3a (8.9 g, 40
mmol). 3b: colorless crystals; 13.5 g (80%, yield); mp ) 89
°C; 1H NMR (250 MHz, CDCl3) δ 7.84-7.63 (m, 4H), 7.60-
7.37 (m, 6H), 7.22 (td, J ) 7 Hz, J PH ) 3.5 Hz, 1H), 7.16 (dd,
J ) 7, 2 Hz,1H), 7.05 (ddd, J PH ) 12.5 Hz, J ) 7, 2 Hz,1H),
3.82 (s, 3H), 3.04 (broad q, J ) 7 Hz, 2H), 2.91 (broad q, J )
(48) Greene, T. W. Protective Groups in Organic Synthesis; Wiley
Interscience. New York, 1982; p 236.
(49) Dhawan, B.; Redmore, D. J . Org. Chem. 1991, 56, 833.
(50) Amatore, C.; Lefrou, C.; Pflu¨ger, F. J . Electroanal. Chem. 1989,
270, 43.

274 J . Org. Chem., Vol. 62, No. 2, 1997
J utand and Mosleh
n-Bu4NBF4 (0.3 M). One mmol of the aryl triflate was added
to the cell followed by 70 mg (0.1 mmol) of PdCl2(PPh3)2. The
electrolysis was conducted at controlled potential and was
stopped when the current was about 5% of its initial value.
The cathodic compartment was hydrolyzed with aqueous HCl
(1 N) and extracted with diethyl ether. The yields of biaryls,
arenes, and ArOH were determined on the crude mixture by
1H NMR spectroscopy (250 MHz), using CHCl2CHCl2 as
internal standard, and by comparison with the authentic
samples, when available. The biaryls were then isolated by
flash chromatography (eluent: petroleum ether/ethylacetate)
and characterized by NMR and mass spectroscopy.
4,4′-Dim eth oxybip h en yl:51 mp ) 179 °C; 1H NMR (250
MHz, CDCl3) δ 7.49 (d, J ) 8.6 Hz, 4H), 6.97 (d, J ) 8.6 Hz,
4H), 3.86 (s, 6H); 13C NMR (CDCl3) δ 158.6 (COCH3), 133.4,
127.7, 114.1, 55.3 (OCH3); m/e (EI) 214 (100, M+), 199, 171.
2,2′-Bip yr id in e:51 mp ) 70-71 °C; 1H NMR (250 MHz,
CDCl3) δ 8.68 (dd, J ) 4.8, 1.6 Hz, 2H), 8.40 (dd, J ) 7.8, 1
Hz, 2H), 7.82 (td, J ) 7.8, 1.6 Hz, 2H), 7.31 (ddd, J ) 7.8, 4.8,
1 Hz, 2H); m/e (EI) 156 (M+).
4,4′-Dica r beth oxybip h en yl:52 mp ) 224-226 °C; 1H NMR
(250 MHz, CDCl3) δ 8.15 (d, J ) 8.4 Hz, 4H), 7.66 (d, J ) 8.4
Hz, 2H), 4.43 (q, J ) 7.1 Hz, 4H), 1.44 (t, J ) 7.1 Hz, 6 H); 13
C
NMR (CDCl3) δ 166.3 (CdO), 144.2 (CCdO), 130.1, 123.0,
127.1, 61.0 (CH2), 14.3 (CH3); m/e (EI) 298 (M+), 253 (100),
225, 152.
Gen er a l P r oced u r e for th e Syn th esis of Bia r yls in th e
P r esen ce of Zin c P ow d er . Zinc powder was activated by
treatment of 20 g in 100 mL of acetic acid for 1 h. After
filtration, the powder was washed three times with distilled
water. It was dried under vacuum for 6 h at 120 °C. NiCl2-
(dppe) (0.158 g, 0.3 mmol), zinc powder (3.9 g, 60 mmol), KI
(2 g, 12 mmol), and the aryl triflate (3 mmol) were stirred in
2 mL of degassed solvent (DMF or THF) at the desired
temperature. The reaction was monitored by TLC or HPLC.
The mixture was hydrolyzed with aqueous HCl 10% and
extracted with ethyl ether. The organic phase was washed
with water and dried over MgSO4. The crude mixture was
treated as described above for the electrosyntheses.
4,4′-Diflu or obip h en yl:51 mp ) 88-90 °C; 1H NMR (200
MHz, CDCl3) δ 7.5 (dd, J ) 8.8, 5.2 Hz, 4H), 7.1 (t(dd), J )
8.8, 8.8 Hz, 4H); m/e (CI NH3) 190 (M+), 171, 152.
4,4′-Dim eth ylbip h en yl:56 mp ) 119 °C; 1H NMR (200
MHz, CDCl3) δ 7.48 (d, J ) 8 Hz, 4H), 7.23 (d, J ) 8 Hz, 4H),
2.38 (s, 6H); m/e (EI) 182 (M+), 167 (100), 152.
2,2′-Dim eth ylbip h en yl:57 1H NMR (250 MHz, CDCl3) δ
7.19-7.10 (m, 6H), 7.02 (dd, J ) 6, 1.5 Hz, 2H), 1.97 (s, 6H);
m/e (EI) 182 (M+), 167 (100), 152, 76.
4,4′-Di-ter t-bu tylbip h en yl:51 mp ) 128-129 °C; 1H NMR
(CDCl3) δ 7.5 (d, J ) 8.6 Hz, 4H), 7.44 (d, J ) 8.6 Hz, 4H),
1.36 (s, 18H).
1,1′-Bin a p h th yl:51 mp ) 144-145 °C; 1H NMR (250 MHz,
CDCl3) δ 7.945, 7.943 (2d, J ) 8.2 Hz, 4H), 7.58 (t, J ) 7 Hz,
2H), 7.50-7.37 (m, 6H), 7.31-7.24 (m, 2H); 13C NMR (CDCl3)
δ 138.4, 133.5, 132.8, 128.1, 127.9, 127.8, 126.5, 125.9, 125.7,
125.3; m/e (EI) 254 (100 M+), 253, 252, 239 ,126.
2,2′-Dim eth yl-1,1′-bin aph th yl:5b 1H NMR (200 MHz CDCl3)
δ 7.89 (d, J ) 8 Hz, 2H), 7.87 (d, J ) 8 Hz, 2H), 7.50 (d, J )
8 Hz, 2H), 7.39 (dd, J ) 7, 1.2 Hz, 2H), 7.19 (dd, J ) 7, 1.2 Hz,
2H), 7.04 (d, J ) 8 Hz, 2H), 2.02 (s, 6H); m/e (EI) 282 (100,
M+), 267, 252, 126.
4,4′-Dicya n obip h en yl:52 mp ) 233-234 °C; 1H NMR (250
MHz, CDCl3) δ 7.8 (d, J ) 8.6 Hz, 4H), 7,7 (d, J ) 8.6 Hz, 4H);
13C NMR (CDCl3) δ 144 (-CCN), 133, 128, 118 (-CN), 113;
IR (KBr pellet) 3080, 2210 (νCtN), 1420, 1480, 580 cm-1; m/e
(EI) 204 (100, M+), 177.
2-(Dip h en ylp h osp h in yl)n a p h th a len e:58 mp ) 184-185
1
°C; H NMR (250 MHz, CDCl3) δ 8.30 (d, J PH ) 14 Hz, 1H),
7.88 (d, J ) 8.5 Hz, 2H), 7.72-7.68 (m, 4H), 7.65-7.44 (m,
10H); 31P NMR (CDCl3) δ 29.19; m/e (CI, NH3) 329 (M + 1).
3-(Dip h en ylp h osp h in yl)-1-m eth oxyben zen e: 1H NMR
(250 MHz, CDCl3) δ 7.67-7.56 (m, 4H), 7.48-7.35 (m, 6H),
4,4′-Bis(tr iflu or om eth yl)bip h en yl:53 mp ) 92-93 °C; 1H
NMR (250 MHz, CDCl3) δ 7.78 (d, J ) 8.8 Hz, 4H), 7.71 (d, J
) 8.8 Hz, 4H); 13C NMR (CDCl3) δ 143.2, 130.3 (q, J CF ) 34.5
Hz, -CCF3), 127.6, 125.8 (q, J CF ) 3.4 Hz, -CCCF3), 124 (q,
J CF ) 272 Hz, CF3); m/e (EI) 290 (M+, 100) 271, 221, 152.
4,4′-Dich lor obip h en yl:54 mp ) 147-148 °C; 1H NMR (250
MHz, CDCl3) δ 7.49 (d, J ) 8.5 Hz, 4H), 7,42 (d, J ) 8.5 Hz,
4H); m/e (EI) 224,222 (100, M+), 188, 190, 152.
7.29 (td, J ) 7.8 Hz, J PH ) 4 Hz, 1 H), 7.22 (broad d, J PH
)
14.5 Hz, 1 H), 7.07 (dd, J PH ) 11 Hz, 7.5 Hz, 1 H), 7.00 (dd, J
) 7.5 Hz, J PH ) 2.7 Hz, 1 H), 3.72 (s, 3H); 31P NMR (CDCl3) δ
29.37; m/e (CI, NH3) 309 (M + 1).
2,2′-Dich lor obip h en yl:55 mp ) 59 °C; 1H NMR (250 MHz,
CDCl3) δ 7.61 (d, J ) 7 Hz, 2H), 7.46 (td, J ) 7, 1.5 Hz, 2H),
7.39-7.30 (m, 4H); m/e (EI) 224,222 (M+), 205, 187, 154,152
(100).
Ack n ow led gm en t. This work has been supported
in part by the Centre National de la Recherche Scien-
tifique (CNRS, URA 1679, “Processus d’Activation Mo-
le´culaire”), the Ministe`re de l’Enseignement Supe´rieur
et de la Recherche (Ecole Normale Supe´rieure, De´par-
tement de Chimie). Serge Ne´gri is acknowledged for
technical assistance.
Bip h en yl:51 mp ) 70 °C; 1H NMR (250 MHz, CDCl3) δ 7.67
(dd, J ) 8.8, 2 Hz, 4H), 7.33 (dd, J ) 8.8, 7.5 Hz, 4H), 7.33
(dd, J ) 7.5, 2 Hz, 2H); m/e (EI) 154 (100, M+), 76, 57.
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