Organometallics
Article
transition metal catalyzed coupling reactions, only (Ph2P)2
could be observed from the reaction of Ph2PNa with PhOTf,
and no Ph3P could be detected at all (runs 17 and 18).
Therefore, it is concluded that for the couplings of Ph2PNa
with PhX, the generally recognized inert PhCl, not the more
reactive PhBr and PhI, is the starting material of choice. By using
PhCl, the desired coupling product can be readily generated in
almost quantitative yields. PhF is as good as PhCl. However, the
use of a chloride, ArCl, rather than a floride, ArF, apparently has
more advantages considering the cost and availability of the two
chemicals.
yield of Ph3P was obtained from Ph2PK under similar
conditions! Therefore, Ph2PNa is the best among the three alkali
phosphides Ph2PM (M = Li, Na and K)!
HMPA(0.2 mmol)
Ph2PM+ PhCl ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ph3P
THF, 67°C,16h
0.3 mmol
(5)
0.21 mmol
where M = Li, not detected; M = Na, 78% yield; and M = K, 43%
yield
2.3.1. Generality for the Preparation of Ph2PR by Reactions
of Ph2PNa with Organochlorides RCl. To disclose the scope
and limitations of the model reaction of Ph2PNa with PhCl
(Table 3, run 11), we investigated the reactions with a series of
aryl chlorides, alkenyl chloride and alkyl chlorides (Table 4). As
shown in Table 4, a range of substituted aryl chlorides having
electron-donating groups such as Me, n-Bu, t-Bu, Bz, t-BuO, and
PhO (1b−g) and electron-withdrawing groups like CO2Me,
CF3, and CN (1h−j) at the para-position of the chlorobenzene
rings, all were readily phosphinated by Ph2PNa affording the
corresponding diphenylarylphosphines (2b−j) in high to
excellent yields. However, compared to PhCl, an alkyl group
at the benzene ring could substantially reduce the reactivity.
Thus, compared to the high yield of the product with PhCl (1a),
4-chlorotoluene (1b) and 1-butyl-4-chlorobenzene (1c) gave 81
and 82% of corresponding products 2b and 2c, respectively, after
heating at 67 °C for 80 h. Interestingly, 1-tert-butyl-4-
chlorobenzene (1d) is more reactive than 1-butyl-4-chlorobenze
(1c), to give almost a quantitative yield of the coupling product
2d after 48 h at 67 °C. 4-Chlorodiphenylmethane (1e) is even
more reactive than 1-tert-butyl-4-chlorobenzene (1d) to give 2e.
Thus, the reactivity approximately follows a decreasing order of
H, Bz > t-Bu > n-Bu, Me. A competing reaction by running a
reaction of Ph2PNa (0.21 mmol) with a mixture of p-G-C6H4Cl
(G = H, Me, t-Bu, and Bn; 0.2 mmol each) in THF (0.6 mL) in
the presence of 15-crown-5-ether at 67 °C for 16 h also
confirmed this phenomena, i.e., the ratio of the coupling
products: H/Me/t-Bu/Bn = 45/7/13/35. In contrast, 1-tert-
butoxy-4-chlorobenzene (1f) showed similar reactivity with
PhCl, and 4-chlorodiphenyl ether (1g) even reacts at room
temperature to give 2g in 81% yield.
2.2.2. Comparison of the Reactivity between Two Halogen
Atoms. To further discriminate the behavior of different halogen
atoms in the reactions with Ph2PNa, the reactions of 1,4-
dihalobenzens with Ph2PNa were studied (Scheme 5).
Scheme 5. Comparison of Reactivity of Two Halogen Atoms
Surprisingly, the reaction of an excess amount of 1,4-
dichlorobenzen with Ph2PNa at room temperature predom-
inantly gave the disubstituted 1,4-bis(diphenylphosphino)-
benzene in 91% yield, and the monosubstituted one was only
obtained in 9% yield. A similar result was obtained by
conducting the reaction at −30 °C. These results indicated
that the monosubstituted 1,4-chloro(diphenylphosphino)-
benzene is more reactive than 1,4-dichlorobenzene. With 1,4-
fluorochlorobenzene, however, only two products were
o b t a i n e d : t h e m o n o s u b s t i t u t e d 1 , 4 - c h l o r o -
(diphenylphosphino)benzene by the substitution of F and the
bis-substituted 1,4-(diphenylphosphino)benzene, in 41 and
59% yield, respectively. The monosubstituted phosphine 1,4-
fulororo(diphenylphosphino)benzene expected by the replace-
ment of Cl could not be detected. Similarly, when it comes to p-
bromo- and p-iodo-substituted chlorobenzenes, we only
detected the monosubstituted phosphine 1,4-chloro-
(diphenylphosphino)benzene via the replacement of the Br
and I atoms and the disubstituted 1,4-(diphenylphosphino)-
benzene. The yields of the coupling products were low for the
bromo and iodo compounds because, as described above, a side
product Ph2PPPh2 was also generated. Therefore, taken together
with the results shown in Table 3, the reactivity (not the
selectivity) of Ar−X with Ph2PNa seems follow an increasing
order of ArCl < ArF < ArBr < ArI, while the selectivity to the
desired coupling products ArPPh2 follows a reversed order ArCl
= ArF > ArBr > ArI.
As also described in Table 3 (run 13), in THF, the reactions
with simple alkyl-substituted chlorobenzenes 1b−d required a
relative long time. However, by changing the solvent to DMF,
the corresponding phosphines 2b−d could be generated in a
short time without adding a crown ether.
Aryl chlorides with an electron-withdrawing group at the
benzene ring react faster than chlorobenzene. Thus, 4-
chlorobenzotrifluoride (1i) and 4-chlorobenzonitrile (1j) all
reacted rapidly at room temperature to give high yields of
corresponding coupling products 2i and 2j. Interestingly,
chloronaphthalenes are also more reactive than the correspond-
ing chlorobenzene, and both 1-chloronaphthalene and 2-
chloronaphthalene all produced the corresponding phosphines
in high yields at room temperature (2k and 2l). In addition,
heteroaryl chlorides chloropyridines (1m,n) and chlorothio-
phenes (1p,q) were tolerable under similar conditions to give
the desired phosphines in good to high yields (2m−q). Multiply
substituted chloroarenes (1r−t) also smoothly underwent
coupling with Ph2PNa with good selectivity and efficiency. For
example, with 1,2-dichlorobenzene (1r), 1,3-dichlorobenzene
(1s), and 2,4,6-trichlorobenzene (1t), one Cl atom was
selectively replaced by Ph2P to give the corresponding
monodentate phosphines (2r−t). Of course, all the chloro
atoms could be replaced by Ph2P by using an excess amount of
2.3. Reactivity of Ph2PM (M = Li, Na, and K): Na Is
Superior to Li and K. As described in Table 1, the reaction of
Ph2PNa with PhCl in the presence of HMPA could gave 78%
yield of Ph3P (Table 3, run 9). Surprisingly, under similar
conditions, the reaction of Ph2PLi with PhCl did not give a trace
amount of Ph3P even under heating for 16 h (eq 5)! More
surprisingly, Ph2PNa is even better than Ph2PK, since only 43%
E
Organometallics XXXX, XXX, XXX−XXX