Nickel(0) complex-catalysed detelluration of diorganyl tellurides and ditellurides with phosphines

Article abstract of DOI:10.1039/a706985e

Nickel(0) complexes efficiently catalyse detelluration of diaryl tellurides and diaryl ditellurides in the presence of phosphines to afford biaryls in high yields.

Full text of DOI:10.1039/a706985e

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Nickel(0) complex-catalysed detelluration of diorganyl tellurides and
ditellurides with phosphines
Li-Biao Han and Masato Tanaka*
National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan
a
Table 1 Detelluration of dianisyl telluride 1a catalysed by Ni(PEt3)4
Nickel(0) complexes efficiently catalyse detelluration of
diaryl tellurides and diaryl ditellurides in the presence of
phosphines to afford biaryls in high yields.
Solvent/quantity of 1a (mmol)/
additive (equiv. relative to 1a)
GC yield of
2a (%)
Run
Organotellurium compounds are becoming increasingly im-
portant in organic synthesis.1 As reported by Bergman et al.
more than two decades ago,2 aryltelluriums undergo detellu-
ration upon treatment with degassed Raney nickel to afford
biaryls. The high accessibility of the aryltelluriums1,2 has made
the detelluration a convenient method for the synthesis of
biaryls. However, attempted transition metal-catalysed detel-
luration has been unsuccessful to data.2,3 Hence, although the
reaction is interesting and synthetically useful, the necessity of
more than a stoichiometric amount of the metal is still a serious
drawback. Herein reported are (i) highly successful nickel-
catalysed detelluration of diaryl tellurides with phosphines
affording biaryls and (ii) its mechanism involving an unex-
pected disproportionation of the complexes formed by oxidative
addition of C–Te bonds to Ni(PEt3)4.4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
C6H6/2/none
C6H6/2/PEt3 (1.2)
C6H6/2/PEt3 (3.0)
MeCN/2/PEt3 (1.2)
13
30
65
41
42
73
93
40
29
MeCN/0.8/PEt3 (1.2)
MeCN/2/PEt3 (3.0)
MeCN/1.2/PEt3 (5.0)
MeCN/2/PBu3 (1.2)
MeCN/0.8/Pi-Pr3 (1.2)
MeCN/0.8/PMePh2 (1.2)
MeCN/0.8/P(NEt2)3 (1.2)
MeCN/0.8/P(pip)3 (1.2)b
MeCN/0.8/P(pyrro)3 (1.2)b
MeCN/0.8/P(pyrro)3 (0.5)b
MeCN/0.8/P(pyrro)3 (1.2)b
MeCN/0.8/P(pyrro)3 (1.2)b
20
41
56
ca. 100
70
95c
61d
Heating a mixture of dianisyl telluride (1a, 684 mg, 2 mmol)
and Ni(PEt3)4 (106 mg, 10 mol% relative to 1a) in benzene (4
ml) at 80 °C slowly formed black precipitates to give the
detelluration product 4,4A-bianisyl 2a in 13% GC yield after 20
h.† Prolonged heating did not improve the yield, indicating a
very poor turnover of the complex catalyst. However, a higher
turnover was achieved when a free phosphine was added to the
reaction system [eqn. (1)]. Thus, the yield of 2a increased to
a All reactions were run in a Schlenk flask under argon atmosphere at 80 °C
for 20 h, using 10 mol% of Ni(PEt3)4 and 4 ml of a solvent. Yields of 2a
b
were determined by GC using biphenyl as an internal standard. pip =
Piperidinyl; pyrro = pyrrolidinyl. c 3 mol% Ni(PEt3)4 was used. d 3 mol%
Ni(PPh3)4 was used as the catalyst.
functionality was also tolerant towards the reaction, and a good
yield of the corresponding product was obtained. Moreover, the
new procedure could be successfully applied to diaryl ditellu-
rides. Thus, the reactions of (PhTe)2 and (4-MeOC6H4Te)2 with
2.5 equiv. of P(pyrro)3 catalysed by Ni(PEt3)4 (10 mol%) in
acetonitrile under similar conditions readily afforded high
yields of the corresponding biaryls in both cases. The reaction of
Ni(PEt3)4
(10 mol%)
Te(C6H4OMe-4)2 + PR3
(4-MeOC6H4)2 + Te=PR3 (1)
80 °C, 20 h
1a
2a
30% and further to 65% when the reaction was conducted under
the same conditions in the presence of 1.2 and 3.0 equiv.
(relative to 1a) of free PEt3, respectively (Table 1). Although
reactions in acetonitrile gave slightly higher yields than those in
benzene, the difference was marginal. The effect of the
concentration of 1a on the yield was not significant either at
concentrations ranging from 0.8 to 2 mmol of 1a in 4 ml of the
solvent. However, the nature of the phosphine employed
exerted a decisive effect. PBu3 was as effective as PEt3, but the
use of more sterically demanding or less electron-donating
phosphines such as PPri3 and PMePh2 reduced the yield. As
compared to these triorganophosphines, however, amino-
phosphines proved much more efficient to achieve significant
improvement in the product yield. In the presence of 1.2 equiv.
of P(pyrro)3 (pyrro = pyrrolidinyl), for example, starting
telluride 1a disappeared completely after 20 h (80 °C) to afford
2a quantitatively. Note that a yield as high as 95% was obtained
even when the amount of the nickel catalyst was reduced to 3
mol%. Obviously, the yield of 2a increases in the order PMePh2
< PEt3 ≈ PBu3 < P(NEt2)3 < P(pip)3 (pip = piperidinyl) <
P(pyrro)3. The trend is in good agreement with the reactivity of
phosphines toward tellurium to form phosphine tellurides
(TeNPR3), P(pyrro)3 being most reactive.5‡
(4-ClC6H4Te)2
was
particularly
noticeable;
4,4A-dichlorobiphenyl was selectively formed in 79% yield
despite the potential reactivity of chloroarenes toward Ni0
species. Since diaryl ditellurides are more readily synthesized
then tellurides, successful application of the procedure to
ditellurides significantly expands the scope of the reaction.
i
N
P
Ar–Ar +
TeAr2 [or (ArTe)2] +
N
P = Te
3
3
1a–h
2a–d
R1
2a: R1 = H, R2 = MeO, 92%
2b: R1 = R2 = H, 96%
2c: R1 = R2 = MeO, 86%
2d: R1 = H, R2 = Me2N, 83%
R2
Te
Te
From
2
1a–d
2e: (= 2a): R = MeO, 87%
2f: (=2b): R = H, 89%
2g: R = Me, 90%
R
From
2h: R = Cl, 79%
2
1e–h
The Ni-catalysed detelluration could be readily applied to
other diaryl tellurides affording the corresponding biaryls in
good yields (Scheme 1).§ Besides methoxy group, the amino
Scheme 1 Reagents and conditions: i, telluride or ditelluride (2.0 mmol),
Ni(PEt3)4 (10 mol%), P(pyrro)3 (2.4 or 5.0 mmol for telluride or ditelluride
respectively), MeCN, 80 °C, overnight
Chem. Commun., 1998
47

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palladium analogues of which have been proposed.7 A suppor-
tive observation was made in a catalytic reaction [1.2 equiv.
P(pyrro)3, 3 mol% Ni(PEt3)4, acetonitrile, 80 °C, 10 h] of an
unsymmetrical telluride, TeAr1Ar2 (Ar1 = C6H4NMe2-4, Ar2 =
C6H4OMe-4), which gave not only Ar1–Ar2 but also cross-over
products, Ar1–Ar1 and Ar2–Ar2 (Ar1–Ar1 :Ar1–Ar2 :Ar2–Ar2 =
1:3:1) in 73% total yield. The telluride recovered in ca. 10%
was also a mixture of TeAr1 , TeAr1Ar2 and TeAr2 .
Further synthetic application of the facile oxidative addition
of C–Te bonds to Ni, Pd and Pt complexes4 is now under
extensive study.
Financial support from the Japan Science and Technology
Corporation (JST) through the CREST (Core Research for
Evolution Science and Technology) program is gratefully
acknowledged.
Ar
L
Te
Ni Ni
Te
L
Ar
L
–L
L
Ni
Ar
TeAr
TeAr2 + NiLn
Ar
L
Ar
4
3
L
2
2
+
TeAr2
+
(Te=NiLn)
Ni
Ar
Ar
6
L
5
PR3
Te=PR3 + NiLn
Ar–Ar + NiLn
Scheme 2
The detelluration of tellurides is likely to proceed, as depicted
in Scheme 2, via oxidative addition of a telluride to the nickel
complex, disproportionation of the resulting complex 3 to
generate diarylnickel species 5, and its reductive elimination.
The mechanism is substantiated by the following observations.
As previously reported,4 treatment of TePh2 (340 mg, 1.206
mmol) with Ni(PEt3)4 (427 mg, 0.804 mmol) in C6D6 at room
temp. gave complex 3b in 10 min, which displayed a singlet at
d 10.3 in its 31P NMR spectrum [eqn. (2)]. Complex 3b was
Footnotes and References
* E-mail: mtanaka@ccmail.nimc.go.jp
† Most of 1a (83% based on GC) remained unreacted. In the absence of the
nickel catalyst, no coupling product was formed under similar conditions,
indicating that the nickel catalyst was essential for the reaction.
‡ Formation of TeNP(pyrro)3 in the detelluration reaction with P(pyrro)3
was confirmed by 31P NMR. See ref. 5.
§ Typical experimental procedure: A mixture of telluride 1a (684 mg, 2
mmol), P(pyrro)3 (579 mg, 2.4 mmol) and Ni(PEt3)4 (106 mg, 0.2 mmol) in
acetonitrile (5 ml) was heated at 80 °C overnight (20 h). The reaction
mixture was poured into 10 ml of 1m HCl to liberate metallic tellurium
instantly. Extraction using CH2Cl2, drying over MgSO4 and concentration
afforded crude 2a, which was subsequently passed through a short silica gel
column (ethyl acetate–chloroform–hexane = 0.5:1:8) to give pure product
2a as a white solid (394 mg, 1.84 mmol, 92%).
PEt3
Ni
Ar
TeAr
TeAr2 + Ni(PEt3)4
(2)
1
PEt3
3
a, Ar = C6H4OMe-4; b, Ar = Ph; c, Ar = C6H3(OMe)2-3,4
¶ A mixture of 1c (371 mg, 0.923 mmol) and Ni(PEt3)4 (500 mg, 0.941
mmol) in benzene (6 ml) was stirred at room temp. for 15 min.
Concentration of the reaction mixture to ca. 0.5 ml resulted in the
precipitation of analytically pure complex 3c as a black solid in 87% yield
(560 mg, 0.803 mmol). Selected data for 3c: 1H NMR (300 MHz, C6D6) d
7.78 (d, 1 H, J 8.0 Hz), 7.67 (s, 1 H), 7.11 (s, 1 H), 6.89 (d, 1 H, J 7.8 Hz),
6.66 (d, 1 H, J 7.8 Hz), 6.41 (d, 1 H, J 8.0 Hz), 3.67 (s, 3 H), 3.52 (s, 3 H),
liquid and rather unstable so prohibiting further purification.
Telluride 1a also reacted similarly to form oily complex 3a.
However, oxidative addition of Te[C6H3(OMe)2-3,4]2 1c with
the nickel complex gave analytically pure complex 3c (87%
yield) as a black solid.¶
3.50 (s, 3 H), 3.40 (s, 3 H), 1.34–1.42 (m, 12 H), 0.90–1.09 (m, 18 H); 31
P
These nickel complexes 3 gradually decomposed even at
room temp. For example, NMR spectroscopy revealed that the
spontaneous decomposition of 3b in C6D6 resulted in precipita-
tion of a black solid (presumably a nickel telluride like 6) to
generate trans-NiPh2(PEt3)2 5b6 and TePh2 (approximately 1:1
ratio; the conversion of 3b was ca. 50% after 2 d) in solution. A
very similar decomposition process was observed with complex
3c, where the diarylnickel complex 5c and the telluride 1c were
found by 1H and 31P NMR spectroscopy to be formed in
addition to the black precipitates. As expected, the decomposi-
tion was faster at elevated temperatures. Thus, while only ca.
50% of complex 3c decomposed at room temp. over 10 h, it
disappeared completely within 3 h at 50 °C to afford 5c and 1c
in ca. 1:1 ratio, which accounted for over 93% of the total aryl
groups as estimated by 1H NMR spectroscopy. Heating to even
higher temperatures induced a secondary decomposition (re-
ductive elimination); diarylnickel complex 5c generated in situ
through the decomposition of 3c at 50 °C in a sealed NMR tube
disappeared after overnight heating at 100 °C and the
corresponding biaryl was obtained in 91% NMR yield (based on
the quantity of 1c). This reductive elimination process ob-
viously results in regeneration of Ni0 species, which carries the
catalysis. Interaction of 6 with a phosphine molecule forming a
phosphine telluride presumably is another route to the Ni0
species. Further mechanistic detail of the decomposition of 3
leading to the generation of 5 is ambiguous at the moment. It
may involve a dimeric intermediate such as 4 (Scheme 2),
(121.5 MHz, C6D6) d 11.3. Anal. Calc. for C28H48NiO4P2Te: C, 48.26; H,
6.94. Found: C, 48.21; H, 7.07%.
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6 The formation of 5b was confirmed spectroscopically by comparing with
an authentic sample synthesized by an established method. J. Chatt and
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Received in Cambridge, UK, 29th September 1997; 7/06985E
48
Chem. Commun., 1998