Phenyltellanyl triflate (PhTeOTf) as a powerful tellurophilic activator in the friedel-crafts reaction

Article abstract of DOI:10.1246/cl.2008.650

A powerful electrophilic activator for organotellurium compounds was developed. Phenyltellanyl triflate (PhTeOTf) prepared in situ from dibromophenyl(phenyltellanyl)telluride and AgOTf selectively activated various organotellurium compounds in the presence of aromatic compounds yielding the corresponding Friedel-Crafts reaction products. Polymer-end organotellurium compounds were also activated by PhTeOTf providing the corresponding end-functionalized polymers. Copyright

Full text of DOI:10.1246/cl.2008.650

650
Chemistry Letters Vol.37, No.6 (2008)
Phenyltellanyl Triflate (PhTeOTf) as a Powerful Tellurophilic Activator
in the Friedel–Crafts Reaction
Takeshi Yamada,1 Eri Mishima,2 Kazuya Ueki,2 and Shigeru Yamagoꢀ2
1Pioneering Research Unit for Next Generation, Kyoto University, Kyoto 611-0011
2Institute for Chemical Research, Kyoto University, Kyoto 611-0011
(Received March 17, 2008; CL-080289; E-mail: yamago@scl.kyoto-u.ac.jp)
A powerful electrophilic activator for organotellurium
Table 1. Friedel–Crafts acylation of acyltelluride 2 and 1,3,5-
trimethoxybenzene (3) with a tellurophilic activatora
compounds was developed. Phenyltellanyl triflate (PhTeOTf)
prepared in situ from dibromophenyl(phenyltellanyl)telluride
and AgOTf selectively activated various organotellurium com-
pounds in the presence of aromatic compounds yielding the
corresponding Friedel–Crafts reaction products. Polymer-end
organotellurium compounds were also activated by PhTeOTf
providing the corresponding end-functionalized polymers.
OMe
O
OMe
PhTe(Br)2TePh (1)
O
AgX
Ph
MeO
+
CH2Cl2
Ph
TeMe
0 oC, 1 h
MeO
OMe
OMe
2
3
Entry
Activator
Yield/%b
1
2c
3
4
5
6
7
1/AgOTf
1/AgOTf
1/AgNTf2
1/AgSbF6
1/AgB(C6F5)4
PhSeOTf
PhSOTf
66
91
53
49
24
50
44
Organoheteroatom compounds have been widely used as
the precursors for reactive carbon species, such as carbanions,
carbocations, and carbon-centered radicals, which are essential
intermediates in organic synthesis. Among these, organotellu-
rium compounds have been recognized as some of the most ver-
satile precursors due to their simple preparation methods, con-
siderable stabilities, and high reactivities.1 For example, various
organometallic reagents are prepared from organotellurium
compounds by the tellurium–metal exchange reaction.2 Car-
bon-centered radicals generated from organotellurium com-
pounds are also used for the controlled synthesis of small mole-
cules3 and macromolecules.4 We recently showed that a dual use
of organotellurium compounds as the precursors of both carbon-
centered radicals and carbanions offered unique synthetic trans-
formations, which were unattainable with a single carbon-reac-
tive species.5 By contrast, the generation of carbocations from
organotellurium compounds has been limited,6 especially in ap-
plications with carbon–carbon bond-formation reactions.7 These
results prompted us to investigate the use of organotellurium
compounds as the precursors of carbocationic species. We hy-
pothesized that this situation could be attributed to the lack of
a suitable activator of organotellurium compounds, and that
the development of an efficient activator would open new possi-
bilities for their use in electrophilic reactions. Here, we report
on a new and efficient tellurophilic activator, and its utilization
in the Friedel–Crafts reaction.
We focused on dibromophenyl(phenyltellanyl)telluride (1)8
as a precursor of an electrophilic activator. A recent study
indicated that 1 reversibly generated PhTeBr at ambient temper-
ature. Thus, 1 prepared by mixing an equimolar amount of
diphenylditelluride and bromine was treated with 2 equiv of
AgOTf at 0 ꢁC in CH2Cl2 for 0.5 h, and a mixture of acyl tellu-
ride 2 and 1,3,5-trimethoxybenzene (3) was added to this solu-
tion at 0 ꢁC. After aqueous workup, the Friedel–Crafts product
was identified in 66% yield by 1H NMR analysis (Table 1,
Entry 1). The yield increased to 91% when the acylation was
carried out at room temperature (Entry 2). As neither 1 nor
AgOTf activated 2 under similar conditions (<6% yield),
PhTeOTf generated from these reagents must be the real activat-
ing species (see below). The synthetic advantages of acyl tellu-
aThe activator prepared from 1 (0.15 mmol) and
AgOTf (0.41 mmol) was added to a CH2Cl2 solu-
tion of 2 (0.26 mmol) and 3 (0.29 mmol) at 0 ꢁC.
bDetermined by 1H NMR based on 2. cThe reaction
was carried out at room temperature.
rides are also worth mentioning, because they are more hydrolyt-
ically stable and easier to purify than acyl halides, which are rou-
tinely used as substrates in the Friedel–Crafts reaction.
Activators in situ prepared from 1 and AgNTf2, AgSbF6,
and AgB(C6F5)4 also served as the activators (Entries 3–5),
but their efficiency was lower than that of PhTeOTf. PhSeOTf9
and PhSOTf10 also activated 2 with moderate efficiency (Entries
7
.
6 and 7). Conventional activators, such as BF3 Et2O, NIS,
NIS/TMSOTf, t-BuOCl, MeOTf,6b and Br2,6g were inefficient
(<14% yield in all cases) under similar reaction conditions.
The generality of the new activator was examined by em-
ploying various organotellurium compounds and electrophiles,
and the results are summarized in Table 2. The reaction of
acyl telluride 2 with substituted benzene derivatives proceeded
smoothly to give the desired products in excellent yields (Entries
1–3). The 1,4-disubstituted product predominantly formed over
the 1,2-disubstituted product in the reaction with methoxyben-
zene (Entry 2). Heteroaromatic compounds, such as thiophene,
also reacted with 2 to give the 3-substituted product in good
yield (Entry 4). The observed regioselectivities were consistent
with those in the conventional Friedel–Crafts reaction, suggest-
ing that the reaction indeed generated carbocationic species
from organotellurium compounds. Carbazole-derived acyl
telluride 6d and alkyl tellurides 7e and 8f were also activated
with PhTeOTf in the presence of 3 to give the desired products
in moderate-to-good yields (Entries 5–7).
The synthetic efficiency was further examined in the poly-
mer-end functionalization using polystyrene 9e, which was
prepared by living radical polymerization of styrene (30 equiv)
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.6 (2008)
651
Table 2. Phenyltellanyl triflate mediated the Friedel–Crafts
reactiona
To confirm the activating species, the intermediate was
analyzed by 125Te NMR spectroscopy. While 1 did not show
125Te NMR resonance in CD2Cl2 at room temperature,8 a new
signal at 1119 ppm appeared upon treatment of 1 with 2 equiv
of AgOTf at 0 ꢁC. The signal at 1155 ppm was also observed
when dichlorophenyl(phenyltellanyl)telluride was treated with
AgOTf.11 These results, combined with the established protocol
for the halogen–triflate exchange reaction using AgOTf, strongly
suggest the formation of PhTeOTf as the activating species.
However, attempts to isolate the intermediate have so far been
unsuccessful. Further synthetic and mechanistic studies are
underway, and will be reported in due course.
O
R1
O
O
R4
S
Ph
Ph
R3
R2
5
4
N
C6H4C8H17-4
Ph
a: R1, R2 = OMe, R3 = H
b: R1, R3 = H, R2 = OMe
c: R1, R2 = Me, R3 = H
R4
6
R4
8
7
d: R4 = TeBu-n, e: R4 = TeMe, f: R4 = TePh, g: R4 = 2,4,6-(MeO)
3C6H2
Entry Substrate
Nucleophile
Product Yield/%
1
2
3
4c
5
6
7
8
2
2
2
1,3-(MeO)2C6H4
MeOC6H5
1,3-Me2C6H4
Thiophene
4a
4b
4c
5
6g
7g
8g
9g
92
91b
94
This work was partly supported by a Program of Improve-
ment of Research Environment for Young Researchers (to TY)
and a Grant-in-Aid for Scientific Research (to SY) from the
Ministry of Education, Culture, Sports, Science and Technology
of Japan. We thank Messrs. T. Kameshima and H. Umemoto of
Otsuka Chemical Co. for the MALDI-TOF mass-spectroscopy
measurements.
2
74
79
6d
7e
8f
9e
3
3
3
3e
63d
74
100f
aA substrate and a nucleophile (1.5 equiv) were treated with the
activator prepared from 1 (0.55 equiv) and AgOTf (1.1 equiv)
in CH2Cl2 at room temperature for 1 h. bThe product formed as
References and Notes
1
a) N. Petragnani, H. A. Stefani, Tellurium in Organic Synthesis,
2nd ed., Elsevier, Amsterdam, 2007. b) N. Petragnani, H. A.
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c
a 96:4 mixture of 1,4-isomers and 1,2-isomers. 2,6-Di-tert-
butyl-4-methylpyridine (1.5 equiv) was added to avoid decom-
position of 5. dDialkylated product formed in 14% yield.
e10 equiv of 3 was used. fPolystyrene with Mn ¼ 3400,
Mw=Mn ¼ 1:14. See text for details for the efficiency of the
end functionalization.
2
with 7e (Entry 8).5a Thus, 9e was treated with PhTeOTf in the
presence of 3 (10 equiv). The gel permeation chromatography
of the purified polymer 9g indicated the formation of a monodis-
perse polystyrene with a number-averaged molecular weight
(Mn) of 3440 and a molecular weight distribution (Mw=Mn,
where Mw is the weight-averaged molecular weight) of 1.14.
The 1H NMR analysis revealed the nearly quantitative efficiency
(97%) of the end-group transformation. MALDI-TOF mass
spectrometry also indicated the predominant formation of 9g
together with a small amount of 10 as judged from a series of
molecular ion peaks (Figure 1, 9g:10 = 93:7). The formation
of 10 is also consistent with the generation of carbocationic
species at the polymer-end.
3
4
a) S. Yamago, Synlett 2004, 1875, and references therein. b)
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M. Okubo, Macromolecules 2007, 40, 9208. c) S. Yusa,
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Morishima, Macromolecules 2007, 40, 5907. d) E. Kayahara,
S. Yamago, Y. Kwak, A. Goto, T. Fukuda, Macromolecules
2008, 41, 527.
5
6
a) S. Yamago, K. Iida, J. Yoshida, J. Am. Chem. Soc. 2002,
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2775.31 (n = 23)
2671.25 (n = 22)
2567.20 (n = 21)
2879.36 (n = 24)
2983.42 (n = 25)
3087.95 (n = 26)
Major compound
Ph Ph
OMe
Intensity
3191.98 (n = 27)
n
3092.68 (n = 28)
2463.16 (n = 20)
3400.37 (n = 29)
3504.47 (n = 30)
3608.60 (n = 31)
MeO
OMe
9g
2359.12 (n = 19)
2255.08 (n = 18)
Minor compound
Ph Ph
3712.75 (n = 32)
3816.92 (n = 33)
3921.06 (n = 34)
4025.19 (n = 35)
Ph
n-1
10
7
8
9
2000
3000
4000
5000
6000
10 F. Effenberger, W. Russ, Chem. Ber. 1982, 115, 3719.
11 The marginal difference of the chemical shift might be due to
the interaction of PhTeOTf with AgX (X = Br or Cl).
12 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/.
(m/z)
Figure 1. MALDI-TOF mass spectrum of polystyrene 9g and
10 (=93:7). A series of peaks, as indicated by the average mass
number, was observed in the silver ion added form (M + Ag)þ.
Published on the web (Advance View) May 17, 2008; doi:10.1246/cl.2008.650