Methyl ester function: An intramolecular electrophilic trap for the isolation of aryltellurenyl hydroxide and diorganotellurium dihydroxide

Article abstract of DOI:10.1021/om2001553

The formation of an aryltellurenyl hydroxide analogue proposed in the telluroxide elimination reaction has been studied. The reaction of the monotelluride dimethyl 5-tert-butyl-2-(butyltellanyl)isophthalate (6) with H₂O₂ afforded the corresponding aryldiacyloxy-n- butyltellurane (8). The reaction of 6 with oxygen in the presence of moisture resulted in the formation of 8 as the major product along with traces of dimethyl 5-tert-butyl-2-(butyltellurinyl)isophthalate (7) and methyl 5-tert-butyl-3-oxo-3H-benzo[c][1,2]oxa tellurole-7-carboxylate (9). The telluroxide intermediate 7 readily undergoes a facile elimination reaction to give the elusive aryltellurenyl hydroxide, which is immediately trapped by the adjacent methyl ester functionality to give the cyclic tellurenate ester 9. Similarly, the hydrolysis of 7 with H₂O affords the corresponding dihydroxytellurane 10, which is immediately converted to tellurane 8. Both compounds 8 and 9 have been characterized by single-crystal X-ray crystallographic studies.

Full text of DOI:10.1021/om2001553

NOTE
pubs.acs.org/Organometallics
Methyl Ester Function: An Intramolecular Electrophilic
Trap for the Isolation of Aryltellurenyl Hydroxide and
Diorganotellurium Dihydroxide
K. Selvakumar,† Harkesh B. Singh,*,† Nidhi Goel,‡ and Udai P. Singh‡
†Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India
‡Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247667, India
S Supporting Information
b
ABSTRACT: The formation of an aryltellurenyl hydroxide
analogue proposed in the telluroxide elimination reaction has
been studied. The reaction of the monotelluride dimethyl 5-tert-
butyl-2-(butyltellanyl)isophthalate (6) with H2O2 afforded the
corresponding aryldiacyloxy-n-butyltellurane (8). The reaction
of 6 with oxygen in the presence of moisture resulted in the
formation of 8 as the major product along with traces of
dimethyl 5-tert-butyl-2-(butyltellurinyl)isophthalate (7) and
methyl 5-tert-butyl-3-oxo-3H-benzo[c][1,2]oxa tellurole-7-car-
boxylate (9). The telluroxide intermediate 7 readily undergoes a
facile elimination reaction to give the elusive aryltellurenyl
hydroxide, which is immediately trapped by the adjacent methyl
ester functionality to give the cyclic tellurenate ester 9. Similarly,
the hydrolysis of 7 with H2O affords the corresponding dihydroxytellurane 10, which is immediately converted to tellurane 8. Both
compounds 8 and 9 have been characterized by single-crystal X-ray crystallographic studies.
’ INTRODUCTION
proposed on the basis of the observation of the disproportionation
product, diaryl ditelluride.11
Organotellurium reagents have received considerable interest in
organic transformations due to their facile incorporation into organic
molecules via both electrophilic or nucleophilic pathways and further
functional group manipulation.1 The important organic transforma-
tions include halogenation of olefins,2 oxidation of alcohols,3 and
dehydration of nucleosides to 20,30-didehydro-20,30-dideoxynucleo-
sides.4 They have also attracted wide interest in biology due to their
interesting redox properties.5 Organotellurium compounds such as
ditellurides and monotellurides have been used as glutathione per-
oxidase (GPx) mimics.6,7 Glutathione peroxidase is a selenoenzyme
that catalyzes the reduction of hyroperoxides by using glutathione
(GSH). The diorganyl telluroxide 6,60-telluroxybis(6-deoxy-β-cyclo-
dextrin) (6-TeOdiCD), obtained from 6-TediCD, has been found to
show hydrolase mimetic activity.8 Therefore, a detailed understanding
of the mechanism of tellurium-centered redox reactions would lead to
the design of potential reagents and redox catalysts.
Uemura and Fukuzawa described the synthesis of some olefins,
allylic alcohols, and allylic ethers via facile elimination reaction of
sec-alkyl phenyl telluroxide.12 They proposed that CꢀTe bond
fission could probably occur via an intermediate state, 2.
The elimination reaction of terminal β-oxy selenoxides is common-
place in olefin synthesis.9 This reaction occurs in a syn stereospecific
manner to give the corresponding selenenic acid and olefin. Similarly,
β-elimination reaction of unsymmetrical diorganyl telluroxides having
sec-alkyl and aryl groups has been considered to be an important step
in olefin synthesis1 and bioactivation of Te-phenyl-L-tellurocysteine.10
Lee and Cava, for the first time, studied the nature of the telluroxide
elimination reaction using dodecyl-(4-methoxyphenyl) telluroxide.11
The formation of aryltellurenyl hydroxide 1 as an intermediate was
To our knowledge, there has been no report on the isolation and
characterization of the aryltellurenyl hydroxide intermediates
formed during the β-elimination reaction of diorganyltelluroxides.
In a different context, McWinnie and co-workers, for the first time,
Received: February 18, 2011
Published: June 22, 2011
r
2011 American Chemical Society
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|

Organometallics
NOTE
Scheme 1. Bertozzi’s Version of the Staudinger Reaction
’ RESULTS AND DISCUSSION
Synthesis. The SNAr reaction of aryl bromide 5 with n-BuTeLi
afforded the monotelluride 6 (Scheme 2). Attempts to purify 6 were
unsuccessful due to its facile oxidation, especially under ambient
conditions. This compound is reasonably stable under nitrogen in a
Schlenk flask at ꢀ22 °C. The oxidation of 6(leads to the formation of
7 and 8, vide infra) in open air is clearly evident from its 1H NMR
spectrum. The three expected singlets at 7.74, 3.91, and 1.30 ppm (see
Supporting Information, Figure S2) were broadened. The aerial
oxidation of telluride 6 is not surprising, and similar aerial oxidations
of diaryltelluride have been known in the literature.19 The oxidation
process was also revealed from the mass spectral studies. The oxidized
species [7 + H]+ (m/z 453.09) and its corresponding tellurone
[tellurone Te(VI) + H]+ (m/z 469.09) have been observed in the
HRMS(ES) spectrum of 6 ([6 + H]+, m/z 437.10). The relative
abundance of the m/z corresponding to the tellurone is ∼35%. The
m/z peak for 7 was observed as a base peak with 100% relative
abundance. However, the formation of the tellurone was not observed
under ambient conditions. Generally, tellurones are prepared under
relatively stronger oxidizing conditions using NaIO4 as an oxidizing
agent in place of peroxides/molecular oxygen20 or photochemical
conditions using singlet oxygen as an oxidizing source.21
isolated and characterized an acyclic tellurenate ester, 2-(phenylazo)-
phenyl-C,N0)tellurium acetate, using intramolecular Te
N
3 3 3
interaction.13 It was obtained by the metathesis of the corresponding
aryltellurenyl chloride with sodium acetate. This tellurenate ester
could be considered as a protected form of the aryltellurenyl
hydroxide 2-(phenylazo)phenyl-C,N0)tellurium hydroxide. Minkin
and co-workers have also reported the synthesis of such stable
tellurenate esters and aryltellurenyl hydroxide anhydrides using
intramolecular secondary bonding Te N interaction.14 Very
3 3 3
recently, our group has succeeded in the isolation and characteriza-
The reaction of monotelluride 6 with H2O2 did not afford the
expected telluroxide 7 (vide infra). Instead, it resulted in the
formation of the hypervalent diorganyldiacyloxytellurane 8
(85%). Tellurane 8 was characterized by routine spectroscopic
techniques (1H, 13C, and 125Te NMR).
tion of an anhydride, bis[(2-(phenylazo)phenyl-C,N0)tellurium]
oxide, which is also stabilized by Te N secondary bonding
3 3 3
interaction.15 This results from the hydrolysis of 2-(phenylazo)-
phenyl-C,N0)tellurium chloride followed by the condensation of the
resultant (2-(phenylazo)phenyl-C,N0)tellurenyl hydroxide.
Monotelluride 6, when kept for one week in open air, afforded 8 as
the major product along with a trace amount of the telluroxide 7 and
the tellurenate ester 9 (7 and 9 were identified by TLC) (Scheme 3).
The reaction of 6 with oxygen at 80 °C in the absence of moisture
allowed the isolation and purification of the telluroxide 7 and the
tellurenate ester 9 by column chromatography. At 110 °C, the
reaction afforded the cyclic tellurenate ester 11 in addition to 9.
Compound 11 could probably result from the telluroxide 7 via the
intermediate 10. The 125Te NMR spectrum of compounds 7 and 9
showed peaks at 1207 and 2280 ppm, respectively. The chemical shift
of 7 is slightly shifted upfield as compared to that of the related
telluroxide bis[2-(phenylazo)phenyl-C,N0]tellurium(IV) oxide
(1228 ppm).15 However, other spectral data for 7 could not be
obtained due to its strong propensity to form 8in open air. Isolation of
7 is not reproducible all the time. Available literature data suggest that
the aryltellroxides with reactive functional groups such as amide and
carboxylic acid around the tellurium center are unstable and undergo
rapid hydrolysis followed by intramolecular cyclization to afford the
corresponding telluranes.19,22 Compound 11 was characterized by
1H, 13C, and 125Te NMR and IR spectroscopic techniques. In the
125Te NMR spectrum, the resonance signal was observed at 2263
ppm, which is in agreement with the chemical shift observed for the
tellurenate ester 9 (2280 ppm). The molecular ion ([M + H]+, m/z
407.05) peak of 11 was found as a base peak in its HRMS spectrum.
This observation leads us to propose the following mechanism
(Scheme 4) for the formation of 8 and 9. Unlike sulfoxides and
selenoxides, the telluroxides formed during oxidation are seldom
isolated.21 They exist usually in the hydrated form.23 Hence, tell-
uroxide 7 formed during oxidation has the capacity to associate with
water molecules,24 leading to the formation of diorganotellurium
dihydroxide 12. Tellurane 12 undergoes an intramolecular cycliza-
tion to form the diorganyldiacyloxytellurane 8. This clearly indicates
that path A is more favored than path B in the presence of moisture.
Strict exclusion of moisture diverts the reaction into a different path
Bertozzi’s versions of the Staudinger reaction illustrate the
intramolecular aza-ylide transfer as an effective tool for the amide
bond formation reaction in aqueous environment (Scheme 1).16
In the presence of water, the aza-ylide undergoes facile hydrolysis
to form an amine and the phosphine oxide, which is known as the
Staudinger reaction. Bertozzi and co-workers used a suitably
placed methyl ester functionality as an electrophilic trap to
capture the aza-ylide to form an amide bond.
We envisaged that an appropriately positioned electrophilic trap,
such as a methyl ester function, around the tellurium center would
capture the nucleophilic aryltellurenyl hydroxide by an intramole-
cular cyclization. At the same time, the unstable cyclic tellurenate
ester would be stabilized by an additional functional group that is
capable of providing a Te O secondary bonding interaction.
3 3 3
Recently, we have succeeded in the isolation of unstable selenenic
acid as cyclic selenenate esters 317 and 418 using the formyl and ester
groups respectively as an electrophilic trap and an intramolecular
donor group. These compounds (3 and 4) are highly stable under
ambient conditions due to the presence of strong secondary
bonding Se O intramolecular interactions. This prompted us
3 3 3
to test this hypothesis to isolate the unstable organotellurium
analogues, such as aryltellurenyl hydroxide and diorganotellurium
dihydroxide. Herein, we describe the isolation and characterization
of an aryltellurenyl hydroxide, the presumed intermediate in the
telluroxide elimination reaction, as cyclic tellurenate ester. In
addition, the trapping of the diorganotellurium dihydroxide as a
hypervalent diorganyldiacyloxytellurane is also described.
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Organometallics
NOTE
Scheme 2. Synthesis of 8a
a Reagents and conditions: (i) n-BuTeLi, THF, rt, 3 h; (ii) aqueous H2O2, MeOH, 5 h.
Scheme 3. Formation of 9 in Open Air and Aerobic Conditionsa
a Reagents and conditions: (i) open air, rt, 1 week; (ii) toluene, O2, 80 °C, 24 h; (iii) toluene, O2, 110 °C, 24 h.
(path B), in which the telluroxide undergoes β-elimination to form
aryltellurenyl hydroxide 13, which in turn cyclizes to form the low-
valent tellurenate ester 9. The formation of 9 under moisture-free
conditions suggests that the CꢀTe bond fission does not require a
diorganotellurium dihydroxide intermediate such as that proposed
for the β-elimination reaction (2).12 Although β-elimination is one
of the reaction paths (path B) for the formation of 9 from 7, the
possibility of 1,2-shift of the n-butyl group from tellurium to oxygen
cannot be ignored (path C). This reaction would lead to the
formation of 10. It is speculated that the n-butanol liberated from
10 upon its exposure to moisture may produce 13, which undergoes
facile intramolecular cyclization to form 9. This corroborates the
observation of Uemura et al., who have shown the formation of
small amounts of 2-phenylethanol from phenylethyl phenyl telluride
upon its oxidation with m-chloroperbenzoic acid.25 Compound 9
probably undergoes tellurium-assisted transesterification reaction
with n-butanol to form 11. At present we do not know the detailed
mechanism of this transestrification process.
The butene formed from 7 could not be detected due to its low
boiling point and its high volatility. To figure out the formation of
olefin, the mononotelluride 14 was synthesized (Scheme 5). The
reaction of sodium 2-phenylethanetellurolate with 5 afforded 14.
Aerobic oxidation of 14 under anhydrous conditions at 80 °C led
to the formation of 9 and styrene. Styrene could be detected in the
GC-MS (m/z 104.0, M+; see Supporting Information, p S38).
Compound 14 was characterized by routine spectroscopic tech-
niques. HRMS studies on 14 are of particular interest. All the reac-
tion products, styrene ([M + H]+, m/z 105.07), cyclic tellurenate
ester 9 ([M + H]+, m/z 365.10), and hypervalent tellurane 15
([M + H]+, m/z 455.06), and the telluroxide 16 ([M + H]+, m/z
499.11) intermediate could be identified in the HRMS spectrum.
X-Ray Crystallographic Study. Molecular Structure of 8. The
structureofcompound8 was determined byX-raycrystallographic
analysis (Figure 1). It crystallizes in a monoclinic crystal system.
The n-Bu and t-Bu groups are disordered due to the availability of
the rotational degree of freedom around the CꢀC bonds.
The molecular structure of tellurane 8 is similar to that of the
spirotellurane 1719 and has a pseudotrigonal-bipyramidal geo-
metry about the central tellurium atom with the more electro-
negative acyloxy ligands in the axial positions and the two aryl
carbons and the lone electron pair in the equatorial plane.
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Organometallics
NOTE
Scheme 4. Plausible Mechanism for the Formation of 8, 9, and 11
Scheme 5. Synthesis of 14 and Its Aerobic Oxidationa
a Reagents and conditions: (i) PhCH2CH2TeꢀNa+, THF, rt, 6 h;
(ii) toluene, O2, 80 °C, 24 h.
Figure 2. Molecular structure of 9. Thermal ellipsoid are drawn at 50%
probability. Selected bond lengths (Å) and angles (deg): Te(1)ꢀC(2)
2.045(4), Te(1)ꢀO(2) 2.094(5), Te(1)ꢀO(1) 2.504(4), C(2)ꢀTe-
(1)ꢀO(2) 78.64(16), C(2)ꢀTe(1)ꢀO(1) 71.17(16), O(2)ꢀTe(1)ꢀ
O(1) 149.81(12).
The Te(1)ꢀO(1) and Te(1)ꢀO(3) bond lengths are 2.133(2)
and 2.173(2) Å, respectively. These distances are slightly longer than
the sum of the tellurium and oxygen covalent radii (2.04 Å)26 and
indicate the hypervalent nature of the TeꢀO axial bonds. The nature
of the carboxylate group 8 was studied by FT-IR spectroscopy. The
stretching frequencies for 8 were observed at 1664 (νas (CꢀO))
and 1330 (νs (CꢀO)) cmꢀ1 and could be assigned as the absor-
ptions resulting from the CꢀO bonds in the carboxylate groups.
The difference between νas and νs is 334 cmꢀ1. It is in agreement
with the IR stretching frequencies (1640 (νas (CꢀO)) and 1298
Figure 1. Molecular structure of 8. Thermal ellipsoids are drawn at 50%
ꢀ
probability. Selected bond lengths (Å) and angles (deg): TeꢀC(1) 2.047(2),
TeꢀC(13B) 2.059(10), TeꢀO(1) 2.133(2), TeꢀO(3) 2.173(2), Teꢀ
C(13A) 2.204(9), C(1)ꢀTeꢀC(13B) 94.3(2), C(1)ꢀTeꢀO(1)
77.05(8), C(13B)ꢀTeꢀO(1) 91.4(3), C(1)ꢀTeꢀO(3) 76.40(8),
C(13B)ꢀTeꢀO(3) 85.7(3), O(1)ꢀTeꢀO(3) 153.00(6), C(1)ꢀTeꢀ
C(13A) 97.3(2), C(13B) ꢀTeꢀC(13A) 8.8(4), O(1)ꢀTeꢀC(13A)
84.1(2), O(3)ꢀTeꢀC(13A) 94.4(2).
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Organometallics
NOTE
(νs (CꢀO)) cmꢀ1) reported for 2-(phenylazo)phenyl-C,N0)-
tellurium acetate, (C12H9N2)Te(O2CMe), which has a differ-
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ence of 342 cm .
ꢀ1 13 The antisymmetric and symmetric stretches
for the related triaryltelluronium carboxylate 18 were observed at
1564 (νas (CꢀO)) and 1379 (νs (CꢀO)) cmꢀ1, respectively,
with a deference of 185 cmꢀ1 27
They indicate that acyloxy
.
groups in 8 and 2-(phenylazo)phenyl-C,N0)tellurium acetate
have an ester character and the acyloxy group in 18 has a
carboxylate character.28 The O(1)ꢀTeꢀO(3) bond angle
(153.00(6)°) in compound 8 is considerably compressed in
comparison to that in spirotellurane 17, in which it was found to
be 161.3°.19 The presence of a compressed O(1)ꢀTeꢀO(3)
angle in 8 is presumably due to the geometrical constraints.
Molecular Structure of 9. The molecular structure of cyclic
tellurenate ester 9 was also confirmed by single-crystal X-ray
crystallography (Figure 2). It crystallizes in an orthorhombic
crystal system. The molecular geometry around the tellurium
atom is distorted T-shaped. The distance between Te(1) and
ꢀ
O(1) is 2.094(5) Å. The presence of a T(1) O(1) interaction
3 3 3
affects the stretching frequency of the interacting carbonyl group.
The IR bond stretching frequency for the carbonyl group
coordinated to Te was observed at 1624 cmꢀ1, which is lower
than the corresponding stretching frequency observed for the
selenenate ester 4 (1642 cmꢀ1).18 This indicates that the Te
O
3 3 3
(9) interaction is stronger than the Se O interaction (4).
3 3 3
’ CONCLUSIONS
In conclusion, the internal protecting group (COOMe) serves as
a good electrophilic trap for the isolation of unstable aryltellurenyl
hydroxide and diorganotellurium dihydroxide in their protected
form. Tellurenate ester 9 was stabilized by strong intramolecular
coordination. Unlike the β-elimination reaction, the formation of 9
does not require diorganotellurium dihydroxide 12 as the inter-
mediate. This study clearly demonstrates that the TeꢀC bond
fission can occur via both β-elimination and 1,2-shift reaction.
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental Section, X-ray
b
crystallographic data for 8 and 9 and the spectroscopic character-
ization data (1H, 13C, 125Te NMR and MS) for 6, 8, 9, 11, and 14.
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: chhbsia@chem.iitb.ac.in.
(21) Oba, M.; Okada, Y.; Endo, M.; Tanaka, K.; Nishiyama, K.;
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’ ACKNOWLEDGMENT
H.B.S. gratefully acknowledges the Department of Science and
Technology, New Delhi, for the Ramanna Fellowship. K.S. is
thankful to CSIR, New Delhi, for SRF and SAIF, IITB, for
spectral data analysis.
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