The Reaction of Acetic Acid 2-Selenoxo-2H-pyridin-1-yl Esters with Benzynes: A Convenient Route to Benzo[b]seleno[2,3-b]pyridines

Article abstract of DOI:10.1002/jhet.5570410103

Benzyne and its 3,4,5,6-tetraphenyl, 3- and 4-methyl, 3-methoxy and 4,5-difluoro derivatives react with acetic acid 2-selenoxo-2H-pyridin-1-yl esters 4a-e to give benzo[b]seleno[2,3-b]pyridines 10-15 in modest yields. The benzynes were generated by one or more of the following methods: diazotization of anthranilic acids 5a-g with isoamyl nitrate; mild thermal decomposition of 2-diazoniobenzenecarboxylate hydrochlorides 6a-d; treatment of (phenyl)[o-(trimethylsilyl)phenyl]iodonium triflate (7) with tetrabutylammonium fluoride; and treatment of 2-trimethylsilylphenyl triflates 8a-c with cesium fluoride. In all the reactions, the corresponding 2-(methylselenenyl)pyridines 16a-d were also obtained suggesting that these reactions may involve selenium addition to benzyne via a SET (single electron transfer).

Full text of DOI:10.1002/jhet.5570410103

Jan-Feb 2004
13
The Reaction of Acetic Acid 2-Selenoxo-2H-pyridin-1-yl Esters with
Benzynes: A Convenient Route to Benzo[b]seleno[2,3-b]pyridines
U. Narasimha Rao, Ramadas Sathunuru, John. A. Maguire and Ed Biehl*
Department of Chemistry, Southern Methodist University, Dallas, TX 75275
Ebiehl@mail.smu.edu
Received February 18, 2003
Received Revised August 8, 2003
Benzyne and its 3,4,5,6-tetraphenyl, 3- and 4-methyl, 3-methoxy and 4,5-difluoro derivatives react with
acetic acid 2-selenoxo-2H-pyridin-1-yl esters 4a-e to give benzo[b]seleno[2,3-b]pyridines 10-15 in modest
yields. The benzynes were generated by one or more of the following methods: diazotization of anthranilic
acids 5a-g with isoamyl nitrate; mild thermal decomposition of 2-diazoniobenzenecarboxylate hydrochlo-
rides 6a-d; treatment of (phenyl)[o-(trimethylsilyl)phenyl]iodonium triflate (7) with tetrabutylammonium
fluoride; and treatment of 2-trimethylsilylphenyl triflates 8a-c with cesium fluoride. In all the reactions, the
corresponding 2-(methylselenenyl)pyridines 16a-d were also obtained suggesting that these reactions may
involve selenium addition to benzyne via a SET (single electron transfer).
J. Heterocyclic Chem., 41, 13 (2004).
This benzyne synthesis and the recently reported novel
ring transformation of dihydroselenines to selenabicy-
clo[3.1.0]hexenes [3] are the only significant examples in
the literature involving the addition of an organoselenium
reagent to benzyne. Additionally, the only reported synthe-
sis of benzoseleno[2,3-b]pyridines involves a multi-step
reaction that requires starting materials that are not readily
available [4]. These results prompted us to investigate the
possibility of synthesizing benzo[b]seleno[2,3-b]pyridines
by the reaction of acetic acid 2-selenoxo-2H-pyridin-1-yl
esters (hereafter referred to as Barton 2-selenoxo esters)
with benzynes. Such a study should allow ready access to
these relatively under-investigated selenium heterocycles
and could provide additional information on the nature of
selenium addition to benzynes.
Introduction.
We [1] have recently reported the synthesis of
benzo[4,5]thieno[2,3-b]pyridines and naphtho[2',3':4,5]-
thieno[2,3- b]pyridines by the reaction of Barton esters
(acetic acid 2-thioxo-2H-pyridin-1-yl ester) with benzyne,
3,4,5,6-tetraphenylbenzyne, 2,3-dehydronaphthalene, and
3-methylbenzyne. The benzyne intermediates were pre-
pared by diazotization of the appropriate anthranilic acid
with isoamyl nitrite in refluxing dichloromethane (40°) in
the presence of the Barton ester . The first three aforemen-
tioned symmetric benzynes gave a single benzyne product,
while the unsymmetric 4-methylbenzyne gave a 1:1 iso-
meric mixture of 7- and 8-methyl regioisomers. Attempts
to generate 3,4-difluoro- or 3,4-dimethoxybenzyne under
similar conditions failed. Although no direct evidence for
free-radical addition of Barton esters to benzyne was
found, such an addition appears likely based on the well-
known nucleophilicity of sulfur. These esters could react
with benzynes by SET (single electron transfer) to give a
radical intermediate, which could subsequently undergo
intramolecular cyclization to give the product via a tri-
cyclic ring radical.
Results and Discussion
As shown in Scheme 1, the Barton 2-selenoxo esters 4a-
e were prepared from the corresponding 2-chloropyridines
1a-e by a previously described method [5]. The percentage
yields of the pyridine N-oxide 2 a - e , N- h y d r o x y - 2 -
selenones 3a-e, and Barton 2-selenoxo esters 4a-e are
listed in Table 1. Unfortunately, the nitro ester 4e decom-
posed rapidly during the esterification of 3e, thus negating
its use in this study. In actual practice, the esters 4a-d were
prepared by esterification of the N-hydroxy-2-selopyri-
dones 3a-d just prior to use.
Okuma and coworkers [2] recently showed that the reac-
tion of benzyne with thiones, which gives 2H- b e n z o-
thietes, could be extended to the preparation of 2H-ben-
zoselenates by the reaction of benzyne with 2-selones.

14
U. N. Rao, R. Sathunuru, J. A. Maguire and E. Biehl
Vol. 41
Table 1
2-diazoniobenzenecarboxylate hydrochlorides 6 [7],
(phenyl)[o-(trimethylsilyl)phenyl]iodonium triflates 7
[8], and 2-(trimethylsilyl)phenyl triflate (8) [9]. The
structures of the individual members in each group and
the benzynes 9 generated from these precursors are
shown in Figures 1 and 2 respectively.
As shown in Scheme 2, the reaction of benzynes 9, gener-
ated from the precursors 5-8, with Barton 2-selenoxo esters 4
gave benzo[b]seleno[2, 3-b]p yridines 10-15 along with 2-
(methylselanyl)pyridines 16 as side products. The structures
of 10-15 and 16 are shown in Figures 3 and 4, respectively.
The yields of the desired seleno[2,3-b]pyridines 1 0-1 5,
which are shown in Table 2, depend upon the nature of the
benzyne precursors and the conditions used in generating the
benzyne intermediates. Thus, the reaction ofbenzyne (9a)
generated from 1 equivalent of the benzyne precursors 5a,
Yields of Compounds 2a-e-4a-e
2
3
4
a
b
c
d
e
97
96
94
99
92
90
95
81
98
74
98
82
85
79
0
Since Barton 2-selenoxo esters are thermally labile and
base sensitive, we sought readily available benzyne pre-
cursors that would be capable of generating benzynes
under mild, aprotic, and non-basic conditions. Four types
of benzyne precursor met these conditions and were
selected for study. They include: anthranilic acids 5 [6],

Jan-Feb 2004
The Reaction of Acetic Acid 2-Selenoxo-2H-pyridin-1-yl Esters with Benzynes
15
On the other hand, the reaction of 7 carried out at room tem-
perature using 5 equivalents of 4a-d per equivalent of 7 gave
10a-d in increased yields of 42-49%. Attempts to improve
the yield of compounds 10a-d by performing the reaction of
7 with 4a-d in refluxing dichloromethane afforded complex
mixtures of selenium decomposition products.
The proposed structures of 10a-d are consistent with
their 1H and 13C nmr spectra. The structure of the 4-methyl
derivative 14b was further verified by single crystal X-ray
crystallographic analysis.
6a, 7 and 8a in the presence of 1.1 equivalents of the Barton
2-selenoxo esters 4a-d (entries 5-8) gave the corresponding
benzo[b]seleno[2,3-b]pyridines 1 0 a - d. Compounds 10a-d
were obtained in highest yields (51-67%) when benzyne (9a)
was generated from 2-diazoniobenzenecarboxylate
hydrochloride salts 6a in tetrahydrofuran at 45°. Slightly
lower yields of 10a-d were obtained when 9a was generated
from anthranilic acid (5a) and isoamyl nitrite in refluxing
dichloromethane (entries 1-4, 41-51%) and from the reac-
tion of 8a with cesium fluoride (entries 13-16, 43-54%) in
acetonitrile. The products 10a-d were obtained in low yields
(20-35%) from the reaction of the Barton 2-selenoxo esters
4 a - d with benzyne (9 a ) generated from (phenyl)-
[o-(trimethylsilyl)phenyl]iodonium triflate (7) with tetra-
butylammonium fluoride at room temperature (entries 9-12).
The reactions of substituted benzynes 9b-9g with the
Barton 2-selenoxo esters 4a-d were next studied. The ben-
zynes were initially generated by diazotization of the com-
mercially or readily available anthranilic acids 5b-f. Of
these, anthranilic acids 5b-d formed benzyne (9 a), 3,4,5,6-
tetraphenylbenzyne (9 b), 4-methyl- (9 c), and 3-methylben-
zyne (9 d), which reacted with Barton 2-selenoxo esters 4 a -
d to give the desired products. For example, the tetraphenyl
compounds 11a-d were formed in yields ranging from 20-
29% (entries 19-22). The low yields probably reflect a
steric effect of the phenyl groups. The reaction of the
unsymmetrical benzynes 9c and 9d with 4-methyl Barton
2-selenoxo ester 4b gave a 1:1 inseparable mixture of

16
U. N. Rao, R. Sathunuru, J. A. Maguire and E. Biehl
Vol. 41
Table 2
Reaction of Benzynes 9a-g with Barton 2-selenoxo Esters 4a-d
Entry
No.
Benzyne
Precursor
Benzyne
Method
Ester
4
Product
10-15
Yield,
% [e]
Product
16
Yield, %
1
2
3
4
5
6
7
8
9
5a
5a
5a
5a
6a
6a
6a
6a
7
9a
9a
9a
9a
9a
9a
9a
9a
9a
A [a]
A
A
A
B [b]
B
B
B
C [c]
a
b
c
d
a
b
c
10a
10b
10c
10d
10a
10b
10c
10d
10a
51
46
45
41
51
62
67
64
30 [f]
(45)[g]
35 [f]
(49)[g]
20 [f]
(42)[g]
26 [f]
(47)[g]
54
48
52
43
<2%
<2%
29
24
20
16a
16b
16c
16d
16a
16b
16c
16d
16a
20
25
29
28
32
26
35
24
16 [f]
(38 )[g]
18 [f]
(39)[g]
15 [f]
(35)[g]
15 [f]
(33)[g]
19
20
17
14
_
_
30
33
36
36
31
27
29
21
28
22
15
13
16
18
d
a
10
11
12
7
7
7
9a
9a
9a
C
C
C
b
c
10b
10c
10d
16b
16c
16d
d
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
8a
8a
8a
8a
5d
5e
5b
5b
5b
5b
5e
5f
6d
6d
6d
6d
8b
8b
8b
8b
9a
9a
9a
9a
9f
9g
9b
9b
9b
9b
9c
9d
9e
9e
9e
9e
9f
D [d]
D
D
D
C
a
b
c
d
a
b
a
b
c
d
b
b
a
b
c
d
a
b
c
10a
10b
10c
10d
14a
15
11a
11b
11c
11d
12a,b
12c,d
13a
13b
13c
13d
14a
14b
14c
14d
16a
16b
15c
16d
16a
16a
16a
16b
16c
16d
16b
16b
16a
16b
16c
16d
16a
16b
16c
16d
C
A
A
A
A
A
A
B
B
B
B
22
26,26
16,32
56
49
64
56
56
49
64
D
D
D
D
9f
9f
9f
d
56
[a] Method A; isoamyl nitrite/dichloromethane/40°; [b] Method B; tetrahydrofuran/45°; [c] Method C; tetrabutylam-
monium fluoride/dichloromethane/room temperature; [d] Method D; cesium fluoride/ acetonitrile/room temperature;
[e] Unless indicated otherwise, 1 equivalent of 1 and 1.1 equivalents of 4 were used; [f] refluxing dichloromethane
resulted in decomposition of 1 and 4 as indicated by gcms; [g] one equivalent of 1 and 5 equivalents of 4 were used.
4,6-dimethyl 12a and 4,7-dimethyl products 1 2 b (entry 23)
and a 1:2 mixture of the 5-methyl- 1 2 c and 8-methyl deriv-
atives 1 2 d (entry 24), respectively. The product ratios were
obtained by gcms analysis and integration of the methyl
ester 4a gave complex reaction mixtures (entries 17 and
18) in which the desired products 14a and 15 were present
in <2% yields as determined by gcms analysis of the reac-
tion mixtures. Similar treatment of 3-methoxyanthranilic
acid (5 e ) and Barton 2-selenoxo ester 4 a gave no
detectible amount of 13a. That the benzynes 9e-g are not
readily formed in these reactions could be a consequence
of an alternate decomposition mode [10]. In such cases,
Wege [10] has shown that this pathway can be avoided by
using diazonium carboxylate hydrochloride salts.
Subsequently we were able to generate 3-methoxybenzyne
(9e) from the hydrochloride salt 6d to which the Barton
2-selenoxo esters 4a-d added regioselectively to give
7-methoxybenzo[b]seleno[2, 3-b]pyridines 1 3 a - d in modest
yields (entries 29-32, 49-64%). An analytically pure sam-
ple of 5-methoxy-2-methylbenzo[b]seleno[2, 3-b]pyridine
1
signals in H nmr spectrum of the reaction mixtures. The
lack of selectivity in the ester addition to 4-benzyne 9d is
expected since the 4-methyl group is located at a remote
location relative to the "triple bond" where its steric and
electronic effects are weak. On the other hand, some
degree of regioselectivity was obtained in the reaction of
3-methylbenzyne (9d) with 4b since the weakly electron-
releasing 3-methyl group is located at a site adjacent to the
"triple bond". A similar products distribution for addition
to 3-methylbenzyne has been previously observed [12].
However, the reaction of the 3,4-difluoro- 5f and 3,4-
dimethoxyanthranilic acids 5g with the Barton 2-selenoxo

Jan-Feb 2004
The Reaction of Acetic Acid 2-Selenoxo-2H-pyridin-1-yl Esters with Benzynes
17
1
(1 3 d) could not be obtained. However, the H nmr spec-
trum of 13c was useful in assigning the position of the
5-methoxy group. For example, the chemical shift of the
4-H singlet in the 5-methoxy-3-methyl derivative 13c is
shifted upfield to δ 7.2 from the 4-H chemical shift in the
formation of 16, which was not considered by Barton, is
shown in Scheme 4 that involves a pericyclic rearrange-
ment of the Barton ester 4a with loss of carbon dioxide.
1
3-methyl derivative 1 0 c. Additionally, the H nmr of the
4-methyl derivative 13b showed that the 4-methyl group
resonance occurred at 1.3 ppm, which is upfield by 0.9 ppm
from the corresponding signal in 1 0 b. However, the major-
ity of the spectrum consisted of highly complex multiplets.
The methyl upfield shift observed in 1 3 c is probably due to
the electronic shielding provided by the lone pair of elec-
trons on the oxygen atom of the 5-methoxy group.
Attempts to extend this method to the synthesis of the
dimethoxy 15 and difluoro14a-d derivatives by heating the
respective benzyne precursors 6c and 6b in a variety of
refluxing solvents failed. Under these conditions, the
esters decomposed rapidly to yield complex mixtures of
selenium side products. 2-Trimethylsilylphenyl triflates
have recently been shown to generate a wide variety of
substituted benzynes including 4,5-difluorobenzyne (9 f)
[11]. We subsequently were able to prepare the difluoro
products 14a-d in 49-64% yields (entries 28-32) by treat-
ing the Barton 2-selenoxo esters 4a-d with 4,5-difluo-
robenzyne (9f), which was generated from the reaction of
cesium fluoride and 4,5-difluoro triflate 8b at room tem-
perature. To date, we have not been able to prepare 4,5-
dimethoxy triflate 8c.
We cannot offer evidence to challenge or support either
pathway. However, it would be difficult to account for the
formation of the benzo[b] s e l e n o [ 2 , 3 -b]py ridines 10-14 if
selenium free radicals were not involved. For example,
attempts to carryout reactions of benzynes 9a-c with
2-selenone 3a at room temperature failed. Thus the
N-acetyloxy group in 4a appears to be a necessary require-
ment for the benzyne reaction. A tentative explanation for
a possible mechanism of the synthesis of 10-14 is given in
Scheme 5 using the observed regioselective addition of the
Barton 2≠selenoxo ester 4a to 3-methoxybenzyne (9 e). As
shown, initial attack by the selenium atom onto 9e via SET
gives the radical adduct 17 that then cyclizes to the tricyclic
ring 1 8 which eventually is converted to product 1 3 a.
As previously mentioned 2-(methylselanyl)pyridines
16a-d were obtained as side products in these reactions.
There are at least two possible mechanisms for the forma-
tion of 1 6. Scheme 3shows a free-radical chain mechanism
based on that proposed by Barton [5] for the rearrangement
of the sulfur ester to 1 6. However, Barton was not able to
trap carbon radicals formed in the rearrangement as they
were able to do so with radicals derived from the Barton
sulfur esters. He thus concluded that the 2-selenoxo esters
must be considered either as better "traps" for carbon radi-
cals than the sulfur esters or as undergoing rearrangement
via a "tight" radical cage. An alternate mechanism for the
The isomer distributions shown in Table 2 are dictated by
both steric and electronic factors . One such factor may be
the inherent stabilities of the intermediate radicals formed
after selenium addition (17 in Scheme 5). Table 3 lists the
relative energies for the isomers in both the CH3C6H4• and
CH3OC6H4• radical systems derived from ab initio molecu-
lar orbital calculations. The calculations show that the ener-
gies of both the anisole and toluene radicals increase in the
order, o- < m- < p-, with the greatest energy differences

18
U. N. Rao, R. Sathunuru, J. A. Maguire and E. Biehl
Vol. 41
and 2-(trimethylsilyl)phenyl triflate (8 a) were obtained from
commercial sources. 5,6,7,8-Tetraphenylanthranilic acid (5b)
[13], 2-diazoniobenzenecarboxylate hydrochloride salts 6a-d [7],
4,5-difluoro- (8b) and 4,5-dimethoxy-(2-trimethylsilyl)phenyl
triflate (8c) [11] were prepared by literature procedures. All ben-
found in the anisole system. These results are consistent
with the data shown in Table 2. The increased stability of
the o-anisole over its m-isomer would favor the formation
of compounds 13a-d in the reaction of 9e with the Barton
esters. Since the methyl group of the anisole ether can be
rotated away form the ester, steric factors should not play a
significant role in determining the site of selenium attack,
and electronic factors should determine the isomer distrib-
ution. On the other hand, steric factors seem to play a
dominant role in the reactions with toluene radicals. Table
3 shows that there is very little difference between the
energies of the o- and m-radicals (~1.1 kJ/mol) or between
the m- and p-radical (~1.3 kJ/mol). Therefore the equal
molar ratio of products 12a and 12b in the reaction of the
ester 4b with 9c is not unexpected. On the other hand, the
zyne reactions were carried out under an atmosphere of dry O -
2
free N via balloon and under subdued lighting.
2
Preparation of Acetic Acid 2-Selenoxo-2 H-pyridin-1-yl Esters
4a-d.
These compounds were prepared by the Barton method [5].
Physical and spectral data are given below.
Acetic Acid 2-Selenoxo-2H-pyridin-1-yl Ester (4a).
1
This compound was obtained as a viscous liquid; H nmr (deu-
teriochloroform): δ 2.5 (s, 3 H), 6.87 (td, J = 6.9, 1.7 Hz, 1 H),
7.25 (td, J = 8.6, 1.4 Hz, 1 H), 7.77 (dd, J = 6.9, 1.4 Hz, 1 H), 8.08
13
(dd, J = 8.6, 1.7 Hz, 1 H); C nmr (deuteriochloroform): δ 18.7,
115.4, 132.7, 138.8, 141.7, 165.6, 173.5.
Table 3
Relative Energies Calculated for Arene Radicals [a,b]
Acetic Acid 4-Methyl-2-selenoxo-2H-pyridin-1-yl Ester (4b).
1
This compound was obtained as a viscous liquid; H nmr (deu-
Radical
∆E [c]
Radical
∆E [c]
teriochloroform): δ 2.19 (s, 3 H), 2.47 (s, 3 H), 6.67 (dd, J = 7.0,
2.1 Hz, 1 H), 7.62 (d, J = 7.0 Hz, 1 H), 7.92 (d, J = 2.1 Hz, 1 H);
C nmr (deuteriochloroform): δ 18.4, 20.4, 117.5, 137.5, 140.0,
145.6, 165.5, 171.4.
o-CH OC H •
6 4
0
4.6
6.2
o-CH C H •
0
1.1
2.4
3
3 6 4
m-CH OC H •
m-CH C H •
3
6 4
3 6 4
13
p-CH OC H •
p-CH C H •
3 6 4
3
6 4
[a] Geometries optimized and energies calculated at the UHF/6-31G**
level of theory using the SPARTAN series of programs; [b] SPARTAN
version 5.0, © 1997 Wavefunction, Inc.; 18401 Von Karman Ave., #370;
Irvine, CA 92715; [c] ∆E (kJ/mol) values are relative to the ortho-radical.
Acetic Acid 5-Methyl-2-selenoxo-2H-pyridin-1-yl Ester (4c).
1
This compound was obtained as a viscous liquid; H nmr (deu-
teriochloroform): δ 2.21 (s, 3 H), 2.49 (s, 3 H), 7.09 (d, J = 8.6
13
Hz, 1 H), 7.58 (s, 1 H), 7.95 (d, J = 8.6 Hz, 1 H); C nmr (deu-
teriochloroform): δ 17.3, 18.8, 126.3, 135.6, 136.9, 140.7, 165.7,
170.2.
preponderance of 12d over 12c can be traced to steric fac-
tors. Ab initio molecular orbital calculations show that the
relative energies of 12a-d, in kJ/mol, are: 5.4 (12a); 3.6
(12b); 54.7 (12c); and 0.0 (12d). The near energy equiva-
lence of 12a and 12b, as well as their respective precursor
radicals is quite consistent with the results in Table 3. On
the other hand, 12c lay some 54.7 kJ/mol in energy above
12d, which would favor production of latter compound. In
addition, the dihedral angle between the pyridine and ben-
zene rings was calculated to be 27° for 12c, compared to 0°
in the other compounds.
In conclusion, we have shown that Barton 2-selenoxo
esters can react with benzynes generated from a variety of
commercially or readily available benzyne precursors to
give benzo[b]seleno[2, 3-b]pyridines in modest yields.
This method is superior to the best non-benzyne method
previously reported [4], which requires several steps.
Acetic Acid 6-Methyl-2-selenoxo-2H-pyridin-1-yl Ester (4d).
1
This compound was obtained as a viscous liquid; H nmr (deu-
teriochloroform): δ 2.42 (s, 3 H), 2.51 (s, 3 H), 6.68 (d, J = 8.2
Hz, 1 H), 7.12 (dd, J = 8.2, 6.1 Hz, 1 H), 7.93 (d, J = 6.1 Hz, 1 H);
13
C nmr (deuteriochloroform): δ 18.2, 18.5, 116.0, 133.2, 138.3,
148.9, 165.1, 172.9.
General Procedure for the Benzyne Reactions.
Using Anthranilic Acids 5a-g to Generate Benzyne Precursors.
Method A.
A solution containing 1.1 mmol of anthranilic acid 5a-g dis-
solved in 5 mL acetone was added dropwise over 45-60 minutes
to a refluxing mixture of 1 mmol or 5 mmol of O-ester 4a-d and
isoamyl nitrite in dichloromethane. After the resulting solution
was refluxed for an additional 3 hours it was cooled to room tem-
perature and the solvent evaporated to give a crude product mix-
ture. The residue was separated on a silica gel column,
prepacked in hexane. The desired products were eluted off the
column with hexane:ethyl acetate (95:5 v/v).
EXPERIMENTAL
General Data.
Using 2-Diazoniobenzenecarboxylate Hydrochlorides 6a-c as
Benzyne Precursors.
Melting points were taken on a capillary melting point appara-
1
tus, and are uncorrected with respect to stem correction. The H
13
and C nmr spectra were recorded on a multi-nuclear nmr spec-
Method B.
A stirred mixture containing 0.325 mmol of the diazonium salt
6 and 0.355 mmol of ester 4 in 20 mL of tetrahydrofuran was
gradually warmed to 45° and kept there for 3 hours. The mixture
trometer; chemical shifts were referenced to TMS as internal
standard. Elemental analyses were obtained from SMU
Analytical Services Laboratories. The anthranilic acids 5 a,c,d

Jan-Feb 2004
The Reaction of Acetic Acid 2-Selenoxo-2H-pyridin-1-yl Esters with Benzynes
19
was then cooled to room temperature and the solvent evaporated
to give crude product residue. The residue was separated on a sil-
ica gel column, prepacked in hexane. The desired products were
eluted off the column with hexane:ethyl acetate (95:5 v/v).
3-Methylbenzo[b]seleno[2,3-b]pyridine (10c).
This compound was obtained as a colorless solid, mp 174-176°;
1
(hexane:ethyl acetate [9:1 v/v]) 0.47; H nmr (deuteriochloro-
R
f
form): δ 2.51 (s, 3 H), 7.44-7.49 (m, 2 H), 7.90 (dd, J = 7.9, 1.5 Hz,
1 H), 8.09 (dd, J = 7.5, 1.6 Hz, 1 H), 8.13 (d, J = 1.4 Hz, 1 H), 8.45
Using (Phenyl)[o-(trimethylsilyl)phenyl]iodonium Triflate (7) as
Benzyne Precursor.
13
(d, J = 1.4 Hz, 1 H); C nmr (deuteriochloroform): δ 18.8, 123.4,
125.5, 126.8, 127.9, 129.9, 130.6, 133.1, 135.6, 139.6, 149.6, 160.8;
Method C.
+
ms: m/z 247 (M , 100), 219 (10), 167 (15), 139 (20).
Anal. Calcd. for C H NSe: C, 58.55; H, 3.69; N, 5.69. Found
The reactions were similar to that described using anthranilic
acid as benzyne precursor with the following exceptions. To a
solution of compound 7 (251 mg, 0.5 mmol) and 5 mL anhydrous
dichloromethane was added the Barton 2-selenoxo ester 4a-d
(0.55 mmol or 2.5 mmol in 5 mL) and the resulting solution
cooled to 0° under an ice bath. After the dropwise addition of
tetrabutylammonium fluoride (0.75 mL of 1.0 M solution in
tetrahydrofuran) the ice-bath was removed and the reaction mix-
ture was allowed to warm to room temperature where it was
stirred for 3 hours. An additional 10 mL of dichloromethane was
then added to the reaction mixture and the resulting solution was
washed with water (2 X 10 mL), dried (sodium sulfate) and evap-
orated to give an oily residue. The residue was separated on a sil-
ica gel column prepacked in hexane. The desired products were
eluted from the column with hexane:ethyl acetate (95:5 v/v).
12
: C, 58.52; H, 3.55; N, 5.61.
9
2-Methylbenzo[b]seleno[2,3-b]pyridine(10d).
This compound was obtained as a colorless solid, mp 78-79°
(lit.,[4a] mp 79 - 80°); R (hexane:ethyl acetate [9:1 v/v]) 0.40;
f
1
H nmr (deuteriochloroform) δ 2.71 (s, 3 H), 7.25 (d, J = 8.0 Hz,
1 H), 7.42-7.48 (m, 2 H), 7.91 (dd, J = 7.0, 1.2 Hz, 1 H), 8.06 (dd,
13
J = 7.9, 1.1 Hz, 1 H), 8.19 (d, J = 8.0 Hz, 1 H); C nmr (deuterio-
chloroform): δ 24.5, 119.8, 122.7, 125.1, 126.3, 127.2, 129.9,
+
130.3, 135.6, 138.4, 157.8, 163.3; ms: m/z 247 (M , 100); hrms:
80
Calcd. for C H N Se: 246.9900, found: 246.9901.
12
9
5,6,7,8-Tetraphenylbenzo[b]seleno[2,3-b]pyridine (11a).
This compound was obtained as a colorless solid, mp 302-
1
304°; R (hexane:ethyl acetate [9:1 v/v]) 0.37; H nmr (deuterio-
f
chloroform): δ 6.69 (dd, J = 8.3, 1.6 Hz, 1 H), 6.89 - 6.91 (m, 11
Using 2-(Trimethylsilyl)phenyl Triflates 8a-c as Benzyne
Precursor.
H), 7.23 - 7.36 (m, 10 H), 8.42 -8.44 (dd, J = 4.6 Hz, 1.6 Hz,1 H);
13
C nmr (deuteriochloroform): δ 119.0, 125.5, 125.7, 126.5,
Method D.
126.7, 127.2, 127.4, 128.2, 128.5, 129.5, 129.9, 131.1, 131.3,
132.3, 133.4, 134.2, 138.2, 139.0, 139.3 139.4, 139.5, 139.6,
To a mixture of cesium fluoride (0.152 g, 1 mmol) and ester
4a-d in acetonitrile (10 mL) was added the appropriate benzyne
precursor (0.50 mmol) and the resulting mixture was stirred at
room temperature for 5 hours. Ethyl acetate (10 mL) was added
to the mixture and the resulting solution was washed with water.
The reaction mixture dried (sodium sulfate) and evaporated to
give an oily residue. The residue was separated on a silica gel
column prepacked in hexane. The desired products were eluted
off the column with hexane:ethyl acetate (95:5 v/v). The spectral
and analytical data for the benzyne products 10-15 are shown
below.
+
139.6, 139.9, 141.2, 141.5, 147.2; hrms: [M ] Calcd. for
80
C H N Se: 537.0096, found: 537.1006.
80
35 23
Anal. Calcd for C H N Se: C, 73.35; H, 4.32; N, 2.61.
35 23
Found: C, 73.47; H, 4.30; N, 2.69.
4-Methyl-5, 6, 7, 8-tetraphenylbenzo[ b]seleno[2, 3-b]pyridine
(11b).
This compound was obtained as a colorless solid, mp 265–267°;
1
R ([hexane:ethyl acetate (9:1 v/v]) 0.40; H nmr (deuteriochloro-
f
form): δ 1.43 (s, 3 H), 6.75-7.32 (m, 21 H), 8.39 (d, J = 4.3 Hz, 1
13
H); C nmr (deuteriochloroform): δ 22.8, 122.3, 125.4, 125.6,
Benzo[b]seleno[2,3-b]pyridine (10a).
126.4, 126.5, 126.7, 126.9, 127.4, 127.6, 128.2, 129.8, 130.9,
130.9, 131.4, 131.5, 131.7, 131.7, 133.8, 134.1, 137.1, 139.1,
+
139.6, 139.7, 139.7, 140.0, 141.5, 141.7, 146.0, 146.6; hrms: [M ]
This compound was obtained as a colorless solid, mp 61-63°;
1
R (hexane:ethyl acetate [9:1 v/v]) 0.35; H nmr (deuteriochloro-
f
form): δ 7.42-7.54 (m, 3 H), 7.96 (dd, J = 7.9, 1.5 Hz, 1 H), 8.14
(dd, J = 7.5, 1.6 Hz, 1 H), 8.35 (dd, J = 8.0, 1.4 Hz, 1 H), 8.63 (dd,
80
Calcd. for C H N Se 551.1152, found: 551.1160.
80
36 25
A n a l. Calcd. for C H N Se: C, 78.54; H, 4.58; N, 2.54.
36 25
Found: C, 78.44; H, 4.66; N, 2.51.
13
J = 4.2, 1.4 Hz, 1 H); C nmr (deuteriochloroform): δ 119.9,
123.1, 125.2, 126.3, 127.7, 129.6, 135.3, 138.8, 148.2, 163.7; ms:
3-Methyl-5, 6, 7, 8-tetraphenylbenzo[ b]seleno[2, 3-b]pyridine
(11c).
+
m/z 233, (M , 100), 153 (40), 126 (10); hrms: [M ] Calcd. for
+
80
C H N Se: 232.9744, found: 232.9746.
11
7
Anal. Calcd. for C H NS: C, 56.91; H, 3.04; N, 6.03. Found:
This compound was obtained as a colorless solid, mp 280-
1
282°; R (hexane: ethyl acetate [9:1 v/v]) 0.31; H nmr (deuteri-
11
7
C, 56.99; H, 3.06; N, 6.10.
f
ochloroform): δ 2.36 (s, 3 H), 6.38 (d, J = 1.3 Hz, 1 H), 6.86-6.89
(m, 10 H), 7.21 - 7.36 (m, 10 H), 8.26 (d, J = 1.3 Hz, 1 H); hrms:
4-Methylbenzo[b]seleno[2,3-b]pyridine (10b).
This compound was obtained as a colorless solid, mp 160-162°
1
[lit [4a] 158 - 162°]; Rf (hexane:ethyl acetate [9:1 v/v]) 0.28; H
nmr (deuteriochloroform): δ 2.90 (s, 3 H), 7.18 (d, J = 4.7 Hz, 1
H), 7.44-7.52 (m, 2 H), 7.95 (dd, J = 7.4, 2.2 Hz, 1 H), 8.30 (dd, J
+
[M ] Calcd. for C H N Se: 551.1152, found: 551.1148.
80
36 25
Anal. Calcd. for C H N Se: C, 78.54; H, 4.58; N, 2.54.
80
36 25
Found: C, 78.57; H, 4.56; N, 2.48.
2-Methyl-5, 6, 7, 8-tetraphenylbenzo[ b]seleno[2, 3-b]pyridine
(11d).
13
= 7.7, 2.2 Hz, 1 H), 8.43 (d, J = 4.7 Hz, 1 H); C nmr (deuterio-
chloroform): δ 22.4, 122.9, 125.0, 126.3, 126.7, 126.9, 131.5,
+
136.7, 138.8, 144.2, 147.1, 164.1; ms: m/z 247 (M , 100); hrms:
This compound was obtained as a colorless solid, mp 295-
1
298°; R hexane:ethyl acetate [9:1 v/v]) 0.28; H nmr (deuterio-
+ 80
[M ]: Calcd. for C H N Se: 246.9900, found: 246.9995.
12
9
f (

20
U. N. Rao, R. Sathunuru, J. A. Maguire and E. Biehl
Vol. 41
6,7-Difluoro-4-methylbenzo[b]seleno[2,3-b]pyridine (14b).
chloroform): δ 2.55 (s, 3 H), 6.56 (d, J = 7.9 Hz, 1 H), 6.78 (d, J =
13
7.9 Hz, 1 H), 6.87-6.89 (m, 11 H), 7.22 - 7.34(m, 9 H); C nmr
(deuteriochloroform): δ 24.5, 119.5, 128.8, 126.0, 126.9, 127.1,
127.3, 127.5, 127.7, 127.9, 128.6, 128.8, 130.0, 130.2, 130.47,
130.6, 131.6, 137.6, 131.7, 131.9, 133.0, 134.0, 138.6, 138.9,
139.3, 139.9, 140.0, 140.1, 140.2, 141.2, 142.0, 157.0, 164.8;
This compound was obtained as a colorless solid, mp 170°;
1
(hexane:ethyl acetate [9:1 v/v]) 0.27; H nmr (deuterio-
R
f
chloroform): δ 2.87 (s, 3 H), 7.24 (d, 4.8 Hz, 1 H), 7.73 (dd,
J = 9.3, 7.6 Hz, 1 H), 8.12 (dd, J = 11.9, 7.4 Hz, 1 H), 8.46 (d,
13
J = 4.8 Hz, 1 H); C nmr (deuteriochloroform): δ 21. 9, 114.1
+
80
hrms: [M ] Calcd. for C H N Se: 551.1152, found: 551.1155.
36 25
(d, J = 19.7 Hz), 114.8 (d, J = 20.0 Hz), 123.1, 130.5, 132.7
(dd, J = 7.0, 3.1 Hz), 133.7 (dd, J = 6.8, 2.7 Hz), 143.9, 147.3,
148.8 (dd, J = 243.2, 13.4 Hz), 149.3 (dd, J = 251.1, 14.3 Hz),
80
A n a l. Calcd. for C H N Se: C, 78.54; H, 4.58; N, 2.54.
36 25
Found: C, 78.40; H, 4.68; N, 2.55.
+
8 0
164.4; hrms: [M ] Calcd. for C H F N Se: 282.9712,
1 2
5-Methoxybenzo[b]seleno[2,3-b]pyridine (13a).
7 2
found: 282.9712.
Anal. Calcd. for C H F N Se: C, 51.08; H, 2.50; N, 4.96.
Found: C 51.15; H, 2.75; N, 5.10.
1
This compound was obtained as a viscous liquid; H nmr (deu-
teriochloroform): δ 3.82 (s, 3 H), 6.95 (m, 1 H), 7.05 (dd, J = 6.4,
1.2 Hz, 1 H), 7.30 (dd, J = 7.2, 2.0 Hz, 1 H), 7.33 (t, J = 5.7 Hz,
80
12
7 2
13
1 H). 7.44 (m, 1 H). 8.46 (d, J = 4.5 Hz, 1 H); C nmr (deuterio-
chloroform): δ 55.7, 115.3, 120.8, 121.5, 124.8, 128.6, 129.0,
6,7-Difluoro-3-methylbenzo[b]seleno[2,3-b]pyridine (14c).
This compound was obtained as a colorless solid, mp
+
130.8, 137.0, 150.3, 159.1, 160.5; lcms: m/z 266 (M , 100), 264
(52), 263 (30), 246 (10), 242 (15).
0
192–193 : R (hexane:ethyl acetate [9:1 v/v]) 0.30; H nmr (deu-
1
f
teriochloroform): δ 2.50 (s, 3 H), 7.66 (dd, J = 9.2, 7.5 Hz, 1 H),
80
Anal. Calcd. for C H NO Se: C, 54.98; H, 3.46; N, 5.34.
13
12
Found: C, 54.95; H, 3.44; N, 5.32.
9
7.82 (dd, J = 10.4, 7.4 Hz, 1 H), 7.98 (s, 1 H), 8.44 (s, 1 H);
C
nmr (deuteriochloroform): δ 18. 3, 111.1 (d, J = 18.7 Hz), 114. 5
(d, J = 20.2 Hz), 129.8, 130.2, 131.5 (dd, J = 6.4, 3.3 Hz), 133.7
(dd, J = 6.8, 2.8 Hz), 149.6 (dd, J = 245.3, 14.1 Hz), 150.9, (dd,
5-Methoxy-4-methylbenzo[b]seleno[2,3-b]pyridine(13b).
1
This compound was obtained as a viscous liquid; H nmr (deu-
teriochloroform): δ 1. 27 (s, 3 H), 3.83 (s, 3 H), 6.87 (m, 1 H),
6.96 (dd, J = 6.4, 1.2 Hz, 1 H), 7.33 (m, 2 H). 8.32 (d, J = 4.5 Hz,
+
80
J = 250.9, 14.4 Hz), 160.7; hrms: [M ] Calcd. for.C H F N Se:
12
7 2
282.9712, found: 282.9711.
Anal. Calcd. for C H F NSe: C, 51.08; H, 2.50; N, 4.96.
Found: C, 51.13; H, 2.61; N, 5.01
12
7 2
13
1 H); C nmr (deuteriochloroform): δ 21.3, 55.7, 115.2, 121.3,
122.3, 125.7, 128.5, 129.2. 130.7, 148.5, 149.9, 158.4, 160.5;
+
lcms: m/z 280 (M ,100), 278 (70), 276 (25), 264 (10).
6,7-Difluoro-2-methylbenzo[b]seleno[2,3-b]pyridine (14d).
80
Anal. Calcd. for C H NO Se: C, 56.53; H, 4.01; N, 5.07.
13 11
Found: C, 56.51; H, 3.98; N, 5.02.
This compound was obtained as a colorless solid, mp 125-
1
127°: R (hexane:ethyl acetate [9:1 v/v]) 0.39; H nmr (deuterio-
f
chloroform): δ 2.71 (s, 3 H), 7.28 (d, J = 7.9 Hz, 1 H), 7.68 (dd,
J = 9.4, 7.3, 1 H), 8.30 (dd, J =10.3, 7.3 Hz, 1 H), 8.08 (d, J = 7.9
5-Methoxy-3-methylbenzo[b]seleno[2,3-b]pyridine (13c).
1
This compound was obtained as a liquid; H nmr (deuterio-
chloroform): δ 2. 27 (s, 3 H), 3.81 (s, 3 H), 6.91 (d, J = 7.3 Hz 1
H), 7.03 (dd, J = 7.3, 2.1 Hz, 1 H), 7.25-7.29 (m, 2 H), 8.32 (d,
+
Hz, 1 H); hrms [M ] Calcd. for C H F N Se: 282.9712,
80
12
7 2
found: 282.9711.
Anal. Calcd. for C H F N Se: C, 51.08; H, 2.50; N, 4.96.
Found: C, 51.04; H, 2.55; N, 5.03.
80
12
7 2
13
J = 1.1 Hz, 1 H); C nmr (deuteriochloroform): δ 18.2, 55.7,
115.0, 121.0, 125.1, 128.1, 129.7, 130.7, 137.9, 150.7, 154.7,
2-(Methylselenenyl)pyridine (16a).
+
160.5; lcms: m/z 280 (M , 100) 278 (60), 276 (30), 254 (15).
80
Anal. Calcd. for C H NO Se : C, 56.53; H, 4.01; N, 5.07.
This compound was obtained a pale yellow oil; R
13 11
Found: C, 56.50; H, 3.96; N, 5.01
f
1
(hexane:ethyl acetate [9:1 v/v]) 0.63; H nmr (deuteriochloro-
form): δ 2.09 (s, 3 H), 6.84 (dd, J = 8.1, 4.1 Hz, 1 H), 7.12 (dd,
J = 7.8, 2.1 Hz, 1 H), 7.54 (dd, J = 8.1, 7.8 Hz, 1 H), 8.46 (dd,
5-Methoxy-2-methylbenzo[b]seleno[2,3-b]pyridine (13d).
1
This compound was obtained as a liquid; H nmr (deuteri-
ochloroform) : δ 2. 54 (s, 3 H), 3.82 (s, 3 H), 6.81-6.97 (m, 2 H),
13
7.29 - 7.32 (m, 3 H), C nmr (deuteriochloroform): δ 30.7, 55.7,
114.88, 115.3, 120.4, 121.7, 128.6, 129.3, 130.8, 137.2, 158.4,
+
J = 4.1, 1.8 Hz, 1 H); ms: m/z 173 (M , 30), 93 (100), 78 (30),
51(20).
4-Methyl-2-(methylselenenyl)pyridine (16b).
+
159.3 160.5; lcms: m/z 280(M , 100), 276 (40), 263 (15). A pure
compound could not be obtained for elemental analysis.
This compound was obtained as a pale yellow oil; R
f
1
(hexane:ethyl acetate [9:1 v/v]) 0.66; H nmr (deuteriochloro-
form): δ 2.19 (s, 3 H), 2.47 (s, 3 H), 6.82 (dd, J = 4.9, 1.6 Hz, 1
H), 7.15 (d, J = 1.6 Hz, 1 H), 8.33 (d, J = 4.9 Hz, 1 H); ms: m/z
6,7-Difluorobenzo[b]seleno[2,3-b]pyridine (14a).
+
This compound was obtained as a colorless solid, mp 184°: R
1
187 (M , 40), 107 (100), 93 (40), 65 (30).
80
A n a l. Calcd. for C H N Se: C, 45.17; H, 4.87; N, 7.53.
f
(hexane:ethyl acetate [9:1 v/v]) 0.34; H nmr (deuteriochloro-
7
Found: C, 45.27; H, 4.92; N, 7.50.
9
form): δ 7.43 (dd, J = 7.9, 4.7 Hz, 1 H), 7.71 (dd, J = 9.4, 7.3 Hz, 1
H), 7.88 (dd, J = 9.4, 7.3 Hz, 1 H), 8.21 (dd, J = 7.9, 1.6 Hz, 1 H),
13
8.61 (dd, J = 4.7, 1.6 Hz, 1 H); C nmr (deuteriochloroform): δ
5-Methyl-2-(methylselenenyl)pyridine (16c).
111.2 (d, J = 19.5 Hz), 114.5, (d, J = 20.1Hz), 120.1, 129.8, 133.1
(dd, J = 7.2, 2.8Hz) 134.5 (dd, J = 6.9, 3.0 Hz), 148.4, 149.0 (dd,
J = 247.4, 14.6 Hz), 150.0 (dd, J = 245.2, 14.6 Hz), 164.1; hrms:
This compound was isolated as a pale yellow oil; R
1
f
(hexane:ethyl acetate [9:1 v/v] 0.65; H nmr (deuteriochloro-
form): δ 2.28 (s, 3 H), 2.46 (s, 3 H), 7.22 (d, J = 8.1 Hz, 1 H), 7.30
13C
+
80
[M ] Calcd. for C H F N Se: 268.9550, found: 268.9555.
80
(d, J = 8.1 Hz, 1 H), 8.32 (s, 1 H); nmr (deuteriochloroform):
δ 5.62, 17.84, 124.13, 129.53, 136.81, 150.34, 152.04; ms: m/z
11 5 2
Anal. Calcd. for C H F N Se: C, 49.28; H, 1.88; N, 5.22.
11 5 2
Found: C, 49.32; H, 1.93; N, 5.26.
+
187 (M , 30), 107 (100), 93 (25).

Jan-Feb 2004
The Reaction of Acetic Acid 2-Selenoxo-2H-pyridin-1-yl Esters with Benzynes
21
80
Anal. Calcd. for C H N Se: C, 45.17; H, 4.87; N, 7.53.
REFERENCES AND NOTES
7
Found: C, 45.10; H, 4.80; N, 7.58.
9
[1] U. N Rao and E. Biehl, J. Org. Chem., 6 7, 3409 (2002).
[2] See: K. Okuma, A. Okada, Y. Koga and Y. Yokomori, J.
Amer. Chem. Soc., 1 2 3, 7166 (2001) and references therein.
[3] E. Honda, S. Watanabe, T. Iwamura and T. Kataoka,
Heterocycles, 55, 465 (2001).
[4a] P. I. Abramenko, V. G. Zhiryakov, L. A. Balykova and T.
K., Ponomareva, Khim. Geterotsiki. Soedin., 796, 1 9 7 4; Chem. Abstr. ,
81, 505347 (1974); [b] P. I. Abramenko and V. G. Zhiryakov, Khim.
Geterotsiki. Soedin., 1541 (1972); Chem. Abstr., 7 8, 58269 (1973).
[5] D. H. R. Barton, D. Crich, Y. Herve, P. Potier and J.
Thierry, Tetrahedro n, 4 1, 4347 (1985).
6-Methyl-2-(methylselenenyl)pyridine (16d).
This compound was obtained as a pale yellow oil; R
1
f
(hexane:ethyl acetate [9:1 v/v]) 0.63; H nmr (deuteriochloro-
form): δ2.46 (s, 3 H), 2.53 (s, 3 H), 6.89 (dd, J = 7.5, 1.7 Hz, 1 H),
13
7.11 (dd, J = 7.8, 1.7 Hz, 1 H), 7.36 (dd, J = 7.8, 7.5 Hz, 1 H);
nmr (deuteriochloroform): δ 5.45, 13.99, 119.29, 121.05, 135.99,
C
+
154.95, 158.74; ms: m/z 187 (M , 100), 107 (40), 92 (50).
80
Anal. Calcd. for C H N Se: C, 45.17; H, 4.87; N, 7.53.
7
9
Found: C, 45.33; H, 4.90; N, 7.45.
[6] L. Friedman and F. M. Logullo, J. Am. Chem. Soc., 85,
1549 (1963).
[7] H. Hart and A. Oku, J. Org. Chem., 3 7, 4269 (1972).
[8] T. Kitamura, M. Yamane, K. Inoue, M. Todaka, N.
Fukatsu, Z. Meng and Y. Fujiwara, J. Am. Chem. Soc., 1 2 1, 11674
(1999).
[9] Y. Himeshima, T. Sonoda and H. Kobayashi, Chem. Lett.,
1211 (1983).
[10] W. M. Best and D. Wege, Aust. J. Chem., 3 9, 635 (1986).
[11] D. Pena, S. Escudero, D. P'erez, E. Guiti'an and L. Castedo,
Angew. Chem. Int. Ed., 3 7, 2659 (1998).
X-ray Analysis of Compound 14c.
Crystal data are as follows. C H F NSe, formula weight
12
282.15, triclinic, space group a P?1, a = 8.371(2), b = 11.217(3),
7 2
o
c = 11.820(3) Å, α = 94.18(2), β = 96.35(2), γ= 109.42(2) , v =
-1
3
1033.2(5) Å , z = 4, d = 1.814 g/cm, µ = 3.630 mm . Data were
collected on a Bruker-AXS P4 diffractometer, MoKα, 2θ 3.5
_
50.0°. Data were corrected for Lorentz and polarization effects,
as well as for absorption. The structure was solved by direct-
methods and refined with SHELX97 [14]. There are two inde-
pendent molecules in an asymmetric unit. H atoms are located in
[12] See, for example: A. M. D. R. R. Gonsalves, T. M.V.D.
Pinho e Melo and T. L. Gilchrist, Tetrahedro n, 48, 6821 (1992).
[13] X. Qiao, M. A. Padula, D. M. Ho, N. J. Vogelaar, C. E.
Schutt and R. A. Pascal, Jr., J. Am. Chem. Soc., 11 8, 741 (1996).
[14] (Sheldrick, G.M., "SHELX97, Program for Crystal
Structure Solution and Refinement", 1998, Institute fur Anorg .
Chemie, Göttingen, Germany.
calculated positions. The final refinement converged at R =
1
0.065, wR (all data) = 0.183.
2
Acknowledgments.
We thank the Robert A. Welch Foundation, TX for generous
financial support of this research.