The first ammonium aromatic diselenoates: Stable heavy congeners of aromatic carboxylic acid salts

Article abstract of DOI:10.1021/ja026110i

Ammonium aromatic diselenoates were synthesized by reacting aromatic diselenoic acid 2-(trimethylsilyl)ethyl esters derived from aluminum 2-(trimethylsilyl)ethyl selenolate with aromatic selenoic acid O-methyl esters with ammonium fluorides. The results o

Full text of DOI:10.1021/ja026110i

Published on Web 05/07/2002
The First Ammonium Aromatic Diselenoates: Stable Heavy Congeners of
Aromatic Carboxylic Acid Salts
Kazuyasu Tani, Toshiaki Murai,* and Shinzi Kato
Department of Chemistry, Faculty of Engineering, Gifu UniVersity, Yanagido, Gifu 501-1193, Japan
Received March 6, 2002
Since the first synthesis1 of sulfur isologues of aromatic
carboxylic acid salts, that is, dithiobenzoic acid salts, many studies
on their synthesis, reactions, and applications have been reported.2
In contrast, the corresponding selenium isologues, that is, diselenoic
acid salts, have been almost totally ignored until very recently. With
regard to their aliphatic derivatives, Jensen proposed that zinc
diselenoates are putative intermediates in the reaction of dialkylzinc
Figure 1. ORTEP representation of 5a. Hydrogen atoms have been omitted
for clarity. Selected bond lengths (Å): Se1-C1, 1.828(4); Se2-Cl, 1.831-
(4); C1-C2, 1.482(5). Selected bond angles (deg): Se1-C1-Se2, 124.3-
(2); Se1-C1-C2, 118.0(3); Se2-C2, 117.7(3).
with carbon diselenide.3 Very recently, the inner salts with a
diselenocarboxyl group were isolated as stable compounds and
characterized.4 Nevertheless, no examples of aromatic diselenoic
acids and their salts have been reported.5 We report here the first
successful isolation and characterization of ammonium aromatic
diselenoates.
Scheme 1
The key starting materials, that is, aromatic diselenoic acid
2-(trimethylsilyl)ethyl esters 4 leading to ammonium diselenoates,
were obtained as green or blue-green oils by reacting aluminum
2-(trimethylsilyl)ethyl selenolate 2 derived from the diselenide 16
and i-Bu2AlH with aromatic selenoic acid O-methyl esters 3
(Scheme 1).7
The oily esters 4 were unstable and decomposed to the corre-
sponding selenoic acid esters (ArCOSeCH2CH2SiMe3) under Ar
within 12 h, even at -20 °C, except for 2-methylphenyl derivative
4b.
bonds (1.74-1.80 Å)12 than to those of C-Se single bonds (1.94
Å).13
Representative spectroscopic data for 4 and 5 are listed in Table
1.
The reaction of 4 with Me4NF gave the corresponding am-
monium aromatic diselenoates 5 as green solids in yields of 47-
78% and in high purity.8 Interestingly, the salt 5a was more stable
than the starting ester 4a. For example, the salt 5a could be handled
in air without any appreciable change, although it decomposed when
it was dissolved in CH3CN and THF. Furthermore, the introduction
of a methyl group to the ortho position of the aromatic ring
increased the stability of the salt 5b. No appreciable change was
observed for 5b, even when it was dissolved in CH3CN at -20
°C.
The molecular structure of 5a was determined by X-ray crystal-
lography.10 An ORTEP drawing of 5a is shown in Figure 1.
This is the first X-ray molecular analysis of aromatic diselenoic
acid salts. Several characteristic features can be noted. First, the
distances between the selenium atoms and the hydrogen atoms of
the ammonium ion are longer than the sum of the van der Waals
radii of both atoms,11 and no interaction was observed between
the diselenocarboxyl and ammonium groups. Second, the salt 5a
is monomeric in the solid state. Third, the dihedral angle of the
phenyl group and diselenocarboxyl group was 48.0(2)°, and the
phenyl group does not appear to resonate with the diselenocarboxyl
group. Fourth, the average length of the two C-Se bonds is 1.830-
(4) Å, which is closer to the lengths of ordinary C-Se double-
In the 13C NMR spectra, signals due to the selenocarbonyl carbon
atoms in 4 were observed at 237 ( 5 ppm, whereas the corre-
sponding signals in salts 5 were shifted to lower fields by 22.5 (
0.5 ppm. Two resonances at about 900 and 1800 ppm in 77Se NMR
spectra were observed for the selenium atoms in C-Se single- and
CdSe double-bonds in the case of 4. Coupling constants typical
of C-Se (172 ( 3 Hz) and CdSe (224 ( 1.2 Hz) bonds were also
detected. On the other hand, only one signal appeared at 1360-
1493 ppm, which is in the region of the signals for CdSe bonds,14
when esters 4 were converted to salts 5. Interestingly, the signals
of 5 [δ5(Se)] are linearly correlated to the signals of the CdSe
groups of 4 [δ4(Se)]:15 δ5(Se) ) 379.3 + 0.595 × δ4(Se) (r ) 1.00).
More importantly, the coupling constants between the selenium
and carbon atoms in 5 were larger than 200 Hz, which indicates
that both the carbon-selenium bonds in 5 possess a double-bond
character. Thus, when the resonance hybrid 5 is depicted with three
resonance forms 5I-5III, the forms 5I and 5III contribute more
to 5 than form 5II, and the electrons are efficiently delocalized on
the diselenocarboxyl group in 5.16,17
* To whom correspondence should be addressed. E-mail: mtoshi@
cc.gifu-u.ac.jp.
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5960
J. AM. CHEM. SOC. 2002, 124, 5960-5961
10.1021/ja026110i CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Spectroscopic Data of Compounds 4 and 5
UV−vis [nm]c
π−π* n−π*
Acknowledgment. This research was supported in part by a
Grant-in-Aid for Scientific Research from the Ministry of Education,
Science, Sports and Culture of Japan.
cmpd 13C NMR δ [ppm] 77Se NMR δ [ppm] 1JSe-Cb [Hz]
a
a
4a
4b
4c
4d
5a
5b
5c
5d
236.9
241.5
234.2
233.9
259.3
263.6
256.1
256.6
897.9, 1770.2 171.9, 223.3 391
954.6, 1846.7 174.5, 225.2 375
901.3, 1798.1 171.6, 222.8 394
863.4, 1652.4 169.2, 222.8 366
622
609
627
618
Supporting Information Available: Experimental procedures,
characterization data for 4-6, 8, crystallographic data, molecular orbital
calculations of model compounds (PDF). An X-ray crystallographic
file in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
1433.3
1493.0
1449.4
1362.6
213.5
208.7
214.9
211.6
447 (423)d 684 (654)d
417 (385)d 634 (611)d
453 (433)d 670 (671)d
452 (448)d 690 (692)d
References
a CDCl3 was used as a solvent for 4, whereas CD3CN and DMSO-d6
were used for 5. b The coupling constants were determined in the 13C NMR
spectra. c THF was used as a solvent. d The UV-vis spectra were measured
in the solid state.
(1) Engelhardt, A.; Latschinoff, P. Z. Chem. 1868, 455-460.
(2) (a) Scheithauer, S.; Mayer, R. In Topics in Sulfur Chemistry; Senning,
A., Ed.; Georg Thime Publischers: Stuttgart, Germany, 1979; Vol. 4. (b)
Mayer, R.; Scheithauer, S. In Methoden der Organishen Chemie; Falbe,
J., Ed.; Georg Thime Verlag: Stuttgart, Germany, 1985; Band E5, Teil
2, pp 891-930. (c) Kato, S.; Murai, T. In Supplement B: The Chemistry
of Acid DeriVatiVes; Patai, S., Ed.; John Wiley & Sons: New York, 1992;
Vol. 2, pp 803-847. (d) Murai, T.; Kato, S. In ComprehensiVe Organic
Functional Group Transformations; Katritzky, A. R., Meth-Cohn, O.,
Rees, C. W., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 5, pp 545-563.
Scheme 2
(3) Jensen, K. A. Q. Rep. Sulfur Chem. 1970, 5, 45-52.
(4) Nakayama, J.; Kitahara, T.; Sugihara, Y.; Sakamoto, A.; Ishii, A. J. Am.
Chem. Soc. 2000, 122, 9120-9126.
(5) The incorporation of a selenium atom into Mo and W carbyne complexes
has been reported to result in the formation of Mo and W diselenoates,
see: Gill, D. S.; Green, M.; Marsden, K.; Moore, I.; Orpen, A. G.; Stone,
F. G. A.; Williams, I. D.; Woodward, P. J. Chem. Soc., Dalton Trans.
1984, 1343-1347.
(6) 2-(Trimethylsilyl)ethaneselenol, or bis[2-(trimethylsilyl)ethyl] diselenide
1, which was not previously known, was the key precursor to synthesize
the esters 4. Several attempts to prepare diselenide 1 failed. We finally
realized the reproducible synthesis of 1 from Li2Se2 and in situ generated
2-(trimethylsilyl)ethyl bromide. For details, see the Supporting Information.
(7) (a) Murai, T.; Mizutani, T.; Kanda, T.; Kato, S. J. Am. Chem. Soc. 1993,
115, 5823-5824. (b) Murai, T.; Mizutani, T.; Kanda, T.; Kato, S. Heteroat.
Chem. 1995, 6, 241-246.
(8) First, the tetrabutylammonium salt was prepared from ester 4a with Bu4-
NF in THF using a procedure to that used to prepare ammonium
selenothioates.9 However, the stability of the resulting salts was fairly
low.
The UV-visible spectra of 5 further support the double-bond
character between the carbon atom and selenium atoms in 5. For
4, absorptions ascribed to π-π* and n-π* transitions were seen
at 366-394 and 609-627 nm, respectively. The corresponding
absorptions for 5 were substantially shifted to a longer wavelength.
The suitability of ammonium diselenoate 5 as a starting material
that could lead to a variety of compounds bearing a diselenocarboxyl
group was demonstrated by the following reaction. First, alkylation
of 5b with p-phenylphenacyl bromide gave the corresponding
phenacyl ester 6 as stable blue needles in 68% yield (Scheme 2).
Second, to generate diselenoic acids, TfOH or HCl/Et2O solution
was added to a THF-d8 suspension of 5b in an NMR tube at -70
°C. Insoluble salt 5b quickly dissolved in THF-d8, and the light
green suspension quickly changed to green, which reflected the
formation of diselenoic acid 7; however, the low-temperature NMR
spectra of the reaction mixture did not show signals that could be
ascribed to 7. To a homogeneous reaction mixture of TfOH with
5b in Et2O was added methyl vinyl ketone at -70 °C, and the
mixture was stirred for 30 min at 30 °C to afford γ-oxabutyl
diselenoate 8 as a labile blue oil in 21% yield. These results suggest
that aromatic diselenoic acid 718 is generated by the protonation of
5b but easily decomposes.
(9) Murai, T.; Kamoto, T.; Kato, S. J. Am. Chem. Soc. 2000, 122, 9850-
9851.
(10) Crystal data of 5a: See Supporting Information.
(11) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
(12) Murai, T.; Kato, S. In Topics in Current Chemistry; Wirth, T. Ed.;
Springer-Verlag: Berlin, 2000; Vol. 208, pp 177-199.
(13) Pauling, L. The Chemical Bond; Cornell University Press: Ithaca, NY,
1976; pp 135-155.
(14) (a) Duddeck, H. Prog. Nucl. Magn. Reson. Spectrosc. 1995, 27, 1. (b)
Klapotke, T. M.; Broschag, M. Compilation of Reported 77Se NMR
Chemical Shifts; John Wiley & Sons: New York, 1996.
(15) The plot of δ5(Se) against δ4(Se) is shown in the Supporting Information.
(16) A resonance structure analogous to 5II has been suggested to be important
for acetate ion based on theoretical investigations. See: Wiberg, K. B. J.
Am. Chem. Soc. 1990, 112, 4177-4182.
(17) Bond orders of three model compounds 9-11 were calculated at the
B3LYP/6-311+G(d,p) level. The polar nature of the CdO group in 9 is
evident, since the value is on the order of 1.25 for CdO. In contrast, the
values are on the order of 1.81 for CdS in 10 and 1.78 for CdSe in 11.
This result suggests that the double bond character of the two carbon-
selenium bonds in diselenoate ions is more important than that of the
carbon-oxygen bonds in carboxylate ions. See the Supporting Information.
In summary, we have succeeded in the first synthesis and
structural analysis of ammonium aromatic diselenoates. The
electronic properties of the selenocarboxyl group in ammonium salts
5 were also elucidated by analyzing various NMR and UV-visible
spectra.
(18) Theoretical studies on diselenoformic acid, see: Jemmis, E. D.; Giju, K.
T.; Leszczynski, J. J. Phys. Chem. A 1997, 101, 7389-7395.
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