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Experimental Evidence for the [H2SSH2] Radical Cation in Solution.

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have not been as completely investigated as sulfonium and
phosphonium ylides.
We report here on reactions of methyl(pheny1)selenoniomethanide ( I ) with carbonyl compounds (2), which afford oxiranes (31 in good yields (Table 1).
PhSe
@
p
3
s
'HZ
::c
+
R" 'R2
B
R'
+ PhSeCH,
R2
Table 1. Selected data of the oxiranes (3) obtained from methyl(pheny1)selenoniomethanide ( I ) and carbonyl compounds (2). (3a), (36). (3e) and (3fl
show molecular ions in the mass spectrum.
Cpd.
la1
R'
130)
(3b)
(3c)
(34
13e)
(38
Ph
H
p-NO,C,H,
H
n-C6H I3
H
n-C4H.,
CH,
n-C,Hq
CZH,
4CHd-
R'
Yield
19/01[b]
B. p.
["C/torr]
94
82
83-85/15
Icl
-
76
88
83
55- 57/75
62-65/25
46-48/23
Cpd.
'H-NMR (6,T M S int., in C D C I )
(30)
2.70 (dd, J = 5.5 and 2.7 Hz, 1 H), 3.05 (dd, J = 5.5 and 4.0 Hz, 1 H),
3.73 (dd, J=4.0 and 2.7 Hz, 1 H), 7.03-7.40 (m. S H )
(3b)
2.73(dd,J=5.8and2.8Hz,lH),3.20(dd,J=5.8and4.0Hz,IH),
(3d)
(3e)
(38
[I] H. J. Reich, Acc. Chem. Res. 12, 22 (1979).
[2l N. N. Magdesieua. L. N. Ngu. N . M. Koloskoua. J. Org. Chem. USSR 13,
928 (1977).
[31 B. M. Trost. L. S. Melvin. Jr.: Sulfur Ylides, Academic Press, New York
1975.
[4] D. L. Klayman, W. H.H. Ciinrher: Organic Selenium Compounds: Their
Chemistry and Biology, Wiley-Interscience, New York 1973.
151 H. M. Gilow. G. L. Walker, J. Org. Chem. 32,2580 (1967).
3.93(dd,J=4.0and2.8Hz, lH),7.40(d,J=8.4Hz,2H),S.l5(d,
J = 8.4 Hz, 2 H)
0.73-1.63 (t and m , J = 7 . 0 Hz, IZH), 2.30-2.60 (m. ZH)
0.77-1.87 (t and m, J=7.0 Hz, 14H),2.20-2.67 (m,ZH)
0.83-2.06 (m. lOH), 2.33-2.60 (m, 2 H )
[a] The products (3) were identified by comparison with authentic samples
prepared from dimethylsulfoniomethanide and the corresponding carbonyl
compounds 131. [b] Yields were determined by NMR. [c] M.p.=83-85"C.
The ylide ( I ) was generated in situ from dimethylphenylselenonium methyl sulfate and sodium hydride in the presence of carbonyl compounds (2), since selenium ylides
are relatively unstable and undergo fission of the ylide
bondsi4I. Generally, these reactions are mild and result in
good yields of ketones and aromatic aldehydes. In the
reaction with heptanal (2c) GC analysis of the reaction
mixture showed a small peak, in addition to those of the
educts, which may be assigned to the expected oxirane (3c),
the isolation of which was, however, difficult. These epoxidations are the first examples, to o u r knowledge, of reactions of non-stabilized selenium ylides with "enolizable"
carbonyl compounds.
Experimental Evidence for the IH,SSH,I* Radical
Cation in Solution
By Shamim A . Chaudhri and Klaus-Dieter Asmus"'
The H,S molecule and radical species derived from it
have been the subjects of numerous investigations. The
ionization and excitation energies, as well as geometrical
parameters, for example, have been calculated using various theoretical models"] and the results compared with
experimental values[*]. Recently,
has used theoretical calculations (HF/4-31G, MP2/4-31G and MNDO) to
consider some thermodynamic and optical characteristics
of an [H2SSHz]tradical cation.
A significant feature of [H2SSH2]i is assumed to be a
three-electron bond; two p-electrons forming a o-bond
and the third, unpaired electron being promoted into an
antibonding o*-orbital. A corresponding three electron
bond has been experimentally well established irr the analogous [R,SSR,]t radical cations formed as intermediates
in the oxidation of t h i o e t h e r ~ ' ~ .We
~ ' . are now able to present experimental evidence on the formation, stability and
optical absorption spectrum of [H,SSH,jt for purpose of
comparison with Clark's calculations.
The experiments were carried out using a pulse radiolysis technique. 'OH and H' react with H2S according to
HZS
+ 'OH/H
+
H S + HZO/HI
to yield H S radicals (k.o,=lO'o and kH.=109 mol-' L
s - ' ) ~ ~The
] . H S radicals exhibit no detectable optical absorption in the UV/VIS-region as is apparent from, for example, pulse radiolysis investigations (aqueous solutions,
p H = 3 , lo-, mol/L H,S). At higher H,S concentrations,
( e . 9 . lo-' mol/L and lower p H values ( e . g . with 0.2 mol/L
HCIO,), however, a well characterized transient optical absorption band can be observed in the UV (A,,,=370 nm;
Fig. la).
General Procedure
A solution of dimethyl(pheny1)selenonium methyl sulfate['] (12 mmol) and the aldehyde or ketone (10 mmol) in
dry dimethylformamide (30 mL) is added dropwise over a
period of 30 min to a stirred suspension of NaH (12 mmol)
in dry tetrahydrofuran (30 mL); all operations were conducted under nitrogen and the mixture was cooled on an
ice bath. The mixture was then warmed to room temperature for 4 h and held at 50°C for 30 min. After cooling,
10% aqueous NH,Cl (20 mL) was added and the organic
layer extracted with ether, washed with NaCl solution, and
dried over Na2S04. Evaporation of the solvent in uacuu
gave a residue which contained the oxirane (3). methyl
phenyl selenide (4), and a small amount of unreacted (2).
Distillation of the residue afforded analytically pure (3).
Received: September 8, 1980 [Z 804 IE]
German version: Angew. Chem. 93, 707 (1981)
672
0 Verlag Chernre GmbH, 6940 Weinheim, 1981
Afnm
Fig. I . a ) UVNIS-absorption spectrum of the [H>SSH:]C radical cation, measured immediately after a 4 ps pulse in an aqueous solution of lo-' mol/L
H2S and 0.2 mol/L HCIO,, b) UV/VIS-absorption spectrum after decay of
[H2SSHz]r(ca. 0.5 ms after pulse).-Optical density (OD) in arbitary units.
[*) Prof. Dr. K.-D. Asmus, Dr. S. A. Chaudhri
Hahn-Meitner-lnstitut fur Kernforschung Berlin GmbH,
Bereich Strahlenchemie
Postfach 3901 28, D-1000 Berlin 39 (Germany)
0570-0833/81/0808-0672
$02.50/0
Angew. Chem. Inr. Ed. Engl 20 (1981) No. 8
10
0
I
7
20 6
E
X
u 2
0
10.‘
10‘~
lo-*
IH, S 1/M
lo-’
Fig. 2. Yield of [H2SSH2]f measured from the absorption at 370 nm as a function of H2S concentration in aqueous solutions containing 0.2 mol/L
HCIO,.
The absorption is fully developed at the end of the 4 ps
pulse. This species decays with a first half-life of ca. 60 ps
to a non-identified product (Fig. Ib). The decay kinetics follow a second order decay process i. e . a radical-radical reaction, with a rate constant of 2k=(3.0-+ 1 . 0 ) ~lo9 mol-’ L
s - I , based o n the assumption of quantitative conversion of
each ‘OH and H into this transient absorbing species.
The yield of the 370 nm transient depends markedly on
the H2S concentration as can be seen from the plot in Figure 2 (all solutions contained 0.2 mol/L HCIO,); a saturation value was reached first at ca. 0.1 mol/L H2S[’].
The yield of the species absorbing at 370 nm is practically constant at high proton concentrations (>
moll
L HCIO,). At lower proton concentrations its yield decreases and the already known [HSSH]: radical anion
(A,,, = 380 nm)16b1appears. The latter is formed by complexation of HS’ by HS- and thus its yield strongly depends on the actual HS- concentration and o n the pHvalue of the solution. At p H = 3, for example, the yield of
[HSSHJ; is low, as indicated by a short-lived “spike” absorption which practically decays already during the 4 ps
pulse. To further distinguish between our new 370 nm-species and the [HSSH]; absorption we also performed corresponding experiments in methanol as solvent, in which the
pK of H2S is considerably higher. As anticipated, no absorption attributable to [HSSH]’ was observed. The absorption at 370 nm also appeared in methanolic solutions
containing HC104. (In methanol HS’ radicals are formed
in the reaction of H2S with ‘CH20H radicals.)[6b1
H\
H’
@
S.‘.S\
/
charge separation formed by addition of HS’ addition to
H,S. Such a conclusion is also in accordance with the fact
that the analogous [R2SSRr radicals apparently cannot be
stabilized in solution. Evidence for the latter has only been
found in some low temperature (77 K) rigid matrices and
with electron withdrawing substituents Rf8].The lower stability of the asymmetrical radical can also be understood
from a MO picture. Thus, the separation of the energy levels should be smaller in [H2SSH]’ than in [H2SSH]+ ESR
data on analogous [R2SSRr radicals further support this
picture by invoking a much weaker sulfur-sulfur bond in
such species relative to the symmetrical [R2SSR2]Cradical
cations[8b1.
The optical absorption of the [R2SSR2]+radical cation
has been attributed to a o+o* transition[’] and this should,
in principle, also apply to absorptions of the [RSSRI-,
[HSSHI-, and [H2SSh2]+ species. Clark has essentially substantiated this assignment in his theoretical calculations on
[HZSSH2]?I3]. He indicated, however, that the newly established ag(o) level may interact with the ag(n-) level of the
non-bonding s-electrons of the two sulfur atoms leading to
a split into ag(cr-n-) and a , ( o t n - ) levels. The optically
excited electron would then be a o-electron with more o r
less non-bonding character. Using the MP2/4-3 1G-method
Clark
has
calculated
the
o+o*
transition
(a,(o-n-)+b,(o*))
to be 3.25 eV or 380 nm and this is in
excellent agreement with our experimental value of 3.35
eV or 370 nm. Finally, the extinction coefficient of
[H2SSH2]t is estimated to be E = 1600+-300mol-’ L cm-’,
based on our experimental results.
Received: December I I , 1980 [ Z 806 IE]
German version: Angew. Chem. 93. 690 (1981)
H. Sakai, S . Yamabe. T. Yamabe. K . Fukui. H. Kato. Chem. Phys. Lett.
25, 541 (1974); A. Hinchclijjfe, J. Mol. Struct. 55, 127 (1979); J B . CoNins.
P. u. R . Schleyer, J . S. Binkley. J . A . Pople. J. Chem. Phys. 64, 5142
(1976).
H. C. Allen, E. Plyler, J. Chem. Phys. 25, 1132 (1956); G . Duxbury, M . Horani, J . Roslas, Proc. R. SOC.A 331, 109 (1972); D . D. Wagman, w. H.
Euans, V. B. Barker, I . Halow, S . M . Bailey. R . H . Schumm, Natl. Bur.
Stand. Techn. Note, No. 270-3 (1968). H. Bock. G. Wagner. Angew
Chem. 84, 119 (1972); Angew. Chem. Int. Ed. Engl. 11. 150 (1972).
T. Clark. J. Cornput. Chem., in press.
G. Meissner, A . Henglein, G . Beck. 2. Naturforsch. 822. 13 (1967). B . C.
Gilbert, D. K . C. Hodgeman. R . 0. C . Norman, J. Chem. SOC. Perkin
Trans. 2 1973, 1748; M . C. R . Symons, ibid. 1974. 1618; M . Bonijjafic. H
Mockel, D. Bahnemann. K.-D. Asmus. ibid. 1975, 657.
K . - D . Asmus. Acc. Chem. Res. 12, 436 (1979).
a) G . Meissner. A . Henglein. Ber. Bunsenges. Phys. Chem. 69, 3 (1965); b)
W. Karmann, G. Meissner, A. Henglein. Z . Naturforsch. B 22, 273
(1967).
Acidic solutions containing more than lo-’ mol/L H2S became somewhat turbid, presumably due to formation of colloidal sulfur. It was
found, however, that this did not significantly influence the results of
pulse experiments.
a) D . J. Nelson. R . L. Peterson. M . C. R . Symons. J. Chem. SOC.Perkin
Trans. 2 1977, 2005; b) J . R . M . Giles, B. P. Roberts. ibrd. 1980, 1497.
H
H
On the basis of our experimental findings and by analogy to the [R2SSR2]t radical cations obtained from
thioethers, the 370 nm absorption (Fig. la) is assigned to
the complex radical cation [H,SSH2] formed in the overall
reaction :
*
HS’ + H2S + Ha:
--+
[HZSSH,]?
The stabilization of the three-electron bond structure in
[H2SSH2]tand in [R,SSR,]t is considered to be facilitated
by the symmetrical resonance structures which are provided by charge- and spin-delocalization over both sulfur
atoms. A similar situation would also explain the relative
stability of the radical anions [HSSH]:, [RSSR];. The radical cation [H2SS)i2)tshould result from addition of two
protons to [HSSH]:, and this is indicated by the corresponding pH-dependent decay of the absorption of
[HSSH]:.
We exclude the possibility of an asymmetrical [H,SSH]’
radical, since one of the resonance forms would require
Angew. Chem. Int. Ed. Engl. 20 (19x1) No. 8
An ab initio Investigation
of the Mechanism of Ester Reduction[**]
By Pietro Cremaschi, Gabrieie Morosi, and
Massimo Simonetta[’’
The dissolving metal reduction of alkyl carboxylic esters
affords, predominantly, the corresponding alkanes rather
[*I Prof. Dr. M. Simonetta, P. Cremaschi, G. Morosi
[**I
Centro del C N R e Istituto di Chimica Fisica
Via Golgi 19, 1-20 133 Milano (Italy)
We are indebted to Prof. D. H . R . Barton for communicating his results
prior to their publication.
0 Verlag Chemie GmbH, 6940 Weinheim, 1981
0570-0833/81/0808-0673 $02.50/0
673
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