Electrophile-promoted carbon - Sulfur bond cleavage in η2-thiophene complexes of pentaammineosmium(II)

Article abstract of DOI:10.1021/ja970941k

Several S-alkylthiophenium complexes of the type [Os(NH₃)₅(4,5-η²-L)](OTf)₃ (where L = S-alkylthiophenium, S-methylbenzo[b]thiophenium) are prepared by alkylation of the corresponding thiophene complexes. The S-

Full text of DOI:10.1021/ja970941k

J. Am. Chem. Soc. 1997, 119, 8843-8851
8843
Electrophile-Promoted Carbon-Sulfur Bond Cleavage in
η2-Thiophene Complexes of Pentaammineosmium(II)
Michael L. Spera and W. Dean Harman*
Contribution from the Department of Chemistry, UniVersity of Virginia,
CharlottesVille, Virginia 22901
ReceiVed March 24, 1997X
Abstract: Several S-alkylthiophenium complexes of the type [Os(NH3)5(4,5-η2-L)](OTf)3 (where L ) S-alkyl-
thiophenium, S-methylbenzo[b]thiophenium) are prepared by alkylation of the corresponding thiophene complexes.
The S-alkylthiophenium species are proposed to undergo rapid and reversible cleavage of the C5-S bond, forming
highly electrophilic metallacyclopropene intermediates. Although not directly observable, these Vinyl cation
intermediates may be trapped with both anionic and neutral nucleophiles affording η2-4-(alkylthio)-1,3-butadiene
complexes. Treatment of selected 4-(alkylthio)-1,3-butadiene complexes with an oxidant (e.g., DDQ) affords the
organic ligand in good yield.
Introduction
been extensively examined,3 their reactivity toward electrophiles
is much less developed.6 Most thiophene complexes contain
cationic metal centers (e.g., Mn(CO)3+, RuCp+, Cp*Rh2+), and
as such are best suited to participate in reactions with nucleo-
philes. We find that 4,5-η2-thiophene complexes of osmium-
(II) react with carbon electrophiles at either C2, C3, or the sulfur
depending on the electrophile and the substitution pattern of
the thiophene.7 Herein, we focus on the synthesis, characteriza-
tion, and reactions of S-alkylated η2-thiophenium complexes of
pentaammineosmium(II).8 We find that in the presence of a
suitable nucleophile, S-alkylthiophenium complexes undergo
cleavage of the C5-S bond to yield η2-4-(alkylthio)-1,3-
butadiene complexes which are readily demetalated to give the
corresponding organic (alkylthio)butadienes (Scheme 1).
Transition metal complexes of thiophenes have received
considerable attention in recent years as homogeneous models
for hydrodesulfurization (HDS) catalysis.1 Studies on the
inorganic and organometallic chemistry of coordinated thiophenes
have focused mainly on reactions which lead to C-S activation,
the key step in the catalytic hydrodesulfurization process.
Consequently, there are numerous reports of C-S bond cleavage
promoted by transition metals, and the most common of these
involve direct insertion of the metal into a C-S bond2 or
nucleophile-induced C-S bond cleavage.3
In the course of our exploration of the ability of pentaam-
mineosmium(II) to promote electrophilic addition reactions on
η2-bound arenes and aromatic heterocycles, we have initiated
an investigation into the chemistry of η2-thiophene complexes
with electrophiles. Although η2-coordination for thiophenes is
rare,4 such a species has been postulated as one of the
intermediates in the hydrodesulfurization process,5 and while
the chemistry of coordinated thiophenes with nucleophiles has
Results
S-Alkylation. S-Alkylated thiophene complexes 8-16 and
the 2,3-dihydrothiophenium complex 17 are prepared from the
corresponding η2-thiophene precursors 1-6 and the 2,3-dihy-
drothiophene precursor 7 through a direct S-alkylation (Table
1). Alkylation of each with methyl triflate provides the
S-methylated compounds 8-13 and 17; alkylation of the parent
thiophene complex 1 with triethyloxonium hexafluorophosphate
in acetonitrile provides the S-ethylated compound 14, while
benzylation of 1 with 3,5-dimethylbenzyl bromide in the
presence of thallium triflate delivers 15 (Table 1). For all cases
examined, the alkylation is regiospecific for sulfur and proceeds
in 85-95% yield. S-Methylation occurs for a number of
substituted thiophenes. Even methyl groups at the R-positions
do not hinder the alkylation process (e.g., 11). Benzo[b]-
thiophene is readily methylated to form 13, and the 3-methoxy-
thiophene complex 5 combines with methyl triflate to form 12.
An excess of trimethyl orthoformate reacts with 1 in the presence
of tert-butyldimethylsilyl triflate (TBSOTf) to afford the S-
X Abstract published in AdVance ACS Abstracts, September 1, 1997.
(1) For reviews of transition metal thiophene complexes, see: (a)
Angelici, R. J. Coord. Chem. ReV. 1990, 105, 61 and references therein.
(b) Rauchfuss, T. B. Prog. Inorg. Chem. 1991, 39, 259.
(2) For recent examples of C-S insertion reactions, see: (a) Myers, A.
W.; Jones, W. D. Organometallics 1996, 15, 2905. (b) Chen, J.; Young, V.
G.; Angelici, R. J. Organometallics 1996, 15, 2727. (c) Chen, J.; Daniels,
L. M.; Angelici, R. J. Organometallics 1996, 15, 1223. (d) Bianchini, C.;
Meli, A. J. Chem. Soc., Dalton Trans. 1996, 801. (e) Myers, A. W.; Jones,
W. D.; McClements, S. M. J. Am. Chem. Soc. 1995, 117, 11704. (f)
Bianchini, C.; Frediani, P.; Herrera, V.; Jimenez, M. V.; Meli, A.; Rincon,
L.; Sanchez-Delgado, R.; Vizza, F. J. Am. Chem. Soc. 1995, 117, 4333. (g)
Bianchini, C.; Jimenez, M. V.; Meli, A.; Vizza, F. Organometallics 1995,
14, 4858.
(3) For examples of nucleophilic additions with or without C-S
activation, see: (a) Krautscheid, H.; Feng, Q.; Rauchfuss, T. B. Organo-
metallics 1993, 12, 3273. (b) Hachgenei, J. W.; Angelici, R. J. J. Organomet.
Chem. 1988, 355, 359. (c) Hachgenei, J. W.; Angelici, R. J. Angew. Chem.,
Int. Ed. Engl. 1987, 26, 909. (d) Spies, G. H.; Angelici, R. J. Organome-
tallics 1987, 6, 1897.
(4) To our knowledge, there are three known η2-thiophene complexes.
(a) For {2,3-η2-[ReCp(NO)(PPh3)]-benzo[b]thiophene}+, see: Robertson,
M. J.; Day, C. L.; Jacobson, R. A.; Angelici, R. J. Organometallics 1994,
13, 179. (b) For {2,3-η2-[Os(NH3)5]thiophene}2+, see: Cordone, R.;
Harman, W. D.; Taube, H. J. Am. Chem.Soc. 1989, 111, 5969. (c) For an
example of a bridging η2-thiophene in a triosmium cluster, see: Arce, A.;
De Sanctis, Y. J. Organomet. Chem. 1986, 311, 371.
(5) (a) Dong, L.; Duckett, S. B.; Ohnman, K. F.; Jones, W. D. J. Am.
Chem. Soc. 1992, 114, 151. (b) Kwart, H.; Schuit, G. C. A.; Gates, B. C.
J. Catal. 1980, 61, 128.
(6) (a) Luo, S.; Rauchfuss, T. B.; Gan, Z. J. Am. Chem. Soc. 1993, 115,
4943. (b) Luo, S.; Rauchfuss, T. B.; Wilson, S. R. J. Am. Chem. Soc. 1992,
114, 8515.
(7) (a) Other electrophiles that react at C(2) include acetals and Michael
acceptors: Spera, M. L.; Harman, W. D. Manuscript in preparation. (b)
For an account describing the protonation of η2-thiophene complexes, see:
Spera, M. L.; Harman, W. D. Organometallics 1995, 14, 1559.
(8) For the synthesis and characterization of some S-alkylthiophenium
salts, see: Acheson, R. M.; Harrison, D. R. J. Chem. Soc. (C) 1970, 1764
and references therein.
S0002-7863(97)00941-4 CCC: $14.00 © 1997 American Chemical Society

8844 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Spera and Harman
Table 2. Yield Data for 4-(Methylthio)-1,3-butadiene Complexes
Scheme 1
Table 1. Yield Data for S-Alkylthiophenium Complexes
in CD3CN is similar to that of the thiophene complex 1 except
that the four ring proton signals of 8 are shifted downfield ∼0.5
to 1 ppm from those of 1. The cis- and trans-ammine
resonances are also shifted downfield, indicating the electron-
deficient nature of the organic ligand. For 8, a new singlet is
observed at 3.22 ppm, corresponding to the S-methyl protons.
The 13C NMR and DEPT spectra (CD3CN) show signals for
two coordinated methine carbons at 61.75 ppm and 51.33 ppm,
signals for two uncoordinated methine carbons at 155.69 and
113.76 ppm, as well as a new feature for the S-methyl carbon
at 31.19 ppm. A cyclic voltammogram (CH3CN/n-Bu4PF6/100
mV/s) for complex 8 exhibits a reVersible oxidation wave at
1.37 V (NHE), ∼800 mV more positive than for its precursor
1. An irreversible ligand-centered reduction wave at -1.38 V
is also observed.
alkylated complex 16 in ∼90% yield, but this reaction is
sometimes compromised by trace amounts of 2H-thiophenium
imputities.7b Running the reaction in the presence of a mild
amine base (i-Pr2EtN) and a large excess of electrophile
minimizes this problem. The resulting product (16) fails to
undergo hydrolysis when treated with water.9 In contrast to
what is known for η2-pyrrole and furan complexes10 of
pentaammineosmium(II), there is no indication of a competing
alkylation of the thiophene ring. But electrophilic addition at
sulfur appears to be a general reaction only for certain alkylating
reagents. While alkyl triflates undergo addition at the sulfur
atom, acetals and Michael acceptors react with η2-thiophene
complexes at C(2).7a
The S-alkylthiophenium complexes (8-17) are exceptionally
stable compounds. While organic S-alkylthiophenium salts are
strong alkylating agents and readily react with water and
alcohols,11 complexes 8-17 are stable in water and exhibit
alkylating properties only in the presence of very polarizable
nucleophiles such as PPh3 (Vide infra). For example, a solution
of 8 is stable in D2O for a week at 22 °C with no evidence of
ring opening or hydrolysis. In CD3CN in the absence of suitable
nucleophiles, complex 8 is stable at elevated temperatures (80
°C) for as long as 24 h before decomposition products are
detected.
As a class, the S-alkylated thiophenium complexes 8-17
show similar spectroscopic and electrochemical characteristics.
The 1H NMR spectrum of the S-methylthiophenium complex 8
(9) In contrast, an η3-allyl complex of pentaammineosmium(II) synthe-
sized via the analogous electrophilic addition of trimethyl orthoformate to
the 2,3-η2-cyclopentadiene complex readily undergoes hydrolysis in wet
acetonitrile or acetone to afford the formyl-substituted allyl complex. Spera,
M. L.; Lopez, K. W.; Harman, W. D. Unpublished results.
Nucleophilic Additions. In contrast to the lack of reactivity
between the η2-thiophene complex 1 and nucleophiles, treatment
of S-methylthiophenium complexes with nucleophiles affords
1-substituted η2-4-(methylthio)-1,3-butadiene complexes in good
yield (Table 2; 55-93%). The products, the net result of
cleavage of the C5-S bond and nucleophilic addition to C5,
(10) For examples of electrophilic additions to analogous η2-pyrrole
complexes, see: Hodges, L. M.; Gonzales, J.; Myers, W. H.; Koontz, J. I.;
Harman, W. D. J. Org. Chem. 1995, 60, 2125. For furan, see: (a) Chen,
H.; Hodges, L. M.; Liu, R.; Stevens, W. C.; Sabat, M.; Harman, W. D. J.
Am. Chem. Soc. 1994, 116, 5499. (b) Liu, R.; Chen, H.; Harman, W. D.
Organometallics 1995, 14, 2861.
(11) For a review of thiophenium salts, see: Porter, A. E. A. AdV.
Heterocycl. Chem. 1989, 45, 151 and references therein.

η2-Thiophene Complexes of Pentaammineosmium(II)
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8845
exhibit cis/trans ammine signals upfield from those of the
starting thiophenium compounds; for the case of hydride
additions, the resulting diene complexes show reversible oxida-
tion couples near 0.60 V (NHE). For example, when the
S-methylthiophenium complex 8 is treated with an acetonitrile
solution of tetra-n-butylammoniumborohydride, a single new
product, 18, is formed. The 1H NMR spectrum of 18 exhibits
an upfield shifted methyl at 2.28 ppm, and signals corresponding
to five olefinic protons ranging from 3.4 to 6.1 ppm. 13C NMR
and DEPT data indicate two uncoordinated methine carbons
(135.28 and 123.28 ppm), one coordinated methine carbon
(51.24 ppm), and a coordinated methylene (43.63 ppm). The
cyclic voltammogram of the product exhibits a reversible
oxidation wave at 0.65 V (NHE) which is characteristic of an
η2-bound olefin or diene.12 On the basis of these data, we assign
18 to be a η2-coordinated 4-(methylthio)-1,3-butadiene (Table
2).
Stereochemical Assignment of Dienes. For all cases
examined, a cis vicinal coupling constant is observed (J ∼9
Hz) for the uncoordinated protons H3 and H4 and establishes
the geometry of this portion of the diene. A somewhat smaller
vicinal coupling constant is observed (J ∼5-7 Hz) for the
coordinated olefinic protons H1 and H2. However, we have
observed for Os(II)-η2-olefin and vinyl ether complexes that
the magnitude of both the cis and trans vicinal coupling constant
on the coordinated olefin is often considerably lower than that
of the uncoordinated olefin and varies depending on substitu-
tion.12,13 Thus, we were unable to assign the geometry of the
coordinated double bond using coupling constants alone. 1D-
NOE and proton decoupling experiments on selected diene
complexes (20, 21, 25, and 36) allow full assignment of all
proton resonances and stereochemistry. In each of the four cases
that we investigated in detail, the coordinated olefin is trans
while the uncoordinated olefin is cis (Figure 1). For example,
with the pyridinium diene complex 21, irradiation of the ortho
ring protons (8.47 ppm) produces a 13.1% NOE enhancement
for H1 and a 6.1% enhancement with H2. If the doublet (e.g.,
H1) at 6.92 ppm (J ) 7.3 Hz) is irradiated, a 13% NOE is
observed with the ortho protons of the pyridinium ring, as well
as an 8.8% NOE with the doublet of doublets at 5.41 ppm (H3).
Irradiation of H3 (5.41 ppm) causes an 11% enhancement with
H(4). Irradiation of H4 (6.59 ppm; J ) 9.5 Hz) causes an 9.1%
enhancement with the doublet of doublets at 5.41 ppm (H3)
and a 3.0% enhancement with the S-methyl protons at 2.40 ppm.
Similar results are obtained for the other (e.g., 20, 25, and 36)
diene complexes. To further verify our claim, removal of the
metal from two such diene complexes (25 and 36) affords the
organic dienes 42 and 43 whose geometry is identical to that
determined for the corresponding osmium(II) complexes. For
example, treatment of 36 with 1.0 equiv of 2,3-dichloro-5,6-
dicyanoquinone (DDQ) in acetone affords the free diene ligand
43 with the same trans,cis geometry (J ) 12.5, 9.5 Hz) in good
(80%) yield. The trans geometry of the coordinated olefin is
observed even in the styrene derivative 31. In this case a 9.0
Hz vicinal coupling constant is observed for the coordinated
olefin while a 16.5 Hz coupling constant is observed for the
free ligand.
Figure 1. Establishment of diene geometry via 1D-NOE enhancements
on selected 4-(methylthio)-1,3-butadiene complexes. [Os]2+ ) [Os-
(NH3)5]2+. Triflate counteranions omitted for clarity.
nol. However, the only carbon nucleophile which was suc-
cessfully added was cyanide ion (19; 81%). In general, all
nucleophilic addition reactions are carried out in an acetonitrile
solution or slurry. Some reagents require the addition of a
nonnucleophilic base (i-Pr2EtN) to promote the reaction (e.g.,
methanol, phenol). Primary amines such as n-propyl amine
require the addition of excess reagent to deprotonate the resulting
n-propylammonium diene. In contrast to the single isomer
observed for other nucleophilic additions, a mixture of cis/trans
isomers results from methanol addition to 8. Thiobutadiene 36
is isolated as a 7.5:1 mixture of trans,cis and cis,cis diene
complexes. If 8 (0.061 M) is dissolved in CD3OD in the
absence of base and the solution monitored by 1H NMR
spectroscopy over several days, 36 is slowly formed as a 1:1
mixture of isomers. The reaction eventually reaches an equi-
librium mixture of 8:36cis,cis:36trans,cis of 1.2:1:1.
A competing demethylation reaction is observed upon treat-
ment of 8 with PPh3. Triphenylphosphine reacts with 8 in CD3-
CN to afford a mixture of 1, diene 24, and methyltriphenylphos-
phonium triflate. The ratio of 1 to diene 24 varies with reaction
conditions. Treatment of a CD3CN solution of 8 with 2.0 equiv
of PPh3 affords a 4:1 mixture of 1 and diene 24 after 36 h at 22
°C. Repeating this reaction at 70 °C results in only a minor
change in the product ratio (5:1). However, we find that by
increasing the concentration of nucleophile to ∼1 M (10 equiv;
22 °C), an increase in the production of the phosphonium diene
complex occurs such that the ratio of 1 to 24 is 1.9:1. If the
order of addition is switched such that the thiophenium complex
8 is added slowly to a high concentration of PPh3 in CD3CN,
then the diene complex 24 becomes the dominant product (4:
1). Workup of the reaction mixture affords pure 24 in 58%
isolated yield. In no case does the ratio of thiophene complex
to diene complex change over time.
The scope of nucleophilic addition with the osmium thiophe-
nium complexes is broad (Table 2). In addition to hydride,
nucleophiles that react at the R-carbon of η2-thiophenium
complexes include aliphatic and aromatic amines, alcohols,
acetate ion, azide ion, triphenylphosphine, phenol, and thiophe-
Similar to the reaction with PPh3, the rate of pyridine addition
is sufficiently slow that it may be monitored by 1H NMR
spectroscopy or cyclic voltammetry (CV). The reaction half-
life is found to vary with concentration of pyridine. For
(12) Chen, H.; Harman, W. D. J. Am. Chem. Soc. 1996, 118, 5672.
(13) The trans- 3JH,H for several η2-olefin and vinyl ether complexes of
pentaammineosmium(II) vary from 6.0 to 10.2 Hz. Cis-coupling constants
vary from 4.8 to 9.0 Hz.

8846 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Spera and Harman
Scheme 2
example, for a pyridine concentration of 1.2 M, the half-life is
∼24 min; if the concentration of nucleophile is decreased to
0.12 M, the half-life shows a corresponding increase to ∼2.8
h. At very high pyridine concentrations (e.g., 3.5 M), the
reaction is complete within 1 min, judging by a cyclic
voltammogram. No reaction is observed when 8 is treated with
an excess (10-20 equiv) of the silyl ketene acetal 1-methoxy-
2-methyl-1-(trimethylsiloxy)propene (MMTP),14 or vinyl ethers
such as 1-ethoxypropene or 2-(trimethylsiloxy)propene. Eno-
lates such as potassium acetylacetonate and sodium diethylma-
lonate are also unreactive at 22 °C over a period of 1 day and
moderate heating (80 °C) results in intractable mixtures of
paramagnetic products. Dimethylzinc fails to react with 8 after
1 day in acetonitrile solution, even at 80 °C and in the presence
of CuOTf2.15
Figure 2. Reactivity of 1-(dimethylsulfonio)-3-propene vs S-methyl-
2,3-dihydrothiophenium and S-methylthiophenium complexes with
pyridine.
We have recently shown that η2-vinyl ether complexes of
pentaammineosmium(II) react with nucleophiles via an SN1-
type elimination-addition mechanism involving a metallacy-
clopropene intermediate (i.e. an η2-vinyl cation; Scheme 2).12
Consequently, we explored the possibility of using the alkoxy-
substituted diene complex 20 as a precursor to functionalized
dienes. However, treatment of a solution of 20 and MMTP
(∼3 equiv) with TBSOTf in acetonitrile at either -40° or 22
°C affords only thiophenium complex 8. Increasing the
concentration of nucleophile to ∼1.5 M does not affect these
results. For diene 21, the pyridinium substituent may also be
eliminated at elevated temperatures. Although the pyridinium
diene complex (21) does not undergo exchange in the presence
of pyridine-d5 at 22 °C, heating a CD3CN solution of 21 at 80
°C affords a mixture of free pyridine and S-methylthiophenium
8. If the reaction is repeated at 80 °C in the presence of a large
excess of pyridine-d5 (6.09 M), complex 8 is not observed, but
the pyridine substituent is exchanged by pyridine-d5 without
any change in olefin geometry. At 22 °C in CD3CN, no such
exchange is detected after 24 h.
Figure 3. Desulfurization of η2-benzothiophene and η2-S-methyl-
thiophenium complexes.
As part of our investigation into the mechanism of the
nucleophilic addition to thiophenium complexes, the reaction
of both 1-(dimethylsulfonio)propene (32) and S-methyl-2,3-
dihydrothiophenium (17) complexes with pyridine was also
explored under conditions similar to those reported above for
8. Complex 32 (8:1 trans/cis) reacts immediately (t1/2 < 1 s)
with pyridine in CD3CN at 22 °C under dilute conditions ([Nu]
) 0.073 M) to give the pyridinium-substituted propene complex
33 as an 8:1 mixture of geometric isomers (Figure 2). These
reaction conditions were similar to those of the reaction of the
thiophenium complex 8 with pyridine except that the latter
reaction proceeded more slowly with a reaction half-life of over
8 h at similar pyridine concentrations (Figure 2). For the
S-methyl-2,3-dihydrothiophenium complex 17, the reaction with
pyridine ([Nu] ) 0.078 M) affords the pyridinium adduct 35
with a half-life of 8 h, a reaction rate that is similar to that seen
for the thiophenium complex 8.
organosulfur compounds.16 Thiophenes are desulfurized with
difficulty and the resulting products undergo rapid hydrogenation
to various hydrocarbons. The η2-thiophene complex 1 is
unreactive toward Raney nickel in MeOH (45 °C).17 However,
the η2-benzo[b]thiophene complex 6 is readily desulfurized to
form the olefin-bound η2-styrene complex 34 (Figure 3).18 The
thiophenium complex 8 also readily reacts with Raney nickel
in MeOH to give two different C-S cleavage products
depending on reaction time. Treatment of 8 with a MeOH slurry
of Raney nickel affords after 30 min 37 as a pale yellow solid
(59%), the product of addition of methoxide to 8 followed by
(16) Mozingo, R. Org. Syn. 1941, 21, 15. For a review describing the
synthetic applications of Raney Nickel, see: Pizey, J. S. Synthetic Reagents;
Wiley: New York, 1974; Vol 2, pp 175-311.
(17) The Raney nickel was purchased as a 50% wt/wt slurry in water
from Aldrich. The water was removed (in a glovebox under nitrogen!) by
repeatedly washing with anhydrous methanol and drying under a stream of
nitrogen. The material should be used immediately since its activity
decreases rapidly when stored as a dry solid. The slurry is usually stable
for ∼6 months. Unwanted portions of this highly pyrophoric material may
be conveniently destroyed by refluxing in a large volume of acetone
overnight followed by dilution with water.
Ring Opening/Desulfurization by Raney Nickel. Raney
nickel is known to desulfurize and hydrogenate a variety of
(14) This silyl ketene acetal undergoes facile nucleophilic addition to
various pentaammineosmium(II) areneium complexes.
(15) Copper triflate promotes the addition of dimethyl zinc to an η3-
allyl species derived from an η2-naphthalene complex. Winemiller, M. D.;
Harman, W. D. Unpublished results.
(18) When 34 is prepared from styrene, a mixture of linkage isomers
results which favors the olefin. Upon warming to 80 °C in CD3CN, the
metal migrates to the olefin.

η2-Thiophene Complexes of Pentaammineosmium(II)
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8847
Table 3. Yield Data for Functionalized
4-(Methylthio)-1,3-butadienes and 2-(Methylthio)styrenes
osmium salt, and free (methylthio)butadiene 41 is obtained in
55% isolated yield. For the [2-(methylthio)phenyl]cyanoethene
complex 31 (Ep,a ) 1.23 V), stronger oxidizing conditions are
required. Addition of triflic acid (∼2 equiv) to a solution of
31 in CH3CN followed by addition of 1.1 equiv of DDQ affords
the organic 40 in 82% isolated yield.
For some of the dienes a decomplexation procedure was
required that was nonacidic in order to prevent polymerization
of the organic product. While the potential of AgOTf in CH3-
CN is only ∼0.59 V (NHE),21 in acetone it is ∼0.73 V, which
makes it a practical oxidant for several of the diene complexes
described herein. For example, addition of 1.2 equiv of AgOTf
in acetone to a solution of 25 affords the phenoxythiobutadiene
42 in 58% yield. Dienes 39-43 are stable toward polymeri-
zation in dilute CDCl3 solution for several hours at 22 °C,
allowing 1H NMR characterization; however, attempts to purify
these materials for microanalysis resulted in polymerization.
Discussion
Examples of C-S activation that do not involve direct
insertion of the metal into the carbon-heteroatom bond2 have
been generally limited to nucleophilic addition reactions.3 For
example, Angelici has shown that the complex CpRu(η5-
thiophene)+ undergoes C-S cleavage upon hydride addition to
the R-carbon via an associative mechanism.3d The nucleophile
attacks the heterocycle directly at an R-carbon affording an
unobserved allyl sulfide intermediate, which then undergoes
C-S cleavage to afford the observed ring-opened product. In
contrast, the reaction of Mn(CO)3(η5-thiophene)+ and hydride
affords only the corresponding allyl intermediate;3a,d the lack
of C-S activation in this complex is in agreement with the much
lower activity of Mn as an HDS catalyst as compared to Ru.22
Rauchfuss et al. have demonstrated that the electron-rich
thiophene complex (η6-C6Me6)Ru(η4-thiophene) reacts with
protons at an R- carbon to form a ring-opened complex, but
this process has been shown to occur by protonation at the metal
followed by hydride transfer to the R-carbon.6b In contrast to
most of the transition metal fragments that coordinate thiophene,
the pentaammineosmium(II) moiety is strongly electron-donating
but yet it resists oxidative addition reactions. Thus, pentaam-
mineosmium(II) complexes of unsaturated organic molecules
such as heterocycles, arenes, and dienes undergo electrophilic
addition reactions directly on the ligand.23 In contrast to pyrrole
or furan complexes, the η2-thiophene systems show reactivity
with hard electrophiles primarily at the heteroatom. While other
electrophiles (e.g., protons,24a anhydrides, acetals, and Michael
acceptors24b) react at C(2) or C(3), trialkyloxonium salts,
orthoesters, and alkyl triflates react to form S-alkylated products.
Whereas pyridine, water, and alcohols are readily alkylated by
S-alkylthiophenium salts, the pentaammineosmium(II) com-
plexes of these ligands exhibit completely different reactivity
desulfurization. If the reaction is carried out for longer reaction
times (> 3h), the only product isolated is the desulfurized and
hydrogenated 1-methoxybutene complex 38 as an off-white solid
in 52% yield. The residual base (NaOH) used during prepara-
tion of the commercial nickel catalyst generates the methoxide
nucleophile.16 The resulting methoxy diene 36 undergoes rapid
desulfurization to 37 and eventual hydrogenation to 38. The
analogous reaction of 8 with methanol in the presence of strong
bases such as NaOMe or DBU (1,8-diazabicyclo[5.4.0]undec-
7-ene) affords an uncharacterized paramagnetic brown solid.
Decomplexation of 4-(Methylthio)-1,3-butadienes. Libera-
tion of the organic product is achieved by oxidation of the
pentaammineosmium(II) moiety with a suitable oxidant (Table
3). For complexes 18 and 30, the half-wave oxidation potentials
are 0.65 and 0.72 V (NHE), respectively, and are readily
oxidized by DDQ.19 For the styrene derivative 30, addition of
0.6 equiv of the two electron oxidant DDQ immediately forms
a dark brown precipitate, and [Os(NH3)5(CH3CN)]3+ is observed
by electrochemistry as an oxidation product (E1/2 ≈ -0.10 V).20
Workup of the solution affords 2-(methylthio)styrene (39) as a
yellow oil in 78% isolated yield. For complex 18, addition of
1.2 equiv of DDQ to an acetonitrile solution precipitates an
(21) The oxidation potential of many oxidants, particularly small cations,
varies considerably with the coordinating ability of the solvent. For a review
of the use of various redox reagents in organometallic chemistry, see:
Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
(22) For a comparison the activity of several transition metals in HDS
catalysis, see Pecoraro, T. A.; Chianelli, R. R. J. Catal. 1981, 67, 430.
(23) For examples of activation of arenes and heterocycles toward
electrophilic additions, see for phenol: Kopach, M. E.; Gonzalez, J.;
Harman, W. D. J. Am. Chem. Soc. 1991, 56, 4321. For aniline, see: Kolis,
S. P.; Gonzalez, J.; Bright, L. M.; Harman, W. D. Organometallics 1996,
15, 245. For activation of pyrrole and furan, see ref 10.
(24) (a) Protonation is believed to occur initially at sulfur, followed by
rearrangement of the metal and transfer of the proton to C(2). See ref 7b.
(b) Both substitution and addition products are observed at either C(2) or
C(3) depending on the electrophile and substitution pattern of the thiophene
ring. Spera, M. L.; Harman., W. D. Preliminary results.
(19) DDQ is a two-electron oxidant whose potential is 0.71 V (NHE)
for the first electron transfer. In theory, only 0.5 equiv of oxidant is required
to oxidize Os(II) to Os(III), however, we have found that ∼1.0 equiv is
required for many Os(II) oxidations.
(20) Observation of this product indicates solvent substitution on Os-
(III) for the desired ligand.

8848 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Spera and Harman
plexes such as 32 undergo nucleophilic substitution at the bound
R-carbon via a metallacyclopropene intermediate that behaves
functionally as a metal-stabilized vinyl cation.12 The thiophe-
nium complex 8 can be considered an analog of 32 except that
the sulfide leaving group is now tethered to the coordinated
olefin (e.g., C4). In a similar manner, cleavage of the C5-S
bond of a thiophenium complex such as 8 would afford an η2-
vinyl cation species similar to that observed for vinyl ethers12
(Figure 4). The implications of this tethered leaving group
manifest themselves in the reactivity of the complex. We have
found the rate of nucleophilic addition to be considerably faster
for the sulfonium substituted olefin 32 than that for the
thiophenium complex 8 (Figure 2) at the same temperature, but
this difference can be attributed to a competition in the latter
case between the rates of external nucleophilic addition (i.e.,
k2 in Figure 4) and reversible addition of the tethered sulfur
(i.e., k-1). The failure to directly observe the proposed vinyl
cation intermediate for the thiophenium complexes, even at low
temperatures (e.g., -80 °C), or to carry out substitution reactions
at C5 with mild carbon nucleophiles that were successfully
added to simple substituted olefins is also likely due to this
competing back rate (k-1). We have shown that for the
analogous reaction with the dihydrothiophenium complex 17,
the rate of pyridine substitution is also considerably more
sluggish than the simple vinylsulfonium complex 32 (Figure
2), yet, this rate is virtually identical to that of the thiophenium
system (8). This observation suggests that the uncoordinated
double bond of the η2-thiophenium has a minimal bearing on
the nucleophilic substitution chemistry at C5. For an associative
C5 substitution mechanism, the substitution rates for the olefin
complex 32, and the dihydrothiophenium complex 17 should
be similar since there are no steric or electronic differences in
the proximity of C5.
Figure 4. Proposed “vinyl cation” (metallacyclopropene) intermediate
from Os(NH3)5(2,3-η2-1-methylthiophenium)3+
.
toward these reagents. The general increase in stability of these
η2-S-alkylthiophenium complexes most likely results from a
π-back-bonding interaction with the metal, similar to how
organic S-alkylthiophenium salts are stabilized by addition of
electron-donating alkyl groups to the thiophene ring.11
Pioneering work by Angelici et al. has shown that highly
reducing η4-thiophene complexes exhibit increased nucleophi-
licity at the heteroatom, and the complex Cp*Ir(η4-2,5-dimeth-
ylthiophene) has been shown to displace dimethyl sulfide from
Me2S‚BH3 to afford a BH3-thiophene adduct.25 An example
of chalcogen alkylation in a coordinated heteroaromatic complex
has also been reported26 where treatment of Cp*(CO)2Re(2,3-
η2-selenophene) with Me3O[BF4] affords the 1-methylsele-
nophenium complex. Most recently, the addition of a Grignard
reagent to the sulfur atom in [(η5-thiophene)Mn(CO)3]+ has been
reported to afford η4-S-alkylthiophenium complexes.27 To our
knowledge, a link between any of these thiophenium complexes
and carbon-chalcogen bond activation has not been shown,28
although Rauchfuss et al. have demonstrated reversible C-S
activation in {(ring)-Ru(η5-tetramethylthiophene)}2+ complexes
by a nucleophilic attack at sulfur.3a Thus, the present study
establishes an important link between electrophilic addition to
sulfur and C-S activation for thiophenes. Furthermore, the
participation of an η2-thiophene in C-S activation has been
demonstrated for the first time.
Mechanism. Heteroatom-substituted olefins (e.g., vinyl
ethers) coordinated to pentaammineosmium(II) readily undergo
nucleophilic substitution in acid.12 This substitution proceeds
through an elimination-addition sequence where the reactive
intermediate is an η2-vinyl cation (i.e., a metallacyclopropene)
which has been observed and characterized by NMR spectros-
copy for selected cases (Scheme 2). Noting similarities in both
the regiochemistry and stereochemistry in the nucleophilic
substitution reactions of η2-vinyl ether12 and the η2-thiophenium
complexes examined in this account, we propose that the
nucleophilic additions observed at C5 for these η2-thiophenium
complexes occur through a similar reaction mechanism (Figure
4). Our rationale for a dissociative C5 substitution mechanism
follows. We have recently reported that vinylsulfonium com-
For most of the reaction products listed in Table 2, the trans
stereochemistry observed for the coordinated double bond is
proposed to be the result of a significant kinetic preference for
â-alkylated η2-vinyl cations to add a nucelophile trans to the
alkyl substituent (Figure 4), and this stereochemical feature was
recently identified for nucleophilic substitutions of η2-coordi-
nated furan as well as η2-vinyl ether complexes.12 For the
reaction of 8 with methanol in which acid is a byproduct, the
formation of both isomers is observed. Consistent with this
observation, acid is known to catalyze a similar cis/trans
equilibration in vinyl ether complexes,12 and when a moderate
base (i-Pr2EtN) is included in the reaction between 8 and
methanol to neutralize the acid, the trans isomer is the dominant
product (7.5:1). Isomerization is also observed when the
methoxy-substituted 4-(alkylthio)-1,3-butadiene complex 36 is
treated with 0.1 equiv of HOTf: here, compound 8 is slowly
regenerated, along with a 1:1 mixture of cis/trans isomers of
36. The formation of thiophenium complex 8 from a ring-
opened product (e.g., 20, 21, and 36) demonstrates the revers-
ibility of the reaction. When the 4-(alkylthio)-1,3-butadiene
complex contains a good leaving group (e.g., pyridine) ring
closure of the tethered sulfide nucleophile occurs via elimination
of the leaving group, the microscopic reverse of the initial ring
opening reaction.
Route to 4-(Alkylthio)-1,3-butadienes. (Alkylthio)buta-
dienes are useful intermediates for organic synthesis but are
difficult to prepare via traditional organic methods.2f Bianchini
and co-workers recently prepared a series of (alkylthio)-
butadienes from the reaction of the 16e- fragment (triphos)-
RhH with various thiophenes.2f Rhodium-promoted C-S
insertion followed by electrophilic addition at the sulfur and
subsequent removal of the metal affords the thiobutadiene
(25) Chen, J.; Daniels, L. M.; Angelici, R. J. J. Am. Chem. Soc. 1990,
112, 199.
(26) Choi, M.-G.; Angelici, R. J. J. Am. Chem. Soc. 1991, 113, 5651.
(27) Lee, S. S.; Lee, T. Y.; Choi, D. S.; Lee, J. S.; Chung, Y. K.; Lee,
S. W.; Lah, M. S. Organometallics 1997, 16, 1749.
(28) An Os(II)-η2-selenophene complex has been Se-alkylated in our
laboratories in high (>95%) yield and undergoes a similar C-Se bond
cleavage in the presence of nucleophiles. Spera, M. L.; Harman, W. D.
Unpublished results.

η2-Thiophene Complexes of Pentaammineosmium(II)
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8849
{4,5-η2-[Os(NH3)5]-1-methylthiophenium}(OTf)3 (8). Procedure
A. To a solution of 1 (397 mg, 0.604 mmol) in 1.97 g of CH3CN was
added a solution of CH3OTf (297 mg, 1.81 mmol) and the reaction
was monitored by CV. After 1 h, the solution was precipitated into
50 mL of stirring diethyl ether and filtered through a fine fritted funnel,
yield 455 mg (92%) of a yellow-orange solid: 1H NMR (CD3CN) δ
7.59 (dd, J ) 5.4 Hz, 2.6 Hz, 1H), 6.60 (dd, J ) 5.4 Hz, 1.6 Hz, 1H),
6.09 (dd, J ) 4.5 Hz, 1.6 Hz, 1H), 5.52 (dd, J ) 4.5 Hz, 2.6 Hz, 1H),
4.67 (br s, 3H), 3.39 (br s, 12H), 3.22 (s, 3H); 13C NMR (CD3CN) δ
155.69 (CH), 113.67 (CH), 61.75 (CH), 51.33 (CH), 31.19 (CH3); CV
E1/2 ) 1.37 V (NHE); Ep,c ) -1.38 V (NHE). Anal. Calcd for
C7H22N5S4O9F9Os: C, 11.69; H, 2.70; N, 7.52. Found: C, 11.90; H,
2.63; N, 7.51. Procedure B: To a vigorously stirred slurry of 1 (508
mg, 0.773 mmol) in 1.43 g of DME was added methyl triflate (330
mg, 2.32 mmol). After 12 h, the resulting dark-brown slurry was added
to 100 mL of stirring diethyl ether and filtered through a 15 mL medium
frit. The filter cake was washed with ether (2 × 10 mL) and dried in
Vacuo to afford 599 mg (0.730 mmol) of a golden-brown powder, 95%.
{4,5-η2-[Os(NH3)5]-1-ethylthiophenium}(OTf)3 (14). To a solution
of 1 (125 mg, 0.19 mmol) in CH3CN (279 mg) was added solid
triethyloxonium hexafluorophosphate (71 mg, 0.29 mmol). The
resulting red-brown solution was allowed to stand for 12 h, and then
triflic acid (38 mg, 0.25 mmol) was added.31 After 10 min, the solution
was added to 40 mL of stirring diethyl ether, the resulting precipitate
was filtered, washed with ether (2 × 5 mL), and dried in Vacuo to
afford 139 mg of a red-brown powder, 88%: 1H NMR (CD3CN) δ
7.57 (dd, J ) 5.1 Hz, 2.9 Hz, 1H), 6.35 (dd, J ) 5.1 Hz, 2.2 Hz, 1H),
5.84 (dd, J ) 4.8 Hz, 2.9 Hz, 1H), 5.45 (dd, J ) 4.8 Hz, 2.2 Hz, 1H),
3.64 (m, 1H), 3.51 (m, 1H), 4.52 (br s, 3H), 3.24 (br s, 12H), 1.25 (t,
J ) 7.3 Hz, 3H); 13C NMR (CD3CN) δ 157.02 (CH), 109.67 (CH),
57.70 (CH), 52.56 (CH2), 37.29 (CH), 7.90 (CH3); CV E1/2 ) 1.34 V
(NHE); Ep,a ) -1.62 V (NHE). Anal. Calcd for C9H24N5S4O9F9Os:
C, 12.93; H, 2.89; N, 8.38. Found: C, 12.63; H, 2.73; N, 8.38.
ligands in good yield. The osmium methodology described
herein is complementary to this approach in that it provides a
general route to functionalized thiobutadienes in which func-
tionality can be adjusted at virtually all positions by choice of
electrophile, substituted thiophene, and nucleophile. Thus,
thiobutadienes may be prepared with both electron-donating and
electron-withdrawing substituents (Table 3), and the metal may
be recovered as the solvent-pentaammineosmium(III) complex.
Concluding Remarks
Central to the coordination chemistry of thiophenes is the
search for a discrete homogeneous model for the mechanism
of catalytic hydrodesulfurization, where the two main unresolved
issues regard the initial coordination of the thiophene to the
catalyst surface and whether the initial step is hydrogenation
of the ligand or C-S cleavage.5b This account illustrates for
the first time chemistry associated with the η2-coordination mode
of thiophenes (a coordination mode implicated in the hetero-
geneous HDS process),5b novel reactions of bound thiophenes
with electrophiles, and a general nucleophilic addition process
for S-alkyl-η2-thiophenium species involving a proposed met-
allacyclopropene intermediate. Although this intermediate is
only postulated in the thiophenium system, a close analog has
been recently characterized.12 More importantly, we have
demonstrated the first example of electrophile initiated C-S
activation in an η2-thiophene complex using a metal known to
form an highly reactive heterogeneous HDS catalyst.22,29 An
increased understanding of the interactions of thiophene with
transition metals will ultimately further the development of more
efficient HDS systems.
{4,5-η2-[Os(NH3)5]-1-(3,5-dimethylbenzyl)thiophenium}(OTf)3
(15). To a solution of 1 (163 mg, 0.25 mmol) in CH3CN (402 mg)
was added solid 3,5-dimethylbenzyl bromide (247 mg, 1.24 mmol) and
thallium triflate (437 mg, 1.24 mmol). The resulting slurry was stirred
for 12 h and filtered through a 15 mL fine porosity frit into 50 mL of
stirring diethyl ether to afford a golden brown precipitate. The
precipitate was filtered, washed with ether, and dried in Vacuo to afford
203 mg (88%) of crude 15: 1H NMR (CD3CN) δ 7.42 (m, 1H), 7.10
(s, 1H), 7.03 (s, 2H), 6.29 (m, 1H), 6.10 (m, 1H), 5.13 (m, 1H), 5.01
(d, J ) 12.5 Hz, 1H), 4.69 (br s, 3H), 4.65 (d, J ) 12.5 Hz, 1H), 3.39
(br s, 12H), 2.31 (s, 6H); 13C NMR (CD3CN) δ 157.48 (C), 139.45
(CH), 131.94 (CH), 129.66 CH), 128.04 (CH), 128.04 (CH), 127.63
Experimental Section
General. The synthesis and characterization of the thiophene (1),
2-methylthiophene (2), 3-methylthiophene (3), 2,5-dimethylthiophene
(4), 3-methoxythiophene (5), and benzo[b]thiophene (6) complexes have
been reported previously.7b All 1H NMR spectra were recorded at 300
MHz and are referenced vs TMS using residual CD2HCN, acetone-d5,
or CHCl3 as an internal standard. All 13C NMR spectra were recorded
at 75 MHz and are referenced vs TMS using the same internal standards.
All cyclic voltammograms were recorded in CH3CN using n-Bu4PF6
as electrolyte with a scan rate of 100 mV/s and are referenced to the
normal hydrogen electrode (NHE) using an internal standard (Cp2Fe,
E1/2 ) 0.55 V (NHE) or [Cp2Co][PF6], E1/2 ) -0.78 V (NHE).
Thiophene ligands were purchased from Aldrich, distilled from CaH2,
and deoxygenated prior to use with the exception of benzo[b]thiophene
which was used as received. All other reagents were purchased from
Aldrich and used as received unless otherwise noted, with the exception
of [Os(NH3)5OTf](OTf)2, which was synthesized as described by Lay
et al.30
(C), 110.16 (CH), 58.42 (CH2), 48.46 (CH), 21.06 (CH3); CV E1/2
)
1.30 V (NHE). A sample of 15 was recrystalized from acetonitrile/
ether prior to microanalysis. Anal. Calcd for C16H30N5O9S4F9Os: C,
20.76; H, 3.27; N, 7.56. Found: C, 20.70; H, 3.25; N, 7.54.
{4,5-η2-[Os(NH3)5]-1-(1,1-dimethoxymethyl)thiophenium}-
(OTf)3 (16). To a solution of 1 (218 mg, 0.226 mmol) in 1.71 g CH3-
CN was added trimethylorthoformate (218 mg, 2.26 mmol, dried, and
distilled from Na°) and TBSOTf (594 mg, 2.26 mmol). After 1 h the
resulting red-brown solution was added to ether (50 mL), and the
precipitate was filtered, washed with Et2O (2 × 10 mL) and dried in
Vacuo to afford an intractable tacky brown solid consisting of
approximately 5:1 of S-alkylated to ring protonated material: 1H NMR
(CD3CN) δ 7.56 (dd, J 5.9 Hz, 2.2 Hz, 1H), 6.56 (dd, J ) 5.9 Hz, 1.5
Hz, 1H), 6.07 (dd, J ) 4.5 Hz, 1.5 Hz, 1H), 5.51 (dd, J ) 4.5 Hz, 2.2
Hz, 1H), 4.65 (br s, 3H), 4.16 (s, 1H), 3.38 (br s, 12H), 3.20 (s, 6H);
13C NMR (CD3CN) δ 155.59 (CH), 113.62 (CH), 61.74 (CH3 X 2),
{4,5-η2-[Os(NH3)5]-2,3-dihydrothiophene}(OTf)2 (7). Pd (10% on
carbon, 36 mg) was suspended in 865 mg of MeOH in a 50 mL round
bottom flask fitted with a gas inlet and balloon. The flask was charged
with hydrogen (∼1 atm) and the suspension was stirred vigorously for
1 h. The flask was vented, and a solution of 1 (211 mg, 0.321 mmol)
in 1.1 g of MeOH was added. The flask was again charged with
hydrogen and stirred vigorously for 12 h. The flask was vented, and
the solvent was concentrated under reduced pressure and filtered through
a bed of Celite into 100 mL of stirring diethyl ether. The resulting
ivory precipitate was filtered, washed with ether (3 × 10 mL), and
dried in Vacuo, affording 175 mg (0.266 mmol) of an pale yellow
powder, 83%: 1H NMR (CD3CN) δ 4.75 (d, J ) 5.9 Hz, 1H), 4.09 (br
s, 3H), 3.89 (m, 1H), 3.01 (br s, 12H), 2.89 (m, 2H), 2.22 (m, 1H),
1.80 (m, 1H).
61.08 (CH), 51.34 (CH), 27.92 (CH); CV E1/2 ) 1.31 V (NHE); Ep,c
)
-1.33 V (NHE). This is a mixture of S-acetal and 2H-thiophenium
complexes.
{1,2-η2-[Os(NH3)5]-(Z)-4-(methylthio)-1,3-butadiene}(OTf)2 (18).
To a solution of 8 (232 mg, 0.282 mmol) in 342 mg of acetonitrile
was added a solution of tetra-n-butylammonium borohydride (80 mg,
0.310 mmol) in 190 mg of acetontrile. After 5 min, the reaction mixture
was added to stirring CH2Cl2, and the precipitate was filtered, washed
(29) Mitchell, P. C. H. Catalysis (London) 1981, 4, 175.
(30) (a) Lay, P. A.; Magnuson, R. H.; Taube, H. Inorg. Chem. 1989, 28,
3001. (b) Lay, P. A.; Magnuson, R. H.; Taube, H. Inorg. Synth. 1986, 24,
269. For an improved detailed procedure for the synthesis of this reagent,
see: Gonzalez, J. Ph. D. Dissertation, University of Virginia, May, 1995.
This compound is currently available from Aldrich.
(31) Addition of excess triflic acid exchanges the PF6 counteranion for
triflate.

8850 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Spera and Harman
with CH2Cl2 (2 × 5 mL), and dried in Vacuo affording 146 mg (0.217
mmol) of a yellow powder, 77%: 1H NMR (CD3CN) δ 6.09 (d, J )
9.5 Hz, 1H), 5.18 (overlapping m, 2H), 4.25 (m, buried, 1H), 4.20 (br
s, 3H), 3.43 (dd, J ) 8.1 Hz, 1.8 Hz, 1H). 3.16 (br s, 12H), 2.29 (s,
3H). 13C NMR (CD3CN): δ 135.28 (CH), 123.28 (CH), 51.24 (CH),
43.63 (CH2), 17.3 (CH3); CV E1/2 ) 0.65 V (NHE). Anal. Calcd for
C7H23N5S3O6F6Os: C, 12.48; H, 3.44; N, 10.40. Found: C, 12.36; H,
3.40; N, 10.69.
2H), 6.96 (m, 2H), 6.57 (d, J ) 5.9 Hz, 1H), 6.33 (d, J ) 9.5 Hz, 1H),
5.30 (dd, J ) 10.5 Hz, 9.5 Hz, 1H), 4.53 (dd, J ) 5.9 Hz, 10.5 Hz,
1H), 4.30 (br s, 3H), 3.27 (br s, 12H), 2.31 (s, 3H); 13C NMR (CD3-
CN) δ 162.40 (C), 130.17 (CH), 129.95 (CH), 126.39 (CH), 122.03
(CH), 116.17 (CH), 91.23 (CH), 44.83 (CH), 17.23 (CH3); CV Ep,a
)
1.05 V (NHE). Anal. Calcd for C13H27N5O7F6S3Os: C, 20.54; H, 3.37;
N, 9.13. Found: C, 20.39; H, 3.55; N, 9.15.
trans-{η2-[Os(NH3)5]-1-(dimethylsulfonio)-propene}(OTf)3 (32).
To a solution of the ethyl propenyl ether complex12 (296 mg, 0.448
mmol) in 535 mg of CH3CN was added dimethyl sulfide (62 mg, 0.987
mmol) and TBSOTf (130 mg, 0.493 mmol). After 5 min the reaction
mixture was added to 50 mL of diethyl ether under stirring, and the
resulting yellow precipiate was filtered, washed with ether (2 × 10
mL), and dried in Vacuo to afford 324 mg (0.393 mmol) of a yellow
powder, 88%: 1H NMR (CD3CN) δ 4.66 (d, J ) 7.3 Hz, 1H), 4.55 (q,
J ) 6.7 Hz, 1H), 4.45 (br s, 3H), 3.38 (br s, 12H), 3.06 (s, 3H), 2.92
(s, 3H), 1.50 (d, J ) 6.7 Hz, 3H). Anal. Calcd for C8H26N5O9F9S4-
Os: C, 11.64; H, 3.17; N, 8.48. Found: C, 11.71; H, 3.22; N, 8.49.
cis-{η2-[Os(NH3)5]-3-pyridinio-2-propene}(OTf)3 (33). Pyridine
(8 mg, 0.101 mmol) was added to a solution of 32 (56 mg, 0.067 mmol)
in 489 mg CH3CN causing an immediate color change from golden
brown to red. The solution was immediately added to 50 mL of CH2-
Cl2 under stirring, filtered, washed with CH2Cl2 (2 × 5 mL), and dried
in Vacuo to afford 41 mg (0.049 mmol) of a pink powder, 72%: 1H
NMR (CD3CN) δ 8.67 (d, J ) 6.5 Hz, 2H), 8.52 (d, J ) 6.9 Hz, 1H),
7.94 (t, J ) 7.1 Hz, 2H), 6.61 (d, J ) 7.3 Hz, 1H), 5.43 (m, 1H), 4.46
(br s, 3H), 3.29 (br s, 12H), 1.49 (d, J ) 6.0 Hz, 3H). Anal. Calcd
for C11H25N6O9S3F9Os: C, 15.68; H, 2.99; N, 9.97. Found: C, 15.76;
H, 3.03; N, 10.01.
2
{1,2-η -[Os(NH3)5]-(1E,3Z)-1-acetoxy-4-(methylthio)-1,3-butadiene}-
(OTf)2 (20). To a solution of 8 (255 mg, 0.311 mmol) in 677 mg
acetonitrile was added solid ammonium acetate (29 mg, 0.370 mmol),
and the resulting slurry was stirred vigorously for 15 min. The slurry
was filtered through 15 mL fine porosity frit into 50 mL of stirring
diethyl ether affording a yellow-orange precipitate. The precipitate was
filtered, washed with ether, and dried in Vacuo to afford 211 mg (0.289
mmol) of a yellow-orange powder, 93%: 1H NMR (CD3CN) δ 7.09
(d, J ) 5.9 Hz, 1H), 6.28 (d, J ) 9.5 Hz, 1H), 5.17 (dd, J ) 10.3 Hz,
9.5 Hz, 1H),4.63 (dd, J ) 9.5 Hz, 5.9 Hz, 1H), 4.19 (br s, 3H), 3.17
(br s, 12H), 2.31 (s, 3H), 2.01 (s, 3H). 13C NMR (CD3CN) δ 173.26
(CO), 131.27 (CH), 124.81 (CH), 88.74 (CH), 42.29 (CH), 20.88 (CH3),
17.06 (CH3); CV Ep,a ) 1.10 V (NHE). Anal. Calcd for C9H25N5S3O8F6-
Os: C, 14.77; H, 3.44; N, 9.57. Found: C, 14.42; H, 3.68; N, 9.39.
2
{1,2-η -[Os(NH3)5]-(1E,3Z)-4-(methylthio)-1-(propylamino)-1,3-
butadiene}(OTf)2 (22). Propylamine (59 mg, 0.360 mmol) was added
to a solution of 8 (134 mg, 0.163 mmol) in 399 mg of CH3CN. After
1 h, the reaction mixture was added to 50 mL of stirring CH2Cl2, the
precipitate was collected on a fine frit, and washed with CH2Cl2 (2 ×
10 mL) to afford 101 mg (0.138 mmol) of a yellow powder, 85%: 1H
NMR (acetone-d6) δ 6.10 (d, J ) 9.5 Hz, 1H), 5.29 (dd, J ) 7.3 Hz,
10.3 Hz, 1H), 5.18 (d, J ) 7.3 Hz, 1H), 4.73 (br s, 3H), 4.28 (dd, J )
10.3 Hz, 9.5 Hz, 1H), 3.79 (br s, 12H), 2.88 (m, 2H), 2.28 (s, 3H),
1.58-1.51 (m, 2H), 0.92 (t, J ) 7.3 Hz, 3H), N-H not assigned; 13C
NMR (acetone-d6) δ 134.65 (CH), 121.85 (CH), 75.86 (CH), 55.41
cis-{η2-[Os(NH3)5]-1-(methylthio)-4-pyridinio-3-butene}(OTf)3
(35). To a solution of 17 (32 mg, 0.039 mmol) in CH3CN (416 mg)
was added pyridine (17 mg, 0.215 mmol). After 16 h, the resulting
red solution was added to 50 mL of CH2Cl2 under stirring, filtered,
washed with CH2Cl2 (2 × 5 mL), and dried in Vacuo to afford 29 mg
(0.032) of a pink powder, 81%: 1H NMR (CD3CN) δ 8.72 (d, J ) 5.9
Hz, 2H), 8.53 (d, J ) 8.1 Hz, 1H), 7.87 (t, J ) 7.3 Hz, 2H), 6.72 (d,
J ) 7.4 Hz, 1H), 5.45 (m, 1H), 4.54 (br s, 3H), 3.36 (br s, 12H), 2.84
(m, 2H), 2.07 (s, 3H), 1.47 (m, 2H). Anal. Calcd for C13H29O9S4F9N6-
Os: C, 17.30; H, 3.24; N, 9.31. Found: C, 17.33; H, 3.00; N, 9.70.
(CH2), 43.85 (CH), 24.13 (CH2), 17.13 (CH3), 11.92 (CH3); CV Ep,a
)
0.57 V (NHE). Anal. Calcd for C10H30N6S3F6O6Os: C, 16.44; H, 4.14;
N, 11.50. Found: C, 16.45; H, 4.20; N, 11.47.
2
{1,2-η -[Os(NH3)5]-(1E,3Z)-1-azido-4-(methylthio)-1,3-butadiene}-
(OTf)2 (23). Solid sodium azide (30 mg, 0.461 mmol) was added to a
solution of 8 (192 mg, 0.234 mmol) in 773 mg of acetonitrile. Water
(∼5 drops) was added to help solubilize the azide salt. The slurry
was stirred vigorously for 2 h, filtered through a 15 mL fine frit to
remove undisolved sodium azide, and precipitated in ether to afford
2
{3,4-η -[Os(NH3)5]-(1E,3Z)-4-methoxy-1-(methylthio)-1,3-
butadiene}(OTf)2 (36). To a solution of 8 (160 mg, 0.195 mmol) in
methanol (916 mg) was added i-Pr2EtN (27 mg, 0.209 mmol). The
solution was allowed to stand for 1 h and added to 50 mL of CH2Cl2,
the precipitate (1) was removed by filtration, and the filtrate diluted
with diethyl ether. The resulting yellow precipitate was filtered, washed
1
137 mg (0.192 mmol) of a tan solid, 82%; H NMR (CD3CN) δ 6.18
(d, J ) 9.2 Hz, 1H), 5.82 (d, J ) 6.6 Hz, 1H), 5.09 (overlapping dd,
J ) 10.2 Hz, 9.2 Hz, 1H), 4.41 (dd, J ) 6.6 Hz, 10.2 Hz, 1H), 4.32 (br
s, 3H), 3.22 (br s, 12H), 2.84 (s, 3H); 13C NMR (CD3CN) δ 130.78
with Et
2O, and dried in Vacuo to afford 104 mg, 76%, of a yellow
powder, as a 7.5:1 mixture of geometric isomers: 1H NMR (CD3CN)
δ 6.16 (d, J ) 9.5 Hz, 1H), 6.01 (d, J ) 5.5 Hz, 1H), 5.07 (dd, J )
10.3 Hz, 9.5 Hz, 1H), 4.37 (dd, J ) 10.3 Hz, 5.5 Hz, 1H), 4.13 (br s,
3H), 3.55 (s, 3H), 3.11 (br s, 12H), 2.31 (s, 3H); 13C NMR (CD3CN)
δ 130.12 (CH), 122.61 (CH), 94.94 (CH), 62.01 (CH3), 42.16 (CH),
(CH), 125.62 (CH), 71.73 (CH), 43.88 (CH), 17.01 (CH3); CV Ep,a
)
0.91 V (NHE). Anal. Calcd for C7H22N8O6F6S3Os: C, 11.76; H, 3.10;
N, 15.68. Found: C, 12.09; H, 3.19; N, 15.41.
{1,2-η2-[Os(NH3)5]-(1E,3Z)-4-(methylthio)-1-(triphenylphospho-
nio)-1,3-butadiene}(OTf)3 (24). A solution of 8 (118 mg, 0.143 mmol)
in 365 mg of CH3CN was added to a solution of triphenylphosphine
(302 mg, 1.14 mmol) in 968 mg of CH3CN. After 30 min, the reaction
mixture was added to 50 mL of CH2Cl2, the resulting thiophene complex
impurity was removed by filtration, and the filtrate diluted with 50
mL of diethyl ether. The resulting yellow precipitate was filtered,
washed with ether, and dried in Vacuo, affording 90 mg (0.083 mmol)
of a yellow powder, 58%: 1H NMR (CD3CN) δ 7.91-7.68 (m, 15H),
6.68 (d, J ) 9.5 Hz, 1H), 5.60 (overlapping dd, J ) 9.5 Hz, 10.5 Hz,
1H), 5.23 (m, 1H), 5.00 (overlapping dd, J ) 9.5 Hz, 9.5 Hz, 1H),
4.57 (br s, 3H), 3.15 (br s, 12H), 2.36 (s, 3H); 13C NMR (CD3CN) δ
135.96 (CH), 134.39 (d, J ) 9.2 Hz, CH), 131.51 (d, J ) 12.8 Hz,
CH), 129.95 (d, J ) 5.5 Hz, C), 133.47 (CH), 123.02 (CH), 49.99
(CH), 20.41 (CH), 17.36 (CH3); CV Ep,a ) 1.39 V (NHE); Ep,c ) -1.45
V (NHE). Anal. Calcd for C27H37N5O9F9PS4Os: C, 28.81; H, 3.44;
N, 6.46. Found: C, 28.80; H, 3.60; N, 6.58.
15.85 (CH3); CV Ep,a
) 0.79 V (NHE). Anal. Calcd for
C10H25N5O7F6S3Os: C, 16.51; H, 3.46; N, 9.62. Found: C, 16.51; H,
3.44; N, 9.70.
{3,4-η2-[Os(NH3)5]-(E)-4-methoxy-1,3-butadiene}(OTf)2 (37). Pro-
cedure A: Under an inert atmosphere, dry Raney nickel32 (600 mg,
∼5 wt/wt equiv, Aldrich, W-2) was added to a solution of 8 (120 mg,
0.146 mmol) in methanol and the resulting slurry was vigorously stirred
for 30 min. The slurry was filtered through a 15 mL fine frit and added
to 50 mL of ether to afford a pale yellow precipitate which was filtered,
washed with diethyl ether (2 × 5 mL) and dried in Vacuo affording 72
mg (0.110 mmol) of a pale yellow powder, 75%: 1H NMR (CD3CN)
δ 6.03 (d, J ) 5.9 Hz, 1H), 5.28-5.11 (m, 3H), 4.13 (br s, 3H), 4.02
(dd, J ) 9.5 Hz, 5.9 Hz, 1H), 3.57 (s, 3H), 2.99 (br s, 12H). Anal.
Calcd for C7H23N5O7F6S2Os: C, 13.09; H, 1.26; N, 10.90. Found: C,
13.00; H, 1.27; N, 11.18. Procedure B: Under an inert atmosphere,
716 mg of dry Raney nickel (Aldrich, W-2) was added to a solution of
36 (141 mg, 0.201 mmol) in 2.71 g methanol. The slurry was stirred
vigorously for 1h and the workup from proceudre A was followed:
yield 96 mg, 72%.
2
{1,2-η -[Os(NH3)5]-(1E,3Z)-4-(methylthio)-1-phenoxy-1,3-
butadiene}(OTf)2 (25). Phenol (36 mg, 0.380 mmol) was added to a
solution of 8 (104 mg, 0.127 mmol) in 608 mg of CH3CN. i-Pr2EtN
(49 mg, 0.381 mmol) was added, and the reaction mixture allowed to
stand for 10 min. The workup for 24 was followed: yield 80 mg (0.105
mmol), 83%; 1H NMR (CD3CN) δ 7.31 (m, 1H), 7.06 (d, J ) 8.1 Hz,
(32) This operation is most easily carried out under dry nitrogen in a
glovebox.

η2-Thiophene Complexes of Pentaammineosmium(II)
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8851
{η2-[Os(NH3)5]-(E)-4-methoxy-3-butene}(OTf)2 (38). Procedure
A: Under an inert atmosphere, dry Raney nickel (Aldrich, W-2) was
added to a solution of 8 in methanol and the resulting slurry was
vigorously stirred for 10 h. The slurry was filtered through a 15 mL
fine frit and added to 50 mL of ether to afford a yellow precipitate:
1H NMR (CD3CN) δ 5.62 (d, J ) 5.9 Hz, 1H), 4.02 (br s, 3H), 3.53
(s, 3H), 3.39 (m, 1H), 3.01 (br s, 12H), 1.28 (t, J ) 7.1 Hz, 3H); 13C
NMR (CD3CN) δ 96.03 (CH), 63.03 (CH3), 47.40 (CH), 22.99 (CH2),
18.56 (CH3); CV Ep,a ) 0.83 V (NHE); Ep,c ) 0.54 V (NHE). Anal.
Calcd for C7H25N5O7F6S2Os: C, 13.05; H, 1.56; N, 10.87. Found: C,
12.96; H, 1.51; N, 10.97. Procedure B: Under an inert atmosphere,
362 mg dry Raney nickel (Aldrich, W-2) was added to a solution of
36 (71 mg, 0.101 mmol) in 1.97 g of methanol. The slurry was stirred
vigourously for 8h and the workup from procedure A was followed:
yield 35 mg of ivory powder, 52%.
2-(Methylthio)styrene (39).33 A solution of DDQ (103 mg, 0.46
mmol) in CH3CN (252 mg) was added to a solution of 30 (549 mg,
0.76 mmol) in 1.12 g CH3CN. After 5 min, the dark reaction mixture
was added to 50 mL of CH2Cl2. The resulting slurry was filtered
through a pad of silica gel, and the frit was washed with CH2Cl2 (2 ×
10 mL). The solvent was removed under reduced pressure affording
88 mg of a golden-brown oil, 88%.
290 mg of CH3CN. The dark slurry was stirred for 5 min and then
filtered through a bed of basic alumina. The solvent was removed
under reduced pressure, affording 41 as a yellow oil, 19 mg, 55%.
(1E,3Z)-4-(Methylthio)-1-phenoxy-1,3-butadiene (42). To a solu-
tion of 25 (315 mg, 0.412 mmol) in acetone (1.3 g) was added solid
AgOTf (127 mg, 0.490 mmol), and the slurry was stirred at ambient
temperature for 15 min. The slurry was added to 30 mL of diethyl
ether and filtered through a plug of silica gel. The filtrate was removed
under reduced pressure to afford 46 mg of a yellow oil, 58%: 1H NMR
(CDCl3) δ 7.33 (t, J ) 8.5 Hz, 2H), 7.07 (t, J ) 8.5 Hz, 1H), 7.00 (d,
J ) 8.8 Hz, 2H), 6.79 (d, J ) 11.9 Hz, 1H), 6.26 (dd, J ) 11.9 Hz,
10.1 Hz, 1H), 6.08 (dd, J ) 10.1 Hz, 9.5 Hz, 1H), 5.84 (d, J ) 9.5 Hz,
1H), 2.31 (s, 3H); 13C NMR (CDCl3) δ 156.71 (C), 146.13 (CH), 129.56
(CH), 125.71 (CH), 123.14 (CH), 122.00 (CH), 116.74 (CH), 109.93
(CH), 17.32 (CH3).
(1E,3Z)-4-(Methylthio)-1-methoxy-1,3-butadiene (43). To a solu-
tion of 36 (245 mg, 0.349 mmol) in acetone (800 mg) was added a
solution of DDQ (0.349 mmol) in 896 mg of acetone. The resulting
slurry was stirred for 15 min and filtered through a bed of basic alumina,
and the filtrate was evaporated to a red-brown residue. The residue
was partitioned between 10% Na2CO3(aq) and CH2Cl2 and extracted with
CH2Cl2, and the organic layers was dried over Na2SO4. The solvent
was removed under reduced pressure to afford 43 as a yellow oil, (36
mg, 0.277 mmol), 80%: 1H NMR (CDCl3) δ 6.64 (d, J ) 12.5 Hz,
1H), 6.02 (dd, J ) 10.2 Hz, 9.5 Hz, 1H), 5.72 (dd, J ) 12.5 Hz, 10.2,
1H), 5.63 (d, J ) 9.5 Hz, 1H), 2.28 (s, CH3).
(E)-1-[2-(Methylthio)phenyl]-2-cyanoethene (40). To a solution
of 31 (200 mg, 0.267 mmol) in 1.01 g CH3CN was added HOTf (120
mg, 0.80 mmol) and DDQ (67mg, 0.294 mmol). After 15 min, DBU
(130 mg, 0.086 mmol) was added affording a dark gummy precipitate.
The reaction mixture was added to 50 mL of diethyl ether and filtered
through a pad of silica gel. The silica was washed with ether (3 × 10
mL), and the solvent removed under reduced pressure to afford 38 mg
(0.217 mmol) of a yellow oil, 82%: 1H NMR (CDCl3) δ 7.91 (d, J )
16.5 Hz, 1H), 7.45-7.21 (m, 4H), 5.83 (d, J ) 16.5 Hz, 1H), 2.47 (s,
3H); 13C NMR (CDCl3) δ 147.54 (CH), 139.08 (C), 133.22 (C), 131.08
(CH), 127.72 (CH), 126.31 (CH), 125.78 (CH), 116.28 (CN), 97.75
(CH), 16.79 (CH3).
Acknowledgment is made to the Camille and Henry Dreyfus
Foundation, and the National Science Foundation (NSF Young
Investigator program) and the A. P. Sloan Foundation for their
generous support of this work, and to Dr. Huiyuan Chen for
his helpful discussions.
(Z)-1-(Methylthio)butadiene34 (41). A solution of DDQ (88 mg,
0.387 mmol) was added to a solution of 18 (237 mg, 0.352 mmol) in
Supporting Information Available: Text giving experi-
mental procedures and characterizations for all compounds
described in this account (15 pages). See any current masthead
page for ordering and Internet access instructions.
(33) This compound has been previously reported. See Crow, W. D.;
McNab, H. Aust. J. Chem. 1979, 32, 123.
(34) This compound has been previously reported. See: (a) Everhardus,
R. H.; Gra¨fing, R.; Brandsma, L. Synth. Commun. 1983, 623. (b) Crumbie,
R. L.; Ridley, D. D. Aust. J. Chem. 1981, 34, 1017.
JA970941K