Sulfoxide carbon-sulfur bond activation

Article abstract of DOI:10.1021/ja050160g

The alkynylsulfoxide, TMSC≡CSO(p-tolyl) (TMS = trimethylsilyl, tolyl = C₆H₄Me), undergoes reaction with (η⁵-C₅H₅)Co(PPh₃)₂ at room temperature to give the cobaltosulfoxide complex, (C₅H₅)Co(PPh₃)(η-C≡CTMS)[η¹-(S)-SO(p-tolyl)], which was characterized by X-ray crystallography. Exposure of this cobaltosulfoxide complex to oxygen gas leads to the formation of the corresponding metallosulfone complex, (C₅H₅)Co(PPh₃)(η¹-C≡CTMS)[η¹-(S)-SO₂(p-tolyl)], which was characterized by X-ray crystallography. Alternatively, in solution at room temperature, the metallosulfoxide is converted to a 1:4 mixture of the equatorial-equatorial and equatorial-axial bridging cobalt-thiolato dimers, {(C₅H₅)Co[μ-S(p-tolyl)]}₂, respectively. The equatorial-equatorial isomer was characterized by X-ray crystallography. Copyright

Full text of DOI:10.1021/ja050160g

Published on Web 03/08/2005
Sulfoxide Carbon-Sulfur Bond Activation
Joseph M. O’Connor,* Kevin D. Bunker, Arnold L. Rheingold,* and Lev Zakharov
Department of Chemistry and Biochemistry, UniVersity of California, San Diego, La Jolla, California 92093
Received January 10, 2005; E-mail: jmoconnor@ucsd.edu
Transition metal insertion into the carbon-sulfur bond of sulfides
to give well-characterized oxidative addition products has been
achieved for cases involving sp-, sp2-, and sp3-hybridized carbon
atoms.1 The widespread use of sulfoxides in synthesis makes
carbon-sulfur insertion reactions particularly desirable for higher
oxidation state sulfur. Despite the existence of numerous metal-
sulfoxide complexes,2 carbon-sulfur bond activation has not
previously been directly observed for sulfoxides. However, indirect
evidence for transition metal insertion into the carbon-sulfur bond
of sulfoxides is found in the sulfinylzincation reactions of alkynoates
(eq 1)3 and the conversion of phenylsulfinyl ethene and Fe2(CO)9
to [Fe2(µ-CHdCH2)(µ-SPh)(CO)6] (eq 2).4
with a ν(CtC) shift of 338 cm-1 upon going from 2-Ph to 3-Ph.
In the 13C{1H} NMR spectrum (CDCl3) of 6, the alkynyl sp-carbons
resonate at 90.2 (d, JCP ) 62.0 Hz) and 113.1 (d, JPC ) 40.4 Hz)
ppm, shifted from 90.1 and 100.1 ppm in 5. For comparison, the
sp-carbons of the alkyne ligand in 3-Ph are observed at 105.5 (d,
JPC ) 3.5 Hz) and 118.1 (d, JPC ) 11.6 Hz) ppm. The observation
of an sp-carbon resonance at 90 ppm and the large JPC coupling
constants indicated that the expected η2-alkyne complex (7), if
formed, had not been isolated. The spectroscopic data and the
known insertion of metals into the silicon-carbon bond of
trimethylsilyl acetylenes8 raised the possibility of a (Cp)Co(PPh3)-
(TMS)(CtCSOTol) structure for 6. However, in the mass spectrum
(FAB) of 6, the base peak at 483 m/z [M+ - SOTol] and a fragment
at 386 m/z [483 - CCTMS] suggested that cobalt had selectively
inserted into the (sp)carbon-sulfur bond of 5.
The metallosulfoxide structure of 6 was conclusively established
by an X-ray crystallographic analysis (Figure 1, Table 1). Data col-
We previously reported that (η5-C5H5)Co(PPh3)2 (1) undergoes
reaction with alkynyl sulfone 2-Ph at room temperature to give the
η2-alkyne complex 3-Ph in 75-80% yield (Scheme 1).5 There was
Scheme 1
Figure 1. Solid-state molecular structures of 6-P1h (bottom) and 4 (top).
Ellipsoids at the 30% probability level.
no evidence for conversion of either 3-Ph or 3-Tol to a cobalto-
sulfone complex (e.g., 4). We now report that reaction of 1 and an
alkynylsulfoxide leads to the first direct observation of carbon-
sulfur bond activation involving a sulfoxide.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 4 and 6
complex
6-P2h(1)/n
6-P1
h
4-P2h(1)/n
Co(1)-S(1)
2.2196(7)
2.2108(7)
1.508(2)
2.2438(9)
2.2047(9)
1.513(2)
2.2014(8)
2.2391(9)
1.4588(17)
1.4698(16)
93.54(7)
104.11(11)
105.29(10)
107.34(8)
114.27(7)
109.47(7)
95.40(3)
When the reaction of 1 (0.013 mmol) and alkynyl sulfoxide 5
(0.013 mmol) was monitored by 1H NMR spectroscopy in benzene-
d6, the cobaltosulfoxide complex 6 was observed to form in 72%
yield.6 In a larger scale reaction, 5 (0.30 mmol) was added to a dry
benzene solution (20 mL) of 1 (0.30 mmol), and the mixture was
allowed to stir at room temperature under a nitrogen atmosphere
for 18 h. Removal of the volatiles and recrystallization of the residue
led to isolation of 6 as dark purple, slightly air-sensitive crystals
(54% yield). In the IR spectrum (NaCl) of 6, ν(CtC) is observed
at 2030 cm-1 and a strong band at 1064 cm-1 is assigned to ν(Sd
O).7 The carbon-carbon triple bond stretch occurs 264 cm-1 wave-
numbers lower than that for noncoordinated 5, and this compares
Co(1)-P(1)
S(1)-O(1)
S(1)-O(2)
C(1)-Co(1)-S(1)
Co(1)-S(1)-O(1)
Co(1)-S(1)-O(2)
Co(1)-S(1)-C(6)
O(1)-S(1)-C(6)
O(2)-S(1)-C(6)
S(1)-Co(1)-P(1)
Co(1)-C(1)-C(2)
C(1)-Co(1)-P(1)
88.30(8)
108.61(8)
88.91(10)
109.54(9)
106.33(8)
105.03(12)
103.92(10)
104.43(13)
91.97(3)
176.1(2)
87.30(8)
90.73(3)
176.1(3)
91.98(9)
174.5(2)
89.84(7)
lected on a needle of 6, obtained as described above, refined in the
centrosymmetric triclinic space group P1h, indicating that the sample
9
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10.1021/ja050160g CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
was a racemate.9 Structural analysis of a second crystal, grown by
slow evaporation of a benzene solution containing 6, belonged to
the centrosymmetric monoclinic space group P2(1)/n. Both poly-
morphs exhibited the same SCo,RS/RCo,SS relative configurations.
The sulfur atom in both structures of 6 exhibits a trigonal pyramidal
geometry and is situated an average of 0.686 Å from the plane
defined by Co(1), C(6), and O(1). This displacement is typical of
other sulfenato ligands and organic sulfoxides (0.7-0.8 Å range).
The average S(1)-O(1) [1.510(2) Å] and S(1)-C(6) [1.804(3) Å]
distances in 6 are remarkably similar to those found in MeS(dO)-
Tol [1.493(6) and 1.797(6) Å]. However, the Co(1)-S(1)-C(6)
angle of 105.1° in 6 is 7.5° larger than the Me-S-Tol angle for
MeS(dO)Tol,10 which may be attributed to greater steric congestion
in 6.
observed only as an eq,ax-isomer, which is reported to rapidly
interconvert with the ax,eq-isomer at higher temperature (Eact
60 kJ/mol).19
)
Acknowledgment. Support of the National Science Foundation
(CHE-9975939) is gratefully acknowledged.
Supporting Information Available: Experimental procedures,
characterization data for all new compounds, and crystallographic data
for 4, 6, and 9-eq,eq. This material is available free of charge via the
Internet at http://pubs.acs.org.
There are four structurally characterized cobaltosulfoxide com-
plexes, all of which involve chelating [η2-(N,S)-H2N(CHR)(CH2)nS-
(O)] sulfenate ligands.11-13 The average Co-S and S-O distances
in this set of structures is 2.232 and 1.540 Å, respectively. More
specifically, (NH2CH2CH2NH2)2Co(NH2CH2CH2SO)][SCN]2 (8)
exhibits a 96.51° Co-S-C angle, a S-O distance of 1.552(3) Å,
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and ν(SdO) of 986 cm-1 11a
The sulfur-oxygen bond in 6 is
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of 6 to oxygen gas led to the formation of the metallosulfone 4,
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1).14 Ground state O2 is known to oxidize both thiolato and cis-
disulfenato ligands;12b,15 however, we are unaware of a precedent
for conversion of a single sulfenato ligand to a sulfinato ligand
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direct O2 oxidation of sulfur, the current data do not allow for such
a conclusion to be made.17
A comparison of the solid-state structure of 4 (Figure 1) to that
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C(1) angles in the more congested metallosulfone. The sulfur
ligands adopt very similar conformations with Cp-Co(1)-S(1)-
O(1) and C(7)-C(6)-S(1)-O(1) torsion angles of -59.6 and
-20.5° for 6 versus Cp-Co(1)-S(1)-O(2) and C(7)-C(6)-S(1)-
O(2) torsion angles of +51.2 and -12.4° in 4.
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(16) Conversion of a bis(sulfenate) complex to a bis(sulfinate) upon reaction
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across the two cis-sulfur atoms (see ref 15).
In benzene-d6, 6 is slowly (months) transformed into a 4:1 ratio
of thiolato-bridged cobalt dimers, 9-eq,ax and 9-eq,eq, in 85%
combined yield (eq 3).18 In addition, OPPh3 (77%) and TMSCt
CCtCTMS (43%) are observed as byproducts. Chromatographic
workup of a large-scale reaction led to the collection of two distinct
fractions (orange and green), but with the orange fraction rapidly
1
turning green over the course of 15 min. H NMR spectroscopic
analysis of the two fractions indicated the same 4:1 ratio of dimers.
Dark-green crystals of 9-eq,eq were grown by slow evaporation of
solvent, and an X-ray crystallographic analysis provided the first
structural characterization of an eq,eq-[CpM(SR)]2 complex. In
benzene-d6 at 70 °C, the two isomers of 9 interconvert rapidly on
the NMR time scale, and the Eact for conversion of 9-eq,ax to
9-eq,eq was determined to be 74.9 ( 0.5 kJ/mol. For comparison,
both at low temperature and in the solid state, [CpCo(SBut)]2 is
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