Aerobic oxidation of methyl p-Tolyl sulfide catalyzed by a remarkably labile heteroscorpionate Ru(II)-aqua complex, fac-[RuII(H2O)(dpp)(tppm)]2+

Article abstract of DOI:10.1021/ja0124111

fac-[Ru^II(Cl)(dpp)(L₃)]⁺ (L₃ = tris(pyrid-2-yl)methoxymethane (tpmm) = [1A]⁺ and tris(pyrid-2-yl)pentoxymethane (tppm) = [1B]⁺ and dpp = di(pyrazol-1-yl)propane) rapidly undergo ligand substitution with water to form fac-[Ru^II(H₂O)(dpp)(L₃)]²⁺ (L₃ = tpmm = [2A]²⁺ and tppm = [2B]²⁺). In the structure of [2A]²⁺, the distorted octahedral arrangement of ligands around Ru is evident by a long Ru(1)-O(40) of 2.172(3) A and a large (40)-Ru (1)-N(51) of 96.95(14)° The remarkably short distance between O (40) of H ₂O and H (45a) of dpp confirms the heteroscorpionate ligand effect fo dpp H ₂O [2B] ²⁺ aerobically catalyzes methl p-toyl sulfide to methyl p-toyl sulfoxide in 1,2 dichlorobenzene at 25.0 ± 0.1 ° C under 11.4 psi of O₂.Experimental facts in support of this aerobic sulfide oxidation are the absence of H ₂O₂ and the oxidative readctive of the putative Ru(IV)-oxo intermediate toward methl p-tolyl Sulfide, 2- propanol, and allyl alcohal This study provides the first documented example of aerobic-sulfide oxidation catalyzed by the remarkably labile heteroscorpionate Ru (III)- aqua complex without the formation of a highly reactive peroxide as an intemediate. Copyright

Full text of DOI:10.1021/ja0124111

Published on Web 12/13/2002
Aerobic Oxidation of Methyl p-Tolyl Sulfide Catalyzed by a Remarkably Labile
Heteroscorpionate Ru(II)-Aqua Complex, fac-[RuII(H2O)(dpp)(tppm)]2+
My Hang V. Huynh,*,†,‡ Laura M. Witham,† Joanne M. Lasker,† Modi Wetzler,† Brendan Mort,†
Donald L. Jameson,§ Peter S. White,| and Kenneth J. Takeuchi*
Department of Chemistry, State UniVersity of New York at Buffalo, Buffalo, New York 14260, the Chemistry
DiVision, C-SIC Group and the Dynamic Experimentation DiVision, DX-2: HE Science and Technology Group,
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and the Department of Chemistry,
Gettysburg College, Gettysburg, PennsylVania 17325
Received October 23, 2001; E-mail: huynh@lanl.gov, takeuchi@nsm.buffalo.edu
Catalytic and stoichiometric oxidations of sulfide by peroxides,
periodate salts, enzymatic systems, and transition-metal-based
oxidants are important from biochemical,1 environmental,2 and
industrial perspectives.3 There are only a few examples of transition-
metal-based catalysts using molecular oxygen as the oxidant because
sulfide has been well-known for many years as a poor ligand4 and
is, therefore, not able to displace peroxide from an inner-sphere
peroxide catalysts. Its sulfoxide analogue, on the other hand, is not
a good leaving group after binding to a catalyst that contains an
oxygen donor ligand. To avoid suffering from a very slow oxidation
rate or no catalytic reactivity, transition-metal-based catalysts must
be used together with highly reactive peroxides such as H2O2 and
tert-BuO2H as the ultimate oxidants5 or have a highly reactive
peroxide involved as an intermediate, sometimes generated via an
outer-sphere electron-transfer pathway.6
We previously reported that the aerobic oxidation of cyclohexene
catalyzed by cis-[RuII(H2O)(bpy)2(PR3)]2+ (bpy ) 2,2′-bipyridine
and PR3 ) tertiary phosphine) involved a putative cis-[RuIV(bpy)2-
(PR3)(O)]2+ intermediate without the formation of H2O2.7a We
recently reported the remarkable heteroscorpionate ligand effect on
rate enhancement (1.9 × 107) of ligand substitution kinetics for
fac-[RuII(H2O)(dpp)(tpmm)]2+ (dpp ) di(pyrazol-1-yl)propane and
tpmm ) tris(pyrid-2-yl)methoxymethane).8
Figure 1. ORTEP diagram of the fac-[RuII(H2O)(dpp)(tpmm)]2+ cation.
Aerobic oxidation of methyl p-tolyl sulfide catalyzed by [2B]-
(PF6)2 in 1,2-dichlorobenzene (ODCB ) o-dichlorobenzene) at 25.0
( 0.1 °C was monitored by GCI-MS.9
This aerobic sulfide oxidation study is reminiscent of those by
Riley,6a-b Mestroni,6c Fergusson,6d Espenson,5a Kagan,5b and
Seraglia.5c However, these reactions require vigorous conditions
and are known to occur only when highly reactive peroxides such
as H2O2 or tert-BuO2 are involved as the intermediate or are used
as the oxidants. For example, alkylarylsulfides were catalytically
oxidized by cis- or trans-[RuII(X)2(DMSO)4] (X ) Cl or Br),6a fac-
or mer-[RuII(Br)2(THT)(BEPS)] (THT ) tetrahydrothiophene and
BEPS ) bis(3-(ethylsulfinyl)propyl)-sulfide),6b trans-[RuIII(Cl)4-
(DMSO)2]-, or mer-[RuIII(Cl)3(DMSO)3]6c with the formation of
H2O2 as the intermediate. In the catalytic oxidations of alkylaryl-
sulfides and chiral sulfides, [ReVII(CH3)(O)3]5a and [TiVI(i-
OC3H7)4],5b [VV(O)(i-OC3H7)3],5c [MoVI(O)2(acac)] (acac ) acetyl-
acetonato), or [MoVI(O)(O2)2]5c must be used with H2O2 and tert-
BuO2H as the oxidants, respectively.
We now report a unique combination of the two studies in which
the low-oxidation state heteroscorpionate RuII-H2O2+ complex
having a remarkable steric ligand effect is used as a catalyst for
aerobic oxidation of methyl p-tolyl sulfide to methyl p-tolyl
sulfoxide. This novel reactivity is the first documented example of
aerobic sulfide oxidation catalyzed by a transition-metal complex
without the formation of a highly reactive peroxide as an intermedi-
ate. Remarkably, this aerobic oxidation of sulfide occurs due
primarily to the steric effect of the heteroscorpionate dpp ligand.
The X-ray crystal structure of [2A](PF6)2 with crystals grown by a
slow diffusion of CH3C(O)CH3 out of a 1:1 (v/v) H2O:CH3C(O)-
CH3 solution is shown in Figure 1.
In addition, our sulfide oxidation study is also reminiscent of
those by James10a and Meyer.10b However, the aerobic sulfide
oxidation catalyzed by Ru(VI)-dioxo porphyrin reported by James
and co-workers was suppressed after 20 min with a turnover of
∼5 because the more labile O-bound sulfoxide RuII-(OSR2)2
2+
complex underwent isomerization to form the substitutionally inert
S-bound sulfoxide RuII(S(O)R2)22+ complex which terminated the
catalytic process. This is a typical problem for unhindered transition-
metal catalysts. Although the sulfide oxidation by cis-[RuIV(bpy)2-
(py)(O)]2+ reported by Meyer had a much faster rate of forming
† State University of New York at Buffalo.
‡ Los Alamos National Laboratory.
§ Gettysburg College.
the O-bound sufoxide RuII-OSR2 product and a much slower
2+
| University of North Carolina at Chapel Hill.
9
308
J. AM. CHEM. SOC. 2003, 125, 308-309
10.1021/ja0124111 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
2+
rate of isomerization to the corresponding S-bound RuII-S(O)R2
the heteroscorpionate RuII-H2O2+ complex possessing a remarkable
product, unfortunately, this Ru(IV)-oxo complex did not possess
the catalytic reactivity toward thioether oxidation.
In our study, methyl p-tolyl sulfide was readily oxidized by [2B]-
(PF6)2 to methyl p-tolyl sulfoxide. The turnover number was
calculated as 53 after a period of 36 h, eq 2.9
steric effect of the heteroscorpionate dpp ligand.
Acknowledgment. K.J.T. acknowledges the National Science
Foundation and ARCO Chemical for partial financial support.
M.H.V.H. gratefully acknowledges the Laboratory Directed Re-
search and Development Program as well as the postdoctoral
fellowship support from the Director’s Office of Los Alamos
National Laboratory for support of this research. Los Alamos
National Laboratory is operated by the University of California for
the U.S. Department of Energy under Contract W-7405-ENG-36.
(CH3)(p-CH3C6H4)S [O ] (11.4 psi), [2B](PF ) Cat.8
2
6 2
ODCB, 25.0 ( 0.1 °C
(CH3)(p-CH3C6H4)SO (2)
Since the reaction in eq 2 occurs under mild conditions, it pre-
sumably proceeds via a mechanism involving the putative fac-[RuIV-
(dpp)(O)(tppm)]2+ intermediate, reminiscent of our aerobic oxida-
Supporting Information Available: Text containing product
analysis and product distribution, Supporting Information Table 1,
Supporting Information Figures 1-8 are included (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
tion of cyclohexene catalyzed by cis-[RuII(H2O)(bpy)2(PR3)]2+ 7b
.
Experimental facts in support of this mechanism are the absence
11
of detected H2O2 and the reactions in which fac-[RuIV(dpp)(O)-
References
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[3A](PF6)2 stoichiometrically reacts with methyl p-tolyl sulfide
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2
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(9) See Supporting Information.
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sulfoxide produced. This mass balance studies show that sulfide is
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13b
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