A facile route to Ru-alkylidenes

Article abstract of DOI:10.1039/c3dt53525h

Reactions of dithioacetal E(CH₂CH₂S)₂CHR (E = O, S, R = Ph, C₆F₅) and O(C₆H ₄S)₂CHPh with either Ru(cod)(cot) or Ru(PPh ₃)₃(H)₂ are used to prepare the Ru-alkylidene complexes E(CH₂CH₂S)₂RuCHPh(L), O(C ₆H₄S)₂RuCHPh(L) (L = PCy₃, SIMes) in high yields.

Full text of DOI:10.1039/c3dt53525h

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A facile route to Ru-alkylidenes†
Adam M. McKinty and Douglas W. Stephan*
Cite this: Dalton Trans., 2014, 43,
2710
Received 16th December 2013,
Accepted 17th December 2013
DOI: 10.1039/c3dt53525h
www.rsc.org/dalton
Reactions of dithioacetal E(CH2CH2S)2CHR (E = O, S, R = Ph, C6F5) reactions of Ru-synthons with alkynes, propargylchlorides,13
and O(C6H4S)2CHPh with either Ru(cod)(cot) or Ru(PPh3)3(H)2 are and propargylalcohols.14 Generally these latter compounds
used to prepare the Ru-alkylidene complexes E(CH2CH2S)2RuCHPh(L), exhibit lower initiation efficiencies, however Dixneuf and co-
O(C6H4S)2RuCHPh(L) (L = PCy3, SIMes) in high yields.
workers11a have shown that Ru-piano-stool complexes exhibit
high metathesis activity.
Olefin metathesis as a synthetic strategy has impacted broadly
on organic chemistry, polymer science and materials chem-
istry. Thus the discovery, mechanism and applications of this
powerful methodology were recognized with the award of the
Nobel Prize in 2005 to Chauvin, Grubbs and Schrock.1 Despite
recent advances,2 the Schrock catalyst systems are generally
perceived as more active although they are also more sensitive
to functional groups. It is for this reason that the Ru-based
catalysts of Grubbs’, (Cy3P)2Cl2RuCHPh and (Cy3P)Cl2RuCHPh
(SIMes), are most commonly used.3 Nonetheless a variety of
closely related derivatives have also been prepared and
studied. A variety of derivatives of the Grubbs catalysts have
also been probed. For example, a pendant oxygen atom donor
on the arene of the benzylidene fragment is incorporated in
the “Hoveyda–Grubbs” catalyst.4 On the other hand, Fogg
and coworkers5 have developed active systems in which the
halides are replaced with aryloxide ligands, while we have
recently reported the dithiolate derivative of Grubbs catalyst
(O(CH2CH2S)2)(SIMes)RuCHPh.6 A variety of synthetic routes
to the Ru-alkylidene (Cy3P)2Cl2RuCHPh have also been
explored. While use of cyclopropene was initially used to
install the alkylidene fragment,7 sulfur-ylides,8 dihalo-
methanes9 and diazomethanes10 provide synthetic alternatives
and indeed the latter reagent is synthetically easier and thus
more widely used to produce Grubbs’ catalysts. Related com-
plexes incorporating allenylidene, indenylidene,11 vinylidene9c
or cumulenylidene12 fragments have also been prepared from
Despite these developments, the most commonly used and
effective avenue to the installation of an alkylidene fragment
remains the use of diazomethane. This methodology is
however not without its problems as diazomethanes are
unstable, potentially explosive, toxic and react with phosphine
donors. Herein, we describe a new synthetic strategy to Ru-
alkylidenes exploiting the reactivity of dithioacetals. This strat-
egy offers a safe, low-cost, high yielding route which is readily
adaptable to give a family of Ru-dithiolate alkylidene com-
plexes. Moreover, these readily accessible compounds are
shown to act as highly convenient synthons for Grubbs’
catalysts.
The cyclic thioacetals were prepared employing a modifi-
cation of the literature procedure described by Hu et al.15
Thus, to a solution of p-toluenesulfonic acid in MeOH heated
to 55 °C, was added a mixture of 2-mercaptoethyl ether and
benzaldehyde in a slow drop-wise fashion. Following stirring
overnight at 55 °C, subsequent work-up afforded the cyclic
thioacetal O(CH2CH2S)2CHPh 1 in 95% isolated yield. 1H NMR
data for 1 and an X-ray structural determination confirmed the
formation of cyclic species (see ESI†). This reaction proved
amenable to variation and was adapted to give the species
S(CH2CH2S)2CHPh 215 and O(C6H4S)2CHPh 3 each isolated in
greater than 90% yield.
The dithioacetal 1 was reacted with Ru(cod)(cot)16 in the
presence of 1.1 equiv. of PCy3 in C6H6 at 50 °C for 2 h. Cooling
the solution afforded a red solid 4 in 73% yield. Compound 4
exhibits a single 31P{1H} NMR signal at 65.6 ppm. The 1H
NMR spectrum was consistent with the presence of the dithio-
late ligand and PCy3 in a 1 : 1 ratio. In addition, a doublet
resonance at 13.68 ppm with P–H coupling of 11.3 Hz was con-
sistent with the presence of a ruthenium–alkylidene. This was
further supported by the observation of the corresponding
13C{1H} resonance at 208.0 ppm. Although compound 4
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto,
Ontario, Canada, M5S 3H6. E-mail: dstephan@chem.utoronto.ca
†Electronic supplementary information (ESI) available: Synthetic and spectro-
scopic data and spectra are deposited. Crystallographic data have been deposited
in the Cambridge database. CCDC 971468–971470. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3dt53525h
2710 | Dalton Trans., 2014, 43, 2710–2712
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could not be crystallographically characterized, these data
infer the formulation as O(CH2CH2S)2RuCHPh(PCy3) 4. This
product is analogous to the recently reported species
O(CH2CH2S)2RuCHPh(SIMes) 7,6 prepared from reactions of
the dilithiodithiolate ligand with (Cy3P)Cl2RuCHPh(SIMes).
Seeking a more convenient Ru-synthon and thus an overall
higher yielding pathway, the dithioacetal 1 was reacted with
17
Ru(PPh3)3(H)2 in the presence of PCy3. During heating to
50 °C for 2 h, the yellow solution became dark red with the
evolution of gas. Subsequent work-up afforded 4 in 89% iso-
lated yield. Following a similar process, the dithioacetals 2 and
3 were treated in a similar fashion with Ru(PPh3)3(H)2 in the
presence of PCy3 to give the species S(CH2CH2S)2RuCHPh-
(PCy3) 5 and O(C6H4S)2RuCHPh(PCy3) 6 in yields of 87, and
84%, respectively (Scheme 1). The 31P resonance for these com-
pounds were seen at 41.7 and 68.6 ppm respectively while the
1
expected resonances from the alkylidenes giving rise to H NMR
signals at 13.48 (JP–H = 19 Hz), 14.69 (JP–H = 15 Hz) and the corres-
ponding 13C signals at 235.2 and 192.2 ppm, respectively. X-ray
structural data for 5 and 6 confirmed the above formulations
(Fig. 1(a) and (b)) and demonstrate the distorted trigonal bipyra-
midal structures in which the two thiolate sulfur atoms and
the alkylidene occupy the equatorial plane. The axial phos-
phine and thioether in 5 or ether in 6 donors complete the
coordination sphere. The Ru–S(thiolate) and Ru–S(thioether)
Fig. 1 POV-ray depictions of (a) 4, (b) 5, (c) 8. Ru: teal, S: yellow, O: red;
P: orange; N blue-green, C: black.
distances in 5 were found to be 2.2916(3) Å, 2.2981(3) Å and distances were also observed to be 1.864(1) Å and 1.848(3) Å in
2.3711(3) Å, respectively, while the Ru–S and Ru–O distances 5 and 6, respectively.
in 6 were 2.2870(8) Å, 2.3080(8) Å and 2.193(2) Å respectively.
Replacement of PCy3 with SIMes18 in a similar reaction of
These Ru–S distances are similar to those observed in the the thioacetals with Ru(PPh3)3(H)2 readily afforded the corres-
structural analog we have previously reported. The Ru–C ponding alkylidene complexes E(CH2CH2S)2RuCHPh(SIMes)
(E = O 7, S 8) and O(C6H4S)2RuCHPh(SIMes) 9.6 Again
the spectral data were consistent with these formulations
and X-ray structure was also obtained for 8 (Fig. 1(c)). The
Ru–C(carbene), Ru–C(alkylidene), Ru–S(thiolate) and Ru–
S-(thioether) distances were determined to be 1.860(5) Å, 2.084(5)
Å, 2.313(1) Å, 2.336(1) Å and 2.380(1) Å similar to those
previously reported for 8.6
This synthetic route to Ru-alkylidene is thought to proceed
by oxidation addition of the S–C bonds to Ru followed by
α-thiolate transfer. In these reactions Ru(PPh3)3(H)2 reacts as a
Ru(0) source, presumable with loss of H2. This latter supposi-
tion is consistent with the observation of gas evolution upon
addition of the dithioacetal to the Ru-synthon. This reaction is
conceptually related to the oxidation addition of dihalo-
methanes to Ru(0),9 although the present strategy affords the
simultaneous delivery of a tridentate ligand and an alkylidene
to Ru, affording access to a family of compounds.
Of the Ru-alkylidenes 4–9 derived herein, compound 7 has
been previously shown to be inactive in ring-closing, ring-
opening and cross metathesis. However, it is noteworthy that
reacting 7 with two equivalents of BCl3 results in an active cat-
ionic metathesis catalyst.6 In addition, the products described
herein can also act as further synthons. Reaction of 7 with a
2 : 1 mixture of PhCOCl and PCy3 cleanly liberates the bis-
thioester O(CH2CH2SC(O)Ph)2 and generates the Grubbs’ gen-
Scheme 1 Synthesis of 4–10.
eration II catalyst (Cy3P)Cl2RuvCHPh(SIMes) (Scheme 2).3
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Conclusions
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2712 | Dalton Trans., 2014, 43, 2710–2712
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