Coupling of alkynols and a phenyl group to a novel η5- dihydronaphthalenide ligand on a ruthenium template

Article abstract of DOI:10.1039/b412052c

We report the highly selective assembly of unprecedented η⁵-1-methylene-1,2-dihydronaphthalenide ligands from the stoichiometric coupling of a phenyl group and two equivalents of disubstituted propargylic alcohols; in this reaction, tetraphenylborate acts as a phenylating agent.

Full text of DOI:10.1039/b412052c

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Coupling of alkynols and a phenyl group to a novel g5-
dihydronaphthalenide ligand on a ruthenium template{
ˇ
Jadranka Cubrilo, Rainer F. Winter* and Dietrich Gudat
Received (in Cambridge, UK) 5th August 2004, Accepted 23rd September 2004
First published as an Advance Article on the web 2nd December 2004
DOI: 10.1039/b412052c
We report the highly selective assembly of unprecedented g5-1-
methylene-1,2-dihydronaphthalenide ligands from the stoichio-
metric coupling of a phenyl group and two equivalents of
disubstituted propargylic alcohols; in this reaction, tetraphe-
nylborate acts as a phenylating agent.
and anisochronic methyl groups in the isopropyl substituent of the
cymene unit indicates that the plane of symmetry through this
ligand has been lost. The observation of four additional methyl
signals suggests that two equivalents of the propargylic alcohol
have been incorporated into the product. The single OH proton
signal (d 5 1.78) in the 1H NMR and strong bands at 3545 cm21
and 1175 cm21 arising from the OH and out-of-phase-C–C–O
stretches in the IR spectrum are characteristic of a tertiary alcohol.
The presence of just one OH group in the product suggests that
one equivalent of water was lost. The remaining 1H and 13C
resonances comprise the signals of four quaternary carbon atoms
and two more olefinic CH-units that resonate at rather high field.
These are attributable to ruthenium coordinated LCH moieties.
Four additional LCH-signals are partially overlapped by the
resonances of the counterion and are characteristic of non-
coordinated arenes.
The coupling of alkynes is of pivotal importance for the assembly
of a broad variety of unsaturated cyclic and linear molecules.
Typical examples include their cyclotrimerization to substituted
arenes1,2 and their linear coupling to butenynes, butatrienes,
hexadienynes or octatetraenes.3–7 Amongst the wealth of such
coupling reactions efficient transformations of propargylic alcohols
are extremely rare. This is because the free hydroxy group strongly
interacts with many of the cyclisation catalysts employed to date,
thus preventing efficient transformations. The few reported
examples include their linear tail-to-tail dimerization to give
hydroxy substituted butadienones,8 their cyclodimerization to
alkylidene cyclobutenes in the presence of a carboxylic acid9 and
the insertion of alkynols into a ruthenacyclopentatriene to give
The two-dimensional C,H and H,H correlation spectra allowed
us to establish the ligand structure as it is depicted in Chart 1. Two
methyls and the OH group are bonded to an aliphatic carbon
atom which resonates at 70.0 ppm. The two remaining methyl
groups and two quaternary olefinic carbon atoms (d(13C) 5 121.5,
123.0) form a CLCMe2 fragment that presumably originates from
the incorporation of a second propargylic alcohol with concomi-
tant loss of one equivalent of water. The analysis of further one-
bond and long-range correlations in H,C HSQC and HMBC
spectra suggests that the terminal carbon atom of the isobutylidene
unit, two p-coordinated CH carbon units (d(13C) 5 48.5, 77.4),
and the three remaining quaternary carbon atoms form a six-
membered ring. One of the latter (d(13C) 5 113.8) is substituted by
the CMe2OH group while the two remaining ones (d(13C) 5 81.7,
101.4) and the four residual LCH-units comprise a second
vinylbutatrienyl ligands.10 The cyclotrimerization of 2-methylbut-
11,12
3-yn-2-ol to either 1,2,4- or 1,3,5-C6H3(CMe2OH)3
and the
cyclotrimerization of but-2-yne-1,4-diol to the corresponding
benzene have also been observed.13
We report here on the co-cyclisation of two equivalents of
disubstituted propargylic alcohols and a phenyl group to afford
unprecedented
g5-1-methylene-1,2-dihydronaphthalen-2-ide
ligands with tetraphenylborate as a phenylating agent. We
discovered this unusual coupling reaction during our investigations
on catalytic transformations of propargylic alcohols with ruthe-
nium complexes such as [(p-cymene)RuCl2]2, 1. Treatment of 1
with NaSbF6 and an excess of 2-methylbut-3-yn-2-ol gave only the
2
known trichloro bridged dimer [{(p-cymene)Ru}2(m-Cl)3]+ SbF6
which was characterized by NMR spectroscopy and X-ray
analysis. When other halide abstracting agents were employed,
intractable product mixtures were obtained. In the presence of
tetraphenylborate, however, a single clean product 2a was formed
as was indicated by NMR spectroscopy.
Since all our attempts to grow X-ray quality crystals of this
product have failed up to now, its identification rests on the results
from NMR spectroscopy (1D and 2D NMR), IR, mass spectro-
1
metry and analytical data.{ The H and 13C{1H} NMR spectra
2
display the resonances of an intact BPh4 counterion and a
p-conjugated p-cymene ligand that are present in a 1 : 1 ratio. The
appearance of four distinguishable sets of p-coordinated CH-units
{ Electronic supplementary information (ESI) available: experimental
section. See http://www.rsc.org/suppdata/cc/b4/b412052c/
*winter@iac.uni-stuttgart.de
Chart 1 1H and 13C shifts (in italics) of the g5-dihydronaphthalenide
ligand of 2a; observed NOEs are indicated as wavy lines.
510 | Chem. Commun., 2005, 510–512
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six-membered ring that is annellated with the first one to give a
naphthalene skeleton (see Chart 1). The positions and spatial
arrangement of the exocyclic propylidene and CMe2OH moieties
were substantiated by the detection of NOE correlations between
the CH proton at 4.70 ppm and the methyl protons at 1.57 ppm,
the CH proton at 6.70 ppm and the CH3 protons at 1.44 and
1.77 ppm, and between one of the aromatic protons at 7.48 ppm
and the nuclei of the second isopropylidene CH3 group at
1.66 ppm. The observation of characteristic upfield shifts for five
of the six atoms in the disubstituted ring (cf. Chart 1) suggests that
the naphthalene framework is bonded in an g5-coordination mode
and, since all carbon atoms in the fused ring system and the
exocyclic methylene unit are three-coordinate, carries a negative
charge. The whole complex cation may thus be described in terms
of a Ru(II) atom that is coordinated by a neutral g6-bound cymene
and a uninegative benzannellated 1-methylene-1,2-dihydrocyclo-
hexadienide ligand which behaves essentially as a pentadienyl
equivalent. Positive ion EI (70 eV) and CI MS spectra (NH3
reactand gas) gave the molecular ion peak at m/z 462.1 in 88%
intensity with the correct isotope pattern (see Supporting
Information). The base peak at m/z 444.1 results from the loss
of water from the CMe2OH entity.
concomitant aromatization followed by dehydration. While all
intermediates along the proposed reaction path are speculative, we
could show that the uncoordinated phenyl ring of the naphtha-
lenide skeleton as well as the hydrogen atom lost in the
dehydration step both arise from the BPh4 anion.14 When
2
15
BPh42-d20 was employed, all of the CH resonances of the non
coordinated part of the naphthalenide ring appeared as non-
binomial quartets in 13C NMR spectra and none of the
corresponding proton resonance signals could be observed.
Likewise, the OH signal at 1.78 ppm and the IR OH band at
3545 cm21 were considerably weakened, and the latter is partially
replaced by a sharp intense band at 2242 cm21. This points to
partial H/D exchange with the water liberated as DOH in the
dehydration process. In the EI MS the molecular ion peak shifts
by five mass units, attesting to the incorporation of five D atoms.
We note that action of the BPh42 anion as a phenylating agent,
although rare, is not without precedent, especially in ruthenium
chemistry. Thus, [{CpRu(CO)2}2(m-X)]+ (X 5 Cl, Br) reacts with
NaBPh4 to give
a
mixture of [CpRu(CO)2X] and
[CpRu(CO)2(Ph)].16 Any of the other reaction steps in Scheme 1
are elementary processes in many transition metal catalyzed or
mediated conversions of alkynes. We also note the high
regioselectivity observed in each of the addition/insertion steps.
In fact, we have not been able to detect any other regioisomer of 2a
in the crude product by NMR spectroscopy.
As to the formation of the dihydronaphthalenide ligand we
suggest the reaction sequence outlined in Scheme 1. In the first step
NaBPh4 acts as a phenylating agent toward the p-cymene
ruthenium dimer, giving [(p-cymene)RuCl2(Ph)]2. Substitution of
one chloride by one equivalent of the propargylic alcohol would
then render [(p-cymene)RuCl(Ph)(g2-HCCCMe2OH)]. Migratory
insertion of the alkyne into the Ru–phenyl bond, possibly via the
corresponding vinylidene [(p-cymene)Cl(Ph)RuLCLCHCMe2OH],
would give the unsaturated vinyl intermediate B. Coordination of
one further equivalent of the alkynol followed by another insertion
step would then give the 4-phenylpentadienyl intermediate
C. Electrocyclic ring closure would generate intermediate D which
transforms into the final product via a 1,3 hydrogen shift with
In order to widen the scope of this reaction we also examined
other alkynols. Essentially the same results were obtained for
1-ethynylcyclohexanol and 1-ethynylcyclopentanol, giving com-
plexes 2b,c (see Supporting Information){. 2-Phenylbutynol and
1,1-diphenyl-2-propyn-1-ol, on the other hand, gave a complex
mixture from which no clean products could be obtained or
identified. 1,1-Dimethylpropyne also failed to react. The related
dimer [(g6-C6Me6)RuCl2]2 gave only small amounts of a complex
corresponding to 2a when treated with NaBPh4 and 2-methylbut-
3-yn-2-ol either at room temperature or under reflux conditions.
We then considered that electrophilic addition to the negatively
charged site(s) of the coordinated naphthalenide ring might induce
a haptotropic rearrangement to a symmetrical dicationic sandwich
structure where the ruthenium atom is coordinated to the benzene
ring of the dihydronaphthalenide ligand. Treatment of 2a with
+
2
various electrophiles such as MeI, Meerwein’s salt OMe3 BF4
,
CF3SO3Me, HBF4 and CF3SO3H however resulted in no
detectable changes of the cation’s NMR signals even after
prolonged reaction times. This attests to the low electrophilicity
of the coordinated dienide ring.
In summary, we have disclosed a novel co-cyclization of a
phenyl group and 2 equivalents of an alkynol to an unprecedented
2-methylene-2,3-dihydronaphthalenide ligand which acts as a Cp2
equivalent. The uncoordinated phenyl ring originates from the
BPh42 counterion as has been shown by deuterium labeling. This
establishes a further example of this anion acting as a phenylating
agent.
Financial support of this work by the Deutsche
Forschungsgemeinschaft (grant Wi1262/4-1) is gratefully
acknowledged.
ˇ
Jadranka Cubrilo, Rainer F. Winter* and Dietrich Gudat
Institut fu¨r Anorganische Chemie der Universita¨t Stuttgart,
Pfaffenwaldring 55, D-70569, Stuttgart, Germany.
Scheme 1 Proposed reaction sequence in the formation of complexes 2.
This journal is ß The Royal Society of Chemistry 2005
E-mail: winter@iac.uni-stuttgart.de; Fax: +49 711 685 4165
Chem. Commun., 2005, 510–512 | 511

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Notes and references
{ Selected spectroscopic data: compound 2a: 1H-NMR (250 MHz,
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3
CD2Cl2), d 1.28, 1.41 [each 3H, d, CH3(iPr), JH–H 5 6.88 Hz], 1.44,
1.57 [each 3H, s, C(CH3)2OH], 1.66, 1.77 [each 3H, s, CH3(naph)], 1.78
[1H, s(br.), OH], 2.05 [3H, s, CH3(cym)], 2.65 [1H, hept, CH (iPr),
3
3JH–H 5 6.88 Hz], 4.18 [1H, d, CH(cym), JH–H 5 6.2 Hz], 4.7 [1H, s,
CH(naph)], 4.82, 5.31, 5.36 [each 1H, d, CH(cym), JH–H 5 6.2 Hz], 6.7
3
3
[1H, s, CH(naph)], 7.31 [1H, t, JHH 5 7.12 Hz, CH(naph)], 7.34 [1H, t,
3
CH(naph), JH–H 5 7.18 Hz], 7.42 [1H, t, CH(naph)], 7.48 [1H, m,
CH(naph)]. 13C-NMR (250 MHz, CD2Cl2), d 18.9, 19.8 [each s,
CH3(naph)], 19.2 [s, CH3(cym)], 21.95, 31.3 [each s, C(CH3)2OH], 24.2,
31.8 [s, CH3(iPr)], 33.85 [s, CH(iPr)], 48.5 [s, CH(naph)], 70.0 [s, CMe2OH],
77.4 [s, CH(naph)], 81.7 [s, Cq(naph)], 84.7, 88.2, 86.7, 90.6 [each s,
CH(cym)], 101.4 [s, Cq(naph)], 105.6, 116.2 [each s, Cq(cym)], 121.5, 123.0
[each s, CLC(naph)], 127.2, 127.6, 130.6, 134.4 [each s, CH(naph)]. IR
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calculated (measured): C 76.81 (75.78), H 6.83 (6.61). MS: positive ion EI,
70 eV, 470 K: 462.1 (M+, 13%), 444.1 (M+ 2 H2O, 31%); positive ion CI,
400 K: 462.1 (M+, 88%), 444.1 (M+ 2 H2O, 100%).
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512 | Chem. Commun., 2005, 510–512
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