The mechanism of the Ru-assisted C-C bond cleavage of terminal alkynes by water

Article abstract of DOI:10.1021/ja9601393

The hydration of phenylacetylene in the presence of the complex mer,trans-(PNP)RuCl₂(PPh₃) in THF at 60°C leads to the cleavage of the C-C triple bond with formation of the carbonyl complex fac,cis-(PNP)RuCl₂(CO) and toluene [PNP = CH₃CH₂CH₂N(CH₂CH₂PPh₂)₂]. A study under different experimental conditions, the use of model and isotope labeling experiments, and the detection of several intermediates, taken altogether, show that the C-C bond cleavage reaction comprises a number of steps, among which the most relevant to the mechanism are 1-alkyne to vinylidene tautomerism, conversion of a vinylidene ligand to hydroxycarbene by intramolecular attack of water, deprotonation of hydroxycarbene to σ-acyl, deinsertion of CO from the acyl ligand, and hydrocarbon elimination by protonation of the metal-alkyl moiety. The following intermediate species have been isolated and characterized: the vinylidene fac,cis-(PNP)RuCl₂{C=C(H)Ph}, the (aquo)(σ-alkynyl) complex fac-(PNP)RuCl(C≡CPh)(OH₂), and the (benzyl)carbonyl mer-(PNP)RuCl(η¹-CH₂Ph)(CO). Other intermediates such as the σ-acyl mer-(PNP)RuCl(η¹-COCH₂Ph)(CO) have been intercepted by addition of appropriate reagents, while the independent synthesis of the aminocarbene complex fac,cis-(PNP)RuCl₂{C(NC₅H₁₀)(CH₂Ph)} and its reaction with water have provided evidence for the intermediacy of a hydroxycarbene species in the C-C bond cleavage reaction.

Full text of DOI:10.1021/ja9601393

J. Am. Chem. Soc. 1996, 118, 4585-4594
4585
The Mechanism of the Ru-Assisted C-C Bond Cleavage of
Terminal Alkynes by Water
Claudio Bianchini,* Juan A. Casares, Maurizio Peruzzini,* Antonio Romerosa, and
Fabrizio Zanobini
Contribution from the Istituto per lo Studio della Stereochimica ed Energetica dei Composti di
Coordinazione, CNR,Via Jacopo Nardi 39 50132, Firenze, Italy
ReceiVed January 16, 1996X
Abstract: The hydration of phenylacetylene in the presence of the complex mer,trans-(PNP)RuCl2(PPh3) in THF at
60 °C leads to the cleavage of the C-C triple bond with formation of the carbonyl complex fac,cis-(PNP)RuCl2(CO)
and toluene [PNP ) CH3CH2CH2N(CH2CH2PPh2)2]. A study under different experimental conditions, the use of
model and isotope labeling experiments, and the detection of several intermediates, taken altogether, show that the
C-C bond cleavage reaction comprises a number of steps, among which the most relevant to the mechanism are
1-alkyne to vinylidene tautomerism, conversion of a vinylidene ligand to hydroxycarbene by intramolecular attack
of water, deprotonation of hydroxycarbene to σ-acyl, deinsertion of CO from the acyl ligand, and hydrocarbon
elimination by protonation of the metal-alkyl moiety. The following intermediate species have been isolated and
characterized: the vinylidene fac,cis-(PNP)RuCl2{CdC(H)Ph}, the (aquo)(σ-alkynyl) complex fac-(PNP)RuCl-
(CtCPh)(OH2), and the (benzyl)carbonyl mer-(PNP)RuCl(η1-CH2Ph)(CO). Other intermediates such as the σ-acyl
mer-(PNP)RuCl(η1-COCH2Ph)(CO) have been intercepted by addition of appropriate reagents, while the independent
synthesis of the aminocarbene complex fac,cis-(PNP)RuCl2{C(NC5H10)(CH2Ph)} and its reaction with water have
provided evidence for the intermediacy of a hydroxycarbene species in the C-C bond cleavage reaction.
Introduction
summarized by reporting the following comment from the most
recent edition of ComprehensiVe Organometallic Chemistry:
“The hydrolysis of Vinylidene ligands typically leads to acyl or
alkyl-carbonyl complexes and accordingly Vinylidene intermedi-
ates, while not isolated, are almost certainly inVolVed in the
synthesis of such complexes from terminal alkynes and halide
complexes of ruthenium(II)”.5
In this article we present a mechanistic study of the reaction
between water and the model 1-alkyne, phenylacetylene, in the
presence of a Ru(II) promoter. We are confident that the results
obtained may constitute the final chapter for the mechanism of
the metal-assisted C-C bond cleavage of 1-alkynes by water
as well as provide new insight into many other reactions in
which C-C bond cleavage is believed to occur Via metal-
hydroxycarbene intermediates (i.e., Fischer-Tropsch chemistry).
The hydration of alkynes to give carbonyl compounds is a
well-known reaction1 that can be promoted by either electro-
philes (H+, Hg2+ 2 or transition metal complexes.3 In the
)
peculiar case of terminal alkynes and iron-group metal promot-
ers, the binary combination of the unsaturated hydrocarbon and
water may lead to the cleavage of the C-C triple bond with
formation of CO and the saturated hydrocarbon with one less
carbon atom (generally in the form of carbonyl and alkyl
ligands).4
Although the first example of ruthenium-assisted C-C triple
bond cleavage by water was reported more than ten years ago,4a
the overall mechanism of this important reaction, which involves
two plentiful molecules of nature, is still rather obscure. From
the mechanistic viewpoint, all of what is known may be
X Abstract published in AdVance ACS Abstracts, April 1, 1996.
(1) (a) March, J. AdVanced Organic Chemistry, 4th ed.; Wiley, New
York, 1992; pp. 762-763. (b) Schmid, G. H. in The Chemistry of the Carbon-
Carbon Triple Bond, Patai, S. Ed.; Wiley: UK, 1978; pp 281-288.
(2) (a) Bassetti, M.; Floris, B. J. Chem. Soc., Perkin Trans. 2 1988, 227.
(b) Janout, V.; Regen, S. J. Org. Chem. 1982, 47, 3331. (c) Olah, G. A.;
Meidar, D. Synthesis 1978, 671. (d) Noyce, D. S.; Schiavelli, M. D. J. Org.
Chem. 1968, 33, 845. (e) Noyce, D. S.; Matesich, A. M.; Peterson, E. J.
Am. Chem. Soc. 1967, 89, 6225. (f) Menashe, N.; Reshof, D.; Shvo, Y. J.
Org. Chem. 1991, 56, 2912. (g) Larock, S. SolVation/Demercuriation
Reactions in Organic Synthesis; Springer: New York, 1986; pp 123-148.
(3) (a) Khan, M. M. T.; Martell, A. E. Homogeneous Catalysis by Metal
Complexes; Academic Press, New York, 1974; Vol. II, p 91. (b) Hiscox,
W.; Jennings, P. W. Organometallics 1990, 9, 1997. (c) Harman, W. D.;
Dobson, J. C.; Taube, H. J. Am. Chem. Soc. 1989, 111, 3061. (d) Utimoto,
K. Pure Appl. Chem. 1983, 55, 1845. (e) Halpern, J.; James, B. R.; Kemp,
L. W. J. Am. Chem. Soc. 1961, 83, 4097. (f) Halpern, J.; James, B. R.;
Kemp, L. W. J. Am. Chem. Soc. 1966, 88, 5142.
Experimental Section
General Procedures. Tetrahydrofuran (THF), chloroform, and
dichloromethane were purified by distillation under nitrogen over
LiAlH4 and P2O5, respectively. Piperidine and triethylamine were
purchased from Carlo Erba, dried over KOH and distilled from BaO
under an N2 atmosphere prior to use. Oxygen-18 labeled water (95
atom % 18O) was purchased from Aldrich. All the other reagents and
chemicals were reagent grade and, unless otherwise stated, were used
as received by commercial suppliers. All reactions and manipulations
were routinely performed under a dry nitrogen atmosphere by using
standard Schlenk-tube techniques. The solid complexes were collected
on sintered glass-frits and washed with light petroleum ether (b.p.
40-60 °C) or n-pentane before being dried in a stream of nitrogen.
The ligand CH3CH2CH2N(CH2CH2PPh2)2 (PNP)6 and the complexes
(4) (a) Bruce, M. I.; Swincer, A. G. Aust. J. Chem. 1980, 33, 1471. (b)
Abu Salah, O. M.; Bruce, M. I. J. Chem. Soc., Dalton Trans. 1974, 2302.
(c) Sullivan, B. P.; Smythe, R. S.; Kober, E. M.; Meyer, T. J. J. Am. Chem.
Soc. 1982, 104, 4701. (d) Mountassir, C.; Ben Hadda, T.; Le Bozec, H. J.
Organomet. Chem. 1990, 388, C13. (e) Knaup, W.; Werner, H. J.
Organomet. Chem. 1991, 411, 471. (f) Bruce, M. I. Pure Appl. Chem. 1986,
58, 553. (g) Davies, S.; McNally, J. P.; Smallridge, A. J. AdV. Organomet.
Chem. 1990, 30, 30.
(5) Hill, A. F. in ComprehensiVe Organometallic Chemistry II. Abel, E.
W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, UK,
1995; Vol. 7, Chapter 6, p 351.
(6) Sacconi, L.; Morassi, R. J. Chem. Soc. A 1969, 2904. An improved
synthesis of the PNP ligands is reported: Bianchini, C.; Farnetti, E.;
Glendenning, L.; Graziani, M.; Nardin, G.; Peruzzini, M.; Rocchini, E.;
Zanobini, F. Organometallics 1995, 14, 1489.
S0002-7863(96)00139-4 CCC: $12.00 © 1996 American Chemical Society

4586 J. Am. Chem. Soc., Vol. 118, No. 19, 1996
Bianchini et al.
mer,trans-(PNP)RuCl2(PPh3) (1)7 and fac,cis-(PNP)RuCl2{CdC(H)-
Ph} (4)8 were prepared as described in the literature. Deuterated
solvents for NMR measurements (Merck and Aldrich) were dried over
molecular sieves (0.4 nm). 1H and 13C{1H} NMR spectra were recorded
on Varian VXR 300 or Bruker AC200 spectrometer operating at 299.94
or 200.13 MHz (1H) and 75.42 or 50.32 MHz (13C), respectively. Peak
positions are relative to tetramethylsilane and were calibrated against
the residual solvent resonance (1H) or the deuterated solvent multiplet
(13C). 13C-DEPT experiments were run on the Bruker AC200 spec-
trometer. 1H,13C-2D HETCOR NMR experiments were recorded on
either the Bruker AC200 spectrometer using the XHCORR pulse
program or a Bruker AVANCE DRX 500 spectrometer equipped with
a 5-mm triple resonance probe head for 1H detection and inverse
detection of the heteronucleus (inverse correlation mode, HMQC
Synthesis of mer-(PNP)RuCl(η1-CH2Ph)(CO) (3) (Open Reactor).
Neat phenylacetylene (0.50 mL, 4.50 mmol) was pipetted into a well
stirred THF slurry (50 mL) of 1 (0.40 g, 0.44 mmol) containing 80 µL
(4.44 mmol) of distilled water and the mixture was refluxed under a
stream of nitrogen for 5 h with stirring. During this time, all 1 dissolved
to give a pale yellow solution which, after cooling to room temperature
separated pale yellow crystals of 3 by addition of ethanol (50 mL)
and slow concentration under nitrogen. Yield 93%. Anal. Calcd for
C39H42NClOP2Ru: C, 63.37; H, 5.73; N, 1.89. Found: C, 63.29; H,
5.65; N, 1.76. IR: ν(CtO) 1899 (s) cm-1 31P{1H} NMR (21 °C,
.
CD2Cl2, 121.42 MHz) 30.64 (s); 1H NMR (21 °C, CD2Cl2, 200.15
MHz): δCH2Ph 3.00 (t, 3JHP 4.2 Hz, 2H). 13C{1H} NMR (21 °C, CD2-
2
Cl2, 50.32 MHz): δCtO 204.50 (t, JCP 14.1 Hz), δCipso-CH2Ph 159.20
3
(t, JCP 2.9 Hz), δCH3CH2CH2N 58.71 (s), δNCH2CH2P 52.17 (s), δNCH2CH2P
1
experiment) with no sample spinning. The H,1H-2D COSY NMR
33.48 [vt, N ) JCP + JCP′ ) 12.0 Hz], δCH3CH2CH2N 13.96 (s), δCH3CH2CH2N
3
experiments were routinely conducted on the Bruker AC200 instrument
in the absolute magnitude mode using a 45° or 90° pulse after the
incremental delay or were acquired on the AVANCE DRX 500 Bruker
spectrometer using the phase-sensitive TPPI mode with double quantum
filter. 31P{1H} NMR spectra were recorded on either the Varian VXR
300 or Bruker AC200 instruments operating at 121.42 and 81.01 MHz,
respectively. Chemical shifts were measured relative to external 85%
H3PO4 with downfield values taken as positive. The proton NMR
spectra with broad-band phosphorus decoupling were recorded on the
Bruker AC200 instrument equipped with a 5-mm inverse probe and a
BFX-5 amplifier device using the wideband phosphorus decoupling
sequence GARP. Infrared spectra were recorded as Nujol mulls on a
Perkin-Elmer 1600 series FT-IR spectrometer between KBr plates.
Reactions under controlled pressure were performed with a Parr 4565
reactor equipped with a Parr 4842 temperature and pressure controller.
A Shimadzu GC-14A/GCMS-QP2000 instrument was employed for
all GC-MS investigations. Elemental analyses (C, H, N) were
performed using a Carlo Erba Model 1106 elemental analyzer.
11.95 (s), δCH2Ph 11.04 (t, JCP 6.2 Hz).
In a separate experiment, the outlet of the Schlenk flask was
connected to another flask containing a solution of NEt3 in diethyl ether.
As the gases coming from the reactor bubbled into the latter solution,
precipitation of [NEt3H]Cl occurred.
Synthesis of mer-(PNP)RuCl(η1-CD2Ph)(CO) (3-d2). Replacing
H2O with D2O and EtOH with EtOD in the above reaction gave the
isotopomer mer-(PNP)RuCl(η1-CD2Ph)(CO) (3-d2) in which the selec-
tive replacement of the methylenic hydrogen atoms of the benzylic
1
group by deuterium was shown by H and 13C-DEPT NMR spectros-
copy.
Stepwise Reaction of 1 with Phenylacetylene and Water (in Situ
NMR Experiment). Neat phenylacetylene (28 µL, 0.25 mmol) was
added via syringe to a solution of 1 (23 mg, 0.025 mmol) in a CDCl3/
THF-d8 mixture (1.0 mL 4:1 v/v) in a screw-cap 5 mm NMR tube.
The tube was placed into the probe of a NMR spectrometer preheated
at 60 °C. Within ca. 2 h, all 1 selectively transformed into the
known vinylidene complex fac,cis-(PNP)RuCl2{CdC(H)Ph}8 (4) (31P,
1H NMR) and free PPh3. At this point, water (4.50 µL, 0.25 mmol)
was syringed into the NMR tube cooled to room temperature. The
tube was again inserted into the NMR probe at 60 °C, and 31P{1H}
Reaction of mer,trans-(PNP)RuCl2(PPh3) (1) with Phenylacet-
ylene and Water (in situ NMR Experiment). Neat phenylacetylene
(28 µL, 0.25 mmol) and water (4.50 µL, 0.25 mmol) were added via
syringe to a solution of 1 (23 mg, 0.025 mmol) in a CDCl3/THF-d8
mixture (1.0 mL 4:1 v/v) in a 5-mm NMR tube. The tube was flame-
sealed under nitrogen and then placed into the probe of a NMR
spectrometer preheated at 60 °C. The reaction was followed at this
1
and H NMR spectra were acquired every 30 min. The practically
immediate transformation of 4 into 3 was primarily observed, followed
by slow conversion of 3 into 2 (quantitative transformation in ca. 5 h).
Reaction of 3 with HCl. A slight excess of HCl (1 M solution in
H2O) was added via syringe into a 5-mm screw cap NMR tube
containing a THF-d8 (0.8 mL) solution of 3 (26 mg, 0.035 mmol).
31P{1H} and 1H NMR spectroscopy showed the quantitative transforma-
tion of 3 into 2. GC-MS analysis of the solution confirmed the
formation of 1 equiv of toluene.
1
temperature by 31P{1H} and H NMR spectroscopy over a period of
14 h. Already after the acquisition of the first spectrum, 1 had partially
converted to two species, namely the known carbonyl complex
fac,cis-(PNP)RuCl2(CO) (2)8 and the new complex mer-(PNP)RuCl-
(η1-CH2Ph)(CO) (3) (see below) with formation of free PPh3 (31P
NMR: singlet at -4.57 ppm). As an example, 82% of 1 transformed
into a 63:19 mixture of 2 and 3 in 4 h. After 7 h, all 1 was converted
into a 84:16 mixture of 2 and 3; further heating for ca. 2 h transformed
completely 3 into 2 that is thermally stable in these reaction conditions.
1H NMR and GC-MS analysis showed increasing formation of toluene
with the disappearance of 3. After total disappearance of 3, a careful
NMR integration of the toluene singlet (δ 2.33) with respect to the
n-propyl CH3 resonance of the PNP ligand in complex 2 (δ 0.96),
confirmed the formation of about 1 equiv of toluene.
Reaction of mer,trans-(PNP)RuCl2(PPh3) (1) with Phenylacet-
ylene and Water (Parr Reactor Experiment). A solution of 1 (0.60
g, 0.65 mmol) and a tenfold excess of both phenylacetylene (0.75 mL,
6.70 mmol) and water (0.12 mL, 6.66 mmol) in a CHCl3/THF mixture
(40 mL 4:1 v/v) was placed into the Parr reactor and pressurized with
nitrogen (1 atm). After 9 h at 60 °C, the reactor was cooled to room
temperature. The contents of the reactor were transferred into a Schlenk
flask and a sample of the solution, analyzed by GC-MS, showed the
formation of toluene. The remaining solution was concentrated to
dryness in vacuo to give a yellow powder which was analyzed by 1H,
31P{1H} and 13C{1H} NMR spectroscopy, which showed the quantitative
transformation of 1 into 2.
Replacing 3 with 3-d2 in the above reaction, yielded 1 equiv of
monodeuterated toluene, PhCH2D (1H NMR and GC-MS analysis).
Reaction of fac,cis-(PNP)RuCl2{CdC(H)Ph} (4) with Water (in
situ NMR Experiment). A 5-mm NMR tube was charged with a
solution of the vinylidene complex 4 (20 mg, 0.026 mmol) in a CDCl3/
THF-d8 mixture (1.0 mL 4:1 v/v) containing a tenfold excess of water
(4.70 µL, 0.26 mmol). The tube was flame sealed under nitrogen and
then inserted into the probehead of a NMR spectrometer preheated at
60 °C. The reaction was followed by 31P{1H} and 1H NMR
spectroscopy at this temperature. The rapid conversion of 4 to 3 was
observed, followed by slow conversion of 3 into 2 (complete trans-
formation in ca. 5 h). 1H NMR and GC-MS analysis showed the
formation of toluene during the secondary transformation of 3 into 2.
Reaction of mer,trans-(PNP)RuCl2{CdC(H)Ph} (4) with Water
(Parr Reactor Experiment). A solution of 4 (0.40 g, 0.52 mmol)
and a tenfold excess of water (0.10 mL, 5.55 mmol) in a CHCl3/THF
mixture (30 mL 4:1 v/v) was placed into a Parr reactor, pressurized
with one atm of nitrogen, and heated to 60 °C with stirring. After 6 h,
the reactor was cooled to room temperature and opened. The contents
of the reactor were transferred into a Schlenk flask and a sample of
the solution, analyzed by GC-MS, showed the formation of toluene.
The rest of the solution was concentrated to dryness in vacuo to give
a yellow powder which was authenticated by 31P{1H} and 13C{1H}
NMR spectroscopy as 2.
(7) Bianchini, C.; Innocenti, P.; Masi, D.; Peruzzini, M.; Zanobini, F.
Gazz. Chim. It. 1992, 122, 461. The X-ray structure of mer,trans-(PNP)-
RuCl2(PPh3) has been determined: Bianchini, C.; Masi, D.; Peruzzini, M.;
Romerosa, A.; Zanobini, F. Acta Crystallogr. 1995, C51, 2514.
(8) Bianchini, C.; Innocenti, P.; Peruzzini, M.; Romerosa, A.; Zanobini,
F. Organometallics 1996, 15, 272.
Reaction of mer,trans-(PNP)RuCl2{CdC(H)Ph} (4) with Water
(Open Reactor). A solid sample of 4 (0.20 g, 0.26 mmol) was
suspended in THF (40 mL) containing 47.0 µL (2.61 mmol) of distilled

C-C Bond CleaVage of Terminal Alkynes by H2O
J. Am. Chem. Soc., Vol. 118, No. 19, 1996 4587
water. The mixture was slowly brought to the boiling point and then
refluxed with stirring for 3 h under a stream of nitrogen. During this
time all 4 dissolved to give a pale yellow solution which after cooling
to room temperature separated pale yellow crystals of 3 by addition of
ethanol (30 mL) and slow concentration under nitrogen. Yield 90%.
Reaction of fac,cis-(PNP)RuCl2{CdC(H)Ph} (4) with D2O
(Schlenk-Flask Experiment). Replacing H2O with D2O in the above
reaction at room temperature gave fac,cis-(PNP)RuCl2{CdC(D)Ph}
(4-d1) via H/D exchange of the vinylidene proton (disappearance of
the vinylidene proton triplet at 5.32 ppm). Heating at the constant
temperature of 60 °C in the spectrometer probehead produced initially
3-d2 and ultimately 2 together with 1 equiv of toluene-d3 (GC-MS
analysis).
a pale yellow powder. 31P{1H} NMR spectroscopy of the crude product
showed the formation of 6 (60%) and of its geometric isomer mer-
(PNP)RuCl(COCH2Ph)(CO) (7) in which the acyl ligand is located trans
to the chloride (40%). Heating this mixture at 60 °C for 12 h in the
NMR tube transformed all 7 into 6, which results thermally stable at
the experimental temperature. Compound 7: IR: ν(CtO) 1916 (vs)
cm-1; ν(COCH2Ph) 1605 (br s) cm-1 31P{1H} NMR (22 °C, CDCl3,
.
81.15 MHz) 38.96 (s). 1H NMR (21 °C, CD2Cl2, 200.15 MHz):
δCOCH2Ph ca. 3.1 (s, partially masked by the aliphatic signals of the
PNP ligand). 13C{1H} NMR (21 °C, CDCl3, 50.32 MHz): δCOCH2Ph
257.43 (t, 2JCP 10.9 Hz), δCtO 206.75 (t, 2JCP 11.9 Hz), δCipso-CH2Ph 159.37
(t, 3JCP 2.3 Hz), δCH3CH2CH2N 70.80 (s), δCH2Ph 55.35 (s), δNCH2CH2P 52.79
(s), δNCH2CH2P 32.21 [vt, N ) JCP + JCP′ ) 11.6 Hz], δCH3CH2CH2N 13.93
(s), δCH3CH2CH2N 12.24 (s).
Reaction of 4 with H218O (Schlenk-Flask Experiment). A solid
sample of 4 (0.10 g, 0.13 mmol) was dissolved in THF (25 mL)
containing 25 µL of H218O (95 atom % 18O, 1.25 mmol). Work up as
described above for the reaction of 4 with water, gave mer-(PNP)-
RuCl(η1-CH2Ph)(C18O) (3-18O). Yield 88%. IR: ν(Ct18O) 1853 (s)
Synthesis of fac,cis-(PNP)RuCl2{C(NC5H10)(CH2Ph)} (8).
A
threefold excess of piperidine, HNC5H10 (0.20 mL, 2.02 mmol), was
added at room temperature to a stirred suspension of 4 (0.50 g, 0.66
mmol) in 30 mL of CH2Cl2. The reaction mixture was stirred in the
dark overnight during which time all the starting vinylidene complex
dissolved to afford a canary yellow solution which was evaporated to
ca. 5 mL. Addition of light petroleum ether gave canary yellow
microcrystals of the aminocarbene 8, which were recrystallized from
CH2Cl2 and light petroleum ether. Yield 96%. Anal. Calcd for
C44H52N2Cl2P2Ru: C, 62.70; H, 6.22; N, 3.32. Found: C, 62.48; H,
6.17; N, 3.12. 31P{1H} NMR (22 °C, C6D6, 81.01 MHz) 50.07 (s). 1H
NMR (22 °C, C6D6, 200.15 MHz): δCH2Ph 4.62 (s, 2H). 13C{1H} NMR
(22 °C, C6D6, 50.32 MHz): δRu)C 252.40 (t, 2JCP 14.2 Hz), δCH3CH2CH2N
62.94 (s), δCH2Ph 57.92 (s), δNCH2CH2P 57.41 (s), δpiperidine(CH2)R 51.85,
δNCH2CH2P 31.64 [vt, N ) JCP + JCP′ ) 13.2 Hz], δpiperidine(CH2)â 27.71,
δpiperidine(CH2)γ 25.37, δCH3CH2CH2N 16.05 (s), δCH3CH2CH2N 12.89 (s).
cm-1
.
Reaction of 4 with H218O (in situ NMR Experiment). A 5-mm
NMR tube was charged with a solution of 4 (20 mg, 0.026 mmol) in
CDCl3/THF-d8 mixture (1.0 mL 4:1 v/v) and with a tenfold excess of
H218O (5.20 µL, 0.26 mmol). The tube was flame sealed under nitrogen
and then inserted into the probehead of the spectrometer preheated at
60 °C. The reaction was followed by 31P{1H} and 1H NMR
spectroscopy at this temperature over a period of 6 h. A clean
transformation of 4 into 3-18O and a secondary conversion of 3-18O
into fac,cis-(PNP)RuCl2(C18O) (2-18O) was observed. 1H NMR and
GC-MS analysis confirmed the formation of toluene during the
secondary transformation of 3-18O into 2-18O. IR: ν(Ct18O) 1899 (s)
cm-1
.
Reaction of 8 with Water. (A) In Situ NMR Experiment in THF-
d8. A fivefold excess of H2O (3.5 µL, 0.19 mmol) was added via
syringe into a 5-mm screw cap NMR tube containing a THF-d8 (0.8
mL) solution of 8 (30 mg, 0.036 mmol). The reaction was monitored
High-Pressure Carbonylation Reaction of the Vinylidene Com-
plex 4 in the Presence of Water. A solution of 4 (0.20 g, 0.26 mmol)
in THF (20 mL) containing 50 µL of water (2.77 mmmol) was prepared
in the Parr reactor, which was pressurized with 30 atm of carbon
monoxide. The reactor was heated to 60 °C and then stirred for 2 h.
After this time, the reactor was cooled to room temperature and slowly
depressurized under a nitrogen stream. The contents of the reactor
were analyzed by GC-MS (which showed the presence of both free
benzylaldehyde and phenylacetylene) and then transferred into a Schlenk
flask. After the solvent was removed under reduced pressure, a pale
yellow solid was obtained. 31P{1H} NMR spectroscopy of the crude
product showed the formation of several ruthenium-PNP complexes:
the new (η1-acyl)carbonyl complex mer-(PNP)RuCl(COCH2Ph)(CO)
(6) (61%) (see below), 2 (14%), and mer,trans-(PNP)RuCl2(CO) (5)
(12%) (some unidentified compounds were also produced). Recrys-
tallization of the crude product from CH2Cl2/n-hexane gave 6 in almost
analytically pure form (traces of 2 and 5 which may occasionally
contaminate the product, can be removed by further recrystallization).
Compound 6: Anal. Calcd for C40H42NClO2P2Ru: C, 62.57; H, 5.52;
N, 1.83. Found: C, 62.40; H, 5.38; N, 1.65. IR: ν(CtO) 1939 (vs)
1
by 31P{1H} and H NMR spectroscopy in the temperature range from
-40 °C to 45 °C. A slow reaction occurred already at 30 °C which
transformed 8 in a 40:60 mixture of 3 and 2 within 4 h. No intermediate
species was detected in the course of the transformation of 8. After
12 h, when all the ruthenium was present in the form of 2, GC-MS
analysis of the solution showed the formation of 1 equiv of both toluene
and piperidine.
(B) Preparative Experiment with H218O. A solid sample of 8 (0.20
g, 0.24 mmol) was dissolved in THF (25 mL) containing 25 µL of
H218O (95 atom % 18O, 1.25 mmol). The resulting solution was stirred
at room temperature for 12 h. Addition of ethanol (30 mL), followed
by a slow evaporation under a stream of nitrogen, gave 2-18O in 93%
yield.
(C) Preparative Experiment in CD2Cl2/H2O. The reaction between
8 (0.20 g, 0.24 mmol) and H2O was carried out in liquid-biphase system
(CH2Cl2/H2O) (40 mL, 5:1 v/v) under vigorous stirring for 4 h. After
phase separation, 3 was selectively obtained from the organic phase,
while piperidinium chloride was found in the water phase.
cm-1; ν(COCH2Ph) 1601 (br s) cm-1 31P{1H} NMR (22 °C, CDCl3,
.
81.15 MHz) 32.56 (s). 1H NMR (21 °C, CD2Cl2, 200.15 MHz):
δCOCH2Ph 3.43 (s, 2H). 13C{1H} NMR (21 °C, CDCl3, 50.32 MHz):
Reaction of 8 with Water under Carbon Monoxide. (A) In Situ
NMR Experiment at Room Temperature. A THF-d8 (1.0 mL)
solution of 8 (30 mg, 0.036 mmol), prepared in a screw cap 5-mm
NMR tube, was saturated with carbon monoxide (1 atm) at room
temperature and analyzed by 31P{1H} NMR spectroscopy. The NMR
study showed no reaction within 3 h. Addition of H2O (3.5 µL, 0.19
mmol) Via syringe almost immediately transformed 8 into a mixture
of the two complexes fac-(PNP)RuCl(COCH2Ph)(CO) (9) (see below)
and 7 in a kinetic ratio of 89:11 [31P{1H} NMR integration after 10
min]. With time, 9 transformed into 7 (complete transformation in 3
h at room temperature). After N2 was substituted for CO, the NMR
tube was heated at 60 °C for 12 h during which time 7 completely
transformed into 6.
2
2
δCOCH2Ph 256.39 (t, JCP 8.7 Hz), δCtO 201.33 (t, JCP 13.1 Hz), δCipso-
3
CH2Ph 159.20 (t, JCP 3.3 Hz), δCH3CH2CH2N 64.19 (s), δCH2Ph 56.78 (s),
δNCH2CH2P 51.28 (s), δNCH2CH2P 32.86 [vt, N ) JCP + JCP′ ) 12.0 Hz],
δCH3CH2CH2N 13.72 (s), δCH3CH2CH2N 12.12 (s).
In order to confirm the presence of an -C(O)CH2Ph ligand in 6, a
THF-d8 solution of this complex in an NMR tube was treated with a
slight excess of concentrated HCl solution. The NMR tube was sealed
under nitrogen and then placed into the probehead of a spectrometer.
No reaction occurred below 60 °C. At this temperature, 6 slowly but
selectively converted to 5 while benzylaldehyde was liberated (GC-
MS).
High-Pressure Carbonylation Reaction of 3. A solution of 3 (0.20
g, 0.27 mmol) in THF (20 mL) was placed in the Parr reactor and
pressurized with 30 atm of carbon monoxide. The reactor was heated
to 60 °C and then stirred at this constant tremperature for 1 h. The
reactor was then cooled to room temperature and slowly depressurized
under a nitrogen stream. The contents of the reactor were transferred
into a Schlenk flask and then concentrated to dryness in vacuo to yield
(B) In Situ NMR Experiment at Low Temperature. A THF-d8
(1.0 mL) solution of 8 (90 mg, 0.11 mmol), prepared as above, was
saturated with carbon monoxide (1 atm) at -78 °C and treated with 5
equiv of H2O (9.9 µL, 0.55 mmol). Monitoring the reaction by
31P{1H} and 1H NMR spectroscopy at -20 °C, revealed that 8
immediately disappeared to give 9. After 12 h at this temperature,

4588 J. Am. Chem. Soc., Vol. 118, No. 19, 1996
Bianchini et al.
Scheme 1
only a small amount of 7 (< 5%) was present in the solution [31P{1H}
NMR integration].
Reaction of 1 with Benzylaldehyde. A solution of 1 (0.20 g, 0.25
mmol) and a tenfold excess of freshly distilled benzylaldehyde (0.3
mL, 2.55 mmol) in THF (30 mL) were refluxed with stirring for 2 h.
After the reactor was cooled to room temperature, addition of n-hexane
gave orange crystals of the dimer [Ru2(µ-Cl)3(PNP)2]Cl, which can be
obtained by simple thermolysis of 1 in refluxing THF.7,8
(C) Synthesis of fac-(PNP)RuCl(η1-COCH2Ph)(CO) (9). A steady
stream of carbon monoxide (1 atm) was bubbled throughout a THF
solution (20 mL) of 8 (0.30 g, 0.36 mmol) cooled at -20 °C. After
15 min, 5 equiv of water (32 µL in 1.0 mL of THF) were added under
vigorous stirring. Stirring was continued for 30 min. Addition of light
petroleum ether (30 mL) and concentration of the resulting solution
under a brisk current of CO gave off-white microcrystals of 9. Yield
76%. Anal. Calcd for C40H42NCl2O2P2Ru: C, 59.85; H, 5.27; N, 1.74.
Results
Reaction of mer,trans-(PNP)RuCl2(PPh3) (1) with Pheny-
lacetylene and Water. Treatment of the complex mer,trans-
(PNP)RuCl2(PPh3) (1)7 in THF with a mixture of phenylacet-
ylene and water in a closed reactor at a constant temperature of
60 °C results in the quantitative formation of the known carbonyl
complex fac,cis-(PNP)RuCl2(CO) (2),8 PPh3, and toluene.
Monitoring the reaction in a sealed NMR tube (CDCl3/THF-d8
Found: C, 59.58; H, 5.11; N, 1.67. IR: ν(CtO) 1932 (s) cm-1
;
ν(COCH2Ph) 1630 (br s) cm-1 31P{1H} NMR (-20 °C, THF-d8, 121.42
.
2
MHz) AM spin system: δA 54.01, δM 38.46, JPP 17.0 Hz. 1H NMR
(-20 °C, THF-d8, 200.15 MHz): δCH2Ph (ABX system): δA 5.34, δB
2
5.13, 2JAB 12.6 Hz, 2JAX ≈ JBX 4.9 Hz. 13C{1H} NMR (-20 °C, THF-
2
2
d8, 75.42 MHz): δCOCH2Ph 249.20 (dd, JCPtrans 94.1, JCPcis 11.2 Hz),
2
δCtO 208.22 (dd, JCP 16.0 and 14.1 Hz), δCH3CH2CH2N 63.38 (s),
1
mixture) by NMR spectroscopy (31P{1H}, H) shows that 2 is
δNCH2CH2P 58.99 (s), δCH2Ph 56.00 (s), δNCH2CH2P 30.53 [vt, N ) JCP
JCP′ ) 11.6 Hz], δCH3CH2CH2N 15.70 (s), δCH3CH2CH2N 12.24 (s).
+
formed at the expense of the intermediate (benzyl)carbonyl
complex mer-(PNP)RuCl(η1-CH2Ph)(CO) (3). The NMR analy-
sis shows also that the formation of 3 is accompanied by
formation of free PPh3, while the formation of 2 is accompanied
by formation of toluene. Once formed, 2 is indefinitely stable
under the experimental conditions (Scheme 1).
Synthesis of fac-(PNP)RuCl(CtCPh)(OH2) (10) and fac-(PNP)-
RuCl(CtCPh)(OD2) (10-d2). Compound 4 (0.40 g, 0.53 mmol) was
dissolved in 25 mL of CH2Cl2. Three equiv of triethylamine, (0.22
mL, 1.59 mmol) in water (25 mL) were added at room temperature,
and the resulting biphase system was vigorously stirred for 2 h. The
organic layer was separated, dried over magnesium sulfate, and
concentrated to ca. 5 mL under vacuum. Addition of light petroleum
ether (10 mL) gave orange microcrystals of fac-(PNP)RuCl(CtCPh)-
(OH2) (10). The crude product was recrystallized from CH2Cl2/n-
hexane solution. Yield 72%. Anal. Calcd for C39H42NClOP2Ru: C,
63.37; H, 5.73; N, 1.89. Found: C, 63.06; H, 5.65; N, 1.70. IR:
ν(OH) 3374 cm-1 (br w), ν(CtC) 2074 cm-1 (s). 31P{1H} NMR (22
°C, CD2Cl2, 81.01 MHz) AM system: δA 77.57 (br), δM 64.84 (br);
The (benzyl)carbonyl complex 3 can be isolated in analyti-
cally pure form by performing the reaction in an open reactor
under a constant bubbling of dinitrogen. This experiment also
shows that the formation of 3 is accompanied by evolution of
HCl, which is removed from the reaction mixture by the stream
of inert gas. If HCl is not removed, as occurs in the reaction
carried out in a closed system, it reacts with 3 producing 2 and
toluene (Vide infra). Once isolated, 3 is thermally stable in THF.
Unambiguous characterization of 3 is provided by IR and NMR
spectroscopies. In particular, a singlet at 30.64 ppm in the
31P{1H} NMR spectrum (CD2Cl2, 20 °C) is consistent with a
meridional arrangement of the PNP ligand,8,9 while a triplet
2
(-50 °C, CD2Cl2, 81.01 MHz) AM system: δA 76.85, δM 62.72, JPP
41.1 Hz. 1H NMR (-50 °C, CD2Cl2, 200.15 MHz): δOH2 4.69 (br t,
3JHP 8.8 Hz, 2H). 13C{1H} NMR (20 °C, CD2Cl2, 50.32 MHz): δCtCPh
2
108.93 (t, 2JCPA ≈ JCPM 12.6 Hz), δCtCPh 114.12 (s), δCH3CH2CH2N 62.25
(s), δNCH2CH2P 53.45 (s), δNCH2CH2P 31.09 [vt, N ) JCP + JCP′ ) 11.5
Hz], δCH3CH2CH2N 14.87 (s), δCH3CH2CH2N 12.06 (s).
1
resonance at δ 3.00 (3JHP 4.2 Hz) in the H NMR spectrum
After concentration under reduced pressure, the water phase gave
the ammonium salt [NEt3H]Cl. Substitution of H2O for H2O in the
above reaction gave 10-d2 (contaminated by ca. 10% of 10).
Reaction of 10 and 10-d2 with HCl. A stoichiometric amount of
gaseous HCl (1.3 mL, 0.06 mmol) was syringed into a screw cap 5-mm
NMR tube containing a CDCl3 (1.0 mL) solution of 10 (42 mg, 0.057
mmol). Monitoring the reaction by 31P{1H} and 1H NMR spec-
troscopies at room temperature showed the transformation of 10 into
the (dichloride)carbonyl complex 2 and toluene Via the intermediacy
of the (benzyl)carbonyl complex 3. Some vinylidene complex 4 (20%)
is also formed as a result of protonation of the Câ carbon atom of the
σ-alkynyl ligand in 10, followed by displacement of the coordinated
water by chloride. Just the HCl consumed in this reaction allows the
formation of a comparable amount of 3. As a matter of fact, the use
of an excess of HCl produces only 2 and 4.
shows that the two benzylic protons are magnetically equivalent.
This signal collapses to a singlet in the broad-band decoupled
1H{31P} NMR spectrum and does not show any cross-peak in
the 1H,1H-2D COSY spectrum. A 1H,13C-2D HETCOR experi-
ment clearly correlates this signal with an upfield triplet at 11.04
ppm in the 13C{1H} NMR spectrum which is assigned to the
Ru-CH2 carbon atom of the benzyl group. The relatively low
magnitude of the carbon to phosphorus geminal coupling (2JCP
6.2 Hz) agrees with the meridional geometry of PNP and
indicates also that the benzyl and chloride ligands are mutually
trans. A 13C-DEPT experiment, unambiguosly confirms this
assignment.
Finally, the 13C{1H} NMR spectrum of 3 shows the presence
of a carbonyl resonance at δ 204.50 and of a narrower triplet at
When the isotopomer 10-d2 was used in the place of 10, the reaction
gave 3-d2 and toluene-d2 (GC-MS).
(9) Meek, D. W.; Mazanec, T. Acc. Chem. Res. 1981, 14, 266.

C-C Bond CleaVage of Terminal Alkynes by H2O
J. Am. Chem. Soc., Vol. 118, No. 19, 1996 4589
Scheme 2
159.21 ppm (3JCP 2.9 Hz) due to the ipso carbon of the benzylic
group. It is worth mentioning that both the position and the
2JCP value (14.1 Hz) of the carbonyl resonance suggest that the
CO group is trans to the N donor of PNP8,10 (the X-ray
authenticated carbonyl 2 exhibits a similar triplet signal at δ
2
203.61 ppm with JCP 15.9 Hz).8
Reactions of the Vinylidene Complex fac,cis-(PNP)RuCl2-
{CdC(H)Ph} (4) with H2O, D2O, or H218O. In the absence
of water, the reaction between 1 and HCtCPh in THF at 60
°C gives exclusively the vinylidene complex fac,cis-(PNP)-
RuCl2{CdC(H)Ph} (4).8 In keeping with this previous finding,
the vinylidene 4 is the first compound which is formed in the
stepwise reaction of 1 with HCtCPh and H2O illustrated in
Scheme 2. However, 4 cannot be intercepted in the course of
the one-pot reaction of 1 with HCtCPh and H2O as it rapidly
reacts with H2O to give 3 which slowly converts to 2.
The independent synthesis of the vinylidene complex 48 has
allowed us to use this key compound in the elucidation of the
mechanism of the C-C triple bond cleavage of phenylacetylene.
In particular, valuable information has been obtained by a variety
of isotope labeling experiments (Scheme 3).
Figure 1. Infrared spectra (Nujol mull between KBr plates) of 3 (A)
and 3-18O (B) in the carbonyl stretching region.
ligand, thus confirming that the C-C bond scission of the
vinylidene ligand is caused by water and not by adventitious
O2. In fact, vinylidene metal complexes are known to react
with O2 yielding carbonyl derivatives and aldehydes Via
oxidative cleavage of the C-C bond.8,11
When the reaction between 4 and H218O is carried out in an
open system, the only Ru product obtained is mer-(PNP)RuCl-
(CH2Ph)(C18O) (3-18O) as shown also by the red-field shift
(Figure 1) exhibited by the stretching vibration of the carbonyl
group [ν(CtO) ) 1853 cm-1 (s)]. The frequency-change in this
Reaction of 4 with D2O in an open reactor in the experimental
conditions used for H2O gives the isotopomer mer-(PN-
P)RuCl(η1-CD2Ph)(CO) (3-d2) in which the selective incorpora-
tion of deuterium into the benzylic group is unequivocally
demonstrated by 1H NMR spectroscopy. A standard 13C-DEPT
experiment confirms this result demonstrating about 100%
incorporation of deuterium into the benzylic multiplet at ca. 11
ppm which, in fact, disappears in the DEPT spectrum of 3-d2.
Deuterium labeling experiments carried out directly in the
NMR tube (closed system) show that 2 and 1 equiv of PhCD3
(1H NMR spectroscopy; GC-MS) are selectively formed.
Incorporation of deuterium occurs neither into the phenyl ring
of the benzyl ligand nor the PNP ligand. The formation of
PhCD3 instead of the expected isotopomer PhCD2H occurs
because of a fast intermolecular exchange of the vinylidene
hydrogen atom with deuterium which precedes the incorporation
of D2O into the complex. Indeed, at room temperature 4
completely transforms into fac,cis-(PNP)RuCl2{CdC(D)Ph}
(4-d1) when a drop of D2O is added to a CDCl3/THF-d8 solution
(4:1 v/v).
absorption, ∆ν(CtO) ) ν(Ct O) - ν(Ct O) ) (1899-1853) cm-1
) 46 cm-1, is in excellent agreement with the value that may
be figured out from the change in reduced mass [∆ν(CtO) ) 47
cm-1].12
16
18
Addition of H218O to a solution of 4 (CDCl3/THF-d8 4:1 v/v)
in an NMR tube, followed by heating to 60 °C leads to formation
of fac,cis-(PNP)RuCl2(C18O) (2-18O) [IR: ν(Ct18O) 1899 cm-1
(s)]. Again, the energy change of the CtO stretching frequency,
∆ν(CtO) ) 43 cm-1 as compared to 2, is in accord with the
incorporation of 18O into the carbonyl ligand.
Modeling Studies Relevant to the Mechanism of the Ru-
Assisted Cleavage of the C-C Bond of Phenyacetylene by
Water. Reaction of the Vinylidene Complex 4 with Water
in the Presence of CO. When 4 is reacted with a slight excess
(11) Several examples of oxidation of Ru(II) vinylidene complexes by
molecular oxygen have been reported, see for example: (a) Bruce, M. I.;
Swincer, A. G.; Wallis, R. C. J. Organomet. Chem. 1979, 171, C5. (b)
Oro, L. A.; Ciriano, M. A.; Foces-Foces, C.; Cano, F. M. J. Organomet.
Chem. 1985, 289, 117. (c) Mezzetti, A.; Consiglio, G.; Morandini, F. J.
Organomet. Chem. 1992, 430, C15. (d) Le Lagadec, R.; Roman, E.; Toupet,
L.; Mu¨ller, U.; Dixneuf, P. H. Organometallics 1994, 13, 5030.
(12) Mingos, D. M. P. in ComprehensiVe Organometallic Chemistry Abel,
E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, UK,
1982; Vol. 3, p 1.
The reaction of 4 with H218O in either open or closed reactors
results in the selective incorporation of 18O in the carbonyl
(10) See for example, Hommeltoft, S. J.; Baird M. C. Organometallics
1986, 5, 190.

4590 J. Am. Chem. Soc., Vol. 118, No. 19, 1996
Bianchini et al.
Scheme 3
Scheme 4
of H2O in the Parr reactor pressurized with 30 atm of CO at 60
°C for 2 h, several products are formed. Among these, the most
abundant (61%) is the new (η1-C-phenylacetyl)carbonyl complex
mer-(PNP)RuCl(η1-COCH2Ph)(CO) (6). The (carbonyl)dichlo-
ride 2 and its meridional isomer mer,trans-(PNP)RuCl2(CO) (5)8
are also produced in 14 and 12% yield, respectively, together
with phenylacetylene, benzylaldehyde, and some unidentified
Ru-PNP products (overall yield 13%).
the analogous resonance in 3 in which there is a direct Ru-
CH2Ph bond (δCH2 11.04). Conclusive experimental evidence
for the presence of a phenylacetyl group in 6 is finally provided
by the reaction with HCl which gives benzylaldehyde and 5.
This result indicates that, at a certain stage of the reaction of 4
with water in the presence of CO, HCl is evolved which later
reacts with 6 to give 5. The formation of both 2 and free
phenylacetylene are consistent with the occurrence of a side-
reaction, namely the displacement of the vinylidene ligand in 4
by CO, which has previously been observed8 (once displaced,
the vinylidene moiety spontaneously tautomerizes to alkyne).14
Recrystallization of the reaction mixture from CH2Cl2/n-
hexane gives 6 sufficiently pure to allow its unambiguous
spectroscopic characterization. The presence of both a terminal
carbonyl ligand and an η1-C-acyl ligand in this complex is
shown by IR bands at 1939 cm-1 [νCtO] and 1601 cm-1
[νC)O] as well as the 13C{1H} NMR spectrum which contains
the typical resonances for Ru-C(O)R [256.39 ppm, 2JCP 8.7 Hz]
(14) A sizable number of metal assisted acetylene to vinylidene tau-
tomerization have been reported. Key references include the following: (a)
Bruce, M. I. Chem ReV. 1991, 91, 197. (b) Antonova, A. B.; Ioganson, A.
A. Russ. Chem. ReV. 1989, 58, 593. (c) Bruce, M. I.; Swincer, A. G. AdV.
Organomet. Chem. 1983, 22, 59. (d) Fryzuk, M. D.; Huang, L.; McManus,
N. T.; Paglia, P.; Rettig, S. J.; White, G. S. Organometallics 1992, 11,
2979. (b) Lumprey, J. R.; Selegue, J. P. J. Am. Chem. Soc. 1992, 114, 5518.
(c) Werner, H. Angew. Chem., Int. Ed. Engl. 1990, 29, 1077. (d) Bianchini,
C.; Peruzzini, M.; Vacca, A.; Zanobini, F. Organometallics 1991, 10, 463.
(e) Xiao, J.; Cowie, M. Organometallics 1993, 12, 463. (f) Bianchini, C.;
Marchi, A.; Marvelli, L.; Peruzzini, M.; Romerosa, A.; Rossi, R.; Vacca,
A. Organometallics 1995, 14, 3203.
2
and Ru-CtO [201.33 ppm, JCP 13.1 Hz] carbon atoms.13 In
addition, the resonance of the Ru-CH2Ph carbon atom is
remarkably shifted to lower field (56.78 ppm) as compared to
(13) See for example, McCooey, K. M.; Probitts, E. J.; Mawby, R. J. J.
Chem. Soc., Dalton Trans. 1987, 1713.

C-C Bond CleaVage of Terminal Alkynes by H2O
J. Am. Chem. Soc., Vol. 118, No. 19, 1996 4591
Scheme 5
Scheme 6
chemistry of the PNP ligand in the precursor 4 is confirmed
by the 31P{1H} NMR spectrum of 8 which consists of a singlet
at 50.07 ppm (indeed, the chemical shift of the PNP phosphorus
atoms can be used as a diagnostic tool for discriminating
between mer and fac stereochemistries).8,15,16 The presence of
the aminocarbene ligand trans to the PNP-nitrogen atom is
unequivocally demonstrated by the 13C{1H} NMR spectrum
which contains a well-resolved triplet in the low-field region
2
[δ 252.40, JCP 14.2 Hz] typical of Ru(II) carbene carbon
atoms.15,17
Unlike the analogous aminocarbene complexes obtained by
reaction of primary amines with 4 (Vide infra), 8 is thermally
stable in refluxing THF.
Reaction of the Aminocarbene 8 with H2O or H218O.
Treatment of 8 in THF with H2O (or H218O) at room tempera-
ture for 12 h results in the quantitative formation of the
(carbonyl)dichloride complex 2 (or 2-18O), piperidine and
toluene (Scheme 6).
Intermediate in this reaction is the (benzyl)carbonyl complex
3 (or 3-18O) which can be isolated when the reaction is carried
out in a liquid-biphase system (CH2Cl2/H2O) to extract piperi-
dinium hydrochloride. In fact, the latter species is the primary
product, and, if not removed from the reaction mixture, it reacts
with the benzyl complex 3 to give 2 and toluene. An isotope
labeling experiment with H218O confirms that the oxygen atom
of the carbonyl ligand in 2 is provided by water.
The overall reactivity pathway toward H2O exhibited by 8 is
thus quite similar to that observed for the vinylidene 4, the only
difference being that the evolved HCl is trapped by the
piperidine.
The aminocarbene complexes fac,cis-(PNP)RuCl2{C(NHR)-
(CH2Ph)} obtained from primary amines15 do not exhibit this
type of reactivity toward H2O. Due to the acidic character of
the NH hydrogen they are deprotonated by water to give
iminoacyl complexes.18
Carbonylation Reaction of the (Benzyl)carbonyl Complex
3. In order to exclude that, in the reaction described above, 6
is formed by the straightforward carbonylation of the (benzyl)-
carbonyl complex 3 (which, in fact, is an intermediate in the
reaction of 4 with water, Vide supra), a THF solution of isolated
3 was reacted with CO (30 atm) at 60 °C in the Parr reactor for
1 h. As a result, two isomeric (phenylacetyl)carbonyl complexes
of the formula mer-(PNP)RuCl(η1-COCH2Ph)(CO) are obtained
in a ratio of 40:60 (Scheme 5).
The minor product 7 is a geometric isomer of 6 in which the
phenylacetyl group is still located trans to chloride as shown
by 13C NMR spectroscopy. Also, 7 is thermodynamically
unstable and spontaneously converts to 6 at the working
temperature. This experiment thus shows that the added CO
forces the migratory insertion of the carbonyl ligand of 3 into
the Ru-CH2Ph bond to give the acyl complex 7, which
subsequently rearranges to the thermodynamic isomer 6. The
driving force for this isomerization is most likely electronic in
nature and originated by the need of eliminating the strong trans
interaction between the Cl and acyl ligands.
Reaction of the Vinylidene Complex 4 with Piperidine.
Treatment of 4 in CH2Cl2 with a slight excess of piperidine at
room temperature gives the aminocarbene complex fac,cis-
(PNP)RuCl2{C(NC5H10)(CH2Ph)} (8) in almost quantitative
yield (Scheme 6).
Recently, Ru(II) aminocarbenes of the general formula
fac,cis-(PNP)RuCl2{C(NHR)(CH2Ph)} [R ) n-Pr, cyclo-C6H11,
(R)-(+)-CHMePh, (R)-(-)-CHMeEt, (S)-(-)-CHMeNaphthyl]
have been prepared by reaction of 4 with primary amines.15
From a comparison of the spectroscopic characteristics of 8
with those of the primary aminocarbenes mentioned above, one
may readily infer that all the complexes exhibit the same primary
geometry.15 In particular, the retention of the facial stereo-
Reaction of the Aminocarbene 8 with H2O in the Presence
of CO. In the presence of a CO atmosphere, 8 in THF reacts
with H2O at -20 °C to give a new isomer of the (phenylacetyl)-
carbonyl complexes of the general formula (PNP)RuCl(COCH2-
Ph)(CO) (Scheme 7). This new isomer, 9, contains a fac-PNP
ligand as shown by the magnetic inequivalence and chemical
shifts of the two phosphorus nuclei [31P{1H} NMR AM pattern
with δP 54.01 and δP 38.46]. The presence of an η1-C-
A
M
phenylacetyl ligand trans to a P donor is unequivocally
confirmed by 13C{1H} NMR spectroscopy (δCOCH2Ph 249.20,
2
2
dd, JCPtrans 94.1 Hz, JCPcis 11.2 Hz).
(15) Bianchini, C.; Peruzzini, M.; Romerosa, A; Zanobini, F. Organo-
metallics 1995, 14, 3152.
(16) Bianchini, C.; Glendenning, L.; Peruzzini, M.; Romerosa, A;
Zanobini, F. J. Chem. Soc., Chem. Commun. 1994, 2219.
(17) (a) Gamasa, M. P.; Gimeno, J.; Mart´ın-Vaca, B. M.; Borger, J.;
Garc´ıa-Granda, S.; Perez-Carren˜o, E. Organometallics 1994, 13, 4045. (b)
Singh, M., M.; Angelici, R. J. Inorg. Chem. 1984, 23, 2691. (c) McCormick,
F. B.; Angelici, R. J. Inorg. Chem. 1981, 20, 1111.
(18) Bianchini, C.; Peruzzini, M. manuscript in preparation.

4592 J. Am. Chem. Soc., Vol. 118, No. 19, 1996
Bianchini et al.
Scheme 7
Treatment of 10 in anhydrous chloroform (NMR experiment)
with a stoichiometric amount of HCl at room temperature
transforms the starting complex into the (dichloride)carbonyl
complex 2 and toluene (Scheme 8). The reaction proceeds
through the intermediacy of the (benzyl)carbonyl complex 3
which is found as a minor product in the final reaction mixture
only because part of the added HCl is consumed to form the
vinylidene complex 4 (Via protonation of the Câ carbon atom
of the σ-alkynyl in 10, followed by displacement of the
coordinated water molecule by chloride).
Consistently, the reaction of the isotopomer fac-(PNP)RuCl-
(CtCPh)(OD2) (10-d2) with HCl gives 3-d2 and PhCHD2.
Discussion
For a better understanding of the overall mechanism of the
present C-C bond cleavage of phenylacetylene by water, it may
be helpful to discuss some relevant literature precedents as well
as summarize the results of the modeling studies reported above.
The Ru(II) aminocarbene complexes obtained by treatment
of the vinylidene 4 with primary amines are thermally unstable
in refluxing THF in which they degrade to the (dichloride)-
isonitrile complexes fac,cis-(PNP)RuCl2(CNR) (R ) n-Pr, cyclo-
C6H11, (R)-(+)-CHMePh, (R)-(-)-CHMeEt, (S)-(-)-CHMeNaph-
thyl) Via toluene elimination (Scheme 9).15 Indeed, this reaction
parallels that of 4 with water and supports the intermediacy of
carbene species (i.e., hydroxycarbene) also in the latter reaction.
As shown in Scheme 9, the formation of the aminocarbene
complexes require the use of 2 equiv of primary amine as 1
equiv serves to deprotonate the vinylidene ligand to alkynyl.
The resulting σ-alkynyl complexes are coordinatively unsatur-
ated because of the removal of a chloride ligand as alkylam-
monium salt, and thus pick up a second molecule of primary
amine to form stable (σ-alkynyl)(primary amine) derivatives
(these can be isolated using a CH2Cl2/H2O biphase system).18
Only at this stage, the ammonium chloride (if not removed)
reprotonates the Câ carbon of the alkynyl ligand to form a
vinylidene ligand cis to the coordinated amine. This rapidly
attacks the vinylidene CR atom to give the aminocarbene product
(unequivocal evidence for intramolecular amine migration has
been provided by treatment of isolated (σ-alkynyl)(primary
amine) complexes with different alkylammonium chlorides; see
Scheme 9).18
The reaction sequence shown in Scheme 9 is thus quite similar
to that illustrated in Scheme 8: dehydrohalogenation of 4 by
NEt3 gives a coordinatively unsaturated σ-alkynyl complex
which coordinates a water molecule (instead of NEt3 for steric
reasons). Like primary amines, the coordinated water (see the
deuterium labeling experiment), after protonation of the σ-alky-
nyl ligand, apparently attacks the CR atom of the resulting
vinylidene and ultimately leads to the C-C bond cleavage.
By analogy with the reaction of 4 with primary amines, an
intermediate hydroxycarbene complex of the formula fac,cis-
(PNP)RuCl2{C(OH)(CH2Ph)} (B) is proposed to form upon
intramolecular migration of the coordinated H2O to the CR atom
of the vinylidene ligand (Scheme 10).
Scheme 8
Compound 9 is stable and isolable at low temperature,
whereas it spontaneously converts to the mer isomer 7 at room
temperature, which in turn transforms into the thermodynamic
isomer 6 at 60 °C.
A low energy barrier to isomerization of fac-PNP complexes
to mer-derivatives has previously been observed and interpreted
in terms of the minor steric congestion in the meridional
complexes.8
Reaction of the Vinylidene Complex 4 with NEt3 in a
CH2Cl2/H2O Biphase System. Under vigorous stirring, com-
plex 4 reacts with NEt3 in a CH2Cl2/H2O biphase system at
room temperature to give the (aquo)σ-phenylethynyl complex
fac-(PNP)RuCl(CtCPh)(OH2) (10) (Scheme 8). This com-
pound is obtained as orange crystals from the organic phase,
while [NEt3H]Cl is found in the aqueous phase.
Complex 10 is fluxional on the NMR time scale. The
31P{1H} NMR spectrum in CD2Cl2 at room temperature displays
two broad humps that resolve at -50 °C into an AM pattern
with 2JPP of 41 Hz. At this temperature, the 1H NMR spectrum
contains a broad triplet at 4.69 ppm (2JHP 8.8 Hz) which
disappears by adding D2O and is thus assigned to the coordi-
nated water molecule. Finally, a strong IR absorption at 2052
The key role of the hydroxycarbene B in the Ru-assisted C-C
bond cleavage of phenylacetylene is also supported by the
reaction of the secondary aminocarbene 8 with water (Scheme
6). Unlike the primary aminocarbene analogs, 8 is thermally
stable in THF (the absence of an NH group impedes the
rearrangement to isonitrile and toluene). However, it reacts with
water to give toluene, piperidine, and the carbonyl 2 (or the
benzyl complex 3 and piperidinium chloride in a liquid biphase
system, see Scheme 6). Although not seen in the course of the
reaction of either 8 or 4 with water, the intermediacy of a
cm-1 as well as 13C NMR resonances at 108.93 ppm (t, JCP
12.6 Hz, CR) and 114.12 ppm (s, Câ) are consistent with the
presence in 10 of a σ-alkynyl ligand19 as a result of the
deprotonation of the vinylidene ligand of 4 by the tertiary amine.
2
(19) See for example: Bianchini, C.; Frediani, P.; Masi, D.; Peruzzini,
M.; Zanobini, F. Organometallics 1994, 13, 4616.

C-C Bond CleaVage of Terminal Alkynes by H2O
J. Am. Chem. Soc., Vol. 118, No. 19, 1996 4593
Scheme 9
Scheme 10
hydroxycarbene species is thus highly probable in both cases.
Indeed, it is well-known that vinylidene metal complexes may
react with alcohols to give alkoxycarbene derivatives.4,20
If one takes for granted the intermediacy of the hydroxycar-
bene B in the reaction of 4 with water, the following stepwise
degradation to acyl (deprotonation of the hydroxycarbene) and
benzyl(carbonyl) complexes (C-C bond cleavage) can readily
be rationalized in light of several literature reports. In fact,
hydroxycarbene metal complexes21 are generally unstable and
thermally degrade to give acyl derivatives (Via HCl elimination
when chloride ligands are present in the complex framework).22
Formyl complexes are also formed by deprotonation of hy-
droxycarbene complexes with weak bases.21c In some cases,
the thermal degradation of hydroxycarbenes yields carbonyl
complexes (Via acyl intermediates).21ac,23 Finally, it is worth
mentioning that Fischer carbenes react with water to give
aldehydes Via hydroxycarbene intermediates formed by nucleo-
philic substitution of the alkoxy group (eliminated as alcohol).24
In the case at hand, we exclude that the hydroxycarbene ligand
in B rearranges to benzylaldehyde also because the reaction of
the thermally generated [(PNP)RuCl2] fragment with benzyla-
ldehyde does not lead to formation of either 3 or 2. Thus, the
conversion of B to the σ-acyl product (intercepted with CO as
6) proceeds Via HCl elimination which, in fact, has been detected
in the “open reactor” experiment.
Incorporation of all the experimental evidence leads to the
mechanism shown in Scheme 11 for the reaction between water
and phenylacetylene in the presence of 1 in THF at 60 °C.
The dissociation of PPh3 from 1 occurs as a thermal step.8
As a result, a free coordination site becomes available at the
Ru center for interaction with phenylacetylene, which is rapidly
tautomerized to vinylidene (step a) to give 4. For Ru(II)
systems, this step generally occurs Via a 1,2-hydrogen shift
mechanism involving π-alkyne intermediates.14 The alternative
mechanism involving C-H oxidative addition, followed by 1,3-
hydrogen shift, in fact, would require a much more electron-
rich metal center.25 At this stage, water comes into play: a
water molecule deprotonates the vinylidene ligands to σ-alkynyl
and also promotes the elimination of one chloride as H3O+Cl-
(step b) (the acidic character of the Câ-H hydrogen of
vinylidene ligands is well-known;14a-c the acidic character of
this hydrogen atom in 4 is shown by the rapid H/D exchange
observed when a solution of 4 is treated with D2O at room
temperature). A second water molecule occupies the resulting
coordination vacancy to give the alkynyl complex 10 (step b),
which is transformed into the cationic vinylidene intermediate
A (step c) by the proton released in the preceding step. Complex
A cannot be intercepted due to fast intramolecular attack by
the coordinated H2O to the vinylidene CR atom (step d). The
hydroxycarbene intermediate B thus is formed, which eliminates
HCl (or H3O+Cl-) to give an unsaturated σ-acyl complex (step
e), intercepted as the saturated carbonyl complex 6 when the
reaction is carried out in the presence of CO (Schemes 4 and
5). From intermediate C the formation of the (benzyl)carbonyl
complex 3 and subsequently of the carbonyl 2 are readily
(20) See for example: (a) Boland-Lussier, B. E.; Hughes, R. P.
Organometallics 1982, 1, 635. (b) Consiglio, G.; Moradini, F.; Ciani, G.
F.; Sironi, A. Organometallics 1986, 5, 1976. (c) Bruce, M. I.; Duffy, D.
N.; Humphrey, M. G.; Swincer, A. G. J. Organomet. Chem. 1985, 282,
383. (d) Parlier, A.; Rudler, H. J. Chem. Soc., Chem. Commun. 1986, 514.
(e) Chisholm, M. H.; Clark, A. C. Acc. Chem. Res. 1973, 6, 202. (f) Le
Bozec, H.; Ouzzine, K.; Dixneuf, P. H. Organometallics 1991, 10, 2768.
(g) Devanne, D.; Dixneuf, P. H. J. Organomet. Chem. 1990, 390, 371.
(21) (a) Barrett, D. S.; Glidewell, C.; Cole-Hamilton, D. J. J. Chem. Soc.,
Chem. Commun. 1988, 1079. (b) Casey, C. P.; Czerwinski, C. J.; Hayashi,
R. K. J. Am. Chem. Soc. 1995, 117, 4189. (c) Lilga, M. A.; Ibers, J. A.
Organometallics 1985, 4, 590. (d) Guerchais, V.; Lapinte, C. J. Chem. Soc.,
Chem. Commun. 1986, 663. (e) Green, M. L. H.; Hurley, C. R. J.
Organomet. Chem. 1967, 10, 188. (f) Buhro, W. E.; Wong, A.; Merrifield,
J. H.; Lin, G.-Y.; Constable, A. C.; Gladysz, J. A. Organometallics 1983,
2, 1852. (g) Tam, W.; Lin, G.-Y.; Wong, W.-K.; Kiel, W. A.; Wong, V.
K.; Gladysz, J. A. J. Am. Chem. Soc. 1982, 104, 141. (h) Barratt, D. S.;
Cole-Hamilton, D. J. J. Organomet. Chem. 1986, 306, C41. (i) Fischer, E.
O.; Riedel, A. Chem. Ber. 1968, 101, 156. (j) Moss, J. R.; Green, M. L. H.;
Stone, F. G. A. J. Chem. Soc., Dalton. Trans. 1973, 975. (k) Fischer, E.
O.; Kreis, G.; Kreissl, F. R. J. Organomet. Chem. 1973, 56, C37. (l) Darst,
K. P.; Lukehart, C. M. J. Organomet. Chem. 1979, 171, 65.
(22) Grundy, K. R.; Jenkins, J. J. Organomet. Chem. 1984, 265, 77.
(23) Fischer, E. O.; Maasbo¨l, A. Chem. Ber. 1967, 100, 2445.
(24) (a) Bernasconi, C. F.; Flores, F. X.; Sun, W. J. Am. Chem. Soc.
1995, 117, 4875. (b) Aumann, R.; Hinterding, P.; Kru¨ger, C.; Goddard, R.
J. Organomet. Chem. 1993, 459, 145.
(25) de los R´ıos, I.; Jime´nez Tenorio, M.; Puerta, M. C.; Valerga, P. J.
Chem. Soc., Chem. Commun. 1995, 1757.

4594 J. Am. Chem. Soc., Vol. 118, No. 19, 1996
Bianchini et al.
Scheme 11
explained by CO deinsertion (step f), followed by reaction with
the previously generated HCl which removes the benzyl ligand
as toluene and saturates the metal coordination sphere with the
chloride (step g).
mediates in the hydrogenation of CO catalyzed by iron-group
metals, to give a variety of organic products. In light of the
results described in this paper, one cannot disregard the
possibility that water (produced in Fischer-Tropsch reactions)30
may interact with vinylidene intermediates to give either
hydrocarbons and further CO or aldehydes and ketones.
Conclusions
In this paper we have shown that the hydration of phenyl-
acetylene in the presence of a Ru(II) complex may lead to the
cleavage of the C-C triple bond with formation of CO and
toluene.
Acknowledgment. Thanks are due to the EC for supporting
a fellowship to A.R. (contract ERBCHRXCT930147) and the
Ministerio de Educacio´n y Ciencia of Spain for providing a
postdoctoral grant to J.A.C. Financial support from the Progetto
Strategico “Tecnologie Chimiche Innovative”, CNR, Italy, is
also gratefully acknowledged.
A study under different experimental conditions, the use of
model and isotope labeling experiments, and the detection of
several intermediates, taken altogether, show that the C-C bond
cleavage reaction comprises a number of steps: 1-alkyne to
vinylidene tautomerism, conversion of a vinylidene ligand to
hydroxycarbene by intramolecular attack of water, deprotonation
of hydroxycarbene to σ-acyl, deinsertion of CO from the acyl
ligand, and hydrocarbon elimination by protonation of the metal-
alkyl moiety. While most of these elemental reactions have
precedents in organometallic chemistry and catalysis, the present
system is unique in that they all occur at the same metal-ligand
fragment. This allows one to draw similar mechanistic conclu-
sions on other relevant reactions such as the recently reported
Ir-assisted hydrolysis of allenylidene ligand to CO and σ-vinyl,26
and, more in general, the Fischer-Tropsch chemistry.27 In fact,
both vinylidene28 and hydroxycarbene29 species are key inter-
JA9601393
(27) (a) Fischer, F.; Tropsch, H. Chem. Ber. 1923, 56, 2428. (b) Fischer,
F.; Tropsch, H. Chem. Ber. 1926, 59, 830. (c) Olah, G. A.; Molna´r, A.
Hydrocarbon Chemistry; Wiley: New York, 1995; Chapter 3, pp 66-73.
(d) Turner, M. L.; Long, H. C.; Shenton, A.; Byers, P. K.; Maitlis, P. M.
Chem. Eur. J. 1995, 1, 549 and references therein.
(28) (a) Hoel, E. L.; Anselt, G. B.; Leta, S. Organometallics 1986, 5,
585. (b) Hoel, E. L. Organometallics 1986, 5, 587.
(29) (a) Starch, H. H.; Golumbic, H.; Anderson, R. B. "The Fischer-
Tropsch and Related Syntheses"; Wiley, New York, 1951. (b) Kummer, J.
T.; Emmet, P. H. J. Am. Chem. Soc. 1953, 75, 5177. (c) Nijs, H. H.; Jacobs,
P. A. J. Catal. 1980, 66, 401.
(30) (a) Brady, R. C.; Pettit, R. J. Am. Chem. Soc. 1980, 102, 6181. (b)
Brady, R. C.; Pettit, R. J. Am. Chem. Soc. 1981, 103, 1297. (c) Biloen, P.
J. Roy. Neth. Chem. Soc. 1980, 99, 33. (d) Biloen, P. J.; Sachtler, W. M. H.
AdV. Catal. 1981, 30, 165.
(26) O’Connor, J. M.; Hiibner, K. J. Chem. Soc., Chem. Commun. 1995,
1209.