Hydration of Alkynes by a PtCl4-CO Catalyst
J . Org. Chem., Vol. 62, No. 3, 1997 671
Sch em e 1
in experiment 24 (Table 1), 3% of 1-cyclohexylethanone
has been obtained as the result of transfer hydrogenation
of the major unsaturated product. During the hydration
of the alkynones PhCtCCOEt and PhCtCCOPh in
aqueous THF (Table 1, experiments 18 and 20) traces of
chlorinated alkenones have been isolated. The origin of
the halogen is the HCl formed by slow hydrolysis of the
starting PtCl4.9
The necessity of the CO in the hydration process
suggests that the acting catalyst is a platinum carbonyl
complex. Calabrese et al.20 have reported that chloro-
platinates undergo reductive carbonylation to form plati-
num(0) carbonyl clusters of general formula [Pt3(CO)3(µ2-
-2
CO)3]n among which the well-characterized complexes
[Pt3(CO)3(µ-CO)3]5-2 and [Pt3(CO)3(µ-CO)3]6-2 form green
solutions. The resemblance of the IR bands of these
complexes with those of neutralized CO-treated PtCl4 in
aqueous THF (strong bands at 2065, 2055, 1875, and
CH2OH, formed from O(CH2CtCPh)2 has been shown to
undergo cyclization in the presence of the platinum
complex to form 1-indanone. The thiol PhCOCH2CH2-
SH generated from S(CH2CtCPh2) undergoes water-
assisted hydrogenolysis rather than cyclization and forms
exclusively 1-phenyl-1-propanone, PhCOCH2CH3, and
H2S.
As an example for the hydration of a triyne we chose
1,1′-(1,2-ethynediyl)bis[2-ethynylbenzene], (C6H4-2-CtCH)-
CtC(C6H4-2-CtCH).17 The compound adds three mol-
ecules of H2O, and the resulting trione undergoes an in-
tramolecular condensation by which 1-[2-(3-methyl-1-oxo-
1H-indan-2-yl)phenyl]ethanone is formed, according to
eq 3 (Table 1, experiments 33 and 34).18
1875 cm-1 20,21 suggests structural similarity between the
)
reported platinum carbonyl complexes and our catalyst
precursor. Extraction of the green platinum compounds
into CH2Cl2 with the aid of [Bu4N]Cl gave a green
solution that showed a 195Pt NMR signal at -3234 ppm
that is characteristic of Pt(0) complexes.22
-2
Studies on the chemical behavior of the [Pt3(CO)6]n
clusters by Longoni and Chini21 revealed that the com-
plexes with relatively large n values are good electro-
philes. Since we assume that in our case n is 5 or 6,
interaction of our clusters with HCl (formed during the
hydrolysis of the starting PtCl4) is feasible (eq 4).
H2[Pt3(CO)6]n + 3HCl h
H2[Pt3(CO)6]n-1 + 3HPtCl(CO)2 (4)
Thus, we believe that HPtCl(CO)2 is the actual catalyst
in the hydration process. Although we were unable to
isolate this complex and to detect a distinguished metal-
1
hydride signal in the H NMR spectrum, we were able
to prove the presence of a Pt-H moiety by applying the
green solution as a hydrogen source for stoichiometric
hydroformylation of 1-hexene. (Both 1- and 2-heptanal
were formed). Furthermore, upon addition of excess
PPh3 to the reaction mixture of PtCl4 and CO in aqueous
THF the known trans-HPtCl(PPh3)2 (compared with an
authentic sample23 and determined by X-ray diffraction
analysis) was obtained. The triphenylphosphine complex
proved, however, to be completely inactive as a hydration
catalyst.
The activity of the green solution as a hydration
catalyst is lost upon treatment with NEt3. When the
halogen-free platinum carbonyl complexes were extracted
into CH2Cl2, (vide supra) washed with water, freed from
the organic solvent, and the green residue dissolved in
wet THF, the resulting solution failed to hydrate phen-
ylacetylene but catalyzed polymerization of the alkyne.
In conclusion, it seems that our PtCl4-CO catalyst is
not only environmentally more friendly than the conven-
tionally used mercury-promoted process but is more
efficient than all other transition metal-catalyzed hydra-
tions of alkynes reported in the literature. In light of
the data at hand, we propose the mechanism outlined in
Scheme 2 for the PtCl4-CO-catalyzed addition of water
The presence of CO in the platinum-catalyzed process
proved absolutely essential. In its absence no hydration
has been observed. However, the carbon monoxide only
rarely caused any carbonylation of the alkyne. Only
during the hydration of diphenylacetylene were small
quantities of carbonylation products obtained. Under
phase-transfer conditions the alkyne gave up to 5% of
PhCHdC(Ph)CO2H (Table 1, experiment 7), and in wet
THF some reductive double carbonylation resulted in the
formation of up to 2% of 3,4-diphenyl-2-(5H)furanone19
(Table 1, experiment 8). Hardly any other side products
have been isolated in the hydration reactions, although
(17) Diercks, R.; Armstrong, J . C.; Boese, R.; Vollhardt, K. P. C.
Angew. Chem., Int. Ed. Engl. 1985, 25, 268.
(18) Experimental data: yellow semi solid; IR (Nujol) 1687, 1715
cm-1 (CdO); 400-MHz 1H NMR (CDCl3) δ 2.22 (s, 3), 2.58 (s, 3), 6.99
(d, 1, J ) 7.6 Hz), 7.21-7.29 (m, 2), 7.42-7.49 (m, 3), 7.54 (dt, 1, J d
)
7.6 Hz, J t ) 1.1 Hz), 7.79 (dd, 1, J 1 ) 7.6 Hz, J 2 ) 1.1 Hz); 100-MHz
13C NMR (CDCl3) δ 12.33, 28.66, 119.47, 122.33, 127.90, 128.62, 128.70,
130.11, 130.62, 131.24, 131.32, 133.62, 135.32, 139.99, 146.07, 153.52,
195.52, 201.34; GC-MS (70 eV, 130 °C) m/z (rel intensity) 262 (M•+
,
100), 247 (C17H11O2+, 65), 233 (C16H9O2+, 38), 220 (C15H18O2•+, 24),
219 (C15H7O2+, 47), 191 (C14H7O+, 13), 189 (C14H5O+, 23). Anal. Calcd
for C18H14O2: C, 82.42; H, 5.38. Found: C, 82.28, H, 5.67.
(19) (a) Tsuji, J .; Nogi, T. J . Am. Chem. Soc. 1966, 88, 1289. (b)
Toda, F.; Takahira, Y.; Kataoka, Y.; Mori, K.; Sato, T.; Segawa, M. J .
Chem. Soc., Chem. Commun. 1984, 1234.
(20) Calabrese, J . C.; Dahl, L. L.; Chini, P.; Longoni, G.; Martinengo,
S. J . Am. Chem. Soc. 1974, 96, 2614.
(21) Longoni, G.; Chini, P. J . Am. Chem. Soc. 1976, 98, 7225.
(22) Pregosin, P. S. Coord. Chem. Rev. 1982, 44, 247.
(23) Cariati, F.; Ugo, R.; Bonati, F. Inorg. Chem. 1966, 5, 1128.