1974, 74, 563; R. Noyori and S. Hashiguchi, Acc. Chem. Res., 1997, 30,
97.
2 T. A. Smith and P. M. Maitlis, J. Organomet. Chem., 1985, 289, 385.
3 (a) J. C. Bailar and H. Itatani, J. Am. Chem. Soc., 1967, 89, 1592, 1600;
J. Am. Oil Chem. Soc., 1967, 43, 377; 1967, 44, 147; (b) H. Imai, T.
Nishiguchi and K. Fukuzumi, J. Org. Chem., 1974, 39, 1622; (c) T. A.
Smith and P. M. Maitlis, J. Organomet. Chem., 1984, 269, C7; (d) D.
Milstein, J. Mol. Cat., 1986, 36, 387.
4 K. Tani, N. Ono, S. Sakamoto and F. Sato, J. Chem. Soc., Chem.
Commun., 1993, 386; B. M. Trost and R. Braslau, Tetrahedron Lett.,
1989, 30, 4657; M. E. Vol’pin, V. P. Kukolev, V. O. Chernyshev and
I. S. Kolomnikov, Tetrahedron Lett., 1971, 4435. Stoichiometric
transfer hydrogenation of diphenylacetylene to trans-stilbene with
ethanol mediated by ‘Ir(PPh3)2H3’ has been reported: R. Zanella, F.
Canziani and M. J. Graziani, J. Organomet. Chem., 1974, 67, 449. For
transfer hydrogenation of activated alkenes, see: M. Saburi, M. Ohnuki,
M. Ogasawara, T. Takahashi and Y. Uchida, Tetrahedron Lett., 1992,
39, 5783; H. Brunner and W. Leitner, Angew. Chem., Int. Ed. Engl.,
1988, 27, 1180.
5 T. Yamagata, A. Iseki and K. Tani, Chem. Lett., 1997, 1215.
6 K. Tani, A. Iseki and T. Yamagata, Angew, Chem., Int. Ed., 1998, 37,
3381; Angew, Chem., 1998, 110, 3590.
7 Catalytic transfer hydrogenation: an alkyne (2 mmol) and complex 2
(0.04 mmol) were dissolved in a mixture of methanol (2 mL) and
toluene (2 mL) in a glass ampoule under argon and sealed under reduced
pressure at 2197 °C. The ampoule, placed in a steel pipe, was heated at
80 °C for 18 h. The reaction products were analyzed by GLC, 1H NMR
and GC–MS.
Scheme 1
8 J. P. Collman, L. S. Hegedus, J. R. Norton and R. G. Finke, Principles
and Applications of Organotransition Metal Chemistry, University
Science Books, Mill Valley, CA, 1987, p. 531.
9 Complex 3: orange powder. Anal. Calc. for C45H32ClIrOP2: C, 61.53;
H, 3.67. Found: C, 61.42; H, 3.84%; Mp > 120 °C (decomp.); MS
(FAB) m/z 878 (M+), 850 (M+ 2 CO); IR 1986 (film, nCO), 304w cm21
(Nujol, nIr–Cl); dP(CDCl3, 121 MHz) 16.5 (d, J 28 Hz), 22.7 (d, J 28 Hz);
dC(CDCl3, 75 MHz) 180.7 (dd, J 11, 123 Hz, CO). Complex 3 can be
quantitatively prepared from the reaction of 1 and CO.
vacuo, two iridium complexes, IrCl(CO)(BINAP) 39 (major)
and ‘{Ir(Cl)(H)(BINAP)}2(m-H)2’ 4 (minor),10 were isolated.
Although complex 3 showed comparable catalytic activity to
that of 2a for the transfer hydrogenation of diphenylacetylene,
complex 4 was far less efficient. Methyl formate, the
Tishchenko product of the by-product formaldehyde, was also
detected by GC–MS in the reaction mixture. Based on these
experimental results, we propose a plausible reaction pathway
of the transfer hydrogenation of alkynes as well as alkenes and
the isomerisation of alkenes catalysed by complex 2 (Scheme
1).11 The real catalyst may be a nascent mononuclear dihydride
species such as 5, which dimerises to give the catalytically
inactive dimer 4. The possibility that coordination of alkyne to
the monomeric hydrido(methoxo)complex 6 is the first step
followed by hydride insertion to give a methoxo(vinyl)
complex, however, can not be eliminated, though such species
could not be detected by NMR of the reaction mixture of
complex 2a and diphenylacetylene.
10 Complex 4: yellowish orange powder, IR (Nujol) 2279 (nIr–H ), ca.
t
1620w br cm21 (nIr–H ); dH(CDCl3, 300 MHz) 222.38 (dd, J 15, 21 Hz,
b
Ir–Ht) and 211.57(tt, J 8, 64 Hz, Ir–Hb); dP(CDCl3, 121.5 MHz) 3.6 (dd,
J 9, 10 Hz) and 5.9 (dd, J 9, 10 Hz); MS (FAB): group of peaks centered
at m/z 1703 resembling the simulated pattern for (M+ 2 1) (M =
C88H6835Cl2193Ir2P2). Although complex 4 could not be isolated as a
pure state, on the basis of these spectral data we tentatively propose the
structure ‘{Ir(H)(Cl)(BINAP)}2(m-H)2’. Complex 4 can also be ob-
tained as the main product by pyrolysis of complex 2a at 80 °C in
methanol–toluene.
11 For isomerisation as well as hydrogenation of alkenes, the same
dihydride 5 can also act as a catalyst. Insertion of cis-alkene into Ir–H
and subsequent b-hydrogen elimination from the hydrido(alkyl) com-
plex gives trans-alkene and 5, or reductive elimination from the
hydrido(alkyl) complex gives alkane and ‘Ir(diphosphine)Cl’, re-
spectively.
This work was partly supported by the Grant-in Aid for
Scientific Research from the Ministry of Education, Science,
Sports, and Culture of Japan. BINAP was a generous gift from
Takasago Perfumery Co. Ltd.
Notes and references
1 For reviews see: R. A. W. Johnstone, A. H. Wilbi and I. D. Entwisth,
Chem. Rev., 1985, 85, 129; G. Brieger and T. J. Nestricle, Chem. Rev.,
Communication 9/05765J
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