Efficient transfer hydrogenation of alkynes and alkenes with methanol catalysed by hydrido(methoxo)iridium(III) complexes

Article abstract of DOI:10.1039/a905765j

Hydrido(methoxo)iridium(III) complexes, [{Ir(H)(diphosphine)}₂(μ-OMe)₂(μ-Cl)]⁺Cl^- [diphosphine = (R)-BINAP 2a or 2,2'-bis(diphenylphosphino)-1,1'-biphenyl (BPBP) 2b] catalysed transfer hydrogenation of alkynes with methanol to give trans-alkenes selectively; plausible reaction pathways are also proposed.

Full text of DOI:10.1039/a905765j

Efficient transfer hydrogenation of alkynes and alkenes with methanol
catalysed by hydrido(methoxo)iridium(iii) complexes
Kazuhide Tani,* Aika Iseki and Tsuneaki Yamagata
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531,
Japan. E-mail: tani@chem.es.osaka-u.ac.jp
Received (in Cambridge, UK) 15th July 1999, Accepted 10th August 1999
Hydrido(methoxo)iridium(iii) complexes, [{Ir(H)(diphos-
phine)}2(m-OMe)2(m-Cl)]+Cl2 [diphosphine = (R)-BINAP
2a or = 2,2A-bis(diphenylphosphino)-1,1A-biphenyl (BPBP)
2b] catalysed transfer hydrogenation of alkynes with metha-
nol to give trans-alkenes selectively; plausible reaction
pathways are also proposed.
give 83% trans-stilbene and 17% 1,2-diphenylethane. Forma-
tion of cis-stilbene was not detected. When the reaction was
stopped after 1 h, the conversion of diphenylacetylene was 29%
and 12% trans-stilbene and 16.5% cis-stilbene were obtained,
accompanied with 0.5% 1,2-diphenylethane. Reaction after 2 h
gave the trans-alkene as the main product with 52% conversion.
Although the reaction also proceeded under reflux > 36 h were
required before complete consumption of diphenylacetylene.
Hydrogenation of trans-stilbene gave 29% of 1,2-diphenyl-
ethane after 18 h, whereas hydrogenation of cis-stilbene gave
the alkane in 39% yield, with almost all starting cis-stilbene
being isomerised to trans-stilbene. Thus, cis-stilbene was
hydrogenated faster than trans-stilbene and isomerisation of the
cis-alkene to the trans-alkene proceeded much faster than its
hydrogenation to the alkane in the presence of the iridium
catalyst. The rate difference of the reduction between the cis and
the trans isomers may reflect the difference of their binding
constants,8 resulting in high selectivity for the trans-alkene in
the product. In other words, the initial hydrogenation product of
diphenylacetylene should be cis-stilbene, which is isomerised to
trans-stilbene by the iridium catalyst under the reaction
conditions. Diphosphine complexes 1 can also be employed as
the catalyst precursor for transfer hydrogenation. Interestingly,
ethanol or propan-2-ol, generally more favorable sources of
hydrogen,2 were far less effective for the present transfer
hydrogenation, and ketones were poor substrates. Acetophen-
one was hydrogenated in only 32% yield to give phenethyl
alcohol with 2a in toluene–methanol at 80 °C after 18 h. When
the transfer hydrogenation was conducted in CD3OD–toluene
(1+1) using 2a as a catalyst precursor, the deuterium contents of
the olefinic as well as the methylene hydrogens of the products
amounted to 100%. For the reaction performed in a CH3OD–
toluene (1+1), however, the corresponding values were 43 and
41%, respectively, as determined by 500 MHz 1H NMR
spectroscopy and GC–MS. Thus, during transfer hydrogenation
hydrogens of both CH3 and OH groups of methanol were
incorporated in the products.
Catalytic transfer hydrogenation of unsaturated compounds
using organic hydrogen donors such as alcohols and formic acid
has widely been studied as an attractive method of reduction
owing to its operational simplicity and environmentally friendly
properties of the hydrogen donors.1 Although methanol could
be an exceedingly useful source of hydrogen from many
viewpoints,2 only a few reports of its use in homogeneous
catalysis are available.2,3 In addition, in contrast to the transfer
hydrogenation of ketones, that of simple alkynes and alkenes by
homogeneous catalysis remains relatively undeveloped.3b,d,4
Here, we report efficient selective catalytic transfer hydro-
genation of alkynes to trans-alkenes with soluble iridium
complexes by using methanol as a hydrogen donor.
Recently we have reported that [IrCl(diphosphine)]2 [di-
phosphine = BINAP 1a, or 2,2A-bis(diphenylphosphino)-1,1A-
biphenyl (BPBP) 1b]5 carrying a peraryldiphosphine readily
activates methanol at ambient temperature to give iridium(iii)
hydrido(methoxo) complexes, [{IrCl(H)(diphosphine)}2(m-
Cl)(m-OMe)2]Cl [diphosphine = BINAP 2a or BPBP 2b].6 In
the course of the study concerning the reactivity of complexes
2 we have found that these hydrido(methoxo) complexes serve
as efficient catalyst precursors for reduction of alkynes to give
trans-alkenes using methanol as a source of hydrogen. Over-
hydrogenation to alkanes was also observed. Hydrogenation
proceeded in toluene–methanol (1+1, v/v) solution under fairly
mild conditions at 80 °C in the presence of a catalytic amount of
2 [eqn. (1)].7
From the solid residue obtained from the catalytic reduction
product of 1-phenylpropyne with methanol using complex 2a as
a catalyst precursor after removing all volatile materials in
The results are summarized in Table 1. After 18 h with
catalyst 2a, diphenylacetylene was consumed completely to
Table 1 Transfer hydrogenation of alkynes and alkenes with MeOH catalysed by 1 and 2a
Substrate
Catalyst
t/h
Conversion (%) trans-Alkene (%)
cis-Alkene (%) Alkane (%)
PhC·CPh
2a
2a
2a
2b
2a
1a
1ab
1bc
2a
2a
2a
1
2
29
52
12
36
83
90
50
60
3
63
71
60
ndd
16.5
14.5
0
0
0
0.5
1.5
17
18
18
18
18
18
18
18
18
48
100
100
100
100
25
95
29
> 99
100
10
50
37
0
12
29
39
ndd
PhC·CMe
3
22
20
0
< 1
ndd
trans-PhCHNCHPh
cis-PhCHNCHPh
Me(CH2)4C·C(CH2)4Me
a
[Substrate] = 0.5 M, [Substrate]+[Ir+] = 25+1, solvent: MeOH–toluene (1+1), reaction temperature = 80 °C. Yields and conversion were determined
by GLC, 1H NMR and GC-MS. b Solvent: EtOH–toluene (1+1). c Solvent: PriOH–toluene (1+1). d nd = not determined. Mixtures of dodecenes and dodecane
were detected by GC–MS.
Chem. Commun., 1999, 1821–1822
1821

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
1822
Chem. Commun., 1999, 1821–1822