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

Abstract

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.

References

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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 –197 °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.
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⁺– CO); IR 1986 (film, ν_CO), 304w cm^–1(Nujol, ν_Ir–Cl); δ_P(CDCl₃, 121 MHz) 16.5 (d, J 28 Hz), 22.7 (d, J 28 Hz); δ_C(CDCl₃, 75 MHz) 180.7 (dd, J 11, 123 Hz, CO). Complex 3 can be quantitatively prepared from the reaction of 1 and CO.
10. Complex 4: yellowish orange powder, IR (Nujol) 2279 (ν_Ir–Ht), ca. 1620w br cm^–1(ν_Ir–Hb); δ_H(CDCl₃, 300 MHz)–22.38 (dd, J 15, 21 Hz, Ir–H_t) and –11.57(tt, J 8, 64 Hz, Ir–H_b); δ_P(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⁺– 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(µ-H)₂’. Complex 4 can also be obtained 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 β-hydrogen elimination from the hydrido(alkyl) complex gives trans-alkene and 5, or reductive elimination from the hydrido(alkyl) complex gives alkane and ‘Ir(diphosphine)Cl’, respectively.