New metathesis catalyst bearing chromanyl moieties at the N-heterocyclic carbene ligand

Summary The synthesis of a new type of Hoveyda–Grubbs 2nd generation catalyst bearing a modified N-heterocyclic carbene ligands is reported. The new catalyst contains an NHC ligand symmetrically substituted with chromanyl moieties. The complex was tested in model CM and RCM reactions. It showed very high activity in CM reactions with electron-deficient α,β-unsaturated compounds even at 0 °C. It was also examined in more demanding systems such as conjugated dienes and polyenes. The catalyst is stable, storable and easy to purify.


Testing of the catalyst 9
General CM procedure for allylbenzene and ethyl acrylate ( Table 1, entries 1, 2) a) To a mixture of allylbenzene (0.2 mmol) and ethyl acrylate (0.4 mmol) in CH 2 Cl 2 (1.5 mL) a solution of catalyst 1, 2, 3 or 9 (2.5 mol %) in CH 2 Cl 2 (0.5 mL) was added. The resulting mixture was stirred at 0 C under argon atmosphere and controlled by 1 H NMR after 1, 2 and 3 hours. b) To a mixture of allylbenzene (0.2 mmol) and ethyl acrylate (0.4 mmol) in CH 2 Cl 2 (1.5 mL) a solution of catalyst 3 or 9 (1 mol %) in CH 2 Cl 2 (0.5 mL) was added. The resulting mixture was stirred at 0 °C under argon atmosphere and analysed by 1 H NMR after 3 hours. The spectral data obtained for CM product are consistent with those given in the literature [1]. Yield was determined by comparison of integration of CM product proton signal (dd, 2H) at 3.53 ppm or (dt, 1H) at 5.83 ppm and SM product of allylbenzene ((E)-1,4-diphenylbut-2-ene) proton signal (m, 2H) at 5.67-5.71 ppm and/or substrate (allylbenzene, since acrylate was used in excess) proton signal (m, 1H) at 5.81-6.17 ppm. Amount of protons corresponding to these signals was taken into account in the calculations.
General CM procedure for hex-5-enyl acetate and methyl vinyl ketone ( Table 1,

entry 3)
To a mixture of the hex-5-enyl acetate (0.2 mmol) and methyl vinyl ketone (0.4 mmol) in CH 2 Cl 2 (1.5 mL) a solution of catalyst 1, 2, 3 or 9 (1 mol %) in CH 2 Cl 2 (0.5 mL) was added. The resulting mixture was stirred at 0 C under argon atmosphere and controlled by 1 H NMR after 3 hours. The spectral data obtained for CM product are consistent with those given in the literature [2]. Yield was determined by comparison of integration of CM product proton signal (dt, 1H) at 6.07 ppm and SM product of hex-5-enyl acetate ((E)dec-5-ene-1,10-diol diacetate) proton signal (m, 2H) at 5.40-5.42 ppm and/or substrate (hex-5-enyl acetate, since ketone was used in excess) proton signal (m, 1H) at 5.74-5.84 ppm. Amount of protons corresponding to these signals was taken into account in the calculations.

entry 4)
To a mixture of the hex-5-enyl acetate (0.2 mmol) and acrylonitrile (0.4 mmol) in CH 2 Cl 2 (1.5 mL) a solution of catalyst 1, 2, 3 or 9 (1 mol %) in CH 2 Cl 2 (0.5 mL) was added. The resulting mixture was stirred at 0 C for 1 h under argon atmosphere and analysed by 1 H NMR. The spectral data obtained for CM product are consistent with those given in the literature [3]. Yield was determined by comparison of integration of CM product proton signal (d, 1H) at 5.35 ppm and SM product of hex-5-enyl acetate ((E)dec-5-ene-1,10-diol diacetate) proton signal (m, 2H) at 5.40-5.42 ppm and/or substrate (hex-5-enyl acetate, since nitrile was used in excess) proton signal (m, 1H) at 5.74-5.84 ppm. Amount of protons corresponding to these signals was taken into account in the calculations.
General CM procedure for allyloxybenzene and hex-5-enyl acetate (Table 2, entry 4) To a mixture of the alkenes (0.2 mmol, both) in CH 2 Cl 2 (1.5 mL) a solution of catalyst 1, 2 or 9 (2.5 mol %) in CH 2 Cl 2 (0.5 mL) was added. The resulting mixture was stirred at 20 C for 3 h under argon atmosphere. The crude product was purified by FC (hexaneethyl acetate v/v 20:1) and analysed by 1 H NMR. The spectral data obtained for CM product are consistent with those given in the literature [4].
General RCM procedure for diethyl diallylmalonate (Table 3, entry 1) To a solution of the alkene (0.2 mmol) in CH 2 Cl 2 (1.5 mL) a solution of catalyst 1, 2 or 9 (1 mol %) in CH 2 Cl 2 (0.5 mL) was added. The resulting mixture was stirred at room temperature for 60 min under argon atmosphere. The crude product was analysed by 1 H NMR. The spectral data obtained for RCM product are consistent with those given in the literature [5].

entry 2)
To a solution of the alkene (0.2 mmol) in CH 2 Cl 2 (1.5 mL) a solution of catalyst 1, 2 or 9 (1 mol %) in CH 2 Cl 2 (0.5 mL) was added. The resulting mixture was stirred at room temperature for 60 min under argon atmosphere. The crude product was analysed by 1 H NMR. The spectral data obtained for RCM product are consistent with those given in the literature [5].

entry 3)
To a solution of alkene (0.2 mmol) in toluene (2 mL) a solution of catalyst 1, 2 or 9 (5 mol %) in toluene (1 mL) was added. The resulting mixture was stirred at 80 C for 16 h under argon atmosphere. The crude product was analysed by 1 H NMR. The spectral data obtained for RCM product are consistent with those given in the literature [5].
General CM procedure for diene and alkene ( Table 4,