Quinoline transfer hydrogenation by a rhodium bipyridine catalyst

Abstract

Dedicated to celebrate Brian JamesÕs scientific career and 70th birthday. AbstractThe catalytic activity of the rhodium complex cis-[Rh(bipy) 2 Cl 2 ]Cl AE 2H 2 O in the transfer hydrogenation of different unsaturated substrates is reported. This complex, if pre-activated, is very active in the transfer hydrogenation of ketones (i.e., cyclohexanone is reduced with a 38.1% conversion at 283 K and 100% at 313 K) while in the case of hex-1-ene, a 36.8% conversion was reached at 293 K. A cyclic olefin (cyclohexene) was also reduced with a lower, but still significant, conversion.It is interesting to note the catalytic activity of this complex in the transfer hydrogenation of a C@N double bond belonging to imides or nitrogen-containing heterocycles. For instance, N-benzylidenaniline was hydrogenated to N-benzylaniline at 303 K with a conversion of 27.3%. Increasing the temperature to 353 K, the conversion rised to 91.8%. A nitrogen containing heterocycle, quinoline, was also reduced by transfer hydrogenation at 353 K with a 11.7% conversion giving 1,2,3,4-tetrahydroquinoline (selectivity of 96.6%). The conversion rised up to 54.2% with a still high selectivity (84.5%) when the temperature was 383 K. Almost the same activity was shown in the reduction of pyridine to pyperidine (conversion, 51.1% at 383 K), while 2-methylpyridine was hydrogenated with a 24.7% conversion.

Selective partial reduction of quinolines: Hydrosilylation vs. transfer hydrogenation

“…The rhodium analogue also gave similar results, but the use of the iridium catalyst is preferable due to its lower cost. We achieved a TON of 67 which is comparable to a rhodium bis-bipyridine complex (TON = 54, T = 82°C) [36]. Catalyst 1, [Rh(cod)(PPh 3 ) 2 ]PF 6 , was not active under these conditions (<5% conversion to 1,2,3,4-tetrahydroquinoline).…”

Homogeneous metal catalysis for conversion between aromatic and saturated compounds

“…Under an atmosphere of syngas (H 2 /CO), Fish found Co 2 (CO) 6 (PPh 3 ) 2 catalyzed the hydrogenation of acridine and quinoline to 9,10-dihydroacridine and 1,2,3,4tetrahydroquinoline respectively (Scheme 73) [82]. 19 Kispert [105] was also able to reduce quinoline with a Ziegler-type Co(stearate) 2 •AlEt 3 catalyst at room temperature (Scheme 74).…”

Transfer Hydrogenation in Water

“…In contrast to the situation of ketone reduction, the TH of heterocycles has been much less explored 62–65. The iridacycle catalysts again show their utility by being capable of reducing an ample variety of N‐heterocycles, including quinolines, indoles, isoquinolinium and pyridinium salts, at a high S/C ratio in water 55.…”