The catalytic behaviour of the half-sandwich iridium triazenide complex 45 in the
hydrogenation of non-conjugated 5-alken-2-ones depends on the presence of a base.
If KOH is present, then the carbonyl group will be selectively hydrogenated in the
first place. However, in the absence of the base, the C¼C bond is preferably
hydrogenated [85].
The carbonyl group of the dihydrocarvone G60 was chemoselectively reduced
with the bis(phosphine) iridium(I) complex 75c. At 0.5 mol% of catalyst loading, in
iPrOH, using 10 equiv. of KOH, at 80
C, 90% yield was obtained within 24 h. The
isolated olefin moiety was not hydrogenated under these conditions [191].
However, the isolated C¼C bond of both 11-dodecen-2-one G59 and
4-acetylstyrene G53 was chemoselectively hydrogenated to the corresponding
unsaturated ketone using the pincer iridium complex 72, containing a rigid
benzoquinoline backbone, as a catalyst. At 1 mol% of catalyst loading, in EtOH,
using tBuONa (2,2 mol%) as a base, 95% yields for G59 and G53 were obtained
after 18 and 30 min, respectively [181].
A mixture of the dimer [IrCl(COD)] 2 (1 mol%) and the diphosphine 1,2-bis
(dicyclohexylphosphino)ethane (4 mol%) promoted the highly chemoselective TH
of the isolated alkene group of unsaturated ketones [194]. In 1,4-dioxane, at 130
C,
terminal alkene groups bearing an acetophenone (G53-G56) or cyclohexanone
(G61) underwent selective reduction of the alkene moiety. The isolated C¼C bond
of the estrone derivative G63 was also chemoselectively reduced [194].
7 Transfer Hydrogenation of N-Heterocycles
Reduced N-heterocycles are frequently found in drugs, agrochemicals and dyes
[120, 202], and, therefore, having efficient methods for their synthesis is highly
desirable. Hydrogenation of N-heterocycles is the most obvious route to that type of
compounds, and heterogeneous [203] and homogeneous [204, 205] catalytic hydrogenation has been the most widely used methodology. Despite the advantages of TH
in terms of sustainability, simplicity of application and safety, this approach has been
much less investigated for the reduction of N-heterocycles [206–211]. Here we
present recent contributions in this area. An example of selective reduction of
benzofurans by TH protocols is also included in this Section.
The iridacycle complex 81 (Scheme 56) reduced quinolines H1–H20 and H22–
H28 (Scheme 57) to tetrahydroquinolines in an aqueous HCOOH/HCOONa solution, using 0.1 mol% of catalyst. The solution pH is critical for the catalytic activity
pH 4.5 giving the best yield. Typically, isolated yields higher than 90% were
obtained within 14 h [212].
Complex 81 also catalyses the reduction of activated isoquinolines and pyridines
by quaternisation. Thus, isoquinolinium H29–H34 and pyridinium H35–H44
(Scheme 58) were hydrogenated in 24–36 h, under the same conditions but at
refluxing temperature. Again, isolated yields higher than 90% were achieved [212].
126
M. Pilar Lamata et al.
hydrogenation of non-conjugated 5-alken-2-ones depends on the presence of a base.
If KOH is present, then the carbonyl group will be selectively hydrogenated in the
first place. However, in the absence of the base, the C¼C bond is preferably
hydrogenated [85].
The carbonyl group of the dihydrocarvone G60 was chemoselectively reduced
with the bis(phosphine) iridium(I) complex 75c. At 0.5 mol% of catalyst loading, in
iPrOH, using 10 equiv. of KOH, at 80
C, 90% yield was obtained within 24 h. The
isolated olefin moiety was not hydrogenated under these conditions [191].
However, the isolated C¼C bond of both 11-dodecen-2-one G59 and
4-acetylstyrene G53 was chemoselectively hydrogenated to the corresponding
unsaturated ketone using the pincer iridium complex 72, containing a rigid
benzoquinoline backbone, as a catalyst. At 1 mol% of catalyst loading, in EtOH,
using tBuONa (2,2 mol%) as a base, 95% yields for G59 and G53 were obtained
after 18 and 30 min, respectively [181].
A mixture of the dimer [IrCl(COD)] 2 (1 mol%) and the diphosphine 1,2-bis
(dicyclohexylphosphino)ethane (4 mol%) promoted the highly chemoselective TH
of the isolated alkene group of unsaturated ketones [194]. In 1,4-dioxane, at 130
C,
terminal alkene groups bearing an acetophenone (G53-G56) or cyclohexanone
(G61) underwent selective reduction of the alkene moiety. The isolated C¼C bond
of the estrone derivative G63 was also chemoselectively reduced [194].
7 Transfer Hydrogenation of N-Heterocycles
Reduced N-heterocycles are frequently found in drugs, agrochemicals and dyes
[120, 202], and, therefore, having efficient methods for their synthesis is highly
desirable. Hydrogenation of N-heterocycles is the most obvious route to that type of
compounds, and heterogeneous [203] and homogeneous [204, 205] catalytic hydrogenation has been the most widely used methodology. Despite the advantages of TH
in terms of sustainability, simplicity of application and safety, this approach has been
much less investigated for the reduction of N-heterocycles [206–211]. Here we
present recent contributions in this area. An example of selective reduction of
benzofurans by TH protocols is also included in this Section.
The iridacycle complex 81 (Scheme 56) reduced quinolines H1–H20 and H22–
H28 (Scheme 57) to tetrahydroquinolines in an aqueous HCOOH/HCOONa solution, using 0.1 mol% of catalyst. The solution pH is critical for the catalytic activity
pH 4.5 giving the best yield. Typically, isolated yields higher than 90% were
obtained within 14 h [212].
Complex 81 also catalyses the reduction of activated isoquinolines and pyridines
by quaternisation. Thus, isoquinolinium H29–H34 and pyridinium H35–H44
(Scheme 58) were hydrogenated in 24–36 h, under the same conditions but at
refluxing temperature. Again, isolated yields higher than 90% were achieved [212].
126
M. Pilar Lamata et al.