9 Mechanistic Aspects
Despite the variety of substrates, i.e. carbonyl compounds, imines, alkenes and
alkynes, used in catalytic TH, mainly the mechanism of the TH of carbonyl compounds using 2-propanol as the hydrogen donor has been systematically studied by
means of both experimental methods and theoretical calculations. For an overview
of mechanistic aspects of TH, see Refs. 228 and 232–234. In general, the different
basicity and nucleophilicity of the substrates makes it difficult to apply the reaction
map depicted for carbonyl compounds to imines, alkenes or alkynes. In addition, as
shown in the previous sections, besides the ubiquitous 2-propanol, different hydrogen donors have been successfully used as well, e.g. formic acid, 1,4-dioxane,
glycerol, ethanol and diols.
As far as TH of carbonyl compounds using 2-propanol is concerned, overall, the
formal transfer of a hydride and a hydrogen ion from the donor to the substrate takes
place.
Three scenarios have been depicted so far:
1. In an inner sphere mechanism, a metal alkoxide is the entry species of the
catalytic cycle (Scheme 70), eventually undergoing a β-hydrogen elimination
rendering a metal monohydride complex. The hydride moiety is transferred by
means of the insertion of the substrate into the metal hydride bond, whereas the
protonolysis of the resulting alkoxide intermediate enables the transfer of the
hydrogen ion.
Alternatively a metal dihydride species has been proposed as the active species
along the catalytic cycle. In this case both hydride moieties come from the
hydrogen donor thanks to the oxidative addition of the O-H bond followed by
the β-hydrogen elimination in the resulting alkoxide (Scheme 70). In this case, the
hydrogen atoms of the metal dihydride moiety transfer to the substrate in a
sequential way by means of the insertion of the substrate into the metal hydride
bond and the following reductive elimination of the hydrogenated substrate from
the hydride alkoxide intermediate.
As a result, in the monohydride route, the C-H hydrogen of the donor
exclusively turns into the C-H hydrogen of the product, while in the dihydride
route, a scrambling of the C-H and O-H hydrogen atoms of the donor takes place.
2. The hydride moiety and the hydrogen ion are transferred to the substrate in an
outer sphere mechanism implying both a metal hydride moiety and a protic end of
one ligand at the metal centre (Scheme 71). This metal-ligand bifunctional
mechanism was initially described by Noyori and co-workers [235] for ruthenium
catalysts and has been later observed in a number of structurally related transition
metal complexes as well as in metal-ligand platforms in which the ligand act as a
non-innocent fragment [236]. Notably recent reviews on this mechanism have
raised concerns about the concertedness of the H
+
/H
– transfer to the substrate and
the genuine non-innocence of the ancillary ligand [236–238].
134
M. Pilar Lamata et al.
Despite the variety of substrates, i.e. carbonyl compounds, imines, alkenes and
alkynes, used in catalytic TH, mainly the mechanism of the TH of carbonyl compounds using 2-propanol as the hydrogen donor has been systematically studied by
means of both experimental methods and theoretical calculations. For an overview
of mechanistic aspects of TH, see Refs. 228 and 232–234. In general, the different
basicity and nucleophilicity of the substrates makes it difficult to apply the reaction
map depicted for carbonyl compounds to imines, alkenes or alkynes. In addition, as
shown in the previous sections, besides the ubiquitous 2-propanol, different hydrogen donors have been successfully used as well, e.g. formic acid, 1,4-dioxane,
glycerol, ethanol and diols.
As far as TH of carbonyl compounds using 2-propanol is concerned, overall, the
formal transfer of a hydride and a hydrogen ion from the donor to the substrate takes
place.
Three scenarios have been depicted so far:
1. In an inner sphere mechanism, a metal alkoxide is the entry species of the
catalytic cycle (Scheme 70), eventually undergoing a β-hydrogen elimination
rendering a metal monohydride complex. The hydride moiety is transferred by
means of the insertion of the substrate into the metal hydride bond, whereas the
protonolysis of the resulting alkoxide intermediate enables the transfer of the
hydrogen ion.
Alternatively a metal dihydride species has been proposed as the active species
along the catalytic cycle. In this case both hydride moieties come from the
hydrogen donor thanks to the oxidative addition of the O-H bond followed by
the β-hydrogen elimination in the resulting alkoxide (Scheme 70). In this case, the
hydrogen atoms of the metal dihydride moiety transfer to the substrate in a
sequential way by means of the insertion of the substrate into the metal hydride
bond and the following reductive elimination of the hydrogenated substrate from
the hydride alkoxide intermediate.
As a result, in the monohydride route, the C-H hydrogen of the donor
exclusively turns into the C-H hydrogen of the product, while in the dihydride
route, a scrambling of the C-H and O-H hydrogen atoms of the donor takes place.
2. The hydride moiety and the hydrogen ion are transferred to the substrate in an
outer sphere mechanism implying both a metal hydride moiety and a protic end of
one ligand at the metal centre (Scheme 71). This metal-ligand bifunctional
mechanism was initially described by Noyori and co-workers [235] for ruthenium
catalysts and has been later observed in a number of structurally related transition
metal complexes as well as in metal-ligand platforms in which the ligand act as a
non-innocent fragment [236]. Notably recent reviews on this mechanism have
raised concerns about the concertedness of the H
+
/H
– transfer to the substrate and
the genuine non-innocence of the ancillary ligand [236–238].
134
M. Pilar Lamata et al.