stronger Rh-CO π-backbonding for more electron-rich hydroformylation catalysts.
The higher overall activity of the monocationic dirhodium catalyst species in water/
acetone solvent is due to the higher concentration of active catalyst present due to
considerably reduced catalyst fragmentation and deactivation that occurs for the
dicationic catalyst 11r/11r*.
Although the computed bimetallic mechanisms for the monocationic and
dicationic catalysts follow an analogous series of fundamental reaction steps and
rely heavily on bimetallic cooperativity throughout each cycle, there are some
substantial differences in the energetics of the reactions steps. For example, the
activation barrier for the initial alkene-hydride migratory insertion step for the
dicationic catalyst 11r* is about half (8 vs. 17 kcal) that for the monocationic
catalyst 15r. The dicationic alkyl intermediate C is quite a bit more stable than the
corresponding species for the monocationic system. The other dramatic difference
between the two systems is that the CO migratory insertion step for the
monocationic catalyst has a very small barrier of 5 kcal vs. a 13 kcal barrier for
the dicationic catalyst.
The other significant difference between the two bimetallic catalysts is that the
monocationic monohydride dirhodium catalyst needs to oxidatively add H 2 in order
to gain the hydride(s) to allow the reductive elimination of aldehyde. The dicationic
dihydride system has the second hydride already present and “ready to go” for the
acyl reductive elimination step. H 2 then oxidatively adds to the dicationic catalyst
to regenerate the dihydride 11r/11r*. But since the monocationic catalyst system
has a low activation barrier for H 2 oxidative addition (8.7 kcal), this is not a
bottleneck in the catalysis cycle.
The need to oxidatively add H 2 to enable the reductive elimination of aldehyde
makes the monocationic dirhodium catalyst 15r somewhat similar to monometallic
hydroformylation catalysts that have the same requirement. The presence of a
formal cationic charge, however, helps compensate for the strongly donating,
mainly alkylated phosphine ligands present that would normally dramatically
reduce the activity of a monometallic hydroformylation catalyst via too strong
coordination of the CO ligands and saturation of the metal center. Most of the
proposed and computed intermediates in the cycle based on 15r have a localized
cationic charge residing on a single Rh center, weakening the Rh–CO bonding and
enabling coordination and oxidative addition of H 2 .
We believe that the presence of free H
+ in the acetone/water solvent system
plays a role in the monocationic system. The rate determining step, once again, is
the reductive elimination of aldehyde with a calculated barrier of 21.6 kcal
(Fig. 14). Protonation of the monocationic dirhodium acyl is an alternate and likely
pathway for eliminating aldehyde and forming the dicationic dirhodium catalyst
11r. Due to the very low activation barrier for the monocationic alkyl-CO migratory insertion step, protonation of Rh-alkyl species to produce alkane is far less
likely and consistent with the much lower alkane side reactions for 15r.
Addition of 2 equivalents of NEt 3 to either the dicationic catalyst in acetone or
the monocationic catalyst in water/acetone dramatically slows the hydroformylation. In acetone the initial TOF is reduced by ~34%, while in water/acetone
22
R.G. Fernando et al.
The higher overall activity of the monocationic dirhodium catalyst species in water/
acetone solvent is due to the higher concentration of active catalyst present due to
considerably reduced catalyst fragmentation and deactivation that occurs for the
dicationic catalyst 11r/11r*.
Although the computed bimetallic mechanisms for the monocationic and
dicationic catalysts follow an analogous series of fundamental reaction steps and
rely heavily on bimetallic cooperativity throughout each cycle, there are some
substantial differences in the energetics of the reactions steps. For example, the
activation barrier for the initial alkene-hydride migratory insertion step for the
dicationic catalyst 11r* is about half (8 vs. 17 kcal) that for the monocationic
catalyst 15r. The dicationic alkyl intermediate C is quite a bit more stable than the
corresponding species for the monocationic system. The other dramatic difference
between the two systems is that the CO migratory insertion step for the
monocationic catalyst has a very small barrier of 5 kcal vs. a 13 kcal barrier for
the dicationic catalyst.
The other significant difference between the two bimetallic catalysts is that the
monocationic monohydride dirhodium catalyst needs to oxidatively add H 2 in order
to gain the hydride(s) to allow the reductive elimination of aldehyde. The dicationic
dihydride system has the second hydride already present and “ready to go” for the
acyl reductive elimination step. H 2 then oxidatively adds to the dicationic catalyst
to regenerate the dihydride 11r/11r*. But since the monocationic catalyst system
has a low activation barrier for H 2 oxidative addition (8.7 kcal), this is not a
bottleneck in the catalysis cycle.
The need to oxidatively add H 2 to enable the reductive elimination of aldehyde
makes the monocationic dirhodium catalyst 15r somewhat similar to monometallic
hydroformylation catalysts that have the same requirement. The presence of a
formal cationic charge, however, helps compensate for the strongly donating,
mainly alkylated phosphine ligands present that would normally dramatically
reduce the activity of a monometallic hydroformylation catalyst via too strong
coordination of the CO ligands and saturation of the metal center. Most of the
proposed and computed intermediates in the cycle based on 15r have a localized
cationic charge residing on a single Rh center, weakening the Rh–CO bonding and
enabling coordination and oxidative addition of H 2 .
We believe that the presence of free H
+ in the acetone/water solvent system
plays a role in the monocationic system. The rate determining step, once again, is
the reductive elimination of aldehyde with a calculated barrier of 21.6 kcal
(Fig. 14). Protonation of the monocationic dirhodium acyl is an alternate and likely
pathway for eliminating aldehyde and forming the dicationic dirhodium catalyst
11r. Due to the very low activation barrier for the monocationic alkyl-CO migratory insertion step, protonation of Rh-alkyl species to produce alkane is far less
likely and consistent with the much lower alkane side reactions for 15r.
Addition of 2 equivalents of NEt 3 to either the dicationic catalyst in acetone or
the monocationic catalyst in water/acetone dramatically slows the hydroformylation. In acetone the initial TOF is reduced by ~34%, while in water/acetone
22
R.G. Fernando et al.