144
J.L. Margitfalvi, E. Tálas / Catalysis Communications 46 (2014) 142–145
3.2. Enantioselective hydrogenation of other activated ketones
Enantioselective hydrogenation of 2,3-BD and 3,4-HD showed mod-
erate or low ee values (see entries 18, 20, 22 in Table 2). The type of the
solvent significantly influenced both the reaction rate and the ee (com-
pare entries 20 and 22 in Table 2). In ethanol poorer results were obtain-
ed than in toluene due to the possibility of condensation type side
reactions [4]. However, the addition of ATAs resulted in slight rate-
and significant ee-increase even in ethanol (compare entries 22 and
23 in Table 2). Accordingly, the observations found for EtPy are maintained
completely.
Upon hydrogenation of MBF, the CD itself caused fourfold increase in
k1 (compare entries 24 and 25 in Table 2). Addition of quinuclidine
(entry 26 in Table 2) or quinoline (entry 27 in Table 2) to the reaction
mixture further increased the k values, but no decrease in the
enantioselectivity was observed.
4. Discussion
Results given in Table 2 and Fig. 4 clearly show that in the
enantioselective hydrogenation of four different substrates neither the
addition of ATAs nor the addition of nitrogen containing aromatic com-
pounds has any negative effect on the ee values and the reaction rates.
The most pronounced positive effect on the ee values and the reaction
rates was obtained when quinuclidine and quinoline were added to-
gether. The lack of the negative effect of all additives contradicts to the
proton transfer mechanism depictured in Fig. 3.
The above argumentation would be questionable since amine addi-
tives have no anchoring moieties, and consequently they cannot form
strongly adsorbed surface species. Although the generally accepted
view that strongly bonded CD to platinum via π-bonding with its quino-
line ring is the “relevant” species in these asymmetric hydrogenation re-
actions, the weakly bound tilted form of CD is a “spectator” [26].
However, our previous [23] and present results (see entries 15–17, 27
in Table 2) clearly demonstrate that the strong adsorption of CD is not
a prerequisite for obtaining both high enantio-discrimination and reac-
tion rates. Data shown in Ref. [23], in accordance with those of Table 2
and Fig. 4, provide evidence that nitrogen containing aromatics have
no negative effect either on the reaction rate or on the enantioselectivity.
Pre-adsorbed quinoline and acridine cannot be fully replaced from the
surface of platinum by CD [23], i.e. the strong adsorption of CD via its
aromatic ring is not a requirement to get high ee values in the
enantioselective hydrogenation of EtPy. Similar conclusion was obtained
upon using MBF. The lack of ee decrease means that the pre-adsorbed
quinoline has no influence on the enantio-differentiation ability of CD.
These experimental findings suggest that neither Scheme 2 in Ref.
[5] nor the concept of “strongly bonded to the platinum CD via its quin-
oline ring” is working adequately.
Fig. 3. Reaction scheme for reaction mixture containing both CD and ATAs.
evidenced by MS. The effect of ATAs can be related to the shift of the CD
dimer–monomer ratio [21,22].
Enantioselective hydrogenation of EtPy was also investigated in the
presence of nitrogen containing aromatic additives. Pyridine and quino-
line increased both rate constants and ee (compare entries 13 and
15–16 in Table 2), too. In an earlier study a similar phenomenon attrib-
uted to some sort of base effect of quinoline [25].
Over CD-Pt/Al2O3 catalyst the highest ee value was obtained when
aromatic basis (quinoline) and achiral tertiary amine (quinuclidine)
were introduced together (entry 17 in Table 2). Fig. 4 represents the
kinetic behavior of this experiment. The co-presence of quinuclidine
and quinoline does not alter the general kinetic patterns (Fig. 4a),
but increases the ke/kr ratio (Fig. 4b) in accordance with the ee-
enhancement. This increase can be attributed to the virtual increase of
CD concentration [21,23].
5. Conclusion
In this contribution experimental evidences are summarized, which
strongly contradict to the proton transfer mechanism [5,15] proposed
for the catalytic system Pt–cinchona alkaloids. If the quinuclidine
Table 1
Reaction conditions.
Substrate
[Substrate]0, M
Catalyst
mcatalyst, g
Vreaction, cm3
treaction, min
Agitation, rpm
EtPy (batch1)
EtPy (batch2)
EtPy (batch3)
2,3-BD
3,4-HD
MBF
1.00
0.85
1.00
1.00
1.00
0.25
E4759
CatASium F214
E4759
E4759
E4759
0.125
0.040
0.125
0.125
0.125
0.125
100
40
100
100
100
50
90
60
90
240
240
180
500
1000
500
500
500
E4759
1000