now contains a geometrically flat metallocycle formed by the imidazolyl-imine
chelate, and allows for a shorter intermetallic distance through alignment of the
complex planes. Complex 46 achieved the highest rate enhancements yet observed
for any bimetallic catalysed reaction, over 6,800 times higher turnover frequency
than that of the analogous monometallic catalyst 49. Increasing the RhÁ Á ÁRh
distance (complex 47) or increasing the flexibility of the C 2 bridge (complex 48)
drastically reduced the effectiveness of the bimetallic catalysts, consistent with
previous observations.
N
N
N
N
Rh
N N
N
N
Rh
flip
rotate
Large conformational freedom
=
poorly defined M-M distance
Fig. 3 Conformational flexibility of bimetallic complexes containing the bis(1-pyrazolyl)
methane ligand donor
OH
2 mol% Rh
1 atm CO
25 o C, C 2 D 2 Cl 4
OH
O
O
O
O
+
[BAr F
4 ] 2
46, 1370 h -1
N Rh
N
N
N
Rh
N
N
>>
N
Rh
N
N
N
Rh
N
N
47, 2.2 h -1
N
Rh
N
N
[BAr F
4 ] 2
N
Rh
N
N
[BAr F
4 ] 2
48, 1.0 h -1
>
N
Rh
N
N
49, 0.2 h -1
[BAr F
4 ] 2
Scheme 17 Rh(I) mono- and bimetallic catalysts containing the imidazolyl-imine ligand (46–49)
used for the intramolecular dihydroalkoxylation reaction
Alkyne Activation Using Bimetallic Catalysts
123