Hydrogen-bond supported heterocyclic

Espinet and coworkers examined gold complexes of ADC ligands containing a hydrogen-bonded backbone - denoted hydrogen-bond supported heterocyclic carbenes (HBHCs) - in cyclizations of 1,6- and 1,7-enynes and observed unusual selectivities [27a]. Catalysts 24a and 24b were found to fevor the endo cycliza-tion product in reactions of enyne 23, whereas previously known phosphite-(25) and NHC-based (26) catalysts favored the exo product (Scheme 16.9). Notably, the differing steric bulk of 24a and 24b did not appear to impact the exo endo ratio, suggesting that the observed selectivity depends primarily on the donor properties of the HBHCs. 

The use of supported (i.e., heterogenized) homogeneous catalysts offers another possibility for easy catalyst separation. New examples include polymer-anchored Schiff-base complexes of Pd(TT),446 PdCl2(PhCN)2 supported on heterocyclic polyamides,447 various Pd complexes supported on crosslinked polymers 448 sol-gel-encapsulated Rh-quatemary ammonium ion-pair catalysts,449 and zwitterionic Rh(T) catalysts immobilized on silica with hydrogen bonding.450... 

On the one hand, x-ray crystallography data could support this idea because the heterocycles are coplanar relative to the B-H bonds. Moreover, in the case of the compound in Strncture 5.5, this conformation remains in solution, according to NOE NMR measnrements. On the other hand, the solid-state CH- -HB distances are too long (< 2.65 A), and the HF/6-31G calculations carried ont for the componnd have shown, that where the heterocyclic rings are oriented orthogonally, the conformation is more stable. Thns, this question is still open. It is quite probable that the interaction between protonic hydrogens on the a-carbons and hydridic BH hydrogens is electrostatically attractive. 

The partial hydrogenation of dienes was successfully carried out by Michalska et al. [21] using palladium supported on heterocyclic polyamides. Under the reaction conditions employed (MeOH, atmospheric pressure, 25 °C) the resin-sup-ported catalyst was able to selectively hydrogenate one of the two double bonds present. Recycling experiments proved the high stability of the used catalysts. For example, in 11 hydrogenation runs with 2-methyl-l,3-pentadiene, which is equivalent to 4300 catalytic cycles per palladium atom, neither loss of activity nor changes in selectivity were observed. 

---