Reduction reactions with NHC-bearing complexes

Ege University, Department of Chemistry, 35100, Bornova, Izmir, Turkey 

The aim of this Chapter is to examine the application of well-defined N-hetero-cyclic carbene (NHC) complexes as well as the systems prepared in situ which involve free NHCs or the precursor salt for the reduction of imsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reductions contain electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types aeeording to reductants used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

N-HeteroeyeKe Carbenes From Laboratory Curiosities to Efficient Synthetic Tools 

The earliest homogeneous hydrogenation catalyst was Wilkinson catalyst [Rh(PPh3)3Cl] 1 which was active at 1 atm of H2 pressure at room temperature for monosubstituted and cw-disubstituted alkenes such as cyclohexene. The second, [Rh(COD)(PPh3)2]PF6 2, and third, [Ir(COD)(PCy3)(pyr)]PF6 3, generation catalysts were developed by Osbom-Schrock and Crabtree, respectively. 

Buriak and co-workers chose a different modification strategy to Crabtree s catalysts and they replaced the pyridine with various alkyl phosphines. Optimization studies revealed that complex [(IMes)Ir(COD)(PBu3)]BARF 5 (BARF = tetrakis[3,5-bis(triflouromethylphenyl)borate]) was the most stable and displayed similar activity to 3. Furthermore, 5 was active for highly hindered tri- and tetrasubstituted alkenes at 1 atm pressure of H2 in the presence of air. 

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