Metal-Free Catalytic Hydrogenation

B-H [40] and N-H bonds. The role of carbenes as the base in FLP activation of H2 and NFl bonds has also been explored [41 3]. As these works are beyond the scope of the present chapter, they are not detailed herein. 

The impact of steric efforts on this reduction process is evident from the data. Sterically less encumbered imines are reduced more slowly, which in the case of the imine PhCH=NCH2Ph leads to it being reduced only stoichiometrically (45). 

Using a similar strategy, the research groups of Repo and Rieger reported the catalytic hydrogenation of imines and enamines using a linked amine-borane species derived from 2,2,6,6-tetramethylpiperidine [57]. In this case, the link 

2721 77 Frustrated Lewis Pairs A Metal-Free Strategy for Hydrogenation Catalysis HB(CeF5)2 

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The majority of metal-based Lewis acid catalysts used in the HDA reaction are moisture sensitive and are thus usually prepared in situ. The stable and storable zirconium-BINOL Lewis acids developed by Kobayashi and coworkers, effective in the aldol reaction (see Section 7.1) can also be used as asymmetric catalysts in the HDA reaction of aliphatic and aromatic aldehydes with dioxygenated dienes. Metal-free catalytic asymmetric HDA reactions have been developed, utilising enantiomerically pure protic molecules that activate the aldehyde component by hydrogen bonding to the carbonyl group. Rawal and coworkers have achieved up... 

In the present chapter the three metal-free catalytic methodologies will be presented while for FLP methodology, basically all the most relevant examples of hydrogenation processes will be reported, for the other two methodologies, where several reviews have been recently published, only the more recent contributions to the field will be discussed, except those that are generally considered the milestones achievements in the field. 

The actual spacings of the metal atoms in the surface will clearly be of importance in making one face of a metal crystal catalytically effective, and another not, depending on how closely the actual atom spacings approximate to the bond distances in alkene and hydrogen molecules. In practice only a relatively sma l proportion of the total metal surface is found to be catalytically effective—the so-called active points . These adsorb alkene strongly, and then desorb immediately the resultant alkane, thus becoming free for further alkene adsorption. 

Catalytic evaluations were conducted using microactivity tests (MAT) ( ) at 910 F initial temperature, 15 WHSV, 6.0 g catalyst, and a 5.0 cat-to-oil ratio. The feedstock was a metals-free mid-continent gas oil. Each data point shown is the average of two MAT runs. Only MAT runs with acceptable mass balance were used (96 to 101%). Additionally, MAT data was normalized to 100% mass balance. Extensive error analysis of conversion, coke, and hydrogen yields indicates the following respective standard deviations 1.62, 0.29, 0.025. The effects of nickel and vanadium on the hydrogen and coke make were calculated by obtaining the difference between the yields obtained with uncontaminated catalysts and that of the contaminated catalyst at the same conversion. 

Recently, we established that several proton acids catalyze the metal-free reduction of ketimines under hydrogen-transfer conditions with Hantzsch dihydropyridine as the hydrogen source.Additionally, we were able to demonstrate a catalytic enantioselective procedure of this new transformation by employing a chiral Br0nsted acid as catalyst.(see Chapter 4.1). 

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