Olefin hydrogenation unfunctionalized olefins

In the area of the asymmetric hydrogenation of C=C double bonds, the reduction of unfunctionalized olefins has been scarcely explored, in comparison with... 

Substrate 2 has also been used as a test substrate HPLC separation methods exist for 2, while ee-value determination of 1 is more difficult [6, 17]. Reflecting the general recent interest in the hydrogenation of unfunctionalized olefins, the past few years have seen the publication of a number of results for this substrate [15, 18-26]. The highest enantioselectivities were achieved using catalysts 12b [22] and 14a [26],... 

Recently, a breakthrough in the hydrogenation of unfunctionalized olefins was made [51]. For the first time, high enantioselectivities with purely alkyl-substituted alkenes such as 72-74 could be achieved using pyridine-phosphinite catalysts 75 and 76. 

Tetrasubstituted alkenes are challenging substrates for enantioselective hydrogenation because of their inherently low reactivity. Crabtree showed that it was possible to hydrogenate unfunctionalized tetrasubstituted alkenes with iridium catalysts [46]. Among the iridium catalysts described in the previous section, several were found to be sufficiently reactive to achieve full conversion with al-kene 77 (Table 30.14). However, the enantioselectivities were significantly lower than with trisubstituted olefins, and higher catalyst loadings were necessary. 

During recent years, substantial progress has been made in the hydrogenation of unfunctionalized alkenes. With iridium complexes derived from chiral phos-phino-oxazolines and related ligands, excellent enantioselectivities and high TON/TOF values can now be obtained for a wide range of unfunctionalized olefins. Most substrates studied to date have at least one aryl substituent at the... 

Scheme 36.4 Lib raries of P,N- and C,N-ligands in the Ir-catalyzed enantioselective hydrogenation of unfunctionalized olefins.

The reaction was carried out by addition of 1.95 equivalents of -BuLi to a THF solution of 22 at 0°C to generate the active catalyst, which was then combined with substrate (S/C about 20 1) under an inert atmosphere using phenylsilane as the stabilizing agent. Trisubstituted unfunctionalized olefins can be hydrogenated in good yield with high ee. Representative results are listed in Table 6-3. 

Figure 10. Buchwald s catalyst system for the enantio-selective hydrogenation of unfunctionalized olefins such as 52 and 53.

Scheme 15. Ir-Catalysed hydrogenation of unfunctionalized olefins and imines using PHOX-ligands.

Recently, Buchwald has developed an analogous, cationic zirconium system which is, to date, the most effective catalyst for the hydrogenation of tetrasubsti-tuted, unfunctionalized olefins.1461... 

Historically, reaction of simple olefins in the presence of chiral phosphine-Rh complexes in 1968 marked the first examples of homogeneous asymmetric hydrogenation [6], However, only a few successful results have been reported for asymmetric hydrogenation of unfunctionalized olefins. Some examples with late and early transition-metal complexes are illustrated in Schemes 1.27-28 and Schemes 1.29-30, respectively. 

As shown in Scheme 1.30, the chiral titanocene catalyst 34 hydrogenates unfunctionalized, disubstituted styrenes under 136 atm of hydrogen at 65°C to give the saturation products with 83 to >99% ee [156]. A high enantioselectivity is now realized only with aryl-substituted olefins. The enantioselectivity of 41% ee attained 2-ethyl-1-hexene and 34 as catalyst is the highest for hydrogenation of non-aromatic olefins. 

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