Unsaturated alcohols, asymmetric hydrogenation

Complexes containing one binap ligand per ruthenium (Fig. 3.5) turned out to be remarkably effective for a wide range of chemical processes of industrial importance. During the 1980s, such complexes were shown to be very effective, not only for the asymmetric hydrogenation of dehydroamino adds [42] - which previously was rhodium s domain - but also of allylic alcohols [77], unsaturated acids [78], cyclic enamides [79], and functionalized ketones [80, 81] - domains where rhodium complexes were not as effective. Table 3.2 (entries 3-5) lists impressive TOF values and excellent ee-values for the products of such reactions. The catalysts were rapidly put to use in industry to prepare, for example, the perfume additive citronellol from geraniol (Table 3.2, entry 5) and alkaloids from cyclic enamides. These developments have been reviewed by Noyori and Takaya [82, 83]. 

This ligand, MeO-BIPHEP (96a), has shown similar reactivities and enantioselectivities to catalysts that contain BINAP.117 Ruthenium catalysts that contain MeO-BIPHEP have been used in several asymmetric hydrogenations from bench scale to multi-ton scale, which include the large-scale preparation of a P-keto ester, an aryl ketone, allylic alcohol, and several oc,P-unsaturated carboxylic acid substrates, which are shown in Figure 12.5. 

Ruthenium and rhodium complexes that contain TMBTP have shown utility in the asymmetric hydrogenation of allylic alcohols,155,156 P-keto esters,155,157 and a,P-unsaturated carboxylic acids.155... 

A considerable success has been realized for asymmetric hydrogenation of functionalized alkenes since the discovery of BINAP-Ru complexes in the mid-1980s [5]. The details are described in each of the following substrates, enamides, alkenyl esters and ethers, a,/3- and /3,y-unsaturated carboxylic acids, a,/3-unsaturated esters and ketones, and allylic and homoallylic alcohols. 

Noyori asymmetric hydrogenation Formation of enantio-enriched carboxylic acids, alcohols and amino acids from unsaturated carboxylic acids, allylic alcohols and enamides, respectively. 316... 

Fig. 40. Recent asymmetric hydrogenation work involving unsaturated alcohols...

Two ruthenium complexes, binap 3.43-Ru(OCOR)2(R = Me,CF3) [892] and binap 3.43-RuX2 (X = Cl, Br, I) [893, 894], are quite useful. The acetate and trifluoroacetate complexes of 3.43 induce selective asymmetric hydrogenations of classes of prochiral olefins that are poorly selective with rhodium complexes. These classes include a,(3- or fcy-unsaturated acids and esters, ally alcohols, j3-acylaminoacrylates and enamide precursors of isoquinoline alkaloids [752, 853, 859, 881, 883, 895]. 

Asymmetric hydrogenation of allylic and homoallylic unsaturated alcohols was not very efficient until the discovery of the BINAP-Ru catalyst. With Ru(BrNAP)(OAc)2 as catalyst, geraniol 70 and nerol 72 are successfully hydrogenated to give ([S)- or (i )-citronellol (71 and 73, respectively) in high overall yield with good enantioselectivity of 98 and 99% ee.59... 

More recently Noyori developed asymmetric hydrogenation of simple ketones with BlNAP/diamine-ruthenium complexes.In this system the catalytic process contrasted with the conventional mechanism of asymmetric hydrogenation of unsaturated bonds which requires metal-substrate 7t-complexation. In fact with BlNAP/diamine-ruthenium neither the ketone substrate nor the alcohol product interacted with the metallic centre during the catalytic cycle. The enantiofaces of the prochiral ketones were differentiated on the molecular surface of the coordinatively saturated RuH intermediate. 

Mechanistic insight into Noyori asymmetric hydrogenations of enamides, allylic alcohols, Q, )S-unsaturated carboxylic acids, o ,jS-keto esters, and aromatic ketones using Ru(OAc)2[(5)-binap], (3), as the catalyst has been highlighted ... 

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