Asymmetric Hydrogenation of Cyclic Enamides

In comparison with the hydrogenation of acyclic enamides, there are fewer sue cessful examples on the catalytic asymmetric hydrogenation of cyclic enamides 

More recently, Bruneau found that the tri substituted cyclic enamides derived from (3 tetralone and its analogues can be efficiently hydrogenated in 

By using an Rh/(R,R) Me DuPHOS catalytic system, Zhou has successfully hydrogenated a new dass of exocyclic enamides, (Z) 3 arylidene 4 acyl 3,4 dihydro 2H benzoxazines (76) [11]. Hydrogenation gave high enantioselectiv ities (up to 98.6% ee) regardless of the substituents on the aromatic ring of 

In the catalytic asymmetric hydrogenation of enamides, which are the most used central metals (For undergraduate student) 

There are still some challenging enamide substrates for the catalytic asym metric hydrogenation. Can you give some examples and describe their recent advances  

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Modification of the electronic and steric properties of BINAP, BIPHEMP, and MeO-BI-PHEP led to the development of new efficient atropisomeric ligands. Although most of them are efficient for ruthenium-catalyzed asymmetric hydrogenation [3], Zhang et al. have recently reported an ortho-substituted BIPHEP ligand, o-Ph-HexaMeO-BIPHEP, for the rhodium-catalyzed asymmetric hydrogenation of cyclic enamides (Scheme 1.2) [31]. 

Scheme 8.5 Asymmetric hydrogenation of cyclic enamides with monophosphoramidite ligands.

Scheme 9.13 Asymmetric hydrogenation of cyclic enamides with an endocyclic double bond derived from a tetralone and a indanone.

Scheme 9.17 Synthesis of N acetylcolchinol via asymmetric hydrogenation of cyclic enamide...

Asymmetric hydrogenation of a cyclic enamide (Approach B) had very sparse literature precedents [7]. It should also be noted that preparation of these cyclic imines and enamides is not straightforward. The best method for the synthesis of cyclic imines involves C-acylation of the inexpensive N-vinylpyrrolidin-2-one followed by a relatively harsh treatment with refluxing 6M aqueous HC1, which accomplishes deprotection of the vinyl group, hydrolysis of the amide, and decarboxylation (Scheme 8.6) [8]. 

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]. 

Scheme 9.15 Asymmetric hydrogenation of tetra substituted cyclic enamides using the Ru catalyst containing Me DuPHOS or Me BPE.

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