BINAP double hydrogenation

Double Hydrogenation of 1,3- and 1,2-Diketones. Scheme 57 illustrates another example of highly enantioselective formation of alcoholic products. The BINAP-Ru(II)-catalyzed hydrogenation of prochiral 1,3-diketones produces diastereomeric 1,3-diols, for which the dl or anti isomers are always dominant and ee values are uniformly high (92,105). For example, the reaction of 2,4-pentanedione catalyzed by an (R)-BINAP-Ru complex produces a 99 1 mixture of almost enantiomer-... 

Benzamido-cinnamic acid, 20, 38, 353 Benzofuran polymerization, 181 Benzoin condensation, 326 Benzomorphans, 37 Benzycinchoninium bromide, 334 Benzycinchoninium chloride, 334, 338 Bifiinctional catalysts, 328 Bifiinctional ketones, enantioselectivity, 66 BINAP allylation, 194 allylic alcohols, 46 axial chirality, 18 complex catalysts, 47 cyclic substrates, 115, 117 double hydrogenation, 72 Heck reaction, 191 hydrogen incorporation, 51 hydrogen shift, 100 hydrogenation, 18, 28, 57, 309 hydrosilylation, 126 inclusion complexes, oxides, 97 ligands, 19, 105 molecular structure, 50, 115 mono- and bis-complexes, 106 NMR spectra, 105 olefin isomerization, 96... 

By contrast, a recent, detailed mechanism of the enantiomeric hydrogenation of a-(acylamino)acrylic esters catalyzed by Ru((S)-binap)(OAc)2 follows that of Scheme 3.3, where both H atoms from the dihydrogen add to the C=C double bond [85]. The high enantioselectivity of the process is produced, in part, by the chelation of the alkene substrate via the C=C double bond and by a carbonyl oxygen of the substrate [86]. 

Enantioselective hydrogenation of unsaturated alcohols such as allylic and homoallylic alcohols was not very efficient until the discovery of the BINAP-Ru catalyst. With Ru(BINAP)(OAc)2 as the catalyst, geraniol and nerol are successfully hydrogenated to give (S)- or (R)-citronellol in near-quantitative yield and with 96-99% ee [3 c]. A substratexatalyst ratio (SCR) of up to 48 500 can be applied, and the other double bond at the C6 and C7 positions of the substrate is not reduced. A high hydrogen pressure is required to obtain high enantioselec-... 

A range of other terminal alkenes has been hydrogenated with ruthenium-diphosphine catalysts. The first set of substrates (Fig. 30.7 Table 30.5) was hydrogenated with Ru-BINAP in dichloromethane (DCM) at 30°C. Products of double bond migration were also detected [5]. 

Takaya and co-workers46 found that BINAP-based Ru(II) dicarboxylate complexes 31 can serve as efficient catalyst precursors for enantioselective hydrogenation of geraniol (2E)-32 and nerol (2Z)-32. (R)- or (iS )-citroncllal 33 is obtained in nearly quantitative yield with 96-99% ee. The nonallylic double bonds in geraniol and nerol were intact. Neither double bond migration nor (fi)-/(Z)-isomerization occurred during the catalytic process. Furthermore, the S/C ratio was extremely high, and the catalyst could easily be recovered (Scheme 6-18). This process can be applied to the asymmetric synthesis of a key intermediate for vitamin E. 

Many methods have been reported for the enantioselective synthesis of the remaining PG building block, the (J )-4-hydroxy-cyclopent-2-enone. For example, the racemate can be kinetically resolved as shown in Scheme 7-28. (iS )-BINAP-Ru(II) dicarboxylate complex 93 is an excellent catalyst for the enantioselective kinetic resolution of the racemic hydroxy enone (an allylic alcohol). By controlling the reaction conditions, the C C double bond in one enantiomer, the (S )-isomer, will be prone to hydrogenation, leaving the slow reacting enantiomer intact and thus accomplishing the kinetic resolution.20... 

Ru(II)-BINAP complexes (1) can effect enantioselective hydrogenation of pro-chiral ally lie and homoallylic alcohols, without hydrogenation of other double bonds in the same substrate.1 The alcohols geraniol (2) and nerol (3) can be reduced to either (R)- or (S)-citronellol (4) by choice of either (R)- or (S)-l. Thus the stereochemical outcome depends on the geometry of the double bond and the chirality... 

BINAP has been extensively used for the asymmetric hydrogenation, transfer hydrogenation and isomerisation of double bonds using both ruthenium and rhodium complexes. 

A further example of ion-exchange of an organometallic complex onto a layered support has been provided by the anion exchange of a sulfonated ruthenium BINAP complex onto the external surface of layered double hydroxides [119]. Although achvihes and enantioselechvities for the hydrogenation of dimethyl itaconate were comparable to the homogeneous catalyst, and catalyst deactivation was not detected, with geraniol as substrate, catalyst deactivation was unavoidable. 

Reaction Conditions versus Selectivity. [Rh(binap)(CH30H)2]C104 is an excellent chiral catalyst for asymmetric hydrogenation (13, 16). Scheme 5 relates the double bond geometry of the starting materials, the configuration of the BINAP ligand, and the stereochemistry of the products. The optical yield and the sense of asymmetric induction are... 

SCHEME 57. Double stereodifferentiation in BINAP-Ru-catalyzed hydrogenation of 2,4-pentanedione. 

In contrast to the high enantioselectivities obtained for the Z substrates, hydrogenation of the E isomers usually proceeds very slowly and in a poor optical yield partly due to the E/Z double-bond isomerization. The enantioselectivity in the BINAP-Rh-catalyzed hydrogenation of E enamides is enhanced in an aprotic solvent THF to minimize the isomerization [15,56],... 

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