Asymmetric transfer hydrogenations dimer

Hydrogenation of dienes with up to 20 1.0 diastereoselectivity and 99% ee is mediated by carbene complexes. The scope and limitations of these reactions were investigated.288 Asymmetric transfer hydrogenation to prochiral ketones, catalysed by a Ru(II) complex (10) or its dimer, with formic acid-triethylamine has been reported, (0 The protocol leads to high yields and enantioselectivity up to 96%. It has been suggested that 16-electron Ru(II) and the Ru-H intermediates are involved in this reaction.289... 

The Noyori asymmetric transfer hydrogenation was utilized in the synthesis of the chiral 1,2,3,4-tetrahydroisoquinolines by R.A. Sheldon et al. These compounds are important intermediates in the Rice and Beyerman routes to morphine. The "Rice imine" was exposed to a series of chiral Ru " complexes, which was prepared from r -arene-Ru " chloride dimeric complexes and A/-sulfonated 1,2-diphenylethylenediamines along with the azeotropic mixture of HCOOH/NEts. With the best catalyst the desired tetrahydroisoquinoline was isolated in 73% yield and the enantiomeric excess was 99%. 

The mode of asymmetric induction can be rationalized from the mechanism of the photopinacolization in the presence of aliphatic amines. The electron transfer from the amine to the excited triplet ketone furnishes charge transfer complex 5, from which a radical pair is formed by protoirtransfer. The weakly coordinated chiral amine seems to favor the dimerization of radical 6 from the si face leading to the (/ , ft)-enantiomer 3. The much lower selectivities observed with methanol as the cosolvent (3% ee at 27°C) indicate dipolar or hydrogen bonding interactions between the chiral diamine and the prochiral radical (Scheme 4). 

Homer and Klaus examined the same photopinacolization reaction in the presence of chiral lactates [16]. Radical 6 is formed by hydrogen transfer from the lactic acid hydroxy group to the triplet excited acetophenone. At room temperature, 4% ee were observed in the presence of an equimolar amount of /-menthyl /-lactate. With /-menthyl dj-lactate no asymmetric induction was obtained. This shows that the stereocenter of the lactic acid influences the enantiodifferentiating dimerization step. 

Hence, the double-proton transfer was assumed to proceed via a concerted mechanism, with the one-dimensional kinetic-energy matrix appropriate to one particle of twice the proton mass. A similar model is widely used to analyze the vibrations of cyclic hydrogen-bonded dimers (for example, see Ref. [59]). The O -O coordinate in this case symbolizes the intermolecular stretching of a doubly H-bonded system. The problem of the double-proton transfer for the benzoic acid dimer is thereby reduced to a problem conceptually similar to that for a single O -H -O entity except for the denoted light atom mass and consequences of the asymmetric 2D PES. 

The other dimers in Fig. 29.7 show asymmetry. If the proton transfer potential is asymmetric, there will be no synchronous transfer and at most accidental level splittings. In the formamide dimer [44] the two hydrogen bonds are the same but the donor and acceptor groups are different. The available calculations indicate that the structure of the equilibrium configuration is intermediate between the equilibrium configurations of dimeric formic acid and dimeric formamidine, as... 

Based on PES calculations on the isolated complexes, the existence of further cyclic ion pairs has been suggested, among them for ammonium salts of zeolite-clusters or ammonium tetrafluoroborate (Ref.46, 3-21G and 6-3IG ab initio basis sets). In these cases, an asymmetric hydrogen bonded form did not occur as a second minimum on the PES. Surprisingly, the proton transfer in the eight-membered formamidine dimer is supposed to pass a transition structure of lower symmetry to form an ion-pair like intermediate... 

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