Asymmetric crossover

Asymmetric crossover Because the monomer can be of variable length, there is no point in choosing the same crossover point for the two parents. The only requirement is that the crossover operator must leave realizable offspring, so a crossover point in the backbone is always chosen. 

A tandem 1,4-addition-Meerwein-Ponndorf-Verley (MPV) reduction allows the reduction of a, /i-unsaturated ketones with excellent ee and in good yield using a camphor-based thiol as reductant.274 The 1,4-addition is reversible and the high ee stems from the subsequent 1,7-hydride shift the overall process is thus one of dynamic kinetic resolution. A crossover experiment demonstrated that the shift is intramolecular. Subsequent reductive desulfurization yielded fiilly saturated compounds in an impressive overall asymmetric reductive technique with apparently wide general applicability. 

A possible rationale for the asymmetric a-methylation of 40 involves the participation of mixed aggregate E in which the undeprotonated starting material functions as a chiral ligand of the potassium cation of the achiral enolate. To test the feasibility of E, a crossover experiment between 40 and 44 was conducted (Scheme 3.12). A 1 1 mixture of racemic 40 and (5)-44 (>99% ee) was treated with KHMDS (1.1 equiv of the total amount of 40 and 44) followed by methyl iodide according to the protocol in Table 3.5, to give racemic 41 (79% yield) and (5)-45 (74% ee, 79% yield). This means that transfer of chirality between substrates did not occur. Thus mixed aggregate E is not responsible for the present asymmetric induction. 

It follows from this that Bernoulli and Markov mechanisms differ in whether the transition probabilities of the crossover, or hetero, steps are the same as or different from the homo steps (see Table 15-6). In addition, both types of mechanism can be subclassified as to whether the transition probabilities for the homo linkages are symmetric or asymmetric. In copolymerization, a symmetric Bernoulli mechanism with constitutionally different monomers is called azeotropic copolymerization with configurationally different monomers, it is called random flight polymerization and in stereocontrolled polymerization with nonchiral monomers, it is also called ideal atactic polymerization. ... 

In another investigation, a series of asymmetric 3-arm PEP stars was used. [305] The samples were obtained by hydrogenation or deuteration of the corresponding asymmetric poly (isoprene) stars to smdy the crossover from linear to branched... 

In this expression T, are and are universal functions with Pq and Pi representing an asymmetric and a first correction-to-scaling function, respectively, while A, are universal exponents with A2 = 2Ai and A3 = A4 = y-fjS — 1. In eqs 10.97 and 10.99 R q) plays the role of a crossover function defined... 

Scheme 1.46 A revised catalytic cycle for the asymmetric transfer hydrogenation of aromatic ketones in propan-2-ol by the Noyori-Ikariya (pre)catalyst 2 demonstrates crossover of the reaction pathways the product is obtained via a H"/H+ outer-sphere hydrogenation mechanism and/or step-wise metal-ligand bifunctional mechanism (see text). Formation of the major enantiomeric product is shown. (Adapted from Dub, P. A. et al., /. Am. Chem. Soc., 135, 2604-2619. Copyright 2013 American Chemical Society.)...

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