Catalytic cycle of asymmetric

Figure 9 The proposed catalytic cycle of asymmetric aminohydroxylation.

Fig. 4.104 Catalytic cycle of asymmetric epoxidation via chiral dioxiranes.

Scheme 1.29 Intermediates involved in the catalytic cycle of asymmetric hydrogenation. (Gridnev, I. D. and Imamoto, T., Chem. Commun., 7447-7464, 2009. Reproduced by permission of The Royal Society of Chemistry.)...

Similar analysis was made for the catalytic cycle of asymmetric hydrogenation of 4 with the Rh-TangPhos catalyst (Figure 1.10). In the case of formation of the chelate cycle in the less hindered quadrant, the... 

Horeau principle provides the mathematical foundation for rationahzing the enan-tioenrichment observed along successive catalytic cycles of asymmetric MCRs [7]. The simple calculations shown in Figure 42.1 reveal that this strategy can provide the major diastereomer of a chiral product that has two stereogenic centers with exquisite levels of enantiocontrol. This is despite combining two consecutive catalytic processes that might be only moderately selective (e.g., 80% ee + 80% ee = 97.5% ee). 

The subsequent steps in the catalytic cycle of asymmetric hydrogenation reaction are similar to that of nonchiral hydrogenation. Structures 5.2 and 5.3 have a diastereomeric relationship because the chirality of the phosphine is the same in 5.2 and 53, but the alkene parts of the two structures are mirror images of each other. 

Scheme 5-14 Stoichiometric reactions of Pt(Me-Duphos) complexes relevant to the proposed catalytic cycle for asymmetric hydrophosphination...

Figure 3.16. Catalytic cycle of Rh/(5)-binap-catalyzed asymmetric 1,4-addition of orga-noboronic acids to a,P-enones.

Figure 3.53. Catalytic cycle of Ni/61-catalyzed asymmetric ring-opening of 60 with Grignard reagents.

Hayashi and co-workers established the catalytic cycle of the asymmetric conjugate addition in 2002 [16]. An example is outlined in Scheme 3.4 for the reaction of phenylboronic acid 2m with 2-cyclohexenone la. The reaction has three main intermediates hydroxo-rhodium (A), phenylrhodium (B), and oxa- j-allylrhodium (C) complexes. They are related in the catalytic cycle by (1) transmetallation of a phenyl group from boron to hydroxo-... 

Oxometalloporphyrins were taken as models of intermediates in the catalytic cycle of cytochrome P-450 and peroxidases. The oxygen transfer from iodosyl aromatics to sulfides with metalloporphyrins Fe(III) or Mn(III) as catalysts is very clean, giving sulfoxides, The first examples of asymmetric oxidation of sulfides to sulfoxides with significant enantioselectivity were published in 1990 by Naruta et al, who used chiral twin coronet iron porphyrin 27 as the catalyst (Figure 6C.2) [79], This C2 symmetric complex efficiently catalyzed the oxidation... 

Figure 8D.5. Proposed Catalytic Cycle of the Asymmetric Michael Reaction Promoted by LSB.

Aggarwal et al. have proposed a catalytic cycle for asymmetric epoxidation of olefins by chiral amines (Scheme 7.13), which involves the initial formation of ammonium... 

Figure 36. Catalytic cycle of direct catalytic asymmetric aldol reactions.

Figure 9.7 Catalytic cycle for asymmetric epoxidation of allyl alcohol with 9.35 as the precatalyst. The precatalyst is generated in situ and undergoes conversion to 9.36 in the presence of allyl alcohol and r-butyl hydroperoxide. S is a solvent molecule. Conversion of 9.36 to 9.37 involves more than one step. This is not shown for clarity (see Problem 10).

A hypothetical catalytic cycle for asymmetric hydroformylation reaction is shown in Fig. 9.13. The precatalyst Rh(acac)(P-P) reacts with H2 and CO to give the square planar catalytic intermediate 9.47. Alkene addition to 9.47 can lead to the formation of 9.48, 9.49, and 9.50. The steric requirements of the chelating ligand would have to be such that the formation of 9.50 is avoided. This is because alkene insertion into the Rh-H bond in this case would lead to the formation of the linear rather than the branched alkyl. Both 9.48 and 9.49, which differ in the coordination positions of the phosphorus atoms, can give 9.51, which has the desired branched alkyl ligand. 

Figure 9.15 Catalytic cycle for asymmetric nitroaldol condensation reaction with 9.12 as the chiral catalyst. The La-O bond marked by an arrow opens up due to protonation of the O atom by nitromethane.

Fig. 4.98 The catalytic cycle of the Sharpless catalytic asymmetric epoxidation.

Scheme 16. Catalytic cycle of the direct heterobimetallic asymmetric aldol reaction.

The Morita-Baylis-Hillman (MBH) reaction is an important 100% atom economic transformation that allows the formation in one step of a flexible allylic alcohol motif. Efforts in this field have been directed recently to the solution of two problems to enhance the generally sluggish reaction rate and to achieve asymmetric catalytic versions. Scheme 1.15 gives the catalytic cycle of the MBH reaction. The catalyst is a highly nucleophilic tertiary amine, generally DABCO, or a tertiary phosphine, which adds to the oc,P-unsaturated electrophile in a 1,4 fashion to deliver an enolate that, in turn, adds to the aldehyde. A critical step is the proton transfer from the enolizable position to the oxygen atom this process is catalysed by an alcohol that plays the role of a proton shuttle between the two positions. Water has also been reported to strongly speed up the reaction at a well-defined concentration. Moreover, the... 

Fig. 7. The full catalytic cycle for asymmetric hydrogenation of enamides with rhodium DI-PAMP complexes...

Asymmetric catalysis is one of the most economical processes for the production of chiral compounds, considering the high turnover levels of most homogeneous catalysts and the fact that the optically active catalyst introduces its chiral information during each new catalytic cycle. The asymmetric catalyst molecules are mainly synthesized by coordination of optically active ligands to a metal rather than resolution of complexes in which the optical activity lies at the metal, and which are prone to racemization. These chiral complexes involve only a few metals. 

Scheme 12. Catalytic Cycle of Direct Catalytic Asymmetric Aldol Reactions.

A detailed NMR study of the catalytic cycle of the Rh-catalyzed 1,4-conjugate addition was performed by Hayashi and co-workers. An important finding was that the acetylacetonato (acac) ligand used as a catalyst precursor in the original asymmetric transformation reported by Takaya et retarded the... 

It is well known that certain diamines increase the reactivity of organolithium compounds by complexation to the Li atom. Consequently, with the appropriate chiral diamines it should be possible to perform catalytic asymmetric deprotonations. In the original report of Evans and co-workers it is mentioned that the enantioselection can be maintained with only 0.7 equivalents of ( )-sparteine, with no further details. These ideas have been recently exploited for t-butyldimethylphosphine borane and the analogous sulfide with sparteine and its surrogates. It can be understood with the catalytic cycle of Scheme 5.54, reported by O Brien and co-workers. 

The examples of direct observation of solvate dihydrides together with the computational data collected in the Table 1.1 illustrate the fact that these species are kinetically competent intermediates in the catalytic cycle of Rh-catalyzed asymmetric hydrogenation. 

Figure 1.7 Schematic phophile of potential energy for the catalytic cycle of the Rh-catalyzed asymmetric hydrogenation. For explanation see text and Scheme 1.29.

Hayashi, T. Takahashi, M. Takaya, Y. Qgasawara, M. Catalytic Cycle of Rhodium-Catalyzed Asymmetric 1,4-Addition of Organoboronic Acids. Arylrhodium, Oxa-Jt-allylrhodimn, and Hydroxorhodimn Intermediates. /. Am. Chem. Soc. 2002,124,5052-5058. 

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