Asymmetric possible mechanisms

Scheme 4. Possible mechanism of catalytic asymmetric nitroaldol reaction.

Scheme 6. Possible mechanism of direct catalytic asymmetric aldol reaction.

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

The system Ru(PPh3)(H20)2(SB )/Phl0/CH3Cy4°C asymmetrically epoxidised styrene [917]. Related complexes made from L-histidine with salicylaldehyde, 5-chloro and 5-methoxysalicylaldehyde as Ru(PPh3)(H30)3(SB )/(PhIO/CH2Cl2 epoxidised non-functionalised styrenes at unspecified low temperatures in the dark. Possible mechanisms were discussed [918]. 

This asymmetric photosynthesis might be explained by three possible mechanisms ... 

When (2S)-1-(1-cyclohexene-l-yl)-2-(methoxymethyl)pyrrolidine (206), enamine from cyclohexanone, and (S)-proline-derived (2S)-(methoxymethyl)pyrrolidine is added to the Knoevenagel condensation products (207), mainly one of the possible four diastereomers is formed. The diastereomeric purity was found to be excellent (d.s. > 90%) 203). The stereochemical course of this highly effective asymmetric synthesis allowed the synthesis of the optically active target molecules (208). A possible mechanism discussed by Blarer and Seebach 203). 

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

The silatropic ene pathway, that is, direct silyl transfer from an silyl enol ether to an aldehyde, may be involved as a possible mechanism in the Mukaiyama aldol-type reaction. Indeed, ab initio calculations show that the silatropic ene pathway involving the cyclic (boat and chair) transition states for the BH3-promoted aldol reaction of the trihydrosilyl enol ether derived from acetaldehyde with formaldehyde is favored [60], Recently, we have reported the possible intervention of a silatropic ene pathway in the catalytic asymmetric aldol-type reaction of silyl enol ethers of thioesters [61 ]. Chlorine- and amine-containing products thus obtained are useful intermediates for the synthesis of carnitine and GABOB (Scheme 8C.26) [62],... 

After these results, the same authors found that asymmetric acetylenes gave this type of reaction with very high regioselectivity (Scheme 13) (71JOC3755). Two other possible mechanisms were proposed. 

Possible Mechanism for Asymmetric Induction in the Step Corresponding... 

Figure 24. A possible mechanism for catalytic asymmetric nitroaldol reactions.

Scheme 6.2.4 summarises the possible mechanisms by which (-)-sparteine induces asymmetry into organolithium reactions, highlighting organolithiums whose reactions typify of each type of asymmetric induction. 

Since longitudinally spin polarized electrons (SPEs) [6] are truly chiral particles it is reasonable to expect that they could induce asymmetric reactions in chiral molecules analogous to CPL. This chapter is devoted to reviewing this area with emphasis on recent advances in the field. In the next section we will discuss natural occurring sources of SPEs followed by a review of experiments aimed at discovering their role in chiral-selective chemistry. The following section will address possible mechanisms and we will conclude with a discussion of future research in this area. 

In the early twentieth century Leuchs reported a surprising example of the a-chlorination of chiral ketone 73, which gave optically active 74 in the absence of additional chiral sources.36 From a mechanistic point of view, however, there remains some ambiguity. Possible mechanisms for the formation of optically active 74 include (1) asymmetric chlorination via an enol intermediate (i.e., memory of chirality), (2) direct electrophilic chlorination of the C-H bond at the stereogenic carbon center, (3) complex formation of an achiral enol intermediate with optically active 73, (4) resolution of dl-74 by co-crystallization with optically active 73, and (5) simultaneous resolution of dl-74. 

Matsumura and co-workers reported a memory effect of chirality in the electrochemical oxidation of 95 to give 96, although the enantioselectivity was modest (Scheme 3.25). The reaction is assumed to proceed via carbenium ion intermediate Q.46 The mechanism for asymmetric induction is not clear. A possible mechanism involves chiral acid (95)-mediated deracemization of racemic 96 produced by the electrochemical oxidation of 95. However, this suggestion may be eliminated based on the finding that treatment of racemic 96 with 95 in methanol containing 5% formic acid did not produce optically active 96. 

Catalytic asymmetric nitroaldol reactions promoted by LLB or its derivatives require at least 3.3 mol % asymmetric catalyst for efficient conversion, and even then the reactions are rather slow. To enhance the activity of the catalyst, consideration of the possible mechanism of catalytic asymmetric nitroaldol reactions is clearly a necessary prerequisite to formulation of an effective strategy. One possible mechanism of catalytic asymmetric nitroaldol reactions is shown at the top of Sch. 10. We strove to detect the postulated intermediate I by use of a variety of methods, but were unsuccessful, probably owing to the low concentrations of the intermediate, which we thought might be ascribed to the presence of an acidic OH group in close proximity. 

Scheme 10.20 The possible mechanism of Ir catalyzed asymmetric hydrogenation of 2,3...

Asymmetric protonation of a metal enolate basically proceeds catalytically if a coexisting achiral acid A-H reacts with the deprotonated chiral acid A -M faster than with the metal enolate, a concept first described by Fehr et al. [44]. A hypothesis for the catalytic cycle is illustrated in Scheme 2. Reaction of the metal enolate with the chiral acid A -H produces (R)- or (S)-ketone and the deprotonated chiral acid A -M. The chiral acid A -H is then reproduced by proton transfer from the achiral acid A-H to A -M. Higher reactivity of A -M toward A-H than that of the metal enolate makes the catalytic cycle possible. When the achiral acid A-H protonates the enolate rapidly at low temperature, selective deprotonation of one enantiomer of the resulting ketone by the metallated chiral acid A -M is seen as an alternative possible mechanism. 

Later, the same group showed that an asymmetric protonation of preformed lithium enolate was possible by a catalytic amount of chiral proton source 23 and stoichiometric amount of an achiral proton source [45]. For instance, when hthium enolate 44, generated from ketene 41 and -BuLi, was treated with 0.2 equiv of 23 followed by slow addition of 0.85 equiv of phenylpropanone, (S)-enriched ketone 45 was obtained with 94% ee (Scheme 4). In this reaction, various achiral proton sources including thiophenol, 2,6-di-ferf-butyl-4-methylphenol, H2O, and pivalic acid were used to provide enantioselectivity higher than 90% ee. The value of the achiral acid must be smaller than that of 45 to accomplish a high level of asymmetric induction. The catalytic cycle shown in Scheme 2 is the possible mechanism of this reaction. 

To discuss in more detail the possible peculiarities of photovoltaics in an asymmetrical D-A multilayer structure, we will make a few remarks concerning possible mechanisms of photo-voltaic effect in multilayered structures containing organic dyes (see (34) and also a recent issue of Chemical Reviews devoted to organic electronics and optoelectronics (35)). 

Blackmond, Donna G. Asymmetric Autocatalysis and its Implications for the Origin of Homochirality. Proceedings of the National Academy of Sciences 101 (2004) 5,732-36. This review describes the kinetics of autocatalysis and possible mechanisms for spontaneous resolution of mixtures having a miniscule excess of one enantiomer. 

Scheme 2.13 Asymmetric CDC reactions of THIQs with ybutyrolactams and a possible mechanism reported by Wang.

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