Zimmerman-Traxler model Ivanov reaction

The application of the Zimmerman-Traxler model to the specific case of the Ivanov reaction of Figure 13.45 is illustrated in Figure 13.46. The reaction proceeds preferentially through... 

Fig. 13.46. Explanation of the anti-selectivity of the Ivanov reaction of Figure 13.45 by means of the Zimmerman-Traxler model. The stereodescriptors Re and Si are defined as follows. Suppose you are looking down on the plane of an alkene, in which an sp2-hybridized C atom is connected to three different substituents. You are on the Re side of the double bond if the Cahn-Ingold-Prelog priorities of these substituents decrease going clockwise, and on the Si side otherwise.

The key idea of the Zimmerman-Traxler model is that aldol additions proceed via six-membered ring transition state structures. In these transition states, the metal (a magnesium cation in the case of the Ivanov reaction) coordinates both to the enolate oxygen and to the O atom of the carbonyl compound. By way of this coordination, the metal ion guides the approach of the electrophilic carbonyl carbon to the nucleophilic enolate carbon. The approach of the carbonyl and enolate carbons occurs in a transition state structure with chair conformation. C—C bond formation is fastest in the transition state with the maximum number of quasi-equatorially oriented and therefore sterically unhindered substituents. 

Fig. 10.41. Explanation of the anh-selectivity of the Ivanov reaction of Figure 10.40 by means of the Zimmerman-Traxler model. The...

A few years ago, Blagonev and Ivanov described the bis-deprotonation of aryl acetic acids by Grignard reagents. These magnesinm dianions, known as Ivanov reagents, react with aldehydes and ketones. Reaction between dianions of phenylacetic acid and benzaldehyde yields the anti S-hydroxy acid as the major diastereomer anti/syn 69/22) (equation 113). This result is in agreement with the formation of a cyclic chair-like transition state according to the model of Zimmerman-Traxler . 

The simple diastereoselectivity of aldol reactions was first studied in detail for the Ivanov reaction (Figure 13.45). The Ivanov reaction consists of the addition of a carboxylate enolate to an aldehyde. In the example of Figure 13.45, the diastereomer of the /1-hydroxycarboxylic acid product that is referred to as the and-diastereomer is formed in a threefold excess in comparison to the. vy/j-diastereoisomer. Zimmerman and Traxler suggested a transition state model to explain this selectivity, and their transition state model now is referred to as the Zimmer-man-Traxler model (Figure 13.46). This model has been applied ever since with good success to explain the simple diastereoselectivities of a great variety of aldol reactions. 

Scheme 5.1 illustrates the transition structure most often invoked to explain the selectivities observed in TC-transfer 1,2-carbonyl additions (cf. Figure 4.1) the so-called Zimmerman-Traxler transition structure [1]. This model, which was originally proposed to rationalize the selectivity of the Ivanov reaction, has its shortcomings (as will be seen) and suffers from an oversimplification when applied to enolates, in that it illustrates a monomeric enolate cf. section 3.1 and ref. [2-4]). Nevertheless, it serves the very useful purpose of providing a simple means to rationalize relative and absolute configurations in almost all of the asymmetric 1,2-additions we will see. The favored transition structure has Ik topicity (Si/Si... 

Scheme 5.1. The Zimmerman-Traxler transition state model for the Ivanov reaction [1],...

In this reaction, the onfr-product is preferentially formed.The Ivanov reaction is known to proceed through the Zimmerman-Traxler transition state model. 

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