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Chemoselective SN reaction

Chemoselective SN reactions of nucleophiles with carboxylic acid derivatives are guaranteed to take place without the risk of an overreaction when the substitution mechanism of Figure 6.4 applies. This is because as long as the nucleophile is present, only one reaction step is possible the formation of the negatively charged tetrahedral intermediate. Figure 6.40 summarizes this addition in the top line as Reaction 1 (— B). [Pg.309]

Chemoselective SN reactions at the carboxyl carbon in which hydride donors function as nucleophiles can be carried out using strategy 1 or strategy 2 from Figure 6.41. [Pg.311]

The Weinreb amide syntheses in Figure 6.50 proceeding via the stable tetrahedral intermediates B and F are chemoselective SN reactions at the carboxyl carbon atom of carbon acid derivatives that are based on strategy 1 of the chemistry of carboxylic acid derivatives outlined in Figure 6.41. Strategy 2 of the chemistry of carboxylic acid derivatives in Figure 6.41 also has a counterpart in carbon acid derivatives, as is demonstrated by a chemoselective acylation of an organolithium compound with chloroformic acid methyl ester in this chapter s final example ... [Pg.318]

Following strategy 2 from Figure 6.32, chemoselective SN reactions of hydride-donors with carboxylic acid derivatives also succeed starting from carboxylic chlorides. For the reasons mentioned further above, weakly nucleophilic hydride donors are used for this purpose preferentially and should be added dropwise to the acylating agent in order to achieve success ... [Pg.265]

Fig. 6.42. Preparation of Weinreb amides through SN reactions at the carboxyl carbon. Chemoselective reduction of Weinreb amides to aldehydes. Fig. 6.42. Preparation of Weinreb amides through SN reactions at the carboxyl carbon. Chemoselective reduction of Weinreb amides to aldehydes.
The SN reaction under consideration is not terminated until water, a dilute acid, or a dilute base is added to the crude reaction mixture. The tetrahedral intermediate B is then protonated to give the compound E. Through an El elimination it liberates the carbonyl compound C (cf. discussion of Figure 6.4). Fortunately, at this point in time no overreaction of this aldehyde with the nucleophile can take place because the nucleophile has been destroyed during the aqueous workup by protonation or hydrolysis. In Figure 6.32 this process for chemoselective acylation of hydride donors, organometallic compounds, and heteroatom-stabilized carbanions has been included as strategy 1. ... [Pg.263]

The chemoselectivity of the reaction depends on the Sn/Rh ratio (Table 2 and Figure 3). [Pg.141]

Another very interesting example from the same group is the use of Sn-Beta for the synthesis of melonal (2,6-dimethyl-5-hepten-l-al), a fragrance that is produced industrially by a Darzens reaction from 6-methyl-5-hepten-2-one, with ethyl chlor-oacetate as reagent. A novel halogen-free synthesis involves the chemoselective... [Pg.173]

In summary, the Sn-beta catalyst has been shown to be highly selective and active for a large number of catalytic reactions, for example BV oxidation, glucose isomerization, and Meerwein—Ponndorfr-Verley reactions. It is believed that the extraordinarily high chemoselectivity that is observed in some of these reactions is the result of a single type of catalytic site in Sn-beta. [Pg.61]


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