Reagents, unstable, reactions nucleophilic

Derivatives of azete are only known as unstable reaction intermediates. Oxetane and azetidine are considerably less reactive than their three-membered connterparts (oxetane reacts with hydroxide anion 10 times more slowly than does oxirane), but nonetheless do undergo similar ring-opening reactions, for example oxetane reacts with organolithium reagents in the presence of boron triflnoride, or with cuprates, and azetidine is opened on heating with concentrated hydrochloric acid. Azetidininm ions react mnch more easily with nucleophiles. ... 

The reaction of an aldehyde or a ketone with a Grignard reagent is a nucleophilic addition reaction—the nucleophile adds to the carbonyl carbon. The tetrahedral alkoxide ion is stable because it does not have a group that can be eliminated. (Recall that a tetrahedral compound is unstable otdy if the sp carbon is attached to an oxygen and to another electronegative atona.)... 

The reaction of nucleophiles with 1,3-dioxanes containing three double bonds is mainly confined to the group of 5-alkylidene-l,3-dioxane-4,6-diones. The parent compound, 5-methylene-l,3-dioxane-4,6-dione, is, however, quite unstable. Two reagents 170 and 171 have been developed to prepare 5-methylene-l,3-dioxane-4,6-dione in situ (Figure 8) <19968215, 2002SC2009>. 

In principle, pyrylium salts can be transformed to pyrans by the addiiion of an appropriate reagent to the 2-, 4-, or 6-position of the heterocycle. Although most transformations seem to be nucleophilic additions (because of the electrophilic character of the pyrylium substrates) some reactions, such as hydrogenation or one-electron reductions, are of a radical nature. Many nucleophilic additions to pyrylium salts proceeding via unstable pyran intermediates seem to be of general synthetic interest, especially in the fields of aromatic and other conjugated systems, l97 200 but are of little interest in pyran chemistry. 

The oxidation of phenols with hypervalent iodine compounds has been used frequently and nucleophilic additions can be performed as well as cyclization reactions using this technique. The resulting quinone derivatives show high reactivity and they have been used in a various subsequent reactions. Substituted phenols like 32 [78] or 34 [79] have been oxidized by hypervalent iodine reagents and, depending on the substitution pattern, cyclizations have taken place as shown in Scheme 16. Product 33 is unstable and undergoes subsequent... 

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