Nucleophilicity acids

Ring expansion of tetrahydrofurans to dihydropyrans results when their 2-W-aziiidinyl imines are heated <96CC909> and when their 2-ca-alkyl bromides are treated with Ag20 in a nucleophilic acidic solvent <96JCS(P1)413>. Alkyl carbenes and bicyclic oxonium ion intermediates are invoked, respectively. 

In general, 3-hydroxy-l,2,4-thiadiazoles react with hard nucleophiles (acid chlorides, sulfonyl chlorides) at the oxygen atom, whereas soft nucleophiles (isocyanates, acid anhydrides) react at the N-2 position yielding 1,2,4-thiadiazolin-3-ones. Nucleophiles react at the N-4 position of 5-hydroxy-l,2,4-thiadiazoles <1996CHEC-II(4)307>. There have been no new publications on O-linked substituents since the publication of CHEC-II(1996). 

Classification of Reagents as Electrophiles and Nucleophiles. Acids and Bases... 

CLASSIFICATION OF REAGENTS AS ELECTROPHILES AND NUCLEOPHILES. ACIDS AND BASES... 

As regards their chemical properties, halodisilanes are (as expected) to be very reactive thermally labile nucleophiles. Acid hydrolysis occurs to give polymeric compounds (see p. 92) retaining Si-Si bonds alkaline hydrolysis yields silicic acid with disruption of the Si-Si bonding. Partial hydrolysis carried out with Si2Cl6 led to S OCljo, Si602Cl4 and Si803Cli8 (see p. 93). 

Alkylation or acylation of a ketone or aldehyde using its enamine derivative as the nucleophile. Acidic hydrolysis regenerates the alkylated or acylated ketone or aldehyde, (p. 1053)... 

Table 3-1 Some hard and soft acids (electrophiles) and bases (nucleophiles) Bases (Nucleophiles) Acids (Electrophiles)...

Excited alkenes and alkynes are highly reactive towards nucleophiles, acids and electron donors. Nucleophilic addition and photoreduction (entries 5 and 8) predominate with alkenes carrying electron-withdrawing substituents. Some electron-rich alkenes or alkynes readily undergo photoprotonation (entries 6 and 7). 

The attacking nucleophile can be negatively charged (Nu ) or neutral (Nu). With neutral nucleophiles, acid catalysis is common. 

Ease of Use and User Friendliness. Once a chemical stmcture has been entered, the user must select a specific mechanistic module (i.e., Carbenoid, Basic/Nucleophilic, Acidic/Electrophilic, Electrophilic Aromatic, Radical, Heterocyclic, Oxidative/Reductive, or Pericyclic) for evaluation of the reactants and define the reaction conditions, reagent, and solvent. Then the user instructs CAMEO to run the reaction. Within approximately 10 seconds, the results are displayed on the screen. 

Ease of Use and User Friendliness. CAMEO was found to be easy to use. CAMEO S menu screens are well designed and easy to follow, and greatly facilitate use and operation of the program, particularly structure entry. To explore fully all potential reactions and the products of these reactions, however, the user must separately evaluate a given set of starting materials and reaction conditions under several if not all of the available reaction modules (i.e., Carbenoid, Radical, Heterocyclic, Basic/Nucleophilic, Acidic/Electrophilic, Electrophilic Aromatic, Oxidative/Reductive, and Pericyclic). Thus, the user can enter reactants and reaction conditions, and, depending upon which module is selected, CAMEO may predict different results. For example, CAMEO correctly predicted carbaryl as the product from the reaction of methyl isocyanate with 1-naphthol only if the Acidic/Electrophilic mechanistic module was selected no product was predicted when the Basic/Nucleophilic module was selected. 1-Naphthol is clearly the nucleophile in this reaction, and it seems that CAMEO should have recognized it as such. 

Stork reaction Alkylation or acylation of a ketone or aldehyde using its enamine derivative as the nucleophile. Acidic hydrolysis regenerates the alkylated or acylated ketone or aldehyde, (p. 1055) tautomerism An isomerism involving the migration of a proton and the corresponding movement of a double bond. An example is the keto-enol tautomerism of a ketone or aldehyde with its enol form. (p. KM2)... 

Dehydration (loss of H2O) of an alcohol is promoted by treatment with a strong, non-nucleophilic acid (such as H2SO4 or H3PO4) and heat. Alcohol dehydration involves an El mechanism with a carbocation intermediate. Since carbocations can rearrange, the double bond can end up anywhere on the carbon chain, and the most stable, most highly substituted alkene is expected as the major product (follows Zaitsev s rule, with possible rearrangement of the carbon skeleton). The planar carbocation intermediate also results in a loss of stereochemistry so the more stable stereoisomer will be produced as the major product (trans or E isomer). The dehydration reaction is most useful when only a single product is possible otherwise, a mixture of alkene products is likely to be obtained. 

Based on the premise established for click reactions, the interaction of thiols with reactive C—C double bonds, or simply enes, is indicated as another method able to be implemented and exploited for synthetic and materials development. This reaction, already well known in the early 1900s, involves the hydrothiolation of the C—C double bond and presents some advantages that make it particularly attractive for researchers it is a facile and versatile process. It can proceed under a variety of conditions, including a radical pathway via catalytic processes mediated by nucleophiles, acids. 

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