The Reactions of Stable Nucleophilic

The Reactions of Stable Nucleophilic Carbenes with Main Group Compounds... 

THE REACTIONS OF STABLE NUCLEOPHILIC CARBENES WITH MAIN GROUP COMPOUNDS... 

The first silicon-organophosphorus betaine with a thiolate center (15a) was synthesized by the reaction of stable silanethione (14) with trimethyl-methylenephosphorane (Scheme 8) and characterized by multinuclear NMR spectroscopy.14 Compound 15a is formed under kinetic control and is transformed, under the thermodynamically controlled conditions, into the silaacenaphthene salt (16). The processes presented in this scheme reflect the competition of the basicity and nucleophilicity of phosphorus ylides. Betaine 15b prepared from less nucleophilic and less basic ylide with phenyl substituents at the phosphorus atom is much less resistant toward retro-decomposition compared to the alkyl analog. Its equilibrium concentration does not exceed 6%. 

The reactions of stable carbocations with water are generally base catalyzed,109,110 and their reactions with hydroxide ion are slower than their reactions with azide ion and sulfhydryl ions. The less favorable reaction of hydroxide ion with carbocations has been attributed to the fact that deprotonation of a water molecule by hydroxide ion is not a thermodynamically favorable reaction, and activation energy to generate a desolvated hydroxide ion is required.110 These factors would also account for the less favorable reaction of hydroxide ion as a nucleophile with 82 to form tetrols instead of as a base to bring about epoxide ring closure. 

The nonstereospecific products are formed in both the reaction of stable thiiranium ions with nucleophiles and the addition of arenesulfenyl chlorides to alkenes. The reaction of the isomeric stable ions (158) and (159) with acetic acid at room temperature gives the same mixture of erythro-(160) and threo-( 161) products (Scheme 59) <75TL245i, 76IZV1318). Nonstereospecific products are also formed by the addition of 2,4-dinitrobenzenesulfenyl chloride to ( )- and (Z)-anisole <72JOC3086> and 4-methoxy-/ -deutereostyrene <84TL4983>. 

Only product (168) of the reaction with chloride ion is reported for the reaction of the stable ion (170) formed by the reaction of the epoxysulfide (169) with HC1 in acetonitrile (Scheme 61) <84IZV2652>. Thus, stable ions, like their counterparts in the addition reaction, react preferentially with the stronger nucleophile. But the reactions of stable ions are not necessarily good models for the intermediates in the addition of arenesulfenyl chlorides to alkenes, because in the addition of arenesulfenyl chlorides the ratio of products depends on the reaction conditions. Thus the reaction of thiiranium ion intermediate competes with that of an open -substituted alkyl carbocation <87CJC1945>. 

There have been few studies on the reaction of stable cations of this class with nucleophiles, but... 

This study reports on the reactions of ambident nucleophiles with electron-deficient nitroaromatic and heteroaromatic substrates anionic complex formation or nucleophilic substitution result. Ambident behavior is observed in the case of phenoxide ion (O versus C attack) and aniline (N versus C attack). O or N attack is generally kinetically preferred, but C attack gives rise to stable thermodynamic control. Normal electrophiles such as 1,3,5-trinitrobenzene or picryl chloride are contrasted with superelectrophiles such as 4,6-dinitrobenzofuroxan or 4,6-dinitro-2-(2,4,6-trinitrophenyl)benzotriazole 1-oxide (PiDNBT), which give rise to exceptionally stable a complexes. Further interesting information was derived from the presence in PiDNBT of two electrophilic centers (C-7 and C-l ) susceptible to attack by the ambident nucleophilic reagent. The superelectrophiles are found to exhibit lesser selectivity toward different nucleophilic centers of ambident nucleophiles compared with normal electrophiles. 

The reaction of a nucleophilic alkyl radical R with benzene affords the a-complex 1, a fairly stable cyclohexadienyl radical, which under oxidizing conditions leads to cation 2 (Scheme 1). Depending on the stability of the attacking radical, the formation of 1 is a reversible process. Deprotonation eventually affords the homolytic aromatic substitution product 3. If the reaction is performed under non-oxidizing conditions, cyclohexadienyl radical 1 can dimerize (—> 4), disproportionate to form cyclohexadiene 5 and the arene 3, or further react by other pathways [3]. 

Why is the reaction of a nucleophile with a phosphoanhydride bond so exergonic In other words, why is the AG° value large and negative A large negative AG° means that the products of the reaction are much more stable than the reactants. Let s look at the reactants and products of the following reaction to see why this is so. 

The cyclic carbamate (oxazoIidin-2-one) 313 is formed by the reaction of phenyl isocyanate (312) with vinyloxirane[I92]. Nitrogen serves as a nucleophile and attacks the carbon vicinal to the oxygen exclusively. The thermodynamically less stable Z-isomer 315 was obtained as a major product (10 I) by the reaction of 2-methoxy-l-naphthyI isocyanate (314) with a vinyloxir-... 

It should be pointed out that the existence of stable structures of the intermediate-complex type (also known as a-complexes or Wheland complexes) is not of itself evidence for their being obligate intermediates in aromatic nucleophilic substitution. The lack of an element effect is suggested, but not established as in benzene derivatives (see Sections I,D,2 and II, D). The activated order of halogen reactivity F > Cl Br I has been observed in quantita-tivei36a,i37 Tables II, VII-XIII) and in many qualitative studies (see Section II, D). The reverse sequence applies to some less-activated compounds such as 3-halopyridines, but not in general.Bimolecular kinetics has been established by Chapman and others (Sections III, A and IV, A) for various reactions. 

Katritzky and co-workers studied the mechanism of this reaction in detail. His work involved a NMR study of 16 reactions of methyl-, phenyl-, 1,2-dimethyl-, and l-methyl-2-phenylhydrazine with /3-keto esters. In many cases starting materials, intermediates, and products were detected simultaneously. Most reactions proceed by nucleophilic addition of the less hindered hydrazine nitrogen atom to the keto carbon of the keto ester. For example, the pathway given in Scheme 3 for the reaction of methyl 3-oxobutanoate 9 with methyl- or phenyUiydrazine 2 (R = Me or Ph) was found to be dominant. The initially formed addition product 10 dehydrates to hydrazone 11, which then isomerizes to hydrazone 12. Intermediate 12 then cyclizes to pyrazol-3-one 13, which tautomerizes to the kinetically more stable pyrazol-3-otie 14 [87JCS(P2)969]. 

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