Nucleophilic attack regiospecificity

When A-tosylaziridines were reacted with amines or thiols in acetonitrile in the presence of 5 mol% DABCO,50 the nucleophile attacked regiospecifically at the least substituted carbon of the aziridine ring. When a benzyl carbon was present in the aziridine ring, both possible products were obtained although the major product was from attack at the benzyl carbon. The catalytic role of DABCO (which may form an ammonium zwitterion which can deprotonate the nucleophile) is under investigation. 

The regioselectivity of nucleophilic attack for the formation of G-G bonds depends on the nature of the nucleophile as well as the substituents attached to the dienyl ligand. Reactions of 1,2-disubstituted 42a and 42b and 1,4-disubstituted 43a and 43b cations with methanol, triphenylphosphine, lithium dimethylcuprate, and sodium dimethylmalonate nucleophiles have been investigated. It was found that cations 42a and 42b direct nucleophilic attack regiospecifically to the unsubstituted C5 position whereas cations 43a and 43b undergo G1 addition, although in a less predictable manner (Scheme 3). Weak nucleophiles such as methanol, allyltrimethylsilane, furan, and allylic organozirconiums are found to attack exclusively at the less substituted pentadienyl terminus and on the less stable, but more reactive, transoid form. Malonate anion attack on 1-substituted acyclic (77 -pentadienyl)-... 

There are two methods for the introduction of a hydroxyalkyl group at position 5 of the pyrazol-3-one ring. Schmidt and Zimmer converted furanediones 258a-k into arylmethylenepyrazol-3-reaction with hydrazine hydrate or methylhydrazine (83Jmechanism proposed for the reaction involves nucleophilic attack of the hydrazine on the ketone carbonyl, followed by attack on the ester carbonyl and ring opening of the... 

The Tsuji-Trost reaction is the palladium-catalyzed allylation of nucleophiles [110-113]. In an application to the formation of an A-glycosidic bond, the reaction of 2,3-unsaturated hexopyranoside 97 and imidazole afforded A-glycopyranoside 99 regiospecifically at the anomeric center with retention of configuration [114], Therefore, the oxidative addition of allylic substrate 97 to Pd(0) forms the rc-allyl complex 98 with inversion of configuration, then nucleophilic attack by imidazole proceeds with a second inversion of configuration to give 99. 

The base-catalyzed condensation of azides with activated methylene compounds is a well-established route to IJT-triazoles. In particular, it is the best route to triazoles bearing a 5-amino or hydroxy substituent and an aryl or carbonyl-containing function in the 4-position. The addition is regiospecific. The reaction is a stepwise one, since anomerism of glycosyl azides has been observed in their reaction with activated methylene compounds, indicating the presence of an intermediate. The mechanism can be envisaged as a nucleophilic attack by the car-banion on the terminal nitrogen of the azide, followed by cyclization to a... 

Fields et al. showed that a variety of unsymmetrical alkenes added regiospecifically to the acridizinium nucleus and pointed out that the great majority of cases could be rationalized by the assumption that the more negatively polarized end of the alkene was preferentially attracted toward position 6, the previously demonstrated center for nucleophilic attack on the acridizinium ring. At the same time they reported that the addition of acrylonitrile to yield a 12- rather than a 13-cyano adduct is the reverse of what would be expected from the polarization of the acrylonitrile molecule. Possible explanations for this exception are offered in Section V. 

As conjugated systems with alternating 7T-charges, the polymethine dyes are comparatively highly reactive compounds (3). Substitution rather than addition occurs to the equalized 7T-bond. If the nucleophilic and electrophilic reactions are charge-controlled, reactants can attack regiospecifically. 

When styrene oxide was reacted in alcohols in the presence of an Al(OTf)3 catalyst, the nucleophile attacked at the benzyl carbon.28 The effect of catalyst loading and the effect of changing the structure of the reacting alcohols was investigated. The reactions of oxiranes with alkyl substituents were not regiospecific. More catalyst was needed when bulkier alcohols were used. 

With these considerations in mind it was decided to investigate phosphinic-carboxylic mixed anhydrides in peptide methodology. .Mechanistic consideration of the reactants (2) and (3) and products shown above would suggest regiospecific nucleophilic attack by the amine component due to the formation of an amide bond with concomitant generation of a new P-0 bond.As in the study of phosophinamidates discussed above,a series of phosphinic acids were selected for preparation of the mixed anhydrides (2) because of the intimate steric and electronic... 

Indium(m) triflate is an efficient catalyst for the intramolecular cyclisation of y,8-unsaturated thiols although the nucleophilic attack can occur at either of the doubly bonded carbon atoms, leading to thiophene and thiopyran ring formation. It appears that the substitution pattern at the double bond controls the regiospecificity, with the product conforming to Markovnikov addition (Equation 139) <2006CC332>. 

These regiospecific photocyclizations to the root of the o-methoxyl group can be explained in terms of a nucleophilic attack from the enamine portion to the somewhat electron-deficient position at the root of the methoxyl group, as in the case of nucleophilic aromatic photosubstitution (33), and followed by a 1,5-sigmatropic shift of the substituent and its elimination to afford the didehydrolactam (1) as the final product (Scheme 5). 

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