Nucleophilic vinylic substitution and vinyl

Nucleophilic vinylic substitution and vinyl cation intermediates in the reactions of vinyl iodonium salts. 37, 1... 

Nucleophilic Vinylic Substitution and Vinyl Cation Intermediates in the Reactions of Vinyl lodonium Salts... 

Nucleophilic substitution, in phosphate esters, mechanism and catalysis of, 25,99 Nucleophilic substitution, single electron transfer and, 26, 1 Nucleophilic vinylic substitution, 7,1... 

Under more basic conditions, a-elimination predominates and insertion of the carbene 40 to the solvent gives racemic 22. Non-basic and poorly nucleophilic conditions allow neighboring group participation to form the rearranged substitution product 23 with complete chirality transfer. The participation can be considered as an intramolecular nucleophilic substitution, and does occur only when it is preferable to the external reactions. Under slightly basic conditions with bases in HFIP, participation is allowed, and the weak base can react with the more electrophilic vinylic cation 21 (but not with the iodonium ion 19). A suitably controlled basicity can result in the formation of cycloalkyne 39, which is symmetrical and leads to racemization. These reactivities are illustrated in Scheme 6. 

Activation of one the double bonds of the allene by coordination to an electrophilic metal center such as Hg(II), Ag(I), Pd(II), Rh(I), Cu(I) or Au(III). Then an intramolecular nucleophile can attack and the product is formed by protodemetallation of the intermediate (Scheme 15.1). Depending on electronic and steric factors, either the proximal or the distal Jt-bond of the allene 1 is activated in that way (2 and/or 3). For each of these two possibilities now an exo or endo attack of the nucleophile is conceivable, leading to intermediates 4—7. An equilibrium between both 5 and 6 and 9 is possible. Finally, from 4 the vinyl-substituted 8 is formed. From 5, 6 or 9 the exocyclic alkene 10 and/or the endocyclic alkene 11 can be observed. Compound 7 would deliver the endocyclic alkene 12. 

Evidence for a Michael addition of a nucleophile to alkenyl(phenyl)iodonium salts at the Cp atom has now been reported for the first time. Nucleophilic vinylic substitutions of (Z)-(/3-bromoalkenyl)iodonium tetrafiuoroborates (161) and its (Z)-(/3-chloroalkenyl) analogue with sodium benzenesulfinate in THE afforded stereoselectively (Z)-l,2-bis(benzenesulfonyl)alkene (163) with retention of configuration. Intermediate formation of (Z)-[/3-(benzenesulfonyl)alkenyl]iodonium salt (162) in these reactions was established by NMR experiments in CDCI3. The formation of (Z)-(162) involves a hitherto unobserved Michael addition of benzenesulfinate anion to the alkenyliodonium salts at the Cp atom, followed by halogen extrusion. ... 

The nucleophilic vinylic substitution (S mV) of heteroatom-substituted alkylidene Meldrum s acids has been intensively studied and kinetics of the reaction <1998JOC6266, 1999CJC584, 2004JOC9248> as well as synthetic applications have been reported <19978567, 2002JHC15, 2005EJ04870> (cf. Section 8.11.4.2, Scheme 10). The preparation of the substrates and a sample application is shown in Scheme 90 <2001J(P2)1534>. 

In 2008 Yang and coworkers have reported an efficient synthesis of substituted a-alkilidene-fi-lactams via aNaOH- promoted intramolecular aza-Michael addition of a-carbamoyl, a-(l-chlorovinyl) ketene-S,S-acetals and subsequent nucleophilic vinylic substitution reaction in alcoholic media (Scheme 90), [198]. 

Numerous reactions of alkenyl(phenyl)iodonium salts leading to the formation of new C-C bond have been reported in the literature. The most important and synthetically useful reactions include the generation and subsequent cyclization of alkylidenecarbenes, alkenylation of carbon substrates via nucleophilic vinylic substitution, and transition metal-mediated coupling reactions. 

The reaction of vinylic phenyliodium salts (57) with cyanide anions could be mistaken for a simple substitution reaction.59 However, the presence of both allylic (58) and vinylic (59) nitrile products suggests a more complex picture. Deuterium labelling experiments show that the allylic product is formed via the Michael addition of cyanide to the vinylic iodonium salt, followed by elimination of iodobenzene and a 1,2-hydrogen shift in the 2-cyanocycloalkylidene intermediate (60). H-shift occurs from the methylene carbon in preference to the methine carbon. The effects of substitution and different nucleophiles were examined. 

Nucleophilic vinylic substitutions of 4//-pyran-4-onc and 2,6-dimethyl-4//-pyran-4-one with a hydroxide ion in aqueous solution were calculated by the density functional theory (B3LYP) and ab initio (MP2) methods using the 6-31+G(d) and 6-31G (d) basis sets. The aqueous solution was modelled by a supermolecular approach, where 11 water molecules were involved in the reaction system. The calculations confirmed a different addition-elimination mechanism of the reaction compared with that in the gas phase or non-polar solution. Addition of OH- at the C(2) vinylic carbon of the pyranone ring with an activation barrier of 10-11 kcalmol-1 (B3LYP) has been identified as the rate-determining step, in good quantitative and qualitative agreement with experimental kinetics. Solvent effects increase the activation barrier of the addition step and, conversely, decrease the barrier of the elimination step.138... 

Nucleophilic catalysis of ester hydrolysis and related reactions, 4, 237 Nucleophilic vinylic substitution, 7, 1... 

Nucleophilic vinylic substitution has previously been reviewed by de la Mare (1958), Patai and Rappoport (1964) and Rybinskaya (1967). 

Nucleophilic vinylic substitutions are closely related to nucleophilic aromatic substitutions, as in both the leaving group leaves from an unsaturated carbon atom. However, the vinylic substitution routes are much more diverse, and disclose more of the details of the reaction. Stereochemical study of the reaction can give information on the lifetime of the intermediate and about the structure of the transition state... 

This review attempts to provide an overview of microwave-promoted metal-catalyzed transformations of aryl and vinyl halides (or pseudo halides), providing a personal selection of both pioneering and very recently published work. Covered areas include carbonylative transformations, Heck and Sono-gashira reactions, nucleophilic substitutions and cross-couplings. Because of the diversity of the microwave systems used, the reader should consult the original references for detailed descriptions of settings and instrumentation. 

Enamines with good 7r-acceptor substituents at Cp, including conjugated enamines of the type just described, can hydrolyze by a base-catalyzed mechanism probably stepwise nucleophilic vinylic substitution at Ca (see equations 34 and 41). The generality of this mechanism has yet to be demonstrated, but it is strongly implied for compounds 35 and 37 (R = N02), and less dramatically for 37 (R = Br). 

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