Leaving group effects vinylic substitution

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

The effect of the nature of ion pairs as nucleophiles in a metal-catalysed substitution reaction has been investigated by determining product ratios for the Pd-catalysed allylic alkylations of substrates (9)-(ll) under various conditions, particularly with respect to catalyst ligands, nucleophiles, and counterions. Each dienyl acetate ionizes to form initially the vinyl (7r-allyl)-Pd intermediate corresponding most closely to the leaving group, i.e. (12) from (9), (13) from (11), but (12) and (13) from (10). The initial intermediate can then either be trapped by the nucleophile or it can begin to equilibrate to some mixture of vinyl 7r-allyl intermediates. If nucleophilic addition occurs before full equilibration, the product ratio is different for each substrate if... 

In contrast to the installation of the nucleobase via nucleophilic substitution of a suitable leaving group on the isoxazolidinyl cycloadduct, Colacino et al. (51) and Sindona and co-workers (52,53) prepared isoxazolidinyl nucleosides using vinyl nucleobases as the dipolarophile (Scheme 1.5). In Sindona s work, while a three-component reaction of hydroxylamine, formaldehyde, and 20 afforded a complex mixture of cycloadducts and byproducts, the known dipole 21 reacted with N-9-vinyladenine (20) in benzene at reflux to afford a racemic mixture of adduct 22 and its enantiomer (45%). The ester function was then used to effect a resolution by pig... 

In-plane, backside, vinylic nucleophilic substitution of an appropriate halide leaving group can also be effected. Thus, the ( )-P-alkylvinyl(phenyl)iodonium salt, which can be prepared from the corresponding trimethylsilyl alkene and iodoso-benzene, undergoes both Sn2 substitution to produce the inverted chloroalkene and elimination to the corresponding alkyne (Scheme 7.11). 

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