Solvent effects vinylic substitution

A unimolecular ionization was shown to be the mechanism of solvolysis by means of rate studies, solvent effects, salt effects, and structural effects (179,180). The products of reaction consist of benzo [bjthiophen derivatives 209 or nucleophilic substitution products 210, depending upon the solvent system employed. By means of a series of elegant studies, Modena and co-workers have shown that the intermediate ion 208 can have either the open vinyl cation structure 208a or the cyclic thiirenium ion 208b, depending... 

An alternative procedure to effect elimination resolves this problem. Opening the oxaspiropentane 26 with selenide anion in a non-protic solvent effects a direct elimination via a merged substitution — elimination mechanism to give the vinyl-... 

A third mechanistically distinct [3 -1- 2] cycloaddition between vinyl ethers and vinyl-carbenoids was discovered and reported in 2001 [26]. This reaction is remarkable because when Rh2(S-DOSP)4 is used as the catalyst, the cis-cyclopentenes 142 are formed in up to 99% enantiomeric excess. The reaction occurs between vinylcarbenoids unsubstituted or alkyl-substituted at the vinyl terminus and vinyl ethers substituted with an aryl or vinyl group. Some illustrative examples are shown in Tab. 14.12. The reaction is considered to be a concerted process, which would be consistent with the highly stereoselective nature of the reaction [26]. Contrary to the [3-1-2] cycloaddition derived by means of vinylogous carbenoid reactivity, this latest [3 -1- 2] cycloaddition is not influenced by solvent effects. Due to steric demands on the carbenoid, the [3-1-2] cycloaddi-tion only occurs with cis-vinyl ethers. 

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... 

In this mode, an umpolung of reactivity for the nucleophile occurs, leading to many useful transformations. More complex pathways are not unusual these include addition to multiple bonds, vinylic and acetylenic substitution, rearrangements - some involving ring-expansion or ring-contraction - generation of reactive intermediates, etc. Solvent effects, especially solvent participation, may add new dimensions to reactivity. 

A solvent effect on the rate of the rearrangement was also determined (Scheme 11.21). The significance of the solvent effect depended on the position of the methoxy substituent. For the 6-methoxy-substituted allyl vinyl ether 9c, the rearrangement proceeded 68 times faster in methanol-d4 than in benzene-dg. 

The proton-transfer reactions of the vinyl cation H3CC=CH2 have been studied in a flow tube and theoretically. The lifetimes and UV-VIS absorption spectra of many aryl-substituted vinyl cations are reported. The solvent effects on the protonation of acetylene and ethylene are the subject of continuum solvent quantum-chemical calculations. The structures chosen were the symmetrically bridged non-classical vinyl and ethyl cations apparently the different hydration energies of these structures affect the energetics of their protonation in water. ... 

Reactions of methyl(vinyl)iodonium ion and its jS-substituted derivatives with chloride ion have been treated by ab initio molecular-orbital calculations (MP2, double-zeta -I- d level). Transition states for 5n2, ligand-coupling substitution (LC), and -elimination ifE) were found. In the gas phase, the barrier to LC is usually the lowest, but the relative barriers for S 2 and change with the substituents. Solvent effects were treated in terms of a dielectric continuum model and found to be large on 5 n2 but small on LC. 

Nucleophilic substitutions on vinylic systems. Polar and dipolar aprotic solvents effect on the reaction rates. Parameters of solvent polarity. 

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