Olefins electron-withdrawing group regioselectivity

D. Regioselectivity with Olefins Bearing an Electron-withdrawing Group at... 

To Study electronic effects on the regioselectivity (a- vs. p-addition), a systematic variation of R and R (selected by their assigned para-Hammett parameters [113]) was conducted. It revealed a kinetic and thermodynamic preference for the p-product. This is in good agreement with the experimental observation that the nitrogen atom usually ends up at the less substituted carbon of the olefin and away from the most electron-withdrawing group [20, 83, 96]. 

The application of this catalyst was extended to the use of Michael acceptors containing two different electron-withdrawing groups at the two ends of the olefin moiety, such as 4-oxoimides (Scheme 4.34). This results in a very regioselective reaction, due to the better efficiency of the ketone moiety to activate the Michael acceptor compared with the imide group. This effect could... 

Tlie insertion of alkenes is often less regioselective than the insertion of olefins possessing an electron-withdrawing group. Insertion of alkenes into Schwartz s reagent CpjZrHCl forms the linear alkyl complex with high regioselectivity (vide supra), but... 

Similar studies of the regioselectivity in the HDA reaction between 2-substituted norbomadienes and unactivated terminal acetylenes catalyzed by a cobalt catalyst were also undertaken. Unfortunately, 2-sub-stituted norbomadienes are much less reactive with unactivated acetylenes in the cobalt-catalyzed HDA reaction than with electron-deficient olefins in the Ni-catalyzed HDA cycloadditions. When an electron-withdrawing group (Y = COOMe) or an electron-donating group (Y = OMe) is attached to the 2-position of the NBD, no desired [2 + 2 + 2] cycloadduct is observed with 1-hexyne, even in refluxing toluene for three days under the usual cobalt-catalyzed conditions (Scheme 12). Instead, some NBD dimers and acetylene trimers are detected. With a... 

Halofluorinations take place, as a rule, regioselectively (Markovnikov addition), the olefinic carbons can be substituted with a variety of substituents ranging from alkyl or aryl groups to different electron-withdrawing functions see for example refs 31 and 178-180. Bromo-fluorination of 4-/m-butyl-l-methylcyclohexene with /V-bromosuccinimide in 70% hydrogen fluoride/pyridine gave two stereoisomers 1 and 2.181... 

The high specificity of the Shi epoxidation permits the regioselective epoxidation in some polyene compounds. McDonald and co-workers employed ketone 2 in the total synthesis of nakorone and abudinol. Triene-yne 72 was selectively epoxidized on the two more electron-rich double bonds, leaving the olefin next to the electron-withdrawing sulfone group intact. Bis-epoxide 73 was transformed into both ew/-nakorone 75 and ent-abudinol B 76. 

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