The Wittig reaction of nonstabilized ylides with benzaldehyde and benzophenone was investigated in detail by means of carbonyl-14C kinetic isotope effects, substituent effects, and isotope-scrambling and probe experiments. The reaction with benzophenone gave the carbon isotope effects and the Hammett p values of considerable magnitude both in Li salt-free and salt-present conditions. In contrast, they are quite small for the reaction with benzaldehyde.Enone-isomerization and dehalogenation probe experiments indicated that the nonstabilized ylide has enough ability to transfer an electron to benzaldehyde and benzophenone. These results were interpreted in a self-consistent manner by the mechanism that the Wittig reaction of nonstabilized ylides proceeds via initial electron transfer from the ylide to the carbonyl compounds. The electron-transfer step is rate-determining for benzaldehyde, while radical coupling following the electron-transfer step is rate determining for benzophenone. From the probe experiments together with the isotope effects and the substituent effects reported previously, the reaction of semistabilized ylides was concluded to proceed through a polar nucleophilic addition mechanism.
The reactions of o-, m-, or p-bromobenzophenone and o-iodobenzophenone with various Grignard reagents were carried out in diethyl ether at 0 °C. The product analysis showed that these halobenzophenones can be a good chemical probe to evaluate the electron-transfer ability of nucleophiles to the aromatic ketones.
Substituent effects were measured for the reactions of substituted benzaldehydes with allyltributyltin (1) and allyltriethyllead (2) reagents in the presence of BF,*OEt, in CH,CI,. The Hammett p values were small and positive at 25°C and negative at -78°C for both 1 and 2. These could be interpreted in terms of the contribution of electrophilic complexation between the aldehyde function and BF, as a rate-limiting step. A large negative p value was observed for the complex-formation equilibria between substituted benzaldehydes and BF,.
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