Theoretical study of the mechanism of the Wittig reaction: Ab initio and MNDO-PM3 treatment of the reaction of unstabilized, semistabilized, and stabilized ylides with acetaldehyde

Synthesis, Properties and Stereochemistry of 2-Halo-1,2λ⁵-oxaphosphetanes

“…The NMR spectra of these compounds reveal signals at 0.2 ppm, singlet (Me3Si), at 2 ppm, doublet, 3 JPH 18.0 Hz (С 3 Н), and at 4.5 ppm (С 4 Н). 13 С-NMR signals of С-3 and С-4 carbons were found at 30 and 90 ppm, correspondingly. The 31 P-NMR signals of 2-chloro-1,2λ 5 -oxaphosphetanes 19 at δP +48 ppm (R 1 = iPrO) and at δP = 60 ppm (R 1 = Et2N) correspond to tetracoordinate phosphorus included in fourmembered phosphetane cycle (Table 1) [25,32].…”

“…The conversion was completed at −20 °С. The threo-and erythro diastereomers were separated and isolated in pure state by chromatography and crystallization (mp 100 and 123 °С, see Table 3, entries 13,14). Although at present many of the puzzling features about the reaction mechanism of ylides phosphorus with carbonyl compounds have been clarified, it seems there is no general mechanism, which could explain the progress of the reaction, transition states and stereochemistry.…”

“…The reaction of ylides 14 with aldehydes is regioselective. For example, the oxaphosphetanes obtained by reaction of silylated P-chloroylide 5 with terephthalic aldehyde depending on a ratio of initial reactants led to the formation of 1,4-bis-vinylphosphonobenzene or phosphonovinylbenzaldehyde, that represent interest as reactants for organic synthesis (See Table 6, entries 10,11) [13,[18][19][20]. The 2-fluoro-1,2λ 5 -oxaphosphetanes enter readily into a number of interesting organophosphorus compounds proceeding without Р-С bond cleavage [26].…”

Wittig and Wittig-Horner reactions under phase transfer catalysis conditions

“…(1) a reversible step which consists of nucleophilic addition of the phosphorus ylide to the carbonyl compound giving betaine or oxaphosphetane intermediates; (2) an irreversible decomposition of the betaine or oxaphosphetane to give Z and E alkenes and trialkylphosphine oxide (Scheme 2) [3]. In order to elucidate the mechanism of the Wittig reaction, it is necessary to understand formation of the two intermediates: the betaine and the oxaphosphetane.…”

“…Thus, extensive investigations of the e®ect of the ylides, of the carbonyl compounds, and the reaction conditions on the Z=E ratio have been carried out. According to their general reactivity, which is dictated by the substituents on the ylide carbon, phosphonium ylides have been classi¯ed into three categories [3]: tabilized ylides -bearing substitutes with a large conjugate capacity, such as -COOMe, -CN, -SO 2 R, preferentially form E alkenes;°s emistabilized ylides -bearing substitutes with a small conjugation capacity, such as -phenyl, propargyl, vinyl,°uorine, represent a more complex situation because a mixture of Z and E alkenes is formed. The high degree of Z-or E-stereoselectivity depends upon the reaction conditions.°n onstabilized ylides -bearing substitutes without conjugation capacity such as -CH 3 , -H, preferentially form Z alkenes.…”