77613-61-7Relevant articles and documents
(E)-Selective Wittig Reactions between a Nonstabilized Phosphonium Ylide Bearing a Phosphastibatriptycene Skeleton and Benzaldehydes
Uchiyama, Yosuke,Ohtsuki, Takemaru,Murakami, Rikiya,Shibata, Munenori,Sugimoto, Jun
, p. 159 - 174 (2017/01/14)
Wittig reactions between benzaldehyde derivatives and a nonstabilized phosphonium ylide bearing a phosphastibatriptycene skeleton, regarded as a tridentate aryl ligand, gave (E)-alkenes with high selectivity in the presence of both lithium and sodium salts. As previously reported, reactions between a triphenylphosphonium ylide and benzaldehyde derivatives under the same conditions afforded mainly (Z)-alkenes. Variable-temperature (VT)31P{1H} NMR spectra showed two signals, assigned to cis- and trans-1,2-oxaphosphetanes, which were observed at different temperatures (–80 °C and –40 °C, respectively) in the Wittig reaction between benzaldehyde and the nonstabilized phosphonium ylide bearing the phosphastibatriptycene skeleton, in the presence of both lithium and sodium salts, and showed the existence of equilibrium between these products at –40 °C. On the other hand, this equilibrium was not clearly observed in the reaction between the triphenylphosphonium ylide and benzaldehyde, for which only one signal was detected. The observed intermediates were confirmed to be 1,2-oxaphosphetanes by deprotonation of the isolated β-hydroxyalkylphosphonium salts bearing a phosphastibatriptycene skeleton and a triphenylphosphine moiety, respectively. Crossover reactions were conducted in the deprotonations of β-hydroxyalkylphosphonium salts with TMS2NNa in the presence of p-chlorobenzaldehyde, resulting in the observation of signals corresponding to 1,2-oxaphosphetanes containing phenyl and p-chlorophenyl groups at the 4-positions, indicating the exchange process between benzaldehyde and p-chlorobenzaldehyde at –40 °C for the phosphastibatriptycene system and at 0 °C for triphenyl derivatives. These results clearly indicated that stereochemical drift occurred at those temperatures, even in reactions using nonstabilized phosphonium ylides. The stereochemical drift in the phosphastibatriptycene system occurred at a lower temperature than in the case of the triphenyl derivative, thus explaining the (E)-selective Wittig reactions between the benzaldehyde derivatives and the nonstabilized phosphastibatriptycene-based phosphonium ylide in the presence of lithium and sodium salts.
Low-temperature characterization of the intermediates in the Wittig reaction
Vedejs,Meier,Snoble
, p. 2823 - 2831 (2007/10/02)
Nonstabilized salt-free ylides react with aldehydes and nonhindered or strained ketones at -78°C to give oxaphosphetanes. The Wittig intermediates can be observed by 31P and 1H NMR techniques. In the presence of LiBr, betaine-lithium bromide adducts often precipitate from solution. The oxaphosphetane from PhCHO + CH2=PPh3 reacts rapidly with LiBr to give a betaine·LiBr adduct, and the corresponding salt Ph3P+CH2CHOHPh Br- reacts with KH at -40°C to form the oxaphosphetane. No salt-free betaine has been detected. Lithium bromide is shown to decrease cis selectivity (CH3CH=PPh3 + PhCH2CH2CHO) in the condensation step and not by oxaphosphetane equilibration. Oxaphosphetane reversal to ylide + aldehyde is confirmed for aryl aldehydes but not for aliphatic aldehydes or ketones according to three types of crossover experiments. Rationales for cis selectivity of aldehyde-ylide reactions are discussed. A "crisscrossed" cycloaddition rationale is proposed, aldehyde and ylide planes tilted toward an orthogonal arrangement to minimize steric interactions, to explain cis-alkene formation. Other transition-state geometries having carbonyl and ylide planes roughly parallel are considered more likely for trans-olefin formation or for Wittig reactions of ketones.
