The Wittig reaction of benzyltriphenylphosphonium salt and benzaldehyde in organic solvent/water (NaOH) two-phase medium was studied, focusing on the influence of substituents and organic solvents. Substituents selected for study include CH3, F, Cl, Br, CH3O, NO2, and CF3. Organic solvents include polar solvents (CHCl3 and CH2Cl2) and non-polar solvents (n-C6H14, C6H6 and CCl4). It was found that the stirring rate does not matter, and the reaction of benzyltriphenylphosphonium chloride and benzaldehyde in the organic phase is the decisive step in determining the Z/E ratio of the product stilbene. In general, polar solvents exhibit more favorable Z-selectivity. By exchanging the substituents on the benzyl group of the phosphorus atom and the phenyl group of the aromatic aldehyde, the Z/E ratio of the product stilbene can be significantly changed. In contrast to meta- and para-substituted benzaldehydes as well as ortho-substituted benzylidene ylides, ortho-substituted benzaldehydes bearing heteroatom substituents exhibit a significant enhancement of Z-selectivity in the order of effectiveness of the substituents. It is CF3>(Cl, Br)>CH3O>F>NO2. A cooperative asynchronous cycloaddition mechanism involving a four-center transition state is proposed to operate in these systems.
The Wittig reaction is one of the most important and versatile reactions in organic chemistry, used for the synthesis of alkenes with well-defined double bond positions. Since its discovery in the early 1950s1,2, the Wittig reaction has been extensively studied theoretically and synthetically3,4,5,6,7,8,9,10,11,12 and due to its simplicity And highly recognized, efficient and versatile. The prototypical Wittig reaction involves the reaction of a phosphonium ylide with an aldehyde or ketone, as shown in reaction (R1).
Stereoselectivity is highly dependent on the substituents bonded to the phyllocarbon and phosphorus atoms, as well as the precise reaction conditions. Phosphorus ylides are divided into three categories based on their general reactivity, namely, non-stable ylides, semi-stable ylides and stable ylides. The Wittig reaction has been shown to preferentially produce E alkenes for stable ylides with strongly conjugated substituents (e.g., COOMe or CN); A mixture of Z and E alkenes that are semistable ylides (base or allyl); whereas the major anti-thermodynamic Z alkenes of non-stable ylides lack such conjugated functional groups (e.g., alkyl groups).
