Old and new ylid chemistry

Abstract

A. The mechanism of the Wittig reaction is re-interpreted on the basis of new experimental and theoretical results. B. Reactions of trimethylsilylated phosphorus ylides are described. They allow a reversal of the stereochemistry of the Wittig reaction. C. The preparative possibilities of the reaction of N-phenyl-ketenylidenetriphenylphosphorane with carbon acids are presented and the synthesis of the tetramer of phenyl isonitrile is reported. D. The results of investigations into the reactions of Hexaphenylcarbodiphosphorane are presented. A. The Wittig Reaction Detailed investigations have led us to propose a new mechanism for the Wittig reaction.123 Ylides, (1), and aldehydes, (2), combine to give oxaphosphetanes, (3), in which the 0-atom occupies an apical position on the pentavalent phosphorus and in which the substituents on the four membered ring are B to one another. Cleavage of the original. ylid C-P bond, which is necessary for formation of the olefin, requires a ligand reorganisation process (pseudorotation) which brings this bond to an apical position,The opening of the C-P bond to give the betain (5) occurs during or after the conversion to the trigonal bipyramidal structure (u).The electronic nature of the substituents R1 and R2 in (5) is instrumental in determining the stereochemistry of the product olef in. When R1 phenyl electron donating R2 substituents cause a very fast elimination of the phosphine oxide, giving B olefins (7). Electron-withdrawing R2 groups (R1phenyl) extend the lifetime of (5), which can now isomerise to the thermodynamically more stable (6), from which phosphine oxide elimination gives E-olef ins (8). If the ligands R1 on phosphorus are electron-donating the rate of the phosphine oxide elimination is also decreased. In this case strongly increased formation of E-olefins is observed, compared to R1C6H5, even when R2 is an electron donor.

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

“…The mechanism presented above is reminiscent of the one proposed by Bestmann to explain the stereoselectivity of the Wittig reaction [12]. According to Bestmann, the stereoselectivity determining step might occur with unstable intermediates of the type (D ↔ E), because bond rotation around the C-C bond in such intermediates is presumed to be faster than alkene elimination.…”

“…Part of the problem is the lack of information concerning the nature of the transition states or transient intermediates that might be present in the Wittig reaction. Several theoretical studies have been undertaken in order to provide an insight on this matter [10][11][12][13][14][15][16][17][18]. For the most part, these studies make use of the mythical Wittig reaction [10,11,[13][14][15]18] (the reaction of methylidenephosphorane with formaldehyde) to model this olefination process.…”

The modern interpretation of the Wittig reaction mechanism

“…84 Betaines derived from nonstabilised ylides were supposed to be very short-lived and thus to 20 quickly decompose to Wittig products, whereas betaines with a stabilising group on the carbanion carbon (i.e. derived from a stabilised ylide) were thought to be longer lived due to the greater stabilisation of the negative charge, so that rotation could occur about the C-C bond before C-O bond breakage occurred, thus 25 giving rise to E-alkene.…”

Molecular modeling of oxaphosphetane intermediates of wittig olefination reactions

“…This result was unexpected owing to previous results found with this method [8, 16,171, which is known not to treat well four-member rings and hypervalent atoms (the d orbital contribution is not explicitly treated in MNDO).…”