Mechanism Of The Benzoin Condensation And Related Studies

Abstract

Chapter 1: An assessment of the mechanism of the benzoin condensation. The reaction under non-hydroxylic conditions. The generally accepted mechanism for the well known benzion condensation is unviable for the following reasons: 1) No experimental evidence for formation of carbanion from oxyanion. 2) No experimental reports on pKa of ‘C-H” in intermediate oxyanion. 3) From previous reports, these types of carbanion are stable only at -78° C. 4) Carbanions possessing nucleofuges generally form carbenes. It was observed that the O- protected benzaldehyde cyanohydrin (1) and O-MOM protected ethyl mandelate (2) are possibly less acidic than benzyl cyanide (3). Attempts to affect hydrogen-deuterium exchange in the O-methyl ether of benzaldehyde cyanohydrin (1) did not yield clear-cut results; neither could the analogous carbanion from O-MOM protected ethyl mandelate be formed, under analogous conditions. O-Protected benzaldehyde cyanohydrin and O-MOM protected ethyl mandelate (2) did not condense with electrophiles like benzaldehyde (4); however benzyl cyanide (3) was condensed with benzaldehyde to form the stilbene cyanide (5) under similar conditions to the benzoin condensation. Scheme 1: Reactions under benzoin conditions. All these evidences indicate that carbanions derived from 1 and 2 are not formed under the conditions of the benzoin reaction. An alternative mechanism via intramolecular participation in the cyanohydrin oxyanion leads initially to an imino-oxirane intermediate; electrophilic capture of this in the key step finally leads to benzoin (Scheme 2). This is an attractive possibility that avoids many of the problems of the earlier mechanism, and is also not incompatible with most of the available experimental evidence. Further experimental and theoretical work is indicated before an acceptable mechanism for the benzoin condensation finally emerges. Scheme 2: A possible alternative mechanism of benzoin condensation Scheme 2: A possible alternative mechanism of benzoin condensation From the studies on mechanism of benzoin condensation, it seems possible to form imino-oxirane from oxyanion of benzaldehyde cyanohydrin instead of carbanion by participation of cyanide ion. To prove the cyanide ion participation in mechanism of benzoin condensation and to avoid ambiguities from O-H peak, the reaction was performed in 1,4 dioxane solvent with benzaldehyde (4) and cyanide with the phase transfer catalyst 18-crown-6, instead of EtOH and H2O as solvent. After mixing the IR spectra for the crude product, indicated the formation of benzoin (8), which was confirmed upon work up. This led to the developing of a novel method for the benzion under anhydrous conditions. Scheme 3: Formation of benzoin under anhydrous conditions Chapter 2: Stereochemical stability of benzion. Generally α-hydroxy ketones undergo tautomerism to the enediol form, which are stabilized by intramolecular hydrogen bonding. Because of this, they undergo racemisation. Benzoin is also a α-hydroxy ketone, but it can be resolved into its enantiomeric forms. It indicates enediol form of benzion is unstable possibly due to steric interference. It was observed from the crystal structure of the carbonate analog of ene-diol form of benzoin that there is steric interference between the two phenyl rings. These were twisted out of the plane of the carbonate moiety by 19.92° and -47.32°. (The crystal structure of 4,5-diphenyl-1,3-dioxol-2-one (9) was reported first time.) This structure also indicated the existence of atropisomerism in the crystalline lattice. Chapter 3: Polymerisation of benzaldehyde. α-Hydroxy esters can be viewed as surrogates of cyanohydrins. To prove the cyanohydrin anion intermediate in classical benzoin condensation mechanism is very difficult. An interesting alternative is to employ α-hydroxy esters instead. It was observed that methyl 2-phenylglyoxylate and methyl 2-(methoxymethyl)-2-phenylglyoxylate (2) failed to react with benzaldehyde in aqueous methanol with sodium carbonate as base. IR spectra indicated that the carbonyl peak of benzaldehyde has disappeared, but NMR spectra showed a mixture of methyl 2-(methoxymethyl)-2-phenylglyoxylate (2) and benzaldehyde (4). This seems to indicate the polymerization of benzaldehyde (10) (Scheme 4). However, the product was not stable enough to be isolated and purified. Scheme 4: Reaction between benzaldehyde and MOM-protected methyl mandalate (For structural formula pl see the abstract file.)