Mercedes C. Cabaleiro

Kinetics and mechanism of oxymercuriation of αβ-unsaturated carbonyl compounds

The products and the kinetics of methoxymercuriation of a series of αβ-unsaturated ketones and esters have been studied. Whereas the methoxymercuriation of αβ-unsaturated esters is stereospecific, the methoxymercuriation of the ketones leads to mixtures of diastereoisomers. The configuration of the products has been deduced from their 1H n.m.r. spectra on the basis of chemical shifts and proton coupling constants. The results are consistent with the formation of a bridged ‘mercurinium’ ion intermediate. It is suggested that the mixtures of diastereoisomers obtained from the αβ-unsaturated ketones are the result of equilibria established after the oxymercuriation reaction.

Thermal reaction of cinnamic acid and of β-styrylphosphonic acid with urea

The thermal reaction of b-styrylphosphonic acid under experimental conditions similar to those applied to cinnamic acid and to methyl cinnamate to provide 6-phenyl-5,6-dihydrouracil, led to the formation of the novel 6-phenyl-5,6-dihydro-4-phosphorylamide-(1H,3H)-2-pyrimidinone.

Triethylamine-induced Reactions of Methyl 2,3-Dibromo-2,3-diarylpropanoates in Methanol†

The title compounds undergo elimination with methanolic triethylamine to afford the corresponding debrominated olefins, most of them through an E2 stereoconvergent process.

Methyl 2,3-dibromo-2,3-diarylpropanoates. Debromination and dehydrobromination reactions

Rate and product studies of the iodide- and methoxide-mediated reactions of methyl (R,R)- and (R,S)-2,3-dibromo-2,3-diarylpropanoates and some of their 2- and 3-(4-substitutedphenyl)-derivatives have been carried out. The results indicate that the reaction of iodide with most of the substrates affords only debrominated olefins, whereas the course of the methoxide-promoted reactions seems to depend mainly on the configuration of the starting substrate. Thus, in the eliminations induced by methoxide ion most of the (R,R)-compounds led exclusively to dehydrobrominated olefins. On the other hand, the dehydrobrominations of most of the (R,S)-isomers can be accompanied by debromination. Mechanistic aspects are discussed.

Mechanism of dehydroacetoxylation of methyl 3-acetoxy-3-aryl-2-halogenopropanoates

The rate of sodium methoxide-induced dehydroacetoxylation of the diesters p-RC6H4CH(OAc)CXHCO2Me (X = Br or Cl) has been studied as a function of the para-substituent. The results correspond to the para-substituent effects expected for proton abstraction from C-2, as estimated from the diastereoisomerisation of model compounds [p-RC6H4CH(OMe)CXHCO2Me] under similar conditions. This suggests that the elimination occurs via a carbanion mechanism, with C(2)–H bond-breaking rate-determining. The apparent insensitivity of the kinetic behaviour to configuration differences, together with the response of the reactions to the effect of the 3-substituent, is interpreted in terms of inductive stabilisation by the 2-halogen atom, which tends to localise the incipient negative charge at C-2, inhibiting distortion of its tetrahedral geometry.

Chlorination of αβ-unsaturated carbonyl compounds. Part V. Mechanism of chlorination of αβ-unsaturated ketones in methanol and in trifluoroacetic acid

The kinetics and products of the reaction of chlorine with 1-phenyl-3-arylprop-2-enones and with 4-arylbut-3-en-2-ones in methanol have been studied. The reactions involve predominant trans-addition of the elements of methyl hypochlorite to the double bond with little formation of dichlorides, and are interpreted in terms of the formation of an open or partially-bridged carbocation which is captured by the solvent mainly prior to appreciable rotation about the carbon–carbon bond. Some catalysis by hydrogen chloride is apparent in the reaction with 4-phenylbut-3-en-2-one and is believed to involve the formation of acetals prior to reaction with chlorine. In contrast the reaction of 1,3-diphenylprop-2-enone and of 4-phenylbut-3-en-2-one with chlorine in trifluoroacetic acid gives appreciable yields of the excepted dichlorides, probably through a mechanism involving ion-pairs.

Isomerisation of Methyl (E) 2-Bromo-3-(4-XC6H4)-propenoates

The geometric isomerisation of the title compound induced by bromine or chlorine appears to involve ionic pair species resulting from the nucleophilic attack of the halogen.

Triethylamine-promoted Elimination from (R,R)-PhCHORCHClCO2Me (R=MeCO, PhCO, 4-MeC6H4, 4-MeOC6H4, MeSO2 and 4-MeC6H4SO2) in Various Solvents

The rates and stereochemistry of elimination from a number of (R,R)-PhCHORCHClCO2Me (R=MeCO, PhCO, 4-MeC6H4, 4-MeOC6H4, MeSO2 and 4-MeC6H4SO2) with triethylamine in tetrahydrofuran, acetone, methanol, dimethylformamide and acetonitrile as solvents led to the conclusion that the elimination process occurs through an (E1cB)I mechanism.

Elimination from diastereoisomeric methyl 2-acetoxy-1-bromo- and 1,2-dibromo-2-phenylethylphosphonates

The examination of the kinetics and stereochemistry of dehydroacetoxylation of erythro-and threo- PhCHOAcCHBrPO3Me2 promoted by triethylamine in various solvents, and comparison with those for erythro- and threo- PhCHOAcCHBrCO2Me, leads to the conclusion that the acetoxybromophosphonates undergo elimination through a carbanion process of the irreversible type. The similarity of the elimination behaviour of the threo-PhCH BrCH BrPO3Me2 and that of threo-PhCHBrCH BrCO2Me is interpreted as indicating that the concerted pathway is also operative for the former compound. In the case of erythro- PhCHBrCHBrPO3Me2 the kinetics suggest that elimination of the latter occurs viaa concerted mechanism.

Methoxide-induced fragmentation of 2,2,3-trihalogeno- and 2,2-dihalogeno-3-methoxy-1,3-diphenylpropanones

The reaction of 2,2,3-trihalogeno-1,3-diphenylpropanones with sodium methoxide in methanol occurs with exclusive formation of fragmentation products. Kinetic and stereochemical evidence is interpreted in terms of a concerted intramolecular process promoted by initial attack of methoxide anion at the carbonyl carbon. In the case of 2,2-dihalogeno-3-methoxy-1,3-diphenylpropanones the reaction gives fragmentation, rearrangement, and elimination products. The former products are believed to be formed through competing reactions involving an intermediate formed on addition of methoxide to the substrate. For the fragmentation pathway the results are consistent with a carbanionic-intermediate mechanism.