The kinetics of Wittig's reaction of α-ethoxycarbonylalkylidenephosphoranes with aromatic aldehydes

Abstract

The kinetics of Wittig's reaction of p-and m-substituted benzaldehydes with a:-ethoxycarbonylalkylidenephosphoranes (C6Hs)3P=C(R)COOC2Hs (R = CH 3 , C 2 H s , n-C3H7) was studied and correlation analysis applied to the evaluation of the effect of substituents in benzaldehydes and of alkylation at the C" atom in the phosphoranes on the reaction rate. The conclusions show that the effect of alkyl residues in the phosphoranes of this type is not determined solely by the inductive effect and by steric reasons. The NMR and IR spectra of the a:-ethoxycarbonylalkylidenephosphoranes were recorded and it was found that the splitting of proton signals due to spinspin interaction with the phosphorus atom is transferred all the way to the protons on the carbon atom which is bound to the a:-carbanion.In connection with the preparation of branched arylaliphatic carboxylic acids with the potential anti-inflammatory effect 1 -3 we set out to synthesize <x-alkylcinnamic acids. The preparation and the biological properties of these compounds will be described elsewhere. For this purpose we made use of Wittig's reaction 4 ,5 of substituted benzaldehydes with <x-ethoxycarbonylalkylidenephosphoranes and obtained a series of ethyl esters of cinnamic acids substituted in the <x-position with respect to the carboxyl with methyl, ethyl and n-propyl groups. The required phosphoranes II were prepared by dehydrohalogenation of the corresponding phosphonium salts I (equation A) which had been obtained by a reaction of triphenylphosphine with ethyl esters of <x-bromopropionic, <x-bromo-n-butyric and cx-bromo-n-valeric acids.Determination of NMR spectra of the cx-ethoxycarbonylalkylidenephosphoranes (Table I) revealed a splitting of the signals belonging to the protons at the C atom attached to the cx-carbanion, due to a spin-spin interaction with the phosphorus atom. The splitting is marked with phosphoranes lIb and IIc* while with phosphorane lId it is covered by the signals of protons of the two methylene groups in the n-propyJ residue. This result is in agreement with the NMR spectrum of cx-methoxycarbonylethylidenephosphorane reported by Bestmann and coworkers 6 , 7 • Protons at the other C-atoms are not affected by this interaction at the experimental temperature. Protons in the ethyl group of the ester are characterized by normal signals, i.e. a quadruplet and a triplet, and only in the case of the a:-n-propyl derivative lId are they hard to distinguish_ This splitting was also observed in phosphorane II (R = CH 2 COO-C 2 H s ) and cx-acetylf3-ethoxycarbonylethylidenephosphorane.

Mechanism of Formation of Aromatic Vinylphosphonium Salts via the Peterson Olefination

“…It should be remembered that two stereoisomers are generated in the olefination reaction, and thus there is a system of two parallel reactions forming the E‐ and Z‐ stereoisomers at k E and k Z reaction rates, respectively. It was assumed that these types of reactions possess very similar energy profiles and may be determined by the same values of rate constants , formally k E + k Z = k . Analogously, changes in these parallel reactions with different p ‐substituted benzene rings in the aromatic aldehyde are also very similar, so they might be represented by one ρ value .…”

“…It was assumed that these types of reactions possess very similar energy profiles and may be determined by the same values of rate constants , formally k E + k Z = k . Analogously, changes in these parallel reactions with different p ‐substituted benzene rings in the aromatic aldehyde are also very similar, so they might be represented by one ρ value .…”