The Effect Of Additives In Pd-catalyzed Alkoxycarbonylation Of Alkenes

Alkoxycarbonylation of alkenes with CO and alcohols has become a significant protocol for the synthesis of high-value added carboxylic esters with advantage of 100%atom economy.Due to the versatile applications of carboxylic esters in the production of pharmaceuticals medicines,surfactants and perfumes ect.,alkoxycarbonylation of alkenes?ie.?-alkenes,dienes and functional alkenes?has been one of the most important homogeneous catalytic processes.Transition metal such as Co,Ru,Rh,Pd can be used to catalyze this reaction,in which Pd complex is widely applied due to its high catalytic activity and selectivity,and the broadly substrate availability.The well-established mechanism for Pd-catalyzed alkoxycarbonylation of alkenes is fundamentally based on the formation and stability of palladium-hydride?Pd–H?active species,which is the real catalyst to initiate the catalytic cycle.Usually Br?nsted acid is applied as an H⁺reservoir to promote the formation of Pd–H active species via the reaction of Pd⁰+H⁺?[Pd ^II-H]⁺,which suffers from the deactivation of the involved phosphine due to its alkylation under acidic condition and the potential quenching of Pd-H active species by excess H⁺along with the release of H₂([Pd^II-H]⁺+H⁺?Pd ^II+H₂?).Considering the above mentioned drawbacks of using Br?nsted acid additives in alkoxycarbonylation of alkenes,in this thesis work,H₂O was applied as the additive instead of Br?nsted acid.The promoting effect of H₂O additive for Pd-catalyzed alkoxycarbonylation of alkenes was fully investigated.It was found that,with H₂O as additive,the methoxycarbonylation of styrene performed smoothly over Pd?MeCN?₂Cl₂ catalyst precursor with involvement of the moisture-insensitive phosphine of Xantphos.The target products of?linear/branched?methyl phenylpropionates were obtained in the yield of 97%?L/B=4.3/1?with the TOF of 192h^-1.The H₂O-promoted Xantphos-Pd?MeCN?₂Cl₂ system also exhibited good generality to the alkoxycarbonylation with wide range of alkene substrates.In addition,the side-reaction which universally happened over acid catalyst was completely inhibited over H₂O-invovled Xantphos-Pd?MeCN?₂Cl₂ system.Then the technique of in situ high pressure spectroscopy was applied to study the derivation and stability of Pd-H hydride species over H₂O-invovled Xantphos-Pd?MeCN?₂Cl₂ system.It was indicated that the presence of H₂O could significantly facilitate the formation and stability of Pd-H hydride species with the characteristic IR-vibration at?1944 cm^-1,which definitely proved that H₂O indeed could serve as an alternative additive as H⁺-reservoir instead of Br?nsted acid to promote alkoxycarbonylation of alkenes over robust Xantphos modified Pd-catalyst.The kinetic study of activity energy was calculated in presence of H₂O.The methoxycarbonylation of ethlylene was also investigated over Xantphos-Pd?MeCN?₂Cl₂ catalytic system.It was found that only the use of CH₃SO₃H as a Br?nsted acid additive could effectively improve the catalytic performanc of Xantphos-Pd?MeCN?₂Cl₂ whereas H₂O additive exhibited poor promoting effect on this reaction,due to the relatively low reactivity of ethlylene compared to alkyl-sussutoted?-alkenes.Over CH₃SO₃H involved Xantphos-Pd?MeCN?₂Cl₂ system,the methoxycarbonylation of ethlylene with MeOH afford the target product?methyl propionate?in the yield of65%without the formation of any by-product.