A DFT study on the mechanism for polymerization of δ-valerolactone initiated by N-heterocyclic carbene (NHC) catalysts

Polymerization reactions using renewable raw material as substrate (namely, δ-valerolactone) and N-heterocyclic carbenes (NHC) as organocatalysts to form polyesters were investigated by computational approaches. Two routes were investigated for the reaction, either with the NHC acting as a Brønsted base and activating an alcohol used as co-initiator or by direct nucleophilic attack of the NHC on the carbonyl carbon of the lactone, forming a zwitterionic intermediate. In agreement with previous studies, the lowest energy pathway leading to polymerization is that where the NHC activates the alcohol co-initiator, yielding a partially charged alkoxide that then performs a nucleophilic attack on the lactone. The proton affinity of the NHCs shows a high correlation with the activation enthalpy for the first reaction step. Thus, NHCs with high proton affinity stabilize the first intermediate and make the lactone ring-opening the rate-determining step for the reaction

A stereoselective, base-free, palladium-catalyzed heck coupling between 3-halo-1,4-naphthoquinones and vinyl-1H-1,2,3-triazoles

A stereoselective, base-free Heck coupling between 1,4-naphthoquinone and 1H-1,2,3-triazole derivatives was reported for the first time. This study shows that depending on the 1,4-naphthoquinone, the use of an additional base is unnecessary to produce the naphthoquinone-triazole conjugates. This is also the first example of a Heck coupling of these two cores without using a base as an additive. In this work, sixteen new naphthoquinone-triazole hybrids were stereoselectively synthetized in good to excellent yields. The reaction mechanism was discussed based on DFT CAM−B3LYP calculations. The first step is the coordination of the arene to the palladium catalyst to form a palladacycle intermediate. After reorganization in this intermediate, the double bond in the arene is restored to proceed with the coupling step and formation of the C−C bond in the rate determining step. The Kozuchi-Shaik span model was employed to rationalize substituent effects

Characterisation of microbial attack on archaeological bone

As part of an EU funded project to investigate the factors influencing bone preservation in the archaeological record, more than 250 bones from 41 archaeological sites in five countries spanning four climatic regions were studied for diagenetic alteration. Sites were selected to cover a range of environmental conditions and archaeological contexts. Microscopic and physical (mercury intrusion porosimetry) analyses of these bones revealed that the majority (68%) had suffered microbial attack. Furthermore, significant differences were found between animal and human bone in both the state of preservation and the type of microbial attack present. These differences in preservation might result from differences in early taphonomy of the bones. © 2003 Elsevier Science Ltd. All rights reserved