Electronegativity Effects in Organic-synthesis - Differences in Stability of Isomeric Substituted Alkanes

Selectivity is such an intellectually, economically, and ecologically important matter in organic synthesis that it is highly desirable to know all the factors, both kinetic and thermodynamic, which influence it. We were surprised by the high selectivity obtained in an equilibration of isomeric beta-thioalkyl trifluoroacetates, because the thermodynamic difference implied by this and other, similar, rearrangements did not appear to have been explained. It now appears that this difference is due to the thermochemical effect whereby polar substituents bind more strongly to alkyl groups of complementary polarity; in other words electronegative atoms prefer more ''procationic'' tertiary and secondary alkyl groups, electropositive ones prefer the more ''proanionic'' primary alkyl and methyl groups. This small but significant (1 to 7 kcal . mol(-1)) effect is well documented by thermochemical data, and has been described several times; it can be predicted qualitatively by Pauling's formula relating bond energy, bond polarisation, and electronegativity. We first describe how the electronegativity effect explains our observations: they are due to the higher electronegativity of oxygen and chlorine relative to that of sulfur. We then go on to show that it is useful for the interpretation of many other literature reports, ranging from the acid-catalysed isomerization of alkyl halides, to the carbocyclization of organozinc reagents, and the isomerization of alkyl-transition metal complexes (a key step for the selectivity of important industrial processes such as alkene hydroformylation)

Stereocontrolled Epoxy-epimination of 1,3-cyclohexadiene

Placement stability is sought for children who enter care and need a place to call home. This is deemed to be necessary for the formation and continuation of secure and loving relationships with parents and carers. However, the term placement stability does not capture the quality of the placement or the subjective experience of the young person. In contrast, the term relational permanence denotes an enduring and supportive relationship between a young person and a caring adult. Research studies have tended to focus on placement stability, or legal and physical permanence, and overlook relational permanence. Within the current study, we found high levels of long-term placement stability for the study population, 354 young people who were under five and in care in Northern Ireland on the 31st March 2000. Placements for those who were adopted, on Residence Order, and rehabilitated with birth parents were more likely to be stable than those in long-term foster care and kinship foster care. However, early interview data with 30 young people and/or their parents/carers revealed high levels of relational permanence, irrespective of placement type, and that placement disruption did not necessarily mean a breakdown in the relationship. Implications for policy and practice are discussed.</jats:p

Spiroannelation reactions of 3-(1-hydroxy-2,2,2-trifluoroethylidene)-1-methyl-pyrrolidin-2-one with alkyn- and allenyl methyl ketones

3-(1-Hydroxy-2,2,2-trifluoroethylidene)-1-methyl-pyrrolidin-2-one 1 reacts with alkyn- and allenyl methyl ketones 2 and 3 to give Michael adducts 4a,b and spirocyclic dihydropyrane 5