Is Aromatic Nitration Spin Density Driven?

The mechanism of aromatic nitration is critically reviewed with particular emphasis on the paradox of the high positional selectivity of substitution in spite of low substrate selectivity. Early quantum chemical computations in the gas phase have suggested that the retention of positional selectivity at encounter-limited rates could be ascribed to the formation of a radical pair via an electron transfer step occurring before the formation of the Wheland intermediate, but calculations which account for the effects of solvent polarization and the presence of counterion do not support that point of view. Here we report a brief survey of the available experimental and theoretical data, adding a few more computations for better clarifying the role of electron transfer for regioselectivity.

Entropy driven preference for alkene adsorption at the pore mouth as the origin of pore-mouth catalysis for alkane hydroisomerization in 1D zeolites

This work presents a theoretical basis for a pore-mouth catalysis model developed to explain the positional selectivity of skeletal isomerization on bifunctional metal acid-zeolite catalysts.

External oxidant-compatible phosphorus(III)-directed site-selective C–H carbonylation

The first development of an external oxidant-compatible system involving a phosphorus(III)-directed C–H functionalization has been uncovered. An efficient C–H esterification of indoles with CO and alcohols has been reported in which the high reactivity and the exclusive C7-selectivity derives from the selection of a P(III)–directing group and the utilization of benzoquinone as an external oxidant with palladium catalysis. This strategy shows many advantages, involving an easily accessible and removable directing group, the use of cheap carbonylation sources, a broad substrate scope, and excellent positional selectivity. Two cyclopalladated intermediates were confirmed by x-ray analysis, uncovering key mechanistic features of this P(III)-directed C–H metalation event.

Evaluation of the Candida sp. 99-125 Lipase Positional Selectivity for 1,3-Diolein Synthesis

In this study, comparative experiments were carried out to investigate the positional selectivity of Candida sp. 99–125 lipase in preparing 1,3-diolein by using medium engineering strategy. The results indicated that the diolein yield was markedly enhanced from 56.5% to 86.7% with increasing log⁡P values of the solvents, while the selectivity of the examined lipase for the sn-1 over the sn-2 hydroxyl of glycerol was decreased, thus leading to a reduced 1,3-diolein to 1,2-diolein ratio. To evaluate the possibility of industrial enzymatic production of 1,3-diolein, larger-scale experiments were assessed. After being used repeatedly for eight batches, the diolein content reached 95.1%, while the 1,3-diolein to 1,2-diolein ratio was 7:1 following purification. Results of the kg level experiments significantly demonstrated the practicability of the enzymatic process and the efficiency of the purification strategy for the product.

Catalyst-controlled positional-selectivity in C–H functionalizations

C–H bonds are ubiquitous in organic molecules and typically these bonds are chemically indistinct from each other and it would be highly advantageous for a synthetic chemist to have the ability to choose which C–H bond is functionalized in a given molecule.

An insight into the solvent effect on the positional selectivity of the immobilized lipase from Burkholderia cepacia in 1,3-diolein synthesis

The diolein yield was higher while the ratio of 1,3-diolein to 1,2-diolein was smaller in solvents with higher log P values.