Structure, Reactivity, and Chemistry of Fluoroalkyl Radicals.

Exploiting electron spectroscopies to probe the structure and organization of self-assembled monolayers: a review

The prenyl transfer reaction. Enzymic and mechanistic studies of the 1'-4 coupling reaction in the terpene biosynthetic pathway

In 1972, M. T. Reetz defined dyotropic (from the greek dyo, meaning two) rearrangements as a new class of pericyclic valence isomerizations in which two σ-bonds simultaneously migrate intramolecularly.1,2 Reactions in which the two migrating groups interchange their relative positions were designated as type I (Scheme 1A) whereas those of type II involve migration to new bonding sites without positional interchange (Scheme 1B,C). The uncatalyzed and concerted nature of these processes was also proposed.3,4 Nowadays, dyotropic transformations are standard tools for the construction of organic and organometallic molecules. Frequently, this unique rearrangement is the single entry to an efficient preparation of target molecules. Moreover, the discovery of new dyotropic reactions involving transition metals and excited states had widened the mechanistic scope of these processes to new reaction pathways well away from the original definition of uncatalyzed and concerted processes. This review comprehensively reports the dyotropic reactions studied since the seventies, accounts for the different reaction mechanisms for these rearrangements, and discusses their synthetic applications in organic and organometallic chemistry.

Dyotropic Reactions: Mechanisms and Synthetic Applications†

The synthesis, characterization, and electrochemical behavior of some acridinedione derivatives are reported. Cyclic voltammetric studies show that all the dyes undergo irreversible oxidation irrespective of the substitution on the nitrogen. The product formed on oxidation is the aromatic derivative in the case of N-H compounds and the acridinium salt in the case of the N-substituted compounds, which have been isolated and characterized. Formation of an intermediate carbon-centered radical is observed as evidenced by ESR spin-trapping experiments. A mechanistic scheme for the electrochemical oxidation is proposed. On carrying out reduction after oxidation, different products are formed depending on the substitution on the nitrogen. There is no reduction of the oxidized product in the case of N-H compounds, and compounds with substitution on nitrogen undergo reduction consistent with the observation in N-alkylpyridinium salts.

Synthesis, Characterization, and Electrochemistry of Some Acridine-1,8-dione Dyes

Previous attempts to prepare the pentacyclic triene 17 for comparison of the rate of intramolecular dyotropy with the kinetics of similar irreversible rearrangements of norbornene ring-substituted analogues had given only dyotropomer 18 with an estimated minimum ratio K1(17)/K1(5)∼ 2 × 105 at 36 °C. In the following it is shown that the steric proximity, dCH, of transferring H atoms to receptor sp2 carbons in the reaction zone cavity together with M M-calculated π-energy differences between dyotropomers can rationalise the large rate enhancement observed for the triene 17 compared with 5 and its analogues. For a series of compounds modelled on 5, in which dCH variations are quite small, observed differences in dyotropic rate are identified as arising from the interplay of molecular geometry changes and small changes in π-energy at the receptor alkene site occasioned by proximate polar groups, the electronic changes associated with aromatisation of the appended donor-site ring remaining essentially constant across the series. When the electronic energy changes associated with dyotropy for a pair of analogous structures are very closely similar, a rate-spread of ca. 104 can be identified with a change in dCH of 0.1–0.17 A. Similar kinetic effects concomitant on small parallel structural variations, virtually identical in relative-rate terms to those in the triene series, are seen in the irreversible dyotropy of a series of analogous pyrazolines modelled on compound 36 and may be likewise rationalised. Kinetic comparisons for a group of aryl-ring substituted analogues of pyrazoline 36 reveal quite modest substituent effects, consistent with reactant-like transition-states for these quantitative, exothermic rearrangements. Inter-series comparison of alicyclic trienes with pyrazolines indicate that when dCH values are essentially identical in representative examples, a rate-differential of 102–103 between the two series can be identified principally with the differing electronic requirements for triene and (slower) pyrazoline rearrangements. Primary deuterium kinetic isotope effects (K12H/K12D, dln[K12H/K12D]/dt and especially A2H/A2D) reveal strong evidence for non-classical behaviour especially for pyrazoline 38.

On intramolecular dyotropy: structural effects on reaction rates, crystal structure–molecular mechanics correlations and primary deuterium kinetic isotope effects. (Parameters for intramolecular recognition)

On intramolecular dyotropy: Structural effects on reaction rates and x-ray crystal structure - molecular mechanics correlations

Influencing reactivity by monolayer compression: an alcohol dehydration

Conformer selection by monolayer compression

In an earlier paper we discussed the effect on reaction rate for thermal intramolecular dyotropy exhibited for a series of trienes, 1–4 and pyrazolines 6–9 having identical structural elements proximate to the 2 H receptor π-bond. Both sets of compounds displayed closely similar rate-ratios across the series correlating with structural modification. The set of homopyrazolines 14–17 analogous to 6–9, now reported, correlate similarly and all three sets of compounds obey a linear free energy relationship log k1A=m log k1B+C, suggestive of a common reaction mechanism for H dyotropy for these compounds. We also report on the kinetic behaviour of new, oxygen-bridged trienes 22 and 23, analogues of triene 1; over quite large temperature ranges, translating into a rate-spread of ∼ 105, Arrhenius plots for rearrangement of 1 and 23 are essentially linear, indicating that these compounds behave classically with scant evidence for quantum tunnelling from this result and the previously reported indecisive PDKIE data. We also describe solvent effects on the rearrangement rates for representative analogues of triene 1, and for trienes 22 and 23, and briefly report on the solid-state thermochemical behaviour of the triene 3, analogous to 1. A new molecular framework exhibiting relatively slow thermal H dyotropy is also reported. The information we have obtained rather strongly suggests a concerted, but non-synchronous transfer of 2 H in the dyotropic process.

K. Brian Astin

Papers

On intramolecular dyotropy: structural effects on reaction rates, crystal structure–molecular mechanics correlations and primary deuterium kinetic isotope effects. (Parameters for intramolecular recognition)

On intramolecular dyotropy: Structural effects on reaction rates and x-ray crystal structure - molecular mechanics correlations

Influencing reactivity by monolayer compression: an alcohol dehydration

Conformer selection by monolayer compression

On intramolecular dyotropy: structural effects on reaction rates and crystal structure-molecular mechanics correlations for some new examples