1,x-Elimination Reactions: Extending the Limits of a Classical Organic Reaction

Synthesis of 2,24-Diene-12,13,15,16,34,35,37,38-octaphenyl[4.4]triphenylparacyclophane

A new octaphenyl[4.4]triphenylparacyclophanediene was readily synthesized in six steps from p-xylene via the installment of bromine atoms, replacement with a vinyl group, carbonylative coupling, intermolecular followed by intramolecular double Grubbs olefin metathesis, Knoevenagel condensation, and Diels–Alder cycloaddition. The belt-shaped structure and trans-stereochemistry of the alkene moieties of the octaphenyl[4.4]triphenylparacyclophane and a synthetic intermediate, 2,21-dioxo-11,30-diene[3.4.3.4]paracyclophane, were determined by X-ray crystallography. The synthetic methodology leading to octaphenyl[4.4]triphenylparacyclophane is applicable for the synthesis of substituted triphenylparacyclophanes and possibly their corresponding bis-hexabenzocoronenylparacyclophanes via a Scholl–Mullen oxidative aryl-aryl coupling reaction.

Intermolecular Three-Component Synthesis of Fluorene Derivatives by a Rhodium-Catalyzed Stitching Reaction/Remote Nucleophilic Substitution Sequence

A three-component synthesis of multisubstituted fluorene derivatives has been developed by devising a rhodium-catalyzed stitching reaction/remote nucleophilic substitution sequence. A variety of nucleophiles can be installed in the second step including both heteroatom and carbon nucleophiles. An efficient synthesis of 5H-benzo[a]fluoren-5-ones has also been realized using N-(2-alkynyl)benzoylpyrrole as the reaction partner through a new reaction pathway.

Odd [n]Cumulenes (n = 3, 5, 7, 9): Synthesis, Characterization, and Reactivity

In comparison to the omnipresent two- and three-dimensional allotropes of carbon, namely, graphite and diamond (as well as recent entries graphene, carbon nanotubes, and fullerenes), a detailed understanding of the one-dimensional carbon allotrope carbyne is not well established, and even the existence of carbyne has been a matter of controversy over the past decades. Composed of sp-hybridized carbon, carbyne could potentially exist in two forms, either as a polyyne (alternating single and triple bonds, expected to show a semiconducting behavior) or as a cumulene (all carbon atoms are connected via double bonds, predicted to show metallic behavior). Although a number of publications are available on the hypothetical structure and properties of carbyne, specific knowledge about its physical and spectroscopic characteristics is still unclear. In order to predict the properties of carbyne, the synthesis and study of model compounds, namely, polyynes and cumulenes, has been a promising avenue. The synthesis of polyynes has been extensively explored in the last decades, culminating with the isolation of a polyyne with 22 acetylene units, which allows extrapolation to the properties of carbyne. Extended cumulenes, on the other hand, have remained much less well-known, and specific studies of properties versus molecular length are quite limited. A limiting factor to the study of [n]cumulenes has been their dramatically increased reactivity, especially in comparison to polyynes of comparable length. For example, most known [7]cumulenes can only be handled in solution, while the polyynes of equivalent length (i.e., a triyne with three acetylene units) are quite stable. [9]Cumulenes are the longest derivatives studied to date. In this Account, we describe our efforts to design and synthesize odd [n]cumulenes (i.e., n = 3, 5, 7, 9) that are sufficiently persistent under ambient conditions to allow in depth characterization of physical and spectral properties. This goal has been achieved through modification of the end-capping groups by increasing the steric bulk and thereby shielding the reactive cumulene framework to provide stable [7]- and [9]cumulenes. An alternative route to stabilization is accomplished via encapsulation of the cumulene skeleton in a macrocycle, that is, formation of cumulene rotaxanes. The new sterically encumbered cumulenic products are reasonably stable under normal laboratory conditions, although some readily undergo cycloaddition reactions to give interesting products. We have explored preliminary trends for the reactivity of long [n]cumulenes. Finally, trends in the series of [n]cumulene model compounds are now discernible, including a thorough consideration of bond length alternation (BLA) in long [n]cumulenes using X-ray crystallographic analyses, as well as electronic properties via UV-vis spectroscopy and cyclic voltammetry.

Thermal dimerization of [n]cumulenes (n = 5, 7, 9)

The dimerization of tetraaryl[n]cumulenes gives a radialene (n = 5) and expanded radialenes (n = 7 or 9).

Synthesis and properties of long [n]cumulenes (n ≥ 5)

This review highlights the synthesis and reactivity of longer [n]cumulenes (n ≥ 5) and summarizes their electronic and structural properties.

Heuristic chemistry--elimination reactions

Besides additions and substitutions, elimination reactions play a fundamental role in organic synthesis. However, conceptual reviews of known 1,x-elimination patterns that go beyond the typical olefin-forming 1,2-eliminations are scarce. To develop a broader understanding of elimination reactions, we follow a heuristic approach and deduce recurrent reaction patterns from traditional and specific elimination reactions. Our work demonstrates that 1,x-elimination reactions and their outcomes can be easily rationalized by defined mnemonic categories.

Heuristic chemistry--addition reactions

Organic chemistry is often considered a difficult subject requiring great effort to achieve expert status. Only once this level has been reached, judging, deciding, and recalling chemical information will be fast and guided by heuristic strategies. These principles, used intuitively, improve the efficiency and speed of problem-solving and decision-making procedures. Establishing these strategies in teaching and learning chemistry should significantly help students acquire well-structured procedural knowledge early in their education. In an attempt to improve conceptual thinking in teaching organic chemistry, in this contribution we develop a heuristic view of addition reactions and propose a new way of perceiving this class of organic reactions.