Strained hydrocarbon structures reactivity

The strained hydrocarbon [1,1,1] propellane is of special interest because of the thermodynamic and kinetic ease of addition of free radicals (R ) to it. The resulting R-substituted [ 1.1.1]pent-1-yl radicals (Eq. 3, Scheme 26) have attracted attention because of their highly pyramidal structure and consequent potentially increased reactivity. R-substituted [1.1.1]pent-1-yl radicals have a propensity to bond to three-coordinate phosphorus that is greater than that of a primary alkyl radical and similar to that of phenyl radicals. They can add irreversibly to phosphines or alkylphosphinites to afford new alkylphosphonites or alkylphosphonates via radical chain processes (Scheme 26) [63]. The high propensity of a R-substituted [1.1.1] pent-1-yl radical to react with three-coordinate phosphorus molecules reflects its highly pyramidal structure, which is accompanied by the increased s-character of its SOMO orbital and the strength of the P-C bond in the intermediate phosphoranyl radical. 

With the initial synthesis of cyclopropane in 1882 , and the report of its thermal structural isomerization to propene in 1896, this simplest of cyclic hydrocarbons began its extraordinarily fruitful stimulation of fresh insights on fundamental problems in organic chemistry, ranging from basic concepts of ring strain and structural isomerism to questions of thermochemistry and reactivity and of a aromaticity And from the beginning there was controversy, extending a few years before suitably authoritative commentators confirmed the fact that cyclopropane is indeed converted thermally to propene. ... 

P. Politzer and J. S. Murray, in Structure and Reactivity (Molecular Structure and Energetics), Vol. 7. J, F. Liebman and A. Greenberg, Eds., VCH Publishers, New York, 1988, Chap. 1. Bond Deviation Indices and Electrostatic Potentials of Some Strained Hydrocarbons and Their Derivatives. 

Centropolyindanes constitute a complete family of arylaliphatic polycyclic hydrocarbons containing several indane units. Mutual fusion of the five-membered rings leads to three-dimensional, carbon-rich molecular frameworks bearing a central carbon atom, such as benzo-annelated [3.3.3]propellanes, triquinacenes, and [5.5.5.6]- and [5.5.5.5]fenestranes. In this review, the structural concept of centropolyindanes is contrasted to other fused indane hydrocarbons. Besides the syntheses of the parent centropolyindanes and recently described related indane hydrocarbons, the preparation of a large variety of bridgehead and arene substituted centropolyindanes is presented including strained, heterocyclic, and centrohexacyclic derivatives. In appropriate cases, the particular reactivity and some structural features of these unusual, sterically rigid polycyclic compounds are pointed out. 

Whether the nonplanarity of single-boundary three-dimensional aromatic hydrocarbons is reflected in predictable changes in physical or chemical properties remains to be established. Good test cases could be the rates of electrophilic aromatic substitutions (39) or the relative rates of Diels-Alder reactions (40). A comparison of the predicted rates with experimental measurements, perhaps by using the procedures of Szentpaly and Herndon (17) summarized in this book, might provide some new insights into the relationships among molecular structure, strain, and reactivity. 

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