Strained hydrocarbon structures

A crystallographic scale of acidity has been developed. Measuring the mean C—H O distances in crystal structures correlated well with conventional P a(DMSO) values. An ab initio study was able to correlate ring strain in strained hydrocarbons with hydrogen-bond acidity. ... 

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. 

As to the cation-radical version of this isomerization, there are testimonies on the transition of the norcaradiene carcass into the cycloheptatriene skeleton. Calculations at the B3LYP level shows that cycloheptatriene cation-radical is more stable than norcaradiene cation-radical by ca. 29 kJ mol (Norberg et al. 2006). Hydrocarbon ion-radicals with strained ring structures have a tendency to undergo facile rearrangement to enforce the unpaired electron delocalization and release their strain energy. 

Highly strained hydrocarbons such as quadricyclane (structure see below) may serve as high-performance aviation fuels (Hill etal., 1997). It is, therefore, important to know the environmental behavior of such compounds, particularly with respect to spills. In this context, Hill et al. (1997) have studied the hydrolysis of quadricyclane in aqueous solution at pH values between 3 and 4 as well as in soil slurries exhibiting pH values between 4.6 and 6.4. They found that in homogeneous aqueous solution at a given pH, the disappearance of quadricyclane followed pseudo-first-order kinetics, and that two major products (i.e., nortncyclyl alcohol and exo-5-norbomen-2-ol) were formed at a ratio of about 15 ... 

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. 

HBT is a strained hydrocarbon that, to avoid the van der Waals interaction between the external aromatic rings, adopts a non-planar structure. It is interesting that while Pascal characterized his product as the more stable of its two possible conformers, the D3-symmetric form I (Fig. 5) [45], the product of the palladium-catalyzed cyclotrimerization is the less stable kinetic product, the C2-symmetric conformer II [47] (which has also been prepared from a 9,10-didehydrophenanthrene-nickel complex, and characterized by X-ray crystallography [48]). 

The Jcc and Jcc couplings of allene and two sterically strained hydrocarbons, cyclopropane and cyclopropene, have been calculated by Ruden et at different levels of electronic-structure theory and compared with each other and with the experimental equilibrium constants obtained from experiment by subtracting the calculated vibrational contributions. 

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