Small Ring Hydrocarbons Research Articles

Conquering the Synthesis and Functionalization of Bicyclo[1.1.1]pentanes.

Bicyclo[1.1.1]pentanes (BCPs) have become established as attractive bioisosteres for para-substituted benzene rings in drug design. Conferring various beneficial properties compared with their aromatic "parents," BCPs featuring a wide array of bridgehead substituents can now be accessed by an equivalent variety of methods. In this perspective, we discuss the evolution of this field and focus on the most enabling and general methods for BCPs synthesis, considering both scope and limitation. Recent breakthroughs on the synthesis of bridge-substituted BCPs are described, as well as methodologies for postsynthesis functionalization. We further explore new challenges and directions for the field, such as the emergence of other rigid small ring hydrocarbons and heterocycles possessing unique substituent exit vectors.

Energy additivity in branched and cyclic hydrocarbons

This paper considers the degree to which branched hydrocarbons obey a group additivity scheme for energy and population, extending the study of the known experimental and theoretical transferability of the methyl and methylene groups of the linear hydrocarbons. The chemical groups are defined and their properties are determined using the quantum theory of atoms in molecules (QTAIM). The calculations are carried out with a large basis set at the restricted Hartree–Fock and MP2(full) levels of theory. The deviations from additivity, noted for small ring hydrocarbons leading to the definition of strain energy, are also investigated, showing that the QTAIM energies recover the experimental values. The particular delocalization of the electron density over the surface of the cyclopropane ring, responsible for its “homoaromatic” properties, is discussed in some detail. The calculations reported here satisfy the virial theorem as required for the atomic definition of energy. The problems associated with the use of DFT theory arising from its failure to satisfy the virial theorem are discussed with reference to the study of group transferability.

Photoinduced electron transfer reactions of chloranil with small-ring hydrocarbons. Contrasting reactivities of radical cation intermediates

Products that result from the irradiation of chloranil with phenylcyclopropane and with 1,2-diphenylcyclobutene in various solvents have been identified. The cyclopropane gave products that derive from nucleophilic attack on the small-ring radical cation that is formed on electron transfer quenching of the quinone excited triplet state. Contrastingly, the cyclobutene ring remains intact following an analogous radical cation formation, but undergoes an addition in which (formally) the quinone is added across an allylic CH bond. The results are consistent with the formation of radical ion pairs (triplet excited complexes, TECs) consisting of quinone radical anions and cations associated with the small-ring derivatives. Intra-ion-pair proton transfer and radical (ion) coupling reactions account for observation of the various products. Different pathways for photolysis of quinone and small-ring hydrocarbons in more polar media involving formation of free-radical ions were also established. The quenching of quinone excited triplet states and the appearance and decay of TEC-type and other intermediates were followed by laser flash photolysis.

Self-ignition of small-ring hydrocarbons behind reflected shock waves

Self-ignition of stoichiometric airlike argon mixtures of monocyclic and bicyclic small-ring hydrocarbons was investigated behind reflected shock waves at 1200–1600 K and 0.6 ± 0.1 MPa. The reactivity of small-ring hydrocarbons has been found to be determined by the properties of their thermal decomposition products. New high-temperature decomposition channels for spiropentane, methylenecyclobutane, and methylenecyclopropane have been discovered.

Thermolysereaktionen gespannter Verbindungen: Mechanistische Deutung und synthetische Nutzung

Thermolysis Reactions of Strained Compounds: Mechanistic Interpretation and Synthetic Use Actual applications of thermolysis reactions in organic synthesis are described with emphasis on pericyclic reactions of small ring hydrocarbons such as cyclopropanes (vinylcyclopropanes, 1,2-divinylcyclopropanes), cyclobutanes, cyclobutenes, and benzocyclobutenes. Several examples for the application of retro Diels-Alder reactions for the preparation of reactive intermediates as well as for the synthesis of interesting target molecules are given.

Structural variations in small ring hydrocarbons bridged by allenic double bonds

Variations in the length of the allenic type double bond in angularly strained molecules have been predicted by semi-empirical (IMO, MINDO/3 and MNDO) methods and by ab initio calculations. It has been found that the CC bond adjacent to the ring systems in ethylidenecyclobutane (3) and ethylidenecyclopropane (4) is significantly shorter than the corresponding bond in 1,1-dimethylallene (2). In the molecules substituted at both ends, i.e. cyclobutylidenemethylenecyclobutane (6), cyclopropylidenemethylenecyclobutane (7) and cyclopropylidenemethylenecyclopropane (8), the shrinkage of each of the two CC bonds is found to be dependent only on the characteristics of the adjacent substituent. The bond length variations can be interpreted in terms of changes in hybridization at the bridgehead carbon atoms and by the energy partitioning technique. Conjugative interaction of the small ring with the adjacent in-plane p Π-orbitals of the double bond is also operative, but is much less important than the rehybridization effect. The same holds for the hyperconjugation of the double bond perpendicular to the small ring and the pseudo-π orbitals of the CH 2 groups.

Molecular Mechanics Calculations in Organic Chemistry: Examples of the Usefulness of this Simple Non‐Quantum Mechanical Model

AbstractVarious problems arising in experimental organic chemistry can be clarified by molecular mechanics or force field calculations: molecular dynamics (conformational analysis and internal rotation), the search for the most stable isomer of various polycyclic hydrocarbons, reactivity calculations, including solvolysis of bridgehead substituted systems. Such calculations are shown to be useful in elucidating the mechanism of multistep carbonium ion rearrangement, for predicting the structure and stability of anti‐Bredt olefins, and also offer an explanation for the selectivity observed in the hydrogenolysis of strained polycyclic smallring hydrocarbons, for the identification of long bonds and electronic effects, for the analysis of late transition states, and for the product distribution in complex reactions.

Configuration Analysis and Electronic States Properties

The excited-states electronic properties of unsaturated small-ring hydrocarbons with four- and six-π-electrons have been studied by the “Molecules in Molecules” method and compared with experimental data and PPP results interpreted through a “configuration analysis” procedure. The results show that the MIM method is applicable with good reliability.

Gold catalysed rearrangements of strained small ring hydrocarbons

Gold catalysed rearrangements of strained small ring hydrocarbons

Hybridization in highly strained small ring hydrocarbons. V. Methylene and isopropylene substituted spiropentanes

Using the method of maximum overlap, the hybrids, the bond overlaps and the deviation angles as well as the interhybrid angles have been calculated for several methylene and isopropylene substituted spiropentanes. Considerably different hybrids are found for the spiro carbon when the two C 3 rings have a different number of substituants. They deviate appreciably from the usually assumed sp 3 type, being as different as sp 2.70 and sp 3.34. A brief discussion and comparison of some experimental molecular properties and the corresponding calculated parameters is presented. A discussion of factors which influence the hybridization in small rings has been given with some critical remarks on the reliability of predictions of molecular geometry (bond lengths) and molecular shapes (bond angles) by the maximum overlap method.

Hybridization in highly strained small ring hydrocarbons: IV. Unsaturated polycyclopropyl spiranic compounds

The electronic structure is considered of three strained unsaturated cyclopropyl spiranic molecules. These molecules have some intrinsic interest since the stringent asymmetric perturbations of small spiro-rings can lead to unusual hybrids. The presence of cyclopropyl substituents as well as exo or endo CC double bonds introduces considerable additional strain to the already highly strained four membered rings. Comparison of hybrids and bond overlaps with other molecules containing CC double bonds is given and a qualitative correlation between CC bond overlap and v(CC) vibrational stretching frequencies is discussed.

Configuration analysis and electronic states properties

The excited-states electronic properties of unsaturated small-ring hydrocarbons with four- and six-π-electrons have been studied by the “Molecules in Molecules” method and compared with experimental data and PPP results interpreted through a “configuration analysis” procedure. The results show that the MIM method is applicable with good reliability.

Hybridization in highly strained small ring hydrocarbons—III : Unsaturated spiro-compounds

Hybridization in spiro[2,4]hepta-4,6-diene (I), spiro[2,4]hepta-2,4,6-triene (II) and spiro[4,4]-nona-2,4,6,8-tetraene (III) is considered by applying the method of maximum overlap. The hybrids of the spiro carbon in I and II differ considerably from the sp 3-case. The presence of the carbon rings of different size leads to nonequivalent hybrids for the central carbon: sp 2·60 and sp 3·49 and sp 2·58 and sp 3·53 for I and II, respectively. The calculated exponents n of the sp n-hybrids for CH bonds are correlated with the τ values (chemical shifts) using a previously observed linear correlation τ = 5(n − 1). The reasons for some deviations from the above correlation are discussed.

Small-Ring Compounds. X. Molecular Orbital Calculations of Properties of Some Small-Ring Hydrocarbons and Free Radicals1

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSmall-Ring Compounds. X. Molecular Orbital Calculations of Properties of Some Small-Ring Hydrocarbons and Free Radicals1John D. Roberts, Andrew Streitwieser Jr., and Clare M. ReganCite this: J. Am. Chem. Soc. 1952, 74, 18, 4579–4582Publication Date (Print):September 1, 1952Publication History Published online1 May 2002Published inissue 1 September 1952https://doi.org/10.1021/ja01138a038RIGHTS & PERMISSIONSArticle Views632Altmetric-Citations153LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (498 KB) Get e-Alerts Get e-Alerts