Geometry and conformation of cyclopropane derivatives having σ-acceptor and σ-donor substituents: a theoretical and crystal structure database study

Abstract

The structures of cyclopropane rings which carry σ-acceptor or σ-donor substituents have been studied using density-functional theory (DFT), and mean bond lengths and conformational parameters retrieved from the Cambridge Structural Database. It is confirmed that σ-acceptor substituents, e.g. halogens, generate positive ring bond-length asymmetry in which there is lengthening of the distal bond (opposite to the point of substitution), and shortening of the two vicinal bonds. This is due to withdrawal of electron density from the cyclopropane 1e′′ orbitals, which are bonding for the distal bond and antibonding for the vicinal bonds. For σ-donor substituents such as SiH3 or Si(CH3)3, the DFT and crystal structure data show negative ring bond-length asymmetry (distal bond shortened, vicinal bonds lengthened), owing to electron donation into the 4e′ ring orbital, which are also bonding for the distal bond and antibonding for the vicinal bonds. The results also show that —OH substituents induce weak positive asymmetry, but that the effects of methyl or amino substituents are either non-existent or extremely small, certainly too small to measure using crystal structure information.

A Million Crystal Structures: The Whole Is Greater than the Sum of Its Parts

“…The conjugation causes the ring bond distal to the substituent to shorten and the vicinal ones to lengthen. In contrast, the reverse happens if the ring is substituted with a σ-acceptor (e.g., halogen) …”

“…In contrast, the reverse happens if the ring is substituted with a σ-acceptor (e.g., halogen). 30 (d) Comparison of Conformations in Dif ferent Environments. Raghavender compared the backbone conformations of amino acid residues in (a) small peptides bound to proteins in Protein Data Bank (PDB) 31 structures and (b) unbound peptides in CSD structures.…”

Dicationic Ring-Opening Reactions of trans-2-Phenylcyclopropylamine·HCl: Electrophilic Cleavage of the Distal (C2–C3) Bond of Cyclopropanes

“…This theoretical prediction has been confirmed by crystallographic data from cyclopropanes having strong σ-acceptor groups. For example, 1,1-difluorocyclopropane has vicinal and distal C–C bond lengths of 1.468 and 1.540 Å, respectively . The lengthened and weakened distal bond of 1,1-difluorocyclopropane is well-known for its tendency to undergo bond homolysis reactions .…”

“…For example, 1,1-difluorocyclopropane has vicinal and distal C−C bond lengths of 1.468 and 1.540 Å, respectively. 11 The lengthened and weakened distal bond of 1,1-difluorocyclopropane is well-known for its tendency to undergo bond homolysis reactions. 12 In this note, we describe the ringopening reactions of trans-2-phenylcyclopropylamine•HCl in superacid and trapping of the resulting ammonium−carbenium dication with arene nucleophiles.…”