Cyclopropyl Carbinyl Research Articles

MINDO-Forces calculation of molecular geometries and reaction paths

A new method for the minimization of molecular energies is described, based on the Murtagh-Sargent procedure and the MINDO/2(3) semiempirical MO method. The derivative of energy is calculated according to the Pulay's Force method. This method was applied to the calculation of heats of formation and geometries of different organic molecules. The results agree well with the experimental and theoretical values known in the literature. The MINDO-Forces method was applied to the calculation of the internal rotation barrier of the benzyl carbonium ion. The calculated value 18.79 kcal/mol agrees well with the theoretical values known in the literature. The MINDO-Forces method was applied to the calculation of the internal rotation barrier of cyclopropyl carbinyl cation. The calculated rotation barrier, 21.28 kcal/mol is in agreement with the known theoretical values and with the expectation based on the NMR measurement of the barrier height in cyclopropyl dimethyl-carbinyl cation.

Acid Catalyzed Rearrangements of Conjugated Cyclopropyl and Epoxy Ketones

A series of spiro[n.2]alkan-2-ones and their 6 and 7 oxa derivatives were subjected to acid catalysis. In the former series, products arising from hydroxyl(cyclopropyl)carbinyl cations are observed, while in the latter series epoxide oxygen protonation appears to be the dominant reaction pathway. A novel oxavinylcyclopropane rearrangement is reported.

Isomerization of vinylcyclopropanes by a homogeneous rhodium catalyst

Vinyl and phenyl cyclopropane derivatives are shown to be generally reactive in the presence of di-μ-chlorotetracarbonyldirhodium (I) in chloroform solution to yield mainly trans-1,3-pentadiene compounds. The most unusual feature of the reaction is that the vinylcyclopropanes possess an uncatalyzed “thermally allowed” concerted reaction path (to yield cyclopentene derivatives), but the rhodium catalyst alters the route and yields a new group of products by a step-wise mechanism. The reaction is subject to steric and conformational effects which influence the rate of the reaction. The most reasonable mechanism is one involving a rhodium coordinated cyclopropyl carbinyl cation.

A test for cyclic conjugation involving the cyclopropyl carbinyl cation

A test for cyclic conjugation involving the cyclopropyl carbinyl cation

Electron-Diffraction Study of the Structure and Internal Rotation of Cyclopropyl Carboxaldehyde

The structure and conformational behavior of cyclopropyl carboxaldehyde in the gas phase have been investigated by electron diffraction. Rotational isomers did not correspond to the trans and gauche isomers observed in a parallel study of the isopropyl derivative, but rather to s-trans and s-cis. The latter isomers were present in nearly equal concentrations. The rotational barrier appears to be greater than 2.5 kcal/mole. Results are interpreted in terms of Walsh's π-electron picture, and various comparisons of cyclopropyl and vinyl groups are made and discussed. The diffraction experiment yielded normal values for bond angles and amplitudes of intramolecular vibrations. No tendency was detected for the aldehyde to assume a nonclassical structure of the sort postulated for its analog, the cyclopropyl carbinyl cation.

Some theoretical observations on cyclopropane

Roald Hoffmann Department of Chemistry, Cornell University, Ithaca, N. Y. (Received 21 August 1965) The pecularities of the electronic structure of cyclopropanes have often been noted. In a previous communication (1) extended Hiickel calculationr were used to obtain a molecular orbital representation of cyclopropane in essence very similar to the Walsh picture of bonding in this molecule (2). Also studied was the preferred edge-on protonation of the molecule and the favored “bi.sected” (I) conformation of a classical cyclopropyl carbinyl cation (3). Some further results of a predictive nature obtained by the name method

Extended Hückel Theory. IV. Carbonium Ions

The conformations, relative stabilities, and electronic distribution of a sample of the more important carbonium ions and positively charged hypothetical transition states are examined. The species studied include the methyl, ethyl, isopropyl, tertiary butyl, and higher alkyl carbonium ions; protonated ethylene, acetylene, benzene, cyclopropane; the cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl carbinyl, allyl, and benzyl cations; the carbonium ions based on norbornane, norbornene, norbornadiene. Significant charge delocalization for a classical carbonium ion geometry is observed—the extent of this phenomenon is wider than anticipated. For the alkyl carbonium ions it is shown that the order of stabilities may be obtained from a calculation in which the polarity of the C–H bond is C—–H+. Protonated ethylene and acetylene show local minima for a symmetrical complex, but with rearrangement to an unsymmetrical cation favored. Protonated cyclopropane prefers an unsymmetrical three-center bonded structure, protonated benzene stabilizes in the familiar benzenium. The orientation of the empty carbonium p orbital with respect to other π-type orbitals determines the conformation in cyclopropyl carbinyl, benzyl, and allyl. The peculiar nature of the cyclopropane electron distribution is studied. The carbonium ions based on the bicyclo[2.2.1]-heptane structure show some nonclassical features; confirming experimental conclusions, the unusual 7-norbornadienyl cation is calculated to prefer an unsymmetrical geometry. Difficulties in applying the extended Hückel theory to charged species make some of the conclusions from the calculations less certain.