Joseph J. Gajewski

Deuterium kinetic isotope effects in the 1,4-dimethylenecyclohexane boat Cope rearrangement

In order to examine the extent of bond making in the boat-like 3,3-sigmatropic shift transition states, trans-2,3-dimethyl-1,4-dimethylenecyclohexane (T) and its exomethylene tetradeuteria derivative (TXD) were prepared. The 3,3-shift of TXD at 305/sup 0/C results in interconversion of starting material, 5,5,6,6-tetradeuterio-trans-2,3-dimethyl-1,4-dimethylene-cyclohexane (TND), and 2,2,3,3-tetradeuterio-anti-1,4-diethylidenecyclohexane (AD). A kinetic analysis of the first-order rate equations for the three-component system in both protio and deuterio species by numerical integration of the data and simplex minimization of the rate constants with symmetry and the assumption of no equilibrium or kinetic isotope effect on the TND-AD reaction gives a bond making kinetic isotope effect of 1/1.04 (0.04). The equilibrium isotope effects observed are 1/1.16 (0.04) so that the extent of bond formation in this boat-like bicyclo(2.2.2)octyl transition state is roughly 25%, a value to be compared with ca. 67% in chair-like acyclic 3,3-shift transition states. This rules out significant intervention of a bicyclo(2.2.2)octane-1,4-diyl intermediate or transition state. 30 references, 6 figures, 4 tables.

General molecular mechanics approach to transition metal complexes

Transition metal complexes have been treated by a molecular mechanics approach using an MM2‐like valence force field for the ligands and a Urey–Bradley force field about the metal. Metal–ligand atom bonds are parameterized with dependence on the metal covalent radius. The current metal–ligand distances and covalent radii were determined by a Simplex minimization and visual optimization of the deviations in bond distance between 230 structures from the Cambridge Crystallographic Structure Database and the force field calculated values. Moderately good reproducibility of structures is obtained with a variety of metal types with different oxidation states, and degree of unsaturation and with square‐planar compounds. The current model also includes coordination of π ligands to the metal center with a 1/r6 attractive term whose minimum is 25 kcal/mol per p orbital at a distance dependent on the covalent radius of the metal and an angular attentuation factor. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1167–1178, 1998

Cross-conjugated biradicals: Kinetics and kinetic isotope effects in the thermal isomerizations of cis- and trans-2,3-dimethylemethylnecyclopropane, cis- and trans-3,4-dimethyl-1,2-dimethylenecyclobutane, and of 1,2,8,9-decatetraene

Abstract cis- and trans - 2,3 - Dimethylenemethylenecyclopropane ( C and T ) interconvert at 160.0° with a small normal kinetic isotope effect (KIE) when the exo-methylene is deuterated, but the 1,3-shift products, 2-methylethylidenecyclopropane, show a large normal KIE, 1.35 and 1.31, when formed from C and T , respectively. This data can be interpreted in terms of either parallel reactions or a common trimethylenemethane diradical intermediate formed with a normal KIE of 1.11 and closing to 1,3-shift product with a normal KIE of 1.29 due to the effect of deuterium in the required 90° rotation of the exo-methylene carbon. The kinetics of the thermal 1,3- and 3,3-shifts of cis- and rans-3,4-dimethyl-1,2-dimethylenecyclobutane ( CB and TB ) were determined in a flow reactor. The first order rate constants are log kCB (sec−1) = 13.7 − 42,200/2.3 RT and log kTB (sec−1) = 13.6 − 41,900/2.3 RT (Ea in kcal/m) which compare favorably to that from the parent hydrocarbon. 1,2-dimethylenecyclobutane, after reasonable correction for dimethyl substitution. Rearrangement of TB and its bis(dideuteriomethylene) derivative at 230.0° revealed a normal KIE of 1.08. This KIE could be interpreted in terms of either a methylene rotational isotope effect in a concerted reaction or formation of a bisallyl diradical with the expected normal rotational IE on closure to the 1,3-shift product of 1.12 with no IE in the ring opening when the result is corrected for return of the biradical to starting material. The kinetics of intramolecular 2 + 2 cycloaddition of 1,2,8,9-decatetraene were determined in a flow reactor. The first order rate constant is log k(sec−1) = 9.4 − 30,800/2.3 RT (Ea in kcal/m). These energetics are compared with those of other 2 + 2 cycloadditions. The major product is 3,4-dimethylenecyclooctene ( DC ) which is also found from the minor product, cis-7,8-dimethylenebicvyclo[4.2.0]octane ( CO ), at higher temperatures. The trans isomer, TO , also gives DC at about the same rate as CO .

Geometry optimization within a modified extended HÜckel formalism : modifications to the ased program

Abstract The ASED program by Anderson and coworkers was modified to include a full geometry optimizer. It was discovered that, even with this modification, the ASED program could not be used to perform geometry optimizations on organic molecules without further modification. These modifications included changing the Wolfsberg-Helmholz equation used in the ASED program back to the original equation found in the FORTICON8 program as well as changing the Slater orbital exponents. A general parameter set for C, H, O and N has been developed, which has been used to perform geometry optimizations on selected organic molecules. The calculated geometries compared reasonably with the experimental values and with previously published AM1 geometries.

Numerical methods: An implementation of difsub on an ibm-pc

Abstract An implementation of the Gear program for numerical integration, DIFSUB, has been done on an IBM-PC microcomputer. A Simplex optimization subroutine has been added and the program is parameterized for six species reacting by first order reversible processes. The accuracy and speed of the program, SMPLXKS, is compared to that of the Runge-Kutta programs that have previously been used in studies of hydrocarbon isomerizations. Two examples of the use of SMPLXKS in the study of the Cope rearrangement are given.

Thermal degenerate rearramement of 1,2-dimethylenecyclobutane iron tricarbonyl

Abstract Vapor phase pyrolysis of bis -1,2-(dideuteriomethylene) cyclobutane iron tricarbonyl at 170° resulted in hydrogen scrambling. Degradation of the rearrangement product indicated that either one exomethylene group was exchanging with one ring methylene or hydrogens were being reversibly transferred from the ring carbons to the exocyclic ones. Mechanisms that would explain either possibility involve the intermediacy of a sigma bonded iron with a π-allyl ligand.

A rotational kinetic isotope effect in the cyclopropylallene to 3-methylenecyclopentene thermal rearrangement

Abstract The deuterium kinetic isotope effect at the exo-methylene carbon of allenyl-cyclopropane in its thermal 1,3-sigmatropic shift to 3-methylenecyclopentene was found to be 1.15/D 2 at 311.5°C. This is a striking example of a rotational effect.

Photolysis of chloride precursors as a method to prepare and characterize the triplet state of fluorenyl-substituted m-xylylene diradicals

Fluorenyl substituted m-xylylene triplet diradicals are conveniently generated from the corresponding dichloride precursors upon photolysis at <100 K in 2-MeTHF glass. Under similar conditions tert-butyl substituted Schlenk type m-xylylene could not be generated from the dichloride precursor 6. Compound 6, however could be dehalogenated with zinc dust in both toluene and 2-MeTHF to produce the triplet diradical.

Possible reversible intersystem crossing in a geminal biscyclopropyl substituted m-xylylene

Abstract Reaction of 6,6-dicyclopropylfulvene with isobutylidene gives rise to products derived from a m-xylylene. Both cyclopropane ring opened monomers with o- and p-isocumene-like structures and dimers with intact cyclopropane rings are formed. Since m-xylylene would appear to be a relatively stable triplet and would not be expected to give cyclopropane ring opened monomeric products, it is suggested that steric effects retard the formation of dimers from the triplet state thereby allowing ring opening to occur out of the singlet state.

The estimation of electron affinities fromab initio 1s orbital energies

SummaryIt has been found that the electron affinities of alkoxy-radicals can be estimated using a correlation with the 1s orbital energy of the oxygen on the associated alkoxy-anion, EA=−0.64503 * (1s orbital energy) −351.58. The method assumes that the species of interest accepts the electron into an orbital which is localized on the oxygen.