Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1966 Syntheses and eliminations of cyclopentyl derivatives David John Rausch Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Organic Chemistry Commons Recommended Citation Rausch, David John, "Syntheses and eliminations of cyclopentyl derivatives " (1966). Retrospective Theses and Dissertations. 2875. https://lib.dr.iastate.edu/rtd/2875 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. This dissertation has been microfilmed exactly as received 66—6996 RAUSCH, David John, 1940- SYNTHESES AND ELIMINATIONS OF CYCLOPENTYL DERIVATIVES. Iowa State University of Science and Technology Ph.D., 1966 Chemistry, organic University Microfilms, Inc., Ann Arbor, Michigan SYNTHESES AND ELIMINATIONS OF CYCLOPENTYL DERIVATIVES by David John Rausch A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Major Subject: Organic Chemistry Approved : Signature was redacted for privacy. Signature was redacted for privacy. Head of Major Department Signature was redacted for privacy. Iowa State University Of Science and Technology Ames, Iowa 1966 ii TABLE OF CONTENTS VITA INTRODUCTION HISTORICAL Conformation of Cyclopentanes Elimination Reactions RESULTS AND DISCUSSION Synthetic Elimination Reactions EXPERIMENTAL Preparation and Purification of Materials Procedures and Data for Beta Elimination Reactions SUMMARY LITERATURE CITED ACKNOWLEDGEMENTS iii VITA The author was born in Aurora, Illinois, on October 24, 1940, to Mr. and Mrs. John Rausch. He attended high school at Marmion Military Acad­ emy in Aurora and was graduated in June, 1958. In September of the same year he enrolled at St. Procopius College at Lisle, Illinois. The author was awarded an undergraduate National Science Foundation research grant during the summer of 1961 and academic year 1961-62. In June, 1962, he received his Bachelor of Science degree from St. Procopius College. In July of 1962, the author began his graduate studies at Iowa State University as a teaching assistant in organic chemistry under Dr. C. H. DePuy. In August of that year he married Gertrude Miller. During his residence in graduate school, his family increased with the addition of a girl in November, 1963 and a boy in July, 1965. After teaching for two years, the author received a Shell Companies Foundation fellowship in September, 1964. The author was graduated with a Ph.D. degree from Iowa State University of Science and Technology in February, 1966. The author has accepted a post-doctoral position to further his stud­ ies under Professor H. L. Goering at the University of Wisconsin at Madi­ son. INTRODUCTION Many Investigations have been carried out in the last 10 to 15 years seeking to elucidate a detailed three-dimensional structure of substi­ tuted cyclohexyl compounds. At the same time, relatively little consider­ ation has been given to the correlation of conformation with the physical and chemical properties in cyclopentane derivatives. Two general methods have been employed in conformational analysis to determine the preferred spatial arrangements of organic molecules. Contained in the first cate­ gory are physical methods such as electron and X-ray diffraction measure­ ments, calorimetric determination of energy contents of compounds, and measurements of dipole moments and spectra (Raman, infrared, microwave, ultraviolet, nuclear magnetic resonance, and optical rotatory dispersion). In the second category are the chemical methods. These concern experi­ ments from which predictions on the confoirmation of a molecule can be made, based on certain reactions of model compounds. Thus far only a few of the many available reactions have been investigated for their usefulness in conformational analysis. The primary purpose of this work was to synthesize substituted cy- clopentyl derivatives and determine a relationship between the conforma­ tions of the five-membered rings using as criteria the chemical reactiv­ ity and product ratios of the derivatives. Equilibration of the substi­ tuted alcohols, epoxidation of the olefins with subsequent reduction to tSh alcohols, hydroboration of the olefins, vapor phase pyrolysis of the acetates, and base catalyzed elimination and solvolysis of the de­ rived bromides and £-toluenesulfonate8 were reactions studied to deter­ 2 mine the significance of conformational effects on reaction mechanisms. The effect of the base/solvent system, effect of the leaving group, and effect of changing the dihedral angle between the beta-proton and the leaving group on the transition state in the base catalyzed elimination reactions (E2) in these systems were also studied. 3 HISTORICAL Conformation of Cyclopentanes Originally, the cyclopentane ring was believed to be a planar mole­ cule, since in this form the internal angle of the regular pentagon (108°) is very close to the tetrahedral carbon bond angle (109°28*) (1). Planar cyclopentane would, however, have five eclipsed methylene groups, leading to a bond opposition strain (2) (Fitter strain) of 14.0 kcal./ mole (3). The origin of Pitzer strain lies in a repulsion of neighboring, nonbonded atoms. If one compares the heat of combustion per CH2 group of cyclopentane (158.7 kcal./mole) with that of cyclohexane (157.4 kcal./mole) or the noirmal value per CB^-group in open-chain aliphatic compounds (157.5 kcal./ mole), a definite increase of the heat of combustion of cyclopentane is observed. From the heats of combustion data a higher energy content is estimated for cyclopentane in comparison to open chain compounds which amounts to 6-7 kcal./mole (4). This higher energy content cannot be re­ lated to Baeyer strain (angle strain), but seems to result from the nearly completely eclipsed hydrogen atoms of the adjacent methylene groups. In order to minimize this eclipsing strain, cyclopentane assumes a puckered form, the increase in angle strain thus arising (4.3 kcal./ (1) Figure 1. Different representations of the two forms of puckered cyolopentane. Envelope Half-chair 6 mole) being more than compensated for by the considerable drop In eclips­ ing strain (7.8 kcal./mole) (5). The over-all reduction In strain, esti­ mated (6) at about 3.6 kcal./mole, brings the calculated residual strain In cyclopentane down to 10.4 kcal./mole. The puckering of cyclopentane has been detected experimentally by several methods, of which the earliest (7) was based on entropy measure­ ments, and the most recent (8) on electron diffraction. Recent calcula­ tions have shown (6, 9) that the energy minimum of the cyclopentane mole­ cule is attained when one carbon atom twists out of the plane by O.sX. This puckering is not fixed byt rotates around the ring by a successive up and down motion of the five methylene groups in what has been termed "pseudorotation" (5). The nonplanar cyclopentane cannot be represented by a single structure. In the course of this pseudorotation the internal energy of the molecule changes by less than RT (600 cal./mole at room temperature), so that, unlike the chair form of cyclohexane, no definite energy minima and maxima come into evidence. Of the various puckered forms of the cyclopentane ring the "enve­ lope" form (Cg) and the "half-chair" form (C2) have the greater symmetry and do not vary greatly in their energy levels. Representations of these forms are given in Figure 1, One carbon atom projects out of the plane of the other four in the envelope form, whereas in the half-chair form the three neighboring carbon atoms lie in one plane, while the other two are twisted such that one lies above and the other below this plane. The numbers in the third diagram Indicate the displacement in angstroms of the atoms above or below the plane of the paper (10). From the diagram three types of bonds can be recognized 1.) the classical axial (a) and 7 equatorial (e) bonds found In cyclohexane, 2.) the quasiaxlal (a*) and quaslequatorlal (e*) bonds, and 3.) the so-called blsectlonal bonds (b) which take up a position between the axial and equatorial bonds and form an angle of 54°44' with the plane of the ring (4). Whereas the nonsubstituted cyclopentane has free pseudorotation, either the envelope or half-chair conformation is stabilized by replace­ ment of a hydrogen atom with a bulkier substituent. The energy barrier for pseudorotation then is not zero. Infrared spectra of halocyclopen- tenes indicated pseudorotation was not present in this series (11). The half-chair form of the five membered ring is preferred by molecules whose energy barrier between neighboring bonds is lowered by substitution. In cyclopentanone, methylenecyclopentane, and heterocyclic analogs of cyclo­ pentane, such as tetrahydrofuran, pyrrolidine and tetrahydrothlophene, two pairs of hydrogen-hydrogen ecllpsings are removed. These molecules are most stable in the half-chair form with the sp^ carbon or heteroatom located on the axis of symmetry. The conformations of these compounds were confirmed by calculations of the enthalpy and free-energy change in the cyclopentanone —- cyclopentanone cyanohydrln equilibrium (6, Î2).