Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1992 The generation and fate of strained, unsaturated, bicyclic hydrocarbons Nathan L. Anderson Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Organic Chemistry Commons Recommended Citation Anderson, Nathan L., "The eg neration and fate of strained, unsaturated, bicyclic hydrocarbons " (1992). Retrospective Theses and Dissertations. 10092. https://lib.dr.iastate.edu/rtd/10092 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]. INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. 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Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6" x 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. University Microfilms international A Bell & Howell Information Company 300 North Zeeb Road. Ann Arbor. Ml 48106-1346 USA 313/761-4700 800/521-0600 Order Number 9311472 The generation and fate of strained, unsaturated, bicyclic hydrocarbons Anderson, Nathan L., Ph.D. Iowa State University, 1992 UMI 300N.ZeebRA Ann Arbor, MI 48106 The generation and fate of strained, unsaturated, bicyclic hydrocarbons by Nathan L. Anderson A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of the Requirement for the Degree of DOCTOR OF PHILOSOPHY Department: Chemistry Major: Organic Chemistry Approved: Signature was redacted for privacy. In Chai^ of Major Work Signature was redacted for privacy. Foiythe M^jor Department Signature was redacted for privacy. For the Graduate College Iowa State University Ames, Iowa 1992 ii TABLE OF CONTENTS GENERAL INTRODUCTION 1 PAPER I. THE GAS-PHASE GENERATION AND DIMERIZATION OF BICYCLO[2.2.2]OCT-l-ENE 2 LITERATURE REVIEW 3 RESULTS AND DISCUSSION 25 EXPERIMENTAL 38 REFERENCES 53 PAPER 2. THE GENERATION AND REARRANGEMENTS OF BICYCLIC ALKYLIDENE CARBENES 56 LITERATURE REVIEW 57 RESULTS AND DISCUSSION 75 EXPERIMENTAL 100 REFERENCES 131 GENERAL SUMMARY 135 GENERAL REFERENCES 136 ACKNOWLEDGMENTS 137 1 GENERAL INTRODUCTION Format explanation Two separate studies in bicyclic hydrocarbon rearrangements will be included in this dissertation. Each paper has its own literature review, discussion, and references and is, in that respect, a complete and separate entity. A general summary and references for the introduction follow the second paper. Subject matter The first paper in this dissertation is a study of the gas-phase generation of bicyclo[2.2.2]oct-l-ene and the structural determination of its dimers. The literature review gives a historical perspective of Bredt olefins^ in general and studies of the [2.2.2] system 1 in particular. Additionally, approaches to generating bicyclo[2.2.2]octa-l,2-diene, a "Bredt aliéné" are discussed. The second paper is a study of the reactions of bicyclic vinylidenes. The literature review is a survey of vinylidenes and their reactions.^ Ring contraction and ring expansion reactions^ are discussed in terms of ring strain. Synthetic approaches to alternative vinylidene precursors are discussed. 2 PAPER I. THE GAS-PHASE GENERATION AND DIMERIZATION OF BICYCLO[2.2.2]OCT-l-ENE 3 LITERATURE REVIEW Since Bredt's studies of the dehydrobromination of constrained hydrocarbons in the first quarter of this century^ and his recognition of the importance of geometric deformation necessary for a double bond to exist at the bridgehead of systems such as camphane and pinane, the study of bridgehead olefins has received special attention. In recognition of the importance of Bredt's work, olefins terminating at the bridgehead of a strained bicyclic system have come to be known as Bredt (or anti-Bredt) olefins and the general rule forbidding such olefins to be known as Bredt's rule. A A typical statement of Bredt's rule is: "On the basis of our conceptions of the positions of atoms in space, in the systems of the camphane and pinane series, as well as in similarly constituted compounds, a carbon double bond cannot occur at the bridging positions A and B (above) of the carbon bridge."2 Subsequent to the publication of the general rule several studies ensued with the purpose being to more clearly define the limits of "similarly constituted compounds ". In his 1950 review Fawcett^ attempted to establish a 4 lower limit on the system size in which a bridgehead double bond would be stable. Fawcett introduced the concept of the S number to predict bridgehead olefin stability. The S number is the sum of the carbons in the bridges of the bicycle, excluding the olefin carbons and bridgehead carbon (S = a + b + c). Although it was eventually proven wrong (vide infra),^ the suggestions that compounds with S>7 should be stable enough to exist as transient intermediates were important bench marks in the study of Bredt olefins. Additionally, Fawcett suggested the exclusion of zero atom bridges from the discussion of Bredt olefins. As pointed out in Warner's later review,^ there are three major types of bridgehead alkenes; A, B, and C. 5 Type A Type B Type C Type A olefins are not twisted, except in special cases, and type B olefins, with zero-atom bridges, are often excluded and are treated as a special case. In 1967 two independent syntheses of bicyclo[3.3.1]non-l-ene 1 were reported by Wiseman^ and by Marshall and Faubl.5 Both routes afforded 1 isomer 1 rather than 2. Wiseman suggested that 1 is more stable than 2 because the ira/is-double bond occurs in the eight-membered ring in 1 but in a six-membered ring in 2, relating the stability of the bicyclic systems to the simpler monocycles. 6 Wiseman and Fletcher® proposed that other Bredt olefins would also display stability and reactivity parallel to that of the corresponding trans cycloalkene. Bredt olefins with trans cyclooctenes should be the smallest systems to be isolated at room temperature, because trans cyclooctene is isolable at room temperature, trans Cycloheptene, on the other hand, is only observable at reduced temperature and "certain bicyclic compounds may possibly be isolable with the trans double bond in a seven membered ring. Wiseman's stability criterion seems to be the simplest and most generally applicable. A shortcoming of Wiseman's system is that it fails to predict differences in reactivity of isomeric compounds such as 3 and 4 below. The deformation of 3 4 double bonds which leads to instability can be quantitated by force field calculations.^ This is the approach used by Ermer"^ and by Maier and Schleyer.8 Two principle forces are at work in Bredt olefins which contribute to the decrease in bond order and decrease in stability of the double bond.9 The "pinning back" of the groups substituent to the double bond at once causes: 1) twisting of the % system from planarity (Figure 1.) and 2) out-of-plane bending of the sp2 centers (Figure 2.). These deformations are accompanied by 7 a = degree of twist of olefin Figure 1. Olefin twist IIIJIIlll P = degree of bend of olefin Figure 2. Olefin bend asymmetric rehybridization of the olefinic carbons. It is suggested^P that the bridgehead carbon will have greater sp3 character while the opposite terminus remains relatively sp2 hybridized (Figure 3.) and, at least in extreme cases, the deformed olefin may be better represented as a dipolar structure rather than a diradical.^0 8 Figure 3. Olefin rehybridization Ermer's calculations^^ give an interesting view of the separate contributions (twisting and bending) to the overall destabilization of a variety of Bredt olefins. The sum of the twisting and bending Vp is an indicator of the reactivity of the Bredt olefin, non-planar deformation energy Voop. IfV^op ^ 15 kcal mol'^ room temperature stability is suggested and Voop > 15 kcal mol-i indicates instability at room temperature. As an example consider the structure of bicyclo[2.2.2]oct-l-ene 5 predicted by Ermer's calculations (Figure 4.). Voop is calculated to be 35.1 kcal mol 1 and V^ and Vp are calculated to be 17.9 kcal mol-l and 17.1 kcal mol-^, respectively. The non-planar deformation energy Voop indicates the degree to which the Bredt olefin resembles an olefiin "which has made progress on the reaction coordinate of some reaction leading to saturation of the double bond." While the separation of twisting and bending doesn't allow better prediction of the overall reactivity or stability of Bredt olefins it does give a clear view of the orbital geometry.