Doctoral Dissertations

Spring 1998

Dissertation

Chemistry

Degree Name

Doctor of Philosophy

Abstract

I. The synthesis, trapping, and strain energy estimates for cyclic butatrienes are described. Four fundamental questions have been investigated: kinetic stability limitations, molecular strain, structural limitations, and the development of a general synthetic route applicable to different ring sizes.

1,2,3-Cyclooctatriene (9) has been generated by magnesium induced 1,2-elimination on 2-bromo-3-chloro-1,3-cyclooctadiene (4). Synthesis of the eight-membered ring completes the C$\sb6$-C$\sb{10}$ series of cyclic butatrienes. This substance shows moderate kinetic stability, but is readily trapped in a ($\rm\sb\pi2\sb{s}{+}\sb\pi4\sb{s}\rbrack$ cycloaddition with diphenylisobenzofuran or 2,5-dimethylfuran. The total strain energy and the strain in the butatriene moiety in cumulene 9 have been assessed by ab initio calculations. The estimates are 17.7 kcal/mol and 12.4 kcal/mol, respectively, at the MP2/6-31G*//HF/3-21G level.

1,2,3-Cycloheptatriene (0) and its isomer cyclohepten-3-yne (9) are readily accessible by magnesium induced 1,2-elimination on appropriate precursors. The calculated total strain energy in the cyclic butatriene, and in the seven-membered enyne are 31.8 kcal/mol and 30.8 kcal/mol, respectively, at the MP2/6-31G*//HF/3-21G level.

5,5-Dimethyl-1,2,3-cyclopentatriene (5) remains elusive. Five likely precursors have been prepared and studied; no evidence for the existence of this compound was found.

Intramolecular vinylidene coupling has been explored as a possible ring-size-independent synthetic route to cyclic butatrienes. In the cases of tetrabromo-olefins 22 and 27 a 1,2-migration occurs faster than the ring closure which would give the cumulene.

II. A general route to cleanly photogenerate reactive carbenes by photolysis of cyclopropanated phenanthrenes has been studied. The adduct of dichlorocarbene and phenanthrene has been modified to produce shelf-stable substances that serve as photochemical precursors to vinylcarbene, and acyclic and cyclic vinylidenes.

Low temperature irradiation of 7-endo-ethylenedibenzo (a;b) bicyclo (4.10) heptane (41) at 254 nm cleanly gives phenanthrene (35) and vinylcarbene (36); the latter rapidly rearranges to cyclopropene. The strained alkene is efficiently trapped by cycloadditions with cyclopentadiene or diphenylisobenzofuran.

Fragmentation of a C$\sb9$ vinylidene precursor 58 leads to efficient formation of 1-nonyne (60) by 1,2-shift.

Cyclobutylidenecarbene (63) rearranges readily to cyclopentyne (3). Cycloalkyne 3 is trapped by cyclohexene to give tricyclo (6.3.0.0$\sp{2,7}\rbrack$undec-1(8)-ene (68) and a cyclohexyl derivative 72 which has not been previously described.

Rearrangements of cyclopentylidenecarbene (63) and cyclohexylidenecarbene (64) are not observed. These carbenes are trapped in (2+1) cycloadditions with cyclohexene.

COinS