Date of Award

2019

Document Type

Honors Thesis (Open Access)

Department

Colby College. Chemistry Dept.

Advisor(s)

Dasan M. Thamattoor

Second Advisor

Greg T. Drozd

Abstract

This study aimed to generate cyclopropyl methyl vinylidene carbene and cyclopropyl phenyl vinylidene carbene via photochemical routes. The precursors to these two carbenes were synthesized from phenanthrene by a two-step procedure. However, these precursors could not be fully purified because the second step generated many side products with similar properties as the precursors. A crude sample of each precursor were photolyzed with UV light in the range of 315 to 400 nm. Vinylidene carbenes are known to rapidly rearrange into alkynes, so the formation of these cyclopropyl vinylidene carbenes was monitored through the formation of their rearrangement products, cyclopropyl methyl acetylene and cyclopropyl phenyl acetylene. Photolysis of the methyl-substituted precursor did not result in cyclopropyl methyl acetylene, either because the carbene was not generated or because the carbene failed to rearrange into the alkyne. In addition, two isomers of the methyl-substituted precursor were detected, which possibly formed via a 1,5-sigmatropic carbon shift and ring opening of the precursor. Photolysis of the phenyl-substituted precursor, in contrast, produced cyclopropyl phenyl acetylene, indicating that the carbene was successfully generated. Computational results show that both carbenes are singlets in their ground states, with singlet-triplet gaps of 45.4 kcal/mol for the methyl-substituted carbene and 41.9 kcal/mol for the phenyl-substituted carbene. Regarding the rearrangement of cyclopropyl methyl vinylidene carbene into the corresponding alkyne, the cyclopropyl shift is more favorable than the methyl shift by 3.9 kcal/mol. However, for the cyclopropyl phenyl vinylidene carbene, the cyclopropyl shift is less favorable than the phenyl shift by 5.4 kcal/mol.

Keywords

carbene, Takeda reaction, photochemistry, organic synthesis

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