The synthesis and reactivity of 2-benzylidenebenzocyclobutenones and derivatives
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Jean-Claude Bradley Ph.D. Thesis at the University of Ottawa under Tony Durst (1993) Chapter 1. A brief overview of the preparation and uses of o-quinodimethanes and benzocyclobutenes is presented. Chapter 2. The preparation of benzylidenebenzocyclobutene analogs from acetylenic precursors is presented. The strategy initially involves addition of tributyltin hydride to acetylenes via free-radical or palladium catalysed processes. The resulting vinylstannanes then undergo an intramolecular Stille coupling to yield benzocyclobutenes. For (o-bromoaryl)phenylalkynones, both steps were mediated by palladium catalysis, thus leading to a convenient one-pot synthesis of 2-benzylidenebenzocyclobutenones. Chapter 3. In order to overcome difficulties in scaling up the preparation of 2-benzylidenebenzocyclobutenones as described in Chapter 2, the reaction in question is examined in greater detail. Although the problems involved were not ultimately eliminated, a greater understanding of the parameters involved points to areas for further experimentation. Attempts to prepare benzocyclobutene derivatives via a double Stille coupling strategy using a vinyldistannane are then discussed. Several other miscellaneous attempts at benzocyclobutene syntheses are also investigated. Chapter 4. A proposed scheme for utilizing 2-benzylidenebenzocyclobutenones as precursors to regioisomeric anthraquinones is presented. The key step in this scheme involves oxidative removal of the benzylidene functionality. Although this transformation was not successful, an interesting rearrangement is found to occur upon ozonolysis of various benzylidenebenzocyclobutene derivatives. Complete assignment of the $\sp $C and $\sp1$H NMR resonances of one of these rearrangement products is described using the FLOCK pulse sequence. Chapter 5. The thermolysis of aryl, vinyl and alkynyl benzylidenebenzocyclobutenols is described. Oxidative removal of the benzylidene functionality after thermolytic ring expansion of the phenyl and vinyl precursors provides anthraquinone and naphthoquinone, respectively. Chapter 6. The anionic cleavage of several benzylidenebenzocyclobutenols is presented. The stereochemistry of the benzylidene group is used as a mechanistic probe to differentiate between a carbanionic and an electrocyclic pathway for the opening of the cyclobutene ring. For all derivatives studied, the carbanionic mechanism is shown to operate. The Z to E isomerization of lithium benzylidenebenzocyclobutenoxide is postulated to proceed via a carbanionic ring opening-vinyl anion isomerization-ring closure sequence. Trapping of the putative aldehyde intermediate with excess methyl lithium supports this mechanism. Chapter 7. The photoisomerization and photodimerization of benzylidenebenzocyclobutenones is discussed. Trapping studies and laser flash photolysis experiments point to the involvement of a ketene-allene as a major intermediate in the isomerization process. Chapter 8. A method is described which allows visualization of column chromatography by use of a quartz column and addition of a fluorescent indicator to commercial adsorbents.
Transcript of The synthesis and reactivity of 2-benzylidenebenzocyclobutenones and derivatives
