A Mechanistic Probe for Nonstatistical Diradical Intermediates of Thermal [1,3] Sigmatropic Rearrangements in Bicyclic Vinylcyclobutane Derivatives Utilizing the CPC Radical Clock
Nocket, Anthony Joseph
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Bicyclo[4.2.0]oct-2-enes and bicyclo[3.2.0]hept-2-enes can undergo thermal [1,3] sigmatropic rearrangements to yield, respectively, bicyclo[2.2.2]oct-2-ene and bicyclo[2.2.1]hept-2-ene products, a process that we and others propose occurs via a singlet diradical intermediate. A dichotomous mechanistic debate has modulated between concerted and stepwise diradical pathways since the discovery of [1,3] carbon shifts of vinylcyclopropanes and vinylcyclobutanes almost sixty years ago. Whereas Woodward-Hoffmann Conservation of Orbital Symmetry (COS) theory has provided a theoretical rationale for a concerted pericyclic process, the classical argument for a stepwise mechanistic analysis has relied upon experimental evidence including high activation barriers and variable stereochemical outcomes. The current study attempts to utilize the cyclopropylcarbinyl to homoallylic (CPC) radical clock reaction, which has seen extensive usage in related fields for free radical detection, as a potential mechanistic probe of a singlet alkyl allyl diradical intermediate. The putative singlet diradical intermediates that result from C1-C7 bond cleavage of 7-exo-cyclopropylbicyclo[3.2.0]hept-2-ene (7-exo-CBH, exo-34) and C1-C8 bond cleavage of 8-exo-cyclopropylbicyclo[4.2.0]oct-2-ene (8-exo-CBO, exo-35) have become potential substrates for both a [1,3] sigmatropic rearrangement and a CPC radical clock reaction by way of the strategically placed cyclopropyl substituent at the migrating carbon of a bicyclic vinylcyclobutane. Due to greater ease and lower cost, the bicyclic vinylcyclobutane derivative 7-exo-CBH was first prepared as a synthetic model system and subjected to a qualitative thermal study in order to determine the degree to which association between the radical centers of its intermediate on C1-C7 bond cleavage affects the degree of [1,3] sigmatropic rearrangement and CPC radical clock progress. The exit channels of the gas phase thermal reactions of 8-exo-CBO include [1,3] sigmatropic rearrangement, [1,1] stereomutation, direct fragmentation, and formation of an isomeric “indirect” product that may have arisen via the CPC radical clock. While the latter process comprises only 2% of the overall rate constant (kobs), structural determination of this product provides strong, albeit indirect, evidence for a diradical mechanism.
Franklin and Marshall College Archives, Undergraduate Honors Thesis 2009
- F&M Theses Collection