Small-ring strained cycloalkynes pose valuable synthetic targets from both a theoretical and a practical standpoint, due to the considerable distortion imparted upon the two sp-hybridized carbons in the ring and the high degree of reactivity that results from this distortion. Exocyclic alkylidene carbenes such as 3 are useful precursors to cycloalkynes, due to their ease of generation by photolysis of cyclopropanated phenanthrene derivatives such as 1. Once formed, alkylidene carbenes undergo 1,2 carbon shifts, resulting in formation of the cyclic alkyne 4. As 3 is unsymmetrical, however, the 1,2 shift can involve two different carbon atoms, proceeding through pathway 3a or 3b, leading to the same alkyne. Computational studies predict that pathway 3b is favored in the case of alkylidene carbene 3, but experimental evidence to support this prediction has heretofore been lacking. Generation of the 13C-labelled alkylidene carbene 3 through photolysis of the labelled cyclopropanated phenanthrene derivative 1 provides a method for experimentally distinguishing between rearrangement pathways 3a and 3b. Results from these experiments will be presented.

Investigation into the rearrangement pathways of alkylidene carbenes with the use of 13C isotope labelling

ACS Spring 2022

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