Investigation into the Stereoelectronics of the Thiol-Promoted Fragmentation of Ylidenenorbornadienes (YNDs)

A simplified synthesis of 6,6-diaryl-substituted fulvenes from substituted benzophenones is reported, providing excellent yields for all substrates except those with strong electron-donating substituents. Symmetric diaryl-ylidenenorbornadienes (YNDs) were prepared via the reaction between 6,6-diaryl-substituted fulvenes and dimethyl acetylenedicarboxylate (DMAD). These YNDs were then reacted with propanethiol (PT) to afford four isolable diastereomers of YND-PT adducts, with trends in diastereomer ratios being attributed to the directing capabilities of aryl substituents during thiolate addition and protonation steps. Linear-free energy correlations were obtained for the retro-[4 + 2] fragmentation of substituted diaryl-YND-PTs. Electron-donating groups on the phenyl substituents on the ylidene of YNDs increased fragmentation rates. The half-lives of fragmentation typically increased by approximately 10-fold from the most electron-donating substituent (NMe₂) to the most electron-withdrawing (NO₂). The individual diastereomers were observed to fragment at markedly different rates. In one substrate, the fastest diastereomer fragmented 100 times faster than the slowest. A series of thiophenols (TPs) were used as the nucleophiles with a model YND system and provided a separable mixture of three diastereomers of YND-TP adducts. Linear-free energy correlations were obtained for the retro-[4 + 2] fragmentation of YND-TPs. Electron-withdrawing substituents on thiophenyl ethers were observed to increase the fragmentation rates of all diastereomers.