Alkenes as alkyne equivalents, or radical fragmentations in self-terminating cascades
The application of Dynamic Covalent Chemistry (DCC) with kinetic self-sorting for the control of chemical reactivity is further amplified by combination of Bu3Sn-mediated reactions of enynes with radical fragmentations. Example shown below illustrates how radical cycle-forming cascade is terminated by C-C bond scission with the formation of resonance-stabilized CH2OR radicals.
The key “self-terminating” C-C fragmentation is assisted by a new electronic effect in radical chemistry, the three-electron through bonding (TB) interaction. This interaction provides a conduit for selective transition state stabilization in the fragmentation process.
The overall sequence renders alkenes synthetic equivalents of alkynes by coupling cyclization/rearrangement cascade with an aromatizing C-C bond fragmentation. The net result is a convenient transformation of readily available enyne reactants to α-Sn substituted naphthalenes that can serve as a launching platform for the preparation of extended distorted polyaromatics.
Alkenes as Alkyne Equivalents in Radical Cascades Terminated by Fragmentations: Overcoming Stereoelectronic Restrictions on Ring Expansions For the Preparation of Expanded Polyaromatics. Mohamed, R.; Mondal, S.; Gold, B.; Evoniuk, C. J.; Banerjee, T.; Hanson, K.; Alabugin, I. V. J. Am. Chem. Soc., 2015, 137, 6335-6349. http://pubs.acs.org/doi/abs/10.1021/jacs.5b02373.
Coupling cyclizations with fragmentations for the preparation of heteroaromatics: a new route to quinolines from o-alkenyl arylisocyanides. Evoniuk, C. J.; Ly, M.; Alabugin, I. V. Chem. Commun. 2015, 51, 12831 - 12834.
Design of Leaving Groups in Radical C-C Fragmentations: Through-Bond 2c-3e Interactions in Self-Terminating Radical Cascades. Mondal, S.; Gold, B.; Mohamed, R. K.; Alabugin, I. V. Chem. Eur. J., 2014, 20, 8664-8669. http://onlinelibrary.wiley.com/doi/10.1002/chem.201402843/abstract.