Alkenes as alkyne equivalents in radical cascades terminated by fragmentations: overcoming stereoelectronic restrictions on ring expansions for the preparation of expanded polyaromatics

Rana K Mohamed; Sayantan Mondal; Brian Gold; Christopher J Evoniuk; Tanmay Banerjee; Kenneth Hanson; Igor V Alabugin

doi:10.1021/jacs.5b02373

Alkenes as alkyne equivalents in radical cascades terminated by fragmentations: overcoming stereoelectronic restrictions on ring expansions for the preparation of expanded polyaromatics

J Am Chem Soc. 2015 May 20;137(19):6335-49. doi: 10.1021/jacs.5b02373. Epub 2015 May 6.

Authors

Rana K Mohamed¹, Sayantan Mondal¹, Brian Gold¹, Christopher J Evoniuk¹, Tanmay Banerjee¹, Kenneth Hanson¹, Igor V Alabugin¹

Affiliation

¹ Florida State University, Tallahassee, Florida 32306, United States.

PMID: 25906261
DOI: 10.1021/jacs.5b02373

Abstract

Chemoselective interaction of aromatic enynes with Bu3Sn radicals can be harnessed for selective cascade transformations, yielding either Sn-substituted naphthalenes or Sn-indenes. Depending on the substitution at the alkene terminus, the initial regioselective 5-exo-trig cyclizations can be intercepted at the 5-exo stage via either hydrogen atom abstraction or C-S bond scission or allowed to proceed further to the formal 6-endo products via homoallylic ring expansion. Aromatization of the latter occurs via β-C-C bond scission, which is facilitated by 2c,3e through-bond interactions, a new stereoelectronic effect in radical chemistry. The combination of formal 6-endo-trig cyclization with stereoelectronically optimized fragmentation allows the use of alkenes as synthetic equivalents of alkynes and opens a convenient route to α-Sn-substituted naphthalenes, a unique launching platform for the preparation of extended polyaromatics.