Alkenes as Alkyne Equivalents in Radical Cascades Terminated by Fragmentations: Overcoming Stereoelectronic Restrictions on Ring Expansions for the Preparation of Expanded Polyaromatics

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.

Alkyne Versus Ynamide Reactivity: Regioselective Radical Cyclization of Yne‐Ynamides

“…[9] Exposing 52 to p-t Buphenol in presence of TFAA delivered the arylation product 56 through aP ummerer-type transformation. [11,12] In principle, four sites of the two yne units of the yne-ynamide scaffold are available for attack by the thiyl radical, [13] which would eventually result in eight different products on the basis of aspecific exo/endo mode of cyclization. [11,12] In principle, four sites of the two yne units of the yne-ynamide scaffold are available for attack by the thiyl radical, [13] which would eventually result in eight different products on the basis of aspecific exo/endo mode of cyclization.…”

1,3-Dien-5-ynes: Versatile Building Blocks for the Synthesis of Carbo- and Heterocycles

“…1, 2). 109,110,111 Interestingly, a critical influence of the olefin substitution in the outcome of the cyclization is observed, as the presence of radical-stabilizing groups leads to indene adducts (see Scheme 68), whereas a terminal alkene or one substituted with a simple alkyl, such as propyl, affords dihydronaphthalenes as major products (Scheme 25, eq. 3).…”

Alkynes as Linchpins for the Additive Annulation of Biphenyls: Convergent Construction of Functionalized Fused Helicenes

“…One of the key building blocks is provided by the selective Sn-mediated cyclization of aromatic orthoenynes. [24] Sonogashira coupling followed by ICl-induced cyclization provide an iodinated precursor for the final photocyclization in yields consistently exceeding 80 %. The dehydrohalogenative photocyclization of iodinated o-terphenyls is viable and offers an efficient synthetic route to 4,5-and 5,5-fused double helicenes with differently substituted aromatic peripheries.T he crystal packing is strongly influenced by the peripheral substituents,s uggesting that their presence can be leveraged to impact photophysical and electrochemical properties of these materials.…”